Title of Invention	A NEW DYE FOR APPLICATION IN DYE LASER AND NONLINEAR OPTICS AND AS A FLUORESCENCE LIFETIME STANDARD
Abstract	Applicant"s Name: Indian Institute of Technology. ABSTRACT This invention discloses a method for preparing a novel organic dye molecule which is represented by the general fornnula wherein R1, R2 is same or different, each represents lower alkyl group and X may be aryl group. The synthesized compound shows the properties of (i) stimulated emission sufficient for it to act as a laser dye (for application in dye laser technology) on pumping at 355nm (third harmonic of Nd:YAG laser), (ii) efficient self-diffraction in the laser induced transient grating experiments at 355nm indicating to be a standard for the nonlinear optical applications in UV region and (iii) high photo stability at the 355nm for it to be a fluorescence-lifetime standard in the excitation region till 408nm and fluorescence standard in the near UV region (Excitation wavelength 310—408nm, Emission wavelength = 435nm ±30nm).

Title of Invention

A NEW DYE FOR APPLICATION IN DYE LASER AND NONLINEAR OPTICS AND AS A FLUORESCENCE LIFETIME STANDARD

Abstract

Applicant"s Name: Indian Institute of Technology. ABSTRACT This invention discloses a method for preparing a novel organic dye molecule which is represented by the general fornnula wherein R1, R2 is same or different, each represents lower alkyl group and X may be aryl group. The synthesized compound shows the properties of (i) stimulated emission sufficient for it to act as a laser dye (for application in dye laser technology) on pumping at 355nm (third harmonic of Nd:YAG laser), (ii) efficient self-diffraction in the laser induced transient grating experiments at 355nm indicating to be a standard for the nonlinear optical applications in UV region and (iii) high photo stability at the 355nm for it to be a fluorescence-lifetime standard in the excitation region till 408nm and fluorescence standard in the near UV region (Excitation wavelength 310—408nm, Emission wavelength = 435nm ±30nm).

Full Text	FIELD OF INVENTION The present invention relates to the preparation of laser dyes. More particularly, this invention relates to an organic dye molecule and its application as laser dyes with high fluorescent quantum yield and in nonlinear optics. DESCRIPTION OF PRIOR ART: A dye laser is a laser which uses an organic dye as the lasing medium generally as a liquid solution (also as solid polymer). The laser wavelengths are easily tunable within the broad gain bandwidth of the dye. Certain dye laze is a known art. There are many patents relating to such laser dyes and preparation of such laser dyes. There have been many organic dyes that have been preferred and used as laser dyes. End substituted distyryl benzenes and their derivatives have been widely used for electro-optic and solar applications. There are several molecules used as fluorescence standards as well as used as a dye laser media. In addition, there are many molecules that show nonlinear optical behaviour with varying strengths. Generally Pie (7T)-conjugated organic materials of well-defined chain length are used as better and efficient dye lasers at various wavelength, nonlinear optical material for photonic application, light-emitting diodes, field-effect transistors or sensors devices in electro optics and also potential energy transporters in one dimensional channel. US 3,781,711, US 3,891,569, US 3,976,655, US 4,026,898, US 4,202,981, US 4,453,252, US 4646309, US 4,603,422, US 4933948 and US 5,142,545 discloses various classes of laser dyes effective in different wavelength regions. US 7354694 discloses a process for producing styryl dyes which have absorption maxima at a wavelength of 400 nm or less and are sensitive to a laser beam with a wavelength of 450 nm or less. US Patent 4,510,311 discloses azolystyryl compounds for use as optical brightner and/or laser dye. US 4,371,475 discloses 1,4 bis styryl benzene derivatives and its use as optical brightening agent. US 5,001,253, discloses a fluorescent brightner consisting of bis-styryl benzene compound and its use as laser dye. However, it deals with unsymmetrical bis-styryl compounds with ester, aldehyde or cyano as functional groups. OBJECT OF THE INVENTION The field of lasers and non-linear optics Is always looking for new dye molecules with high fluorescence quantum yield and high stability with large optical non-llnearlty. The first object of the present invention Is to prepare a novel organic dye molecule for use as a laser dye. Another object of the present Invention Is to provide a better and efficient photo stable dye laser on pumping at 355nm (third harmonic of Nd:YAG laser). Yet another object of the present Invention Is to utilize the synthesized dye as nonlinear optical material with application In photonics. A further object of the present invention is to make use of the synthesized dye as a fluorescence standard In the near UV region and fluorescence lifetime standard in the excitation region till 408 nm. These and other objects, advantages of the present Invention will be apparent by the forthcoming description. BRIEF DESCRIPTION OF INVENTION A new organic dye Is prepared which is an efficient and stable medium for the dye laser in the violet region at the pump wavelength of 355nm and the other pump lasers (such as Nitrogen laser, Excimer laser and second harmonic of femto second TkSapphire laser and with other pump wavelengths within Its absorption spectrum). It is highly photo-stable at the laser pump wavelength of 355 nm. The dye exhibits efficient self-diffraction property in the transient grating experiment at 355nm which indicate the dye to be a standard for the nonlinear optical applications in UV region and also as a nonlinear optical material with application In photonics. Fluorescence lifetime for the synthesized dye Is found to be 0.95±0.04ns which is constant within 5% with concentration and hence the dye can be used as a fluorescence-lifetime standard in the excitation region till 408nm and fluorescence standard In the near UV region (Excitation wavelength 310—408nm, Emission wavelength = 435nm ±30nm). BRIEF DESCRIPTION OF DRAWINGS Fig. 1. Steady state absorption (a) and fluorescence (b) spectrum of [INVENTED DYE] , absorption (c) and fluorescence (d) spectrum of [POPOP]. Solvent is benzene and Conc.=10"^M in both the cases. Fig. 2. Concentration dependence of the fluorescence spectra disclosed in the current patent. Fig. 3 (A) Geometry of lasing dye used in the current invention Fig. 3. (B). Photograph of the lasing for dye POPOP (standard) (1mM in cyclohexane). (Left spot is the pump laser, while the right spot is the lasing spot from the dye). Fig. 3. (C). Photograph of the lasing for the current invention (1 mM in benzene) (The spot on the left is the pump laser, while that on the right is the lasing spot from the dye). The pump spot has a diffraction pattern due to the geometry of placing the dye cell and is not important for the discussion here.) Fig. 3. (D). Photograph of the lasing for the current invention (0.1 mM in benzene) (The spot on the left is the pump laser, while that on the right is the lasing spot from the dye). The dye cuvette is also seen in the photograph. Fig. 4. Comparison for the spectra obtained at low and high pump powers for the current invention. At low pump powers the fluorescence spectrum is seen. At high pump power the strong stimulated emission results in the sharp spectrum due to lasing form the current invention in the dye cell geometry. The scattered pump (355 nm) and residual second harmonic from the Nd:YAG laser is also sensed by the detector and provide the calibration for the experiment. Fig. 5. Schematic diagram of the box geometry of DFWM process. The two pump beams (P1, P2) intersect at an angle 2q. The probe beam (Pr) makes an angle q with the plane containing the pump beams. Pd is the diffracted probe beam. Fig. 6. Schematic diagram of self-diffraction geometry. P and Q are the two pump beams, 29 is the angle between them. Subscripts represent the self-diffraction orders of the beams. Fig. 7. Photo of self-diffraction of the third harmonic (355 nm) of Nd.YAG laser by the new dye dissolved in benzene. D2 (2""^ order) to D5 (5*^ order) are the diffracted spots on either side of the two pump beams (P1 and P2). The first order diffracted spot of the beam P1 overlaps with the spot of the beam P2 and, vice versa. Fig.8. Calculated third order nonlinear susceptibility (x ^^^) of the current invention in benzene. The concentration is 1x 10'^ M, Cell thickness = 1 mm. Energy density (Wp) = 8.7x10"^ J/cm^. Fig. 9. Schematic of single- (A) and two-photon (B) absorptions. Dashed line represents the virtual state. Fig. 10. Fluorescence decay profile of compound INVENTED DYE [10 "^ M ] in chloroform (Aexc = 408 nm) (Panel A). The data points are indicated by circles (o) and the thick line, continuous line shows the fit with the single exponential decay function. Panel B gives the residual for the fit. Fluorescence lifetime is 0.95 +/-0.04 ns. Fig 11. Schematic Representation of patenting of the current invention. DESCRIPTION OF INVENTION This invention relates to the preparation of compounds (1) of the general structure (1) wherein Ri, R2 is same or different, each represents lower alkyl group and X may be aryl group. Ri, Ramay be methyl or ethyl or propyl group. Suitable aryl groups may be Phenyl or naphthyl or anthracenyl or phenanathrenyl. The compound of formula (I) can be prepared by condensation of compound A r^^; RiO p R; A where W may be: A o o ® Q -PPhaBr >' \ OEt /-OEt where X is arylene moiety H H B wherein X may be aryl group, in organic solvent like DMF in the presence of K2CO3 The reaction is carried out in the temperature range of 60°C to 100°C preferably at 70°C under nitrogen atmosphere for at least 16h. The reaction was quenched with water and extracted with chloroform. The product was purified by silica gel column chromatography. The styryl benzene derivative 4, 4'-(1£, 1'E)-2, 2'-(1, 4-phenylene)bis(ethene-2,1-diyl)bis(N-methoxy-N-methyl benzamide) of formula (la) was prepared by condensation of sulphone (A') with terephthaldehyde(B')in the presence of K2CO3 in dry DMF medium for atleast 16h at70°C under nitrogen atmosphere. The reaction was quenched with water and extracted with chloroform. The product was purified by silica gel column chromatography. The molecular weight of the dye was found to be 452. CHO CHO K2CO3 DMF 70 "C A' B' 1a The starting compound (A') can be prepared by reported method from 4-methyl-N-methoxy-N-methyl benzamide by facile benzylic bromination with NBS followed by nucleophilic substitution with 2-mercaptobenzothiazole and oxidation with Na2W04.2H20 and 30% H2O2. The synthesized dye dissolves easily in chloroform. It is also soluble in benzene and toluene. The sample used for the present study was prepared by dissolving the Invented dye in benzene. The steady state absorption spectra were recorded using a UV-visible spectrometer (JASCO, V-570) in a quartz cell of 1 mm thickness. Absorption scan of solvent alone was taken to ensure absence of impurities in it in the pump wavelength region. Fluorescence spectra were recorded using a fluorescence spectrometer (JASCO, FL-6600) in the quartz cell of thickness of 10 mm. Fig.1 gives the absorption and fluorescence spectra of the dye of current invention and that of a standard laser dye 2,2'-(1,4-phenylene)bis(5-phenyl)oxazole (POPOP) in benzene. It is evident from the figure that the value of the molar extinction coefficient (E) is larger for the synthesized dye when compared to that of standard dye POPOP. Fig 2 gives the concentration dependence of the fluorescence spectra of the present invention at the excitation wavelength of 350nm. It can be seen that for the lower concentration of the dye, the spectrum shows a vibrational spectrum with a shoulder at 415nm followed by the peak at 434nm. With increase in the concentration the vibrational spectrum disappears with a subsequent shift of the peak to 439nm. This kind of behavior is usual when the absorption spectra show a tail extending to the emitting wavelengths. The first and the most important application of the invention is as lasing dye. Organic dyes fall under the category of easily accessible laser gain media in UV-visible-near IR regions. Lasing spectra from the samples were recorded using a spectrometer (Ocean Optics, S2000). Photographs of dye lasing and self diffraction experiments were taken with the help of a KODAK digital camera (Easyshare C340). Fig 3A gives the geometry of the lasing dye used in current invention. Fig 3B and 3C gives the photograph of the lasing from the standard POPOP and the synthesized dye at 1mM concentration with the third harmonic (355nm) of a pico second Nd:YAG laser (Quanta system, Himalaya series, 10Hz, 30mJ at 1064nm) as pump laser respectively. From the figures 3A, 3B and 3C it is evident that the synthesized dye has better lasing property than the standard POPOP. It is also noted that the current invention is capable of lasing even at 0.1M concentration (Fig. 3D) showing more efficiency than the standard POPOP at this wavelength range. Fig 4 depicts the fluence dependence of the emission from the new invention. Its evident from the figure that at lower values of fluence the invented dye exhibits spontaneous emission and at higher fluence it shows stimulated emission. The stimulated emission can be obtained even from the dilute concentration of 0.1 mM. More intense stimulated emission and lasing are observed at 1mM. Thus, the synthesized dye can exhibit stimulated emission that is sufficient enough, to act as a laser dye (for applications in dye laser technology) on pumping at 355 nm (third harmonic of Nd:YAG laser). The invented compound also finds useful application in non- linear optics. The synthesized dye was subjected to transient grating experiment at 355nm to check the self diffraction property of the current invention so as to be used as an efficient nonlinear medium. Degenerate four wave mixing (DFWM) is a widely used technique to measure nonlinear optical parameters. It is also used to study fast molecular relaxation processes of the electronic excited state. Solvation dynamics and relaxation phenomena have been studied by various geometries of DFWM technique by several authors. Femtosecond real-time probing of reactions and measurement of dephasing time of molecules also have been done by photon echo experiments. Laser induced transient grating (LITG) is a four wave mixing (FWM) technique. When two coherent light beams pass through a medium within the coherence time, the interference between the beams creates a sinusoidal spatial modulation of the light intensity. The radiation-matter interaction and subsequent photophysical and photochemical processes change the optical properties of the medium at the position of the bright fringe. This includes the changes due to the real and imaginary parts of the refractive index. Such a grating diffracts another probe beam while satisfying the phase matching condition. The schematic diagram of the box geometry of the degenerate four wave mixing (DFWM) process is shown in Fig. 5. The intensity of the diffracted probe beam (Pa) carries the information about the nonlinear response of the sample. The intensity of the diffracted beam with respect to the delay time of the probe beam gives the information about the excited-state relaxation of the sample. The various geometries of DFWM technique was applied for measuring nonlinear parameters of organic dyes, porphyrins and carbon nanotubes. The effect of excitation energy transfer on (i) optical nonlinear parameters of the dye-pair and (ii) excited state relaxation dynamics of the donor molecules has been studied. The effect of the metal core ion and various groups at p-pyrrole position in porphyrins and their subsequent modifications of the structure on the nonlinear optical parameters have been studied by the phase conjugation geometry of the LITG technique by Kailash and Bisht. In the self-diffraction geometry (Fig. 6) of DFWM, the first-order diffracted signal is related to the third order nonlinear susceptibility (x^'). Similarly, the s'^ diffracted order represents the 2s+1 order nonlinear susceptibility (x^^®""^^) in 2s+2 wave mixing process. In general, the measured values of the higher order nonlinear susceptibility contain contributions from both, the direct 2s+1 order process, as well as cascaded contributions from the interaction between the fundamental field and, the fields generated via lower order processes. Both the contributions diffract along the same direction. However, the second order diffracted output is smaller by 3 orders of magnitude and does not affect the obtained value of first order diffraction. The experimental set up for the DFWM was calibrated by measuring the standard sample of R6G in ethylene glycol. The calculated value of x^* from this measurement was 2.1 x 10'^° m^A/^, which is consistent with the value reported by Costela et al. Diffraction efficiency (ri) of a thin grating is measured by taking the ratio of the intensity of the diffracted signal (Iditfr) to that of the transmitted signal (Itrans) of one of the laser beams producing the grating. Theoretically, the diffraction efficiency of a sample is given by ri=- -(Aa'+k'An') 25 4 Where m = 2 (Iil2)^'^/(lil2) is the visibility of the interference fringes (here \^ and I2 are the intensities of the pump beams), k is the angular wave vector, Aa and An are the changes in the absorption coefficient and the refractive index, respectively. As seen from above equation there are two contributions to the total efficiency: (i) amplitude part and (ii) phase part. The first part corresponds to the diffraction efficiency due to changes in the absorption, whereas the later is due to the refractive index changes, Fig 7 shows the photograph of the self diffraction of the third harmonic (355nm) of Nd.YAG laser by the invented dye dissolved in benzene. From the figure it is inferred that the invented dye exhibits strong self diffraction and has optical nonlinearity. Table 1. depicts the photo physical parameters for the synthesized dye and standard POPOP. The third order optical non-linearity x^^' can be obtained from the efficiency of self diffraction. The third order optical non-linearity (x'^) of standard carbon-di-sulphide (CS2) is 5 x 10"^^ mV. Table 1 Photophysical parameters for invented dye and POPOP (d = 0.1mm, Wp = 8.7 x lO'^ J/cm^ [Invented dye] = 1 x lO'^M [POPOP] = 1 x lO'^M) Sample Aabs^ '»ems x^^^im^/y^^ Measured Calculated Invented dye 368 nm 346 nm (63±6)x10-^^ (21±3)x10"^' POPOP 358 nm 420 nm (24±4)x10^^ (16±3)xl0-'' ^'Wavelength of absorption maximum., '' Wavelength of emission maximum. "^ At 355 nm. Fig. 8 shows the variation of the X^^^ values as a function of the wavelength for the current invention. The calculated curve of fC^' shows a peak at 370 nm. It decreases on either side of the peak and for values greater than 410 nm, it vanishes. It can be seen that the sample can be used as a nonlinear optical standard for the wavelength region of approximately 300 to 400 nm. The X ^ values were also calculated as a function increasing energy density for the current invention in benzene at 355 nm. It was found that the X^^^ values remain constant at lower energy densities but decrease after about 10"^ J/ cm^. A comparison with calculated values of POPOP shows that they are smaller than the current invention. Data are given in Table 1. The theoretically simulated (calculated) values are sensitive to the molar extinction coefficient, thickness and energy density and the duration of the pump pulse. The calculate values are apparently smaller than the measured values. This is due to working at higher energy densities where errors are large due to the saturation of the diffraction efficiency. A saturated value of the diffraction efficiency will lead to smaller value of the actual efficiency as it is the ratio of the diffracted to that of the transmitted energy. Nevertheless, the plot indicates a trend of the nonlinear optical nature of the current invention as a function of the wavelength. Multiphoton absorption processes have several applications in medical science, three-dimensional spatially-resolved optical data storage, optical switching and micro fabrication. These processes require high peak powers which are available from ultrashort pulsed lasers. Multiphoton excitation of a fluorophore occurs when it absorbs two or more photons simultaneously to arrive at a higher electronically excited state. The fluorescence emission and hence, the lasing can occur as a result of the multiphoton absorption, also provided that the quantum yield of fluorescence is very high and the molecule shows lasing under single photon excitation. Fig. 9 shows the schematic diagram of single- and two-photon absorption processes. In a single-photon absorption, the molecule absorbs a single photon of energy (hv) to raise itself from its ground state to an excited state whereas in a two-photon absorption, the molecule absorbs two photons simultaneously, each of half the energy (hv/2) to attain the excited state. Two-photon absorption (TPA) is a third-order nonparametric process in which simultaneous absorption of two photons takes place via virtual state in a medium. The transition strengths for TPA and three-photon absorption (ThPA) are very weak and display an optical power-squared and power-cubed dependence, respectively. TPA cross-sections are measured in GM (after Maria Goeppert-Mayer who first predicted two-photon absorption as a single quantum event) in which 1 GM = 10"^° cm^'s. Since the emission after the multiphoton excited occurs at higher frequencies than the excitation frequencies, it is also known as fluorescence upconversion. The invented dye is also expected to exhibits multiphoton absorption and hence exhibits fluorescence upconvertion under higher concentrations and high pump fluence. The second harmonic generation (SHG) is a nonlinear optical process in which due to second order optical nonlinearity two low frequency photons (foi) are converted in to a high frequency photon (coa). It is a special case of sum frequency generation. 0)1 + ©1 = 0)2 A large number of crystals and powders possess SHG property. 6 Barium Borate and Urea are typical examples of such crystals. The invented dye in solid crystalline form is also expected to produces SHG which makes it an excellent non linear optical material. Higher order optical nonlinearities of organic dyes are useful in optical communication, switching, 3D data storage and optical limiting. Dyes are also useful as saturable absorbers. The refractive and absorptive nonlinear parameters are needed to assess a material for its response to the ultra fast pulses. Optical nonlinear studies for the cases such as that at high absorbance region (on-resonant) and at low or negligibly small absorbance regions (above-resonant) are useful in saturable absorption (SA) and excited state and multi-photon absorptions, respectively. If the working wavelength is near the peak of the absorption, the effect of SA may be observed due to high absorbance of the sample. If the working wavelength is far from the peak of the absorption i.e. in the low absorbance region, then due to the effects of multi-photon absorption or excited state absorption reverse saturation of absorption (RSA) effects may also be observed in samples. Thus the invented dye can be used as standard for non-linear optical application in UV region since it is as efficient as popular standard CS2, Hence the invented dye can be used as non- linear optical medium. Another vital application of the invented compound is as fluorescence life time standard. Fluorescence standards are organic molecule which gives fluorescence emission. The fluorescence emission spectrum should not change drastically as a function of the concentration which shows absence of formation of any other species. The fluorescence quantum yield should be high. If the fluorescence lifetime is also independent of the wavelength and concentration, these molecules are used for standardization of the measuring instruments. For the time resolved measurements, a 40 ps pulsed laser at 408 nm (Pilas, Advanced Photonics) was used as an excitation source. A fluorescence microscope was used for time-resolved measurements. The fluorescence from the samples was collected with a 20X objective lens, passed through a monochromator and detected by a photomultiplier tube (Hamamatsu, R928). A barrier filter of > 420 nm was used to block the scatter from the emission. The fluorescence decays were recorded with the help of time-correlated single photon counting technique. The decay profiles were deconvolated with the instrument response function (IRF) of the excitation pulse and were fitted with theoretical functions using standard software (TC900, Edinburgh Instruments). The goodness of the fit was estimated by reduced X ^ and weighted residuals. For a good fit, the value of iC^ was around one and the residuals were distributed evenly on both sides of the fitted curve. Fig 10 gives a typical fluorescence decay profile of the current invention measured by single photon counting technique under fluorescence microscopy. From the Fig 10 the fluorescence lifetime was found to be 0.95±0.04 ns. Table 2. Concentration dependence of fluorescence lifetime of the current invention. Concentration (mM) Fluorescence lifetime (ns) 0.05 0.97 0.10 0.99 5.00 0.91 Table 2. gives the concentration dependence of the fluorescence lifetime of the current invention. It can be inferred from the table that the fluorescence lifetime remains within 0.91-0.97 ns even with a 100 fold increase in the concentration. This indicates that there are no other species present even at higher concentration. This is required condition for the fluorescence standard (that the decay time does not change with concentration). The two procedures for estimating the quantum yield of fluorescence (Q) of the current invention are. (i). With the help of the Stickler and Berg formula By using the Stickler and Berg formula for the radiative rate constant, the quantum yield (Q) is given by Q = r _ ^ _ measured(T) K +K r calculated (T I W) J 1/ Where ri : refractive index of the solvent., u : wave number. , £ (u) : molar extinction coefficient., F(u): fluorescent intensity. The value of Q using these values was obtained to be 0.93 ± 0.02 The values calculated using the absorption and emission spectrum of Invented dye in benzene JF(V>/V = 3095±19 f£(^ = (2.60±0.01)xl0-'' r^(vVv_/,o^^A,x,.in3 :(12.6±0.1)xl0' - V « = 1.49 ±0.01 The calculated value of Kr is = (0.95±0.02) xio^ Relation between quantum yield and radiative decay constant Quantum yield(Q) = -i— = ^ = -^easuredij) K +K T„ calculatedyr) Where T= Fluorescent life time (measured) = (0.98 ± 0.1)x10"® s To = Natural life time (calculated) = (1.05±.02)xl0"^ s Kr = Radiative rate constant (calculated) = (0.95±0.02)x10^ s'"' Knr = Non-radiative rate constant (calculated) = (0.07±0.01)x10^ s"^ (ii). Relative yield calculations by using the quantum yield of POPOP The quantum yield of POPOP (0.93) was used for the relative yield calculations. The following formula is used std •cfu i'i A,. \ ^sid) ( T,KAJ ^nl^ .^std ) Where A is absorbance, F is integrated fluorescent intensity, r\ is the refractive index. The subscripts u and std are unknown (current invention) and the standard (POPOP), respectively. By this method the quantum yield of the current invention was found to be 0.96 ± 0.03. The average quantum yield was found to be 0.94. The high quantum yield of 0.94, absence of other species at high concentration, and constant fluorescence lifetime as a function of the concentration makes the synthesized dye as a fluorescence standard in the UV region (Excitation wavelength 310—408nm, Emission wavelength = 435nm ±50nm) and a fluorescence lifetime standard in the excitation region till 408nm Since, there are no popular lifetime standards in the excitation wavelength region near 400 nm, the new dye can be used as fluorescence life time standard in spectroscopy. Schematic Representation (Fig 11) is given for easy understanding. The invented dye can be used firstly and most importantly as an efficient laser dye as it exhibits stimulated emission comparatively more than the standard laser dye POPOP. Secondly, it can also find applications in non-linear optics since it possess more efficient self diffraction in laser induced transient grating experiments than the standard laser dye POPOP. Thirdly the fluorescence life time of the invented dye does not vary with concentration and thus makes the new dye a potential fluorescence life time standard. Example 1. (0.2 g, 0.53129 mmol) of sulfone (A'), (0.034 g, 0.2530 mmol) of terephthalaldehyde(B') and (0.441 g, 3.1878 mmol) of anhydrous K2CO3 was mixed with(9 mL/mmol of sulfone A) dry DMF and heated at 70°C under N2 atmosphere. The reaction course was monitored by TLC (4:6; EA/Hexane). After 16 h, the reaction mixture was quenched with water and extracted with CHCI3 thoroughly. The crude product was purified by silica-gel column chromatography. Isolated yield of the product I, E-E isomer was 57.3% (0.066 g). The structure of the compound has been confirmed by ^H-NMR (400 MHz), ^^C-NMR (100MHz), DEPT, UV and IR studies. 4,4'-(1 E, 1 'E)-2,2'-(1,4-phenylene)bis(ethene-2,1 -diyl)bis(N-methoxy-N-methyl benz-amide):Yield: 57.3%, Rf = 0.125, (hexane-Ethylacetate, 6:4), yellow crystalline solid. ^H NMR [CDCI3, 400 MHz]: 8 = 3.37 (s, 6 H), 3.57 (s, 6 H), 7.18 (d, 2 H, J = 16.4 Hz), 7.13 (d, 2 H, J = 16.4Hz), 7.52 (s, 4 H), 7.54 (d, 4 H, J = 8.4 Hz), 7.71 (d, 4 H, J = 8.4 Hz). ■'^C NMR [CDCI3, 100 MHz]: 6 = 33.9, 61.2, 126.2, 127.2, 128.0, 129.0, 129.9, 133.0, 136.8, 139.7, 169.6. IR (CH2CI2): 1378, 1420, 1623 cm ^ References; [1]. Burroughes. J. H, Bradley. D. D. C, Brown. A. R, Marks. R. N, Mackay. K, Friend. R. H. Burns. P. L, Holmes. A. B. Nature (London) 347 (1990) 539. [2]. Gustafsson. H, Cao. Y. Treacy. G. M. Klavetter. F, Colanerl. N, Heeger. A. J. Nature (London) 357 (1992) 477. [3]. Halls. J. J. M. , Walsh. C. A., Greenham N. C. , Marseglia. E. A. , Friend. R. H. , Moratti. S. C. , Holmes. A. B., Nature (London) 376 (1995) 498. [4]. Friend. R. H., Gymer. R. W., Holmes. A. B., Burroughes. J. H., Marks. R. N., Taliani. C, Bradley. D. D. C, Dos Santos. D. A., Bredas. J. L., Logdiund. M.and Salaneck W. R. Nature (London) 397 (1999) 121. [5]. Sancho-Garcla.J.C, Bredas. J.L., Beljonne. D., Cornil. J, Martinez-Alvarez. R., Hanack. M., Poulsen. L., Gierschner.J., Mack. H.G., Egelhaaf. H.J., and Oelkrug. D., J. Phys. Chem. B 109,2005 , 4872-4880. [6]. Fleming. G. R., Chemical applications of ultrafast spectroscopy, Oxford Univ. Press, 1986, p. 95. [ 7]. Brackmann. U., Lambdachrome Laser Dyes data sheets , Lambda Physik, Germany, 1986. [8]. Sailaja. R., Bisht. P. B., Singh. G. P., Bindra. K. S. and Oak. S. M., Optics Communications 277 (2007) 433-439. [9] Sancho-Garcia. J. C., Poulsen. L., Gierschner. J., Martinez-Alvarez. R., Henneblcq. E.,Kanack. M., Egelhaff. H.-J., Oelkrig. D., Beljonne. D., Bredas. J.-L, Cornil. J. Adv. Mater 16 (2004) 1193-1197. [10] Balasubramaniam. S., Annamalai. S., Aidhen. I. S., Synlett 18, (2007), 2841- 2846. [11] Moog. R. S., Ediger. M. D., Boxer. S. G., Payer. M. D., J. Phys. Chem. 86 (1982) 4694. [12] Rajesh. R. J., Bisht. P. B., Chem. Phys. Lett. 357 (2002) 420. [13] Rajesh. R. J., Jose. M., Bisht. P. B., Indian J. Phys. B 76 (2002) 737. [14] Jena. K. C, Bisht. P. B., Chem. Phys. 314 (2005) 179. [15]. Jena. K. C, Bisht. P. B., Optics Communications, 273 (2007) 153-158 [16]. Eichler. H. J., Gunter. P. and Pohl. D. W., (1986), "Laser Induced Transient Grating", Springer-Verlag, Berlin. [17]. Gumy. J. C, Nicolet. O., and Vauthey. E., (1999), "Investigation of the solvation ynamics of an organic dye in polar solvents using the femtosecond transient grating technique", J. Phys. Chem. A, 103, 10737-10743. [18]. Rajesh. R. J., and Bisht. P. B., "Theoretical and experimental studies on photophysical parameters of N, N' - bis(2,5,-di-tert-butylphenyl)-3,4,9-10- perylenebis (dicarboximide) (DBPI) by using transient grating technique", Chem. Phys. Lett, 357, (2002), 420-425. [19]. Bragg. A. E., Verlet. J. R. R., Kammrath. A., Cheshnovsky. O., Neumark. D. M., (2005), "Electronic relaxation dynamics of water cluster anions", J. Am. Chem. Soc, [20]. Huxter. V. M., and Scholes. G. D., , "Nonlinear optical approach to multiexciton relaxtion dynamics in quantum dots", J. Chem. Phys., 125, (2006) 144716-12. [21]. Brown E. J., Zhang. Q,. and Dantus. M., , "Femtosecond transient grating techniques: Population and coherence dynamics involving ground and excited states", J. Chem. Phys., 110, (1999)5772-5788. [22] Zewail A. H., , "Femtochemistry: atomic-scale dynamics of the chemical bond", J. Phys. Chem. A, 104, (2000) 5660-5694. [23]. Zimdars. D., Tokmakoff. A., Chen. S., Greenfield. E. R., Fayer. M. D., Smith. T. Land Schwettman. H. A., , "Picosecond infrared vibrational photon echoes in a liquid and glass using a free electron laser", Phys. Rev. Lett, 70, (1993) 2718-2721. [24]. Thompson. D. E., Merchant. K. A., and Fayer. M. D., "Solute-solvent interaction; two-dimensional ultrafast infrared vibrational echo experiment", Chem. Phys. Lett, 340, (2001), 267-274. [25]. Acioli. L. H., Gomes. A. S. L. and Leite. J. R. R., , "Measurement of higher-order optical nonlinear susceptibilities in semiconductor-doped glasses", Appl. Phys. Lett., 53,(1988)1788-1790. [26] Costela. A., Figuera .J. M., and Florido. F., , "Thermal and acoustic effets in optical phase conjugation in R6G solutions with nanosecond pulses". Opt Commun., 100, (1993)536-544. [27]. Stickler. S.J. and Berg. R.A. J. Phys. 37 (1962) 814-822. [28]. Berlman. B., Handbook of Fluorescence Spectra of Aromatic Molecules, Academic Press, 1971. WE CLAIM: 1. A compound exhibiting laser dye properties having a general formula (I), wherein R1, R2 is same or different, each represents lower alkyl group and X is an aryl moiety 2. The compound as claimed in claim 1 wherein R1 and R2 being same, representing the lower alkyl group. 3. The compound as claimed in claim 1 wherein the lower alkyl group contains 1-4 carbon atoms. 4. The compound as claimed in claim 1 wherein the said lower alkyl group is methyl group. 5. The compound as claimed in claim 1 wherein the said aryl moiety is phenylene(1,4) residue. 6. The compound as claimed in claim 1 which is 4, 4'-(1E, 1'E)-2, 2'-(1, 4- phenylene)bis(ethene-2,1-diyl)bis(N-methoxy-N-methyl benzamide) of formula (la). 7. A method of preparing the compound as claimed in claiml comprising the steps of a. preparing a reaction mixture comprising of compound of formula (A), aromatic di-aldehyde of formula (B), anhydrous potassium carbonate and dry dimethyl formamide (DMF); d. purifying the said reaction product with silica-gel column chromatography. 10. A compound prepared by the method claimed in claim 9. 11. A laser dye medium essentially consisting of the compound of claim 1, homogenized in a non-polar solvent exhibiting an excellent lasing property in the violet region at a pump wavelength ranging from 300nm to 410nm. 12. A laser dye medium essentially consisting of the compound of claim 6, homogenized in a non-polar solvent exhibiting an excellent lasing property in the violet region at a pump wavelength ranging from 300nm to 410nm preferably at 355nm. 13. The laser dye medium as claimed in claim 11 and 12 wherein the said non- polar solvent is benzene or chloroform. 14. The laser dye medium of claim 13 wherein the dye concentration ranges from 0.05-5.00 mM. 15. A non-linear optically active dye medium at third harmonic of Nd:YAG and second harmonic of Ti-sapphire laser comprising the compound as claimed in claim 1. 16. A non-linear optically active dye medium at third harmonic of Nd:YAG and Second harmonic of Ti-sapphire laser comprising the compound as claimed in claim 6. 17. A fluorescence life-time standard comprising the compound as claimed in claim 1, at a pump wavelength of 300 nm to 410 nm. 18. A fluorescence life-time standard comprising the compound as claimed in claim 6, at a pump wavelength of 300 nm to 410 nm. 19. An up conversion fluorescence emitting, lasing dye medium formed with the compound claimed in claiml on multi photon excitation. 20. An up conversion fluorescence emitting, lasing dye medium formed with the compound claimed in claims on multi photon excitation. 21. An up converted second harmonic Ti sapphire and Nd:Yag laser generating, dye medium formed with the compound claimed in claiml on multi photon excitation. 22. An up converted second harmonic Ti sapphire and Nd:Yag laser generating, dye medium formed with the compound claimed in claim 6 on multi photon excitation. 23. An optical limiter comprising the compound claimed in claim 1. 24. An optical limiter comprising the compound claimed in claim 6. 25. A saturable absorber comprising the compound claimed in claim 1. 26. A saturable absorber comprising the compound claimed in claim 6. 27. A reverse saturable absorber comprising the compound claimed in claim 1. 28. A reverse saturable absorber comprising the compound claimed in claim 6.

Full Text

FIELD OF INVENTION
The present invention relates to the preparation of laser dyes. More particularly, this invention relates to an organic dye molecule and its application as laser dyes with high fluorescent quantum yield and in nonlinear optics.
DESCRIPTION OF PRIOR ART:
A dye laser is a laser which uses an organic dye as the lasing medium generally as a liquid solution (also as solid polymer). The laser wavelengths are easily tunable within the broad gain bandwidth of the dye. Certain dye laze is a known art. There are many patents relating to such laser dyes and preparation of such laser dyes. There have been many organic dyes that have been preferred and used as laser dyes. End substituted distyryl benzenes and their derivatives have been widely used for electro-optic and solar applications. There are several molecules used as fluorescence standards as well as used as a dye laser media. In addition, there are many molecules that show nonlinear optical behaviour with varying strengths. Generally Pie (7T)-conjugated organic materials of well-defined chain length are used as better and efficient dye lasers at various wavelength, nonlinear optical material for photonic application, light-emitting diodes, field-effect transistors or sensors devices in electro optics and also potential energy transporters in one dimensional channel.
US 3,781,711, US 3,891,569, US 3,976,655, US 4,026,898, US 4,202,981, US 4,453,252, US 4646309, US 4,603,422, US 4933948 and US 5,142,545 discloses various classes of laser dyes effective in different wavelength regions.
US 7354694 discloses a process for producing styryl dyes which have absorption maxima at a wavelength of 400 nm or less and are sensitive to a laser beam with a wavelength of 450 nm or less. US Patent 4,510,311 discloses azolystyryl compounds for use as optical brightner and/or laser dye. US 4,371,475 discloses 1,4 bis styryl benzene derivatives and its use as optical brightening agent. US 5,001,253, discloses a fluorescent brightner consisting of bis-styryl benzene compound and its use as laser dye. However, it deals with unsymmetrical bis-styryl compounds with ester, aldehyde or cyano as functional groups.

OBJECT OF THE INVENTION
The field of lasers and non-linear optics Is always looking for new dye molecules with high fluorescence quantum yield and high stability with large optical non-llnearlty.
The first object of the present invention Is to prepare a novel organic dye molecule for use as a laser dye.
Another object of the present Invention Is to provide a better and efficient photo stable dye laser on pumping at 355nm (third harmonic of Nd:YAG laser).
Yet another object of the present Invention Is to utilize the synthesized dye as nonlinear optical material with application In photonics.
A further object of the present invention is to make use of the synthesized dye as a fluorescence standard In the near UV region and fluorescence lifetime standard in the excitation region till 408 nm.
These and other objects, advantages of the present Invention will be apparent by the forthcoming description.
BRIEF DESCRIPTION OF INVENTION
A new organic dye Is prepared which is an efficient and stable medium for the dye laser in the violet region at the pump wavelength of 355nm and the other pump lasers (such as Nitrogen laser, Excimer laser and second harmonic of femto second TkSapphire laser and with other pump wavelengths within Its absorption spectrum). It is highly photo-stable at the laser pump wavelength of 355 nm. The dye exhibits efficient self-diffraction property in the transient grating experiment at 355nm which indicate the dye to be a standard for the nonlinear optical applications in UV region and also as a nonlinear optical material with application In photonics. Fluorescence lifetime for the synthesized dye Is found to be 0.95±0.04ns which is constant within 5% with concentration and hence the dye can be used as a fluorescence-lifetime standard in the excitation region till 408nm and fluorescence standard In the near UV region (Excitation wavelength 310—408nm, Emission wavelength = 435nm ±30nm).

BRIEF DESCRIPTION OF DRAWINGS
Fig. 1. Steady state absorption (a) and fluorescence (b) spectrum of [INVENTED
DYE] , absorption (c) and fluorescence (d) spectrum of [POPOP]. Solvent is
benzene and Conc.=10"^M in both the cases.
Fig. 2. Concentration dependence of the fluorescence spectra disclosed in the
current patent.
Fig. 3 (A) Geometry of lasing dye used in the current invention
Fig. 3. (B). Photograph of the lasing for dye POPOP (standard) (1mM in
cyclohexane). (Left spot is the pump laser, while the right spot is the lasing spot from
the dye).
Fig. 3. (C). Photograph of the lasing for the current invention (1 mM in benzene)
(The spot on the left is the pump laser, while that on the right is the lasing spot from
the dye). The pump spot has a diffraction pattern due to the geometry of placing the
dye cell and is not important for the discussion here.)
Fig. 3. (D). Photograph of the lasing for the current invention (0.1 mM in benzene)
(The spot on the left is the pump laser, while that on the right is the lasing spot from
the dye). The dye cuvette is also seen in the photograph.
Fig. 4. Comparison for the spectra obtained at low and high pump powers for the
current invention. At low pump powers the fluorescence spectrum is seen. At high
pump power the strong stimulated emission results in the sharp spectrum due to
lasing form the current invention in the dye cell geometry. The scattered pump (355
nm) and residual second harmonic from the Nd:YAG laser is also sensed by the
detector and provide the calibration for the experiment.
Fig. 5. Schematic diagram of the box geometry of DFWM process. The two pump
beams (P1, P2) intersect at an angle 2q. The probe beam (Pr) makes an angle q
with the plane containing the pump beams. Pd is the diffracted probe beam.
Fig. 6. Schematic diagram of self-diffraction geometry. P and Q are the two pump
beams, 29 is the angle between them. Subscripts represent the self-diffraction
orders of the beams.
Fig. 7. Photo of self-diffraction of the third harmonic (355 nm) of Nd.YAG laser by the
new dye dissolved in benzene. D2 (2""^ order) to D5 (5**^ order) are the diffracted

* spots on either side of the two pump beams (P1 and P2). The first order diffracted spot of the beam P1 overlaps with the spot of the beam P2 and, vice versa. Fig.8. Calculated third order nonlinear susceptibility (x ^^^) of the current invention in benzene. The concentration is 1x 10'^ M, Cell thickness = 1 mm. Energy density (Wp) = 8.7x10"^ J/cm^.
Fig. 9. Schematic of single- (A) and two-photon (B) absorptions. Dashed line represents the virtual state.
Fig. 10. Fluorescence decay profile of compound INVENTED DYE [10 "^ M ] in chloroform (Aexc = 408 nm) (Panel A). The data points are indicated by circles (o) and the thick line, continuous line shows the fit with the single exponential decay function. Panel B gives the residual for the fit. Fluorescence lifetime is 0.95 +/-0.04 ns. Fig 11. Schematic Representation of patenting of the current invention.
DESCRIPTION OF INVENTION
This invention relates to the preparation of compounds (1) of the general structure

(1)
wherein Ri, R2 is same or different, each represents lower alkyl group and X may be aryl group. Ri, Ramay be methyl or ethyl or propyl group. Suitable aryl groups may be Phenyl or naphthyl or anthracenyl or phenanathrenyl.
The compound of formula (I) can be prepared by condensation of compound A

r^^;

RiO p
R; A

where W may be:

A
o o

® Q -PPhaBr >'

\ OEt /-OEt

where X is arylene moiety
H H
B wherein X may be aryl group, in organic solvent like DMF in the presence of K2CO3 The reaction is carried out in the temperature range of 60°C to 100°C preferably at 70°C under nitrogen atmosphere for at least 16h. The reaction was quenched with water and extracted with chloroform. The product was purified by silica gel column chromatography.
The styryl benzene derivative 4, 4'-(1£, 1'E)-2, 2'-(1, 4-phenylene)bis(ethene-2,1-diyl)bis(N-methoxy-N-methyl benzamide) of formula (la) was prepared by condensation of sulphone (A') with terephthaldehyde(B')in the presence of K2CO3 in dry DMF medium for atleast 16h at70°C under nitrogen atmosphere. The reaction was quenched with water and extracted with chloroform. The product was purified by silica gel column chromatography. The molecular weight of the dye was found to be 452.

CHO
CHO

K2CO3
DMF 70 "C

A' B' 1a
The starting compound (A') can be prepared by reported method from 4-methyl-N-methoxy-N-methyl benzamide by facile benzylic bromination with NBS followed by

nucleophilic substitution with 2-mercaptobenzothiazole and oxidation with Na2W04.2H20 and 30% H2O2.
The synthesized dye dissolves easily in chloroform. It is also soluble in benzene and toluene. The sample used for the present study was prepared by dissolving the Invented dye in benzene. The steady state absorption spectra were recorded using a UV-visible spectrometer (JASCO, V-570) in a quartz cell of 1 mm thickness. Absorption scan of solvent alone was taken to ensure absence of impurities in it in the pump wavelength region. Fluorescence spectra were recorded using a fluorescence spectrometer (JASCO, FL-6600) in the quartz cell of thickness of 10 mm.
Fig.1 gives the absorption and fluorescence spectra of the dye of current invention and that of a standard laser dye 2,2'-(1,4-phenylene)bis(5-phenyl)oxazole (POPOP) in benzene. It is evident from the figure that the value of the molar extinction coefficient (E) is larger for the synthesized dye when compared to that of standard dye POPOP.
Fig 2 gives the concentration dependence of the fluorescence spectra of the present invention at the excitation wavelength of 350nm. It can be seen that for the lower concentration of the dye, the spectrum shows a vibrational spectrum with a shoulder at 415nm followed by the peak at 434nm. With increase in the concentration the vibrational spectrum disappears with a subsequent shift of the peak to 439nm. This kind of behavior is usual when the absorption spectra show a tail extending to the emitting wavelengths.
The first and the most important application of the invention is as lasing dye. Organic dyes fall under the category of easily accessible laser gain media in UV-visible-near IR regions. Lasing spectra from the samples were recorded using a spectrometer (Ocean Optics, S2000). Photographs of dye lasing and self diffraction experiments were taken with the help of a KODAK digital camera (Easyshare C340).
Fig 3A gives the geometry of the lasing dye used in current invention. Fig 3B and 3C gives the photograph of the lasing from the standard POPOP and the synthesized dye at 1mM concentration with the third harmonic (355nm) of a pico second Nd:YAG laser (Quanta system, Himalaya series, 10Hz, 30mJ at 1064nm) as pump laser

respectively. From the figures 3A, 3B and 3C it is evident that the synthesized dye has better lasing property than the standard POPOP. It is also noted that the current invention is capable of lasing even at 0.1M concentration (Fig. 3D) showing more efficiency than the standard POPOP at this wavelength range.
Fig 4 depicts the fluence dependence of the emission from the new invention. Its evident from the figure that at lower values of fluence the invented dye exhibits spontaneous emission and at higher fluence it shows stimulated emission. The stimulated emission can be obtained even from the dilute concentration of 0.1 mM. More intense stimulated emission and lasing are observed at 1mM.
Thus, the synthesized dye can exhibit stimulated emission that is sufficient enough, to act as a laser dye (for applications in dye laser technology) on pumping at 355 nm (third harmonic of Nd:YAG laser).
The invented compound also finds useful application in non- linear optics. The synthesized dye was subjected to transient grating experiment at 355nm to check the self diffraction property of the current invention so as to be used as an efficient nonlinear medium.
Degenerate four wave mixing (DFWM) is a widely used technique to measure nonlinear optical parameters. It is also used to study fast molecular relaxation processes of the electronic excited state. Solvation dynamics and relaxation phenomena have been studied by various geometries of DFWM technique by several authors. Femtosecond real-time probing of reactions and measurement of dephasing time of molecules also have been done by photon echo experiments.
Laser induced transient grating (LITG) is a four wave mixing (FWM) technique. When two coherent light beams pass through a medium within the coherence time, the interference between the beams creates a sinusoidal spatial modulation of the light intensity. The radiation-matter interaction and subsequent photophysical and photochemical processes change the optical properties of the medium at the position of the bright fringe. This includes the changes due to the real and imaginary parts of the refractive index. Such a grating diffracts another probe beam while satisfying the

phase matching condition. The schematic diagram of the box geometry of the degenerate four wave mixing (DFWM) process is shown in Fig. 5. The intensity of the diffracted probe beam (Pa) carries the information about the nonlinear response of the sample. The intensity of the diffracted beam with respect to the delay time of the probe beam gives the information about the excited-state relaxation of the sample.
The various geometries of DFWM technique was applied for measuring nonlinear parameters of organic dyes, porphyrins and carbon nanotubes. The effect of excitation energy transfer on (i) optical nonlinear parameters of the dye-pair and (ii) excited state relaxation dynamics of the donor molecules has been studied. The effect of the metal core ion and various groups at p-pyrrole position in porphyrins and their subsequent modifications of the structure on the nonlinear optical parameters have been studied by the phase conjugation geometry of the LITG technique by Kailash and Bisht.
In the self-diffraction geometry (Fig. 6) of DFWM, the first-order diffracted signal is related to the third order nonlinear susceptibility (x*^'). Similarly, the s'^ diffracted order represents the 2s+1 order nonlinear susceptibility (x^^®""^^) in 2s+2 wave mixing process. In general, the measured values of the higher order nonlinear susceptibility contain contributions from both, the direct 2s+1 order process, as well as cascaded contributions from the interaction between the fundamental field and, the fields generated via lower order processes. Both the contributions diffract along the same direction. However, the second order diffracted output is smaller by 3 orders of magnitude and does not affect the obtained value of first order diffraction.
The experimental set up for the DFWM was calibrated by measuring the standard sample of R6G in ethylene glycol. The calculated value of x*^* from this measurement was 2.1 x 10'^° m^A/^, which is consistent with the value reported by Costela et al.
Diffraction efficiency (ri) of a thin grating is measured by taking the ratio of the intensity of the diffracted signal (Iditfr) to that of the transmitted signal (Itrans) of one of

the laser beams producing the grating. Theoretically, the diffraction efficiency of a sample is given by

ri=-
-(Aa'+k'An')
25 4
Where m = 2 (Iil2)^'^/(lil2) is the visibility of the interference fringes (here \^ and I2 are the intensities of the pump beams), k is the angular wave vector, Aa and An are the changes in the absorption coefficient and the refractive index, respectively.
As seen from above equation there are two contributions to the total efficiency: (i) amplitude part and (ii) phase part. The first part corresponds to the diffraction efficiency due to changes in the absorption, whereas the later is due to the refractive index changes,
Fig 7 shows the photograph of the self diffraction of the third harmonic (355nm) of Nd.YAG laser by the invented dye dissolved in benzene. From the figure it is inferred that the invented dye exhibits strong self diffraction and has optical nonlinearity.
Table 1. depicts the photo physical parameters for the synthesized dye and standard POPOP. The third order optical non-linearity x^^' can be obtained from the efficiency of self diffraction. The third order optical non-linearity (x'^*) of standard carbon-di-sulphide (CS2) is 5 x 10"^^ mV.
Table 1
Photophysical parameters for invented dye and POPOP
(d = 0.1mm, Wp = 8.7 x lO'^ J/cm^ [Invented dye] = 1 x lO'^M [POPOP] = 1 x lO'^M)

Sample Aabs^ '»ems x^^^im^/y^^

Measured Calculated
Invented dye 368 nm 346 nm (63±6)x10-^^ (21±3)x10"^'
POPOP 358 nm 420 nm (24±4)x10^^ (16±3)xl0-''

^'Wavelength of absorption maximum., '' Wavelength of emission maximum. "^ At 355 nm.
Fig. 8 shows the variation of the X^^^ values as a function of the wavelength for the current invention. The calculated curve of fC*^' shows a peak at 370 nm. It decreases on either side of the peak and for values greater than 410 nm, it vanishes. It can be seen that the sample can be used as a nonlinear optical standard for the wavelength region of approximately 300 to 400 nm. The X *^* values were also calculated as a function increasing energy density for the current invention in benzene at 355 nm. It was found that the X^^^ values remain constant at lower energy densities but decrease after about 10"^ J/ cm^. A comparison with calculated values of POPOP shows that they are smaller than the current invention. Data are given in Table 1.
The theoretically simulated (calculated) values are sensitive to the molar extinction coefficient, thickness and energy density and the duration of the pump pulse. The calculate values are apparently smaller than the measured values. This is due to working at higher energy densities where errors are large due to the saturation of the diffraction efficiency. A saturated value of the diffraction efficiency will lead to smaller value of the actual efficiency as it is the ratio of the diffracted to that of the transmitted energy. Nevertheless, the plot indicates a trend of the nonlinear optical nature of the current invention as a function of the wavelength.
Multiphoton absorption processes have several applications in medical science, three-dimensional spatially-resolved optical data storage, optical switching and micro fabrication. These processes require high peak powers which are available from ultrashort pulsed lasers. Multiphoton excitation of a fluorophore occurs when it absorbs two or more photons simultaneously to arrive at a higher electronically excited state. The fluorescence emission and hence, the lasing can occur as a result of the multiphoton absorption, also provided that the quantum yield of fluorescence is very high and the molecule shows lasing under single photon excitation.
Fig. 9 shows the schematic diagram of single- and two-photon absorption processes. In a single-photon absorption, the molecule absorbs a single photon of

energy (hv) to raise itself from its ground state to an excited state whereas in a two-photon absorption, the molecule absorbs two photons simultaneously, each of half the energy (hv/2) to attain the excited state.
Two-photon absorption (TPA) is a third-order nonparametric process in which simultaneous absorption of two photons takes place via virtual state in a medium. The transition strengths for TPA and three-photon absorption (ThPA) are very weak and display an optical power-squared and power-cubed dependence, respectively. TPA cross-sections are measured in GM (after Maria Goeppert-Mayer who first predicted two-photon absorption as a single quantum event) in which 1 GM = 10"^° cm^'s. Since the emission after the multiphoton excited occurs at higher frequencies than the excitation frequencies, it is also known as fluorescence upconversion. The invented dye is also expected to exhibits multiphoton absorption and hence exhibits fluorescence upconvertion under higher concentrations and high pump fluence.
The second harmonic generation (SHG) is a nonlinear optical process in which due to second order optical nonlinearity two low frequency photons (foi) are converted in to a high frequency photon (coa). It is a special case of sum frequency generation.
0)1 + ©1 = 0)2
A large number of crystals and powders possess SHG property. 6 Barium Borate and Urea are typical examples of such crystals. The invented dye in solid crystalline form is also expected to produces SHG which makes it an excellent non linear optical material.
Higher order optical nonlinearities of organic dyes are useful in optical communication, switching, 3D data storage and optical limiting. Dyes are also useful as saturable absorbers. The refractive and absorptive nonlinear parameters are needed to assess a material for its response to the ultra fast pulses.
Optical nonlinear studies for the cases such as that at high absorbance region (on-resonant) and at low or negligibly small absorbance regions (above-resonant) are useful in saturable absorption (SA) and excited state and multi-photon absorptions, respectively.

If the working wavelength is near the peak of the absorption, the effect of SA may be observed due to high absorbance of the sample. If the working wavelength is far from the peak of the absorption i.e. in the low absorbance region, then due to the effects of multi-photon absorption or excited state absorption reverse saturation of absorption (RSA) effects may also be observed in samples.
Thus the invented dye can be used as standard for non-linear optical application in UV region since it is as efficient as popular standard CS2, Hence the invented dye can be used as non- linear optical medium.
Another vital application of the invented compound is as fluorescence life time standard. Fluorescence standards are organic molecule which gives fluorescence emission. The fluorescence emission spectrum should not change drastically as a function of the concentration which shows absence of formation of any other species. The fluorescence quantum yield should be high. If the fluorescence lifetime is also independent of the wavelength and concentration, these molecules are used for standardization of the measuring instruments.
For the time resolved measurements, a 40 ps pulsed laser at 408 nm (Pilas, Advanced Photonics) was used as an excitation source. A fluorescence microscope was used for time-resolved measurements. The fluorescence from the samples was collected with a 20X objective lens, passed through a monochromator and detected by a photomultiplier tube (Hamamatsu, R928). A barrier filter of > 420 nm was used to block the scatter from the emission. The fluorescence decays were recorded with the help of time-correlated single photon counting technique. The decay profiles were deconvolated with the instrument response function (IRF) of the excitation pulse and were fitted with theoretical functions using standard software (TC900, Edinburgh Instruments). The goodness of the fit was estimated by reduced X ^ and weighted residuals. For a good fit, the value of iC^ was around one and the residuals were distributed evenly on both sides of the fitted curve.

Fig 10 gives a typical fluorescence decay profile of the current invention measured by single photon counting technique under fluorescence microscopy. From the Fig 10 the fluorescence lifetime was found to be 0.95±0.04 ns.
Table 2.
Concentration dependence of fluorescence lifetime of the current invention.

Concentration (mM) Fluorescence lifetime (ns)
0.05 0.97
0.10 0.99
5.00 0.91
Table 2. gives the concentration dependence of the fluorescence lifetime of the current invention. It can be inferred from the table that the fluorescence lifetime remains within 0.91-0.97 ns even with a 100 fold increase in the concentration. This indicates that there are no other species present even at higher concentration. This is required condition for the fluorescence standard (that the decay time does not change with concentration).

The two procedures for estimating the quantum yield of fluorescence (Q) of the current invention are.
(i). With the help of the Stickler and Berg formula
By using the Stickler and Berg formula for the radiative rate constant, the quantum yield (Q) is given by
Q =
r _ ^ _ measured(T)
K +K r calculated (T
I

W)

J 1/

Where ri : refractive index of the solvent., u : wave number. , £ (u) : molar extinction
coefficient., F(u): fluorescent intensity.
The value of Q using these values was obtained to be 0.93 ± 0.02

The values calculated using the absorption and emission spectrum of Invented dye in
benzene
JF(V>/V = 3095±19
f£(^ = (2.60±0.01)xl0-''
r^(vVv_/,o^^A,x,.in3
:(12.6±0.1)xl0'
- V
« = 1.49 ±0.01
The calculated value of Kr is = (0.95±0.02) xio^
Relation between quantum yield and radiative decay constant
Quantum yield(Q) = -i— = ^ = -^easuredij) K +K T„ calculatedyr)
Where
T= Fluorescent life time (measured) = (0.98 ± 0.1)x10"® s
To = Natural life time (calculated) = (1.05±.02)xl0"^ s
Kr = Radiative rate constant (calculated) = (0.95±0.02)x10^ s'"' Knr = Non-radiative rate constant (calculated) = (0.07±0.01)x10^ s"^
(ii). Relative yield calculations by using the quantum yield of POPOP
The quantum yield of POPOP (0.93) was used for the relative yield calculations. The
following formula is used

std
•cfu i'i

A,.
\ ^sid)

( T,KAJ

^nl^
.^std )

Where A is absorbance, F is integrated fluorescent intensity, r\ is the refractive index. The subscripts u and std are unknown (current invention) and the standard (POPOP), respectively. By this method the quantum yield of the current invention was found to be 0.96 ± 0.03. The average quantum yield was found to be 0.94.

The high quantum yield of 0.94, absence of other species at high concentration, and constant fluorescence lifetime as a function of the concentration makes the synthesized dye as a fluorescence standard in the UV region (Excitation wavelength 310—408nm, Emission wavelength = 435nm ±50nm) and a fluorescence lifetime standard in the excitation region till 408nm Since, there are no popular lifetime standards in the excitation wavelength region near 400 nm, the new dye can be used as fluorescence life time standard in spectroscopy.
Schematic Representation (Fig 11) is given for easy understanding. The invented dye can be used firstly and most importantly as an efficient laser dye as it exhibits stimulated emission comparatively more than the standard laser dye POPOP. Secondly, it can also find applications in non-linear optics since it possess more efficient self diffraction in laser induced transient grating experiments than the standard laser dye POPOP. Thirdly the fluorescence life time of the invented dye does not vary with concentration and thus makes the new dye a potential fluorescence life time standard.
Example 1.
(0.2 g, 0.53129 mmol) of sulfone (A'), (0.034 g, 0.2530 mmol) of terephthalaldehyde(B') and (0.441 g, 3.1878 mmol) of anhydrous K2CO3 was mixed with(9 mL/mmol of sulfone A) dry DMF and heated at 70°C under N2 atmosphere. The reaction course was monitored by TLC (4:6; EA/Hexane). After 16 h, the reaction mixture was quenched with water and extracted with CHCI3 thoroughly. The crude product was purified by silica-gel column chromatography. Isolated yield of the product I, E-E isomer was 57.3% (0.066 g).
The structure of the compound has been confirmed by ^H-NMR (400 MHz), ^^C-NMR (100MHz), DEPT, UV and IR studies.
4,4'-(1 E, 1 'E)-2,2'-(1,4-phenylene)bis(ethene-2,1 -diyl)bis(N-methoxy-N-methyl benz-amide):Yield: 57.3%, Rf = 0.125, (hexane-Ethylacetate, 6:4), yellow crystalline solid. ^H NMR [CDCI3, 400 MHz]: 8 = 3.37 (s, 6 H), 3.57 (s, 6 H), 7.18 (d, 2 H, J = 16.4 Hz), 7.13 (d, 2 H, J = 16.4Hz), 7.52 (s, 4 H), 7.54 (d, 4 H, J = 8.4 Hz), 7.71 (d, 4 H, J = 8.4 Hz). ■'^C NMR [CDCI3, 100 MHz]: 6 = 33.9, 61.2, 126.2, 127.2, 128.0, 129.0, 129.9, 133.0, 136.8, 139.7, 169.6. IR (CH2CI2): 1378, 1420, 1623 cm ^

References;
[1]. Burroughes. J. H, Bradley. D. D. C, Brown. A. R, Marks. R. N, Mackay. K, Friend.
R. H. Burns. P. L, Holmes. A. B. Nature (London) 347 (1990) 539. [2]. Gustafsson. H, Cao. Y. Treacy. G. M. Klavetter. F, Colanerl. N, Heeger. A. J.
Nature (London) 357 (1992) 477. [3]. Halls. J. J. M. , Walsh. C. A., Greenham N. C. , Marseglia. E. A. , Friend. R. H. ,
Moratti. S. C. , Holmes. A. B., Nature (London) 376 (1995) 498. [4]. Friend. R. H., Gymer. R. W., Holmes. A. B., Burroughes. J. H., Marks. R. N.,
Taliani. C, Bradley. D. D. C, Dos Santos. D. A., Bredas. J. L., Logdiund. M.and
Salaneck W. R. Nature (London) 397 (1999) 121. [5]. Sancho-Garcla.J.C, Bredas. J.L., Beljonne. D., Cornil. J, Martinez-Alvarez. R.,
Hanack. M., Poulsen. L., Gierschner.J., Mack. H.G., Egelhaaf. H.J., and Oelkrug.
D., J. Phys. Chem. B 109,2005 , 4872-4880. [6]. Fleming. G. R., Chemical applications of ultrafast spectroscopy, Oxford Univ.
Press, 1986, p. 95. [ 7]. Brackmann. U., Lambdachrome Laser Dyes data sheets , Lambda Physik,
Germany, 1986. [8]. Sailaja. R., Bisht. P. B., Singh. G. P., Bindra. K. S. and Oak. S. M., Optics
Communications 277 (2007) 433-439. [9] Sancho-Garcia. J. C., Poulsen. L., Gierschner. J., Martinez-Alvarez. R.,
Henneblcq. E.,Kanack. M., Egelhaff. H.-J., Oelkrig. D., Beljonne. D., Bredas. J.-L,
Cornil. J. Adv. Mater 16 (2004) 1193-1197.
[10] Balasubramaniam. S., Annamalai. S., Aidhen. I. S., Synlett 18, (2007), 2841-
2846.
[11] Moog. R. S., Ediger. M. D., Boxer. S. G., Payer. M. D., J. Phys. Chem. 86 (1982)
4694.
[12] Rajesh. R. J., Bisht. P. B., Chem. Phys. Lett. 357 (2002) 420.
[13] Rajesh. R. J., Jose. M., Bisht. P. B., Indian J. Phys. B 76 (2002) 737.
[14] Jena. K. C, Bisht. P. B., Chem. Phys. 314 (2005) 179.
[15]. Jena. K. C, Bisht. P. B., Optics Communications, 273 (2007) 153-158

[16]. Eichler. H. J., Gunter. P. and Pohl. D. W., (1986), "Laser Induced Transient Grating", Springer-Verlag, Berlin.
[17]. Gumy. J. C, Nicolet. O., and Vauthey. E., (1999), "Investigation of the solvation ynamics of an organic dye in polar solvents using the femtosecond transient grating technique", J. Phys. Chem. A, 103, 10737-10743.
[18]. Rajesh. R. J., and Bisht. P. B., "Theoretical and experimental studies on photophysical parameters of N, N' - bis(2,5,-di-tert-butylphenyl)-3,4,9-10- perylenebis (dicarboximide) (DBPI) by using transient grating technique", Chem. Phys. Lett, 357, (2002), 420-425.
[19]. Bragg. A. E., Verlet. J. R. R., Kammrath. A., Cheshnovsky. O., Neumark. D. M., (2005), "Electronic relaxation dynamics of water cluster anions", J. Am. Chem. Soc, [20]. Huxter. V. M., and Scholes. G. D., , "Nonlinear optical approach to multiexciton relaxtion dynamics in quantum dots", J. Chem. Phys., 125, (2006) 144716-12. [21]. Brown E. J., Zhang. Q,. and Dantus. M., , "Femtosecond transient grating techniques: Population and coherence dynamics involving ground and excited states", J. Chem. Phys., 110, (1999)5772-5788.
[22] Zewail A. H., , "Femtochemistry: atomic-scale dynamics of the chemical bond", J. Phys. Chem. A, 104, (2000) 5660-5694.
[23]. Zimdars. D., Tokmakoff. A., Chen. S., Greenfield. E. R., Fayer. M. D., Smith. T. Land Schwettman. H. A., , "Picosecond infrared vibrational photon echoes in a liquid and glass using a free electron laser", Phys. Rev. Lett, 70, (1993) 2718-2721. [24]. Thompson. D. E., Merchant. K. A., and Fayer. M. D., "Solute-solvent interaction; two-dimensional ultrafast infrared vibrational echo experiment", Chem. Phys. Lett, 340, (2001), 267-274.
[25]. Acioli. L. H., Gomes. A. S. L. and Leite. J. R. R., , "Measurement of higher-order optical nonlinear susceptibilities in semiconductor-doped glasses", Appl. Phys. Lett., 53,(1988)1788-1790.
[26] Costela. A., Figuera .J. M., and Florido. F., , "Thermal and acoustic effets in optical phase conjugation in R6G solutions with nanosecond pulses". Opt Commun., 100, (1993)536-544.
[27]. Stickler. S.J. and Berg. R.A. J. Phys. 37 (1962) 814-822. [28]. Berlman. B., Handbook of Fluorescence Spectra of Aromatic Molecules, Academic Press, 1971.

WE CLAIM:
1. A compound exhibiting laser dye properties having a general formula (I),

wherein R1, R2 is same or different, each represents lower alkyl group and X is an aryl moiety
2. The compound as claimed in claim 1 wherein R1 and R2 being same, representing the lower alkyl group.
3. The compound as claimed in claim 1 wherein the lower alkyl group contains 1-4 carbon atoms.
4. The compound as claimed in claim 1 wherein the said lower alkyl group is methyl
group.
5. The compound as claimed in claim 1 wherein the said aryl moiety is
phenylene(1,4) residue.
6. The compound as claimed in claim 1 which is 4, 4'-(1E, 1'E)-2, 2'-(1, 4-
phenylene)bis(ethene-2,1-diyl)bis(N-methoxy-N-methyl benzamide) of formula (la).

7. A method of preparing the compound as claimed in claiml comprising the steps of
a. preparing a reaction mixture comprising of compound of formula (A),
aromatic di-aldehyde of formula (B), anhydrous potassium carbonate and
dry dimethyl formamide (DMF);

d. purifying the said reaction product with silica-gel column chromatography.
10. A compound prepared by the method claimed in claim 9.
11. A laser dye medium essentially consisting of the compound of claim 1,
homogenized in a non-polar solvent exhibiting an excellent lasing property in the
violet region at a pump wavelength ranging from 300nm to 410nm.
12. A laser dye medium essentially consisting of the compound of claim 6,
homogenized in a non-polar solvent exhibiting an excellent lasing property in the
violet region at a pump wavelength ranging from 300nm to 410nm preferably at
355nm.
13. The laser dye medium as claimed in claim 11 and 12 wherein the said non- polar solvent is benzene or chloroform.
14. The laser dye medium of claim 13 wherein the dye concentration ranges from 0.05-5.00 mM.
15. A non-linear optically active dye medium at third harmonic of Nd:YAG and second harmonic of Ti-sapphire laser comprising the compound as claimed in claim 1.
16. A non-linear optically active dye medium at third harmonic of Nd:YAG and Second harmonic of Ti-sapphire laser comprising the compound as claimed in claim 6.
17. A fluorescence life-time standard comprising the compound as claimed in claim 1, at a pump wavelength of 300 nm to 410 nm.
18. A fluorescence life-time standard comprising the compound as claimed in claim 6, at a pump wavelength of 300 nm to 410 nm.
19. An up conversion fluorescence emitting, lasing dye medium formed with the compound claimed in claiml on multi photon excitation.
20. An up conversion fluorescence emitting, lasing dye medium formed with the compound claimed in claims on multi photon excitation.
21. An up converted second harmonic Ti sapphire and Nd:Yag laser generating, dye medium formed with the compound claimed in claiml on multi photon excitation.
22. An up converted second harmonic Ti sapphire and Nd:Yag laser generating, dye medium formed with the compound claimed in claim 6 on multi photon excitation.
23. An optical limiter comprising the compound claimed in claim 1.
24. An optical limiter comprising the compound claimed in claim 6.
25. A saturable absorber comprising the compound claimed in claim 1.

26. A saturable absorber comprising the compound claimed in claim 6.
27. A reverse saturable absorber comprising the compound claimed in claim 1.
28. A reverse saturable absorber comprising the compound claimed in claim 6.

Documents:

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Patent Number

279387

Indian Patent Application Number

2643/CHE/2008

PG Journal Number

03/2017

Publication Date

20-Jan-2017

Grant Date

19-Jan-2017

Date of Filing

30-Oct-2008

Name of Patentee

INDIAN INSTITUTE OF TECHNOLOGY, MADRAS

Applicant Address

INDIAN INSTITUTE OF TECHNOLOGY, MADRAS DEPARTMENT OF PHYSICS, CHENNAI-600 036

Inventors:

#	Inventor's Name	Inventor's Address
1	DR. PREM B BISHT	INDIAN INSTITUTE OF TECHNOLOGY, MADRAS DEPARTMENT OF PHYSICS, CHENNAI-600 036
2	DR. INDRAPAL SINGH AIDHEN	INDIAN INSTITUTE OF TECHNOLOGY, MADRAS DEPARTMENT OF PHYSICS, CHENNAI-600 036

PCT International Classification Number

H01S3/20

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA