Comparative Study of Coumarin-120 (C-120) and Stilbine-3 (STB-3) Laser Dyes Doped in Sol-Gel Glasses

In 1967 the first work in solid state dye laser was performed by doping rhodamine dyes in Polymethylmethacrylate (PMMA) materials. Since then some materials like various types of polymers, co-polymers, poly composite glasses have been used as host matrices for doping different laser dyes. Polymers suf-fer from limited mechanical and thermal stability. Hence glasses can be the alternative hosts. However, because of high processing temperature leading to permanent destruction of dye molecules, the conventional glass preparation technique is not suitable for the introduction of organic laser dyes. This dif-ficulty can be overcome by introducing the laser dye molecules in sol-gel glass which is prepared at low temperature. Recent work with sol-gel glasses shows that these glasses may prove to be better materials compared to polymeric materials because glasses being hard, best optically transparency in Near UV-UV and Visible region and show better photostability. In this research work we reported, comparative study of the photophysical properties of Coumarin-120 (C-120) belonging to 7-aminocoumarin family having two hydrogen atoms attached to the N atom at the 7-position, with Stilbene-3 (STB-3) in three types of HCl catalyzed SiO 2 sol-gel matrices prepared by Method I, Method II and Method III respectively.


Introduction
The first demonstration of dye laser was reported by Sorokin and Lankard in 1966 [1]. They observed stimulated emission from the alcoholic solution of 3,3' di-After about 45 days from the date of preparation, solid blocks were obtained in the form of parallelopipeds with dimensions (0.8 × 0.8 × 2.0) cm 3 . These glass samples were immersed in MeOH/H 2 O (50:50 by volume) for 16 hours and then subsequently in 15 ml methanolic solution of C-120 and STB-3 laser dye of known concentration for one hour. After removing the samples from the solution they were dried at room temperature. After 15 days of drying the surface of the samples gets dried so that it is handable and can be subjected to various measurements.
2) Method II: using HCl as catalyst at 60˚C and drying at room tempera- 3) Method III: using HCl as catalyst at 60˚C and heated at 600˚C temperature for 3 hours A sol was prepared by mixing 78 ml TEOS, 102 ml H 2 O, 2.4 ml HCl as catalyst under magnetic stirring at 60˚C temperature for 1 hour. After stirring, 3.5 ml sol was poured in the rectangular polystyrene cuvette and then sealed with teflon tape. Drying and aging of gel were carried out at 60˚C temperature in heating blocks. After about 4 days from the date of preparation, solid blocks were obtained in the form of parallelopipeds with dimensions (0.6 × 0.6 × 1.7) cm 3 .
These glass samples were given heat treatment by heating in programmable microwave furnace at 600˚C for 3 hours and then subsequently they were cooled and kept at room temperature for one day and then the blocks were immersed separately in 15 ml methanolic solution of C-120 and STB-3 dye of known concentration for one hour. After removing the samples from the solution, they were dried at room temperature. After 3 days of drying the surface of the samples gets dried so that it is handable and can be subjected to various measurements.
The number density of dye doped molecules in the solid host was calculated by difference method from the Optical Density (OD) of absorption of dye solution before and after dipping of the glass sample. The desired number density of dye molecules in solid host can be obtained by dipping of glass samples in varied concentration of methanolic solution of dye. The dried solids obtained by all the above methods were visually of good surface finish with plane parallel side faces.
They were used directly for spectroscopic and laser studies without any polishing of the faces.

Absorption Spectroscopy Study
Absorption/Transmission spectra of undoped glasses were recorded with air as reference and absorption spectra of dye-doped solids were recorded with undoped glass as reference. Absorption wavelength maximum (λ a ) and molar absorptivity (ε) of the dye in solid were determined and were compared with the respective properties in solution.

Fluorescence Spectroscopy
Fluorescence is a type of energy emission which is produced when a molecule returns to the ground state after having been raised to an excited state by absorption of energy. Fluorescence of the organic compounds is measured by fluorimeter. The experimental setup of a fluorimeter in our research laboratory is shown in Figure 1. It consists of a xenon lamp as an excitation source, an excitation monochromator, a sample compartment, an emission monochromator, a photomultiplier tube to detect signal, a photon counter and a XY/t recorder to get fluorescence spectrum or fluorescence data from photon counter can be stored in computer through Microprocessor Data Acquisition System (MIDAS). Fluorescence spectroscopy in the present study involves scanning of fluorescence spectra and measurement of fluorescence quantum yield. The fluorescence quantum yield of an emissive species is defined as the ratio of the number of emitted photons to the number of absorbed photons by the species. The two well known methods [56] for the measurement of fluorescence quantum yield value are given below:

1) Transverse Method
This method is used for optically dilute samples having optical density at λ a less than 0.1. In this geometry the excitation direction and detector are at right angles to each other as shown in Figure 2. Fluorescence spectra for the sample and reference are recorded by exciting them at appropriate wavelengths. Fluorescence spectra are corrected for monochromator and detector sensitivity. Fluorescence quantum yield of the sample is then calculated according to the formula:   2) Front Surface Method This method is useful for the measurement of quantum yield with high dopant concentration. The concentration of dopant is normally selected so that 99 % or more exciting light is absorbed in a first few millimeters of the sample. The experimental arrangement for the front surface geometry is shown in Figure  In the present work, front surface excitation emission geometry is used for the fluorescence measurement of dye/sol-gel samples and dye in solution phase.

Fluorescence Lifetime Measurement
Fluorescence lifetime is defined as the average time interval between the absorption of exciting light and the emission of fluorescence by a molecule. It is typi-cally of the order of a few nanoseconds. There are mainly two methods used for the measurement of fluorescence lifetime, namely the pulse method and phase modulation. Pulse method can be of two types, direct method and indirect method. Direct method involves single pulse measurement by storage oscilloscope or Streak camera, while indirect measurement is the single photon counting technique [4]. In the present work, an indirect method known as single photon counting technique is used for the measurement of fluorescence lifetime. The experimental setup is shown in Figure 4. The principle involved in this method is the measurement of time interval between excitation pulse and the arrival of first photon at the detector.  Fluorescence lifetime of dye/sol-gel samples are measured on the model SP-80 nanosecond fluorescence spectrometer by using single photon counting technique. The experimental set-up consists of flashlamp (hydrogen filling at 0.5 atm) with 100 kHz frequency, as excitation source, excitation/emission monochromators, start and stop photomultipliers, time to amplitude converter, multichannel analyzer and a computer. The deconvolution technique has been used to find lifetime value. They are single exponential decay with chi square value from 0.9 to 1.1.

Absorption/Fluorescence Properties of C-120
The absorption and fluorescence spectra of C-120 dye doped sol-gel glass samples prepared by Method I, Method II and Method III are shown in  Table 1 and Table 2.      increases. This is accompanied by three more peaks appearing at shorter wavelengths viz 311 nm, 278 nm and 269 nm. Their shapes are similar to their respective counterparts in solid sol-gel glass samples prepared by method I and II. These peaks are not due to absorption of HCl at lower wavelengths because they are absent in case of undoped samples though all the solids contain equal concentration of acid. The fluorescence maximum in solution is initially constant at about 429 nm, only the fluorescence intensity decreases as the amount of acid in solution increases. Fluorescence at longer wavelength, namely 478 nm starts appearing at moderate acid concentrations only. At intermediate concentration of acid both peaks appear in the emission spectrum and finally only a single peak at 478 nm with diminished intensity is observed (Figure 8(b)). As the drying time of sol-gel samples (Method I and II) and dipping time of sol-gel samples (Method II) dipped in MeOH/H 2 O increases the absorption spectra of dried sol-gel glass samples resemble to the spectra of C-120 methanolic solution containing HCl acid. From the Figure 8(a) it can also be seen that there exists an isobestic point indicating that the increase in one peak is at the loss of OD at the other peak. This confirms the conversion of monomer species into some other species, probably protonated species. Table 3 and Table 4 presents the changes in the absorption and fluorescence properties of the C-120 dye in the sol-gel glass blocks with drying time prepared by the three methods respectively.
The parameter listed in Table 3 and Table 4    almost no change in value with drying time. This shows that the nonradiative deactivation rate is maximum in Method I and Method II samples due to greater protonation as explained earlier which goes on decreasing after drying and it is least in Method III samples. The fluorescence quantum yield (Ф f ), lifetime of fluorescence (τ f ) and nonradiative deactivation rate constant (Knr) of C-120/ sol-gel glass solids prepared by Method III are found to remain almost unchanged after a period of 90 days. This behaviour of C-120 dye can be explained by considering the ground state and excited state equilibria of the dye in the presence of acid as reported earlier by Drexhage [57]. These are depicted in Figure 9(a), Figure 9(b). The ground state of C-120 is represented by non-polar form (Figure 9(a)) which absorbs at 352 nm. As mentioned earlier, in excited state C-120 predominantly exists in polar form (Figure 9(b)) which emits at 429 nm. Being polar in excited state the carbonyl group has greater affinity towards proton. On the contrary, in the ground state basicity is associated with amino group. On addition of strong acid like HCl this amino group gets protonated.
This protonated form does not absorb at 352 nm hence the OD decreases [57]. The protonation increases with the acid concentration and the OD at 352 nm goes on decreasing. At the higher concentration of acid in addition to this protonated form two more species (may be higher protonated forms) are observed at lower wavelengths (311 nm, 279 nm and 270 nm). When excited at these wavelengths they were found to be non-emitting species.
Due to proton transfer from surrounding solvent molecules to the carbonyl group in excited state (Figure 9(b)) the fluorescence shifts towards green, namely at 478 nm. The shift of fluorescence is observed clearly in solution phase. R. Srinivasan [26] has reported that when one or two drops of hydrochloric acid are added to the methanolic solution of coumarin 120, the fluorescence of C-120 shifts to greenish side as against its usual blue emission. This is because of the abundance of protons in acidified solution which leads to increased protonation at the carbonyl group in the excited state. Due to loss of energy during proton transfer the fluorescence shifts to longer wavelength. This can be used to improve tunability by changing pH of the solution. As acid catalysed sol-gel process is widely used for preparing host matrices for the laser dyes, the effect of method of preparation on spectroscopic properties of dopant molecules is of great importance as regard to the application of these dye embedded sol-gel glasses as laser materials.
But studied dried C-120 dye doped sol-gel solid glass samples show only single emission peak at 430 nm, 432 nm and 427 nm prepared by Method I, II and Method III respectively, which gets slightly red shifted with increase in intensity with drying time for the method I and II samples. Thus appearance of carbonyl protonated form cannot be seen in absorbance and has not been observed in fluorescence spectrum of all C-120 sol gel glass samples. As the fluorescence spectra of all C-120 sol-gel glass samples do not have any emission in 478 nm region, one can say that the corresponding carbonyl protonated species is not formed in solid sol-gel glass samples. Therefore the absorption peaks at 311 nm, 278 nm and 269 nm of C-120 in dried sol-gel glass samples prepared by Method I and II can be due to amino-protonated and higher protonated form and cannot be due to carbonyl protonated form.
In case of Method I and Method II samples in present work, after 15 days of drying the local environment of C-120 molecules in glass may be having HCl containing water and MeOH surrounding them as the sample is not completely dried. Hence in addition to neutral coumarin form which exists in ground state absorbing at 354 nm, there may be protonated, higher protonated and even some higher protonated species may be present which show absorption peaks in shorter wavelength region of the spectrum. But as the drying time increases the sol-gel glass matrix gets rigidised separating the molecules of C-120 in pores. The molecules of C-120 exist in various forms depending on the environment in pores. During this process of drying and separation, some protonated molecules may be converted into neutral form depending upon the environment in the pores as supported by decrease in OD value of absorption peaks at 311 nm, 269 nm and 278 nm as seen from Figure 5. This is supported by observation of increase in OD at 354 nm peak and increase in FI with drying time with some shift of fluorescence peak wavelength. Also increase in OD at 354 nm confirms increase in neutral form concentration with drying time. As the dipping time of the sol-gel host samples (Method II) increases the maximum residual solvent containing HCl coming out from sol-gel glass matrix and therefore existence of neutral form of coumarin dyes is high in longer dipped samples as compared to low dipped glass samples. This automatically reduces the possibility of formation of protonated form with longer dipped matrix samples. This is supported by decrease in OD value of absorption peaks at 311 nm, 269 nm and 278 nm and increasing in FI at main peak as seen from Figure 7(a), Figure 7(b). Hence an in-crease in OD/FI occurs only because of conversion of protonated species into the neutral monomer species. It was also observed that absorption/fluorescence peaks of C-120/sol-gel glass samples (Method I and II) initially were almost same as that of C-120/MeOH (352/429nm), which is shifted towards longer wavelength after 90 days of drying. In case of samples prepared by method III, the unwanted residual solvent and other chemicals are coming out of the solid during heating the glass sample at high temperature and therefore the environment of dye molecule is only solid SiO 2 cage resulting in almost no change (very little change in fluorescence intensity with time) in spectroscopic properties of samples with increasing drying time. The red shift in absorption/fluorescence peak of coumarin containing sol-gel glass samples with respect to its methanolic solution may be the combined effect of increased refractive index of sol-gel compared to methanol, the local environment of the dye molecules in the sol-gel and polar nature of the host [38] [39]. Figures 11(a)-(c) show the absorption and fluorescence spectra of Stilben-3 (STB-3) dye doped sol-gel glass samples prepared by Method I, Method II and Method III respectively with increasing drying time. It represents the typical behavior of these solids which is observed for all the concentrations of the dye studied in the present work. The shapes of the spectra are similar to those in alcoholic solution suggesting existence of similar forms of dye molecule in the solution and solids.

Figures 10(a)-(c) and
All STB-3 doped solid glass samples prepared by Method I, II and Method III having number densities of the order of 1015/cm3 to 1017/cm3 show single absorption peak peaking at 345 nm, 344 nm and 348 nm respectively. As the drying time increases there is a decrease in the optical density (OD) value of main absorption peak at 345 nm and 344 nm for Method I and Method II samples respectively. But in case of samples prepared by Method III no such change in OD value is observed as drying time increases.
Also slight blue shift (2 nm) is observed in absorption maximum wavelength after 240 days of time of drying for all three types of STB-3 embedded sol-gel glasses. The Fluorescence Intensity (FI) also decreases with the lapse of time of preparation for Method I and Method II prepared samples, while no change in fluorescence intensity is observed in samples prepared by Method III as the drying time increases as shown in Figure 11(c). However, the observed changes in absorption and fluorescence properties with increasing drying time are very less in samples prepared by Method II as compared to samples prepared by Method I. This observation is similar as that observed in coumarin dyes. Table 5 lists the absorption properties of STB-3 in sol-gel blocks as well as in methanol with low concentration of the dye (number density of the order of 10 15 per cm 3 ). The absorption wavelength maxima (λ a ) for sample prepared by all the three methods are little blue shifted as compared to that in methanol. The extinction coefficient (ε) of STB3 in solid prepared by Method I and Method II are less compared to that in solution, where as in solids prepared by Method III it is more than that in solution. But the order of magnitude of ε value is same for all the samples.    The absorption and fluorescence properties of STB-3 in sol-gel glass samples after drying for 240 days along with that in methanol are summarized in Table 7 and Table 8 respectively. The spectroscopic properties have been compared on the basis of following parameters namely absorption wavelength maximum (λ a ), extinction coefficient (ε), fluorescence wavelength maximum (λ f ), fluorescence quantum yield (Φ f ), fluorescence lifetime (τ f ) and nonradiative deactivation rate constant (K nr ).   It can be seen from Table 7 and Table 8

Comparison between C-120 and Stilbene-3 (STB-3) in Sol-Gel Glass
For all studied of coumarin-120 (C-120) dye doped and Stilbene-3 (Stileben-3) dye doped glass samples, for all three methods; the extinction coefficient (ε) at the longest absorption peak is least for C-120 is highest for method III samples

Conclusions
C-120 and STB-3 were successfully embedded in HCl catalyzed sol-gel glasses prepared by all the three different methods. The spectroscopic properties of the dye have been found to be highly dependent on the method of dye trapping.
1) In addition to usual molecular form of C-120 (λ a = 352 nm, λ f = 430 nm) that exists in methanol, the dye has been found to exist in two new and distinct molecular forms mentioned as amino-protonated or higher protonated forms (λ a = 311 nm, 278 nm, 269 nm and non-fluorescent) in the dried state of samples prepared by Method I and II. The acidic environment of the cage, created by HCl used as catalyst, and unwanted residual molecules, produced even in the initial stage of drying are responsible for the protonation at the amino group in sol-gel samples prepared by Method I and II respectively. But it has been identified that the neutral species increase at the cost of protonated form with time of drying for Method I and II prepared sol-gel glass samples. Thus together with environment of dye molecules, drying of sol-gel samples also plays an important role.
2) The photophysical behavior of C-120 in sol-gel host matrix prepared by Method I is the same as that in the Method II samples with better enhancement in photophysical properties with drying time than that of Method II samples.
3) On the other hand, the spectroscopic behaviour of C-120 in Method III sol-gel samples is similar to that of methanolic dye solution. The non-acidic environment and absence of unwanted residual species in the pores are responsible factors for the non-existance of protonated and any other unwanted residuals.
The absence of any degradation in absorption/fluorescence properties of C-120 in these samples with drying time confirms that the dye molecules are very stable in these solids.
4) The comparative study of C-120 in sol-gel matrices prepared by all the three methods shows that the photophysical performance of dye in Method III sol-gel samples is better than that in the Method I and II samples resulting in enhanced longevity and photophysical properties in sol-gel matrix.
5) The STB-3 in sol-gel matrices prepared by all the three methods shows that the excellent photophysical properties in sol-gel matrix.