Effect of method of preparation on photophysical properties of Rh-B impregnated sol–gel hosts

Effect of method of preparation on photophysical propertiesof Rh-B impregnated sol–gel hosts

Aparna V. Deshpande *, Uday Kumar

Physics Division, University Department of Chemical Technology, Matunga, Nathalal Parikh Marg, Mumbai 400 019, India

Received 16 January 2000; received in revised form 7 January 2002

Abstract

Rhodamine-B (Rh-B) impregnated sol–gel samples have been prepared using two different methods – mentioned as

the dope and dip methods. Two types of dye impregnated samples were prepared using the dope method and one type

using the dip method. A comparative study of photophysical properties of the three types of samples were carried out

with the lapse of time of preparation up to about an year. The two types of samples prepared by dope method showed

degradation in absorption/fluorescence properties with the lapse of time whereas no such degradation was observed

with those samples prepared by dip method. The change in the absorption/fluorescence properties of Rh-B in dope

samples has been associated with the increasing acid environment during sol–gel–xerogel transitions resulting in con-

version of molecular form from cation to weakly fluorescent higher protonated form of Rh-B. On the other hand, Rh-B

in dip samples have very stable absorption/fluorescence properties with high fluorescence quantum yield value equal to

0.98(�0.1). On the basis of a comparative study of the three types of Rh-B impregnated samples, Rh-B dipped samples

were found to have the best performance and may be useful as laser active materials in solid state dye lasers. � 2002

Elsevier Science B.V. All rights reserved.

1. Introduction

Organic dye molecules in solutions have beenwidely used as amplifying media in tunable dyelasers [1]. But, there are various drawbacks tousing liquid dye lasers, such as the volatile sol-vents, cumbersome system design, poor thermalstability of the dye solutions. The possibility ofincorporating these molecules in a solid host leads

to a new field of applications of these lasers. Asolid matrix for laser dyes has technical advanta-ges such as compactness, better manageability andsuitability for field measurements. A number ofsolid polymeric matrices have been used in the past[2–14]. However, these host materials have beenshown to be inherently lacking in mechanical andthermal properties and photostability [2–4]. Avnirand co-workers demonstrated the possibility andapplications of doping a gel with an organic dye in1984 [15,16]. The recent research work with silicagel has attracted a great deal of attention becauseof its potential utility [17–23] indicated higherstability and better lasing properties than thosebased on polymers [24–26]. The sol–gel reaction

Journal of Non-Crystalline Solids 306 (2002) 149–159

www.elsevier.com/locate/jnoncrysol

* Corresponding author. Tel.: +91-22 414 5616; fax: +91-22

414 5614.

E-mail address: [email protected] (A.V. Deshpande).

0022-3093/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

PII: S0022-3093 (02 )01054-2

mail to: [email protected]

occurs at room temperature, therefore, organic dyemolecules can be incorporated in the gel net-work with no risk of thermal degradation, whereasconventional glass production usually involveshigh temperature rendering organic dyes impreg-nation impossible due to dye thermal degradation.Also the host matrices are highly porous and canbe prepared in the form of rectangular blocks,films or cylinders. Therefore, the sol–gel routeprovides a method for incorporation of organicdye molecules into an inorganic silica host. Thelasing action of dye depends on its photophysicalproperties [27,28]. Rhodamine-B (Rh-B) is a con-troversial laser dye for its deactivation processesand it exists in three distinct monomeric molecularforms as shown in Fig. 1 (cation, zwitterion andlactone) depending on the solvent environment[29–31]. The zwitterion (Z) form of Rh-B exists inmethanol and it dominates the equilibrium inprotic solvents with the lactone (L) form. The ca-tion (C) form occurs in methanol (acidic) [31]. Zand C exhibit similar absorption and fluorescencebands with an exception for a small red shift (6–8nm) in the absorption and fluorescence spectrumof C with respect to the corresponding maximumfor Z form. Lactone form has no contribution tothe absorbance or fluorescence in the visible regionand is colorless. Among these molecular forms of

Rh-B, only cationic form is the most preferentialspecies for the lasing action.

Many reports on photophysical properties ofacid catalyzed Rh-B contained sol–gel glass areavailable [32–36]. The published reports related tophotophysical properties of Rh-B in SiO2 sol–gel matrices show the existence of dimers [33,37]or lactone [34,36]. The reason for Rh-B dimerformation has been associated to water contentin SiO2 sol–gel matrix [33]. On the other hand,decrease in polarity from sol to xerogel [34]and increase in acid concentration during Rh-Bdoped SiO2 sol–gel glass preparation [36] havebeen reported for Rh-B lactone formation. Thesereports mention the poor photophysical behav-iour of Rh-B in SiO2 sol–gel glass with time ofpreparation and consequently, such materialscannot be used as amplifying media in solid statedye laser.

Although, various reports are available on Rh-B impregnated sol–gel glasses; there is no clearinformation about the best method of preparationthat would retain the photophysical properties ofthe dye as in solution and have better fluorescenceproperties and high longevity which would beuseful in application of these materials in dye la-sers. Hence, in this paper we report the role ofmethod of preparation and the best method ofpreparation of Rh-B encapsulated SiO2 matricesfrom the point of their usability as solid state dyelaser materials. With this objective, the three typesof sol–gel samples containing Rh-B have beenprepared. In the first two types of sol–gel samples,the dye molecules are incorporated into sol–gelmatrices by predoping (dope method), where thedye is mixed with the precursor components ofsol–gel glass, and in the third type of samples bypostdoping (dip method), where the dye is al-lowed to diffuse into the pores of the glass struc-ture after synthesis [38]. The role of method ofpreparation has been discussed by studying thefluorescence/absorption properties of the dye em-bedded materials so that the results could be usefulin the application of these samples as dye lasermaterials. All the samples were studied for at leastone year from the date of preparation of thesamples.Fig. 1. Molecular forms of Rh-B.

150 A.V. Deshpande, U. Kumar / Journal of Non-Crystalline Solids 306 (2002) 149–159

2. Experimental

2.1. Sample preparation

Laser grade Rh-B was obtained. Its purity waschecked (by UV visible absorption spectroscopy)and then it was used as it was received. Tetra-ethylorthosilicate,methanol (GRgrade), potassiumhydroxide pellets purified, potassium dihydrogenphosphate, nitric acid and formamide were usedwithout further purification.

2.1.1. Dope method(A) The sol was prepared by hydrolysis

and polycondensation of tetraethylorthosilicate(TEOS), using distilled water and methanol underacidic condition using HNO3. TEOS 11.15 ml,distilled water 9 ml, methanol 6 ml and nitric acid1 ml (0.035 M) were used for the sol preparation.After 17 h stirring, glass sol and methanolic solu-tion of Rh-B of known concentration were mixedin appropriate proportion. The coloured glass solwas poured in polystyrene cuvettes for the samplepreparation. Drying and aging were carried out atroom temperature under controlled environment.The concentration of dye solution or volumetricratio of dye solution and glass sol were adjusted toobtain the desired dopant concentration.

(B) Glass sol was prepared in the same manneras mentioned above in (A). After stirring, alkalinesolution (pH ¼ 6) was mixed with sol in 1:2volumetric ratio. The term alkaline solutionstands for a mixture of KOH/water (0.01 M) andKH2PO4/water (0.01 M) throughout the paper. Anappropriate amount of methanolic solution of Rh-B was mixed in the sol (alkaline solution mixed)and the resultant homogeneous dye doped sol wasused for the sample preparation. Drying and agingwere carried out in the same manner as mentionedin (A).

2.1.2. Dip methodThe molar ratios of TEOS:water:nitric acid:

methanol:formamide were 1:10:0.45:3:3. Forma-mide was added after six hours of stirring and thesol was used for the sample preparation. Twenty

days after preparation, samples were used to im-merse in methanolic solution of Rh-B of knownconcentration for different intervals of time. Theno. density of Rh-B molecules in the solid host wascalculated by difference method from the knowl-edge of absorption spectrum of the Rh-B solutionbefore and after the dipping of glass sample.Drying of samples was carried out at room tem-perature. The concentration of dye in methanoland time interval of dipping were adjusted to ob-tain desired no. density in solid host. All thesamples prepared by this method were in the formof block.

2.2. Measurement of absorption/fluorescence

Optical absorption spectra of all the preparedsamples were recorded by a conventional Spec-trophotometer. The fluorescence spectra werescanned using an assembled fluorimeter with afront surface excitation emission geometry [5,6,14].Rh-B in methanol (acidic) was used as the refer-ence with fluorescence quantum yield (Qf ) equal to0.4 [31] for the determination of the quantum yieldfor all the samples. Fluorescence spectra of sam-ples and reference were scanned at the sameexcitation wavelength (530 nm). Recorded spec-tra were corrected for the photomultiplier andmonochromator sensitivities. The area under curveof the emission spectra were used for the quantumyield measurement.

Fluorescence lifetime of Rh-B/sol–gel sampleswere measured at RSIC, IIT, Bombay by usingsingle photon counting technique. The experi-mental set-up consists of flashlamp excitationsource, excitation/emission monochromators,start and stop photomultipliers, time to amplitudeconverter, multichannel analyzer and a computer.The deconvolution technique has been used to findlifetime value. They are single exponential decaywith chi square value between 0.9 and 1.1.

The dye leaching test was performed after oneyear from the date of preparation of the samples.The complete dried samples were dipped in dis-tilled water at room temperature and then corre-sponding absorption spectra of sample and waterbefore and after dipping were compared.

A.V. Deshpande, U. Kumar / Journal of Non-Crystalline Solids 306 (2002) 149–159 151

3. Results

3.1. Doped samples

3.1.1. (A) Rh-B doped samples prepared by usingonly HNO3

The transmission of undoped dried sample was90% in the wavelength region of 800–350 nm and63% at 300 nm. The dimensions of dried samplewere 0:6 cm� 0:6 cm� 1:5 cm. The absorptionspectra of the dye doped samples prepared bymethod-(A) mentioned above were recorded dur-ing sol–gel–xerogel transitions and effect of drying(time) from the date of preparation of samples wasstudied. The absorption peak was at 557 nm dur-ing sol/gel state and it was constant up to 90 days.Afterward, a continuous blue shift was seen inabsorption peak showing the peak at 531 nm afterabout an year. The optical density (OD) of all thesamples was found to be decreasing with time.The OD at peak versus time graph is as shown inthe Fig. 2, this trend is observed for all the studiedsamples with no. density ranging from 5:6� 1014

to 1:6� 1016 molecules per cm3. The OD decreaseslinearly up to 210 days. The slope of graph chan-ges afterward but still the nature is linear. Thefluorescence spectra were also studied with thelapse of time of preparation of the samples.

The results are listed in Table 1 for a typicalsample with no. density 7:03� 1015 molecules percm3. A gradually increasing blue shift in fluores-cence maximum wavelength was observed withtime and it was found that the peak was at 565 nmafter about a year.

For Rh-B/sol–gel samples with lower concen-tration, the blue shift in the fluorescence maximum

Table 1

Change in absorption/fluorescence peaks wavelength of Rh-B in

type (A) sample with drying time for a typical no. density

7:03� 1015 per cm3

Drying time

(days)

Physical

state

ka (nm) kf (nm)

0 Sol 557 588

2 Gel 557 588

30 Solid 558 589

40 Solid 558 590

90 Solid 557 580

150 Solid 553 581

250 Solid 540 571

370 Solid 531 565

Extinction coefficient (e) value in sol state ¼ 1:07� 105

lm�1 cm�1 at ka ¼ 557 nm.

Extinction coefficient (s) value in gel state ¼ 1:06� 105


Extinction coefficient (e) value in MeoH ðacidicÞ ¼ 1:12� 105


Uncertainty ka, kf ¼ �1 nm.

Fig. 2. Degradation in OD of Rh-B in type (A) sample with drying time for a typical no. density 3:45� 1015 per cm3.


was up to 549 nm and for higher concentration, itwas up to 565 nm after one year of preparation.Also fluorescence intensity goes on decreasing withthe lapse of time of preparation. The absorption/fluorescence spectra of the dried sample after anyear of the preparation are shown in the Fig. 3.

3.1.2. (B) Rh-B samples prepared by using HNO3

and alkaline solutionThe transmission of dried undoped solid sample

was 90% in the wavelength range 800–400 nm and45% at 300 nm. The dimensions of the sample were0:5 cm� 0:5 cm� 1:5 cm. The absorption/spectrawere recorded for Rh-B glass doped samples pre-pared using HNO3 and alkaline solution. ODvalues are found to show variation with the time oflapse from the preparation of these samples. Atypical plot of OD at peak versus time is shown inFig. 4. The OD increases initially, reaches maxi-mum (after about 180 days) and decreases withtime as shown in Fig. 4. This behavior is observedfor all the samples studied with the concentrationranging from 3:3� 1015 to 3:3� 1016 molecules percm3. The higher concentrated samples are alsoprepared but the absorbance value is very high.

The absorption and fluorescence spectra of thesample after 180 days (called as Tc curing time) areshown in Fig. 5. It has been found that the curingtime does not depend on the no. density of thesamples for the studied concentration range. Theabsorption/fluorescence peaks 556/590 nm werefound to be constant with drying time for all thesamples in the studied concentration range up to180 days. For lower concentrations a blue shift inabsorption/fluorescence has been observed after180 days. This can be seen from Table 2 in whichchanges in absorption/fluorescence properties ofthe samples with time of preparation for typicalconcentration 9:9� 1015 molecules per cm3 arelisted. Also fluorescence intensity goes on de-creasing with drying time in all these samples.

3.2. Rh-B samples prepared by Dip method

The dried solid sample (without dye) was 90%transparent from 800–400 nm and 60% at 330 nm.The dimensions of sample were 0:5 cm� 0:5 cm�1:5 cm in the form of block. The absorptionmeasurements were recorded after three monthsfrom the date of dipping which is the drying period

Fig. 3. Absorption/fluorescence spectra of Rh-B in type (A) sample (after 380 days) for a typical no. density 7:03� 1015 per cm3.


of these samples. Absorption spectra shows peakat 557 nm which is not much different from theabsorption peak of Rh-B/methanol in acidic so-

lution. It has been observed that the OD of thedipped Rh-B samples increases initially with timewhich becomes constant after 230 days. This is

Fig. 4. Degradation in OD (at peak) of Rh-B in type (B) sample with drying time for a typical no. density 3:3� 1015 per cm3.

Fig. 5. Absorption/fluorescence spectra of Rh-B in type (B) sample (after 180 days) with a typical no. density 3:3� 1015 per cm3.


defined as curing time (Tc) of the sample. Thecuring time (Tc) does not vary with variation in no.density of the sample. The molar extinction coef-ficients ðeÞ of the cured samples were calculatedfrom a plot of OD versus no. density of the sam-ples. The extinction coefficients of Rh-B dippedsamples has been found at various wavelengthssuch as (337, 445, 460, 532, 545, 553, 595, and 610nm) to check and confirm the formation of anyaggregates. They were compared with the extinc-tion coefficients of very dilute acidic samples ofRh-B in solution. Even the absorption spectra ofthin films with high no. density (matching with

blocks) were studied and no such aggregation isobserved. The absorption/fluorescence spectra ofdipped samples are shown in Fig. 6 (after curingtime). The fluorescence peak of all these sampleswas observed at 591 nm which is slightly redshifted compared to Rh-B/methanol (acidic).

The values of absorption cross-section of Rh-Bin different sol–gel host prepared by various routesare shown in Table 3. The result of leachingtest showed no change in absorbance value atpeak before and after dipping the dried samples inwater for all types of samples prepared by variousroutes.

4. Discussion

4.1. Doped samples

4.1.1. (A) Rh-B doped samples prepared by usingonly HNO3

The absorption/fluorescence peaks at 557/588nm in sol and gel state show a slight red shiftcompared to 553/578 nm of Rh-B/methanol(acidic) respectively. This red shift is due to re-fractive index. The extinction coefficient of Rh-B

Fig. 6. Absorption/fluorescence spectra of Rh-B in dip sample (after 230 days) with a typical no. density 2:6� 1016 per cm3.

Table 2

Change in absorption/fluorescence peaks wavelength of Rh-B

in type (B) sample with drying time for a typical no. density

9:9� 1015 per cm3

Drying time (days) ka (nm) kf (nm)

0 556 589

45 556 590

80 556 590

115 556 589

190 556 589

240 551 586

260 550 586

380 540 575


in sol or gel state at peak is comparable to that ofRh-B cation (Table 1). The comparision showsthat Rh-B exists in cationic form in sol and gelstate of samples. The reason behind large reduc-tion in OD after gelation can be understood byLambert–Beer’s law. Putting c ¼ N=lbh, equationOD ¼ ebc takes the form OD ¼ eN=lh, where Nand l, b, h are total number of dopant moleculesand length, breadth, height of the sample respec-tively. If molecular species of Rh-B(cation) remainunaltered then absorbance should increase withdrying and should show constant value after cur-ing time. But experimental results show differentbehaviour. Therefore, the large decrease in OD aswell as fluorescence intensity and blue shift in ab-sorption (557–531 nm)/fluorescence (588–549 nm)peak positions are indication of decrease of Rh-Bcationic species and formation of new species atthe cost of cation. Because of large volumeshrinkage of sample, the first possibility is of dimerformatiom. Generally, Rh-B dimers absorb andfluoresce in the wavelengths 558–576 and 610–650nm respectively [33,39–41]. Considering these re-ported results, clearly the blue shift in absorptionand fluorescence are not associated with dimerformation. Also there is no possibility of zwitte-rion formation because of acidic cage environmentproduced by HNO3. It is also well known that thethird monomeric species lactone of Rh-B has nocontribution to absorption or fluorescence spec-trum in the visible region of interest [34,37,42].Takahashi et al. have reported Rh-B lactoneemission around 450 nm in addition to the emis-sion at 586–590 nm region, when they irradiatedRh-B impregnated sol–gel samples with kex: ¼ 310nm (probably lactone absorption) [36]. But oursamples did not show emission band at 450 nm butonly a single peak at 549 nm with kex: ¼ 310 nm.

This shows that lactone is not responsible for theobserved blue shift.

To identify the reason behind degradation inspectroscopic properties, one can consider theinitial stage of drying in which percentage of waterand alcohol gets reduced and volume shrinkage inthe sample takes place rapidly resulting in an in-crease in acidic level. Therefore, it is reasonable toclaim that the effect is only due to formation ofsome Rh-B species in highly acidic medium of thecage. To verify it, Rh-B was studied in water-methanol (70% and 30% by volume) mixed solventwith dye concentration 7:9� 10�6 mole per litre byincreasing acid to solution. The study shows adecrease in absorption and fluorescence intensityat ka ¼ 556 nm and kf ¼ 585 nm respectively withincreasing acid concentration. For pH6 0:1, themain peak at 556 nm was found to vanish andmultiple peaks at 531, 496, and 467 nm wereobserved while the fluorescence study evenwith varying excitation wavelength showed broademission band near 585 nm. These results showthat fluorescent cation species with kf ¼ 585 nmare converted into a weakly fluorescent species ofRh-B with increasing acid concentration. In dyedoped solids, highly acidic cage environment iscapable to supply more protons (Hþ) to Rh-Bcation. These (Hþ) ions are attached to the nitro-gen atom (uncharged) carrying a lone pair ofelectron in the structure of Rh-B cation resultinghigher protonated form. Due to Coulombic re-pulsive interaction, the attachment of Hþ ion atthe nitrogen atom carrying positive charge is notpossible. Thus, the absorption/fluorescence peakof Rh-B in the sol–gel host at 531/549 nm respec-tively is due to formation of higher protonatedform of Rh-B. A large decreased quantum yieldvalue 0.32 of Rh-B in these solids compared to

Table 3

Absorption cross-section of Rh-B in methanol/various types of solid sol–gel samples

Host Absorption cross-section (ra)

(N � 10�18) cm2

Absorption peak

(ka) (nm)Drying time

(days)

Concentration (mole per litre)

MeoH 4:081� 0:025 545 – 4:9� 10�7 to 2:06� 10�6

MeoH (acidic) 4:312� 0:025 553 – 4:9� 10�7 to 2:06� 10�6

Type (A) 2:356� 0:024 556 110 9:29� 10�7 to 2:65� 10�5

Type (B) 4:034� 0:026 556 180 ð1:74–6:77Þ � 10�6

Dipped 3:938� 0:022 557 230 ð2:86–4:31Þ � 10�6


0.98 in dipped samples shows the weakly fluores-cent characteristic of higher protonated form(Table 4). The protonation in other xanthene dyeRh-6G has also been reported earlier in acidicsolution phase and dried Nafion (Hþ) membrane[43,44]. Thus, the reason for the large blue shift inabsorption (556–531 nm) and fluorescence (590–549 nm) peak positions with drying time is asso-ciated with the increase in relative concentration ofhigher protonated form of Rh-B with cation in thesol–gel host. The increasing red shift in absorption(531–535 nm) and fluorescence (549–566 nm) withincreasing dye concentration in sol–gel samples isdue to an increase in the relative cation concen-tration with the protonated form. A similar type ofblue shift in absorption and fluorescence peaks ofRh-B in sol–gel glasses with increasing acid (HC1used as a catalyst) concentration has also beenreported [36]. Thus the changes in the cage solventcomposition and acid concentration around Rh-Bmolecules, play an important role to bring agradual change in the molecular form of Rh-B inthe preparation of Rh-B/sol–gel glasses. The life-time s ¼ 3:55 ns of Rh-B in our system shows thepresence of a single fluorescent form of Rh-B andis comparable to that of reported monomer life-time 3.98 ns in SiO2 matrix, and s ¼ 3:7 ns in Si–Tibinary oxide by earlier workers [34]. The smallervalue of lifetime in these samples is due to thepresence of protonated Rh-B species in compara-

ble number which is also reflected from extinctioncoefficient value (Table 4).

4.1.2. (B) Rh-B samples prepared by using HNO3

and alkaline solutionOn the basis of earlier discussion and reports

[33,39–41], Rh-B species with absorption and flu-orescence peaks 556 and 590 nm respectively hasbeen attributed to cation. This is also supported bythe results shown in Tables 4 and 5. Based uponearlier analysis, the reason for a blue shift in ab-sorption and fluorescence peaks (Table 2) anddecrease of OD beyond curing point has been as-sociated with the formation of higher protonatedform of Rh-B. However, this conversion of Rh-Bcation is at slower rate and hence initial increase inOD is observed in these samples followed by de-crease in OD with no. of days (Fig. 4). The lifetime4.36 ns of Rh-B in these samples shows the pres-ence of a single fluorescent species Rh-B cationwith comparatively less no. of protonated speciescompared with samples of type (A).

4.2. Dip method

The absorption and fluorescence spectra of Rh-B in dipped samples are shown in the Fig. 6. Fromthe earlier discussion Rh-B species in sol–gel dip-ped samples with absorption/emission peaks 557/591 nm are cation [33,39–41]. The sol–gel host

Table 4

Photophysical properties of Rh-B in methanol/solid sol–gel hosts

Host ka (nm) kf (nm) e (N � 105)

(lmol�1 cm�1)

Qf No. density

(N � 1016) (cm�3)

Lifetime

(ns)

Drying

time (days)

MeoH (acidic) 553 578 1:12� 0:025 0.40 – – –

Type (A) 540 572 0:33� 0:025 0.32 1.1 3.55 245

Type (B) 556 590 1:03� 0:025 0.81 5.3 4.36 180

Dipped 557 591 1:02� 0:025 0.98 9.5 4.55 230

Uncertainty Qf ¼ �0.1.

Table 5

Extinction coefficient values (in lmol�1 cm�1) for Rh-B in methanol/solid sol–gel hosts at various wavelengths

Rh-B/host e337 e445 e460 e532 e545 e556=553 e595 e610

MeoH 4:16� 103 0:99� 103 2:07� 103 7:32� 104 1:03� 105 8:39� 104 0:59� 103 0:09� 103

MeoH (acidic) 4:02� 103 1:05� 103 1:85� 103 5:13� 104 8:75� 104 1:12� 105 0:97� 103 0:1� 103

Type B 4:37� 103 0:81� 103 1:60� 103 4:64� 104 8:35� 104 1:03� 105 1:05� 103 0:21� 103

Dipped 3:41� 103 1:16� 103 1:79� 103 4:44� 104 7:80� 104 1:02� 105 1:84� 103 0:13� 103


(prepared using formamide) has large pore dia-meter and pore volume [45] making it suitable forthe dipping. During dipping, the dye molecules arereaching in pores by diffusion process. In thisprocess, part of remaining solvent in pores iscoming out.

It was observed that some dye impregnatedsamples with lower time of dipping (1–2 h) wereshowing hygroscopic nature. This was becausepores were not emptied completely from solventcontaining formamide. Thus, only appropriateacidic environment in the cage is the reason for theexistence of Rh-B cation. The slight red shift inabsorption/emission wavelength of Rh-B in sol–gelcompared to methanol (acidic) is related to therefractive index of the host. The reason for high Qf

value of Rh-B in dipped samples compared toacidified methanolic dye solution (Table 4) hasbeen correlated to the intramolecular rotationmodel of deactivation [1]. According to this model,rotation of diethyl amino group about the xanth-ene amine bond causes internal conversion. Therate of internal conversion decreases with the sol-vent shear viscosity. In dipped sol–gel samples, themolecule is in solid cage reducing the rotationalmotion of the diethyl amino group resulting highQf value for Rh-B. This is supported by thephotophysical properties of Rh-B in solvent/host[14]. The plot of OD verses time shows no degra-dation in OD with time. The single lifetime 4.55 nsof Rh-B in these samples is an indication ofthe existence of a single fluorescent species. Theleaching of dye was not detected in any type ofdried sample prepared by various routes.

5. Comparison

The comparison of photophysical properties ofRh-B by various methods shows that

(I) degradation in photophysical properties ishighest in type (A) host and it just starts aftergelation, moderate in type (B) host and it startsafter 180 days, whereas dipped sample does notshow any degradation.

(II) The longevity is the least of type (A) sam-ples, moderate of type (B) sample and best ofdipped sample (more than a year).

(III) The less curing time of type-(B) samples(180 days) compared to those of dipped samples(230 days) is the advantage of the use of alkalinesolution.

(IV) The band overlapping area in case of op-tical absorption and fluorescence is maximum intype (A) samples, minimum in dipped sample andit is in between the two for type (B) sample. Theincrease in band overlapping area increases thereabsorption effect resulting decrease in quantumefficiency.

6. Conclusion

The highly acidic micromolecular environmentin the cage is responsible for the formation ofhigher protonated form of Rh-B in type (A) and(B) samples resulting in degradation in photo-physical properties of dye molecule. Rh-B existsin cationic form in dipped samples even up to thelate stage of drying. These samples show highlongevity and very good photophysical propertiesof Rh-B in SiO2 cage. It is clear that the stability ofdye molecules in host matrix and photophysicalproperties are greatly influenced by the method ofpreparation. The dip method is the best method ofembedding Rh-B in inert matrices among all themethods used resulting enhanced longevity andphotophysical properties in sol–gel matrix. Theseresults lead to the possibility that dipped samplescan be used as active materials in solid state dyelasers.

Acknowledgements

The authors thank Department of Science andTechnology for financial support for the presentwork.

References

[1] F.P. Schafer, Topics in applied physics, in: Dye Lasers, vol.

1, Springer, Berlin, 1990.

[2] M.V. Bondar, O.V. Przhonskaya, E.A. Tikhonov, Sov. J.

Quant. Electron. 19 (11) (1989) 1415.

[3] K.M. Dyumaev, A.A. Manenkov, A.P. Maslyukov, G.A.

Matyushin, V.S. Nechitoilo, A.M. Prokhorov, J. Opt. Soc.

Am. B 9 (1) (1992) 143.


[4] Sergei Popov, Appl. Opt. 37 (27) (1998) 6449.

[5] A.V. Deshpande, E.B. Namdas, J. Photochem. Photobiol.

A 110 (1997) 117.

[6] A.V. Deshpande, E.B. Namdas, Chem. Phys. Lett. 263

(1996) 449.

[7] J. Huang, J. Lumin. 8 (4) (1999) 285.

[8] S.M. Giffin et al., Opt. Commun. 161 (1–3) (1999) 163.

[9] E.L Malitsev et al., Appl. Phys. Lett. 73 (25) (1998) 3641.

[10] A. Ramalingm, G. Somasundaram, Proc. SPIE-Int. Soc.

Opt. Eng. (1998) 3417.

[11] M. Ahmad, M.D. Rahn, T.A. King, Appl. Opt. 38 (30)

(1999) 6337.

[12] N.A. Bakr, A.F. Mansour, M. Hammam, J. Appl. Polym.

Sci. 74 (4) (1999) 3316.

[13] A. Costela, I. Garcia-Moreno, J. Barroso, R. Sastre, Appl.

Phys. B 67 (1998) 167.

[14] A.V. Deshpande, E.B. Namdas, Appl. Phys. B 64 (1997)

419.

[15] D. Avnir, D. Levy, R. Reisfeld, J. Phys. Chem. 88 (1984)

5956.

[16] D. Avnir, V.R. Kauiman, R. Reisfeld, J. Non-Cryst. Solids

74 (1985) 395.

[17] Jun Chen, Jianhong Ge, Zhi Hong, Chengfong Bao,

Wenga Qiu, Xiuping Wang, Proc. SPIE––Int. Soc. Opt.

Eng. (1999) 3862.

[18] K. Sio, D. Lo, Chem. Phys. Lett. 297 (3 and 4) (1998) 329.

[19] D. Lo, K.S. Lam, S.K. Lam, C. Ye, Opt. Commun. 156

(4–6) (1998) 316.

[20] M.J. Garcia, J.E. Ramirez, M.A. Mondragon, R. Ortega,

P. Loza, A. Campero, J. Sol–Gel Sci. Technol. 13 (1–3)

(1998) 657.

[21] S.K. Lam, D. Lo, Appl. Phys. B: Laser Opt. 68 (6) (1999)

1147.

[22] T. Suratwala, Z. Gardlund, K. Davidson, D.R. Uhlmann,

J. Watson, S. Bonilla, N. Peyghambarian, Chem. Mater. 10

(1998) 199.

[23] T. Suratwala, Z. Gardlund, K. Davidson, D.R. Uhlmann,

J. Watson, S. Bonilla, N. Peyghambarian, Chem. Mater. 10

(1998) 190.

[24] M. Canva, P. Georges, A. Brun, D. Larrue, J. Zarzycki,

J. Non-Cryst. Solids 147&148 (1992) 636.

[25] D. Lo, J.E. Parris, J.L. Lawless, Appl. Phys. B 55 (1992)

365.

[26] J.M. Mckiernan, S.A. Yamanaka, B. Dunn, J.I. Zink,

J. Phys. Chem. 94 (1990) 5652.

[27] A.V. Deshpande, V.B. Mhatre, J. Lumin. 65 (1996) 313.

[28] Arbeloa et al., Appl. Phys. B 64 (1997) 651.

[29] I. Lopez Arbeloa, K.K. Rohatgi-Mukherjee, Chem. Phys.

Lett. 128 (1986) 474.

[30] K.G. Cesey, E.L. Quitevis, J. Phys. Chem. 92 (1988)

6590.

[31] T.-L. Chang, H.C. Cheung, J. Phys. Chem. 96 (1992)

4874.

[32] T. Fujii, Trends Photochem. Photobiol. 3 (1994) 243.

[33] H. Nishikiori, T. Fujii, J. Phys. Chem. B 101 (19) (1997)

3680.

[34] N. Negishi, M. Fujino, H. Yamashita, M.A. Fox, M.

Anpo, Langmuir 10 (6) (1994) 1772.

[35] Y. Sorek, R. Reisfeld, A.M. Weiss, Chem. Phys. Lett. 224

(1995) 371.

[36] Y. Takahashi et al., J. Lumin. 60&61 (1994) 451.

[37] D.A. Hinckley, P.G. Saybold, D.P. Borris, Spectrochim.

Acta 42A (1986) 747.

[38] M.D. Rahn, T.A. King, Appl. Opt. 34 (1995) 36.

[39] T. Fujii, H. Nishikiori, T. Tamura, Chem. Phys. Lett. 223

(1995) 424.

[40] I. Yamazaki, N. Tamai, T. Yamazaki, J. Phys. Chem. 94

(1990) 516.

[41] K. Kemniz, N. Tamai, I. Yamazaki, N. Nakashima, K.

Yoshihara, J. Phys. Chem. 90 (1986) 5094.

[42] I. Rosenthal, P. Peretz, K.A. Muszkat, J. Phys. Chem. 83

(3) (1979) 350.

[43] K.H. Drexhage, in: F.P. Schaffer (Ed.), Topics in Applied

Physics, vol. 1, Springer, New York, 1973, p. 144.

[44] H. Mohan, R.M. Iyer, J. Chem. Soc. Faraday Trans. 88 (1)

(1992) 41.

[45] G. Orcel, L.L. Hench, I. Artaki, J. Jones, T.W. Zerda,

J. Non-Cryst. Solids 105 (1988) 223.


Effect of method of preparation on photophysical properties of Rh-B impregnated sol–gel hosts

Documents

Transcript of Effect of method of preparation on photophysical properties of Rh-B impregnated sol–gel hosts