Contrasting Solvent Polarity Effect on the Photophysical Properties of

J. Phys. Chem. A 2010, 114, 4507–4519

4507

Contrasting Solvent Polarity Effect on the Photophysical Properties of Two Newly Synthesized Aminostyryl Dyes in the Lower and in the Higher Solvent Polarity Regions M. Shaikh,† J. Mohanty,† P. K. Singh,† A. C. Bhasikuttan,† R. N. Rajule,‡ V. S. Satam,‡ S. R. Bendre,‡ V. R. Kanetkar,‡ and H. Pal*,† Radiation & Photochemistry DiVision, Bhabha Atomic Research Center, Trombay, Mumbai-400085, India, and Department of Technology of Dyestuff and Intermediates, Institute of Chemical Technology (ICT), Mumbai-400019, India ReceiVed: NoVember 13, 2009; ReVised Manuscript ReceiVed: January 25, 2010

Solvent polarity effect on the photophysical properties of two newly synthesized aminostyryl-thiazoloquinoxaline dyes, one with a flexible diphenylamino group, namely, N,N-diphenyl-4-[2-(thiazolo[4,5-b]quinoxalin2-yl)vinyl]aniline (TQ1), and the other with a rigid julolidinylamino group, namely, (9-[2-(thiazolo[4,5b]quinoxalin-2-yl)vinyl]julolidine) (TQ2), have been investigated in different aprotic solvents and solvent mixtures. From the polarity dependent changes in the absorption and fluorescence spectral properties, it is indicated that the fluorescent states of the dyes are of intramolecular charge transfer (ICT) character. For both the dyes, the photophysical properties like fluorescence quantum yields (Φf), fluorescence lifetimes (τf), radiative rate constants (kf ) Φf/τf), and nonradiative rate constants (knr ) 1/τf - Φf/τf) show clearly contrasting solvent polarity effects in the lower and in the higher solvent polarity region, causing an interesting reversal in the properties below and above an intermediate solvent polarity. It is inferred that the domination of the cis-trans isomerization in the lower solvent polarity region and that of the twisted intramolecular charge transfer (TICT) state formation in the higher solvent polarity region are responsible for the observed contrasting solvent polarity effects on the photophysical properties of the two dyes. As both isomerization and TICT state formation causes an enhancement in the nonradiative decay rate of the excited dyes and both the processes become less significant at the intermediate solvent polarity region, the two dyes show their largest Φf and τf values at intermediate solvent polarities. Suitable mechanistic schemes have been proposed and qualitative potential energy diagrams have been presented to explain the observed results with the changes in the polarity of the solvents used. 1. Introduction Styryl dyes are an important class of molecules having potential applications in different areas like dye lasers and optical materials,1-6 molecular electronics,7-10 biology,11-16 etc. The characteristic structural feature of these molecules is the presence of both electron donating and electron accepting chromophores in the same molecule joined by a suitable π conjugation involving either single or multiple enyl groups (styryl double bonds). Due to this distinctive structural feature, styryl dyes show very high first-order hyperpolarizability, which in turn makes these dyes very efficient materials for applications in molecular electronics,7-10 and in second harmonic generation.7-10,17-19 Styryl dyes are also known to have very good photoelectroconversion efficiency.20,21 The role of the styryl dyes as the free-radical photoinitiators has also been reported in the literature with possible applications in polymerization processes.22-25 The presence of donor and acceptor moieties joined by π-conjugation in a single molecule makes the styryl dyes to display strong ICT character, especially in the excited electronic states. Due to this strong ICT character, these molecules show prominent solvent polarity dependent changes in their emission characteristics, undergoing large Stokes’ shifts in the fluorescence spectra and significant changes in the fluorescence * Author for correspondence. E-mail: [email protected]. Fax: 91-2225505151 /25519613. † Bhabha Atomic Research Center. ‡ Institute of Chemical Technology (ICT).

quantum yields and lifetimes when the solvent polarity changes.26-33 The solvatochromic shifts in the fluorescence spectra of the styryl dyes has been utilized extensively in understanding the dynamics of ultrafast solvation processes followingthewell-knowndynamicStokes’shiftmeasurements.31-33 Understanding the excited state properties of the styryl dyes has also been the subject of extensive research for a long time.26-38 Though the photophysical behavior of many styryl dyes has been reported in the literature from both experimental and theoretical perspectives,26-40 there are still many unresolved issues related to the excited state properties of these dyes. From the viewpoint of the structural constitution of the styryl dyes, it is expected that the photoexcitation of these molecules will trigger a number of intramolecular relaxation processes to modulate their excited state properties.26-40 Further, these relaxation processes are also expected to be influenced strongly by the solvent environment present around the excited dye. For most organic dyes, it is normally perceived that the immediate act of photoexcitation is to populate the molecules in their locally excited (LE) state.41,42 Since in the styryl dyes the electron donor and the electron acceptor moieties are linked by π conjugation, the LE states of these dyes are expected to be the intramolecular charge transfer (ICT) states.26-40 The two important processes that these excited ICT states of the styryl dyes can subsequently undergo are (i) the cis-trans isomerization due to the rotation of the two chromophoric moieties around the styryl double bonds (the π-conjugations) and (ii) the rotation of the electron

10.1021/jp9107969  2010 American Chemical Society Published on Web 02/19/2010

4508

J. Phys. Chem. A, Vol. 114, No. 13, 2010

donating groups around a suitable single bond resulting in the formation of highly polar twisted intramolecular charge transfer (TICT) state, where an electron from the donor moiety is effectively transferred to the acceptor moiety.26-40 The main difficulties in understanding the detailed photophysical properties of styryl dyes are related to the fact that for most of these dyes there are more than one isomerization and/or TICT formation channels possible and in most of these cases it often remains obscured as to which channel is actually related to what effect.26-40 In many of the aminostyryl dyes, the electron donating chromophores are composed of the N,N-dialkyl (-NR2) or N,N-diaryl (-NAr2) aniline moieties. For these molecules, the TICT state formation can logically take place either by the rotation of the N,N-disubstituted amino group (NR2/ NAr2) around the Φ-N single bond (Φ represents the aromatic ring of the aniline moiety) or by the rotation of the whole N,Ndisubstituted aniline moiety around the single bond connecting this group to the styryl double bond. For such molecules, it is often difficult to resolve from photophysical studies which of the two single bond rotations are actually responsible for their TICT state formation. In the present work, we have judiciously selected two homologous aminostyryl dyes, one with a flexible N,N-diarylamino group in the aniline moiety capable of undergoing free rotation around the Φ-N single bond and the other with a structurally rigid amino group (using julolidinyl ring) in the aniline moiety imposing restrictions on the rotation around the Φ-N single bond. For both the dyes, however, the electron accepting moiety as well as the styryl conjugation remains the same. Thus, it is expected that a comparison of the photophysical properties of the two dyes will show some light on the interesting issue of which single bond rotation is actually responsible for the TICT state formation in the aminostyryl dyes. The two aminostyryl derivatives thus investigated in the present study are N,N-diphenyl-4-[2-(thiazolo[4,5-b]quinoxalin-2-yl)vinyl]aniline (TQ1) and (9-[2-(thiazolo[4,5-b]quinoxalin-2-yl)vinyl]julolidine) (TQ2) dyes, the former one with the flexible N,Ndiarylamino group and the latter one with the structurally rigid julolidinylamino group. The photophysical properties of the two dyes have been investigated systematically in different aprotic solvents and solvent mixtures, using ground state absorption and steady state and time-resolved fluorescence measurements. The aim of the present study is mainly to understand the effect of solvent polarity on the photophysical properties of the two dyes investigated and to use these solvent polarity dependent modulations in the photophysical properties in realizing the possible structural changes and the relaxation pathways involved in the excited states of these molecules following their photoexcitation in different solvent environments. The chemical structures of the two aminostyryl dyes investigated in the present work are shown in Chart 1. 2. Materials and Methods The N,N-diphenyl-4-[2-(thiazolo[4,5-b]quinoxalin-2-yl)vinyl]aniline (TQ1) and (9-[2-(thiazolo[4,5-b]quinoxalin-2-yl)vinyl]julolidine) (TQ2) dyes were synthesized following the procedures given in the Supporting Information.43 The samples were purified by column chromatography using silica gel as a stationary phase and toluene/ethyl acetate 4:1 (for TQ1) and toluene (for TQ2) as the eluents. The purity of the samples was checked by 1H NMR studies and microanalysis results.43 All the solvents used in the present study were of spectroscopic grade and obtained either from Spectrochem (Mumbai, India) or from Fluka (Buchs, Switzerland). The solvent acetonitrile (ACN) was used as received and cyclohexane (CHX) and ethyl

Shaikh et al. CHART 1: Chemical Structures of TQ1 and TQ2 Dyes

acetate (EA) were used after further purification by the distillation method. Dielectric constants (ε) and refractive indices (n) of the pure solvents were obtained from the literature reports,44 and those of the mixed solvent systems (εMS and nMS, respectively) were calculated using the following equations:45-54

εMS ) fAεA + fBεB

(1)

nMS2 ) fAnA2 + fBnB2

(2)

where the subscripts A and B represent the cosolvents used and the factors fA and fB represent their respective volume fractions. In the present study, the Lippert-Mataga polarity parameter (∆f) has been considered as the measure of the polarity of different solvents and solvent mixtures used and was calculated using following equation.41,42,55,56

∆f )

ε-1 n2-1 - 2 2ε + 1 2n + 1

(3)

It should be mentioned here that the solvent polarity parameter ∆f as defined in eq 3 is actually the measure of the solvent orientational polarization only, as the electronic polarization part {(n2 - 1)/(2n2 + 1)} has been subtracted from the total polarization {(ε - 1)/(2ε + 1)} of the solvent.41,42,55,56 Such a consideration for the solvent polarity parameter ∆f is based on the fact that the electronic polarization is an instantaneous process and accordingly it occurs simultaneously along with the electronic transitions (absorption and emission of photon). Hence the effect of the electronic polarization will not be reflected on the measured photophysical parameters of a chromophoric dye, as the latter are determined by the reactions and interactions of the excited dye molecule during its excited state lifetime. It is thus evident that it is only the solvent orientational polarization that will be responsible for the observed solvent polarity dependent changes in the photophysical properties of a dye molecule. Accordingly, in most solvatochromic studies, the parameter ∆f is generally considered as the solvent polarity parameter to correlate the effect of solvent polarity on the observed photophysical parameters of the chromophoric dye molecules.41,42,55,56 Further, we should mention here that in the present work we have used only the aprotic solvents and solvent mixtures such that the parameter ∆f can be used suitably as the measure of the solvent polarity in

Photophysical Properties of Two Aminostyryl Dyes

J. Phys. Chem. A, Vol. 114, No. 13, 2010 4509

correlating the observed results.45-52 In the present study, therefore, we have intentionally avoided the use of any protic solvent or solvent mixture to avoid the complications that normally arise due to the specific solute-solvent hydrogen bonding interactions involving protic solvents.57-61 Absorption spectra were recorded using a JASCO (model V-530, Tokyo, Japan) UV-vis spectrophotometer. Steady state (SS) fluorescence spectra were recorded using a Hitachi (model F-4010, Tokyo, Japan) spectrofluorometer. The fluorescence quantum yield (Φf) values were estimated by a comparative method,41,42 using the dye neutral red in acetonitrile solution as the reference (Φf ) 0.038).62,63 For all the fluorescence measurements, the absorbance values of the solutions were kept quite low (∼0.2) at the excitation wavelength. Time-resolved (TR) fluorescence measurements were carried out using an IBH (Scotland, U.K.) fluorescence spectrometer based on the time-correlated-single-photon-counting technique.64,65 The samples were excited by 455 nm light pulses from a nanoLED source operated at a repetition rate of 1 MHz. The fluorescence decays were detected at a right angle to the excitation path using a PMT based detection module (model TBX4, IBH, Scotland, U.K.). The typical instrument response function for the present setup is ∼1.2 ns at the full-width at half-maximum. All the measurements were carried out at the magic angle configuration to eliminate the contribution of the rotational anisotropy of the probes on the observed fluorescence decays. The measured fluorescence decays were analyzed by following a reconvolution procedure.64,65 The instrument response function (IRF) was obtained by replacing the sample with a dilute scatterer solution (suspended TiO2 particles in water) and the IRF function thus obtained was used for the reconvolution analysis of the measured fluorescence decays. In all the solvents studied, the fluorescence decays of both the dyes were seen to fit well with a single-exponential function as

I(t) ) B exp (-t/τf)

TABLE 1: Absorption and Fluorescence Spectral Characteristics of the TQ1 Dye in Different Solvents and Solvent Mixtures solventsa

∆f

max λabs (nm)

λflmax (nm)

λflfit (nm)

∆νj (cm-1)

CHX CHX95EA05 CHX90EA10 CHX85EA15 CHX80EA20 CHX70EA30 CHX60EA40 CHX40EA60 CHX20EA80 EA EA95ACN5 EA90ACN10 EA80ACN20 EA60ACN40 EA40ACN60 EA20ACN80 ACN

0 0.025 0.046 0.058 0.081 0.108 0.128 0.160 0.183 0.201 0.210 0.240 0.259 0.280 0.292 0.300 0.305

470 469 469 468 466 466 465 464 463 464 464 463 465 467 467 466 465

495 508 524 530 539 551 558 591 598 609 613 624 630 641 647 655 662

525 531 542 549 563

2229 2454 2838 3153 3538 3334 3620 4608 4867 5131 5239 5573 5632 5813 5957 6215 6400

a Abbreviations for the solvents: CHX, cyclohexane; EA, ethyl acetate; ACN, acetonitrile. For mixed solvents, the subscripts represent the volume percentages of the respective cosolvents.

(4)

where τf is the fluorescence lifetime and B is the pre-exponential factor. The reduced χ2 values for all the accepted fits were close to unity and the weighted residuals were randomly distributed among the data channels.64,65 3. Results 3.1. Absorption and Fluorescence Spectral Characteristics of TQ1 and TQ2 Dyes. The absorption and fluorescence spectra of TQ1 and TQ2 dyes were recorded in different aprotic solvents and solvent mixtures listed in Table 1. Parts A and B of Figure 1 show the representative absorption spectra (normalized OD at the absorption maxima), and parts A and B of Figures 2 show the representative fluorescence spectra (normalized intensity at the emission maxima) of TQ1 and TQ2 dyes, respectively, recorded in some of the selected solvent systems. The absorption max (λmax abs ) and fluorescence (λfl ) maxima of the two dyes measured in different solvents and solvent mixtures are listed in Tables 1 and 2, respectively, along with the polarity parameter ∆f of the different solvent systems calculated using eq 3.44-56 As already mentioned, the use of protic solvents has been avoided in this study to exclude the specific solute-solvent interactions.61-63 max For the TQ2 dye, as indicated from Table 2, both λabs and λflmax values gradually undergo a red shift when the solvent polarity increases. The red shift is, however, much larger for max the λflmax values than the λabs values, suggesting the fluorescent state to be more polar in nature than the ground state.41,42,45-52

Figure 1. Absorption spectra of (A) TQ1 and (B) TQ2 dyes in some representative solvents of different polarities. Solvents 1-7 correspond to CH, CH9.5EA0.5, CH8EA2, CH6EA4, EA, EA8ACN2, and ACN, respectively. The solvent abbreviations are as in the footnote of Table 1.

For the TQ1 dye, as indicated from Table 1, the λflmax values undergo a large red shift with solvent polarity, quite similar to that observed for the TQ2 dye. Thus, the fluorescent state of the TQ1 dye is also indicated to be of polarity similar to that max values for the TQ2 dye. Unlike the TQ2 dye, however, the λabs for the TQ1 dye display somewhat unusual behavior, showing a small blue shift initially with ∆f in the lower solvent polarity region (∆f < 0.1) but at the higher solvent polarity region the max values show the usual red shift, as observed for the TQ2 λabs dye, though this red shift for the TQ1 dye is relatively much

4510


Shaikh et al.

Figure 2. Fluorescence spectra of (A) TQ1 and (B) TQ2 dyes in some representative solvents of different polarities. Solvents 1-7 correspond to CH, CH9.5EA0.5, CH8EA2, CH6EA4, EA, EA8ACN2, and ACN, respectively. The solvent abbreviations are as in the footnote of Table 1.

TABLE 2: Absorption and Fluorescence Spectral Characteristics of the TQ2 Dye in Different Solvents and Solvent Mixtures solventsa

∆f

max λabs (nm)

λflmax (nm)

λflfit (nm)

∆νj (cm-1)


0 0.025 0.046 0.058 0.081 0.108 0.128 0.160 0.183 0.201 0.210 0.240 0.259 0.280 0.292 0.300 0.305

491 492 492 493 494 497 499 501 505 504 506 508 511 512 516 519 518

511 539 552 560 572 585 590 606 618 617 626 633 643 653 663 668 673

544 549 556 563 570

1984 2110 2340 2522 2699 3027 3091 3458 3621 3634 3788 3887 4017 4217 4297 4298 4446

a Abbreviations for the solvents: CHX, cyclohexane; EA, ethyl acetate; ACN, acetonitrile. For mixed solvents, the subscripts represent the volume percentages of the respective cosolvents.

less than that of the TQ2 dye. A close inspection of the absorption spectra indicates that in nonpolar solvent cyclohexane (CHX) the absorption spectrum of the TQ1 dye shows two clear peaks, corresponding to the two reasonably resolved vibrational bands in the spectrum. As the solvent polarity is slowly increased, the resolution of the two vibrational bands initially decreases such that in the intermediate solvent polarity region the observed spectra become very broad and blunt. Once the solvent polarity is made reasonably high, however, the bluntness in the absorption spectra of TQ1 dye decrease again and the absorption peak becomes similarly sharp as that observed for

Figure 3. Plots of the νjabs and νjfl values for (A) TQ1 and (B) TQ2 dyes against the solvent polarity parameter ∆f. The data points represented by symbol (k) correspond to the λflfit values in the lower polarity solvents as discussed in the text.

TQ2 dye in all the solvents studied (cf. Figure 1A,B). From max values noted for these observations, it is evident that the λabs the TQ1 dye in different solvents and solvent mixtures might not correspond to the same vibrational band,66-68 and accordmax values may not show a regular red shift with ingly these λabs solvent polarity as otherwise observed for the TQ2 dye. max values for the TQ1 However, since the variations in the λabs dye in different solvents are always quite small (cf. Figure 1 and Table 1), in our forthcoming analysis and correlations we do not confer much emphasis on these changes except that these max values λmax abs values are used as such in combination with the λfl in estimating the values of the other important spectral properties, i.e., the Stokes’ shift for the dye in different solvents studied. Obviously, in the estimation of the Stokes’ shift values max it was assumed that the small abnormality observed in the λabs values will not cause any major effect in the correlations of the Stokes’ shift values of the dye with the polarity of the solvents used. Parts A and B of Figure 3 show the correlations of the absorption and fluorescence maxima (νjabs and νjfl, respectively, in cm-1) of the TQ1 and TQ2 dyes, respectively, with the solvent polarity function ∆f. For the TQ2 dye, the νjabs values apparently correlate linearly with ∆f (cf. Figure 3B), suggesting that the ground state of the dye is moderately polar in nature. For the max values do not shift systematically with TQ1 dye, since the λabs ∆f for the whole solvent polarity range studied, no clear correlation could be suggested for the νjabs vs ∆f plot (cf. Figure 3A). Moreover, for this dye, since the effective span of the changes in the νjabs values in different solvents is quite small, it is also difficult to comment much on the deviations observed on the νjabs values. Apparently, the small abnormality in the λmax abs values for the TQ1 dye in the lower to the intermediate solvent polarity region is partly due to the bluntness in the absorption max values do not correspond to spectra such that the observed λabs



the same vibrational band. In the higher solvent polarity region, max values of the however, since the bluntness is removed, the λabs dye show a clear red shift with solvent polarity used. Following this observation, which is quite similar to that of the TQ2 dye, we expect that the ground state of the TQ1 dye is also of polarity similar to that of the TQ2 dye. As shown in Figure 3A,B, for both the dyes, the νjfl values show a reasonably good linear correlation with ∆f for the moderate to higher solvent polarity region. The νjfl values, however, show a clear positive deviation from the above linearity in the lower solvent polarity region. A close inspection of the fluorescence spectra of the two dyes in this solvent polarity region indicates the presence of more than one vibrational bands (either clearly resolved or indicated as the shoulders) though the spectra are quite broad and structureless in the moderate to higher solvent polarity region (cf. Figure 2). Therefore, it is likely that a higher energy vibrational band might correspond to the observed λflmax values in the low polarity solvents than the one associated with the λflmax values in the moderate to higher polarity solvents. To explore this possibility further, we employed the Gaussian multiple peak fitting of the observed fluorescence spectra (considering either two or three peaks, as required) in the lower polarity solvents and selected the most suitable fitted peak (λflfit) that correlates best with the linear correlation observed in the νjfl vs ∆f plots for the two dyes for the moderate to higher solvent polarity region. The most suitable fitted peak positions thus selected for the TQ1 and TQ2 dyes in the lower polarity solvents are listed in Tables 1 and 2, respectively. Corresponding data points in the νjfl vs ∆f plots for the two dyes are shown by different symbols in Figure 3 for their easy identification. It is seen that with this approach the νjfl vs ∆f plots for both the dyes effectively show quite acceptable linear correlation for the whole range of the solvent polarity studied. This observation thus suggests that for both TQ1 and TQ2 dyes the nature of the fluorescent states remains essentially unchanged in all the solvents used, though the spectral feature changes quite significantly due to the solvent induced modulation in the vibrational bands. The reasonably high slopes of the νjfl vs ∆f plots for both the dyes further suggest that the fluorescent states of these dyes are highly polar in nature, possibly of intramolecular charge transfer (ICT) character. In different solvents, considering that the absorption and fluorescence spectra involve the electronic transitions between the same two ground and excited electronic states, the Stokes’ shift (∆νj) is expected to follow a linear relation with ∆f, as suggested by the Lippert and Mataga equation as41,42,55,56

∆ν¯ ) ∆ν¯ 0 +

2(µe - µg)2 hcr3

∆f

(5)

where µe and µg are the excited (fluorescent) state and the ground state dipole moments of the dye, h is Planck’s constant, c is the velocity of light, and r is the Onsager radius of the dipole-solvent interaction sphere. The ∆νj values for TQ1 and TQ2 dyes in different solvents were estimated (in cm-1) max considering either the λabs and λflmax values (for moderate to max and λflfit values (for lower higher polarity solvents) or the λabs polarity solvents) as applicable. The ∆νj values thus estimated for the two dyes are listed in Tables 1 and 2, respectively. Parts A and B of Figure 4 show the ∆νj vs ∆f plots for the two respective dyes. It is seen from these plots that the ∆νj values correlate quite linearly with the ∆f values for all the solvents studied.

Figure 4. Plots of the ∆νj values for (A) TQ1 and (B) TQ2 dyes against the solvent polarity parameter ∆f. The data points represented by symbol (k) correspond to the λflfit values in the lower polarity solvents as discussed in the text.

To analyze the ∆νj vs ∆f plots following eq 5, it is required to have an estimate for the Onsager radius r for the two dyes. For large molecules like the present TQ1 and TQ2 dyes, it is difficult to find an estimate for the Onsager radius. However, considering the oxidation and reduction centers in the TQ1 and TQ2 dyes,69 it is expected that in their ICT structures, the positive and the negative charge centers will effectively be localized on the nitrogen atom of the anilino group and the pyrazine ring of the thiazolo-quinoxalin moiety, respectively. Based on this consideration, the separation between the positive and negative charge centers in the two dyes is approximately estimated as about 11.2 Å (obtained from geometry optimized structures using Chem3D ultra 8.0) and accordingly the value of the Onsager radius r in the present cases is assumed to be about 5.6 Å. Using this r value, the (µe - µg) values for TQ1 and TQ2 dyes are estimated using the slopes of the respective ∆νj vs ∆f plots following eq 5 and are found to be about 15.3 and 12.0 D, respectively. These (µe - µg) values are substantially high, suggesting that the fluorescent states of both the dyes are of strong ICT character, as inferred earlier. That the (µe - µg) value for TQ1 dye is somewhat higher than that of the TQ2 dye is possibly due to the higher µg for the latter dye compared to that of the former dye. This we expect because for the TQ2 max values showed a significant red shift with solvent dye the λabs polarity but not for TQ1 dye. It is possible that compared to the two flexible phenyl substituents in the anilino group in TQ1 dye the rigid julolidinyl group favors the TQ2 dye to undergo higher ICT from the anilino group to the thiazolo-quinoxalin moiety. This is in fact further emphasized in the nonradiative deexcitation processes of the two dyes as will be discussed latter (cf. section 4). 3.2. Fluorescence Quantum Yields and Fluorescence Lifetimes of TQ1 and TQ2 Dyes. Fluorescence quantum yields (Φf) of TQ1 and TQ2 dyes were estimated in different solvents at ambient temperature (25 ( 1 °C) by using a comparative

4512


Shaikh et al.

TABLE 3: Fluorescence Quantum Yields and Fluorescence Lifetimes of the TQ1 Dye in Different Solvents and Solvent Mixturesa solventsb

∆f

Φf

τf (ns)

kf (108 s-1)

knr (109 s-1)


0 0.025 0.046 0.058 0.081 0.108 0.128 0.160 0.183 0.201 0.210 0.240 0.259 0.280 0.292 0.300 0.305

0.045 0.078 0.114 0.155 0.203 0.234 0.273 0.300 0.280 0.240 0.200 0.155 0.105 0.073 0.054 0.030 0.018

0.45 0.76 1.05 1.22 1.42 1.64 1.91 1.97 1.83 1.59 1.35 1.02 0.76 0.52 0.39 0.25 0.16

1.00 1.03 1.09 1.27 1.43 1.43 1.43 1.52 1.53 1.51 1.48 1.52 1.38 1.40 1.38 1.20 1.13

2.12 1.21 0.84 0.69 0.56 0.47 0.38 0.36 0.39 0.48 0.59 0.83 1.18 1.78 2.43 3.88 6.14

a The radiative and nonradiative rate constants of the dye in different solvents and solvent mixtures are also listed. b Abbreviations for the solvents: CHX, cyclohexane; EA, ethyl acetate; ACN, acetonitrile. For mixed solvents, the subscripts represent the volume percents of the respective cosolvents.

TABLE 4: Fluorescence Quantum Yields and Fluorescence Lifetimes of the TQ2 Dye in Different Solvents and Solvent Mixturesa solventsb

∆f

Φf

τf (ns)

kf (107 s-1)

knr (1010 s-1)


0 0.025 0.046 0.058 0.081 0.108 0.128 0.160 0.183 0.201 0.210 0.240 0.259 0.280 0.292 0.300 0.305

0.009 0.023 0.038 0.061 0.073 0.103 0.123 0.14 0.133 0.123 0.102 0.072 0.055 0.022 0.014 0.008 0.004

0.12 0.29 0.44 0.63 0.88 1.11 1.22 1.49 1.46 1.44 1.21 0.92 0.78 0.40 0.27 0.17 0.11

7.50 8.04 8.72 9.68 8.30 9.28 10.10 9.40 9.11 8.54 8.43 7.83 7.05 5.54 5.24 4.73 3.57

8.33 3.49 2.29 1.58 1.13 0.89 0.81 0.66 0.68 0.69 0.82 1.08 1.27 2.51 3.74 5.91 8.93

a

The radiative and nonradiative rate constants of the dye in different solvents and solvent mixtures are also listed. b Abbreviations for the solvents: CHX, cyclohexane; EA, ethyl acetate; ACN, acetonitrile. For mixed solvents, the subscripts represent the volume percents of the respective cosolvents.

method (cf. section 2) and the values thus obtained for the two dyes are listed in Tables 3 and 4, respectively. The solvent polarity dependent changes in the Φf values for the two dyes are shown in Figure 5A,B, respectively. It is seen that for both the dyes the Φf values show effectively a linear increase with ∆f at the lower solvent polarity region and effectively a linear decrease with ∆f at the higher solvent polarity region, causing a clear reversal in the trends at an intermediate ∆f value of ∼0.14 for TQ1 and ∼0.16 for TQ2 dye. Such a reversal in the trends for the Φf values clearly suggests that for the fluorescent states of the TQ1 and TQ2 dyes there are in fact two different nonradiative deexcitation processes, of which one process dominates in the lower solvent polarity region and the other process dominates in the higher solvent polarity region.

Figure 5. Plots of the Φf values for (A) TQ1 and (B) TQ2 dyes against the solvent polarity parameter ∆f. The Φf values increase with ∆f in the lower polarity solvents and decrease with ∆f in the higher polarity solvents showing a transition at ∆f ∼ 0.14 and 0.16 for the two respective dyes.

To understand the further details of the excited state processes, the fluorescence decays of the two dyes were measured in different solvents at ambient temperature (25 ( 1 °C). All these decays were measured at the respective emission maxima of the dyes in different solvents. In all these cases, the observed decays were seen to fit well with a single-exponential function (cf. eq 4). The τf values obtained for TQ1 and TQ2 dyes in different solvents are listed in Tables 3 and 4, respectively. Changes in the τf values of the two dyes as a function of the solvent polarity function ∆f are shown in Figure 6A,B, respectively. It is seen that just like the Φf values, the τf values for the TQ1 and TQ2 dyes also show a linear increase in the lower solvent polarity region, attain a maximum at an intermediate ∆f value (∼0.14 for TQ1 and ∼0.16 for TQ2), and then decrease again following a linear correlation in the higher solvent polarity region. These observations thus support our proposition of the existence of two different nonradiative deexcitation processes for the fluorescent states of TQ1 and TQ2 dyes, one dominating in the lower polarity solvents and the other dominating in the higher polarity solvents. To understand more about the deexcitation processes in the fluorescent states of TQ1 and TQ2 dyes, the radiative (kf) and nonradiative (knr) decay rate constants in different solvents were estimated using the following two photophysical relations.41,42

kf ) Φf/τf

(6)

knr ) (1/τf) - (Φf/τf)

(7)

The kf and knr values thus estimated in different solvents for TQ1 and TQ2 dyes are listed in Tables 3 and 4, respectively. Parts A and B of Figure 7 show the kf vs ∆f plots for the two respective dyes. Although the scattering in the data points are


Figure 6. Plots of the τf values for (A) TQ1 and (B) TQ2 dyes against the solvent polarity parameter ∆f. The τf values increase with ∆f in the lower polarity solvents and decrease with ∆f in the higher polarity solvents showing a transition at ∆f ∼ 0.14 and 0.16 for the two respective dyes.

Figure 7. Plots of the kf values for (A) TQ1 and (B) TQ2 dyes against the solvent polarity parameter ∆f. The kf values increase with ∆f in the lower polarity solvents and decrease with ∆f in the higher polarity solvents showing a transition at ∆f ∼ 0.14 and 0.16 for the two respective dyes.

somewhat higher, the kf vs ∆f plots show similar trends of increase in the lower solvent polarity region and decrease in the higher solvent polarity region, as observed with Φf and τf values. Since the fluorescent states of both TQ1 and TQ2 dyes were indicated to remain the same (ICT character) in all the


Figure 8. Plots of the knr values for (A) TQ1 and (B) TQ2 dyes against the solvent polarity parameter ∆f. The insets show the expanded plots for the intermediate solvent polarity region. The knr values increase with ∆f in the lower polarity solvents and decrease with ∆f in the higher polarity solvents showing a transition at ∆f ∼ 0.14 and 0.16 for the two respective dyes.

solvents studied (linear ∆νj vs ∆f plots with high slope; cf. Figure 4), the kf values of the two dyes were expected to follow a single linear correlation for the whole solvent polarity range, as observed earlier for a number of coumarin, anthraquinone, and phenazine dyes.45-52,60-63,66-68 That in the present cases the kf vs ∆f plots show two different correlations in the lower and the higher solvent polarity regions is possibly due to the modulations in the fluorescent probability from ICT states of these dyes by the influence of the kinetically coupled trans-cis isomerization and TICT state formation (discussed latter) in the two solvent polarity regions, respectively; though from the present results we cannot comment further on this issue. The knr vs ∆f plots for TQ1 and TQ2 dyes are shown in Figure 8A,B, respectively. For both the dyes, when the ∆f value of the solvents increases, the knr value initially decreases, goes through a minimum at an intermediate solvent polarity (∆f ∼ 0.14 for TQ1 dye and ∼ 0.16 for TQ2 dye), and then increases again very sharply in the higher solvent polarity region. These observations clearly indicate that the fluorescent sates of these dyes are at least coupled to two distinctly different nonradiative decay channels, one dominating in the lower solvent polarities and the other dominating in the higher solvent polarities. Moreover, it is evident from the present results that the propensity of the nonradiative channel operating in the lower solvent polarity region decreases when the solvent polarity increases whereas that in the higher solvent polarity region increases when the solvent polarity increases. As the results indicate, in the intermediate solvent polarity (∆f ∼ 0.14 for TQ1 dye and ∼ 0.16 for TQ2 dye), apparently the effects of both of these nonradiative decay channels become quite less or negligible such that the knr value goes through its minimum and accordingly the Φf and τf values pass through their respective maxima, displaying the interesting inversion in the photophysical properties of these dyes when the solvent polarity changes.

4514


4. Discussion The most fascinating observation in the photophysical properties of the TQ1 and TQ2 dyes is the contrasting behavior in the trends for the Φf and τf values in the lower and higher solvent polarity region and consequently the reversal in the parameters at an intermediate ∆f value of the solvents. Though the photophysical properties of large number of aminostyryl derivatives have been reported in the literature,26-39 such an interesting reversal in the Φf and τf values when the solvent polarity changes is not reported so explicitly except in the report of Gruen and Gorner69 on some 4-nitro-4′-(dialkylamino)stilbenes in some selected solvents where the Φf values have apparently indicated a behavior qualitatively similar to the present observation, though not much has been discussed by the authors on this observation. For the present dyes, following the observation that the Stokes’ shift (∆νj) effectively changes linearly with ∆f for the whole solvent polarity range studies, it is suggested that the basic nature of the fluorescent states of the TQ1 and TQ2 dyes remains unchanged on changing the solvent polarity. When the solvent polarity changes, the νjfl values change much more sharply than νjabs values, suggesting that the fluorescent states of these dyes are much more polar in nature than their ground states. The large slopes obtained from the ∆νj vs ∆f plots and consequently the large changes in the dipole moments (µe µg) calculated clearly suggest that the fluorescent states of the TQ1 and TQ2 dyes are of strong ICT character. Such a strong ICT character is possible if the dye molecules in the excited state adopt a planar or quasi-planar conformation such that a large extent of charge is transferred from the strong electron donating N,N-disubstituted aniline moiety to the strong electron accepting thiazolo[4,5-b]quinoxaline moiety via the participation of the styryl π-conjugation. It should be mentioned that a planar or quasi-planar conformation for the fluorescence states of different styryl dyes with strong ICT character has also been suggested in the literature on the basis of the photophysical properties of such dyes.26-40,70,71 As discussed in the literature, the fluorescence of the styryl dyes is mainly from their trans isomers, because the cis isomers of these dyes are either nonfluorescent or very weakly fluorescent in nature.26-40,70,71 Drawing an analogy, we also propose that the fluorescence of the TQ1 and TQ2 dyes are due to the emission from their trans isomers in the excited state {trans(S1)}. It is also reported in the literature that the styryl dyes undergo very efficient trans a cis isomerization in the excited state via the rotation around the styryl double bond of the molecules.1-19,26-40,70,71 As indicated in the literature, the trans a cis isomerization in the styryl dyes mainly occurs in the singlet excited state, because the triplet yields for these dyes are usually very low.26-40,70-72 It is further understood that the trans a cis isomerization in the styryl dyes is not a single step process but occurs via the involvement of an intermediate state {perp(S1)} with perpendicular or near perpendicular configuration of the substituents (dihedral angle ∼90°) at the styryl double bond and for this {perp(S1)} state the potential energy minimum is quite close to the potential energy maximum of the corresponding ground state configuration {perp(S0)}.26-40,70-72 Therefore, as the energy gap between the {perp(S1)} and {perp(S0)} states is very small, there is a very fast nonradiative deexcitation of the {perp(S1)} state to the {perp(S0)} state, from where the molecules quickly relax either to {trans(S0)} or to {cis(S0)} configurations with almost equal probability. Considering this mechanistic scheme, the deactivation of the fluorescent {trans(S1)} states of the present dyes can be qualitatively represented by Scheme 1 where hνa and hνf represent the

Shaikh et al. SCHEME 1: Deexcitation Pathways of the Fluorescent {trans(S1)} States of the TQ1 and TQ2 Dyes in the Nonpolar to Intermediate Solvent Polarity Regiona

a The propensity of the channel ktwist followed by kd gradually decreases when the solvent polarity increases. The channel effectively becomes nonoperative for TQ1 and TQ2 dyes in the intermediate solvent polarity region (∆f ∼ 0.15 ( 0.01).

absorption and fluorescence photons, kf and kIC represent the fluorescence and the internal conversion (IC) rates of the {trans(S1)} state, ktwist represents the activation controlled conversion of {trans(S1)} to {perp(S1)} configuration (intermediate state), kd represents the fast nonradiative deexcitation from the {perp(S1)} to {perp(S0)} state, and R and (1 - R) represent the fractions of the molecules relax to the trans(S0) and the cis(S0) configurations, respectively, from the energy maximum of the {perp(S0)} state. From the earlier studies it has been indicated that the trans a cis isomerization process in the styryl dyes is very efficient in a nonpolar solvent and the propensity of the process decreases when the solvent polarity increases.70-72 Thus, following Scheme 1, the photophysical properties of the TQ1 and TQ2 dyes in the nonpolar to medium polarity solvents (until the Φf and τf values show the inversion) can be rationalized by considering the effect of solvent polarity on the isomerization process. As the observed results indicate, in the nonpolar to medium polarity solvents, the overall nonradiative deexcitation rate (knr) decreases almost exponentially with ∆f. As transpires from Scheme 1, the rate constant kd for the deactivation of the {perp(S1)} state to the {perp(S0)} state is unusually high and hence the {trans(S1)} state to the {perp(S1)} state conversion (rate constant ktwist) becomes the rate determining step. Since the conversion of {trans(S1)} to {perp(S1)} requires a twisting of the molecule around the styryl double bond, it is evident that this process would be strongly activation controlled. On the contrary, the IC process in the {trans(S1)} state is not expected to involve any activation barrier. Further, as the results indicate (cf. Figure 8A,B), the activation barrier (Ea) associated with ktwist is supposed to be very small in a nonpolar solvent but it increases quite sharply as the solvent polarity is increased. Considering Figure 8A,B, it is expected that at an intermediate solvent polarity, which appears to be ∆f ∼ 0.14 for TQ1 and ∼ 0.16 for TQ2 dyes, the value of Ea is so high that ktwist becomes quite negligible compared to kIC. Accordingly, the overall nonradiative deexcitation rate constant knr for the {trans(S1)} state effectively approaches the kIC value for the TQ1 and TQ2 dyes at the intermediate solvent polarities. In general, however, for the nonpolar to the intermediate solvent polarity region, the overall knr for the {trans(S1)} state should be expressed as

knr ) kIC + ktwist

(8)

The role of increasing solvent polarity in the nonpolar to intermediate polarity region on the stabilizations of different



Figure 9. Qualitative potential energy diagram for the TQ1 and TQ2 dyes, as applicable for the nonpolar to intermediate solvent polarity region. With increasing solvent polarity, as the Ea increases the propensity of the twisting process decreases accordingly.

configurations of the TQ1 and TQ2 dyes and the consequent modulations in their photophysical properties can be visualized by using the conceptual potential energy diagrams shown in Figure 9. In these presentations, the solvent stabilizations on potential energy surfaces have been considered only for the excited states, keeping the relative ground state energies unchanged. Such a presentation is adequate enough because our interest is to rationalize the solvent polarity effect on the excited state properties of the two dyes studied. In a nonpolar solvent, since there is almost no dielectric interaction, the strongly polar (high ICT character) trans(S1) states of the dyes hardly undergo any solvent stabilization and hence the activation barrier (Ea) associated with ktwist will be very small or negligible. As the solvent polarity is gradually increased, the trans(S1) state undergoes gradually increasing solvent stabilization, causing an increase in the Ea value. To understand how Ea changes with solvent polarity, we can use the Lippert-Mataga concept (cf. eq 5),41,42,55,56 for which the difference in the solvent stabilization energies between two electronic states is proportional to the solvent polarity parameter ∆f, with the condition that the dipole moments of the two states concerned do not change when the solvent polarity changes. In the present context, the activation barrier Ea is the difference in the solvent stabilization energies between the transition state (TS) of the twisting process (cf. Figure 9) and the {trans(S1)} state, and it is quite logical to assume that the dipole moments of the above two states will not vary significantly when the solvent polarity changes. Accordingly, we can express Ea as equal to B∆f, where the proportionality constant B is a function of the dipole moments of the above two states concerned and hence the rate constant ktwist for the twisting process in Scheme 1 can be expressed as 0 ktwist ) ktwist exp(-B∆f/RT)

(9)

0 where ktwist is the preexponential factor. Using eqs 8 and 9, we can thus get the more explicit expression for the overall nonradiative rate constant knr for the {trans(S1)} state as

0 knr ) kIC + ktwist exp(-B∆f/RT)

(10)

Thus, it was interesting to analyze the changes in the knr values for the TQ1 and TQ2 dyes in the nonpolar to the intermediate polarity region by using eq 10. For this purpose we considered

Figure 10. Fitting of the overall knr values for (A) TQ1 and (B) TQ2 dyes in the nonpolar to intermediate solvent polarity region by using eq 10.

TABLE 5: Parameters for TQ1 and TQ2 Dyes As Obtained by Fitting the knr Values Following Eq 10 for the Nonpolar to Intermediate Polarity Region and Following Eq 11 for the Intermediate to Higher Polarity Region dye

B 0 ktwist (s-1) (kcal mol-1)

TQ1 1.75 × 109 TQ2 7.52 × 1010

18.4 24.9

kIC (s-1)

B′ 0 kTICT (s-1) (kcal mol-1)

3.61 × 108 1.56 × 104 7.48 × 109 4.66 × 104

24.7 27.8

kIC in eq 10 to be a constant, assuming that kIC does not vary significantly when the polarity of the solvents changes.41,42,48-52 Accordingly, the observed knr values for the two dyes in the nonpolar to the intermediate solvent polarity region were fitted according to eq 10. The fitted curves thus obtained for the two dyes are shown in Figure 10A,B, respectively. As indicated in Figure 10, for both TQ1 and TQ2 dyes the knr values in the nonpolar to the intermediate polarity region fit very nicely 0 , B, and kIC values thus obtained for following eq 10. The ktwist the two dyes are listed in Table 5. As indicated from Table 5, the B values obtained for both the dyes are very high, suggesting that the Ea for the twisting process is a strong function of the solvent polarity. The high B values also suggest that the TS for the twisting process is much less polar in character in comparison to the polarity of the {trans(S1)} state of the dyes. Accordingly, the twisting process becomes very efficient in a nonpolar solvent (due to very low Ea) and the propensity of the process decreases sharply as the polarity of the solvent is gradually increased (due to sharply increasing Ea with solvent 0 values suggest that in the intermediate polarity). The low ktwist solvent polarities of ∆f ∼ 0.14-0.16, the ktwist hardly contributes to knr and hence knr effectively becomes similar to kIC. Thus it is evident that the strong solvent polarity dependent changes in the twisting process effectively causes the Φf and τf values of the two dyes to increase gradually when the solvent polarity increases in the nonpolar to intermediate solvent polarity region.

4516


The observation that the B value for the TQ2 dye is significantly higher than that of the TQ1 dye suggests that the presence of the julolidinyl ring favors the {trans(S1)} state of the TQ2 dye to acquire higher ICT character than the {Trans(S1)} state of the TQ1 dye having two flexible phenyl substituents at its anilino group. Though the changes in the Φf and τf values for TQ1 and TQ2 dyes with ∆f values can be suitably accounted for the nonpolar to intermediate polarity solvents by considering the participation of the styryl double bond twisting process, the observation that the Φf and τf values follow a reverse trend in the intermediate to higher polarity solvents (cf. Figures 5 and 6) suggests that, in the latter cases, there is a new nonradiative deexcitation process other than the styryl double bond twisting process, and the propensity of this process increases almost exponentially when the ∆f values of the solvents increase. It is apparent from the observed results (cf. Figure 8) that this new nonradiative process starts operating for the two dyes as the solvent polarity exceeds ∆f ∼ 0.14 for TQ1 and ∆f ∼ 0.16 for the TQ2 dye. In the literature, it is reported that the chromophoric dyes consisting of both donor and acceptor moieties in the single molecule often undergo conversion from the initially photoproduced ICT state to a highly polar twisted intramolecular charge transfer (TICT) state, if the solvent polarity is made reasonably high.48,60,61,73-79 In the TICT state, there is a complete electron transfer (ET) from the donor moiety to the acceptor moiety, and this ET process is assisted by the twisting of the molecule around a suitable single bond. As evident, a TICT state is highly polar in nature and hence it undergoes large stabilization in a polar solvent. Though in nonpolar or in moderately polar solvents the TICT state is higher in energy than the initially photoproduced ICT state and hence cannot participate in deexciting the excited molecules, but when the solvent polarity increases beyond a certain critical limit, the TICT state can be stabilized substantially and hence start participating in the deexcitation mechanism of the excited dyes. The propensity of the process, however, being to increase largely when the solvent polarity increases further. Since the TQ1 and TQ2 dyes also belong to the above class of donor-acceptor molecules, participation of the TICT states is suggested to be responsible for the new nonradiative deexcitation process for the two dyes in the intermediate to higher solvent polarity region. In many donor-acceptor molecules, their TICT states are known to be either nonfluorescent or very weakly fluorescent in nature.48,60,61,73-79 In the photophysics of these dyes, the main role of their TICT states is to introduce a strong nonradiative deexcitation for the fluorescent ICT state, causing a large reduction in their Φf and τf values in high polarity solvents. This happens because the ICT to TICT conversion increases largely when the solvent polarity increases, and the TICT state deexcites (by nonradiative process) very quickly to the ground state due to a very small energy gap between the latter two states. For the TQ1 and TQ2 dyes, we have already discussed that the initially photoproduced excited state {trans(S1)} is of ICT character and the fluorescence of these dyes arises mainly from this {trans(S1)} state. Since no new fluorescence band corresponding to the TICT state could be observed for the present dyes in any of the solvents studied, we infer that the TICT states in the present cases are nonfluorescent in nature. We thus propose that the drastic reduction in the Φf and τf values for the TQ1 and TQ2 dyes in the intermediate to higher polarity solvents is due to the involvement of a nonfluorescent TICT state to cause very fast nonradiative deexcitation of excited dye molecules.

Shaikh et al. SCHEME 2: Deexcitation Pathways of the Fluorescent {trans(S1)} States of the TQ1 and TQ2 Dyes in the Intermediate to Higher Solvent Polarity Regiona

a The propensity of the channel kTICT followed by kd′ increases very drastically when the solvent polarity increases. The channel is, however, nonoperative for TQ1 and TQ2 dyes in the solvents below ∆f ∼ 0.15 ( 0.01.

In donor-acceptor molecules containing N,N-dialkyl/diarylamino substituent (NR2/NAr2) in the donor moiety, it is often indicated that the TICT formation actually involves the rotation of this NR2/NAr2 group around the single bond connecting this group to its adjacent chromophoric moiety (Φ′).48,60,61,73-79 Though such a rotation for the NAr2 group around the N-Φ′ single bond is quite possible for the TQ1 dye to form its TICT state, a similar rotation around the N-Φ′ single bond is impossible in the TQ2 dye because of the presence of the julolidinyl structure (cf. Chart 1). There are, however, reports in the literature that in some donor-acceptor molecules the TICT state formation can also involve the rotation of the complete anilino group (PhNR2/PhNAr2) around the C-C single bond that connects this group to the rest of the chromophore rather than rotation of just the NR2/NAr2 substituent.26-29 In the present study, since both the TQ1 and TQ2 dyes behave quite similarly for the intermediate to higher solvent polarity region, and since the rotation around the N-Φ′ single bond is not possible for the TQ2 dye, it is possible that for both the dyes the rotation of the complete anilino group around the C-C single bond connecting the rest of the molecule is mainly responsible for their TICT state formation. It is, however, not very unlikely as well that the TICT state formation for the TQ1 dye involves the rotation around the N-Φ′ single bond and that for the TQ2 dye involves the rotation around the C-C single bond, though to resolve such an issue one needs to carryout ultrafast studies, which are presently under consideration. In the present context, however, it is evident that the deactivation of the fluorescent states of the TQ1 and TQ2 dyes in the intermediate to higher polarity solvents is mainly due to the involvement of the nonfluorescent TICT states of these molecules, and such a mechanism can suitably be represented by the following generalized Scheme 2. In this scheme, the parameters kf and kIC are the same as in Scheme 1, kTICT represents the {trans(S1)} to TICT conversion rate and kd′ represents the fast nonradiative deexcitation of the TICT state to the ground state of the dyes. As discussed earlier, for the intermediate to higher polarity solvents the involvement of the styryl double bond twisting process appears to be negligible (because of very high Ea for the process) and, accordingly, in this solvent polarity region the main deexcitation rate constants to be considered for the {trans(S1)} state are kf, kIC, and kTICT. On the basis of the TICT mechanism and considering that the {trans(S1)} to TICT formation is an activation controlled process with activation barrier Ea′, it is expected that the Ea′



Figure 11. Qualitative potential energy diagram for the TQ1 and TQ2 dyes, as applicable for the intermediate to higher solvent polarity region. With increasing solvent polarity, as the activation barrier Ea′ for the {trans(S1)} to TICT conversion decreases, the propensity of the TICT mediated nonradiative deexcitation process increases accordingly.

value should gradually decrease when the solvent polarity increases. With this consideration, the changes in the photophysical properties of the TQ1 and TQ2 dyes in the intermediate to higher solvent polarity region can conceptually be visualized by the qualitative potential energy diagrams, shown in Figure 11. In this figure, for the convenience of presentation, the effect of increasing solvent polarity is shown only for the excited state, keeping the relative positions of the ground state energies unchanged. Following the observed results, until the solvent polarity reaches a critical ∆f value of ∼0.14 for the TQ1 dye or ∼0.16 for the TQ2 dye, the TICT state energy is higher than that for the {trans(S1)} state and, accordingly, does not participate in the deexcitation mechanism. Once the solvent polarity exceeds these critical ∆f values, the TICT states of the respective dyes stabilize substantially so that the {trans(S1)} to TICT conversion starts contributing in the deexcitation mechanism of the excited molecules. When the solvent polarity increases further, the TICT state gradually becomes more stabilized and the activation barrier Ea′ decreases largely, resulting an exponential increase in the TICT mediated nonradiative deexcitation process of the excited molecules, and accordingly, there is a gradual decrease in the Φf and τf values of the two dyes in the intermediate to higher solvent polarity region when the solvent polarity increases. In line with the discussion related to the activation barrier Ea for the styryl double bond twisting process, the activation barrier Ea′ for the TICT state formation can also be related to the solvent polarity by Ea′ ) -B′∆f, where the proportionality constant B′ is dependent on the dipole moments of the {trans(S1)} state and the TS for the {trans(S1)} to TICT conversion.41,42,55,56 Accordingly, the overall nonradiative rate constant knr for the {trans(S1)} state in the intermediate to the higher solvent polarity regions can be expressed as 0 knr ) kIC + kTICT exp(B′∆f/RT)

(11)

where k0TICT is the preexponential factor. Thus, for the intermediate to higher polarity solvents, the overall knr values for the TQ1 and TQ2 dyes were fitted following eq 11. As discussed earlier, in the present analysis also we consider kIC as a constant and its value is fixed as that obtained in the earlier analysis of the knr values in the nonpolar to intermediate region following eq 10. The fitted curves thus obtained for the two dyes are shown in Figure 12A,B, respectively. As indicated from this figure, 0 and B′ values thus the fits are reasonably good. The kTICT obtained for the two dyes are listed in Table 5. As indicated

Figure 12. Fitting of the overall knr values for (A) TQ1 and (B) TQ2 dyes in the intermediate to higher solvent polarity region by using eq 11.

from Table 5, the B′ values obtained for both the dyes are exceptionally high, suggesting that the {trans(S1)} to TICT conversion is very strongly dependent on the solvent polarity. Very high B′ values also suggest that the TS for the {trans(S1)} to TICT conversion process is very polar in nature in comparison to the polarity of the {trans(S1)} state of the dyes. The low k0TICT values suggest that in the intermediate solvent polarities of ∆f ∼ 0.14-0.16, the kTICT hardly contributes to knr and hence knr effectively becomes similar to kIC. It is thus evident that the unusual reversal in the photophysical properties of TQ1 and TQ2 dyes at the intermediate solvent polarity is suitably explained by invoking the participation of the styryl double bond twisting mechanism in the nonpolar to the intermediate solvent polarity region and that of the TICT state of the dyes in the intermediate to higher solvent polarity region. To the best of our knowledge, such an interesting reversal in the photophysical properties of the styryl or other dyes with solvent polarity is not so explicitly analyzed in any of the reported studies. 5. Conclusion Solvent polarity effects on the photophysical properties of two newly synthesized donor-acceptor dyes, namely, N,Ndiphenyl-4-[2-(thiazolo[4,5-b]quinoxalin-2-yl)vinyl]aniline (TQ1) and 9-[2-(thiazolo[4,5-b]quinoxalin-2-yl)vinyl]julolidine (TQ2), have been investigated in different aprotic solvents and solvent mixtures. Structurally, the dye TQ1 contains a flexible diphenylamino group at the donor moiety while the dye TQ2 contains a rigid julolidinylamino group at the equivalent position. From the absorption and fluorescence spectral studies, it is indicated that the fluorescent states of both the dyes are of intramolecular charge transfer (ICT) character. In the photophysical parameters like fluorescence quantum yields (Φf), fluorescence lifetimes (τf), radiative rate constants (kf ) Φf/τf), and nonradiative rate constants (knr ) 1/τf - Φf/τf), both dyes show interesting and

4518


quite unusual changes when the solvent polarity changes. Thus, the solvent polarity causes a contrasting effect on the photophysical properties of the two dyes in the lower and in the higher solvent polarity region, resulting in the photophysical parameters displaying an interesting reversal in their values below and above an intermediate solvent polarity of ∆f ∼ 0.14 for the TQ1 dye and ∆f ∼ 0.16 for the TQ2 dye. It is inferred from the present study that two excited state processes are actually responsible for the above unusual effects: one is the twisting of the molecule around the styryl double bond, which dominates in a nonpolar solvent and its effect gradually decreases when the solvent polarity increases, and the other is the formation of the twisted intramolecular charge transfer (TICT) state of the molecule, which starts participating only at an intermediate solvent polarity (∆f ∼ 0.14 for TQ1 dye and ∆f ∼ 0.16 for TQ2 dye) and its effect gradually increases as the solvent polarity is increased further. As indicated from the observed results, at ∆f ∼ 0.14 for the TQ1 dye and ∆f ∼ 0.16 for the TQ2 dye, both the styryl double bond twisting and the TICT state formation processes apparently contribute very negligibly in the deexcitation of the excited dye molecules and, accordingly, the Φf and τf values pass through their maximum values at these intermediate solvent polarities. Observed results of the two dyes have been discussed and rationalized using suitable mechanistic schemes for the excited state processes. Qualitative potential energy diagrams have also been presented to understand and rationalize the observed solvent polarity effect on the photophysical properties of the TQ1 and TQ2 dyes. The interesting reversals of the photophysical properties of the TQ1 and TQ2 dyes as observed in the present study are very unusual observations and to the best of our knowledge such observations are not so far reported, discussed, or analyzed so explicitly for any of the chromophoric dyes including the styryl dyes. Acknowledgment. We are thankful to Dr. T. Mukherjee, Director, Chemistry Group, and Dr. S. K. Sarkar, Head, Radiation & Photochemistry Division, BARC, for their constant encouragement and support during the course of this work. Supporting Information Available: Synthesis and characterization of TQ1 and TQ2 dyes. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Zhou, G.; Wang, D.; Ren, Y.; Yang, S.; Xu, X.; Shao, Z.; Cheng, X.; Zhao, X.; Fang, Q.; Jiang, M. Appl. Phys. B 2002, 74, 147. (2) Qin, C.; Zhang, W.; Wang, Z.; Zhou, M.; Wang, X.; Chen, G. Opt. Mater. 2008, 30, 1607. (3) Ruland, G.; Gvishi, R.; Prasad, P. N. J. Am. Chem. Soc. 1996, 118, 2985. (4) Zhao, C. F.; Gvishi, R.; Narang, U.; Ruland, G.; Prasad, P. N. J. Phys. Chem. 1996, 100, 4526. (5) He, G. S.; Bhawalkar, J. D.; Zhao, C. F.; Prasad, P. N. Appl. Phys. Lett. 1995, 67, 2433. (6) Zheng, Q.; He, G. S.; Lin, T.-C.; Prasad, P. N. J. Mater. Chem. 2003, 13, 2499. (7) Abraham, U. An introduction to ultrathin organic films: From Langmuir-Blodgett to self-assembly; Academic Press: Boston, 1991. (8) Messier, J., Kajzar, F., Prasad, P., Eds. Organic molecules for nonlinear optics and photonics; Klumer Academic Publishers: Dordrecht, Boston, 1991. (9) Chemla, D. S.; Zyss, J. Nonlinear optical properties of organic molecules and crystals; Academic Press: Orlando, FL, 1987. (10) Dtirr, H.; Bouas-Laurent H. Photochromism-molecules and systems; Elsevier: Amsterdam, 1990. (11) Clarke, R. J.; Kane, D. J. Biophys. J. 2007, 93, 4187. (12) Clarke, R. J.; Apell, H.-J.; Kong, B. Y. Biochemistry 2007, 46, 7034. (13) Jones, M. A.; Bohn, P. W. Anal. Chem. 2000, 72, 3776.

Shaikh et al. (14) Zhang, J.; Davidson, R. M.; Wei, M. D.; Loew, L. M. Biophys. J. 1998, 74, 48. (15) Canepari, M.; Campani, M.; Spadavecchia, L.; Torre, V. Eur. Biophys. J. 1996, 24, 359. (16) Cornelius, F.; Fedosova, N. U.; Klodos, I. Biochemistry 1998, 37, 16686. (17) Ashwell, G. J.; Jackson, P. D.; Crossland, W. A. Nature 1994, 368, 438. (18) Duan, X. M.; Konami, H.; Okada, S.; Oikawa, H.; Matsuda, H.; Nakanishi, H. J. Phys. Chem. 1996, 100, 17780. (19) Kim, O. K.; Choi, L.-S.; Zhang, H. Y.; He, X. H.; Shih, Y. H. J. Am. Chem. Soc. 1996, 118, 12220. (20) Wang, Z. S.; Li, F. Y.; Huang, C. H.; Wang, L.; Wei, M.; Jin, L. P.; Li, N. Q. J. Phys. Chem. B 2000, 104, 9676. (21) Li, F.; Zheng, J.; Huang, C.; Jin, L.; Zhuang, J.; Guo, J.; Zhao, X.; Liu, T. J. Phys. Chem. B 2000, 104, 5090. (22) Soko1owska, J.; Podsiad1y, R.; Stoczkiewicz, J. Dyes Pigments 2008, 77, 510. (23) Kabatc, J.; Jedrzejewska, B.; Paczkowski, J. J. Polym. Sci. A 2006, 44, 6345. (24) Paczkowski, J.; Kabatc, J.; Pietrzak, M. Macromolecules 1998, 31, 4651. (25) Chatterjee, S.; Davis, P. D.; Gottschalk, P.; Kurz, M. E.; Sauerwein, B.; Yang, X.; Schuster, G. B. J. Am. Chem. Soc. 1990, 112, 6329. (26) Pham, T. H. N.; Clarke, R. J. J. Phys. Chem. B 2008, 112, 6513. (27) Huang, Y.; Cheng, T.; Li, F.; Huang, C.; Hou, T.; Yu, A.; Zhao, X.; Xu, X. J. Phys. Chem. B 2002, 106, 10020. (28) Huang, Y.; Cheng, T.; Li, F.; Luo, C.; Huang, C.; Cai, Z.; Zeng, X.; Zhou, J. J. Phys. Chem. B 2002, 106, 10031. (29) Huang, Y.; Cheng, T.; Li, F.; Huang, C.; Wang, S.; Huang, W.; Gong, Q. J. Phys. Chem. B 2002, 106, 10041. (30) Koti, A. S. R.; Bhattacharjee, B.; Haram, N. S.; Das, R.; Periasamy, N.; Sonawane, N. D.; Ragnekar, D. W. J. Photochem. Photobiol. A: Chem. 2000, 137, 115. (31) Bingemann, D.; Ernsting, N. P. J. Chem. Phys. 1995, 102, 2691. (32) Kovalenko, S. A.; Ernsting, N. P.; Ruthmann, J. J. Chem. Phys. 1997, 106, 3504. (33) Zeglinski, D. M.; Waldeck, D. H. J. Phys. Chem. 1988, 92, 692. (34) Song, Q.; Evans, C. E.; Bohn, P. W. J. Phys. Chem. 1993, 97, 13736. (35) Fromherz, P. J. Phys. Chem. 1995, 99, 7188. (36) Sertova, N.; Nunzi, J.-M.; Petkov, I.; Deligeorgiev, T. J. Photochem. Photobiol. A. Chem 1998, 112, 187. (37) Panigrahi, M.; Dash, S.; Patel, S.; Behera, P. K.; Mishra, B. K. Spectrochim. Acta, Part A 2007, 68, 757. (38) Shim, T.; Lee, M. H.; Kim, D.; Ouchi, Y. J. Phys. Chem. B. 2008, 112, 1906. (39) Strehmel, B.; Seifert, H.; Rettig, W. J. Phys. Chem. B 1997, 101, 2232. (40) Kim, J.; Lee, M. J. Phys. Chem. A 1999, 103, 3378. (41) Lakowicz, J. R. Principle of Fluorescence Spectroscopy; Plenum Press: New York, 2006. (42) Birks, J. B. Photo Physics of Aromatic Molecules; Wiley-Interscience: New York, 1970. (43) The synthesis, purification and characterization of the TQ1 and TQ2 dyes are given in the Supporting Information. (44) Dean, J. A., Ed. Lange’s Handbook of Chemistry, 13th ed.; McGraw-Hill: New York, 1987. (45) Dahiya, P.; Maity, D. K.; Nayak, S. K.; Mukherjee, T.; Pal, H. J. Photochem. Photobiol. A: Chem. 2007, 186, 218. (46) Dahiya, P.; Kumbhakar, M.; Mukherjee, T.; Pal, H. J. Mol. Struct. 2006, 798, 40. (47) Dahiya, P.; Kumbhakar, M.; Maity, D. K.; Mukherjee, T.; Tripathi, A. B. R.; Chattopadhyay, N.; Pal, H. J. Photochem. Photobiol. A: Chem. 2006, 181, 338. (48) Senthilkumar, S.; Nath, S.; Pal, H. Photochem. Photobiol. 2004, 80, 104. (49) Nad, S.; Pal, H. J. Phys. Chem. A 2003, 107, 501. (50) Barik, A.; Nath, S.; Pal, H. J. Chem. Phys. 2003, 119, 10202. (51) Nath, S.; Pal, H.; Sapre, A. V. Chem. Phys. Lett. 2002, 360, 422. (52) Nad, S.; Pal, H. J. Phys. Chem. A 2001, 105, 1097. (53) Masuhara, H.; Hino, T.; Mataga, N. J. Phys. Chem. 1975, 79, 994. (54) Masuhara, H.; Mataga, N. Acc. Chem. Res. 1981, 14, 312. (55) Lippert, V. Z. Z. Naturforsch. 1957, 10a, 541. (56) Mataga, N.; Kaifu, Y.; Koizumi, M. Bull. Chem. Soc. Jpn. 1956, 29, 465. (57) Marcus, Y. J. Soln. Chem. 1991, 20, 929. (58) Kamlet, M. J.; Abbound, J. L. M.; Taft, R. W. An examination of linear solVation energy relationships. In Progress in physical organic chemistry.; Taft, R. W., Ed.; John Wiley & Sons: New York, 1981; Vol. 13, pp 485-630. (59) Arbeloa, T. L.; Arbeloa, F. L.; Tapia, M. J.; Arbeloa, I. L. J. Phys. Chem. 1993, 97, 4704.

Photophysical Properties of Two Aminostyryl Dyes (60) Dahiya, P.; Kumbhakar, M.; Mukherjee, T.; Pal, H. Chem. Phys. Lett. 2005, 414, 148. (61) Barik, A.; Kumbhakar, M.; Nath, S.; Pal, H. Chem. Phys. 2005, 315, 277. (62) Singh, M. K.; Pal, H.; Bhasikuttan, A. C.; Sapre, A. V. Photochem. Photobiol. 1998, 68, 32. (63) Singh, M. K.; Pal, H.; Bhasikuttan, A. C.; Sapre, A. V. Photochem. Photobiol. 1999, 69, 529. (64) O’Connor, D. V.; Philips D. Time correlated single photon counting; Academic Press: New York, 1984. (65) Becker, W. AdVanced Time Correlated Single Photon Counting Technique; Springer: New York, 2005. (66) Dahiya, P.; Kumbhakar, M.; Maity, D. K.; Mukherjee, T.; Mittal, J. P.; Tripathi, A. B. R.; Chattopadhyay, N.; Pal, H. Photochem. Photobiol. Sci. 2005, 4, 100. (67) Satpati, A.; Senthilkumar, S.; Kumbhakar, M.; Nath, S.; Maity, D. K.; Pal, H. Photochem. Photobiol. 2005, 81, 270. (68) Dahiya, P.; Dutta Choudhury, S.; Maity, D. K.; Mukherjee, T.; Pal, H. Spectrochim. Acta, Part A 2008, 69, 134.

J. Phys. Chem. A, Vol. 114, No. 13, 2010 4519 (69) Bard, A. J.; Lund, H. Encyclopedia of Electrochemistry of the Elements; Marcel Dekker: New York, 1984; Vol. XV. (70) Gruen, H.; Gorner, H. J. Phys. Chem. 1989, 93, 7144. (71) Bartocci, G.; Mazzucato, U.; Massetti, F.; Galiazzo, G. J. Phys. Chem. 1980, 84, 847. (72) Foldiak, G.; Schuler, R. H. J. Phys. Chem. 1978, 82, 2756. (73) Satpati, A. K.; Kumbhakar, M.; Nath, S.; Pal, H. Photochem. Photobiol. 2009, 85, 119. (74) Nad, S.; Kumbhakar, M.; Pal, H. J. Phys. Chem. A 2003, 107, 4808. (75) Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Chem. ReV. 2003, 103, 3899. (76) El-Gezawy, H.; Rettig, W.; Lapouyade, R. J. Phys. Chem. A 2006, 110, 67. (77) Murali, S.; Kharlanov, V.; Rettig, W.; Tolmachev, A. I.; Kropachev, A. V. J. Phys. Chem. A 2005, 109, 6420. (78) Cornelissen-Gude, C.; Rettig, W. J. Phys. Chem. A 1999, 103, 4371. (79) Braun, D.; Rettig, W.; Delmond, S.; Le’tard, J.-F.; Lapouyade, R. J. Phys. Chem. A 1997, 101, 6836.

JP9107969

Contrasting Solvent Polarity Effect on the Photophysical Properties of

Recommend Documents