Aggregation Studies of Dipolar Coumarin-153 Dye in Polar Solvents

Article pubs.acs.org/JPCA

Aggregation Studies of Dipolar Coumarin-153 Dye in Polar Solvents: A Photophysical Study Poonam Verma† and Haridas Pal*,‡ †

Radioanalytical Chemistry Division, and ‡Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India ABSTRACT: Photophysical studies have been carried out to explore the aggregation behavior of coumarin-153 (C153) dye in polar organic solvents of both aprotic and protic nature, namely acetonitrile (ACN) and ethanol (EtOH). No unusual behavior is observed in aprotic ACN solvent, suggesting only the monomers of the dye as the single emitting species in the solution. In protic EtOH solvent, however, the dye shows the presence of multiple emitting species in the solution. The concentration-, temperature- and wavelength-dependent changes in the fluorescence decays, and the time-resolved emission spectra (TRES) and area-normalized emission spectra (TRANES) suggest the coexistence of dye aggregates along with the dye monomers in the EtOH solution. Observed results indicate that the emission spectra of the aggregates are substantially blueshifted compared to the spectra of the monomers, suggesting the H-aggregation of the dye in the present cases. Time-resolved fluorescence anisotropy, ultrafast fluorescence up-conversion measurements and scanning electron microscopy studies support the aggregation of the dye in EtOH solution. Strong dipole−dipole interaction is supposedly responsible for the aggregation of C153 dye (dipole moment ∼6.4 D) and the polar protic solvent EtOH apparently stabilizes the aggregates through solute−solvent hydrogen bonding interaction, which is not possible in aprotic ACN solvent. This is further supported by the time-resolved fluorescence results in a strongly hydrogen bond donating solvent, 2,2,2-trifluoroethanol. Aggregation of C153 dye observed in the present study in polar protic organic solvent is an intriguing finding, as the dye is widely used as a fluorescent probe for various photochemical studies, where overlooking such aggregation will definitely mislead the observed results.

1. INTRODUCTION Since the initial report by Jelley,1 understanding the intricate details of aggregation of the chromophoric dyes in solution has been the subject of numerous investigations. For many chromophoric dyes, as the dye concentration increases in the solution, apart from dye monomers, some dimers and higher dye aggregates also coexist in the solution. Various physicochemical and biological processes are significantly influenced by the presence of such aggregates in the system. For example, in biological systems, molecular aggregates play a vital role in guiding the energy channels for sunlight in the solar energy conversion process.2 Aggregates of dye molecules have also found many technological applications as in solar cells,3,4 electronic devices,5,6 light emitting diodes,7 optical communication,8 etc. Though aggregation of the dyes favors many of their applications, however, there are uses of the chromophoric dyes, for example, in dye lasers, in fluorescent sensing/probing and also in many photophysical and photochemical studies, where dye aggregation is detrimental for the desired effects. In such cases, the solution behavior of dyes should be scrutinized thoroughly and appropriate strategies should be adopted to control the aggregation process. Thus, understanding the possible aggregation of a dye for its usefulness as a fluorescent probe is extremely important. Coumarin dyes (1,2-benzopyrone derivatives) are well-known laser dyes in the blue-green region.9,10 7-Aminocoumarins © 2014 American Chemical Society

(with differently substituted 7-amino groups to the basic coumarin moiety) have wide range of applications in diverse research areas, as they show quite remarkable excited-state properties, e.g., strong intramolecular charge transfer (ICT) character,11,12 large solvatochromism,11,12 very high fluorescence quantum yields (Φf), long fluorescence lifetimes (τf),11−14 etc. These favorable properties along with their high photostability have made these dyes as useful fluorescence probes in investigating various chemical and physiochemical processes.15−19 Coumarin dyes have also found useful applications as chemosensors20,21 as well in biological and biomedical sciences.22−24 Though coumarin dyes are extensively used in different studies and the reported literature on their photophysical behavior is also quite rich, many of the atypical and unusual behaviors of the coumarin dyes in different solvent environments are yet to be explored completely.11−14,25−29 In the quest of our understanding on the behavior of the 7-aminocoumarin dyes in solution phase, recently we have reported an intriguing behavior of a 7-(N,N-dialkylamino)coumarin dye, namely coumarin-481 (C481), undergoing H-type of dimer and higher aggregate formation in aqueous solution30 as well as in polar organic solvents.31 Most importantly, unlike the majority of the Received: June 20, 2014 Revised: July 31, 2014 Published: August 5, 2014 6950

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other dyes forming H-aggregates with nonfluorescent character,32,33 the H-aggregates of C481 dye are found to be quite emissive in nature.30,31 With these intriguing results on the aggregation of C481 dye in solution, we were curious to know if similar aggregation is also possible for other coumarin derivatives. This inquisitiveness was further stimulated by the results of aggregation and/or self-assembly formation for some of the coumarin derivatives, reported under the environments like organic solvents,34,35 silica/methanol interfaces,36 mesoporous molecular sieves,37 solvent mixtures,38,39 Langmuir−Blodgett films,40,41 etc. In the present study, therefore, we have carried out a systematic investigation involving an important homologue of the 7-aminocoumarin dyes, namely coumarin-153 (C153), a dye that has been used most extensively among different 7-aminocoumarin dyes as a fluorescent probe to explore the microenvironments of various confined media. The selection of C153 dye in the present study is also from the consideration of the structural rigidity for the 7-amino substituent of the dye, which is a part of the rigid julolidinyl rings, in comparison to the flexible 7-N,N-diethylamino substituent present in the C481 dye studied earlier.30,31 Thus, one of the objectives of the present study is to understand if the flexibility of the amino substituent in the 7-aminocoumarin derivatives has any role in realizing the aggregation behavior of these dyes. It should be mentioned that due to the presence of flexible 7-N,N-diethylamino group, the C481 dye undergoes unusually fast excited-state relaxation in polar solvents via the formation of nonfluorescent twisted intramolecular charge transfer (TICT) state,12,14,26,29,42 causing its fluorescence quantum yield to be very low and thus allowing the weakly fluorescent H-aggregates of the dye to be observed easily in the solution. Because TICT formation is absent in C153 dye due to its rigid 7-amino substituent, the fluorescence quantum yield of the dye is much higher in comparison to that of C481.12,14,26,29,42 It is therefore interesting to see if the signature of any possible aggregation of the dye can still be realized, even though the high fluorescence yield of the dye monomers would undoubtedly suppress the effect of the weakly fluorescent dye aggregates formed in the solution. Though photophysical behavior of C153 dye in different protic and aprotic organic solvents of various polarities have been reported earlier, each of these reports considered the presence of monomeric dyes in the solution and accordingly interpreted the observed results.42,43 With the anticipation of a possible aggregation of the dye in some of the solvent environments, a revisiting on the behavior of the C153 dye in the conventional organic solvents seemed very essential. Exploration on the aggregation behavior of C153 dye in conventional polar organic solvents is also very important because this dye has been very extensively used as an extrinsic fluorescent probe in perceiving the mechanisms and dynamics of various physicochemical processes15−24 where dye aggregation can adversely affect the observed results. In the present study, therefore, we have systematically investigated the photophysical properties of C153 dye in two conventional polar organic solvents, namely polar aprotic solvent acetonitrile (ACN) and polar protic solvent ethanol (EtOH), to understand the possible aggregation behavior of the dye and the consequent changes in its fluorescence behavior. The chemical structure of the studied dye is shown in Chart 1 for a quick visualization.

Chart 1. Chemical Structure of Coumarin-153 (C153) Dye

recrysallization from the MeOH−water mixture (50:50 by v/v). Spectroscopic grade EtOH and ACN were obtained from Les Alcools de Commerce Inc. (Brampton, Ontario, Canada) and S D Fine Chem. Ltd. (Mumbai, India) respectively. ACN was used without further purification, and EtOH was additionally purified by distillation before use. Stock solution of C153 in EtOH and ACN were prepared by adding a small amount of solid dye sample in the respective solvents. The dye solutions thus obtained were centrifuged at 10 000 rpm for 10 min. The supernatant solutions were then collected and further sonicated for 10 min before use to avoid any undissolved dye particle in the solution. Until otherwise stated, present measurements were carried out at ambient temperature (25 ± 1 °C). 2.2. Experimental Measurements and Data Analysis. Absorption spectra were recorded using a JASCO UV−vis spectrophotometer, model V-650 (Tokyo, Japan). Steady-state (SS) fluorescence spectra were recorded using a Hitachi spectrofluorometer, model F-4500 (Tokyo, Japan). Time-resolved (TR) fluorescence was measured using a time-correlated single photon counting (TCSPC)44−46 spectrometer from Horiba Jobin Yvon IBH, (Scotland, U.K.), where samples were excited with a 374 nm diode laser (repetition rate 1 MHz, pulse width ∼100 ps) and fluorescence decay was collected at right angle geometry using a MCP PMT. All the measurements were carried out with analyzer at magic angle configuration with respect to the vertically polarized excitation beam to avoid any rotational depolarization effect of the dye on the observed fluorescence decays. Measured decays were analyzed using DAS-6 reconvolution analysis software obtained from IBH. The instrument response function (IRF) for the experimental setup was recorded by replacing the sample cell with a dilute scatterer solution (suspended TiO2 particles in water) and used in the reconvolution analysis of the observed decays. A typical IRF for the present setup is ∼110 ps at full-width half-maximum (fwhm) and the shortest fluorescence lifetime measurable following reconvolution analysis of the decays is about 30 ps.44−46 In the present study, observed decays were fitted in general as a sum of exponentials, I (t ) =

∑ Bi exp(−t /τi)

(1)

where τi is the fluorescence lifetime and Bi is the pre-exponential factor for the ith component of the decay. The quality of the fits was judged from the reduced χ2 value and the distribution of the weighted residuals among the data channels.44−46 Percentage contributions (ai) of the individual lifetime components (τi) in the multiexponential decays were calculated by using the following relation. ai = {Bi τi /∑ Bi τi)} × 100

(2)

TR fluorescence anisotropy was measured using the same TCSPC spectrometer. In these measurements, the fluorescence decays with parallel I∥(t) and perpendicular I⊥(t) emission polarizations with respect to the vertically polarized excitation

2. MATERIALS AND METHODS 2.1. Materials and Solution Preparation. Laser grade C153 sample was obtained from Exciton, USA, and purified by 6951

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2.3. Construction of Time-Resolved Emission Spectra. Time-resolved emission spectra (TRES) were constructed following the measurements of the fluorescence decays at regular wavelength intervals, covering the whole emission spectrum of the dye.49−51 Observed decays were first analyzed following a triexponential function uniformly and the decay parameters thus obtained were used to construct the TRES following the standard procedure given by Maroncelli and Fleming.49 Briefly, in this procedure, the intensity normalized decays DN(λ,t) at each of the measuring wavelengths are first calculated using the fitted parameters of the experimental decay at the respective wavelengths, following eq 4.

light were collected and the anisotropy decay function r(t) was then constructed as44−46 r (t ) =

I (t ) − GI⊥(t ) I (t ) + 2GI⊥(t)

(3)

where G is the correction factor for the polarization bias of the detection setup. The G-factor was obtained independently by measuring the two perpendicularly polarized fluorescence decays while the sample was excited with the horizontally polarized excitation light.44−46 TR fluorescence decays in the ultrafast time domain were measured using a femtosecond fluorescence up-conversion system (model FOG 100, from CDP Inc., Russia). The details of this instrument are given elsewhere.47,48 Briefly, the second harmonic light (390 nm) of a 50 fs Ti:sapphire laser was used for the excitation of the sample taken in a thin (0.4 mm) rotating cell and the residual fundamental (780 nm) of the Ti:sapphire laser was used as the gate pulses to up-convert the fluorescence light in a 0.5 mm BBO crystal. The up-converted light was detected using a Hamamatshu PMT (model 5000U-09) operated in the photon counting mode. The IRF of the present setup is about 220 fs at fwhm. Dynamic light scattering (DLS) measurements were carried out using a Malvern Autosizer 4800 instrument (Malvern, Worcestershire, U.K.), employing 7132 digital correlator and PCS0078 software. A 25 mW He−Ne laser operated at 633 nm was used as the light source and the scattered light was detected at 90° with respect to the laser light beam, using an avalanche photodiode (APD) as the detector. The time autocorrelation function, g1(τ), for the scattered light was constructed from the recorded time traces of the scattered light intensities. The minimum particle size measurable with the present instrument is about 5 nm. Scanning electron microscope (SEM) measurements were carried out using Zeiss Auriga SEM instrument. Microfilms for the SEM studies were prepared by putting a drop of the C153 solution in the studied solvent on a highly polished copper stub and allowing the solvent to get dried-off by evaporation at ambient condition.

D N(λ ,t ) =

∫0 D(λ ,t ) dt

(4)

where D(λ,t) = ∑3t=1{Bi(λ)} exp[−t/{τi(λ)}] and the parameters {Bi(λ)} and {τi(λ)} are the pre-exponential factor and the decay time, respectively, for the ith decay component at wavelength λ. The normalized decays are then used in combination with the SS emission spectrum ISS(λ) of the dye to calculate the time-dependent changes in the fluorescence intensity at different wavelengths I(λ,t), by using the following relation. I(λ ,t ) = D N(λ ,t ) × ISS(λ)

(5)

The TRES thus obtained using eqs 4 and 5 are in the wavelength scale and these spectra are finally converted to the wavenumber scale (energy scale, ν)̅ by using the following relation. I(ν ̅ ,t ) = λ 2I(λ ,t )

(6)

Because the number of data points in the TRES thus constructed is limited due to the limited number of wavelengths where fluorescence decays are actually measured experimentally, the TRES data obtained from the above procedure is further fitted using a log-normal line shape function, defined by eq 7, to obtain the smooth TRES for the system studied.49

⎧ ⎛ ln[1 + 2b(ν ̅ − νp̅ )/w] ⎞⎫ ⎪ ⎪ ⎜⎜ ⎟⎟⎬ ln(2) − I(ν ̅ ) = a exp⎨ ⎪ b ⎝ ⎠⎪ ⎩ ⎭ =0

D(λ ,t ) ∞

when {2b(ν ̅ − νp̅ )/w} ≤ 1

where the amplitude a, the peak wavenumber νp̅ , the width parameter w, and the asymmetry parameter b, are used as the adjustable parameters.

when {2b(ν ̅ − νp̅ )/w} > 1 (7)

the possibility of any aggregation of C153 dye in ACN and EtOH solutions, we systematically carried out the concentration-dependent studies on the absorption and SS fluorescence spectra of the dye by gradually diluting the respective stock solutions. It is observed that in either of the solvents the shape of both absorption and fluorescence spectra remain effectively unchanged, even on sufficient dilution of the initial solutions (Figure 1). To realize this behavior more evidently, the absorption and the fluorescence spectra of the dye for its highest and lowest concentrations in the respective solvents were normalized at their respective maxima, as are shown in Figure 1A,B. In both the solvents, the very similarity in the spectral characteristics for the peak normalized absorption and fluorescence spectra for the lowest and the highest dye concentrations used seemingly indicate no unusual solution

3. RESULTS AND DISCUSSION 3.1. Ground-State Absorption and Steady-State Fluorescence Measurements. Ground-state absorption and SS fluorescence spectra were measured for C153 dye in ACN and EtOH solutions, as are shown in Figure 1A,B, respectively. In ACN solution, the absorption maximum (λmax abs ) and emission ) appear at 418 and 515 nm, whereas in EtOH maximum (λmax fl max and λ appear at 424 and 525 nm, ressolution, the λmax abs fl pectively. As shown in Figure 1, both absorption and fluorescence spectra are very broad and structureless. To investigate 6952

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Figure 1. (A) Absorption and fluorescence spectra of C153 dye in ACN solutions. Spectra in red and green correspond to the solutions with 14.1 and 0.8 μM dye concentrations, respectively. The spectrum in blue is the normalized spectrum for 0.8 μM dye solution to the peak height of the spectrum for 14.1 μM dye solutions. (B) Absorption and fluorescence spectra of C153 dye in EtOH solution. Spectra in red and green correspond to the solutions with 15.2 and 1.8 μM dye concentrations, respectively. The spectrum in blue is the normalized spectrum for 1.8 μM dye solution to the peak height of the spectrum for 15.2 μM dye solution.

aggregation behavior of the dye in a solution. Present observation is thus quite intriguing and obviously demands more detailed investigations, especially when a number of recent reports suggest the clear aggregation of some of the coumarin derivatives under different solvent conditions and/or microenvironments.30,31,34−41 Because TR fluorescence measurements are more sensitive in detecting multiple emissive species than SS fluorescence measurements,44−46,52 to explore more on the possible aggregation of C153 dye in the studied solvents, we resorted to the TR fluorescence measurements for the present systems, as are discussed in the next section. 3.2. Time-Resolved Fluorescence Measurements. Fluorescence decays of C153 dye in ACN and EtOH solutions were measured at different emission wavelengths, covering the whole SS emission spectra of the dye (Figure 1), as are shown in Figure 3A,B respectively. It is observed that the fluorescence decays in ACN solution are always single exponential in nature with a long lifetime component (∼5.6 ns) and are also independent of the monitoring emission wavelength used (Figure 3A). On the contrary, fluorescence decays in EtOH solution are found to be nonsingle exponential at the blue edge of the emission spectrum, though the decays are effectively single exponential at the longer emission wavelengths, beyond about 500 nm (Figure 3B). As shown in Figure 3B, a long lifetime component always dominates in EtOH solution throughout the emission wavelengths, but the presence of some additional shorter lifetime components in the decays at the shorter emission wavelengths is clearly indicated, albeit with relatively lower contributions. Observed wavelength-dependent changes in the fluorescence decays evidently suggest that there is more than one emitting species present in the EtOH solution. To indicate the dissimilarities in the fluorescence characteristics in ACN and EtOH solutions, the initial parts of the decays that manifest these characteristic differences in the two solvents are specifically marked with blue circles in Figure 3A,B. To know the behavior of C153 dye in ACN and EtOH solutions further, fluorescence decays of the dye in the two solvents were measured at different emission wavelengths as a function of the dye concentration used in the solutions. In ACN solution, the fluorescence decays measured at different dye concentrations show no obvious changes at any of the monitoring emission wavelengths for the whole emission spectrum of the dye. Typical of these decays measured in ACN solution at an emission wavelength of 460 nm with varying dye concentrations

behavior of the dye in the studied solvents. However, as the ground-state absorption and the SS fluorescence are not very sensitive techniques, it is possible that the subtle changes in the spectra due to small extent of dye aggregation remain undetected by these measurements. Because excitation spectra are known to provide better information in relation to the multiple emissive species present in a solution,44−46,52 in the present study we also recorded the excitation spectra as a function of the emission wavelengths, covering both the blue and the red edge of the dye emission spectra, in both ACN and EtOH solutions. In ACN solution, the excitation spectra recorded for different emission wavelengths do not show any observable spectral changes. In EtOH solution, however, the excitation spectra are found to be significantly blue-shifted when emission wavelengths are kept at the blue edge of the spectrum, though the differences in the excitation spectra are not that appreciable when the monitoring emission wavelengths are kept at the emission maximum or at the red edge of the spectrum, as are shown in Figure 2.

Figure 2. Peak normalized fluorescence excitation spectra of C153 dye in EtOH. The emission wavelengths were 460 nm (purple), 470 nm (blue), 525 nm (green), and 600 nm (red). The dye concentration is 31.8 μM.

These results are quite indicative that there is more than one emitting species present for the dye in EtOH solution.44−46,52 We feel that such multiple emitting species arise due to small extent of aggregation of the dye, where the aggregates are also weakly emissive in nature. Important to mention here that though extensive literature exits on C153 dye, both in terms of its photophysical properties14,29,42,43 and also in relation its uses as a fluorescence probe,16,17,47,48 there is no report so far on the 6953

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Figure 3. (A) Fluorescence decays of C153 dye (2.6 μM) in ACN solution measured at different emission wavelengths. (B) Fluorescence decays of C153 dye (2.7 μM) in EtOH solution measured at different emission wavelengths. Excitation was done at 374 nm. The blue circles at the initial part of the decays in the two panels are to specifically indicate the characteristic differences in the fluorescence decays of the dye in ACN and EtOH solutions.

effectively independent of the dye concentration used. Typical such decays measured for the dye in EtOH solutions at 525 nm (fluorescence maximum of the dye) and at 460 nm (blue edge of the emission spectrum) are shown in Figure 5A,B, respectively, displaying the clear difference in the concentration dependence in the decays at the two region of the emission spectrum. The decays measured at the emission maximum (525 nm) and longer wavelengths are always single exponential in nature, with no effective change in the decay component with the changing dye concentration (Figure 5A). At the blue edge of the spectrum, however, the decays not only are multiexponential in nature but also display an observable change in the contributions of the decay components with the changing dye concentrations (Figure 5B). As seen from Figure 5B, there is an obvious decrease in the shorter lifetime components in the decays as the dye concentration is gradually decreased in the solution. Analysis of these decays at the blue edge of the emission spectrum in EtOH solution requires a triexponential function in general to obtain acceptable fits (χ2 values around 1.0 and weighted residuals distributed randomly among data channels).44−46 Typical results obtained from triexponential analysis of the decays at 460 nm with different dye concentrations in EtOH solution are listed in Table 1. As indicated from Table 1 and Figure 5B, with the decrease in the dye concentration, there is a clear decrease in the contribution of the shortest lifetime component (τ1) with the concomitant increase in the contribution of the intermediate (τ2) lifetime component in EtOH solution. The contribution of

Figure 4. Fluorescence decays of C153 dye in ACN solution measured for different dye concentrations at a blue-shifted emission wavelength (460 nm). The dye concentrations for the decays shown in red and blue are 0.8 and 14.1 μM, respectively. Excitation was done at 374 nm. Results were very similar at other emission wavelengths of the dye in ACN solution.

are shown in Figure 4. These results conclusively suggest that the dye displays no unusual behavior in polar aprotic solvent ACN. We infer from the present observation that the dye exclusively exists in its monomeric form in polar aprotic solvent ACN. Contrary to the observations in ACN solution, for C153 dye in EtOH solution, the concentration-dependent fluorescence decays measured at the blue edge of the spectra are found to change very significantly, though the decays measured at the longer wavelengths (above about 500 nm) are found to be

Figure 5. Fluorescence decays of C153 dye in EtOH solution for different dye concentrations. (A) Decays measured at the emission maximum (525 nm) and (B) decays measured at a blue-shifted emission wavelength (460 nm). Dye concentrations for the decays shown in black, violet, blue, magenta, green, and red are 30.7, 15.9, 7.9, 4.2, 2.7, and 1.5 μM, respectively. Excitation was done at 374 nm. The blue circles at the initial part of the decays in the two panels are to indicate the characteristic differences in the decays of the dye in ACN and EtOH solutions. 6954

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Thus, following Kasha’s exciton theory (Appendix),46,53,54 we assign these aggregates of C153 dye in EtOH solution as the H-aggregates. The important point to be noted here is that the H-aggregates of C153 dye are found to be emissive in nature (albeit with low emission yields, as SS emission measurements cannot distinguish these emissions), which is evidently an interesting observation. According to Kasha’s exciton theory, the H-aggregates are supposed to be nonfluorescent in nature, because excitation to an allowed higher excitonic state of H-aggregates (Scheme A1, Appendix) is followed by a rapid nonradiative relaxation to the lower excitonic state (due to internal conversion, IC), from where a radiative transition to the ground state is forbidden. Accordingly, a major decay channel for the excited H-aggregates is expected to go through a triplet state, formed by the intersystem crossing (ISC) process of the lower excitonic state.46,52−56 Though the nonfluorescent nature of the H-aggregates is quite a common feature, there are cases where fluorescent H-aggregates are also reported for some of the chromophoric dyes.30,31,38,57−69 Importantly, however, in most of the cases of the fluorescent H-aggregates, the emissions are found to be very weak as well as red-shifted in comparison to the monomer emission spectra, suggesting that these emissions actually originate from the lower excitonic states. It is proposed that in these cases a nonvanishing electronic transition probability for the lower excitonic states of the H-aggregates arises due to combination of vibronic coupling and a small extent of rotational twist of the stacked dyes than their perfect orientation required for parallel transition dipoles.66−69 In the literature, there are very limited reports where blueshifted emissions from H-aggregates (arising from the higher (allowed) excitonic state) have been observed.30−32,38,57−63 A rapid back-conversion of the forbidden lower excitonic state to the allowed higher excitonic state achieved through dephasing of intrachromophoric transition dipoles caused by solvent perturbations has been suggested as one of the probable mechanism for the blue-shifted emissions from H-aggregates.60,61 In addition to this, it is likely that in the H-aggregates formed with highly dipolar dyes the IC process from higher excitonic state to lower excitonic state is suppressed to a substantial extent due to the extra rigidity attained by these aggregates through Coulombic interactions of the permanent dipoles arranged in an antiparallel configuration and thus allowing a weak blue-shifted emission to be observed from the higher excitonic states. It is thus suggested that the weak fluorescence emission observed for C153 dye in EtOH solution is due to the combined effects of the above two factors. It should be mentioned here that, in any of these cases, the IC from the higher to the lower excitonic states will always be quite efficient (due to small energy gap between the two states) and accordingly the emissions from the higher excitonic states will have very short fluorescence lifetimes,62,63 as observed in the present study. From the discussions above and considering the TR fluorescence results of C153 dye in EtOH solution, we assign the shortest lifetime component (τ1 ∼ 0.04 ns) to relatively larger H-aggregates and the intermediate lifetime component (τ2 ∼ 1.4 ns) to relatively smaller H-aggregates. As it is understandable, the longest lifetime component (τ3 ∼ 4.8 ns) in the present system is assigned to the monomeric dye present in the solution. The reasoning behind the presumption that the larger aggregates have a lifetime shorter than that of the smaller aggregates is the fact that the increased density of the excitonic states in the larger aggregates will facilitate the nonradiative IC

Table 1. List of the Fluorescence Decay Parameters for C153 Dye in EtOH Solution as a Function of the Dye Concentration Useda [C153]/μM

a1/%

τ1/nsb

a2/%

τ2/ns

a3/%

τ3/ns

χ2

30.7 15.9 7.9 4.2 2.7 1.5

12.4 11.5 10.5 10.1 8.8 7.7

0.04 0.04 0.04 0.04 0.04 0.04

4.4 5.5 5.5 6.4 6.2 7.2

1.57 1.64 1.30 1.38 1.49 1.35

83.2 83.0 84.0 83.5 85.4 85.1

4.75 4.81 4.81 4.87 4.82 4.79

1.02 1.10 1.03 1.05 1.08 1.10

a

The decays were measured at 460 nm with 374 nm excitation. All the decays were fitted with a tri-exponential function following reconvolution analysis. bThe τ1 component of the decays was always close to 0.04 ns. For the results listed in the table, the τ1 component was fixed to 0.04 ns to obtain consistent results for the other decay components and their relative contributions.

the longest (τ3) lifetime component, however, remains more or less similar, indicating only a small increase with the decreasing dye concentration. It should be mentioned here that the effect of dye concentration on the observed decays at the other wavelengths in the blue region of the spectrum are qualitatively very similar to that observed at 460 nm (Table 1), except that the overall contributions of both τ1 and τ2 components gradually decrease on increasing the monitoring emission wavelength, causing the decays to eventually become singleexponential beyond about 500 nm. These results clearly suggest that there are at least three different emitting species present for C153 dye in EtOH solution. Drawing an analogy with the literature reports on the aggregation of some other coumarin derivatives in different solvent environments,30,31,34−41 the present results can be rationalized by assuming a small extent of aggregation of C153 dye in EtOH solution. From the observed results it is suggested that the dye monomers along with the dye aggregates jointly contribute in the observed decays in EtOH solution and the contributions of different emitting species change quite significantly on changing the dye concentration as well as the monitoring emission wavelength. That the absorption and SS fluorescence results presented in section 3.1 could not reveal this aggregation is certainly due to the less sensitive nature of these steady-state measurements.30,31,44−46 Moreover, the degree of aggregation being small and the fluorescence yields and lifetimes of the aggregates being much lower than the dye monomers (Table 1) the detection of these aggregates by the usual absorption and SS fluorescence measurements seems not possible. In the literature, the reported photophysical properties on the coumarin dyes, including that of C153 dye, in different organic solvents have been realized mainly from the observations made at the emission and absorption maxima of the dyes, suggesting that these dyes mainly exist as the monomeric species in the solution.12−29,42 On the basis of these reports, we infer that the longest lifetime component (τ3) of about 4.8 ns, which dominates the decays for C153 dye in EtOH solution, is due to the monomeric species.29,42 Accordingly, the two shorter lifetime components (τ1 and τ2) observed for the dye in EtOH solution are ascribed to the dye aggregates formed in the solution. From the observed decays in Figure 3B and Figure 5B it is very evident that the contributions of these aggregates (τ1 and τ2) are always comparatively higher at the shorter emission wavelengths, suggesting that the aggregate emissions are largely blue-shifted relative to the monomer emission. 6955

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work,57 the authors have also employed the PM6 level of quantum-chemical calculations to show that the H-dimer of the studied dye is substantially stabilized (by about 42 kJ mol−1) when the two dipolar dye molecules are stacked one above the other with their permanent dipoles oriented in an antiparallel configuration. Thus, following detailed absorption and fluorescence studies which are supported by quantum chemical calculations these authors have concluded that the H-dimers/ aggregates of the dipolar dyes are actually driven by the combined effects of the dipole−dipole interaction and the donor−acceptor π−π interaction between the stacked dyes.57 It is also reported by Cigan et al.38,57 that a strong electrostatic interaction actually helps in overcoming the restriction imposed from Kasha’s exciton theory on the electronic transitions involving the excitonic states of the aggregates and thus making it possible to observe emissions from the higher excitonic states of the H-aggregates. We strongly feel that similar binding interactions are also responsible for the formation of the fluorescent H-aggregates of highly dipolar C153 dye (dipole moment, μ ∼ 6.4 D)14 in EtOH solution. In our earlier studies on the photophysical characteristics of different ICT dyes it has been observed that the dipolar character of these dyes is often increased due to involvement of the dye-solvent intermolecular hydrogen bonding interaction in protic solvents.12,75−79 Because C153 dye has a strong ICT character, it is suggested that in EtOH solution the dipolar character of the dye increases quite significantly due to dyesolvent hydrogen bonding interaction,38,57,70−74 which cannot happen in aprotic solvent ACN. Therefore, we feel that the aggregation of C153 dye in EtOH solution is supported through enhanced dipole−dipole interaction among the stacked dyes, which is not possible in ACN solution. Moreover, for the aggregates in EtOH solution, a solvent molecule hydrogen bonded to a particular C153 dye (preferably at its CO group) can additionally participate in the electrostatic interaction with the positive pole of the adjacent dye arranged in an antiparallel dipole orientation with respect to the first dye, resulting an extra stabilization for the H-aggregates. Thus, H-aggregate formation for C153 dye seems feasible in polar protic EtOH solvent but not in polar aprotic ACN solvent. 3.3. Time-Resolved Emission Spectra (TRES) and Their Analysis. To understand the spectral features of different emitting species for C153 dye in EtOH solution, we measured the fluorescence decays at regular wavelength intervals covering the whole emission spectrum of the dye and consequently constructed the TRES following the reported procedure49−51 discussed in section 2. Typical of these fluorescence decays measured at some representative wavelengths are shown in Figure 3B. It is clearly indicated from this figure that the contributions of the short decay components are significantly larger at the shorter wavelengths and these contributions gradually decrease with a concomitant increase in the contribution of the longer decay component as the monitoring wavelength is increased. Figure 6 shows some selected TRES constructed for C153 dye (2.7 μM) in EtOH solution for the time span of 0.005−25 ns. As indicated from these TRES, the fluorescence intensity decreases very sharply at the initial time windows but the decrease is quite slow at the longer time windows. The behavior is more evidently shown by plotting the integrated intensity under these TRES against the respective time, as shown in the inset of Figure 6. The sharp decrease in the integrated intensity at the initial time windows corroborates well with the very short τ1 value of ∼0.04 ns assigned to larger dye

process and accordingly will accelerate the deexcitation process for the excited dye aggregates. Considering the presence of smaller and larger H-aggregates along with dye monomers in EtOH solution, it is expected that on decreasing the dye concentration, the contribution of larger aggregates (τ1) would decrease with a concomitant increase in the contribution of the smaller aggregates (τ2), whereas contribution of the monomeric species (τ3) might not change that substantially. As evident from Table 1, with a reduction in the dye concentration in EtOH solution, there is an expected change in the contributions of the τ1 and τ2 components, clearly indicating the disintegration of larger aggregates into smaller aggregates. Interestingly, however, the contributions of both of these aggregates still remain quite appreciable even when the dye concentration is made exceedingly low, only about 1.5 μM (Figure 5B and Table 1). These results thus suggest that in EtOH solution even at a very low C153 concentration one cannot get a purely monomeric solution. Thus, the presence of aggregates seems inevitable for C153 dye in protic EtOH solution, even though no aggregation is indicated for the dye in polar aprotic solvent ACN (Figures 3A and 4). An important point to be mentioned here is that the dye C153 apparently shows quite high solubility in both ACN (∼76 mM) and EtOH (∼30 mM) solvents. Considering such high solubility of the dye in EtOH solution, its aggregation in this solvent seems quite intriguing, whereas there is no aggregation in ACN solvent. It is thus indicated from the present results that there is an additional effect than a simple solvent polarity which assists the dye to undergo aggregation in EtOH solution. We feel that this effect is by some means related to the protic nature of EtOH, providing an extra stability for the H-aggregates through dye−solvent hydrogen bonding interactions. Chromophoric dyes having large permanent dipole moments are often susceptible to undergo aggregation in solution. Aggregation of such strongly dipolar dyes is usually driven by electrostatic interaction between the permanent dipoles, where the Gibbs binding energy is proportional to the square of the ground-state dipole moment of the dyes.70 In the H-aggregates of such dyes, as the dye molecules are stacked together with their dipole moments oriented in an antiparallel configuration, the electron-rich and electron-deficient parts of the dyes come on top of each other and hence maximize the electrostatic interactions,38,57,70−74 as shown in Scheme 1 for the present dye. Scheme 1. π−π Stacking of C153 Dye Molecules in the H-Aggregatesa

a

The antiparallel arrangement of the static dipoles of the molecules stabilizes the H-aggregates further through dipole−dipole (Coulombic) interactions.

In the present context, it is important to draw an attention to the work of Cigan et al. where the role of Coulombic interaction between permanent dipoles of the dyes have been highlighted for the formation of H-aggregates for some of their studied coumarin derivatives.38,57 In fact, in one of their latest 6956

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indicated from Figure 7, there is a substantial red shift at the initial time span of about 0.2 ns, beyond which the shift is not that appreciable (left inset of Figure 7). This feature is further appreciated by plotting the peak wavenumber (νm ̅ ax) for the TRES against time, as shown in the right inset of Figure 7, clearly showing a very sharp decrease in ν̅max at the initial about 0.2 ns time scale and then it remains more or less constant. The large red shift in the peak position for the TRES at the initial windows is due to the large decrease in the contributions from the blue-shifted emissions of larger H-aggregates, having a very short lifetime of about 0.04 ns. Though we expected a further red shift at the intermediate time spans due to smaller H-aggregates, this could, however, not be realized from the observed TRES. As the lifetimes of larger aggregates (∼0.04 ns) are about 120 times shorter than those of the monomers (∼4.8 ns), this drastic difference in the lifetimes makes the spectral shift to be observed clearly at the very initial time windows. However, because the lifetime of the smaller aggregates (∼1.4 ns) is not that drastically different from that of the dye monomer (differ only by about 3.4 times) and the contribution of these smaller aggregates in the integrated intensity for the TRES is quite less, the emission from the smaller aggregates cannot modulate the spectral features any significantly relative to that of the strongly emissive monomer emission at the intermediate time windows. Accordingly, the spectral feature in the TRES at the intermediate time windows in effect represents the monomer emission of the dye. Time-resolved area-normalized emission spectra (TRANES) often provide better insight for the multiple emissive species exist in the solution.80,81 Therefore, for the C153 dye (2.7 μM) in EtOH solution we also constructed the TRANES by normalizing the integrated area under each of the TRES in the time span of 0.005−25 ns, as are shown in Figure 8. As expected,

Figure 6. Time-resolved emission spectra (TRES) of C153 dye (2.7 μM) in EtOH solution. The time snaps for the spectra with decreasing intensity are from 0.005 to 25.0 ns, respectively. The inset shows the decrease in the integrated fluorescence intensity with time. Circles are the data points, and the continuous curve is the triexponential fit to the data.

aggregates. The significantly slow decrease in the integrated intensity at very longer times also agrees well with the long τ3 value of ∼4.8 ns attributed to dye monomers. In the intermediate time region, the rate of decrease in the integrated intensity is also intermediary in nature as expected from the intermediate τ2 value of ∼1.4 ns for the smaller dye aggregates. In fact, the data points for the plot in the inset of Figure 6 could very well be fitted with a triexponential function giving decay parameters as 0.04 ns (0.44%), 1.40 ns (3.80%) and 4.80 ns (95.76%) with an R2 value of 0.999, which are quite consistent with the τ1, τ2, and τ3 values estimated from the fluorescence decay analysis at the blue edge of the emission spectrum. Therefore, the results from the TRES analysis strongly support our proposition that along with the dye monomers some smaller and larger H-aggregates of C153 dye are also present in its EtOH solution. For the TRES in Figure 6, one more interesting point to be noticed that there is a large time-dependent red shift, which becomes very evident as we plot the TRES following their peak intensity normalization, as are shown in Figure 7. As clearly

Figure 8. Time-resolved area normalized spectra (TRANES) of C153 dye (2.7 μM) in EtOH solution in the time span from 0.005 to 25 ns. For easy visualization of isoemissive point and spectral shifts, the TRANES for the initial time span are shown separately in the inset (0.005−0.1 ns). Only the log-normal fits of the TRANES are shown for the cleanliness of presentation and better visualization of their time-dependent shifts. Figure 7. Peak intensity normalized time-resolved emission spectra (TRES) of C153 dye (2.7 μM) in EtOH solution. The time snaps for the spectra until 0.2 ns show appreciable red shift with time. The left inset shows the time snaps for the spectra from 0.1 to 25 ns indicating no significant shift beyond 0.2 ns. The right inset shows changes in the peak maxima with time. Only the log-normal fits to the calculated data points (Figure 6A) are shown for the cleanliness of the presentation.

in accordance with the observation in TRES, the reconstructed TRANES also show a clear red shift in the spectra with an increase in the time span. The interesting observation from the TRANES in Figure 8 is that the spectra show a quite clear isoemissive point during the initial time span of 0.005−0.1 ns (inset of Figure 8). The presence of such an iso-emissive point in the 6957

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Figure 9. Temperature-dependent fluorescence decays of C153 dye (15.3 μM) in EtOH solution (A) measured at the emission maximum (525 nm) and (B) measured at a blue-shifted emission wavelength (460 nm). The temperature is increased from 25 to 65 °C. The inset of Figure 9B shows the fluorescence decays measured at 25 °C, before (red) and after (blue) the heating and cooling cycle, as discussed in the text. The single-exponential behavior of the decays at 525 nm and non-single-exponential behavior of the decays at 460 nm are clearly indicated at the initial regions of the decays, marked by the blue circles. Excitation wavelength for all the decays was 374 nm.

TRANES indicates the simultaneous presence of at least two emitting species in the solution.80,81 As discussed by Periasamy and co-workers,80,81 for a three-component system an isoemissive point in the TRANES can be clearly observed at the very initial time windows where τ1 ≪ τ3, because in this situation the emission contribution from τ3 component will be quite negligible and hence the system would effectively behave as a two component system, i.e., τ1 and τ2. As the time progresses, the monomer emission gradually starts overtaking the overall emission and thus the iso-emissive point eventually disappears for the TRANES at the longer time (Figure 8). For the present system, as a whole, the observations from the TRES and TRANES for C153 dye in EtOH solution are in accordance with our proposition that in the present system there are emission contributions from both H-aggregates, having short fluorescence lifetimes and hence modulate the TRES/TRANES at the shorter time spans, and dye monomers having longer fluorescence lifetime that dominates the TRES/TRANES at the longer time spans. 3.4. Temperature Effect on the Fluorescence Decay of C153 in EtOH Solution. To understand the behavior of C153 dye in EtOH solution further, the temperature-dependent TR fluorescence of the dye was measured both at the emission maximum (525 nm) and at a blue-shifted emission wavelength (460 nm), varying solution temperature from 25 to 65 °C. These results are shown in Figure 9A,B, respectively. It is evident from Figure 9A that there is no observable temperaturedependent change in the decays, measured at the emission maximum of the dye, indicating that the monomer emission is effectively independent of temperature, at least for the studied temperature range. Unlike at emission maximum, there is a substantial temperature-dependent change in the decays at the blue edge of the emission spectrum of the dye. The temperaturedependent fluorescence decay parameters at 460 nm, as obtained by triexponential analysis of the observed decays, are listed in Table 2. As expected, with increasing temperature, there is a gradual increase in the monomer contribution (τ3) with the concomitant decrease in the aggregate contributions (τ1 and τ2), suggesting the deaggregation of the dye at elevated temperatures. As indicated from Table 2, the decrease in the contribution of the larger aggregates (τ1) is more prominent than that of the smaller aggregates (τ2). Thus, temperature-dependent results are qualitatively in support of our proposition that the dye C153 undergoes aggregation in EtOH solution. To understand if the dye monomers and aggregates maintain any

Table 2. List of the Fluorescence Decay Parameters for C153 Dye (15.3 μM) in EtOH Solution as a Function of Temperaturea temp/°C

a1/%

τ1/ns

a2/%

τ2/ns

a3/%

τ3/ns

χ2

25 35 45 55 65

8.27 6.89 5.26 4.39 3.83

0.04 0.04 0.04 0.04 0.04

6.57 6.79 6.59 5.43 5.63

1.31 1.36 1.40 1.26 1.33

85.16 86.32 88.16 90.18 90.54

4.94 4.92 4.88 4.80 4.79

1.07 1.12 1.17 1.32 1.37

a

The decays were measured at 460 nm with 374 nm excitation and analyzed using a triexponential function.

equilibrium in the solution, the fluorescence decays were measured at 25 °C before and after the heating and cooling cycle. Interestingly, the two decays thus measured match quite nicely (Figure 9B inset), suggesting that the dye monomers and aggregates apparently maintain a kind of quasi-equilibrium in the solution. 3.5. Time-Resolved Fluorescence Anisotropy Studies. To obtain additional evidence for the aggregates of C153 dye in EtOH solution, we carried out TR fluorescence anisotropy measurements in both ACN and EtOH solutions. These measurements were carried both at the emission maxima (515 nm for ACN and 525 nm for EtOH) and at the blue edge of the emission spectra (460 nm) of the dye. Observed anisotropy decays in the two solvents are shown in Figure 10A,B, respectively. As indicated from Figure 10A, the anisotropy decays in ACN solution are very similar both at the emission maximum and at the blue-shifted wavelength. These decays fit satisfactorily with a single exponential function giving a rotational correlation time (τr) of about 0.1 ns. Observed results clearly indicate that the dye exists only as the monomers in ACN solution. Unlike in ACN, in EtOH solution (Figure 10B), the anisotropy decay at the blue-shifted wavelength is evidently much slower than that at the emission maximum. The decays at both the wavelengths fit reasonably well with a single-exponential function, giving τr values as about 0.13 ns at the emission maximum and as about 0.21 ns at the blue-shifted wavelength. Though, on the basis of the multiexponential fluorescence decay at 460 nm (Figures 3B and 5B), the anisotropy decay at this wavelength was also expected to be multiexponential in nature, a similar multiexponential analysis, however, did not give meaningful anisotropy parameters. Thus, we relied on the single-exponential analysis of the anisotropy 6958

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Figure 10. Fluorescence anisotropy decays of C153 dye: (A) in ACN solution (27.8 μM), measured at the blue edge (460 nm) and at the emission maximum (515 nm) of the emission spectrum; (B) in EtOH solution (28.9 μM), measured at the blue edge (460 nm) and emission maximum (525 nm) of the emission spectrum.

Figure 11. Fluorescence decays of C153 dye recorded at the blue edge (470 nm) of the emission spectra in (A) ACN and (B) EtOH solutions using fluorescence up-conversion measurements. Dye concentrations were 760 and 190 μM in ACN solution and 300 and 75 μM in EtOH solution.

decay even at 460 nm in EtOH to obtain an average τr value for the fluorescent species contributing at this wavelength. The τr value of 0.13 ns as observed at the emission maximum in EtOH solution, which is also quite similar to that observed in ACN solution both at 460 nm and at the emission maximum undoubtedly indicate the fluorescence species as the monomers in the solution. Distinctly higher τr value (∼0.21 ns) observed at 460 nm in EtOH solution clearly suggests that it is due to the contribution of the dye aggregates, because the τr value is directly related to the effective volume of the fluorescence species, as given by following Stokes−Einstein relation.44−46

τr =

Vη RT

in the solutions. Figure 11A shows the kinetic traces measured in ACN solution at 470 nm employing 760 and 190 μM dye concentrations. As we can see, there is no obvious change in the kinetic traces even on using largely different dye concentrations, confirming that the dye does not display any unusual behavior in ACN solution. In Figure 11A, the initial unusually fast decay component is understandably due to the ultrafast solvation dynamics reported in ACN solvent (average solvation time ⟨τs⟩ ∼ 0.26 ps).82 The flat fluorescence signal with no apparent decay observed during the latter time span shown in Figure 11A nicely corroborate with the very long single-exponential fluorescence decay of the dye in ACN solution (lifetime ∼5 ns), as estimated using TCSPC measurement (section 3.2). Figure 11B shows the kinetics traces measured for C153 dye in EtOH solution at 470 nm, keeping the dye concentrations as 300 and 75 μM, respectively. As observed from this figure, like in ACN solution, there is no obvious change in the traces even for using two largely different dye concentrations. The initial significantly fast decay in the kinetic traces in EtOH solution is necessarily due to solvation dynamics (⟨τs⟩ ∼16 ps).82 Interestingly, in EtOH solution, the kinetic traces show a slow continuously decaying feature even for the longer time span (until about 230 ps) covered in the present measurements, which is distinctly different from the flat fluorescence signal observed in ACN solution at the longer time span. Thus, the observed up-conversion traces in EtOH solution indicate the presence of the emitting species having much shorter decay times than the fluorescence lifetime (τ ∼ 4.8 ns; Table 1) of the monomeric dye in this solvent. These ultrafast fluorescence kinetic traces, however, could not be analyzed convincingly, due

(8)

where V is the effective volume of the fluorophore and η is the viscosity of the solvent. Considering the extent of increase in the τr value at 460 nm, which is just about 1.6 times the τr value observed at 525 nm, we suggest that the size of the aggregates in the present system is not very large, possibly dominated mainly by the dye dimers, trimers, etc. In brief, observed fluorescence anisotropy results are in accordance with our proposition that C153 dye undergoes small size H-aggregate formation in polar protic solvent EtOH, which is not possible in polar aprotic solvent ACN. 3.6. Femtosecond Fluorescence Up-Conversion Measurements. To understand the fluorescence characteristics of C153 dye in ACN and EtOH solutions at the ultrafast time scales, we carried out fluorescence up-conversion measurements, recording the kinetic traces at the blue edge of the emission spectra (470 nm) employing largely different dye concentrations 6959

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Figure 12. SEM images of the microfilms formed by using the C153 solutions in (A) polar aprotic ACN and (B) polar protic EtOH solvents.

Figure 13. (A) Comparison of the fluorescence decays of C153 dye (∼8 μM) in TFE and EtOH solutions, measured at 470 nm. (B) Wavelengthdependent fluorescence decays for C153 dye (∼13 μM) in TFE solution, covering the whole steady-state fluorescence spectrum of C153 dye.

to the strong influence of solvent relaxation dynamics, to extract any meaningful decay parameters for the dye aggregates present in the solution. In any case, the observed up-conversion traces in EtOH solution are qualitatively in accordance with the inferences drawn from the TR fluorescence results obtained from TCSPC measurements, supporting the aggregation of the dye in the protic solvent. 3.7. Dynamic Light Scattering and Scanning Electron Microscopy (SEM) Measurements. Dynamic light scattering (DLS) measurements were carried out to understand the presence of any scattering particles in the EtOH solution of C153 dye. The scattering signal from the dye solution was not only very weak but also quite similar to that of the blank EtOH solvent. Moreover, no reasonable correlation function could be constructed from the scattering signals, for either the dye solution or the bulk solvent. These results apparently suggest that the dye aggregates in the solution are quite smaller in size, possibly dominated by dye dimers, trimers, etc., as also indicated earlier from fluorescence anisotropy measurements (section 3.5), and hence not detectable by DLS measurements. To explore the present system further, we also carried out the SEM measurements for the dye solutions in both ACN and EtOH solvents. The SEM pictures thus obtained for the microfilms from ACN and EtOH solutions are shown in Figure 12A,B, respectively. As indicated from Figure 12A, there are large islands of dye aggregates in the microfilms formed by using ACN solution of the dye. This is likely because during evaporation of the solvent from the dye solution on the copper stub there must be some micro bubble formation at latter stages of the drying process where the dye molecules are eventually deposited, forming islands of varying sizes. Interestingly, the SEM image formed from the EtOH solution of the dye shows

unique threadlike structures of the deposited dyes on the microfilm, which are distinctively very different than the SEM image obtained in the case of ACN solution of the dye. Though in the present study it is not possible to exactly understand how these typical threadlike structures are formed during the evaporation of EtOH from the dye solution, the observation clearly suggests that all along the evaporation process the protic EtOH solvent plays an active role in associating the dyes in a specific manner such that the dyes are eventually deposited on the copper stub forming the threadlike structures. From the present observation one can thus infer that even in the dilute solutions of C153 dye in EtOH solvent, as are used in the photophysical studies, there are also formation of small dye aggregates, possibly in the forms of dye dimers, trimers, etc., which are supported by the dye-solvent hydrogen bonding interaction. 3.8. Time-Resolved Fluorescence Studies in Strongly Protic Trifluoroethanol Solvent. As it is indicated from the observed results and discussion presented earlier, the protic nature of EtOH solvent apparently favors the aggregation of C153 dye in the solution. Considering this, we carried out some time-resolved fluorescence measurements in a strongly protic solvent, namely, 2,2,2-trifluoroethanol (TFE). Figure 13A shows the comparison of the fluorescence decays of C153 dye in TFE and EtOH solutions, measured at 470 nm. As indicated from this figure, the decay in TFE shows larger contribution for the faster decay component than that in EtOH, suggesting a better aggregation of the dye in the former solvent of having relatively stronger protic character. Figure 13B shows the wavelengthdependent fluorescence decays in TFE solution, covering the whole steady-state fluorescence spectrum of C153 dye in this solvent. The changes in the decays with changing wavelength 6960

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interaction the excited-state energy level of the chromophore undergoes a splitting, generating a higher and a lower energy states relative to the monomeric excited state, as shown in Scheme A1. As suggested in this theory, the nature of this

are qualitatively very similar to that observed in EtOH solution (Figure 3B). Present observations in TFE solvent in general provide an additional support to the fact that the aggregation of C153 dye in solution is largely favored by the protic nature of the solvent studied.

Scheme A1. Energy Diagram for Aggregated Dimers with Coplanar Inclined Transition Dipolesa

4. CONCLUSION Photophysical properties of coumarin-153 (C153) dye do not show any unusual behavior in polar aprotic organic solvent, namely acetonitrile (ACN), whereas unusual aggregation of the dye is observed in polar protic solvent, ethanol (EtOH). Wavelength-dependent changes of the fluorescence decays in EtOH solution clearly indicate the presence of multiple emitting species of the dye. Concentration-dependent changes in the fluorescence decay characteristics suggest that multiple emitting species in EtOH solution arises due to small extent of aggregation of the dye. In this solvent, the fluorescence decays show triexponential behavior at the blue edge of the emission spectrum, with two shorter decay components due to dye aggregates (τ1 ∼ 0.04 ns for larger aggregates and τ2 ∼1.4 ns for smaller aggregates) and a longer decay component (τ3 ∼ 4.8 ns) due to monomeric dye. Contributions of the shorter decay components decrease with an increase in the monitoring emission wavelength, signifying the largely blue-shifted aggregate emissions as compared to the monomer emission and hence suggesting the formation of H-type of aggregates of the dye. Time-dependent red shift in the TRES and TRANES are also in support of the blue-shifted emissions from the short-lived H-type aggregates as compared to the emission from the dye monomers in the solution. Temperature-dependent fluorescence decays and time-resolved fluorescence anisotropy results directly support the aggregation of the dye in EtOH solution. Supportive evidence for the aggregates in EtOH solution is also obtained from ultrafast fluorescence up-conversion studies. Though DLS results are apparently inconclusive, possibly due to the smaller size of the aggregates, the SEM studies show clear differences in the images for microfilms formed from ACN and EtOH solutions of the dye, indicating the obvious role of protic EtOH solvent in assisting the aggregation process. Aggregate formation of C153 dye in EtOH is a very intriguing result because the dye apparently shows quite high solubility in this polar protic solvent. We infer that the highly dipolar characteristic of C153 dye (dipole moment ∼6.4 D) and the enhancement of this dipolar character via dye-solvent hydrogen bonding interaction in EtOH solvent drives the aggregation of the dye through Columbic interactions. That the aggregation is not observed in polar aprotic solvent ACN but in polar protic solvent EtOH certainly suggest that the protic nature of the solvent helps in the stabilization of the dye aggregates, through hydrogen bonding interaction. A supportive evidence for this proposition is also obtained from TR fluorescence results in a strongly hydrogen bond donating solvent, TFE. The present results are extremely important finding, because the dye C153 is a well utilized fluorescence probe for various photochemical studies where dye aggregation, if overlooked, can adversely affect the interpretation of the observed results.

■

The geometry and angle θ are illustrated above. Arrows indicate allowed transitions.

a

splitting for the excited state depends on the angle θ between the direction of the transition dipoles of the dye molecules and the axis of the dye aggregates (i.e., the line passing through the centers of the dye molecules in the aggregate; Scheme A1). Thus, when θ = 90°, as it happens in the case of H-type of aggregate formation where molecular planes of the dyes are stacked one above other (i.e., sandwich-type arrangement with face-to-face stacking of the dyes), the electronic transition between the ground state and the higher excitonic state is the most allowed transition whereas that involving the ground and the lower excitonic states is the highly forbidden transition. Accordingly, the absorption band for the H-type aggregates of the dyes is seen to be hypsochromically shifted relative to that of the dye monomer. On the contrary, if θ = 0° (i.e., head-totail stacking of the dyes; Scheme A1), the transition between the ground and the lower excitonic states is the most allowed transition and accordingly there is a bathochromic shift for the absorption band of the J-type of dye aggregates relative to that of the dye monomer. As Kasha’s theory suggests and one can anticipate from Scheme A1, for aggregates having θ > 54.7°, the electronic transition from the ground state to the excitonic state would effectively lead a hypsochromic shift for the absorption band of the dye. Similarly, for aggregates with θ < 54.7°, the electronic transition from the ground state to the excitonic state would effectively lead a bathochromic shift for the dye absorption band. Interestingly, for the aggregates with θ = 54.7° (magic angle), there will be no shift expected for the absorption spectrum of the dye and such unique aggregates are characteristically designated as the I-type (intermediate type) aggregates.

■

*H. Pal. E-mail: [email protected]. Fax: 022-25505151 & 02225519613. Tel: 022-25595396. Notes

The authors declare no competing financial interest.

■

APPENDIX

The electronic transitions dyes are best explained by basis of the interactions monomeric units present

AUTHOR INFORMATION

Corresponding Author

ACKNOWLEDGMENTS We are thankful to Dr. A. K. Mora and Dr. S. Nath of Radiation & Photochemistry Division, BARC, for their help in fluorescence up-conversion measurements, Dr. R. Ganguly of Chemistry Division, BARC, for his help in DLS measurements,

in the aggregates of chromophoric Kasha’s exciton theory given on the of the transition dipoles of the in the aggregates.48,49 Due to this 6961

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and Dr. C. B. Basak of Glass and Advanced Materials Division, BARC, and Dr. V. Sudarsan of Chemistry Division, BARC, for their help in the SEM measurements. We are also thankful to Dr. B. S. Tomar, Head, Radioanalytical Chemistry Division, Dr. D. K. Palit, Head, Radiation & Photochemistry Division, Dr. K. L. Ramakumar, Director, Radiochemistry & Isotope Group, and Dr. B. N. Jagatap, Director, Chemistry Group, for their constant encouragement and support.

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