J. Phys. Chem. 1995,99, 11860- 11866
11860
Photophysics of a Thiacarbocyanine Dye in Organic Solvents Dimitri Noukakis, Mark Van der Auweraer,* Susanne Toppet, and Frans C. De Schryver Laboratory for Molecular Dynamics, Department of Chemistry, KU kuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium Received: February 2, 1995; In Final Form: May 22, 1 9 9 9
The photophysical properties of the triethylammonium salt of 3,3’-bis(sulfopropyl)-5,5’-dichloro-9-ethylthiacarbocyanine (THIATS) and other related dyes have been studied by means of steady-state and time-resolved spectroscopic techniques. Absorption and fluorescence maxima and bandwidths suggest, together with NMR data, the existence of more than one ground-state conformation for this dye. Analysis of the fluorescence decays by time-correlated single-photon counting and time-resolved fluorescence spectroscopy further supports the steady-state spectroscopy results. An additional short-lived fluorescent species is observed in some polar solvents such as methanol and 1-octanol. The dependence of the spectral features and the fluorescence kinetics on the nature of the solvent is explained by a model invoking ion-pairing and conformational changes in the ground state of the dye. It is shown that polarity and polarizability of the solvent control the photophysical properties of THIATS.
1. Introduction The importance of cyanine dyes in the spectral sensitization of photographic emulsions and semiconductors has motivated a large amount of scientific work in the past three decades. Most of the effort has focused on understanding the aggregation properties of these molecules and the factors controlling the growth of the aggregates in aqueous solutions or on various substrates.’ The spectral features of the H- and J-aggregates of cyanine dyes have been successfully described through the Davydov molecular exciton model? and the intriguing spectral properties of the J-aggregates are still giving rise to theoretical and experimental r e s e a r ~ h . ~ . ~ However, less attention has been accorded to the photophysical properties of the cyanine dyes in their monomeric form. In an early work, West and Geddes5reported the absorption spectra of several cyanine dyes in a series of solvents. It has been shown that the position of the absorption maximum is shifted to longer wavelengths by dispersive interactions between the transition dipole of the dye and the electronic polarizability of the solvent. The same group showed that, at low temperatures in alcoholic solvents, the meso-substituted thiacarbocyanines exhibit two absorption maxima, which are attributed to the alltrans and mono-cis isomers.6a In an X-ray study of a series of thiacarbocyanine crystals by Steiger et al.,’ the effect of the meso substituent on the stereoisomerism of these dyes has been demonstrated. More recently, advances in excited state lifetime measurements allowed the determination of the fluorescence decays of some cyanine dyes.* For trimethine indolocarbocyanines, deviations from monoexponential decay kinetics have been observed and have been attributed to the presence of monocis and all-trans isomer^.^ Recently, the photoisomerization of some carbocyanine dyes has been studied by means of flash photolysis,IOsuggesting that the solvent affects only the cistrans isomerization rate on the excited-state surface due to viscosity changes. Although it is generally accepted that radiationless relaxation of the excited state of cyanine dyes is coupled with torsional motion around the polymethine chain,I’ the details of this process are still not fully understood. In this contribution we present an investigation of the photophysical properties of a meso-substituted thiacarbocyanine @
Abstract published in Advance ACS Abstracts, July 1, 1995.
R,, R3 = Me, Et, (CH2)3S03‘,
...
& = H, Me, Et, Phenyl, ... R5, R6 = CI, Me, Et, OMe,
...
X = C, N, 0,S
THlATS
: 3 . 3 ’ ~ s ~ l f o p r o p y 1 - ~ . 5 ” - 9 ~ lthiacarbo~yanine hyl
triethyl amonnium
Figure 1. General chemical formula of thiacarbocyanine dyes and of 3,3’-bis(sulfopropyl)-5,5’-dichloro-9-ethylc~bocyaninetriethylamonnium (THIATS).
dye by means of N M R , steady-state absorption, and emission spectroscopy and time-correlated single-photon counting.
2. Experimental Methods The chemical structure of different cyanine dyes under investigation is given in Figure 1, together with that of THIATS, on which dye this contribution is focused.
0022-365419512099-11860$09.0010 0 1995 American Chemical Society
Photophysics of a Thiacarbocyanine Dye NMR experiments were carried out using a Bruker WM 250 spectrometer for the high- and low-temperature experiments and a Bruker AMX 400 for the room temperature experiments. Steady-state absorption spectra were obtained with a PerkinElmer 1 6 spectrophotometer;steady-state-correctedfluorescence spectra, with a SPEX Fluorolog spectrophotometer. All the solvents used were of spectroscopic quality. Fluorescence quantum yields (Of) were measured relative to that of Rhodamine-B in ethanol (Of= 0.65). The differences in refractive index and optical densities of the various solutions were taken into account in the calculation of the integrated fluorescence intensity. Fluorescence decay kinetics were measured using a timecorrelated single-photon counting (TC-SPC) setup described elsewhere.I2 The excitation light was produced by a modelocked, cavity-dumped, synchronously pumped pyromethane dye laser. A Hamamatsu (R2809U) Multi Channel Plate photomultiplier was used to detect the fluorescence signal. Several cutoff filters were used in front of the monochromator, and to ensure that no scattered excitation light was passing through, a blank experiment was performed before each series of measurements. This was done by placing a diluted LUDOX solution in the sample holder and measuring the scattered light passing through the cutoff filters and the monochromator. Then the filters and the entrance slit of the monochromator were adjusted to completely extinguish the scattered light at the desired emission wavelength. The experiment was run at a rate of 800 KHz in the "inverted" mode, and the resulting instrument response function had a width between 34 and 55 ps. Analysis of the kinetic traces was made using the 6 function convolution method.I3 In this way it was possible to resolve the decay of pinacyanol chloride in methanol and propanol with decay times of 7 and 24 ps, respe~tive1y.l~ Time-resolved fluorescence spectra were obtained by gating the multichannel analyzer at a predetermined time window and scanning the emission monochromator, with typical dwell times of 5- 10 dchannel. The fluorescence spectra obtained this way were slightly biased by the wavelength dependence of the response of the detection system.
3. Results and Data Analysis 3.1. NMR Spectroscopy. Proton NMR spectra of THIATS were obtained in deuterated methanol (CD30D), chloroform (CDC13), and tetrachloroethane (CDChCDCL). The spectra in CDCl3 and CDChCDCl:! were very similar, so the latter was chosen for the low-temperature studies because of the better solubility of the dye. In Figure 2 the 'H-NMR spectra of THIATS in CD30D at -25 and +55 "C and those in CDClzCDCh at -20 and +lo0 "C are presented. It can easily be seen that at 250 MHz and at low temperature the two vinyl protons are equivalent in methanol (one peak at -6.5 ppm), while in tetrachloroethane they give rise to two distinct peaks (at -6.4 and 7.2 ppm). Raising the temperature of the latter solvent gradually brings the signal of the two vinyl protons to coalescence at -6.86 ppm. A list of the chemical shifts of the relevant protons in these two solvents at different temperatures is given in Table 1. It is noticeable that the protons at positions 7 and 6 equally give rise to two distinct signals at low temperatures, although with a less important splitting of 0.41 and 0.05 ppm, respectively. 3.2. Steady-State Absorption and Emission Spectroscopy. The absorption and emission spectra of THIATS in some solvents are presented in Figure 3. All spectra were taken at room temperature and are normalized at the maximum of absorptiodemission. The spectroscopic data together with the
J. Phys. Chem., Vol. 99, No. 31, 1995 11861
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fluorescence quantum yields in different solvents are summarized in Table 2. It is striking that the absorption spectra exhibit large differences in different solvents, while the fluorescence spectra seem to change very little. Indeed, for the fluorescence the full width at two-thirds of the maximum (FW2/3M) is -590 f 40 cm-' in all solvents, while for the absorption the FW2/3M can double by changing the solvent, passing from 550 to 1200 cm-I. The spectra presented in Figure 3 suggest that there are at least two different absorbing species in the ground state and that their relative contribution to the absorption spectrum depends on the nature of the solvent. In solvents of high dielectric constant (er) the absorption is mainly due to a blue-absorbing species, while in solvents of low cr only a red-absorbing species is observed. These changes are very different from the red solvatochromic shift (due to the refractive index changes) observed for the absorption spectra of other carbocyanine dyes.5 In solvents of intermediate dielectric constant both species are observable, and their relative contribution seems to depend on the concentration of the dye. To check whether this concentration dependence is due to aggregation of the dye or to pairing between the dye and its counterion, two different experiments were performed in 1-octanol and 1,2-dichloroethane (DCE). First the absorption spectra of THIATS were taken at different dye concentrations, and second, the absorption of a dilute solution of THIATS (-2 x M) was monitored while varying the concentration of the counterion by adding triethylammonium hydrochloride (TEA). These experiments yielded the same results in both solvents, so only those in DCE are presented in Figure 4. In Figure 4a it can be seen that increasing the concentration
Noukakis et al.
11862 J. Phys. Chem., Vol. 99, No. 31, 1995
TABLE 1: '€I-NMR Chemical Shifts of Some Relevant Protons of THIATS in CDJODand CDC13CDC12, Obtained with a 250 MHz Spectrometer at Various Temperatures in CD3OD temp ("C)
HvlnydS)a
55 30 15 -5 -25 a
in CDClzCDClz
H7(D)a
HdD)
&(SI
8.00 8.00 8.00 8.00 8.02
1.40 7.40 7.40 7.38 7.38
7.89 7.91 1.92 7.93 1.93
6.59 6.59 6.58 6.53 6.48
temp ("c) 100 60 25 -5 -20
HvinydS) 6.86 6.82 6.46t7.20 6.4117.18 6.3917.16
H7(D) 1.17 7.19 1.5817.92 1.5817.99 7.5918.00
HdD) 7.28 1.32 1.33 1.3617.32 1.3711.32
H4(S) 1.54 1.54 7.55 1.54 1.54
(S) and (D) stand for singlet and triplet.
TABLE 2: Absorption and Emission Maxima (v. and vr, respectively); Fuil Widths at Two-Thirds of the Maximum (F'WY in Different Solvents, Together with the Solvent's 3M);a Stoke's Shifts (Avp-f)p and Fluorescence Quantum Yields (a?) ParametersAn2)" and Static Dielectric Constant E, solvent methanol acetonitrile 1-octanol 1,2-dichloroethane chloroform chlorobenzene In cm-I.
Va
Vf
18 116 18 149 17 921111 554d 17 953111 513d 11 391 17 212
17 331 17 271 11 065 11 036 11 007 16 892
Because of the 0
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FW213Ma 933 957 1207 1127 780 556
FW213Mi 637 595 610 592 62 1 549
Ava-i 785 878 8651479 9 171477d 385 320
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0.0048 0.0094 0.0970
0.338 0.350 0.410 0.420 0.421 0.469
32.6 37 10.3 10.4 4.8 5.1
0.0940 0.2800
1 vibrational structure, it was not possible to measure the full width at half-maximum. 'An2)= 2(nZ- 1)/(2n2 In these solvents two absorption maxima are observed depending on the concentration of the dye or
+ 1) is a modified refractive index function. its counterion (see text).
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of THIATS shifts the maximum of absorption from 557 to 571 nm. The same effect is observed upon addition of TEA in a dilute solution of THIATS (Figure 4b), thus suggesting that ionpairing and not dye aggregation is the process responsible for the anomalous absorption solvatochromic shift observed for this dye. Interestingly, the excitation spectra in DCE and 1-octanol
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Figure 4. Absorption spectra of THIATS in 1,2-dichloroethane(a) at two different THIATS concentrations and (b) [THIATS] = 2 x M and at two different concentrations of TEA. are red-shifted and much narrower than the absorption spectra, corresponding more closely to the mirror image of the fluorescence band. The fluorescence spectra of THIATS exhibit the normal solvatochromic red shift observed with the absorption spectra of other cyanine dyes.5 Since this shift is due to dispersive interactions, it should be possible to correlate the energy of the fluorescence maximum to the solvent's refractive indext5 (see Figure 5 ) .
J. Phys. Chem., Vol. 99, No. 31, 1995 11863
Photophysics of a Thiacarbocyanine Dye 18200
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Figure 5. Absorption (0)and fluorescence (m) maxima of THIATS against the An2) function of the refractive index of the solvent (see Table 2). The vertical bars represent the error made when reading the absorption and fluorescence maximum (& 1 nm). In the insets are given the goodness of fit (R) and the slope (a)for a linear regression on each set of data. The solvatochromic shift of the fluorescence, as described in Figure 5, together with the fact that the bandwidth of the fluorescence changes very little with the solvent, suggests that either the steady-state emission spectrum is dominated by one species or, if different species are present in the excited state, they all have very similar fluorescence spectra. This is also supported by the observation that, in all solvents, changing the excitation wavelength did not affect significantly the fluorescence spectrum. On the other hand, the position of the absorption maximum does not correlate with the refractive index (see Figure 5 ) or with any other physical constant of the solvent, indicating that there are at least two absorbing species and that their relative concentration is solvent dependent. 3.3. Time-Resolved Single-Photon Counting. The fluorescence decay of thiacarbocyanine dyes without any meso group could be analyzed, as expected, as monoexponential decays (e.g. for the thiacarbocyanine with R5 = R6 = R5' = %' = C1, R9 = H, and RI = R3 = RI' = R3' = Et in chloroform, robs = 110 ps). On the other hand, the fluorescence kinetics of THIATS in solution was found to be very complicated, and the decay traces could only be described as the sum of at least two or three exponentials. These results are in contradiction with those obtained by Kemnitz et al.? where the fluorescence decay of the same dye in methanol was thought to be singleexponential with a lifetime of 70 ps. To obtain a better insight into the excited-state kinetic behavior of THIATS in various solvents, a systematic study by time-resolved fluorescence spectroscopy and TC-SPC was undertaken, varying the excitation and the emission wavelengths. Time-resolved fluorescence spectra of THIATS were obtained in all the solvents, using the same experimental setup as for the TC-SPC experiments. Uncorrected fluorescence spectra were recorded at different time windows and at different excitation wavelengths. Figure 6 shows the most dramatic effect observed during these experiments. In 1-octanol, the fluorescence spectrum of THIATS changes with time when the sample is excited at 540 nm but is independent of the time window when excited at 585 nm. At A,,, = 540 nm and at short times, the fluorescence spectrum becomes much broader and the maximum is slightly blue-shifted. Similar results were obtained in methanol. Conversely, in chloroform and in chlorobenzene, no significant time or excitation wavelength dependence of the fluorescence spectrum could be observed, as shown in Figure 7.
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Figure 6. Time-resolved fluorescence spectra of THIATS in 1-octanol at different excitation wavelengths (Aexc). Broken lines: spectra obtained during the first 300 ps, 100 ps after the excitation pulse. Solid lines: spectra obtained at a time window spanning from 2.8 to 6.4 ns. All the spectra have been normalized to their maximum of emission intensity.
Although the fluorescence kinetics of THIATS was too complex to be quantitatively analyzed, we have attempted to treat the set of data obtained at different excitation and emission wavelengths in a qualitative way, using the global analysis technique.I6 The results of this analysis are presented in the form of graphs (see Figure 8), where the normalized (&i = 1) pre-exponential factors of each decay component are plotted against the emission wavelength at which the decay was recorded. At this point it should be obvious that these preexponential factors can be associated with different decaying species only if one assumes that the interconversion rate from one species to the other in the excited state is very slow compared to the decay rate of each species.
4. Discussion The 'H-NMR results demonstrate the existence of at least two different isomers in the ground state, in agreement with earlier observations with similar molecules, where different isomers have been separated by chromatographic methods." In a detailed 'H-Nh4R study of meso-substituted carbocyanines in deuterated acetone,I8 similar results have been obtained at low temperatures, and the splitting of the protons Hs and Hg' has been explained in terms of decrease of the rate of cis-trans interconversion as the temperature decreases. This would also explain the splitting of the signal of the aromatic protons H7 and H6. According to ref 18, the 8 and 8' protons of the mono-
Noukakis et al.
11864 J. Pkys. Ckem., Vol. 99, No. 31, 1995
600
550
700
650
Emission Wavelength [nm]
Figure 7. Time-resolved fluorescence spectra of THIATS in chloroform at different excitation wavelengths &A. Broken lines: spectra obtained during the first 200 ps, 100 ps after the excitation pulse. Solid lines: spectra obtained at a time window spanning from 1.3 to 3.2 ns. All the spectra have been normalized to their maximum of emission intensity.
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A, max
0.6
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0.2
-
temperatures at which the experiments were carried out (-25 "C). However, in low-polarity solvents such as chloroform or tetrachloroethane, lowering the temperature allows the signals from the different isomers to be fully resolved. This difference would be due to the different isomerization rates ( h ) between the cis and trans conformations of THIATS in these two solvents. These differences in ki,, may originate from two distinct effects. First, since the transition state for the cistrans isomerization of thiacarbocyanines is believed to be characterized by a more localized positive charge,'ICit should be stabilized in the more polar solvents, and thus the energy barrier for the isomerization reaction would be lowered. The second effect might be due to ion-pairing between the dye and its counterion. In polar solvents where the dye exists most probably in the form of solvated ions, k,,, should be larger than in nonpolar solvents, where the dye is mainly paired with its counterion and hence is less flexible (in regard to the rotational motions around the polymethine chain). The results from the steady-state absorption spectra agree with the interpretation of the NMR data. At least two different species coexist in most of the solvents: one with an absorption maximum at -550 nm and the another with a maximum at -580 nm. The relative concentration of these species varies greatly with the solvent, as can be seen in the spectra of THIATS in methanol, l-octanol, and chlorobenzene. This is most probably the same effect as the one observed by West et when similar dyes where studied in alcoholic solutions at low temperature. According to these authors, the blue-absorbing species could be assigned to the mono-cis isomer and the redabsorbing one to the all-trans. The work of Scheibe et al.6bon the absorption spectra of rigid structures of the different isomers of carbocyanine dyes further supports the above characterization of the mono-cis and all-trans isomers. It is then reasonable to assume that in the ground state the two species are in equilibrium and that the equilibrium constant is solvent dependent. At this point, it should be noticed that the parameter that controls this equilibrium is not the viscosity but the polarity of the solvent. For instance, the spectral features of the absorption spectrum of THIATS in l-octanol and 1,2dichloroethane are identical (see Table 2). These two solvents have very different viscosity but identical dielectric constants. Ion-pairing between the dye and its counterion would also explain the solvent dependence of the equilibrium constant between the two isomers. In low-polarity solvents, the dye, bearing an overall negative charge, will most likely be paired with the positively charged counterion. This should force the equilibrium toward the all-trans conformation (one could conclude that the negatively charged sulfonate groups share the positive charge of the triethylammonium). On the contrary, in polar solvents the unpaired dye would adopt the less sterically hindered mono-cis conformation. This argument based on the dependence of the ion-pairing on the polarity of the solvent (in terms of static dielectric constant) cannot be used to explain the spectroscopic differences between solutions of THIATS in chloroform and chlorobenzene. However, as has already been proposed,6a the mono-cis isomer would have a lower polarizability than the all-trans. Therefore, the all-trans isomer will be more stabilized in the more polarizable solvent, chlorobenzene, than in chloroform (see Table 2 underAn2)). In fact, it is very probable that in chlorobenzene mainly the all-trans isomer of THIATS is present. Although the decay kinetics of THIATS is too complex to be quantitatively resolved, some useful conclusions can be drawn from the kinetic analysis. In very polar solvents, the excited state of the dye decays faster than in low-polarity solvents, as
22 0
530
550
570
590
610
Emission Wavelength
630
6.54
670
[nm]
Figure 8. Fluorescence decay curve analysis of THIATS in l-octanol
at ,a, = 535 nm, using a three-exponential model in the global analysis fit. The pre-exponential factors are normalized such that &, = 1 and are plotted against the fluorescence detection wavelength. The time increment is 8 pskhannel and X I = 44 ps, t2 = 176 ps. t3 = 1.05 ns. 1, max and max correspond to the maximum of the steady-state absorption and emission spectrum.
cis isomer would absorb at higher field than the 8 and 8' protons
of the all-trans isomer. Furthermore, the decrease of the magnitude of this splitting (Ad(H,in,l) > AB(&)) is consistent with the picture of the mono-cis isomer. This assignment of the vinyl protons suggest that in methanol the equilibrium between the mono-cis and the all-trans isomers is strongly shifted toward the mono-cis. In polar solvents such as methanol, the time resolution of these experiments does not allow the resolution of the signals from the different isomers at the
Photophysics of a Thiacarbocyanine Dye can be seen from the fluorescence quantum yield (@f) values. Since the fluorescence rate constants of the different isomers should not depend on the solvent, the increase of the
