7717
J . Phys. Chem. 1989, 93, 7711-7725 sistent with A-type Franck-Condon scattering. We conclude that the process which underlies the resonantly enhanced intensity of the C=C stretching mode of the monomer and dimer is quite different.
Conclusions The profile for the resonance-enhanced intensity of marker bands of the Raman spectrum of chlorophyll a in hexane is interpreted in terms of the chemical structure of an approximately T-shaped dimer of this large molecule. The Raman data are consistent with a chemical linkage between the C=O group on position 9 of ring V of one molecule to the Mg atom of another. The assignment of the resonance Raman spectrum in terms of this particular linkage is facilitated by the fact that the intensity of a C=O mode associated with the ester group on position 10 of ring V is weak and falls outside the detection limit of the pulsed laser spectrometer used in the present experiments. The resonance
Raman data also reveal that the process which governs the intensity enhancements is quite different for monomeric and dimeric chlorophyll a. Whereas Franck-Condon factors as given in Albrecht's A terms appears to determine the intensities for the dimer, those for the monomer are related to the B term in order to describe the Raman excitation profile of a band due to C=C vibrations of the macrocycle. The relative orientation of the two Chla molecules of the dimer can in principle be obtained from a comparison of the depolarization ratio of corresponding bands of the monomer and the dimer (non-resonance-enhanced intensities). Such experiments are now in progress and their results shall be published in future publications from this laboratory.
Acknowledgment. This research was supported by the Natural and Engineering Research Council of Canada. Registry No. Chlo, 479-61-8; Chla.H,O, 57957-30-9; chla-lutidine, 122491-51-4.
Fluorescence and Photoisomeriration of an Amphiphiiic Aminostiibazoiium Dye As Controlled by the Sensitivity of Radiationless Deactivation to Polarity and Viscosity Heinz Ephardt and Peter Fromherz* Abteilung Biophysik der Universitat Ulm,D- 7900 Ulm-Eselsberg, FRG (Received: March 30, 1989)
-
The dynamics of the first excited singlet state of (dibuty1amino)stilbazolium butylsulfonate is investigated in various solvents. The lifetime and the quantum yield of fluorescence and the quantum yield of trans cis photoisomerization change by 2 orders of magnitude whereas the yield of cis trans photoisomerization is invariant. The environment affects an activated radiationlessdeactivation which competes with radiative decay and photoisomerization. The effect is assigned to the stabilization of a polar transition state in media of high dielectric constant (Born energy) and to friction along the reaction path in media of high viscosity (Kramers equation). Within such a model the isothermal data are compatible with the Arrhenius energies. The radiationless process is assigned tentatively to the formation of a twisted internal charge transfer state with fast internal conversion. Probing of fast changes of the electrical potential in biological membranes is discussed in the light of the photophysical properties of the dye.
-
The change of fluorescence-at a given wavelength and polarization of excitation and emission-may be due to a spectral shift, to a reorientation of the transition moments, or to a change of the quantum yield of fluore~cence.~'A change of quantum yield may be due to an intramolecular effect or due to aggregation. Spectral shift (electrochromism) has been suggested as a principal mechanism of the potential sensitivity of stilbazolium
dyes.8-10 The fluorescence spectra in artificial membranes, however, indicate a distinct potential-dependent component that must be assigned to a change of quantum yield.lO*ll It appears that the physical mechanism of these important probes is unclear. A precise knowledge of the origin of potential sensitivity is the prerequisite to optimize the dyes with respect to sensitivity, stability, staining, and toxicity. Distinctly improved probes are required for high-resolution studies in neuron dendrites.I2 The most extensive literature on the photochemistry of stilbene and stilbene derivatives includes only little information on aminostilbazolium dyes.13*14 We have started thus a characterization of these potential-sensitive dyes with respect to their spectra and to the dynamics of their first excited singlet state which determines fluorescence intensity. In this paper we present our observations on (dibuty1amino)stilbazolium butylsulfonate with respect to the yield of fluorescence and of trans-cis photoisomerization in organic solvents and amphiphilic assemblies. We assign the effect of the environment to a modulation of an activated radiationless deactivation by polarity and viscosity. We discuss the results within the concept of twisted internal charge transfer (TICT). The implications with respect
(1) Cohen, L. B.; Salzberg, B. M.; Davila, H. V.; Ross, W. N.; Landowne, D.; Waggoner, A. S.; Wang, C. H. J . Membr. Biol. 1974, 19, I . (2) Grinvald, A,; Hildesheim, R.; Farber, I. C.; Anglister, L. Biophys. J . 1982, 39, 301. (3) Orbach, H. S.; Cohen, L. B. J . Neurosci. 1983, 3, 2251. (4) Grinvald, A.; Salzberg, B. M.; Lev-Ram, V.; Hildesheim, R. Biophys. J . 1987, 51, 643. ( 5 ) Conti, F. Annu. Rev. Biophys. Bioeng. 1975, 4, 287. (6) Waggoner, A. S.; Grinvald, A. Ann. N . Y . Acad. Sci. 1977, 303, 217. (7) Dragsten, P. R.; Webb, W. W. Biochemistry 1978, 17, 5228.
(8) Loew, L. M.; Simpson, L. L. Biophys. J . 1981, 34, 353. (9) Loew, L. M.; Bonneville, G. W.; Surow, J. Biochemistry 1978, 17, 4065. (10) Loew, L. M. J . Biochem. Biophys. Methods 1982,6, 243. (11) Dambacher, K. H.; Fromherz, P., in preparation. (12) Fromherz, P.; Vetter, T. Proceedings of the 17rh Gdttingen Neurobiology Conference; Elsner, N., Singer, W., Eds.; Thieme: Stuttgart, 1989; paper 216. (13) Abdel-Mottaleb, M. S. A. Loser Chem. 1984, 4, 305. (14) Gorner, H.; Gruen, H. J . Photochem. 1985, 28, 329.
Introduction Aminostilbazolium dyes are used most frequently as fluorescence probes to follow the fast change of the electrical membrane potential during an action potential in neurons.I4 A typical example of this class of dyes is (dibuty1amino)stilbazolium butylsulfonate.
/ - -
0022-3654/89/2093-7717$01.50/0
0 1989 American Chemical Society
1718 The Journal of Physical Chemistry, Vol. 93, No. 22, 1989
Ephardt and Fromherz
-
The quantum yield of cis trans photoisomerization 4cr is given by the yield P of decay to the trans state from the perpendicular state if each excited cis isomer reaches the perpendicular state according to
-
Figure 1. Reaction scheme for the excited (dibuty1amino)stilbazolium butylsulfonate. Ground states: trans conformation T and cis conformation C. Excited singlet states: fluorescent state F, perpendicular state P, and I state (with fast internal conversion). Kinetic parameters: radiative decay constant kF,rate constants kp and klof horizontal transitions, and yield p of formation of trans isomer from the P state by internal conversion.
to the potential sensitivity of the dye are considered.
(4) We evaluate the rate constant kF of radiative decay from the lifetime 7 and the yield 4Fof fluorescence by using eq 5 as obtained from eq 1 and 2 (5)
Reaction Scheme Our experiments are guided by a minimal reaction scheme as shown in Figure 1. It is suggested by numerous studies in the field of aminostilbazolium d y e ~ , of ~ ~amin~stilbenenitriles,~~J~ J~ of other stilbene derivative^,'^-^^ and of stilbene i t ~ e l f . ~ ~ ; ~ ~ We distinguish five molecular states: the ground state in trans conformation (T), the ground state in cis conformation (C), a fluorescent excited state (F), and two nonfluorescent excited states (P and I). The P state is on the path of photoisomerization with the double bond in perpendicular conformation. It decays by internal conversion to the ground state in trans or cis conformation. The I state-probably with twisted single bonds-decays by internal conversion to the ground state in trans conformation. The hypothetical adiabatic potential surfaces drawn in Figure 1 are singlet states. They are relaxed with respect to solvation. The reaction scheme is characterized by four rate parameters: the rate constant of radiative decay of the F state (kF),the rate constants of horizontal radiationless transitions26to the P state ( k p )and to the I state ( k I ) ,and the yield of internal conversion to the trans conformation from the perpendicular state (P). The reaction scheme neglects intersystem crossing, internal conversion from the F state, fluorescence from the P state and I state, and back reactions to the F state from P state and I state. We discuss the contraction of kinetics to the minimal scheme of Figure 1 below. We measure four parameters: the fluorescence lifetime ( 7 ) , the quantum yield of fluorescence (4F),the quantum yield of trans cis photoisomerization (hC), and the quantum yield of cis trans photoisomerization (+m). The four experimental quantities 7 , &, 4 T ~ and , ~ C are T defined by the four rate parameters kF, kp, kI, and P and vice versa. The lifetime 7 and the quantum yield 4Fof fluorescence are determined by the rate constants kF, kp, and kl according to
-
Trans cis photoisomerization is assigned to the formation of the P state with subsequent forward isomerization. Its quantum yield 4Tcis the product of the yield of horizontal transition to the P state and of the yield 1 - P of decay to the cis state from the P state according to
-
(15) Gruen, H.; GBrner, H. Z. Naturforsch., A: Phys., Phys. Chem., Kosmophys. 1983, 38A. 928. (1 6) Gilabert, E.; Lapouyade, R.; Rulliere, C. Chem. Phys. Lett. 1988, 245, 262. (17) Pisanias, M. N.; Schulte-Frohlinde, D. Ber. Bunsen-Ges.Phys. Chem. 1975, 79, 662. (18) Gorner, H.; Schulte-Frohlinde, D. J. Photochem. 1978.8, 91. (19) Steiner, U.; Abdel-Kader, M. H.; Fischer, P.; Kramer, H. E. A. J . Am. Chem. SOC.1978, 100, 3 190. (20) Corner, H. J . Photochem. 1980, 23,269. (21) Corner, H.; Schulte-Frohlinde, D. J. Phys. Chem. 1985, 89, 4105. (22) Gorner, H.; Fojtik, A.; Wroblewski, J.; Currell, L. J. Z. Naturforsch., A: Phys., Phys. Chem., Kosmophys. 1985,40A, 525. (23) Saltiel, J.; Charlton, J. L. In Rearrangements in Ground and Excited States; deMayo, P., Ed.; Academic Press: New York, 1980; Vol. 3, p 25. (24) Hochstrasser, R. M. Pure Appl. Chem. 1980.52, 2683. (25) Troe, J.; Weitzel, K. M. J. Chem. Phys. 1988, 88, 7030. (26) Birks, J. B. Chem. Phys. Lett. 1978, 54,430.
Consider that the ratio kp/kFof the rate constants of formation of the P state and of fluorescence is identical with the ratio of the corresponding quantum yields. The quantum yield for the transition to the P state is defined by the yields ~ $ and ~ c 4cTof photoisomerization according to eq 4 with eq 3. We derive thus eq 6, which we use to evaluate the ratio k p / k Ffrom the experimental yields. To evaluate the rate constant kI of internal conversion from 7 , 4F,$TC, and 4CT,we use eq 7-which is eq 2 inverted-substituting kF and kp/kF according to eq 5 and 6. (7)
Materials and Methods Dye. We have synthesized (dibuty1amino)stilbazoliumbutylsulfonate by aldol condensation of (dibuty1amino)benzaldehyde and sulfonatobutylpicolinium betaine.27 The melting point of the product after recrystallization from ethanol (twice) was 293-294 "C. The purity was checked by thin-layer chromatography (silica gel Merck F254, ethanol/water (7:3)). The product was identified by mass spectrometry. With NMR spectroscopy we have measured the coupling constant of the olefinic protons as 16.0 Hz. This value is characteristic for derivatives of trans-~tilbene.'~ We have synthesized the cis isomer by photochemical trans cis isomerization of the protonated trans isomer and subsequent deprotonation.28 An 80-mg portion of the trans isomer was dissolved in 40 mL of 0.25 N aqueous HCl. The solution was irradiated with a xenon lamp (150 W, 6 h, undetermined quantum flux). The reaction mixture was handled in dim red light. It was neutralized by 1 N NaOH. Purification by chromatography or crystallization was not possible due to thermal back reaction. Cis and trans isomers were separated by filtration as the cis isomer is considerably more soluble in water than the trans isomer. The quality of the separation was checked by absorption spectrometry. The coupling constant of the olefinic protons was 12.5 Hz, which is typical for derivatives of cis-~ti1bene.I~In addition we have observed a characteristic shift of the olefinic protons to higher fields in the NMR spectrum by about 1 ppm as compared to the trans isomer.I9 A contribution of the trans isomer was not visible in the NMR spectrum. All experiments with the cis isomer were performed immediately after synthesis. Soluents. Acetonitrile, butanol, chloroform, dichloromethane, dimethylformamide,ethanol, ethylene glycol, methanol, propanol, and pentanol were obtained from Merck (Darmstadt, FRG). Benzyl alcohol, cyclohexanol, cyclopentanol, decanol, hexanol, and
-
(27) Hassner, A.; Birnbaum, D.; Loew, L. M. J. Org. Chem. 1984, 49, 2546. (28) Williams, J. L. R.; Carlson, J. M.; Adel, R. E.; Reynolds, G. A. Can. J . Chem. 1965,43, 1345.
Fluorescence of Aminostilbazolium Dye octanol were obtained from Fluka (Neu-Ulm, FRG). In all cases the best available quality was used. Water was purified by Milli-Q columns (Millipore, Bedford, MA). To investigate spectra, lifetime, and quantum yields of fluorescence and of photoisomerization, an aliquot of a stock solution of the dye in ethanol was added to the various solvents. Thus all solutions contain 2% ethanol. As the cis isomer was prepared as an aqueous solution, the samples used for cis trans isomerization contain 2% water except for the solutions in chloroform and dichloromethane. Oxygen was not removed from the solutions. In test experiments an effect of oxygen on the quantum yields of fluorescence and photoisomerization was not observed. Vesicles/ Micelles. Sodium dodecyl sulfate (SDS) from Fluka was recrystallized from ethanol/chloroform (1:l). A micellar solution of 35 mM was used. Dodecyltrimethylammonium bromide (DTAB) from Sigma, Heidelberg, FRG, was recrystallized from toluol/ethanol. A 60 mM solution was used. Small lipid vesicles were prepared from a dispersion of egg lecithin (type 111-E,Sigma) by sonication in a titanium vessel with a Branson sonifier (1.25 mM, 100 mM NaCI, 20 mM Tris buffer, pH 8.1). Titanium and large fragments of lipid were removed by centrifugation at 10000 rpm. The diameter of the vesicles (around 30 nm) was checked by quasielastic light scattering, Absorption Spectra. The extinction coefficients of the trans isomer (eT) were obtained from absorption spectra of solutions of known concentration (usually 20 pM) as recorded in a Cary 219 spectrophotometer a t 25 OC using Beer's law. The wavelengths of the absorption maxima are 518 and 480 nm in chloroform and water, respectively. The extinction coefficients are 6.27 X lo4 and 3.68 X IO4 (M cm)-l. (The spectrum is broader in water. The integrated oscillator strengths are similar.) The absorption spectra of solutions of freshly prepared cis isomer were measured similarly. To obtain the extinction coefficient of the cis isomer ( e c ) , the concentration of a solution of cis isomer was determined as follows: The solution was irradiated up to the photostationary state. Its absorbance was measured. A solution of trans isomer of known concentration was irradiated also up to the same photostationary state. From this solution an effective extinction coefficient was determined of a dye mixture as given by the composition of the photostationary state. By use of this effective extinction coefficient, the concentration of the photostationary solution was evaluated as obtained from the cis isomer. The absorption maxima of the cis isomer are 526 and 480 nm in chloroform and water, respectively. The extinction coefficients are 2.4 X IO4 and 1.18 X lo4 ( M cm)-' (10% error). Fluorescence Spectra. The fluorescence spectra were measured in a Schoeffel RRS-1000spectrofluorophotometer in 90' geometry. The fluorescence spectra were found to be independent of the excitation wavelength. Usually an excitation wavelength of 497 nm was used. The spectra were calibrated by a MgO scattererz9 using a quantum counter (basic blue 3 in ethylene glycolM)to normalize the Xe lamp. Photostationary solutions were used to avoid light-induced drift. The fluorescence maxima in chloroform and water are 599 and 626 nm, respectively. Lifetime. The fluorescence lifetimes T were obtained by excitation at 425 nm by a double-cavity dye laser with two stages of amplification which was pumped by a terphenyl laser as driven by an excimer laser.31 The fluorescence was detected above 450 nm with a photodiode, using a sampling technique. After deconvolution the decay curves could be fitted by single exponentials with slight deviations in micelles and vesicles. The estimated error is 10% for lifetimes above 100 ps and 20 ps for shorter lifetimes. Quantum Yield of Fluorescence. The quantum yields of fluorescence of the trans isomer $F were determined with respect to the quantum yield of a reference (4FREF) according to eq 8.
-
(29) Siadards in fluorescence speciromeity; Miller, J. N., Ed.;Chapman and Hall: London, 1981; p 56. (30) Kopf, U.; Heinze, J. Anal. Chem. 1984, 56, 1931. (31) Bor, Z.; Racz, 9. Appl. Opt. 1985, 24, 1910.
The Journal of Physical Chemistry, Vol. 93, No. 22, 1989 7719 Photostationary samples were used. S and SREF are the integrated calibrated fluorescence spectra of the sample and of the reference. ET, Es, and EREF are the extinctions of the sample in the trans state and in the photostationary state and of the reference. n and nREF are the refractive indices of the solutions.32 xTis the molar fraction of trans isomer in the photostationary state. As standards we have used quinine sulfate (Fluka) with $FREF = 0.546 in 1 N HzSO4 excited at 350 nm33*34 and Rhodamin-101 (Lambda Physik, Gottingen, FRG) with $FREF = 1.0 in ethanol excited at 540 nm1.35 The deviations with respect to the two standards were found to be negligible. The quantum yield of fluorescence was measured as a function of temperature from 5 to 85 OC if possible. The absorbance of the solutions was measured for each temperature to correct for a temperature-dependent extinction coefficient. Ratio of Quantum Yields of Photoisomerization. At a given wavelength of illumination the concentrations of trans and cis isomers in the photostationary state (CTand Cc) are defined by ~ C C T C= T $at&. With the total concentration CToT = CT + CC and the extinction Es = ccCc)d (where d is path length) we obtain eq 9 and 10 for the ratio of the quantum yields and the fraction of the trans-isomer in the photostationary state. The photostationary state was investigated coming from both the trans- and from the cis-isomer.
+
(9)
XT
=
E s / d - TOT CTOT(~T - CC)
(10)
Quantum Yields of Photoisomerization. The dynamics of trans cis photoisomerization at a local quantum intensity (ZQ) is governed by the kinetic equation dCT/dt = [-$TCCTCT + $,--ec X (CTOT - cT)]IQif negligible stationary concentrations of excited states are taken into account. The change of the quantum intensity in time across a cuvette has to be taken into account to evaluate the quantum yields (pa and $a.The complete dynamics has been considered in ref 36 and 37 for a homogeneous solution-as attained by stirring-in a cuvette with parallel windows, at constant, homogeneous, monochromatic illumination without scattering and reflection at the exit window. We use eq 11 to evaluate the quantum yield 4Tcfrom the absorbances El and Ez at times tl and t2, from the absorbance Es of the photostationary state, from the extinction coefficients 9and ec, from the ratio $TC/$CT, from the quantum flux FQ (quanta/time), from volume V, and from path length d. An analogous formula is used for the cis trans isomerization. We have measured the quantum flux by
-*
-
El
- Es
(El*-E2*)
I/
(E2- E , )
A (11)
A = F Q ( ~W/ t z - t i ) ( e ~ + ~C$CT/$TC)
El* = E1/(1 - 10-El) E2* = E2/(1 - 10-E2) potassium ferrioxalate a c t i n ~ m e t r y . ~Monochromatic ~-~~ light (32) Parker, C. A. Photoluminescence of Solutions; Elsevier: Amsterdam, 1968; p 261. (33) Melhuish, W. H. J . Phys. Chem. 1961, 65, 229. (34) Demas, J. N.; Crosby, G. A. J . Phys. Chem. 1971, 75,991. (35) Karstens, T.; Kobs, K. J . Phys. Chem. 1980,84, 1871. (36) Mauser, H. Formale Kinetik; Bertelsmann Verlag: Diisseldorf, 1974. (37) Gauglitz, G. J . Photochem. 1976, 5 , 41. (38) Murov, S.L. Handbook ofPhoiochemisiry; Marcel Dekker: New York. 1973; p 119.
7720 The Journal of Physical Chemistry, Vol. 93, No. 22, 1989
Ephardt and Fromherz
TABLE I: Fluorescence Lifetime ( 7 ) and Quantum Yield (&) of Fluorescence of (DibutvlPmino)sHlbPzoliumButylsulfonate at 25 O c a solvent r/ns 6F water 0.04 0.0053 ethylene glycol 0.49 0.10 dimethylformamide 0.13 0.028 acetonitrile 0.03 0.009 methanol 0.06 0.013 0.19 0.044 ethanol dichloromethane 1.09 0.31 chloroform 1.56 0.56 0.45 0.1 1 SDS 0.45 0.1 I DTAB 0.82 0.19 lecithin "The relative error of the lifetimes is about 10% for times above 100 ps. For shorter times the error is around 20 ps. The relative error of the quantum yields is about 10%.
was obtained by interference filters (bandwidth 15 nm, Schott, Mainz, FRG). We have used 430 nm for water, ethanol, and lecithin and 460 nm otherwise considering the quantum yields of the actinometer of 4(430 nm) = 1.0439and 4(460 nm) = 0.98.39 Quenching. Ferrocene (Merck) was recrystallized from ethanol. Azulene (Aldrich, Steinheim, FRG) was used without further purification. To a 20 pM solution of the dye in chloroform we have added small volumes of a 100 mM solution of ferrocene in chloroform. The quantum yield of trans-cis photoisomerization was measured as described above. The fluorescence intensity was measured at an excitation wavelength of 540 nm and an emission wavelength of 600 nm. To correct for dilution and for light absorption by ferrocene at 540 nm, we have multiplied the measured intensities by the factor K according to eq 12, where V and Voand E and Eo are volume and extinction of the solution with and without ferrocene. Fluorescence quenching by azulene was not studied due to the spectral overlap of azulene and the stilbazolium dye. E V(1 - 1O-EO) K = EOVO(1 - 10-E)
Results Fluorescence. We have measured the fluorescence lifetime T and the quantum yield $F of fluorescence in eight solvents and three amphiphilic assemblies. The results are shown in Table I. A change by almost 2 orders of magnitude is observed for both quantities. Lifetime T and quantum yield +F correlate as illustrated in Figure 2. The data are compatible with a linear relation according to eq 5 with an invariant rate constant of radiative decay kF = 0.24 ns-I. Photoisomerization. We have measured the ratio & c / $ a of the quantum yield of photoisomerization in six solvents and three amphiphilic assemblies. Identical values were obtained from the trans or cis isomer. The results are shown in Table 11. We have measured the quantum yield 4a of cis trans photoisomerization in six solvents and three amphiphilic assemblies and the quantum yield C$TC of trans cis photoisomerization in two solvents and three amphiphilic assemblies. The results are also shown in Table 11. The cis trans photoisomerization is almost invariant. The yield of the trans cis photoisomerization changes by almost 2 orders of magnitude. For the yield of cis trans photoisomerization we calculate an average = 0.57 excluding the value for water. According to eq 3 the yield of formation of the trans state from the perpendicular state is fl = 0.57. Considering the invariant yield of formation of the cis state from the P state as (1 - p) = 0.43, we see that the sensitivity of the quantum yield C$TC of trans-cis
- -
-
-
(39) Hatchard, C. G.;Parker, C. A. Proc. R. Soc. London, A 1956,235, 518. (40) Demas, J. N.; Bowman, W. D.; Zalewski, E. F.; Velapoldi, R. A. J . Phys. Chem. 1981,85, 2766.
Figure 2. Quantum yield $ J ~of fluorescence versus fluorescence lifetime T of (dibuty1amino)stilbazolium butylsulfonate in 11 solvents and amphiphilic assemblies at 25 OC (data from Table I). The line $JF = kFT is drawn with a radiative decay constant kF = 0.24 ns-I.
0,l
1 0.01
-
-
I
0.1 @F
Figure 3. Ratio bTC/(1 - $m) of the quantum yields hcand q5m of trans cis and cis trans photoisomerization versus the quantum yield @F of fluorescence for nine solvents and amphiphilic assemblies (data 1 - $m) = ( k p / k F ) 6 is ~ drawn for a ratio from Table 11). The line he/( of rate constants k p / k F= 0.5. TABLE 11: Molar Fraction xT of the Trans Isomer of (Dibuty1amino)stilbazoliumButylsulfonate in the Photostationary State at 25 OC, Ratio &/$= of the Quantum Yields of Trans Cis Trans Photoisomerization from Measurements in the and Cis Photostationary State, and Individual Quantum Yields 6cr and & of Cis Trans and Trans Cis Photoisomerization from Kinetic Data" solvent 6CT 6TC XT ~TC/~CT 0.30 0.002. water 0.99 0.006 0.60 0.01* 0.016 ethylene glycol 0.96 0.003; 0.54 0.006 methanol 0.98 0.013 0.007; 0.50 ethanol 0.98 0.61 0.033 0.078 dichloromethane 0.84 0.58 0.13 0.26 chloroform 0.59 0.029 0.58 SDS 0.87 0.054 0.030 DTAB 0.87 0.056 0.58 0.040 0.61 0.8 1 0.087 lecithin
-
-
-
-
"The relative errors are around 20%. The values of hc with aster. error isks are calculated from the ratio &c/$m and from $ J ~ Their is around 50%.
photoisomerization is not caused by a modulation of the process of internal conversion in the P state. The modulation of the yield 4Tcof trans cis photoisomerization correlates with the yield 4Fof fluorescence. The yield hc--divided by (1 - @)= (1 - q5cr)-is plotted in Figure 3 versus the yield & of fluorescence. The data are compatible with a linear relation according to eq 6 with an invariant ratio kp/kF of the rate constants of P formation and fluorescence as kp/kF = 0.5. cis It appears that the sensitivity of the yield $TC of trans
-
-
The Journal of Physical Chemistry, Vol. 93, No. 22, 1989 7721
Fluorescence of Aminostilbazolium Dye
TABLE 111: Quantum Yield 4Fof Fluorescence of (Dibuty1amino)stilbazolium Butylsulfonate at 25 OC'
no.
1
0,OO
0.01
Co/M
0,02
0,OJ
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
I
-
0.04
Figure 4. (Upper) Relative reciprocal yield of trans cis isomerization of the (dibuty1amino)stilbazolium butylsulfonate versus the
&c0/&c
concentration CQ of ferrocene (circles) and of azulene (triangles). The = 23 M-'for ferrocene lines are drawn for Stern-Volmer constants eTC and 110 M-'for azulene. (Lower) Relative reciprocal fluorescence intensity lo/lversus concentration cQ of ferrocene. The line is drawn for = 46 M-I. a Stern-Volmer constant eF
solvent
dE
water
0.0053 0.1 dimethylformamide 0.028 acetonitrile 0.009 methanol 0.013 ethanol 0.044 propanol 0.091 cyclopentanol 0.25 butanol 0.13 cyclohexanol 0.34 pentanol 0.2 hexanol 0.22 benzyl alcohol 0.26 octanol 0.35 dichloromethane 0.31 decanol 0.41 chloroform 0.56 SDS 0.1 1 DTAB 0.1 1 lecithin 0.19
ethylene glycol
EA/ k,/ns-l (kJ/mol) 45.0 2.04 8.2 26.3 18.1 5.1 2.27 0.61 1.43 0.36 0.84 0.73 0.56 0.32 0.41 0.23 0.07 1.82 1.82 0.9
15.6 23.4 12.7 7.6 11.3 12.0 12.3
An/103 119 114 6.1 2.1 7.3 2.7 2.0
12.3
1.2
7.1 18.7
0.035 1.5
'The error is around 10%. The rate constants kI of radiationless deactivation are obtained as k , = k~(&-' - 1 - kp/kF) with kF = 0.24 ns-' and k p = 0.12 ns-'. Apparent Arrhenius energy EAand preexponential factor A . describe the temperature-dependentquantum yield of fluorescence as $F-' - 1 = A , exp(-EA/RT). TABLE I V Parameters of the Solvents at 25 "C: Dielectric Constant e, Logarithmic Increment d log r/dT of Dielectric Constant with Respect to Temperature T, Viscosity q, and Activation Energy of Viscosity E,"
no. 1
solvent water
2 ethylene glycol 3 dimethylformamide 4 acetonitrile 5 methanol 6 ethanol 7 propanol 8 cyclopentanol 9 butanol Figure 5. Logarithm of the rate constant kI of radiationless deactivation at 25 OC versus the reciprocal of the dielectric constant c for 17 solvents. The number code of the solvents is given in Table IV.
IO cyclohexanol 1 I pentanol I2 hexanol 13 benzyl alcohol 14 octanol 15 dichloromethane 16 decanol 17 chloroform
e
78.4 37.7 37.0 36.7 32.7 24.55 20.33 18.0 17.51 15.0 13.9 13.3 13.1 9.85 8.93 8.1 4.72
1 0 3x ~
E,/
d log c/dT q/cP (kJ/mol) -2.0 0.89 17.87 -2.24 16.79 28.54 0.796 -1.9 0.345 7.74 -2.64 0.547 10.38 -2.7 1.678 13.98 -2.93 1.947 17.37 9.6 -3.35 2.607 19.75 56.5 3.19 4.592 5.48 7.215 -1.8 0.422 6.61 11.8 -1.6 0.542 7.37
isomerization is not caused by a sensitivity of the horizontal transition to the P state. With kF = 0.24 ns-' we obtain as an invariant rate constant of the transition kp = 0.12 ns-'. "The values of the dielectric constant are from ref 41-43. (The data Intersystem Crossing. The quantum yield 4Tcof trans cis of cyclopentanol, benzyl alcohol, and decanol are for 20 "C.) The photoisomerization was not affected by deoxygenation. We have values of d log e/dT are taken from ref 41-43 or evaluated from temmeasured hcas a function of the concentration (cQ) of ferrocene perature functions c ( T ) given there. The data for the viscosity are and azulene, respectively, in chloroform. The data-normalized from ref 42 and 44. The activation energies E, are evaluated from by the quantum yield 4OTC without quencher-are shown in Figure tabulated values q(7'). 4. They are described by Stern-Volmer relations 4"Tc/$Tc = 1 + KQTccQ with "quenching" constants of KQTC= 23 M-' for conclude that intersystem crossing may be ruled out as an imferrocene and KQTc = 110 M-' for azulene. portant process of deactivation of the fluorescent state. For comparison we have measured the quenching of relative Internal Conversion. As the rate constants kF and kp for fluorescence intensity ( I / I o )by ferrocene. The data are shown deactivation of the singlet state by fluorescence and by horizontal in Figure 4. They are described by a Stern-Volmer relation Io/[ transition to the P state are invariant, we assign the solvent effects = 1+ The quenching constant KQF = 46 M-' is very close to a modulation of a third path of deactivation which competes to the quenching constant with respect to isomerization. Sternwith both processes. Volmer constants on the order 10 M-' are usual for quenching We evaluate the rate constant kI of this process from the of singlet states by ferrocene in stilbene derivatives,21 whereas quantum yield +F of fluorescence according to eq 7 by using the Stern-Volmer constants in the order of lo3 M-' are expected for average rate parameters kF = 0.24 ns-' and kp = 0.12 ns-l. quenching of a triplet state by ferrocene and azulene.21 We have measured the quantum yield 4Fof fluorescence in 17 The values of the Stern-Volmer constants indicate that a triplet solvents and 3 amphiphilic assemblies. The results are shown in state is not involved in the process of photoisomerization. From Table 111. The rate constant kI changes by almost 3 orders of the fact that the triplet mechanism of photoisomerization does magnitude. For illustration k , is plotted in Figure 5 versus the not occur-in contrast to numerous stilbene d e r i v a t i v e ~ ~ ~ . ~ ~ * ~ ~ -reciprocal we of the dielectric constant t (Table IV4Id3) and in Figure
-
7722 The Journal of Physical Chemistry, Vol. 93, No. 22, 1989
Ephardt and Fromherz entiation of eq 13 and 14 with respect to (RT)-' at invariant kF leads to a relation between the apparent Arrhenius energy EAand the Arrhenius energies EAiand EApaccording to eq 15. The activation energies and the rate constants are to be taken at a given temperature.
If the transition to the P state is slow as compared to the transition to the I state with kp
