J . Phys. Chem. 1987, 91, 3524-3530
3524
observed in solution and in the solid state are very similar, we also assign the emission observed in solution as originating from an excited state of the zwitterionic or keto form. However, it is evident that, at in least M e 2 S 0 and ethanol, the major photoproduct is the anion and not the zwitterion. We believe that the anion, as the zwitterion, is produced adiabatically from the enol form. However, since all fluorescence traces were fitted by a single exponential, we believe that the anion is weakly fluorescent compared to the zwitterion. Note that in dichloromethane, where no photochemistry is observed by transient absorption spectroscopy for the 3’-nitro derivative and only very little is observed for the 5’-nitro derivative, we still observed emission from the zwitterionic form. Given these results, it would appear that the quantum yield of zwitterion production is 60.01. The absence of emission of the enol form makes us believe that the excited-state proton transfer is very fast with a rate constant greater than 10” s-’ as found for salicylideneaniline or 2-(2’-
hydroxyphenyl)benzothiazole.7-9~‘ Conclusion For all the studied compounds, there are at least three species which can be present in solution. In a nonpolar non-hydrogenbonding solvent such as methylcyclohexane, only one species is clearly observed which is the intramolecular hydrogen-bonded enol. In solvents capable of hydrogen bonding as ethanol or dimethyl sulfoxide, two additional species exist. At high concentration (M), an aggregate is observed with a maximum of absorption near 430 nm. At low concentration (-5 X 10” M), the aggregate dissociates and we observe the anion of the nitro derivatives. The aggregate is proposed to be an enol-zwitterion mixed dimer. In polar but non-hydrogen-bonding solvents, the mixed dimer does not dissociate since the enol cannot transfer its proton to the solvent. PPP and INDO/S semiempirical calculations help confirm the assignment for the enol and anionic forms for the nitro derivatives in solution. After laser excitation at 355 nm, 3’4tro- and 5’-nitro derivatives of salicylideneaniline undergo excited-state intramolecular proton transfer on the picosecond time scale ( k m > 10” s-I) to form adiabaticaly a twisted zwitterion. The twisted zwitterion subsequently isomerizes in the excited state on the picosecond time scale to give one (in alcohol) or two (in all the other sdvents) conformers. Only the emission of the twisted zwitterion is observed. The thermal reverse back proton transfer is observed to be
slower than the ESIPT by several orders of magnitude as observed for other classes of molecules showing ESIPT. The activation energy for the reverse proton transfer is found to be dependent upon the solvent. We believe that the principal component of the overall activation energy requirement probably is that required for conformational changes. Competitive photochemistry is observed in M e 2 S 0 for (5’-nitrosalicy1idene)aniline to form the anion and in dichloromethane for (3’-nitro salicy1idene)aniline to produce a presently unidentified photoproduct. In ethanol or Me,SO where aggregation is present, it is proposed that within the mixed dimer the enol molecule undergoes anti-syn isomerization leading to the dissociation of the mixed dimer. After laser excitation of 2-(2’-hydroxy-.5’-nitrophenyl)benzothiazole in ethanol or Me#O, the anion is the major photoproduct and not the zwitterion. In dichloromethane, any proton transfer is not the dominant deactivation path of the excited state since we observed the triplet-triplet transient of the enol. No such transient is observed in ethanol or Me2S0. The absence of the zwitterion can be explained by the lack of change in basicity of the nitrogen atom in the excited state compared to the ground state; thus, intramolecular proton transfer is not favored (as found by PPP calculations). 2-(2’-Hydroxy-3’-nitrophenyl)benzothiazole is found to be photochromic only in M e 2 S 0 , forming the anion after laser excitation, but not in dichloromethane and ethanol. It is obvious from this work that the photochromism of the nitro derivatives of salicylideneaniline or 2-(2’-hydroxypheny1)benzothiazole is not a topochemically determined phenomenon but that the electronic structure of the singlet excited state (charge densities) compared to the ground state and the presence or absence of intra- and intermolecular H bonding are of importance in the photochromism of such compounds. We believe that it is true for any derivatives of salicylideneaniline or 2-(2’-hydroxypheny1)benzothiazole. Competitive photophysical and photochemical processes (to ESIPT) occur depending upon the nature of the solvent.
Acknowledgment. The laser flash experiments were performed at the Center for Fast Kinetic Research at the University of Texas at Austin, which is supported by N I H Grant RR-00886, the Biotechnology Branch of the Division of Research Resources, and the University of Texas. We also thank Dr. Michael Zerner at the University of Florida- at Gainesville for kindly supplying us with one theoretical program.
Photosensitized Electron-Transfer Reaction in the First Excited Singlet State of a Polymethine-Cyanine Dye D. Doizi and J. C. Mialocq* CEA-CENISACLAY IRDIIDESICPIDPCISCM U A 331 CNRS, 91 191 GiflslYvette Cedex, France (Received: November 11, 1986) The electron transfer from the first excited singlet state of a polymethine-cyanine dye, 3,3’-diethyloxadicarbocyanine iodide (DODCI), to methylviologen leads to redox products, which have been fully characterized by using an electron acceptor, p-benzoquinone @-BQ), and an electron donor, ascorbic acid. The quantum yields of the redox products have been measured by using the DODCI photoisomerization process as an internal actinometer. A complete reaction scheme is proposed and the various reaction rate constants have been determined. The photosystem DODCI-MVC1,-p-BQ-ascorbicacid is presented as a new system for the modelling of solar energy conversion. Introduction Photoinduced electron-transfer reactions have been the subject of many theoretical and experimental studies related to their possible use in the photochemical conversion of solar energy.’-4 (1) Photochemical Conversion and Storage of Solar Energy: Connolly, John, S.,Ed.; Academic: New York, 1981.
0022-3654/87/2091-3524$01.50/0
In the case of organic sensitizers, most of the studies have been devoted to electron-transfer reactions in the triplet excited state. ( 2 ) Photogeneration of Hydrogen: Harriman, A,, West, M. A., Eds.; Academic: New York, 1982. ( 3 ) Structure and Bonding 49, Solar Energy Materials: Springer-Verlag: New York, 1982.
0 1987 American Chemical Society
Electron-Transfer Reaction of DODCI Intersystem crossing (isc) is indeed very efficient in porphyrin, phthalocyanine, flavine, and thionine molecules. The participation of the triplet state of polymethine-cyanines or carbocyanines in photochemical electron-transfer reactions has also been considThe interest in these dyes is largely due to their current use as optical sensitizers in photography8 and as saturable absorbers for the Q-switching and mode-locking of lasers. Consideration of the molecular structure and of the nature of the heterocyclic moieties is of a particular importance since they greatly determine the efficiency of the isc process7 In thia- and selenacarbocyanines, isc is very efficient and Chibisov has already studied electron-transfer reactions involving the triplet state of thiacarbocyanines and various electron acceptors and donor^.^,^ Fluorescence quenching due to electron transfer (ET) from the excited electron donor (N,N’-dioctadecyloxacyanine) to an electron acceptor (N,N’-dioctadecyl-4,4’-bipyridinium)has been observed in monolayer assemblies9and evidence for a tunneling mechanism of electron transfer has been presented by Mobius.Io Charge transfer in the system oxacarbocyanine dyelp-chloranil crystal has also been observed.” The influence of the multiplicity of the sensitizer excited state on the efficiency of charge separation is well established. Although it is well-known that the primary electron transfer in bacterial photosynthesis takes place from the excited singlet state of the primary donor in the picosecond time range, charge separation following E T in the first singlet excited state appears to be inefficient in solution. For example, Seely12 has shown that the efficiency of photoreduction of nitro compounds sensitized by the singlet excited state of pyrochlorophyll is less than that sensitized by the triplet excited state as a consequence of the spin multiplicity of the radical-ion pair created by the electron transfer. Similarly the quenching of the first excited singlet state of bacteriopheophytin and chlorophyll a by quinones in solution does not lead to the formation of the oxidized species but rather to the ground state before separation of the ion pair can However, significant yields of the separated ions are formed from the triplet state of the sensitizer in the case of bacteriopheophytin and p ben~oquinone.’~ The same behavior was observed in the acridine orange-methylviologen system.I7 Therefore it appeared to us of interest to investigate electron-transfer reactions in the first excited singlet state of cyanine dyes, particularly oxa- or quinocarbocyanines which are characterized by a very low quantum yield of isc.18-21 However, dyes possessing too short-lived an excited singlet state in a low-viscosity solvent should be avoided because of the high reactant concentration needed for the quenching: p i n a c y a n 0 1 , ~1,l ~ ~’-diethyl-2,2’-dicarbocyanine ~~~~~ iodide (DDI), and crypt~cyanine.’~ Therefore DODCI or 3,3’diethyloxadicarbocyanine iodide possessing well-established photo physic^^^,^'-^^ appeared to us a good candidate for a pre(4) Chibisov, A. K. High Energy Chem. 1976, 10, I . (5) Lifanov, Yu. I.; Kuz’min, V. A.; Chibisov, A. K.; Levkoev, I. I. Khim. Vys. Energ. 1973, 7, 321. (6) Lifanov, Yu. I.; Korobov, V. E.; Karyakin, A. V.; Chibisov, A. K. Khim. Vys. Energ. 1975, 9, 265. (7) Chibisov, A. K. J . Photochem. 1976177, 6, 199. (8) Chimie et Physique photographiques; GlafkidBs, P., Ed.; Paul Montel: Paris, 1976. (9) Mobius, D. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 848. (IO) Mobius, D. Acc. Chem. Res. 1981, 14, 63. ( 1 1 ) Killesreiter, H. 2.Naturforsch. 1979, 340, 737. (12) Seely, G. R. J . Phys. Chem. 1969, 73, 125. (13) Holten, D.; Gouterman, M.; Parson, W. W.; Windsor, M. W.; Rockley, M. G. Photochem. Photobiol. 1976, 23, 415. (14) Gouterman, M.; Holten, D. Photochem. Photobiol. 1977, 25, 85. ( 1 5 ) Huppert, D.; Rentzepis, P. M.; Tollin, G. Biochim. Biophys. Acta 1976, 440, 356. (16) Andreyeva, N. Ye.; Barashkov, B. I.; Zakharova, G. V.; Shubin, V. V.; Chibisov, A. K. Biophysics 1977, 22, 789. (17) Chan, Man Sze; Bolton, J. R. Photochem. Photobiol. 1981,34, 537. (18) Dempster, D. N.; Morrow, T.; Rankin, R.; Thompson, G. F. J . Chem. Soc., Faraday Trans. 2 1972, 6 8 , 1479. (19) Dempster, D. N.; Morrow, T.; Rankin, R.; Thompson, G. F., Chem. Phys. Lett. 1973, 18, 488. (20) Arvis, M.; Mialocq, J. C. J . Chem. SOC.,Faraday Trans. 2 1979, 75, 415. (21) Mialocq, J. C.; Goujon, P.; Arvis, M. J . Chim. Phys. 1979, 76, 1067.
The Journal of Physical Chemistry, Vol. 91, No. 13, 1987 3525
TABLE I Emid
dye quinocarbocyanine n = 0 (pseudoisocyanine, C1-) n = 1 (pinacyanol, C1-) n = 2 (PTS-) oxacarbocyanine n=O n = 1 (DOCI) n = 2 (DODCI) thiacarbocyanine n=O n = 1 (DTCI) n = 2 (DTDC)
(vs. Ag/AgCI), V 0.99 0.58 0.28
ref 41 41 42
>1.0 0.94 0.67
38 38, 41 calcd. this work
>1.0 0.78 0.48
38 38, 41 38
liminary study of the electron transfer in the first singlet excited state of a polymethine-cyanine dye.34,35 The present report analyzes the complete reaction scheme of the photoactivated electron transfer from DODCI to MVC12.
Experimental Section DODCI (Eastman Kodak), p-BQ (Merck), p-dinitrobenzene @-DNB) (Merck), MVClz (Sigma), L(+)-ascorbic acid (Merck), and methanol (Merck UVASOL for spectroscopy) were of reagent grade and were used without further purification. All absorption spectra were run on a Beckman UV 5240 UVvisible IR spectrophotometer. The fluorescence intensities of dilute solutions of DODCI were measured under continuous excitation at 514.5 nm with a Raman Coderg T800 spectrometer equipped with an argon ion lasers3’ The DODCI fluorescence lifetimes were measured under picosecond single-pulse excitation at 600 nm delivered by a flashlamp pumped rhodamine 6G dye laser mode-locked with DODCI.27 The emitted fluorescence was analyzed with a streak camera device optically coupled to an optical multichannel analyzer (OMA, Princeton Applied Research). The data were transferred to a Hewlett-Packard 9845 T computer which enabled the analysis of fluorescence decay^.^^,^^ The time-resolved picosecond absorption spectra were obtained by using the experimental arrangement already d e ~ c r i b e and d~~~~~ similar to that described by Magde and W i n d ~ o r .Briefly, ~~ the sample was excited at 532 nm in a 1-mm-thick cell by using a single picosecond pulse (27 ps fwhm) delivered by a frequencydoubled Nd:YAG laser. The unconverted infrared light was retarded by an optical delay line and focused in the middle of a 4-cm-thick cell containing an aqueous solution of orthophosphoric acid (30 vol 9%). This enabled the less structured and the most intense continuum of white light to be generated. After the 1064-nm light was filtered out, the continuum was focused onto a diffuser. The unpolarized light so obtained was independent of the small geometrical variations introduced by the delay line. The analyzing light was focused by a cylindrical lens to probe simultaneously the two cylindrical volumes of 1 mm diameter, (22) Arthurs, E. G.; Bradley, D. J.; Roddie, A. G. Opt. Commun. 1973, 8 , 118. (23) Arthurs, E. G.; Bradley, D. J.; Roddie, A. G. Chem. Phys. Lett. 1973, 22, 230. (24) Magde, D.; Windsor, M. W. Chem. Phys. Lett. 1974, 27, 31. (25) Shank, C. V.; Ippen, E. P. Appl. Phys. Lett. 1975, 26, 62. (26) Mialocq, J. C.; Jaraudias, J.; Boyd, A. W.; Sutton, J. Lasers in Physical Chemistry and Biophysics; Elsevier: Amsterdam, 1975; p 345. (27) Mialocq, J. C.; Boyd, A. W.; Jaraudias, J.; Sutton, J. Chem. Phys. Lett. 1976, 37, 236. (28) RulliBre, C. Chem. Phys. Lett. 1976, 43, 303. (29) Jaraudias, J.; Goujon, P.; Mialocq, J. C. Chem. Phys. Lett. 1977, 45, 107.
(30) Adams, M. C.; Bradley, D. J.; Sibbett, W.; Taylor, J. R. Chem. Phys. Lett. 1979, 66, 428. (31) Jaraudias, J. J . Photochem. 1980, 13, 35. (32) Velsko, S. P.; Fleming, G. R. Chem. Phys. 1982, 65, 59. (33) Doizi, D.; Mialocq, J. C. C.R. Acad. Sci. Paris 1983, 297, 109. (34) Mialocq, J. C.; Doizi, D.; Gingold, M. P. Chem. Phys. Let?. 1983, 103, 225. (35) Doizi, D.; Mialocq, J. C. Ultrafast Phenomena IV; Auston, D. H., Eisenthal, K. B., Eds.; 1984; p 377. (36) Doizi, D. These de Docteur-Ingtnieur, UniversitE Paris-Sud, Dec 22, 1983.
3526 The Journal of Physical Chemistry, Vol. 91, No. 13, 1987 D.0
Doizi and Mialocq
1
+
*+
+
1
+
t'
+ +
+
+
+ * + +
-0,2
2 3 -+
0,Ol
I
1
I
1
8
'
'
300
I
1
I
'
400
I
'
1
'
420
' * 440' ' ' 480' ' ' nm1
Figure 1. Differential absorption spectrum at methanolic solution.
1
= 6 ns in a DODCI
one being excited by the 532-nm actinic pulse and the other being the reference. The resulting two beams were then focused on the entrance slit of an Instruments S A . , UFS 200 spectrometer coupled to an optical multichannel analyzer (OMA, Princeton Applied Research) equipped for two-dimensional analysis. The spectral data were transferred as indicated above for the fluorescence measurements to an HP computer which gave the calculated differential absorption spectrum at the instant determined by the time delay between the exciting pulse and the analyzing pulse. The spectral range was calibrated with a highpressure mercury lamp and the spectral resolution was better than 2 nm as verified on the separated 577- and 579-nm lines. The nanosecond absorption spectroscopy setup was described previo~sly.~'The sample was excited at 532 nm with a 6-11s pulse. The spectrometer provided a time resolution of about 3 ns.
Results and Discussion the fluorescence quenching In our previous picosecond of DODCI by three electron acceptors (methylviologen (MVCI,), p-benzoquinone (p-BQ), and p-dinitrobenzene (p-DNB)) was investigated in methanolic solution. In order to estimate the energetics of the electron-transfer reactions involved, it was necessary to calculate the DODCI excited singlet oxidation potential from the ground-state redox potential and the excitation energy of DODC+* (S,). Whereas the polarographic reduction potential of DODCI is known (Erd, = -0.92 V vs. an aqueous silver/silver chloride reference electrode, its polarographic oxidation potential Eoxid is unknown.38 A tentative measurement was unsuccessful and can be attributed to the irreversibility of the DODC'Z+/DODC+ system.3g A value of .Eoxidcan be estimated from the Sturmer and Gaugh correlation38between the polarographic oxidation potentials measured by Large in methanolw2 for several other cyanine dyes and their theoretical highest filled energy levels cHF: Eoxid (VS. Ag/AgCI) = 6.2tHF - 0.93 Using cHF = 0.258,33a value Eoxld= 0.67 V (vs. a siIver/silver chloride electrode) was calculated, Le., 0.89 V (NHE). The 0.67 V value follows the general trend observed in an homologous cyanine series as quoted in Table I. The excitation energy of the first excited singlet state of DODC' is the energy of the maximum of the fluorescence spectrum in methanol (XFmax = 602.5 nm) Le., 2.05 eV. Therefore, the oxidation potential of its excited state is E(DODC'*+/DODC+*(S,)) = -2.05 + 0.89 = -1.16 V (NHE) Although the DODCI fluorescence is similarly quenched by the three electron acceptors under investigation and despite favorable thermodynamics for the photosensitized electron-transfer reactions,34no charge separation could be observed in the case (37) (38) (39) (40) (41) (42)
-0,3
1
I
Mialocq, J . C. Chem. Phys. 1982, 73, 107. Sturmer, D. M.: Gaugh, W. S . Photogr. Sci. Eng. 1973, 17, 146. Reverdy, G . , Grenoble University, France. Reference 29 in ref 38. Penner, T. L.; Gilman, P. B. Photogr. Sci. Eng. 1975, 19, 102. Leubner, I . H . Photogr. Sri. Eng. 1976, 20, 61.
-0,4
I
;
+. 'Y
Figure 2. Differential absorption spectra at t = 2 f i s in a M DODCI M MVCI, solution (*). solution (+) and in a M DODCI, 5 X Difference spectrum (0).
. I
I
0
350
400
450
Figure 3. Differential absorption spectrum at t = 2 DODCI, 5 X IOw2 M MVCI, solution.
nm fis
in a
M
of p-BQ and p-DNB with nanosecond absorption spectroscopy, but new short-lived species were observed in the case of MVCI,. The beneficial influence of Coulombic repulsion between the radical ions when MV2+is used as an electron acceptor47is again verified. Therefore the DODCI-MVCI, photosystem necessitated a more detailed study. DODCI-MVC12 System. Figure 1 shows the differential absorption spectrum obtained at time t = 6 ns after excitation of a DODCI methanolic solution using a 6-ns laser pulse at 532 nm. The buildup and the rapid decay of this absorption are within the response time of the apparatus. The wavelength of the maximum = 430 nm) is in good agreement of this absorption spectrum (A, with that attributed to the first singlet excited state of DODCI . ~ differential ~ ~ ~ ~ absorption spectrum in our picosecond s t ~ d y This is very similar to the real S, absorption spectrum since the ground-state absorption spectrum is negligible below 480 nm. Figure 2 shows the differential absorption spectra at time t = 2 I.LS obtained after excitation of a pure mol dm-3 methanolic DODCI solution and of a mol dm-3 methanolic DODCI solution containing 5 X lo-, mol dm-3 MVCI,. A difference spectrum is also superimposed. In the pure DODCI solution, the bleaching is stable for several hundreds of microseconds governed by the lifetime of the DODCI p h o t o i ~ o m e r . ' ~ ~In~the ' ~ ~presence ~~~' of MVCI,, the photobleaching decreases due to a smaller yield of the photoisomerization process which is in competition with the excited-state quenching by MVCI,. Figure 3 shows the differential absorption spectrum between 350 and 485 nm obtained at t = 2 ws in the same solution. This spectrum is the true absorption spectrum of the species created by the reaction between DODC+* and MVCI, since the DODC' ground-state absorbance and MVClz absorbance are negligible in this region. The band with a maximum around 395 nm is reminiscent of the well-known absorption spectrum of the reduced MV" specie^^^-^^ with cg:; (43) Calderbank, A. Adv. Pest. Control. Res. 1968, 8, 127. (44) Evans, A. G.: Dodson, N. K.: Rees, N . H . J . Chem. SOC.,Perkin Trans. 2 1976, 859.
The Journal of Physical Chemistry, Vol. 91, No. 13, 1987 3527
Electron-Transfer Reaction of DODCI
3t I
2or '
///
0 -
1
..-..
I l l l l l l l l l l l i I I I I I
l l 1 I 1 1 1 1 1 1 1 1
350 400 450 nm Figure 5. Differential absorption spectra at t = 2 0 ns (0) in a 2 X M DODCI, 6 X lo-, M MVCl, solution and at t = 560 ns (+) in a 2 X M DODCI, 6 X lo-, M MVCI,, 2.0 X lo-' M p-BQ solution.
I
/
I-
/
/
p-BQ, DODCI fluorescence quenching is negligible.34 After electron transfer to methylviologen, two-thirds of the 395-nm absorbance disappear in the presence of p-BQ whereas the 460-nm absorption band is little affected. Reduction of p-BQ by the reduced viologen radical cation is thermodynamically feasible (see further below)
+ p-BQ
MV"
t (xlO%)
Figure 4. Plots of the reciprocal of the optical densities observed at 395 and 460 nm.
= (4.8 f 0.1) X lo4 dm3 mol-' cm-' in methanol.44 The other band with a maximum around 460 nm is attributable to the oxidized DODCo2+radical cation as already inferred in our picosecond This interpretation is based on the similarity of the decay kinetics of the two species which recombine with a second-order rate constant k, in the bulk solution as expected on thermodynamic grounds. Plots of the reciprocal of the optical densities at 395 and 460 nm are linear (Figure 4) giving kr/c395nm = 5.2 X lo4 cm s-' and kr/c460nm= 5.0 X lo4 cm s-I. The exact determination of k , is not possible at the present stage since the 395-nm absorbance is the sum of both MV" and DODCo2+ absorbances (see below). The exact contribution of the species to the absorbance can be ascertained by using an electron acceptor or an electron donor. It is worthwhile to notice that the presence of oxygen in aerated methanol has no effect on the reduced viologen decay. Although the reaction MV"
+ O2
-
MV2+
+ 0,'-
is very rapid in water, its rate constant decreases by a factor of ca. 200 from water to methanol: k = 3.3 X lo6 dm3 mol-' S - I . ~ A half-life of -10 ms of MV" in methanol for [O,] = 3.47 X loT3mol dm-3 at 40 "C has even been reported.@ Our observation of a pure second-order reaction for the recombination of MV" and DODC2+ which are present at a very low concentration confirms that oxygen in pure aerated methanol is not very reactive toward MV". DODCI-MVCl,-p-BQ System. Figure 5 shows the differential absorption spectra obtained at t = 20 ns in a 2 X lom5mol dm-3 DODCI, 6 X lo-* mol dm-2 MVCI, solution and at t = 560 ns mol dm-3 DODCI, 6 X lo-, mol dmw3MVCI,, and in a 2 X 2.0 X mol dm-3 p-BQ solution. As already stated above, no charge separation arises from the quenching of the DODCI mol dm-3 fluorescence by p-BQ. Furthermore, below 5 X (45) Farrington, J. A,; Ebert, H.; Land, E. J. J . Chem. Soc., Faraday Trans. I 1978, 3, 665. (46) Patterson, L. K.; Small, R. D.; Scaiano, J. C . Radiat. Res. 1977, 72, 218.
-
+
MV2+ p-BQ'-
The remaining 395-nm absorbance is attributable to the oxidized DODC'*+ species. From the already known molar extinction coefficient of the reduced viologen (see above), an estimate of cy$::'2+ = 2.3 X lo4 dm3 mol-' cm-I is therefore possible. In the ' *: : ; :c = 8.5 X lo4 dm3 same way we deduce the estimate +of mol-' cm-I. We have neglected in this calculation the absorbance of the p-BQ'- radical anion which is maximum around 425 nm, 4;:; = 6.9 X lo3 dm3 mol-] As will be seen later, in the presence of ascorbic acid, one-third of the 395-nm absorbance disappears but the 460-nm absorption band disappears totally at t = 70 gs as evidenced in Figure 5. This observation confirms the oxidizing properties of the species absorbing at 460 nm. From the above measured value kr/c395nmand that of the sum c395nm of MV" and DODCo2+,the value of k , is deduced as k , = 3.7 X IO9 dm3 mol-' s-I. In the same way from the kr/c460nm and eDODC'2+ values, we find k, = 4.3 X lo9 dm3 mol-' s-l. 460nm Therefore the more probable value of the second-order rate constant for the recombination reaction is k, = (4.0 f 0.5) X lo9 dm3 mol-' s-I. The standard free energy change AG for the reaction of the reduced viologen cation with p-BQ can be calculated by using the reduction potentials of MV2+and p-BQ. For MVCI, the reduction potential is expected to be less negative in methanol than in water due to the difference in dielectric constant and a value -0.41 V (SCE) has been found in acetonemethanol (7:3) by Holten et al.47 For p-BQ, a value -0.22 V (SCE) has been measured in acetonemethanol (7:3),14 although other values have been published: 0.29 to 0.22 V (NHE)48and 0.099 V (NHE)49in neutral aqueous solution. Assuming the following half-reactions in methanolic solution
+ ep-BQ + eMV2+
-
MV"
E o = -0.17 V (NHE)
(ref 47)
p-BQ'-
Eo = f 0 . 0 2 V (NHE)
(ref 14)
we calculate for the electron-transfer reaction from MV" to p-BQ in methanolic solution MV"
+ p-BQ
-
MV2+
+ p-BQ'-
AG = -0.19 eV
The fast component of the 395-nm absorbance decay follows a pseudo-first-order kinetics and the plot of its rate constant vs. (47) Holten, D.; Windsor, M . W.; Parson, W. W.; Gouterman, M. Photochem. Photobiol. 1978, 28, 95 1. (48) Morton, R. A. Biochemisrry of Quinones; Academic: New York, 1965; p 4. (49) Swallow, A. J. Functional Quinones, Energy Conseroing Systems. Trumpower, B. L. Ed.; Academic: New York, 1982; pp 59-72.
The Journal of Physical Chemistry, Vol. 91, No. 13, 1987
3528
D. 0
Doizi and Mialocq
1
( X1O2)
_ _ _ Ac.APC ... MV2+
5~10% 5x10m2M
- A c . k +MV2+
sdvmt: CH,OH
',
090I
I , I , I , /
I
*.'... *...
I I 1 I 1
I
l
.'
\ e*
% .C . 4. I ~
l
1
1
1
I I
L
I
1
i
095
-
-
090-
-. .-,-
- < * a -
-
~. .-........- -.? A. * + * -, l , I I / , I . I . l . l , I , I , '*.
nm
Figure 7. Absorption spectrum of ascorbic acid (- - -), methylviologen (O), and the mixture (-) in methanol.
such complex formation is the bathochromic shift of the absorption spectrum of a methanolic solution of 5 X mol dm-3 methylviologen in the presence of an equimolar concentration of ascorbic acid, as shown in Figure 7. A spectrophotometric measurement of the optical density at 380 nm enabled us to calculate the equilibrium constant K, and the molar extinction coefficient e380nm of the complex. Using the Benesi-Hildebrand m e t h ~ d , ~we ~ . 'found ~ K, = 3.5 dm3 mol-' and 6380nm = 95 dm3 mol-' cm-' whereas the Scatchard treatment7zgave K, = 2.0 dm3 mol-' and t380nm = 150 dm3 mol-' cm-I K, =
C ([MV2+lO- c)([H,AIo -
0
where C i s the complex concentration and [MV2+]oand [HZA], are the initial concentrations of methylviologen and ascorbic acid. In both treatments, ([H2AIO- c)is replaced by [H2A],. A plot of [H2A], vs. the optical density measured at 380 nm using the rigorous expression
(50) (51) (52) (53)
Creutz, C. Inorg. Chem. 1981, 20, 4449. White, B. G. Trans. Faraday Soc. 1969, 65, 2000. Kosower, E. M. J . A m . Chem. Soc. 1958, 80, 3253. Nakahara, A.; Wang, J. H . J . Phys. Chem. 1963, 67, 496
(54) Ledwith, A,; Iles, D. H . Chem. Br. 1968, 4 , 266. (55) Haque, R.; Coshow, W. R.; Johnson, L. F. J . Am. Chem. Soc. 1969, 91, 3822. (56) Macfarlane, A. J.; Williams, R. J. P. J . Chem. Soc. A 1969, 1517. (57) Ledwith, A,; Woods, H . J. J . Chem. Soc. C 1970, 1422. (58) Haque, R.; Lilley, S. J . Agric. Food Chem. 1972, 20, 57. (59) Brown, N. M. D.; Cowley, D. J.; Murphy, W. J. J . Chem. Soc., Chem. Commun. 1973, 592. (60) Barnett, J. R.; Hopkins, A. S.; Ledwith, A. J . Chem. Soc., Perkin Trans. 2 1973, 80. (61) Verhoeven, J. W.; Verhoeven-Schoff, A. M. A,; Masson, A,; Schwyzer, R. Helu. Chim. Acta 1974, 57, 2503. (62) Brown, N . M. D.; Cowley, D. J.; Hashmi, M. J . Chem. SOC.,Perkin Trans. 2 1979, 462. (63) Poulos, A. T.; Kelley, C. K.; Simone, R. J . Phys. Chem. 1981,85, 823. (64) Poulos, A. T.; Kelley, C. K. J . Chem. Soc., Faraday Trans. 1 1983, 79, 55. (65) Ebbesen, T. W.; Levey, G.; Patterson, L. K. Nature (London) 1982, 298, 545. (66) Ebbesen, T. W.; Ferraudi, G. J . Phys. Chem. 1983, 87, 3717. (67) Ebbesen, T. W.; Ohgushi, M. Photochem. Photobiol. 1983,38, 251. (68) Ebbesen, T. W.; Manring, L. E.; Peters, K. S. J . Am. Chem. Soc. 1984, 106, 7400. (69) Le Roux, D.; Mialocq, J. C.; Anitoff, 0.;Folcher, G.J . Chem. Soc., Faraday Trans. 2 1984, 80, 909. (70) Prasad, D . R.; Hoffman, M. Z . J . Phys. Chem. 1984, 88, 5660. (71) Benesi, H. A,; Hildebrand, J. H . J . A m . Chem. Sor. 1949, 7 1 , 2703. (72) Scatchard, G. Ann. N . Y . Acad. Sci. 1949, 51, 660.
The Journal of Physical Chemistry, Vol. 91, No. 13, 1987 3529
Electron-Transfer Reaction of DODCI [HzAIo =
d380' - d380(%80[MVZ+lo+ e 3 8 0 / K a )
h.4
do, a2
- [MVz+10980z
d380t380
however shows a better fit of the experimental data with the calculated curve until [HzAIo= 0.4 mol dm-3 using Ka = 2.0 dm3 mol-' and e380 = 150 dm3 mol-' cm-'. Reaction Scheme. To account for our observations, we propose the following set of reactions: DODC'
-
DODC+*(SI) DODC+*(S1)
-
+ hd
DODC'
(4)
k
(encounter complex)
(5)
+
[DODC*Z+ (MV*+, ci-)I [DODC'*+ + (MV", Cl-)] (MV", C1-)
k6 +
[DODC'"
--
+ (MV",
+ + (MV2+,Cl-)]
(7)
[DODC'
(8)
MV2+ + DODC'
k9 = (4.0 f 0.5) X lo9 dm3 mol-l (MV", CI-)
+ p-BQ
Cl-)] (6)
DODC*~+ (MV*+,ci-)
+ DODCo2+
(MV", C1-)
(9)
s-I
+ p-BQ'-
+ p-BQ'-
-
DODC'
+
-
+ p-BQ
(1 1)
+ H~A*+
(12)
The bimolecular DODC+* excited singlet quenching rate constant kQ is expressed in the form
where kQMaois the quenching rate constant at zero ionic strength. ~ Our experimental value, k9 =. 1.05 X 1O'O dm3 mol-l s - ' , ~ has thus to be corrected for the ionic strength effect. Methylviologen is expected to be incompletely dissociated in methanolic solution mol dm-3 MVC12),the equilibrium being displaced ((5-6) X toward the (MV2+,C1-) specie^:^^^^^ MV2+
+ C1-
;=t
(MVZ+,Cl-)
K
'
I
I
'
-
>2X
1 2 3 4 5 0 7 0 9 10' [MVCI,]
Figure 8. Plot of the ratio of the optical densities d395/d620 as a function
of [MVClZ].
ZB= 1, E is the dielectric constant of the solvent (e = 32.6), and rAB= 1.0 nma7, The values AG 0.044 eV and ks/k-s = 0.18 are thus obtained. Using k5 = 6.6 X lo9 dm3 mol-' s-',~, we find k4 = 3.7 X 1O'O s-l. Since kQfi" k5, examination of eq a leads us to conclude that the rate constant for electron transfer in the encounter complex k6 is thus certainly higher than The decay of the radical ion pair leads to the formation of the separated radical ions (k7) and to the spin-allowed back electron transfer in the pair (k8). The yield of redox products $t is thus controlled by the partition between these two decay routes:
-
k7 'I
k l l = (2.3 f 0.3) X lo9 dm3 mol-' s-l
DODC*Z+ H ~ A DODC+
I
-
(10)
kIo = (3.4 f 0.5) X lo9 dm3 mol-' s-I DODC'"
'
-
(3)
+ ( M V ~ +ci-) , 2 [DODC+*...(MVZ+, ci-)I
[DODC+*--(MV2+, Cl-)]
I
(2)
DODC' (photoisomer) k-s
'
(1)
DODC+(So) internal conversion
DODC+*(SI) DODC+*(SJ
-
DODC'"(S1)
I
lo2 dm3 mol-I (ref 66)
a
The recombination of the radical ions in the bulk solution occurs with a rate constant k9 slightly smaller than the diffusion rate constant in methanol. A calculation of k9 using the Debye equation is unrealistic because we do not know the charge of the reduced viologen species, (MV", CI-) or MV'+ even though the species (MV'+, C1-) has been considered as the end product in the photolysis of MVZ+(C1-)2 in methanol.66 Relative Yields of Redox Products and Photoisomer. As indicated above the photoisomer formation decreases with MVC12 concentration. A plot of the ratio of the optical densities d39Snm/d62Onm as a function of MVClz concentration is given in Figure 8. The quantum yield of ionic products can be written
whereas the photoisomer quantum yield is
-
Therefore at zero ionic strength, a Debye-Huckel calculation leads to kQfigo 6.3 X lo9 dm3 mol-' s-I which is close to the diffusion From the measurement of the optical density at 620 nm due to rate constant kS = 6.6 X lo9 dm3 mol-' s-l, previously calculated the photoisomer formation = 1.84 X los dm3 mol-' cm-I)l8 by using the Debye equation for charged ions.34 as a function of methylviologen concentration [MVCl,] and the This calculation did not take into account the ionic radii. The quantum yield of DODCI photoisomerization in pure methanol radius of the DODC' species is indeed difficult to define3, and (4: = 0.07),31one can plot the ratio $:/+, as a function of the radius of the viologen cation is also questionable since a weak [MVCl,] (Figure 9). A linear regression allows us to construct complex (MV2+C1--CH30H) has been hypothesized to account the straight line of best fit with a slope for the methylviologen photochemistry in methanolic s o l ~ t i o n . ~ ~ ~ ~ ~ ~ ~ k ~ / k , k3 k, = 8.7 dm3 mol-' The calculation of the equilibrium constant k5/k4 is possible using the free energy change associated with the formation of the enSince ( k 2 + k3 + k4)-' is the DODCI excited singlet lifetime in counter complex74 pure methanol 7 = (910 f 100) one can calculate kQ = 9.6 X lo9 dm3 mol-' s-I, in good agreement with the value k, = 1.05 X 1Olodm3 mol-' s-' obtained by using Stern-Volmer plots of the DODCI fluorescence quenching.34 Then the ratio where ZAe and ZBe are the charges of the reactants, with ZA= 4i[MVC12] - kq[MV2+, CI-] k7 4,[MVC121 k4 k7 + k8 (73) Rodgers, M. A. J. Photochem. Photobiol. 1979, 29, 1031.
+ +
(74) Harriman, A.; Porter, G.; Richoux, M. C . J . Chem. SOC.,Faraday Trans. 2 1981, 77, 1175.
can be plotted by using the measured optical density at 395 nm
J . Phys. Chem. 1987, 91, 3530-3536
3530
47 1 : a
culation of the redox products concentration was based on the sole absorbance of reduced viologen at 395 nm. Even though the electron transfer from DODC+* to (MV2+, C1-) leads to a globally neutral reduced viologen species ( M Y + , Cl-), the radical-ion pair [DODC2+(MV'+, Cl-)] will be more affected by the Coulombic repulsion of the nearest DODCo2+and MV" species. Therefore, separation of the redox products is made possible in the case of MVCl, whereas Coulombic attraction precludes separation in the case of p B Q or p-DNB. Finally, back electron transfer in the radical pair which is a spin allowed So internal conversion is never1(2DODC'2+,*(MV'+, CI-)) theless the predominant process.
-
1 2 3 4 5 6 7 8 9 10' [MVCI,]
Figure 9. Plot of the ratios
and @,/@p as a function of [MVCI2].
and the sum of the molar extinction coefficients of the radical ions c395nm= 7.1 X lo4 dm3 mol-] cm-' to determine the radical ions concentration. The quantum yield of the ions +i is calculated by using the photoisomer formation in pure methanol as an actinometer. A linear regression (Figure 9) allows us to construct the straight line of best fit with a slope
kQ ---
k7
- 5.4 dm3 mol-] k4 kl + k8 Using k4 = c$:(~/T) = 7.7 X lo7 dm3 mol-I SKIand k , = 1.0 X 1Olo dm3 mol-' s-I, one obtains --k7 - 0.04 kl + k8 a value which reflects the efficiency of separation of the redox products in the radical pair. This ratio is certainly negligible in the case of p-BQ and p-DNB. The separation of the redox products is thus easier in the case of MVC1,. The present analysis is more rigorous than in the preliminary study34where the cal-
Conclusion The fluorescence quenching of the first singlet excited state of DODCI via electron transfer to electron acceptors MVCl,, p-BQ, and p D N B is equally efficient, but a significant formation of redox products is only observed in the case of MVCl,, due to Coulombic repulsion of the radical ions in the pair. Redox properties of these radical ions have been used to attribute the visible absorption bands = 395 nm) and the respectively to the reduced viologen (A, oxidized DODCo2+species (A, = 460 nm). A complete reaction scheme has been presented including electron transfer in the encounter complex, redox products separation in the radical pair, recombination in the bulk solution, and electron-transfer reactions with an electron acceptor p-BQ and an electron donor ascorbic acid. Even though direct electron transfer from DODC+* to p-BQ does not lead to redox products, the use of a relay MVCI, enables significant reduction of the quinone. The DODCI-MVCl,-p-BQ photosystem with ascorbic acid as a sacrificial donor is therefore an interesting model for the study of electron-transfer reaction in the first singlet excited state of polymethine-cyanine dyes and for the modelling of solar energy conversion. Registry No. DODCI, 14806-50-9; p-BQ, 106-51-4; p-DNB, 10025-4; MVC12, 1910-42-5; L-(+)-ascorbic acid, 50-8 1-7; D O D C 2 + , 95272-87-0; MV", 25239-55-8
Picosecond Excited-State Solvation Dynamics of 9,9'-Bianthryl in Alcohol Solutions D. W. Anthon* and J. H. Clarkt Laboratory of Chemical Biodynamics, Lawrence Berkeley Laboratory, and Department of Chemistry, University of California, Berkeley, California 94720 (Receioed: December 5, 1986)
Picosecond time-resolved emission spectroscopy has been used to measure the rate of charge transfer and solvation of 9,9'-bianthryl in several solvents and solvent mixtures. In nonpolar and aprotic polar solvents no rise times were detected, indicating that all charge transfer and solvation occur within the 10-ps experimental resolution. In linear alcohols, complex biexponential fluorescence rise times are seen. These are interpreted in terms of an initial charge transfer (-30-60 ps) forming a weakly solvated charge-transfer state which subsequently (-S0-500 ps, increasing with solvent viscosity) reacts with the solvent to form a hydrogen-bonded exciplex. Evidence for hydrogen bonding comes from deuterium exchange measurements; excited-state proton transfer can occur at the 10 and 10' positions. Solvation times in alcohol-alkane mixtures are slower than in comparable alcohols. This is interpreted as being due to the locally low alcohol concentrations associated with solvation of the nonpolar BA ground state. The role of dielectric relaxation in these systems is also discussed.
Introduction Charge transfer and solvation are fundamental processes which play an important role in a wide range of chemical and biological phenomena. On short time scales these two processes are closely related, as the rate of charge transfer generally depends on the rate at which the solvent polarizability can react to changes in
the molecular dipole moment.' The dynamics of these processes can be readily observed with picosecond resolution in systems undergoing excited-state relaxation. Large populations Of excited-state molecules can be rapidly created with a short laser Pulse, and their subsequent evolution can be monitored with time-resolved emission spectroscopy.* In many excited molecules,
'Present address: Ammo Research Center, Naperville, IL 60566. Address all correspondence to this author. 'Alfred P. Sloan Research Fellow and Henry and Camille Dreyfus Teacher-Scholar. Present address: Amoco Research Center, Naperville, IL 60566.
( 1 ) Kosower, E. M. J. Am. Cfiem. SOC.1985, 107, 11 14. Kosower, E. M.; Huppert, D. Cfiem. Phys. Lett. 1983, 96, 433. Huppert, D.; Kanety, H.; Kosower, E. M. Discuss. Faraday SOC.1982, 7 4 , 161.
~~~~~~
0022-3654/87/209 1-3530$01.50/0
~
~~~~~~~~~~~~
(2) Campillo, A. J.; Shapiro, S. L. IEEE J . Quantum Electron. 1983, QE-19, 5 8 5 .
0 1987 American Chemical Society
