J. Phys. Chem. 1992,96, 10119-10124 adequate for the flexible D-A molecules. The rate constant of S-S energy transfer of dmphthylalkanes reported by Ikeda et aL20 is also plotted in Figure 8. The rate anstant of T-T energy transfer is much smaller than that of S-S energy transfer and is strongly dependent on chain length. This difference is related to the respective distance dependence of the two forementioned mechanisms. These results are also in good agreement with data previously collected in fluid solutions.2b The buildup of the naphthalene phosphorescence was too fast to be detected in this investigation. Use of nanosecond T-T absorption might be a better method to determine the rate of energy transfer in the shorter chains. These experiments are currently in progress.
Concllrsioa The intramolecular T-T energy transfer of bichromophoric compounds connected with a methylene chain was directly measured by phosphorescence decay measurement. The donoracceptor distance was calculated by a conformational analysis, and the phosphorescence decay was simulated using Dexter's equation. The result of simulation was in fairly good agreement with the experimental values. The value obtained for T-T energy transfer in this study is very close to the value obtained previously for another system in which benzophenone and dibenz[bflazepine were connected by a methylene chain. The flexible methylene chain is common to both systems. The present study reconfirms our previous report that the 'through-space" mechanism governs the intramolecular T-T energy transfer in the flexible D-A molecules.
References and Notes (1) Dexter, D. L. J. Chem. Phys. 1953, 21, 836. (2) (a) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: New York, 1970. (b) T w o , N. J. Modern Molecular Photochemistry; Benjamin: Menlo Park, CA, 1978. (c) Wilkinaon, F. Q. Rev. 1966, 20, 403. (3) (a) Pemn, F. Compr. Rend. 1924, 178, 1978. (b) Strambini, G. B.; Galley, W. C. J. Chem. Phys. 1975,63, 3467. (c) Kobashi, H.; Morita, T.;
10119
Mataga, N. Chem. Phys. Lett. 1973, 20, 376. (4) (a) Lamola, A. A.; Leermaken, P. A.; Byen, G. W.; Hammond, G. S.J. Am. Chem. Soc. 1965,87,2322. (b) Keller, R. A.; Dolby, L. J. J. Am. Chem. Soc. 1%7,89,2768. (c) Breen, D. E.; Keller, R. A. J. Am. Chem. Soc. 1968,90, 1935. (d) Keller, R. A. J. Am. Chem. Soc. 1968,90, 1940. (e) Thiery, C. Mol. Photochem. 1970,2,1. ( f ) Zimmaman, H. E.; M c h h , R. D. J. Am. Chem. Soc. 1971,93,3638. (8) Amrein, W.; scheffner, K. Heh. Chim. Acta 1975,58, 397. (h) Rauh, R. D.; Evans, T. R.; Leermaken, P. A. J. Am. Chem. Soc. 1968,90,6891. (i) Gust, D.; Moore, T. A.; h"OI, R. V.; Mathii, P.; Land, E. J.; Chachoty, C.; Moore, A. L.; Liddell, P. A.; Nemeth, G. A. J. Am. Chem. Soc. 1985, 107,3631. (5) (a) Keller, R. A.; Dolby, L. J. J. Am. Chem. Soc. 1%9,91, 1293. (b) Maki, A. H.; Ween, J. G.; Hilinski, E. F.; Milton, S. V.; Rentzepis, P. M. J. Chem. Phvs. 1984.80.2288. (6) Clw,-G. L.; fiotrowiak, P.; MacInnis, J. M.;Fleming, G. R. J. Am. Chem. Soc. 1988,110,2652. (7) Katayama, H.; Maruyama, S.; Ito, S.; Tsujii, Y.; Tsuchida, A,; Yamamoto, M.J. Phys. Chem. 1991. 95. 3480. (8) Tamao, K.;Sumitani, K.; Kiao, Y.; Zembayashi, M.;Fujioka, A.; Kodama, S.; Nakajima, I.; Minato, A.; Kumada, M. Bull. Chem. Soc. Jpn. 1976,49, 1958. (9) Ito, S.; Katayama, H.; Yamamoto, M. Macromolecules 1988, 21, 2456. (IO) Ito, S.; Tahmi, K.; Tsujii, Y.; Yamamoto, M. macromolecule,^ 1990, 23, 2666. (1 1) (a) Flory, P. J. Statistical Mechanics of Chain Molecules; Wiley: New York, 1969. (h) Hopfinger, A. J. Conformational Properties of Macromolecules; Academic: New York, 1973. (12) Popova, E. G.; Chetkina, L. A. Zh. Strukr. Khim. 1979, 20, 665. (13) Brock, C. P.; Dunitz, J. D. Acta Crystallogr. 1982, 838, 2218. (1 4) Abe, A.; Jemigan, R. L.; Rory, P. J. J. Am. Chem. SOC.1966,88, 631. (1 5) Gilmore, E. H.; Gibum, G. E.; McClure, D. S.J . Chem. Phys. 1952, 20, 829. (16) (a) Ennolaev, V. L. Opt. Spectrosk. 1961, IZ, 266. (b) Berman, I. B. J . Chem. Phys. 1970,52, 5616. (17) Ennoleav, V. L. Sou.Phys.--Dokl. (Engl. Transl.) 1962, 6, 600. (18) (a) Overing, H.; Paddon-Row, M. W.; Hepperer, M.;Oliver, A. M.; Cotaaris, E.; Verhoeven, J. W.; Hush, N. S.J . Am. Chem. Soc. 1987, 109, 3258. (b) Overing, H.; Verhoeven, J. W.; Paddon-Row, M. W.; Cotsaria, E.; Hush, N. S.Chem. Phys. Lett. 1988,143,488. (c) Kroon, J.; Oliver, A. M.; Paddon-Row, M.W.; Verhoeven,J. W. J. Am. Chem. SOC.1990,112,4688. (19) Inai, Y.; Siido, M.;Imanishi, Y. J. Phys. Chem. 1991, 95, 3847. (20) Ikeda, T.; Lee, B.; Kurihara, S.; Tazuke, S.;Ito, S.;Yamamoto, M. J . Am. Chem. Soc. 1988, 110, 8299.
Dependence of the Benzophenone Anion Solvatlon on Solvent Structure Y.Lin* and C.D.Jonah* Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: June 8, 1992; In Final Form: September 17, 1992)
The solvation of the benzophenone anion has been studied at room temperature using the pulse radiolytic pumpprobe technique. The time-dependent benzophenone anion absorption spectra have been monitored in several different solvents ranging from linear alcohols to branched alcohols to acetonitrile. The maximum of the steady-state spectrum shifts to the red as the solvent is changed from linear alcohols to branched alcohols to acetonitrile. Computer Monte Carlo simulations indicate that the observed spectral shift can be assigned to the position and the orientation of the dipole functional group. The experimental dynamics of the anion solvation were also studied. By fitting the time-dependent absorption data to a multistate evolution kinetic model, the solvation time for these systems is obtained.
I. latroduction There are many experiments that probe the dynamics of the solvation in polar fluids, including dielectric relaxation, nuclear magnetic resonance, etc. In the past few years, the application of the picosecond spectroscopy to the study of molecular dipole solvation and electron solvation proceapes has provided an exciting new microscopic probe for the relaxation processes in polar fluids.'-' In such studies, the central focus has been on measuring how rapidly a solvent rqonds to changes in the charge distribution of a solute molecule and on understanding what solvent and/or solute attributes determine this response time. These studies have primarily probed either the solvation of a large molecular dipole14 0022-3654/92/2096-10119$03.00/0
or the solvation of a quasi-free Because of the differences in physical structure and charge distribution, solvation around a charged species will be different from the solvation around a dipole. Early experimental work on ion solvation has observed the solvent reorganization around benzophenone anions in low-temperature alcohol solutions using pulse radiolysis?*1° With an improvement in time resolution of approximately 3 orders of magnitude, we are able to study the benzophenone solvation in a variety of alcohols at room temperature.' A charged species in solution such as benzophenone anion exerts a strong local field on its environment, forcing the surrounding '9'
0 1992 American Chemical Society
10120 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992
Lin and Jonah
solvent molecules to change from a configuration that would solvate a neutral molecular dipole to a configuration that solvates the anion. In the system where the anion is ultimately stabilized over a time scale comparable to local environmental change, the anion can be perceived as a microscopic probe of its environment. The interaction between the solvent molecules and the anion will manifest itself in the anion absorption and emission spectra. Therefore, the time evolution of the anion absorption spectrum provides us with detailed dynamic information on the solvation process. The benzophenone anion was selected as the microscopic probe for several reasons. The anion spectrum is separated from that of the triplet and the excited state and is strongly shifted by a polar solvent. The anion can also be produced very quickly by a reaction with the “dry electron”. When a high-energy electron beam is injected into a dense polar medium, it ionizes the solvent molecules and generates the “dry electron”, which undergoes multiple scattering events before its localization. Benzophenone captures dry electrons, forming the corresponding anion. As was shown in previous work, the solvation process of the benzophenone anion is characterized by the blue shift of the transient absorption spectrum of the anion.’* For the linear alcohol systems we studied, the solvation is faster for the smaller alcohols. However, the final spectral position and width are very similar for different linear alcohols. This similarity indicates that the (relatively few) nearest neighbors determine the energy of the electronic states rather than the energy shift occurring through long-ranged interactions. The time scale of the anion solvation appears to be longer than the electron solvation in the same solvent, and the familiar “two-state” solvation m ~ d e l , ~which - ~ has been successful in describing the electron solvation in water or in alcohols at room temperature, does not describe anion solvation. How do the structures of the solvent molecules and solute molecules determine the dynamics and energetics of the solvation of an ion in solution? There are a few publications where the spectral changes in the solvation process caused by varying molecular structure of the solvent have been investigated.I3-l6 In particular, a systematic exploration of the solvated electron spectrum in a variety of alcohols has been made by Hentz and Kenney-Wallace.’5-16 They found that the peak of the final solvated electron absorption spectrum is sensitive to the number and size of branches and distance of the branch point from OH group. In a recent p~blication,’~ Fujisaki et al. also found that the absorption spectra of the solvated electron in cis and trans isomers of methylcyclohexanol can be different by as much 100 nm, irrespective of the position where a hydrogen atom on the cyclohexanol ring is substituted for a methyl group. In the present work the transient absorption spectra of benzophenone anion have been studied in several linear and branched alcohol systems. The absorption spectrum of the fully solvated benzophenone was found to be virtually independent of the chain length for primary alcohols; however, the conformal structure of the solvent molecules plays a major role in the solvation energetics. The branched alcohols were found to have different solvation behavior and solvation energy from the linear alcohols. A preliminary analysis of the systems has been carried out using computer simulation, and the observed solvated benzophenone anion spectra can be related to the structure conformations of the alcohol solvents.
the transmission of the sample as a function of time. The pulse width of this system is approximately 30 ps, and the width is limited by space-charge broadening. The limitation is not serious in this system, since this is approximately equal to the difference in time (18 ps) that it takes a light pulse and an electron pulse to traverse a 2-cm cell. All solvents were either “distilled-in-glass quality” from Burdick and Jackson or “spectroscopic” grade from Fisher Scientific and were used without further purification. The water used in the hydrated electron measurement was purified by a deionizing water system that included a charcoal filter and a submicron final filter. Benzophenone was purchased from Aldrich (specified as 99+%) and used as received. Solvation experiments were run using 0.25 or 0.5 M benzophenone solutions depending on the solubility. The sample solution was flowed through a Suprasil cell to avoid the degradation due to the radiation, and the amount of the radiation damage was constantly checked to be ” a l . All pulse radiolysis pumpprobe experiments were run at room temperature. If the solvated electron were present, its absorption spectrum would overlap with the absorption of the benzophenone anion and could interfere with the experiment. However, only few solvated electrons are formed at 0.25 M benzophenone concentration. The contribution of the solvated electron to the anion absorption has been shown to be minimal.” The role of solute concentration on the final spectrum of the benzophenone anion was found to be negligible. The final spectrum of the anion was found to be the same in 0.05 M benzophenone solutions and in 0.5 M benzophenone solutions.” T h e results suggest strongly that there is minimal anionsolute interactions on the time scale of these measurements. The overall instrument response time was determined from the hydrated electron absorption at 600 nm. This electron absorption was monitored before and after each experiment to ensure no signifcant beam drift occurred during the experiment. Because the electron solvation process in water is much faster than our pump and probe pulses, the fast rise of the hydrated electron absorption can be used to determine a response function that can be used for subsequent data analysis. The initial hydrated electron concentration was approximately 20 pM. The time resolution of the measurements was always around 30-40 ps.
II. Erperimeatal Section
III. Results
The Argonne stroboscopic pulse radiolysis system was used for the measurements. Because this system has been described completely in the p a ~ t , ” 9 ~only ~ a short summary of its capabilities will be given in the present publication. A 20-MeV electron linac generates a short pulse of electrons. Approximately 30% of the electrons are intercepted by a cell filled with 1 atm of xenon to generate Cerenkov light for an analyzing light pulse. The remainder of the electrons are focused into a 2-cm radiolysis cell. The delay of the light beam can be varied so that it reaches the irradiation cell before, during, or after the pulse of ionizing radiation. By varying the delay of the light beam, one can determine
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1. Benzophenoneanion absorption at 50 ps, 100 ps, and 3 ns after the pulse excitation in 2-octanol solution. The center of the absorption spectra shifts from 680 to 650 nm as solvent reorganization procecds.
Both time- and wavelength-dependent absorption spectra were recordedfor the benzophenone solutions. In Figure 1, the transient
absorption spectra of the benzophenone anion in 2-octanol solution are shown. Similar to what has been observed in linear alcohols,,”2 the solvation p r m in 2octanol is charactmized by the blue shift of the anion absorption spectrum with time. The peak of the absorption spectrum shifts from 680 nm at 50 ps to 650 nm at 3 ns. The ahrption band has been assigned to the benzophenone anion. Furthermore the width of the absorption spectra narrows as the reorganization process. To display the differenas between the primary and secondary alcohols, Figure 2 plots the absorption
The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 10121
Solvation of the Benzophenone Anion
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in 2-octanol at various wavelengths. The intensities are rcscaled to show the rate of the decay.
TABLE I: Benzophenone Anion Solvation Time and Characteristic Lonpitndld Relaxation Times for the Solvent 0.021
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Wavelength (nm) Figure 2. Transient absorption spectra of benzophenone anion in n-octanol and 2-octanol solutions at 50 ps (upper) and 3 ns (bottom) after the pulse excitation.
0.10-
Wavelength (nm) Figwe 3. Transient absorption spectra of benzophenone anion in acetonitrile solution at 50 p, 300 p, and 3 ns after the pulse excitation. The dynamics of the solvation in this solvent are too fast to be observed. The center of the absorption peak is at 720 nm.
of the benzophenone anion in n-octanol and 2-octanol solutions. As shown in Figure 2, the absorption spectra are very similar for the primary and secondary alcohols at times prior to the solvation procxss. Both reflect a newly created ion species in random solvent configurations(unrelaxed con@urations). The absorption spectra become very different for the fully solvated species in linear and branched alcohols. The shift of the spectrum of the benzophenone anion in 2-octanol is smaller than in the n-octanol, and final spectral position is about 35 nm red-shifted from what is observed in the nonnal alcohol. S i solvation behavior was also obsetved in other branched alcohol systems, such as 2-butanol and 2propanol. The transient absorption of the benzophenone anion in acetonitrile is shown in Figure 3. Because the peak of the absorption is at the same place at 50 ps, 300 p,and 3 ns, the timedependent solvation process in this system is probably too fast for us to observe. Using the time-dependent fluorescence Stokes shift, previous studies of molecular dipole solvation in acetonitrile indicate that the characteristic solvation time f Lfor the acetonitrile is approximately 0.3 p,19*20 which is faster than our instrument
compound n-octanol 2-octanol acetonitrile n-decanol
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