10115
J. Phys. Chem. 1992,96, 10115-10119 95,8337. (c) Whitney, S.G.; Coolbaugh, M. T.; Vaidyanathan, G.; Garvey, W. R.,Jr. J. Phys. Chem. 1991,95,9625. (d) Alexander, M. L.; Levinger, N. E.; Johnson, M. A.; Ray, D.;Lineberger, W. C. J. Chem. Phys. 1988,88, 6200. (e) Ray,D.; Levinger, N. E.;Papanikolas, J. M.; Lineberger, W. C. J. Chem. Phys. 1989,91,6533. (0 Posey, L. A.; Campagnola, P. J.; Johnson, M. A.; Lee, G. H.; Eaton, J. G.; Bowm, K.H. J. Chem.Phys. 1989,91,6536. (g) Papanikolas, J. M.;Gord, J. R.; Levinger, N. E.; Ray, D.; Vorsa, V.; Lineberger, W. C. 1.Phys. Chem.1991,95,8028. (h) Posey, L. A,; Deluca, M.J.; Campagnola, P. J.; Johnson, M.A. J. Phys. Chem. 1989, 93, 1178. (1 1) (a) Shida, T.Annu. Rev. Phys. Chem.1991,42,55. (b) Mikami, N.; Sasaki, T.; Sato, S. Chem. Phys. k t r . 1991, 180, 431. (c) Maeyama, T.; Mikami. N. J. Phvs. Chem. 1991.95.7197: Ibid. 1990.94.6973. (d) Rieln. C.; Lahr;la~,C.;*Brutschy,B. J . Phys. Chem. 1992, %, 3626. (e) Bhtschy, B. J. Phys. Chem. 1990, 94, 8637. (12) Bernstein, E.R.;Law, K.; Schauer, M.J . Chem. Phys. 1984,80,207, 634. (13) Disselkamp, R.;Bernstein, E. R. J. Chem. Phys., to be published.
(14) Li, S.; Bernstein, E. R. J . Chem. Phys. 1992, 97, 792, 804, oo00. (IS) Newton, M. D.;Sutin, N. Annu. Rev. Phys. Chem. 1984,35,437. Sum, N.Pmg. Inwg. Chem. 1986,30,441. Closs, G. L.; Mier, J. R. ScleNv 1988,240,440. Mar-, R.A.; Sutin, N . Biochim. Biophys. Acta lW, 811, 265. (16) (a) Foster, S.C.; Miller, T. A. J. Phys. Chem. 1989,93, 5586 and references therein. (b) Lm,T. Y. D.; Damo, C. P.; Dunlop, J. R.;Miller,T. A. Chem. Phys. Lcrr. 1990,168,349. (c) Fukushm, M.; Obi, K.J. Chem. Phys. 1990,93,8488. (d) Im,H. S.;Bemtein, E. R. J . Chem. Phys. 1991, 95, 6326. (1 7) Hmberg, G. Spectra and Structure of Simple Free Radicals; Cornell University Press: Ithaca, NY, 1971; Electronic Spectra and Electronic Structure of Polyatomic Molecules; Van Nostrand New York, 1966. (18) (a) Goumari, A.; Pauwels, J. F.; Sawerysyn,J. P.; Dwolder, P. Chem. Phys. k r t . 1990,171,303. Ebata, T.;Obi,K.;Tanaka, I. Chem. Phys. k r r . 1981, 77, 480. Nelson, H. H.; McDonald, J. R.J. Phys. Chem. 1982, 86, 1242. (b) Kochi, J. Free Radicals; Wiley: New York, 1973.
ARTICLES Intramolecular Triplet Energy Transfer of the System Having Donor and Acceptor at the Chain Ends. 2. The Carbazole-Naphthalene System Hideaki Katayama, Shinzaburo Ito, and Masahide Yamamoto* Department of Polymer Chemistry, Faculty of Engineering, Kyoto University, Sakyo- ku, Kyoto 606, Japan (Received: April 27, 1992; In Final Form: July 29, 1992)
Intramolecular triplet-triplet (T-T) energy transfer in a series of polymethylene chains having a carbazole group as an energy donor and a naphthalene group as an energy acceptor has been studied by phosphorescence measurement. The phosphorescence decay curves were analyzed by Dexter's equation in which the distribution of donor-acceptor (D-A) distance was calculated by the conformational energy analysis. The results of the simulation were in fairly good agreement with the experimentally observed decay curves. We conclude that the through-space mechanism is adequate for intramolecular T-T energy transfer of the flexible D-A molecules.
Introduction Triplet-triplet (T-T) energy transfer is a basic photophysical process which has been extensively studied since the mechanism was proposed by Dexter.' T-T energy transfer is forbidden by the dipole-dipole mechanism but is allowed by the exchange mechanism? Therefore, the rate constants are highly sensitive to the distance of separation between donor and acceptor.3 several studies on intramolecular T-T energy transfer have been but few have measured the rate constantsof T-T energy transfer.57 Recently, Closs et al. investigated the bichromophoric compounds connected by a rigid spacer and estimated the rate constant of T-T energy transfer in benzene solution at room temperature, by using picosecond laser photolysis.6 They concluded that the T-T energy-transfer process occurs via the u-bond of the spacer (through-bond mechanism). We previously studied the intramolecular T-T energy transfer of the bichmnophoriccompounds connected by flexible methylene units using a system having a benzophenone (BP)group as an energy donor and dibenz[bflazepine (DBA)group as an energy acceptor.' Analysis of the phosphorescence decay curves by Dexter's equation in which the distribution of the donoracceptor distance was calculated by the conformational energy analysis revealed that the T-T energy-transfer prows of flexible I S A molecules obeys Dexter's equation (through-space mechanism). In the present work, another system of intramolecular T-T energy transfer will be discussed. The sample used here is a series 0022-3654/92/2096-10115$03.00/0
of polymethylene chains having a carbazole (Cz) group as an energy donor and naphthalene (Np) group as an energy acceptor.
w Cz-n-Np
Bp-m-DBA
These compounds are denoted by Cz-n-Np (n = 8-12), each numeral representing the number of methylene units for each polymethylene chain. This system has the following characteristics: (a) the spacer is composed of simple methylene units, while the previous one has ether and carbonyl group, (b) the decay profile of the isolated donor (Cz) is single exponential, (c) both the donor and the acceptor reactivities are much less than those of the previous syatem, and (d) the acceptor (Np) emits phosphoresoance, Ca 1992 American Chemical Society
10116 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992
Katayama et al.
while DBA used in the previous work is nonphosphorescent.
Experimental Seetion Materials. A series of polymethylene compounds having a carbazole group and naphthalene group as chain terminals (Czn-Np) were synthesized by the Grignard reaction of 9-(wbrom0alkyl)carbazolewith 2-bromonaphthaleneaccording to the procedure of Tamao et a1.* 9-(w-Bromoalkyl)carbazole was synthesized from the sodium salt of carbazole and cu,w-dibromoalkane. For example, Cz-8-Np was prepared as follows. To a mixture of sodium hydride (2.2 g) and dry THF (50 mL) was added dropwise a solution of carbazole (8.0 g) in THF (10 mL), and the mixture was refluxed at 50 OC for 2 h. After the mixture was cooled to room temperature, 1,I-dibromooctane (25 g) was added dropwise and refluxed at 70 OC for 2 h. The product was extracted with ether and washed twice with water and with aqueous saturated sodium chloride. Then the solvent was evaporated. The product mixture was purified by column chromatography on silica gel eluted with a mixed solvent of hexane and benzene (21) to give 9-(8-bromooctyl)carbamle(12 g). One tenth of the solution of 9-(8-bromooctyl)carbazolein ether was added to a mixture of magnesium (0.8 g) and dry ether (10 mL), which was then stirred at room temperature. The remaining ether solution was added at 0 OC, and the mixture was refluxed for 30 min. After the mixture was cooled to room temperature, it was added dropwise to a mixture of 2-bromonaphthalene (7 g), dichloro[ 1,3-bis(diphenylphosphino)propane]nickel(II) (20 mg), and ether (10-15 mL). The mixture was stirred at room temperature for 2 h and then refluxed for 20 h. Then 2 N hydrochloric acid (10 mL) was added at 0 OC. The product was extracted with ether and washed with water, with aqueous saturated sodium hydrogen carbonate and again with water. Then it was dried over sodium sulfate. After evaporation of the solvent, the product was purified by column chromatography on silica gel eluted with a mixed solvent of hexane and benzene (3:l) and recrystallized twice from ethanol. A series of other Cz-n-Np compounds were prepared by the same procedure as for Cz-8-Np. The obtained products were identified by IR and NMR spectra. The spectra for all the obtained products showed the same characteristic spectra. The data were as follows: IR (KBr) 2920,2850,1324,854,810,746, and 720 cm-'; 'NMR (CDCI,) 6 1.4-2.1 (m, methylene H), 2.8 (t, methylene H), 4.3 (t, methylene H), and 7.0-8.1 (m, Ar H). The melting points of the products were 60 OC for Cz-8-Np, 5 1 OC for Cz-g-Np, 71 OC for Cz-lO-Np, and 66 OC for Cz-12-Np. Sample Preparation. 2-Methyltetrahydrofuran (MTHF) was used for a rigid glass at 77 K. Before the sample preparation, Cz-n-Np was repurified by liquid chromatography (Japan Spectroscopic Co.Ltd) to remove any trace amount of impurities. The final product showed a single peak on the chromatogram. The concentration of Cz-n-Np was adjusted to be 1.2 X lo4 mol L-' to avoid the intermolecular interaction. The solution was degassed by several freezethaw cycles and sealed in vacuo. Spectroscopic Measurement. The absorption spectra were measured by a Shimadzu UV-200s spectrophotometer. Phosphorescence spectra in a photostationary condition were measured with a Hitachi 850 spectrophotometer fitted with a phosphorescence attachment. Phosphorescencedecays were measured with a phosphorimeter assembled in our laboratory. Details of the system have been described el~ewhere.~The temperature was regulated to be 77 K. Conformatid Analysis. Conformational analysis was performed by an empirical potential energy calculation which was ~ ~ ~calculation, ~' the same method as reported p r e v i ~ u s l y . ~In~this the van der Waals interaction energy between nonbonded atoms and the intrinsic torsional potential energy of the rotation about C-C bond were considered. The van der Waals energies were calculated by a Lennard-Jones type function. Figure 1 shows the structural parameters used in this calculation. The parameters for the catbazole and naphthalene groups were obtained from data of X-ray analysis of N-methylcarbazole12 and na~hthalene,'~ and those of skeletal alkane chain were taken from references by Abe et al.I4 N + 1 rotational angles I$', I$*, ..., I$"+, were taken into
Figure 1. Structural parameters of Cz-n-Np used for conformational analysis.
> I-
J,
z
w
d
[L
500 600 WAVELENGTH I n m Figure 2. Phosphorescence spectra of (a) Cz-12-Np, (b) Cz-lO-Np, (c) Cz-9-Np, and (d) Cz-8-Np in MTHF at 77 K. The excitation wavclength is 337 nm. Concentration of chromophore is 1.2 X lo4 mol L-I. A broken line and a dotted line indicate the phosphorescencespectra of N-ethylcarbamle and 2-methylnaphthalene,respectively. Spectra arc normalized at the maximum of each spectrum peak. 400
account to generate a given conformation for the Cz-n-Np compound.
Results md Discussion The absorption spectra of Cz-n-Np compounds correspond to the sum of N-ethylcarbazole and 2-methylnaphthalene spectra within experimental error,and no additional absorption bands were detected. This mans that chromophores of Cz-n-Np compounds do not interact with each other at the ground state. The quenching efficiency for the Cz-n-Np compounds was estimated at a photostationary condition at 77 K. The relative phosphorescxnce intensity of Cz-n-Np compounds ( I c ~ . , N ~was ) determined in reference to that of EtCz ( I E a ) , which was chosen as the isolated carbazole model compound, and the quenching efficiency was evaluated by Ic~-,,.N~/IE~c~. Figure2showstheIK"d ' phoL3plk"lce spectra for each sample. Emission from naphthalene was o k e d for all samples. In this measurement, only the carbazole chromophore was 8clectively excited at 337 nm. The spectra shown in F i i 2 indicate that energy transfer from carbazole to naphthalene occurs in all samples. The quantum yield of naphthalene phosphorescenceis much smaller (q = 0.060)1sthan that of carbazole (0 = 0.21).16 Therefore, for Cz-10-Np for example, the intensity of carbazole phosphorescence is comparable to that of naphthalene phosphorescence, but the quenching efficiency is 0.964. For all Czn-Np compounds, most of the carbazole in triplet state was quenched intramolecularly by naphthalene. Table I shows the quenching efficiency for each sample. The table also lists the maximum distances between Carbazole and naphthalene calculated from the all-trans conformation. The critical radius of phosphoracence quenching between carbazole and naphthalene (&) was reported to be 1.5 31m.l' The maximum distances between carbazole and naphthalene of Cz-PNp, Cz-IO-Np, and Cz-12-Np
The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 10117
Energy Transfer of the Carbazole-Naphthalene System
180
TABLE I: Quenchg Efflcieacy md Donor-Acceptor Distance of
,
I
I
I\ \ I
, . I
Cz-a-NDC o m m d s ~~
sample Cd-ND
R,' nm
quenching ef&iency 0.999 0.997 0.964 0.930
1.47 1.57 1.71 1.97
OR was calculated under the assumption that the Cz-n-Np compound takes an all-trans conformation.
90
n 0
180
- 90
90
n 0
-
- 180
0
€P
0
-90
180
90 O
1
Figure 5. Conformational energy map of 2-propylnaphthalene. The symbols are the same as in Figure 4. -90
- _-Rn -180 1
-90
0
90
180
61(O) Figure 3. Conformational energy map of N-propylcarbazole. Numerals on the contours are the energy values (kcal mol-') relative to the minimum energy.
cI
t
Figure 4. Structural parameters of N-propylcarbazole used for the calculation of conformational energy map.
are larger than &. The values shown in Table I suggest that a considerable fraction of the Cz-n-Np molecules take a contracted form rather than extended all-trans conformation. Detailed analysis of the conformation will be discussed later. The decay curves of c a r h l e phosphorescence of the Cz-n-Np compounds were nonexponential and strongly depended on the chain length. Analysis of these nonexponential decay curves was carried out with Dexter's equation, in which the distance between carbazole and naphthalene was estimated from conformational energy calculation. The calculations were performed by the empirical potentials described in the Experimental Section. Figure 3 shows the calculated conformational energy map of N-propylcarbazole. Figure 4 shows the structural parameters of N-propylcarbazole. The map was drawn from the potential energies calculated at intervals of 10' for 9, and d2,which were taken as 0' at the trans conformation. The contours are drawn every 1 kcal mol-' relative to the minimum. The minima of the energy are located at 90' and 270° of the qil angle when & is 0'; the propyl chain takes a trans conformation. The plane of the carbazole ring falls at right angles with the plane of the propyl group taking a trahs conformation. On the other hand, when the propyl chain takes a gauche conformation, the minima of the energy are located at the 9, angle rotated by 10' from 90'; in the case of 92 = 1 10'. +1 = 100' and in the &se of & = -1 loo,
t 0
1
2
Rlnm Figure 6. Distribution function of R for the (a) Cz-8-Np, (b) Cz-9-Np, (c) Cz-lGNp, and (d) Cz-12-Np at 136 K. The ordinates show the fraction of conformers (O.l/division) at a distance increment of 0.02 nm. e were Obtained for 2propylnaphthalene. 41 80'. The s ~ m values
Figure 5 shows the calculated conformational energy map of 2-propylnaphthalene. In the following calculation of Cz-n-Np, three rotational isomers, trans, gauche(+), and gauche(-), were considered for &, &, ...,&, and the angles and &l rotate coincident with the angles of $z and &,reapectively, taking the energy minima from the result of the calculation for N-propylcarbazole and Zpropylnaphthalene. Then,3*' amformations were generated (see Figure l), and the fraction of the ith conformation, fi, was calculated by q 1 under the assumption that the distribution of conformation obeys the Boltzmann relation, where E, is the calculated potential energy for the Ith conformation. In q 1, T = 136 K was used for MTHF glass. This temperature
fi = e x p ( - E I / K T ) / ~1 e x p ( - E , / K T )
(1)
is the freezing point of MTHF, and therefore the conformation of Cz-n-Np compounds will be fixed at this temperature. The
10118 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992
Katayama et al.
t 0
05
. m=3
T-T
1
TlMEls Figure 7. Phosphorescence decay simulation in MTHF at 77 K (a) Cz-12-Np, (b) Cz-IO-Np, (c) Cz-9-Np, and (d) Cz-8-Np. The solid lines are the calculated curves. The dots are observed points. The employed parameters are 0.117 nm for L and 1.3 X 10I2s-I for ko.
interchromophore distance, R, was calculated for all conformations: R was defined as the distance between the center of the carbazole moiety and the center of the naphthalene moiety. The distribution function of R for each Cz-n-Np compound is shown in Figure 6, in which the distribution was calculated at every 0.02-nm increment. The ensemble-averaged interchromophore distance of each Cz-n-Np compound, (R), was calculated by eq 2, where R, is the interchromophore distance of the ith conformer ( R ) = CRJ;/W;
(3)
where ko is constant, R is the donor-acceptor distance, and L is a constant called the effective average Bohr radius. The energy-transfer rate for the ith conformation is expressed by kTTl= ko exp(-2R,/L)
(4)
where R, is the donoracceptor distance of the ith conformation. Then phosphorescence decay is expressed by the s u m of the decay in the ith conformation multiplied by the fraction of the conformation I = Io exp(-t/7o)U exp(-krr,t)
1 .o
1.5
/nm
Figure 8. Chain length dependence of the rate constants of the intramolecular T-T energy transfer and of the intramolecular S-S energy transfer. The rate constants of T-T energy transfer were calculated with the obtained parameters, and the rate constants for S-S energy transfer are the values reported for dinaphthylalkanes.20
(2)
andf; is the fraction of the ith conformation given by eq 1. The distribution functions spread wider for the samples having a longer methylene chain. A considerable fraction of the molecules take a more contracted conformation. Theoretical treatment of triplet energy transfer was performed using the rate constant expressed by Dexter. In this system, the donor is carbazole and the acceptor is naphthalene for all samples. Then, Dexter's equation is expressed by
krr = ko exp(-ZR/L)
0.5
(5)
I
where T~ is the intrinsic lifetime of carbazole. This was determined to be 6.4 s from the measurement of N-ethylcarbazole in MTHF rigid solution at 77 K. This equation was used for carbazole decay simulation where the Bohr radius, L, and ko were employed as the variable parameters. The best fit parameters were 0.117 nm for L and 1.3 X 1OI2 for ko. These values are in fairly good agreement with those of the intermolecular system from carbazole to naphthalene, studied by Galley et al.:3b L = 0.107 nm and ko = 1.8 X 10" s-l. Figure 7 shows the results of this simulation. The dots indicate the observed points, and the solid lines indicate the calculated decay curves. The simulated curves are in good agreement with the experimental data. Here, the experimental data for Cz-8-Np was corrected, because strong naphthalene phosphorescence contaminated and affected the data at a later stage. The naphthalene component was eliminated from the experimental data under the assumption that naphthalene phosphorescencedecays with its intrinsic lifetime. The simulation deviates slightly at the early stage of Cz-12-Np. The reasons for the deviation are as follows. The calculation is based on the distance between two
spatial points representing the distance of the center of each chromophore. However, the real system consists of large aromatic rings, and r electrons are delocalized throughout the aromatic planes. Some of the geometrical arrangements may take partial overlapping of the aromatic rings. The rate under such an arrangement cannot be evaluated by the present calculation. Figure 8 shows a comparison of the chain length dependence of the rate constants for the present intramolecular T-T energy-transfer system and the previous system: the intramolecular T-T energy-transfer system having benzophenone and dibenz[bflampine at the chain ends in sec-BuC1 rigid solution at 77 K.7 The rate constant of T-T energy transfer is similar for both systems. The coincidence is accidental because the parameters which determine the rate constant of T-T energy transfer, L and ko,differ from one pair of donor and acceptor to another. The values for L and ko are almost the same for this system and the one previously mentioned. These values are L = 0.1 17 nm and ko = 1.3 X 10I2s-l for the current system and L = 0.1 1 nm and ko = 6 X 10l2s-I for the previous system. The values for k,,are determined from the overlap integral of the donor's emission and the acceptor's absorbance. The close agreement between the parameters is probably due to the similarity of the overlap integrals. As Figure 8 shows, the donor-acceptor distance, R,influences the rate constant of T-T energy transfer rather than the kind of donor-acceptor pair. We have analyzed the triplet energy transfer in the intramolecular D-A system with a flexible spacer on the basis of the through-space mechanism. However, in several reports the through-bond mechanism has been used for analyzing the energy transfer in the intramolecular D-A system with a rigid spacer.6J8 Which is predominant for intramolecular T-T energy-transfer system remains to be determined. Recently, Inai et al. reported an intramolecular electron-transfer system in which a donor and an acceptor are ~onnectedwith a peptide chain.19 They assumed that both mechanisms of electron transfer take place: the through-space mechanism occurs when the donor and the acceptor are close, and the through-bond mechanism occurs when the donor and the acceptor are separated. Which mechanism takes place depends on the distance of donor and acceptor, molecular structure, and molecular conformation. The situation may be the same for an intramolecular .T-Tenergy-transfer system. However, the present results indicate that the through-space mechanism is
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
