4492
J. Phys. Chem. 1992,96,4492-4496
Substituent Effects on Carbanion Photophysics. 9-Arylfluorenyl Anions Laren M. Tolbert,*vt Susan M. Nesselroth,t Thomas L. Netzel,*v*Narciso Raya,’ and Michael Stapletons School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, and Department of Chemistry, Georgia State University, Atlanta, Georgia 30303 (Received: December 16, 1991)
In an attempt to better understand the photophysics of resonance-stabilized carbanions, substituent effects on the excited-state properties of 9-arylfluorenyl anions have been examined. These properties include excited-state lifetimes, quenching rate constants, fluorescence quantum yields and rate constants, and radiationless decay rate constants. While the resultant data reveal a parallel between excited-state lifetime and the energy of the T * orbital associated with the substituents obtained by electron transmission spectroscopy, time-resolved picosecond absorption spectroscopy fails to indicate the presence of an intramolecular charge-transfer state. The excited-state lifetimes can be rationalized by an appeal to the energy gap law, relating the nonradiative decay rate to the energy difference between the first excited state and the ground state.
Substantial progress has been made toward understanding the excited-state structure and photophysics of neutral hydrocarbons. The structure and photophysics of resonancestabilized carbanions is, however, further complicated by ion-pairing effects,’ photooxidation,*and photoisomerization.’ Electron-transfer processes initiated by irradiation may include direct photoejection, autodetachment, intramolecular charge-transfer, as well as more conventional collisional electron transfer! In the presence of these competing processes, a description of the excited state of a single carbanion, particularly of the way in which charge is distributed, remains problematic. For this reason, we elected to investigate the intrinsic decay rates for a photoexcited carbanion unencumbered by counterion effects, which might provide some insight into the nature of the lowest energy excited state. We chose 9-arylfluorenyl anions because of the well-behaved nature of the parent hydrocarbon anion and the ease with which substitution of the 9-aryl group would perturb the excited state so as to probe the excited-state charge distribution. In fact, we discovered that in dimethyl sulfoxide, 9-arylfluorenyl anions exhibit strong fluorescence with monoexponential decay rates that are in accord with the energy gap law, and we propose an excited-state model to account for this behavior. Fluorenyl anions, in general, exhibit a strong counterion-dependent fluorescence in ethereal solvents.s This fluorescence is in contrast to the weak to nonexistent fluorescence of the structurally related triarylmethyl anions.6 Prompted by Bordwell’s pioneering work on ground-state carbanions,’ in which hydrocarbon acidities were cation independent, we carried out measurements on the excited-state properties of 9-arylfluorenyl anions, including excited-state lifetimes, quenching rate constants, fluorescence quantum yields and rate constants, and radiationless decay constants. We reasoned that substituents on the aryl ring should perturb the electron distribution in the excited state and affect the photophysics in, perhaps, predictable ways. In particular, we hoped to determine if the lowest excited state of 9-phenylfluorenyl anion involved a fluorene-centered qpPnexcited state or a fluorene to phenyl T ~ - T *excited ~ ~ state-more precisely, a “twisted intramolecular charge transfer” (TICT)state.
Results Steadystate and Time-Resolved Emission Spectroscopy. 9Phenylfluorenes (9ArF) substituted in the meta or para position by chloro, methyl, methoxy, or tert-butyl groups were synthesized by literature procedures. Anions were generated in Me2S0 by deprotonation of the 9-arylfluorenes with potassium methylsulfinylmethide (dimsyl) in thoroughly deoxygenated solutions under an argon atmosphere.’ Steady-state fluorescence spectra were obtained and, in all cases, showed emission spectra indeGeorgia Institute of Technology. *Georgia State University. ‘Department of Chemistry, University of Kentucky, Lexington, KY 40506.
0022-3654/92/2096-4492$03.00/0
pendent of excitation wavelength. Both excitation and emission spectra showed similar vibrational structure. The low-energy portion of the excitation spectra consisted of three absorptions centered at ca. 490 nm with a 1200-cm-’ vibrational spacing, and the emission spectra also showed three peaks, although the lowest energy band appeared as a shoulder on the strongest emission peak centered at ca. 590 nm (see Figure 1). In contrast, the 944bipheny1)fluorenyl anion (9BF) absorption spectrum showed weak emission with a maximum at 620 nm and no apparent fine structure. Addition of biphenyl to 9-phenylfluorenyl anion resulted in fluorescence quenching at the rates shown in Table I. The emission of the substituted 9ArF compounds was quenched by a number of hydrocarbon acceptors, including anthracene, chrysene, tetracene, and higher aromatics. However, for more quantitative studies the acceptor was limited to biphenyl. Quenching rate constants were determined either under steadystate conditions using Stern-Volmer techniques or directly by time-correlated singlephoton counting. Singlet decay rates (kdt) were determined by single photon counting. The data are listed in Table I. Time-ResolvedAbsorption Spectroscopy. The picosecond excited-state absorption spectrum of 9-phenylfluorenyl anion was characterized by a strong bleach of the anion and a featureless absorption at longer wavelengths (see Figure 2). The absorption measured at 455 nm decayed over the time window of the experiment with a time constant equal, within experimental error, to the lifetime T (= l/kdt) determined by single-photon counting, Le., 44 ns (seeFigure 3). The excited-state absorption spectrum of 9-(4-biphenyl)fluorenylanion (9BF) was quite similar, again showing a strong absorption at ca. 410 nm, a strong bleach of the ground-state absorption at 475 nm, and a broad featureless a b sorption at longer wavelengths (see Figure 4). The bleach at 475 nm decayed with a lifetime of 90 ps. Calculations. Vertical vs TICT State. According to AM1 calculations, the phenyl group in 9-phenylfluorenyl anion is twisted 37O out of planarity, presumably because of severe steric interactions between the 0-phenyl protons and protons at C-1 and (2-8. Since photoexcitation formally removes an electron from an orbital centered at C-9, the bond order between C-9 and the ipso phenyl atom should be severely reduced, producing a dihedral bond angle nearer to 90°. Indeed, excited-state AM1 calculations yield ( I ) Hogen-Esch, T. E.; Plodinec, J. J. Am. Chem. Soc. 1978,100,7633. (2) Fox, M.A,; Owen, R. C. J. Am. Chem. Soc. 1980, 102, 6559. (3) Parkes, H. M.;Young,R. N. J . Chem. Soc.,Perkin Trans. 2 1978,249. (4) (a) Tolbert, L. M.Org. Photochem. 1983,6, 177. (b) Tolbert, L.M. Acc. Chem. Res. 1986,19,268. (c) Tolbert, L. M . Comp. Carbanion Chem. Purr C; Buncel, E.; Ed.; Elsevier: Amsterdam, 1987; pp 223-270. (d) Fox, M. A. Chem. Reo. 1979, 79, 253. (5) Hogen-Esch, T. E.; Plodinec, M.J. J. Phys. Chem. 1976,80, 1090. (6) Chandross, E. A. Trans. N.Y. Acad. Sci. 1969, 31, 571. (7) Bordwell, F. G.;Imes, R. H.; Steiner, E. C. J. Am. Chem. Soc. 1967, 89, 3905.
0 1992 American Chemical Society
The Journal of Physical Chemistry, Vo1. 96, No. 11, 1992 4493
Substituent Effects on Carbanion Photophysics TABLE I: Excited-State Properties of 9-Arylfluorenyl Anions substituent
kd,:b
p-OMe P-I-BU p-Me m-Me m-C1 p-Cl H (9PF) m-OMe p-Ph (9BF) a
107 s-1
3.19 2.58 2.58 2.38 2.54 2.34 2.21 2.20 1100'
@f
k,,, 107 s-I
0.18 0.24 0.12 0.22
3.09 1.97 2.26 1.86
0.22 0.36 0.22
