Transient absorption spectra of 9,9'-bianthryl (BA) in heptane, in acetonitrile, and in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (bmimTf(2)N) are observed with a nanosecond time-resolved near-IR absorption spectrometer for the wavenumber range of 4500-10,500 cm(-1) (2200-950 nm). In nonpolar heptane solution, a broad absorption band is observed at 6700 cm(-1) (1500 nm), in addition to a strong absorption band of the locally excited (LE) state centered at 9800 cm(-1) (1020 nm). The broad band is assigned to a partial charge transfer (PCT) band. The decay time constants of the PCT band and the LE band are both (13 +/- 1) ns. The agreement of the two decay constants strongly suggests that the PCT state is in equilibrium with the LE state in heptane. In acetonitrile, an absorption band of the charge transfer (CT) state is observed at 8000 cm(-1) (1250 nm). This band decays in (41 +/- 2) ns. In bmimTf(2)N, the CT band appears at 8500 cm(-1) (1180 nm) and decays in (34 +/- 1) ns. The difference in peak position for the CT bands in acetonitrile and in bmimTf(2)N, and the PCT bands in heptane, is explained well by the model based on the charge resonance between the two equivalent electronic structures of the CT state.

Nanosecond time-resolved infrared spectra of photoexcited 9,9¤-bianthryl (BA) and its deuterated derivative ) were recorded in acetonitrile-d 3 and cyclohexane-d 12 . The chargetransfer (CT) bands observed at the 12001400 cm ¹1 region are contributed mostly by the intra-and inter-ring CC stretch vibrations. The intrinsic frequency of the inter-ring CC stretch falls within the 12001400 cm ¹1 region, which is consistent more with a twisted structure than with a fully conjugated planar structure of the CT state.Photoinduced charge transfer (CT) is a fundamental chemical reaction. The intramolecular CT state formation in 9,9¤-bianthryl (BA) has attracted attention since its proposal in 1968. 1A large number of spectroscopic experiments on the CT reaction in BA have been performed in the UVvisible region in clusters, 24 in solutions, 512 or in ionic liquids. 13,14 When BA is irradiated by UV light, it is excited to the locally excited (LE) state in which one of the anthracene moieties is electronically excited. The CT reaction proceeds between the two anthracene moieties after the UV irradiation in polar solvents, while it is widely accepted that the complete CT reaction does not proceed in nonpolar solvents. Our previous time-resolved near-IR absorption experiments 1518 have revealed that an intermediate, "partial CT (PCT)" state, is formed in the CT reaction. The PCT state has a transition dipole moment parallel to the inter-ring CC bond as the CT state does. 17 The PCT state is also observed in nonpolar solvents, where it is in equilibrium with the LE state. 17,18 Structural information on BA in the CT and PCT states is required for elucidating the detailed mechanism of the CT reaction. Internal rotation is proposed as one of the key factors that determine the reaction kinetics. The internal rotation process around the central CC bond is presumable, because a nonperpendicular dihedral angle is favorable for effective inter-ring interaction. For examining the molecular structure of BA in the CT and PCT states, time-resolved IR and Raman spectroscopy is an effective method, because the internal rotation will affect the position and the strength of the inter-ring CC stretch band. To the best of our knowledge, however, there has been no study reported on time-resolved vibrational spectroscopy of the excited state of BA in solution, except for a time-resolved CARS/CSRS experiment. 19 The CSRS signals of BA in the CT state were all assigned to ring vibrations, whereas no vibrational band predominated by the inter-ring CC stretch was reported. In this letter, nanosecond time-resolved IR spectra of BA and its deuterated derivative, BA-d 18 , are reported. We assign the observed spectra to the CT state of BA and discuss the origin of the IR bands.Transient absorption spectra were measured with a nanosecond time-resolved IR absorption spectrometer. 20,21 Briefly, sample solutions were photoexcited by the third harmonic of a pulsed Q-switched Nd:YAG laser (IB LASER DiNY pQ355, 355 nm, 250 Hz, 40¯J/pulse, 7 ns) or by the se...