RADIATIVE DEPOPULATION OF THE EXCITED INTRAMOLECULAR CHARGE-TRANSFER STATE OF 9-(4-(N,N-DIMETHYLAMINO)PHENYL)PHENANTHRENE

Intramolecular photoinduced electron transfer in 9-(p-N,N-dimethylanil ino)phenanthrene (9DPhen) has been studied in solution. The solvent de pendence of the fluorescence spectra of 9DPhen indicates that the emis sion occurs from a highly polar excited state. The quantum yield of fl uorescence (Phi(f)) of 9DPhen is quite high and increases with increas ing solvent polarity. The radiative rate constant (k(f)), however, sho ws a maximum for solvents of intermediate polarity, e.g., in butyl ace tate a value of 2.3 x 10(8) s(-1) is attained. These results are diffi cult to explain within the ''TICT'' (twisted intramolecular charge tra nsfer) model, which predicts a strongly forbidden fluorescence caused by a minimum overlap of the orbitals involved in the transition. The a bove-mentioned trend as a function of the solvent polarity is observed in particular donor-acceptor substituted arenes where the L(b) State of the corresponding arenes is lower in energy than the L(a) state. Th e quantum chemical calculations actually could explain this behavior o n the basis of an ICT state which interacts with the lower lying (1)L( a) and (1)L(b) states of the acceptor. The quantum mechanical mixing o f states can occur by two pathways, namely orbital mixing and mixing o f configurations, and is modified by geometrical changes and by solven t polarity. The single exponential fluorescence decay, obtained with t ime-correlated single-photon-timing, suggests emission from an excited charge-transfer state, resulting from a solvent-induced rapid relaxat ion of the initial delocalized excited state of 9DPhen, obtained immed iately after picosecond pulsed excitation. Picosecond transient absorp tion spectra in acetonitrile show a rapid decay within a few picosecon ds from a less polar but delocalized excited state toward a more polar ICT state. Even the triplet state of 9DPhen in isopentane at 77 K sho ws a significant polar character. As a reference compound, 9-phenylphe nanthrene (9PhPhen) was also examined by means of stationary and time- resolved fluorescence measurements as well as transient absorption exp eriments.