Radiative Depopulation of the Excited Intramolecular Charge-Transfer State of 9-(4-(N,N-Dimethylamino)phenyl)phenanthrene

Abstract

Intramolecular photoinduced electron transfer in 9-(p-N,N-dimethylanilino)phenanthrene (9DPhen) has been studied in solution. The solvent dependence of the fluorescence spectra of 9DPhen indicates that the emission occurs from a highly polar excited state. The quantum yield of fluorescence (Φf) of 9DPhen is quite high and increases with increasing solvent polarity. The radiative rate constant (kf), however, shows a maximum for solvents of intermediate polarity, e.g., in butyl acetate a value of 2.3 × 108 s-1 is attained. These results are difficult to explain within the “TICT” (twisted intramolecular charge transfer) model, which predicts a strongly forbidden fluorescence caused by a minimum overlap of the orbitals involved in the transition. The above-mentioned trend as a function of the solvent polarity is observed in particular donor−acceptor substituted arenes where the Lb state of the corresponding arenes is lower in energy than the La state. The quantum chemical calculations actually could explain this behavior on the basis of an ICT state which interacts with the lower lying 1La and 1Lb states of the acceptor. The quantum mechanical mixing of states can occur by two pathways, namely orbital mixing and mixing of configurations, and is modified by geometrical changes and by solvent polarity. The single exponential fluorescence decay, obtained with time-correlated single-photon-timing, suggests emission from an excited charge-transfer state, resulting from a solvent-induced rapid relaxation of the initial delocalized excited state of 9DPhen, obtained immediately after picosecond pulsed excitation. Picosecond transient absorption spectra in acetonitrile show a rapid decay within a few picoseconds 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 shows a significant polar character. As a reference compound, 9-phenylphenanthrene (9PhPhen) was also examined by means of stationary and time-resolved fluorescence measurements as well as transient absorption experiments.