Femto- to Microsecond Excited State Relaxation of 9-(4-(N,N-Dimethylamino)phenyl)phenanthrene and 4-(9-Phenanthryl)-3,5-N,N-tetramethylaniline

Abstract

This paper discusses how the solvent-induced rapid relaxation of the initial delocalized excited state of 9-(4-N,N-dimethylaminophenyl)phenanthrene (9DPhen), obtained immediately after picosecond pulsed excitation, can be resolved by means of femtosecond transient absorption experiments. The results obtained for 9DPhen are compared to the results of a sterically hindered compound 4-(9-phenanthryl)-3,5-N,N-tetramethylaniline (3,5Me9DPhen) in order to get more information about the possible conformational relaxation process suggested for these compounds. From the results of the femtosecond transient absorption experiments, a possible model is proposed to characterize the kinetic behavior of these molecules. After photoexcitation of 9DPhen and 3,5Me9DPhen, the distribution of higher excited states shows a fast transition within a femtosecond timescale to a “hot” charge transfer state. This state looses excess energy by a relaxation process (electronic and/or vibrationally and/or conformationally relaxation) on picosecond timescale. From this relaxed excited charge transfer state, fluorescence and intersystem crossing to a triplet state originate simultaneously and in competition. From the comparison of the steady state absorption spectrum of 9DPhen and 3,5Me9DPhen, as well as the transient absorption spectra of the triplet state, one can distinguish the quite different nature of the ground- and the triplet state in both compounds. The bathochromic shift of the emission spectrum of both compounds suggests a larger excited-state dipole moment for 3,5Me9DPhen compared to 9DPhen. The lower values of the radiative rate constant 〈kf〉 and the longer decay times of 3,5Me9DPhen correlate with a less allowed radiative transition compared to that of 9DPhen. It is suggested that for 3,5Me9DPhen, the emissive state mixes to a smaller extent with a state with a strongly allowed transition and/or that the average angle between the phenyl and phenanthrene moieties of the excited state is larger (farther away from 0) than in the unsubstituted molecule, leading to a less allowed transition and a smaller value of the rate constant of fluorescence.