FEMTOSECOND TO MICROSECOND EXCITED-STATE RELAXATION OF 9-(4-(N,N-DIMETHYLAMINO)PHENYL)PHENANTHRENE AND 4-(9-PHENANTHRYL)-3,5-N,N-TETRAMETHYLANILINE

This paper discusses how the solvent-induced rapid relaxation of the i nitial delocalized excited state of 9-(4N,N-dimethylaminophenyl)phenan threne (9DPhen), obtained immediately after picosecond pulsed excitati on, can be resolved by means of femtosecond transient absorption exper iments. The results obtained for 9DPhen are compared to the results of a sterically hindered compound 4-(9-phenanthryl)-3,5-N,N-tetramethyla niline (3,5Me9DPhen) in order to get more information about the possib le conformational relaxation process suggested for these compounds. Fr om the results of the femtosecond transient absorption experiments? a possible model is proposed to characterize the kinetic behavior of the se molecules. After photoexcitation of 9DPhen and 3,5Me9DPhen, the dis tribution of higher excited states shows a fast transition within a fe mtosecond timescale to a ''hot'' charge transfer state. This state loo ses excess energy by a relaxation process (electronic and/or vibration ally and/or conformationally relaxation) on picosecond timescale. From this relaxed excited charge transfer state, fluorescence and intersys tem crossing to a triplet state originate simultaneously and in compet ition. 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 s hift of the emission spectrum of both compounds suggests a larger exci ted-state dipole moment for 3,5Me9DPhen compared to 9DPhen. The lower values of the radiative rate constant [k(f)] and the longer decay time s of 3,5Me9DPhen correlate with a less allowed radiative transition co mpared to that of 9DPhen. It is suggested that for 3,5Me9DPhen, the em issive state mixes to a smaller extent with a state with a strongly al lowed transition and/or that the average angle between the phenyl and phenanthrene moieties of the excited state is larger (farther away fro m 0) than in the unsubstituted molecule, leading to a less allowed tra nsition and a smaller value of the rate constant of fluorescence.