| Abstract
| - This paper discusses how the solvent-induced rapid relaxation ofthe initial delocalized excited state of 9-(4-N,N-dimethylaminophenyl)phenanthrene(9DPhen), obtained immediately after picosecond pulsedexcitation,can be resolved by means of femtosecond transient absorptionexperiments. The results obtained for 9DPhenare compared to the results of a sterically hindered compound4-(9-phenanthryl)-3,5-N,N-tetramethylaniline(3,5Me9DPhen) in order to get more information about the possibleconformational relaxation process suggestedfor these compounds. From the results of the femtosecond transientabsorption experiments, a possible modelis proposed to characterize the kinetic behavior of these molecules.After photoexcitation of 9DPhen and3,5Me9DPhen, the distribution of higher excited states shows afast transition within a femtosecond timescaleto a “hot” charge transfer state. This state looses excessenergy by a relaxation process (electronic and/orvibrationally and/or conformationally relaxation) on picosecondtimescale. From this relaxed excited chargetransfer state, fluorescence and intersystem crossing to a tripletstate originate simultaneously and in competition.From the comparison of the steady state absorption spectrum of9DPhen and 3,5Me9DPhen, as well as thetransient absorption spectra of the triplet state, one can distinguishthe quite different nature of the ground-and the triplet state in both compounds. The bathochromic shift ofthe emission spectrum of both compoundssuggests a larger excited-state dipole moment for 3,5Me9DPhencompared to 9DPhen. The lower values ofthe radiative rate constant 〈kf〉 and thelonger decay times of 3,5Me9DPhen correlate with a lessallowedradiative transition compared to that of 9DPhen. It is suggestedthat for 3,5Me9DPhen, the emissive statemixes to a smaller extent with a state with a strongly allowedtransition and/or that the average angle betweenthe phenyl and phenanthrene moieties of the excited state is larger(farther away from 0) than in the unsubstitutedmolecule, leading to a less allowed transition and a smaller value ofthe rate constant of fluorescence.
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