RELAXATION PATHWAYS IN PHOTOEXCITED ELECTRON-RICH STILBENES (D-D STILBENES) AS COMPARED TO D-A STILBENES

Spectra and shifts, fluorescence quantum yields and lifetimes of 4,4'-dimethylaminostilbene (DDS) and the bridged derivatives DDS-B15 and DDS-B24 were measured as a function of solvent polarity and temperature. The behavior of DDS and DDS-B15 is interpreted in terms of the intramolecular rotational relaxation around the single bond connecting the dialkylanilino group to the ethylene group (bond 2). The high quantum yield of fluorescence of DDS derivatives is discussed and the activation energy between the emissive state (E*) and the perpendicular double bond state (P*) for DDS-B24 in diethyl ether and in ethanol is estimated. CNDO/S calculations have been applied in an attempt to obtain a closer understanding of the multidimensional photochemistry of DDS. The lowest singlet excited state presenting a large ICT character has been found to be the S4 State in the ps phase for a perpendicularly twisted conformation of the dimethylanilino group (TICT state) but according to the solvation energy it can already be the S1 state in moderately polar solvents.