Electronic relaxation processes of anthracene derivatives
The fluorescence lifetimes observed for 9,10-dichloroanthracene (DCA) and 9,10-dicyanoanthracene (DCNA) at their electronic origins are regarded as the natural radiative lifetimes. The nonradiative transitions, intersystem crossing (ISC) from the S1 to the second triplet T2 electronic state, are not available for these molecules. The S1 levels of these molecules are considered to be lower than the levels of T2 which has the most effective interaction with the S1 state.
On the other hand, in the electronic relaxation processes of 9,10-diphenylanthracene (DPA), the ISC from any vibronic levels of the S1 state to the T2 state can occur due to the higher energy level of S1 relative to T2, so that the fluorescence lifetimes are significantly shorter (≤ 10.2 ns) than the radiative lifetime.
