PHOTOPHYSICAL PROPERTIES OF TETRACENE DERIVATIVES IN SOLUTION .2. HALOGENATED TETRACENE DERIVATIVES

Photophysical properties of the halogenated tetracene derivatives 5-chloro- (MCT), 5,11-dichloro- (DCT), 5,6,11,12-tetrachloro-(TCT), 5-bromo- (MBT), and an isomeric mixture of 5,11- and 5,12-dibromotetracene (DBT) were studied in toluene solution. The influences of the internal heavy-atom effect and of the close proximity of substituents in TCT were investigated. For the molecules investigated internal conversion (IC) with a quantum yield of Q(IC) = 0.4-0.5 at room temperature was found to be an important pathway for the deactivation from the S1 state. From the temperature dependence of the quantum yields of fluorescence (Q(F)) and intersystem crossing (Q(ISC)) the experimental activation energies E(a) for the rate constants k(ISC) of (S1-T2) ISC were obtained according to an Arrhenius-type relationship. Approximate Arrhenius parameters were also determined for k(IC). In a combined evaluation of Q(F) and Q(ISC) with the measured triplet lifetime tau-T, the effect of the heavy-atom perturbation in the molecules on Q(ISC) and Q(F) can be accounted for. A consistent description of the photophysical processes is presented that allows the determination of E(a) for the halotetracenes investigated from the results of room-temperature measurements of Q(ISC), Q(F), and tau-T.