The photophysical properties of the monosubstituted methylperylenes and bromoperylenes and the temperature dependence of the quantum yields of fluorescence (&) and intersystem crossing (&-) of l-bromoperylene
(1-BrPer) and 3-bromoperylene (3-BrPer) were investigated. It was found that, relative to perylene, substitution in position 2 or 3 leads to small red spectral shifts of the S, state energies E(S,), whereas substitution in position 1 results in blue spectral shifts of E&S,). While I-methylperylene is highly fluorescent (&= 0.96 in cyclohexane at room temperature), as is typical for perylene derivatives, internal conversion with a quantum yield of &=0.88f0.04 is unusually efficient in 1-BrPer. As in earlier studies, we have interpreted the experimental activation energies E,(ISC) of 1-BrPer and 3-BrPer as the energy gaps for thermally activated (S1-T2) transitions to the second excited triplet states T,. In 1-BrPer the experimental activation energy E,(IC)=550f 100 cm-' is significantly smaller than values of E,(IC) found for perylene and other perylene derivatives (1200 <E,(IC) < 5500 cm-').
The spectroscopic results of this work suggest that due to steric effects, both 1-MePer and 1-BrPer are non-planar. The observation that IC is strongly enhanced in 1-BrPer only is possibly related to the presence of higher excited singlet states or the occurrence of promoting C-Br vibrational modes.