Meta-positioning effect in sterically hindered N-phenyl-pyrroles: A photophysical study

The photophysical properties of the dimethyl derivative of N-pyrrolo-4-benzonitrile (DPBN), with a change in the position of the acceptor moiety (m-DPBN), have been investigated and compared with the parent compound (p-DPBN). The values of the Stokes shift and of the excited-state dipole moment indicate that both meta- and para-DPBN possess similar excited-state properties regardless of the meta-positioning of the cyano group. The low values for the radiative rate constant suggest the presence of a strongly forbidden transition supporting the model of twisted intramolecular charge transfer states.