Photophysical properties of pyrrolobenzenes with different linking and substitution pattern: The transition between charge transfer states with large (MICT) and small (TICT) resonance interaction

Pyrrolobenzenes, with different linking and substitution patterns, 2′-(4-cyanophenyl)-methylpyrrole (MP2-BN) and 2′-(2,5-cyanophenyl)-methylpyrrole (MP2-B25CN), are investigated by steady-state and time-resolved UV–Vis spectroscopy and compared to the parent compound N-pyrrolobenzonitrile (PBN). Both the electron donor–acceptor linking sites and the strength of the electron acceptor moiety are found to influence the emission characteristics of these compounds. The large radiative rate constant of MP2-BN indicates an allowed emission due to mesomeric interaction between the donor and acceptor moieties (MICT), whereas in the case of PBN and MP2-B25CN, the reduced radiative rate constant indicates a forbidden emission from a twisted intramolecular charge transfer (TICT) state.