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.