Dual Fluorescence and Multiple Charge Transfer Nature in Derivatives of N-Pyrrolobenzonitrile

N-Pyrrolobenzonitrile (PBN), its ester derviative PBAEE, and a more twisted model compound DPBN have been compared regarding absorption and fluorescence properties. A long wavelength absorption shoulder in DPBN has been assigned to the charge transfer (CT) state. In polar solvents, single-fluorescence bands with a strong solvatochromic effect establish a CT nature of the emission for all compounds. The long radiative lifetime (ca. 130 ns for PBN, 700 ns for DPBN in CH 2 Cl 2 ) points to a forbidden emission, and the 5-fold longer value for DPBN indicates a difference in CT nature tentatively assigned to conformations differing in the twist angle. Even in low-temperature polar solvent glasses, the single-fluorescence band of PBN is of CT nature and develops into a dual fluorescence by thermal activation. Also in alkanes, the increased fluorescence rate constants and the temperature effect on spectral structuring indicate emission from an equilibrium involving a CT state with an unstructured spectrum and a less polar locally excited state with structured emission.