N-Aryl substituents have an influence on the photophysics of tetraaryl-pyrrolo[3,2-b]pyrroles

Abstract

The photophysics of two series of 1,4-dihydro-tetraaryl-pyrrolo[3,2-b]pyrroles possessing N-aryl substituents with various electronic characteristics was investigated systematically. The molecular structure of these compounds was designed so that their solubility enabled us to study their absorption and emission in a broad range of solvents. The presence of N-4-nitrophenyl substituents is responsible for a weak charge-transfer absorption band and shifts the emission band hypsochromically. At the same time, their presence quenches fluorescence; although if electron-withdrawing substituents are present at positions 2 and 5, this effect is reduced by an order of magnitude. In the case of less electron-withdrawing N-4-cyanophenyl and N-3-cyanophenyl groups, strong emission is present only if the electron-withdrawing groups are located at positions 2 and 5. The combined experimental and computational study points out the existence of a barrier between a_u (bright) and a_g (dark) CT states, the height of which is the key factor governing the fate of these molecules in the excited state. Weaker electronic communication at positions 1 and 4 of the DHPP core is responsible for strong charge separation. Polar solvents favor the formation of transient dipole moments due to excited-state symmetry-breaking, which amplifies the nonradiative deactivation of nitro-TAPPs. A large increase in fluorescence intensity at 77 K suggests that internal conversion is a key channel for non-radiative electronic relaxation. Conversely, moderate to weak electron-donating groups favor strong LE emission.