N-Aryl substituents have the influence on 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 of various electronic character was investigated systematically. The molecular structure of these compounds was designed so that their solubility enabled us to study absorption and emission in a broad range of solvents. The presence of N-4-nitrophenyl substituents is responsible for 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 point out the existence of a barrier between au (bright) and ag (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 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. Conversely, moderate to weak electron-donating groups favor strong LE emission.