Excited-state relaxation of bridged and unbridged anilino-pyridinium dyes

Several anilino-pyridinium-type hemicyanines including flexible and bridged model compounds are compared using experimental and quantum chemical methods. The flexible compounds show broadened fluorescence band and exhibit strong fluorescence quenching connected with a barrierless viscosity-controlled transition from the planar to the perpendicular geometry. The latter is connected with a small S1-S0 and a vanishing S1-T1 energy gap, with charge localization and with forbidden emissive character.