A peculiar dependence of intersystem crossing of p-nitro-2,5-distyrylfuran on the dielectric properties of the solvent

The excited state behaviour of EE-2-(4'-nitrostyryl),5-styryl-furan (NSF) has been studied with different stationary and pulsed spectrometric techniques in solvents of different polarity and polarizability. The interpretation of previous results on the fluorescence and intersystem crossing of NSF Phys. Chem. Chem. Phys., 2011, 13, 4519] was found to be complicated by an uncommon effect of solvent polarity on the competitive relaxation pathways. To answer the open questions, the photobehaviour was revisited in solvents with a restricted range of the dielectric constant, also under conditions of constant polarizability, and in a large temperature range. The results thus obtained, supported by parallel quantum-mechanical calculations on the singlet/triplet properties, allowed a reasonable interpretation of the photobehaviour to be reached. This includes the role of an activated photoisomerization above room temperatures and the play of small changes of dielectric properties of the solvent in favouring ICT, thus affecting the efficiency of the ISC process, where an "inverse" energy gap trend was found to be operative.