Proton Effects on Diazafluorene Derivatives with Sulfur-Rich Substituents, a Structural, Spectroscopic and Theoretical Study

Abstract

Proton effects on an intramolecular donor-acceptor molecular system 4',5'-diaza-9'-(4,5-dialkylthio-1,3-dithiol-2-ylidene)fluorenes (methylthio = L(1) (1), ethylthio = L(2) (2)) have been experimentally and theoretically studied. Structures of L(2) (2) and three protonated salts, [L(2) x H]CF(3)SO(3) (3), [L(1) x H]CF(3)SO(3) (4), and [L(1) x H]ClO(4) (5), were characterized by single-crystal X-ray determination. In all these salts the cations are found to be monoprotonated even in an excessive amount of acid. The protonated imine group forms strong N-H...O or N-H...N hydrogen bonds, which enhance the interaction between cations and anions and play an important role in the molecular packing. The measurement of UV-vis spectra indicates a two-step red shift of the absorption bands, which corresponds to the two-step protonation of L in solution. The intramolecular charge-transfer (ICT) property is enhanced when the compounds are protonated. Compound L is also a proton sensible fluorescent system with two-step remarkable changes in band energy and intensity. The study on the effect of protonation is further complemented by theoretical calculation with the DFT/TDDFT method.