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

Proton effects on an intramolecular donor−acceptor molecular system 4′,5′-diaza-9′-(4,5-dialkylthio-1,3-dithiol-2-ylidene)fluorenes (methylthio = L¹ (1), ethylthio = L² (2)) have been experimentally and theoretically studied. Structures of L² (2) and three protonated salts, [L²·H]CF₃SO₃ (3), [L¹·H]CF₃SO₃ (4), and [L¹·H]ClO₄ (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.