Pure space steric hindrance effect on intramolecular proton transfer in excited state and photophysics of conjugated new dyes under near-IR laser irradiation

This study presented a variety of new extended conjugated dyes bearing internal proton transfer segments, which were characterized with different extent of space steric hindrance. Besides, the dyes lack of proton transfer moieties were synthesized as the references. 1H NMR spectra and X-ray single crystal showed presence of internal hydrogen bond in the target dyes. The study suggested that the target dyes with small or moderate space steric hindrance exhibited internal proton transfer in excited state, and thus yielded dual emission bands under one-photon and near-infrared two-photon irradiation respectively, while the dyes with heavy steric hindrance displayed quite weaker or even no intramolecular proton transfer emission in excited state. Molecular modeling to understand inhibition effect of space steric hindrance on internal proton transfer in excited state for the target dyes was further performed.