Influence of Sulfur Oxidation States on the Photophysical Properties of Naphthalimide-Based Donor–Acceptor Systems

This study explores the correlation between the photophysical properties and the oxidation state of the sulfur bridge in NI-SO_<i>n</i>-PhOMe (<i>n</i> = 0, 1, 2), which connects naphthalimide and <i>p</i>-methoxyphenyl units. The investigations were conducted in solution and in the solid state, with comparisons made to NI-PhOMe, where the two moieties are directly linked via a carbon-carbon bond. In the sulfur-bridged derivatives, aggregation-induced emission (AIE) was observed, the characteristics of which are influenced by intermolecular interactions and the nature of the medium in which insoluble aggregates form. In contrast, NI-PhOMe exhibited conventional solvatochromism typical of donor-acceptor <b>(</b>D-A) push-pull systems. A detailed analysis is presented on the influence of the oxidation state of the sulfur atom on key properties, including the D-A character induced by naphthalimide substitution, as well as the charge transfer (CT) contribution that governs its photophysical behavior.