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-SOn-PhOMe (n = 0, 1, 2), which connects naphthalimide and p-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 (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.