Unexpected fluorescence from polymers containing dithio/amino-succinimides

Abstract

Fluorescent polymers without conventional fluorescent units are attracting extensive interest because of their excellent biocompatibility. Succinimides are generally known as common fluorescence quenchers of proteins due to some degree of electron transfer between the excited state and the quencher. In this work, we observed unexpected fluorescence from succinimide-containing polymers. We synthesized a series of succinimides and found that 2,3-dithio-succinimide and 2-amino-succinimide were fluorescent, presenting solvent-dependent emission. This study is the first to report that nonfluorescent succinimides become fluorescent only by thiol and amine group substitutions, without employing any fluorescent units. The unusual fluorescence of dithio/amino-succinimides resulted from the spatial separation of HOMO and LUMO based on density functional theory calculations. Compared with monomers, polymers containing 2-amino-succinimide exhibited substantial fluorescence enhancement (maximum of ∼200-fold) because of the isolation effect, in which nonradiative decay among succinimide fluorophores was significantly suppressed.