Unprecedented light induced aggregation of cationic 1,4,5,8-naphthalenediimide amphiphiles

Abstract

Naphthalenediimide amphiphiles (NDI-as) with quaternary ammonium groups (DC4, DaP, and DaO) display unprecedented UV light-induced aggregation in solutions of water, acetonitrile and THF. Light-induced aggregation brings significant spectral modification by which unusual absorption band distribution is assigned to H-type aggregates. Such aggregation can be reversibly broken by heating and reassembled by photoirradiation. The irradiation/heating cycles that provoke assembly/disassembly process can be confirmed by ¹H-NMR spectroscopy. The possibility of a chemically promoted aggregation by or radical anion either anion–π interaction may be ruled out by UV-Vis, EPR and electrochemistry measurements. These photoinduced aggregates can sensitize dissolved O₂ to produce singlet oxygen, a surprising observation when it comes to sensitization efficiency for dyes as aggregation usually decreases sensitization yield. Laser-induced luminescence measurements corroborate the existence of monomeric and aggregate forms, as well as confirm triplet states as phosphorescence was detected, being particularly intense in concentrated THF solutions.