Reaction induced supramolecular gelation with the evolution of circularly polarized luminescence

Abstract

Chemical reaction-guided supramolecular gelation has been attracting attention due to its dynamic response and adaptability. We demonstrated here that the direct reduction of the CN bond to a C–N bond by NaBH₄ caused sol-to-gel and gel-to-gel transitions of amphiphilic Schiff bases. 1-Hydroxy-1-naphthaldehyde or 2-hydroxy-1-naphthaldehyde was reacted with an amine-terminated dialkyl glutamide amphiphile (LG) to afford Schiff base 1-NLG or 2-NLG, respectively. These amphiphilic Schiff bases couldn’t self-assemble into supramolecular gels in ethanol, while the introduction of NaBH₄ led to the formation of a supramolecular gel constituted by 2-NLG via reducing the imine to a secondary amine group. It was worth noting that 2-NLG was non-fluorescent due to the occurrence of excited-state intramolecular proton transfer (ESIPT). Interestingly, as the reduction reaction proceeded, the emission of 2-NLG was enhanced and it emitted green fluorescence, and then the green fluorescence emitted by the 2-NLG gel gradually turned into a blue emission. In contrast, only an enhanced blue emission was observed in the case of the 1-NLG assembly upon reduction, instead of sol-to-gel transitions. Accompanied by the formation of a supramolecular gel, the circularly polarized luminescence (CPL) of 2-NLG was also observed, whose colour evolved from green to blue. Furthermore, when 8-hydroxy-1,3,6-pyrenetrisulfonate (HPTS) was doped into the 2-NLG system, both an enhanced emission and an amplified CPL signal of HPTS were found upon the formation of the supramolecular co-gel. This work provided an attractive strategy for fabricating dynamic adaptable soft materials with amplified CPL signals.