Reversible fluorescence and Förster resonance energy transfer switching behaviours of bistable photo-switchable [c2] daisy chain rotaxanes and photo-patterning applications

Abstract

A series of bistable photo-switchable [c2] daisy chain rotaxanes containing photochromic spiropyran (SP) and triphenylamine-chalcone (TPA-C) chromophores were synthesized via a self-sorting strategy and click reactions, which could be controlled by acid–base shuttling processes to achieve the extended and contracted forms of [c2]-SP-Ext and [c2]-SP-Con, respectively. The molecular structures of these analogues were characterized and confirmed by nuclear magnetic resonance (NMR) and high resolution mass spectroscopy (HRMS). Both [c2] daisy chain rotaxanes ([c2]-SP-Ext and [c2]-SP-Con) and on-interlocked analogue (Cy-SP) could undergo reversible photo-isomerization to form the corresponding [c2]-MC-Ext, [c2]-MC-Con and Cy-MC upon UV/Vis exposure and their aggregation-induced emission enhancement (AIEE) properties were examined in semi-aqueous solutions (>60% H₂O). Besides, these [c2] daisy chain and non-interlocked analogues consisting of yellow emissive TPA-C donors and orange-red emissive merocyanine (MC) acceptors induced interesting ratiometric photoluminescence (PL) and reversible Förster resonance energy transfer (FRET) characteristics upon UV/Vis irradiation, which were investigated by PL measurements and further confirmed by theoretical studies. Accordingly, [c2] daisy chain rotaxanes ([c2]-MC-Con and [c2]-MC-Ext) exhibited better energy transfer processes than those of the non-interlocked analogue (Cy-MC) with respective FRET efficiencies of 78.2, 73.1 and 63.9%. Various responsive effects of external stimuli on the FRET-ON/OFF behaviours for [c2] daisy chain and non-interlocked analogues, including water fractions, UV/Vis exposures, temperature controls and acid/base treatments were studied in this report. Furthermore, the interesting reversible FRET ON-OFF switching behavior and fluorescence photo-patterning applications of photo-switchable [c2]-SP-Ext-based coated papers under different environmental conditions were also explored and demonstrated.