Surface coating induced room-temperature phosphorescence in flexible organic single crystals

Materials exhibiting room temperature phosphorescence (RTP) are in high demand for signage, information encryption, sensing, and biological imaging. Due to weak spin-orbit coupling and other non-radiative processes that effectively quench the triplet excited states, RTP is sparsely observed in organic materials. Although the incorporation of a heavy atom through covalent or non-covalent modification circumvents these drawbacks, heavy-atom-containing materials are undesirable because of their deleterious side effects. Here, we designed and synthesized a new naphthalidenimine-boron complex as a coating material for the single crystals of 4,4′-dimethoxybenzophenone. The coated surface was observed to exhibit yellowish-green phosphorescence with ms lifetimes at ambient conditions through Förster resonance energy transfer (FRET). Importantly, the mechanical flexibility of the single crystals was observed to be retained after coating. The fluorescence-phosphorescence dual emission was utilised for colour-tunable optical waveguiding and anti-counterfeiting applications. As organic single crystals that can sustain mechanical deformations are emerging as the next-generation materials for electronic device fabrication, the flexible RTP organic crystals showing colour-tuneable optical waveguiding could be omnipotent in electronics. A simple and facile surface coating protocol imparts visual room-temperature phosphorescence to flexible organic single crystals, and allows their application in colour-tunable optical waveguiding and anti-counterfeiting.