Molecular barrier-assisted multi-step FRET enabling full-color-emissive cocrystals based on ACQ chromophores

Realizing full-color emission from polycyclic aromatic hydrocarbon (PAH) chromophores that encounter severe aggregation-caused quenching (ACQ) is highly challenging. Herein, molecular barrier-assisted multi-step Förster resonance energy transfer (FRET) achieved via a facile grinding method was firstly applied to achieve full-color-emissive cocrystals based on ACQ chromophores. Phenanthrene (Phe), bis(phenylethynyl)anthracene (BPA) and bis(phenylethynyl)naphthacene (BPN) were selected as representative ACQ chromophores, and weakly emissive octafluoronaphthalene (OFN) acted as the "molecular barrier". The solid state Phe, BPA and BPN emit weak blue, orange, and dark (100% quenched) fluorescence, respectively. Thanks to cocrystallization with OFN, the solid-state photoluminescence quantum yield (PLQY) of Phe can be increased by 82%, and the green emission of BPA and the red fluorescence of BPN in their single-molecule (solution) states were successfully unlocked in the solid state. Based on these results, we were able to achieve full-color emission, including dark blue, blue, cyan, green, yellow, orange-yellow, orange, and red, via multi-step FRET from Phe-OFN to BPA-OFN and then to BPN-OFN. The mechanisms of the "molecular barrier" strategy and multi-step FRET were elucidated via analyzing the single-crystal structures and steady-state spectra. This contribution provides an efficient strategy for achieving full-color emission from ACQ chromophores and will create more opportunities for fundamental research and exploration of their application.