Revisiting Organic Charge-transfer Cocrystals for Wide-range Tunable, Ambient Phosphorescence

Simple and efficient designs that enable a wide range of phosphorescence emission in organic materials have ignited scientific interest across diverse fields. One particularly promising approach is the cocrystallization strategy, where organic cocrystals are ingeniously formed through relatively weaker and dynamic non-covalent interactions. In our present study, we push the boundaries further by extending this cocrystal strategy to incorporate donor-acceptor components, stabilized by various halogen bonding interactions. This non-covalent complexation triggers ambient, charge-transfer phosphorescence (CT), which can be precisely tuned across a broad spectrum by a modular selection of components with distinct electronic characteristics. At the core of our investigation lies the electron-deficient phosphor, pyromellitic diimide, which, upon complexation with different donors based on their electron-donating strength, manifests a striking array of phosphorescence emission from CT triplet states, spanning from green to yellow to reddish orange accompanied by noteworthy quantum yields. Through a systematic exploration of the electronic properties using spectroscopic studies and molecular organization through single-crystal X-ray diffraction, we decisively establish the molecular origin of the observed phosphorescence. Notably, our work presents, for the first time, an elegant demonstration of tunable CT phosphorescence emission in intermolecular donor-acceptor systems, highlighting their immense significance in the quest for efficient organic phosphors.