Stoichiometry ControlsPacking and Luminescence: TwoCocrystals of 2-Hydroxymethylbenzimidazole with TCNB

Two charge-transfer cocrystals of 2-hydroxymethylbenzimidazole (2HMEBI) and 1,2,4,5-tetracyanobenzene (TCNB), with stoichiometric ratios of 1:1 (2HMETC-1) and 2:1 (2HMETC-2), were successfully prepared via a solvent-induced method. Single-crystal X-ray diffraction revealed distinct hydrogen-bonding networks and π–π stacking motifs: 2HMETC-1 features a rigid alternating donor–acceptor layered structure held together by hydrogen bonds, whereas 2HMETC-2 exhibits a continuous hydrogen-bonded layer of 2HMEBI with TCNB molecules embedded as guests. Despite both cocrystals displaying similar steady-state emission peaks at approximately 497 nm, their fluorescence lifetimes differ dramatically─39.34 ns for 2HMETC-1 versus 0.72 ns for 2HMETC-2, a difference of about 55-fold. Combined Hirshfeld surface analysis, thermogravimetric-infrared analysis, solid-state UV–vis absorption, time-resolved fluorescence, and quantum yield measurements revealed the origin of this extreme difference: the rigid, preorganized donor–acceptor packing in 2HMETC-1 effectively suppresses nonradiative decay (with a nonradiative rate constant knr = 2.31 × 107 s–1), whereas the lack of rigid anchoring in 2HMETC-2 leads to a dominant nonradiative channel (knr = 1.36 × 109 s–1). Furthermore, 2HMETC-1 exhibits reversible fluorescence switching upon exposure to acid–base vapors. This work elucidates how stoichiometry governs the hydrogen-bonding network and thereby controls donor–acceptor packing rigidity and luminescence dynamics, providing a new strategy for the rational design of organic optoelectronic materials with tunable fluorescence lifetimes through crystal engineering.