Hydrogen Bonding-Induced Morphology Dependence of Long-Lived Organic Room-Temperature Phosphorescence: A Computational Study

Organic room-temperature phosphorescence (RTP) is generally only exhibited in aggregate with strong dependence on morphology, which is highly sensitive to the intermolecular hydrogen bonding interaction. Here, 4,4'-bis(9H-carbazol-9-yl)methanone (Cz2BP), emitting RTP in a cocrystal consisting of chloroform but not in the amorphous nor in the crystal phase, was investigated to disclose the morphology dependence through molecular dynamics simulations and first-principles calculations. We find that the strong intermolecular C=O center dot center dot center dot H-C hydrogen bonds between Cz2BP and chloroform in cocrystals decrease the nonradiative decay rate of T-1 -> S-0 by 3-6 orders of magnitude due to the vibronic decoupling effect on the C=O stretching motion and the increase of (pi,pi*) composition in the T-1 state. The former is responsible for high efficiency and the latter for long-lived RTP with a calculated lifetime of 208 ms (exp. 353 ms). Nevertheless, the weak hydrogen bonds cannot cause any appreciable RTP in amorphous and crystal phases. This novel understanding opens a way to design organic RTP materials.