Unraveling the Origin of High-Efficiency Photoluminescence in Mixed-Stack Isostructural Crystals of Organic Charge-Transfer Complex: Fine-Tuning of Isometric Donor-Acceptor Pairs

Charge-transfer (CT) crystals show unique transport and emission properties which are substantially different from those of individual electron donor (D) and acceptor (A) molecules constituting them. While the transport properties of CT crystals are well established as to enable the interpretation and prediction of their electrical properties, their photophysical processes-particularly the emission-structure relationship in CT crystals-are much less explored; this is because of the often weak photoluminescence (PL), attributed to the small oscillator strength (f) of the emitting state and the arbitrary molecular design of CT complexes reported so far. In this work, we demonstrate that the novel isometric design of D-A molecules with appropriate CT interactions in the mixed-stack organic charge-transfer crystals can produce extremely strong CT emission in a predictable way. It was found that isometric D-A interactions in the mixed-stack isostructural CT crystals can generate highly increased oscillator strength within the slipped stack intermolecular arrangement via favorable configuration interaction, effective suppression of the nonradiative processes, and also triplet harvesting via reverse intersystem crossing. Based on the synergy of these effects, our mixed stack CT crystals marks a record high PL quantum yield of 83%. Notably, four different CT pairs made of isometric D and A molecules all showed the isomorphic/quasi-isostructural intrastack (p-stack) crystal, enabling us to find the sole effect of electronic CT interaction on their photophysical properties by decoupling the complicated morphological effect. Based on this specifically designed study of emission-structure relationship using the X-ray structure analysis, photophysical properties investigations, and time-dependent density functional theory (TD-DFT) calculations, the mechanism of CT luminescence of mixed stack organic charge-transfer crystal was unraveled in terms of the modulated oscillator strength, nonradiative deactivation, and triplet harvesting.