Triplet State Engineering in Regioisomeric Cocrystals Enabling Dual Phosphorescence and Efficient Thermally Activated Delayed Fluorescence

ABSTRACT Organic donor–acceptor cocrystals have emerged as a versatile class of functional materials for modulating excited‐state processes through supramolecular assembly. In this work, a series of regioisomeric D‐A cocrystals (oDCB, mDCB, and pDCB) exhibiting both thermally activated delayed fluorescence (TADF) and dual phosphorescence from the 3 CT and 3 LE states are reported. Structural variations of acceptors enable systematic control over intermolecular charge transfer interactions, resulting in tunable singlet‐triplet energy gaps (ΔE ST ) and hence reverse intersystem crossing (RISC) rates. The oDCB cocrystal, owing to the smallest ΔE ST , exhibits the fastest RISC rate (1.29 × 10 5 s − 1 ), whereas pDCB, with the largest ΔE ST (0.23 eV), displays the longest excited state lifetime (2.12 ms) and the slowest RISC rate (6.97 × 10 2 s − 1 ). At 77 K, all the cocrystals demonstrate dual phosphorescence from 3 CT and 3 LE states, assisted by vibronic coupling. The energy landscape of the triplet state of the cocrystals depends on the triplet energy of the parent components, and hence, determines the extent of vibronic coupling between 3 LE and 3 CT. These results elucidate how the interplay between the 3 CT and 3 LE states controls TADF and dual phosphorescence, underscoring that cocrystal design is an effective way to tune triplet state dynamics in organic cocrystals.