Spin–Vibronic Coupling From Higher‐Lying Triplets is Crucial for Intersystem Crossing and Room‐Temperature Phosphorescence

ABSTRACT Achieving highly efficient purely organic room‐temperature phosphorescence (RTP) remains a formidable challenge due to inherently weak spin–orbit coupling and slow intersystem crossing (ISC) between the singlet and triplet manifolds. To address this limitation, the involvement of higher‐lying triplet states ()—either through direct ISC () or indirect nonadiabatic spin–vibronic coupling (NA‐SVC) enhanced ISC ( mediates without being populated)—has emerged as a powerful strategy to facilitate triplet harvesting. However, it remains unclear exactly how participates in these two distinct pathways, and under which conditions one pathway dominates over the other. To address these questions, we develop a theoretical framework integrating the time‐dependent generating function (TD‐GF) algorithm with the multilayer energy‐based fragment (MLEBF) method, enabling evaluation of NA‐SVC contributions to ISC rate constants in the crystalline phase at the full quantum mechanical (QM) level. This computational protocol is applied to elucidate the competing ISC pathways in a series of carbazole derivatives ( BeCbz , AcCbz , and PhCbz ) in solution and crystalline phases. Quantitative evaluations reveal that the superior RTP performance of BeCbz is decisively governed by a direct, ‐mediated pathway. Moreover, the significant NA‐SVC enhancement drives an ultrafast ISC that effectively outcompetes fluorescence. In contrast, AcCbz primarily utilizes a direct spin–orbit coupling (DSO)‐dominated pathway, whereas PhCbz exhibits solely fluorescence due to severely hindered ISC channels. Overall, this study demonstrates that explicit full QM modeling of environmental effects and NA‐SVC is of great importance, providing a rigorous predictive tool for the rational design of high‐performance organic phosphors.