Phase Transition‐Induced Regulation of Room Temperature Phosphorescence and Delayed Fluorescence in Doping System

Abstract Switching between triplet‐involved room temperature phosphorescence (RTP) and delayed fluorescence (DF) is pivotal for advancing molecular encryption application but remains a significant challenge in multicomponent material systems. In this study, we successfully realized the selective modulation of RTP and DF emissions in a donor‐acceptor doping system by inducing conformational changes in the donor through solid‐liquid transition. In the solid state, the donor adopts a highly twisted quasi‐axial conformation, which effectively suppresses non‐radiative decay, resulting in efficient green RTP. In contrast, in the liquid state, the donor transitions to a relatively planar conformation, forming the intermolecular charge transfer state with the acceptor, significantly reducing the energy gap between the lowest singlet and triplet excited states, thereby facilitating reverse intersystem crossing (RISC) and generating yellow DF. Furthermore, the coexistence of multiple emissive triplet states demonstrates promising applications in information encryption, ink‐free rewritable paper, and thermal sensing.