n/π Orbital Decoupling via Heavy Selenium Atoms toward Efficient Red Room-Temperature Phosphorescence in Purely Organic Systems

Heavy atoms are widely used to induce efficient room-temperature phosphorescence (RTP) in purely organic materials by enhancing spin-orbit coupling (SOC). However, the heavy-atom effect is conditional. Herein, we incorporate flexible folded units (phenylselenyl, phenylthio, or phenoxyl) into the luminescent core benzo[c][1,2,5]thiadiazole (BZT) to design a series of isomeric molecules. Interestingly, the isomer pair 4,7-2Se and 5,6-2Se, differing only in the substitution site of the selenium-containing folded units on the BZT core, exhibits fluorescence and RTP, respectively. Our findings reveal that the substitution site dictates the photophysical behavior by influencing the orientation between the nonbonding n-orbitals on the selenium atoms in the folded units and the π-orbitals of the BZT core, which results in the order of magnitude differences in SOC coefficients. Specifically, 5,6-2Se exhibits efficient red RTP emission (λ_P = 640 nm, Φ_P = 10.13%), owing to the significantly enhanced SOC induced by the almost orthogonal orientation between the n- and π orbitals, which we term n/π orbital decoupling. In sharp contrast, 4,7-2Se displays pure fluorescence due to n/π orbital coupling. Through experimental and theoretical analyses, we validate the reasonability of n/π orbital decoupling and further explore its synergistic relationship with the heavy-atom effect in enhancing SOC. Furthermore, we demonstrate the application potentials of 5,6-2Se in pattern anticounterfeiting and aqueous oxygen sensing. This work not only establishes a strategy of n/π orbital decoupling for designing efficient red RTP materials, but also provides an in-depth understanding of the chalcogen-based heavy-atom effect on SOC.