Thiocarbonyl‐Bridged N‐Heterotriangulenes for Energy Efficient Triplet Photosensitization: A Theoretical Perspective

Abstract

AbstractStructurally‐rigid metal‐free organic molecules are of high demand for various triplet harvesting applications. However, inefficient intersystem crossing (ISC) due to large singlet‐triplet gap ( ) and small spin‐orbit coupling (SOC) between lowest excited singlet and triplet often limits their efficiency. Excited electronic states, fluorescence and ISC rates in several thiocarbonyl‐bridged N‐heterotriangulene ( S‐HTG) with systematically increased thione content ( 0–3) are investigated implementing polarization consistent time‐dependent optimally‐tuned range‐separated hybrid. All S‐HTGs are dynamically stable and also thermodynamically feasible to synthesize. Relative energies of several low‐lying singlets ( ) and triplets ( ), and their excitation nature (i. e., or ) and SOC are determined for these S‐HTGs in dichloromethane. Low‐energy optical peak displays gradual red‐shift with increasing thione content due to relatively smaller electronic gap resulted from greater degree of orbital delocalization. Significantly large SOC due to different orbital‐symmetry and heavy‐atom effect produces remarkably high ISC rates ( ~1012 s−1) for enthalpically favoured ( ) channel in these S‐HTGs, which outcompete radiative fluorescence rates (~108 s−1) even directly from higher lying optically bright singlets. Importantly, high energy triplet excitons of ~1.7 eV resulting from such significantly large ISC rates from non‐fluorescent make these thiocarbonylated HTGs ideal candidates for energy efficient triplet harvest including triplet‐photosensitization.