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

Rigid metal‐free organic molecules are of high demand for various triplet harvesting applications. Inefficient intersystem crossing (ISC) due to large singlet‐triplet gap ([[EQUATION]]) and small spin‐orbit coupling (SOC) between lowest excited singlet and triplet in these molecules often limits their efficiency. We computationally investigate excited‐state electronic structures, fluorescence and ISC rates in several thiiocarbonyl‐bridged N‐heterotriangulene ([[EQUATION]]S‐HTG) with systematically increased S‐content ([[EQUATION]]0‐3). All [[EQUATION]]S‐HTGs are dynamically stable and also thermodynamically feasible to synthesize. Relative energies of several low‐lying singlets ([[EQUATION]]) and triplets ([[EQUATION]]), and their excitation nature ([[EQUATION]] or [[EQUATION]]) and SOC for these [[EQUATION]]S‐HTGs in dichloromethane are determined implementing time‐dependent optimally‐tuned range‐separated hybrid. Low‐energy optical peak displays gradual red‐shift with increasing S‐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 ([[EQUATION]] ~1012 s‐1) for enthalpically favoured [[EQUATION]]([[EQUATION]]) channel in these [[EQUATION]]S‐HTGs, which outcompete radiative fluorescence rates (~108 s‐1) even directly from higher lying optically bright [[EQUATION]] singlets (anti‐Kasha’s emission). Importantly, high energy triplet excitons of ~1.7 eV resulting from such significantly large ISC rates from non‐fluorescent [[EQUATION]] make these thionated molecules ideal candidates for energy efficient triplet harvest including triplet‐photosensitization.