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.