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

Abstract

Structurally-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.