Executive Summary : | Triplet harvesting is crucial for advanced applications in lighting, photovoltaics, photocatalytic, sensing, bioimaging, and biomedical fields. The efficiency of triplet harvest relies on nonradiative intersystem crossing (ISC), which competes with radiative fluorescence (F) and nonradiative internal conversion (IC). Heavy-metal-containing molecules typically exhibit high ISC rates due to large spin-orbit coupling (SOC), but these systems are expensive and toxic. Metal-free organic molecules are generally highly fluorescent but show limited ISC due to large singlet-triplet gap (?E(S-T)) and small SOC. Molecular design strategies that improve ISC and suppress F and IC in metal-free organic molecules and materials are significant for developing eco-friendly triplet harvestors. The presence of molecular-twist and/or hetero atoms or donor-acceptor building blocks in organic molecular systems can lower ?E(S-T) and induce substantial SOC between the excited singlet and triplet states. However, this strategy can introduce several intermediate triplets (Tn) between the S1 and lowest excited triplet (T1), which is predicted to boost triplet formation efficiency. The proposed study aims to develop reliable and robust electronic structure methods based on time-dependent range-separated hybrid and double-hybrid, as well as develop theoretical models and protocols for accurately determining low-lying optical excitations, ?E(S-T), SOC, and nonadiabatic coupling (NAC) relevant to light absorption and excited-state deactivation. Metal-free structurally rigid molecules, such as azulene (AZ), heptazine (HZ), triangulene (TG), and hetero-triangulene (HTG), and their derivatives with different functional groups in the condensed phase, will be explored for their potential in triplet harvesting. |