Unraveling the Efficiency of Thioxanthone Based Triplet Sensitizers: A Detailed Theoretical Study

Photochemical activation by triplet photosensitizers is highly expedient for a green focus society. In this work, we have theoretically probed excited state characteristics of thioxanthone and its derivatives for their triplet harvesting efficiency using density functional theory (DFT) and time‐dependent density functional theory (TDDFT). Absorption and triplet energies corroborate well with the available experimental data. Our results predict that both the S1 and T1 states are π‐π* in nature, which renders a high oscillator strength for S0 to S1 transition. Major triplet exciton conversion occurs through intersystem crossing (ISC) channel between the S1 (1π‐π*) and high energy 3n‐ π* state. Apart from that, there is both radiative and non‐radiative channel from S1 to S0, which competes with the ISC channel and reduces the triplet harvesting efficiency. For thioxanthones with −OMe (Me=Methyl) or −F substitution at 2 or 2’ positions, the ISC channel is not energetically feasible, causing sluggish intersystem crossing quantum yield (ΦISC). For unsubstituted thioxanthone and for isopropyl substitution at 2’ position, the S1‐T1 gap is slightly positive ( ΔES1-3nπ* ${\Delta {E}_{{S}_{1}-{}^{3}n{\rm \pi }{\rm {^\ast}}}$ ), rendering a lower triplet harvesting efficiency. For systems with −OMe or −F substitution at 3 or 3’ position of thioxanthone, because of buried π state and high energy π* state, the S1‐3nπ* gap becomes negative. This leads to a high ΦISC (>0.9), which is key to being an effective photocatalyst. Thioxanthone and its derivatives are highly efficient triplet photosensitizers. But unsubstituted thioxanthone shows low intersystem crossing quantum yield (ΦISC). DFT and TDDFT calculations show that −OMe or −F substitution at 3 or 3’ position stabilizes the π‐orbital energies leading to higher 1π‐π transition energy, which favors the S1 to 3n‐π* ISC transitions leading to higher ΦISC.