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 S 1 and T 1 states are π-π * in nature, which renders a high oscillator strength for S 0 to S 1 transition. Major triplet exciton conversion occurs through intersystem crossing (ISC) channel between the S 1 ( 1 π-π * ) and high energy 3 n- π * state. Apart from that, there is both radiative and non-radiative channel from S 1 to S 0 , 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 (Φ ISC ). For unsubstituted thioxanthone and for isopropyl substitution at 2' position, the S 1 -T 1 gap is slightly positive ( Δ E S 1 - 3 n π * ${\Delta {E}_{{S}_{1}-{}^{3}n{\rm \pi }{\rm {^\ast}}}}$ ), 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 S 1 - 3 nπ * gap becomes negative. This leads to a high Φ ISC (>0.9), which is key to being an effective photocatalyst.