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

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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 3 n-π* ISC transitions leading to higher ΦISC.


Abstract

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 3 n- 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 ( ), 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 S1-3 nπ* gap becomes negative. This leads to a high ΦISC (>0.9), which is key to being an effective photocatalyst.

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