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

Thioxanthone and its derivatives are highly efficient triplet photosensitizers. But unsubstituted thioxanthone shows low intersystem crossing quantum yield (Φ_ISC). 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 Φ_ISC.

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 (Φ_ISC). 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 Φ_ISC (>0.9), which is key to being an effective photocatalyst.

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