Chandra Garain B, Pati SK. Unraveling the Efficiency of Thioxanthone Based Triplet Sensitizers: A Detailed Theoretical Study.
Chemphyschem 2022;
24:e202200753. [PMID:
36495016 DOI:
10.1002/cphc.202200753]
[Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/11/2022] [Revised: 12/09/2022] [Accepted: 12/09/2022] [Indexed: 12/14/2022]
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 (1 π-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 ( 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 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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