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Thiocoumarin (TC), however, is an exceptional aromatic thioketone whose excited-state dynamics are distinctly different from those of common aromatic thioketones. Although many experimental and theoretical investigations have been devoted to TC and its derivatives, the intrinsic origin of their unusual photophysical properties remains unclear. In this work, high-level XMS-CASPT2 calculations were performed for the five lowest-lying electronic states, S0, T1, S1, T2, and S2, of TC and benzopyranthione (BPT), with BPT used as a representative common aromatic thioketone for comparison. The calculated results show that the small difference between the molecular structures of BPT and TC leads to substantial changes in the excited-state structures, electronic characters, and energy-level ordering in going from BPT to TC. These changes are identified as the intrinsic origin of the anomalous photophysical properties of TC. Meanwhile, the XMS-CASPT2 calculations reveal that the T1 state of TC can be populated through the two ultrafast S2 \u2192 S1 \u2192 T1 and S2 \u2192 T2 \u2192 T1 processes, whereas the subsequent T1 \u2192 S0 radiative and nonradiative processes are predicted to be inefficient. The theoretical predictions are in excellent agreement with the experimental findings that the long-lived T1 state of TC is formed with near-unity quantum yield. The T1-state features make TC and its derivatives metal-atom-free photosensitizers applicable in the field of photodynamic therapy.<\/jats:p>","DOI":"10.1021\/acs.jpclett.6c02742","type":"journal-article","created":{"date-parts":[[2026,9,23]],"date-time":"2026-09-23T17:02:06Z","timestamp":1790182926000},"page":"11606-11613","source":"Crossref","is-referenced-by-count":0,"title":["Probing Unusual\nIntrinsic Photophysical Properties\nof Thiocoumarin from XMS-CASPT2 Calculations"],"prefix":"10.1021","volume":"17","author":[{"ORCID":"https:\/\/orcid.org\/0009-0002-4271-1954","authenticated-orcid":true,"given":"Ye-Guang","family":"Fang","sequence":"first","affiliation":[{"name":"Beijing Normal University , , ,","place":["Beijing, P. R. China, 100875"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Lei","family":"Zhao","sequence":"additional","affiliation":[{"name":"Beijing Normal University , , ,","place":["Beijing, P. R. China, 100875"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3532-7770","authenticated-orcid":true,"given":"Haobin","family":"Wang","sequence":"additional","affiliation":[{"name":"University of Colorado Denver , , , ,","place":["Denver, Colorado, United States, 80204"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8316-1816","authenticated-orcid":true,"given":"Chongqin","family":"Zhu","sequence":"additional","affiliation":[{"name":"Beijing Normal University , , ,","place":["Beijing, P. R. China, 100875"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6878-8963","authenticated-orcid":true,"given":"Lin","family":"Shen","sequence":"additional","affiliation":[{"name":"Beijing Normal University , , ,","place":["Beijing, P. R. China, 100875"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1668-465X","authenticated-orcid":true,"given":"Wei-Hai","family":"Fang","sequence":"additional","affiliation":[{"name":"Beijing Normal University , , ,","place":["Beijing, P. R. China, 100875"]}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"316","published-online":{"date-parts":[[2026,9,23]]},"reference":[{"key":"2026100811170081000_cit1","doi-asserted-by":"publisher","first-page":"6738","DOI":"10.1021\/ja00360a046","article-title":"Photophysics of the Second Excited Singlet States of\nXanthione and Related Thiones in Perfluoroalkane Solvents","volume":"105","author":"Maciejewski","year":"1983","journal-title":"J. Am. Chem. 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Soc."},{"key":"2026100811170081000_cit30","doi-asserted-by":"publisher","first-page":"16243","DOI":"10.1021\/jacs.9b09220","article-title":"An Emerging Molecular Design Approach\nto Heavy-Atom-Free Photosensitizers for Enhanced Photodynamic Therapy\nunder Hypoxia","volume":"141","author":"Nguyen","year":"2019","journal-title":"J. Am. Chem. Soc."},{"key":"2026100811170081000_cit31","doi-asserted-by":"publisher","first-page":"6423","DOI":"10.1039\/D0SC00063A","article-title":"Real-time\ntracking of the entangled pathways in the multichannel photodissociation\nof acetaldehyde","volume":"11","author":"Yang","year":"2020","journal-title":"Chem. 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