{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,8,24]],"date-time":"2026-08-24T11:21:25Z","timestamp":1787570485310,"version":"build-2736575974"},"reference-count":57,"publisher":"AIP Publishing","issue":"8","funder":[{"DOI":"10.13039\/501100001843","name":"Science and Engineering Research Board","doi-asserted-by":"publisher","award":["CRG\/2023\/002306"],"award-info":[{"award-number":["CRG\/2023\/002306"]}],"id":[{"id":"10.13039\/501100001843","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":["pubs.aip.org"],"crossmark-restriction":true},"short-container-title":[],"published-print":{"date-parts":[[2026,8,28]]},"abstract":"<jats:p>We investigate the intersystem crossing (ISC) mechanism of three isomeric molecules, namely, methyl 2-(9H-carbazol-9-yl)benzoate (o-MCBA), methyl 3-(9H-carbazol-9-yl)benzoate (m-MCBA), and methyl 4-(9H-carbazol-9-yl)benzoate (p-MCBA). Our analyses highlight the role of direct spin\u2013orbit (DSO) coupling, Herzberg\u2013Teller (HT) coupling, and spin\u2013vibronic (SV) coupling in facilitating the ISC processes in these molecules. We have found that the dominant ISC pathway between the lowest excited singlet state (SCT1o\/p) having charge-transfer (CT) character and the triplet state (TCT1+LE1o\/p) with mixed CT and locally excited (LE) characters is primarily governed by both DSO and HT interactions in o-MCBA and p-MCBA, whereas for m-MCBA, the ISC pathway between the lowest excited singlet state (SLE1m) and the triplet state (TLE1m) is mainly assisted by HT and SV interactions. The time-dependent correlation function based method has been employed for the calculations of ISC rate constant (kISC). The computed DSO\/HT assisted kISCs for the ISC pathway involving the lowest excited singlet and the triplet states of ortho, para, and meta isomers are found to be 0.87 \u00d7 106\/4.16 \u00d7 106, 1.18 \u00d7 106\/2.24 \u00d7 106, and 5.67 \u00d7 103\/4.02 \u00d7 106 s\u22121, respectively, while the SV-aided kISC of m-MCBA is turned out to be 5.45 \u00d7 106 s\u22121. Our theoretical analysis reveals that while the SV interaction plays a crucial role on the ISC mechanism of meta isomer, the larger energy gaps between the lowest excited singlet state (SCT1o\/p) and the higher lying triplet state (TCT2+LE2o\/p) as well as that between (TCT2+LE2o\/p) and the lowest excited triplet state (TCT1+LE1o\/p) suppress the SV-driven ISC processes in ortho and para isomers. This study emphasizes that the origins of the same order of the experimental kISCs of the structural isomers are completely different.<\/jats:p>","DOI":"10.1063\/5.0343043","type":"journal-article","created":{"date-parts":[[2026,8,24]],"date-time":"2026-08-24T10:35:21Z","timestamp":1787567721000},"update-policy":"https:\/\/doi.org\/10.1063\/aip-crossmark-policy-page","source":"Crossref","is-referenced-by-count":0,"title":["Role of spin\u2013orbit and nuclear motion assisted spin\u2013orbit coupling on the structural isomerism dependent intersystem crossing mechanism"],"prefix":"10.1063","volume":"165","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-3414-4358","authenticated-orcid":false,"given":"Pijush","family":"Karak","sequence":"first","affiliation":[{"name":"Department of Chemistry, University of Calcutta 1 , 92\u00a0A. P. C. Road, Kolkata 700009, West Bengal,","place":["India"]},{"name":"Department of Chemistry, KTH Royal Institute of Technology 2 , SE-10044 Stockholm,","place":["Sweden"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0005-7525-822X","authenticated-orcid":false,"given":"Keniya","family":"Basu","sequence":"additional","affiliation":[{"name":"Department of Chemistry, University of Calcutta 1 , 92\u00a0A. P. C. 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