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Full-dimensional quantum wavepacket simulations indicate that the wavepacket initiated on S1 would move through the S1\u2013S2 conical intersections and accumulate in the S2 state. This nonadiabatic population transfer feature, combined with small barrier energies on S1 and S2, allows the molecule to display proton transfer from both states. The efficient S1\u2013T3 intersystem crossing is the third event, where the rate of this event increases with the elongation of the proton transfer coordinate (O\u2013H distance). Vibronic coupling in the triplet manifold enables an ultrafast T3 to T1 decay. In addition, the proton transfer and triplet formation features are studied in nonpolar and aprotic polar solvents to interpret the experimental findings.<\/jats:p>","DOI":"10.1063\/5.0310249","type":"journal-article","created":{"date-parts":[[2026,2,12]],"date-time":"2026-02-12T13:00:47Z","timestamp":1770901247000},"update-policy":"https:\/\/doi.org\/10.1063\/aip-crossmark-policy-page","source":"Crossref","is-referenced-by-count":0,"title":["Uncovering the excited-state intramolecular proton transfer and triplet formation pathways of 3-methyl-6-hydroxy-m-phthalic acid"],"prefix":"10.1063","volume":"164","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-2703-2152","authenticated-orcid":false,"given":"Anshuman","family":"Bera","sequence":"first","affiliation":[{"name":"School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram , Maruthamala PO, Vithura, Thiruvananthapuram, Kerala 695551,","place":["India"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8252-9586","authenticated-orcid":false,"given":"Sivaranjana Reddy","family":"Vennapusa","sequence":"additional","affiliation":[{"name":"School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram , Maruthamala PO, Vithura, Thiruvananthapuram, Kerala 695551,","place":["India"]}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"317","published-online":{"date-parts":[[2026,2,12]]},"reference":[{"key":"2026021208004013700_c1","doi-asserted-by":"publisher","first-page":"3799","DOI":"10.1021\/jo402756y","article-title":"Switching off the charge transfer and closing the S1\u2013T1 isc channel in excited states of quinolizinium derivatives: A theoretical study","volume":"79","year":"2014","journal-title":"J.\u00a0Org. 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