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Previous studies have led to conflicting assignments of excited states and relaxation pathways. Here, we combine femtosecond broadband time-resolved fluorescence, transient absorption spectroscopy, and theoretical calculations to delineate the radiative and nonradiative excited-state cascades of 5cadCyd in water. We reveal an unusual two-state-three-component fluorescence dynamics and an ultrafast bifurcation of the initially excited population into two competing relaxation channels. The dominant channel undergoes sub-picosecond internal conversion through an emissive \u03c0\u03c0* state with a strongly distorted geometry, providing an efficient photo-stabilizing pathway. A second, less populated yet substantial channel proceeds through a carboxylate-derived n\u03c0* state. Contrary to the prevailing view that n\u03c0* states in pyrimidine nucleobases and their derivatives are optically dark, this state in 5cadCyd is weakly emissive and contributes appreciably to its fluorescence. More importantly, it serves as an efficient gateway for intersystem crossing through spin\u2013orbit coupling, generating a long-lived triplet state with an 18.3\u00a0ps formation time. This triplet-forming pathway may enhance the susceptibility of 5cadCyd to photochemical reactions and oxidative photodamage. These results resolve earlier controversy and show that the C5-carboxylate group markedly reshapes the balance between photostability and photoreactivity relative to canonical pyrimidine nucleosides. More broadly, they establish 5cadCyd as a representative of an underrecognized class of nucleobase derivatives in which substituent-derived n\u03c0* states contribute to fluorescence while mediating efficient triplet formation, suggesting an intrinsic epigenetic hotspot for photoinduced oxidative damage and genome instability.<\/jats:p>","DOI":"10.1063\/5.0349772","type":"journal-article","created":{"date-parts":[[2026,9,23]],"date-time":"2026-09-23T11:12:19Z","timestamp":1790161939000},"update-policy":"https:\/\/doi.org\/10.1063\/aip-crossmark-policy-page","source":"Crossref","is-referenced-by-count":0,"title":["Direct observation of triple-component fluorescence and triplet formation in epigenetic nucleoside 2\u2032-deoxy-5-carboxylcytidine"],"prefix":"10.1063","volume":"165","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-5588-7521","authenticated-orcid":false,"given":"Qingwu","family":"Xiong","sequence":"first","affiliation":[{"name":"College of Chemistry and Environmental Engineering, Shenzhen University 1 , Shenzhen, Guangdong 518071,","place":["People\u2019s Republic of China"]},{"name":"College of Physics and Optoelectronic Engineering, Shenzhen University 2 , Shenzhen, Guangdong 518060,","place":["People\u2019s Republic of China"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0005-8232-1311","authenticated-orcid":false,"given":"Tao","family":"Li","sequence":"additional","affiliation":[{"name":"College of Chemistry and Environmental Engineering, Shenzhen University 1 , Shenzhen, Guangdong 518071,","place":["People\u2019s Republic of China"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0005-0529-7602","authenticated-orcid":false,"given":"Yicheng","family":"Huang","sequence":"additional","affiliation":[{"name":"College of Chemistry and Environmental Engineering, Shenzhen University 1 , Shenzhen, Guangdong 518071,","place":["People\u2019s Republic of China"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-1935-0695","authenticated-orcid":false,"given":"Mingliang","family":"Wang","sequence":"additional","affiliation":[{"name":"College of Chemistry and Environmental Engineering, Shenzhen University 1 , Shenzhen, Guangdong 518071,","place":["People\u2019s Republic of China"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-1407-0061","authenticated-orcid":false,"given":"Chensheng","family":"Ma","sequence":"additional","affiliation":[{"name":"College of Chemistry and Environmental Engineering, Shenzhen University 1 , Shenzhen, Guangdong 518071,","place":["People\u2019s Republic of China"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-3673-1182","authenticated-orcid":false,"given":"Wai-Ming","family":"Kwok","sequence":"additional","affiliation":[{"name":"Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University 3 , Hung Hom, Kowloon, Hong Kong 999077,","place":["People\u2019s Republic of China"]}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"317","published-online":{"date-parts":[[2026,9,23]]},"reference":[{"issue":"6047","key":"2026092307102537700_c1","doi-asserted-by":"publisher","first-page":"1300","DOI":"10.1126\/science.1210597","article-title":"Tet proteins can convert 5-methylcytosine to 5-formylcytosine and 5-carboxylcytosine","volume":"333","year":"2011","journal-title":"Science"},{"issue":"1","key":"2026092307102537700_c2","doi-asserted-by":"publisher","first-page":"20","DOI":"10.1021\/cb2002895","article-title":"The curious chemical biology of cytosine: Deamination, methylation, and oxidation as modulators of genomic potential","volume":"7","year":"2012","journal-title":"ACS Chem. 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