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We investigated this using halogenated 7\u2010hydroxycoumarin, unsubstituted 4\u2010methylumbiliferone, and ortho\u2010chloro and bromo substitutions on the phenolic ring. Charge density data from X\u2010Ray diffraction and computational methods revealed that halogenation at the ortho position significantly reduced quantum yield (QY). Specifically, 7\u2010hydroxycoumarin (<jats:bold>1<\/jats:bold>) had a QY of 70\u2009%, while ortho\u2010chloro (<jats:bold>2<\/jats:bold>) and ortho\u2010bromo (<jats:bold>3<\/jats:bold>) had QYs of 61\u2009% and 30\u2009%, respectively. Experimental data showed that these molecules excited similarly, but the electrostatic potential and dipole moments indicated that <jats:bold>2<\/jats:bold> and <jats:bold>3<\/jats:bold> dissipated excitation energy more easily due to charge separation. The heavy\u2010atom effect of Cl and Br did not fully explain the QY reductions, suggesting other radiative decay processes were involved. By incorporating spin\u2010orbit coupling (SOC) effects, we estimated intersystem crossing (ISC) and phosphorescence rates, providing theoretical QYs of 78\u2009% for <jats:bold>1<\/jats:bold>, 59\u2009% for <jats:bold>2<\/jats:bold>, and 15\u2009% for <jats:bold>3<\/jats:bold>. The large deviation for <jats:bold>3<\/jats:bold> was attributed to its higher SOC potential derived in computational calculations. Our overall findings indicate that <jats:bold>3<\/jats:bold>\u2019s reduced QY results from a mix of SOC\u2010induced ISC and charge dissipation due to the electronegativity of Br atom, while <jats:bold>2<\/jats:bold>\u2019s reduction is primarily due to charge separation caused by Cl alone. Further studies are needed to validate this approach with other scaffolds.<\/jats:p>","DOI":"10.1002\/cphc.202400812","type":"journal-article","created":{"date-parts":[[2024,11,30]],"date-time":"2024-11-30T13:59:07Z","timestamp":1732975147000},"update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":1,"title":["Influence of Halogen Substituents on the Photophysical Properties of 7\u2010Hydroxycoumarin: Insights from Experimental and Theoretical Studies"],"prefix":"10.1002","volume":"26","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-0280-039X","authenticated-orcid":false,"given":"Bryson A.","family":"Hawkins","sequence":"first","affiliation":[{"name":"Sydney Pharmacy School Faculty of Medicine and Health The University of Sydney  NSW 2006 Australia"},{"name":"Current Address: Antimicrobial Discovery Centre College of Science Northeastern University  Boston, MA 02115 USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Liam D.","family":"Adair","sequence":"additional","affiliation":[{"name":"School of Chemistry Faculty of Science The University of Sydney  NSW 2006 Australia"},{"name":"Australian Research Council Centre of Excellence for Innovations in Peptide and Protein Science The University of Sydney  NSW 2006 Australia"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"William G.","family":"Ryder","sequence":"additional","affiliation":[{"name":"School of Chemistry Faculty of Science The University of Sydney  NSW 2006 Australia"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Jonathan J.","family":"Du","sequence":"additional","affiliation":[{"name":"Sydney Pharmacy School Faculty of Medicine and Health The University of Sydney  NSW 2006 Australia"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Elias","family":"Najib","sequence":"additional","affiliation":[{"name":"Sydney Pharmacy School Faculty of Medicine and Health The University of Sydney  NSW 2006 Australia"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Elizabeth J.","family":"New","sequence":"additional","affiliation":[{"name":"School of Chemistry Faculty of Science The University of Sydney  NSW 2006 Australia"},{"name":"Australian Research Council Centre of Excellence for Innovations in Peptide and Protein Science The University of Sydney  NSW 2006 Australia"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Philip A.","family":"Gale","sequence":"additional","affiliation":[{"name":"Faculty of Science University of Technology Sydney  City Campus, Broadway NSW 2007 Australia"},{"name":"School of Chemistry Faculty of Science The University of Sydney  NSW 2006 Australia"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"James A.","family":"Platts","sequence":"additional","affiliation":[{"name":"School of Chemistry Cardiff University  Cardiff CF10 3AT UK"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Paul W.","family":"Groundwater","sequence":"additional","affiliation":[{"name":"Sydney Pharmacy School Faculty of Medicine and Health The University of Sydney  NSW 2006 Australia"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"David E.","family":"Hibbs","sequence":"additional","affiliation":[{"name":"Sydney Pharmacy School Faculty of Medicine and Health The University of Sydney  NSW 2006 Australia"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"311","published-online":{"date-parts":[[2024,12,12]]},"reference":[{"key":"e_1_2_7_1_1","doi-asserted-by":"publisher","DOI":"10.1007\/s11696-021-01955-7"},{"key":"e_1_2_7_2_1","doi-asserted-by":"publisher","DOI":"10.1016\/j.saa.2018.12.022"},{"key":"e_1_2_7_3_1","doi-asserted-by":"publisher","DOI":"10.3390\/molecules15010352"},{"key":"e_1_2_7_4_1","doi-asserted-by":"crossref","unstructured":"Y. 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