Influence of Halogen Substituents on the Photophysical Properties of 7-Hydroxycoumarin: Insights from Experimental and Theoretical Studies

The benzopyrone molecule coumarin is a popular fluorescent scaffold, but how chemical modifications affect its properties is not well understood. We investigated this using halogenated 7-hydroxycoumarin, unsubstituted 4-methylumbiliferone, and ortho-chloro and bromo substitutions on the phenolic ring. Charge density data from X-Ray diffraction and computational methods revealed that halogenation at the ortho position significantly reduced quantum yield (QY). Specifically, 7-hydroxycoumarin (1) had a QY of 70 %, while ortho-chloro (2) and ortho-bromo (3) had QYs of 61 % and 30 %, respectively. Experimental data showed that these molecules excited similarly, but the electrostatic potential and dipole moments indicated that 2 and 3 dissipated excitation energy more easily due to charge separation. The heavy-atom effect of Cl and Br did not fully explain the QY reductions, suggesting other radiative decay processes were involved. By incorporating spin-orbit coupling (SOC) effects, we estimated intersystem crossing (ISC) and phosphorescence rates, providing theoretical QYs of 78 % for 1, 59 % for 2, and 15 % for 3. The large deviation for 3 was attributed to its higher SOC potential derived in computational calculations. Our overall findings indicate that 3's reduced QY results from a mix of SOC-induced ISC and charge dissipation due to the electronegativity of Br atom, while 2's reduction is primarily due to charge separation caused by Cl alone. Further studies are needed to validate this approach with other scaffolds.