Dipole Moment of the S0 → S1 Chlorophyll a Transition in Solvents with a Varied Refraction Index

The dependence of the dipole moment of chlorophyll a’s (Chl) S0 → S1 transition on the value of the solvent refractive index n is calculated. The interactions between the electric field of a light wave, the electronic transition of the pigment to an excited state, and the dielectric polarization of an optical medium are analyzed. The reactive changes in Chl’s transition dipole moment in solvents with different refractive index values are calculated in the time-dependent density functional theory (TD–DFT) using the LC-ωPBE hybrid functional and the polarizable continuum model. The ab initio calculations are approximated by the Onsager reactive field model with an effective polarizability of Chl equal to 21 Å3. The model quantitatively describes the experimental dependence of Chl’s extinction coefficient in solvents with a refractive index of 1.3 < n < 1.7. In a protein environment with a refractive index of n = 1.4, the transition dipole moment of Chl is 5.5 D. For this environment, the distributions of the electrostatic potential in the ground and excited states of Chl are calculated; the ab initio calculations are approximated by a set of partial transient charges located on the heavy atoms of the π-conjugated system of the Chl molecule.