Photophysical Properties of Free, Zinc‑, and Pyridine- Chlorin e6 Trimethyl Ester: A Computational Study with...

Chlorophyll derivatives, such as chlorin e6, are natural products with promising properties as photosensitizers (PS); they are highly abundant, and their extraction is a relatively straightforward process. Therefore, they serve as an ideal starting point for the rational design of new photosensitizers with enhanced properties. In this project, the photophysics of chlorin e6 trimethyl ester (TMEe6), along with its derivatives with zinc (ZnTMEe6) and pyridine (PyrTMEe6), have been computationally characterized. Spectra and transition rate constants for absorption, fluorescence, phosphorescence, and intersystem crossing were computed using the implementation of the analytical solution of Fermi's golden rule via the path integral formalism including vibrational effects. The Adiabatic Hessian model was employed in conjunction with TDDFT CAM-B3LYP/def2-SVP geometries, Hessians, and energies of the involved singlet and triplet states. The Tamm-Dancoff approximation (TDA) was used to optimize the triplet states due to triplet instabilities. The computed spectra are reasonably close to the experimental ones, with some differences in vibrational bands, but good agreement with (0-0) bands, with deviations of less than 0.05 eV in band maxima, and up to 0.2 eV if TDA/TDDFT is used. Calculated rates for fluorescence and ISC are of the same order of magnitude as the experimental data available, which is approximately 10⁸ s^-1. Our results suggest that the main intersystem crossing (ISC) channel for TMEe6 and PyrTMEe6 involves coupling between S₁ ⇝ T₂, and for ZnTMEe6 between S₁ ⇝ T₃. Particularly, the addition of zinc leads to a destabilization of the T₃ state, making the S₁ and T₃ states almost isoenergetic. This change results in an enhancement of the ISC rate by a factor of 2.19 when compared to TMEe6. Pyridine addition enhances ISC rates for channels S₁ ⇝ T₁ and S₁ ⇝ T₃, but not for the main channel S₁ ⇝ T₂. This ultimately increases the total ISC rate by 41%. This methodology provides a better understanding of the photophysics of these molecules that could not be observed with the usual energy gaps and spin-orbital coupling matrix elements. It could therefore aid in the development of a rationally designed synthetic protocol for porphyrinoid photosensitizers.