Bimolecular Quenching Rate of Electron Transfer Reactions from Aromatic Amines to Coumarin Dyes: A Conceptual Density Functional Theory-Based Approach.

In the current work, a qualitative trend of the quenching rate of a series of bimolecular electron transfer reactions is computed through an alternative and cost-effective approach. The electron transfer reactions from eight aromatic amines in their ground state to a series of six substituted coumarin dyes in their singlet excited states (S₁) are chosen as representative quenching processes. The acetonitrile solvent is used as the reaction medium. While the reaction free energy values are evaluated through conceptual density functional theory (CDFT) [or CDASE-scheme, to be more specific]─based stabilization energy, the reorganization energy values are calculated using a conventional method already prescribed in the literature. The reactions, being diffusion-controlled ones, only outer-sphere reorganization energy (or, solvent reorganization energy) is considered, neglecting the intramolecular reorganization energy (as suggested by the corresponding experimental study available in the literature). The generated data demonstrate that as the absolute values of the CDFT-based stabilization energy become closer to the corresponding reorganization energy, the experimental quenching rate constant values of the chosen bimolecular electron transfer reactions increase. This observation exactly correlates with the Marcus theory of electron transfer reaction. Thus, the authors would like to claim that the Marcus theory is validated through an unconventional approach based on conceptual density functional theory (CDFT).