Excitation of a photosensitizer-reagent complex: 4BBA-nonanoic acid complex in an atmospheric environment

Organic chromophores and photosensitizers present in the marine boundary layer and atmospheric aerosols play a crucial role in daytime photochemical processes. Despite the recognized importance of photosensitizers in atmospheric chemistry, a detailed microscopic understanding of their mechanisms of action is in the early stages of development. This study explores the possibility that the formation of complexes between a photosensitizer and reactive molecules plays a crucial role in the initial excitation process. In this work, we investigate the effect of a representative fatty acid, nonanoic acid (NA), on a model chromophore, 4-benzoylbenzoic acid (4BBA), using quantum chemical calculations. This study aims to elucidate how fatty acid and chromophore interactions influence the excited state characteristics of organic photosensitizers in atmospheric environments. The key findings of this study are as follows: (i) complexation of 4BBA with NA leads to the absorption peak within the tropospherically relevant solar spectral region (>300 nm), which is absent in isolated 4BBA. (ii) Upon hydration of the 4BBA and NA complex, the ordering of the excited states changes, resulting in a shift of the dominant absorption from the S₃ state to the S₂ state. (iii) Under acidic conditions, the S₂ state remains dominant, with enhanced transition probability and a redshift of up to 0.65 eV observed relative to isolated 4BBA. These findings demonstrate that complexation between the photosensitizer and the reactive molecule can significantly influence photoexcitation, while microenvironmental conditions, e.g., neighboring water molecules and protons, also play a critical role.