Quinonile Photobasicity is Mediated by Hole Injection

Photoacids and photobases drive acid-base reactions by photon energy. Although known since 1940, the large-scale development of photoacids and photobases into efficient molecular light harvesting devices has been confounded by a lack of mechanistic understanding. A new mechanism explaining the photobasicity of quinolines is proposed based on nonadiabatic molecular dynamics simulations using time-dependent density functional theory (TDDFT). Rather than changing the basicity of the quinoline, photoexcitation is found to induce oxidation of adjacent water molecules. We develop a mechanistic understanding of the process in the molecular orbital theory framework, and discuss the limitations of our methods arising from using TDDFT to simulate excited state. Further support for the proposed mechanism is provided by comparison to experimental time-resolved fluorescence spectroscopy and correlated wavefunction calculations. These results suggest that the conventional picture of excited acid-base equilibrium may need to be revisited.