Photobasicity in Quinolines: Origin and Tunability via the Substituents' Hammett Parameters

Coupling between electronic excitation and proton transfer is relevant to the kinetics of redox reactions, in particular those involved in solar-to fuel light harvesting. A prime example of such coupling occurs in photoacids, where electronic excitation leads to proton release in the excited state. Here, we systematically study the inverse of this effect, photobasicity, in which a molecule becomes more basic in the excited state compared to the ground state. This endows the molecule with light induced proton removal capability which is anticipated to be of use in driving reactions where proton transfer is kinetically challenging. To investigate the origins and tunability of photobasicity, a set of 5-R-quinoline derivatives (R = {NH2, CH3O, H, Br, Cl, CN}) were selected and their changes in pK(a) upon electronic excitation in aqueous solutions were determined. The Hammett parameters sigma(p) of these substituents, indicative of their electron withdrawing capability, span a range of -0.7 to +0.7. Using Forster cycle analysis, the acid dissociation equilibria in the ground and first excited state were determined. The ground state pK(a) obeys an expected linear relationship with respect to the Hammet parameter sigma(p). An important finding of our work is that the excited state pK(a)* also obeys a linear relationship with respect to sigma(p). Interestingly, the excited state pK(a)* is similar to 5 times more sensitive to the electron-withdrawing power of the substituent than the ground state pK(a). We attribute this difference to the larger polarizability of the excited state charge density. Increase in pK(a) due to optical excitation ranging between 2.2 (R = CN) and 10.6 (R = NH2) units were observed within the set. This substantial range of Delta pK(a) values may find use in applications such as oxidation catalysis, in which optically induced removal of protons could speed up reaction kinetics. Finally, we comment on the correlation between photobasicity and enhancement of electronic charge density on the heterocyclic nitrogen upon optical excitation.