Structure-Photochemical Function Relationships in Nitrogen-Containing Heterocyclic Aromatic Photobases Derived from Quinoline

Photobases are compounds which become strong bases after electronic excitaton into a charge-transfer excited state. Recent experimental studies have highlighted the photobasicity of the 5-R quinoline compounds, demonstrating a strong substituent dependence to the pK_a^*. Here we describe our systematic study of how the photobasicity of four families of nitrogen-containing heterocyclic aromatics are tuned through substituents. We show that substituent position and identity both significantly impact the pK_a^*. We demonstrate that the substituent effects are additive and identify many disubstituted compounds with substantially greater photobasicity than the most photobasic 5-R quinoline compound identified previously. We show that the addition of a second fused benzene ring to quinoline, along with two electron-donating substituents, lowers the vertical excitation energy into the visible while still maintaining a pK_a^* > 14. Overall, the structure-function relationships developed in this study provide new insights to guide the development of new photocatalysts that employ photobasicity.