Photobases are compounds that become strong bases after electronic excitation. Recent experimental studies have highlighted the photobasicity of the 5-R quinoline compounds, demonstrating a strong substituent dependence to the pK(a)*. In this paper, we describe our systematic study of how the thermodynamic driving force for photobasicity is tuned through substituents in four families of nitrogen-containing heterocyclic aromatics. 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 S-0 -> S-PBS vertical excitation energy into the visible region 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.