Regiochemistry and substituent effects on pyrylogen and thiopyrylogen stability and electronic character

A physical organic analysis of a collection of 57 data sets generated by B3LYP/6-311+G(2d,p) geometry optimizations and TD-DFT calculations is presented. The analysis engenders recommendations for syntheses of new pyrylogen electron-transfer sensitizers that are potentially optimized for low energy absorbance and high extinction coefficients. It is argued that bathochromically shifted intense absorbance is especially useful to avoid competitive absorbance by electron-transfer substrates and for use at the low concentrations likely achievable in experimentally attractive nonpolar solvents.