Electronic Excited State Redox Properties for BODIPY Dyes Predicted from Hammett Constants: Estimating the Driving Force of Photoinduced Electron Transfer

Here we formulate equations based solely on empirical Hammett substituent constants to predict the redox potentials for the electronic excited state of boron-dipyrromethene (BODIPY) dyes. We utilized computational, spectroscopic, and electrochemical techniques toward characterizing the effect of substitution at the positions C2, C6, and C8 of the 1,3,5,7-tetramethyl BODIPY core. Working with a library of 100 BODIPY dyes, we found that highest occupied molecular orbital (HOMO) energies calculated at the B3LYP 6-31g(d) level correlated linearly with the Hammett sm value for substituents at position C8 and with Hammett sp values for substituents at positions C2 and C6. In turn, we observed that LUMO energies correlated linearly with Hammett sp at position C8 and with Hammett sm at positions C2 and C6. Focusing on a subset of 26 dyes for which reduction potentials were either previously available or measured herein and ranged from -1.84 to -0.52 V (a full 1.3 V), we found a linear relationship between redox potentials in acetonitrile and HOMO and lowest unoccupied molecule orbital (LUMO) energies determined via density functional theory (DFT). A linear correlation was thus ultimately established between redox potentials in acetonitrile and Hammett substituent constants. Combining this with equations derived for the linear relationship existing between the zero vibrational energy of the excited BODIPY and Hammett substituent constants enabled us to provide the parameters toward predicting the oxidizing/reducing power of photoexcited 1,3,5,7,-tetramethyl BODIPY dyes in their singlet excited state.