Singlet Excited-State Lifetimes of Nitro- and Dimethylamino-Substituted Bacteriochlorins Vary in Their Sensitivity to Solvent Polarity

Naturally occurring porphyrins, chlorophylls, and bacteriochlorophylls are conjugated organic molecules essential to biological processes such as ligation, catalysis, and photosynthesis. Synthetic porphyrins, chlorins, and bacteriochlorins (BCs) have found widespread use in photodynamic therapy, photocatalysis, bioimaging, nonlinear optics, and, most recently, quantum information science. Many of these applications benefit from their strong light absorption, and it can also be advantageous to vary their permanent excited-state dipole moment (d_e) or their difference dipole moment (Δd). One way to achieve this is by adding electron-donating or electron-withdrawing groups, such as dimethylamino and nitro substituents. However, these substituents are known to shorten the singlet excited-state lifetime (τ_S1) and little is known about their impact on τ_S1 of porphyrins, chlorins, and BCs. In this work, we show that dimethylamino substitution of BCs does not shorten τ_S1, somewhat surprisingly, while the addition of a single nitro substituent causes τ_S1 to be sensitive to solvent polarity. Specifically, we characterize four 5-methoxy-substituted bacteriochlorins featuring 3,13-phenylethynyl substituents with dimethylamino and/or nitro groups via a combination of steady-state and time-resolved optical spectroscopy in toluene and dimethylformamide. We use steady-state spectroscopy to characterize the optical properties of the BCs, evidencing their rigid nature and moderately strong light absorption, and the impact the solvent has on their line broadening. We find that the dimethylamino-substituted BC exhibits long (∼3 ns) τ_S1 in both nonpolar and polar solvents, while the addition of a single nitro substituent causes τ_S1 to be short (<200 ps) in polar solvent. We attribute the shortened τ_S1 to relaxation of the lowest singlet excited state directly to the ground state via internal conversion. These results indicate that dimethylamino-substituted BCs are suitable for applications that benefit from long τ_S1 independent of their environment, such as photodynamic therapy, photocatalysis, fluorescence imaging, and quantum information science, while nitro-substituted BCs may be better suited as a probe of environment polarity, for example, via fluorescence lifetime imaging.