Photophysics and redox properties of aza-BODIPY dyes with electron-withdrawing groups

Abstract

The optical and electrochemical properties are compared for aza-BODIPY dyes that differ by virtue of the substituents at 1,7- and 3,5-positions of the aza-BODIPY backbone. In particular, the impact of highly electron-withdrawing nitro substituents on these properties is of consideration. The aza-BODIPYs studied exhibit a broad absorption band spanning from ca. 600 nm to ca. 700 nm – a spectral region that is highly sought after for biomedical applications – and possess high molar absorption coefficients (ε) ranging from 64 000 [M⁻¹ cm⁻¹] to 85 000 [M⁻¹ cm⁻¹] in THF. The compounds studied are weakly emissive as non-radiative decay is the predominant way of the excited state energy dissipation. Exemplarily, the fluorescence quantum yields (Φ_fl) measured in THF were in the range of 0.03 to 0.06 for the aza-BODIPYs studied. Transient absorption experiments revealed T₁ → T_n absorption spanning from ca. 400 nm to ca. 600 nm and allowed determination of the triplet state lifetimes. The estimated triplet lifetimes (τ_T) in deaerated THF ranged from 77 µs to 130 µs. As estimated by the CV/DPV measurements, all aza-BODIPYs studied exhibited one irreversible oxidation and two quasi-reversible reductions. Estimation of the E_HOMO gave a value of ca. −5.8 eV while the E_LUMO was found to be located at ca. −4.5 eV. Extremely high photostability and thermal robustness up to approximately 300 °C were found for the nitro-substituted aza-BODIPYs.