Theoretical study on the photophysical properties of thiophene-fused-type BODIPY series molecules in fluorescence imaging and photodynamic therapy

Abstract

As a class of photosensitizers (PSs) with dual functions of photodynamic therapy (PDT) and fluorescence imaging, the relationship between the structure and dual-function of thiophene-fused-type BODIPY dyes has not been studied in depth before. We found that the thiophene-fused-type BODIPY triplet photosensitizer is produced according to the energy level matching rule and the introduction of the thiophene ring significantly reduces the energy gap ΔE_ST between singlet and triplet states, as revealed by our investigation of the excited state structures and energies of thieno-fused BODIPY dyes. At the same time, a tiny ΔE_ST also results in a greatly enhanced intersystem crossing (ISC) rate, k_ISC. The k_ISC value of MeO-BODIPY, having the highest singlet oxygen quantum yield (Φ_Δ), is the largest. Substitution with a strong electron donor N,N-dimethylaminophenyl (DMA) leads to the vertical configuration in the T₁ state. The small ΔE (0.0029 eV) between the HOMO and HOMO−1 triggers the photo induced electron transfer (PET) of inhibiting ISC and fluorescence. When thieno-fused BODIPYs react with pyrrole, the increase of π-conjugation and smaller ΔE_HOMO–LUMO explain the redshift in emission wavelength of thieno-pyrrole-fused BODIPY. The more planar configuration of the S₁ state and the stronger oscillator intensity reflect a higher fluorescence quantum yield (Φ_F). The extension of π-conjugation can cause molecules to transition to higher-level singlet excited states (S_n states, n ≥ 1) after absorbing energy and reduce the energy level of the excited state, resulting in multiple channels and favoring ¹O₂ production for thieno-pyrrole-fused BODIPYs with electron-withdrawing groups at the para-position of the phenyl groups. Due to ΔE_S0–T1 < 0.980 eV, the substitution of electron-donating groups cannot produce ¹O₂. In this work, we have revealed the mechanism of ISC and the fluorescence emission process in the thiophene-fused-type BODIPY dye, which has provided a theoretical foundation and guidance for the future design of BODIPY-based heavy-atom-free PSs for molecular applications in PDT.