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

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 Δ E 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 Δ E 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 T 1 state. The small Δ E (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 Δ E HOMO-LUMO explain the redshift in emission wavelength of thieno-pyrrole-fused BODIPY. The more planar configuration of the S 1 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 1 O 2 production for thieno-pyrrole-fused BODIPYs with electron-withdrawing groups at the para -position of the phenyl groups. Due to Δ E S0-T1 < 0.980 eV, the substitution of electron-donating groups cannot produce 1 O 2 . 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.