Thienyl-substituted BODIPYs with strong visible light-absorption and long-lived triplet excited states as organic triplet sensitizers for triplet-triplet annihilation upconversion

Thienyl-substituted BODIPY derivatives were prepared as organic triplet photosensitizers for triplet-triplet annihilation (TTA) upconversion. The photophysical properties of the sensitizers were fully studied with steady state and time-resolved spectroscopy, as well as density functional theory (DFT) calculations. Sensitizers with both 2-monothienyl substituted (BI-1) and 2,6-dithienyl substituted BODIPY cores (BI-2) were prepared. These sensitizers show strong absorption in the visible range. Interestingly, the sensitizers show large Stokes shifts (up to 86 nm) vs. small Stokes shifts (ca. 15 nm) for the normal BODIPY derivatives. DFT/TDDFT calculations show that the large Stokes shifts are due to the remarkable geometry relaxation of the sensitizers upon photoexcitation. The sensitizers show long-lived triplet excited states (up to 95.2 ms at room temperature), which is the longest T-1 state lifetime of organic sensitizers used for TTA upconversion. DFT calculations indicate that the energy level T1 state does not decrease, although the absorption-emission wavelengths of the thiophene-substituted sensitizers are red-shifted compared to the unsubstituted BODIPY, which is beneficial for TTA upconversion. The organic sensitizers were used for TTA upconversion and upconversion quantum yields up to 16.5% were observed, compared to the quantum yields of 0.6% and 6.1% observed previously with organic triplet sensitizers. The efficient TTA upconversion is attributed to the enhanced triplet-triplet-energy-transfer (TTET) process, confirmed by the lifetime quenching experiments of the photosensitizers. Our results are useful for the design of new efficient organic triplet photosensitizers to replace the currently used phosphorescent transition metal complex sensitizers for TTA upconversion and other appropriate photophysical processes, such as photocatalysis, photodynamic therapy, etc.