The Triplet Photosensitizer Of Flavin Derivatives: Design,Synthesis,and Applications

Photosensitizer is a special kind of molecule with special photochemical and photophysics properties.They are widely used in Triplet–Triplet Annihilation based Up-Conversion?TTA-UC?,photocatalysis,photodynamic therapy?PDT?,oxygen sensing,etc.The applications of transition metal complexes are always limited by the weak absorption in visible light region and short triplet lifetime.Rational design of molecular structure by introduction of suitable chromophores may help to improve the absorption in the visible light region and at some level contribute to prolonged triplet lifetime and so as promoted energy transfer efficiency of the transition metal complexes.Organic dyes are always companied with short excited lifetime and unreachable high triplet energy levels,so their efficiencies for energy transfer are low.It is possible to turn on the interconversion pathways for these organic dyes,if heavy atoms can be introduced into their conjugated frameworks.Flavin and its derivatives are of excellent performance as nontoxic organic dyes and are also the building blocks and functional groups within many enzymes,with strong adsorption in visible light region.In this work,we used flavin as the chromophore and bonded it directly to Ru^II complex and halogen atoms to activate the intersystem crossing pathways via heavy atom effect.Several photosensitizers were synthesized with their photophysics properties characterized and their performance in TTA-UC and photocatalysis tested.The key findings are as the following:?1?Using tris?bipyridine?Ru^II complex?Ru-0?as electron donor and flavin as electron acceptor,a Ruthenium^II complex?Ru-1?with flavin moiety connected to one of the bipyridine ligand via acetylene bond was designed and synthesized,and the photophysics properties were investigated.Comparing with Ru-0 which has?=1.36×10⁴ M^-1 cm^-1 at 453 nm,the introduction of flavin moiety facilitates Ru-1 with strong absorption in the visible range(?=2.34×10⁴ M^-1 cm^-1 at 456 nm).The emission of Ru-1 can be quenched by O₂ which proves the emission of Ru-1 is phosphorescence.The emission wavelength of Ru-1 at 651 nm is red shifted 56 nm than Ru-0.This is the first time that the phosphorescence was observed within Ru^II complexes with flavin moiety.We proposed that the excited state of Ru-1 is from low lying triplet excited state of ³IL nature,according to the time-resolved transient difference absorption spectra and DFT/TDDFT calculations.Ru-1 was used for TTA-UC,showing a fair quantum yield of 0.7%.The findings pave the way for rational design of phosphorescence transition metal complexes.?2?Using Br for introduction of heavy atom effect to facilitate the intersystem crossing to reach the triplet excited state of flavine by directly bond Br into the conjugated framework of flavin,a dibro-flavin was synthesized and the structure and photophysics properties were characterized.Comparing with the alternative compound flavin which has?=1.01×10⁴ M^-1cm^-1 at 441 nm,dibro-flavin showed stronger absorption in the visible range(?=1.90×10⁴M^-1 cm^-1 at 450 nm).The heavy atom effect of Br enhances the singlet oxygen quantum yield from 55.3%to 99.2%at the expense of decreased luminance quantum yield from 37.7%to5.5%.This suggests that at least a portion of the excited dibro-flavin molecules evolve to triplet excited states as expected.The compounds were used in photocatalytic oxidation of various sulfoethers.As expected,dibro-flavin exhibits a superior photocatalytic performance with respect to flavin in sensitizing O₂ for photocatalytic oxidation.In controlled experiments,2⁵ fold performance enhancements with respect to flavin were observed.Further to these,the photocatalytic oxidation with dibro-flavin was found highly selective.Detailed mechanism investigation showed that both singlet oxygen and superoxide anion radical are formed as oxidative species.The findings pave the way for design and application of novel organic sensitizers for photocatalytic oxidation.