Bodipy Dimerization Strategy For Elevating Triplet-triplet Annihilation Upconversion Performance

Triplet-triplet annihilation upconversion（TTA-UC）has broad application prospects in solar photovoltaics,bioimaging,photocatalysis,and other fields due to its advantages of high upconversion quantum efficiency,low-power excitation density,and adjustable excitation/emission wavelengths.Upconversion quantum efficiency is a crucial parameter for TTA-UC in practical applications,for example,upconversion quantum efficiency affects the photoelectric conversion efficiency of photovoltaics.However,the development of TTA-UC systems with high upconversion quantum efficiency remains extremely challenging due to the low fluorescence quantum yield of annihilators at high concentrations,etc.Boron-dipyrromethene（Bodipy）dyes as annihilator is a crucial method to improve the upconversion quantum efficiency of TTA-UC,because Bodipy has the advantages,such as high fluorescence quantum yield,robust photostability,and simplicity of chemical functionalization.TTA-UC generally requires high concentration of annihilator for its application.However,conventional Bodipy dyes have large molar extinction coefficients and small Stokes shifts（<20 nm）,which make them undergo severe internal filtration effects at high concentrations,resulting in lower upconversion quantum efficiency greatly limits the application of Bodipy dyes in TTA-UC.This thesis aims to enhance the upconversion quantum efficiency of TTA-UC by increasing the fluorescence quantum yield of Bodipy annihilators at high concentrations.In this thesis,Bodipy dimer（B-2）with a greater degree ofπ-conjugation was synthesized by the strategy of dimerization of Bodipy monomer（B-1）.Palladium（II）meso-tetraphenyl-tetrabenzoporphyrin（Pd TPBP）,which absorbs red light with a long triplet state lifetime,was used as the photosensitizer to form the TTA-UC system with B-2.UV-Vis absorption spectroscopy,steady-state and transient photoluminescence spectroscopy,and electrochemical and density functional theory calculations（DFT）were used to investigate the photophysical properties of B-2 and the upconversion luminescence performance of the Pd TPBP/B-2 system.The experimental results showed that B-2 has a large Stokes shift（41 nm）and a high fluorescence quantum yield（100%）at high concentration（1 m M）.The upconversion quantum efficiency of Pd TPBP/B-2（10.7%）was higher than the upconversion quantum efficiency of Pd TPBP/B-1（4.0%）by 2.7 times.The results of DFT calculations indicate that the large Stokes shift of B-2 is due to the significant difference in the dihedral angles between the two Bodipy parts of its ground state（S₀state）and the lowest singlet excited state（S₁ state）molecular geometries,which show 71°and 54°,respectively.The large Stokes shift allows B-2 to maintain 100%fluorescence quantum yield at high concentration（1 m M）.The experimental and DTF calculations showed that the significantly higher upconversion quantum efficiency of Pd TPBP/B-2 than Pd TPBP/B-1 was due to the higher fluorescence quantum yield of B-2 than B-1 at high concentrations.Secondly,the larger effective triplet state collision surface of B-2 than B-1 resulted in a higher triplet-triplet energy transfer efficiency of Pd TPBP/B-2（66.9%）than Pd TPBP/B-1（49.1%）.This thesis achieves remarkable improvement in upconversion quantum efficiency at high annihilator concentrations using the Bodipy dimerization strategy,which has important implications for the further development of Bodipy dyes in TTA-UC.