Sythesis And Application Of Triplet Photosensitizers Based On BODIPY

Triplet photosensitizers have attracted much attention due to their wide applications in organic electroluminescent, biological imaging, photocatalytic hydrogen production, and triplet-triplet annihilation upconversion. As transition metal compounds, transition metal complexes though have high intersystem crossing effect, their use is greatly restricted because of the low ability of light capturing and the short triplet lifetime. Thus, it is an important but challenging work to design new triplet photosensitizers meeting the requirements. BODIPY(Boron-dipyrromethene) is widely concerned as a new chromophore because of its excellent physical properties. Owing to the advantages of BODIPY chromophores(the stronge ability to capture light, well photostability, sufficient chemical modification sites and suitable intersystem crossing effect), we designed and synthesized a series of novel triplet photosensitizers with intersystem crossing effect. Their properties were investigated via the steady, transient spectroscopys and theoretical calculation methods.Based on FRET mechanism, we obtained two BODIPY compounds(B-BDP and S-BDP) linking two different BODIPY chromophores with flexible chain. These two compounds have high light absorption ability and show broadband absorption. S-BDP shows larger Stokes shift and certain singlet oxygen quantum yield. With steady spectroscopys, we proved the intramolecular singlet energy transfering from unsubstituted BODIPY part to benzene-/thiophene-substituted BODIPY part in these compounds. With transient spectrum, we observed S-BDP had triplet absorption ability with a long triplet lifetime of 484.9 μs. The riplet signal of compound B-BDP hasn’t been observed.Conjugating the thiophene groups with BODIPY, we obtained heavy-atom free compound BDP-1 which show efficient intersystem crossing and a singlet oxygen quantum yield of 58.9%. With transient spectrum, we observed compound BDP-1’s triplet singal, with a very high triplet state quantum yield of 63.7%. Two reference BODIPY derivatives [containing no sulfur atom(BDP-2) and thiophene groups just participating in single bond with the BODIPY core(BDP-3)] do not show any significant ISC(ΦT is negligible for BDP-2, and ΦT = 6.1% for BDP-3). Theoretical computations demonstrate that the reasons of the increased ISC for heavy-atom free compound BDP-1 as compared with BDP-2 and BDP-3 are due to i) the participation of the sulfur atom on the lowest-lying excited states, leading to moderate SOCs and ii) a small singlettriplet energy gap in BDP-1.Linking iodo-BODIPY chromophore with perylene, we obtained compound P-B with a broadband visible light absorption ability. Its UV/vis absorption spectrum almost overlays iodo-BODIPY and Perylene parts’ absorption spectra, with maximum absorption at 538 nm(ε = 55400 M-1 cm-1). With steady spectroscopys, we proved that the intramolecular singlet energy transfered from iodo-BODIPY part to perylene part. Compound P-B has been successfully applied in TTA upconversion and the upconversion fluorescence of Perylene(acceptor) was observed(the quantum yield was 3.9%).