Arylated Bipyridyl Pt~Ⅱ Complexes With Long-Lived Triplet States

With good photostability,efficient intersystem crossing(ISC)and feasible derivatization,bipyridyl PtⅡ complexes have attracted extensive attention in photocatalysis,photodynamic therapy(PDT),molecular sensors and triplet-triplet annihilation(TTA)upconversion.However,traditional bipyridyl PtⅡ complexes have disadvantages such as weak absorbance in the visible range,short triplet lifetime(usually less than 10 μs,which is assigned to the metal-to-ligand charge transfer state,i.e.3MLCT state)and narrow absorption band(with only one kind of chromophore),which leads to their low utilization of broadband natural light source(e.g.sunlight)and limits their application prospect.In this thesis,organic chromophores with strong visible light absorption were introduced into PtⅡ coordination center to enhance the absorbance in visible light region.By selecting appropriate ligands,intraligand triplet state(3IL state)becomes the lowest triplet excited state,which prolongs the triplet state lifetime.Broadband absorption and intramolecular energy transfer(FRET)were achieved by introducing polychromophore(mixed coordination).Herein,three groups of strong visible light harvesting bipyridyl PtⅡ photosensitizers with long-lived triplet states have been designed and synthesized,and their photophysical properties were studied by steady-state and time-resolved spectroscopies.The specific research results are as follows:Using different substituted ethynyl Bodipy ligands,strong visible light harvesting(the molar extinction coefficient at 500 nm exceeds 1.0 × 105M-1 cm-1)heteroleptic bipyridyl PtⅡcomplex 2-Ptl with broad-band visible light absorbing(from 400 nm to 650 nm)were synthesized,which overcomes the disadvantages of traditional PtⅡ complexes such as weak absorption in the visible region and narrow absorption band.The triplet-state lifetime of 2-Ptl(54 μs)is 57-fold of the bipyridyl PtⅡ complex with phenylacetylene(0.94 μs),and 2-Ptl has a high triplet quantum yield as 91%.By introducing pyrene and naphthalimide(NI)into the bipyridyl PtⅡ coordination framework,heteroleptic bipyridyl PtⅡ complex 3-Ptl with efficient intramolecular energy transfer was synthesized.The TTA upconversion quantum yield(34%)of 3-Ptl is higher than the reference complexes with only one kind of chromophore 3-Pt2(28%)and 3-Pt3(15%).Taking advantage of the two closely-lying triplet states,i.e.the NI localized emission state(3LE,2.0 eV)and 3MLCT state(2.2 eV)of the bipyridyl PtⅡ complexes,4-Ptl and 4-Pt2(with steric hindrance groups)with reversible triplet-triplet energy transfer(triplet equilibrium)were designed and synthesized,and the equilibrium was effectively tuned by solvent polarity and temperature.The equilibrium constant(Keq)of 4-Ptl(5.2 x 103)in benzonitrile is 530-fold of 4-Pt2(9.8)in toluene.Moreover,to improve the application of PtⅡ complex in TTA upconversion,a series of anthracene derivatives were prepared as triplet acceptors/emitters for TTA upconversion with 5-Ptl as photosensitizer.With different triplet energy levels,the upconversion quantum yields of A-2(1.32 eV)and A-4(1.26 eV)are the same(11.8%).A-5 has a similar triplet energy level(1.21 eV)as A-4,but its upconversion quantum yield is as low as 1.3%.These differences come from the TTA efficiencies of the acceptors by the study of intermolecular photophysical processes between the photosensitizer and acceptors.Thus,the energy gap between the sensitizer and acceptor,as well as the relative Ti and Si energy levels of the acceptor should both be considered to improve the TTA upconversion quantum yield.The former determines the efficiency of intermolecular energy transfer,while the latter directly affects the yield of TTA emission.These studies will be useful for future development and application of bipyridyl PtⅡ photosensitizers.