Synthesis And Application Of The Photosensitizers With Long-lived Triplet Excited State

Transition metal Pt(II) complex is a kind of triplet photosensitizer with superior performance, which has a wide range of applications such as in the upconversion, luminescent oxygen sensing and singlet oxygen (1O2) sensitizing. It is important to enhance the visible light absorption and to prolong the triplet excited state lifetimes of these triplet photosensitizers, and the applications of these compounds in upconversion, oxygen sensing can be improved and the triplet energy transfer efficiency can be increased. Therefore, it is very important to enhance the visible light absorption and to prolong the triplet excited state lifetime of triplet photosensitizers.Triplet-triplet-annihilation upconversion has attracted much attention due to its advantages of low lower excitation (sunlight is sufficient), strong absorption of visible light and high upconversion quantum yields. We used Pt(II) complexes with different ligands as triplet sensitizers and perylene or DPA as energy acceptors, different upconversion effect were observed. The result show that the3IL excited state with longer excited state lifetime is much more efficient in sensitizing the TTA-based upconversion than the3MLCT excited states.We found that the excited states of triplet complexes can be readily tuned by selection of different acetylide ligand of the N∧N Pt(II) bisacetylide complexes (N∧N is4,4’-di-tert-butyl-2,2’-bipyridine). We prepared complex Pt-1with ethynylated naphthalenediimine(NDI) as the acetylide ligand. Pt-1shows a long-lived triplet excited state (3IL, τT=22.3μs), which is20times of the model complex N∧N Pt(II) bis(phenylacetylide). With DFT/TDDFT calculations and nanosecond time-resolved spectroscopy, we found that the triplet excited state of Pt-1is3IL state rather than the traditional3MLCT state, which is the main reason of the long-lived triplet excited state lifetime of Pt-1. It is also the first time that room temperature phosphorescence of NDI (in near-infrared region at784nm) was observed. In addition, through the introduction of NDI, Pt-1shows intense absorption in the visible range, with red shift wavelength and increased molar extinction coefficient, these photophysical properties of Pt-1is ideal for triplet-triplet-annihilation upconversion. Using perylene as energy acceptor, TTA upconversion quantum yield of9.5%was observed for Pt-1.Using BF2-bound ligand, which has excellent light absorption and light stability, we regulated the excited state of Pt(II) complex effectively. We studied the emitting mechanism of this Pt(II) complex by steady-state and time-resolved spectroscopy, emission at77K and DFT theoretical calculations. The complex was also used as a triplet sensitizer for triplet-triplet-annihilation based upconversion and upconversion quantum yield was observed as8.9%.We also designed and synthesized a cyclometalated Pt(II) complex. Its triplet state lifetime is up to34.6μs, which made its upconversion efficiency further improved compared with the previous two complexes. Its upconversion quantum yield was23.3%.Next, we used the efficient intersystem crossing (ISC) capability of fullerene to access the triplet state of perylenebisimide and avoided the use of precious metal atom for the ISC effect. This method provides a new approach for the design of heavy-atom-free triplet photosensitizers. Our results show that visible light-harvesting perylenebisimide reaches the singlet excited state upon photoexcitation and the energy was transferred to fullerene (the energy transfer efficiency up to98%), then fullerene reach the triplet excited state (the ISC efficiency up to95%) via the intrinsic ISC ability and in turn the backward triplet state energy transfer from fullerene to perylenebisimide occurs. The lifetime of the T1state of this compound is105.9μs. Based on this property, our molecule can be used for1O2sensitizing. We demonstrated that the photooxidation of1,5-dihydroxylnaphthalene (DHN) with C60dyads as the1O2photosensitizer is more efficient than that with the conventional Ir(III) complex1O2photosensitizer.