Syntheses Of Triplet Sensitizers And Their Applications For TTA Upconversion

Triplet—triplet annihilation (TTA) upconversion has attracted much attention owing to its potential applications in bioimaging, photovoltaics and photocatalysis. Triplet sensitizer is the crucial component of TTA upconversion, and their photophysical properties (absorption wavelength, molar extinction coefficient and triplet lifetime, etc.) significantly affect the efficiency of TTA upconversion. So far, the commercial available sensitizers are scarce and limited to only a small number of Ru(Ⅱ) polyimine complexes, Pt(Ⅱ)—Pd(Ⅱ) porphyrin complexes. As TTA sensitizer, these complexes have the shortage of weak absorption at visible wavelength or short triplet lifetime. The present thesis aims to synthesize triplet photosensitizers (PSs) with intense absorption in visible range and long-lived triplet excited states and to investigate their applications in TTA upconverion.Based on the reported results and DFT/TDDFT calculations, two molecular design rationales for developing new transition-metal-complex PSs have been proposed which are:(1) when dangle, or tether a visible light-harvesting antenna to the central atom, i.e. there is no π-conjugation between the central atom and the light harvesting ligand, the1IL/3IL energy level of the ligand should be higher or close to the respective energy level of the metal-to-ligand-charge-transfer (’MLCT and3MLCT) states, otherwise, the cascade energy transfer process1IL�'3IL�'3MLCT will be perturbed.(2) When a visible light-harvesting ligand has been attached to the central atom via a conjugation bond, such as a C≡C bond, or through direct metalation of a fluorophore (the spin density centre of the antenna as the carbon donor of the C-metal bond), will facilitate the direct ISC of1IL�'3IL, and the wavelength of the sensitizer can be extended to the red-end of the spectrum.The first visible-light-harvesting Ru(Ⅱ)-coumarin array Ru-2has been synthesized and the energy of the donor excited state is efficiently converted to acceptor excited state (i.c., efficient energy transfer) without losses in the acceptor quantum yield. We have also demonstrated that the light-harvesting effect can be used to improve the upconversion quantum yield to15.2%, which is improved by16-fold compared to the model complexes Ru-1and Ru(dmb)3without visible-light-harvesting ability. The overall upconversion capability or apparent brightness (η=ε×ΦUC) is improved by nearly90-fold compared to Ru-1. Direct metalation of the fluorophore Bodipy with NΛN Pt、 NΛCΛN Pt and linear platinum(Ⅱ) bis(aryleneethynylene) with phosphine ligands, has significantly improved the visible-light absorbability of these complexes. The wavelength of these complexes with Bodipy antenna has been shifted to the red-end of the spectrum, for Pt-10,λabs=643nm, ε=42300M-1cm-1. Room temperature long-lived near-IR phosphorescence of Bodipy was observed for the first time (Pt-6, Φp=3.5%, τT=128.4μs), which demonstrated that3IL states localized on Bodipy in these complexes were populated efficiently. The complexes have been used as efficient triplet photosensitizers for TTA upconversion, upconversion quantum yields up to19.0%and anti—Stokes shift up to0.84eV were observed.The drawbacks of the transition-metal-complex triplet PSs are apparent. Herein, for the first time a small library of organic triplet sensitizers derived from a single chromophore for which the intense UV-vis absoiptions cover a wide range of510nm-629nm (ε is up to180000M-1cm-1) have been prepared. And for the applications of these sensitizers in TTA upconversion, anti-Stokes shift range to0.37-0.74eV and ΦUC up to28.5%were observed.In order to get heavy-atom free (without metal, without Br, I, etc.) organic triplet PSs that show predictable ISC, C60was used as a spin converter, Bodipy as the antenna, and a universal strategy for devising heavy-atom free PSs was proposed. Two visible light-harvesting C60-bodipy dyads B-9, B-10have been prepared. The dyads have been used as triplet photosensitizers for TTA upconversion and an upconversion quantum yield up to7.0%was observed. Therefore, replacement of the currently used phosphorescent transition metal complex triplet photosensitizers with heavy atom-free organic triplet photosensitizers has been achieved.