Efficient Enhancement of the Visible-Light Absorption of Cyclometalated Ir(III) Complexes Triplet Photosensitizers with Bodipy and Applications in Photooxidation and Triplet Triplet Annihilation Upconversion
We report molecular designing strategies to enhance the effective visible light absorption of cyclometalated Ir(III) complexes. Cationic cyclometalated Ir(III) complexes were prepared in which boron-dipyrromethene (Bodipy) units were attached to the 2,2'-bipyridine (lapy) ligand via -C C- bonds at either the meso-phenyl (Ir-2) or 2 position of the pi core of Bodipy (Ir-3). For the first time the effect of pi conjugating (Ir-3) or tethering (Ir-2) of a light-harvesting chromophore to the coordination center on the photophysical properties was compared in detail. Ir(ppy)(2)(bpy) (Ir-1; ppy = 2-phenylpyridine) was used as model complex, which gives the typical weak absorption in visible range (epsilon < 4790 M-1 cm(-1) in region > 400 nm). Ir-2 and Ir-3 showed much stronger absorption in the visible range (epsilon = 71 400 M-1 cm(-1) at 499 nm and 83 000 M-1 cm(-1) at 527 nm, respectively). Room temperature phosphorescence was only observed for Ir-1 (lambda(em) = 590 nm) and Ir-3 (lambda(em) = 742 nm). Ir-3 gives RT phosphorescence of the Bodipy unit On the basis of the 77 K emission spectra, nanosecond transient absorption spectra, and spin density analysis, we proposed that Bodipy-localized long-lived triplet excited states were populated for Ir-2 (tau(T) = 231 mu s) and Ir-3 (87.2 mu s). Ir-1 gives a much shorter triplet state lifetime (0.35 mu s). Complexes were used as singlet oxygen (O-1(2)) photosensitizers in photooxidation. The O-1(2) quantum yield of Ir-3 (Phi(Delta) = 0.97) is ca. 2-fold of Ir-2 (Phi(Delta) = 0.52). Complexes were also used as triplet photosensitizer for TTA upconversion; upconversion quantum yields of 1.2% and 2.8% were observed for Ir-2 and Ir-3, respectively. Our results proved that the strong absorption of visible light of Ir-2 failed to enhance production of a triplet excited state. These results are useful for designing transition metal complexes that show effective strong visible light absorption and long-lived triplet excited states, which can be used as ideal triplet photosensitizers in photocatalysis and TTA upconversion.
American Chemical Society
NSFC [20972024, 21073028, 21273028]; Royal Society (UK); NSFC China-UK Cost-Share Science Networks ; Science Foundation Ireland (SFI E.T.S. Walton Program) [11/W.1/E2061]; Fundamental Research Funds for the Central Universities [DUT10ZD212]; Ministry of Education [NCET-08-0077, 20120041130005]; Dalian University of Technology