Page - 43 - in Photovoltaic Materials and Electronic Devices

X-ray crystal structure of 43b shows a typical geometry with both ligands coordinated in a meridional fashion; bond distances between Ru and the coordinated N or C are similar and the carboxyl function is deprotonated (Figure 34). Overall efficiencies were far from N719 tested in the same conditions, a result that was mainly attributed to low charge injection. Figure 34. ORTEP drawing of complex 43b [142] (Reprinted with permission from Park, H.-J.; Kim, K. H.; Choi, S. Y.; Kim, H.-M.; Lee, W. I.; Kang, Y. K.; Chung, Y. K. Unsymmetric Ru(II) Complexes Figure 34. ORTEP drawing of complex 43b [142] (Reprinted with permission from Park, H.-J.; Kim, K. H.; Choi, S. Y.; Kim, H.-M.; Lee, W. I.; Kang, Y. K.; Chung, Y. K. Unsymmetric Ru(II) Complexes with N-Heterocyclic Carbene and/or Terpyridine Ligands: Synthesis, Characterization, Ground- and Excited-State Electronic Structures and Their Application for DSSC Sensitizers. Inorg. Chem. 2010,49,7340–7352. Copyright2010AmericanChemicalSociety). Kinoshita et al. [144,145]spentefforts inorder to furtherextendtheabsorption of BD. In conventional Ru(II) complexes, short-lived 1MLCT states immediately relax to long-lived 3MLCTstates throughintersystemcrossing. Thespin-forbidden singlet-to-triplet transition from HOMO to 3MLCT has been observed for a phosphine-coordinated Ru(II) sensitizer (44 in Figure 35), providing light conversion up to1000 nm andunprecedented charge injection (26.8 mA/cm´2). Unfortunately noevidenceabout long-termstabilityof thiscomplexwasreported. Materials 2016, 9, 137 23 of 37 with N-Heterocyclic Carbene and/or Terpyridine Ligands: Synth sis, Charac erization, Ground- and Excited-State Electronic Structures and Their Applicat on for DSSC Sensitizers. Inorg Chem. 2010, 49, 7340–7352. Copyright 2010 American Che ical Society). Bonacin et al. [143] propo ed a complex of Ru(II) with carboxyphenyl tpy, thiocyanate, and 8-hydroxy quinoline in order to host a carboxy ethyl cyclodextrin anchored to TiO2. Even if poor results were reported (ascribed to high HOMO potential and low regeneration), the host-guest interaction of the dye with the cyclodextrin increased the performances by preve ting dye aggregation and limiting the dark current. Kinoshita et al. [144,145] spent efforts in order to further extend the absorption of BD. In conventio al Ru(II) complexes, short-lived 1MLCT states immediately relax to long-lived 3MLCT states through intersystem crossing. The spin-forbidden singlet-to-triplet transition from HOMO to 3MLCT has been observed for a phosphine-coordinated Ru(II) sensitizer (44 in Figure 35), providing light conversion up to 1000 nm and unprecedented charge injection (26.8 mA/cm-2). Unfortunately no evidence about long-term stability of this complex was reported. Figure 35. Phosphine-coordinated Ru(II) sensitizer by Kinoshita et al. [144]. Recently, Li used 2,2’-dipyrromethanes as N^N’ bidentate ligand in order to substitute thiocyanates in the BD structure. The dipyrromethanes having 5-pentafluorophenyl and 2-thienyl substituents gave IPCE curves showing a sensitization up to 950 nm (45, Figure 36) [146]. Figure35. Phosphine-coordinatedRu(II) sensitizerbyKinoshita et al. [144]. 43