Page - 28 - in Photovoltaic Materials and Electronic Devices

thiocyanate ligand has the role to tune the spectral and redox properties of the sensitizersactingonthedestabilizationof themetal t2gorbital [93]. Byexchanging these ligands with σ-donor groups, it was possible to tune the photochemical properties of the complex, and to minimize the drawbacks associated with these monoanchored ligands. In fact, the possible formation of isomers, owing the bidentate character of the thiocyanate ligand causes a decrease in the synthetic yield[21,78,94]. Moreover theweakRu-NCSbonditself leads toadecreasedstability of the complex and, more importantly, thiocyanate lacks of an effective chromophore that could improve IPCE, particularly at shorter wavelengths. All these features encouraged the engineeringof new heteroleptic cyclometalated complexesstarting from Black Dye, by exchanging one or more thiocyanate ligands. A drawback affectingthiskindofmodification is thedestabilisationofHOMOorbitals thatcan leadtoa lowerdrivingforce in thedyeregenerationbytheelectrolyte. Strategies for the design of Ru tridentate heterocyclic ligands tailored to tune the properties of the excited state were recently reviewed by Pal et al. [95]. Medlycott [96] in 2005 surveyed the strategies for improving the photophysical properties of tridentate ligands commonly considered weaker than bipyridine ones, and Hammarstrom et al., in 2010 [97], investigated the possibility to expand their bite angle. In the following paragraphs we will report an overview of ancillary ligands properly synthesizedto tune the photoelectrochemical properties of tpy for applications inDSCs. 3.2.1. Bipyridines Ancillary ligandexchangewaspioneeredin1997byZakeeruddin etal. [25]who substituted two of the three thiocyanates with a 4,4’-dimethyl-2,2’-bipyridine. In this case, the tpy ligand was not represented by tctpy, but by a simpler tpy with a phosphonicacidanchoringgroup(Figure14). Materials 2016, 9, 137 12 of 37 photochemical properties of the complex, and to minimize the drawbacks associated with these monoanchored ligands. In fact, the possible formation of isomers, owing the bidentate character of the thiocyanate ligand causes a decrease in the synthetic yield [21,78,94]. Moreover the weak Ru–NCS bond itself leads to a decreased stability of the complex and, more importantly, thiocyanate lacks of an effective chromophore that could improve IPCE, particularly at shorter wavelengths. All these features encouraged the engineering of new heteroleptic cyclometalated complexes starting from Black Dye, by exchanging one or more thiocyanate ligands. A drawback affecting this kind of modification is the destabilisation of HOMO orbitals that can lead to a lower driving force in the dye regeneration by the electrolyte. Strategies for the design of Ru tridentate heterocyclic ligands tailored to tune the properties of the excited state were recently reviewed by Pal et al. [95]. Medlycott [96] in 2005 surveyed the strategies for improving the photophysical properties of tridentate ligands commonly considered weaker than bipyridine ones, and Hammarstrom et al., in 2010 [97], investigated the possibility to expand their bite angle. In the following paragraphs we will report an overview of ancillary ligands properly synthesized to tune the photoelectrochemical properties of tpy for applications in DSCs. 3.2.1. Bipyridines Ancillary ligand exchange was pi ne red in 1997 by Zakeeruddin et al. [25] who substituted two of the three thiocyanates with a 4,4’-dimethyl-2,2’-bipyridine. In this case, the tpy ligand was not represented by tctpy, but by a simpler tpy with a phosphonic acid anchoring group (Figure 14). Figure 14. First example of tpy Ru-complex showing a bipyridine instead of two thiocyanates [25]. This research topic became of interest again when, in 2011, Chandrasekharam et al. [98] proposed to substitute two thiocyanate ancillary ligands with a bipyridine having electron donor styryl moieties in 4,4’- position (20a-b, Figure 15). Worse panchromatic behavior was observed with respect to BD, but also better performances in device, owing to an increased molar extinction coefficient in the visible region. A low value of fill factor led to a 3.36% best efficiency, higher with respect to that of BD evaluated in the same conditions (TiO2: 9 + 4.8 μm, ethanol solution, Z580 electrolyte: 0.2 M I2, 0.5 M GuSCN, 0.5 M N-methylbenzimidazole in [bmim] [I] / 1-ethyl-3-methylimidazolium tetracyanoborate 65:35). Similar bipyridines, slightly modified in the Figure14. Firstexampleof tpyRu-complexshowingabipyridine insteadof two thiocyanates [25]. 28