Page - 20 - in Photovoltaic Materials and Electronic Devices

case N-methyliminodiacetic acid (MIDA [66]) boronates have been successfully applied as key reagents to obtain quaterpyridine ligands in good yields [67] through Suzuki-Miyaurareaction. Inorder toobtainRu(II) complexesofpolypyridines,Adeloye et al. [18]usedRu p-cymene or Ru(III)Cl3 as starting materials and they substituted the chlorines with thiocyanatesorotherancillary ligands. Exploitingmicrowave-assistedsynthesis, a facileprocedure toobtainafunctionalizedqtpyligandandits trans-dithiocyanato rutheniumcomplexhasbeenreported[68] (Scheme3). Materials 2016, 9, 137 6 of 37 Scheme 3. Microwave-assisted synthesis of the trans-Ru (II) complex.[68] 3. Modifications of Black Dye and Structure–Properties Relationships on Devices 3.1. Terpyridine modification In this section, tpy based ruthenium complexes bearing three thiocyanates as ancillary ligands will be reviewed, outlining structural modifications on tpy ligand and their effects on DSCs performances. Molecular engineering on tpy ligands has commonly the aim to extend π-conjugation in order to increase the molar extinction coefficient and further stabilize the LUMO level. In this way more photons can be harnessed and converted thanks to a simultaneous hyperchromic effect and bathochromic shift in the absorption spectra, respectively. Other common structural modifications are the substitution of one of the three pyridines with either a donor group (such as triphenyl amine), in order to enhance the push-pull system character, or a hydrophobic group, in order to reduce recombination with the electrolyte. Particularly interesting are the structural variations related to the anchoring moieties. The tctpy used in BD offers three possible anchoring points, allowing a proper sensitizer-semiconductor coupling and improving the stability of the device. Moreover, alternative anchoring groups, with respect to the carboxylic acid functionality, have been tested. Zakeeruddin [25] proposed a terpyridine functionalized with a phosphonic acid group on 4’- position with the purpose of overcoming the slow desorption of the carboxyl anchoring group from the semiconductor surface in presence of water. Waser [69] proposed a tpy bearing a phosphonic acid functionality, coupled with TiO2 for DSCs and water splitting applications, while Anthonysamy et al. [70] proposed a 4’-methacryloyloxymethylphenyl moiety as an anchoring group. As far as the carboxyl anchoring group is concerned, in 2002 Wang et al. [71] tested a 4’-carboxyphenyl substitution (Figure 4), obtaining an appreciable bathochromic shift with respect to N3 (cis-diisothiocyanato-bis(2,2’-bipyridyl-4,4’-dicarboxylic acid) ruthenium(II)), but a sensible loss in short circuit current in comparison with BD occurred, which can be explained by the fewer grafting points on the structure. Scheme3. Microwave-assistedsynthesisof the trans-Ru(II) complex [68]. 3. ModificationsofBlackDyeandStructure-PropertiesRelationships onDevices 3.1. Terpyridin m dification In thissection, tpybasedrutheniumcomplexesbearingthree thiocyanatesas ancillary ligandswillbereviewed,outliningstructuralmodificationsontpy ligand andtheireffectso DSCsperf rmances. Molecular engineering on tpy ligands has commonly the aim to extend pi-conjugationinordertoincreasethemolarextinctioncoefficientandfurtherstabilize theLUMOlevel. I thiswaymorephotonsc nbeharnessedandconvertedthanks to simultaneous hyperchromic effect and bath chromic shift in the absorption spectra, respectively. Other common structural modifications are the substitution of one of the three pyridines with either a donor group (such as triphenyl amine), in order to enhance the push-pull system character, or a hydrophobic group, in order tore cerecombi ationwithth electrolyte. Particularl interestingare the structuralvariationsrelatedto theanchoringmoieties. ThetctpyusedinBDoffers threepossibleanchoringpoints,allowingapropersensitizer-semiconductorcoupling andimprovingthestabilityof thedevice. Moreover,alternativeanchoringgroups, with respect to the carboxylic acid functionality, have been tested. Zakeeruddi [25] proposedaterpyridine functionalizedwithaphosphonicacidgroupon4’-position 20