Page - 19 - in Photovoltaic Materials and Electronic Devices

Materials 2016, 9, 137 5 of 37 Scheme 2. Example of the Kröhnke pathway. The second strategy exploits recent advances in organometallic reactions (cross-coupling in Scheme 1). The electron poor pyridines are less effective in the Suzuki reaction [52] due to the weaker electrophilicity of pyridyl-boronates with respect to other organometallic reagents, such as the organo-tin involved in Stille reaction [53]. Noteworthy, the synthetic pathway used to achieve 4,4’,4’’-tricarboxy-2’,6’-terpyridine (tctpy) for Black Dye [54] involves the formation of the terpyridine core starting from 4-ethyl pyridine refluxed with Pd/C over nine days. This procedure was further improved by Dehaudt et al. [55]. Among the other possible strategies to obtain a tpy core, it is worth noting an inverse Diels-Alder reaction on 1,2,4-triazine that uses 2,5-norbornadiene as dienophile [56]. 2.2. Functionalization of Terpyridines In order to design complexes suitable for DSCs applications a series of modifications has to be taken into consideration, with the aim of introducing anchoring moieties, donor groups, bulky alkyl chains, or extending the π-conjugation. Cross-coupling reactions represent the most frequently used synthetic tool, while more specific pathways include the formation of carboxylic acid by furan degradation [57–60]. Other common syntheses are dealing with pyridine functionalizations; for example, the pyridine N-oxide is used as an intermediate to obtain halogen and pyrrolidinyl functionalizations [61,62], while 4-pyridones analogues are used to have access to halogens or triflates derivatives [63]. Husson et al. reviewed the derivatizations with thienyl [56] and furanyl [64] moieties while recently Woodward et al. [65] reported a synthetic strategy to further extend the scope and number of the anchoring moieties on oligopyridines. 2.3. Quaterpyridine Synthesis and Complex Formation The synthesis and functionalization of qtpy usually exploit the same synthetic strategies used for tpy, namely Kröhnke and coupling reactions. In the latter case N-methyliminodiacetic acid (MIDA [66]) boronates have been successfully applied as key reagents to obtain quaterpyridine ligands in good yields [67] through Suzuki-Miyaura reaction. In order to obtain Ru(II) complexes of polypyridines, Adeloye et al. [18] used Ru p-cymene or Ru(III)Cl3 as starting materials and they substituted the chlorines with thiocyanates or other ancillary ligands. Exploiting microwave-assisted synthesis, a facile procedure to obtain a functionalized qtpy ligand and its trans-dithiocyanato ruthenium complex has been reported [68] (Scheme 3). Scheme2. Exampleof theKröhnkepathway. The second strategy exploits recent advances in organometallic reactions (cross-coupli g in Scheme 1). The electron poor pyridines are less effective in the Suzuki reaction [52] due to the weaker electrophilicity of pyridyl-boronates with respect to other organometallic reagents, such as the organo-tin involved in Stille reaction[53]. N teworthy, the synthetic pathway used to achieve 4,4’,4”-tricarboxy-2’,6’-terpyridine (tctpy) forBlackDye[54] involves the formation of the terpyridinecorestartingfrom4-ethylpyridinerefluxedwithPd/Covernine days. This procedure was further improved by Dehaudt et al. [55]. Among the other possible strategies to obtain a tpy core, it is worth noting an invers Diels-Al er reactionon1,2,4-triazine thatuses2,5-norbornadiene sdienophile [56]. 2.2. FunctionalizationofTerpyridines In order to design complexes suitable for DSCs applications a series of modifications has to be taken into consideration, with the aim of introducing anchoringmoieties,donorgroups,bulkyalkylchains,orextendingthepi-conjugation. Cross-coupling reactions represent the most frequently used synthetic tool, while more specific pathways include the formation of carboxylic acid by furan degradation [57–60]. Other common syntheses are dealing with pyridine functionalizations; for example, the pyridine N-oxide is used as an intermediate to obtain halogen and pyrrolidinyl functionalizations [61,62], while 4-pyridones analoguesareusedtohaveaccesstohalogensortriflatesd rivatives[63]. Hussonetal. reviewedthederivatizationswiththienyl[56]andfuranyl[64]moietieswhilerecently Woodward et al. [65] reported a synthetic strategy to further extend the scope and numberof theanchoringmoietiesonoligopyridines. 2.3. QuaterpyridineSynthesis andComplexFormation Thesynthesisandfunctionalizationofqtpyusuallyexploit thesamesynthetic strategies used for tpy, namely Kröhnke and coupling reactions. In the latter 19