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2. Synthesis The terpyridine structure was first synthesized in 1932 by Morgan and Burstall [27]asabyproductofbipyridinesynthesis,obtainedbydehydrogenationof pyridine in the presence of anhydrous ferric chloride. Nowadays, several synthetic pathwayshavebeendeveloped[28–30],allowingthisligandtoreachlargeapplications such as uses in the preparation of Co(II) [31], Os(II) [32], Ru(II) [33] Ir(II) [34,35], Pd(II),Pt(II),andAu(III)complexes[36],supramolecularcomplexes[37–40],molecular wires [41],polymers [42], in thesurface functionalizationofnanostructures [43], in the conjugation with amino acids [44], biomacromolecules [45], in the coupling with inorganicnanoparticles [46],andhaveshowntheirremarkableactivity inotherfields suchassensing[47]andcatalysis [48,49]. Wewill reportbrieflythemainstrategies used to obtain tpy ligands focusing on the structure–properties relationship in DSCs. 2.1. TerpyridineCore Tpy structures are mainly prepared through two basic synthetic approaches, which involveeitherringassemblyorcouplingmethodologies,assummarizedin Scheme1. Materials 2016, 9, 137 4 of 37 The state of the art of polypyridine structures designed to further improve BD performances is summarized in the next sections. After a survey on the synthetic pathways to obtain tpy and qtpy structures, the three main types of changes underlined before (metal centre, ancillary, and tpy ligands) and their effect on DSCs performances will be taken into account in order to outline a structure-property relationship. Moreover, we remind that DSCs are a complex multivariate system [26], with different components and variables, and that a direct correlation between the photosensitizers’ molecular structures and related efficiencies can sometimes lead to inaccurate conclusions. For this reason, we selected literature examples where an internal standard reference (BD, 719 or N3) is reported in order to compare the characteristics of the novel structures. Moreover, specific conditions have be n added to selected refere ces. 2. Synthesis The terpyridine structure was first synthesized in 1932 by Morgan and Burstall [27] as a byproduct of bipyridine synthesis, obtained by dehydrogenation of pyridine in the presence of anhydrous ferric chloride. Nowadays, several synthetic pathways have bee developed [28–30], allowing this ligand to reach large applications such as uses in the preparation of Co(II) [31], Os(II) [32], Ru(II) [33] Ir(II) [34,35], Pd(II), Pt(II), and Au(III) complexes [36], supramolecular complexes [37–40], molecular wires [41], polymers [42], in the surface functionalization of nanostructures [43], in the conjugation with amino acids [44], biomacromolecules [45], in the coupling with inorganic nanoparticles [46], and have shown their remarkable activity in other fields such as sensing [47] and catalysis [48,49]. We will report briefly the main strategies used to obtain tpy ligands focusing on the structure–properties relationship in DSCs. 2.1. Terpyridine Core Tpy structures are mainly prepared through two basic synthetic approaches, which involve either ring assembly or coupling methodologies, as summarized in Scheme 1. Scheme 1. Retrosynthetic pathways to tpy core. The first route has been formerly reviewed in 1976 by Kröhnke [50], who reported the synthesis of α,β-unsaturated ketones from 2-acetyl derivatives of pyridine and aldehydes. Then, the intermediate reacts with another 2-acetylpyridine to form a 1,5-diketone that can undergo cyclization to pyridine thanks to ammonia sources such as AcONH4 (Scheme 2). A series of modifications to this procedure has been proposed in order to increase yields or improve the synthetic pathway sustainability [28,51]. Scheme1. Retrosyntheticpathways to tpycore. The first route has been formerly reviewed in 1976 by Kröhnke [50], who reported the s nthesis of α,β-unsaturated ketones from 2-ac tyl derivat ves of pyridine and aldehydes. Then, the intermediate reacts with another 2-acetylpyridine to forma1,5-diketonethatcanundergocyclizationtopyridine thanks toammonia sources such as AcONH4 (Scheme 2). A series of modifications to this procedure has been proposed in order to increase yields or improve the synthetic pathway sustainability [28,51]. 18