Aromatic Azo-coupled Zinc Phthalocyanines: Synthesis And Properties

Phthalocyanines (Pcs) and their metal complexes are typical NIR dyes. Pcs have strongabsorptions on two distinct edges of visible region, namely, the Q-band at around680nm andthe Soret-band at around350nm. The facile modification chromophore skeleton ofphthalocyanines provides potential for molecular design. Up to now, various chromophoreshave been introduced into the phthalocyanine core to tune the absorptions. However, almostall the chromophores were introduced through oxygen bridge, thioetherbondor C C bond andthe characteristic absorptions of these chromophores were overlapped with the Soret-band ofphthalocyanines as well. Moreover, the absorption bands of these phthalocyanines red shiftedslightly.In the paper, azo dyes were chosen as chromophores and were connected onto the Pc ringthrough N=N double bond. Pcs and azo dyes show great attractiveness due to their broadavailability and perfectly complementary absorption regions. The synthesis and properties ofthe azo-coupled Pcs were researched. The absorptions in the spectral window between400and600nm of four azo-coupled phthalocyanines were remarkable.The coupling reactions between arenediazonium ion and phenol derivatives was usedto prepare the azo-coupled phthalonitriles:4-(4-hydroxyphenylazo)phthalonitrile (2),4-(4-hydroxy-naphthalen-1-ylazo)phthalonitrile (3),4-(2-hydroxyl-5-ethyl-oxycarbonyl-phenyla-zo)phthalonitrile (4) and4-[2-hydroxyl-3-methoxyl-4-(4-carboxyl-benzylidene-amino)phen-ylazo]phthalonitrile (6). Then, the self-cyclotetramerization of the azo-coupled phthaloni-triles in n-pentanol under the protection of nitrogen atmosphere afforded the four azo-coupled zinc phthalocyanines: tetrakis-(4-hydroxyphenylazo)phthalocyaninato zinc(Ⅱ)(7),tetrakis-(4-hydroxynaphthalen-1-ylazo)phthalocyaninato zinc(Ⅱ)(8), tetrakis-(2-hydroxy-5 -carboxy-pheny-lazo)phthalocyaninato zinc(Ⅱ)(10) and tetrakis-[2-hydroxy-3-methoxy-4-(4-carboxy-N-phenylmethanimino)phenylazo]phthalocyani-nato zinc(Ⅱ)(11).The structures of the target compounds were characterized by elemental analysis togetherwith the spectral data (FT-IR,1H NMR, and UV-Vis). The characterization data of the newcompounds was consistent with the assigned formula.The electronic absorption spectra of phthalonitrile precursors were recorded in THFsolution, and azo-coupled zinc phthalocyanines were tested both in THF solution andadsorbed on anatase TiO2thin films. The phthalonitrile precursors showed intense absorptionsin the range of300-600nm. The four compounds showed impressive absorptions in all thevisible and near-IR regions as a result of lowering of the energy gaps. It was clear thatabsorption characteristics of both azobenzene and Pc moieties were displayed whenconjugated into one molecule. In addition, thermal stability studies showed that the foursensitizers were stable above200oC.Electrochemical studies were made to investigate the HOMO and LUMO levels. TheLUMO levels of the four sensitizers were high enough to achieve efficient electron injectionfrom the excited dyes to the conduction band of the TiO2. In addition, the HOMO levels offour dyes could ensure efficient regeneration of the oxidized dyes.Both the four azo-coupled phthalocyanines were tested in DSSC. In contrast, compounds8sensitized cell exhibited the maximum η value.The low efficiency of DSSC using the azo-coupled phthalocyanines is probably due to theaggregation of azo-coupled phthalocyanine molecules on the TiO2surface and thesymmetrical structures of the dyes, which are not good enough for intramolecular chargetransfer and directional electron injection.