Synthesis and properties of carboxyl-substituted coumarin-phthalocyanine dyad dyes: effect of linker unit

Two novel coumarin-substituted phthalocyanine dyes (CAPc and CSPc) with different linking units, using carboxylic groups as electron acceptor and injector, were designed, synthesized, and applied as panchromatic organic sensitizers in I-/I (3) (-) -based dye-sensitized solar cells (DSSCs). The asymmetric coumarin-phthalocyanine dyad analogue (OAPc) and the symmetric coumarin-phthalocyanine dyad analogue (COPc) were also synthesized for comparison. The structure-property relationship for these dyes was investigated by absorption spectroscopy, cyclic voltammetry, and thermogravimetric analysis. The ultraviolet-visible (UV-Vis) absorption was dramatically affected by the nature of the linker. Notably, modulation of the pi-conjugated coumarin-phthalocyanine dyad CAPc and CSPc resulted in a large red-shift (Q-band above 710 nm); in particular, the dyad CAPc showed remarkably intense absorption in the spectral window of 450-650 nm. Electrochemical data for the dyes indicated that the azo double-bond-linked coumarin-phthalocyanine dyad dyes possessed relatively low-lying E (HOMO) values, which may be beneficial to suppress electron back-transfer from the conduction band of TiO2 to oxidized dyes due to facile regeneration of the oxidized dye by the electrolyte. Thermogravimetric analysis showed that the three coumarin-phthalocyanine dyad sensitizers were stable above 200 A degrees C. When the dyads were applied as light-harvesting sensitizers in dye-sensitized solar cells, the cell sensitized by OAPc showed the best power conversion efficiency of 2.4%.