Engineering Subphthalocyanine dyes for DSSC applications

In the past two decades, much attention has been focused on the design and synthesis of chromophoric systems for their application in the field of dye sensitized solar cells (DSSCs). Recently, one class of "small molecule" materials of great interest to researchers is boron (III) Subphthalocyanine (SubPc, scheme 1). SubPcs are attractive dyes in DSSCs because of their strong absorption properties in the visible region 500-700 nm. Also, SubPcs are non planar conical shaped dyes displaying higher solubility and reduced aggregation tendency compared to their homologues phthalocyanines.1 We focused on axially substituted SubPc that can bind TiO2 through a carboxylic or cyanoacrylic acid function present on the axial group. Just recently a prototype of such substituted SubPc has been employed as sensitizer. The reported cell performances are low but promising, leaving plenty of space to design and engineer SubPc scaffold.2 In this communication, we report on the design of axially substituted SubPcs capable of vectorial electron transfer, from the dye to the TiO2, through the short conjugated chain bearing the anchoring group (Scheme 1). DFT calculations have been employed to model the structure of the conjugated chain. The chromophores are synthesized by nucleophilic substitution on chloro-SubPc with a proper phenol derivative. The investigated dyes display a strong and intense absorption picked at 565 nm together with high energy transitions below 350 nm. Whereas the former pick belongs to the SubPc core absorption (Q-band), the latter bands are influenced by the axial substituent and its conjugation. Preliminary DSSC device testing, employing dye A, is on-going.