New Push-Pull, Multicomponent Phthalocyanine-Based Photsensitizers for DSSCs

In the last few years, phthalocyanines (Pcs) have attracted strong interest in the field of dye-sensitized solar cells (DSSCs).¹ A₃B phthalocyanines such as TT40,² functionalized with non-aggregating bulky substituents at three isoindole rings (i.e. A) and a highly directional carboxylic acid linker at the other isoindole (i.e. B), have reached photovoltaic power conversion efficiencies over 6% in hybrid devices with mesoporous TiO₂. Although Pcs are more stable and strongly absorbing than porphyrin (Por) analogues, the latter have shown much more efficient as dyes for DSSCs, especially those with a “push-pull” substitution pattern, such as SM315.³ In the search for new hints for the rational molecular design and optimization routes, we aim to explore if this “push-pull “ pattern also applies to Pcs as a tool to achieve efficient sensitization of TiO₂. Therefore, we have synthesized donor-π-acceptor Zn(II)Pcs such as 2, holding one electron-donor, aminophenyl–type unit and a carboxyethynyl moiety as acceptor and anchoring group, starting from ABAB Pc 1 recently reported by us.⁴ This Pc consist of two opposite isoindoles functionalized with bulky bis(trifluoromethyl)phenyl groups, which afford solubility to the molecule and hamper its aggregation, and two faced isoindoles containing iodine atoms, which can be used  for further chemical transformation into the target donor-π-acceptor system 2.  Additionally, we have undertaken the preparation of Zn(II)Por-Zn(II)Pc dyads 3, which absorb a broad part of the visible spectrum and show cascade-charge transfer capabilities, for the sensitization of TiO₂.  In these ensembles, the Zn(II)Por unit, functionalized with three arylamino moieties, features as the ultimate electron donor unit, and the carboxy Zn(II)Pc is responsible of the direct  injection of electrons to the semiconducting film.

¹ Martinez-Diaz, M. V.; de la Torre, G.; Torres, T.  Chem. Commun., 2010, 46, 7090.

² Ragoussi, M.-E. ; Cid, J.-J. ; Yum, J.-H. ; de la Torre, G. ; Di Censo, D. ; Grätzel, M. ; Nazeeruddin, M. K.; Torres, T. Angew. Chem. Int. Ed. 2012, 51, 4375.

³ Mathew, S.;  Yella, A.;  Gao, P.;  Humphry-Baker, R.;  Curchod, B. F. E.;  Ashari-Astani, N.; Tavernelli, I.;  Rothlisberger, U.;  Nazeeruddin, M. K. ; Grätzel, M.;  Nature Chem. 2014, 6, 242.

⁴ Fazio, E. Jaramillo-Garcia, J.; de la Torre, G.; Torres, T. Org. Lett., 2014, 16, 4706.