Rationalizing the electronic properties of two classes of push–pull DSSC sensitizers based on Zn(II) di- or tetrarylporphyrinates substituted in meso or beta-pyrrolic positions: an electrochemical investigation

Push–pull Zn(II)-porphyrinates have recently shown attracting performances as light harvesting systems in dye-sensitized solar cells (DSSCs). To fully exploit their intrinsically high efficiency it is important to finely tune their HOMO and LUMO levels, which can be achieved by proper choice of the push and pull substituents. Of course such target-oriented molecular design requires the availability of reliable relationships between molecular structure and electronic properties; therefore we have carried out a detailed electrochemical investigation, also supported by spectroscopy and theoretical computations, on a wide, systematic range of Zn(II)-porphyrinates: (a) a family of 5,15-meso substituted ones with phenylethynyl linkers, including a first "benchmark" symmetric series carrying on the opposite terminals the same substituent (N(CH3)2, OCH3, COOCH3, COOH, NO2); and a second push–pull one, with the terminal positions carrying one donor and one acceptor group belonging to the series above. Moreover, two suitably modified porphyrins allowed evaluation of the effects of (i) the presence or absence of the phenyl group in the linker between the porphyrin core and the acceptor group, and (ii) the effect of perfluorination on the same phenyl group; (b) a family of mono and, for the first time, disubstituted push-pull ZnIIporphyrinates bearing a variety of ethynyl-phenyl moieties in β-pyrrolic position, which were compared with their meso analogues, producing evidence that, although the HOMO-LUMO energy gap of the meso substituted push-pull dyes is lower, the β mono or disubstituted push-pull porphyrinic dyes show comparable or better efficiencies when acting as sensitizers in DSSCs, possibly on account of a more facile charge injection into TiO2. A rationalization scheme is proposed concerning relationship between structure and redox properties, affording inter alia interesting clues on the different localization of the redox centres and the effective conjugation between the porphyrin core and the side chains as a function of the molecular design. References: [1] P. R. Mussini, A. Orbelli Biroli, F. Tessore, M. Pizzotti, C. Biaggic, G. Di Carlo, M. G. Lobello, F. De Angelis, Electrochimica Acta 85 (2012) 509– 523 [2] G. Di Carlo, A. Orbelli Biroli, M. Pizzotti, F. Tessore, V. Trifiletti, R. Ruffo, A. Abbotto, A. Amat, F. De Angelis, P. R. Mussini, Chemistry-a European Journal, in press (2013).