A Multitechnique Physicochemical Investigation of Various Factors Controlling the Photoaction Spectra and of Some Aspects of the Electron Transfer for a Series of Push-Pull Zn(II) Porphyrins Acting as Dyes in DSSCs

A multitechnique physicochemical comparative investigation involving TDDFT theoretical calculations, steady-state and time-resolved electronic absorption spectra, and electrochemical and photoelectrochemical investigations was carried out on a family of push-pull porphyrinic sensitizers ([5-(4′-carboxy- phenylethynyl)-15-(4′-methoxy-phenylethynyl)-10,20-bis(3, 5-di-tert-butylphenyl)porphyrinate]Zn(II) (1) and [5-(4′-carboxy- phenylethynyl)-15-(4′-N,N-dimethylamino-phenylethynyl)-10,20-bis(3, 5-di-tert-butylphenyl)porphyrinate]Zn(II) (2) and the new fluorinated porphyrinic dye [5-(4′-carboxy-2′,3′,5′,6′- tetrafluorophenylethynyl)-15-(4′-N,N-dimethylamino-phenylethynyl)-10, 20-bis(3,5-di-tert-butylphenyl)porphyrinate]Zn(II) (3)) with the aim of identifying the structurally related electronic properties at the basis of efficient interfacial charge separation. We found for all dyes a photoconversion nearly twice more effective for the B band than for the Q band, which could not be explained only by considerations based on the electron collection efficiency but also by a more energetically favorable electron injection from the S2 excited state. The lower photoconversion of the fluorinated dye 3, when compared to dyes 1 and 2, was explained not only by a more difficult absorption on the TiO2 photoanode but also by a lower electron injection efficiency and a less successful hole transfer to the electrolyte, leading to increased charge recombination.