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Physicochemical Investigation of the Panchromatic Effect on β‑Substituted ZnII Porphyrinates for DSSCs: The Role of the π Bridge between a Dithienylethylene Unit and the Porphyrinic Ring
journal contribution
posted on 2014-04-10, 00:00 authored by Gabriele Di Carlo, Alessio Orbelli Biroli, Francesca Tessore, Maddalena Pizzotti, Patrizia Romana Mussini, Anna Amat, Filippo De Angelis, Alessandro Abbotto, Vanira Trifiletti, Riccardo RuffoThree novel dyes based on ZnII porphyrinates combined, in β-pyrrolic position, with
the π unit dithienylethylene (DTE) have been synthesized and
investigated for application in DSSCs. The panchromatic effect due
to elongation of the π-delocalized system through a bridge between
the porphyrinic ring and the DTE unit such as the 4-ethynylstyryl
(1), ethynyl (2), and ethenyl (3) bonds have been investigated by computational, electrochemical,
and photoelectrochemical methods. For all three dyes the π conjugated
substituents in the β position produced the expected panchromatic
effect with broadened electronic absorption spectra over a wide range
of wavelengths and IPCE spectra featuring a broad plateau in the region
430–650 nm. In addition both DFT computational and electrochemical
data have shown a smaller HOMO–LUMO energy gap for dye 3, when compared to dye 2 suggesting a slightly
more facile conjugation between the porphyrinic core and the DTE unit
through the ethenylic bond. Conversely the photoelectrochemical investigation
showed improved DSSC performances from 3 to 1. These results have been rationalized by an in-depth DFT computational
study of dyes 2 and 3 interacting with a
cluster of 82 TiO2 units. The small energetic overlap between
the LUMO and the TiO2 conduction band characterizing the
more structurally distorted dye 3 would suggest low quantum
yield of electron injection, while dye 2 shows a greater
interaction between the LUMO of the dye and the semiconductor. Consequently
the increased linearity and planarity of the structure of dye 1 seems to be the origin of its best performance in DSSC.
Therefore it appears that the nature of the bridge between the DTE
unit and the porphyrinic ring is quite relevant for the efficiency
of these dyes for DSSC, due to distortion from the planarity and linearity
of the structure of the dye and the consequent changes on the dye
π conjugation.