A striking case of connectivity impact on molecular semiconductor electrochemistry and chirality: a,a’ vs b,b’ central link in spider-like oligothiophene Isomers
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Data di Pubblicazione:
2015
Citazione:
A striking case of connectivity impact on molecular semiconductor electrochemistry and chirality: a,a’ vs b,b’ central link in spider-like oligothiophene Isomers / V. Marino, S. Arnaboldi, T. Benincori, P.R. Mussini, S. Rizzo, F. Sannicolò. ((Intervento presentato al convegno Electrochemistry of Electroactive Materials tenutosi a Bad Herrenalb nel 2015.
Abstract:
We have recently introduced a family of inherently chiral molecular semiconductors, where chirality does not stem from a localized stereocentre, but from a tailored torsion of the whole conducting backbone1,2. This unique coincidence of the source of both chirality and electroactivity with the main molecular scaffold results in huge chirality manifestations, like impressive circularly polarized luminescence2 and outstanding enantiorecognition ability3.
The first strategy applied to obtain the high torsional angle providing the stereogenic element was based on the insertion of bi-heteroaromatic scaffolds (e.g. bi-benzothiophene ones) along the conjugated backbone.1 However, a development of our former study on spider-like oligothiophenes4,5 demonstrates that the same target can be achieved even in an all-thiophene spider-like monomer, by proper connectivity choice. A clear example is provided by the comparison of octathiophene isomers in the Figure 1, featuring the same number of thiophene rings, but with different connectivity (a,a’ vs b,b’) between the two central ones. The torsional energy barrier, low in T84, dramatically increases in T83 so that it can no more be overcome at room temperature. This implies that the effective conjugation significantly decreases for T83 with respect to T84, with the two moieties becoming more independent. But above all, it implies T83 to be chiral, since it can be separated into two stable antipodes.
The electrochemical characterization of monomer T83 and the corresponding electrooligomerization products has been carried out in three different media (ACN, DCM and BMIMPF6) and on three different electrode surfaces (GC, Pt and Au). The CV features of the monomer show two neat oxidation peaks both in ACN and DCM solvent. The two redox peaks correspond to two equivalent but slightly interacting redox sites, the processes appear chemically irreversible and electrochemically quasi reversible. In the case of DCM, peaks are more distant than those recorded in ACN due to the shielding effect of this solvent on the positive charges of the generated radical cation.
Electrooligomerization is observed upon potential cycling, and, albeit only linear oligomers can be obtained in this case, unlike the former cited benzothiophene one1, the oligomer films retain the chirality of the starting monomers, and display outstanding enantioselectivity as electrode surfaces, comparable to inherently chiral materials including bi-heteroaromatic scaffolds. This observation is important since it confirms the general validity of the inherent chirality strategy to obtain materials with outstanding enantioselection properties.
(T8)3 oligomers show a particurarly neat electrochromic effect, displaying a green colour in the doped state becoming orange in the undoped one. This phenomenon also corresponds to a net and reversible variation of the UV-vis spectrum of the (T8)3 oligomer between the neutral and polaronic state, observed in spectroelectrochemistry tests. The intensity of the band associated with the π-π* transition in the neutral state at low wavelength decreases. This decrease starts at the electrochemical potential where anodic current onset occurs in the corresponding CV trace. Increasing the potential results in further and progressive blenching of the band, accompanied by loss of its vibronic fine structure. Conversely, a growth of two doping-induced bands is observed at higher wavelengths corresponding to more conjugated polaronic and bipolaronic states respectively. Enantiorecognition tests were carried out by means of CV and EIS. The enantiorecognition ability of enantiopure oligo-T83 films was tested with two different protocols. The first test was performed on (R)- or (S)-oligo-T83 films electrodep
Tipologia IRIS:
14 - Intervento a convegno non pubblicato
Elenco autori:
V. Marino, S. Arnaboldi, T. Benincori, P.R. Mussini, S. Rizzo, F. Sannicolò
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