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 electrodeposited on Au screen printed electrodes (SPE) from a drop of monomer solution in BMIMPF6 ionic liquid, testing the modified SPE with (R)- and (S)-N,Ndimethyl-1-ferrocenylethylamine probes, both enantiopure and as 1:1 racemate. As Figure 2 shows, an impressive and specular discrimination is obtained for the enantiopure probes on the enantiopure oligo-T83 electrode surfaces, with a peak separation of about 250 mV. Good enantioselection is also observed with the probe racemate; however, in this case the separation is significantly less (about 150 mV) than in the single antipode case. Moreover, working with the racemate the peak potentials are quite shifted to more positive potentials with respect to the single enantiomer cases, pointing to some reciprocal interaction of the two antipodes in the sensing process within the chiral film. The second test (Figure 3) was performed on L- and D-DOPA enantiopure solutions in aqueous hydrogen chloride, using (R)- or (S)-oligo-T83 films electrodeposted on Au SPE from monomer solutions in BMIMPF6 ionic liquid. A neat discrimination was again achieved, although with a peak separation (60-70 mV) less spectacular than in the former case. These outstanding enantiorecognition results are very important; both tests not only demonstrate the high enantioselectivity of oligo-T83 surfaces, but strongly confirm the general validity of the inherent chirality approach, which does not depend on the chemical nature of the atropoisomeric scaffold. In the same conditions of the first enantiorecognition test, EIS measurements were carried out: they have proved a useful tool to estimate the enantiorecognition capability of these inherently chiral materials. Acknowledgements This work was supported by Fondazione Cariplo, grant no.2011-0417 References [1] F. Sannicolò, P.R. Mussini, T. Benincori, R. Birilli, S. Abbate, S. Arnaboldi, S. Casolo, E. Castiglioni, G. Longhi, R. Martinazzo, M. Panigati, M. Pappini, E. Quartapelle Procopio, S. Rizzo Inherently chiral macrocyclic oligothiophenes: easily accessibile electrosensitive cavities with outstanding enantioselection performances Chem. Eur. J., 20 (2014), p. 15298 [2] F. Sannicolò, S. Arnaboldi, T. Benincori, V. Bonometti, R. Birilli, L. Dunsch, W. Kutner, G. Longhi, P.R. Mussini, M. Panicati, M. Pierini, S. Rizzo Potential-driven chirality manifestations and impressive enantioselectivity by inherently chiral electroactive organic films Angew. Chem. Int. Ed., 53 (2014), p. 2623 [3] S. Arnaboldi, T. Benincori, R. Cirilli, W. Kutner, M. Magni, P.R. Mussini, K. Noworyta, F. Sannicolò Inherently chiral electrodes: the tool for chiral voltammetry Chem. Sci., 6 (2015), p. 1706 [4] T. Benincori, M. Capaccio, F. De Angelis, L. Falciola, M. Muccini, P. Mussini, A. Ponti, S. Toffanin, P. Traldi, F. Sannicolò Spider-like oligothiophenes Chem. Eur. J., 14 (2008), p. 459 [5] T. Benincori, V. Bonometti, F. De Angelis, L. Falciola, M. Muccini, P. R. Mussini, T. Pilati, G. Rampinini, S. Rizzo, S. Toffanin, F. Sannicolò Towards molecular design rationalization in branched multi-tiophenes semiconductors: the 2-thienyl-persubstituted α-oligothiophenes Chem. Eur. J., 16 (2010), p. 9086

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.

A striking case of connectivity impact on molecular semiconductor electrochemistry and chirality: a,a’ vs b,b’ central link in spider-like oligothiophene Isomers

S. Arnaboldi;P.R. Mussini;F. Sannicolò
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 electrodeposited on Au screen printed electrodes (SPE) from a drop of monomer solution in BMIMPF6 ionic liquid, testing the modified SPE with (R)- and (S)-N,Ndimethyl-1-ferrocenylethylamine probes, both enantiopure and as 1:1 racemate. As Figure 2 shows, an impressive and specular discrimination is obtained for the enantiopure probes on the enantiopure oligo-T83 electrode surfaces, with a peak separation of about 250 mV. Good enantioselection is also observed with the probe racemate; however, in this case the separation is significantly less (about 150 mV) than in the single antipode case. Moreover, working with the racemate the peak potentials are quite shifted to more positive potentials with respect to the single enantiomer cases, pointing to some reciprocal interaction of the two antipodes in the sensing process within the chiral film. The second test (Figure 3) was performed on L- and D-DOPA enantiopure solutions in aqueous hydrogen chloride, using (R)- or (S)-oligo-T83 films electrodeposted on Au SPE from monomer solutions in BMIMPF6 ionic liquid. A neat discrimination was again achieved, although with a peak separation (60-70 mV) less spectacular than in the former case. These outstanding enantiorecognition results are very important; both tests not only demonstrate the high enantioselectivity of oligo-T83 surfaces, but strongly confirm the general validity of the inherent chirality approach, which does not depend on the chemical nature of the atropoisomeric scaffold. In the same conditions of the first enantiorecognition test, EIS measurements were carried out: they have proved a useful tool to estimate the enantiorecognition capability of these inherently chiral materials. Acknowledgements This work was supported by Fondazione Cariplo, grant no.2011-0417 References [1] F. Sannicolò, P.R. Mussini, T. Benincori, R. Birilli, S. Abbate, S. Arnaboldi, S. Casolo, E. Castiglioni, G. Longhi, R. Martinazzo, M. Panigati, M. Pappini, E. Quartapelle Procopio, S. Rizzo Inherently chiral macrocyclic oligothiophenes: easily accessibile electrosensitive cavities with outstanding enantioselection performances Chem. Eur. J., 20 (2014), p. 15298 [2] F. Sannicolò, S. Arnaboldi, T. Benincori, V. Bonometti, R. Birilli, L. Dunsch, W. Kutner, G. Longhi, P.R. Mussini, M. Panicati, M. Pierini, S. Rizzo Potential-driven chirality manifestations and impressive enantioselectivity by inherently chiral electroactive organic films Angew. Chem. Int. Ed., 53 (2014), p. 2623 [3] S. Arnaboldi, T. Benincori, R. Cirilli, W. Kutner, M. Magni, P.R. Mussini, K. Noworyta, F. Sannicolò Inherently chiral electrodes: the tool for chiral voltammetry Chem. Sci., 6 (2015), p. 1706 [4] T. Benincori, M. Capaccio, F. De Angelis, L. Falciola, M. Muccini, P. Mussini, A. Ponti, S. Toffanin, P. Traldi, F. Sannicolò Spider-like oligothiophenes Chem. Eur. J., 14 (2008), p. 459 [5] T. Benincori, V. Bonometti, F. De Angelis, L. Falciola, M. Muccini, P. R. Mussini, T. Pilati, G. Rampinini, S. Rizzo, S. Toffanin, F. Sannicolò Towards molecular design rationalization in branched multi-tiophenes semiconductors: the 2-thienyl-persubstituted α-oligothiophenes Chem. Eur. J., 16 (2010), p. 9086
giu-2015
Settore CHIM/01 - Chimica Analitica
Settore CHIM/02 - Chimica Fisica
Settore CHIM/06 - Chimica Organica
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.
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