Inherently Chiral Spider-Like Oligothiophenes

Article abstract of DOI:10.1002/chem.201504899

The racemate of an inherently chiral “spider-like” octathiophene monomer T8₃, in which chirality is generated by torsion in its backbone, was synthesized. The racemate was resolved into configurationally stable antipodes by HPLC on a chiral stationary phase. Electrooxidation of the enantiomers resulted in materials displaying high enantiorecognition ability towards the antipodes of some chiral probes. Moreover, the T8₃racemate demonstrated great aptitude to stimulate formation of 3D rigid architectures if used as a cross-linking monomer for molecular imprinting. This feature was exploited to devise a molecularly imprinted polymer-based chemosensor selective for a thymine–adenine oligonucleotide.

Full text of DOI:10.1002/chem.201504899

DOI: 10.1002/chem.201504899
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Chirality |Hot Paper|
Inherently Chiral Spider-Like Oligothiophenes
Francesco Sannicolꢀ,[a] Patrizia R. Mussini,*[a] Tiziana Benincori,*[b] Rocco Martinazzo,*[a]
Serena Arnaboldi,[a] Giulio Appoloni,[b] Monica Panigati,[a] Elsa Quartapelle Procopio,[a]
Valentina Marino,[a] Roberto Cirilli,[c] Simone Casolo,[a] Wlodzimierz Kutner,[d]
Krzysztof Noworyta,[d] Agnieszka Pietrzyk-Le,[d] Zofia Iskierko,[d] and Katarzyna Bartold[d]
Abstract: The racemate of an inherently chiral “spider-like”
octathiophene monomer T83, in which chirality is generated
by torsion in its backbone, was synthesized. The racemate
was resolved into configurationally stable antipodes by
HPLC on a chiral stationary phase. Electrooxidation of the
enantiomers resulted in materials displaying high enantiore-
cognition ability towards the antipodes of some chiral
probes. Moreover, the T83 racemate demonstrated great ap-
titude to stimulate formation of 3D rigid architectures if
used as a cross-linking monomer for molecular imprinting.
This feature was exploited to devise a molecularly imprinted
polymer-based chemosensor selective for a thymine–ade-
nine oligonucleotide.
Introduction
We recently devised an innovative strategy for the design of
chiral functional materials based on the concept of the “inher-
ent chirality”: it implies that the same molecular moieties con-
stituting the stereogenic elements responsible for chirality also
provide the material with its specific properties.[1]
We have applied this approach to prepare chiral Dn symmet-
ric macrocyclic oligothiophenes by chemical or electrochemical
oxidation of monomers (R)-1 and (S)-1, (Scheme 1),[2] where
chirality is not introduced by attaching chiral pendants to the
electroactive backbone, according to the most popular strat-
egy,[3] envisaged by Roncali nearly thirty years ago,[4] but it is
inherent to it. The conjugated oligothiophene system responsi-
ble for the optical and electrochemical properties also consti-
tutes the atropisomeric scaffold responsible for chirality. We
found that these materials exhibit impressive chiroptical prop-
Scheme 1. Structural formulas of the inherently chiral monomers (R)- and
(S)-1 and the spider-like oligothiophenes (R)- and (S)-T83.
[a] Prof. F. Sannicolꢀ, Prof. P. R. Mussini, Prof. R. Martinazzo, Dr. S. Arnaboldi,
Dr. M. Panigati, Dr. E. Quartapelle Procopio, V. Marino, Dr. S. Casolo
Dipartimento di Chimica Universitꢁ degli Studi di Milano
Via Golgi 19, 20133 Milano (Italy)
erties correlated to the electrochemical ones and unprecedent-
ed ability to pronouncedly separate voltammetry peaks of
enantiomers of quite different chiral probes.[5]
In this paper we report the results of a research aiming to
apply the inherent chirality concept to the “spider-like” oligo-
thiophenes, a class of branched all-thiophene molecules we in-
vestigated a few years ago, characterized by an inner core,
from thiophene to pentathiophene (the trunk of the spider),
persubstituted with 2-thienyl[6] or 2-(5,2’-bithienyl)[7] units (the
legs).
E-mail: patrizia.mussini@unimi.it
rocco.martinazzo@unimi.it
[b] Prof. T. Benincori, G. Appoloni
Dipartimento di Scienza e Alta Tecnologia
Universitꢁ degli Studi dell’Insubria Via Valleggio 11, 22100 Como (Italy)
E-mail: tiziana.benincori@uninsubria.it
[c] Dr. R. Cirilli
Dipartimento del Farmaco
Istituto Superiore di Sanitꢁ Viale Regina Elena 299, 00161 Roma (Italy)
Multi-branched oligothiophene molecules have attracted
considerable attention on account of their 3D conjugated ar-
chitectures endowed with isotropic charge transport and opti-
cal properties, coupled with high chemical stability and
remarkable solubility in organic non-polar solvents.[8] We con-
sidered it to be of interest to couple these properties with
[d] Prof. W. Kutner, Dr. K. Noworyta, Dr. A. Pietrzyk-Le, Z. Iskierko, K. Bartold
Institute of Physical Chemistry Polish Academy of Sciences
Kasprzaka 44/52, 01-224 Warsaw (Poland)
Supporting information and ORCID number(s) for the author(s) of this arti-
cle are available on the WWW under
http://dx.doi.org/10.1002/chem.201504899.
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chirality and, in this light, we synthesized the C2 symmetric
2,2’,4,4’,5,5’-hexa(2-thienyl)-3,3’-bithiophene (T83).
We focused our attention not only on the enantiorecogni-
tion ability of the enantiopure antipodes of the T83, but we
also exploited its racemate to synthesize a co-polymer molecu-
larly imprinted with the TATAAA (T-thymine, A-adenine) oligo-
nucleotide, employed as a recognition unit of the extended-
gate field-effect transistor (EG-FET) chemosensor.
Results and Discussion
T83 was easily prepared in acceptable yields by Stille reaction
of the hexabromo-3,3’-bithiophene with 2-tributylstannylthio-
phene.
T83 structure differs from the “swivel cruciform” a-terthio-
phene dimer (BT3), a p-type semiconductor investigated by
Scherf, for the presence of the 2-thienyl units in position ortho
to the bond containing the stereogenic axis responsible for
the atropisomerism.[9] They also formally result from doubling
Figure 1. Calculated preferred conformation for (R)-T83.
the 5’-(2-thienyl)-2,2’,3’,2“-terthiophene (4T),
a
monomer
widely used by Ludwigs for preparing hyperbranched polythio-
phenes by electrochemical or chemical oxidation.[8e–g] This mo-
nomer is also the constitutional isomer of the T’84, a member
of the class of the spider-like oligothiophenes cited above
(Scheme 2).[6]
sity of both the radical cation and radical anion.This result sug-
gests that the preferred electrooxidation-electrooligomeriza-
tion sites in T83 are the four a-positions of the two homotopic
a-conjugated fragments. Considering that the terminal a-posi-
tions of each terthiophene unit are constitutionally heterotopic
and the architecture of the molecule is tridimensional and
rigid, we expected a very stiff cross-linked network of intercon-
nected heterotopic thiophene units when T83 was subjected
to oxidative electrooligomerization.
The above structural characteristics are confirmed by the
electronic spectrum showing absorption lmax at 371 nm in tolu-
ene solution (e=27000mꢀ1 cmꢀ1, Figure 2), attributable to the
spin-allowed p–p* transition. This band is redshifted by 10 nm
compared to that of the analogous BT3 (lmax =361 nm)[10] and
redshifted by 16 nm compared to that of a-terthiophene
(lmax =355 nm, e=19000mꢀ1 cmꢀ1) in chloroform solution.[11]
Noticeably, the same redshift of about 10 nm is observed for
T’53 (lmax =361 nm)[6] (Scheme 2), formally resulting from the
addition of a second thiophene ring in the b position of the
Scheme 2. Structural formulas of oligothiophenes structurally related to T83.
DFT calculations provided interesting information on the
conformational properties of T83, on the configurational stabili-
ty of the enantiomers, and on the p-electron distribution on
the different thiophene rings. First of all, the calculated racemi-
zation energy barrier for the enantiomers is very high
(178.74 kJ molꢀ1), thereby granting their configurational stabili-
ty.
central ring of the a-terthiophene moiety of 4T (lmax
=
352 nm).[12] This behaviour suggests a weak electronic interac-
The equilibrium geometry of the most stable conformer ex-
hibits two homotopic a-terthiophene units orthogonal to each
other (dihedral angle Ca-Cb-Cb’-Ca’=ꢀ97.38), with the three
rings substantially coplanar and the 2-thienyl groups in b-posi-
tion nearly perpendicular to them (dihedral angle Ca-Cb-Ca’’-
Cb’’=109.58). Similar structures were found for different con-
formers when turning each of the thienyl groups around their
interannular CꢀC links. They lie slightly higher in energy than
the example shown in Figure 1 (ꢁ0.25–1.4 kJ molꢀ1 depending
on the specific thienyl unit considered) and are easily accessi-
ble under normal conditions.
tion through the a–b connection.
DFT calculations revealed that both the HOMO and the
HOMO-1 orbitals have sizable amplitude on the a-terthiophene
moieties and modest weight on the b-connected units, a result
which is further confirmed by the distribution of the spin-den-
Figure 2. The UV/Vis (dark line) and emission (light line) spectra of T83 in
aerated toluene solution at 298 K.
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The same absorption maximum observed for T83 is shown
also by T’84, the isomeric spider-like oligothiophene (lmax
=
370 nm).[6] This means that the decrease in efficiency of the a–
a conjugation observed in the case of T’84 has the same influ-
ence on the optical properties of the b–b connection in T83.
The value of the extinction coefficient determined for T83,
which is higher than that expected for a single terthiophene
chain, but lower than that expected for two chains, can be as-
cribed to the branched structure of the molecule.
Upon optical excitation with UV light (lex =380 nm), T83 dis-
plays a broad and featureless emission band at 467 nm in the
toluene aerated solution (Figure 2), with the photolumines-
cence quantum yield f=0.012. The emission maximum lies
between those observed for the analogous BT3 (lem =478 nm)
and the a-quarterthiophene (lem =453 nm, with a shoulder at
478 nm, in dry THF, f=0.24).[13] However, as in the case of BT3,
the emission band is broad and no vibrational structure is ob-
served, indicating a reduced planarization in the excited state.
The higher conformational reorganization, required in the elec-
tronic excitation for T83 and BT3 with respect to the linear a-
terthiophene or a-quaterthiophene, accounts for the higher
Stokes shift observed (ꢁ100 nm for T83 and BT3 vs. 50–60 nm
for the linear oligothiophenes). The very low value of the pho-
toluminescence quantum yield is connected to the geometry
of the excited state, because non-radiative deactivation path-
ways, favored by torsional vibration motions induced by the
presence of the thiophenes in b positions, are easily accessible.
The resolution of racemic (ꢂ)-T83 by HPLC on a chiral sta-
tionary phase presents some difficulties related to the low res-
olution factor (Figure 3a), so that only minute amounts of
enantiopure antipodes could be obtained.
Figure 3. a) HPLC resolution of (ꢂ)-T83. CSP: Chiralpak IB-3, 250ꢂ4.6 mm
I.D.+Chiralpak IB-3, 250ꢂ4.6 mm I.D.; eluent: n-hexane/dichloromethane/
ethanol 100/5/0.2; flow-rate: 1.0 mLminꢀ1; temperature: 258C; detection: UV
(top) and CD (bottom) at 350 nm. b) CD spectra (CHCl3) of (+)-T83 (black)
and (ꢀ)-T83 (grey).
The configuration assignment to the enantiomers was per-
formed by comparison of the experimental CD curves (Fig-
ure 3b) with that calculated for the (S)-antipode. The latter cor-
responded to the first eluted dextrorotatory enantiomer (Fig-
ure SI3 in the Supporting Information).
The redox properties of T83 have been studied by cyclic vol-
tammetry (CV), in dichloromethane and acetonitrile (ACN),
both with 0.1m (TBA)PF6 supporting electrolyte, and in the bu-
tylmethylimidazolium hexafluophosphate, (BMIM)PF6, ionic
liquid. A comparison of CV features in the three media on
glassy carbon (GC) electrode, plotted versus the intersolvental
Fc+ jFc reference redox couple, is shown in Figure 4 (Au and
Pt were also tested, with similar results, see the Supporting In-
formation SI5).
In ACN and (BMIM)PF6 both oxidation and reduction peaks
can be observed, affording to estimate HOMO–LUMO gaps for
the molecule, 2.93 and 2.89 eV respectively, only slightly lower
than that of linear a-terthiophene (3.1 eV).[14] This is in agree-
ment with the T83 structure consisting of two a-terthiophene
moieties, with low or null conjugation on account of the very
high torsional angle (in branched oligothiophenes electronic
properties are essentially determined by the longest conjugat-
ed chain).[6,7,8c]
Figure 4. CV patterns at 0.2 Vsꢀ1 for 0.0005m monomer T83 on the GC elec-
trode (0.031 cm2) in different media. Top curve: dichloromethane + 0.1m
(TBA)PF6; middle curve: acetonitrile + 0.1m (TBA)PF6; bottom curve:
(BMIM)PF6 ionic liquid (ꢂ5).
As in the case of the inherently chiral monomers 1,[1] on the
oxidation side, a two-peak system is clearly observed in the
two conventional media, and perceived in the ionic liquid too,
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in spite of poorer signal resolution. Again, such patterns could
account for two equivalent redox centers reciprocally interact-
ing in space, corresponding to the two identical moieties. In
fact, on one hand, the second oxidation peak could hardly be
assigned to the oxidation of a product of the first oxidation,
because the current ratio of the two peaks appears ~1 and
nearly independent of the scan rate (Figure SI6 in the Support-
ing Information). On the other hand, te second oxidation on
a terthiophene chain occurs at a potential of ꢁ1.0 V higher
than that of the first oxidation (Table 1), so each oxidation pro-
cess should take place on a different moiety. This assumption
is also supported by the potential difference between the two
peaks, which is larger in CH2Cl2 with respect to ACN (0.23 V vs.
0.19 V), possibly accounting for higher interaction between the
two charges, on account of the lower solvent screening effect.
Both oxidative peaks are only moderately shifted to more
positive potentials as the scan rate increases (Figure SI6 in the
Supporting Information), pointing to rather facile electron
transfers. On the other hand, the absence of symmetrical
return peaks and the presence of irregular negative peaks at
less positive potentials point to the electron transfer products
undergoing a chemical reaction, in particular, coupling of radi-
cal cations in a-terminal positions. Actually, no less than four
a-terminal positions (two identical pairs of constitutionally het-
erotopic sites with nearly equivalent properties) are available
for oligomerization on the T83 monomer (instead of the two
homotopic ones of identical properties available for 1), al-
though, following the above considerations, it is improbable
that more than one position per terthiophene chain would be
activated at the same time as the radical cation at the first two
oxidation potentials.
Figure 5. Multicycle curves of potentiodynamic electropolymerization
(36 cycles, 0.05 Vsꢀ1 scan rate) of 0.01m T83 (above) and film stability
(below) in (BMIM)PF6 ionic liquid on Au SPE (geometric surface ꢁ0.11 cm2).
Scan numbers are indicated with Roman numerals.
tion). Slow growth, affording very regular films, is observed if
the scan is reversed at a potential in between the first and the
second oxidation peak; on the contrary, much faster growth is
observed when the scan is reversed after the second oxidation
peak.
Electroactive films regularly grow on the electrode by poten-
tial cycling (Figure 5 and Figure SI7 in the Supporting Informa-
Film growth occurs on all supports and in all solvents. Cy-
cling around the first oxidation peak (Figure SI7 in the Sup-
porting Information), the deposition rate follows the sequence
ACN>CH2Cl2 >IL. This sequence could arise from a combina-
tion of factors. The growing oligomers could be more soluble
in the more apolar solvent; moreover, in the more polar sol-
vent, reduced Coulombic repulsion of radical cations should fa-
cilitate coupling;[15] besides, a polar solvent typically increases
reaction rate if charged species are formed.[16] In the IL case,
however, the extremely high viscosity should prevail on the
former effects. Remarkably, including the second oxidation
peak, the scan rate greatly increases (the effect is particularly
conspicuous in the CH2Cl2 case) and the oligomerization pat-
tern changes remarkably (an example is provided in Fig-
ure SI7b in the Supporting Information), a feature which could
be the object of further investigation.
Table 1. Electrochemical data of some oligothiophenes and their electro-
oligomerization films, with corresponding HOMO and LUMO energies and
gaps
EIc
Ep,aI
Ep,aII
ELUMO
EHOMO
[eV]
EG
[eV]
vs. FcjFc+ vs. FcjFc+ vs. FcjFc+ [eV]
[V]
[V]
[V]
MONOMERS
1.53[b]
[a]
a-T3
ꢀ2.50[b]
ꢀ2.46[d]
0.58[b]
0.63[d]
ꢀ2.30 ꢀ5.38 3.08
ꢀ2.34 ꢀ5.43 2.88
[c]
T’84aa
0.84[d]
[e]
T83bb
T83bb
T83bb
n.d.
0.62[d]
0.74[d]
0.81[d]
OLIGOMER FILMS
0.20[h]
0.85[d]
0.93[d]
1.06[d]
n.d.
ꢀ5.42 n.d.
[f]
[g]
ꢀ2.38[d]
ꢀ2.42 ꢀ5.54 3.12
ꢀ2.54 ꢀ5.61 3.07
ꢀ2.26[d]
[c]
T’84aa
ꢀ1.40[h]
ꢀ3.40 ꢀ5.00 1.60
[e]
T83bb
T83bb
T83bb
ꢀ1.33[h]
ꢀ1.84[h]
n.d.
0.20[h]
0.38[h]
0.43[h]
ꢀ3.47 ꢀ5.00 1.53
ꢀ2.96 ꢀ5.18 2.22
[f]
[g]
All the electrodeposited films are stable upon subsequent
n.d.
ꢀ5.23 n.d.
oxidative potential cycling in
a monomer-free solution
(Figure 5). Moreover, neat charge-trapping effects are observed
in the traditional media (Figure SI9 in the Supporting Informa-
tion).
[a] Ref. [14]. [b] Formal potentials E8’. [c] CH2Cl2 +0.1m TBAP, GC electrode
(Ref. [6]; Ep,aII unpublished data). [d] Peak potentials Ep. [e] CH2Cl2 +0.1m
(TBA)PF6, GC electrode. [f] ACN+0.1m (TBA)PF6, GC electrode.
[g] (BMIM)PF6, GC electrode. [h] Onset potentials Eonset. n.d.=not deter-
mined.
It is interesting to compare the electrochemical properties of
T83 and its films, deposited by electrooxidation, with those ex-
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hibited by constitutional isomer T’84 and its films (Table 1). Evi-
dently, the electrochemical data are nearly identical, even
though the a-conjugated thiophene sequence for the latter
compound is longer. However, as already discussed, the torsion
around the bond connecting the central thiophene units
halves the a-tetrathiophene chain of T’84 into two bithiophene
moieties with small reciprocal conjugation, whereas such dis-
tortion does not decrease the longest a-terthiophene se-
quence in T83. The combination of these effects smoothes the
differences in the electronic properties of these monomers, as
also confirmed by the UV/Vis absorption spectra (Figure 2).
The enantiorecognition ability of enantiopure oligo-T83 films
was tested with two structurally different probes and two dif-
ferent protocols, previously developed for the oligo-1 films.[1,5]
The first test was performed with (R)- and (S)-N,N-dimethyl-
1-ferrocenylethylamine probes ((R)-2 and (S)-2), both enantio-
pure and as the racemate. They have been chosen for electro-
chemical reversibility and commercial availability. (R)- or (S)-
oligo-T83 films electrodeposited on Au screen-printed electro-
des (SPE) from a drop of monomer solution in (BMIM)PF6 ionic
liquid were used (36 oligomerization potential cycles, T83
12 mm, scan rate=50 mVsꢀ1).
Moreover, enantioselectivity was observed with the racemic
probe even though, in this case, peak separation was signifi-
cantly lower (ꢁ150 mV), unlike the oligo-1 modified SPEs, for
which peak separation was higher in the case of the racemate.
Furthermore, for the racemate of the probe, the peak poten-
tials are remarkably shifted to more positive potentials with re-
spect to those observed for the pure enantiomers of the
probe. This suggests the occurrence of some reciprocal interac-
tion of the two antipodes in the sensing process within the
chiral film (Figure 6). A similar three-actor effect was previously
observed for the oligo-1 case, even though it was far less pro-
nounced.
The second test was performed on enantiopure l- and d-
DOPA (0.016m) in 0.05m HCl, using (R)-oligo-T83 film deposit-
ed by electrooxidation on the Au SPE.
Similarly, a neat enantiodiscrimination was achieved, al-
though with a less spectacular peak separation (60–70 mV)
than in the former case (but similar to the ꢁ90 mV separation
achieved with the electrodes modified with oligo-1 films)
(Figure 7).[5]
An impressive and specular discrimination was obtained for
the enantiopure probes on the enantiopure oligo-T83 elec-
trode surfaces, with a peak separation of ꢁ250 mV (Figure 6),
that is, even larger than that observed in the case of oligo-
1 with the same probes.[1]
Figure 7. Enantiorecognition CV tests with d-DOPA and l-DOPA chiral redox
probes (0.016m) on the oligo-(ꢀ)-(R)-T83 film coated electrode. Scan rate:
50 mVsꢀ1
.
Based on the peculiar structural features of T83 described
above, we envisaged application of this octathiophene as an
efficient cross-linking monomer for molecular imprinting in
polymers.[17] If used as such a monomer, T83 should promote
formation of a polymer of a structure very advantageous for
molecularly imprinted polymers (MIPs). In the MIP synthesis,
a cross-linking and functional monomer is co-polymerized in
the presence of a selected template, which is subsequently re-
moved from the resulting MIP, leaving empty molecularly im-
printed cavities capable of recognizing the original imprinter.[18]
Electrochemical co-polymerization of T83 would result in an
MIP with a structure facilitating analyte permeation through
the MIP matrix on the one hand and stabilizing tailored molec-
ular cavities on the other.[19]
To test the usefulness of this T83 application, we used this
oligothiophene as a cross-linking monomer for deposition by
electropolymerization of an MIP imprinted with the 5’-TATAAA-
3’ oligonucleotide, that is, a TATA box promoter (Figure 8). In-
Figure 6. Enantiorecognition CV tests with (R)-2 and (S)-2 chiral redox
probes (8 mm); a) oligo-(+)-(S)-T83 film coated electrode; b) oligo-(ꢀ)-(R)-T83
film coated electrode; c) with the (ꢂ)-2 racemic redox probe on the oligo-
(ꢀ)-(R)-T83 film coated electrode. Scan rate: 50 mVsꢀ1
.
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Figure 8. Structural formulas of the templating molecule, 5’-TATAAA-3’ 3, as
well as functional co-monomers 4 and 5.
formation contained within the TATAAA is critical for proper in-
itiation of transcription in eukaryotes. Sensitive and selective
nucleotide determination is important in several fields[20] in-
cluding clinical diagnostics,[21] environmental monitoring,[22] de-
tection of biological threats,[23] and food safety.[24] The devised
MIP was subsequently used as a recognition unit of the EG-FET
selective TATAAA chemosensor.
Figure 9. a) The background-corrected drain current changes with time for
the EG-FET chemosensor with the gate area of the Au-glass slide coated
with (1) the MIP-TATAAA thin film and (2) the NIP film. Extended-gate surface
area: 21 mm2; gate voltage: 1.50 V. b) The TATAAA calibration plots for the
EG-FET chemosensor with the recognition unit of (1) the TATAAA-extracted
MIP film and (2) the NIP film.
For the TATAAA imprinting, we used bis[2-(5,2’-bithienyl)]me-
thane functional monomers 4 and 5, bearing thymine and ade-
nine pendant, respectively, to form in solution a pre-polymeri-
zation complex with 3 through Watson–Crick type hydrogen
bonds (Figure 8).
es of the TATAAA chemosensors with either the MIP or NIP rec-
ognition unit are presented in Figure 9a. After each addition of
the TATAAA analyte solution, the source-drain current in-
creased for both the MIP and NIP film-coated EG-FET gate.
However, changes observed for the MIP were much more sig-
nificant than those for the NIP-coated electrode.
The EG-FET based technique is an interesting extension of
the field-effect transistor transduction method developed in
1970.[25] As the conventional FET transductor, the EG-FET is
based on the measurement of changes of the source-drain cur-
rent at constant gate voltage when it is modulated by the
gate voltage changes incurred by sorption of charged species
within the gate region of the transistor. This effect can readily
be used as a transduction method in the chemosensor.[26] In
the EG-FET construction, the gate region can be electrically “ex-
tended” to outside metallic electrode, which greatly facilitates
the modification of the sensor surface. This technique has al-
ready been successfully used for fabrication of various sen-
sors,[27] including MIP-based ones.[28,29]
From these curves, calibration plots for the TATAAA-extract-
ed MIP and the NIP film coating the EG-FET were constructed
(Figure 9b). For the former, the linear dynamic concentration
range extended from 0.1 to 7.5 mm TATAAA (Curve 1 in Fig-
ure 9b). This behaviour obeys the linear regression equation:
Id/nA=[16.2 (ꢂ5.7)ꢂ10ꢀ5/nA+13.2(ꢂ1.6)]ꢂ10ꢀ5 nAmMꢀ1. The
sensitivity and correlation coefficient were (13.2ꢂ1.6) mMꢀ1
and 0.96, respectively. Of note, the NIP-coated sensor response
to TATAAA was much lower, indicating effective formation of
selective cavities in the MIP film.[29]
For TATAAA determination with the EG-FET chemosensor,
either the MIP-TATAAA or the NIP-coated Au-glass electrode
and the Pt grid as pseudo-reference electrode were immersed
in a test aqueous solution and connected to the gate of the
MOSFET. Gate voltage was set to 1.5 V and source-drain current
changes were monitored. Increasing amounts of TATAAA ana-
lyte aqueous solution were consecutively added to the test
aqueous solution under batch conditions. The EG-FET respons-
Conclusion
The results obtained, combining the peculiar properties of the
all-thiophene branched materials with the enantiorecognition
ability deriving from chirality, are very promising. The first “in-
herently chiral” spider-like oligothiophene T83, in which the
electroactive backbone coincides with the stereogenic axis, is
synthetically accessible in three steps from inexpensive and
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hybrid B3LYP functional of Handy and co-workers (aka CAM-
B3LYP)[33] for both structural and optical calculations.
commercially available compounds. Its electronic properties
have been studied by means of spectroscopic and electro-
chemical investigations, as well as of theorethical calculations,
and compared with other popular achiral, linear, and branched
multi-thiophene systems.
Geometry optimizations were performed for the most reasonable
isomers, and the transition state for enantiomer conversion of the
most stable isomer was located with the three-structure STQN
transition state searching method of Schlegel and co-workers.[34]
The electrooxidation of the enantiomers produces stable
enantiopure films displaying remarkable enantiorecognition
ability towards the enantiomers of some chiral redox probes,
clearly differentiating their oxidation potentials.
Optical calculations were of the linear-response Time-Dependent
DFT type, and computed using the above functional in the adia-
batic approximation, within the Casida’s formulation of the prob-
lem of computing the longitudinal dielectric function. Calculations
were performed both in vacuum and in the presence of a solvent
reaction field. In the latter case, the Polarizable Continuum Model
was adopted, using the solvent parameters appropriate for chloro-
form.[35]
Finally, T83, when used as the monomer or co-monomer in
electrooligomerization processes, is able to weave rigid cross-
linked 3D frameworks, sufficiently porous for analyte penetra-
tion into the polymer matrix. An MIP film for the selective de-
termination of an artificial nature-mimicking a single-stranded
oligonucleotide was successfully developed with potential ap-
plications in clinical analysis.
Photophysical Characterization
Photophysical measurements were carried out in air-equilibrated
toluene solutions at 298 K. Electronic absorption spectra were re-
corded on an Agilent Model 8543 spectrophotometer at room
temperature and using quartz cells with 1.0 cm path length.
Experimental Section
Steady-state emission spectra were obtained with an Edinburgh
FLS980 spectrofluorometer equipped with a 450 W xenon arc
lamp. Emission spectra are corrected for source intensity (lamp and
grating) and emission spectral response (detector and grating) by
standard correction curves.
Organic synthesis
Melting points were measured by a Bꢃchi B-540 instrument. NMR
spectra were recorded on a Bruker AV 300 spectrometer. Chemical
shifts are given in ppm (d, dd=double doublet). Mass spectra
were recorded on Bruker Daltonics high-resolution FT-ICR model
APEXTM II (4.7 T Magnex cryomagnet supplied with ESI source).
Hexabromo-3,3’-bithiophene:[30] Br2 (5.0 mL; 96 mmol) was drop-
ped into a solution of 3,3’-bithiophene[31] (0.50 g; 3.0 mmol) in dry
CS2 (15 mL) and the solution was refluxed for 36 h. A saturated so-
lution of Na2S2O5 in water was added portionwise to the mixture
until the brown color completely disappeared. The crude product,
separated as a brown-colored solid, was repeatedly treated with
hot ethanol to remove all soluble impurities (1.56 g; 80% yield;
m.p. 180–1838C); M.S. (E.I.+) 640 m/z.
Photoluminescence quantum yields (f) were measured in optically
dilute solution (typically 1.0ꢂ10ꢀ5 m) and compared to the quinine
sulfate in 0.1m H2SO4 solution at room temperature used as the
standard reference emitter (f=0.58). All the f values fall within
a ꢂ5% range.[36]
Enantioselective HPLC
HPLC enantioseparations were performed by using the stainless-
steel Chiralpak IB-3 and Chiralpak IB analytical (250 mm 4.6 mm
i.d.) and semipreparative (250 mmL.ꢂ10 mm i.d.) columns (Chiral
Technologies Europe, Illkirch, France). All chemicals, solvents for
HPLC, and syntheses and spectral grade solvents were purchased
from Sigma–Aldrich and used without further purification.
2,2’,4,4’,5,5’-Tetra(2-thienyl)-3,3’-bithiophene (T83): A mixture of
hexabromo-3,3’-bithiophene (0.50 g, 0.8 mmol), 2-(tributylstannyl)
thiophene (2.4 g, 6.4 mmol, Sigma–Aldrich) and 99% Pd(PPh3)4
(0.185 g, 1.6 mmol, Sigma–Aldrich) and p-xylene (25 mL), was re-
fluxed under nitrogen atmosphere for 20 h. The reaction mixture
was filtered on a silica gel cake (Merk 60, 230–70 mesh) to remove
the catalyst, the solvent was removed under reduced pressure and
the crude residue chromatographed on a silica gel column (Merk
60, 325–230 mesh) using a 4:1 mixture of n-hexane: CH2Cl2. Com-
bined final fractions gave T83 as a pale-yellow solid by removal of
the solvent (0.15 g; 26% yield; m.p.=120–1258C (synt.)). 1H NMR
(400 MHz, CDCl3) d=7.24 (2H, dd, J1 =5.1 Hz, J2 =1.2 Hz), 7.17 (2H,
dd, J1 =5.1 Hz, J2 =1.1 Hz), 7.16 (2H, dd, J1 =5.1 Hz, J2 =1.0 Hz),
7.13 (2H, dd, J1 =3.7 Hz, J2 =1.1 Hz), 7.06 (2H, dd, J1 =3.7 Hz, J2 =
1.1 Hz), 6.95 (2H, dd, J1 =5.1 Hz, J2 =3.7 Hz), 6.92 (2H, dd, J1 =
5.1 Hz, J2 =3.7 Hz), 6.87 (2H, dd, J1 =5.1 Hz, J2 =3.5 Hz), 6.56 ppm
(2H, dd, J1 =3.5 Hz, J2 =1.1 Hz); 13C NMR (400 MHz, CDCl3): d=
135.7 (C), 135.6 (C), 135.0 (C), 134.9 (C), 133.3 (C), 132.6 (C), 131.9
(C), 128.9 (CH), 127.0 (CH), 126.9 (CH), 126.8 (CH), 126.6 (CH), 126.2
(CH), 126.1 (CH), 126.0 (CH), 125.1 (CH). MS (EI): 658 [M+].
The analytical HPLC apparatus consisted of a PerkinElmer 200 LC
pump equipped with a Rheodyne injector, a 20 mL sample loop,
a HPLC Dionex CC-100 oven and a Jasco Model CD 2095 Plus UV/
CD detector. For semi-preparative separations, a PerkinElmer 200
LC pump equipped with a Rheodyne injector, a 500 mL sample
loop, a PerkinElmer LC 101 oven and Waters 484 detector were
used. The signal was acquired and processed by the Clarity soft-
ware of DataApex.
Circular dichroism
The circular dichroism (CD) spectra of the (S)-(+)-T83 and-(R)-(ꢀ)-
T83 were recorded by using a Jasco Model J-700 spectropolarime-
ter. The spectra were average-computed over three instrumental
scans and the intensities are presented in terms of ellipticity values
(mdeg).
Electrochemistry
Theoretical Computations
Monomer characterization: Cyclic voltammetry (CV) experiments
were performed using Autolab PGSTAT potentiostats of Eco-
Chemie, controlled by a PC with the GPES software of the same
manufacturer.
Density Functional Theory calculations were performed with the
atomic basis-set implementation available in Gaussian 09,[32] using
the Pople’s 6-31+ +G** basis-set and the long-range corrected
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The voltammetric investigations on the T83 redox features and oli-
gomerization ability were carried out in traditional solvents aceto-
nitrile (ACN, Sigma–Aldrich, anhydrous, ꢃ99.8%) and dichlorome-
thane (CH2Cl2, Sigma–Aldrich) in both cases with 0.1m tetrabuty-
lammonium hexafluorophosphate as the supporting electrolyte
(Fluka, ꢃ98%), or in ionic liquid 1-butyl-3-methylimidazolium hexa-
fluorophosphate ((BMIM)PF6, for catalysis, ꢃ98.5%, Fluka). A three-
electrode V-shaped minicell (with 3 cm3 of solution) was employed,
including a glass-embedded glassy carbon disk (GC, Metrohm, S=
0.031 cm2) or a gold disk (Au, Amel, S=0.033 cm2) or platinum disk
(Pt, Amel, S=0.031 cm2) as the working electrode, a Pt disk or wire
as the counter electrode, and an aqueous saturated calomel (SCE)
as the operating reference electrode, inserted in a double bridge
filled with the working medium, to avoid water and KCl leakage
into the working solutions. The optimized preliminary polishing
procedure for working disk electrodes consisted of treatment with
a diamond powder of 1 mm diameter (Aldrich) on a wet DP-Nap
cloth (Struers).
Chemosensor preparation: The MIP-TATAAA film was deposited
on the Au-glass electrode by potentiodynamic electropolymeriza-
tion from the 40 mm TATAAA, 0.1 mm 4-[bis(2,2’-bithien-5-yl)metha-
ne]phenyl thymine-1-acetic acid ester (functional monomer 4),
0.05 mm 4-[bis(2,2’-bithien-5-yl]methane] phenyl 2-adenine ethyl
ether (functional monomer 5), 0.2 mm 2,2’,4,4’,5,5’-tetra(2-thienyl)-
3,3’-bithiophene T83 (cross-linking monomer), and 0.1m (TBA)ClO4
(supporting electrolyte) solution of the volume ratio of acetonitrile-
to-water-to-toluene-to-isopropanol equal to 7.5:1:1:0.5. For this
deposition, the potential was scanned from 0.5 to 1.25 V at the
rate of 50 mVsꢀ1 for six current-potential cycles, which resulted in
deposition of the MIP-TATAAA film. Then, the TATAAA template
was extracted from this film with 0.1m NaOH for 45 min at room
temperature. In the same manner, a control non-imprinted poly-
mer (NIP) film was prepared in the absence of the TATAAA tem-
plate in the solution for the electropolymerization. The TATAAA-ex-
tracted MIP and NIP films served as recognition units of the EG-FET
chemosensors.
Oligomer film deposition and characterization: Depositions of
conducting T83 oligomer films by potentiodynamic electrooxida-
tion were performed through 36 consecutive oxidative potential
cycles at 0.2 Vsꢀ1 around the first or the second oxidation peaks
(with a monomer concentration of 0.5 mm), followed by repeated
potential cycling in the same potential range in a monomer free
solution; then the potential range was widened on the negative
side, to look for charge trapping and other reduction processes. In
the case of the (BMIM)PF6 ionic liquid, depositions and film stability
on SPEs were carried out at the 0.05 mVsꢀ1 potential scan rate with
the monomer concentration of 12 mm.
Acknowledgements
The financial support of Fondazione Cariplo (grants 2011-0417
and 2011-1851) is gratefully acknowledged. The research on in-
herently chiral materials and surfaces is developed in the con-
text of the SmartMatLab Centre project, cofunded by Regione
Lombardia (EU FESR and FSE funds), Fondazione Cariplo, Uni-
versitꢄ degli Studi di Milano and ISTM-CNR. Laboratori Alchem-
ia s.r.l is acknowledged for supporting the cover cost.
Enantiorecognition tests: For the first enantioselection test, (R)-
and (S)-conducting sensing oligomer films were electrodeposited
from a drop (0.02–0.03 cm3) of 12 mm (S)- and (R)-T83 monomer
solutions, dispensed on screen-printed gold working electrodes
(Au-SPEs, Metrohm, 61208210) by 36 consecutive oxidative CV
cycles at 0.05 Vsꢀ1 scan rate, followed by several potential cycles in
a monomer-free blank medium. The voltammetric characteristics of
(R)- and (S)-N,N-dimethyl-1-ferrocenylethylamine probes (Sigma–Al-
drich, ꢃ97%) were then recorded, both enantiopure and in 1:1
ratio, dissolved in a drop of (BMIM)PF6 at 0.18 mm concentration.
Keywords: chirality · enantiorecognition · hyperbranching ·
molecularly imprinted polymers · oligothiophenes
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MIP Experimental
Instrumentation: An SP-300 Modular Research Grade Potentiostat/
Galvanostat/FRA of Bio-Logic, controlled by the EC-lab software of
the same manufacturer was used for deposition of the molecularly
imprinted with TATAAA (T=thymine, A=adenine) polymer (MIP-
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Received: December 4, 2015
Published online on && &&, 0000
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FULL PAPER
Hyperbranched oligothiophenes dis-
play outstanding physicochemical prop-
erties. Hindered rotation around some
specific interannular bonds provides
these systems with additional relevant
attributes: inherent chirality and high
framework stiffness. The former endows
the molecules with chiral discrimination
properties, the latter with the unfading
memory of templating species.
&
Chirality
F. Sannicolꢀ, P. R. Mussini,* T. Benincori,*
R. Martinazzo,* S. Arnaboldi, G. Appoloni,
M. Panigati, E. Quartapelle Procopio,
V. Marino, R. Cirilli, S. Casolo, W. Kutner,
K. Noworyta, A. Pietrzyk-Le, Z. Iskierko,
K. Bartold
&& – &&
Inherently Chiral Spider-Like
Oligothiophenes
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