Effect of regioregularity and role of heteroatom on the chiral behavior of oligo(heteroalkyl thiophene)s

Article abstract of DOI:10.1002/chir.23282

Novel optically active oligothiophenes bearing electron-donating chiral side chains have been prepared by synthetic methods suitable to achieve regioregular head-to-tail and head-to-head/tail-to-tail derivatives. In particular, the chiral (S)-(2-methyl)butyl moiety was linked at position 3 of the thiophene ring through heteroatoms, such as S or O, to evaluate its effect on the macro molecular aggregation and, consequently, on the chiroptical properties of the material in the solid state. The materials have been fully characterized and investigated by optical and chiroptical methods upon aggregation both from the solution and as cast films. Compared with the related head-to-tail and head-to-head/tail-to-tail poly(3-alkyl)thiophene derivatives, with the same optically active moiety directly linked to the ring and possessing a higher polymerization degree, the chiroptical properties of the newly synthesized oligomers were significant, or even better, and provided insight into the role of intrachain–interchain interactions between the heteroatom and the thienyl sulfur atom.

Full text of DOI:10.1002/chir.23282

Received: 4 August 2020
DOI: 10.1002/chir.23282
Revised: 2 October 2020
Accepted: 7 October 2020
R E G U L A R A R T I C L E
Effect of regioregularity and role of heteroatom on the
chiral behavior of oligo(heteroalkyl thiophene)s
1
1
1
Martina Marinelli | Luigi Angiolini | Massimiliano Lanzi
|
2
1
Francesca Di Maria | Elisabetta Salatelli
1
Department of Industrial Chemistry
Abstract
“Toso Montanari”, University of Bologna,
Bologna, Italy
Novel optically active oligothiophenes bearing electron-donating chiral side
chains have been prepared by synthetic methods suitable to achieve
regioregular head-to-tail and head-to-head/tail-to-tail derivatives. In particular,
the chiral (S)-(2-methyl)butyl moiety was linked at position 3 of the thiophene
ring through heteroatoms, such as S or O, to evaluate its effect on the macro
molecular aggregation and, consequently, on the chiroptical properties of the
material in the solid state. The materials have been fully characterized and
investigated by optical and chiroptical methods upon aggregation both from
the solution and as cast films. Compared with the related head-to-tail and
head-to-head/tail-to-tail poly(3-alkyl)thiophene derivatives, with the same
optically active moiety directly linked to the ring and possessing a higher poly-
merization degree, the chiroptical properties of the newly synthesized oligo-
mers were significant, or even better, and provided insight into the role of
intrachain–interchain interactions between the heteroatom and the thienyl
sulfur atom.
2
Istituto per la sintesi organica e
fotoreattività (ISOF), CNR-ISOF and
Mediteknology s.r.l., Via Gobetti 101,
Bologna, 40129, Italy
Correspondence
Elisabetta Salatelli, Department of
Industrial Chemistry “Toso Montanari”,
University of Bologna, Viale Risorgimento
4, Bologna 40136, Italy.
Email: elisabetta.salatelli@unibo.it
Funding information
UE project INFUSION, Grant/Award
Number: 734834; University of Bologna
K E Y W O R D S
chiral heteroalkyl thiophene, circular dichroism, regioregular oligothiophenes, solvatochromism
1
| INTRODUCTION
active moiety in the side chain, obtained by electro-
chemical polymerization, was investigated in 1988 by
3
Chiral conjugated polymers, owing to the combination of
chirality with electrical conductivity, are potentially use-
ful in many areas such as polarization-sensitive
electrooptical devices, polarized photoluminescence
Lemaire et al. In early investigations, however, the
optically active derivatives were frequently mixtures of
regioisomers involving steric interactions, as in the case
of head-to-head (HH) couplings, determining a decrease
of conjugation length as well as lower chiroptical prop-
erties. A decisive breakthrough in the research was rep-
resented by the possibility to prepare regioregular
derivatives starting from monomeric 3-alkylthiophene
through the development of regiospecific polymeriza-
(PL) and electroluminescence, or chiral enantioselective
1,2
sensors, and therefore, they have long been the object
of intense research work.
Chiral polythiophenes, in particular, were initially
prepared by chemical or electrochemical oxidation of
monomers with a chiral substituent covalently attached
4
tion routes, such as those of McCullough et al. and
2
5
to the 3-position of the thiophene ring. The first chiral
Chen and Rieke, which allowed to achieve the desired
poly(3-alkyl)thiophene (P3AT) bearing the optically
regioregular head-to-tail (HT) couplings, giving access
Chirality. 2020;32:1361–1376.
wileyonlinelibrary.com/journal/chir
© 2020 Wiley Periodicals LLC.
1361

1362
MARINELLI ET AL.
to a wide range of chiral P3ATs with markedly
enhanced chiroptical properties. Circular dichroism
inactive monomolecular phase present in good solvents
was first converted into
a chiral monomolecular
(
CD) studies allowed to demonstrate that in HT P3ATs,
(or loosely aggregated) phase and then into one or more
associate phases upon treatment with poor solvent or
obtained as cast film.
the aggregation involves essentially planar polymer
chains that have a helical interchain order, rather than
an intramolecular helical conformation. In particular,
HT poly{3-[(S)-(2-methylbutyl)]thiophene} (PMBT) was
deeply investigated and was found to display chiral
20
Chiral HT and HH/TT poly(3-alkoxythiophene)s
bearing the optically active 3,7-dimethyloctyloxy moiety
in the side chain, as spin-coated films or upon aggre-
gation from the solution, exhibited CD spectra charac-
terized by large bisignate Cotton effects and high
g factors values as originated by the formation of
−2
anisotropy factor g values as high as 1.0ꢀ10
at
22
6,7
6
12 nm and a high degree of crystallinity (65%)
upon aggregation from the solution by gradual addition
of a weak solvent. During the ordering process, a kind
of sergeants-and-soldiers principle acts, in which the
longest chains determine the chirality of the aggregated
form, that is, the aggregation of π-conjugated polymers
is a multichain event even in the case of very dilute
solutions. Indeed, HT PMBT in the solid state consists
mainly of stacks of nearly coplanar extended chains,
and reasonable values for the distances of interacting
thiophene units have been found by applying the exci-
ton coupling scheme.
15,16
coplanar strands stacked in a chiral way.
Similar
results were also obtained with diblock copolymers
constituted by 3-alkyl and 3-alkoxy polythiophene
blocks bearing the (S) and/or (R)-3,7-dimethyloctyl
moiety linked directly to the ring and/or through the
oxygen atom, upon addition of poor solvent to the
chloroform solution, giving rise to aggregation as heli-
cal supramolecular structures again characterized by
8
9
21
high g values.
10
In this context, we have considered of interest the
preparation of optically active oligomeric polythiophene
derivatives bearing a chiral 3-sulfanyl or 3-alkoxy moi-
ety in the side chain and possessing known reg-
ioregularity in order to investigate and assess by
chiroptical methods their behavior upon aggregation
from the solution and in the solid state as cast films.
The same (S)-(2-methyl)butyl moiety was used as the
chiral component, with the aim to obtain results com-
parable with those reported for the above-mentioned
More recently, we reported the synthesis of the
regioregular HH/tail-to-tail (TT) isomeric homologue of
PMBT, obtained by polymerization of the bithiophenic
0
11
monomer 3,3 -bis[(S)-(2-methylbutyl)]thiophene.
As
evidenced by CD, the polymer displays both in solution
and in the solid state chain sections having helical struc-
ture, as a consequence of steric interactions between side
chain alkyl groups, that disfavor coplanarity of thiophene
rings, and therefore, the obtainment of supramolecular
chiral aggregates upon aggregation from the solution. For
this reason, the introduction of a heteroatom, such as sul-
fur or oxygen, between the thiophene ring and the chiral
alkyl group should provide a reduction of the steric hin-
drance and promote conformations suitable to produce
improved rings coplanarity with a longer extent of conju-
gation in the main chain, even in HH/TT regioregular
polymeric derivatives. In addition, it is widely
11,22
optically active P3ATs
and better elucidate the
effect of the sulfur and oxygen spacers on aggregation
process and functional properties. The synthetic routes
to monomers TSR*Br , TOR*Br , T2SR*, and T2OR*
2
2
as precursors to the derivatives PTSR*, PTOR*,
PT2SR*, and PT2OR*, respectively, are reported in
Scheme 1.
12–19
reported
that the presence of intramolecular inter-
actions between the thienyl sulfur atom and the pendant
sulfur or oxygen atom located in the adjacent moiety
appears to provide a strong contribution to planarity of
the backbone.
2 | MATERIALS AND METHODS
2.1 | Materials
However, if compared with chiral P3ATs, less
research concerning optically active 3-sulfanyl or
All commercial reagents and solvents were purchased
from Sigma-Aldrich and used as received unless other-
wise stated. N-Bromosuccinimide (NBS) was rec-
rystallized from hot water, and pyridine (Fluka) was
freshly distilled before use. Anhydrous solvents were pre-
3
-alkoxy thiophene polymeric derivatives can be found in
2
the literature. In particular, the presence of more than
one single phase upon aggregation was reported for 4,4 -
0
0
23
bis[(S)-2-methylbutylsulfanyl]-2,2 -bithiophene polymer-
pared following literature procedures and stored over
ized in the presence of ferric chloride, providing the
corresponding TT/HH regioregular polythiophene. This
resulted optically active when the random and optically
molecular sieves. All manipulations involving air- or
moisture-sensitive reagents were performed under nitro-
gen in dried glassware.

MARINELLI ET AL.
1363
SCHEME 1 Synthetic route to optically
active heteroalkyl thiophene monomers and
oligomers. Reagents and conditions: (i) 2
equiv of NBS, CH
hexylmagnesiumbromide in Et
Cl , 2-MeTHF, reflux; (iii) 1 equiv of NBS,
DMF, 0 C; (iv) 0.5 equiv of bis(pinacolato)
diboron, Pd(dppf)Cl , NaHCO , THF/H
/1, MW, 90 C for T2SR*; n-butyllithium in
hexane, Fe(acac) , THF for T2OR*; (v) 4
equiv of FeCl , CHCl room temp
2 2
Cl , reflux; (ii) 1 equiv of
2
O, Ni(dppp)
2
ꢁ
2
3
2
O
ꢁ
2
3
3
3
ꢁ
ꢁ
2.2 | Methods and characterization
operating under nitrogen in the −50 C to 200 C temper-
ꢁ
ature range at a heating rate of 10 C/min was used
Microwave (MW) irradiation was performed in a Mile-
stone MicroSYNTH Labstation operating at 2,450 MHz
and equipped with pressure and temperature sensors.
The H-NMR spectra were recorded with a Varian
Mercury 400 (400 MHz) spectrometer at room
to determine the glass transition temperature (T ) of
the oligomers.
g
Ultraviolet–visible (UV–Vis) and PL spectra were car-
ried out on a PerkinElmer Lambda 20 and PerkinElmer
1
ꢁ
−3
LS50B spectrophotometer, respectively, at 25 C on 10 /
−
4
temperature in CDCl solutions, using tetramethylsilane
10 M CHCl solutions in 1 cm quartz cells. Thin film
3
3
as an internal reference. Chemical shifts are given
in ppm.
Molecular mass and dispersity index of the oligomers
were determined at room temperature by gel permeation
chromatography (GPC) in tetrahydrofuran (THF) solu-
tion on a high-performance liquid chromatography
measurements were made on samples cast from chloro-
benzene solutions on quartz slides by drop-casting.
Infrared (IR) spectra were carried out on Ge disks
using
a
PerkinElmer 1750 or
a
Spectrum One
spectrophotometer.
High-resolution
mass
spectroscopy (HRMS)
(
HPLC) Lab Flow 2000 apparatus equipped with a
(electrospray ionization [ESI]) spectra were recorded on a
Waters XEVO Q-TOF instrument.
Rheodyne 7725i injector, a Phenomenex Phenogel mixed
bed 5μ MXL type column and an RI K-2301 KNAUER
detector. Calibration curves were obtained by using
monodisperse polystyrene standards. Before being mea-
sured, the materials were dissolved in THF (ca. 1 mg/ml)
and filtered over a 0.2-μm pore size filter.
Optical activities were measured on CHCl solutions
3
with a PerkinElmer 341 digital polarimeter, equipped
with a Toshiba sodium bulb, using a cell path length of
0.1 cm. Specific rotation values at the sodium D line are
expressed as degree dm−
1
g
−1
dl.
The decomposition temperature (T ) of the oligo-
mers was determined in the 20–800 C temperature
range on a thermogravimetric analysis (TGA) TA Instru-
The CD spectra were recorded at the same solute
molar concentration as the UV–Vis spectra in 1cm quartz
cells, at room temperature, on CHCl or CHCl /CH CN
d
ꢁ
3
3
3
ments Q600 apparatus operating at a heating rate of
solutions, by a Jasco 810 dichrograph; Δε values, expressed
as L mol cm , were calculated by the following equa-
ꢁ
−1
−1
2
0 C/min under nitrogen atmosphere. A differential
scanning calorimetry (DSC) TA Instruments Q2000
tion: Δε = [Θ]/3,300, where the molar ellipticity Θ is in

1364
MARINELLI ET AL.
2
−1
0
degree cm dmol . The chiral anisotropy ratio g (Δε/ε)
was calculated by the ratio between the maximum Δε (in
absolute value) and the ε value at the same wavelength.
The molar concentration in terms of one substituted thio-
phene repeating unit was equal for each sample both in
pure CHCl and in all the CHCl /CH CN mixtures and
2.3.3 | (+)-3,3 -Bis[(S)-(2-methylbutyl)
0
sulfanyl]-2,2 -bithiophene (T2SR*)
A mixture of TSR*Br (0.342 g, 1.3 mmol), bis(pinacolato)
diboron (0.197 g, 0.8 mmol), Pd(dppf)Cl (0.053 g, 5%
mol), and NaHCO (0.325 g, 3.9 mmol) in THF/water 2:1
(3.0 ml) was irradiated by MW at 90 C for 40 min. The
reaction mixture was then cooled to room temperature,
diluted with dichloromethane, and washed with water;
the solvent was evaporated under reduced pressure, and
the product purified by flash chromatography with cyclo-
hexane as eluent to afford T2SR* (0.433 g, 90% yield) as a
2
3
3
3
3
ꢁ
assumed to remain constant within the range of solvent
composition tested. The molar concentrations of the sam-
ples were 1.6ꢀ10 , 1.7ꢀ10 , 1.6ꢀ10 , and 1.8ꢀ10 M for
PTSR*, PTOR*, PT2SR*, and PT2OR*, respectively.
−4
−4
−4
−4
25
1
2
.3 | Synthetic methods
yellowish oil: [α]D = +17.3 (c = 0.3 in CHCl ); H-NMR
3
400 MHz, CHCl , δ): 7.36 (d, J = 5.3 Hz, 2H, Ar 5-H),
3
(
2
.3.1 | (+)-2,5-Dibromo-3-[(S)-(2-
7.08 (d, J = 5.3 Hz, 2H, Ar 4-H), 2.80 (dd, 2H, CH S), 2.62
α
methylbutyl)sulfanyl]thiophene (TSR*Br2)
(dd, 2H, CH S), 1.60–1.38 (m, 4H, CHCH and CH CH ),
β
3
α
3
1
.24–1.10 (m, 2H, CH CH ), 0.92 (d, 6H, CHCH ), 0.83 (t,
β
3
3
+ +
To a two-necked flask containing a solution of TSR*
6H, CH CH ); HRMS (ESI , m/z): [M + H] calcd for
2 3
C H S , 371.0995; found: 371.0993.
18 27 4
+
(0.246 g, 1.3 mmol) in 7.0 ml of CH Cl , NBS (0.518 g,
2
2
2
.9 mmol) was added portionwise under stirring and pro-
ꢁ
tection from light. The mixture was heated at 45 C and
maintained at this temperature for 24 h. After the organic
layer was cooled and washed with water, the crude prod-
uct was purified by column chromatography on silica gel
2.3.4 | (+)-3-[(S)-(2-Methylbutoxy)]
thiophene (TOR*)
with cyclohexane to obtain TSR*Br (0.419 g, 92% yield)
To a solution of 3-methoxythiophene (Alfa Aesar)
(0.760 g, 6.7 mmol) in 8.4 ml of toluene, (S)-
(−)-2-methyl-1-butanol (1.4 ml, 13.3 mmol) and
p-toluenesulfonic acid monohydrate (0.189 g, 1.0 mmol)
were sequentially added under stirring and inert atmo-
sphere. After 24 h at reflux, the mixture was cooled to
room temperature, poured into 100 ml of water, and then
extracted with CH Cl . The organic phase was washed
2
25
1
as a colorless oil: [α]D = +14.1 (c = 0.5 in CHCl ); H-
3
NMR (400 MHz, CHCl , δ): 6.90 (s, 1H, Ar 4-H), 2.83 (dd,
3
1
H, CH S), 2.67 (dd, 1H, CH S), 1.62–1.46 (m, 2H,
α
β
CHCH and CH CH ), 1.31–1.19 (m, 1H, CH CH ), 1.01
3
α
3
β
3
+
(d, 3H, CHCH ), and 0.89 (t, 3H, CH CH ); HRMS (ESI ,
3 2 3
+
+
m/z): [M + H] calcd for C H Br S , 342.8825; found:
9
13 2 2
3
42.8823.
2
2
with water, dried over Na SO , and concentrated by evap-
2
4
oration under reduced pressure. The crude product was
purified by column chromatography on silica gel with
cyclohexane/CH Cl 80:20 v/v as eluent to afford TOR*
2
.3.2 | 2-Bromo-3-[(S)-(2-methylbutyl)
sulfanyl]thiophene (TSR*Br)
2
2
25
(
0.764 g, 67% yield) as a yellow liquid: [α]D = +8.7
1
A solution of NBS (0.497 g, 2.8 mmol) in 4.0 ml of
dimethylformamide (DMF) was added dropwise at 0 C in
1
(c = 0.4 in CHCl ); H-NMR (400 MHz, CHCl , δ): 7.15
3 3
ꢁ
(dd, J = 5.5 Hz, J = 3.2 Hz, 1H, Ar 5-H), 6.76 (dd,
J = 5.5 Hz, J = 1.6 Hz, 1H, Ar 4-H), 6.22 (dd, J = 3.3 Hz,
h, under stirring and in the dark, to a solution of TSR*
(0.520 g, 2.8 mmol) in 4.0 ml of DMF. The reaction mix-
J = 1.6 Hz, 1H, Ar 2-H), 3.81 (dd, 1H, CH O), 3.73 (dd,
α
ture was stirred for 24 h protected from the light, then
1H, CH O), 1.94–1.88 (m, 1H, CHCH ), 1.62–1.50 (m, 1H,
β
3
diluted with water, and finally extracted with Et O. After
CH CH ), 1.36–1.20 (m, 1H, CH CH ), 1.01 (d, 3H,
2
α
3
β
3
+
being dried over Na SO4 and solvent evaporation at
CHCH ), and 0.94 (t, 3H, CH CH ); HRMS (ESI , m/z):
3 2 3
2
+
+
reduced pressure, the crude product was purified by col-
umn chromatography on silica gel with hexane/ethyl ace-
[M + H] calcd for C H OS , 171.0843; found: 171.0842.
9 15
tate 19:1 v/v as eluent to give TSR*Br (0.723 g, 98% yield)
1
as a colorless liquid: H-NMR (400 MHz, CHCl , δ): 7.24
(
2.3.5 | (+)-2,5-Dibromo-3-[(S)-2-
methylbutoxy]thiophene (TOR*Br2)
3
d, J = 5.6 Hz, 1H, Ar 5-H), 6.92 (d, J = 5.6 Hz, 1H, Ar 4-
H), 2.87 (dd, 1H, CH S), 2.68 (dd, 1H, CH S), 1.60–1.42
α
β
(
m, 2H, CHCH3 and CH CH ), 1.32–1.20 (m, 1H,
The same procedure described for TSR*Br was followed
α
3
2
CH CH ), 1.00 (d, 3H, CHCH ), and 0.89 (t, 3H,
starting from TOR* (0.199 g, 1.2 mmol) and NBS
(0.464 g, 2.6 mmol) in CH Cl to give TOR*Br (0.328 g,
β
3
3
CH CH ).
2
3
2
2
2

MARINELLI ET AL.
1365
2
5
+
+
5
4% yield) as a colorless oil: [α]
= +4.0 (c = 0.6 in
CH CH ); HRMS (ESI , m/z): [M + H] calcd for
2 3
D
1
+
CHCl ); H-NMR (400 MHz, CHCl , δ): 6.76 (s, 1H, Ar
C H NaO S , 361.1271; found: 361.1270.
3
3
18 26
2 2
4
1
1
0
-H), 3.85 (dd, 1H, CH O), 3.77 (dd, 1H, CH O),
α β
.90–1.76 (m, 1H, CHCH ), 1.60–1.48 (m, 1H, CH CH ),
3
α
3
.32–1.18 (m, 1H, CH CH ), 1.01 (d, 3H, CH CH), and
2.3.8 | Oligo{3-[(S)-(2-methylbutyl)
sulfanyl]thiophene} (PTSR*)
β
3
3
+
+
.94 (t, 3H, CH CH ); HRMS (ESI , m/z): [M + H] calcd
3
+
2
for C H Br OS , 326.9053; found: 326.9052.
9
13
2
To a three-necked flask containing TSR*Br (0.313 g,
2
0.9 mmol), dried 2-methyltetrahydrofuran (5 ml) and 2 M
2
.3.6 | 2-Bromo-3-[(S)-2-methylbutoxy]
of hexylmagnesium bromide in Et O (0.5 ml, 1.0 mmol)
were sequentially added under stirring and nitrogen
atmosphere. The solution was heated to reflux in the
2
thiophene (TOR*Br)
The same procedure described for TSR*Br was followed
starting from TOR* (0.565 g, 3.3 mmol) and NBS
dark for 1 h. Then, Ni(dppp)Cl (0.024 g, 5 mol%) was
2
added, and the mixture left under stirring and reflux
overnight. After being dried, chloroform was added, and
the mixture was washed with water, followed by drying
over Na SO . The organic phase was concentrated to
(
9
0.591 g, 3.3 mmol) in DMF obtaining TOR*Br (0.740 g,
0% yield) as a light brown oil: H-NMR (400 MHz,
1
CHCl , δ): 7.18 (d, J = 5.9 Hz, 1H, Ar 5-H), 6.73 (d,
3
2
4
J = 5.9 Hz, 1H, Ar 4-H), 3.89 (dd, 1H, CH O), 3.81 (dd,
small volume and finally treated with hot MeOH to
afford PTSR* (0.121 g, 73% yield) as a dark red solid; H-
α
1
1
H, CH O), 1.88–1.78 (m, 1H, CHCH ), 1.62–1.50 (m, 1H,
β
3
CH CH ), 1.32–1.20 (m, 1H, CH CH ), 1.01 (d, 3H,
CH CH), and 0.95 (t, 3H, CH CH ); HRMS (ESI , m/z):
NMR (400 MHz, CHCl , δ): 7.38 (s, 1H, backbone Ar
α
3
β
3
3
+
4-H), 3.00–2.60 (m, 2H, CH S), 1.80–1.15 (m, 3H, CH CH
3
3
2
2
3
+
+
[
M + H] calcd for C H BrOS , 248.9948; found:
and CH CH ), 1.10–0.95 (m, 3H, CH CH), and 0.95–0.78
9
14
3
2
3
2
48.9947.
(t, 3H, CH CH ).
3 2
0
2
2
.3.7 | (+)-3,3 -Bis[(S)-(2-methylbutoxy)]-
2.3.9 | Oligo{3-[(S)-(2-methylbutoxy)]
thiophene} (PTOR*)
0
,2 -bithiophene (T2OR*)
A solution of n-butyl lithium in hexane 2.5 M (0.4 ml,
To a three-necked flask containing TOR*Br (0.296 g,
2
1
.0 mmol) was added dropwise under inert atmosphere
0.9 mmol), dry THF (5 ml) and 2 M of hexylmagnesium
ꢁ
at −70 C to a solution of TOR*Br (0.215 g, 0.9 mmol) in
bromide in Et O (0.5 ml, 1.0 mmol) were sequentially
2
dry THF (3 ml). After 30 min, Fe(acac) (Alfa Aesar)
added under stirring and nitrogen atmosphere. The solu-
tion was heated to reflux in the dark for 1 h. Then,
3
(0.494 g, 1.4 mmol) was added to the reaction mixture,
and the mixture allowed to reach room temperature
under stirring overnight before quenching with water.
Ni(dppp)Cl (0.024 g, 5 mol%) was added, and the mix-
2
ture was left under stirring and reflux for 1 h. After being
dried, chloroform was added, and the mixture was
washed with water, followed by drying over Na SO . The
The aqueous phase was extracted with Et O, and the
2
combined organic phases were washed with brine. After
2
4
being dried over Na SO4 and removal of the solvent
organic phase was concentrated to small volume and
2
under reduced pressure, the crude product was purified
by column chromatography on silica with hexane as elu-
ent to afford T2OR* (0.050 g, 16% yield) as a dark green
oil constituted by a 60:40 mixture of HH and HT
finally treated with hot MeOH to afford PTOR* (0.079 g,
53% yield) as a dark blue powder: H-NMR (400 MHz,
1
CHCl , δ): 7.02–6.80 (bm, 1H, backbone Ar 4-H),
3
4.10–3.90 (m, 2H, CH O), 2.08–1.90 (m, 1H, CH CH),
2
3
25
1
regioisomers: [α]D = +42.7 (c = 0.4 in CHCl ); H-NMR
and 1.78–0.86 (m, 8H, CH CH, CH CH and CH CH ).
3
3
3
2
3
2
(
5
400 MHz, CHCl , δ): 7.07 (d, J = 5.7 Hz, 2H, Ar 5-H and
3
0
-H, HH), 7.02 (d, J = 5.7 Hz, 1H, Ar 5-H, HT), 6.88 (d,
0
0
J = 1.8 Hz, 1H, Ar 3 -H, HT), 6.83 (d, J = 5.7 Hz, 2H, Ar
2.3.10 | Oligo{3,3 -bis[(S)-(2-methylbutyl)
0
0
4
6
4
2
-H and 4 -H, HH), 6.81 (d, J = 5.7 Hz, 1H, Ar 4-H, HT),
sulfanyl]-2,2 -bithiophene} (PT2SR*)
0
.10 (d, J = 1.8 Hz, 1H, Ar 5 -H, HT), 4.01 and 3.94 (m,
0
H, 3- and 3 -OCH and 3-OCH , HH), 4.01 and 3.94 (m,
To a solution of T2SR* (0.075 g, 0.2 mmol) in anhydrous
α
β
0
H, 3-OCH and 3-OCH , HT), 3.81 and 3.73 (m, 2H, 4 -
CHCl (8.1 ml), FeCl (0.131 g, 0.8 mmol) was added
α
β
3
3
0
OCH and 4 -OCH , HT), 2.00–1.80 (m, 4H, CHCH ),
1
1
under a weak stream of nitrogen at room temperature.
The suspension was left under stirring for 24 h, turning
from greenish to dark blue color. Then, 50 ml of THF
α
β
3
.70–1.50 (m, 4H, CH CH ), 1.44–1.18 (m, 4H, CH CH ),
α
3
β
3
.12–1.06 (m, 12H, CHCH3), 1.02–0.90 (m, 12H,

1366
MARINELLI ET AL.
and 100 ml of CHCl3 were added; the mixture was
washed several times with 2% v/v aqueous HCl up to
complete elimination of the iron(III) ion (negative essay
3-methoxythiophene with (−)-(S)-2-methyl-1-butanol,
according to the procedure reported by Xu for the related
27
3-O-hexyl thiophene derivative.
Treatment of TSR* and TOR* with NBS according to
with NH SCN) and finally with water to neutrality. The
4
28,29
organic layer was dried over Na SO , concentrated under
established procedures
allowed to obtain the related
2
4
reduced pressure, and finally treated with MeOH to
2,5-dibromo derivatives TSR*Br (dextrorotatory enantio-
meric isomer of the previously reported compound)
2
3
0
afford PT2SR* (0.071 g, 97% yield) as a brownish waxy
1
solid: H-NMR (400 MHz, CHCl , δ): 7.38 (d, 1H, termi-
and TOR*Br , as well as the 2-bromo derivatives
3
2
0
24
nal Ar 5-H), 7.22–7.12 (ms, 2H, backbone Ar 4-H and 4 -
TSR*Br and TOR*Br, subsequently used for the prepa-
H), 7.08 (d, 1H, terminal Ar 4-H), 2.96–2.82 (m, 2H,
CH S), 2.76–2.62 (m, 2H, CH S), 1.70–1.40 (m, 2H,
ration of the HH bithiophene monomers T2SR*
and T2OR*.
α
β
0
CH CH), 1.34–1.10 (m, 2H, CH CH ), 1.04–0.98 (m, 2H,
In
particular,
(+)-3,3 -bis[(S)-(2-methylbutyl)
3
α
3
0
CH CH ), 0.98–0.92 (m, 6H, CH CH), 0.92–0.78 (m, 6H,
sulfanyl]-2,2 -bithiophene T2SR* was obtained in high
yield following the synthetic protocol based on MW-
assisted Suzuki–Miyaura reaction recently adopted for
β
3
3
CH CH ).
2
3
31
similar, optically inactive, derivatives. By contrast, the
preparation of the related oxyalkyl disubstituted
bithiophene monomer T2OR* through this route was
unsatisfactory. Moreover, despite the expected higher
reactivity due to the presence of oxygen directly con-
nected to the thiophene ring, the synthesis by oxidative
0
2
(
(
.3.11 | Oligo{3,3 -bis[(S)-
0
2-methylbutoxy)]-2,2 -bithiophene}
PT2OR*)
The same procedure described for PT2SR* was followed
starting from the HH and HT mixtures of regioisomers
labeled as T2OR* (0.050 g, 0.1 mmol) and FeCl (0.065 g,
31
coupling with BuLi/CuCl2 was also inconclusive. How-
0
0
ever, (+)-3,3 -bis[(S)-(2-methylbutoxy)]-2,2 -bithiophene
T2OR* could be obtained from TOR*Br with BuLi in the
3
0
.4 mmol) in CHCl to obtain PT2OR* (0.032 g, 95%
3
1
yield) as a black solid: H-NMR (400 MHz, CHCl , δ):
presence of Fe(acac) according to the method adopted
3
3
0
32
7
7
3
.18–7.08 (m, 2H, backbone Ar 4-H and 3 -H, HH),
for similar derivatives, although mixed with 40% of the
1
.08–6.92 and 6.92–6.68 (m, 2H, backbone Ar 4-H and
HT regioisomer, as established by H-NMR spectroscopy
0
0
-H, HH and backbone Ar 4-H and 4 -H, HT), 4.2–3.6
(see Section 2 and Figure S2B).
The optically active monothiophene and bithiophene
monomers were then polymerized, respectively, by
(
m, 4H, OCH ), 2.0–1.8 (m, 2H, CHCH ), 1.7–1.5 (m, 2H,
2
3
CH CH ), 1.4–1.2 (m, 2H, CH CH ), 1.1–1.0 (m, 6H,
α
3
β
3
CHCH ), and 1.0–0.9 (m, 6H, CH CH ).
regiospecific
organometallic
coupling
to
the
3
2
3
corresponding HT derivatives and by simple oxidative
coupling with FeCl , leading to HH/TT derivatives in
3
3
3
| RESULTS AND DISCUSSION
.1 | Synthesis
consequence of the chemical equivalence of the
bithiophenic reactive sites.
3
3,34
The GRIM (Grignard Metathesis) procedure,
involving a magnesium/bromine exchange after the ini-
tial treatment with a Grignard reagent, was adopted for
the polymerization of TSR*Br and TOR*Br , in order to
The monothiophene and bithiophene monomers
TSR*Br , TOR*Br , T2SR*, and T2OR* (Scheme 1) used
2
2
2
2
for the preparation of the corresponding oligomeric
derivatives were obtained respectively from the starting
materials TSR* and TOR*.
obtain highly regioregular HT derivatives, owing to the
selectivity of the sterically hindered catalyst Ni(dppp)Cl2
that does not promote HH or TT couplings. However,
similar to what was obtained by Goldoni et al. in the
polymerization of 3-(butylthio)thiophene, in our case, the
2
9
The synthesis of (+)-3-[(S)-(2-methylbutyl)sulfanyl]
24
thiophene (TSR*) was carried out following the method
12
reported for the related 3-S-hexyl thiophene by nucleo-
polymerization of TSR*Br , carried out under the same
2
12
philic reaction of 3-mercaptothiophene with (+)-(S)-1-
reaction conditions, gave a low molecular mass product
probably due to unfavorable interactions between the cat-
alyst and the thioethereal moiety. Consequently, the
bromo-2-methylbutane, prepared in turn by reaction with
25
LiBr of the p-toluensulfonate ester of enantiomerically
pure (−)-(S)-2-methyl-1-butanol, obtained through a
modified literature procedure (see Supporting Informa-
tion for details).
polymerization of TOR*Br was performed with the aim
2
26
to obtain a macromolecular derivative with a comparable
polymerization degree.
0
(
+)-3-[(S)-(2-Methylbutoxy)]thiophene (TOR*) was
The polymerization of (+)-3,3 -bis[(S)-(2-methylbutyl)
0
0
obtained by transetherification of commercial
sulfanyl]-2,2 -bithiophene (T2SR*) and (+)-3,3 -bis[(S)-

MARINELLI ET AL.
1367
1
0
(
2-methylbutoxy)]-2,2 -bithiophene (T2OR*) to the
3.2 | IR and H-NMR characterizations
corresponding PT2SR* and PT2OR* oligomeric deriva-
tives was carried out following the simple and well-
established oxidative coupling with iron(III) trichloride.
The symmetrical structure and chemical equivalence of
the coupling positions in the HH bithiophenic monomer
allow to obtain a completely regioregular derivative that
displays only HH/TT junctions between the thiophene
rings, despite the use of a nonregiospecific method.
T2OR*, actually a mixture of the HH and HT
regioisomers (60:40 molar ratio), was anyhow submitted
The IR spectrum of PTSR* displays the expected absorp-
tions characteristic of 2,5-coupled 3-substituted
−1
polythiophenes: aromatic C H stretching at 3,071 cm
and one out-of-plane deformation of aromatic C H at
803 cm− due to the trisubstituted ring. The alkyl chain
1
−
1
gives stretching vibrations in the region 2,963–2,854 cm
−1
and deformation modes below 1,475 cm .
In PT2SR*, the alkylsulfanyl chain gives rise to C H
−
1
stretching vibrations in the region 2,963–2,846 cm and
to oxidative coupling with FeCl , in order to obtain a
to deformation modes around 1,478, 1,458, 1,434
3
−
1
product (PT2OR*) at least partially comparable with the
related sulfanyl derivative PT2SR*.
(S CH ), and 1,378 cm . The aromatic C H stretching
2
and out-of-plane deformation are found at 3,079 and
−
1
All the macromolecular samples were characterized
for molecular mass, thermal, and spectral properties.
As reported in Table 1, the average polymerization
degrees of PTSR* and PTOR* were of comparable
magnitude. The average molecular mass of PT2SR*
appears significantly lower with respect to the HT
814 cm , respectively.
The alkoxy pendants of PTOR* give C H stretching at
−
1
2,959–2,850 cm (CH and CH ). Ring vibrational modes
2
3
−1
are seen at 1,523, 1,446, and 1,350 cm . The band at
−1
1,068 cm is assigned to C (ring) O C stretch. The
vibrational band at 802 cm− is attributable to C H out-
1
β
derivative,
being
substantially
constituted
by
of-plane deformation mode of thiophene rings. Similarly,
1
bithiophenic tetramers, as confirmed also by the H-
NMR spectrum (see below). The related polymeric
derivative with TT/HH regioregularity, obtained under
PT2OR* gives the corresponding absorptions at 2,966–
−1
2,850, 1,523, 1,398, and 1,353 cm . The C (ring) O C
stretch appears at 1,064 cm− and the C H out-of-plane
1
β
−1
the same conditions as PT2SR* by polymerization of
deformation mode of thiophene rings at 802 cm .
The chemical structure and regioregularity degree of
PTSR* and PTOR* were evaluated by H-NMR spectros-
0
the
TT
monomer
(+)-4,4 -bis[(S)-(2-methylbutyl)
0
20
1
sulfanyl]-2,2 -bithiophene, displayed a Mn value of
7,000 g/mol reasonably in consequence of deeper frac-
1
copy in CDCl . The spectrum of PTSR* (Figure S3A) dis-
3
tionation of the crude polymeric product with respect
to PT2SR*, as inferred by the 59% reaction yield
plays a major singlet at 7.38 ppm that is assigned to 4-H
of the backbone, in accordance to the chem. shift value
reported for HT poly(3-butylsulfanylthiophene) obtained
(vs. our 97%) reported in the above paper. By contrast,
2
9
PT2OR*, obtained with yield similar to PT2SR*, dis-
played higher DP value, which is to be related to a
positive role towards polymerization played by the alkoxy
group, although the presence of a fraction with lower
molecular weight, that is, approximately bithiophenic
dimers, was also detected by GPC.
by similar regiospecific Kobayashi procedure, along
with other low intensity resonances, attributed to the ter-
minal protons of oligomeric chains, and to minor HH/TT
connections at around 7.2 ppm, lowering the HT reg-
1
ioregularity degree to about the 90%, as measured by H-
NMR. By contrast, the spectrum of PTOR* displays a
TABLE 1 Yields and characterization data of macromolecular derivatives
a
b
c
d
ꢁ
e
ꢁ
d
T ( C)
f
Sample
PTSR*
Yield (%)
M
n
(g/mol)
Ð
DP
14
21
8
T
g
( C)
73
53
97
95
2,600
3,600
1,500
700
1.8
2.0
1.2
1.1
1.3
0
5
288
263
139
114
PTOR*
PT2SR*
PT2OR*
−26
−16
4
3,400
20
a
Weight of product/weight monomer × 100.
Number average molar mass determined by gel permeation chromatography (GPC) in tetrahydrofuran (THF).
b
c
Dispersity.
d
Average polymerization degree expressed as monothiophenic repeating units.
e
f
Glass transition temperature determined by differential scanning calorimetry (DSC) (second heating cycle).
Decomposition temperature determined by thermogravimetric analysis (TGA).

1368
MARINELLI ET AL.
broad resonance of the backbone 4-H proton at
.02–6.80 ppm (Figure S3B), upfielded with respect to the
reduced planarity of the backbone caused by the presence
of HH junctions. The alkoxy derivatives PTOR* and
PT2OR* exhibit a slightly higher rigidity than PTSR*
and PT2SR*, respectively, ascribable to larger conjuga-
tion extent in the backbone induced by the electron
donor alkoxy group that increases the polarity of the aro-
matic system, favoring a more compact arrangement of
the macromolecules in the solid state.
Both PT2SR* and PT2OR* show remarkably lower
thermal stability with respect to the related HT
regioregular samples (Figure S5) to be again attributed to
higher flexibility of the main chain, with the oxygen-
containing derivatives PTOR* and PT2OR* becoming
more unstable upon heating with respect to PTSR* and
PT2SR*.
7
corresponding proton of PTSR*, as a consequence of
increased electron density induced by the oxygen atom
directly linked to thiophene ring. No accurate evaluation
of the regioregularity degree can be made for PTOR*, as
the resonances of the 4-H proton related to the four possi-
ble triad sequences (HT/HT, TT/HT, HT/HH, and
TT/HH), reported as ranging in the intervals
3
5
36
7
(
.04–6.84 ppm or 6.95–6.83 ppm for various poly
3-alkoxythiophene)s obtained under nonregiospecific
conditions, are clearly overlapped. However, as the maxi-
mum intensity of the 4-H resonance of PTOR* is cen-
tered at 6.90 ppm, very close to the 6.91-ppm value
reported for highly regioregular HT linkage in poly
(
3-decyloxythiophene) obtained by the same GRIM
The absence of endothermic peaks related to melting,
and the presence of glass transitions only in the DSC
thermograms (Figure S6), suggests an overall amorphous
character in the solid state of all the oligomeric
derivatives.
37
method, we can conclude that PTOR* is characterized
by highly predominant HT regioregularity.
The expected structure for PT2SR* is confirmed by
1
the H-NMR spectrum (Figure S4A) that displays at
7
.2 ppm the resonance of the 4-H proton of the backbone,
as well as the presence of intense signals related to termi-
nal groups, as evidenced by the GPC measurements indi-
cating a low molecular mass of this sample. As expected,
the spectrum of PT2SR* appears identical to that
3.4 | Optical properties
The UV–Vis spectra of the monomers (Table 2 and
Figure 1A) display, as expected, a remarkable red shift of
the absorption maxima of the alkoxy derivatives with
respect to the related sulfanyl compounds, with this effect
being more evident in the bithiophene compounds (52 vs.
35 nm) and is attributed to higher coplanarity between
the thiophene rings induced by the O atom as compared
with S. Accordingly, the aromatic conjugation originated
by connecting two thiophene rings in the bithiophenic
monomers is higher in the alkoxy derivatives (28 nm
when passing from TOR* to T2OR*) with respect to the
sulfanyl derivatives (11 nm when passing from TSR* to
T2SR*). However, it has to be noted that a significant
electron-donating ability is also displayed by the sulfanyl
monomers compared with the related 3-alkylthiophene
0
reported for the above-mentioned poly{4,4 -bis[(S)-
0
(
2-methylbutyl)sulfanyl]-2,2 -bithiophene}
possessing
20
TT/HH regioregularity. On the other hand, the clear
presence of signals related to the 4-H and 5-H hydrogens
of terminal thiophene rings, in addition to the low-molec-
ular-weight value determined by GPC (Table 1), confirms
that the polymerization of T2SR* actually occurred to a
limited extent, yielding a tetrameric derivative consti-
tuted by bithiophenic counits.
1
The H-NMR spectrum of PT2OR* (Figure S4B) con-
firms the obtainment of a material having a substantially
regiorandom structure, with the presence of three broad
signals centered at 7.13, 6.97 (main), and 6.83 ppm,
related, respectively, to the presence of prevalently
HH/TT oligomers and to HH/TT/HT and HT/HT
0
and 3,3 -dialkyl bithiophene derivatives, exhibiting maxi-
35,36,38
39
junctions.
mum absorptions at lower wavelengths, 241
and
11
2
48 nm, respectively.
The UV–Vis spectra of the macromolecular deriva-
3
.3 | Thermal properties
tives in solution (Table 2 and Figure 1B) show absorption
maxima related to the π–π* electronic transition of the
backbone aromatic system. The HT regioregular deriva-
tives confirm the electron-releasing mesomeric effect of
oxygen directly linked to the thiophene ring, displaying a
stronger red shift of the maximum wavelength passing
from TOR* to PTOR* (274 nm) with respect to the shift
shown passing from TSR* to PTSR* (218 nm). In addi-
tion to its lower polymerization degree, it is possible that
steric effects of the bulky thioalkyl moiety distorting the
As shown in Table 1, both PTSR* and PTOR* display a
similar thermal behavior, with a slightly higher value of
the glass transition temperature and a lower decomposi-
tion temperature for PTOR* with respect to PTSR*. The
values for the glass transition temperature of the
bithiophene derivatives PT2SR* and PT2OR* appear
lower with respect to their counterparts PTSR* and
PTOR*, attributable to their lower molecular mass and

MARINELLI ET AL.
1369
3
TABLE 2 Maximum absorption (λmax) of monomers and absorption/emission (λem) wavelengths (nm) of oligomers in CHCl solution
and as films obtained from chlorobenzene solution
Solution
Film
max (nm)
Monomer
TSR*
λ
max (nm)
Oligomer
PTSR*
λ
max (nm)
λ
em (nm)
Stokes shifts
λ
a
265
300
276
328
483
574
416
522
601
665
582
612
118
509
580
469
529
b
4,579
a
TOR*
PTOR*
PT2SR*
PT2OR*
91
b
b
b
2
,250
a
T2SR*
T2OR*
166
7,333
a
90
2,834
a
In nm.
b
In cm−1
.
FIGURE 1 Normalized absorption spectra in CHCl
3
of monomers (A) and oligomers (B) (red line, TSR* and PTSR*; green line, TOR*
and PTOR*; blue line, T2SR* and PT2SR*; black line, T2OR* and PT2OR*)
main chain are also contributing to reduce the bat-
at similar wavelength as HT poly[(3-hexylthio)thiophene]
(485 nm), despite its much lower polymerization degree
(14 vs. 22 and 76, respectively).
40
40
hochromic shift in PTSR* with respect to PTOR*. The
maximum absorption wavelengths of PTOR* (574 nm in
solution, 580 nm as film) appear blue shifted compared
with those observed for the optically active HT poly
Both the λmax values of the bithiophene derivatives
PT2SR* and PT2OR* in CHCl solution are blue shifted
3
(
3-alkoxythiophene) containing the 3,7-dimethyloctyloxy
compared with PTSR* and PTOR* as a consequence
of their lower polymerization degree and backbone
configuration, which is expected to disfavor coplanar
arrangements of the conjugated aromatic rings. As
expected, the λmax value of PT2SR* appears lower than
moiety and characterized by a much higher PD value of
5
absorption maxima in CHCl3 solution of HT poly
(
(
(
of PTOR*, thus suggesting high regioregularity and suffi-
ciently extended aromatic conjugation in PTOR* despite
its oligomeric structure. The same consideration also
15
7 (602 nm in solution, 635 nm as film). However, the
4
0
3-hexyloxythiophene) (580 nm)
and HT poly
37
20
3-decyloxythiophene) (565 nm) having also high PD
71 and 45, respectively) are of similar magnitude to that
the value (469 nm) reported for the polymeric TT/HH
0
0
derivative poly{4,4 -bis[(S)-(2-methylbutyl)sulfanyl]-2,2 -
bithiophene} but not much dissimilar from the chemically
pure oligomer HH/TT octithiophene bearing the side
12
chain thiohexyl moiety (434 nm in CH Cl ).
2
2
holds for PTSR*, absorbing in CHCl at a wavelength
In addition to a weak absorption at 345 nm, ascribable
to the presence of a shorter oligomeric fraction, as
suggested by GPC, it is noteworthy that the λmax (522 nm)
3
(
483 nm) not much different from the absorption maxi-
30
mum of HT poly[(3-butylthio)thiophene] (502 nm) and

1370
MARINELLI ET AL.
of PT2OR* is strongly red shifted compared with both
the derivatives PTSR* and PT2SR*. Indeed, despite that
PT2OR* is actually a mixture of oligomers with an over-
all regiorandom structure, as suggested by its lower maxi-
mum absorption wavelength with respect to regioregular
to the sulfanyl derivatives, indicative of reduced energy
level difference of the excited states of the formers. As
deduced by the UV–Vis absorption spectra, and in agree-
ment with the results obtained with the above-mentioned
1
6
optically active regioregular poly(3-alkoxythiophene)s,
0
0
HH/TT poly(3,3 -dibutoxy-2,2 -bithiophene) and TT/HH
this confirms the increased extent of aromatic resonance
and hence coplanarity of the thiophenic rings in PTOR*
and PT2OR*. Interestingly, as Stokes shift values ranging
0
0
poly(4,4 -dibutoxy-2,2 -bithiophene) with similar poly-
38
merization degrees (545 and 574 nm, respectively), as
well as to the optically active HH/TT poly[3,3 -bis(3,7-
dimethyloctyloxy)-2,2 -bithiophene] (583 nm) and other
regioregular mono-alkoxy substituted polythiophenes
583–588 nm), it is confirmed that the oxygen atom pro-
0
−1
around 2,000 cm are considered typical of rigid conju-
0
16
41,42
gated polymers in solution,
it appears that PTOR*
and PT2OR* are characterized by significantly higher
stiffness with respect to their sulfanyl counterparts.
Solvatochromism experiments were carried out on
the chloroform solutions of the oligomers upon gradual
addition of a poor solvent, such as acetonitrile, in order
to promote a transition from the disordered random coil
conformation to self-assembling and microaggregation of
the macromolecules.
The addition of acetonitrile to the chloroform solu-
tions of HT PTSR* and PTOR* (Figure 2A,C) pro-
duces a gradual decrease of light absorption with a
broadening of the band related to the electronic π–π*
transition, as well as a slight red shift of the maximum
36
(
motes a remarkable electron delocalization, even in the
absence of prevalent regioregularity and high polymeriza-
tion degree. It is again to be noted that the maximum
absorption values in solution of all the sulfanyl and alk-
oxy oligomeric derivatives are in any case red shifted with
22
respect to the related maxima of HT (431 nm) and TT/HH
11
P3ATs (392 nm) materials bearing the same chiral moiety
in the side chain.
PL spectra carried out on the macromolecular deriva-
tives (Table 2 and Figure S7B) allowed to assess lower
Stokes shift values of the alkoxy derivatives with respect
FIGURE 2 Absorption (A) and circular dichroism (CD) (B) spectra of PTSR* in CHCl
dashed line), CHCl /CH CN 30:70 (red dotted line) and CHCl /CH CN 20:80 (red dashed-dotted line); absorption (C) and CD (D) spectra of
PTOR* in CHCl (green solid line), CHCl /CH CN 60:40 (green dashed line), CHCl /CH CN 40:60 (green dotted line) and CHCl /CH CN
0:80 (green dashed-dotted line)
3 3 3
(red solid line), CHCl /CH CN 40:60 (red
3
3
3
3
3
3
3
3
3
3
3
2

MARINELLI ET AL.
1371
wavelength (ꢂ10 nm) without particularly evident
vibronic features. Upon a similar treatment, PT2OR*
3.5 | CD spectroscopy
(
Figure 3C) behaves similarly to PTSR* and PTOR* as
The CD spectrum of PTSR* in pure chloroform
(Figure 2B) does not display any optical activity in the
spectral region related to the π–π* electronic transition of
polythiophene, as expected, owing to the random coil
conformation of the macromolecules. By contrast, a weak
CD signal is present in the spectrum of PTOR*
(Figure 2D) at around 580 nm, in correspondence to the
UV–Vis absorption, suggesting an intrinsically chiral
structure of short main chain sections, even in dilute
solution, to be reasonably ascribed to the increased rigid-
ity of the backbone induced by the mesomeric effect of
the side chain alkoxy group. The gradual addition of the
poor solvent acetonitrile produces in both PTSR* and
PTOR* the appearance of bisignate dichroic signals of
opposite sign, with crossover points related to the UV–
Vis maxima (at around 520 nm for PTSR* and 580 nm
for PTOR*) (Table 3).
far as the main absorption is concerned, with the
absorption at 345 nm, related to a shorter, more solu-
ble, oligomeric fraction insensitive to the change of
solvent. By contrast, HH/TT PT2SR* (Figure 3A)
exhibits the appearance of a new band close to
4
70 nm at a CHCl /CH CN composition of 10/90 v/v,
3 3
suggesting the presence of structures possessing more
extended electronic conjugation upon aggregation,
regardless of the presence of unfavorable HH
junctions.
The UV–Vis spectra of thin films of PTSR*, PTOR*,
PT2SR*, and PT2OR*, obtained by drop-casting from
chlorobenzene solution, are reported in Figure 4 and the
related data in Table 2. In addition to a sequence of
absorption maxima similar to the solution in chloroform,
all samples exhibit broader bands and appear furtherly
red shifted with respect to those of the aggregated struc-
tures obtained from solvatochromism experiments, with
PT2SR* confirming the presence in the solid state of
multiple aggregated structures.
A bisignated CD signal is characteristic of exciton
coupling between transition dipole moments of chromo-
phores on adjacent polymer chains in the aggregate state,
whereas for individual polymer chains with a helical
FIGURE 3 Absorption (A) and circular dichroism (CD) (B) spectra of PT2SR* in CHCl
dotted line), and CHCl /CH CN 10:90 (blue dashed line); absorption (C) and CD (D) spectra of PT2OR* in CHCl
CH CN 60:40 (black dashed line), CHCl /CH CN 40:60 (black dotted line), and CHCl /CH CN 20:80 (black dashed-dotted line)
3
(blue solid line), CHCl
3 3
/CH CN 30:70 (blue
3
3
3
(black solid line), CHCl /
3
3
3
3
3
3

1372
MARINELLI ET AL.
FIGURE 4 Absorption and circular dichroism (CD) spectra of PTSR* (A, B), PTOR* (C, D), PT2SR* (E, F), and PT2OR* (G, H) as
films cast from chlorobenzene

MARINELLI ET AL.
1373
ꢁ
TABLE 3 Circular dichroism (CD) spectra at 25 C of oligomers in the microaggregated state
a
b
c
a
b
Sample
PTSR*
Solvent CHCl
20:80
3
:CH
3
CN (v/v)
Δε
1
λ
1
λ
0
Δε
2
λ
2
+3.21
−6.09
−0.63
+0.15
563
650
523
561
512
579
486
-
−1.80
+5.59
+0.55
-
474
523
458
-
PTOR*
PT2SR*
PT2OR*
20:80
10:90
20:80
a
Δε expressed in L mol−1 cm−1
Wavelength (in nm) of the maximum dichroic absorption.
.
b
c
Wavelength (in nm) of the crossover point of dichroic bands.
conformation, the CD effect is not expected to be
lower polymerization degree, disfavoring the formation
of large chiral macromolecular aggregates. In addition, a
shoulder around 600 nm is present in the CD spectrum,
similarly to PTSR*, to be attributed to an optically active
phase constituted by oligomeric strands of one prevailing
helicity. Similar UV–Vis and CD spectra were also
observed upon aggregation of the above-mentioned poly-
22,43
bisignated.
Thus, the bisignated CD spectra observed
in π-conjugated oligomers with optically active pendant
side chains such as PTSR* and PTOR* are due to a chi-
ral supramolecular structure of predominantly planar
44
macromolecules.
In particular, the alkoxy derivative PTOR* displays
0
higher optical activity, as well as higher chiral anisotropy
meric TT/HH derivative poly{4,4 -bis[(S)-(2-methylbutyl)
−
3
0
factor g (Δε/ε) (2 ꢀ 10 at 651 nm) with respect to PTSR*
sulfanyl]-2,2 -bithiophene} by addition of methanol or
−
3
(
1 ꢀ 10 at 562 nm), which exhibits, in addition to the
bisignate band, a positive CD signal as a shoulder around
30 nm. The g value of PTOR* is close to the reported
n-hexane to its chloroform solution, or by evaporation
from a good solvent, and attributed to the formation of
more than one single aggregate phase, depending on the
6
−3
20
value of 3 ꢀ 10 at 568 nm found for the optically active
HT poly(3-alkoxythiophene) bearing the3,7-dim-
experimental conditions.
It therefore appears that the possibility of coplanar
arrangement of thiophene rings exists also in the case
of disfavoring HH connections originated by the pres-
16
ethyloctyloxy moiety in the side chain. These findings
can be interpreted as due to aggregation of macromole-
cules giving rise to supramolecular arrangements of
opposite helicity both in PTOR* (levo helix sense) and, to
a less extent, in PTSR* (dextro helix sense), where aggre-
gates possessing intrinsic chiral structure are also present.
Even at a very low polymerization degree, the oligomer
HT PTOR* is able to give chiral supramolecular aggre-
gates characterized by a significant g value, similarly to
the related HT poly{3-[(S)-(2-methylbutyl)]thiophene}
0
0
ence of 3,3 -disubstituted 2,2 -bithiophene counits. The
extra sulfur atom linked to each thiophene ring may
in fact induce self-aggregation via SꢀꢀꢀS interactions
12–14
and weak CHꢀꢀꢀS hydrogen bonding.
The direction-
ality of the above interactions, as well as the large
polarizability of sulfur in thioether fragments, would
favor the formation of highly anisotropic supramolecu-
lar systems characterized by antiplanar conformations
involving reduced steric hindrance between the side
6,22
possessing remarkably higher molecular mass.
31
The CD spectrum (Figure 3B) of HH/TT sulfanyl
derivative PT2SR* in dilute CHCl3 solution shows a
weak signal in the spectral region 300–400 nm, indicative
of the presence of a small amount of chain sections
possessing one-handed helical sense. By gradual addition
of acetonitrile, a moderate but clear bisignate dichroic
signal related to exciton coupling between the thiophene
rings in the backbone appears in consequence of chiral
supramolecular aggregation, similar and with opposite
helicity, with respect to PTSR*. Indeed, the crossover
point of CD spectrum is centered at 486 nm,
corresponding to the wavelength of the shoulder dis-
played in the corresponding UV–Vis spectrum and
related to aggregates of PT2SR* (Figure 3A). However,
chain substituents even in the case of oligomeric
constitution of the material.
By contrast, the CD spectrum in chloroform
(Figure 3D) of the alkoxy oligomer PT2OR*, possessing
actually a regiorandom configuration, shows weak CD
−
4
positive signals (g 2ꢀ10 at 555 nm) with maxima related
to the UV–Vis main chain absorptions, indicative of
intrinsic chirality of the backbone limited to short sec-
tions, insensitive to the addition of poor solvent. Thus,
the absence of exciton coupling in the CD spectrum of
PT2OR*, in contrast to what observed for PTOR*, can be
attributed to the absence of regioregular sections capable
to promote sufficiently extended coplanarity of the thio-
phene rings, so as to favor the development of chiral
supramolecular structures upon aggregation of the
macromolecules.
−
4
the maximum optical activity (g 3ꢀ10 at 522 nm) results
to be lower than in PTSR* mainly in consequence of its

1374
MARINELLI ET AL.
It is worth noting that both side chain heteroatom
formation of chiral supramolecular aggregates upon
aggregation from the solution, although with lower opti-
cal activity originated by their low polymerization degree.
Indeed, it is confirmed that the S or O heteroatom inter-
posed between the chiral residue and the thiophene ring
favors coplanarity of aromatic rings in the backbone,
owing to interactions between the thienyl sulfur and the
S or the O atom connecting the side chain.
and regioisomerism appear to affect the CD sign of the
aggregates obtained by addition of poor solvent to the
chloroform solution of the oligomeric samples: indeed,
the CD sign is inverted upon heteroatom change in the
samples possessing the same kind of regioregularity, the
same occurring when the HT oligomers are compared
with the corresponding HH/TT samples possessing equal
heteroatom. We may tentatively attribute this behavior to
the presence of slightly different conformations leading
to aggregates of opposite helicity. In the case of the
thioalkyl derivatives, where the presence of the
above-mentioned S‧‧‧S interactions favors coplanarity of
the thiophene rings both in the HT and HH/TT compounds,
the different regioisomerism may give rise to the difference
in the helical sense of the π-stacking structures, although the
chiral substituent has identical configuration. In the case of
oxyalkyl derivatives, it is the mesomeric effect of oxygen
that favors coplanarity and PTOR* behaves similarly to
the related HT PMBT bearing the same chiral alkyl group
in the side chain, but displays opposite CD sign with
respect to PTSR*. Accordingly, PT2OR*, although not
producing any exciton couplet, due to its low degree of
HH/TT regioregularity, however, displays a positive CD
signal of opposite sign with respect to both PTOR* and
PT2SR*. An inversion of the CD sign in HT and HH/TT
poly(3-alkoxythiophene)s bearing the optically active
Notwithstanding its unfavorable regioregularity, even
the behavior of the HH/TT sulfanyl derivative PT2SR*
appears substantially analogous to that of PTSR* and dis-
plays in the microaggregated state a relevant amount of
chains with supramolecular chirality with respect to mac-
romolecular strands possessing a prevailing one-handed
helical sense. In comparison with the previously investi-
gated optically active HH/TT P3AT bearing the same
(S)-(2-methyl)butyl chiral residue directly linked to the
thiophene ring, which is characterized by the presence of
helical intrachain conformations disfavoring the
11
formation of chiral supramolecular structures, PT2SR*
appears more prone to produce helical interchain order
even at a much lower polymerization degree (8 vs. 172 in
terms of one thiophenic repeating unit). This can be
attributed to the presence of the thioalkyl sulfur
atom that promotes a more coplanar arrangement of
thiophene rings with respect to the HH/TT optically
active P3AT derivative.
3
,7-dimethyloctyloxy moiety in the side chain was also
16
observed.
It appears, however, that experimental procedure of
aggregation and molecular structure of the material sig-
nificantly affect the chiral behavior of the samples. In
fact, when thin films of PTSR* and PTOR* are produced
by slow evaporation of dilute chlorobenzene solutions,
the resulting CD spectra differ from those obtained from
solvatochromism experiments.
4 | CONCLUSION
HT and HH/TT optically active oligothiophenes bearing
the (S)-(2-methyl)butyl residue linked at position 3 of the
thiophene ring through sulfur or oxygen atom have been
prepared with the aim to compare their chiroptical prop-
erties with those of the related HT and HH/TT P3AT
derivatives possessing the same chiral moiety. Compared
with the polymers, both the sulfanyl and alkoxy oligo-
mers display red-shifted absorption maxima indicative of
enhanced aromatic conjugation in the backbone due to
the electron-releasing properties of the heteroatom. Upon
aggregation from the solution by means of poor solvent
addition, both the HT oligomers, and particularly the HT
3-alkoxy derivative, exhibit chiroptical properties, in con-
sequence of the formation of supramolecular chiral
aggregates, similarly to the related HT P3AT derivative
possessing higher molecular mass. In addition, the HT
3-sulfanyl derivative displays the presence of individual
chains possessing helical conformation. The HH/TT
3-sulfanyl derivative behaves similarly to its HT counter-
part, although with lower chiral anisotropy, showing
however a greater tendency to give chiral supramolecular
aggregates than the related optically active HH/TT P3AT
In particular, PTSR* (Figure 4B), PTOR*
(Figure 4D), and PT2OR* (Figure 4G) actually display
low optical activity with respect to that given by addition
of acetonitrile to the chloroform solution. By contrast,
the CD spectrum of a thin film of PT2SR* resulting from
slow evaporation of dilute chlorobenzene solutions
(Figure 4F) is similar to that given by the micro-
aggregates formed upon acetonitrile addition to the chlo-
roform solution, likely in consequence of its more
homogeneous molecular composition (Table 1) with
respect to the other derivatives. Thus, low dispersity
value and regioregularity degree appear as relevant fac-
tors affecting the ordering process of the macromolecules
from the solution to the solid state.
In conclusion, both the HT oligomeric derivatives
PTSR* and PTOR* are able to display a behavior similar
to HT poly{3-[(S)-(2-methylbutyl)]thiophene}, with

MARINELLI ET AL.
1375
polymeric derivative. By contrast, the HH/TT 3-alkoxy
oligomer, actually constituted by macromolecules with a
mixture of HH/TT and HT connections, appears unable
to produce relevant chiroptical properties upon aggrega-
tion and exhibits only the presence of short sections
possessing chiral conformation in the aggregated state. It
is also to be noted that these materials do not display rel-
evant chiroptical properties as cast films obtained by
direct evaporation from the solution, thus suggesting, as
previously observed, that the aggregation of these
π-conjugated materials is a stepwise event which requires
to start from very dilute solutions and gradually add a
weak solvent so that the process may take place with sig-
nificant formation of chiral aggregates. To this respect,
high regioregularity degree and low dispersity values
appear to be favorable parameters positively affecting the
film formation from the solution. In addition to favoring
the extent of conjugation in the main chain, of interest
for application to polarization-sensitive devices in photo-
voltaic cells and organic light-emitting diodes (OLEDs),
the presence of oxygen or sulfur as connecting atom
between the thiophene ring and the chiral alkyl side
chain allows to achieve supramolecular chiral aggrega-
tions, of interest for CPL applications, not dissimilar, or
better, with respect to those obtained with the related
chiral P3ATs even in the case of a low polymerization
degree.
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ACKNOWLEDGMENTS
The authors thank Mr. Luca Zuppiroli (University of
Bologna) for the high-resolution mass spectra. The finan-
cial support from the University of Bologna is gratefully
acknowledged. Francesca Di Maria acknowledges finan-
cial support from the UE project INFUSION
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synthesis from 3,3 -dialkyl 2,2 -bithiophene monomer. Synth
Met. 2015;202:169-176.
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tunable shape and function. J am Chem Soc. 2011;133(22):
(Engineering optoelectronic INterfaces: a global action
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DATA AVAILABILITY STATEMENT
The data that supports the findings of this study are avail-
able in the supplementary material of this article.
ORCID
Elisabetta Salatelli https://orcid.org/0000-0002-4965-
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