Poly(arylene vinylene) Synthesis via a Precursor Step-Growth Polymerization Route Involving the Ramberg-B?cklund Reaction as a Key Post-Chemical Modification Step

Article abstract of DOI:10.1021/acs.macromol.8b00676

The synthesis of conjugated copolymers based on poly(fluorene vinylene) [PFV] and poly(fluorene vinylene-co-carbazole vinylene) [PFVCV] was achieved via a previously unexplored precursor three-step synthetic route involving the Ramberg-B?cklund reaction. The resulting π-conjugated (co)polymers proved highly soluble in common organic solvents, such as DCM, THF, or CHCl₃. The solution step-growth polymerization between 2,7-bis(bromomethyl)-9,9′-dihexyl-9H-fluorene [F-Br] and 2,7-bis(mercaptomethyl)-9,9′-dihexyl-9H-fluorene [F-SH] was carried out under basic conditions at 100 °C in a mixture of MeOH and THF. The resulting polysulfides were then subjected to an oxidation reaction using m-CPBA, which was followed by the Ramberg-B?cklund reaction in the presence of CF₂Br₂/Al₂O₃-KOH, thus achieving the desired PFV. Similarly, PFVCV could be synthesized through the same three-step sequence employing, in this case, 2,7-bis(mercaptomethyl)-9-(tridecan-7-yl)-9H-carbazole (C-SH) and F-Br. Conjugated polymers with apparent molecular weights up to 15 kg mol^-1 and exhibiting promising optical features were obtained following this convenient synthetic strategy.

Full text of DOI:10.1021/acs.macromol.8b00676

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Cite This: Macromolecules XXXX, XXX, XXX−XXX
Poly(arylene vinylene) Synthesis via a Precursor Step-Growth
̈
Polymerization Route Involving the Ramberg−Backlund Reaction as
a Key Post-Chemical Modification Step
Kunche Aravindu,†,‡ Eric Cloutet,‡ Cyril Brochon,‡ Georges Hadziioannou,‡ Joan Vignolle,‡
†
Frederic Robert, Daniel Taton,*,‡ and Yannick Landais*,†
†
́
́
́
Institute of Molecular Sciences, UMR-CNRS 5255, University of Bordeaux, 351 Cours de la liberation, 33405 Talence cedex,
France
‡
̀
́
Laboratoire de Chimie des Polymeres Organiques (LCPO UMR 5629), CNRS-Universite de Bordeaux-Bordeaux INP, 16 Avenue
Pey-Berland, 33607 Pessac cedex, France
S
* Supporting Information
ABSTRACT: The synthesis of conjugated copolymers based
on poly(fluorene vinylene) [PFV] and poly(fluorene vinylene-
co-carbazole vinylene) [PFVCV] was achieved via a previously
unexplored precursor three-step synthetic route involving the
̈
Ramberg−Backlund reaction. The resulting π-conjugated
(co)polymers proved highly soluble in common organic
solvents, such as DCM, THF, or CHCl3. The solution step-
growth polymerization between 2,7-bis(bromomethyl)-9,9′-
dihexyl-9H-fluorene [F-Br] and 2,7-bis(mercaptomethyl)-9,9′-dihexyl-9H-fluorene [F-SH] was carried out under basic
conditions at 100 °C in a mixture of MeOH and THF. The resulting polysulfides were then subjected to an oxidation reaction
̈
using m-CPBA, which was followed by the Ramberg−Backlund reaction in the presence of CF2Br2/Al2O3−KOH, thus achieving
the desired PFV. Similarly, PFVCV could be synthesized through the same three-step sequence employing, in this case, 2,7-
bis(mercaptomethyl)-9-(tridecan-7-yl)-9H-carbazole (C-SH) and F-Br. Conjugated polymers with apparent molecular weights
up to 15 kg mol−1 and exhibiting promising optical features were obtained following this convenient synthetic strategy.
like materials can be synthesized by a wide variety of synthetic
routes following either a step-growth or a chain-growth
polymerization pathway. Step-growth polymerization methods
lead directly to PPV oligomers and rely on well-established
elementary reactions of molecular chemistry, such as the
Knoevenagel condensation,8 the Wittig or Horner−Wads-
worth−Emmons olefination,9−,11 the palladium-catalyzed
Stille, Suzuki, or Heck coupling,12 and the Siegrist reaction.13
Alternatively, chain-growth polymerization methods allow
higher molecular weights to be achieved. However, the final
PPV material is often obtained by post-chemical modification
of a polymer precursor as, for instance, in Gilch,14 Wessling
sulfonium,15,16 dithiocarbamate (or xanthate), and sulfinyl
routes.17,18 In order to alleviate solubility issues and facilitate
their processability, specific groups can be introduced in 2- and
5-positions of the aromatic ring of PPV-type materials.
Controlled living polymerization has also been considered
for PPV synthesis.19 With the exception of PPVs grown both
by the latter method under specific conditions and by ring-
opening metathesis polymerization (ROMP), relatively poor
control over molecular weights and dispersities generally
INTRODUCTION
■
In recent years, thin-film device technologies employing π-
conjugated polymers have attracted a great deal of attention
owing to their properties suitable for electronic and
optoelectronic applications, including organic transistors,1,2
bulk heterojunction solar cells for organic photovoltaics
(OPVs),3,4 or organic light-emitting diodes (OLEDs).5 The
solution-processability of π-conjugated polymers, combined
with their optoelectronic properties, such as light absorption,
carrier mobility, and exciton dynamics, can be fine-tuned
through subtle changes in their structure. Molecular parame-
ters include molecular weight, bond length alternation,
substituent effects, aromaticity, planarity, regioregularity,
intermolecular interactions, etc. Hence, simple synthetic
modifications can dramatically impact optoelectronic and
thin-film self-assembled properties. Most π-conjugated poly-
mers consist of sp2-hybridized carbon atoms and are based on
aryl−aryl or aryl−vinyl repeating units, mostly through
alternation between CC and CC bonds.6 Heteroatoms
can be introduced in the π-conjugation system: for instance,
the -CN- double bond is somehow electrically equivalent to
the -CC- one.
In this context, poly(p-phenylenevinylene) (PPV)-type π-
conjugated polymers represent a special class of highly
fluorescent, highly stable polymeric semiconductors.7,8 PPV-
Received: March 30, 2018
Revised: July 3, 2018
© XXXX American Chemical Society
A
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characterize these polymerization routes. More generally, these
synthetic methods to PPVs, although straightforward, still
show some limitations, including (a) structural defects due to
incomplete post-chemical modification by thermal conversion
or oxidation, (b) modest molecular weights, (c) generation of
toxic side products during the elimination process, (d) gelation
or precipitation of the polymeric products, which may occur
during polymerization, (e) limited control over the E/Z
content of the vinyl bridge, (f) use of costly and non-renewable
metal catalysts, and, last but not least, (g) the presence of
residual metallic traces, which require lengthy and tedious
purification steps.20
On this basis, we envisioned developing novel PPV-like
materials following a previously unexplored precursor step-
growth polymerization route/post-chemical functionalization
sequence, involving a three-step sequence of reactions (Figure
1). The first step (i) is achieved by step-growth polymerization
opens a general and straightforward access to symmetrical
PPV-like materials bearing identical aromatic fragments (Route
A, Figure 1). By varying the nature of aryl groups, the
optoelectronic properties could be tuned depending on the
targeted application (OPVs, OLEDs, etc.).25,26 Furthermore,
̈
the conjunctive nature of the Ramberg−Backlund process
should be particularly appealing for the design of alternating
monomer units along the π-conjugated backbone (Route B,
Figure 1). For example, it could allow the formation of
“donor−acceptor” alternating low-band-gap copolymers by
using electron-rich and electron-poor monomer units. Such
copolymers are characterized by an extended optical
absorption to the red region and allow for a better
manipulation of the HOMO/LUMO levels, providing
conjugated systems with relatively high charge carrier
mobility.27−31
As an example of the potentiality of these arylene−vinylene
polymers, recently, Nomura and colleagues have obtained
PPVs with different end-groups by combining acyclic diene
metathesis polymerization with Wittig-type couplings.32,33
Here we describe the synthetic developments validating both
strategies, by reporting the synthesis of poly(fluorene vinyl-
ene)- and poly(fluorene-carbazole vinylene)-type conjugated
(co)polymers.
EXPERIMENTAL SECTION
All reactions were carried out under an argon atmosphere with
solvents dried over activated alumina columns on MBraun Solvent
Purification System (SPS-800), unless otherwise noted.
■
All reagent-grade chemicals were obtained from commercial
suppliers and were used as received, unless otherwise stated. 1H
NMR and 13C NMR were recorded on various spectrometers: a
Bru ker
ker Avance 300 (1H, 300 MHz; 13C, 75.46 MHz), a Bru
̈ ̈
Avance 400 (1H, 400 MHz; 13C, 100.6 MHz) using CDCl3 as internal
reference unless otherwise indicated. The chemical shifts (δ) and
coupling constants (J) are expressed in ppm and Hz, respectively. The
following abbreviations were used to explain the multiplicities: s =
singlet, d = doublet, t = triplet, q = quartet, m = multiplet, br = broad.
FT-IR spectra were recorded on a Perkin-Elmer Spectrum 100 using a
diamond ATR accessory. High-resolution mass spectra (HRMS) were
recorded with a Waters QTOF 2 spectrometer in the electrospray
ionization (ESI) mode. Melting points are not corrected and were
determined by using a Stuart Scientific SMP3 apparatus. Analytical
thin layer chromatography was performed using silica gel 60 F254
pre-coated plates (Merck) with visualization by ultraviolet light,
potassium permanganate. Flash chromatography was performed on
silica gel (0.043−0.063 mm) with ethyl acetate (EtOAc) and
petroleum ether (PE) as eluents unless otherwise indicated.
Figure 1. Synthetic strategies toward poly(arylene vinylene)s (PPVs)
̈
based on the Ramberg−Backlund reaction.
in solution of dibromides and dithiols as monomers, forming
polysulfides. The second step (ii) consists in oxidizing these
polymer precursors into polysulfones, which is followed by an
ultimate step (iii) of elimination, via the so-called Ramberg−
21
̈
̈
Backlund reaction. The Ramberg−Backlund reaction is very
versatile for the construction of carbon−carbon double bonds
and has been largely applied in the context of total synthesis of
complex natural products. Only recent reports have described
its application in dendrimer synthesis.22 This reaction involves
a suitably designed sulfone precursor, which upon α-
halogenation under basic conditions provides the correspond-
ing olefin generally in high yields. It is thus a simple way to
create CC double bonds under mild conditions, starting
from readily available substrates. Another advantage of this
process is that it allows the coupling around the sulfonyl group
of identical or differentiated substituents, on purpose, in a
limited number of steps. Starting materials are easily available
and provide olefins without alkene rearrangements under mild
alkaline conditions. E/Z-stereocontrol mainly relies on the
nature of the base, with mild bases favoring the Z-isomer, while
the E-isomer is generally formed in the presence of stronger
bases. Many examples have demonstrated that the Ramberg−
2,7-Bis(hydroxymethyl)-9,9′-dihexyl-9H-fluorene (2). To a
stirred solution of 9,9′-dihexyl-9H-fluorene-2,7-dicarbaldehyde 134
(4.29 g, 11 mmol) in a 3:1 THF/MeOH mixture (100 mL), NaBH4
(3.34 g, 88 mmol) was added portionwise at 0 °C, and then the
reaction mixture was allowed to warm to room temperature for 4 h.
The reaction was quenched with aqueous NH4Cl solution (10 mL),
and the aqueous layer was extracted with ethyl acetate (2 × 50 mL).
The organic layer was then dried over anhydrous Na2SO4, and
solvents were evaporated to give a crude mixture, which was purified
by flash column chromatography (silica, EtOAc/PE 1:9), affording
the desired compound as a white solid (3.46 g, 80%). The spectral
data are consistent with those reported in the literature.35
Mp = 74−76 °C; IR (ATR) νmax (cm−1): 3285, 2925, 2854, 1457;
1H NMR (300 MHz, CDCl3): δ 0.52−0.68 (m, 4H), 0.70−0.82 (m,
6H), 0.94−1.18 (m, 12H), 1.88 (br, 2H), 1.90−2.02 (m, 4H), 4.75 (s,
4H), 7.27−7.36 (m, 4H), 7.62−7.70 (m, 2H); 13C NMR (75 MHz,
CDCl3): δ 13.97, 22.56, 23.71, 29.67, 31.47, 40.32, 54.99, 65.72,
119.64, 121.49, 125.73, 139.74, 140.35, 151.30; HRMS: calcd for
C27H38O2 [M]+• 394.28718, found 394.28862.
̈
Backlund process can be applied to the preparation of
polysubstituted alkenes, including tetrasubstituted ones as
well as polyenes.21 Iterative synthesis of oligo-PPVs has been
described,23,24 but to the best of our knowledge, this reaction
has never been applied to the synthesis of polyolefins starting
from polysulfones. The synthetic approach we devise here
B
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2,7-Bis(bromomethyl)-9,9′-dihexyl-9H-fluorene (F-Br). To a
stirred solution of 2 (1.54 g, 3.91 mmol) in benzene (20 mL), PBr3
(1.09 mL, 11.7 mmol) was slowly added at room temperature, then
the reaction mixture was heated to 45 °C for 6 h. The reaction was
quenched with water (10 mL) and extracted with dichloromethane (2
× 25 mL). The organic layer was dried over anhydrous Na2SO4, and
solvents were evaporated to give a crude mixture, which was purified
by flash column chromatography (silica, CH2Cl2/PE 1:9) to afford
the compound F-Br as a white solid (1.21 g, 59%). The spectral data
are consistent with literature data.35
DMF (2.5 mL) was stirred at room temperature for 3 days. Water (5
mL) was then added to the reaction mixture. The resulting solid was
filtered and washed with water (10 mL) and MeOH (10 mL). The
crude compound thus obtained was purified by precipitation in a large
excess of cold MeOH from a CHCl3 solution, to obtain the PF-SO
(0.078 g, 37%) as a white solid. The spectral data (1H NMR) are
consistent with the earlier data.
Synthesis of PFV. In a sealed round-bottom flask, a stirred
solution of PF-SO (0.20 g, 0.47 mmol) in a 1:1 THF/t-BuOH
mixture (10 mL) was treated with freshly prepared Al2O3−KOH (1
g) at 0 °C, and then CF2Br2 (1 mL) was added at the same
temperature. The reaction mixture was then stirred at room
temperature for 24 h. The reaction mixture was filtered through
Celite and washed with CH2Cl2. The solvent was evaporated, and the
crude compound thus obtained was purified by precipitation in a large
excess of cold MeOH from a CHCl3 solution, to afford the desired
polymer as a yellow solid (89 mg, 53%).
1
Mp = 51−53 °C; IR (ATR) νmax (cm−1): 2924, 1468, 1291; H
NMR (300 MHz, CDCl3): δ 0.53−0.68 (m, 4H), 0.70−0.82 (m, 6H),
0.95−1.18 (m, 12H), 1.90−2.02 (m, 4H), 4.59 (s, 4H), 7.32−7.39
(m, 4H), 7.59−7.68 (m, 2H); 13C NMR (75 MHz, CDCl3): δ 13.98,
22.46, 23.61, 29.51, 31.34, 34.43, 40.05, 55.11, 120.03, 123.64, 127.98,
136.86, 140.71, 151.63; HRMS: calcd for C27H36Br2 [M]+•
518.11838, found 518.12470.
1
IR (ATR) νmax (cm−1): 2924, 2853, 1466, 1048, 790; H NMR
2,7-Bis(mercaptomethyl)-9,9′-dihexyl-9H-fluorene (F-SH).
Following a slightly modified literature procedure,36 thiourea (0.334
g, 4.40 mmol) was added to a stirred solution of F-Br (1.04 g, 2
mmol) in EtOH (10 mL), and the reaction mixture was heated to
reflux for 4 h. The solvent was then evaporated under vacuum, and
the solid obtained was hydrolyzed with 50% aqueous sodium
hydroxide solution (10 mL) and extracted with EtOAc (2 × 25
mL). The organic layer was dried over anhydrous Na2SO4, and
solvents were evaporated to give a crude mixture, which was purified
by flash column chromatography (silica, CH2Cl2/PE 3:7) to afford
the desired compound F-SH as a white solid (0.42 g, 49%).
(300 MHz, CDCl3): δ 0.71−0.83 (m, 8H), 0.94−1.21 (m, 14H),
1.89−2.15 (m, 4H), 7.30 (s, 2H), 7.48−7.63 (m, 4H), 7.70 (d, 2H, J
= 7.8 Hz); λmax: 426, 450 nm.
2,7-Diformyl-9-(tridecan-7-yl)-9H-carbazole (4). Following a
slightly modified literature procedure,37 a stirred solution of 3 (5.07g,
10 mmol) in THF (50 mL) was cooled to −78 °C, and a 1.85 M
solution of n-BuLi in hexanes (11.4 mL, 21 mmol) was added. The
mixture was stirred at this temperature for 1 h. Next, DMF (2.3 mL,
30 mmol) in THF (10 mL) was added dropwise, and the reaction
mixture was allowed to warm to room temperature and stirred for 1 h.
The reaction mixture was quenched with aq. NH4Cl and extracted
with EtOAc (2 × 50 mL). The organic layer was dried over anhydrous
Na2SO4, and solvents were evaporated to give a crude mixture, which
was purified by column chromatography (silica gel, EtOAc/PE 1:9) to
afford 4 as a yellow solid (3.26 g, 81%).
1
Mp: 39−41 °C; IR (ATR) νmax (cm−1): 2925, 2556, 1520; H
NMR (300 MHz, CDCl3): δ 0.54−0.68 (m, 4H), 0.76 (t, 6H, J = 6.9
Hz), 0.95−1.18 (m, 12H), 1.77 (t, 2H, J = 7.5 Hz), 1.86−1.98 (m,
4H), 3.82 (d, 4H, J = 7.5 Hz), 7.22−7.30 (m, 4H), 7.54−7.62 (m,
2H); 13C NMR (75 MHz, CDCl3): δ 13.98, 22.49, 23.64, 29.37,
29.57, 31.37, 40.15, 54.95, 119.63, 122.48, 126.69, 139.69, 139.91,
151.34; HRMS: calcd for C27H38S2 [M]+• 426.24149, found
426.24213.
Mp = 88−90 °C; IR (ATR) νmax (cm−1): 2917, 2850, 1676, 1227;
1H NMR (300 MHz, CDCl3): δ 0.76 (t, 6H, J = 6.6 Hz), 0.89−0.92
(m, 2H), 1.04−1.34 (m, 14H), 1.95−2.12 (m, 2H), 2.24−2.42 (m,
2H), 4.72 (septet, 1H, J = 5.1 Hz), 7.80 (d, 2H, J = 7.8 Hz), 8.03 (s,
1H), 8.16 (s, 1H), 8.23−8.37 (m, 2H); 13C NMR (75 MHz, CDCl3):
δ 13.88, 22.38, 26.73, 28.90, 31.44, 33.75, 57.37, 110.48, 113.14,
121.08, 121.87, 126.40, 135.21, 143.24, 192.42; HRMS: Calcd for
C27H35NO2 [M]+• 405.26678, found 405.26691.
Synthesis of PF-S. In a sealed tube, a solution of F-Br (1.04 g, 2
mmol) and F-SH (0.85 g, 2 mmol) in a 1:1 THF/MeOH mixture (20
mL) was treated with powdered KOH (0.34 g, 6 mmol), and the
reaction mixture was heated to 100 °C for 14 h. The reaction mixture
was then cooled to room temperature and the solvent evaporated
under vacuum. The crude reaction mixture was washed with H2O (20
mL) and MeOH (20 mL) and the solid filtered. The latter was then
dissolved in CHCl3 (5 mL) and purified by precipitation using MeOH
(100 mL) to provide the desired compound (PF-S) as a pale yellow
solid (1.27 g, 78%).
2,7-Bis(mercaptomethyl)-9-(tridecan-7-yl)-9H-carbazole (C-
SH). To a stirred solution of 4 (3.24 g, 8.0 mmol) in a 3:1 THF/
MeOH mixture (60 mL), NaBH4 (2.43 g, 64 mmol) was added
portionwise at 0 °C, and the reaction mixture was allowed to warm to
room temperature for 4 h. The reaction was quenched with aqueous
NH4Cl solution (100 mL) and extracted with EtOAc (2 × 100 mL),
and the organic layer was dried over anhydrous Na2SO4. The solvent
was then evaporated to afford the desired compound as a pale viscous
liquid and used without further purification in the next step. To a
stirred solution of the crude compound (2.96 g, 7.24 mmol) in
benzene (30 mL), PBr3 (1.0 mL, 21.8 mmol) was slowly added at
room temperature, and then the reaction mixture was heated to 45 °C
for 6 h. The reaction was quenched with water (25 mL) and extracted
with CH2Cl2 (2 × 50 mL). The organic layer was dried over
anhydrous Na2SO4 and the solvent evaporated. The oily residue, 2,7-
bis(bromomethyl)-9-(tridecan-7-yl)-9H-carbazole (5), was used in
the next step without purification, since the compound was unstable
under ambient conditions.
To a stirred solution of 5 (3.5 g, 6.5 mmol) in EtOH (30 mL),
thiourea (1.5 g, 19.5 mmol) was added, and the reaction mixture was
heated to reflux for 8 h. The solvent was then evaporated under
vacuum, and the solid obtained was hydrolyzed with 50% aqueous
sodium hydroxide and extracted with EtOAc (2 × 50 mL). The
organic layer was dried over anhydrous Na2SO4, and solvents were
evaporated to give a crude mixture, which was purified by flash
column chromatography (silica, CH2Cl2/PE 3:7) to afford C-SH as a
colorless liquid (1.56 g, 55%). The overall yield for the three steps is
47%.
IR (ATR) νmax (cm−1): 2923, 2852, 1465, 817, 748; 1H NMR (300
MHz, CDCl3): δ 0.52−0.83 (m, 10H), 0.90−1.21 (m, 12H), 1.88−
2.12 (m, 4H), 3.68 (s, 4H), 7.19−7.35 (m, 4H), 7.56−7.70 (m, 2H);
13C NMR (75 MHz, CDCl3): δ 13.98, 22.52, 23.82, 29.74, 31.56,
35.50, 40.54, 54.96, 119.56, 123.42, 127.89, 136.84, 139.87, 150.97;
λmax: 290, 316 nm.
Synthesis of PF-SO. A stirred solution of PF-S (0.98 g, 2.5
mmol) in CHCl3 (50 mL) was treated with m-CPBA (2.16 g, 12.5
mmol), and the reaction mixture was stirred at room temperature for
8 h. The reaction mixture was then filtered and washed with CHCl3.
The filtrate was washed with saturated aqueous NaHCO3 (2 × 25
mL), brine, and water, successively. The organic solvent was
evaporated, and the crude compound thus obtained was purified by
precipitation in a large excess of cold MeOH from a CHCl3 solution,
to afford the desired polymer as a brown solid (0.67 g, 63%).
1
IR (ATR) νmax (cm−1): 2924, 2853, 1467, 1316, 1150, 612; H
NMR (300 MHz, CDCl3): δ 0.57−0.77 (m, 10H), 0.95−1.15 (m,
12H), 1.96−2.13 (m, 4H), 4.20 (s, 4H), 7.35 (d, 2H, J = 7.8 Hz),
7.44 (s, 2H), 7.76 (d, 2H, J = 7.8 Hz); 13C NMR (75 MHz, CDCl3):
δ 13.93, 22.46, 23.91, 29.62, 31.50, 40.24, 55.40, 58.18, 120.43,
125.37, 126.70, 129.89, 141.15, 151.67; λmax: 290, 316 nm.
PF-SO via the Rongalite Route. A mixture of F-Br (0.26 g, 0.5
mmol), rongalite (0.3 g, 2.5 mmol), K2CO3 (0.34 g, 2.5 mmol), and
tetra-n-butylammonium bromide (TBAB) (0.08 g, 0.25 mmol) in
C
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1
IR (ATR) νmax (cm−1): 2923, 2853, 2561, 1457, 1435; H NMR
(300 MHz, CDCl3): δ 0.78 (t, 6H, J = 6.6 Hz), 0.91−1.31 (m, 16H),
1.84 (t, 2H, J = 7.5 Hz), 1.87−1.98 (m, 2H), 2.18−2.35 (m, 2H),
3.94 (d, 4H, J = 7.5 Hz), 4.53 (septet, 1H, J = 5.1 Hz), 7.16 (d, 2H, J
= 7.8 Hz), 7.34 (s, 1H), 7.50 (s, 1H), 7.91−8.06 (m, 2H); 13C NMR
(75 MHz, CDCl3): δ 14.05, 22.58, 26.77, 29.09, 30.02, 31.60, 33.70,
56.35, 108.25, 110.95, 119.00, 120.21, 120.44, 121.36, 138.21, 138.68;
HRMS: Calcd for C27H39NS2 [M]+• 441.25239, found 441.25219.
Synthesis of PFC-S. In a sealed tube, a solution of F-Br (0.52 g, 1
mmol) and C-SH (0.44 g, 1 mmol) in a 1:1 THF/MeOH mixture (10
mL) was treated with powdered KOH (0.17 g, 3 mmol) and the
reaction mixture was heated to 100 °C for 14 h. The reaction mixture
was then cooled to room temperature and the solvent evaporated
under vacuum. The crude reaction mixture was washed with H2O and
MeOH and the solid filtered. The latter was then dissolved in CHCl3
(5 mL) and purified by precipitation using MeOH (100 mL) to
provide the desired compound (PFCS) as a pale yellow solid (0.62 g,
83%).
Scheme 1. Synthesis of Monomers 2,7-Bis(bromomethyl)-
9,9′-dihexyl-9H-fluorene (F-Br) and 2,7-
Bis(mercaptomethyl)-9,9′-dihexyl-9H-fluorene (F-SH)
1
IR (ATR) νmax (cm−1): 2922, 2849, 2354, 1599, 1440; H NMR
Figure 1. 9,9-Dihexylfluorene has been widely employed as a
building block for the synthesis of electroluminescent
polymers.38−40 Moreover, fluorene-based polymers exhibit
excellent solubility, good charge carrier mobility, and high
solid-state photoluminescence (PL) quantum yield.41 Reduc-
tion of fluorene-dicarbaldehyde34 (1) with NaBH4 in THF/
MeOH42 afforded the 2,7-bis(hydroxymethyl)-9,9′-dihexyl-
9H-fluorene (2) in 80% yield. Treatment of 2 with PBr3 in
benzene35 at 45 °C gave the desired 2,7-bis(bromomethyl)-
9,9′-dihexyl-9H-fluorene monomer (F-Br) in 59% yield.
Subsequent treatment of monomer F-Br with thiourea in
refluxing EtOH for 4h followed by basic hydrolysis provided
the desired monomer, F-SH (Scheme 1).36
The general protocol for the preparation of poly(fluorene
vinylene) (PFV) is illustrated in Scheme 2. After screening
various conditions for the step-growth polymerization of
stoichiometric amounts of F-Br and F-SH (see Supporting
Information, Tables S1−S4), soluble polymers of rather high
apparent molecular weight were obtained in the presence of 3
equiv of powdered KOH in THF/MeOH (1:1) at 100 °C for
14 h. These conditions led to poly(fluorene-sulfide)s (PF-S) in
78% final yield. Strong, non-nucleophilic 1,1,3,3-tetramethyl-
guanidine (TMG) afforded higher molecular weights and
narrower dispersities (see Supporting Information). Due to
their lower molecular weight, however, polymers obtained
from KOH also proved more soluble. Initial studies on the
oxidation of PF-S using oxone as the oxidizing agent in
MeOH/H2O were unsuccessful, resulting in a mixture of
sulfoxide and sulfone products. In contrast, treatment of 1
equiv of PF-S with 5 equiv of m-CPBA in CHCl3 at room
temperature for 8 h afforded the polyfluorene-sulfone (PF-SO)
in 63% yield. The latter polymer was finally subjected to the
(400 MHz, CDCl3): δ 0.57−0.80 (m, 16H), 0.92−1.32 (m, 28H),
1.88−2.08 (m, 6H), 2.25−2.44 (m, 2H), 3.71 (s, 4H), 3.80 (s, 4H),
4.52−4.67 (m, 1H), 7.07−7.16 (m, 2H), 7.21−7.32 (m, 4H), 7.40 (s,
1H), 7.57 (s, 1H), 7.63 (d, 2H, J = 7.8 Hz), 7.90−8.10 (m, 2H); 13C
NMR (100 MHz, CDCl3): δ 13.96, 22.52, 23.82, 26.85, 29.14, 29.74,
31.54, 31.60, 33.83, 35.84, 36.24, 40.48, 54.96, 56.39, 109.11, 111.78,
119.50, 119.83, 120.08, 121.36, 122.73, 123.49, 127.89, 135.18,
135.58, 136.97, 139.13, 139.87, 142.55, 151.04; λmax: 245, 276, 317
nm.
Synthesis of PFC-SO. A stirred solution of PFC-S (0.54 g, 0.73
mmol) in CHCl3 (15 mL) was treated with m-CPBA (0.63 g, 3.6
mmol), and the reaction mixture was stirred at room temperature for
8 h. The reaction mixture was then filtered and washed with CHCl3.
The filtrate was washed with saturated aqueous NaHCO3 (2 × 25
mL), brine, and water, successively. The organic solvent was
evaporated, and the crude material thus obtained was purified by
precipitation in a large excess of cold MeOH from a CHCl3 solution,
to afford the desired polymer as a dark-green solid (0.43 g, 76%).
1
IR (ATR) νmax (cm−1): 2930, 2823, 1480, 1308, 1110; H NMR
(400 MHz, CDCl3): δ 0.58−0.83 (m, 16H), 0.92−1.29 (m, 28H),
1.88−2.14 (m, 6H), 2.22−2.43 (m, 2H), 4.21 (s, 4H), 4.33 (s, 4H),
4.51−4.71 (m, 1H), 7.15 (d, 2H, J = 8.2 Hz), 7.30−7.48 (m, 4H),
7.61 (s, 1H), 7.75 (d, 2H, J = 7.6 Hz), 7.80 (s, 1H), 8.05−8.21 (m,
2H); 13C NMR (100 MHz, CDCl3): δ 13.91, 13.94, 22.44, 22.47,
23.91, 26.80, 29.01, 29.63, 31.50, 31.55, 33.74, 40.23, 111.45, 114.03,
120.38, 120.63, 120.84, 121.60, 122.47, 123.83, 124.91, 125.50,
126.73, 129.89, 139.29, 141.14, 142.69, 151.65; λmax: 249, 259, 275,
287, 317 nm.
Synthesis of PFVCV. In a sealed round-bottom flask, a stirred
solution of PFC-SO (0.2 g, 0.26 mmol) in a 2:1 THF/t-BuOH
mixture (12 mL) was treated with freshly prepared Al2O3−KOH (0.3
g) at 0 °C, then CF2Br2 (1.5 mL) was added at the same temperature.
The reaction mixture was stirred at room temperature for 24 h, then
filtered through Celite and washed with CH2Cl2. The solvent was
evaporated and the crude material purified by precipitation in a large
excess of cold MeOH from a CHCl3 solution, to afford the desired
polymer as a yellow solid (32 mg, 43%).
̈
Ramberg−Backlund reaction using Chan’s modified procedure
(CF2Br2, Al2O3−KOH, t-BuOH).43 The reaction was however
sluggish under these conditions, very likely because of the low
solubility of PF-SO in t-BuOH. We noted that a mixture of
THF and t-BuOH (1:1) was more suitable for this reaction.
Treatment of PF-SO with an excess of CF2Br2 and freshly
prepared Al2O3−KOH in THF/t-BuOH (1:1), at room
temperature for 24 h, thus gave polymer PFV in 53% yield.
The three polymers, PF-S, PF-SO, and PFV were purified by
precipitation using CHCl3 and MeOH at room temperature.
Characterization of the monomers and related polymers
(PF-S, PF-SO, PFV) through IR and 1H NMR spectroscopies
revealed the expected structures with high purity (Figure 2).
The overlay of IR spectra of the different polymers is shown in
Figure S1 (Supporting Information). Comparison between IR
1
IR (ATR) νmax (cm−1): 2924, 2853, 1466, 1048, 790; H NMR
(400 MHz, CDCl3): δ 0.64−0.78 (m, 16H), 0.93−1.16 (m, 28H),
1.86−2.06 (m, 6H), 2.23−2.39 (m, 2H), 4.52−4.62 (m, 1H), 7.22−
7.32 (m, 3H), 7.40−7.54 (m, 6H), 7.57−7.67 (m, 3H), 7.94−8.06
(m, 2H); λmax: 265, 427, 452 nm.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Symmetrical Poly-
(arylene vinylene)s. For our initial studies, we have selected
specific monomers, namely, 2,7-bis(bromomethyl)-9,9′-dihex-
yl-9H-fluorene (F-Br) and 2,7-bis(mercaptomethyl)-9,9′-di-
hexyl-9H-fluorene (F-SH) (Scheme 1). The general synthetic
strategy to symmetrical PPVs following Route A is outlined in
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Scheme 2. Synthesis of Poly(fluorene vinylene) (PFV) via a Three-Step Procedure Involving Step-Growth Polymerization,
̈
Oxidation, and Ramberg−Backlund Reaction
Figure 2. 1H NMR spectra of (i) PFV, (ii) PF-SO2, (iii) PF-S, (iv) F-SH, and (v) F-Br in CDCl3 (400 MHz). The n-hexyl peaks are omitted for
clarity.
spectra of PF-S and PF-SO showed additional absorption
bands at 1319, 1100 cm−1, corresponding to the SO group.
During post-functionalization of PF-SO into PFV, disappear-
ance of the band at 1319 cm−1 and a new absorption band at
860 cm−1 due to the olefinic moiety was observed, confirming
mentioning that we did not observe any residual benzylic
protons at δ 4.20 ppm, attesting to the complete conversion of
dibenzylic sulfones into olefinic moieties, and to the formation
of fully π-conjugated polymers. Average molecular weights
were evaluated by size exclusion chromatography (SEC) in
THF using polystyrene standards. Results are summarized in
Table 1, and SEC traces of PFV, PF-S, and PF-SO are shown
in Figure 3. The fluorene-based copolymer, PF-S, is
characterized by a moderate molecular weight for the parent
compounds (Table 1, entry 1), increasing after sulfonation
(Table 1, entry 2). The final polymer, PFV, has a similar
molecular weight than its precursor PF-S (Table 1, entry 3).
After post-chemical modification forming the sulfone and the
π-conjugated polymers, Mn and Mw values changed signifi-
cantly, most likely owing to the variation of solubility of the
different polymers. Finally, the high thermal stability of the
different polymers (PF-S, PF-SO, PFV) was established by
TGA with a loss less than 10% upon heating up to 400 °C (see
Figure S6, Supporting Information).
1
the complete transformation of the sulfone group. H NMR
spectra revealed the presence of methylene protons for
monomers F-Br and F-SH appearing at δ 4.59 (s) and 3.82
(d) ppm, respectively. These signals vanished in the spectrum
of PF-S, where a single peak at δ 3.68 ppm (Figure 2) was
noted. Upon oxidation, the methylene protons of PF-SO were
shifted downfield to δ 4.20 ppm. Importantly, no signal around
δ 3.90 ppm, resulting from a partial oxidation of the sulfide
into the sulfoxide was detected.44 After treating the PF-SO
1
̈
under modified Ramberg−Backlund reaction conditions, H
NMR of PFV revealed a (E)-stereoselectivity for the double
bonds, a result consistent with observations made for model
dibenzylic sulfones reported earlier,43,45 and that is in good
agreement with spectroscopic data reported by Nomura et al.
concerning PFV synthesis by ADMET.32,33 It is also worth
E
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synthesis of unsymmetrical poly(arylene vinylene)s (PAr1,2V’s;
Route B, Figure 1). A dithiol-functionalized carbazole
derivative (C-SH) was thus selected as a co-monomer, the
latter being synthesized as shown in Scheme 4.37 Compound 5
Table 1. Size Exclusion Chromatography Analysis of
Polymers in THF at 25 °C Using PS Standards for
Calibration
a
n
a
a
entry
polymer
M
̅
(g/mol)
7 500
M
̅
(g/mol)
Đ
w
1
2
3
4
5
6
PF-S
14 300
32 000
13 200
82 300
61 200
11 400
1.90
2.10
1.70
2.02
2.01
2.05
Scheme 4. Synthesis of Carbazole Monomer C-SH
PF-SO
PFV
15 000
7 600
PFC-S
PFC-SO
PFVCV
40 700
30 400
5 500
a
Number- and weight-average molar mass (in g mol−1) and dispersity
determined by SEC in THF (PS calibration).
being not stable at room temperature, the crude compound
was used in the next step without further purification, leading
to the desired C-SH in 55% yield, after treatment with thiourea
and hydrolysis under basic conditions.
To validate our strategy for the synthesis of alternated
poly(arylene vinylene)s (Route B, Figure 1), 1 equiv of F-Br
was treated with 1 equiv of C-SH in the presence of 3 equiv of
powdered KOH in THF/MeOH at 100 °C for 14 h (Scheme
5). This gave a polysulfide, PFC-S, in 83% yield after
purification. Oxidation of PFC-S with 5 equiv of m-CPBA in
CHCl3 at room temperature for 8 h provided the
corresponding polysulfone (PFC-SO) in 76% yield. Finally,
Figure 3. SEC traces in THF (PS standards; RI detector) of PFV, PF-
SO, and PF-S.
Synthesis of Poly(fluorene-sulfone)s (PF-SO) via the
Rongalite Route. With a view at developing a simpler and
inexpensive protocol for the synthesis of polysulfone-type
̈
treatment of PFC-SO under Ramberg−Backlund conditions
produced the desired poly(fluorene vinylene-co-carbazole
vinylene) (PFVCV) in 43% yield.
̈
precursors of the Ramberg−Backlund reaction, we envisioned
Monomers and polymers (PFC-S, PFC-SO, and PFVCV)
that commercially available rongalite,46 i.e., sodium hydroxy-
methanesulfinate, could serve as a source of sulfoxylate dianion
(SO22−). Rongalite is inexpensive and is used as a less oxygen-
sensitive bleaching agent than sodium dithionite (Na2S2O4) for
the synthesis of symmetrical sulfones. Best results were
obtained when 1 equiv of fluorene-Br (F-Br) was treated
with rongalite (5 equiv) in the presence of K2CO3 (5 equiv)
and tetra-n-butylammonium bromide (TBAB) (0.5 equiv) as a
phase-transfer catalyst in DMF at room temperature for 3 days
(Scheme 3). This gave the desired polyfluorene-sulfone (PF-
1
were characterized by SEC, IR, and H NMR spectroscopy.
1
Similar patterns were observed by H NMR, as illustrated in
Figure 4. Polysulfides PFC-S exhibited high apparent
molecular weights (Table 1, entries 4−6). Again, this could
be due to an increase of the hydrodynamic volume in THF
after sulfonation. Higher molecular weights were achieved for
the parent PVCS (Table 1, entry 4), but lower values were
noted after the two modification steps forming PFC-SO and
PFVCV (Table 1, entries 5 and 6), respectively (see SEC
traces in Figure 5). This could not only be explained by a
different solution behavior, but also by possible fractionation
during purification by filtration, in agreement with the decrease
in dispersity. As above for PFV, high thermal stability of the
polymers (PFC-S, PFC-SO, and PFVCV) was shown by TGA
(see Figure S6, Supporting Information).
Scheme 3. Access to PF-SO Using the Rongalite Route
Overall, this three-step reaction sequence to poly(arylene
vinylene)s is efficient and stereoselective, although the
̈
Ramberg−Backlund reaction step gives moderate yields,
particularly when steric hindrance around the olefinic moiety
is increasing. This limitation is likely inherent to the Ramberg−
SO) with M = 2700 g/mol and Đ = 1.7. Despite a modest
isolated yield of 37% and low molecular weight, this method
opens up an easy entry to PF-SO, and its application affords an
access to symmetrical PPV-like materials in only two-step from
readily available benzylic bromides.
Synthesis of Unsymmetrical Poly(arylene vinylene)s.
Based on the above findings, we extended our strategy to the
̅
n
̈
Backlund reaction, which proceeds through an episulfone
intermediate resulting from the displacement of the halogen α′
to the sulfonyl group by an α-carbanion (Scheme 6). This
process is believed to occur through a semi-W-conformation,
the elimination step being rate-limiting.47,48 While this
conformation is easily attained in small acyclic systems, this
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Scheme 5. Synthesis of Poly(fluorene vinylene-co-carbazole vinylene) (PFVCV)
Figure 4. 1H NMR spectra of (i) PFCV, (ii) PFC-SO, (iii) PFC-S, (iv) C-SH, and (v) F-Br in CDCl3 (400 MHz). The n-hexyl peaks are omitted
for clarity.
might not be the case with polymeric chains, in which strain
and folding might retard the elimination and the subsequent
SO2 extrusion. This might also explain the long reaction time
needed in our polymerization experiments, compared to those
noted with molecular compounds, and the low conversion with
substrates having substituents on the arenes ortho to the
sulfonyl chain (not described here).
Optical Properties. The optical properties of the final
(co)polymers were analyzed by UV−vis absorption and
fluorescence spectrophotometry (PFV, PFVCV) both in
solution in chloroform and in bulk on solid thin films. UV−
vis absorptions in chloroform are shown for PFV (and its
precursors PF-S and PF-SO) in Figure 6, and for PFVCV (and
its precursors PFC-S and PFC-SO) in Figure 7. In both cases,
a strong red-shift for the two final (co)polymers can be
observed. It can be attributed to the extension of the
conjugation for PFV and PFVCV, while their precursors are
not π-conjugated. Surprisingly, the two (co)polymers, PFV
and PFVCV, showed almost the same UV−vis pattern in
chloroform solution (Figures 6 and 7), exhibiting in both cases
two absorption maxima around 425 and 455 nm in solution.
These UV data are very similar to those of PFV synthesized by
the ADMET method, as reported by Namura et al.33 In other
words, alternation with carbazole subunits did not affect optical
properties of the material. No significant change was eventually
noted in the UV−vis absorption from thin-films of (co)-
polymers PFV and PFVCV (see Supporting Information,
Figure S5). The absorption bands were slightly red-shifted
relative to that of PFV derivatives.49 Such small differences can
be assigned to the effect of alkyl chains from fluorene repeating
units that are shorter in our case, thus favoring π−π stacking
and extension of conjugation length. The optical band gaps
(Eg) for both polymers, calculated from the onset of the
absorption, were approximately 2.6 eV.
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Figure 7. UV−vis absorption of PFC-S, PFC-SO, and PFVCV.
Figure 5. SEC traces in THF of PFVCV, PFC-SO, and PFC-S.
Scheme 6. Conformational Requirements in the Ramberg−
polymer exhibit two maxima of emission at 475 and 500 nm.
When excitation was done at 450 nm (Figures 8b and 9b), the
second emission peak was more prominent than when
excitation was performed at 425 nm (Figures 8a and 9a). In
solid thin films, both polymers (PFV and PFVCV) exhibited a
broad emission band, even when they were excited at 425 nm
(Figures 8c and 9c) or at 450 nm (Figures 8b and 9b).
However, two main differences can be highlighted between
PFV and PFVCV emission spectra in thin film. First, a more
prominent emission peak was noted around 512 nm for
PFVCV (Figure 9c,d), in agreement with reported data on
poly(carbazolyl-2,7-vinylene), which is characteristic of a more
bluish emitted light as compared to greenish-blue emitted light
from PFV.50 Second, the emission band was extended up to
700 nm in the case of PFV, compared to 650 nm for PFVCV,
emphasizing the impact of the carbazolyl vinylene group
insertion. This strongest red-shift in solid state for PFV could
also be explained by a better organization (e.g., π−π stacking)
comparing to PFVCV. Finally, the quantum yield of the
purified PFV material, measured upon excitation at 400 nm in
THF, reached 40.3% (Supporting Information). Although
lower than those found by Nomura et al.,33 our values are very
satisfactory, indicating that the PFV prepared through our
method is also free of defects, as further supported by the
similar absorption and emission maxima (vide supra).
̈
Backlund Reaction
For quenched poly(fluorene vinylene), PFVQ, despite a
slight variation of molar masses, there is no significant change
in the absorption and emission spectral properties (Figures S3
and S4, Supporting Information).
Thermal Stability. The thermal stability of PFV and its
precursors (PF-S, PF-SO) as well as the stability of PFVCV
were finally analyzed by TGA (see Supporting Information,
Figure S6). The two conjugated polymers PFV and PFVCV
are stable until 450 °C. Their precursors (of sulfide- and
sulfone-type) are reacting at lower temperatures (between 300
and 350 °C) and finally start to decompose at 450 °C.
Figure 6. UV−vis absorption of PF-S, PF-SO, and PFV.
Figures 8 and 9 show the fluorescence spectra recorded with
the excitation wavelength corresponding to the two maxima of
absorption of the polymers at 425 and 450 nm. In solution,
two maxima are observed for both polymers (as well as a slight
shoulder), and the corresponding wavelengths are slightly
dependent on the wavelength of excitation. For instance, a
PFV excitation at 425 nm (Figure 8a) led to a strong emission
band at 463 nm, then a shoulder at 494 nm and a slight
shoulder at 534 nm, in excellent agreement with Nomura’s
emission spectrum recorded at 426 nm (465, 496, and 530 nm,
respectively).33 However, in the solid state, broad emission
peaks can be observed for both, in almost all the visible region
and whatever the wavelength of excitation. In solution, both
CONCLUSION
A novel synthetic strategy involving a sulfone-based precursor
step-growth polymerization route followed by post-chemical
■
̈
modification via the Ramberg−Backlund reaction provides an
easy access to poly(arylene vinylene)-like semiconductors. The
sulfone precursors are easily obtained, allowing the connection
between electronically differentiated arenes. Symmetrical
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Figure 8. Emission spectra of PFV: (a) excited at 425 nm in CHCl3, (b) excited at 450 nm in CHCl3, (c) excited at 420 nm on thin film, and (d)
excited at 450 nm on thin film.
Figure 9. Emission spectra of PFVCV: (a) excited at 425 nm in CHCl3, (b) excited at 450 nm in CHCl3, (c) excited at 425 nm on thin film, and
(d) excited at 450 nm on thin film.
polysulfones may also be elaborated starting from benzylic
bromides precursors and rongalite as a source of sulfoxylate
dianion (SO22−). Polysulfones and their polysulfide precursors
can be equipped with apolar alkyl chains, thus securing their
solubility in organic solvents during polymer elaboration,
allowing the final purification of the material through
precipitation. Further optimization may be needed, in
particular to reduce the environmental impact of the reaction
sequence.45 However, such a methodology used to synthesize
copolymers composed of carbazolyl and fluorenyl subunits
paves the way for the preparation of a plethora of copolymer
structures and/or various architectures. We are, for instance,
currently investigating alternation of electron-rich and
electron-poor subunits, along with the integration of such
materials free of transition metal sources in optoelectronic
devices.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.8b00676.
■
S
Tables S1−S5, giving data for the optimization of the
step-growth polymerization and synthesis of PF-S and
SEC analysis; Figures S1−S6, giving IR, UV−vis, and
emission spectra, SEC analysis, and TGA thermograms;
1
and H and 13C NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: taton@enscbp.fr
*E-mail: yannick.landais@u-bordeaux.fr
■
ORCID
Eric Cloutet: 0000-0002-5616-2979
I
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(18) Henckens, A.; Colladet, K.; Cleij, T. J.; Lutsen, L.; Gelan, J.;
Vanderzande, D.; Fourier, S. Synthesis of 3,4-Diphenyl-Substituted
Poly(Thienylene Vinylene), Low-Band-Gap Polymers via the
Dithiocarbamate Route. Macromolecules 2005, 38, 19−26.
́
́
Frederic Robert: 0000-0003-2469-2067
Daniel Taton: 0000-0002-8539-4963
Yannick Landais: 0000-0001-6848-6703
Notes
(19) Zaquen, N.; Lutsen, L.; Vanderzande, D.; Junkers, T.
Controlled/Living Polymerization Towards Functional Poly(p-
phenylene vinylene) Materials. Polym. Chem. 2016, 7, 1355−1367.
(20) Junkers, T.; Vandenbergh, J.; Adriaensens, P.; Lutsen, L.;
Vanderzande, D. Synthesis of Poly(p-phenylene vinylene) Materials
via the Precursor Routes. Polym. Chem. 2012, 3, 275−285.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the University of Bordeaux for financial
support through the Idex program (2014 post-doctoral
fellowship). CNRS is also gratefully acknowledged for financial
support.
̈
(21) Taylor, R. J. K.; Casy, G. The Ramberg-Backlund Reaction.
Org. React. 2003, 62, 359−475 and references cited therein.
(22) Chow, H. F.; Ng, M. K.; Leung, C. W.; Wang, G. X. Dendrimer
Interior Functional Group Conversion and Dendrimer Metamorpho-
sis-New Approaches to the Synthesis of Oligo(dibenzyl sulfone) and
Oligo(phenylenevinylene) Dendrimers. J. Am. Chem. Soc. 2004, 126,
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