New chiral π-conjugated polymers based on a (1R,2R)-diiminocyclohexane chiral unit with weak interchain π stacking

Article abstract of DOI:10.1039/b208869j

New chiral π-conjugated polymers consisting of alternating conjugated segments and (1R,2R)-diiminocyclohexane units with C2 symmetry were prepared by a palladium-catalyzed coupling reaction, they exhibited very high specific optical rotations ([α] up to -3000°) and strong Cotton effects ([θ] 10⁶ deg cm² mol^-1).

Full text of DOI:10.1039/b208869j

New chiral p-conjugated polymers based on a
(1R,2R)-diiminocyclohexane chiral unit with weak interchain p stacking†
Jean-Pierre Lère-Porte,* Joël J. E. Moreau, Françoise Serein-Spirau and Salem Wakim
UMR CNRS 5076 Hétérochimie moléculaire et macromoléculaire, E.N.S.C.M, 8 rue de l’Ecole Normale,
34096 Montpellier Cedex 05, France. E-mail: lere@cit.enscm.fr; Fax: +33(0)467144353;
Tel: +33(0)467147213
Received (in Cambridge, UK) 11th September 2002, Accepted 29th October 2002
First published as an Advance Article on the web 15th November 2002
New chiral p-conjugated polymers consisting of alternating
conjugated segments and (1R,2R)-diiminocyclohexane units
with C2 symmetry were prepared by a palladium-catalyzed
coupling reaction, they exhibited very high specific optical
rotations ([a] up to 23000°) and strong Cotton effects ([q]
106 deg cm2 mol21).
alcohols and CH3CN. THF solutions of polymers were purified
by preparative gel permeation chromatography (GPC), over
polystyrene bio-beads S-X1 (200–400 mesh) with THF as the
eluent. The average molecular weights of the collected fractions
of (5–7) were evaluated by analytical GPC column calibrated
with polystyrene standards; they varied from 12000 to 23000
suggesting an average degree of polymerization (DPn) from 13
to 19. Solutions of polymers in THF showed specific optical
rotations up to 3200°, four to five times higher than those of the
isolated chiral unit.
The absorption and emission properties of the polymers 5–7
are indicated‡. For comparison, those of the isolated conjugated
segments 5A, 6A, 7A (Scheme 1) have been evaluated. The
absorption and emission properties of 5A–7A (cf. ESI†) are
identical to those of 5–7‡ in agreement with a confinement of
the conjugation length between two chiral units.18 Under visible
irradiation in dilute THF solutions the polymers emitted strong
fluorescence in the blue region and showed two bands around
425–470 nm for lexcitation 400 nm. Thin films of polymers 5–7
and of repeating units 5A–7A on a glass substrate were realized by
spin-coating from saturated solutions in CHCl3 and the optical
properties were studied and compared to the solution. The
absorption of the isolated conjugated units shows a red-shift of
the wavelength maximum in the solid state 5A–7A. This
behaviour can be attributed to an enhanced conjugation in the
solid state, due to favoured p-stacking interchain interactions.
Interestingly the polymers 5–7 showed absorption bands
exactly identical in CHCl3 solutions and in the solid state (Fig.
1a,b). This result is consistent with the absence of p-stacking
interactions of the conjugated units in the polymeric solid
material. Moreover X-ray diffraction diagrams of 7 showed
p-Conjugated organic polymers have attracted considerable
attention owing to their interesting properties as materials for
charge transport,1–4 for non linear optics,5 for electronics and
optoelectronics.1–3,6 Particularly, they are attractive emissive
materials for light emitting diodes (LEDS) because they are
easily processed and their optical properties can be easily tuned
by chemical engineering.7 The stacking of the p-conjugated
polymer chains plays a determining role in the physical
properties of these materials.8,9 The introduction of a chiral unit
into a p-conjugated polymer can lead to a backbone with a
helical secondary structure and has been explored as a route to
enantioselective sensors and catalysts.10,11 In addition, the
secondary structure of the chain prevents p-stacking of the
conjugated segments leading to enhanced electroluminescence
efficiency.7,12 Circularly polarized electroluminescence was
also reported for these materials.13 p-Conjugated polymers
bearing optically active side groups have been extensively
studied, but few examples of p-conjugated polymers possessing
chiral units in the main chain have been reported. The latter
polymers were shown to have a more stable helical secondary
structure with less dependence on temperature and solvent.14,15
In this paper we report a versatile synthesis of a new class of
chiral polymers which have a backbone consisting of alternating
conjugated segments and (1R,2R)-1,2-diiminocyclohexane. We
explored the use of the more rigid diiminocyclohexane
compared to diaminocyclohexane as a means to induce stable
helical secondary structure in the chain. The optical and electro-
optical properties associated with a tunable p-conjugated
segment located between the chiral units are reported.
Chiral conjugated polymers containing (1R,2R)-diiminocy-
clohexane were obtained as shown in Scheme 1. 1,4-Diethynyl-
(2,5-dioctoxy)benzene (1) and 2,5-diethynyl (3-octyl)thiophene
(2) were prepared by palladium-catalyzed coupling of trime-
thylsilacetylene with the appropriate aromatic derivative fol-
lowed by a KOH-promoted deprotection. The reaction of
(1R,2R)-diaminocyclohexane and 5-bromo-2-thiophene car-
boxaldehyde or 4-bromobenzaldehyde in CH2Cl2 using 3 Å
molecular sieves16 afforded (1R,2R)-N,NA-bis(5-bromothio-
phene-2-ylmethyl)diiminocyclohexane (3) and (1R,2R)-N,NA-
bis(bromobenzylidene)diaminocyclohexane (4). The polycon-
densation reaction was achieved using equimolar quantities (1
mmol) of chiral units (3) or (4) and diethynyl conjugated
moieties (1) or (2) in the presence of palladium tetra-
kistriphenylphosphine (4 mol%) and cuprous iodide (4 mol%)
in a toluene–diisopropylamine mixture (25 mL/10 mL)17
(Scheme 1). The resulting polymers‡ (5–7) were soluble in
toluene, DMF, THF, CHCl3 and CH2Cl2 and insoluble in
† Electronic supplementary information (ESI) available: selected character-
ization data for 2–4, and 5A–7A. See http://www.rsc.org/suppdata/cc/b2/
b208869j/
Scheme 1
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only one diffraction signal at small angles (2q = 3.4°) and a
broad signal (2q = 21°) characteristic of semi-crystalline
material. Conversely 7A revealed several diffraction peaks
significative of a crystalline product. This behaviour is
consistent with lower order and stacking of the conjugated
segment in the polymer compared to the isolated segment in the
monomer.
Notes and references
‡
Selected characterization data for 5: Yield: 79%; GPC: Mw = 22830
and Mn = 10870 (PDI = 2.1), DPn = 19; [a]D = 23194° (c = 0.72, THF);
1H NMR (d, ppm, CDCl3): 0.86 (3H, t, J = 5 Hz, CH3), 1.28 (12 H, broad
singlet, Hoctyl and Hcyclohexyl), 1.6 (2H, m, Hoctyl), 1.84 (6H, broad singlet,
Hcyclohexyl), 2.65 (2H, t, J = 6.6 Hz, th-CH2), 3.28 (2H, broad singlet, –CH–
N), 7.02 (1H, s, Hth), 7.05 (2H, broad singlet, Hth), 7.10 (2H, broad singlet,
Hth), 8.17 (2H, s, –CHNN–). nmax (KBr)/cm21: 1627 (CNN); absorption
(CHCl3) lmax (nm) (emax/l mol21 cm21) 403 (39075), 280 (14890), 242
(14681); absorption (film on glass) lmax (nm) 403; emission (THF) lmax
(nm) 447, 474 (excitation lmax = 400 nm); solid state emission lmax (nm)
584 (excitation lmax = 405 nm); CD(CHCl3) lmax (nm) (q/10 deg cm2
mol21) 415.8 (22.87 3 105), 363.2 (6.82 3 104), 305.9 (1.58 3 104), 280.9
(5.58 3 104).
Selected characterization data of 6: Yield: 79%; GPC: Mw = 25800 and
Mn = 12500 (PDI = 2.06), DPn = 18; [a]D = 22200° (c = 0.95, THF);
1H NMR (d, ppm, CDCl3): 0.85 (6H, broad singlet, CH3), 1.20–1.5 (22 H,
multiplet, Hoctyl and Hcyclohexyl), 1.78–1.85 (10H, broad singlet, Hoctoxy and
Hcyclohexyl), 3.31 (2H, broad singlet, –CH–N), 3.98 (4H, t, J = 6Hz, O–
CH2), 6.93 (2H, s, Hphenyl), 7.05 (2H, s, Hth), 7.11(2H, s, Hth), 8.17 (2H, s,
–CHNN–); nmax (KBr)/cm21: 2196 (CNC), 1627 (CNN); absorption (CHCl3)
lmax (nm) (emax/l mol21 cm21) 403 (41270), 342 (24300), 261 (15333), 241
(16700); absorption (film on glass) lmax (nm) 404; emission (THF) lmax
(nm) 449, 473 (excitation lmax = 402 nm); solid state emission lmax (nm)
505 (excitation lmax = 400 nm); CD(CHCl3) lmax (nm) (q/10 deg cm2
mol21) 425.4 (23.36 3 105), 374.4 (6.23 3 104), 351.7 (21.69 3 104),
322.5 (1.11 3 105), 271.5 (4.30 3 104).
Selected characterization data of 7 : Yield: 79%; GPC: Mw = 13600 and
Mn = 8500 (PDI = 1.45), DPn = 13 ; [a]D = 2825° (c = 0.78, THF); 1H
NMR (d, ppm, CDCl3): 0.83 (6H, broad singlet, CH3), 1.20–1.5 (22 H,
multiplet, Hoctyl and Hcyclohexyl), 1.78–1.85 (10H, broad singlet, Hoctoxy and
Hcyclohexyl), 3.41(2H, broad singlet, –CH–N), 3.98 (4H, t, J = 6Hz, O–
CH2), 6.97 (2H, s, Hphenyl), 7.46 (4H, d, J = 8.6 Hz, Hph), 7.57 (4H,d, J =
8.3 Hz, Hph), 8.18 (2H, s, –CHNN–); nmax (KBr)/cm21: 2198 (CNC), 1641
(CNN); absorption (CHCl3) lmax (nm) (emax/l mol21 cm21) 379 (39154),
323 (37667), 243 (24534) ; absorption (film on glass) lmax (nm) 323, 382;
emission (THF) lmax (nm) 428, 446 (excitation lmax = 380 nm); solid state
Fig. 1 Absorption spectrum of 7 in CHCl3 (a) and in solid state (b).
Fluorescence spectrum of 7 in the solid state (c).
The circular dichroism spectra have been recorded. Chiral
conjugated copolymers (5–7) display strong Cotton effects and
the molar ellipticity values [q], calculated using the molecular
weight of the polymer repeating unit, are very high from 1.2 to
3.4 3 106 deg cm2 mol21, an example is given in Fig. 2. The CD
spectrum of the polymer chain may arise from a contribution of
the helical secondary structure which could be responsible for
the observed low degree of interchain interactions in the
material.
emission lmax (nm) 505 (excitation lmax
= 381 nm); g = 1.04°;
CD(CHCl3) lmax (nm) (q/10 deg cm2 mol21) 394.5 (21.21 3 105), 333
(27.97 3 104), 304 (4.09 3 104), 247.5 (6.16 3 104).
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In the solid state, the polymers emitted strong photo-
luminescence in the green region (lmax = 505 nm) under visible
irradiation (lexcitation = 381 nm) (Fig. 1c). Photoluminescence
quantum yields were measured for the three copolymers.
Moderate or low values were observed for 5 (2%) and 6 (3%),
it can be explained by the heavy-atom effect of sulfur atoms
contained in the polymeric chain.19 Interestingly, a quite high
value (32%) was obtained for 7 and it is as high as those
reported for phenylene–vinylene based copolymers currently
used to develop electroluminescent diodes.9 Very large Stokes
shifts from 100 to 180 nm are observed for all polymers. The
intense photoluminescence can be related to the non-planar
structure of the polymeric chain leading to lower chain
interactions and preventing photoluminescent centres from
packing.12
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In summary, chiral conjugated polymers containing the rigid
(1R,2R)-diiminocyclohexane showed physical characteristics
and properties consistent with a low degree of interaction
between the conjugated segments. They are highly solubles and
show very high specific optical rotation, strong Cotton effect,
low crystallinity and similar absorption spectra in solution and
in the solid state. These results support low p-stacking
interactions which can be derived from the existence of a helical
conformation of the polymer chain. The high photolumines-
cence quantum yield (30%) observed for 7, associated with its
good ability to form homogeneous films on glass, opens
interesting possibilities to fabricate green light emittting
diodes.
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Dekkers and E. W. Meijer, J. Am. Chem. Soc., 1997, 119, 9909.
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W. Meijer, Macromolecules, 1998, 31, 6702.
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