Convenient tautomeric forms in an amino-anthraquinone diimine for the generation of a mixed-valent push-pull conjugated polymer

Article abstract of DOI:10.1039/c2cc36768h

The use of NH₂ on the (Pt(CC)₂(PBu₃) ₂)_n/anthraquinone diimine)_n polymer induces a tautomeric species which leads to a mixed-valent form reminiscent of emaraldine in polyaniline. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc36768h

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COMMUNICATION
Convenient tautomeric forms in an amino-anthraquinone diimine
for the generation of a mixed-valent push–pull conjugated polymerw
Tommy Kenny, Shawkat M. Aly,z Daniel Fortin* and Pierre D. Harvey*
Received 17th September 2012, Accepted 12th October 2012
DOI: 10.1039/c2cc36768h
R
The use of NH2 on the (Pt(C C)2(PBu3)2)n/anthraquinone
diimine)n polymer induces a tautomeric species which leads to a
mixed-valent form reminiscent of emaraldine in polyaniline.
Polyaniline (PANI) is a redox active p-conjugated 1D material
exhibiting applications in anticorrosive coatings, solar energy
conversion, electronic devices, rechargeable batteries, artificial
muscles and sensors.1,2 It exists under three different redox
forms: the fully reduced leucoemeraldine, the fully oxidised
perigraniline, and the mixed-valent emeraldine (Fig. 1).2 Emaraldine
becomes a conductive polymer when protonated.
Fig. 2 Structures of the quinone diimine-containing polymers.
in the chain. We report herein an original way to produce the
mixed-valent form (6) using a unique precursor, amino-
anthraquinone (7b), taking advantage of the formation of a
tautomeric form. Its detection is made using 1H NMR, IR and
emission spectroscopy.
In parallel, platinum(II)polyyne-containing polymers are
subjects of current and intense interest as they offer the possibility
of low-band gap excited states and many studies on their bulk
hetero-junction solar cells have been reported.3 Similarly, these
materials are also found to be electroluminescent and light emitting
diodes were studied.4 Recently, our group explored the qui-
none diimine-containing conjugated polymers 1–4 where the
push–pull process is defined by the donor ability of the trans-
Pt(PR3)2 units and the acceptor properties of the quinone
substituents X (Fig. 2).5
The fully oxidized 5 and 6 are prepared in three steps from
the commercially available 7a,b and p-Me3SiCRCC6H4NH2
(Scheme 1; see Experimental section in the ESIw). 8a,b are
prepared from 7a,b using a condensation method with TiCl4
which are then deprotected using a weak base (K2CO3) to
1–3 are the oxidized version, whereas 4 is the mixed-valent
form. The synthesis of the latter requires the mixing of both
the reduced and oxidized precursors prior to polymerization,5c
leading to an irregular sequence of oxidized and reduced units
Fig. 1 The three different redox forms of PANI.
´
Departement de Chimie, Universite de Sherbrooke, 2550 Boulevard de
l’Universite, Sherbrooke, Quebec, Canada J1K 2R1.
´
´
E-mail: Pierre.Harvey@USherbrooke.ca; Fax: 001-819-821-8017;
Tel: 001-819-821-7092
w Electronic supplementary information (ESI) available: Experimental
details, X-ray and photophysical data. CCDC 901520 and 901521. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2cc36768h
z On leave from the Chemistry Department, Assiut University, Assiut,
Egypt.
Scheme 1 Synthesis of 5, 6, 10a,b and 13.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 11543–11545 11543

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Table 1 Gel permeation chromatography data for 5 and 6
Polymer
Mn
nb. of units
Mw
PD
5
6
15 200
18 100
14.6
17.2
25 800
51 400
1.70
2.93
form 9a,b. The characterization is performed by 1H NMR, IR
(n(CRC), n(CQN), n(N–H)), mass spectrometry (EI) and
chemical analysis. The X-ray structure of 8a was also determined
(ESIw), which reveals the common butterfly structure.6 9a,b react
with 1 or 7 eq. of trans-PtCl2(PBu3)2 in the presence of CuI and
iPr2NH to respectively form 5 and 6, or models 10a,b. GPC, (gel
permeation chromatography; Table 1) reveals an oligomeric
1
nature of B14 to 17 units for the polymers. The J(31P–195Pt)
coupling constants of 2352–2383 Hz in the 31P NMR spectra
confirm the trans-geometry about the Pt atom.5b A fully reduced
model, 13, is prepared in three steps from 7a, which was first
coupled with p-BrC6H4NH2 to afford 11. The latter is then reduced
with HI to form 12, which is then coupled with Me3CRCH to
form 13 confirmed by X-ray crystallography (ESIw).
The UV-vis spectra of the fully oxidized species 8a, 10a, and
5 exhibit the characteristic low energy charge transfer (CT) band
CRCC6H4 - quinone (ESIw).5 The position and absorptivity
data (see 5 in Table 2) compare well with that of the
poly(anthraquinone diimine phenylene) material (lmax = 468 nm;
e = 6200 Mꢀ1 cmꢀ1).7 The high energy bands in the 300–350 nm
window stem from the CRCC6H4 p - p* transitions. The
emission properties of 8a, 10a, and 5 (ESIw) were studied at
77 K since they are not luminescent at 298 K. They exhibit the
fluorescence (ns) and phosphorescence (ms) bands arising from
the pp* manifolds of the CRCC6H4 unit (i.e. S2 - S0 and
T2 - S0), not from the CT ones.5 These are used as fingerprint
for the presence of the oxidized moiety. Indeed, the mono-
amino-anthraquinone diimine-containing models 8b and 10b
exhibit the expected structured S2 - S0 fluorescence and T2 -
S0 phosphorescence associated with the CRCC6H4 unit
(Fig. 3; red traces). However, a new emission at B535 nm is
detected. A fluorescence of the CT excited state (i.e. S1 - S0)
was first suspected due to its apparent proximity with the CT
band (Stoke shift: B4700 (8b), B3500 cmꢀ1 (10b)) and is short
lifetime (i.e. tF; ns time scale; the photophysical detail is in
the ESIw). As the relative emission intensity depends on lexc
(i.e. the maxima of the excitation lowest energy band does
not match well the absorption one, neither in position as in 8b
nor in bandwidth as in 10b) another species is therefore
responsible for the extra emission at B535 nm. Its identifi-
cation is readily made using the fully reduced model 13, which
exhibits an unstructured fluorescence at B510 nm (Stoke shift =
2600 cmꢀ1; tF = 6.09 ꢁ 0.09 ns).
Fig. 3 Absorption (black), fluorescence (red, orange), and excitation
(blue, turquoise) spectra of 13, 8b, 10b and 6 in 2MeTHF at 77 K.
The tF data for this signal are 4.52 ꢁ 0.06 (13), 4.49 ꢁ 0.05
(8b), and 4.64 ꢁ 0.02 ns (10b). The similarity in lmax and tF
provides a strong proof that the B535 nm emission is the
fluorescence arising from an amine unit. The absence of heavy
atom effect on the fluorescence (tF should decrease when Pt is
introduced) is explained by the long Pt/amine-containing
chromophore distance. The possibility of the anthraquinone
diimine fragment be reduced into 1,4-diamino-benzene during
the synthesis is ruled out since under the same conditions 8a,
10a and 5 were never reduced. Consequently, the presence of
these two emissions is due to tautomers (Fig. 5). Tautomerism
in anthraquinone imines and anthraquinones is indeed well
known.8 Forms A and B must co-exist in the ground state since
they are both detected at 77 K and long range proton transfer is
13, 8b, and 10b were also studied at 298 K (Fig. 4) and the
same comments described above apply. No phosphorescence
is noted leaving only the fluorescence appearing in the spectra,
notably the B535 nm signal common to 13, 8b, and 10b.
Table 2 Absorption data for 5 and 6 in 2MeTHF at 298 K
most unlikely to occur in frozen media. The presence of
1
tautomers is evidenced by the presence of two H NMR NH
Polymer
lmax (nm)
e (Mꢀ1 cmꢀ1
)
lmax (nm)
e (Mꢀ1 cmꢀ1
)
ꢀ
5
6
295
290
15 700
14 400
449
428
4230
6600
resonances at B4 ppm (8b and 10b) and by three IR signals in
the 3220–3475 cmꢀ1 window (8b, 9b, 10b and 6; detail in ESIw)
c
11544 Chem. Commun., 2012, 48, 11543–11545
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the same fingerprint as those measured for 8b, 9b and 10b.
Attempts to prepare the corresponding fully reduced model
complex and polymer from 13 stubbornly failed for unknown
reasons. The resulting materials were always the fully oxidized
polymer 6 and complex 10b. Therefore, this tautomer property
now appears to be an appealing alternative to prepare a
polymer exhibiting features approaching that of the reduced
form of 5. Indeed, the tautomeric form B of 6 (Fig. 5) contains
both imine and amine units along the polymer chain, so it
exists as a mixed-valent polymer, similar to the organometallic
version of PANI illustrated in Fig. 1 (4). To the best of our
knowledge, this strategy has never been used for organic PANI
and its analogues and may open the door to new developments
in functional polymers.
This research was supported by the Natural Sciences and
Engineering Research Council of Canada (NSERC), le Fonds
Que
(FQRNT), the Centre d’Etudes des Mate
Photoniques de l’Universite de Sherbrooke (CEMOPUS).
´
be
´
cois de la Recherche sur la Nature et les Technologies
´
riaux Optiques et
´
Notes and references
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Fig. 4 Absorption (black; arbitrary units), fluorescence (red, orange), and
excitation (blue, turquoise) spectra of 13, 8b, and 10b in 2MeTHF at
298 K. The shoulder at B480 nm (red trace in 8b) is due to an impurity.
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Fig. 5 Top: Tautomer forms in 8b, 10b and 6. Bottom: structure of a
fragment of the mixed-valent amine–imine polymer 6 (not necessarily
alternated) stressing its emission behaviour at 77 K.
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whereas only one (1H NMR) and two (IR) signals are expected
for form A.
No fluorescence arising from the amino-benzene unit at
B535 nm is confidently observed in 6 at 298 and 77 K,
although a shoulder at this position is noted at 77 K
(Fig. 4). The absence or weakness of this signal does not exclude
the possibility of tautomeric forms as 10b exhibits a strong
amino-benzene fluorescence and that the IR spectra of 6 exhibit
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 11543–11545 11545