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Ladder Polymers with a Heteroacene Skeleton.

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Ladder Polymers with a Heteroacene Skeleton**
R’
R’
Torsten Freund, Ullrich Scherf, and K l a u s Miillen*
Dedicated to Professor Wolfgang Luttke
on the occasion of his 75th birthdaji
The synthesis of macromolecular ladder structures of type 1
is a particular challenge for polymer chemistry. With ladder
polymers such as these it is possible to compare the form and
persistence length of doublestranded polymers with that of
their linear analogues.[’] More1
over, in the presence of extended n-conjugation two-dimensional structures have electronic advantages, for example, a
lower band gap.[*]
Here we describe the synthesis of the high molecular mass
heteroacene 2 (nDec = n-decyl), which is completely soluble in
R’
8
6a: R’ = nDec
66: R1 = tBu
4a: R1 = nDec
= tBu
4b: R I
R1
S
S
2
0
3
F
~
8F%
H S P S H
5a:R3 = H
5b:R3 = Me
organic solvents and which, according to spectroscopic investigations, exhibits a defect-free ladder structure. Studies on
oligomeric model compounds are particularly important for
the investigation into the formation of the polymer and also
the subsequent generation of extended n-conjugation in the
acene.
In principle, there are two methods available for the synthesis
of double-stranded polymers : a “concerted” route with repetitive cycloadditions and a multistep route, in which the linear
precursor can undergo polymer-analogous cyclization reactions. In the case of acenes 3 the repetitive Diels-Alder cycloaddition is the preferred method,[31whereas for the sulfur-containing “polyhydroacene” 2 a multistep synthetic route is required.
The starting compound was the terephthalophenone 4a (see
Scheme 2) ,14] which can be converted into single-stranded
poly(phenylenesu1fide)s with benzoyl substituents by nucleophilic aromatic substitution with 1,3-benzenedithiol (“dithioresorcin”) (5 a) or 2-methyl-l,3-benzenedithiol
(5 b). Subsequent
cyclization in a sequence of Friedel -Crafts reactions should
then yield ladder structures. The phenyl substituents in 4 function as stabilizing substituents on the perimeter of the acene,
while the n-alkyl groups are essential to achieve sufficient solubility of the rigid polymer backbone.
In model reactions (Scheme 1 ) 4a and 4b can be treated with
thiophenolate to give 6 a and 6 b (each in 95 YOyield), respectively. The diketones 7a and 7b are obtained analogously from
2-fluorobenzophenone and the dithioresorcins 5 a and 5 b.[5,61
Phenylenesulfide formation of 6, which proceeds without any
detectable side reactions, suggests polycondensation of 4 a with
[*I
[**I
Prof. Dr. K . Miillen. Dip1.-Chem. T. Freund. Dr. U. Scherf
Max-Planck-lnstitut fur Polyrnerforschung
Ackermannweg 10, D-55128 Mainz (FRG)
Telefax: Int. code + (6131)379-350
T. F. thanks the Verband der Chemischen Industrie for a doctoral stipend
2424
$>
VCH Verlagsgrselbchuff mhH, 0-69451 Weinheim, 1994
7a: R3 = H
JJd)
7b: R3 = Me
l l a : R3 = H
l l b : R3 = Me
Scheme 1. a) K,CO,/dimethylacetamide, 120‘C; b) 1. LiAIHJTHF; 2 . BF;OEt,/
CH,Cl,; c) DDQ/THF 0.5% H,O; d) 7 a : 1. LiAIHJTHF; 2. BF,.OEt,/CH,CI,,
76: 1. LiAIHJTHF; 2. SnCI,/CH,CI,.
5a (Scheme 2). Thus, the dithiolate formed by deprotonation
with potassium carbonate in dimethylacetamide replaces both
chlorine atoms in 4 a at 320°C. In this way poly(pheny1enesulfide) 8a is accessible, for example, with molecular weights
M , = 150000 and M , = 670000, determined by membrane osmometry and light-scattering, respectively.
If polymer 8 is precipitated by pouring the reaction mixture
into 10 % acetic acid, the gel permeation chromatograms show
a bimodal distribution. The maximum in the low molecular
mass range is attributed to the formation of cyclic oligomers 9,
which could be shown to contain up to eight repeating units by
FD-mass spectrometry. The macrocyclic oligomers 9 can, however, be easily removed from the crude material (dissolved in
dichloromethane) by fractional precipitation with acetone. In
this way polymer samples of 8a with narrower molecular mass
distribution and number average molecular weights M , =
50000-110000 are obtained; these samples can be used for
investigations into the molecular weight dependence of the subsequent polymer-analogous Friedel- Crafts reactions.
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Angru. Chem. Int. Ed. Engl. 1994, 33, No. 23/24
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polymer does not take
place, either during the reduction or during the sub0
sequent ring closure. AcHS
SH
R1
R2
cording to N M R spectroscopic analysis, cyclization
5a: R2 = H
4a
products of similar struc5b: R2 = Me
ture are obtained independent of the molecular
weight of the sample. The
NMR signals of the methine bridges are shown to be
of particular diagnostic
&
s
H
H
value, as already seen for
the model reactions leading
to 10. 11, and 12. No indication of incomplete polymer-analogous cyclization
is observed.[', O1
The multistep sequences
described thus lead to a
2a
2b
high molecular weight,
Scheme 2 . it) K,CO,,dimethylacetamide. 120'C; b) 1. LiAIH,,'THF; 2. SnClJCHCI,. R ' = p-C,H,-n-C,,H,,
completely cyclized, and
soluble ladder polymer 2 b.
This product should now permit the desired comparison of the
Dithiol5b reacts with diketone 4 a to give the poly(pheny1eneform behavior and the persistence length of one- and two-disulfide) 8b (see Scheme 2). In contrast to 8a, neither a bimodal
mensional structures, for example, on the basis of light-scatterdistribution in the G P C elution profile nor the presence of cyclic
ing and viscosity measurements.[']
oligomers in the FD-mass spectrum could be detected. The isolated material also shows a lower molecular weight and a lower
Furthermore, polymer 2 b and model compound 10 are suitable precursors for the preparation of heteropolyacenes with
polydispersity ( M , = 65000 and M , = 240000) than that of 8a.
extended conjugated x-electron systems by dehydrogenation or
The Friedel- Crafts cyclization route towards ladder strucionization methods. The reaction of 10 b with 2,3-dichloro-5,6tures was initially investigated by using the model compounds
dicyano-p-benzoquinone (DDQ) in T H F in the presence of
6a, 6b, 7a, and 7b (see Scheme 1). It was shown that the cyclization of the diketones is significantly inferior to the analogous
0.5vol.% water led to diol 13 in 96% yield (see Scheme 1).
Treatment of 13 with trifluoromethanesulfonic acid in direaction of the diols. This result confirms observations in the
synthesis of ladder-type p~lyphenylenes.[~]
The reaction of the
chlormethane or acetonitrile (Scheme 3) gave a quantitative
diols obtained from 6 a and 6 b (LiAlH, in T H F ) with BF, 'OEt,
in dichloromethane gave a quantitative yield of 10a and 10b as
a mixture of the respective diastereomers.
In principle, unlike the cyclization of 6, regioisomers can be
formed by the cyclization of 7. Although the isomer mixture
precipitated after reduction of 7 a and subsequent FriedelCrafts alkylation with BF;OEt, in dichloromethane no longer
contains any uncyclized diarylmethanol units ('H and 'jC
NMR evidence). the bent structures 12 (as a mixture of
diastereomers), formed by Friedel - Crafts alkylation in the 2position of the benzenedithiol group, were detected in addition
13
to the linear annelated systems 11 a . After column chromatography (alumina, CH,Cl,/petroleum ether 4: 1) and crystallization (CH,Cl,/hexane) a pure cis isomer of 12 was obtained and
c10;
its structure determined by crystallography. Thus, it is understandable that 2-methyl-I ,3-benzenedithiol (5 b) is always used
15
as a bifunctional nucleophile, rather than 5a. to avoid formation of bent substructures during ring closure on route to acenes.
Indeed. reaction of the diol obtained from 7 b with SnCI, in
Scheme 3 a) Trifluoromethanesulfonic dcid/CHiCl2 or CH ,CN
dichloromethane results in a cis-trans mixture of diastereomers
of the lineur nnneluted structure 11 b (see Scheme I) .['I
The reduction of the polymers 8 a and 8 b to the corresponding polyalcohols with LiAlH, in THF was quantitative in both
yield, within seconds, of the stable dication 14, which was charcases. Cyclization (see Scheme 2) was also carried out with three
acterized by NMR and electronic absorption spectroscopy
equivalents of SnCI, per reactive group using chloroform as the
(Fig. 1 ; see also Experimental Procedure). A close relationship
preferred solvent. This reaction can be conducted in gram-quanof dication 14 with the isoelectronic pentacene could be demontities; a polymer concentration of about 4 g polyalcohol per liter
strated by the electronic absorption spectrum.[' ' 1 The longest
of chloroform was chosen, which guarantees that the reaction
wavelength absorption of 14 shows, in contrast to that of 9can take place in an homogeneous medium. Degradation of the
phenylthioxanthenylium perchlorate (15) (?.,,,ax = 515 nm),["' a
&
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(m. 4H). 8.57 (m, 4H). 8.18 (t. 'J(H.H) =7.8 Hz. 2H), 7.94 (d. 'J(H,H) =7.9 Hz.
~H).~.~~(~.'J(H,H)=~.~HZ.~H).~.~~(S,~~H);~~C
263 K): 6 = 175.2.157.8. 154.1, 142.8. 139.4, 13X.8,137.8. 132.9, 131.7. 131.5, 131.2,
M in
130.6, 128.3. 127.0. 35.6, 30.9. UV;VIS spectrum of 14: r(13) = 2 . 9 ~
0.05 M trifiiioromethanesulfonic acid in acetonitrile: A,, , ( F [Lmol-' cm-'1) = 123
(64200). 439 (12100).648 (1280), 706 (2920). 778 (4530).
'4
8
Received: July 15, 1994 [27121IE]
German version: Angew. Chem. 1994. 106. 2547
tBu
I
L
250
450
6.kO
h [nrnl
111 a) S. A. Jenekhe. P. 0 . Johnson. Mrr[).O)?iO/rLlf/(,,\.1990. 23. 4419; b) C. P.
Wong. G . C. Berry, P d v m r r 1979. 211. 229; c) T. F. Helminiak, G . C . Berry. J.
P u / w ? Scr. Pulrm. S v n p . 1978. 65. 107; d ) C. P. Wong. H. Ohnuma.
G. C Berry. rhrd. 1978. 65. 143; e) U . Scherf. K . Mullen. Adv. Po/yrii. Sci.. in
press.
12) a) L. Yu, M. Chen. L. R Dalton, C h n i . Mutrr- 1990. 2. 649; b) L. Yu.D. W.
Polis, F. Xiao. L. S. Sapochak. M. R. McLean. L. R. Dalton, C. W. Spangler,
T. J. Hall. K . 0 . Havelka. P u / j n i w 1992, 33. 3239; c) K. Miillen. U. Scherf.
Sjnrhesi5 1992, 23; d) H. Naarmnnn, Adiz. M u r e r . 1990, 2. 345.
[31 a) A:D. Schluter. Ad?. Muter. 1991.3.282; b) M. Loffier, A. D. Schluter. GIT
Furl?;. Luh. 1992. f9Y-7, 1101; c) J P. Mathias. J. F. Stoddart, Chrm. Sor. Rn.
850
Fig 1. UV'VIS spectrum of the dication 14
marked bathochromic shift, which can be assigned to the
smaller band gap of the higher homologue 14.
Polymer 2 b can be converted, analogous to the formation of
13, into the polyalcohol 16 by treatment with DDQ in T H F /
0.5voI.% H,O (Scheme4). The lack of a ' H N M R signal at
2b
i
a)
S'
16
CH3
17
Scheme 4. a ) DDQ/THF/0.5% H,O; b) 1. Trifluoromethanesulfonic acid/CH ,CN;
2. Washing with anhydrous CH,CN. R ' = p-C,H,-n-C,,H,,
6 = 5.27, which stems from the methine bridge in 2 b, substantiates the complete conversion. Characterization of the polycation 17 (analogous to 14) is prevented by its low solubility (e.g.,
in dichloromethane, acetonitrile, nitromethane, or trifluoroacetic anhydride). Thus, 16 (in T H F ) was poured into a thin
film from T H F solution and this was ionized by using trifluoromethanesulfonic acid in acetonitrile. After washing the product
with dry acetonitrile an electronic absorption spectrum was obtained that, based on the longest wavelength absorption at
978 nm, reveals the presence of extended n-conjugated cationic
subunits. Examination of the extent of the conversion in the
immobilized phase and a more detailed characterization of the
ionic polyacene structure 17 will be reported elsewhere.
1992. 21. 215: d ) S. Wegener. K. Mullen. M a r r o n ~ o / ~ , c r r1993.
/ ~ ~ s 26, 3037: e) S
Wegener. K . Mullen, Chem. Bo.. 1991. 124. 2101
[4] Diketones 4a and 4b were prepared, according to ref. [7], from 2.5-dichloro-pxylene. which was oxidized with KMnO, in pyridine to give 2.5-dichloroterephthalic acid (72% yield). The bis-acid chloride obtained with SOCI, in
benzene (85%) then gave 4 a (69% yield) or 4b (75% yield). respectively. in a
Friedel Crafts acylation with 1-phenyldecane or with tert-butylbenzene.
[5] Compound 6 b was synthesized from 2-methylresorcin, according to ref. [6].
[6] H.-J. Kurth. U . Kradtz, F. Korte, Chent. Err. 1973. 106, 2419.
[7] U.Scherf, K. Mullen. Makromol. Chrm. RupidConzmun. 1991. 12, 489.
[8] The trityl protons of 11 b show signals at 6 = 5.55 and 6 = 5.50. The signal for
the corresponding carbon atom appears at 6 = 53.5 and is not split. in spite of
the diastereomer formation observed in the ' H NMR spectrum.
[9] In the ' H N M R spectrum of 2 a the protons of the methine bridges show the
signal expected f o r a triphenyhnethane structure at 6 = 5.25. as well as an extra
signal at 6 = 5.95. This result. however, is not attributed to the presence of
uncyclized diphenylinethanol groups, since the signals of the alcohol structures
at 6 =72.8 in the I3C N M R spectrum have completely disappeared and new
signals have appeared at 6 = 52.0 and 6 = 49.0. Taking into account that the
proton of the methine bridge formed by cyclization at the 2-position of the
dithioresorcin subunit in the model compound 12 shows a resonance signal at
= 6.0, it becomes evident that cyclization of the polyalcohol leads to a
product Za, in which bent substructures also exist. On the other hand, the
cyclization ofthe polyalcohol obtained from 8 b gave, asexpected. an exclusively linear annekated product 2b, the NMR spectrum of which shows only a
single signal for the trityl protons at 6 = 5.27 and the corresponding I3C N M R
signal at 6 = 52.9.Even at a signal-to-noise ratio S / N =150 for the ' H NMR
signal of the methine bridge, no signals were detected in the range 5.8 > 6 > 6.3.
as would be expected for an incompletely cyclized diphenylmethanol structure.
Nevertheless, in the electronic absorption spectrum a band with low intensity
was found at I,,, = 602 nm, which can be attributed to partial dehydrogenation during cyclization and thus formation of quinodimethane subunits. This
assignment is based on a comparison with the corresponding model compounds and related polymers [lo]. The fraction of such structures, which are
not detectable in the N M R and IR spectra, is below 1 %.
[lo] a) U. Scherf. K. Miillen, Polymer 1992. 33, 2443; b) U. Scherf, Synrh. Met.
1993, 55-57? 767.
[l 11 D M S - U V - A / / u s 0r.yzni.wher Vrrhindungen. V d . 3, Butterworth. London, and
Verlag Chemie. Weinheim. 1966. spectrum E4,'l.
1121 a) C. C . Price, M. Hori, T. Parasaran, M . Polk, J. Am. Chem. Soc. 1963, HS,
2278; b) 0.Convert, J.-P. Le Roux, P.-L. Desbene, A. Defoin, Bull. SOC.Chm.
Fr. 1975. 2023.
~
Experimental Procedure
Dication 14: 10 mgofarnixture ofdiastereomers ofdiol 13 wasdissolvedin CD,CI,
(1 mL) and treated with one drop of trifluoromethanesulfonic acid. 'H NMR
(500MH~,CD,C1~,263K):S=9.72(5.2H),8.75(d,'J(H,H)=8.4Hz.2H),8.75
2426
VCH Yerlu~.~~esr//.~chuft
nibH. 0.69451 Weinhrnn, 1994
0570-083319412323-2426 S 10.00+ .25!'0
Angca.. Chem. Inr. Ed. Engl. 1994, 33, Nu. 23/24
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