Synthesis of twist-twist πconjugated di-sec-alkylstilbenes and stilbene polymers

Article abstract of DOI:10.1021/jo0204505

Twist-twist π-conjugated (TTPC) π systems promise unique properties with their 90° twist angles. Di-sec-alkyl substituted stilbenes, 5, were prepared by low-valent titanium coupling of phenyl ketones, 4. Long alkyl chains stopped the coupling reaction. Stilbenes 5 were shown to be ～90% TTPC. Inserting TTPC units into poly(p-phenylene) polymers created highly fluorescent, soluble, TTPC π systems with weak electronic segmentation for organic light emitting diode (OLED) studies.

Full text of DOI:10.1021/jo0204505

Syn th esis of Tw ist-Tw ist π-Con ju ga ted
Di-sec-a lk ylstilben es a n d Stilben e P olym er s
J ames E. Gano,* D. J . Osborn, III, Nagendra Kodali,
Padmanabhan Sekher, Min Liu, and Eddie D. Luzik, J r.
Department of Chemistry, Bowman-Oddy Laboratories,
University of Toledo, Toledo, Ohio 43606
jgano@frii.com
Received J uly 8, 2002
F IGURE 1. HOMO molecular orbital in a TTPC stilbene
showing the σ-π orbital mixing.
Abstr a ct: Twist-twist π-conjugated (TTPC) π systems
promise unique properties with their 90° twist angles. Di-
sec-alkyl substituted stilbenes, 5, were prepared by low-
valent titanium coupling of phenyl ketones, 4. Long alkyl
chains stopped the coupling reaction. Stilbenes 5 were shown
to be ∼90% TTPC. Inserting TTPC units into poly(p-
phenylene) polymers created highly fluorescent, soluble,
TTPC π systems with weak electronic segmentation for
organic light emitting diode (OLED) studies.
On the horizon are flexible displays,21 microdisplays,15
polarized displays,11 organic lasers,23 and organic sen-
sors.13
New materials fueled OLED advances. Herein is
described the first incorporation of the TTPC stilbene unit
into soluble polymers. Of fundamental theoretical inter-
est, conjugation in such a polymer is regular but weakly
segmented by the TTPC unit. Furthermore, blue light
emission, good stability, and minimal interchain quench-
ing are expected in OLED applications.
Sterically congested stilbenes with connected, perpen-
dicular, multiple, π systems, such as 1, are described
herein as twist-twist π conjugated (TTPC). The TTPC
HOMO depicted in Figure 1 reveals σ mixing with
adjacent π systems.29 Novel electronic properties are
expected.30-38 Introduction of TTPC features into poly-
mers promises to further expand the varied array of
materials that are facilitating rapid advances in OLED
technology.25,28,39,40
Stilbenes have played a central role in many aspects
of organic chemistry.1-3 Sufficiently large substituents
on the double bond rotate the phenyl groups 90° out of
the molecular plane creating twist-twist π conjugation
(TTPC).4,5 Novel features can result. tert-Butyl-substi-
tuted TTPC stilbenes have allowed unique mechanistic
investigations of bromine addition6,7 and phenyl rotation
in stilbene photochemistry.8
Electroluminescence in organic compounds was thrust
into prominence with the discovery of polymer lumines-
cence in a stilbene relative, poly(p-phenylene vinylene).9
Subsequent, rapid advances in organic light-emitting
diodes (OLED) led to commercial, flat-panel displays.10-28
(18) Scott, J . C.; Malliaras, G. G. The Chemistry, Physics And
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Based Light-Emitting Diodes; Academic Press: San Diego, 2001; p 1.
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Phys. Chem. 2001, 52, 391.
* To whom correspondence should be addressed: Voice mail: (970)-
493-3328. FAX: (419)530-4033.
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10.1021/jo0204505 CCC: $25.00 © 2003 American Chemical Society
Published on Web 04/05/2003
3710
J . Org. Chem. 2003, 68, 3710-3713

SCHEME 1. Stilben e Syn th esis
Investigation of the properties of extended TTPC
systems 2a ,b revealed solubilities rapidly decreased as
n increased.41 Preparation of 2c-h with increased phen-
ylene units only modestly raised the solubilities.42
This Note describes TTPC compounds of high molec-
ular weight, which retain the rigid TTPC geometry, have
structural flexibility, and are readily soluble.42,43
Previous attempts to prepare versions of 2a ,b with
extended alkyl chains to increase solubility failed. Modi-
fied TTPC systems with increased solubilities could be
prepared, in principle, by attachment of long alkyl groups
to 2 either on the benzene rings or on the tert-butyl
groups. TTPC systems are typically prepared by the low-
valent titanium44-46 coupling of alkyl phenyl ketones,
Scheme 1.47 However, 1-(2-methylphenyl)-2,2-dimethyl-
1-propanone failed to couple under low-valent titanium
conditions, demonstrating an alkyl group cannot be
extended from the ortho position of the benzene ring.
Although 1-(3-methylphenyl)-2,2-dimethyl-1-propanone
readily coupled,8 in subsequent steps the presence of the
meta substituent prevented low-valent titanium coupling
to extend the TTPC system to analogues of 2a ,b.41
Finally, adding one methyl or one heptyl to the tert-butyl
group produced other ketones that failed to couple.
The necessity to simultaneously reduce steric conges-
tion as extended chains are added to increase solubility
became evident. A reasonable compromise could be
achieved by replacing the tert-butyl group with a sec-alkyl
group. The simplest example of this series is diisopro-
pylstilbene.
TABLE 1. Ca lcu la ted Geom etr ies a n d En er giesa
a
Calculations used MacSpartan/AM1.
(E)-Diisopropylstilbene, 5a , has been prepared by low-
valent titanium coupling.48 Evidence indicated it pos-
sessed the desired 90° twisted TTPC geometry. Its UV
spectrum was blue shifted, compared to stilbene, with
NMR spectrum showed the characteristic upfield shifted
methyl resonance at δ 0.77.49 However, the smaller
isopropyl group may not lock the geometry of 5a at 90°
as effectively as the tert-butyl group in 1. This was
suggested by the low-valent titanium coupling results.
The preparation of 1 produced no cis isomer, but the
similar coupling to produce 5a produced 12% of cis isomer
6a . Unlike 1, additional low-energy conformers must be
considered for 5a . To gain insight into this issue, a
computational study of 5a and 6a was undertaken.
AM1 provided consistent structures and reasonable
relative energies for TTPC systems.29,50,51 AM1 calculated
heats of formation are shown for the conformers of 5a
and 6a in Table 1.52 The lowest energy conformer is
1
maxima at 225 (λ ) 7120) and 258 nm (λ ) 550).49 Its H
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Schoo, H. Synth. Met. 1997, 91, 109.
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M.; Wu, W. W.; Woo, E. P. Appl. Phys. Lett. 1999, 73, 629.
(41) Gano, J . E.; Kirschbaum, K.; Luzik, E. D.; Sekher, P. Tetrahe-
dron Lett. 1998, 39, 6641.
(42) Liu, M. MS thesis, University of Toledo, 1998.
(43) Osborn, D. MS thesis, University of Toledo, 2001.
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1996, 35, 2442.
5a syn,syn. Its phenyl groups were perpendicular,
dihedral
) 90.4°, to the double bond.
(45) McMurry, J . E. Chem. Rev. 1989, 89, 1513.
(46) Lenoir, D. Synth.-Stuttgart 1989, 883.
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4715.
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27, 5339.
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1987, 52, 5636.
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R. J . Org. Chem. 1991, 56, 6,
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J . Am. Chem. Soc. 1985, 107, 7,
J . Org. Chem, Vol. 68, No. 9, 2003 3711

Conformer 5a syn,anti is 1.84 kcal/mol less stable than
5a syn,syn. Furthermore, both 5a syn,anti and 6a syn,anti can be
formed in two ways, syn,anti and anti,syn. Thus, in any
equilibrium 5a syn,anti and 6a syn,anti enjoy an additional
entropy advantage over 5a syn,syn and 6a syn,syn. Although
one phenyl was still perpendicular to the double bond in
5a syn,anti, the other, flanked by small hydrogens on both
sides, was twisted only 70.6°. Calculated energies for
conformers of 6a showed a similar preference for the
syn,syn geometry.
Calculations performed with ZINDO predicted UV
absorptions at 217 (ꢀ 43 000) and 274 nm (ꢀ 1500) for
5a syn,syn and 221 (ꢀ 28 000) and 250 nm (ꢀ 17 000) for
5a syn,anti. The presence of 5a syn,anti in 5a syn,syn would be
revealed by its strong absorption band between the
measured bands for 5a syn,syn at 225 and 258 nm. The
absence of a major feature in this region suggested only
a small amount of 5a syn,anti could be present.
F IGURE 2. IV absorption (A, B, E) and fluorescence emission
(C, D) spectra for 5e (E), 9e (B, D), and p-quaterphenyl (A,
C). The UV absorption data are plotted, for comparison
purposes, at identical “chromophor” concentrations.
In the 240-nm region, 1, which has only one conforma-
tion, and 5a showed similar trends. Variable-tempera-
ture, UV absorption measurements of 5a revealed a small
absorbance increase at 240 nm from 0.37 to 0.47 as the
temperature was raised from 11 to 80 °C. Isosbestic
points were evident at 225 and 271 nm. Since the molar
absorptivity of 5a syn,anti was calculated to be 17 000 in the
240-nm region, these results supported the view that 5a
existed primarily as the syn,syn conformer with <10%
as the syn,anti conformer. Thus, any polymer containing
these TTPC units would have the syn,syn conformation
occasionally interspersed with syn,anti sites.
SCHEME 2. P olym er Syn th esis
A series of di-sec-alkylstilbenes was prepared. Since
replacing the methyl groups on 5a with C18 chains would
give excellent solubility, 5f was the initial target. Con-
sidering the versatility of the low-valent titanium reac-
tion, the coupling of 4f to produce 5f would seem to be
straightforward. However, numerous trials failed to
produce any 5f. Therefore, the molecular size was gradu-
ally increased by adding longer alkyl chains to ketone
4a to find suitable conditions. New stilbenes 5b-e were
prepared.
upon reaching the terminal -CH2CH3 units. Polymer 9c
showed low solubility. It was not investigated further.
GPC measurements on 9d ,e in THF with polystyrene
standards gave Mn ) 8.8 × 104 and Mw ) 3.1 × 105 for
9d and Mn ) 4.5 × 104 and Mw ) 1.6 × 105 for 9e. In
contrast, the molecular weight of tert-butyl-substituted
Ketones 4b-e were prepared by direct acylation of
benzene or alkylation of ketone 3. Low-valent titanium
coupling of these ketones in alumina-dried THF gave
stilbenes 5b-e in 12-80% yields. As observed for 5a ,
small amounts of 6b-e were evident before purification.
1
polymer 2h was estimated to be 6.7 × 103 from H NMR
determination of the number of repeat units, n ) 14.42
Polymers 9d ,e were readily soluble in organic solvents
and gave clear films of readily adjustable thickness when
coated on glass plates. Since the two different alkyl
substituents dictated 9d to be a ∼1:1 mixture of two
diastereomers, its high solubility was expected.
Thermal stability of stilbenes 5d ,e and polymers 9d ,e
was estimated by simultaneous TG-DTA under a flowing
stream of nitrogen. Following melting, 5d and 5e evapo-
rated cleanly at 320 and 340 °C, respectively. Polymer
9e showed no weight change until a 70% weight loss,
centered on 460 °C, occurred above 400 °C. Polymer 9d
showed a 5% weight loss at 130-380 °C followed by a
weight loss of 73% centered on 460 °C.
1
The UV and H NMR data supported the TTPC geom-
etries for 5b-e. The melting points for compounds 5a -e
were 165-168, 108-110, 110-112, 75-88, and 81-82
°C, respectively, which suggested increased solubilities
for derivatives of 5d ,e.
Polymers incorporating 5c-e units were prepared
according to Scheme 2.
Utilizing iodination in moist methylene chloride,53
iodostilbenes 7c-e were prepared from stilbenes 5c-e
in 79-82% yields. These were copolymerized with di-
borane 8 with use of the Suzuki coupling to give polymers
9c-e.54 Optimum polymer yields and highest molecular
weights utilized a toluene, DMF, water (2:2:1) mixed
solvent system.
The polymeric nature of 9d ,e was readily evident by
the broadened signals observed in the 13C NMR spectrum.
This broadening extended to the methylene units nearest
the polymer backbone. It only disappeared completely
The UV absorption spectra of 9e, 5e, and p-quater-
phenyl are compared in Figure 2. Introduction of the
p-quaterphenyl unit into a TTPC polymer created a 16-
(53) Sekher, P.; Gano, J . E.; Luzik, E. D. Synth. Commun. 1997,
27, 3631.
(54) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
3712 J . Org. Chem., Vol. 68, No. 9, 2003

(m, 2H), 1.50 (m, 2H), 1.24 (m, 16H), 0.84 ppm (t, J ) 6.40 Hz,
6H); 13C NMR (100 MHz) δ 204.7, 137.8, 132.7, 128.6, 128.1,
46.2, 32.5, 31.7, 29.5, 27.6, 22.6, 14.0 ppm; HRMS (Electrospray)
calcd for C20H320 + Na 311.2351, found 311.2356.
nm red shift (294 f 310 nm) and 8% increase in the
absorptivity. When tert-butyl groups replaced the sec-
alkyl groups as in 2h , a similar, 10-nm shift (294 f 304
nm) was observed.42 Comparison of p-quaterphenyl with
model compound 2g showed attachment of the C(tert-
butyl)C(tert-butyl)Ph group to the para and para′ posi-
tions of p-quaterphenyl produced an 8-nm red shift and
an increased absorptivity of 75%.
The fluorescence emission spectrum of 9e is compared
to the spectrum of p-quaterphenyl in Figure 2. Introduc-
tion of the p-quaterphenyl unit into a TTPC polymer
created an 18-nm red shift (363 f 381 nm) with some
broadening of the vibrational bands. Stilbene 5e (not
shown) did not fluoresce significantly. Polymer 9e gave
φ ) 0.55. This compared to φ ) 0.89 for p-quaterphenyl.55
When tert-butyl groups replaced the sec-alkyl groups as
in 2h , the fluorescence showed a 15-nm red shift (∼362
f ∼377 nm). Comparison of p-quaterphenyl with model
compound 2g showed the attachment of the C(tert-
butyl)C(tert-butyl)Ph group to the para and para′ posi-
tions of p-quaterphenyl produced an 8-nm red shift and
a negligible decrease in quantum yield.42
(E)-7,10-Dih exyl-8,9-diph en yl-8-h exadecen e, 5e. Zinc pow-
der (21 g, 0.32 mol) was added to a slurry of TiCl4 (18 mL, 0.16
mol) in dried THF (300 mL) at 0 °C. Under argon, 4e (23.0 g,
0.798 mol) in dry THF (30 mL) was added dropwise. The mixture
was refluxed for 48 h. The reaction was quenched with aqueous
K2CO3. Methylene chloride (200 mL) was added and the organic
layer was separated and filtered through a bed of alumina. The
solvent was removed under vacuum to obtain a viscous oil.
Addition of methanol (∼3 mL) led to the crystalline product (8.4
g, 39%); mp 81.0-81.5 °C; FTIR (KBr) 3058 (m), 736 (m), 696
1
(s) cm-1; MS (m/z) 544 (M+), 91 (100%); H NMR (400 MHz) δ
7.33 (t, J ) 7.20 Hz, 4H), 7.26 (t, J ) 8.00 Hz, 2H), 7.07 (d, J )
6.80 Hz, 4H), 2.16-2.06 (m, 2H), 1.40 (m, 4H), 1.26 (m, 16H),
1.14 (m, 16H), 0.99 (m, 4H), 0.89 ppm (t, J ) 6.80 Hz, 12H); 13
C
NMR (100 MHz) δ 142.2, 139.8, 130.2, 127.3, 126.0, 42.8, 33.7,
32.0, 29.6, 28.0, 22.7, 14.1 ppm; HRMS ESI calcd for C40H64
Na 567.4906, found 567.4903.
+
(E)-7,10-Dih exyl-8,9-d i(4-iod op h en yl)-8-h exa d ecen e, 7e.
To a solution of 5e (373 mg, 0.685 mmol) in 80 mL of wet
methylene chloride, prepared as described,53 was added iodine
(697 mg, 2.74 mmol), silver sulfate (430 mg, 1.37 mmol), and
sodium trifluoromethanesulfonate (8.0 mg, 0.040 mmol). The
reaction mixture was stirred at 30-35 °C for 4 h, cooled to room
temperature and quenched with 5% aqueous sodium meta-
bisulfite until the violet color disappeared. The mixture was
filtered through a silica gel column (60-80 mesh, 9 cm × 2.5
cm), and the layered filtrate was separated. The aqueous layer
was extracted with hexanes. The combined organic layer was
washed with water and saturated aqueous sodium chloride and
dried over anhydrous sodium sulfate. The solvent was removed
under vacuum, and the crude product was purified by flash
column chromatography (2.5 cm × 19 cm, 230-425 mesh silica
gel, hexanes) to give (447 mg, 82%) an off-white product: mp
82-83 °C; MS (m/z) 796 (M+, 100%); 1H NMR (400 MHz,) δ 7.66
(d, J ) 8.40 Hz, 4H), 6.79 (d, J ) 8.40 Hz, 4H), 2.06 (m, 2H),
1.28 (m, 20H), 1.12 (m, 12H), 1.02 (m, 8H), 0.89 ppm (t, J )
6.80 Hz, 12H); 13C NMR (100 MHz) δ 141.8, 139.1, 136.6, 132.0,
91.8, 42.9, 33.6, 32.0, 29.6, 28.0, 22.7, 14.1 ppm; HRMS calcd
for C40H62I2 796.2945, found 796.2923.
P r ep a r a tion of P olym er 9e. Recrystallized iodostilbene 7e
(398 mg, 0.500 mmol), biphenyl boronate ester 8 (203 mg, 0.500
mmol), Pd(PPh3)4 (19.3 mg, 0.0167 mmol), and K2CO3 (1.38 g,
10.0 mmol) were placed in a flask under argon and charged with
100 mL of degassed 40:40:20 toluene/DMF/H2O. After 78 h at
78 °C, the mixture was quenched with water and the organic
layer was separated. The polymer was precipitated into metha-
nol and then purified by Soxhlet extraction for 9 days with
toluene. The solution was reduced to 25 mL and precipitated in
500 mL of methanol. The precipitation was repeated with
acetone, providing 308 mg of a white solid: TGD/DTA mp 300-
800 °C dec; 1H NMR (400 MHz) δ 7.79, 7.68, 7.18, 2.27, 1.32,
1.23, 0.93 ppm (all peaks were broad); 13C NMR (100 MHz) δ
142.2, 140.0, 139.4, 139.1, 138.2, 130.7, 127.4, 125.9, 43.2, 33.9,
32.1, 29.7, 28.1, 22.8, 14.2 ppm.
In conclusion, the TTPC unit was incorporated into
readily soluble polymers with large fluorescence quantum
yields. Whereas two 7-tridecyl substituents on the stil-
bene units maintained good geometric integrity and
provided good solubility, two 5-nonyl substituents gave
poorly soluble materials. Very long substituents such as
octadecyl could not be incorporated by this route.
Exp er im en ta l Section
Routine mass spectra were obtained by using EI. Melting
points were uncorrected. The mode of THF purification was
critical to success during McMurry coupling reactions. HPLC
(0.005% water) grade was further dried by storing over 4 Å
molecular sieves and passing through a basic alumina column
directly into the reaction vessel.
Ketones 4c-e are known but not well described. Ketone 4b56
was prepared by direct acylation. Ketones 4c,57-59 4d ,60and 4e61
were prepared in 65-78% yield by alkylation of ketones 3c-e,
respectively. A typical procedure is described for 4e.
2-Hexyl-1-p h en yl-1-octa n on e, 4e. To a solution of 3e (40.8
g, 0.200 mol) and sodium amide (16 g of 50% w/w solution in
toluene, 0.21 mol, transferred quickly in open air) in 300 mL of
toluene was added 1-bromohexane (38.5 g, 0.232 mol) dropwise
under argon over 40 min. This mixture was refluxed for 12 h
and quenched with dilute HCl. The organic layer was separated
and passed through a basic alumina column (1:1 mixture, CH2-
Cl2 and hexanes). The solvent was removed under vacuum and
the crude product was purified by flash column chromatography
(silica gel, 230-425 mesh) with use of methylene chloride and
hexanes (1:19) followed by fractional distillation under vacuum
mm
mm
to obtain 4e (37.3 g, 64.6%): bp0.27
125-135 °C (lit.61 bp8
192 °C); FTIR (Neat) 1681 (s) cm-1; MS (m/z) 288 (M+), 105
(100%); 1H NMR (400 MHz) δ 7.95 (d, J ) 6.80 Hz, 2H), 7.55 (t,
J ) 7.20 Hz, 1H), 7.46 (t, J ) 8.00 Hz, 2H), 3.41 (m, 1H), 1.75
Ack n ow led gm en t. Financial Support was received
from the Ohio Board of Regents Research Challenge
Program and the University of Toledo URAFP Program.
HRMS measurements were supplied by the CCIC at
OSU. We thank D. C. Neckers, M. Coleman, D. Dolli-
more, T. Mueser, and W. Lee for helpful discussions and
measurements on GPC, spin coating, TGA, fluorescence,
and chemical precedent, respectively.
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Su p p or tin g In for m a tion Ava ila ble: Characterization of
new compounds and 1H NMR and 13C NMR spectra of
compounds 4e, 5b-e, and 7c-e. This material is available
free of charge via the Internet at http://pubs.acs.org.
J O0204505
J . Org. Chem, Vol. 68, No. 9, 2003 3713