Introduction of methyl groups on vinylene segments of phenylene vinylene systems: Synthesis and properties

Article abstract of DOI:10.1039/b909500d

A facile synthetic route towards phenylene vinylene systems with methyl substituents at the vinyl linkages is demonstrated through palladium catalyzed Heck coupling of 1,4-diisopropenylbenzene with aryl bromides. The coupling leads to a series of model co

Full text of DOI:10.1039/b909500d

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Introduction of methyl groups on vinylene segments of phenylene vinylene
systems: synthesis and properties†
Khai Leok Chan and Alan Sellinger*‡
Received 14th May 2009, Accepted 30th June 2009
First published as an Advance Article on the web 5th August 2009
DOI: 10.1039/b909500d
A facile synthetic route towards phenylene vinylene systems with methyl substituents at the vinyl
linkages is demonstrated through palladium catalyzed Heck coupling of 1,4-diisopropenylbenzene with
aryl bromides. The coupling leads to a series of model compounds that feature, in some cases, direct
access to a trans-configuration at the double bonds. The oxidative stability of these systems is
surprisingly unaffected by the presence of allylic C-H bonds in the methyl substituents. The methyl
substituents modulate the electronic properties of the phenylene vinylene systems by inducing a
significant twist to the conjugated backbone. The study suggests that the introduction of methyl groups
at vinylene sites is a viable alternative to functionalising and tuning the properties of phenylene vinylene
systems.
products consisting of largely inseparable mixtures of cis- and
Introduction
trans-stereoisomers19,24 or only the cis-isomers.25
Phenylene vinylene oligomers and the corresponding polymers
In this contribution, we demonstrate the viability of introducing
(PPVs) have been extensively investigated over the past decade as
methyl substituents at the vinyl linkages of phenylene vinylene
they exhibit favorable properties such as electroluminescence,1–4
through successful one-step synthesis and isolation of a series
photoconductivity,5 charge transport6 and nonlinear optical
behavior.7 To date, a large pool of work is devoted to improving
the electronic properties, color purity and processibility of these
of methyl-substituted diisopropenylbenzenes 1–6 using the Heck
coupling reaction of arylbromides with the stable and commer-
cially available 1,4-diisopropenylbenzene 7. We use 2D NOESY
materials via the introduction of functional or solubilizing groups
NMR experiments to confirm the trans,trans¢-configuration at the
on the phenyl rings and the vinyl linkages of the phenylene vinylene
vinyl linkage for selected products. We show that the oxidative
backbone. On the other hand, the color stability of these systems
stabilities of these model compounds are not affected by the
has also been vastly improved through appropriate substitutions
presence of the methyl groups, that are in an allylic position. This is
at the vinyl sites to reduce the double bond’s susceptibility towards
particularly interesting as it is contrary to the common assumption
photooxidation.8–11 For example, the introduction of cyano or
that allylic C-H bonds are highly susceptible to oxidation and
trifluoromethyl moieties has been found to reduce the probability
should be avoided in polymer semiconductors. We also explore
of oxygen attack on double bonds during excitation.12–14 Bulky
moieties such as phenyl groups have also been introduced at vinyl
sites to provide steric protection against the attack of oxygen.15
A natural extension of this strategy would be to introduce alkyl
groups at the vinyl linkages as this should, in principle, improve sol-
the effects of the methyl groups on the geometries and electronic
properties of the model oligomers via experimental and theoretical
modeling.
Results and discussion
ubility and offer an additional parameter for electronic- and color-
tuning of the phenylene vinylene system. However, there have been
very few reports of systems having such alkyl substitutions. This
is primarily due to synthetic difficulty, as conventional coupling
procedures for phenylene vinylene systems (such as Horner–
Wadsworth–Emmons,7,16 Siegrist,17 Heck,18,19 Suzuki-Miyaura20
and McMurray21–25 coupling) either do not offer the possibility
of alkyl addition at the vinyl site, or when they do, give rise to
Synthesis
The synthesis of alkyl substituted phenylene vinylenes was pre-
viously studied using Mukaiyama–McMurray coupling of alkyl
phenyl ketones, but the reactions were not amenable to hetero-
coupling, and gave mixtures of cis- and trans-stereoisomers that
were difficult to separate.24 A separate study using the Heck
reaction also yielded a mixture of cis- and trans-isomers and/or
internal and external olefins.19 Here, we employed Heck coupling
of 1,4-diisopropenylbenzene 7 with two molar equivalents of a
range of bromoarenes 8–13, and successfully obtained oligomers
1–6, as shown in Table 1. Compound 7 is an excellent starting
material to work with as it is a stable, inexpensive and pure
commercially available crystalline solid,26 as opposed to its
1,4-divinylbenzene analogue which is a very reactive, expensive
liquid available only in 85% purity.27 Reactions were performed
Institute of Materials Research and Engineering (IMRE), and the Agency
for Science, Technology and Research (A*STAR), 3 Research Link
Singapore 117602, Republic of Singapore
† Electronic supplementary information (ESI) available: Experimental
procedures, synthetic and spectroscopic data, density functional theory
calculation parameters. See DOI: 10.1039/b909500d
‡ Present address: Department of Materials Science and Engineering and
the Center for Advanced Molecular Photovoltaics (CAMP), Geballe Lab-
oratory for Advanced Materials, 476 Lomita Mall, Stanford University,
Stanford, CA 94305-4045, USA. E-mail: aselli@stanford.edu
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Table 1 Synthesis of diisopropenylbenzene derivatives 1–6 via Heck
reaction of 1,4-diisopropenylbenzene 7 with bromoarenes 8–13
that the isomers correspond to the four possible permutations
of double substitution, as illustrated in Fig. 1. Purification of
the crude reaction mixtures by column chromatography (using
a mixture of dichloromethane and hexane as the eluent) yielded
fractions containing mixtures of isomers. In all cases, we suspect
the desired trans,trans¢-isomers could be isolated from the isomer
mixtures by precipitation or recrystallisation in carefully selected
solvents. The products were successfully isolated in high purity
in yields of 28% (3), 8% (4), 14% (5) and 22% (6). In order to
confirm the trans,trans¢-isomer we ran 2D NOESY NMR on
(6) as it has the best solubility (see ESI). The results clearly
show that the methyl hydrogens only have interactions with the
hydrogens on the adjacent phenyl rings, and no interactions with
the adjacent vinylene hydrogen. Although this result only confirms
the trans,trans¢-isomer for (6), we highly suspect the isolated
fractions of the other compounds also have the same trans,trans¢-
isomer.
Entry
1
Ar–Br
Product
Yield
66
2
71
3
4
28
8
5
6
14
22
Fig. 1 The four possible stereo- and regio-isomers of di-substituted
diisopropenylbenzene from Heck coupling reaction.
The marked difference in the stereo- and regio-selectivity of
the Heck coupling between diisopropenylbenzene 7 and the two
groups of monomers is surprising. We surmise that in the case of
the bulky 9-bromoanthracene 9, steric factors may play a part in
directing the coupling for the trans-orientation. The relatively low
solubility of 1 and 2 in toluene also suppresses further substitutions
of the di-substituted products once they are formed.
◦
in toluene at 100 C using bis(tri-t-butylphosphine)palladium(0)
[Pd[P(tBu)3]2] (1.0 mol%) as catalyst and dicyclohexylmethylamine
as base/HBr scavenger.28 The reactions were monitored by thin
layer chromatography, gas chromatography mass spectrometry
(GCMS) and matrix assisted laser desorption ionization-time-of-
flight (MALDI-TOF) spectrometry.
Physical and optical properties
The thermal behavior and stability of the materials were
investigated by differential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA), respectively. The results are
summarized in Table 2. All the materials melt above 215 ◦C
We observed markedly different degrees of coupling for the
various reactions. For example, the coupling between 7 and either
1-bromonaphthlene 8 or 9-bromoanthracene 9 were found to
be highly stereo- and regio-selective. The desired trans,trans¢-
isomers 1 and 2 were formed almost exclusively, with little or no
trace of other isomers as observed in GCMS. Upon cooling the
reaction mixture, both 1 and 2 were found to precipitate from their
respective mixtures. Filtration of the reaction mixture followed by
sequential washing of the residues with copious amounts of water
and hexane furnished 1 and 2 in 66% and 71% yields respectively.
The yields were satisfactory considering that they reflect the overall
percentages from two individual Heck reactions.
In contrast, the coupling of 7 with monomers 10–13 was found
to give products with molecular weights corresponding to mono-,
di-, and tri-substituted diisopropenylbenzenes, as evidenced from
MALDI-TOF and GCMS analyses. In all cases, di-substituted
diisopropenylbenzenes were the major products. GCMS analysis
further revealed that the di-substituted components typically
consisted of four distinct stereo- and regio-isomers. We believe
Table 2 Optical and physical properties of compounds 1–6
UV-vis
Bandgap PL lmax
Compound lmax (nm)a
(eV)b
(nm)c
Tm (◦C)d
Td (◦C)d
1
2
3
4
5
6
323
372, 391
379
356
342
3.30
2.92
2.78
2.97
3.13
2.97
430
470
446
467
437
460
215–218
300–305
59–62
234–237
248–250
243–245
306
380
428
390
353
362
362
a Measured in a chloroform solution. b Measured from the absorption
onset of the UV-vis spectrum. c Obtained from DSC measurement.
d Obtained from TGA measurement (temperature at 5% mass loss under
nitrogen, 10 ◦C min-1 ramp rate).
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with the exception of compound 3 (59–62 ◦C) which con-
tains long alkyl chains. The thermal decomposition temper-
atures (5% mass fraction loss) of all compounds are above
350 ◦C, indicating that the materials have relatively high thermal
stabilities.
measurement, indicating that degradation due to photoexcitation
of the films occurred during each measurement.
The intensities of the emissions, measured at wavelengths cor-
responding to the emission peaks, were plotted for each successive
measurement relative to that of the first measurement (Fig. 3, see
Supporting Information for details). The rates of degradation in
compounds 6 and 14 were found to be comparable, which suggests
that the degradation mechanisms in both compounds may be
similar. Given that compound 14 does not contain allylic C–H
bonds, this suggests that allylic hydrogens in 6 do not contribute
significantly (if at all) to the degradation process of the compound,
and that the degradation pathway of 6 may be independent of the
additional methyl group.
The photophysical properties of the compounds were measured
by UV-visible and fluorescence spectroscopy in chloroform solu-
tion. The compounds show strong absorption and emission in the
320–400 nm and 430–470 nm ranges, respectively (Fig. 2, Table 2).
The relatively narrow ranges and high energy band gaps across the
series of compounds suggest that p-conjugation in the phenylene
vinylene backbones may have been attenuated as the backbones
take on a more non-planar conformation to accommodate the
methyl substituents. For example, the photoluminescence spec-
trum of compound 6 shows a single band with lmax = 464 nm
whereas that of the corresponding non-methyl-substituted p-bis(p-
N,N-diphenylaminostyryl)benzene 1429 (see structure in Fig. 4)
shows two red-shifted peaks at lmax = 470 nm (major) and 530 nm
(minor).30 These observations are also consistent with previous
reports of sterically congested stilbenes.23
Fig. 3 Relative photoluminescence intensities of emission peaks recorded
over successive photoluminescence measurements of compounds 6 (blue
square) and 14 (red circle) in solid state (excitation at 400 nm). The plots
show that the rates of photo-oxidative degradation of vinyl-alkylated (6)
and non-alkylated (14) phenylenevinylene systems are comparable.
Theoretical study on geometry
To further investigate the effect of the methyl group on the
geometries of the compounds, we performed density functional
theory calculations on compound 6 and the corresponding non-
substituted p-bis(p-N,N-diphenylaminostyryl)benzene 14.29 The
geometries of 6 and 14 were fully optimized in the generalized
gradient approximation with nonlocal exchange and correla-
tion functions according to Perdew–Wang (GGA–PW91).33 The
phenylene vinylene backbone of 6 was found to be significantly
twisted from planarity, with dihedral angles between the benzene
rings and the double bonds ranging from 34.1◦ [U(3,4,5,6)] to 26.5◦
[U(7,8,9,10)]. In contrast, the backbone of DSA-Ph 14 was almost
planar, with the largest dihedral angle found to be U(1¢,2¢,3¢,4¢) =
1.6◦ (Fig. 4a).
Fig. 2 (a) UV-vis absorption and (b) photoluminescence spectra of
compounds 1–6 in chloroform solution.
Oxidative stability
Studies have shown that allylic C–H bonds can undergo facile
proton abstraction and trigger oxidative degradation processes in
organic molecules.31,32 The introduction of methyl groups on the
vinyl linkages of compounds 1–6, and the implicit introduction
of allylic protons, may therefore undermine the oxidative stabil-
ities of these compounds. We investigated the effect of methyl
substitution on photo-oxidative stability by subjecting drop-cast
thin films of 6 and 14 (without allylic C–H bonds) to repeated
photoluminescence measurements at an excitation wavelength of
400 nm under ambient conditions. The emission intensities of
both compounds were found to decrease with each successive
Analysis of the HOMO and LUMO density maps of the systems
further highlighted the effects of methyl substituent on the degree
of conjugation in 6. Compared with 14, the p-orbital contribution
from the benzene rings to the HOMO frontier orbitals in 6
is reduced whereas the electron density on the double bonds
is enhanced, as illustrated in Fig. 4b. The skewed distribution
of electron clouds among the conjugated segments in 6 clearly
illustrates an appreciable disruption in the conjugation of the
system. This readily explains the wider bandgap observed in
compound 6 (2.97 eV, Table 2) compared with the bandgap
reported for 14 (2.70 eV).29
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Fig. 4 The (a) optimized geometries, and (b) HOMO and (c) LUMO density maps of 6 and 14, calculated using density functional theory in generalized
gradient approximation with nonlocal exchange and correlation functions according to Perdew–Wang (GGA–PW91).
chromatography-mass spectrometry (GCMS) was carried out on
a Varian Chrompack 3800 gas chromatograph coupled to a Varian
Conclusion
We have demonstrated a novel method of introducing methyl
4000MS mass spectrometer detector.
substituents at the vinylene segments in phenylene vinylene
All commercially available materials were used as received unless
systems through a one-step Heck coupling reaction between
otherwise noted. All reactions were carried out using Schlenk
the stable and commercially available 1,4-diisopropenylbenzene
techniques in an argon or nitrogen atmosphere with anhydrous
with arylbromides, while achieving the trans-configuration at the
double bond. This method opens the door to new methods
solvents. Column chromatography was carried out on silica gel
[Merck Kieselgel 60 (230–400 mesh)]. TLC was performed on
of tuning the color and electronic properties, while potentially
0.25 mm thick plates precoated with Merck Kieselgel 60 F254 silica
enhancing the solubility and color stability of phenylene vinylene
gel, and visualised by UV light (254 and 366 nm) and cerium(IV)
systems. We are currently studying these materials for potential
sulfate solution.
application in organic electronic devices such as light emitting
diodes (OLED), solar cells, (OPV) and thin film transistors
(OTFT).
General procedure for the synthesis of compounds 1–6
To a mixture of 1,4-diisopropenylbenzene (7) (1,4-di(prop-1-
en-2-yl)benzene) (200 mg, 1.26 mmol, 1 equiv.), aryl bromide
(2.52 mmol, 2 equiv.) and Pd[P(t-Bu)3]2 (32 mg, 63 mmol) in
argon was added a degassed solution of dicyclohexylmethylamine
(0.68 cm3, 3.16 mmol, 2.5 equiv.) in toluene (15 cm3). The resulting
mixture was stirred at 100 ◦C for 48 hours, cooled to room
temperature and quenched with distilled water (20 cm3). For
compounds 1 and 2, precipitates were observed upon cooling
of the reaction mixture. The product was obtained by filtration
of the mixture followed by washing of the residue with copious
amount of distilled water. For compounds 3–6, the product was
extracted into dichloromethane (3 ¥ 30 cm3) and the organic
layer washed with dried with anhydrous MgSO4 and removed
via rotary evaporation. The product was then purified by column
chromatography followed by recrystallisation in a mixture of
hexane and dichloromethane.
Experimental
General
1H and 13C NMR data were performed on a Bruker DPX
400 MHz spectrometer with chemical shifts referenced to residual
CHCl3 in CDCl3. Matrix assisted laser desorption/ionization
time-of-flight (MALDI-TOF) mass spectra were obtained on a
Bruker Autoflex TOF/TOF instrument without the use of matrix.
Differential scanning calorimetry (DSC) was carried out under
nitrogen on a TA Instrument DSC Q100 instrument (scanning
◦
rate of 20 C min-1). Thermal gravimetric analysis (TGA) was
carried out using◦ a TA Instrument TGA Q500 instrument
(heating rate of 10 C min-1). UV–vis spectra were recorded on a
Shimadzu model 2501-PC. Photoluminescence (PL) spectra were
measured on a Perkin-Elmer (LS50B) spectrofluorimeter. Gas
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1,4-Bis((E)-1-(naphthalen-1-yl)prop-1-en-2-yl)benzene (1)
4,4¢-(1E,1¢E)-2,2¢-(1,4-Phenylene)bis(prop-1-ene-2,1-
diyl)bis(N,N-diphenylaniline) (6)
Off-white solid (345 mg, 66%); mp 215–218 ◦C; Anal. Calcd. for
C32H26: C, 93.62; H, 6.38. Found: C, 93.59; H, 6.41; H NMR
Pale green solid (177 mg, 22%); mp 243–245 ◦C; Anal. Calcd. for
C48H40N2: C, 89.40; H, 6.25; N, 4.34. Found: C, 89.53; H, 6.37; N,
4.10; 1H NMR (400 MHz, CDCl3) d 1.52 (6H, s, CH3), 6.82 (2H, s,
1
=
(400 MHz, CDCl3) d 2.19 (6H, s, CCH3), 7.37 (2H, s, C CH),
7.46–7.54 (8H, m, ArH), 7.72 (4H, s, ArH), 7.82 (2H, d, J = 8.0,
ArH), 7.89–7.91 (2H, m, ArH), 8.05–8.08 (2H, m, ArH); 13C NMR
(100 MHz, CDCl3) d 17.8, 125.6, 125.7, 125.8, 126.2, 126.3, 126.4,
127.1, 127.7, 128.9, 132.6, 134.0, 136.1, 138.8, 142.6; MALDI-
TOF-MS m/z: 410.1780; calcd for C32H26 = 410.2035.
=
C CH), 7.00–7.18 (16H, m, ArH), 7.25–7.30 (12H, m, ArH), 7.52
(4H, s, ArH); 13C NMR (100 MHz, CDCl3) d 18.0, 123.3, 123.7,
124.8, 126.2, 12.5, 129.7, 130.5, 133.0, 136.2, 143.2, 136.6, 148.1;
MALDI-TOF-MS m/z: 644.3190; calcd for C48H40N2 = 644.3191.
1,4-Bis((E)-1-(anthracen-9-yl)prop-1-en-2-yl)benzene (2)
Acknowledgements
Yellow solid (456 mg, 70%); mp 300–305 ◦C; Anal. Calcd. for
1
The authors acknowledge the Institute of Materials Research and
Engineering (IMRE) and the Visiting Investigatorship Programme
(VIP) of the Agency for Science, Technology and Research
(A*STAR), Republic of Singapore, for financial support. We
thank Prof. Ananth Dodabalapur and Dr. Prashant Sonar for
valuable discussions, Dr. Kian Soon Yong for help in DFT calcu-
lations, and Dr. Pramoda Kumari Pallathadka and Ms Nguyen
Thi Thuy Linh for running 2D NOESY NMR experiments. We
also thank Dr. Christopher Kohl for his early contributions on
this work.
C40H30: C, 94.08; H, 5.92. Found: C, 94.08; H, 5.92; H NMR
=
(400 MHz, CDCl3) d 1.91 (6H, s, CCH3), 7.59 (2H, s, C CH),
7.48–7.53 (8H, m, ArH), 8.06–8.08 (8H, m, ArH), 8.20–8.22 (8H,
m, ArH), 8.46 (2H, s, ArH); 13C NMR (100 MHz, CDCl3) d
18.0, 124.2, 125.6, 125.9, 126.5, 126.6, 129.2, 130.2, 131.9, 133.3,
140.9, 142.2; MALDI-TOF-MS m/z: 510.2010; calcd for C40H30
510.2348.
=
4,4¢-(1E,1¢E)-2,2¢-(1,4-Phenylene)bis(prop-1-ene-2,1-diyl)bis(N-
hexadecyl-1,8-naphthalimide) (3)
Pale green solid (350 mg, 28%); mp 59–62 ◦C; Anal. Calcd. for
C68H88N2O4: C, 81.88; H, 8.89; N, 2.81. Found: C, 81.45; H, 8.73;
References
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CH2CH3), 1.20–1.48 (56H, m, CH2), 1.70–1.80 (4H, s, NCH2CH2),
2.22 (6H, s, CCH3), 4.20 (4H, d, J = 7.6, NCH2), 7.39 (2H, s,
=
C CH), 7.51–7.78 (8H, m, ArH), 8.40 (2H, d, J = 8.8, ArH),
8.63–8.68 (4H, m, ArH); 13C NMR (100 MHz, CDCl3) d 14.5, 18.1,
23.1, 27.6, 28.6, 29.8, 32.3, 40.9, 121.9, 123.6, 124.3, 126.7, 127.1,
128.1, 128.9, 131.0, 131.1, 131.6, 131.7, 141.6, 142.5, 142.9, 164.5,
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996.6744.
1,4-Bis((E)-1-(pyren-1-yl)prop-1-en-2-yl)benzene (4)
Yellow solid (56 mg, 8%); mp 234–237 ◦C; Anal. Calcd. for
1
C44H30: C, 94.59; H, 5.41. Found: C, 94.22; H, 5.78; H NMR
=
(400 MHz, CDCl3) d 2.26 (6H, s, CCH3), 7.69 (2H, s, C CH),
7.83 (4H, s, ArH), 7.00–7.04 (4H, m, ArH), 8.10–8.14 (6H, m,
ArH), 8.20–8.23 (6H, m, ArH), 8.31 (2H, d, J = 8.0, ArH); 13C
NMR (100 MHz, CDCl3) d 17.9, 124.9, 125.2, 125.3, 125.4, 125.5,
126.3, 126.4, 126.5, 127.6, 127.7, 127.8, 129.6, 130.8, 131.5, 131.9,
133.9, 139.2, 142.7; MALDI-TOF-MS m/z: 558.1760; calcd for
C44H30 = 558.2348.
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1,4-Bis((E)-1-(9,9-dimethyl-9H-fluoren-2-yl)prop-1-en-2-
yl)benzene (5)
Pale green solid (112 mg, 14%); mp 248–250 ◦C; Anal. Calcd. for
1
C42H38: C, 92.94; H, 7.06. Found: C, 92.93; H, 7.07; H NMR
(400 MHz, CDCl3) d 1.53 (12H, s, C(CH3)2), 2.39 (6H, s, CCH3),
=
7.00 (2H, s, C CH), 7.30–7.48 (8H, m, CH2), 7.59 (4H, s, ArH),
7.74 (2H, d, J = 8.0, ArH); 13C NMR (100 MHz, CDCl3) d 18.0,
27.6, 47.2, 120.1, 120.4, 123.0, 123.8, 126.3, 127.4, 127.5, 128.5,
128.6, 137.0, 137.9, 138.1, 139.4, 143.2, 154.0, 154.3; MALDI-
TOF-MS m/z: 542.2780; calcd for C42H38 = 542.2974.
3980 | Org. Biomol. Chem., 2009, 7, 3976–3981
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