Electron-accepting conjugated materials based on 2-Vinyl-4,5- dicyanoimidazoles for application in organic electronics

Article abstract of DOI:10.1021/jo802720m

We report the Heck coupling of 2-vinyl-4,5-dicyanoimidazole (vinazene) with selected di- and trihalo aromatics in an effort to prepare linear and branched electron-accepting conjugated materials for application in organic electronics. By selecting the sui

Full text of DOI:10.1021/jo802720m

Electron-Accepting Conjugated Materials Based on
2-Vinyl-4,5-dicyanoimidazoles for Application in Organic Electronics
Richard Y. C. Shin, Prashant Sonar, Pei Sann Siew, Zhi-Kuan Chen,* and Alan Sellinger†,*
Institute of Materials Research and Engineering (IMRE) and the Agency for Science, Technology and
Research (ASTAR), 3 Research Link, 117602, Republic of Singapore
aselli@stanford.edu; zk-chen@imre.a-star.edu.sg
ReceiVed December 18, 2008
We report the Heck coupling of 2-vinyl-4,5-dicyanoimidazole (vinazene) with selected di- and trihalo
aromatics in an effort to prepare linear and branched electron-accepting conjugated materials for application
in organic electronics. By selecting the suitable halo-aromatic moiety, it is possible to tune the
HOMO-LUMO energy levels, absorption, and emission properties for a specific application. In this
regard, materials with strong photoluminescence from blue f green f red are reported that may have
potential application in organic light-emitting diodes (OLEDs). Furthermore, derivatives with strong
absorption in the visible spectrum, coupled with favorable HOMO-LUMO levels, have been used to
prepare promising organic photovoltaic devices (OPVs) when combined with commercially available
semiconducting donor polymers.
Introduction
effect transistors (OFETs),8-17 and photovoltaic cells (OPVs).18-27
Both electron-donor (hole-transporting) and electron-acceptor
(electron-transporting) materials with thermal/chemical stability,
solution processability, charge carrier mobility, and controllable
HOMO-LUMO energy levels are essential for these applica-
tions. With regard to new materials development, to date most
studies have focused on donor semiconductors based on
aromatic amines and thiophenes, with significantly less work
being reported on new acceptor materials.28-35 For example,
Over the past two decades there have been extensive efforts
to develop organic semiconductors due to their potential for
application in organic light-emitting diodes (OLEDs),1-7 field
† Present address: Geballe Laboratory for Advanced Materials, Department
of Materials Science and Engineering, 476 Lomita Mall, Stanford University,
Stanford, CA 94305-4045.
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10.1021/jo802720m CCC: $40.75
Published on Web 04/03/2009
2009 American Chemical Society
J. Org. Chem. 2009, 74, 3293–3298 3293

Shin et al.
cyano phenylvinylene polymers (CN-PPVs), perylene dianhy-
drides and diimides, and fullerenes are the most widely studied
acceptor materials in the areas of OPVs and OFETs. Since the
mid-1990s, fullerene compounds have been the most utilized
acceptor materials for use in solution processable OPVs provid-
ing power conversion efficiencies (PCE) in the range of 2-5%
when combined with selected donor materials.26,36-38 Despite
their success, fullerenes tend to have low absorption coefficients
in the visible range, are difficult to synthesize and purify, are
very expensive, and lead to devices with relatively low open
circuit voltages (Voc).
Recently, several groups have reported high-performance
accepting materials by introducing cyano or fluoro electron-
withdrawing substituents into π-conjugated systems mostly for
application in OFETs39-45 with very few new acceptors being
reported for application in OPVs. Along these lines, our group
has previously reported on novel solution processable accepting
materials based on 2-vinyl-4,5-dicyanoimidazoles (vinazene).46,47
As an extension of this work, we herein report a more complete
synthesis and characterization study of this family of n-type
conjugated materials based on vinazene where by simply
changing the central aromatic segments we could easily tune
the optical properties and HOMO-LUMO energy levels of the
resultant products. Previously, vinazene has been used as a
monomer and/or comonomer in the preparation of dicyanoimi-
dazole-containing polymers with high nitrogen content.48-50 To
the best of our knowledge, we have been the first to synthesize
4,5-dicyanoimidazole-based derivatives for use in organic
electronic applications.
Results and Discussion
Synthesis. The vinazene-based n-type materials were syn-
thesized through the Heck coupling reaction between 1-alkyl-
2-vinyl-4,5-dicyanoimidazole (1-alkylvinazenes) with selected
di- and tribromoaromatics. The synthetic procedure is shown
in Scheme 1a-1b.
Alkylation of vinazene with various alkyl bromides and
iodides (butyl and hexyl) in DMF or acetone using K2CO3 as
base gave the desired products in good yield after recrystalli-
zation in ethanol. Vinazene with a branched alkyl group at the
1-position (ethylhexyl) was also synthesized to further enhance
the solubility of the final n-type products. Ethylhexyl vinazene
was obtained as an oil after purification by column chromatog-
raphy, which solidified on cooling. This material was found to
be unstable if stored at room temperature for a few days, readily
converting into a brown insoluble product due to photoinitiated
polymerization of the active vinyl group on vinazene.49
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The Heck coupling reaction between 1-alkylvinazene and
aromatic bromides was carried out in anhydrous DMF using
bis(tri-tert-butylphosphine)palladium(0) [Pd[P(tBu)3]2] as cata-
lyst and dicyclohexylmethylamine (NCy2Me) as base/HBr
scavenger (Scheme 1b).51 The final products were obtained by
precipitation of the reaction mixture into ethanol followed by
recrystallization of the crude product in DMF/ethanol or THF/
ethanol. The yields range from 75 to 88%, except for compounds
2, 7, 9, 12, 13, and 14. The lower yields (41% to 55%) for
these compounds are mainly due to the relatively poor solubility.
In selected cases, we used ethylhexylvinazene to increase the
product solubility as compared to their hexylvinazene analogues.
Trivinazene substituted products 15 and 16 have also been
synthesized using similar reaction conditions with 1,3,5-
tribromobenzene and 1,3,5-tris(4-bromophenyl)benzene as the
core moiety in yields of 58% and 87%, respectively, as shown
in Scheme 2.
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The structure and purity of all the final products have been
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3294 J. Org. Chem. Vol. 74, No. 9, 2009

Electron-Accepting Conjugated Materials
SCHEME 1. (a) Alkylation of Vinazene and (b) Heck Couplings of Aryl Dibromides with Alkylvinazene
SCHEME 2. Heck Couplings of Aryl Tribromides with Alkylvinazene
Optical Properties. The optical properties of all the com-
pounds were measured by UV-vis and fluorescence spectros-
copy in toluene and are shown in Table 1. The absorption spectra
of the compounds provide information on the extent of
π-conjugation between the central aromatic segment and the
linked alkylvinazenes. The representative absorption and emis-
sion spectra for compounds 6, 11, 12, and 15 are shown in
Figure 1.
The π-π* transition bands of these compounds are located
in the 300-600 nm range with the remaining absorption peaks
located in 331-520 nm region. These values are strongly
dependent on the nature of the central aromatic ring. For
example, compound 1 with a phenylene bridge showed a
maximum absorption peak at 376 nm. When the phenylene
group is replaced with biphenylene (compound 2) or binaph-
thalene (compound 7), the absorption peaks are blue-shifted to
369 and 362 nm, respectively, due to the twisted conformation
between the two phenyl/naphthyl rings resulting in disruption
of conjugation. However, when the biphenyl rings are fixed in
a more planar configuration, such as 3 (fluorene), the overall
conjugation length is increased thereby causing a red shift of
λmax by 21 nm (λmax: 390 nm) compared to compound 2. When
the linkage of the central group is shifted from the para to the
meta position (compounds 4 and 5), the absorption bands are
also significantly blue-shifted.
In addition to planarity, the electron-donating nature of the
central aromatic group also plays a crucial role in the optical
properties. For example, when the bithienylene or thienothiophene
groups are used as building blocks (compound 12 and 13), red
shifts of 40-60 nm are observed compared to compound 3.
J. Org. Chem. Vol. 74, No. 9, 2009 3295

Shin et al.
TABLE 1. Physical Properties of Vinazene Derivatives
a
a
b
c
Tg
(°C)
Tm
(°C)
Td
(°C)
UV-visλmax
UV-vis onset
HOMO/LUMOf
(eV)
bandgap (eV)
(from CV)f
bandgap (eV)
(from UVonset)
c
g
compd
(nm)
(nm)
PL λmax (nm)
Φd
1
2
3
4
5
6
7
8
59
na
na
na
na
na
89
53
86
na
na
na
na
45
na
115
250
208
238
226
257
215
na
382
402
387
410
414
396
393
376
343
381
400
382
400
300
405
400
376
369
390 (409)
321, 386
331
392
362
422
516
337, 448
390, 520
450
430, 448
379
421
419
435
431
400
468
418
508
596
517
628
519
482
430
380
396
(413) 437
(417) 442
427, 453
426 (455)
397, 411
467, 496
445
592
638
516
606
506, 541
476, 507
443
399 (425)
(408) 436
0.56
0.76
0.74
0.05
0.02
0.57
0.09
0.29
0.07e
0.48
0.13e
0.19
0.09
0.69
0.36
0.12
-6.00/-2.98
-5.95/-2.85
-5.82/-2.85
-5.47/-
3.02
3.10
2.97
2.95
2.96
2.85
2.88
3.10
2.65
2.97
2.44
2.08
2.40
1.97
2.39
2.58
2.91
3.26
3.13
-5.83/-2.76
-5.83/--3.10
-5.85/-2.78
-5.61/-3.26
-5.34/-3.41
-6.02/-3.50
-5.52/-3.45
-5.57/-3.09
-5.70/-3.10
-5.60/-2.90
-/-2.93
3.07
2.73
3.07
2.35
1.93
2.52
2.07
2.48
2.60
2.70
279
na
9
10
11
12
13
14
15
16
205
266
244
264
na
247
na
331
351
-5.99/-2.79
3.20
a Obtained from DSC measurements. b Obtained from TGA measurements (temperature at 5% weight loss). c Measured in a toluene solution. d Using
quinine sulfate as standard. e Using rhodamine 6G as standard. f Calculated from CV data. g Calculated from UV-vis absorption spectral band edge. na
) not detected. Data in parentheses are shoulders in UV and PL spectra.
acceptor structure. The lower band gap of such an alternating
structure is attributed to the increase of quinoidal character of
the ground state in the molecule.54-56 Following this strategy,
a central segment of 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole
was incorporated to form a donor-acceptor (D-A-D) structure
(compound 11), which showed a maximum absorption of 520
nm. The band gap calculated from the absorption onset of this
compound (11) is 1.97 eV (which is the lowest band gap among
all materials in this study). From the above discussion, it is clear
that by changing the central moiety one can effectively tune
the band gap of materials using structure-property relationships.
The range of absorption bands as well as energy levels (from
wide band gap to low band gap materials) of all these new n-type
materials (acceptor) provide more opportunities to match p-type
materials (donor) for OPV applications, as will be demonstrated
later in this manuscript.
The fluorescence spectra of selected materials are shown in
Figure 1. Compounds 9 and 11 exhibit strong red photolumi-
nescence (PL) emission at 638 and 606 nm, respectively. This
is attributed to the interaction of the HOMO and LUMO of the
donor and acceptor segments respectively within the molecule.
The PL quantum efficiency also depends strongly on the central
aromatic group (see values in Table 1). For example, high PL
efficiencies were obtained with the central aromatic groups such
as 2,5-pyridyl, biphenyl, and fluorene (69-76%), moderate
efficiencies with phenylene, benzothiadiazole, and naphthalene
(48-57%), and low efficiencies with 3,6-carbazole, 3,6-diben-
zothiophene, 2,2′-dimethylbinaphthyl, and tetracene (2-9%).
Electrochemical Properties. Cyclic voltammetry (CV) was
used to determine the electrochemical properties of the materials.
All CV measurements were recorded (Figures S1-S16 Sup-
porting Information) in dichloromethane with 0.1 M tetrabuty-
lammonium hexafluorophosphate as supporting electrolyte (scan
rate of 100 mV s-1). The experiments were performed at room
temperature with a conventional three-electrode configuration
consisting of a platinum wire working electrode, a gold counter
electrode, and a Ag/AgCl in 3 M KCl reference electrode. The
measured potentials were converted to SCE (saturated calomel
FIGURE 1. UV-vis and PL spectra of selected compounds in toluene.
This large red shift is ascribed to the electron-donating effect
of the sulfur atom and the more coplanar configuration of the
bithienyl/thienothiophene rings. The effective conjugation length
can be also controlled by using fused aromatic groups such as
naphthalene, anthracene, and tetracene. For example, when 1,4-
naphthalene (compound 6), 9,10-anthracene (compound 8), or
5,11-tetracene (compound 9) is used as the central groups, the
absorption peaks are red-shifted by 16, 46, and 140 nm,
respectively, in comparison with compound 1.
The various combinations of central cores with the vinazene
acceptor offer a unique and versatile method to prepare materials
with tunable band gaps for application in organic photovoltaics.
For example, lower band gap materials have a higher potential
to harvest photons from sunlight.52,53 For the materials synthe-
sized here, band gaps are tuned from 2.95 eV (wide band gap
material) to 2.08 eV (low band gap material) by selecting the
appropriate central aromatic moiety. It is well known that the
band gaps of conjugated molecules can be lowered by mixing
different electron-donating and electron-withdrawing blocks in
the conjugated backbone to form an alternating donor and
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E. W. Mater. Sci. Eng. R. 2001, 32 (1), 1–40.
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3296 J. Org. Chem. Vol. 74, No. 9, 2009

Electron-Accepting Conjugated Materials
TABLE 2. Organic Solar Cell Device Data Using Compound 10
with Selected Donor Polymers
P3HT:10
(hexyl)
A-PPE-PPV:10
PCz:10
(ethylhexyl)
(hexyl)
anneal temp (°C)
Voca (V)
140
0.67
1.79
37.3
14.0
0.45
80
0.98
1.20
35.0
12.2
0.42
80
1.36
1.14
49.0
18.5
0.75
Isca (mA/cm2)
fill factor (%)
IPCEa max (%)
PCEa (%)
a Voc is open circuit voltage; Isc is short-circuit current density; IPCE
is incident photon to conversion efficiency; PCE is power conversion
efficiency.
FIGURE 2. Selected donor polymers used with compound 10 for
The first-order screening process involved selecting materials
with suitable HOMO and LUMO levels to match with com-
mercially available donor polymers such as regioregular poly(3-
hexylthiophene) (P3HT), which has reported HOMO-LUMO
levels of -5.2 and -3.3 eV, respectively. Based on this
criterion, suitable vinazene-based compounds included com-
pounds 8-13, which were all tested in OPV devices with P3HT.
Of these six materials, compound 10 showed the best perfor-
mance and was selected for further optimization. For the
convenience of the reader, we have compiled published OPV
device data based on compound 10 in Table 2. Details of the
device preparation and testing can be found in the provided
references. Although the initial PCEs reported here are roughly
5× lower than those achieved by C60-based fullerene-based
acceptors (PCBM) combined with donor polymers such as
regioregular poly(3-hexylthiophene) P3HT, our materials show
some very promising properties. For example, a majority of the
white light illumination (solar simulator, AM 1.5 and 100mW/
cm2) is absorbed by compound 10 and not the donor polymer.
This is in contrast to the traditional P3HT:PCBM system where
P3HT absorbs most of the light. Furthermore, the LUMO level
of compound 10 (-3.50 eV) is 0.5 eV lower than that of PCBM
(-4.00 eV) resulting in higher Voc values for OPV devices.
This can be seen in Table 2 where the Voc values are tuned
from 0.67-1.36 V depending on the donor polymer. For the
compound 10:PCz system where 1.36 V is achieved, almost all
the visible light is absorbed by compound 10 as PCz is a high
band gap polymer and absorbs primarily in the UV region. In
this case, derivatives such as PCBM could not be efficiently
utilized with PCz as both materials primarily absorb in the UV
region. As a result, compound 10 is a very versatile acceptor
material that can be used with many donor systems. Further
optimization of processing conditions, active layer morphology,
donor polymer, and materials design will likely lead to improv-
ing the ultimate PCEs. In fact, improved device efficiencies of
1.1% have recently been prepared using compound 10 with other
donor polymers and the results will be reported shortly. Finally,
we are currently building a solar cell lifetime setup to compare
devices prepared with these new acceptors with the more
traditional fullerene based materials.
preparing organic solar cell devices.
electrode), and the corresponding ionization potential (IP) and
electron affinity (EA) values were derived from the onset redox
potentials, based on -4.4 eV as the SCE energy level relative
to vacuum (EA ) Ered-onset + 4.4 eV, IP ) Eox-onset + 4.4 eV).
Cyclic voltammograms of vinazene compounds 1-16 were
used to estimate the LUMO (electron affinity) and HOMO
(ionization potential) values which are shown in Table 1.
Compounds 1, 2, 10, and 16 showed the lowest HOMO values
(furthest from vacuum) compared to the other derivatives of
the series. The lower HOMO values indicate the higher oxidative
stability of the materials, which is a key requirement for organic
electronics. The LUMO values vary from -2.76 to -3.50 eV
and again demonstrate the wide range of materials for band gap
engineering obtained using this strategy. The optical band gaps
calculated from the UV-vis absorption onset,5,57 and the
electrochemical band gaps obtained from cyclic voltammetry,
are found to be in good agreement as shown in Table 1. The
lowest band gap calculated for the compound 9 and 11 are
around 2.0 eV due to higher conjugation length of blocks used
as a central moiety.
Thermal Properties. The thermal behavior of all the materi-
als was determined by a repeating heating-cooling cycle using
differential scanning calorimetry (DSC) (see Table 1). Com-
pounds 1, 7-9, 14, and 16 showed glass transition temperatures
varying from 45 to 115 °C. The highest Tg was reported for
compound 16 due to the two phenyl rings that enhance the rigid
nature of the branched material. All compounds showed melting
peaks except 7, 9, 14, and 16 despite heating to 300 °C. This
observation also indicates the stiff nature of the material due to
the fused aromatic structure of the central moiety. Thermal
decomposition (Td) analysis of all the materials was carried out
using thermogravimetric analysis (TGA). Td, 5 wt % loss, for
all compounds were observed in the 300-414 °C range. These
values clearly indicate the high thermal stability of the materials,
which is a prerequisite for eventual device lifetime and various
processing conditions of organic devices.
Photovoltaic Properties. After careful screening of all the
materials prepared in this study, the hexyl and ethylhexyl
versions of compound 10 were found to be the most promising
for application in OPV devices when combined with the selected
donor polymers shown in Figure 2.46,47,58,59
Conclusions
We have reported the synthesis and characterization of a new
family of n-type conjugated materials based on the Heck reaction
of 1-alkylvinazenes with selected bromoaromatics. The materials
are easily prepared in high yields from minimal step reactions
while the optoelectronic properties, such as UV-vis, PL,
HOMO-LUMO are easily tuned by selecting the proper central
aromatic segment. The materials may be tuned for solution
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E.; Thompson, M. E. Org. Elect. 2005, 6 (1), 11–20.
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J. Org. Chem. Vol. 74, No. 9, 2009 3297

Shin et al.
J ) 7.6 Hz), 1.20-1.42 (m, 8 H), 1.80 (septet, 1 H, J ) 6.4 Hz),
3.98 (d, 2 H, J ) 7.6 Hz), 5.81 (dd, 1 H, J ) 8.4, 3.6 Hz),
6.45-6.55 (m, 2 H). 13C NMR (CDCl3): δ 10.6, 13.9, 22.9, 23.8,
28.4, 30.4, 41.0, 50.7, 108.7, 111.9, 112.9, 120.5, 122.6, 126.5,
149.8.
processability and high thermal stability which are key param-
eters for processing and device lifetime. We are currently
investigating their applications as emissive and/or electron
transporting materials for OLEDs as well as continued studies
on related materials for use in organic photovoltaic cells.
General Procedure for Heck reactions. In a glovebox, the aryl
halide, Cy2NMe, Pd[P(t-Bu)3]2, DMF, and the olefin (alkyl vina-
zene) were added to an oven-dried Schlenk flask equipped with a
stir bar. The Schlenk flask was taken out of the glovebox and
connected to the nitrogen line, and the reaction mixture was stirred
at 85 °C for 20 h. At the end of the reaction, the mixture was cooled
to room temperature and filtered through a glass sinter (Por. 4).
Ethanol was added to the filtrate while stirring to precipitate out
the product which was collected and washed with EtOH followed
by hexane. In cases where the product could not be precipitated
out, EtOH was added and the solution kept at 4 °C until precipitation
occurred. The product obtained was then recrystallized from DMF/
EtOH or THF/EtOH followed by cooling.
Experimental Section
Materials. Commercially available materials were used as
received unless noted otherwise. Dicyclohexylmethylamine
(Cy2NMe) was distilled prior to use. N-Hexyl-3,6-dibromocarbazole,
5,11-dibromotetracene,60 4,4′-dibromo-2,2′-dimethyl-1,1′-binaph-
thyl,61 and 4,7-bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole62 were
prepared according to literature procedures. All reactions were
carried out using Schlenk techniques in an argon or nitrogen
atmosphere with anhydrous solvents.
Synthesis. The synthetic routes for the alkylation of vinazene
and Heck couplings are shown in Schemes 1 and 2, and their
synthetic procedures are provided as follows:
Compound 1. Reaction system: 1,4-dibromobenzene (71 mg,
0.30 mmol), 1-hexylvinazene (171 mg, 0.75 mmol), Cy2NMe (0.161
mL, 0.75 mmol), Pd[P(t-Bu)3]2 (3.1 mg, 0.006 mmol), and DMF
(2 mL). After workup, the product was obtained as yellow solids
Synthesis of 1-Alkylvinazenes. To a solution of vinazene (2.00
g, 0.014 mol) in 30 mL of DMF was added 2.80 g (0.020 mol) of
anhydrous K2CO3 and 1-iodohexane (2.80 mL, 0.019 mmol). The
solution was stirred at 80 °C for 24 h and then quenched with 30
mL of water. The aqueous layer was extracted three times with 50
mL of diethyl ether. The organic layer was dried over magnesium
sulfate, and the solvent was removed under vacuum. The oily
residue was recrystallized in EtOH to give pale yellow crystalline
solids (2.22 g, 70% yield) after 24 h at 4 °C. 1H NMR (CDCl3): δ
0.90 (t, 3 H, J ) 6.8 Hz), 1.33 (unresolved m, 6 H), 1.80 (q, 2 H,
J ) 6.8 Hz), 4.11 (t, 2 H, J ) 7.2 Hz), 5.82 (t, 1 H, J ) 6.4 Hz),
6.52 (d, 2 H, J ) 6.0 Hz). 13C NMR (CDCl3): δ 14.0, 22.4, 26.1,
30.7, 31.1, 46.7, 108.5, 111.9, 112.3, 120.2, 122.5, 126.6, 149.4.
The analogue 1-(2-ethylhexyl)vinazene was synthesized from
1-bromo-2-ethylhexane using similar conditions. After being stirred
for 24 h, the solution was filtered and DMF removed under vacuum.
The oily residue was purified by column chromatography (silica
gel, 25% ethyl acetate in hexane as eluent). A pale yellow oil was
obtained after removal of solvent. The oil solidified upon cooling
4 °C for 24 h to give the product as a cream-colored solid (34%
1
(120 mg, 75% yield). H NMR (CDCl3): δ 0.90 (t, 6 H, J ) 7.2
Hz), 1.36 (unresolved m, 12 H), 1.86 (q, 4 H, J ) 6.8 Hz), 4.19 (t,
4 H, J ) 7.6 Hz), 6.80 (d, 2 H, J ) 16.0 Hz), 7.60 (s, 4 H), 7.87
(d, 2 H, J ) 15.6 Hz). 13C NMR (CDCl3): δ 14.0, 22.5, 26.2, 30.8,
31.2, 46.7, 108.6, 111.2, 111.8, 112.5, 123.0, 128.4, 136.6, 139.5,
149.7. Anal. Calcd for C32H34N8: C, 72.43; H, 6.46; N, 21.12.
Found: C, 72.27; H, 6.79; N, 21.29. MS (MALDI-TOF): m/z 531.37
(M + 1), calcd for C32H34N8 530.29.
Acknowledgment. Financial support was provided by the
Agency for Science, Technology and Research (A-STAR)
through the Visiting Investigator Program (VIP) in Organic
Electronics with Professor Ananth Dodabalapur and the Institute
of Materials Research and Engineering (IMRE), Republic of
Singapore.
1
Supporting Information Available: Detailed experimental
procedures and compound characterization data for all products,
NMR spectra (1H), and cyclic voltammogram traces. This
material is available free of charge via the Internet at
http://pubs.acs.org.
yield). H NMR (CDCl3): 0.89 (t, 3 H, J ) 6.4 Hz), 0.92 (t, 3 H,
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