Comparison of CC triple and double bonds as spacers in push-pull chromophores

Article abstract of DOI:10.1002/ejoc.201100378

We report the synthesis and properties of two series of homologous donor-acceptor (D-A) chromophores in which N,N-dimethylanilino (DMA) or N,N-dihexylanilino (DHA) donors and dicyanovinyl acceptors are separated by up to four C≡C triple-bond spacers or up

Full text of DOI:10.1002/ejoc.201100378

FULL PAPER
DOI: 10.1002/ejoc.201100378
Comparison of CC Triple and Double Bonds as Spacers in Push–Pull
Chromophores
Brian B. Frank,[a] Philip R. Laporta,[b] Benjamin Breiten,[a] Mark C. Kuzyk,[b]
Peter D. Jarowski,[a] W. Bernd Schweizer,[a] Paul Seiler,[a] Ivan Biaggio,[b] Corinne Boudon,[c]
Jean-Paul Gisselbrecht,[c] and François Diederich*[a]
Keywords: Alkynes / Charge transfer / Conjugation / Donor–acceptor systems / Non-linear optics / Optoelectronic
properties / Chromophores
We report the synthesis and properties of two series of ho-
mologous donor–acceptor (D–A) chromophores in which
N,N-dimethylanilino (DMA) or N,N-dihexylanilino (DHA)
donors and dicyanovinyl acceptors are separated by up to
four CϵC triple-bond spacers or up to three C=C double-
lations. In both series, the optical and electrochemical
HOMO–LUMO gaps decrease with increasing spacer length.
The HOMO–LUMO gaps for the D–A oligoynes and oligo-
enes with a given spacer length are nearly identical. The ef-
fect of the spacer length was found to level-off for spacers
bond spacers. The intramolecular charge-transfer (CT) inter- with more than six carbon atoms. The third-order optical
actions of the new D–A oligoynes and the known all-trans
D–A oligoenes were investigated by X-ray crystallography,
nonlinearity of both series of molecules was determined by
measuring the rotational averages of the third-order polariz-
electrochemistry, UV/Vis spectroscopy, and theoretical calcu- abilities γrot by degenerate four-wave mixing.
It has long been known that the off-resonant, that is,
Introduction
nonabsorptive, third-order polarizability strongly increases
when the energy associated with the transition from the
ground to the first excited state (HOMO–LUMO gap) de-
creases.[10] Furthermore, it depends on the extension of the
linear D–A conjugation pathways.[11,12] Stronger D–A cou-
pling across shorter π-conjugated spacers leads to higher-
energy absorptions, whereas weaker D–A coupling across
longer spacers results in lower HOMO–LUMO
gaps.[3b,4b,13–15] When the size of the spacer between the do-
nor and acceptor groups is maintained constant,[16] the
HOMO–LUMO gap decreases with the π-conjugation effi-
ciency. Some of these postulates were recently further vali-
dated in the investigation of the linear and third-order non-
linear optical properties of small push–pull chromophores
bearing DMA donor and C(CN)2 acceptor units.[17,18] The
best of these molecules have a third-order optical nonlinear-
ity, represented by the rotational average of the third-order
polarizability, γrot, which is very large relative to their size[18]
and approached the theoretically predicted fundamental li-
mit.[19]
Donor–acceptor (D–A) chromophores continue to at-
tract considerable interest due to their electronic and optical
properties, in particular, their second- and third-order op-
tical nonlinearities.[1] We have reported high third-order op-
tical nonlinearities for four classes of small push–pull chro-
mophores, linearly conjugated donor–acceptor-substituted
tetraethynylethenes (TEEs),[2] donor-substituted cyano-
ethynylethenes (CEEs),[3] donor-substituted 1,1,4,4-tetracy-
anobuta-1,3-dienes (TCBDs),[4,5] and recently also for
homoconjugated systems.[6] In addition, we have shown that
one of the N,N-dimethylanilino (DMA)-substituted non-
planar TCBDs (DDMEBT, {2-[4-(dimethylamino)phenyl]-
3-[4-(dimethylamino)phenyl]ethynyl}buta-1,3-diene-1,1,4,4-
tetracarbonitrile) forms high-optical-quality, homogeneous
thin films by vapor-phase deposition.[7] These films have
found applications as materials in integrated nonlinear op-
tics (NLO).[8,9]
[a] Laboratorium für Organische Chemie, ETH-Hönggerberg,
HCI,
Popular π-conjugated spacers connecting the D and A in
push–pull chromophores comprise trans-substituted C=C
double bonds, CϵC triple bonds, or mixed enyne systems.
However, direct comparisons under identical experimental
conditions of the efficiency of olefinic and acetylenic spa-
cers in enhancing third-order optical nonlinearities of
push–pull chromophores are rare.
8093 Zürich, Switzerland
Fax: +41-44-632-1109
E-mail: diederich@org.chem.ethz.ch
[b] Lehigh University, Department of Physics, 415 Lewis Lab,
16 Memorial Dr. East, Bethlehem, PA 18015, USA
[c] Laboratoire d’Electrochimie et de Chimie Physique du Corps
Solide, Institute de Chimie-UMR 7177, CNRS, Université de
Strasbourg,
4, rue Blaise Pascal, CS 90032, 67081 Strasbourg Cedex, France
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201100378.
Comparisons of the electrochemical and optoelectronic
properties of C=C double and CϵC triple bond spacers
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F. Diederich et al.
FULL PAPER
have been reported for D–π–D systems (π = π-conjugated interest for its experimental contribution to the ongoing de-
spacer) and for systems in which electroneutral substituents bate about the efficiency of CC double and triple bonds in
are linked by oligoenes and -ynes.[20–22] The electronic and transmitting conjugative effects.[41]
nonlinear optical properties of D–A-substituted oligoenes
and -ynes have been compared in theoretical studies.[23,24]
Results and Discussion
D–A-substituted allenes have also been included in one
theoretical comparison.[25]
Synthesis and Structures of D–A Oligoynes
In experimental studies, a wide variety of D–A-substi-
tuted oligoenes have been prepared to investigate their sec-
ond-order NLO behavior.[26–30] The second-order NLO
properties of D–A oligoynes have also been reported.[31–33]
Tykwinski and co-workers studied the third-order NLO
properties of triisopropylsilyl (TIPS) and phenyl end-
capped oligoynes of various lengths.[19b,34,35] Only a few
limited studies have been published on the third-order NLO
properties of D–A-substituted oligoenes.[36]
Herein we report a comparative investigation of the lin-
ear and third-order nonlinear optical properties of D–A oli-
goynes 1[4a] and 2–6 and oligoenes 7–11 (Figure 1) under
identical conditions. The synthesis of pentayne 6 was also
pursued but the compound was too unstable to isolate. Oli-
goenes 7 and 11 were only considered in the computational
section of this work. The investigation also included com-
pound 12 with a mixed enyne spacer. Although the oli-
goynes are new compounds, the oligoenes have previously
been extensively studied as second-order NLO chromo-
phores by Marder[37] and Kawasaki[38] and their co-workers.
Blanchard-Desce and co-workers reported the second- and
third-order NLO properties of the N,N-dibutylanilino
(DBA)-substituted analogues of oligoenes 7–11.[36,39] Blan-
chard-Desce et al. also investigated the third-order NLO
properties of different anilino-substituted oligoene series,
up to a tetraene spacer and bearing N,NЈ-diethyl-3-thiobar-
D–A-substituted diyne 2 was prepared from DMA-sub-
stituted 2,4-pentadiyn-1-ol 13[42] by oxidation to the corre-
sponding aldehyde with Dess–Martin periodinane (DMP)
followed by Knoevenagel condensation with malononitrile
in a “one-pot” set-up (Scheme 1).[17] It is a dark-red solid,
which is stable up to its melting point at around 145 °C,
and was characterized by X-ray analysis [for details, see the
Supporting Information (SI)]. For the preparation of enyne
12, diyne 13 was regioselectively reduced to enyne 14 with
sodium bis(2-methoxyethoxy)aluminium hydride (Red-
Al®).[43] Only the trans double bond was formed under
these conditions, as confirmed by X-ray analysis
(Scheme 1). Enyne 12 was subsequently obtained as a red
solid by the “one-pot” oxidation/Knoevenagel condensa-
1
tion and is stable up to its melting point at 119–121 °C. H
NMR spectroscopy showed partial isomerization of the
trans double bond to the cis double bond for CDCl3 solu-
tions of 12 on exposure to light. No isomerization was ob-
served when CDCl3 or CH2Cl2 solutions of 12 were kept in
the dark. As a solid, 12 is photochemically stable. Thus,
enyne 12 was synthesized, purified, and characterized in the
absence of light.
bituric
acid
and
3-(dicyanomethylidene)-2,3-dihy-
drobenzo[b]thiophene 1,1-dioxide acceptor moieties.[40]
However, the third-order polarizabilities were measured by
using third-harmonic generation (THG) under increasingly
resonant conditions for the longer molecules. Direct com-
parison with the literature data is generally rendered diffi-
cult by the fact that the optical nonlinearities are strongly
solvent-dependent and also dependent on the method of
measurement. All NLO data in this work were obtained by
degenerate four-wave mixing (DFWM) at the off-resonant
wavelength of 1.5 μm with CH2Cl2 used as the solvent
throughout the entire investigation. The study was also of
Scheme 1. Synthesis and ORTEP plots of chromophores 2 and 12.
Reagents and conditions: a) 1. Dess–Martin periodinane (DMP),
CH2Cl2, 1.5 h, 25 °C; 2. CH2(CN)2, Al2O3, CH2Cl2, 1 h, 50 °C,
47%; b) Red-Al®, THF, 4 h, 0–25 °C, 52%; c) 1. DMP, CH2Cl2,
1 h, 25 °C; 2. CH2(CN)2, Al2O3, CH2Cl2, 6 h, 50 °C, 38%. For fur-
ther details of the crystal structures, see the SI.
The synthesis of 3 and 4 started with the oxidative Hay
heterocoupling of diynes 15[4b] and 16[44] with an excess of
prop-2-yn-1-ol to give triynes 17 and 18, respectively
(Scheme 2), which were transformed, as described above,
into 3 and 4. The DHA substituent in 4 enhances its solu-
bility, whereas DMA-substituted 3 exhibits poor solubility
in common organic solvents (e.g., CHCl3 or CH2Cl2).
Figure 1. D–A-substituted oligoynes 1–6, oligoenes 7–11, and en-
yne 12.
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CϵC and C=C Bonds as Spacers in Push–Pull Chromophores
Scheme 2. Synthesis of the D–A triynes 3 and 4 and ORTEP plot of chromophore 3. Reagents and conditions: a) prop-2-yn-1-ol (4 and
15 equiv. for 17 and 18, respectively), CuCl, N,N,NЈ,NЈ-tetramethylethylenediamine (TMEDA), CH2Cl2, 25 °C, 53% (17), 68% (18);
b) 1. DMP, CH2Cl2, 1 h, 25 °C; 2. CH2(CN)2, Al2O3, CH2Cl2, 1 h, 50 °C, 48% (3), 19% (4). For details of the crystal structure, see the
SI.
Dark-blue 4 was stable when subjected to severe agitation
and was characterized by X-ray analysis (Scheme 2). How-
ever, explosive decomposition was observed at temperatures
over 180 °C. Triyne 4 seems to be stable up to its melting
point at around 65 °C.
The D–A-substituted tetrayne 5 was prepared from
alcohol 19[45] by the “one-pot” oxidation/Knoevenagel con-
densation method (Scheme 3). Similarly to its precursor, 5
exhibits very low thermal stability: Decomposition was ob-
served at 40 °C during evaporation of the solvent. Thus, the
Knoevenagel condensation with malononitrile was per-
formed at 25 °C. DMP oxidation of 19 was completed after
10 min.
Scheme 4. Synthesis of pentayne 22 and hexayne 23.[45] Reagents
and conditions: a) 1. K2CO3, MeOH/THF (1:1), 2 h, 25 °C;
2. TMEDA, MeOH/THF/CH2Cl2 (1:1:1), 16 h, 25 °C, 23% (22),
24% (23).
The known D–A oligoenes 8–10 were synthesized ac-
cording to literature procedures.[37,38] The three chromo-
phores are thermally and photochemically stable as solids
over periods of months. However, CDCl3 and CH2Cl2 solu-
tions show partial isomerization after exposure to light for
24 h (see Figures S1 and S2). To prevent the photochemical
trans/cis isomerization of the double bonds of the oligoene
Scheme 3. Synthesis of D–A-substituted tetrayne 5. Reagents and
chromophores and their precursors, they were synthesized,
conditions: a) 1. DMP, CH2Cl2, 10 min, 25 °C; 2. CH2(CN)2,
purified, and characterized in the absence of light.
Al2O3, CH2Cl2, 1 h, 25 °C, 38%.
Electrochemistry
For the synthesis of D–A-substituted pentayne 6, com-
pounds 20[46] and 21,[45] upon deprotection of the TMS
The electrochemical investigations of the new D–A oli-
group, were subjected to oxidative Hay heterocoupling to goynes and the known D–A oligoenes were carried out by
afford 22 together with the symmetric hexayne 23 cyclic voltammetry (CV) and rotating disk voltammetry
(Scheme 4).[45] Pentayne 22 was handled and stored at tem- (RDV) in CH2Cl2 with nBu4NPF6 (0.1 m) as the supporting
peratures below 40 °C because of its low thermal stability. electrolyte. The redox potentials versus Fc+/Fc (ferricinium/
The synthesis of 6 was attempted by the “one-pot” oxi- ferrocene couple) for the D–A oligoynes and oligoenes are
dation/Knoevenagel condensation method. However, the listed in Table 1. Comparisons of the redox properties of
target compound could not be obtained in pure form due the oligoynes and -enes have previously been reported and
to its low thermal stability.
are further discussed in the SI.[20–22] For systems with the
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F. Diederich et al.
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general structure D–π–D, the first reduction potentials of dicyanovinyl moiety is also increasingly facilitated upon in-
the oligoynes are anodically shifted relative to oligoenes creasing the length of the spacer [8 (–1.63 V), 9 (–1.52 V),
containing the same number of unsaturated CC and 10 (–1.43 V); a difference of ca. 200 mV], but not as
bonds.[20–22,47] The data reported below also validate these much as in the corresponding oligoyne series (a difference
trends for D–π–A systems. Also, the redox properties re- of ca. 480 mV). Also, the potential shift of the oxidation
corded for oligoenes 8–10 are in good agreement with those process in the oligoene series is more pronounced (ca.
reported by Marder and co-workers for 8 and 9,[48] and by 280 mV difference) than for the reduction process (ca.
Blanchard-Desce and co-workers for the corresponding 200 mV difference).
DBA derivatives of 8–10.[36]
The electrochemical HOMO–LUMO gap, determined
from the RDV results, decreases along the oligoyne series
from 2.20 V for 1 down to 1.62 V for 3 (see Table 2). Note
that the values of the electrochemical HOMO–LUMO gaps
are rather approximate due to the irreversible nature of the
reduction steps in the series. However, these results clearly
show that up to the D–A triyne there is less efficient cou-
pling between the aniline donor and the dicyanovinyl ac-
ceptor with increasing spacer length.
In the oligoene series, the electrochemical HOMO–
LUMO gap decreases from 2.20 (8) to 1.70 V (10, RDV).
Interestingly, the electrochemical HOMO–LUMO gaps of
D–A oligoynes and oligoenes are quite similar or even iden-
tical, as in the case of 1 and 8 [2.20 V (RDV)] even though
their individual oxidation and reduction potentials differ
considerably. The similarity between the HOMO–LUMO
gaps in both series is also seen in the optical gaps, deter-
mined by UV/Vis spectroscopy (see below).
Table 1. Cyclic voltammetry (CV; scan rate v = 0.1 Vs–1) and rotat-
ing disk voltammetry (RDV) data in CH2Cl2 (+0.1 m nBu4NPF6).[a]
CV
RDV
Slope [mV][f]
E° [V][b]
ΔEp [mV][c] Ep [V][d]
E1/2 [V][e]
1[g]
2
+0.66[h]
+0.70 (1e–)
–1.50 (1e–)
+0.63 (1e–)
–1.24 (1e–)
+0.55
70
100
65
75
60
–1.50
+0.62
+0.59
+0.58
+0.55
+0.53
+0.36
+0.25
100
–1.33
100
–1.02
60
–1.07
80
–1.09
100
–1.63
100
–1.52
75
3
–1.07
+0.59
–1.06
75
60
4
60
60
60
65
100
65
220
60
5
+0.56 (1e–)
–1.09 (1e–)
+0.54 (1e–)
–1.66 (1e–)
+0.36 (1e–)
–1.56 (1e–)
+0.25 (1e–)
–1.45 (1e–)
8
9
10
The enyne 12 exhibits an irreversible 1e– oxidation of the
aniline moiety at +0.56 V. This value is much closer to the
potential recorded for tetrayne 5 (+0.55 V) than for diyne 2
(+0.62 V) and shows a strong influence of the spacer struc-
ture on the redox properties. Also, the first reduction poten-
tial is strongly cathodically shifted upon exchanging a CC
triple bond in diyne 2 [Ered,1 = –1.24 V (RDV)] to a CC
–1.43
120
[a] All potentials are given vs. the Fc+/Fc couple used as internal
standard. [b] E° = (Epc + Epa)/2 in which Epc and Epa correspond
to the cathodic and anodic peak potentials, respectively. [c] ΔEp =
Epa – Epc. [d] Ep = irreversible peak potential. [e] E1/2 = half-wave
potential. [f] Logarithmic analysis of the wave obtained by plotting
E vs. log[I/(Ilim – I)]. [g] See ref.[4a] [h] Scan rate v = 2.0 Vs–1.
All D–A-substituted oligoynes show an irreversible 1e– double bond in enyne 12 [Ered,1 = –1.43 V (RDV)].
reduction step occurring at the dicyanovinyl moiety. The
first reduction potentials of triynes 3 [–1.07 V (RDV)] and
4 [–1.06 V (RDV)] in CV and RDV are nearly identical.
When going from monoyne 1 to tetrayne 5, the value of
UV/Vis Spectroscopy
Ered,1 shifts anodically from –1.50 to –1.09 V in CV. Inter-
Both D–A-substituted oligoyne and -ene chromophores
estingly, the most anodically shifted reduction potentials in feature one intramolecular CT band in their UV/Vis spectra
the entire series are found for triynes 3 [–1.02 V (CV)] and recorded in CH2Cl2. The longest wavelength maximum λmax
4 [–1.07 V (CV)] and not for the longer tetrayne 5. This of the D–A oligoynes appears between 477 (2.60 eV, 1[4a]
)
indicates that the effect of the spacer length on the re- and 573 nm (2.17 eV, 5; Figure 2) and the intensity of the
duction potential of D–A oligoynes is negligible in com- CT band drops with increasing spacer length from 1 (ε =
pounds with more than three CϵC triple bonds (see below). 44100 m–1 cm–1) to 5 (ε = 10100 m–1 cm–1). The CT character
The first oxidation potentials in CV are cathodically of the longest-wavelength absorption was confirmed by
shifted from +0.66 V in monoyne 1[4a] to +0.55 V in tet- protonation/neutralization experiments (see the SI). The
rayne 5 due to the reduced transfer of electron density from spectra of oligoenes 8–10 are depicted in Figure S7 and are
the donor to the acceptor with increasing spacer length. in good agreement with previously reported data.[36,37,48]
However, the potential shift for the oxidation process is less The electrochemical HOMO–LUMO gaps and the optical
pronounced (ca. 100 mV difference) than for the reduction gaps determined either from the longest wavelength max-
process (ca. 400 mV difference). The donor properties of ima or the end-absorption (λend) in the UV/Vis spectra are
the DMA and DHA moieties are quite similar, as seen in presented in Table 2. Plots of the optical gap against the
the comparison of the two triynes 3 and 4.
electrochemical HOMO–LUMO gap in both series gave a
In the series of oligoenes, the oxidation potentials are strong linear correlation (R2 = 0.86 for λmax and R2 = 0.96
cathodically shifted when increasing the spacer length from for λend) between the two quantities (Figures S8 and S9),
monoene 8 (+0.53 V)[48] to diene 9 (+0.36 V)[48] and triene which indicates that the same HOMO and LUMO orbitals
10 (+0.25 V). The irreversible (CV) 1e– reduction on the are involved.
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CϵC and C=C Bonds as Spacers in Push–Pull Chromophores
antees an accurate determination of the molecular concen-
tration that leads to the four-wave mixing signal even in
those cases in which it was not possible to accurately con-
trol solvent evaporation and solute molecular mass for each
dilution step, like for example when only limited quantities
of a given compound were available and multiple filtration
steps were needed when preparing the solution. In addition,
comparison of the absorption spectrum of the solution used
for the DFWM measurements with the reference spectrum
was used to check the possibility of any decomposition,
contamination, or isomerization during the NLO measure-
ments.
For real-value third-order polarizability and low concen-
trations, the third-order susceptibility of the solution is pro-
portional to the square root of the DFWM signal, which
depends linearly on the concentration in units of mass per
Figure 2. UV/Vis spectra of D–A oligoynes 1–5 in CH2Cl2 at
298 K.
–1/2
volume with a slope of f4S
γ
ρ/m in which f = (n2 +
rot
2)/3 is a Lorentzian local field factor that depends on the
refractive index n = 1.4242 of CH2Cl2, S is the DFWM
signal of a cell containing only CH2Cl2, ρ is the mass den-
sity of CH2Cl2, and m is the mass of the molecule under
investigation. We obtained absolute values for γrot by a ref-
erence measurement that established the DFWM signal S
for a cell filled with pure CH2Cl2 to be 6Ϯ1 times larger
than for a 1 mm thick fused silica sample. For final cali-
Table 2. Optical and electrochemical gaps determined by UV/Vis
spectroscopy and RDV in CH2Cl2 and computed values based on
the TD-B2LYP/6-31G(d) method.
Experimental
Computed
λmax
λmax
ε
λend
Δ(Eox,1 – Ered,1
)
f[a]
[nm] [m–1 cm–1
]
[nm (eV)]
[V]
[nm]
bration, we used a third-order susceptibility of χ(3)
(–ω, –ω, ω, ω) = 1.9ϫ10–22 for fused silica at 1.5 μm.
1[b] 477
44100
33300
16100
12700
10100
47000
35000
41700
35900
590 (2.11)
647 (1.92)
782 (1.59)
769 (1.62)
762 (1.63)
590 (2.11)
670 (1.85)
793 (1.57)
639 (1.94)
2.20
1.87
1.62
1.65
1.65
2.20
1.92
1.70
1.99
416
472
529
1.0
1.10
1.08
1111
2
512
538
560
573
488
519
539
505
Alkyne 1 has also been measured previously[3a] but we
have since discovered a calibration error for the experimen-
tal set-up used in refs.[3a,4a,17,18] that caused the values of
the third-order polarizabilities γrot reported there to be too
large by a factor 1.5. The new value for 1 is in agreement
with the value reported in ref.[3a] divided by 1.5. Oligoynes
2 and 3 were increasingly difficult to dissolve, delivering
solutions polluted by scattering particles that required mul-
tiple filtering steps, especially for 3, which affected the accu-
racy of the DFWM measurements, which needed to be re-
peated multiple times. Molecule 4 delivered a better solu-
tion, but it could only be tested once because of the small
quantity available. The problems observed with molecules 2
and 3 were greatly exacerbated for molecule 5, which pro-
duced such a high level of scattering in solution that it made
the DFWM measurements impossible. In addition, the
spectrum of molecule 5 showed clear signs of decomposi-
tion, possibly originating from its intercontinental transfer,
3
4
5
8
9
584[c]
408
456
1.02
1.20
1.54
1.84
10
12
502
[a] f = oscillator strength. [b] See ref.[4a] [c] Calculated for the
DMA-substituted derivative.
In line with the electrochemical data, the effect of the
spacer on the optical gap slowly levels off on going from
the triyne 3 (λmax = 538 nm) to the tetrayne 5 (λmax
=
573 nm). Also, the optical gaps of the D–A oligoynes and
-enes with spacers of equal length are quite similar.
Nonlinear Optical Properties
The rotational average of the third-order polarizability which led us to discard all nonlinear optical measurements
γrot of oligoynes 1–4, oligoenes 8–10, and enyne 12 were for this molecule.
determined by measuring the third-order susceptibility χ(3)
Diene-spaced 9 also showed some dissolution problems
1111 (–ω, –ω, ω, ω) of CH2Cl2 solutions of varying molecular relative to both ethylene-linked 8 and triene-spaced 10 with
concentration. The measurements were taken by degenerate the DFWM data from solutions of 9 generally having more
four-wave mixing (DFWM) at 1.5 μm using 1 ps laser noise. The absorption spectra of molecules 8–10 and 12
pulses obtained from a TOPAS travelling wave optical para- during and after the nonlinear optical measurements did
metric amplifier system pumped by a Clark MXR amplified not show any significant change though, from which we
Ti:Sapphire laser. We used 1 mm thick solutions in fused conclude that there was no change in the isomer distribu-
silica spectroscopy cells, the concentrations of which were tion (all-trans) during the optical measurements.
varied by successive dilutions and determined for each solu-
The third-order NLO properties of the DBA analogues
tion from the absorption spectrum calibrated to a reference of DMA-substituted oligoenes 8–10 had previously been
spectrum of the molecular extinction coefficient. This guar- measured by Blanchard-Desce and co-workers.[36] When re-
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F. Diederich et al.
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scaled to take into account an incorrect calibration,[39] the tively say that triple bonds are less effective in generating
values for the third-order polarizabilities γrot for the DBA high third-order optical nonlinearities in the studied series.
analogues of 8–10 were 2.2ϫ10–48
, , and Another observation is that although the HOMO–LUMO
10ϫ10–48
43ϫ10–48 m5 V–2, respectively. However, these values were gap in the series of oligoynes decreases with the size of the
obtained by third-harmonic generation (THG) from a spacer in a similar way to the oligoenes, the strength of the
wavelength of 1.9 μm, which means that the values for the transition, as described by the molar extinction coefficient,
longer molecule are three-photon resonant, which explains decreases as the length of the molecules increases, whereas
the steeper increase of third-order polarizability with length for the double-bond molecules the extinction coefficient re-
that is seen in that study[36,39] compared with the present mains approximately constant (see Figure S7). The smaller
work in which the measurements are definitely more off- cross-section for the excitation of the larger oligoynes would
resonant by virtue of the DFWM measurements at 1.5 μm, then lead to a lesser enhancement of the third-order polar-
which give a two-photon resonance at 750 nm instead of izability when compared with the oligoenes. However, a
the three-photon resonance at 633 nm for the THG experi- note of caution is necessary here because of the aforemen-
ments. Overall, and considering also the different solvents tioned stability problems that affect the nonlinear optical
that have been used, we are in remarkable agreement with characterization of the oligoynes.
the previous results[36,39] published for the nonlinearities of
the oligoenes 8–10. Only diene-bridged 9 gave us some ex-
perimental difficulties that may have caused an underestim-
ation of its third-order polarizability, but it is clear that the
third-order polarizability for D–A oligoenes increases
rapidly with the number of C=C moieties in the spacer. This
is caused by a significant redshift of the longest-wavelength
optical absorption accompanied by the increase in the size
of the molecule.
Note also that we previously determined the third-order
polarizability of pentayne 24[45] (Table 3) to be quite large,
proving that triple bond spacers can be very effective in
creating a large nonlinearity, in contrast to what we see here
for molecules 1–5.
Table 3 also lists two other quantities that are useful for
judging the nonlinear optical properties of a molecule. The
first is a simple figure of merit, the specific third-
~
order polarizability γ, obtained by dividing the experimen-
Despite the initial tripling of the third-order polarizabil- tal value for the third-order polarizability γrot by the molec-
ity on going from acetylene-spaced 1 to diyne-spaced 2, ular mass: It determines the potential bulk third-order
there is no further comparable steep increase in the third- susceptibility of a dense supramolecular assembly of mole-
order polarizability with the number of spacers in the oli- cules. The second is the intrinsic third-order polarizability
goynes. Molecule 3 shows a third-order polarizability that γI. The concept of intrinsic hyperpolarizability has been
is only at the same level as 2, and longer molecules do not proposed as a scale-invariant measure of a molecule’s non-
show any measurable increase. Although the nonlinear op- linear optical properties.[49] For our third-order molecules,
tical results for the triple-bond molecules with longer spa- we define it as the ratio between the experimental rotational
cers should not be considered very accurate because of the average of the third-order polarizability γrot and the corre-
issues of solubility and decomposition mentioned above, sponding fundamental limit in the centrosymmetric case,
from the behavior of the shorter molecules one could tenta- γk,[19a] which is also listed in Table 3.
Table 3. Summary of the NLO relevant characteristics of the molecules investigated in this study.[a]
~
Mmol
λmax
ε
γrot
γ
γI
γk
Nπ
n
Type
[gmol–1]
[nm]
[103 m–1 cm–1] [10–48 m5V–2]
[10–23 m5V–2 kg–1]
[γrot/γk]
[10–48 m5V–2]
1
2
3
4
221.26
245.28
269.30
409.57
433.59
625.30
247.29
218.80
264.27
275.35
14
16
18
18
20
24
16
14
16
18
1
2
3
3
4
5
2
1
2
3
T
T
T
T
T
477
512
538
560
573
624
505
488
519
539
44.1
33.3
16.1
12.7
10.1
15.1
35.9
47.0
35.0
41.7
3
10
9
7
–
60
5
3.5
6
Ϯ1
Ϯ3
Ϯ5
Ϯ3
–
Ϯ20
Ϯ1.5
Ϯ1.5
Ϯ1
0.82
2.46
2.01
1.03
–
5.78
1.22
0.96
1.37
4.37
0.017
0.031
0.017
0.011
–
0.031
0.017
0.018
0.017
0.038
174
323
524
623
886
1954
290
195
346
529
5
24[b]
12
8
T
D + T
D
D
9
10
D
20
Ϯ2
[a] For each molecule, we give the molecular mass Mmol, the number of conjugated electrons Nπ, the number of CC spacers n, the type
of spacers (T = triple bond, D = double bond), the longest wavelength absorption maximum λmax and the corresponding molar extinction
~
coefficient ε, the experimental value of the third-order polarizability γrot (rotational average), the specific third-order polarizability γ
derived from it, and the intrinsic third-order susceptibility γI defined in terms of the fundamental limit for centrosymmetric molecules,
γk. [b] See ref.[45]
4312
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 4307–4317

CϵC and C=C Bonds as Spacers in Push–Pull Chromophores
We see that pentayne 24[45] is a very efficient molecule, acceptors are separated by up to four CϵC triple bonds as
with its intrinsic third-order polarizability almost reaching spacers. The DMA-substituted mono-, di-, and triynes are
the largest value we observed (10) for the oligoenes and the thermally stable up to their melting points, whereas the tet-
high value of oligoyne 2. Thus, the triple-bond spacers are rayne decomposes above 40 °C. Explosive decomposition at
potentially very effective, at least in the case of this mole- 180 °C was observed in the case of the DMA-triyne. The
cule, which is more stable than oligoynes 3–5 and also has D–A pentayne could not be obtained due to its low thermal
a different and stronger acceptor system.
stability. The corresponding all-trans D–A oligoenes were
synthesized following known procedures. These com-
pounds, as well as enyne 12, exhibit good thermal stability,
but are sensitive to light, especially in solution. Partial pho-
tochemical trans/cis isomerization after 24 h of light expo-
sure was observed by 1H NMR spectroscopy for all the oli-
goenes. Therefore, they were synthesized, purified, and
characterized in the absence of light or under reduced illu-
mination. The all-trans configuration was confirmed by X-
ray crystallographic analysis and NMR spectroscopy. X-ray
crystallographic analysis revealed that the oligoyne and en-
yne chromophores are nearly fully planar in the solid state.
The UV/Vis spectra of the highly colored chromophores
feature intense intramolecular charge-transfer bands that
shift bathochromically with increasing spacer length. Cyclic
voltammetry and rotating disk voltammetry showed 1 e–
oxidations centered on the DMA or DHA moieties and 1 e–
reductions on the dicyanovinyl moieties. In both series, the
electrochemical HOMO–LUMO gap decreases from 2.2 V
for both monoene and -yne to values between 1.62 (triyne)
and 1.70 V (triene). Thus, the optical and electrochemical
gaps of D–A oligoynes and -enes with a given spacer length
are almost identical. In the oligoyne series, the electrochem-
ical gap is mainly influenced by the change in the reduction
potentials, whereas the shift in the oxidation potentials is
more pronounced for the oligoenes. The effect of spacer
length levels off, that is, reaches saturation beyond six car-
bon atoms in both series.
The conclusions from the analysis of CC triple- and
double-bond spacers with respect to enhancement of third-
order optical nonlinearities are ambivalent: The oligoenes
have a better ability to produce good homogeneous solu-
tions for the nonlinear optical measurements and show a
clear increase in third-order polarizability with the size of
the spacer. This is not the case for the oligoynes studied
here; the oligoynes were also much less capable of produc-
ing a good homogeneous solution with signs of decomposi-
tion for the larger molecules and the third-order polarizabil-
ity was already saturated at the diyne. This instability pre-
vented an extension of the chromophoric series to longer
spacer lengths. On the other hand, pentayne 24 with a five-
triple bond spacer and a much stronger acceptor moiety did
show a very strong third-order nonlinearity. This result
hints at a strong NLO potential for the acetylene spacers in
suitably designed chromophores.
Computational Studies
The molecular structures of the D–A oligoyne and -ene
series were optimized at the B3LYP/6-31G(d) level of
theory by using the software package Gaussian 03 (Fig-
ure S10).[50] For 5, the DMA moiety was used instead of
the DHA moiety. All structures are confirmed ground-state
minima according to the analysis of their analytical fre-
quencies computed at the same level, which show no imagi-
nary frequencies. Time-dependent density functional theory
single-point calculations [TD B3LYP/6-31G(d)] were per-
formed on these minima. The predicted transition energies
(λmax) and oscillator strengths (f) are given in Table 2. These
transitions are composed mostly of HOMO-to-LUMO ex-
citations and thus can best be described as intramolecular
charge-transfer processes involving the transfer of electron
density from the DMA donor to the dicyanovinyl acceptor,
as depicted in the molecular orbital representations of these
levels (Figure S10).
The calculated transition energies tend to be higher than
the experimental data for both the oligoene and -yne series.
However, the calculated values are in line with the expected
accuracy of intramolecular charge-transfer (ICT) systems
using this methodology, which can have an error up to and
beyond Ϯ1 eV[51] (for the complete set of compounds, there
is a mean absolute deviation of 0.24 eV). Given that ICT
systems are comparatively difficult to calculate accurately,
the results here are quite good.[52] In contrast, the calcu-
lated oscillator strengths do not agree qualitatively with the
experimental results. Discrepancies occur for compound 1,
which has a lower calculated oscillator strength than com-
pound 2, whereas in the oligoene series, compound 9 has a
larger value than 8. The accuracy of the treatment of oscil-
lator strength (f) is more difficult to assess, as quality exper-
imental data is lacking.[51] However, the TD B3LYP method
has been benchmarked against various ab initio methods
for the calculation of oscillator strengths and performs simi-
larly to DFT/MRCI.[53] Thus, the calculated data are of the
highest quality reasonably attainable for treatment of the
largest systems in these series. Suffice it to say, there is
clearly an issue in the calculation of oscillator strength for
the oligoene and -yne series in going from one CC multiple
bond (n = 1) to two (n = 2) in which the former tends to
be underestimated compared with the latter. Beyond this
kink in the data, the predicted values are in qualitative
agreement with the experimental results.
Experimental Section
Materials and General Methods: Reagents and solvents were pur-
chased as reagent grade from Acros, ABCR, Aldrich, and Fluka
and used as received. THF was freshly distilled from Na/benzo-
Conclusions
A series of homologous D–A oligoynes have been pre-
pared in which DMA or DHA donors and dicyanovinyl phenone, toluene from Na, and CH2Cl2 from CaH2 under N2. For
Eur. J. Org. Chem. 2011, 4307–4317
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4313

F. Diederich et al.
FULL PAPER
all aqueous solutions, deionized water was used. The Hay catalyst
refers to a freshly prepared solution of CuCl (65 mg, 0.66 mmol)
and N,N,NЈ,NЈ-tetramethylethylenediamine (TMEDA; 80 mg,
0.69 mmol) in CH2Cl2 (5 mL). All reactions, except Hay couplings,
were performed under an inert atmosphere by applying a positive
pressure of argon. Drying was performed in vacuo at 10–2 Torr.
Solvents for flash chromatography (FC), medium-pressure liquid
chromatography (MPLC), plug filtrations, and extractions were of
technical quality and distilled before use. The chromatographic sep-
arations were carried out on SiO2 60 (particle size 0.040–0.063 mm,
230–400 mesh; Silicycle). FC was carried out at an overpressure of
0.1–0.6 bar. Thin-layer chromatography (TLC) was conducted on
SiO2-layered glass plates (60 F254, Merck) and visualization was
performed with UV light (254 or 366 nm). Compounds 8,[37,38] 9,[37]
10,[37] 13,[42] 15,[4b] 16,[44] 19,[45] 20,[46] and 21[45] were prepared ac-
cording to literature procedures.
97).[55] All non-hydrogen atoms were refined anisotropically, hydro-
gen atoms were refined isotropically.
X-ray Crystal Structure of 2: Crystal data at 223 K for C16H11N3,
Mr = 245.29, monoclinic, space group P21/c, Dx = 1.227 gcm–3, Z
= 8, a = 14.8880(5), b = 6.9699(3), c = 25.5845(11) Å, α = 90.00, β
= 90.539(2), γ = 90.00°, V = 2654.7(2) Å3. Bruker–Nonius Kappa-
CCD diffractometer, Mo-Kα radiation,
λ = 0.7107 Å, μ =
0.075 mm–1.
A
black crystal of (linear dimensions ca.
2
0.33ϫ0.09ϫ0.07 mm) was obtained by slow evaporation of a solu-
tion of 2 in CH2Cl2 at 25 °C. The numbers of measured and unique
reflections are 7433 and 4079, respectively (Rint = 0.081). Final
R(F) = 0.0609, wR(F2) = 0.1539 for 431 parameters and 4079 re-
flections with IϾ2σ(I) and 2.753Ͻ θϽ 24.108° (the corresponding
R values based on all 7433 reflections are 0.0927 and 0.1748,
respectively).
X-ray Crystal Structure of 3: Crystal data at 220 K for C18H11N3,
Melting points were measured with a Büchi B-540 melting-point
apparatus in open capillaries. Decomp. refers to decomposition. 1H
and 13C NMR spectra were measured with Varian Mercury 300,
Bruker ARX 300, Bruker DRX400, Bruker AV400, or Bruker
DRX 500 instruments at 298 K unless otherwise stated. Chemical
shifts are reported in ppm relative to the signal of tetramethylsilane.
Residual solvent signals in the 1H and 13C NMR spectra were used
as internal references. Coupling constants (J) are given in Hz. The
apparent resonance multiplicity is described as s (singlet), br. s
(broad singlet), d (doublet), dd (doublet of doublets), dt (doublet
of triplets), t (triplet), q (quartet), sept. (septet), and m (multiplet).
Infrared spectra (IR) were recorded neat with a Varian 800 FT-IR
instrument. Absorption bands are reported in cm–1. UV/Vis spectra
were recorded with a Varian Cary-5 spectrophotometer. The spec-
tra were recorded in CH2Cl2 in a quartz cuvette (1 cm) at 298 K.
The absorption wavelengths are reported in nm with the extinction
coefficient ε, given in m–1 cm–1, in parentheses; shoulders are indi-
cated as sh. High-resolution (HR) EI-MS spectra were recorded
with a Waters Micromass AutoSpec-Ultima spectrometer, HR-ESI-
TOF-MS spectra with a Bruker maXis ESI-Q-TOF spectrometer,
and HR-FT-ICR-MALDI-MS and ESI-MS spectra with a Varian
IonSpec Fourier Transform (FT) ICR instrument with 3-hydroxy-
pyridine-2-carboxylic acid (3-HPA) or {(2E)-3-[4-(tert-butyl)-
phenyl]-2-methylprop-2-enylidene}malononitrile (DCTB) as ma-
trix. The most important signals are reported in m/z units with
M+ representing the molecular ion. The names of compounds were
generated with ACD Name 9 by Advanced Chemistry Develop-
ment Inc.
Mr = 269.30, triclinic, space group P1 (no. 2), Dx = 1.254 gcm–3,
¯
Z = 2, a = 8.9841(12), b = 9.0879(12), c = 10.2202(13) Å, α =
68.626(18), β = 76.636(19), γ = 67.430(11)°, V = 713.49(16) Å3.
Bruker–Nonius Kappa-CCD diffractometer, Mo-Kα radiation, λ =
0.7107 Å, μ = 0.076 mm–1. A blue crystal of 3 (linear dimensions
ca. 0.10ϫ0.05ϫ0.03 mm) was grown by slow diffusion of hexanes
into a CH2Cl2 solution of 3 at 25 °C. The numbers of measured
and unique reflections are 2993 and 1863, respectively (Rint
=
0.095). Final R(F) = 0.089, wR(F2) = 0.196 for 193 parameters and
1005 reflections with IϾ2σ(I) and 5.58Ͻ θϽ 22.99° (the corre-
sponding R values based on all 1863 reflections are 0.167 and
0.238, respectively).
X-ray Crystal Structure of 12: Crystal data at 223 K for C16H13N3,
Mr = 247.30, monoclinic, space group P21/c, Dx = 1.199 gcm–3, Z
= 4, a = 7.9783(5), b = 13.3236(10), c = 13.4951(8) Å, α = 90.00, β
= 107.212(4), γ = 90.00°, V = 1370.3(2) Å3. Bruker–Nonius Kappa-
CCD diffractometer, Mo-Kα radiation,
λ = 0.7107 Å, μ =
0.073 mm–1. A dark-brown crystal of 12 (linear dimensions ca.
0.24ϫ0.09ϫ0.09 mm) was obtained by slow evaporation of a solu-
tion of 12 in CDCl3 at 25 °C. The numbers of measured and unique
reflections are 4164 and 2451, respectively (Rint = 0.044). Final
R(F) = 0.0462, wR(F2) = 0.1303 for 224 parameters and 2451 re-
flections with IϾ2σ(I) and 2.425Ͻ θϽ 25.350° (the corresponding
R values based on all 4164 reflections are 0.0688 and 0.1513,
respectively).
CCDC-802809 (for 2), -802810 (for 3), and -802811 (for 12) contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Electrochemistry: Electrochemical measurements were carried out
at 20 °C in CH2Cl2 containing 0.1 m nBu4NPF6 in a classic three-
electrode cell. CH2Cl2 was purchased in spectroscopic grade from
Merck, dried with molecular sieves (4 Å), and stored under Ar
prior to use. Electrochemical grade nBu4NPF6 was purchased from
Fluka and used as received. The working electrode was a glassy
carbon disk electrode (3 mm in diameter) used either motionless in
cyclic voltammetry (CV; 0.1–10 Vs–1) or as a rotating-disk elec-
trode in rotating disk voltammetry (RDV). The auxiliary electrode
was a Pt wire and the reference electrode was either an aqueous
Ag/AgCl electrode or a platinum wire used as a pseudo-reference
electrode. All potentials are referenced to the ferricinium/ferrocene
(Fc+/Fc) couple, used as an internal standard, and are uncorrected
from ohmic drop. The cell was connected to an Autolab
PGSTAT30 potentiostat (Eco Chemie BV, Utrecht, The Nether-
lands) driven by GPSE software running on a personal computer.
General Method for the “One-Pot” Oxidation/Knoevenagel Conden-
sation: Dess–Martin periodinane (1–3 equiv. of a 15% solution in
CH2Cl2) was added to a solution of the appropriate propargyl
alcohol derivative in CH2Cl2. The resulting solution was stirred at
25 °C for the indicated time, filtered through a plug of SiO2
(CH2Cl2), and concentrated in vacuo (without heating) to about
50% of its original volume. Malononitrile (1–3 equiv.) and Al2O3
(2–12 equiv.) were added and the solution was stirred at the indi-
cated temperature and time. The solvents were evaporated in vacuo
and the residue subjected to FC.
{5-[4-(Dimethylamino)phenyl]-2,4-pentadiyn-1-ylidene}malononitrile
(2): The title compound was prepared from 13 (300 mg, 1.51 mmol)
and Dess–Martin periodinane (958 mg, 2.26 mmol) in CH2Cl2
(50 mL) according to the general method. The mixture was stirred
for 1.5 h. Then malononitrile (176 mg, 2.66 mmol) and Al2O3
X-ray Analysis: The structures were solved by direct methods (SIR-
97)[54] and refined by full-matrix least-squares analysis (SHELXL- (1.23 g, 12.05 mmol) were added. The mixture was stirred for 1 h
4314
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 4307–4317

CϵC and C=C Bonds as Spacers in Push–Pull Chromophores
at 50 °C and purified by FC (CH2Cl2/heptane, 8:2, then CH2Cl2] to
0.22 mmol) and Dess–Martin periodinane (143 mg, 0.34 mmol) in
CH2Cl2 (40 mL) according to the general method. The mixture was
stirred for 1 h. Then malononitrile (18 mg, 0.27 mmol) and Al2O3
afford 2 (173 mg, 47%) as a dark-red solid. Rf = 0.2 (SiO2, heptane/
1
AcOEt, 8:2); m.p. 142–145 °C. H NMR (300 MHz, CDCl3): δ =
3.06 (s, 6 H), 6.63 (d, J = 9.0 Hz, 2 H), 7.00 (s, 1 H), 7.45 (d, J = (74 mg, 0.73 mmol) were added. The mixture was stirred for 1 h at
9.0 Hz, 2 H) ppm. 13C NMR (75 MHz, C2D2Cl4): δ = 39.93, 73.37,
50 °C and purified by FC (CH2Cl2/heptane, 8:2, then CH2Cl2) to
93.40, 100.19, 101.53, 105.13, 111.56, 112.66, 135.05, 140.31,
afford 3 (29 mg, 48%) as a dark-blue metallic solid; m.p. Ͼ 180 °C
(caution! explosive decomposition). H NMR (300 MHz, CDCl3): δ
1
151.72 (11 out of 13) ppm. IR (neat): ν = 3008, 2905, 2353, 2232,
˜
2154, 1591, 1546, 1520, 1446, 1371, 1312, 1260, 1224, 1198, 1166,
1141, 1122, 1061, 942, 864, 815 cm–1. UV/Vis (CH2Cl2): λmax (ε)
= 234 (20300), 330 (21800), 343 (21300), 512 nm (33300 m–1 cm–1).
HRMS (EI): calcd. for C16H11N3 [M]+ 245.0953; found 245.0945.
= 3.05 (s, 6 H), 6.61 (d, J = 9.1 Hz, 2 H), 6.97 (s, 1 H), 7.45 (d, J
= 9.1 Hz, 2 H) ppm. 13C NMR (75 MHz, C2D2Cl4): δ = 40.51,
65.91, 72.91, 82.73, 91.07, 96.25, 100.09, 104.79, 111.70, 112.04,
112.76, 120.58, 135.55, 140.47, 151.70 ppm. IR (neat): ν = 3001,
˜
2918, 2229, 2104, 2061, 1598, 1543, 1397, 1372, 1329, 1232, 1184,
1067, 899, 806 cm–1. UV/Vis (CH2Cl2): λmax (ε) = 269 (sh, 19400),
276 (21400), 328 (15800), 375 (20500), 538 nm (16100 m–1 cm–1).
HRMS (EI): calcd. for C18H11N3 [M]+ 269.0948; found 269.0947.
(2E)-5-[4-(Dimethylamino)phenyl]-2-penten-4-yn-1-ol (14): A solu-
tion of Red-Al® (1.36 mL, 898 mg, 4.44 mmol, 65% in toluene) was
added to a solution of 13 (385 mg, 1.93 mmol) in THF (40 mL) at
0 °C. After 1 h, the mixture was warmed to 25 °C and stirred for
3 h. After cooling to 0 °C, AcOEt (30 mL), H2O (30 mL), and 1 m
aq. NaOH (30 mL) were added. The aqueous phase was extracted
with Et2O (2ϫ100 mL) and the combined organic phases were
washed with sat. aq. NaCl (150 mL) and dried with MgSO4. The
solvents were evaporated in vacuo and the residue purified by FC
(heptane/AcOEt, 95:5, then heptane/AcOEt, 9:1, then heptane/Ac-
7-[4-(Dihexylamino)phenyl]-2,4,6-heptatriyn-1-ol (18): A solution of
the Hay catalyst in CH2Cl2 (40 mL) was added to alkyne 16
(413 mg, 1.33 mmol) and prop-2-yn-1-ol (1.18 mL, 20.22 mmol) in
CH2Cl2/MeOH/THF (1:1:1; 60 mL). The resulting mixture was
stirred, exposed to air, for 2 h at 25 °C. The solvent was evaporated
in vacuo and the residue purified by FC (heptane then heptane/
AcOEt, 8:2, then heptane/AcOEt, 1:1) to afford 18 (330 mg, 68%)
1
OEt, 1:1) to afford 14 (203 mg, 52%) as an orange oil. H NMR
1
(300 MHz, CDCl3): δ = 1.41 (br. s, 1 H), 2.98 (s, 6 H), 4.23–4.25
(m, 2 H), 5.96 (dt, J = 15.7, 1.7 Hz, 1 H), 6.27 (dt, J = 15.7, 5.5 Hz,
1 H), 6.63 (d, J = 9.1 Hz, 2 H), 7.29–7.34 (m, 2 H) ppm. 13C NMR
as a brown oil. H NMR (300 MHz, CDCl3): δ = 0.85–0.92 (m, 6
H), 1.24–1.31 (m, 12 H), 1.54–1.64 (m, 5 H), 3.26 (t, J = 7.7 Hz, 4
H), 4.39 (d, J = 5.5 Hz, 2 H), 6.5 (d, J = 9.3 Hz, 2 H), 7.35 (d, J
(75 MHz, CDCl3): δ = 40.25, 63.28, 85.23, 91.47, 109.82, 111.36, = 9.1 Hz, 2 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 13.93, 22.56,
111.70, 132.52, 139.56, 149.89 ppm. IR (neat): ν = 3306, 2859,
26.64, 27.01, 31.56, 50.83, 51.62, 64.59, 65.28, 71.27, 72.48, 79.78,
˜
2358, 2188, 1888, 1601, 1519, 1445, 1359, 1226, 1189, 1170, 1120,
1083, 1035, 985, 946, 814 cm–1. HRMS (ESI): calcd. for
C13H16NO+ [MH]+ 202.1226; found 202.1226.
104.91, 111.00, 134.59, 148.80 (16 out of 17) ppm. IR (neat): ν =
˜
3366, 2924, 2852, 2361, 2172, 1597, 1517, 1465, 1402, 1363, 1293,
1253, 1186, 1000, 810 cm–1. HRMS (EI): calcd. for C25H33NO
[M]+ 363.2557; found 363.2556.
{(2E)-5-[4-(Dimethylamino)phenyl]-2-penten-4-yn-1-ylidene}malono-
nitrile (12): The title compound was prepared from 14 (185 mg,
0.92 mmol) and Dess–Martin periodinane (585 mg, 1.38 mmol) in
CH2Cl2 (20 mL) according to the general method. The mixture was
stirred for 1 h. Then malononitrile (172 mg, 2.60 mmol) and Al2O3
(225 mg, 2.21 mmol) were added. The mixture was stirred for 6 h
at 50 °C and purified by FC (CH2Cl2/heptane, 8:2) to afford 12
{7-[4-(Dihexylamino)phenyl]-2,4,6-heptatriyn-1-ylidene}malono-
nitrile (4): The title compound was prepared from 18 (92 mg,
0.25 mmol) and Dess–Martin periodinane (161 mg, 0.38 mmol) in
CH2Cl2 (35 mL) according to the general method. The mixture was
stirred for 1.5 h. Then malononitrile (25 mg, 0.38 mmol) and Al2O3
(116 mg, 1.14 mmol) were added. The mixture was stirred for 1 h
at 50 °C and purified by FC (CH2Cl2/pentane, 1:1, then CH2Cl2)
(87 mg, 38%) as a red solid. Rf = 0.2 (SiO2, heptane/AcOEt, 8:2);
1
m.p. 119–121 °C. H NMR (300 MHz, CDCl3): δ = 3.05 (s, 6 H), to afford 4 (20 mg, 19%) as a blue metallic solid; m.p. 65–67 °C.
6.59–6.67 (m, 3 H), 6.99 (dd, J = 11.8 Hz, 1 H), 7.37–7.45 (m, 3
1H NMR (300 MHz, CDCl3): δ = 0.9 (t, J = 6.5 Hz, 6 H), 1.26–
1.32 (m, 12 H), 1.55–1.58 (m, 4 H), 3.29 (t, J = 7.8 Hz, 4 H), 6.53
H) ppm. 13C NMR (75 MHz, CDCl3): δ = 40.10, 81.31, 89.69,
107.38, 110.27, 111.58, 113.70, 130.69, 131.12, 134.23, 151.31, (d, J = 9.0 Hz, 2 H), 6.97 (s, 1 H), 7.40 (d, J = 9.0 Hz, 2 H) ppm.
158.59 (12 out of 13) ppm. IR (neat): ν = 3038, 2918, 2851, 2360,
13C NMR (75 MHz, CDCl3): δ = 14.12, 22.73, 26.78, 27.2, 31.69,
˜
2221, 2155, 1734, 1606, 1557, 1538, 1444, 1369, 1259, 1222, 1184,
51.04, 65.48, 72.42, 73.59, 82.50, 90.76, 95.98, 99.87, 103.53,
1093, 1066, 1018, 963, 942, 812, 792 cm–1. UV/Vis (CH2Cl2): λmax 111.22, 112.29, 135.28, 139.63, 149.62 (19 out of 20) ppm. IR
(ε) = 294 (sh, 12000), 321 (14800), 505 nm (35900 m–1 cm–1). HRMS
(neat): ν = 2924, 2854, 2340, 2174, 2116, 2074, 1593, 1552, 1520,
˜
(EI): calcd. for C16H13N3 [M]+ 247.1104; found 247.1105.
1409, 1362, 1294, 1257, 1180, 1016, 812, 795 cm–1. UV/Vis
(CH2Cl2): λmax (ε) = 278 (17000), 332 (13200), 382 (16000), 560 nm
(12700 m–1 cm–1). HRMS (FT-MALDI, 3-HPA): calcd. for
C28H32N3 [MH]+ 410.2591; found 410.2584. C28H31N3 (409.57): C
82.11, H 7.63, N 10.26; found C 81.65, H 7.63, N 9.78.
7-[4-(Dimethylamino)phenyl]-2,4,6-heptatriyn-1-ol (17): A solution
of the Hay catalyst in CH2Cl2 (2.5 mL) was added to the alkyne
15 (76 mg, 0.45 mmol) and prop-2-yn-1-ol (0.11 mL, 1.91 mmol) in
CH2Cl2 (30 mL). The resulting mixture was stirred, exposed to air,
for 16 h at 25 °C. The solvent was evaporated in vacuo and the
residue purified by FC (heptane/AcOEt, 8:2, then heptane/AcOEt,
1:1) to afford 17 (53 mg, 53 %) as a light-brown oil. 1H NMR
(300 MHz, CDCl3): δ = 1.63 (t, J = 6.3 Hz, 1 H), 3.0 (s, 6 H), 4.39
(d, J = 5.9 Hz, 2 H), 6.59 (d, J = 8.7 Hz, 2 H), 7.39 (d, J = 9.0 Hz,
2 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 40.08, 51.64, 64.52,
65.33, 71.35, 72.71, 79.50, 106.22, 111.49, 134.37, 150.72 (11 out of
12) ppm. HRMS (EI): calcd. for C15H13NO [M]+ 223.0992; found
223.0991.
11-[4-(Dihexylamino)phenyl]-2,4,6,8,10-undecapentayn-1-ol (22) and
4,4Ј-(1,3,5,7,9,11-Dodecahexayne-1,12-diyl)bis(N,N-dihexylaniline)
(23)[45]
TMS Cleavage of 20: K2CO3 (995 mg, 7.20 mmol) was added to a
solution of the TMS-protected alkyne 20 (141 mg, 0.93 mmol) in
MeOH/THF (1:1; 10 mL). The mixture was stirred for 2 h at 25 °C.
CH2Cl2 (70 mL) was added and the organic layer was washed with
H2O (70 mL), dried with MgSO4, and filtered. The resulting solu-
tion of deprotected alkyne 20 was concentrated to around 30% of
{7-[4-(Dimethylamino)phenyl]-2,4,6-heptatriyn-1-ylidene}malono- its initial volume in vacuo and used directly in the oxidative cou-
nitrile (3): The title compound was prepared from 17 (50 mg, pling step.
Eur. J. Org. Chem. 2011, 4307–4317
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4315

F. Diederich et al.
FULL PAPER
TMS Cleavage of 21: K2CO3 (86 mg, 0.62 mmol) was added to a
solution of the TMS-protected triyne 21 (30 mg, 0.074 mmol) in
MeOH/THF (1:1; 6 mL). The resulting mixture was stirred for 2 h
at 25 °C. CH2Cl2 (40 mL) was added and the organic layer was
washed with H2O (40 mL), dried with MgSO4, and filtered. The
resulting solution of deprotected triyne 21 was concentrated to
around 50% of its initial volume in vacuo and used directly in the
next step.
Economic Development through the Ben Franklin Technology De-
velopment Authority.
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Oxidative Hay Coupling: A solution of the Hay catalyst in CH2Cl2
(3 mL) was added to a solution of the deprotected alkyne 21
(25 mg, 0.074 mmol) and deprotected 20 (75 mg, 0.93 mmol) in
MeOH/THF/CH2Cl2 (1:1:1; 30 mL) and the resulting mixture was
stirred, exposed to air, for 16 h at 25 °C. The mixture was pre-
adsorbed on SiO2 and purified by FC (pentane, then pentane/Ac-
OEt, 8:2, then pentane/AcOEt, 1:1, then AcOEt) to afford 22
(7 mg, 23%) as a red-brown solid and 23 (6 mg, 24%) as an orange
solid (yields over two steps).
22: Rf = 0.2 (SiO2, heptane/AcOEt, 8:2). 1H NMR (300 MHz,
CDCl3): δ = 0.90 (t, J = 6.6 Hz, 6 H), 1.21–1.31 (m, 12 H), 1.56–
1.68 (m, 5 H), 3.27 (t, J = 7.7 Hz, 4 H), 4.38 (s, 2 H), 6.50 (d, J =
9.1 Hz, 2 H), 7.37 (d, J = 8.8 Hz, 2 H) ppm. 13C NMR (125 MHz,
CDCl3): δ = 14.13, 22.73, 26.80, 27.18, 31.71, 50.99, 51.64, 62.19,
63.67, 63.83, 64.47, 64.53, 67.28, 70.96, 73.19, 80.18, 103.96,
111.11, 135.09, 149.05 (20 out of 21) ppm. IR (neat): ν = 3362,
˜
2955, 2922, 2852, 2360, 2342, 2177, 2087, 1700, 1595, 1519, 1464,
1401, 1364, 1258, 1224, 1187, 1091, 1018, 863, 810 cm–1. UV/Vis
(CH2Cl2): λmax (ε) = 239 (8700), 256 (9100), 266 (12400), 309
(4000), 328 (4200), 347 (4600), 370 (4600), 384 (sh, 3900), 423
(4700), 456 nm (4000 m–1 cm–1). HRMS (FT-MALDI, 3-HPA):
calcd. for C29H34NO [MH]+ 412.2635; found 412.2635.
23: Analytical data were identical to those reported for 23.[45]
{9-[4-(Dihexylamino)phenyl]-2,4,6,8-nonatetrayn-1-ylidene}malono-
nitrile (5): The title compound was prepared from 19[45] (19 mg,
0.049 mmol) and Dess–Martin periodinane (31 mg, 0.074 mmol) in
CH2Cl2 (6 mL) according to the general method. The mixture was
stirred for 10 min. Then malononitrile (5 mg, 0.079 mmol) and
Al2O3 (16 mg, 0.159 mmol) were added. The mixture was stirred
for 1 h at 25 °C and purified by FC (pentane, then pentane/AcOEt,
95:5, then pentane/AcOEt, 9:1) to afford 5 (8 mg, 38%) as a dark-
blue solid. Rf = 0.5 (SiO2, heptane/AcOEt, 8:2); m.p. Ͼ 40 °C (de-
comp.). 1H NMR (300 MHz, CDCl3): δ = 0.9 [t, 3J(H,H) = 6.2 Hz,
6 H], 1.24–1.31 (m, 12 H), 1.55 (br. s, 4 H), 3.29 [t, 3J(H,H) =
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3
7.6 Hz, 4 H], 6.52 [d, J(H,H) = 9.3 Hz, 2 H], 6.95 (s, 1 H), 7.40
3
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14.04, 22.67, 26.75, 27.16, 29.74, 31.66, 66.84, 70.35, 73.48, 73.56,
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˜
2342, 2178, 2153, 2069, 1729, 1595, 1554, 1524, 1464, 1425, 1402,
1362, 1293, 1259, 1191, 1096, 1016, 884, 796 cm–1. UV/Vis
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Supporting Information (see footnote on the first page of this arti-
cle): UV/Vis spectra, X-ray structure and electrochemistry details.
Acknowledgments
This work was supported by the ETH Research Council and the
European Research Council (ERC Advanced Investigators, grant
number 246637, “OPTELOMAC”). We also acknowledge partial
support from the Pennsylvania Department of Community and
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Received: March 18, 2011
Published Online: June 7, 2011
Eur. J. Org. Chem. 2011, 4307–4317
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4317