Property tuning in charge-transfer chromophores by systematic modulation of the spacer between donor and acceptor

Article abstract of DOI:10.1002/chem.200601735

A series of donor-acceptor chromophores was prepared in which the spacer separating 4-dimethylanilino (DMA) donor and C(CN)₂ acceptor moieties is systematically varied. All of the new push-pull systems, except 4b, are thermally stable molecules

Full text of DOI:10.1002/chem.200601735

DOI: 10.1002/chem.200601735
Property Tuning in Charge-Transfer Chromophores by Systematic
Modulation of the Spacer between Donor and Acceptor
Filip Buresˇ,[a] W. Bernd Schweizer,[a] Joshua C. May,[b] Corinne Boudon,[c]
[a]
Jean-Paul Gisselbrecht,[c] Maurice Gross,[c] Ivan Biaggio,[b] and FranÅoisDiederich*
Abstract: A series of donor–acceptor
chromophores was prepared in which
the spacer separating 4-dimethylanilino
(DMA) donor and C(CN)2 acceptor
moieties is systematically varied. All of
the new push–pull systems, except 4b,
are thermally stable molecules. In
series a, the DMA rings are directly at-
tached to the central spacer, whereas
in series b additional acetylene moiet-
ies are inserted. X-ray crystal structures
were obtained for seven of the new, in-
tensely colored target compounds. In
series a, the DMA rings are sterically
forced out of the mean plane of the re-
sidual p system, whereas the entire
conjugated p system in series b is
nearly planar. Support for strong
donor–acceptor interactions was ob-
tained through evaluation of the qui-
noid character of the DMA ring and
by NMR and IR spectroscopy. The
UV/Vis spectra feature bathochromi-
cally shifted, intense charge-transfer
bands, with the lowest energy transi-
tions and the smallest optical gap being
measured for the two-dimensionally
extended chromophores 6a and 6b.
The redox behavior of the push–pull
molecules was investigated by cyclic
voltammetry (CV) and differential
pulse voltammetry (DPV). In the series
1b, 2b, 4b, 5b, in which the spacer be-
tween donor and acceptor moieties is
systematically enlarged, the electro-
chemical gap decreases steadily from
1.94 V (1b) to 1.53 V (5b). This de-
crease is shown to be a consequence of
a reduction in the D–A conjugation
with increasing spacer length. Degener-
ate four-wave mixing experiments
reveal high third-order optical nonli-
nearities, pointing to potentially inter-
esting applications of some of the new
chromophores in optoelectronic devi-
ces.
Keywords:
N
N
optics · charge transfer · donor–
acceptor chromophores · electro-
T
chemistry
Introduction
third-order optical nonlinearities.[2] We reported strong in-
tramolecular charge-transfer (CT) interactions and high
third-order optical nonlinearities for three classes of small
push–pull chromophores, donor–acceptor-substituted tetrae-
thynylethenes (TEEs),[3] and, more recently, donor-substitut-
ed cyanoethynylethenes (CEEs)[4] and donor-substituted
1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs).[5] Useful struc-
ture–activity relationships with predictive power were estab-
lished, highlighting the dependence of third-order optical
nonlinearity from the energy of the longest-wavelength CT
transition and the extension of the linear donor–acceptor
conjugation pathways and demonstrating the beneficial ef-
fects of low molecular symmetry, and two-dimensional
donor–acceptor (D–A) conjugation.[3,4a] In particular, donor-
substituted CEEs showed some of the strongest third-order
nonlinear responses observed today.[4a] Optical and electro-
chemical band-gap tuning[4b,d,e,5a] through structural variation
revealed that the position of the longest-wavelength CTab-
sorption in the UV/Vis spectra is not a good measure for
the efficiency of D–A conjugation: stronger D–A coupling
Molecular and polymeric donor–acceptor (push–pull) chro-
mophores[1] continue to attract large interest for their opto-
electronic properties, in particular their high second- and
ˇ
[a] Dr. F. Bures, Dr. W. B. Schweizer, Prof. Dr. F. Diederich
Laboratorium für Organische Chemie, ETH-Zürich
Hçnggerberg, HCI, 8093 Zürich (Switzerland)
Fax : (+41)1-632-1109
E-mail: diederich@org.chem.ethz.ch
[b] J. C. May, Prof. Dr. I. Biaggio
Department of Physics and Center for Optical Technologies
Lehigh University, Bethlehem, PA 18015 (USA)
[c] Prof. C. Boudon, Dr. J.-P. Gisselbrecht, Prof. Dr. M. Gross
Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide
Institut de Chimie, UMR 7177, C.N.R.S
UniversitØ Louis Pasteur, 4, rue Blaise Pascal, 67000 Strasbourg
(France)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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Chem. Eur. J. 2007, 13, 5378 – 5387

FULL PAPER
leads to higher energy absorptions, whereas weaker coupling
between strong donor and acceptor moieties results in lower
HOMO–LUMO gaps.[1e,4b] The coupling between strong
donors and acceptors, and thus the optoelectronic proper-
ties, is modulated by variation of the spacer between
trates the strong electrophilic nature of the dicyanovinyl-ac-
tivated chromophore; similar conjugate amine additions to
sterically well accessible, activated alkynes had previously
been observed in our laboratory.[10] When the conditions of
the Knoevenagel condensation were changed, using Al2O3
as catalyst instead of piperidine, the desired butadiene 2b
was obtained in good yield.
On the way to chromophores 4a,b with a central hexa-
1,5-dien-3-yne-1,6-diyl spacer, the intermediate enynes
10a,b were prepared from 3-(trimethylsilyl)propargyl alco-
hol by PCC oxidation[12] and Corey–Fuchs dibromoolefina-
tion[8c,13] to give 9, followed by Suzuki cross-coupling[14] with
4-(dimethylamino)phenylboronic acid[15] or Sonogashira
A
influence of spacers on the optoelectronic properties of
small push–pull chromophores. Here we report the synthesis
and properties of molecules 1–6 featuring different conju-
gated spacers between the DMA donor and the C(CN)2 ac-
ceptor functions (Figure 1). With their high third-order opti-
cal nonlinearities, as revealed in preliminary measurements,
and their high thermal stability, some of these compounds
should become interesting chromophores for thin film for-
mation by vapor deposition techniques and applications in
optoelectronic devices.
cross-coupling
with
4-ethynyl-N,N-dimethylaniline
(Scheme 2). Removal of the Me3Si group in 10b and direct
lithiation was accomplished with MeLi·LiBr,[16] and reaction
with DMF, followed by pouring the mixture into aqueous
phosphate buffer,[17] surprisingly provided the remarkably
stable aldehyde 11, which was characterized by X-ray analy-
sis (see Supporting Information). In contrast, enyne 10a did
not afford any aldehyde in the same transformation. Buta-
diene 3b with the additional alkynyl substituent in the
spacer was subsequently formed by Al2O3-catalyzed Knoe-
venagel condensation.
Removal of the Me3Si protecting group in 10b with
nBu4NF and oxidative Hay cross-coupling of the obtained
terminal alkyne with propargyl alcohol afforded the moder-
ately stable alcohol 12 (Scheme 3). Subsequent Dess–Martin
oxidation[18] and Knoevenagel condensation provided 5b
with a central octa-1,7-diene-3,5-diyne-1,8-diyl spacer. While
10a could also be deprotected, the resulting terminal alkyne
did not undergo oxidative cross-coupling with propargyl al-
cohol, which prevented the preparation of 5a.
Figure 1. “Expanded” DMA-substituted cyanoethynylethenes with differ-
ent spacers between donor and acceptor moieties.
Results and Discussion
Synthesis of donor–acceptor
chromophoreswith different
spacers: The preparation of
compounds 1a,[6] 1b,[4b] and
2a[7] has been previously de-
scribed. The synthesis of 2b
started
from
ketone
7
(Scheme 1).[8] Deprotonation of
acetaldehyde tert-butylimine[9]
with LDA and addition to 7,
followed by imine hydrolysis,
provided the unsaturated alde-
hyde, which was treated with
malononitrile and piperidine in
a Knoevenagel reaction. This
sequence, however, did not pro-
vide 2b but rather hexatriene 8.
In contrast, 2a can be prepared
in 81% yield by this route,
starting from Michlerꢁs ketone.
Conjugate addition of piperi-
dine at intermediately formed
Scheme 1. Synthesis of chromophore 2b with a central 1,3-butadiene-1,4-diyl spacer. a) 1. LDA, THF ꢀ788C
! 208C, 14 h, then AcOH; 2. (CN)2CH2, piperidine, AcOH, CH2Cl2, 208C, 3 h; 37%; b) 1. LDA, THF ꢀ788C
! 208C, 14 h, then aq. ZnCl2; 2. (CN)2CH2, Al2O3 (act. II-III), CH2Cl2, D, 1 h; 71%. LDA=lithium diisopro-
2b under generation of 8 illus- pylamide.
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5379

F. Diederich et al.
X-ray crystal structure analysis
and spectroscopic evidence for
the efficiency of donor–accept-
or conjugation: All target com-
pounds, except 4b, were ob-
tained as stable, intensely col-
ored solids, readily soluble in
chlorinated solvents. Accord-
ingly, crystals suitable for X-ray
analysis were prepared by slow
diffusion of hexane into CH2Cl2
solutions of the chromophore at
208C. The ORTEP plots in
Figure 2 confirm the molecular
structures assigned to 1a, 1b,
2a, 2b, 3b, 5b, and 6b. T he
DMA moieties are roughly in
plane with the residual p chro-
mophore if separated from the
central spacer by an acetylene
Scheme 2. Synthesis of trisalkynylated dicyanobutadiene 3b. a) 1. PCC, Celite, mol. sieves (4) Š), CH2Cl2,
208C, 2.5 h; 2. CBr4, PPh3, Zn, CH2Cl2, 208C, 14 h; 69%; b) for 10a: 4-(dimethylamino)phenylboronic acid,
[PdCl2
N
G
2
CuI, Et3N, 20 8C, 14 h; 79%; c) MeLi.LiBr, THF, DMF, 08C, 1 h, then KH2PO4, H2O, Et2O, 5 8C; 67%; d)
(CN)2CH2, Al2O3, CH2Cl2, D, 15 min; 93%. PCC=pyridinium chlorochromate; DMF=N,N-dimethylform-
A
moiety
(n=1,
series
b,
Figure 1). In contrast, DMA
rings directly attached to the
central core are sterically
forced out of the mean plane of
the residual
p chromophore
(n=0, series a) and the two ar-
omatic rings in 1a and 2a adopt
nonplanar conformations typi-
cal for 1,1-diarylethenes.[21] Ad-
ditional crystal structures of in-
termediates 7 and 8 and side
product 11 are reported in the
Supporting Information.
Scheme 3. Synthesis of 5b with a central octa-3,5-diene-2,7-diyne-1,8-diyl spacer. a) nBu4NF, THF, 208C, 1 h,
then propargyl alcohol, CuCl, air, TMEDA, CH2Cl2, 208C, 14 h; 81%; b) Dess–Martin periodinane, CH2Cl2,
208C, 1 h, then (CN)2CH2, Al2O3, D, 1 h; 87%.
Several experimental data
evidencing the efficiency of
ground-state CTin the new
tBuMe2Si-protected propargyl alcohol served as a suitable
starting material for the synthesis of chromophores 4 and 6
(Scheme 4). Formylation provided aldehyde 13, which was
transformed by addition of lithiated tBuMe2Si-protected
propargyl alcohol and oxidation into bispropargylic ketone
14. Dibromoolefination of 13 and 14 yielded the dibromo-
chromophores are summarized in Table 1.
The reduction in bond length alternation in the DMA
rings seen in the crystal structures is a good indication for
the efficiency of the intermolecular ground-state CTfrom
the DNA donor to the cyano acceptor moieties. It can be
expressed by the quinoid character (dr) of the ring accord-
ing to Equation (1):
olefins 15 and 16, respectively. Subsequent Suzuki or Sono-
gashira cross couplings led to 17a,b and 18a,b, respectively,
from which the targeted CTchromophores 4b and 6a,b
were obtained by removal of the tBuMe2Si protecting
groups, Dess–Martin oxidation, and Knoevenagel condensa-
tion. Target molecule 4a could not be prepared since the al-
cohol, obtained by deprotection of 17a, was found to de-
compose under all attempted oxidation conditions (Dess–
Martin periodinane, MnO2,[8c] PCC,[12] tetrapropylammoni-
um perruthenate (TPAP),[19] or Swern oxidation[20]). D–A
chromophore 4b is only stable in solution; upon removal of
the solvent (CH2Cl2), rapid decomposition to a black soot
was observed.
dr ¼ ½ðaꢀbÞ þ ðcꢀbފ2 ꢁ ½ða0ꢀb0Þ þ ðc0ꢀb0ފ2
ð1Þ
where a, b, c and a’, b’, c’ are the bond lengths in the DMA
ring (shown for 1a in Figure 2).[4b,22] In benzene, dr equals 0,
whereas fully quinoid rings show values between 0.08 and
0.1. The data in Table 1 (dr between 0.037 and 0.070) are de-
rived from the experimental bond lengths in the X-ray crys-
tal structures and reveal high quinoid character for the
DMA rings in the new chromophores, even exceeding previ-
ously measured values for simpler DMA-substituted CEEs
and TCBDs. It should, however, be noted that the dr values
bear considerable uncertainties resulting from the uncertain-
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Chem. Eur. J. 2007, 13, 5378 – 5387

Donor–Acceptor Chromophores
FULL PAPER
Most probably, the distortion of
the DMA rings out of the plane
of the residual p-conjugated
chromophores accounts for the
reduced D–A conjugation in
series a.
The stretching frequency of
ꢂ
the cyano groups, n(C N), pro-
A
vides another indication for the
efficiency of the intramolecular
CT. This frequency shifts to
lower energy when passing
from series
a
to series
b
(2209 cmꢀ1 (2a) vs. 2138 cmꢀ1
(2b) or 2145 cmꢀ1 (6a) vs.
2115 cmꢀ1 (6b)), again reflect-
ing mainly the reduced D–A
conjugation in the chromo-
phores lacking full planarity.
Scheme 4. Synthesis of D–A chromophores 4b and 6a,b. a) MeLi·LiBr, THF, DMF, 208C, 1 h, then KH2PO4,
Electrochemistry: Electrochem-
ical investigation of chromo-
phores 1a–6b carried out by
cyclic voltammetry (CV) and
ꢀ ꢂ
H2O, Et2O, 5 8C; 89%; b) tBuMe2SiOCH2 C CH, nBuLi, THF, ꢀ208C, 15 min, then MnO2, CH2Cl2, 208C,
1 h; 97%; c) CBr4, PPh3, Zn, CH2Cl2, 208C, 2–24 h; 46% (15), 11% (16); d) for 17a,18a: 4-(dimethylamino)-
phenylboronic acid, [PdCl2
H
ethynyl-N,N-dimethylaniline, [PdCl2(PPh3)2], CuI, Et3N, 20 8C, 14 h; 83% (17b), 74% (18b); e) 1. nBu4NF,
C
THF, 208C, 1 h; 2. Dess–Martin periodinane, CH2Cl2, 208C, 1 h; 3. (CN)2CH2, Al2O3, CH2Cl2, D, 1 h; ꢁ70%
(4b), 42% (6a), 64% (6b).
rotating
disc
voltammetry
(RDV) gave, for two consecu-
tive scans, quite often irrepro-
ducible curves due to the insu-
Table 1. Quinoid character and spectral properties evidencing the effi-
ciency of ground-state D–A conjugation.
lating film formation either during oxidation or reduction.
However, on polished electrodes, reproducible results could
be observed. For these reasons, the working electrode was
polished before each scan. The acquired data are summar-
ized in Table 2. As a general trend, the first oxidation occurs
on the DMA groups, whereas the first reduction involves
the conjugated core bearing the cyano moieties as observed
in previous studies on donor-substituted CEEs.[4b,e]
Series a, bearing DMA groups directly on the ethylene
bridge, gave two well-separated reversible one-electron oxi-
dations. The observed potential splitting results from elec-
trostatic interactions occurring in the oxidized form. For
chromophores 1b–6b (series b), the oxidation of the DMA
groups occurs in a single, irreversible two-electron step. At
scan rates higher than 2 Vsꢀ1, the electron transfer became
reversible. For these compounds, no electrostatic interac-
tions could be observed. The irreversible behavior at low
scan rates denotes a lower stability of the generated bis(rad-
ical cation) than for serie a, presumably due to the more ef-
ficient electronic communication in the more planar chro-
mophores.
[c]
ꢂ
Compound
dr [Š][a]
dmeta [ppm][b]
n
N
A
]
1a
1b
2a
2b
3b
4b
5b
6a
6b
0.046/0.070
7.37
7.52
2193
2140
2209
2138
[d]
0.067/0.037
7.35/7.11
7.46
7.47/7.44
7.46/7.41
7.45/740
7.36
[d]
0.069/0.064
2133
[d]
[e]
0.072/0.062
2125
2145
2115
[f]
0.046/0.057
7.51
[a] Quinoid character of the DMA rings (right/left according to Figure 1).
[b] 1H NMR chemical shifts (CDCl3, 300 MHz) of the DMA protons
ꢂ
meta to the Me2N group. [c] Frequency of the C N stretch. [d] The dr
values are not reliable due to disorder in the crystal structure. [e] Not de-
termined due to instability of the solution. [f] No single crystal prepared.
ties of the individual experimental bond lengths entering
Equation (1). Nevertheless, the data support strong intramo-
lecular CTinteractions in the ground state.
A comparison of the 1H NMR chemical shifts of the
DMA protons in meta position to the Me2N substituent, dm,
nicely demonstrates the beneficial contributions of the addi-
Reduction occurs as a reversible one-electron step for
chromophores 1a, 1b, 2a, 2b, and 3b, whereas for 4b, 5b,
6a, and 6b the electron transfers were irreversible. Only the
reduction of 4b became reversible at scan rates higher than
1 Vsꢀ1. Along the entire series (a and b), the reduction po-
tentials were shifted to more anodic potentials, with Ered,1 in
the range from ꢀ1.90 to ꢀ1.38 V for the three chromo-
phores 1a, 2a, 6a and from ꢀ1.34 to ꢀ1.00 V for chromo-
tional acetylene units in series b to the efficiency of ground-
[4b]
state CTconjugation.
In the pairs of chromophores with
identical central spacer (1a,b, 2a,b, 6a,b), this resonance
appears much more downfield shifted in series b (Table 1),
which mainly reflects a larger reduction of the electron den-
sity in the DMA rings as a result of better D–A conjugation.
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5381

F. Diederich et al.
Figure 2. ORTEP representations of the new D–A chromophores a) 1a, b) 1b, c) 2a, d) 2b, e) 3b, f) 5b, and g) 6b with vibrational ellipsoids obtained
shown at the 50% probability level (for recording temperatures, see Supporting Information).
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Donor–Acceptor Chromophores
FULL PAPER
Table 2. Electrochemical data of chromophores 1a-6b observed by cyclic
voltammetry (CV) at a scan rate of 0.1 Vsꢀ1 and rotating disk voltamme-
try (RDV) in CH2Cl2 (+0.1m nBu4NPF6). All potentials are given versus
the ferricinium/ferrocene couple.
chromphores 6a and 6b feature two particularly strongly
bathochromically shifted bands at 500/632 (6a) and 497/629
(6b) nm, respectively, reflecting two different CTtransi-
tions. The CT character of the longest-wavelenth absorption
of all chromophores was confirmed by protonation/neutrali-
zation experiments (see Supporting Information). Upon pro-
tonation of the Me2N groups with p-toluenesulfonic acid
(PTSA), all of the CH2Cl2 solutions turned from intensely
colored (for the colors see the Supporting Information) to
nearly colorless, and the longest-wavelength absorption
bands disappeared almost completely. Full regeneration of
the original CTband occurred upon neutralization with
Et3N (Supporting Information).
Table 3 reports both the position of the longest-wave-
length CTtransitions ( lmax) as well as the optical gaps corre-
sponding to the end-absorption lend. Also shown are the
electrochemical HOMO–LUMO gaps, calculated as the dif-
ference between first oxidation and reduction potentials
D(Eox,1ꢀEred,1). A plot of the optical band gap, calculated
from lmax of the CTband or the end-absorption lend, for
chromophores 1a, 1b, 2a, 2b, 3b, 4b, and 5b, with the same
number of DMA donor and CN acceptor moieties, against
the electrochemical band gap shows a linear correlation
(R=0.95 for lmax and R=0.97 for lend) between the two
quantities (see Supporting Information).[4b] This means that
in both quantities, the same orbitals (HOMO and LUMO)
are involved.
The influence of the spacer on the efficiency of the D–A
conjugation is best evaluated in the series 1b, 2b, 4b, 5b, in
which the donor–acceptor separation is systematically ex-
panded. In this series, the electrochemical gap decreases
steadily from 1.94 V (1b) to 1.53 V (5b) (Table 3). This de-
crease is a consequence of a reduction in the D–A conjuga-
tion with increasing spacer length.[4b] In DMA-substituted
tetraethynylethene (TEE), the first DMA-centered oxida-
tion occurs at +0.44 V,[23] whereas in bis-(iPr)3Si-protected
1,1-dicyano-2,2-diethynylethene, the first reduction is ob-
served at ꢀ1.15 V.[4b] The first of the two control compounds
features an “unperturbed” DMA donor, the second an “un-
perturbed” C(CN)2 acceptor. The first oxidation and reduc-
tion potentials, measured for compounds 4b and 5b with
the larger spacers, are similar to those of the two control
Compound
Cyclic voltammetry
Rotating disk
voltammetry
E8
DEp
Ep
E1/2
Slope
[V][a]
[mV][b]
[V][c]
[V][d]
[mV][e]
1a
+0.82[f]
+0.65
ꢀ1.85
100
65
90
+0.65 (1eꢀ)
ꢀ1.90 (1eꢀ)
ꢀ2.45
70
75
ꢀ2.38
+0.75
75
[g]
1b
2a
+0.60 (2eꢀ)
ꢀ1.34 (1eꢀ)
ꢀ2.05
ꢀ1.27
80
65
100
100
75
ꢀ2.06
+0.71[h]
+0.72 (1eꢀ)
+0.52 (1eꢀ)
ꢀ1.71 (1eꢀ)
ꢀ2.45
+0.51
ꢀ1.61
100
70
75
ꢀ2.28
+0.54
100
[g]
2b
3b
4b
+0.53 (2eꢀ)
ꢀ1.25 (1eꢀ)
ꢀ2.07
ꢀ1.23
ꢀ1.21
70
90
160
ꢀ1.89
+0.57
150
+0.55 (2eꢀ)
ꢀ1.26 (1eꢀ)
ꢀ1.86
[g]
90
100
[g]
+0.43
ꢀ1.24
ꢀ1.74
+0.46
ꢀ1.07
+0.40
ꢀ1.18 (1eꢀ)
70
ꢀ1.68 (1eꢀ)
70
[g]
5b
6a
ꢀ1.06 (1eꢀ)
+0.70 (1eꢀ)
+0.51 (1eꢀ)
60
75
70
+0.70
+0.55
60
100
ꢀ1.35
+0.53
ꢀ0.96
ꢀ1.38 (2eꢀ)
110
[g]
6b
ꢀ1.00
ꢀ1.52
60
150
[a] E8=(Epc +Epa)/2, where Epc and Epa correspond to the cathodic and
anodic peak potentials, respectively. [b] DEp =EoxꢀEred, where the sub-
scripts ox and red refer to the conjugated oxidation and reduction steps,
respectively. [c] Epc =Irreversible peak potential. [d] E1/2 =Half-wave po-
tential. [e] Slope of the linearized plot of E versus log[I/
A
Ilim is the limiting current and I the current. [f] Reversible at scan rates
>0.5 Vsꢀ1. [g] Insulating film formation at the electrode surface during
oxidation [h] Reversible at scan rates >1.0 Vsꢀ1
.
phores 1b–6b. This reflects the reduced transfer of electron
density from the DMA donor to the dicyanovinyl acceptor
moieties with increasing spacer length (see below). Four
cyano groups in chromophores
6a and 6b cause the largest
anodic shift of the first reduc-
tion potential along both series
(Ered,1 =ꢀ1.38 V for 6a and
ꢀ1.00 V for 6b).
UV/Vis spectroscopy: All new
chromophores exhibit an in-
tense intramolecular CTband,
with the lmax of compounds 1a–
5b appearing between 425 and
559 nm (Figure 3 and Figure 4,
Table 2). Interestingly, the two-
dimensionally
conjugated Figure 3. UV/Vis spectra of the chromophores in series a in CH2Cl2.
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5383

F. Diederich et al.
acceptor groups (Table 4): The
grot obtained for chromophore
5b was significantly larger even
though its more one-dimension-
al geometry has
a negative
effect on the rotational averag-
ing. Using chromophore 3b as
an example, we find that its ro-
tationally averaged third-order
polarizability corresponds to
about 1”10ꢀ48 m5 Vꢀ2 for each
delocalized electron (3b has
eight double and five triple
bonds), which is one measure
for the efficiency of the molecu-
Figure 4. UV/Vis spectra of the chromophores in series b in CH2Cl2. The molar extinction coefficients of 4b
have substantial uncertainties due to the instability of the solution.
Table 3. Optical and electrochemical gaps of 1a–6b determined from
UV/Vis spectroscopy and CV or RDV in CH2Cl2.
Table 4. Third-order polarizabilities grot measured by degenerate four-
wave mixing at l=1.5 mm in CH2Cl2.
Compound
lmax [nm (eV)]
lend [nm (eV)]
D(Eox,1ꢀEred,1) [V]
Compound
grot ”10ꢀ48 m5 Vꢀ2
1a
1b
2a
2b
3b
4b
5b
6a
6b
434 (2.88)
524 (2.37
510 (2.43)
655 (1.89)
590 (2.10)
695 (1.78)
730 (1.70)
740 (1.68)
785 (1.58)
755 (1.64)
845 (1.47)
2.55
1.94
2.23
1.78
1.81
1.58
1.53
1.89
1.49
1a
1b
2a
2b
3b
4b
5b
6a
6b
2ꢃ2
14ꢃ3
4ꢃ3
498 (2.49)
534 (2.32)
559 (2.22)
534 (2.32)[a]
530 (2.34)[a]
632 (1.96)
629 (1.97)[a]
18ꢃ5
25ꢃ3
13ꢃ3[a]
33ꢃ15
8ꢃ4[b]
47ꢃ20[b]
[a] Peak fitting by Gaussian deconvolution of the broad absorption bands
of 4b, 5b, and 6b gave hidden peak maxima (observed as shoulders on
the broad absorptions) at lmax =580 nm (2.14 eV, 4b), 600 nm (2.07 eV,
5b) and 700 nm (1.77 eV, 6b).
[a] Only stable in dichloromethane solution. [b] Low solubility in di-
chloromethane.
lar nonlinearity. Another relevant figure of merit that de-
scribes the nonlinear efficiency of a molecule is its specific
third-order polarizability (grot per molecular mass),[4a] which
for this compound is 3”10ꢀ23 m5 Vꢀ2 Kgꢀ1. Both these values
hint at a very efficient third-order nonlinearity. In fact, the
third-order polarizability of 3b is only about 50 times small-
er than the fundamental maximum limit for an idealized
molecule containing the same number of delocalized elec-
trons and with the same HOMO–LUMO gap.[24] A full dis-
cussion of chromophores 6a and 6b is not possible at this
stage because their low solubility in dichloromethane hin-
dered a precise determination of their grot, and because of
enhancements in the experimental grot that are caused by
their 2D structure and, particularly in the case of 6b, by the
proximity of the 1.5 micrometer wavelength we used to the
first two-photon absorption transition. The large off-reso-
nant third-order polarizabilities observed in such small mol-
ecules as 2b, 3b, 5b rival what was seen in the most efficient
molecules measured to date.[4a] From their specific third-
order polarizability one can expect that the off-resonant
third-order susceptibility for a dense assembly of these mol-
ecule would be more than 3 orders of magnitude larger than
for fused silica. With these nonlinear optical properties, the
mentioned chromophores are attractive candidates for
compounds in which donor and acceptor are “unperturbed”
(Table 2). In contrast, in 1b (and to a lesser extent also in
2b), efficient D–A coupling across the shorter spacer shifts
electron density from the DMA donor to the C(CN)2 ac-
ceptors. As a result, the DMA-centered HOMO becomes
lowered, and the C(CN)2-centered LUMO is raised, leading
to a larger HOMO–LUMO gap. Thus, the obtained results
confirm in an impressive way previous findings that the
band gap can be tuned by insertion of acetylenic and olefin-
ic spacers that “insulate” strong donors and acceptors and
reduce the efficiency of their conjugation.[4b,5a]
Nonlinear optical properties: The third-order optical nonli-
nearity of the new D–A chromophores was determined by
measuring the third-order susceptibility of a CH2Cl2 solution
with varying molecular concentration, which gave us the ro-
tational average of the third-order polarizability grot.[4a] We
used degenerate four-wave mixing at a wavelength of 1.5 mm
and fused silica with c(3) =1.9”10ꢀ22 m2 Vꢀ2 as a reference; a
detailed account of these investigations will be published
elsewhere. Unless otherwise noted, the third-order polariza-
bilities measured in this way are off-resonant, valid in the
long-wavelength limit. We found a general trend by which
grot increases with the size of the spacer between donor and
5384
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ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 5378 – 5387

Donor–Acceptor Chromophores
FULL PAPER
(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 presure of N2. Column chromatography and plug filtrations were
carried out with SiO2 60 (particle size 0.040–0.063 mm, 230–400 mesh;
Fluka) and distilled technical solvents. Thin-layer chromatography (TLC)
was conducted on Al2O3 sheets coated with SiO2 60 F254 obtained from
Macherey–Nagel; visualization with a UV lamp (254 or 366 nm). Melting
points (M.p.) were measured on a Büchi B-540 melting-point apparatus
in open capillaries and are uncorrected. 1H NMR and 13C NMR spectra
were measured on a Varian Gemini 300 at 208C. Chemical shifts are re-
ported in ppm relative to the signal of Me4Si. Residual solvent signals in
the 1H and 13C NMR spectra were used as an internal reference. Cou-
pling constants (J) are given in Hz. The apparent resonance multiplicity
is described as s (singlet), br s (broad singlet), d (doublet), t (triplet), q
(quartet), and m (multiplet). Infrared spectra (IR) were recorded on a
Perkin–Elmer Spectrum BX instrument. UV/Vis spectra were recorded
on a Varian Cary-5 spectrophotometer. The spectra were measured in
CH2Cl2 in a quartz cuvette (1 cm). The absorption wavelenghts are re-
ported in nm with the molar extinction coefficient e (molꢀ1 dm3 cmꢀ1) in
brackets; shoulders are indicated as sh. High-resolution (HR) EI-MS
spectra were measured on a Hitachi-Perkin-Elmer VG-Tribrid spectrom-
eter. HR FT-MALDI spectra were measured on an IonSpec Ultima Four-
ier transform (FT) instrument with [(2E)-3-(4-tert-butylphenyl)-2-methyl-
prop-2-enylidene]malononitrile (DCTB) or 3-hydroxypicolinic acid (3-
HPA) as matrix. The most important signals are reported in m/z units
with M as the molecular ion. Elemental analyses were performed by the
Mikrolabor at the Laboratorium für Organische Chemie, ETH Zürich,
with a LECO CHN/900 instrument.
device integration, in particular since many of them feature
the thermal robustness required in fabrication techniques.[4a]
Conclusion
A series of new donor–acceptor chromophores was prepared
in which DMA donors and C(CN)2 acceptors are separated
by unsaturated spacers of different size and shape. The com-
pounds can be divided in two series: in series a, the DMA
rings attached directly to the central spacer, whereas in
series
b they are linked through additional acetylene
bridges. X-ray crystallographic analysis showed that the
DMA rings in series a are forced out of the mean plane of
the residual chromophore, whereas the entire chromophores
are nearly planar in series b. The high quinoid character of
the DMA rings in the crystal structures as well as NMR and
IR data support efficient donor–acceptor conjugation in the
ground state. The UV/Vis spectra of the highly colored
chromophores feature intense bathochromically shifted
charge-transfer bands, with two lowest-energy transitions
being observed for 6a and 6b with multiple donor–acceptor
conjugation paths. Cyclic voltammetry (CV) and differential
pulse voltammetry (DPV) showed one-electron reductions
centered on the DMA moieties and one-electron reductions
occurring on the residual electron-accepting chromophore
with the cyano moieties. This picture is in full agreement
with time-dependent calculations of HOMO and LUMO in
DMA-substituted cyanoethynylethenes (CEEs) such as
1b.[4d] In the series 1b, 2b, 4b, 5b, in which the spacer be-
tween donor and acceptor moieties is systematically expand-
ed, the electrochemical gap decreases steadily from 1.94 V
(1b) to 1.53 V (5b). In agreement with previous investiga-
tions[4b,5a] this decrease is a direct consequence of a reduc-
tion in the D–A conjugation with increasing spacer length.
This paper clearly demonstrates that the band gap in a
series of structurally related donor–acceptor chromophores
is conveniently tuned by insertion of acetylenic and/or ole-
finic spacers between donor and acceptor moieties. Weaker
electronic donor–acceptor coupling leads to lower-energy
CTabsorptions and this should serve as an extremely useful
guideline for chromophore design targeting optoelectronic
applications. Finally, degenerate four-wave mixing experi-
ments revealed that some of the new chromophores display
very high third-order polarizabilities grot. In view of their
considerable thermal robustness, these chromophores are
now investigated as components for nonlinear optics-based
device fabrication.
Acetaldehyde tert-butylimine was synthesized from acetaldehyde and
tert-butylamine in 85% yield and purified by distillation (b.p. 77–808C)
[lit.[9] b.p. 66–678C]. tBuMe2Si-protected propargyl alcohol was prepared
from propargyl acohol and tBuMe2SiCl in 93% yield following literature
procedure.[25] Final purification was carried out by vacuum distillation
(b. p. 27–298C at 0.7 T orr) [lit[2.5] b.p. 48–50 at 11 Torr]. 4-(Dimethylamino)-
phenylboronic acid was synthesized from 4-bromo-N,N-dimethylaniline
following literature procedure in 61% yield.[15] 4-Ethynyl-N,N-dimethyl-
aniline was prepared from 4-iodo-N,N-dimethylaniline and trimethylsilyl-
acetylene by Sonogashira cross-coupling reaction and final Me3Si group
removal (K2CO3, MeOH) in 92% overall yield. Compounds 1a,[6] 1b,[4b]
2a,[7] and 7[8] were prepared according to literature procedure.
Electrochemistry: Electrochemical measurements were carried out in
CH2Cl2 containing 0.1m nBu4NPF6 in a classical three-electrode cell by
cyclic voltammetry (CV) and rotating-disk voltammetry (RDV). The
working electrode was a glassy C disk (2 mm in diameter), the auxiliary
electrode a Pt wire, and the reference electrode an aqueous Ag/AgCl
electrode. The cell was connected to an Autolab PGSTAT20 potentiostat
(Eco Chemie, Holland) driven by a GPSE software running on a person-
al computer. All potentials are given versus Fc+/Fc used as internal refer-
ence and are uncorrected from ohmic drop.
Degenerate four-wave mixing: A full account on the NLO properties of
the chromophores described in this paper will be given elsewhere. For
[4a]
the method, see reference
.
X-ray analysis: CCDC-622730 (1a), CCDC-622731 (1b), CCDC-622732
(2a), CCDC-622733 (2b), CCDC-622734 (3b), CCDC-622735 (5b),
CCDC-622736 (6b), CCDC-622737 (7), CCDC-622738 (8), and CCDC-
622739 (11) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data request.cif.
2-{(2Z,4E)-5-[4-(Dimethylamino)phenyl]-3-[4-(dimethylamino)phenyl-
G
Experimental Section
stirred solution of (iPr)2NH (0.07 mL, 0.53 mmol) in THF (1 mL) at 08C
under N2, nBuLi (0.3 mL, 0.48 mmol, 1.6m solution in hexane) was
added, and the mixture stirred for 15 min. Acetaldehyde tert-butylimine
(37.5 mg, 0.38 mmol) was added dropwise at 08C, and the solution was
transferred to a suspension of ketone 7 (100 mg, 0.32 mmol) in THF
(5 mL) at ꢀ788C. The mixture was allowed to reach 208C and stirred
overnight. The orange solution was concentrated in vacuo and the resi-
due treated (15 min) with AcOH (2 mL, 8m solution), diluted with water
Materialsand general methods : Reagents and solvents were purchased at
reagent grade from Acros, Aldrich, and Fluka, and used as received.
THF was freshly distilled from Na/benzophenone under N2 atmosphere.
Evaporation and concentration in vacuo was performed at water aspira-
tor pressure. Hay catalyst refers to a freshly prepared solution of CuCl
(65 mg,
0.66 mmol)
and
N,N,N’,N’-tetramethylethylenediamine
Chem. Eur. J. 2007, 13, 5378 – 5387
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5385

F. Diederich et al.
(30 mL), and extracted with Et2O (3”50 mL). The combined organic
layers were washed with saturated aqueous NaHCO3 (2”50 mL), saturat-
ed aqueous NaCl, dried (MgSO4), and evaporated in vacuo to leave an
orange solid of crude unsaturated aldehyde. The aldehyde was taken up
into CH2Cl2 (5 mL), and malononitrile (21 mg, 32 mmol), acetic acid (1
drop), and piperidine (1 drop) were added. The mixture was stirred for
3 h and concentrated in vacuo. Column chromatography (SiO2; CH2Cl2)
752 cmꢀ1
;
UV/Vis (CH2Cl2): lmax (e)=297 (sh, 30000), 479 nm
(44200 molꢀ1 dm3 cmꢀ1); HR-FT-MALDI-MS (DCTB) m/z: calcd for
C28H30N2OSi+: 438.2122; found 438.2129 ([M]+).
2-{5-[4-(Dimethylamino)phenyl]-3-[4-(dimethylamino)phenylethynyl]-2-
[(trimethylsilyl)ethynyl]pent-2-en-4-ynylidene}malononitrile (3b): To the
solution of 11 (30 mg, 0.068 mmol), malononitrile (6.6 mg, 0.1 mmol) in
CH2Cl2 (50 mL) and Al2O3 (50 mg, 0.5 mmol) were added and the mix-
ture heated to reflux for 15 min. Filtration through a plug (SiO2; CH2Cl2)
afforded 3b (31 mg, 93%) as a golden metallic solid. Rf =0.8 (SiO2;
CH2Cl2); m.p. 219–2208C; 1H NMR (300 MHz, CDCl3, 208C, TMS): d=
0.33 (s, 9H; SiCH3), 3.06 (s, 6H; NCH3), 3.07 (s, 6H, NCH3), 6.65 (d, 3J-
afforded
8 (48 mg, 37%) as a dark metallic solid. Rf =0.32 (SiO2;
CH2Cl2); m.p. 255–2568C; 1H NMR (300 MHz, CDCl3, 208C, TMS): d=
1.60–1.75 (m, 6H; pip.), 2.97 (s, 6H; NCH3), 2.98 (s, 6H; NCH3), 3.30–
3.45 (m, 4H; pip.), 5.67 (s, 1H; CH), 6.40 (d, 3J
ACHTREUNG
CH), 6.47 (d, 3J
Ar), 6.91 (d, 3J
A
ACHTREUNG
A
(H,H)=9.0 Hz, 2H; Ar), 7.47 (d, 3J-
C
ACHTREUNG
AHCTREUNG
ACHTREUNG
CDCl3, 208C, TMS): d=ꢀ0.28, 40.16, 79.74, 86.59, 91.35, 96.90, 106.97,
107.73, 109.97, 110.62, 111.56, 111.62, 111.81, 112.17, 115.90, 122.16,
127.17, 133.91, 134.54, 151.36, 151.45, 152.26 ppm; IR (neat): n˜ =2900,
2133, 1594, 1525, 1439, 1363, 1246, 1185, 1103, 997, 943, 841, 810, 760,
CDCl3, 208C, TMS): d=24.26, 26.23, 40.02, 50.89, 63.14, 77.10, 84.37,
105.45, 108.39, 110.99, 111,50, 116.19, 118.63, 119.71, 121.85, 131.71,
133.58, 144.49, 150.48, 151.97, 154.42, 164.74 ppm; IR (neat): n˜ =2930,
2200, 2163, 1604, 1473, 1436, 1358, 1185, 1062, 993, 940, 873, 815,
668 cmꢀ1
;
UV/Vis (CH2Cl2): lmax (e)=303 (sh, 37200), 559 nm
676 cmꢀ1
; UV/Vis (CH2Cl2): lmax (e)=290 (29500), 580 nm (sh,
(59400 molꢀ1 dm3 cmꢀ1); HR-FT-MALDI-MS (DCTB) m/z: calcd for
C31H30N4Si+: 486.2234; found 486.2230 ([M]+).
59500 molꢀ1 dm3 cmꢀ1); HR-FT-MALDI-MS (3-HPA) m/z: calcd for
C31H34N5+: 476.2809; found 476.2820 ([MH]+).
“One-pot” silyl deprotection/oxidation/Knoevenagel condensation—gen-
eral method: To a solution of tBuMe2Si-protected alcohol (0.21 mmol) in
THF (20 mL), nBu4NF (0.5 mL, 0.5 mmol, 1m THF solution) was added.
The mixture was stirred for 1 h, diluted with CH2Cl2 (30 mL) and water
(50 mL), and the layers were separated. The aqueous layer was extracted
with CH2Cl2 (2”30 mL), and the combined organic layers were dried
(MgSO4) and concentrated to 5 mL. CH2Cl2 (30 mL) and Dess–Martin
periodinane (0.85 g, 0.3 mmol, 15% solution in CH2Cl2) were added. The
resulting colored solution was stirred for 1 h at 208C, filtered through a
plug (SiO2; CH2Cl2), and concentrated to 50 mL. Malononitrile (14 mg,
0.21 mmol) and Al2O3 (50 mg, 0.49 mmol) were added, and the solution
was heated to reflux for 1 h. Evaporation of the solvent and CC (SiO2;
CH2Cl2) afforded the desired products.
2-{5-[4-(Dimethylamino)phenyl]-3-[(4-(dimethylamino)phenylethynyl]-
pent-2-en-4-ynylidene}malononitrile (2b): Into
a stirred solution of
(iPr)2NH (0.4 mL, 3.05 mmol) in THF (5 mL) at 08C under N2, nBuLi
(1.55 mL, 2.5 mmol, 1.6m hexane solution) was added and the solution
stirred for 15 min. Acetaldehyde tert-butylimine (187.5 mg, 1.9 mmol)
was added dropwise at 08C and the solution transferred to 7 (500 mg,
1.6 mmol) in THF (40 mL) at ꢀ788C. The resulting mixture was allowed
to reach 208C and stirred overnight. The solution was concentrated in
vacuo and the residue poured in
a stirred solution of ZnCl2 (1 g,
7.3 mmol) in water (10 mL). Et2O (30 mL) was added and the suspension
stirred for 1 h and filtered through a plug (Celite, Et2O). The aqueous
layer was extracted with Et2O (2”25 mL), and the combined organic
layers were washed with water (50 mL), dried (MgSO4), and evaporated
in vacuo. The residue was taken up into CH2Cl2 (30 mL), and malononi-
trile (150 mg, 2.27 mmol) followed by Al2O3 (600 mg, 5.9 mmol, activity
II-III) were added. The mixture was heated to reflux for 1 h, filtered
through a plug (SiO2; CH2Cl2), and concentrated in vacuo. Column chro-
matography (SiO2; CH2Cl2/hexane 1:1) afforded 2b (0.44 g, 71%) as a
golden metallic solid. Rf =0.6 (SiO2; CH2Cl2); m.p. 245–2478C; 1H NMR
(300 MHz, CD2Cl2, 208C, TMS): d=3.04 (s, 6H; NCH3), 3.05 (s, 6H;
2-[7-(4-Dimethylaminophenyl)-5-(4-dimethylaminophenylethynyl)hepta-
4-en-2,6-diynylidene]malononitrile (4b): The title compound was pre-
pared from protected alcohol 17b (100 mg, 0.21 mmol), following the
general method. Dinitrile 4b revealed to be unstable and, upon evapora-
tion of the solvent, gave a black soot. Therefore the analytical characteri-
zation was carried out in CH2Cl2 solution. Yield 63 mg (ꢁ70%). Rf =
0.72 (SiO2; CH2Cl2); 1H NMR (300 MHz, CDCl3, 208C, TMS): d=3.02 (s,
6H; NCH3), 3.03 (s, 6H; NCH3), 6.25 (d, 5J
6.64 (d, 3J(H,H)=9.0 Hz, 2H; Ar), 7.16 (d, 5J
7.41 (d, 3J(H,H)=9.0 Hz, 2H; Ar), 7.46 ppm (d, 3J
ACHTREUNG
NCH3), 6.67–6.71 (m, 4H; Ar), 7.01 (d, 3J
7.44–7.49 (m, 4H; Ar), 8.02 ppm (d, 3J
ACHTREUNG
N
ACHTREUNG
AHCTREUNG
G
ACHTREUNG
13C NMR (75 MHz, CD2Cl2, 208C, TMS): d=40.03, 79.25, 85.51, 90.09,
103.41, 106.22, 106.87, 107.19, 111.81, 111.84, 112.81, 115.03, 123.65,
130.28, 134.09, 134.38, 151.77, 155.19 ppm; IR (neat): n˜ =2894, 2294,
2181, 2138, 1597, 1514, 1360, 1323, 1209, 1183, 1088, 926, 860, 811,
Ar); 13C NMR (75 MHz, CDCl3, 208C, TMS): d=39.95, 86.29, 86.78,
88.33, 90.08, 96.86, 100.18, 103.18, 107.78, 111.48, 111.52, 111.67, 111.83,
113.27, 113.48, 115,97, 124.93, 133.64, 134.14, 139.89, 150.90, 150.95 ppm;
UV/Vis (CH2Cl2): lmax (e)=300 (32000), 329 (sh, 33000), 534 nm
(41000 molꢀ1 dm3 cmꢀ1); HR-FT-MALDI-MS (3-HPA) m/z: calcd for
C28H22N4+: 414.1839; found 414.1832 ([M]+).
639 cmꢀ1
;
UV/Vis (CH2Cl2): lmax (e)=299 (sh, 32000), 534 nm
(55000 molꢀ1 dm3 cmꢀ1); HR-FT-MALDI-MS (3/HPA) m/z: calcd for
C26H23N4+: 391.1917; found 391.1923 ([MH]+).
2-[9-(4-Dimethylaminophenyl)-7-(4-dimethylaminophenylethynyl)nona-
6-en-2,4,8-triynylidene]malononitrile (5b): The title compound was pre-
pared from alcohol 12 (100 mg, 0.25 mmol) following the general method
for the oxidation and Knoevenagel condensation. Yield 97 mg (87%).
Dark metallic solid. Rf =0.76 (SiO2; CH2Cl2); M.p. >1848C (decomp);
1H NMR (300 MHz, CD2Cl2, 208C, TMS): d=3.01 (s, 6H; NCH3), 3.02
5-[4-(Dimethylamino)phenyl]-3-{[4-(dimethylamino)phenyl]ethynyl}-2-
[(trimethylsilyl)ethynyl]pent-2-en-4-ynal (11): To
a solution of 10b
(100 mg, 0.24 mmol) in THF (10 mL), MeLi·LiBr (0.12 mL, 0.25 mmol,
2.2m solution in Et2O) was added under N2 at 08C. The mixture turned
deep red immediately and was stirred for 2 h at 08C. DMF (0.1 mL,
1.3 mmol) was added and stirring continued for 1 h at 08C. The mixture
was poured into a vigorously stirred biphasic system of saturated aqueous
KH2PO4 and Et2O at 58C (10/10 mL). The organic layer was separated
and the aqueous layer extracted with Et2O (2”20 mL). The combined or-
ganic layers were dried (MgSO4), the solvents evaporated, and the resi-
due purified by CC (SiO2; CH2Cl2) to give 11 (71.7 mg, 67%) as a red
solid. Rf =0.66 (SiO2; CH2Cl2); m.p. >1978C (decomp); 1H NMR
(300 MHz, CDCl3, 208C, TMS): d=0.30 (s, 9H; SiCH3), 3.04 (s, 6H;
(s, 6H; NCH3), 6.14 (d, 7J
Ar), 7.11 (d, 7J
(H,H)=1.5 Hz, 1H; CH), 7.40 (d, J=9.0 Hz, 2H; Ar),
A
AHCTREUNG
7.44 ppm (d, J=9.0 Hz, 2H; Ar); 13C NMR (75 MHz, CD2Cl2, 208C,
TMS): d=40.04, 80.02, 82.87, 86.72, 87.34, 95.29, 95.51, 98.72, 99.79,
101.99, 107.66, 107.73, 111.70, 111.81, 111.95, 112.77, 113.76, 124.87,
133.64, 133.95, 140.20, 151.30, 151.42 ppm; IR (neat): n˜ =2853, 2163,
2125, 1599, 1520, 1355, 1165, 1102, 943, 811 cmꢀ1; UV/Vis (CH2Cl2): lmax
(e)=293 (34500), 364 (25300), 530 nm (28500 molꢀ1 dm3 cmꢀ1); HR-FT-
NCH3), 3.05 (s, 6H; NCH3), 6.64 (d, 3J
ACHTREUNG
+
3
3J
(H,H)=9.0 Hz, 2H; Ar), 7.48 (d, J
ACHTREUNG
MALDI-MS (3-HPA) m/z: calcd for C30H22N4
: 438.1839; found
438.1846 ([M]+).
(s, 1H; CHO); 13C NMR (75 MHz, CDCl3, 208C, TMS): d=ꢀ0.02, 39.97,
84.22, 88.50, 98.89, 106.19, 107.56, 107.81, 108.06, 108.81, 111.45, 111.51,
128.75, 128.01, 133.56, 134.09, 151.06, 151.14, 189.13 ppm; IR (neat): n˜ =
2904, 2151, 1654, 1599, 1529, 1440, 1358, 1247, 1184, 1101, 833, 805,
5-[Bis(4-dimethylaminophenyl)methylene]nona-1,8-dien-3,6-diyne-
1,1,9,9-tetracarbonitrile (6a): The title compound was prepared from pro-
tected alcohol 18a (50 mg, 0.083 mmol) following the general method
5386
www.chemeurj.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 5378 – 5387

Donor–Acceptor Chromophores
FULL PAPER
using twice the amount of nBu4NF, Dess-Martin periodinane, and malo-
nonitrile. Yield 16 mg (42%), dark metallic solid. Rf =0.5 (SiO2;
CH2Cl2); m.p. 210–2118C; 1H NMR (300 MHz, CD2Cl2, 208C, TMS): d=
[3] a) R. R. Tykwinski, U. Gubler, R. E. Martin, F. Diederich, C. Bos-
shard, P. Günter, J. Phys. Chem. A 1998, 102, 4451–4465; b) U.
Gubler, R. Spreiter, C. Bosshard, P. Günter, R. R. Tykwinski, F. Die-
derich, Appl. Phys. Lett. 1990, 73, 2396–2398.
3.10 (s, 12H; NCH3), 6.69 (d, 3J
CH), 7.37 ppm (d, 3J
ACHTREUNG
N
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CD2Cl2, 208C, TMS): d=40.33, 87.84. 89.61, 111.14, 112.45, 114.05,
120.76, 125.52, 134.94, 140.09, 153.96 ppm (2C missing); IR (neat): n˜ =
2915, 2145, 2078, 1589, 1530, 1355, 1164, 941, 892, 819, 735 cmꢀ1; UV/Vis
(CH2Cl2): lmax (e)=267 (17700), 313 (sh, 22200), 500 (49200), 632 nm
(47400 molꢀ1 dm3 cmꢀ1); HR-FT-MALDI-MS (3-HPA) m/z: calcd for
C30H23N6+: 467.1979; found 467.1971 ([MH]+).
5-[1,5-Bis(4-dimethylaminophenyl)penta-1,4-diyn-3-ylidene]nona-1,8-
dien-3,6-diyne-1,1,9,9-tetracarbonitrile (6b): The title compound was pre-
pared from protected alcohol 18b (50 mg, 0.077 mmol) following the gen-
eral method using twice the amount of nBu4NF, Dess–Martin periodi-
nane, and malononitrile. Yield 25 mg (64%), dark metallic solid. Rf =0.5
(SiO2; CH2Cl2); M.p. >2478C (decomp); 1H NMR (300 MHz, CD2Cl2,
3
208C, TMS): d=3.08 (s, 12H; NCH3), 6.70 (d, J
(H,H)=9.3 Hz, 4H; Ar),
7.24 (s, 2H; CH), 7.51 ppm (d, 3J(H,H)=9.3 Hz, 4H; Ar); 13C NMR
A
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(75 MHz, CD2Cl2, 208C, TMS): d=40.22, 91.44, 93.60, 106.83, 111.87,
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IR (neat): n˜ =2904, 2115, 1591, 1526, 1422, 1360, 1155, 1112, 942, 811,
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Received: December 4, 2006
Published online: February 16, 2007
Chem. Eur. J. 2007, 13, 5378 – 5387
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5387