1,1-Dicyano-4-[4-(diethylamino)phenyl]buta-1,3-dienes: Structure-property relationships

Article abstract of DOI:10.1002/ejoc.201200111

We report the synthesis and physical study of a series of 1,1-dicyano-4-[4-(diethylamino)phenyl]buta-1,3-dienes in which the number and position of additional CN substituents along the 1,1-dicyanobuta-1,3-dienyl fragment is systematically varied. While X-

Full text of DOI:10.1002/ejoc.201200111

FULL PAPER
DOI: 10.1002/ejoc.201200111
1,1-Dicyano-4-[4-(diethylamino)phenyl]buta-1,3-dienes: Structure–Property
Relationships
Francesca Tancini,[a] Yi-Lin Wu,[a] W. Bernd Schweizer,[a] Jean-Paul Gisselbrecht,[b]
Corinne Boudon,[b] Peter D. Jarowski,[c] Marten T. Beels,[d] Ivan Biaggio,[d] and
François Diederich*[a]
Keywords: Donor–acceptor systems / Charge transfer / Conjugation / Chromophores / Nonlinear optics
We report the synthesis and physical study of a series of 1,1-
dicyano-4-[4-(diethylamino)phenyl]buta-1,3-dienes in which
the number and position of additional CN substituents along
the 1,1-dicyanobuta-1,3-dienyl fragment is systematically
varied. While X-ray analysis provided unambiguous infor-
mation about molecular geometries in the crystal, UV/Vis
and electrochemical measurements, by cyclic voltammetry
(CV) and rotating disk voltammetry (RDV), revealed that in-
troduction of additional cyano groups in the C2- and C4-posi-
tions most affected the optical properties of these molecules
in solution, in terms of intramolecular charge-transfer ab-
sorption energy and intensity. A comparison with structurally
related chromophores indicates that the shift of the anilino
donor from position 2/3 to 4 along the butadiene scaffold re-
sults in a remarkable bathochromic shift of the ICT absorp-
tion maxima, mainly due to the higher planarity in the pres-
ent series. These findings are further corroborated by density
functional theory calculations. Preliminary nonlinear optical
(NLO) measurements confirm the promise of the new push-
pull chromophores as third-order nonlinear-optical molecular
materials.
however, a clear indication of the effective and/or superflu-
ous outcome of each cyano substitution as a function of
position is imperative for the further design and application
of such systems.
Introduction
π-Conjugated donor–acceptor (D–A) chromophores
have been investigated for quite some time,[1,2] but have re-
cently attracted renewed interests for potential applications
in the fabrication of opto-electronic materials.[2–4] The en-
ergy and intensity of their characteristic intramolecular
charge-transfer (ICT) transitions depend on the strength of
the electron donor and acceptor moieties and the nature of
the π-conjugated spacer.[5–8] While the nature of the donor,
acceptor, and π-conjugated spacer have been systematically
varied,[9] the number and positioning of the push/pull sub-
stituents along the π-conjugated spacer backbone has only
been addressed in a few cases.[10] Our group has observed
strong electro-optical effects associated with the increasing
number of cyano groups in push–pull chromophores;[10e,11]
The synthesis and optical properties of 4-[4-(dimeth-
ylamino)phenyl]buta-1,3-dienes with 1,1-dicyano (1)[12] and
1,1,2-tricyano (2)[13] substituents (Figure 1) had been pre-
viously reported. The additional cyano group in 2 produced
a bathochromic shift of the ICT-band in the UV/Vis spec-
trum by 0.4 eV, accompanied by a modest increase of the
molar extinction coefficient (see Table 2 below). Stimulated
by these early findings, we decided to prepare the complete
series of 1,1-dicyano-4-[4-(dialkylamino)phenyl]buta-1,3-
dienes 3–8, with additional CN substituents in positions 2,
3, 4, and combinations thereof (Figure 1).
Herein, we report the synthesis, structural characteriza-
tion, and opto-electronic properties of the new push–pull
chromophores 3–8. This systematic investigation would al-
low the identification of the relative importance of each po-
sition along the butadiene backbone on chromophoric
properties. Identification of both the most and the least ef-
fective position was anticipated to be of importance. For
the latter, such ineffective position(s) could then be used to
further extend the chromophores by judicious functionali-
zation or branching to optimally tune materials properties
such as stability, sublimability, solubility, or self-assembly
behavior without deleteriously affecting opto-electronic at-
tributes.
[a] Laboratorium für Organische Chemie, ETH Zürich,
Hönggerberg, HCI, 8093 Zürich, Switzerland
Fax: +41-44-632-1109
E-mail: diederich@org.chem.ethz.ch
[b] Laboratoire d’Electrochimie et de Chimie Physique du Corps
Solide UMR 7177, CNRS, Université de Strasbourg,
4, rue Blaise Pascal, CS 90032, 67081 Strasbourg Cedex, France
[c] Advanced Technology Institute and Department of Physics,
University of Surrey,
Stag Hill, Guildford GU2 7XH, Surrey, United Kingdom
[d] Department of Physics and Center for Optical Technologies,
Lehigh University, 415 Lewis Lab,
16 Memorial Dr. East, Bethlehem, PA 18015, USA
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201200111.
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π-Conjugated Donor–Acceptor Chromophores
Figure 1. a) Known[12,13] and b) new push–pull chromophores with a 1,1-dicyano-4-[4-(dialkylamino)phenyl]buta-1,3-diene scaffold.
CuCN/KCN[18] to give the target molecules in low yields
Results and Discussion
(5–32%). In the case of 3 and 5, heating 9 and 10 with 4-
Synthesis
(diethylamino)cinnamonitrile (17)[19] in acetonitrile gave the
desired products in 47% and 41% yield, respectively.
Target chromophores 3–8 were prepared by short syn-
thetic routes, starting from ethynyl- or ethenyl-N,N-dieth-
A variety of dicyanovinyl (DCV) and tricyanovinyl
ylanilines (Scheme 1; for the synthesis of the aniline precur- (TCV) derivatives have been shown to react with N,N-dialk-
sors, see the Supporting Information). The dimethylamino ylanilino-substituted acetylenes in a formal [2+2] cycload-
groups in 1 and 2 were replaced by diethylamino groups to dition (CA) to give a common cyclobutene intermediate,
enhance the solubility of the target dipolar chromophores which rapidly undergoes cycloreversion (CR) with complete
without much sacrifice in their crystallinity.[14] To introduce torquoselectivity to provide 2-anilino-substituted 1,1-dicya-
the cyano functionality in position 4 in compounds 4, 6, 7, nobuta-1,3-dienes (Scheme 2).[20] With DCV and TCV
and 8, dicyanovinyl (9[15]) and tricyanovinyl chloride 10[16] chlorides, however, this product formation via a CA–CR
were heated with appropriate ethynylanilines 11[17] and 12 cascade is not observed; rather 4-anilino-4-chloro-1,1-dicy-
to give the 4-chlorinated buta-1,3-diene precursors 13–16 in anobuta-1,3-dienes 13–16 are formed and isolated exclu-
46–93% yield. Substitution of the chloro substituent in 13– sively.[21] Their formation, which is proven by X-ray analysis
16 by cyano was achieved by Pd-catalyzed reaction with (see the Supporting Information), points at a slightly dif-
Scheme 1. Synthesis of target chromophores 3–8: a) i) CH3CN, 90 °C, 2 h, 93% [(Z)-13], 85% [(E)-14 + (Z)-14], 65% [(Z)-15], 46% [(Z)-
16]; ii) dry toluene/DMF (5:1), 18-crown-6, KCN, CuCN, [Pd(PPh3)4], 95 °C, overnight, 28% (4), 20% [6 from (E)-14 + (Z)-14], 32% (7),
5% (8); b) i) CH3CN, 90 °C, 3 h, 47% (3), 41% (5).
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Scheme 2. Proposed mechanisms for the reactions of DCV and TCV chlorides 9 and 10 with a) ethynylanilines 11 and 12 and b) 4-
(diethylamino)cinnamonitrile (17).
ferent mechanism in the reaction with DCV and TCV and Supporting Information). Although a clear under-
chlorides (Scheme 2).
standing of the observed stereoselectivity requires the
The formation of 13–16 may start with an addition–eli- knowledge about the reaction barriers, the geometries of
mination process followed by conjugate re-addition of the molecules isolated are in good agreement with theoreti-
chloride to give the isolated products (route a, Scheme 2a). cal calculations (see Supporting Information for details),
An alternative route b could involve formation of the cyclo- confirming that each push–pull system adopts the energeti-
butene, followed by 1,3-chloride migration[22,23] and cyclo- cally favored configuration. When the energy difference be-
reversion. Likewise, the formation of 3 and 5 may occur tween (E) and (Z) isomers is low, as in the case of 14, both
through the ionic route aЈ[24] or involve a cyclobutane inter- compounds were obtained from synthesis as an inseparable
mediate (route bЈ), from which elimination of HCl, followed mixture. The final Pd-catalyzed substitution of Cl by
by
CR, generates
the isolated
buta-1,3-dienes CuCN/KCN occurs with retention of configuration. Inter-
(Scheme 2b).[13a] Given the strong polarization of the start- estingly buta-1,3-diene 6 was obtained exclusively as (E)-
ing materials and the isolation of 14 as an (E)/(Z) mixture, isomer, even if a mixture of (E)-14 and (Z)-14 precursors
the routes a and aЈ are conceivably likely; however, it is was employed.
not clear at this stage whether these addition–elimination
processes involve zwitterionic intermediates.
Target N,N-diethyl compounds 3–8 are soluble in the
millimolar concentration range (0.1–20 mm) in common or-
The (E) and (Z) configurations depicted in Scheme 1 are ganic solvents, such as CH2Cl2, CHCl3, CH3CN, acetone,
all supported by single-crystal X-ray analysis (see below and toluene, giving rise to deeply colored solutions. These
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π-Conjugated Donor–Acceptor Chromophores
solutions are stable for extended periods under normal lab- included in the Supporting Information.[25] Except for
oratory conditions (3 months, at room temperature, under pentacyano derivatives 8 (torsion angle about C1–C2–C3–
daylight exposure). They are thermally stable, with melting C4 153°), the buta-1,3-diene backbones in all other chro-
points ranging from 180 °C (for 8) to 220 °C (for 5).
mophores (3–7) are highly planar (torsion angles 175–180°).
We evaluated the extent of donor–acceptor interactions
in the ground state from the bond length alternation in the
X-ray Crystal Structures and Bond-Length Alternation
The target push–pull chromophores 3–8 and chloro-pre- anilino rings.[26] Table 1 shows that these rings feature re-
cursors 13–16 were all crystallized by slow diffusion of n- markably high quinoid character δr (for definition, see the
pentane into a CH2Cl2 solution. The crystal structures of caption to Figure 2) with δr values ranging from 0.043 for
3–8 are depicted in Figure 2, whereas those of 13–16 are 4 to 0.072 for 8. These values are noticeably higher than
Figure 2. ORTEP plots of a) 3, b) 4, c) 5, d) 6, e) 7, and f) 8 with vibrational ellipsoids shown at the 50% probability level, T = 100 K.
Arbitrary numbering. Selected bond lengths [Å] and torsion angles [°]: 3: C11–C16 1.414(3), C11–C12 1.422(3), C12–C13 1.372(3), C13–
C14 1.417(3), C14–C15 1.425(3), C15–C16 1.365(3), C1–C2–C3–C4 –179.8(3); 4: C1–C6 1.405(6), C1–C2 1.418(5), C2–C3 1.371(5), C3–
C4 1.403(6), C4–C5 1.425(5), C5–C6 1.368(5), C7–C8–C9–C10 175.3(6); 5: C5–C10 1.427(2), C5–C6 1.428(2), C6–C7 1.362(2), C7–C8
1.428(2), C8–C9 1.428(2), C9–C10 1.363(2), C1–C2–C3–C4 176.8(2); 6: for one of the two different molecules in the unit cell: C6–C11
1.425(4), C6–C7 1.424(4), C7–C8 1.363(4), C8–C9 1.424(4), C9–C10 1.422(4), C10–C11 1.350(4), C2–C3–C4–C5 178.8(3); 7: C5–C10
1.407(3), C5–C6 1.403(3), C6–C7 1.369(3), C7–C8 1.431(4), C8–C9 1.425(3), C9–C10 1.364(3), C1–C2–C3–C4 179.9(4); 8: C6–C11
1.422(4), C6–C7 1.418(4), C7–C8 1.354(4), C8–C9 1.428(4), C9–C10 1.427(4), C10–C11 1.349(4), C2–C3–C4–C5 152.5(3). Quinoid charac-
ter of the anilino rings δr = [(a – b) + (c – b)]/2 ≈ [(aЈ – bЈ) + (cЈ – bЈ)]/2; for the definition of bond lengths a, aЈ, b, bЈ, c, and cЈ, see
the figure. For benzene, δr = 0.00, for fully quinoid chromophores, δr = 0.10–0.12 Å.
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F. Diederich et al.
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those of planar N,N-dialkylanilino-substituted cyanoethyn- bands was corroborated by reversible protonation/depro-
ylethenes[27] (up to 3 CN groups, δr values of 0.03–0.04) or tonation experiments, and the Lambert–Beer law was fully
highly non-planar 2-N,N-dialkylanilino-substituted di-, obeyed under the experimental concentration range (see
tri-, or tetracyanobuta-1,3-dienes[20,28,29] (δr values of 0.03– Supporting Information). Table 2 summarizes the UV/Vis
0.063) indicating stronger intramolecular D–A interactions results including the data for the known derivatives 1[12] and
in 3–8. The much higher planarity of the 4-anilino-substi- 2[13] as well as those previously recorded for the related
tuted cyanobutadienes (Figure 1), as compared to 2(3)-anil- push–pull chromophores 18 and 19.[11,28]
ino-substituted analogues, such as 18 and 19,[2a,2b,20,28,29]
Table 2. UV/Vis data for 1–8 and 18 and 19.[a]
evidently promotes more efficient intramolecular CT conju-
gation. According to the measured quinoid character, CT
efficiency increases in the series 4 Ͻ 7 ≈ 3 Ͻ 5 ≈ 6 Ͻ 8.
λmax [nm] (eV)
ε [m–1 cm–1]
1[12]
490 (2.53)
582 (2.13)[b]
524 (2.37)
567 (2.19)
594 (2.09)
610 (2.03)
678 (1.83)
698 (1.78)
343 (3.61)
481 (2.58)
450 (2.76)
643 (1.93)
46000
63900[b]
105500
54100
97600
64000
70300
24500
10200
7000
2[13]
Table 1. Quinoid character (δr, standard uncertainties shown in pa-
renthesis) of compounds 3–8 calculated from the experimental X-
ray data, and 1H NMR chemical shifts (300 MHz, CDCl3) for their
Hmeta protons.
3
4
5
6
7
δr [Å]
δmeta [ppm]
8
3
4
5
6
7
8
0.051(3)
0.043(4)
0.065(2)
7.96
7.66
8.10
8.24
7.75
7.85
18[28]
19[11]
30000
3200
0.067(3), 0.059(4)
0.050(3)
0.072(3)
[a] Except for 2, data were measured in CH2Cl2. [b] Measured in
acetone;[13a,13b] λmax = 585 nm in CHCl3.[13c]
This polarization also causes downfield shifts of the anil-
ino protons (δmeta) in the meta position relative to the Et2N
group.[27b] These shifts generally increase as the δr values
increase, although local anisotropic effects of neighboring
CN groups prevent a complete agreement between the two
quantities (Table 1).
The energy of the single dominant ICT absorption in the
spectra of 1–8 is clearly affected by the number of the CN
UV/Vis Spectroscopy and Preliminary NLO Data
The UV/Vis spectra of compounds 3–8 in CH2Cl2 feature groups along the butadienyl backbone. Moving from two
ICT absorptions in the visible range with high molar (1, λmax: 490 nm, 2.53 eV), to three (e.g. 3, λmax: 524 nm,
extinction coefficients (ε), obtained by at least three inde- 2.37 eV), to four (e.g. 6, λmax: 610 nm, 2.03 eV), and to five
pendent measurements, between 24500 m–1 cm–1 and (8, λmax: 698 nm, 1.78 eV) cyano groups results in a ba-
105500 m–1 cm–1 (Figure 3). The ICT character of these thochromic shift of 208 nm (0.75 eV). The dependence of
Figure 3. UV/Vis spectra of 3–8 in CH2Cl2.
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π-Conjugated Donor–Acceptor Chromophores
the ICT absorption maximum on the position of the CN Table 3. Electrochemical data observed by cyclic voltammetry (CV)
at a scan rate of ν = 0.1 Vs–1 and rotating disk voltammetry (RDV)
groups is even more remarkable. Tricyano derivatives 2
in CH2Cl2 + 0.1 m nBu4NPF6. All potentials are given vs. the fer-
ricinium/ferrocene couple, used as internal standard.
(λmax: 585 nm, 2.12 eV, in CHCl3), 3 (λmax: 524 nm,
2.37 eV), or 4 (λmax: 567 nm, 2.19 eV) show substantially
different transition energies, and the same holds for the tet-
CV
RDV
E1/2
[V]
[b]
[c]
E°[a]
[V]
ΔEp
Ep
Slope[d]
[mV]
racyano derivatives 5 (λmax: 594 nm, 2.09 eV), 6 (λmax
:
[mV]
[V]
610 nm, 2.03 eV), and 7 (λmax: 678 nm, 1.83 eV). This com-
parison shows that introduction of additional CN groups
in positions 2 and 4 more effectively induce bathochromic
shifts than in position 3 (compare 2, 4 with 3, or 7 with 5
and 6).
The intensities of the ICT band are very high, except for
the case of 8 (ε: 24500 m–1 cm–1) which features stronger
non-planarity in the buta-1,3-diene backbone. Substitution
in position 3 results in particularly intense absorptions (3:
ε = 105500 m–1 cm–1 and 5: ε = 97600 m–1 cm–1).
The comparison between the absorption spectra of 3–8
and 18/19 reveals that moving the anilino donor substituent
from position 2 (in 18) or position 3 (in 19) to position 4
(in 3–8) leads to a strong bathochromic shift in the UV/Vis
spectra (compare λmax of the tetracyano derivatives 5–7
with 18). This correlates well with the higher ground-state
intramolecular CT interaction, as determined by the qui-
noid character (see above), and reflects the higher planarity
of the π-conjugated butadiene backbone in 4-anilino-substi-
tuted 3–8 as compared to 2- or 3-anilino-substituted deriva-
tives.[30]
3
4
+0.66
90
+0.70 (1 e–)
–1.50
65
80
95
100
70
70
60
90
70
60
70
60
60
60
70
60
70
–1.49
+0.71
–1.19
140
140
+0.75 (1 e–)
–1.23 (1 e–)
–1.95
–1.84
–0.88
5
6
7
8
+0.91
60
+0.95 (1 e–)
–0.88 (1 e–)
–1.57
+0.93
–0.87
70
70
+0.96 (1 e–)
–0.89 (1 e–)
–1.57
–1.51
+0.81
–0.70
–1.41
+1.04
–0.47
–1.15
70
70
80
60
70
70
+0.82 (1 e–)
–0.70 (1 e–)
–1.42 (1 e–)
+1.06 (1 e–)
–0.49 (1 e–)
–1.17 (1 e–)
[a] E° = (Epc + Epa)/2, where Epc and Epa correspond to the cath-
odic and anodic peak potentials, respectively. [b] ΔEp = Epa – Epc.
[c] Ep = Irreversible peak potential at sweep rate ν = 0.1 Vs–1. [d]
Logarithmic analysis of the wave obtained by plotting E vs. log[I/
(Ilim – I)].
A distinct positive solvatochromism was measured for all
chromophores (see Supporting Information).[31,32] Upon
changing from n-pentane to more polar nitrobenzene, the
ICT band (λmax) of the tricyano derivatives shifts by 42–
50 nm whereas the absorption maxima of compounds with
four or five CN groups shift by 68–97 nm.
First experiments to determine the third-order optical
nonlinearities of the new chromophores by degenerate four-
wave mixing at wavelength of 1.5 μm were undertaken in
CH2Cl2 (see Supporting Information). For 4, the rotational
average of the third-order polarizability was determined as
γrot = (11Ϯ3)ϫ10–48 m5 V–2, a value in line with those mea-
sured for 2(3)-anilino-substituted cyanobutadienes.[33]
(ΔE1/2 = 0.36 V), resulting in the overall decrease in the
HOMO–LUMO gaps. While the first redox potentials for
1,1,2,3- and 1,1,3,4-tetracyano substituted 5 and 6 are prac-
tically identical (E1/2(ox) ≈ +0.95 V and E1/2(red) ≈ –0.88 V),
a significantly different redox behavior was observed for
1,1,2,4-tetracyano derivative 7 (E1/2(ox) +0.82 V and E1/2(red)
–0.70 V). A similar difference was found in the UV/Vis and
bond length alternation studies, and this is rationalized
based on the computational studies reported below. Elec-
trochemical data for the chlorinated series 13–16 and a brief
discussion on their redox properties can be found in the
Supporting Information.
Calculations
Electrochemistry
For the ease of computation, the N-ethyl substituents on
Electrochemical measurements were carried out in the anilino moieties in 3–8 were changed to N-methyl
CH2Cl2 containing 0.1 m nBu4NPF6 in a classical three- groups. Vertical transition energies between S0–S1 states
electrode cell by cyclic voltammetry (CV) and rotating-disk were calculated by time-dependent density functional
voltammetry (RDV) (Table 3).
theory (TD-DFT) at the B3LYP/6-311+G(2d,p)//B3LYP/6-
Chromophores 3–8 featured a reversible one-electron 31G(d) level; solvation in CH2Cl2 was applied using the po-
oxidation occurring on the N,N-diethylanilino moiety, larizable continuum model (PCM) (for details of the com-
whereas the reductions behave either as irreversible or re- putational studies, see the Supporting Information).[34,35]
versible electron transfers. Increasing the number of CN
In all cases, the computed transition energies are slightly
substituents causes anodic shifts in both the oxidation and larger than the experimental values. Differences between
reduction potentials. These shifts are due to the electron- computed excitation energies and experimental absorption
withdrawing effect as a result of the increasing CN-substitu- maxima are in the range of 0.12–0.34 eV, well within the
tion and are more pronounced for the first reduction events expected error.[35] A minor part of these differences results
[RDV (Table 3): E1/2 = –1.50 V for 3 and –0.49 V for 8, from the Et Ǟ Me replacement, which should lead to small
ΔE1/2 = 1.01 V] than for the first, aniline-centered oxidation shifts of 4–6 nm.[30]
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Figure 4. Visualization of the frontier molecular orbitals of a) 1, b) 2, c) the N,N-dimethyl analog of 3, and d) the N,N-dimethyl analog
of 4, computed at the B3LYP/6-31G(d) level. The orientation of each molecule is according to Figure 1, and the HOMO is shown at the
left- and the LUMO at the right-hand side in each graph.
The effect of the addition of CN groups along the buta- level has dropped roughly by 0.4 eV and the HOMO level
dienyl chain, in positions 2, 3 and/or 4, was regarded as a only by 0.2 eV, compared to 1. This results ultimately in a
perturbation of compound 1, bearing two CN groups in 79 nm red-shift of the absorption maximum of 4 relative to
position 1. Since the low-energy absorption bands of 1–8 1. A similar distortion of the orbital lobes in the C2 and
are characterized by a HOMO–LUMO transition, we fo- C4 positions towards the attached CN groups was also
cused on these two frontier MOs, where the HOMO of 1 found in the LUMO plots of 5–8 (see Supporting Infor-
has a large coefficient on C3, and the LUMO large coeffi- mation). The reduction in excitation energy, therefore, is
cients on C2 and C4. The frontier MOs of 1 and 2, and the found to be affected by the C2- and/or C4-cyanations,
N,N-dimethyl derivatives of 3 and 4 are visualized in Fig- which lower the LUMO level to a larger extent than the
ure 4; MO energies and the plots for 5–8 can be found in HOMO level. Combining the discussed effects, we observed
the Supporting Information.
for the tricyano-substituted butadienes experimentally: 2
While a ca. 90 nm (0.4 eV) bathochromic shift upon C2- (585 nm) Ͼ 4 (567 nm) Ͼ 3 (524 nm), calculated: 4 (538 nm)
cyanation of 1 was found for 2, the C3-cyanation was esti- Ն 2 (537 nm) Ͼ 3 (459 nm); and for the tetracyano-substi-
mated to have a relatively small effect on the S0–S1 gap of tuted butadienes experimentally: 7 (678 nm) Ͼ 6 (610 nm)
34 nm (or 0.16 eV); exptl.: 3 (524 nm) Ͼ 1 (490 nm); calcd. Ͼ 5 (594 nm), calculated: 7 (597 nm) Ͼ 6 (538 nm) Ͼ 5
3(458) Ͻ 1 (466 nm). Such findings reflect that both the (522 nm).
HOMO and LUMO levels of 3 are stabilized to similar
amounts by cyanation at C3. Our electrochemical studies
for 3 (E1/2,ox = +0.70, E1/2,red = –1.50 V) and the previously
Conclusions
reported data for 1 (E1/2,ox = +0.55, E1/2,red = –1.60 V under
similar conditions)[12b] confirm this quantitatively. Also, the
A complete series of 1,1-dicyano-4-(N,N-diethylanilino)-
3-substituted CN group has significant orbital coefficients substituted buta-1,3-dienes 3–8 was synthesized, and the
in both the HOMO and LUMO levels. This additional spa- configuration and geometry of each compound unambigu-
tial overlap results in the higher extinction coefficients when ously determined by X-ray analysis. The quinoid character,
this position is CN-substituted: oscillator strength f for 3 calculated from the X-ray crystal data, of the new chromo-
(1.57) Ͼ 2 (1.29) and 4 (1.19), and 5 (1.29) and 6 (1.24) Ͼ phores is high, reflecting the high planarity of their π-conju-
7 (1.13), almost parallel to the trend in ε, Table 2. For 2 and gated buta-1,3-diene backbone.
4, there is little orbital coefficient on the additional CN
UV/Vis measurements and theoretical calculations show
group in the HOMO, whereas significant orbital coeffi- that by increasing the number of electron-accepting CN
cients are observed for these groups in the LUMO (Fig- groups along the π-conjugated bridge of these chromo-
ure 4). This suggests that the LUMO levels have been per- phores, remarkable red-shifts of the ICT absorption max-
turbed to a greater extent than the HOMO levels. This un- ima can be achieved, and good control over the extinction
equal perturbation is clearly seen in the electrochemistry of coefficients of these transitions is possible. Particularly cru-
4 (E1/2,ox = +0.75, E1/2,red = –1.23 V), where the LUMO cial are C2 and C4-cyanation, which result in the largest
2762
www.eurjoc.org
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2012, 2756–2765

π-Conjugated Donor–Acceptor Chromophores
m/z. C17H16N4 (276.34): calcd. C 73.89, H 5.84, N 20.27; found C
73.68, H 5.86, N 20.08.
bathochromic shifts, as the LUMO level is more strongly
lowered than the HOMO level. In contrast, cyanation at C3
lowers both HOMO and LUMO levels to a similar extent
which explains why the smallest bathochromic shifts of the
ICT band are observed. Consequently, position C3 is in fu-
ture work best used for further functionalization and
branching. Modification at this site, as an additional posi-
tion to the anilino nitrogen, with alkyl or substituted eth-
ynyl groups could improve physical and molecular materials
properties, such as solubility, without considerably affecting
the chromophore planarity and opto-electronic behavior.
Preliminary NLO measurements for compound 4 suggest
that this additional modification is worthwhile and holds
promise for producing new advanced functional materials
for optical device applications.[36]
(Z)-4: This compound was synthesized according to General Pro-
cedure A with (Z)-13 (150 mg, 0.52 mmol). Flash chromatography
(SiO2; CH2Cl2/pentane, 8:2) gave (Z)-4 (40 mg, 28%) as a violet
solid. Rf = 0.57 (CH2Cl2/pentane, 8:2); m.p. 204.1–205.0 °C. 1H
NMR (300 MHz, CDCl3, 25 °C): δ = 1.25 [t, J = 7.2 Hz, 6 H,
N(CH2Me)2], 3.49 [q, J = 7.2 Hz, 4 H, N(CH2Me)2], 6.70 (d, J =
9.3 Hz, 2 H, ArHortho), 7.28 (d, J = 12.3 Hz, 1 H, ArClC=CH),
7.66 (d, J = 9.3 Hz, 2 H, ArHmeta), 7.88 [d, J = 12.3 Hz, 1 H,
HC=C(CN)2] ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ = 12.85,
45.16, 81.67, 111.96, 112.12, 113.81, 114.71, 118.16, 122.02, 127.84,
130.639 151.41, 153.84 ppm. IR (ATR): ν = 2974 (m), 2925 (m),
˜
2211 (s), 1608 (s), 1543 (s), 1507 (m), 1471 (w), 1448 (w), 1414 (m),
1377 (w), 1357 (w), 1333 8 (w), 1291 (m), 1270 (m), 1195 (m), 1152
(m), 1093 (w), 1071 (m), 1009 (w), 972 (w), 926 (w), 902 (w), 880
(m), 820 (s), 794 (m), 754 (m), 729 (m) cm–1. UV/Vis (CH2Cl2):
λmax (ε) 294 (18000), 567 nm (54100 m–1 cm–1). HR-ESI-MS: calcd.
for C17H17N4 [M + H]+ 277.1448; found 277.1445 m/z. C17H16N4
+
Experimental Section
(276.34): calcd. C 73.89, H 5.84, N 20.27; found C 73.95, H 5.97,
N 20.34.
Only the synthesis of the target chromophores 3–8 is reported here,
whereas all other details and protocols can be found in the Sup-
porting Information.
(Z)-5: This compound was synthesized according to General Pro-
cedure B with 17 (160 mg, 0.8 mmol). Flash chromatography (SiO2;
CH2Cl2) gave (Z)-5 (96 mg, 41%) as a blue-green solid. Rf = 0.68
(CH2Cl2); m.p. 231.0–232.0 °C. 1H NMR (300 MHz, CDCl3,
25 °C): δ = 1.31 [t, J = 7.2 Hz, 6 H, N(CH2Me)2], 3.59 [q, J =
7.2 Hz, 4 H, N(CH2Me)2], 6.79 (d, J = 9.0 Hz, 2 H, ArHortho), 7.81
(s, 1 H, C=CH), 8.10 (br. s, 2 H, ArHmeta) ppm. 1H NMR
(300 MHz, CD3CN, 25 °C): δ = 1.24 [t, J = 7.4 Hz, 6 H,
N(CH2Me)2], 3.61 [q, J = 7.4 Hz, 4 H, N(CH2Me)2], 6.93 (d, J =
9.1 Hz, 2 H, ArHortho), 7.83 (s, 1 H, C=CH), 8.08 (br. s, 2 H,
ArHmeta) ppm. 13C NMR (75 MHz, CD3CN, 25 °C): poor quality
CCDC-863131 (for 3), -863132 (for 4), -863133 (for 5), -863134 (for
6), -863135 (for 7), -863136 (for 8), -863137 (for 13), -863138 (for
14), -863139 (for 15), -863140 (for 16) 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.
General Procedure A (ClǞCN Substitution): A solution of chlori-
nated buta-1,3-diene (1 equiv.) in dry toluene (20 mL) was treated
with 18-crown-6 (1.3 equiv.), KCN (2.1 equiv.), and [Pd(PPh3)]4
(0.07 equiv.). The mixture was stirred at 95 °C for 1.5 h, treated
with a solution of CuCN (2.0 equiv.) in dry DMF (2 mL), stirred
at 95 °C for 1 h, treated with another portion of CuCN (2.0 equiv.)
in dry DMF (2 mL), and finally stirred at 95 °C overnight. After
evaporation, a solution of the residue in CH2Cl2 was filtered and
the solvents evaporated. Flash chromatography (SiO2) gave the de-
sired compound as a colored solid.
spectrum because of solubility reasons. IR (ATR): ν = 2971 (m),
˜
2930 (m), 2207 (s), 1606 (s), 1561 (m), 1514 (m), 1488 (w), 1457
(w), 1416 (m), 1367 (m), 1339 (w), 1302 (w), 1262 (w), 1188 (m),
1162 (m), 1006 (w), 918 (m), 830 (s), 809 (m), 795 (m), 784 (m),
737 (w), 676 (w), 616 (m) cm–1. UV/Vis (CH2Cl2): λmax (ε) 293
(6900), 375 (8900), 398 (9100), 594 nm (97600 m–1 cm–1). HR-
MALDI-MS (3-HPA): calcd. for C18H15N5 [M]– 301.1333; found
–
301.1333 m/z. C18H15N5 (301.35): calcd. C 71.74, H 5.02, N 23.24;
found C 71.74, H 5.23, N 22.93.
General Procedure B [Reaction of 17 with Dicyanovinyl (DCV) or
Tricyanovinyl (TCV) Chlorides]: A solution of 17 (1.00 mmol) and
cyanovinyl chloride (1.50 mmol) in acetonitrile (9 mL) was heated
at 90 °C for 3 h and subsequently evaporated. Flash chromatog-
raphy (SiO2) gave the desired compound as a colored solid.
(E)–6: This compound was synthesized from a mixture of (E)–14
and (Z)–14 (200 mg, 0.64 mmol) following General Procedure A.
Flash chromatography (SiO2; CH2Cl2) gave (E)-6 (38 mg, 28%) as
a dark green solid. Rf = 0.28 (CH2Cl2); m.p. 188.3–189.7 °C. 1H
NMR (300 MHz, CDCl3, 25 °C): δ = 1.32 [t, J = 7.2 Hz, 6 H,
N(CH2Me)2], 3.58 [q, J = 7.2 Hz, 4 H, N(CH2Me)2], 6.79 (d, J =
7.2 Hz, 2 H, ArHortho), 7.81 (s, 1 H, C=CH), 8.23 (d, J = 7.2 Hz,
2 H, ArHmeta) ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ = 13.02,
45.91, 80.91, 99.54, 110.27, 112.83, 114.02, 114.42, 114.80, 132.72,
(Z)-3: This compound was synthesized according to General Pro-
cedure B with 17 (200 mg, 1.01 mmol). Flash chromatography
(SiO2; CH2Cl2) gave (Z)-3 (130 mg, 47%) as a purple solid. Rf =
1
0.55 (CH2Cl2); m.p. 208.0–209.0 °C. H NMR (300 MHz, CDCl3,
25 °C): δ = 1.27 [t, J = 7.2 Hz, 6 H, N(CH2Me)2], 3.52 [q, J =
7.2 Hz, 4 H, N(CH2Me)2], 6.73 (d, J = 9.3 Hz, 2 H, ArHortho), 7.24
(s, 1 H, ArC=CH), 7.37 [s, 1 H, HC=C(CN)2], 7.96 (br. d, J =
9.3 Hz, 2 H, ArHmeta) ppm. 13C NMR (75 MHz, CDCl3/CD3CN,
8:2, 25 °C): δ = 12.61, 45.26, 94.93, 111.29, 111.96, 115.02, 115.05,
119.29, 135.86, 153.04, 156.41, 157.80 ppm (one signal missing be-
cause of overlap with the CD3CN resonance at 116.52). IR (ATR):
134.46, 150.34, 153.45 ppm. IR (ATR): ν = 2971 (m), 2923 (m),
˜
2212 (s), 1606 (s), 1551 (s), 1492 (w), 1438 (s), 1347 (m), 1315 (m),
1260 (m), 1194 (m), 1158 (w), 1095 (w), 1070 (w), 1014 (w), 928
(w), 821 (s), 796 (m), 696 (m), 666 (m), 609 (m) cm–1. UV/Vis
(CH2Cl2): λmax (ε) 262 (6400), 329 (7100), 610 nm (64000 m–1 cm–1).
HR-MALDI-MS (3-HPA): calcd. for C18H15N5 [M]– 301.1333;
–
found 311.1328. C18H15N5 (301.35): calcd. C 71.74, H 5.02, N
23.24; found C 71.56, H 5.00, N 23.07.
ν = 2980 (m), 2940 (m), 2217 (s), 1606 (s), 1532 (m), 1497 (m),
˜
1470 (w), 1448 (w), 1411 (m), 1335 (w), 1281 (w), 1227 (w), 1186
(m), 1156 (m), 1082 (m), 1009 (w), 975 (w), 923 (w), 893 (m), 824 (Z)-7: This compound was synthesized from (Z)-15 (130 mg,
(s), 794 (m), 779 (m), 762 (m), 680 (m), 630 (m) cm–1. UV/Vis 0.42 mmol) following General Procedure A. Flash chromatography
(CH2Cl2): λmax (ε) 289 (6500), 524 nm (105500 m–1 cm–1). HR-ESI- (SiO2; CH2Cl2) gave (Z)-7 (40 mg, 32%) as a green solid. Rf =
+
1
MS: calcd. for C17H20N5 [M + NH4]+ 294.1713; found 294.1704
0.31 (CH2Cl2); m.p. 221.0–222.0 °C. H NMR (300 MHz, CDCl3,
Eur. J. Org. Chem. 2012, 2756–2765
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2763

F. Diederich et al.
FULL PAPER
Ratner, T. J. Marks, Chem. Rev. 1994, 94, 195–242; c) J. Y. Lee,
K. S. Kim, B. J. Mhin, J. Chem. Phys. 2001, 115, 9484–9489.
25 °C): δ = 1.29 [t, J = 7.2 Hz, 6 H, N(CH2Me)2], 3.55 [q, J =
7.2 Hz, 4 H, N(CH2Me)2], 6.75 (d, J = 9.6 Hz, 2 H, ArHortho), 7.24
(s, 1 H, C=CH), 7.75 (d, J = 9.6 Hz, 2 H, ArHmeta) ppm. 13C NMR
(75 MHz, CDCl3/CD3CN, 8:2, 25 °C): δ = 12.79, 45.59, 85.51,
111.15, 111.58, 112.59, 112.88, 113.08, 120.68, 125.71, 131.56,
132.74, 152.75 ppm (one signal missing because overlapped with
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CD CN resonances at 116.52). IR (ATR): ν = 2972 (m), 2931 (m),
˜
3
2210 (s), 1607 (s), 1508 (s), 1477 (m), 1420 (m), 1346 (m), 1312 (m),
1296 (m), 1276 (m), 1213 (m), 1192 (m), 1092 (w), 1077 (w), 1009
(w), 961 (m), 901 (m), 856 (m), 830 (s), 797 (m), 746 (m) cm–1. UV/
Vis (CH2Cl2): λmax (ε) 341 (8800), 402 (5500), 442 (3000), 678 nm
(70300 m–1 cm–1). HR-MALDI-MS (3-HPA): calcd. for C18H15N5
–
[M]– 301.1333; found: 301.1329 m/z. C18H15N5 (301.35): calcd. C
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(Z)-8: This compound was synthesized from (Z)-16 (170 mg,
0.51 mmol) following General Procedure A. Flash chromatography
(SiO2; CH2Cl2) gave (Z)-8 (9 mg, 9%) as a green solid. Rf = 0.13
(CH2Cl2); m.p. 187.3–188.1 °C. 1H NMR (300 MHz, CDCl3,
25 °C): δ = 1.36 [t, J = 7.2 Hz, 6 H, N(CH2Me)2], 3.65 [q, J =
7.2 Hz, 4 H, N(CH2Me)2], 6.86 (d, J = 9.6 Hz, 2 H, ArHortho), 7.85
(br. s, 2 H, ArHmeta) ppm. 13C NMR (75 MHz, CDCl3/CD3CN,
8:2, 25 °C): poor quality spectrum because of solubility reasons. IR
(ATR): ν = 2973 (m), 2931 (m), 2212 (s), 1602 (s), 1508 (s), 1471
˜
(w), 1416 (w), 1345 (w), 1308 (m), 1180 (m), 1157 (m), 1094 (w),
1076 (w), 1002 (w), 894 (m), 829 (s), 794 (m), 742 (m) cm–1. UV/
Vis (CH2Cl2): λmax (ε) 259 (6100), 379 (5400), 431 (3800), 698 nm
–
(24500 m–1 cm–1). HR-MALDI-MS (3-HPA): calcd. for C19H14N6
[M]– 326.1285; found: 326.1278 m/z. C19H14N6 (326.36): calcd. C
69.92, H 4.32, N 24.75; found C 69.51, H 4.31, N 24.65.
Supporting Information (see footnote on the first page of this arti-
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cluding oscillator strength and transition dipole moment), electro-
[10]
1
chemsitry (13–16), X-ray data, DFT calculations, and H and 13C
NMR spectra of new compounds.
Acknowledgments
The work at the Eidgenössische Technische Hochschule (ETH) was
supported by European Commision through an ERC Advanced
Grant No. 246637 (OPTELOMAC). The authors are grateful for
access to the high-performance Brutus computer cluster (ETH).
Y.-L. W. acknowledges finacial support by the Stipendienfonds der
Schweizerischen Chemischen Industrie (SSCI).
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Received: January 31, 2012
Published Online: March 22, 2012
Eur. J. Org. Chem. 2012, 2756–2765
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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