Donor-acceptor (D-A)-substituted polyyne chromophores: Modulation of their optoelectronic properties by varying the length of the acetylene spacer

Article abstract of DOI:10.1002/chem.201301642

A series of donor-acceptor-substituted alkynes, 2 a-f, was synthesized in which the length of the π-conjugated polyyne spacer between the N,N-diisopropylanilino donor and the 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) acceptor was systematically changed. The

Full text of DOI:10.1002/chem.201301642

FULL PAPER
Spacer length matters: The influence
of the length of polyyne spacers on the
optoelectronic properties of push–pull
chromophores was comprehensively
investigated in a combined experimen-
tal and theoretical study. The high sol-
ubility of chromophores allowed for
INADEQUATE NMR measurements
without 13C labeling, thus providing
unique insight into the p-electron
delocalization along the polyyne chain.
Very high, off-resonant specific third-
order optical nonlinearities were meas-
ured at a spacer length of two and
three triple bonds.
Donor–Acceptor Chromophores
ˇ
M. Stefko, M. D. Tzirakis, B. Breiten,
M.-O. Ebert,* O. Dumele,
W. B. Schweizer, J.-P. Gisselbrecht,
C. Boudon, M. T. Beels, I. Biaggio,*
F. Diederich* .................. &&&& — &&&&
Donor–Acceptor (D–A)-Substituted
Polyyne Chromophores: Modulation
of Their Optoelectronic Properties by
Varying the Length of the Acetylene
Spacer
Chem. Eur. J. 2013, 00, 0 – 0
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DOI: 10.1002/chem.201301642
Donor–Acceptor (D–A)-Substituted Polyyne Chromophores:
Modulation of Their Optoelectronic Properties by Varying
the Length of the Acetylene Spacer
[a]
[a]
[a]
[a]
ˇ
Martin Stefko, Manolis D. Tzirakis, Benjamin Breiten, Marc-Olivier Ebert,*
Oliver Dumele,[a] W. Bernd Schweizer,[a] Jean-Paul Gisselbrecht,[b] Corinne Boudon,[b]
Marten T. Beels,[c] Ivan Biaggio,*[c] and FranÅois Diederich*[a]
Dedicated to Professor Martin Quack on the occasion of his 65th birthday
Abstract: A series of donor–acceptor-
substituted alkynes, 2a–f, was synthe-
sized in which the length of the
p-conjugated polyyne spacer between
the N,N-diisopropylanilino donor and
INADEQUATE NMR measurements
without 13C labeling, which, in turn, re-
sulted in a complete assignment of the
carbon skeleton in chromophores 2a–f
and the determination of the 13C–13C
coupling constants. This body of data
provided unprecedented insight into
characteristic 13C chemical shift pat-
terns in push–pull-substituted polyynes.
Electrochemical and UV/Vis spectro-
scopic studies showed that the
HOMO–LUMO energy gap decreases
with increasing length of the polyyne
spacer, while this effect levels off for
spacers with more than four acetylene
units. The third-order optical nonli-
nearity of this series of molecules was
determined by measuring the rotation-
al averages of the third-order polariza-
bilities (grot) by degenerate four-wave
mixing (DFWM). These latter studies
revealed high third-order optical nonli-
nearities for the new chromophores;
most importantly, they provided funda-
mental insight into the effect of the
conjugated spacer length in D–A poly-
ynes, that can be exploited in the
future design of suitable charge-trans-
fer chromophores for applications in
optoelectronic devices.
the
1,1,4,4-tetracyanobuta-1,3-diene
(TCBD) acceptor was systematically
changed. The effect of this structural
change on the optoelectronic proper-
ties of the molecules and, ultimately,
their third-order optical nonlinearity
was comprehensively investigated. The
branched N,N-diisopropyl groups on
the anilino donor moieties combined
with the nonplanar geometry of 2a–f
imparted exceptionally high solubility
to these chromophores. This important
property allowed for performing
Keywords: charge-transfer · INAD-
EQUATE · nonlinear optics · poly-
ynes · push–pull chromophores
Introduction
Donor–acceptor (D–A) chromophores continue to attract
considerable interest due to their electronic and optical
properties, and particularly their second- and third-order op-
tical nonlinearities.[1] Most of the known D-p-A chromo-
phores feature a planar molecular scaffold to ensure effi-
cient p-conjugation between the donor and the acceptor
moieties.[2–10] These planar chromophores tend to form crys-
talline films by virtue of strong p–p stacking interactions
and antiparallel molecular dipole alignment in the solid
state.[11] Their employment in optoelectronic devices, howev-
er, requires the formation of amorphous thin films which
can achieve high homogeneity and optical quality over large
areas.[12] Thus, the design of suitable nonplanar chromo-
phores[13] that avoid the formation of crystalline films, is of
upmost importance for their practical applications.
ˇ
[a] Dr. M. Stefko, Dr. M. D. Tzirakis, Dr. B. Breiten, Dr. M.-O. Ebert,
O. Dumele, Dr. W. B. Schweizer, Prof. Dr. F. Diederich
Laboratorium fꢁr Organische Chemie, ETH Zꢁrich
Hçnggerberg, HCI, 8093 Zurich (Switzerland)
Fax : (+41)44-632-1109
Fax : (+41)44-632-1475
E-mail: ebert@org.chem.ethz.ch
diederich@org.chem.ethz.ch
[b] Dr. J.-P. Gisselbrecht, Prof. Dr. C. Boudon
Laboratoire d’Electrochimie et de Chimie Physique du Corps
Solide, Institute de Chimie-UMR 7177, CNRS
Universitꢂ de Strasbourg, 4, rue Blaise Pascal
CS 90032, 67081 Strasbourg Cedex (France)
[c] M. T. Beels, Prof. Dr. I. Biaggio
Lehigh University, Department of Physics
415 Lewis Lab, 16 Memorial Dr. East, Bethlehem, PA 18015 (USA)
Fax : (+1)610-758-5730
Since the mid-1990s, we reported high third-order optical
nonlinearities for five classes of small push–pull chromophores:
donor–acceptor-substituted
tetraethynylethenes
(TEEs),[8]
E-mail: biaggio@lehigh.edu
donor-substituted cyanoethynylethenes (CEEs),[9,10] donor-
substituted 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs),[14,15]
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301642.
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Donor–Acceptor (D–A)-Substituted Polyyne Chromophores
FULL PAPER
homoconjugated push–pull systems,[16] and more recently
N,N’-dicyanoquinone diimide-derived
acterization.[39] On the other hand, a donor-substituted
TCBD with a five-triple-bond spacer displayed good solubil-
ity and showed a very strong third-order optical nonlinearity
for self-phase modulation at a wavelength 1.5 mm, which
suggested a high potential for NLO applications of alkyne
spacers in suitably designed chromophores.[40]
In this work, we present a systematic study on the influ-
ence of the acetylene spacer length in TCBD-derived push–
pull chromophores. To this end, we have prepared a series
of push–pull chromophores 2a–f, in which the number of
the acetylene bonds in the polyyne spacer between the anili-
no donor and the TCBD acceptor varies systematically from
0 to 5 (Figure 1b).
D–A chromophores.[17] Especially in one of these studies we
showed that a nonplanar N,N-dimethylanilino (DMA)-sub-
stituted TCBD, namely 2-[4-(dimethylamino)phenyl]-3-
{[4(dimethylamino)phenyl]ethynyl}buta-1,3-diene-1,1,4,4-tet-
racarbonitrile; DDMEBT (1)) exhibits a large third-order
nonlinear optical response despite its small number of delo-
calized p-electrons (Figure 1a).[14] Most importantly, it is this
The presence of the branched N,N-diisopropylamino moi-
eties combined with the nonplanar geometry of 2a–f enhan-
ces the solubility of these chromophores, thus allowing for
performing NMR (INADEQUATE) and NLO measure-
ments in common organic solvents such as CH2Cl2 (CD2Cl2)
or CHCl3 (CDCl3). Furthermore, it should be noted that the
TCBD unit exhibits enhanced electron-accepting ability
with respect to the dicyanovinyl moiety used in the planar,
donor-substituted CEEs.[7]
Figure 1. a) Structure of the DDMEBT chromophore (1) employed in
previous studies and device fabrication.[14,18,19] b) D–A chromophores 2a–
f with polyyne spacers of different length investigated in this project.
very nonplanar geometry of DDMEBT that allows it to
form dense, high-optical quality homogeneous films upon
vapor deposition; these films have been elaborated into
nonlinear optical silicon-organic hybrid slot waveguides for
all-optical high-speed signal processing with excellent per-
formance.[18,19]
The use of polyyne spacers in D–A conjugates and, in par-
ticular, the intramolecular charge-transfer (CT) and the non-
linear optical (NLO) properties of such systems have long
Results and Discussion
Synthesis of oligoynes 3a–f and D–A chromophores 2a–f:
All TCBD chromophores 2a–f were prepared in moderate
to excellent isolated yields (50–92%) by the thermal [2+2]
cycloaddition–retroelectrocyclization (CA–RE) reaction[41]
between the corresponding N,N-diisopropylanilino-end-
capped oligoynes 3a–f and tetracyanoethylene (TCNE) in
CH2Cl2 at 258C (Scheme 1).
been studied on both
a theoretical and experimental
basis.[20–28] Typically, the off-resonant, that is, nonabsorptive,
third-order polarizability strongly increases as the energy as-
sociated with the transition from the ground to the first ex-
cited state (HOMO–LUMO gap) decreases.[29] More specific
investigations on the third-order NLO properties of poly-
ynes without D–A substitution have shown that their molec-
ular second hyperpolarizabilities (g) increase as a function
of the number of acetylene units (n) according to the power-
law gꢀn4.28Æ0.13 [30–32]
.
Similarly, in D–A-substituted polyynes,
polarizability depends on the length of the linear D–A con-
jugation pathways.[33,34] Thus, stronger D–A coupling across
shorter p-conjugated spacers leads to hypsochromic shifts,
whereas weaker D–A coupling across longer spacers results
in smaller HOMO–LUMO gaps and hence bathochromical-
ly shifted CT bands.[35,36] In D–A chromophores of given
spacer size, the HOMO–LUMO gap decreases with increas-
ing p-conjugation efficiency.[37]
The role of the acetylenic groups as spacers in push–pull
chromophores has been comprehensively studied for donor-
substituted cyanoethynylethenes (CEEs).[9,38,39] The intro-
duction of alkynyl spacers (n=0–3) in these chromophores
resulted in less readily homogeneous solutions due to the re-
sulting planar molecular geometry and concomitant low sol-
ubility.[39] Also, there were indications that saturation of the
third-order polarizability could already be reached at the
diyne stage, but the weak solubility did not allow a full char-
Scheme 1. Synthesis of D–A chromophores 2a–f by the CA-RE reaction
of oligoynes 3a–f with TCNE. a) TCNE, CH2Cl2, 258C, 6 h. TCNE=tet-
racyanoethylene.
The synthesis of oligoynes 3a–f started from 4-ethynyl-
N,N-diisopropylaniline (4) prepared on a multigram scale by
a conceptually new protocol (see Scheme 1SI in the Sup-
porting Information). Monoyne 3a was prepared by Sonoga-
shira cross-coupling reaction of 4-bromo-N,N-diisopropyl-
AHCTUNGTREGaNNNU niline with terminal acetylene 4 in 58% yield (see Sche-
me 2SI in the Supporting Information). Diyne 3b and tet-
rayne 3d were obtained by following well-established syn-
thetic protocols[42,43] by an oxidative homocoupling reaction
of the corresponding terminal mono- and di-acetylenes in
88% and 45% overall yield, respectively, based on 4 (see
Scheme 3SI and 5SI in the Supporting Information). Triyne
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M.-O. Ebert, I. Biaggio, F. Diederich et al.
3c was prepared from 4 in 28% yield over 4 steps; the last
critical step included a Fritsch–Buttenberg–Wiechell (FBW)
rearrangement[44–47] of the corresponding symmetrical gem-
dibromoolefin[48] (see Scheme 4SI in the Supporting Infor-
mation). The synthesis of pentayne 3e started from terminal
acetylene 4 which, after treatment with nBuLi at À188C,
was allowed to react with triisopropylsilylpropargyl alde-
hyde[49] to give pentadiynol 5 in 90% yield (Scheme 2). Sub-
sequently, the oxidation of alcohol 5 by BaMnO4 provided,
Scheme 2. Synthesis of intermediate 8: a) nBuLi, Et2O, À788C, 20 min,
Scheme 3. Synthesis and ORTEP plot of pentayne 3e: a) nBuLi, À188C,
THF, 10 min, then DMF, À188C, 30 min, 76%; b) 8, LDA, À788C, Et2O,
30 min, then 9, À788C, 30 min, 84%; c) BaMnO4, CH2Cl2, 258C, 1 h,
À ꢁ À
then TIPS C C CHO, À788C, 30 min, 90%; b) BaMnO4, CH2Cl2, 258C,
30 min, 98%; c) PACHTUNGTRENNUNG(OiPr)3, CBr4, CH2Cl2, 08C, 1.5 h, 83%; d) TBAF,
THF, 08C, 20 min, 96%. TIPS=triisopropylsilyl; TBAF=tetra-n-buty-
lammonium fluoride; THF=tetrahydrofuran.
98%; d) PACTHUNGTRNEUNG(OiPr)3, CBr4, CH2Cl2, 08C, 1 h, 75%; e) nBuLi, toluene,
À788C to À108C, 30 min, 46%. THF=tetrahydrofuran; DMF=dime-
thylformamide; LDA=Lithium diisopropylamide. A second independent
molecule in the unit cell of 3e is shown in Figure 3SI in the Supporting
Information.
after a simple filtration of manganese salts, pure ketone 6 in
quantitative yield (Scheme 2). The Horner–Wadsworth–
Emmons modification of the Ramirez gem-dibromoolefina-
tion[50] was then used to transform ketone 6 into gem-dibro-
moolefin 7. Thus, a solution of ketone 6 was treated at 08C
13C NMR chemical shifts and assignments: Due to the lack
of protons in large parts of chromophores 2a–f, the 13C
chemical shift assignment by HSQC/HMBC is not possible.
Assignments of longer polyynes available in the literature
have been achieved so far by specific 13C labeling.[32] The ex-
cellent solubility of chromophores 2a–f in CDCl3, which re-
sults from the presence of the branched N,N-diisopropylami-
no moieties and is further enhanced by the nonplanar geom-
etry of the push–pull systems, allowed us to prepare NMR
samples concentrated enough for chemical shift assignment
by INADEQUATE spectroscopy, without 13C labeling.[51–53]
All assigned spectra are shown in Figure 2. A summary of
the chemical shifts for the carbon atoms in the acetylenic
linkers of chromophores 2b–f is provided in Table 1 (for the
list of all carbons, see Table 1SI in the Supporting Informa-
tion). Alongside the resonances of C3’ and C18’ carbons in
the aniline rings, the other two carbon signals at lowest field
belong to C7’ and C9’ of the TCBD moiety. The resonances
of the neighboring C8’ and C10’ carbons in a-position rela-
tive to the cyano groups are shifted to higher field by more
than 50 ppm. This pattern is characteristic for the shifts of
sp2 carbons adjacent to a dicyanomethylene group (e.g., in
benzylidene malononitrile)[54] and can be rationalized by a
favorable resonance structure bearing the positive charge at
with CBr4 in the presence of PACTHNUGTRENUNG(OiPr)3 furnishing 7 as a
bench-stable orange oil in 83% yield (Scheme 2). Cleavage
of the TIPS-protecting group was accomplished upon treat-
ment with nBu4NF giving terminal acetylene 8 in 96% yield.
Lithium acetylide, generated from 8 upon treatment with
LDA in Et2O at À788C, was subsequently allowed to react
with aldehyde 9 to give alcohol 10 in 84% yield,[32] which
was further oxidized by BaMnO4 furnishing ketone 11 in
quantitative yield (Scheme 3). Aldehyde 9 was prepared in
76% yield by the reaction of lithium acetylide (generated
form 4 upon treatment with nBuLi at À188C) with DMF
(Scheme 3). Subsequent gem-dibromoolefination of 11 fur-
nished bis(gem-dibromoolefin) 12 in 75% yield (Scheme 3).
In the final step, a two-fold FBW rearrangement of 12 gave
pentayne 3e in 46% yield (Scheme 3). Stable crystals, suita-
ble for X-ray analysis, were obtained by slow diffusion of
hexanes into a solution of 3e in CH2Cl2 at 258C (Scheme 3;
see also Figure 3SI in the Supporting Information).
The final member of this series of chromophores, hexayne
3 f, was prepared in 50% yield over two steps by oxidative
homocoupling of terminal acetylene 8 and subsequent two-
fold FBW rearrangement of the resulting bis-dibromoolefin
(see Scheme 6SI in the Supporting Information).
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Donor–Acceptor (D–A)-Substituted Polyyne Chromophores
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Figure 2. Assignment of the 13C NMR resonances for chromophores 2a–f. Parts of the spectrum below 60 ppm are omitted for clarity. The colored lines
connect the acetylenic carbons with the same distances from the aniline donor (D) or the TCBD acceptor (A). The following changes in chemical shift
as a function of increasing polyyne length are specifically highlighted: i) resonances of C1 directly attached to the donor (green line), ii) C-atoms in a-po-
sition to the acceptor (dotted green line), iii) C2 atoms next to the donor (magenta line), iv) C-atoms in b-position to the acceptor (dotted magenta line),
v) C3 atoms next to the donor (orange line), and iv) C-atoms in g-position to the acceptor (dotted orange line).
the b-carbon, whereas the negative charge is positioned at
one of the cyano groups. In the series 2a–f, there is a rather
large difference in chemical shift between carbons at posi-
tion 8’ (more than 20 ppm), whereas the 13C shifts at posi-
tion 10’ are unaffected by the length of the polyyne linker
(see Figure 4SI in the Supporting Information). This finding
can be accounted for by the existence of two independent
intramolecular CT pathways: one pathway involves CT from
the distant aniline to the dicyanovinyl acceptor via the ace-
tylene spacer, whereas the other pathway involves a CT
from the second diisopropylanilino donor directly to the ad-
jacent dicyanovinyl acceptor. This latter pathway, which in-
volves the dicyano group at C10’ and the adjacent aniline, is
decoupled from the polyyne and therefore not markedly af-
fected by the length of the linker (vide infra).
Figure 2 also highlights the change in the chemical shift of
the polyyne carbons with the same distance from the anilino
donor or the TCBD acceptor, respectively, as a function of
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M.-O. Ebert, I. Biaggio, F. Diederich et al.
Table 1. Summary of the chemical shifts (d, ppm) for the acetylenic carbons in chromophores 2b–f.[a]
Chemical shifts (d, ppm)[a]
resonances, respectively. The
intersection of the curves leads
to a breakdown of a strict al-
n[b]
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
2b
2c
2d
2e
2 f
1
2
3
4
5
125.81
104.78[c]
93.02
87.27
84.62
90.55
74.92[c]
74.22
73.91
73.77
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
ternation for 2 f (still intact in
2e) and will obscure it further
for longer chains. For donor–
acceptor systems, such as 2a–f,
the alternating chemical shifts
may be rationalized by a series
of resonance structures that all
locate the positive charge on
carbons 1, 3, 5, 7, 9 with odd
104.86
85.63
75.25
70.28
78.25
66.24
67.15
67.47
102.23
84.18
74.57
72.9
62.39
63.96
100.7
83.03
70.46
60.66
99.60
69.44
[a] Chemical shifts are given in ppm and are referenced to the signal of CDCl3 (77.16 ppm). [b] n=number of
ꢁ
C C units in the spacer between the anilino donor and the TCBD acceptor. [c] The assignment of C1 and C2
was achieved by combination of INADEQUATE and HMBC (400 MHz) measurements (cross-peak between
H5’ and C1).
chain length. With growing chain, the resonances at each po-
sition are gradually shifted to higher field, with the odd-
numbered C-atom resonances (C-atoms 1, 3) with respect to
the donor affected more than the even-numbered ones (C-
atoms 2) (solid lines, Figure 2). Similarly, the resonances of
carbons at odd distance to the acceptor shift more than
those at even distance (dashed lines, Figure 2). These indi-
vidual trends, which are in agreement with those reported
by Tykwinski and Luu for symmetrically substituted deriva-
tives,[55] may be explained with the inductive effect of the
polyyne chain. The same authors have also reported a
steady decrease in chemical
distances to the donor group and the negative charge on car-
bons 2, 4, 6, 8, 10 with even positions from the donor group.
Together, this result would suggest an alternating partial
charge separation leading to deshielded odd carbons and
shielded even carbons, thus explaining the downfield shift of
the odd versus the even positions. Indeed, the Hirshfeld
atomic charges[56] of 2d calculated in Gaussian 09[57] at the
B3LYP/6-311+GACTHNUTRGNEUNG(d,p) level of theory are weakly positive
for the odd and negative for the even positions (values:
0.002, À0.05, 0.006, À0.02, 0.003, À0.03 for C1 to C6 of the
polyyne linker, respectively).
shift of the carbon atoms in
the center of the chain with
growing chain length together
with
a polarization of the
chemical shifts at the level of
the individual carbon chains,
that is, chemical shifts at even
and odd positions are grouped
together.[55] An analogous plot
for 2a–f is shown in Figure 3a.
Because of the asymmetric
substitution of the polyyne
chain, here two individual de-
caying curves are obtained.
The curves suggest a conver-
gence of the chemical shifts of
the central atoms of this type
of donor–acceptor system in a
region around 70 ppm. If we
only plot the chemical shifts of
the polyyne linker of 2 f with-
out the terminal atoms 1 and
10 (Figure 3b), the increase of
the chemical shifts at the odd
positions with increasing dis-
tance to the donor, together
with a significant, albeit less
pronounced decrease of the
chemical shifts at the even po-
sitions with increasing distance
to the donor, leads to two indi-
vidually decaying curves con-
necting even and odd carbon
Figure 3. a) Chemical shift (d, ppm) of the C-atoms at the chain center plotted versus chain length. The black
line connects the shifts of the central acetylenic C-atoms on the side of the anilino donor. The red line con-
nects the shifts of the central acetylenic C-atoms on the side of the dicyanovinyl acceptor. b) Chemical shifts
(d, ppm) of the C-atoms in chromophore 2 f. C-atom numbering according to Figure 2.
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1
Table 2. Summary of the J
G
In agreement with this ex-
planation, for the 13C shifts of
two
di-tert-butyl-end-capped
polyynes reported by Bohl-
mann and Brehm (C16H18 and
C18H18)[58] where neither donor
nor acceptor is present, no al-
ternation in chemical shift but
only a decay towards the cen-
tral carbon atoms was ob-
served. This simple argument,
however, does not explain the
sense of polarization observed
Coupling constants[a]
n[b]
J1
J2
J3
J4
J5
J6
J7
J8
J9
J10
J11
2b
2c
2d
2e
2 f
1
2
3
4
5
95
187
194
198
195
195
100
–
–
–
–
–
–
–
206
217
–
–
–
101
169
–
–
–
–
–
–
–
–
[c]
[c]
–
–
202
215
213
101
167
172
170
–
–
94
94
95
162
166
205
215
215
99
168
171
[c]
[c]
–
–
205
102
ꢁ
[a] Coupling constants are given in Hz. [b] n=number of C C units in the spacer between the anilino donor
for
a,w-diphenylpolyynes.[55]
and the TCBD acceptor. [c] Not determined.
Here, one would expect the p-
donor effect of the two phenyl
groups (at least close to the polyyne termini) to lead to
higher shielding of the even positions compared to the odd
ones. Conversely, Tykwinski and Luu have found that for
these molecules the odd positions are shifted towards higher
field.[55] Clearly, a better understanding of the influence of
the electronic structure on the 13C chemical shifts is still
needed.
10 Hz. The coupling constants corresponding to the inner
triple-bonds are around 215 Hz. The ones corresponding to
the formal Csp–Csp single bonds are between 160 and
170 Hz. The only exception is 2b with a coupling constant
of only 187 Hz between the carbon atoms forming the triple
bond.
UV/Vis spectroscopy: Figure 5 depicts the UV/Vis absorp-
tion spectra of CT chromophores 2a–f recorded in CH2Cl2
at 258C. A detailed discussion of the UV/Vis spectra of the
oligoyne precursors 3a–f is provided in the Supporting In-
formation. All chromophores 2a–f feature a broad, low-
energy transition that is assigned to the intramolecular
charge-transfer (ICT) transition resulting from excitation
from the anilino donor at the polyyne termini to the TCBD
acceptor. The CT nature of this band was confirmed by
treating a solution of these chromophores in CH2Cl2 with
CF3COOH, leading to attenuation of the CT band; the band
re-appeared when the acidified solution was treated with
NEt3 (see Figure 9SI in the Supporting Information). The
energy of this CT band decreases with increasing spacer
13C–13C coupling constants: Inspection of the cross-peak fine
structure in the INADEQUATE spectra of 2a–f allowed for
ACTHNUTRGNEUNG
a straightforward extraction of 1J(13C–13C) coupling con-
stants (for 2e, see Figure 4; for all compounds of the series,
ACHTUNGTRENNUNG
Figure 4. 1J(13C–13C) coupling constants; chromophore 2e is shown as a
representative example (complete data for the whole series 2a–f are
shown in Figure 5SI in the Supporting Information).
length; for example, the longest-wavelength maximum (lmax
)
see Figure 5SI in the Support-
ing Information). Outside the
polyyne spacer, the coupling
constants are nearly constant
throughout the series 2a–f. In
contrast to the chemical shifts,
there is also very little varia-
tion among coupling constants
between spacer atoms at the
same
relative
position
(Table 2). The coupling con-
stants corresponding to the
two outermost triple bonds are
close to 200 Hz with the one
between the two carbons di-
rectly adjacent to the TCBD
moiety being the larger one of
the two by approximately
ꢁ
Figure 5. UV/Vis spectra (CH2Cl2, 258C) of chromophores 2a–f with different number of C C units in the
spacer between the diisopropylanilino donor and the TCBD acceptor.
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M.-O. Ebert, I. Biaggio, F. Diederich et al.
increases from 482 nm (2.57 eV) for 2a (n=0) to 629 nm
(1.97 eV) for 2 f (n=5) (Table 3). More specifically, the D–
A chromophore 2a (n=0) displays an intense CT band at
confirms that this latter CT is almost independent on the
spacer length, and that both CT interactions are essentially
independent from each other. Similar conclusions have been
drawn on the basis of the NMR investigations presented
above.
482 nm with molar absorption coefficient
eACHTUNGTRENNU(G lmax)=
Finally, the UV/Vis spectra of 2a–f show that
the CT band at the longest wavelength (lmax) is
Table 3. UV/Vis data (CH2Cl2, 258C) for chromophores 2a–f with different number
ꢁ
Entry
of C C units in the spacer between the anilino donor and the TCBD acceptor.
[b]
Compd
n[a]
l1,max
e
A
)
lmax
[nm (eV)]
e
U
)
broader than the one at lower wavelength (l1,max
)
A
R
]
G
U
and broadens with increasing the spacer length.
This effect should be ascribed to the flexibility of
the acetylene spacer in solution, which increases
with increasing length and results in a larger vari-
ety of low-frequency molecular vibrational modes
in the longer acetylene linkers.
1
2
3
4
5
6
2a
2b
2c
2d
2e
2 f
0
1
2
3
4
5
–
–
–
–
482 (2.57)
539 (2.30)
586 (2.12)
613 (2.02)
628 (1.97)
629 (1.97)
66800
49200
36200
28000
22300
17900
460 (2.70)
462 (2.68)
469 (2.64)
473 (2.62)
31800
44900
60300
68800
To further support the above-mentioned find-
ings, we prepared the corresponding series of
N,N-dimethylamino analogues 13a (n=0), 1 (n=
1), and 13c,d (n=2, 3) (Figure 6) and their UV/
ꢁ
[a] n=number of C C units in the acetylene spacer. [b] l1 values for compounds 2c–f
refer to the high-energy transition.
66800mÀ1 cmÀ1 (Table 3, entry 1), whereas the corresponding
CT band of chromophore 2b (n=1) is bathochromically
shifted by 57 nm with concomitant decrease in absorption
[eACHTUNGTRENNUNG
(lmax)=49200mÀ1 cmÀ1] (Table 3, entry 2). Starting from 2c
(n=2) the absorption spectrum is split into two separated
CT bands, including, in particular, one high-energy band
with l1,max =460 nm (2.70 eV, 31800mÀ1 cmÀ1) and one low-
energy band with lmax =586 nm (2.12 eV, 36200mÀ1 cmÀ1)
(Table 3, entry 3). With increasing acetylenic spacer length,
the low-energy band in 2d (n=3) is bathochromically shift-
ed to lmax =613 nm (2.02 eV) accompanied by a decrease in
intensity (28000mÀ1 cmÀ1). On the other hand, the high-
energy band is bathochromically shifted by only 2 nm to
l1,max =462 nm (2.68 eV) accompanied—in contrast to the
low-energy band—by an increase in intensity from e-
Figure 6. N,N-Dimethylanilino-derived chromophores 13a,[14] 1,[14] 13c,
and 13d.[42]
Vis spectra were directly compared to those of 2a–d (n=0–
3) (see Figure 20SI and 21SI; see also Figure 16SI–19SI in
the Supporting Information). These measurements support-
ed fully the aforementioned conclusions regarding the effect
of the spacer length on the optoelectronic properties of (D–
A)-substituted acetylene chromophores. A further compari-
son between the UV/Vis spectra of these two series of chro-
mophores revealed a small bathochromic shift of all ICT
bands of 2a–d with respect to those of 13a, 1 and 13c,d, ac-
companied by a small increase in intensity (see Figure 21SI
in the Supporting Information). This effect should be ascri-
bed to the stronger electron-donating ability of the N,N-dii-
sopropylanilino donor as compared to its dimethylanilino
analogue.[43]
ACHTUNGTRENNUNG
(l1,max)=31800mÀ1 cmÀ1 (2c) to 44900mÀ1 cmÀ1 (2d). The
same trend continues for the rest of the series (Table 3,
Figure 5; see also Figure 22SI in the Supporting Informa-
tion). The lmax values in this series of chromophores level
off at 628 nm (1.97 eV) for 2e (n=4) and 2 f (n=5), thus
suggesting a saturation of the D–A interactions. Further-
more, chromophores 2c (n=2) and 2d (n=3) display posi-
tive solvatochromism; the change of solvent from n-hexane
to CH2Cl2 resulted in a bathocromic shift of the longer-
wavelength CT band by 56 and 59 nm for 2c and 2d, respec-
tively (see Figures 23SI–24SI in the Supporting Informa-
tion).
The slight differences observed for the l1,max values of 2c–
f compared to the large bathochromic shift of the lmax
values (see Figure 22SI in the Information), can be ascribed
to the different influence of the spacer length on the two
CT pathways. Thus, an increase in the spacer length influen-
ces only the ICT from the distant anilino donor via the
spacer to the adjacent dicyanovinyl group; this results in a
Computational studies: The vertical optical transitions of
the optimized molecular structures of 2a–f were calculated
by time-dependent density functional theory (TD-DFT; for
further details on the computational methods, see the Sup-
porting Information). In all cases, the computed transition
energies are slightly larger than the experimental values
(see Table 4SI–9SI in the Supporting Information). Differen-
ces between computed excitation energies and experimental
absorption maxima are in the range of 0.15–0.37 eV (mean
absolute deviation of 0.27 eV), well within the expected
error.[59–61] The comparison of the computed and the experi-
remarkable bathochromic shift of the ICT absorption (lmax
)
by 147 nm upon moving from 2a to 2 f. On the other hand,
the CT from the anilino donor to the directly connected di-
cyanovinyl group (l1,max) displays only a slight bathochromic
shift (from 460 nm in 2c to 473 nm in 2 f). This observation
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Donor–Acceptor (D–A)-Substituted Polyyne Chromophores
FULL PAPER
mental absorption in the low-energy transitions shows a sim-
ilar trend (see Figure 25SI in the Supporting Information);
with increasing spacer length, the absorption maxima show
bathochromic shifts reaching a plateau starting with the tet-
rayne spacer. The plateau energy is due to the centering of
HOMO and LUMO on the donor and acceptor, which is a
characteristic of these (D–A)-substituted polyynes. Without
D–A substitution, the lmax values would be expected to keep
growing for longer spacers as it was observed for the corre-
sponding non-(D–A)-substituted polyynes 3a–f (see Fig-
ure 8SI in the Supporting Information).[62] Also without
D–A substitution, the absolute value of lmax would have
been much smaller (see Figure 8SI in the Supporting Infor-
mation).[30]
effect is also reflected in the decrease of the transition
dipole moment (M) (see the Supporting Information, Sec-
tion 6.3), the decrease of the extinction coefficient (e) of the
low-energy absorptions (lmax), and the decrease of the off-
resonant third-order polarizability for the longer-spacer
chromophores 2e,f (vide infra). Further detailed discussion
of the frontier orbitals of 2a–f and the resulting effect on
the transition dipole moments (M) is given in the Support-
ing Information (Section 6.3).
The higher-energy transitions (at 460–473 nm) mainly con-
sist of CT from HOMOÀ1-to-LUMO (for 2a–c) and
HOMOÀ1-to-LUMO+1 (2d–f). This computational finding
suggests that this CT results from the transition from the
anilino donor adjacent to the TCBD acceptor; the orbital
The lowest-energy transitions are composed mostly of
HOMO-to-LUMO excitations and thus can best be descri-
bed as intramolecular charge-transfer processes involving
the transfer of electron density from the anilino donor to
the dicyanovinyl acceptor in compound 2a (n=0) and from
the distant anilino donor via the acetylene linker to the ad-
jacent dicyanovinyl acceptor moiety in compounds 2b–f
(n=1–5) as depicted in the molecular orbital representa-
tions of these levels (see Table 4SI–9SI in the Supporting In-
formation). Interestingly, with increasing length of the acety-
lene spacer all orbitals involved in the discussed excitations
become less diffuse and get clearly separated, reaching
almost no overlap in the electron density on the donor and
acceptor moieties for the case of the pentayne spacer (2 f),
although a certain overlap of the HOMO and LUMO densi-
ty on the acetylenic spacer is still observed (Figure 7). This
pictures confirm this trend to a certain extent. The
HOMOÀ1 electron density is mainly on the anilino moiety
adjacent to the TCBD moiety and transfers to the conjugat-
ed dicyano group. However, from the diyne to the pentayne
spacer in 2c–f, a significant electron density on the acetylen-
ic units is observed for the LUMO+1 orbital (which is,
however, only significantly involved in that high-energy
transition in the case of 2d–f). This behavior clearly shows
the electronic communication between the two twisted intra-
molecular donor–acceptor parts and shows the influence of
the acetylenic spacer on the neighboring D–A system; this
influence explains particularly the small bathochromic shift
of the higher-energy excitations (460–473 nm) with increas-
ing spacer length.
Electrochemistry: Electrochemical investigations were car-
ried out by cyclic voltammetry (CV) (see Figure 27SI in the
Supporting Information) and rotating disk voltammetry
(RDV) in CH2Cl2 containing 0.1m nBu4NPF6 on a glassy
carbon electrode (Table 4). All potentials are given versus
Fc+/Fc (ferrocenium/ferrocene couple), which was used as
an internal reference.
All chromophores 2a–f gave rise to two reversible one-
electron reductions located on the TCBD moiety. Only in
the case of pentayne 2 f, a third irreversible reduction was
observed. The reduction potentials are anodically shifted as
a function of increasing number of acetylenic units between
the donor and the acceptor. Thus, the first two reversible
one-electron reductions are shifted from À1.15 and À1.31 V
for 2a to À0.59 and À0.94 V for 2 f (Table 4; see also Fig-
ure 28SI in the Supporting Information), respectively, as a
result of the spacer extension.
The first oxidation, located on the anilino unit separated
from the TCBD acceptor by the alkynyl spacer, is facilitated
with increasing spacer length. Thus, the aniline ring transfers
less electron density into the acceptor moiety which results
in a dramatic cathodic shift of the first oxidation potential
from 0.81 V in 2a to 0.56 V in 2 f (Table 4; see also Fig-
ure 29SI in the Supporting Information). On the other hand,
the second oxidation of the aniline ring adjacent to the
TCBD moiety did not differ much throughout the whole
series (0.92 V in 2a to 0.89 V in 2 f, Table 4; see also Fig-
ure 29SI in the Supporting Information). This is in agree-
Figure 7. The HOMO (bottom) and LUMO (top) orbitals of 2 f. Further
information for the other orbitals involved in 2 f, as well as for 2a–e, is
given in the Supporting Information.
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M.-O. Ebert, I. Biaggio, F. Diederich et al.
Table 4. Electrochemical data observed by cyclic voltammetry (CV; scan
rate n=0.1 VsÀ1) and rotating disk voltammetry (RDV) in CH2Cl2 (+
0.1m nBu4NPF6).[a]
in the semi-transparent spectral region between cells with
neighboring concentrations. This guarantees an accurate de-
termination of the molecular concentration responsible for
the four-wave mixing signal even in those cases in which it
was not possible to accurately control solvent evaporation
and solute molecular mass for each dilution step. In addi-
tion, comparison of the absorption spectrum of the weakest
concentration solution with the reference spectrum before
and after the DFWM experiments confirmed the absence of
any decomposition or contamination during the measure-
ments.
CV
DEp
RDV
E8
Ep
E1/2
slope
[V][b]
[mV][c]
[V][d]
[V][e]
[mV][f]
+0.92
+0.81
À1.15
À1.31
+0.90
+0.69
À0.91
À1.16
+0.90
+0.64
À0.76
À1.07
+0.91
+0.59
À0.67
À1.05
+0.90
+0.57
À0.62
À0.97
+0.89
+0.56
À0.59
À0.94
60
60
110
100
80
+0.95 (1eÀ)
+0.84 (1eÀ)
À1.19 (1eÀ)
À1.36 (1eÀ)
+0.93 (1eÀ)
+0.71 (1eÀ)
À0.92 (1eÀ)
À1.19 (1eÀ)
60
60
90
80
60
60
80
80
70
70
70
90
70
70
110
120
70
70
70
2a
2b
2c
2d
2e
80
110
100
100
60
100
100
80
For low concentrations, the third-order susceptibility of
the solution depends on the concentration C in units of
mass of the solute divided by mass of the solution as
c(3)1111(C)=c(3)1111(0) + C f4 grot 1/m, where f=(n2 +2)/3 is a
Lorentz local field factor that depends on the refractive
index n=1.42 of CH2Cl2, 1 is the mass density of CH2Cl2,
and m is the mass of the molecule under investigation. The
DFWM measurement determines the absolute value
squared of c(3)1111(C) for different values of C. We obtained
absolute values for grot by a reference measurement that es-
+0.90
(1eÀ)
+0.62 (1eÀ)
À0.78 (1eÀ)
À1.11 (1eÀ)
+0.91 (1eÀ)
+0.62 (1eÀ)
À0.71 (1eÀ)
À1.07 (1eÀ)
+0.91 (1eÀ)
+0.58 (1eÀ)
À0.65 (1eÀ)
À1.04 (1eÀ)
+0.85
70
110
120
90
75
100
100
80
60
60
80
tablished the value of c(3)
1111AHCTUNRGTEG(NNUN C=0) (for a cell filled with
[g]
pure CH2Cl2) to be 6Æ1 times larger than for a 1 mm thick
fused silica sample, and using a third-order susceptibility of
c(3)1111(Àw, Àw, w, w)=1.9ꢃ10À22 for fused silica at 1.5 mm.
Table 5 lists the results for the third-order polarizability of
molecules 2a–f, as well as 13a, 1, and 13c,d. In Table 5 we
also list one more quantity that is useful for judging the non-
linear optical properties of a molecule, namely the specific
third-order polarizability (gS), obtained by dividing the ex-
perimental value for the third-order polarizability grot by the
molecular mass;[10] this quantity determines the potential
bulk third-order susceptibility of a dense supramolecular as-
sembly of molecules.
+0.53 (1eÀ)
À0.58 (1eÀ)
À0.95 (1eÀ)
À2.05 (1eÀ)
75
60
60
110
2 f
60
À2.15
[a] All potentials are given versus Fc+/Fc, which was used as internal
standard. [b] Eo =(Epc +Epa)/2, where Epc and Epa correspond to the
cathodic and anodic peak potentials, respectively. [c] DEp =EpaÀEpc.
[d] Ep =irreversible peak potential. [e] E1/2 =half-wave potential.
[f] Slope=slope of the linearized plot of E versus log[I/
Ilim is the limiting current and I the current. [g] Electrode inhibition.
G
ment with the small wavelength shift observed by UV/Vis
spectroscopy for the CT from the anilino donor directly con-
nected to the dicyanovinyl acceptor group.
The nonlinear response of each molecule measured at
1.5 mm increases strongly with each addition of a triple bond
in the spacer. The extended TCBD 2 f exhibits a large, reso-
Nonlinear optical properties: The rotational aver-
age of the third-order polarizability grot of 2a–f
Table 5. Summary of the NLO relevant characteristics (CH2Cl2, 258C) of the mole-
was determined by measuring the third-order sus-
ceptibility c(3)1111(Àw, Àw, w, w) of CH2Cl2 solu-
tions of varying molecular concentration. The
measurements were taken by degenerate four-
wave mixing (DFWM) at 1.5 mm using 1 ps laser
pulses obtained from a TOPAS travelling wave op-
tical parametric amplifier system pumped by a
Clark MXR Ti:Sapphire amplifier. We used 1 mm
thick solutions in fused silica spectroscopy cells,
the concentrations of which were varied by succes-
sive dilutions and determined for each solution
from the absorption spectrum calibrated to the
spectrum of the molecular extinction coefficient,
accurately measured by using solutions with vary-
ing molecular concentration (Figure 5; see also
Figure 10SI–19SI in the Supporting Information).
In addition, the higher concentration cells were
calibrated by comparing the absorption spectrum
cules investigated in this study.[a]
Compound
Np
n
lmax
e
(lmax
)
grot
[10À48 m5 VÀ2
gS
[nm]
[103 mÀ1 cmÀ1
]
]
[10À23 m5 VÀ2 KgÀ1
]
2a
2b
2c
2d
2e
2 f
24
26
28
30
32
34
24
26
28
30
0
1
2
3
4
5
0
1
2
3
482
539
586
613
628
629
471
526
570
595
66.8
49.2
36.2
28.0
22.3
17.9
58.5
45.3
33.6
25.7
2Æ1
0.24
0.68
1.6
3.1
6.0
7.7
0.31
0.9
1.6
3.1
6Æ2
15Æ5
30Æ10
60Æ20
80Æ30
2Æ1
13a[14]
1[12,14]
13c
13d
6Æ1
12Æ4
24Æ8
[a] For each molecule, we give the total number of electrons Np in the two separate
ꢁ
conjugated subsystems, the number of C C spacers n, the longest wavelength absorp-
tion maximum lmax, the corresponding molar extinction coefficient e, the experimental
value of the third-order polarizability grot (rotational average), and the specific third-
order polarizability (gS). The third-order polarizabilities have been measured at a
fixed wavelength of 1.5 mm and are increasingly resonantly enhanced for molecules
2d–2 f.
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Donor–Acceptor (D–A)-Substituted Polyyne Chromophores
FULL PAPER
nantly-enhanced grot value of (80Æ30)ꢃ10À48 m5 VÀ2. For
comparison, 1 (DDMEBT) currently used in non-linear
optics devices, displays under the same conditions an off-res-
onant grot value of (6Æ1)ꢃ10À48 m5 VÀ2.[12]
optical response, was provided by the two- and three-triple-
bond spacers. Compounds 2d and 13d in particular, feature
a remarkably high, non-resonantly enhanced third-order op-
tical nonlinearity.
In essence, this investigation provided important insight
into the effect of the conjugated spacer length in D–A poly-
ynes, which can be exploited in the future design of suitable
CT chromophores with potential applications in optoelec-
tronic devices.
It is important to note that, as more acetylene moieties
are added to the spacers to obtain molecules 2d–f, the wave-
length of maximum absorption shifts towards the red, with
the corresponding absorption peak widening towards wave-
lengths longer than 700 nm. As a consequence, the first two-
photon resonance for DFWM approaches the wavelength
we used for the measurements (1.5 mm), and the third-order
polarizability values obtained for molecules 2d–f are in-
creasingly enhanced by the closeness to the two-photon ab-
sorption condition.
Experimental Section
General materials and physical characterization data are found in the
Supporting Information.
In fact, DFWM experiments performed at longer off-reso-
nant wavelengths confirm that the third-order polarizability
values start saturating or even decreasing for the longest oli-
goyne chains. As a consequence, the specific off-resonant
third-order polarizability reaches a maximum for the com-
pounds with two and three triple-bond spacers (2c, d) and
then decreases for the longer molecules. This behavior is
consistent with the decrease in the HOMO–LUMO transi-
tion dipole moment mentioned above.
2D NMR spectra: INADEQUATE spectra were measured on a Bruker
Avance III 600 MHz spectrometer equipped with a 5 mm CPDCH cryo-
genic probe optimized for carbon sensitivity. Sample concentration in
CDCl3 was between 100 and 400 mm. Pulse sequence and phase cycle
were taken from reference [53]. The echo delay (1/ACTHUNRTGNEUNG(4 JCC)) was set to
2.5 ms. Recycle delay (including acquisition time) was between 2 and 3 s.
Total experiment time was between 19 and 80 h.
2,3-Bis[4-(diisopropylamino)phenyl]buta-1,3-diene-1,1,4,4-tetracarboni-
trile (2a): A solution of 3a (112 mg, 0.30 mmol) in CH2Cl2 (3 mL) was
treated with TCNE (46 mg, 0.36 mmol) and stirred at 258C for 8 h (the
reaction color changed from brown-yellow to dark red upon addition of
TCNE). Evaporation and column chromatography (SiO2, CH2Cl2) gave
2a (135 mg, 90%) as a black metallic solid. Rf =0.45 (SiO2, CH2Cl2); m.p.
2538C; 1H NMR (300 MHz, CDCl3, 258C): d=1.35 (d, J=6.8 Hz, 24H,
H-C(1’)), 4.06 (hept, J=6.8 Hz, 4H, (H-C(2’)), 6.82 (d, J=9.6 Hz, 4H,
H-C(4’)), 7.76 ppm (d, J=9.5 Hz, 4H, H-C(5’)); 13C NMR (75 MHz,
CDCl3, 258C): d=20.75 and 20.83, 48.33 (C(2’)), 73.45 (C(8’)), 113.98
Conclusion
A series of push–pull chromophores 2a–f, with alkyne
spacers of systematically varying length between one anilino
donor ring and the TCBD acceptor moiety was synthesized.
The effect of this structural change on the spectroscopic and
electronic properties of the molecules and, ultimately, on
the measured third-order optical nonlinearity was evaluated.
The two, independent charge-transfer conjugation pathways
in these molecules were comprehensively investigated by
means of NMR and UV/Vis spectroscopy, electrochemistry,
and theoretical calculations. The branched N,N-diisopropy-
lanilino groups and the non-planarity of the donor–acceptor
moiety originating from the [2+2] cycloaddition–retroelec-
trocyclization cascade strongly increased the solubility of
these chromophores, thereby allowing for INADEQUATE
NMR measurements without 13C labeling. These measure-
ments enabled a complete assignment of the carbon skele-
ton in this series of chromophores and provided unique in-
sight into the electronic properties of these chromophores,
such as the p-electron delocalization along the polyyne
chain. UV/Vis and electrochemical measurements revealed
that the HOMO–LUMO gaps decrease with increasing
length of the polyyne chain spacer, while the effect of the
spacer length levels off for spacers with more than four ace-
tylene units (saturation). These findings were further corro-
borated by density functional theory calculations. Also,
these conclusions were in complete agreement with the
NLO studies, which, in turn, showed a decrease of the spe-
cific off-resonant third-order polarizability for chromo-
phores with four- and five-triple-bond spacers (2e,f). Impor-
tantly, the optimal conjugation length, in terms of nonlinear
(CN), 114.52 (C(4’)), 115.07 (CN), 118.20 (CACTHUNRGTNEUNG(15’)), 132.26 (C(5’)), 153.06
(C(3’)), 164.93 ppm (C(7’)); IR (ATR): n˜ =2975 (w), 1935 (w), 2215 (m),
1599 (s), 1480 (s), 1445 (s), 1373 (w), 1340 (m), 1309 (s), 1278 (m), 1213
(m), 1187 (m), 1162 (m), 1124 (m), 1016 (w) cmÀ1; UV/Vis (CH2Cl2): lmax
(e)=260 (13300), 286 (sh, 10400), 413 (sh, 22800), 482 (66800mÀ1 cmÀ1);
HR-MALDI-MS (DCTB): m/z (%): 504.2997 (100, [M]+, calcd for
C32H36N6+: 504.2996).
2-[4-(Diisopropylamino)phenyl]-3-{[4-(diisopropylamino)phenyl]ethy-
nyl}buta-1,3-diene-1,1,4,4-tetracarbonitrile (2b): A solution of diyne 3b
(105 mg, 0.26 mmol) in CH2Cl2 (2 mL) was treated with TCNE (40 mg,
0.31 mmol) and stirred at 258C for 8 h (the reaction color changed from
brown-yellow to dark red upon addition of TCNE). Evaporation and
column chromatography (SiO2, CH2Cl2) gave 2b (120 mg, 92%) as a
black metallic solid. Rf =0.47 (SiO2, CH2Cl2); m.p. 2048C; 1H NMR
(300 MHz, CDCl3, 258C): d=1.34 (d, J=6.9 Hz, 12H, H-C(1’)), 1.37 (d,
J=6.8 Hz, 12H, H-C
(hept, J=6.8 Hz, 2H, H-C
(d, J=9.5 Hz, 2H, H-C(17’)), 7.42 (d, J=9.2 Hz, 2H, H-C(5’)), 7.79 ppm
(d, J=9.5 Hz, 2H, H-C
(16’)); 13C NMR (75 MHz, CDCl3, 258C): d=
20.78 and 20.86 (C(20’)), 20.78 (C(1’)), 48.03 (C(2’)), 48.39 (C(19’)), 72.61
(C(10’)), 86.87 (C(8’)), 90.55 (C(2)), 104.79 (C(6’)), 111.78 and 112.81 (2
C; C(13’,14’)), 113.69 (C(11’or 12’)), 114.61 (C(17’), 114.77 (C(4’)), 114.92
(C(11’or 12’)), 117.00 (C(15’)), 125.81 (C(1)), 132.21 (C(16’)), 135.73
(C(5’)), 149.95 (C(7’)), 151.79 (C(3’)), 153.26 (C(18’)), 160.78 ppm
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
(C(9’)); IR (ATR): n˜ =2971 (w), 2937 (w), 2880 (w), 2215 (m), 2119 (s),
1595 (s), 1525 (m), 1484 (s), 1447 (m), 1431 (s), 1382 (m), 1329 (s), 1314
(m), 1293 (m), 1218 (m), 1190 (m), 1166 (w), 1153 (w), 1109 (s), 1011
(m), 988 (w), 827 (m) cmÀ1; UV/Vis (CH2Cl2): lmax (e)=284 (17100), 324
(sh, 9600), 441 (sh, 27200), 486 (sh, 35300), 539 (49200mÀ1 cmÀ1);
HR-MALDI-MS (DCTB): m/z (%): 528.2997 (100, [M]+, calcd for
C34H36N6+: 528.2996).
2-[4-(Diisopropylamino)phenyl]-3-{[4-(diisopropylamino)phenyl]buta-
1,3-diyn-1-yl}buta-1,3-diene-1,1,4,4-tetracarbonitrile (2c): A solution of
Chem. Eur. J. 2013, 00, 0 – 0
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M.-O. Ebert, I. Biaggio, F. Diederich et al.
triyne 3c (110 mg, 0.26 mmol) in CH2Cl2 (16 mL) was treated with TCNE
(37 mg, 0.29 mmol) and stirred at 258C for 8 h (the reaction color
changed from orange to dark blue upon addition of TCNE). Evaporation
and column chromatography (SiO2, CH2Cl2) gave 2c (119 mg, 83%) as a
black metallic solid. Rf =0.48 (SiO2, CH2Cl2); m.p. 1338C; 1H NMR
(400 MHz, CDCl3, 258C): d=1.32 (d, J=6.9 Hz, 12H, H-C(1’)), 1.38 (d,
469 (60300), 628 (22300mÀ1 cmÀ1); HR-MALDI-MS (DCTB): m/z (%):
600.2997 (100, [M]+, calcd for C40H36N2+: 600.2996).
2-[4-(Diisopropylamino)phenyl]-3-{[4-(diisopropylamino)phenyl]deca-
1,3,5,7,9-pentayn-1-yl}buta-1,3-diene-1,1,4,4-tetracarbonitrile (2 f): A sol-
ution of hexayne 3 f (166 mg, 0.33 mmol) in CH2Cl2 (37 mL) was treated
with TCNE (52 mg, 0.40 mmol) and stirred at 258C for 8 h (the reaction
color changed from red to dark green upon addition of TCNE). Evapora-
tion and column chromatography (SiO2, CH2Cl2) gave 2 f (105 mg, 50%)
J=6.9 Hz, 12H, H-C
(hept, J=6.9 Hz, 2H, H-C
(d, J=9.5 Hz, 2H, H-C(17’)), 7.38 (d, J=9.0 Hz, 2H, H-C(5’)), 7.75 ppm
(d, J=9.4 Hz, 2H, H-C
(16’)); 13C NMR (101 MHz, CDCl3, 258C): d=
20.80 (C(20’)), 20.92 (C(1’)), 47.83 (C(2’)), 48.52 (C(19’)), 72.80 (C(10’)),
74.92 (C(2)), 78.25 (C(4)), 92.86 (C(8’)), 104.39 (C(6’)), 104.78 (C(1)),
104.86 (C(3)), 110.93 and 111.81 (2 C; C(13’,14’), 113.64 and 114.64 (2 C;
(11’,12’)), 114.75 (C(17’)), 114.81 (C(4’)), 116.75 (C(15’)), 132.15 (C-
(16’)), 135.10 (C(5’)), 150.01 (C(7’)), 150.84 (C(3’)), 153.44 (C(18’)),
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
as
(decomp); 1H NMR (400 MHz, CDCl3, 258C): d=1.30 (d, J=6.9 Hz,
12H, H-C(1’)), 1.39 (d, J=6.9 Hz, 12H, H-C(20’)), 3.93 (hept, J=6.9 Hz,
2H, H-C(2’))), 4.10 (hept, J=6.9 Hz, 2H, H-C(19’)), 6.72 (d, J=9.2 Hz,
2H, H-C(4’)), 6.87 (d, J=9.5 Hz, 2H, H-C(17’)), 7.37 (d, J=9.2 Hz, 2H,
H-C(5’)), 7.68 ppm (d, J=9.5 Hz, 2H, H-C
(16’)); 13C NMR (101 MHz,
CDCl3, 258C): d=20.76 (C(20’)), 20.93 (C(1’)), 47.73 (C(2’)), 48.69 (C-
(19’)), 60.66 (C(8)), 63.96 (C(6)), 67.47 (C(4)), 69.44 (C(10)), 70.28
(C(3)), 72.48 (C(10’)), 73.77 (C(2)), 74.57 (C(5)), 83.03 (C(7)), 84.62
(C(1)), 98.40 (C(8’)), 99.60 (C(9)), 103.77 (C(6’)), 110.13 and 111.02 (2 C;
(13’,14’)), 113.49 and 114.37 (2 C; C(11’,12’)), 114.88 (C(17’)), 114.89
(C(4’)), 116.38 (C(15’)), 132.15 (C(16’)), 135.14 (C(5’)), 149.13 (C(7’)),
150.13 (C(3’)), 153.65 (C(18’)), 157.71 ppm (C(9’)); IR (ATR): n˜ =2972
a black metallic solid. Rf =0.57 (SiO2, CH2Cl2); m.p.> 3408C
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
C
G
E
N
ACHTUNGTRENNUNG
A
U
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
159.48 ppm (C(9’)); IR (ATR): n˜ =2971 (m), 2933 (w), 2872 (w), 2214
(m), 2125 (s), 1586 (s), 1509 (s), 1482 (s), 1446 (s), 1383 (m), 1331 (s),
1298 (s), 1218 (m), 1204 (m), 1188 (m), 1153 (s), 1118 (s), 1062 (m), 1017
(m), 821 (m) cmÀ1; UV/Vis (CH2Cl2): lmax (e)=352 (19800), 460 (31800),
586 (36200mÀ1 cmÀ1); HR-MALDI-MS (DCTB): m/z (%): 552.2996 (100,
[M]+, calcd for C36H36N2+: 552.2996).
AHCTUNGTRENNUNG
C
N
N
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
(m), 2934 (w), 2872 (w), 2215 (m), 2173 (m), 2010 (s), 1587 (s), 1519 (s),
1484 (s), 1445 (s), 1372 (m), 1333 (m), 1297 (s), 1218 (m), 1202 (m), 1188
(m), 1153 (s), 1118 (s), 1085 (m), 1016 (m), 816 (m) cmÀ1; UV/Vis
(CH2Cl2): lmax (e)=256 (50100), 269 (sh, 47000), 318 (sh, 44100), 333
(49300), 349 (sh, 46100), 409 (43000), 473 (68800), 629 (17900mÀ1 cmÀ1);
HR-MALDI-MS (DCTB): m/z (%): 624.2997 (100, [M]+, calcd for
C42H36N2+: 624.2996).
2-[4-(Diisopropylamino)phenyl]-3-[(4-(diisopropylamino)phenyl)hexa-
1,3,5-triyn-1-yl]buta-1,3-diene-1,1,4,4-tetracarbonitrile (2d): A solution of
tetrayne 3d (116 mg, 0.26 mmol) in CH2Cl2 (7 mL) was treated with
TCNE (40 mg, 0.310 mmol) and stirred at 258C for 8 h (the reaction
color changed from dark yellow to dark green upon addition of TCNE).
Evaporation and column chromatography (SiO2, CH2Cl2) gave 2d
(88 mg, 60%) as a black metallic solid. Rf =0.53 (SiO2, CH2Cl2); m.p.
1268C; 1H NMR (400 MHz, CDCl3, 258C): d=1.31 (d, J=6.9 Hz, 12H,
H-C(1’)), 1.39 (d, J=6.9 Hz, 12H, H-C
C(2’)), 4.10 (hept, J=6.9 Hz, 2H, H-C(19’)), 6.74 (d, J=9.2 Hz, 2H, H-
C(4’)), 6.88 (d, J=9.5 Hz, 2H, H-C(17’)), 7.37 (d, J=9.0 Hz, 2H, H-
C(5’)), 7.71 ppm (d, J=9.5 Hz, 2H, H-C
(16’)); 13C NMR (101 MHz,
CDCl3, 258C): d=20.82 (C(20’)), 20.96 (C(1’)), 47.75 (C(C2’)), 48.60 (C-
(19’)), 66.24 (C(4)), 72.77 (C(10’)), 72.90 (C(6)), 74.22 (C(2)), 85.63
(C(3)), 93.02 (C(1)), 96.05 (C(8’)), 102.23 (C(5)), 104.03 (C(6’)), 110.49
and 111.34 (2 C; C(13’,14’)), 113.57 and 114.51 (2 C; C(11’,12’)), 114.83
(C(17’), 114.88 (C(4’)), 116.62 (C(15’)), 132.14 (C(16’)), 135.07 (C(5’)),
149.68 (C(7’)), 150.41 (C(3’)), 153.54 (C(18’)), 158.67 ppm (C(9’)); IR
ACHTUNGTNER(NUNG 20’)), 3.95 (d, J=6.9 Hz, 2H, H-
AHCTUNGTRENNUNG
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Acknowledgements
AHCTUNGTRENNUNG
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This work was supported by the ERC Advanced Grant No. 246637 (“OP-
TELOMAC”). B.B. and O.D. acknowledge the receipt of a Kekulꢂ fel-
lowship from the Fonds der Chemischen Industrie. M.B. and I.B. ac-
knowledge support from a Faculty Innovation Grant from Lehigh Uni-
versity.
A
ACHTUNGTRENNUNG
E
ACHTUNGTRENNUNG
G
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
(ATR): n˜ =2974 (w), 2935 (w), 2876 (w), 2216 (w), 2174 (w), 2111 (s),
2064 (s), 1590 (s), 1523 (m), 1486 (m), 1448 (m), 1373 (w), 1335 (m), 1300
[1] S. Barlow, S. R. Marder in Functional Organic Materials (Eds.:
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(m), 1267 (m), 1219 (m), 1190 (m), 1154 (m), 1119 (m), 823 (w) cmÀ1
;
UV/Vis (CH2Cl2): lmax (e)=275 (31100), 342 (23300), 395 (31600), 462
(45000), 613 (28000mÀ1 cmÀ1); HR-MALDI-MS (DCTB): m/z (%):
576.2997 (100, [M]+, calcd for C38H36N6+: 576.2996).
2-[4-(Diisopropylamino)phenyl]-3-[(4-(diisopropylamino)phenyl]octa-
1,3,5,7-tetrayn-1-yl)buta-1,3-diene-1,1,4,4-tetracarbonitrile (2e): A solu-
tion of pentayne 3e (102 mg, 0.22 mmol) in CH2Cl2 (9 mL) was treated
with TCNE (33 mg, 0.26 mmol) and stirred at 258C for 8 h (the reaction
color changed from red to dark green upon addition of TCNE). Evapora-
tion and column chromatography (SiO2, CH2Cl2) gave 2e (72 mg, 56%)
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as
(decomp); 1H NMR (400 MHz, CDCl3, 258C): d=1.31 (d, J=6.9 Hz,
12H, H-C(1’)), 1.39 (d, J=6.9 Hz, 12H, H-C(20’)), 3.94 (hept, J=6.9 Hz,
2H, H-C(2’)), 4.10 (hept, J=6.9 Hz, 2H, H-C(19’)), 6.72 (d, J=9.2 Hz,
2H, H-C(4’)), 6.88 (d, J=9.5 Hz, 2H, H-C(17’)), 7.38 (d, J=9.2 Hz, 2H,
H-C(5’)), 7.69 ppm (d, J=9.5 Hz, 2H, H-C
(16’)); 13C NMR (101 MHz,
CDCl3, 258C): d=20.85 (C(20’)), 21.01 (C(1’)), 47.73 (C(2’)), 48.66 (C-
(19’)), 62.39 (C(6)), 67.15 (C(4)), 70.46 (C(8)), 72.89 (C(10’)), 73.91
(C(2)), 75.25 (C(3)), 84.18(C(5), 87.27(C(1)), 97.56 (C(8’)), 100.70 (C(7)),
104.01 (C(6’)), 110.26 and 111.14 (2 C; C(13’,14’)), 113.53 and 114.43 (2
C; C(11’,12’)), 114.91 (C(17’)), 114.95 (C(4’)), 116.58 (C(15’)), 132.14 (C-
(16’)), 135.10 (C(5’)), 149.46 (C(7’)), 150.23 (C(3’)), 153.61 (C(18’)),
a black metallic solid. Rf =0.56 (SiO2, CH2Cl2); m.p.> 3408C
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
158.21 ppm (C(9’)); IR (ATR): n˜ =2973 (w), 2937 (w), 2876 (w), 2215
(w), 2178 (w), 2141 (w), 2057 (s), 1592 (s), 1518 (m), 1486 (m), 1448 (m),
1372 (w), 1335 (m), 1300 (m), 12158 (m), 1197 (m), 1155 (m), 1119 (m),
1018 (w), 821 (w) cmÀ1; UV/Vis (CH2Cl2): lmax (e)=298 (sh, 40200), 305
(40900), 324 (sh, 36900), 361 (sh, 28900), 377 (32600), 434 (sh, 47900),
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Donor–Acceptor (D–A)-Substituted Polyyne Chromophores
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Received: April 29, 2013
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