N, N ′-dicyanoquinone diimide-derived donor-acceptor chromophores: Conformational analysis and optoelectronic properties

Article abstract of DOI:10.1021/ol202815q

A formal [2 + 2] cycloaddition-cycloreversion (CA-CR) between N,N′-dicyanoquinone diimides (DCNQIs) and electron-rich alkynes was explored, providing a new class of π-conjugated donor-acceptor chromophores. These DCNQI adducts exist in the solid state as

Full text of DOI:10.1021/ol202815q

ORGANIC
LETTERS
N,N0-Dicyanoquinone Diimide-Derived
DonorꢀAcceptor Chromophores:
Conformational Analysis and
Optoelectronic Properties†
2012
Vol. 14, No. 1
54–57
Melanie Chiu,‡ Bernhard Jaun,‡ Marten T. R. Beels,§ Ivan Biaggio,§
Jean-Paul Gisselbrecht, Corinne Boudon, W. Bernd Schweizer,‡
Milan Kivala,‡ and Franc-ois Diederich*,‡
€
€
Laboratorium fu€r Organische Chemie, ETH Zu€rich, Honggerberg, 8093 Zurich,
Switzerland, Center of Optical Technologies and Department of Physics, Lehigh
University, Bethlehem, Pennsylvania 18015, United States, and Laboratoire
d’Electrochimie et de Chimie Physique du Corps Solide, Institut de Chimie-UMR 7177,
ꢀ
CNRS, Universite de Strasbourg, 4 rue Blaise Pascal, 67081 Strasbourg Cedex, France
diederich@org.chem.ethz.ch
Received October 20, 2011
ABSTRACT
A formal [2 þ 2] cycloadditionꢀcycloreversion (CAꢀCR) between N,N0-dicyanoquinone diimides (DCNQIs) and electron-rich alkynes was
explored, providing a new class of π-conjugated donorꢀacceptor chromophores. These DCNQI adducts exist in the solid state as single
diastereoisomers, but as two interconverting diastereoisomers in solution. Solid- and solution-state evidence for intramolecular charge transfer
(CT) was obtained; additionally, the DCNQI adducts exhibit positive solvatochromism and significant solution-state third-order polarizabilities.
Formal [2 þ 2] cycloadditionꢀcycloreversion (CAꢀCR)
between electron-rich alkynes and electron-deficient al-
kenes, such as tetracyanoethene (TCNE) or 7,7,8,8-tetra-
cyano-p-quinodimethane (TCNQ), has been developed as
a convenient and robust method for preparing nonplanar,
π-conjugated, donorꢀacceptor chromophores that exhibit
intense, low-energy, intramolecular charge-transfer (CT)
bands.1 These features render the 1,3-butadiene CAꢀCR
products attractive for nonlinear optical (NLO) applic-
ations.2 We therefore sought to expand the scope of accep-
tors for this reaction to include heteroatom analogues.
Dicyanoquinone diimides (DCNQIs), first reported by
† Dedicated to Prof. Dr. Dr. h. c. Siegfried H€unig on the occasion of his
90th birthday.
‡ ETH Z€urich.
€
Hunig and co-workers in 1984, emerged as prime CAꢀCR
acceptor candidates,3 as they are easily accessible in one step
by condensation of commercially available 1,4-benzoqui-
nones with bis(trimethylsilyl)carbodiimide.4 Extensive stud-
ies have been performed on the conducting and magnetic
properties of their intermolecular charge-transfer salts,4ꢀ6
§ Lehigh University.
ꢀ
Universite de Strasbourg.
(1) For a review on the CAꢀCR reaction, see: Kivala, M.; Diederich,
F. Acc. Chem. Res. 2009, 42, 235–248.
(2) (a) Koos, C.; Vorreau, P.; Vallaitis, T.; Dumon, P.; Bogaerts, W.;
Baets, R.; Esembeson, B.; Biaggio, I.; Michinobu, T.; Diederich, F.;
Freude, W.; Leuthold, J. Nat. Photonics 2009, 3, 216–219. (b) Barlow, S.;
Marder, S. R. Functional Organic Materials; M€uller, T. J. J., Bunz, U. H. F.,
Eds.; Wiley-VCH: Weinheim, 2007; pp 393;437. (c) Segura, J. L.; Martın,
´
N. Angew. Chem., Int. Ed. 2001, 40, 1372–1409. (d) Szablewski, M.;
Thomas, P. R.; Thornton, A.; Bloor, D.; Cross, G. H.; Cole, J. M.;
€
(3) Hunig, S; Herberth, E. Chem. Rev. 2004, 104, 5535–5563.
€
€
(4) Aumuller, A.; Hunig, S. Liebigs Ann. Chem. 1986, 142–164.
ꢀ
o, C.; Gonzalez, A.;
(5) Martı
ꢀ
´
n, N.; Segura, J. L; Seoane, C.; Torı
´
ꢀ
Howard, J. A. K.; Malagoli, M.; Meyers, F.; Bredas, J.-L.; Wenseleers, A.;
Pingarron, J. M. Synth. Met. 1994, 64, 83–89.
Goovaerts, E. J. Am. Chem. Soc. 1997, 119, 3144–3154. (e) Tykwinski,
€
R. R.; Gubler, U.; Martin, R. E.; Diederich, F.; Bosshard, C.; Gunter, P.
ꢀ
ꢀ
(6) Martın, N.; Perez, I.; Sanchez, L.; Seoane, C. J. Org. Chem. 1997,
62, 870–877.
´
J. Phys. Chem. B 1998, 102, 4451–4465.
r
10.1021/ol202815q
Published on Web 11/29/2011
2011 American Chemical Society

but little has been reported on the reactivity of DCNQIs.
Armed with the precedents that Lewis acids catalyze
CAꢀCR reactions between N-tosylaldimines and alkyny-
lated amines (ynamines),7 sulfides,8 and selenides,9 how-
ever, we sought to investigate the reactivity of DCNQIs with
electron-rich alkynes as well as to characterize the resulting
products in the context of optoelectronic properties.
In contrast, reactions using more activated substrates were
completely inhibited by competing radical polymerization
of the DCNQI starting materials.11 Despite these constraints
on substrate scope, several notable structural and optoelec-
tronic features can be gleaned from the characterization of
adducts 3aꢀd.
Table 1. Synthesis of DCNQI CAꢀCR Adductsa
alkyne DCNQI (equiv) temp/°C time/h product yield/%
1a
1a
1a
1b
2a (1.2)
2b (1.5)
2c (1.5)
2a (2.0)
25
45
80
80
1
4
4
8
3a
3b
3c
3d
95
88
85
83
Figure 1. Third-order polarizabilities and ORTEP representa-
tions of the crystal structures of 3aꢀd with thermal ellipsoids
shown at the 50% probability level.
a Reaction concentration was 0.05 M with respect to alkyne.
The identities and structures of DCNQI adducts 3aꢀd
were studied by NMR experiments and X-ray crystal-
lography (Figure 1). Notably, the crystal structure of
adduct 3d confirms the absolute regioselectivity of the
imine CAꢀCR reaction that is also observed by NMR
spectroscopy: the iminoquinoid moiety is adjacent to the
aniline moiety, analogous to regioselectivity observed in
the CAꢀCR reaction using TCNQ as an acceptor.12 For
all four compounds, the same space group and cell dimen-
sions were found when g10 crystals of each were dif-
fracted, suggesting that only one diastereoisomer of each
compound exists in the solid state.13 NMR spectra of these
compounds, on the other hand, indicated two fluxional
species in solution, in a 1:1 ratio from 193 to 273 K. When
crystals of the DCNQI adducts were redissolved, NMR
spectra identical to those of the bulk solids were obtained.
These results suggest that the two isoenergetic species are
interconverting in solution but that crystallization of one is
favored.14
Internal, N,N-dimethylanilino-susbstituted alkynes 1a,b
underwent CAꢀCR with DCNQIs 2aꢀc10 to yield dicya-
noimine chromophores 3aꢀd in good yields as dark blue-
green solids (Table 1). Compounds 3aꢀd are surprisingly
robust and do not hydrolyze after heating for 12 h in 6 M
HCl (353 K). They are also competent electron acceptors:
reduction potentials of 3aꢀd measured by cyclic voltam-
metry(CH2Cl2 þ 0.1Mn-Bu4NPF6, against Fcþ/Fc) range
from ꢀ0.81 to ꢀ0.95 V, compared to ꢀ0.25 to ꢀ0.45 V for
the parent DCNQIs (for electrochemistry details, see the
Supporting Information, section 2). Lewis acids did not
catalyze the transformation, but those reactions using less
electron-rich alkynes (e.g., 1b) or less electron-deficient or
sterically hindered DCNQIs (e.g., 2b,c) required addition
of excess DCNQI and heating to reach full conversion.
(7) Kuehne, M. E.; Sheehan, P. J. J. Org. Chem. 1968, 33, 4406–4413.
(8) Ishitani, H.; Nagayama, S.; Kobayashi, S. J. Org. Chem. 1996, 61,
1902–1903.
(9) Ma, Y.; Qian, C. Tetrahedron Lett. 2000, 41, 945–947.
(10) Parent DCNQI 2a exists at room temperature as a mixture of
syn/anti isomers. The barrier of inversion between these two isomers is
ca. 15 kcal molꢀ1. In contrast, naphthoquinone-derived 2b and 2,5-
dimethylbenzoquinone-derived 2c exist exclusively in the syn and anti
Several dynamic processes occur simultaneously in
3aꢀd (see the Supporting Information, section 5, for
spectra and further details). Rotation about the C1ꢀC2
single bond and inversion of the imine moiety3 were not
conformations, respectively, at room temperature. See ref 5 and Cossı
F. P.; de la Cruz, P.; de la Hoz, A.; Langa, F.; Martın, N.; Prieto, P.;
´
o,
(12) Kivala, M.; Boudon, C.; Gisselbrecht, J.-P.; Seiler, P.; Gross,
M.; Diederich, F. Chem. Commun. 2007, 4731–4733.
(13) Bulk DCNQI adducts 3aꢀd are amorphous solids, so powder
diffraction on the bulk solids was not possible.
(14) DFT calculations (B3LYP/6-31G(d)) show that ΔGf,298K be-
tween 3b-cis and 3b-trans is e1 kcal molꢀ1. See the Supporting Informa-
tion, section 6, for details.
´
ꢀ
Sanchez, L. Eur. J. Org. Chem. 2000, 2407–2415 for further information.
(11) When radical inhibitors (e.g., 2,6-di-tert-butyl-p-cresol) were
added to these reaction mixtures, no consumption of the starting
materials was observed. Addition of radical inhibitors to reaction
mixtures that yielded the desired DCNQI adducts did not affect the
rates or yields of those reactions.
Org. Lett., Vol. 14, No. 1, 2012
55

Scheme 1. Torquoselective Formation of Observed Isomers 3b-trans and 3b-cis and Their Rotational and Interconversion Dynamics
observed, as they are too rapid and slow, respectively, on
the NMR time scale. The most pertinent of the observed
processes, illustrated for 3b in Scheme 1, is slow intercon-
version between two isomers, which were identified as cis/
trans isomers about the exocyclic, quinoidal C2ꢀC3 bond
(3b-cis and 3b-trans, Scheme 1); isomer 3b-trans was the
isomer identified by X-ray crystallography. Preferential
formation of 3b-cis and 3b-trans can be attributed to
torquoselectivity.15,16 There are two presumed isomeric
azetine intermediates in the CAꢀCR reaction,16,17 4-syn
and 4-anti, arising from inversion of the pyramidal azetine
nitrogen (Scheme 1), in which the cyano groups occupy the
same or opposite sides of the azacyclobutene ring, respec-
tively. Because the cyano substituents on those nitrogens
are electron-withdrawing, inward rotations of the cyano
groups are favored in the thermally allowed, conrotatory
ring-openings of 4-syn and 4-anti to yield the observed
isomers, 3b-cis and 3b-trans (Scheme 1).16,17 Based on
comparisons between ROESY cross-peak volumes and
line-shape analysis of variable temperature 1H NMR
spectra, the rate of interconversion between 3b-cis and
3b-trans is estimated to be <1 sꢀ1 at 223 K. This estimate is
consistent with the fact that the naphthoquinoid proton
signals of both isomers show no line broadening beyond
3 Hz even at 373 K.
slightly longer than a typical CdC double bond (1.34 A for
ethylene18). Rotation about the C2ꢀC3 bond at relatively
low temperatures might therefore be possible because of
this single-bond character, which is apparent in the
diradical19 and zwitterionic, charge-separated resonance
forms of 3b, 3b-rad and 3b-CT, respectively. In adducts 3a
and 3d, which are derived from symmetric parent DCNQI
1a, quinoidalimine inversion would alsoyield the observed
cis/trans isomers.
Having established the structural dynamics of 3aꢀd, we
sought to examine their optoelectronic properties. Struc-
tural parameters derived from X-ray crystallographic data
suggest a rough correlation between steric bulk and the
degreeof intramolecularCTin3aꢀd. Inadducts 3aand 3d,
the dihedral angle, θ, defined by C3ꢀC2ꢀC4ꢀC5 is
relatively large at ca. 170° (Table 2). In adducts 3b and
3c, which are derived from bulkier DCNQIs, the analo-
gous dihedral angle is smaller (ca. 150°). Increasing steric
bulk about the quinoid moiety therefore induces the
quinoid and adjacent DMA moieties to deviate from
coplanarity, thereby decreasing the conjugation between
them. Conjugation between these moieties is, in turn,
responsible for the intramolecular CT in the chromo-
phores, which is manifested in the solid state as bond-
length alternation in the aniline rings (δr, see Table 2
footnote for definition).20 In sterically encumbered
3b and 3c, δr for ring B is small (0.03 A) compared to
Interconversion between the two cis/trans isomers in
adducts 3bꢀc results from apparent bond rotation about
the C2ꢀC3 bond (Scheme 1). For all four adducts, this
C2ꢀC3 bond length ranges from 1.38ꢀ1.41 A, which is
(18) Allen, F. H.; Kennard, O.; Watson, D. G. J. Chem. Soc., Perkin
Trans. 2 1987, S1–S19.
(19) (a) Doering, W.; von, E.; Roth, W. R.; Bauer, F.; Boenke, M.;
Breuckmann, R.; Ruhkamp, J.; Wortmann, O. Chem. Ber. 1991, 124,
1461–1470. (b) Doering, W.; von, E.; Kitagawa, T. J. Am. Chem. Soc.
1991, 113, 4288–4297.
(15) For a review on torquoselectivity, see: Dolbier, W. R.; Koroniak,
H.; Houk, K. N.; Sheu, C. Acc. Chem. Res. 1996, 29, 471–477.
(16) For the torquoselectivity of azetine ring-openings, see: (a) Kallel,
A. Ph.D. Thesis, University of California, Los Angeles, 1991.
(b) Mangelinckx, S.; Van Speybroeck, V.; Vansteenkiste, P.; Waroquier,
M.; De Kimpe, N. J. Org. Chem. 2008, 73, 5481–5488.
ꢀ
(20) (a) Dehu, F.; Meyers, F.; Bredas, J.-L. J. Am. Chem. Soc. 1993,
115, 6198–6206. (b) Hilger, A.; Gisselbrecht, J.-P.; Tykwinski, R. R.;
€
Boudon, C.; Schreiber, M.; Martin, R. E.; Luthi, H. P.; Gross, M.;
(17) Warrener, R. N.; Kretschmer, G.; Paddon-Row, M. N. J. Chem.
Soc., Chem. Commun. 1977, 806–807.
Diederich, F. J. Am. Chem. Soc. 1997, 119, 2069–2078. (c) Moonen,
N. N. P.; Diederich, F. Org. Biomol. Chem. 2004, 2, 2263–2266.
56
Org. Lett., Vol. 14, No. 1, 2012

δr = 0.06ꢀ0.07 A in unhindered adducts 3a and 3d.
Thus, incorporation of DCNQIs with less sterically demand-
ing quinoid moieties enables more effective conjugation and
CT in the resulting CAꢀCR adducts.
TCNE-derived CAꢀCR chromophore that was used in an
integrated NLO device for all-optical switching.2a,21
Table 2. Selected Structural Parameters of 3aꢀd
b
b
θa/deg
δrringA /A
δrringB /A
˚
˚
compd
3a
3b
3c
3d
165.3(3)
147.1(4)
152.0(7)
169(2)
0.04
0.03
0.04
0.03
0.06
0.03
0.03
0.07
Figure 2. Representative UV/vis spectra of 3a in varying
ratios of DCM/n-hexane (T = 298 K). DCM = CH2Cl2,
hex = n-hexane.
a θ is the dihedral angle defined by C3ꢀC2ꢀC4ꢀC5. b δr = {[(a þ a0)/
2 ꢀ (b þ b0)/2] þ [(c þ c0)/2 ꢀ (b þ b0)/2]}/2; for benzene, δr = 0.00 A, and
for fully quinoid rings, δr = 0.08ꢀ0.12 A.
In summary, the CAꢀCR reactivity of DCNQIs with
electron-rich alkynes has been developed, yielding a new
class of cyanoimine-based, donorꢀacceptor chromo-
phores. The CAꢀCR reaction proceeds torquoselec-
tively to give two isomers that are cis- and trans-
oriented about the exocyclic, quinoidal CdC bond.
These two isomers interconvert in solution, but crystal-
lization of one is favored. The cyanoimine CAꢀCR
adducts exhibit intramolecular CT and positive solvato-
chromism. They also function as competent third-order
nonlinear optical chromophores. This work therefore
expands the scope of the CAꢀCR reaction, extends the
known reactivity of DCNQIs, and demonstrates the
potential utility of cyanoimine chromophores in opto-
electronic applications.
UV/vis spectroscopy provided further evidence for in-
tramolecular CT in 3aꢀd. A main absorption band with
λmax = 600ꢀ640 nm in CH2Cl2 (Figure 2) is observed; in
3aꢀc, a second absorption band occurs with λmax
=
390ꢀ430 nm. The lower energy transition is attributed to
CT between DMA ring B and the quinoid moiety, and the
higher energy transition is derived from CT between DMA
ring A and the adjacent cyanoimine moiety. Since 3d lacks
DMA ring A, the higher energy band is absent. Adducts
3aꢀd also exhibit positive solvatochromism. Less polar
solutions of 3aꢀd (e.g., toluene) are green, and more polar
solutions (e.g., MeOH) are purple (Figure 2); correspond-
ing bathochromic shifts of ca. 50ꢀ120 nm for both absorp-
tion maxima in 3aꢀc, and of ca. 50 nm for the single CT
band in 3d were observed. The excited-state dipole mo-
ments of DCNQI adducts 3aꢀd are therefore greater than
those of their ground states.
Acknowledgment. We thank Dr. Dee-Hua Huang
(Scripps Research Institute) for help with acquisition
of NMR data. Igor Pochorovski and Yi-Lin Wu (ETH
Having established that adducts 3aꢀd exhibit intramole-
cular CT, we turned to investigating their third-order NLO
properties. The rotational average of the third-order molec-
ular polarizability (γrot) for each cyanoimine adduct was
measured near the zero-frequency limit in degenerate four-
wave mixing experiments. A narrow range of γrot values was
observed ((5ꢀ9) ꢁ 10ꢀ48 m5 Vꢀ2, Figure 1) for DCNQI
adducts 3aꢀd, which are significant given the small molecular
weights of these compounds and the energies of their absorp-
tion maxima.2e Their high γrot values and their nonplanar
structures make adducts 3aꢀd promising for nonlinear
optical applications, as has been shown for a similar
€
Zurich) are acknowledged for reviewing the manu-
script. This work was supported by ERC Advanced
Grant No. 246637 and an ETH Research Fellowship
(M.C.).
Supporting Information Available. Synthesis, NMR
spectra, X-ray crystallography details, CIF files, elec-
trochemical data, UV/vis spectra, DFT calculations,
and degenerate four-wave mixing experimental details.
This material is available free of charge via the Internet
at http://pubs.acs.org
(21) See the Supporting Information, section 8, for details.
Org. Lett., Vol. 14, No. 1, 2012
57