A new class of organic donor-acceptor molecules with large third-order optical nonlinearities

Article abstract of DOI:10.1039/b417393g

Donor-acceptor molecules with 4-(dimethylamino)phenyl donor and 1,1,4,4-tetracyanobuta-1,3-diene acceptor moieties are readily prepared by short, high-yielding routes. The quite small chromophores are characterised by X-ray crystallography and feature int

Full text of DOI:10.1039/b417393g

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A new class of organic donor–acceptor molecules with large third-order
optical nonlinearities{
Tsuyoshi Michinobu,a Joshua C. May,b Jin H. Lim,b Corinne Boudon,c Jean-Paul Gisselbrecht,c Paul Seiler,a
Maurice Gross,c Ivan Biaggio*b and Franc¸ois Diederich*a
Received (in Cambridge, UK) 18th November 2004, Accepted 6th December 2004
First published as an Advance Article on the web 17th January 2005
DOI: 10.1039/b417393g
between the two dicyanovinyl planes in 1 (with two independent
molecules) is expressed by the torsion angles h (C(11)–C(10)–
Donor–acceptor molecules with 4-(dimethylamino)phenyl
donor and 1,1,4,4-tetracyanobuta-1,3-diene acceptor moieties
are readily prepared by short, high-yielding routes. The quite
small chromophores are characterised by X-ray crystallography
and feature intense intramolecular charge-transfer bands,
substantial quinoid character in the donor rings, reversible
electrochemical reductions and oxidations and powerful third-
order optical nonlinearities.
C(16)–C(17))
5
2144.3u and
h
(C(119)–C(109)–C(169)–
C(179)) 5 146.5u. The corresponding h values obtained for 2
(C(28)–C(1)–C(13)–C(23)) and 3 (C(11)–C(1)–C(16)–C(26)) are
296.7u and 62.3u, respectively (for atom numbering, see Fig. 1).
Note that the TCBD moiety of 3 has s-cis configuration. Clearly,
sterically enforced deconjugation of the two dicyanovinyl moieties
is the least pronounced in 1 and this is consistent with the UV/Vis
and electrochemical data described below. Bond length alternation
in the DMA rings is a good indication for the efficiency of the
charge-transfer from the donor to the acceptor moieties, which can
be expressed by the quinoid character (dr) of the ring (for a
definition,7 see caption to Fig. 1, for bond lengths, see
Organic donor–acceptor molecules are extensively studied as active
nonlinear optical (NLO) materials with a wide range of potential
applications such as all-optical computing and signal processing.1,2
We have in the past shown that donor–acceptor substituted
tetraethynylethenes (3,4-diethynylhex-3-ene-1,5-diynes, TEEs) fea-
ture high third-order optical nonlinearities, in particular in the case
of low molecular symmetry,3 and, more recently, provided
evidence for the high potential of 4-(dimethylamino)phenyl-
(‘‘dimethylanilino’’, DMA) substituted cyanoethynylethenes
(CEEs) as efficient NLO chromophores.4 Here we introduce a
new class of low-molecular-weight chromophores featuring high
third-order optical nonlinearities. They contain DMA donor and
1,1,4,4-tetracyanobuta-1,3-diene (TCBD) acceptor moieties and
are prepared by very short and high-yielding synthetic routes.
TCBD-containing donor–acceptor molecules have in the past been
investigated as second-order NLO materials,5 but there has been,
to our knowledge, no report on third-order optical nonlinearities.
The new donor-substituted TCBD derivatives 1–3 were
obtained in nearly quantitative yield (96–100%) by [2 + 2]
cycloaddition between tetracyanoethylene (TCNE) and DMA-
substituted alkynes 4–6 (see ref. 6), followed by electrocyclic ring
opening of the initially formed cyclobutenes (Scheme 1).{ For
comparison, we also prepared gem-dinitrile 7, a sub-unit of 2, by
Knoevenagel condensation between 8 and malononitrile.
Single-crystals of 1–3 and 7 suitable for X-ray structure analysis
were grown by slow diffusion of hexane into CH2Cl2 solutions of
the compounds at 215 uC (Fig. 1; for crystal packings, see
Fig. 2(ESI)).§ Whereas chromophore 7 is fully planar, the TCBD
moieties in 1–3 are nonplanar. However, each dicyanovinyl moiety
and the directly attached DMA ring in 1–3 are nearly co-planar,
which ensures efficient intramolecular charge-transfer (CT) inter-
actions. Distortion from planarity occurs mainly by rotation
around the central single bond of the TCBD moiety. The twist
{ Electronic Supplementary Information (ESI) available: Syntheses,
crystal structures and electrochemical data for 1–3 and 7. See http://
www.rsc.org/suppdata/cc/b4/b417393g/
Scheme 1 Synthesis of donor-substituted TCBD derivatives: i, C6H6 or
THF, 20 uC; 97% (1), 96% (2), 100% (3); ii, CH2(CN)2, Al2O3 (act. II-III),
CH2Cl2, 40 uC; 87%.
*diederich@org.chem.ethz.ch (Franc¸ois Diederich)
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DMA donor group, the intensity of the CT band increases
dramatically. All molecules show a distinct solvatochromism in
hexane–CHCl3 mixtures. The largest solvent effect is observed for
2, with the CT absorption shifting from 477 nm (2.60 eV) in
hexane to 526 nm (2.36 eV) in CHCl3.
Compounds 1–3 and 7 all displayed reversible first oxidation
and reduction steps (except for the irreversible reduction of 7) in
the cyclic voltammograms (CVs) recorded in CH2Cl2 (+0.1M
Bu4NPF6) at room temperature (see Table 2(ESI)). Thus, 1 gave
two well-resolved, reversible, TCBD-centered reduction steps at
20.69 and 21.26 V (vs. Fc+/Fc). The first of the two reversible
1e2-reductions in 2 and 3 are more cathodically shifted (20.89 in 2
and 21.06 in 3), which reflects the presence of two DMA donor
groups. With a first DMA-centered oxidation potential of 0.86 V,
the electrochemical HOMO–LUMO gap for 1 was estimated as
1.55 V; it is the lowest in the series. The two DMA groups in 2 are
oxidized at separate potentials (0.72 and 0.90 V; HOMO–LUMO
gap: 1.61 V), while 3 shows a single oxidation peak for a 2e2-
transfer at ca. 0.90 V (HOMO–LUMO gap: 1.92 V). In com-
parison, oxidation of the DMA moiety in 7 takes place at 0.66 V.
In investigations of the third-order nonlinear optical properties,
the rotational average of the second hyperpolarisability c was
measured in the zero-frequency limit by degenerate four-wave
mixing in CH2Cl2 at a wavelength of 1.5 mm, using fused silica,
with a third-order susceptibility x(3) 5 1.9 6 10222 m2 V22 at
1.5 mm, as a reference. Whereas compound 1 showed a c-value of
the order of 4 6 10248 m5 V22, the more extended chromophore 2
with its two, nearly orthogonal, donor–acceptor pathways, gave
c 5 (12 ¡ 2) 6 10248 m5 V22. This value is comparable to the
highest values obtained for donor–acceptor substituted TEEs and
other potent NLO chromophores1c and quite significant consider-
ing the small number of delocalised p-electrons in 2.
Fig. 1 ORTEP plots of 1 (a), 2 (b), 3 (c) and 7 (d) with vibrational
ellipsoids shown at the 30% probability level. For selected bond lengths
˚
(A) and angles (u), see Fig. 1(ESI).{ Quinoid character: dr 5 (((a + a9)/2–
(b + b9)/2) + ((c + c9)/2–(b + b9)/2))/2.7
Fig. 1(ESI){). In benzene, the dr value equals 0, whereas values
between 0.08 and 0.10 are found in fully quinoid rings. For the
DMA rings in the two different molecules in the unit cell of 1, dr
averages to 0.036. The DMA rings in 3 exhibit a very high dr value
of 0.050, which is unprecedented for molecules with anilino donor
and cyano acceptor groups (see Table 1(ESI)).8 The quinoid
character of 2 shows the same trend, but because of the reduced
accuracy the results are less reliable.
In conclusion, we have prepared, by elegant one- or two-step
protocols shown in Scheme 1, a series of new chromophores in
which DMA donors undergo efficient intramolecular CT interac-
tions with TCBD acceptors. The low energy of the CT bands and
the low electrochemical HOMO–LUMO gaps, in particular of 1
and 2, demonstrate substantial p-conjugative effects (beside
s-effects) despite the twist of the planes of the two dicyanovinyl
moieties as observed by X-ray analysis. The compounds are
thermally stable up to the melting points (1: 144–145 uC; 2: 191–
193 uC; 3: 273.5–274.6 uC) and can be sublimed without
decomposition, which opens perspectives for device construction
by vapor phase deposition techniques that are currently pursued.
We thank the ETH Research Council and the NCCR
‘‘Nanoscale Science’’ for support of this work.
Dicyanovinyl derivative 7 displays a single CT band at
lmax 5 477 nm (2.60 eV), whereas the TCBD derivatives show
multiple transitions (Fig. 2), resulting from different donor–
acceptor pathways. One of these transitions is always close in
energy to the CT band in 7. For example, the spectrum of 1
exhibits two rather weak CT bands at 481 and 570 nm (2.58 and
2.18 eV, respectively) with the end absorption reaching into the
near infrared (960 nm, 1.29 eV). Upon introduction of a second
Tsuyoshi Michinobu,a Joshua C. May,b Jin H. Lim,b Corinne Boudon,c
Jean-Paul Gisselbrecht,c Paul Seiler,a Maurice Gross,c Ivan Biaggio*b
and Franc¸ois Diederich*a
aLaboratorium fu¨r Organische Chemie, ETH-Ho¨nggerberg, CH-8093,
Zu¨rich, Switzerland. E-mail: diederich@org.chem.ethz.ch
bCenter of Optical Technologies and Department of Physics, Lehigh
University, Bethlehem, Pennsylvania, 18015, USA
cLaboratoire d’Electrochimie et de Chimie Physique du Corps Solide,
UMR 7512, C.N.R.S., Universite´ Louis Pasteur, 4, rue Blaise Pascal,
67000, Strasbourg, France
Notes and references
{ All new molecules were fully characterised by IR, UV/Vis, H and 13C
NMR, mass spectrometry and elemental analysis.
1
Fig. 2 Electronic absorption spectra of 1, 2, 3 and 7 in CH2Cl2.
738 | Chem. Commun., 2005, 737–739
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§ Crystal data of 1 at 173 K: C16H11N5, [Mr 5 273.30]: triclinic, space
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group P1 (no. 2), Dc 5 1.236 g cm23, Z 5 4, m 5 0.079, a 5 10.7459(2),
¯
˚
b 5 11.0754(3), c 5 15.1601(5) A, a 5 70.935(1), b 5 71.461(1),
3
˚
c 5 61.437(2)u, V 5 1469.05(7) A . Bruker-Nonius Kappa-CCD
˚
diffractometer, Mo-Ka radiation, l 5 0.7107 A. Number of reflections
measured 5 11122, number of independent reflections 5 6573, Rint5 0.03,
final R(F) 5 0.046, wR(F2) 5 0.096 for 394 parameters and 5258 reflections
with I . 2s(I) and 3.68 , h , 27.45u (corresponding R-values based on all
6573 reflections are 0.061 and 0.105, respectively). Crystal data of 2 at
¯
263 K: C26H20N6, 2(CH2Cl2)[Mr 5 586.33]: triclinic, space group P1 (no.
2), Dc 5 1.332 g cm23, Z 5 2, m 5 0.433, a 5 7.3661(1), b 5 9.5653(2),
˚
c 5 20.8690(4) A, a 5 91.580(8), b 5 95.014(7), c 5 92.986(9)u,
3
V 5 1462.03(5) A . Bruker-Nonius Kappa-CCD diffractometer, Mo-Ka
˚
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T. Kawai, A. Kishioka, M. Gross, M. Irie and F. Diederich, Org.
Biomol. Chem., 2003, 1, 2032.
˚
radiation, l 5 0.7107 A. Number of reflections measured 5 3850, number
of independent reflections 5 2595, Rint 5 0.03, final R(F) 5 0.100,
wR(F2) 5 0.235 for 348 parameters and 2223 reflections with I . 2s(I) and
2.38 , h , 20.81u (corresponding R-values based on all 2595 reflections are
0.114 and 0.245, respectively). Crystal data of 3 at 220 K: C24H20N6,
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6 (a) The adduct structure of 5 with TCNE proposed by the authors is
wrong: J. G. Rodr´ıguez, S. Ramos, R. Mart´ın-Villamil, I. Fonseca and
A. Albert, J. Chem. Soc., Perkin Trans. 1, 1996, 541 and J. G. Rodr´ıguez,
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[Mr 5392.46]: orthorhombic, space group Pbca(no. 61), Dc 51.270gcm23
,
Z 5 8, m 5 0.079, a 5 15.0448(7), b 5 15.9323(8), c 5 17.1300(8) A,
˚
3
V 5 4106.0(3) A . Bruker-Nonius Kappa-CCD diffractometer, Mo-Ka
˚
˚
radiation, l 5 0.7107 A. Number of reflections measured 5 7171, number
of independent reflections 5 3962, Rint 5 0.05, final R(F) 5 0.050,
wR(F2) 5 0.107 for 296 parameters and 2861 reflections with I . 2s(I) and
3.02 , h , 26.01u (corresponding R-values based on all 3962 reflections are
0.079 and 0.119, respectively). Crystal data of 7 at 273 K: C14H11N3,
[Mr 5 221.26]: monoclinic, space group P 21/n (no. 14), Dc 5 1.200 g cm23
,
Z 5 4, m 5 0.074, a 5 4.0006(3), b 5 17.9661(10), c 5 17.145(20) A,
˚
3
b 5 96.308(3), V 5 1224.85(9) A . Bruker-Nonius Kappa-CCD
˚
˚
diffractometer, Mo-Ka radiation, l 5 0.7107 A. Number of reflections
measured 5 2749, number of independent reflections 5 1465, Rint 5 0.03,
final R(F) 5 0.044, wR(F2) 5 0.120 for 167 parameters and 1176 reflections
with I . 2s(I) and 2.27 , h , 21.98u (corresponding R-values based on all
1465 reflections are 0.057 and 0.128, respectively). CCDC 256337–256340.
See http://www.rsc.org/suppdata/cc/b4/b417393g/ for crystallographic data
in .cif format.
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