Homoconjugated push-pull and spiro systems: Intramolecular charge-transfer interactions and third-order optical nonlinearities

Article abstract of DOI:10.1002/anie.201002236

Homoconjugated push-pull chromophores are obtained by [2+2] cycloaddition of 2,3- dichloro-5,6-dicyano-p-benzoquinone to anilino or ferrocene donor-substituted alkynes, and in one case a spiro compound (see picture: examples with corresponding electron adsorption spectra; C gray, Cl green, N blue, O red). Significant third-order optical nonlinearities could be measured for the first time for homoconjugated push-pull systems.

Full text of DOI:10.1002/anie.201002236

Angewandte
Chemie
DOI: 10.1002/anie.201002236
Push–Pull Chromophores
Homoconjugated Push–Pull and Spiro Systems: Intramolecular
Charge-Transfer Interactions and Third-Order Optical
Nonlinearities**
Shin-ichiro Kato, Marten T. Roberts Beels, Philip La Porta, W. Bernd Schweizer,
Corinne Boudon, Jean-Paul Gisselbrecht, Ivan Biaggio, and Franꢀois Diederich*
Planar and non-planar push–pull chromophores (D-p-A),
consisting of strong electron donors (D) and acceptors (A)
connected by a p-conjugated spacer, feature intense intra-
molecular charge-transfer (CT) interactions[1,2] and have
recently received considerable attention as functional com-
ponents in molecular electronics and nonlinear optics
(NLO).[3] Transannular CT interactions between nonconju-
gated donors and acceptors in rigid molecular frameworks
have also been extensively investigated. Through-space CT
interactions in cyclophane derivatives, in which the donor and
acceptor moieties are parallel to one another, were estab-
lished by Staab and Rebafka[4] and Misumi and Otsubo,[5] and
further explored by others.[6] When donor and acceptor are
separated by appropriately aligned, rigid s-bond frameworks,
long-range through-bond CT interactions were also observed
in the UV/Vis spectra.[7] Only a few examples of transannular
CT chromophores with a non-parallel arrangement, so-called
homoconjugated push–pull systems, have been reported,[8–11]
since Nakazawa and Murata published the first example of
homoconjugated CT interactions in 9,10-dihydro-9,10-(1,2-
tropylio)anthracene tetrafluoroborate in 1977.[8] In general,
the reported homoconjugative CT interactions have been
mostly restricted to bicyclic [2.2.2] p systems, and in particular
triptycene derivatives.
tion between donor-activated electron-rich alkynes and the
=
electron-deficient C C double bonds in DDQ could take
place to give new push–pull systems.[2b] Very recently, during
the completion of the present study, Trofimov et al. indeed
described [2+2] cycloadditions of DDQ with pyrrole- or
indole-activated alkynes to give a variety of cycloadducts.[14]
Herein, we describe a new class of homoconjugated push–pull
chromophores (ꢀ )-1 to (ꢀ )-9 by [2+2] cycloaddition of N,N-
dialkylanilino (DAA) and ferrocene (Fc)-substituted alkynes
to DDQ. They feature surprisingly strong intramolecular CT
interactions and, to the best of our knowledge, are the first
homoconjugated push–pull chromophores with promising
third-order nonlinear optical (NLO) properties. We also
report the synthesis and characterization of spiro compound
(ꢀ )-10 by an electrocyclic ring-opening reaction of (ꢀ )-1
followed by a transannular rearrangement.
Good to excellent yields (63–94%) of DAA-substituted
chromophores (ꢀ )-1 to (ꢀ )-6 were obtained by [2+2]
cycloaddition of DDQ with the acetylenic precursors 11 to
16 in 1,2-dichloroethane at 258C (Scheme 1). Analogously,
the Fc-substituted chromophores (ꢀ )-7 to (ꢀ )-9 were formed
in 62–90% yield by reaction of DDQ with alkynes 17 to 19,
respectively. The chromophoric products are deeply colored
solids that are stable at ambient temperature in air. Thermal
gravimetric analysis (TGA) did not show any significant
weight loss (< 5%) below 1608C. However, upon heating a
1,2-dichloroethane solution of (ꢀ )-1 at 808C for 36 h, we
discovered the formation of (ꢀ )-10, in which an unsaturated
g-lactone is spiro-annulated, in 16% yield as a purple solid. It
originates from electrocyclic ring-opening reaction of (ꢀ )-1
to the intermediate cyclooctatrienedione 20, followed by
transannular reaction of the two carbonyl groups and migra-
tion of one cyanovinyl moiety to close the spiro system
(Scheme 2).[15] Indeed, density functional theory (DFT)
calculations for the three isomers (ꢀ )-1, (ꢀ )-10, and 20 at
the PBE1PBE/6-311 + G(d)//PBE1PBE/6-31G(d) level pre-
dict that (ꢀ )-10 is the most stable and (ꢀ )-1 and 20 are less
stable by 32.2 and 34.2 kcalmolꢁ1, respectively.[16]
2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is a
popular strong oxidizing and dehydrogenating agent with
many uses.[12,13] We envisaged that thermal [2+2] cycloaddi-
[*] Dr. S.-i. Kato, Dr. W. B. Schweizer, Prof. Dr. F. Diederich
Laboratorium fꢀr Organische Chemie, ETH Zꢀrich
Hꢁnggerberg, HCI, 8093 Zꢀrich (Switzerland)
Fax: (+41)446-321-109
E-mail: diederich@org.chem.ethz.ch
M. T. R. Beels, P. La Porta, Prof. Dr. I. Biaggio
Department of Physics and Center for Optical Technologies
Lehigh University
Bethlehem, PA 18015 (USA)
Prof. Dr. C. Boudon, Dr. J.-P. Gisselbrecht
Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide
Institut de Chimie-UMR 7177, C.N.R.S., Universitꢂ de Strasbourg
4, rue Blaise Pascal, 67000 Strasbourg (France)
The X-ray analyses of (ꢀ )-1, (ꢀ )-2, and (ꢀ )-7 to (ꢀ )-9
revealed that the cycloaddition was regioselective with
respect to DDQ, and occurred at the dicyano-substituted
[**] This work was supported by a grant from the ETH research council
and in part by the ERC Advanced Grant No. 246637 (“OPTELO-
MAC”). S.K. acknowledges the receipt of a JSPS Postdoctoral
Fellowship for Research Abroad. We thank Dr. B. Bernet (ETHZ) for
useful discussions.
=
C C bond (Figure 1, and Figures S1–S5 in the Supporting
Information). Interestingly, distinct bond length alternations
were observed in the benzene rings in DAA-substituted
cycloadducts (ꢀ )-1 and (ꢀ )-2. The DAA rings in both (ꢀ )-1
and (ꢀ )-2 display quinoid character dr (for definition and
bond lengths, see caption to Figure 1) of 0.031 ꢀ, which is
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002236.
Angew. Chem. Int. Ed. 2010, 49, 6207 –6211
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6207

Communications
Scheme 1. Synthesis of homoconjugated push–pull chromophores
(ꢀ)-1 to (ꢀ)-9. Reagents and conditions: a) DDQ, 1,2-dichloroethane,
258C; b) DDQ, 1,2-dichloroethane, 258C ((ꢀ)-7 and (ꢀ)-8) or 608C
((ꢀ)-9).
Figure 1. ORTEP plots of a) (ꢀ)-1, b) (ꢀ)-7, and c) (ꢀ)-10 with
vibrational ellipsoids at 173 K at the 50% probability level.
Arbitrary numbering. Quinoid character:
dr=(((a+a’)/2ꢁ(b+b’)/2)+((c+c’)/2ꢁ(b+b’)/2))/2).
In benzene, dr equals 0; in fully quinoid rings, dr=0.08—00.1 ꢃ. For
details of the X-ray analyses, including CCDC numbers, see the
Supporting Information.
dione acceptor moiety are seen in the crystal structures
(Supporting Information, Figures S3–S5). These observations
suggest that through-bond interactions are dominant for the
intramolecular CT transition in DAA-substituted cycload-
ducts (ꢀ )-1 to (ꢀ )-6, whereas through-space CT interactions
probably make an additional important contribution in Fc-
substituted cycloadducts (ꢀ )-7 to (ꢀ )-9. Spiro compound
(ꢀ)-10, in which donor and part of the acceptor are
p-conjugated, was also characterized by X-ray analysis
(Figure 1). As a result of the direct p conjugation, its DAA
ring features a higher dr value of 0.038 ꢀ relative to
homoconjugated (ꢀ )-1 and (ꢀ )-2.
Scheme 2. Synthesis of donor-substituted spiro compound (ꢀ)-10.
Reagents and conditions: a) 1,2-dichloroethane, 808C.
indicative of a substantial contribution of the quinoidal
structures as a result of efficient homoconjugative CT
interactions. In Fc derivatives (ꢀ )-7 to (ꢀ )-9, however,
clear bond-length alternations in the substituted cyclopenta-
dienyl (Cp) rings are not observed, which hints at additional
mechanisms of donor–acceptor interactions.[17] Short intra-
The efficiency of the intramolecular CT interactions in the
new chromophores was further evaluated by UV/Vis spec-
troscopy, computational studies, and electrochemistry. Com-
ꢁ
ꢁ
molecular C H···O and/or C H···C contacts at van der Waals
distances between the Fc donor moiety and the dichloroene-
6208 www.angewandte.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 6207 –6211

Angewandte
Chemie
pounds (ꢀ )-1 to (ꢀ )-9 display very broad CT absorption
bands of moderate intensity, with end absorptions reaching in
some cases into the near-infrared region in CH2Cl2 (Figure 2).
The CT character of these bands in (ꢀ )-1 to (ꢀ )-6 with DAA
donor results in a substantial bathochromic shift of the CT
absorption bands in 5 and (ꢀ )-6 relative to that of (ꢀ )-4,
which contains only one DAA donor. Thus, the lowest-energy
CT band of 5 is seen at 677 nm (1.83 eV) with an end-
absorption near 980 nm (1.26 eV; Figure 2a; for solvato-
chromism, see the Supporting Information, Figure S17).
The electronic-transition analysis of (ꢀ )-1 to (ꢀ )-3 by
time-dependent DFT (TD-DFT) at the TD-PBE1PBE/cc-
pVDZ//PBE1PBE/6-31G(d) level compares well with exper-
imental values (Supporting Information, Tables S5–S7). The
longest-energy absorptions in all of the chromophores are
derived from the transition between the DAA-centered
HOMO and the enedione-centered LUMO. The HOMO
and LUMO orbitals in all three chromophores partially
overlap on the cyclobutene moieties (Supporting Informa-
tion, Figure S20), suggesting a main pathway for the observed
homoconjugative CT interactions through the cyclobutene
rings.
The UV/Vis absorption spectrum of spiro compound
(ꢀ)-10 is dominated by a CT band at lmax = 514 nm (2.41 eV)
with a large e value of 23500 Lmolꢁ1 cmꢁ1 (Figure 2a). DFT
calculations reveal that the intramolecular CT in (ꢀ )-10
predominantly arises from the p-conjugated D–A interaction
between the DAA donor and the Cp acceptor moieties
(Supporting Information, Figure S22). The electron-with-
drawing, unsaturated g-lactone moiety, however, should also
contribute to the acceptor potency owing to the spiroconju-
gative effect.[18]
Cyclic voltammetry (CV; Table 1) and rotating-disk
voltammetry (RDV; Supporting Information, Table S3) in
CH2Cl2 (+ 0.1molLꢁ1 nBu4NPF6, internal standard Fc+/Fc)
revealed that all of the cycloadducts (ꢀ )-1 to (ꢀ )-9 display
two 1eꢁ-reduction couples between ꢁ0.69 and ꢁ1.53 V that
are centered on the quinone core. Each donor moiety (DAA
or Fc) undergoes a 1 eꢁ-oxidation step. The effect of
homoconjugative CT interactions in the ground state is
notably visible in the DAA-centered oxidation potentials of
(ꢀ )-4 and 5. The H-substituted derivative (ꢀ )-4 is oxidized at
+ 0.65 V. In (ꢀ )-4, the DAA moiety retains nearly full
planarity with the cyclobutene moiety (torsion angle C8-C7-
C17-C18 = 10.4(8)8 in (ꢀ )-1), thereby rendering the intra-
molecular CT interactions quite efficient. In contrast, sub-
stitution with a second DAA donor in 5 induces a twist around
Figure 2. Electronic absorption spectra of a) DAA-substituted cyclo-
adducts (ꢀ)-1 to (ꢀ)-3, 5, (ꢀ)-6, and (ꢀ)-10 and b) Fc-substituted
cycloadducts (ꢀ)-7 to (ꢀ)-9 (b) in CH2Cl2 at 298 K.
ꢁ
the C C single bond connecting the DAA moiety to the
cyclobutene moiety. This twisting causes a reduced transfer of
electron density from the DAA donor to the dichloroene-
dione acceptor moiety, resulting in a significant cathodic shift
of the first oxidation potential in 5 by 360 mV (+ 0.65 V in
(ꢀ)-4 and + 0.29 V in 5) and of the second oxidation potential
by 130 mV (+ 0.52 V). A good linear correlation exists
between the optical gap (DEopt; energy of the longest-
donors was confirmed by protonation experiments (Support-
ing Information, Figures S9–S15). When a CH2Cl2 solution of
(ꢀ )-1 was acidified with CF3COOH, the color changed from
purple to colorless and the broad absorption band at 529 nm
completely disappeared. Neutralization with Et3N regener-
ated the original spectra almost quantitatively. DAA-substi-
tuted cycloadducts (ꢀ )-1 to (ꢀ )-3 feature similar maxima of
their lowest-energy intramolecular CT bands at lmax = 529 nm
(2.34 eV, (ꢀ )-1), 541 nm (2.29 eV, (ꢀ )-2), and 528 nm
(2.35 eV, (ꢀ )-3). The CT bands of Fc-substituted cyclo-
adducts (ꢀ )-7 to (ꢀ )-9 (620 nm ((ꢀ )-7), 635 nm ((ꢀ )-8),
635 nm ((ꢀ )-9)) are weaker in intensity but bathochromically
shifted by about 100 nm relative to those in the spectra of
(ꢀ)-1 to (ꢀ )-3 (Figure 2b). Incorporation of a second DAA
wavelength maxima) and the electrochemical gap (DEredox
in (ꢀ )-1 to (ꢀ )-6 (Supporting Information, Figure S8).
)
High third-order optical nonlinearities have in the past
been mostly reported for planar and non-planar p-conjugated
chromophores. Consequently, this has limited the structural
diversity that can be used to generate desirable nonlinear
optical functionalities. We measured the rotational average of
the third-order polarizability of (ꢀ )-1 to (ꢀ )-4, (ꢀ )-6, and
Angew. Chem. Int. Ed. 2010, 49, 6207 –6211
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 6209

Communications
Table 1: CV data[a] and a summary of electrochemical and optical energy
gaps.
framework. Transannular CT interactions presumably also
contribute in the Fc-derived push–pull systems. Remarkable
third-order polarizabilities suggest new design opportunities
for the development of NLO chromophores, which we are
now fully exploiting.
Cyclic voltammetry
E8
DEp
Ep
DEredox
DEopt
[V][b]
[mV][c]
[V][d]
[V][e]
[eV][f]
(ꢀ)-1
+0.67
75
1.51
1.13
2.34
1.83
ꢁ0.84[g]
ꢁ1.48[g]
Received: April 15, 2010
Published online: July 20, 2010
5
+0.52
+0.29
ꢁ0.84
ꢁ1.53
+0.52
+0.36
ꢁ0.81
90
60
80
100
80
60
Keywords: charge transfer · conjugation · cycloadditions ·
.
nonlinear optoelectronic properties · spiro compounds
(ꢀ)-6
1.17
1.98
[1] R. Gompper, H.-U. Wagner, Angew. Chem. 1988, 100, 1492 –
1511; Angew. Chem. Int. Ed. Engl. Ed. 1988, 27, 1437 – 1455.
[2] a) M. Kivala, F. Diederich, Acc. Chem. Res. 2009, 42, 235 – 248;
b) S.-i. Kato, F. Diederich, Chem. Commun. 2010, 46, 1994 –
2006.
[3] a) S. Barlow, S. R. Marder in Functional Organic Materials (Eds.:
T. J. J. Mꢂller, U. H. F. Bunz), Wiley-VCH, Weinheim, 2007,
pp. 393 – 437; b) Special issue on Organic Electronics and
Optoelectronics, Chem. Rev. 2007, 107, 923 – 1386, edited by
S. R. Forrest and M. E. Thompson.
[4] a) W. Rebafka, H. A. Staab, Angew. Chem. 1973, 85, 831 – 832;
Angew. Chem. Int. Ed. Engl. 1973, 12, 776 – 777; b) H. A. Staab
in New Aspects of Organic Chemistry I (Eds.: Z. Yoshida, T.
Shiba, Y. Oshiro), VCH, Weinheim, 1989, pp. 227 – 236.
[5] S. Misumi, T. Otsubo, Acc. Chem. Res. 1978, 11, 251 – 256.
[6] a) Cyclophanes (Ed.: F. Vꢃgtle), Springer, Heidelberg, 1983;
b) F. Diederich, Cyclophanes, Royal Society of Chemistry,
Cambridge, 1991; c) Modern Cyclophane Chemistry (Eds.: R.
Gleiter, H. Hopf), Wiley-VCH, Weinheim, 2004.
70
ꢁ1.51
(ꢀ)-7
+0.32
ꢁ0.76
65
75
1.08
1.76
1.99
2.41
ꢁ1.53[g]
ꢁ1.00
(ꢀ)-10
+0.76
75
[a] Recorded in CH2Cl2 (+0.1 molLꢁ1 nBu4NPF6). All potentials are given
versus the Fc+/Fc couple used as internal standard. Working electrode:
glassy carbon electrode, counter electrode: Pt, pseudo reference
electrode: Pt. Scan rate: 0.1 Vsꢁ1. The complete set of data for (ꢀ)-1
to (ꢀ)-10, including those from rotating-disk voltammetry (RDV), are
included in the Supporting Information. [b] E8=(Epc +Epa)/2, where Epc
and Epa correspond to the cathodic and anodic peak potentials,
respectively. [c] DEp =EoxꢁEred, where the subscripts ox and red refer to
the conjugated oxidation and reduction steps, respectively. [d] Ep =irre-
versible peak potential. [e] The electrochemical gap, DEredox, is defined as
the potential difference between the first oxidative and first reductive
redox potentials. [f] The optical gap, DEopt, is defined as the energy
corresponding to the energy of the CT absorption (lmax). [g] Reversible
[7] H. Oevering, J. W. Verhoeven, M. N. Paddon-Row, J. M.
Warman, Tetrahedron 1989, 45, 4751 – 4766.
electron transfer at scan rates higher than 1 Vsꢁ1
.
[8] a) T. Nakazawa, I. Murata, J. Am. Chem. Soc. 1977, 99, 1996 –
1997; b) T. Nakazawa, N. Abe, K. Kubo, I. Murata, Tetrahedron
Lett. 1979, 20, 4995 – 4998; c) K. Yamamura, T. Nakazawa, I.
Murata, Angew. Chem. 1980, 92, 565 – 566; Angew. Chem. Int.
Ed. Engl. 1980, 19, 543 – 545; d) T. Nakazawa, Y. Niimoto, K.
Kubo, I. Murata, Angew. Chem. 1980, 92, 566 – 567; Angew.
Chem. Int. Ed. Engl. 1980, 19, 545 – 546; e) T. Nakazawa, K.
Kubo, I. Murata, Angew. Chem. 1981, 93, 195 – 196; Angew.
Chem. Int. Ed. Engl. 1981, 20, 189 – 190; f) K. Yamamura, K.
Nakasuji, I. Murata, S. Inagaki, J. Chem. Soc. Chem. Commun.
1982, 396 – 397; g) K. Yamamura, K. Nakasuji, H. Yamochi, I.
Murata, S. Inagaki, Chem. Lett. 1983, 627 – 630; h) K. Nakasuji,
H. Yamochi, I. Murata, N. Yasuoka, M. Kusunoki, Chem. Lett.
1986, 969 – 972.
[9] H. Iwamura, K. Makino, J. Chem. Soc. Chem. Commun. 1978,
720 – 721.
[10] R. Gleiter, T. Doerner, H. Irngartinger, Liebigs Ann. 1996, 381 –
391.
[11] Y. Kim, Z. Zhu, T. M. Swager, J. Am. Chem. Soc. 2004, 126, 452 –
453.
(ꢀ)-10 by degenerate four-wave mixing (DFWM) at a
wavelength of 1.5 micrometers in CH2Cl2 solutions (see the
Supporting Information). Homoconjugated chromophores
(ꢀ)-1 to (ꢀ )-4 and (ꢀ )-6 displayed moderate to large third-
order polarizabilities (grot values), which must be ascribed to
efficient intramolecular CT interactions (Supporting Infor-
mation, Table S4). Whereas (ꢀ )-4 showed a grot value of
(2.0 ꢀ 1.0) ꢁ 10ꢁ48 m5 Vꢁ2, the more extended (ꢀ )-6, which
incorporates two DAA donors, gave grot = (5.0 ꢀ 0.5) ꢁ
10ꢁ48 m5 Vꢁ2. Remarkably, this value is comparable to those
obtained for the most efficient planar and nonplanar D-p-A
chromophores of similar size (Supporting Information,
Table S4).[19,20] Substituents (H, Me, and CN) on the cyclo-
butene ring only slightly affected the grot value in (ꢀ )-1 to
(ꢀ)-3: < 1.0 ꢁ 10ꢁ48 m5 Vꢁ2 in (ꢀ )-1, (0.8 ꢀ 0.8) ꢁ 10ꢁ48 m5 Vꢁ2
in (ꢀ )-2, and (1.6 ꢀ 1.0) ꢁ 10ꢁ48 m5 Vꢁ2 in (ꢀ )-3). The spiro
system is also NLO-active; (ꢀ )-10 showed a grot value of
(2.5 ꢀ 0.5) ꢁ 10ꢁ48 m5 Vꢁ2.
[12] D. Walker, J. D. Hiebert, Chem. Rev. 1967, 67, 153 – 195.
[13] a) J. S. Lindsey, I. C. Schreiman, H. C. Hsu, P. C. Kearney, A. M.
Marguerettaz, J. Org. Chem. 1987, 52, 827 – 836; b) F. Diederich,
H. A. Staab, Angew. Chem. 1978, 90, 383 – 385; Angew. Chem.
Int. Ed. Engl. 1978, 17, 372 – 374.
[14] B. A. Trofimov, L. N. Sobenina, Z. V. Stepanova, I. A. Ushakov,
L. M. Sinegovskaya, T. I. Vakulꢄskaya, A. I. Mikhaleva, Syn-
thesis 2010, 470 – 476.
In summary, we prepared a new family of homoconju-
gated push–pull chromophores by [2+2] cycloadditions of
DDQ to donor-substituted alkynes, and exploited their
unique reactivity to access the first member of a new class
of D–A-functionalized spiro compounds. These cycloadducts
feature intense intramolecular CT interactions as a result of
effective homoconjugation that arises from the rigid bicyclic
[15] E. Ghera, Y. Gaoni, S. Shoua, J. Am. Chem. Soc. 1976, 98, 3627 –
3632. An alternative mechanistic proposal was kindly made by
=
ꢁ
one of the referees, involving a homolytic cleavage of a C( O)
C(CN) bond in 20, followed by a radical cyclization cascade.
6210 www.angewandte.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 6207 –6211

Angewandte
Chemie
[16] All calculations were conducted with Gaussian 09 (Revision
A.02): M. J. Frisch et al. (see the Supporting Information).
[17] Bond-length alternations in the substituted Cp rings indicate a
contribution of fulvene-type resonance structures in the ground
state. For some recent work, see: a) T. Mochida, S. Yamazaki,
J. Chem. Soc. Dalton Trans. 2002, 3559 – 3564; b) S.-i. Kato, M.
Kivala, W. B. Schweizer, C. Boudon, J.-P. Gisselbrecht, F.
Diederich, Chem. Eur. J. 2009, 15, 8687 – 8691; c) T. L. Kinni-
brugh, S. Salman, Y. A. Getmanenko, V. Coropceanu, W. W.
Porter III, T. V. Timofeeva, A. J. Matzger, J.-L. Brꢅdas, S. R.
Marder, S. Barlow, Organometallics 2009, 28, 1350 – 1357.
Burkey, J. Am. Chem. Soc. 1990, 112, 5359 – 5360; c) P. Maslak,
A. Chopra, J. Am. Chem. Soc. 1993, 115, 9331 – 9332; d) Y.-Y.
Chien, K.-T. Wong, P.-T. Chou, Y.- M. Cheng, Chem. Commun.
2002, 2874 – 2875; e) P. Sandꢆn, A. Marꢆnez-Grau, L. Sꢇnchez, C.
Seoane, R. Pou-Amꢅigo, E. Ortꢆ, N. Martꢆn, Org. Lett. 2005, 7,
295 – 298.
[19] a) J. C. May, J. H. Lim, I. Biaggio, N. N. P. Moonen, T. Michi-
nobu, F. Diederich, Opt. Lett. 2005, 30, 3057 – 3059; b) J. C. May,
ˇ
I. Biaggio, F. Bures, F. Diederich, Appl. Phys. Lett. 2007, 90,
251106.
[20] T. Michinobu, J. C. May, J. H. Lim, C. Boudon, J.-P. Gisselbrecht,
P. Seiler, M. Gross, I. Biaggio, F. Diederich, Chem. Commun.
2005, 737 – 739.
[18] For a review on spiroconjugation, see: a) H. Dꢂrr, R. Gleiter,
Angew. Chem. 1978, 90, 591 – 601; Angew. Chem. Int. Ed. Engl.
1978, 17, 559 – 569; See also: b) P. Maslak, M. P. Augustine, J. D.
Angew. Chem. Int. Ed. 2010, 49, 6207 –6211
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6211