motifs in proximity may provide a unique type of memory
device with dual mode permutation in a synergistic fashion.
There are a few reports on azobenzenes bearing photoinert,
axially chiral cores8a-c or photoswitchable stilbenes.8d,e So
far, azobenzenes combined with photoresponsive chiral
helicenes as dual mode, chiro- and photochromic optical
switches have been relatively unexplored.8f,g
As an extension of our ongoing program on the uses of
C2-symmetric, dibenzosuberene (DBE)-derived helicenes as
potential chirochromic LC optical switches,3d,e we recently
sought to realize the dual-mode optical switch by appending
two azobenzene units onto the C3 and C7 positions of a
DBE-based helicene bearing an 8-naphthylmenthol-derived
chiral ester (i.e., compound 1 in Scheme 1). Upon judicious
eomeric mixture (26:74) of 3,7-dinitroepisulfides (1′R)- and
(1′S)-2 can be prepared by a Staudinger-type, diazothioketone
coupling reaction.9 The isolated, pure (1′S)-2 diastereomer
was reduced by SnCl2·2H2O to afford (1′S)-3 in 85% yield.
Both the amino moieties in (1′S)-3 were reacted with in situ
generated HNO2 in aqueous acetone to give the correspond-
ing bis-diazonium salt, which was further coupled with
phenol under basic conditions to furnish the corresponding
bis-azobenzene (1′S)-4 in 84% yield. Alkylation of both the
phenolic moieties in (1′S)-4 with 1-bromoheptane in the
presence of K2CO3 and KI led to bis-azobenzene (1′S)-5 in
85% yield. Reductive desulfurization of respective (1′S)-2
and (1′S)-5 by hexamethylphosphorous triamide (HMPT) at
0 °C gave 3,7-dinitrohelicene 11 and 3,7-bisazohelicene 1
of (M)-helicity (>99% yield, >99% de). Similarly, (1′R)-2
can lead to 3,7-bisazo-helicene(P)-1 by the same sequence
of reactions.
Scheme 1. Synthesis of (M)-11 and (M)-E,E-1
The 3,7-dinitro-substituted helicene 11 synthesized from
the episulfide (1′S)-2 was confirmed to possess (M)-form
helicity as determined by X-ray crystallographic analysis
(Figure 1a). The split Cotton effects in its CD spectrum in
Figure 1. (a) X-ray crystal structure of (M)-11 (hydrogens omitted
for clarity); (b) stacked CD plots of (M)-11, (M)-E,E-1 and (P)-
E,E-1 in hexanes.
selection of four different irradiation wavelengths, P-M
isomerization of the helicene moiety or double E-Z isomer-
ization of both the azobenzenes in 1 can be collaboratively
carried out. In principle, a nematic LC phase can be induced
to a cholesteric one with modulable CD, helical pitch, and
handness readouts by using 1 as the chiral dopant. Therefore,
four metastable cholesteric mesophases in a single nematic
LC host may be identified through the dual-mode photo-
switching with a dual-mode binary logic.
hexanes indicate a positive exciton chirality (227 nm (+), 215
nm (-)). Similarly, (M)-E,E-1 obtained from episulfide (1′S)-2
also shows a positive exciton chirality of its split Cotton effects
(230 nm (+), 207 nm (-)) in its CD spectrum.
Conversely, (P)-E,E-1 synthesized from episulfide (1′R)-2
shows a negative exciton chirality (249 nm (-), 204 nm (+))
based on its split Cotton effects in CD spectrum, suggesting
an opposite helical chirality from (M)-11 (Figure 1b).10
The synthetic routes to a model helicene 11 and the dual-
mode optical switch 1 are shown in Scheme 1.9 A diaster-
(7) (a) Yokoyama, Y. Chem. ReV. 2000, 100, 1717. (b) Yokoyama, Y.
In Molecular Switches; Feringa, B. L., Ed.; Wiley-VCH: Weinheim,
Germany, 2001; pp 107-121.
(4) (a) Carreno, M. C.; Garcia, I.; Ribagorda, M.; Merino, E.; Pieraccini,
S.; Spada, G. P. Org. Lett. 2005, 7, 2869. (b) Tamaoki, N.; Wada, M. J. Am.
Chem. Soc. 2006, 128, 6284. (c) Henzl, J.; Mehlhorn, M.; Gawronski, H.;
Rieder, K. H.; Morgenstern, K. Angew. Chem., Int. Ed. 2006, 45, 603. (d)
Kondo, M.; Yu, Y.; Ikeda, T. Angew. Chem., Int. Ed. 2006, 45, 1378. (e)
Mathews, M.; Tamaoki, N. J. Am. Chem. Soc. 2008, 130, 11409.
(5) (a) Denekamp, C.; Feringa, B. L. AdV. Mater. 1998, 10, 1080. (b)
Morimoto, M.; Irie, M. Chem. Commun. 2005, 3895. (c) Higashiguchi, K.;
Matsuda, K.; Tanifuji, N.; Irie, M. J. Am. Chem. Soc. 2005, 127, 8922. (d)
Tian, H.; Yang, S. J. Chem. Soc. ReV. 2004, 33, 85.
(8) (a) van Delden, R. A.; Mecca, T.; Rosini, C.; Feringa, B. L.
Chem.sEur. J. 2004, 10, 61. (b) Pieraccini, S.; Gottarelli, G.; Labruto, R.;
Masiero, S.; Pandoli, O.; Spada, G. P. Chem.sEur. J. 2004, 10, 5632. (c)
Li, Q.; Green, L.; Venkataraman, N.; Shiyanovskaya, I.; Khan, A.; Urbas,
A.; Doane, J. W. J. Am. Chem. Soc. 2007, 129, 12908. (d) Qu, D.-H.; Ji,
F.-Y.; Wang, Q.-C.; Tian, H. AdV. Mater. 2006, 18, 2035, and references
therein. (e) See also: Andr´easson, J.; Straight, S. D.; Moore, T. A.; Moore,
A. L.; Gust, D. J. Am. Chem. Soc. 2008, 130, 11122. For other dual-mode
switches, see: . (f) Saravanan, C.; Senthil, S.; Kannan, P. J. Polym. Sci.,
Part A: Polym. Chem. 2008, 46, 7843. (g) Inouye, M.; Akamatsu, K.;
Nakazumi, H. J. Am. Chem. Soc. 1997, 119, 9160.
(6) (a) Berkovic, G.; Krongauz, V.; Weiss, V. Chem. ReV. 2000, 100,
1741. (b) Raymo, F. M.; Giordani, S. J. Am. Chem. Soc. 2001, 123, 4651.
(c) Giordani, S.; Raymo, F. M. Org. Lett. 2003, 5, 3559. (d) Zhou, W.;
Chen, D.; Li, J.; Xu, J.; Lu, J.; Liu, H.; Li, Y. Org. Lett. 2007, 9, 3929. (e)
Andersson, J.; Li, S.; Lincoln, P.; Andre´asson, J. J. Am. Chem. Soc. 2008,
130, 11836.
(9) See the Supporting Information for details.
(10) Harada, N.; Nakanishi, K. Circular Dichroic Spectroscopy: Exciton
Coupling in Organic Photochemistry; University Science Books: Mill
Valley, CA, 1983.
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