zethrenebis(dicarboximide) (e.g., 1 in Scheme 1) would show
substantial near-infrared (NIR) absorption and emission,5
which is much longer than that of the common rylene diimide
dyes.6 In 2006, Maksic´ et al. reported that zethrene as well
as its longitudinal homologues would exihibit large absolute
proton affinity (APA) and second-order hyperpolarizability
(γ) based on semiempirical AM1 calculations, and this was
further supported by Nakano’s calculations that zethrene will
possess a significant singlet diradical character at the ground
state (Figure 1). These predictions suggest that zethrene and
its derivatives can be used as useful building blocks for
nonlinear optical materials and NIR dyes.7
Different from previous work, we chose 4,6-dibromo-1,8-
naphthalimide (5) as precursor with bulky 2,6-diisopropy-
lphenyl as substituent, which can improve the solubility as
well as suppress aggregation of the zethrene chromophore.
As shown in Scheme 1, 1,8-dibromonaphthoic anhydride (4)
was first prepared by oxidation of 1,8-dibromoacenaphthene-
dione (3)9 with oxone,8a and subsequent imidization of 4
with 2,6-diisopropylaniline afforded 4,6-dibromo-1,8-naph-
thalimide (5).10 Stille cross coupling reaction11 between 5
and bis(tributylstannyl)acetylene (1:1 ratio) and subsequent
in situ transannular cyclization reaction gave the desired
zethrenebis(dicarboximide) 1 in one pot. It is essential to
carefully control the reaction conditions to obtain the target
compound. This reaction must be performed in dilute solution
to favor intramolecular cyclization over intermolecular
polymerization.12 Oxygen has to be strictly excluded from
the reaction system. The optimized temperature is 80 °C to
avoid incomplete conversion at lower temperature or com-
plicated products at higher temperature. Although we did
rigid control on the experimental conditions, the separation
yield for this step was still low (13-20%) due to the
existence of other oligomers, which complicated the column
chromatography purification.
Figure 1. Structural characteristics of zethrene.
Despite all of these attracting properties and promising
applications, zethrene and its derivatives were seldom
synthesized and studied deeply due to their low accesibility
and high sensitivity in the presence of oxygen and light
especially in dilute solution.3a Tobe et al. have successfully
synthesized 7,14-substituted zethrene by blocking the most
reactive 7,14-positions. In parallel to that work, we have been
working on the synthesis of electron-withdrawing dicar-
boxylic imide group substituted zethrene derivative (1 in
Scheme 1). Such an approach will not only stabilize the
highly reactive zethrene by lowering its HOMO energy level,
but also will result in obvious red-shift of the absorption
and emission spectra to the far-red or NIR region owing to
the acceptor-donor-acceptor structure. This concept has
also been proved to be efficient to prepare soluble and stable
NIR dyes by using very unstable hydrocarbons such as
bisanthene as building blocks.8 In addition, according to
calculations, zethrene exihibits a central butadiene moiety
flanked by two naphthalene rings and the central butadiene
unit shows significant bond length alternation (1.368 and
1.468 Å, Figure 1).4 So we can also study the reactivity of
the butadiene subunit and perform further modifications at
the reactive 7,14-positions. Herein we report an efficient
synthesis of a soluble and stable zethrene derivative 1 and
its unusual reaction.
Scheme 1. Synthetic Route Toward Compounds 1 and 2
To further modify the zethrene at the 7,14-positions,
bromination of 1 was attempted by using N-bromosuccin-
imide (NBS) in DMF.13 Interestingly, the oxidized product
zethrenebis(dicarboximide)quinone (2) rather than bromi-
nated product was formed. The structure of 2 was confirmed
by 1H NMR, 13C NMR, HR-ESI MS, MALDI-TOF MS, and
(5) De´silets, D.; Kazmaier, P. M.; Burt, R. A. Can. J. Chem. 1995, 73,
319–324.
(6) (a) Wu¨rthner, F. Chem. Commun. 2004, 40, 1564–1579. (b) Bhosale,
S. H.; Jani, C. H.; Langford, S. J. Chem. Soc. ReV. 2008, 37, 331–342. (c)
Sakai, N.; Mareda, J.; Vauthey, E.; Matile, S. Chem. Commun. 2010, 46,
4225–4237.
(9) Yan, J.; Travis, B. R.; Borhan, B. J. Org. Chem. 2004, 69, 9299–
(7) (a) Knezˇevic´, A.; Maksic´, Z. B. New J. Chem. 2006, 30, 215–222.
(b) Nakano, M.; Kishi, R.; Takebe, A.; Nate, M.; Takahashi, H.; Kubo, T.;
Kamada, K.; Ohta, K.; Champagne, B.; Botek, E. Comput. Lett. 2007, 3,
333–338.
9302.
(10) Ro¨ger, C.; Wu¨rthner, F. J. Org. Chem. 2007, 72, 8070–8075.
(11) Zimmermann, E. K.; Stille, J. K. Macromolecules 1985, 18, 321–
327.
(8) (a) Yao, J.; Chi, C.; Wu, J.; Loh, K. Chem.sEur. J. 2009, 15, 9299–
9302. (b) Li, J.; Zhang, K.; Zhang, X.; Huang, K.; Chi, C.; Wu, J. J. Org.
Chem. 2010, 75, 856–863. (c) Zhang, K.; Huang, K.; Li, J.; Luo, J.; Chi,
C.; Wu, J. Org. Lett. 2009, 11, 4854–4857.
(12) Knops, P.; Sendhoff, N.; Mekelburger, H. B.; Vo¨gtle, F. Top. Curr.
Chem. 1991, 161.
(13) Mitchell, R. H.; Lai, Y. H.; Williams, R. V. J. Org. Chem. 1979,
44, 4733–4735.
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