Syntheses and photophysical properties of boomerang-shaped Bis(dehydrobenzo[12]annulene) and trapezoid-shaped tris(dehydrobenzo[12] annulene)

Article abstract of DOI:10.1246/cl.2007.838

A boomerang-shaped bis(dehydrobenzo[12]annulene) and a trapezoid-shaped tris(dehydrobenzo[12]annulene) were synthesized, and their ground- and excited-state properties were investigated. Copyright

Full text of DOI:10.1246/cl.2007.838

838
Chemistry Letters Vol.36, No.7 (2007)
Syntheses and Photophysical Properties of Boomerang-shaped Bis(dehydrobenzo[12]annulene)
and Trapezoid-shaped Tris(dehydrobenzo[12]annulene)
Kazukuni Tahara, Takashi Yoshimura, Mariko Ohno, Motohiro Sonoda, and Yoshito Tobeꢀ
Division of Frontier Materials Science, Graduate School of Engineering Science,
Osaka University, Toyonaka, Osaka 560-8531
(Received April 9, 2007; CL-070381; E-mail: tobe@chem.es.osaka-u.ac.jp)
A boomerang-shaped bis(dehydrobenzo[12]annulene) and
a trapezoid-shaped tris(dehydrobenzo[12]annulene) were syn-
thesized, and their ground- and excited-state properties were
investigated.
Synthesis of the multiply fused dehydrobenzoannulenes
becomes current interests because of their unique electronic
and optical properties, local aromaticity as well as potential util-
ities in supramolecular chemistry.1 Though the parent dehydro-
benzo[12]annulene (1, DBA, Chart 1) has been extensively in-
vestigated since 50 years ago,2 its multiply fused homologues
have still been unknown expect for rhombic-shaped and bow-
tie-shaped bisDBA derivatives3 and a trefoil-shaped trisDBA
derivative.4 It is therefore of interest to know how the number
and mode of fusion of DBAs would affect their local aromaticity
and electronic properties.5 In this respect, we report herein
the syntheses and photophysical properties of novel multiply
fused DBAs; boomerang-shaped bisDBA derivative 2b and tra-
pezoid-shaped trisDBA derivative 3b, the latter being one of the
remaining fragments of the full-wheel-shaped hexakisDBA.
We recently reported on the theoretical studies based on the
DFT calculations for the parent compounds 2a and 3a at the
B3LYP/6-31Gꢀ level of theory; 2a adopts a nonplanar confor-
mation while 3a is planar, the nucleolus independent chemical
shift (NICS) calculations indicate that aromaticity of the central
benzene ring reduces as the number of fused DBA ring increases
(NICS = ꢁ9:13 for 2a and NICS = ꢁ8:49 for 3a calculated
by GIAO method at the HF/6-31Gꢀ//B3LYP/6-31Gꢀ level
of theory), and there is little difference between their HOMO–
LUMO gaps (2.99 eV for 2a and 3.02 eV for 3a).5
Our approach to the synthesis of the multiply fused DBAs
2b and 3b relies on a double elimination strategy which was
employed for the synthesis of trefoil-shaped trisDBA to con-
struct the peripheral triple bonds.4 First, the peripheral benzene
rings are connected by single bonds with appropriate leaving
groups. Then, elimination of the leaving groups leads to the
formation of triple bonds at the periphery.
Scheme 1. Syntheses of bisDBA 2b and trisDBA 3b.
The synthetic routes for 2b and 3b are shown in Scheme 1.6
Reaction of 1,2,3,4-tetrabromobenzene and an excess amount
of formylalkyne 5 under the Sonogashira coupling conditions
afforded tetraaldehyde 6. Intramolecular pinacol coupling of 6
using a low-valent vanadium reagent gave tetraol 7 as a mixture
of isomers, which was transformed to tetrachloride 8 by treat-
ment with thionyl chloride. Finally, a double elimination reac-
tion of 8 using potassium tert-butoxide furnished boomerang-
shaped bisDBA 2b in 60% yield.
Similarly, the synthesis of trapezoid-shaped trisDBA
3b began with 1,2,3,4-tetrabromobenzene. Regio-selective
Sonogashira reaction at the 1,4-positions of tetrabromobenzene
Chart 1.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.7 (2007)
839
energy transitions at 400–450 nm as shown in Figure 1a. While
2b and 3b show absorption maxima at 362 and 338 nm, respec-
tively, attributable to the p-bis(phenylethynyl)benzene chromo-
phore,9 they do not show intense absorptions at shorter wave-
length region which are observed in 1 and 4, indicating that
absorptions of 2b and 3b in the UV region are dominated by
the linearly conjugated chromophore.
DBAs 2b and 3b emit bright blue fluorescence (ꢀmax
¼
462 nm for 2b and ꢀmax ¼ 510 nm for 3b) in chloroform
(Figure 1b). Whereas the 0–0 vibrational transition band of 2b
is the most intense like the alternately fused DBA reported
previously,4 the fluorescence profile of 3b with strong 0–1 band
is similar to those of 1 and 4. The reason for the difference in
the vibrational profile between the twisted and planar [12]DBAs
is not certain.
In summary, we synthesized two novel multiply fused
DBAs, boomerang-shaped bisDBA 2b and trapezoid-shaped
trisDBA 3b by the double elimination strategy. As theoretically
predicted, aromaticity of the central benzene ring reduces with
increasing number of DBA ring. On the other hand, 3b maintains
better the characteristic of the [12]DBA chromophore than 2b
judging from the absorption and fluorescence spectra. These
insights are useful for the design and synthesis of novel fused
DBAs and their application in organic materials.10
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
Figure 1. Electronic absorption spectra (a) and normalized
fluorescence emission spectra (b) of 1 (black line), 2b (red line),
3b (blue line), and 4 (green line) in CHCl3 at 30 ꢂC. The symbols
indicated the respective ꢁ-conjugated core of the [12]DBAs.
References and Notes
1
For recent reviews on DBAs, see: a) Y. Tobe, M. Sonoda, in
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M. M. Haley, in Acetylene Chemistry: Chemistry, Biology and
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with two equivalents of formylalkyne 5 gave dialdehyde 9.7
Then, two-fold cross-coupling reaction of 9 with trimethylsilyl-
acetylene afforded tetraethynyl derivative 10. Introduction of the
latter two alkylne units required a high catalyst loading (25
mol %) because of the low reactivity of 9 probably owing to
steric hindrance around the reaction center. In situ deprotection
of the silyl groups of 10 and the cross-coupling reaction with io-
doformylarene 11 gave hexaaldehyde 12. Pinacol coupling reac-
tion of 12, forming hexaol 13, and subsequent chlorination gave
hexachloride 14. Elimination of HCl from 14 produced a trace
amount of the desired trapezoid-shaped trisDBA 3b (<1%).
The yield of 3b was much lower than those of 2b and trefoil-
shaped tris[12]DBA4 owing to the formation of side products
possessing double bond(s) at the periphery which were not easily
separated from 3b.
Comparison of the 1H NMR spectra of 2b and 3b reveals the
decrease of the diamagnetic ring current at the central benzene
ring of 3b compared with that of 2b. Indeed, protons attached
to the central benzene ring of 2b and 3b resonate at 7.10 ppm
for 2b and 6.91 ppm for 3b. These results qualitatively agree
with the theoretical predictions by GIAO calculations for model
compounds 2a (7.34 ppm) and 3a (7.15 ppm).8 In addition, NICS
values of 2a and 3a also indicate decrease of diamagnetic ring
current at the central benzene ring (vide supra).5 Another
possibility to induce the upfield shift of the benzene protons is
the effect of paratropic 12-membered ring.
2
3
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GIAO calculation was performed by using the GIAO-HF/
6-31Gꢀ//B3LYP/6-31Gꢀ level ab intio calculation. Tetramethyl-
silane was used as a standard.
9
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The absorption spectra of 2b and 3b exhibit distinctly
different profiles from those 1 and 4, except for the lowest
Published on the web (Advance View) May 31, 2007; doi:10.1246/cl.2007.838