Multibridged chiral naphthalene oligomers with continuous extreme-cisoid conformation

Article abstract of DOI:10.1021/ol100582v

Axially chiral 2,2′-methylenedioxy-bridged-1,1′-binaphthyls, quaternaphthalenes, and octinaphthalenes were synthesized and their optical properties researched. 2,2′-Methylenedioxy bridges led to a continuous extremely cisoid conformation and subsequent extensive conjugation in the rod direction. Therefore, the absorption and fluorescence regions were red-shifted as the number of naphthalene rings increased. These oligonaphthalenes fluoresced in both solution and the solid state. Furthermore, DFT calculations showed that the LUMO and HOMO of these bridged oligonaphthalenes were spread over a wide range.

Full text of DOI:10.1021/ol100582v

ORGANIC
LETTERS
2010
Vol. 12, No. 8
1832-1835
Multibridged Chiral Naphthalene
Oligomers with Continuous
Extreme-Cisoid Conformation
Kazuto Takaishi,*,† Masuki Kawamoto,† and Kazunori Tsubaki‡
Supramolecular Science Laboratory, RIKEN, Wako, Saitama 351-0198, Japan, and
Graduate School of Life and EnVironmental Science, Kyoto Prefectural UniVersity,
Sakyo, Kyoto 606-8522, Japan
ktakaishi@riken.jp
Received March 10, 2010
ABSTRACT
Axially chiral 2,2′-methylenedioxy-bridged-1,1′-binaphthyls, quaternaphthalenes, and octinaphthalenes were synthesized and their optical properties
researched. 2,2′-Methylenedioxy bridges led to a continuous extremely cisoid conformation and subsequent extensive conjugation in the rod
direction. Therefore, the absorption and fluorescence regions were red-shifted as the number of naphthalene rings increased. These
oligonaphthalenes fluoresced in both solution and the solid state. Furthermore, DFT calculations showed that the LUMO and HOMO of these
bridged oligonaphthalenes were spread over a wide range.
π-Conjugated linear oligomers such as oligothiophenes,
oligophenylenes, and oligofluorenes are typically synthesized
and designed as wire-type functional materials.1 Previously,
we have synthesized optically active 1,1′-oligonaphthalenes
1 (4mers to 32mers) as representative axially chiral oligomers
by the bottom-up method, which maintains asymmetry and
precisely constructs oligomers without a molecular weight
distribution, to elucidate the asymmetric and optical proper-
ties.2 Although energy transfer via the axes was detected, 1
did not realize extensive conjugation because the dihedral
angles of adjacent naphthalene units were nearly 90°.2c,g
(2) (a) Tsubaki, K.; Miura, M.; Morikawa, H.; Tanaka, H.; Kawabata,
T.; Furuta, T.; Tanaka, K.; Fuji, K. J. Am. Chem. Soc. 2003, 125, 16200.
(b) Tsubaki, K.; Tanaka, H.; Takaishi, K.; Miura, M.; Morikawa, H.; Furuta,
T.; Tanaka, K.; Fuji, K.; Sasamori, T.; Tokitoh, N.; Kawabata, T. J. Org.
Chem. 2006, 71, 6579. (c) Tsubaki, K.; Takaishi, K.; Tanaka, H.; Miura,
M.; Kawabata, T. Org. Lett. 2006, 8, 2587. (d) Takaishi, K.; Tsubaki, K.;
Tanaka, H.; Miura, M.; Kawabata, T. Yakugaku Zasshi 2006, 126, 779. (e)
Tsubaki, K.; Takaishi, K.; Sue, D.; Kawabata, T. J. Org. Chem. 2007, 72,
4238. (f) Tsubaki, K. Org. Biomol. Chem. 2007, 5, 2179. (g) Tsubaki, K.;
Takaishi, K.; Sue, D.; Matsuda, K.; Kanemitsu, Y.; Kawabata, T. J. Org.
Chem. 2008, 73, 4279. (h) Sue, D.; Takaishi, K.; Harada, T.; Kuroda, R.;
Kawabata, T.; Tsubaki, K. J. Org. Chem. 2009, 74, 3940. (i) Takaishi, K.;
Sue, D.; Kuwahara, S.; Harada, N.; Kawabata, T.; Tsubaki, K. Tetrahedron
2009, 65, 6135.
† RIKEN.
‡ Kyoto Prefectural University.
(1) Recent reviews and articles: (a) Mishra, A.; Ma, C.-Q.; Bau¨erle, P.
Chem. ReV 2009, 109, 1141. (b) Banerjee, M.; Shukla, R.; Rathore, R. J. Am.
Chem. Soc. 2009, 131, 1780. (c) Lovett, J. E.; Hoffmann, M.; Cnossen, A.;
Shutter, A. T. J.; Hogben, H. J.; Warren, J. E.; Pascu, S. I.; Kay, C. W. M.;
Timmel, C. R.; Anderson, H. L. J. Am. Chem. Soc. 2009, 131, 13852. (d)
Grimsdale, A. C.; Chan, K. L.; Rainer, E.; Martin, R. E.; Jokisz, P. G.;
Holmes, A. B. Chem. ReV. 2009, 109, 897. (e) Ikeda, T.; Aratani, N.; Osuka,
A. Chem. Asian J. 2009, 4, 1248. (f) Mareda, J.; Matile, S. Chem.sEur. J.
2009, 15, 28. (g) Liu, Y.; Di, C.- a.; Du, C.; Liu, Y.; Lu, K.; Qiu, W.; Yu,
G. Chem.sEur. J. 2010, 16, 2231.
10.1021/ol100582v  2010 American Chemical Society
Published on Web 03/25/2010

Therefore, the potential of these orthogonal type oligomers has
yet to be fully exploited. Thus, we speculated that oligonaph-
thalenes consisting of nearly planar, tiered naphthalene rings
will behave as oligomers by a strong interaction between adja-
cent naphthalene rings and will show unique properties.
The dihedral angles for adjacent naphthalene rings in the
optimized structure of (R,S,R)-2 are 74.5° (C(1)-C(2)-
C(3)-C(4)), 97.2° (C(5)-C(6)-C(7)-C(8)), and 89.6° (C-
(9)-C(10)-C(11)-C(12)) (Figure 1). In contrast, each 1,1′-
the simplest tetramer, (R,S,R)-4, which is the bridged form
of (R,S,R)-2, the dihedral angles are 48.9° (C(1′)-C(2′)-
C(3′)-C(4′)), 48.6° (C(5′)-C(6′)-C(7′)-C(8′)), and 49.0°
(C(9′)-C(10′)-C(11′)-C(12′)),5 suggesting the possibility
of the extended conjugation of 4 or higher order bridged
oligonaphthalene 3. Based on the calculation results, we
synthesized 2,2′-bridged-1,1′-oligonaphthalenes and analyzed
their optical behaviors. These 1,1′-oligonaphthalenes have
continuous axial chiralities. The oxy-functional groups of
the alternating-chiral (R,S,R,S,R,S,...) form are arranged
linearly, whereas those of the homochiral (R,R,R,R,R,... or
S,S,S,S,S,...) form are arranged helically. Although examining
the homochiral form would be structurally interesting, herein
we selected the alternating-chiral form because it has the
potential to form ꢀ-barrel structures6 in the future (this point
is not discussed in this paper).
2,2′-Bridged naphthalene oligomers 6, 8, and 10 were
synthesized by the double Williamson ether synthesis using
dibromomethane or bromochloromethane and diols 5, 7, and
9, respectively (Scheme 1). These diols were prepared via
Scheme 1. Synthesis of Bridged Oligonaphthalenes
Figure 1. Optimized geometries obtained by DFT calculations at
the B3LYP/6-31G(d) level for the (A) open- and (B) bridged-form
oligonaphthalenes; Red axes denote R configuration and blue axes
denote S configuration.
axial rotation of oligonaphthalene 3 is limited, and these
naphthalene rings form an extreme-cisoid conformation by
the 2,2′-methylenedioxy bridges3,4 and peri-hydrogens. For
(3) For examples of 2,2′-bridged-1,1′-binaphthyls, see: (a) Ernesstimp-
son, J. E.; Daub, G.; Hayes, F. N. J. Org. Chem. 1973, 38, 1771. (b) Zhang,
M.; Schuster, G. B. J. Am. Chem. Soc. 1994, 116, 4852. (c) Deussen, H.-
J.; Hendrickx, E.; Boutton, C.; Krog, D.; Clays, K.; Bechgaard, K.; Persoons,
A.; Bjørnholm, T. J. Am. Chem. Soc. 1996, 118, 6841. (d) Schafer, L. L.;
Tilley, T. D. J. Am. Chem. Soc. 2001, 123, 2683. (e) Minatti, A.; Do¨tz,
K. H. Tetrahedron: Asymmetry 2005, 16, 3256. (f) Tu, T.; Maris, T.; Wuest,
J. D. J. Org. Chem. 2008, 73, 5255
.
(4) For examples of Gela¨nder-type molecules, see: (a) Kiupel, B.;
Niederalt, C.; Nieger, M.; Grimme, S.; Vo¨gtle, F. Angew. Chem., Int. Ed.
1998, 37, 3031. (b) Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.;
Moore, J. S. Chem. ReV. 2001, 101, 3893. (c) Hu, Z.-Q.; Chen, C.-F.
Tetrahedron 2006, 62, 3446. (d) Modjewski, M.; Lindeman, S. V.; Rathore,
repeated oxidative homodimerization reactions with a CuII-
amine complex and subsequent optical resolutions, and their
absolute configurations were determined by the exciton chi-
R. Org. Lett. 2009, 11, 4656
.
Org. Lett., Vol. 12, No. 8, 2010
1833

rality method2c,h,7 with respect to the two TPP(Zn) (tet-
raphenylporphyrin zinc complex) adducts (see the Supporting
Information). As the number of naphthalene units increased,
the solubility decreased. Among the diastereomers, alternat-
ing-chiral naphthalenes with the oxy-functional groups
concentrated on one side were less soluble because the dipole
moment of the entire molecule is larger than that of the other
diastereomers. Octamer 10 was the upper limit without
solubilizing substituted groups. These methylenedioxy bridges
were stable under heat, at least up to 80 °C, as well as under
moderate acidic or basic conditions. Moreover, epimerization
did not occur under these conditions.8
nm) became broader and the spectra was red-shifted, while
the change in the maximum absorption wavelength of the
short wavelength side (220-240 nm) was insignificant. These
results indicate that 1La (transition moment of the short axes
of the naphthalene rings) rather than 1Bb (transition moment
of the long axes of the naphthalene rings) is affected,9 and
adjacent naphthalene rings interact strongly in the rod
direction. Each diastereomer displayed similar characteristics,
but the spectral shape varied slightly (Figure S2, Supporting
Information). Open-form compounds 1, which are even
higher order oligonaphthalenes, did not exhibit a red-shift.2a,g
In Figure 2B, the CD intensity of (R)-6 was much higher
than that of (R)-5, although both had identical Cotton effect
patterns. Moreover, debenzylated analogues of (R)-6 showed
similar spectra (Figure S4, Supporting Information). There-
fore, the optical properties directly reflect differences in the
torsion angle of the naphthalene rings. Higher order (R,S,R)-8
and (R,S,R,S,R,S,R)-10 had CD intensities almost identical
to that of (R)-6. Because each naphthalene ring of 8 and 10
interacted with distant naphthalene rings, the CDs of S and
R binaphthalene units canceled each other on a whole. Figure
2C shows the emission spectra in solution. Along with the
absorption, the emission shifted bathochromically with the
number of naphthalene rings. Each compound showed rela-
tively high FL quantum yields in solution; φ310 of (R)-6 was
0.44, φ330 of (R,S,R)-8 was 0.79, and φ330 of (R,S,R,S,R,S,R)-
10 was 0.64. In contrast, φfl of the open-form dimers was
relatively low; φ310 of (R)-5 was 0.045,10 and φ310 for
compounds with methoxy groups instead of hydroxy groups
of (R)-5 was 0.16. The reason that bridged oligonaphthalenes
exhibited a high fluorescence is unknown. However, plausible
explanations include that the bridges provide the entire
molecule with rigidity, especially the constraint axial rotation,
or as suggested by the relatively small Stokes shifts (for
example, the Stokes shift of (R,S,R,S,R,S,R)-10 is 36 nm),
the dihedral angles of the naphthalene rings of the S0 state
are inherently similar to that of the S1 state,11 tand hus, the
excitation energy is more likely to be converted into
fluorescence than axial vibration. Moreover, the bridged
oligonaphthalenes strongly fluoresced in the solid state under
UV light (Figure 2D, φ ) 0.08-0.21); (R)-6 was pale blue
Figure 2 depicts the optical properties. Figure 2A shows
the absorption spectra; as the number of naphthalene rings
increased, the longer wavelength absorption region (320-400
(5) Dihedral angles of the two naphthalene rings in the X-ray crystal
structure of the bridged dimers are observed slightly above 50°; see :(a)
Deussen, H.-J.; Boutton, C.; Thorup, N.; Geisler, T.; Hendrickx, E.;
Bechgaard, K.; Persoons, A.; Bju`rnholm, T. Chem.sEur. J. 1998, 4, 240.
(b) Schneider, J. F.; Nieger, M.; Na¨ttinen, K.; Lewall, B.; Niecke, E.; Do¨tz,
K. H. Eur. J. Org. Chem. 2005, 1541.
(6) Litvinchuk, S.; Bollot, G.; Mareda, J.; Som, A.; Ronan, D.; Shah,
M. R.; Perrottet, P.; Sakai, N.; Matile, S. J. Am. Chem. Soc. 2004, 126,
10067.
(7) (a) Harada, N.; Nakanishi, K. Acc. Chem. Res. 1972, 5, 257. (b)
Koslowski, A.; Sreerama, N.; Woody, R. W. In Circular Dichroism:
Principles and Applications; Berova, N., Nakanishi, K., Harada, N., Eds.;
Wiley-VCH: New York, 2000; pp 55-95.
(8) Racemization barriers of a simple compound have been reported
(∆Gq ) 33 kcal/mol in decalin), see: Park, J.-W.; Ediger, M. D.; Green,
M. M. J. Am. Chem. Soc. 2001, 123, 49.
Figure 2. Optical properties of bridged oligonaphthalene (R)-6 (red
line), (R,S,R)-8 (black line), (R,S,R,S,R,S,R)-10 (aqua line), and
nonbridged (R)-5 (green line): (A) absorption spectra in 1,4-dioxane;
(B) CD spectra in 1,4-dioxane; (C) fluorescence spectra in 1,4-
dioxane (excitation wavelengths are 310 nm for 5 and 6 and 330
nm for 8 and 10); (D) pictures of solid-state fluorescence under
UV light (254 nm).
(9) Rosini, C.; Superchi, S.; Peerlings, H. W. I.; Meijer, E. W. Eur. J.
Org. Chem. 2000, 61.
(10) Hydroxy groups of BINOL are involved in its fluorescence
quenching, see: Wang, Q.; Chen, X.; Tao, L.; Wang, L.; Xiao, D.; Yu,
X.-Q.; Pu, L. J. Org. Chem. 2007, 72, 97.
(11) Dihedral angles of the S0 and S1 state have been studied; see:
Fujiyoshi, S.; Takeuchi, S.; Tahara, T. J. Phys. Chem. A 2004, 108, 5938.
1834
Org. Lett., Vol. 12, No. 8, 2010

Figure 3 shows the calculated HOMO, LUMO, and
dihedral angles of (R,S,R,S,R,S,R)-10. The dihedral angles
held steady at 48-49°, which is between planar and orthog-
onal, but the oligonaphthalene skeleton had minor deviations
toward the 2,2′-bridges. Except for the bridges and terminal
benzyloxy groups, both the HOMO and LUMO are spread
throughout the molecule12 but gravitate somewhat toward
the central naphthalene rings. These bridged oligonaphtha-
lenes show a peculiar conjugation, that is, molecular-wide
graduated-twisted π-conjugation.
In conclusion, we developed multibridged, chiral oligo-
naphthalenes. The 2,2′-methylenedioxy bridges gave them
a continuous extremely cisoid conformation and, conse-
quently, a notably extensive conjugation in the rod direction
and high fluorescence. Although a lengthy process is
currently required to synthesize these types of oligonaph-
thalenes with a specified number of naphthalene units and
axial chirality, these compounds and their analogues may
contribute to the development of photo- and electrochemistry
with asymmetric properties.
Acknowledgment. This work was supported in part by
the Mitsubishi Chemical Corp. Fund. We are also grateful
to members of the molecular characterization team at RIKEN
and JASCO Corp. for the spectral measurements.
Supporting Information Available: Synthetic details,
determination of the absolute configuration, and structural
and optical characterization of various compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Figure 3. Optimized structure with the HOMO, LUMO, and
consistentdihedralanglesofadjacentnaphthaleneringsof(R,S,R,S,R,S,R)-
10 obtained by DFT calculations at the B3LYP/6-31G(d) level.
(λmax,emm ) 370 nm), (R,S,R)-8 was light blue (λmax,emm
)
OL100582V
416 nm), and (R,S,R,S,R,S,R)-10 was yellow-green (λmax,emm
) 443 nm). Figure S5 (, Supporting Information) shows the
solid-state fluorescence spectra.
(12) Both the HOMO and LUMO of an open form octinaphthalene are
localized on about three naphthalene rings (Figure S6, Supporting Informa-
tion).
Org. Lett., Vol. 12, No. 8, 2010
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