refluxing dichloromethane or THF for several days resulted
only in the recovery of 8a without the formation of 6a. In
contrast, with reflux in 1,2-dichloroethane in the presence
of the same acid catalyst for 10 days, desired 6a was suc-
cessfully obtained along with recovered 8a. The purification
of the reaction mixture by column chromatography (silica
gel) and GPC led to the isolation of 6a as a single isomer in
13% yield. As described below, isolated 6a is the anti-isomer,
and no syn-isomer was detected in the reaction mixture.
isomer they are partially overlapped, in the manner suggested
by the H NMR spectra as mentioned above (Figure 2).
1
The intramolecular dehydration of 8b under similar condi-
tions gave 6b as a mixture of syn- and anti-isomers in 5%
yield, in contrast with 6a. The syn/anti-isomer ratio was de-
termined as 2:5 on the basis of 1H NMR integration. Under
the same conditions, diol 9 also produced both syn- and anti-
isomers of 7 in a ratio of 1:4 (total 6% yield). It was quite
difficult to separate and isolate each isomer of 6b and 7,
especially the syn-isomers, but a small amount of both
isomers was isolated by repeated preparative HPLC (ODS
column, methanol).
Figure 2. Optimized structures of (a) anti-6b and (b) anti-7
determined by MM2 calculations.
The absorption spectra of 6a, 6b, and 7, measured in
cyclohexane at room temperature, exhibit considerable
broadening and shift relative to those of phenanthrene and
3,9-dimethylphenanthrene. These observations indicate that
the two phenanthrene nuclei interact electronically with each
other in the ground state. Roughly speaking, the spectral
features of syn-6b and syn-7 with full overlap are similar to
each other, and those of anti-6a, anti-6b, and anti-7 with
partial overlap are also similar to one another.
The fluorescence spectra of 6a, 6b, and 7 in cyclohexane
at room temperature are shown in Figure 3, which also shows
the spectrum of 3,9-dimethylphenanthrene as a reference. The
fluorescence excitation spectra, on monitoring these emis-
sions, were in good agreement with the absorption spectra,
obviously indicating that these emissions originate from the
phenanthrenophanes. No decomposition was observed after
fluorescence measurements.
The fluorescence spectrum of anti-7 is considerably
different from those of the other phenanthrenophanes and
relatively similar to that of 3,9-dimethylphenanthrene, indi-
cating monomer-like emission. The quite small overlap
between phenanthrene rings in anti-7 cannot be sufficient
for the excimer formation, as also demonstrated in anti-5
reported previously.10
In contrast, anti-6a, syn- and anti-6b, and syn-7 exhibit
red-shifted broad emission without vibrational structures.
Since the large peak shift is characteristic of excimer
fluorescence for most aromatic compounds,15 it is reasonable
to assign these broad structureless emissions to intramolecular
excimer fluorescence.
The structures of oxa[3.n]phenanthrenophanes 6a, 6b, and
1
7 obtained were mainly determined by H NMR spectros-
copy. In both syn-isomers with Cs symmetry and anti-isomers
with C2 symmetry, eight sets of aromatic proton peaks are
observed. However, their spectral patterns are remarkably
different from each other.
The aromatic protons of syn-6b and syn-7 are almost equally
high-field shifted compared to those of precursors such as
8b and 9, as observed in the syn-phenanthrenophanes report-
ed previously.7-10 This observation demonstrates that the two
phenanthrene nuclei are wholly overlapped with each other.
In contrast, the aromatic protons of anti-6a,b and anti-7
range over a wider region. Among the aromatic protons of
anti-6a,b, H6-8 are hardly shifted relative to 8a,b, while
H1 and H10 are extremely high-field shifted, indicating that
these protons are located above the opposite phenanthrene
ring. Therefore, anti-6a,b are expected to adopt a conforma-
tion where one six-membered ring of one phenanthrene
nucleus is mainly overlapped with that of the other phenan-
threne nucleus, as similar to anti-4a,b reported previously.9
The 1H NMR spectrum of anti-7 exhibits a pattern rather
different from anti-6a,b. Among the aromatic protons in anti-
7, H5-9 are hardly shifted relative to precursor 9, whereas
the H1 and H2 protons resonate at much higher fields than
those in 9 and syn-7, indicating that these protons are located
above the opposite phenanthrene ring. These observations
clearly contrast with those for anti-6a,b, and suggest that
anti-7 possesses a less overlapped structure than anti-6a;
similar to anti-5,10 the overlap in anti-7 is only about half
of the six-membered ring on the C1-C4 side, as also
demonstrated by MM2 calculations (see below).
The excimer fluorescence of syn-7 is the most red-shifted
(λmax ) 432 nm) among the excimers of phenanthrenophanes
so far observed.7-10 This large peak shift in syn-7 is ascribed
to its arrangement where the distance between the two fully
overlapped phenanthrene nuclei is moderately short due to
the bridging by two trimethylene-type linkages leading to
larger stabilization energy in the excited state and larger
repulsion energy in the ground state. The trimethylene-type
According to the optimized molecular structures of 6a,
6b, and 7 by MM2 calculations, the two phenanthrene rings
are arranged almost in parallel for all of 6a,b and 7, though
the distance between C9 atoms is relatively long in 6b carry-
ing a tetramethylene linkage. Apparently, syn- and anti-isomers
differ in the extent of overlap of the phenanthrene rings. They
are fully overlapped in the syn-isomer, whereas in the anti-
(15) Klo¨pffer, W. In Organic Molecular Photophysics; Birks, J. B., Ed.;
John Wiley and Sons: New York, 1973; Vol. 1, pp 357-401.
Org. Lett., Vol. 7, No. 15, 2005
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Nakamura, Yosuke
