Synthesis and fluorescence emission behavior of novel anti-[2.n] (3,9)phenanthre nophanes

Article abstract of DOI:10.1016/S0040-4039(99)02306-0

Novel syn- and anti-[2.n](3,9)phenanthrenophanes (n=3,4) were successfully prepared by the intramolecular [2+2] photocycloaddition of α,ω-bis(3-vinyl-9-phenanthryl)alkanes. The anti-isomer of n=3 was found to give excimer fluorescence in spite of the only

Full text of DOI:10.1016/S0040-4039(99)02306-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 1419–1423
Synthesis and fluorescence emission behavior of novel
anti-[2.n](3,9)phenanthrenophanes
Yosuke Nakamura, Takahiro Fujii and Jun Nishimura ∗
Department of Chemistry, Gunma University, Tenjin-cho, Kiryu, Gunma 376-8515, Japan
Received 26 October 1999; accepted 3 December 1999
Abstract
Novel syn- and anti-[2.n](3,9)phenanthrenophanes (n=3,4) were successfully prepared by the intramolecular
[2+2] photocycloaddition of α,ω-bis(3-vinyl-9-phenanthryl)alkanes. The anti-isomer of n=3 was found to give
excimer fluorescence in spite of the only partially overlapping phenanthrene rings, although that of n=4 gave
monomer fluorescence. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: cyclophane synthesis; [2+2] photocycloaddition; photochemistry; fluorescence; phenanthrenophanes.
In contrast with aromatic hydrocarbons such as benzene, naphthalene, and pyrene, phenanthrene
fails to give excimer fluorescence in solution.1 Even 1,3-diphenanthrylpropanes mainly afford
monomer fluorescence,2 contrary to Hirayama’s rule.3 We have succeeded in the first observation
of excimer fluorescence almost free from monomer fluorescence at room temperature for syn-
[2.2](1,6)phenanthrenophanes 1 whose phenanthrene rings are held almost in parallel.4 For phenanthrene,
however, the relationship between the arrangement of aromatic nuclei and fluorescence behavior has so
far hardly been clarified, mainly due to the difficulty in the synthesis and separation of model compounds.
In particular, the fluorescence for anti-phenanthrenophanes in which the phenanthrene rings only partially
overlap had been unknown until our recent reports on anti-2,5 although Staab et al. observed excimer
fluorescence from a mixture of syn- and anti-[2.2](2,7)phenanthrenophane in a fluorene host crystal
at 4.2 K.6,7 Surprisingly, anti-2 also provided excimer fluorescence with a slight blue-shift relative to
syn-2.5 This result prompted us to examine anti-phenanthrenophanes consisting of phenanthrene rings
which overlap even less with each other and to clarify the minimum overlap essential for the formation
of excimer. Here we report the preparation of anti-[2.n](3,9)phenanthrenophanes 3 (n=3, 4), regarded
as a candidate for such phenanthrenophanes, by the intramolecular [2+2] photocycloaddition of the
corresponding vinylphenanthrene derivatives 4 and their fluorescence behavior at room temperature.
∗
Corresponding author.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)02306-0
tetl 16234

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α,ω-Bis(3-vinyl-9-phenanthryl)alkanes 4a and b were prepared by a synthetic sequence entirely
different from that for the precursor of 2, as shown in Scheme 1, which utilizes the photochemical
transannular reaction of stilbene derivatives as a key step. The intramolecular photoreaction of 4a and b,
carried out in benzene (2 mM) using a 400 W high-pressure mercury lamp through a Pyrex filter under
a nitrogen atmosphere, gave 3a and b in 44 and 40% yields, respectively, as a mixture of syn- and anti-
isomers. The syn:anti-isomer ratios were determined as ca. 1:6 and 1:3 for 3a and b, respectively, on the
basis of the peak areas of 1H NMR spectra. No interconversion between the two isomers was observed
at room temperature for at least several months.
Scheme 1.
Each isomer of 3a and b was successfully separated by repeated GPC and/or HPLC. Their structures
1
were mainly determined by H NMR spectroscopy.8 In syn-3a, only eight sets of aromatic proton
peaks are observed and are generally high-field shifted compared to those of 4a. The two methine
protons of the cyclobutane ring appear as an equivalent peak. These results apparently indicate the syn-
conformation where the two chromophores are well overlapped. The configuration of the cyclobutane
ring was determined as depicted in Scheme 1 on the basis of a NOESY experiment; NOE interaction
is detected between the cyclobutane methylene protons and H4 protons of the phenanthrene ring.9 On
1
the contrary, the H NMR spectrum of anti-3a suggest a lower symmetrical structure; sixteen peaks
are observed for the aromatic protons and two peaks for the methine protons. Among the aromatic
protons, H1, H2, H4, H5, and H10 are high-field shifted relative to 4a, while H6–8 are hardly shifted.
Especially, the H10 (δ 5.99, 6.03) and H1 protons (δ 6.47, 6.63) in anti-3a resonate at higher fields than
those in syn-3a (H10: δ 6.99; H1: δ 6.98), indicating that these protons are located above the opposite
phenanthrene ring. Therefore, anti-3a is expected to adopt a conformation where one six-membered ring

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Fig. 1. Fluorescence spectra of: (a) syn-3a; (b) anti-3a; (c) syn-3b; and (d) anti-3b in cyclohexane at room temperature upon
280 nm excitation

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of one phenanthrene nucleus is mainly overlapped with that of the other phenanthrene nucleus. The
1H NMR spectral patterns of syn- and anti-3b are similar to those of syn- and anti-3a, respectively.
The optimized structures of 3a and b were determined by the MM2 calculations,10 since single crystals
suitable for X-ray crystallographic analysis have not been obtained. The two phenanthrene rings are
arranged almost in parallel for both isomers of 3a, but they differ in the extent of their overlap; in anti-
3a, they are partially overlapped in the manner described above, whereas they are fully overlapped in
syn-3a.
The absorption spectra of syn- and anti-3a,b in cyclohexane exhibit considerable broadening and a
red-shift in comparison with that of phenanthrene.11 Among these phenanthrenophanes, however, we
cannot find marked differences corresponding to the structural differences.
Fig. 1 illustrates the fluorescence spectra of syn- and anti-3a,b in cyclohexane at room temperature.11
The spectrum of syn-3a is composed of a broad structureless emission, similar to that of 1 reported
previously,4 with a maximum at 420 nm red-shifted compared to 1 (410 nm). Interestingly, anti-3a
also exhibits a similar broad emission without any vibrational structures, where the maximal position
(407 nm) is remarkably blue-shifted relative to syn-3a. As far as can be judged from the appearance,
it is reasonable to interpret these emissions as excimer fluorescence rather than monomer fluorescence,
although some kinetic investigation is necessary for the precise assignment. The blue-shift in anti-3a
relative to syn-3a is ascribed to the lower stabilization in the excimer due to the smaller overlap of the
two aromatic rings. For syn-3b, similar broad structureless emission is observed with further blue-shift
(λmax=395 nm). This is also assignable as excimer fluorescence. The blue-shift in syn-3b relative to syn-
3a is apparently derived from the difference in the bridging length; in the excited state of syn-3b, the
two phenanthrene rings cannot approach each other as closely as in syn-3a. In contrast, anti-3b affords
apparent vibrational structures characteristic of monomer fluorescence. The arrangement of anti-3b no
longer appears to be favorable for the excimer formation.
In summary, we have succeeded in the preparation of syn- and anti-[2.n](3,9)phenanthrenophanes 3a
(n=3) and 3b (n=4) by the intramolecular [2+2] photocycloaddition and the separation of both isomers.
Surprisingly, excimer fluorescence was observed even for anti-3a, where the phenanthrene nuclei overlap
mainly at the single six-membered ring.
Acknowledgements
This work was financially supported by a Grant-in-Aid from the Ministry of Education, Science, Sports
and Culture, Japan.
References
1. Chandross, E. A.; Thomas, H. J. Am. Chem. Soc. 1972, 94, 2421.
2. Zachariasse, K. A.; Busse, R.; Schrader, U.; Kühnle, W. Chem. Phys. Lett. 1982, 89, 303.
3. Hirayama, F. J. Chem. Phys. 1965, 42, 3163.
4. Nakamura, Y.; Tsuihiji, T.; Mita, T.; Minowa, T.; Tobita, S.; Shizuka, S.; Nishimura, J. J. Am. Chem. Soc. 1996, 118, 1006.
5. Nakamura, Y.; Fujii, T.; Sugita, S.; Nishimura, J. Chem. Lett. 1999, 1039.
6. Staab, H. A.; Haenel, M. Chem. Ber. 1973, 106, 2190.
7. Schweitzer, D.; Colpa, J. P.; Behnke, J.; Hausser, K. H.; Haenel, M.; Staab, H. A. Chem. Phys. 1975, 11, 373.
8. The spectral data of 3a and 3b are as follows: syn-3a: m.p.>300°C; 1H NMR (CDCl3, 500 MHz) δ 8.09 (2H, d, J=7.3 Hz,
H5), 7.74 (2H, s, H4), 7.65 (2H, d, J=7.0 Hz, H8), 7.15 (4H, m, H6 and H7), 6.99 (2H, s, H10), 6.98 (2H, d, J=8.0 Hz,
H1), 6.59 (2H, d, J=8.0 Hz, H2), 4.43 (2H, m), 3.65 (2H, m), 3.01 (1H, m), 2.94 (2H, m), 2.78 (4H, m), 2.34 (1H, m);

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13C NMR (CDCl3, 125 MHz) δ 137.60, 133.67, 130.30, 129.83, 129.68, 129.59, 127.53, 126.62, 126.58, 125.18, 124.96,
124.08, 122.58, 121.79, 47.15, 34.53, 29.70, 26.82, 20.86; HRMS (FAB) calcd for C35H28 (M+): 448.2191; found: 448.2189.
anti-3a: m.p. >300°C; 1H NMR (CDCl3, 500 MHz) δ 8.37 (1H, d, J=7.3 Hz, H5), 8.22 (1H, d, J=8.0 Hz, H5), 8.14 (1H, d,
J=7.6 Hz, H8), 8.09 (1H, d, J=7.9 Hz, H8), 7.61 (4H, m, H6 and H7), 7.37 (1H, s, H4), 7.18 (1H, d, J=8.0 Hz, H2), 7.01
(1H, s, H4), 6.72 (1H, d, J=7.9 Hz, H2), 6.63 (1H, d, J=8.2 Hz, H1), 6.47 (1H, d, J=8.2 Hz, H1), 6.03 (1H, s, H10), 5.99
(1H, s, H10), 4.37 (1H, m), 4.24 (1H, m), 3.56 (2H, m), 2.80 (4H, m), 2.55 (4H, m); 13C NMR (CDCl3, 125 MHz) δ 137.09,
136.66, 134.44, 134.26, 131.17, 131.04, 130.66, 130.46, 129.45, 129.39, 129.34, 129.19, 127.53, 125.80, 125.53, 125.32,
125.24, 125.16, 124.76, 124.57, 124.25, 124.23, 124.00, 123.33, 123.26, 121.30, 48.28, 46.76, 31.67, 31.50, 28.86, 21.09,
19.91; HRMS (FAB) calcd for C35H28 (M+): 448.2191; found: 448.2184. syn-3b: m.p. >300°C; 1H NMR (CDCl3, 500 MHz)
δ 8.27 (2H, d, J=8.3 Hz, H5), 8.07 (2H, s, H4), 7.56 (2H, d, J=8.3 Hz, H8), 7.21 (2H, t, J=7.5 Hz, H6), 7.13 (2H, t, J=7.5
Hz, H7), 7.03 (2H, d, J=7.9 Hz, H1), 6.93 (2H, s, H10), 6.69 (2H, d, J=7.9 Hz, H2), 4.43 (2H, m), 3.15 (2H, m), 2.94 (2H,
m), 2.78 (2H, m), 2.40 (2H, m), 1.84 (2H, m), 1.64 (2H, m); 13C NMR (CDCl3, 125 MHz) δ 138.12, 135.96, 130.42, 130.00,
129.60, 128.77, 128.39, 126.73, 125.63, 125.33, 125.26, 123.81, 122.41, 121.26, 46.80, 31.80, 29.70, 30.46, 21.27; HRMS
(FAB) calcd for C36H30 (M+): 462.2348; found: 462.2351. anti-3b: m.p. 290°C (dec); 1H NMR (CDCl3, 500 MHz) δ 8.42
(1H), 8.28 (1H), 7.95 (1H), 7.92 (1H), 7.58 (6H, m), 7.40 (1H, d, J=8.3 Hz, H2), 6.94 (1H, d, J=8.2 Hz, H2), 6.92 (1H, d,
J=7.9 Hz, H1), 6.77 (1H, d, J=7.9 Hz, H1), 6.30 (1H, s, H10), 6.29 (1H, s, H10), 4.43 (1H, m), 4.29 (1H, m), 3.15 (2H, m),
2.99 (1H, m), 2.78 (1H, m), 2.65 (1H, m), 2.53 (1H, m), 2.23 (2H, m), 1.78 (2H, m), 1.55 (2H, m); 13C NMR (CDCl3, 125
MHz) δ 137.80, 137.47, 135.96, 135.85, 131.61, 131.58, 130.11, 129.91, 129.44, 129.36, 128.57, 128.30, 127.49, 125.63,
125.59, 125.53, 125.50, 125.47, 125.14, 125.08, 124.68, 124.48, 124.45, 123.94, 123.79, 122.92, 122.86, 120.27, 47.94,
46.44, 31.29, 30.78, 30.63, 21.39, 20.28; HRMS (FAB) calcd for C36H30 (M+): 462.2348; found: 462.2343.
9. The isomer possessing a cyclobutane ring directed to the H2-side seems to be only slightly formed, but its isolation and
characterization has been unsuccessful.
10. MM2 calculations were performed by CS Chem 3D Pro Version 4.0 (Cambridge Soft Corporation).
11. The absorption and fluorescence spectra were measured for the solutions in the range of 10−5–10−4 M. The fluorescence
excitation spectra are in good agreement with the corresponding absorption spectra in all cases.