J. Vicario thanks the Departamento de Educacio´n,
Universidades e Investigacio´n del Gobierno Vasco for a post-
doctoral fellowship.
Notes and references
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2 Molecular Switches, ed. B. L. Feringa, Wiley-VCH, Weinheim, 2001.
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4 (a) A. M. Stevens and C. J. Richards, Tetrahedron Lett., 1997, 38, 7805;
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W. D. Hounshell, C. A. Johnson, A. Guenzi, F. Cozzi and K. Mislow,
Proc. Natl. Acad. Sci. USA, 1980, 77, 6961; (d) F. Cozzi, A. Guenzi,
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Scheme 2 Full 360u cycle for the non-symmetrical fluorene motor 6.
5 T. C. Bedard and J. S. Moore, J. Am. Chem. Soc., 1995, 117,
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6 (a) A. R. Pease, J. O. Jeppesen, J. F. Stoddart, Y. Luo, C. P. Collier and
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(366 nm, Hg-lamp) at 240 uC of a toluene-d8 solution of stable cis-
alkene 6 afforded a PSS comprising trans-unstable/cis-stable with a
ratio 77/23 (Scheme 2), determined by integration of pseudoequa-
torial and pseudoaxial methyl doublet signals. No other possible
cis-unstable or trans-stable isomers were observed. Heating the
sample to rt resulted exclusively in transformation of trans-
unstable 6 to trans-stable 6 (Scheme 2) (monitored by integration
of the methoxy singlets, since the ratio cis-stable : trans-stable is
equivalent to the ratio cis-stable : trans-unstable at the PSS).
Thermal cis–trans isomerisation was excluded, which confirms the
unidirectionality of the thermal irreversible isomerisation. Similar
results were obtained when a second irradiation of the resulting cis-
stable–trans-stable mixture was carried out at 240 uC. The ratio
trans-unstable : cis-stable at the PSS is known from the previous
irradiation experiment and the ratio cis-unstable : trans-stable can
be calculated from the integration of the doublet signals for the
pseudoequatorial and pseudoaxial methyl substituents. Heating of
the irradiated sample to complete the 360u cycle showed in this
case transformation of cis-unstable exclusively to cis-stable,
demonstrating the unidirectionality of the overall 360u rotary cycle.
It should be noted that fast rotation is observed in spite of using
a fluorene lower half in motor 5, which due to the rigid planar
structure does not allow bending and therefore makes difficult for
the upper naphthalene unit to pass along the lower half. It is
envisaged that the use of more flexible lower halves might increase
the speed of rotation even further. The new light driven molecular
motor described here is not only the fastest, but also the smallest
reported so far.
7 T. Muraoka, K. Kinbara, Y. Kobayashi and T. Aida, J. Am. Chem.
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´
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17 Chemical formula: C27H20; M: 344.46 g mol21; unit cell dimensions
3
a 5 32.404(2) A, b 5 8.9496(4) A, c 5 8.9496(4) A, V 8138.3(8) A ;
˚
˚
˚
˚
T 100(1) K; trigonal R3; Z 5 8; m (Mo Ka), cm21 5 0.71073 A;
˚
2
wR(F2) 5 0.1029 for 3991 reflections with Fo ¢ 0 and R(F) 5 0.0412
for 3399 reflections with Fo ¢4.0 s(Fo) and 324 parameters. CCDC
in CIF or other electronic format.
18 Note added in proof: for a recent comprehensive review on molecular
rotors, see: G. S. Kottas, L. I. Clarke, D. Horinek and J. Michl, Chem.
Rev., 2005, 105, 1281.
5912 | Chem. Commun., 2005, 5910–5912
This journal is ß The Royal Society of Chemistry 2005