C O M M U N I C A T I O N S
nπ* transition of the 1E isomer, has its peak absorption at λ )
90 nm. Additional spectral changes are seen in the regions of the
4
ππ* transitions of both isomers at λ < 350 nm, where the absorption
by the (E) isomer is again stronger than that of the (Z) isomer.
Figure 3. Measured absorbances of a solution of 1 at λ ) 400 nm (blue)
1
and λ ) 490 nm (red) in the photostationary states after alternating
2
irradiation at λ ) 385 and λ ) 520 nm in repeated switching cycles.
AB requires irradiation in the nπ* band in the visible in one
direction and in the ππ* band in the UV in the other. The
photoisomerization quantum yields of 1 are substantially higher in
both directions than in the case of AB, where ΦZfE ) 53% and
9
Φ
EfZ ) 24%. Moreover, a >90% photoconversion yield cannot
Figure 2. UV/vis absorption spectra of 1Z, 1E, and the photostationary
state (PSS385) in n-hexane (blue, red, and green, respectively). The inset
be achieved in the case of AB to our knowledge. Unlike AB, the
shows the S
1
(nπ*) region on an enlarged scale, with the recorded spectra
nπ* absorption of the (E) isomer of 1 is stronger than that of the
(blue and green circles), the fitted log-normal spectra (blue and green lines),
(Z) isomer; this was reconciled by time-dependent density functional
and the extracted (E) isomer spectrum (red line). The thin black lines show
the evolution of the spectrum in the course of the thermal back-isomerization
from the PSS385 after ∆t ) 2, 4, 6, 10, and 15 h.
calculations on the excited states. The 4.5 h thermal lifetime of 1E
at room temperature is not a drawback especially for applications
involving fast repeated forward and backward switching cycles or
at lower temperatures.
In the observed photostationary state at λ ) 385 nm (PSS385),
the (E) isomer is clearly predominant. By monitoring the absorption
in the dark, its thermal lifetime was found to be τ ) (4.5 ( 0.1) h
at 28.5 °C. To elucidate the spectrum for pure 1E and to determine
the quantitative (Z) to (E) conversion yield, we fitted the 1Z
spectrum using an appropriate log-normal model function (see
Cyclic ABs, which attract much attention as shape-switchable
1
0
molecules, show a tendency for unconventional properties when
1
1
subject to strain. The ring structures alter the quantum yields,
and distorted nonplanar geometries shift the nπ* bands of the
1
2
isomers. Except for 1, however, we are aware of only one cyclic
(Z)-AB that is thermodynamically favored over its (E) isomer
without a badly compromised photoresponse, a highly constrained
7
Figure 2). We then fitted the spectrum of the PSS385 using a
superposition of the suitably scaled 1Z spectrum and a second log-
normal model function for the 1E absorption. The resulting
spectrum for the pure 1E isomer is displayed in Figure 2. From
the absorption coefficients in the PSS385 compared to the pure
isomers, the (Z) to (E) conversion was derived to be Γ ) (92 ( 3)
1
3
azobenzenophane. Considering its distinctive structural change
and the favorable properties of its nπ* states, 1 appears to be an
1
4
ideal functional unit for a molecular tweezer, which would be
closed in its off-form (Z), opened by photoconversion to the (E)
isomer, and returned to closed by the reverse photoconversion.
%
. Conversely, using light at λ ) 520 nm, the (E) isomer was
quantitatively (∼100%) switched back to the (Z) isomer. Moreover,
Acknowledgment. This work has been supported by the
Deutsche Forschungsgemeinschaft within the Sonderforschungs-
bereich 677 “Function by Switching” (subproject A1).
the photoisomerization quantum yields in the forward and back
8
directions, which we investigated using the method of Rau et al.,
are ΦZfE ) (72 ( 4)% and ΦEfZ ) (50 ( 10)%. The severe
constraints by the ethylenic bridge of our title compound do not
appear to hinder its E-Z photoisomerization.
Supporting Information Available: Experimental protocol, X-ray
diffraction results for 1Z, and calculated electronic ground state
structures of 1Z and 1E. This material is available free of charge via
the Internet at http://pubs.acs.org.
For a sustainable application as a molecular photoswitch, a
compound must have low photochemical fatigue to allow for large
numbers of switching cycles. Monitoring the UV/vis absorptions
of 1 after many repeated alternating irradiation cycles at λ ) 385
and at λ ) 520 nm, respectively, we were unable to detect any
signs for photodegradation (Figure 3). The excellent photostability
of the compound is vividly demonstrated moreover by the fact that
we could use one and the same batch for all our photoswitching
experiments made in the course of half a year.
References
(1) Feringa, B., Ed. Molecular switches; Wiley-VCH: Weinheim, 2001.
(
2) Balzani, V.; Credi, A.; Venturi, M. Molecular deVices and machines; Wiley-
VCH: Weinheim, 2008.
(
(
(
(
3) Duval, H. Bull. Soc. Chim. Fr. 1910, 7, 727.
4) Paudler, W. W.; Zeiler, A. G. J. Org. Chem. 1969, 34, 3237.
5) Tauer, E.; Machinek, R. Liebigs. Ann. 1996, 1213.
6) Ahlrichs, R.; B a¨ r, M.; H a¨ ser, M.; Horn, H.; K o¨ lmel, C. Chem. Phys. Lett.
1989, 162, 165.
In conclusion, the bridged AB derivative 1 has much superior
spectroscopic properties compared to the parent AB molecule and
other commonly used AB derivatives. Most importantly, the
(
7) Siano, D. B.; Metzler, D. E. J. Chem. Phys. 1969, 51, 1856.
8) Rau, H.; Greiner, G.; Gauglitz, G.; Meier, H. J. Phys. Chem. 1990, 94,
6523.
(
(
9) Rau, H. J. Photochem. 1984, 26, 221.
respective S
absorptions at λ
1
(nπ*) bands of 1Z and 1E are well resolved (peak
) 404 and λ ) 490 nm), in striking contrast to
(
10) M u¨ ri, M.; Schuermann, K. C.; De Cola, L.; Mayor, M. Eur. J. Org. Chem.
Z
E
2009, 2562.
(
(
11) Norikane, Y.; Tamaoki, N. Eur. J. Org. Chem. 2006, 1296.
12) Janus, K.; Sworakowski, J. J. Phys. Chem. B 2005, 109, 93.
AB, where the (Z) and (E) nπ* bands are practically coincident.
Efficient switching of 1 in the (Z) f (E) and (E) f (Z) directions
can therefore be accomplished with visible light via one or the other
nπ* band, whereas conversion between the (E) and (Z) isomers of
(13) Norikane, Y.; Katoh, R.; Tamaoki, N. Chem. Commun. 2008, 1898.
(
14) Kl a¨ rner, F. G.; Kahlert, B. Acc. Chem. Res. 2003, 36, 919.
JA906547D
J. AM. CHEM. SOC. 9 VOL. 131, NO. 43, 2009 15595