Communications
On the basis of these data, we can speculate about the
overall light-induced unfolding process. The E!Z isomer-
ization of an interior azobenzene unit would cause immediate
unfolding, that is, only one switching event would denature
the helix completely. However, this isomerization seems to
constitute a less likely pathway in view of the slower kinetics
of unfolding, despite the increased number of internal
azobenzenes for longer oligomers, as well as the obtained
average number of roughly two Z-azobenzenes in the PSS.
Instead, it appears that unfolding is predominantly induced by
unwinding the ends of the helix. If E!Z isomerization occurs
at one terminus in a given oligomer with n repeat units, it
leads to an oligomer with an effectively decreased chain
length (nꢀ1), thereby resembling its shorter “cousin” (see
Figure S16[8]). Hence, only one isomerization event is neces-
sary to completely unfold the shortest oligomer 105, while on
average two isomerization events are necessary to unfold the
longer oligomers 126 and 147. Note that in the case of 147, even
two isomerization events at the termini would result in
formation of a quasi-105, which should still be partially folded,
thus explaining the residual CD signal in the PSS.[15] The
increased efficiency of E!Z isomerization at the helix
termini can be explained by the lower enthalpic cost, that is,
breaking of only one p–p stacking contact, while in the case of
isomerizing internal as well as central azobenzene units, at
least two such interactions, either to adjacent intraturn or
interturn neighbors, have to be broken. In view of these
arguments, it seems reasonable to assume that the isomer-
ization events depend on one another, which means that
isomerization is facilitated in proximity to a Z isomer.
Considering that there should be no preference for exciting
a particular azobenzene unit, the most likely overall process
involves initial excitation of any azobenzene moiety in the
helix, followed by rapid energy transfer between the helically
stacked chromophores to a trap site, that is, the termini,
undergoing the most efficient subsequent E!Z isomeriza-
tion. Consequently, this event shifts the trap site towards the
inside and leads to the progression of isomerization events
towards the helix interior.
Figure 2. UV/Vis absorption (left) and CD spectra (right) of photo-
chemical E!Z isomerization of oligomers 105 (top), 126 (middle), and
147 (bottom) during the course of irradiation at lirr =358 nm in CH3CN
at 258C. The insets in the UV/Vis spectra show an enlargement of the
increasing n!p* absorption band of the azobenzene units.
dramatic decrease in signal intensity upon irradiation asso-
ciated with the loss of excess helicity and therefore indicating
a depopulation of the overall helical conformation, that is,
helix denaturation.[13] While in the case of the longest
oligomer 147 a small Cotton effect remains upon reaching
the photostationary state (PSS), the CD signals for the shorter
oligomers 126 and 105 vanish almost completely; however, in
the case of 105 the denaturation occurs on a shorter time scale.
To better understand the complex unfolding process in
our oligomers, the E!Z isomerization event, which is evident
from UV/Vis absorption spectra, and the denaturation event,
which is reflected in the CD spectra, were analyzed separately
(Table 1), although both processes occur in concert. Kinetic
analysis shows that both the rates of the E!Z isomerization
as well as the denaturation increase with decreasing oligomer
length, that is, in the order 147!126!105.[14] As the number
and location of the isomerized azobenzene moieties should be
critical for the denaturation process, the exact local compo-
Importantly, upon irradiation with visible light (lirr
>
405 nm), the foldamers can be readily converted back into
their all-E isomers, which consequently re-adopt the initial
helical conformation as shown by CD spectroscopy. Repet-
itive switching cycles by employing alternating irradiation
with UV and visible light demonstrate the full reversibility of
our photoswitchable foldamers (see Figure S17 and S18[8]).
Furthermore, the temporal evolution of both the UV/Vis and
CD spectra during the switching cycles coincides and hence
no significant time lag for both the folding and unfolding
events was observed.
In summary, we have designed a new family of photo-
switchable foldamers composed entirely of azobenzene
repeat units and have investigated their light-induced unfold-
ing as a function of chain length. Oligomer 126 shows the best
performance as it is completely folded in the dark, yet rapidly,
quantitatively, and reversibly unfolds upon UV irradiation.
Further investigations concerned with the variation of the
relative orientation of the azobenzene photochromes and the
implementation of energy gradients,[16] that is, incorporation
1
sition of the PSS was investigated by H NMR spectroscopy
(Table 1). In oligomers 126 and 147, the content of Z-
azobenzene in the PSS is between 17% and 43% depending
on the location, that is, on average two azobenzene moieties
are converted into their Z isomers. In both cases, the extent of
E!Z isomerization was significantly higher for the terminal
azobenzene units (Zterm) as compared to the internal and core
azobenzene moieties (Zint and Zcore), which show rather
similar Z contents. Note that in contrast in the PSS in
acetonitrile solution, model compound 63 has almost the
same Z-azobenzene content in different locations, that is,
67% for the core and 69% for the terminal photochromic
units, (see Figure S8 and S15[8]).
1642
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 1640 –1643