S. Hecht and Z. Yu
Within the helix, the azobenzenes are placed at different
sites with distinct local environments, that is, p,p stacking in-
teractions and exposure to solvent. To gain further insight
into the effect of the exact location of the azobenzene units
in the helix structure on the photoisomerization process, the
quantum yields of each individual photoisomerization event
were determined. As shown in Table 1, all of the individual
quantum yields upon irradiation with UV light (365 nm) are
ranging from 0.7% to 15.9%. It is evident that in acetoni-
trile solution, foldamers show a noticeable trend of decrease
of FEE!EZ from terminal toward core azobenzene position,
that is, from 7.8% for 122-term to 1.6% for 122-core.
The efficiencies of the subsequent photoisomerization
step (FEZ!ZZ) are similar to the initial step (FEE!EZ) in the
cases of 122-core and 122-int, but substantially improve for
122-core. From these values (Table 1, bottom) it is apparent
that photoisomerization occurs more efficiently when the re-
spective azobenzene moiety is located at the helix terminus
or in an already (partially) unfolded structure. In the case of
122-term, the two photoisomerization events are seemingly
independent of one another, as the initial step does not
affect the efficiency of the subsequent step. On the other
hand, when the azobenzene unit is placed in the interior of
the helix, its ability to isomerize is largely reduced, which is
clearly due to the local environment that presumably causes
additional steric and/or electronic barriers. Importantly,
once the initial photoisomerization in such structure, that is,
122-core, has occurred, the subsequent photoisomerization
event is greatly facilitated and thus the system exhibits
Figure 2. Representative UPLC traces during the course of irradiation
showing baseline separation of all three isomers for all foldamers, that is,
122-core (top), 122-int (center), and 122-term (bottom).
proximately 10–12 repeat units,[16] it is apparent that in 122-
int and 122-core the helix will be completely denatured even
if only one azobenzene photoisomerization event is occur-
ring, which means the formed E,Z isomer should display no
CD signal. Therefore, in the cases of 122-int and 122-core,
the E,E isomer is the sole contributor to helicity in the CD
spectra, and indeed a good correlation with the E,E isomer
contents determined from the UPLC traces was found (Fig-
ure 1b, top and center). In contrast, in the case of 122-term
the first isomerization event is not sufficient to fully dena-
ture the helix, that is, the E,Z isomer has a residual CD
signal, and both terminal azobenzene moieties have to be
isomerized for complete unfolding to occur. Therefore, for
122-term, both the E,E isomer as well as the E,Z isomer
give rise to a Cotton effect, as revealed by overlaying the
CD and UPLC data (Figure 1b, bottom). The molar circular
dichroism DeE,Z associated with the E,Z isomer was calculat-
ed by subtracting the molar circular dichroism of the domi-
nating E,E isomer (DeE,E) from the overall molar circular di-
chroism (De) of the mixture by taking into account the con-
tents of the isomers determined by UPLC.[11] Consequently,
it is worth noting that in all series of the photoswitchable
foldamers, the decrease of the CD signal is directly and
quantitatively correlating to the extent of denaturation; that
is, the CD signal decay is a precise index of the unfolding
process (Scheme 2). This finding implies that there are no
significant changes on the twist sense bias in these foldamers
during the photoisomerization process and furthermore no
particular preference for untwisting either one of the P or
M helices.
AHCTUNGTRENNUNG
(positive) cooperativity[18] with regard to its switching ability.
The transduction mechanism to confer such cooperative be-
havior is based on the conformational transition, that is, un-
folding, which creates a more flexible structure with en-
hanced photoreactivity. This effect, yet to a smaller extent,
is also observed for the other isomer with interior azoben-
zene moieties, namely 122-int.
In chloroform solution, where oligomers adopt a random
coil conformation, the efficiencies of the individual photoi-
somerization events are depending neither on azobenzene
location nor on the isomerization order (with an exception
of FEZ!ZZ in the case of 122-term). This clearly supports our
findings related to the influence of the folded structure on
the photoisomerization of the azobenzene moieties. Note
that in general the quantum yields of E!Z photoisomeriza-
tion in our foldamers are intrinsically limited, which is pre-
sumably due to the unfavorable electronic properties of
such rather extended p-conjugated systems.[19] Therefore,
the absolute observed effect is rather small; quantum yields
increase from 1.6% to 3.7% for 122-core, yet the relative
magnitude of the effect is significant, that is, the quantum
yield more than doubles during the process.
The use of a photoswitchable unit confers reversibility to
the system and thus the photochemically triggered refolding
process of the denatured structure has also been investigat-
ed. All of the foldamers can readily be converted from the
Z isomers into the corresponding E isomers upon irradiation
with visible light (lirr >405 nm) and consequently readopt
As mentioned before, the efficiency of photoisomerization
is dependent on the solvent, forcing the backbone to adopt
either compact helical or loose random coil conformations.
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ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
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