FIGURE 5 Normalized fluorescence intensities (λex = 555
nm, λem = 657 nm,
CHCl3/CH3CN, 1:1, v/v, 298 K) of the
isomers of host Zn3 as a function of time after the addition
(at t = 50 s) of: (A) no guest, (B) 3.6 equivalents of guestV4 , (C) 1.2 equivalents of guest V5 , and (D) 13
equivalents of guest V6 , and subsequent irradiation of the
motor substituents (λ = 365 nm for 10 seconds) at t = 250, 500, 750 s,
see bars.
effects of the photochemical isomerization of the motor substituents on
the fluorescence emission of the zinc porphyrins. Figure 5A displays the
normalized fluorescence intensity of all isomers of Zn3 in the
absence of guest, with three intermediate irradiation events of 10
seconds (λmax = 365 nm) at t = 250, 500, 750 s. These
irradiation events turned out to not substantially change the
fluorescence intensity of the zinc porphyrins of the hosts.
Subsequently, we carried out the light-gated binding experiments. Figure
5B depicts the fluorescence intensities of the three isomeric hosts upon
the addition of viologen guest V4 (at t = 50 s, 3.6 equiv of
guest was added). In all cases, an instant drop in fluorescence
intensity indicated the formation of the host-guest complexesZn3·V4 , with host Zn3a having the highest host-guest
occupancy, followed by Zn3b and then Zn3c . This trend
is in line with the decrease in binding constant as determined by the
fluorescence titrations (Table 1). The subsequent in situphotochemical isomerization of the motor substituents of Zn3awas associated with an instant increase in fluorescence intensity,
indicating that the occupancy, and thus the binding affinity, dropped.
Over time, thermal helix inversion of the motor substituents took place,
resulting in the slow recovery of the original – higher – binding
affinity. In sharp contrast, the photochemical isomerization ofZn3c resulted in an instant decrease in fluorescence intensity,
which is associated with an increase in binding affinity. In analogy
with the behavior of Zn3a , the thermal relaxation of the
metastable motors regenerated the original – lower – binding affinity.
For the non-symmetric host Zn3b , the effect of irradiation was
the smallest. Only a small drop in fluorescence intensity was observed,
which is in line with a small increase in binding affinity. This effect
may be caused by differences in PSS ratios of the two non-identical
motor substituents of Zn3b . If the bound motor has a higher PSS
ratio than the loose motor, an increase in binding affinity is to be
expected. Successive isomerization-THI cycles showed that the processes
are repeatable for all isomers of Zn3 (Figure 5B–D). In a next
series of experiments, we evaluated the binding between the isomers ofZn3 and benzyl viologen V5 (Figure 5C), which has a
higher affinity for the porphyrin cages compared to viologen guestsV4 and V6 . The addition of a small excess ofV5 (at t = 50 s, 1.2 equiv of guest was added) to the solutions
of the three isomers of Zn3 caused a similar drop in
fluorescence as was observed when 3.6 equiv of V4 was added to
the same hosts, highlighting the differences in binding affinity.
Repeated isomerization-THI cycles indicated that the host-guest mixtures
containing guest V5 displayed similar light- and heat-responses
as those containing guest V4 . Finally, we examined whether the
weakest binding viologen V6 would also be susceptible to
light-gated binding in the double-motorized porphyrin cages Zn3(Figure 5D). The addition of a large excess of V6 (at t = 50 s,
13 equiv of guest was added) resulted in a similar decrease in
fluorescence intensity as observed for V4 (3.6 equiv) andV5 (1.2 equiv). Again, the host-guest mixtures containingV6 responded similarly to the photochemical isomerization-THI
cycles as those containing V4 and V5 .
In summary, we have successfully synthesized light-responsive
double-motorized porphyrin cage compounds Zn3a , Zn3b ,
and Zn3c . The structures and assignments of the absolute
configurations of these compounds were established by 1D and 2D NMR, and
by circular dichroism analysis. UV-vis and 1H NMR
analysis showed that cage compounds Zn3 efficiently undergo
photochemical isomerization and thermal helix inversion steps, in
agreement with what we have shown previously for the single-motorized
cage compounds Zn2 .30 Furthermore, the
different stereoisomers of Zn3 presented in this study show
significant differences in binding affinity for viologen guests
(K a of Zn3a > Zn3b> Zn3c ). These differences are attributed to
differences in the three-dimensional structures of the isomeric hosts.Zn3a possesses two loosely bound motor fragments, whereas one
of the motors of Zn3b and both motors of Zn3c display
intramolecular supramolecular interactions with the cavity part of the
hosts, which competes with viologen guest binding inside the porphyrin
cage. Photochemical isomerization of the motor substituents leads to the
unique phenomenon that two diastereomers can become pseudo-identical.
This phenomenon leads to orthogonal behavior in light-gated binding of
guest molecules. Whereas the photo-isomerization of theC 2-symmetric diastereomer Zn3a (two
loose motors) significantly lowers its binding affinity for viologen
guests, the opposite is true for the C 2-symmetric
diastereomer Zn3c (two bound motors), i.e. its binding affinity
increases. These studies show that a combination of fixed chiral
elements, e.g. point chirality and planar chirality, with a
light-responsive chiral element, e.g. helicity, gives rise to
light-responsive functional molecular systems that can operate in an
orthogonal fashion.