FIGURE 4 UV-vis spectra (CHCl3/CH3CN,
1:1, v/v, 298 K) of: (A) Zn3a , (B) Zn3b , and (C)Zn3c before irradiation (black), after irradiation to
PSS365 (red), after partial THI (pink dashed), and after
full THI (blue dashed). Partial 1H NMR spectra (500
MHz, CDCl3/CD3CN, 1:1, v/v, 277 K) of:
(D) Zn3a , (E) Zn3b , and (F) Zn3c before
irradiation (black) and after irradiation to PSS365(red) showing the signals of methyl protons H-c . See Figure 1
for the proton assignment. Note that the apparent triplet observed for
the methyl protons of Zn3b-st-st is the result of two
overlapping doublets originating from the two non-identical molecular
motor substituents.
did not provide information about the relative abundance of the
metastable and stable species in solution. In order to measure these PSS
ratios, we resided to 1H NMR spectroscopy (Figure
4D–F). The isomerization of the motor substituents of all isomers ofZn3 with UV light (λmax = 365 nm) resulted in
similar deshieldings of the methyl protons H-c (Δδ = +0.26
ppm). After the subsequent thermal helix inversion, the original1H NMR spectra were restored. The decays of the
metastable states were determined by monitoring the signals of methyl
protons H-c (Supporting Information, Figures S41–S44) over
time, and the corresponding apparent rate constants were in reasonable
agreement with those obtained by UV-vis spectroscopy (Table 2, second
and third column). NMR spectroscopy was also used to probe the
structural changes, as proposed in Figure 3B. The irradiation ofZn3a (two loose motors) resulted in metastable species with one
or two bound motors (Δδ = −0.91 ppm for H-a , and Δδ = −0.84 ppm
for H-b ). As expected, the opposite effect was observed whenZn3c (two bound motors) was irradiated: new signals for loose
motors (Δδ = +0.84 ppm for H-a , and Δδ = +1.24 ppm for
H-b ) were observed. For Zn3b , the situation was more
complex, because of the absence of symmetry in the stable isomer and
each of its three possible metastable isomers. Nevertheless, by
monitoring the chemical shift changes of the aromatic protons attached
to the xylylene sidewalls of the porphyrin cages, we could detect all
proposed isomers (Supporting Information, Figures S49–S51). Moreover,1H NMR analysis enabled us to determine the PSS ratios
for the different diastereomers, i.e. the ratios between the
total amount of metastable and stable motors (Table 2). These appeared
to be nearly identical (metastable:stable ~1:3) for all
investigated double-motorized cages Zn3 and also comparable to
those found for the single-motorized cages Zn2 . It has to be
noted though that the measured PSS ratios, which are relatively low,
cannot be considered to be accurate PSS ratios. In order to avoid
aggregation and precipitation of the porphyrin cages, the measurements
were performed at 4 °C. At this temperature, the photochemical
isomerization is still in competition with the thermal helix inversion.
Lower temperatures would tackle this issue, but could not be applied as
it results in precipitation of the porphyrin cages.
The differences in 3D structure between the stable isomers ofZn3a , Zn3b , and Zn3c caused significant
differences in their binding affinities for viologen guests
(Zn3a > Zn3b >Zn3c , see Table 1). These 3D structures could be changed by
photochemical isomerization of the motor substituents (Figure 3). For
instance, the stepwise isomerization of Zn3a-st-st (two loose
motors) gives Zn3a-ms-st (one loose motor, one bound motor),
and eventually Zn3a-ms-ms (two bound motors). The latter two
isomers are pseudo-identical to Zn3b-st-st andZn3c-st-st , respectively, both of which have a lower affinity
for viologen guests. Therefore, we expected that the photochemical
isomerization of Zn3a-st-st would also result in a stepwise
lower binding affinity for viologen guests. Similarly, the stepwise
isomerization of Zn3c-st-st to metastable isomers that are
pseudo-identical to Zn3b-st-st and Zn3a-st-st should
lead to an increase in the binding affinity for viologen guests. For the
non-symmetric isomer Zn3b-st-st , photochemical isomerization to
the mixture of metastable isomers is expected to have a minor influence
on the binding affinity for viologen guests, as the isomerization
effects of the two motor substituents would cancel out. We studied the
light-gated binding of viologen guests V4 –V6 in the
double-motorized porphyrin cages Zn3 with the help of
time-resolved fluorescence quenching spectroscopy (Figure 5). The
host-guest complexes are non-fluorescent, and therefore the normalized
fluorescence intensity originating from the zinc porphyrin after
addition of guest is a measure for the amount of free host present in
solution relative to the initial amount of free host. A normalized
fluorescence intensity of 0.5 resembles a 1:1 mixture of free host and
host-guest complex (50% occupancy). Based on the binding constants
(Table 1), we calculated the amount of each guest that should be added
to Zn3b (medium affinity for V4 –V6 ) in order
to reach ~50% occupancy with an initial host
concentration of 10−5 M (Supporting Information, Table
S13). We first investigated the possible