UV–Vis Experiments on dOH-PE-THR and dOH-PE-RTX

dOH-NI unit that undergo a rapid ESPT upon excitation. According to the proposed operation mechanism of the dihydroxyrotaxane, the shuttling in this case is expected to occur only via the excited-state pathway. Strong bases, on the other hand, are able to deprotonate one of the hydroxyl groups in the ground state, but the protonated base remains hydrogen bonded to the deprotonated hydroxyl oxygen. If the interaction of the macrocycle toward the deprotonated dOH-NI unit is significantly stronger than that of the protonated DBU, strong bases are expected to result in translocation of the macrocycle already in the ground state. The shuttling is additionally expected to increase the acidity of the hydroxyl group due to the additional stabilization of deprotonated dOH-NI by the macrocycle. The absorption spectra of dOH-PE-THR and dOH-PE-RTX upon addition of a weak (NMI) and a strong base (DBU) in MeCN and PhCN are presented in We have shown in a previous publication (64) that weak bases form neutral ground-state complexes with the hydroxyl groups of theunit that undergo a rapid ESPT upon excitation. According to the proposed operation mechanism of the dihydroxyrotaxane, the shuttling in this case is expected to occur only via the excited-state pathway. Strong bases, on the other hand, are able to deprotonate one of the hydroxyl groups in the ground state, but the protonated base remains hydrogen bonded to the deprotonated hydroxyl oxygen. If the interaction of the macrocycle toward the deprotonatedunit is significantly stronger than that of the protonated DBU, strong bases are expected to result in translocation of the macrocycle already in the ground state. The shuttling is additionally expected to increase the acidity of the hydroxyl group due to the additional stabilization of deprotonatedby the macrocycle. The absorption spectra ofandupon addition of a weak (NMI) and a strong base (DBU) in MeCN and PhCN are presented in Figures 1 and S4 (Supporting Information), respectively.

Figure 1 Figure 1. Steady-state absorption spectra of (top) dOH-PE-THR and (bottom) dOH-PE-RTX (c ≈ 15 μM) upon addition of (A) NMI and (B) DBU in MeCN. The absorption of the bases has been subtracted from the overall absorption spectra. The colored solid lines represent the spectra of the pure species obtained from the global fits.

K 1 = 4K 2 ; see A H ), 1:1 complex (A HG ), and 1:2 complex (A HG 2 ) are depicted in ref-dOHNI, which could indicate some competition in the binding of NMI toward the initial succ station or the macrocycle.dOH-NI unit and NMI is not significantly influenced by the presence of the thread or the macrocycle. The absorption spectra of the rotaxane and the thread exhibit a red shift and a broadening upon addition of NMI. The spectra were analyzed globally with a 1:2 host/guest association model assuming noncooperative binding (i.e.,= 4; see Supporting Information for additional details, spectra, and fits), (64) and the association constants are summarized in Table 1 . The obtained species spectra for the free host (), 1:1 complex (), and 1:2 complex () are depicted in Figure 1 by the colored lines. The association constants of both compounds are slightly smaller than those of the model compound,, which could indicate some competition in the binding of NMI toward the initial succ station or the macrocycle. (64) The spectral changes are similar in all cases. Nevertheless, formation of the ground-state complexes between theunit and NMI is not significantly influenced by the presence of the thread or the macrocycle.

Table 1. Association Constants for Complex Formation between the Dihydroxy Compounds and NMI and the Equilibrium Constant for Deprotonation by DBU compounda solvent base K 1 (M–1) K 2 (M–1) THR MeCN NMI 50 ± 3 12 ± 1 RTX 55 ± 4 14 ± 1 ref-dOHNI 58 ± 10b 9 ± 2b THR PhCN NMI 73 ± 6 18 ± 2 RTX 83 ± 6 21 ± 2 ref-dOHNI 95 ± 9b 19 ± 2b THR MeCN DBU (1.8 ± 0.1) × 105 RTX (5.5 ± 0.2) × 105 ref-dOHNI (1.9 ± 0.1) × 105b THR PhCN DBU (2.1 ± 0.4) × 104 RTX (9.6 ± 0.7) × 105 ref-dOHNI (4.1 ± 0.2) × 104b

dOH-NI + protonated DBU). Clear changes are observed between the thread and the rotaxane. First, the rotaxane responds much more readily to the addition of DBU (notice the different concentration ranges in dOH-NI presumably via hydrogen bonding to the deprotonated hydroxyl oxygen. Moreover, the narrowing of the band could be due to exclusion of the solvent around the dOH-NI station by the macrocycle in the shuttled state. We expect to observe similar spectral changes upon shuttling of the macrocycle in the time-resolved experiments. The absorption spectra upon addition of DBU exhibit a drastic decrease in the absorption band of the neutral form with a concomitant rise of a new long-wavelength absorption band attributed to the 1:1 complex of the ground-state ion pair (deprotonated+ protonated DBU). Clear changes are observed between the thread and the rotaxane. First, the rotaxane responds much more readily to the addition of DBU (notice the different concentration ranges in Figure 1 B). Second, the spectrum of the ground-state anion of the rotaxane is significantly blue-shifted (463 nm vs 486 nm in MeCN), narrower and higher in intensity. The differences must originate from the presence of the macrocycle. We have shown in a previous publication that hydrogen-bonding interactions to deprotonated hydroxyl oxygen atoms in related 1,8-naphthalimide photoacids greatly stabilize the ground-state anion, causing a blue shift of the spectrum. (64,67) The blue shift observed for the rotaxane suggests that the macrocycle stabilizes the ground-state anion ofpresumably via hydrogen bonding to the deprotonated hydroxyl oxygen. Moreover, the narrowing of the band could be due to exclusion of the solvent around thestation by the macrocycle in the shuttled state. We expect to observe similar spectral changes upon shuttling of the macrocycle in the time-resolved experiments.

K 1 value of ref-dOHNI. In both solvents, K 1 of dOH-PE-THR is comparable to that of the reference compound. On the contrary, dOH-PE-RTX exhibits a significantly higher K 1 in both solvents. Similarly to the above case, this can be explained by stabilization of the ground-state anion by the macrocycle resulting in an increased equilibrium constant for deprotonation (i.e., decreased pK a value). Similar remote control of acid–base properties has been recently reported in the case of other bistable [2]rotaxanes. To quantify the observed differences between the thread and the rotaxane, the spectra upon addition of DBU were analyzed with a modified 1:2 association model. Due to the low concentration range of DBU, the concentration of 1:2 complexes, where one DBU is associated with each of the hydroxyl groups, is very low and does not need to be considered. However, we account for a competitive association channel which presumably corresponds to a binding of one DBU molecule to the initial succ station without significant spectral changes (see Supporting Information for additional details, spectra, and fits). Hence, we only determined the equilibrium constant for the deprotonation of one of the hydroxyl group by DBU, which can be compared with the correspondingvalue of. In both solvents,ofis comparable to that of the reference compound. On the contrary,exhibits a significantly higherin both solvents. Similarly to the above case, this can be explained by stabilization of the ground-state anion by the macrocycle resulting in an increased equilibrium constant for deprotonation (i.e., decreased pvalue). Similar remote control of acid–base properties has been recently reported in the case of other bistable [2]rotaxanes. (33,68)

c = 60 mM) and NMI (c = 200 mM) in MeCN and PhCN. As discussed in the previous section, weak bases form neutral ground-state complexes with the hydroxyl groups which undergo sub-nanosecond ESPT and eventually produce the ground-state anion in approximately 14 ns.mc toward the deprotonated dOH-NI station. Based on the steady-state absorption measurements in the presence of DBU, translocation of the mc induces a significant blue shift and narrowing of the ground-state anion absorption band due to hydrogen bonding of the mc to the deprotonated hydroxyl oxygen. Representative time-resolved transient absorption spectra of dOH-PE-RTX in the presence of 60 mM DABCO in MeCN are presented in The shuttling rates were measured using a time-resolved UV–vis transient absorption setup in the presence of weak bases, DABCO (= 60 mM) and NMI (= 200 mM) in MeCN and PhCN. As discussed in the previous section, weak bases form neutral ground-state complexes with the hydroxyl groups which undergo sub-nanosecond ESPT and eventually produce the ground-state anion in approximately 14 ns. (64) The shuttling is expected to occur due to the increased hydrogen-bonding affinity of thetoward the deprotonatedstation. Based on the steady-state absorption measurements in the presence of DBU, translocation of the mc induces a significant blue shift and narrowing of the ground-state anion absorption band due to hydrogen bonding of the mc to the deprotonated hydroxyl oxygen. Representative time-resolved transient absorption spectra ofin the presence of 60 mM DABCO in MeCN are presented in Figure 2 . The remaining time-resolved spectra, together with the fits, are presented in the Supporting Information (Figures S23–S28).

Figure 2 Figure 2. (A) Time-resolved UV–vis transient absorption spectra of dOH-PE-RTX (c ≈ 100 μM) in the presence of DABCO (c = 60 mM) in MeCN. Negative intensities due to emissive processes are removed for clarity. (B) 2D representation of the same spectra. (C) Decays monitored at 465 nm (red squares) and 524 nm (green circles) together with multiexponential fits (black solid lines). The residuals are given in the top panel. The excitation wavelength was 385 nm.

dOH-PE-RTX in the presence of weak bases contain both negative and positive contributions from emissive and absorptive processes, respectively (see ES ≈ 14 ns and centered initially around 500 nm. During the first 100 ns, this band undergoes a significant narrowing from the red side around 480–570 nm and an increase in intensity around 440–480 nm, in excellent agreement with the observed spectral differences between the thread and the rotaxane in the presence of DBU. Moreover, the spectral evolution observed for the rotaxane could be qualitatively reproduced either from the steady-state spectra of the thread and the rotaxane in the presence of DBU or from the transient absorption spectra of the ground-state anions (Figures S4 and S5, dOH-NI end station. The transient spectra ofin the presence of weak bases contain both negative and positive contributions from emissive and absorptive processes, respectively (see Figure 2 A,B and Figure S1 for the full spectra). Excitation of the ground-state complex results in formation of excited-state ion pair species with wavelength-dependent multiexponential dynamics. (64) We therefore focused on the broad transient absorption band of the ground-state anion formed with τ≈ 14 ns and centered initially around 500 nm. During the first 100 ns, this band undergoes a significant narrowing from the red side around 480–570 nm and an increase in intensity around 440–480 nm, in excellent agreement with the observed spectral differences between the thread and the rotaxane in the presence of DBU. Moreover, the spectral evolution observed for the rotaxane could be qualitatively reproduced either from the steady-state spectra of the thread and the rotaxane in the presence of DBU or from the transient absorption spectra of the ground-state anions (Figures S4 and S5, Supporting Information ). Therefore, the observed spectral changes can be attributed to an association of the macrocycle with the deprotonatedend station.