Unidirectional Photoinduced Shuttling in a Rotaxane with a Symmetric Stilbene Dumbbell

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1 opyright WLY-V Verlag mb, Weinheim, 200. Supporting nformation for ngew. hem. nt. d. Z8537 Unidirectional Photoinduced Shuttling in a Rotaxane with a Symmetric Stilbene umbbell arol. Stanier, Sara. lderman, Tim. W. laridge and arry L. nderson Section. NMR measurements. Mixtures of the and Z-isomers of 3 and 3 α- were generated by irradiating solutions of each -isomer in N 2 -saturated water (ca mm) in 0 mm silica cuvettes at 340 and 347 nm respectively using monochromated light from a xenon lamp in a obin Yvon Ltd. luoromax-2 spectrofluorometer (5 nm excitation bandwidth). t this concentration it was necessary to continue irradiation for 2 days to obtain a substantial mole-fraction of the Z-isomer. fter irradiation, samples were evaporated and made up in 2 for NMR analysis. The resonances of the and Z-isomers are well resolved, as illustrated for the rotaxane in igure S. The :Z mole ratio was measured by NMR integration using a 90 pulse and a s relaxation delay to obtain accurate integrals (spectra were also acquired with a 2 s relaxation delay but this did not change the ratio of the integrals). The NMR spectra of the and Z- isomers of 3 and 3 α- were assigned from the SY, TSY, SQ and NSY spectra of both the pure -isomer and the /Z mixture in each case. The 2 NSY spectra of the rotaxanes were recorded with a mixing time of 400 ms and all observed Ns were negative. These NSY spectra reveal many dumbbell-to-cyclodextrin Ns, as illustrated in igure S2. The integrals of these N cross-peaks for both the and Z isomers were divided by the concentrations of these species, and divided by the number of equivalent protons involved in each interaction, to give the normalized N strengths listed in Table S and displayed graphically in Scheme (values listed for 6 are averages of 6 and 6'). The weakest Ns, -6 in Z-3 α- and -3 in -3 α-, do not show in the 2 NSY spectra but were reliably detected in PS NSY experiments; these Ns are arbitrarily assigned integrals of 0. in Table S. S

2 Na Na Na Na -, Na Na Na Na, ' 6 ' ,6' igure S. NMR spectrum of a mixture of -3 α- (assignments in red) and Z-3 α- (assignments in blue) recorded in 2 at 298 K, 500 Mz. S2

3 Na Na Na Na Na Na Na Na 5 6' ,6' ' -6/6' ' ' -6/6' , -3,-5,-3,-6' , igure S2. n expanded region from the NSY spectrum of a mixture of -3 α- (assignments in red) and Z-3 α- (assignments in blue) recorded in 2 at 298 K, 500 Mz. S3

4 Na Na Na Na Na Na Na Na Table S. Normalized N cross-peak integrals for the and Z- isomers of 3 α- on the same relative scale. Section 2. etermination of /Z-photoisomerization quantum yields Φ Z and Φ Z for 3 and 3 α-. (a) alculation of absorption spectra of the Z-isomers. We were unable to isolate the Z-isomers of 3 and 3 α- so their extinction coefficient spectra (ε Z vs. λ) were deduced from the absorption spectra of /Z-mixtures of known composition (from NMR integration) and known concentration, by subtraction of the spectrum of the pure -isomer (ε vs. λ). Spectra for 3 α- are shown in igure S3. (b) nalysis of photostationary mixtures to give Φ Z /Φ Z. The ratio Φ Z /Φ Z can be determined from the composition of a photostationary reaction mixture using the equation Φ Z /Φ Z (S) = ε Z ε ([Z-isomer]/[-isomer]) PSS where ε Z ε is the ratio of the extinction coefficients of the two isomers and ([Z-isomer]/[-isomer]) PSS is the ratio of their concentrations at photostationary equilibrium. [] This relationship is only accurate for dilute solutions where 0 ln 0, so all our experiments were carried out at low concentration (absorbance 0. at excitation wavelength; conc. 0 µm in N 2 -saturated water S4

5 in silica cuvettes). We chose to irradiate each pure -isomer at the isosbestic wavelength for its /Z isomerization (300 and 307 nm for 3 and 3 α- respectively) so that ε Z ε =. rradiation was conducted in a luoromax-2 spectrofluorometer (5 nm excitation bandwidth). series of absorption spectra were recorded at minute intervals over 25 minutes of irradiation. The changes in absorption fit first order decay curves, and by the end of the irradiation photoisomerization was >97.5% complete, so in each case the final spectrum could be used as a spectrum of the photostationary state, PSS. Longer irradiation times were avoided to minimize irreversible photochemistry. The final absorption is related to the composition of the mixture by PSS (S2) = ε Z [Z-isomer] PSS + ε [-isomer] PSS Rearrangement of quation (S2) allows the mole fraction of Z- isomer in this photostationary mixture, f Z, to be calculated using f Z = [Z-isomer] PSS / tot = ( PSS / tot ε )/(ε Z ε ) (S3) where tot is the total concentration ( tot = [-isomer] PSS + [Zisomer] PSS = [-isomer] initial ). This calculation was performed at nm intervals across the absorption band, to give a curve as illustrated in igure S3; f Z is undefined at the isosbestic point (where ε Z ε ) and at long wavelengths (where ε Z ε 0) but other wavelengths give consistent values and averaging over many wavelengths gave f Z = ± for 3 α- and f Z = 0.7 ± 0.02 for 3. rom quation (S), these mole ratios are related to the quantum yields by Φ Z /Φ Z = f Z /( f Z ) (S4) This gives Φ Z /Φ Z = for 3 α- and Φ Z /Φ Z = 0.2 for 3. These values were found to be reproducible to within 0%. S5

6 60 f Z = ε / mm cm 20 ε f Z f Z ε Z λ / nm 0.00 igure S3. bsorption spectra of rotaxanes -3 α- and Z-3 α- (ε and ε Z respectively) and values of the Z-mole fraction of the photostationary state, f Z, calculated using quation (S3) with data points at nm intervals across the spectrum. (c) Measurement of initial rates to give Φ Z. The initial rates of Z photoisomerization of -3, -3 α- and -stilbene were compared during irradiation with the same monochromated light source (λ = 325 nm, band width =.0 nm), in dilute solution ( 0 0.; conc. 0 µm) under identical conditions (except that the stilbene was in cyclohexane whereas the other solutions were aqueous). These dilute conditions essentially eliminate competition from bimolecular photochemistry. bsorption spectra were recorded at minute intervals, and in each case the absorption at the wavelength of maximum absorption change (λ 2 = 340, 347 and 308 nm for 3, 3 α- and stilbene) was found to decrease linearly with time over the first 6 minutes of irradiation, as illustrated in igure S4. S6

7 rotaxane 3 α- / dumbbell t / min igure S4. Normalised absorption vs. time during irradiation of 3 and 3 α- in N 2 -saturated water in under identical conditions. Straight lines show initial gradients, fitted through data for the first 6 minutes in each case. rom the eer-lambert law, the absorbed light intensity abs is abs = 0 ( 0 o ) (S5) where 0 is the absorption at the irradiation wavelength (λ = 325 nm). uring this initial stage of reaction, Z photoisomerization can be ignored, because [Z-isomer] 0, so the rate of change of -isomer concentration is d[-isomer]/dt = abs Φ Z (S6) and the initial rate of change of absorption at λ 2 is d/dt (S7) = l(ε ε Z )d[-isomer]/dt = l(ε ε Z ) 0 ( 0 o )Φ Z where l is the pathlength and ε and ε Z are the extinction coefficients of the and Z-isomers at probe wavelength λ 2. Thus the values of d/dt for the three compounds were used to determine S7

8 values of Φ Z for 3 and 3 α- using the known value (Φ Z = 0.50) [2] for stilbene. This gave Φ Z = 0.7 ± 0.02 for 3 and Φ Z = 0.06 ± 0.0 for 3 α-. This whole experiment was carried out twice with excellent (± 5%) reproducibility, but there is also an uncertainty of at least 0% in the standard value of Φ Z for stilbene in cyclohexane. [3] The larger value of Φ Z for the dumbbell 3 is immediately evident from its faster rate of Z photoisomerization in igure S4. References []. Saltiel,.. Sears,.-. Ko, K.-M. Park in R andbook of rganic Photochemistry and Photobiology, ed. W. M. orspool, P.-S. Song, R Press, oca Raton, 995, hapter. [2] S. Malkin,. ischer,. Phys. hem. 964, 68, [3] T.-. o, T.-M. Su, T.-. wang,. Photochem. Photobiol. hem. 988, 4, ;. egiou, K.. Muszkat,. ischer,. m. hem. Soc. 968, 90, 2 8;. üsten, L. Klasinc, Tetrahedron Lett. 968, S8