Unravelling the Photoswitching Mechanism in Donor Acceptor Stenhouse Adducts

Transcription

1 Supporting Information Unravelling the Photoswitching Mechanism in Donor Acceptor Stenhouse Adducts Michael M. Lerch, Sander J. Wezenberg, Wiktor Szymanski,, Ben L. Feringa*, Centre for Systems Chemistry, Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands Department of Radiology, University of Groningen, University Medical Center Groningen, Hanzeplein 1, 9713 GZ, Groningen, The Netherlands Corresponding Author: * b.l.feringa@rug.nl S1

2 Table of Contents 1 Materials and Methods... S4 3 Synthesis and Characterization... S6 4 Light Sources... S8 5 UV/vis Absorption Spectra and Photoswitching:... S9 5.1 Photochemical characterization of compounds 1 and 2... S Compound 1... S Compound 2... S Spectroscopy and photochemistry... S Time-drives at different temperatures... S Appearance of an absorption band upon photoswitching... S Photoswitching at lower temperature... S Effect of light-intensity... S Phases of photoswitching... S Photoswitching at 195 K (Changes in Color)... S24 6 Measurement of Reaction Rates... S PSS at different wavelengths... S Determination of k 2 at higher temperatures (258 K up to 278 K)... S Measurement and Eyring analysis of k -1,thermal... S Measurement and Eyring analysis of k S36 7 Estimation of the Photochemical Quantum Yield... S38 8 Absorption Spectra of Cyclized-1... S40 9 UV/vis Photoswitching in Water... S H-NMR Photoswitching Studies in Water... S Compound 1... S D 2O... S Compound 2... S D 2O... S Comparison: No irradiation... S44 11 Photoswitching in Dichloromethane... S UV/vis absorption spectra and photoswitching... S H-NMR irradiation experiments... S Setup... S Measurement... S47 S2

3 Discussion... S Photoswitching at 195 K (changes in color)... S48 12 TD-DFT Calculations... S49 13 Kinetic Modelling... S Experimental data... S Kinetic Model... S Irradiation... S Decay... S Implicit assumptions for the kinetic model... S Fitting... S Fitted data... S K... S K... S K... S K... S Discussion... S H- and 13 C-NMR Spectra... S Compound 1... S Compound 2... S62 S3

4 1 Materials and Methods General Reagent Information: Preparation of commercially unavailable compounds: unless stated otherwise, all reactions were carried out in oven- and flame-dried glassware using standard Schlenk techniques and were run under nitrogen atmosphere. The reaction progress was monitored by TLC. Starting materials, reagents and solvents were purchased from Sigma Aldrich, Acros, Fluka, Fischer, TCI, J.T. Baker or Macron and were used as received, unless stated otherwise. Solvents for the reactions were of quality puriss., p.a.. Anhydrous solvents were purified by passage through solvent purification columns 1 (MBraun SPS-800). For aqueous solutions, deionized water was used. Furfural and diethylamine were purchased from Sigma Aldrich. 1,3-Dimethylbarbituric acid was purchased from TCI Europe. General Considerations: Thin Layer Chromatography analyses were performed on commercial Kieselgel 60, F254 silica gel plates with fluorescence-indicator UV 254 (Merck, TLC silica gel 60 F 254). For detection of components, UV light at λ = 254 nm or λ = 365 nm was used. Alternatively, oxidative staining using aqueous basic potassium permanganate solution (KMnO 4) or aqueous acidic cerium phosphomolybdic acid solution (Seebach s stain 2 ) was used. Drying of solutions was performed with MgSO 4 and volatiles were removed with a rotary evaporator. General Analytical Information: Nuclear Magnetic Resonance spectra were measured with an Agilent Technologies 400-MR (400/54 Premium Shielded) spectrometer (400 MHz). All spectra were measured at room temperature (22 24 C). Chemical shifts for the specific NMR spectra were reported relative to the residual solvent peak [in ppm; CDCl 3: H = 7.26; CDCl 3: C = 77.16; CD 2Cl 2: H = 5.32; CD 2Cl 2: C = 53.84; d 6-DMSO: H = 2.50; d 6-DMSO: C = 39.52; toluene-d 8: H = 2.08, 6.97, 7.01, 7.09; toluened 8: C = , , , , 20.43; CD 3OD: H = 3.31; CD 3OD: C = 49.00] 3. The multiplicities of the signals are denoted by s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), br (broad signal), app (apparent). All 13 C-NMR spectra are 1 H-broadband decoupled. High-resolution mass spectrometric measurements were performed using a Thermo scientific LTQ OrbitrapXL spectrometer with ESI ionization. The molecule-ion M +, [M + H] + and [M X] + respectively are given in m/z-units. Melting points were recorded using a Stuart analogue capillary melting point SMP11 apparatus. For spectroscopic measurements, solutions in Uvasol grade solvents were measured in a 10 mm quartz cuvette. UV/vis absorption spectra were recorded on an Agilent 8453 UV/vis absorption Spectrophotometer. UV/vis absorption spectra of temperature-dependent processes were recorded on a Hewlet-Packard HP 8543 diode array or a Analytik Jena Specord S600 diode array equipped with a Peltier based temperature controlled cuvette holder (QuantumNorthwest). Irradiation at 561 (100 mw, Cobolt Lasers, Jive) and 532 nm (300 mw, Cobolt Lasers, Samba) was carried out with defocusing by a 5 cm planoconvex length to provide a beam with of ca. 1 cm diameter at sample. Power at sample was determined using a PM160T Power meter (Thorlabs). For non-coherent light-sources, the power was measured using a PM160 Power Meter with Photodiode Sensor (Thorlabs) at their corresponding peak-wavelength (λ max). The distance from the light source is indicated. The obtained UV/vis spectra were baseline corrected. S4

5 Data-analysis of kinetic measurements was performed using GraphPad Prism and Origin Software. Kinetic modeling was performed with R software ( S5

6 3 Synthesis and Characterization Synthesis of donor-acceptor Stenhouse adduct (DASA, 1): 4 Compound S1 has been prepared according to a reported procedure. 4a Spectral properties matched previously reported values. 5-((2Z,4E)-5-(diethylamino)-2-hydroxypenta-2,4-dien-1-ylidene)-2,2-dimethyl-1,3-dioxane-4,6- dione (1): 4 S1 (1.00 g, 4.50 mmol) was suspended in tetrahydrofuran (10 ml). Subsequently, diethylamine (465 µl, 4.50 mmol) was added to the suspension at room temperature, followed by water (few drops). The reaction mixture was stirred for 10 min. at room temperature and 20 min. at 0 C. The color of the reaction mixture turned dark purple. Upon cooling, a precipitate was formed which was then collected by filtration and washed with cold diethyl ether and cold pentane and dried. The final product (1) was obtained as dark red crystals (1.08 g, 81% yield). Mp C; 1 H NMR (400 MHz, CDCl 3) δ 1.29 (t, J = 7.3 Hz, 3H, NCH 2CH 3), 1.33 (t, J = 7.2 Hz, 3H, NCH 2CH 3), 1.70 (s, 6H, C(CH 3) 2), 3.49 (q, J = 7.2 Hz, 4H, 2 x NCH 2CH 3), 6.05 (t, J = 12.3 Hz, 1H, vinylh), 6.73 (dd, J = 12.4, 1.5 Hz, 1H, vinylh), 7.28 (d, J = 12.3 Hz, 1H, vinylh), (s, 1H, OH); 13 C NMR (101 MHz, CDCl 3) δ 12.4, 14.6, 26.8, 44.2, 52.0, 90.6, 102.4, 103.5, 139.0, 145.0, 151.5, 157.2, 165.4, 167.2; HRMS (ESI+) calc. for C 15H 22NO 5 [M + H] + : , found: Synthesis of donor-acceptor Stenhouse adduct (DASA, 2): 4 Compound S2 has been prepared according to a reported procedure. 4a Spectral properties matched previously reported values. 5-((2Z,4E)-5-(diethylamino)-2-hydroxypenta-2,4-dien-1-ylidene)-1,3-dimethylpyrimidine- 2,4,6(1H,3H,5H)-trione (2): 4 S2 (234 mg, 1.00 mmol) was suspended in tetrahydrofuran (10 ml). Subsequently, diethylamine (104 µl, 1.00 mmol) was added to the suspension at room temperature, followed by water (few drops). The reaction mixture was stirred for 30 min. at room temperature. The color of the reaction mixture turned dark purple. Upon consumption of the starting material (TLC), additional tetrahydrofuran (10 ml) was added and the product was precipitated from the reaction mixture with cold pentane. Compound 2 was filtered to obtain dark purple crystals (275 mg, 89% yield) that can be further purified by recrystallization from warm acetonitrile if needed. Mp C; 1 H NMR (400 MHz, CDCl 3) δ 1.31 (t, J = 7.2 Hz, 3H, NCH 2CH 3), 1.34 (t, J = 7.2 Hz, 3H, NCH 2CH 3), 3.34 (s, 3H, NCH 3), 3.35 (s, 3H, NCH 3), 3.48 (q, J = S6

7 7.2 Hz, 2H, NCH 2CH 3), 3.50 (q, J = 7.4 Hz, 2H, NCH 2CH 3), 6.07 (t, J = 12.4 Hz, 1H, vinylh), 6.75 (dd, J = 12.3, 1.5 Hz, 1H, vinylh), 7.15 (s, 1H, vinylh), 7.22 (d, J = 12.3 Hz, 1H, vinylh), (s, 1H, OH); 13 C NMR (101 MHz, CDCl 3) δ 12.5, 14.7, 28.4, 28.6, 44.2, 51.9, 98.7, 102.7, 139.5, 146.7, 151.1, 152.1, 156.4, 163.5, 165.2; HRMS (ESI+) calc. for C 15H 22N 3O 4 [M + H] + : , found: S7

8 4 Light Sources Sources for irradiation have been purchased from commercial suppliers. Technical details (λ em or λ max, intensity, type and supplier) are summarized in Table S1 to S3. Table S1 Comparison of used commercial laser-systems. intensity entry color [a] λ em. (nm) [b] (mw) [b] type supplier Cobolt Samba Cobolt Cobolt Jive Cobolt [a] Color code used throughout the manuscript and supporting information; [b] As indicated by the supplier. Table S2 Comparison of used commercial light sources. intensity entry color [a] λ max (nm) [b] (mw) [c] [810] type LED light source 3x 530 nm supplier Sahlmann Photochemical Solutions [9.6] M530F2 Thorlabs [d] 604-APTD1608SYC/J3 Mouser [1200] 5 6 white light (wl) white light (halogen) LED light source 3x 627 nm Sahlmann Photochemical Solutions ND OSL1-EC Thorlabs ND Plusline ES Small 160W 3100 lm Philips [a] Color code used throughout the manuscript and supporting information; [b] Absorption maximum (λmax) of the light source as indicated by the supplier. [c] Values in [] denote intensities reported by the supplier. ND = not determined [d] Distance from light source: 5 cm. The optical band-pass filters were purchased from Andover Coporation (Table S3). The filter was used in combination with the high-intensity white light source (Table S2, entry 5; OSL1-EC, ). Table S3 Used optical band-pass filter. entry color [a] λ center (nm) [b] bandwidth (nm) [b] transmission (%) [b] type / / FS / / FS10-50 [a] Color code used throughout the manuscript and supporting information; [b] As indicated by the supplier. The optical cut-off filter was purchased from OptoSigma (Molenaar Optics; SCF-50S-44Y, transmittance limit: λ T = 440±5 nm, wavelength slope width Δλ < 25 nm). S8

9 5 UV/vis Absorption Spectra and Photoswitching: 5.1 Photochemical characterization of compounds 1 and Compound 1 Figure S1 Absorption spectra for the photoisomerization of compound 1 (λ max = 545 nm; ~4 µm in toluene; room temperature): a) cyclization with λ = 546 nm (Table S3, entry 1; 546FS10-50, ) and b) thermal relaxation. Irradiation times are indicated. Figure S2 Reversible photochromism for repeated switching cycles of compound 1 (~4 µm in toluene; room temperature) observed at λ max = 545 nm: Switching with λ = 546 nm (Table S3, entry 1; 546FS10-50, ) and thermal relaxation. S9

10 Figure S3 a) No significant shift of the λ max is observed in a series of UV/vis absorption spectra of increasing concentrations. b) Determination of the molar extinction coefficient ε 545 of compound 1 in triplicates (linear regression, consider each replicate value as single point; 95% confidence intervals are indicated with dashed lines). S10

11 5.1.2 Compound 2 Figure S4 Absorption spectra for the photoisomerization of compound 2 (λ max = 570 nm; ~4 µm in toluene; room temperature): a) cyclization with λ = 590 nm (Table S2, entry 3; 604-APTD1608SYC/J3, ) and b) thermal relaxation. Irradiation times are indicated. Figure S5 Reversible photochromism for repeated switching cycles of compound 2 (~4 µm in toluene; room temperature) observed at λ max = 570 nm: Switching with λ = 590 nm (Table S2, entry 3; 604- APTD1608SYC/J3, ) and thermal relaxation. S11

12 Figure S6 a) No significant shift of the λ max is observed in a series of UV/vis absorption spectra of increasing concentrations. b) Determination of the molar extinction coefficient ε 570 of compound 2 in triplicates (linear regression, consider each replicate value as single point; 95% confidence intervals are indicated with dashed lines). S12

13 5.2 Spectroscopy and photochemistry Time-drives at different temperatures Figure S7 Photoswitching at 323 K and 293 K: Reversible photochromism plot of DASA 1 (4 8 µm in toluene; normalized for comparison) (a) and enlarged view of the irradiation period (b). Temperatures of experiments are indicated (323 K and 293 K). Irradiation of samples with white light (Table S2, entry 5; OSL1-EC, ). Figure S8 Photoswitching at 283 K, 278 K and 273 K: Reversible photochromism plot of DASA 1 (4 8 µm in toluene; normalized for comparison) (a) and enlarged view of the irradiation period (b). Temperatures of experiments are indicated (283 K, 278 K and 273 K). Irradiation of samples with white light (Table S2, entry 5; OSL1-EC, ). S13

14 Figure S9 Photoswitching at 273 K, 268 K and 263 K: Reversible photochromism plot of DASA 1 (4 8 µm in toluene; normalized for comparison) (a) and enlarged view of the irradiation period (b). Temperatures of experiments are indicated (273 K, 268 K and 263 K). Irradiation of samples with white light (Table S2, entry 5; OSL1-EC, ). Figure S10 Photoswitching at 258 K and 253 K: Reversible photochromism plot of DASA 1 (4 8 µm in toluene; normalized for comparison) (a) and enlarged view of the irradiation period (b). Temperatures of experiments are indicated (258 K and 253 K). Irradiation of samples with white light (Table S2, entry 5; OSL1-EC, ). S14

15 5.2.2 Appearance of an absorption band upon photoswitching Below the time-dependent behavior of the transient absorption band upon photoswitching of compound 1 and 2 were observed. The source of irradiation used is indicated Compound White light Figure S11 Photoswitching of compound 1 (4 8 µm in toluene). Absorption spectra for the photoisomerization of compound 1 (λ max = 545 nm; room temperature): a) cyclization with white light (Table S2, entry 5; OSL1-EC, ) and b) thermal relaxation. c) Photoswitching monitored at λ max = 545 nm (dark blue line) and λ absorption band = 600 nm (light blue line) of the kinetic run depicted in (a) and (b). S15

16 nm Figure S12 Photoswitching of compound 1 (4 8 µm in toluene). Absorption spectra for the photoisomerization of compound 1 (λ max = 545 nm; room temperature): a) cyclization with λ = 530 nm (Table S2, entry 1; LED light source 3x 530 nm, ) and b) thermal relaxation. c) Photoswitching monitored at λ max = 545 nm (dark blue line) and λ absorption band = 600 nm (light blue line) of the kinetic run depicted in (a) and (b). S16

17 nm Figure S13 Photoswitching of compound 1 (4 8 µm in toluene). Absorption spectra for the photoisomerization of compound 1 (λ max = 545 nm; room temperature): a) cyclization with λ = 546 nm (Table S3, entry 2; 546FS10-50, ) and b) thermal relaxation. c) Photoswitching monitored at λ max = 545 nm (dark blue line) and λ absorption band = 600 nm (light blue line) of the kinetic run depicted in (a) and (b). S17

18 Compound White light Figure S14 Photoswitching of compound 2 (4 8 µm in toluene). Absorption spectra for the photoisomerization of compound 1 (λ max = 570 nm; room temperature): a) cyclization with white light (Table S2, entry 5; OSL1-EC, ) and b) thermal relaxation. c) Photoswitching monitored at λ max = 570 nm (dark blue line) and λ absorption band = 625 nm (light blue line) of the kinetic run depicted in (a) and (b). S18

19 nm Figure S15 Photoswitching of compound 2 (4 8 µm in toluene). Absorption spectra for the photoisomerization of compound 1 (λ max = 570 nm; room temperature): a) cyclization with λ = 577 nm (Table S3, entry 2; 577FS10-50, ) and b) thermal relaxation. c) Photoswitching monitored at λ max = 570 nm (dark blue line) and λ absorption band = 625 nm (light blue line) of the kinetic run depicted in (a) and (b). S19

20 nm Figure S16 Photoswitching of compound 2 (4 8 µm in toluene). Absorption spectra for the photoisomerization of compound 1 (λ max = 570 nm; room temperature): a) cyclization with λ = 530 nm (Table S2, entry 1; LED light source 3x 530 nm, ) and b) thermal relaxation. c) Photoswitching monitored at λ max = 570 nm (dark blue line) and λ absorption band = 625 nm (light blue line) of the kinetic run depicted in (a) and (b). S20

21 5.2.3 Photoswitching at lower temperature a) b) Figure S17 Absorption spectra for the photoisomerization of compound 1 (λ max = 545 nm; ~7 µm in toluene; 253 K): a) cyclization with white light (Table S2, entry 5; OSL1-EC, ) and b) reversible photochromism plot with the time-points of the spectra in (a) depicted with grey lines. An isosbestic point at 567 nm is maintained during switching Effect of light-intensity Figure S18 Reversible photochromism plot for the photoswitching of DASA 1 (λ max = 545 nm; 4 8 µm in toluene; room temperature). Irradiation of the sample was performed at λ max = 546 nm (Table S3, entry 1; 546FS10-50, ). UV/vis absorption spectra are normalized for comparison. S21

22 Measured light-intensities: entry Description Intensity (10 2 µw) 1 high 17 2 medium 6 3 low 0.7 Measured with a PM160 Power Meter with Photodiode Sensor (Thorlabs) at 546 nm Figure S19 Reversible photochromism plot for the photoswitching of DASA 2 (λ max = 570 nm; 4 8 µm in toluene; room temperature). Irradiation of the sample was performed at λ max = 577 nm (Table S3, entry 2; 577FS10-50, ). UV/vis absorption spectra are normalized for comparison. Measured light-intensities: entry Description Intensity (10 2 µw) 1 high 15 2 medium low 0.7 Measured with a PM160 Power Meter with Photodiode Sensor (Thorlabs) at 577 nm Phases of photoswitching We propose to divide the photoswitching of compound 1 and 2 into five phases (i to v, as apparent with UV/vis absorption spectroscopy). phase i phase ii phase iii phase iv phase v Before irradiation. Irradiation; generation of A from A (reaching a PSS). Irradiation: - Low temp.: PSS maintained, k 2,obs. is negligible. - High temp.: PSS maintained, but generation of B from A. After irradiation; A reverts back to A (fast), B reverts back to A (slow). After irradiation; B reverts back to A via A. S22

23 Figure S20 Schematic drawing of phases of photoswitching of DASAs 1 and 2. At 293 K (a) and 253 K (b). Figure S21 Phases of photoswitching at 253 K (a), 283 K (b) and 323 K (c). Time-drives are taken from Time-drives at different temperatures. A solution of 1 in toluene (4 8 µm, normalized) was irradiated with white light (Table S2, entry 5; OSL1-EC, ) and the time-depedent change of absorption was monitored at λ max = 545 nm (navy line) and λ absorption band = 600 nm (dotted line, light blue). S23

24 5.2.6 Photoswitching at 195 K (Changes in Color) A solution of compound 1 (2 mm in toluene) at 195 K (EtOH, liq. N 2 bath) was irradiated at λ = 530 nm (Table S2, entry 1; LED light source 3x 530 nm, ) for 5-10 min, which resulted in a color change from red to dark purple. Upon warming the sample, the color changes back to the original red. The irradiated samples (λ = 530 nm, 5-10 min) can be irradiated with λ = 627 nm (Table S2, entry 4; LED light source 3x 627 nm, ) for 2 3 min which results also in a color-change back to the original red. Under identical conditions, similar observations were made for compound 2 (color changes from dark purple to blue/navy and back). Important remarks: - The solubility of compound 1 and 2 at 195 K is quite low, preventing UV/vis absorption experiments. - Samples usually tend to become darker in the first 60 s after taking out of the cooling bath, which we attributed to a solubilizing effect. Discussion: Importantly, the observed color change is likely to be a result of the generation of A, which is responsible for the bathochromically shifted absorption band. At 195 K, this intermediate is expected to be relatively stable on the time-scale of the experiment (max. 30 min.). Irradiation with 627 nm leads to backswitching supporting the presence of the photochemical reaction rate k Compound Effect of low temperature (195 K) without irradiation Figure S22 Effect of low temperature on the sample color of compound 1 (2 mm in toluene): Sample A was kept at 293 K, whereas sample B was cooled to 195 K for 15 min and then taken out of the cooling bath and monitored during the warming process. S24

25 Irradiation at 530 nm (at 195 K) Figure S23 Effect of irradiation with 530 nm on the sample color of compound 1 (2 mm in toluene): Sample A was kept at 293 K, whereas sample B was cooled to 195 K and irradiation at 530 nm (Table S2, entry 1; LED light source 3x 530 nm, ) for min. Then the sample was taken out of the cooling bath and monitored during the warming process. A dark color changes upon warming to the original redish color Irradiation at 530 nm and 627 nm (at 195 K) Figure S24 Effect of irradiation with 627 nm on the sample color of compound 1 (2 mm in toluene) after switching with 530 nm: Sample A and B were cooled to 195 K and irradiation at 530 nm (Table S2, entry 1; LED light source 3x 530 nm, ) for min. Then sample B was irradiated at 627 nm (Table S2, entry 4; LED light source 3x 627 nm, ) for 2 3 min, while sample A was kept at 195 K in the dark. After this time, both samples were taken out of the cooling bath and monitored during the warming process. S25

26 Compound Effect of low temperature (195 K) without irradiation Figure S25 Effect of low temperature on the sample color of compound 2 (2 mm in toluene): Sample A was kept at 293 K, whereas sample B was cooled to 195 K for 15 min and then taken out of the cooling bath and monitored in the warming process Irradiation at 530 nm (at 195 K) Figure S26 Effect of irradiation with 530 nm on the sample color of compound 2 (2 mm in toluene): Sample A was kept at 293 K, whereas sample B was cooled to 195 K and irradiation at 530 nm (Table S2, entry 1; LED light source 3x 530 nm, ) for min. Then the sample was taken out of the cooling bath and monitored during the warming process. A dark color changes upon warming to the original redish color Irradiation at 530 nm and 627 nm (at 195 K) Figure S27 Effect of irradiation with 627 nm on the sample color of compound 2 (2 mm in toluene) after switching with 530 nm: Sample A and B were cooled to 195 K and irradiation at 530 nm (Table S2, entry 1; LED light source 3x 530 nm, ) for min. Then sample B was irradiated at 627 nm S26

27 (Table S2, entry 4; LED light source 3x 627 nm, ) for 2 3 min, while sample A was kept at 195 K in the dark. After this time, both samples were taken out of the cooling bath and monitored during the warming process. Remark: Sample A was partially frozen (in the middle panel). S27

28 6 Measurement of Reaction Rates PSS at different wavelengths Measurement: A solution of compound 1 (4 8 µm in toluene at 253 K) is saturated with irradiation at either 532 nm laser (300 mw, Table S1, entry 1; Cobolt Samba, ) or 561 nm laser (100 mw, Table S1, entry 2; Cobolt Jive, ). Under irradiation conditions, the system is almost immediately pumped into the photostationary state of A and A and remains stable (the system gets saturated above 20 mw). The decay of A after cessation of irradiation was monitored at 600 nm. Figure S28 Determination of PSS values at 253 K in triplicates of a solution of 1 in toluene (2 4 µm): a) c) Irradiation at λ = 532 nm. d) f) Irradiation at λ = 561 nm. PSS 532 = 69:31 (A':A) PSS 561 = 51:49 (A':A) Remarks: - Comparison with PSS-values reached in experiments for the determination of k 2 are comparable. - PSS calculation assumes ε 545 = 0 for species A and that it is a PSS of A:A and no other species are involved. S28

29 Determination of k -1 (relaxation of A ) at 253 K from the reached PSS. Figure S29 Phase iv was used for fitting. Schematic drawing of phases of photoswitching of DASA 1 and 2 at 253 K. Results obtained by single exponential decay fitting of the decay curve at 600 nm after reaching the photostationary state. Table S4 Fitting of single exponential decays. 532nm 561nm entry fitted τ1/2 (s) R 2 fitted τ1/2 (s) R ,6 0,999 9,2 0, ,3 0,999 8,9 0, ,1 0,999 11,4 0,999 S29

30 6.1.2 Determination of k2 at higher temperatures (258 K up to 278 K) Measurement: A solution of compound 1 (4 8 µm in toluene at different temperatures, as indicated) is saturated with irradiation at 532 nm laser (300 mw, Table S1, entry 1; Cobolt Samba, ). Under the irradiation conditions, the system is immediately pumped into the photostationary state of A and A and remains stable (the system gets saturated above 20 mw). In the measured temperature range: 278 K to 258 K, formation of B is possible (from A by k 2). Thus under irradiation, the rate of formation of B can be extracted by single exponential decay fitting after reaching the photostationary states. For every temperature (278 K, 273 K, 268 K, 263 K and 258 K), triplicate measurements were performed. In the following, all decay curves of the triplicates are shown and selected absorption spectra of the first measurement to illustrate the measurement K Figure S30 Determination of k 2 at 278 K: a) Selected absorption spectra of the irradiation experiment depicted in (b) to illustrate the measurement. Irradiation at λ = 532 nm (300 mw, Table S1, entry 1; Cobolt Samba, ) of a solution of 1 in toluene (4 8 µm). b) to d) Triplicate measurements of the decay curve under irradiation monitored at characteristic wavelengths for A and A (λ = 545 nm for A and λ = 600 nm for A). S30

31 K Figure S31 Determination of k 2 at 273 K: a) Selected absorption spectra of the irradiation experiment depicted in (b) to illustrate the measurement. Irradiation at λ = 532 nm (300 mw, Table S1, entry 1; Cobolt Samba, ) of a solution of 1 in toluene (4 8 µm). b) to d) Triplicate measurements of the decay curve under irradiation monitored at characteristic wavelengths for A and A (λ = 545 nm for A and λ = 600 nm for A) K Figure S32 Determination of k 2 at 268 K: a) Selected absorption spectra of the irradiation experiment depicted in (b) to illustrate the measurement. Irradiation at λ = 532 nm (300 mw, Table S1, entry 1; Cobolt Samba, ) of a solution of 1 in toluene (4 8 µm). b) to d) Triplicate measurements of the decay curve under irradiation monitored at characteristic wavelengths for A and A (λ = 545 nm for A and λ = 600 nm for A). S31

32 K Figure S33 Determination of k 2 at 263 K: a) Selected absorption spectra of the irradiation experiment depicted in (b) to illustrate the measurement. Irradiation at λ = 532 nm (300 mw, Table S1, entry 1; Cobolt Samba, ) of a solution of 1 in toluene (4 8 µm). b) to d) Triplicate measurements of the decay curve under irradiation monitored at characteristic wavelengths for A and A (λ = 545 nm for A and λ = 600 nm for A) K Figure S34 Determination of k 2 at 258 K: a) Selected absorption spectra of the irradiation experiment depicted in (b) to illustrate the measurement. Irradiation at λ = 532 nm (300 mw, Table S1, entry 1; Cobolt Samba, ) of a solution of 1 in toluene (4 8 µm). b) to d) Triplicate measurements of the decay curve under irradiation monitored at characteristic wavelengths for A and A (λ = 545 nm for A and λ = 600 nm for A). S32

33 entry Temp. ( C) Table S5 Fitting of single exponential decays at different temperatures. Temp. (K) Triplicate (1 ) (s) R 2 (1 ) Triplicate (2 ) (s) R 2 (2 ) Triplicate (3 ) (s) R 2 (3 ) ,5 0,999 29,1 0,999 27,8 0, ,8 0,999 51,5 0,999 48,1 0, ,9 0,999 79,4 0,998 99,3 0, , , , , , ,999 Table S6 Mean and standard deviation of the triplicate measurements. entry Temp. ( C) mean Standard deviation ,1 1, ,5 2, ,5 11, , ,7 Eyring Plot -33 ln(k*h)/k B T y = (-8438 ± 112.9)*x - (2.419 ± ) R 2 = /T Figure S35 Eyring plot analysis for k 2 of 1 in toluene (4 8 µm): Reaction-rates were determined at five different temperatures (278 K, 273 K, 268 K, 263 K and 258 K) in triplicates, by monitoring the decrease in absorption at 600 nm during irradiation. The rate constant were determined by single exponential decay fitting (R 2 -values are given in Table S5). ΔG = 76.0 kj mol -1, ΔH = 70.2 kj mol -1, ΔS = J mol -1 K -1, t 1/2(293 K)= 4.05 s, k(293 K) = 0.17 s -1. S33

34 6.1.3 Measurement and Eyring analysis of k-1,thermal Measurement: A solution of compound 1 (4 8 µm in toluene at different temperatures, as indicated) was irradiated with white light (Table S2, entry 5; OSL1-EC, ) for 60 s and the photoswitching was monitored at λ max = 545 nm and λ absorption band = 600 nm (see data at Time-drives at different temperatures). To determine k -1,thermal, phase iv (see Figure S36) was identified and the single exponential decay at 600 nm fitted. Fitting parameters and quality of fit are indicated in Table S7. Figure S36 Phase iv was used for fitting. Schematic drawing of phases of photoswitching of DASAs 1 and 2 at 293 K (a) and 253 K (b). Table S7 Fitting of single exponential decays at 600 nm in phase iv at different temperatures. entry Temp. ( C) Temp. (K) fitted value (τ1/2) (s) R ,158 0, ,250 0, ,647 0, ,060 0, ,545 0, ,305 0, ,776 0,999 S34

35 -29 Eyring Plot ln(k*h)/k B T y = (-4657 ± 437.9)*x - (13.31 ± 1.637) R 2 = /T Figure S37 Eyring plot analysis for k -1,thermal of 1 in toluene (4 8 µm): Reaction-rates were determined at seven different temperatures (283 K, 278 K, 273 K, 268 K, 263 K, 258 K and 253 K) on single measurements by monitoring the decrease in absorption at 600 nm during irradiation with white light (Table S2, entry 5; OSL1-EC, ). The rate constant were determined by single exponential decay fitting (R 2 -values are given in Table S7). ΔG = 70.6 kj mol -1, ΔH = 47.9 kj mol -1, ΔS = J mol -1 K -1, t 1/2(293 K)= 0.43 s, k(293 K) = 1.62 s -1. S35

36 6.1.4 Measurement and Eyring analysis of k-2 Measurement: A solution of compound 1 (4 8 µm in toluene at different temperatures, as indicated) was irradiated with white light (Table S2, entry 5; OSL1-EC, ) for 60 s and the photoswitching was monitored at λ max = 545 nm and λ absorption band = 600 nm (see data at Time-drives at different temperatures). To measure k -2, phase v (see Figure S38) was identified and the single exponential association at 545 nm fitted. Fitting parameters and quality of fit are indicated in Table S8. Figure S38 Phase v was used for fitting. Schematic drawing of phases of photoswitching of DASAs 1 and 2 at 293 K. Table S8 Fitting of single exponential association at 545 nm in phase v at different temperatures. entry Temp. ( C) Temp. (K) fitted value (τ1/2) (s) R , , , ,8 0,999 General remark: Below 278 K, the reaction rate becomes too slow for a reasonable fit on the time-scale of the experiment. S36

37 Eyring Plot ln(k*h)/k B T y = (-6261 ± 606.6)*x - (13.49 ± 2.071) R 2 = /T Figure S39 Eyring plot analysis for k -2 of 1 in toluene (4 8 µm): Reaction-rates were determined at four different temperatures (323 K, 293 K, 283 K and 278 K) on single measurements by monitoring the increase in absorption at 545 nm during thermal reversion. The rate constant were determined by single exponential association fitting (R 2 -values are given in Table S8). ΔG = 84.9 kj mol -1, ΔH = 52.1 kj mol -1, ΔS = J mol -1 K -1, t 1/2(293 K)= 154 s, k(293 K) = s -1. S37

38 7 Estimation of the Photochemical Quantum Yield The quantum yield (φ) for the photoswitching process (Z E isomerization) of compound 1 at the wavelength λ = 532 nm (laser, 300 mw, Table S1, entry 1; Cobolt Samba, ) was determined in toluene (17.2 µm) at 253 K. The determined value is a result of seven distinct experiments. The switching process was followed by UV/vis spectroscopy and the change in absorbance determined at λ max = 532 nm. Care was taken that the toluene solution containing 1 absorbs 90% of the incident light (Absorbance > 1.0). Samples were irradiated until the PSS was reached. For analysis, only the initial conversion was considered (~10%, linear regime of Δt and ΔA). The incident light intensity at 532 nm was determined using a PM160 Power Meter with Photodiode Sensor (Thorlabs). All light was focused on the cuvette. The quantum yield was determined as φ = 0.17 ± Absorbance Absorbance at 532 nm Linear Regression y = *x R 2 = Time (s) Figure S40 Determination of the photochemical quantum yield (φ 532) 253 K: Every data point represents an average of seven measurements. The initial three measurements were fitted with a linear regression. S38

39 Figure S41 Determination of the photochemical quantum yield (φ) at 253 K: a) Selected absorption spectra of the irradiation experiment depicted in (b) to illustrate the measurement. Irradiation at λ = 532 nm (300 mw, Table S1, entry 1; Cobolt Samba, ) of a solution of 1 in toluene (17.2 µm). The decay curve under irradiation was monitored at characteristic wavelengths for A and A (λ = 532 nm and λ = 545 nm for A and λ = 600 nm for A). S39

40 8 Absorption Spectra of Cyclized-1 Cyclization of donor-acceptor Stenhouse adduct (DASA, 1): 4 Compound cyclized-1 has been prepared according to a reported procedure 4a using white light (halogen, Table S2, entry 6; Plusline ES Small 160W 3100 lm, ). Spectral properties matched previously reported values. Figure S42 Absorption spectra for cyclized-1 (B) in different concentrations and solvents: a) in methanol, 4 55 µm and b) in water 4 77 µm. No photoswitching. S40

41 9 UV/vis Photoswitching in Water Figure S43 Photoswitching of compound 1 (4 8 µm) in ddh 2O: Absorption spectra for the photoisomerization of compound 1 (λ max =480 nm; room temperature) with white light (Table S2, entry 5; OSL1-EC, ). S41

42 10 1 H-NMR Photoswitching Studies in Water NMR-samples were prepared in the corresponding deuterated solvent as saturated solutions of DASA 1 (or DASA 2, respectively) and irradiated for the indicated time-period with white light (halogen, Table S2, entry 6; Plusline ES Small 160W 3100 lm, ) Compound D 2O before 1 h white light 2 h white light 18 h white light Figure S44 1 H-NMR-studies of the photochemical cyclization of DASA 1 in D 2O under white light irradiation (halogen, Table S2, entry 6; Plusline ES Small 160W 3100 lm, ). Remark: Appearing peak of acetone at 2.22 ppm. S42

43 10.2 Compound D 2O before 1 h white light 2 h white light 3 h white light 4 h white light 18 h white light Figure S45 1 H-NMR-studies of the photochemical cyclization of DASA 2 in D 2O under white light irradiation (halogen, Table S2, entry 6; Plusline ES Small 160W 3100 lm, ). S43

44 10.3 Comparison: No irradiation Remark: Both DASA 1 and 2 cyclize slowly in water without irradiation. D2O, DASA 1, before D2O, DASA 1, 18 h dark D2O, DASA 1, 50 h dark D2O, DASA 2, before D2O, DASA 2, 18 h dark Figure S46 1 H-NMR-studies of the cyclization of DASA 1 and 2 in D 2O (2 mm) in the dark. S44

45 11 Photoswitching in Dichloromethane Rationale: Halogenated solvents generally favor A, as has been shown by the back-extraction of cyclized DASAs from a water layer to the dichloromethane layer. 4b A proposed Z E isomerization would still be expected to take place in dichloromethane, even though formation of B is not favored UV/vis absorption spectra and photoswitching Figure S47 Absorption spectrum of compound 1 (~8 µm in dry dichloromethane; room temperature) without and with application of an optical cut-off filter (< 440 nm, SCF-50S-44Y). high med. low high = high intensity white light med. = medium intensity white light low = low intensity white light Figure S48 Photoswitching of compound 1 (~8 µm in dry dichloromethane; room temperature) using a 440 nm optical cut-off filter (SCF-50S-44Y). Irradiation with white light (Table S2, entry 5; OSL1- EC, ) of different intensity (high, medium and low) and λ = 546 nm (Table S3, entry 1; 546FS10-50, S45

46 ) as indicated. Photoswitching monitored at λ max = 539 nm (black line) and λ absorption band = 590 nm (light green line). Figure S49 Photoswitching of compound 1 (~8 µm in dry dichloromethane; room temperature) using a 440 nm optical cut-off filter (SCF-50S-44Y ) for prolonged irradiation with white light (Table S2, entry 5; OSL1-EC, ). Photoswitching monitored at λ max = 539 nm (black line) and λ absorption band = 590 nm (dark green line). Discussion: Formation of a transient intermediate is observed, that behaves similarly to A in toluene at low temperature. The photostationary state reached depends on the light-intensity and light-source used H-NMR irradiation experiments Setup The LED-based irradiation setup for NMR spectroscopic measurements was built according to a reported system. 5 The fiber-optic cable and the LED was purchased from Thorlabs: 530 nm Fiber-coupled LED (Table S2, entry 2; M530F2, ) M28L05; Ø400 μm, 0.39 NA, SMA-SMA Fiber Patch Cable, 5 Meters The NMR tubes were purchased from Wilmad-LabGlass (SP Scienceware): WGS-5BL Coaxial Insert for 5 mm NMR Sample Tube 535-PP-7 5 mm Thin Wall Precision NMR Sample Tube 7" L, 600MHz The measured optical output to the NMR sample amounts to about 4-5 mw. S46

47 Measurement Conditions: CD 2Cl 2, 4 mm; 203 K. Figure S50: 1 H-in-NMR isomerization studies at 203 K in CD 2Cl 2: Overlay of the non-irradiated (orange) and irradiated (cyan) sample Discussion The following can be concluded: - Single photogenerated isomer - Polyene region is affected - Very low PSS obtained (combination of weak irradiation source, high concentration (4 mm) and high molar absorptivity of DASA 1 S47

48 11.3 Photoswitching at 195 K (changes in color) A solution of compound 1 (2 mm in dichloromethane) at 193 K (EtOH, liq. N 2 bath) was irradiated at λ = 530 nm (Table S2, entry 1; LED light source 3x 530 nm, ) for 15 min, which resulted in a color change from red to dark red/purple. Upon warming the sample, the color changes back to the original red. Discussion: Importantly, the observed color change is likely to be a result of the generation of the observed transient intermediate A (Figures S48 S50), which is responsible for the bathochromically shifted absorption band. At 195 K, this intermediate is expected to be relatively stable on the time-scale of the experiment (max. 30 min.) Compound Effect of low temperature (195 K) without irradiation Figure S51 Effect of low temperature on the sample color of compound 1 (2 mm in dichloromethane): Sample A was kept at 293 K, whereas sample B was cooled to 195 K for 10 min and then taken out of the cooling bath and monitored during the warming process Irradiation at 530 nm (at 195 K) Figure S52 Effect of irradiation with 530 nm on the sample color of compound 1 (2 mm in dichloromethane): Sample A was kept at 293 K, whereas sample B was cooled to 195 K and irradiation at 530 nm (Table S2, entry 1; LED light source 3x 530 nm, ) for 15 min. Then the sample was taken out of the cooling bath and monitored during the warming process. The observed dark red/purple color changes upon warming to the original reddish color. S48

49 12 TD-DFT Calculations The Gaussian 09 program 6 was used. Input geometries were first optimized at the semi-empirical PM6 level to find the global minima. DFT and TD-DFT calculations were carried out by using a similar computational approach as the one recently described by the group of Jacquemin for DASA switches. 7 Hence, further geometry optimizations were performed at the DFT MO6-2X/6-31+G(d) level of theory using tight convergence criteria. All calculated structures were found to have zero imaginary frequencies. Zero-point energy and TD-DFT calculations were carried out using the larger G(2df,2p) atomic basis set. Solvent effects were accounted for by using an IEFPCM toluene solvent model. The geometries and energies of three possible photogenerated intermediates were calculated (Scheme S1). The total energies (ΔE) relative to the initial state A are 20.9, 20.4, and 4.6 kj mol 1 for A (I), A (II) and A (III), respectively. Previous calculations of the cyclized product B revealed an energy difference (ΔE in MeOH) of 7.1 kj mol 1, 7 which indicates that cyclization of A (III) is energetically uphill. Scheme S1 Possible intermediates A I-III upon phototriggered Z E isomerization of A. The HOMO and LUMO plots of A and A (I-III) are shown in Figure S53 and the calculated absorbance spectra in Figure S54. The calculated absorption maximum for A in toluene (λ max = 465 nm) corresponds with that previously calculated in MeOH (λ max = 449 nm) using the same DFT method which has a discrepancy with the experimental trend. 7 The absorption maxima for the intermediates A (I) and A (II) are bathochromically shifted to 502 nm and 482 nm, respectively, whereas the absorption of A (III) remains virtually the same. These TD-DFT calculations therefore support the notion that Z E isomerization of the neutral form of DASA A can cause a bathochromic shift which is in line with what is observed experimentally. S49

50 Figure S53. LUMO plots for the initial state A and photogenerated states A I-III. Figure S54. Calculated absorption spectra for A and A I-III showing the nm range. S50

51 Table S9. Cartesian coordinates of A [MO6-2X/ G(2df,2p)] atom X Y Z C C C C C C C C C C C C C C C N O O O O O H H H H H H H H H H H H H H H H H H H H H E(RM062X) = S51

52 Table S10. Cartesian coordinates of A (I) [MO6-2X/ G(2df,2p)] atom X Y Z C C C C C C C C C C C C C C C N O O O O O H H H H H H H H H H H H H H H H H H H H H E(RM062X) = S52

53 Table S11. Cartesian coordinates of A (II) [MO6-2X/ G(2df,2p)] atom X Y Z C C C C C C C C C C C C C C C N O O O O O H H H H H H H H H H H H H H H H H H H H H E(RM062X) = S53

54 Table S12. Cartesian coordinates of A (III) [MO6-2X/ G(2df,2p)] atom X Y Z C C C C C C C C C C C C C C C N O O O O O H H H H H H H H H H H H H H H H H H H H H E(RM062X) = S54