The micron-scale movement of biomolecules along supramolecular pathways, mastered by nature, is a remarkable system requiring strong yet reversible interactions between components under the action of a suitable stimulus. Responsive microscopic systems using a variety of stimuli have demonstrated impressive relative molecular motion. However, locating the position of a movable object that travels along self-assembled fibres under an irresistible force has yet to be achieved. Here, we describe a purely supramolecular system where a molecular ‘traveller’ moves along a ‘path’ over several microns when irradiated with visible light. Real-time imaging of the motion in the solvated state using total internal reflection fluorescence microscopy shows that anionic porphyrin molecules move along the fibres of a bis-imidazolium gel upon irradiation. Slight solvent changes mean movement and restructuring of the fibres giving microtoroids, indicating control of motion by fibre mechanics with solvent composition. The insight provided here may lead to the development of artificial travellers that can perform catalytic and other functions.
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All data supporting the findings are included in the manuscript and Supplementary Information.
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All the authors thank the School of Life Sciences Imaging (SLIM) in Nottingham for access to the optical microscope and the Nanoscale and Microscale Research Centre (nmRC) for facilitating access to electron microscopes. D.B.A. thanks the Telluride Conference on Molecular Rotors, Motors, and Switches for inspiring this research. We warmly thank M. Amabilino i Pérez for assistance with graphics (D.B.A.). A.R.M. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under Marie–Skłodowska–Curie grant agreement no. 793424. The microscope facility was established using the BB/L013827/1 fund. This work was supported by the Engineering and Physical Sciences Research Council (EPSRC) under grants EP/M005178/1 and EP/N024818/1 (D.B.A.), EU ERDF (FEDER) funds, Spanish Government grants TEC2017-85059-C3-2-R and PID2020-115663GB-C3-2 (L.P.-G.) and the University of Nottingham (B.B., M.S.) including work under the Anne McLaren fellowship scheme (L.P.-G.) and the Propulsion Futures Beacon of Excellence (D.B.A.).
The authors declare no competing interests.
Peer review information Nature Chemistry thanks the anonymous reviewers for their contribution to the peer review of this work.
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(a) A picture of gel samples of 1·2Br obtained with water:ethanol ratio 5:5 without guest molecules (Gel), with incorporated azobenzene (Gel@Azo), porphyrin (Gel@TCPP) and both (Gel@TCPP@Azo). The scale bar represents 1 cm. Gel samples were always prepared by addition of MilliQ water to an ethanolic solution of 1·2Br, giving a final amphiphile concentration of 8 mM. For TCPP-containing gels, aqueous solutions of the TCPP sodium salt were prepared by dispersing the desired amount of solid TCPP in water followed by the addition of 4 equivalents of sodium hydroxide (from a 0.1 M stock solution). For Azo-containing gels, an ethanolic solution of 4-(phenylazo)benzoic acid (Azo)was premixed with 1·2Br, and the equimolar equivalent of sodium hydroxide was added in water in order to incorporate the Azo as its sodium salt. The final concentration of TCPP and Azo within the gels was always 60 μM and 2 mM, respectively. (b) Gelation time of the samples at room temperature (approximately 22 °C) obtained with solvent ratio 5:5 (blue bars) and 7:3 (red bars).
Shear stress profiles displaying storage (G’, black curve) and loss (G”, red curve) moduli obtained for gels in water-ethanol ratio 5:5 (top box) and 7:3 (bottom box).
X-ray powder diffractograms of xerogels made from gel with no guests (black lines), Gel@TCPP (red lines), Gel@Azo (green lines) and Gel@TCPP@Azo (blue lines) in water-ethanol ratio 5:5 (a) and 7:3 (b).
UV-Visible absorption spectra of: Top. Azo 50 μM in homogeneous ethanol solution as prepared (black line) and sequential irradiation under light at 405 and 365 nm, and; Below. Gel@TCPP@Azo in 5:5 water:ethanol under light irradiation at 405 and 365 nm.
TIRF images of Gel@TCPP@Azo in water-ethanol 5:5 before (a-b-c-d) and after (e-f-g-h) irradiation (for a total of 8.3 minutes) with light at 405, 488, 561, 642 nm in the central region. Scale bar represents 10 μm. The difference maps below show the areas where changes of intensity can be appreciated (note that the white and black intensities on the four maps have different intensities and do not indicate amount of change). Some fibres in the irradiated area are darker in the difference map (so there is more porphyrin after irradiation). Those fibres are apparently above the focal plane, deeper into the sample compared with the layer imaged. In principle, this can occur because laser intensity is higher at the slide-sample interface, then decreases as we look deeper into the sample. It is evidence for movement of TCPP into the sample as well as to the sides of the irradiated area.
Zoomed SRRF images of Gel@TCPP@Azo in water-ethanol 7:3 during irradiation at 405 nm. The frames are taken from Supplementary Video 9. Scale bar represents 2 μm, all images are to the same scale.
The steady state fluorescence spectra (excitation wavelength 405 nm) in both 5:5 and 7:3 water:ethanol gels show (top row) that there is a quenching of the porphyrin fluorescence in Gel@TCPP@Azo compared with Gel@TCPP for both solvent mixtures, as a result of energy transfer from the TCPP to the Azo chromophore. The middle row shows the time evolution of the same Gel@TCPP samples under continuous irradiation at 405 nm, where a modest bleaching is observed over time for both solvent systems. The bottom row shows the evolution of the fluorescence of Gel@TCPP@Azo under continuous irradiation, where at 5:5 water:ethanol a slight bleaching is observed (similar to Gel@TCPP), while for the 7:3 mixture an increase in fluorescence is observed, so that after 60 minutes the intensity is actually higher than that for Gel@TCPP after the same time.
TIRF micrographs from two regions at time zero with irradiation at 405 nm (top) and after the end of Supplementary Movie 13 (left) and Supplementary Video 15 (right) of sample Gel@TCPP@Azo where Azo was enriched in the cis isomer by photoisomerization prior to gel preparation. Scale bar represents 5 μm on the left hand series and 2 μm in the right hand series, all images in a series are to the same scale.
List of videos, Supplementary Figs. S1–S19, Tables S1–S6 and Synthetic Procedures and Characterization.
TIRF video of an irradiation experiment performed on sample Gel@TCPP@Azo obtained in water:ethanol ratio 5:5. Real time 8.3 min, 20 frames per second (fps).
TIRF video of an irradiation experiment performed on sample Gel@TCPP@Azo obtained in water:ethanol ratio 7:3. Real time 8.3 min, 20 fps.
TIRF video of the central irradiation experiment performed on sample Gel@TCPP@Azo obtained in water:ethanol ratio 5:5. Real time 8.3 min, 20 fps.
TIRF video of the central irradiation experiment performed on sample Gel@TCPP@Azo obtained in water:ethanol ratio 6:4. Real time 8.3 min, 20 fps.
Video of an irradiation experiment performed on sample Gel@TCPP@Azo 5:5 using light at 405 nm. Real time 8.3 min, 20 fps.
Video of the irradiation experiment performed on sample Gel@TCPP@Azo 5:5 using light at 488 nm. Real time 5 min, 10 fps.
Video of the irradiation experiment performed on sample Gel@TCPP@Azo 5:5 using light at 561 nm. Real time 8.3 min, 20 fps.
Video of the irradiation experiment performed on sample Gel@TCPP@Azo 5:5 using light at 642 nm. Real time 8.2 min, 20 fps.
SRRF video of the irradiation experiment performed on sample Gel@TCPP@Azo obtained in water:ethanol ratio 7:3. Real time 1.7 min, 10 fps.
TIRF video of the irradiation experiment performed on sample Gel@TCPP obtained in water:ethanol ratio 5:5. Real time 8.3 min, 10 fps.
TIRF video of the irradiation experiment performed on sample Gel@TCPP obtained in water:ethanol ratio 7:3. Real time 1.7 min, 10 fps.
TIRF video of the irradiation experiment performed on sample Gel@TCPP@AzoH (prepared from the sodium salt of TCPP but with no additional base to deprotonate the Azo compound) obtained in water:ethanol ratio 5:5. Real time 8.3 min, 10 fps.
TIRF video of an irradiation experiment performed on sample Gel@TCPP@Azo in 5:5 water:ethanol with a starting trans:cis ratio of approximately 35:65 (prepared by irradiating the ethanol stock solution containing the Azo used for the preparation of the gel at 365 nm). Real time 50 s, 5 fps.
TIRF video of the irradiation experiment performed on sample Gel@TCPP@Biph obtained in water:ethanol ratio 7:3. Real time 8.3 min, 20 fps.
TIRF video of an irradiation experiment performed on sample Gel@TCPP@Azo in 5:5 water:ethanol with a starting trans:cis ratio of approximately 35:65 (prepared by irradiating the ethanol stock solution containing the Azo used for the preparation of the gel at 365 nm). Real time 120 s, 10 fps.
TIRF video of an irradiation experiment performed on sample Gel@TCPP@Azo obtained in water:ethanol ratio 5:5. Real time 8.3 min, 20 fps.
TIRF video of an irradiation experiment performed on sample Gel@TCPP@Azo obtained in water:ethanol ratio 7:3. Real time 8.3 min, 10 fps.
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Samperi, M., Bdiri, B., Sleet, C.D. et al. Light-controlled micron-scale molecular motion. Nat. Chem. 13, 1200–1206 (2021). https://doi.org/10.1038/s41557-021-00791-2
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