Ultrafast dynamics of an azobenzene-containing molecular shuttle based on a rotaxane

An ultrafast spectroscopic study was carried out for a photoisomerizable, rotaxane-based molecular shuttle, in which photoisomerization of the azobenzene moiety of the thread-like guest drives a shuttling motion of a...

Resolving photoisomerization dynamics via ultrafast UV-visible transient absorption spectroscopy

<p>Transient absorption spectroscopy has been employed to investigate three photo–active compounds; azobenzene, foldamer controlled by azobenzene, and oxazine. These compounds all have absorption in the ultra–violet regions responsible for their photo–active behavior. Due to this, the current transient absorption setup has been modified to extend the probing wavelength range to 320–650 nm, with the possibility of exciting the photo–active molecule in the ultra–violet. Azobenzene is valuable in benchmarking and optimizing the transient absorption setup, it shows that the detection window has been extended out to 320 nm. By resolving the ground state bleach we have added support for the assignment of the final decay to thermalization in the ground state. Comparison of relaxation lifetime in acetonitrile and tetrahydrofuran shows no noticeable change in the photophysics of isomerization between the two solvents. The foldamer family excited state relaxation is similar to azobenzene. There is an extension in the S₁ branching lifetime from 1.1 ps in azobenzene to 1.7 ps for foldamer 1 and 4.2 ps for foldamer 2. The separation of branching on the S₁ surface and relaxation through the S₁ to electronic ground state intersection was possible by comparison of azobenzene and foldamer family. The solvent effects show little difference for all members of the foldamer family expect for foldamer 2, suggesting that the dynamics of the azobenzene moiety are not affected by the larger macro–structure of the foldamer. For oxazine it has been established, by varying solvent polarity, that isomerization happens through three states; bond breakage, transfer to a dark state, and the final photo–isomer. This is confirmed by further studies completed after the introduction of electron withdrawing fluorine atoms. Carbon–oxygen bond cleavage occurs on the picosecond timescale, with solvent dependent rotation occurring in hundreds of picoseconds. Fluorinated oxazine shows a strong solvent dependence with rotation suppressed for all but the most polar of solvents.</p>

Resolving photoisomerization dynamics via ultrafast UV-visible transient absorption spectroscopy

<p>Transient absorption spectroscopy has been employed to investigate three photo–active compounds; azobenzene, foldamer controlled by azobenzene, and oxazine. These compounds all have absorption in the ultra–violet regions responsible for their photo–active behavior. Due to this, the current transient absorption setup has been modified to extend the probing wavelength range to 320–650 nm, with the possibility of exciting the photo–active molecule in the ultra–violet. Azobenzene is valuable in benchmarking and optimizing the transient absorption setup, it shows that the detection window has been extended out to 320 nm. By resolving the ground state bleach we have added support for the assignment of the final decay to thermalization in the ground state. Comparison of relaxation lifetime in acetonitrile and tetrahydrofuran shows no noticeable change in the photophysics of isomerization between the two solvents. The foldamer family excited state relaxation is similar to azobenzene. There is an extension in the S₁ branching lifetime from 1.1 ps in azobenzene to 1.7 ps for foldamer 1 and 4.2 ps for foldamer 2. The separation of branching on the S₁ surface and relaxation through the S₁ to electronic ground state intersection was possible by comparison of azobenzene and foldamer family. The solvent effects show little difference for all members of the foldamer family expect for foldamer 2, suggesting that the dynamics of the azobenzene moiety are not affected by the larger macro–structure of the foldamer. For oxazine it has been established, by varying solvent polarity, that isomerization happens through three states; bond breakage, transfer to a dark state, and the final photo–isomer. This is confirmed by further studies completed after the introduction of electron withdrawing fluorine atoms. Carbon–oxygen bond cleavage occurs on the picosecond timescale, with solvent dependent rotation occurring in hundreds of picoseconds. Fluorinated oxazine shows a strong solvent dependence with rotation suppressed for all but the most polar of solvents.</p>

Photoinduced Collective Motion of Oil Droplets and Concurrent Pattern Formation in Surfactant Solution

Collective motion is ubiquitous in living systems. Although various biomimetic artificial systems have been constructed, there have been few studies reported on collective motion induced by the coupling of chemical reactions, diffusion and convection in a far-from-equilibrium state. In this study, we report an artificial system of oil droplets in a surfactant solution wherein the collective motion of multiple droplets and pattern formation occurred concurrently. Using photo-responsive surfactants with an azobenzene moiety, the assembly of droplets and the formation of circular patterns around the formed droplet clusters occurred under UV illumination, whereas the disassembly of droplets and disappearance of the patterns occurred under subsequent visible light illumination. The observed dynamics were induced by Marangoni flows based on the reversible photoisomerisation of azobenzene-containing surfactants. The phenomena were considered analogous to the bioconvection of microorganisms. These findings could be useful for understanding the mechanism of motion of life in terms of physicochemical aspects.

Photoinduced Collective Motion of Oil Droplets and Concurrent Pattern Formation in Surfactant Solution

Collective motion is ubiquitous in living systems. Although various biomimetic artificial systems have been constructed, there have been few studies reported on collective motion induced by the coupling of chemical reactions, diffusion and convection in a far-from-equilibrium state. In this study, we report an artificial system of oil droplets in a surfactant solution wherein the collective motion of multiple droplets and pattern formation occurred concurrently. Using photo-responsive surfactants with an azobenzene moiety, the assembly of droplets and the formation of circular patterns around the formed droplet clusters occurred under UV illumination, whereas the disassembly of droplets and disappearance of the patterns occurred under subsequent visible light illumination. The observed dynamics were induced by Marangoni flows based on the reversible photoisomerisation of azobenzene-containing surfactants. The phenomena were considered analogous to the bioconvection of microorganisms. These findings could be useful for understanding the mechanism of motion of life in terms of physicochemical aspects.<br>

Photoinduced Collective Motion of Oil Droplets and Concurrent Pattern Formation in Surfactant Solution

Collective motion is ubiquitous in living systems. Although various biomimetic artificial systems have been constructed, there have been few studies reported on collective motion induced by the coupling of chemical reactions, diffusion and convection in a far-from-equilibrium state. In this study, we report an artificial system of oil droplets in a surfactant solution wherein the collective motion of multiple droplets and pattern formation occurred concurrently. Using photo-responsive surfactants with an azobenzene moiety, the assembly of droplets and the formation of circular patterns around the formed droplet clusters occurred under UV illumination, whereas the disassembly of droplets and disappearance of the patterns occurred under subsequent visible light illumination. The observed dynamics were induced by Marangoni flows based on the reversible photoisomerisation of azobenzene-containing surfactants. The phenomena were considered analogous to the bioconvection of microorganisms. These findings could be useful for understanding the mechanism of motion of life in terms of physicochemical aspects.<br>

Overtemperature-protection intelligent molecular chiroptical photoswitches

Stimuli-responsive intelligent molecular machines/devices are of current research interest due to their potential application in minimized devices. Constructing molecular machines/devices capable of accomplishing complex missions is challenging, demanding coalescence of various functions into one molecule. Here we report the construction of intelligent molecular chiroptical photoswitches based on azobenzene-fused bicyclic pillar[n]arene derivatives, which we defined as molecular universal joints (MUJs). The Z/E photoisomerization of the azobenzene moiety of MUJs induces rolling in/out conformational switching of the azobenzene-bearing side-ring and consequently leads to planar chirality switching of MUJs. Meanwhile, temperature variation was demonstrated to also cause conformational/chiroptical inversion due to the significant entropy change during the ring-flipping. As a result, photo-induced chiroptical switching could be prohibited when the temperature exceeded an upper limit, demonstrating an intelligent molecular photoswitch having over-temperature protection function, which is in stark contrast to the low-temperature-gating effect commonly encountered.

A Photoresponsive Artificial Viral Capsid Self-Assembled from an Azobenzene-Containing β-Annulus Peptide

Photoinduced structural changes in peptides can dynamically control the formation and dissociation of supramolecular peptide materials. However, the existence of photoresponsive viral capsids in nature remains unknown. In this study, we constructed an artificial viral capsid possessing a photochromic azobenzene moiety on the peptide backbone. An azobenzene-containing β-annulus peptide derived from the tomato bushy stunt virus was prepared through solid-phase synthesis using Fmoc-3-[(3-aminomethyl)-phenylazo]phenylacetic acid. The azobenzene-containing β-annulus (β-Annulus-Azo) peptide showed a reversible trans/cis isomerization property. The β-annulus-azo peptide self-assembled at 25 μM into capsids with the diameters of 30–50 nm before UV irradiation (trans-form rich), whereas micrometer-sized aggregates were formed after UV irradiation (cis-form rich). The artificial viral capsid possessing azobenzene facilitated the encapsulation of fluorescent-labeled dextrans and their photoinduced release from the capsid.

A photoswitchable helical peptide with light-controllable interface / transmembrane topology in lipidic membranes

According to the three-step model, the spontaneous insertion and folding of helical transmembrane (TM) polypeptides into lipid bilayers is driven by three sequential equilibria: solution-to-membrane interface (MI) partition, unstructured-to-helical folding, and MI-to-TM helix insertion. However, understanding these three steps with molecular detail has been challenged by the lack of suitable experimental approaches to rapidly and reversibly perturb membrane-bound hydrophobic polypeptides out of equilibrium. Here, we report on a 24-residues-long hydrophobic α-helical polypeptide, covalently coupled to an azobenzene photoswitch (KCALP-azo), which displays a light-controllable TM/MI equilibrium in hydrated lipid bilayers. FTIR spectroscopy shows that dark-adapted KCALP-azo (trans azobenzene) folds as a TM α-helix, with its central TM region displaying an average tilt of 36 ± 4° with the membrane normal (TM topology). After trans-to-cis photoisomerization of the azobenzene moiety with UV light (reversed with blue light), spectral changes by FTIR spectroscopy indicate that the helical structure of KCALP-azo is maintained but the peptide experiences a more polar environment. Interestingly, pH changes induced similar spectral alterations in the helical peptide LAH4, with a well-characterized pH-dependent TM/MI equilibrium. Polarized experiments confirmed that the membrane topology of KCALP-azo is altered by light, with its helix tilt changing reversibly from 32 ± 5° (TM topology, blue light) to 79 ± 8° (MI topology, UV light). Further analysis indicates that, while the trans isomer of KCALP-azo is ~100% TM, the cis isomer exists in a ~90% TM and ~10% MI mixture. Strategies to further increase the perturbation of the TM/MI equilibrium with the light are briefly discussed.