Mina Han

Light-driven molecular switches in azobenzene self-assembled monolayers: effect of molecular structure on reversible photoisomerization and stable cis state

Both the reversible trans<-->cis photoisomerization and slow thermal back cis-to-trans isomerization of azobenzene-functionalized self-assembled monolayers on gold surfaces have been achieved by rationally designed single-component azobenzene thiol.

Rational design of light-directed dynamic spheres

We created light-directed dynamic spheres based on simple azobenzene monomers showing (i) a high yield of reversible trans↔cis photoisomerization and (ii) noticeable phase transition from crystalline to isotropic states under UV light irradiation at ambient temperature.

Realization of highly photoresponsive azobenzene-functionalized monolayers

We report the successful fabrication of azobenzene-functionalized self-assembled monolayers (SAMs) exhibiting high and reversible photoswitching between trans and cis states on a flat gold surface. Azobenzene thiols (MeSH and EtSH) containing meta and/or ortho substituents were chosen based on the occupied area per molecule as well as intermolecular interactions between the azobenzene aromatic rings (formation of H-aggregates). Theoretical predictions of the geometrical structures were performed to clarify the correlation between the molecular structure and photoisomerization characteristics in monolayer systems. The striking difference in absorption spectra of a trans-EtSH SAM and a cis-EtSH SAM by alternating UV and visible light irradiation was in good agreement with that in their contact angles for water, strongly indicating that the structural changes were controlled by light wavelength. By contrast, despite there being sufficient free space for each MeSH molecule, the strong tendency of the planar azobenzene units to generate H-aggregates even during cis-MeSH SAM formation lessened the trans-to-cisphotoisomerization yield in a monolayer.

A reversibly photoswitchable mononuclear palladium(II) complex with ortho-diethylated azobenzene ligands

A mononuclear palladium(II) complex (PdCl2(2Et-Azo)2), with sterically bulky ortho-diethylated azobenzenes (2Et-Azo) in the long-lived cis form, was designed to achieve reversible photoswitching originating from a reversible change in its molecular structure. An X-ray crystallographic study indicated that the metal center in the complex has a trans square-planar structure, with two 2Et-Azo ligands coordinated to a palladium ion in a monodentate manner. Alternating UV and visible light irradiation of PdCl2(2Et-Azo)2 gave rise to reversible changes in the molecular structure between the trans and cis forms, confirmed by UV-vis absorption and NMR spectroscopic measurements. Neither significant decomposition nor fast dissociation of PdCl2(2Et-Azo)2 was observed by UV and visible light irradiation. Furthermore, slow thermal back-isomerization occurred over the course of 2 days at ambient temperature, which may be closely related to the stability of the cis form of the ortho-alkylated azobenzene ligand.

One-dimensional molecular zippers.

We synthesized an azobenzene derivative to demonstrate a one-dimensional molecular zipper. The formation and underlying mechanism of the molecular zipper formed by combined hydrogen-bonding and van der Waals interactions between adjacent molecules were investigated on a Au(111) surface using scanning tunneling microscopy and density functional theory calculations.

Effect of the steric molecular structure of azobenzene on the formation of self-assembled monolayers with a photoswitchable surface morphology.

The growth processes of self-assembled monolayers (SAMs) of two azobenzene disulfides formed on flat gold surfaces were studied to confirm the effect of the intermolecular interactions between azobenzene molecules on the light-triggered surface morphologies of the SAMs. Scanning tunneling microscopy (STM), atomic force microscopy (AFM), thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), and ultraviolet-visible (UV-vis) absorption spectroscopy were employed to study the SAMs and their growth processes. The SAMs studied were of bulky-substituted azobenzene disulfide (Et-2S), and nonsubstituted azobenzene disulfide (Me-2S), formed on a gold-covered substrate, and had a twisted and a planar structure, respectively. STM-based imaging of the initial stage of the self-assembly of the Et-2S molecules revealed that cleavage of the disulfide bond occurred on the gold surface, and phase-separated domains composed of azobenzenethiolate and dodecanethiolate were formed. Time-dependent AFM-based imaging illustrated the mechanism through which the Et-2S SAM grew-it was through the formation of dendritic aggregates and islands-eventually resulting in phase-separated domains with a wormlike structure. This wormlike structure showed noticeable changes in its surface morphology upon irradiation with UV and visible light. On the other hand, while the growth process for the Me-2S SAM was similar to that of the Et-2S SAM, the final Me-2S SAM had smooth domains whose morphology did not exhibit photoswitchability. The TD and XP spectra of the SAMs showed that the number of adsorbed Et-2S molecules reached a point of saturation after a 24 h long immersion while the number of Me-2S molecules increased even after a 336 h long immersion. Furthermore, the area occupied by the azobenzene moiety in the Et-2S SAM was constant regardless of the immersion time, whereas that in the Me-2S SAM decreased with the immersion time. These results indicated that the strength of the interactions between the azobenzene molecules significantly influenced the aggregate-forming ability in SAMs.

A design strategy for stable light-sensitive palladium complexes

We describe the important determinants of stabilizing/destabilizing azobenzene-based palladium complexes capable of undergoing repeated light-triggered conformation changes. Inspired by highly warped palladium complexes with distorted azobenzene ligands, three different types of azobenzene ligands (1–3) were chosen to modify both the molecular structure (from a ubiquitous planar to a twisted trans-azobenzene) and the solubility of the azobenzenes in organic solvents. Single-crystal X-ray structure analyses were carried out for ortho-alkylated azobenzene (ortho-Az: 1b), meta-alkylated azobenzene (meta-Az: 2b) and PdCl2(ortho-Az)2 (1-Pd). Whereas both 2b and 3 lacking the meta and ortho substituents had planar structures, ortho-alkylated 1b was highly distorted by 68.06° from planarity. In stark contrast to PdCl2(meta-Az)2 (2-Pd) and PdCl2(Az)2 (3-Pd), which quickly dissociated in organic solvents, 1-Pd was very stable, especially in nonpolar solvents, so that it was possible to successfully purify and characterize them. The results suggest that unusually distorted trans-azobenzene is hardly influenced by the complexation reaction which requires considerable distortion of the azobenzene unit, thus stabilizing mononuclear palladium complexes. In nonpolar solvents, 1-Pd underwent repeated conformation changes under alternating UV and visible light irradiation. However, in polar solvents, the UV-triggered conformation change was accompanied by facile light-assisted breaking of the N:→Pd bond. Even dark incubation in polar organic solvents caused the dissociation of azobenzene ligands from the complexes. The breaking rate of the N:→Pd bond increased in the order of benzene ≈ dichloromethane < acetone < DMF, with more polar solvents inducing faster dissociation. The solvent polarity effect on the stability of azobenzene-based complexes can be interpreted in terms of the degree of polarization of the metal–ligand bond formed as a consequence of interactions between the palladium ion as a soft acid and azobenzene nitrogen as a hard base.

Light-driven modulation of fluorescence color from azobenzene derivatives containing electron-donating and electron-withdrawing groups

We report a simple preparation and color-tunable fluorescence of a series of azobenzene derivatives. The introduction of an electron-withdrawing or electron-donating group at the X position of azobenzenes (1–8) containing a biphenyl unit makes it possible to modulate the fluorescence color of the UV-exposed azobenzene solutions from blue to yellow, which correlates with the electron-donating abilities of the respective substituents. Theoretical calculations suggest that changes in both the dihedral angles between two phenyl rings of the biphenyl unit and the dipole moments between the trans and cis forms depending on the substituents seem to be important factors in determining the photochemical properties of chromophores.

Facile morphological control of fluorescent nano/microstructures via self-assembly and phase separation of trigonal azobenzenes showing aggregation-induced emission enhancement in polymer matrices

We report a facile and mild strategy for constructing diverse fluorescent nano/microstructures via self-assembly and phase separation of trigonal azobenzene chromophores (3N1s) showing aggregation-induced emission enhancement (AIEE) in polymer matrices [poly(methyl methacrylate) (PMMA) and/or poly(4-chlorostyrene) (PSCl)]. Thermal treatment above the glass transition temperature enhances the large-scale molecular motions of the polymer chains, which causes AIEE-active 3N1 molecules to assemble into fluorescent nanorods and long nanosticks in the confined homopolymer (PMMA and PSCl, respectively) matrices. Strikingly, as-prepared 3N1–PMMA–PSCl ternary mixtures exhibit a splendid raspberry-like morphology. In other words, the uneven island-like surfaces with micrometer-scale round protuberances are decorated with red fluorescent nanospheres. This result can be interpreted as surface-directed phase separation of immiscible PMMA and PSCl during quick solvent evaporation, which would help the 3N1 components instantaneously assemble into nanospheres on uneven surfaces. By annealing above the glass transition temperature, a distinct morphological transformation from a raspberry-like to a bead-like structure could readily be visualized via (i) the inherent assembly of 3N1 molecules into red fluorescent spherical or 1D aggregates and (ii) the selective fluorescence marking due to the difference in compatibility between 3N1 and PMMA or PSCl.

Light-responsive microstructures capable of pyrene monomer fluorescence switching

We synthesized an amphiphilic fluorescent compound (PyAzEG) having a pyrene fluorophore directly attached to twisted ortho-alkylated azobenzene (i) to suppress pyrene excimer formation and (ii) to facilitate pyrene monomer fluorescence switching. PyAzEG molecules in a dilute solution underwent azobenzene-based trans ↔ cis photoisomerization under UV and visible light irradiation. When such amphiphilic molecules dispersed in an aqueous solution, they could assemble into spheres through a delicate balance between π–π stacking among aromatic rings and hydrophilic interactions among tetra(ethylene glycol) chains. For a PyAzEG suspension containing micrometer-sized spheres, typical pyrene monomer fluorescence was observed in the range of 370–460 nm, whereas excimer fluorescence did not emerge from the fluorescence spectrum. Monomer fluorescence switching as well as structural transformation from spheres to less packed vesicle-like structures were achieved by changing the wavelength of light. Furthermore, PyAzEG spheres exhibited enhanced fluorescence relative to the monomeric solution, which is probably due to moderate intermolecular interactions among chromophores in a head-to-tail fashion.