Jetsuda Areephong

Lateral Self-Sorting on Surfaces: A Practical Approach to Double-Channel Photosystems

We report that self-sorting during self-organizing surface-initiated copolymerization (co-SOSIP) provides facile access to oriented multicomponent architectures. Alternate lateral and uniform axial self-sorting into formal supramolecular n/p-heterojunction photosystems is found to generate up to 40 times more photocurrent. More or less topological matching gives rise to alternate axial self-sorting into inactive charge-transfer complexes or uniform lateral sorting into the less active macrodomains, respectively. Experimental support for self-repair during co-SOSIP is reported. Initiators on the surface are shown to serve as templates for the self-sorting into multichannel architectures of freely variable composition.

Symmetric Six-Fold Arrays of Photo- and Electrochromic Dithienylethene Switches

The direct synthesis of six-fold symmetric hexaphenylbenzenes with multiple photochromic dithienylethene units via a cobalt-catalyzed cyclotrimerization is reported. This approach allows for six photochromic units to be held in proximity with a well-defined spatial separation without affecting the photochromic properties of each unit.

Photoresponsive dithienylethene-urea-based organogels with “reversed” behavior

Dithienylperhydrocyclopentene-bisurea-based low molecular weight gelators are described that function as photoresponsive organogels that show a remarkable gel-to-liquid transition upon irradiation. The two series of derivatives, with and without alkyl spacers between the urea hydrogen bonding groups and the photochromic unit, show different gelation behavior. Upon UV irradiation of the gels, a gel liquified at only 1.4% conversion of the photochromic unit. Transmission electron microscopy (TEM) shows that the gel fibres consist of thin ribbons. Semi-empirical (PM3) calculations indicate that the hydrogen bonding between the open-ring isomer (o) molecules is weak, and that formation of the closed-ring isomer (c) destabilises the hydrogen bonding further. The results indicate that a small amount of the closed-ring isomer will disrupt the intermolecular hydrogen-bonding, leading to disintegration of the gel fibre ribbons and hence reversible liquification.

Photoswitchable Sexithiophene-Based Molecular Wires

Photochromic sexithiophenes were prepared by oxidative electrochemical coupling of terthiophenes. The redox properties in the open state are typical of sexithiophenes. Ring closure of both photochromic units leads to a decrease in the energy of the LUMO orbitals with little affect on the energy of the HOMO orbitals. The photochemical tuning of the conjugation of a molecular wire is achieved by combining dithienylethene units with a sexithiophene.

On/Off Photoswitching of the Electropolymerizability of Terthiophenes

In this contribution the polymerization of terthiophene, to form an alkene bridged alpha,alpha-coupled sexithiophene polymer, is controlled by light; i.e. the electropolymerizability of the monomer 1F is switched off and on with UV and visible light, respectively. The system comprises of both bis-terthiophene and photochromic dithienylethene units. The presence of a light-switchable unit allows on-off switching of the electropolymerization of the monomer with light. Furthermore the incorporation of the dithienylethene in the polymer backbone increases dramatically the homogenity of the polymer formed (i.e., only sexithiophene units are formed). The derived films are robust and fully retain electrochromic behavior as has been demonstrated through cyclic voltammetry while spatial control (patterning) is readily achieved by applying simple optical masking techniques.

Light-controlled formation of vesicles and supramolecular organogels by a cholesterol-bearing amphiphilic molecular switch

A new responsive material composed of an amphiphilic light-switchable dithienylethene unit functionalized with a hydrophobic cholesterol unit and a hydrophilic poly(ethylene glycol)-modified pyridinium group has been designed. This unique single-molecule system shows responsive light-switchable self-assembly in both water and organic solvents. Light-triggered reversible vesicle formation in aqueous solutions is reported. The molecule shows a different behavior in apolar aromatic solvents, in which light-controlled formation of organogel fibers is observed. The light-triggered aggregation behavior of this molecule demonstrates that control of a supramolecular structure with light can be achieved in both aqueous and organic media and that this ability can be present in a single molecule. This opens the way toward the effective development of new strategies in soft nanotechnology for applications in controlled chemical release systems.

A Chiroptical Photoswitchable DNA Complex

The interesting structural, electronic, and optical properties of DNA provide fascinating opportunities for developing nanoscale smart materials by integrating DNA with opto-electronic components. In this article we demonstrate the electrostatic binding of an amine-terminated dithienylethene (DET) molecular switch to double-stranded synthetic polynucleotides. The DET switch can undergo photochemical ring-closure and opening reactions. Circular dichroism (CD) and UV-vis spectroscopy show that both the open, 1o, and the closed, 1c, forms of the switch bind to DNA. Upon addition of DNA to a solution of 1o or 1c, the UV-vis spectrum displays a hypochromic effect, indicative of an interaction between the switch and the DNA. The chirality of the DNA double-helix is transmitted to the switching unit which displays a well-defined CD signal upon supramolecular complexation to the DNA. Additionally, the CD signal of the DNA attenuates, demonstrating that both components of the complex mutually influence each others structure; the DNA induces chirality in the switch, and the switch modifies the structure of the DNA. Modulation of the chiroptical properties of the complex is achieved by photochemically switching the DET between its ring open and closed isomers. A pH dependence study of the binding shows that when the pH is increased the switches lose their binding ability, indicating that electrostatic interactions between protonated amines and the negatively charged phosphate backbone are the dominant driving force for binding to the DNA. A comparison of poly(deoxyguanylic-deoxycytidylic) acid [poly(dGdC)(2)] polynucleotides with poly(deoxyadenylic-deoxythymidylic) acid [poly(dAdT)(2)] shows distinct differences in the CD spectra of the complexes.

Reversible photochemical control of cholesteric liquid crystals with a diamine- based diarylethene chiroptical switch

Upon addition of a chiral dopant to a nematic liquid crystal, amplification of molecular chirality can occur and consequently a cholesteric liquid crystal is formed. A major challenge in materials science consists in designing efficient chiral dopants that allow for control over chiral amplification by use of an external trigger, for example by irradiation with light, and thereby achieving the control of the dynamic and responsive structure of cholesteric liquid crystals. Here, a chiral photochromic switch bearing two chiral imine units connected viaphenyl spacers was synthesized and characterized in solution, where it can be photo-chemically converted from a colourless ring-opened form 1o to a coloured ring-closed form 1c, reversibly. We show that a small amount of 1o used as a dopant induces the formation of a stable cholesteric liquid crystal. The retention of the photochromic properties of 1, when used as a chiral dopant, allows for reversible photocontrol over the period of the cholesteric helix, and shows the highest values of helical twisting power achieved so far with diarylethene-based photoswitchable dopants.

Photochromism and Electrochemistry of a Dithienylcyclopentene Electroactive Polymer

A bifunctional substituted dithienylcyclopentene photochromic switch bearing electropolymerisable methoxystyryl units, which enable immobilization of the photochromic unit on conducting substrates, is reported. The spectroscopic, electrochemical, and photochemical properties of a monomer in solution are compared with those of the polymer formed through oxidative electropolymerization. The electroactive polymer films prepared on gold, platinum, glassy carbon, and indium titanium oxide (ITO) electrodes were characterized by cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The thickness of the films formed is found to be limited to several monolayer equivalents. The photochromic properties and stability of the polymer films have been investigated by UV/vis spectroscopy, electrochemistry, and XPS. Although the films are electrochemically and photochemically stable, their mechanical stability with respect to adhesion to the electrode was found to be sensitive to both the solvent and the electrode material employed, with more apolar solvents, glassy carbon, and ITO electrodes providing good adhesion of the polymer film. The polymer film is formed consistently as a thin film and can be switched both optically and electrochemically between the open and closed state of the photochromic dithienylethene moiety.

Tunable aggregation and luminescence of bis(diarylethene)sexithiophenes.

Diarylethenes with two different side groups (phenyl and chloro) were appended to both alpha-ends of a sexithiophene unit. The temperature dependent aggregation properties for both compounds were characterized by steady state and transient absorption spectroscopy. The peripheral side groups show an unexpectedly significant influence on the electronic properties of the sexithiophene core. Furthermore, the relative influence of the phenyl and chloro substituents on the aggregation behavior observed is remarkable. The phenyl compound exhibits formation of H-aggregates over a narrow temperature range, between 240 and 200 K, typical of strong intermolecular interactions. In contrast, the chloro compound shows gradual aggregation over a wide temperature range, forming H-aggregates albeit with weaker intermolecular interactions. The results demonstrate that minor changes in the structure lead to tunability of the aggregation and corresponding luminescence properties of sexithiophenes in solution and hold particular relevance to supramolecular and polymer systems based on sexithiophene units.