Light-Induced Dyotropic Rearrangement of Diarylethenes: Scope, Mechanism, and Prospects

Irreversible two-photon photorearrangement of 1,2-diarylethenes is a unique process providing access to complex 2a1 ,5a-dihydro-5,6-dithiaacenaphthylene (DDA) heterocyclic core. This reaction was serendipitously discovered during studies on photoswitchable diarylethenes and was initially considered as a highly undesired process. However, in recent years, it has been recognized as an efficient photochemical reaction, interesting by itself and as a promising synthetic method for the synthesis of challenging molecules. Herein, we discuss the state-of-the-art in studies on this notable process.

Bis(thienyl)ethenes with α-methoxymethyl groups. Syntheses, spectroscopic Hammett plots, and stabilities in PMMA films

A series of bis(thienyl)ethenes (BTEs) possessing perfluorocyclopentene backbones and methoxymethyl groups (MOM) in the 2/2'-positions of the thiophenes was prepared and examined. The substitution pattern of the 5/5'-positions was varied, covering the range from electron-donating to electron-withdrawing. The substituent effects of the absorption wavelengths of the ring-opened and the ring-closed isomers, which are interconverted by reversible 6π-electrocyclizations and cycloreversions, are studied by means of the spectroscopic Hammett equation and the Hammett-Brown equation. Excellent correlations of these linear free energy relationships were found, when the σp values of the Hammett equation, which summarize inductive, mesomeric and field effects, were replaced to the Hammett-Brown σp+ and σp- values which also take direct conjugation into account. We studied solvent effects on the spectroscopic properties and embedded the BTEs into polymethylmethacrylate (PMMA) coatings to examine their fatigue resistance. By our studies, the spectroscopic properties of BTEs can be adjusted by variation of the substitution pattern to a desired excitation wavelength for switching processes.

Photochromic Diarylethenes Designed for Surface Deposition: From Self-Assembled Monolayers to Single Molecules

The efficient switching that can occur between two stable isomers of diarylethenes makes them particularly promising targets for opto- and molecular electronics. To examine these classes of molecules for electronics applications, they have been subjected to a series of scanning tunneling microscopy (STM) experiments, which are the focus of this Review. A brief introduction to the chemical design of diarylethenes in terms of their switching capabilities along with the basics of STM are presented. Next, initial STM studies on these compounds under ambient conditions are discussed. An overview of how molecular design affects the isomerization and self-assembly of diarylethenes at the solid-liquid interface as investigated by STM is then presented, as well as single-molecule studies under ultrahigh vacuum. The last section presents further prospects for molecular design in the field.

Self-assembly of photochromic diarylethene-peptide conjugates stabilized by β-sheet formation at the liquid/graphite interface

Two-dimensional (2-D) self-assembly of diarylethene (DAE)-peptide conjugates at the octanoic acid/graphite interface was investigated by scanning tunnelling microscopy (STM). The open-ring isomer of a DAE-peptide conjugate formed a stable 2-D molecular assembly with an antiparallel β-sheet structure. Quantitative analysis of surface coverage depending on concentration revealed a stronger stabilization effect of the oligopeptide than that of the alkyl group with a similar side chain length.

Light-Induced Contraction/Expansion of 1D Photoswitchable Metallopolymer Monitored at the Solid-Liquid Interface

The use of a bottom-up approach to the fabrication of nanopatterned functional surfaces, which are capable to respond to external stimuli, is of great current interest. Herein, the preparation of light-responsive, linear supramolecular metallopolymers constituted by the ideally infinite repetition of a ditopic ligand bearing an azoaryl moiety and Co(II) coordination nodes is described. The supramolecular polymerization process is followed by optical spectroscopy in dimethylformamide solution. Noteworthy, a submolecularly resolved scanning tunneling microscopy (STM) study of the in situ reversible trans-to-cis photoisomerization of a photoswitchable metallopolymer that self-assembles into 2D crystalline patterns onto a highly oriented pyrolytic graphite surface is achieved for the first time. The STM analysis of the nanopatterned surfaces is corroborated by modeling the physisorbed species onto a graphene slab before and after irradiation by means of density functional theory calculation. Significantly, switching of the monolayers consisting of supramolecular Co(II) metallopolymer bearing trans-azoaryl units to a novel pattern based on cis isomers can be triggered by UV light and reversed back to the trans conformer by using visible light, thereby restoring the trans-based supramolecular 2D packing. These findings represent a step forward toward the design and preparation of photoresponsive "smart" surfaces organized with an atomic precision.

Frontiers of supramolecular chemistry at solid surfaces

Supramolecular chemistry on solid surfaces represents an exciting field of research that continues to develop in new and unexpected directions.

Effects of Alkyl Chain Length and Hydrogen Bonds on the Cooperative Self-Assembly of 2-Thienyl-Type Diarylethenes at a Liquid/Highly Oriented Pyrolytic Graphite (HOPG) Interface

An appropriate understanding of the process of self-assembly is of critical importance to tailor nanostructured order on 2D surfaces with functional molecules. Photochromic compounds are promising candidates for building blocks of advanced photoresponsive surfaces. To investigate the relationship between molecular structure and the mechanism of ordering formation, 2-thienyl-type diarylethenes with various lengths of alkyl side chains linked through an amide or ester group were synthesized. Their self-assemblies at a liquid/solid interface were investigated by scanning tunneling microscopy (STM). The concentration dependence of the surface coverage was analyzed by using a cooperative model for a 2D surface based on two characteristic parameters: the nucleation equilibrium constant (Kn) and the elongation equilibrium constant (Ke). The following conclusions can be drawn. 1) The concentration at which a stable 2D molecular ordering is observed by STM exponentially decreases with increasing length of the alkyl chain. 2) Compounds bearing amide groups have higher degrees of cooperativity in self-assembly on 2D surfaces (i.e., σ, which is defined as Kn/Ke) than compounds with ester groups. 3) The self-assembly process of the open-ring isomer of an ester derivative is close to isodesmic, whereas that of the closed-ring isomer is cooperative because of the difference in equilibrium constants for the nucleation step (i.e., Kn) between the two isomers.

Diarylethene Self-Assembled Monolayers: Cocrystallization and Mixing-Induced Cooperativity Highlighted by Scanning Tunneling Microscopy at the Liquid/Solid Interface

Stimulus control over 2D multicomponent molecular ordering on surfaces is a key technique for realizing advanced materials with stimuli-responsive surface properties. The formation of 2D molecular ordering along with photoisomerization was monitored by scanning tunneling microscopy at the octanoic acid/highly oriented pyrolytic graphite interface for a synthesized amide-containing diarylethene, which underwent photoisomerization between the open- and closed-ring isomers and also a side-reaction to give the annulated isomer. The nucleation (Kn) and elongation (Ke) equilibrium constants were determined by analysis of the concentration dependence of the surface coverage by using a cooperative model at the liquid/solid interface. It was found that the annulated isomer has a very large equilibrium constant, which explains the predominantly observed ordering of the annulated isomer. It was also found that the presence of the closed-ring isomer induces cooperativity into the formation of molecular ordering composed of the open-ring isomer. A quantitative analysis of the formation of ordering by using the cooperative model has provided a new view of the formation of 2D multicomponent molecular ordering.

Photoinduced Four-State Three-Step Ordering Transformation of Photochromic Terthiophene at a Liquid/Solid Interface Based on Two Principles: Photochromism and Polymorphism

We have investigated photoinduced ordering transformation of a photochromic terthiophene derivative by scanning tunneling microscopy (STM) at the trichlorobenzene (TCB)/highly oriented pyrolytic graphite (HOPG) interface. The open-ring and annulated isomers of the terthiophene formed two-dimensional molecular orderings with different patterns while the closed-ring isomer did not form any ordering. The ordering of the open-ring isomer exhibited polymorphism depending on the concentration of supernatant solution. Upon UV light irradiation to a solution of the open-ring isomer or the closed-ring isomer, ordering composed of the annulated isomer was irreversibly formed. Upon visible light irradiation or thermal stimulus to the closed-ring isomer, the two kinds of polymorph composed of the open-ring isomer were formed due to the polymorphism. By controlling photochromism and polymorphism among four states made of three photochemical isomers, four-state three-step transformation was achieved by in situ photoirradiation from a solution of the closed-ring isomer (no ordering) into the ordering composed of the open-ring isomer (ordering α and β) followed by the orderings composed of the annulated isomer (ordering γ).

Investigation on the Surface-Confined Self-Assembly Stabilized by Hydrogen Bonds of Urea and Amide Groups: Quantitative Analysis of Concentration Dependence of Surface Coverage

Formation of a hydrogen-bond network via an amide group is a key driving force for the nucleation-elongation-type self-assembly that is often seen in biomolecules and artificial supramolecular assemblies. In this work, rod-coil-like aromatic compounds bearing an amide (1 a-3 a) or urea group (1 u-3 u) were synthesized, and their self-assemblies on a 2-D surface were investigated by scanning tunneling microscopy (STM). According to the quantitative analysis of the concentration dependence of the surface coverage, it was revealed that the strength of the hydrogen bond (i.e., amide or urea) and the number of non-hydrogen atoms in a molecular component (i.e., size of core and length of alkyl side chain) play a primary role in determining the stabilization energy during nucleation and elongation processes of molecular ordering on the HOPG surface.

Surface-induced selection during in situ photoswitching at the solid/liquid interface

Here we report for the first time a submolecularly resolved scanning tunneling microscopy (STM) study at the solid/liquid interface of the in situ reversible interconversion between two isomers of a diarylethene photoswitch, that is, open and closed form, self-assembled on a graphite surface. Prolonged irradiation with UV light led to the in situ irreversible formation of another isomer as by-product of the reaction, which due to its preferential physisorption accumulates at the surface. By making use of a simple yet powerful thermodynamic model we provide a quantitative description for the observed surface-induced selection of one isomeric form.

Improving the fatigue resistance of diarylethene switches

When applying photochromic switches as functional units in light-responsive materials or devices, an often disregarded yet crucial property is their resistance to fatigue during photoisomerization. In the large family of diarylethene photoswitches, formation of an annulated isomer as a byproduct of the photochromic reaction turns out to prevent the desired high reversibility for many different derivatives. To overcome this general problem, we have synthesized and thoroughly investigated the fatigue behavior of a series of diarylethenes, varying the nature of the hetaryl moieties, the bridging units, and the substituents. By analysis of photokinetic data, a quantification of the tendency for byproduct formation in terms of quantum yields could be achieved, and a strong dependency on the electronic properties of the substituents was observed. In particular, substitution with 3,5-bis(trifluoromethyl)phenyl or 3,5-bis(pentafluorosulfanyl)phenyl groups strongly suppresses the byproduct formation and opens up a general strategy to construct highly fatigue-resistant diarylethene photochromic systems with a large structural flexibility.

Phototriggered formation and disappearance of surface-confined self-assembly composed of photochromic 2-thienyl-type diarylethene: a cooperative model at the liquid/solid interface

A photoresponsive self-assembly on a 2-D surface was investigated by STM and the behavior was analyzed by a cooperative model.