Reductive Electrochemical Cyclization of a Photochromic 1,2-Dithienylcyclopentene Dication

Ion valence-gated photochromism of an aza-crowned diarylethene

A non-photochromic diarylethene 2o with an N-phenylaza-15-crown-5 was synthesized. When the nitrogen atom in the aza-crown ring was protonated, it became photochromic due to the prevention of a twisted intramolecular charge transfer (TICT). Although addition of a monovalent metal cation (Li+, Na+, K+, Rb+, Cs+, Cu+, Ag+) in acetonitrile could not stop the TICT so that it was not photochromic, the addition of a multivalent metal cation (Mg2+, Ca2+, Sr2+, Ba2+, Fe2+, Ni2+, Al3+, Sb5+) changed 2o to be photochromic due to the strong attraction of the lone pair on the nitrogen atom. In the presence of excess Cu2+, 2o was oxidized to be EPR-detectable 2o·+, which was thermally unstable as well as inert towards visible-light irradiation. However, 2o·+ was further oxidized to be fairly stable 2o2+ by the irradiation of 365-nm light in the presence of Cu2+. ESI-MS measurements strongly suggested the generation of 2o·+ by mixing 2o with Cu(ClO4)2 in acetonitrile, and the transformation of 2o·+ to 2o2+ by successive 365-nm light irradiation. Fe3+ similarly worked as the oxidant, but the two-step oxidation of 2o to 2o2+ occurred more easily.

All Visible Light Photoswitch Based on the Dimethyldihydropyrene Unit Operating in Aqueous Solutions with High Quantum Yields

Molecular systems and devices whose properties can be modulated using light as an external stimulus are the subject of numerous research studies in the fields of materials and life sciences. In this context, the use of photochromic compounds that reversibly switch upon light irradiation is particularly attractive. However, for many envisioned applications, and in particular for biological purposes, illumination with harmful UV light must be avoided and these photoactivable systems must operate in aqueous media. In this context, we have designed a benzo[e]-fused dimethyldihydropyrene compound bearing a methyl-pyridinium electroacceptor group that meets these requirements. This compound (closed state) is able to reversibly isomerize under aerobic conditions into its corresponding cyclophanediene form (open isomer) through the opening of its central carbon-carbon bond. Both the photo-opening and the reverse photoclosing processes are triggered by visible light illumination and proceed with high quantum yields (respectively 14.5% yield at λ = 680 nm and quantitative quantum yield at λ = 470 nm, in water). This system has been investigated by nuclear magnetic resonance and absorption spectroscopy, and the efficient photoswitching behavior was rationalized by spin-flip time-dependent density functional theory calculations. In addition, it is demonstrated that the isomerization from the open to the closed form can be electrocatalytically triggered.

New insights into the redox properties of pyridinium appended 1,2-Dithienylcyclopentenes

The optical and redox properties of a methyl pyridinium appended 1,2-dithienylethene photochromic derivative have been thoroughly investigated. A complex multi-step photo/redox mechanism is proposed for the closed isomer on the ground of spectro-electrochemical and theoretical data. The generated compounds are not stable over the time because of chemical reactions associated to the redox processes and a new dithienylethene derivative incorporating a seven-membered ring has been isolated and characterized.

A Photo- and Redox-Driven Two-Directional Terthiazole-Based Switch: A Combined Experimental and Computational Investigation

Terthiazoles with redox-active substituents like an N-methyl pyridinium group and ferrocene have been synthesized and their photo- and electro-chromic behaviors were investigated. The presence of two lateral N-methyl pyridinium substituents in the structure of terthiazole proved to be effective in inducing not only the reductive ring-closure of the terthiazole core but also its oxidative ring-opening reaction, leading to the first terarylene-based switch able to fully operate both photochemically and electrochemically. Moreover, the large increase in the redox potential between its open and closed form (700 mV) means that a part of the photon energy necessary to trigger the cyclization is stored in the form of chemical potential available for other works. Introduction of a second redox-active unit such as ferrocene onto the central thiazolyl moiety is found to inhibit the photochromism of the switch but not its redox switchability, which, instead, got improved for the ring-opening reaction via the redox properties of the ferrocenyl unit. The optical and redox properties of the switch in its different oxidation states are analyzed with the aid of DFT calculations in order to rationalize different 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.

Switchable second-order nonlinear optical response of platinum-sensitized dithienylethenes

The electronic structure and the nonlinear optical (NLO) properties of a series of platinum-sensitized dithienylethenes (DTEs) were investigated by using the density functional theory (DFT) method. DFT calculations reveal that the second-order NLO properties of complexes significantly increase with the DTE ligand being directly linked with terpyridine–Pt(II) complexes. Due to the good [Formula: see text]-conjugated characteristics, closed-ring complexes possess much larger second-order NLO properties than the corresponding open-ring complexes. The computational [Formula: see text] values are in the order of 3c ([Formula: see text] esu) [Formula: see text] 3o ([Formula: see text] esu) [Formula: see text] 4c ([Formula: see text] esu) [Formula: see text] 4o ([Formula: see text] esu) [Formula: see text] 1 ([Formula: see text] esu) [Formula: see text] 2c ([Formula: see text] esu) [Formula: see text] 2o ([Formula: see text] esu). Among all calculated platinum-sensitized dithienylethenes, 3c has the largest second-order NLO properties. The 4c and 4o have almost the same [Formula: see text] values and the [Formula: see text] value of 4o is slightly larger than that of 2o because the ether bonds can significantly prevent charge transfer within the complexes.

Unusual structural changes as a result of weathering benzofuran-based diarylethenes in simulated sunlight

Two unusual products are observed when benzofuran-based diartylethenes are weathered in simulated sunlight.

Theoretical analysis of electrochromism under redox of bis(3-thienyl)/(2-thienyl)hexafluorocyclopentene: effects of charged and substituted systems

Electrochromism with the ring-closing or ring-opening isomerization of substituted and unsubstituted bis(3-thienyl)/(2-thienyl)hexafluorocyclopentene is discussed using the DFT method. In the neutral ground state, bond making and breaking between two reactive C atoms on thienyls are thermodynamically forbidden. Under redox conditions, the gain or loss of electrons can have a significant effect on the frontier molecular orbital distribution of both open- and closed-ring isomers, particularly in reactive sites. Corresponding structural changes show a trend toward isomerization. The reaction energy barrier shows greater reduction for dication than monocation and even becomes barrierless for dianion. During the isomerization in different states, the conjugated system switches distinctively, which is attributed to the special redistribution of molecular orbitals and spin population in each state. In monocation and monoanion, for the involvement of a single electron, isomerization is inclined to proceed sequentially between right and left thienyls, whereas it becomes synchronous in dication. The direction depends on the stabilization achieved by the formation of a global conjugated system and more average spin population on the molecule. The effect of substituents on thienyls is demonstrated in the promotion of the extent of conjugation and the determination of the spin population level on the reactive C atoms. Moreover, according to their electron-donating and withdrawing abilities, they can kinetically support or suppress the electron transfer pattern in the process from isomer to transition state, which leads to the control of reaction efficiency.

Electrochemical control of the switching process of photochromic dimethyldihydropyrene derivatives

The electrochemical control of the isomerization process of a photo switchable dimethyldihydropyrene (DHP) derivative has been investigated.

Isolation and crystal structure of a dithiophene dication: controlling covalent connection and disconnection with temperature and phase

A dithienylethene (1o) undergoes a two-electron chemical oxidation to a singlet diradical as an open-isomer (1o(2+)) in solution, which cyclizes to a closed-form (1c(2+)) upon cooling. The latter crystallizes out and its structure is analyzed by single crystal X-ray diffraction. Equilibrium between 1o(2+) and 1c(2+) in solution is observed by NMR and UV spectroscopy at various temperatures and is further supported by reduction reactions with Zn powder.

Substituent effects on the properties of photochromic hybrid diarylethenes with a naphthalene moiety

Four new unsymmetrical photochromic diarylethenes bearing both naphthalene and thiophene moieties were synthesized, and the structures of two diarylethenes were determined by single-crystal X-ray diffraction analysis. The naphthalene ring was connected directly to the central perfluorocyclopentene ring as an aryl moiety and available to participate in photoisomerization reaction. All the diarylethenes exhibited favorable photochromism and functioned as fluorescence switches in both solution and poly(methyl methacrylate) films. The electron-withdrawing substituent significantly shifted the absorption maxima to a longer wavelength and evidently suppressed the cycloreversion quantum yield, whereas the electron-donating substituents enhanced the fluorescence quantum yield of diarylethenes with a naphthalene moiety. Furthermore, cyclic voltammograms suggested that the oxidation onsets and band-gaps of the open-ring isomers were much bigger than those of the closed-ring isomers. The results indicated that the substituents at the 5-position of thiophene ring could availably modulate their optical and electrochemical behaviors.

Novel photochromic and electrochromic diarylethenes bearing triphenylamine units

Novel diarylethenes containing triphenylamine units have been synthesized and characterized, and possessed both photochromic and electrochromic properties.

Fluorescence modulation with photochromic switches in nanostructured constructs

This tutorial review illustrates the structural design, photochemical and photophysical properties of nanostructured constructs incorporating luminescent and photochromic components. In these systems, the pronounced structural and electronic modifications that accompany the transformations of the photochromic components can be exploited to modulate the emission intensity of the luminescent components on the basis of electron and energy transfer processes. These photoresponsive systems can be assembled by: (1) integrating fluorescent and photochromic components within the main chain of the same polymer; (2) attaching multiple photochromes to a fluorescent organic polymer or luminescent inorganic nanoparticle; (3) appending either independent fluorophores and photochromes or fluorophore-photochrome dyads to a common polymer scaffold; (4) trapping distinct fluorophores and photochromes within the hydrophobic interior of the same cross-linked polymer. In all instances, the changes in absorbance and/or redox potentials associated with the reversible interconversion of the two states of each photochromic component regulate the radiative deactivation of the luminescent components. As a result, the emission intensity of these nanoscaled assemblies can reversibly be switched between high and low values under the influence of optical stimulations. Thus, these clever operating principles for fluorescence modulation can lead to the development of innovative functional and nanostructured materials with photoresponsive character. In particular, protocols for the optical writing and reading of data as well as luminescent probes for bioimaging applications might ultimately emerge from these fundamental studies on photoresponsive molecular switches.

Syntheses and Substituent Effects on the Properties of Unsymmetrical Photochromic Diarylethenes Bearing a Benzothiophene Unit

Photochromic unsymmetrical diarylethenes 1a–5a that bear different electron-donating or electron-withdrawing substitutes have been synthesized, and the structures of 2a–5a determined by single-crystal X-ray diffraction analysis. Substituent effects on their optoelectronic properties, including photochromism, fluorescence, and electrochemical properties are investigated in detail. The strong electron-donating substituents have effective contributions to the absorption maxima of the closed-ring isomers and the molar coefficients, whereas the electron-withdrawing substituents can shift significantly the absorption maxima of the diarylethenes to a longer wavelength and increase their cyclization quantum yield. Diarylethenes 2, 3, and 4 show good photochromism both in solution and in the single crystalline phase. However, diarylethenes 5 shows no photochromism in the crystalline phase. Diarylethenes 1, 2, and 3 exhibit good fluorescent switching upon alternating irradiation with UV and visible light, and their fluorescent conversions in the photostationary state are all larger than 80% in hexane. In addition, cyclic voltammogram tests show that different electron-donating and electron-withdrawing substituents have a remarkable effect on the electrochemical behaviours of these diarylethenes.

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.