Control of the photoreactivity of diarylethene derivatives by quaternarization of the pyridylethynyl group

Photochromic behavior of diarylethene derivatives with (4-pyridyl)ethynyl group directly attached to the 6-pi hexatriene moieties of the diarylethenes was investigated. Upon quaternarization of the pyridine moieties, the photoreactivity was strongly suppressed. On the other hand, diarylethene derivatives with nonconjugated (4-pyridyl)ethyl group exhibited the photochromic reactivity, regardless of whether pyridyl rings are quaternarized or not. In the case of the (4-pyridyl)ethynyl-substituted compounds, the photochromic reactivity was suppressed by the addition of trifluoroacetic acid and was restored by diethylamine.     

Gated photochromism in triarylborane-containing dithienylethenes: a new approach to a "lock-unlock" system

Photochromic inactive dithienylethene derivatives appended with 3- or 5-dimesitylboryl-2,2'-bithiophene have been synthesized. Upon fluoride ion-binding, the photochromic reactivity is "unlocked", displaying a novel gated photochromic property.     

Regulation of folding and photochromic reactivity of terarylenes through a host-guest interaction

The photochromic reactivity of terarylenes is integrated with molecular folding that is controlled through a host-guest interaction. A thieno[3,2,b]pyridine unit is introduced into a photochromic terarylene structure as an aryl unit to form a guest-interacting site. Thienopyridine-containing terarylenes showed solvent-dependent photochromic reactivity in solution. A terarylene moiety that contains two thienopyridyl units showed significantly high photocoloration reactivity as high as 88% of photocyclization quantum yield in methanol, whereas that value was only 24% in hexane. A temperature-dependent (1)H NMR spectroscopic study in different solvents indicated an interconversion between photochromic-reactive and unreactive conformations. In methanol, the intermolecular interaction between terarylene species and the solvent molecule slows the rate of interconversion and increases the population of the photochromic-active form, whereas the unreactive conformation is dominant in hexane. Crystal-structural studies demonstrated the perfect regulation of molecular folding between a photochromic-active form and an unreactive conformation by changing the solvents for recrystallization. Single crystals prepared from solutions in methanol showed reversible photochromic reactivity, whereas recrystallization from solutions in hexane did not show this reactivity. X-ray crystallographic studies of single crystals from solutions in methanol demonstrated that the photochromic molecules bind a solvent methanol molecule at the guest-interacting site to regulate the molecular conformation into a photochromic-active form in collaboration with specific intramolecular interactions, whereas crystals from solutions in hexane possess the photochromic-unreactive conformation.     

Photochromic diarylethenes for molecular photonics

1,2-Diarylethenes with heterocyclic aryl groups, such as benzothiophene rings, underwent thermally irreversible and fatigue resistant photochromic reactions. The photocyclization response time of a dithienylethane was measured to be 1 ps. Gated photochromic reactivity was given to a 1,2-bis(benzothiophen-3-yl) perfluorocyclopentene by introducing intramolecular hydrogen-bond forming groups.     

New photoswitching unit for magnetic interaction: diarylethene with 2,5-bis(arylethynyl)-3-thienyl group

Photoswitching of the intramolecular magnetic interaction was demonstrated using diarylethenes with 2,5-bis(arylethynyl)-4-methyl-3-thienyl side group. Two nitroxide radicals were placed at each end of the 2,5-bis(arylethynyl)-4-methyl-3-thienyl group. Three kinds of aryl groups, 2,5-thienylene, p-phenylene, and m-phenylene groups, were used in the arylethynyl moiety. The diarylethene photoswitching units have an extended pi-conjugated chain on one side of the diarylethene. The photochromic reactivity was dependent on the arylethynyl group. Diarylethenes with m-phenylene group showed an efficient photochromic reactivity. Along with the photochromic reaction the diarylethenes showed photoswitching of an ESR spectrum originating from the change in the magnetic interaction between two unpaired electrons. The open-ring isomer showed stronger exchange interaction than the photogenerated closed-ring isomer. The magnetic interaction between two radicals via the pi-conjugated chain was altered by photocyclization due to the change of the hybrid orbital at the 2-position of the thiophene ring from sp(2) to sp(3).     

Photochromism of bis(2-alkyl-1-benzofuran-3-yl)perfluorocyclopentene derivatives

[reaction: see text] Photochromic diarylethene derivatives having benzofuran heteroaryl groups, bis(2-methyl-1-benzofuran-3-yl)perfluorocyclopentene and bis(2-butyl-1-benzofuran-3-yl)perfluorocyclopentene, were synthesized, and their photochromic performance was examined in hexane solution as well as in the single-crystalline phase. The compounds exhibited photochromic reactivity even in the single-crystalline phase.     

A novel gated photochromic reactivity controlled by complexation/dissociation with BF3

A photochemically active dithiazolethene BN was designed and synthesized, exhibiting a specific gated photochromism. That is, the photochromic reactivity of BN is prevented to a great extent by BF(3), showing a "Lock" gate.     

Characterization of the photochromism of dihydropyrenes with photophysical techniques

Dihydropyrenes are negative photochromes and their photophysical characterization, achieved in the past decade, is reviewed. Special emphasis is given to the theoretical background and to the experimental details which are relevant for the characterization of bi-stable systems, such as photochromic ones. In contrast to most photochromic compounds, dihydropyrenes have long lifetimes for their photoreactive excited states making it possible to tune their reactivity with the introduction of substituents. This knowledge is applicable for the design of more efficient photochromic molecules and for multichromophoric systems.     

Photoswitching of intramolecular magnetic interaction using diarylethene with oligothiophene pi-conjugated chain.

Diarylethenes having two nitronyl nitroxide radicals at both ends of the molecule with oligothiophene spacers were synthesized. The diradicals underwent photochromic reactions upon alternate irradiation with UV and visible light. ESR spectra of the diradicals reversibly changed with the photochromism. The magnetic interaction between spins through oligothiophene spacers was stronger than that through oligophenylene spacers, and the photochromic reactivity of the diradical diarylethenes with oligothiophene spacers was much reduced. The difference of the exchange interaction between open- and closed-ring form isomers was estimated to be more than 150-fold.     

Light-controlled gas permeability of mesoporous silica glass bearing photochromic spironaphthoxazine on its surface

N2 and CO2 gas permeability of mesoporous silica glass bearing photochromic indolinospironaphth[2,1-b][1,4]oxazine through a covalent linkage was controlled by photo-irradiation: the photo-isomerization of the spironaphthoxazine to the photomerocyanine form suppressed the gas permeation of the glass.     

Molecular design for a write-by-light/erase-by-heat recording system using photochromic diarylethenes with thermal cycloreversion

We designed and synthesized photochromic diarylethenes for a write-by-light/erase-by-heat recording system. The introduction of polar substituents at both sides of the diarylethene maintained the photocyclization and photocycloreversion reactivities, but significantly affected the thermal cycloreversion reactivity. The introduction of electron-withdrawing substituents accelerated the thermal reaction and the introduction of electron-donating substituents suppressed the thermal reaction. The rate constants of the thermal reaction were well correlated with Brown-Okamoto's substituent constant σ⁺ that is a modified value of Hammett's substituent constant σ. The large rate constants are ascribed to the lower activation energy for the thermal reaction. These results provide new knowledge for the molecular design of diarylethenes for a write-by-light/erase-by-heat recording system.     

Photochromic Dithienylethene‐Containing Triarylborane Derivatives: Facile Approach to Modulate Photochromic Properties with Multi‐addressable Functions

A series of dithienylethene‐containing triarylboranes has been designed, synthesized, and characterized. The electrochemistry, photophysics, and photochromic behavior have also been studied. The photophysical and photochromic properties could be facilely tuned in this system by varying the thiophene spacers (thiophene, thienothiophene, and bithiophene) between the dithienylethene and the dimesitylboron (BMes₂) or the position of the BMes₂ substitution in the thiophene spacers. The absorption of closed form has been found to be more sensitive towards the structural modification upon incorporation of the BMes₂ unit. Moreover, multi‐addressable photochromic reactivity is obtained upon addition of Lewis base (F^?), which is due to the formation of boron–Lewis base adduct. The dependence of the photophysical and photochromic properties on the thiophene spacers and the position of the BMes₂ substitution has been further supported by computational studies.     

Peptide Conjugated Dihydroazulene/Vinylheptafulvene Photoswitches in Aqueous Environment

Light-responsive molecules have seen a major advance in modulating biological functions in recent years. Especially photoswitches are highly attractive building blocks due to the reversible nature of their light-mediated reactivity. They are frequently used to affect both the properties of small bioactive compounds and biomacromolecules if incorporated suitably. Despite their success in a plethora of applications, only a limited set of photochromic core structures is routinely employed and a large number of photochromic couples are under-investigated in biological context. Broadening the toolbox of photoswitches available to modulate biological activity would open new avenues and unlock the full potential of photoswitchable molecules for biological studies. In this work, we explore the photochemical and thermal properties of the dihydroazulene/vinylheptafulvene photochromic couple as peptide conjugates in aqueous environment.     

Synthesis, crystal structure and optoelectronic properties of a new unsymmetrical photochromic diarylethene

A new unsymmetrical photochromic diarylethene, 1-(2-methyl-5-phenyl-3-thienyl)-2-[2-methyl-5-(3-trifluoromethylphenyl)-3-thienyl]perfluorocyclopentene (1a), was synthesized and its structure was determined by single-crystal X-ray diffraction analysis. Its optical and electrochemical properties, including photochromic reactivity both in solution and in the solid state (PMMA film and the single-crystalline phase), fluorescence and electrochemical properties were investigated in detail. The compound showed excellent photochromism even in the single-crystalline phase by photo-irradiation. In acetonitrile, the open-ring isomer of diarylethene 1 exhibited relatively strong fluorescence at 470nm when excited at 300nm, and its emission intensity decreased along with the photochromism upon irradiation with 313nm light. Its closed-ring isomer showed almost no fluorescence. The electrochemical properties of diarylethene were investigated by performing cyclic voltammetry experiment and its HOMO and LUMO energy level were calculated.     

Photochromic Dithienylethene‐Containing Boron(III) Ketoiminates: Modulation of Photo‐Responsive Behavior through Variation of Intramolecular Motion

A series of dithienylethene‐containing boron(III) ketoiminates, as well as their corresponding β‐ketoimine ligands, have been synthesized and characterized. The photophysical, electrochemical, and photochromic properties of the compounds have been studied. Photocyclization has been found to be suppressed upon introduction of a phenyl substituent on the nitrogen atom of the β‐ketoiminate core, whereas photochromism could be observed by replacement of the phenyl substituent with a bulky mesityl group. It is believed that the steric effect of the mesityl unit restricts molecular rotation, resulting in such a prominent difference in the photochromic properties.     

Effect of imino nitroxyl and nitronyl nitroxyl groups on the photochromic reactivity of diarylethenes

Diarylethene derivatives having imino nitroxide and nitronyl nitroxide have been prepared to examine the effect of the radical substituents on the photochromic reactivity of 1,2-bis(2-methyl-1-benzothiophen-3-yl)perfluorocyclopentene. These radical substituents reduce the quantum yields of both cyclization and cycloreversion reactions. The nitronyl nitroxyl moiety is more effective to suppress the reactivity in comparison with the imino nitroxide moiety. [reaction: see text]     

Wavelength-dependent reactivity of a quinolinone: toward a photochromic three-state system

The photochemical reactivity of the quinolinone 3 was investigated using NMR by monitoring its reactions under appropriate irradiation wavelengths. Besides the irreversible formation of degradation products which were structurally identified, the reversible formation of the enol 4 and cyclobutenol 5 was also observed. The enol and cyclobutenol can be switched or reversed back to the quinolinone 3, resulting in a photochromic three-state system in which the relative ratio of the three components largely depends on the irradiation wavelength used.     

Photon‐Quantitative 6π‐Electrocyclization of a Diarylbenzo[b]thiophene in Polar Medium

The high reactivity of 6π‐electrocyclization in polar solvents has remained one of the important challenges for diarylethenes because of the emergence of a twisted intramolecular charge transfer (TICT) state at the excited state in such polar media, which usually quenches the photocyclization reaction. Herein we report on the preparation and highly efficient photocyclization of 2,3‐diarylbenzo[b]thiophenes with nonsymmetric side‐aryl units in a polar solvent. While the dithiazolylbenzo[b]thiophene showed a suppressed quantum yield of 6π‐electrocyclization of 54 % in methanol, the replacement of a thiazole unit with a thiophene ring led to a photon‐quantitative 6π‐cyclization reaction. The nonsymmetrical modification into the side‐aryl units was considered to enhance the CH/π interactions between side‐aryl units to support a photoreactive conformation in methanol. The stabilization of the photochromic reactive conformation is expected to suppress the formation of the TICT state at the excited state, leading to highly efficient photoreactivity.     

Photochromic Benzo[b]phosphole Alkynylgold(I) Complexes with Mechanochromic Property to Serve as Multistimuli‐Responsive Materials

A series of novel benzo[b]phosphole alkynylgold(I) complexes has been demonstrated to display photochromic and mechanochromic properties upon applying the respective stimuli of light and mechanical force. Promising multistimuli‐responsive properties of this series of gold(I) complexes have been successfully achieved through judicious molecular design, which involves incorporation of the photochromic dithienylethene‐containing benzo[b]phosphole into the triphenylamine‐containing arylethynyl ligand that is susceptible to mechanical force‐induced color changes via gold(I) complexation. With excellent thermal irreversibility and robust fatigue resistance of this series of gold(I) complexes, multicolor states controlled by the photochromism and mechanochromism have been realized. Repeatable photochromic and mechanochromic cycles without apparent loss of reactivity have also been observed under ambient conditions. The present work provides important insight and an alternative strategy for the molecular design of multistimuli‐responsive materials, paving the way for further development of the underexplored photoresponsive gold(I) complexes and the multistate photocontrolled system.