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Abstract: Energy gaps between a biradical triplet (BR-T) state and a zwitterionic singlet (ZI-S) state in tetramethyleneethane (TME) π-systems stabilized by hetero-atoms are discussed. Unsymmetrical introduction of both electron-donating and -withdrawing groups into the TME system caused substantial stabilization of the ZI-S. Thus, the energy separation between ZI-S and BR-T states in 2-dicyanomethylene-3-(1,3-dithiolenylidene)cyclopenta-1,3-diene was as small as 0.62kcalmol−1, in contrast to that in 2,3-dimethylenecyclopenta-1,3-diene at 22.5kcalmol−1, using the B3LYP/6-31G* level of calculation. In a series of 2,3-dimethylene-5-heterocyclopentadiene, the ZI-S state was stabilized by donor-atoms in the order of X (hetero-atoms)=O>S>NH>NMe. The ZI-S states of 3,4-dimethylenepyrroles (NH and N–Me) and 3,4-dimethylene-1,2-diazole lie below the BR-T by 3.1, 4.4 and 10.3kcalmol−1, respectively. Cyclic 10-π-delocalization in hetero-pentalene further stabilizes the ZI-S, and 2,5-dimethylpyrazolo[3,4-c]pyrazole is a ground state singlet molecule lying below the triplet by 49.5kcalmol−1. Time dependent-density functional theory (TD-DFT) calculations have successfully predicted the absorption spectra of these TMEs in the ZI-S state. [Copyright &y& Elsevier]

Nine 1- [2-(1-substituted)ethyl-3-benzothienyl]-2-(2-methyl-3-benzothienyl)hexafluorocyclopentenes (4-substituted benzoyloxy, N-arylcarbamoyloxy, and aryloxycarbonyloxy) were synthesized. They showed diastereoselective photochromic ring closure upon UV irradiation. Among them, 4-nitro- phenoxycarbonyloxy-substituted compound recorded a remarkable 94% diastereomer excess with 91% conversion to the closed form. [ABSTRACT FROM AUTHOR]

The carbonylation of the substituted 2-butyne-1,4-diols, RArC(OH)&z.tbnd6;C(OH)ArR (Ar, 2,5-dimethylthienyl; R, alkyl) was studied by using Pd(OAc)2/I2 as the catalyst in benzene under pressure. The diols bearing methyl or isopropyl group as the alkyl substituent gave photochromic fulgide (4a or 4b). On the other hand, the carbonylation of the diol bearing bulky t-butyl group did not afford the corresponding fulgide but lactone 5, together with butatriene 6. [Copyright &y& Elsevier]

A bisthienylethene with hitherto unprecedented facial chirality imposed by a triethyleneglycol bridge on a thiophene ring was synthesized and its photochromic ring closure was shown to occur with 100% diastereoselectivity upon UV-light irradiation. [ABSTRACT FROM AUTHOR]

Controlled construction of molecular chirality is central to modern molecular organic chemistry. In their Communication on page 9434 ff., Y. Yokoyama et al. demonstrate that human serum albumin (HSA), a nonenzymatic protein in blood, acts as a mold to fix the conformation of thermally irreversible photochromic diarylethenes and transfers chirality in photoreactions. A bisthienylperfluorocyclopentene with a hydroxy group at each end of the molecule, incorporated in the HSA pocket, yields the colored cyclic form in 63 % ee at 25 °C and 71 % ee at −4 °C. The X‐ray image of HSA was provided courtesy of the RCSB PDB (Code 4K2C with Simple Viewer 4.1.0). [ABSTRACT FROM AUTHOR]

A novel chiral photoswitch composed of a binaphthyl unit and a hexafluorocyclopentene ring has been synthesized. This chiral photoswitch exhibited thermally reversible photochromism between the binaphthyl and helicenoid forms based on 6π-electrocyclization. The helicity of the binaphthyl moiety was reversed upon stereospecific photocyclization and reverted back during the thermal ring opening.

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 (Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ , Fe 2+ , Ni 2+ , Al 3+ , Sb 5+ ) changed 2o to be photochromic due to the strong attraction of the lone pair on the nitrogen atom. In the presence of excess Cu 2+ , 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 2o 2+ by the irradiation of 365-nm light in the presence of Cu 2+ . ESI-MS measurements strongly suggested the generation of 2o ·+ by mixing 2o with Cu(ClO 4 ) 2 in acetonitrile, and the transformation of 2o ·+ to 2o 2+ by successive 365-nm light irradiation. Fe 3+ similarly worked as the oxidant, but the two-step oxidation of 2o to 2o 2+ occurred more easily., (© 2023. The Author(s), under exclusive licence to European Photochemistry Association, European Society for Photobiology.)

Photochromic 1,2-bis(5-carboxy-3-methyl-2-thienyl)hexafluorocyclopentene and its dimethyl ester incorporated in human serum albumin (HSA) showed highly enantioselective photochromic ring-closing reactions upon 366 nm light irradiation. The absolute stereochemistry of the major ring-closed form of the dicarboxylic acid at the newly formed sp 3 carbon atoms was determined to be ( S,S ) by the process of docking simulation of the diarylethene molecule and HSA followed by molecular dynamics calculations and comparison of the measured and calculated CD spectra. Esterification of the major closed form of the diacid gave the minor closed form of the diester. The absolute stereochemistry of the major diester was thus determined to be ( R,R ).

A chiral and thermally irreversible photochromic fulgide derivative incorporating an (R)-binaphthol unit in its acid anhydride moiety was used for the photoswitching of the pitch length of cholesteric liquid crystals. Since the absorption maximum wavelengths of both thermally stable photoisomers are nearly in the UV region (quasi-stealth photochromism), it can be exposed to visible light without inducing photochromic reactions. Therefore, when the photoswitching molecule is added to a permanent cholesteric liquid crystal whose reflection light wavelength is in the visible region, the UV light-induced photochromic reaction of the photoswitching molecule changes the wavelength of the reflection light in the visible light region. We have succeeded in regulating the color of cholesteric liquid crystalline cells between red and blue upon UV light irradiation. Attempts to introduce this system in polymer dispersed cholesteric liquid crystals are also described., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A thermoresponsive fluorophore based on a photochromic diarylethene possessing donor and acceptor moieties in close proximity to each other has been synthesized. The fluorophore exhibits dual fluorescence, where photoresponsive turn-off switching as well as a thermoresponsive relative intensity change are observed.

Fine-tuning of the molecular structure of organic bistable compounds to improve their photochromic performance or to introduce additional functions remains an important issue in the development of photoresponsive materials. Diarylethenes bearing heterocyclic moieties belong to the most intensively studied class of organic photochromes due to their excellent photochemical properties. A huge number of diarylethenes have been synthesized so far. Analysis of the literature data shows that there are very worthy examples of diarylethenes developed by the Irie and Feringa groups, which can be the common starting material for a number of diarylethenes functionalized in hetaryl moieties. We refer to these structures as photochromic diarylethene precursors. These diarylethenes have proved to be very useful in the construction of functional molecules with desired properties. On the other hand, in our groups, we have elaborated on diarylethene precursors with modifiable ethene bridges. In this review, we have collected examples of such structures and their chemical modifications, leading to the improvement or fine-tuning of photochromic switching., (© 2019 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Three new diarylethenes were synthesized from 1,2-bis(5-methyl-2-(4-substituted-phenyl)thiazol-4-yl)ethyne and benzyl azide through Ru(I)-catalyzed Huisgen cyclization reactions. The 4,5-bisthiazolyl-1,2,3-triazoles thus prepared, which belong to the terarylene family, showed thermally reversible photochromism. The absorption maximum wavelengths of the closed forms are longer than other terarylenes reported so far. The thermal back reactions are much faster when the substituents on the terminal phenyl groups are electron-withdrawing cyano groups than when they are electron-donating methoxy groups., (Copyright © 2019, Zhang et al.; licensee Beilstein-Institut.)

The synthesis of thermally irreversible photochromic arylbutadienes has been demonstrated with a one-step reaction using commercially available ketones by McMurry cross-coupling. Although ( Z)-5-(1-([1,1'-bi(cyclopentylidene)]-2-ylidene)ethyl)-4-methyl-2-phenylthiazole, a 1-(4-methyl-2-phenyl-5-thiazolyl)butadiene, showed poor photochromism, ( Z)-5-(1-([1,1'-bi(cyclopentylidene)]-2-ylidene)ethyl)-2,4-dimethylthiazole, a 1-(2,4-dimethyl-5-thiazolyl)butadiene, showed stealth photochromic transformations by 313 and 366 nm light irradiation between the moderate conversion ratios of the open and closed forms.

Two photochromic bisbenzothienylethenes with two fluorescent (4-((2,5-bis(dodecyloxy)-4-(phenylethynyl)phenyl)ethynyl)phenyl) units and their disulfone derivatives were synthesized, and their photochromic and fluorescence properties were examined. Bisbenzothienylethenes showed photochromism and turn-off type fluorescence by UV-light irradiation, while their disulfone derivatives showed turn-on type fluorescence and the one-way isomerization to the closed form.

A spiro-functionalized photochromic diarylethene derivative showed multi-colour fluorescence modulation with a photon-quantitative photocyclization reactivity and high thermal stability.

Upon visible light irradiation, a thermally reversible photochromic acid generator merocyanine dye gives a proton to the dimethylamino group of a non-photochromic diarylethene in acetonitrile. The protonation rendered the non-photochromic diarylethene photochromic. Thus, the thermally irreversible photochromic nature of the diarylethene is activated without any physical contact or addition of any compound but only by photoirradiation to the merocyanine dye.

A facile method for the fabrication of a highly sensitive surface relief is demonstrated, which operates on the principle of spatially patterned photopolymerization-induced mass transport in the amorphous films of a series of bisanthracene compounds. The stability of the resultant colorless transparent relief structure is dramatically improved owing to the polymerization of the bisanthracene.

Thermally irreversible photochromic 1-tert-butyl-substituted 2,3-bisthiazolylindenol has been synthesized. It showed perfect diastereoselectivity and high ring-closing quantum yield with high conversion ratio to the closed form. The collaborative interaction of two intramolecular hydrogen bonds and the steric restriction fixed the conformation in favour of cyclization in a highly diastereoselective manner.

Three new photochromic coumarins were synthesized. Fluorescence of the open form of 7-hydroxy-3,4-bisthiazolyl-coumarin increased to 1400% by changing the pH only slightly from 6.05 to 7.58. This was subsequently quenched to 1.5% of the maximum intensity at the UV photostationary state in water-methanol media.

The first demonstration of photoinduced surface relief grating formation using amorphous thin films composed of a radical dissociative photochromic compound, 2,2'-di(ortho-chlorophenyl)-4,4',5,5'-tetraphenylbiimidazole, show that mass transfer occurred from the UV-light-irradiated area to the shaded area by patterned light irradiation (365 nm).

Spironaphthooxazine (SNO) was attached to a three-dimensional network of polysiloxane through the allyloxy group on C-7 of the naphthalene ring of SNO, and the photochemical coloration and thermal decoloration behaviors of this material (Film-P) were examined by comparing their properties with those of model SNO having a 7-propyloxynaphthyl group in solution, in a doped polysiloxane film (Film-D), and in a doped PMMA film. For Film-P, the SNO units were connected to a part of the vinyl groups on the Si atoms of a linear polysiloxane by hydrosilylation, and the remaining vinyl groups were bridged by the bifunctional linear polysiloxane possessing two hydrosilyl groups at both ends of the chain. While Film-P containing 1.7 x 10(-3) mol dm(-3) of the SNO unit formed a transparent film, Film-D containing more than 1.2 x 10(-3) mol dm(-3) could not form a transparent film but segregation of SNO occurred. A transparent Film-D was prepared at 0.54 x 10(-3) mol dm(-3) SNO concentration. The decoloration reaction rate of Film-D was faster than those for Film-P, solutions, and PMMA. However, due to the low concentration, the coloration of Film-D was weak. When the concentration of SNO in Film-P was increased 5.7 times, the decoloration rate became 3.6 times faster. Its half-life time was 0.9 s, which is 3.1 times as fast as that of corresponding solution in hexamethyldisiloxane.

Bisarylindenones and an acetal derivative, a novel thermally irreversible photochromic family based on 6pi-electrocyclization with three easily modifiable functional groups, have been synthesized. 2,3-Bis(5-methyl-2-phenyl-4-thiazolyl)indenone showed photochromic back-and-forth reactions with two different visible lights. Its ethylene acetal recorded 0.81 as the photocyclization quantum yield in hexane, which is the largest value known to date for 6pi-electrocyclization in solution.

Into my arms: Photochemical cyclization of diarylethenes that have two chiral side arms showed up to 100% de (see scheme). Introduction of these chiral side arms onto the carbon atoms where ring closure occurs is a general strategy for the highly diastereoselective cyclization of diarylethenes.

Photochromic dendrimer diarylethenes with a C-2-connected bisbenzothienylethene core were synthesized; the most notable feature of them is the strong memory effect of cyclizable conformation of the open form when generated from the closed form by visible light irradiation in polycarbonate films.

A novel photochromic helicenoid diarylethene (R)-1-[1-(1-methoxymethoxyethyl)-2-naphtho[2,1-b]thienyl]-2-(2,4,5-trimethyl-3-thienyl)hexafluorocyclopentene was synthesized enantioselectively. It showed highly diastereoselective photocyclization (90% de) and a large change (950 degrees) in the specific optical rotation value at 633 nm upon UV light irradiation in ethyl acetate.

A diarylethene possessing one [4]thiaheterohelicene and one benzothiophene, the latter with a chiral methoxymethoxyethyl group on its C-3 position, was proved to work as a switch of specific optical rotation at a wavelength at which both colored and colorless forms have no absorption in solution. The difference of the specific optical rotation was 1300 degrees between the open form and the photostationary state. The specific optical rotation of one of the isolated optically active major colored forms was -4680 degrees. The conversion to the colored form was 64%, and the diastereomeric excess of photocyclization was 47%.

The concept "introduction of allylic 1,3-strain (A(1,3)-strain) to photochromism" originated from the experience of total synthesis of natural products about 18 years ago, when the author was a post doc. It was first applied to synthetic organic reactions, then it was successfully employed in the diastereoselective photochromic reactions of diarylethenes. A diastereomeric excess of the photoreaction was 88 % for the first example, and it was improved to 94 % by the recent operation of rational molecular tailoring.

Introduction of an asymmetric center to C-2 of one of the benzothiophene rings of bisbenzothienylperfluorocyclopentene results in a highly diastereoselective photochromic system. The stereogenic center bears a hydrogen atom, a methyl group, and a methoxymethoxy group. The steric as well as the electronic repulsions gave an 87-88% diastereomer excess in various solvents at room temperature with 80-85% conversion to the colored form. The enantioselective synthesis was also carried out. Upon photoirradiation in hexane, a change in optical rotation at 820 nm, where neither the open form nor the colored form absorbs light, was observed repeatedly.