Photochromic compounds can be classified into two categories. One is thermally irreversible (P-type) and the other thermally reversible (T-type). Although diarylethenes are generally P-type, a certain environmental modification such as the addition of specific chemicals can change it into the T-type. For example, the addition of Brønsted acid to a diarylethene possessing an amino group is known to change the reaction mode of photochromism. However, in order to restore its P-type photochromism, a scavenger of the acid is required. We have disclosed here that the P-type/T-type switching of diarylethenes can be achieved by the photochromic reaction of a coexisting spiropyran which can generate a strong acid by photoirradiation. When it takes the thermally stable merocyanine (mc) form, it works as a weakly acidic phenol. On the other hand, when it takes the spiropyran (sp) form generated by visible light irradiation, it works as a strongly acidic iminium sulfonate. Protonation to the accompanying diarylethene changes it from P-type to T-type. When the sp form thermally returns to the mc form, the P-type nature of the diarylethene is restored again. Thus, we have achieved the all-optical switching between the P-type/T-type photochromic reaction modes of diarylethenes. These procedures do not require the addition or removal of any agents from the photochromic system.

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 (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.

A new dipeptide with an acridone chromophore, namely 2-acridonylalanine-phenylalanine (AAP) was synthesized, and a fluorescent hydrogel was produced under physiological pH conditions by the AAP self-assembly process. The hydrogel exhibited a blue emission and reasonable thermoresponsiveness. Therefore, dipeptides containing an acridone side chain of the first residue were found to be effective hydrogelators.

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 sp3 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).

The design and control of self-assembling biomaterials have significantly attracted attention over the last decades because of their broad ranges of applications. Here, we introduce the self-assembled fibers of the pyrene connected dipeptides, L-pyrenylalanine-L-phenylalanine (L-Pyr-L-Phe) and D-pyrenylalanine-L-phenylalanine (D-Pyr-L-Phe), and their structural analysis using experimental and computational techniques. While L-Pyr-L-Phe self-assembled into solid fibers, D-Pyr-L-Phe self-assembled into hydrogels with different morphologies. Fluorescence spectroscopy revealed monomer and red-shifted excimer emissions of the self-assembled L-Pyr-L-Phe and D-Pyr-L-Phe dipeptide nanostructures, respectively. This result was related to different C-13 and N-15 solid-state nuclear magnetic resonance (NMR) data on the backbone and side-chains of the self-assembled dipeptides. Molecular dynamics simulations demonstrated detailed information about the chirality effects of the dipeptides on their self-assembled structures.

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.

This paper examines the role of protonation on the photochromic reactions of multi-responsive phenyl amine diarylethene derivatives (PA-DAEs). Reversible protonation and deprotonation provide a secondary stimulus for controlling the properties of light-responsive PA-DAEs, including solubility and thermal stability. For this reason, the phenylamine substituted DAEs were synthesized using a novel and efficient microwave-assisted synthetic route. Steady-state spectroscopy results indicated that the photocyclization and photocycloreversion reactions were reversible at different pH values. Even though, the acidic condition caused red-shift of the visible light absorption bands and generated a new absorption band at near-IR to IR region in the closed-ring structure but made blue-shift in the absorption spectra of the open-ring isomers. Also, it was showed that the excess amount of proton locked back the photochromic reaction. Results also indicated that the protonated closed-ring isomers of PA-DAEs are hydrophilic, whereas the deprotonated forms and protonated open-ring  forms are hydrophobic. In addition to photoswitching and solubility of PA-DAEs under irradiation at different light wavelengths, the thermal stability of P-type PA-DAEs was also monitored in the presence of trifluoroacetic acid at different temperatures as external stimuli.

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.

The Front Cover shows the generation of a colored image, whose origin is the UV-controlled selective reflection of visible light from a cholesteric liquid crystal containing chiral photochromic molecules. More information can be found in the Article by Yasushi Yokoyama and co-workers.

1. Department of Organic Colorants, Institute for Color Science and Technology, P. O. Box: 16765-654, Tehran, Iran 2. Department of Chemistry, University of Kansas, Lawrence, KS 66045, United States 3. Department of Advanced Materials Chemistry, Graduate School of Engineering, Yokohama National University, Yokohama, Japan 4. Department of Resin and Additives, Institute for Color Science and Technology, P. O. Box: 16765-654, Tehran, Iran 5. Center of Excellence for Color Science and Technology, Institute for Color Science and Technology, P.O. Box: 16765-654, Tehran, Iran

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.

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.

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.

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.

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

The attachment of 3H-naphthopyran to a three-dimensional network of polysiloxane elastomer was carried out by hydrosilylation reactions using 3H-9-(5-hexenyloxy) naphthopyran. The photochemical coloration and subsequent thermal decoloration behaviors of polysiloxane-bound 3H-naphthopyran molecules were examined by comparing their properties with those of a model 9-hexyloxy-3H-naphthopyran compound doped in a polysiloxane film, a PMMA film, and in solutions. The decoloration reaction rate of polysiloxane-bound naphthopyran was basically the same as that in the polysiloxane-doped system and in solutions. However, the polysiloxane-bound system is superior to the doped system due to: (1) more concentrated incorporation of naphthopyran molecules in the polysiloxane film; (2) less degradation of naphthopyran molecules during film preparation; and (3) less temperature dependence in the thermal decoloration rate.

Two new bithiazole-containing diarylethenes, 1[5-methyl-2-(5-methyl-2-thiazolyl)-4-thiazolyl]-2,5-dimethy1-3-thienyl-3,3,4,4,5,5-hexafluorocyclopentene and 1[5-methyl-2-(5-methyl-2-thiazolyl)-4-thiazolyl]1-2-methyl-3-benzothienyl-3,3,4,4,5,5-hexafluorocyclopentene were synthesized and their photochromic and fluorescent properties have been investigated. They exhibited thermally irreversible photochromism and fluorescent switching in toluene. The effects of complexation of these two compounds with various metal cations such as Na+, Ag+, Hg+, Co2+, Ni2+, Cu2+, Ba2+, Zn2+, Cr3+, Pb2+, Cd2+, Sn2+, Al3+, Fe3+ on photochromic and fluorescent properties were also studied. Metal cations caused small effects on the absorption bands of their ring closed forms. However, fluorescence intensities of both compounds were strongly suppressed by Ni2+, Cu2+ and Fe3+.

The mechanism of the cycloreversion reaction of a fulgide derivative in the higher excited states (S-n states) pumped by the stepwise two-photon excitation was investigated by femtosecond transient absorption spectroscopy. The first photon excitation of the ground state by the visible light led to the production of the Franck-Condon 1B state, which underwent internal conversion to the 2A state with 100 fs time constant and the structural reorganization accompanied with the vibrational cooling with 1.5 ps time constant. After these relaxation processes, the 2A state underwent the cycloreversion reaction (11% reaction yield) with competitive nonradiative as well as radiative deactivation processes into the ground state with a time constant of 10 ps. Under the double-pulse pump condition, whereas the excitation of the 1B state induced no remarkable cycloreversion reaction, the excitation of the 2A state resulted in the marked enhancement of the cycloreversion reaction with a reaction quantum yield of ca. 80% in the Sn state. By summarizing the excitation wavelength dependence of the reaction amount on wavelengths of the second pump pulse, we discussed the mechanism of the effective multiphoton-gated reaction in highly excited states.