Polarity-Driven Isomerization of a Hydroxynaphthalimide-Containing Spiropyran at Room Temperature

Design of spiropyrans showing spontaneous isomerization driven by the polarity of solvents is an important consideration for the synthesis of optical sensory materials. Although some spiropyrans undergo polarity-driven isomerization, they must be heated owing to the high activation energy required for isomerization. In this study, we describe that a spiropyran containing a hydroxynaphthalimide unit (1) exhibits a polarity-driven isomerization at room temperature. It exists as a colorless spirocyclic (SP) form in less polar solvents but is isomerized to a colored merocyanine (MC) form in polar solvents. The equilibrium amount of the MC form increases with an increase in the polarity of solvents. The MC form involves two resonance structuresthe quinoidal and zwitterionic forms. In polar media, the zwitterionic form dominates mainly owing to solvation by polar molecules. Solvation stabilizes the negative charge of the zwitterionic form and decreases its ground state energy, thereby enhancing SP → MC isomerization. The SP ⇌ MC isomerization terminates within barely 30 s even at room temperature because the naphthol moiety with high π-electron density lowers the activation energy for the rate-determining rotational step

Phenylbenzoxazole–Amide–Cyclen Linkage as a Ratiometric Fluorescent Receptor for Zn(II) in Water

A phenylbenzoxazole–amide–cyclen linkage (<b>L</b>) behaves as a ratiometric fluorescent receptor for Zn²⁺ in water. The receptor dissolved in water at neutral pH shows fluorescence at 383 nm. The addition of Zn²⁺, however, leads to a decrease in this emission, along with an appearance of red-shifted emission at 445 nm. This thus facilitates ratiometric Zn²⁺ sensing. Other metal cations do not promote such spectral change. Complexation of <b>L</b> with Zn²⁺ involves the coordination with four cyclen nitrogens and amide oxygen. IR and potentiometric analysis revealed that strong coordination of Zn²⁺ with amide oxygen leads to a deprotonation of the amide moiety and creates red-shifted fluorescence. Ab initio calculation indicated that the deprotonation of the amide moiety allows rotational motion of the benzoxazole moiety in the excited state and stabilizes the twisted intramolecular charge transfer (TICT) excited state. This results in the creation of red-shifted fluorescence from the TICT excited state

Mechanism for Different Fluorescence Response of a Coumarin–Amide–Dipicolylamine Linkage to Zn(II) and Cd(II) in Water

A coumarin–amide–dipicolylamine linkage (<b>L</b>) was synthesized and used as a fluorescent receptor for metal cations in water. The receptor dissolved in water with neutral pH shows almost no fluorescence due to the photoinduced electron transfer (PET) from the amide and amine nitrogens to the excited state coumarin moiety. Coordination of Zn²⁺ or Cd²⁺ with <b>L</b> creates strong fluorescence at 437 or 386 nm, respectively, due to the suppression of PET. In contrast, other metal cations scarcely show fluorescence enhancement. IR, NMR, and potentiometric analysis revealed that both Zn²⁺ and Cd²⁺ are coordinated with two pyridine N, amine N, and amide O; however, the Zn²⁺ center is also coordinated with a hydroxide anion (OH^–). The structure difference for Zn and Cd complexes results in longer- and shorter-wavelength fluorescence. Ab initio calculations revealed that π electrons on the excited state Cd complex are delocalized over the molecules and the Cd complex shows shorter-wavelength emission. In contrast, π electrons of OH^–-coordinated Zn complex are localized on the coumarin moiety. This increases the electron density of coumarin moiety and shows longer-wavelength fluorescence

Photocatalytic Conversion of Nitrogen to Ammonia with Water on Surface Oxygen Vacancies of Titanium Dioxide

Ammonia (NH₃) is an essential chemical in modern society. It is currently manufactured by the Haber–Bosch process using H₂ and N₂ under extremely high-pressure (>200 bar) and high-temperature (>673 K) conditions. Photocatalytic NH₃ production from water and N₂ at atmospheric pressure and room temperature is ideal. Several semiconductor photocatalysts have been proposed, but all suffer from low efficiency. Here we report that a commercially available TiO₂ with a large number of surface oxygen vacancies, when photoirradiated by UV light in pure water with N₂, successfully produces NH₃. The active sites for N₂ reduction are the Ti³⁺ species on the oxygen vacancies. These species act as adsorption sites for N₂ and trapping sites for the photoformed conduction band electrons. These properties therefore promote efficient reduction of N₂ to NH₃. The solar-to-chemical energy conversion efficiency is 0.02%, which is the highest efficiency among the early reported photocatalytic systems. This noble-metal-free TiO₂ system therefore shows a potential as a new artificial photosynthesis for green NH₃ production

Selective Nitrate-to-Ammonia Transformation on Surface Defects of Titanium Dioxide Photocatalysts

Ammonia (NH₃) is an essential chemical in modern society, currently manufactured via the Haber–Bosch process with H₂ and N₂ under extremely high pressure (>200 bar) and high-temperature conditions (>673 K). Toxic nitrate anion (NO₃^–) contained in wastewater is one potential nitrogen source. Selective NO₃^–-to-NH₃ transformation via eight-electron reduction, if promoted at atmospheric pressure and room temperature, may become a powerful recycling process for NH₃ production. Several photocatalytic systems have been proposed, but many of them produce nitrogen gas (N₂) via five-electron reduction of NO₃^–. Here, we report that unmodified TiO₂, when photoexcited by ultraviolet (UV) light (λ > 300 nm) with formic acid (HCOOH) as an electron donor, promotes selective NO₃^–-to-NH₃ reduction with 97% selectivity. Surface defects and Lewis acid sites of TiO₂ behave as reduction sites for NO₃^–. The surface defect selectively promotes eight-electron reduction (NH₃ formation), while the Lewis acid site promotes nonselective reduction (N₂ and NH₃ formation). Therefore, the TiO₂ with a large number of surface defects and a small number of Lewis acid sites produces NH₃ with very high selectivity

Noble-Metal-Free Deoxygenation of Epoxides: Titanium Dioxide as a Photocatalytically Regenerable Electron-Transfer Catalyst

Catalytic deoxygenation of epoxides into the corresponding alkenes is a very important reaction in organic synthesis. Early reported systems, however, require noble metals, high reaction temperatures (>373 K), or toxic reducing agents. Here, we report a noble-metal-free heterogeneous catalytic system driven with alcohol as a reducing agent at room temperature. Photoirradiation (λ <420 nm) of semiconductor titanium dioxide (TiO₂) with alcohol promotes efficient and selective deoxygenation of epoxides into alkenes. This noble-metal-free catalytic deoxygenation is facilitated by the combination of electron transfer from surface Ti³⁺ atoms on TiO₂ to epoxides, which promotes deoxygenation of epoxides, and photocatalytic action of TiO₂, which regenerates oxidized surface Ti atoms with alcohol as a reducing agent

Coumarin–Spiropyran Dyad with a Hydrogenated Pyran Moiety for Rapid, Selective, and Sensitive Fluorometric Detection of Cyanide Anion

We synthesized a coumarin–spiropyran dyad with a hydrogenated pyran moiety (<b>2</b>), behaving as an off–on type fluorescent receptor for rapid, selective, and sensitive detection of cyanide anion (CN^–) in aqueous media. The receptor itself shows almost no fluorescence with a quantum yield < 0.01, due to the delocalization of π-electrons over the molecule. Selective nucleophilic addition of CN^– to the spirocarbon of the molecule rapidly promotes spirocycle opening within only 3 min. This leads to localization of π-electrons on the coumarin moiety and exhibits strong light-blue fluorescence at 459 nm with very high quantum yield (0.52). As a result of this, the receptor facilitates rapid, selective, and sensitive fluorometric detection of CN^– as low as 1.0 μM

Highly Efficient and Selective Hydrogenation of Nitroaromatics on Photoactivated Rutile Titanium Dioxide

We report that photoactivated rutile titanium dioxide (TiO₂) catalyzes a highly efficient and selective hydrogenation of nitroaromatics with alcohol as a hydrogen source. Photoirradiation (λ >300 nm) of rutile TiO₂ suspended in alcohol containing nitroaromatics at room temperature and atmospheric pressure produces the corresponding anilines with almost quantitative yields, whereas common anatase and P25 TiO₂ show poor activity and selectivity. The Ti³⁺ atoms located at the oxygen vacancies on the rutile surface behave as the adsorption site for nitroaromatics and the trapping site for photoformed conduction band electrons. These effects facilitate rapid and selective nitro-to-amine hydrogenation of the adsorbed nitroaromatics by the surface-trapped electrons, enabling aniline formation with significantly high quantum yields (>25% at <370 nm). The rutile TiO₂ system also facilitates chemoselective hydrogenation of nitroaromatics with reducible substituents; several kinds of functionalized anilines are successfully produced with >94% yields

Pt–Cu Bimetallic Alloy Nanoparticles Supported on Anatase TiO₂: Highly Active Catalysts for Aerobic Oxidation Driven by Visible Light

Visible light irradiation (λ > 450 nm) of Pt–Cu bimetallic alloy nanoparticles (∼3–5 nm) supported on anatase TiO₂ efficiently promotes aerobic oxidation. This is facilicated <i>via</i> the interband excitation of Pt atoms by visible light followed by the transfer of activated electrons to the anatase conduction band. The positive charges formed on the nanoparticles oxidize substrates, and the conduction band electrons reduce molecular oxygen, promoting photocatalytic cycles. The apparent quantum yield for the reaction on the Pt–Cu alloy catalyst is ∼17% under irradiation of 550 nm monochromatic light, which is much higher than that obtained on the monometallic Pt catalyst (∼7%). Cu alloying with Pt decreases the work function of nanoparticles and decreases the height of the Schottky barrier created at the nanoparticle/anatase heterojunction. This promotes efficient electron transfer from the photoactivated nanoparticles to anatase, resulting in enhanced photocatalytic activity. The Pt–Cu alloy catalyst is successfully activated by sunlight and enables efficient and selective aerobic oxidation of alcohols at ambient temperature

Sunlight-Driven Nitrate-to-Ammonia Reduction with Water by Iron Oxyhydroxide Photocatalysts

The photocatalytic reduction of harmful nitrates (NO3–) in strongly acidic wastewater to ammonia (NH3) under sunlight is crucial for the recycling of limited nitrogen resources. This study reports that a naturally occurring Cl–-containing iron oxyhydroxide (akaganeite) powder with surface oxygen vacancies (β-FeOOH(Cl)-OVs) facilitates this transformation. Ultraviolet light irradiation of the catalyst suspended in a Cl–-containing solution promoted quantitative NO3–-to-NH3 reduction with water under ambient conditions. The photogenerated conduction band electrons promoted the reduction of NO3–-to-NH3 over the OVs. The valence band holes promoted self-oxidation of Cl– as the direct electron donor and eliminated Cl– was compensated from the solution. Photodecomposition of the generated hypochlorous acid (HClO) produced O2, facilitating catalytic reduction of NO3–-to-NH3 with water as the electron donor in the entire system. Simulated sunlight irradiation of the catalyst in a strongly acidic nitric acid (HNO3) solution (pH ∼ 1) containing Cl– stably generated NH3 with a solar-to-chemical conversion efficiency of ∼0.025%. This strategy paves the way for sustainable NH3 production from wastewater