27 research outputs found
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 Zn<sup>2+</sup> in water. The receptor dissolved in water at neutral
pH shows fluorescence
at 383 nm. The addition of Zn<sup>2+</sup>, however, leads to a decrease
in this emission, along with an appearance of red-shifted emission
at 445 nm. This thus facilitates ratiometric Zn<sup>2+</sup> sensing.
Other metal cations do not promote such spectral change. Complexation
of <b>L</b> with Zn<sup>2+</sup> involves the coordination with
four cyclen nitrogens and amide oxygen. IR and potentiometric analysis
revealed that strong coordination of Zn<sup>2+</sup> 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 Zn<sup>2+</sup> or Cd<sup>2+</sup> 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 Zn<sup>2+</sup> and Cd<sup>2+</sup> are coordinated
with two pyridine N, amine N, and amide O; however, the Zn<sup>2+</sup> center is also coordinated with a hydroxide anion (OH<sup>–</sup>). 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<sup>–</sup>-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 (NH<sub>3</sub>) is an essential
chemical in modern society.
It is currently manufactured by the Haber–Bosch process using
H<sub>2</sub> and N<sub>2</sub> under extremely high-pressure (>200
bar) and high-temperature (>673 K) conditions. Photocatalytic NH<sub>3</sub> production from water and N<sub>2</sub> 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 TiO<sub>2</sub> with a large number
of surface oxygen vacancies, when photoirradiated by UV light in pure
water with N<sub>2</sub>, successfully produces NH<sub>3</sub>. The
active sites for N<sub>2</sub> reduction are the Ti<sup>3+</sup> species
on the oxygen vacancies. These species act as adsorption sites for
N<sub>2</sub> and trapping sites for the photoformed conduction band
electrons. These properties therefore promote efficient reduction
of N<sub>2</sub> to NH<sub>3</sub>. 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 TiO<sub>2</sub> system therefore shows a potential as a new artificial photosynthesis
for green NH<sub>3</sub> production
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 (TiO<sub>2</sub>) 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 Ti<sup>3+</sup> atoms on TiO<sub>2</sub> to
epoxides, which promotes deoxygenation of epoxides, and photocatalytic
action of TiO<sub>2</sub>, 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<sup>–</sup>) 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<sup>–</sup> 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<sup>–</sup> as low as 1.0 μM
Pt–Cu Bimetallic Alloy Nanoparticles Supported on Anatase TiO<sub>2</sub>: 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 TiO<sub>2</sub> 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
Highly Efficient and Selective Hydrogenation of Nitroaromatics on Photoactivated Rutile Titanium Dioxide
We report that photoactivated rutile titanium dioxide
(TiO<sub>2</sub>) catalyzes a highly efficient and selective hydrogenation
of nitroaromatics with alcohol as a hydrogen source. Photoirradiation
(λ >300 nm) of rutile TiO<sub>2</sub> suspended in alcohol
containing
nitroaromatics at room temperature and atmospheric pressure produces
the corresponding anilines with almost quantitative yields, whereas
common anatase and P25 TiO<sub>2</sub> show poor activity and selectivity.
The Ti<sup>3+</sup> 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
TiO<sub>2</sub> system also facilitates chemoselective hydrogenation
of nitroaromatics with reducible substituents; several kinds of functionalized
anilines are successfully produced with >94% yields
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