Shigehiro Sumiya

A Rhodamine−Cyclen Conjugate as a Highly Sensitive and Selective Fluorescent Chemosensor for Hg(II)

A rhodamine-cyclen conjugate (1) behaves as a highly sensitive and selective fluorescent chemosensor for Hg(2+). The high emission selectivity is due to the formation of 1-Hg(2+) 1:2 complex leading to spirocycle opening of 1.

Thermoresponsive Copolymer Containing a Coumarin–Spiropyran Conjugate: Reusable Fluorescent Sensor for Cyanide Anion Detection in Water

A simple copolymer consisting of N-isopropylacrylamide and coumarin-conjugated spiropyran (CS) units, poly(NIPAM-co-CS), has been synthesized. This polymer enables selective fluorometric detection of cyanide anion (CN(-)) in water at room temperature. The polymer itself shows almost no fluorescence, but shows a strong blue fluorescence in the presence of CN(-) under irradiation of UV light. The fluorescence enhancement occurs via a nucleophilic interaction between CN(-) and the photoformed merocyanine form of the CS unit, leading to a localization of π-electrons on the coumarin moiety. The polymer enables accurate determination of very low levels of CN(-) (>0.5 μM). The polymer can be recovered from water by simple centrifugation at high temperature (>40 °C), due to the heat-induced aggregation of the polymer. In addition, the polymer is regenerated by simple acid treatment, and the resulting polymer is successfully reused for further CN(-) sensing without loss of sensitivity.

Mechanism for different fluorescence response of a coumarin-amide-dipicolylamine linkage to Zn(II) and Cd(II) in water.

A coumarin-amide-dipicolylamine linkage (L) 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(2+) or Cd(2+) with L 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(2+) and Cd(2+) are coordinated with two pyridine N, amine N, and amide O; however, the Zn(2+) 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.

Multicolor Fluorescence of a Styrylquinoline Dye Tuned by Metal Cations

A styrylquinoline dye with a dipicolylamine (DPA) moiety (1) has been synthesized. The dye 1 in acetonitrile demonstrates multicolor fluorescence upon addition of different metal cations. Compound 1 shows a green fluorescence without cations. Coordination of 1 with Cd(2+) shows a blue emission, while with Hg(2+) and Pb(2+) exhibits yellow and orange emissions, respectively. The different fluorescence spectra are due to the change in intramolecular charge transfer (ICT) properties of 1 upon coordination with different cations. The DPA and quinoline moieties of 1 behave as the electron donor and acceptor units, respectively, and both units act as the coordination site for metal cations. Cd(2+) coordinates with the DPA unit. This reduces the donor ability of the unit and decreases the energy level of HOMO. This results in an increase in HOMO-LUMO gap and blue shifts the emission. Hg(2+) or Pb(2+) coordinate with both DPA and quinoline units. The coordination with the quinoline unit decreases the energy level of LUMO. This results in a decrease in HOMO-LUMO gap and red shifts the emission. Addition of two different metal cations successfully creates intermediate colors; in particular, the addition of Cd(2+) and Pb(2+) at once creates a bright white fluorescence.

Entropy-Driven Thermal Isomerization of Spiropyran in Viscous Media

Effects of solvent viscosity on the thermal isomerization properties of a spiropyran derivative have been studied in glycerol, ethylene glycol, 1,4-butanediol, and ionic liquid solutions. Thermal isomerization of the colorless spirocyclic (SP) form to the colored merocyanine (MC) form is enhanced with an increase in the concentrations of viscous solvents in solution. Equilibrium absorption analysis revealed that the enhanced SP → MC isomerization in viscous media is due to the strong solvent-solvent interaction, which suppresses the ordering of solvent molecules around the MC form. This results in a positive entropy change for isomerization and, hence, promotes entropy-driven isomerization. Kinetic absorption analysis revealed that the solvent viscosity scarcely affects the thermal activation process for isomerization, where the activation enthalpy and entropy parameters are solely affected by the solvent polarity.

Spiropyran–cholesterol conjugate as a photoresponsive organogelator

We synthesized a spiropyran–cholesterol conjugate (1) and studied its gelation and photochromic properties. The compound 1, when dissolved in carbon disulfide (CS2), forms a homogeneous gel at −40 °C due to the van der Waals interaction between the cholesterol moieties. UV or visible light irradiation of the CS2 sol or gel promotes a reversible change in color between yellow and purple without phase transition. This is promoted by reversible photoisomerization of the spiropyran moiety between the spirocyclic (SP) and the spirocycle-opened merocyanine (MC) form. In contrast, 1 forms a gel in benzyl alcohol (BA) at room temperature. The BA gel also exhibits a reversible color change upon UV/visible light irradiation, but the BA sol always shows a purple color due to the stabilization of the MC form by strong H-bonding interaction with solvents. The BA gel is successfully applied as a material for information storage, easily writable and erasable by light irradiation.