Spiropyran as a reusable chemosensor for selective colorimetric detection of aromatic thiols.

Design of optical molecular probes for selective detection of aromatic thiols has attracted much attention. Although several types of probes have been proposed, all of them exhibit colorimetric or fluorometric response via irreversible reaction with aromatic thiols and cannot be reused. Here we report that a spiropyran dye is the first example of a reusable chemosensor for aromatic thiols. A colorless spiropyran dye () dissolved in aqueous media containing aromatic thiols is selectively isomerized to the colored merocyanine form in the dark. In contrast, visible light irradiation of the merocyanine form promotes successful reversion to the colorless spirocyclic form. Kinetic absorption analysis and ab initio calculations of the transition states revealed that this colorimetric response in the dark is ascribed to the decrease in activation energy for isomerization via the nucleophilic interaction between the aromatic thiol and the olefinic carbon of the dye.

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.

A coumarin-thiourea conjugate as a fluorescent probe for Hg(II) in aqueous media with a broad pH range 2-12.

A coumarin-thiourea conjugate (1) behaves as a highly selective fluorescent probe for Hg(2+) in aqueous media. The probe 1 shows selective and quantitative fluorescence decrease upon Hg(2+) addition in aqueous media with a broad pH range, 2-12. Ab initio molecular orbital calculations reveal that the fluorescence decrease of 1 upon Hg(2+) addition is promoted by a Hg(2+)-induced desulfurization of the thiourea moiety, leading to a decrease in an intramolecular charge transfer (ICT) character of the excited-state coumarin moiety.

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.

Selective fluorometric detection of aromatic thiols by a chemosensor containing two electrophilic sites with different local softness.

A resorufin­dinitrophenyl ether conjugate (1) shows emission enhancement for aromatic thiols in aqueous media with a neutral­basic pH, while being nonemissive for aliphatic thiols. This is achieved by two electrophilic sites with different local softness on compound 1; the respective sites selectively react with aromatic or aliphatic thiolate anions.

Highly sensitive cyanide anion detection with a coumarin-spiropyran conjugate as a fluorescent receptor.

A coumarin-spiropyran conjugate (2) shows a CN(-)-selective fluorescence enhancement under UV irradiation. This enables accurate determination of very low levels of CN(-) (>0.5 µM).

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.