Minjoong Yoon

PHOTOCATALYTIC ACTIVITIES OF THE NANO-SIZED TIO2-SUPPORTED Y-ZEOLITES

Abstract Scientific studies of TiO 2 as photocatalyst started more than two and half decades ago. High efficiency in the photocatalytic reactions was achieved by increasing the surface area of the photocatalyst by supporting fine TiO 2 particles on porous materials. Among various supports, zeolites seem to be an attractive candidate. So, the present review focuses on the photocatalytic reactivities and their mechanisms of various Ti-based photocatalysts for various different types of reactions in order to achieve as highly efficient photocatalytic reactivities as possible.

Photoregulated fluorescence switching in axially coordinated tin(IV) porphyrinic dithienylethene.

Photochromic fluorophore Sn(TTP)(DTE)2 , in which two phenolic derivatives of 1,2-dithienylethene are axially coordinated to (5,10,15,20-tetratolylporphyrinato)tin(IV) in trans position, has been synthesized and fully characterized by various spectroscopic methods. We have also investigated the photoregulated fluorescence switching behavior of Sn(TTP)(DTE)2 . The fluorescence of the porphyrin macrocycle in Sn(TTP)(DTE) 2 greatly depends on the state of the 1,2-dithienyletene photochromic switch. In the open state (Sn(TTP)(o-DTE)2), the porphyrin exhibits high fluorescence intensity at 609 and 664 nm when excited at 410 nm. When the photocyclization reaction was carried out by irradiating Sn(TTP)(o-DTE)2 with the UV light (approximately 365 nm), the fluorescence intensity of the porphyrin macrocycle decreased. Back irradiation with visible light at wavelengths greater than 500 nm regenerated Sn(TTP)(o-DTE)2 and almost restored the original fluorescence spectrum. The fluorescence intensity of the porphyrin fluorophore is efficiently regulated by photochromic switching between Sn(TTP)(o-DTE)2 and Sn(TTP)(c-DTE)2 in several cycles, clearly demonstrating that the Sn(TTP)(DTE)2 can act as a system for reversible data processing using fluorescence as the detection method.

Cyclodextrin effects on intramolecular charge transfer of 2-biphenylcarboxylic acid: a pre-twisted molecule

Intramolecular charge transfer (ICT) of a pre-twisted 2-biphenylcarboxylic acid (2BPCA) in aqueous cyclodextrin (CD) solution has been studied by using the steady-state and time-resolved fluorescence techniques, and it has been demonstrated that the ICT interaction is accompanied by a further twist of the biphenyl moiety in order to be orthogonal in the excited state. The ICT emission of 2BPCA at 390 nm in aqueous solution is quenched upon addition of α- or β-CD, followed by an enhancement of a new emission at 330 nm. In parallel with this phenomenon, the 330 nm fluorescence decay is resolved into two decay components upon addition of CD in contrast to the single exponential decay of the ICT emission. These results, and AM1 calculations, suggest that the ICT of 2BPCA in a CD solution is inhibited by a restraint on the further twist of the biphenyl moiety of the photoexcited 2BPCA in the CD cavity. The restraint of the conformational change is due to the reduced intermolecular hydrogen bonding of 2BPCA compared with when in water, as well as to the reduced polarity of the CD cavity.

Heteropolytungstic acid (H3PW12O40)—encapsulated into the titanium-exchanged HY (TiHY) zeolite: a novel photocatalyst for photoreduction of methyl orange

Abstract The preparation, characterization and photoreactivity of heteropolytungstic acid (H 3 PW 12 O 40 )—encapsulated into the titanium-exchanged HY (TiHY) zeolite is presented. In the photoreaction study of methyl orange in the presence of HPA-encapsulated TiO 2 zeolite, a 20-fold increase in the photoreaction rate is observed as compared to the rate observed in the presence of HPA-encapsulated HY zeolite without TiO 2 .

Optical Waveguiding and Lasing Action in Porphyrin Rectangular Microtube with Subwavelength Wall Thicknesses

Lasing action by planar-, fiber-, or ring-type waveguide has been extensively investigated with different types of microcavities such as thin films, wires, cylindrical tubes, or ribbons. However, the lasing action by sharp bending waveguide, which promises efficient interconnection of amplified light in the photonic circuits, remains unexplored. Here, we report the first observation of microcavity effects in the organic rectangular microtubes (RMTs) with sharp bends (ca. 90°) and subwavelength nanoscale wall thicknesses, based on single crystalline and themostable tetra(4-pyridyl)porphyrin (H(2)TPyP)-RMTs synthesized by the VCR process. A bright tip emission is observed from the sharp bending edges of a single RMT upon laser excitation, demonstrating a clear waveguiding behavior in RMT. The appearance of a peak from the (0-1) band at a threshold tube length and the gradual decrease of its full width at half-maximum (fwhm) suggest that amplification of spontaneous emission (ASE) is developed by stimulated emission along the walls of the RMTs. The ehancement of the ASE peak together with the narrowing of its fhwm over a threshold pump power and the tube size (width and length) dependence of the mode spacing strongly support vibronic lasing action in the RMTs. The stimulated emission by the subwavelength bending waveguide demonstrates that the organic RMTs can be applied as new building blocks for micromanipulation of optical path and amplification in the integrated circuits for efficient photonic devices.

Excited-state intramolecular charge transfer of p-N,N-dimethylaminobenzoic acid in Y zeolites: hydrogen-bonding effects

Abstract The excited-state intramolecular charge transfer (ICT) properties of p - N , N -dimethylaminobenzoic acid (DMABA) have been investigated in NaY and HY zeolites by steady-state and time-resolved fluorescence. The ratio of the ICT emission intensity to the normal ICT/LE is greatly enhanced in the zeolites compared to the homogeneous polar solvent. In accordance with steady-state fluorescence properties, the decay times of the ICT emission are increased from 1.4 to 2.1 ns with a rise time of about 20 ps. The ICT/LE is reduced upon illumination of DMABA in NaY zeolites while no illumination effects are observed in HY or CaY zeolites. Observation of the marked differences between the diffusive reflectance spectrum and the fluorescence excitation spectrum of DMABA in the zeolite imply that hydrogen bonding between the zeolite surface and the carboxylic acid group is stimulated in the excited state, supporting the importance of hydrogen bonding in the ICT process.

SIO2 COLLOIDAL EFFECTS ON THE TWISTED INTRAMOLECULAR CHARGE TRANSFER OF P-N,N-DIMETHYLAMINOBENZOIC ACID IN ACETONITRILE

Abstract The twisted intramolecular charge transfer (TICT) in the excited state of p -N,N-dimethylaminobenzoic acid (DMABA) has been investigated in the presence of SiO 2 colloids in acetonitrile by using steady-state and time-resolved fluorescence techniques. The SiO 2 -dependent intensity ratio of the TICT emission to the normal emission clearly demonstrates a distinct phenomenon as a function of SiO 2 concentration: an enhancement of the TICT emission occurs upon the addition of 0.3 μM SiO 2 whereas a reduction occurs in the presence of excess SiO 2 colloids. These results suggest that the formation of the excited TICT state is enhanced by the hydrogen bonding between the carboxylic acid of DMABA and SiO 2 colloidal surface.

Fabrication of ZnO nanoplates for visible light-induced imaging of living cells

Visible light-sensitive ZnO nanoplates (ZnO NPls) were successfully synthesized using a hydrothermal sol-gel method and their structures were characterized by using techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmitting electron microscopy (TEM), atomic force microscopy (AFM) and FT-IR analysis. From these studies it was found that the nanoplates have excellent crystallinity and a perfect nanoplate morphology with diameter ranging from 50 nm to 250 nm and a thickness of ∼10 nm. Surfaces of the ZnO NPls were further conjugated with hydrophilic amino groups such as aminopropyl triethoxysilane (APTES) to enhance the biocompatibility and cell penetrations. The resultant APTES-modified ZnO NPls showed excellent colloidal stability in various aqueous media, exhibiting stable and strong red fluorescence emission (∼650 nm) under visible light-excitation at 405 nm. They also exhibited strong red emission even after being penetrated into living cells with negligible cytotoxicity. Therefore, the APTES-modified ZnO NPls should be promising alternative nanomaterials to the traditional quantum dots as well as previously reported ZnO cellular labelling agents which exhibit green emission by UV-excitation.

Photoinduced electron transfer studies of Nile red in the presence of TiO2 colloidal nanoparticles

Photoinduced electron transfers between Nile red (NR) with TiO2 colloidal nanoparticles are studied using picosecond transient absorption and time resolved fluorescence spectroscopy. The dynamics of electron transfer from the dye molecule to the semiconductor were understood from the transient, and also the formation of conduction band electron and Nile red cation radical were detected.

Protic solvent effects on the photophysical properties of OTiIVTSPP: photoinduced electron transfer

The photophysical properties of oxotitanium(IV)meso-tetra(4-sulfonatophenyl) porphyrin (O=Ti(IV)TSPP) have been investigated in water and methanol by laser spectroscopic techniques. The fluorescence emission spectrum of O=Ti(IV)TSPP in methanol exhibits two strong emission bands at 610 and 670 nm at room temperature with the decay time of ca. 310 +/- 10 ps and the rise time shorter than 30 ps, in contrast to the extremely weak emission with the decay time of ca. 27 +/- 4 ps in water, indicating that the fluorescence emissive states are different in the two solvents as supported by the solvent dependences of the excitation spectrum. The transient Raman spectra of O=Ti(IV)TSPP in water has been observed to exhibit a remarkable enhancement of phenyl-related mode at 1599 cm(-1), while in methanol, the Raman frequencies of the porphyrin skeletal modes (upsilon2 and upsilon4) are down-shifted without any apparent enhancement of the phenyl-related mode, indicating different interactions of the two solvents with the excited O=Ti(IV)TSPP. These Raman studies reveal that methanol molecule interacts with the photoexcited O=Ti(IV)TSPP more strongly than water, forming the exciplex, O=Ti(IV)TSPP(MeOH)*, suggesting that the two different emissive states are the singlet Franck-Condon state and the exciplex state in methanol and water, respectively. A broad triplet transient absorption of O=Ti(IV)TSPP has been also observed at 480 nm in water as well as in methanol, which is decreased upon addition of methyl viologen (MV2+) with appearance of a new absorption band at 620 nm. This indicates that the photoinduced electron transfer (PET) takes place from the porphyrin to MV2+ in both solvents. The kinetic analysis of the transient absorption band exhibits the PET rate constants of 4.76 x 10(5) s(-1) and 3,03 x 10(4) s(-1) in methanol and water, respectively. All these results infer that the PET takes place from the (d,pi) CT state and the triplet state of the excited porphyrin in methanol and water, respectively.