Haeng-Boo Kim

Photocontrolled aggregation of colloidal silica

Spirobenzopyran having a decamethylene spacer has been chemisorbed on the surface of colloidal silica particles of diameter 150 nm. The silica particles covered with the photochromic units displayed normal photochromism when they were dispersed in organic solvents having refractive indices matched with that of the silica. The fact that normal photochromism was observed even in highly polar ethylene glycol, in which a low-molecular-weight spiropyran shows reversed photochromism, indicates that the highly hydrophilic character of the silica surface is converted to a relatively hydrophobic one owing to the long hydrocarbon chain. When the photochromic particles were dispersed in carbon tetrachloride, the UV irradiation induced flocculation, causing sedimentation of the colloidal silica. Subsequent visible light irradiation recovered the dispersed state, showing the photocontrol of the dispersibility and offering a novel photoresponsive system.

Photoisomerizability of an azobenzene covalently attached to silica-gel matrix

Abstract Sol-gel glass thin films were prepared from tetraethyl orthosilicate in the presence of 4-methoxy-4′-(2-hydroxyethoxy)azobenzene (MHAB), 4-methoxy-4′-(N-(3-triethoxysilylpropyl)carbamoylmethoxy)azobenzene (MTAB) and 4,4′-bis(N-(3-triethoxysilylpropyl)carbamoylmethoxy)azobenzene (DTAB). The optical absorption spectra of the three azobenzenes entrapped in the films were investigated. The trans-cis photoisomerization and the thermal reversion of the azobenzene moiety were studied in the glass films and compared with those in solutions as well as in polymethyl methacrylate (PMMA) thin films. Triethoxysilyl groups linked with the azobenzene moiety had no effect on isomerization reactions. The cis fraction of the three azobenzenes in a photostationary state in PMMA was quite similar to that in solution whereas the photoisomerization was strongly suppressed in sol-gel films due to the rigidity of the matrix. The introduction of one triethoxysilyl group has a slight effect on both photoisomerization and thermal reversion in sol-gel films. Photoisomerization of DTAB with two silylating units was depressed in a sol-sol film because both sides of the azobenzene moiety were fixed to the sol-gel glass matrix.

Triboluminescence in N-alkyl and N-alkyl-3-substituted carbazole crystals

Abstract The triboluminescence (TL) of crystals of four carbazole derivatives was investigated by means of a multichannel plate/photodiode array detector system. 3,9-diethylcarbazole and N-isopropyl-3-vinylcarbazole were demonstrated for the first time to be triboluminescent. The TL active carbazole crystals belong to polar space groups, while the non-TL active crystals belong to non-polar ones. This suggests that TL activity in carbazole derivatives is closely related to crystal structure, in particular, to the space group polarity and thus to the generation of piezo- or pyro- electric charges.

Time-Resolved Observation of Isothermal Phase Transition of Liquid Crystals Induced by Photoisomerization of Azobenzene Dopant

Abstract The isothermal phase transition behavior of azobenzene guest/liquid crystal host mixtures induced by nanosecond laser palse was investigated. Two kinds of azobenzene derivatives were employed as dopant, 4-butyl-4′-methoxyazobenzene (BMAB) and 4-hydroxyazobenzene (HOAB), in order to examine the heat effect on the phase transition. BMAB showed a trans to cis photoisomerization while HOAB showed little isomerization even by prolonged photoirradiation. In the BMAB/5CB mixture, the photochemical nematic (N) → isotropic (I) phase transition was found to be completed within ∼ 100 ms at the reduced temperature of 0.998 on irradiation of a single laser pulse of 10 ns FWHM which gave the cis isomer of BMAB. Contrary to the BMAB/5CB mixtare, no phase transition of the HOAB/5CB mixture was observed by the laser flash, indicating that the N-I phase transition observed for the BMAB/5CB mixture is ascribed to the photochemical reaction of the dopant, not to the heat-mode process. The photochemical phase transit...

Excited state relaxation of Ru(bpy)32+ at low temperature. Time evolution of the emission quantum yield

Abstract Decay of emission from the metal-to-ligand charge transfer (MLCT) excited state of Ru(bpy)32+ (bpy=2,2′-bipyridine) was studied by single photon counting as well as by nanosecond time-resolved emission spectroscopy in 4:1 ethanol-methanol. While the MLCT emission decay of *Ru (bpy)32+ could be fitted by a double exponential function at 125 K, the decay profile was strongly dependent on the monitoring wavelength. This dependence, and a rise component detected for the emission at the lower energy edge of the spectrum, were interpreted on the basis of a time-dependent shift of the emission spectrum.

Photocontrol of dispersibility of colloidal silica

Abstract The surface of colloidal silica particles was chemisorbed with spirobenzopyran molecule having a long hydrocarbon chain. The spiropyran silica particles (SP-CS) displayed normal photochromism. When SP-CS were dispersed in carbon tetrachloride, UV irradiation induced flocculation of the particles to bring about markedly enhanced sedimentation. Subsequent visible irradiation recovered the dispersed state again.

Time-resolved observation of isothermal phase transition of liquid crystals triggered by photochemical reaction of dopant

A mixture of 4-butyl-4′-methoxyazobenzene (BMAB) and 4-n-pentyl-4′-cyanobiphenyl (5CB)([BMAB]/[5CB]= 0.03) undergoes an isothermal phase transition on pulse irradiation at 355 nm (10 ns fwhm), in which the relaxation from the nematic to the isotropic phase was completed within ∼ 100 ms as shown by time-resolved birefringence measurements.

Photochemical behaviour of tethered dianthryl compounds in sol–gel glass

Sol–gel films and monoliths were prepared by hydrolysis of tetraethyl orthosilicate (TEOS) or tetramethyl orthosilicate (TMOS) in the presence of 1,3-bis(9-anthryl)propan-1-ol (BAP), bis(9-anthrylmethyl)amine (BAA) and the intramolecular photodimer of BM (DBAA). The intramolecular photodimerization of these dianthryl compounds as photoprobes was examined in sol–gel glasses and compared with those in solutions as well as in poly(methyl methacrylate)(PMMA) films. The intramolecular photodimerization of both BAP and BAA was severely suppressed in sol–gel glasses and PMMA films to lead to exclusive photo-oxidization, indicating the steric restriction of these matrices. A marked difference in the matrix effect between the two solids was found in the reversible photodimerization of DBAA. Photogenerated anthryl units of DBAA disappeared upon exposure to 365 nm light faster in PMMA than in sol–gel silica, suggesting that the face-to-face conformation of photodissociated DBAA is more readily relaxed rather than in the inorganic matrix. This was supported by the fact that further exposure to 285 nm light gave a lower regeneration of anthryl units in the silica bulk material than in the PMMA matrices.

Versatile setup for pressure and temperature‐controlled time‐resolved emission spectroscopy

A versatile instrumental setup for time‐resolved emission spectroscopy under variable pressure (1 atm–11 GPa) and variable temperature (77–400 K) was described. This setup, consisting of a diamond (or sapphire) anvil cell, was successfully interphased to a conventional fluorescence spectrometer, a gated diode‐array system with nanosecond laser‐pulse excitation, and a photon counting system excited with a cw YAG/dye laser system. The form of the sample for measurements can be either solid or liquid. Furthermore, the anvil cell can be used for IR spectroscopy by means of a FTIR spectrometer as well as for qualitative absorption spectroscopy. The performance was checked with a number of examples such as pressure and temperature effect on the time‐dependent shift of emission from Ru(bpy)2+3 in viscous solution, pressure effect on a solid binuclear complex of platinum, and fluorescence behaviors of several organic molecules. Scope and limitation of this novel setup were discussed.

Pressure- and temperature-controlled time-resolved emission spectroscopy applied to coordination compounds

Abstract Pressure effects on emission spectra of [Pt 2 (P 2 O 5 H 2 ) 4 ] 4- (Pt 2 4- ) crystals and Ru(bpy) 3 2+ solution (RuB 3 2+ , in ethanol-methanol) were studied. In Pt 2 4- crystals, reduction of both intra- and intermolecular Pt-Pt distances under high pressure resulted in lower energy shifts of fluorescence and phosphorescence, emergence of eximer-like emission, as well as in efficient T-T annihalation. In RuB 3 2+ solution, pressure induced high energy shift and sharpening of the emission were observed at 110 - 200 K, and were interpreted by viscosity dependent solvent relaxation.