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The evidence for magnetic susceptibility of synthetic libethenite [Cu.sub.2]P[O.sub.4]OH by using an isolated square-spin cluster model defined by the CuI-O-Cu2 superexchange is reported.

The structural grounds of the decrease of point and lattice symmetries coupled with switching of the exchange interaction in single crystals of a highly strained, coordinationally unsaturated bisdiaminecopper(II) cation are described. The interplay between the inherent vibrionic instability and ligand-field strain enables effective switching of the pseudo-Jahn-Teller [d.sup.9] centers in the low-temperature regime and valence orientation and coordination geometry in the high-temperature regime.

The nitro-assisted proton transfer (NAPT), responsible for the photoactivity of ortho-nitrobenzylpyridines and a model for the nitro-based caged compounds, is studied along with the parent compound 2-(2',4'-dinitrobnzyl)pyridine (DNBP) with polarized optical spectroscopy and theoretical calculations. The transition dipole moments of a DNBP single-crystal identified oriented molecules of the long-lived enamine tautomer, rather than of the aci-nitro tautomer.

The anticholinergic agent oxitropium bromide has displayed a strong thermosalient effect ('ju mping crystal' effect) and it is examined by a combination of structural, microscopic and thermoanalytical methods. The effect in the single crystal and structure determination of both phases has shown that the jumping of the crystals is a macroscopic manifestation of a highly anisotropic change in the cell volume.

The effects of the environment on the photophysics and photochemistry of the green fluorescence protein (GFP) are studied by determining the eight crystal structures of six synthetic aryl-substituted analogues of the GFP chromophore and correlating them with their two-dimensional steady-state and time-resolved solid-state excitation-emission spectra. The reaction pathways are dependent on the excitation energy, resulting either in [2+2] photodimerization at the bridging double bond or flipping of the imidazolone ring and the meta-hydroxy chromophore is the only GFP-chromophore analogue that is obtained as more than one stable polymorph in the pure state.

A series of new bimetallic gold-silver vapochromic structures were exposed to solvent vapors to effectively switch the luminescence among different colors. The emission colors in the system were controlled by both aurophilic interactions and halogen bonding, which affected the emission energy through different orbitals.

Several studies are conducted to explain the crystal structures of oxyluciferin (oxy[LH.sub.2]), the unsubstituted light emitter of firefly bioluminescence. Their steady-state absorption, as well as emission spectra are also determined.

The limiting topochemical criteria for photodimerization are analyzed by the modulation of the [pi]-[pi] stacking interactions between self-templating planar molecules with hydrogen bonding. The product yield and composition are controlled by the distance between the reactive bonds.

The extraordinary stability of the photoinduced red form of a cationic spiropyran is used for obtaining in situ X-ray diffraction evidence of its molecular structure. The density functional theory calculations have shown that the stabilization of the red form of the cationic relative to the neutral spiropyran is thermodynamically controlled, through compensation of the charge within the zwitterion by the methylpyridinium group.

The reaction mechanism underlying the photoinduced linkage isomerization of discrete arsenic-sulfur clusters in the realgar form of tetraarsenic tetrasulfide to its pararealgar form was studied using Raman spectroscopy and infrared spectroscopy. The formation of intermediate [As.sub.4][S.sub.5] clusters and its linkage isomerization was observed.

A photoinduced analogue of the thermal Orton rearrangement reaction by which an N-chlorine atom from a side amino group is transferred to a phenyl ring is analyzed in the solid state. Based on the change of the crystal structure and the physicochemical data, a three-center five-atom mechanism involving homolytic cleavage of the N-Cl bond followed by hydrogen abstraction by the carbonyl group is indicated for the solid-state photoinduced Orton rearrangement.

The first direct atomic-level photodiffraction evidence for domain formation by doping with charge photocarriers of the stable organic radical TTTA (1,3,5-trithia-2,4,6-triazapentalenyl) which exhibits unusually wide thermal hysteresis of the paramagnetic-to-diamagnetic phase transition around ambient temperature is reported.

The structure of the blue NH isomer produced by two-photon excitation in a single crystal of DNBP was studied. The results stated that the photocoloration of solid DNBP was attributed to a nitro-assisted proton transfer mechanism.