The mechanism for negative photochromism of spiropyran in silica was investigated. Prior to our study, the chemical origin of the high thermal stability of the photomerocyanine form (PMC-form) dispersed in perhydropolysilazane (PHPS), which is converted to silica at ambient temperature, had been investigated. The high thermal stability of the PMC-form is attributed to the protonated PMC-form (HÁÁÁPMC-form), which is produced by intermolecular hydrogen bonding between oxide anions generated by the cleavage of the C À O bonds and the partially uncondensed Si À OH and O À H bonds of silica. Furthermore, the HÁÁÁPMC-form could be thermally isomerized from the SPform without UV light irradiation. This specific phenomenon is caused by the so-called negative photochromism. In this study, we proposed a mechanism for negative photochromism according to the relationship of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The relationship between the HOMOs was determined using cyclic voltammetry (CV) and ultraviolet photoelectron spectroscopy (UPS). On the other hand, the relationship between the LUMOs was determined from the respective optical bandgap. As a result, the HOMO level of HÁÁÁPMC-form was À6.1 eV and that of SP-form was À5.3 eV. Accordingly, the thermodynamic stabilization of HÁÁÁPMC-form was attributed to the thermal isomerization through negative photochromism from the SP-form.
We have carried out the photoemission study of various alkanethiolate-͑AT-͒ passivated Au nanoparticles. From the detailed line-shape analyses of Au 4f core-level photoemission spectra, it is found that the interface chemical states are independent on the surface passivants of AT molecules among the AT-passivated Au nanoparticles with the same size. Moreover, the interface electronic structures of AT-passivated Au nanoparticles have been characterized. It is found that the surface-potential shifts due to the interface dipoles accompanying the adsorption of AT molecules are about 0.36 eV and are independent on the surface passivants of AT molecules. The detailed interface electronic state of AT-passivated Au nanoparticle is discussed.
The energy band-lineup and the electronic structure of NO2-exposed H-terminated diamond/Al2O3 heterointerface have been investigated by synchrotron radiation photoemission and x-ray absorption near-edge structure (XANES) measurements. It is found that the energy band-lineup is stagger-type, so-called type-II, with its valence band discontinuity of as high as 3.9 eV and its conduction band discontinuity of 2.7 eV. The valence band maximum of the H-terminated diamond surface is positioned at Fermi level as a result of high-density hole accumulation on the diamond side. The XANES measurement has shown that the oxygen-derived interface state locates at about 1–3 eV above the Fermi level.
Along with the great interest in two-dimensional elemental materials that has emerged in recent years, atomically thin layers of bismuth have attracted attention due to physical properties on account of a strong spin-orbit coupling. Thickness dependent electronic band structure must be explored over the whole Brillouin zone in order to further explore their topological electronic properties. The anisotropic band structures along zig-zag and armchair directions of α-bismuthine (α-Bi) were resolved using the two-dimensional mapping of angle-resolved photoemission spectra. An increase in the number of layers from 1- to 2-bilayers (BLs) shifts a small hole pocket on Γ-X1 line to high wavenumber regions. Subsequently, this hole pocket transforms into an electron pocket, and another hole pocket centred at Γ point appears in the 3 BL α-Bi. Gapless Dirac-cone features with a large anisotropy were clearly resolved on X2 point in the 1-BL and 2-BL α-Bi, which can be attributed to the strong spin-orbit coupling and protection by the nonsymmorphic symmetry of the α-Bi lattice.
We have demonstrated in this and previous studies that the reactivity of the photoexcited triplet state of porphyrins and chlorophylls with radicals is substantially higher than that of the corresponding ground states. This behavior, not limited to the porphyrin moiety only, was found also with pyrene,1 2345 whose photoexcited triplet exhibits similar chemical reactivity toward transient radicals. The experimental results clearly indicate that PRAP spec-troscopy is most suitable in studying reactions of photoexcited states with short-lived radicals. With regard to stable radicals such as MV+•, the advantage of employing this method, over conventional laser photolysis, is less substantial. Nevertheless, in the PRAP mode the interference due to a possible absorption overlap between the photoexcited triplet and that of the stable radical can be avoided as the triplet state is prepared prior to the radical production.
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