Toshihiko Hoshi

Fluorescence spectra of Fe(II) spin crossover complexes with 2,6-bis(benzimidazole-2′-yl)pyridine

Abstract Temperature dependent fluorescence, excitation and absorption spectra of two spin crossover Fe(II) complexes with 2,6-bis(benzimidazole-2 ′ -yl) pyridine, Febz-H 2 (containing 2(ClO 4 − ) as counter ions) and Febz-H (uncharged), have been observed. The origin of the unexpected observed fluorescence is discussed. Polarized absorption spectra in stretched polymer films are described and aid to the interpretation as well as extended PPP–MO calculations. The observed fluorescence spectra are not only the assignment of electronic transitions but also shown to be useful for the extraction of high-spin (HS) and low-spin (LS) contents in temperature dependent spin crossover systems.

Assignment of the π—π* Transitions of some Tropone Derivatives by Dichroism Analysis

The absorption spectra of tropolone (abbreviated to HT) and 2-aminotropone (AT) were observed in various solutions. In alkaline solution, HT becomes tropolone anion (HT-) by dissociation of hydrogen of the phenolic hydroxyl group. In acid solution, AT is protonated at the amino group, (AT+). The dichroic spectra of the above compounds in the stretched polyvinylalcohol (PVA) sheet were observed and the direction of the transition moment of each band was determined by the dichroism analysis. Two oxygen atoms of HTare not equivalent. All the above compounds possess four — * transitions above 220 μ. Calculations about the electronic transitions of tropone, HT and AT were made by means of the semi-empirical LCAO ASMO SCF CI method. It was shown that the transition direction obtained by the dichroism analysis can provide an important information to determine the parameters for the semi-empirical calculations.

Electronic transitions and electronic structure of bis(tropolonato)copper(II)

The electronic structure of bis(tropolonato)copper(II) (abbreviated to troCu(II)) has been investigated by use of a polarized electronic absorption spectrum in the stretched PVA film together with unpolarized spectra in fluid and rigid media. Comparison of the polarized absorption spectrum and MO calculations shows that troCu(II) takes a transoid structure in the PVA film. The complex troCu(II) exhibits shorter molecular axis polarized bands at 387 (), 267 () and 220 () nm and longer axis polarized ones at 338 () and 252.9 () nm. All these bands are due to ππ∗ transitions localized on the ligand moieties. In addition to these bands, troCu(II) shows a band around 316 nm which can be assigned to ligand-to-metal (or metal-to-ligand) intramolecular charge transfer transition. A single crystal X-ray diffraction analysis shows that troCu(II) forms a sandwich type van der Waals dimer in the crystalline state. In the dimer state, a considerable charge migration occurs from the oxide ion to the carbonyl oxygen, but each monomer still maintains a transoid structure.

Application of the stretched polymer-film method to the electron spin resonance study of phosphorescent triplet states

Abstract Using a stretched polyvinyl alcohol film as a host, electron spin resonance of phosphorescent triplet states of several aromatic compounds has been studied. Since the guest molecules have a tendency to orient biaxially, the determination of the fine structure tensor is rather straightforward. The advantage of the present method is discussed in relation with various conventional methods.

Polarized Absorption Spectra of 1,10-Phenanthroline and Phenanthrene in Stretched PVA Sheets

The divided spectra of 1,10-phenanthroline and phenanthrene were obtained, and the polarization direction of each band in the near ultraviolet region was determined. By comparison of the results of the divided spectra and those of the Pariser-Parr-Pople calculations including a variable ß approximation, assignment of the electronic bands of these compounds was established. It has been found that 1,10-phenanthroline has the a;-polarized bands (polarized perpendicularly to the (72-axis) at 30.7 kK, 34.5 kK, 37.3 kK and 43.7 kK, and the 2/-polarized bands (parallel to the C2-axis) at 29.4 kK, 39.2 kK and 42.7 kK. The a;-polarized band at 30.7 kK might be assigned to be due to an

Localized and Delocalized Electronic Transitions in Diphenylacetylene, Stilbene and Diphenylbutadiene

Dichroic spectra of diphenylacetylene, stilbene and diphenylbutadiene were measured in stretched polyvinylalcohol sheets, and semiempirical SCF-CI calculations for these compounds were carried out. The first bands of these compounds appeared above 300 μ are considered to be delocalized bands and the relatively weak second bands at around 230 μ to be localized ones. The polarization of the first band of each compound is along a line connecting two centers of the ben-

Higher excited state fluorescence of mixed ligand lanthanide(III) complexes with acetylacetonato, nitrate and terpyridine

Abstract Electronic absorption, polarized absorption and fluorescence spectra were measured to clarify the electronic structures and the nature of electronic transitions of [Nd(acac)(terpy)(NO 3 ) 2 ] and [Pr(acac)(terpy)(NO 3 ) 2 H 2 O]. It is found for both complexes that the ππ * 1 transition is localized on the terpyridine moiety whereas the ππ * 2 transition can be assigned to the intramolecular CT transition from the terpyridine moiety (πHOMO) to the acetylacetone (πLUMO). [Nd(acac)(terpy)(NO 3 ) 2 ] (or [Pr(acac)(terpy)(NO 3 ) 2 H 2 O]) fluoresces from the upper excited state localized on the terpyridine moiety. This emission phenomenon is considered in detail because emissions from higher excited states are rarely observed.

Electronic Spectra of Phenolate and Naphtholate Anions

The transition energies, oscillator strengths and polarization directions of phenolate, ocand ^-naphtholate ions were calculated by use of the PPP procedure including a variable electronegativity (VESCF) and variable ß methods. As for the naphtholate ions, the calculated results were compared with the results of the polarized aborption spectra using stretched PVA (polyvinyl alcohol) sheets. The calculated results are in good agreement with the experimental data including polarization directions. In the semiempirical calculations, the second ionization potential of the 0~ ion was approximated by In--+o+

Solid surface enhancement effects on chemiluminescence. II. Diaryloxalate and polymers as media solids

Abstract Solid surface enhancement effects on chemiluminescence were studied using several natural and synthetic polymers. The influence of surface conditions of cellulose on the chemiluminescence of powder and filter paper was also examined. Intensities for powder were relatively low compared to paper. The influence of packed powder and transparent film with the same particle size was examined for PVA, PVC and acetylcellulose. Each polymer showed similar chemiluminescent properties for film state, irrespective of their different chemical structures. Also studied were the effects of functional groups on the chemiluminescence intensity and lifetime of cellulose derivatives (cellulose, acetylcellulose, chitin and chitosan) in powder states. Finally, this paper discusses the relationship among intensities, lifetime and chemiluminescence energies for the respective samples.

Behaviour of triplet benzil and a related compound in polyhydric matrices. Formation of cisoid geometry and phosphorescence from the second triplet state

Benzil shows dual phosphorescence in polyhydric solvent matrices at 77 K. The short-lived phosphorescence (τ= 5.8 ms) has a maximum at 504 nm and the long-lived one (τ= 20 ms) has maxima at 402, 427 and 460 nm. Unlike the former, the latter phosphorescence appears only in specific matrices which have at least two hydroxylic hydrogen atoms per molecule, such as ethylene glycol. The 504 nm phosphorescence arises from the lowest triplet state of benzil having skewed geometry. From a comparison of the emission band in the short-wavelength region with that of 10,11-dihydro-5H-dibenzo[a,d]cycloheptene-10,11-dione (‘bridged benzil’), which has a rigid near-planar cisoid conformation, the shorter-wavelength phosphorescence observed for the first time in this study can be assigned to a medium imposed near-planar cisoid conformer of benzil. Furthermore, bridged benzil has been found to show phosphorescence from the second triplet state (T2) as well as T1 emission (530 nm). The lifetimes of the T1 and T2 emissions are 9.0 and 21 ms, respectively. Phosphorescence of cisoid benzil corresponds to the T2 emission of bridged benzil, thus indicating that the shorter-wavelength emission of benzil can also be assigned to T2 phosphorescence.