Tsuneaki Koike

Ultraviolet photoelectron spectroscopy of poly(pyridine-2,5-diyl), poly(2,2′-bipyridine-5,5′-diyl), and their K-doped states

Ultraviolet photoelectron spectra were measured using synchrotron radiation for two kinds of π‐conjugated polymers, poly(pyridine‐2,5‐diyl) (PPy) and poly(2,2′‐bipyridine‐5,5′‐diyl) (PBPy) which exhibit n‐type electrically conducting properties. The two compounds show similar spectra and they were analyzed with MO calculations and the comparison with the data of related molecules. The ionization threshold energies of PPy and PBPy were found to be 6.3 and 6.35 eV, respectively. These values are higher than those of π‐conjugated conducting polymers capable of p doping. Upon potassium doping of PBPy, two new states appeared in the originally empty energy gap and the intensity of the state at 0.65 eV from EF grows as the doping proceeds. This finding and the change of optical absorption spectra upon doping indicate that bipolaron bands are formed in K‐doped PBPy. While K‐doped PPy also shows similar gap states, it requires higher dopant concentration to create bipolaron bands than in the case of K‐doped PBPy. The difference of the dependence on dopant concentration between K‐doped PPy and K‐doped PBPy is discussed based on the conformational difference between these polymers.Ultraviolet photoelectron spectra were measured using synchrotron radiation for two kinds of π‐conjugated polymers, poly(pyridine‐2,5‐diyl) (PPy) and poly(2,2′‐bipyridine‐5,5′‐diyl) (PBPy) which exhibit n‐type electrically conducting properties. The two compounds show similar spectra and they were analyzed with MO calculations and the comparison with the data of related molecules. The ionization threshold energies of PPy and PBPy were found to be 6.3 and 6.35 eV, respectively. These values are higher than those of π‐conjugated conducting polymers capable of p doping. Upon potassium doping of PBPy, two new states appeared in the originally empty energy gap and the intensity of the state at 0.65 eV from EF grows as the doping proceeds. This finding and the change of optical absorption spectra upon doping indicate that bipolaron bands are formed in K‐doped PBPy. While K‐doped PPy also shows similar gap states, it requires higher dopant concentration to create bipolaron bands than in the case of K‐doped PBPy....

Electronic structure of polyperylene

Abstract The electronic structure of polyperylene is studied on the basis of the one-dimensional tight-binding SCF-CO (self consistent field-crystal orbital) method. The geometry of this polymer is optimized from the energetic point of view. The analysis of the electronic properties of polyperylene reveals that this polymer will be a promising member of the one-dimensional graphite as a new electrically conductive or semi-conductive material.

X-ray diffraction studies of pristine and heavily-doped polyacenic materials

Abstract An X-ray diffraction study was performed for pristine and heavily-doped polyacenic materials prepared by pyrolytic treatment of phenolformaldehyde (PF) resin. Seven kinds of pristine samples prepared under different pyrolysis temperatures were studied, as well as the moulded PF resin itself. The result of the analyses shows that the structural change of the samples in the heavily-doped regime differs in a peculiar manner from what is expected in conventional intercalated graphite. An attempt is made to interpret the obtained results in terms of the amorphous structure of the present polyacenic materials.

Electronic structures of polyacenacene and polyphenanthrophen anthrene. Design of one-dimensional graphite

Abstract The electronic structures of polyacenacene (PAA) and its geometrical isomer, polyphenanthrophenanthrene (PPhP) have been studied using the tight-binding SCF-CO (self consistent field-crystal orbital) method with respect to a design of one-dimensional (1D) graphite. The geometry of each polymer is optimized from the energetic point of view. It has been found that from the analysis of the electronic structure of its optimized geometry, PAA favours the structure without the carbon-carbon bond alternations, yielding no band gap. In this sense, PAA is a typical 1D-graphite. On the other hand, it is predicted that the magnitude of the band gap of PPhP in its optimized geometry is almost equal to that of trans -polyacetylene.

Intersoliton hopping in lightly doped polyacetylene: assistance by vibrational motion of the dopant

The rate of intersoliton hopping associated with the interchain conduction mechanism in lightly doped polyacetylene has been studied explicitly including a certain vibrational motion of the dopant molecule. The model system which was used consists of a neutral soliton and a charged soliton alternating between these chains. Conventional time‐dependent perturbation theory was applied to the effective Hamiltonian developed here to study the system. We find that, under the assistance of the vibrational motion of the dopant along the axis between the two soliton sites, the rate of this intersoliton hopping is largely accelerated up to the order of that of the vibrational motion.

Structural change in the photoexcited cis-polyacetylene

Abstract The structural change in the cis-polyacetylene photoexcited with the energy corresponding to its band gap was examined. The growth of four new infrared absorption peaks at 802, 1062, 1112 and 1259 cm−1 were found after the irradiation. The appearance of these peaks strongly suggests the formation of the trans-cisoid segment in the cis-transoid chain of polyacetylene.

Photoemission Study on Poly(pyridine-2,5-diyl), Poly(2,2′-bipyridine-5,5′-diyl), and their K-doped States

Abstract Ultraviolet photoelectron spectra were measured using synchrotron radiation for two kinds of π-conjugated polymers, poly(pyridine-2,5-diyl) (PPy) and poly(2,2′-bipyridine-5,5′-diyl) (PBPy) which exhibit n-type electrically conducting properties. The two compounds show similar spectra, and they were analyzed with MO calculations. Upon potassium doping of PBPy, two new states appeared in the originally empty energy gap. This finding indicates that bipolaron bands are formed in K-doped PBPy. While K-doped PPy also shows similar gap states, it requires higher dopant concentration to create bipolaron bands than in the case of K-doped PBPy. The difference of the dependence on dopant concentration between K-doped PPy and K-doped PBPy is discussed based on the conformational difference between these polymers.

Theoretical studies on the structural changes of conjugated conductive polymers in the heavily-doped regime

Abstract Structural changes of simple π-conjugated organic polymers such as trans-polyacetylene, polyyne and polydiacetylene in their cationic states have been studied in detail with a view to examining the carbon-carbon bond deformation in their heavily-doped regime. It has been theoretically bound for the first time that although the well-known bond alternations and band gaps in these polymers tend to reduce in this regime, they never actually vanish within realistic doping levels. These findings agree well with the optical observations reported with respect to heavily-doped polyacetylenes. In this sense, the ‘counter Peierls’ transition, the concept of which was employed in a preliminary study, is unlikely to be completed and, hence, this terminology should be used with certain reservations.

Electronic Structures of Conjugated Polymers and Conducting Mechanism

Abstract Concerning the conduction mechanism of conjugated polymers, two selected subjects are studied. First, the occurrence of the counter-Peierls distortion in the heavily doped trans-polyacetylene of p-type is pointed by the semi-empirical Hartree-Fock crystal orbital calculation. Second, the energy balance of bipolaron separation in several conjugated polymers are discussed.

ESR studies of polyacenic semiconductive material

Abstract ESR measurements have been performed on the polyacenic material prepared by the pyrolytic treatment of phenol-formaldehyde resin. Six kinds of samples (A)-(F) with various electrical conductivity (σ) were employed for the measurements. Each sample shows a single narrow line, the g -values of which suggest the existence of unpaired electrons of π-type. If the sample (F) ( σ = 1 × 10° S cm −1 ) the single narrow line becomes drastically diminished in the presence of a small amount of oxygen.