Masaharu Hirohata

Optical Properties and Blue and Green Electroluminescence in Soluble Disubstituted Acetylene Polymers

Intense photoluminescence (PL) was observed in soluble disubstituted polyacetylenes such as poly(1-phenyl-2- p-adamantylphenyl acetylene (PDPA-Ad) at 530 nm, poly(1-phenyl-2- p-triphenylsilylphenyl acetylene) (PDPA-TPSi) at 530 nm and poly(1-hexyl-2-phenyl acetylene) (PHxPA) at 455 nm, although they are considered to be derivatives of trans-polyacetylene. In monosubstituted acetylene polymers such as poly( o-trimethylsilylphenyl acetylene) (PTMSiPA), PL was negligible. Green and blue light emissions were observed from an electroluminescent (EL) device having soluble poly(diphenyl acetylene) (PDPA) derivatives (PDPA-Ad and PDPA-TPSi) and PHxPA as emission layers, respectively. EL emission is clearly observable under normal room illumination conditions. The electronic energy structures of the disubstituted PA derivatives are also discussed to interpret PL and EL characteristics.

Donor polymer (PAT6) — acceptor polymer (CNPPV) fractal network photocells

Abstract Unique characteristics such as quenching of photoluminescence and improvement of photovoltaic effect were observed in donor polymer-acceptor polymer composites, PAT6-CNPPV system and PDPATPSi-CNPPV system. Electronic energy structures of these polymers were determined and these characteristics are interpreted in terms of photoinduced charge transfer between donor polymer (PAT6 and PDPATPSi) and acceptor polymer (CNPPV) and formation of fractal network .

Optical properties and electroluminescence characteristics of polyacetylene derivatives dependent on substituent and layer structure

Abstract Intense photoluminescence (PL) is observed in di-substituted polyacetylene derivatives even though solitonic mid-gap absorption is observed upon doping, contrary to non-substituted trans-polyacetylene and mono-substituted polyacetylene in which strong PL is not observed. Intense green and blue electroluminescence (EL) is realized utilizing poly(diphenylacetylene) derivatives and poly(1-alkyl-2-phenylacetylene) derivatives, respectively. Greenish-blue emission is also observed in poly (1-chloro-2-phenylacetylene) derivatives. The dependence of wavelength and intensity of PL and EL on the molecular structure of substituents is clarified in detail. The effects of molecular alignment and layer structure on the EL characteristics are also discussed. Upon intense light excitation, remarkable spectral narrowing due to stimulated emission is also observed in these di-substituted polyacetylene derivatives.

Electroluminescence in conducting polymers based on poly(phenylene ethynylene)

Abstruct Electroluminescence (EL) in various conducting polymers (CP) based on poly(phenylene ethynylene) (ROPPE) which have C-C triple bond in their main chains were studied. EL in Al/CP/ITO structure utilizing copolymer based on ROPPE and pyridine as CP was blue-green and stronger than that in same structure utilizing well-known poly(dialkoxy-p-phenylene vinylene) (ROPPV). However, weaker red EL was observed in same structure utilizing copolymer based on ROPPE and anthracene. These result suggests that the increased band gap energy and improve exciton confinement efficiency, due to shortened conjugate length, were realized by introduction of C-C triple bonds in main chain of conducting polymers such as ROPPV, while these effects of C-C triple bonds are suppressed by introduction of electron-rich moiety such as anthracene which should increase effective conjugation length. Electrochemical studies on these copolymers also confirmed this interpretation.

Blue-Green Electroluminescence in Copolymer Based on Poly(1,4-phenylene ethynylene)

Optical characteristics of poly(2,5-dialkoxy-1,4-phenylene diethynylene- co-2,5-pyridinylene) (ROPPE-Py) were investigated. Intense blue-green electroluminescence (EL) emission was observed in light emitting diode (LED) with Al/ROPPE-Py/indium-tin-oxide (ITO) structure. This result suggests that triple bonds in the main chain are resopnsible for the blue shift and enhancement of EL, due to the shortening the effective conjugation length and effective confinement of excitons or exciton-polarons. However, the effects of triple bonds were suppressed by the introduction of electron-rich moieties in the main chain such as poly(2,5-dialkoxy-1,4-phenylene diethynylene- co-9,10-anthracenylene) (ROPPE-An). Electronic energy structures obtained from optical absorption spectra and cyclicvoltammetry measurements confirmed the effect of the electron-rich moieties, which is due to the delocalization of π-electrons and the enhancement of interchain interactions.

Spectral narrowing of emission in di-substituted polyacetylene

We studied spontaneous and stimulated optical emission in films of di-substituted trans-polyacetylene (t-PA). We found that di-substitution of t-PA drastically changes the polymer optical emission properties, so that photogeneration of exciton and photoluminescence (PL) quantum yields become much higher than those measured in unsubstituted and mono-substituted t-PA. The strong PL is accompanied by high optical gain and emission spectral narrowing at high excitation intensities; the emission spectrum changes from a broad PL band of 100 nm at low intensities to a narrow stimulated emission band of ~ 8 nm at high intensities. These properties show that di-substituted t-PA films can be good candidates for laser medium in the green and blue spectral ranges.

Effect of Alkyl and Aromatic Substituents on Blue Electroluminescence in Polyacetylene Derivatives.

Substituent dependences of electroluminescence (EL) and photoluminescence (PL) in disubstituted polyacetylene derivatives such as poly(1-methyl-2-phenylacetylene) (PMePA), poly(1-ethyl-2-phenylacetylene) (PEtPA), poly(1-n-hexyl-2-phenylacetylene) (PHxPA) and poly(1-methyl-2-naphthylacetylene) (PMeNA) have been studied. Blue light emission was observed from EL devices utilizing PEtPA and PHxPA, and blue-green light emission was observed from those utilizing PMePA and PMeNA. EL intensity, relative EL efficiency and PL intensity increase by one or two orders of magnitude on increasing the alkyl chain length. EL intensity, EL efficiency and PL intensity of PMeNA are higher than those of PMePA. The results are discussed in terms of substituent-dependent interchain interaction and electronic energy states.

Effect of Molecular Structure of Substituents on Green Electroluminescence in Disubstituted Acetylene Polymers

Intense green photoluminescence (PL) in the wavelength range of 520 to 540 nm was observed in many poly(diphenylacetylene) (PDPA) derivatives. The band gap energy and the highest occupied molecular orbital (HOMO) energy of these polymers were estimated to range from 2.6 to 2.7 eV and -5.6 to -5.9 eV depending on the substituent. Green electroluminescence (EL) was observed from EL devices fabricated utilizing these polymers. Among these PDPA derivatives, poly(1-phenyl-2-m-(trimethylsilyl)phenylacetylene) (PDPA-mSiMe3) with the smallest band gap energy exhibited the weakest PL and EL of longest wavelength. Polymers with bulkier and/or longer substituents exhibited stronger PL and EL. The results are interpreted in terms of a steric hindrance effect and the interchain interaction depending on the molecular structure of the substituents. In addition, upon electrochemical doping, these light emissive polyacetylene derivatives exhibited evolution of new single midgap absorption suggesting formation of solitons.

Electronic Properties and Electroluminescence of Monosubstituted Polyacetylenes and Their Mixtures with Disubstituted Polyacetylene

Monosubstituted polyacetylenes (PA), that is, poly(phenylacetylene) (PPA) derivatives with various substituents attached at the ortho-position of the phenyl ring, have been studied. The valence band top and the bandgap energy of these polymers were estimated in the range from -5.5 eV to -5.9 eV (vs vacuum level) and from 1.9 eV to 2.3 eV, respectively, depending on the substituents. Although PL of these PPA derivatives is very weak, we have observed electroluminescence (EL), which is considered to be related to the nature of the self-trapped excitonic states of these polymers. The EL characteristics were found to depend on the substituents of polymers. In the mixture of poly(o-trimethylsilylphenylacetylene) (PPA-oSiMe3) with the strong luminescent disubstituted PA, poly(1-phenyl-2-p-n-butylphenylacetylene) (PDPA-nBu), the enhancement of PL of PPA-oSiMe3 was not observed. However, EL of PPA-oSiMe3 was enhanced in the mixture with PDPA-nBu, which is explained in terms of excitation energy transfer.

Novel properties of molecularly doped conducting polymers and junction devices

Abstract Properties of conducting polymers, especially optical properties, have been found to be strongly influenced by doping of molecular dopants such as C 60 , aromatic molecules, photo chromic dyes and different structures of conducting polymers, which are interpreted in terms of dynamics of photo-excited electrons, holes and excitons by taking electronic energy states of both conducting polymer and molecular dopants into consideration. Novel characteristics have also been observed upon double doping, superconductivity and ferromagnetism upon doping of Alkali metal and TDAE into C 60 doped conducting polymer, respectively. Molecular doped conducting polymers also have exhibited novel junction characteristics such as high efficiency of photovoltaic effects, light sensitive FET characteristics and voltage and its polarity dependent electroluminescence