Yasuhisa Manda

Anomalous dependences of luminescence of poly (3-alkylthiophene) on temperature and alkyl chain length

Abstract Photo-luminescence of poly(3-alkylthiophene) film is found to increase with increasing temperature and after indicating maximum at a melting point it again decreases in the liquid phase. Photo-luminescence also increases with increasing alkyl chain length. These results are discussed in terms of the changes of the effective conjugation length due to the introduction of a large torsion angle between neighbouring thiophene rings and the inter-chain interaction which influence on the dynamics of excited spices and recombination.

Temperature Depenndence of the Doping Effect of Poly(3-alkylthiophene) and Polaronic State

The absorption spectrum of poly(3-alkylthiophene) such as poly(3-docosylthiophene) changes at the solid-liquid phase transition. This phenomenon can be interpreted in terms of the change of the effective conjugation length due to the existence of some twisted bonds between neighbouring thiophene rings on the polymer main chain in the liquid state. From measurements of the cyclic voltammogram in solid and liquid phases, the top of the valence band is confirmed to be located at a higher energy in the liquid state. Formation of polaronic states, even in the liquid state, has been clarified from the drastic change of absorption spectrum upon doping. A simple energy band scheme of poly(3-alkylthiophene) in the solid and liquid states is indicated.

Electrical and Optical Properties of Poly(3,4-dialkylthiophene)

Poly(3,4-dialkylthiophene) films prepared by both electrochemical and chemical methods utilizing FeCl 3 as catalyst exhibit much larger band gap compared with non-substituted and 3-substituted polythiophene films. Poly(3,4-dialkylthiophene)s with long alkyl chains are soluble in several solvent. However, thermochromism has not been found contrary to poly(3-alkylthiophene), which are explained in terms of large torsion angle between neighbouring thiophene rings due to the steric hindrance by bulky alkyl group. Poly(3,4-dimethylthiophene) with large band gap also demonstrates drastic spectral, electron spin resonance (ESR) and conductivity changes upon doping due to an insulator-metal transition.

Fusible Poly(3-alkylselenophene) and Its Properties

The electrical conductivity of poly(3-alkylselenophene) is low and exhibits the range of an insulator. The band gap of poly(3-alkylselenophene) is evaluated at around 2.4 eV, which is larger than that of poly(3-alkylthiophene). The results are interpreted in terms of the decrease of the coplanary of selenophene rings due to a steric hindrance between the alkyl group and the large Se atom. Poly(3-alkylselenophene) is found to be soluble in several solvents. However, the color of the solution does not change drastically with temperature. Poly(3-dodecylselenophene) is found to be fusible at around 250°C, which is higher than the corresponding temperature for poly(3-dodecylthiophene) with the same alkyl chain length.

Optical and Magnetic Properties of Electrochemically Doped p- and n-Type Poly(3-hexylthiophene)

The electrochemical, optical and magnetic properties of poly(3-hexylthiophene) (PAT-6) in comparison with those of polythiophene (PT) during electrochemical p- and n-type doping have been investigated by cyclic voltammetry, optical absorption spectroscopy and electron spin resonance measurements. The results are discussed in terms of polaron and/or bipolaron models and lead to a better understanding of the electronic levels. Specifically, as the top of the valence band of PAT-6 is located at a higher energy state than that of PT by about 0.3 eV, the p-type dopant in PAT-6 is relatively stable compared with that in PT. However, because the bottom of the conduction band of PAT-6 was evaluated to be located at a higher energy state by about 0.3 eV than that of PT, the n-type doping in PAT-6 is more difficult than in PT.

Novel Temperature-Dependent Junction Characteristics of Poly(3-alkylthiophene) Schottky Diodes

Electrical characteristics of poly(3-alkylthiophene) and metal interfaces have been studied. Good Schottky type diodes have been fabricated using aluminum and gold-tin electrodes vacuum-evaporated onto poly(3-alkylthiophene) films. Markedly large temperature dependences are observed for poly(3-alkylthio-phene) Schottky diodes and discussed in terms of a temperature-dependent electronic band scheme.

A photoelectron emission study of polythiophene derivatives

The difference between the energy of the top of the valence band in polythiophene and those in poly(3-alkylthiophene)s has been evaluated by photoelectron spectroscopy in air. The top of the valence band becomes higher in energy on gradually increasing alkyl chain length. This indicates that an acceptor dopant is relatively stable in poly(3-alkylthiophene)s with longer alkyl chain lengths, as observed. In doped polythiophene and poly(3-methylthiophene), the difference in the threshold energy of photoelectron emission is explained tentatively by electron emission from the polaron and bipolaron levels formed by the dopant.

Electronic States in Polythiophene and Poly(3-methylthiophene) Studied by Photoelectron Spectroscopy in Air

The difference of the energy of the top of the valence band in polythiophene and poly(3-methylthiophene) has been evaluated by photoelectron spectroscopy in air to be about 0.2 eV in coincidence with that obtained by cyclic voltammetry in electrolytes. In the doped state of poly(3-methylthiophene) with BF4-, the threshold energy of the photoemission has been found to decrease by about 0.4 eV compared with the undoped sample. This is discussed in terms of polaron state formation by doping.

In Situ Electron Spin Resonance Measurements of Electrochemically Doped n-Type Polythiophene

The electrochemical, optical and magnetic properties in polythiophene during electrochemical n -type doping have been investigated by cyclic voltammetry, optical absorption spectrum and ESR measurements. The spin susceptibility increases by about one order of magnitude from 1.7×10 -7 to 5.1×10 -6 emu/mol up to dopant concentration of about 8 mol%. With further increasing dopant concentration up to about 20 mol%, the spin susceptibility decreases, followed by the slight increase of spin susceptibility again. The ESR linewidth decreases by doping from about 7.0 G to about 2.0 G and then again increases slightly with increasing dopant concentration. The g value of 2.0039 at the reduced state shifts upon doping to smaller value of 2.0027 which corresponds to the nearly free electron g value. The evolution of ESR spectrum in electrochemically doped n -type polythiophene is essentially the same behaviour as that of p -type doping. These results were discussed in terms of polaron and bi-polaron models.

DEPENDENCE OF LUMINESCENCE IN FIVE MEMBERED HETEROCYCLIC CONDUCTING POLYMERS ON MOLECULAR STRUCTURE AND TEMPERATURE

Dependencies of photoluminescence in five membered heterocyclic conducting polymers on heteroatoms and temperature are studied. Luminescence intensity of poly(3‐alkylthiophene)s increases with temperature and luminescence intensity of poly(3‐alkylfuran)s is slightly enhanced with increasing temperature. However, luminescence intensity of poly(3‐hexylselenophene) is even suppressed with temperature. These difference of properties among heterocyclic conducting polymers with different heteroatoms can be interpreted in terms of the difference of steric hindrance effect between heteroatoms and alkyl side chain and the interchain interaction which influence on the dynamics of excited species and recombination.