Stanislav Nešpůrek

Electrochemical preparation and study of thin poly(N-vinylcarbazole) layers

Abstract The growth of poly(N-vinylcarbazole) (PVCZ) films was followed under the conditions of potentiostatic square wave multipulse electropolymerization of N-vinylcarbazole in a medium of LiClO4 in acetonitrile. The time dependence of the polymer mass during the initial film growth and the changes in the oxidation level of PVCz during this process were derived from oscilloscopic I-t curves. Two polymerization stages were found: in the first, a thin layer of very compact (PVCZ) is formed, on which, in the second stage, a polymer with a globular structure grows. The thickness and morphology of the film is controlled by the frequency of the square wave potential pulses. The transition to the globular polymer is caused by polymerization via the vinyl groups of N-vinylcarbazole, initiated by H+ ions. Photogeneration of charge carriers was studied on non-conducting PVCz (polymerized only through the vinyl groups) formed in the bulk of a non-aqueous polymerization solution as a side product of electropolymerization. The intrinsic charge carrier photogeneration is depicted as a multistage process which can be described within the framework of the ballistic model. The dissociation of electron-hole pairs is in good agreement with predictions based on the Onsager theory of geminate recombination. The pair separation distance was determined as 2.8 nm.

Charge Carrier Photogeneration in Poly(phenylacetylene)

Abstract Pulsed photoconduction in poly(phenylacetylene) (PPA) has been studied between 220–700 nm. The photoconduction threshold has been detected at 410 nm, although the film absorption extends up to 550 nm. The action spectrum of the photocarrier generation efficiency shows a steep rise in the range 300–400 nm and a beginning of a plateau at shorter wavelengths. It is suggested that the mechanism of photogeneration is intrinsic by nature. The thermal activation of photogeneration suggests that the experimental results can be described within the framework of the ballistic model. The dissociation of electron-hole pairs is in a good agreement with predictions based on the Onsager theory of geminate recombination. From the temperature dependence as well as from the field dependence of the quantum yield, the pair separation distance was obtained as ca 2.2 nm. The fraction of absorbed photons which produce thermalized electron-hole pairs is close to 2 × 10−2 charges per photon.

A study of photochromic sydnones: structure of the sydnone ring

Abstract The chemical structure of the sydnone ring O(1)-N(2)-N(3)-C(4)-C(5) is discussed using X-ray structural data and results of theoretical calculations obtained by the INDO method. The ring is semiaromatic, the N(2)-N(3)-C(4)-C(5) part being conjugated. The exocyclic C(5)-0(6) bond is close to a double bond and participates in the conjugation.

Charge carrier photogeneration in polymers

The charge carrier photogeneration in polymers can be depicted as a multistage process that involves i) photoexcitation to a neutral molecular electronic state, ii) autoionization of the excited state, iii) thermalization of the hot electron, leading to the formation of a Coulomb field-bound geminate electron-hole pair, and iv) thermal dissociation of the electron-hole pair into free carriers. This model is proposed for all types of polymers with a large ocnjugated pendant group and both the saturated and non-saturated main chain. The separation distance of the electron-hole pairs ranges from 2 to 3 nm, and the primary quantum efficiency of the electron-hole pairs formation assumes values of several tenths of charge per photon. In intrinsic materials like substituted polyacetylenes both the interchain and intrachain charge transfer excitons can be formed. Onsagers theory can provide a satisfactory explanation of the dissociation step. In polymers with the saturated main chain and large conjugated pendant groups the charge-transfer excitons are very often stabilized by self-trapping in dimer deep traps.

Electrical, photoelectrical and nonlinear optical properties of a new form of soluble polyacetylene

Abstract The electrical, photoelectrical and nonlinear optical properties of a new type of soluble graft copolymer of polyacetylene with poly(methyl methacrylate) were studied. The polyacetylene chains in this material are electrically isolated and the conductivity is limited by the transport of carriers through the PMMA barriers. The mobility is influenced by the trap-controlled transport. The electrical conductivity can be increased by iodine doping up to 10 −6 S cm −1 . The material showed nonlinear optical behavior with a very fast response within hundreds of femtoseconds. The value of the third-order optical susceptibility obtained was χ (3) = 2 × 10 −12 e.s.u.

A photochemical synthesis of bis(ethylenedithiolo)tetrathiafulvalene

Bis(ethylenedithiolo)tetrathiafulvalene was prepared by a photochemical desulphurization of 4,5-ethylenedithiolo-1,3-dithiole-2-thione (1), and the effect of the type of solvent, nucleophilic agent and its concentration, and temperature on the yield of the reaction is discussed.

Organic semiconductors: optical magnetic and electrical properties of 1-methyl-3-propylimidazolium 7,7,8,8-tetracyano-p-quinodimethane

Abstract Electrical conductivity and spectral properties (UV VIS absorption and especially the temperature dependence of IR absorption) are studied for the simple salt 1-methyl-3-propyl-imidazolium 7,7,8,8-tetracyano-p-quinodimethane (MPI+ TCNQ− . The temperature dependence of the IR absorption coefficient for the bands due to activation of the mode Ag is discussed in terms of changes in energy level population, geometry, and electron interactions. The temperature dependence of the absolute number of the TCNQ dimers in the triplet state is measured independently using the EPR method. The temperature dependences of the ground (singlet) state population of the TCNQ dimers and of the singlet—triplet energy separation J(T) = [(0.18 ± 0.01)-(2.5 ± 0.5) × 10−4 T] eV were determined from this measurement. These values lead to an unrealistically large value of the linear temperature expansion coefficient within the framework of the simple isolated model of dimers. Therefore, the weakly interacting model of dimers should be preferred. The activation energy of the triplet state exchange frequency. EW = 0.27 eV, and the room temperature jumping frequency of the exciton along the TCNQ stacks, νj = 5 × 1010 s−1, were determined from EPR measurements of the width of triplet lines.

Crystalline domains as trapping states in amorphous materials

SummaryThe photoconductivity and thermostimulated currents were measured on amorphous and spherulitic layers of oligo(ethyleneoxy) 2,5-di(anilino)terephthalate. The crystalline domains in the amorphous phase form new traps, c. 0,6 eV deep.

Photodielectric Effect in Thin Films of Ethyl 2,5-Dianilinoterephthalate

Abstract The photodielectric effect in ethyl 2,5-dianilinoterephthalate has been studied. The experimental data could be explained using a conductivity hypothesis in which charge carrier trapping is included.

Photoelectrical properties of high density polyacetylene

Abstract Photoelectrical properties of high density trans -polyacetylene prepared by using a new type of catalyst have been studied. A strong dependence of photogeneration efficiency on the external electric field and the time evolution of the photoelectrical sensitivity after application of voltage was observed. The influence of uncompensated ionized impurities (O 2 − ) is discussed. The spectral behaviour of the polyacetylene/aluminium Schottky barrier solar cell was studied and explained by a new model based on the Onsager dissociation of the Coulomb-bound charge carriers in the field of the barrier.