Krzysztof Janus

Kinetics of Photochromic Reactions of Substituted Azobenzenes in Solutions, and in Liquid Crystalline and Polymer Matrices

Abstract Kinetics of photochromic reactions was studied in azobenzene derivatives: azobenzene-containing crown ethers, and 4-methoxy-4′-halogen-substituted azobenzenes capable of forming liquid crystalline phases. The photoactive materials were dissolved in alkanes, and in polymer and liquid-crystalline matrices. The isomerisation reactions follow the first order kinetics, in some polymer matrices, however, a dispersion of the rate constants was found. Thermal cis-trans isomerisation rates of halogen-substituted azobenzenes in n-heptane were found practically independent of the substitutent. Kinetics of the same reactions in liquid crystalline matrices seems to exhibit anomalies near phase transitions of the matrices.

Kinetics of photochromic reactions in a 10-membered dibenzoazo crown ether

The paper reports on results of measurements of the kinetics of the photochromic reaction in a member of the series of azobenzene-containing crown ethers. The 10-member crown (referred to as O2N2) undergoes a reversible photo- and thermally driven isomerisation, similarly as parent azobenzene. It is expected that constraints imposed by the crown on conformations of azobenzene-containing crown ethers may modify the energetics of the isomerisation process. The experiments were performed in pMMA matrices. The isomerisation was monitored spectrophotometrically. The trans– cis isomerisation was driven photochemically (k ¼ 365 or 500 nm) and the reverse cis–trans isomerisation was driven by the excitation in the visible (413 nm) or occurred spontaneously in the dark. The pre-exponential factor of the latter process amounts to ca. 6 � 10 11 s � 1 and the activation energy is equal to ca. 118 kJ/mol, being higher than in other azobenzene-containing crown ethers and in parent azobenzene. The stability of the cis isomer at room temperature is much higher than that in other crown ethers, the extrapolated ambient-temperature time constant being of the order of several years. The stability of the cis form of O2N2 in the darkness, combined with a good reversibility is a promising feature for future applications of the material in optical data storage. 2002 Elsevier Science B.V. All rights reserved.

Comparison of Z⇄E isomerization in Langmuir–Blodgett layers and in solution

Abstract Z⇄E isomerization processes in the Langmuir–Blodgett (LB) monolayer and in the bulk of the solution were compared. The molecules studied are crown ethers with the electro- and photoactive azobenzene moiety as part of the macrocyclic ring. The rates of the photoinduced E→Z isomerization and of the spontaneous thermal Z→E reaction were calculated on the basis of the electrochemical and spectroscopic measurements. Isomerization was found to be faster in the monolayer assembly than in the bulk of the solution. On the other hand, the activation energies of the thermal Z→E isomerization in the film and in the heptane solution were comparable. Entropy of the isomerization reaction is larger in the film indicating the lower stability of the Z form upon immobilization. The higher rate of isomerization in monolayers compared to what was observed in solutions was explained as due to shorter intermolecular distances and uniform orientation of molecules in the monolayer. These features, unique for well-organized monolayers, allow for the cooperative “domino type” behavior of molecules leading to a faster isomerization process than observed in the bulk of the solution.

An approximate non-isothermal method to study kinetic processes controlled by a distribution of rate constants: the case of a photochromic azobenzene derivative dissolved in a polymer matrix

An approximate method is put forward allowing one to extract information on the parameters determining rate constants (activation energies and frequency factors) from non-isothermal measurements of quantities proportional to the concentrations of reactants (e.g., absorbance). The method, based on an approximate solution of a first-order kinetic equation in a non-isothermal regime, has been extended to fractional-heating experiments—a technique often used to extract parameters of the distributions of rate constants. The method was employed to obtain information on the parameters controlling the thermally driven cis–trans isomerisation of a novel photochromic azobenzene derivative: azobenzene-containing crown ether dissolved in a polystyrene matrix. The kinetics of the thermal isomerisation is controlled by a distribution of rate constants, with the activation energy amounting to ca. 97 kJ mol−1 and the frequency factor of the order of 1010 s−1 The results obtained are in a good agreement with results of isothermal experiments performed on similar systems.

Kinetics of a photochromic reaction in a dibenzoazo crown ether in solution and in polymer matrices

The kinetics of thermal cis–trans isomerisation of photochromic dibenzoazo 16-crown-4 ether was studied in several matrices (isooctane, polystyrene and pMMA samples fabricated in various ways) using UV-vis spectroscopy. The rate constants were found to be almost independent of the nature of the matrix and the method of its fabrication. The average activation energy of the isomerisation amounts to 93±5 kJ mol−1, the pre-exponential factor being of the order of 1010 s−1. The independence of activation energy on the matrix can be explained by taking into account the relatively small difference in the molecular volumes of the two isomers. In polymerised pMMA samples the decays were non-exponential, probably owing to a distribution of rate constants associated with a distribution of microenvironments of photoactive molecules. Copyright

Study of self-diffraction process in photoconducting polymer-nematic liquid crystal hybrid structure

The authors present results of the measurements of self-diffraction process inside the nematic liquid crystal cell with a photoconducting polymeric layer poly(N-vinylcarbazole) doped with trinitrofluorenone molecules. The dynamics of diffraction grating fading process versus applied voltage and different Ar+ laser power has been studied. Three-step process of grating’s fading was observed, connected with the charge carrier generation and their mobility in the photoconducting polymer. Theoretical calculations assuming proposed mechanism responsible for the grating decay are in a very good agreement with experimental results.

Local states in organic materials: charge transport and localization

The origin of local states capable of localizing charge carriers in molecular materials is discussed. The classical electrostatic model is employed to estimate the energies of chemical and structural traps in molecular materials, with particular attention being paid to polycrystalline and amorphous low-molecular weight and polymeric materials

Nanoaggregation of p3ht in chloroform-anisole solution: relationship between morphology and electrical properties

Recent studies of poly(3-hexylthiophene) solutions demonstrate that „sample history” (solution preparation, storage conditions) affects the aggregation process(es) and, consequently, electrical properties. In this paper, we demonstrate the evolution of UV-Vis spectra and the formation of additional red-shifted peaks which visualize the aggregation of p3ht chains. We report comparative study of p3ht based organic field effect transistors fabricated by spin coating using fresh and aged (for 6 months) chloroform solutions with anisole addition. Three order of magnitude improvement in the oFET charge carrier mobility is exhibited. These results provide new insight into p3ht crystallization process(es) and nanofiber-based oFETs.

Hole transport in organic field-effect transistors with active poly(3-hexylthiophene) layer containing CdSe quantum dots

Hybrid field-effect transistors (FETs) based on poly(3-hexylthiophene) (P3HT) containing CdSe quantum dots (QDs) were fabricated. The effect of the concentration of QDs on charge transport in the hybrid material was studied. The influence of the QDs capping ligand on charge transport parameters was investigated by replacing the conventional trioctylphosphine oxide (TOPO) surfactant with pyridine to provide closer contact between the organic and inorganic components. Electrical parameters of FETs with an active layer made of P3HT:CdSe QDs blend were determined, showing field-effect hole mobilities up to 1.1×10−4 cm2/Vs. Incorporation of TOPO covered CdSe QDs decreased the charge carrier mobility while the pyridine covered CdSe QDs did not alter this transport parameter significantly.

Photoswitchable OFETs controlled by optical bistability in the dielectric layer

Interest in electronic properties of organic materials has been stimulated by emerging prospects of their application in electronics and photonics [1–4]. Among potential applications, one may envisage use of organic materials as active elements of molecular switches. According to a general definition, a switch is a mechanical, electrical, electronic, or optical device for opening or closing a circuit, or for diverting energy or charge from one part of a circuit to another. In this context, photochromic materials have been extensively studied.