Martin Cigl

Photosensitive self-assembling materials as functional dopants for organic photovoltaic cells

New photosensitive liquid crystalline compounds with a specific molecular structure have been designed in order to use them as functional dopants for organic photovoltaic devices. The main idea is to establish the effect of doping on the bulk-heterojunction solar cell properties and characteristics. As a part of basic characterisation, the self-assembling behaviour of three new organic compounds has been determined. The resulting photovoltaic devices with a P3HT:PCBM active layer doped by the designed photosensitive materials were investigated by UV-Vis spectroscopy, electrochemical measurements, cyclic voltammetry experiments, AFM surface morphology studies and impedance spectroscopy. The obtained results are summarised and discussed in terms of power conversion efficiency improvement considering the specific chemical structure of the compounds used as functional dopants and the annealing temperature of an active layer. The molecular structure of compounds used as a dopant has a strong effect on the value of short circuit current density. However, no significant effect of the dopant in the P3HT:PCBM active layer on the open circuit voltage and fill factor has been found.

New azobenzene-based chiral-photochromic substances with thermally stable Z-isomers and their use for the induction of a cholesteric mesophase with a phototunable helix pitch

Novel photosensitive azobenzene-containing substances with sorbitol- and lactate-based chiral fragments were synthesized to serve as chiral-photochromic dopants in cholesteric mixtures. Owing to the presence of chlorine or methyl lateral substituents in the azobenzene fragment of the dopants, the photoinduced Z-form of their molecules is highly stable. The photooptical properties of the prepared cholesteric mixtures were studied and it was shown that UV-irradiation of the planarly-oriented films results in a shift of the selective light reflection wavelength to a long-wavelength spectral region due to E–Z isomerization of the azobenzene moieties. This process is thermally and photooptically reversible and the rate of blue-light-induced helix twisting is comparable with the rate of the untwisting process, whereas the thermal relaxation completes only after ca. 40 days at room temperature (for a chlorine-substituted chiral-photochromic dopant). It was found that this value is almost two orders of magnitude less in comparison with the non-substituted chiral-photochromic dopant also studied in this work. The obtained results demonstrate the promising properties of the synthesized compounds for creation of novel photoswitchable cholesteric materials.

Photosensitive chiral self-assembling materials: significant effects of small lateral substituents

Novel azobenzene-based photosensitive mesogens with lactate chiral units were synthesized. In order to modify the rate of the thermal Z–E isomerization of these compounds, small lateral substituents were introduced into their core in the proximity of the azo group. The influence of lateral substitution on the kinetics of the Z–E isomerization, mesomorphic behaviour, and UV-Vis absorption spectra was studied. It was found that the position of the substituents in the azobenzene core significantly affects the rate of their thermal isomerization. The stability of Z-isomers of several studied compounds is comparable to that of compounds with complex molecular structures designed for optical data storage. Although lateral substitution influences the breadth/length ratio of the core, liquid-crystalline properties of the studied materials have been preserved.

Photochromic LC–polymer composites containing azobenzene chromophores with thermally stable Z-isomers

Novel types of photochromic liquid crystalline (LC) polymer-based composites containing chiral photochromic azobenzene moieties were elaborated. For this purpose, a new chiral azobenzene-containing methacrylic monomer was synthesized. On one of the benzene rings of the azobenzene chromophore, there are methyl substituents in both positions ortho to the azo group. This substitution significantly increases the thermal stability of the photoinduced Z-form. The first type of novel photosensitive composite is a glass cell filled with cholesteric polymer-stabilized layers produced by thermal polymerization of a mixture containing a synthesized chiral photochromic azobenzene monomer, a nematic mixture of cyclohexane derivatives, mesogenic diacrylate and a thermal initiator. UV-irradiation of such samples leads to E–Z isomerization of the photochromic groups of the azobenzene monomer units, accompanied by a shift of the selective light reflection peak to a long wavelength spectral region. This process is thermally and photochemically reversible; the kinetics of the selective light reflection shift was studied. The second type of novel LC composite was obtained by the introduction of the same cholesteric photochromic mixture into porous stretched polyethylene (PE) films. Due to the highly anisotropic porous structure of PE, a uniaxially aligned nematic phase was obtained inside the pores. Irradiation with UV and visible light provides the possibility of dichroism and birefringence photocontrol in the obtained LC composite films. The possibility of photo-optical image recording on the prepared composite was demonstrated. It is noteworthy that the images recorded by the novel composite types have extremely high thermal stability (weeks) in comparison with those based on other azobenzene derivatives.

Communication: Experimental proof of symmetry breaking in tilted smectics composed of molecules with axial chirality.

For the first time, domains with twisted structures have been established in planar samples of achiral compounds in tilted smectic C phase. This evidences separation of molecular conformers differing in the sense of axial chirality and confirms polar C(2) symmetry of these domains. A simple model considering polar surface anchoring energy and bulk energy of the twist can account for this finding. Conditions for coexistence of twisted and homogeneous domains are discussed.

Design of calamitic self-assembling reactive mesogenic units: mesomorphic behaviour and rheological characterisation

ABSTRACT Several calamitic reactive mesogens containing only two benzene rings in the molecular core and with or without lateral substitution by the methyl/methoxy groups have been designed and their mesomorphic behaviour was characterised. Depending on the molecular structure, some of the materials exhibit the nematic and the orthogonal smectic mesophases. The reactive mesogens are aimed for further design of the macromolecular materials like polysiloxane-based polymers and elastomers. Beyond the mesomorphic and structural properties, the electrorheological properties within the temperature range of the nematic and smectic A mesophases were studied with and without applied electric field for the selected reactive mesogen. The increase of viscosity was found not to be higher than three times under applied electric field strength of 2 kV/mm. The mesomorphic, structural and rheological properties of the newly designed reactive mesogens are discussed in order to contribute to better understanding of the molecular architecture–nano-organisation properties relationship of such mesogenic materials. GRAPHICAL ABSTRACT

Effect of lactate group in the chiral chain of new compounds exhibiting short-pitch cholesteric or TGBA phase

ABSTRACT We have synthesised and studied lactic acid derivatives based on chlorine substituted molecular core, which is created from two biphenyls linked by an ester, and terminated with one or two lactic units in a chiral chain. The compounds with one lactate group exhibit cholesteric phase with rather short helix pitch (200 nm) in a broad temperature range. On contrary, compound with two lactate units reveals a stable TGBA phase, with transition temperatures substantially lower than those for one-lactate derivatives. We have studied mesophase behaviour and electro-optical properties, mostly based on the texture observation in polarising microscope. Additionally, we have used AFM and x-ray techniques to confirm mesophase identification and establish structural properties. GRAPHICAL ABSTRACT

Mesomorphic and structural properties of liquid crystalline side-chain polymethacrylates: from smectic C* to columnar phases

ABSTRACT The chiral methacrylate monomers with photosensitive azobenzene group possessing the orthogonal smectic A* and tilted smectic C* (Sm-C*) phases have been synthesised and characterised. The monomers have been used as functional side chains for the design of corresponding polymethacrylates. X-ray diffraction has been applied to elucidate the structure and phase behaviour of liquid-crystalline side-chain polymethacrylates with azobenzene-containing central core, chiral fragments and aliphatic spacers and tails of different length. X-ray patterns of polymethacrylates oriented fibres impose the tilted Sm-C* order as a basic structure of these materials. This is complemented by a regular pattern of small-angle diffuse spots, which implies complex positional order on the local scale and serves as a precursor for the formation of a columnar phase. The increase of the total length of the aliphatic tail and spacer of the side-chain fragments leads to formation of the tilted columnar phase (Coltilt*) with two-dimensional monoclinic lattice. For the polymer containing 10 methylene units in both, spacer and aliphatic tail, the Coltilt* precedes the formation of the Sm-C* phase. The observed structural changes are explained as due to coupling between the smectic ordering of the mesogenic side groups and the polymer backbone conformation. Graphical Abstract