Dietmar Janietz

Nanoscale segregation, molecular recognition and fluorophobic effect – from molecular design towards complex mesophase morphologies

Mesomorphic structure formation of hydrogen-bonded complexes of amino substituted 1,3,5-triazines with complementary (semiperfluoro)alkoxybenzoic acids is presented. The substitution pattern of both components was modified systematically in order to elucidate the influence of molecular parameters on the mesophase morphologies of the binary mixed systems. The phase sequence of the triazines, and of their associates with the acids, spans the range from smectic layer structures to discontinuous cubic phases composed of closed inverted micelles. Columnar phases with various two-dimensional lattice symmetries and bicontinuous cubic phases were found as intermediates. The mesophase morphologies are discussed in terms of the microsegregation of rigid polar, lipophilic and fluorinated molecular blocks in different sub-spaces along with tailoring the shape of (curved) aggregates by the space requirement of incompatible molecular fragments.

Mesomorphic complexes through hydrogen bonding between alkoxy‐substituted triarylmelamines and semiperfluorinated benzoic acids

The thermal behaviour of two 2,4,6‐triarylamino‐1,3,5‐triazines carrying either three or six peripheral alkoxy chains have been investigated in binary mixtures with two‐chain and three‐ chain partially fluorinated benzoic acids by means of polarizing microscopy, differential scanning calorimetry and X‐ray scattering. The melamines form hydrogen‐bonded aggregates with the complementary carboxylic acids. Each investigated equimolar mixture exhibits a hexagonal columnar mesophase. In the case of the six‐fold alkoxy‐modified triazine the individual columns are built up by descrete hetero‐dimers with a circular cross‐sectional shape. The segregation of fluorinated from lipophilic side chain regions leads to a superstructure within the hexagonal lattice. The hexagonal columnar phases of 1:1 compositions of the triazine, incorporating just three aliphatic chains with the semiperfluorinated benzoic acids, are formed by pairs of H‐bonded dimeric supermolecules.

Hydrogen‐bonded block mesogens derived from semiperfluorinated benzoic acids and the non‐mesogenic 1,2‐bis(4‐pyridyl)ethylene

Thermal properties of benzoic acids carrying one or two semiperfluorinated alkoxy tails on the aromatic core have been investigated in binary mixtures with the non‐liquid crystalline bidirectional trans‐1,2‐bis(4‐pyridyl)ethylene. The hydrogen bonded complexes built from the complementary molecular species show a significantly enhanced mesophase stability compared with the fluorinated acids in their pure states. The mesophase morphologies of the complexes are governed mainly by the number of the partially fluorinated chains grafted to the acid component. Mixed systems comprising the one‐chain acids exhibit a smectic C phase followed by a smectic A phase at more elevated temperatures. Incorporation of a second semiperfluorinated chain into the acid leads to the formation of columnar mesophases. These columnar phases of the H‐bonded complexes should represent ribbon phases resulting from the collapse of the smectic layers.

Structure-related differences in the temperature-regulated fluorescence response of LCST type polymers

We demonstrate new fluorophore-labelled materials based on acrylamide and on oligo(ethylene glycol) (OEG) bearing thermoresponsive polymers for sensing purposes and investigate their thermally induced solubility transitions. It is found that the emission properties of the polarity-sensitive (solvatochromic) naphthalimide derivative attached to three different thermoresponsive polymers are highly specific to the exact chemical structure of the macromolecule. While the dye emits very weakly below the LCST when incorporated into poly(N-isopropylacrylamide) (pNIPAm) or into a polyacrylate backbone bearing only short OEG side chains, it is strongly emissive in polymethacrylates with longer OEG side chains. Heating of the aqueous solutions above their cloud point provokes an abrupt increase of the fluorescence intensity of the labelled pNIPAm, whereas the emission properties of the dye are rather unaffected as OEG-based polyacrylates and methacrylates undergo phase transition. Correlated with laser light scattering studies, these findings are ascribed to the different degrees of pre-aggregation of the chains at low temperatures and to the extent of dehydration that the phase transition evokes. It is concluded that although the temperature-triggered changes in the macroscopic absorption characteristics, related to large-scale alterations of the polymer chain conformation and aggregation, are well detectable and similar for these LCST-type polymers, the micro-environment provided to the dye within each polymer network differs substantially. Considering sensing applications, this finding is of great importance since the temperature-regulated fluorescence response of the polymer depends more on the macromolecular architecture than the type of reporter fluorophore.

Mesomorphic Hydrogen-Bonded Complexes of Complementary Semiperfluorinated Components

2,4-Diamino-6-phenyl-1,3,5-triazines carrying one or two semiperfluorinated chains at the phenyl substituent have been investigated in binary mixtures with partially fluorinated benzoic acids by polarizing microscopy, differential scanning calorimetry, and X-ray diffraction. The number and positions of the fluorinated tails of the acid component were systematically modified. Equimolar mixtures of the triazines with the aromatic acids form discrete hydrogen-bonded heterodimers with an elongated central core. Mesomorphic properties are observed only if at least three terminal fluoroalkyl chains are grafted to the hydrogen-bonded rigid core. The dimeric supermolecules exhibit columnar mesophases with rectangular or oblique two-dimensional lattice symmetry or bicontinuous cubic phases. The mesophases of the dimers are ribbon phases resulting from the collapse of smectic layers. Thereby, the phase type is defined by the number and positions of the fluorinated tails of the two components. Docking of two or three equivalents of two-chain benzoic acids to the diaminotriazine core leads to the formation of cylindrical aggregates, which organize into hexagonal columnar phases, whereas 1:2 and 1:3 mixtures of the two-chain triazine with a three-chain fluorinated aromatic acid display micellar cubic phases.

Coumarin-based emissive liquid crystals

Calamitic luminophores are presented based on an elongated aryl-substituted coumarin core. The substitution pattern at terminal and lateral positions were systematically modified. The new compounds exhibit smectic C and/or nematic liquid crystalline phases. The chromophores show fluorescence in the blue spectral region. The benzopyranones are miscible with acrylate-substituted reactive nematic mesogens. Uniaxial orientation of the mixed systems was achieved by surface-assisted alignment on polyimide orientation layers. The film anisotropy was stabilised by subsequent photocrosslinking. The crosslinked oriented films display linear polarised photoluminescence upon isotropic excitation.

Multiple hydrogen bonded mesomorphic complexes between complementary 1,3,5-triazine and pyrimidine derivatives

A 2,4-diamino-6-phenyl-1,3,5-triazine grafted with two semiperfluorinated chains at the phenyl substituent has been investigated in binary mixtures with complementary orotic acid and barbituric acid derivatives carrying either a lipophilic alkyl or a semiperfluoroalkyl tail. Equimolar mixtures of the triazine with nucleobases form triple hydrogen-bonded heterodimers with an elongated central core. Mesomorphic properties are observed only if the pyrimidinone component provides an additional free NH function not involved in triple H-bonding with the triazine nucleus. In that case, additional H-bond interactions orthogonal to the rod-shaped core of the heterodimers initiate the formation of cyclotrimeric rosette-type associates with circular geometry of the polar core region. Rosettes of heterodimers involving an alkyl substituted nucleobase organize to hexagonal columnar mesophases. Replacing the alkyl tail of the pyrimidine with a semiperfluorinated fragment leads to the formation of a micellar cubic phase. The individual micelles are composed of segmented columns three-dimensionally surrounded by a continuum of the semiperfluorinated segments.

Structure Formation Control of Disc-Shaped Molecules

Supramolecular assemblies of radial multialkynylbenzene derivatives are presented which arise either from specific interactions with interfaces or from the covalent linkage of different molecular fragments. Surface-assisted molecular orientation control was investigated at the air/water interface, on photo-oriented azobenzene films and on rubbed polyimide orientation layers, respectively. Surface alignment generates edge-on or tilted orientations of the aromatic discs. Two- and three-dimensional architectures are formed which differ remarkably from the nematic-discotic bulk phases of the compounds. On the other hand, linking chemically a pentaalkynylbenzene donor moiety with an intramolecular acceptor function leads to charge-transfer mesogens, displaying novel types of mesophases which cannot be achieved with binary mixtures of flat donor and acceptor components.

Hydrogen-Bonded Block Mesogens with Fluorinated Molecular Fragments

An alkoxyphenyl substituted melamine derivative has been investigated in binary mixtures with a two-chain semiperfluorinated benzoic acid. The two components are non-liquid crystalline in their own right. Equimolar mixtures of the melamine with the complementary benzoic acid form discrete hydrogen-bonded heterodimers. The dimeric supermolecules exhibit an induced rectangular columnar mesophase with two-dimensional c2 mm lattice symmetry. The columnar phase represents a ribbon phase resulting from the collapse of smectic bilayers. Docking of two or three equivalents of the two-chain benzoic acid to the melamine core leads to the induction of hexagonal columnar mesophases.

Surface controlled orientation of a novel star‐shaped discotic oligomesogen

The alignment behaviour of a nematic‐columnar phase forming discotic liquid crystal was explored on various substrates. The mesogen employed was a new star‐shaped monodisperse oligomesogen characterized by three flat radial pentaalkynylbenzene moieties grafted to a central benzene linking unit via flexible alkyl spacers. It is shown that suitable polyimide orientation layers along with the mesomorphic properties give rise to a tilted alignment of the disc‐shaped pentayne cores. The photoluminescence spectra of oriented films were lineary polarized upon isotropic excitation and unambigiously reveal that, beside the tilted alignment of the rigid cores, the molecular directors of the discotic building blocks preferentially are oriented along the rubbing direction of the orientation layer. The three‐dimensional ordering is stable even at room temperature due to the glass forming properties of the applied discotic mesogen.