Gaël Labat

Alignment and relaxation dynamics of dye molecules in host-guest inclusion compounds as probed by dielectric spectroscopy.

The alignment and relaxation dynamics of a polar dye molecule, N,N-dimethyl-4(4-nitrophenylazo)aniline (DNAA), in zeolite L and perhydrotriphenylene (PHTP) channels were investigated by means of a combination of optical, dielectric, and quantum-chemical methods. Both the zeolite L and PHTP channels enable the dye molecules to align along the channel axis. An amplified net dipole moment of DNAA in PHTP is observed and attributed to enhanced 1D close alignment of dye molecules. In zeolite L channels, a concentration gradient is found with aggregation at the channel entrances. The dynamics of the dye in zeolite L channels reveals localized conical rotational fluctuation modes following Arrhenius-type activation with energy of 0.31 eV, which we assign to small noninteracting fluctuating polar units of the dyes being loosely aligned or isolated. Unlike zeolite L, relaxations in PHTP are characterized by cooperative wobbling motions interpreted as increased intermolecular dipole interaction due to a closely packed one-dimensional array. Temperature-dependent activation energies of 0.25 eV below 0 degrees C and 0.37 eV at ambient temperature reflect the role of the soft channel walls in the activation process.

New symmetrically substituted 1,3,5-triazines as host compounds for channel-type inclusion formation

A new synthetic procedure for the production of symmetrically substituted 1,3,5-triazines by cyclization from aromatic nitriles in the molten state with zinc chloride as a catalyst has been developed and applied for a number of aromatic and fluoro aromatic nitriles. The new triazine 8 [2,4,6-tris(4′-bromobiphenylyl)-1,3,5-triazine] is shown to form channel-type inclusion compounds with several aromatic solvents, featuring two different channels in the same crystal.

Polymorphism, polar morphology and absolute structure determination of 4-iodo-4′-nitrobiphenyl (INBP)

4-Iodo-4′-nitrobiphenyl (INBP), a typical nonlinear optical molecule, features three polymorphs: Fdd2 as known before and two new forms, P2/c and P2. Monopolar crystals of the Fdd2 structure express a polar morphology and allowed an absolute structure determination. Iodo substituents cover the (001) and related faces. A negative pyroelectric coefficient was observed. The P2/c form was obtained from sublimation. Heating INBP (Fdd2) in the range of 150 to 180 °C ended in a second polar form (P2). DSC and dielectric measurements allowed us to assume a diffuse type of phase change from Fdd2 into P2. Lattice energy calculations for Fdd2 and P2/c yielded very similar energies. The twinning state observed for crystals not expressing polar symmetry is explained as a 50 : 50 % intergrowth of Fdd2 and P2/c. Periodic fragments allowed the close packing mode of all three polymorphs to be understood. A Hirshfeld surface analysis makes evident that apart from a strong linear synthon (I⋯NO2) other interactions in all three forms are very similar.

Influence of the solvent of crystallization on the orientational disorder of (trans)-4-chloro-4′-nitrostilbene

The polar molecule (trans)-4-chloro-4′-nitrostilbene, which shows significant second harmonic generation in the solid state, has been crystallized from 12 different solvents. Careful X-ray diffraction studies on 18 different crystals revealed varying deviations from an exact 50 : 50 head-to-tail orientational disorder and thus from the centrosymmetric average structure reported previously. The degree of disorder varies somewhat with the solvents used for crystallization. A weak correlation is found between the deviations from a centrosymmetric structure and the polarity of the solvent. No polymorphs were observed.

Pyroelectric and piezoelectric scanning microscopy applied to reveal the bipolar state of 4-iodo-4′-nitrobiphenyl (INBP)

Two recent scanning probe techniques were applied to investigate the bipolar twin state of 4-iodo-4′-nitrobiphenyl (INBP) crystals. Solution grown crystals of INBP show typically a morphology which does not express that of a mono-domain polar structure (Fdd2, mm2). From previous X-ray diffraction a twinning volume ratio of ∼70 : 30 is now explained by two unipolar domains (Flack parameter: 0.075(29)) of opposite orientation of the molecular dipoles, joined by a transition zone showing a width of ∼140 μm. Scanning pyroelectric microscopy (SPEM) demonstrates a continuous transition of the polarization P from +P into −P across the zone. Application of piezoelectric force microscopy (PFM) confirms unipolar alignment of INBP molecules down to a resolution of ∼20 nm. A previously proposed real structure for INBP crystals built from lamellae with antiparallel alignment is thus rejected. Anomalous X-ray scattering was used to determine the absolute molecular orientation in the two domains. End faces of the polar axis 2 are thus made up by NO2 groups. Using a previously determined negative pyroelectric coefficient pc leads to a confirmation also by a SPEM analysis. Calculated values for functional group interactions (D⋯A), (A⋯A), (D⋯D) and the stochastic theory of polarity formation allow us to predict that NO2 groups should terminate corresponding faces. Following the present analysis, INBP may represent a first example undergoing dipole reversal upon growth to end up in a bipolar state.

A new core topology in pentanuclear complexes

Two members of a new family of pentanuclear FeIII complexes have been synthesized and found to possess competing antiferromagnetic exchange interactions that result in a spin ground state of S = 5/2.

Peculiar orientational disorder in 4-bromo-4′-nitrobiphenyl (BNBP) and 4-bromo-4′-cyanobiphenyl (BCNBP) leading to bipolar crystals

Two dipolar biphenyls show significant 180° orientational disorder resulting in bipolar as-grown crystals. An added symmetrical biphenyl interferes with polarity formation and inverts the initial bipolar state.

Hydrogen-bond patterns in the congruent complex 4-nitrophenol-acetamide (1/1).

Fusion of 4-nitrophenol and acetamide in a 1:1 molar ratio gives the title product, C6H5NO3.C2H5NO, (III), which has the character of a pure covalently bonded compound, having a high sharp melting point. Complex (III) (m.p. 371.9-372.9 K) can be recrystallized from various solvents and forms eutectics with either acetamide or 4-nitrophenol. Similar fusion of mixtures of acetamide and 2-nitrophenol yields no complex similar to (III) and mixtures of acetamide and 3-nitrophenol produce only a weak low-melting complex. The significance of this study lies in its demonstration, via graph set analysis, that some of the patterns found individually in crystalline acetamide or 4-nitrophenol have been preserved in crystals of complex (III), while several higher order graph sets are produced in (III) due to new hydrogen bonds involving the nitro group. In particular, large hydrogen-bonded rings are formed together with helical chains.

1,5‐Bis(3‐thien­yloxy)‐3‐oxapenta­ne: a thio­phene‐based precursor for thio­phene‐based aza­cryptand Mannich bases

# 2005 International Union of Crystallography Printed in Great Britain – all rights reserved The title compound, C10H12O2S2, is composed of two thiophene rings bridged by an –O(CH2)2O– chain in a trans arrangement. The molecule possesses C2 symmetry with the twofold axis bisecting the central C—C bond. In the crystal structure, molecules related by a centre of symmetry are bridged by C—H O hydrogen bonds, forming a zigzag onedimensional chain extending in the c-axis direction.

A heterodifunctionalised ferrocene derivative that self-assembles in solution through complementary hydrogen-bonding interactions

A heterodifunctionalised ferrocene, containing a carboxylic acid and an amidopyridine unit, self-assembles in organic solvents through complementary hydrogen bonds.