Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

Molecular self-assembly and nanochemistry: A chemical strategy for the synthesis of nanostructures

Photoinduced motions in azo-containing polymers.

Photoalignment of Liquid-Crystal Systems.

Big is beautiful--"aromaticity" revisited from the viewpoint of macromolecular and supramolecular benzene chemistry.

Linear and Nonlinear Optical Properties of Photochromic Molecules and Materials.

The molecular motions underlying the dielectric and dynamic-mechanical β relaxation in poly(methyl methacrylate) (PMMA) have been elucidated in detail by means of two-dimensional (2D) and three-dimensional (3D) 13 C exchange NMR of the carbonyl moiety and 2D 2 H exchange NMR of the methoxy group. The identity of the motions observed by NMR and the β-relaxation dynamics is proved by the agreement of the measured correlation times. The selective-excitation «3D» NMR spectrum proves that, for every mobile side group, a relatively well-defined motion between two potential-energy minima occurs. The 2D spectral pattern shows that the OCO plane of the side group undergoes 180° (±20°) flips

Molecular nature of the beta-relaxation in poly(methyl methacrylate) investigated by multidimensional nmr

The conformational order and the molecular mobility of the alkyl side chains in the polymers [COPh(OR) 2 CO-X-Ph-X] n and [N(CO) 2 Ph(CO) 2 N-Ph(OR) 2 ] n (R= C 6 H 13 , X= O, NH) are investigated. The side chains, which are phase-separated from the main chain in a layer-type structure, can form crystalline as well as amorphous phases. The sizes of these domains depend on the nature of the main chains and their organization. In the polyimide and the polyester with regular main chain packing, crystalline as well as amorphous regions are observed extending over more than one layer spacing

Stiff macromolecules with aliphatic side chains: side-chain mobility, conformation, and organization from 2D solid-state NMR spectroscopy

The occurrence and rate of 180° chain-flip motions in the crystalline regions of two polyethylenes were studied by 13C NMR. In high-density polyethylene (HDPE) and in ultradrawn ultrahigh molecular weight polyethylene (UHMWPE) fibers, the changes in the 13C−13C dipolar couplings brought about by the reorientations of the 13C−13C internuclear vectors in the crystallites were observed. In the HDPE sample, which was labeled with 4% of 13C−13C pairs, the rotational motion was observed directly via two-dimensional exchange spectroscopy, stimulated-echo decays, and 1D line shape changes monitoring the 13C−13C dipolar coupling. The data show that the jumps occur between two sites, with a rotation angle of 180° and with a jump rate of ∼10/s at ambient temperature. The correlation function of the motion was found to be slightly nonexponential, with a stretched-exponential β parameter of 0.8 ± 0.1. The data yield an activation energy of 93 ± 10 kJ/mol for the 180° chain flips. In the fibers, the narrowing of natura...

Chain flips in polyethylene crystallites and fibers characterized by dipolar 13C NMR

The coupling of the α and β processes in poly(ethyl methacrylate) has been investigated in detail by multidimensional 13 C solid-state NMR of the carboxyl moiety. In the glassy state the underlying molecular motion is anisotropic and involves a π flip of the side group coupled to a rocking motion around the local chain axis with a ±20° amplitude. Above the T g the molecular motion remains highly anisotropic. The geometry of the molecular motion is similar to that in the glass; however, the rocking amplitude increases upon raising the temperature above T g

Coupling of the alpha and beta-processes in poly(ethyl methacrylate) investigated by multidimensional nmr

Infrared spectroscopy is used to study simultaneously the orientational behaviour of different segments of dye containing liquid-crystalline side group copolymers in sandwich type films of about 2 μm thickness. Under continuous irradiation with polarized light above and below T g of the polymers both azobenzene and phenyl benzoate side groups reorient preferentially normal to the film plane leading to a strongly biaxial orientation distribution. The analysis of the kinetics reveals that the reorientation is essentially a mono-exponential process with an additional faster process only found for the azobenzene dye and assigned to the initial trans to cis isomerization step. Investigation of an isotropic copolymer system containing azobenzene in the side groups shows that an anisotropy can be induced through irradiation with polarized light that is strongly dependent on temperature.

C. Boeffel

Papers

Molecular nature of the beta-relaxation in poly(methyl methacrylate) investigated by multidimensional nmr

Stiff macromolecules with aliphatic side chains: side-chain mobility, conformation, and organization from 2D solid-state NMR spectroscopy

Chain flips in polyethylene crystallites and fibers characterized by dipolar 13C NMR

Coupling of the alpha and beta-processes in poly(ethyl methacrylate) investigated by multidimensional nmr

An infrared spectroscopic study of photo-induced reorientation in dye containing liquid-crystalline polymers