The miniaturization of components used in the construction of working devices is being pursued currently by the large-downward (top-down) fabrication. This approach, however, which obliges solid-state physicists and electronic engineers to manipulate progressively smaller and smaller pieces of matter, has its intrinsic limitations. An alternative approach is a small-upward (bottom-up) one, starting from the smallest compositions of matter that have distinct shapes and unique properties-namely molecules. In the context of this particular challenge, chemists have been extending the concept of a macroscopic machine to the molecular level. A molecular-level machine can be defined as an assembly of a distinct number of molecular components that are designed to perform machinelike movements (output) as a result of an appropriate external stimulation (input). In common with their macroscopic counterparts, a molecular machine is characterized by 1) the kind of energy input supplied to make it work, 2) the nature of the movements of its component parts, 3) the way in which its operation can be monitored and controlled, 4) the ability to make it repeat its operation in a cyclic fashion, 5) the timescale needed to complete a full cycle of movements, and 6) the purpose of its operation. Undoubtedly, the best energy inputs to make molecular machines work are photons or electrons. Indeed, with appropriately chosen photochemically and electrochemically driven reactions, it is possible to design and synthesize molecular machines that do work. Moreover, the dramatic increase in our fundamental understanding of self-assembly and self-organizational processes in chemical synthesis has aided and abetted the construction of artificial molecular machines through the development of new methods of noncovalent synthesis and the emergence of supramolecular assistance to covalent synthesis as a uniquely powerful synthetic tool. The aim of this review is to present a unified view of the field of molecular machines by focusing on past achievements, present limitations, and future perspectives. After analyzing a few important examples of natural molecular machines, the most significant developments in the field of artificial molecular machines are highlighted. The systems reviewed include 1) chemical rotors, 2) photochemically and electrochemically induced molecular (conformational) rearrangements, and 3) chemically, photochemically, and electrochemically controllable (co-conformational) motions in interlocked molecules (catenanes and rotaxanes), as well as in coordination and supramolecular complexes, including pseudorotaxanes. Artificial molecular machines based on biomolecules and interfacing artificial molecular machines with surfaces and solid supports are amongst some of the cutting-edge topics featured in this review. The extension of the concept of a machine to the molecular level is of interest not only for the sake of basic research, but also for the growth of nanoscience and the subsequent development of nanotechnology.

Artificial Molecular Machines.

Azobenzene undergoes trans → cisisomerization when irradiated with light tuned to an appropriate wavelength. The reverse cis →transisomerization can be driven by light or occurs thermally in the dark. Azobenzene's photochromatic properties make it an ideal component of numerous molecular devices and functional materials. Despite the abundance of application-driven research, azobenzene photochemistry and the isomerization mechanism remain topics of investigation. Additional substituents on the azobenzene ring system change the spectroscopic properties and isomerization mechanism. This critical review details the studies completed to date on the 3 main classes of azobenzene derivatives. Understanding the differences in photochemistry, which originate from substitution, is imperative in exploiting azobenzene in the desired applications.

Photoisomerization in different classes of azobenzene

Photoinduced motions in azo-containing polymers.

This Account reports the synthesis and characterization of dendrimer-encapsulated metal nanoparticles and their applications to catalysis. These materials are prepared by sequestering metal ions within dendrimers followed by chemical reduction to yield the corresponding zerovalent metal nanoparticle. The size of such particles depends on the number of metal ions initially loaded into the dendrimer. Intradendrimer hydrogenation and carbon−carbon coupling reactions in water, organic solvents, biphasic fluorous/organic solvents, and supercritical CO2 are also described.

http://rcrooks.cm.utexas.edu/research/resources/Publications/rmc113.pdf

Dendrimer-Encapsulated Metal Nanoparticles: Synthesis, Characterization, and Applications to Catalysis

Photoalignment of Liquid-Crystal Systems.

A mean-field model of photoinduced surface reliefs in dye containing side-chain polymers is presented. It is demonstrated that photoinduced ordering of dye molecules subject to anisotropic intermolecular interactions leads to mass transport even when the intensity of the incident light is spatially uniform. Theoretical profiles are obtained using a simple variational method and excellent agreement with experimental surface reliefs recorded under various polarization configurations is found. The polarization dependence of both period and shape of the profiles is correctly reproduced by the model.

Mean-field theory of photoinduced formation of surface reliefs in side-chain azobenzene polymers

New side-chain liquid crystalline polyesters have been prepared by melt transesterification of diphenyl tetradecanedioate and a series of mesogenic 2-[ω-[4-[(4-cyanophenyl)azo]phenoxy]alkyl]-1,3-propanediols, where the alkyl spacer is hexa-, octa-, and decamethylene in turn. The polyesters have molecular masses in the range 5000-89 000. Solution 13 C NMR spectroscopy has been employed to identify carbons of polyester repeat units and of both types of end groups. Polyester phases and phase transitions have been investigated in detail by polarizing optical microscopy and differential scanning calorimetry for the hexamethylene spacer architecture with different molecular masses. Using FTIR polarization spectroscopy, the segmental orientation in unoriented polyester films induced by argon ion laser irradiation has been followed and an irradiation-dependent order parameter for the cyanoazobenzene mesogens calculated. FTIR is also utilized to follow the temperature-dependent erasure of the induced orientation. Optical storage properties of thin unoriented polyester films are examined through measurements of polarization anisotropy and holography. A resolution of over 5000 lines/mm and diffraction efficiencies of about 40% have been achieved. Lifetimes greater than 30 months for information stored have been obtained, even though the glass transition temperatures are about 20 °C. Complete erasure of the information can be obtained by heating the films to about 80 °C, and the films can be reused many times without fatigue.

Novel Side-Chain Liquid Crystalline Polyester Architecture for Reversible Optical Storage

SEVERAL classes of organic materials (such as photoanisotropic liquid-crystalline polymers1–4 and photorefractive polymers5–7) are being investigated for the development of media for optical data storage. Here we describe a new family of organic materials—peptide oligomers containing azobenzene chromophores— which appear particularly promising for erasable holographic data storage applications. The rationale for our approach is to use the structural properties of peptide-like molecules to impose orientational order on the chromophores, and thereby optimize the optical properties of the resulting materials. Here we show that holographic gratings with large first-order diffraction efficiencies (up to 80%) can be written and erased optically in oligomer films only a few micrometres thick. The holograms also exhibit good thermal stability, and are not erased after heating to 180 °C for one month. Straightforward extension of this peptide-based strategy to other molecular structures should allow the rational design of a wide range of organic materials with potentially useful optical properties.

Peptide oligomers for holographic data storage

A tutorial review is presented to inform and inspire the reader to develop and integrate strong scientific links between liquid crystals and holographic data storage, from a materials scientist's viewpoint. The principle of holographic data storage as a means of providing a solution to the information storage demands of the 21st century is detailed. Holography is a small subset of the much larger field of optical data storage and similarly, the diversity of materials used for optical data storage is enormous. The theory of polarisation holography which produces holograms of constant intensity, is discussed.
 Polymeric liquid crystals play an important role in the development of materials for holographic storage and photoresponsive materials based on azobenzene are targeted for discussion due to their ease of photo-reversion between trans- and cis-states. Although the final polymer may not be liquid crystalline, irradiation can induce ordered domains. The mesogens act in a co-operative manner, enhancing refractive indices and birefringences. Surface relief gratings are discussed as a consequence of holographic storage.
 Cholesteric polymers comprising azobenzene are briefly highlighted. Irradiation causing cis–trans-isomerisation can be used to control helix pitch. A brief mention of liquid crystals is also made since these materials may be of future interest since they are optically transparent and amenable to photo-induced anisotropy.

Liquid crystals for holographic optical data storage.

Potential holographic materials are obtained from the azobenzene-functionalized dendrimers described herein (shown schematically on the right). Even in the largest species presented, which contains up to 32 azobenzene chromophores, each chromophoric unit behaves independently compared to “monomer” model compounds. Diffraction efficiencies larger than 20 % were achieved with holographic films of the azobenzene dendrimers – the second-generation dendrimer exhibits good thermal stability.

P. S. Ramanujam

Papers

Mean-field theory of photoinduced formation of surface reliefs in side-chain azobenzene polymers

Novel Side-Chain Liquid Crystalline Polyester Architecture for Reversible Optical Storage

Peptide oligomers for holographic data storage

Liquid crystals for holographic optical data storage.

Azobenzene-Functionalized Cascade Molecules: Photoswitchable Supramolecular Systems