Signaling Recognition Events with Fluorescent Sensors and Switches.

Ru(II) polypyridine complexes: photophysics, photochemistry, eletrochemistry, and chemiluminescence

Self-Assembly of Discrete Cyclic Nanostructures Mediated by Transition Metals

Lanthanide-based luminescent hybrid materials.

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

Molecular meccano. 1. [2]Rotaxanes and a [2]catenane made to order

The concept of machine can be extended to the molecular level by designing and synthesizing (supra)molecular species capable of performing mechanical movements. The energy needed to make a machine work can be supplied as chemical energy, electrical energy, or light. When a chemical “fuel” is used, waste products are formed, whereas this is not the case when suitable photochemical or electrochemical energy inputs are employed. A number of elementary functions performed by molecular-level machines are illustrated, and more complex ones are foreseen.

Artificial Molecular-Level Machines: Which Energy To Make Them Work?†

Two novel [2]rotaxanes, comprised of a dibenzo[24]crown-8 (DB24C8) macroring bound mechanically to a chemical “dumbbell” possessing two different recognition sitesviz., secondary dialkylammonium (NH2+) and 4,4‘-bipyridinium (Bpym2+) unitshave been synthesized by using the supramolecular assistance to synthesis provided by, inter alia, hydrogen bonding interactions. One of these rotaxanes bears a fluorescent and redox-active anthracene (Anth) stopper unit. NMR spectroscopy and X-ray crystallography have demonstrated that the DB24C8 macroring exhibits complete selectivity for the NH2+ recognition sites, i.e., that the [2]rotaxanes exist as only one of two possible translational isomers. Deprotonation of the rotaxanes' NH2+ centers effects a quantitative displacement of the DB24C8 macroring to the Bpym2+ recognition site, an outcome that can be reversed by acid treatment. The switching processes have been investigated by 1H NMR spectroscopy and, for the Anth-bearing rotaxane, by electrochemical and photophys...

Acid−Base Controllable Molecular Shuttles†

A molecular-level abacus-like system driven by light inputs has been designed in the form of a [2]rotaxane, comprising the pi-electron-donating macrocyclic polyether bis-p-phenylene-34-crown-10 (BPP34C10) and a dumbbell-shaped component that contains 1) a Ru(II) polypyridine complex as one of its stoppers in the form of a photoactive unit, 2) a p-terphenyl-type ring system as a rigid spacer, 3) a 4,4'-bipyridinium unit and a 3,3'-dimethyl-4,4'-bipyridinium unit as pi-electron-accepting stations, and 4) a tetraarylmethane group as the second stopper. The synthesis of the [2]rotaxane was accomplished in four successive stages. First of all, the dumbbell-shaped component of the [2]rotaxane was constructed by using conventional synthetic methodology to make 1) the so-called "west-side" comprised of the Ru(II) polypyridine complex linked by a bismethylene spacer to the p-terphenyl-type ring system terminated by a benzylic bromomethyl function and 2) the so-called "east-side" comprised of the tetraarylmethane group, attached by a polyether linkage to the bipyridinium unit, itself joined in turn by a trismethylene spacer to an incipient 3,3'-dimethyl-4,4'-bipyridinium unit. Next, 3) the "west-side" and "east-side" were fused together by means of an alkylation to give the dumbbell-shaped compound, which was 4) finally subjected to a thermodynamically driven slippage reaction, with BPP34C10 as the ring, to afford the [2]rotaxane. The structure of this interlocked molecular compound was characterized by mass spectrometry and NMR spectroscopy, which also established, along with cyclic voltammetry, the co-conformational behavior of the molecular shuttle. The stable translational isomer is the one in which the BPP34C10 component encircles the 4,4'-bipyridinium unit, in keeping with the fact that this station is a better pi-electron acceptor than the other station. This observation raises the question- can the BPP34C10 macrocycle be made to shuttle between the two stations by a sequence of photoinduced electron transfer processes? In order to find an answer to this question, the electrochemical, photophysical, and photochemical (under continuous and pulsed excitation) properties of the [2]rotaxane, its dumbbell-shaped component, and some model compounds containing electro- and photoactive units have been investigated. In an attempt to obtain the photoinduced abacus-like movement of the BPP34C10 macrocycle between the two stations, two strategies have been employed-one was based fully on processes that involved only the rotaxane components (intramolecular mechanism), while the other one required the help of external reactants (sacrificial mechanism). Both mechanisms imply a sequence of four steps (destabilization of the stable translational isomer, macrocyclic ring displacement, electronic reset, and nuclear reset) that have to compete with energy-wasteful steps. The results have demonstrated that photochemically driven switching can be performed successfully by the sacrificial mechanism, whereas, in the case of the intramolecular mechanism, it would appear that the electronic reset of the system is faster than the ring displacement.

Roberto Ballardini

Papers

Molecular meccano. 1. [2]Rotaxanes and a [2]catenane made to order

Artificial Molecular-Level Machines: Which Energy To Make Them Work?†

Acid−Base Controllable Molecular Shuttles†

A photochemically driven molecular-level abacus

Phosphorescent 8-quinolinol metal chelates. Excited-state properties and redox behavior