Absorption, circular dichroism, and nuclear magnetic resonance studies show the conformation of gramicidin A' (GA'), a commercial preparation (Nutritional Biochemicals) of gramicidin, and hydrogenated gramicidin A' to be highly solvent dependent. Infrared data in dimethyl sulfoxide indicate no change of backbone conformation on hydrogenation of gramicidin A'. In trifluoroethanol it is proposed, on the basis of a marked hypochromism and a circular dichroism with positive bands at 224 and 212 nm and a negative band at 194 nm, that in this solvent hydrogenated G ramicidin A is a linear pentadecapeptide with the following sequence : HCO-L-Val-Gly-L-Ala-D-Leu-L-Ala-D-
The introduction of photoactive metal centres onto cyclodextrin receptors opens up new possibilities for the design of sensors, wires and energy conversion systems. This tutorial review focuses on strategies involving such metallocyclodextrins for the construction of supramolecular arrays with light-activated functions. The assembly procedures for building such arrays are presented, together with the features required for their functions both as sensors for ion or small molecule detection and as wires for photoinduced long-range energy or electron transport. Systems for metal ion sensing are described where the cyclodextrin plays a mediating role in influencing the luminescence properties of an organic probe, responsive to metal binding. Small molecule sensing by the cyclodextrin cavity is realised using luminescent lanthanide or transition metal functionalised cyclodextrins. The light signal of the photoactive metal is switched on or off upon binding an analyte in the cyclodextrin cavity. The metallocyclodextrin systems that function as wires are distinguished by the controlled assembly of transition metal polypyridine and metalloporphyrin units. These units have inherent photoactivity that defines the vectorial direction of energy or electron transfer processes through the wire.
In recent years, the importance of miniature photomolecular devices in nanoscale technology has led to the design of polymetallic supramolecular assemblies. [1][2][3] The employment of polyelectronic metal centers in a single supramolecular structure provides the basis for the development of molecular energy-conversion systems and wires in macromolecular systems. [4][5][6][7] Multistep reactions are usually required to link metallic building blocks together by means of covalent bonds; such multistep processes result in poor yields and synthetic complexity. Noncovalently assembled systems, in contrast, may allow control of the photoinduced processes by a simple choice of the assembled photoactive components. The noncovalent assembly of units in water presents other opportunities, not only to mimic natural processes, but also to provide easily accessible architectures. Self-assembled systems between ruthenium-bipyridine centers and various electron or energy acceptors have recently attracted much interest. [8][9][10] We are interested in studying photoinduced processes between metal units assembled in water through noncovalent interactions. We have previously introduced photoactive metals onto cyclodextrin rims for the assembly of photoactive units brought together by the cyclodextrin cavity. [11][12][13][14] Herein, we present a ruthenium tris(bipyridyl) cyclodextrin "wheel", [Ru(b-CD-mbpy) 3 ] 2+ (1; b-CD-mbpy = 6-mono[4-methyl(4'-dimethyl-2,2'-bipyridyl] permethylated b-cyclodextrin which was isolated as the PF 6 salt 1-(PF 6 ) 2 and which could be converted into the Cl salt 1-Cl 2 to enhance its solubility) that acts as both energy donor and acceptor leading to a versatile system for communication between the inner metal core and the outer guest unit by energy transfer; metal-complex guests based on osmium(ii) and iridium(iii) terpyridine species with biphenyl or adamantyl tails attached to one of the ligands have been employed to examine the importance of the included tail in the photoinduced process (Figure 1).Compound 1, is an attractive luminescent receptor molecule with three cyclodextrin cups available for recognition. Solutions of 1 in water with 10 % acetonitrile exhibit luminescence at room temperature at 622 nm (F = 0.027, t aerated = 460 ns, t degassed = 640 ns) upon excitation at the metalto-ligand charge transfer (MLCT) band at 436 nm. Comparison of the photophysical properties of this complex with the parent complex [Ru(mbpy) 3 ][PF 6 ] 2 , (mbpy = 4,4'-dimethyl-2,2'-bipyridine) which bears no cyclodextrins, indicates that cyclodextrin substitution does not have a significant effect.Osmium(ii)-based guests, have been employed to play the role of energy acceptors whereas iridium(iii)-based guests were used as energy donors for the Ru II center. The guests have hydrophobic biphenyl and adamantyl tails to ensure high binding constants in the cyclodextrin cavity. The nature of the hydrophobic tail, aliphatic versus aromatic, is selected to examine its effect in the photoinduced processes. Considerin...
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