Polymerization-induced color changes of polydiacetylene-containing liposomes and peptide amphiphile fibers.

Polydiacetylenes have received much attention due to their intrinsic optical properties. Their inclination to change color in response to environmental factors has been extensively exploited in the sensing of analytes. In this study we functionalized diacetylene-containing peptide amphiphiles and phospholipids with α-bromo esters so that they could be used as initiators in atom transfer radical polymerization (ATRP) reactions. Subsequently, the supramolecular assemblies formed by these molecules upon their addition to water, namely peptide amphiphile fibers and liposomes, were stabilized by polymerizing the diacetylene moieties present in the molecules. As a result, highly colored, disassembly resistant, macro initiators were created. To investigate whether steric crowding on the surface of these assemblies could influence the color of the polydiacetylenes, we utilized the initiator functionality that had been introduced prior to assembly in ATRP. We found that the chromatic properties of the polydiacetylenes were directly related to the formation of polymer on the surface of peptide amphiphile fibers as well as liposomes. Furthermore, we were able to demonstrate that the progress of this color change could be monitored with UV-visible spectroscopy.

Cooperative double deprotonation of bis(2-picolyl)amine leading to unexpected bimetallic mixed valence (M(-I), M(I)) rhodium and iridium complexes.

Cooperative reductive double deprotonation of the complex [Rh(I)(bpa)(cod)](+) ([4](+), bpa = PyCH(2)NHCH(2)Py) with one molar equivalent of base produces the bimetallic species [(cod)Rh(bpa-2H)Rh(cod)] (7), which displays a large Rh(-I),Rh(I) contribution to its electronic structure. The doubly deprotonated ligand in 7 hosts the two "Rh(cod)" fragments in two distinct compartments: a "square planar compartment" consisting of one of the Py donors and the central nitrogen donor and a "tetrahedral π-imine compartment" consisting of the other pyridine and an "imine C═N" donor. The formation of an "imine donor" in this process is the result of substantial electron transfer from the {bpa-2H}(2-) ligand to one of the rhodium centers to form the neutral imine ligand bpi (bpi = PyCH(2)N═CHPy). Hence, deprotonation of [Rh(I)(bpa)(cod)](+) represents a reductive process, effectively leading to a reduction of the metal oxidation state from Rh(I) to Rh(-I). The dinuclear iridium counterpart, complex 8, can also be prepared, but it is unstable in the presence of 1 mol equiv of the free bpa ligand, leading to quantitative formation of the neutral amido mononuclear compound [Ir(I)(bpa-H)(cod)] (2). All attempts to prepare the rhodium analog of 2 failed and led to the spontaneous formation of 7. The thermodynamic differences are readily explained by a lower stability of the M(-I) oxidation state for iridium as compared to rhodium. The observed reductive double deprotonation leads to the formation of unusual structures and unexpected reactivity, which underlines the general importance of "redox noninnocent ligands" and their substantial effect on the electronic structure of transition metals.