Reversible open-closed conformational switching of nano-size metalloporphyrin dimers triggered by light and temperature.

The current work demonstrates the reversible control of substantial molecular motion in 'nano-sized' molecules, where two structural isomers can 'open' and 'close' their cavities in response to light or heat. The isomers differ widely in their photophysical properties, including colour, polarity, two-photon absorption and π-conjugation, and can easily be separated through column chromatography and thus have wide applicability.

Stable Dication Diradicals of Triply Fused Metallo Chlorin-Porphyrin Heterodimers: Impact of the Bridge on the Control of Spin Coupling to Reactivity.

We report an unexpected rearrangement, controlled by the nature of the bridge, leading to the formation of novel, remarkably stable triply fused dinickel(II)/dicopper(II) chlorin-porphyrin dication diradical heterodimers in excellent yields. Here, a dipyrromethene bridge gets completely fused between two porphyrin macrocycles with two new C-C and one C-N bonds. The two macrocycles exhibit extensive π-conjugation through the bridge, which results in an antiferromagnetic coupling between the two π-cation radicals.

Control of spin coupling through a redox-active bridge in a dinickel(II) porphyrin dimer: step-wise oxidations enable isolations of a chlorin-porphyrin heterodimer and a dication diradical with a singlet ground state.

A dinickel(II)porphyrin dimer has been used here in which the redox-active pyrrole-moiety, similar to the tryptophan residue in diheme enzymes such as MauG and bCcP, has been placed between two Ni(II)porphyrin centers connected a flexible, but unconjugated methylene bridge. This arrangement provides a large physical separation between the two metal centers and thus displays almost no communication between them through the bridge. Upon treatment with DDQ as an oxidant, the dinickel(II) porphyrin dimer slowly gets converted into an indolizinium-fused chlorin-porphyrin heterodimer.

Long-Range Intramolecular Spin Coupling through a Redox-Active Bridge upon Stepwise Oxidations: Control and Effect of Metal Ions.

Dinickel(II) and dicopper(II) porphyrin dimers have been constructed in which two metalloporphyrin units are widely separated by a long unconjugated dipyrrole bridge. Two macrocycles are aligned somewhat orthogonally to each other, while oxidation of the bridge generates a fully π-conjugated butterfly-like structure, which, in turn, upon stepwise oxidations by stronger oxidants result in the formation of the corresponding one- and two-electron-oxidized species exhibiting unusual long-range charge/radical delocalization to produce intense absorptions in the near-infrared (NIR) region and electron paramagnetic resonance (EPR) signals of a triplet state due to interaction between the unpaired spins on the Cu(II) ions. Although the two metal centers have a large physical separation through the bridge (more than 16 Å), they share electrons efficiently between them, behaving as a single unit rather than two independent centers.

Heme-Heme Interactions in Diheme Cytochromes: Effect of Mixed-Axial Ligation on the Electronic Structure and Electrochemical Properties.

Diheme cytochromes, the simplest members in the multiheme family, play substantial biochemical roles in enzymatic catalysis as well as in electron transfer. A series of diiron(III) porphyrin dimers have been synthesized as active site analogues of diheme cytochromes. The complexes contain six-coordinated iron(III) having thiophenol and imidazole at the fifth and sixth coordination sites, respectively.

Equatorial ligand plane perturbations lead to a spin-state change in an iron(iii) porphyrin dimer.

A complete reversal of the spin state of iron(iii) is observed upon a small change to the diporphyrin bridge from ethane to ethene by keeping all other factors intact. Combined analysis using single crystal X-ray structure determination, Mössbauer, variable-temperature magnetic, 1H NMR and EPR studies has confirmed the spin states of iron(iii) complexes both in solid and solution phases.