Sriparna Mukherjee

Synthetic model of the asymmetric [Mn3CaO4] cubane core of the oxygen-evolving complex of photosystem II

The laboratory synthesis of the oxygen-evolving complex (OEC) of photosystem II has been the objective of synthetic chemists since the early 1970s. However, the absence of structural information on the OEC has hampered these efforts. Crystallographic reports on photosystem II that have been appearing at ever-improving resolution over the past ten years have finally provided invaluable structural information on the OEC and show that it comprises a [Mn3CaO4] distorted cubane, to which is attached a fourth, external Mn atom, and the whole unit attached to polypeptides primarily by aspartate and glutamate carboxylate groups. Such a heterometallic Mn/Ca cubane with an additional metal attached to it has been unknown in the literature. This paper reports the laboratory synthesis of such an asymmetric cubane-containing compound with a bound external metal atom, [(1)] . All peripheral ligands are carboxylate or carboxylic acid groups. Variable-temperature magnetic susceptibility data have established 1 to possess an S = 9/2 ground state. EPR spectroscopy confirms this, and the Davies electron nuclear double resonance data reveal similar hyperfine couplings to those of other MnIV species, including the OEC S2 state. Comparison of the X-ray absorption data with those for the OEC reveal 1 to possess structural parameters that make it a close structural model of the asymmetric-cubane OEC unit. This geometric and electronic structural correspondence opens up a new front in the multidisciplinary study of the properties and function of this important biological unit.

Cyclometalated Fe(II) Complexes as Sensitizers in Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) often utilize transition metal-based chromophores for light absorption and semiconductor sensitization. Ru(II)-based dyes are among the most commonly used sensitizers in DSSCs. As ruthenium is both expensive and rare, complexes based on cheaper and more abundant iron could serve as a good alternative. In this study, we investigate Fe(II)-bis(terpyridine) and its cyclometalated analogues, in which pyridine ligands are systematically replaced by aryl groups, as potential photosensitizers in DSSCs. We employ density functional theory at the B3LYP/6-31G*,SDD level to obtain the ground state electronic structure of these complexes. Quantum dynamics simulations are utilized to study interfacial electron transfer between the Fe(II) photosensitizers and a titanium dioxide semiconductor. We find that cyclometalation stabilizes the singlet ground state of these complexes by 8-19 kcal/mol but reduces the electron density on the carboxylic acid attached to the aryl ring. The results suggest that cyclometalation provides a feasible route to increasing the efficiency of Fe(II) photosensitizers but that care should be taken in choosing the substitution position for the semiconductor anchoring group.

Onset of a Propagating Self-Sustained Spin Reversal Front in a Magnetic System

The energy released in a magnetic material by reversing spins as they relax toward equilibrium can lead to a dynamical instability that ignites self-sustained rapid relaxation along a deflagration front that propagates at a constant subsonic speed. Using a trigger heat pulse and transverse and longitudinal magnetic fields, we investigate and control the crossover between thermally driven magnetic relaxation and magnetic deflagration in single crystals of Mn(12)-acetate.

Partial spin reversal in magnetic deflagration

The reversal of spins in a magnetic material as they relax toward equilibrium is accompanied by the release of Zeeman energy which can lead to accelerated spin relaxation and the formation of a well-defined self-sustained propagating spin-reversal front known as magnetic deflagration. To date, studies of Mn

Tuning the Electronic Structure of Fe(II) Polypyridines via Donor Atom and Ligand Scaffold Modifications: A Computational Study

_{12}

Linker dependence of interfacial electron transfer rates in Fe(II)-polypyridine sensitized solar cells

-acetate single crystals have focused mainly on deflagration in large longitudinal magnetic fields and found a fully spin-reversed final state. We report a systematic study of the effect of transverse magnetic field on magnetic deflagration and demonstrate that in small longitudinal fields the final state consists of only partially reversed spins. Further, we measured the front speed as a function of applied magnetic field. The theory of magnetic deflagration, together with a modification that takes into account the partial spin reversal, fits the transverse field dependence of the front speed but not its dependence on longitudinal field. The most significant result of this study is the finding of a partially spin-reversed final state, which is evidence that the spins at the deflagration front are also only partially reversed.