The Nocera group focuses on basic mechanisms of energy conversion in biology and chemistry. A signature of the program is the ability to make and measure. The group is well versed in synthetic methodologies of inorganic, organometallic, organic, materials and biological chemistry. Expertise in a host of steady state (electronic, Raman) and time-resolved spectroscopies (from femtoseconds to milliseconds), augmented by computational chemistry, permits us to define critical physical and chemical phenomena. These insights in turn guide us in the further design of new systems with targeted properties and/or reactivity. Four current research areas are summarized here. More detail can be found on our group home page.

Chemical energy conversion.
Multielectron reactions are fundamental to promoting energy conversion transformations such as the oxidation of water and the reduction of hydrohalic acid to hydrogen. The basic redox chemistry of excited states is single electron transfer. By itself, single electron transfer is limited inasmuch as most important reactions including the small molecule activation reactions of energy conversion are multielectron processes. Can excited states directly promote multielectron reactions critical to energy conversion? The answer is yes, but only when new types of electronic excited state molecules are designed. For example, we have recently elaborated two-electron mixed valence excited states of transition metal complexes, which are capable of effecting a host of discrete multielectron reactions upon the absorption of a photon; one such transformation involves the photocatalytic generation of hydrogen from homogeneous acidic solutions. Other light-to-chemical energy conversion reactions of these novel excited states are currently under investigation. Emphasis areas: synthesis of inorganic/organometallic complexes, spectroscopy, laser chemistry, photochemistry and computational chemistry.

Biological energy conversion.
Biological energy conversion is predicated on the coupling between protons and electrons. Small-molecule activation processes, redox-driven proton pumps and radical initiation and transport in biology can all involve the coupling of electron transfer to proton motion. A mechanistic framework with which to interpret these processes is being developed by examining proton-coupled electron transfer (PCET) reactions in model and natural systems. Specifically, PCET investigations are underway on the following three fronts:

1. The elucidation of the mechanism of PCET by time-resolved laser spectroscopy of electron donors (D) and acceptors (A) juxtaposed by a proton transfer interface. Emphasis areas: synthesis of novel D---[H+]---A supramolecules, fast to ultrafast laser spectroscopy.
2. Define the role of amino acid and substrate-derived radicals in biological catalysis with the radical initiation and transport processes of ribonucleotide reductase as a focal point (this project is performed in collaboration with the Stubbe group). Emphasis areas: biophysical studies of radicals, laser spectroscopy, biochemical synthesis techniques.
3. Exploit PCET in small molecule activation reactions with emphasis on bond-breaking and bond-making processes involving oxygen and water, respectively, at biomimetic platforms. Emphasis areas: synthesis of novel biomimetic active sites, bioinorganic chemistry, catalysis, time-resolved kinetics methods.

Chemosensing on the nanoscale.
When the flow of energy in a molecule can be precisely controlled, new photophysical schemes may be developed for a variety of applications. One scheme of particular interest to the Nocera group is to synthesize supramolecular active sites that optically sense chem- and biomolecules by the “3R” approach – recognize, relay and report. We have extended this 3R approach to small length scales by fabricating microfluidic platforms containing optical chemosensing active sites. The miniaturization offered by microFluidic Optical Chemosensors allows for the detection of species at trace concentrations, in minute volumes and with high fidelity and has the possibility for massive parallelism. However, our studies show that as the size of sensors moves toward micro- and nanodimensions, the sensitivity of the device is compromised because there are simply too few active sites available for sensing. We see this issue as a fundamental challenge confronting the design of sensors on the nanoscale. One approach to addressing this challenge is to convert the signal transduction mechanism of the 3R scheme from a linear, single photon response to an extremely nonlinear one. To achieve this objective, we are replacing current reporter sites (e.g., emissive metal ions) with a lasing medium. Recognition of an analyte at the surface of the lasing medium produces a high gain response by perturbing lasing action. The techniques and principles developed here can be applied to target many problems in microsensor development and materials applications. Emphasis areas: supramolecular chemistry, laser spectroscopy, nanofabrication.

Magnetic layered materials.
Highly correlated behavior of spin-frustrated systems in extended solids is an intensely studied subject of contemporary condensed matter physics. Magnetic ions arranged at the corners of corner-sharing triangles produce one type of lattice (i.e., a kagomé lattice) that displays spin frustration. We recently developed new methods for preparation of a pure and highly crystalline kagomé lattice of a compound called jarosite. The synthetic methods permit magnetic metal ions of various d-electron counts to be introduced into the triangular lattice. In addition to d3 and d5 electron counts of spin frustration, ferromagnetic ordering has also been observed for d2 metal ions. Our new synthetic methods also allow for greater control over the kinetics of crystal growth. Crystal sizes have been increased by more than 4 orders of magnitude with respect to the previously known techniques, making the newly prepared specimens amenable to neutron diffraction single-crystal measurements. From these studies (and single crystal susceptibility measurements), the mechanism for spin frustration and magnetic ordering in the kagomé lattice is under investigation. Emphasis areas: hydrothermal materials synthesis, magnetism, neutron diffraction.