Person:

Nocera, Daniel

Two nanocrystal-osmium(II) polypyridyl (NC-Os(II)PP) conjugates have been designed to detect oxygen in biological environments. Polypyridines appended with a single free amine were linked with facility to a carboxylic acid functionality of a semiconductor NC overlayer to afford a biologically stable amide bond. The Os(II)PP complexes possess broad absorptions that extend into the red spectral region; this absorption feature makes them desirable acceptors of energy from NC donors. Fluorescence resonance energy transfer (FRET) from the NC to the Os(II)PP causes an enhanced Os(II)PP emission with a concomitant quenching of the NC emission. Owing to the large two-photon absorption cross-section of the NCs, FRET from NC to the Os(II)PP can be established under two-photon excitation conditions. In this way, two-photon processes of metal polypyridyl complexes can be exploited for sensing. The emission of the NC is insensitive to oxygen, even at 1 atm, whereas excited states of both osmium complexes are quenched in the presence of oxygen. The NC emission may thus be used as an internal reference to correct for fluctuations in the photoluminescence intensity signal. These properties taken together establish NC-Os(II)PP conjugates as competent ratiometric, two-photon oxygen sensors for application in biological microenvironments.

Personalized energy (PE) is a transformative idea that provides a new modality for the planet’s energy future. By providing solar energy to the individual, an energy supply becomes secure and available to people of both legacy and non-legacy worlds, and minimally contributes to increasing the anthropogenic level of carbon dioxide. Because PE will be possible only if solar energy is available 24 hours a day, 7 day a week, the key enabler for solar PE is an inexpensive storage mechanism. HX (X = halide or OH−) splitting is a fuel-forming reaction of sufficient energy density for large scale solar storage but the reaction relies on chemical transformations that are not understood at the most basic science level. Critical among these are multielectron transfers that are proton-coupled and involve the activation of bonds in energy poor substrates. The chemistry of these three italicized areas is developed, and from this platform, discovery paths leading to new HX and H2O splitting catalysts are delineated. For the case of the water splitting catalyst, it captures many of the functional elements of photosynthesis. In doing so, a highly manufacturable and inexpensive method has been discovered for solar PE storage.

Hydrogen-bonded interfaces are essential structural elements in biology. Furthermore they can mediate electron transport by coupling the electron to proton transfer within the interface. The specific hydrogen-bonding configuration and strength have a large impact on the proton transfer, which exchanges the hydrogen bonded donor and acceptor species (i.e. NH---O → N---HO. Modulations of the hydrogen-bonding environment, such as the hydrogen-bond stretch and twist modes, affect the proton transfer dynamics. Here we present transient grating and echo peak shift measurements of the NH stretch vibrations of four doubly hydrogen-bonded cyclic dimers in their electronic ground state. The equilibrium vibrational dynamics exhibit strong coherent modulations that we attribute to coupling of the high-frequency NH vibration to the low frequency inter-dimer stretch and twist modes and not to interference between multiple Fermi-resonances that dominate the substructure of the linear spectra.

A Zn(II) porphyrin-amidinium is the excited state electron donor (D) to a naphthalene diimide acceptor (A) appended with either a carboxylate or sulfonate functionality. The two-point hydrogen bond (---[H+]---) formed between the amidinium and carboxylate or sulfonate establishes a proton-coupled electron transfer (PCET) pathway for charge transfer. The two D---[H+]---A assemblies differ only by the proton configuration within the hydrogen bonding interface. Specifically, the amidinium transfers a proton to the carboxylate to form a non-ionized amidine-carboxylic acid two-point hydrogen network whereas the amidinium maintains both protons when bound to the sulfonate functionality forming an ionized amidinium-sulfonate two-point hydrogen network. These two interface configurations within the dyads thus allow for a direct comparison of PCET kinetics for the same donor and acceptor juxtaposed by an ionized and non-ionized hydrogen-bonded interface. Analysis of PCET kinetics ascertained from transient absorption and transient emission spectroscopy reveal that the ionized interface is more strongly impacted by the local solvent environment, thus establishing that the initial static configuration of the proton interface is a critical determinant to the kinetics of PCET.

Two-point hydrogen bonding between acid and base functionalities provides a convenient method for the modular assembly of proton-coupled electron transfer (PCET) networks, especially when that interface comprises an amidinium and two-point anionic partner; a system is presented that permits the proton configuration within the interface to be determined when pKa values of the conjugate acids are known.

Photochemical ribonucleotide reductases (photoRNRs) have been developed to study the proton-coupled electron transfer (PCET) mechanism of radical transport in class I E. coli ribonucleotide reductase (RNR). The transport of the effective radical occurs along several conserved aromatic residues across two subunits: β2(•Y122→W48→Y356)→ α2(Y731→Y730→C439). The current model for RNR activity suggests that radical transport is strongly controlled by conformational gating. The C-terminal tail peptide (Y- βC19) of β2 is the binding determinant of β2 to α2 and contains the redox active Y356 residue. A photoRNR has been generated synthetically by appending a Re(bpy)(CO)3CN ([Re]) photo-oxidant next to Y356 of the 20-mer peptide. Emission from the [Re] center dramatically increases upon peptide binding, serving as a probe for conformational dynamics and Y356 protonation state. The diffusion coefficient of [Re]-Y-βC19 has been measured (kd1 = 6.1 × 10−7 cm−1 s–1), along with the dissociation rate constant for the [Re]-Y-βC19:α2 complex (7000 s−1 > koff > 400 s−1). Results from detailed time-resolved emission and absorption spectroscopies reveal biexponential kinetics, suggesting a large degree of conformational flexibility in the α2:[Re]-Y-βC19 complex that partitions the N-terminus of the peptide into both bound and solvent-exposed fractions.