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This project was a modeling and synthesis effort that examined metal and metal-complexing ligand dynamics in a major US Naval harbor, the Elizabeth River (VA). It built on our past work (funded through the ONR Harbor Processes program) studying the biogeochemical cycling of Cu, Cd, Zn, and Mn in the water column and sediments of the Elizabeth River.

The Fe(III) --> Fe(II) reduction of the heme iron in aquomet-myoglobin, induced by x-rays at cryogenics temperatures, produces a thermally trapped nonequilibrium state in which a water molecule is still bound to the iron. Water dissociates at T > 160 K, when the protein can relax toward its new equilibrium, deoxy form. Synchrotron radiation x-ray absorption spectroscopy provides information on both the redox state and the Fe-heme structure. Owing to the development of a novel method to analyze the low-energy region of x-ray absorption spectroscopy, we obtain structural pictures of this photo-inducible, irreversible process, with 0.02-0.06-A accuracy, on the protein in solution as well as in crystal. After photo-reduction, the iron-proximal histidine bond is shortened by 0.15 A, a reinforcement that should destabilize the iron in-plane position favoring water dissociation. Moreover, we are able to get the distance of the water molecule even after dissociation from the iron, with a 0.16-A statistical error.

Dye molecules with pyridyl side substituents (porphyrins and heterocyclic perylene diimides) coordinatively attached to semiconductor CdSe/ZnS quantum dots (QDs) surface form quasi-stable “QD-Dye” nanocomposites of various geometry in the competition with capping molecules (tri-n-octyl phosphine oxide or long chain amines) exchange. This results in photoluminescence (PL) quenching of the QDs both due to Foerster resonance energy transfer and formation of non-radiative surface states. QD surface is inhomogeneous with respect to the involved attachment and detachment processes. The formation of “QD-Porphyrin” nanocomposites is realized at least two time scales (60 and 600 s), which is attributed to a reorganisation of tri-n-octylphosphine oxide capping shell. In a low temperature range of 220÷240 K related changes in QD absorption and emission reveal a phase transition of the capping shell (tri-n-octyl phosphine oxide and amine). In “QD-Dye” nanocomposites, this phase transition is enhanced considerably by only a few attached dye molecules and has impact on the QD core structure followed by changes of PL quenching and exciton-phonon coupling. A combination of ensemble and single molecule spectroscopy of “QD-Dye” nanocomposites reveals that few or even only one attached dye molecule change the surface distribution and energy of dye related surface trap states considerably.

Optical spectroscopy on ensembles and single CdSe/ZnS semiconductor quantum dots (QDs) demonstrates a competition of trap and near band edge photoluminescence (PL). This competition can be markedly influenced by a few surface attached pyridyl functionalized dye molecules (porphyrins or perylene diimides) forming nanoassemblies with well defined geometries. Temperature variation and related changes in absorption and emission reveal sharp changes of the ligand shell structure in a narrow temperature range for organic (TOPO and amine) surfactants (phase transition). The effects on QD PL at this transition become considerably pronounced upon attachment of only a few dye molecules to QD surface. Moreover, under ambient conditions amine capped QDs are photodegraded in the course of time. This process is enhanced by attached dye molecules both on the ensemble and single particle/dye level. This investigation elaborates the importance of (switchable) surface states for the characterization of the PL of QDs. © 2012 Elsevier B.V. All rights reserved.