Photosynthesis produces molecular oxygen from water catalyzed by an enzyme whose active site contains a tetramanganese−oxo core of incompletely established structure. The first functional mimic of this core has been synthesized containing a cubical [Mn4O4] n + core, surrounded by six facially bridging bidentate chelates to the manganese ions ((dpp)6Mn4O4 (1); dpp- = diphenylphosphinate anion). Bond enthalpy data predict that the Mn4O4 6+ core is thermodynamically capable of releasing molecular O2, but is kinetically prevented from doing so by an activation barrier. UV light absorption into a Mn−O charge-transfer excited state (but not excitation of a Mn ligand-field excited state) efficiently releases an O2 molecule if performed in the gas phase and concomitantly releases a bridging dpp- anion and the cationic species (dpp)5Mn4O2 + (presumed Mn4O2-butterfly core type). All species were identified by high resolution mass spectrometry. This reaction proceeds with high quantum efficiency (>50%) and is the only observable reaction channel. The O2 product derived exclusively from the corner oxo's of the cube based on photochemistry of the 18O-isotopomer, ((dpp)6Mn4(18O)4. Neither O2 release nor dpp- dissociation are observed individually to occur in the excited state, indicating that O−O bond formation and O2 release require dissociation of one of the six dpp- chelates (“Jack-in-the-Box” mechanism for O2 formation). By contrast, neither O2 production nor chelate photodissociation are observed in condensed phases, presumably due to either quenching of the photoexcited state or rapid recombination of dpp- and (dpp)5Mn4O4 + in the solvent cage. Previous results show that chemical reduction of (1) in solution using hydrogen atom donors produces the deoxygenated (dpp)6Mn4O2 core and releases two water molecules as the only products. Thus the [Mn4O4] n + cubane core is an intrinsically reactive core topology that facilitates both the selective chemical reduction of two of the four oxygen atom bridges to water molecules and their photorearrangement to an O2 molecule under the control of chelation of the manganese ions by dpp-. These results may offer insight into the possible nature of the photosynthetic O2-evolving mechanism.

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A high-resolution time-of-flight secondary ionization mass spectrometer (TOF-SIMS) has been used to investigate chain length effects in hydrocarbon seff-assembled monolayer (SAM) surfaces on gold substrates. A wide range of n-alkanethiols was used to make homogeneous SAM surfaces, which included both odd and even hydrocarbon chain length thiols. Variations in coverage, extent of oxidation, and high-mass cluster formation as a function of hydrocarbon chain length of the alkanethiol SAM surfaces were investigated. Long-short chain length effects were observed for the relative coverage of the SAM surfaces, which directly influences the extent of oxidation for the thin films. The formation of gold-sulfur and gold-adsorbate cluster ions was also observed, since the mass range of the TOF-SIMS made it possible to monitor all of the cluster ions that were formed following the high-energy ion/surface interactions.

A high-resolution time-of-flight secondary ionization mass spectrometer (TOF-SIMS) was used to investigate chain length effects for the formation of cluster ions. The clusters were formed from high energy monatomic ion collisions with hydrocarbon self-assembled monolayer (SAM) surfaces that were deposited onto gold substrates. A wide range of n-alkanethiols was used to make the SAM surfaces, which included both odd and even hydrocarbon chain length thiols. The mass range of the TOF−SIMS made it possible to monitor all of the cluster ions that were formed by the high energy ion/surface interactions. The focus of this work was on the formation of high mass gold−sulfur and gold−adsorbate cluster ions. Mechanisms for the formation of these large cluster ions are proposed, which build upon a precursor cluster mechanism for high energy collisions with hydrocarbon SAM surfaces. The formation of high mass cluster ions is a function of the chain length of the alkanethiol chemisorbed to the gold surface. It has been demonstrated that both the attenuation of the ions by steric effects and van der Waals interactions dictate their formation.

The low-energy reactive collisions of two independent probe ions, pyrazine and d 6-benzene, illustrate an odd−even hydrocarbon chain length effect for a wide range of hydrocarbon self-assembled monolayer (SAM) surfaces. SAM surfaces prepared from alkanethiols ranging from CH3(CH2)10SH to CH3(CH2)17SH chemisorbed to polycrystalline gold are shown to exhibit an odd−even effect where the orientation of the terminal methyl group determines the reaction behavior of the thin film. X-ray photoelectron spectroscopy shows the surfaces to be homogeneously covered and provides evidence for the presence of a thiolate (Au−SR) on the SAM surface. When 30 eV incident ions were used, the extent of hydrogen addition to the incident probe ion was larger for odd carbon chain length SAM surfaces when compared to the even chain length films. For an odd chain length SAM surface, the terminal methyl group exposes a C−H bond perpendicular to the surface, increasing hydrogen addition reactivity for the probe ions. The low-energy probe ions also reacted with the surfaces to form alkyl addition products. The alkyl addition products also showed an odd−even effect. In this case, the even chain length SAM surfaces were more reactive than the odd chain length surfaces. For an even SAM surface, the terminal C−C bond is oriented quasi-perpendicular to the Au surface, allowing more direct access to the carbon atom on the terminal methyl group and increasing its reactivity.