Richard F. Dallinger

THE LIFETIME OF THE EXCITED SINGLET STATE OF β‐CAROTENE: CONSEQUENCES TO PHOTOSYNTHETIC LIGHT HARVESTING

Abstract The mechanism of singlet‐singlet energy transfer to chlorophyll from carotenoid auxiliary pigments in photosynthetic apparatus is considered. Transmittance studies and resonance‐enhanced Raman spectroscopy on a picosecond time scale lead to the conclusion that the de–excitation lifetime of the β‐carotene singlet state is not greater than one picosecond. This would require close contiguity on the part of the transferring partners.

Electronic, Redox, and Photophysical Consequences of Metal-for-Carbon Substitution in Oligo-Phenylene-Ethynylenes

The electronic structures, redox chemistry, and excited-state properties of tungsten-containing oligo-phenylene-ethynylenes (OPEs) of the form W[C(p-C6H4CC)n-1Ph](dppe)2Cl (n = 1-5; dppe =1,2-bis(diphenylphosphino)ethane) are reported and compared with those of organic analogues in order to elucidate the effects of metal-for-carbon substitution on OPE bonding and electronic properties. Key similarities between the metallo- and organic OPEs that bear on materials-related functions include their nearly identical effective conjugation lengths, reduction potentials, and π* orbital energies and delocalization. In addition to these conserved properties, the tungsten centers endow OPEs with reversible one-electron oxidation chemistry and long-lived emissive triplet excited states that are not accessible to organic OPEs. The electronic similarities and differences between metallo- and organic OPEs can be understood largely on the basis of π/π* orbital energy matching between tungsten and organic PE fragments and the introduction of an orthogonal mid-π/π*-gap d orbital in metallo-OPEs. These orbital energies can be tuned by varying the supporting ligands; this provides a means to rationally implement and control the emergent properties of metallo-OPE materials.

Effects of cation-anion interactions on the structures and photophysical properties of anionic d0 tungsten-benzylidyne complexes

Abstract The structures and photophysical properties of anionic tungsten–benzylidyne complexes of the type [Na(L)][W(CPh)(OBut)4] ([Na(L)]1; L=blank, 15-crown-5, crypt-2,2,2) are strongly dependent on the nature of the [Na(L)]+ ion. The structures of the 1− ions are qualitatively similar, consisting of square-pyramidal tungsten centers with short WC bonds, but differ as a function of cation in the extent of their Na–OBut interactions, the geometries of their OBut ligands, and their WO bond distances. The 1− ions exhibit long-lived luminescence (τ>1 μs) in fluid solution and the solid state at room temperature. The emission lifetimes and energies are cation dependent even in polar solvents, indicating the presence of [Na(L)]+/1− ion pairs in solution for some L. The emissive state is a spin triplet of either [π(WCPh)]1[dxy]1 or [π(WCPh)]1[π*(WCPh)]1 configuration.

Picosecond Resonance Raman Spectroscopy

We report the first picosecond-timescale resonance Raman spectra obtained using single-pulse and repetitive large-pulse laser excitation. Our observations on solutions of β-carotene and cytochrome c are close to the ultimate limits both of time resolution and high dilution in Raman spectroscopy.

Methylidyne Complexes: Structures, Spectra, and Bonding

The nature of the M≡C bond has been probed by single-crystal X-ray diffraction and NMR, electronic, and Raman spectroscopic studies of methylidyne complexes of the type trans-W(≡CH)L4X.

Synthesis and Characterization of Potassium Tris(oxalato)ferrate(III) Trihydrate: A Spectrophotometric Method of Iron Analysis

A previous Journal article [J. Chem. Educ. 1984, 61, 1098--1099] described a potassium tris(oxalato)ferrate(III) trihydrate empirical formula experiment that offered an excellent integrative experience in synthesis and characterization for general chemistry laboratory students. However, we have introduced a fast and accurate spectrophotometric method for the determination of iron in the product that takes the place of the photochemical-gravimetric procedure described in the article. Besides the pedagogic interest of bringing three different types of chemical analysis (titrimetric, gravimetric, and spectrophotometric) to bear on one compound, the new iron determination allows students to complete the experiment in 2, 3-hr laboratory periods rather than the 5 periods allotted in the original experiment.

A colorimetric titration experiment with laser excitation and computer-interfaced endpoint detection

88. Students experience the use and handling of laser beams, the construction of a simple and inexpensive photodetector, and the use of a commercially available computer interface system all within the framework of the familiar titration curve.