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Polarization-dependent X-ray absorption spectroscopy is combined with density functional calculations and atomic multiplet calculations to determine the crystal field parameters 10Dq, Ds, and Dt of transition metal phthalocyanines and octaethylporphyrins (Mn, Fe, Co, Ni). The polarization dependence facilitates the assignment of the multiplets in terms of in-plane and out-of-plane orbitals and avoids ambiguities. Crystal field values from density functional calculations provide starting values close to the optimum fit of the data. The resulting systematics of the crystal field can be used for optimizing electron-hole separation in dye-sensitized solar cells. [ABSTRACT FROM AUTHOR]

The photodissociation of the amide bond by UV light and soft x-rays is investigated by x-ray absorption spectroscopy at the C, N, and O 1s edges. Irradiation leaves a clear and universal signature for a wide variety of amides, ranging from oligopeptides to large proteins and synthetic polyamides, such as nylon. As the π* peak of the amide bond shrinks, two new π* peaks appear at the N 1s edge with a characteristic splitting of 1.1 eV. An additional characteristic is the overall intensity reduction of both the π* and σ* features at the O 1s edge, which indicates loss of oxygen. The spectroscopic results are consistent with the release of the O atom from the amide bond, followed by the migration of the H atom from the N to one of its two C neighbors. Migration to the carbonyl C leads to an imine, and migration to the Cα of the amino acid residue leads to a nitrile. Imine and nitrile produce the two characteristic π* transitions at the N 1s edge. A variety of other models is considered and tested against the N 1s spectra of reference compounds. [ABSTRACT FROM AUTHOR]

A significant obstacle to organic photovoltaics is radiation damage, either directly by photochemical reactions or indirectly via hot electrons. Such effects are investigated for biomimetic dye molecules for solar cells (phthalocyanines) and for a biological analog (the charge transfer protein cytochrome c). Both feature a central transition metal atom (or H2) surrounded by nitrogen atoms. Soft x-ray absorption spectroscopy and photoelectron spectroscopy are used to identify three types of radiation-induced changes in the electronic structure of these molecules. (1) The peptide bonds along the backbone of the protein are readily broken, while the nitrogen cage remains rather stable in phthalocyanines. This finding suggests minimizing peptide attachments to biologically inspired molecules for photovoltaic applications. (2) The metal atom in the protein changes its 3d electron configuration under irradiation. (3) The Fermi level EF shifts relative to the band gap in phthalocyanine films due to radiation-induced gap states. This effect has little influence on the optical absorption, but it changes the lineup between the energy levels of the absorbing dye and the acceptor/donor electrodes that collect the charge carriers in a solar cell. [ABSTRACT FROM AUTHOR]

Recently,single-walled carbon nanotubes as well as graphene functionalizedwith azobenzene chromophores have drawn attention for applicationsin optoelectronics due to their ability to undergo cis–transisomerization when exposed to light.The electronic properties of the nanocarbon materials at these unconventionalinterfaces can be tailored by gaining structural insight into theorganic monolayers at the molecular level. In this work, we use polarization-dependentX-ray absorption spectroscopy to probe the orientation of three chromophoreson graphene, all identical except for their terminal groups. All threeterminal groups (methyl, nitro, and nitrile) are well-oriented, witha tilt angle of about 30° from the substrate for the shared azobenzenegroup. Density functional theory calculations are in good agreementwith experimental results and give two similar, stable configurationsfor the orientation of these molecules on graphene. [ABSTRACT FROM AUTHOR]

Monolayer-thickness two-dimensional layers of α,ω-dihexylsexithiophene (α,ω-DH6T) exhibit field-effect hole mobility of up to 0.032 cm2 V−1 s−1, higher than previously reported for monolayers of other small-molecule organic semiconductors. In situ measurements during deposition show that the source-drain current saturates rapidly after the percolation of monolayer-high islands, indicating that the electrical properties of α,ω-DH6T transistors are largely determined by the first molecular monolayer. The α,ω-DH6T monolayer consists of crystalline islands in which the long axes of molecules are oriented approximately perpendicular to the plane of the substrate surface. In-plane lattice constants measured using synchrotron grazing-incidence diffraction are larger in monolayer-thickness films than the in-plane lattice constants of several-monolayer films and of previously reported thick-film structures. Near-edge X-ray absorption fine structure spectroscopy (NEXAFS) reveals that the larger in-plane lattice constant of single-monolayer films arises from a larger tilt of the molecular axis away from the surface normal. NEXAFS spectra at the C 1s and S 2p edges are consistent with a high degree of molecular alignment and with the local symmetry imposed by the thiophene ring. The high mobility of holes in α,ω-DH6T monolayers can be attributed to the reduction of hole scattering associated with the isolation of the thiophene core from the interface by terminal hexyl chains. [ABSTRACT FROM AUTHOR]

X-ray absorption spectroscopy is used to investigatethe photodissociationof the imides PMDI (pyromellitic diimide) and SSMCC (sulfosuccinimidyl4-(N-maleimidomethyl) cyclohexane-1-carboxylate).PMDI contains only one type of imide, and its photodissociation canbe explained by a simple conversion from imide to a mix of imine andnitrile after desorption of the oxygens from the imide. SSMCC containstwo different imides. One reacts like PMDI, the other in a more complexmultistep process. Eventually, N2is formed in the bulkof the sample at high radiation density. The sequence of reactionsis inferred from the π* peaks in total electron yield and fluorescenceyield absorption spectra at the N 1s and O 1s edges. First-order rateequations are used to model the evolution of the peak areas versusradiation dose. [ABSTRACT FROM AUTHOR]