Orbital-resolved partial charge transfer from the methoxy groups of substituted pyrenes in complexes with tetracyanoquinodimethane--a NEXAFS study.

It is demonstrated that the near-edge X-ray absorption fine structure (NEXAFS) provides a powerful local probe of functional groups in novel charge transfer (CT) compounds and their electronic properties. Microcrystals of tetra-/hexamethoxypyrene as donors with the strong acceptor tetracyano-p-quinodimethane (TMP/HMP-TCNQ) were grown by vapor diffusion. The oxygen and nitrogen K-edge spectra are spectroscopic fingerprints of the functional groups in the donor and acceptor moieties, respectively. The orbital selectivity of the NEXAFS pre-edge resonances allows us to precisely elucidate the participation of specific orbitals in the charge transfer process. Upon complex formation, the intensities of several resonances change substantially and a new resonance occurs in the oxygen K-edge spectrum. This gives evidence of a corresponding change of hybridization of specific orbitals in the functional groups of the donor (those derived from the frontier orbitals 2e and 6a(1) of the isolated methoxy group) and acceptor (orbitals b(3g), a(u), b(1g), and b(2u), all located at the cyano group) with π*-orbitals of the ring systems. Along with this intensity effect, the resonance positions associated with the oxygen K-edge (donor) and nitrogen K-edge (acceptor) shift to higher and lower photon energies in the complex, respectively. A calculation based on density functional theory qualitatively explains the experimental results. NEXAFS measurements shine light on the action of the functional groups and elucidate charge transfer on a submolecular level.

The light emission observed from small palladium particles during passage of electronic current.

Light emission spectra were measured from separate Pd particles excited by the passage of electrical current through them. As the particle size decreases or input power increases the intensities of peaks increase more strongly in the high-energy part of the spectra than in the low-energy part. Moreover, new peaks can appear in the high-energy region. The color of an emission center changes from red to blue upon an increase in electric power fed into the Pd particle. This process is reversible and can be observed visually. On a further increase in feeding power, the Pd particle can be brought to a premolten state and can even melt. The integral intensity of light emission then increases dramatically, and the light emission spectrum changes and corresponds to thermal radiation. This latter process is irreversible.