The interaction of rubidium atoms with a model molecule for polyacetylene: a photoelectron spectroscopy investigation

The interaction of rubidium, a first group metal, with (alpha,omega-diphenyltetradecaheptaene) (DP7), a diphenylpolyene having seven double bonds in the polyene chain, has been studied by means of X-ray and UV photoelectron spectroscopy. DP7 can be considered as a model molecule for polyacetylene since the frontier pi-orbitals are mainly localized on the polyene part of the molecule. The experimental data are interpreted with the help of related experimental data from sodium-doping of DP7 and quantum chemical calculations. The interaction with rubidium results in n-type doping of the DP7, i.e., an electron-transfer process from the metal to the pi-conjugated system occurs. The doping process affects mainly the polyene part of the molecule, and new electronic states are created in the previously forbidden energy gap. At low doping levels, it appears that one new broad peak is developed in the band gap and, most important, a finite density of states at the Fermi energy can be detected. Upon further doping, there is a slight shift of the single peak, and a second peak is formed. The evolution of the new doping-induced states can be explained in terms of a transition from a polaron-like situation to a soliton-antisoliton pair formation

Polaron to bipolaron transition in a conjugated polymer: Rubidum-doped poly(para-phenylenevinylene)

Photelectron spectroscopy has been used to study the evolution of the electronic structure induced by n-type doping of two pi-conjugated systems: poly(p-phenylenevinylene), or PPV, and trans,trans-1,4-distyrylbenzene, a model molecule for PPV. Doping was carried out in UHV by physical vapor deposition of rubidium atoms. In both systems, two different doping regimes are observed. At low doping levels, for both materials, one new electronic state appears in the originally forbidden energy gap, and a finite density of states is observed at the Fermi level. Upon increased doping, the observed state moves to lower binding energy and a second state appears within the original energy gap. The evolution of the gap states, together with the appearance of a finite density of states at Fermi level at low doping levels, indicates a transition from polaron to bipolaron charge storage states in these conjugated systems. It should be stressed that these results constitute the first direct measurements of a polaron to bipolaron transition in a conjugated polymer using ultraviolet photoelectron spectroscopy

The electronic and geometric structures of neutral and potassium-doped poly[3-(4-octylphenyl)thiophene] studied by photoelectron spectroscopy

The electronic and geometric structures of poly [3-(4-octylphenyl)thiophene] have been studied by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS, respectively). Thermochromic effects, and new charge induced states generated by potassium doping, have been observed by direct UPS measurements. The experimental results are in very good agreement with the results of theoretical quantum chemical calculations performed with the Austin Model 1 semi-empirical model and the valence-effective Hamiltonian pseudo-potential model

Experimental and theoretical studies of the electronic structure of poly(p-phenylenevinylene) and some ring-substituted derivatives

The electronic structure of a conjugated polymer of current interest in organic LED's, poly(p-phenylenevinylene), or PPV, has been studied by ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy. The focus of this work is on the nature of the pi-electronic band structure nearest the Fermi level and the physical influence of finite torsion angles, the geometry of the polymer backbone, on the electronic properties of the system. Details of the ct-electronic bands, as reflected in the associated density-of-states, are observed clearly in the spectra, from which some underlying geometrical details of the polymer system can be deduced. The experimental spectra have been analyzed theoretically using band structure calculations based upon the valence effective Hamiltonian (VEH) model. In addition, in order to control the band structure, three ring-substituted derivatives of PPV, each of which induces a different bonding geometry in the backbone, have been studied. The changes in the experimental results can be explained on the basis of both physical and chemical interactions of the substituents with the backbone, which lead to geometrical changes along the backbone, which influence the ct-bandwidths and contribute to differences in both the optical absorption threshold and the binding energy of the valence band edge

Experimental and theoretical studies of the electronic structure of substituted and unsubstituted poly(para-phenylenevinylene) (PPV)

The electronic structure of poly(p-phenylenevinylene) and that of its ring-substituted derivatives, poly(2,5-diheptyl-1,4-phenylenevinylene), poly(2,5-dimethoxy-1,4-phenylenevinylene), and poly(2-methoxy-5-(2'-ethylhexoxy)-1,4-phenylenevinylene), are studied by Ultraviolet Photoelectron Spectroscopy, UPS, and X-ray Photoelectron Spectroscopy, XPS. It is observed by UPS that the pi-bands closest to the valence band edge are strongly affected by the presence of the substituents. The influence of the side groups on the experimental spectra is studied theoretically using the Valence Effective Hamiltonian, VEH, model. Calculations are carried out on isolated polymer chains, including full treatment of the aliphatic side groups. Particular attention is paid to the effect of chain torsion angles on the pi-band edge. For the diheptyl derivative, the experimental results can be explained on the basis of side-group-induced torsions of the phenylene rings along the backbone, which influence the pi-band widths and contribute to differences in both optical absorption threshold and binding energy of the valence band edge. For the alkoxy derivatives, the side groups cause strong modifications in the shape of the upper two occupied pi-bands, which results in significant changes in the electronic density of states