THEORETICAL AND EXPERIMENTAL STUDIES OF THE INTERACTION BETWEEN SODIUM AND OLIGOTHIOPHENES

Quantum-chemical calculations and ultraviolet photoelectron spectrosco py (UPS) measurements have been performed in order to study the intera ction between sodium and oligothiophenes. with a focus on the origin o f experimentally observed relaxation energy effects in alkali-metal-do ped conjugated molecules. Upon doping of alpha-sexithienylene (alpha-6 T) with sodium atoms, (1) a broad feature appears in the valence band, in an energy region corresponding to the band gap in pristine alpha-6 T, and (2) certain structural features in the valence band shift towar ds lower binding energies in the doped material. In particular, upon d oping, a structural peak related to electronic levels mainly localized to the sulfur and beta-carbon atoms destabilizes to an energy corresp onding to that of the valence-band edge in pristine alpha-6T. The resu lts of ab initio Hartree-Fock and local-spin-density calculations on a lpha-trithienylene and bithiophene are consistent with the experimenta l data, and allow for an assignment of these destabilization effects i n terms of initial-state relaxations. We stress that similar destabili zation effects, reported for other alkali-metal-doped conjugated syste ms, had previously been proposed to be associated with final-state ele ctronic screening, i.e., a dynamic artifact within the UPS measurement s; this is in show that all structural features in the UPS data are co ntained in the results of sufficiently complete quantum chemical calcu lations.