Charge-transfer shake-up satellites accompanying core ionization in organic donor/acceptor molecules: bis(4-dimethylaminophenyl)squaraine and its derivatives

Abstract

Solid state XPS spectra of bis(4-dimethylaminophenyl)squaraine and a number of derivatives are presented. The complex multipeak N1s and O1s satellite spectra are analyzed in terms of intra and inter-molecular contributions and related to corresponding electronic transitions of the neutral species. It is concluded that a distinctive low-energy O1s satellite characterizes the intermolecular excited state charge transfer between the donor and acceptor fragments on adjacent overlapping sites. A CNDO/S(S + DES CI) equivalent-core computation on O1s ionization of the parent monomer supports this interpretation. Spectral comparisons indicate that chemical substitution to yield intramolecular hydrogen bonding may provide a more favorable pathway for O1s core-hole screening, and thus inhibit intermolecular charge-transfer. The broad asymmetric C1s signals are shown to reflect a wide spread of chemically shifted carbon sites characteristic of a highly charged polarized monomer unit. The consequences of adsorbed O2/H2O on the spectral properties and electronic conductivity are discussed.