Two-photon states in squaraine monomers and oligomers Original Research Article

The excited states of novel monomeric and oligomeric squaraines of the indole series were studied by two-photon fluorescence (TPF). Two-photon excitation scans have been performed in an energy range from 1.75 to 3.2 eV by linearly and circularly polarized light. In addition, results from linear absorption and fluorescence measurements are presented. For the monomers a two-photon transition into the first excited state (Bu-symmetry) allowed by vibronic coupling was found. The interpretation of this transition is different to results of other groups. A second transition to an electronic state in an energy range of 2.5–3.5 eV was observed experimentally for the first time. The optical properties of this 2Ag state strongly depend on the molecular structure. The experimental results are compared to a four-state model. It is shown theoretically and experimentally that both, the linear absorption and the two-photon spectra of squaraine oligomers can be explained by a state interaction between excitons localized at the monomeric subunits (exciton coupling theory).