Molecular environment effects on two-photon-absorbing heterocyclic chromophores

Over the past several years, organic molecules exhibiting significant two-photon absorbance and subsequent up-converted fluorescence have been of intense interest for a wide variety of applications including data storage, imaging, and optical limiting. However, the establishment of structure-property relationships for some asymmetric molecules has been hindered by the sensitivity of these nonlinear optical properties to the local molecular environment and to the pulse width of the incident radiation. To understand the influence of the local molecular environment on the excited states of these two-photon-absorbing molecules, the linear absorbance, the single-photon-excited photoluminescence, and the two-photon-excited photoluminescence of a series of heterocyclic dyes are examined. The stabilization of the longest-lived one-photon-excited state by the local molecular environment can be described by mean field interactions with solvent molecules as given by the Lippert equation. Because the same stabilization dominates the two-photon-induced longest-lived excited state, the influence of the local molecular environment on the two-photon luminescence can be predicted using the Lippert equation and one-photon experiments. These results support models that suggest excited-state absorption is the primary cause of sensitivity of the "effective" two-photon cross-section to the pulse-width and the local molecular environment.