Molecular environment effects on two-photon-absorbing heterocyclic chromophores

Over the past several years, organic molecules exhibiting significant two-p hoton absorbance and subsequent up-converted fluorescence have been of inte nse interest for a wide variety of applications including data storage, ima ging, and optical limiting. However, the establishment of structure-propert y relationships for some asymmetric molecules has been hindered by the sens itivity of these nonlinear optical properties to the local molecular enviro nment and to the pulse width of the incident radiation. To understand the i nfluence of the local molecular environment on the excited states of these two-photon-absorbing molecules, the linear absorbance, the single-photon-ex cited 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 de scribed by mean field interactions with solvent molecules as given by the L ippert equation. Because the same stabilization dominates the two-photon-in duced longest-lived excited state, the influence of the local molecular env ironment on the two-photon luminescence can be predicted using the Lippert equation and one-photon experiments. These results support models that sugg est excited-state absorption is the primary cause of sensitivity of the "ef fective" two-photon cross-section to the pulse-width and the local molecula r environment.