Influence of molecular environment on single photon behavior of heterocyclic NLO chromophores and its implications to two-photon behavior

Over the past several years, organic molecules exhibiting significant two-photon absorbance have been of intense interest for a wide variety of applications including high speed communications, data storage, imaging, and optical limiting. However, it has been commonly observed that the local molecular environment can significantly affect the linear and nonlinear optical properties of the chromophores. In an effort to examine these effects, the influence of the solvent environment on the linear absorbance and photoluminescence of a series of donor-acceptor heterocyclic chromophores was examined. The Stoke's shift associated with one-photon absorbance and photoluminescence was observed to increase with increasing solvent polarity. This behavior is adequately described by the Lippert equation and is related to relaxation of the solvent molecules around an excited molecule. Additionally, it was observed that the spectral shape, as well as the solvent dependence, of two-photon and one-photon pumped photoluminescence were similar, thus indicating that the longest-lived luminescing excited state is independent of the method of excitation. These results have direct implications to two-photon applications which rely on up-converted fluorescence. They also yield insight into the structure-property relationships governing their linear and multi-photon behavior including the potential contributions to the effective two-photon cross-section from excited state absorbance.