Organic compounds that undergo strong nonlinear, multiphoton absorption have been gaining greater interest, mainly in the developing fields of multiphoton fluorescence imaging, optical data storage, 3-D microfabrication, and photodynamic therapy. Systematic studies have shown that conjugated organic molecules with large delocalized π electron systems show very large nonlinear optical effects. Two-photon absorbing chromophores have also been incorporated into dendrimers to increase two-photon absorptivity. A cooperative enhancement of two-photon absorption (2PA) has been observed, such as in the linkage of branched chromophores through a common amine group and chromophore-metal complexes. This enhancement may be related to extensive two-dimensional π-delocalization in these molecules. Herein, we describe the synthesis, structural characterization and photophysical study of a series of compounds (model, oligomer, and polymer) with symmetric molecular structure of the D-π-D motif and branched D-π-D dendrimeric structures based on substituted fluorene derivatives. Femtosecond 2PA cross sections were very large for some derivatives (over 10,000 GM) and often exhibited substantial solvent effects.