Syntheses, characterization, and photophysics studies of photoactive chromophore 2-naphthyl-labeled [n]-ladderanes

The syntheses and characterization of a series of saturated ladderanes 3, oligomers of 1,3-cyclobutadiene-1,2-dicarboxylate diesters with pendant 2-naphthyl groups, are described. Absorption and fluorescence spectra provide evidence for a ground state through-space interchromophore interaction that becomes stronger with an increasing number of pendant aryl groups. A new fluorescence band (lambda(max) = 303 nm) observed in the tetramer (3c) and pentamer (3d) as blue-shifted from the monomer (lambda(max) = 335 nm) was assigned as emission from an imperfectly stacked chromophore array. Nonexponential fluorescence decays of all four oligomers, obtained by single photon counting, are accommodated well by conformations attained from a molecular mechanics geometry optimization. In the solid state, the monomeric precursor of the family, tricarbonyl[di-2-naphthylmethyl 1,2,3,4-eta-1,3-cyclobutadiene-1,2-dicarboxylate]iron (2), exists as highly stable channel solvate under ambient conditions.