  The excited state electronic structure of $\pi$ conjugated
phenylene-acetylene oligomers is calculated using time-dependent density
functional theory (TD-DFT) approaches. The theoretical fluorescence spectra are
analyzed in terms of Frank-Condon active nuclear normal modes and shown to
compare well with experiment. Theoretical and experimental results for the
optical absorption and emission spectra of these molecules indicate that the
conjugation length can be significantly reduced by conformational rotations
about the triple-bonded carbon links. This has serious implications on the
electronic functionalities of polyphenylene-acetylene based molecular wires and
their possible use as charge/energy conduits in nano-assemblies.
