We report the observation and analysis of the high resolution fluorescence excitation spectrum of tolane (or diphenylacetylene, DPA) in a molecular beam. This analysis shows that the first one-photon-allowed state in DPA has B-1(1u) symmetry, a consequence of exciton-like coupling between the two La-1 states of the connected phenylacetylene, (PA) units. The resulting long-axis polarization of the B-1(1u)-(1)A(g) transition of each linear segment is responsible for the diverse optical properties of PA dendrimers.