Identification of the light-absorbing states in tolane with potential relevance to self-similar phenylacetylene dendrimers

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 1B1u symmetry, a consequence of exciton-like coupling between the two 1La states of the connected phenylacetylene (PA) units. The resulting long-axis polarization of the 1B1u–1Ag transition of each linear segment is responsible for the diverse optical properties of PA dendrimers.