A tight-binding model of phenylene molecules with meta-connections – implications for phenylacetylene dendrimers Original Research Article

A simple tight-binding model can describe the most essential features of the optical properties of phenylacetylene dendrimers. We investigate two kinds of chain models which are obtained by simplifying two series of dendrimers and are composed of linear segment of phenyl rings connected to each other at meta-positions. These chain models exhibit different size dependence of the gap between HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital), corresponding to that of the observed optical gap in dendrimers. In the chain model in which the gap decreases with molecular size, the HOMO and its consecutive orbitals are localized within each segment. In the other kind of chain in which the decrease of the gap is much more moderate, the orbitals form a fairly narrow band instead of the localization, although each orbital seems delocalized in appearance. These features are ascribed to the small absolute values of coefficients of the HOMOs at meta-positions in the constructing segments.