A tight-binding model of phenylene molecules with meta-connections - implications for phenylacetylene dendrimers

A simple tight-binding model can describe the most essential features of th e optical properties of phenylacetylene dendrimers. We investigate two kind s of chain models which are obtained by simplifying two series of dendrimer s and are composed of linear segment of phenyl rings connected to each othe r at meta-positions. These chain models exhibit different size dependence o f the gap between HOMO (highest occupied molecular orbital) and LUMO (lowes t unoccupied molecular orbital), corresponding to that of the observed opti cal gap in dendrimers. In the chain model in which the gap decreases with m olecular size, the HOMO and its consecutive orbitals are localized within e ach 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 H OMOs at meta-positions in the constructing segments. (C) 1999 Elsevier Scie nce B.V. All rights reserved.