Electronic structure of fully conjugated dendritic oligomers of beta,beta-dibromo-4-ethynyl styrene

Quantum-mechanical calculations of fully conjugated dendritic oligomers car ried out at B3LYP/3-21G//HF/3-21G (d) and B3LYP/3-21G//PM3 levels of theory showed that loose dendritic architecture of beta,beta-dibromo-4-ethynyl st yrene oligomers contributes little to the instability and conjugation disru ption compared to 1 --> 2 branched polyacetylene, while Br terminal atoms i n dendrimers strongly affect the electronic density distribution in studied molecules. On the one hand the bulky bromine atoms decrease the conjugatio n in Br-terminated dendrimers caused by steric hindrances, on the other han d, highly polarisable bromine atoms reduced significantly adiabatic ionisat ion potentials (IPa) to be up to 1.5 eV lower than corresponding vertical p otentials (IPv). Another phenomenon contributing to the reducing of IPa's o f all dendrimers is the flattening of molecular geometry accompanying the i onisation thus allowing better delocalisation of positive charge over the c onjugated system while all aromatic ring except the very outer layer lost t heir aromaticity becoming essentially quinone by nature. (C) 1999 Elsevier Science B.V. All rights reserved.