Effects of Disorder-Induced Symmetry Breaking on the Electroabsorption Properties of a Model Dendrimer

Disorder-induced symmetry breaking is studied in a model dendrimer that consists of three arms arranged with C3 symmetry. Electroabsorption spectroscopy measurements in the accompanying paper (Bangal, P. R.; Lam, D. M. K.; Peteanu, L. A.; Van der Auweraer, M. J. Phys. Chem. B 2004, 108, 16834) show that the dipole moment change of the dendrimer is similar to that of the monomer, suggesting a completely symmetry-broken dendrimer with the excitation localized on one arm of the structure. In this work, we model the symmetry breaking of the dendrimer as a function of its structural disorder. Several collections of disordered dendrimers are created. The excited states of the dendrimer and of the three arms that make up the dendrimer are calculated using the intermediate neglect of differential overlap/singles configuration interaction (INDO/SCI) approach. These data are used to verify and parametrize exciton models that relate the properties of the dendrimer to those of the arms. A binning-and-averaging procedure is introduced so that the calculated electroabsorption properties of the dendrimer can be studied as functions of the energetic disorder in the structure. The excellent agreement between the INDO/SCI method and the exciton models validates the latter models for symmetry-broken structures and demonstrates that diagonal disorder is the dominant form of disorder in the dendrimer. A thorough derivation of the electroabsorption spectrum for C3-symmetric molecules indicates that the dipole moment change ratio between the dendrimer and the arm is a sensitive measure of disorder and symmetry breaking. This ratio is 1/ in the absence of disorder, 1/2 at intermediate disorder, and 1 at large disorder. These results indicate that the experimental dendrimer sample is symmetry-broken.