Disorder-induced symmetry breaking is studied in a model dendrimer that consists of three arms arranged with C 3 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 symmetrybroken 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 symmetrybroken structures and demonstrates that diagonal disorder is the dominant form of disorder in the dendrimer. A thorough derivation of the electroabsorption spectrum for C 3 -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 / 2 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.
Disorder plays an important role in the photophysics of conjugated polymers such as poly(para-phenylene vinylene) (PPV). The dipole moments measured by electroabsorption spectroscopy for a centrosymmetric system such as PPV provide a direct quantitative measure of disorder-induced symmetry breaking. Although inner-sphere (structural) disorder is present, outer-sphere (environmental) disorder dominates the symmetry breaking in PPV. This paper develops and compares six models of outer-sphere disorder that differ in their representation of the electrostatic environment of PPV in glassy solvents. The most detailed model is an all-atom description of the solvent glass and this model forms the basis for comparison of the less detailed models. Four models are constructed in which multipoles are placed at points on a lattice. These lattice models differ in the degree to which they include correlation between the lattice spacings and the orientations of the multipoles. A simple model that assigns random Gaussian-distributed electrostatic potentials to each atom in the PPV molecule is also considered. Comparison of electronic structure calculations of PPV in these electrostatic environments using the all-atom model as a benchmark reveals that dipole and quadrupole lattices provide reasonable models of organic glassy solvents. Including orientational correlation among the solvent molecules decreases the effects of outer-sphere disorder, whereas including correlation in the lattice spacings increases the effects. Both the dipole and quadrupole moments of the solvent molecules can have significant effects on the symmetry breaking and these effects are additive. This additivity provides a convenient means for predicting the effects of various glassy solvents based on their multipole moments. The results presented here suggest that electrostatic disorder can account for the observed symmetry breaking in organic glasses. Furthermore, the lattice models are in general agreement with the dipole and quadrupole lattice models used to explain the Poole-Frenkel behavior in charge transport through disordered organic materials.
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