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

Abstract

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.

Charge Instability in Quadrupolar Chromophores: Symmetry Breaking and Solvatochromism

“…The coupling between electronic and vibrational degrees of freedom indeed governs structural instabilities in a variety of extended systems with delocalized electrons and/or extended charge resonance, including polymers, charge transfer salts, and mixed valence chains. 71 Instabilities in finite molecular systems are also an active research field, 32,40,72,73 where, however, the subtle distinction between static and dynamic effects makes the problem much more complex than in extended systems. 32,55 Here we focus on the spectroscopic consequences of brokensymmetry states in quadrupolar organic dyes for NLO applications.…”

“…16,40,43,44,46,47 Due to their highly symmetric structure, all these systems have no permanent dipole moment and of course vertical (unrelaxed) states are non-dipolar as well. However for many of these chromophores, experimental data suggest the existence of polar excited states.…”

Simultaneous Control of Emission Localization and Two-Photon Absorption Efficiency in Dissymmetrical Chromophores

“…Excitation localization in branched systems is not uncommon and has been reported in various branched systems built from a triphenylamine core 40,58 or different cores. 42,56,[59][60] In the case of octupolar derivatives, excitation localization on single branches occurs after excitation prior to emission for any (even slightly polar) solvent, because of the unconditional nature of symmetry breaking for these chromophores. 55 This is the reason why we observe that the emission solvatochromism of the octupolar-type derivatives O1-O3 follows the Lippert-Mataga relationship with a linear dependency of the Stokes shift as a function of a polarity/polarizability parameter [72][73] (Fig.…”

Electronic States of Tris(bipyridine) Ruthenium(II) Complexes in Neat Solid Films Investigated by Electroabsorption Spectroscopy

“…The next important example is an orbitally degenerate (by symmetry) electronic state as in s-triazine where the first-order (linear) Stark effect should be considered [ 33 ]. The electroabsorption behavior of orbitally degenerate E states of molecular systems of C 3 symmetry was thoroughly analyzed in the literature [ 34 , 35 , 36 ]. In short, when an excited state is degenerate, the energy level of this eigenstate is split in the electric field into two sublevels and, consequently, the absorption spectrum is also split into two sub-bands.…”

“…An important message is that an excited degenerate state without a permanent dipole moment can give rise to the EA signal of the second-derivative lineshape with a scaling factor dependent on the dipole coupling between the respective components of the degenerate state (μ 12 ). The detailed quantitative analysis requires taking into account the degree of electronic delocalization of the trimer molecular systems considered [ 34 , 35 ].…”