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

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

Spectroscopic and MD Study of Dynamic and Structural Heterogeneities in Ionic Liquids

The structure of ionic liquids (ILs) surrounding solute dyes and the effects of solvent structure on solute diffusion have been investigated using molecular dynamics (MD) and the experimental tools of confocal and fluorescence correlation spectroscopies. Although confocal microscopy and simulations show that the local environment around solutes in ILs is heterogeneous and that the structural heterogeneity is rather long-lived, the local polarity and the diffusion constant were found to be uncorrelated. Moreover, the complex diffusion observed experimentally is not due to the structural heterogeneity of the IL but rather due to the dynamic heterogeneity arising from the viscous glassy nature of the IL environment. MD simulations show that the degree of dynamic heterogeneity depends on the first nonvanishing electric multipole moment of the solute. The dynamics of a cationic solute are the least heterogeneous, whereas those of a solute without an electric multipole moment are the most heterogeneous. This indicates that the length scale over which the solute–solvent interactions occur, and thus the number of solvent degrees of freedom that couple to the solute, are the key factors governing the dynamic heterogeneity of the solute

Electric Field Effects on Internal Conversion: An Alternative Mechanism for Field-Induced Fluorescence Quenching of MEH-PPV and Its Oligomers in the Low Concentration Limit

In a previously published study (J. Phys. Chem. B 2006, 110, 7732−7742), we reported field-induced fluorescence quenching in both poly[2-methoxy,5-(2‘-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and several model oligomers in solvent glass matrices at high dilution (<0.1% by weight). The observed quenching is not readily explained by field-induced exciton dissociation or by the presence of free charges, two mechanisms that have been invoked to explain this phenomenon by previous authors. A model is developed here that ascribes the observed fluorescence quench in dilute samples to an energetic shift of the relaxed excited state caused by the electric field resulting in increased nonradiative relaxation. To determine whether the relevant nonradiative pathway is intersystem crossing or internal conversion, analytical expressions are derived for each of these two mechanisms. Only the expression derived for the Stark effect on the rate of internal conversion quantitatively predicts the magnitude of quench observed in MEH-PPV and in the oligomeric species

Modeling Field-Induced Quenching in Poly(<i>p</i>‑phenylene vinylene) Polymers and Oligomers

Field-induced fluorescence quenching of poly­(<i>p</i>-phenylene vinylene) (PPV) oligomers due to nonradiative relaxation through free electron–hole pair (FEHP) states is modeled using singles configuration interaction computations with the intermediate neglect of differential overlap Hamiltonian. The computations find FEHP states with energies that drop linearly with applied field and undergo avoided crossings with the fluorescent state. The coupling between the FEHP and fluorescent state, computed for multiple FEHP states on a variety of oligomer lengths, is found to depend primarily on the field strength required for the state to cross the fluorescent state. The rate of decay to these dark FEHP states is then calculated from Marcus theory, which is modified to take into account dielectric in addition to other bulk measurement considerations. The results predict that individual molecules go from being emissive to fully quenched over a small range of applied field strengths. Phenomenological introduction of inhomogeneous broadening for the energies of the FEHP states leads to a more gradual dependence on applied field. The fluorescence quenching mechanism considered here is found to be important for applied fields above about 1 MV cm^–1, which is similar in magnitude to those present in light-emitting diodes

Exciton–Exciton Annihilation as a Probe of Interchain Interactions in PPV–Oligomer Aggregates

One measure of exciton mobility in an aggregate is the efficiency of exciton–exciton annihilation (EEA). Both exciton mobilities and EEA are enhanced for aggregate morphologies in which the distances between chromophores and their relative orientations are favorable for Förster energy transfer. Here this principle is applied to gauge the strength of interchain interactions in aggregates of two substituted PPV oligomers of 7 (OPPV7) and 13 (OPPV13) phenylene rings. These are models of the semiconducting conjugated polymer MEH–PPV. The aggregates were formed by adding a poor solvent (methanol or water) to the oligomers dissolved in a good solvent. Aggregates formed from the longer-chain oligomer and/or by addition of the more polar solvent showed the largest contribution of EEA in their emission decay dynamics. This was found to correlate with the degree to which the steady-state emission spectrum of the monomer is altered by aggregation. The wavelength dependence of the EEA signal was also shown to be useful in differentiating emission features due to monomeric and aggregated chains when their spectra overlap significantly

The Electronic Properties of a Model Active Site for Blue Copper Proteins as Probed by Stark Spectroscopy

A complex that mimics many of the properties of the blue copper protein center that was synthesized by Holland and Tolman is studied using Stark spectroscopy to determine the values of two electronic properties, the change in the dipole moments (| |) and the average change in the polarizability (〈Δα〉) for excitation into the ligand-to-metal charge transfer (LMCT) band. Measurements at 77 K in methylcyclohexane yield a value for | | of between 1.3 and 1.9 D for the lowest energy LMCT band, which has been assigned as a predominantly thiolate (Sπ) → Cu(dx2-y2) transition. The value of | | is remarkably similar to that we have measured earlier for the type-1 blue copper protein azurin in a glycerol−water glass. The sensitivity of Stark spectroscopy to charge-transfer transitions is utilized to identify and, in some cases, to confirm the assignment previously made via magnetic circular dichroism (MCD)2 of several higher-energy charge-transfer bands. Values for the electron-transfer matrix element (Hab) and the effective charge-transfer distance (Rab) derived from our measurements on this complex are also reported. These parameters are likewise found to be quite similar to those previously determined for azurin

Electroabsorption of Dimers Containing MM (M = Mo, W) Quadruply Bonded Units: Insights into the Electronic Structure of Neutral Coupled Redox Centers and Their Relationship with Mixed Valence Ions

The electroabsorption spectra for the metal-to-ligand charge transfer transition in complexes containing oxalate and terephthalate bridged MM quadruply bonded units, [(MM)(pivalate)3]2-μ2-BR, where M = Mo or W and BR = oxalate or terephthalate, are reported. The measured magnitude of the change in dipole moment (|Δμ⃗|) and the change in polarizability (Δα) that accompany this electronic transition are found to be small and not to follow the behavior expected on the basis of the two-state model. In addition, the trend in the value of Δα for the neutral states is mirrored by the trend in the degree of electronic coupling (HAB) for the strongly coupled mixed valence states formed by the same complexes in their singly oxidized states

The Effects of Structural and Microenvironmental Disorder on the Electronic Properties of Poly[2-methoxy,5-(2‘-ethyl-hexoxy)-1,4-phenylene vinylene] (MEH−PPV) and Related Oligomers

In this study, electroabsorption (Stark) spectroscopy is used to determine the trace of the change in polarizability (tr ) and the absolute value of the change in dipole moment (|Δμ|) of the electroluminescent polymer poly[2-methoxy,5-(2‘-ethyl-hexoxy)-1,4-phenylene vinylene] (MEH−PPV) and several model oligomers in solvent glass matrixes. We find a value of tr of ∼2000 Å3 for the polymer and for a 9-ring substituted oligomer in both toluene and 2-methyl tetrahydrofuran matrixes at 77 K with smaller values being obtained for 3- and 5-ring unsubstituted oligomers. Although gas-phase calculations of tr using INDO/SCI yield values that are about a factor of 8 smaller than the experiment, excellent agreement is obtained when the effects of solid-state dielectric screening are included. Screening increases tr by bringing the energy gap between the 1Bu and mAg states into agreement with solid-state measurements. Substantial values of |Δμ| are observed experimentally both for the polymer and for the oligomers (6−11 D). Because in a planar (C2h) geometry the oligomer and polymer are centrosymmetric, the observed |Δμ| is an indication of disorder-induced symmetry breaking in the material. Calculations indicate that disorder in the ground-state geometry of the polymer (inner-sphere disorder) can account for nearly half of the observed |Δμ|. Disorder in the glassy environment (outer-sphere disorder) leads to a nonuniform electrostatic environment, and calculations show that this is a substantial contributor, accounting for the remainder of the observed |Δμ|

Chain Length and Substituent Effects on the Formation of Excimer-Like States in Nanoaggregates of CN-PPV Model Oligomers

The effects of aggregate formation on the photophysical properties of alkoxy and cyano-substituted polyphenylene phenylene vinylene oligomers (CN-PPVs) were studied in bulk solution to better understand the consequences of aggregation for the emission properties of the polymer. Nanoaggregates of oligomers from 5 to 13 repeat units in length were formed using a solvent reprecipitation method. The propensity for these aggregates to exhibit excimer-like emission in solution was found to be a strong function of oligomer chain length and the solvents used in the reprecipitation process. Short-chain oligomers produced nanoaggregates with absorption and fluorescence spectra and emission lifetimes essentially identical to those of the monomer. The aggregates of long-chain oligomers have broad and red-shifted emission spectra and relatively long emission lifetimes, both of which are characteristic of excimer states. However their absorption spectra are also perturbed suggesting that the oligomer chains in these aggregates interact strongly in their electronic ground states as well. For intermediate chain lengths, dual monomer-like (green) and excimer-like (red) emission is observed. Single aggregate dispersed emission spectra from aggregates deposited onto glass coverslips demonstrate that, in the absence of solvent, the predominant emitters are monomer-like rather than excimer-like. Moreover, the monomer-like emitters are found to be far more photostable than the analogous non-CN substituted aggregates, whereas the photostability of the excimer-like emitters is exceptionally poor under the illumination conditions used for microscopy. Comparisons between the properties of these nanoaggregates and the corresponding CN-substituted polymer are drawn