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

Molecular Packing in Unsubstituted Semiconducting Phenylenevinylene Oligomer and Polymer

p-Bis(p-styrylstyryl)benzene, C38H30, a five-ring phenylene−vinylene model compound for unsubstituted PPV, has been obtained in single-crystal form and the crystal structure has been resolved. The arrangement of the molecules is of the herringbone-type, and its characteristic dimensions correspond remarkably well to those of the structure proposed for PPV in 1986. This packing mode is entirely different from what has been found for substituted oligo(phenylenevinylene)s but similar to that observed in other types of conjugated oligomers without lateral substituents. Thin films of the unsubstituted oligomer show electroluminescence at a lower onset voltage and blue-shifted relative to substituted oligo(phenylenevinylene)s

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

Molecular Packing in Unsubstituted Semiconducting Phenylenevinylene Oligomer and Polymer

p-Bis(p-styrylstyryl)benzene, C38H30, a five-ring phenylene−vinylene model compound for unsubstituted PPV, has been obtained in single-crystal form and the crystal structure has been resolved. The arrangement of the molecules is of the herringbone-type, and its characteristic dimensions correspond remarkably well to those of the structure proposed for PPV in 1986. This packing mode is entirely different from what has been found for substituted oligo(phenylenevinylene)s but similar to that observed in other types of conjugated oligomers without lateral substituents. Thin films of the unsubstituted oligomer show electroluminescence at a lower onset voltage and blue-shifted relative to substituted oligo(phenylenevinylene)s

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

Detection of Ultralow Concentrations of Non-emissive Conjugated Polymer Aggregates via Fluorescence Correlation Spectroscopy

The aggregation of conjugated polymers in common organic solvents is investigated using fluorescence correlation spectroscopy (FCS), burst analysis, and microscopy. Poly­(3-hexylthiophene) and poly­[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] are both shown to form weakly bonded non-emissive aggregates in toluene that persist even at picomolar concentrations. These aggregates decrease the bulk emission intensity in solution but do not affect the fluorescence spectra or lifetimes, consistent with a static quenching mechanism. Passing the solutions through a syringe filter causes an increase in the number of emitters as measured by FCS, indicating that this process dissociates the aggregates. Films cast from solutions that have been filtered are more uniform and significantly more emissive than those made from unfiltered solutions. These results show that FCS is a highly sensitive probe of non-emissive aggregates in solution that have a deleterious effect on the emission properties and overall quality of spin-cast thin films, even at sub-nanomolar concentrations

Electric-Field-Induced Fluorescence Quenching in Polyfluorene, Ladder-Type Polymers, and MEH-PPV: Evidence for Field Effects on Internal Conversion Rates in the Low Concentration Limit

Electric field-induced fluorescence quenching has been measured for a series of conjugated polymers with applications in organic light-emitting diodes. Electrofluorescence measurements on isolated chains in a glassy matrix at 77 K show that the quenching efficiency for poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) is an order of magnitude larger than that for either a ladder-type polymer (MeLPPP) or polyfluorene (PFH). This effect is explained in terms of the relatively high probability of field-enhanced internal conversion deactivation in MEH-PPV relative to either MeLPPP or PFH. These data, obtained under dilute sample conditions such that chain−chain interactions are minimal, are contrasted with the much higher quenching efficiencies observed in the corresponding polymer films, and several explanations for the differences are considered. In addition, the values of the change in dipole moment and change in polarizability on excitation (|Δμ⃗| and tr(Δα⃡), respectively) are reported, and trends in these values as a function of molecular structure and chain length are discussed

Visualizing Core–Shell Structure in Substituted PPV Oligomer Aggregates Using Fluorescence Lifetime Imaging Microscopy (FLIM)

The use of fluorescence lifetime imaging microscopy (FLIM) is introduced as a means of directly imaging core–shell structured organic aggregates through the gradient observed in their emission wavelength and lifetime as a function of distance from their center to their exterior. The aggregates studied consist of alkoxy-substituted oligomeric PPVs (OPPVs) 7 and 13 rings in length that are formed via reprecipitation in a mixture of methyl tetrahydrofuran (MeTHF) and methanol (MeOH). Prior bulk fluorescence spectroscopy and wavelength-dependent lifetime measurements on these aggregates (J. Phys. Chem. C 2009, 113, 18851–18862) showed that their properties are consistent with the presence of two types of emitters, one that behaves identically to the monomer with the other having the longer emission wavelengths and shorter lifetimes characteristic of aggregated chains. These two emitters were postulated to be the components of “core-shell”-like structures in which the core consists of aggregated chains and the shell consists of monomer-like chains that are in direct contact with the surrounding solvent. The FLIM images of individual aggregates presented here are consistent with this model which had been developed on the basis of measurements on bulk samples. The uniformity of the emission spectra of these aggregates is also demonstrated using single-aggregate dispersed emission spectroscopy

Aggregation Effects on the Emission Spectra and Dynamics of Model Oligomers of MEH-PPV

The effects of aggregate formation on the photophysical properties of oligomers of MEH-PPV were studied in bulk solution to better understand the effects of aggregation on the emission properties of the polymer. Nanoaggregates of oligomers from 3 to 17 repeat units in length were formed using a solvent reprecipitation method. The spectra are not readily modeled using the classical dipole−dipole coupling picture of interchain interactions. A strong dependence of the photophysics on the oligomer chain length is also observed. Short-chain oligomers produce nanoaggregates with absorption and emission spectra essentially identical to those of the monomer. Long-chain oligomers form aggregates having more strongly perturbed absorption and fluorescence spectra and decreased emission yields. In these aggregates, the size of the 0−0 band relative to that of the vibronic replicates is a sensitive function of aggregate size and solvent precipitation conditions. Their fluorescence lifetimes are also strongly wavelength dependent. These trends are explained in terms of a core−shell model that postulates the existence of “single-chain-like” and “aggregate-like” emitters within a single aggregate