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.

Conjugated systems are frequently studied in their nanoaggregate form to probe the effects of solvent and of film formation on their spectral and dynamical properties. This article focuses on the emission spectra and dynamics of nanoaggregates of alkoxy-substituted PPV oligomers with the goal of interpreting the vibronic emission envelopes observed in these systems (J. Phys. Chem. C2009, 113, 18851–18862). The aggregates are formed by adding a nonsolvent such as methanol (MeOH) or water to a solution of the oligomers in a good solvent such as methyl tetrahydrofuran (MeTHF) or tetrahydrofuran (THF). The emission spectra of aggregates formed using either of these combinations exhibit a vibronic pattern in which the ratio of the intensity of highest-energy band to that of the lower energy peaks depends strongly on the ratio of good to poor solvent. In aggregates formed from MeTHF:MeOH, this was shown to be due to the presence of both aggregate-like and monomer-like emitters forming a “core” and surrounding “shell”-like structure, respectively, within a single aggregate (J. Phys. Chem. C2011, 115, 15607–15616). In support of this model, the monomer-like emission is shown here to be significantly decreased by changing the solvent pair to the more polar THF:water. This suggests that nanoaggregates formed in THF:water contain a much smaller proportion of monomer-like chains than those formed in MeTHF/MeOH, as would be expected from using a more highly polar nonsolvent. Results from bulk steady-state and time-resolved emission measurements as well as fluorescence lifetime imaging microscopy (FLIM) of the aggregates are shown to be consistent with this interpretation.

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. C2009, 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.

The efficiency of the glutathione monolayer-protected gold nanocluster (NC) Au25 (1.2 nm metal core diameter (d)) in quenching the emission of dyes intercalated into DNA is compared with that of 2 and 4 nm gold nanoparticles (NPs). In all cases, the DNA/dye moieties and the gold particles are not covalently attached but rather form noncovalent ground state complexes. Under these conditions, steady-state measurements reveal that the quenching efficiency of Au25 is a factor of 10 lower than that of plasmonic 4 nm gold NPs but comparable to that of 2 nm particles, which do not show a distinct plasmon band. Nonetheless, significant emission quenching is observed even at very low (nanomolar) concentrations of Au25. The quenching efficiency of the 4 nm NPs is significantly higher for dyes emitting near the wavelength of the plasmon peak, whereas that of the 2 nm gold NPs is well-described by the nanosurface energy transfer (NSET) model proposed by the Strouse group (J. Am. Chem. Soc.2005, 127, 3115). Interestingly, for Au25, the maximum quenching efficiency occurs for dyes emitting in the same wavelength range as that of the 2 and 4 nm NPs (490–560 nm), where it shows no discrete absorption features, rather than for wavelengths coincident with its HOMO–LUMO, intraband, or interband transitions. The fluorescence quenching properties of Au25 NCs are therefore found to be distinct from those of larger NCs and NPs but do not appear to conform to theoretical predictions advanced thus far.

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.

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.

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.

Electroabsorption (Stark spectroscopy) is used to measure the electronic properties of o-hydroxybenzaldehyde (oHBA) and o-hydroxyacetophenone (oHAP), two molecules that undergo excited-state intramolecular proton transfer (ESIPT) in a non-interacting organic glass matrix. We report the change in dipole moment, , and the average change in polarizability, , for both oHBA and oHAP as well as for a molecular analog of oHAP, o-methoxyacetophenone (oMAP), in which proton transfer is blocked by methyl substitution. The experimental results for the two ESIPT molecules are found to compare well to the results of ab initio calculations. In contrast, the measured values of and for these systems compare poorly to the INDO/SCI results. Preparation for ESIPT in the Franck–Condon region of the potential energy surface in oHAP and oHBA is apparent from the calculated atomic charge distributions as an increase of the intramolecular hydrogen bond strength and a decrease in the aromaticity of the phenyl ring.

The polaron band at ∼1.4 eV of d,l-camphorsulfonic acid-doped polyaniline ((±)-HCSA-PANI) in a poly(methyl methacrylate) (PMMA) matrix was studied using electroabsorption (Stark-effect) spectroscopy at 298 K. A very small change in dipole moment on excitation () in the order of 1.4±0.2 D was measured as well as an average change in polarizability () of only −1.3±0.4 Å3. The electroabsorption signal of (±)-HCSA-PANI in this region showed evidence for heterogeneity in the absorption band.