Fluorescence quantum yields of conjugated polymer films are systematically lower than their counterparts in dilute solution. Films also exhibit a long “temporal tail” in their fluorescence decay dynamics not present in solution. We study the spectroscopy, excitation wavelength dependence, temperature dependence, and electric field quenching of the temporal tail of the photoluminescence in MEH-PPV on a nanosecond time scale to elucidate the relationship between those observations. We conclude that the tail represents emission from H-like aggregated regions in the polymer. Using a simple model of the photophysics, we estimate the formation yield of the aggregates responsible for the tail emission to be <20% so that they cannot account for the large reduction in fluorescence observed in densely packed films relative to that in solution.

Enhancement of solar photovoltaics by integrating cells with metal nanoparticles is of potential interest to reduce the usage of semiconductor material. Appropriately shaped metal particles to optimize spectral response have inadequate thermal stability to withstand standard semiconductor processing. We synthesize silica-capped gold nanorods that maintain nonspherical shapes to over 600 °C and show that they can increase photoconductivity in thin films of amorphous silicon by much more than a factor of 2 across the entire visible spectrum. We also report mechanistic studies of this phenomenon that show that much of this effect is primarily due to strong near-field light concentration rather than scattering as has often been assumed.

A variety of rapid biomolecular assays under development rely on the selective adsorption of single-stranded DNA onto unfunctionalized, negatively charged, citrate-stabilized gold nanoparticles. We investigate the adsorption mechanism with a study of the binding kinetics and find strong evidence for the dominance of hydrophobic effects including linear compensation between the activation energy and the natural log of the Arrhenius prefactor and the correlation of the adsorption rate in the presence of various salts with the Hofmeister series. These results explain the selectivity for single-stranded over double-stranded DNA adsorption and contradict previous work citing an electrostatic DLVO-like mechanism. Our understanding should facilitate improvements to the selective-adsorption-based assays and, more generally, contribute to the understanding of interactions between like-charged species in aqueous solution.

Dramatic localization of optical fields by interactions with surface plasmons on nanotextured metal surfaces allows us to record Raman spectra of individual chromophores on single chains of a high-molecular-weight model conjugated polymer, poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene. The data are broadly consistent with two configurations of the chromophore that are characteristic of packed and loose conformations of the backbone. Within these types, fluctuations of the chromophore planarity are observed, and we show how they can be analyzed by studying the temporal correlation of successive spectra. Chromophores exhibit some configurational memory on the time scale of minutes. We also observe substantial spectral fluctuations and associate these mostly with thermally induced motions of the chromophore in the “hot spot”. However, we also provide instances of data representing irreversible photochemistry as well as charging and discharging of the chromophore. Finally, we show that single chromophores embedded in the polymer reorganize in response to poor solvents. Keywords: conformational fluctuations; conjugated polymer vibrational spectroscopy; nanotextured silver; solvation

Under certain conditions, single-stranded DNA adsorbs to negatively charged gold nanoparticles in a colloid whereas double-stranded DNA does not. We present evidence that this phenomenon can be explained by the difference in their electrostatic properties that in turn reflects conformational differences. The ability to discriminate the hybridization state of DNA on the basis of adsorption behavior can be utilized to design simple colorimetric and fluorimetric assays that take advantage of plasmon resonance in the gold nanoparticles. We present examples where we detect specific target sequences in oligonucleotides and in genomic DNA. Because conformational changes in special DNA sequences can also be induced by analytes such as potassium, we report a potassium ion detection scheme based on the same principle.

We present fluorescence spectra, quantum yields and structural data from 1H NMR studies on solutions of a di-alkoxy substituted pentamer of p-phenylenevinylene (5PV) and the corresponding polymer poly-(p-phenylenevinylene) (PPV). We vary the solvent quality continuously using miscible good and poor solvents to study the effects of polymer aggregation as is relevant to properties of films and of the concentrated solutions from which films are formed. We observe a large drop in quantum yield with reduced solvent quality that we attribute to formation of interchain excitations with negligible luminescence when the polymer aggregates. Concomitant with the drop in fluorescence, we observe a red shift in the spectra. The NMR data suggest the red shift is due to an increase in the steric hindrance of the backbone motion that leads to an increase in effective conjugation length. In each material, our data can be described as a linear combination of only two species with distinct spectroscopy and quantum yield, an isolated and aggregated form of the polymer. Studies of the properties of mixtures of the pentamer and polymer demonstrate conclusively that these species mix and that the spectroscopic changes are due to aggregation and not conformational changes of single chains. They also provide insight into recent results from electroluminescent devices based on analogous conjugated polymer blends.

We find that single- and double-stranded oligonucleotides have different propensities to adsorb on gold nanoparticles in colloidal solution. We use this observation to design a hybridization assay based on color changes associated with gold aggregation. Because the underlying adsorption mechanism is electrostatic, no covalent functionalization of the gold, the probe, or the target DNA is required. Hybridization conditions can be optimized because it is completely separated from the detection step. The assay is complete within 5 min, and <100 femtomoles of target produces color changes observable without instrumentation. Single-base-pair mismatches are easily detected.

We report transient photoluminescence (PL) measurements in a conjugated polymer over more than six decades in time and intensity. Long-lived luminescence by the singlet exciton arising from charge photogeneration and subsequent recombination represents a significant fraction of the total emission. Comparison between neat films and dilute films in polystyrene illustrates the importance of interchain charge separation in the photophysics of these materials. The data permit quantitative estimates of polaron pair and excimer formation.