Oligomeric thiophenes are commonly used components in organic electronics and solar cells. These molecules stack and/or aggregate readily under the processing conditions used to form thin films for these applications, significantly altering their optical and charge-transport properties. To determine how these effects depend on the substitution pattern of the thiophene main chains, nanoaggregates of three sexithiophene oligomers having different alkyl substitution patterns were formed using solvent-poisoning techniques and studied using steady-state and time-resolved emission spectroscopy. The results indicate the substantial role played by the side-chain substituents in determining the emissive properties of these species. Both the measured spectral changes and their dependence on substitution are well-modeled by combined quantum chemistry and molecular dynamics simulations. The simulations connect the side-chain-induced disorder, which determines the favorable chain-packing configurations within the aggregates, with their measured electronic spectra.
A synthetic
route to mixed composition particles based on different
Prussian blue analogues containing a gradient in either the divalent
metal or the hexacyanometalate components is explored. Synthetic conditions
and combinations of components that favor kinetically trapping the
gradient structures are identified, and these are contrasted with
cases for which gradients in composition are harder to achieve. By
an exploration of several combinations, the relative rate of precipitation
of the PBA components is shown to be the crucial determinant for achieving
control over the gradient synthesis, a parameter that is complicated
by differing crystallization mechanisms within the PBA family. For
one combination, cobalt hexacyanoferrate with nickel hexacyanoferrate,
a complete series of particles is demonstrated, including particles
with differing divalent metal ion gradients, core particles with a
gradient shell, and particles with discrete core and shell components
separated by a gradient. The structural characteristics of the gradient
heterostructures are compared to the individual single phases and
to more standard core–shell particles.
Applications of conjugated polymers in photovoltaics and displays drive the need to understand how morphology affects emission and charge migration. Due to the inherent complexity of polymers, parallel studies of oligomer aggregates are required to 'build-up' an understanding of the polymer features. Fluorescence lifetime imaging microscopy (FLIM) is used to probe variations in vibronic patterns and emission lifetime between individual aggregates and trends in these properties as a function of aggregate size. This technique yields insight into the structure and packing properties of these materials in the aggregated state.
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