Molecular Aggregation Research

Aggregating Dyes

Certain water –soluble dyes aggregate spontaneously. The aggregates of many of these dyes are stacks of molecules, with a spacing of 0.34 nm between the molecules in the stacks. These types of systems are also characterized by isodesmic aggregation, meaning that the energy advantage in adding a molecule to a stack does not depend on the size of the stack. Such systems aggregate at all concentrations with a distribution in aggregate size. As the concentration of the dye increases, the size distribution shifts to larger aggregate size. Below are the results of a simple theoretical calculation of the size distribution for two concentrations of a typical aggregating dye.

The 0.34 nm spacing between the molecules in a stack produces a strong x-ray reflection that can be observed in all aggregated dye systems. Below is what the reflection looks like for a sample of Bordeaux dye in the liquid crystal phase. For many aggregated dye systems, the reflection is much broader than this. The scattering wavevector is equal to (4π/l)sin(q/2), where l is the x-ray wavelength and q is the scattering angle.

The aggregation is sometimes evident by a change in the absorption spectrum of the dye. Exciton theory predicts that the absorption coefficient should decrease and shift as the number of molecules in an aggregate increases. This is certainly true for the food dye Sunset Yellow FCF, as can be seen from the data shown below. When exciton theory is used with these data, an estimate of the stacking free energy of 22 kT results.

Small angle xray scattering experiments reveal that the stacks are made of single columns for Sunset Yellow FCF. Broad x-ray scattering peaks representing the aggregate - aggregate distance are observed (see below). The dependence of the peak angle on volume fraction indicates that the aggregates are stacks with a cross-sectional area about equal to the size of the Sunset Yellow FCF molecule.

When x-ray data from many research groups is analyzed in a consistent manner, it is possible to ascertain the number of molecules in the cross-sectional area of the aggregate. As the following chart displays, some systems have one molecule in the aggregate cross-section, some have two molecules in the aggregate cross-section, and some have three or more molecules in the aggregate cross-section.

Resonance Light Scattering

When molecules that absorb light aggregate, electronic communication among the molecules can cause extraordinary optical effects. One example is resonance light scattering, i.e., enhanced scattering for light in the absorption band of the aggregated molecule. For aggregates of sufficient size, the enhanced scattering overwhelms the absorption that is taking place and a peak in the scattering spectrum appears. This is shown in the following figure, where the solid red line is the measured scattering spectrum showing this peak in the absorption band of the aggregated molecules (around 490 nm for these porphyrin derivatives). The amount of absorption is considerable, as shown by the dashed blue line showing the scattering spectrum corrected for absorption.

Just as important, resonance light scattering can be used to determine the size and shape of these aggregates. Below is shown how the scattering intensity depends on angle for a specific solution of porphyrin derivatives. The solid blue line is the scattering expected from a solution of very thin rods of length 0.58 µm. The nice fit indicates that these aggregates are much longer than they are wide and, when combined with other data, reveals that the aggregates must include on the order of a hundred thousand molecules.

The kinetics of this aggregation process is also quite interesting. There is often an induction period, after which rapid aggregation occurs. The rate of this rapid aggregation depends on the concentration of aggregating molecules and the solvent conditions. A theoretical model that accurately describes the kinetic data is one in which the formation of an aggregation nucleus is rate determining, but is catalyzed by the fractal array of aggregates produced by the process. The process is therefore autocatalytic, with both a non-catalytic and catalytic pathways.