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The goal of the grant was to investigate two new dyes that form chromonic liquid crystals, Bordeaux Dye and Benzopurpurin 4B (top and bottom, respectively, in the figure below).  The former has a structure quite different from the other dyes previously studied and the latter is reported to have a liquid crystal phase at very low concentrations.  These studies were completed a year ago, but a final year was necessary to finish work done on an infrared absorbing dye and begin experiments on the kinetics of the aggregation/disaggregation process.

An Infrared Absorbing Dye

By this time a year ago, we had mapped out the phase diagram of IR-806, an infrared absorbing dye that forms a liquid crystal at very low concentrations.  Its structure is given below.

We had also observed that its absorption spectrum is much more sensitive to aggregation that the other chromonic systems.  This year we extended the absorption measurements to higher concentration, only to discover that the last stage of aggregation, the one that results in the liquid crystal phase changes the absorption spectrum even more.  Thus this compound allows us to follow the aggregation process with more specificity that ever before.  It is clear that well below the liquid crystal phase, aggregation occurs, probably to a dimer or small aggregate.  Then at a higher concentration, large aggregates form resulting in the liquid crystal phase.  Three representative spectra are shown below, illustrating the various changes that take place with concentration.  The concentrations are, from left to right, 0.0005 wt%, 0.101 wt%, and 0.966 wt%.

Also shown in the figure are the results of decomposing the spectra into individual peaks.  Spectra over the entire concentration range could be fit with 6 peaks (center wavelength and width fixed), with 3 of these peaks being very sensitive to aggregation.  The concentration dependence of the amplitudes of the peaks representing the monomer, dimer or small aggregate, and large aggregate are shown in the figure below.

The fall of the monomer peak, rise and fall of the dimer peak, and rise of the aggregate peak all follow the prediction of a simple theory in which the equilibrium constant for dimer formation is much larger than the equilibrium constants for larger aggregates, which are independent of aggregate size.  We will submit these results for publication as soon as the full analysis is completed.

Viscosity Measurements

In a collaboration with David Pines at New York University, we have measured the viscosity of two chromonic liquid crystals as aggregation proceeds and the liquid crystal phase is formed.  The measurements were taken with a unique apparatus, built completely by the Pines group, and the experimental precision is outstanding.  We chose two systems for which there is evidence that the aggregation process is very different.  As can be seen from the following figure, the viscosity data also show the great difference between these two compounds as the temperature approaches the liquid crystal transition (about 20°C in the figure).

There has been little experimental work on the viscosity of chromonic liquid crystals, and we intend to continue this collaboration.

Kinetic Experiments

Investigations into the kinetics of large molecules have revealed a great deal about their behavior and structure, yet there has been no experimental work on the kinetics of aggregation and disaggregation in systems that form chromonic liquid crystals.  For this reason, we decided to perform some preliminary experiments to see if an extensive investigation would be worthwhile.  Slow kinetics experiments can be done by changing the conditions and monitoring a sensitive parameter such as absorption or fluorescence.  Fast kinetics requires specialized equipment such as a stop-flow or temperature-jump apparatus.  When we realized that the kinetics in some of the aggregating dyes was too fast for our apparatus, we entered into a collaboration with Dr. Heinrich Roder at the Fox Chase Cancer Institute, which is located on the other side of Philadelphia from Swarthmore College.  What we found was a huge variation in kinetics among different aggregating systems.  For Benzopurpurin 4B, which forms a very large aggregate, reaching a new equilibrium after dilution took hours.  For Sunset Yellow FCF, with an aggregate structure consisting of a stack of molecules, the time to reach equilibrium after salt is added to promote aggregation was about a millisecond.  For IR-806, aggregation took about half a second and the absorption vs. time data are fit so perfectly by a stretched exponential that the fit cannot be distinguished from data.

A stretched exponential is indicative of a system in which many time constants contribute to the kinetics.  This is very reasonable for the IR-806 system, in which there are many different sized aggregates, perhaps with varying time constants.  We feel studying the kinetics of these aggregating dyes is a wide open avenue of investigation and plan to assemble the resources for an extended project.