59th Annual Report on Research 2014

The Second Step of Assembly in IR-806

            To finish up the investigation of the assembly process in the chromonic liquid crystal IR-806, attention was directed at theoretical models that might explain the second step in the assembly process (the first step is consistent with an isodesmic model).  The fundamental finding of this work is that the second step is highly cooperative.  In fact, both the equilibrium and kinetic results are consistent with several assembly models (e.g., nucleation and growth, activation and growth), since these models are identical in the limit of high cooperativity.[1]

            Below are absorption data for the 830 nm peak (due to large assemblies) as a function of temperature for a 0.3 wt% solution of IR-806.  The line is a theoretical prediction in the highly cooperative limit and gives a value for the growth step enthalpy change of (-65 ± 0.9) kJ/mole.

            Below is a plot of the absorption at 870 nm of a 0.2 wt% solution of IR-806 vs. time after a 10:1 dilution with water.  The noise in the experimental data is due to the very small change in absorption.  The line is the prediction of a nucleation and growth model for the change in the number of assemblies due to such a dilution.  Both the data and the prediction show two processes with very different rates, a fast process whereby the assemblies decrease in size to close to the size of the nucleus, and a slow process as the nuclei break apart into smaller assemblies.

            The assembly process can also be modeled successfully.  Below is a graph showing the change in absorbance at 560 nm due to the addition of a NaCl solution (1:10 ratio) causing assemblies to form.  The IR-806 concentrations were chosen so each solution started with the same absorption spectrum verifying that no large assemblies were present.  The concentrations of the added NaCl solutions were adjusted so that the sample showed the same absorption spectrum after the addition of NaCl.  The theoretical line is the prediction of a nucleation and growth model with parameters selected to give a curve similar to the experimental data for the 0.2 wt% solution.

            The investigation of the assembly process in IR-806 is now complete, and a manuscript is in preparation for publication (submission date: October, 2014).

The Assembly Processes in Pinacyanol Acetate

            Taking a hint from a recent article describing assembly phenomena in pinacyanol chloride[2], experiments were conducted on pinacyanol acetate solutions at higher concentrations and after they had been prepared some time beforehand.  As shown below, the signature of large assembly formation is a set of peaks at 600 and 640 nm (visible in the 0.5 wt%, 0.6 wt%, and 0.7 wt% solutions).

Whereas solutions freshly prepared from powder with concentrations at or above 0.3 wt% showed absorption changes upon dilution (indicating that large assemblies were present), after waiting a day, only solutions with concentrations at or above 0.5 wt% showed absorption changes upon dilution.  This is consistent with the results of the equilibrium experiments shown above.  Also, unlike IR-806 that has kinetic constants on the order of a fraction of a second, both the disassembly and assembly processes in pinacyanol acetate are quite slow, with kinetic constants closer to a fraction of an hour.  These are shown below for a 10:1 dilution with water of various pinacyanol acetate solutions and for the addition of NaCl solutions (1:10 ratio) of varying concentrations to a 0.1 wt% pinacyanol acetate solution.  The disassembly and assembly data are close to being exponential, with the highest salt concentration revealing some non-exponential behavior in the first ten minutes.

            This completes the study of the assembly process in pinacyanol acetate and demonstrates that in many ways it is similar to the assembly behavior of benzopurpurin 6B, an investigation completed earlier with support of this grant.  Preparation of a manuscript reporting on both compounds is planned for later this calendar year.

[1] Smulders, M. M. J.; Nieuwenhuizen, M. M. L.; de Greed, T. F. A.; van der Schoot, P.; Schenning, A. P. H. J.; Meijer, E. W. Chem. Eur. J. 2010, 16, 362.

[2] v. Berlepsch, H.; Ludwig, K.; Bottcher, C.; Phys. Chem. Chem. Phys. 2014, 16, 10659.