60th Annual Report on Research 2015

During PRF support, the Blanchard group has made a number of advances in the area of energy flow and dynamics in fluid systems, with an eye toward understanding the details of energy relaxation pathways in fluid or other disordered systems. This work has spanned direct measurements of vibrational population relaxation and state-dependent molecular rotation through the monitoring of energy dissipation processes mediated by metal ion complexation. As a result of this support, we have published 22 papers listed as having PRF support. The work described below summarizes some of the more interesting findings made during the support period.

The first area is State-Dependent Rotational Diffusion of Tetracene in n-Alkanes. Evidence for a Dominant Energy Relaxation Pathway. This work was published in the Journal of Physical Chemistry B in 2013.

We are interested in understanding the effects of transient heating on local organization. We have measured the rotational diffusion time constants of tetracene in n-alkanes C₈ through C₁₆, and for excitation of the tetracene S₁ and S₂ states. Emission from the S₁ state was monitored for both excitation conditions. Data were collected using a time-correlated single photon counting instrument that is capable of providing the requisite excitation wavelengths and recording polarized fluorescence transients with an instrument response function of ca. 35 ps. The experiment is to perform rotational diffusion measurements for excitation in the two different states, S₁ and S₂, and monitor any difference in rotational diffusion for the two excitation conditions. Differences are due to transient heating associated with the radiationless dissipation of ca. 1.6 eV of excess energy during rapid relaxation from the tetracene S₂ state to the S₁ state. The transient heating effect is expected and is especially pronounced in this system because of the characteristically fast dynamics of tetracene in alkanes and the modest thermal conductivity of the alkane solvents.

The data exhibit several interesting features. The first is that for S₂ excitation, the tetracene reorientation time is almost independent of solvent aliphatic chain length. The second interesting feature of these data is that for excitation of the S₁ state of tetracene, solvent-dependent reorientation is observed that exhibits an odd-even aliphatic chain length effect. This is most probably due to the relative proximity of the chromophore and the solvent terminal methyl groups. Based on our experimental data, it appears the chromophore itself is exerting an influence on the n-alkane solvent molecules in closest proximity, which accounts for the pronounced odd-even effects we observe. The fact that analogous odd-even effects are not seen for other polycyclic aromatic hydrocarbon chromophores in the same alkane solvents argues for the shape of the chromophore “templates” the surrounding solvent molecules into a preferential conformation.

A second area of interest is Evidence for Molecular-Scale Heterogeneity in the Cyclohexane / n-Butanol Binary Solvent System. This work was published in the Journal of Physical Chemistry B in 2015.

It is critically important to develop tools to probe molecular-scale compositional heterogeneity because of its importance in processes ranging from the solubility properties of mixed solvent systems to the organization of the plasma membrane in mammalian cells. At the present time there are a very limited number of ways to investigate such transient organization, and our combination of molecular motion (rotational diffusion) measurements with energy transfer behavior (vibrational relaxation measurements) has proven to give substantial insight into such transient order. We have found that the rotational diffusion dynamics of the chromophore perylene and the transfer of vibrational energy from the ring breathing mode of this chromophore at 1375 cm^-1 to the surrounding bath provide collectively a self-consistent picture of compositional heterogeneity in the cyclohexane/n-butanol binary solvent system. Our data demonstrate that there is a preferential solvation effect for perylene with n-butanol which leads to compositional non-uniformity as well as anomalous chromophore dynamics under conditions where only a relatively small fraction of the solvent system is n-butanol. These data point to the fallacy of uniform distribution of species in the solution phase and the importance of transient compositional heterogeneity in determining the behavior of a binary solvent system. The virtue of this approach is that it interrogates processes that are operative at the molecular scale rather than bulk system properties.