59th Annual Report on Research 2014

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.

The second area is Orientational and Vibrational Relaxation Dynamics of Perylene in the Cyclohexane-Ethanol Binary Solvent System.  This work was published in the Journal of Physical Chemistry B in 2014.

We are interested in understanding local organization in complex binary solvent systems in order to quantitate and control vibrational energy flow, and provide insights on tailoring solvent systems for optimization of specific properties.  In this work we have used a pump-probe laser spectroscopic technique based on stimulated emission that is capable of measuring both vibrational population relaxation time for specific vibrational modes and rotational diffusion of the ground state chromophore.  We have examined the rotational dynamics and vibrational population relaxation of the polycyclic aromatic hydrocarbon perylene in a series of ethanol-cyclohexane binary solvent mixtures.  The goal of this work is to relate solvent system composition to local organization.  Steady-state spectroscopic data reveal a discontinuous dependence of the chromophore spectroscopic origin on binary solvent system composition.  Both rotational diffusion and vibrational population relaxation time constants show a clear discontinuity between 5% and 7.5% (v/v) ethanol in cyclohexane, indicative of a discontinuous change on molecular scale rearrangement in the chromophore local environment.  We understand these results in the context of the chromophore residing in an environment that is heterogeneous on the molecular scale, with changes in its average conformation being seen for different amounts of the two solvent components. The two measurements sense different but complementary aspects of the chromophore local environment. Both sets of data exhibit a discontinuous change in the perylene local environment for ethanol concentrations between 5% and 7.5% (v/v).  The reorientation data demonstrate that for low concentrations of ethanol the chromophore is constrained to reorient primarily about the axis normal to the plane of its π system.  For higher ethanol concentrations, motion around a chromophore in-plane axis dominates, suggesting less confining environment being formed by the solvent.  The vibrational population relaxation data show that at the same time that the chromophore is more able to reorient about an in-plane axis, its proximity to ethanol terminal methyl groups is enhanced.  These two findings suggest that for ethanol concentrations above ca. 5%, the perylene local environment becomes dominated by the presence of ethanol, and the cyclohexane is displaced.  This behavior is consistent with the steady state absorbance and emission data, showing a change in the 0-0 band frequency in this same range.  Taken collectively, the data point to the formation of regions of relatively high ethanol concentration once enough ethanol is present to allow for their formation.  An issue that remains to be resolved is whether the chromophore plays a role in nucleating the regions of high ethanol concentration in cyclohexane.  We are pursuing this question at present.