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Fused polycyclics as solvent probes June 14, 2009

Posted by fetzthechemist in Uncategorized.
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The fluorescence spectra of many cata-condensed alternant polycyclic aromatic hydrocarbons (PAHs with only six-member rings where many are joined through more than one face) are often mirror images. The excitation spectrum is a series of increasingly larger and evenly-spaced peaks, while the emission spectrum is a series of decreasingly smaller and evenly-spaced peaks.

An exceptions are several PAHs that do not have mirror imaged pairs. In these, the spectral peaks show somewhat random peak heights and spacing between peaks, as well. The patterns in the excitation spectra are not similar in any way to that in the emission spectra.

PAHs that show this effect include pyrene, benzo[e]pyrene, benzo[ghi]perylene, coronene, and ovalene (in a long series of papers from a collaboration with William Acree, lots more were studied).

The excitation spectrum of coronene follows:

Coronene excitation in nonpolar solvent

Coronene excitation in nonpolar solvent

 The emission spectra in nonpolar and polar solvents are below:

coronene emission in hexane

coronene emission in ethanol

The heights of certain bands change drastically relative to each other! Why is that?

For the “normal” PAHs, the ground and excited states are the same geometry. What goes up in energy comes back down in reciprocal fashion. But for coronene and those others, the excited state has a different geometry than the ground state. The solvation volume and arrangement changes to deal with the geometric and electron distribution changes.

These changes result in different degrees of interaction with solvents. That changes the energy distribution in the excited state, which gives a different spectrum.

The bottomline is that differences in solvation strength can be seen by the spectral changes. Nonpolar solvents give one pattern. Polar ones give another. Intermediate interacting solvents show intermediate changes. Thus, these PAHs are good probes of the solvent chemistry and environment.

Lots of potential uses can be thought of since differing solvent conditions happen in many areas of science.

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