Background:
Summary of Methods Used
Simple Example:
Theory
The Vibrations
Research Topics:
Fatty Acid Methyl Esters (FAMEs)
School of Chemistry
The Queen's University of Belfast
Non-Planar Porphyrins
The vast majority of porphyrin molecules are flat or near-to–flat and recent work has shown that in natural systems subtle tuning can be induced by small distortions from planarity and although there are several different ways to introduce distortion, the method that we use is to add on bulky substituents at the meso positions highlighted here.
Typically, in the natural biological compounds the distortions from planarity are fairly small but we have chosen to study a much more extreme example which is this tetra-tert-butyl porphine (TtBP) shown above.
By turning the molecule so that you’re looking at it from the side you can see that the extent of the so-called ruffling distortion here really is enormous and this affects lots of it’s chemistry but the thing that’s interesting is that both its ground and excited state properties are very markedly perturbed such that the excited state lifetime of non-planar TtBP is measured in ps whereas planar near-analogues are measured in ns.
You can see that TtBP is unusual because we found these strong low frequency bands in its ground state spectrum which are absent from the spectrum of the very similar, but near-planar, porphyrin tetra-iso-propyl porphine. The only difference in the IPP molecule is that one methyl group has been removed from each of the substituents.
Unfortunately, extensive experimental studies weren’t able to explain the vibrational motions that give rise to these bands so we wanted to carry out calculations to try to find out what they were.
But – as was the case with our work on FAMEs, we couldn’t just start doing calculations on TtBP itself because we wouldn't be sure that our results would have any validity.
