How Peripheral Substitutions Affect Porphyrin Basicity

How Peripheral Substitutions Affect Porphyrin Basicity

Using H2TPP as a central reference, a set of proton competition studies of free base octa alkyl and meso substituted porphyrins were carried out. The H2TPP was selected since it has a large absorption for dication at 650 nm. The octa alkyl substituted porphyrins used in this study were H2OEP, H2OMP and H2etio1; selected because they are a good representation of natural occurring porphyrins. H2TPP, H2TBPP, H2DPP, and H2TTFMPP, using different arrangements on substitution, where also investigated to further study their meso substitution effect on basicity. The results against the H2OEP produces the free base H2OEP, the dication H4OEP+2 and the monocation H3OEP+1. The monocation is easily seen in solution but it is difficult to isolate since a type of disproportionation reaction seems to take place upon crystallization, where the dication is formed at the expense of the monocation. In each experiment, the resultant spectra were obtained by mixing and analyzing an equimolar solution of two free-bases, added to compete.

Additionally, meso phenyl substituted porphyrins were tested against H2TPP. It was suspected that the porphyrins studied would not be as basic as H2TPP, because of the electron withdrawing substituents which might increase acidity, thus decreasing basicity. We concluded that the H2TPP was again preferentially protonated, and hence more basic than the octa alkyl substituted porphyrins H2OEP, H2OMP and H2etio1. However, H2TBPP and H2TTFMPP were more basic than H2OEP, H2OMP H2etio1 and H2DPP. Using MM+ and semi empirical methods, computations studies were carried out to validate porphyrin basicity.