1.1 Porphyrins - Friedrich-Alexander-Universität Erlangen-Nürnberg

Discussion and Results 3
coefficients, the behavior is not uniform as the ε value for the SORET band decreases by
nearly 30 % while the ones for the Q-bands stay comparable or even increased. For example,
the value for QIV appears surprisingly high – nearly triple the one observed in 51.
Additionally, the whole Q-band region shows up quite badly resolved as if different band
sets overlaid each other what might indicate the presence of different species like e.g.
tautomers.
To explain those observations, it seemed worth to compute the shapes of the molecule’s
frontier orbitals. Calculations were performed on the PM3 level using Materials Studio® 99
and resulted amongst others the pictures presented in Figure 16.
HOMO LUMO
Figure 16. Calculated shapes of HOMO and LUMO in 53.
Therewith, the explanation for the detected red-shift of all bands is straightforward since the
ketone in the exocycle as well as the tethered phenyl ring become to some extend
incorporated into to porphyrin’s π-system leading to the observed effect. The displayed
dissymmetry of the orbitals hints to a loss of degeneracy being also deducible from the
calculated orbital energies. As consequence, the number of energy levels and therewith the
number of possible π → π * transitions should rise resulting in additional bands in close
proximity to each other. That would explain the observed seemingly overlaid Q-bands. The
increase in the corresponding extinction coefficients is considered to be due to a significant
enhancement of the oscillator strengths of the Q-bands arising from the conjugated keto
group. Computational studies on higher levels support these assumptions. 100
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