Abstract
It is known from previous work that porphine dication, porphine dianion and the ethanol adduct of magnesium porphine are exceptions to the “golden rule” of MCD spectroscopy that positive A terms are associated with the Q and B (Soret) electronic transitions of perimeter symmetric centrally substituted porphyrins since for them a negative A term is found for the Q 0 transition, giving rise to an overall electronic MCD band sign pattern of + − − +. The present work uncovers additional examples in a series of zinc tetrakis( ortho-halophenyl)porphyrins. Michl's perimeter model is used to provide insight into structural effects which might lead to the condition of ΔHOMO ≲ ΔLUMO required to explain the sign inversions on a purely electronic basis. The possibility that the sign variations have a more general vibronic origin is also considered.
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