A quantitative analysis of excitonic superhyperchromism in porphyrin J-/H-aggregates

We argue that the mixing between energetically well-separated bands should have a magnified importance in the aggregates exhibiting an excitonic structure with a collective superstate, for which the J-/H-aggregates of porphyrins represent excellent examples. We present a quantitative account of the excitonic intensity borrowing in such systems and reveal a remarkably simple explanation of its enhancement: superhyperchromism is manifested by the mediation of the interband (Q–B) coupling between the two superstates (in Q and B) of such aggregates. The significance of the superhyperchromism is conveyed through its effects on the absorption spectrum of a natural `porphyrin J-aggregateʹ – the chlorosome.