On the role of excitonic interactions in carotenoid-phthalocyanine dyads and implications for photosynthetic regulation.

In two recent studies, energy transfer was reported in certain phthalocyanine-carotenoid dyads between the optically forbidden first excited state of carotenoids (Car S(1)) and phthalocyanines (Pcs) in the direction Pc → Car S(1) (Kloz et al., J Am Chem Soc 133:7007-7015, 2011) as well as in the direction Car S(1) → Pc (Liao et al., J Phys Chem A 115:4082-4091, 2011). In this article, we show that the extent of this energy transfer in both directions is closely correlated in these dyads. This correlation and the additional observation that Car S(1) is instantaneously populated after Pc excitation provides evidence that in these compounds excitonic interactions can occur. Besides pure energy transfer and electron transfer, this is the third type of tetrapyrrole-carotenoid interaction that has been shown to occur in these model compounds and that has previously been proposed as a photosynthetic regulation mechanism. We discuss the implications of these models for photosynthetic regulation. The findings are also discussed in the context of a model in which both electronic states are disordered and in which the strength of the electronic coupling determines whether energy transfer, excitonic coupling, or electron transfer occurs.