Tuning the Triplet-Triplet Energy Transfer Between Phthalocyanine and Carotenoid by Methyl Groups on the Conjugated Chain.

Mimicking light-harvesting and photoprotective processes of natural photosynthesis by artificial supramolecular systems is of considerable interest for artificial photosynthesis. The authors of the highlighted paper report on synthesis and spectroscopic characterization of a novel Pd-phthalocyanine-carotenoid dyads that allow to directly follow the triplet-triplet energy transfer between Pd-phthalocyanine and carotenoid. Unexpectedly, the T-T energy transfer does not follow the dependence on conjugation length of the acceptor carotenoid. Instead, the donor-acceptor coupling and resulting T-T energy transfer rate is controlled by the presence or absence of a methyl groups on the conjugated chain in the vicinity of the carotenoid keto-oxygen. This reveals yet another level of tuning the spectroscopic properties of carotenoids having a conjugated keto group in their structure, underlining their potential for tailoring specific supramolecular complexes carrying out either light-harvesting or photoprotective functions.