Ultrafast energy transfer and excitonic coupling in an artificial photosynthetic antenna

In photosynthetic organisms carotenoids (Car) and (bacterio)chlorophylls harvest sunlight in peripheral antennas and subsequent transfer the energy to the reaction center, with almost 100% efficiency in some systems [1]. In this work we studied an artificial supramolecule composed of a Car with 10 conjugated double bonds linked to a phthalocyanine (Pc) [2], mimicking the light harvesting unity [3]. We follow the selective excitation of Car, monitoring the ultrafast rise of the Pc population. The Car→Pc ET process occurs from the bright Car S₂ excited state and competes with an internal conversion (IC) process towards the lower-lying dark Car S₁ excited state. To establish the relative weight of these two deactivation pathways, we performed pump-probe experiments with sub-20-fs temporal resolution [4] on the isolated Car and on the dyad and compared the excited state dynamics. In addition, following selective excitation of the Pc, we have identified the spectral signatures of the S₁ excited state of the Car which appear within the ≈50-fs time resolution of our measurement. This strongly indicates excited state coupling between the S₁ state of Car and the S₁ state of Pc, with important implications for the regulation of photosynthetic activity.