Abstract

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.

© 2013 IEEE