Combined Experimental and Theoretical Study of Efficient and Ultrafast Energy Transfer in a Molecular Dyad

We have characterized the dynamics and the efficiency of electronic energy transfer (EET) in a newly synthesized molecular dyad, composed of a styryl-pyridinium donor and a BODIPY acceptor. The kinetics of the process has been studied with femtosecond transient absorption spectroscopy in different solvents. In all the analyzed media EET is quantitative and very fast, as we find that almost 70% of the overall excitation energy is transferred from the donor to the acceptor on a subpicosecond time scale. The experimental measurements have been supported by a theoretical analysis; the electronic couplings between the donor and acceptor moieties have been calculated at the (TD)DFT level and complemented by a conformational analysis of the full dyad. The computed energy transfer times are in good agreement with the experimental values; this allowed us to verify the correctness of the Forster equation, demonstrating that, although EET in the examined system occurs on an ultrafast time scale, the approximations introduced in the case of the weak coupling regime remain valid.