Steric hindrances and spectral distributions affecting energy transfer rate: A comparative study on specifically designed donor-acceptor pairs

In this work the photophysics of four bichromophoric units was studied by means of static and time resolved spectroscopy, with the aim of disentangling the contribution of steric and electronic factors in regulating the efficiency of electronic energy transfer (EET). The newly synthesized dyads share the same acceptor moiety, a substituted BODIPY chromophore, and differ either in the donor or in the molecular bridge connecting the two units. The use of different linkers allows for tuning the conformational flexibility of the dyad, while changing the donor has an influence on the electronic coupling and spectral overlap between the two chromophores. The efficiency of energy transfer is extremely high in all the four dyads and can be modelled within the frame of the F€orster equation. In the special case of a dimeric donor, a theoretical analysis was performed to further support the experimental findings. Geometry optimization at DFT level indicated that different conformations with similar energy can exist in solution, explaining the observed multi-exponential EET. Furthermore, energy transfer rates, computed at DFT level, resulted in optimal agreement with the experimental ones. Our analysis allowed to conclude that, in case of the studied systems, steric hindrance and donor-acceptor relative orientations plays a prominent role in regulating the EET dynamics, even overcoming electronic effects.