Solvent-dependent photo-induced dynamics in a non-rigidly linked zinc phthalocyanine-perylenediimide dyad probed using ultrafast spectroscopy

In this work, we characterize the energy and electron transfer kinetics of a zinc phthalocyanine-perylenediimide dyad (ZnPc-PDI) in various solvents using steady-state and tunable narrowband pump-probe spectroscopy. We fit the ultrafast data with global analysis techniques and find that upon excitation of the PDI moiety (pump pulse at 540 nm), the excitation energy transfer (EET) rate to the ZnPc moiety displays a solvent sensitivity that we attribute to changes in the relative equilibrium moiety orientation. We rationalize these observations by considering the nature of the non-rigid bridge used to link the two moieties as well as the degenerate nature of the Q band transitions in the ZnPc species. By tuning the pulse into resonance with the ZnPc Q band (685 nm) we can directly photo-induce an electron transfer (ET) process back to the PDI moiety. Employing the same global analysis, we find that the dynamics of the ultrafast electron transfer are completely kinetically controlled according to the Bixon-Jortner model of barrierless solvent-controlled curve crossing, while the recombination to reform the ground state is well-described using the static energetic picture according to Marcus theory.