Speaker
Description
Three examples of charge transfer processes in rylene diimide molecules and macromolecules will be discussed using ultrafast transient absorption spectroscopy (TAS) results. First, the photoinduced charge transfer dynamics in luminescent donor-acceptor polymers and copolymers1 that use a single naphthalene diimide (NDI) as acceptor, polystyrene chains as primary donor, and secondary stronger-donor chains, will be presented. Here, TAS shows that electron transfer (ET) process occurs mostly intrachain when in solution, while in the spin-coated films the ET occurs mostly interchain. Our results also show that excitation at the red-edge of the NDI absorption band prompts the population of highly coupled pairs. This is the first study on this type of polymers.
In the second part, we will examine charge-transfer processes within three perylene diimide (PDI) chromophores forming a cage that undergoes photoinduced symmetry-breaking charge separation (SB-CS) i.e., ET occurs between two PDIs yielding the PDI•- and PDI•+ radicals2. We will show how a six-parameter kinetic model allows us to successfully interpret the femtosecond to microsecond TAS data while ensuring a single solution for the time constants using the biexponential fluorescence lifetimes and amplitudes as constraints. This approach can be extended to the analysis of similar systems that also display thermally-activated delayed fluorescence (TADF).
Finally, we will dive into the quenching of PDI•- in the excited state, which can be used as a photocatalyst for the reduction of aryl halides in the controversial con-PET (consecutive-photoinduced electron transfer) mechanism. Our TAS results were obtained using a combination of pump-probe and pump-pump probe3,4 strategies. They demonstrate that static quenching between PDI•- and the aryl halide 4-bromoacetophenone (BAP) is operative and plays a major role in this system, yielding a small (1%) quantum yield of this mechanism. Our molecular dynamics simulations show that preassociation is not necessary for static quenching, and rather, a high probability of finding a BAP molecule in the vicinity of PDI•- (a condition fulfilled at high concentrations of this quencher) is sufficient to have electron transfer without the need for diffusion. This study brings into question the challenges faced using radical species as photocatalysts.