Interrogating Conformationally Dependent Electron-Transfer Dynamics via Ultrafast Visible Pump/IR Probe Spectroscopy

We demonstrate for the first time the utility of time-resolved visible pump/mid-infrared (IR) probe spectroscopy to interrogate directly, and provide unique infomation regarding, conformationally dependent photoinduced ET dynamics and the subsequent structural evolution of the resulting charge-separated (CS) state. Exemplary polarized visible pump/IR probe experiments involving N-[5-(10,20-diphenylporphinato)zinc(II)]-N‘(octyl)pyromellitic diimide (PZn-PI) and [5-[4‘-(N-(N‘-octyl)pyromellitic diimide)phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) (PZn()PI) show that it is possible to assess the mean PZn-to-PI interplanar torsional angle of electronically excited structural conformers that undergo ET within the sub-ps time domain for both of these donor−spacer−acceptor (D-Sp-A) systems. Further transient specroscopic experiments carried out on PZn()PI determine how this angle evolves with time. Because vibrational transition moments are often known and typically localized, this work underscores that pol...