Excited-State Charge Distribution Of A Donor-Pi-Acceptor Zn Porphyrin Probed By N K-Edge Transient Absorption Spectroscopy

Zinc porphyrin solar cell dyes with donor-pi-acceptor architectures combine light absorber (pi), electron-donor, and electron-acceptor moieties inside a single molecule with atomic precision. The donor-pi-acceptor design promotes the separation of charge carriers following optical excitation. Here, we probe the excited-state electronic structure within such molecules by combining time-resolved X-ray absorption spectroscopy at the N K-edge with first-principles time-dependent density functional theory (TD-DFT) calculations. Customized Zn porphyrins with strong-donor triphenylamine groups or weak-donor tri-tert-butylbenzene groups were synthesized. Energetically well-separated N K-edge absorption features simultaneously probe the excited-state electronic structure from the perspectives of the macrocycle and triphenylamine N atoms. New absorption transitions between the macrocycle N atoms and the excited-state HOMO vacancy are observed, and the triphenylamine associated absorption feature blue-shifts, consistent with partial oxidation of the donor groups in the excited state.