Spectroscopic Analysis of a Biomimetic Model of Tyr_Z Function in PSII

Using natural photosynthesis as a model, bio-inspired constructs for fuel generation from sunlight are being developed. Here we report the synthesis and time-resolved spectroscopic analysis of a molecular triad in which a porphyrin electron donor is covalently linked to both a cyanoporphyrin electron acceptor and a benzimidazole-phenol model for the Tyr_Z-D₁His190 pair of PSII. A dual-laser setup enabled us to record the ultrafast kinetics and long-living species in a single experiment. From this data, the photophysical relaxation pathways were elucidated for the triad and reference compounds. For the triad, quenching of the cyanoporphyrin singlet excited state lifetime was interpreted as photoinduced electron transfer from the porphyrin to the excited cyanoporphyrin. In contrast to a previous study of a related molecule, we were unable to observe subsequent formation of a long-lived charge separated state involving the benzimidazole-phenol moiety. The lack of detection of a long-lived charge separated state is attributed to a change in energetic landscape for charge separation/recombination due to small differences in structure and solvation of the new triad.