Phthalocyanine as Bioinspired Model for Chlorophyll f Containing Photosystem II Pushing Photosynthesis in the Far-Red.

Textbook explanation that P680 pigments are the red limit to drive oxygenic photosynthesis must be reconsidered by the recent discovery that chlorophyll f (Chlf) containing Photosystem II (PSII) absorbing in the far-red limit (FRL) at 727 nm can drive the four-electron water oxidation. The almost 110 meV energy drop from 680 to 727 nm is argued as the spare energy that is used to buffer the pulses in delta psi (the charge across the membrane) under variable light intensities. Two different families of unsymmetrically substituted Zn phthalocyanines (Pc) absorbing in the 700-800 nm spectral window and containing a fused imidazole-phenyl substituent (ZnPc I and ZnPc II) or a fused imidazole-hydroxyphenyl group (ZnPc I-OH and ZnPc II-OH) have been synthetized and characterized as bioinspired model of the Chlf /TyrosineZ/Histidine190 cofactors of PSII. Laser flash photolysis studies in presence of an electron acceptor and irradiating in the far-red region evidenced an intramolecular electron transfer process at an isoenergetic landscape scenario. Transient absorption and FT-IR signatures indicate the formation of a hydrogen-bonded phenoxyl radical in ZnPc II-OH. This study sets the basis for the utilization of a broader spectral window for multielectronic catalytic processes with one of the most robust and efficient commercial dyes.