Subporphyrazine scaffolds as emerging electron acceptors for long-lived charge separation

Using subporphyrazines (SubPzs) as electron acceptors in the modular design, via metal-ligand axial coordination with ruthenium(ii) phthalocyanine (Ru(CO)Pc), was key to dictating the different photophysical evolution in four electron donor-acceptor Ru(CO)Pc-SubPz conjugates (1-4). Complementary absorptions of SubPzs and Ru(CO)Pc allowed a nearly panchromatic absorption across the visible range. The oxidizing ability of the hexasubstituted SubPz acceptors in 1-4 was tuned through their peripheral functionalization with propyl and sulfanyl groups, as well as with strong electron-accepting (E)-acrylate- and (E)-4-nitrostyryl groups, respectively. Intramolecular Förster Resonance Energy Transfer (i-FRET) and intramolecular charge separation (i-CS) upon SubPz excitation were corroborated by means of absorption, fluorescence, and electrochemical measurements. Excited-state deactivations were established using time-resolved pump-probe transient absorption spectroscopy. In 1, photoexcitation is followed by a rapid i-FRET and Ru(CO)Pc singlet excited state formation, which decays via its triplet state. For 2, excitation triggers i-FRET and i-CS and generates a singlet charge-separated state, decaying through Ru(CO)Pc triplet state. Excitation in 3 sparks i-FRET and i-CS to afford a triplet charge-separated state, where the spin evolution was confirmed by magnetic-field-dependent studies. Lastly, for 4, i-CS outcompetes i-FRET, forming a singlet charge-separated state that spin flips to a long-lived triplet charge-separated state featuring lifetimes of several microseconds.