Subporphyrazine Scaffolds as Emerging Electron Acceptors for Long-Lived Charge Separation

Abstract

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 governing 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.