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

## TL;DR

Researchers designed new molecules using subporphyrazines to control how electrons behave after being excited by light, leading to long-lasting energy states.

## Contribution

The study introduces subporphyrazines as tunable electron acceptors for creating long-lived charge-separated states in donor-acceptor conjugates.

## Key findings

- Subporphyrazine-based conjugates showed different excited-state deactivation pathways depending on their structure.
- One conjugate formed a long-lived triplet charge-separated state with lifetimes of several microseconds.
- Functionalization of subporphyrazines allowed tuning of their electron-accepting properties.

## 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 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.

Using subporphyrazines (SubPzs) as tunable electron acceptors, four donor–acceptor conjugates were prepared. The nature of each SubPz governed excited-state deactivation, yielding either singlet, or long-lived triplet charge-separated states.