Exploring the Reversible Equilibrium State between CS and CSS in a Ru(phen)-Naphthalene Diimide Dyad.

This study explores the dynamics of charge separation (CS) and recombination in the photoinduced electron transfer of the [Ru(phen)(pNDIp)] dyad, focusing on the thermal equilibrium between rapid charge separation (CS) and the slower charge-separated state (CSS). The pNDIp component is a naphthalene diimide linked to one of the phen ligands, providing nearly unrestricted orthogonal freedom between the {[Ru(phen)]} and {pNDIp} units. The investigation employs steady-state and time-resolved spectroscopic techniques, electrochemical methods, and DFT/TD-DFT computational calculations. The results show that selective excitation of the {[Ru(phen)]} at 450 nm partially quenches the MLCT emission due to thermal equilibrium with the CSS state, {Ru(phen)(pNDIp)} ⇌ {Ru(phen)(pNDIp)}. This equilibrium is attributed to a combination of nonradiative forward (τ = 10 ps) and reverse (τ = 140 ps) time decays, driven by the intramolecular charge transfer. The long-lived MLCT state, the reduced distance between the donor and acceptor, and the vibrational structure of the dyad provide sufficient time for CS⇌CSS equilibrium. These findings support Marcus theory and highlight key parameters such as -Δ = 0.279 eV, λ = 0.49 eV, and H = 0.28 eV. Additionally, the dyad's ability to generate singlet oxygen under 450 nm light suggests potential applications in photodynamic therapy and oxidative processes. Its ability to form radical anion RupNDIp upon 350 nm light exposure further demonstrates its versatility in photocatalytic applications.