Exploring the Reversible Equilibrium State between ³CS and ³CSS 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)₂(pNDIp)]²⁺ 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)₃]²⁺} 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)₃]²⁺} at 450 nm partially quenches the ³MLCT emission due to thermal equilibrium with the ³CSS state, ³{Ru³⁺(phen^{•^-})₂(pNDIp)} ⇌ ³{Ru³⁺(phen)₂(pNDIp^{•^-})}. This equilibrium is attributed to a combination of nonradiative forward (τ_CT = 10 ps) and reverse (τ_-CT = 140 ps) time decays, driven by the intramolecular charge transfer. The long-lived ³MLCT state, the reduced distance between the donor and acceptor, and the vibrational structure of the dyad provide sufficient time for ³CS⇌³CSS equilibrium. These findings support Marcus theory and highlight key parameters such as -Δ<i>G</i>_CS = 0.279 eV, λ = 0.49 eV, and H_DA = 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.