Abstract
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 (3)MLCT emission due to thermal equilibrium with the (3)CSS state, (3){Ru(3+)(phen(•(-)))(2)(pNDIp)} ⇌ (3){Ru(3+)(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 (3)MLCT state, the reduced distance between the donor and acceptor, and the vibrational structure of the dyad provide sufficient time for (3)CS⇌(3)CSS equilibrium. These findings support Marcus theory and highlight key parameters such as -ΔG(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.