# Exploring the Reversible Equilibrium State between 3CS and 3CSS in a Ru(phen)–Naphthalene Diimide Dyad

## TL;DR

This paper studies how a ruthenium-naphthalene diimide complex balances fast and slow charge-separated states after light exposure, revealing insights into electron transfer dynamics.

## Contribution

The study reveals a reversible equilibrium between 3CS and 3CSS states in a Ru(phen)–naphthalene diimide dyad, supported by experimental and computational methods.

## Key findings

- The dyad exhibits a thermal equilibrium between 3CS and 3CSS states with forward and reverse decay times of 10 ps and 140 ps.
- Marcus theory is supported with parameters −ΔGCS = 0.279 eV, λ = 0.49 eV, and HDA = 0.28 eV.

## 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 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
(τCT = 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 −ΔGCS =
0.279 eV, λ = 0.49 eV, and HDA = 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.

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).