From Energy Matching to Lifetime Control: Tunable Excited-State Equilibria in an Iron-Pyrene Dyad

Replacing noble-metal-based photosensitizers with earth-abundant alternatives remains a central challenge in photoredox catalysis, particularly for iron complexes where ultrafast deactivation pathways limit excited-state lifetimes. Iron(III) N-heterocyclic carbene complexes have recently emerged as promising photoactive systems featuring nanosecond-lived ligand-to-metal charge-transfer (2LMCT) states, yet further lifetime extension is required for efficient energy storage and reactivity. Herein, we report a rationally designed iron(III)-polyaromatic hydrocarbon dyad in which a pyrene energy acceptor was introduced, yielding [Fe(LPhPy)2]+. Femtosecond to nanosecond spectroscopic analysis reveals a sequential population of 3*PhPy via a charge-separated state (CSS) intermediate through a spin-allowed electron-transfer cascade. In this system, the energies of the 2LMCT state, the CSS, and the pyrene triplet (3*PhPy) lie within ∼0.2 eV, enabling detailed investigation of excited-state equilibria and near-isoenergetic excited-state manifolds. Solvent polarity and ionic strength are shown to effectively modulate the CSS energy, thereby tuning triplet yields and lifetimes. The dyad exhibits an over 50-fold increase in the excited-state lifetime relative to the parent iron complex by populating a triplet state that stores comparable energy but offers distinct excited-state redox potentials. Temperature-dependent kinetic analysis provides mechanistic insights into the competing electron-transfer pathways and highlights entropy-dominated processes. For proof-of-principle applications, the current dyad is benchmarked in energy versus electron transfer reactivity from different excited states and compared to the unsubstituted iron complex. These findings establish external stimuli like solvent or salt as powerful tools for controlling excited-state dynamics in iron-based molecular dyads and highlight clear pathways to tune excited states and increase the efficiency of triplet state population for a rational design of next-generation iron photosensitizers.