From Energy Matching to Lifetime Control: Tunable Excited-State Equilibria in an Iron–Pyrene Dyad
(2026) Journal of the American Chemical Society — Vol. 148, n° 30, p. 32318-32334 (2026)
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- Authors
Glaser, Felix 
Beneventi, Giovanni Cadranel, Alejandro
- Abstract
- Replacing noble-metal-based photosensitizers with earth-abundant alternatives remains a central challenge in photo-redox 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 (2 LMCT) 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(L PhPy) 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 2 LMCT 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. ■ INTRODUCTION In the past decade, considerable interest has been dedicated to replacing noble metals by earth-abundant alternatives. 1−4 The field of photoredox catalysis is not an exception, and as such, avenues to replace the well-established metal-based complexes based on precious and rare 4d and 5d elements like ruthenium, osmium, or iridium were actively investigated. While considerable success was made using Mn, 5−8 Co, 9,10 Cr, 11−14 or Cu, 15−17 which reached excited-state lifetimes that covered the nanosecond to millisecond time scales, iron complexes reaching these properties still remain scarce. 18−23 A main caveat in developing photoactive Fe II complexes lies in their low-lying metal-centered states with triplet and quintet multiplicity that deactivates the excited states on the ultrafast time scale, 24,25 thereby preventing diffusion-limited photoredox catalysis. Facing these challenges, several concepts were investigated. 18−22,26−31 After a decade of research by pioneers such as Warnmark, Sundstrom, Persson, and Lomoth, the change from Fe II to Fe III in combination with strong sigma-donating NHC ligands turned out to be a very promising strategy. 18,29,32 This led to the development of photoactive Fe III complexes with nanosecond-lived ligand-to-metal charge transfer excited states (2 LMCT) instead of metal-to-ligand charge transfer excited states (MLCT) afforded by Fe II analogues. 18,33 While this impressive success enabled diffusional bimolecular quenching to drive photoredox catalysis, 33−38 elongation of the excited-state lifetime would still be highly beneficial as it would allow for efficient excited-state diffusional quenching with low quencher concentrations or even with small quenching rate constants. 39 Due to the spin-allowed nature to transition from the 2 LMCT to the doublet ground state (2 GS), the radiative
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Citations
Glaser, F., De Kreijger, S., Beneventi, G., Cadranel, A., & Troian-Gautier, L. (2026). From Energy Matching to Lifetime Control: Tunable Excited-State Equilibria in an Iron–Pyrene Dyad. Journal of the American Chemical Society, 148(30), 32318-32334. https://doi.org/10.1021/jacs.6c08190 (Original work published 2026)