The Carbene Cannibal: Photoinduced Symmetry-Breaking Charge Separation in an Fe(III) N-Heterocyclic Carbene.

Nidhi Kaul,Reiner Lomoth

doi:10.1021/jacs.1c03770

Abstract

Photoinduced symmetry-breaking charge separation (SB-CS) processes offer the possibility of harvesting solar energy by electron transfer between identical molecules. Here, we present the first case of direct observation of bimolecular SB-CS in a transition metal complex, [FeIIIL2](PF6) (L = [phenyl(tris(3-methylimidazol-1-ylidene))borate]−). Photoexcitation of the complex in the visible region results in the formation of a doublet ligand-to-metal charge transfer (2LMCT) excited state (E0–0 = 2.13 eV), which readily reacts with the doublet ground state to generate charge separated products, [FeIIL2] and [FeIVL2]2+, with a measurable cage escape yield. Known spectral signatures allow for unambiguous identification of the products, whose formation and recombination are monitored with transient absorption spectroscopy. The unusual energetic landscape of [FeIIIL2]+, as reflected in its ground and excited state reduction potentials, results in SB-CS being intrinsically exergonic (ΔGCS° ∼ −0.7 eV). This is in contrast to most systems investigated in the literature, where ΔGCS° is close to zero, and the charge transfer driven primarily by solvation effects. The study is therefore illustrative for the utilization of the rich redox chemistry accessible in transition metal complexes for the realization of SB-CS.

Highlights

Photoinduced symmetry-breaking charge separation (SB-CS) processes offer the possibility of harvesting solar energy by electron transfer between identical molecules
Photoinduced symmetry-breaking charge separation (SBCS) processes are characterized by preferential activity of one of two or more seemingly equivalent charge separation pathways.[1−3] Intramolecular SB-CS has received widespread attention in the literature,[4−12] with many tailored synthetic architectures serving as models to understand design principles endemic in natural systems[13] for efficiently harvesting solar energy
SB-CS can be expected to be substantially more endergonic in transition metal complexes (TMCs) compared to organic systems, with deleterious nonradiative decay processes such as intersystem crossing or rapid deactivation to low-lying metal-centered states[18] making it a priori energetically untenable. This is true for recently reported TMCs where self-quenching products have been hypothesized as intermediates in photocatalytic reactions.[19−22] Reported values indicate that the charge separation step can be expected to be fairly endergonic;[23] one may postulate that the overall process might be driven by the exothermicity of ensuing reactions

Summary

Introduction

Photoinduced symmetry-breaking charge separation (SB-CS) processes offer the possibility of harvesting solar energy by electron transfer between identical molecules. (a) Molecular Structure of [FeIIIL2]+, Excited State Energy (E0‐0) and Free Energies (b) Latimer Diagram,a (c) Thermodynamics of SB-CS for a of Charge Separation (ΔGCS°) and Recombination (ΔGCR°)b

Results

Conclusion