Excitation Energy-Dependent Branching Dynamics Determines Photostability of Iron(II)–Mesoionic Carbene Complexes

Abstract

Photoactive metal complexes containing earth-abundant transition metals recently gained interest as photosensitizers in light-driven chemistry. In contrast to the traditionally employed ruthenium or iridium complexes, iron complexes developed to be promising candidates despite the fact that using iron complexes as photosensitizers poses an inherent challenge associated with the low-lying metal-centered states, which are responsible for ultrafast deactivation of the charge-transfer states. Nonetheless, recent developments of strongly σ-donating carbene ligands yielded highly promising systems, in which destabilized metal-centered states resulted in prolonged lifetimes of charge-transfer excited states. In this context, we introduce a series of novel homoleptic Fe–triazolylidene mesoionic carbene complexes. The excited-state properties of the complexes were investigated by time-resolved femtosecond transient absorption spectroscopy and quantum chemical calculations. Pump wavelength-dependent transient absorption reveals the presence of distinct excited-state relaxation pathways. We relate the excitation-wavelength-dependent branching of the excited-state dynamics into various reaction channels to solvent-dependent photodissociation following the population of dissociative metal centered states upon excitation at 400 nm.

Chemistry of Compounds Based on 1,2,3-Triazolylidene-Type Mesoionic Carbenes

“…These complexes have been synthesized from the corresponding triazolium salts of type 80 via deprotonation using KO t Bu in the presence of iron(II) bromide. 104 …”

“…In this complex, destabilized metal-centered states resulted in prolonged lifetimes of charge-transfer excited states. 104 …”

Ultrafast Spectroscopy of Fe(II) Complexes Designed for Solar‐Energy Conversion: Current Status and Open Questions

“…[47] Finally, ligand dissociative reaction channels were very recently reported in tridentate Fe(II) complexes bearing mesoionic triazolylidene ligands, when these were excited into higher lying excited states. [48] Cyclometalated ligands (family E) are known to be even stronger σ donors than carbenes, but the synthesis of Fe(II) complexes with pyridylbenzene ligands is yet more challenging than with NHCs. Taking up on theoretical predictions [49][50][51] of exceptionally strong LFSE and destabilisation of the MC state, first heteroleptic complexes were published very recently with a phenylÀ bipyridine (pbpy) ligand and a terpyridine (tpy) ligand (family E).…”

Mesoionic Carbenes in Low- to High-Valent Vanadium Chemistry

“…Almost two decades after the first report of an abnormal 5-imidazolinylidene carbene complex, mesoionic carbenes have been developed into a distinguished ligand class. , Among them, 1,2,3-triazole derived mesoionic carbenes, namely 1,2,3-triazolinylidenes, stand out by their modular synthesis via the copper-catalyzed [3 + 2] cycloaddition between azides and alkynes. − After their initial reporting by Albrecht et al , they quickly became prominent synthetic targets for (electro-)catalysis, − supramolecular chemistry, − magnetism, and photochemistry due to their versatile synthesis and comparatively straightforward handling. − Throughout these studies, a great effort has been made to decipher their electronic structure. Mesoionic carbenes are commonly believed to be strong σ-donor ligands paralleling heteroaryls; however, recent reports emphasize their π-accepting properties − as demonstrated by the isolation of a reduced triazolinylidene ligand .…”