Design and Synthesis of Photoactive Iron N-Heterocyclic Carbene Complexes

Abstract

The use of iron in photoactive metal complexes has been investigated for decades. In this respect, the charge transfer (CT) states are of particular interest, since they are usually responsible for the photofunctionality of such compounds. However, only recently breakthroughs have been made in extending CT excited state lifetimes that are notoriously short-lived in classical polypyridine iron coordination compounds. This success is in large parts owed to the use of strongly σ-donating N-heterocyclic carbene (NHC) ligands that help manipulating the photophysical and photochemical properties of iron complexes. In this review we aim to map out the basic design principles for the generation of photofunctional iron NHC complexes, summarize the progress made so far and recapitulate on the synthetic methods used. Further, we want to highlight the challenges still existing and give inspiration for future generations of photoactive iron complexes.

Photophysical Investigation of Iron(II) Complexes Bearing Bidentate Annulated Isomeric Pyridine-NHC Ligands

“…[16][17][18][19] However, the combination of the -deficiency of pyridine and the -donation of NHC in pyridyl-NHC iron(II) complexes is effective in altering the energy order of the excited state manifold. Thus, important breakthroughs in increasing the MLCT lifetime have been achieved, 8,[20][21][22][23][24][25] from tens of picoseconds for azinyl-NHC iron(II) complexes with tridentate C^N^C [26][27][28][29][30] and bidentate C^N ligands 31,32 to the nanosecond domain for scorpionate-like C^C^C ligands, this latter even showing luminescent properties. 33 We have recently shown that for C^N^C ligands, framing the carbene moiety with extended conjugated systems notably stabilizes the MLCT excited state, and prevents spin crossover into the MC quintuplet 5 MC, by enhancing the -back donation from the iron center to the ligand, hence leading to prolonged excited state lifetimes.…”

Iron’s Wake: The Performance of Quantum Mechanical-Derived Versus General-Purpose Force Fields Tested on a Luminescent Iron Complex

“…However, the synergic effect of deficiency of pyridine and -donation capacity of N -heterocyclic carbenes (NHCs) in pyridyl-NHC iron(II) complexes, has been proven effective in altering the energy order of the excited state manifold, inducing a destabilization of MC states with respect to MLCT ones and thus suppressing their accessibility. Thus, important breakthroughs in increasing the MLCT lifetime have been reported in the last years [ 28 , 29 , 30 , 31 , 32 ], spanning from tens of picoseconds [ 33 , 34 , 35 , 36 ] to the nanosecond time scale [ 37 ]. Even though simple concepts based on the ligand field strength [ 38 ] can provide general guidelines to increase the MLCT states lifetime of iron TMCs, a more subtle understanding of the various electronic, structural and environmental effects ruling the excited states relaxation process has been recognized as of pivotal importance [ 39 , 40 ].…”

Photophysics and Photochemistry of Iron Carbene Complexes for Solar Energy Conversion and Photocatalysis

“…Given the rapid development of this research area, the ambition here has been to include a comprehensive survey of key developments without unduly repeating lengthy discussions from previous reviews mentioned for some of the early work, which is referenced when beneficial. This review is also accompanied in this issue by a review by Kaufhold and Wärnmark focused on the design and synthesis of the iron carbenes [34].…”