Bidentate Pyridyl‐NHC Ligands: Synthesis, Ground and Excited State Properties of Their Iron(II) Complexes and the Role of the fac/mer Isomerism

Abstract

Iron complexes are promising candidates for the development of sustainable molecular photoactive materials as an alternative to those based on precious metals such as Ir, Pt or Ru. These compounds possess metal‐ligand charge transfer (MLCT) transitions potentially of high interest for energy conversion or photocatalysis applications if the ultrafast deactivation via lower‐lying metal‐centred (MC) states can be impeded. Following an introduction describing the main design strategies used so far to increase the MLCT lifetimes, we review some of our latest contributions to the field regarding bidentate Fe(II) complexes comprising N‐heterocyclic carbene ligands. The discussion covers all aspects from their synthesis to their characterization via photophysical, electrochemical and computational techniques. The impact of bidentate coordination together with the configuration (facial and meridional isomers) is analysed, finally highlighting the current challenges in this promising area.

Ligand-Field Spectroscopy of Co(III) Complexes and the Development of a Spectrochemical Series for Low-Spin d6Charge-Transfer Chromophores

“…The most successful approaches have exploited the strong σ-donating characteristics of carbene and isocyanide ligands. The pioneering example was published by Wärnmark and co-workers in 2013, but this has since been followed by and expanded upon through work from a number of groups including Wenger, Heinze, Bauer, , Herbert, , and Gros , to name a few. A less well-developed but potentially promising approach of exploiting information from quantum coherence was recently demonstrated by us, but for both of these research threads, the problem ultimately distills down to synthetic design.…”

Photophysical Integrity of the Iron(III) Scorpionate Framework in Iron(III)–NHC Complexes with Long-Lived 2LMCT Excited States

“…However, the introduction of strongly σ-donating N -heterocyclic carbene (NHC) ligands in the field of photoactive iron complexes , has significantly increased the excited state lifetime of Fe­(II) MLCT states, reaching up to 528 ps . By increasing the ligand field strength, the MC states increase in energy, thus slowing down the deactivation of the MLCT state. ,, The photophysical properties of Fe–NHC metal complexes have been further improved using different approaches. − …”

“… 6 , 14 , 15 The photophysical properties of Fe–NHC metal complexes have been further improved using different approaches. 16 − 20 …”

A multiscale free energy method reveals an unprecedented photoactivation of a bimetallic Os(ii)–Pt(ii) dual anticancer agent

“…48 After photoexcitation to the singlet manifold, triplet states are accessed through readily operative ultrafast intersystem crossing (ISC) processes characteristic of complexes based on heavy metals. [50][51][52][53][54] Typically, the excited-state cooling (occurring in a few hundreds of fs) relax the chromophore to its lowest-lying excited states, which ultimately govern the photophysical and photochemical decay routes. [55][56][57][58][59][60] Therefore, the ISC will be dictated by the interplay between the lowest-lying singlet excited states and energetically close triplet states.…”