Iron(ii) complexes with diazinyl-NHC ligands: impact of π-deficiency of the azine core on photophysical properties

Abstract

Boosting iron(ii) complex excited-state lifetime by combining pyrazine and benzimidazolylidene NHC ligands.

Design and Synthesis of Photoactive Iron N-Heterocyclic Carbene Complexes

“…This is also reflected in the electrochemical bandgap -the difference between the first oxidation and reduction wave in CV, which is often used to estimate the HOMO-LUMO gap by electrochemical means. This gap is smaller for the newly reported complexes in comparison to the parental complex [Fe(pbmi) 2 ] 2+ as the reduction is more significantly anodically shifted (= to more positive potentials) than the oxidation [35]. significantly than the energy of the t2g orbitals und thus decreases the HOMO-LUMO gap (c.f., Figure 6).…”

“…In the complexes containing n-hexyl instead of methyl substituents, such puckering motion could be sterically hindered to some extent, explaining the sequential decay of the excited state and longer lifetimes found for complexes with n-hexyl substituents. Further, the benzimidazolylidene moieties in [Fe(pzbhbi) 2 ] 2+ can accept additional electron density, minimizing the detrimental effects for this complex and explaining its superior lifetime [35] compared to the other complexes presented in this paragraph. A further example of parallel decay will be explained in some more detail below (see Section 4).…”

“…Altering the central six-membered ring of CNC ligands is another approach to modifying the π properties. A series of complexes following this concept by replacing the pyridine ring with pyrazine or pyrimidine was recently published and led to the structures shown in Figure 7 [35]. Steady state absorption spectra show a bathochromic shift (sometimes also called redshift) of the low-energy MLCT band of 30-40 nm for all these complexes compared to [Fe(pbmi) 2 ] 2+ .…”

“…The involved states have triplet multiplicity, however, undergoing a MLCT → MC conversion when progressing along the reaction coordinate. Introduction of the pyrazine and n-hexyl moieties quite significantly increases the longest lifetime compared to parental [Fe(pbmi) 2 ] 2+ (around 9 ps) and [Fe(pbmbi) 2 ] 2+ (16 ps) [21,33,35]. As the long time component of [Fe(pzbmi) 2 ] 2+ (pzbmi = (pyrazine-2,6-diyl)bis(1-methyl-imidazol-2-ylidene)) ( Figure 7) was found to be in the range of 21 to 25 ps, most of the influence seems to stem from the changes induced by alteration of the six-membered ring [35].…”

Recombination and regeneration dynamics in FeNHC(ii)-sensitized solar cells

“…One of the most successful strategies employed to delay the deactivation is to increase the ligand field strength. [10][11][12][13][14][15][16] Significant advances have been recently reported by the use of Pyridyl-Nheterocyclic carbenes (NHC), 10,[13][14][15][16] resulting in notable improvement of 3 MLCT lifetimes up to 16 ps 13 for complex C1 (inset Figure 1) combining -donating NHCs and -accepting carboxylic groups to be covalently grafted to the TiO 2 surface. The first photovoltaic characterization of a Fe(II)-NHC complex, reported by some of us, 13,17 revealed a weak power conversion efficiency (0.13%).…”