Extended π-Conjugated Ligands Tune Excited-State Energies of Iron(II) Polypyridine Dyes

Abstract

Over the past decade, iron(II) polypyridines have gained a lot of attention as potential chromophores and sensitizers due to the low cost and high abundance of iron. Unfortunately, most iron(II) polypyridines are poor chromophores since their initially excited, photoactive metal-to-ligand charge transfer (MLCT) states quickly decay into non-photoactive metal-centered (MC) states. Many strategies to increase their lifetime have been pursued, built mainly around increasing the ligand field strength of these complexes and thus destabilizing the MC states. In this work, we aim to design a new class of Fe(II) complexes by stabilizing the energies of their MLCT states. To this end, we employ density functional theory (DFT) and time-dependent DFT to investigate a series of Fe(II) complexes, [Fe(L/X) 2,4 (N^N)] 2+/2− where L/X represents either cyanide, isocyanide, or bipyridine ligands and N ̂N stands for bidentate-extended π-conjugated ligands derived from the bipyridine. The L/X ligands tune the energetics of the Febased t 2g molecular orbitals, while the amount of π-conjugation on the N^N ligand impacts the energies of its π and π* orbitals, thus tuning the energetics of the MLCT and the ligand-centered (LC) states. Overall, our results suggest that the use of N^N ligands with the extended π-conjugation is a viable strategy to tune the relative energies of MLCT, LC, and MC states.

Photoactive Metal-to-Ligand Charge Transfer Excited States in 3d6 Complexes with Cr0, MnI, FeII, and CoIII

“…34 Further MLCT excited- state tuning in iron(II) cyanido complexes is possible through variation of the α-diimine ligands. 35 N-Heterocyclic carbene (NHC) ligands emerged as better alternatives to polypyridines as far as long 3 MLCT lifetimes are concerned, largely owing due their stronger σ-donor properties. 6,7,36,37 Following an initial study published in 2013, 38 a series of conceptually related Fe II compounds (Figure 4a-f) with chelating NHC ligands featuring MLCT lifetimes on the order of a few picoseconds were discovered (Table 2), 39−45 which represented record lifetimes at that point.…”

“…Borylation of the peripheral nitrogen atoms of the cyanido ligands presumably further strengthens the ligand field, yet the detectable excited-state lifetime of 28 ps in [Fe­(bpy)­(BCF) 4 ] 2– (BCF = tris­(pentafluorophenyl)) (Figure c) was attributed to an MC rather than an MLCT state . Further MLCT excited-state tuning in iron­(II) cyanido complexes is possible through variation of the α-diimine ligands …”

Molecular Engineering to Tune Functionality: The Case of Cl-Substituted [Fe(terpy)2]2+

“…Theoretical chemistry-based ligand design can be a great asset for such efforts, yet high-level wave function-based computations are too expensive for applying them routinely to systems as large as transition-metal complexes. However, DFT offers a viable alternative, and the recent developments in understanding and designing certain properties of similar complexes computationally − as well as the vast accumulated knowledge on the Fe­(II) spin state transition systems ,− provide us with a great opportunity to use such complexes as a testing ground in evaluating the impact of different ligand modifications. We selected a prototypical Fe­(II) polypyridine complex to assess how electronic substitution effects can modify the potential energy landscape, in particular the singlet–quintet energy gap and the transition rate between these states.…”

Molecular engineering to tune functionality: the case of Cl substituted [Fe(terpy)2]2+

“…Theoretical chemistry based ligand design can be a great asset for such efforts, yet high-level wave-function-based computations are too expensive for applying them routinely to systems as large as transition metal complexes. How-ever, DFT offers a viable alternative, and the recent developments in understanding and designing certain properties of similar complexes computationally [8][9][10][11][12][13][14][15][16][17] , as well as the vast accumulated knowledge on the Fe(II) spin state transition systems 12,[18][19][20] provide us with a great opportunity to use such complexes as a testing ground in evaluating the impact of different ligand modifications. We selected a prototypical Fe(II) polypyridine complex to assess how electronic substitution effects can modify the potential energy landscape, in particular the singlet-quintet energy gap and the transition rate between these states.…”