Azaullazine Meets Iron: Orbital Design Unlocking Long-Lived Emission In Fe(II) Complexes

The search for earth-abundant alternatives to ruthenium photosensitizers has positioned iron complexes as highly attractive candidates in photophysical applications. Recent advances have shown that replacing classical polypyridyl ligands with pyridyl-based N-heterocyclic carbenes (NHCs) can significantly enhance Fe(II) photoactivity, yielding excited-state lifetimes of several tens of picoseconds. [1] Yet, achieving efficient solar-energy conversion demands not only longer-lived excited states but also broad, intense, and ideally panchromatic absorption. Strategies such as extended π-conjugation and HOMO inversion have recently proven effective in expanding spectral coverage. [2] In this work, we introduce a new ligand platform based on azaullazine, a fused and highly conjugated heterocycle which synthetic access has been developed by our team. [3] By substituting the central pyridine ring of pyridyl-NHC ligands with azaullazine, we sought to reshape the frontier orbitals of Fe(II) complexes and unlock new photophysical behavior. This design allowed us to synthesize the C2 complex exhibiting one of the broadest absorption spectrum reported for Fe(II) systems, effectively achieving panchromatic light harvesting (Fig.1A). [4] A combination of transient-absorption spectroscopy, spectroelectrochemistry, DFT calculations and photoluminescence studies reveals how the extended conjugation enables frontier-orbital engineering. Most remarkably, complex C2 exhibits an unprecedented, long-lived emissive excited state, with an average lifetime of 3.5 ns, and a high fluorescence quantum yield, arising from a mixed MLCT / ILCT singlet state (Fig. 1B). This behavior, not observed in the analogous complexes C0 and C1, is enabled by the distinctive azaullazine-based ligand framework. Overall, this work demonstrates how rational frontier-orbital design, enabled by azaullazine ligands, can overturn long-standing limitations of iron photophysics and pave the way toward next-generation Fe(II) photosensitizers with performance approaching that of precious metals.