Ligand‐Induced Donor State Destabilisation – A New Route to Panchromatically Absorbing Cu(I) Complexes

Abstract

The intense absorption of light to covering a large part of the visible spectrum is highly desirable for solar energy conversion schemes. To this end, we have developed novel anionic bis(4H‐imidazolato)Cu(I) complexes (cuprates), which feature intense, panchromatic light absorption properties throughout the visible spectrum and into the NIR region with extinction coefficients up to 28,000 M−1 cm−1. Steady‐state absorption, (spectro)electrochemical and theoretical investigations reveal low energy (Vis to NIR) metal‐to‐ligand charge‐transfer absorption bands, which are a consequence of destabilized copper‐based donor states. These high‐lying copper‐based states are induced by the σ‐donation of the chelating anionic ligands, which also feature low energy acceptor states. The optical properties are reflected in very low, copper‐based oxidation potentials and three ligand‐based reduction events. These electronic features reveal a new route to panchromatically absorbing Cu(I) complexes.

Trendbericht: Photochemie

“…Schulz konzipierte neue, anionische, homoleptische Cu I -Komplexe des Typs [Cu(N^N) 2 ] -, die auf dem 4H-Imidazolatliganden beruhen (Abbildung 2). 20,21) Die beiden anionischen Liganden destabilisieren die kupferbasierten Donororbitale, sodass intensive MLCT-Übergänge vom Sichtbaren bis zirka 1000 nm auftreten.…”

Photophysics of Anionic Bis(4H‐imidazolato)CuI Complexes

“…Recently, we reported a novel class of Cu I complexes, namely homoleptic, anionic bis(4 H ‐imidazolato)Cu I cuprates. [26] This Cu I complex class features an unprecedented broad and intense absorption spectrum ranging from the UV into the NIR region and exhibits low‐energy metal‐to‐ligand charge‐transfer (MLCT) transitions. The low‐energy MLCT bands are due to the destabilization of the copper‐based donor states by the two anionic 4 H ‐imidazolate ligands and a stabilization of the ligand‐based acceptor states.…”

“…The low‐energy MLCT bands are due to the destabilization of the copper‐based donor states by the two anionic 4 H ‐imidazolate ligands and a stabilization of the ligand‐based acceptor states. [26] Besides MLCT transitions, the main absorption band also contains ligand‐centered (LC) bands. Computational investigations of the electronic properties of these homoleptic bis(4 H ‐imidazolato)Cu I complexes indicated the existence of just two acceptor orbitals (LUMO and LUMO+1) for the NIR and visible transitions.…”

“…Detailed electrochemical investigations showed that the energy levels of the ligand‐based electron acceptor states can be controlled by the substitution pattern of the N ‐aryl substituents while the energy levels of the copper‐based oxidation states is controlled by the steric demand of the N ‐aryl substituents. [26] …”

“…The complexes Cu − F, Cu − Cl, and Cu − CF 3 (Figure 1 ) were selected due to the increasing electron‐withdrawing properties of the N ‐aryl moieties. [26] Additionally, complex Cu −2C F 3 with increased steric demand was synthesized and studied. Our investigations reveal excited‐state dynamics, which fundamentally differ from the related neutral, heteroleptic [Cu I (xantphos)(4 H ‐imidazolato)] complexes[ 27 , 28 ] as well as from homoleptic, cationic Cu I bis(phenanthroline)‐ or Cu I bis(bipyridine)‐type complexes.…”