Symmetry-Broken Charge-Transfer Excited State in Homoleptic Zinc(II) Imidazo[1,2-a]pyridine Complexes

A series of four homoleptic cationic Zn-II complexes (Zn1-Zn4) coordinated by two substituted bis-imidazo[1,2-a]pyridine (ImPy) ligands (L1-L4) is herein presented. The ligands are functionalized with either electron acceptor or donor functional groups at the C6 positions of the two ImPy moieties. In DMF solution, all of the complexes display intense near-UV to blue emission (lambda(em)=379-450 nm) with high photoluminescence quantum yields (PLQY) up to 0.50 and short-lived excited states. Additionally, complex Zn4, containing the -NPh2 functionalized ligand, displays an interesting solvatochromic behavior. The absorption spectrum is characterized by electronic transitions with mainly ligand-centered ((LC)-L-1) and intraligand charge-transfer ((ILCT)-I-1) character, which involves a symmetric electron density redistribution at Franck-Condon (FC). In stark contrast, the subsequent excited-state dynamics relaxes the molecular symmetry and allows symmetry-breaking hole-electron pair redistribution on the bichromophoric system. As a consequence, an efficient radiative de-excitation process takes place that is ascribed to a singlet-manifold excited state with symmetry-broken charge transfer ((SBCT)-S-1) character, as supported by an in-depth photophysical, electrochemical and time-dependent density functional theory (TD-DFT) investigation. Remarkably, the (SBCT)-S-1 nature provides structureless green photoluminescence in the solid state (lambda(em) up to 520 nm for Zn4), which is rather uncommon for Zn-II complexes.