Modified bibenzimidazole ligands as spectator ligands in photoactive molecular functional Ru-polypyridine units? Implications from spectroscopy

Abstract

The photophysical properties of Ruthenium-bipyridine complexes bearing a bibenzimidazole ligand were investigated. The nitrogens on the bibenzimidazole-ligand were protected, by adding either a phenylene group or a 1,2-ethandiyl group, to remove the photophysical dependence of the complex on the protonation state of the bibenzimidazole ligand. This protection results in the bibenzimidazole ligand contributing to the MLCT transition, which is experimentally evidenced by (resonance) Raman scattering in concert with DFT calculations for a detailed mode assignment in the (resonance) Raman spectra.

Influence of the Protonation State on the Excited-State Dynamics of Ruthenium(II) Complexes with Imidazole π-Extended Dipyridophenazine Ligands

“…For bim and bbim systems, deprotonation of the ligands caused a complete loss of the emission and a bathochromic shift of the MLCT absorption maximum (by 0.45 eV). , Contrarily, the excited-state lifetimes of 4 H -im-derived complexes shorten upon protonation of the 4 H -imidazole ligand. , This limits their applicability in systems for solar fuel generation, where drastic pH changes might occur during the catalytic turnover. In case of Ru­(II) complexes bearing ip-derived ligands, it was found that changes in the protonation state of the imidazole induce changes in the electronic configuration of the lowest excited-state, ultimately affecting the excited-state lifetime and emission properties. − However, it was found that protection of the acidic protons of the imidazole unit, e.g ., the substitution of the N,N ′-acidic protons of bim and bbim ligands by a propylene group or substitution of the N -acidic proton of ip derivatives, yielded complexes with pH-invariant properties and prolonged excited-state liefetimes. − Hence, this strategy allowed us to keep the favorable properties of the complexes ( i.e. , high molar absorptivity in the visible region and sufficiently long-lived excited states) at various pH values.…”

Molecular Scylla and Charybdis: Maneuvering between pH Sensitivity and Excited-State Localization in Ruthenium Bi(benz)imidazole Complexes

“…Instead, the initially excited state was also localized on the protected R 2 bbim ligands; i.e., their favorable property of acting as spectator ligands is lost upon protection with an ethylene or a phenylene group (Figure 2). 33 Following the hypothesis that these rigid protection groups might impair optimal ligand interaction with the Ru core, 33 2). The synthesis followed established protocols 34−39 and started from either 1H,1′H-2,2′-biimidazole (bim), 1H,1′H-bibenzo[d]imidazole (bbim), or its respective 4,4′,5,5′-tetramethylated derivative (tmbbim).…”

Extended charge accumulation in ruthenium–4H-imidazole-based black absorbers: a theoretical design concept

“…ruthenium(II) bipyridyl and ruthenium(II) phenanthrolyl complexes (and related systems) function as non-electron-accepting ligands and thus do not participate in low-lying excited states such as MLCT and IL states. 15,[54][55][56][57][58][59] Hence, unidirectional CT can be achieved upon photoexcitation, however, the (photo)physical and (photo)chemical properties of ruthenium(II) bibenzimidazolyl, and structurally-related complexes, are known to be highly pH-dependent, i.e., with protonation of the non-coordinating nitrogen atoms of bibenzimidazole. 15,56,58,60,61 Such pH-dependency is of interest for applications in molecular sensors but it is undesired in the scope of molecular photocatalysis.…”

“…Several attempts have been proposed to overcome this shortcoming by protecting the non-coordinating nitrogen atoms with phenylene or 1,2-ethanidyl groups. 59,62 In this contribution we present a priori quantum chemical evaluation of the photophysical and photochemical properties of four 4H-imidazole-ruthenium(II)-bi(benz)imidazolyl complexes (complexes Ru1-Ru4 in Fig. 2) upon multi-photoexcitation.…”