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

Abstract

Bi­(benz)­imidazoles (b­(b)­im) acting as N,N-chelates in ruthenium complexes represent a unique class of ligands. They do not harbor metal-to-ligand charge-transfer (MLCT) excited states in ruthenium polypyridyl complexes upon visible-light excitation provided that no substitution is introduced at the N atoms. Hence, they can be used to steer light-driven electron-transfer pathways in a desired direction. Nonetheless, the free N atoms are susceptible to protonation and, hence, introduce highly pH-dependent properties into the complexes. Previous results for ruthenium complexes containing R 2 bbim ligands with alkylic or arylic N,N′-substitution indicated that, although pH insensitivity was accomplished, unexpected losses of spectator ligand features incurred simultaneously. Here, we report the synthesis and photophysical characterization of a series of differently N,N′-alkylated b­(b)­im ligands along with their corresponding [(tbbpy)2Ru­(R 2 b­(b)­im)]­(PF6)2 complexes (tbbpy = 4,4′-tert-butyl-2,2′-bipyridine). The data reveal that elongation of a rigid ethylene bridge by just one methylene group drastically increases the emission quantum yield, emission lifetime, and photostability of the resultant complexes. Quantum-chemical calculations support these findings and allow us to rationalize the observed effects based on the energetic positions of the respective excited states. We suggest that N,N′-propylene-protected 1H,1′H-2,2′-biimidazole (prbim) is a suitable spectator ligand because it stabilizes sufficiently long-lived MLCT excited states exclusively localized at auxiliary bipyridine ligands. This ligand represents, therefore, a vital building block for next-generation photochemical molecular devices in artificial photosynthesis.

Outpacing conventional nicotinamide hydrogenation catalysis by a strongly communicating heterodinuclear photocatalyst

“…At more negative potentials, i.e., between −1.6 and −2.2 V vs. Fc + /Fc, 1 – 4 feature three reduction waves. At −1.9 V and −2.2 V vs. Fc + /Fc the terminal bipyridine ligands become reduced 34 , 35 , while the first reduction in the mononuclear complexes is localized on the BL. For the alkynyl complexes 1 and 2 , this reduction (−1.6 V vs. Fc + /Fc) is shifted anodically by ~100 mV compared to the triazole complexes 3 and 4 at −1.7 V vs. Fc + /Fc.…”

Light‐Driven Multi‐Charge Separation in a Push‐Pull Ruthenium‐Based Photosensitizer – Assessed by RASSCF and TDDFT Simulations

“…However, structural modification of the bpy‐ligands, such as with electron donating groups, might tunes the relative electron acceptor potentials in Ru sufficiently to allow a stepwise double reduction on the target ligand. Alternatively, the electron deficient bipyridyl ligand can be replaced by electron rich ligands, such as, for example, carbene or biimidazole ligands, in order to localize the excited‐state relaxation and redox processes on the desired ligand sphere as shown very recently in the literature [67–71] …”

“…Alternatively, the electron deficient bipyridyl ligand can be replaced by electron rich ligands, such as, for example, carbene or biimidazole ligands, in order to localize the excited-state relaxation and redox processes on the desired ligand sphere as shown very recently in the literature. [67][68][69][70][71]…”

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.…”