Excited State Interaction of Ruthenium (II) Imidazole Phenanthroline Complex [Ru(bpy)₂ipH]²⁺ with 1,4‐Benzoquinone: Simple Electron Transfer or Proton‐Coupled Electron Transfer?

Abstract

The unidirectional proton coupled electron transfer (PCET) from the excited state of Ru(II) imidazole phenanthroline complex [Ru(bpy) ipH] to 1,4-benzoquinone, was studied by steady-state (SS) and time-resolved (TR) fluorescence and transient absorption (TA) measurements. The pK (9.7) and pK * (8.6) values of the complex suggest that it behaves as a photoacid on excitation. The difference in the quenching rates obtained from SS and TR fluorescence studies indicate participation of both dynamic quenching and static quenching involving the hydrogen bonded ipH ligand of [Ru(bpy) ipH] with the 1,4-benzoquinone quencher, formed in the ground state. Within the hydrogen bonded complex, the ruthenium centre acts as the electron donor, while the ipH ligand acts as the proton donor to the hydrogen bonded 1,4-benzoquinone that acts simultaneously both as the electron and proton acceptor. It is proposed that the static quenching in the hydrogen bonded [Ru(bpy) ipH] -1,4-benzoquinone pairs occurs involving the PCET mechanism, while the dynamic quenching occurs through the simple ET mechanism, on diffusional encounter of the isolated 1,4-benzoquinone with the excited [Ru(bpy) ipH] complex. The occurrence of broad TA bands around 420-430 nm suggests formation of both 1,4-benzoquinone radical anion as well as the 1,4-benzosemiquinone radical by the interaction of excited [Ru(bpy) ipH] with 1,4-benzoquinone, thus supporting the ET process in the studied system.

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

“…From the inflection point of the titration curve obtained by plotting the absorbance changes at 287, 316, 349, and 405 nm as a function of the proton concentration in the solution (cf. Figure S1b,c), 7,9,13,34 a pK a,1 value of approximately 2 for both complexes, corresponding to a deprotonation of the imidazolium cation, is obtained. Previous studies on Ru(II) complexes bearing an ip ligand have reported similar pK a values.…”

“…Both the absorptivities at 288 nm and at around 470 nm (shoulder) increase, while the absorption band at 316 nm vanishes. 7,20,35 The existence of three isosbestic points at 327, 390, and 435 nm during the spectrophotometric base-titration (from 1 up to 500 molar equivalents of base) demonstrates that only two absorbing species, namely, Ru-im and Ru-im − , are observed and thus a single deprotonation step occurs. The obtained pK a,2 value of 10, corresponding to the deprotonation of the secondary amine (cf.…”

“…Generally, both the electronic ground and excited states of Ru(II) complexes bearing imidazole-containing ligands are pH responsive. [7][8][9]14,19,20 Hence, the rR experiments are complemented by sub-ps (fs-TA) and ns-time-resolved transient absorption spectroscopy (ns-TA). Moreover, the long-lived excited states are studied by means of time-resolved emission spectroscopy.…”

“…Figure S2), and observed changes in the ground-state absorption spectra are similar to observations on Ru(II) complexes bearing ip-derived ligands. 7−9,20,35−37 As the imidazolate acts as a stronger π-donor than the neutral imidazole unit, 7 the electron density at the metal center increases, yielding a destabilization of phen-and tbbpycentered unoccupied molecular orbitals. Therefore, the corresponding MLCT features are blue-shifted upon deprotonation.…”

Proton-controlled Action of an Imidazole as Electron Relay in a Photoredox Triad

“…The GS pK a value of the imidazole group is about 19.8 ± 0.3 in the three complexes. Furthermore, a photoacid behavior of complexes bearing an imidazole unit fused to a ruthenium chromophore has been reported in the literature [14,27]. This property appears amplified in presence of ester-substituted bipyridine ligands, as clearly evidenced by the observed reversible flash-induced deprotonation of complex 3 in presence of a base, visible as a red-shift of the MLCT band of the ruthenium chromophore (Fig.…”