Hydrogen-Bond Strengthening upon Photoinduced Electron Transfer in Ruthenium–Anthraquinone Dyads Interacting with Hexafluoroisopropanol or Water

Abstract

Quinones play a key role as primary electron acceptors in natural photosynthesis, and their reduction is known to be facilitated by hydrogen-bond donors or protonation. In this study, the influence of hydrogen-bond donating solvents on the thermodynamics and kinetics of intramolecular electron transfer between Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridine) and 9,10-anthraquinone redox partners linked together via one up to three p-xylene units was investigated. Addition of relatively small amounts of hexafluoroisopropanol to dichloromethane solutions of these rigid rodlike donor-bridge-acceptor molecules is found to accelerate intramolecular Ru(bpy)(3)(2+)-to-anthraquinone electron transfer substantially because anthraquinone reduction occurs more easily in the presence of the strong hydrogen-bond donor. Similarly, the rates for intramolecular electron transfer are significantly higher in acetonitrile/water mixtures than in dry acetonitrile. In dichloromethane, an increase in the association constant between hexafluoroisopropanol and anthraquinone by more than 1 order of magnitude following quinone reduction points to a significant strengthening of the hydrogen bonds between the hydroxyl group of hexafluoroisopropanol and the anthraquinone carbonyl functions. The photoinduced intramolecular long-range electron transfer process thus appears to be followed by proton motion; hence the overall photoinduced reaction may be considered a variant of stepwise proton-coupled electron transfer (PCET) in which substantial proton density (rather than a full proton) is transferred after the electron transfer has occurred.

Proton‐Coupled Electron‐Transfer Processes in Ultrafast Time Domain: Evidence for Effects of Hydrogen‐Bond Stabilization on Photoinduced Electron Transfer

“…[26] Despite several literaturer eports, the majority of studies on the mechanistic understanding of PCET processes have focused on the electronic ground state; [27,28] there are only limited reports on the PCET process associated with the electronically excited state. [34,35] In general,t he electronic absorption spectra of transition metal complexes depend on the oxidation state of the corresponding metal ion, and such complexes with well-defined redox and excited-state properties are ideal for studying the PCET process. Although it has been argued that studies on excited-state dynamics and femtosecond transienta bsorption spectroscopy are crucial to gain ab etter insight into PCET processes, systematic studies in the ultrafast time domain are still lacking.…”

“…The two subsequent reversible AQ/AQC À and AQC À /AQ 2À couples appeared at À0.95 and À1.24 V( vs. Fc/Fc + ), respectively.T he Ru II/III couple provides an estimate of the HOMO energy level.F or the RuQ complex, two more bpy-based reductionsw ere observeda tÀ1.77 and À1.97 V( vs. Fc/Fc + ). [35] In the present study,t he AQ/AQC À couple appeared at À0.95 V( vs. Fc/Fc + ); this facile reduction is attributedt ot he coordination of L 2 to the cationic Ru II center. [36] Note that the second bpy reduction was quasireversible;d ifferential pulsed voltammetry (DPV) plots for these two redox processes are shown in Figure 1.…”

“…[36] The presenceo ft he extended conjugation and the electronwithdrawing groupss uch as imidazole/anthraquinonei s expectedt ol ower the L 2 -based LUMO energy in RuQ, and this furthers upports the observed longer-wavelength absorption band maximum for RuQ compared with [Ru(bpy) 3 ] 2+ .S teadystate luminescence spectra for [Ru(bpy) 3 ] 2+ and RuQ in acetonitrile (at 298 K) are shown in Figure 2a' and 2b',r espectively. [35] For RuQ, intramolecular H-bonding betweeno ne of the two carbonyl moieties of the anthraquinone fragment and NH imidazole further lowerst he energy level of the anthraquinone LUMO, which, in turn, favors the possibility of the electrontransfer process. This emission is attributedt o 3 MLCT-based emission.…”

Probing the Effect of the Solution Environment around Redox-Active Moieties Using Rigid Anthraquinone Terminated Molecular Rulers

“…More recently, it was demonstrated by Hankache et al 42 Another approach that was employed to change the position of the AQ moieties relative to the distal surface of the SAM is by comparing the S-NB and L-NB systems using the same diluent (Figure 7). It was shown in the SFG section that the L-NB molecule situate the AQ moiety almost tangential to the surface of the diluent while in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 24 the S-NB the AQ moiety is almost 45 ° from the surface normal.…”

Photoinduced Electron and Proton Transfer with Metal Complexes and Organic Molecules

“…Interestingly, this lifetime correlates with the hydrogen-bond donor strength of the solvent rather than with the solvent dielectric constant. [19,21] For example, in molecule 14 τ CR increases from 50 ns in CH 2 Cl 2 to 2 µs in hexafluoroisopropanol. [20] This suggests that the anthraquinone radical anion is stabilized by hydrogen-bond formation, and this may be considered a variant of PCET.…”