Dual Emission and Excited-State Mixed-Valence in a Quasi-Symmetric Dinuclear Ru–Ru Complex

Abstract

The synthesis and characterization of the new dinuclear dipeptide [(EtOOC-tpy)Ru(tpy-NHCO-tpy)Ru(tpy-NHCOCH3)](4+) 3(4+) of the bis(terpyridine)ruthenium amino acid [(HOOC-tpy)Ru(tpy-NH2)](2+) 1(2+) are described, and the properties of the dipeptide are compared to those of the mononuclear complex [(EtOOC-tpy)Ru(tpy-NHCOCH3)](2+) 4(2+) carrying the same functional groups. 3(4+) is designed to serve a high electronic similarity of the two ruthenium sites despite the intrinsic asymmetry arising from the amide bridge. This is confirmed via UV-vis absorption and NMR spectroscopy as well as cyclic voltammetry. 4(2+) and 3(4+) are emissive at room temperature, as expected. Moreover, 3(4+) exhibits dual emission from two different triplet states with different energies and lifetimes at room temperature. This is ascribed to the presence of a unique thermal equilibrium between coexisting [Ru(II)(tpy-NHCO-tpy(·-))Ru(III)] and [Ru(III)(tpy-NHCO-tpy(·-))Ru(II)] states leading to an unprecedented excited-state Ru(II)Ru(III) mixed-valent system via the radical anion bridge tpy-NHCO-tpy(·-). The mixed-valent cation 3(5+), on the other hand, shows no measurable interaction of the Ru(II)Ru(III) centers via the neutral bridge tpy-NHCO-tpy (Robin-Day class I). Reduction of 3(4+) to the radical cation 3(3+) by decamethylcobaltocene is bridge-centered as evidenced by rapid-freeze electron paramagnetic resonance spectroscopy. Interestingly, all attempts to observe 3(3+) via NMR and UV-vis absorption spectroscopy only led to the detection of the diamagnetic complex 3-H(3+) in which the bridging amide is deprotonated. Hence 3-H(3+) (and 4-H(+)) appear to reduce protons to dihydrogen. The ease of single and double deprotonation of 4(2+) and 3(4+) to 4-H(+), 3-H(3+), and 3-2H(2+) was demonstrated using a strong base and was studied using NMR and UV-vis absorption spectroscopies. The equilibrating excited triplet states of 3(4+) are reductively quenched by N,N-dimethylaniline assisted by hydrogen bonding to the bridging amide.

Understanding the Excited State Behavior of Cyclometalated Bis(tridentate)ruthenium(II) Complexes: A Combined Experimental and Theoretical Study

“…At 77 K, a much more intense emission is observed with an emission maximum at 716 nm and a band shape typical for a ruthenium-based emission arising from a single vibronic progression (see Supporting Information Figure S27). 27,88 The temperature dependence of the phosphorescence of 2(PF 6 ) is shown in Figure 5. The emission intensity rapidly increases upon lowering the temperature.…”

“…The oxidation steps in the potential range of −1.0 to −0.5 V for 1(PF 6 ) following the irreversible reduction processes are similar to those observed for bis(terpyridine)ruthenium(II) complexes bearing amide functional groups and might be associated with reduction of the NH proton at the terpyridine moiety to hydrogen. 88 Electron Paramagnetic Resonance Studies on Redox Products. As evidenced from the cyclic voltammograms, both complexes can be oxidized to 1 2+ and 2 2+ using tris(4-bromophenyl)aminium hexachloridoantimonate as oxidant (0.67 V vs FcH/FcH + in acetonitrile, 0.70 V vs FcH/FcH + in dichloromethane) and reduced to 1 and 2 using decamethylcobaltocene Co(Cp*) 2 as reductant (−1.91 V vs FcH/FcH + in acetonitrile, −1.94 V vs FcH/FcH + in dichloromethane).…”

Excited state decay of cyclometalated polypyridine ruthenium complexes: insight from theory and experiment

“…2). 118,152 Excitation into the tpy-centered LUMO is only possible from the HOMO−1, while the dpb-centered LUMO+2 can be reached from the HOMO as evidenced from timedependent DFT calculations. Both excitations occur at very similar energy and contribute comparably to the absorption band at 500 nm.…”

Strongly Coupled Cyclometalated Ruthenium Triarylamine Chromophores as Sensitizers for DSSCs

“…Additionally, two reversible reduction waves are observed for the ester‐substituted complexes [ 1 a ][PF 6 ]–[ 1 c ][PF 6 ]. As these reductions are tpy‐centered, their potentials are essentially independent from the substitution pattern at the dpb‐X ligand with the first reduction occurring at −1.5 V and the second at −1.85 V vs. FcH/FcH + for all three complexes.…”

“…The [62] The trend of the first andt he second oxidation potentials towards higher values in the order N(4-C 6 H 4 OCH 3 ) 2 < N(C 6 H 5 ) 2 < N-carbazole is in agreement with the decreasing + Ie ffect of the respectivea mine substituent X. Additionally, the unpairede lectron of the mixed-valent compounds[ 1] 2 + is substantially delocalized between the metal center and the triarylamine fragment via the 1,4-phenylene bridge. [62, 63a] The electron donor strength of the dpb substituent [62, 63a] strongly affects degree of delocalization as evidenced by DFT calculations (Figure 3b 87,88] their potentials are essentially independentf rom the substitution pattern at the dpb-X ligand with the first reduction occurring at À1.5 Va nd the second at À1.85 Vv s. FcH/FcH + for all three complexes. Deprotectiona nd deprotonation of the ester functionalities, however,s hifts the first ands econd reduction potentials by 600 mV towards more negative values (Figure 1, Ta ble 1; Supporting Information, Figure S5).…”