Bis(tridentate) Ruthenium(II) Complexes Containing Quinone Substituents or N-Heterocyclic Carbene Ligands

Abstract

Part I. Photoinduced Electron Transfer in Rigid, Cofacially Aligned p- Stacked Ruthenium(II)-Bridge-Quinone Systems

Section 1. Study on Unsymmetric 6-Benzoquinonyl-2,2,6,2-terpyridine Based on Ru(II) Complexes

New π-stacked [Ru(tpy)2]2+ (T_T)-benzoquinone (Q) donor-acceptor (D-A) systems, [Ru(6-(2-cyclohexa-2,5-diene-1,4-dione)-2,2:6,2-terpyridine) (2,2:6, 2-terpyridine)][PF6]2 (TQ_T), and [Ru(6-(2-cyclohexa-2,5-diene-1,4-dione)- 2,2:6,2-terpyridine)(4-phenyl-2,2:6,2-terpyridine)] [PF6]2 (TQ_TPh) have been synthesized and characterized. Orthogonal alignment of Q to the tpy ligand imposes this unit juxtaposed cofacially on the central pyridyl ring in another tpy with a typical van der Waals distance. The low-energy electronic absorptions of these complexes are mainly metal-to-ligand charge transfer (MLCT) in nature, similar to that observed in T_T benchmark system, and do not exhibit distinguishable metal-to- Q charge transfer (MQCT) absorption in spite of the proximal location of the electron acceptor unit (Q) to the electron donor unit (T_T). TD-DFT calculation supports the experimental results that the collective oscillator strength of MQCT bands remains ∼0.002. Due to the negligible intensity of MQCT bands, evaluation of HDA between the ground and the lowest energy MQCT states are not available through conventional Mulliken-Hush analysis. For such systems, HDA values were successfully evaluated from the relative difference (ξ) of the carbonyl stretching frequency between the neutral Q and its one-electron radical anion, which was determined by an ultrafast visible-pump/mid-IR-probe (TrIR) spectroscopic method. TrIR results showed that the partial charge localized on the Q moiety in the MQCT state was ca. -0.97e, and the corresponding HDA was ∼1600 cm-1. This value was in good agreement with that estimated by the Mulliken population analysis of the ground-state geometry.

Section 2. Study on 1-Benzoquinonyl NN^C Type N-Heterocyclic Carbene Based on Ru(II) Complexes

A new p-stacked donor-acceptor (D-A) system, [Ru(1-([2,2'-bipyridine]-6-yl- methyl)-3-(2-cyclohexa-2,5-diene-1,4-dionyl)-1H-imidazole)(2,2:6,2-terpyridine)][PF6]2 (ImQ_T), has been synthesized and characterized. Similar to its precedent, [Ru(6-(2-cyclohexa-2,5-diene-1,4-dione)-2,2:6,2-terpyridine) (2,2:6,2-terpyridine)][PF6]2 (TQ_T), this system has a cofacial alignment of terpyridine (tpy) ligand and quinonyl (Q) group, which facilitates an electron transfer through p-stacked manifold. Despite the presence of lowest-energy charge transfer transition from the Ru-based-HOMO-to-Q-based-LUMO (MQCT) predicted by theoretical calculations by using time-dependent density functional theory (TD-DFT), the experimental steady-state absorption spectrum does not exhibit such a band. The selective excitation to the Ru-based occupied orbitals-to-tpy-based virtual orbital MLCT state was thus possible, from which charge separation (CS) reaction occurred. The photo-induced CS and thermal charge recombination (CR) reactions were probed by using ultrafast visible-pump/mid-IR-probe (TrIR) spectroscopic method. Analysis of decay kinetics of Q and Q- state CO stretching modes as well as aromatic C=C stretching mode of tpy ligand gave time constants of <1 ps for CS, 1-3 ps for CR, and 10-20 ps for vibrational cooling processes. The electron transfer pathway was revealed to be Ru-tpy-Q rather than Ru-bpy-imidazol-Q.

Part II. Manipulation of Absorption Maxima by Controlling the Oxidation Potentials in Bis(tridentate) Ru(II) N-Heterocyclic Carbene Complexes

Homoleptic and heteroleptic Ru complexes have been of great interest as sensitizers for a dye-sensitized solar cell (DSSC). Manipulation of redox potential of such complexes is important for improving efficiencies of DSSCs. Here, a series of seven Ru(II) complexes bearing N-heterocylic carbene (NHC) ligands (NNC or NN^C, where NN = bipyridyl, C = NHC, and ^ = methylene spacer) have been newly designed and synthesized. These complexes are fully characterized by 1H and 13C NMR spectroscopy, high-resolution mass spectrometry and elemental analysis. The electronic structures of these complexes were analyzed by spectroscopic and electrochemical methods and further confirmed by theoretical calculations. Voltammetric data show that all complexes possessing NHC ligand exhibited lower RuII/III oxidation potentials relative to Ru(tpy)2 benchmark mainly due to strong s- donating properties of NHC ligands. The oxidation potentials of Ru complexes studied in this work are in the following order

Ru(tpy)2 > Ru(tpy)(bzim) > Ru(bzim)2 > Ru(tpy)(im) > Ru(tpy)(^bzim) > Ru(im)2 > Ru(^im)2 > Ru(tpy)(^im) (bzim = 3- ([2,2-bipyridin]-6-yl)-1-methyl-1H-benzimidazolyl, im = 3-([2,2-bipyridin]-6-yl)- 1-methyl-1H-imidazolyl, ^bzim = 3-(2,2-bipyridine-6-ylmethyl)-1-methyl-1H- benzimidazolyl, ^im = 3-(2,2-bipyridine-6-ylmethyl)-1-methyl-1H-imidazolyl). Disruption of conjugation between bipyridyl and NHC groups (NN^C type ligands) rather than the mere increase of the number of carbene ligands appear to be more efficient to destabilize the HOMO energy level of these complexes. Theoretical calculation results indicate that the electron densities of HOMOs in Ru(NNC)-type complexes are delocalized over Ru metal and NNC ligand but those in Ru(NN^C)- type complexes are localized within Ru metal and NHC moiety, which is major background of observed oxidation potentials. This work provide an insight in the design motif of Ru complexes in DSSC and other photocatalyst application.

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We have synthesized new π-stacked quinone (Q) containing [Ru(tpy)2]2+, donor (D)-acceptor (A) systems. Orthogonal alignment of Q to the terpyridine (tpy) ligand imposes this unit juxtaposed cofacially on the central pyridyl ring in another tpy with a typical van der Waals distance. We anticipated strong D-A interaction for their short distance, however it doesnt exhibit a distinguishable charge transfer band. For this particular system, electronic coupling matrix element (HDA) which cannot be obtained by conventional Mulliken-Hush analysis, were successfully evaluated by the TrIR spectroscopic method. Although electron transfer was verified by measuring the Q/Q- mode of CO stretching frequencies, the detailed electron transfer pathway could not be determined. In order to identify the pathway, we have changed the tpy-Q ligand to N-heterocyclic carbene (NHC) ligand with methylene spacer which has strong σ-donating power and block the electronic delocalization of the ligand. TrIR analysis with the aid of ground and excited state frequency calculation confirmed the electron transfer through π-stacked manifold not through covalent bond. In addition, we have newly synthesized a series of Ru NHC complexes for studying the origin of structure-property relationships of Ru(NHC) systems. By spectroscopic and electrochemical measures, we found that the λ_max of the complexes are correlated with only the oxidation potentials and could modulate λ_max values of Ru(NHC) complexes to some extent.