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Exploring the non-innocent character of electron rich π-extended 8-oxyquinolate ligands in ruthenium(II)
bipyridyl complexes
Stephanie Bellinger-Buckley,
†Tse-Cing Chang,
‡Seema Bag,
†David Schweinfurth,
¥Weihong
Zhou,
†Bela Torok,
†Biprajit Sarkar,
¥Ming-Kang Tsai,
‡* Jonathan Rochford
†*
†Department of Chemistry, University of Massachusetts Boston, 100 Morrissey Boulevard, Boston,
MA 02125. ‡Department of Chemistry, National Taiwan Normal University, Taipei, Taiwan ROC.
¥Institut für Chemie und Biochemie, Anorganische Chemie, Freie Universität Berlin, Fabeckstraβe 34-36, Berlin, Germany.
KEYWORDS
ruthenium polypyridyl / electronic structure / non-innocent ligand / EPR spectroscopy
SYNOPSIS
The influence of π-extension and electron rich n→π donation has been explored at the non- innocent Ru(dπ)−OQN(π) system using a combination of optical, electrochemical, spectroelectrochemical and computational analysis. Covalent mixing of the Ru(dπ) atomic and OQN(π) molecular orbitals results in a breakdown of d-orbital degeneracy and significant destabilization of the HOMO level relative to the classic [Ru(bpy)3]2+ system. Upon one-electron oxidation, extensive charge delocalization is observed using a combination of UV/Vis/NIR, EPR spectroelectrochemistry and Mulliken spin-density analysis, giving strong evidence for hole- delocalization across the delocalized Ru(dπ)−OQN(π) system, in particular for the electron rich
5,7-bis(4-methoxyphenyl) and 5,7-bis(4-(diphenylamino)phenyl) systems 72+ and 82+. The potential of this class of ruthenium chromophore in a photovoltaic device is discussed as is the application of the presented bonding model to a wider range of isoelectronic non-innocent ligand transition metal systems to better understand their redox, photophysical and photoelectrochemical properties.
中文摘要
在 此 介 紹 一 系 列 含 有 多 電 子π- 共 軛 的 8-oxyquinolate(OQN) 配 位 基 在 Ruthenium polypyridyl 錯合物上。與傳統的Ruthenium trisbipyridyl [Ru(bpy)3]
2+
錯合物相比，推電子基的 OQN配位使得Ru(dπ)−OQN(π)較具共價性質，且會增加其吸收光能力及具有較多樣化的氧化 還原特性。在本研究中將探討這些合成出的錯合物，如: [Ru(bpy)2(R-OQN)](PF6), where bpy = 2,2’-bipyridine and R = 5-phenyl; 5,7-diphenyl; 2,4- diphenyl; ,7-bis(4-methoxyphenyl);
5,7-bis(4-(diphenylamino)phenyl)。
透過研究並分析Ru(bpy)2(OQN)]+之間的鍵結模式，並和[Ru(bpy)3]2+相對比較後，來了解 其氧化還原及光譜性質，並利用上述的研究模式，進一步解釋將8-oxyquinolate(OQN) 配位基 上換成含有推電子π-共軛官能基的[Ru(bpy)2(R- OQN)](PF6)時的氧化還原及光譜性質。實驗方 面,透過電化學與光譜(ex. UV/Vis/NIR吸收及放射光譜，EPR光譜)探討此系列錯合物的電子 結構，並利用DFT及TD-DFT B3LYP/6-31g(d,p)確認其具有(metal-ligand)-to-ligand (MLLCT)
電荷轉移的特性。在實驗上觀察到錯合物[Ru(bpy)2(OQN)] spin-density分析法觀察單電子氧化的Extensive charge delocalization 現象，如此便可證明hole- delocalization across the delocalized Ru(dπ)−OQN(π)，尤其是在多電子官能基的錯合物 5,7-bis(4-methoxyphenyl) 及 5,7-bis(4-(diphenylamino)phenyl)系統中。
ABSTRACT
A series of ruthenium polypyridyl complexes are presented incorporating π-extended electron rich derivatives of the 8-oxyquinolate (OQN) ligand. The π-donating property of the OQN ligand introduces covalent character to the Ru(dπ)−OQN(π) bonding scheme enhancing its light harvesting properties and diversifying its redox properties, relative to the classic ruthenium(II) trisbipyridyl complex [Ru(bpy)3]2+. Synthesis and characterization is presented for the complexes [Ru(bpy)2(R-OQN)](PF6), where bpy = 2,2’-bipyridine and R = 5-phenyl; 5,7-diphenyl; 2,4- diphenyl; 5,7-bis(4-methoxyphenyl); 5,7-bis(4-(diphenylamino)phenyl). A comprehensive bonding analysis is presented for the [Ru(bpy)2(OQN)]+ system illustrating the origin of its unique spectroscopic and redox properties relative to [Ru(bpy)3]2+. This model is then extended to enable a consistent interpretation of spectra and redox properties for the π-extended [Ru(bpy)2(R- OQN)](PF6) series. The electronic structures have been probed experimentally by a combination of electrochemical and spectroscopic techniques (UV/Vis/NIR absorption, emission, EPR spectroscopy) where (metal-ligand)-to-ligand (MLLCT) charge-transfer properties are found consistent with density functional theory (DFT) and time dependent-density functional theory (TD-DFT) analysis, at the B3LYP/6-31g(d,p) level of approximation. Substantial mixing, due to bonding and antibonding combinations of Ru(dπ) and OQN(π) orbitals, is observed at the HOMO and HOMO-3 levels for the ruthenium-oxyanion bond in [Ru(bpy)2(OQN)]+, which is responsible for the low-energy MLLCT based electronic transition and destabilization of the HOMO level viz.
cyclic voltammetry. This non-innocent π-bonding phenomenon is consistent throughout the series which allows for controlled tuning of complex redox potentials while maintaining panchromatic
the one-electron oxidized species using a combination of UV/Vis/NIR, EPR spectroelectrochemistry and Mulliken spin-density analysis, giving strong evidence for hole- delocalization across the delocalized Ru(dπ)−OQN(π) system, in particular for the electron rich 5,7-bis(4-methoxyphenyl) and 5,7-bis(4-(diphenylamino)phenyl) systems.
INTRODUCTION
Ruthenium polypyridyl complexes, such as [Ru(bpy)3]2+, are very well established as highly diverse photochemical and redox systems due to their strong visible absorption, long excited state lifetimes, redox stability and efficient electron transfer properties. Studies have ranged from purely dark redox chemistry taking advantage of its electron/hole mediating and catalytic properties,1-7 to light-harvesting of diffuse solar radiation8-11 and multiphoton nonlinear photonics.12-14 The unique photophysical properties of [Ru(bpy)3]2+ are characterized by its strong 1MLCT visible absorption (λmax = 454 nm, ε = 14,600 M-1 cm-1), metastable 3MLCT excited state (τ ∼ 1 µs) and reversible redox chemistry (E° = 1.29 V vs. SCE).15, 16, 34 These attractive properties of ruthenium(II) polypyridyl systems stem from the localized ruthenium dπ and polypyridyl π*
orbitals, each of which show independent redox active behavior, i.e. Ru(III/II) and bpy(0/•−) redox states.17, 18, 32, 33 Recently there has been a renewed and expanding interest in redox active transition
metal systems which incorporate metal-ligand π-bonding, or so called, “non-innocent” ligands.
30 The term “non-innocent ligand” implies that the ligand in question forms hybrid metal(dπ)−ligand(π) molecular orbitals which preclude formal assignment of the central metal oxidation state.24, 31 Such a bonding scenario opens the opportunity to explore diverse electronic distributions in π-conjugated metal-ligand frameworks where the photophysical and redox properties may be tailored by design for a wide variety of applications.38 This body of work aims
to investigate the non-innocent character of the 8-oxyquinolate (OQN) ligand at the d6 ruthenium center and, in addition, to explore the influence of π-extension and peripheral redox active groups on its electronic properties. There remain limited reports of Ru-OQN systems in the literature 39-42 with most recent studies demonstrating their application as a photosensitizer in TiO2 based solar cells and even as water oxidation catalysts using the tridentate 2-carboxy-8-oxyquinolate ligand at a ruthenium(II) aqua center.43, 44 To develop a greater understanding of the electronic structure of the Ru(dπ)−OQN(π) system towards application in photoelectrochemical and electrocatalytic systems a series of [Ru(bpy)2(R-OQN)](PF6) complexes, where bpy = 2,2’-bipyridine and R = 5- phenyl; 5,7-diphenyl; 2,4-diphenyl; 5,7-bis(4-methoxyphenyl); 5,7-bis(4- (diphenylamino)phenyl), hereafter referred to as TPA2OQN, have been designed to explore the impact of π-delocalization and electron-donation on their photophysical and redox properties (Fig.
1).
Results and Discussion
Synthesis.
The unsubstituted 8-hydroxyquinoline and 5,7-dimethyl-8-hydroxyquinoline ligands are commercially available whereas their 5,7-substituted derivatives with π-extended functional groups are less common. Fortunately there has been some interest in the development of π- extended 8-hydroxyquinoline ligands in recent years due to its significance in the tailoring of organoboron based light-emitting diode devices. In particular the combined works of Hormi, Anzenbacher and Jaekle have independently demonstrated the influence of alkenyl, ethynyl and aryl substitution at the 2, 4, 5 and 7 positions of the OQN ligand when bound to redox inert boron, aluminum or zinc centers.45-51 In this study we have adapted standard Suzuki coupling conditions for the introduction of the aryl substitutents phenyl, 4-methoxyphenyl and 4- (diphenylamino)phenyl groups at 5,7-positions of the OQN ligand using the appropriate boronic acids (Scheme 1).38 The monosubstituted 5-phenyl-8-hydroxyquinoline ligand was prepared according to a literature method where the 5-chloro position of the tosylate protected 5-chloro-8- oxyquinoline reagent is selectively activated to Suzuki coupling by virtue of the para electron withdrawing tosyl substitutent, often itself active under Suzuki conditions.47 Key to success of 5,7- substitution is the benzyl protected 5,7-diodo-8-oxyquinoline intermediate where the benzyl group plays a dual role in passivation of the ligand chelation properties and aiding its solubility, thus facilitating optimum reactivity. This step is followed by reductive cleavage of the benzyl protecting group using cyclohexadiene as a hydrogen source in the presence of a Pd/C catalyst to generate the neutral 8-hydroxyquinoline ligand precursor.52 The 2,4-diphenyl-8-hydroxyquinoline ligand was prepared following a convenient one-pot microwave synthesis to allow comparison of OQN π-extension at either the pyridyl or phenoxy side of the ligand.53 Once isolated, the 8-
hydroxyquinoline ligand precursors underwent a straight forward complexation at the ruthenium(II) bis(2,2-bipydidyl) core using a mixture of methanol and aqueous potassium hydroxide. Analytically pure [Ru(bpy)2(R-OQN)](PF6) salts were isolated via KPF6 mediated metathesis resulting in analytically pure dark reddish-brown solids.
Scheme 1. (a) Synthetic procedure employed in the synthesis of the 2,4-diphenyl-8-hydroxyquinoline [i-Ph2OQN(H)] ligand. (b) Suzuki-Miyaura coupling conditions employed in the synthesis of the 5,7-bisaryl-8-hydroxyquinoline ligands. (c) Synthesis of the Ru(bpy)2(R-OQN)]+ complexes.
Computational analysis
To aid in the assignment of electronic transitions and redox transformations presented below for the [Ru(bpy)2(R-OQN)]+ series of complexes a prior theoretical analysis of their frontier orbitals is warranted. Introduction of the π-donating OQN ligand at the Ru(II) center dramatically alters the classical D3 σ-bonding scenario of [Ru(bpy)3)]2+ (hereafter denoted as 12+),32 as the reduced
C
1 symmetry of [Ru(bpy)2(OQN)]+ causes a breakdown of degeneracy in its electronic structureRu d(π) OQN(π) R bpy (total) Energy (eV)
85 0 0 15 -6.07
32 63 0 5 -4.99
whose mixing results in a covalent Ru(dπ)−OQN(π) interaction which generates a bonding/anti- bonding pair of occupied molecular orbitals. These energy levels are of significant interest to this study due to their non-innocent character derived from covalent mixing of both the central metal
dπ and ligand π manifolds (Fig. 2).
Figure 2. Aerial and side-on perspective views of both the HOMO and HOMO-3 levels for 2+ illustrating the π-bonding/anti-bonding combination of Ru(dπ) and OQN(π) systems. For complete DFT results please refer to the supporting information. An acetonitrile polarizable continuum model was employed using the B3LYP functional and 6-31g(d,p) (C,H,N,O) and LANL08 (Ru) basis sets.
A summary of metal-ligand contributions to the HOMO orbital for complexes 12+ - 8+ is presented in Table 1 alongside the calculated energies. In contrast to the localized 12+ system, the HOMO of complexes 2+
- 8
+ is dominated by > 60 % contribution from the R-OQN ligands with an increasing influence evident for electron-donating substituents with the triphenylamine and OQN components each contributing over 40% for to the HOMO for complex 8+.Table 1. Metal-ligand contributions (%) to the HOMO of complexes 12+ − 8+ as determined by DFT analysis including calculated HOMO energies where R illustrates the increasing contribution from substituents at the OQN ligand when ascending the series.
1
2+ [Ru(bpy)3]2+2
+ [Ru(bpy)2(OQN)]+[Ru(bpy)2(Me2OQN)]+ 22 75 0 3 -4.81
Molecular Orbital Energy (eV)
2 theoretical viewpoint by the comprehensive molecular orbital contribution plot provided for 8+ in Figure 3 (remaining plots are provided in the supporting information).
Ru OQN TPA
Percentage contribution (%)
Figure 3. Percentage contributions of Ru, OQN, TPA and bpy fragments to frontier molecular orbitals of 8+. Filled and valence levels are separated
3.48 eV 2.69 eV 2.52 eV 2.63 eV 2.63 eV 2.59 eV 2.48 eV 2.38 eV
To summarize the frontier orbital energies for all complexes 12+ - 8+ a molecular orbital energy diagram is presented in Figure 4. Consistent with the UV/Vis absorption data and electrochemical data discussed below, a larger HOMO−LUMO band gap of 3.48 eV is observed for 12+ with the R-OQN series displaying a narrower band gap ranging from 2.52 − 3.00 eV depending upon the nature of the OQN ring substituents. The bpy(π∗) orbital remains consistent as the LUMO level in the series 12+ - 2+ with the lowest energy HOMO→LUMO electronic transition therefore described as (metal-ligand)-to-ligand charge-transfer (MLLCT). highlighted in blue and red, respectively.
ε (M-1 cm-1 )
UV/Vis electronic absorption spectra and TDDFT.
The [Ru(bpy)2(R-OQN)]+ complexes 2+ − 8+ have a dark reddish-brown appearance in contrast to the bright orange color of 12+. As discussed in the computational analysis above, introduction of the π-donating R-OQN ligand is responsible for a significant reduction in HOMO−LUMO band- gap for these complexes with an associated breaking of degeneracy. All oxyquinolate complexes display analogous absorption bands in the visible region with comparable maxima and extinction coefficients. The UV/Vis spectra for select complexes are shown in Figure 5 with the complete set of spectra for complexes 12+ − 8+ available in the supporting information (Fig. SI-2). The low energy maximum absorption observed for 2+ at 496 nm (ε = 12 100 M-1 cm-1) undergoes a slight bathochromic shift of 10 – 15 nm with substitution either at the 2,4 (4+), 5 (5+) or 5,7 (3+, 6+, 7+,
8
+) positions with the 5,7-bis(4-(diphenylamino)phenyl)OQN complex 8+ showing the lowest energy maximum absorption at 511 nm (ε = 13 200 M-1 cm-1).The computed TD-DFT spectra including molecular orbital contributions and relevant molecular orbital images for 8+ are shown in Figures 6 and 7 with the complete series of data included in the supporting information.
Most notable for all oxyquinolate substituted complexes is a splitting of the characteristic 450 nm 1MCT absorption band of 12+ into higher and lower frequency absorptions of comparable intensity. The lowest energy absorption maximum is in fact due to numerous contributing electronic transitions including HOMO-2→LUMO and HOMO-1→LUMO+1 but the greatest contribution to this absorption band, across the series 2+ − 8+, is consistently from the HOMO→LUMO+2 electronic transition. This excitation is unique to this class of complex in that it originates from the highest energy, filled and delocalized Ru(dπ)−OQN(π) HOMO with the destination orbital for the transition being R-OQN(π*) in nature. As such this absorption band is most accurately described as a singlet (metal-ligand)-to-ligand charge transfer (1MLLCT)
Absorption λmax (nm); (ε x 104 M-1 cm-1) a Emission λmax (nm) b
beyond 700 nm due to additional underlying electronic transitions. These are assigned to the weak oscillator strength HOMO→LUMO+1 and HOMO→LUMO transitions again originating from the same delocalized Ru(dπ)−OQN(π) orbital and are therefore also 1MLLCT in character with lower energy bpy(π*) destination orbitals. The higher energy visible absorption band occurring in the range 360 – 400 nm is more typical of traditional ruthenium polypyridyl systems being derived from localized Ru(dπ)→bpy(π*) 1MLCT transitions. However, even here there exists significant contribution from [Ru(dπ)−OQN(π)]→R-OQN(π*) electronic transitions involving higher energy R-OQN(π*) orbitals, for example HOMO→LUMO+7 in 8+ (Fig. 6). The strong enhancement of absorption for 8+ in the UV region at 331 nm is assigned to a combination of [Ru(dπ)−OQN(π)]→bpy(π*) (HOMO-3→LUMO+5) and [Ru(dπ)−OQN(π)]→(TPA)2OQN(π*) (HOMO→LUMO+7) 1MLLCT transitions. A complete database comparing experimental and theoretical TD-DFT derived spectra, including assignments of all electronic transitions and electron density maps of contributing molecular orbitals is provided in the supporting information for all complexes. A summary of spectral data for complexes 12+ − 8+ recorded in acetonitrile is provided in Table 2.
Table 2. Electronic absorption and phosphorescence emission data for complexes 12+ − 8+.
1
2+2
+3
+4
+[Ru(bpy)2(PhOQN)]+ 246 (3.99), 256 (3.82), 293 (4.99), 369 (1.11), 702, 758 (sh)
[a] Recorded at room temperature in acetonitrile.[b] recorded at 77K in an ethanol:methanol (4:1) frozen glass.
In addition to its characteristic visible absorption properties, 12+ has been widely studied for its phosphorescent properties with an absolute quantum yield of Φphos = 0.095 reported in acetonitrile at room temperature.55 The low temperature phosphorescence spectrum of 12+ in an ethanol:methanol (4:1) 77 K frozen glass is illustrated in Figure SI-3 displaying three closely resolved 3MLCT excited states with maxima at 581, 630 and a shoulder at 680 nm (∆ν ∼ 1340 cm
-1).56 In comparison, all oxyquinolate complexes 2+ − 8+ show very weak emission only observable at reduced temperature. Emission maxima are all shifted by over 3000 cm-1 relative to 12+ occurring in the range 702 – 730 nm for the series 2+ − 8+. Similar to 12+, fine structure is observed with a weaker shoulder peak present on the low energy side of the principle emission peak (∆ν ∼ 1040 cm-1). Attempts to record accurate quantum yields were in vain due to the weak nature of the emission with no signal observed at room temperature. Furthermore, radiative decay constants were beyond the time resolution of our instrument (∼200 ps) which is in accordance with the weak intensity of the steady state spectra. Invoking Kasha’s rule, as confirmed by excitation spectra (Figs. SI-4 to SI-10), and assuming efficient intersystem crossing typical of d6 ruthenium polypyridyl complexes, it is likely that the poor photoluminescence response of these complexes is due to rapid 1MLLCT→3MLLCT intersystem crossing followed by population of a metal
37, 57, 58 While such behavior is typical of ruthenium polypyridyl complexes,34-37 it is also possible that the 3MC state in [Ru(bpy)2(R-OQN)]+ complexes actually has substantial ligand contribution via strong dπ−pπ mixing. This would serve to increase the electron-vibrational coupling constant (SM) and excited-state/ground-state vibrational overlap ultimately increasing the non-radiative rate constant (knr) consistent with our observations.35
Electrochemistry
Redox potentials for the series 12+− 82+ were recorded by cyclic voltammetry in an acetonitrile electrolyte and the data is summarized in Table 3 in reference to SCE. A selection of cyclic voltammograms are also presented in Figure 9 to illustrate influence of the OQN ligand on redox potentials at the Ru(II) bis(bipyridyl) core and to inform on the redox properties of the π-extended (6+) and electron donating (7+, 8+) derivatives. All R-OQN complexes show reversible first oxidations within a potential range of E° = +0.40 V to +0.52 V demonstrating a strong cathodic shift relative to 12+ (E° = +1.29 V). This is unsurprising considering the introduction of an anionic ligand at the Ru(II) center however, in agreement with the DFT electronic assignments, this first oxidation is assigned to redox of the delocalized Ru(dπ)−OQN(π) HOMO energy level, i.e. a [Ru(OQN)]2+/+ redox couple as opposed to the localized Ru(III/II) couple typical of classical ruthenium polypyridyl systems.39 In fact DFT calculations show almost identical molecular orbital contributions for the HOMO and HOMO(β) of the monocationic and one-electron oxidized systems, respectively (Table 1, supporting information). Also common to the R-OQN series is an irreversible anodic peak occurring at potentials E° > +1.20 V vs. SCE which, at least for complexes series 2+− 6+ , is most likely due to oxidation of the singly occupied Ru(dπ)−OQN(π) orbital of the dicationic derivatives. Complexes 7+ and 8+ display a more complex electrochemical response with additional redox couples observed as a result of the electron-donating p-methoxyphenyl a
triphenylamine substituents. A single additional reversible redox couple is observed for complex