Article pubs.acs.org/IC
Aryl-Decorated RuII Polypyridyl-type Photosensitizer Approaching NIR Emission with Microsecond Excited State Lifetimes Tina Schlotthauer,† Benedikt Suchland,† Helmar Görls,‡ Giovanny A. Parada,∥ Leif Hammarström,∥ Ulrich S. Schubert,*,†,§ and Michael Jag̈ er*,†,§ †
Laboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Humboldtstraße 10, 07743 Jena, Germany ‡ Laboratory of Inorganic and Analytical Chemistry, Friedrich Schiller University Jena, Lessingstraße 8, 07743 Jena, Germany ∥ Department of Chemistry - Ångström Laboratory, Uppsala University, Box 523, SE-75120 Uppsala, Sweden § Center for Energy and Environmental Chemistry Jena (CEEC Jena), Friedrich Schiller University Jena, Philosophenweg 7a, 07743 Jena, Germany S Supporting Information *
ABSTRACT: Bis-tridentate RuII complexes based on the dqp scaffold (dqp is 2,6-di(quinolin-8-yl)pyridine) with multiple aryl substituents were explored to tailor the absorption and emission properties. A synthetic methodology was developed for the facile synthesis and purification of homo- and heteroleptic bis-tridentate Ru complexes. The effect of the aryl substituents in the para positions of the pyridine and quinoline subunits was detailed by X-ray crystallography, steady state and time-resolved spectroscopy, electrochemistry, and computational methods. The attachment of the aryl groups results in enhanced molar extinction coefficients with the largest effect in the pyridine position, whereas the quinoline substituent leads to red-shifted emission tailing into the NIR region (up to 800 nm). Notably, the excited state lifetimes remain in the microsecond time scale even in the presence of O2, whereas the emission quantum yields are slightly increased with respect to the parental complex [Ru(dqp)2]2+. The peripheral functional groups (Br, Me, OMe) have only a minor impact on the optical properties and are attractive to utilize such complexes as functional building blocks.
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INTRODUCTION Ruthenium(II) polypyridyl complexes combine beneficial photophysical and redox properties,1−3 resulting in a broad application in photocatalysis4−6 and molecular photosynthesis,7 in medical applications,8,9 as sensors,10 or in molecular multicomponent systems.11,12 During the last few decades, numerous complexes have been investigated based on the archetypical ligands 2,2′-bipyridine (bpy) and 2,2′:6′,2″terpyridine (tpy), aiming at tailored absorption, emission, and electrochemical characteristics.1 Tris-bidentate complexes based on bpy exhibit beneficial photochemical properties of the populated triplet metal-to-ligand charge transfer state (3MLCT),3 whereas the bis-tridentate congeners provide a topology more suitable for the construction of linear donorphotosensitizer-acceptor (D-P-A) systems via functionalization of the axial 4′-pyridine positions.11 Such geometrical features are desirable to control and tune electron transfer rates, particularly to minimize recombination rates of the fully chargeseparated state (D+-P-A−). However, the excited state lifetime of the prototypical [Ru(tpy)2]2+ complex (0.25 ns) is significantly shorter than that of [Ru(bpy)3]2+ (850 ns)1 due to efficient thermal deactivation via metal-centered (3MC) states.3,13 Hence, substantial work has been devoted to the © XXXX American Chemical Society
design of new ligand platforms for light-induced charge separation that simultaneously improves both photochemical and geometrical properties. The attachment of electron-withdrawing and -donating substituents at the axial position of tpy affects the energetic positioning of the involved triplet states and leads to prolonged excited state lifetimes up to several tens of nanoseconds.14,15 The introduction of π-conjugated substituents is also reported to prolong the excited state lifetimes,16 e.g., alkynes,17,18 vinylidenes,16 pyrimidyls,19 and to a lesser extent also phenyls.20−22 An extended delocalization leads to the energetic stabilization of the ligand-based lowest-unoccupied orbitals (LUMOs) and, thus, to an increased barrier for deactivation from the 3MLCT via the 3MC state. In the case of an attached organic triplet reservoir (bichromophoric approach), luminescence lifetimes of several microseconds are reported and are assigned to the fractional repopulation of the emissive 3MLCT state.23,24 Consequently, such compounds display pronounced quenching by energy transfer and are particularly attractive for sensing applications (e.g., O2), yet the capability to undergo Received: February 19, 2016
A
DOI: 10.1021/acs.inorgchem.6b00420 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 1. (a) Molecular structure of [Ru(dqp)2]2+ (1). (b) Tridentate ligands based on the tpy or dqp framework with para-arylation of the central pyridine (R) and peripheral pyridine/quinoline (R′). (c) Hexa-arylated [Ru(dqp)2]2+-based complexes (this work). tpy is 2,2′:6′,2″-terpyridine, dqp is 2,6-di(quinolin-8-yl)pyridine, anth is 9-anthracenyl, and xylyl is a 4-substituted 1,3-dimethylbenzen-2-yl.
Scheme 1. Optimized Synthetic Route for Functionalized Multi-Arylated Ligands
a
Acetic acid, reflux, 2 h, 10%. bSn(II)Cl2, EtOH, reflux, 1 h, 83%. c(i) TEA, THF, reflux, 8 h, (ii) (Ph)3CCl, P2O5, H3PO4, 90 °C, 3 days, 58 to 68%.
inherent helical symmetry of enantiomerically pure 1 for DNA intercalation has been suggested.45 Recently, related tridentate RuII complexes containing 8-quinolyl-pyridine fragments were investigated as catalysts for water oxidation46,47 or as versatile photosensitizers in dye-sensitized solar cells with high efficiencies (>10%).48,49 In this contribution, we explore the effect of arylation of the [Ru(dqp)2]2+ core on the photosensitizer’s characteristics (Figure 1), which is referred to as aryl-decoration in the following. The extended delocalization of the ligand’s π system leads generally to red-shifted absorption and emission profiles without changing the advantageous geometrical properties of the ligand scaffold, so that the rates of nonradiative deactivation should be governed by thermally activated pathways and/or the energy gap law.3 Multiarylation at only the two peripheral para positions (R′) of tpy-based ligands has been reported, e.g., to study the influence on the photophysical processes21,50 or to construct molecular threaded rings.51 To the best of our knowledge, the only hexa-arylated bis-tridentate RuII complex has already been reported three decades ago and featured an increased molar extinction coefficient but only slightly prolonged emission lifetimes.21,52 As a consequence of the elaborate synthesis and purification of the ligands and their complexes, the hexa-arylated bis-tridentate RuII complexes remained unexplored to date. In contrast, the related trisbidentate congeners have been functionalized by multiarylation for a wide range of applications, e.g., as triplet sensors.53 The conventional synthetic routes have been revisited and subsequently optimized, in order to systematically develop a synthetic platform of highly functionalized [Ru(dqp)2]2+-based complexes. A novel synthetic protocol is developed featuring significantly increased yields and simplified purification efforts. The electrochemical and photophysical properties of three model complexes are presented, including time-resolved emission and transient absorption data. The effect of hexaarylation is further detailed by quantum chemical calculations using density functional theory (DFT). Since the seminal
light-induced electron transfer is still determined by the 3 MLCT state. As discussed before,25 the 3MLCT reactivity is not increased by the bichromophoric approach because the gain in observed emission lifetime is accompanied by a corresponding loss in population of the 3MLCT state, as the excited state equilibrium is shifted on the appended organic chromophore. Recent strategies have focused on further extending the intrinsic 3MLCT lifetime through systematic ligand design.24−26 Substantial progress has been made, and several RuII complexes with novel bis-tridentate ligands are reported that exhibit excited state lifetimes in the microsecond time scale, exceeding even that of the prototypical [Ru(bpy)3]2+ complex. In general, a higher ligand-field splitting is necessary to diminish the deactivation via 3MC states. In this regard, microsecond excited state lifetimes can be achieved by virtue of very strong σdonating ligands, e.g., N-heterocyclic and mesoionic carbenes,27−30 or by inducing an improved octahedral geometry of the coordinating polyhedron around the metal center.25 The latter strategy has been demonstrated using tridentate ligands featuring 6-membered chelating rings either by the insertion of keto31 or amino26 groups into the tpy skeleton or by using peripheral 8-quinolines and related anellated heterocyclic rings.32−36 This approach has been explored in detail in a series of functionalized complexes based on 2,6-di(quinolin-8yl)pyridine (dqp), which display longer excited state lifetimes and enhanced photostability vs [Ru(bpy)3]2+.32,34 The success of this strategy prompted the incorporation of the archetypal [Ru(dqp)2]2+ complex (1, Figure 1a) into various supramolecular architectures, e.g., in donor-photosensitizer-acceptor triads displaying highly efficient and long-lived light-induced charge separation,37 a bichromophoric anthracene-based system that prolongs the luminescence lifetime,38 as well as functional polymers such as dye-labeled polycaprolactones,39 photoredoxactive macromolecular architectures,40,41 and electropolymerized responsive metallopolymers.42,43 Additionally, the biomedical application of a [Ru(tpy)(dqp)]2+ photosensitizer for 1 O2 generation was reported,44 and the utilization of the B
DOI: 10.1021/acs.inorgchem.6b00420 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Scheme 2. Improved Stepwise Synthesis of Multi-Arylated RuII Complexes Based on Parental Complex 1 (R, R′, R″, R‴ = H)
a b
(i) Optimized conditions (see also Table 1): 5, MeCN, 120 °C, 10 h, microwave; (ii) AgNO3, MeCN/EtOH/H2O (4:2:1), 80 °C, 16 h, 46 to 54%. Ligand 4, ethylene glycol, 140 °C, 48 h, 14 to 63%.
Table 1. Screening of the Reaction Conditions for the Arylated [Ru(dqp)(MeCN)3]2+ Intermediates entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Ru source III
[Ru Cl3]*xH2O
[RuIIICl3]*xH2O
[RuIIICl3]*xH2O 5
5 6a 6b 6b 6e
ligand
product
4a 4b 4c 4d 4e 4e
6a 6b 6c 6d 6e 6e
4a 4b 4d
6a 6b 6d
4a 4b 4a 4b 4a 4fd
6a 6b 7 8 9 10
T [°C]
time [h]
yield [%]
EtOH
120
16
EtOH EtOH/toluene (1/1) EtOH/THF (4/1) ethylene glycol ethylene glycolb ethylene glycolb ethylene glycolb MeCN ethylene glycol MeCN MeCN MeCN ethylene glycol ethylene glycol ethylene glycol ethylene glycol
80 120
72 16
120
16
90 120 120c 120c 120c 140 140 140 140
9 10 10 10 10 24 24 14 14
9 12 7
