Article pubs.acs.org/Organometallics
Postsynthetic Systematic Electronic Tuning of Organoplatinum Photosensitizers via Secondary Coordination Sphere Interactions Hanah Na, Ayan Maity, and Thomas S. Teets* Department of Chemistry, University of Houston, 3585 Cullen Boulevard, Room 112, Houston, Texas 77204-5003, United States S Supporting Information *
ABSTRACT: In this work we show that postsynthetic addition of borane Lewis acids to Lewis base decorated organoplatinum photosensitizers induces significant changes in the optical and electrochemical properties. In particular, the charge transfer (CT) energies of these chromophores are significantly modified by these outer-sphere interactions. The direction of the CT shift depends on the site of Lewis acid binding, which occurs either at the diimine ligand in bipyrazine-linked molecules or at an ancillary acetylide ligand in pyridyl-substituted bis(acetylide) molecules. The magnitude of the shift depends on the Lewis acidity of the borane and the number of equivalents added and is comparable to the perturbation brought on by covalent substituent modification of supporting ligands in related complexes. This approach offers a new means of tuning the properties of organometallic phosphors that complements the traditional approach of covalent modification and other postsynthetic modification strategies.
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enhanced reactivity. The “weak-link approach” has been popularized in supramolecular chemistry, whereby coordination-driven structural changes engender synthetic catalysts with allosteric properties.14 In the context of olefin polymerization catalysis, remote Lewis acid triggers have been used to alter the electron density at the metal center and enhance reactivity.15−19 Along the same lines, rhenium hydrogenation catalysts have been enhanced by coordination to Lewis acids through a nitrosyl oxygen,20−22 and the reactivity of hydrogenase model complexes can be altered when boranes are coordinated to terminal cyanide ligands.23,24 In other recent accounts, secondary coordination sphere interactions in macrocyclic cation receptors have led to altered reactivity patterns brought on either by interaction of the bound Lewis acid with substrate molecules or by decoordination of the hemilabile crown receptor from the metal center.25−29 In a similar vein, the interaction of a bipyrazine-ligated platinum diaryl complex with B(C6F5)3 (abbreviated herein as BArF3) gives rise to a substantial increase in the rate of biaryl reductive elimination.30 Careful control of the redox, absorption, and emission properties of photosensitizer molecules is a requisite for their continued development and implementation in devices, and there are multiple examples where covalent substituent modification has led to pronounced changes in fundamental properties and in the performance of the photosensitizer in a device7 or photochemical process.10 There are also many noteworthy examples, mostly in the context of metal ion,
INTRODUCTION Molecular inorganic and organometallic complexes with structurally and electronically tunable supporting ligands play prominent roles in a number of important chemical subfields, including catalysis,1−3 small-molecule activation,4,5 and photosensitization.6,7 A unifying theme in all of these examples is the central role played by the supporting ligands in controlling the electronic structure, reactivity, and photophysical properties of the metal complexes. Traditionally, targeted modification of a complex is brought on by one or more covalent modifications to the supporting ligand environment, requiring ground-up synthesis of the modified ligand and complex. Systematic ligand modification has been used to understand reaction mechanisms and develop improved homogeneous catalysts,8,9 as well as to elucidate the electronic structure and tailor the photophysical properties of photosensitizer molecules.7,10,11 This controlled ligand modification is particularly fruitful when ligands containing aromatic groups are involved, where the wellknown Hammett substituent constants12,13 can be leveraged to quantitatively rationalize and predict the effects of the structural perturbations. In many cases such alterations to the supporting ligands are synthetically trivial, but in others this approach is derailed by synthetic limitations or results in only marginal gains after much synthetic effort. An alternative strategy would be to incorporate functional groups into supporting ligands that allow for the facile postsynthetic modification of the metal complex, permitting subtle, systematic changes to the ligand environment that can be rapidly implemented. There are examples where postsynthetic modification of transition-metal complexes and ligands leads to altered or © XXXX American Chemical Society
Received: April 25, 2016
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DOI: 10.1021/acs.organomet.6b00332 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
perturbations of the electrochemical and optical properties, (iii) the magnitudes of the observed effects are influenced by the stoichiometry of the borane additive, and (iv) the pyridine− borane interactions form instantly and cleanly, allowing for the properties to be tuned rapidly with no additional workup or purification needed.
solvent, and pH sensing, of phosphorescent metal complexes with properties that can be modified in response to external additives.31,32 Platinum acetylide complexes33−40 and complexes of other transition metals41,42 can be solvatochromic40,42 and have also been designed with various functional groups that can interact with cationic Lewis acids35−37,39,41,42 or protons.33,36 In many of these examples, the colorometric and luminescence response is brought on by controlling aggregation, which turns on or off absorption/emission from an aggregated state, or by inhibiting photoinduced electron transfer (PET), which quenches the emission in the unbound state.31 In these mechanisms the identity of the Lewis or Brønsted acid additive does not affect the wavelength of absorption/emission, and the response is on/off in nature. Furthermore, sensors are often designed to be reversible, such that the adducts with Lewis acids are often not able to be isolated. We propose here that secondary coordination sphere Lewis acid−base interactions involving pyridines and substituted boranes could be a complementary approach for modulating the physical properties of photosensitizer molecules and offer some potential advantages of this strategy. Unlike alkali-metal and alkaline-earth-metal Lewis acids, boranes are available with numerous organic substituents, giving them a potentially wider range of available Lewis acidities and engendering much more precise tunability over the electronic structure than is possible with simple metal cations. In addition, in contrast to the cation receptors, which often utilize crown ethers that are size-matched to only certain cations, pyridine receptors should be able to coordinate to virtually any borane Lewis acid (and many others), allowing the design of versatile platforms for studying the effects of Lewis acid additives on the electrochemical and photophysical properties. Finally, pyridine−borane interactions are thermodynamically stable,43 permitting isolation of the adducts and their further manipulation and processing. For these reasons, we believe that phosphors with strategically placed pyridines would permit careful postsynthetic alteration of both redox potentials and excited-state energies upon interaction with boranes, which could allow for rapid optimization of the properties for a given application. In this work, we demonstrate the ability of borane−pyridine Lewis acid−base interactions in the secondary coordination sphere to measurably alter the electrochemical and photophysical properties of organometallic platinum diimine photosensitizers. In complexes of this type, the luminescent excited states are triplet charge transfer (3CT) states, involving Pt d orbitals and diimine π* orbitals, and it has been shown that covalent modifications to both the diimine ligand and the acetylide or aryl ancillary ligand can induce profound changes in the excited-state properties.10,44 We describe one complex, Pt(bpy)(CCpy)2 (1; bpy = 2,2′-bipyridine, CCpy = 4pyridylacetylide), where the Lewis acidic boranes interact with the acetylide ancillary ligands, and a second, Pt(bpz)(4C6H4CF3) 2 (2; bpz = 2,2′-bipyrazine), where boranes coordinate to the diimine ligand. For both of these complexes, we study the effects of triarylborane additives on the electrochemical and optical properties of the platinum complexes, which are perturbed with magnitudes rivaling those obtainable by covalent substituent modification. Our investigations reveal several attractive features of this strategy: (i) the site of borane binding determines the direction of the shift of the excited-state energy, (ii) the identity of the borane and its Lewis acidity determine the magnitude of the
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RESULTS AND DISCUSSION Synthesis of Complexes and Their Borane Adducts. The complexes Pt(bpy)(CCpy)2 (1)45 and Pt(bpz)(4C6H4CF3)2 (2)30 were prepared following previously published procedures. The crystal structure of complex 2, which was not previously reported, has been determined and is presented in Figure S1 in the Supporting Information, showing the expected square-planar coordination environment of PtII. Whereas complex 1 has been shown to bind lanthanide ions in the construction of platforms for sensitized emission,45 its coordination chemistry with boranes has not been explored. As summarized in Scheme 1, complex 1 reacts cleanly and Scheme 1. Synthesis of Borane Adducts of Complex 1
successively with 2 equiv of the boranes BPh3 and BArF3 to furnish adducts where either 1 or 2 equiv of the borane is bound to the pyridyl nitrogen. 1H NMR studies demonstrate quantitative and irreversible binding of both BPh3 and BArF3. Figures S2−S4 in the Supporting Information show stacked NMR spectra for the successive addition of 1, 2, and 2.5 equiv of BPh3 and BArF3 to complex 1. With 1 equiv, the C2 symmetry is lost, and an asymmetric spectrum results that is consistent with one bound pyridine and one free pyridine. The asymmetry is evident in 1H NMR peaks assigned to both the bipyridyl ligand and to the pyridylacetylide. For example, the most downfield resonance in complex 1, assigned to the bpy H6 position (ortho to nitrogen) occurs at 9.64 ppm. Addition of 1 equiv of BPh3 splits this resonance, resulting in peaks at 9.62 ppm, minimally perturbed from complex 1, and 9.48 ppm, B
DOI: 10.1021/acs.organomet.6b00332 Organometallics XXXX, XXX, XXX−XXX
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Organometallics substantially upfield. The upfield shift is even more pronounced when BArF3 is added, and with 1 equiv added the H6 protons resonate at 9.61 and 9.41 ppm, consistent with stronger binding of the more Lewis acidic borane. With 2 equiv of borane only one upfield-shifted H6 resonance is observed. For both BPh3 and BArF3, no further changes in the 1H spectra are observed after addition of more than 2 equiv of borane. The borane adducts of complex 1 can all be isolated. The NMR spectra of the isolated products are identical with the spectra obtained by treating 1 with BAr3 in situ (see Figure S5 in the Supporting Information for a representative example), and the isolated powders gave satisfactory elemental analyses for all four 1(BAr3)n (n = 1, 2) complexes. As observed by Tilley and co-workers, bipyrazine complex 2 binds up to 2 equiv of BArF3 at the free bipyrazine nitrogens, summarized in Scheme 2.30 Addition of >2 equiv results in no Scheme 2. Synthesis of Borane Adducts of Complex 2
Figure 1. X-ray crystal structure of the complex 1-(BArF3)2, depicted with ellipsoids at the 50% probability level. Hydrogen atoms and dichloromethane solvent molecules are omitted. Selected bond lengths (Å): Pt(1)−N(1) 2.057(2), Pt(1)−N(2) 2.065(2), Pt(1)−C(20) 1.944(3), Pt(1)−C(30) 1.947(3), N(3)−B(1) 1.624(4), N(4)−B(2) 1.613(4).
the pyridyl rings of the acetylide. In complex 1 both pyridine rings are nearly orthogonal to the PtN2C2 coordination plane, with dihedral angles of 83.4 and 79.1°, whereas in 1-(BArF3)2 one of the pyridine rings shows a shallow dihedral angle of 30.06° and the other a nearly orthogonal angle of 87.36°. This is likely a crystal-packing phenomenon, as one of the two dichloromethane solvate molecules is sandwiched between the two BArF3 moieties, and in solution the apparent C2 symmetry in the 1H and 19F NMR spectra are consistent with free rotation of the borane-bound pyridine rings. Effect of Borane Binding on Electronic Absorption and Emission. Diimine platinum acetylide complexes akin to 1 have been long known as photosensitizers, with absorption that tails into the visible region and efficient triplet emission.10,48,49 The low-energy absorption and emission features in these complexes arise from charge transfer (CT) excited states. Previous experimental and theoretical work on closely related bis-acetylide complexes has demonstrated that the CT transitions involve occupied orbitals of mixed Pt dπ/ acetylide π parentage (HOMO and HOMO-1) and the bpy π* LUMO50,51 and as such can be characterized more precisely as mixed metal−ligand to ligand charge transfer states (MMLL′CT). Binding of boranes to the pyridylacetylide ligands perturbs the frontier orbital manifold by influencing the mixing between the acetylide orbitals and the Pt dπ orbitals, resulting in changes in the absorption spectra as complex 1 is titrated with boranes. Figure 2 displays the evolution of the absorption spectra as complex 1 is titrated with BPh3 and BArF3. For the latter, the borane was added in increments of 0.25 equiv, showing the gradual change that occurs as complex 1 converts first to 1-(BArF3) and then to 1-(BArF3)2. In both cases, addition of borane induces a hypsochromic shift in the low-energy 1CT absorption band, and no further changes are noted after addition of >2 equiv. The low-energy band appears at 389 nm (ε = 6600 M−1 cm−1) in complex 1 (CH2Cl2 solvent), shifting to 376 nm (ε = 9100 M−1c m−1) in 1-(BArF3) and 370 nm (ε = 13000 M−1 cm−1) in 1-(BArF3)2. The results are qualitatively similar when BPh3 is added, although with this weaker Lewis acid additive the magnitudes of the shifts are not
further changes in the NMR spectra of the adduct, but some additional aromatic peaks grow in which suggest decomposition in the presence of excess BArF3. However, addition of BPh3 to bipyrazine complex 2 only resulted in minimal shifts of the 1H NMR peaks ([2] ≈ 6 mM), and no asymmetry was observed after addition of 1 equiv, suggesting weak binding of BPh3 that is rapidly reversible. This observation of weak binding of BPh3 by bipyrazine complex 2 was further verified by UV−vis titrations (see below), and for this reason adducts of bipyrazine complex 2 with BPh3 were not studied further. One of the borane adducts, 1-(BArF3)2, was crystallized by layering hexane into a concentrated dichloromethane solution and characterized by single-crystal X-ray diffraction. The structure is shown in Figure 1. The N(pyridine)−B bond distances are 1.624(4) and 1.613(4) Å, very similar to the 1.628(2) Å N−B distance in C6H5N→BArF346 and the 1.620(3) Å distance in an ethynylpyridine−BArF3 adduct,47 suggesting strong interaction between the pyridine and borane that is not significantly weakened by the presence of the PtII center. In a comparison of bond metrics to those previously determined for complex 1,45 nearly identical Pt−C distances are observed1.939(8) and 1.956(8) Å for 1 and 1.944(3) and 1.947(3) Å for 1-(BArF3)2. Similarly, the Pt−N distances in 1 are negligibly perturbed when 2 equiv of BArF3 is bound. One difference between the structures of 1 and 1-(BArF3)2 is the dihedral angle between the platinum’s coordination plane and C
DOI: 10.1021/acs.organomet.6b00332 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Figure 3. Emission spectra for complex 1 on titration with BArF3 (top) and BPh3 (bottom). Spectra were recorded in CH2Cl2 at room temperature with λex 388 nm. For the upper graph, the borane was added successively in 0.25 equiv aliquots, and the spectra for 1-(BArF3) and 1-(BArF3)2 are shown as thicker lines.
Figure 2. Electronic absorption spectra for complex 1 when titrated with BArF3 (top) and BPh3 (bottom). Spectra were recorded in CH2Cl2 at room temperature. For the upper graph, the borane was added successively in 0.25 equiv aliquots, and the spectra for 1-(BArF3) and 1-(BArF3)2 are shown as thicker lines.
emission maximum occurs at 465 nm (Φ = 0.0036) with vibronic structure observed. The vibronic spacing of ∼1300 cm−1 between the first two maxima is consistent with aromatic stretching vibrations52 and suggests that when 2 equiv of borane is bound a 3LC state mixes with the low-energy 3CT state, demonstrating that these secondary coordination sphere interactions can not only perturb the energy of the excited state but also influence the excited-state character. Similar shifts are observed upon addition of BPh3, but again the magnitude is not as large. The complex 1-(BPh3) has a featureless emission spectrum with λmax 523 nm (Φ = 0.025), and the spectrum for 1-(BPh3)2 shows poorly resolved vibronic structure, with overlapping maxima at 485 and 508 nm (Φ = 0.0074). One potential concern with this approach is the instability of the borane adducts relative to covalent substituent bonds, which would limit the utility of the strategy described here. Acknowledging that pyridine−borane interactions (ΔH°f for Ph3B·NC5H5 from pyridine and BPh3 is −17.9 kcal/mol)43 are considerably weaker than covalent carbon−heteroatom bonds, all of our experiments indicate good stability of the adducts of complex 1 under the experimental conditions. In addition to the titration experiments described in Figures 2 and 3, we carried out a number of experiments on isolated borane adducts of 1. As shown in Figures S6 and S7 in the Supporting Information, when isolated solids of 1-(BArF3) and 1-(BArF3)2 are dissolved in CH2Cl2, the absorption and emission spectra are nearly identical with those of the samples prepared by titration. In addition, as depicted in Figures S8 and S9 in the Supporting Information, the absorption spectra of the isolated compounds obey Beer’s law, and the emission wavelengths are largely concentration independent, although there is some red shifting in complex 1-(BArF3) at higher concentration which is
as pronounced. The complex 1-(BPh3) has an absorption maximum of 382 nm (ε = 7600 M−1 cm−1), and for 1-(BPh3)2 the value is 379 nm (ε = 11000 M−1 cm−1). The magnitude of the effect depends on the strength of the Lewis acid, with BArF3 perturbing the absorption band by ca. 1300 cm−1 when 2 equiv is bound, as opposed to a perturbation of only ca. 680 cm−1 when 2 equiv of BPh3 is coordinated. These shifts are consistent with a stabilization of the Pt dπ orbital brought on by coordination of the borane to the pyridyl acetylide. The higher-energy region of the absorption spectrum, λ
