Photophysical Properties of Tetracationic Ruthenium Complexes and

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Photophysical Properties of Tetracationic Ruthenium Complexes and Their Ter-Ionic Assemblies with Chloride Ludovic Troian-Gautier,‡ Sara A. M. Wehlin,‡ and Gerald J. Meyer* Department of Chemistry, University of North Carolina at Chapel Hill, Murray Hall 2202B, Chapel Hill, North Carolina 27599-3290, United States

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S Supporting Information *

ABSTRACT: The synthesis of seven ruthenium(II) polypyridyl complexes bearing one dicationic bis-4,4′-(trimethylaminomethyl)-2,2′-bipyridine (tmam) ligand is reported. The ancillary ligands of each complex were 2,2′-bipyrazine (bpz), 2,2′-bipyridine (bpy), 4,4′-tert-butyl-2,2′-bipyridine (dtb), 4,4′dimethyl-2,2′-bipyridine (4,4′-dmb), 5,5′-dimethyl-2,2′-bipyridine (5,5′-dmb), 4,4′-nonyl-2,2′-bipyridine (nonyl), and 4,4′methoxy-2,2′-bipyridine (MeO). The metal-to-ligand charge transfer excited state was localized on the tmam ligand in all instances with the exception of [Ru(bpz)2(tmam)]4+, where it was localized on the bpz ligand. All [PF6]− complexes were shown to form strong ion pairs with chloride in a Ru/Cl 1:2 stoichiometry in acetone, as evidenced by 1H NMR and UV− visible titrations. With the exception of [Ru(bpz)2(tmam)]4+, ion pairing with chloride anions resulted in excited states that were ∼25% longer-lived and with an ∼50% increase in the photoluminescence quantum yields compared to the [PF6]− ion pairs. It was shown that the quantum yield enhancements originated from a decreased nonradiative rate constant and an increased radiative rate constant. [Ru(bpy)2(tmam)]4+ showed curious excited state quenching behavior at higher equivalents of chloride, the origin of which is not understood.

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the nanosecond instrument response time.27 Such strong ion pairs could only be achieved through the use of the dicationic ancillary tmam ligands, where tmam stands for 4,4′-bis(trimethylaminomethyl)-2,2′-bipyridine, which was the ligand used for ion pairing in this study as well. Similar ligands bearing ammonium groups29,30 have been reported recently for a number of transition metal complexes and used for different applications such as lysosome-localized photosensitizers for therapeutic agents,31 for their DNA binding properties in biomedical applications,32−37 and as electro-chemiluminescent compounds.38,39 The tmam ligand, in particular, has also been shown to promote iodide binding in acetonitrile solution,40 where ruthenium(II) tmam complexes bearing overall charges of 4+, 6+, and 8+ were used. Ruthenium chromophores have been observed to ion pair with halides in organic solvents, and even in cases where very little or no driving force for electron transfer from the halide to the ruthenium was reported, the oxidized halide product has been observed.41−43 A supramolecular approach to excited-state chemistry offers a number of ways to control, improve, and direct the reactivity.41−46 Common to all of these applications is the requirement that ion-pairing induces a significant change in the photophysical properties of the transition metal complex. Photoluminescence

INTRODUCTION In his 1987 Nobel laureate lecture, Jean-Marie Lehn described the field of supramolecular chemistry as “chemistry beyond the molecule, bearing on the organized entities of higher complexity that result from the association of two or more chemical species held together by intermolecular forces”.1 Indeed, some of the strengths of supramolecular chemistry is the relative ease with which subtle changes in the molecular assembly can be introduced that can lead to dramatic changes in their photophysical, electrochemical, and catalytic properties. These properties depend on the system as a whole, and not the individual entities. Supramolecular chemistry has found applications in several fields of chemistry and photochemistry. Classical examples include host−guest chemistry in cyclodextrins,2−4 the “lock and key principle”,5,6 crown-ethers,7−9 cryptands and azacryptands,10−17 and calixarenes.11,12,17−23 Supramolecular interactions have also been commonly used in chromatography,24 catalysis,25,26 and solar cells.27,28 In devices using charged redox mediators, such as dyesensitized solar cells, these supramolecular assemblies are often associated “ion pairs”. Indeed, ion pairs have been observed between ruthenium polypyridyl complexes and an iodide redox mediator or a cobalt complex.27,28 For example, the formation of ground-state ion pairs between hexacationic ruthenium(II) complexes and anionic cobalt mediators resulted in electron injection in TiO2 and dye regeneration that occurred within © XXXX American Chemical Society

Received: July 10, 2018

A

DOI: 10.1021/acs.inorgchem.8b01921 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

1.8 Hz, 2H), 4.90 (s, 4H), 3.40 (s, 18H). HRMS (ESI-MS) m/z: [M]+ (C34H40N12P3F18Ru) calcd. 1153.14677; found 1153.14245. [Ru(bpy)2(tmam)]4+·4PF6−. Yield: 89%. 1H NMR (600 MHz, acetone-d6) δ 8.95 (d, J = 1.4 Hz, 2H), 8.82 (d, J = 8.1 Hz, 4H), 8.30 (d, J = 5.8 Hz, 2H), 8.26−8.19 (m, 4H), 8.08 (d, J = 5.6 Hz, 2H), 7.99 (d, J = 5.6 Hz, 2H), 7.85 (dd, J = 5.9, 1.8 Hz, 2H), 7.59 (ddd, J = 7.6, 5.6, 1.3 Hz, 2H), 7.52 (ddd, J = 7.7, 5.6, 1.3 Hz, 2H), 4.88 (s, 4H), 3.40 (s, 18H). HRMS (ESI-MS) m/z: [M]+ (C38H44N8P3F18Ru) calcd. 1149.16578; found 1149.16203. [Ru(dtb)2(tmam)]4+·4PF6−. Yield: 85%. 1H NMR (600 MHz, acetone-d6) δ 9.07 (s, 2H), 8.87 (s, 4H), 8.21 (d, J = 5.8 Hz, 2H), 7.92 (d, J = 6.0 Hz, 2H), 7.82 (dd, J = 5.9, 1.8 Hz, 2H), 7.78 (d, J = 6.0 Hz, 2H), 7.58 (dd, J = 6.1, 2.1 Hz, 2H), 7.54 (dd, J = 6.0, 2.1 Hz, 2H), 4.90 (s, 4H), 3.42 (s, 18H), 1.39 (s, 18H), 1.38 (s, 18H). HRMS (ESI-MS) m/z: [M]+ (C54H76N8P3F18Ru) and [M]2+ (C54H76N8P2F12Ru1) calcd. 1373.41617 and 614.22599; found 1373.42179 and 614.22610. [Ru(4,4′-dmb)2(tmam)]4+·4PF6−. Yield: 81%. 1H NMR (600 MHz, acetone-d6) δ 8.93 (d, J = 1.9 Hz, 2H), 8.67 (d, J = 5.0 Hz, 4H), 8.28 (d, J = 5.8 Hz, 2H), 7.84 (d, J = 5.8 Hz, 2H), 7.82 (dd, J = 5.9, 1.9 Hz, 2H), 7.77 (d, J = 5.8 Hz, 2H), 7.43−7.39 (m, 2H), 7.34−7.30 (m, 2H), 4.87 (s, 4H), 3.40 (s, 18H), 2.57 (s, 6H), 2.55 (s, 6H). HRMS (ESI-MS) m/z: [M]2+ (C42H52N8P2F12Ru) calcd. 530.13210; found 530.13275. [Ru(5,5′-dmb)2(tmam)]4+·4PF6−. Yield: 85%. 1H NMR (600 MHz, acetone-d6) δ 8.88 (d, J = 1.9 Hz, 2H), 8.61 (dd, J = 8.4, 3.3 Hz, 4H), 8.25 (d, J = 5.8 Hz, 2H), 8.04−7.97 (m, 4H), 7.83 (dd, J = 5.9, 1.8 Hz, 2H), 7.78 (s, 2H), 7.74−7.70 (m, 2H), 4.94−4.84 (m, 4H), 3.42 (s, 18H), 2.21 (s, 6H), 2.18 (s, 6H). HRMS (ESI-MS) m/z: [M]2+ (C42H52N8P2F12Ru) calcd. 530.13210; found 530.13248. [Ru(nonyl)2(tmam)]4+·4PF6−. Yield: 71%. 1H NMR (500 MHz, methanol-d4) δ 8.82 (s, 2H), 8.57 (d, J = 7.3 Hz, 4H), 8.01 (d, J = 5.8 Hz, 2H), 7.71 (d, J = 5.8 Hz, 2H), 7.62 (dd, J = 5.7, 1.5 Hz, 2H), 7.56 (d, J = 5.8 Hz, 2H), 7.38−7.28 (m, 4H), 4.65 (s, 4H), 3.23 (s, 18H), 2.83 (q, J = 7.6 Hz, 8H), 1.71 (q, J = 7.7 Hz, 8H), 1.53−1.06 (m, 44H), 1.00−0.74 (m, 12H). HRMS (ESI-MS) m/z: [M] 2+ (C74H116N8P2F12Ru) calcd. 754.38250; Found 754.38414. [Ru(MeO)2(tmam)]4+·4PF6−. Yield: 88%. 1H NMR (600 MHz, acetone-d6) δ 8.99 (s, 2H), 8.37 (ddd, J = 10.9, 5.6, 3.0 Hz, 6H), 7.84−7.80 (m, 2H), 7.76 (ddd, J = 14.2, 6.5, 3.0 Hz, 4H), 7.17 (dt, J = 6.2, 2.9 Hz, 2H), 7.05 (dt, J = 6.2, 2.9 Hz, 2H), 4.88 (s, 4H), 4.05 (s, 6H), 4.02 (s, 6H), 3.40 (s, 18H). HRMS (ESI-MS) m/z: [M]+ (C42H52N8O4P3F18Ru) calcd. 1269.20803; found 1269.20626. Nuclear Magnetic Resonance. Characteristic NMR spectra and halide titration experiments were obtained at room temperature on a Bruker Avance III 600 MHz spectrometer equipped with a broadband inverse (BBI) probe. Solvent residual peaks were used as internal standards for 1H (δ = 2.05 ppm for (CD3)2CO) and δ = 3.31 ppm for CD3OD) chemical shift referencing. NMR spectra were processed using MNOVA. Mass Spectrometry. Samples were analyzed with a hybrid LTQ FT (ICR 7T) (ThermoFisher, Bremen, Germany) mass spectrometer. Samples were introduced via a micro-electrospray source at a flow rate of 3 μL/min. Xcalibur (ThermoFisher, Breman, Germany) was used to analyze the data. Each mass spectrum was averaged over 200 time domains. The mass range was set to 150−2000 m/z. All measurements were recorded at a resolution setting of 100 000. Solutions were analyzed at 0.1 mg/mL or less based on responsiveness to the ESI mechanism. UV−Visible Absorption. UV−vis absorption spectra were recorded on a Varian Cary 60 UV−vis spectrophotometer with a resolution of 1 nm. The molar absorption coefficient of the different complexes was determined by diluting a stock solution of ruthenium complex and represent averages of at least three independent measurements. Steady-State Photoluminescence. Room temperature and 77 K steady-state photoluminescence (PL) spectra were recorded on a Horiba Scientific-FL-1000 fluorimeter and were corrected by calibration of the instrument’s response with a standard tungstenhalogen lamp. During chloride titrations, ruthenium complexes were

quenching is a common effect occurring with the addition of redox active anions, either through electron or energy transfer or by activating some other pathway for nonradiative decay.47 Contrary to this expectation, the complexes reported here displayed photoluminescence whose intensity drastically increased with chloride addition. Indeed, photophysical studies revealed that ion pairing resulted in decreased nonradiative rate constants and increased radiative rate constants. We report herein a detailed, systematic study of the supramolecular assemblies of seven tetracationic ruthenium sensitizers with chloride. The ruthenium(II) complexes all shared a common tmam ligand, while the ancillary ligands were modified to tune the excited state properties as well as solubility. Contrary to previous work performed in acetonitrile where a 1:1 stoichiometry with iodide was observed, the use of acetone allowed for the seven complexes to form strong ion pairs with a 1:2 stoichiometry with chloride in acetone. The present work is relevant for the design of ruthenium sensitizers for solar energy conversion.

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EXPERIMENTAL SECTION

Materials. Acetone (Burdick and Jackson, 99.98%), acetonitrile (Burdick and Jackson, 99.9%), methanol (Fisher, 99.9%), dichloromethane (Spectroscopic grade, Fisher, 99.9%), and n-butyronitrile (BuCN, Acros Organics, 99%) were used as received. Argon gas (Airgas, 99.998%) was passed through a Drierite drying tube before use. Tetrabutylammonium chloride (TBACl, Sigma-Aldrich, ≥99.0%), tetrabutylammonium hexafluorophosphate (TBAPF6, Sigma-Aldrich, for electrochemical analysis, ≥ 98%), and ruthenium trichloride hydrate (Oakwood Chemicals, 97%), were used as received. Deuterated acetone was purchased from Cambridge Isotope Laboratories, Inc. Trimethylamino-methyl-2,2′-bipyridine (tmam) and [Ru(cymene)(tmam)(Cl)]3+·Cl−,2PF6− were synthesized according to a literature procedure.40 All other chemicals were obtained from commercial distributors with a minimum purity of 98% and used as received. All solutions were sparged with argon for at least 30 min before all electrochemical, photoluminescence, and titrations experiments. Procedure for the Synthesis of [Ru(L)2(tmam)]4+ Complexes. In a typical experiment, [Ru(cymene)(tmam)(Cl)]3+· Cl−,2PF6− (100 mg, 0.111 mmol), AgNO3 (75 mg, 0.44 mmol), and the ancillary ligand L (0.234 mmol, 2.1 equiv) were placed in 15 mL of absolute ethanol. The reaction was heated in a sealed vessel under microwave irradiation. The microwave parameters were set such that the vessel would reach 150 °C in 5 min, and this temperature was held for an additional 20 min. After the reaction, the mixture was cooled to 55 °C using an external nitrogen flux. Once at room temperature, the mixture was filtered on a fine porosity frit to remove precipitated AgCl, and the filtrate was evaporated under reduced pressure. The residue was dissolved in the minimum amount of methanol and purified by size exclusion (LH20) column chromatography, using methanol as the eluent. The desired orange band was collected and evaporated under reduced pressure. The residue was finally dissolved in water (with the exception of [Ru(nonyl)2(tmam)]4+ that was dissolved in MeOH) and precipitated using a saturated aqueous solution of NH4PF6 (3 mL). The residue was collected by filtration, and washed with water, a minimum amount of cold ethanol, and diethyl ether. Final products were obtained as orange to brown solids with yields that ranged between 71 and 89%. Note that the products can also be purified by column chromatography (7 cm height, 1.5 cm wide) on neutral Al2O3 using CH3CN/H2O 100:0 to 80:20 as the eluent. In that case, the yields are systematically 20−30% lower due to compound adsorption on Al2O3. [Ru(bpz)2(tmam)]4+·4PF6−. Yield: 79%. 1H NMR (600 MHz, acetone-d6) δ 10.13 (s, 4H), 9.01 (d, J = 2.3 Hz, 2H), 8.71 (d, J = 3.3 Hz, 2H), 8.68 (d, J = 3.1 Hz, 2H), 8.40 (d, J = 5.8 Hz, 2H), 8.33 (dd, J = 3.2, 1.2 Hz, 2H), 8.25 (dd, J = 3.2, 1.2 Hz, 2H), 7.89 (dd, J = 5.9, B


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Inorganic Chemistry

Figure 1. Structures of the seven ruthenium(II) complexes included in this study. All complexes share a common 4,4′-bis(trimethylaminomethyl)2,2′-bipyridine (tmam) ligand introduced for ion pairing purposes, while the ancillary ligands help tune the photophysical and photochemical properties of the ruthenium(II) complexes. excited at a ground-state isosbestic point, which ensured no contributions from ground-state absorption changes to the emission changes. The photoluminescence intensity was integrated for 0.1 s at 1 nm resolution and averaged over three scans. The PL quantum yield, ΦPL, was measured by comparative actinometry using [Ru(bpy)3]2+·2PF6− in acetonitrile (Φ = 0.062) as a quantum yield standard.48 77 K emission measurements were performed using a Janis Dual Reservoir VPF system with a four-way fused quartz windows. 77 K emission spectral measurements were obtained using the same parameters as the room temperature emission measurements and were performed on the complexes in 4:1 ethanol/methanol or BuCN glasses depending on solubility. Time-Resolved Photoluminescence. Time-resolved PL data were acquired on a nitrogen dye laser with excitation centered at 445 nm, as described previously.49 Pulsed light excitation was achieved with a Photon Technology International (PTI) GL-301 dye laser that was pumped by a PTI GL-3300 nitrogen laser. The PL was detected by a Hamamatsu R928 PMT optically coupled to a ScienceTech model 9010 monochromator terminated into a LeCroy Waverunner LT322 oscilloscope. Decays were monitored at the photoluminescence maximum and averaged over 180 scans. Nonradiative and radiative rate constants were calculated from the quantum yields, ΦPL = kr/(kr + knr) and lifetimes, τ = 1/(kr + knr). Electrochemistry. Cyclic voltammetry was performed with a BASi Epsilon potentiostat in a standard three-electrode-cell, i.e., a platinum working electrode, platinum or glassy carbon counter electrodes and a nonaqueous silver/silver chloride (Pine) reference electrode that was referenced to an external decamethyl ferrocene (Fe(Cp*)2, +243 mV vs NHE)50 or ferrocene (Fe(Cp)2, +630 mV vs NHE)51 standard, depending on the electrolyte used.50 Experiments were performed in 0.1 M TBAPF6 acetone or acetonitrile electrolyte, each used depending on complex solubility. Chloride Titrations. UV−vis, steady-state PL, and time-resolved chloride titration experiments were performed in acetone using approximately 10 μM of complex. Titration measurements were performed for each of the spectroscopies with TBACl through additions of approximately 0.3 equiv of TBACl. TBACl was dried under a vacuum for a minimum of 30 min prior to use. Stock solutions were initially prepared for all of the complexes, and 5 mL of the stock solution was transferred to a quartz cuvette. Five milliliters titration solutions was then prepared from the same stock solution by

adding TBACl to obtain the desired concentrations. The solutions were then titrated into the quartz cuvette in 10 or 20 μL additions. The concentration of complex remained unchanged throughout the titrations by preparing the stock solutions, except for the [Ru(bpz)2(tmam)]4+, which formed a precipitate in the presence of high concentrations of TBACl. All solutions were purged with argon for 30 min prior to experiments. Titrating solutions containing TBACl were kept in the dark for the duration of the experiment. The 1H NMR titrations were performed using Bruker Avance III 600 MHz spectrometer equipped with a broadband inverse (BBI) probe using 600 μL of a 500 μM [Ru(L)2(tmam)]4+·4PF6− solution in deuterated acetone. Titration was performed by 0.25 equiv additions of TBACl (10 μL additions). Each spectrum was averaged over 32 scans and was not corrected for dilution. Equilibrium Constants. Equilibrium constants were determined by plotting the change in absorbance at five different wavelengths, around the metal-to-ligand charge transfer (MLCT) absorption, as a function of chloride concentration. The binding isotherm was then fit according to a 1:2 stoichiometry expression. ΔAbs =

εΔRu:IK1[Ru]0 [I−] + εΔRu:2IK1K 2[Ru]0 [I−]2 1 + K1[I−] + K1K 2[I−]2

The delta extinction coefficients were not known for the two ion pairs, but the changes were assumed to be small and identical for each species over the wavelength range used in the fit. Data analysis for all experiments was performed using OriginLab, version 9.0. Data fitting was preformed using a Levenberg−Marquardt iteration method. Franck−Condon line-shape analysis was performed by using a single mode fitting procedure as has been described previously.52 Fitting was performed in Mathematica 10.4. Density Functional Theory and Time-Dependent Density Functional Theory. Quantum mechanical calculations were carried out using the Gaussian 09 program package.53 The structures of complexes with no counteranions, one chloride, and two chloride anions located at the positions indicated by 1H NMR were optimized to a minimum energy, and frequency calculations were performed to verify no contributions from imaginary frequencies (no negative frequencies). In all calculations, the B3LYP functional was used,54−57 using the LANL2DZ basis set58−60 with an added f-polarization function applied to ruthenium,61 6-311+G* applied to the chlorides, C


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Inorganic Chemistry and 6-311G* applied to the other elements.62 A conductive polarizable continuum model (CPCM) solvent model with acetone as the solvent of choice and an ultrafine grid was used for all calculations.63,64 The optimized ground-state geometries were used as the starting coordinates for the time-dependent density functional theory (TD-DFT) calculations.65−67 The first 20 lowest excitations were evaluated for each of the complexes for their corresponding energies, transitions coefficients, and oscillators strengths, and among those the MLCT transition was identified for each structure and the orbital contributions were evaluated.

of similar ruthenium polypyridyl complexes.68 Additional absorptions bands were observed between 200 and 300 nm in solvents that had transmittance at these wavelengths, which were assigned as ligand centered (LC) π → π* transitions (see Supporting Information). These absorption values and their corresponding molar absorption coefficients are gathered in Table 1.

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Table 1. Photophysical Properties of the Indicated Complexes Recorded under Argon in Acetone Solutionsa

RESULTS Synthesis. The synthesis of the seven [Ru(L)2(tmam)][PF6]4 complexes, where tmam is 4,4′-bis(trimethylaminomethyl)-2,2′-bipyridine and L is 2,2′-bipyrazine (bpz), 2,2′-bipyridine (bpy), 4,4′-tert-butyl-2,2′-bipyridine (dtb), 4,4′-dimethyl-2,2′-bipyridine (4,4′-dmb), 5,5′dimethyl-2,2′-bipyridine (5,5′-dmb), 4,4′-nonyl-2,2′-bipyridine (nonyl), and 4,4′-methoxy-2,2′-bipyridine (MeO) respectively (Figure 1) was achieved via the common [Ru(pcymene)(tmam)(Cl)][Cl][PF6]2 precursor.40 The reactions were carried out in ethanol in the presence of silver nitrate, and 2 equiv of the ancillary ligands under microwave irradiation for 20 min. The final products were obtained after size exclusion column chromatography and were isolated as the hexafluorophospate ([PF6]−) salts after ion metathesis. Photophysical, Photochemical, and Electrochemical Characterization. The photophysical properties of all seven complexes were characterized in water (H2O), acetonitrile (CH3CN), methanol (MeOH), acetone (DMK), and dichloromethane (DCM) under an argon atmosphere. The dielectric constants of the five different solvents varied from 80.1 (H2O) to 8.91 (DCM). The ground state absorption spectra for the seven complexes displayed absorption features characteristic of ruthenium polypyridyl complexes, i.e., intense absorption bands between 400 and 550 nm with values of molar absorption coefficients, ε, that were between 10,300 to 14,400 M−1 cm−1 (Figure 2). These visible absorption bands were assigned as metal-toligand charge transfer (MLCT) based on previous assignments

complex

MLCT (nm)

ε (M−1 cm−1)

PLmax (nm)

τ (ns)

[Ru(bpz)2(tmam)]4+ [Ru(bpy)2(tmam)]4+ [Ru(dtb)2(tmam)]4+ [Ru(4,4′-dmb)2(tmam)]4+ [Ru(5,5′-dmb)2(tmam)]4+ [Ru(nonyl)2(tmam)]4+ [Ru(MeO)2(tmam)]4+

450 453 480 450 475 450 490

12,600 14,400 12,750 11,650 10,900 14,250 10,300

640 660 680 685 677 685 710

1590 700 450 460 560 420 190

a The counterions for all complexes are [PF6]−. Photophysical properties in other solvents are summarized in Tables S1−S5.

Room temperature excitation of the MLCT transitions resulted in photoluminescence, PL (Figure 2). The PL maxima varied over a range of 70 nm, or 190 meV, between the different complexes. The PL maxima shifted to lower energy (red-shifted) in more polar solvents, with shifts spanning 39− 114 meV. Time-resolved PL data were well described by a firstorder kinetic model for all complexes in all solvents. The excited-state lifetimes, τ0, ranged from 1.5 μs for [Ru(bpz)2(tmam)]4+ in acetone to 60 ns for [Ru(MeO)2(tmam)]4+ in water. Quantum yields were measured by the optically dilute method using [Ru(bpy)3]2+ in acetonitrile (Φ = 0.062) as a standard.48 The quantum yields ranged from 1 to 8% in the different solvents investigated, with the highest quantum yields generally being recorded in DCM. Values of quantum yields, PL shifts, and excited-state lifetimes are tabulated in Supporting Information (Tables S1−S5). The steady-state PL spectra were also recorded at 77 K in 4:1 ethanol/methanol mixture or BuCN glasses (Figure 3). The PL maxima at this temperature ranged from 659 to 593 nm, which corresponds to a shift of approximately 200 meV. The corrected PL spectra showed the expected vibronic progressions and blue-shifted PL maxima and were fit according to eq 1.69 ÄÅ T Å ÅÅi E − v ℏω y3i S v y Åj m mz zz jjj M m zzz PLI(v )̃ = ∑ ÅÅÅjjj 0 zz jj v ! zz ÅÅj E0 vm Å {k m { ÅÇk É 2 Ñ i v ̃− E0 + vmℏωm zy Ñ zz )Ñ (−4ln(2)jjjj Ñ z Ñ Δv1/2 ̃ k { Ñ e ÑÑ ÑÑ ÑÑÖ (1) In this equation, νm is the vibrational quantum number for the medium frequency acceptor mode, SM is the Huang−Rhys factor, also known as the coupling factor and is a measure of the geometric distortion between the ground and excited states, Δν1/2 is the full-width at half-maximum of the transition, ℏωm corresponds to the vibrational energy spacing in the ground-state potential energy surface and E0 corresponds to the energy difference between the ground and the luminescent MLCT excited-state from which the free energy in the excited

Figure 2. Ground-state absorption and corresponding photoluminescence spectra for [Ru(bpz)2(tmam)]4+ (black), [Ru(bpy)2(tmam)]4+ (red), [Ru(dtb)2(tmam]4+ (blue), [Ru(4,4′-dmb)2(tmam)]4+ (pink), [Ru(5,5′-dmb)2(tmam)]4+ (orange), [Ru(nonyl)2(tmam)]4+ (green) and [Ru(MeO)2(tmam)]4+ (yellow) in acetone. D


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Inorganic Chemistry

complexes, in accordance with eq 3, in which SM is the Huang−Rhys factor and ωm is the medium frequency vibration, as described above. C is a collection of other terms and γ0 is defined further in eq 3.74,75 The nonradiative rate constants in water resulted in a linear relationship that was offset above the trend for the organic solvents (Figure 4b). The nonradiative rate constant of [Ru(bpz)2(tmam)]4+ in DCM was found to be higher than expected, which is represented by the data point which is offset from the rest in the plot. ln k nr = C −

state was calculated, ΔGES.46 The spectra were fit according to the same method outlined in previous publications.52,70−72 The values extracted from fits of the 77 K PL measurements using eq 1 are gathered in Table 2. It is worth noting that the values obtained for the medium frequency acceptor modes were all very similar and that the SM values extracted trend with the E0 values obtained.52,73 Table 2. Fitting Parameters Obtained from Franck-Condon Line Shape Analysis of the PL Spectra ℏωm (cm−1)

E0 (cm−1)

SM

Δν1/2 (cm−1)

[Ru(bpz)2(tmam)]4+ [Ru(bpy)2(tmam)]4+ [Ru(4,4′-dmb)2(tmam)]4+ [Ru(dtb)2(tmam)]4+ [Ru(5,5′-dmb)2(tmam)]4+ [Ru(nonyl)2(tmam)]4+ [Ru(MeO)2(tmam)]4+

1,260 1,270 1,260 1,280 1,250 1,260 1,370

16,830 16,280 16,280 15,930 15,920 15,830 15,170

0.82 0.83 0.80 0.79 0.75 0.75 0.73

960 1,120 1,000 1,030 1,000 1,050 1,180

E°(Ru 4 +* /3 +) = E°(Ru 4 + /3 +) + ΔG ES

PL3max 3πε0ℏ4c 3

μ2

μ2 = |⟨ψgs|μ|̂ ψes⟩|2

(3)

(4)

Chloride Ion-Pairing in Acetone. In acetone, the UV− visible absorption spectra of all the complexes were sensitive to the presence of tetrabutylammonium chloride (TBACl). The MLCT absorption sharpened and increased in intensity with increased TBACl concentration. The changes to the MLCT saturated beyond two equivalents of chloride. In the case of [Ru(bpz)2(tmam)]4+, the MLCT was observed to red-shift. A typical example for [Ru(4,4′-dmb)2(tmam)]4+ is represented in Figure 5a) while UV−vis titrations for the other complexes are gathered in the Supporting Information (Figures S1−S6). Chloride titrations with a 1H NMR assay were performed for each of the complexes in deuterated acetone using 0.25 equiv increments of TBACl. In all cases, large downfield shifts (∼1 Δppm) were observed for the 3,3′ H atoms of the tmam ligand as well as for the methylene linker resonances in the 1H NMR spectra. A typical example for [Ru(4,4′-dmb)2(tmam)]4+ is presented in Figure 5b, while 1H NMR titrations for the other complexes are gathered in the Supporting Information (Figures S7−S10). Upon chloride binding, the methylene protons resolved into a roofed doublet due to their enantiotopic nature,

The radiative rate constants were plotted against the cube of the PLmax, the maximum wavelength of the PL converted to eV, according to eq 2, which also shows the relationship with the transition moment integral, μ, where ε0 is the permittivity of free space and c is the speed of light.74,75 Overall, the trend was linear for the radiative rate constant with the cube of the PLmax for these complexes, with [Ru(bpz)2(tmam)]4+ displaying different behavior than the other complexes (Figure 4a). kr =

ℏωm

ji PLmax zyz γ0 = lnjjj −1 j S ℏω zzz k M m{

Electrochemical characterization was performed in 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) acetone or acetonitrile electrolyte using decamethylferrocene (+243 mV vs NHE)50 or ferrocene (+630 mV vs NHE)51 as an external reference for all complexes. The RuIII/II reduction potential shifted positive with the electron withdrawing ancillary ligand bpz, and [Ru(bpz)2(tmam)]4+ had the most positive RuIII/II potential of 2.10 V vs NHE. The four complexes substituted with aliphatic carbon chains all exhibited similar RuIII/II redox potential. A small shift to less positive values in the RuIII/II values was observed for complexes with more donating ligands, such as nonyl and MeO. With [Ru(bpz)2(tmam)]4+, a reversible first reduction at −0.50 V vs NHE was measured and assigned as the first ligand reduction. However, scanning further reductively revealed a broad, irreversible peak and led to discoloration of the working electrode. A similar broad, irreversible reduction wave was observed for all other complexes at approximately the same potential (−0.8 V vs NHE) with the same discoloration of the working electrode after applying reducing voltages. This was true in both acetone and acetonitrile electrolytes. The nature of the adsorption was not examined, as such irreversible behavior has been previously reported for similar compounds.76 The potential associated with this irreversible reduction peak was used to estimate the first ligand reduction. All relevant potentials are summarized in Table 3, and excited-state potentials were estimated (eq 4) using the ground-state first reduction and the excited-state free energy (ΔGES).

Figure 3. Steady-state photoluminescence of [Ru(bpz)2(tmam)]4+ (black), [Ru(bpy)2(tmam)]4+ (red), [Ru(dtb)2(tmam]4+ (blue), [Ru(4,4′-dmb)2(tmam)]4+ (pink), [Ru(5,5′-dmb)2(tmam)]4+ (orange), [Ru(nonyl)2(tmam)]4+ (green), and [Ru(MeO)2(tmam)]4+ (yellow) recorded in frozen 4:1 ethanol/methanol or BuCN glass at 77 K. Overlaid in dashed blue are calculated fits from a single mode FC line shape analysis.

compound

γ0 PLmax

(2)

A plot of the natural logarithm of the nonradiative decay constant (ln knr) against the PLmax was linear for all seven E


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Inorganic Chemistry

Figure 4. (a) The radiative rate constants measured in dichloromethane, acetone, methanol, acetonitrile, and water as a function of the PL maxima (eV). (b) The natural logarithm of the nonradiative rate constant measured in dichloromethane, acetone, methanol, acetonitrile, and water as a function of the PL maxima (eV). In both graphs, rate constants are color coded accordingly: [Ru(bpz)2(tmam)]4+ (black), [Ru(bpy)2(tmam)]4+ (red), [Ru(dtb)2(tmam]4+ (blue), [Ru(4,4′-dmb)2(tmam)]4+ (pink), [Ru(5,5′-dmb)2(tmam)]4+ (orange), [Ru(nonyl)2(tmam)]4+ (green) and [Ru(MeO)2(tmam)]4+ (yellow). See Supporting Information for tabulated values (Tables S1−S5).

the 5,5′ and 6,6′ proton resonances on the tmam and 6 and 5 proton resonances on the ancillary ligands, indicated a second binding site near the metal center of all seven complexes. The UV−vis spectra measured during chloride titration were used to estimate the ion pair equilibrium constants. The data were fit to an expression describing a 1:2 ion pair, as the absorption increases could not be satisfactorily fit by a 1:1 binding model, and the 1H NMR data indicated two different binding motives in the complex. The first equilibrium constants were found to be very large (∼105 M−1) for all of the complexes, and the values for the second equilibrium constants were generally at least an order of magnitude smaller. Chloride Ion Pairing Effects on Excited States. Addition of chloride to solutions of the [PF6]− complexes affected the excited-state photophysical properties. As it has been previously reported that [Ru(bpz)2(tmam)]4+ oxidizes chloride in acetone,43 this complex will not be further discussed here. Considering the other complexes, ion pairing with chloride led to both hypsochromic shifts (blue-shifts) in the excited state and greater PL quantum yields. The blue-shift corresponded to an increased energy gap, which were between 30 and 40 meV for each complex. The corresponding excited-

Table 3. Reduction Potentials and the Gibbs Free Energy Stored in the Excited state of the Indicated Ru Complexes complex

ΔGES (eV)

Ru4+/3+ (V vs NHE)

Ru4+*/3+d (V vs NHE)

RuIII/II (V vs NHE)

[Ru(bpz)2(tmam)]4+a [Ru(bpy)2(tmam)]4+a [Ru(4,4′-dmb)2(tmam)]4+a [Ru(dtb)2(tmam)]4+b [Ru(5,5′-dmb)2(tmam)]4+a [Ru(nonyl)2(tmam)]4+a [Ru(MeO)2(tmam)]4+b

2.09 2.02 2.02 1.97 1.97 1.96 1.88

−0.50 −0.80c −0.86c −0.77c −0.81c −0.80c −0.76c

1.59 1.22 1.16 1.20 1.16 1.16 1.12

2.10 1.59 1.55 1.60 1.57 1.50 1.40

a Measured in 0.1 M TBAPF6 acetonitrile electrolyte. bMeasured in 0.1 M TBAPF6 acetone electrolyte. cElectrochemically irreversible peak. dThe approximate value of the reduction potentials due to irreversible redox chemistry introduces significant uncertainty into the tabulated excited-state reduction potentials.

as has been previously shown with complexes bearing the tmam ligand.40,43,77 The atomic resolution of 1H NMR allowed for the determination of the tmam ligand as the primary binding “pocket”, but spectral shifts associated with

Figure 5. (a) Absorption spectra of [Ru(4,4′-dmb)2(tmam)]4+ upon titration of chloride from 1 to 4.6 equiv in acetone. Inset shows the difference between the spectra after each addition of chloride and the initial spectrum. (b) 1H NMR titration of [Ru(4,4′-dmb)2(tmam)]4+ with chloride in acetone-d6. The black spectrum shows the 1H NMR of the [PF6]− complex in acetone-d6, and the circles show the shifts of the peaks upon chloride addition in 0.25 equiv additions up to 2 equiv. F


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Inorganic Chemistry

Figure 6. (a) Time-resolved PL decays of [Ru(nonyl)2(tmam)]4+* (10 μM) with the addition of up to 4.9 equiv of TBACl. Inset shows the corresponding steady-state PL spectra with the addition of 4.9 equiv of TBACl. (b) Ratios of the quantum yields (ϕ), kr, and knr for 10 μM [Ru(nonyl)2(tmam)]4+ with the indicated TBACl concentration in acetone. (c) Time-resolved PL decays of [Ru(bpy)2(tmam)]4+* (10 μM) with the addition of up to 8.5 equiv of TBACl. Inset shows the corresponding steady-state PL spectra with the addition of 8.5 equiv of TBACl. (d) Ratios of the quantum yields (ϕ), kr and knr for 10 μM [Ru(bpy)2(tmam)]4+ with the indicated TBACl concentration in acetone.

Figure 7. (a) The radiative rate constants measured for the [PF6]− complexes, {Ru4+, 4PF6−} and the ion paired complexes, {Ru4+:2Cl−}, in acetone as a function of the PL maxima (eV). (b) The natural logarithm of the nonradiative rate constant measured for the [PF6]− complexes, {Ru4+, 4PF6−} and the ion paired complexes, {Ru4+:2Cl−}, in acetone as a function of the PL maxima (eV). Values are tabulated in Tables S2 and S6 in the Supporting Information.

as did the UV−vis spectra, which was observed to be the case for the other complexes as well. During all measurements, care was taken to excite the complexes at an isosbestic point, so that the fraction of absorbed light was constant throughout the titration. The ratios of the quantum yield, ΦPL, kr, and knr, divided by their initial value, are shown in Figure 6b, and showed the overall increase in ΦPL with the addition of chloride. Interestingly, the data shows that both a decrease in

state lifetimes also increased with the addition of chloride. The quantum yield increase ranged between 31% for [Ru(bpy)2(tmam)]4+ and 73% for [Ru(nonyl)2(tmam)]4+, while the excited-state lifetimes increases ranged from 11% for [Ru(MeO)2(tmam)]4+ to 38% for [Ru(nonyl)2(tmam)]4+. Figure 6a shows the steady-state and time-resolved PL data for [Ru(nonyl)2(tmam)]4+ with TBACl. The increase in PL intensity saturated at the same number of chloride equivalents G


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Inorganic Chemistry

Figure 8. (a) Simplified Jablonski-type diagram for ruthenium polypyridyl complexes indicating the electronic states of importance to the discussion. (b) Suggested structure of the {Ru4+:2Cl−} ion pair with one chloride located in the tmam pocket and the second chloride located closer to the ruthenium metal center.

knr and an increase in kr accompanied chloride ion pair formation. The [Ru(bpy)2(tmam)]4+* excited state exhibited curious behavior at higher chloride equivalents, compared to the other complexes. Initially, the same blue-shift and increase was observed in the PL spectra, Figure 6c (blue traces), as well as, similar trends in the ratios of the ΦPL, kr, and knr, Figure 6d. However, when the number of equivalents of chlorides exceeded 2, the PL instead decreased in intensity, with no further spectral shifts, and this was accompanied by a decreased lifetime Figure 6c (red traces). The decrease in ΦPL was reflected in the ratios in Figure 6d that was attributed to the dramatic increase in the knr values. The increase in the energy gap, as observed by the blue-shift, was accompanied by slower excited state decay. Similar to the previous data (Figure 4) showing the solvent dependency, Figure 7a,b demonstrates similar behavior with the ion pairing. Worth noticing is that both plots show linear relationships and that the slopes remain remarkably similar between the [PF6]− complexes and the ion pairs. In Figure 7b, the slopes for the linear fit of both the [PF6]− and the ion paired complexes resulted in slopes of −9.2 and −12.0 eV−1, respectively.

and with an exponential increase in the nonradiative rate constant, knr, and hence a decreased excited state lifetime. The electrochemical measurements were complicated by a clear discoloration of the electrode observed after the first reduction. This discoloration was attributed to either adsorption onto the electrode, or decomposition of the tmam ligand.38,39,76,84 Nonetheless, considering the position of the first ligand centered reduction, the shape, as well as the irreversibility, it was concluded that the first reduction occurred on the tmam ligand and that the excited state electron localized on this ligand as well. However, in the case of [Ru(bpz)2(tmam)]4+, a reversible reduction was observed at a ∼260 mV more positive potential which indicated that one of the bpz ligands was first reduced. Therefore, the excited-state charge transfer complex formed upon light excitation was assumed to be located on a bpz ligand for this complex. This would explain the difference in excited-state behavior of this complex as the excited-state was located away from the tmam ion binding pocket. Upon light excitation, the initial Franck−Condon transition of ruthenium polypyridyl complexes located the charge in a 1 MLCT state, which rapidly undergoes intersystem crossing to the 3MLCT state.79,81,85−88 There is another state that should be taken into account for these complexes, a 3MC (metalcentered) state, also known as a ligand field (LF) state.79,89,90 It is from this 3MC state, which is antibonding with respect to metal−ligand bonds, that photochemical loss of ligand occurs. Depending on the relative energies of this state relative to the 3 MLCT state, it may be thermally populated at room temperature and contribute to the nonradiative decay pathways of the complex. The relative energies of the electronic states were explored by the study of PL in different solvents (Figure 8). As the solvent polarity increased, a bathochromic shift of the PLmax was observed with an increased knr. This was indicative of a stabilization of the 3MLCT state in the more polar solvents, which resulted in a smaller energy gap and red-shifted PL. The linear fit obtained when plotting the natural logarithm of the nonradiative rate constant as a function of the PL maxima confirmed that solvent dependency was consistent with expectations from the energy gap law.78,91 The intercept for the nonradiative rate constant measured in water was offset compared to that in organic solvents, although the slopes remained the same in all cases. This was attributed to the strong O−H vibrational modes of water, which provided additional nonradiative pathways.92 The relative independence of the radiative rate constants when plotted against PLmax was

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DISCUSSION A series of ruthenium(II) complexes bearing a dicationic ligand designed for supramolecular interactions with halides such as chloride and ancillary ligands that tune the photophysical properties have been successfully synthesized using one common intermediate, i.e., [Ru(p-cymene)(tmam)(Cl)][Cl][PF6]2 precursor. All complexes formed ter-ionic assemblies with chloride in acetone with a 1:2 stoichiometry, whose excited-state behavior were further characterized. The chloride ion-pairs showed a remarkable increase in PL quantum yield for dialkyl bpy complexes that was absent for the bpy and bpz complexes. Photophysical Scheme of the Seven Ru(II)-tmam Complexes. The complexes all exhibited photophysical behavior in agreement with the thoroughly characterized electronic states of ruthenium polypyridyl complexes.68,78−82 All seven complexes exhibited classical MLCT absorption features centered between 400 and 500 nm, with a red-shift in absorption for complexes bearing more electron-donating ligands. The PL maximum was directly correlated to the excited state lifetime in accordance with the energy gap law as formulated by Jortner et al.83 Essentially, as more electron donating ligands were introduced to the complexes, the energy gap between the excited state and the ground state decreased, H


Article

Inorganic Chemistry also expected, as kr varied with the cube of the energy gap, in accordance with eq 3 as was shown in Figure 7a. From the 77 K PL measurements, and the subsequent FC line shape analysis, the parameters extracted informed on the decay pathways of the complexes. The vibrational modes, ℏωm, were found to be between 1250 and 1380 cm−1, which was similar to that reported for other ruthenium polypyridyl complexes and consistent with C−C and C−N acceptor modes that are typically reported at ∼1300 cm−1. The Huang−Rhys factor, SM, were reported with values less than one (0.8) that indicated less distortion in the excited-states.73,93 The SM values were also found to decrease with decreasing energy gap, E0, which is in accordance to what has been previously reported.73,93 Theory indicated that as the energy gap decreased, an increased mixing of the ground and excitedstates occurred, which resulted in smaller perturbations upon excitation.73 The high knr value of [Ru(bpz)2(tmam)]2+ in DCM, on the other hand, was suggestive of an excited-state localization on a the bpz, instead of on tmam as was the case for the other complexes. Ruthenium complexes bearing π-deficient ancillary ligands (such as 2,2′-bipyrazine or 1,4,5,8-tetraazaphenanthrene) are known to be photolabile and undergo ligand loss through population of the metal-centered (3MC) state.81,94−96 In DCM, the 3MLCT was destabilized to such an extent that crossing between the 3MLCT and the 3MC was easily achieved. Deactivation through the 3MC state was a nonradiative process, hence the large value of knr. Chloride Ion Pairing. Spectroscopic data showed that ion pairing between all seven complexes and chloride occurred in acetone. Evolution of the UV−visible spectra upon binding of chloride led to increased and narrowed visible absorption bands. 1H NMR titrations provided evidence that the ion pair structures were essentially identical for all seven complexes. The tmam ligand has previously been shown to bind halides in both acetonitrile and acetone.27,40,43 The presence of a chloride anion close to a ligand in a transition metal complex changes the local magnetic environment97 and can therefore be used to determine the structure of the ion pair.49 The 1H NMR shifts induced by chloride influenced the 3,3′ H atoms and methylene linker resonances on the tmam ligand dramatically. Chloride resided in a “pocket” created by the two positively charged substituents on the tmam ligand and the 3,3′ H atoms (Figure 8b). Moreover, the shifts of proton resonances on the ancillary ligands and 6,6′ H atoms of the tmam ligand indicated a second ion pairing event, resulting in 1:2 ion pairs at higher chloride concentrations. Indeed, beyond 1 equiv of chloride, the data indicated that a second chloride anion was bound above or below the tmam ligand. Spectral changes in the UV−vis absorption were observed to plateau beyond 2 equiv and was used to determine the equilibrium constants (Table 4). The equilibrium constants were shown to be similar for all seven complexes, which was expected as the same binding motive was involved for all complexes. Importantly, the 1H NMR signal of the 3,3′ H atoms of the ancillary ligands (Figure 5 and S7−S12) on all complexes were not affected by the chloride titration, indicating that these acidic H atoms did not participate in ion-pairing. Excited-State Decay Pathways. The hypsochromic shift (blueshift) observed in the steady state PL spectra upon chloride titration was consistently accompanied by an increase in the radiative rate constant and a decrease in the nonradiative rate constant. The presence of chloride in the tmam pocket

Table 4. Equilibrium Constants for Chloride Ion-Paring in Acetone UV vis −1

complex 4+

[Ru(bpz)2(tmam)] [Ru(bpy)2(tmam)]4+ [Ru(dtb)2(tmam)]4+ [Ru(4,4′dmb)2(tmam)]4+ [Ru(5,5′dmb)2(tmam)]4+ [Ru(nonyl)2(tmam)]4+ [Ru(MeO)2(tmam)]4+

K1 (M )

K2 (M−1)

5 ± 1 × 10 7 ± 1 × 104 1.3 ± 0.8 × 106 5 ± 1 × 104 2 ± 1 × 105 2.4 ± 0.8 × 105 5.1 ± 0.4 × 105

4 ± 1 × 103 5 ± 1 × 104 3 ± 1 × 104 1.0 ± 0.5 × 104 1.0 ± 0.4 × 105 5.0 ± 0.7 × 104 7.1 ± 0.3 × 103

5

restricted this ligand translationally and torsionally due to strong interactions with the alkyl H atoms through H-bonding, as shown by 1H NMR. It was postulated that the positive charge on the trimethylammonium group also interacted with the negatively charged anions. The result was a decreased number of nonradiative decay pathways, as the vibrations of the molecule were restricted. This was also in agreement with the data for [Ru(bpz)2(tmam)]4+ with TBACl titrations, where the excited-state location on the bpz instead of the tmam ligand precluded this effect. The increased ΦPL and excited-state lifetime were also accompanied by increased radiative rate constants, kr, in all complexes with the addition of chloride. The UV−vis spectra saturated at the same number of equivalents of chloride and the species present were assigned to the ter-ionic assemblies. Furthermore, the new species followed energy gap law expectations with behavior similar to the [PF6]− complexes. This suggests new excited-state behavior could be viewed as a property of the ion paired species, the ter-ionic assemblies, which should be considered as unique luminophores. The increase in radiative rate constants were also consistent with the enhanced ground-state extinction coefficients in accordance with the theory laid out by Stickler and Berg.98 In the case of [Ru(bpy)2(tmam)]4+, the interpretation of the excited-state behavior was more complicated. The titration of TBACl established that a series of equilibria, forming first the mono-ion paired and then the bis-ion paired species in solution (eq 5) were responsible for the different species present in solution. [Ru]4+ + Cl− F {Ru 4 +:Cl−}3 + F {Ru 4+:2Cl−}2+

(5) 4+

− 3+

The experimental evidence suggested that {Ru :Cl } behaved like the 1:1 ion pairs of the other heteroleptic bpy complexes, while {Ru4+:2Cl−}2+ displayed decreased PL and ϕPL with a smaller extinction coefficient in the ground state. Once the ter-ionic species was the prevailing species in solution, the PL intensity decreased. One possible explanation for the ter-ionic species possessing a smaller PL quantum yield was that the bpy and tmam ligand were electronically very close in energy, and ion pairing influenced the ligand on which the excited-state was localized.99 Time dependent density functional theory (TD-DFT) was performed on optimized structures of [Ru(bpy)2(tmam)]4+ with zero, one, and two chloride anions present in an effort to understand the difference observed in excited-state behavior. The MLCT transitions were identified and were in good agreement with the experimental results, i.e., the energy of the MLCT transition increased as the complex ion-paired chloride with a calculated shift from 450 nm to 442 nm. However, the calculations did not show any substantial shift in the lowest I


Inorganic Chemistry

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ACKNOWLEDGMENTS The University of North Carolina (UNC) authors acknowledge support from the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy (Grant DE-SC0013461). L.T.-G. acknowledges the Belgian American Educational Foundation (BAEF) as well as the Bourse d’Excellence Wallonie-Bruxelles International (WBI.World) for postdoctoral funding. The authors acknowledge Wesley B. Swords for the gift of the tmam ligand, Andrew B. Maurer for helpful discussions about DFT, as well as the University of North Carolina’s Department of Chemistry Mass Spectrometry Core Laboratory, especially Brandie Ehrmann, for their assistance with mass spectrometry analysis.

unoccupied molecular orbital upon ion-pairing chloride (Figure S13) which remained on the tmam ligand throughout the calculations. Nonetheless, larger contributions from bpy orbitals to the MLCT transition were observed for {Ru4+:2Cl−}2+ compared to {Ru4+:2PF6−}2+. Indeed, calculations for {Ru4+:2Cl−}2+ revealed two bpy based and one tmam based transitions that were reversed for {Ru4+:2PF6−}2+. These preliminary calculations suggest closer energy spacings with more mixing in the case of {Ru4+:2Cl−}2+ relative to {Ru4+:2PF6−}2+.

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CONCLUSION The synthesis of seven ruthenium(II) complexes from a common [Ru(p-cymene)(tmam)(Cl)][Cl][PF6]2 precursor that share a specifically introduced common ligand, tmam, with a binding pocket for chloride was described. The photophysical and electrochemical properties of the complexes were tuned by varying the nature of the ancillary ligands. All complexes were shown to form strong ion pairs with chloride in a 1:2 stoichiometry in acetone. The excited-state properties were influenced by chloride ion pairing, largely due to excitedstate localization on the tmam ligand. The ter-ionic excitedstates exhibited larger quantum yields of photoluminescence and longer-lived excited-state lifetimes than the [PF6]− analogues, in full agreement with the energy gap law. In addition, an increase in the radiative rate constant was observed with Cl− ion pairing that could be utilized when designing new chromophores for energy applications. [Ru(bpz)2(tmam)]4+ was the most potent photo-oxidant and possessed an excited-state that was localized on the ancillary bpz, leading to different photophysical behavior than the other complexes. [Ru(bpy)2(tmam)]4+ exhibited unusual excitedstate behavior, where up to 2 equiv of chloride induced an increase in PL quantum yield and additional equivalents led to quenching. The data show that molecular synthetic design provides complexes that recognize and report on chloride ions for possible applications such as solar energy conversion.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01921.

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Article

Tables of photophysical properties of the complexes; TD-DFT analysis of MLCT transitions; figures of UV−vis absorption changes of complexes upon titration; 1 H NMR titration of complexes; frontier orbital surfaces of the optimized excited-states of complexes (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ludovic Troian-Gautier: 0000-0002-7690-1361 Sara A. M. Wehlin: 0000-0002-7265-2644 Gerald J. Meyer: 0000-0002-4227-6393 Author Contributions ‡

L.T.-G. and S.A.M.W. contributed equally.

Notes

The authors declare no competing financial interest. J


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