Unusual Role of Excited State Mixing in the Enhancement of Photoinduced Ligand Exchange in Ru(II) Complexes

Abstract

Four Ru(II) complexes were prepared bearing two new tetradentate ligands, cyTPA and 1-isocyTPQA, which feature a piperidine ring that provides a structurally rigid backbone and facilitates the installation of other donors as the fourth chelating arm, while avoiding the formation of stereoisomers. The photophysical properties and photochemistry of [Ru(cyTPA)(CH₃CN)₂]²⁺ (1), [Ru(1-isocyTPQA)(CH₃CN)₂]²⁺ (2), [Ru(cyTPA)(py)₂]²⁺ (3), and [Ru(1-isocyTPQA)-(py)₂]²⁺ (4) were compared. The quantum yield for the CH₃CN/H₂O ligand exchange of 2 was measured to be Φ₄₀₀ = 0.033(3), 5-fold greater than that of 1, Φ₄₀₀ = 0.0066(3). The quantum yields for the py/H₂O ligand exchange of 3 and 4 were lower, 0.0012(1) and 0.0013(1), respectively. DFT and related calculations show the presence of a highly mixed ³MLCT/³ππ* excited state as the lowest triplet state in 2, whereas the lowest energy triplet states in 1, 3, and 4 were calculated to be ³LF in nature. The mixed ³MLCT/³ππ* excited state places significant spin density on the quinoline moiety of the 1-isocyTPQA ligand positioned trans to the photolabile CH₃CN ligand in 2, suggesting the presence of a trans-type influence in the excited state that enhances ligand exchange. Ultrafast spectroscopy was used to probe the excited states of 1–4, which confirmed that the mixed ³MLCT/³ππ* excited state in 2 promotes ligand dissociation, representing a new manner to effect photoinduced ligand exchange. The findings from this work can be used to design improved complexes for applications that require efficient ligand dissociation, as well as for those that require minimal deactivation of the ³MLCT state through low-lying metal-centered states.

Graphical Abstract

INTRODUCTION

The excited states of ruthenium polypyridyl complexes have been used extensively in the areas of dye-sensitized solar cells, photocatalysis, synthetic chemistry, photochromic switching, photochemotherapy, and as biological sensors, among others.^1–32 The utility of these complexes for these applications relies on their relatively strong absorption and emission intensity in the visible region, long-lived triplet metal-to-ligand charge transfer (³MLCT) excited state lifetimes, and high stability under various experimental conditions.^1–33 In contrast, the efficient photoinduced ligand dissociation for the release of drugs or biological probes with spatiotemporal control necessitates the population of the dissociative metal-centered, ligand field states (³LF), known to lead to nonradiative deactivation of the emissive ³MLCT state.^14,34–41 Therefore, understanding the interplay between the ³MLCT and ³LF states is important to improve the function of Ru(II) complexes, both when the long-lived ³MLCT state is desired, as well as for cases where increased ³LF population is required.^14,42 Therefore, the improvement of agents for this wide range of applications requires understanding of the molecular and electronic factors that result in ³MLCT deactivation and ligand photodissociation through population of the ³LF state(s).

The excited state ligand exchange photochemistry of Ru(II) complexes has long been believed to arise from ³LF states with electron configurations that result in electron density in molecular orbitals of Ru–L(σ*) character.¹⁴ In complexes with low-lying ³MLCT and ³LF states, such as in ruthenium polypyridyl compounds, it is generally accepted that those with lower energy ³LF states should exhibit greater ligand photo-dissociation quantum yields.^{14,27,30,39,42–44} This trend was shown by Ford and co-workers on the series [Ru(NH₃)₅(py-X)]²⁺ (py-X = substituted pyridine) and trans-[Ru(NH₃)₄(py)-(L)]ⁿ⁺ (py = pyridine; L = pyrazine, pyrazinium, 4-acetylpyridine), which resulted in quantum yields of ligand substitution that spanned over 3 orders of magnitude.⁴⁵ The incorporation of electron donating substituents on pyridine led to mainly ³LF character in the lowest triplet excited states and efficient photosubstitution, whereas electron withdrawing substituents resulted in mostly ³MLCT character and decreased photosubstitution efficiency.^34,45,46 In addition, it has been shown that in complexes where the ³MLCT state remains the lowest energy triplet state, reducing the ³MLCT–³LF energy gap by either raising the ³MLCT state or lowering the ³LF state, can also lead to increased population of the dissociative ³LF states.^39,47–50 On the basis of this understanding of photoinduced ligand dissociation in Ru(II) polypyridyl complexes, it follows that the photochemical ligand exchange should be more efficient in complexes where the lowest energy excited state is metal-centered, ³LF, and not ³MLCT. The latter condition should hold provided that the energy of the ³LF states are not so low that intersystem crossing back to the ground state becomes too rapid and competes with ligand dissociation in accordance with the energy gap law.^44,50

The majority of studies concerning the photochemical ligand exchange of polypyridyl Ru(II) complexes have focused on complexes containing planar bi- or tridentate ligands, such as the prototypical 2,2′-bipyridine or 2,2′:6′,2″-terpyridine, respectively, and their derivatives.^{27,41,48,51–55} Recently, the photochemistry of Ru(II) complexes bearing tetradentate ligands based on tris(2-pyridylmethyl)amine (TPA) was examined. In both [Ru(TPA)(CH₃CN)₂]²⁺ and [Ru(TPA)-(py)₂]²⁺, the monodentate ligand cis to the basic amine nitrogen of the TPA ligand was shown to undergo preferential ligand photodissociation, with quantum yields (Φ) in H₂O of 0.012(1) with λ_irr = 350 nm and 0.0097(8) with λ_irr = 400 nm, respectively.^56,57 The photochemistry of a related nitrile complex containing the tetradentate ligand tris(2-quinolinylmethyl)amine (TQA), [Ru(TQA)(CH₃CN)₂]²⁺, resulted in Φ₄₀₀ = 0.027(1) in H₂O, ~2-fold greater than that of [Ru(TPA)(CH₃CN)₂]²⁺. Like [Ru(TPA)(CH₃CN)₂]²⁺ and [Ru(TPA)(py)₂]²⁺, irradiation of [Ru(TQA)(CH₃CN)₂]²⁺ only results in ligand exchange of the CH₃CN positioned cis to the basic amine nitrogen of the TQA ligand.⁵⁸ Calculations provided an explanation for the increase and selectivity of ligand exchange, showing the presence of favorable orbital overlap in the excited state between the coplanar quinoline arms of the TQA ligand and the photolabile CH₃CN ligand that is not present in the TPA complex.⁵⁹

In the present work, four new Ru(II) complexes were prepared bearing two new tetradentate ligands related to TPA, cyTPA (2,2′-((2R,6S)-1-(pyridin-2-ylmethyl)piperidine-2,6-diyl)dipyridine) and 1-isocyTPQA (1-(((2R,6S)-2,6-di-(pyridin-2-yl)piperidin-1-yl)methyl)isoquinoline), depicted in Figure 1. These ligands feature a piperidine ring, which locks the adjacent pyridine arms into the trans position to form a planar tridentate moiety within the tetradentate ligand. This structurally rigid backbone facilitates the installation of other donors as the fourth chelating arm, while avoiding the formation of stereoisomers. The photophysical properties and photochemistry of [Ru(cyTPA)(CH₃CN)₂]²⁺ (1) and [Ru(1-isocyTPQA)(CH₃CN)₂]²⁺ (2) were compared, as well as to those of the corresponding pyridine complexes, [Ru(cyTPA)-(py)₂]²⁺ (3) and [Ru(1-isocyTPQA)(py)₂]²⁺ (4). Photo-induced ligand exchange was found to be significantly more efficient for 2, a property that was investigated using ultrafast transient absorption spectroscopy and computational studies. The findings provide a new mechanism that leads to ligand dissociation that does not involve a low-lying ³LF state and can now be exploited in the design of new complexes for efficient photoinduced drug delivery with low energy excitation. These results also provide insight into an additional mode of deactivation of Ru(II) complexes important for solar energy conversion applications and the use of these systems as emissive probes, all of which require long-lived ³MLCT excited states and minimum deactivation through nonradiative channels.

Figure 1.

(a) Synthetic route of the ligands cyTPA and 1-isocyTPQA and (b) molecular structures of complexes 1–4.

EXPERIMENTAL SECTION

Materials

All materials for synthesis were used as received without further purification and the solvents were of reagent grade quality. Dry methanol, absolute ethanol, dry acetonitrile, acetone, ethyl acetate, dichloromethane, hexane, dimethyl sulfoxide (DMSO), diethyl ether and pyridine were purchased from Fisher Scientific. Trifluoromethanesulfonic acid, 2-picolyl chloride hydrochloride, N,N-diisopropylethylamine (DIPEA), and tetrakis(dimethyl sulfoxide)-dichlororuthenium(II) (cis-[Ru(DMSO)₄Cl₂]) were purchased from Sigma-Aldrich. Ammonium hexafluorophosphate was obtained from AK Scientific, and NaI·2H₂O was purchased from ACROS Organics. The precursors (2R,6S)-2,6-di(pyridin-2-yl)piperidine⁶⁰ and 1-(chloromethyl)isoquinoline⁶¹ were synthesized according to literature procedures. All reactions were performed under ambient atmosphere unless otherwise noted. Anaerobic reactions were performed by purging the reaction solutions with Ar or N₂. For transient absorption experiments acetone was dried using anhydrous MgSO₄, and acetonitrile was distilled over CaH₂.

cyTPA (2,2′-((2R,6S)-1-(pyridin-2-ylmethyl)piperidine-2,6-diyl)-dipyridine)

To a solution of (2R,6S)-2,6-di(pyridin-2-yl)piperidine (36 mg, 0.15 mmol) in 11 mL of dry CH₃CN, 2-picolyl chloride hydrochloride (25 mg, 0.15 mmol) was added. To this, DIPEA (52 μL, 0.30 mmol) and NaI·2H₂O (28 mg, 0.15 mmol) were added and the reaction mixture was heated at 50 °C for 16 h. The reaction mixture was cooled to room temperature and concentrated under reduced pressure, and the residue was extracted with dichloromethane and an aqueous solution of saturated NaHCO₃. The organic layer was concentrated under reduced pressure to give the crude product, which was purified by chromatography on silica using ethyl acetate as the eluent to give cyTPA as a yellow solid (26 mg, 52%): ¹H NMR (400 MHz, CDCl₃) δ 8.46 (d, 2H, J = 4.8 Hz), 8.18 (d, 1H, J = 4.8 Hz), 7.57 (d, 2H, J = 8.0 Hz), 7.50 (t, 2H, J = 7.6 Hz), 7.31 (t, 1H, J = 7.6 Hz), 7.04–7.00 (m, 3H), 6.84 (t, 1H, J = 6.0 Hz), 3.86 (d, 2H, J = 9.6 Hz), 3.63 (s, 2H), 1.90–1.81 (m, 4H), 1.64–1.58 (m, 2H); ¹³C NMR (600 MHz, CDCl₃) δ 163.7, 159.4, 148.8, 148.1, 136.2, 135.1, 123.8, 123.0, 122.0, 120.8, 70.2, 59.6, 34.8, 24.3; ESMS calculated for ([M + H]⁺) C₂₁H₂₃N₄: 331.1878, found ([M + H]⁺): 331.1919.

1-IsocyTPQA (1-(((2R,6S)-2,6-di(pyridin-2-yl)piperidin-1-yl)-methyl)isoquinoline)

To a solution of (2R,6S)-2,6-di(pyridin-2-yl)piperidine (20 mg, 0.084 mmol) in 6.0 mL of dry CH₃CN, 1-(chloromethyl)isoquinoline (15 mg, 0.084 mmol) was added. To this, DIPEA (29 μL, 0.17 mmol) and NaI·2H₂O (16 mg, 0.084 mmol) were added and the reaction mixture was heated at 50 °C for 16 h. The reaction mixture was cooled to room temperature and concentrated under reduced pressure, and the residue was extracted with dichloromethane and an aqueous solution of saturated NaHCO₃. The organic layer was dried over Na₂SO₄ and concentrated under reduced pressure to give the crude product, which was purified by chromatography on silica using methanol/ethyl acetate (1:9, v/v) as the eluent to give 1-isocyTPQA as an orange solid (15 mg, 47%): ¹H NMR (500 MHz, CD₃OD) δ 8.19–8.15 (m, 3H), 7.92 (d, 1H, J = 8.5 Hz), 7.67 (d, 1H, J = 8.0 Hz), 7.62 (d, 3H, J = 7.5 Hz), 7.52–7.46 (m, 3H), 7.39 (s, 1H), 7.02–6.99 (m, 2H), 4.09 (s, 2H), 4.03 (dd, 2H, J = 10 Hz), 1.95–1.83 (m, 4H), 1.73–1.70 (m, 2H); ¹³C NMR (500 MHz, CD₃OD) δ 163.0, 157.9, 147.6, 139.4, 136.7, 135.9, 130.2, 126.9, 126.8, 126.6, 125.4, 123.3, 122.1, 120.3, 70.1, 54.5, 34.6, 23.7; ESMS calculated for C₂₅H₂₅N₄ ([M + H]⁺): 381.2079, found ([M + H]⁺): 381.1794.

[Ru(cyTPA)(CH₃CN)₂](PF₆)₂ (1)

To a solution of cyTPA (13 mg, 0.040 mmol) in 4.0 mL of dry MeOH under inert atmosphere in a pressure flask, cis-[Ru(DMSO)₄Cl₂] (19 mg, 0.040 mmol) was added. The solution was then purged with Ar for 10 min at room temperature, and the reaction mixture was allowed to reflux for 5 h under inert atmosphere, during which time the color of the reaction mixture darkened slightly. The reaction mixture was cooled to room temperature and concentrated under reduced pressure. To the flask, a mixture of CH₃CN:H₂O (1:1 v/v, 4.0 mL) was added, and the reaction mixture was refluxed for another 16 h under inert atmosphere. The reaction mixture was again cooled to room temperature. Ice cold water (15 mL) and NH₄PF₆ (15 mg) were then added, resulting in the formation of a yellow precipitate which was filtered, washed with ice cold water (200 mL) and dried under reduced pressure to give 1 as a yellow solid (26 mg, 82%): ¹H NMR (400 MHz, (CD₃)₂CO) δ 9.25 (d, 1H, J = 4.4 Hz), 8.90 (d, 2H, J = 4.4 Hz), 7.95 (t, 2H, J = 7.2 Hz), 7.64 (t, 1H, J = 7.2 Hz), 7.49 (d, 2H, J = 7.6 Hz), 7.40 (t, 2H, J = 7.2 Hz), 7.33 (t, 1H, J = 7.2 Hz), 7.12 (d, 1H, J = 7.6 Hz), 5.53 (t, 2H, J = 7.2 Hz), 4.65 (s, 2H), 2.93 (s, 3H), 2.57 (s, 3H), 2.68–2.42 (m, 6H); IR (KBr) ν_max (cm⁻¹) 3247, 3116, 2943, 2874, 2360, 2342, 2271, 1701, 1608, 1570, 1486, 1443, 1426, 1365, 1317, 1287, 1250, 1211, 1160, 1114, 1084, 1053, 1036, 991, 932, 838, 764, 751, 740, 715, 668; UV–vis (DMSO) λ_max = 390 nm (ε = 9200 M⁻¹ cm⁻¹); ESMS calculated for C₂₅H₂₈F₆N₆PRu ([M – PF₆⁻]⁺): 659.1068, found ([M – PF₆⁻]⁺): 659.0580; Anal. Calcd for C_26.3H₃₂F₁₂N₆P₂O_0.6Ru (1·0.3Et₂O· 0.3H₂O): C, 37.91; H, 3.87; N, 10.07, found: C, 37.91; H, 3.74; N, 10.21.

[Ru(1-isocyTPQA)(CH₃CN)₂](PF₆)₂ (2)

To a solution of 1-isocyTPQA (15 mg, 0.040 mmol) in 4.6 mL of dry MeOH under inert atmosphere in a pressure flask, cis-[Ru(DMSO)₄Cl₂] (19 mg, 0.040 mmol) was added. The solution was then purged with Ar for 10 min at room temperature, and the reaction mixture was allowed to reflux for 5 h under inert atmosphere, during which time the color of the reaction mixture changed from pale yellow to dark red. The reaction mixture was cooled to room temperature and concentrated under reduced pressure. To the flask, a mixture of CH₃CN:H₂O (1:1 v/v, 4.6 mL) was added, and the reaction mixture was refluxed for another 16 h under inert atmosphere. The reaction mixture was again cooled to room temperature. Ice cold water (15 mL) and NH₄PF₆ (15 mg) were then added, resulting in the formation of a yellow precipitate which was filtered, washed with ice cold water (200 mL) and dried under reduced pressure to give 2 as a yellow solid (16 mg, 47%): ¹H NMR (400 MHz, (CD₃)₂CO) δ 9.12 (d, 1H, J = 6.4 Hz), 8.97 (d, 2H, J = 5.2 Hz), 8.03 (d, 1H, J = 8.4 Hz), 7.94–7.86 (m, 3H), 7.81 (d, 1H, J = 6.4 Hz), 7.74 (t, 1H, J = 7.6 Hz), 7.60 (t, 1H, J = 7.6 Hz), 7.50 (d, 2H, J = 8.0 Hz), 7.36 (t, 2H, J = 6.8 Hz), 5.63 (d, 2H, J = 10 Hz), 5.24 (d, 2H, J = 6.4 Hz), 2.98 (s, 3H), 2.60 (s, 3H), 2.56–2.53 (m, 4H), 2.50–2.44 (m, 2H); IR (KBr) ν_max (cm⁻¹) 3422, 2950, 2876, 2276, 1701, 1606, 1563, 1508, 1467, 1421, 1405, 1362, 1319, 1251, 1221, 1160, 1117, 1083, 1050, 992, 966, 840, 768, 758, 740; UV–vis (DMSO) λ_max = 390 nm (ε = 11 900 M⁻¹ cm⁻¹); ESMS calculated for C₂₉H₃₀F₆N₆PRu ([M – PF₆⁻]⁺): 709.1225, found ([M – PF₆⁻]⁺): 709.1170; Anal. Calcd for C_30.5H₃₃F₁₂N₆O_0.5P₂Ru (2·0.5(CH₃)₂CO): C, 41.50; H, 3.77; N, 9.52, found: C, 41.17; H, 3.78; N, 9.73.

[Ru(cyTPA)(py)₂](PF₆)₂ (3)

To a solution of cyTPA (20 mg, 0.061 mmol) in 6.0 mL of dry MeOH under inert atmosphere in a pressure flask, cis-[Ru(DMSO)₄Cl₂] (29 mg, 0.061 mmol) was added. The solution was then purged with Ar for 10 min at room temperature, and the reaction mixture was allowed to reflux for 5 h under inert atmosphere, during which time the color of the reaction mixture darkened slightly. The reaction mixture was cooled to room temperature and concentrated under reduced pressure. To the flask, a mixture of pyridine:H₂O (1:1 v/v, 6.0 mL) was added, and the reaction mixture was refluxed for another 16 h under inert atmosphere. The reaction mixture was then cooled to room temperature. Ice cold water (15 mL) and NH₄PF₆ (15 mg) were added to the reaction mixture, resulting in the formation of an orange precipitate which was filtered, washed with ice cold water (200 mL) and dried under reduced pressure to give 3 as an orange solid (45 mg, 84%): ¹H NMR (400 MHz, (CD₃)₂CO) δ 8.98 (d, 2H, J = 4.8 Hz), 8.85 (d, 2H, J = 5.2 Hz), 8.76 (d, 1H, J = 5.6 Hz), 8.33 (d, 2H, J = 6.0 Hz), 8.14 (t, 1H, J = 7.6 Hz), 8.00 (t, 2H, J = 7.6 Hz), 7.95 (t, 1H, J = 7.6 Hz), 7.70 (t, 2H, J = 7.2 Hz), 7.62 (t, 1H, J = 7.6 Hz), 7.57 (d, 2H, J = 8.0 Hz), 7.48 (t, 2H, J = 6.4 Hz), 7.39 (t, 2H, J = 7.2 Hz), 7.26–7.22 (m, 2H), 5.17 (dd, 2H, J = 11.2 Hz), 4.71 (s, 2H), 2.61–2.41 (m, 6H); IR (KBr) ν_max (cm⁻¹) 3441, 2963, 2350, 1777, 1708, 1605, 1485, 1466, 1445, 1321, 1297, 1217, 1162, 1117, 1066, 993, 838, 770, 759, 711, 701; UV–vis (DMSO) λ_max = 360 nm (ε = 12 800 M⁻¹ cm⁻¹), 410 nm (ε = 9140 M⁻¹ cm⁻¹); ESMS calculated for C₃₁H₃₂F₆N₆PRu ([M – PF₆⁻]⁺): 735.1382, found ([M – PF₆⁻]⁺): 735.1263; Anal. Calcd for C₃₁H₃₂F₁₂N₆P₂Ru: C, 42.33; H, 3.67; N, 9.55, found: C, 42.25; H, 3.71; N, 9.48.

[Ru(1-isocyTPQA)(py)₂](PF₆)₂ (4)

To a solution of 1-isocyTPQA (15 mg, 0.039 mmol) in 4.5 mL of dry MeOH under inert atmosphere in a pressure flask, cis-[Ru(DMSO)₄Cl₂] (19 mg, 0.039 mmol) was added. The solution was then purged with Ar for 10 min at room temperature, and the reaction mixture was allowed to reflux for 5 h under inert atmosphere, during which time the color of the reaction mixture changed from pale yellow to dark red. The reaction mixture was cooled to room temperature and NH₄PF₆ (15 mg) was then added, resulting in the formation of a yellow precipitate which was filtered and washed with ice cold water (200 mL). The solid was collected, distilled water (6.0 mL) and 3 to 4 drops of trifluoromethanesulfonic acid were added, and the reaction mixture was refluxed for another 4 h under inert atmosphere. During this time, the color of the reaction mixture changed from dark red to green. The reaction mixture was cooled to room temperature, and ice cold water (15 mL) and NH₄PF₆ (15 mg) were then added, resulting in the precipitation of green [Ru(1-isocyTPQA)(H₂O)₂](PF₆)₂. The green solid was filtered, washed with ice cold water (200 mL) and dried under reduced pressure. It was then treated with pyridine (31 mg, 0.39 mmol) in 6.0 mL of absolute EtOH under inert atmosphere in a pressure flask. The solution was purged with argon for 10 min at room temperature. The mixture was then allowed to reflux for another 16 h under inert atmosphere, and the color of the reaction mixture was observed to change from green to dark red. The reaction mixture was again cooled to room temperature and concentrated under reduced pressure, washed with toluene (20 mL), and extracted with dichloromethane and water. The organic layer was collected, dried over Na₂SO₄, and concentrated under reduced pressure to give the crude product. The crude product was purified by chromatography on alumina using acetone/hexane (8:2, v/v) as the eluent to give 4 as a red solid (14 mg, 38%): ¹H NMR (400 MHz, (CD₃)₂CO) δ 9.06 (d, 2H, J = 5.2 Hz), 8.93 (d, 2H, J = 5.6 Hz), 8.60 (d, 1H, J = 6.8 Hz), 8.38 (d, 2H, J = 5.2 Hz), 8.18–8.14 (m, 2H), 7.98–7.90 (m, 4H), 7.77–7.71 (m, 4H), 7.63 (t, 1H, J = 7.6 Hz), 7.57 (d, 2H, J = 8.0 Hz), 7.46–7.39 (m, 4H), 5.31–5.25 (m, 4H); IR (KBr) ν_max (cm⁻¹) 3437, 2959, 1607, 1566, 1508, 1487, 1465, 1445, 1404, 1276, 1161, 1085, 839, 759, 740, 704; UV–vis (DMSO) λ_max = 360 nm (ε = 10 600 M⁻¹ cm⁻¹), 415 nm (8020 M⁻¹ cm⁻¹); ESMS calculated for C₃₅H₃₄F₆N₆PRu ([M – PF₆⁻]⁺): 785.1540, found ([M – PF₆⁻]⁺): 785.1445; Anal. Calcd for C₃₉H₄₄F₁₂N₆OP₂Ru (4·Et₂O): C, 46.66; H, 4.42; N, 8.37, found: C, 46.37; H, 4.27; N, 8.25.

Methods and Instrumentation

¹H and ¹³C NMR spectra were recorded on either a Varian FT-NMR Agilent 400 MHz, Oxford 500 MHz, or Agilent DD2 600 MHz spectrometer in CDCl₃, CD₃OD or (CD₃)₂CO, and the spectra were referenced to the residual protonated solvent peaks. Mass spectra were recorded on a time-of-flight Micromass LCT Premier XE Spectrometer, IR spectroscopy was conducted on a Nicolet FT-IR spectrophotometer (KBr pellet), and electronic absorption spectra were recorded on a Varian Cary 50 or an Agilent 8454 spectrophotometer.

Single crystal X-ray diffraction data were collected at 100 K with Mo Kα radiation using a Bruker X8 diffractometer equipped with a kappa geometry goniometer, graphite monochromator, and an APEX-II CCD. The frames were integrated with the Bruker SAINT software package⁶² using a narrow-frame algorithm, and the data were corrected for absorption effects using the multiscan method (SADABS).⁶³ Using Olex2,⁶⁴ structures of 1 and 2 were solved with the SHELXS-1997 structure solution program⁶⁵ using direct methods and refined with the SHELXL refinement package⁶⁶ using least-squares minimization. The structure of 3 was solved with the SHELXT structure solution program⁶⁷ using direct methods and refined as with 1 and 2. During the refinement of structure 1, a disordered PF₆⁻ counterion was removed and modeled using a solvent mask. All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were placed in calculated positions. Crystals of 1 were yellow. 200 837 reflections were collected and merged into 12 344 independent reflections with R_int = 0.0334. The asymmetric unit contains one Ru complex and 1.5 PF₆⁻ counterions. The phosphorus atom of one of the PF₆⁻ ions sits on a special position and is 1/2 occupied, which may be due to an invisible counterion (like H⁺) in the electron density map. Crystals of 2 were also yellow. 23 2068 reflections were collected and merged into 21 952 independent reflections with R_int = 0.0273. The asymmetric unit contains one Ru complex, two PF₆⁻ counterions and one CH₃CN in the solvent area. Crystals of 3 were again yellow, and 91 822 reflections were collected and merged into 6547 independent reflections with R_int = 0.0837. The asymmetric unit contains one Ru complex and two PF₆⁻ counterions. Any counterions or solvent molecules present were not shown in the ORTEP diagrams for clarity.

The irradiation source for photolysis and ligand exchange quantum yield experiments was a 150 W Xe arc lamp (USHIO) in a MilliArc lamp housing unit powered by a LPS-220 power supply equipped with a LPS-221 igniter (PTI). A 395 nm long-pass filter (CVI Melles Griot) was used for the photolysis experiments, while a 400 nm bandpass filter (Thorlabs) with a 335 nm long-pass filter (CVI Melles Griot) was used to measure quantum yields of ligand exchange. For both the photolysis and ligand exchange quantum yield experiments, the samples were dissolved in H₂O with <5% acetone added for solubility. Potassium tris(ferrioxalate) was used as a chemical actinometer to determine the photon flux of the lamp at 400 nm as previously described (1.6 × 10⁻⁷ mol photons/min).⁶⁸ For NMR studies, solutions of 1 and 2 in CD₃CN (~2 mM) were prepared and stored for 1 h in the dark, and then were irradiated in an NMR tube using the 150 W Xe arc lamp and a 395 nm long-pass filter. ¹H NMR spectra were collected using a Bruker DPX 400 MHz spectrometer at the indicated time intervals, and all chemicals shifts were referenced to the residual protonated solvent peak (δ = 1.94 ppm).

Spin restricted and unrestricted density functional theory (DFT) calculations were performed with the Gaussian 09 package.⁶⁹ All geometry optimization and vibrational frequency calculations were performed with the SDD⁷⁰ basis set on Ru and the TZVP⁷¹ basis set on all other atoms with the PBE^72,73 exchange-correlation functional. The geometries of all complexes were fully optimized starting from X-ray crystal structures, when available. All optimized geometries have positive harmonic frequencies, confirming the calculated structures as electronic energy minima. Further calculations of molecular orbitals and time-dependent DFT (TD-DFT) utilized the B3LYP^74–76 hybrid functional, again with the SDD basis set on Ru and with the TZVP basis set on all other atoms with the inclusion of solvation effects using the polarized continuum model (PCM) with acetonitrile as the solvent in the TD calculations.⁷⁷ TD-DFT was used to calculate the electronic transition state energies and intensities of the 75 lowest-energy states. Spin densities were calculated using Mulliken population analysis methods (MPA). Orbitals from the Gaussian calculations were plotted using the Chemcraft program. The analysis of the MO compositions in terms of fragment orbitals, Mayer bond orders, and charge decomposition analyses^78,79 (CDA) were performed using AOMix-FO⁸⁰ within the AOMix program.^80,81 CDA and its applications have been previously described in detail by Gorelsky and co-workers.^82,83

Ultrafast transient absorption data were collected on a broadband UV–vis system (Figure S23, Supporting Information) consisting of a Ti:sapphire oscillator (Vitara-S, Coherent), which generated 20 fs pulses at 80 MHz to seed a high-energy Ti:sapphire regenerative amplifier (Astrella 1K-USP, Coherent) to produce 7 mJ, 35 fs pulses at 1 kHz. The output from the regenerative amplifier was then split, where 1 mJ was used to generate a white light continuum as previously described,⁸⁴ and the remaining 6 mJ was split again using a 50:50 beamsplitter. From the latter, 3 mJ was used to pump an OPA (OPerA Solo, Coherent/Positive Light) to generate a pump pulse tunable from 300–2500 nm. A thermoelectrically cooled CCD camera (Princeton Instruments, 1340 × 100 pixels) and home-developed software, written in LabVIEW 2015, were used to collect the spectral data. An instrument response full width at half-maximum (fwhm) of ~85 fs was measured using the optical Kerr effect in cyclohexane. Typically, a total sample volume of ~10–20 mL was circulated through a Harrick Scientific flow cell (1 mm thick CaF₂ windows, 1 mm optical path length). The sample solutions were prepared with an absorbance at the excitation wavelength of ~0.5–0.6, and the pump pulse energy was set to ~2 μJ at the sample. All other aspects of the optical setup and data collection are as previously described.⁸⁴ The samples were excited with 350 nm, and the polarization angle between the pump and probe beams was set to the magic angle of 54.7° to avoid effects caused by rotational diffusion. Measurements were repeated three times at each time delay by collecting three retroreflector cycles and the spectra were corrected for the chirp in the white light continuum.⁸⁵ The kinetic traces were fit to mono- or biexponential decays as needed.⁸⁶

RESULTS AND DISCUSSION

Synthesis and X-ray Crystallography

The precursor (2R,6S)-2,6-di(pyridin-2-yl)piperidine (structure shown in Figure 1) was synthesized according to a known literature procedure⁶⁰ in which the commercially available starting material 2-pyridinecarboxaldehyde was treated with 1,3-acetonedicarboxylic acid in the presence of ammonium acetate, followed by a Wolff–Kishner reaction. The ligand cyTPA was prepared by treating (2R,6S)-2,6-di(pyridin-2-yl)piperidine with 2-picolyl chloride hydrochloride, NaI·2H₂O and DIPEA in CH₃CN at 50 °C through alkylation. The ligand 1-isocyTPQA was synthesized following a similar procedure using 1-(chloromethyl)isoquinoline (Figure 1).

Treating each ligand with 1 equiv of cis-[Ru(DMSO)₄Cl₂] in MeOH at 70 °C provided a 2:1 mixture of [Ru(L)(DMSO)-Cl]Cl stereoisomers (L = cyTPA, 1-isocyTPQA) as previously described in the literature.⁸⁷ Heating [Ru(L)(DMSO)Cl]Cl in a CH₃CN:H₂O mixture (1:1, v/v) resulted in the formation of the corresponding [Ru(cyTPA)(CH₃CN)₂]²⁺ (1) and [Ru(1-isocyTPQA)(CH₃CN)₂]²⁺ (2) complexes in good yield. Heating [Ru(cyTPA)(DMSO)Cl]Cl at 80 °C under similar conditions in the presence of 10 equiv of pyridine, followed by precipitation with NH₄PF₆ resulted in [Ru(cyTPA)(py)₂]-(PF₆)₂ (3) in 84% yield. The precipitation of [Ru(1-isocyTPQA)(DMSO)Cl]Cl with NH₄PF₆, followed by heating in distilled H₂O with trifluoromethanesulfonic acid at 100 °C afforded the intermediate complex [Ru(1-isocyTPQA)-(H₂O)₂](PF₆)₂ as a green solid. Treating [Ru(1-isocyTPQA)-(H₂O)₂](PF₆)₂ with 10 equiv of pyridine in absolute EtOH at 80°C resulted in [Ru(1-isocyTPQA)(py)₂](PF₆)₂ (4) in moderate yield.

Diffusion of diethyl ether into solutions of 1–3 in acetone provided single crystals suitable for X-ray crystallographic analysis (Figure 2). Additional crystallographic data for 1–3, including data collection parameters, are detailed in Tables S1–S3 (Supporting Information), with selected bond lengths and bond angles listed in Tables S2 and S3. For all three structures, the N6 donor is defined as that which is positioned cis to the basic nitrogen N3, while the N1 atom is positioned trans to N3 (Figure 2).

Figure 2.

ORTEP diagrams of the dications (a) 1 [Ru(cyTPA)-(CH₃CN)₂]²⁺, (b) 2 [Ru(1-isocyTPQA)(CH₃CN)₂]²⁺ and (c) 3 [Ru(cyTPA)(py)₂]²⁺ (thermal ellipsoids shown at 50% probability, hydrogen atoms are omitted for clarity). Selected bond distances (Å) for 1: Ru1–N1, 2.047(2); Ru1–N6, 2.047(2). Selected bond distances (Å) for 2: Ru1–N1, 2.0312(8); Ru1–N6, 2.0331(8). Selected bond distances (Å) for 3: Ru1–N1, 2.13(1); Ru1–N6, 2.12(1).

Complex 1 crystallizes in the monoclinic space group C2/c with Z = 4. The coordination environment around the Ru(II) metal center exhibits a slightly distorted octahedral geometry, with bond angles ranging from 79.36(7)° to 101.15(7)°. In 1, the fourth arm of the tetradentate cyTPA ligand (the N5 arm) is slightly tilted toward the N2 arm, causing the N5–Ru1–N2 angle to be 84.95(7)°, a value smaller than the analogous angle in [Ru(TPA)(CH₃CN)₂]²⁺, 90.4(2)°.⁵⁶ Furthermore, the rigidity afforded by the piperidine ring in 1 appears to cause a small amount of additional steric strain in the cyTPA ligand as evidenced by the slightly smaller N2–Ru1–N4 angle of 159.15(7)° as compared to 164.1(2)° in [Ru(TPA)-(CH₃CN)₂]^2+.56 These small distortions do not cause a significant lengthening of any of the Ru–N bonds in 1, since the bond distances are all within a few hundredths of an Ångstrom to those reported for [Ru(TPA)(CH₃CN)₂]²⁺,⁵⁶ and the Ru–N(CH₃CN) bond lengths in 1 are comparable to those reported for cis-[Ru(bpy)₂(CH₃CN)₂]^2+.88

Complex 2 crystallizes in the monoclinic space group P2₁/n with Z = 4. The coordination environment around the metal center also exhibits a slightly distorted octahedral geometry, with bond angles ranging from 80.98(3)° to 99.80(4)°. As expected, the substitution of a pyridine for an isoquinoline moiety in the N5 arm of the tetradentate ligand does not produce steric strain around the metal center. A comparison between the bond angles of 1 and 2 reveals that the only difference is that in 2 the N6 CH₃CN cis to the basic amine nitrogen is tilted slightly toward the N2 arm, as evidenced by the slightly smaller N6–Ru1–N2 angle of 86.99(3)° in 2 as compared to 93.00(7)° in 1. Again, the Ru–N bond lengths in 2 are comparable to 1, as well as to those of the previously reported [Ru(TQA)(CH₃CN)₂]^2+.58 In addition, the Ru–N(CH₃CN) bond lengths in 2 also compare well with those reported for cis-[Ru(bpy)₂(CH₃CN)₂]^2+.88

Complex 3 crystallizes in the monoclinic space group P2₁/c with Z = 4. The coordination environment around the Ru(II) metal center again exhibits a slightly distorted octahedral geometry with bond angles ranging from 76.8(4)° to 101.9(4)°. In 3, the N5 arm of the cyTPA ligand is bent toward the N4 arm to a larger degree than that in 1 or 2, as evidenced by the smaller N5–Ru1–N4 angle of 80.7(5)° in 3. This difference is attributed to the steric bulk afforded by a py ligand as compared to an CH₃CN in 1 and 2. The N2 and N4 arms of the cyTPA ligand cause the N1 py ligand to be canted rather than coplanar, as is the case in 1 and 2 with CH₃CN ligands, which pushes the N5 arm of cyTPA closer to the N4 arm, as well as the N2 arm closer to the central piperidine ring. This steric clashing between the N1 py and the tetradentate ligand is also observed in [Ru(TPA)(py)₂]^2+.57 The Ru–N bond lengths of the tetradentate ligand in 3 are similar to the corresponding bond lengths in 1 and 2, and the Ru–N(py) bond lengths of 3 are similar to those measured in [Ru(TPA)(py)₂]²⁺,⁵⁷ as well as in cis-[Ru(bpy)₂(py)₂]^2+.89

Electronic Absorption Spectroscopy and Photochemistry

The electronic absorption spectra of the CH₃CN complexes 1 and 2 are displayed in Figure 3 and exhibit ¹MLCT-based absorption features with maxima at 390 nm (ε = 9200 M⁻¹ cm⁻¹ and 11 900 M⁻¹ cm⁻¹, respectively). Ligand-centered ³ππ* transitions of the ligands in 1 and 2 result in strong absorption in the ultraviolet range. The intensities of these transitions correlate well with those of [Ru(TPA)-(CH₃CN)₂]²⁺; however, the ¹MLCT absorption of 2 is red-shifted as compared to those of 1 and [Ru(TPA)-(CH₃CN)₂]²⁺,⁵⁶ attributed to the presence of the quinoline moiety in the tetradentate ligand (Figure 3). The pyridine complexes 3 and 4 exhibit ¹MLCT transitions at 360 nm (ε = 12 800 M⁻¹ cm⁻¹ and 10 600 M⁻¹ cm⁻¹, respectively). Shoulders of this peak in 3 and 4 are observed at ~410 nm (ε ~ 9140 M⁻¹ cm⁻¹) and ~415 nm (ε ~ 8020 M⁻¹ cm⁻¹), respectively, that tail to ~500 nm (Figure S24, Supporting Information). The ¹MLCT absorption maxima of 3 and 4 are red-shifted compared to the analogous CH₃CN complexes, as is well-known for other cis-Ru(II) complexes coordinated by two CH₃CN and py ligands.^37,90 The quinoline moiety in 4 leads to a further red shift as compared to 3. Again, the intensities of these transitions are in good agreement with those of [Ru(TPA)(py)₂]^2+.57

Figure 3.

Electronic absorption spectra of 1 (black, dashed) and 2 (red, solid) in DMSO.

The irradiation of 1–4 in H₂O (<5% acetone) results in changes to the absorption spectra of each complex (λ_irr ≥ 395 nm), whereas no changes are observed under similar conditions when the solutions of each complex are kept in the dark for at least 6 h (Figure S25, Supporting Information). The spectral changes observed for 1 and 2 are shown in Figure 4, and those for 3 and 4 appear in Figure S26 (Supporting Information). The photolysis of 1 is complete within 27 min, as no additional spectral changes are observed after this time. In contrast, the conversion of 2 to product is complete within only 9 min under similar experimental conditions. This difference in photo-chemical activity is reflected in the difference in the measured quantum yields, Φ₄₀₀, for the two complexes (λ_irr = 400 nm, Table 1), where 2 is 5-fold more photoactive than 1. In contrast to the CH₃CN complexes 1 and 2, those containing py at the N1 and N6 positions, 3 and 4, require at least 2 h of irradiation to reach completion under similar experimental conditions. The values of Φ₄₀₀ for 3 and 4 are comparable, but both are ~30-fold lower than that measured for 2 (Table 1). The less efficient photochemistry of py Ru(II) complexes as compared to analogous CH₃CN compounds is well documented in the literature.^14,27,48,51

Figure 4.

Changes to the electronic absorption spectra upon irradiation (λ_irr ≥ 395 nm) of (a) 1 (t_irr = 0–27 min) and (b) 2 (t_irr = 0–9 min) in H₂O (<5% acetone).

Table 1.

Photochemical and Photophysical Data for 1–4

complex	Φ400a	lowest 3ESb	τTA/ps
1	0.0066(3)	3LF	36c
2	0.033(3)	3MLCT/3ππ*	42c
3	0.0012(1)	3LF	160d
4	0.0013(1)	3LF	120d

^aH₂O (<5% acetone) at 298 K.

^bFrom Mulliken Spin Density Calculations.

^cCH₃CN.

^dAcetone.

During the photolysis of 1–4 in H₂O, a decrease in the ¹MLCT absorption band of each complex is observed with a concomitant increase in a peak at lower energies, as well as well-defined isosbestic points indicative of the formation of a single product from the reactant. For example, the irradiation of 2 in H₂O results in a decrease of the ¹MLCT absorption at 383 nm and a growth of a new absorption peak with maximum at 414 nm, resulting in an isosbestic point at 399 nm. On the basis of prior work on related complexes, it is expected that one of the CH₃CN ligands in 1 and 2 and py in 3 and 4 exchanges with a solvent H₂O molecule upon irradiation.

As discussed above, the ligand exchange quantum yield of 2 is 5-fold greater than that of 1, and it was shown in previous work that steric hindrance leads to enhanced ligand dissociation.⁹¹ The major structural difference between 1 and 2 is the substitution of the third pyridine in the cyTPA ligand in 1 for a quinoline moiety in the 1-isocyTPQA ligand in 2. However, the quinoline moiety in 2 is oriented away from the metal center, such that it is not expected to contribute to steric distortions around the metal or to weaken the Ru–N(CH₃CN) bonds. In fact, the ground state bond lengths and angles observed in the X-ray crystal structures are similar between 1 and 2, inconsistent with a weaker Ru–N bond induced by steric clash. Therefore, it may be concluded that the increased quantum yield in 2 must be associated with differences in electronic structure afforded by the presence of the quinoline arm in 2 as compared to a pyridine arm in 1.

In order to gain additional structural information on the products formed upon irradiation, the photolyses of 1 and 2 were also followed using ¹H NMR spectroscopy in CD₃CN (Figure S27 and Figure 5, respectively). No spectral changes are observed for either complex in the dark in CD₃CN (1 h). However, upon irradiation the resonance at 2.39 ppm in 1 (λ_irr ≥ 395 nm, 10 min) associated with the protons of the N6 CH₃CN ligand cis to the basic nitrogen and trans to the pyridine arm of the cyTPA begins to decrease in intensity. A similar decrease is observed for the signal at 2.41 ppm upon irradiation of 2 (λ_irr ≥ 395 nm, 5 min), which corresponds to the bound N6 CH₃CN ligand positioned cis to the basic nitrogen and trans to the quinoline arm of the 1-isocyTPQA ligand. This behavior is associated with the exchange of the bound CH₃CN ligand for solvent CD₃CN and continues until the process is complete. As expected, in both complexes there is an increase in intensity of the peak corresponding to free CH₃CN, which appears at 1.96 ppm in CD₃CN. Continued irradiation leads to the complete exchange of the N6 CH₃CN ligand in 90 min for complex 1, a process that is complete in 25 min in 2. The aromatic regions of the NMR spectra of 1 and 2 remain unchanged throughout the photolysis experiments (Figures S28 and S29, respectively, Supporting Information), indicating that the only reaction is the exchange of CH₃CN for CD₃CN in the complex, thus retaining its electronic and structural characteristics.

Figure 5.

Comparison of the aliphatic region of the ¹H NMR spectrum of 2 in CD₃CN as initially prepared in the dark (black), after 1 h in the dark (blue), and at various irradiation times (λ_irr ≥ 395 nm).

CALCULATIONS

Density functional theory (DFT) calculations were utilized to gain a better understanding of the bonding and electronic structure of 1–4. Geometry optimizations for these complexes in the singlet ground state produced calculated structures in good agreement with crystallographic data (Tables S4–S7, Supporting Information), as well as ν(CN) of the CH₃CN ligands of 1 and 2 that are in good agreement with experimental IR data. The electronic structures of these complexes in the ground state can be described by frontier molecular orbitals that are similar across the series. The HOMOs of 1–4 are mostly composed of Ru d-orbital character with some ligand mixing, featuring 70–81% metal localization in 1, 3, and 4 (Figure 6). Interestingly, the HOMO of 2 exhibits the lowest Ru d character, 62%, with the highest ligand mixing of the series. The LUMOs are exclusively either localized on the cyTPA ligand of 1 and 3 or 1-isocyTPQA of 2 and 4.

Figure 6.

Frontier molecular orbital diagram for complexes 1–4 in the ground state (L = cyTPA, L′ = 1-isocyTPQA).

Time-dependent DFT, TD-DFT, calculations for 1–4 show remarkable similarities for the four lowest energy transitions of the complexes, all of which involve the ruthenium metal center and the tetradentate ligand (Table S8, Supporting Information). The calculated lowest ¹MLCT absorption bands of 1 and 3 exhibit Ru(d_π) → π*(cyTPA) character with energies of 26 500 cm⁻¹ and 24 600 cm⁻¹, respectively, which compare well with the corresponding experimental values of 25 800 cm⁻¹ and 24 400 cm⁻¹. Similarly, the lowest ¹MLCT Ru(d_π) → π*(1-isocyTPQA) transitions are calculated for 2 at 25 600 cm⁻¹ and 4 at 24 400 cm⁻¹, in good agreement with the experimental maxima at 25 000 cm⁻¹ and 24 200 cm⁻¹, respectively. Notably, unlike 1, 3, and 4, one of the low-energy transitions calculated for 2 exhibits some mixing of ligand-centered 1-isocyTPQA ¹ππ* character into the broad ¹MLCT band and another is solely attributable to 1-isocyTPQA ¹ππ* (Table S8). This finding is consistent with the larger amount of ligand mixing in the HOMO of 2, which is absent in 1, 3, and 4, as well as the participation of the ligand-centered HOMO–3 in the low energy transitions of 2.

Geometry optimizations and vibrational frequency calculations were also completed in the triplet states of 1–4. It should be noted that the optimized structures for 1, 3, and 4 yielded structures in which the Ru–N bonds of the tetradentate cyTPA and 1-isocyTPQA ligands were severely elongated in the triplet state, possibly suggesting facile dissociation of one of these arms in the excited state (Tables S4–S7). In contrast, these elongations were not observed in the optimized structure of the lowest triplet state of 2 (Table S5). These differences for 2 relative to the other complexes are of interest given its significantly more efficient ligand exchange upon irradiation.

Mulliken spin density calculations were performed to determine the spin density on the Ru(II) center and to examine the nature of the triplet state. For complexes with a ³MLCT as the lowest energy triplet state, the Mulliken spin density on the ruthenium center should theoretically equal 1, indicative of one unpaired electron on the metal. Those complexes with a ³LF as the lowest energy triplet state should have a spin density of about 2 on the ruthenium center, indicating two unpaired electrons on the metal. Any complex with a spin density on the metal that is not close to values of 1 or 2 suggests some level of ligand mixing. For complexes 1, 3, and 4, the Mulliken spin densities on the Ru(II) center were calculated to be 1.63, 1.75, and 1.73, respectively. These spin densities are consistent with a ³LF state as the lowest energy triplet state with some spin localization on the ligands. Surprisingly, for complex 2, the Mulliken spin density on the metal was calculated to be 0.37, which is extremely low for a ³MLCT state and is more consistent with significant mixing from a ligand-centered state. Figure 7 shows a comparison of the Mulliken spin densities on the atoms of complexes 1 and 2. Notably, the majority of the spin density in 2 is localized on the quinoline arm of the 1-isocyTPQA ligand, with only minimal contributions from other atoms.

Figure 7.

Mulliken spin densities of the lowest energy triplet state calculated for 1 and 2.

Although 2 exhibits the most efficient photoinduced CH₃CN ligand exchange with solvent, it is surprising that it is the only complex in the series that does not feature a ³LF state as the lowest energy triplet state. It should be noted, however, that the ³LF states of 1, 3, and 4 appear to be dissociative taking into account the elongated bonds of the tetradentate ligand. This result indicates that if the ³LF state is accessed in these complexes, then an arm of the tetradentate ligand may be dissociated upon irradiation, which would likely recoordinate without resulting in an overall chemical transformation. Therefore, irradiation of these complexes would not result in the exchange of CH₃CN in 1 or py in 3 and 4.

¹H NMR spectroscopy shows that the CH₃CN ligand that dissociates more readily in 2 is positioned trans to the quinoline moiety in the tetradentate ligand. In general, σ-donation from py- and quinoline-type ligands is stronger than that of nitrile-containing ligands. Instead, the CH₃CN ligand relies heavily on π-back-donation for bonding to ruthenium. Charge decomposition analysis (CDA), a theoretical method used to quantify the total electron donation and back-donation between the metal and each ligand, was used to determine these quantities in 1 and 2. The back-donation from the Ru center to the photolabile N6 CH₃CN ligand in 2 decreases by 23% in the triplet state as compared to the singlet ground state, as the ruthenium center is partially oxidized in the excited state (Table S17, Supporting Information). The corresponding cyTPA complex, 1, possesses a py moiety rather than quinoline in the tetradentate ligand, and shows a decrease in back-donation to the CH₃CN ligand from the singlet state to the triplet state of less than 10%.

This trend was further examined by analyzing the Mayer bond orders (MBOs) of the Ru–N bonds. MBOs offer insight into the relative covalency vs ionicity of a bond and are an extension of Wiberg bond orders, a classical view of bonding, and can be related to the bond strength.⁹² Previous theoretical studies have shown that MBO analysis is a useful tool in describing bonding in transition metal systems.^92–97 For 2, the MBO analysis shows a dramatic 57% increase in Ru–N(quinoline) bond strength and a 14% decrease in Ru–N(CH₃CN) bond strength in the CH₃CN ligand trans to the quinoline moiety (N6 CH₃CN) in the triplet state. The quinoline moiety, therefore, exerts a trans-type influence on this CH₃CN ligand, greatly weakening the Ru–N(CH₃CN) bond, promoting dissociation in the triplet excited state. This weakening does not occur in the corresponding py complex, 4, due to the similar σ–bond strengths between quinoline and the leaving py ligand. In this complex, the MBO of the Ru–N(py) decreases only by ~2% in the triplet state, while the Ru–N(quinoline) MBO increases by 3%. A similar trend is observed for complexes 1 and 3 at the same positions, but with increased MBOs calculated from the ground state to the lowest triplet state for both Ru–N(CH₃CN) and Ru–N(py) bonds, respectively (Table 2). The observation of trans influence and trans effect in ruthenium complexes has been reported previously, specifically with phosphine and py-type ligands in the ground state.^98–101 To our knowledge, there are no reports of trans influence in the excited state that directly results in ligand dissociation. From the results of the Mulliken spin density calculations, in combination with the CDA and MBO analyses, it may be inferred that the high quantum yield of ligand dissociation observed for 2 is associated with the high spin density localized on the quinoline moiety in the mixed ³MLCT/³ππ* excited state. This ³MLCT/³ππ* state must induce a trans-type influence, allowing for efficient ligand dissociation of the N6 CH₃CN ligand.

Table 2.

MBO Analysis for 1–4 in the Ground State Singlet and Lowest Energy Triplet State

complex	Ru–N6a		Ru–N5b
	singlet	triplet	singlet	triplet
1	0.611	0.612	0.434	0.446
2	0.599	0.518	0.415	0.652
3	0.164	0.168	0.397	0.421
4	0.148	0.144	0.369	0.380

^aN atom of CH₃CN in 1 and 2 and py in 3 and 4.

^bN atom of py arm of cyTPA in 1 and 3 and quinoline arm of 1-isocyTPQA in 2 and 4.

Time-Resolved Spectroscopy

Ultrafast transient absorption spectroscopy was used to compare the nature of the excited states of 1 and 2, since the latter undergoes ligand exchange with 5-fold greater efficiency than the former. In addition, the transient absorption spectra of 3 and 4 were also collected and are shown in the Supporting Information. It is well established that intersystem crossing (ISC) from the ¹MLCT to the ³MLCT state occurs within 15–40 fs in [Ru(bpy)₃]²⁺, as well as in other Ru(II) polypyridyl and related complexes.^90,102,103 As such, the transient absorption signals observed for 1 and 2 after excitation of the ¹MLCT state can be attributed to states in the triplet manifold due to the ~85 fs pulse employed in the present experiments.¹⁰⁴

The transient absorption spectra of 2 collected in CH₃CN following 350 nm excitation are shown in Figure 8a (fwhm = 85 fs), and consist of a ground state bleach from 390 to 438 nm, with positive absorption from 438 to 600 nm and an isosbestic point at 438 nm. Both the bleach at 400 nm and the positive signal with maximum at 465 nm decay monoexponentially with τ = 42 ps (Table 1). No spectral changes are observed from 200 fs to 1 ps (Figure S34a, Supporting Information). The intensity and position of the positive absorption is consistent with a ³MLCT state, similar to the spectral features of ³MLCT states ₂₊ of related Ru(II) complexes, including [Ru(bpy)₂(CH₃CN)₂] in CH₃CN.⁹⁰ In addition, the simulated transient absorption spectrum of 2 shown in Figure S35b (Supporting Information), created from the subtraction of the calculated absorption spectrum of ³MLCT/³ππ* state and that of the ground state, agrees fairly well with experiment. The lifetime of 2 in CH₃CN is similar to that reported for [Ru(bpy)₂(CH₃CN)₂]²⁺ in the same solvent, 50 ps.⁹⁰ In CH₃CN, the photoinduced ligand exchange of 2 does not result in a product that is different than the starting material, such that any kinetics that may be associated with that process are not observed. These results are consistent with the formation of the ³MLCT/³ππ* state of 2 within the laser pulse, and that this state decays to regenerate the ground state with τ = 42 ps in CH₃CN. The short lifetime of the ³MLCT/³ππ* state can be attributed to competing ligand dissociation.

Figure 8.

Transient absorption spectra of 2 in (a) CH₃CN collected at 0.4, 8, 20, 51, 99, and 285 ps, and (b) acetone collected at 1, 3, 8, 17, 39 51, 192, 718, and 2681 ps following the laser pulse (λ_exc = 350 nm, fwhm = 85 fs, baseline collected at −20 ps).

When the transient absorption experiment is conducted in acetone under similar conditions (Figure 8b), the irreversible formation of the ligand exchange product, [Ru(1-isocyTPQA)-(CH₃CN)(acetone)]²⁺, is observed at long delay times consistent with the spectral changes measured during steady-state photolysis. At earlier times, 1–17 ps, a broad positive signal is observed in the 450–600 nm range which can be fitted to a monoexponential decay with τ = 36 ps. Although the photoproduct absorbs in the 390–440 nm range, the kinetics of the bleach signal at 400 nm are also monoexponential with τ = 36 ps. It should be noted that there are no spectral changes from 100 fs to 1 ps (Figure S34b, Supporting Information). In addition, a power dependence of the product formed in the ultrafast experiment in acetone is consistent with a one-photon process (Figure S36).

Because of the overlap between the bleach signal and that of the product in Figure 8b, the kinetics of product formation could not be measured independently. There are two possibilities for the observation of the same monoexponential time constant for the bleach signal at 390 nm that overlaps with the product absorption and the positive ³MLCT signal at 500 nm, namely that the product is formed directly from the ³MLCT state or that it is generated within the laser pulse and its signal does not change throughout the experiment. In the latter scenario, the ligand exchange dynamics do not contribute to the transient absorption signal in the 1 ps –2.68 ns range, such that the signal of the last trace of Figure 8b collected at 2.68 ns would have been present at all time delays. In the case where the product is formed from the ³MLCT state, the signal in Figure 8b collected at early times would have little contribution from product, and the absorption of the latter would increase exponentially with the measured time constant of 36 ps. A series of additions of product signal collected at 2.68 ns in Figure 8b to the signal from the pure ³MLCT (Figure 8a) were undertaken as described in detail in the Supporting Information and shown in Figure S37. Two exemplary time points at early and late times in the decay, 8 ps (Figure S37c) and 51 ps (Figure S37d), respectively, are used to compare the simulated signals which are consistent with the formation of the photoproduct from the ³MLCT state and not within the laser pulse.

The transient absorption spectra collected for 1 in CH₃CN are similar to those of 2 in the same solvent and are shown in Figure S38 (Supporting Information, λ_exc = 350 nm, fwhm = 85 fs). The dominant features are the ground state bleach at ~390 nm and a broad positive transient absorption signal in the 430–600 nm range with maximum at ~460 nm. Both features decay monoexponentially with τ = 36 ps to regenerate the ground state (Table 1). Similar absorption and bleach signals were observed for 1 in acetone with τ = 34 ps. The low quantum yield for ligand exchange in this complex precludes the observation of different kinetics in CH₃CN and acetone in 1. Although the lowest energy triplet state of 1 was calculated to be ³LF in nature, from the intensity of the observed excited state absorption, together with spectral and kinetic similarities to 2 and other Ru(II) polypyridyl complexes, it may be concluded that the state observed with 34–36 ps lifetime is the ³MLCT, not the ³LF. In addition, it is evident that the calculated transient absorption spectrum of the ³LF state shown in Figure S39 (Supporting Information) features ~10-fold lower intensity of the positive signal as compared to that calculated for 2, inconsistent with the experimental data.

The transient absorption spectra of 3 and 4 are shown in Figure S40 (Supporting Information) and exhibit similar features that are red-shifted compared to those of 1 and 2, respectively. Complex 3 exhibits a ground state bleach at 420 nm and a weak positive transient absorption signal from 470 to 600 nm with τ = 160 ps (Table 1). The identical, monoexponential kinetics of the positive signal and the bleach measured for 3 are indicative of the population of a single state within the laser pulse that returns to the ground state. Similarly, the spectral features of 4 are red-shifted relative to those of the corresponding CH₃CN complex, 2, and features a ground state bleach at 420 nm and a positive transient absorption signal at >480 nm (Figure S40). These signals decay monoexponentially with τ = 120 ps (Table 1). The ³LF state was calculated as the lowest energy triplet state in both 3 and 4, with simulated transient absorption spectra for these complexes shown in Figures S41 and S42, respectively (Supporting Information). As is the case with 1, the positive signal observed for 3 and 4 is significantly stronger than expected for a metal-centered state and can be assigned as ³MLCT states. The broad spectral profiles throughout the visible range are similar to those previously reported for the ³MLCT excited states of the related complexes [Ru(bpy)₂(py)₂]²⁺ and [Ru(bpy)₂(NA)₂]²⁺ (NA = nicotinamide), respectively.^105–107

Excited State Populations and Ligand Exchange

Although the minimized ³LF state lies at a lower energy than the ³MLCT state in 1, the latter is observed in the time-resolved absorption spectrum. A possible explanation for this behavior is that ISC from the ¹MLCT populates the ³MLCT state in <100 fs, as is typically observed in related Ru(II) polypyridyl complexes.^102,103 Displacement of the ³LF potential energy surface along a given nuclear coordinate, such as elongation of one or more Ru–N bonds in the triplet state, would require that the system overcome an activation barrier to reach the ³LF state (Figure 9). Inspection of Tables S4–S7 reveal that indeed two of the Ru–N bonds to the tetradentate ligand, Ru–N3 and Ru–N4 in 1, and Ru–N2 and Ru–N4 in 3 and 4, are significantly elongated in the ³LF state as compared to the ground state. It is evident from Table S4 that the Ru–N3 and Ru–N4 bonds in 1 are elongated by 0.27 and 0.34 Å, respectively, whereas one of the Ru–N(CH₃CN) bonds lengthens by 0.05 Å and the other contracts by 0.01 Å.

Figure 9.

Schematic representation of the relative energies of the ³MLCT and ³LF potential energy surfaces in 1.

For comparison, the calculated bond changes in the ³MLCT/³ππ* state of 2 show a small contraction of three of the four Ru–N bonds to the 1-isocyTPQA ligand of <0.035 Å, while the Ru–N2 bond lengthens by only 0.015 Å. Therefore, significant structural changes are expected from the ground and ³MLCT states of 1 to the ³LF state, resulting in a high activation barrier to populate the ³LF state. The observation of the ³MLCT instead of the lowest energy ³LF states in the transient absorption of 1, explained by a high activation barrier to populate the ³LF state from the vibrationally cooled ³MLCT state, is consistent with the low quantum yield for ligand exchange in this complex and a lifetime that is similar to that of 2 and related complexes. Similar arguments can be made for complexes 3 and 4, which exhibit elongation of two of the RuN bonds to the tetradentate ligand of ≥0.3 Å.

CONCLUSIONS

A series of Ru(II) polypyridyl complexes containing two different tetradentate-based frameworks were synthesized and their effectiveness for use as photocages to deliver nitrile or pyridine ligands upon irradiation was explored. The quantum yield for the CH₃CN/H₂O ligand exchange of 2 was measured to be Φ₄₀₀ = 0.033(3), 5-fold greater than that of 1, Φ₄₀₀ = 0.0066(3), although this effect cannot be attributed to steric crowding in the former. DFT and related calculations show the presence of a highly mixed ³MLCT/³ππ* excited state as the lowest triplet state in 2, whereas the lowest energy triplet state in 1, 3, and 4 was calculated to be ³LF in nature. The mixed ³MLCT/³ππ* excited state places significant spin density on the quinoline moiety of the 1-isocyTPQA ligand that is trans to the photolabile N6 CH₃CN ligand in 2. Additionally, MBO analysis reveals that in the triplet state of 2, the Ru–N(quinoline) bond strength increases while the Ru–N-(CH₃CN) bond strength of the photolabile N6 CH₃CN ligand decreases compared to the singlet ground state, suggesting the presence of a trans-type influence in the excited state that leads to the higher value of Φ₄₀₀ measured in 2. The result that complexes with ³LF lowest energy excited states were less photolabile than that with ³MLCT/³ππ* lowest energy excited state is counter to the general understanding of Ru(II) photochemistry. Ultrafast spectroscopy was used to probe the excited states of 1–4, which reveal that the ³LF states are not populated in 1, 3, and 4, consistent with the low yields of ligand exchange product upon irradiation in these complexes. This unusual result is explained by the large elongation of two RuN bonds of the tetradentate ligands in these complexes, thus increasing the activation energy from the ³MLCT to the ³LF states in 1, 3, and 4.

Mixed ³MLCT/³ππ* excited states that promote ligand dissociation, such as that of 2, represent a new manner to effect photoinduced ligand exchange. In addition, ³MLCT deactivation through the lower-lying ³LF state was shown to be avoided through significant structural changes around the metal center in the latter. Both concepts can be used to design improved complexes for applications that require efficient ligand dissociation, such as drug delivery, as well as for those that require minimal deactivation of the ³MLCT state through low-lying metal-centered states. As such, these findings are expected to be useful to fields from photochemotherapy to solar energy conversion and sensing.