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Dual-Emissive Monoruthenium Complexes of N(CH3)-Bridged Ligand: Synthesis, Characterization, and Substituent Effect

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19 September 2023

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Abstract
Three monoruthenium complexes 1(PF6)23(PF6)2 bearing a N(CH3)-bridged ligand have been synthesized and characterized. These complexes have a general formula of [Ru(bpy)2(L)](PF6)2, where L is a 2,5-di(N-methyl-N-(pyrid-2-yl)amino)pyrazine (dapz) derivative with various substituents and bpy is 2,2′-bipyridine. The photophyscial and electrochemical properties of these compounds have been examined. The solid-state structure of complex 3(PF6)2 is studied by single crystal X-ray analysis. These complexes show two well-separated emission bands centered at 451 and 646 nm (max = 195 nm) for 1(PF6)2, 465 and 627 nm (max = 162 nm) for 2(PF6)2, and 455 and 608 nm (max = 153 nm) for 3(PF6)2 in dilute acetonitrile solution, respectively. The emission maxima of the higher-energy emission bands of these complexes are similar, while the lower-energy emission bands are dependent on the electronic nature of substituents. These complexes display two consecutive redox couples owing to the stepwise oxidation of the N(CH3)-bridged ligand and ruthenium component. In addition, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations are carried out to elucidate these experimental findings.
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Subject: Chemistry and Materials Science  -   Inorganic and Nuclear Chemistry

1. Introduction

Dual emissions of a single component luminescent material are not in consistence with the Kasha’s rule which states that only the lowest energy excited state of a given spin multiplicity is emissive [1,2]. Besides, the dual-emission phenomena of luminescent materials could be affected by unrecognized impurities, isomers, aggregation behavior or fluctuation of instrument. However, this kind of intriguing behavior has been found in different kinds of luminescent materials such as organic small molecules, polymers, quantum dots, hybrid lead halides, metal-organic frameworks (MOFs), and transition-metal complexes (TMCs). Related phenomenon can be divided into three categories including dual fluorescences [3,4,5,6,7,8,9,10], dual phosphorescences [11,12,13,14,15,16,17], and dual fluorescence/phosphorescence [18,19,20,21,22,23,24,25]. Compared to luminescent materials with a single emission band, dual-emissive materials have been used in ratiometric sensing [26,27,28,29,30,31,32,33,34,35,36], white emitting generation [37,38,39], near-infrared circularly polarized luminescence [40], and multicolor bioimaging [41,42].
Due to the presence of multiple intraligand and charge-transfer excited states, TMCs are good candidates for constructing dual-emissive materials [43,44]. The dual-emissive characteristic of TMCs could be achieved by tuning electronic structures of the ligand [45,46,47,48] or the synthesis of dinuclear bridged complexes with the same or different metal ions [49,50,51]. To date, a number of dual-emissive TMCs have been reported based on monometallic [52,53,54], dimetallic [55,56,57,58], and multimetallic complexes [59,60,61,62]. The dual-emissive multimetallic complexes often demand complicated synthetic procedures with low production yield. Moreover, the existence of intra/intermolecular interaction or energy transfer process might hinder the observation of dual emission properties [63]. In comparison, monometallic complexes possess simpler structures. The structural modification of monometallic complexes may provide a means to design and construct dual-emissive materials with higher production and quantum yields.
In our previous work, a fluorescence/phosphorescence dual-emissive mononuclear ruthenium complex [Ru(bpy)2(dapz)]2+ (22+, Scheme 1) was designed and synthesized, where dapz is 2,5-di(N-methyl-N′-(2-pyridyl)amino)pyrazine and bpy is 2,2′-bipyridine, respectively [64]. This complex contains an electron-rich bidentate ligand dapz with a large bite angle and it shows dual-emissions as a result of energy-separated excited states. The dual-emissive behavior of 2(PF6)2 could be tuned by solvents, oxygen, and metal ions. To extend our work and understand the effect of substituent on dual-emissive behavior, a series of dual-emissive mononuclear ruthenium complexes 1(PF6)2−3(PF6)2 have been designed and prepared by changing the electronic nature of the bidentate N(CH3)-bridged ligand (Scheme 1).

2. Experimental Section

2.1. Synthetic Details

All chemicals were obtained from commercial resources and used as received. A Bruker Avance spectrometer (400 MHz for 1H and 100 MHz for 13C nuclei, respectively) was used to collect NMR spectra in the designated solvents (CD3CN and CDCl3 for complexes and ligands, respectively). A Bruker Daltonics Inc. Apex II FT-ICR and a Autoflex III MALDI-TOF mass spectrometer were employed to record mass spectrometry data. Microanalysis results were performed on a Flash EA 1112 or Carlo Erba 1106 analyzer.

2.1.1. Synthesis of 2,5-di(N-methyl-N′-(4-methoxy-2-pyridyl)amino)pyrazine (L1)

A mixture of 2,5-dibromopyazine (238 mg, 1.0 mmol), N-methyl-4-methoxy-2-pyridinamine (304 mg, 2.2 mmol), tris(dibenzylideneacetone)dipalladium(0) (92 mg, 0.1 mmol), sodium tert-butoxide (384 mg, 4.0 mmol), and 1,1′-ferrocenediyl-bis(diphenylphosphine) (55 mg, 0.1 mmol) was dissolved in toluene (10 mL). The reaction mixture was refluxed at 130 °C for two days under N2 atmosphere in a sealed pressure tube. After cooling room temperature, the solvent was evaporated in vacuo and the crude product was purified by silica gel column chromatography (eluent: petroleum ether/acetic ether = 5/1) to yield 280 mg of L1 as a brown solid (80%). 1H NMR (400 MHz, CDCl3): δ = 3.56 (s, 6H), 3.81 (s, 6H), 6.45 (d, J = 4.0 Hz, 2H), 6.50 (s, 2H), 8.12 (d, J = 4.0 Hz, 2H), 8.44 (s, 2H). 13C NMR (100 MHz, CDCl3): δ = 36.3, 55.4, 97.1, 104.3, 136.4, 148.5, 149.3, 159.2, 167.4. EI-MS (m/z): 352 for [M]+. ESI-HRMS: calcd. for C18H20N6O2 352.1648. Found: 352.1645.

2.1.2. Synthesis of 2,5-di(N-methyl-N′-(4-(trifluoromethyl)-2-pyridyl)amino)pyrazine (L3)

A suspension of 2,5-dibromopyazine (95 mg, 0.4 mmol), N-methyl-4-(trifluoromethyl)-2-pyridinamine (176 mg, 1.0 mmol), tris(dibenzylideneacetone)dipalladium(0) (37 mg, 0.04 mmol), sodium tert-butoxide (153 mg, 1.6 mmol), and 1,1′-ferrocenediyl-bis(diphenylphosphine) (22 mg, 0.04 mmol) was dissolved in toluene (10 mL). The reaction mixture was refluxed at 130 °C for two days under N2 atmosphere in a sealed pressure tube. After cooling to room temperature, the solvent was evaporated in vacuo and the crude product was purified by silica gel column chromatography (eluent: petroleum ether/acetic ether = 5/1) to yield 130 mg of L3 as a yellow solid (76%). 1H NMR (400 MHz, CDCl3): δ = 3.65 (s, 6H), 7.03 (d, J = 4.0 Hz, 2H), 7.26 (s, 2H), 8.42 (d, J = 8.0 Hz, 2H), 8.54 (s, 2H). 13C NMR (100 MHz, CDCl3): δ = 36.4, 107.3, 111.8, 127.3 (q, J = 271 Hz), 136.8, 140.7 (q, J = 33 Hz), 148.6, 149.4, 157.7. EI-MS (m/z): 428 for [M]+. ESI-HRMS: calcd. for C18H14N6F6 428.1184. Found: 428.1190.

2.1.3. Synthesis of 1(PF6)2

A suspension of cis-[Ru(bpy)2Cl2] (51.9 mg, 0.1 mmol) and L1 (35.2 mg, 0.1 mmol) in HOCH2CH2OH (5 mL) was heated under microwave irradiation for 30 min. The resulting deep red solution was cooled and an excess of an aqueous solution of potassium hexafluorophosphate was added. The crude product was purified by flash column chromatography on silica gel (eluent: CH3CN/KNO3(aq) = 200/1) to give 55 mg 1(PF6)2 as an orange solid in 52% yield. 1H NMR (400 MHz, CD3CN): δ = 3.35 (s, 3H), 3.44 (s, 3H), 3.82 (s, 3H), 3.85 (s, 3H), 6.38 (s, 1H), 6.43 (dd, J = 8.0 and 4.0 Hz, 1H), 6.56 (dd, J = 4.0 and 2.0 Hz, 1H), 6.71 (d, J = 4.0 Hz, 1H), 7.15 (t, J = 8.0 Hz, 2H), 7.29 (t, J = 4.0 Hz, 1H), 7.56−7.64 (m, 6H), 7.87 (t, J = 8.0 Hz, 1H), 7.92 (t, J = 8.0 Hz, 1H), 8.10 (t, J = 8.0 Hz, 1H), 8.15 (t, J = 8.0 Hz, 1H), 8.29 (s, 1H), 8.33−8.36 (m, 3H), 8.40 (d, J = 8.0 Hz, 1H), 8.49 (d, J = 8.0 Hz, 1H), 8.58 (d, J = 4.0 Hz, 1H). 13C NMR (100 MHz, CD3CN): δ = 41.0, 57.2, 101.4, 106.6, 109.3, 124.9, 125.0, 125.1, 125.4, 127.4, 128.0, 128.3, 135.9, 137.1, 138.3, 138.5, 138.8, 138.9, 152.1, 152.6, 152.8, 153.5, 154.1, 158.0, 158.1, 158.2, 169.2. MALDI-MS: m/z = 911.3 for [M − PF6]+, 765.3 [M − 2PF6]2+, 609.1 [M − 2PF6 − bpy]2+. Anal. Calcd. For C38H36F12N10O2P2Ru⋅H2O: C, 42.50; H, 3.57; N, 13.04. Found: C, 42.17; H, 3.11; N, 12.96.

2.1.4. Synthesis of 3(PF6)2

A suspension of cis-[Ru(bpy)2Cl2] (51.9 mg, 0.1 mmol) and L3 (42.8 mg, 0.1 mmol) in HOCH2CH2OH (5 mL) was heated under microwave irradiation for 30 min. The resulting deep red solution was cooled and an excess of an aqueous solution of potassium hexafluorophosphate was added. The crude product was purified by flash column chromatography on silica gel (eluent: CH3CN/KNO3(aq) = 300/1) to give 80 mg 3(PF6)2 as an orange solid in 71% yield. 1H NMR (400 MHz, CD3CN): δ = 3.40 (s, 3H), 3.57 (s, 3H), 7.03 (d, J = 8.0 Hz, 1H), 7.13 (d, J = 4.0 Hz, 1H), 7.20−7.24 (m, 2H), 7.33 (t, J = 8.0 Hz, 1H), 7.55−7.67 (m, 6H), 7.80 (s, 1H), 7.89 (d, J = 4.0 Hz, 1H), 7.93 (t, J = 8.0 Hz, 1H), 7.98 (t, J = 8.0 Hz, 1H), 8.17 (t, J = 8.0 Hz, 1H), 8.21 (t, J = 8.0 Hz, 1H), 8.36−8.41 (m, 4H), 8.48 (d, J = 8.0 Hz, 1H), 8.52−8.55 (m, 2H). 13C NMR (100 MHz, CD3CN): δ = 36.1, 41.4, 108.9, 112.9, 113.7, 116.4, 122.0, 122.6, 124.7, 125.2, 125.5, 125.6, 127.9, 128.2, 128.3, 128.5, 136.2, 138.8, 138.9, 139.3, 139.4, 140.3 (q, J = 61.6 Hz), 148.1, 149.7, 149.9, 152.6, 152.7, 153.6, 153.7, 153.9, 157.6, 158.0, 158.2, 160.6. MALDI-MS: m/z = 987.3 [M − PF6]+, 841.3 [M − 2PF6]2+, 685.2 for [M − 2PF6 − bpy]2+. Anal. Calcd. For C38H30F18N10P2Ru: C, 40.33; H, 2.67; N, 12.38. Found: C, 40.23; H, 2.84; N, 12.17.

2.2. X-ray Crystallography

A Rigaku Saturn 724 diffractometer on a rotating anode (Mo Kα radiation, λ = 0.71073 Å) was employed to obtain the X-ray diffraction data which was analysed by SHELXS-97 and Olex2 software. Olex2 software was used to generate the structure graphic shown in Figure 1. CCDC 2107266 for 3(PF6)2 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.

2.3. Spectroscopic Measurements

A TU-1810DSPC spectrometer of Beijing Purkinje General Instrument Co. Ltd. was employed to collect the absorption data at room temperature in chromatographic grade acetonitrile. A F-380 spectrofluorimeter of Tianjin Gangdong Sci. & Tech. Development Co. Ltd was employed to collect the steady-state emission and excitation spectra. The quartz cuvettes of 1 cm path length was used to prepare the samples for absorption and emission measurements. The quinine sulfate in 1.0 M aq H2SO4 (Φ = 55%) and [Ru(bpy)3](PF6)2 (Φ = 9.5%) were used to calculate the relative luminescence quantum yields in degassed acetonitrile solution as the reference. The luminescence decays were measured using a nanosecond flash photolysis setup (Edinburgh FLS920 spectrometer), combined with a picosecond pulsed diode laser and analyzed by the online software of the FLS920 spectrophotometer. Temperature-dependent emission studies were carried out on the FLS1000 spectrophotometer.

2.4. Electrochemical Measurements

A three-electrode system (glassy-carbon as the working electrode, Ag/AgCl electrode as the pseudoreference electrode, and platinum wire as the counter electrode, respectively) based on a CHI660D electrochemical station was employed to collect the cyclic voltammetry (CV) and different pulsed voltammetry (DPV) data. All solutions for electrochemical studies were bubbled with N2 for 3 min before test. All potentials are determined by using a saturated Ag/AgCl electrode as the standard. All measurements were carried out with around 0.3 mM concentration of the corresponding compounds in 0.1 M Bu4NClO4/CH3CN at a scan rate of 100 mV/s.

2.5. DFT and TDDFT Calculations

The Gaussian 09 software package and the B3LYP exchange correlation functional were employed for the density functional theory (DFT) calculations. The input files were generated from the single-crystal X-ray data. The electronic structures were optimized with a general basis set with the Los Alamos effective core potential LANL2DZ basis set for Ru and 6-31G* for other atoms. The solvation effects in CH3CN were included for all calculations. No symmetry constraints were used for all of the calculations. Frequency calculations were performed with the same level of theory to ensure the optimized geometries to be local minima. All orbitals were computed at an isovalue of 0.02 e bohr−3. TDDFT calculations were performed on the DFT-optimized structures on the same level of theory.

2.6. HPLC Analysis

High performance liquid chromatography (HPLC) measurements were carried out on a Shimadzu UFLC system (two LC-20AD pumps, a SPD-M20A diode array detector, a CTO-20A oven, and a SIL-20A autosampler). The analysis data was collected by a Shim-pack XR-ODS column (2.2 μm, 75 mm × 4.6 mm, i.d.). A gradient solvent of CH3CN in water was used to elute these samples (10%−90% over 0−10 min, followed by isocratic elution of 90% CH3CN for 5 min). A 0.1% of trifluoroacetic acid was added in all solvents. The flow rate was set at 1.0 mL/min. The detection wavelengths were set at 450, 451, and 430 nm for 1(PF6)2, 2(PF6)2, and 3(PF6)2, respectively.

3. Results and Discussions

3.1. Studies on Preparation and Single Crystal X-ray Analysis

Three N(CH3)-bridged ligands L1−L3 and corresponding mononuclear ruthenium complexes 1(PF6)2−3(PF6)2 were synthesized as outlined in Scheme 1. The bidentate ligands L1−L3 were obtained through a Pd-catalyzed C-N coupling reaction of 2,5-dibromopyrazine with 2-(N-methylamino)pyridine derivatives in the range of 76%−98% yields [64,65]. The reaction of L1, L2, and L3 with 1 equiv. cis-[Ru(bpy)2Cl2] under microwave irradiation, followed by anion exchange using potassium hexafluorophosphate, provided complexes 1(PF6)2, 2(PF6)2, and 3(PF6)2 in 52%, 88%, and 71% yield, respectively. These new compounds were fully characterized by nuclear magnetic resonance (NMR) (Figures S1), mass spectrometry, and elemental analysis. Furthermore, high performance liquid chromatography (HPLC) analysis results indicate that these complexes have high purities (Figure S9).
The solid-state structure of complex 3(PF6)2 was determined by single-crystal X-ray analysis. Figure 1 shows the ORTEP diagram and Tables S1 in the Supporting Information summarize the crystallographic data. Single crystal of complex 3(PF6)2 was acquired by the slow diffusion of ethyl ether into a solution of the complex in acetonitrile. The coordination geometry of the ruthenium atom has a distorted octahedral with bpy and L3. The N−Ru−N bite angle of the bpy (78.86(11)° and 78.68(11)°) is smaller than that of ligand L3 (88.30(10)°). Similar findings have been recorded in our previously reported N∧N bidentate Ru(II) complexes [64,65]. The Ru−N bond lengths of complex 3(PF6)2 are in the range of 2.056(3)−2.099(3) Å. No distinct length difference is present among the Ru−N bonds associated with bpy and L3. The bidentate ligand bpy has a planar structure. However, ligand L3 shows a severely twisted structure. The torsion angles between the two pyridine planes and pyrazine plane of L3 are 26.38° and 127.73°, respectively.

3.2. Spectroscopic Studies

The absorption, excitation, and steady-state emission spectra, and emission decay studies of complexes 1(PF6)2−3(PF6)2 in acetonitrile are shown in Figure 2, and their photophysical data are summarized in Table 1. Ligands L1−L3 display two absorption bands at 310 and 360 nm, 310 and 361 nm, and 295 and 353 nm, respectively (Figure S10a). These absorption bands in UV and visible region are ascribed to the π−π* excitations and the intraligand charge transfer (ICT) transitions from the amine-unit to pyrazine ring of the N(CH3)-bridged ligand, respectively. Under nitrogen saturated conditions, ligands L1−L3 display structureless emission bands at 457, 445, and 450 nm with quantum yields of 24%, 43%, and 0.06%, respectively, relative to 55% of quinine sulfate in 1.0 M aq H2SO4 (Figure S10b). In the nitrogen saturated solutions, the excited-state lifetimes of these emission bands were determined to be 5.6, 11.5, and 0.4 ns for L1−L3 (Table 1), respectively. These experimental findings indicate that these emission bands of ligands L1−L3 are of singlet charge transfer (1CT) character.
Complexes 1(PF6)2−3(PF6)2 show intense absorption bands in UV region at 289, 288, and 284 nm, respectively (Figure 2a). These intense absorption bands are ascribed to the π−π* excitations of ligands. The intense and broad bands in the visible region are observed at 350 and 467 nm, 375 and 448 nm, and 368 and 427 nm for 1(PF6)2−3(PF6)2, respectively. As indicated by time dependent density functional theory (TDDFT) calculations below, these higher-energy absorption bands are associated with the ligand-to-ligand charge transfer (LLCT) transitions from the N(CH3)-bridged ligand to bpy and ICT of the N(CH3)-bridged ligand, while the lower-energy absorption bands are assigned to the metal-to-ligand charge transfer (MLCT) transitions from the ruthenium component to bpy ligand. By variation of the substituents, 3(PF6)2, with electron-withdrawing –CF3 groups on the pyridine rings, shows 40 nm blue shift of the MLCT absorption band in comparison to 1(PF6)2 containing electron-donating –OMe groups. All three complexes show intriguing dual-emissive behavior in dilute acetonitrile solution under irradiation, with two well-separated emission bands centered at 451 and 646 nm for 1(PF6)2, 465 and 627 nm for 2(PF6)2, and 455 and 608 nm for 3(PF6)2, respectively. The wavelength difference between the dual emission maxima (Δλmax) is 195, 162, and 153 nm for 1(PF6)2−3(PF6)2, respectively (Figure 2d). The excitation spectra of the higher-energy emission bands of these complexes are associated with the LLCT/ICT absorptions, while the longer-wavelength MLCT absorptions are basically in accordance with the excitation spectra of the lower-energy emission bands with slight red-shift (Figure 2b,c, and Figure S11). The emission maxima of the higher-energy emission bands of these complexes have similar emission wavelength, while the lower-energy emission band shows 38 nm blue shift from 1(PF6)2 to 3(PF6)2 with decreasing electron-donating capabilities of substituents. This is consistent with an ascending order of the energy of the MLCT absorption band from 1(PF6)2 to 3(PF6)2. In nitrogen saturated acetonitrile solution, complex 3(PF6)2 has a low quantum yield of 0.53%, relative to 9.5% of [Ru(bpy)3](PF6)2 [66], while complex 1(PF6)2 show a better quantum yield of 2.1%. Compared with the prototype complex 2(PF6)2, complexes 1(PF6)2 and 3(PF6)2 possess lower emission quantum yields. We conjecture that the introduction of substituents may increase the free rotations of single bonds and molecular vibrations and enhance the rate constant of the nonradiative process.
In time resolved emission decay studies, all three complexes 1(PF6)2−3(PF6)2 exhibit distinct excited-state lifetimes for two emission bands in acetonitrile solution (Figure 2e,f, and Table 1). In air-equilibrated solutions, the excited-state lifetimes of the higher-energy emission bands are 14, 10, and 10 ns, while the lower-energy emission bands have longer lifetimes of 89, 58, and 24 ns for 1(PF6)2−3(PF6)2, respectively. In nitrogen saturated solutions, the lifetimes of the higher-energy emission bands show little changes with respect to those in air-equilibrated solutions (16, 13, and 15 ns for 1(PF6)2−3(PF6)2, respectively), while the lifetimes of the lower-energy emission bands are considerably elongated (193, 219, and 189 ns for 1(PF6)2−3(PF6)2, respectively). Moreover, the lifetimes of the representative complexes 1(PF6)2 and 2(PF6)2 were measured in glassy CH3CN at 77 K. The higher-energy emission bands still exhibit nanosecond range lifetimes (5.0 ns for both 1(PF6)2 and 2(PF6)2) and those of the lower-energy emission bands significantly increase to microsecond range (0.53 and 2.5 μs for 1(PF6)2 and 2(PF6)2, respectively). Based on these experimental observations, the higher- and the lower-energy emission bands are ascribed to the admixtures of singlet (N(CH3)-bridged ligand to bpy 1LLCT and N(CH3)-bridged ligand 1ICT charge transfer) and the triplet 3MLCT (ruthenium component to bpy charge transfer) character, respectively.

3.3. Temperature-Dependent Emission Spectral Studies

To investigate the dual emissive properties changes by temperature stimuli, the temperature-dependent emission spectral studies of the representative complexes 1(PF6)2 and 2(PF6)2 have been measured in CH3CN solution and shown in Figure 3. Both two complexes display similar emission spectral changes. Upon decreasing the temperature from 345 to 250 K, the higher-energy emission intensities decrease significantly, while those of the lower-energy emissions gradually increase. During the spectral changes process, a well isoluminescence point was recorded at 576 and 566 nm for 1(PF6)2 and 2(PF6)2, respectively. The excited-state lifetimes of the higher-energy emission bands of these complexes still fall in the nanosecond range from 345 to 250 K, excluding a thermally activated process. This suggests that the Franck–Condon transitions of the higher-energy emission are slowed down upon decreasing the temperature, and thus facilitates the intersystem crossing from 1LLCT/1ICT to 3MLCT state [67]. The relative intensity of the two emission bands of 1(PF6)2−3(PF6)2 vary as a function of excitation wavelength (Figure S12). When excited at shorter wavelength (300−360 nm), the higher-energy emission band is much higher than the lower-energy emission band. When a longer excitation wavelength was applied (380−500 nm), the higher-energy emission band almost disappeared and the lower-energy emission band became dominant.

3.4. Electrochemical Studies

Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were applied to study the electrochemical properties of complexes 1(PF6)2−3(PF6)2 and ligands L1−L3. Figure 4 displays the CV and DPV profiles of 1(PF6)2−3(PF6)2 and their electrochemical data are summarized in Table 1, together with ligands L1−L3. In the anodic scan, 1(PF6)2−3(PF6)2 display an irreversible and a reversible oxidation peaks at +1.12 and +1.26 V, +1.23 and +1.44 V, and +1.35 and +1.53 V, versus Ag/AgCl in CH3CN, respectively. The presence of the electron-donating groups –OMe in complex 1(PF6)2 make both oxidation processes more negative in comparison to 2(PF6)2 (110 and 180 mV shift for the first and second oxidation processes, respectively). In contrast, 120 and 90 mV postive redox shifts are observed for the first and second oxidation processes of 3(PF6)2 relative to that of 2(PF6)2 due to the introduction of electron-withdrawing –CF3 groups. The first irreversible peaks are assigned to the N(CH3)-bridged ligand oxidation while the second reversible signals are ascribed to the RuII/III process. In the cathodic scan, 1(PF6)2−3(PF6)2 show two consective reversible reduction waves at −1.36 and −1.61 V, −1.37 and −1.62 V, and −1.27 and −1.48 V, versus Ag/AgCl in CH3CN, respectively. These peaks are associated with the reductions of bpy ligands. These assignments are consistent with the electrochemical results of the pristine ligands (Figure S13). L1−L3 display the amine-based irreversible oxidation peaks at +0.84, +0.89, and +1.01 V, versus Ag/AgCl in CH3CN, respectively. No obvious cathodic scan signals were observed for these ligands. These assignments are also supported by the DFT calculation results discussed below. The energy gaps calculated by the electrochemical potential difference between the first anodic and first cathodic wave (ΔEechem) of these complexes are 2.48, 2.60, and 2.62 eV for 1(PF6)2−3(PF6)2, respectively.

3.5. DFT and TD-DFT calculations

In order to gain insight into the electronic properties of complexes 1(PF6)2−3(PF6)2. DFT calculations were performed on complexes 12+, 22+, and 32+ on the B3LYP/LANL2DZ/6-31G*/CPCM level of theory. The single-crystal structure of complex 3(PF6)2 was used to generate the input file for geometrical optimizations. Figure 5 displays the calculated energy diagram and isodensity plots of 12+−32+. The frontier energy gap of complex 32+ (3.43 eV) is silghtly larger with respect to those of 12+ and 22+ (3.28 and 3.22 eV, respectively). This is mainly caused by the stabilization of the highest occupied molecular orbital (HOMO) level of 32+. These calculation findings are in agreement with the above experimental results.
The isodensity plots of some representative frontier orbitals of complexes 12+ and 32+ are displayed in Figures S14. The HOMOs of these complexes are dominated by the N(CH3)-bridged ligand, with minor contributions from the ruthenium component. The lower occupied orbitals (HOMO−1, HOMO−2, and HOMO−3) are dominated by the ruthenium ion. The lowest unoccupied molecular orbital (LUMO) and LUMO+1 of these complexes have a bpy character, while the LUMO+2 is dominated by the N(CH3)-bridged ligand.
On the basis of the above DFT-optimized structures of these complexes, TDDFT calculations were carried out at the same level of theory, the predicted excitations from UV to visible region are summarized in Table 2 and shown in Figure S16. The predicted S1 excitation of 12+ is associated with the HOMO → LUMO transition, which is associated with the low-energy absorption extending over 650 nm. The S2 excitation is associated with the HOMO−1 → LUMO+1 transition, which is responsible for the observed MLCT transitions and associated with the lower-energy emission at 646 nm. The higher-energy S3, S4, S7, and S8 transitions have similar MLbpyCT character. The predicted S9, S10, S11, S12, S13, and S19 excitations are mainly responsible for the observed absorption bands from 330 to 400 nm. TDDFT calculation results indicate that these excitations are associated with LLCT (amine unit of L1 to bpy), ICT (amine-unit to pyrazine ring of L1), and L1-targeted MLCT transitions. These states are perturbed by MLCT states [64], and they are associated with the higher-energy emission band, indicating the admixtures of LLCT/ICT character of the excited state. A similar situation was observed for 32+. The predicted S1 excitation of 32+ have very low oscillator strength (f). The predicted S2 excitation of complex 32+ has a MLbpyCT character, which is responsible for the observed MLCT transitions and is associated with the lower-energy emission at 608 nm. The predicted S7, S9, S10, S13, and S16 excitations are mainly responsible for the observed higher-energy charge transfer absorption bands from 350 to 400 nm. These excitations are attributed to LLCT (amine unit of L3 to bpy), ICT (amine-unit to pyrazine ring of L3), and L3-targeted MLCT transitions, and they are attributed to the observed higher-energy emission at 455 nm.

4. Conclusions

In summary, three monoruthenium complexes 1(PF6)2−3(PF6)2 with dual fluorescence/phosphorescence are prepared and characterized. These complexes show well-separated dual emissions that are ascribed to the 1LLCT/1ICT and 3MLCT transitions, respectively. The energy gaps of two emissions can be tuned by introducing different substituent to the N(CH3)-bridged ligand, which are decreased with enhancing electron-withdrawing capabilities of substituents. Future work will focus on the design and application of dual-emissive transition metal complexes as ratiometric photoluminescent probes.

Supplementary Materials

The following supporting information can be downloaded at: Preprints.org, Figures S1: 1H and 13C NMR spectra of L1, L3, 1(PF6)2, and 3(PF6)2; Figure S9: HPLC spectra of 1(PF6)2−3(PF6)2; Tables S1: Single-crystal X-ray data of complex 3(PF6)2; Figure S10: Absorption and emission spectra of ligand L1−L3; Figure S11: Absorption and excitation spectra of 1(PF6)2−3(PF6)2; Figure S12: Emission spectra changes of 1(PF6)2−3(PF6)2 at different excitation wavelength; Figure S13: CVs and DPVs of ligands L1−L3; Figures S14: DFT calculation results of 1(PF6)2 and 3(PF6)2; Figure S16: TD-DFT calculation results of 1(PF6)2−3(PF6)2.

Author Contributions

Conceptualisation, S.-H.W. and Y.-W.Z.; methodology, S.-H.W., Z.-L.G. and Y.-W.Z.; validation, Z.Z. and R.-H.Z.; formal analysis, Z.Z. and R.-H.Z.; investigation, S.-H.W., L.W. and Z.-L.G.; resources, Z.Z., R.Y. and R.-H.Z.; data curation, Z.Z., J.-Y.S. and R.-H.Z.; writing—original draft preparation, S.-H.W. and Y.-W.Z.; writing—review and editing, S.-H.W., L.W., Z.-L.G. and Y.-W.Z.; supervision, S.-H.W.; project administration, Y.-W.Z. and S.-H.W.; funding acquisition, S.-H.W., L.W. and Y.-W.Z. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

We acknowledge the financial support from the National Natural Science Foundation of China (22004041, 21925112, 21975264, and 21872154) and the Fujian Province Natural Science Foundation (2023J01129).

Conflicts of Interest

The authors declare no conflict of interest.

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Preprints 85552 sch001
Scheme 1. Scheme 1. Structures and the Synthetic Route of Complexes 1(PF6)2–3(PF6)2.
Scheme 1. Scheme 1. Structures and the Synthetic Route of Complexes 1(PF6)2–3(PF6)2.
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Figure 1. ORTEP diagram of the single-crystal X-ray structure of complex 3(PF6)2. The thermal ellipsoids are set at 50% probability. Hydrogen atoms, solvent molecules, and anions omitted for clarity.
Figure 1. ORTEP diagram of the single-crystal X-ray structure of complex 3(PF6)2. The thermal ellipsoids are set at 50% probability. Hydrogen atoms, solvent molecules, and anions omitted for clarity.
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Figure 2. (a) UV/vis absorption spectra of complexes 1(PF6)2−3(PF6)2 in acetonitrile at a concentration of 5 × 10−5 M. (b-c) Excitation spectra of complexes 1(PF6)2−3(PF6)2 at higher- and lower-energy emission bands. (d) Emission spectra of complexes 1(PF6)2−3(PF6)2 in acetonitrile on excitation at 350 nm. (e-f) Emission decay profiles of complexes 1(PF6)2−3(PF6)2 at higher- and lower-energy emission bands under N2-saturated condition.
Figure 2. (a) UV/vis absorption spectra of complexes 1(PF6)2−3(PF6)2 in acetonitrile at a concentration of 5 × 10−5 M. (b-c) Excitation spectra of complexes 1(PF6)2−3(PF6)2 at higher- and lower-energy emission bands. (d) Emission spectra of complexes 1(PF6)2−3(PF6)2 in acetonitrile on excitation at 350 nm. (e-f) Emission decay profiles of complexes 1(PF6)2−3(PF6)2 at higher- and lower-energy emission bands under N2-saturated condition.
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Figure 3. Temperature-dependent emission spectra changes of complexes 1(PF6)2 (a) and 2(PF6)2 (b) in acetonitrile on excitation at 350 nm.
Figure 3. Temperature-dependent emission spectra changes of complexes 1(PF6)2 (a) and 2(PF6)2 (b) in acetonitrile on excitation at 350 nm.
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Figure 4. CVs and DPVs of complexes 1(PF6)2−3(PF6)2 in acetonitrile containing 0.1 M Bu4NClO4 at a scan rate of 100 mV/s. The working electrode is a glassy carbon, the counter electrode is a Pt wire, and the reference electrode is Ag/AgCl in saturated aq. NaCl.
Figure 4. CVs and DPVs of complexes 1(PF6)2−3(PF6)2 in acetonitrile containing 0.1 M Bu4NClO4 at a scan rate of 100 mV/s. The working electrode is a glassy carbon, the counter electrode is a Pt wire, and the reference electrode is Ag/AgCl in saturated aq. NaCl.
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Figure 5. Isodensity plots and energies of HOMO and LUMO of 12+, 22+, and 32+ (isovalue = 0.02 e bohr−3).
Figure 5. Isodensity plots and energies of HOMO and LUMO of 12+, 22+, and 32+ (isovalue = 0.02 e bohr−3).
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Table 1. Photophysical and Electrochemical Dataa.
Table 1. Photophysical and Electrochemical Dataa.
Compound λmax,abs [nm] (ε [105 M−1cm−1])b λmax,emi
[nm]c 		τ [ns]d
(air)		 τ [ns]
(N2)		 τ [ns]
(77K)		 Φ (N2)e E1/2,anodic [V]f E1/2,cathodic [V]
L1 310 (0.16), 360 (0.07) 457 ND 5.6 ND 24% +0.84 ND
L2 310 (0.13), 361 (0.07) 445 ND 11.5 ND 43% +0.89 ND
L3 295 (0.08), 329 (0.07), 353 (0.07) 450 ND 0.4 ND 0.06% +1.01 ND
1(PF6)2 289 (1.10), 350 (0.16), 467 (0.12) 451/646 14/89 16/193 5.0/528 2.1% +1.12, +1.26 −1.36, −1.61
2(PF6)2 288 (0.83), 375 (0.14), 448 (0.13) 465/627 10/58 13/219 5.0/2500 6.7% +1.23, +1.44 −1.37, −1.62
3(PF6)2 284 (1.09), 368 (0.21), 427 (0.15) 455/608 10/24 15/189 ND 0.53% +1.35, +1.53 −1.27, −1.48
aAll spectral data were collected in 1.0 cm quartz cell. ND = not determined. bThe absorption spectra were measured in acetonitrile. cThe excitation wavelength is 360 nm and 350 nm for ligands and complexes, respectively. dThe data measured was well simulated by a biexponential decay, the average lifetime τ was calculated by τ = [A1(τ1)2 + A2(τ2)2]/(A1τ1 + A2τ2). eQuantum yield was determined by comparing with that of quinine sulfate in 1.0 M aq H2SO4 (for ligands) or [Ru(bpy)3](PF6)2 (for complexes), which has a quantum yield of 55% and 9.5%, respectively. fAll data were recorded in CH3CN containing 0.1 M tetrabutylammonium perchlorate as the supporting electrolyte. Unless otherwise noted, the potential is determined as the E1/2 value vs Ag/AgCl.
Table 2. TDDFT Resultsa.
Table 2. TDDFT Resultsa.
Comp. Sn E [ev] λ [nm] f Dominant transition(s) (% contributionb) Assignmentc
12+ 1 2.65 468 0.0111 HOMO → LUMO (61) Ldapz-OMeLbpyCT
2 2.72 455 0.0190 HOMO−1 → LUMO+1 (41) MLbpyCT
3 2.73 454 0.0144 HOMO−2 → LUMO (36) MLbpyCT
4 2.81 442 0.0378 HOMO−2 → LUMO+1 (36) MLbpyCT
7 3.04 409 0.0678 HOMO−3 → LUMO (34) MLbpyCT
8 3.11 399 0.0702 HOMO−3 → LUMO+1 (28), HOMO−3 → LUMO (17) MLbpyCT
9 3.19 389 0.0319 HOMO → LUMO+2 (75) ICT
10 3.33 373 0.0150 HOMO−2 → LUMO+2 (59), HOMO−1 → LUMO+2 (22) MLdapz-OMeCT/ICT
11 3.45 359 0.0596 HOMO−1 → LUMO+2 (39) MLdapz-OMeCT/ICT
12 3.53 351 0.0696 HOMO−3 → LUMO+2 (69), HOMO−1 → LUMO+2 (16) MLdapz-OMeCT/ICT
13 3.56 349 0.0102 HOMO → LUMO+3 (18) Ldapz-OMeLbpyCT
19 3.81 325 0.0183 HOMO → LUMO+5 (52) Ldapz-OMeLbpyCT
32+ 2 2.81 441 0.0107 HOMO−1 → LUMO+1 (35) MLbpyCT
3 2.83 438 0.0181 HOMO−1 → LUMO (23), HOMO−1 → LUMO+1 (27) MLbpyCT
4 2.93 423 0.0467 HOMO → LUMO+1 (40) Ldapz-CF3LbpyCT
7 3.13 396 0.0930 HOMO−3 → LUMO (50) MLbpyCT
9 3.17 391 0.0555 HOMO−2 → LUMO+1 (37) MLbpyCT
10 3.27 379 0.0406 HOMO−1 → LUMO+3 (26), HOMO → LUMO+3 (33) MLdapz-CF3CT/ICT
13 3.42 363 0.0945 HOMO−2 → LUMO+2 (67), HOMO → LUMO+2 (14) MLdapz-CF3CT/ICT
14 3.52 352 0.1135 HOMO−3 → LUMO+2 (38), HOMO−1 → LUMO+2 (13) MLdapz-CF3CT/ICT
16 3.58 347 0.0388 HOMO−1 → LUMO+12 (21) Ldapz-CF3LbpyCT
20 3.77 329 0.0310 HOMO−1 → LUMO+4 (22), HOMO → LUMO+4 (12) MLdapz-CF3CT/Ldapz-CF3LbpyCT
aCalculated on the level of theory of B3LYP/LANL2DZ/CPCM (solvent = CH3CN). bThe actual percent contribution = (configuration coefficient)2×2×100%. cL1 = Ldapz-OMe,  L3 = Ldapz-CF3.
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