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

Three monoruthenium complexes 1(PF6)2–3(PF6)2 bearing an 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 photophysical 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 (Δλmax = 195 nm) for 1(PF6)2, 465 and 627 nm (Δλmax = 162 nm) for 2(PF6)2, and 455 and 608 nm (Δλmax = 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. Moreover, these experimental observations are analyzed by computational investigation.


Synthetic Details
All commercial reagents were used directly without further purification.NMR measurements ( 1 H and 13 C NMR) were performed on a Bruker Avance spectrometer in the designated solvents (CD3CN and CDCl3 for complexes and ligands, respectively).Mass spectrometry data were collected by a Thermo Exactive GC and a Bruker Autoflex III MALDI-TOF mass spectrometer.A Flash EA 1112 analyzer was applied to obtain elemental analysis data.

Synthetic Details
All commercial reagents were used directly without further purification.NMR measurements ( 1 H and 13 C NMR) were performed on a Bruker Avance spectrometer in the designated solvents (CD 3 CN and CDCl 3 for complexes and ligands, respectively).Mass spectrometry data were collected by a Thermo Exactive GC and a Bruker Autoflex III MALDI-TOF mass spectrometer.A Flash EA 1112 analyzer was applied to obtain elemental analysis data.

X-ray Crystallography
The X-ray diffraction measurements were performed on a Rigaku Saturn 724 diffractometer on a rotating anode (Mo Kα radiation, λ = 0.71073 Å).Olex2 software (SVN Revision No. 5506) was applied to analyze the data and generate the structure graphic.The crystallographic data of 3(PF 6 ) 2 (CCDC 2107266) were deposited in The Cambridge Crystallographic Data Centre for free.

Spectroscopic Measurements
The electronic absorption measurements were performed on a TU-1810DSPC spectrometer in chromatographic grade acetonitrile at room temperature.An F-380 spectrofluorometer was employed to collect the steady-state emission and excitation spectra.The samples of photophysical analysis were prepared by using the 1 cm pathlength quartz cuvettes.The quinine sulfate in 1.0 M aq H 2 SO 4 (Φ = 55%) and [Ru(bpy) 3 ](PF 6 ) 2 (Φ = 9.5%) were used to calculate the relative luminescence quantum yields in degassed CH 3 CN as the reference.The luminescence decays and temperature-dependent emission studies were carried out on the FLS920 and FLS1000 spectrophotometer, respectively.

Electrochemical Measurements
All electrochemical measurements were carried out on a CHI660D electrochemical station based on a three-electrode system.After bubbling with nitrogen for 5 min, the samples of electrochemical studies were tested at a scan rate of 100 mV/s in 0.1 M Bu 4 NClO 4 /CH 3 CN.The potential of a saturated Ag/AgCl electrode was used to calculate the relative potentials of these compounds as the reference.

DFT and TDDFT Calculations
The B3LYP exchange-correlation functional and the Gaussian 09 software package were employed for the density functional theory (DFT) calculations.The crystallographic data of 3(PF 6 ) 2 were employed to generate the input files.The Los Alamos effective core potential LANL2DZ basis set for Ru and 6-31G* for other atoms were used to optimize the electronic structures of these compounds.All of the calculations were carried out including the solvation effects and using the no symmetry constraints.The DFT-optimized structures were employed for the TDDFT calculations on the same level of theory.In order to make the optimized geometries be local minima, frequency calculations were carried out on the same level of theory.An isovalue of 0.02 e bohr −3 was set for all orbitals on calculations.

HPLC Analysis
All HPLC measurements were performed on a Shimadzu UFLC system.The analysis data were collected by a Shim-pack XR-ODS column (2.2 µm, 75 mm × 4.6 mm, i.d.).A gradient solvent of CH 3 CN in water was used to elute these samples (10−90% over 0-10 min, followed by an isocratic elution of 90% CH 3 CN 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(PF 6 ) 2 , 2(PF 6 ) 2 , and 3(PF 6 ) 2 , respectively.
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 and S2

Spectroscopic Studies
The absorption, excitation, and steady-state emission spectra and the 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 S13).These absorption bands in the 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 S13).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

Spectroscopic Studies
The absorption, excitation, and steady-state emission spectra and the emission decay studies of complexes 1(PF 6 ) 2 -3(PF 6 ) 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 S13).These absorption bands in the 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(CH 3 )-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 H 2 SO 4 (Figure S13).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 results indicate that the substituent effect will increase the non-emissive deactivation pathway of these ligands and thus impair the emission properties.Compared with L3 with electron-withdrawing -CF 3 groups, L1 with electron-donating -OMe groups shows better quantum yield and longer excited-state lifetime due to the inductive effect of an alkoxyl substituent [66,67].These experimental findings indicate that these emission bands of ligands L1-L3 are of singlet charge-transfer ( 1 CT) character.Complexes 1(PF6)2-3(PF6)2 show intense absorption bands in the UV region at 289, 288, and 284 nm, respectively (Figure 2a), which are assigned to π-π* transitions 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 timedependent density functional theory (TDDFT) calculations below, these higher-energy absorption bands are associated with the ligand-to-ligand charge-transfer (LLCT) transitions  a All spectral data were collected in a 1.0 cm quartz cell.ND = not determined.b The absorption spectra were measured in acetonitrile.c The excitation wavelength is 360 nm and 350 nm for ligands and complexes, respectively.d The data measured were well simulated by a biexponential decay, the average lifetime τ was determined by ). e Quantum yield was determined by using the quinine sulfate in 1.0 M aq H 2 SO 4 (55%) and [Ru(bpy) 3 ](PF 6 ) 2 (9.5%) as the reference for ligands and complexes, respectively.f All data were recorded at a scan rate of 100 mV/s in 0.1 M Bu 4 NClO 4 /CH 3 CN.The E 1/2 value was determined by Ag/AgCl as the reference.
Complexes 1(PF 6 ) 2 -3(PF 6 ) 2 show intense absorption bands in the UV region at 289, 288, and 284 nm, respectively (Figure 2a), which are assigned to π-π* transitions 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(PF 6 ) 2 -3(PF 6 ) 2 , respectively.As indicated by time-dependent density functional theory (TDDFT) calculations below, these higherenergy absorption bands are associated with the ligand-to-ligand charge-transfer (LLCT) transitions from the N(CH 3 )-bridged ligand to bpy and ICT of the N(CH 3 )-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(PF 6 ) 2 , with electron-withdrawing -CF 3 groups on the pyridine rings shows a 40 nm blue shift of the MLCT absorption band in comparison to 1(PF 6 ) 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(PF 6 ) 2 , 465 and 627 nm for 2(PF 6 ) 2 , and 455 and 608 nm for 3(PF 6 ) 2 , respectively.The wavelength difference between the dual emission maxima (∆λ max ) is 195, 162, and 153 nm for 1(PF 6 ) 2 -3(PF 6 ) 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 a slight red shift (Figures 2b,c and S14).The emission maxima of the higher-energy emission bands of these complexes have similar emission wavelength, while the lowerenergy emission band shows a 38 nm blue shift from 1(PF 6 ) 2 to 3(PF 6 ) 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(PF 6 ) 2 to 3(PF 6 ) 2 .The solvent effect on the dual-emissive properties of 2(PF 6 ) 2 was reported in our previous work [64].Similar results were observed for 1(PF 6 ) 2 and 3(PF 6 ) 2 .The relative intensities and emission maxima of these two emission bands are also strongly dependent on the solvent polarity.For instance, the lower-energy emission band of 1(PF 6 ) 2 became dominant in DMF, while the two emission bands exhibited comparable intensities in other solvents.For complex 3(PF 6 ) 2 , the higher-energy emission band became dominant in CH 3 CN, acetone, and CH 3 OH, while the lower-energy emission band displayed a much higher intensity in DMSO.In DMF, the two emission bands have comparable intensities (Figure S15).The emission spectra in the solid state of 1(PF 6 ) 2 and 3(PF 6 ) 2 exhibit similar emission maxima (600 and 645 nm for 1(PF 6 ) 2 and 3(PF 6 ) 2 , respectively) with respect to that of the lower-energy emission band in CH 3 CN (Figure S16).Compared with the lower-energy emission band in CH 3 CN, the emission spectrum of pure powder of 2(PF 6 ) 2 shows a 31 nm red shift most likely due to the aggregates formed by intermolecular interaction.In nitrogen saturated acetonitrile solution, complex 3(PF 6 ) 2 has a low quantum yield of 0.53%, relative to 9.5% of [Ru(bpy) 3 ](PF 6 ) 2 [68], while complex 1(PF 6 ) 2 shows a better quantum yield of 2.1%.Compared with the prototype complex 2(PF 6 ) 2 , complexes 1(PF 6 ) 2 and 3(PF 6 ) 2 possess lower emission quantum yields.We conjecture that the introduction of substituents may increase the flexibility of structure and molecular vibrations and enhance the rate constant of the nonradiative process.
In luminescence decay studies, all three complexes 1(PF 6 ) 2 -3(PF 6 ) 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(PF 6 ) 2 -3(PF 6 ) 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(PF 6 ) 2 -3(PF 6 ) 2 , respectively), while the lifetimes of the lower-energy emission bands are considerably elongated (193, 219, and 189 ns for 1(PF 6 ) 2 -3(PF 6 ) 2 , respectively).Compared with ligands, the substituent effect on excited-state lifetimes of these complexes is negligible.Moreover, the lifetimes of the representative complexes 1(PF 6 ) 2 and 2(PF 6 ) 2 were measured in glassy CH 3 CN at 77 K.The higher-energy emission bands still exhibit nanosecond range lifetimes (5.0 ns for both 1(PF 6 ) 2 and 2(PF 6 ) 2 ), and those of the lower-energy emission bands significantly increase to a microsecond range (0.53 and 2.5 µs for 1(PF 6 ) 2 and 2(PF 6 ) 2 , respectively).Based on these experimental observations, the higher-and the lower-energy emission bands are ascribed to the admixtures of the singlet (N(CH 3 )-bridged ligand to bpy 1 LLCT and N(CH 3 )-bridged ligand 1 ICT charge transfer) and the triplet 3 MLCT (ruthenium component to bpy charge transfer) character, respectively.A number of studies suggest that the dual fluorescence/phosphorescence character of a single complex was found in different luminescent TMCs such as platinum [18][19][20]27,29], osmium [21], copper [22], and iridium [24,30], as well as ruthenium complexes [28,32,34].Kasha's rule states that only the lowest energy excited state of a given spin multiplicity is emissive.However, the dual emissions coming from the excited states with the different spin manifold of a single complex are still obeying Kasha's rule.

Temperature-Dependent Emission Spectral Studies
To investigate the changes in dual-emissive properties by temperature stimuli, the temperature-dependent emission spectral studies of the representative complexes 1(PF 6 ) 2 and 2(PF 6 ) 2 have been measured in CH 3 CN solution and shown in Figure 3.Both of the 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(PF 6 ) 2 and 2(PF 6 ) 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 facilitate the intersystem crossing from 1 LLCT/ 1 ICT to the 3 MLCT state [69].The relative intensity of the two emission bands of 1(PF 6 ) 2 -3(PF 6 ) 2 vary as a function of the excitation wavelength (Figure S17).When excited at a shorter wavelength (300-360 nm), the higherenergy 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.
cent TMCs such as platinum [18][19][20]27,29], osmium [21], copper [22], and iridium [24,30], as well as ruthenium complexes [28,32,34].Kasha's rule states that only the lowest energy excited state of a given spin multiplicity is emissive.However, the dual emissions coming from the excited states with the different spin manifold of a single complex are still obeying Kasha's rule.

Temperature-Dependent Emission Spectral Studies
To investigate the changes in dual-emissive properties 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 of the 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 facilitate the intersystem crossing from 1 LLCT/ 1 ICT to the 3 MLCT state [69].The relative intensity of the two emission bands of 1(PF6)2-3(PF6)2 vary as a function of the excitation wavelength (Figure S17).When excited at a 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.

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 collected 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 peak 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 peaks, respectively).In contrast, 120 and 90 mV positive redox shifts are observed for the first and second oxidation peaks 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

Electrochemical Studies
Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were applied to study the electrochemical properties of complexes 1(PF 6 ) 2 -3(PF 6 ) 2 and ligands L1-L3.Figure 4 displays the CV and DPV profiles of 1(PF 6 ) 2 -3(PF 6 ) 2 , and their electrochemical data are collected in Table 1, together with ligands L1-L3.In the anodic scan, 1(PF 6 ) 2 -3(PF 6 ) 2 display an irreversible and a reversible oxidation peak at +1.12 and +1.26 V, +1.23 and +1.44 V, and +1.35 and +1.53 V, versus Ag/AgCl in CH 3 CN, respectively.The presence of the electron-donating groups -OMe in complex 1(PF 6 ) 2 make both oxidation processes more negative in comparison to 2(PF 6 ) 2 (110 and 180 mV shift for the first and second oxidation peaks, respectively).In contrast, 120 and 90 mV positive redox shifts are observed for the first and second oxidation peaks of 3(PF 6 ) 2 relative to that of 2(PF 6 ) 2 due to the introduction of electron-withdrawing -CF 3 groups.The first irreversible peaks are assigned to the N(CH 3 )-bridged ligand oxidation while the second reversible signals are ascribed to the Ru II/III process.In the cathodic scan, 1(PF 6 ) 2 -3(PF 6 ) 2 show two consecutive 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 CH 3 CN, respectively.These peaks are associated with the reductions of bpy ligands.These assignments are consistent with the electrochemical findings of these ligands (Figure S18).L1-L3 display the amine-based irreversible oxidation peaks at +0.84, +0.89, and +1.01 V, versus Ag/AgCl in CH 3 CN, respectively.No obvious reduction processes were found for these ligands.These assignments are also supported by the DFT calculation results discussed below.The difference between the first anodic and first cathodic electrochemical potential (∆E echem ) of these complexes was employed to calculate the energy gaps which were determined to be 2.48, 2.60, and 2.62 eV for 1(PF 6 ) 2 -3(PF 6 ) 2 , respectively.
These assignments are consistent with the electrochemical findings of these ligands (Figure S18).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 reduction processes were found for these ligands.These assignments are also supported by the DFT calculation results discussed below.The difference between the first anodic and first cathodic electrochemical potential (ΔEechem) of these complexes was employed to calculate the energy gaps which were determined to be 2.48, 2.60, and 2.62 eV for 1(PF6)2-3(PF6)2, respectively.

DFT and TDDFT Calculations
In order to further understand the electronic transitions of complexes 1(PF 6 ) 2 -3(PF 6 ) 2 , DFT calculations have been carried out on complexes 1 2+ , 2 2+ , and 3 2+ .The optimized geometries were generated from the single-crystal structure of 3(PF 6 ) 2 .Figure 5 displays the calculated energy diagram and isodensity plots of 1 2+ −3 2+ .The frontier energy gap of complex 3 2+ (3.43 eV) is slightly larger with respect to those of 1 2+ and 2 2+ (3.28 and 3.22 eV, respectively).This is mainly caused by the stabilization of the highest occupied molecular orbital (HOMO) level of 3 2+ .These calculation findings are in agreement with the above experimental results.In the same level of theory, TDDFT calculations were performed on the basis of the DFT-optimized structures of these complexes.These predicted electronic transitions are summarized in Table 2 and shown in Figure S21.The predicted S1 excitation of 1 2+ is associated with the HOMO → LUMO transition, which is associated with the low-energy absorption extending over 650 nm.The lower-energy emission band at 646 nm was assigned to the predicted S2 excitation (HOMO-1 → LUMO+1) which is attributed to the MLbpyCT transitions.The higher-energy S3, S4, S7, and S8 transitions have similar MLbpyCT characters.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 Figures S19 and S20 display the representative frontier orbital isodensity plots of 1 2+ and 3 2+ , respectively.The HOMOs of these complexes are dominated by the N(CH 3 )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(CH 3 )-bridged ligand.
In the same level of theory, TDDFT calculations were performed on the basis of the DFT-optimized structures of these complexes.These predicted electronic transitions are summarized in Table 2 and shown in Figure S21.The predicted S 1 excitation of 1 2+ is associated with the HOMO → LUMO transition, which is associated with the low-energy absorption extending over 650 nm.The lower-energy emission band at 646 nm was assigned to the predicted S 2 excitation (HOMO-1 → LUMO+1) which is attributed to the ML bpy CT transitions.The higher-energy S 3 , S 4 , S 7 , and S 8 transitions have similar ML bpy CT characters.The predicted S 9 , S 10 , S 11 , S 12 , S 13 , and S 19 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 the LLCT/ICT character of the excited state.A similar situation was observed for 3 2+ .The predicted S 1 excitation of 3 2+ has a very low oscillator strength (f ).The lower-energy emission band at 608 nm was assigned to the predicted S 2 excitation (HOMO-1 → LUMO+1) which is attributed to the ML bpy CT transitions.The predicted S 7 , S 9 , S 10 , S 13 , and S 16 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 responsible for the observed higher-energy emission band at 455 nm.

Conclusions
In summary, three monoruthenium complexes 1(PF 6 ) 2 -3(PF 6 ) 2 with dual fluorescence/phosphorescence are prepared and characterized.These complexes show wellseparated dual emissions that are ascribed to the 1 LLCT/ 1 ICT and 3 MLCT transitions, respectively.The substituent and solvent effects in the dual-emissive properties of these complexes have been examined.The energy gaps of two emissions can be tuned by in-troducing different substituents to the N(CH 3 )-bridged ligand, which are decreased by enhancing the electron-withdrawing capabilities of substituents.The relative intensities and emission maxima of two emissions are also strongly dependent on the solvent used.Future work will focus on the design and application of dual-emissive transition-metal complexes as ratiometric photoluminescent probes for the detection of proteins and nucleic acids.
in the Supporting Information summarize the crystallographic data.A single crystal of complex 3(PF 6 ) 2 was acquired by the slow diffusion of (C 2 H 5 )O into a solution of the complex in CH 3 CN.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(PF 6 ) 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 the pyrazine plane of L3 are 26.38 • and 127.73 • , respectively.
in the Supporting Information summarize the crystallographic data.A single crystal of complex 3(PF6)2 was acquired by the slow diffusion of (C2H5)O into a solution of the complex in CH3CN.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 the pyrazine plane of L3 are 26.38° and 127.73°, respectively.
Photon counts
All spectral data were collected in a 1.0 cm quartz cell.ND = not determined.b The absorption spectra were measured in acetonitrile.c The excitation wavelength is 360 nm and 350 nm for ligands and complexes, respectively.d The data measured were well simulated by a biexponential decay, the average lifetime τ was determined by τ = [A1(τ1) 2 + A2(τ2) 2 ]/(A1τ1 + A2τ2).e Quantum yield was determined by using the quinine sulfate in 1.0 M aq H2SO4 (55%) and [Ru(bpy)3](PF6)2 (9.5%) as the reference for ligands and complexes, respectively.f All data were recorded at a scan rate of 100 mV/s in 0.1 M Bu4NClO4/CH3CN.The E1/2 value was determined by Ag/AgCl as the reference. a

Table 1 .
Photophysical and electrochemical data a .