Remarkably Intense Emission from Ruthenium(II) Complexes with

Oct 22, 2015 - Remarkably Intense Emission from Ruthenium(II) Complexes with Multiple Borane Centers. Atsushi Nakagawa†, Eri Sakuda†‡§, Akitaka...
1 downloads 17 Views 1MB Size
Download PDF
Article pubs.acs.org/IC

Remarkably Intense Emission from Ruthenium(II) Complexes with Multiple Borane Centers Atsushi Nakagawa,† Eri Sakuda,†,‡,§ Akitaka Ito,∥ and Noboru Kitamura*,†,‡ †

Department of Chemical Sciences and Engineering, Graduate School of Chemical Sciences and Engineering, and ‡Department of Chemistry, Faculty of Science, Hokkaido University, Kita-10, Nishi-8, Kita-ku, Sapporo 060-0810, Japan § Division of Chemistry and Materials Science, Graduate School of Engineering, Nagasaki University, 1-14, Bunkyo-machi, Nagasaki 852-8521, Japan ∥ Department of Chemistry, Graduate School of Science, Osaka City University, 3-3-138, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan S Supporting Information *

ABSTRACT: The electrochemical, spectroscopic, and phophysical properties of a series of Ru(II) complexes having a triarylborane-appended 2,2′-bipyridine (bpy) ligand(s) (RuBbpys: [Ru(Bbpy) n (bpy) 3−n ] 2+ (B1n) and [Ru(B2bpy)n(bpy)3−n]2+ (B2n), B = (dimesityl)boryldurylethynyl group(s) at the 4- or 4,4′-position(s) in bpy, n = 1−3) are described. In the excited states of the complexes, the intramolecular charge transfer transitions between the πorbital of the aryl group and the vacant p-orbital on the boron atom (π(aryl)−p(B) CT) synergistically interact with the metal-to-ligand charge transfer (MLCT) transitions. The molar absorption coefficient of the MLCT band (ε(MLCT)) of the complex increased with increasing n, and B23 showed extremely intense absorption with ε(MLCT) = 5.6 × 104 M−1 cm−1 at 488 nm. Furthermore, B23 showed the highest emission quantum yield (0.43) among those of the polypyridine Ru(II) complexes hitherto reported. As one of the interesting results, we report that the radiative rate constant of B2n shows the correlation with ε(MLCT). The effects of the synergistic MLCT/ π(aryl)−p(B) CT interactions on the spectroscopic and photophysical characteristics of RuBbpys are discussed in detail.

■

INTRODUCTION Polypyridyl Ru(II) complexes (RuL32+), represented by [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine), have been widely utilized in a variety of photochemical applications in sensing, electroluminescence, artificial photosynthesis, and so forth.1 While they show obvious phosphorescence from the triplet metal-toligand charge transfer (3MLCT) excited states even at room temperature, their emission quantum yields (Φem) are not necessarily high enough among those of other phosphorescent transition metal complexes. In deaerated CH2CN at 298 K, as an example, the Φem value of [Ru(bpy)3]2+ is 0.095,2 whereas that of [Ir(ppy)3] (ppyH = 2-phenylpyridine) or [Pt(L)(PPh3)]+ (HL = 2-phenyl-6-(1H-pyrazol-3-yl)pyridine, PPh3 = triphenylphosphine) is 0.923 or 0.52,4 respectively. Therefore, the development of RuL32+ showing intense emission is of primary importance. We have focused on triarylboranes as a choice for synthetic modulation of the spectroscopic and photophysical properties of transition metal complexes.3,5 Owing to the presence of the vacant p-orbital on the boron atom (p(B)) in a triarylborane, a class of transition metal complexes having a triarylborane group(s) in the periphery of the ligand(s) shows fascinating excited-state properties, characterized by the synergistic © XXXX American Chemical Society

interactions between MLCT in a transition metal complex and the intramolecular CT interaction between the π-orbital of the aryl group (π(aryl)) and p(B) (π(aryl)−p(B) CT)6 in the triarylborane group. As a typical example, we reported that the [Ru(phen)3]2+ (phen = 1,10-phenanthroline) derivative having a (dimesityl)boryldurylethynyl (DBDE) group at the 4- or 5position of one of the three phen ligands showed intense MLCT absorption/emission and a long-lived excited state compared with [Ru(phen)3]2+: 4BRu2+ ([Ru(4-DBDE-phen)(phen)2]2+), ε473 (molar absorption coefficient at 473 nm) = 2.6 × 104 M−1 cm−1 (M = mol/dm3), Φem = 0.11, and τem (emission lifetime) = 12 μs; 5BRu2+ ([Ru(5-DBDE-phen) (phen)2]2+), ε448 = 1.7 × 104 M−1 cm−1, Φem = 0.11, and τem = 1.2 μs; [Ru(phen)3]2+, ε445 = 1.7 × 104 M−1 cm−1, Φem = 0.045, and τem = 0.42 μs in CH3CN at 298 K.5b While it has been known that the 3MLCT excited state of RuL32+ is likely to undergo thermal activation to the nonemissive dd excited triplet state (3dd*) followed by fast deactivation to the electronically ground state,7 such thermal processes do not contribute to 3MLCT excited-state decay in 4BRu2+ and Received: July 20, 2015

A

DOI: 10.1021/acs.inorgchem.5b01626 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Chart 1. Chemical Structures of RuBbpys (B2n and B1n)

5BRu2+, and, thus, these two complexes show quite small nonradiative decay rate constants: knr. Such spectroscopic and photophysical properties of 4BRu2+ and 5BRu2+ are best characterized by the synergistic MLCT/π(aryl)−p(B) CT excited states as described above, and a variety of transition metal complexes (Pt(II),8 Ir(III),9 Ru(II),10 Cu(I),8b and so on11) bearing an arylborane units(s) on the ligand(s) have been hitherto reported aiming at sensing fluoride and cyanide ions, tuning emission colors, and so forth. Since a triarylborane unit is one of the fascinating choices for chemical decoration for transition metal complexes, further understanding of the arylborane effects on the spectroscopic and photophysical properties of the complexes are of primary importance toward the molecular design for future photofunctional materials. In this study, we synthesized a series of polypyridyl Ru(II) complexes having multiple arylborane units (RuBbpys; [Ru(B 2 bpy) n (bpy) 3−n ] 2+ (B2n) and [Ru(Bbpy)n(bpy)3−n]2+ (B1n), n = 1−3, B2bpy = 4,4′-bis(DBDE)-bpy, Bbpy = 4-DBDE-bpy; see Chart 1) and evaluated in detail the arylborane effects on the electrochemical, spectroscopic, and photophysical properties of the complexes. It is worth emphasizing that, although a variety of the transition metal complexes bearing a triarylborane-appended ligand have been reported as mentioned above,8−11 the complex having multiple arylborane-substituted polypyridyl ligands (i.e., multiborane centers in multiple ligands in a complex) has never been reported. In the case of a triarylborane, it has been reported that an introduction of multiborane centers to a molecule brings about very interesting effects on the spectroscopic and photophysical properties of the compound through πconjugation via the boron centers.6c,g Therefore, we expect that an introduction of multiple arylborane centers in a ligand and/or polypyridine Ru(II) complex would give rise to interesting effects on the spectroscopic and photophysical properties of such a type of complex. In this study, we demonstrate the unique photophysical properties of B1n and B2n.

■

Electrochemical and Spectroscopic Measurements. CicaReagent N,N-dimethylformamide (DMF) for nonaqueous titrimetry (Kanto Chemical Co., Inc.) was used for electrochemical measurements. Spectroscopic-grade (Wako Pure Chemical Ind., Ltd.) or Luminasol (Dojindo Molecular Technologies, Inc.) CH3CN was used for absorption or emission measurements, respectively, without further purification. Propylene carbonate for temperature (T)-controlled measurements was distilled prior to use.12 Beside T-controlled experiments, all of the measurements were conducted at 25 ± 2 °C. Cyclic (CV) and differential pulse voltammetries (DPV) in DMF were conducted by using a BAS ALS-701D electrochemical analyzer with a three-electrode system using glassy carbon working, Pt auxiliary, and Hg/Hg2Cl2 (saturated calomel electrode (SCE)) reference electrodes. Tetra-n-butylammonium hexafluorophosphate (TBAPF6, 0.1 M), purified by recrystallizations from ethanol three times, was used as a supporting electrolyte. The concentration of a complex was set at ∼0.3 mM. The sample solutions were deaerated by purging an Ar-gas stream over 20 min prior to measurements. The potential sweep rate was 100 mV/s in CV, and DPV was conducted with 50 mV height pulses (0.06 s duration), being stepped by 4 mV intervals (2.0 s interval between the two pulses). The absorption spectra of the complexes were measured by using a Hitachi U-3900H spectrophotometer. The corrected emission spectra of the complexes were measured by using a Hamamatsu PMA-11 multichannel photodetector (excitation wavelength = 355 nm). The absolute emission quantum yields (Φem) of the complexes were measured by a Hamamatsu C9920-02 system equipped with an integrating sphere and a PMA-12 red-sensitive multichannel photodetector (excitation wavelength = 400 nm).2,13 Emission decay profiles were measured by using a Hamamatsu C4334 streak camera as a photodetector by exciting at 355 nm using a nanosecond Q-switched Nd:YAG laser (LOTIS TII, Ltd. LS-2137, fwhm ≈ 16−18 ns, repetition rate = 10 Hz). A liquid N2 cryostat (Oxford Instruments OptistatDN2 optical Dewar and MercuryiTC temperature controller) was used for T-controlled measurements. For emission spectroscopy, the absorbance of a sample solution was set 400 nm. The spectral comparison between B21, B11, and [Ru(bpy)3]2+ is shown in Figure 1b as an inset. The complex having Bbpy or B2bpy showed a low-energy and intense 1MLCT absorption compared to [Ru(bpy)3]2+: see Table 2. The low-energy absorption of B21 and B11 can be explained by the stabilization of the π*-orbital energy of the bpy-DBDE ligand as discussed later in detail. The MLCT absorption maximum energy of RuL32+ (Eabs) in general correlates proportionally to the difference in the redox potentials (Eox − Ered1) as a measure of the energy difference between the highest-energy occupied molecular orbital C

DOI: 10.1021/acs.inorgchem.5b01626 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (HOMO) and the lowest-energy unoccupied MO (LUMO).19 As shown in Figure 2a, in practice, we found a linear

375 nm) were shifted to lower energy compared to those of B1n (λabs = 363−364 nm). We suppose such triarylborane effects (i.e., π(aryl)−p(B) CT effects) on the electronic structures would reflect the MLCT absorption transition probabilities of RuBbpys. Theoretical Calculations. To reveal more details about the synergistic MLCT/π(aryl)−p(B) CT interactions, we conducted DFT calculations for B21, B11, and [Ru(bpy)3]2+; the MO contours in the frontier molecular orbitals (i.e., HOMO and LUMO) of the complexes are shown in Figure 3. The

Figure 2. Relationship between the MLCT absorption (Eabs, (a)) or emission maximum energies (Eem, (b)) of the Ru(II) complexes and the relevant (Eox − Ered1) values. Green line represents the linear regression line for the data.

Figure 3. Frontier molecular orbitals (contour = 0.02 eÅ−3) and orbital energies of B21, B11, and [Ru(bpy)3]2+.

population percentages of B11 and B21 in the HOMO and LUMO levels are also summarized in Table 3. On the basis of the calculations, the HOMO of B21 or B11 is evaluated to be destabilized in energy compared to that of [Ru(bpy)3]2+ and best characterized by the electron densities on the d-orbital of the Ru(II) center and the π-orbital of the durylethynyl-bpy moiety. On the other hand, the MOs of the complexes in the LUMO levels distribute to the π*-orbitals of the bpy rings in the arylborane-substituted ligands and the p-orbitals on the boron atoms: see Table 3. These results strongly support the assignment of the lowest-energy absorption bands of RuBbpys to the synergistic MLCT/π(aryl)−p(B) CT transitions. Furthermore, it is worth noting that the LUMO of B21 distributes to both boron atoms, with a larger extent of MO at the boron atom than that of B11. Such a contribution of the π(aryl)−p(B) CT to the LUMO enhances the transition dipole moments of the MLCT absorption and, thus, is consistent with the experimental observation that the ε values of the MLCT/ π(aryl)−p(B) CT bands for both B2n and B1n increase with an increase in n as discussed in the previous section. Emission Properties. All of the complexes showed structureless emission in CH3CN at 298 K as shown in Figure 4, and the emission decay profiles of the complexes were fitted satisfactorily by single-exponential functions: see Figure S2 in the Supporting Information. The emission maximum wavelengths (λem), quantum yields (Φem), and lifetimes of the complexes (τem) are summarized in Table 4. It is worth emphasizing that the Φem values of RuBbpys are quite high (Φem > 0.20) and increase with an increase in n for both B1n and B2n. In particular, B23 shows extremely intense emission with Φem = 0.43. To the best of our knowledge, this is the

relationship between Eabs and (Eox − Ered1) for a series of complexes with the slope value being ∼1.1. Since the effects of the DBDE group(s) on Ered1 are much larger than those on Eox as described before, the lower-energy MLCT absorption of RuBbpys compared to that of [Ru(bpy)3]2+ is responsible for the stabilization of the π*(bpy)-orbital energy through the π(aryl)−p(B) CT interaction. Another important aspect of the present study is the increase in the ε value of the MLCT band (ε(MLCT)) of the complex with an increase in n as seen in Table 2 and Figure 1. Although the increase in the ε value of the LC band at 300−400 nm with n is a reasonable consequence, that in ε(MLCT) is very extraordinary, indicating that the presence of the DBDE group(s) in the bpy ligand(s) gives rise to the enhancement of the MLCT transition dipole moment. Since results similar to those of RuBbpys have also been confirmed for 4BRu2+ and 5BRu2+,5b the enhancement of the ε(MLCT) values of RuBbpys compared to that of [Ru(bpy)3]2+ will be due to the synergistic MLCT/π(aryl)−p(B) CT interactions. It is worth emphasizing that the ε(MLCT) value of B2n ((2.4−5.6) × 104 M−1 cm−1) for a given n is larger than the relevant value of B1n ((2.1−4.2) × 104 M−1 cm−1): see Table 2. In particular, B23 showed an extremely large ε value (ε488 = 5.6 × 104 M−1 cm−1). It has been reported that the presence of multiple boron centers in a triarylborane brings about electron conjugation via p(B) in the LUMO,6c and this gives rise to a lower energy spectral band shift and enhancement of the ε value of the π(aryl)−p(B) CT absorption band of the derivative. In practice, the π(aryl)−p(B) CT absorption bands of B2n (λabs = 371− D

DOI: 10.1021/acs.inorgchem.5b01626 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 3. MO Populations of the Frontier Molecular Orbitals in B21, B11, and [Ru(bpy)3]2+ B2bpy or Bbpy complex B21 B11 [Ru(bpy)3]2+

MO

Ru

bpy

ethynyl

duryl

boron

mesityl

bpy

LUMO HOMO LUMO HOMO LUMO HOMO

5.19 44.64 4.29 41.77 1.74 83.65

70.47 10.45 72.02 10.59

7.91 10.96 5.80 11.74

8.03 23.57 6.09 25.39

1.00 0.41 0.84 0.44

1.44 4.38 1.17 4.67

5.96 5.59 9.79 5.40 98.26 16.35

in the molecular size of the complex, this will lead to a decrease in the solvation energy around the 3MLCT excited state, and, thus, the unfavorable solvation structures in the sequence n = 1 < n = 2 < n = 3 for B1n or B2n may render destabilization of the 3MLCT excited-state energy: a shorter wavelength shift of the spectrum. The solvation structure changes around the excited state will reflect the full-width at half-maximum (fwhm) of the emission spectrum. Nevertheless, the fwhm value of the emission spectrum in CH3CN at 298 K was almost constant at 2670−2680 or 2390−2420 cm−1 for B1n or B2n, respectively. Franck−Condon analysis of the emission spectrum21 of the complex, reported in Figure S3 and Table S1 in the Supporting Information, also demonstrated that the outer-sphere reorganization energy around the excited complex (λo) was almost constant at 1270−1280 or 1040−1090 cm−1 for B1n or B2n, respectively. Therefore, we suppose that the solvation structure changes in the excited complex by n will not explain the present results in Figure 2b. Another possibility is the change in the excited singlet (S1)− triplet (T1) splitting energy (ΔEST) of B1n or B2n with n. In practice, free energy content of the excited triplet state (ΔGES, corresponds to the excited-state energy before stabilization by solvation) evaluated by the Franck−Condon spectral fitting of the emission spectrum depends on n, and ΔGES increases with n as seen in Table S1 in the Supporting Information: 16 870 (B11) < 16 960 (B12) < 17 010 cm−1 (B13) and 16 260 (B21) < 16 380 (B22) < 16 480 cm−1 (B23). Furthermore, the (Eabs − Eem) value of the complex as a measure of ΔEST decreases with an increase in n: ΔEST = 6240 (B11) > 5750 (B12) > 5610 cm−1 (B13) and 5620 (B21) > 5080 (B22) ≈ 5130 cm−1 (B23). While the HOMO level of B11 or B21 is characterized by the electron densities on the Ru(II) center and the durylethynyl-bpy moiety, the LUMOs of the complexes extend partly to the boron atom(s) as described before (Figure 3 and Table 3). Although we have not conducted DFT calculations on other B1n and B2n complexes owing to the large molecular sizes, the extension of the excited electron to the boron atom(s) in the LUMO will also be true for B1n and B2n with n = 2 and 3. The participation of p(B) in the LUMO will give rise to the decrease in the electron exchange integral in the excited state of the complex and, therefore, to the small ΔEST values of B1n and B2n. We suppose that the variation of ΔEST with n in B1n and B2n will be the primary reason for the results in Figure 2b. It is worth noting, furthermore, that the variation of ΔEST with n in B1n and B2n explains very well the Φem and kr values of the complexes as described in the following section. Radiative Rate Constant. Table 4 also includes the kr and knr values of the complexes, evaluated by the relations τem = 1/ (kr + knr) and Φem = kr/(kr + knr). The kr values of B2n ((1.6− 2.5) × 105 s−1) and B1n ((0.87−1.0) × 105 s−1) increased with an increase in n. For a series of B2n and B1n, it is important to emphasize that the ε(MLCT) value of the complex increases

Figure 4. Emission spectra of RuBbpys and [Ru(bpy)3]2+ in CH3CN at 298 K. Spectral integrations in a wavenumber scale correspond to the relative values of the emission quantum yields of the complexes. Inset: The spectra whose intensities are normalized at the maximum wavelengths.

Table 4. Emission Properties of RuBbpys in CH3CN at 298 K complex

λem/nm

Φem

τem/μs

kr/105 s−1

B23 B22 B21 B13 B12 B11 [Ru(bpy)3]2+

651 656 659 640 642 647 620

0.43 0.36 0.27 0.26 0.24 0.20 0.095

1.7 1.7 1.7 2.5 2.4 2.3 0.89

2.5 2.1 1.6 1.0 1.0 0.87 1.1

knr/105 s−1 3.4 3.8 4.3 3.0 3.2 3.5 10

highest value among those of RuL32+ hitherto reported.20 While the Φem value varies with n, the τem value was insensitive to n at 2.3−2.5 and 1.7 μs for B1n and B2n, respectively. The results would suggest that the excited-state electron of the complex is localized on one of the arylborane-substituted ligand(s). We will discuss further on the Φem and τem values of the complexes in terms of the radiative (kr) and nonradiative decay rate constants (knr) as described in the succeeding sections. The λem values of B1n or B2n were shifted to longer wavelength compared to that of [Ru(bpy)3]2+, in good accordance with the λabs values of the complexes. For a B1n or B2n series, however, the λem value was shifted to the shorter wavelength with an increase in n: from 647 to 640 nm for B1n and from 659 to 651 nm for B2n. This is unexpected since the emission maximum energy of RuL32+ (Eem) correlates in general proportionally to the (Eox − Ered1) value of the complex, similar to Eabs in Figure 2a.19 As seen in Figure 2b, the Eem value of B1n or B2n depends reversely on the (Eox − Ered1) value. There are two possible reasons for the results in Figure 2b. One possibility is the changes in the solvation structures around the emitting excited state of the complex. Since an introduction of the DBDE group(s) to a bpy ligand(s) results in an increase E

DOI: 10.1021/acs.inorgchem.5b01626 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

well as of [Ru(aryl-substituted-phen)3]2+24 as reported in Figures S4 and S5 in the Supporting Information. More favorably, one must check the correlation between kr and the molar absorption coefficient of the S0−T1 MLCT absorption band of a complex: ε(S0−T1(MLCT)). In the present stage of the investigation, unfortunately, the S0−T1(MLCT) absorption spectra of B1n and B2n are not available. However, the linear relationship between ε(S0−S1(MLCT)) and kr seen in Figure 5a demonstrates that the ε(S0−T1(MLCT)) value of the complex is proportional to the relevant ε(S0−S1(MLCT)) value. This is not strange since the phosphorescence transition probability of a molecule in the T1 state is gained by mixing between S1 and T1 by spin−orbit coupling.25 In practice, one of the important results of the present study is the observation of the linear relationship between Φem and kr for a series of B2n, as shown in Figure 5b and, thus, that between Φem and ε(S0− S1(MLCT)), while such a correlation is not clear enough for the data on B1n. Although further theoretical consideration is absolutely necessary, the results in Figures 5, S4, and S5 suggest that the kr values of these Ru(II) complexes including B1n can be discussed on the basis of the Strickler−Berg-type relation. Another important finding of the present study is the observation of the relationships between kr and S1−T1 splitting energy (i.e., ΔEST = Eabs − Eem) for B1n and B2n as shown in Figure 6, though the correlation for B1n is not necessarily good

with an increase in n as mentioned before, and this may relate to the n dependence of the kr value. As shown in Figure 5a, in

Figure 5. Relationships between (a) kr and ε(MLCT) and (b) Φem and kr for RuBbpys and [Ru(bpy)3]2+ in CH3CN at 298 K.

practice, the kr value of B2n correlates very well with ε(MLCT). Also, one can find a positive correlation between kr and ε(MLCT) values for B1n, although the variation of kr with n is not large enough. This reminds us of the Strickler− Berg relation for the fluorescence transition of a molecule given by eq 1,22 −9

0

k r = 2.880 × 10 Ds

2

gl

∫ I(ν)̃ dν ̃

g u ∫ ν−̃ 3I(ν)̃ dν ̃

∫

ε(ν)̃ dν ̃ ν̃

Figure 6. Relationship between kr and ΔEST (= Eabs − Eem) for RuBbpys and [Ru(bpy)3]2+. (1)

where is the intrinsic (or natural) S1−S0 fluorescence rate constant. I(ν̃) and ε(ν̃) are the fluorescence intensity and the ε value of the absorption band of a molecule responsible for the (S1−S0) fluorescence transition at a given wavenumber (ν̃), respectively. Ds is the refractive index of a medium and, gl and gu are the electronic degeneracies in the ground and excited states, respectively. The Strickler−Berg relation demonstrates that the larger the ε value (i.e., ε(S0−S1)), the larger the relevant fluorescence rate constant (kr0) and, thus, the fluorescence quantum yield: Φem ∝ ε(S0−S1). The results in Figure 5a suggest a Strickler−Berg-type relation, although there is no theoretical proof for the applicability of the relation to a phosphorescence transition. While the Strickler−Berg-type relation between kr and ε(MLCT) for a phosphorescent transition metal complex has never been reported, the present results in Figure 5 for B2n are not fortuitous, and similar results can be found for other ruthenium(II) complexes. As examples, the linear relationships between kr and ε(MLCT)/Φem are found for a series of [Ru(2,2′-bipyrazine)n(bpy)3−n]2+21d,23 as

enough. The data demonstrate that the smaller the ΔEST value, the larger the kr value. Since excited singlet−triplet mixing is more likely for a smaller ΔEST value, the larger kr value for the complex with a smaller ΔEST value is a reasonable consequence. The very large Φem values observed for B23 (0.43) and B22 (0.36) will be thus explained by the smallest ΔEST values (5080−5130 cm−1) among those of other B1n and B2n complexes (ΔEST > 5610 cm−1). It is worth noting that the almost linear relationship between kr and ΔEST can also be found for [Ru(2,2′-bipyrazine)n(bpy)3−n]2+21d,23 and [Ru(arylsubstituted-phen)3]2+,24 as reported in Figure S6 in the Supporting Information. Although a detailed discussion on the kr value of a transition metal complex has been rarely reported, the present results in Figures 5, 6, and S4−S6 demonstrate the importance of the Strickler−Berg-type analysis of the radiative data of the complexes. We are conducting further analyses of the emission properties of transition metal complexes along the line mentioned above, and the results will be reported in a separate publication.

kr0

F

DOI: 10.1021/acs.inorgchem.5b01626 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 8 with the fitting parameters (ΔE, (kr0 + knr0), and k′) summarized in Table 5. The ΔE values of B21 (930 cm−1) and

Nonradiative Decay Rate Constant. The knr values of B1n and B2n ((3.0−4.3) × 105 s−1) were much smaller than that of [Ru(bpy)3]2+ (10 × 105 s−1, see Table 4). In general, the knr values of the polypyridyl complexes of Ru(II), Os(II), Re(I), and so forth follow the energy gap law,21a,d,26 and the ln knr value of a complex linearly correlates with Eem Figure 7 shows the energy gap plot for the present complexes. As clearly seen in the figure, RuBbpys showed a

Figure 8. Temperature dependences (T = 250−330 K) of the emission lifetimes of B21, B11, and [Ru(bpy)3]2+ in propylene carbonate. The curves represent the fits by eq 2. Inset: Enlarged view.

Table 5. Activation Parameters for the T-Dependent Emission Lifetimes of B21, B11, and [Ru(bpy)3]2+ in Propylene Carbonate

Figure 7. Energy gap plot for RuBbpys and [Ru(bpy)3]2+ in CH3CN at 298 K. Green line represents the linear regression for the data of RuBbpys.

complex B21 B11 [Ru(bpy)3]2+

good linear relation between ln knr and Eem. The results demonstrate that the nonradiative decay mode from the MLCT excited state to the ground state is common for all complexes, while the large deviation of the data for [Ru(bpy)3]2+ from the linear relation indicates the difference in the nonradiative decay mode from those of RuBbpys. Since the excited electrons of RuBbpys in the LUMOs distribute more or less to p(B) as described in the preceding section, the large deviation of the data on [Ru(bpy)3]2+ from the linear plot in Figure 7 could also be explained by the participation of the synergistic MLCT/ π(aryl)−p(B) CT interactions in the emitting excited states of RuBbpys.27 To reveal the origin of the small knr values of RuBbpys, we studied temperature (T) dependences of the emission lifetimes of B21, B11, and [Ru(bpy)3]2+ in propylene carbonate (PC). The T dependence of the emission lifetime of RuL32+ (τem(T)) is known to be analyzed by an Arrhenius-type equation, eq 2,7 ⎛ ΔE ⎞ [τem(T )]−1 = (kr 0 + k nr 0) + k′exp⎜ − ⎟ ⎝ kBT ⎠ 0

ΔE/cm−1

(kr0 + knr0)/s−1

k′/s−1

930 2450 3780

6.0 × 10 4.1 × 105 6.3 × 105

1.4 × 107 7.3 × 109 2.9 × 1013

5

B11 (2450 cm−1) are smaller than that of [Ru(bpy)3]2+ (3780 cm−1), suggesting that thermal activation from 3MLCT to 3dd* is more likely for B21 and B11 compared to that of [Ru(bpy)3]2+. On the other hand, the k′ values of B21 (1.4 × 107 s−1) and B11 (7.3 × 109 s−1) are much smaller than that of [Ru(bpy)3]2+ (2.4 × 1013 s−1), demonstrating a minor contribution of the 3dd* state to nonradiative decay from the 3 MLCT excited states of B21 and B11. Such small k′ values have been often obtained for RuL32+-type complexes and, in such cases, the T-dependent τem has been discussed in terms of the contribution of the fourth 3MLCT excited state, locating at higher energy at several hundreds of cm−1 than the emitting 3 MLCT excited state.28 Since both ΔE and k′ values observed for B21 or B11 are closer to those for thermal activation from 3 MLCT to the fourth 3MLCT state, we suppose that the Tdependent emission lifetimes of B21 and B11 will be explained by the contribution of thermal activation from 3MLCT to the fourth MLCT state to nonradiative decay from the 3MLCT excited state of the complex.29 While the contribution of thermal activation to the fourth 3 MLCT excited state is generally minor for the excited-state dynamics of RuL32+, it appears in the excited-state dynamics of B21 and B11, probably due to large stabilization of the emitting 3 MLCT state in energy by the synergistic MLCT/π(aryl)−p(B) CT interactions relative to that of the 3dd* energy. Such characteristics are common for the MLCT/π(aryl)−p(B) CT excited triplet states of RuBbpys. As described in the preceding section, the Eox values assigned to the metal oxidation are almost the same for all RuBbpys and [Ru(bpy)3]2+, while the Ered1 value assigned to the ligand reduction of RuBbpys shifts to the positive direction compared to that of [Ru(bpy)3]2+. This indicates that the 3dd* state energies of the complexes are

(2)

0

where kr and knr are the T-independent radiative and nonradiative decay rate constants from the 3MLCT excited state to the ground state, respectively. The parameter k′ is the frequency factor for thermal activation from the 3MLCT excited state to the upper-lying nonemitting excited state (typically, excited triplet dd state (3dd*) for a polypyridyl Ru(II) complex) with the energy barrier between the two states being ΔE and, kB is the Boltzmann constant. Figure S7 in the Supporting Information shows the T (250− 330 K) dependences of the emission decay profiles of the complexes in PC. While the emission lifetime of [Ru(bpy)3]2+ decreased largely upon T elevation, B21 and B11 showed very small T-dependent emission lifetimes. The emission decay profiles of the complexes at 250−330 K were analyzed by single-exponential decay functions irrespective of T, and the T dependences of τem were adequately fitted by eq 2 as shown in G

DOI: 10.1021/acs.inorgchem.5b01626 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

■

comparable with one another, while the relevant 3MLCT excited-state energies decrease in the presence of the DBDE group(s) in the ligand(s) relative to the 3dd* state energies. The resulting large 3MLCT−3dd* energy gap of RuBbpy would suppress thermal deactivation of the 3MLCT state via the 3dd* state, which will be the origin of the small T dependences of the emission lifetimes and, thus, the knr values of RuBbpys.

ACKNOWLEDGMENTS This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government for the support of the research (Nos. 25107504 (Grant-in-Aid for Scientific Research on Innovative Areas), 25620035 (Grantin-Aid for Exploratory Research), and 26248022 (Grant-in-Aid for Scientific Research (A)) to N.K.

■

■

CONCLUSIONS We synthesized a series of Ru(II) complexes having a triarylborane-appended bpy ligand(s) (RuBbpys: B1n and B2n, n = 1−3) and studied their electrochemical, spectroscopic, and photophysical properties in detail. RuBbpys showed intense absorption at 290−600 nm, and the ε value of the MLCT absorption band (ε(MLCT)) of the complex was largely enhanced by the introduction of a DBDE group(s) to the bpy ligand(s) of the complex, owing to the synergistic MLCT/π(aryl)−p(B) CT interactions. One of the important findings of the present study is the increase in the ε(MLCT) value of B2n with that in n, and the increase in ε(MLCT) of the complex with n brings about a linear increase in the Φem value of the complex: Φem ∝ ε(MLCT), as seen in Figure 5. In particular, B23, with an extremely high ε(MLCT) value (5.6 × 104 M−1 cm−1 at 488 nm), showed the highest Φem (0.43) among those of RuL32+ hitherto reported.20 Furthermore, we found the linear relationship between ε(MLCT) and Φem for a series of B2n suggesting the Strickler−Berg-type relation between ε and kr. The linear correlation between kr and (Eabs − Eem) as a measure of the S1−T1 splitting energy of the complex was also demonstrated in the present study: Figures 6 and S6. These results are essentially due to the synergistic MLCT/π(aryl)−p(B) CT interactions in the emitting excited triplet state of RuBbpy, as demonstrated by the present electrochemical, spectroscopic/photophysical, and DFT calculation studies. Thus, the present study successfully demonstrated synthetic control/tuning of the electrochemical, spectroscopic, and photophysical properties of a polypyridine Ru(II) or other transition metal complex by a triarylboraneappended π-chromophoric ligand(s).

■

REFERENCES

(1) (a) Sasso, M. G.; Quina, F. H.; Bechara, E. J. H. Anal. Biochem. 1986, 156, 239. (b) Hartmann, P.; Leiner, M. J. P.; Lippitsch, M. E. Anal. Chem. 1995, 67, 88. (c) McDonagh, C.; MacCraith, B. D.; McEvoy, A. K. Anal. Chem. 1998, 70, 45. (d) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85. (e) Kalyanasundaram, K. Coord. Chem. Rev. 1982, 46, 159. (f) Schwarz, P.; Bossmann, S.; Guldner, A.; Duerr, H. Langmuir 1994, 10, 4483. (2) Suzuki, K.; Kobayashi, A.; Kaneko, S.; Takehira, K.; Yoshihara, T.; Ishida, H.; Shiina, Y.; Oishi, S.; Tobita, S. Phys. Chem. Chem. Phys. 2009, 11, 9850. (3) Ito, A.; Hirokawa, T.; Sakuda, E.; Kitamura, N. Chem. Lett. 2011, 40, 34. (4) Koo, C.-K.; Ho, Y.-M.; Chow, C.-F.; Lam, M. H.-W.; Lau, T.-C.; Wong, W.-Y. Inorg. Chem. 2007, 46, 3603. (5) (a) Sakuda, E.; Funahashi, A.; Kitamura, N. Inorg. Chem. 2006, 45, 10670. (b) Sakuda, E.; Ando, Y.; Ito, A.; Kitamura, N. Inorg. Chem. 2011, 50, 1603. (c) Ito, A.; Kang, Y.; Sakuda, E.; Kitamura, N. Inorg. Chem. 2012, 51, 7722. (d) Kang, Y.; Ito, A.; Sakuda, E.; Kitamura, N. J. Photochem. Photobiol., A 2015, 313, 107. (6) (a) Ramesey, B.; Leffler, J. E. J. Phys. Chem. 1963, 67, 2242. (b) DuPont, T. J.; Mills, J. L. J. Am. Chem. Soc. 1975, 97, 6375. (c) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2000, 122, 6335. (d) Kitamura, N.; Sakuda, E. J. Phys. Chem. A 2005, 109, 7429. (e) Kitamura, N.; Sakuda, E.; Yoshizawa, T.; Iimori, T.; Ohta, N. J. Phys. Chem. A 2005, 109, 7435. (f) Sakuda, E.; Tsuge, K.; Sasaki, Y.; Kitamura, N. J. Phys. Chem. B 2005, 109, 22326. (g) Sakuda, E. Thesis for doctoral degree, Hokkaido University, 2008. (h) Kitamura, N.; Sakuda, E.; Iwahashi, Y.; Tsuge, K.; Sasaki, Y.; Ishizaka, S. J. Photochem. Photobiol., A 2009, 207, 102. (i) Yamaguchi, S.; Shirasaka, T.; Tamao, K. Org. Lett. 2000, 2, 4129. (j) Kitamura, N.; Sakuda, E.; Ando, E. Chem. Lett. 2009, 38, 938. (k) Sakuda, E.; Ando, Y.; Ito, A.; Kitamura, N. J. Phys. Chem. A 2010, 114, 9144. (l) Ito, A.; Kawanishi, K.; Sakuda, E.; Kitamura, N. Chem. - Eur. J. 2014, 20, 3940. (7) Van Houten, J.; Watts, R. J. J. Am. Chem. Soc. 1976, 98, 4853. (8) (a) Sun, Y.; Ross, N.; Zhao, S.-B.; Huszarik, K.; Jia, W.−L.; Wang, R.−Y.; Macartney, D.; Wang, S. J. Am. Chem. Soc. 2007, 129, 7510. (b) Zhao, S.-B.; McCormick, T.; Wang, S. Inorg. Chem. 2007, 46, 10965. (c) Sun, Y.; Wang, S. Inorg. Chem. 2009, 48, 3755. (d) Hudson, Z. M.; Zhao, S.-B.; Wang, R. Y.; Wang, S. Chem. - Eur. J. 2009, 15, 6131. (e) Rao, Y.-L.; Wang, S. Inorg. Chem. 2009, 48, 7698. (f) Sun, Y.; Wang, S. Inorg. Chem. 2010, 49, 4394. (9) (a) Zhou, G.; Ho, C.-L.; Wong, W.-Y.; Wang, Q.; Ma, D.; Wang, L.; Lin, Z.; Marder, T. B.; Beeby, A. Adv. Funct. Mater. 2008, 18, 499. (b) You, Y.; Park, S. Y. Adv. Mater. 2008, 20, 3820. (c) Zhao, Q.; Li, F.; Liu, S.; Yu, M.; Liu, Z.; Yi, T.; Huang, C. Inorg. Chem. 2008, 47, 9256. (d) Lin, W.; Tan, Q.; Liang, H.; Zhang, K. Y.; Liu, S.; Jiang, R.; Hu, R.; Xu, W.; Zhao, Q.; Huang, W. J. Mater. Chem. C 2015, 3, 1883. (10) (a) Wade, C. R.; Gabbaï, F. P. Inorg. Chem. 2010, 49, 714. (b) Sun, Y.; Hudson, Z. M. Rao, Y.; Wang, S. Inorg. Chem.2011, 50, 3373.10.1021/ic1021966 (11) (a) Smith, L. F.; Blight, B. A.; Park, H.-J.; Wang, S. Inorg. Chem. 2014, 53, 8036. (b) Park, H.−J.; Ko, S.−B.; Wyman, I. W.; Wang, S. Inorg. Chem. 2014, 53, 9751. (12) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory Chemicals, 2nd ed.; Paragamon Press: New York, 1980. (13) Ishida, H.; Tobita, S.; Hasegawa, Y.; Katoh, R.; Nozaki, K. Coord. Chem. Rev. 2010, 254, 2449.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01626. Syntheses, cyclic voltammograms, emission decay profiles, Franck−Condon analyses of RuBbpys, plots of kr−ε(MLCT), Φem−kr, and kr−ΔEST for a series of [Ru(2,2′-bipyrazine)n(bpy)3−n]2+ and [Ru(aryl-substituted-phen)3]2+-type complexes, and emission decay profiles of B21, B11, and [Ru(bpy)3]2+ at 250−330 K (PDF)

■

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. H

DOI: 10.1021/acs.inorgchem.5b01626 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (14) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, M. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision A.1; Gaussian, Inc.: Wallingford, CT, 2009. (15) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 37, 785. (16) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (17) Petersson, G. A.; Al-Laham, M. A. J. Chem. Phys. 1991, 94, 6081. (18) Dennington, R.; Keith, T.; Millam, J. GaussView, Version 5; Semichem Inc: Shawnee Mission, KS, 2009. (19) Kawanishi, Y.; Kitamura, N.; Tazuke, S. Inorg. Chem. 1989, 28, 2968. (20) For example: Montalti, M.; Credi, A.; Prodi, L.; Teresa, M. Handbook of Photochemistry, 3rd ed.; CRC Press: Boca Raton, FL, 2006. (21) (a) Kober, E. M.; Caspar, J. V.; Lumpkin, R. S.; Meyer, T. J. J. Phys. Chem. 1986, 90, 3722. (b) Kestell, J. D.; Williams, Z. L.; Stultz, L. K.; Claude, J. P. J. Phys. Chem. A 2002, 106, 5768. (c) Huang, K.; Rhys, A. Proc. R. Soc. London, Ser. A 1950, 204, 406. (d) Allen, G. H.; White, R. P.; Rillema, D. P.; Meyer, T. J. J. Am. Chem. Soc. 1984, 106, 2613. (e) Fylstra, D.; Lasdon, L.; Watson, J.; Waren, A. Interfaces 1998, 28, 29. (f) Thompson, D. W.; Ito, A.; Meyer, T. J. Pure Appl. Chem. 2013, 85, 1257. (g) Ito, A.; Meyer, T. J. Phys. Chem. Chem. Phys. 2012, 14, 13731. (22) Strickler, S. J.; Berg, R. A. J. Chem. Phys. 1962, 37, 814. (23) Rillema, D. P.; Allen, G.; Meyer, T. J.; Conrad, D. Inorg. Chem. 1983, 22, 1617. (24) Alford, P. C.; Cook, M. J.; Lewis, A. P.; McAuliffe, G. S. G; Skarda, V.; Thomson, A. J.; Glasper, J. L.; Robbins, D. J. J. Chem. Soc., Perkin Trans. 2 1985, No. 5, 705. (25) Yersin, H.; Rausch, A. F.; Czerwieniec, R.; Hofbeck, T.; Fischer, T. Coord. Chem. Rev. 2011, 255, 2622. (26) (a) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5583. (b) Caspar, J. V.; Meyer, T. J. Inorg. Chem. 1983, 22, 2444. (c) Kober, E. M.; Sullivan, B. P.; Dressick, W. J.; Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1980, 102, 7383. (d) Caspar, J. V.; Meyer, T. J. J. Phys. Chem. 1983, 87, 952. (27) Treadway, J. A.; Loeb, B.; Lopez, R.; Anderson, P. A.; Keene, F. R.; Meyer, T. J. Inorg. Chem. 1996, 35, 2242. (28) (a) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1984, 23, 3877. (b) Lumpkin, R. S.; Kober, E. M.; Worl, L. A.; Murtaza, Z.; Meyer, T. J. J. Phys. Chem. 1990, 94, 239. (29) Taheri, A.; Meyer, G. J. Dalton Trans. 2014, 43, 17856.

I

DOI: 10.1021/acs.inorgchem.5b01626 Inorg. Chem. XXXX, XXX, XXX−XXX