Supramolecular CO2–Reduction Photocatalysts - ACS Publications

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Kinetics and Mechanism of Intramolecular Electron Transfer in Ru(II)−Re(I) Supramolecular CO2−Reduction Photocatalysts: Effects of Bridging Ligands Yasuomi Yamazaki,†,|| Kei Ohkubo,† Daiki Saito,† Taiki Yatsu,† Yusuke Tamaki,† Sei’ichi Tanaka,† Kazuhide Koike,*,‡ Ken Onda,*,§ and Osamu Ishitani*,† †

Department of Chemistry, Tokyo Institute of Technology, O-okayama 2-12-1-NE-1, Meguro-ku, Tokyo 152-8550, Japan National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan § Department of Chemistry, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan

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ABSTRACT: The supramolecular photocatalysts in which a Ru(II) complex as a molecular redox photosensitizer unit and a Re(I) complex as a molecular catalyst unit are connected with a various alkyl or ether chain have attracted attention because they can efficiently photocatalyze CO2 reduction with high durability and high selectivity of CO formation, especially on various solid materials such as semiconductor electrodes and mesoporous organosilica. The intramolecular electron transfer from the one-electron reduced photosensitizer unit to the catalyst unit, which follows excitation of the photosensitizer unit and subsequent reductive quenching of the excited photosensitizer unit by a reductant, is one of the most important processes in the photocatalytic reduction of CO2. We succeeded in determining the rate constants of this intramolecular electron transfer process by using subnanosecond time-resolved IR spectroscopy. The logarithm of rate constants shows a linear relationship with the lengths of the bridging chain in the supramolecular photocatalysts with one bridging alkyl or ether chain. In conformity with the exponential decay of the wave function and the coupling element in the long-distance electron transfer, the apparent decay coefficient factor (β) in the supramolecular photocatalysts with one bridging chain was determined to be 0.74 Å−1. In the supramolecular photocatalyst with two ethylene chains connecting between the photosensitizer and catalyst units, on the other hand, the intramolecular electron transfer rate is much faster than that with only one ethylene chain. These results strongly indicate that the intramolecular electron transfer from the one-electron reduced species of the Ru photosensitizer unit to the Re catalyst unit proceeds by the through-bond mechanism.

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INTRODUCTION Supramolecular photocatalysts comprise multinuclear complexes with a molecular redox photosensitizer unit and a molecular catalyst unit. These have attracted attention as one of the key technologies in artificial photosynthesis. Many supramolecular photocatalysts for CO2 reduction1 and for the formation of H22 and O23 exist. Some of them exhibit much better photocatalytic capability compared with mixed systems of the corresponding mononuclear complexes.1e,4 The Ru(II)− Re(I) binuclear complex, for example, consisting of a Ru(II) trisdiimine complex and a Re(I) diimine biscarbonyl bisphosphine complex ([Ru−BL−Re]3+, BL = bridging ligand), acts as an efficient photocatalyst in CO2 reduction. A selectivity of CO formation (SCO) of 94%, a quantum yield (ΦCO) of 15%, a turnover frequency (TOFCO) of 1.2 min−1, and a turnover number (TONCO) of 207 were reported from using [Ru−C2−Re]3+ (Figure 1) as a photocatalyst and 1-benzyl1,4-dihydronicotinamide (BNAH) as a reductant.4 Although a © XXXX American Chemical Society

1:1 mixture of the mononuclear complexes of Ru(II) and Re(I) also acts as a photocatalyst in CO2 reduction, its catalytic properties (SCO = 57%, TOFCO = 0.41 min−1, TONCO = 48) are subordinate to those of the supramolecular photocatalyst. The molecular structure of the supramolecular photocatalyst imparts advantages especially on the surface of solid materials. The distance between a photosensitizer and a catalyst is difficult to control on the surface due to random adsorption of the photosensitizer and the catalyst in mixed systems. This negatively impacts the efficiency of electron transfer between the photosensitizer and the catalyst, which is one of the most important processes in the photocatalytic reactions. The supramolecular photocatalyst resolves this problem because it enables the control of the distance between the photosensitizer and catalyst units through a suitable bridging ligand. For Received: April 30, 2019

A

DOI: 10.1021/acs.inorgchem.9b01256 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Structures and abbreviations for [Ru−BL−Re]3+. The counteranion of all of the complexes was PF6−.

Scheme 1. Elementary Processes in the Photocatalytic Reaction Using [Ru−BL−Re]3+ for CO2 Reduction

electrodes.1k,3a,6 Since the excited metal-complex photosensitizers have relatively weak oxidation power, in many reported systems, a strong reductant such as 1,3-dimethyl-2phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) and 1-benzyl-1,4-dihydronicotinamide (BNAH) is required for constructing photocatalytic systems using only metal complexes. This problem can be solved by hybridizing the supramolecular photocatalysts with semiconductor particles or semiconductor electrodes.1k,3a,6 These hybrid systems can photocatalytically reduce CO2 by using relatively weak electron donors, such as methanol6a and even water,6e,f owing to emergent hybridization of the high selectivity for CO2 reduction (supra-

example, the hybrid of a Ru(II)−Re(I) supramolecular photocatalyst and periodic mesoporous organosilica of which numerous organic groups are embedded in the silica framework as visible-light absorbers results in efficient excitation energy accumulation into the Ru(II) photosensitizer unit and efficient photocatalytic reduction of CO2 to CO.5 On the other hand, the efficiency, selectivity, and durability were much lower in the case using a mixture of the corresponding mononuclear Ru(II) and Re(I) complexes instead of the supramolecular photocatalysts. This advantage of the supramolecular photocatalysts also accounts for the various hybrid systems with semiconductor particles or semiconductor B

DOI: 10.1021/acs.inorgchem.9b01256 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Herein, we report the intramolecular electron transfer rates of a series of Ru(II)−Re(I) supramolecular photocatalysts, containing different bridging ligands (Figure 1). Investigation of the relationship between the structure of the bridging chain and the reaction rate clarified the mechanism of the intramolecular electron transfer.

molecular photocatalysts) and the strong oxidation power (semiconductor materials). The connection of the photosensitizer to the catalyst units is expected to produce fast and efficient intramolecular electron transfers, even when immobilized on the solid surfaces. This partly explains the high photocatalytic activity of the supramolecular photocatalyst on the surface. Scheme 1 illustrates the proposed pathway for the photocatalytic reduction of CO2 using the Ru(II)−Re(I) supramolecular photocatalyst ([Ru−BL−Re]3+). It involves the absorption of light by the Ru photosensitizer unit (process 1) to populate to the lowest energy triplet state. This is characterized by the triplet metal-to-ligand charge transfer (3MLCT) excited state ([3Ru*−BL−Re(I)]3+). Reductive quenching (process 2) of [3Ru*−BL−Re(I)]3+ by an electron donor such as BNAH or BIH generates the one-electron reduced species (OERS) of the photosensitizer unit. The electron inserted into the Ru unit should hop among the peripheral and bridging ligands of the Ru(II) unit because the energy levels of these three ligands are close to each other. Therefore, the OERS of the Ru unit is abbreviated as [{Ru}•−−BL−Re(I)]2+ for simplicity. In hybrids with semiconductor material, electron injection from the semiconductor unit facilitates reductive quenching. The intramolecular electron transfer (process 3) from the OERS of the Ru unit to the Re catalyst unit reduces the Re unit ([Ru(II)−BL− {Re}•−]2+). The CO2 reduction (process 4) takes place on the Re(I) tricarbonyl complex requiring another electron and an O2− acceptor.7 Since the intramolecular electron transfer represents an essential process in supramolecular photocatalysis, understanding its mechanism is crucial. Direct determination of rate constants poses difficulties because the electron transfer process starts immediately after the excitation and reductive quenching processes (processes 1−3 shown in Scheme 1). Owing to this situation, three short-lived intermediates, including [3Ru*−BL−Re(I)]3+, [{Ru}•−−BL− Re(I)]2+, and [Ru(II)−BL−{Re}•−]2+, characterized by broad absorption in the UV−vis region, coexisted in the solution, with continuous variation in concentrations. We reported previously that infrared vibrational bands of the CO ligands (νCO) in the Re catalyst unit are useful “indicators” for the formation of the OERS of the Re catalyst unit.8 The connection of the Re to the Ru unit through the nonconjugated ethylene chain minimizes the electronic interaction between the units. Consequently, the excitation and the reduction of the Ru unit marginally affect the absorption wavenumbers of the νCO’s (≤10 cm−1). Contrarily, the reduction of the Re unit induces greater low-energy shifts of the νCO’s (24−32 cm−1), which is caused by the increases of the electron density of the Re unit, and elevates the π-backdonation to the π* orbital of the carbonyl ligands.9 On the basis of these results, we performed time-resolved infrared (TR-IR) spectroscopy to determine the rate constant of electron transfer from the OERS of the Ru unit to the Re unit for the [Ru−C2−Re]3+. Despite potential rate limitation by the proceeding process 2, the high concentration of the strong reductant, BIH (0.3 M, EOXP = 0.33 V vs SCE10), and subnanosecond TR-IR equipment yielded a rate (kET) of 1.4 × 109 s−1. Investigation of the relationship between the structure of the bridging chain and the reaction rate will clarify the mechanism of the intramolecular electron transfer, i.e., contribution of the through-bond and/or through-space electron transfer.

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RESULTS AND DISCUSSION Photophysical and Electrochemical Properties of [Ru−BL−Re]3+. Table 1 summarizes photophysical and Table 1. Photophysical and Redox Properties of [Ru−BL− Re]3+ and the Corresponding Mononuclear Complexesa E1/2/V vs Ag/ AgNO3e complex [Ru−C2−Re]3+ [Ru−C4−Re]3+ [Ru−C6−Re]3+ [Ru−RC2−Re]3+ [Ru−COC−Re]3+f [Ru(dmb)3]2+g [Re(dmb)(CO) {P(C6H4F)3}2]+

λabs, nmb

λem, nmc

Φemc

τ, nsd

[Re(L/ L•−)]

[Ru(L/ L•−)]

461 461 461 463 460 459 396

648 641 637 661 649 638

0.10 0.10 0.11 0.10 0.09 0.09

746 768 774 659 735 881

−1.65 −1.68 −1.68 −1.53 −1.62

−1.73 −1.75 −1.75 −1.72 −1.73 −1.74

−1.67

a Emission data were measured in DMF at 25 °C. The samples were degassed by the freeze−pump−thaw method before measurement. b The maximum absorption wavelength of the MLCT absorption band. cλex = 480 nm. dλex = 510 nm. eMeasured in MeCN solution containing a complex (0.5 mM) and Et4NBF4 (0.1 M) under an Ar atmosphere by the differential pulse voltammetry method. The first reduction wave was fitted using two Gaussian functions to obtain each peak potential (Ep). Ep obtained by differential pulse voltammetry (DPV) was converted to E1/2 using the equation Ep = E1/2 + ΔEp/2. Pulse amplitude (ΔEp) was 50 mV. Under this condition, the ferrocenium/ferrocene redox couple was observed at 0.08 V vs Ag/ AgNO3. fRef 1m. gRef 4.

redox properties of [Ru−BL−Re]3+. They showed quite similar UV/vis absorption spectra except for [Ru−RC2− Re]3+, and these spectra could be well-simulated by summation spectra of the corresponding model complexes (Figure S1). It strongly indicates that the intramolecular electronic interaction between the Ru and Re units is weak in the ground state. Although the emission properties of all of the [Ru−BL−Re]3+ except [Ru−RC2−Re]3+ are also similar to each other and similar to that of [Ru(dmb)3]2+, a small red-shift of emission caused by the weak interaction through the bridging ligand11 was observed in the cases with the short bridging chain. The redox properties of the complexes did not strongly depend on the bridging ligand. It is noteworthy that the driving force of the intramolecular electron transfer, which is basically determined by the difference between the reduction potential of the Ru unit (E1/2([Ru(L/L•−)])) and that of the Re unit (E1/2([Re(L/L•−)]), are similar to each other in all of the [Ru−BL−Re]3+’s (differences of the standard free energy change were within 0.04 eV). Therefore, the series of [Ru− BL−Re]3+ should be suitable for investigating dependence of the rate of the intramolecular electron transfer on the length of the bridging chain in the BL. In addition, since the driving forces are not so large in all the cases, both forward and reverse electron transfer between the Ru and Re units should proceed. In the case of [Ru−RC2−Re]3+, the photophysical and C

DOI: 10.1021/acs.inorgchem.9b01256 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. TR-IR spectra of (a) [Ru−C6−Re]3+ and (b) [Ru−RC2−Re]3+ (0.2 mM) with BIH (0.3 M) in a DMF-TEOA (5:1 v/v) solution measured with various time delays after excitation at λex = 532 nm. The FT-IR spectra in a DMF-TEOA (5:1 v/v) solution are shown at the bottom.

1 ps after the laser light irradiation, and a decrease of the ground state ([Ru(II)−C6−Re(I)]3+) should simultaneously proceed. The OERS of the Ru unit ([{Ru}•−−C6−Re(I)]2+) should be also produced in the initial stage, and [3Ru*−C6− Re(I)]3+ should simultaneously decrease. However, we did not observe a drastic change of the TR-IR spectrum according to these reactions. This is reasonable because the νCO bands of these three species should be very similar to each other: separation by the long alkyl chain between the Re and Ru units suppresses electronic interaction between the units. Therefore, the bleaching at νCO’s of 1864 and 1939 cm−1 is attributed to the sum of the concentrations of the ground state, the 3MLCT excited state, and the OERS of the Ru unit. This process produced the [{Ru}•−−C6−Re(I)]3+ and should induce the induction period of the bleaching at νCO’s of 1864 and 1939 cm−1 because the increase of the electronic density in the Ru unit does not induce changes of νCO’s as described above. We determined the rate constant of the reductive quenching of the

electrochemical properties were different from those of the other [Ru−BL−Re]3+ and the corresponding model complexes because of relatively strong electronic interaction through the two ethylene chains as we previously reported.1o TR-IR Spectra of [Ru−BL−Re]3+. Figure 2 displays TR-IR spectra of the [Ru−C6−Re]3+ and the [Ru−RC2−Re]3+ measured in an N,N-dimethylformamide (DMF)/triethanolamine (TEOA) solution (5:1 v/v) containing BIH (0.3 M) as the reductant (λεx = 532 nm, pulse duration = 0.6 ns). The Ru unit mostly absorbed the irradiation light (Figure S1), with the TR-IR spectra obtained from the region of the vibrations of the CO ligands (1800−2000 cm−1). In the case of [Ru−C6−Re]3+, after the induction period for several nanoseconds, two new bands resembling those in its ground state emerged at νCO’s of 1832 and 1909 cm−1, with bleaching of the bands at νCO’s of 1864 and 1939 cm−1, with the peaks increasing until 120 ns. The 3MLCT excited state of the Ru unit ([3Ru*−C6−Re(I)]3+) should be produced within D

DOI: 10.1021/acs.inorgchem.9b01256 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry [3Ru*−C6−Re(I)]3+ (kq) from the Stern−Volmer plots using the steady-state emission strength measurement in the presence of various amounts of BIH (I) and that without any BIH (I0, eq 1). I0 = 1 + kqτem[BIH] (1) I Using the emission lifetime without BIH (τem = 774 ns, Table 1), the kq was determined to be 1.2 × 109 M−1 s−1. This indicates completion of the electron injection within several nanoseconds in the presence of 0.3 M BIH (1/kq[BIH] = 2.8 ns). This process produced [{Ru}•−−C6−Re(I)]3+; however, it should not induce spectral change because of the similarity of νCO’s between [3Ru*−C6−Re(I)]3+ and [{Ru}•−−C6− Re(I)]3+ as described above. The observed induction period of the bleaching at νCO’s of 1864 and 1939 cm−1 clearly indicates that the intramolecular electron transfer from the OERS of the Ru unit to the Re unit was slower than the reductive quenching process. In other words, we infer that the enlargement of the

bleaching bands directly highlights the kinetics of the electron transfer processes from the OERS of the Ru unit. Since this spectral change finalized within 120 ns, this forward electron transfer process and also the back-electron transfer from the OERS of the Re unit to the Ru unit should reach equilibrium within 120 ns. It should be noted that TEOA failed as the reductant because of its more positive oxidation potential (EOXP(TEOA/ TEOA•+) = +0.50 V vs Ag/AgNO3)12,13 compared with that of the lowest (3MLCT) excited state of the Ru photosensitizer unit (E1/2(Ru(I)/*Ru(II)) = +0.24 V in the case of [Ru(dmb)3]2+. The BIH acted as the reductant because of its higher reducing power (EOXP(BIH/BIH•+) = +0.03 V vs Ag/AgNO 3 ). However, the TEOA served as a base, accelerating deprotonation from the one-electron oxidized species of the BIH (BIH•+, eq 2) during the TR-IR measurements. This suppressed the back-electron transfer from the reduced Ru unit to the BIH•+ and increased the yield of the reduced species of the complexes.

Conversely, the [Ru−RC2−Re]3+ showed considerably rapid spectral changes (Figure 2b). The two new bands originating from the reduction of the Re unit (1836, 1913 cm−1) and the two bleaching bands (1874, 1948 cm−1) were observed within 0.5 ns. Since the excited [Ru−RC2−Re]3+ was quenched by the BIH on the same time scale (kq = 1.2 × 109 M−1 s−1) as the [Ru−C6−Re]3+, the intramolecular electron transfer process in the [Ru−RC2−Re]3+ proceeded faster than in the [Ru−C6−Re]3+. The electron transfer between the Ru and the Re units reached equilibrium within 9 ns because of the absence of change in the absorption spectra thereafter. Figure 3 shows the TR-IR spectra detected after 1 and 9 ns of

subtraction of the normalized spectrum after 9 ns (blue line) from that after 1 ns (red line) displays absorption bands at 1863 and 1936 cm−1 and bleaching bands at 1879 and 1948 cm−1. The absorption bands are attributed to the formation of the 3MLCT excited state of the Ru unit ([3Ru*−RC2− Re(I)]3+) and/or the OERS of the Ru unit ([{Ru}•−−RC2− Re(I)]3+). This was accompanied by a decrease in the bleaching bands at the ground state of the [Ru−RC2−Re]3+. In the 3MLCT excited state and the OERS of the Ru unit, the electron density on the diimine moiety of the bridging ligand in the Ru unit increases. This increase is expected to induce a slight elevation of the electron density of the diimine moiety in the Re unit, owing to the relatively strong interaction through the two ethylene chains. Such strong interaction caused by the RC2 bridging ligand has also been observed in various properties, e.g., photophysical, electrochemical (Table 1), and photocatalytic properties.1o All spectra obtained until 9 ns after the excitation (Figure 2b) allowed fitting using the linear combination of the green spectrum (eq 3) and the blue spectrum shown in Figure 3. ΔmOD(Xns)(λ) = A × ΔmOD(Green)(λ) + A′ × ΔmOD(Blue)(λ))

(3)

As shown in Figure 4, A, normally proportional to the total concentration of ([3Ru*−RC2−Re(I)]3+) and [{Ru}•−− RC2−Re(I)]3+, increases within 1 ns and then decreases (green line). The quantity A′, usually proportional to the concentration of the OERS of the Re unit of [Ru−RC2−Re]3+ ([Ru(II)−RC2−{Re}•−]2+), increased up to 9 ns, with an induction period of 0.25 ns. [Ru−COC−Re] 3+ and [Ru−C4−Re] 3+ show similar spectral changes to [Ru−C6−Re]3+, with the two transient absorption bands and the bleaching bands of the ground state observed after a period of induction. The weak interaction produced no significant change in the spectra (Figure S2 and S3). However, as explained below, the time to attain the

3+

Figure 3. TR-IR spectra of [Ru−RC2−Re] (0.2 mM) in a DMFTEOA (5:1 v/v) solution containing BIH (0.3 M), measured after 1 ns (red) and 9 ns (blue) of excitation at λex = 532 nm. The ΔmOD is normalized at 1836 and 1912 cm−1. The difference between the blue line and the red line is illustrated as the green line.

excitation, with the strengths of the peaks normalized at 1836 and 1912 cm−1, which are attributed to OERS of the Re unit. The spectra demonstrate changes in the peak shapes of the bleaching bands, whereas the shapes and positions of the two absorption bands attributed to the OERS of the Re unit at 1815−1845 cm−1 and 1896−1924 cm−1 remained essentially unchanged. The spectrum (green line in Figure 3) obtained by E

DOI: 10.1021/acs.inorgchem.9b01256 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Table 2. Quenching Constants and Population Ratios of [Ru−BL−Re]3+ complex 3+

[Ru−C2−Re] [Ru−C4−Re]3+ [Ru−C6−Re]3+ [Ru−RC2−Re]3+ [Ru−COC−Re]3+

kd/106 s−1

kq/109 M−1 s−1

αa

1.34 1.30 1.30 1.52 1.36

1.2 1.2 1.2 1.2 1.2

9 9 9 ≥9 9b

a Population ratios (α) of an added electron between the Ru and Re units were defined by the eq 4. bRef 1m.

s−1) are much higher than the total rate of the radiative and nonradiative decays of the excited state (kd = 1.3−1.5 × 106 s−1, Table 2). The intramolecular oxidative quenching process involving electron transfer from the excited state of the Ru unit to the Re unit observed in the case of [Ru−C2−Re]3+ was significantly slower (k = (3.4−4.8) × 104 s−1)9a than the reductive quenching by BIH. Since the emission lifetimes of other complexes resemble that of the [Ru(dmb)3]2+, the speed of the intramolecular oxidative quenching process is inferior to the radiative and nonradiative decay of the excited state of the Ru unit. These results demonstrate that the reductive quenching by BIH proceeds almost quantitatively, and the intramolecular electron transfer process commences after the quenching. The back-electron transfer processes from the reduced complex to BIH•+, with the rate constant denoted by krec, are much slower compared with the intramolecular forward electron transfer under the reaction conditions. Therefore, these processes affect the behavior of the intramolecular electron transfer negligibly, as discussed in detail below. Due to the low driving force for the intramolecular electron transfer from the OERS of the Ru unit to the Re unit (ΔG° = −0.19 eV for [Ru−RC2−Re]3+;1o ΔG° = −0.11 eV for [Ru− COC−Re]3+,1m ΔG° ≤ −0.08 eV for the other [Ru−BL− Re]3+, Table 3), the backward electron transfer (kRET) should also proceed. These two reversible electron transfer processes produce the equilibrium state between the [{Ru}•−−BL− Re(I)]2+ and the [Ru−BL−{Re}•−]2+. Similar “quasi”-equilibrium observed during the photochemical CO2 reduction using the Ru(II)−Re(I) supramolecular photocatalysts is noteworthy.1m,o,4 Measuring the differential UV−vis absorption of the solution containing 0.3 mM of [Ru−BL−Re]3+ and 0.1 M of 1-benzyl-1,4-nicotinamide (BNAH) as an electron donor during irradiation provided the equilibrium constants (red solid line in Figure 5). The spectrum of the OERS of the

Figure 4. Time courses of A and A′.

equilibrium state varied for the three complexes, with the longer alkyl chains in the bridging ligands requiring the longest times. Considering [Ru−C2−Re]3+,8 two minor absorption bands at 1851 and 1923 cm−1 and two bleaching bands at 1872 and 1934 cm−1 emerged after 0.25 ns of photoexcitation (Figure S4), such as in [Ru−RC2−Re]3+. These bands are also attributable to the 3MLCT excited state and/or OERS of the Ru unit. The considerable weakness of the νCO bands of these transient species clearly indicates a location of the absorptions at wavenumbers similar to those of the ground state of [Ru− C2−Re]3+. This is because the electronic interaction between the Ru unit and the Re in [Ru−C2−Re]3+ is weaker than that in [Ru−RC2−Re]3+. After about 12 ns of excitation, the absorption spectra displayed no change, which was slower compared with [Ru−RC2−Re]3+. The Determination of the Rate Constant of the Intramolecular Electron Transfer. Scheme 2 summarizes the mechanism of the photochemical reduction of the Ru(II)− Re(I) supramolecular photocatalysts [Ru−BL−Re]3+ by the BIH. It also illustrates the electron transfer from the OERS of the Ru unit to the Re unit and vice versa. After excitation of the Ru unit, the lowest excited state represented by the 3MLCT excited state ([3Ru*−BL−Re(I)]3+) forms. This is followed by reductive quenching by BIH, giving OERS of the Ru unit ([{Ru}•−−BL−Re(I)]2+). The intramolecular electron transfer produces the OERS of the Re unit ([Ru−BL−{Re}•−]2+). The formation of [3Ru*−BL−Re(I)]3+ via an intersystem crossing is achievable within several tens to hundreds of femtoseconds.14 The rate constant for the reductive quenching process (kq) determined from the Stern−Volmer plots was 1.2 × 109 M−1 s−1 for [Ru−BL−Re]3+ (Table 2). The quenching rates in the presence of 0.3 M of BIH (kq[BIH] = 3.6 × 108

Scheme 2. Processes of the Photochemical Reaction of [Ru−BL−Re]3+ in the Presence of BIH

F

DOI: 10.1021/acs.inorgchem.9b01256 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

do not have any clear evidence which supports the rigidity of BL affecting the intramolecular electron transfer rate up to now. The results for various parameters including kd, kq, and α of each complex are summarized in Table 2. We determined the rate constant for the electron transfer from fitting of the time profiles of the [Ru−BL−{Re}•−]2+ following the mechanism shown in Scheme 2. The changes in concentration of the intermediates produced photochemically are represented by the following equations:

Table 3. Rate Constants of the Forward and Backward Electron Transfer and the Total Length of the Bonds between the Diimine Moieties in the Bridging Ligand in [Ru−BL−Re]3+ TBLDA/ Åa

−ΔG°/ eVb

≤4.9 ± 0.5

4.50

0.19

14 ± 1 6.0 ± 0.3

1.6 ± 0.1 0.67 ± 0.03

4.50 5.84

0.08 0.11

1.3 ± 0.1 0.23 ± 0.01

0.14 ± 0.01 0.026 ± 0.001

7.52 10.55

0.07 0.07

complex

kET/× 108 s−1

[Ru−RC2− Re]3+ [Ru−C2−Re]3+ [Ru−COC− Re]3+ [Ru−C4−Re]3+ [Ru−C6−Re]3+

47 ± 5

kRET/× 108 s−1

d[[3 Ru * −BL − Re(I)]3 + ] dt = −(kd + kq[BIH])[[3 Ru*−BL − Re(I)]3 + ]

a

Total length of the bonds. bStandard free energy change of the electron transfer reaction from the reduced Ru unit to the Re unit.

(5)

d[[{Ru}• − − BL − Re(I)]2 + ] dt = (kq[BIH])[[3 Ru * −BL − Re(I)]3 + ] + kRET[[Ru(II) − BL − {Re(I)}• − ]2 + ] − (kET + k rec[BIH• +])[[{Ru}• − − BL − Re(I)]2 + ]

(6)

•− 2+

d[[Ru(II) − BL − {Re} ] ] dt = kET[[{Ru}• − − BL − Re(I)]2 + ] − (kRET + k rec[BIH• +][[Ru(II) − BL − {Re(I)}• − ]2 + ]

d[BIH• +] = kq[BIH])[[3 Ru * −BL − Re(I)]3 + ] dt − kdp[TEOA][BIH• +]

Figure 5. Differential UV−vis absorption spectrum of a DMF-TEOA (5:1 v/v) solution containing 0.1 M of BNAH and 0.3 mM of [Ru− C6−Re]3+ during irradiation (red solid line) and simulated spectra using the spectrum of the OERS of the corresponding mononuclear Ru(II) and Re(I) complexes shown in Figure S5 (dotted black line).

− k rec([[{Ru}•− − BL − Re(I)]2 + ] + [[Ru(II) − BL − {Re}•− ]2 + ])[BIH• +]

corresponding mononuclear Ru(II) and Re(I) complexes provided reasonable simulations of the spectrum as shown in Figure 5 (dotted black line). Therefore, except for [Ru−RC2− Re]3+, estimates of the ratios between the two OERSs of the Ru and Re units (α in eq 4) yielded a value of 9 from the simulation (Table 2). This indicates that the rate of the forward electron transfer is 9 times faster than that of the backward electron transfer. α=

k [[Ru(II) − BL − {Re}• − ]2 + ] = ET kRET [[{Ru}• − − BL − Re(I)]2 + ]

(7)

(8)

where [BIH] is the initial concentration of BIH (i.e., 0.3 M and hypothetically time-independent), kd is the total rate constant of the radiative and nonradiative decays of the [3Ru*−BL− Re(I)]3+; kq is the quenching rate constant of the [3Ru*−BL− Re(I)]3+ by BIH, kET is the forward electron transfer rate constant, kRET is the backward electron transfer rate constant, krec is the recombination rate constant between BIH•+ and the OERS, and kdp is the rate constant of the deprotonation of BIH•+ by the TEOA. The produced OERSs of the Ru and the Re units potentially decrease through charge recombination with BIH•+ during these processes. Since the time scales of reactions in this research range from several hundreds of picoseconds to tens of nanoseconds after the excitation, the back-electron transfer in the solvated ion pair can be disregarded because it terminates within 210 ps.15 After the dissociation from the solvent cage, collision between the reduced complex and the BIH•+ perhaps induces the back-electron transfer. However, the speed of the collision is diffusion controlled, and the concentrations of the OERS of the complex and BIH•+ were below 30 μM, even immediately after reductive quenching terminated.16 Therefore, because the kdiff[OERS][BIH•+] should be below 2.5 × 106 s−1,17 the back-electron transfer minimally affects the intramolecular electron transfer in all cases of [Ru−BL−Re]3+. The oxidation processes of BIH are shown in eq 2. BIH reportedly serves as a two-electron donor.1j BIH•+ is deprotonated by the TEOA as a base to form the radical species (BI•) that possesses a reducing power strong enough to

(4)

We could not determine the exact value of α for the [Ru− RC2−Re]3+ because of a poor fit by the simulation for the absorption spectra recorded during photocatalysis using the [Ru−RC2−Re]3+ (Figure S6). This is probably due to the stronger interaction between the two units through the two ethylene chains compared with the other complexes. The introduction of two ethylene chains in the [Ru−RC2−Re]3+ also induces a stronger driving force for the intramolecular electron transfer from the OERS of the Ru unit to the Re unit (ΔG0 ≈ − 0.19 eV, Table 3). Thus, a higher α is expected for the [Ru−RC2−Re]3+ relative to other complexes (i.e., α > 9). In addition, the peak shift caused by the increase of the electron density on the BL in the excited state and/or the reduced state of the Ru unit, which was observed only in the [Ru−RC2−Re]3+ complex, should also clearly indicate that there is electronic interaction between the Ru unit and the Re unit in the [Ru−RC2−Re]3+ complex. On the other hand, we G

DOI: 10.1021/acs.inorgchem.9b01256 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry reduce the ground state of [Ru−BL−Re]3+. Due to the relative slowness (kdp ≈ 1 × 105 M−1 s−1) of the deprotonation step,1j the injection of the second electron to the [Ru−BL−Re]3+ by the BI• can be neglected in the simulation to determine the rates of the intramolecular electron transfer processes. On the basis of the information provided above, we determined the kET by nonlinear least-squares fitting using the rate equations in eqs 5−8, with the kd and kq values listed in Table 2. The fitting data of [Ru−C6−Re]3+ and [Ru−RC2− Re]3+, for example, for an α value of 9, are illustrated in Figure 6. The evaluated rate constant for the forward intramolecular

Figure 7. Time profiles of [Ru(II)−BL−{Re}•−]2+ (•) in the presence of 0.3 M BIH in an argon-saturated DMF-TEOA (5:1 v/v) solution after 532 nm excitation and their fitting results; [Ru−RC2− Re]3+(red), [Ru−C2−Re]3+(orange), [Ru−COC−Re]3+ (green), [Ru−C4−Re]3+ (sky blue), and [Ru−C6−Re]3+ (blue).

2 |V |2 ijj π 3 yzz j z h jjk λkBT zz{

1/2

kET =

l o o (ΔG° + λ)2 | o − expo m } o o o o λ 4 k T B n ~

(9)

where V is the electronic coupling between the donor and acceptor, λ is sum of the reorganization energies of the inner and outer spheres (molecular and solvent), kB is the Boltzmann constant, T is the absolute temperature, and h is Planck’s constant.18 The free energy change (ΔG°) of the electron transfer process from the OERS of the Ru unit to the Re unit evaluated using the electrochemical data (Table 1) and eq 10 is presented in Table 3. −ΔG° = −E1/2(Ru(L/L•−) + E1/2(Re(L/L•−)

(10)

The electronic interaction can be assumed to be proportional to the overlap integral between the corresponding electronic orbitals, and its parameter V is formulated as eq 11 using the distance (r) between the Ru and Re units: |V |2 = |V0|2 exp{−β(r − r0)}

(11)

where r0 is the close contact distance between the Ru and Re units and β is the decay coefficient factor (damping factor).19 Figure 8 illustrates the relation between the kET and the length of the chain in the bridging ligand. This length involves the total length of the bonds between the carbons at the four positions of the bpy moieties in the bridging ligand (TBLDA). For its calculation, a semiempirical optimization with an AM1 parameter in MOPAC620 was utilized, and the data are given in Table 3. We might consider flexibility of the bridging chains, which enables changing the distance between the Ru and Re units. However, both the Ru and Re units have a positive charge and are linked to each other through the short or relatively short bridging chain. Since the electronic repulsion should suppress the collision between them,18a,21 the actual average distance should approximately have a linear relationship to TBLDA. Similar treatments were reported in some other diad systems: the distance dependences of kET were investigated using the total bond length of the bridging chains as the distance parameter.18a,22 Figure 8 clearly indicates that a linear relation exists between ln(kET) and TBLDA in the cases of [Ru−C2−Re]3+, [Ru−C4−Re]3+, and [Ru−C6−Re]3+. The apparent decay coefficient factor (β) determined from the

Figure 6. Time profiles of [Ru(II)−BL−{Re}•−]2+ (•) in the presence of 0.3 M BIH in an argon-saturated DMF-TEOA (5:1 v/v) solution after 532 nm excitation, (a) [Ru−C6−Re]3+ and (b) [Ru− RC2−Re]3+. The kinetic simulation results for [3Ru*−BL−Re(I)]3+, [{Ru}•−−BL−Re(I)]2+, and [Ru(II)−BL−{Re}•−]2+ are also shown (solid lines).

electron transfer, kET, yielded estimates of 0.23 × 108 s−1 for [Ru−C6−Re]3+ and 44−50 × 108 s−1 for [Ru−RC2−Re]3+ (α = 9−∞). Suitable fitting results obtained from other complexes are displayed in Figure 7 and Figures S7−S9. The evaluated rate constants for the forward intramolecular electron transfer, kET, are featured in Table 3. These data demonstrate that the rate of the forward electron transfer from the OERS of the Ru unit to the Re unit depends strongly on the bridging ligand. The Dependence of kET on the Structure of the Bridging Ligands. Based on the Marcus theory of electron transfer reactions, the kET is calculated as follows: H

DOI: 10.1021/acs.inorgchem.9b01256 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

strongly supports the existence of a rapid through-bond electron transfer. Considering the kET and TBLDA values for [Ru−COC− Re]3+ with a C−O bond and slightly higher −ΔG° than those of [Ru−C2−Re]3+, [Ru−C4−Re]3+, and [Ru−C6−Re]3+ because of the stronger electronegativity of the O atom, it fit within the linear line generated by the complexes containing one alkyl chain (Figure 8). Although the higher −ΔG° might cause a larger kET value, this effect should be small in the case of [Ru−COC−Re]3+ because the difference of −ΔG° between [Ru−COC−Re]3+ and the complexes with one alkyl linker are only 30−40 mV. Therefore, these results indicate that the O atom in [Ru−COC−Re]3+ causes a similar degree of electronic coupling to the C atoms in the bridging ligands of the other [Ru−BL−Re]3+’s because both the C−C and C−O bonds are σ-bonds and play a similar role when electron transfer proceeds through the σ-bond.19,25 For the [Ru−RC2−Re]3+, the kET is higher than the “expected” value from the linear line derived from the kET values of other [Ru−BL−Re]3+’s as shown in Figure 8. For example, the kET of the [Ru−RC2−Re]3+ is at least three times higher than that of the [Ru−C2−Re]3+, despite the similarity in the length of the alkyl chain or chains between the Ru and Re units. There are several possible reasons for the acceleration of the electron transfer in the case of [Ru−RC2−Re]3+, including a higher −ΔG° (80−120 mV) compared to those of the other [Ru−BL−Re]3+’s as described in the preceding section. Another is the larger |V|2 derived from the two ethylene chains to cause a larger superexchange interaction between the Ru and Re units. The rigidity of the RC2 bridging ligand should also be noted. The rigidity originating from the two ethylene chains limits the rotation of each bpy moiety in the bridging ligand, and the relative orientation of the two bpy moieties is restricted considerably. Therefore, the average distance between the Ru center and the Re center (RDA) in [Ru−RC2−Re]3+ should be shorter than that in [Ru−C2− Re]3+, even though the TBLDA’s of the two complexes are the same. The shorter RDA will cause a decrease of the λ value (eq 12). In the Marcus normal region, the ket increases (eq 9) as the λ value increases. Moreover, the short RDA can promote the through-space electron transfer owing to the easier approach and/or direct contact between the two units. The main reason for this remains unclear at this stage. Mallouk and his co-workers reported photoinduced forward and backward electron transfer in a Ru tris-diimine complex connected to a methyl-viologen unit through a flexible alkyl chain.18a In systems with alkyl chains shorter than C6H12, the through-bond electron transfer from the excited Ru unit to the viologen unit was also advanced. They reported the contribution of the through-space electron transfer in addition to the through-bond electron transfer in systems with longer alkyl chains (e.g., C8H16). The intramolecular electron transfer for the [Ru−BL−Re]3+ likely proceeds mostly by the throughbond mechanism because of the short chains in the bridging ligands. These short chains allow faster electron transfer compared with the through-space electron transfer via collision between the Ru and the Re units. The large electrostatic and steric repulsion between them also likely contributes to suppression of the collision. This should be an important consideration for increasing the photocatalytic efficiency of supramolecular photocatalysts. Although one of the effective ways to accelerate the intramolecular electron transfer process is introduction of

Figure 8. Relationship between kET and the total length of the bonds between the diimine moieties of the bridging ligand (TBLDA).

gradient of the fitting curve was 0.74 Å−1. This β value is comparable to those reported for donor−acceptor twocomponent systems connected through alkyl chains (β = 0.8−1.0 Å−1), in which photoinduced intramolecular electron transfer proceeds by the through-bond mechanism.18a,22 In these reported systems, the distance dependence of kET was also investigated using the bond length of the bridging chains as a distance parameter; it might be useful to compare the decay coefficients normalized by the number of bonds in the bridging chains because each bond length is often estimated in different ways. The apparent decay coefficient per each bond was calculated to be 1.1 [bond−1]. This value is also in the same range of the reported systems (1.1 [bond−1] (Closs et al.),23 1.3−1.5 [bond−1] (Mallouk and coworkers)18a). In contrast, a decay coefficient factor reported for the throughspace mechanism is approximately 3 Å−1.24 The fact that the three plots for [Ru−Cn−Re]3+ (n = 2, 4, 6) were on a straight line with a low gradient (low β value) supports the idea that the through-bond mechanism should be the main mechanism of intramolecular electron transfer. This mechanism involves electron-transfer acceleration by superexchange tunneling through the C−C bonds.18b The actual decay coefficient factor should be estimated by compensating both differences of the driving forces (ΔG°) and reorganization factors (λ) for the intramolecular electron transfer. As described above, the difference of ΔG° in the three [Ru−Cn−Re]3+ was enough small to be negligible (Table 3). Equation 12 shows the relationship between the distance of the units and λ.18a

1 1 zyz ji 1 λ ∝ jjj + − z j 2rD 2rA RDA zz{ (12) k where rD is the donor radius, rA is the acceptor radius, and RDA is the center-to-center distance between the donor and the acceptor, i.e., RDA ≈ rD + rA + TBLDA. According to the equation, the λ increases with an increase of TBLDA. It is reported that the apparent decay coefficient factor for the electron transfer in the Marcus normal region will be slightly overestimated mainly due to the increase of the λ values. The electron transfer in [Ru−Cn−Re]3+ should be that in the Marcus normal region because of the quite small driving force; Therefore, the actual decay coefficient for [Ru−Cn−Re]3+ will be slightly smaller than the apparent β value (0.74 Å−1). This

I

DOI: 10.1021/acs.inorgchem.9b01256 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

The redox potentials of the complexes were measured in an acetonitrile solution containing a complex (0.5 mM) and Et4NBF4 (0.1 M) as a supporting electrolyte through CV or DPV, with an ALS/CHI CHI-620 electrochemical analyzer comprising an Ag/ AgNO3 (0.01 M) reference electrode, a glassy carbon working electrode, and a Pt counter electrode. The Et4NBF4 was dried under a vacuum at 100 °C for a day, prior to use. Materials. DMF was dried over 4 Å molecular sieves and distilled at low pressure (10−20 Torr). The distilled DMF was stored under Ar and used within a week. The TEOA distilled at low pressure (