Dynamic emission quenching of a novel ruthenium(II) complex by carbon dioxide in solution

Abstract

Emission from the tris(1,10-phenanthroline)ruthenium(II) complex (Ru(phen)₃²⁺) bearing a (dimesityl)boryldurylethynyl (DBDE) charge transfer unit at the 4-position of one of the three phen ligands ([Ru(phen)₂(4-DBDE-phen)] = 4BRu²⁺) was quenched by carbon dioxide in solution with the rate constant of ∼10⁶ M⁻¹ s⁻¹.

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We reported quite recently that a novel ruthenium(II) complex (4BRu²⁺) showed intense emission at the maximum wavelength of 681 nm in CH₃CN at 298 K with the quantum yield (Φ_em) and lifetime (τ_em) being 0.11 and 12 μs, respectively.¹ It is worth emphasizing that, since the Φ_em and τ_em values of [Ru(phen)₃]²⁺ in CH₃CN at 298 K are 0.045 and 0.42 μs, respectively,¹ the values of 4BRu²⁺ are extraordinarily large and long, respectively, compared with the relevant value of [Ru(phen)₃]²⁺. Such emission characteristics of 4BRu²⁺ are attributed to participation of the synergistic interactions between Ru(II)-to-phen charge transfer (MLCT) and CT from the π–orbital of the durylethynyl moiety (π(aryl)) to the vacant p-orbital on the boron atom (p(B)) in the 4-DBDE-phen ligand: synergistic MLCT/π(aryl)-p(B) CT. In practice, our TD-DFT calculations indicate that the highest-energy occupied molecular orbital (HOMO) and HOMO-1 in 4BRu²⁺ are characterized by the electron densities on the Ru(II) atom and the pyridine ring in the durylethynyl-phen system without any contribution of p(B) to the HOMOs, while the excited electron in the lowest-energy unoccupied MO (LUMO) distributes to both p(B) and the durylethynyl system.¹ Owing to MLCT/π(aryl)-p(B) CT nature and the very long τ_em value, the excited electron primarily localized on p(B) in 4BRu²⁺ would react and/or interact with an electrophile. Among various electrophiles, carbon dioxide is of primary importance in respect to the role in earth's climate warming and, thus, its chemical fixation/reduction is one of the urgent issues to be solved. In this communication, we report here that the excited triplet state of 4BRu²⁺ interacts with gaseous CO₂ in solution at room temperature as demonstrated by emission quenching of 4BRu²⁺ by CO₂.

4BRu²⁺ used in the present study was essentially the same with that reported previously.¹Acetonitrile and propylene carbonate (PC) for spectroscopic/photophysical measurements, purchased from Wako Pure Chemicals Ind., were purified by the accepted procedures.² The purity of CO₂ gas (Air Water Co. Ltd.) was 99.8%. Absorption and emission spectra of 4BRu²⁺ were recorded on a Hitachi U-3300 spectrophotometer and a multichannel photodetector (PMA-11, Hamamatsu Photonics), respectively. Emission lifetime measurements were conducted by the system reported previously at 532 nm excitation.¹

Fig. 1 shows the absorption spectrum of an N₂–gas purged acetonitrile solution of 4BRu²⁺ (2.7 × 10⁻⁶ mol dm⁻³ (= M), shown by black curve), while the red curve corresponds to the spectrum observed for the same sample solution after purging CO₂ gas for 20 min. The inset in Fig. 1 displays the relevant emission spectrum. As seen clearly in Fig. 1, although the absorption spectra of 4BRu²⁺ in N₂ and CO₂ gas atmosphere were essentially the same with each other, the emission from 4BRu²⁺ is quenched in the presence of CO₂ without any change in the spectral band shape. Fig. 2 shows the emission decay profile of 4BRu²⁺ in the N₂ (shown by black) or CO₂ atmosphere (shown by red). In both systems, the emission decay profiles were fitted by single exponential functions and the τ_em value in the N₂ (τ_em(N₂)) or CO₂ atmosphere (τ_em(CO₂)) was determined to be 12.3 or 6.3 μs, respectively. The emission quenching efficiency, defined as τ_em(N₂)/τ_em(CO₂), was 2.0, whose value was almost comparable to the value determined by the emission intensity in Fig. 1:1.9. Therefore, it is concluded that the MLCT/π(aryl)-p(B) CT excited state of 4BRu²⁺ is dynamically quenched by CO₂, probably through the interaction between the excited electron (charge) on p(B) and CO₂.

Fig. 1 Absorption spectra of 4BRu²⁺ in acetonitrile under N₂ (black curve) and CO₂ gas atmosphere (red curve). The inset shows the relevant emission spectrum.

Fig. 2 Emission decay profiles of 4BRu²⁺ in the N₂ (black) and CO₂ atmospheres (red) in acetonitrile at 298 K.

In order to accumulate further experimental evidence, we studied a CO₂–gas pressure (P_CO2) dependence of the emission lifetime of 4BRu²⁺. A CH₃CN solution of 4BRu²⁺ (2.7 × 10⁻⁶ M) in a glass tube equipped with a pressure-tight bulb was deaerated by three freeze-pump-thaw cycles and, subsequently, CO₂ gas was introduced to the tube in an ice-cooled bath at a given P_CO2. Emission lifetime measurements were then conducted at room temperature. The emission decay profiles of 4BRu²⁺ at P_CO2 = 0, 4, 20, and 1013 hPa were determined in both CH₃CN and PC solutions at room temperature. The emission decay profile was fitted by a single exponential function irrespective of P_CO2 and a solvent, and the τ_em values determined were summarized in Table 1. The τ_em values in CH₃CN and PC were ranged in 13.2–6.3 and 11.3–8.0 μs, respectively, while those of [Ru(phen)₃]²⁺ in CH₃CN and PC were 0.4 and 0.6 μs, respectively, irrespective of P_CO2 (experimental data are not shown here). It is worth emphasizing that the emission quenching efficiency, τ_em(N₂)/τ_em(CO₂), is linearly correlated with the log P_CO2 value irrespective of a solvent as seen in Fig. 3. When the log P_CO2 value is correlated linearly with the molar CO₂ concentration ([CO₂]) in each solvent, the linear relations in Fig. 3 should be analyzed by a Stern–Volmer type equation: τ_em⁰/τ_em(CO₂) = 1 + k_qτ_em⁰[CO₂] where τ_em⁰ is that at P_CO2 = 0 and k_q is the bimolecular quenching rate constant of the excited state of 4BRu²⁺ by CO₂. Unfortunately, however, there is no available relation between log P_CO2 and [CO₂] in solution. Instead of analyzing the linear relation in Fig. 3, therefore, we evaluated k_q based on the data at P_CO2 = 1013 hPa. The [CO₂] value in CH₃CN or PC evaluated by the weight method was 0.071 or 0.056 M, respectively.³ The τ_em⁰ and τ_em (P_CO2 = 1013 hPa) demonstrate that the k_q value in CH₃CN or PC is 1.2 × 10⁶ or 0.74 × 10⁶M⁻¹s⁻¹, respectively.

Fig. 3 CO₂–gas pressure (P_CO2) dependency of the emission lifetimes of 4BRu²⁺ in acetonitrile (closed circles) and PC (open circles) at 298 K.

Table 1 Emission lifetimes of 4BRu²⁺ under several CO₂–gas pressures

	τ em/μs
CO2–gas pressure/hPa	0	4	20	1013
Acetonitrile	13.2	13.3	9.6	6.3
PC	11.3	10.9	9.4	8.0

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Emission quenching of 4BRu²⁺ by CO₂ is worth discussing in term of the Gibbs free energy change for photoreduction of CO₂ by the excited state of 4BRu²⁺: ΔG^‡. As reported previously, 4BRu²⁺ shows four consecutive reduction waves at −1.09, −1.41, −1.75, and −2.12 V (vs.SCE in CH₃CN) and the three reduction waves observed more positive than −2.0 V are ascribed to consecutive one-electron reductions of the three phen ligands (E(Ru^2+/+), E(Ru^+/0), and E(Ru^0/−)), while the fourth wave at −2.12 V is responsible for the reduction of the 4-DBDE group in the complex: electron occupation by p(B) at E(4-DBDE^0/−).¹ Since the excited state of 4BRu²⁺ is characterized by synergistic MLCT/π(aryl)-p(B) CT interactions as mentioned above, we suppose that the first reduction potential observed electrochemically (E(Ru^2+/+)) for the ground state of 4BRu²⁺ (i.e., −1.09 V) will not correspond to the reduction potential in the excited state of the complex, E(Ru*^2+/+): E(Ru*^2+/+) ≠ E(Ru^2+/+) + E_0,0, where E_0,0 is the excited state energy of 4BRu²⁺ (E₀₀ = 1.91 eV at 80 K). When one assumes that CO₂ interacts with p(B) in the excited state of 4BRu²⁺, the E(Ru*^2+/+) value would be rather similar to E(4-DBDE^0/−) = −2.12 V. If this is the case, the ΔG^‡ value for emission quenching of the excited state of 4RuB²⁺ by CO₂ is evaluated to be −0.02 V through the one-electron reduction potential of CO₂ being −2.1 V (vs.SCE).^4,5 The above discussions indicate that photoreduction of CO₂ by the excited state of 4BRu²⁺ is thermodynamically possible but is not necessarily favourable enough, if one adopts the present quenching reaction to the Rehm-Weller type k_qvs.ΔG^‡ relationship.⁶ This may explain that the present system shows very small k_q value: ∼10⁶ M⁻¹ s⁻¹.

It is worth noting that [Ru(phen)₂(5-DBDE-phen)] (= 5BRu²⁺) which is the 5-position analogue of 4BRu²⁺ shows E(5-DBDE^0/–) at −1.89 V (vs.SCE).¹ The reduction potential of 5BRu²⁺ is more positive than E(4-DBDE^0/−) in 4BRu²⁺ and the excited state lifetime of 5BRu²⁺ is much shorter (τ_em = 1.2 μs in CH₃CN at 298 K) than that of 4BRu²⁺.¹ Thus, the excited state of 5BRu²⁺ is unfavourable for emission quenching by CO₂. In practice, the emission from 5BRu²⁺ is not quenched by CO₂ under analogous conditions to those for 4BRu²⁺. We suppose that the long excited state lifetime and the reduction potential negative enough for the interaction with CO₂ are the necessary conditions for emission quenching by CO₂.

Photoreduction/photofixation of CO₂ has been hitherto explored on the basis of both molecular^4,7–11 and semiconductor systems.¹² In molecular systems, Lehn et al.⁷ and Ishitani et al.^4,8 reported that CO₂ was reduced photochemically to COvia a CO₂–Re(CO)₃LX adduct (L and X were diimine and halogen/monodentate ligands, respectively) in the presence of an electron donor. Alternatively, Tazuke et al. reported carboxylation of an aromatic hydrocarbon (ArH) through the electrophilic attack of CO₂ to the anion radical of ArH produced by photoinduced electron transfer with an aromatic amine as an electron donor.⁹ The present emission quenching of 4BRu²⁺ by CO₂via the electronic interaction between the excited electron/charge on p(B) in the complex and CO₂ might be understood by the mechanism similar to the electrophilic attack of CO₂ to the anion radical of ArH mentioned above and, thus, we suppose that the present system would produce the anion radical of CO₂. To the best of our knowledge, this is the first experimental observation of direct emission quenching of a molecule by gaseous CO₂ in solution. Furthermore, it is worth noting that the present system does not require an electron donor or sacrificial agent and, therefore, is very unique compared with other molecular CO₂ photoreduction/photofixation systems. To confirm the quenching mechanism, we are performing product analysis of the system, which will be reported in a forthcoming paper.

We thank the Global COE program of Hokkaido University (Catalysis as the Basis for Materials Innovation) for a postdoctoral fellowship (2009) and a research fellowship (2008–2010) to E. S. and A. I., respectively. E. S. also thanks the F3 Project of MEXT, Japan (2010–).

References

1. E. Sakuda, Y. Ando, A. Ito and N. Kitamura, Inorg. Chem., 2011, 50, 1603.
2. D. D. Perrin, A. L. Armargo and D. R. Perrin, Purification of Laboratory Chemicals, 2nd edn, Pergamon Press, New York, 1980.
3. The [CO₂] value in CH₃CN or PC at P_CO2 = 1013 hPa was evaluated as follows. A 10 mL solution of CH₃CN or PC in a glass vessel cooled in an ice-water bath was purged by a CO₂ gas stream for 40 min. The solution was then allowed to room temperature under a CO₂ gas stream flowing above the solution in the vessel and sealed with a stop-cock. By measuring the weights of the solution before and after CO₂ gas purging, we evaluated [CO₂] solublized in the solution. The average weight increment of a CH₃CN or PC solution (10 mL) before and after CO₂ gas purging was 0.0312 or 0.0363 g, respectively, corresponding to [CO₂] to be 0.071 or 0.056 M, respectively.
4. H. Takeda and O. Ishitani, Coord. Chem. Rev., 2010, 254, 346.
5. In the cyclic voltammogram of 4BRu²⁺ in a CH₃CN – 0.1 M tetra-n-butylammonium hexafluorophosphate solution, the electrode current at around E(DBDE^0/−) = −2.1 V increased in the presence of CO₂ compared with that under Ar-gas atmosphere, suggesting reduction of CO₂ through that of the DBDE group in 4BRu²⁺. However, since the direct reduction current of CO₂ flew in the same potential range, we could not distinguish two contributions to the observed current.
6. (a) D. Rehm and A. Weller, Ber. Bunsen-Ges. Phys. Chem., 1969, 73, 834; (b) Isr. J. Chem. 1970 8 259.
7. J. Hawecker, J.-M. Lehn and R. Ziessel, Helv. Chim. Acta, 1986, 69, 1990.
8. (a) H. Hori, K. Koike, M. Ishizuka, K. Takeuchi, T. Ibusuki and O. Ishitani, J. Organomet. Chem., 1997, 530, 169; (b) B. Gholamakhass, H. Mametsuka, K. Koike, T. Tanabe, M. Furue and O. Ishitani, Inorg. Chem., 2005, 44, 2326; (c) H. Tsubaki, A. Sekine, Y. Ohashi, K. Koike, H. Takeda and O. Ishitani, J. Am. Chem. Soc., 2005, 127, 15544; (d) S. Sato, K. Koike, H. Inoue and O. Ishitani, Photochem. Photobiol. Sci., 2007, 6, 454; (e) K. Koike, S. Naito, S. Sato, Y. Tamaki and O. Ishitani, J. Photochem. Photobiol., A, 2009, 207, 109.
9. (a) S. Tazuke and H. Ozawa, J. Chem. Soc., Chem. Commun., 1975, 237; (b) S. Tazuke, S. Kazama and N. Kitamura, J. Org. Chem., 1986, 51, 4548.
10. (a) H. Ishida, K. Tanaka and T. Tanaka, Chem. Lett., 1985, 405; (b) H. Ishida, K. Tanaka and T. Tanaka, Organometallics, 1987, 6, 181; (c) H. Ishida, T. Terada, K. Tanaka and T. Tanaka, Inorg. Chem., 1990, 29, 905.
11. (a) J. Hawecker, J.-M. Lehn and R. Ziessel, J. Chem. Soc., Chem. Commun., 1983, 536; (b) R. Ziessel, J. Hawecker and J.-M. Lehn, Helv. Chim. Acta, 1986, 69, 1065; (c) D. Mandler and I. Willner, J. Chem. Soc., Perkin Trans. 2, 1988, 997; (d) I. Willner, R. Maidan, D. Mandler, H. Dürr and K. Zengerle, J. Am. Chem. Soc., 1987, 109, 6080.
12. (a) T. Inoue, A. Fujishima, S. Konishi and K. Honda, Nature, 1979, 277, 637; (b) K. Teramura, T. Tanaka, H. Ishikawa, Y. Kohno and T. Funabiki, J. Phys. Chem. B, 2004, 108, 346; (c) W. Y. Lin and H. Frei, J. Am. Chem. Soc., 2005, 127, 1610; (d) K. Teramura, H. Tsuneoka, T. Shishido and T. Tanaka, Chem. Phys. Lett., 2008, 467, 191; (e) S. Qin, F. Xin, Y. Liu and W. Ma, J. Colloild Intface Sci., 2011, 356, 257.

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