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Light Energy Accumulation from Pyrene Derivative to Tris(bipyridine)ruthenium on Clay Surface Daiki Morimoto,† Haruya Yoshida,† Keita Sato,† Kenji Saito,† Masayuki Yagi,† Shinsuke Takagi,‡ and Tatsuto Yui*,† †

Department of Materials Science and Technology, Faculty of Engineering, Niigata University, 8050 Ikarashi-2, Niigata 950−2181, Japan ‡ Department of Applied Chemistry, Graduate Course of Urban Environmental Sciences, Tokyo Metropolitan University, Minami-ohsawa 1-1, Hachiohji, Tokyo 192−0397, Japan S Supporting Information *

ABSTRACT: A novel type of energy donor−acceptor system on a clay surface has been prepared. The energy transfer between an energy-donating cationic pyrene derivative (AnPy2+) and an energy-accepting tris(bipyridine)ruthenium complex (Ru2+) on the clay surface was investigated using absorption, emission, and lifetime measurements. An obvious energy transfer was observed, and one Ru2+ molecule quenched the emission from five molecules of An-Py2+ with an emission quenching efficiency of 85% on the clay surface. This suggests that the light energies absorbed by five of the An-Py2+ molecules were accumulated in the one Ru2+ molecule. Near-quantitative emission quenching was observed for stoichiometric amounts of An-Py2+ and Ru2+. The apparent quenching rate constant is approximately 1017 L mol−1 s−1, and thus the quenching rate constant is 107−108 times higher than the diffusion rate constant in a homogeneous solution.

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INTRODUCTION Tris(bipyridine)ruthenium (Ru2+; Figure 1a) and its derivatives exhibit visible-light absorption, strong emission, a long lifetime,

to provide a solution. Pyrene derivatives are promising organic molecules for this purpose because they exhibit intense absorption and a high emission quantum yield and consist of common atoms.16−21 We have reported efficient energy transfer from a light-absorbing and energy-donating cationic pyrene derivative (Py4+)22 to energy-accepting Ru2+ on a synthetic clay (Sumecton SA [SSA])23,24 surface.25,26 In this case, the emission from Py4+ was quenched and an apparent emission quenching rate constant (kq,app) of 7.4 ± 0.7 × 1015 L mol−1 s−1 was achieved.25 However, this system requires stoichiometric amounts of Py4+ and Ru2+ to achieve emission quenching of ca. 85% owing to the low overlap integral (J = 0.91 × 1014 M−1 cm−1 nm4) between the fluorescence of Py4+ and the absorption of Ru2+ on the SSA surface. Because, the absorption and emission spectra of Py4+ gradually red-shifted on the SSA surface due to the flattening of pyridinium ring with respect to the pyrene ring.22 This flattening leads to the enhancement of electron withdrawing and apparent extension of the π-conjugate within the Py4+ molecule.27−29 To avoid the redshift of pyrene on the SSA surface, the π-conjugation between the pyridinium ring and the pyrene moiety should be severed. Based on the this point of view, we have synthesized a novel pyrene derivative substituting trimethylanilinium groups

Figure 1. Molecular formula of Ru2+ (a) and An-Py2+ (b).

and unique redox properties and are thus widely used as redox photosensitizers for various photoreactions.1−15 However, the absorption intensity of Ru2+ in the visible region is not particularly high (molar absorption coefficient [ε] ≈ 1.4 × 104 L mol−1 cm−1 at around 460 nm). Moreover, Ru2+ contains atoms of ruthenium, which is rare and expensive. To obtain efficient and effective photoreaction systems, a combination of Ru2+ and an ordinary organic molecule that has strong light absorption and light energy accumulation properties is expected © XXXX American Chemical Society

Received: February 14, 2017 Revised: March 28, 2017 Published: March 28, 2017 A

DOI: 10.1021/acs.langmuir.7b00512 Langmuir XXXX, XXX, XXX−XXX

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Langmuir (An-Py2+; Figure 1b), which exhibits strong absorption (ε ≈ 2.6 × 104 L mol−1 cm−1 at 355 nm) and a high fluorescence quantum yield (Φf ≈ 90%) in water.30 Moreover, An-Py2+ effectively suppresses red shifts in its absorption and emission spectra when adsorbed on the SSA surface in comparison to Py4+.30 From this experimental result, improvements are expected in the value of J and the efficiency of light energy accumulation between pyrene and Ru2+ on the SSA surface. Here we report the hybridization of two photofunctional molecules, namely, modified light-absorbing cationic pyrene (An-Py2+) and energy-accepting Ru2+, on a clay surface and the efficient light energy accumulation and energy transfer behavior of this system. We found that only one Ru2+ molecule quenched the emission from five molecules of An-Py2+ with 85% efficiency, indicating that the light energies absorbed by five of the An-Py2+ molecules were accumulated in the one Ru2+ molecule. Moreover, the apparent quenching rate constant (kq.app) is on the order of 1017 L mol−1 s−1, which is 107−108 times higher than the diffusion rate constants of solutes in common solvents. These results indicate that even if the amount of the quencher, i.e., precious Ru2+ is reduced by a factor of 108 the reaction proceeds with the same efficiency as in the homogeneous solution.

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Figure 2. Absorption spectra of An-Py2+/Ru2+/SSA (blue), An-Py2+/ SSA (black), Ru2+/SSA (red), and simple summation of An-Py2+/SSA and Ru2+/SSA (green): [SSA] = 16 mg L−1, [An-Py2+] = 2 μmol L−1 (25% CEC), [Ru2+] = 2 μmol L−1 (25% CEC).

and Ru2+ under these conditions (Ru2+ and An-Py2+ = 25% CEC).26 The absorption spectra of An-Py2+/SSA and Ru2+/SSA and the calculated spectrum (simple sum of those of An-Py2+/ SSA and Ru2+/SSA) are illustrated in Figure 2 (green line) as a comparison. The experimentally observed absorption of AnPy2+/Ru2+/SSA agreed well with the calculated value, which indicates that adsorbed Ru2+ did not affect the shapes of the absorption spectra of An-Py2+/Ru2+/SSA. These results suggest that neither An-Py2+ nor Ru2+ formed aggregates or complexes in the ground state on the SSA surface. The emission spectra of An-Py2+/Ru2+/SSA with An-Py2+ = 25% CEC and Ru2+ = 0−25% CEC using light at 350 nm for excitation are shown in Figure 3. The ratio of the absorption of

EXPERIMENTAL SECTION

A hybrid of An-Py2+ and Ru2+ adsorbed onto SSA (An-Py2+/Ru2+/ SSA) was prepared by mixing an aqueous dispersion of SSA (16 mg L−1, 0.016 mequiv L−1), an aqueous solution of An-Py2+ (2 μmol L−1), and aqueous Ru2+ (0−2 μmol L−1) for 2 h at room temperature in the dark. An aqueous solution of the An-Py2+ and the Ru2+ were loaded to the SSA dispersion in this order. The loading amounts of An-Py2+ and Ru2+ are expressed as the percentage of cationic charge on the dyes with respect to the cation exchange capacity (CEC = 0.997 mequiv g−1) of SSA,24 which is hereafter denoted by % CEC. Under these conditions, a loading of 25% CEC An-Py2+ was adsorbed onto SSA and the loading of Ru2+ was varied from 0% to 25% CEC. The quantitative adsorption of both An-Py2+ and Ru2+ onto SSA was confirmed by filtration, i.e., neither dye was detected in the filtrate solutions by absorption measurements. Further experimental details, analytical methods, and materials have been reported in previous papers.25,26,30

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RESULTS AND DISCUSSION The normalized emission spectrum of independently prepared An-Py2+ on SSA (An-Py2+/SSA, An-Py2+ = 0.1% CEC) and the absorption spectrum of Ru2+ on SSA (Ru2+/SSA, Ru2+ = 25% CEC) are shown in Figure S1. The spectral overlap integral (J) was estimated to be 1.7 × 1014 M−1 cm−1 nm4, which is approximately twice those observed in earlier studies for Py4+/ SSA and Ru2+/SSA systems.25,26 This J value suggests efficient energy transfer from An-Py2+ to Ru2+ on SSA. The absorption spectra and corresponding Lambert−Beer plot of An-Py2+/ Ru2+/SSA at the maximum absorption wavelength (λmax) of the Ru2+ component (454.5 nm) with different loading amounts of Ru2+ (Ru2+ = 0−25% CEC) are shown in Figure S2. The absorption intensities of Ru2+ increased linearly with the amount of adsorbed Ru2+; the estimated absorption coefficient (ε) was 1.26 ± 0.16 × 104 L mol−1 cm−1. The obtained ε value at λmax was close to those of aqueous Ru2+ (1.42 × 104 L mol−1 cm−1 at 453 nm) and Ru2+/SSA (1.43 × 104 L mol−1 cm−1).25,26 Representative absorption spectra of An-Py2+/ Ru2+/SSA with adsorbed amounts of An-Py2+ and Ru2+ of 25% CEC are shown in Figure 2 (blue line). Based on area coverage, the SSA surface was almost fully covered by An-Py2+

Figure 3. Emission spectra of An-Py2+/Ru2+/SSA: [SSA] = 16 mg L−1, [An-Py2+] = 2 μmol L−1 (25% CEC), [Ru2+] = 0 (black), 0.2 (red), 0.4 (green), 0.5 (blue), 1.0 (dark yellow), 1.5 (brown), and 2.0 (purple) μmol L−1 (0, 2.5, 5, 6.25, 12.5, 18.75, and 25% CEC). Emission intensities were corrected by relative absorbed photon numbers (IA) as following equation; IA = 1−10−A, where A denotes the absorbance at the excitation wavelength. Inset shows the emission spectra normalized by the emission intensity at 428 nm. The excitation wavelength was 350 nm.

photons between An-Py2+ and Ru2+ on SSA was 1:0.15 at 350 nm (see Figure 2, dashed line), even at the largest adsorbed amount of Ru2+. Thus, An-Py2+ molecules on SSA nearly selectively absorbed incident photons under the experimental conditions, and the emission intensities were practically corrected by the amount of photons absorbed by An-Py2+. In B

DOI: 10.1021/acs.langmuir.7b00512 Langmuir XXXX, XXX, XXX−XXX

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Langmuir the absence of Ru2+ (An-Py2+/SSA), a strong emission band was observed from An-Py2+ on SSA. Fluorescence quantum yields (Φf) were measured by absolute emission quantum yield measurement systems equipped with a calibrated integrating sphere,9,25,26,30,31 and Φf of An-Py2+/SSA was estimated to be 27.0 ± 1.0%.30 In contrast, the emission intensities of An-Py2+ drastically decreased with a very small amount of Ru2+, and a new broad emission band from Ru2+ appeared in the region of 550−750 nm (Figure 3, inset). Excitation spectra of Ru2+/SSA and An-Py2+/Ru2+/SSA monitored at 615 nm are shown in Figure S3. Excitation spectrum of Ru2+/SSA is in good agreement with the absorption of Ru2+ component. In contrast, excitation spectrum of the An-Py2+/Ru2+/SSA shows characteristic excitation band at 300−400 nm and it agreed well with the absorption of the An-Py2+ component on SSA. These results strongly suggest that coadsorbed Ru2+ efficiently quenched excited An-Py2+ via energy transfer. The emission spectrum of An-Py2+/Ru2+/SSA was deconvolved for the wavelength region of 390−750 nm by the linear sum of the emission spectra, using the spectra of An-Py2+/SSA (An-Py2+ = 25% CEC) and Ru2+/ SSA (Ru2+ = 25% CEC) as modeled by the least-squares method.25,26 The emission spectra of An-Py2+/Ru2+/SSA with the deconvolved An-Py2+ and Ru2+ components are shown in Figure S4. The sum of the two components accurately reproduced the experimentally observed emission spectrum of An-Py2+/Ru2+/SSA. The emission quenching efficiency (ηq) is expressed by the following equation (1): I −I ηq = 0 I0 (1)

quantitative emission quenching was observed, which was also confirmed by measurements of the emission lifetime (τ), as shown in Figure 4. In the absence of Ru2+ (An-Py2+/SSA, [An-

Figure 4. Emission decay profiles of An-Py2+/SSA (25% CEC) (red), An-Py2+/Ru2+/SSA (blue), and apparatus response (black) monitored at 425 nm after excitation at 376 nm: [SSA] = 16 mg L−1, [An-Py2+] = 2 μmol L−1 (25% CEC), [Ru2+] = 2 μmol L−1 (25% CEC).

Py2+] = 25% CEC), the decay of the emission from An-Py2+ (monitored wavelength =425 nm) was fitted well to two components (τ1 = 0.23 ns [75%] and τ2 = 1.1 ns [25%]) owing to the formation of an excimer.30 However, in the presence of Ru2+ at 25% CEC, no decay of the emission from An-Py2+ was observed within our apparatus response time (pulse width =0.4 ns, full width at half-maximum). Such near-quantitative emission quenching implies that energy transfer from An-Py2+ to Ru2+ is faster than excimer formation of An-Py2+. These results clearly indicate that the increase in J values was directly linked to the increase in ηq values. Based on the Stern−Volmer (SV) relationship (eq 2) and the emission lifetime of An-Py2+/ SSA (τ1 = 0.23 ns was used as the major component), the apparent quenching rate constant (kq.app) was estimated from eq 3.32,33 SV plot (Figure S5) did not obey the liner relationships, due to the ηq values reaching the limit within the concentration. Thus, each SV constants (K) were estimated at every one point. I0 = 1 + K[Q ] (2) I

where I and I0 are the integrated emission intensities of the AnPy2+ component in the region of 370−525 nm in the presence and absence of the quencher (Ru2+), respectively. The relationship between the amount of adsorbed Ru2+ and the value of ηq is summarized in Table 1. The ηq value increased Table 1. Emission Quenching Efficiency (ηq) and Apparent Quenching Rate Constant (kq.app) of An-Py2+/Ru2+/SSA with Various Adsorbed Amount of Ru2+a adsorbed amount of Ru2+/% CEC

molar ratio of An-Py2+: Ru2+

ηq/%

kq.app × 1017/ L mol−1 s−1

2.5 5.0 6.3 12.5 18.8 25.0

10:1 5:1 4:1 2:1 1.3:1 1:1

63 85 87 97 98 99

0.4 0.6 0.6 1.2 1.8 2.1

K (3) τ 2+ where [Q] is the concentration of the quencher (Ru ). The value of kq.app is on the order of 1017 L mol−1 s−1, which is 107− 108 times higher than the diffusion rate constants of solutes in common solvents.34 These results indicate that even if the amount of the quencher is reduced by a factor of 108 the reaction proceeds with the same efficiency as in the homogeneous solution. These results expected that the efficient emission quenching caused by the two types of molecules (AnPy2+ and Ru2+) are both fixed on SSA surface, and the two types molecules take adjoined geometrical arrangement without forming aggregate on SSA surface. Such characteristic situation induces efficient emission quenching and the emission quenching mechanisms on SSA surface might be different for the homogeneous solutions. An increase in the phosphorescence quantum yield of Ru2+ (Φp) on the SSA surface26 was observed, i.e., Φp increased from kq.app =

[SSA] = 16 mg L−1, [An-Py2+] = 2 μmol L−1 (25% CEC), and [Ru2+] = 0−2 μmol L−1.

a

systematically with the adsorbed amount of Ru2+. The emission from An-Py2+ was quenched by Ru2+ with an efficiency of 85% at a relatively low adsorbed amount of Ru2+ (5.0% CEC, AnPy2+:Ru2+ = 5:1). This suggests that the light energies absorbed by five of the An-Py2+ molecules were accumulated in the one Ru2+ molecule. In earlier studies on Py4+/Ru2+/SSA systems, stoichiometric amounts of Ru2+ were required to achieve ca. 85% quenching of the emission of Py4+. Thus, the same quenching efficiency was shown for An-Py2+ as was found for Py4+ in earlier studies, but using one-fifth the amount of Ru2+.25,26 Furthermore, when the loading amount of Ru2+ reached 12.5−25.0% CEC (An-Py2+:Ru2+ = 2−1:1), nearC

DOI: 10.1021/acs.langmuir.7b00512 Langmuir XXXX, XXX, XXX−XXX

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Langmuir 0.084 ± 0.011 to 0.205 ± 0.08 in the absence and presence of An-Py2+ (25% CEC), respectively. Emission decay profile of Ru2+ emission in An-Py2+/Ru2+/SSA is shown in Figure S6. Excitation and monitoring wavelength were set at 376 and 615 nm, respectively. This allows nearly selective excitation of AnPy2+ and monitoring Ru2+ emission. Emission decay was well fitted with two components as follows: 79 ns (53%) and 503 ns (47%). The longer lifetime component is good agreed with emission lifetime of Ru2+ in homogeneous aqueous solution (590 ns).26 These results may indicate that energy transfer from excited An-Py2+ to Ru2+ proceeded on the SSA surface. However, the calculated energy transfer efficiency (ηET)35 of An-Py2+/Ru2+/SSA at a loading amount of Ru2+ of 25% CEC is 15−20%. This result implies that unexpected energy loss processes such as electron transfer may occur in the system. Further details will be reported in the near future.

an NADH model, 1-benzyl-1,4-dihydronicotinamide, with aromatic carbonyl compounds and comparison with thermal reactions. J. Org. Chem. 1987, 52, 2790−2796. (2) Clark, C. D.; Debad, J. D.; Yonemoto, E. H.; Mallouk, T. E.; Bard, A. J. Effect of Oxygen on Linked Ru(bpy)32+-Viologen Species and Methylviologen: A Reinterpretation of the Electrogenerated Chemiluminescence. J. Am. Chem. Soc. 1997, 119, 10525−10531. (3) Myahkostupov, M.; Piotrowiak, P.; Wang, D.; Galoppini, E. Ru(II)-bpy Complexes Bound to Nanocrystalline TiO2 Films through Phenyleneethynylene (OPE) Linkers: Effect of the Linkers Length on Electron Injection Rates. J. Phys. Chem. C 2007, 111, 2827−2829. (4) Duan, L.; Xu, Y.; Zhang, P.; Wang, M.; Sun, L. Visible LightDriven Water Oxidation by a Molecular Ruthenium Catalyst in Homogeneous System. Inorg. Chem. 2010, 49, 209−215. (5) Ozawa, H.; Sakai, K. Photo-hydrogen-evolving molecular devices driving visible-light-induced water reduction into molecular hydrogen: structure-activity relationship and reaction mechanism. Chem. Commun. 2011, 47, 2227−2242. (6) Li, F.; Jiang, Y.; Zhang, B.; Huang, F.; Gao, Y.; Sun, L. Towards A Solar Fuel Device: Light-Driven Water Oxidation Catalyzed by a Supramolecular Assembly. Angew. Chem., Int. Ed. 2012, 51, 2417− 2420. (7) Stoll, T.; Gennari, M.; Serrano, I.; Fortage, J.; Chauvin, J.; Odobel, F.; Rebarz, M.; Poizat, O.; Sliwa, M.; Deronzier, A.; Collomb, M.-N. [RhIII(dmbpy)2Cl2]+ as a Highly Efficient Catalyst for VisibleLight-Driven Hydrogen Production in Pure Water: Comparison with Other Rhodium Catalysts. Chem. - Eur. J. 2013, 19, 782−792. (8) Pullen, S.; Fei, H.; Orthaber, A.; Cohen, S. M.; Ott, S. Enhanced Photochemical Hydrogen Production by a Molecular Diiron Catalyst Incorporated into a Metal−Organic Framework. J. Am. Chem. Soc. 2013, 135, 16997−17003. (9) Yui, T.; Takeda, H.; Ueda, Y.; Sekizawa, K.; Koike, K.; Inagaki, S.; Ishitani, O. Hybridization between Periodic Mesoporous Organosilica and a Ru(II) Polypyridyl Complex with Phosphonic Acid Anchor Groups. ACS Appl. Mater. Interfaces 2014, 6, 1992−1998. (10) Maeda, K.; Sahara, G.; Eguchi, M.; Ishitani, O. Hybrids of a Ruthenium(II) Polypyridyl Complex and a Metal Oxide Nanosheet for Dye-Sensitized Hydrogen Evolution with Visible Light: Effects of the Energy Structure on Photocatalytic Activity. ACS Catal. 2015, 5, 1700−1707. (11) Sahara, G.; Ishitani, O. Efficient Photocatalysts for CO2 Reduction. Inorg. Chem. 2015, 54, 5096−5104. (12) Wang, L.; Mirmohades, M.; Brown, A.; Duan, L.; Li, F.; Daniel, Q.; Lomoth, R.; Sun, L.; Hammarström, L. Sensitizer-Catalyst Assemblies for Water Oxidation. Inorg. Chem. 2015, 54, 2742. (13) Sahara, G.; Kumagai, H.; Maeda, K.; Kaeffer, N.; Artero, V.; Higashi, M.; Abe, R.; Ishitani, O. Photoelectrochemical Reduction of CO2 Coupled to Water Oxidation Using a Photocathode with a Ru(II)−Re(I) Complex Photocatalyst and a CoOx/TaON Photoanode. J. Am. Chem. Soc. 2016, 138, 14152−14158. (14) Phuakkong, O.; Sentic, M.; Li, H.; Warakulwit, C.; Limtrakul, J.; Sojic, N.; Kuhn, A.; Ravaine, V.; Zigah, D. Wireless Synthesis and Activation of Electrochemiluminescent Thermoresponsive Janus Objects Using Bipolar Electrochemistry. Langmuir 2016, 32, 12995− 13002. (15) Adams, R. E.; Schmehl, R. H. Micellar Effects on Photoinduced Electron Transfer in Aqueous Solutions Revisited: Dramatic Enhancement of Cage Escape Yields in Surfactant Ru(II) Diimine Complex/ [Ru(NH3)6]2+ Systems. Langmuir 2016, 32, 8598−8607. (16) Farhangi, S.; Casier, R.; Li, L.; Thoma, J. L.; Duhamel, J. Characterization of the Long-Range Internal Dynamics of PyreneLabeled Macromolecules by Pyrene Excimer Fluorescence. Macromolecules 2016, 49, 9597−9604. (17) Panzuela, S.; Bernabei, M.; Velasco, E.; Delgado-Buscalioni, R.; Tarazona, P. A Novel Technique To Predict the Solubility of Planar Molecules. Energy Fuels 2016, 30, 10747−10757. (18) Sugiura, T.; Ikeda, K.; Nakano, M. Kinetic Analysis of the Methyl-β-cyclodextrin-Mediated Intervesicular Transfer of PyreneLabeled Phospholipids. Langmuir 2016, 32, 13697−13705.

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CONCLUSION A novel type of energy donor−acceptor system on a clay surface was prepared, and the energy transfer between a pyrene derivative (An-Py2+) and a ruthenium complex (Ru2+) on the clay surface was investigated. An obvious energy transfer was observed, and one Ru2+ molecule quenched the emission from five molecules of An-Py2+ with a value of ηq of 85%. Nearquantitative emission quenching was observed for stoichiometric amounts of Py4+ and Ru2+ on a clay surface. This efficient emission quenching may be caused by an increase in the overlap integral (J) in comparison to the earlier system. The estimated value of kq,app is approximately 1017 L mol−1 s−1, and thus the quenching rate constant is 107−108 times higher than the diffusion rate constant in a homogeneous solution. Such efficient photoreaction behavior on a clay surface may be employed to enable efficient photocatalytic reactions.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b00512. Absorption, emission, and excitation spectra and Stern− Volmer plot and decay profiles of An-Py2+/Ru2+/SSA systems. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shinsuke Takagi: 0000-0001-7013-4942 Tatsuto Yui: 0000-0002-7526-3521 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been partly supported by Uchida Energy Foundation, Suzuki-zaidan Research Foundation, and JSPS KAKENHI (Grant-in-Aid for Challenging Exploratory Research, No. 50362281), Japan.

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DOI: 10.1021/acs.langmuir.7b00512 Langmuir XXXX, XXX, XXX−XXX