Subscriber access provided by Caltech Library Communication Improving Photocatalysis for the Reduction of CO2 through non-Covalent Supramolecular Assembly Po Ling Cheung, Savannah C. Kapper, Tian Zeng, Mark E. Thompson, and Clifford P. Kubiak J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 06 Sep 2019 Downloaded from pubs.acs.org on September 6, 2019 Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. 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However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties. Page 1 of 5 Journal of the American Chemical Society 1 2 3 4 5 6 7 Improving Photocatalysis for the Reduction of CO2 Through Non- 8 Covalent Supramolecular Assembly 9 10 Po Ling Cheung†, Savannah C. Kapper‡, Tian Zeng†⊥, Mark E. Thompson‡, Clifford P. Kubiak†* 11 12 † Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 13 92093-0358, United States 14 ‡ Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States 15 16 17 Supporting Information Placeholder 18 19 ABSTRACT: We report the enhancement of photocatalytic per- and the catalyst form hydrogen bonding pairs, to determine the ef- 20 formance by introduction of hydrogen bonding interactions to a Re fects of non-covalent interactions on photocatalytic performance. 21 bipyridine catalyst and Ru photosensitizer system (ReDAC/Ru- 22 DAC) by the addition of amide substituents, with carbon monoxide The system composed of Ru(dac)(bpy)2(PF6)2 (RuDAC) (4,4’- 23 (CO) and carbonate/bicarbonate as products. This system demon- bis(methyl acetamidomethyl)-2,2’-bipyridine = dac, 2,2’-bipyri- strates a more-than-threefold increase in turnover number (TONCO dine = bpy) as the photosensitizer and Re(dac)(CO)3Cl (ReDAC) 24 = 100 ± 4) and quantum yield (ΦCO = 23.3 ± 0.8 %) for CO for- as the catalyst was chosen for this study (Figure 1). A previous 25 mation compared to the control system using unsubstituted Ru pho- electrochemical study of ReDAC indicated the formation of a hy- 20 26 tosensitizer (RuBPY) and ReDAC (TONCO = 28 ± 4 and ΦCO = 7 ± drogen-bonded Re−Re dimer in acetonitrile (MeCN) . In order to 27 1 %) in acetonitrile (MeCN) with 1,3-dimethyl-2-phenyl-2,3-dihy- fairly assess the effects of hydrogen bonding interactions on pho- 28 dro-1H-benzo[d]imidazole (BIH) as sacrificial reductant. In dime- tocatalysis, a control system composed of Ru(bpy)3(PF6)2 (RuBPY) 29 thylformamide (DMF), a solvent that disrupts hydrogen bonds, the and ReDAC was used for comparison (Figure 1). RuBPY has sim- ReDAC/RuDAC system showed a decrease in catalytic perfor- ilar reduction potentials, UV-vis absorption spectrum, and excited 30 mance while the control system exhibited an increase, indicating state lifetime as RuDAC. 31 the role of hydrogen bonding in enhancing the photocatalysis for 32 CO2 reduction through supramolecular assembly. The similar prop- 33 erties of RuDAC and RuBPY demonstrated in lifetime measure- 34 ments, spectroscopic analysis, electrochemical and spectroelectro- 35 chemical studies revealed that the enhancement in photocatalysis is not due to differences in intrinsic properties of the catalyst or pho- 36 tosensitizer, but to hydrogen bonding interactions between them. 37 38 39 40 The efficient storage of solar energy into liquid fuels is a major 41 challenge of renewable energy today. To mimic natural photosyn- Scheme 1. Experimental and control systems for investigation of 42 thesis, photocatalytic reduction of CO2 has been extensively inves- hydrogen bonding effects on photocatalysis. tigated using transition metal complexes that can act as redox pho- 43 tosensitizers (PS) by transferring electrons upon photon excitation 20 21 44 ReDAC and BIH were synthesized as reported and RuDAC and as catalysts (Cat) by accepting electrons and reducing CO2. Re was synthesized by adapting methods in the literature22. In a typical 45 bipyridine catalysts have been shown to be excellent electrocata- mixed system with the Ru photosensitizer and catalyst as separate 1-3 4, 5 46 lysts and photocatalysts when coupled with photosensitizers. components, the Ru photosensitizer (Ru2+) is first excited by light 47 Since 2005, the Ishitani group has studied extensively supramolec- and then reduced by a sacrificial reductant (D), followed by elec- 48 ular photocatalysts by linking different photosensitizers and Re cat- tron transfer from the singly reduced Ru photosensitizer (Ru•+) to alysts using short, covalent bridging ligands6-14. The catalytic per- 23, 24 49 the catalyst (Scheme 2). The latter step (eq. 2 in Scheme 2) is formance of these systems is greatly improved compared to the sep- •+ 50 driven by the difference between the redox potential of Ru and arated systems due to the acceleration and higher yield of intramo- catalyst. To better demonstrate the impact of hydrogen bonding on 51 lecular electron transfer between the two components, with the rate photocatalysis, the first reduction potential of the photosensitizer 9 −1 7 52 of electron transfer as fast as kET = (1.4 ± 0.1) × 10 s . Recently, should be comparable to that of the catalyst so that there is a low 53 our laboratory estimated that the rates of electron transfer (kET) be- driving force for electron transfer. In the previous study, ReDAC 54 tween Ru3O clusters linked by hydrogen bonded pyrimidinones was reported to display its first reduction at −1.77 V (vs. Fc+/0)20. 11 −1 15 55 were on the order of 10 s . There are several reports of the Cyclic voltammograms (CVs) of RuDAC, RuBPY and influence of second coordination sphere effects, such as hydrogen Ru(dmb)3(PF6)2 (RuDMB, where 4,4’-dimethyl-2,2’-bipyridine = 56 16-19 bonding interactions, on catalysis . Consequently, in this work, dmb) were taken under Ar in dry MeCN solutions containing 0.1 57 we investigate a bimolecular system, in which the photosensitizer M tetrabutylammonium hexafluorophosphate (TBAPF6). Within 58 experimental error, the CVs of RuDAC and RuBPY display first 59 60 ACS Paragon Plus Environment Journal of the American Chemical Society Page 2 of 5 reduction potentials at −1.76 V, making them excellent trial sys- solvents. The higher activity of the RuDAC/ReDAC system com- 1 tems comparing hydrogen bonding effects in this study. Another pared to the control system in DMF is unexpected, but it is likely 2 commonly used photosensitizer, RuDMB, has a more negative re- due to increased contact surface area between photosensitizer and 25 3 duction potential at −1.88 V. The increased driving force for elec- catalyst . The reason for higher activity of RuBPY/ReDAC in tron transfer could mask hydrogen bonding effects. Therefore, DMF compared to MeCN remains unclear, but previous studies 4 RuBPY was chosen to be the control for comparison of catalytic show that solvents play important roles in controlling catalytic rates 5 performance without a hydrogen bonding interaction. and selectivity23, 26-30. Since the RuDAC/ReDAC system behaves 6 in a trend opposite to the control, it is reasonable to conclude that 7 hydrogen bonding is the dominant factor influencing differences in 8 performance. 9 10 In addition to gaseous products, the liquid mixtures were ana- lyzed. No formate is detected with 1H NMR (Table S1 and S2) but 11 [2]− appreciable [H]CO3 are observed with IR. It is known that Re- - 12 DAC undergoes the disproportionation mechanism of 2CO2 + 2e 2− 20 13 ⇋ CO and CO3 in electrocatalysis . When a proton source is not 14 used in the photocatalytic experiments, bands are observed in the −1 [2]− 15 IR at 1672 and 1637 cm , consistent with formation of [H]CO3 13 16 species (Figure S5). Repeating these experiments with CO2 showed shifts in these bands (1625 and 1598 cm−1), consistent with 17 20, 31 isotopic labeling . Control studies under Ar reveal no CO or H2 18 production upon irradiation (Table S2). In addition, control reac- 19 tions without ReDAC reveal that RuDAC is a better catalyst than RuBPY in both solvents and it performed better in MeCN than in 20 DMF. These observations are similar to the trend seen in the Ru- 21 Figure 1. Cyclic voltammograms of 1 mM of RuDAC (red), 22 DAC/ReDAC system, but the absence of the Re catalyst signifi- RuBPY (blue) and RuDMB (black) in dry MeCN containing 0.1 cantly slows down the rate of CO formation (Table S2). 23 M TBAPF6 at scan rate 100 mV/s under Ar.