Photocatalysts Based on Cobalt-Chelating Conjugated Polymers for

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Photocatalysts Based on Cobalt-chelating Conjugated Polymers for Hydrogen Evolution from Water Lianwei Li, Ryan G. Hadt, Shiyu Yao, Wai-Yip Lo, Zhengxu Cai, Qinghe Wu, Bill Pandit, Lin X. Chen, and Luping Yu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01477 • Publication Date (Web): 11 Jul 2016 Downloaded from http://pubs.acs.org on July 12, 2016

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Photocatalysts Based on Cobalt-chelating Conjugated Polymers for Hydrogen Evolution from Water Lianwei Li,1∆ Ryan G. Hadt,2∆ Shiyu Yao,2 Wai-Yip Lo,1 Zhengxu Cai,1 Qinghe Wu,1 Bill Pandit,3 Lin X. Chen2,3* and Luping Yu1* 1

Department of Chemistry and the James Franck Institute, The University of Chicago, 929 E 57th Street, Chicago, IL 60637, USA. 2

Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 S Cass Avenue, Lemont, IL 60439, USA.

3

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA.

Supporting Information Placeholder ABSTRACT: Developing photocatalytic systems for water splitting to generate oxygen and hydrogen is one of the biggest chemical challenges in solar energy utilization. In this work, we report the first example of heterogeneous photocatalysts for hydrogen evolution based on in-chain cobalt-chelating conjugated polymers. Two conjugated polymers chelated with earth abundant cobalt ions were synthesized and found to evolve hydrogen photocatalytically from water. These polymers are designed to combine functions of the conjugated backbone as light-harvesting antenna and electron transfer conduit with the in-chain bipyridyl chelated transition metal centers as catalytic active sites. In addition, these polymers are soluble in organic solvents, enabling effective interactions with the substrates as well as detailed characterization. We also found a polymer-dependent optimal cobalt chelating concentration at which the highest photocatalytic hydrogen production (PHP) activity can be achieved.

Introduction Various inorganic molecular- and semiconductor-based systems have been developed for photocatalytic hydrogen production (PHP) from water. Although some of these catalysts are robust and efficient, many suffer from limited tunability in their redox/catalytic properties as well as large band gaps.1-3 Organic semiconductors, such as conjugated polymers, can effectively harvest sunlight due to their chemically tunable energy levels and gaps. Upon light absorption, excitons are generated in the conjugated polymer backbone; these can be split into holes and electrons if a proper electron acceptor or donor is present (e.g., a fullerene derivative4). A single polymer chain can potentially accommodate multiple excitons and hence generate multiple holes and electrons, which could be utilized to drive catalytic reactions that require multiple redox equivalents, such as water splitting (e.g., 2H2O → 2H2 + O2). Additionally, low band gap charge transfer donor-acceptor polymers have been found to exhibit exciton splitting in solution phase even in the absence of fullerene derivatives, and they possess lower exciton binding energies than those of the homopolymers. These properties have been exploited in developing organic photovoltaic (OPV) devices.5 Here, we set out to combine the lightharvesting properties of OPV devices with photocatalytic processes involving redox reactions. Despite their attractive electro-optic properties, organic semiconductors remain relatively unexplored for PHP. So far, the most well-known all-organic system for photo-

catalytic water splitting is based on graphitic carbon nitride (g-C3N4),6-8 but there are very few examples of soft organic polymers that act as photocatalysts for PHP. To date, only some insoluble linear polymers based on poly(p-phenylene)9-11 and poly(2,2’-bipyridine)12,13, and some cross-linked polymers, such as poly(azomethines),14 poly(triazine)-,15 poly(heptazine)-,16,17 poly(hydrazine)-18 and poly(pyrene)-based19 networks, have been studied for PHP. One challenge in the previously reported systems is the insolubility of materials in organic and aqueous solvents, which limits applications and characterization. Additionally, the random spatial distribution of photodeposited noble metal nanoparticles can lead to poor control over their morphologies and relative positions with respect to the light-harvesting chromophore of the polymer, which limits the electronic communication between the randomly deposited nanoparticles and the polymer chains. In OPV devices, residual metals increase trapassisted recombination processes, which leads to poor device performance; however, for photocatalysis, the trap behavior of incorporated metal ions is beneficial for driving redox reactions. However, how to uniformly load the metal cocatalyst onto the semiconductor still needs to be considered. Along these lines, recent developments in polymeric photovoltaic solar cells has inspired us to integrate lightharvesting, charge-carrier transport, and catalytic active sites into soluble metal-chelating conjugated polymers for

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Results and discussion To investigate how the electronic properties of conjugated polymers affect their photocatalytic performances, both the electron donating block benzodithiophene (BDT) and electron accepting block perylene diimide (PDI) were copolymerized with the ligand block bipyridine (bpy) by palladium-catalyzed C-C coupling. These conjugated polymers are soluble in organic solvents and can coordinate with metal ions (vide infra). The BDT- and PDI-based polymers are referred to as PBDT-bpy and PPDI-bpy, respectively; their chemical structures are shown in Figure 1.

be monitored with UV-vis absorption spectroscopy (Figure 2). Upon Co(II) binding, the main optical absorption bands of PBDT-bpy undergo significant red shifts, and saturation is almost reached at a [CoCl2]/[bpy] ratio of 1.0/1.0 (Figure 2a). In direct contrast to PBDT-bpy, the lowest-energy transitions of PPDI-bpy do not change upon Co(II) titration (Figure 2b), rather, the higher-energy transitions (~400 nm) are red-shifted upon Co(II) binding. The UV-vis absorption spectra of the two polymers in the solid state (neat film) are similar as those in chloroform solution (Figure S6). These different spectral behaviors between the two polymers suggest the electronic structures of PBDT- and PPDI-bpy are quite different. The apparent charge transfer band upon Co(II) binding to bpy also shifts the absorption of the polymer due to stronger conjugation in PBDT-bpy unit. In contrast, Co(II) binding only changes bpy absorption while little change in polymer absorption takes place upon Co(II) binding. The latter case originates from the strongly twisted, nonplanar linkage between the bpy and PDI units. These results are complimented by electrochemistry and electronic structure calculations as presented below. Absorbance

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effective PHP. Photogenerated holes and electrons can be intercepted by metal catalytic centers and used directly as redox equivalents for catalytic reactions.20-23 This paper describes our recent development and investigation of the first set of soluble cobalt-chelating conjugated polymers, which are shown here to photocatalytically evolve hydrogen from water.

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Figure 1. Chemical structures of PBDT-bpy and PPDI-bpy conjugated polymers used for hydrogen generation.

Detailed synthetic procedures for the building blocks and copolymers are described in Supporting Information. Successful preparation of these polymers was confirmed by GPC, 1H-NMR (Figures S1 and S2), elemental analysis (Table S1), UV-vis (Figure S3) and FT-IR (Figure S4). GPC results show the number average molecular weights of PBDT-bpy and PPDI-bpy are 12200 and 23800 g/mol, respectively, and the corresponding molecular weight distribution index (Mw/Mn) is 1.84 and 2.46. 1H-NMR spectra (Figures S1 and S2) collected in chloroform-d show a general underestimation of the integrated areas of aromatic protons and broadened peak widths, indicating partial aggregation. Elemental analysis (Table S1) and FT-IR (Figure S4) support the structures as proposed. The thermogravimetric analysis (TGA) (Figure S5) indicates PBDT-bpy is less stable (< 300 ºC) than PPDI-bpy (< 400 ºC), possibly due to the existence of relatively weak carbon-sulfur bonds. Non-noble first-row transition metal cobalt was chosen here as a potential active site as it has been widely used in molecular PHP catalysts.1,24 Co(II) coordination to bpy can

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Figure 2. UV-vis spectra of polymers PBDT-bpy (a) and PPDI-bpy (b) titrated with CoCl2 (in EtOH) in chloroform. Fluorescence spectra of polymers PBDT-bpy (c) and PPDIbpy (d) titrated with CoCl2 (in EtOH) in chloroform.

In order to verify the influence of metal chelation on the exciton properties of the polymer, emission spectra were measured as a function of Co(II) concentration. Figure 2c shows the addition of Co(II) to PBDT-bpy solution effectively quenches the luminescence upon 450-nm excitation. A BDT/biphenyl-copolymer did not exhibit emission quenching or red-shifts in UV/vis absorption spectra upon addition of Co(II), confirming the quenching process involves intramolecular charge/energy transfer due to the formation of Co(II)-bpy moieties. Emission quenching was also observed for the acceptor PDI-based PPDI-bpy (Figure 2d). Transient absorption (TA) data for PBDT-bpy are given in Figure S7. Excitation of cobalt-free PBDT-bpy (Figure S7a) at 491 nm results in a strong ground state bleach (GSB) at ~480 nm and a broad excited state absorption (ESA) centered at ~600 nm (Figure S7a). Stimulated emission (SE) is observed at ~550 nm and appears as a dip on

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the ESA. Upon addition of Co(Il) (~25% in Figure S7b), the SE is quenched, which is consistent with the fluorescence quenching results above and confirms a charge/energy transfer process that occurs upon Co(II) chelation. Kinetics traces at 477 (GSB) and 585 (ESA) nm were fit using three exponential functions and a constant offset step function (A∞) to account for a fraction of longlived species (Tables S2 and S4). Upon metal titration, the three time constants for the GSB remain similar, but A∞ decreases (~15 to 10 % for 0 to 25 % Co(II)). The same is observed for the kinetics fits for the ESA (A∞ ~16 to 9 %; Table S3). However, also for the ESA, the weight of ultrafast τ1 decay component increases in magnitude upon metal titration (~29 to 40 %). Together, the fluorescence and TA data are generally consistent with the quenching of excited states formed on the polymer backbone through charge/energy transfer processes involving Co(II)-bpy moieties. Detailed multi-wavelength, timeresolved spectroscopic study of both polymers under catalytic conditions is ongoing. LUMO

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gy level of PPDI-bpy, however, does not exhibit a metal dependence. These observations indicate the HOMO is mainly localized on the BDT and PDI units of the respective polymers. Additionally, the LUMO of PBDT-bpy must have partial bpy character, as its energy is stabilized upon binding the cationic metal ion. Conversely, the LUMO of BPDI-bpy must be localized on PDI. Moreover, the electrochemically determined band gaps agree well with the optical gaps (Figure 2). The above results are fully reproduced using density functional theory (DFT) and timedependent DFT (TD-DFT) calculations (vide infra). From their LUMO levels, both PBDT-bpy and PPDI-bpy have enough driving force for proton reduction. We note, however, that no obvious electrochemical response has been observed for Co(II)-based reduction processes. This may be due to the large polymer-based currents observed in the CVs (Figures S8 and S9). PHP activity for the polymers was investigated using a 30/70 vol% diethylamine (DEA)/water mixture illuminated under illumination of a 450-W Xenon lamp (Figure S11). Control experiments in the dark showed no hydrogen production, and cobalt-free polymers generated only trace amount of hydrogen under the same illumination condition (Table S5), which confirmed catalysis took place at cobalt-bound active sites. (a)

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[CoCl2]/[bpy] Figure 3. Electrochemical bandgap diagram. HOMO and LUMO energy levels of PDBT-bpy and PPDI-bpy titrated with different contents of CoCl2 were obtained from the oxidation and reduction onset in cyclic voltammetry curves (Figures S8 and S9), where SA represents the sacrificial agent diethylamine.

The electrochemical properties of the polymers with and without Co(II) were investigated using cyclic voltammetry (CV) (Table S4 and Figures S8-S10). The cyclic voltammograms exhibit an irreversible oxidation process for PBDT-bpy and PPDI-bpy in either solid or solution state, while PPDI-bpy presents a quasireversible process for reduction. The corresponding HOMO (oxidation potential) and LUMO (reduction potential) energy levels of PBDT-bpy and PPDI-bpy are summarized in Figure 3. Interestingly, the HOMO energy levels of both PBDT-bpy and PPDI-bpy remain relatively constant (-5.20 eV and -6.0 eV) upon addition of Co(II); however, for PBDT-bpy, the LUMO energy level is strongly dependent on the amount of Co(II) chelation, varying from ~-2.90 to ~-3.70 eV (Figure 3, top). The LUMO ener-

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Figure 4. (a), Time course of produced H2 for polymer PBDT-bpy with different Cobalt contents. (b), Time course of H2 production from water for polymer PBDT-bpy with a [CoCl2]/[bpy] = 0.1. (c), [CoCl2]/[bpy] dependence of photocatalytic hydrogen production rate from water. (d), Wavelength-dependent AQE of water splitting for polymers PBDTbpy and PPDI-bpy. Experimental conditions.

The dependence of PHP activity on Co concentration for polymer PBDT-bpy is shown in Figure 4a. The highest hydrogen evolution rate (0.28 μmol/h) was achieved after only 10 - 20% Co loading, and further increase in [Co(II)] resulted in significantly decreased rates of hydrogen production. The system is robust, with hydrogen being steadily produced over a period of 27 h (Figure 4b). Figure 4c summarizes the dependence of hydrogen generation rates on Co(II) concentration for both PBDT-bpy and PPDIbpy. Interestingly, PPDI-bpy displayed a different rate dependence on Co(II) concentration than PBDT-bpy, with an optimal hydrogen evolution rate of 0.71 μmol/h at

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significantly higher Co(II) loading levels (maximum at ~60%). The difference of the photocatalytic behaviors for PBDT-bpy and PBDT-bpy is also reflected in the wavelength-dependent apparent quantum yields (QYs). PPDIbpy shows a QY profile that closely tracks the UV-vis absorption spectrum, and thus a good photocatalytic activity under visible light excitation (400 – 600 nm) (Figure 4d). In contrast, the donor-BDT-based PBDT-bpy exhibits very limited hydrogen evolution at longer wavelengths and a rapid increase in activity for wavelengths < 400 nm (Figure 4d). The stability of Co-chelation was also investigated by combining ICP-MS and FTIR measurements before and after the reaction. For both PBDT-bpy and PPDI-bpy, nearly 100% of Co was retained with [CoCl2]/[bpy] ratios < 60%, while ~20% of Co was lost when the [CoCl2]/[bpy] ratio reached 0.9 (Figure S12), which implies all Co remains bound before and after reaction under ideal Co concentrations. The stability of these polymeric catalysts is also reflected in the unchanged FTIR spectra after the photocatalytic reaction (Figure S13). The optimal value of cobalt content for PHP can be influenced by several factors, likely reflecting a counterbalance between the effective rates of charge transfer to and hydrogen evolution at the catalytic centers. The aggregation state of the heterogeneous polymer will influence the proximity between the metal centers and polymer-based ET conduits, which will also affect the overall performance. Lastly, increasing the density of metal centers can bring about detrimental effects, as metal centers in close proximity may act as charge recombination sites. These different metal dependencies and differences in QY profiles (Figure 4d) suggest PBDT- and PPDI-bpy likely operate by two different underlying photochemical mechanisms. The QY profile of PBDT-bpy in particular suggests the nature of the excited state may play a strong role in driving photocatalysis. That is, exitonic excited states with greater charge-transfer character may lead to better charge separation and thus better catalysis.25,26 This is further supported below with TD-DFT calculations. The hydrogen generation results also show no positive correlation between the reduction potential (or driving force) and performance. Namely, PPDI-bpy has a lower driving force (LUMO ~ 0.6 V vs. NHE) but exhibits better performance; however, the hydrogen evolution halfreaction can be achieved using semiconductors with small driving forces (0.1 – 0.3 V, V vs. NHE),2,27 which is also the case for the well-known g-C3N4 based materials. Thus, other properties, such as charge separation efficiency, charge mobility, and interfacial properties (morphology and wettability), of these photocatalysts are likely to dominate the performance here. Dynamic light scattering (DLS) was first used to gain insight into the aggregation state of the polymers in chloroform. The average hydrodynamic radius of PBDT-bpy increases from 19.0 nm to 31.0 nm, and the scattering intensity increases by 2.5 times when the ratio of [CoCl2]/[bpy] increases from 0 to 1 (Figure S14). This indi-

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cates that cobalt-linked interchain aggregates may already form in the solution state with Co: bpy ratios < 1; however, the exact molar ratio of different cobalt species such as Co(bpy) and Co(bpy)2 is not distinguishable at this time. Note that for polymer PBDT-bpy, there is a well-defined isosbestic point in the titration curves (Figure 2A). The optical density (OD) vs cobalt ion concentration plots monitored at 451 nm and 540 nm are well-correlated with similar slopes (Figure S15). This result suggests the main species in chloroform (dilute solution) is Co(bpy) based, in accord with our previous study on the photophysical properties of PPV-bpy metal containing polymers.28 Note that, in the absence of water or hydroxide, chloride ions from the metal salt can complete the cobalt coordination sphere. However, during the drying process for photocatalytic experiment, the solution concentration changes dramatically, which may also change the metalligand binding behavior due to the increased polymer/ligand concentration and shortened interchain distance. Therefore, the possibility of forming multiple bpy binding species Co(bpy)n (n =2 or 3) in the final solid state, especially at low cobalt content (Co/bpy < 0.5), cannot be excluded at this time. Contact angle (CA) measurements were further carried out to investigate the surface wettability of polymer films. The results show a monotonic increase in CA values as Co content increases (Figure S16), indicating enhanced surface hydrophobicity. The result suggests that intra- or inter-molecular cross-lined Co(bpy)x species can potentially enhance the surface hydrophobicity of the films. PBDT-bpy presented smaller CA values due to the hydrophilic nature of the TEG side chain. It is noted that the increase in surface hydrophobicity may result in a decrease in the water accessibility and catalytic performance of polymer photocatalysts, which partially explains why there exists an optimal cobalt concentration for a given polymer. As a control, PBDT-bpy-C10 with hydrophobic linear decyl side chains was also prepared and tested for PHP. This reference polymer presents similar optical (Figure S17) and electrochemical (Figure S18) properties as PBDTbpy-TEG. However, PBDT-bpy-C10 does not suspend well in a DEA/water solution (3/7, v/v) due to the increased hydrophobicity of the side-chains (Figures S19 and S20). Nevertheless, a comparison between the photocatalytic behaviors of PBDT-bpy-TEG and PBDT-bpy-C10 was carried out in DEA/water solution (7/3, v/v), and the two polymers show similar Co content-dependent hydrogen evolution curves (Figure S21 and Table S6), indicating the performance is largely dominated by the chemical nature of the conjugated backbone when the polymer is fully wetted. To correlate the experimentally observed PHP activity and electronic structure, DFT and TD-DFT calculations were carried out for both PPDI- and PBDT-bpy. The results are given in Supporting Information along with detailed molecular orbital analyses (Figures S22-S24 and Tables S7-S12). As found experimentally, the HOMO and LUMO levels of PPDI-bpy are both PDI-based, thus giving rise to PDI-based excited states for the lower-energy tran-

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sitions of both metal-free and Co(II)-bound forms. In contrast, the LUMO level of PBDT-bpy is delocalized over the BDT-bpy backbone, reflecting strong π-conjugation. Co(II) binding in PBDT-bpy gives rise to partial localization and a stabilization of the LUMO (Figure S22 and Tables S7/S9). The lowest energy absorption band of PBDTbpy (~500 nm) is calculated as the HOMO →LUMO transition and also red shifts upon Co(II) binding (Figure S23). The experimental data show what appears to be vibronic structure on this electronic transition (vibrational spacing of ~1500 cm-1, Figure S23). Importantly, for PBDT-bpy, transitions above the HOMO/LUMO energy gap are calculated to involve successively more localized donor and acceptor orbitals (Supporting Information Tables S8-S12). This can potentially give rise to excited states with increasing charge-transfer character with increasing energy, which can possibly result in a higher quantum yield at ~400 nm for PBDT-bpy.25,26 Thus, the good agreement between experiment and theory indicates that PPDI- and PBDT-bpy have different electronic structures due to differences in π-conjugation, and this may influence photocatalysis. Given these considerations in electronic structures, we can hypothesize that the mechanisms of H2 generation from these two polymers are different. For PBDT-bpy, the BDT-based HOMO and bpy-Co-based LUMO would favor a mechanism whereby photoinduced electron transfer from BDT to Co(bpy)x can potentially occur to form Co(I), an intermediate shown to be involved in molecular Cobased electrocatalysis.29,30 Conversely, for PPDI-bpy, the PDI-based HOMO and LUMO will result in PDI-based excited states, which is consistent with PDI being traditionally a photooxidant. This favors a reductive quenching of photo-excited PDI by the sacrificial reductant, followed by subsequent ET to Co(II) centers, which would also result in Co(I) formation. The hypothesized mechanisms are currently being investigated using transient spectroscopies and will be discussed in due time.

Conclusions In summary, two conjugated polymers chelated with earth abundant cobalt ions were synthesized and found to evolve hydrogen photocatalytically from water. These polymers exhibit good solubility in organic solvents, which facilitated detailed characterization. Different polymers require different optimal cobalt loadings and solvent compositions to achieve optimized PHP activity. The difference in the critical cobalt site density likely reflects the complex interplay and counterbalance between lightharvesting and the charge transfer and reaction rates at the catalytic center, which can also be influenced by the aggregation state of the heterogeneous polymer. Although the overall quantum efficiency for hydrogen evolution is still low, results reported here clearly indicate metal-chelating conjugated polymers represent a rich opportunity for further rational molecular design of photocatalysts for hydrogen evolution from water.

ASSOCIATED CONTENT

Supporting Information. Full experimental methods, additional characterization, and supporting Figures S1-S27 and Tables S1-S12 as described in the main text. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected] [email protected] [email protected]

AUTHOR CONTRIBUTIONS ∆

Lianwei Li and Ryan G. Hadt contributed equally to this work.

ACKNOWLEDGMENT This work is supported by National Science Foundation (DMR-1263006, LPY) and by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, through Argonne National Laboratory under Contract No. DE-AC0206CH11357 through a collaboration with the University of Chicago (UC) and Northwestern University (NU). This work also benefited from NSF MRSEC at the University of Chicago. The contribution from NU was supported by National Science Foundation (DMR-1230217).

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For Graphical Abstract Use Only Photocatalysts Based on Cobalt-chelating Conjugated Polymers for Hydrogen Evolution from Water Lianwei Li,1 Ryan G. Hadt,2 Shiyu Yao,2 Wai-Yip Lo,1 Zhengxu Cai,1 Qinghe Wu,1 Bill Pandit,3 Lin X. Chen2,3* and Luping Yu1*

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