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Chemistry of Singlet Oxygen with a Cadmium-Sulfur Cluster: Physical Quenching vs. Photooxidation David A. Cagan, Arman C. Garcia, Kin Li, David Ashen-Garry, Abegail Cardenas Tadle, Dong Zhang, Katherine Nelms, Yangyang Liu, Jeffrey R Shallenberger, Joshua J. Stapleton, and Matthias Selke J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b10516 • Publication Date (Web): 21 Dec 2018 Downloaded from http://pubs.acs.org on December 22, 2018
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Journal of the American Chemical Society
Chemistry of Singlet Oxygen with a Cadmium-Sulfur Cluster: Physical Quenching vs. Photooxidation David A. Cagan†, Arman C. Garcia†, Kin Li†, David Ashen-Garry†, Abegail C. Tadle†, Dong Zhang†, Katherine Nelms†, Yangyang Liu†, Jeffrey R. Shallenberger‡, Joshua J. Stapleton‡, Matthias Selke*† †Department ‡
of Chemistry and Biochemistry, California State University, Los Angeles Materials Characterization Laboratory, The Pennsylvania State University, University Park PA
Supporting Information Placeholder ABSTRACT: We investigated the chemistry of singlet oxygen with a cadmium-sulfur cluster, (Me4N)2[Cd4(SPh)10]. This cluster was used as a model for cadmium-sulfur nanoparticles. Such nanoparticles are often used in conjunction with photosensitizers (for singlet oxygen generation or dye-sensitized solar cells), and hence it is important to determine if cadmium-sulfur moieties physically quench and/or chemically react with singlet oxygen. We found that (Me4N)2[Cd4(SPh)10] is indeed a very strong quencher of singlet oxygen with total rate constants for 1O2 removal of 5.8±1.3x108 M-1sec-1 in acetonitrile and 1.2±0.5x108 M-1sec-1 in CD3OD. Physical quenching predominates, but chemical reaction leading to decomposition of the cluster and formation of sulfinate is also significant, with a rate constant of 4.1±0.6x106 M-1sec-1 in methanol. Commercially available cadmium-sulfur quantum dots (“lumidots”) show similar singlet oxygen quenching rate constants, based on the molar concentration of the QDs.
Thiolate ligands are commonly used as surface capping agents for various nanomaterials including cadmium chalcogenide nanoparticles such as CdS, CdSe and CdTe quantum dots (QDs).18 However, such thiolate ligands may undergo several different reactions leading either to ligand dissociation (for example, via protonation of the thiolate moiety)9 or ligand oxidation, either by a Type I (radical) or Type II (energy transfer/singlet molecular oxygen, 1O2) process.10,11 For example, for CdS nanorods with mercaptopropionic acid (MPA) surface capping ligands, there is a decrease in photocatalytic H2 production after several hours of irradiation which was attributed to photooxidation of the MPA surface capping ligand.12 Photodegradation of nanocrystal QDs through photoexcitation of solvent – oxygen ion pairs producing 1O has also been reported.13 Several groups hypothesized that 2 CdTe or CdSe quantum dots coated with thiolate ligands undergo photooxidative damage, possibly via a Type II singlet oxygen mechanism, leading to a blue shift followed by photobleaching and possible destruction of the nanoparticle.14-16 However, no kinetic parameters or photoproducts are known for these systems, and experimental evidence as to whether Type I or Type II mechanisms (or both) are operative in the above examples is scant. In fact, in the absence of any kinetic data on the interaction of 1O2 with these nanoparticles, it is impossible to assess whether or not degradation of QDs by 1O2 is a significant factor as far as the stability of these nanoparticles is concerned. While numerous silicon and gold nanostructures produce 1O2 (via a Dexter electron exchange/tunneling mechanism for silicon, and intermediate formation of superoxide anion in the case of
gold)17-19, singlet oxygen production from cadmium-based QDs has only been observed in a few cases, and the reported quantum yields of 1O2 production are very low, ranging from 0.01 to 0.05 for CdSe quantum dots.20-23 However, there are numerous examples in which sensitizers are attached to QDs, either for 1O2 production for possible use in photodynamic therapy24 or for electron transfer,25 with possible applications for dye-sensitized solar cells (DSSCs).2527 In some cases, the sensitizers are covalently attached to thiolate ligands. Singlet oxygen produced by excitation of sensitizers attached to nanoparticles could subsequently interact with thiolate moieties, leading to degradation of the nanoparticle and/or removal of 1O2 by a physical quenching mechanism without chemical modification or degradation of the QD. Singlet oxygen may either chemically react (kr) and/or be physically quenched (kq) by a substrate without chemical transformation of that substrate. In the case of cadmium-sulfur nanostructures for applications in DSSCs, the former pathway could lead to degradation of the QDs, while the latter pathway could be desirable, as a potentially harmful oxidant – singlet oxygen - would be removed. There are relatively few examples of reactions and/or physical quenching of 1O2 by metal thiolates scattered throughout the literature.29 In most cases, the values of the rate constants kr and kq are unknown, although for some Ni thiolato complexes, physical quenching near the diffusion-controlled rate limit has been observed.30 A few Pt, Ru, Co and Zn complexes with either aliphatic or aromatic thiolato ligands are known to undergo both chemical reaction with and physical quenching of 1O2, with the total rate constant for 1O2 removal, kT (kT=kr+kq), typically being in the range of 107-108 M-1sec-1.31-35 To the best of our knowledge, there have been no reports of kinetic data and/or products for reactions of 1O2 with cadmium-sulfur complexes, clusters, or nanostructures. Understanding the reactivity of 1O2 with cadmium-sulfur moieties is of critical importance to evaluate the stability of cadmium-sulfur surfaces and to rationally predict the photophysical and photochemical behavior of systems in which photosensitizers are attached to such surfaces. We began our investigation of the kinetics of the interactions of 1O with cadmium-sulfur structures by determining the magnitude 2 by which simple, commercially available cadmium-sulfur quantum dots (Lumidot CdS, Sigma-Aldrich) remove singlet oxygen. These QDs do not have an additional surface-capping ligand, and their surface consists of cadmium-sulfide moieties. We conducted timeresolved measurements of the 1O2 near-infrared (NIR) emission signal in the presence of a toluene solution of 5 mg/ml CdS (QDs diameter 1.8-2.3 nm) with the first absorption peak at 372 nm. Singlet oxygen was produced by flash excitation (ex=532 nm) of toluene solutions containing tetraphenylporphine (99+%, Sigma-
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Journal of the American Chemical Society Aldrich) in the presence of various concentrations of the CdS QD. The total rate constant (i.e. the sum of kr and kq) was determined based on the formula weight of CdS (144.48 g/mol). The kT value thus obtained – based on the number of CdS units – was 1.8±0.7x106 M-1sec-1 (Figure 1a). Using empirical methodology developed by Peng et al.36 yields an estimate of the extinction coefficient of ~190,000 cm-1M-1. Based on measuring the absorption of the CdS QD solution in toluene at the first excitonic peak, we obtained a value of ~500 CdS units per nanoparticle. Hence the kT value based on the molar concentration of the QDs is approximately 8.8±3.3x108 M-1sec-1. This large rate constant prompted us to conduct a more general investigation of the interaction of 1O2 with cadmium-sulfur moieties. In particular, we sought to determine whether or not physical quenching or chemical reaction predominates – with obvious implications for the stability and behavior of cadmium nanoparticles to which photosensitizers have been attached. Since sulfur-coated cadmium nanostructures are not homogenous37, we instead chose to investigate the chemistry of singlet oxygen with a clearly defined cadmium-sulfur cluster, allowing for a more rigorous determination of the rate constants kr and kq. We chose the cluster (Me4N)2[Cd4(SPh)10] (1)38 as a model system; unlike the mononuclear complex (Me4N)2[Cd(SPh)4], cluster 1 does not undergo ligand dissociation in solution and is unreactive with ground state (triplet) dioxygen.
Bengal (cut-off filter at 496 nm so that 1 was not excited), the characteristic UV/vis peak of 1 at 265 nm disappeared over a period of 2 hours (Figure S2).41 Disappearance of the cluster was followed both spectrophotometrically and by monitoring the 113Cd NMR signal of 1 (583 ppm, Cd(NO3)2 external reference). The 113Cd NMR peak completely disappeared (Figures S1/S3) concomitant with formation of an insoluble oligomeric precipitate. Due to the complete insolubility of this precipitate in any solvent, we were unable to obtain crystals suitable for single-crystal x-ray diffraction. Instead, we characterized this material by XPS (Figure S4) which (i) revealed the presence of sulfinate groups in the precipitate, and (ii) demonstrated the absence of the tetramethylammonium counterions in the photooxidation product, which implies that the material is not a small anionic cluster. PXRD analyses (Figure S5) of the product is also consistent with a semiamorphous material. Solid state FTIR analyses of the material indicated two very strong peaks at 984 and 942 cm-1 which are not present in the starting cluster 1 (Figure S6). These absorbances are consistent with an O,O-bound sulfinate.42-45 Thus, characterization of the precipitate indicates that some fraction of the interaction of 1O with the model cluster 1 is due to chemical quenching. In 2 contrast, the CdS Lumidots showed no chemical reactivity upon exposure to 1O2 over a period of several hours. We hypothesize that the reaction of 1 with singlet oxygen is initiated by electrophilic attack of 1O2 on the non-bridging Cd-S-Ph moieties, as these groups are sterically most exposed, and have the most electron-rich sulfur atoms; electron-donating groups are known to increase reactivity of 1O2 with phenyl sulfides.35,46
6.10E+04 5.80E+04
Scheme 1. Chemical reaction (kr) and physical quenching (kq) of singlet oxygen by cluster 1. We initially determined the total rate constant of 1O2 removal (kT) by cluster (Me4N)2[Cd4(SPh)10] (1) via time-resolved singlet oxygen luminescence quenching experiments. Singlet oxygen was produced by flash excitation of solutions of Rose Bengal in the presence of various concentrations of cluster 1 (ex=532 nm). Cluster 1 does not absorb at this wavelength, and no 1O2 was detected when a solution of 1 without the sensitizer was flashexcited. We found that 1 removes 1O2 with very large rate constants, namely kT=5.8±1.3x108 M-1sec-1 in CD3CN or CH3CN (no difference was observed for deuterated or non-deuterated acetonitrile) and 1.2±0.5 x 108 M-1sec-1 in CD3OD (Figure 1); the very short lifetime of 1O2 in CH3OH39 precluded measurements in that solvent. Addition of 10% D2O to the CD3OD solution did not affect the kT value. The somewhat lower kT value in a protic environment is very similar to what has been observed for cobalt thiolato complexes; the effect has been attributed to hydrogen bonding to the thiolate and a concomitant decrease in its nucleophilicity.40 The contribution of the cluster’s counterion to the quenching process is insignificant. In control experiments, we measured the kT value of tetramethylammonium chloride to be just 2.5x106 M-1sec-1 in CD3CN, or about two orders of magnitude below the value of cluster 1, and just 2.9x105 M-1 sec-1 for the free thiol ligand (thiophenol). We then investigated if at least part of the very large kT value is due to chemical reaction of 1O2 with the cluster. We again used an external photosensitizer (Rose Bengal) to produce 1O2 in the presence of the cluster under continuous irradiation. When a solution of 1 ([1]=10-6 M) was irradiated in the presence of Rose
kobs (M-1 s-1)
5.50E+04 5.20E+04 4.90E+04 4.60E+04 4.30E+04 4.00E+04 0.00E+00
5.00E-03
[CdS]
4.80E+04
4.00E+04
kobs (M-1 s-1)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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3.20E+04
2.40E+04
1.60E+04
8.00E+03
0.00E+00 0.00E+00
[(Me4N)2[Cd4(SPh)10)]]
1.00E-04
Figure 1. Top: Singlet oxygen luminescence quenching by cadmium-sulfur nanoparticles in toluene. Concentrations are based on the formula weight of the QD (144.48 g/mol). Bottom: Singlet oxygen luminescence quenching by cluster 1 in CD3CN (triangles) and CD3OD (circles).
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Journal of the American Chemical Society In order to evaluate the relative contribution of chemical reaction vs. physical quenching of 1O2 by 1, we conducted competition experiments using a singlet oxygen acceptor with a known value of kr, namely 1,5-dihydroxy naphthalene (2). Compound 2 reacts with singlet oxygen to form the quinone 5-hydroxy-1,4-naphthalene dione (3), with a kr value of 1.5x106 M-1sec-1 in methanol.47-48 Using the methodology originally developed by Foote and Higgins,49 methanol solutions containing Rose Bengal, cluster 1, and the dihydroxy naphthalene 2 were irradiated under steady state conditions (a cut-off filter at 496 nm was used so that only Rose Bengal was irradiated) such that the substrates competed for a common intermediate (i.e., 1O2). Loss of 1 was monitored spectrophotometrically at 277 nm (where the UV/vis spectrum obtained for the transformation of the acceptor 2 to the quinone has an isosbestic point), while appearance of the photooxidation product 3 was monitored at 427 nm where cluster 1 does not absorb (Figure S7). The results were fitted into the Higgins equation (eq. 1).49 kr (1)
=
kr (2)
log
{[1]f /
[1]0}
log {[2]f / [2]0}
(1) (1
) The ratio of the kr values for cluster 1 vs. acceptor 2 thus obtained 6 -1 -1 was 2.7±0.4, which yields a kr value of 4.1±0.6x10 M sec for cluster 1. Since the value of kr for 1 is nearly two orders of magnitude smaller than its kT value, the physical quenching rate constant kq is almost equal to that of kT. The kinetic parameters for the interaction of 1O2 with cluster 1 are summarized in Table 1 below.
thiolato ligands and singlet oxygen when photosensitizers are attached to nanomaterials.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental details including 113 Cadmium NMR, FT-IR, XPS, PXRD, UV-Vis and additional kinetic data (PDF)
AUTHOR INFORMATION Corresponding Author *
[email protected] Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT We gratefully acknowledge support from the NSF-PREM program (No. DRM-1523588). A.G. and M.S. acknowledge support from the NSF-CREST program (No. DRM-1547723), and D.C. from the NIH-NIGMS MARC program (No. T34 GM08228). We would like to thank Dr. Nichole Wonderling (The Pennsylvania State University) for assistance with the PXRD spectra.
Table 1: Kinetic parameters for interaction of singlet oxygen with (Me4N)2[Cd4(SPh)10].
REFERENCES
kT (M-1sec-1) in CD3CN or CH3CNa
kT (M-1sec-1) in CD3ODa
kr (M-1sec-1) in CH3OHb
(5.8 ± 1.3) × 108
(1.2 ± 0.5) × 108
(4.1 ± 0.6) × 106
a = ex
532 nm, sens. = Rose Bengal, average of four to six runs, error is one standard deviation. bCompetition experiments with 1,5dihydroxynaphtalene, average of three runs, error is one standard deviation. In contrast with gold and silicon nanostructures17-19, cluster 1 does not generate 1O2 or undergo self-sensitized photooxidation. In the absence of an external photosensitizer, it undergoes slow photodecomposition (ca. 25 % in two hours of continuous irradiation), but the rates of photodecomposition under N2 and O2 are identical within limits of error, in agreement with previous observations.50 The competition experiments in conjunction with the 1O2 luminescence quenching measurements indicate that physical quenching (kq) of singlet oxygen predominates relative to chemical reaction (kr) for cluster 1. Nevertheless, the value of kr is large enough that exposure of the cluster 1 to 1O2 leads to complete decomposition. This could be a concern if cadmium-sulfur moieties are employed in a nanohybrid where 1O2 generation is the main application (i.e. where singlet oxygen photosensitizers are tethered to the nanomaterial surface). On the other hand, the very large physical quenching rate constant of cluster 1 implies that cadmiumsulfur moieties are highly efficient at removing singlet oxygen. This would be advantageous if 1O2 is an undesired minor byproduct in a system in which cadmium-thiolato moieties are used. Our results indicate that it would certainly be a mistake to a priori neglect the possibility of interactions between surface capping
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Journal of the American Chemical Society 50) Türk, T.; Resch, U.; Fox, M. A.; Vogler, A. Cadmium benzenethiolate clusters of various size: Molecular models for metal chalcogenide semiconductors J. Phys. Chem. 1992, 96, 3818-3922.
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