Evidence for a Through-Space Pathway for Electron Transfer from

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Letter pubs.acs.org/JPCL

Evidence for a Through-Space Pathway for Electron Transfer from Quantum Dots to Carboxylate-Functionalized Viologens Adam J. Morris-Cohen, Mark D. Peterson, Matthew T. Frederick, Judith M. Kamm, and Emily A. Weiss* Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States S Supporting Information *

ABSTRACT: Ultrafast transient absorption measurements reveal that the rate constant for photoinduced electron transfer (PET) from colloidal CdS quantum dots (QDs) to alkylcarboxylate-functionalized viologens is independent of the number of methylene groups in the alkyl chain (n). The rate constant for PET is (1.2 ± 0.3) × 1010 s−1 for n = 1, 2, and 3, and for n = 0 (methylviologen). The insensitivity of the electron transfer rate constant to the length of the functional groups on the viologen suggests that a “throughspace” pathway, where the electron bypasses the alkylcarboxylate and tunnels instead through only the orbitals of the QD and of the bipyridinium core, is the dominant PET pathway. SECTION: Physical Processes in Nanomaterials and Nanostructures

T

Chart 1. Chemical Structures of Methyl Viologen and the Three Viologen Derivatives Used as Electron Acceptors in This Worka

his letter describes the use of photoinduced electron transfer (PET) as a probe of the geometry in which derivatized viologen molecules adsorb to the surfaces of colloidal CdS quantum dots (QDs) to form electron donor− acceptor pairs. Methylviologen is a popular electron-accepting ligand for colloidal QDs due to its low reduction potential (−0.64 V vs Ag/AgCl)1 and the high molar absorptivity and convenient location of the absorption band of its radical cation (∼620 nm). The literature contains many examples of reproducible, picosecond to subpicosecond electron transfer time constants in QD-methylviologen complexes2−10 using both CdS4,6 and CdSe2,3,8,11 QDs prepared through various synthetic methods. These consistently fast dynamics have been observed despite the fact that methylviologen does not include an obvious functional group by which it would coordinate to the surface of the QD. Here, we determine the orbital pathway by which the ultrafast PET from QDs to methylviologen occurs by measuring the rate constant for PET for methylviologen and a series of viologens functionalized, at N and N′, respectively, with a heptane group and a (CH2)nCOOH group (n = 1−3) (Chart 1). By functionalizing the viologens with carboxylic acids, which are known to chemisorb to Cd2+ on the surface of QDs,12 and varying the number of carbons that separate the viologen core from the carboxylate, we distinguish between two possible PET pathways: (i) a “through-bond” pathway where the electron tunnels through the alkylcarboxylate group to the bipyridinium core, and (ii) a “through-space” pathway where PET bypasses the sigma system along the molecular axis of the alkylcarboxylate, and occurs only through nonbonded orbitals of the QD and of the bipyridinium core. Figure 1 shows these two tunneling pathways for two limiting-case adsorption geometries of the viologen. If the PET process occurs via a through-bond pathway, then the rate constant for PET should (i) be different for the carboxylate-functionalized viologens © 2012 American Chemical Society

a

The ligands differ from one another by the number of methylene groups between the carboxylic acid and the viologen core (n = 1, 2, 3). The bromide counter ions are not shown.

than for methylviologen, and (ii) depend on the length of the alkyl chain (n). In contrast, we find that, once we account for the surface coverage of viologen acceptors on the QDs,4,13 the intrinsic (single donor-single acceptor) rate constant for PET is the same for the alkylcarboxylate-functionalized viologens as it is for methylviologen, and is insensitive to the number of Received: August 31, 2012 Accepted: September 18, 2012 Published: September 18, 2012 2840

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excitonic feature of −0.3 OD. Control experiments show that the excitation dynamics at this fluence are independent of pump power, and therefore are below the multiexciton threshold. The solution was stirred with a magnetic stir bar to reduced local heating and charging effects. We have previously shown that illuminating the QD at energies greater than or equal to the energy of the first excitonic state of CdS QDs results in dissociation of the exciton to give an oxidized QD and a reduced viologen, V1+.4 The energetic driving force for the PET reaction does not vary significantly within the viologen series because the reduction potential of the alkyl acid viologen derivatives is only weakly sensitive to the number of methylene groups between the bipyridinium moiety and the carboxylic acid (the range in reduction potentials over the entire series is less than 50 meV). The Supporting Information contains representative cyclic voltammograms of the HVR2+ series. The electron transfer reaction induces the recovery of the ground state bleach of the QD (at 455 nm) and the formation of a new absorption band centered at 620 nm, which we assign to the V1+ radical cation (Figure 2A). We can monitor the dynamics at either wavelength to measure the rate of PET. It is simpler, however, to use the formation of the V1+ transient to monitor the PET process because the dynamics of the ground state bleach are a multiexponential convolution of signals from QDs with no adsorbed V2+ and QDs that participate in PET. The signal at 620 nm also has a small contribution from a

Figure 1. Schematic diagram depicting two possible adsorption geometries of the asymmetric viologen ligands, and two possible PET pathways (through-bond, red, and through-space, blue) for each geometry. If the PET pathway is through-bond, increasing the length of the alkyl chain will decrease the rate of PET by decreasing the donor−acceptor electronic coupling. If the PET pathway is throughspace, then lengthening the alkyl chain will not change the donor− acceptor distance or the rate of PET.

methylene groups between the carboxylic acid and the bipyridinium core (n). Electron transfer therefore must occur via a pathway that not only bypasses the carboxylate group and the alkyl chain, but also is approximately the same tunneling distance regardless of the presence or length of the alkylcarboxylate group. We interpret this result as evidence that the dominant PET pathway is a through-space pathway from the QD to the bipyridinium core physisorbed directly onto the QD surface (Figure 1, geometry 2, blue arrow). We synthesized CdS QDs with a procedure adapted from Yu and Peng12 and three types of viologen derivatives using a modified literature procedure (see Supporting Information).14 The three derivatives are asymmetrically substituted viologens, where one of the nitrogen atoms is functionalized with a heptane group (H) and the other nitrogen atom is functionalized by an alkyl carboxylic acid. The three viologen derivatives differ from one another in the length of the alkyl carboxylic acid substituent (A = acetyl, P = propyl, B = butyl). We denote the generic asymmetric viologen series as HVR2+ (HVA2+, HVP2+, and HVB2+; Chart 1). For comparison, we also studied samples with the commercially available methyl viologen MVM2+. We used a bromide counterion for all viologens in this study. We prepared the QD-viologen complexes by mixing a stock solution of the purified CdS QDs dispersed in THF and a stock solution of the V2+ bromide salt in methanol, such that in the final solutions: (i) the optical density at the band-edge absorption of the QD was 0.2 in a 2-mm cuvette, (ii) there were between 0 and 100 V2+ ligands per QD, and (iii) the solutions were 16:1 THF/methanol. The mixture of THF and methanol was necessary to solubilize both the QDs (passivated with oleate ligands) and the charged viologen molecules. For each viologen derivative, we prepared samples with several different ratios of V2+/QD. We allowed all of the samples to equilibrate overnight before characterization. We measured the rate of PET from CdS QDs to each of the four V2+ derivatives using ultrafast transient absorption (TA) spectroscopy with a 430-nm pump and a visible continuum probe; the setup is described in detail elsewhere.4,15 We depolarized the pump light to prevent contributions to the TA kinetics from rotational dynamics within the sample, and tuned the fluence of the pump beam such that QDs without any viologens added exhibited a maximum bleach of the first

Figure 2. (A) TA spectra of 8.7 × 10−6 M CdS QDs (d = 5.0 nm) in THF/MeOH, 500 ps after photoexcitation (blue), and of the same QDs with 2.6 × 10−4 M HVP2+ (red). Electron transfer quenches the ground state bleach of the QD ensemble and produces a new absorption feature centered at 620 nm, which corresponds to the transient HVP1+ radical cation. Inset: Same spectra zoomed-in on the absorption band of the HVP1+ transient. (B) Kinetic traces at 620 nm of the same two samples in panel A, and best fits to these traces with the fit function in eq 1. The Supporting Information contains representative kinetic traces from complexes of the QDs with all four viologen ligands, and tables summarizing the values of all the fit parameters from eq 1 for each experiment. 2841

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We performed either five or six measurements of kCS,int for each sample: MVM2+ (n = 0), HVA2+ (n = 1), HVP2+ (n = 2), and HVB2+ (n = 3), where the QD-viologen complexes are all prepared from a single synthetic batch of QDs. Figure 3 shows

broad, flat feature attributed to intraband transitions of the electron and hole,15 but these intraband dynamics are easily deconvoluted from those of V1+.4 Figure 2B shows a representative kinetic trace of the peak of the V1+ feature for a mixture of QDs and HVP2+ and, for comparison, a kinetic trace at 620 nm for a sample of QDs with no added viologen. An ensemble of QD-viologen complexes comprises a set of subpopulations, where each subpopulation is characterized by the number of adsorbed viologens, m, per QD.13,16,17 The observed rate constant for PET in colloidal QD-ligand complexes is linearly proportional to the number of redox active ligands adsorbed per QD in ET-active geometries.4,16−19 It is possible that each of the four V2+ derivatives that we studied here has a different affinity for the QD surface and, therefore, a different adsorption constant. In this case, controlling the concentration of V2+ added to each QD sample is insufficient to guarantee that each sample has the same number of adsorbed V2+ per QD, and is insufficient to eliminate differences in binding equilibria as the origin for observed differences in PET rate within the viologen series. In order to account for the influence of adsorption constant on observed PET rate, we perform a procedure, described in detail in previous work,4,13,16,19,20 by which we measure the intrinsic PET rate constant, kCS,int. The intrinsic PET rate constant is that which we would observe if every QD in the ensemble had exactly one adsorbed V2+ in an ET-active configuration. The value of kCS,int is therefore independent of the QD-V2+ binding equilibrium, and instead only depends on the structural, chemical, and electronic factors that govern the rate of PET between the QD and the viologen (such as donor− acceptor distance). In this procedure, we describe the distribution of adsorbed viologens per QD with the binomial equation, and use this distribution to construct a weighted sum of exponential terms, where each exponential term within the sum describes the PET dynamics for the subpopulation of QDs in the ensemble with m adsorbed viologens.19 There exists a closed form of this sum: the term multiplied by ACS in eq 1.

Figure 3. Average and standard deviation of the log of either five or six measurements of the intrinsic charge separation rate (kCS,int) for each of the QD−viologen complexes investigated: MVM2+ (n = 0), HVA2+ (n = 1), HVP2+ (n = 2), and HVB2+ (n = 3). We determined the rate constant for PET from the rate of formation of the V1+ transient at 620 nm, and the distributed rate model described in the text (eq 1). The dotted line represents the best fit of the data with the expected exponential decrease in the rate constant with increasing n, for an attenuation factor, β, of 1.04 Å−1.

the average and standard deviation of the log10(kCS,int) from these measurements. Within the uncertainty of our measurement, kCS,int is the same for each of the four viologen derivatives. The constant value of kCS,int for the different lengths of the alkylcarboxylate substituent suggests that the dominant PET pathway bypasses the carboxylate linking group, and instead occurs through orbitals of the bipyridinium core that overlap those of the QD surface (Figure 1, geometry 2, blue arrow). If the PET process occurred via a through-bond pathway (red arrows in Figure 1), or via a through-solvent pathway in a geometry in which the alkyl linker extended the viologen away from the QD surface (blue arrow in Figure 1, geometry 1), then the rate of PET would decrease with increasing linker length (n). The rate constant for tunneling through a saturated alkyl chain decreases exponentially with the molecular length of the chain.21 We calculated the decrease in PET rate on going from HVA2+ to HVP2+ to HVB2+ for the through-bond pathway with a decay constant for tunneling (β) of 1.04/Å, which was previously measured for alkyl carboxylic acids with n = 1−3.22 The Supporting Information contains a detailed description of this calculation. In doing this calculation, we assume an alkyl C−C bond length of 154 pm, and a tetrahedral bond angle of 109.5° for each sp3 carbon atom. Given these assumptions, the electron tunneling distance for PET is 252 pm longer for HVB2+ than for HVA2+. This increase in tunneling distance results in a 14-fold decrease in the PET rate constant on going from n = 1 to n = 3 for through-bond electron transfer, as illustrated by the dotted line in Figure 3. The data do not follow this trend, so we can conclude that the PET pathway does not occur through the alkylcarboxylate group. The agreement between the value of kCS,int for the HVR2+ series and MVM2+ (n = 0) provides further evidence that the PET occurs through the through-space pathway. Chemically tuning the electronic coupling between redoxactive ligands and inorganic QDs provides a handle by which we can tune the rates of charge transfer across the QD-ligand interface in order to (i) generate (and inhibit the recombina-

I(t ) = IRF ⊗ (A CS(1 + (e−kCS,intt − 1)θ )N i

+

∑ Aje − kjt ) j=1

(1)

In eq 1, ACS is the amplitude of the signal originating from the V1+, IRF is the instrument response function, here an error function, and the last term is a sum of exponentials to describe the dynamics of the intraband absorption at 620 nm and the dynamics of charge recombination (which is on the nanosecond time scale). The parameter θ within the charge separation (CS) term is the mean fractional surface coverage of viologen ligands within the ensemble, and N is the total number of available surface sites on the QD for adsorption of V2+. This model assumes that the distribution of geometries of viologen ligands on the QDs in the ensemble does not change in time, and the ligand changes conformation on the surface of the QD more slowly than the electron transfer time scale (∼50 ps). The Supporting Information and our previous work4,13,20 contain a detailed procedure for measuring θ and N using the response of the photoluminescence of the QDs to added viologen. We used eq 1 to fit the dynamics of the formation and decay of the V1+ transient at 620 nm for each of the QDviologen samples to obtain kCS,int (Figure 2B). 2842

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tion of) mobile charge carriers within photovoltaic materials,23 (ii) initiate redox chemistry within photocatalytic materials in the production of solar fuels,24 or (iii) regulate photoluminescence of the QDs in redox-based ion or biomolecule detection schemes.25 The conformation of ligands adsorbed to surfaces of QDs is determined by complex and dynamic interactions between the ligand and the surface of the QD, the neighboring ligands, and the solvent molecules. In future work, we will investigate how these interactions affect the QD-ligand equilibrium adsorption constant in addition to the rate of PET. Some previous work in dye-sensitized solar cells (DSSCs),26 a closely related molecule-semiconductor nanoparticle system, has shown that modifying the linker between the dye and the nanoparticle strongly influences the rate of electron transfer between the donor (dye) and the acceptor (nanoparticle);22,27 other work with DSSCs has concluded that linker modification produces little effect on the PET dynamics.28 The role of the linker’s chemical structure appears to be system-specific. Although cadmium chalcogenide QDs and titania are not chemically equivalent, our demonstration that the dominant PET pathways in CdS−viologen complexes is the throughspace pathway implies that competing adsorption geometries can nullify the influence of the nominal linker group on the rate and yield of heterogeneous charge transfer. The contribution of these pathways therefore needs to be considered in developing strategies to optimize the electronic coupling between nanoparticles and adsorbed redox-active molecules.

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Photoinduced Electron Transfer Rate for a CdS Quantum Dot− Viologen Complex. J. Am. Chem. Soc. 2011, 133, 10146−10154. (5) Rossetti, R.; Brus, L. E. Picosecond Resonance Raman-Scattering Study of Methylviologen Reduction on the Surface of Photoexcited Colloidal CdS Crystallites. J. Phys. Chem. 1986, 90, 558−560. (6) Logunov, S.; Green, T.; Marguet, S.; El-Sayed, M. A. Interfacial Carriers Dynamics of CdS Nanoparticles. J. Phys. Chem. A 1998, 102, 5652−5658. (7) Matsumoto, H.; Uchida, H.; Matsunaga, T.; Tanaka, K.; Sakata, T.; Mori, H.; Yoneyama, H. Photoinduced Reduction of Viologens on Size-Separated CdS Nanocrystals. J. Phys. Chem. 1994, 98, 11549− 11556. (8) Tagliazucchi, M.; Tice, D. B.; Sweeney, C. M.; Morris-Cohen, A. J.; Weiss, E. A. Ligand-Controlled Rates of Photoinduced Electron Transfer in Hybrid CdSe Nanocrystal/Poly(viologen) Films. ACS Nano 2011, 5, 9907−9917. (9) Ramsden, J. J.; Gratzel, M. Formation and Decay of Methylviologen Radical Cation Dimers on the Surface of Colloidal CdS - Separation of Two-Dimensional and Three-Dimensional Relaxation. Chem. Phys. Lett. 1986, 132, 269−272. (10) Shallcross, R. C.; D’Ambruoso, G. D.; Pyun, J.; Armstrong, N. R. Photoelectrochemical Processes in Polymer-Tethered CdSe Nanocrystals. J. Am. Chem. Soc. 2010, 132, 2622−2632. (11) Harris, C.; Kamat, P. V. Photocatalysis with CdSe Nanoparticles in Confined Media: Mapping Charge Transfer Events in the Subpicosecond to Second Timescales. ACS Nano 2009, 3, 682−690. (12) Yu, W. W.; Peng, X. Formation of High-Quality CdS and Other II−VI Semiconductor Nanocrystals in Noncoordinating Solvents: Tunable Reactivity of Monomers. Angew. Chem. 2002, 41, 2368−2371. (13) Morris-Cohen, A. J.; Vasilenko, V.; Amin, V. A.; Reuter, M. G.; Weiss, E. A. Model for Adsorption of Ligands to Colloidal Quantum Dots with Concentration-Dependent Surface Structure. ACS Nano 2012, 6, 557−565. (14) Mezei, G.; Kampf, J. W.; Pecoraro, V. L. Temperature-, Molar Ratio- and Counterion-Effects on the Crystal Growth of Bipyridinium−Bis(alkylcarboxylic acid)−Crown Ether Pseudorotaxanes. New J. Chem. 2007, 31, 439−446. (15) McArthur, E. A.; Morris-Cohen, A. J.; Knowles, K. E.; Weiss, E. A. Charge Carrier Resolved Relaxation of the First Excitonic State in CdSe Quantum Dots Probed with near-Infrared Transient Absorption Spectroscopy. J. Phys. Chem. B 2010, 114, 14514−14520. (16) Tachiya, M. Kinetics of Quenching of Luminescent Probes in Micellar Systems 0.2. J. Chem. Phys. 1982, 76, 340−348. (17) Song, N.; Zhu, H.; Jin, S.; Zhan, W.; Lian, T. PoissonDistributed Electron-Transfer Dynamics from Single Quantum Dots to C60 Molecules. ACS Nano 2010, 5, 613−621. (18) Boulesbaa, A.; Issac, A.; Stockwell, D.; Huang, Z.; Huang, J.; Guo, J.; Lian, T. Ultrafast Charge Separation at CdS Quantum Dot/ Rhodamine B Molecule Interface. J. Am. Chem. Soc. 2007, 129, 15132−15133. (19) Tachiya, M. Application of a Generating Function to ReactionKinetics in Micelles - Kinetics of Quenching of Luminescent Probes in Micelles. Chem. Phys. Lett. 1975, 33, 289−292. (20) Morris-Cohen, A. J.; Aruda, K. O.; Rasmussen, A. M.; Canzi, G.; Seideman, T.; Kubiak, C. P.; Weiss, E. A. Controlling the Rate of Electron Transfer between a Quantum Dot and a Tri-Ruthenium Molecular Cluster by Tuning the Chemistry of the Interface. Phys. Chem. Chem. Phys. 2012, DOI: 10.1039/C2CP40827A. (21) Weiss, E. A.; Wasielewski, M. R.; Ratner, M. A. In Topics in Current Chemistry; De Cola, L., Ed.; Springer: New York, 2005; Vol. 257, p 103. (22) Asbury, J. B.; Hao, E.; Wang, Y.; Lian, T. Bridge LengthDependent Ultrafast Electron Transfer from Re Polypyridyl Complexes to Nanocrystalline TiO2 Thin Films Studied by Femtosecond Infrared Spectroscopy. J. Phys. Chem. B 2000, 104, 11957− 11964. (23) Semonin, O. E.; Luther, J. M.; Choi, S.; Chen, H.-Y.; Gao, J.; Nozik, A. J.; Beard, M. C. Peak External Photocurrent Quantum

ASSOCIATED CONTENT

S Supporting Information *

Experimental details, procedure for measuring the fractional surface coverage of V2+ on the QDs, cyclic voltammagrams of viologen derivatives, PL spectra of the QDs with added V2+, transient absorption kinetic traces, and tables of fit parameters for eq 1. This material is available free of charge via the Internet http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the DOE through the Office of Science Early Career Research Award (DE-SC0003998), and by the NSF through a Graduate Research Fellowship (for A.J.M.-C.).

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REFERENCES

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