Fluorescence-Based Reconfigurable and Resettable Molecular

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J. Phys. Chem. C 2007, 111, 11706-11711

Fluorescence-Based Reconfigurable and Resettable Molecular Arithmetic Mode Wei Sun, Yao-Rong Zheng, Chun-Hu Xu, Chen-Jie Fang, and Chun-Hua Yan* Beijing National Laboratory for Molecular Sciences, State Key Lab of Rare Earth Materials Chemistry and Applications, and Peking UniVersity-The UniVersity of Hong Kong Joint Lab in Rare Earth Materials and Bioinorganic Chemistry, Peking UniVersity, Beijing 100871, China ReceiVed: February 10, 2007; In Final Form: May 19, 2007

Reconfigurable and resettable arithmetic functions from a dual-emissive molecule, 1,2-di[5-methoxy-2-(2pyridiyl)thiazoyl]ethyne (DMPTE), were realized. On the basis of its chemical-sensitive fluorescence, the half-adder is idealized by the introduction of sodium sulfide and trifluoroacetic acid, while the half-subtractor is constructed with sodium hydroxide and trifluoroacetic acid. Each signal of the fluorescent outputs triggered by the chemical inputs is compensated through the proper reset agents, resulting in the initial solution state being restored. Through the reversible control of the fluorescent on-off states, the arithmetic functions are reconfigured and reset. Thus, both the half-adder and the half-subtractor are idealized in a single cell with a successive introduction of chemical inputs.

1. Introduction

CHART 1: Structure of DMPTE

Miniaturization of modern silicon-based electronic devices by the top-down approach has encountered challenges from both manufacturing and physical limits.1 In contrast, molecular systems carrying out binary logic operations in a bottom-up approach exhibit their potential applications in the information storage and processing devices.2 Research in this field has been extended from the construction of simple logic to combinational logic and even logic devices.3,6g The research is particularly motivated by idealization of algebraic operations at the molecular level, which is an important step toward the construction of molecular computer.4-6 There are two ways to construct algebraic functions: one is the parallel operations of simple logic gates carried out by different supramolecular systems, and the other is the reconfiguration of several simple logic gates loaded in a single supramolecular system. Due to the need for stronger computing power and easier operating modes, reconfigurable molecular systems that can execute either addition or subtraction have been of intense interest ever since the first report of molecular half-adder by de Silva and McClenaghan.4a Shanzer and co-workers6c reported, for the first time, that arithmetic functions could be reconfigured. The half-adder and halfsubtractor could be loaded in a single fluorescent molecular system with different chemical input sets running in parallel. The reconfiguration capacity of arithmetic functions provides a new pathway for promoting the functional integration and computing power at the molecular level. The reset capacity in fluorescein and tetrathiafulvalene with arithmetic functions stimulates further potential of the functional molecules applied in low-dimensional computing devices.4f,6d,e To idealize the reconfigurable arithmetic functions in a unique supramolecular system requires reproducing different output signals in response to multiple input signals, such as the introduction of ions, molecules, irradiation, or electricity.7 A receptor-chromophore-receptor model has been established for the design of supramolecular systems with arithmetic * Corresponding author: tel +86-10-62754179; fax +86-10-62754179; e-mail [email protected].

functions.4a With different chromophores, the outputs are identified as either absorption or photoluminescence. Among various spectral characteristics used as output signals, fluorescence is attractive due to its advantages of high sensitivity and zero background.8 Especially, the all-fluorescence output mode is appreciated in the idealization of solid-state molecular switches, sensors, and logic gates.9 Akkaya and co-workers5b demonstrated an output mode of fluorescent signals at different wavelengths for the idealization of arithmetic functions. There is great interest to exploit supramolecular systems capable of performing reconfigurable arithmetic functions with reset capacity solely in a fluorescent output mode. Recently we prepared a new dual-emissive ligand, 1,2-di[5methoxy-2-(2-pyridiyl)thiazoyl]ethyne (DMPTE) (Chart 1). The compound 5-methoxy-2-pyridylthiazoles (MPTs), first prepared by us, exhibited proton-sensitive fluorescence in water.10 The conjugation of the MPTs through the triple bond makes the current DMPTE a chromophore-bridge-chromophore molecule. Under different coordination and protonation states of the two MPT species, DMPTE exhibits an on-off-on switching behavior. Herein, by investigating its chemical-sensitive fluorescent behaviors, we report the idealization of the reconfigurable arithmetic functions from DMPTE solely with fluorescent outputs. Starting from a copper-quenching-fluorescence DMPTE solution, the introduction of sulfide anion and proton as inputs produces the half-adder function, while the addition of hydroxyl anion and proton as inputs constructed the half-subtractor function. The arithmetic functions can be reconfigured and reset. The reconfigurable 2-bit arithmetic functions assembled in a single cell provide a new prototype for Boolean logic integration in the fluorescent molecules.

10.1021/jp071143b CCC: $37.00 © 2007 American Chemical Society Published on Web 07/13/2007

Reconfigurable and Resettable Molecular Arithmetic

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2. Experimental Section All reagents were purchased from commercial sources and used without further purification. DMPTE was prepared following a typical Sonogashira cross-coupling route (Figure S1, Supporting Information).11 Reaction of 5-methoxy-2-(2-pyridyl)thiazole (2-MPT) with N-iodosuccinate (NIS) in the trifluoroacetic acid/acetic acid mixture produced the iodo-substituted MPT derivative 4-iodo-5-methoxy-2-(2-pyridyl)thiazole (IMPT) as white crystals. The IMPT was then conjugated with acetylene in tetrahydrofuran (THF), producing a monosubstituted ethynyl compound, [5-methoxy-2-(2-pyridiyl)thiazoyl]ethyne, as white crystals. [5-Methoxy-2-(2-pyridiyl) thiazoyl]ethyne was further conjugated with IMPT, leading to the formation of the desired compound DMPTE as a white solid after recrystallization in chlorobenzene. The overall yield was 50%. Characterization of DMPTE: HRMS (EI) calcd for C20H14N4O2S2 [M+] 406.0558, found 406.0555; 1H NMR (400 MHz, CF3COOD, TMS) δ ) 8.71 (d, J ) 5.6 Hz, 2H), 8.58 (t, J ) 8.0 Hz, 2H), 8.14 (t, J ) 8.1 Hz, 2H), 7.93 (t, J ) 6.8 Hz, 2H), 4.25 (s, 6H); 13C NMR (100 MHz, CF3COOD, TMS) δ ) 172.4, 162.5, 162.0, 161.7, 161.7, 161.2, 148.3, 141.4, 126.1, 123.3, 118.6, 115.7, 113.0, 110.1, 84.1, 79.0, 64.2. All spectral characterizations were carried out in acetonitrile (HPLC-grade) solution at 25 °C with a 10 mm quartz cell. The concentration of DMPTE was 2 × 10-5 mol/L. UV-Vis absorption spectra were measured with a Shimadzu UV-3100 spectrometer, and the fluorescence emission spectra were recorded upon excitation at 350 nm on a Hitachi F-4500 fluorescence spectrometer. DFT calculations were carried out with Gaussian 03 software (Gaussian Inc.) at the B3LYP/631G(d, p) level.12 The quantum yield was measured at 25 °C with excitation at 350 nm (Xe lamp in the F-4500 spectrometer). Rhodamine B ethanol solution (φfr ) 0.96) is selected as the reference. Calculation of quantum yield is according to eq 1.13 φfr and φ are the quantum yield of the standard and the test sample, respectively; Ar and A are the absorbance at the excitation wavelength of the standard and the test sample, respectively; Lr and L are the light path length in the absorption cells of the standard and the test sample, respectively; Nr and N are the indexes of refraction of the solvents in standard sample and the test sample, respectively; and Dr and D are the integrated areas of the emission peaks of the standard and the test sample, respectively.

φ ) φfr

(

)( )( )

1 - 10-ArLr N2 D 1 - 10-AL Nr2 Dr

(1)

3. Results and Discussion 3.1. Spectral Properties of DMPTE. In acetonitrile solution, neutral DMPTE exhibited an emission band centered at 454 nm upon excitation at 350 nm. The acid-dependent change in fluorescent spectra of DMPTE is shown in Figure S2 (Supporting Information). The addition of trifluoroacetic acid gradually decreased the peak at 454 nm, while a new peak centered at 470 nm appeared. Further introduction of acid promoted a new emission band centered at 550 nm. The new fluorescent emissions are associated with different protonated states of the pyridine rings in DMPTE. The monoprotonated species (HDMPTE)+ is formed at low acidic concentration, while the diprotonated species (H2DMPTE)2+ is formed at high acidic concentration. Upon protonation, the absorption at 345 nm was red-shifted to 402 nm (Figure 1), indicating the lower energy gap between highest occupied molecular orbital (HOMO) and

Figure 1. Absorption spectra of (a) DMPTE, (b) (HDMPTE)+, and (c) (H2DMPTE)2+ in acetonitrile solution.

Figure 2. Fluorescent intensity (λem ) 454 nm, λex ) 350 nm) of DMPTE (0.02 mM) measured in acetonitrile solution with various divalent metal cations (0.6 mM).

lowest unoccupied molecular orbital (LUMO) in the protonated states. Theoretical calculation reveals that the energy gap is 3.48 eV in the neutral DMPTE and 2.99 eV in the diprotonated state (Figure S3, Supporting Information). The decrease of energy gap between the frontier molecular orbitals in the protonated states also results in the bathochromic shift of fluorescence.10 It is obvious that pyridine rings in the fluorescent DMPTE can also bind with metal ions. The binding features between the metal ions and DMPTE are dependent on the nature and quantity of the metals. In the same concentration of metal ions, DMPTE displays a selective response to the cupric ion rather than some other divalent metal ions, such as zinc, calcium, manganese, and lead (Figure 2; Figure S4, Supporting Information). Upon addition of cupric ion, the emission at 454 nm is gradually quenched due to the coordination interaction between copper and DMPTE. The stoichiometry of cupric ion to DMPTE is determined as 2:1 from the spectral characterization (Figure 3; Figure S5, Supporting Information) and electrospray ionization (ESI) measurement.14 The selective binding of cupric ion also indicates that the possibility that other metal ions’ interference is low when the specific metal ions are chosen as the input signals. Formation of the Cu2DMPTE complex is prohibited by the addition of competing ligands that can also bind strongly with cupric ion. Both the sulfide and the hydroxyl anion precipitate the cupric ion in aqueous solution, and the same precipitation can also be observed in the acetonitrile solution. Upon the respective addition of sodium sulfide or sodium hydroxide to the nonfluorescent Cu2DMPTE solution, the competing ligands

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Figure 5. Schematic representation of the interconversion of DMPTE among different fluorescent states.

Figure 3. (Top) Fluorescent emissions of DMPTE (λem ) 454 nm, λex ) 350 nm) in the presence of different amounts of Cu2+ in acetonitrile solution (the amount of Cu2+ increased from a to p, [DMPTE] ) 0.02 mM) at 25 °C. (Bottom) Fluorescent intensity at 454 nm versus the introduced equivalents of Cu2+.

Figure 4. Normalized fluorescent intensities of Cu2DMPTE upon addition of 0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 M CF3COOH (from a to g).

liberates DMPTE from Cu2DMPTE, resulting in a recovery of the fluorescence at 454 nm, since the conversion from Cu2DMPTE to CuS or Cu(OH)2 is thermodynamically favored. The addition of proton in the Cu2DMPTE solution leads to different fluorescent responses from that in the free DMPTE solution. Figure 4 shows acid-dependent changes in the fluorescence spectra of Cu2DMPTE. In Cu2DMPTE solution, the addition of proton (0.3 M) first restores an emission band centered at 475 nm, and then the introduction of excessive protons (0.6 M) restores the emission band at 550 nm. In contrast to the Cu2DMPTE solution, even 0.1 M trifluoroacetic acid

Figure 6. Molecular logic gates for the half-adder. (Top) Normalized fluorescence spectrum (λex ) 350 nm) for DMPTE (0.02 mM) with CuCl2 (0.06 mM) in acetonitrile in the presence of different chemical inputs. (Bottom) Truth table for the half-adder solely in fluorescent mode. Inputs: 0.06 mM Na2S, 0.3 M CF3COOH. Outputs: fluorescent intensity at 475 (O1) and 575 nm (O2) and the difference between the fluorescent intensities at two wavelengths (O3 ) O1 - O2). (---) Threshold value for different logic-0 and logic-1 states.

triggers a strong emission band at 550 nm in the free DMPTE solution. Due to the presence of two pyridine rings in the DMPTE, the two-stage fluorescence recovery from the initial state Cu2DMPTE is attributed to the stepwise protonation of two pyridine rings. When both the cupric ion and proton are present in DMPTE solution, due to the competing binding of the cupric ion with DMPTE, the protonation of pyridine rings in DMPTE is more difficult than that without cupric ion, resulting in different amounts of proton needed for recovery of the fluorescence at 550 nm to the same extent. 3.2. Half-Adder and Half-Subtractor. Upon the addition of different chemical inputs, the fluorescence of DMPTE exhibits an on-off-on switching behavior (Figure 5; Figure S6, Supporting Information). The chemical-sensitive fluorescence can be used to construct the Boolean logic gates and arithmetic functions at the molecular level. The nonfluorescent complex Cu2DMPTE is selected as the initial state, and the fluorescent signals at 475 (O1) and 575 nm (O2) are selected as the dual-channel outputs. For each single channel, the low and high fluorescent states are denoted as logic-0 and logic-1 in binary algebra, respectively. Starting with the nonfluorescent Cu2DMPTE solution (O1 ) 0, O2 ) 0), the fluorescent changes induced by the addition of sodium sulfide (I1) and trifluoroacetic acid (I2) are shown in Figure 6. The addition of each single input only leads to the increase of the fluorescent intensity only at 475 nm (O1 ) 1,

Reconfigurable and Resettable Molecular Arithmetic

Figure 7. Molecular logic gates for the half-subtractor. (Top) Normalized fluorescence spectrum (λex ) 350 nm) for DMPTE (0.02 mM) with CuCl2 (0.06 mM) in acetonitrile in the presence of different chemical inputs. (Bottom) Truth table for the half-subtractor solely in fluorescent mode. Inputs: 0.6 M NaOH, 0.6 M CF3COOH. Outputs: fluorescent intensity at 475 (O1) and 575 nm (O2). (---) Threshold value for different logic-0 and logic-1 states.

O2 ) 0), while the simultaneous addition of both inputs results in the enhanced emission bands at both 475 and 575 nm (O1 ) 1, O2 ) 1). When the fluorescence at 475 nm is recorded as the output signal, an OR logic gate is constructed; the logic function is then reconfigured to an AND logic gate when the output signal is recorded at 575 nm. An XOR logic gate is constructed through simultaneous observation of the fluorescence at both 475 and 575 nm. The difference of the fluorescent intensities at the two wavelengths is recorded as the third output signal (O3). The output signal is high (O3 ) 1) only when the input signal is introduced separately. An XOR logic gate is thus constructed. When the dual-channel fluorescence is monitored, the half-adder carrying the sum (XOR gate) and the carry (AND gate) operations is realized at the molecular level. When the chemical input is switched from sodium sulfide to sodium hydroxide, the algebraic function is reconfigured from the half-adder to the half-subtractor. The initial state and the output signals setting are kept the same as those in the halfadder. The chemically induced fluorescent changes are shown in Figure 7. The addition of sodium hydroxide results in the increase of the fluorescent intensity at 475 nm (O1 ) 1, O2 ) 0), while the addition of trifluoroacetic acid triggers the increase of the fluorescent intensities at both 475 and 575 nm (O1 ) 1, O2 ) 1). When both inputs are added, the neutralization reaction results in the quenching of fluorescence by cupric ion at both 475 and 575 nm (O1 ) 0, O2 ) 0). Thus by monitoring the

J. Phys. Chem. C, Vol. 111, No. 31, 2007 11709 fluorescence change at 475 nm, an XOR logic gate is constructed, while the logic expression is reconfigured to an INHIBIT logic gate when the output channel is switched to 575 nm. By the choices of chemical-encoded input signals, the molecular half-adder is then reconfigured to half-subtractor executing borrow (INHIBIT gate) and difference (XOR gate) operations. The electronic symbols for the molecular half-adder and half-subtractor are shown in Figure 8. The competing ligands play an important role in the conversion between the half-adder and the half-subtractor. Addition of both sulfide anion and hydroxide anion precipitates the cupric ion, liberating DMPTE from the nonfluorescent complex Cu2DMPTE to promote the fluorescence at 475 nm (O1 ) 1, O2 ) 0). But the stabilities of the as-obtained cupric precipitation are different in acidic solution. CuS is stable in an acidic solution. When sodium sulfide and trifluoroacetic acid are added as the input signals, the free DMPTE, liberated from the nonfluorescent complex by the sulfide anion, is converted to the diprotonated state. Thus the inputs of sulfide anion and proton are in cooperation for the recovery of fluorescence at 575 nm, indicating the idealization of an AND logic gate. However, Cu(OH)2 dissolves in trifluoroacetic acid to release the cupric ion, quenching the fluorescence of DMPTE. The fluorescence at neither 475 nor 575 nm can be detected above a threshold. When the inputs of hydroxide anion and proton are both introduced, they annihilate each other, producing an INHIBIT logic gate. The conversion from AND to INHIBIT logic gate results in the reconfiguration of arithmetic functions between the half-adder and the half-subtractor. 3.3. Reset Capacity. In order to reconfigure the arithmetic functions, the reset capacity of the current system was investigated. In each output channel, the fluorescent intensities below and above the threshold are denoted as logic-0 and logic-1 in binary algebra, respectively. For the cycling and reconfiguration of logic functions, it is important to have all the operation processes executed in a single cell without change of solutions or parallel operations of multiple cells. All the logic operations in the present system are idealized with the introduction of sulfide anion, trifluoroacetic acid, or hydroxide anion as the input signals. The acid and base input signals can be reset by each other, restoring the system to the initial state with the accumulation of only water and CF3COONa. The introduction of cupric ion precipitates the sulfide, regenerating the initial nonfluorescent compound, accumulating only CuS as a stable precipitate and NaCl (Figure S7, Supporting Information). All of the formed salts affect the fluorescent properties of DMPTE weakly. The stepwise reset and reconfiguration processes are shown in Figure 9. In the half-adder cycle with initial state Cu2DMPTE (O1 ) 0, O2 ) 0), the sulfide anion (I1 ) 1, I2 ) 0) is first added, producing the output signal at 475 nm (O1 ) 1, O2 ) 0). The reset agent cupric dichloride is then added to quench the fluorescence signal to the initial state (O1 ) 0, O2 ) 0). The acid input signal (I1 ) 0, I2 ) 1) is added to the above solution, producing the output signal at 475 nm (O1 ) 1, O2 ) 0). After that, sodium hydroxide is added gradually till the neutralization of the acid, resetting the output signal to

Figure 8. Electronic logic circuits for the half-adder and the half-subtractor.

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Figure 9. Reset and reconfiguration processes for the arithmetic functions: (left) schematic representation of the reset processes; (right) normalized fluorescent intensities of different output channels in the reset and reconfiguration processes.

the initial state again (O1 ) 0, O2 ) 0). The presence of both sulfide and acid (I1 ) 1, I2 ) 1) produces the output state with emission at both 475 and 575 nm (O1 ) 1, O2 ) 1), while the consecutive addition of sodium hydroxide and cupric ion compensates the effects made by the chemical inputs, restoring the initial state (O1 ) 0, O2 ) 0). After the entire cycle of the half-adder, the fluorophore DMPTE returns to its bound state Cu2DMPTE, capable of performing other cycles of arithmetic functions. The half-subtractor is then executed in the same solution with the switch of chemical inputs. The input signal, either sodium hydroxide (I1) or trifluoroacetic acid (I2), can also perform as the reset agent for the other input signal. The introduction of trifluoroacetic acid (I1 ) 0, I2 ) 1) triggers the emission at both 475 and 575 nm (O1 ) 1, O2 ) 1). The subsequent addition of sodium hydroxide (I1 ) 1, I2 ) 1) quenches the emission band (O1 ) 0, O2 ) 0) and restores the initial solution state. Further introduction of sodium hydroxide (I1 ) 1, I2 ) 0) produces the emission only at 475 nm (O1 ) 1, O2 ) 0), and then the input signal is reset by the trifluoroacetic acid to the initial state (O1 ) 0, O2 ) 0). After the cycles of the half-adder and the half-subtractor, DMPTE changes to the cupric complex again, demonstrating that the arithmetic functions can be reconfigured and reset in the current system. However, a potential challenge for the reconfiguration and reset processes is the accumulation of chemical wastes, which brings out some difficulties in recovering fluorescent signals after several arithmetic cycles in solution. As shown in Figure 9, the current molecular calculator still works after 10 cycles of set/reset process. The high contrast between the fluorescent output signals provides a sufficient signal-to-noise ratio to distinguish different logic 0/1 states even when a large amount of chemical wastes is accumulated (Figure S8, Supporting Information). 4. Conclusion The fluorescent behaviors of a new dual-emissive molecule, DMPTE, in the presence of different ions are studied in this work. The fluorescent changes corresponding to different protonation and coordination states of DMPTE are introduced to construct the arithmetic functions at the molecular level. The major conclusions are as follows: (1) Both the half-adder and the half-subtractor are assembled in the same supramolecular system. The arithmetic functions are accomplished by the fluorescent changes at different wavelengths in response to the two-input chemical introduction.

(2) The function reconfiguration is achieved based on the different stability of the as-prepared cupric precipitation in the acidic solution. With the precipitation changed from the acidstable cupric sulfide to the acid-unstable cupric hydroxide, the half-adder can be reconfigured to the half-subtractor. (3) The reset capacity of the current system is investigated. The fluorescent states triggered by the introduction of the chemical inputs can be reset to the initial state by addition of the reset agent. All the arithmetic processes, both the half-adder and the half-subtractor, are operated in a single cell. Although the current supramolecular system provides only the prototype for application of the chemical-sensitive dualfluorescence in molecular arithmetic, the reconfigurable arithmetic functions integrated in a single molecule may promote the computing power in future molecular calculators on solid supports. Acknowledgment. We thank the NSFC (20490213, 20221101, and 20423005) and PKU for financial support. Supporting Information Available: Synthetic route for DMPTE, optimized structures and frontier molecular orbitals for DMPTE and (H2DMPTE)2+, and detailed fluorescent spectra of DMPTE in the presence of different chemical inputs. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Packan, P. A. Science 1999, 285, 2079-2081. (b) Schulz, M. Nature 1999, 399, 729-730. (c) Hu, C. M. Nanotechnology 1999, 10, 113116. (2) (a) de Silva, A. P.; McClenaghan, N. D.; McCoy, C. P. MolecularLeVel Electronics, Imaging and Information, Energy and EnVironment, in Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim, Germany, 2001; Vol. 5. (b) Balzani, V.; Venturi, M.; Credi, A. Molecular DeVices and Machines. A Journey into the Nano World; Wiley-VCH: Weinheim, Germany, 2003. (c) Brown, G. J.; de Silva, A. P.; Pagliari, S. Chem. Commun. 2002, 2461-2463. (d) Raymo, F. M. AdV. Mater. 2002, 14, 401-414. (e) Guo, X. F.; Zhang, D. Q.; Zhou, Y. C.; Zhu, D. B. J. Org. Chem. 2003, 68, 5681-5687. (f) Raymo, F. M.; Giordani, S.; White, A. J. P.; Williams, D. J. J. Org. Chem. 2003, 68, 4158-4169. (g) de Silva, A. P.; McClenaghan, N. D. Chem.sEur. J. 2004, 10, 574-586. (h) Wang, Z. X.; Zheng, G. R.; Lu, P. Org. Lett. 2005, 7, 3669-3672. (3) (a) Stojanovic, M. N.; Stefanovic, D. Nat. Biotechnol. 2003, 21, 1069-1074. (b) Macdonald, J.; Li, Y.; Sutovic, M.; Lederman, H.; Pendri, K.; Lu, W. H.; Andrews, B. L.; Stefanovic, D.; Stojanovic, M. N. Nano Lett. 2006, 6, 2598-2603. (c) Margulies, D.; Felder, C. E.; Melman, G.; Shanzer, A. J. Am. Chem. Soc. 2007, 129, 347-354. (4) (a) de Silva, A. P.; McClenaghan, N. D. J. Am. Chem. Soc. 2000, 122, 3965-3966. (b) Stojanovic, M. N.; Stefanovic, D. J. Am. Chem. Soc. 2003, 125, 6673-6676. (c) Guo, X. F.; Zhang, D. Q.; Zhang, G. X.; Zhu,

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