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Intramolecular Hydrogen Bonding Assisted Charge Transport through Single Rectifying Molecule Wei Wang and Luping Yu* Department of Chemistry and The James Franck Institute, The University of Chicago, 929 East 57th Street, Chicago, Illinois 60637, United States
bS Supporting Information ABSTRACT: A novel diode molecule consisting of R-hydroxyphenyl pyridine motif was synthesized. Molecular self-assembly onto gold electrode surface leads to moderate rectifying behavior. The replacement of the hydroxyl proton by a methyl group leads to the disruption of intramolecular hydrogen bonding and yields a twisted molecular conformation. Comparing the current-voltage characteristics between these two compounds reveals that the presence of intramolecular hydrogen bonding substantially improves the molecular conductivity.
’ INTRODUCTION Controlling and predicting of the charge transport through a single molecular component is the central theme in molecular electronics. Therefore, various molecular components with different functions were synthesized and investigated. Since Aviram-Ratner first proposed the concept of molecular diodes with structures of donor-bridge-acceptor (D-σ-A),1 molecules serving as electronic wires,2-4 diodes,5,6 transistors,7,8 and switches9-11 have been demonstrated. Most recently, studies on the effect of subtle structural changes on charge transport have led to new revelation of structural and charge transport correlation. Typical examples include hydrogen tautomerization,12 metapara effect,13,14 metal center,15 torsion angle dependent molecular conductivity, and so forth.16,17 Our group is especially interested in the molecular rectifying behavior and has successfully developed several asymmetric diblock molecules,18-22 which exhibit asymmetric charge transport characteristics under applied bias voltage, similar to p-n junction semiconductor. Further study reveals that the charge transport direction can be controlled by the molecular orientation on the electrode surface.20 Additionally, the protonation on pyrimidine motif in another diode molecule can cause the inversion of the rectifying direction, a molecular switch triggered by a proton.21 These results illustrate the significance of structural modifications on charge transport. To explore new p-n type molecular junctions, the incorporation of functional structure components is of considerable interest. Ideally, the integrated functional group is assumed to r 2011 American Chemical Society
respond for certain external stimulus and the resulting molecular structures will lead to substantially different physical and chemical properties. Thus, the prepared molecular diodes will be able to be operated under appropriate conditions. This general concept is highly desirable for the development of smart stimuliresponsive molecular devices. Herein, we report the synthesis and the characterization of a novel rectifying molecule, the charge transport of which was significantly affected by the presence or absence of an intramolecular hydrogen bonding. The currentvoltage (I/V) characteristics indicated that a simple rotating operation could lead to substantially decreased molecular conductivity. This result is, in principle, quite instructive for the design of switchable molecular devices. Scheme 1 shows the target molecular structures (1 and 2). Detailed synthesis and characterization of molecules 1 and 2 are given in the Supporting Information. The molecule 1 was synthesized by integrating R-hydroxyphenyl pyridine in between two thiolfunctionalized thiophene units. The hydroxyphenyl pyridine unit renders the target molecule a moderate dipole moment. Replacement of the hydroxyl proton on molecule 1 with a methyl group yields the control compound 2. To deposit these molecules onto a gold electrode with a controllable manner, the two molecules are functionalized with two different thiol-protecting groups, cyanoethyl (-SCH 2 CH 2 CN, CNE) and trimethylsilylethyl Received: October 4, 2010 Revised: January 18, 2011 Published: January 27, 2011 2084
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Langmuir Scheme 1. Molecular Structures 1 and 2
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Scheme 2. Graphic Illustration of the Surface Deposition Process of These Two Molecules onto Gold Electrode and the Assemblies A1 and A2a
(-SCH2CH2SiMe3, TMSE), which allow the sequential deprotection and attachment onto the gold electrode surface.
’ EXPERIMENTAL SECTION Substrate Preparation. The gold on mica (Au/mica) substrates used in the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) were prepared by thermal evaporation of gold (99.999%) onto freshly cleaved mica under a vacuum at 2 10-7 Torr. The mica sheet (highest quality V1, Ted Pella, Inc.) was mounted between two copper templates and slowly heated up to ∼450 C overnight to remove the residue water on the surface and the interlayers. The heater was switched off before evaporation and roughly 120 nm gold film was then deposited onto the mica at a temperature of 330 C followed by annealing at 400 C for overnight. The Au/mica substrates were hydrogen flame annealed until the gold film radiated a dim orange color and then were quenched in pure ethanol saturated with argon before the surface deposition with diode molecules. Monolayer Preparation. The molecular self-assembly onto the gold surface was carried out under N2 atmosphere. After hydrogen flame annealing, the gold substrate was treated with 1 mM dodecanthiol (DDT) ethanol solution overnight. The DDT-derivatized gold (DDT SAM/Au) was exhaustively washed with ethanol followed by freshly distilled tetrahydrofuran (THF). A solution of molecular diode in THF (∼2 10-5 M) was reacted with an excess amount of NaOEt solution and the DDT SAM/Au electrode was immersed in this solution for about 3 h. The cyanoethyl (CNE) protecting group was removed and the TMSE protecting group was retained during this procedure; the exposed thiol group will be adsorbed onto the defect sites of the electrode. The diode-modified substrate was further dipped into a 10 mM solution of tetrabutylammonium fluoride (TBAF) in THF to cleave the TMSE group and to release the other thiol. After being rinsed with a large amount of THF, ethanol, and toluene, the sample was immersed in a solution of 5 nm gold nanoparticles in toluene (absorbance ∼0.1 at 520 nm, the ammonium ions stabilized gold nanoparticles are freshly prepared according to a reported procedure23) for 2 h. The final sample was dried with a continuous flow of argon and was used for the subsequent microscopic study. The thiol-gold assembly is robust and stable STM images can be obtained after repeated scans. Scanning Tunneling Microscopy. STM analysis was carried out by utilizing a NanoScope IV standalone (Digital Instruments). The Pt/Ir tip was prepared by electrochemical etching of a Pt/Ir wire (Molecular Imaging, Phoenix, AZ) in 8 M NaOH solution. After being rinsed with deionized water and dried, the tip was mounted in the STM scanner head. The experiments were performed in atmospheric condition at room temperature and constant current operating mode was applied. Scanning Tunneling Spectroscopy. STS data were acquired using the I-V mode of Nanoscope software version 5.12r4 (Digital Instruments). After the tip was positioned at a specific Au NP on the Au(111) surface, the feedback was shut off and a spectroscopic plot was
a
Deposition condition (a) NaOEt/EtOH, (b) DDT SAM, (c) 10 mM TBAF/THF, and (d) AuNP/Tolune. acquired (the tunneling current was measured as the sample voltage ramped at 8 V/s). In all the measurements, the tip was grounded and the bias potential was applied to the sample. In this configuration, a positive bias corresponds to an electron flow from the tip to the sample while a negative bias corresponds to an electron flow from the sample to the tip. The STS data shown here are averaged I-V curves, measured with at least 20 Au Nps attached to the molecules.
’ RESULTS AND DISCUSSION To assemble the compounds onto the Au(111) surface with a controlled orientation, a sequential deprotection and deposition strategy is shown in Scheme 2. To avoid molecular cross “talk”, the gold electrode (Au/mica) was pretreated with the insulating molecule, dodecanethiol, to form a self-assembled monolayer (DDT SAM). The CNE group was removed in situ with sodium ethoxide/ethanol to release one end thiol; subsequently the deprotected target molecules were adsorbed onto the defect sites of the DDT SAM substrate. Afterward, the molecule 1 modified substrate was treated with a solution of TBAF in THF to free the top thiol. The resulting thiol was reacted with a suspension of gold nanoparticles to give the final sample A1 and A2 (Scheme 2) for the STM and STS analysis. Figure 1a shows the constant-current STM image of the insertion of diode molecules into a DDT SAM. The bright spots as shown in Figure 1b, surrounded by DDT SAM, were obtained after the insertion of molecule 1 and gold nanoparticles were attached on the top. Further evidence for the sequential deprotection process was obtained from grazing incidence FTIR (Supporting Information Figure SI6). The diameter of the Au NPs in STM images ranges from 5 to 8 nm.24 After a stable image was obtained, the Pt/Ir STM tip was positioned at the center of the Au NP and a current-voltage plot is recorded. As was expected, both the STS spectra for molecule 1 and 2 showed moderate degree of asymmetric charge-transport behavior. This asymmetry can be quantified by the rectification ratio (RR) defined as I(þ2.0 V)/-I(-2.0 V). The RR values for the 2085
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Figure 1. Constant-current STM topography of (a) diode molecules inserted into dodecanethol/SAM on Au(111). (b) The image of two AuNPs. STM imaging conditions: Vbias = þ1.0 V, It = 5 pA.
Figure 3. Optimized molecular structures of 1 and 2.
Figure 2. Averaged I-V curves measured over approximately 20 different gold nanoparticles for the assembly A1 (dashed curve) and A2 (green solid curve), Vbias ramped from -2.0 to þ2.0 V. Inset: histogram of RR range for different Au NP A1 and A2 assemblies measured. Set point conditions: 10 pA and 1000 mV.
assembly A1 and A2 range from 1.2 to 3.5, which is smaller than that of the previously reported RR of 4.5 to 9 corresponding to the dipyrimidinyl-diphenyl diblock.21 This deviance is in agreement with the calculated dipole moment difference for the compound 1 (3.4 debye), 2 (2.1 debye), and dipyrimidinyldiphenyl molecule (6.3 debye). The average RR value for molecule 1 and 2 is 2.4 and 1.9 respectively. Their statistic distribution is shown in the inset histogram of Figure 2. The above-described phenomenon of molecular dipoles leading to rectification is not unusual, and we have shown in our previous work that the molecular-rectifying behavior is mainly interpreted in terms of the internal molecular dipole moment and the localization of the wave function.18-22 In a particular example, we have shown that a p-n type molecule exhibited rectification behavior under applied bias voltage, and the asymmetric currentvoltage behavior was reversed once the molecular orientation was flipped over. This fact indicated that the asymmetric molecule itself plays the central role for the rectification phenomenon.20 Substantial differences in molecular conductivity was observed when comparing the I/V characteristics of assembly A1 with A2. As is shown in Figure 2, the I(V) curve is the average of about 20 different immobilized single Au NPs for each assembly. The averaged conductance of molecule 1 is roughly ∼10 times higher than that of molecule 2 at the bias voltage less than -1.5 or above 1.5 V.25 A similar pattern follows at different current set point (see Supporting Information Figure SI7). The results suggested that the hydrogen bonding assisted on the charge transport. The hydroxyl group on molecule 1 forms hydrogen bonding with the
neighbor nitrogen, which helps the molecule maintain a planar conformation. The 1 H NMR spectrum of compound 1 in CD2Cl2 clearly verifies this hypothesis, as the hydrogen-bonded OH proton appears in the spectrum (Supporting Information Figure SI1) at 14.2 ppm. UV-vis spectra also support this result (Supporting Information Figure SI5). The maximum absorbance of molecule 1 red shifted 25 nm in comparison to that of molecule 2, indicating a much better conjugated system in molecule 1 than that in molecule 2. The coplanarity and extended conjugation facilitates the charge transportation. Theoretical calculations, based on B3LYP density functional theory (DFT) using the LANL2DZ basis set leads to the optimized molecular structures that are shown in Figure 3. Three dihedral angles — S1C1C2C3, — C4C5C6N1, and — C7C8C9S2 are -14.13, -0.48, and -18.52, respectively, for molecule 1, while the values are -20.34, -53.79, and þ18.57 for molecule 2, respectively. The results indicated that the disruption of the hydrogen bonding leads to the two adjacent aromatic planes ( — C4C5C6N1) rotated off roughly 54. Estimated from the linear relationship of G/G0 versus cos2 Φ (Φ is the interplane torsion angle) developed by Mayor’s group,17 the conductivity of molecule 1 could be roughly 3 times higher than that of molecule 2. Our results are obviously beyond this range. Owing to the similarity between molecule 1 and 2, we believe that the hydrogen bonding made contributions to the further improved charge transport capability.
’ CONCLUSION In summary, we have synthesized a novel molecular electronic component, which consisted of a hydroxyphenyl pyridine motif. The molecular orientation onto electrode surface is fully controllable through sequential deprotection and deposition strategy. STS studies revealed that the assembly in between two gold electrodes shows moderate rectifying effects. Most interestingly, a simple rotating operation leads to substantially decreased molecular conductivity, which is comparable to switching between “on” and “off” states. Because the compound has the potential for photoinduced proton transfer between O and N sites, it will be a candidate for photoswitch.26 ’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental details for the synthesis of molecule 1 and 2, 1H NMR and 13C NMR characterization,
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Langmuir assembly preparation, STS at different set point, GI-FTIR, and UVvisible measurement. This material is available free of charge via the Internet at http://pubs.acs.org.
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longer, which leads to a lower conductance and subsequently a much lower current and (2) the background current noise is large at lower voltage bias (the NanoScope IV standalone detecting limit ∼10-9 A). (26) Weller, A. Z. Electrochem. 1956, 60, 1144–1147.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors gratefully acknowledge the support of this work by the NSF (NSF-0726897) and NSF MRSEC program at the University of Chicago. ’ REFERENCES (1) Aviram, A.; Ratner, M. Chem. Phys. Lett. 1974, 29, 277–283. (2) Venkataraman, L.; Klare, J. E.; Nuckolls, C.; Hybertsen, M. S.; Steigerwald, M. L. Nature 2006, 442, 904. (3) He, J.; Chen, F.; Li, J.; Sankey, O. F.; Terazono, Y.; Herrero, C.; Gust, D.; Moore, T. A.; Moore, A. L.; Lindsay, S. M. J. Am. Chem. Soc. 2005, 127, 1384. (4) Ying, J.; Liu, I.; Xi, B.; Song, Y.; Campana, C.; Zuo, J.; Ren, T. Angew. Chem., Int. Ed. 2010, 49, 954. (5) Metzger, R. M. Chem. Rev. 2003, 103, 3803–3834. (6) Ying, J.; Cordova, A.; Xu, G.; Ren, T. Chem.—Eur. J. 2007, 13, 687. (7) Chen, F.; Hihath, J.; Huang, Z.; Li, X.; Tao, N. J. Annu. Rev. Phys. Chem. 2007, 58, 535. (8) Albrecht, T.; Guckian, A.; Kuznetsov, A. M.; Vos, J. G.; Ulstrup, J. J. Am. Chem. Soc. 2006, 128, 17132. (9) Collier, C. o, J.; P.; Mattersteig, G.; Wong, E. W.; Luo, Y.; Beverly, K.; Sampaio, J.; Raymo, F. M.; Stoddart, J. F.; Heath, J. R. Science 2000, 289, 1172. (10) Flood, A. H.; Stoddart, J. F.; Steuerman, D. W.; Heath, J. R. Science 2004, 306, 2055. (11) Green, J. E.; Choi, J. W.; Boukai, A.; Bunimovich, Y.; JohnstonHalperin, E.; DeIonno, E.; Luo, Y.; Sheriff, B. A.; Xu, K.; Shin, Y. S.; Tseng, H.-R.; Stoddart, J. F.; Heath, J. R. Nature 2007, 445, 414. (12) Pan, S.; Fu, Q.; Huang, T.; Zhao, A.; Wang, B.; Luo, Y.; Yang, J.; Hou, J. Proc. Nat. Acad. Sci. U.S.A. 2009, 106, 15259–15263. (13) van der Molen, S. J.; Liljeroth, P. J. Phys.: Condens. Matter 2010, 22, 133001. (14) Mayor, M.; Weber, H.; Reichert, J.; Elbing, M.; von Hanisch, C.; Beckmann, D.; Fischer, M. Angew. Chem., Int. Ed. 2003, 42, 5834. (15) Lee, Y.; Yuan, S.; Sanchez, A.; Yu, L. Chem. Commun. 2008, 2, 247–249. (16) Venkataraman, L.; Klare, J. E.; Nuckolls, C.; Hybertsen, M. S.; Steigerwald, M. L. Nature 2006, 442, 904–907. (17) Vonlanthen, D.; Mishchenko, A.; Elbing, M.; Neuburger, M.; Wandlowski, T.; Mayor, M. Angew. Chem., Int. Ed. 2009, 48, 8886. (18) Ng, M.-K.; Lee, D.-C.; Yu, L. J. Am. Chem. Soc. 2002, 124, 11862. (19) Ng, M.-K.; Yu, L. Angew. Chem., Int. Ed. 2002, 41, 3598. (20) Jiang, P.; Morales, G. M.; You, W.; Yu, L. Angew. Chem., Int. Ed. 2004, 43, 4471. (21) Morales, G. M.; Jiang, P.; Yuan, S.; Lee, Y.; Sanchez, A.; You, W.; Yu, L. J. Am. Chem. Soc. 2005, 127, 10457. (22) Diez-Perez, I.; Hihath, J.; Lee, Y.; Yu, L.; Adamska, L.; Kozhushner, M. A.; Oleynik, I. I.; Tao, N. Nat. Chemistry 2009, 392, 635–641. (23) Weare, W. W.; Reed, S. M.; Warner, M. G.; Hutchison, J. E. J. Am. Chem. Soc. 2000, 122, 12890. (24) No coulomb blockade effects observed at room temperature. (25) We compared the conductance ratio at higher voltages. Refer to Mayor’s work because (1) the molecular length in this work is much 2087
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