Distinguishing Diketopyrrolopyrrole Isomers in Single-Molecule

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Distinguishing Diketopyrrolopyrrole Isomers in Single-Molecule Junctions via Reversible Stimuli-Responsive Quantum Interference Yu-Peng Zhang, Li-Chuan Chen, Ze-Qi Zhang, Jing-Jing Cao, Chun Tang, Junyang Liu, Lin-Lin Duan, Yong Huo, Xiangfeng Shao, Wenjing Hong, and Hao-Li Zhang J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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Distinguishing Diketopyrrolopyrrole Isomers in Single-Molecule Junctions via Reversible Stimuli-Responsive Quantum Interference Yu-Peng Zhang,†,§ Li-Chuan Chen,†,§ Ze-Qi Zhang,† Jing-Jing Cao,† Chun Tang,‡ Junyang Liu,‡ Lin-Lin Duan,† Yong Huo,† Xiangfeng Shao,† Wenjing Hong‡,* and Hao-Li Zhang†,⊥,* †

State Key Laboratory of Applied Organic Chemistry (SKLAOC); Key Laboratory of Special Function Materials and Structure Design (MOE); College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, P. R. China ‡ State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Collaborative Innovation Center of Chemistry for Energy Materials, Xiamen University, Xiamen 361005, China. ⊥ Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Tianjin University, and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P. R. China. Supporting Information ABSTRACT: Distinguishing structural isomers at singlemolecule level remains a challenge at present. We report the single-molecule recognition of two diketopyrrolopyrrole containing isomers (SDPP and SPPO) employing the mechanically controllable break junction technique. The single-molecule conductances of the two isomers are indistinguishable under normal condition. However, reversible protonation and deprotonation of the SPPO in molecular junction result in more than one order of magnitude conductance change, which dramatically enhances the conductance difference between the two isomers. Theoretical study reveals that the dramatic conductance switching is due to reversible quantum interference effect. It is suggested that combination of stimuli-response and quantum interference can be an efficient strategy to enhance isomer recognition and conductance switching in single-molecule junctions.

in single-molecule devices.6 The concept of quantum interference describes the interference between the electron waves propagating through molecular orbitals in a single-molecule junction.7 Constructive quantum interference enhances conductance, which has been reported in molecular junctions with double-pathway backbones.8 While destructive-quantum-interference suppresses conductance, which was typically found in cross-conjugated molecules,6b,9 like certain anthraquinone structure9b,10 and benzene ring with meta-configuration.11 Quantum interference effects are believed to be useful in the construction of molecular switches,7a,10b,12 transistors7b,13 and thermoelectric devices.14 Stimuli-responsive nature has previously been utilized to enhance the distinction of molecules in single-molecule junction, including using stimulus of light, acid/base, electric/magnetic fields to modulate the molecular properties.15 However, combination of quantum interference and stimuli-response for isomer recognition has yet been achieved.

The recognition and identification of isomers possess very important significance for both fundamental researches and practical applications.1 A wide range of analytical techniques have been developed to identify and isolate isomers, typically various spectroscopic and chromatographic methods. However, the majority of the available techniques for isomer recognition require large amounts of molecules to be tested. The idea of pushing the isomer recognition capability to single-molecule level is fascinating, however also extremely challenging. Distinguishing structural isomers with utilizing molecule intrinsic charge transport properties in single-molecule junction has attracted great attention due to its scientific significance and application prospect. Break junction techniques enable both distinguishing molecular electrical signatures and counting molecular events/collisions; hence is rapidly entering the chemical space.2 Recently, scientists have succeeded in the recognition of carbohydrate,3 DNA4 and peptide5 in single-molecule circuits. However, these approaches generally require sophisticated instruments and specialized data analysis algorithm.5b In order to facilitate isomer recognition from simple conductance measurements, new strategy is required. Recently, quantum interference phenomena have been explored as a new and efficient strategy for tuning molecular conductance

Scheme I. Schematic of MCBJ setup and the two DPP isomers. Herein, we studied the feasibility of the conductance distinction of two small molecule isomers in single molecule junctions using mechanically controllable break junction (MCBJ) technique (Scheme I). These two isomers have identical molecular length and very similar core structure with slightly different alkyl substitute positions. Although the single-molecule conductance measurements of both isomers showed very similar conductance, the conductance difference between the two isomers undergoes more than one order of magnitude changes upon addition of acid or

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the histograms show distinct conductance clouds (red dotted line area), indicating a high junction formation probability. The relative displacement distribution change very lightly from SDPP to SPPO (0.98±0.16 nm to 0.97±0.16 nm by Gauss function), for which the statistical range was from 10-3.0 to 10-4.5G0 for the two molecules. These data suggest that the two isomers possess relatively rigid structures and the same molecule length to endure almost full stretching before junction breakage. Both onedimensional and two-dimensional histograms indicate that the conductance difference between SDPP and SPPO is within the error range of the measurement; therefore, direct recognition of the two isomers from the single-molecule conductance measurement is not possible.

base. Theoretical calculation reveals that the enhanced conductance difference of reversible acid/base response of one of the isomers is attributed to the reversible switching between nonquantum-interference and destructive-quantum-interference states. This work we demonstrate that combination of stimuli-response and quantum interference can be a new and efficient strategy for enhancing single-molecule level isomer recognition. Diketopyrrolopyrrole (DPP) is a classic organic dye, which has been widely used as building blocks in the construction of a variety of organic semiconductors for optoelectronics.16 Scheme I shows the structures of the two DPP derivatives used in this research, named as SDPP and SPPO, respectively. The synthetic details and spectroscopic characterization are provided in the Supporting Information (SI). Methyl sulfide terminal units are used as the anchoring groups, which have been proven to bind well on gold.17 SDPP and SPPO type isomers are simultaneously produced during the alkylation of the DPP units,18 which are very often coexisted in DPP-containing organic materials. Understanding the charge transport through these DPP containing structures is of fundamental importance for future material design.

Figure 2. Conductance histograms of SPPO-H+ (a) and SPPO-H+TEA (b) All-date-point 2D conductance versus relative distance (∆z) histogram and relative displacement distribution (inset) of (c) SPPO-H+ and (d) SPPO-H+-TEA. The only difference between the two isomers is the alkyl substitution on the nitrogen atoms. Both nitrogen atoms in the SDPP have alkyl substitution, whereas only one nitrogen atom in SPPO has alkyl chain. The free nitrogen in SPPO can be reversibly protonated and deprotonated upon addition of acid and base, respectively. Such reversible halochromic effect was confirmed by UVVis and NMR spectroscopic measurements in solutions (Figure S5 and S6). We have utilized the halochromic effect of the SPPO to enhance the conductance recognition of the two isomers. When camphorsulfonic acid (CSA) was added to the SDPP solution during the in-situ MCBJ measurements, a very slight conductance change from 10-3.46±0.29G0 to 10-3.43±0.30G0 was observed, as shown in Figure 1 and S3, which is within the error range of the fitting. In contrast, when CSA was added to the SPPO junction (noted as SPPO-H+), a dramatically decreased conductance (10-4.62±0.30G0) was obtained from the conductance histograms in Figure 2a. Conductance value of the SPPO-H+(2.40×10-5G0) was around seventeen times lower than that of the SPPO (4.17×10-4G0). Figure 1d and 2c suggest that the relative displacement distribution of SPPO after adding CSA is almost unchanged, indicating that conductance difference is not due to the change of junction lengths, hence should be attributed to the change in molecular conductivity.

Figure 1. (a) Typical measured conductance distance traces of SDPP and SPPO. (b) Conductance histogram comparisons between SDPP and SPPO. All-date-point 2D conductance versus relative distance (∆z) histogram and relative displacement distribution (inset) of (c) SDPP and (d) SPPO. Single-molecule conductance measurements were carried out at room temperature using MCBJ technique in the mixed chloroform/mesitylene (CHCl3/TMB, 1:4v/v) solution with a 0.1mM concentration of the SDPP or SPPO, and a 0.1 V bias voltage was applied between the two electrodes.19 Control experiments were performed using the solvent (CHCl3/TMB, 1:4v/v), which suggested that the background noise was around 10-7G0 (Figure S2). Figure 1a shows the typical conductance traces of SDPP and SPPO. Both isomers show clear molecular plateaus after discrete G0 steps (where G0 is the quantum conductance which equals 2e2/h), indicating a gold atomic chain is pulled out then a molecular junction is formed.20 Figure 1b shows the conductance histograms of both the SDPP and SPPO, each was constructed from over two thousands single conductance traces from Figure 1a without any data selection. Statistical results exhibited the typical ± ± conductance is 10-3.46 0.30G0 for SDPP and 10-3.38 0.29G0 for SPPO.

To further verify whether the acid-induced conductance change of SPPO is indeed due to protonation, triethylamine (TEA) was added to the SPPO-H+ (named as SPPO-H+-TEA) during the conductance measurements. Figure 2b shows that conductance value of SPPO-H+-TEA is 10-3.23 G0, very similar to the initial conduct-

Figures 1c and 1d present the two-dimensional conductance– distance histograms for the two isomers,19b,21 with each of their relative displacement distributions shown as the upright insets. All 2

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Journal of the American Chemical Society ance of SPPO (10-3.38G0). Relative displacement distribution of SPPO and SPPO-H+-TEA show same molecular junction length 0.97 nm in Figure 1d and 2d. When adding the 0.5 nm snap-back distance,19b,22 the lengths of the molecular junctions are around 1.47 nm. These results confirm that the observed acid/base response of the SPPO molecular junction is due to the reversible protonation/deprotonation of the SPPO isomer in the singlemolecule junction. Similar acid/base stimulated measurements were also conducted on SDPP (Figure S3), but observed no obvious conductance change.

with the SDPP and SPPO. As mentioned above, the possible reason for the dramatically lower conductance of SPPO-H+ is quantum interference. Destructive interference is characterized by a sharp dip in the transmission probability, indicating antiresonances take place.6b,9b,10c As shown in Figure 3b, a SPPO-H+ shows an obvious transmission drop near Fermi level that is much deeper than that of SPPO, which confirms that destructive interference occurs in the SPPO-H+ junction.

The conductance difference between the SPPO and SPPO-H+ can be understood from the analysis to their conjugated structures. Several works have discussed the enormous configuration impact on molecular conductance for molecular structure with linearly conjugated form or cross-conjugated form9-10c,19a,23 as a typical consequence of quantum interference. Figure 3a shows the possible resonance structures of the SPPO and SPPO-H+. It can be seen that SPPO possesses a linearly conjugated structure, consisting of linearly extended double-single bond alternating structure, propagating through the whole molecule linking the two anchoring groups, forming a good electron transport path. When protonated, the SPPO-H+ has two possible resonance structures, which are linearly conjugated and cross-conjugated forms, respectively. The linearly conjugated form provides continuous charge transport path similar to that in SPPO. In contrast, the cross-conjugated resonance structure will induce typical destructive-quantuminterference effect, because there are two double-single alternating paths that are not conjugated to each other.24 This destructive interference ultimately arises from the nodes jointing the two pyrrole rings that prevent electronic coupling between two linearly conjugated substructures. Therefore, the cross-conjugated structure is expected to show poor conductance due to destructivequantum-interference.

Figure 3. (a) Resonance structure of SPPO and SPPO-H+.(b) Calculated transmission function for the SDPP, SPPO and SPPOH+ containing molecular junctions.

To find out which conjugation form is dominant in SPPO-H+, we have studied the bond lengths obtained from the density functional theory (DFT) calculation. Figure S7 and Table S1 present detailed bond length information for the SPPO and SPPO-H+. From their bond length variation, we conclude that the dominated structure of SPPO-H+ is the cross-conjugated resonance form. Therefore, it is the destructive-quantum-interference that suppresses electron transport, which is responsible to the lower conductance of SPPO-H+ compared with SPPO. These results confirm that we have realized reversible quantum interference in molecular junctions of SPPO by acid/base stimulation.

As demonstrated by our conductance measurement results, when destructive interference occurs upon protonation to SPPO, more than one order of magnitude of conductance change is observed. More interestingly, the low conductance state of SPPO-H+ can be easily recovered to the high conductance state by deprotonation using base. In fact, previous theoretical investigations have proposed molecular designs of switch devices based on quantum interference effect.7 Herein, we have experimentally demonstrated, for the first time, a quantum-interference-based molecular device that its conductance can be reversibly switched on/off on demand. The unique reversible quantum interference switching phenomenon observed on SPPO shines new light to the design of novel isomer chemistry and molecular electronics.

The destructive-quantum-interference within the SPPO-H+ molecular junction was also confirmed by theoretical simulation of the charge transport in molecular junctions using the DFT and non-equilibrium Green's functions (NEGF) in the ATK software package.25 Figure 3b shows the calculated transmission functions for SDPP, SPPO and SPPO-H+ sandwiched between two gold electrodes. The transmission spectra suggest that the main transmission channels of the SDPP and SPPO junctions are their HOMO orbitals, whereas the main channel is LUMO for SPPOH+. The transmission through SDPP and SPPO shows similar features. The energy levels of the major bands of the SDPP are slightly higher than that of SPPO, consistent with the above calculation of the frontier orbitals (Figure S9). The transmission near the Fermi level is almost identical for the SDPP and SPPO junctions, which explains their similar molecular conductance. The energy levels of the major bands of the SPPO-H+ appear in much lower energies, which is in agreement with DFT calculation. It is noted that the SPPO-H+ exhibited a significant transmission decline just below the Fermi level, which brings about the weak transmission capacity and much lower conductance compared

In summary, we have investigated the charge transport through two isomers SDPP and SPPO using MCBJ technique, and realized the recognition of two isomers at the single-molecule level. The two isomers SDPP and SPPO show similar single-molecule conductance at their neutral states. Meanwhile, reversible acid/base response was found in SPPO with more than one order of magnitude conductance difference. DFT simulations show that the conductance change is due to a significant change of the transport path from non-quantum-interference to destructive-quantuminterference when SPPO was protonated. This work reveals that the combination of stimuli-response and quantum interference may lead to a new and efficient strategy for enhancing singlemolecule level isomer recognition and construction of new molecular devices.

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ASSOCIATED CONTENT Supporting Information Experimental details, UV-Vis spectra, mechanically controllable break junction, control experiment and computational details.

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

Author Contributions §

Yu-Peng Zhang and Li-Chuan Chen contributed equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work is supported by the National Key R&D Program China (2017YFA0204903), National Natural Science Foundation of China (NSFC. 51733004, 51525303, 21602093, 21673195), 111 Project and the Fundamental Research Funds for the Central Universities. The authors thank beam line BL14B1 (Shanghai Synchrotron Radiation Facility) for providing the beam time.

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