Oligofluorene Molecular Wires: Synthesis and Single-Molecule

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Oligofluorene Molecular Wires: Synthesis and Single-Molecule Conductance Cole Sagan, Gina M. Florio, Yi Jiang, Francisco Caban, Jordan Snaider, Rene Amell, and Sujun Wei J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07713 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 18, 2017

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The Journal of Physical Chemistry

Oligofluorene Molecular Wires: Synthesis and Single-Molecule Conductance Cole Sagan,†, ¢ Yi Jiang,‡,ǁ Francisco Caban, ‡ Jordan Snaider,†,§ Rene Amell, † Sujun Wei,*,‡ and Gina M. Florio*,†,¥ †

Department of Chemistry, St. John's University, Jamaica, NY 11439

¥

Department of Physics, St. John's University, Jamaica, NY 11439

‡

Department of Chemistry, Queensborough Community College of the City University of New

York, Bayside, NY 11346 ǁ

Department of Chemistry and Biochemistry, Queens College of the City University of New

York, Flushing, NY 11367

§

Current Address: Department of Chemistry, Purdue University, West Lafayette, IN, 47907

¢

Current Address: Department of Chemistry, University of Wisconsin-Madison, Madison, WI,

53706

*Corresponding Authors: [email protected], [email protected]

ACS Paragon Plus Environment

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ABSTRACT

We report the synthesis and molecular junction conductance for a series of oligofluorenes to establish clear structure-property relationships for this electronically important material. We use a scanning tunneling microscopy based break-junction method (STM-BJ) to measure singlemolecule conductance in oligofluorenes that vary in (a) the number of fluorene repeat units (n = 1-3), (b) bridge carbon substitution (dihydrogen, dimethyl, dihexyl, didodecyl), and (c) linkergroup termination (methyl sulfide versus primary amine).

Conductance in oligofluorene

molecular junctions is found to occur via tunneling, with a tunneling decay constant, β, of 0.31 per Å, or equivalently, 2.6 per fluorene unit, consistent with other π-conjugated molecular wires. Simple tunnel coupling calculations for model Au2-oligofluorene molecular clusters are reported to validate experimental conductance measurements.

Finally, molecular conductance

distributions for methyl sulfide terminated oligofluorenes are observed to be extremely broad due to the relatively flat torsional potential energy surface for rotation about the Au-S bond and the strong orientation effect of the conductance through a π-coupled state.

Introduction Polyfluorenes are electroactive and photoactive materials used as active components in organic electronic devices, such as organic light emitting diodes (O-LEDs), organic photovoltaics (OPVs), and organic field effect transistors (OFETs).1-6 Fluorene, the monomeric base unit of polyfluorenes, received the spotlight as an interesting chromophore throughout the early to mid1900s.7 However, its polymer counterpart, polyfluorene, did not receive much attention again


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until mounting interest in electronic properties of conjugated polymers took off in the late 1970s.8 Polyfluorenes are highly photoluminescent, light absorbing, charge transporting, and thermo-mechanically stable.9 At the same time, they are also solution processable and synthetically tunable, either by substitution on fluorene unit or incorporating other comonomers in the polymer’s backbone. Significant improvements in polyfluorene synthetic methodology, from the early oxidative polymerization by strong Lewis acids10 or electropolymerization,11-12 to more recent cross-coupling polymerization by various metal catalysis systems,13-18 provides more uniform homopolymers17 and expands to include many more copolymers, thus paving the way for wider usage of this class of materials. Despite the successful application of polyfluorene in organic electronic devices, there is a significant lack of reliable models to establish clear structure-property relationships. This is primarily because of the complicated process of charge transport19-21 and the large pool of choices in molecular structures.22 Recently Campos, Venkataraman, and co-workers demonstrated the use of the scanning tunneling microscope break junction method23 (STM-BJ) to study charge transport in oligothiophenes at the molecular level to gain insight into polythiophene.24-25 Inspired by these studies, we performed similar junction conductance measurements on oligofluorenes, which we hope will shed light into the behavior of polyfluorene. To the best of our knowledge, there have been few investigations of the use of oligofluorenes (or related compounds) in single molecule devices.28-32 Herein, we present the synthesis of two series of oligofluorene molecules and report on their molecular conductance when wired between gold quantum point contacts in single molecule junctions (Scheme 1). This work builds upon prior work on the relationship between molecular structure and single molecule junction conductance,33-34 and broadens the number of families of


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molecules for which careful, statistically relevant, junction conductance measurements have been made. Oligofluorene molecular wires can be synthesized with high efficacy and purity, and are readily incorporated into single molecule circuits under solution phase, ambient temperature and pressure conditions. Using the STM-BJ method, we have measured the molecular conductance of the five newly synthesized oligofluorene molecules (Scheme 1), differing in either their length (monomer, dimer, or trimer) or their bridge-carbon chemistry (dimethyl, dihexyl, or didodecyl), as well as a benchmark molecule, 2,7-diaminofluorene (DAF). Density function theory calculations on isolated oligofluorenes and analogue molecules, as well as Au2-clusters have been performed to investigate their structural and electronic properties for comparison with the junction conductance measurements. Oligofluorene molecular wires show higher conductance as compared to prototypical wires with extended π-electronic states, such as oligophenyleneethynylene (OPE) and oligophenylenevinylene (OPV),35-38 as well as a correlation between the trends in the conductance and the HOMO–LUMO gap.33-38 Charge transport in oligofluorene molecular junctions is expected to occur via HOMO-mediated tunneling.33 In addition, we have determined that the conductance is controlled by the orientation of the bond formed between gold and the sulfur atoms of the methyl sulfide (–SMe) linker groups relative to the π-system of fluorene series, in keeping with prior work.39


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Scheme 1. The oligofluorenes synthesized and studied in this work, along with a cartoon of molecular junction formation and measurement using the STM-BJ method. Methods 1. Synthesis Details of the synthesis of each compound studied herein are in the supporting information. The compounds are identified according to the numbers and abbreviations given in Scheme 2. 2. Junction Conductance We use the STM-BJ method to measure molecular conductance under room temperature, atmospheric pressure conditions.23, 40 A detailed description of the method and instrumentation can be found in the supporting information. Briefly, in the STM-BJ method, an atomically sharp gold STM tip is crashed into an atomically flat gold substrate under potential control (~25-50 mV bias voltage). When the crash-pull-break sequence (a “pull-out conductance trace”) is


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performed in the presence of properly functionalized analyte molecules, a single molecule may bridge the gap between the electrodes, allowing the molecular junction conductance to be measured as a function of the tip-surface displacement. We record individual pull-out conductance traces using a home-built STM, replicating the system used by Venkataraman and co-workers.33, 40 We monitor the junction current as a function of tip-substrate displacement over a specified distance (5-8 nm). The junction measurement is repeated continuously and used to generate a statistical data set typically comprised of tens of thousands of individual conductance traces. Individual pull-out conductance traces are binned into conductance histograms without manual data selection using automated algorithms developed by Venkataraman and co-workers.33 All conductance traces starting above 5 G0 and dropping below 5 × 10−5 G0 within the designated tip-surface displacement are included in the conductance histogram. For conductance traces measured in the presence of properly functionalized molecules, conductance histograms show peak(s) associated with molecular junction formation below the 1 G0 peak observed for gold quantum point contacts.41 The center position of a Gaussian curve fit to the molecular peak in the conductance histogram is the most probable molecular junction conductance. Herein, we report the molecular junction conductance as the average value of the most probable conductance obtained by fitting the molecular peak in the conductance histograms of multiple data sets (3-5) of the same molecule, measured under identical conditions. Each data set was obtained using a newly cut gold tip, a new and freshly UV/O3 cleaned gold substrate, and a freshly prepared analyte solution. In our STM break-junction measurements, the largest source of uncertainty is derived from day-to-day variations in the measured conductance. The


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uncertainty in the average molecular conductance is reported here as the standard deviation of the individual conductance values measured for each data set, and is typically found to be within ≤ 10% of the most probable conductance. Two-dimensional conductance histograms are also generated without data selection using an automated algorithm,42 allowing us to monitor how the conductance evolves as a function of junction elongation. 3. Computational Two different types of Density Function Theory calculations were carried out in support of the experimental work. In each case, calculations were performed using the PBEPBE/LanL2DZ method43-48 in Gaussian 0349 running on a CPU. First, we mapped out a relaxed potential energy surface for complete, simultaneous rotation of the torsional angles formed between each methyl sulfide group and the phenyl in 1,4bis(methylthio)benzene (see supporting information). This calculation allows us to confirm the global minimum relative orientation of the two methyl sulfide groups. It also provides an estimate of the energy barrier associated with torsional motion that is relevant for our discussion of the wide range of conductance values observed experimentally for methyl sulfide-linked fluorene molecules (v.i.). In addition, we performed a series of calculations on model fluorene molecular junctions in which the junctions are simplified to isolated Au2-molecular clusters (i.e. Au-1FC6-Au). Such simplified molecular junctions have proven to be rather robust as model systems for comparison with experimental single-molecule conductance measurements.33 For each cluster calculation,


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we first optimized the isolated fluorene molecule and then used the optimized structure to generate a starting structure for the associated Au2-fluorene cluster. Optimized Au2-clusters for 1FC1, 1FC6, 1FC12, and 2FC6 were achieved; however, we were unable to converge the trimer-containing cluster, despite numerous attempts with different convergence criteria and structural modifications (e.g. replacement of the side chains with methyl groups or hydrogen atoms). For each optimized cluster, the frontier orbitals were calculated and their energetics assessed for comparison with conductance measurements. Inversion/mirror symmetry was neither included nor enforced for the geometry optimization and molecular orbitals calculations. Results and Discussion 1. Synthesis

Scheme 2 Synthesis of oligofluorene monomer 1FC6, dimer 2FC6 and trimer 3FC6. The monomer 1FC6, dimer 2FC6 and trimer 3FC6 were synthesized as shown in Scheme 2. Oligofluorene materials are all terminated with methyl sulfide at both ends, to bind


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undercoordinated gold atoms during our STM-BJ measurement. The methyl sulfide was introduced on fluorene by lithium-halogen exchange with n-butyl lithium at -78ºC, followed by quenching with dimethyl disulfide, either at two ends of the molecule (1FC6), or only one end (2). The same strategy was applied to synthesis of key intermediate 3, except quenching with PinBop instead of dimethyl disulfide. With 3 in hand, dimer 2FC6 and trimer 3FC6 were quickly obtained by typical Suzuki-Miyaura cross-coupling reactions.50 Dimethyl 1FC1 and didodecyl 1FC12 were synthesized similarly to 1FC6. All the compounds were purified by silica gel chromatography, and characterized by 1H-NMR, 13C-NMR, and mass spectroscopy. 2,7-diaminofluorene (DAF) (>97%) was purchased from Sigma-Aldrich, and used as received without any further purification. 2. Junction Conductance Junction conductance measurements were made for both series of oligofluorenes molecules (Scheme 1). These data are discussed with respect to what is learned by: (1) varying the length of the molecular wire, (2) in comparison with the benchmark molecule 2,7-diaminofluorene (DAF), (3) changing the substituent installed at the bridge carbon, and (4) the choice of linker group contact chemistry. To determine the magnitude and nature of charge transport in oligofluorene molecular wires, we first measured the molecular-junction conductance as a function of the length of the wire. Specifically, we have varied the number of fluorene units, n, and measured the junction conductance for wires with n = 1, 2, and 3 – the monomer, dimer, and trimer, respectively. In each case, dihexyl sidechains (C6) were installed at the bridge carbon (Scheme 1, top left). Example conductance histograms for the monomer 1FC6 and dimer 2FC6 are shown in Figure


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1 and for the trimer 3FC6 in Figure 2. In Figures 1 and 2, histograms measured for gold in air prior to solution deposition are shown along with those obtained using the same tip and substrate under solution conditions containing the analyte. In all cases, we observe peaks at near integer multipliers of the quantum of conductance (only 1 G0 is shown) due to the formation of Au quantum point contacts,41 whereas molecular junction conductance is observed below 1 G0. We find the average most probable molecular conductance of 1FC6 to be (1.9 ± 0.2) × 10‒3 G0, 2FC6 to be (1.43 ± 0.08) × 10‒4 G0, and 3FC6 to be (1.60 ± 0.02) × 10‒5 G0. Table 1 summarizes these data. Two overlapping molecular peaks are observed in the conductance histograms of 3FC6 (Figure 2) under all concentration conditions for which junction formation was present – a large peak near 1.6 × 10‒5 G0 and a shoulder near 1 × 10‒4 G0. We ascribe the high conductance shoulder to the formation of aggregates in solution, as the relative intensity of this peak grows in or wanes as the solution concentration of 3FC6 is increased or decreased, respectively (supporting information Figure S.2). Aggregation effects are common for large or long π-conjugated oligomers in solution, and are likely associated with π-stacking. A secondary explanation could be the presence of a 2FC6 impurity in the 3FC6 sample; however, any impurity is expected to be present in small quantities (

Oligofluorene Molecular Wires: Synthesis and Single-Molecule

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