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Quasi-Ohmic Single Molecule Charge Transport

through Highly Conjugated meso-to-meso Ethyne-

Bridged Porphyrin Wires

Zhihai Li,1,§

Tae-Hong Park,2,§,†

Jeffery Rawson,3 Michael J. Therien,

3* and Eric Borguet

1*

1Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122;

2Department of

Chemistry, 231 South 34th Street, University of Pennsylvania, Philadelphia, Pennsylvania 19104;

3Department of Chemistry, French Family Science Center, 124 Science Drive, Duke University,

Durham, North Carolina 27708

§These authors contributed equally to this work.

†Present Address: Nuclear Chemistry Research Division, Korea Atomic Energy Research Institute

(KAERI), Daejeon, 305-353, Korea

*To whom correspondence may be sent. Email: eborguet@temple.edu; michael.therien@duke.edu

Supporting Information

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Experimental Section

Materials and Methods. Chloroform (anhydrous, >99%), tetrahydrofuran (THF, anhydrous, >

99.9%, inhibitor free), pyridine (anhydrous, 99.8%), mesitylene (1,3,5-trimeththylbenzene, puriss, >

99.0%) were obtained from Sigma-Aldrich. The Au(111) electrode used for preparing the molecule-

modified substrates was a single crystal disc (10 mm diameter, 2.0 mm height) purchased from Mateck

(Germany). Gold wire (0.25 mm in diameter, premion, 99.999%) was obtained from Alfa Aesar.

All manipulations were carried out under argon unless otherwise stated. Tetrahydrofuran (THF)

was distilled over Na/benzophenone under N2. Diisopropylamine (i-Pr2NH), triethylamine (Et3N), and

diisopropylethylamine (DIEA) were distilled over KOH under N2. Pd(PPh3)4 Pd(PPh3)2Cl2,

tris(dibenzylideneacetone)dipalladium(0) (Pd2dba3), triphenylarsine (AsPh3), tris(ortho-tolyl)phosphine

(P(o-tol)3), and CuI were purchased from Strem Chemicals and used as received. N-Bromosuccinimide

(NBS, Aldrich) was recrystallized from water; tetrabutylammonium fluoride (TBAF, 1M THF solution,

Aldrich) was further diluted to 0.1 M in THF. Flash column chromatography and size exclusion

chromatography were performed on the bench top, using silica gel (Silicycle Inc., 200-425 mesh) and

SX-1 Biobeads (Bio-Rad Laboratories, Inc.), respectively.

Design and Synthesis. The synthetic route to S-acetyl-protected dithiol-PZnn compounds

(PZnn-SAc structures) is illustrated in Scheme 1. The synthesis of PZn3-SAc has been reported

elsewhere1.

Scheme 1. Synthesis of PZnn-SAc structures.

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(i) Pd(PPh3)4 , THF/DIEA, 35-40 °C; (ii) Pd2dba3, P(o-tol)3, THF/Et3N, 60 °C; (iii) TBAF, THF, 0 °C;

(iv) Pd2dba3, P(o-tol)3, CuI, n-Bu4NI, DMF/i-PrNH, 70 °C.

General procedure for Pd-catalyzed cross-coupling reactions. All cross-coupling reactions were

performed in a Schlenk tube under Ar. The reagents were placed in the tube, which was then evacuated

and backfilled with Ar three times. A mixture of solvent and base, subjected to three freeze-pump-thaw-

degas cycles, was added to the reaction tube. After the reaction was complete, the reaction mixture was

diluted with CH2Cl2 and washed with sat. NH4Cl (aq) (×2) and water. The combined aqueous layers

were extracted with CH2Cl2 (×3) and the combined organic layers were dried over MgSO4, unless

otherwise stated. The crude mixture was purified by chromatography on silica gel using an appropriate

solvent mixture as eluent. Further purification was performed by size exclusion chromatography on SX-

1 biobeads using THF as eluent, followed by chromatography on silica gel, if necessary.

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Synthesis of 1,2-Bis[(15-(triisopropylsilylethynyl)-10,20-bis(2’,6’-bis(3,3-dimethyl-1-

butyloxy)phenyl)porphinato)zinc(II)-5-yl]ethyne (5). [5-(Triisopropylsilylethynyl)-10,20-bis(2’,6’-

bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)2 (3, 148 mg, 0.125 mmol) and [5-ethynyl-15-

(triisopropylsilylethynyl)-10,20-bis(2’,6’-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)1 (4,

123 mg, 0.104 mmol), Pd2dba3 (14.3 mg, 0.016 mmol), and P(o-tol)3 (38 mg, 0.125 mmol) in 20 mL of

THF/Et3N (5:1) were heated at 60 °C under Ar. Following a 12 h reaction time and isolation as

described above, the residue was chromatographed on silica gel using CHCl3 as eluent. The crude

product was further purified by size exclusion chromatography (SX-1 biobeads, eluent = THF) and

subsequent silica gel chromatography using 1:1 CHCl3:hexanes as eluent, to afford greenish brown solid

5 (210 mg, 90% yield based on 4). 1H NMR (500 MHz, CDCl3): 10.34 (4H, d, J = 4.5 Hz), 9.71 (4H, d,

J = 4.5 Hz), 9.04 (4H, d, J = 4.5 Hz), 8.92 (4H, d, J = 4.5 Hz), 7.74 (4H, t, J = 8.6 Hz), 7.06 (8H, d, J =

8.6 Hz), 3.98 (16H, m), 1.50 (42H, m), 0.95 (16H, t, J = 7.3 Hz), and 0.32 (72H, s) ppm. Vis (λ, (log ε,

THF): 423 (5.20), 438 (5.16), 452 (5.11), 489 (5.63), 575 (4.36), and 739 (5.03) nm. MALDI-TOF

calcd for C136H174N8O8Si2Zn2 (M+): 2231, found 2222.

1,2-Bis[(15-ethynyl-10,20-bis(2’,6’-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato)zinc(II)-

5-yl]ethyne (6). TBAF (0.1 M in THF, 2.8 mL, 0.282 mmol) was added to 5 (210 mg, 0.094 mmol) in

10 mL of THF at 0 °C. After 10 min, the solution was poured onto a plug of silica gel then eluted with

CHCl3. After the solvent was evaporated, the residue was chromatographed over silica gel using 1:5

THF:hexanes as eluent to afford greenish brown solid 6 (186 mg, 99% yield based on 5). 1H NMR (500

MHz, CDCl3): 10.32 (4H, d, J = 4.5 Hz), 9.62 (4H, d, J = 4.5 Hz), 9.00 (4H,d, J = 4.5 Hz), 8.88 (4H, d,

J = 4.5 Hz), 7.73 (4H, t, J = 8.6 Hz), 7.04 (8H, d, J = 8.6 Hz), 4.13 (2H, s), 3.96 (16H, t, J = 7.3 Hz),

0.91 (16H, t, J = 7.3 Hz), and 0.34 (72H, s) ppm. MALDI-TOF calcd for C118H134N8O8Zn2 (M+): 1919,

found 1921.

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[5,15-bis(4’-(S-acetylthio)phenylethynyl)-10,20-bis(2’,6’-bis(3,3-dimethyl-1-

butyloxy)phenyl)porphinato]zinc(II) (PZn1-SAc). 5,15-Diethynyl-10,20-bis(2’,6’-bis(3,3-dimethyl-1-

butyloxy)phenyl)porphinato]zinc(II) (2, 113 mg, 0.116 mmol), 1-(S-acetylthio)-4-iodobenzene (1, 70

mg, 0.256 mmol), and Pd(PPh3)4 (20 mg, 0.017 mmol) in 5 mL of 10:1 THF:DIEA were heated at 35-40

°C for 3 d under Ar. After workup, the residue was chromatographed on silica gel using CHCl3 as

eluent. The crude product was further purified by size exclusion chromatography (SX-1 biobeads in

eluent = THF), and then chromatographed on silica gel using CHCl3 as eluent to afford PZn1-SAc (118

mg, 80% yield based on 2). 1H NMR (500 MHz, CDCl3): 9.65 (4H, d, J = 4.5 Hz), 8.88 (4H, d, J = 4.5

Hz), 8.01 (4 H, d, J = 8.2 Hz), 7.72 (2H, t, J = 8.5 Hz), 7.55 (4H, d, J = 8.2 Hz), 7.01 (8H, d, J = 8.6

Hz), 3.92 (8H, t, J = 7.5 Hz), 2.45 (6H, s, -SCOCH3), 0.89 (8H, t, J = 7.5 Hz), and 0.25 (36H, s) ppm.

Vis (λ, (log ε, THF): 430 (4.81), 452 (5.72), 584 (3.93), and 656 (4.82). HRMS calcd for

C76H80N4O6S2Zn (M+): 1272.4811, found (ESI) 1272.4808.

1,2-Bis[(15-(4’-(S-acetylthio)phenylethynyl)-10,20-bis(2’,6’-bis(3,3-dimethyl-1-

butyloxy)phenyl)porphinato)zinc(II)-5-yl]ethyne (PZn2-SAc). 6 (56.2 mg, 29.2 µmol), 1 (159 mg,

584 µmol), and Pd(PPh3)4 (60 mg, 51.9 µmol) were heated in 10 mL of 10:1 THF:DIEA at 35-40 °C for

3 d under Ar. After workup, the residue was chromatographed on silica gel using CHCl3 as eluent. The

crude product was further purified by size exclusion chromatography (SX-1 biobeads in THF as eluent),

and then chromatographed on silica gel using CHCl3 as the mobile phase to afford PZn2-SAc (54 mg,

83% yield based on 4). 1H NMR (500 MHz, CDCl3): 10.32 (4H, d, J = 4.5 Hz), 9.63 (4H, d, J = 4.5 Hz),

8.97 (4H, d, J = 4.4 Hz), 8.87 (4H, d, J = 4.5 Hz), 8.03 (4H, d, J = 8.2 Hz), 7.74 (4H, t, J = 8.6 Hz), 7.57

(4H, d, J = 8.2 Hz), 7.05 (8H, d, J = 8.7 Hz), 3.97 (16H, t, J = 7.4 Hz), 2.48 (6H, s, -SCOCH3), 0.90

(16H, t, J = 7.4 Hz), and 0.39 (72H, s) ppm. Vis (λ, (log ε, THF): 431 (5.14), 444 (5.17), 458 (5.13), 492

(5.52), 577 (4.28), and 750 (5.01) nm. MALDI-TOF calcd for C134H146N8O10S2Zn2 (M+): 2218, found

2218.

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Molecular solutions and deprotection S-acetyl-protected α,ω-di[(4’-thiophenyl)ethynyl]-

terminated PZnn compounds (PZnn-SAc structures) were constructed via Pd-catalyzed cross-coupling

reactions involving appropriately ethynylated and halogenated building blocks (Scheme 1). Pyridine-

containing CHCl3 or THF solutions of PZnn-SAc compounds were utilized for molecular self assembly

on a gold single crystal surface; pyridine serves to both impede molecular aggregation on the electrode

surface, as well as demask the S-acetyl protecting group3 to provide α,ω-di[(4’-thiophenyl)ethynyl]-

terminated PZnn species (dithiol-PZnn compounds: Figure 1).

Preparation of Self-Assembled Monolayers on Au. Before each experiment, the gold single

crystal was annealed in a hydrogen flame until it became red hot (~2 min). The crystal was then

immediately quenched in hydrogen-saturated clean water (resistivity > 18.2 MΩ.cm), following which it

was dried under an argon stream. Molecular layer assembly utilized 0.05-0.01 mM PZnn-SAc solutions

in CHCl3 or THF solvent; 20 − 50 µM pyridine was added to these solutions to preclude PZnn

aggregation3. The annealed gold electrode was immersed in these solutions, which was subsequently

deaerated with dry argon for 1 min. We chose different assembly times3,4

of either 0.5 – 2 min or 3 – 12

h4 and found that the assembly time did not affect the molecular conductance values (Fig. S2). Modified

electrodes were thoroughly rinsed with solvent (THF or chloroform) following removal from the

assembly solution, prior to single molecule resistance measurements.

STM Break Junction Experiments. STM break junction experiments were carried out using a

Molecular Imaging Picoscan or Picoplus scanning tunneling microscope (Agilent). The STM-tips were

mechanically cut gold wires (wire diameter = 0.25 mm) and the substrate electrode was a Au(111) disk

single crystal as described above. In these studies, the tip was first engaged and the molecule modified

electrode surface was scanned in a constant current imaging mode. Clearly resolved atomically high

surface steps indicated both a sharp tip and a clean surface. Surface terraces were typically observed to

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be 50 - 100 nm wide. Next, the tip was either withdrawn or kept scanning for about 2 hours in order to

allow the system to settle mechanically so as to reduce drifting of the STM tip. The z-direction drift was

checked by switching off the STM feedback loop and simultaneous monitoring the tunneling current

versus time. When the drift was substantially reduced, the STM imaging mode was switched to current-

distance spectroscopy mode, in which the STM was programmed to move the tip in and out of contact

with the surface. The tip movement speed was typically 10-20 nm/s. During the process of the

formation and breaking of junctions, molecules may bridge across the two electrodes (the gold STM tip

and gold substrate) via the anchoring groups of molecules (Figure 1A)5. The displacement of the tip

was automatically repeated to generate a large number of current versus distance traces. Measurements

were made at 3 or 4 biases per molecule; 5,000-10,000 current-distance traces were recorded at a given

bias for statistical analysis. Typically, about 10-15% of the curves show current steps and these curves

were used for constructing the current histograms to determine molecular conductance. (A more detailed

description of data analysis can be found elsewhere.4,6

)

Single molecule break junction experiments were carried out in either mesitylene solvent, or in a

solvent-free environment under ambient atmospheric conditions. Note that these latter experiments

established single molecule conductance values identical to those determined in mesitylene, but evinced

noisier individual current-distance (i-∆s) traces, likely due to diminished damping of vibrations that

arise during the stretching and breaking of molecular junctions.

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Supplemental Schemes and Figures

Figure S1: Electronic absorption spectra PZnn-SAc in THF: n = 1 (black); n = 2 (red); and n =3 (blue).

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Figure S2: Individual current-distance traces for gold electrodes modified with dithiol-PZn1 by

immersing gold electrodes into the molecule-containing chloroform solution (0.05 mM) for 12 hours4

and rinsed with chloroform solvent thoroughly (A, C). Individual current-distance traces for gold

electrodes modified with dithiol-PZn1 by immersing gold electrodes into the molecule-containing

solution (0.05 mM) for 2 minutes3,4

(B, D). The individual traces generated from the samples

prepared by a longer immersion time, e.g., 12 hrs (A, C) are noisier than ones from a shorter time, e.g.,

2 min (B, D). Current-distance traces were measured at bias voltages of 0.05 V (A), 0.1 V (B), 0.2 V

(C) and -0.1 V (D). We did not observe conductance differences between the positive bias and

negative bias, i.e., the bias of 0.10 V (B) and - 0.10 V (D) gives the same molecular conductance for

the symmetrical dithiol-PZn1 molecules

0.0 0.5 1.0

-2.0

-1.5

-1.0

-0.5

0.0

cu

rre

nt

/ n

m

distance / nm

0.0 0.5 1.0

0.0

0.5

1.0

1.5

2.0

Cu

rre

nt / n

A

distance / nm

-0.5 0.0 0.5 1.00.0

0.5

1.0

1.5

2.0

Cu

rre

nt

/ n

A

distance / nm

-1.0 -0.5 0.0 0.5 1.00.00

0.25

0.50

0.75

1.00

Cu

rre

nt /

nA

distance / nm

BA

C

D

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Figure S3: STM image of dithiol-PZn2 modified electrode surface (A), Vbias = 0.10 V, tunneling

setpoint current (iT) = 0.10 nA; sample selected individual current-distance traces (B) at Vbias = 0.10 V,

tip sweeping rate: 10 nm/s, including low conductance (black), high conductance, and mixed low and

high conductance traces, and conductance histogram constructed from 133 curves out of 1000

recorded curves (C), zoom-in of (D) to show low conductance current peaks.

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Figure S4: (A) Conductance histograms of dithiol-PZn3 constructed by 162 stepped traces from

1000 total traces at bias of 0.10 V, bin size: 1 pA; (B) zoom-in of (A). inset: molecule structure of

dithiol-PZn3. Electrodes were modified with PZn3 molecules by immersing the electrode into 0.01

mM dithiol-PZn3 in THF + 1% pyridine solution for 60 second and rinse with THF.

200

150

100

50

0

Co

un

ts

1.00.80.60.40.20.0

Current /nA

200

150

100

50

0

Co

un

ts

0.50.40.30.20.10.0

Current /nA

A

B

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Figure S5: 1H NMR spectra of 1,2-Bis[(15-(triisopropylsilylethynyl)-10,20-bis(2’,6’-bis(3,3-dimethyl-

1-butyloxy)phenyl)porphinato)zinc(II)-5-yl]ethyne (5) in CDCl3.

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Figure S6. 1H NMR spectra of 1,2-Bis[(15-ethynyl-10,20-bis(2’,6’-bis(3,3-dimethyl-1-

butyloxy)phenyl)porphinato)zinc(II)-5-yl]ethyne (6) in CDCl3

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Figure S7. 1H NMR spectra of [5,15-bis(4’-(S-acetylthio)phenylethynyl)-10,20-bis(2’,6’-bis(3,3-

dimethyl-1-butyloxy)phenyl)porphinato]zinc(II) (PZn1-SAc) in CDCl3.

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Figure S8. 1H NMR spectra of 1,2-Bis[(15-(4’-(S-acetylthio)phenylethynyl)-10,20-bis(2’,6’-bis(3,3-

dimethyl-1-butyloxy)phenyl)porphinato)zinc(II)-5-yl]ethyne (PZn2-SAc) in CDCl3.

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Figure S9. 1H NMR spectra of [5,15-Bis[(15’-(4’-(S-acetylthio)phenylethynyl)-10’,20’-bis(2’,6’-

bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato)zinc(II)ethyn-5’-yl]-10,20-bis(2’,6’-bis(3,3-dimethyl-1-

butyloxy)phenyl)porphinato]zinc(II) (PZn3-SAc) in CDCl3.

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