- Email: [email protected]
Molecular scale electronics. Syntheses and Testing
ELSEVIER
Synthetic
Metals 84 (1997)
303-306
Molecular scale electronics. Syntheses and Testing Darren L. Pearson, LeRoy Jones III, Jeffry S. Schumm, James M. Tour* Department of Chemistry and Biochemistry University of South Carolina Columbia, South Carolina 29208, USA
Abstract The syntheses of conjugated molecules that are over 100 A long are presented. They are based on oligo(thiophene-ethynylene)s and oligo(phenylene-ethynylene)s. A method for attachment of molecular-sized alligator clips is presented. These serve as bonding locations to gold probes. Keywords: Coupling reactions, and/or conducting polymers
1.
transition-metal-catalysed
reactions,
Introduction
Future computational system will likely consist of logic devices that are ultra dense, ultra fast, and molecular-sized [I31. The slow step in existing computational architectures is not usually the switching time, but the time it takes for an electron to travel between devices. By using molecular scale electronic interconnects [4], the transmit times could be minimized, resulting in computational systems that operate at far greater speeds than is presently attainable from conventional patterned architectural arrays [l]. Though it is well-documented that bulk conjugated organic materials can be semiconducting or even conducting when doped [5], we only recently determined how thiol-ended rigid rod conjugated molecules orient themselves on gold surfaces [6], and how to record electronic conduction through single undoped conjugated molecules that are end-bound onto a metal probe surface [7]. Here we show the syntheses of some soluble oligo(3-ethyl-2,5-thiophene-ethynylene)s and oligo(phenylene-ethynylene)s, potential molecular scale wires, by a rapid iterative divergent/convergent doubling approach [8,9]. Additionally, the attachments of protected thiol moieties to one or both ends of the oligomers are presented. These thiols serve as molecular scale alligator clips for adhesion of the molecular scale wires to the gold probes [6,7]. There has been considerable recent effort to prepare large conjugated molecules of precise length and constitution [lo]. Our approach to these compounds maintains several key features that make it well-suited for the requisite large molecular architectures needed for molecular scale electronics studies. Specifically, the route involves (1) a rapid construction method that permits doubling molecular length 0379~6779197/%17.00 0 1997 Else&r Science S.k All rights reserved PII SO379-6779(96)04015-5
self-assembly
using surface chemistry,
other
conjugated
at each coupling stage to afford an unbranched 100 A oligomer, the approximate size of present nanopatterned probe gaps [l], (2) an iterative approach so that the same high yielding reactions can be used throughout the sequence, (3) the syntheses of conjugated compounds that are semiconducting in the bulk, (4) products that are stable to light and air so that subsequent engineering manipulations will not be impeded, (5) products that could easily permit independent functionalization of the ends to serve as molecular alligator clips that are required for surface contacts to metal probes, (6) products that are rigid in their frameworks so as to minimize conformational flexibility yet containing substituents for maintaining solubility and processability, (7) alkynyl units (cylindrically symmetric) separating the aryl units so that ground state contiguous ?r-overlap will be minimally affected by rotational variations, (8) molecular systems that do not have degenerate ground state resonance forms and are thus not subject to Peierls distortions [5], and finally, (9) products that serve as useful models for the understanding of bulk polymeric materials [5]. 2.
Results
and
Discussion
The iterative divergent/convergent synthetic approaches to oligo(thiophene-ethynylene)s and oligo(phenyleneethynylene)s are outlined in Schemes 1 and 2. We then sought to affix protected thiol moieties to the ends of the oligomers. These thiols were used as molecular alligator clips for adhesion to gold probe surfaces [6,7]. In some cases, we prepared monothiol-terminated systems for adhesion of these oligomers to single gold surfaces. In other cases, we prepared the cr,o-difunctionalized systems for adhesion between two
D.L. Pearson
304
et al. /Synthetic
Metals
84 (1997)
303-306
Scheme 1
100% b-225A
I
CYCH3
50 A
I-
Reagents: (a) LDA, Et20, -78” to 0°C then 12, -78’. %), THF, iPr#lH, 23 “C.
-I
IOOA
(b) K2C03, MeOH, 23°C. (c) &Pd(PPh&
(2 mol %), Cul (1.5 mol
D.L. Pearson et al. /Synthetic Metals 84 [I 997) 303-306
305
Scheme2 a ldRaSiMe3
SiMe, R = CH,C H, ; 1 ;et;ylheptyl 12
R = CHpC H,, 99% R = 3.ethylheptyl, 92% R = C,*H,j, 94%
EbN,
25
Et,N,
R = Cl-W H3, 96% R = 3-ethylheptyl, 96% R = C~H25q 97%
b
EbN,
SiMe,
3 b
W-4
#=&}:
R = CH2C H3, 100% R = 3-ethylheptyl, 96% R = C&lz5 100%
100%
.R R = CH2C H3, 93% R = 3-ethylheptyl, 44% R = C12H25, 87%
=
R = CH H,, 96%* R = 3-et f ylheptyl, 100% R = C,,HPj, 95%
R E CH CH,, 90% R = 3-e&ylheptyl, 78% R = C,,H,,, 65%
d-H
R = Cl+CH3,98% R = 3-ethylheptyl, R = C,2H25, 92%
@+-iMc
+c=J-}
EbN3 -
@+ziM;
R = CH$H, R = 3-ethylheptyl, R = C,,H,,, 48%
++-j;
b 70%
R = CHzC HO, 55% R = 3-ethylheptyl, 100% R = C12H25, 100%
128
t-
Reagents:
rk
R = 3-ethylheptyl, R = C,,H,j, 90% a. MeI.
b. &CO3
MeOH,
or n-Bu,NF,
THF.
c. Pd(dba),
(5 mol%),
26%
Cul (10 mol%),
PPh3(20
mol %), i-Pr,NHTTHF
(1:6).
Fig. 1. A preciselydefinedmacromoleculewith protectedalligatorclip termini.
gold probe surfaces. We found that the terminal aromatic thiols were most difficult to manipulate since they are unstable,undergoingdisulfide formation in the presenceof oxygen, often resulting in insoluble oligomers. However, the acetyl-protected thiols were resilient enough for manipulationsin air, yet they could be readily hydrolyzed with NH40H, in situ, onceexposedto the gold probe surfaces [6a]. We alsoprepareda,w-difunctionalized oligomersbased on someof our larger conjugatedsystems, one of which is
shownin Figure 1. We arepresently using related molecular systems, starting with benzene-l,4-dithiol, to bridge proximal gold probes of a mechanically controllable break junction wherein a single molecule can straddle two atomically sharpgold tips [ll]. This permitsus to quantitate the resistanceacrosssingle molecules. A Coulombblockade wasobservedwherethe adhesionS-Au contacts appearto be the major resistancepoints with an areaof low resistance between[12].
306
References
D.L. Pearson
and
etal. /SyntheticMetals
84 (1997) 303-306
Notes
For somerecent backgroundwork on molecular scale electronics, see: (a) Aviram, A., (ed.), Molecular Electronics: Science and Technology; Confer. Proc. No. 262, AmericanInstitute of Physics: New York, 1992. (b) Miller, J. S. Adv. Mater., 2 (1990) 378, 495, 601. (c) Birge, R. R., (ed,), .Molecular and Biomolecular Advances in Chemistry Series 240, Electronics, American ChemicalSociety, (1991). (d) Kirk, W. P.; Reed, M. A., (eds.), Nanostructures and Mesoscopic Systems, Academic:New York, (1992). PI For sometheoretical considerationson molecularscale wires, see: (a) Samanta,M. P.; Tian, W.; Datta, S.; Henderson,J. I.; Kubiak, C. P. Phys. Rev. B, 53 (1996) R7626. (b) Mujica, V.; Kemp, M.; Roitberg, A.; Ratner. M. J. Phys. Chem., 104 (1996) 7296. (c) Joachim, C.; Vinuesa,J. F. Europhys. Lett. 33 (1996) 635. [31 For somerecent backgroundwork on the formation of molecular-basedtransporters and devices, see: (a) Grosshenny, V.; Harriman, A.; Ziessel R. Angew. Chem. Int. Ed. Engl. 34 (1995) 1100. (b) Langler, L.; Stockman,L; Heremans,J. P.; Bayor, V.; Olk, C. H.; Van Haesendonck,C.; Bmynseraede,Y; Issi, J. -P. Synth. Met. 70 (1995) 1393. (c) Pascual,J. I.; MCndez, J; G6mez-Herrero,J; Bar6, A. M.; Garcia, N; Landman, U.; Luedtker, W. D.; Bogachek, E. N.; Cheng, H. -P. Science 267 (1995)1793. (d) Purcell,S. T.; Garcia,N.; Binh, V. T.; JonesII, L; Tour, J. M. 1. Am. Chem. Sot. 116 (1994) 11985. (e) Martin, C. R. Science 266 (1994) 1961. (f) Seth,J.; Palaniappan,V.; Johnson,T. E.; Prathapan,S.; Lindsey, J. S.; Bocian, D. F. J. Am. Chem. Sot. 116 (1994) 10578. (g) Gust, D. Nature 372 (1994) 133. (h) Wu, C.; Bein, T. Science (1994) 1013. (i) Wagner,R. W.; Lindsay,J. S.; Seth,J.; Palaniappan, V.; Bocain, D. F. J. Am. Chem. Sot. 266, 118, (1996) 3996. (i) Sailor, M. J.; Curtis, C. L. Adv. Mater. 6 (1994) 688. (k) Brigelletti, F.; Flamigni, L; Balzani, V.; Collin, J.; Sauvage,J.; Sour, A.; Constable,E. C.; Thompson,A. M. W. C. J. Am. Chem. Sot. II6 (1994) 7692. (1)Wu, C.; Bein, T. Science 264 (1994) 1757. (m) Moemer, W. E. Science 265 (1994) 46. (n) Sessler J. L.; Capuano,V. L.; Harriman,A. J. Am. Chem. Sot. Ul
[41
[51 WI
[71 PI PI
UOI Cl11
WI
115 (1993) 4618. (0) Farazdel, A.; Dupuis, M.; Clementi,E.; Aviram. A. J. Am. Chem. Sot. 112 (1990) 4206. (p) Tour, J. M.; Wu, R.; Schumm.J. S. J. Am. Chem. Sot. 113 (1991)7064. (q) Dai, H.; Wong, E. W.; Lieber, C. M. Science 272 (1996) 523. (r) Wu, R.; Schumm,J. S.; Pearson,D. L.; Tour, J. M. J. Org. Chem. in press. “Molecular electronics” is a poorly defined term since someauthorsrefer to it asany molecular-based system such as a film or a liquid crystalline array. Other authors,includingus, have preferredto reservethe term “molecularelectronics” for single moleculetasks, such as single molecule-based devices or single molecular wires. Dueto this confusion, we have chosenhere to follow the Petty et al. (Petty, M. C.; Bryce, M. R.; Bloor, D. (eds.), Introduction to Molecular Electronics, Oxford Univ. Press:NewYork, 1995) terminology by using two subcategories,namely “molecular materials for electronics” for bulk applications and “molecular scaleelectronics”for singlemoleculeapplications. Skotheim, T. A., (ed.), Handbook of Conducting Polymers, Dekker: New York, (1986). (a) Tour, J. M.; Jones,L., II; Pearson,D. L.; Lamba,J . S,; Burgin, T.; Whitesides,G. W.; Allara, D. L.; Parikh, A. N.; Atre, S. J. Am. Chem. Sot. 117 (1995) 9529. (b) Dhirani, A.; Zehner, R. W.; Hsung, R. P.; GuyotSionnest,P., Sita, L. R. J. Am. Chem. Sot. 118 (1996) 3319. Bumm,L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones,L., II, Allara, D. L.; Tour, J. M.; Weiss,P. S. Science 271 (1996) 1705. Schumm,J. S.; Pearson,D. L.; Tour, J. M. Angew. Chem. Int. Ed. Engl. 33 (1994) 1360. Pearson, D. L.; Schumm, J. S.; Tour, J. M. Macromolecules 27 (1994)2348. Tour, J. M. Chem. Rev. 96 (1996)537. M. A. Reed,C. Zhou, C. J. Muller, T. P. Burgin andJ. M. Tour, unpublishedwork. For a diagram of a mechanically controllable break junction, see C. J. Muller, J. M. vanRuitenbeekandL. J. deJongh,Physica c 191 (1992)485. We thank the Defense Advanced ResearchProjects Agency for supportof this work.
Synthetic
Metals 84 (1997)
303-306
Molecular scale electronics. Syntheses and Testing Darren L. Pearson, LeRoy Jones III, Jeffry S. Schumm, James M. Tour* Department of Chemistry and Biochemistry University of South Carolina Columbia, South Carolina 29208, USA
Abstract The syntheses of conjugated molecules that are over 100 A long are presented. They are based on oligo(thiophene-ethynylene)s and oligo(phenylene-ethynylene)s. A method for attachment of molecular-sized alligator clips is presented. These serve as bonding locations to gold probes. Keywords: Coupling reactions, and/or conducting polymers
1.
transition-metal-catalysed
reactions,
Introduction
Future computational system will likely consist of logic devices that are ultra dense, ultra fast, and molecular-sized [I31. The slow step in existing computational architectures is not usually the switching time, but the time it takes for an electron to travel between devices. By using molecular scale electronic interconnects [4], the transmit times could be minimized, resulting in computational systems that operate at far greater speeds than is presently attainable from conventional patterned architectural arrays [l]. Though it is well-documented that bulk conjugated organic materials can be semiconducting or even conducting when doped [5], we only recently determined how thiol-ended rigid rod conjugated molecules orient themselves on gold surfaces [6], and how to record electronic conduction through single undoped conjugated molecules that are end-bound onto a metal probe surface [7]. Here we show the syntheses of some soluble oligo(3-ethyl-2,5-thiophene-ethynylene)s and oligo(phenylene-ethynylene)s, potential molecular scale wires, by a rapid iterative divergent/convergent doubling approach [8,9]. Additionally, the attachments of protected thiol moieties to one or both ends of the oligomers are presented. These thiols serve as molecular scale alligator clips for adhesion of the molecular scale wires to the gold probes [6,7]. There has been considerable recent effort to prepare large conjugated molecules of precise length and constitution [lo]. Our approach to these compounds maintains several key features that make it well-suited for the requisite large molecular architectures needed for molecular scale electronics studies. Specifically, the route involves (1) a rapid construction method that permits doubling molecular length 0379~6779197/%17.00 0 1997 Else&r Science S.k All rights reserved PII SO379-6779(96)04015-5
self-assembly
using surface chemistry,
other
conjugated
at each coupling stage to afford an unbranched 100 A oligomer, the approximate size of present nanopatterned probe gaps [l], (2) an iterative approach so that the same high yielding reactions can be used throughout the sequence, (3) the syntheses of conjugated compounds that are semiconducting in the bulk, (4) products that are stable to light and air so that subsequent engineering manipulations will not be impeded, (5) products that could easily permit independent functionalization of the ends to serve as molecular alligator clips that are required for surface contacts to metal probes, (6) products that are rigid in their frameworks so as to minimize conformational flexibility yet containing substituents for maintaining solubility and processability, (7) alkynyl units (cylindrically symmetric) separating the aryl units so that ground state contiguous ?r-overlap will be minimally affected by rotational variations, (8) molecular systems that do not have degenerate ground state resonance forms and are thus not subject to Peierls distortions [5], and finally, (9) products that serve as useful models for the understanding of bulk polymeric materials [5]. 2.
Results
and
Discussion
The iterative divergent/convergent synthetic approaches to oligo(thiophene-ethynylene)s and oligo(phenyleneethynylene)s are outlined in Schemes 1 and 2. We then sought to affix protected thiol moieties to the ends of the oligomers. These thiols were used as molecular alligator clips for adhesion to gold probe surfaces [6,7]. In some cases, we prepared monothiol-terminated systems for adhesion of these oligomers to single gold surfaces. In other cases, we prepared the cr,o-difunctionalized systems for adhesion between two
D.L. Pearson
304
et al. /Synthetic
Metals
84 (1997)
303-306
Scheme 1
100% b-225A
I
CYCH3
50 A
I-
Reagents: (a) LDA, Et20, -78” to 0°C then 12, -78’. %), THF, iPr#lH, 23 “C.
-I
IOOA
(b) K2C03, MeOH, 23°C. (c) &Pd(PPh&
(2 mol %), Cul (1.5 mol
D.L. Pearson et al. /Synthetic Metals 84 [I 997) 303-306
305
Scheme2 a ldRaSiMe3
SiMe, R = CH,C H, ; 1 ;et;ylheptyl 12
R = CHpC H,, 99% R = 3.ethylheptyl, 92% R = C,*H,j, 94%
EbN,
25
Et,N,
R = Cl-W H3, 96% R = 3-ethylheptyl, 96% R = C~H25q 97%
b
EbN,
SiMe,
3 b
W-4
#=&}:
R = CH2C H3, 100% R = 3-ethylheptyl, 96% R = C&lz5 100%
100%
.R R = CH2C H3, 93% R = 3-ethylheptyl, 44% R = C12H25, 87%
=
R = CH H,, 96%* R = 3-et f ylheptyl, 100% R = C,,HPj, 95%
R E CH CH,, 90% R = 3-e&ylheptyl, 78% R = C,,H,,, 65%
d-H
R = Cl+CH3,98% R = 3-ethylheptyl, R = C,2H25, 92%
@+-iMc
+c=J-}
EbN3 -
@+ziM;
R = CH$H, R = 3-ethylheptyl, R = C,,H,,, 48%
++-j;
b 70%
R = CHzC HO, 55% R = 3-ethylheptyl, 100% R = C12H25, 100%
128
t-
Reagents:
rk
R = 3-ethylheptyl, R = C,,H,j, 90% a. MeI.
b. &CO3
MeOH,
or n-Bu,NF,
THF.
c. Pd(dba),
(5 mol%),
26%
Cul (10 mol%),
PPh3(20
mol %), i-Pr,NHTTHF
(1:6).
Fig. 1. A preciselydefinedmacromoleculewith protectedalligatorclip termini.
gold probe surfaces. We found that the terminal aromatic thiols were most difficult to manipulate since they are unstable,undergoingdisulfide formation in the presenceof oxygen, often resulting in insoluble oligomers. However, the acetyl-protected thiols were resilient enough for manipulationsin air, yet they could be readily hydrolyzed with NH40H, in situ, onceexposedto the gold probe surfaces [6a]. We alsoprepareda,w-difunctionalized oligomersbased on someof our larger conjugatedsystems, one of which is
shownin Figure 1. We arepresently using related molecular systems, starting with benzene-l,4-dithiol, to bridge proximal gold probes of a mechanically controllable break junction wherein a single molecule can straddle two atomically sharpgold tips [ll]. This permitsus to quantitate the resistanceacrosssingle molecules. A Coulombblockade wasobservedwherethe adhesionS-Au contacts appearto be the major resistancepoints with an areaof low resistance between[12].
306
References
D.L. Pearson
and
etal. /SyntheticMetals
84 (1997) 303-306
Notes
For somerecent backgroundwork on molecular scale electronics, see: (a) Aviram, A., (ed.), Molecular Electronics: Science and Technology; Confer. Proc. No. 262, AmericanInstitute of Physics: New York, 1992. (b) Miller, J. S. Adv. Mater., 2 (1990) 378, 495, 601. (c) Birge, R. R., (ed,), .Molecular and Biomolecular Advances in Chemistry Series 240, Electronics, American ChemicalSociety, (1991). (d) Kirk, W. P.; Reed, M. A., (eds.), Nanostructures and Mesoscopic Systems, Academic:New York, (1992). PI For sometheoretical considerationson molecularscale wires, see: (a) Samanta,M. P.; Tian, W.; Datta, S.; Henderson,J. I.; Kubiak, C. P. Phys. Rev. B, 53 (1996) R7626. (b) Mujica, V.; Kemp, M.; Roitberg, A.; Ratner. M. J. Phys. Chem., 104 (1996) 7296. (c) Joachim, C.; Vinuesa,J. F. Europhys. Lett. 33 (1996) 635. [31 For somerecent backgroundwork on the formation of molecular-basedtransporters and devices, see: (a) Grosshenny, V.; Harriman, A.; Ziessel R. Angew. Chem. Int. Ed. Engl. 34 (1995) 1100. (b) Langler, L.; Stockman,L; Heremans,J. P.; Bayor, V.; Olk, C. H.; Van Haesendonck,C.; Bmynseraede,Y; Issi, J. -P. Synth. Met. 70 (1995) 1393. (c) Pascual,J. I.; MCndez, J; G6mez-Herrero,J; Bar6, A. M.; Garcia, N; Landman, U.; Luedtker, W. D.; Bogachek, E. N.; Cheng, H. -P. Science 267 (1995)1793. (d) Purcell,S. T.; Garcia,N.; Binh, V. T.; JonesII, L; Tour, J. M. 1. Am. Chem. Sot. 116 (1994) 11985. (e) Martin, C. R. Science 266 (1994) 1961. (f) Seth,J.; Palaniappan,V.; Johnson,T. E.; Prathapan,S.; Lindsey, J. S.; Bocian, D. F. J. Am. Chem. Sot. 116 (1994) 10578. (g) Gust, D. Nature 372 (1994) 133. (h) Wu, C.; Bein, T. Science (1994) 1013. (i) Wagner,R. W.; Lindsay,J. S.; Seth,J.; Palaniappan, V.; Bocain, D. F. J. Am. Chem. Sot. 266, 118, (1996) 3996. (i) Sailor, M. J.; Curtis, C. L. Adv. Mater. 6 (1994) 688. (k) Brigelletti, F.; Flamigni, L; Balzani, V.; Collin, J.; Sauvage,J.; Sour, A.; Constable,E. C.; Thompson,A. M. W. C. J. Am. Chem. Sot. II6 (1994) 7692. (1)Wu, C.; Bein, T. Science 264 (1994) 1757. (m) Moemer, W. E. Science 265 (1994) 46. (n) Sessler J. L.; Capuano,V. L.; Harriman,A. J. Am. Chem. Sot. Ul
[41
[51 WI
[71 PI PI
UOI Cl11
WI
115 (1993) 4618. (0) Farazdel, A.; Dupuis, M.; Clementi,E.; Aviram. A. J. Am. Chem. Sot. 112 (1990) 4206. (p) Tour, J. M.; Wu, R.; Schumm.J. S. J. Am. Chem. Sot. 113 (1991)7064. (q) Dai, H.; Wong, E. W.; Lieber, C. M. Science 272 (1996) 523. (r) Wu, R.; Schumm,J. S.; Pearson,D. L.; Tour, J. M. J. Org. Chem. in press. “Molecular electronics” is a poorly defined term since someauthorsrefer to it asany molecular-based system such as a film or a liquid crystalline array. Other authors,includingus, have preferredto reservethe term “molecularelectronics” for single moleculetasks, such as single molecule-based devices or single molecular wires. Dueto this confusion, we have chosenhere to follow the Petty et al. (Petty, M. C.; Bryce, M. R.; Bloor, D. (eds.), Introduction to Molecular Electronics, Oxford Univ. Press:NewYork, 1995) terminology by using two subcategories,namely “molecular materials for electronics” for bulk applications and “molecular scaleelectronics”for singlemoleculeapplications. Skotheim, T. A., (ed.), Handbook of Conducting Polymers, Dekker: New York, (1986). (a) Tour, J. M.; Jones,L., II; Pearson,D. L.; Lamba,J . S,; Burgin, T.; Whitesides,G. W.; Allara, D. L.; Parikh, A. N.; Atre, S. J. Am. Chem. Sot. 117 (1995) 9529. (b) Dhirani, A.; Zehner, R. W.; Hsung, R. P.; GuyotSionnest,P., Sita, L. R. J. Am. Chem. Sot. 118 (1996) 3319. Bumm,L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones,L., II, Allara, D. L.; Tour, J. M.; Weiss,P. S. Science 271 (1996) 1705. Schumm,J. S.; Pearson,D. L.; Tour, J. M. Angew. Chem. Int. Ed. Engl. 33 (1994) 1360. Pearson, D. L.; Schumm, J. S.; Tour, J. M. Macromolecules 27 (1994)2348. Tour, J. M. Chem. Rev. 96 (1996)537. M. A. Reed,C. Zhou, C. J. Muller, T. P. Burgin andJ. M. Tour, unpublishedwork. For a diagram of a mechanically controllable break junction, see C. J. Muller, J. M. vanRuitenbeekandL. J. deJongh,Physica c 191 (1992)485. We thank the Defense Advanced ResearchProjects Agency for supportof this work.