Synthesis of Functionalized Molecular Wires : New Materials for Organic Electronics

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Synthesis of Functionalized Molecular Wires

—

New Materials for Organic Electronics

Dissertation submitted for the degree of Doctor of Natural Sciences

Presented by Jannic Sebastian Wolf

at the

Mathematisch‐Naturwissenschaftliche Sektion Fachbereich Chemie der Universität Konstanz

Date of the oral examination: 27.09.2013 First supervisor: Prof. Dr. U. Groth Second supervisor: Prof. Dr. E. Scheer

Third supervisor: Prof. Dr. R. Winter Oral examiner: Priv. Doz. Dr. T. Exner

Konstanzer Online-Publikations-System (KOPS) URL: http://nbn-resolving.de/urn:nbn:de:bsz:352-247603

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I Ein Forscher, wenn er sich, wie sich das gehört, in Neuland begibt,

macht meistens Dinge, die er nicht versteht.

Und wenn er das Neue hat,

kommen die, die es schon immer gewusst haben.

Heinz Maier‐Leibnitz

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II

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Danksagung

III

Danksagung

Die vorliegende Arbeit entstand im Zeitraum von Oktober 2009 bis August 2013 im Fachbereich Chemie der Universität Konstanz in der Arbeitsgruppe von Prof. Dr. Groth.

Mein besonderer Dank gilt Herrn Prof. Dr. Groth für sein stetes Interesse am Fortgang dieser Arbeit, für die Möglichkeit frei auf einem interdisziplinären Forschungsgebiet an der Grenze zwischen Chemie und Physik arbeiten zu können, sowie für die Ausstellung des Erstgutachtens zu dieser Arbeit. Frau Prof. Scheer danke ich für die Möglichkeit in ihrer Arbeitsgruppe Experimente durchführen zu können, sowie für die Übernahme des Zweitgutachtens. Herrn Prof. Winter danke ich für die Ausstellung des Drittgutachtens und die Übernahme des Prüfungsvorsitzes. Herrn Prof. Exner danke ich für interessante Diskussionen zu DFT‐Rechnungen von Molekülorbitalen und angeregten Zuständen von molekularen Schaltern, für die Durchführung der Berechnungen und die Teilnahme an der Prüfung als mündlicher Prüfer.

Herrn Dr. Huhn danke ich für das gemeinsame Durchstehen von etlichen wissenschaftlichen Diskussionen, neue Impulse wenn das Thema mal wieder in einer Sackgasse steckte, Kaffeegespräche, kurzum für die gelungene Betreuung der vorliegenden Arbeit. Mein Dank gebührt Ihm außerdem für das Lösen der Röntgenstrukturen und das kritischen Lektorat dieser Arbeit.

Meinen Bachelorstudenten Timo Witt, Holger Reiner, Corinna Kirchner, Iris Eberspächer und Michael King danke ich herzlich für Ihre Lernbereitschaft und die begeisterte Mitarbeit an meiner Forschung. Oft genug bildeten Ihre Arbeiten den Ansatzpunkt für weitere Ideen, die in dieser Arbeit verwirklicht wurden.

Gleiches gilt für meine Mitarbeiter Patrick Ortmann, Ilona Heckler, Patrick Höring, Timo Witt, Iris Eberspächer, Stivi Schütter, Jennifer Römer und David Siebert, bei denen ich mich immer auf Ihre Ausdauer beim Ausprobieren oder Nachkochen verlassen konnte.

Ein besonderer Dank gebührt meinem langjährigen Kooperationspartner Simon Verleger aus der AG Scheer. Bei der Bearbeitung des „Stempel‐Projekts“ zeigte sich hier immer wieder, wie spannend und vor allem fruchtbar die Zusammenarbeit mit Kollegen anderer Fachbereiche sein kann, wenn es einfach passt.

Durch seine Beiträge verschiedener Modelle zum „Protonierungsgleichgewicht‐

Projekt“ und das Anpassen der Modelle an die experimentellen Daten mit Hilfe von Mathematica‐Programmen konnte Herr Prof. Steiner dieser Arbeit einen besonderen Höhepunkt verleihen. Hierfür gebührt ihm mein herzlicher Dank.

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IV

Der Gruppe um Dr. Artur Erbe (FZ Dresden) sowie Dr. Francesca Moresco (TU Dresden) danke ich für Ihr Interesse an meinen Molekülen und die daran durchgeführten physikalischen Untersuchungen. Dr. James Kingsley (Ossila Ltd, Sheffield) danke ich dafür, dass er mit großem Einsatz die bestmöglichen Ergebnisse aus meinen OLED Experimenten heraus geholt hat. Prof. Dr. Takeya (ISIR, Osaka) danke ich für die Berechnung der Austauschintegrale sowie den Hinweis, das hier potentielle OFET Materialien vorliegen. Artem Fedoseev danke ich für die Einführung in die photokinetische Untersuchung der molekularen Schalter, sowie seine Hilfe bei meinen ersten eigenen Experimenten. Inigo Göttker und Bernhard Weibert danke ich für das Aufnehmen und teilweise Lösen der Röntgenstrukturen. Matthias Hagner danke ich für seine Hilfbereischaft beim Arbeiten im Nanolabor der Uni Konstanz. Patrick Braun danke ich für das Aufnehmen des EPR Spektrums. Torben Seitz danke ich für das Messen der LC‐MS Daten. Dr. Markus Ringwald (MCAT) danke ich für die Unterstützung meiner Forschung durch die mir überlassenen Palladium‐Katalysatoren.

Schließlich möchte ich mich bei all meinen tollen Kollegen, mit denen ich in den letzten Jahren in der AG Groth zusammen arbeiten durfte, bedanken. Das gilt natürlich für alle ehemaligen, aber besonders für die aktuelle Mannschaft. Nicht unerwähnt bleiben sollten darunter Joachim Braun (für sein kritisches Lektorat von Teilen dieser Arbeit, für das tägliche „Käffsche nachher“ und die Möglichkeit, mit Ihm so manche Sache auch einmal humoristisch‐unreflektiert diskutieren zu können), Johannes Drexler (für sein kritisches Lektorat von Teilen dieser Arbeit und für die Kunst‐am‐Labor‐Tafel) und Juliane Leutzow (für Ihr kritisches Lektorat von Teilen dieser Arbeit und für Ihr Vertrauen in mich, wenn es um das Steuern eines Kanus geht). Malin Bein möchte ich für Ihre Unterstützung bei der Benutzung der GC und HPLC Geräte danken.

Zu guter Letzt möchte ich meiner Familie danken, die mich in all den Jahren in Konstanz immer unterstützt hat.

Sollte ich in der obigen Aufzählung einen Namen unerwähnt gelassen haben, so bestimmt ohne Absicht. Während meiner Promotion konnte ich jedoch mit so vielen netten Menschen zusammen Arbeiten und meine Ergebnisse Diskutieren, dass es nicht möglich wäre, hier alle zu nennen.

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Table of Contents

V

Abbreviations

 molar extinction coefficient

Ac acetyl

ADF amsterdam density functional

AFM atomic force microscopy

aq aqueous

ATR attenuated total reflection

bp. boiling point

BuLi buthyllithium

Calcd calculated

CCDC Cambridge Crystallographic Data Centre

cd candela

cf. compare

CMOS complementary metal oxide semiconductor

compd compound

CP‐AFM conductive probe atomic force microscopy

DC direct current

DCM dichloromethane

DFT density functional theory

DI de‐ionized

DMAP dimethylamino pyridine

DMF dimethyl formamide

DMSO dimethylsulfoxide

DSC differential scanning calorimetry

EDC 1‐ethyl‐3‐(3‐dimethylaminopropyl)carbodiimid

EI electron impact

EBJ electromigrated break junction

equiv equivalent

Et Ethyl

et al. and others

ETMS ethyl trimethylsilyl

FAB fast atom bombardement

GC gas chromatography

GGA generalized gradient approximation

HOBt hydroxybenzotriazole

HOMO highest occupied molecular orbital

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X

HPLC high‐performance liquid chromatography

Hz hertz

IETS inelastic electron tunneling spectroscopy

IR infrared

ITO indium tin oxide

LB‐film Langmuir‐Blodgett film

LC liquid chromatography

LDA lithium diisopropylamide

LED light emitting diode

LT low temperature

LUMO lowest unoccupied molecular orbital

M mol/l

max maximum

MCBJ mechanically controlled break junction

Me methyl

MHz megahertz

MPTMS 3‐mercaptopropyl‐trimethoxysilane

mp. melting point

MS mass spectrometry

nTP nanotransfer printing

NMR nuclear magnetic resonance

OFET organic field effect transistor OLED organic light emitting diode

OPI oligophenylenimine

OPE oligophenylene(ethynylene)

PEDOT:PSS poly(3,4‐ethylenedioxythiophene) poly(styrenesulfonate)

PDMS poly(dimethylsiloxane)

ppm parts per million

PSS photostationary state

quant. quantitative

rpm revolutions per minute

rt room temperature

SAM self assembled monolayer

STM‐BJ scanning tunneling microscopy break junction

SEM scanning electron microscopy

SiO2 silica gel

SM starting material

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Abbreviations

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SMW switchable molecular wire

STM scanning tunneling microscopy

TAOPE tetra‐aryl oligophenylene(ethynylene)

TBAF tetrabutylammonium fluoride

TEA triethylamine

TEMPO tetramethyl piperidyl‐1‐oxyl

TEMPOL tetramethyl piperidyl‐1‐oxyl‐alcohol

TFA trifluoroacetic acid

THF tetrahydrofurane

TLC thin layer chromatography

TMS trimethylsilyl

TMSA trimethylsilylacetylene

UV ultraviolet

UV‐vis ultraviolet‐visible

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XII

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Introduction to Molecular Electronics

1 1 Introduction to Molecular Electronics

Molecular Electronics is the incorporation of

functional molecular structures into electronic devices.

This very basic idea has been around for more than 40 years now, initiated by the studies of Kuhn and Mann on conductivity measurements through monolayers of fatty acid salts in 1971.^[1] A few years later, Aviram and Ratner proposed a single molecular rectifier and calculated the transmission properties of a device that incorporated a rectifying molecule between metal electrodes.^[2] Although this is most often discerned as the “big bang” of Molecular Electronics, research didn’t gain momentum till the invention of the scanning tunneling microscope (STM) and the atomic force microscope (AFM) at IBM, Switzerland. Both inventions dramatically influenced the field, since up to that time, contacting small ensembles or even single molecules was merely impossible.

After years, the first single‐molecule transport measurement was conducted by Reed an coworkers at Yale University.^[3] With the mechanically controlled break junction (MCBJ), a device was introduced that nowadays is one of the most valuable tools for single molecule transport measurements.

Initiated by these experiments, ideas of electric circuits, logic and hence computers made of molecules started to emerge. It was the proper time for ideas that promised to fulfill the famous Moore’s Law, which predicts that the number of transistors on the same wafer size doubles every two years.^[4] At that time, new technologies for further miniaturization of complementary metal oxide semiconductor (CMOS) devices were still questionable.^[5] For example, the loss of insulating properties of silicon oxide layers thinner than 1.2 nm was yet to overcome.^[6] The all organic computer, made from functional molecular building blocks such as wires, switches, resistors and diodes was considered as an appropriate bottom up approach to solve this problem.^[7] However, in 2013 the silicon based computer is unlikely to be replaced by its organic counterpart based on Molecular Electronics. The semiconductor industry successfully passed all hurdles on the invention of new technologies for “traditional”

electronics, albeit with new materials and techniques. Consequently, Moore’s Law still holds to some extend, as can be seen in Figure 1. Current industry roadmaps additionally deal with the two concepts “More Moore” and “More‐than Moore”.^[8] The first paradigm involves further shrinking of the digital functionalities (logic and memory storage) in order to improve cost per function and performance.

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Figure 1 Logarithmic plot of transistor packing density on microprocessors. The normalized transistor number per mm²is plotted against the year of introduction, demonstrating the validity of Moore’s Law.

Data taken from ref^[9].

The latter concept breaks with Moore’s Law and includes functionalities that provide additional value, for example the integration of several components into a single chip.

Molecular Electronics is not limited to understanding the basic properties of new materials. The field branched into different specialized areas, and it is important to separate them for a better understanding of actual research. Besides the single molecular or monolayer systems, bulk molecular systems play an important role for applications that are more close to being manufactured. This part is most often described as Organic Electronics, or Plastic Electronics. Thin films of small molecules or polymers that can be processed by either vapor or solution processing act as conductive electrodes, harvest light in organic solar cells or produce light in organic light emitting devices. Most of these devices can be produced on flexible substrates and allow the fabrication of cheap devices for example by roll‐to‐roll printing on plastic foil.

The most striking feature in both areas, however, is the possibility to influence device properties already at the drawing board. As molecules are responsible for the functionality of all of these devices, especially organic chemistry contributes to this highly interdisciplinary field with its countless synthetic possibilities.

In the present work, tailor‐made organic molecules for various applications spanning over both fields, Molecular and Organic Electronics, are presented. The syntheses focus on oligophenylene(ethynylene) (OPE) based molecular wires for charge transport measurements and applications such as organic light emitting devices (OLED).

Besides the testing of new molecules in available devices, a new concept for the formation of metal‐molecule‐metal contacts on molecular ensembles was investigated.

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Contacting Molecules in Molecular Electronics

3 2 Methods and Concepts

2.1 Contacting Molecules in Molecular Electronics

2.1.1 Contacting Ensembles of Molecules

Probing many molecules in parallel has some certain advantages that explains the till now unbroken interest in the fabrication of such junctions.^[10] First of all, as soon as a significant number of molecules are probed, the measured properties of the ensemble are reliably detected, yielding the averaged electronic properties of a single molecule in all of its conformations and contact geometries. Ensemble devices can be probed by optical spectroscopy, and the mass production and incorporation of organic devices in existing CMOS technology is feasible without major technical inventions.^[10] Therefore, a plentitude of techniques has been developed that allow the formation of metal or semiconductor contacts to molecular ensembles, with early examples based on self assembled monolayers (SAM) and Langmuir‐Blodgett (LB) films^[11‐13]. Nowadays, most of the contacting methods are based on SAMs sandwiched between metal electrodes.

This section covers some of the techniques that figured most prominently over the last years. A special focus will be placed on the transfer‐printing technique, since the soft deposition of metal films onto SAMs was also part of the present work.

One of the most severe problems when contacting organic monolayers with a second metal electrode is the penetration of the SAM by metal atoms. Especially thermally evaporated top‐electrodes are prone to the formation of metal filaments through the molecular layer, thereby short circuiting the device.^[14] Introduced in 1997 by Zhou et al.^[15], nanopores are one elegant way to limit the risk for film‐penetration.

A small pore was defined in a free standing silicon nitride membrane, with an approximately 30 nm hole at the bottom of the pore. One side of the sample was then coated with gold, forming the bottom electrode of the device. The SAM was thereafter assembled in the 30 nm well, followed by carefully evaporating the top‐contact on the device. Due to the small diameter of the pores, short circuits are less probable. One important application of the nanopore technique was the unambiguous proof of tunneling as the dominant transport mechanisms in alkanethiol SAMs.^[16]

Despite the enhanced device yield of nanopores compared to top electrodes directly evaporated onto a SAM, the nanopore concept was optimized by polymer based electrodes. The application of a conductive polymer completely evades the direct vapor deposition of metals onto the sensitive SAM. Polymer electrodes were first introduced by Akkerman et al.^[17‐18] and fabricated as shown in Figure 2. The device yields were

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found to be higher than 95%, even for very large junctions with a diameter of 100 µm.

The authors probed the device for alkanedithiols with different length and found the expected temperature independent through‐bond tunneling charge transport. The metal‐like PEDOT:PSS polymer electrode did not induce any significant asymmetry to the junction.

Figure 2 Large‐area polymer electrode junction by Akkerman et al. a) A bottom electrode is defined on a Si wafer. b) A lithographically defined hole in a photoresist is used as well for monolayer assembly. c) PEDOT:PSS is spin coated onto the device. d) Deposition of a top‐electrode onto the conductive polymer.

Among the methods that try to reduce defects due to shorts, a new technique called Nanoskiving allows the investigation of relatively small molecular ensembles.

Nanoskiving was pioneered by the Whitesides group^[19] and brought to application in metal‐molecule‐metal contacts by Pourhossein et al.^[20] The process involves the assembly of a SAM onto a gold electrode that is thereafter covered with a slightly offset vapor deposited gold top‐electrode. This stacked architecture was then embedded into an epoxy matrix and sliced in 50–100 nm thin slabs with an ultramicrotome. Since the resulting area of the SAM in between the metal electrodes is reduced to roughly 50 µm², the usually observed shorts in vapor deposited devices are avoided similar to the Nanopore technique. Furthermore, it is possible to produce hundreds of devices from one epoxy block. The authors reported yields from 36% up to 71% depending on the used alkanedithiol SAM.

Many other ways of soft contacting have been developed; most of them completely avoid the vapor deposition of metals onto the active organic layer. Early junctions involve a SAM‐coated mercury drop electrode that is brought in contact with a SAM or LB‐film on a metal substrate.^[21] The reproducibility of these junctions is rather

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5 bad, and eutectic GaIn alloys^[22] have started to take over their place. A much more reliable technique for the formation of metal‐molecule‐metal contacts are crossed‐wire and crossbar junctions. Crossed‐wire junctions consist of two thin metal wires covered with a SAM that cross each other in close proximity. As soon as a DC voltage is applied to one of the wires, the generated Lorentz‐force deflects the wire in an external magnetic field. By controlling the bias, both wires are brought in contact and form a metal‐

molecule‐metal junction. The pioneering works used the crossed‐wire junction to measure charge transport in different OPEs.^[23‐24]

Crossbar junctions extend this technique to larger electrode areas but suffer from the same metal deposition problems as discussed above. Therefore, new methods for the deposition of large area metal electrodes like lift‐off/float‐on^[25] or transfer printing (see below) of metal electrodes had to be developed. An especially fascinating method for the preparation of large arrays of crossbar junctions from semiconducting materials was reported by Bufon^[26] and is shown in Figure 3. A strained composite semiconducting structure was prepared on top of a sacrificial layer next to a SAM covered gold electrode. As soon as the sacrificial layer was dissolved, the strained multilayer rolled up to a tube‐shaped electrode that rests on top of the SAM covered bottom electrode.

Figure 3 a) Differential resistance of Au/SAM heterojunctions. The characteristics have been derived from the IV‐traces shown in the bottom insert. The top insert presents the device fabrication. b) SEM image of an array of devices. c) Close‐up SEM image of one device. The crossbar junction is shown in the insert. Reprinted with permission from ref^[26]. Copyright 2011, American Chemical Society.

The method allowed a high degree of parallelization and therefore the fabrication of many devices in one run. Alkanethioles with different length gave rise to the expected length‐dependent junction resistance.

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6

Compared to the methods discussed above, nanotransfer printing (nTP) appears to be a rather simple and straightforward method on the first glance. Introduced by Loo et al.^[27], the technique relies on the interplay of the surface chemistry on a substrate and a tailor‐made stamp, respectively. In this seminal publication, a gold film on a patterned poly(dimethylsiloxane) (PDMS) stamp was transferred onto an oxidized titanium film. This concept was extended to the chemical modification of a silicon substrate, as shown in Figure 4.^[28] A cleaned silicon wafer was oxidized to generate surface hydroxyl groups. The substrate was then reacted with the bifunctional 3‐mercaptopropyl‐trimethoxysilane (MPTMS) that carries a silane for condensation with the surface hydroxyl groups and a thiol for the printing process. After the surface had been modified, a gold coated PDMS stamp was brought into contact with the surface.

Thiols on the surface of the SAM form covalent bonds to the gold film on the stamp, which was peeled off from the substrate after a distinct time. The protruding gold pattern was transferred onto the SAM.

Figure 4 Nanotransfer printing by Loo et al. A thiol terminated monolayer captures the gold film on the stamp by formation of covalent Au‐S bonds. The gold on the protruding areas therefore sticks to the surface. Reprinted with permission from ref^[28]. Copyright 2002, American Chemical Society.

This approach is not limited to silicon wafers, but can be reproduced on any hydroxyl group carrying substrate, such as glass or even paper^[29]. By altering the interface chemistry, other substrates such as GaAs are compatible with the method, enabling high resolution pattern transfer on octanedithiol monolayers in GaAs.^[30] Since n⁺(001) GaAs has a high conductivity, the first molecular contacts by nTP for charge transport measurements were fabricated on GaAs with octanedithiol.^[31] However, endeavors to realize symmetric metal‐molecule‐metal contacts quickly lead to the development of novel junctions based on the deposition of for example gold nano‐dots onto a SAM coated gold substrate.^[32] The contact pads were connected by a conductive probe AFM (CP‐AFM) and the typical tunneling behavior of an alkanethiol monolayer was observed. This experiment also revealed that the pressure that is applied to the contact pad strongly influences the conductance of the junctions. This suggests that depending on the method of contacting, different junction behavior has to be expected.

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7 The success of nTP crucially depends on the proper interface conditions, and many tweaks have been added to the method to improve its reproducibility. For example, special treatment of the PDMS stamp has been applied or other stamp‐

materials such as fluoropolymers with low surface energy have been used.^[33] Further considerations to this problem will be discussed in chapter 4.6. Since the nano‐dot connection method always involves direct contact of the junction with the probe (for example a CP‐AFM tip), crossbar structures that allow the noninvasive connection of the junctions have been developed at the University of Konstanz.^[34‐37] The results from ref^[37] are summarized in the following paragraph.

The crossbar samples were prepared on oxide free GaAs (100). The devices comprised of a three‐layered bottom electrode, shown in Figure 5a. Silicon oxide (5 nm) was used as insulating layer between substrate and electrode. A thin layer of chromium (5nm) was applied as adhesion promoter before the gold layer was deposited (15 nm).

The substrates were immersed into a solution of a dithiol to form a SAM on the whole device. In analogy to the experiments by Loo et al., the SAM surrounding the metal electrode was intended as glue that sticks the whole electrode to the substrate during the printing process.

Figure 5 a) Bottom‐electrode structure of the nTP‐substrates in ref^[37]. Silicon dioxide ∎ as insulator, chromium ∎ as adhesion promoter and gold ∎ as electrode material have been thermally evaporated onto an oxide free GaAs ∎ wafer through a shadow mask. The length of the wire between the contact pads was 15 mm; the width of the electrode was 25 µm. b) The cartoon demonstrates the printing process of the top‐electrode with a 15 nm thin gold film on a PDMS stamp. Taken from ref^[37].

After the assembly of the molecules on the device, a patterned PDMS stamp coated with gold (15 nm) was applied to the reactive surface (Figure 5b). The thiol groups terminating the SAM reacted with the gold, which remained on the surface of the device, yielding a large number of metal‐molecule‐metal junctions produced at the same time. An assembled device is shown in Figure 6. Despite the promising macroscopic view of the samples, only one single junction of a particular sample exhibited characteristics of a metal‐molecule‐metal contact. The IV characteristics, however, showed a strong temperature dependence that was not in agreement with the expected tunneling behavior through an alkanedithiol junction.

a b

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Figure 6 a) Cartoon of an assembled crossbar device after the nTP‐printing process. b) Optical micrograph of an assembled device. Taken from ref^[37].

SEM pictures of the fabricated devices revealed that due to geometrical constraints of the evaporating system, the three‐layered stack of the bottom electrode was not properly aligned, with the silicon dioxide systematically displaced to one side of the stack. Most of the time, the printed gold electrodes got ruptured at this displaced silicon dioxide layer. Further experiments without silicon dioxide showed that the transferred top‐electrode is also prone to damage directly at the gold–gold cross‐

section. In order to overcome the problems with the thiol‐limited GaAs substrate and the complicated stacked bottom‐electrode, a new silicon based design was proposed that should rely on the surface chemistry shown in Figure 4. The elaborated concepts from these studies have been used as a starting point for further developments within this work, which are discussed in chapter 4.6.

2.1.2 Contacting Single Molecules

Single molecules are most conveniently contacted by MCBJ, scanning tunneling microscopy break junctions (STM‐BJ), CP‐AFM or electromigrated break junctions (EBJ).

In the following paragraph, these techniques and selected results on OPE type molecular wires are briefly reviewed.

During the fabrication process of a MCBJ, a metal layer is lithographically structured to a thin wire that is attached to larger area gold‐contacts, or a notched wire is spanned over a gap. Underneath the metal, an insulating polymer or oxide layer electrically decouples the system from a flexible substrate. A scanning electron microscope (SEM) image of an unbroken junction is shown in Figure 7. The sample is placed in a three‐point mechanism that allows the controlled bending of the substrate.

Thereby, the metal structure is stretched until the electrodes are formed by the formation of a gap in the structure. A unique feature of the MCBJ is the possibility to adjust the geometry of the gap on the sub Ångstrom scale during the whole experiment, therefore making this method ideal for the investigation of molecules of different length.

Furthermore, the electrodes can be formed at variable temperature in solution, in the gas phase or in vacuum, giving access to a broad range of deposition techniques for

a

b

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9 molecules. Typical measurements in MCBJs are the collection of conductance histograms from repeated opening/closing cycles, current/voltage spectroscopy at a fixed junction position on a single molecule, and more advanced techniques like inelastic tunneling spectroscopy (IETS) or transition voltage spectroscopy (TVS). A good introduction to both tools that also help to separate real molecular junctions from artifacts is found in the review of Song et al.^[38] Due to the adjustable gap size, molecules in contact with the electrodes can be stretched and their electrical response measured.

Figure 7 SEM image of a break junction. The free standing gold bridge between larger metal islands was not broken. The insulating substrate is colored in green. Taken from ref^[39]. Copyright © 2009 by John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.

The first example of a single molecular contact measured in a MCBJ was reported by Reed et al.^[3] In their experiment, a liquid cell around the break junction was used to self assemble 1,4‐benzenedithiol onto a notched gold wire. As soon as the gold wire was broken, the resulting tips got also covered by the SAM. The solvent was evaporated, and the junction was slowly closed until the conductance onset was monitored. From control experiments, the authors deduced that the number of molecules in the junction could be as few as one. Starting from this point, a multitude of different experiments characterizing single molecules has been performed.

A first demonstration of charge transport through single OPEs in a MCBJ was given by Reichert et al.^[40] The molecules were immobilized with thiol anchoring groups in lithographically designed break junctions. The experiment unambiguously revealed the symmetry of the formed junctions due to the IV characteristics obtained for both symmetric and unsymmetric molecular wires. While these results did not deduce any systematic information on the molecules properties in the presence of side chains, a comparative study on four molecules reported that oligophenylene vinylenes (OPV) have a higher conductance than the corresponding OPE.^[41] Additionally, alkoxy side‐

chains that were installed to improve solubility did not alter the conductance significantly. Figure 8 shows the conductance histograms obtained from the MCBJ measurements and the corresponding molecules. It should be noted that the side chains did not hinder the proper formation of the metal‐molecule‐metal contact.

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Figure 8 Conductance histograms of 100 succesive opening cycles for OPEs with or without alkoxy side chains. No significant conductance change was observed due to the presence of the side chains. In contrast, the OPV‐type molecule 1 has a higher conductance. C8 (octanedithiol) was used as reference.

More recently, Kaliginedi et al. presented a comprehensive study of various OPEs in a combined MCBJ/STM‐BJ study.^[42] The transport mechanism for all investigated molecules was found to be coherent tunneling, with a tunneling decay coefficient

β = 3.4 ± 0.1 nm^‐1. An increase of molecular length and of the HOMO/LUMO gap lead to a

decrease in junction conductance, whereas no difference in conductance was observed between an un‐substituted linear OPE and an alkoxy side chain modified one (see chapter 2.5.2).

STM combines the single molecule probe of the MCBJ setup with the possibility to image the molecules under the same conditions they are measured. Additionally, spatially resolved electrical spectroscopy provides the local density of states with atomic resolution.^[43] Compared to the MCBJ setup, the collection of statistics for significant conductance histograms is much simpler when the STM‐BJ technique is used.

To do so, the STM tip is moved in and out of contact with the metal substrate that carries the molecules. An example of this process is shown in Figure 9. Stable conductance values for a single or few gold atoms and organic molecules were obtained. A drawback of this method is the unsymmetric electrode geometry which might hamper the interpretation of the collected data. Nevertheless, the STM‐BJ is competitive to the MCBJ, as can be judged from the aforementioned study by Kaliginedi et al., where the results of both experimental techniques were almost similar.^[42]

Due to the high reliability of the technique, one of the very first proves for molecular wire behavior of OPEs was demonstrated by diluting the conductive molecules in a thiolate SAM and scanning across the surface with the STM.^[44] The

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11 conductive OPEs were apparent as protruding areas with very high conductance compared to the surrounding thiolate SAM.

Figure 9 A) The conductance of gold contacts between substrate and STM tip decreases in units of G0. B) Conductance histogram of 1000 conductance traces as shown in A. C) As soon as the tip is pulled away from the surface in the presence of a solution with molecules, new conductance steps appear that stem from molecules bridging the electrodes. D) Conductance histogram of 1000 conductance traces collected in a bipyridine solution. G values of 1 ×, 2 × and 3 × 0.01 G0 were attributed to one, two or three molecules, respectively. E) and F) In the absence of molecules in the solution, no peaks are observed in the histogram in the same conductance range. From ref^[45]. Reprinted with permission from AAAS.

Other experiments in STM‐BJs dealt with the formation of multiple contact geometries for OPEs at the electrodes. Martín et al. reported on the aromatic coupling of two OPE molecules of the same length via π–π‐stacking.^[46] Thereby, conductive junctions with much longer electrode–electrode distance than usually expected were observed. As soon as bulky side chains hindered the interaction, no current flow was detected. In a study by the Wang group, OPEs of different lengths were found to exhibit the transition from tunneling to hopping transport at a length of 2.75 nm.^[47] The single molecule junction resistance increased from around 5.6 MΩ for the 0.98 nm long 1,4‐diaminobenzene to 217 MΩ for a 5.1 nm long OPE with 7 benzene units.

CP‐AFM represents a similar approach to conductance measurements like STM but gets along with less complicated equipment. The main disadvantage of contacting molecules by an AFM tip, however, is the relatively large metal coated tip that does not allow for single molecule measurements. On the other hand, there is no STM like vacuum tunneling gap between sample and probe tip, resulting in more symmetric and stable contacts. A representative CP‐AFM experiment is discussed in chapter 2.5.1.

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With the break junction techniques presented above, installing a third or gate electrode to the device is difficult. EBJs, on the other hand, are readily prepared on top of additional electrodes on the substrate.^[48‐49] The process was introduced by Park et al.

and makes use of the current‐driven migration of metal atoms in a metallic constriction.^[50] A thin metal wire is defined by lithography before a large current is applied to the wire. This results in the formation of a 1–2 nm wide gap that can be bridged by a molecule. The molecules can be applied to the electrodes before or after breaking the wire. Since the large current also produces heat in the region of the constriction, molecules might get damaged during the formation of the junction.

Furthermore, the junctions formed by electromigration cannot be used for the repetitive collection of data with one and the same junction. Therefore, many devices must be produced to obtain statistics, as summarized in the review of Song et al^[38].

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Functional Molecules for Molecular Electronics

13 2.2 Functional Molecules for Molecular Electronics

2.2.1 Molecular Wires and Insulators

Molecular wires represent linear chains of π‐conjugated structures and are essential for the most basic mechanism in Molecular Electronics, which is charge transport.^[51]

Particularly, molecular wires can provide charge transport over distances that are larger than the typical tunneling distances (1–4 nm).^[52] By attaching molecular wires in between electrodes, “molecular junctions” are formed that can be probed. This junction geometry already suggests that charge transport of has to be preferential along a rod‐

like structure in the molecule. Indeed, many of the commonly investigated π‐conjugated molecules already exhibit a rod‐like structure due to the geometry of the conductive backbone. A selection of structure motives that might be used as molecular wire is shown in Figure 10. These molecules have frontier orbitals that are close to the Fermi level of for example gold electrodes, allowing the efficient charge injection into the molecular wire. This is of great importance since the charge transport for sufficiently long molecular wires is expected to take place through the frontier orbitals.^[53]

Figure 10 Examples of potential molecular wires. Polyenes A, polyphenylenvinylenes B, polythiophenes C, polyphenyleneimines D, polyphenyleneethynylenes E and polyynes F exhibit an extended conjugation along the molecular backbone. Adapted and modified from ref^[52].

Larger π‐systems are even more favorable, since the energy difference between HOMO and LUMO decreases and the frontier orbitals get closer to the Fermi level of the electrodes. In addition, transport characteristics can be tuned by influencing the frontier orbitals by chemical modification.^[54‐55] This was also part of the present work, and a detailed introduction will be given in section 2.5.2.

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Compared to molecules with delocalized π‐electrons, non‐conjugated structure motives such as alkyl chains can serve as insulators (Figure 11).^[53] Tunneling transport through space is still possible in these systems. However, the conformational flexibility of the alky chains limits their application in functional (single molecule) devices. For example, alkyl chains can easily arrange in a way that two π‐systems separated by it are short circuited. This limitation can be overcome by introducing conjugation breaking subunits to rigid conjugated molecules.

Figure 11 Examples of molecular insulators. Alkane A only consists of σ‐bonds and no electron delocalization is possible. In molecule B, the meta‐connected alkynes at a benzene ring are not conjugated^[53]. In biphenyls C, the amount of conjugation is dependent on the twist angle between the two rings. The platinum complex D shows insulating properties due to the σ‐bonds between the alkyne and the metal center^[56].

One of the most instructive examples on how to use tailor‐made rigid systems as insulators has been reported on biphenyls, where the degree of conjugation is readily tuned by introducing torsion along the molecules axis (Figure 12). Depending on the twist angle between the two benzene rings, the conductance changed from its maximum in the planar state to the expected minimum in the perpendicular state.^[57]

Figure 12 Biphenyl junction conductance as a function of molecular twist angle investigated in a STM‐BJ setup at 25 mV bias; a) Structures of a subset of the biphenyl series studied; b) Conductance histograms obtained from measurements using molecule 2, 4, 6 and 8. Arrows point to the peak conductance values;

c) Position of the peaks for all the molecules studied plotted against cos²Θ. Θ is the calculated twist angle for each molecule. Reprinted by permission from Macmillan Publishers Ltd: Nature^[57], copyright 2006.

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Molecular Switches

15 2.3 Molecular Switches

In order to realize functionality besides charge transport with single molecules, bistable structures that can be switched back and forth between two well defined states are crucial. Figure 13 gives a selection from the large area of available molecular switches.

S S

F

N N

Ph Ph Ph

Ph

DiaryletheneA F

S S

N N Ph Ph

Ph Ph

N N

AzobenzeneB

O O

O

O O

O

O O

UV Vis / UV

UV Vis

FulgideC

N N

HN

N N

NH N

H

N N

HN

Cu²⁺ Ni²⁺

O O

O

O O

O

CatenaneD

UV Vis or T

N N

O₂N NO₂

SAc AcS

Dinitro OPEE

tBu

[Fe(Cp)₂]

tBu

[Fe(Cp)₂]

[Fe(Cp)₂] UV Vis

DihydropyreneF or electric filed

tBu

Figure 13 Diarylethene A reversibly modulates hole mobility in a bilayer device.^[58] Azobenzene B was the first azobenzene reversibly isomerized by electric field on a Au(111) surfaces.^[59] Fulgide C represents the prototype of nowadays switches of this type.^[60] Catenane D exhibits intermolecular motion of the crown‐

ether moiety between the two metal centers upon exposing the molecule to different potentials.^[61] This is also exemplarily for rotaxane switches. Dinitro OPE E showed bias‐switchable charge transport in a MCBJ.^[62] Dihydropyrene F represents a photoswitchable mixed valance compound, whose intramolecular crosstalk between the ferrocenyl units is modulated by the degree of conjugation across the switching dihydropyrene part.^[63]

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More detailed information on these and other switches not discussed in this work can be found for example in ref^[64]. Depending on their structure, various stimuli have been applied to induce molecular switching. The switching mechanisms can be coarsely divided into processes that induce an isomerization with changes in the molecule’s three‐dimensional structure or an electron uptake or loss leading to a different charged species.^[65] Current induced switching by vibrational heating was demonstrated for a single Xe atom^[66] and azobenzene^[59]. A redox‐potential‐induced switching process was observed by tuning the molecular energy levels of an Fe(III)‐protoporphyrine in an electrochemical STM set‐up. Adjusting the electrochemical gate properly gave rise to a conductance change of one order of magnitude.^[67]

One has to consider the very high field strengths in the small electrode gaps present during the experiment. A typical molecular‐length gap of 1 nm under a bias of 0.5 V generates a electric field of 5x10⁸Vm^‐1.^[65] This field was strong enough to interact with the molecular dipole of Zn(II)‐ethioporphyrine and induced a conformational change.^[68] Thermal excitation has to be considered as a relatively unspecific driving force for molecular switching, and indeed was a major problem until the invention of thermally stable switches such as diarylethenes. Nevertheless, this mechanism was successfully exploited in so called spin‐switches, where the ligand field allows high‐ and low‐spin state to be present at different temperatures^[69‐71].

One of the most important stimuli for molecular switches is light. It is ubiquitous and does not produce any side products or other waste during switching a molecule.^[72]

Therefore, photochromic molecular building blocks arguably belong to the most fascinating ones in molecular electronics. Photochromism, which is a “reversible transition in a chemical species between two forms having different absorption spectra induced by photoirradiation”,^[73] has therefore attracted considerable attention over the last decades. Especially the application of photochromic substances in optical memories and switches bears a huge potential for miniaturization and new applications.^[74] Since the first observation of photochromism by Fritzsche in 1867,^[75] the consequent development of new photoswitches with improved properties led to a wide variety of bistable molecules that can be switched back and forth between two well defined states by optical activation. The most important discovery probably was the development of fatigue‐resistant spirooxazines and thermally stable diarylethenes.^[73]

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Molecular Switches

17 2.3.1 Diarylethenes

Although a large number of photochromic molecules has been reported, only a small fraction of them exhibit thermally irreversible switching properties. In 1996, Irie et al.

reported about a new class of molecules, the so called diarylethenes.^[73] These molecules resemble an advancement of the long known photochromic switching process of stilbene^[76‐77] that undergoes photocyclization to dihydrophenanthrene (Figure 14). In the absence of air, dihydrophenanthrene readily underwent thermally driven back reaction to stilbene. In the presence of air, the stable oxidation product phenanthrene was formed.

Figure 14. Light induced electrocyclization of stilbene. In the presence of oxygen, phenanthrene is formed from the intermediate. Reproduced from ref^[74].

Kellogg was able to stabilize the cyclization product by exchanging the phenyl groups with thiophenes.^[78] Irie perfected this approach and introduced a new type of molecular switch, the diarylethenes.^[73] Since that time, numerous examples of these materials have been synthesized. To some extent, most of these compounds share the following unique features that make them superior photochromic compounds:^[79]

1) thermally stable (t1/2 up to 470 000 years)

2) fatigue resistant (up to 10⁵ switching cycles in solution)

3) switching quantum yields for the ring closing reaction up to 1 (100%) 4) ultrafast switching on the order of a 10 ps timescale

5) switching also occurs in the single crystalline phase 6) only a small length change occurs during switching

The reasons for these superior properties have been intensively studied by the Irie group and others. The thermal irreversibility was already predicted in a theoretical study in 1988.^[80] Analysis of 1,3,5‐hexatriene according to the Woodward‐Hoffmann rules revealed that a disrotatory electrocyclic reaction is thermally possible, whereas a conrotatory reaction is possible after excitation with light. In the disrotatory case, the reaction in the ground state is unlikely to happen, as depicted in the state correlation diagram derived from modified neglect of diatomic overlap (MNDO) calculations (Figure 15). The ground state energies of the products are higher than those of the

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starting materials. This energy barrier hinders the reaction in both analyzed cases. In the conrotatory regime however, no large energy barriers exist for a transition from a photoexcited state of the open‐ring isomer to the ground state of the closed‐ring form.

Figure 15 Left: State correlation diagram for the disrotatory electrocyclization of model compounds. In the closed‐form, the hydrogen atoms adopt cis‐onfiguration. The high energy barrier prevents a reaction in the ground state of both model compounds. Right: State correlation diagram for the conrotatory electrocyclization of model compounds. In the closed‐form, the hydrogen atoms adopt trans‐

configuration. Cyclization is possible from photoexcited states. Thermal cycloreversion is hindered due to an activation barrier. Reprinted with permission from ref^[74]. Copyright 2001, American Chemical Society.

The observed thermal irreversibility was explained with the aromatic stabilization energy of the aryl moieties. In the case of phenyl, the energy difference between the aromatic ground state and the non‐aromatic structure in the ring‐closed form is rather large. In fact, the ground state of the ring‐closed isomer lies energetically high, and since there is no activation barrier for the back reaction in the ground state, thermal energy is sufficient to affect the cycloreversion. In case of furyl‐ or thienyl‐

residues, the ground state energy was estimated to be significantly lower, resulting in an activation barrier that cannot be crossed by thermal activation (Figure 15). Two limitations to this design principle were found. Strong electron withdrawing groups at the 2 and 2' position seem to decrease the thermal stability due to a weakening of the carbon‐carbon bond formed upon photocyclization.^[74] The same applies for bulky groups in the 2‐position of benzothiophene.^[81]

Fatigue resistance depends on the exact structure of the diarylethene. In order to achieve the above mentioned switching cycles, the side reactions quantum yield has to be < 0.0001. In the presence of oxygen, endoperoxide formation was observed for some dithienylethenes.^[82] Much more important, the main fatigue process in diarylethenes without methyl groups in the 4 and 4' position such as 1,2‐di(2‐dimethyl‐5‐

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Molecular Switches

19 phenylthiophen‐3‐yl)perfluorocyclopentene, has been found to be the formation of a photochemically inactive, six‐membered ring derivate (Figure 16).^[83] The formed product usually exhibits long‐wavelength absorption just as the ring‐closed isomer does.

On the other hand, a single crystal of the same material showed a very high fatigue resistance and no evidence of by‐product formation even after 10 000 switching cycles.^[74] The mechanism of the by‐product formation has been investigated by theory^[84] and experiment^[85] and it was found that the photoactivated reaction takes place from the closed form of the switch.

Figure 16 Proposed intermediate^[84] and photoinactive by‐product^[83] formed from 1,2‐di(2‐dimethyl‐5‐

phenylthiophen‐3‐yl)perfluorocyclopentene.

Switching quantum yields vary over a broad range throughout the diarylethenes synthesized so far. Upon variation of the central ring of the diarylethene, a decrease of the cylization quantum yield was found upon decreasing the ring size.^[86‐89] An increase of the cycloreversion quantum yield was found at the same time. An issue that controls the quantum yield for the ring closing reaction of 2,4‐disubstituted diarylethenes is the coexistance of parallel‐ and anti‐parallel conformations in the open form (Figure 17).

Only the anti‐parallel conformation is capable of photocyclization. Therefore, the cyclization quantum yield is limited to 50% in the case that the molecule is present in a 1:1 ratio of both conformers.^{[74, 90]}

Figure 17. Light induced reactions of photochromic 2,4‐disubstituted dithienylethenes. Only the anti‐

parallel conformer cyclizes. The parallel conformation remains photoinactive.^[91]

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20

An increase in the photoactive anti‐parallel conformation results in an increased cyclization quantum yield. This was successfully demonstrated by Takeshita et al., who designed a bridged thia[2.3](2,4)thiophenophan‐1‐ene, locked in its anti‐parallel conformation. (Figure 18) They observed a 1.6 times increased cyclization quantum yield compared to the not‐tethered molecule.^[92]

Figure 18 A bridged thia[2.3](2,4)thiophenophan‐1‐ene was used to demonstrate the influence of the ration between the two conformers on the cyclization quantum yield. The quantum yield increased 1.6 times compared to the unbridged derivate.^[92]

A similar observation was reported by Yamaguchi et al., who synthesized several 1,2‐bis(2‐n‐alkyl 1‐benzofuran‐3yl)perfluorocyclopentenes.^[93] Longer alkyl chains led to a population shift (determined by low temperature NMR) of the anti‐parallel species from 53% for methyl groups to 73%, 91% and 98% for ethyl‐, propyl‐ and butyl‐groups, respectively. Simultaneously, the cyclization quantum yield increased from 0.38 to 0.49.

Similar results were obtained by incorporating the molecular switch into cyclodextrines^[94‐95] or by polymerization of switchable monomers^[96]. The cycloreversion quantum yields strongly depend on the nature of the substituents attached to the diarylethene and is independent of the cyclization quantum yield.

Different heterorarylic systems have been discussed to influence the geometry of the closed ring isomer. Irie reported a single crystal structure of a furan based switch that clearly shows a lack of co‐planarity in the central ring of the cyclized diarylethene.^[97]

This was thought to decrease the degree of π‐conjugation throughout the molecule in comparison to the planar thiophene based analogue, whose cycloreversion quantum yield is about 6 times lower. In conjunction with this, extended π‐systems have been found to decrease the cycloreversion quantum yields drastically ^[98‐99].

Ultrafast response time of the switching process was confirmed for 2,3‐di(2,4,5‐

trimethyl‐3‐thienyl)maleic anhydride by pump‐probe experiments in hexane solution.

Transient absorbance measurements showed a response time for the cylization on the order of 10 ps, the cycloreversion reaction took place in 2‐3 ps.^[73] Further and more accurate experiments^[100‐102] have been carried out with femtosecond spectrosopy, indicating that the photoreactions took place within 1‐3 ps. As an additional benefit