Samorí , Paolo : Self-assembly of conjugated (macro)molecules: nanostructures for molecular electronics


Kapitel 3. Conjugated molecular systems

3.1 Introduction

While macromolecules and organic materials have been known for many decades [Flo53], their intrinsic conductive properties as semiconductors and metals have only be recently discovered. The report in 1977 about the high electrical conductivity of trans-poly(acetylene) [Chi77] that can be achieved upon p and n doping opened new avenues of exploration for chemistry and physics, and for technology. The key finding by Mac Diarmid and Heeger [Chi77] was a chemical species characterized by a delocalized pi -electron system along the polymer backbone. The semiconductor properties of the materials arise from the overlap of pz orbitals that originates from the double bonds. If the overlap is over several sites, delocalised pi valence and pi conduction bands occur, with a relatively small energy gap. The limitation in the firsts experiments was the chemical stability of the poly(acetylene) in air and the difficult material processing.

Figure 3.1: Chemical formulae of several conjugated polymers.

The synthesis of different conjugated derivatives [Bäu93, Tou96, Kra98, Mar99] allowed later to design new materials with different band gaps and electron affinities and in addition made it possible processing to be carried out under different conditions, i.e. in organic solvents, inorganic solvents and aqueous media. A step forward in the processability of conjugated macromolecules has been made by attaching long flexible aliphatic side-chains to the conjugated skeletons, which give rise to the entropic stabilization of the polymer chain in


solution [Reh89]. The polymeric species obtained have been called “hairy-rods“. They exhibit a remarkable increase in the solubility of the molecules in organic solvents, while the molecular packing in the solid state is tremendously affected. The most well-known conjugated polymers are shown in Figure 3.1.

During the last years a great deal of effort has been devoted to oligomers of conjugated macromolecules because they are good model compounds of their related polymers with respect to their electronic properties and moreover they can much more easily be handled [Mül98a]. In fact their reduced size enables them to be processed in thin films using typical techniques that until few years ago were just used for inorganic materials, like sublimation in ultra-high vacuum (UHV), known also as Molecular Beam Epitaxy (MBE). Furthermore their monodispersity makes it possible for them to self-assemble into 2D and 3D mono-crystalline structures. Typical examples of this synthetic effort are oligothiophenes [Hot93]. Other types of monodisperse systems, which are very interesting because of their high charge carrier mobilities (0.13 cm2/Vs), are hexa-peri-benzocoronenes (HBC)s [Sta95b, Vdc98, Mül98b].

Figure 3.2: Chemical formula of hexa-peri-benzocoronenes (HBC).

They are nanoscale versions of an infinite 2D graphene sheet that, upon functionalization with side chains, can exhibit a good solubility in several organic solvents. HBC is just the first representative of a large family of poly-aromatic hydrocarbons that have been designed by Müllen and co-workers. Recently they have been able to extend this 2D moieties to more than 200 carbon atoms in the aromatic core [Mül98c].

3.2 Application in molecular electronics

Research on conjugated (macro)molecules is particularly appealing because of several


reasons: a remarkable versatility of the materials, the possibility to build large area electro-optical devices, which have not necessarily a flat shape, their easy processability in different environments and also their low cost of production. In particular, materials based on conjugated polymers have great potential for electronic and photo-physical applications such as flat-screen displays [Wed98], light emitting electrochemical cells [Pei95], light emission devices [Bur90], organic transistors [Gar90], and solar cells [Hal95]. A breakthrough was the work of Friend and co-workers who built the first Light Emitting Diode (LED) based on a polydisperse poly(para-phenylenevinylene) (PPV) active layer assembled on an indium tin oxide (ITO) surface by spin coating [Bur90]; this first device exhibits quite moderate quantum efficiencies (photons emitted per electron injected), namely up to 0.05%. The work-up of these devices is strongly dependent on the interplay between electronic structure and molecular arrangement [Bäu95,Bis95]. This latter feature, as reported later in the thesis, arises from the interplay of intra-molecular as well as inter-molecular and interfacial interactions, making it possible to design very precise and reproducible 2D and 3D architectures.

A key question that has not been answered so far, is whether the charge carrier transport does occur also in the case of a single (macro)molecular chain or a well defined assembly of parallel chains. The latter would be necessary in order to insight into the role played by charge carrier hopping, which is required to transfer the charge carrier from one chain end to another. Indeed the long-term goal of the research described here is the fabrication of a molecular nanowire. A novel set-up used for probing the electronic properties of single molecules have been presented recently by Reed and co-workers [Ree97]. Their approach was based on the “Mechanically controllable break junctions“ which offers a way to obtain metallic nanoelectrodes with a gap of 1-2 nm. A critical analysis of this method casts some doubts on the real molecular packing between the two metallic contacts since uncontrolled aggregations of the molecules can be expected; this affects the reproducibility of the measurements. This lack can be overcome by constructing a highly ordered molecular architecture with dimensions in the tens of nanometer scale, and to interface this well-defined assembly with Au nanoelectrodes that exhibit a gap in the same spatial range. The choice to work on these scales enables the structure of both the organic and metallic component to be observed using Scanning Force Microscopy in every step of the device preparation.

The ideal organic moiety for producing this device should posses a good solubility in organic solvents, functional groups that can attach covalently to the Au nanoelectrodes, a high


stiffness and a good conductivity along the unsaturated backbone. Good candidates that belong to the family of conjugated macromolecules are poly(para-phenyleneethynylene)s [Tou96, Gie96, Bun00]. They exhibit a rigid-rod structure along the conjugated backbone [Mor94, Wau96], strongly anisotropic electronic properties, an electroluminescence in the blue green-region [Tad96], and a high and stable photoluminescence quantum yield [Wed96] that made it possible to use them for the development of a liquid-crystal based photoluminescent display [Wed98]. A direct measurement of their molecular conductivity for the case of short oligomers has be carried out using Scanning Tunneling Microscope (STM) probing the average resistance of the self-assembled organic monolayers [Bum96, Dhi97].

3.3 Phenyleneethynylenes

For the project that will be discussed in detail in chapter 6, oligomeric and polymeric derivatives of para-phenyleneethynylenes have been synthesized with a polycondensation route by Dr. Viola Francke in the group of Prof. Dr. Klaus Müllen in the MPI for Polymer Research in Mainz within a collaborative project.

Two different types of PPE (2) alpha-iodo-omega -[(2,5-dihexyl-4-ethynyl)phenyl]-poly[(2,5-dihexylphenylene-1,4)ethynylene)] and (6) alpha-[[4-[(N,N-dimethylcarbamoyl)thio]phenyl] ethynyl]-omega-[4-[(N,N-dimethylcarbamoyl)thio]phenyl]-poly[(2,5-dihexylphenylene-1,4) ethynylene)] were synthesized by efficient Pd-catalysed coupling of the AB-monomer 4-ethynyl-2,5-dihexyliodobenzene (1) [Dra 98] (see Fig. 3.3). The procedure involves the coupling of the AB-monomer under Pd(PPh3)4/CuI catalysis according to Hagihara [Son75, Die75], followed by the addition of an excess of 4-[(N,N-dimethylcarbamoyl)thio]iodobenzene (3) as end-capping reagent. After the work-up procedure, which is necessary to remove the remaining end-capping reagent, the iodine function of the resulting alpha-iodo-omega-[4-[N,N-dimethylcarbamoyl)thio]phenyl]-poly[(2,5-dihexylphenylene-1,4)ethynylene] (4) was coupled with 4-ethynyl-[(N,N-dimethylcarbamoyl)thio]benzene (5) under the same conditions described above to yield (6). The average contour length of the macromolecules are between 8 and 30 nanometer according to 1HNMR analysis on the end groups. Furthermore, alpha-phenylethynyl-omega-phenyl-ter[(2,5-dihexylphenylene-1,4)ethynylene)] (9), has been synthesized also by efficient Pd-catalyzed coupling of 1,4-diethynyl-2,5-dihexylbenzene (7) with 2,5-dihexyl-4-


[(trimethysilyl)ethynyl]iodobenzene (8) under Hagihara conditions (see Fig. 3.4).

Figure 3.3: Synthesis of poly(para-phenyleneethynylene).


Figure 3.4: Synthesis of para-phenyleneethynylene trimer.


3.3.1 Kinetics of the polycondensation reaction

The synthetic route followed for producing the PPE was a polycondensation reaction. Consider the case of a general polycondensation reaction A+Brarr C+D [Bra89], which is characterized by a kinetic of the second order . In the present case

A=B and the reaction is:

Since [A]=[B], then , which means that

where: k = reaction constant;

t = time;

[A]0 = initial concentration;

[A] = concentration at time t;

p = fraction of functional groups initially present that reacted at time t.

It is known that: [A]=(1-p)[A]0 and so we have

3.3.2 Molecular weight distribution

Macromolecules can be classified into 2 different classes:


Monodisperse: Single definite molar mass (used also as an approximation for polymers when U le 0.1, see below).

Polydisperse: Sample is a mixture of molecules with various chain lengths (typical of polymers that usually have Uap2).

The distribution of molecular masses can be described by means of several parameters:

1) The arithmetic average: the Number Average Molar Mass:


where N= total number of molecules

Ni = number of molecules with molar mass Mi

2) The weight average: the Mass Average Molar Mass:


where m = total mass of the sample

mi = total mass of molecules of molar mass Mi

3) The width of the distribution can be expressed in terms of the Polydispersity:


The full distribution functions can be expressed as normalized mole fraction distribution F(r), that is the fraction of molecules of size r:


Alternatively one may use the weight fraction distribution:



The average number of structural units or repeat units in the molecule is the Number average degree of polymerization:



The is related to the number average molar mass by:

Where Mr.u. is the molar mass of a repeat unit

In the case of the polycondensation synthesis, the distribution of molecular weights is according to one of the following theoretical functions that describe particular cases: Schulz-Zimm distribution

Mole fraction distribution:



k is the degree of coupling (i.e. the number of independently growing chains required to form one dead chain) and gives the curve shape. Moreover

In the present case k =2 as for a standard linear polymerization reaction. Therefore the expected polydispersity, assumed in the Schulz-Zimm function, is .

where is the number average degree of polymerization.


gamma(k) is the gamma function: : for k=2

Weight fraction distribution:


Number average degree of polymerization:


(this parameter could be measured by means of Gel Permeation Chromatography investigations or elemental analysis (C,H)).

For k=1: this distribution reduces to the Schulz-Flory distribution (see below).

k = large values: this distribution approximates the Poisson distribution (see below). Schulz-Flory distribution

Known also as “Most Probable Distribution“.

Number Average Molar Mass:


where M0 = mean molecular weight for a structural unit

Mole fraction distribution:


where p = a real number less than 1, indicating the extent to which the reaction goes to, or as the probability that A reacts with B;

r = number of reacted monomers = number of repeating units in the polymer.


Weight fraction distribution:


Number average degree of polymerization:

(3.13) Poisson Distribution

Mole fraction distribution:


where ny = mean main chain length

r = number of reacted monomers = number of repeating units in the polymer

Weight fraction distribution:


Number average degree of polymerization:


The Schulz-Flory distribution is commonly used for low degrees of polymerization and Schulz-Zimm for high ones.


3.4 Hexa-peri-hexabenzocoronenes

Hexa-peri-benzocoronenes (HBC)s are a good candidate to build-up columnar aggregates on Highly Oriented Pyrolytic Graphite (HOPG) where the disc like molecules are packed with a high degree of order thanks to pi-pi interactions as displayed in Fig.3.5 [Vdc98]. In this case the tip of the STM could be used to gain insight into the conductivity along one column.

Figure 3.5: Columnar stacking of HBC-C12.

Previous explorations at the solid-liquid interface on the soluble HBC-C12 with STM have shown that the molecules tend to lie flat on the basal plane of a conductive HOPG substrate. In addition with Scanning Tunneling Spectroscopy (STS) is was possible to detect a diode-like electrical behavior of the aromatic cores while the aliphatic part of the molecules has exhibited a symmetric current vs. voltage (I-V) curve [Sta95b].

Figure 3.6: Synthesis of HBC-C12.

The synthesis of HBC-C12 has been performed by Dr. Johan D. Brand in the group of Prof. Dr. Klaus Müllen (MPI-Mainz) (Fig. 3.6).

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