Semiconductors are materials with intermediate electrical conductivity between those of conductors and insulators, and underpin modern electronics, with multiple components including transistors, diodes, solar cells and integrated circuits being semiconductor devices. Traditional semiconductor devices are commonly fabricated from crystalline solids, usually silicon, though other materials including germanium and silicon carbide are used. In the second half of the twentieth century, research into organic semiconducting materials has grown significantly, and semiconductivity has been shown by single molecules, oligomers and organic polymers, such as those resulting from polymerisation of 3-alkylthiophenes supplied by Carbosynth.
Polymerisation of 3-alkylthiophenes can result in the formation of organic semiconducting materials when, after polymerisation, electrons are added or removed from the conjugated ğ-orbitals of the material via the process of doping. Doping involves the introduction of impurities into extremely pure semiconductor material to remove (p-doping) electrons from or add (n-doping) electrons to the p-system, resulting in a change to the electrical properties. The electrical conductivity of polythiophenes results from the delocalization of electrons along the conjugated backbone of the polymer, through overlap of p-orbitals. Such organic semiconducting materials are frequently referred to as ‘synthetic metals’. Removal or addition of electrons create a charged unit called a bipolaron (illustrated in Scheme 1 for p-doping) which moves along the polymer chain, and is responsible for the conductivity. Numerous reagents have been used to dope poly(3-alkylthiophenes), including iodine, bromine, organic acids and ferric chloride.
The physical properties of poly(3-alkylthiophene) semiconductors are intrinsically linked to structure, with control of regularity and order in the polymer being closely linked to enhancements in the electrical properties. Mo and colleagues have investigated the structure and properties of poly(3-alkylthiophenes) formed from thiophenes (2), (3) and (6) by various techniques including X-ray diffraction, Fourier transform infrared (FTIR) and UV-visible (UV-VIS) spectroscopy and found that the length of the alkyl chain has a significant effect on the packing of the molecular chains. The melting point and thermal stability of the polymers was observed to decrease with an increase in the number of carbon atoms in the alkyl side chains, and analysis of the FTIR and UV-VIS spectra indicated that the conjugation length was longest in material formed from 3-dodecylthiophene (3). Factors such as the nature of the doping agent and the presence of trace metal impurities or residual monomer can also influence the electronic and spectroscopic properties of the material.
There are a number of ways to synthesize poly(3-alkylthiophenes) with control of the regioregularity. McCullough reported the first regioselective preparation of poly(3-alkylthiophenes) in 1992, commencing with bromination at the 2-position of the 3-alkylthiophenes as shown in scheme 2. A one-pot sequence of regiospecific lithiation, transmetallation to magnesium and nickel catalysed cross-coupling yielded poly(3-alkylthiophenes) in yields ranging from 20 to 69%. Thiophenes (1)-(3) bearing hexyl, octyl and dodecyl 3-alkyl groups were all successfully polymerised, yielding material with almost exclusively head-to-tail coupling, where the 2-position of a substituted thiophene is linked to the 5’-position of the next monomer.
In a related process reported by Rieke, 3-alkylthiophenes are dibrominated then treated with highly reactive zinc powder (Rieke zinc), yielding a mixture of isomeric organozinc reagents, which upon treatment with Ni(dppe)Cl2
produces regioregular polymeric material, scheme 3. This technique enabled the preparation of the first completely head-to-tail regioregular poly(3-hexylthiophene-2,5-diyl) material from 3-hexylthiophene (1).
The methods described by McCullough and Rieke have the disadvantageous requirements of low temperature, anhydrous and anaerobic conditions and brominated monomers. An alternative process has been described by Sugimoto, which proceeds via oxidative polymerisation of thiophenes using catalytic ferric chloride under less demanding conditions. The effect of the amount of catalyst on the structure and properties of poly(3-dodecylthiophene) formed from 3-dodecylthiophene (3) has been investigated by Mo and co-workers. The mechanism of polymerisation under these conditions has been the subject of considerable debate, with a radical cation mechanism (Scheme 4), being the most generally accepted. Alternative radical or carbocation mechanisms have also been proposed.
Thiophenes can also be polymerised via electrochemical methods, where a voltage is applied across a solution containing the thiophene monomer and an electrolyte (a substance containing free ions which can conduct electricity, typically an ionic solution) which produces a poly(3-alkylthiophene) film on the anode. Electrochemical synthesis typically results in polymers with structural irregularities, whereas chemical synthesis, for example by the methods of Rieke or McCullough, is advantageous because of the greater control of regioregularity that can be achieved.
Organic semiconductors prepared from 3-alkylthiophenes such as (1)-(6) have utility as p-channel conductors in organic field effect transistors, and p-type materials in high efficiency heterojunction photovoltaic devices
(solar cells). Other potential applications include electroluminescent devices, photochemical resistors, nonlinear optical devices, batteries, diodes and chemical sensors.
As well as semiconductivity, other interesting properties and uses result from delocalisation of electrons in the extended ğ-systems of conjugated poly(3-alkylthiophenes) formed from (1)-(6). Significant alterations of the optical properties of the materials can result from changes in the environment, with factors including solvent, temperature, applied electric field, presence of dissolved ions and binding to other molecules being involved. Conjugation, and the subsequent delocalisation of electrons, requires overlap of the p-orbitals of the thiophene rings, which can only happen if the orbitals are coplanar. Disruption to the conjugation, through twisting of the polymer backbone for instance, can affect colour and conductivity, both of which rely upon an extended p-system. This environmental sensitivity makes poly(3-alkylthioiphenes) useful as chemical sensors which can provide optical and electronic responses to various stimuli.
For example, novel nanotransistors fabricated from polymeric thiophene (1) have recently been reported, and their use as chemical sensors demonstrated by detection of vapour from methanol, isopropyl alcohol, trichloroethylene and acetone. Regioregular polymers formed from thiophene (1) have been evaluated in cell-growth and adhesion studies with mouse fibroblasts to investigate the biocompatibility and biofunctionalization of the system and its potential for use in sensing applications. Langmuir-Blodgett films formed from thiophenes (1) and (3) have been investigated in the context of biosensors for lactose and glucose respectively.