Chapter Three - Synthesis and Applications of Thiophene Derivatives as Organic Materials
Introduction
This chapter covers aspects of the most recent developments in the synthesis and application of thiophene oligomers and polymers for use in a wide range of fields, from very efficient organic solar cells (2015NC10085) to fluorophores specifically binding amyloid fibrils formed inside neural cells in Alzheimer's and other neurodegenerative diseases (2010B6838). Thiophene-based oligomers and polymers are organic semiconductors with a wealth of properties spanning from charge conduction in the oxidized or reduced states (“doped” states) to light emission on irradiation at appropriate wavelength. Research studies involving these compounds are highly interdisciplinary (2005AM2281, 2005AM1581, 2008MI1, 2009MI2). Owing to their multiple functional properties, chemical robustness and versatility, thiophene-based materials play a preeminent role in nanoscience and nanotechnology, which largely rely on the production of new materials both from a scientific and a technological point of view. They exhibit a great amount of structural diversity and their synthesis is mainly oriented toward molecular structures useful for elucidating property–structure relationships for the optimization of specific properties.
Due to the very rapid progression in the last few years and the consequent huge number of publications, it would be a very hard task to give an exhaustive account of basic research studies and applications on thiophene-based materials. Our objective is to arouse curiosity and interest in thiophene derivatives as organic materials and furnish some basic information and references useful to the reader as a trace to deepen the knowledge on the topics that he/she will consider most important. We mainly describe recent papers but earlier papers that have played an important role in the field are also included. We emphasize that the papers we mention only concern a small part of the knowledge available at the present time and that they mostly reflect our personal inclinations.
Before proceeding further, we review a few points concerning thiophene-based materials. Fig. 1 shows the molecular structure and the numbering scheme of thiophene and the functionalization positions of α-linked thiophene rings.
The impressive versatility of thiophene-based materials relies on the numerous possibilities for ring functionalization and chain elongation from a few rings forming monodisperse oligomers to a large number rings forming polydisperse polymers. Sulfur is a medium-sized atom located at the intersection of the 16th group (column) and the 3rd period (row) of the periodic table. It has diffuse orbitals, easily polarizable electrons and formal oxidation states 2, 4, and 6. In the oxidation states 4 and 6, it is hypervalent, i.e., it is surrounded by more than the eight electrons associated with filled s and p shells (1969ACIE54). Thiophene sulfur has a formal oxidation state 2 and two lone pair electrons one of which participates in ring aromatization. So far most synthetic effort has focused on the functionalization of the aromatic backbone and relatively few investigations have taken into account the functionalization of thiophene sulfur (2013CEJ5289, 2013CEJ9699, 2009OL2149, 2015NM426, 2015JMCC7756, 2009OE2557, 2015NC209, 2014OL5870, 2013PC895, 2016AFM6970, 2008ACR1202).
Chart 1 shows a few examples of the wide variety of structures and shapes of thiophene-based molecules synthesized so far. All compounds reported in the chart are functional materials displaying, in particular, photoluminescence and charge transport properties.
Thiophene oligomers and polymers are described by their conformation generated by the rotations around the single bond linking two adjacent thienyl rings, as shown in Fig. 2. Rotation angles (ω) between α,α′-linked thienyl units may differ in magnitude and in sign, giving rise to different conformational isomers. Conformational isomers have slightly different energies and very low energy barriers for conversion from syn to anti forms, so that in solution interconversion between different conformers is rapid and the presence of bulky substituents is not sufficient to hinder conformational mobility (2002JPCA1266, 2002OL2067, 2009ACIE6632).
Variations in the interring angle ω bring about variations in the overlap between the p orbitals of the α-linked carbons of two adjacent thienyls. The overlap is maximum for the planar syn (ω = 0 degree) and anti (ω = 180 degrees) forms. The larger the distortion from planarity, the weaker the extent of π-conjugation and electron delocalization. The electronic properties of oligomers and polymers depend on the degree of π-conjugation, which affects HOMO–LUMO orbital energies, energy gaps, optical, electrochemical, and electrical properties. Rotational distortions decreasing π-conjugation may deeply affect the molar absorption coefficient in the solid state (2016NM746, 2009MRC1323). The torsion angle between thiophene rings is also relevant to the question of chirality observed in the solid state for achiral thiophene oligomers. In this case the optical activity observed in the solid state is the effect of hindered rotations about the thienyl–thienyl interring bonds caused by the freezing of the molecule on the solid support in conformations lacking any symmetry element (2014AFM4943).
Thiophene rings are characterized by easy bond and angle deformability due to the high polarizability of sulfur bound and unbound electrons and the long C
S bonds. As a consequence, the aromatic backbone of oligo- and polythiophenes is capable of adapting its geometry to the surrounding molecules by means of small bond and angle deformations extending over the entire system (1993AM834, 2015ACR2230). This capability—well illustrated by X-ray diffraction data of single crystals of thiophene oligomers showing that thiophene has always slightly different bond angles and lengths and is never a regular pentagon—together with the very low energy barriers to rotation around the interring carbon–carbon bonds confers extreme flexibility to the aromatic backbone of oligo- and polythiophenes. The great flexibility of the aromatic backbone can give rise in the solid state to polymorphism, i.e., the possibility to pack with different modalities and different conformations causing variations in the electronic properties, which depend on the degree of electron delocalization between adjacent rings. Conformational polymorphs slightly differing in torsional angles have been identified for unsubstituted α-quaterthiophene and α-sexithiophene and have been analyzed by single crystal X-ray diffraction (1999ACS209). For α-sexithiophene the existence of four different crystalline phases in thin films vacuum evaporated on different substrates has been demonstrated. The different phases display different charge transport properties that depend on the interplay of supramolecular organization and morphology. Different packing modalities and solid state conformations bring about profound differences in the optical and charge transport properties as recently shown for a substituted octathiophene forming polymorphic crystalline microfibers with very different charge transport and photoluminescence properties (2014AFM4943).
Most applications of thiophene-based materials concern organic (opto)electronics that is expected to replace the current technology based on inorganic semiconductors (2013MI3, 2016APR021302). However, in recent years a few families of brightly fluorescent thiophene oligomers have been developed for a variety of possible medical applications (2013NCB499), as fluorophores for proteins and DNA labeling (2009JACS10892) and for differential staining of various cell types, including cancer cells, by cytofluorimetry (2014C628).
Fluorescent quinquethiophenes with precise regiochemistry of substitution have been exploited for the detection of the manifestations of protein misfolding diseases such as prion and Alzheimer's diseases (2013CBC607). These quinquethiophenes are amyloid protein-specific ligands showing fine variations in absorption and emission spectra upon binding to aggregates of misfolded proteins, thus allowing to associate molecular structure modifications, conformational variations, and consequent optical changes of the probes, to the manifestation of pathogenesis. More generally, these studies have disclosed the possibility of using thiophene-based fluorophores, some of which show efficient crossing of the blood–brain barrier, as tools for pathological and prognostic evaluation of neurological diseases (2013ACSCN1057).
Section snippets
Synthesis of Oligo- and Polythiophenes
The synthesis of oligo- and polythiophenes is aimed at creating new molecular structures with enhanced functional properties. Attention is oriented toward obtaining the purest products and ease of separation from side products. Purity and reproducibility are crucial requirements in conjugated materials chemistry since even parts per million of contaminants can deeply affect properties such as charge transport or light emission.
The synthesis of π-conjugated oligo- and polythiophenes, linear,
Oligo- and Polythiophenes in Organic (Opto)Electronics
The thiophene ring is one of the most widely used building blocks in the synthesis of semiconducting conjugated oligomers and polymers that are incorporated into devices for organic electronics. Organic electronics is expected to replace inorganic solid state electronics, owing to its advantages in terms of flexibility, low costs, easy deposition of materials on large-area substrates, environmentally benign manufacturing, and tunability of electronic properties via chemical synthesis (2013MI3,
Thiophene Oligomers as Fluorescent Probes in Biological Systems
Similar to polythiophenes, thiophene oligomers are characterized by fluorescence frequency tunable from blue to near infrared by judicious choice of molecular structure (2015CC4528). They are optically very stable compounds that, contrary to most commercial organic fluorescent probes, do not undergo photobleaching or blinking but display intense and persistent emission signals. They show large Stokes shifts (difference between absorption and emission signals) mainly due to planarization in the
Oligo/Polythiophene-Based Nanoparticles and Microfibers
It is well established that for thiophene-based materials the morphology in the solid state has a significant impact on the optical and electrical properties hence on the performance of thin film devices employed in organic electronics. Owing to different processing conditions, to the “plasticity” of the thiophene ring and to multiple possible nonbonding interactions (2015ACR2230), there is a great diversity of possible morphologies for the same molecular structure when passing from solution to
Concluding Remarks
Thousands of articles on thiophene-based materials and their applications have been published. The present work is not an exhaustive review of everything published in the field. It is just a short updated account from the point of view of the authors and aimed to give the reader an idea of the countless research opportunities related to thiophene compounds as organic materials.
There is growing interest in developing methods for the scalable cost-effective syntheses of oligo/polythiophenes
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