Przemyslaw Ledwon

Advances in Star‐Shaped π‐Conjugated Systems: Properties and Applications

Until recently π-conjugated organic materials are based mainly on linear systems. Recent years, however, have brought about increasing interest in molecules boasting a dendritic, branched, or star-shaped architecture. This tendency is a direct result of the ongoing search for materials with progressively better properties. Such compounds, featuring novel, 3D architectures, exhibit a multitude of interesting qualities, making them stand out from well-known materials. The direction of star-shaped compound application is determined by whether they are able to form aggregates, π-stacks. This feature is a source of some astounding properties, coveted in numerous applications. Among this class of compounds high charge mobility, high fluorescence efficiency, and good charge separation are all found. Depending on the structure of the core, the molecule may adopt various types of symmetry. Similarly, the conjugation of orbitals may extend over the whole structure or be interrupted at chosen segments. The number of papers pertaining to star-shaped oligomers and polymers is ascending with each year, evidencing a growing interest in them. Consequently, this Review focuses particularly on the most recent reports concerning modification of the structure and properties of the aforementioned type of compounds, as well as on the development of devices based on them.

A novel donor–acceptor carbazole and benzothiadiazole material for deep red and infrared emitting applications

A novel organic material (C1) with the structure D–π–A–π–D was synthesised and characterised. Carbazole was utilised as the electron donor and benzothiadiazole as the electron acceptor unit. The electrochemical, optical and electronic properties of the synthesised compound were studied. Compound C1 exhibits absorption in the visible and ultraviolet range with a high molar absorption coefficient. A strong solvatochromic effect was observed in its emission spectra. Electrochemical and spectroelectrochemical measurements were performed in order to estimate the properties of the molecule in different redox states. Electron paramagnetic resonance (EPR) measurements indicate the delocalisation of radical cations and radical anions over different moieties. Interpretations of the electrochemical and optical results are supported by DFT calculations. OLEDs based on C1 present efficient emission in red and infrared spectral ranges, with a quantum efficiency of 3.13% and a current efficiency of 6.8 cd A−1. The performance is considerably better than what has been reported for analogous devices, based on other carbazole and benzothiadiazole units.

The role of structural and electronic factors in shaping the ambipolar properties of donor–acceptor polymers of thiophene and benzothiadiazole

The influence of different thiophene donor units on electrochemical and spectroscopic properties of benzothiadiazole based donor–acceptor π-conjugated organic materials is studied. Two different structure modification vectors of the donor units are being considered – one addressing the intermolecular interactions through off-conjugation side chain architecture, and the other focusing on intramolecular interactions tuned by in-conjugation substituents. Electrochemical and simultaneous in situ EPR-UV-Vis-NIR spectroelectrochemical studies of the oxidative (p-) and reductive (n-) doping processes, which are responsible for the optoelectronic properties of these materials, revealed their disparate course and dissimilar effects of redox reactions of the conjugated π-bond. While p-doping prevalent species were found to comprise intensively interacting spin bearing and spinless charge carriers, the n-doping state was found to involve only one type of negatively charged carrier, with spin carrying species being selectively generated at due cathodic potentials. No spin pairing of these negative polarons was observed with their increasing population behaving like a collection of localised charge carriers. Qualitative and quantitative comparisons between the p- and n-doping carrier populations provided independent support for the spin pairing phenomena of positive charge carriers. Steric effects of varying alkyl side chain substitution have demonstrated predominant impact on the electrochemical properties of investigated polymers, and, thereto related, stability of n-doped state, while mesomeric effects of different 3,4-ethylenechalcogenide thiophene functionalities have been found to shape the energy level related spectral properties of these polymers, with particular reference to p-doping induced charged states. These findings provide new insights into the factors requiring attention during structure tailoring of donor–acceptor assemblies for organic optoelectronic applications.

Oligo-3-hydroxybutyrate functionalised pyrroles for preparation of biodegradable conductive polymers

We report the synthesis and characterization of a novel polypyrrole material grafted with biodegradable oligo-3-hydroxybutyrate pendants. The polymer was prepared in a two-step process. Firstly, the potassium salt 1–(2-carboxyethyl)pyrrole was reacted with β-butyrolactone affording N-substituted macromonomer. Secondly, the macromonomer was oxidatively polymerised with FeCl3 Lewis acid. The reaction was carried out in solvents of assorted polarity: dimethylsulphoxide, acetonitrile, dimethylformamide and water. Obtained polymers have been characterised comprehensively using a suite of spectral techniques. The material was found to combine the well-known merits of 3-hydroxybutyrate polymers with the electrical conductivity imparted by polypyrrole units.

Synthesis, photophysics and electrochemistry of novel, nitrogen-containing heterocyclic derivatives

A series of new π-conjugated donor (thienyl – T) and acceptor (A) oligomers were prepared by Stille coupling reaction. The T–A oligomers consisting of thiophene/bithiophene as donors and 1,2,4-triazine as an acceptor were prepared to investigate their photophysical and electrochemical properties. These compounds were spectroscopically confirmed to be highly conjugated. These UV-vis data show that the number and position of the thiophene considerably affect the width of the HOMO–LUMO gap and the rigidity of the conjugated system. Compounds A2TA, A3TA, A4TA show photoluminescence in a range 466–543 nm.

Spectral, electrochemical and structural study of aryl derivatives of trans-stilbenes

Aryl-substituted alkenes are promising materials for optoelectronic and photovoltaic applications. In this study we report a new series of stilbene derivatives with aromatic substituents. UV-vis absorption and fluorescence spectra of the studied compounds are presented. The optical properties are found to be dependent on dihedral angle and orbital delocalization between side substituents and the central stilbene moiety, as well as on relaxation of the excited state geometry. Especially the anthracene derivative presents different geometry and excited state properties than other studied compounds. The conclusions are supported by theoretical quantum-chemical analysis at the DFT level. Depending on the type of aromatic side substituents the HOMO orbital is localized either mainly on the central vinylene or the outmost aryl moieties. This affects the reactivity of these compounds in electrochemical reactions as well as the type of oxidation products. The thiophene derivative undergoes polymerization during oxidation and a conjugated polymer is produced while anthracene, phenylene and pentafluorophenylene derivatives undergo irreversible oxidation with creation of unconjugated products.

Synthesis, photophysics and electrochemical properties of 1,1′-(2,2′-bithiophene-5,5′-diyl)bis(cycloalkeno[c]pyridine) as a result of the Diels–Alder reaction of 3-(2-thienyl)-1,2,4-triazine

Two π-conjugated thienylenecycloalkeno[c]pyridine (A–D)2 (cyclopentane, 1, and cycloheptane, 2, units when n = 1 and 3, respectively) compounds were prepared using a palladium C–C coupling reaction. The π–π* absorption peak of compound 1 was observed at the longer wavelength of 392 nm than that of compound 2, which was observed at 364 nm. UV-Vis data reflect the interaction between the thiophene ring (donor – D) and cycloalkeno[c]pyridine (acceptor – A) units. These compounds show photoluminescence (PL) in the range of 466–470 nm and give quantum yields, Φ, of 15%. Furthermore, intermediates 5 and 6 are characterized by their shorter absorption peaks (304 and 292 nm, respectively). The electrochemical parameters of compounds 1 and 2 were characterized using cyclic voltammetry (CV) measurements and theoretical calculations at the DFT/B3LYP/6-311++G(d,p) level.

Synthesis by Stille Cross-Coupling Procedure and Electrochemical Characterization of Branched Polymers Based on Substituted 1,3,5-Triarylbenzenes

A series of various substituted 1,3,5-triarylbenzenes, 2,4,6-triaryl-1-phenoles, 1,3,5-triaryl-1-methoxybenzenes and 2,4,6-triaryl-1,3,5-trimethoxybenzenes were synthesized by a Stille cross-coupling procedure. Their structures were confirmed by 1H NMR, 13C NMR, and elemental analysis. Containing thienyl, furyl and EDOT groups polymers obtained in the process of electropolymerization could be carefully considered as a building material in a wide range of organic-electronic devices. We compare properties of monomers and related polymers depending on aryls moieties and their influence of hydroxyl and methoxyl groups attached to the central benzene core.

The Effect of Aromatic Diimide Side Groups on the π-Conjugated Polymer Properties

The presented study describes the method for the synthesis and characterization of a new class of conjugated copolymers containing a perylenediimide (PDI) and naphthalene diimide (NDI) side groups. The main conjugated backbone is a donor-acceptor polymer poly[3,6-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] containing thiophene and carbazole as donor units and benzothiadiazole as an acceptor unit. The presented compounds were synthesized in a multistep synthesis. The polymerization was carried out by Suzuki or Stille coupling reaction. Redox properties of the studied polymers were tested in different conditions. Electrochemical investigation revealed independent reduction of the main polymer chain and diimide side groups. UV-Vis spectroscopy revealed the overlap of two absorption spectra. The difference between the electron affinity of the polymer main chain and that of the diimides estimated electrochemically is approximately 0.3 eV.

Investigation of the Effects of Non-Conjugated Co-Grafts on the Spectroelectrochemical and Photovoltaic Properties of Novel Conjugated Graft Copolymers Based on Poly(3-hexylthiophene)

A new type of polysiloxane copolymers, with conjugated–regioregular poly(3-hexylthiophene) (P3HT) and non-conjugated-poly(ethylene glycol) (PEG)-grafts have been synthesised, and their properties have been studied alongside those of the parent conjugated polymer (P3HT). Spectroelectrochemical and conductometric analyses revealed an early rise of the conductance of the polymers. Once spectral changes begin taking place, the conductance is stable, implying a loss of mobility of charge carriers, even though standard doping/dedoping patterns are observed. Prototype bulk heterojunction solar cells have been fabricated, based on P3HT/[6,6]-Phenyl-C61-butyric acid methyl ester (PCBM), as well as by substituting P3HT for each of the copolymers. The prototype solar cells achieved PCEs of up to 2.11%. This is one of the highest reported power conversion efficiency (PCE) for devices based on P3HT with low average molecular weight Mn = 12 kDa. Strong correlation between the structure of the copolymer and its photovoltaic performance was found. Elongation of PEG copolymer chain and the use of methyl group instead of terminal hydroxyl groups significantly improved photovoltaic performance.