Fused H-shaped tetrathiafulvalene-oligothiophenes as charge transport materials for OFETs and OPVs

A series of hybrid tetrathiafulvalene-oligothiophene compounds has been synthesised, where the tetrathiafulvalene unit is fused at each side to an end-capped oligothiophene chain of varying length (terthiophene, quinquithiophene and septithiophene). Each hybrid structure (1-3) has been studied by cyclic voltammetry and triple EPR-UV-Vis-NIR spectroelectrochemistry in the case of the quinquithiophene compound (2). Comparison is made with the corresponding half-units, which lack the fulvalene core and contain just one oligothiophene chain. The highest hole mobility of quinquithiophene-TTF 2 was obtained from field effect transistors (8.61 × 10-3 cm 2 V-1 s-1); its surface morphology was characterised by tapping mode atomic force microscopy and a power conversion of 2.5% was achieved from a bulk heterojunction organic solar cell device using PC71BM as the acceptor. This journal is © the Partner Organisations 2014

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

Mono and di-substituted BODIPY with electron donating carbazole, thiophene, and 3,4-ethylenedioxythiophene units

In an effort to extend the conjugation system of boron-dipyrrin (BODIPY), the dye was coupled with polymerisable units of carbazole, alkylthiophene, and 3,4-ethylenedioxythiophene (EDOT). Due to electron donating character of the units connected to the strong electron accepting segment of tri-coordinated boron, the optical and electrochemical properties of the dye changed, and followed the strength of the electron donating substituent. All studied compounds exhibited electroactivity in both anodic and cathodic potential ranges, however, electropolymerisation was only observed for BODIPY modified with EDOT, leading to trimeric and tetrameric substituents. Potential controlled optical changes (spectroelectrochemistry) and in situ spin density distribution measurements, supported by quantum chemical calculations using density functional theory, were undertaken in order to understand the reversibility of the electrochemical processes. Hyperfine split ESR spectra of the radical anion species were successfully modelled revealing intriguing, hyperconjugation type unpaired spin delocalisation patterns

Electrochromic Properties of Novel Selenophene and Tellurophene Derivatives Based on Carbazole and Triphenylamine Core

The 2,7- and 3,6-substituted carbazole and triphenylamine chalcogenophene (Se, Te) derivatives and their electrodeposited polymers are investigated using electrochemical and UV–vis–NIR/ESR spectroelectrochemical methods. Major differences in the case of oxidation and electropolymerization behavior between monomers and related polymers are shown. Se and Te atoms do not conjugate their lone electron pairs with the π-conjugated system and therefore only increase the contribution of the quinoid form of the chalcogenophene unit. The 2,7- substituted carbazole derivatives present stronger carbazole–chalcogenophene conjugation than 3,6-substituted derivatives. One 3,6-substituted carbazole derivative and triphenylamine derived polymers were found to have promising electrochromic properties with black electrochromism

Electrochemically induced synthesis of poly(2,6-carbazole)

The formation of a poly(2,6-carbazole) derivative during an electrochemical polymerization process is shown. Comparison of 3,5-bis(9-octyl-9H-carbazol-2-yl)pyridine and 3,5-bis(9-octyl-9H-carbazol-3-yl)pyridine by electrochemical and UV-Vis-NIR spectroelectrochemical measurements and DFT (density functional theory) calculation prove the formation of a poly(2,6-carbazole) derivative. Both of the compounds form stable and electroactive conjugated polymers

Diquinoline Derivatives as Materials for Potential Optoelectronic Applications.

Here we report the characterization of diquinoline derivatives with ambipolar character. The investigated derivatives show low p- and moderately low n-doping redox systems. The derivatives were investigated and the electrochemical and optical properties of these compounds measured, with attempts to correlate the properties with their pi-electron conjugation lengths. We showed the marked influence of the heteroatom in the bridge (central ring) and geometry of the molecule with its electronic properties. The compounds were compared by UV-vis-NIR and electron paramagnetic resonance (EPR) spectroelectrochemistry indicating very different charge and spin properties dependent on the heteroatoms present. Finally the organic light-emitting diode (OLED) devices were formed and characterized

Thermally Activated Delayed Fluorescence Mediated through the Upper Triplet State Manifold in Non-Charge-Transfer Star-Shaped Triphenylamine–Carbazole Molecules

Thermally activated delayed fluorescence has been found in a group of tricarbazolylamines that are purely electron-donating, non-charge-transfer (CT) molecules. We show that the reverse intersystem crossing step in these materials is mediated through upper triplet states. Reverse internal conversion is shown to be the thermally activated mechanism behind the triplet harvesting mechanism. The strongly mixed n−π*/π–π* character of the lowest energy optical transitions retains high oscillator strength and gives rise to high ΦPL. Organic light-emitting diode devices using these materials were fabricated to show very narrow (full width at half-maximum = 38–41 nm) electroluminescence spectra, clearly demonstrating the excitonic nature of the excited states. This new combination of physicochemical properties of a non-CT molecule yields thermally activated delayed fluorescence, but via a different, physical mechanism, reverse internal conversion delayed fluorescence

Exciplex Enhancement as a Tool to Increase OLED Device Efficiency

Organic electronics, mainly due to advancements in OLED (organic light-emitting diode) technology, is a fast developing research area having already revolutionized the displays market. This direction presents the use of exciplex emitters and thermally activated delayed fluorescence (TADF) in OLEDs, to give efficient, stable emitters that do not use scarce and expensive materials such as iridium. Here, it is shown for the first time several diketopyrrolopyrrole (DPP) derivatives that could be used as emitters in OLED devices. We were able to improve the efficiency of DPP materials by forming exciplex-enhanced OLED devices. These organic materials were also characterized by electrochemical and spectroscopic methods in order to elucidate each molecule's interaction and decrease the photoluminescence efficiency

Symmetrically Disubstituted Bithiophene Derivatives of 1,3,4-Oxadiazole, 1,3,4-Thiadiazole, and 1,2,4-Triazole – Spectroscopic, Electrochemical, and Spectroelectrochemical Properties

Electrochemical and spectroelectrochemical properties of a series of new penta-ring donor–acceptor compounds, comprising 1,3,4-oxadiazole, 1,3,4-thiadiazole, and 1,2,4-triazole central ring, symmetrically connected to substituted bithiophenes, were investigated. Aromaticity and electrophilic–nucleophilic traits of the aza-heterocyclic units, fostering inductive and resonance effects that translate to conjugation enhancement and electron (de)­localization, were found a major factor determining the key electron properties of ionization potential (IP) and electron affinity (EA) of these molecules. Replacing the alkyl thiophene substituent for an alkoxy one afforded certain control over the two parameters as well. All studied compounds were found to undergo electrochemical polymerization giving p- and n-dopable products, featuring good electrochemical reversibility of their oxidative doping process, as demonstrated by cyclic voltammetry and UV–vis–NIR, EPR spectroelectrochemistry. While electropolymerization of entities differing in the heterodiazole unit was found to conserve the EA value, the IP parameter of polymerization products was found to decrease by 0.6–0.7 eV, affording an asymmetric narrowing of the frontier energy levels gap. Aided by quantum chemical computations, the effects of structure tailoring of the investigated systems are rationalized, pointing to conscious ways of shaping the electronic properties of thiophene class polymers using synthetically convenient heterodiazole π-conjugated units