Dithieno[3,2-b:2',3'-e]pyridine (DTP) is a heterocycle consisting of a central pyridine ring fused with two thiophene rings and can be regarded as a thiophene-based analogue of the classic N-heterocycle acridine. Unlike acridine-based dyes, the fundamental chemistry of DTP has received relatively little attention. Here, we report the straightforward synthesis of a library of DTP molecules and an investigation of their electronic properties using UV/vis absorbance and emission spectroscopy supported by computational insights, with particular focus on the influence of protonation and methylation of the central N atom on their properties. While the absorbance of the DTPs studied was confined to ultraviolet wavelengths, protonation or N-methylation of the pyridine group led to significant visible light absorption, which was exploited for the visible light photosensitisation of oxygen.

Understanding the degrees of freedom of movement of molecules in the condensed phase is critical for informing the design of new molecular materials. Here we have applied muon spin resonance spectroscopy (μSR) to study the three-dimensional (3D) polycyclic aromatic hydrocarbon triptycene in the solid phase. This was inspired by the rotational dynamics observed in previous μSR studies of the archetypical 3D molecular electronic materials, the fullerenes. We identified that there is only limited motion of triptycene in its crystals. We also make the intriguing observation that muonium adds to the barrelene core of the molecules which may provide a powerful new method to examine through-space interactions between aromatic rings.

Strategies for tuning the optical properties of organic chromophores generally focus on shifting the edges of the spectrum: this might be red-shifting the longest absorbance band to improve solar absorbance, or blue-shifting of the highest energy emission band towards deep blue emission. In contrast, strategies to enhance molar absorptivity and control excited state rate constants are less obvious, with intermolecular excitons such as J-aggregates providing arguably the most powerful approach. Here, a homologous series of π-extended triptycenes is presented which reveal opportunities to control both aspects. These molecules have electronic spectra consisting of two distinct regimes, a low energy intramolecular charge transfer and a mid-spectral progression which has characteristics similar to that of a J-aggregate in several respects. This reveals that a homoconjugated framework can be utilised rationally to separate and independently control distinct regions of the electronic structure of the molecule, here leading to controllably amplified mid-spectrum absorbance intensities and high fluorescence quantum yields.

Four novel small molecules featuring central 1,2,5,6-tetrathiocin (TTC) units fusing two oligothiophene chains have been prepared, characterised and their structures elucidated through X-ray crystallography. These compounds have been evaluated as p-channel semiconductors in organic field-effect transistor devices, representing the first time that any tetrathiocin-containing compound has been utilised in organic transistors. Through extensive optimisation, involving the screening of different processing solvents, surface treatments, post-processing conditions, dielectric materials, and device configurations, hole mobilities (mu h) in the 10-2 cm2 V-1 s-1 range were obtained for the most optimised devices, with the highest value of 7.3 x 10-2 cm2 V-1 s-1 achieved in 5T-TTC. The double-cable design, established through two parallel oligothiophene chains in the same molecule bridged by a tetrathiocin core, provides improved device characteristics over analogous double-cable oligothiophenes with tetrathiafulvalene and germanium cores.

The formation of effective interphases is crucial to enable high-performance lithium-ion batteries. This can be facilitated by the introduction of electrolyte additives, ensuring improved stability and transport properties. The identification of proper additives requires a comprehensive understanding of the fundamental mechanisms of interfacial reactions governing interphase formation. This study presents a detailed investigation of widely known and less conventional interphase-forming additives in high-voltage LiNi0.6Mn0.2Co0.2O2, NMC622||artificial graphite cells. The electrochemical characterization shows that cells containing vinylethylene carbonate (VEC) significantly outperform all other investigated electrolyte formulations. Surprisingly, gas chromatography-mass spectroscopy measurements of the electrolyte composition after cycling indicate the formation of an ineffective solid-electrolyte interphase (SEI) in the presence of VEC. A thorough analysis of the interfacial composition via operando shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) and surface-enhanced Raman spectroscopy elucidates rather the formation of an effective cathode-electrolyte interphase (CEI). This phenomenon results from the reductive reaction of VEC on the anode, followed by the product transfer and electro-polymerization of reaction products on the cathode. Additionally, focused ion beam secondary ion mass spectrometry (FIB-SIMS) with a time of flight (ToF)-detector is used to analyze the elemental spatial distribution of Li-species and Mn in the respective SEIs. In this work vinylethylene carbonate is introduced as an effective electrolyte additive to improve the performance of high-voltage NMC622|graphite lithium-ion batteries. The cross-talk mechanism of VEC reductive decomposition leads to the formation of an effective CEI captured by operando SHINERS. image

Photocatalyst systems combining donor polymers with acceptor molecules have shown the highest evolution rates for sacrificial hydrogen production from water for organic systems to date. Here, new donor molecules have been designed and synthesised taking inspiration from the structure-performance relationships which have been established in the development of non-fullerene acceptors. While a conventional bulk heterojunction (BHJ) pairing consists of a donor polymer and acceptor small molecules, here we have successfully reversed this approach by using our new molecules in combination with a n-type conjugated polymer to produce non-conventional BHJ nanoparticles and applied these blends to the sacrificial hydrogen evolution from water. The best performing heterojunction displayed high activity for sacrificial hydrogen production from water with a hydrogen evolution rate of 22,321 µmol h−1 g−1 which compares well with the state-of-the-art for conventional BHJ photocatalyst systems.

A new yellow-emitting quadrupolar donor-pi-acceptor-pi-donor (D-pi-A-pi-D) molecule compound has been synthesised featuring benzo-[1,2-c:4,5-c ']dithiophene-4,8-dione as the acceptor. This molecule was prepared for the purpose of elucidating the origins of the very low photoluminescence quantum yield encountered in its thermally activated delayed fluorescent (TADF) red-emitting isomer which used benzo-[1,2-b:4,5-b ']dithiophene-4,8-dione as the acceptor. The molecule was designed to circumvent the energy gap law, by having a wider HOMO-LUMO gap, while retaining a comparable singlet-triplet gap but ultimately demonstrates even weaker photoluminescence than the red isomer. It shows extremely fast intersystem crossing followed by rapid non-radiative decay and no observable TADF. The electronic structure of this new molecule has been studied using cyclic voltammatery alongside steady-state and transient optical spectroscopy, with observations underpinned by computational insights. To identify whether the observations made from the experimental results might be general properties of benzodithiophene-4,8-dione containing emitters, a computational study is extended to the four isomers of benzodithiophene-4,8-dione in comparison with 9,10-anthraquinone. The results suggest that the singlet and triplet manifolds of these systems are strongly coupled via spin-orbit interactions, and explain how the relative electron-accepting strength of these quinones arises from an interplay between the resonance gains or losses of the central benzene and fused thiophene rings upon photoexcitation. This provides valuable insights into the design principles required for efficient organic light-emitting materials. A new yellow emitting donor-acceptor compound is presented, alongside insights into potential limitations imposed on the luminescent properties of ICT emitters when using benzodithiophene-4,8-diones as acceptors.

The synthesis and properties of a series of 11,11,12,12-tetracyano-9,10-anthraquinodimethane (TCAQ) inspired electron acceptors based on thiophene-fused quinone and triptycene motifs is presented. This has yielded insights into structure-property relationships for establishing and modulating simultaneous two-electron reduction processes in TCAQ analogues. These new compounds were synthesised using a Friedel-Crafts acylation between triptycene and thiophene-3,4-dicarbonyl chloride. Isomeric para-quinones featuring a [c]-fused thiophene on one side and a beta,beta- or alpha,beta-fused triptycene on the other were isolated alongside a thiophene-3,4-diketone which bears two triptycene fragments. Knoevenagel condensation of these products with malononitrile produced a quinoidal bis(dicyanomethylene), an oxo-dicyanomethylene and an acyclic bis(dicyanomethylene). This series of new electron accepting molecules has been studied using X-ray crystallography and the implications of their 3D structures on NMR and UV/vis absorbance spectroscopy and cyclic voltammetry results have been ascertained with conclusions underpinned by computational methods. The products of two isomeric thiophene/triptycene para-quinones with malononitrile provide insights into modulating potential inversion in 11,11,12,12-tetracyano-9,10-anthraquinodimethane type electron-acceptors. Intramolecular hydrogen bonding influences both the product distribution of the reaction to make the quinones, and the NMR chemical shift of the bridgehead hydrogens involved. image

The direct transformation of 1,3-dienes into valuable 2,5-diarylfurans using transition metal-free conditions is presented. By employing a simple oxidation - dehydration sequence on readily accessible 1,3-dienes, important 2,5-diarylfuran building blocks frequently used in medicinal and materials chemistry are prepared. The oxidation step is realised using singlet oxygen, and the intermediate endoperoxide is dehydrated under metal-free conditions and at ambient temperature using the Appel reagent. Notably, this sequence can be streamlined into continuous flow, thereby eliminating the isolation of the intermediate, often unstable endoperoxide. This leads to a significant improvement in isolated yields (ca. 27% average increase) of the 2,5-diarylfurans, while also increasing safety and reducing waste. Our transition metal-free synthetic approach to 2,5-diarylfurans delivers several important furan building blocks used commonly in medicinal chemistry and as optoelectronic materials, including short chain linearly conjugated furan oligomers. Consequently, we also complete a short study of the optical and electrochemical properties of a selection of these novel materials.

Narrow bandgap conjugated polymers are a heavily studied class of organic semiconductors, but their excited states usually have a very short lifetime, limiting their scope for applications. One approach to overcome the short lifetime is to populate long-lived triplet states for which relaxation to the ground state is forbidden. However, the triplet lifetime of narrow bandgap polymer films is typically limited to a few microseconds. Here, we investigated the effect of film morphology on triplet dynamics in red-emitting conjugated polymers based on the classic benzodithiophene monomer unit with the solubilizing alkyl side chains C16 and C2C6 and then used Pd porphyrin sensitization as a further strategy to change the triplet dynamics. Using transient absorption spectroscopy, we demonstrated a 0.45 ms triplet lifetime for the more crystalline nonsensitized polymer C2C6, 2-3 orders of magnitude longer than typically reported, while the amorphous C16 had only a 5 μs lifetime. The increase is partly due to delaying bimolecular electron-hole recombination in the more crystalline C2C6, where a higher energy barrier for charge recombination is expected. A triplet lifetime of 0.4 ms was also achieved by covalently incorporating 5% of Pd porphyrin into the C16 polymer, which introduced extra energy transfer steps between the polymer and porphyrin that delayed triplet dynamics and increased the polymer triplet yield by 7.9 times. This work demonstrates two synthetic approaches to generate the longest-lived triplet excited states in narrow bandgap conjugated polymers, which is of necessity in a wide range of fields that range from organic electronics to sensors and bioapplications.

Heterocyclic dimers consisting of combinations of butterfly-shaped phenothiazine (PTZ) and its chemically oxidized form phenothiazine-5,5-dioxide (PTZ(SO2)) have been synthesized. A twist is imposed across the dimers by ortho-substituents including methyl ethers, sulfides and sulfones. X-ray crystallography, cyclic voltammetry and optical spectroscopy, underpinned by computational studies, have been employed to study the interplay between the oxidation state, conformational restriction, and emission mechanisms including thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP). While the PTZ(SO2) dimers are simple fluorophores, the presence of PTZ induces triplet-mediated emission with a mixed PTZ-PTZ(SO2) dimer displaying concentration dependent hallmarks of both TADF and RTP.

Homoconjugation occurs when two aromatic rings are separated by a single non-conjugated atom. This is demonstrated in triptycene by the through-space overlap of the π-clouds of the aryl fins and can be used to enhance optoelectronic properties.

Here a new electron donor is developed to endow a near-IR TADF molecule with good solubility for solution processing and AIE behavior. This is the first AIE TADF material with all PL > 700 nm.

A fundamental problem facing thermally activated delayed fluorescence (TADF) is to overcome the paradox of efficient electronic transitions and a narrow singlet-triplet energy gap (ΔEST) in a single luminophore. We present a quinoxaline-based TADF iptycene as the first clear example that homoconjugation can be harnessed as a viable design strategy toward this objective. Homoconjugation was introduced in an established TADF luminophore by trimerization through an iptycene core. This homoconjugation was confirmed by electrochemistry. As a direct consequence of homoconjugation we observed synergistic improvements to photoluminescence quantum yield (ΦPL), radiative rate of singlet decay (krS), delayed fluorescence lifetime (τTADF), and rate of reverse intersystem crossing (krISC), while narrowing the ΔEST. The cooperative enhancement is rationalised with TD-DFT calculations including spin-orbit coupling (SOC). A facile synthesis of this system, and the ubiquity of the pyrazine motif in state-of-the-art TADF materials across the electromagnetic spectrum, leads to a great potential for generality.

A series of four heterocyclic dimers has been synthesized, with twisted geometries imposed across the central linking bond by ortho-alkoxy chains. These include two isomeric bicarbazoles, a bis(dibenzothiophene-S,S-dioxide) and a bis(thioxanthene-S,S-dioxide). Spectroscopic and electrochemical methods, supported by density functional theory, have given detailed insights into how para- vs. meta- vs. broken conjugation, and electron-rich vs. electron-poor heterocycles impact the HOMO-LUMO gap and singlet and triplet energies. Crucially for applications as OLED hosts, the triplet energy (E-T) of these molecules was found to vary significantly between dilute polymer films and neat films, related to conformational demands of the molecules in the solid state. One of the bicarbazole species shows a variation in E-T of 0.24 eV in the different media-sufficiently large to "make-or-break" an OLED device-with similar discrepancies found between neat films and frozen solution measurements of other previously reported OLED hosts. From consolidated optical and optoelectronic investigations of different host/dopant combinations, we identify that only the lower E-T values measured in neat films give a reliable indicator of host/guest compatibility. This work also provides new molecular design rules for obtaining very high E-T materials and controlling their HOMO and LUMO energies.

Thermally activated delayed fluorescence (TADF) is a current promising route for generating highly efficient light-emitting devices. However, the design process of new chromophores is hampered by the complicated underlying photophysics that requires a number of different pathways to be optimised simultaneously. In this work, four closely related donor-pi-acceptor-pi-donor systems have been investigated, two of which were synthesised previously, with the aim of elucidating their varying effectiveness for TADF. We, first, outline that neither the frontier orbitals nor the singlet-triplet gaps are sufficient in discriminating between the molecules. Subsequently, a detailed analysis of the excited states, performed at a correlated ab initio level, is shown highlighting the presence of a number of closely spaced singlet and triplet states of varying character. Five density functionals are benchmarked against this reference revealing dramatic changes in, both, excited state energies and wavefunctions following variations in the amount of Hartree-Fock exchange included. Excited-state minima are optimised in solution showing the crucial role of structural variations for stabilising locally excited and CT states and of symmetry breaking for producing a strongly emissive S1 state. More generally, this work shows how a detailed analysis of excited-state wavefunctions can provide critical new insight into excited-state electronic structure, helping to reveal the photophysics of existing push-pull chromophores and ultimately guiding the design of new ones.

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The construction of carboxylic acid compounds in a selective fashion from low value materials such as alkenes remains a long-standing challenge to synthetic chemists. In particular, β-addition to styrenes is underdeveloped. Herein we report a new electrosynthetic approach to the selective hydrocarboxylation of alkenes that overcomes the limitations of current transition metal and photochemical approaches. The reported method allows unprecedented direct access to carboxylic acids derived from β,β-trisubstituted alkenes, in a highly regioselective manner.

Greater than the sum of its parts – a 3D ICT molecule displays greatly improved optoelectronic properties over a 2D analogue.

A dual-chain structural motif provides a new promising material for ternary OPV devices.