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The synthesis of a family of alkanethiol molecules with planar aromatic head groups, designed to anchor molecules effectively to graphene electrodes, is reported. Characterisation of self-assembled monolayers of these molecules on a gold surface via conductive atomic force microscopy shows that when an aromatic head group is present, the conductance Ggraphene obtained using a graphene coated probe is higher than the conductance GPt obtained using a platinum (Pt) probe. For Pt probe and graphene probe junctions, the tunnelling decay constant of benzyl ether derivatives with an alkanethiol molecular backbone is determined as 𝛽 = 5.6 nm-1 and 3.5 nm-1, respectively. The conductance ratio Ggraphene/GPt increases as the number of rings present in the aromatic head unit, n, increases. However, as the number of rings increases, the conductance path length increases because the planar head groups lie at an angle to the plane of the electrodes. This means that overall conductance decreases as n increases. Density functional theory-based charge transport calculations support these experimental findings. This study confirms that planar aromatic head groups can function as effective anchoring units for graphene electrodes in large area molecular junctions. However, the results also indicate that the size and geometry of these head groups must be considered in order to produce effective molecular designs.

New cationic mono- and AIE tetra-nuclear Ir(iii) complex-porphyrin conjugates and the corresponding carrier-free NP PSs by self-assembly methods without any auxiliary reagents, [TPP-Ir]+ NPs and [TPP-4Ir]4+ NPs, are obtained for highly efficient PDT.

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.

Photodynamic therapy (PDT) is a promising cancer treatment method. Traditional small-molecule photosensitizers (PSs) suffer from low intersystem crossing (ISC) ability and aggregation-caused quenching (ACQ), which adversely affects the luminous efficiency and singlet oxygen (1O2) yield of PSs in the aggregated state. Ir(III) complexes are promising PSs with long excited-state lifetime, good photophysical and photochemical properties and large Stokes shifts. Aggregation-induced emission (AIE) characteristics could reduce the nonradiative recombination and improve the ISC ability of excited states through the restriction of the intramolecular motions in aggregated states. Accordingly, two AIE-active Ir(III) complexes Ir-1-N+ and Ir-2-N+ were successfully designed and obtained based on Schiff base ligands. Experimental results showed that Ir-1-N+ and Ir-2-N+ have good photophysical properties and the corresponding nanoparticles (NPs) have good water solubility and 1O2 generation ability. Notably, Ir-2-N+ NPs can be efficiently taken up by mouse breast cancer cells (4T1 cells) with good biocompatibility, low dark toxicity and excellent phototoxicity. This work demonstrates a versatile strategy for exploiting efficient transition metal PSs with a cationic ligand in PDT.

The integration of an aggregation induced emission (AIE)-active Ir(III) complex and upconversion nanoparticles (UCNPs) has achieved a NIR-irradiated photosensitizer (PS), UCNPs@Ir-2-N. This PS has satisfactory biocompatibility, excellent phototoxicity, good accumulation in cells and high 1O2 generation ability, thereby effectively killing 4T1 mouse cancer cells in vitro. This work has potential for future photodynamic therapy (PDT) applications.

Here two D–A3 regioisomers, comprising three dibenzothiophene-S,S-dioxide acceptor units attached to a central triazatruxene core, are studied. Both molecules show thermally activated delayed fluorescence (TADF), however, the efficiency of the TADF mechanism is strongly affected by the D–A substitution position. The meta- substituted emitter (1 b) shows a slightly higher-lying singlet charge transfer state and a lower-lying triplet state than that observed in the para- substituted emitter (1 a), resulting in a larger singlet–triplet splitting (ΔEST) of 0.28 eV compared to only 0.01 eV found in 1 a. As expected, this ΔEST difference strongly impacts the reverse intersystem crossing (rISC) rates and the para- isomer 1 a exhibits a much faster delayed fluorescence emission. Calculations show that the triplet energy difference between the two isomers is due to steric hindrance variances along the donor–acceptor rotation axis in these molecules: as 1 b is less restricted, rotation of its acceptor unit leads to a lower T1 energy, further away from the region of high density of states (the region where larger vibronic coupling is found, favouring rISC). Therefore, our results show how the substitution pattern has a marked effect on triplet state energies and character, verifying the key structural designs for highly efficient TADF materials.

Understanding and controlling the orbital alignment of molecules placed between electrodes is essential in the design of practically-applicable molecular and nanoscale electronic devices. The orbital alignment is highly determined by the molecule-electrode interface. Dependence of orbital alignment on the molecular anchor group for single molecular junctions has been intensively studied; however, when scaling-up single molecules to large parallel molecular arrays (like self-assembled monolayers (SAMs)), two challenges need to be addressed: 1. Most desired anchor groups do not form high quality SAMs. 2. It is much harder to tune the frontier molecular orbitals via a gate voltage in SAM junctions than in single molecular junctions. In this work, we studied the effect of the molecule-electrode interface in SAMs with a micro-pore device, using a recently developed tetrapodal anchor to overcome challenge 1, and the combination of a single layered graphene top electrode with an ionic liquid gate to solve challenge 2. The zero-bias orbital alignment of different molecules was signalled by a shift in conductance minimum vs. gate voltage for molecules with different anchoring groups. Molecules with the same backbone, but a different molecule-electrode interface, were shown experimentally to have conductances that differ by a factor of 5 near zero bias. Theoretical calculations using density functional theory support the trends observed in the experimental data. This work sheds light on how to control electron transport within the HOMO-LUMO energy gap in molecular junctions and will be applicable in scaling up molecular electronic systems for future device applications.

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., A new series of high E T heterocyclic dimers has been synthesized with large dihedral angles enforced by steric hindrance. A meta‐conjugated carbazole dimer displays the highest E T of 3.07 eV in dilute polymer film, however in a neat film this falls to 2.83 eV. Similar variation is observed across the series. This work highlights that to reliably estimate host/guest compatibility, E T determined from phosphorescence of neat films must be reported for new host materials. The currently accepted practice of reporting values from frozen solution or polymer films is no substitute.

Intramolecular hydrogen bonding between donor and acceptor segments in thermally activated delayed fluorescence (TADF) materials is now frequently employed to─purportedly─rigidify the structure and improve the emission performance of these materials. However, direct evidence for these intramolecular interactions is often lacking or ambiguous, leading to assertions that are largely speculative. Here we investigate a series of TADF-active materials incorporating pyridine, which bestows the potential ability to form intramolecular H-bonding interactions. Despite possible indications of H-bonding from an X-ray analysis, an array of other experimental investigations proved largely inconclusive. Instead, after examining computational potential energy surfaces of the donor–acceptor torsion angle we conclude that the pyridine group primarily alleviates steric congestion in our case, rather than enabling an H-bond interaction as elsewhere assumed. We suggest that many previously reported “H-bonding” TADF materials featuring similar chemical motifs may instead operate similarly and that investigation of potential energy surfaces should become a key feature of future studies.

Purely organic molecules, which emit light by dual emissive (DE) pathways, have received increased attention in the last decade. These materials are now being utilized in practical optoelectronic, sensing and biomedical applications. In order to further extend the application of the DE emitters, it is crucial to gain a fundamental understanding of the links between the molecular structure and the underlying photophysical processes. This review categorizes the types of DE according to the spin multiplicity and time range of the emission, with emphasis on recent experimental advances. The design rules towards novel DE molecular candidates, the most perspective types of DE and possible future applications are outlined. These exciting developments highlight the opportunities for new materials synthesis and pave the way for accelerated future innovation and developments in this area.

Nonconventional chromophores, without classic conjugation and rigid units, are a hot topic of luminescent materials nowadays because of their unique under-explored fundamental properties and their promising applications. Despite the increased attention recently paid to this exciting area, the structural design and the mechanism of photoluminescence in these materials remain unclear and controversial. Different luminescence mechanisms have been successively proposed in recent years, such as oxidation, aggregation of carbonyl units, interactions with phenyl groups, clustering of chromophores and through-space conjugation, etc. This review summarizes oligomers/polymers with unconventional chromophores reported in recent years, which are classified according to their chemical structures and luminescence mechanisms. We present an interconnected overview of the topic highlighting the opportunities for new materials synthesis, looking forward to accelerated future innovation and developments in this area.

Soft luminescent gel materials have many practical applications due to their excellent solution processability, thixotropic, soft and facile coating performance. Here we have prepared a series of oligourethane (OU)-based gelators with fine-tuned optical properties. Chromophores with different π-conjugated lengths were introduced into the backbone in order to systematically control the energy gaps of the OUs. Blue/green/red (BGR) fluorescent gelators were thus achieved. The gelation behavior was driven by non-covalent bonds, notably hydrogen-bonds and van der Waals interactions. Because of the dynamic character of these non-covalent interactions, a simple mixture of sols could revert back to homogeneous hybrid gels. Upon gelation, the extended network of the gels leads to sequential multichromophoric Förster resonance energy transfer (FRET). Benefiting from the presence of three primary colours, the resultant hybrid gel exhibited almost any desired visible emission colour at different ratios of gelators. Of particular significance, a white-light-emitting diode (WLED) was prepared with CIEx,y coordinates (0.35, 0.33) in the hybrid gel formed by precise control over the OUB/OUG/OUR stoichiometry. This type of supramolecular gel holds promise in designing large-area, flexible optical illumination, fluorescent labels and light-harvesting systems, and could be a suitable scaffold for new-generation soft materials with tunable optical properties in display technologies.

The synthetic methodology to covalently link donors to form cyclophane-based thermally activated delayed fluorescence (TADF) molecules is presented. These are the first reported examples of TADF cyclophanes with “electronically innocent” bridges between the donor units. Using a phenothiazine-dibenzothiophene-S,S-dioxide donor–acceptor–donor (D–A–D) system, the two phenothiazine (PTZ) donor units were linked by three different strategies: (i) ester condensation, (ii) ether synthesis, and (iii) ring closing metathesis. Detailed X-ray crystallographic, photophysical and computational analyses show that the cyclophane molecular architecture alters the conformational distribution of the PTZ units, while retaining a certain degree of rotational freedom of the intersegmental D–A axes that is crucial for efficient TADF. Despite their different structures, the cyclophanes and their nonbridged precursors have similar photophysical properties since they emit through similar excited states resulting from the presence of the equatorial conformation of their PTZ donor segments. In particular, the axial–axial conformations, known to be detrimental to the TADF process, are suppressed by linking the PTZ units to form a cyclophane. The work establishes a versatile linking strategy that could be used in further functionalization while retaining the excellent photophysical properties of the parent D–A–D system.

There are notably few literature reports of electron donor-acceptor oligoynes although they offer unique opportunities for studying charge transport through ‘all-carbon’ molecular bridges. In this context, the current study focuses on a series of carbazole–(C≡C)n-2,5-diphenyl-1,3,4-oxadiazoles (n = 1-4) as conjugated π-systems, in general, and explores their photophysical properties, in particular. Contrary to the behavior of typical electron donor-acceptor systems, for these oligoynes the rates of charge recombination after photoexcitation increase with increasing electron donoracceptor distance. To elucidate this unusual performance, detailed photophysical and time-dependent density functional theory investigations were conducted. Significant delocalization of the electron density along the bridge indicates that the bridging states come into resonance with either the electron donor or acceptor, thereby accelerating the charge transfer. Moreover, the calculated bond lengths reveal a reduction in bond length alternation upon photoexcitation, indicating significant cumulenic character of the bridge in the excited state. In short, strong vibronic coupling between the electrondonating N-arylcarbazoles and the electron-accepting 1,3,4-oxadiazoles accelerates the charge recombination as the oligoyne becomes longer.

We report a new type of mechanically sensitive multicolor luminescent oligourethane gel (OUA-gel). The conformation of the oligomeric chains can be controlled by changing the strength of hydrogen bonds. The optical properties of the oligomers are highly dependent on the conformations which vary in response to mechanical stresses and phase transitions. The design relies on the introduction of a single mechanical chromophore, aurintricarboxylic acid, with propeller-like, spatially crowded, and highly twisted conformations, and the presence of three carboxyl groups, which provide multidirectional hydrogen-bonding opportunities. Introducing dimethylsulfoxide (DMSO) as an additional H-bond acceptor molecule leads to a viscous OUA-gel which exhibits multiemission colors because of changes in the chain conformation within the matrix, which are induced by different strengths of H bonds. The conformation can be adjusted by mechanical force or temperature, both of which influence the H-bonding. The multifunctional and multicolored mechanochromism of the OUA-gel has great promise in sensing applications. The results represent a substantial step toward understanding the mechanism of polychromism in soft materials and the molecular design of advanced smart materials.

A novel type of thermally activated delayed fluorescence (TADF) dendronized polymer was designed and synthesized. Firstly, one side of the asymmetric TADF unit was encapsulated by 3,6-di-tert-butylcarbazole via a conjugated linkage with strong twisted intramolecular charge transfer (TICT) to minimize the energy gap between 1CT and 3CT, and then the peripheral dicarbazole connects in a non-conjugated way on the other side, showing weak TICT and high-lying 3LE&3CT state; finally, a linear dendronized polymer, PDCDC, was obtained by introducing the alkyl polymer backbone as main chains. As far as we know, this is the first blue or greenish-blue-emitting TADF dendronized polymer. For PDCDC, the spin-forbidden 1CT ↔ 3CT transitions are activated by molecular vibrations, which combine with a small energy gap and reorganization energy to enable 1CT ↔ 3CT spin-flip transition rates reaching 106 s−1 with negligible role of a second triplet state. Films of PDCDC show a double exponential decay (prompt fluorescence and delayed fluorescence) in the presence of oxygen due to a low oxygen permeability ascribed to the entangled polymeric backbone which increases film density. Besides, obvious aggregation-enhanced emission (AEE) property can minimize the exciton quenching in aggregated states. A maximum external quantum efficiency of 9.0% for non-doped PDCDC-based organic light-emitting diodes (OLEDs) can be obtained. To the best of our knowledge, these are the most efficient dendronized polymer devices with blue or greenish-blue emission.

Thin films comprising synthetically robust, scalable molecules have been shown to have major potential for thermoelectric energy harvesting. Previous studies of molecular thin-films have tended to focus on massively parallel arrays of discrete but identical conjugated molecular wires assembled as a monolayer perpendicular to the electrode surface and anchored via a covalent bond, know as self-assembled monolayers. In these studies, to optimise the thermoelectric properties of the thin-film there has been a trade-off between synthetic complexity of the molecular components and the film performance, limiting the opportunities for materials integration into practical thermoelectric devices. In this work, we demonstrate an alternative strategy for enhancing the thermoelectric performance of molecular thin-films. We have built up a series of films, of controlled thickness, where the basic units—here zinc tetraphenylporphyrin—lie parallel to the electrodes and are linked via π–π stacking. We have compared three commonly used fabrications routes and characterised the resulting films with scanning probe and computational techniques. Using a Langmuir-Blodgett fabrication technique, we successfully enhanced the thermopower perpendicular to the plane of the ZnTPP multilayer film by a factor of 10, relative to the monolayer, achieving a Seebeck coefficient of −65 μV K−1. Furthermore, the electronic transport of the system, perpendicular to the plane of the films, was observed to follow the tunnelling regime for multi-layered films, and the transport efficiency was comparable with most conjugated systems. Furthermore, scanning thermal microscopy characterisation shows a factor of 7 decrease in thermal conductance with increasing film thickness from monolayer to multilayer, indicating enhanced thermoelectric performance in a π–π stacked junction.

Organic thermoelectric materials have potential for wearable heating, cooling, and energy generation devices at room temperature. For this to be technologically viable, high-conductance (G) and high-Seebeck-coefficient (S) materials are needed. For most semiconductors, the increase inSis accompanied by a decrease inG. Here, using a combined experimental and theoretical investigation, we demonstrate that a simultaneous enhancement ofSandGcan be achieved in single organic radical molecules, thanks to their intrinsic spin state. A counterintuitive quantum interference (QI) effect is also observed in stable Blatter radical molecules, where constructive QI occurs for ameta-connected radical, leading to further enhancement of thermoelectric properties. Compared to an analogous closed-shell molecule, the power factor is enhanced by more than 1 order of magnitude in radicals. These results open a new avenue for the development of organic thermoelectric materials operating at room temperature.

Two asymmetric “half-dendronized” and “half-dendronized-half-encapsulated” TADF emitters eliminate degenerate excited states, assuring a solely thermal equilibrium route for an effective spin-flip process. AEE properties can minimize the exciton quenching. The isolated encapsulating tricarbazole unit protects the TADF unit, reducing nonradiative decay. Non-doped OLEDs with a remarkably high EQEmax of 24.0 % and low roll-off are achieved. The mechanism of thermally activated delayed fluorescence (TADF) in dendrimers is not clear. We report that fully-conjugated or fully-nonconjugated structures cause unwanted degenerate excited states due to multiple identical dendrons, which limit their TADF efficiency. We have synthesized asymmetrical “half-dendronized” and “half-dendronized-half-encapsulated” emitters. By eliminating degenerate excited states, the triplet locally excited state is ≥0.3 eV above the lowest triplet charge-transfer state, assuring a solely thermal equilibrium route for an effective spin-flip process. The isolated encapsulating tricarbazole unit can protect the TADF unit, reducing nonradiative decay and enhancing TADF performance. Non-doped solution-processed devices reach a high external quantum efficiency (EQEmax) of 24.0 % (65.9 cd A−1, 59.2 lm W−1) with CIE coordinates of (0.24, 0.45) with a low efficiency roll-off and EQEs of 23.6 % and 21.3 % at 100 and 500 cd m−2.

Blue-emitting gels are scarce, especially oligomeric/polymeric systems. Here, a series of 4,4′-sulfonyldiphenol (SDP) based oligourethane derivatives (OUs), namely OUHDI, OUHMDI and OUTDI based on 1,6-hexamethylene diisocyanate, 4,4′-methylenebis(cyclohexyl isocyanate) and 2,4-diisocyanatotoluene, respectively, were rationally designed and obtained by simple procedures. The new oligomers all display aggregation-induced blue fluorescence and upon gelation they achieve enhanced bright deep-blue emission as a result of interchain hydrogen bonding and oxygen cluster interactions. The OUs show spontaneous thermoreversible gelation in solvents which possess hydrogen-bond receptor units (C[double bond, length as m-dash]O or S[double bond, length as m-dash]O) which facilitate self-assembly of the OU chains into nanotubes. The oligourethane gels (OUGs) possess excellent ability for resisting external stress while at the same time exhibiting a distinct viscous flow state. The results illustrate that heteroatomic non-conjugated oligomers with advantageous hydrogen bonding interactions and oxygen clusters provide an efficient route to photoluminescent blue-emitting gels. The thermoreversible OUGs with excellent mechanical properties have been exploited to make sticky coatings, transparent films and a blue fluorescent molded shape using simple fabrication processes.

The development of metal complexes for optoelectronic applications is a fertile area of research. In contrast to the rigorous development of mononuclear metal complexes, dinuclear species have been less well studied and their fundamental chemistry and applications are under-explored. However, dinuclear species present special properties and functions compared with mononuclear species as a consequence of tuning the bridging ligands, the cyclometalated ligands or the two metal centers. More recently, dinuclear species have enabled important breakthroughs in the fields of OLEDs, photocatalytic water splitting and CO2 reduction, DSPEC, chemosensors, biosensors, PDT, smart materials and so on. Here we present an overview of recent developments of dinuclear metal complexes, their multifunctional properties and their various applications. The relationship between structure and property of dinuclear species and important factors which influence device performance are discussed. Finally, we illustrate some challenges and opportunities for future research into dinuclear metal complexes. This review aims to provide an up-to-date summary and outlook of functional dinuclear metal complexes and to stimulate more researchers to contribute to this exciting interdisciplinary field.

This work shows that trifluoromethyl (CF3) substituents can be used to increase the rate of thermally activated delayed fluorescence (TADF) in conjugated organic molecules by tuning the excitonic character of the singlet and triplet excited states. The synthesis and detailed photophysical characterization of four new functionalized donor–acceptor–donor (D–A–D) compounds using CF3-substituted-thioxanthene-S,S-dioxide and dimethylacridine units are presented. Several compounds are reported to exhibit rapid blue/green thermally activated delayed fluorescence with major delayed fluorescence lifetime components of ≈1–2 μs. Changes in the orbital character of singlet and triplet states (local vs. charge transfer) result in significant changes in rates of delayed fluorescence, despite similar ΔEST values. The change in orbital character is well supported by TD-DFT calculations. Electrochemical measurements reveal strong shifts in redox potentials that can be induced by σ-electron withdrawing CF3 substituents. Trifluoromethyl substituted compounds with a wider ΔEST also perform more efficiently than might be expected due to the demonstrated changes in excited state character. This study reveals important photophysical and molecular design implications for the future development of TADF emitters.

Developing multifunctional photosensitizers (PSs) is needed to effectively simplify cancer treatment, but it remains a big challenge. Here, two red-emitting AIE-active, donor–acceptor (D–A) PSs with small ΔEST and their AIE nanoparticles, are rationally designed and synthesized. The PS1 NPs exhibit bright red-emission with high quantum yield, appropriate 1O2 generation ability and good biocompatibility. More importantly, PS1 NPs can strongly light up the cytoplasm by gently shaking the cells for only 5 s at room temperature, indicating ultrafast staining and mild incubation conditions. In vitro and in vivo cell tracing demonstrate that PS1 NPs can track cells over 14 days, and effectively inhibit tumor growth upon irradiation. To the best of our knowledge, this work is the first example of a PS that integrates image-guided PDT, ultrafast staining and long-term tracing functions, demonstrating the “all-in-one” concept which offers great advantages for potential clinical applications., Developing multifunctional photosensitizers (PSs) is needed to effectively simplify cancer treatment, but it remains a big challenge.

We report the synthesis and structural and photophysical characterization of two series of molecules with functionalized azatriangulene electron donor cores and three pendant electron acceptor units. The presented donor and acceptor units are joined by C–C bonds, instead of the usual C–heteroatom bonds often found in thermally activated delayed fluorescence (TADF) emitters. The effects of the donor–acceptor strength and donor–acceptor dihedral angle on the emission properties are assessed. The data establish that the singlet–triplet energy gap is >0.3 eV and that delayed emission is present in only specific host matrices, irrespective of host polarity. Specific host behavior is atypical of many TADF materials, and we suggest the delayed emission in this work does not occur by a conventional vibronically coupled TADF mechanism, as the ΔEST value is too large. Detailed photophysical analysis and supporting density functional theory calculations suggest that some presented azatriangulene molecules emit via an upper-triplet state crossing mechanism. This work highlights that several different mechanisms can be responsible for delayed emission, often with highly similar photophysics. Detailed photophysical analysis is required to establish which delayed emission mechanism is occurring. Our results also highlight a clear future direction toward vibronically coupled C–C bonded TADF materials.

Controlling charge transport through molecular wires by utilizing quantum interference (QI) is a growing topic in single-molecular electronics. In this article, scanning tunneling microscopy-break junction techniques and density functional theory calculations are employed to investigate the single-molecule conductance properties of four molecules that have been specifically designed to test extended curly arrow rules (ECARs) for predicting QI in molecular junctions. Specifically, for two new isomeric 1-phenylpyrrole derivatives, the conductance pathway between the gold electrodes must pass through a nitrogen atom: this novel feature is designed to maximize the influence of the heteroatom on conductance properties and has not been the subject of prior investigations of QI. It is shown, experimentally and computationally, that the presence of a nitrogen atom in the conductance pathway increases the effect of changing the position of the anchoring group on the phenyl ring fromparatometa, in comparison with biphenyl analogues. This effect is explained in terms of destructive QI (DQI) for themeta-connected pyrrole and shifted DQI for thepara-connected isomer. These results demonstrate modulation of antiresonances by molecular design and verify the validity of ECARs as a simple “pen-and-paper” method for predicting QI behavior. The principles offer new fundamental insights into structure–property relationships in molecular junctions and can now be exploited in a range of different heterocycles for molecular electronic applications, such as switches based on external gating, or in thermoelectric devices.

Two redox and magnetically active perchlorotriphenylmethyl (• PTM) radical units have been connected as end-capping groups to a bis(phenylene)diyne chain through vinylene linkers. Negative and positive charged species have been generated, and the influence of the bridge on their stabilization is discussed. Partial reduction of the electron-withdrawing • PTM radicals results in a class-II mixed-valence system with the negative charge located on the terminal PTM units, proving the efficiency of the conjugated chain for the electron transport between the two terminal sites. Counterintuitively, the oxidation process does not occur along the electron-rich bridge but on the vinylene units. The • PTM radicals play a key role in the stabilization of the cationic species, promoting the generation of quinoidal ring segments., This work was supported by the MICINN of Spain (Projects CTQ2016-80030-R, PGC2018-099568-B-I00 (EU Feder Funds), PID2019-111682RB-I00, Severo Ochoa FUNFUTURE CEX2019-000917-S, and María de Maeztu CEX2019- 000919-M), the Generalitat de Catalunya (2017-SGR-918), the Generalitat Valenciana (PROMETEO/2020/077), and the Networking Research Center on Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN) Development Fund. P.M.- B. gratefully thanks financial support from the Juan de la Cierva-Formación 2015 programme (FJCI-2015-23577) supported by MICINN. F.B. thanks the MECD for a predoctoral FPU fellowship. J.C. acknowledges the Generalitat Valenciana (APOSTD/2017/081). J.A.d.S. is enrolled in the Materials Science doctoral program of the UAB. J.A.d.S. thanks the MINECO for the FPI-SO fellowship. The authors acknowledge A. Bernabé from ICMAB for the MALDI-ToF measurements and Dr. Mireia Díaz and Dr. Marta Vilaseca, from the IRB Barcelona Mass Spectrometry and Proteomics Core Facility (unity member of ProteoRed, PRB3-ISCIII, supported by Grant PRB3 (IPT17/0019-ISCIII-SGEFI/ERDF), for the ES+ ToF MS experiments. The work in Durham was supported by the EC FP7 ITN project MOLESCO (606728).

Oligo(arylene ethynylene) (OAE) derivatives are the “workhorse” molecules of molecular electronics. Their ease of synthesis and flexibility of functionalisation mean that a diverse array of OAE molecular wires have been designed, synthesised and studied theoretically and experimentally in molecular junctions using both single-molecule and ensemble methods. This review summarises the breadth of molecular designs that have been investigated with emphasis on structure–property relationships with respect to the electronic conductance of OAEs. The factors considered include molecular length, connectivity, conjugation, (anti)aromaticity, heteroatom effects and quantum interference (QI). Growing interest in the thermoelectric properties of OAE derivatives, which are expected to be at the forefront of research into organic thermoelectric devices, is also explored.

Controlling charge transport through molecular wires by utilizing quantum interference (QI) is a growing topic in single-molecular electronics. In this article, scanning tunneling microscopy-break junction techniques and density functional theory calculations are employed to investigate the single-molecule conductance properties of four molecules that have been specifically designed to test extended curly arrow rules (ECARs) for predicting QI in molecular junctions. Specifically, for two new isomeric 1-phenylpyrrole derivatives, the conductance pathway between the gold electrodes must pass through a nitrogen atom: this novel feature is designed to maximize the influence of the heteroatom on conductance properties and has not been the subject of prior investigations of QI. It is shown, experimentally and computationally, that the presence of a nitrogen atom in the conductance pathway increases the effect of changing the position of the anchoring group on the phenyl ring from para to meta, in comparison with biphenyl analogues. This effect is explained in terms of destructive QI (DQI) for the meta-connected pyrrole and shifted DQI for the para-connected isomer. These results demonstrate modulation of antiresonances by molecular design and verify the validity of ECARs as a simple “pen-and-paper” method for predicting QI behavior. The principles offer new fundamental insights into structure–property relationships in molecular junctions and can now be exploited in a range of different heterocycles for molecular electronic applications, such as switches based on external gating, or in thermoelectric devices.