Synthetic Principles for Bandgap Control in Linear pi-Conjugated Systems

Tetraanionic Oligo-Dioxaborines: Strongly Absorbing Near-Infrared Dyes

Polymethine dyes of tetraanionic nature comprising 1,3,2-dioxaborine rings in the polymethine chain and end-groups of different electron-accepting abilities have been synthesized. They can be considered as oligomeric polymethines, where a linear conjugated π-system passes through three 1,3,2-dioxaborine units and a number of tri- and dimethine π-bridges between two end-groups. The obtained dyes exhibit near-infrared absorption and fluorescence, with molar absorption coefficients reaching as high as 564000 M-1 cm-1 in DMF, rendering them among the strongest absorbers known. The novel compounds are bright NIR fluorophores, with fluorescence quantum yields up to 0.13 in DMF. A comparative analysis of the electronic structure of the obtained dyes with respective dianionic and trianionic oligomers was conducted through quantum chemical calculations.

Rational Design of NIR-II Emitting Conjugated Polymer Derived Nanoparticles for Image-Guided Cancer Interventions

Due to the reduced absorption, light scattering, and tissue autofluorescence in the NIR-II (1000-1700 nm) region, significant efforts are underway to explore diverse material platforms for in vivo fluorescence imaging, particularly for cancer diagnostics and image-guided interventions. Of the reported imaging agents, nanoparticles derived from conjugated polymers (CPNs) offer unique advantages to alternative materials including biocompatibility, remarkable absorption cross-sections, exceptional photostability, and tunable emission behavior independent of cell labeling functionalities. Herein, the current state of NIR-II emitting CPNs are summarized and structure-function-property relationships are highlighted that can be used to elevate the performance of next-generation CPNs. Methods for particle processing and incorporating cancer targeting modalities are discussed, as well as detailed characterization methods to improve interlaboratory comparisons of novel materials. Contemporary methods to specifically apply CPNs for cancer diagnostics and therapies are then highlighted. This review not only summarizes the current state of the field, but offers future directions and provides clarity to the advantages of CPNs over other classes of imaging agents.

Reply to the Comments on Planar Tetracoordinate Hydrogen: Pushing the Limit of Multicentre Bonding

Recently, Huo et al. has commented on our communication (Angew. Chem. Int. Ed. 2024, 63, e202317312, DOI: 10.1002/anie.202317312), regarding the multireference character (MRC) of our proposed cluster. Their argument is based on small HOMO-LUMO gap, fractional occupation density (FOD) and CASPT2(12,13) calculations. They also proposed that the singlet planar In4H+ cluster cannot be observed. We present our calculations which reveals that some of their arguments are based on wrong interpretation of data and inadequate use of methodology. While we certainly agree with the strong physical ground of FOD, CASSF and CASPT2 methodology, we believe that such analysis for clusters is not adequate.

α-α Coupling-Dominated PPy Film with a Well-Conjugated Structure for Superlong Cycle Life Supercapacitors

As the electrode of a supercapacitor, polypyrrole (PPy) inevitably suffers from structural rupture during repeated doping/dedoping processes and releases low practical capacitance due to the large amount of aggregation or cross-linking in PPy chains. The coupling mode (α-α, α-β, or β-β coupling) of pyrroles is critical to the conjugated structure, the conductivity, and cycling stability of PPy. Here, we prepared an α-α coupling-dominated PPy film via simple frozen interfacial polymerization. The PPy film with a nanostructure exposes more electrochemical active sites for the electrode, which can enhance the practical capacitance. The high proportion of the α-α coupling mode results in a high degree of large π-conjugation and a planar structure that can effectively improve the reversible ion transport efficiency and promote uniform stress distribution during the charge and discharge process. The assembled symmetric water-based supercapacitor delivers a high specific capacitance of 267.1 F g-1 at 1 A g-1 and 266.7 F g-1 at 5 A g-1 and exhibits an outstanding cycling performance of above 200 F g-1 even after 60,000 cycles.

Continuous Topological Transition and Bandgap Tuning in Ethynylene-Linked Acene π-Conjugated Polymers through Mechanical Strain

By variation of the chemical repeat units of conjugated polymers, only discrete tuning of essential physical parameters is possible. A unique property of a class of π-conjugated polymers, where polycyclic aromatic hydrocarbons are linked via ethynylene linkers, is their topological aromatic to quinoid phase transition discovered recently by Cirera et al. and González-Herrero et al., which is controllable in discrete steps by chemical variations. We have discovered by means of density functional theory computations that such a phase transition can be achieved by applying continuous variations of longitudinal strain, allowing us to tune the bond length alternation and bandgap. At a specific strain value, the bandgap becomes zero due to an orbital level crossing between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Our hypothesis provides a perspective on the design of organic electronic materials and provides a novel insight into the properties of a continuous phase transition in topological semiconducting polymers.

V-Shaped Tröger Oligothiophenes Boost Triplet Formation by CT Mediation and Symmetry Breaking

A new family of molecules obtained by coupling Tröger's base unit with dicyanovinylene-terminated oligothiophenes of different lengths has been synthesized and characterized by steady-state stationary and transient time-resolved spectroscopies. Quantum chemical calculations allow us to interpret and recognize the properties of the stationary excited states as well as the time-dependent mechanisms of singlet-to-triplet coupling. The presence of the diazocine unit in Tröger's base derivatives is key to efficiently producing singlet-to-triplet intersystem crossing mediated by the role of the nitrogen atoms and of the almost orthogonal disposition of the two thiophene arms. Spin-orbit coupling-mediated interstate intersystem crossing (ISC) is activated by a symmetry-breaking process in the first singlet excited state with partial charge transfer character. This mechanism is a characteristic of these molecular triads since the independent dicyanovinylene-oligothiophene branches do not display appreciable ISC. These results show how Tröger's base coupling of organic chromophores can be used to improve the ISC efficiency and tune their photophysics.

NIR-II Absorbing Monodispersed Oligomers Based on N-B←N Unit

Organic molecules with near-infrared II (NIR II) light absorption are essential for many biological and opto-electronic applications. Herein, we report monodispersed oligomers as NIR II light absorber using a new molecular design strategy of resonant N-B←N unit, i.e. balanced resonant boron-nitrogen covalent bond (B-N) and boron-nitrogen coordination bond (B←N). We synthesize a series of monodispersed oligomers with thiophene-fused 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (TB), which contains resonant N-B←N unit, as the repeating unit. The TB pentamer exhibits the maximum absorption wavelength of 1169 nm, which is the longest for oligomers reported so far. Organic photodetectors (OPDs) with the TB tetramer as the electron acceptor shows the specific detectivity of 2.98×1011 Jones at 1180 nm under zero bias. This performance is among the best for NIR II OPDs. These results indicate a new kind of NIR II absorbing molecules as excellent opto-electronic materials.

Tunable full-color emission of stilbazoles containing a 2-halo-3,4-dicyanopyridine acceptor

Synthesis of a series of novel push-pull stilbazole-based chromophores containing a strong 2-halocinchomeronic dinitrile acceptor is reported. The photophysical properties of the compounds are described. Strong positive solvatofluorochromism typical of intramolecular charge transfer (ICT) dyes is observed for the synthesized stilbazoles. Their tunable multicolor emission ranges from 442 nm to 710 nm and covers the whole visible spectrum.

Building Block Engineering toward Realizing High-Performance Electrochromic Materials and Glucose Biosensing Platform

The molecular engineering of conjugated systems has proven to be an effective method for understanding structure-property relationships toward the advancement of optoelectronic properties and biosensing characteristics. Herein, a series of three thieno[3,4-c]pyrrole-4,6-dione (TPD)-based conjugated monomers, modified with electron-rich selenophene, 3,4-ethylenedioxythiophene (EDOT), or both building blocks (Se-TPD, EDOT-TPD, and EDOT-Se-TPD), were synthesized using Stille cross-coupling and electrochemically polymerized, and their electrochromic properties and applications in a glucose biosensing platform were explored. The influence of structural modification on electrochemical, electronic, optical, and biosensing properties was systematically investigated. The results showed that the cyclic voltammograms of EDOT-containing materials displayed a high charge capacity over a wide range of scan rates representing a quick charge propagation, making them appropriate materials for high-performance supercapacitor devices. UV-Vis studies revealed that EDOT-based materials presented wide-range absorptions, and thus low optical band gaps. These two EDOT-modified materials also exhibited superior optical contrasts and fast switching times, and further displayed multi-color properties in their neutral and fully oxidized states, enabling them to be promising materials for constructing advanced electrochromic devices. In the context of biosensing applications, a selenophene-containing polymer showed markedly lower performance, specifically in signal intensity and stability, which was attributed to the improper localization of biomolecules on the polymer surface. Overall, we demonstrated that relatively small changes in the structure had a significant impact on both optoelectronic and biosensing properties for TPD-based donor-acceptor polymers.

Control of Bandgaps and Energy Levels in Water-Soluble Discontinuously Conjugated Polymers through Chemical Modification

Bandgap and energy levels are crucial for developing new electronic and photonic devices because photoabsorption is highly dependent on the bandgap. Moreover, the transfer of electrons and holes between different materials depends on their respective bandgaps and energy levels. In this study, we demonstrate the preparation of a series of water-soluble discontinuously π-conjugated polymers through the addition-condensation polymerization of pyrrole (Pyr), 1,2,3-trihydroxybenzene (THB) or 2,6-dihydroxytoluene (DHT), and aldehydes, including benzaldehyde-2-sulfonic acid sodium salt (BS) and 2,4,6-trihydroxybenzaldehyde (THBA). To control the energy levels of the polymers, varying amounts of phenols (THB or DHT) were introduced to alter the electronic properties of the polymer structure. The introduction of THB or DHT into the main chain results in discontinuous conjugation and enables the control of both the energy level and bandgap. Chemical modification (acetoxylation of phenols) of the polymers was employed to further tune the energy levels. The optical and electrochemical properties of the polymers were also investigated. The bandgaps of the polymers were controlled in the range of 0.5-1.95 eV, and their energy levels could also be effectively tuned.

Vibrational Funnels for Energy Transfer in Organic Chromophores

Photoinduced intramolecular energy transfers in multichromophoric molecules involve nonadiabatic vibronic channels that act as energy transfer funnels. They commonly take place through specific directions of motion dictated by the nonadiabatic coupling vectors. Vibrational funnels may support persistent coherences between electronic states and sometimes delineate the presence of minor alternative energy transfer pathways. The ultimate confirmation of their role on the interchromophoric energy transfer can be achieved by performing nonadiabatic excited-state molecular dynamics simulations by selectively freezing the nuclear motions in question. Our results point out this strategy as a useful tool to identify and evaluate the impact of these vibrational funnels on the energy transfer processes and guide the in silico design of materials with tunable properties and enhanced functionalities. Our work encourages applications of this methodology to different chemical and biochemical processes such as reactive scattering and protein conformational changes, to name a few.

Ru(0)-catalysed cross-dimerisation and -trimerisation of alkynyl- with butadienylheteroarenes

Ru(0)-catalysed cross-dimerisation and -trimerisation give a series of di- and triheteroaryl compounds cross-linked by π-conjugated trienyl groups. Their photochemical behaviour is studied using UV-visible absorption spectra, fluorescence emission spectra, and TD-DFT calculations. The cross-trimer prepared from 2,5-dialkynylthiophene with 2 equiv. of 2-butadienylpyridine shows a longer wavelength shift in the absorption maximum than the cross-trimer prepared from dialkynylbenzene with 1-phenylbutadiene. The solvent effect and the TD-DFT calculations suggest that the planarity of the π-conjugated system contributes more than spontaneous polarization. Namely, in the 5-membered thiophene ring, the conjugated trienyl group extends in the same plane (dihedral angle: -4.0°) as the thienyl group, whereas in the 6-membered benzene ring, the planarity is reduced due to steric hindrance (dihedral angle: -24.1°). Thus, the cross-trimers with a 5-membered heteroaryl centre contribute to longer wavelengths of absorption and fluorescence emission due to the increased planarity of the conjugated trienyl groups.

Synthesis and Properties of Diphenylbenzo[j]naphtho[2,1,8-def][2,7]phenanthrolines

A series of hitherto unknown 5,14-diphenylbenzo[j]naphtho[2,1,8-def][2,7]phenanthrolines, containing a 5-azatetracene and a 2-azapyrene subunit, were prepared by combination of Pd-catalyzed cross-coupling reactions with a one-pot Povarov/cycloisomerization reaction. In the final key step four new bonds are formed in one step. The synthetic approach allows for a high degree of diversification of the heterocyclic core structure. The optical and electrochemical properties were studied experimentally and by DFT/TD-DFT and NICS calculations. Due to the presence of the 2-azapyrene subunit, the typical electronic nature and characteristics of the 5-azatetracene moiety are lost and the compounds are electronically and optically more related to 2-azapyrenes.

Enhancing the Photovoltaic Properties via Incorporation of Selenophene Units in Organic Chromophores with A2-π2-A1-π1-A2 Configuration: A DFT-Based Exploration

Currently, polymer organic solar cells (POSCs) are widely utilized due to their significant application, such as low-cost power conversion efficiencies (PCEs). Therefore, we designed a series of photovoltaic materials (D1, D2, D3, D5 and D7) by the incorporation of selenophene units (n = 1-7) as π₁-spacers by considering the importance of POSCs. Density functional theory (DFT) calculations were accomplished at MPW1PW91/6-311G (d, p) functional to explore the impact of additional selenophene units on the photovoltaic behavior of the above-mentioned compounds. A comparative analysis was conducted for designed compounds and reference compounds (D1). Reduction in energy gaps (∆E = 2.399 - 2.064 eV) with broader absorption wavelength (λ_max = 655.480 - 728.376 nm) in chloroform along with larger charge transference rate was studied with the addition of selenophene units as compared to D1. A significantly higher exciton dissociation rate was studied as lower values of binding energy (E_b = 0.508 - 0.362 eV) were noted in derivatives than in the reference (E_b = 0.526 eV). Moreover, transition density matrix (TDM) and density of state (DOS) data also supported the efficient charge transition origination from HOMOs to LUMOs. Open circuit voltage (V_oc) was also calculated for all the aforesaid compounds to check the efficiency, and significant results were seen (1.633-1.549 V). All the analyses supported our compounds as efficient POSCs materials with significant efficacy. These compounds might encourage the experimental researchers to synthesize them due to proficient photovoltaic materials.

Quinoid Conjugated Polymer Nanoparticles with NIR-II Absorption Peak Toward Efficient Photothermal Therapy

Recently, extensive efforts have been devoted to the development of the second near-infrared bio-window (NIR-II, 1000-1700 nm) theranostic agents owing to the excellent tissue-penetration capability of NIR-II light. The exploration of organic NIR-II photothermal therapy materials, especially those with absorption peak over 1000 nm, is an appealing yet significantly challenging task. Herein, we have designed conjugated polymer nanoparticles (PIS NPs) with NIR-II absorption peak at 1026 nm through a combined strategy of introducing quinoid donor-acceptor (D-A) structures, constructing intramolecular "conformational locks" and extending the conjugation area to narrow the band gap. Irradiated at 1064 nm, PIS NPs showed remarkable photothermal conversion performance for efficient photothermal ablation of tumor cells in vitro and in vivo. This study provides useful insights into the rational design of organic NIR-II photothermal agents based on multiple strategies.

Tunable Photoinduced Charge Transfer at the Interface between Benzoselenadiazole-Based MOF Linkers and Thermally Activated Delayed Fluorescence Chromophore

Structural modifications to molecular systems that lead to the control of photon emission processes at the interfaces between photoactive materials play a key role in the development of fluorescence sensors, X-ray imaging scintillators, and organic light-emitting diodes (OLEDs). In this work, two donor-acceptor systems were used to explore and reveal the effects of slight changes in chemical structure on interfacial excited-state transfer processes. A thermally activated delayed fluorescence (TADF) molecule was chosen as the molecular acceptor. Meanwhile, two benzoselenadiazole-core MOF linker precursors, Ac-SDZ and SDZ, with the presence and absence of a C≡C bridge, respectively, were carefully chosen as energy and/or electron-donor moieties. We found that the SDZ -TADF donor-acceptor system exhibited efficient energy transfer, as evidenced by steady-state and time-resolved laser spectroscopy. Furthermore, our results demonstrated that the Ac-SDZ-TADF system exhibited both interfacial energy and electron transfer processes. Femtosecond-mid-IR (fs-mid-IR) transient absorption measurements revealed that the electron transfer process takes place on the picosecond timescale. Time-dependent density functional theory (TD-DFT) calculations confirmed that photoinduced electron transfer occurred in this system and demonstrated that it takes place from C≡C in Ac-SDZ to the central unit of the TADF molecule. This work provides a straightforward way to modulate and tune excited-state energy/charge transfer processes at donor-acceptor interfaces.

Water-soluble extracellular vesicle probes based on conjugated oligoelectrolytes

We developed a series of transmembrane conjugated oligoelectrolytes (COEs) with tunable optical emissions from the UV to the near IR to address the false-positive problem when detecting nanometer-sized extracellular vesicles (EVs) by flow cytometry. The amphiphilic molecular framework of COEs is defined by a linear conjugated structure and cationic charged groups at each terminal site. Consequently, COEs have excellent water solubility and the absence of nanoaggregates at concentrations up to 50 μM, and unbound COE dyes can be readily removed through ultrafiltration. These properties enable unambiguous and simple detection of COE-labeled small EVs using flow cytometry with negligible background signals. We also demonstrated the time-lapsed tracking of small EV uptake into mammalian cells and the endogenous small EV labeling using COEs. Briefly, COEs provide a class of membrane-targeting dyes that behave as biomimetics of the lipid bilayer and a general and practical labeling strategy for nanosized EVs.

Advances in the Chemistry of 2,4,6-Tri(thiophen-2-yl)-1,3,5-triazine

Heterocyclic systems are now considered to be an integral part of material chemistry. Thiophene, selenophene, furan, pyrrole, carbazole, triazine and others are some such examples worth mentioning. 2,4,6-Tri(thiophen-2-yl)-1,3,5-triazine is a C3h -symmetric system with thiophene as the donor unit and s-triazine as the acceptor unit. This review gives an insight into the advances made in the thienyl-triazine chemistry over the past two to three decades. The synthetic pathways for arriving at this system and all its important derivatives are provided. The major focus is on the materials synthesized using the thienyl-triazine system, including star molecules, linear and hyperbranched polymers, porous materials and their diverse applications. This review will play a catalytic role for new dimensions to be explored in thienyl-triazine chemistry.

Progresses and Perspectives of Near-Infrared Emission Materials with "Heavy Metal-Free" Organic Compounds for Electroluminescence

Organic/polymer light-emitting diodes (OLEDs/PLEDs) have attracted a rising number of investigations due to their promising applications for high-resolution fullcolor displays and energy-saving solid-state lightings. Near-infrared (NIR) emitting dyes have gained increasing attention for their potential applications in electroluminescence and optical imaging in optical tele-communication platforms, sensing and medical diagnosis in recent decades. And a growing number of people focus on the "heavy metal-free" NIR electroluminescent materials to gain more design freedom with cost advantage. This review presents recent progresses in conjugated polymers and organic molecules for OLEDs/PLEDs according to their different luminous mechanism and constructing systems. The relationships between the organic fluorophores structures and electroluminescence properties are the main focus of this review. Finally, the approaches to enhance the performance of NIR OLEDs/PLEDs are described briefly. We hope that this review could provide a new perspective for NIR materials and inspire breakthroughs in fundamental research and applications.

Transition Metal-Free Regio- and Stereo-Selective trans Hydroboration of 1,3-Diynes: A Phosphine-Catalyzed Access to (E)-1-Boryl-1,3-Enynes

We report a transition metal-free, regio- and stereo-selective, phosphine-catalyzed method for the trans hydroboration of 1,3-diynes with pinacolborane that affords (E)-1-boryl-1,3-enynes. The reaction proceeds with excellent selectivity for boron addition to the external carbon of the 1,3-diyne framework as unambiguously established by NMR and X-ray crystallographic studies. The reaction displays a broad substrate scope including unsymmetrical diynes to generate products in high yield (up to 95 %). Experimental and theoretical studies suggest that phosphine attack on the alkyne is a key process in the catalytic cycle.

Electrical Conductivities of Narrow-Bandgap Polymers with Two Types of π-Conjugated Post-Crosslinking

Bandgap energy is one of the most important properties for developing electronic devices because of its influence on the electrical conductivity of substances. Many methods have been developed to control bandgap, one of which is the realization of conducting polymers using narrow-bandgap polymers; however, the preparation of these polymers is complex. In this study, water-soluble, narrow-bandgap polymers with reactive groups were prepared by the addition–condensation reaction of pyrrole (Pyr), benzaldehyde-2-sulfonic acid sodium salt (BS), and aldehyde-containing reactive groups (aldehyde and pyridine) for post-crosslinking. Two types of reactions, aldehyde with p-phenylenediamine and pyridine with 1,2-dibromoethylene, were carried out for the π-conjugated post-crosslinking between polymers. The polymers were characterized by proton nuclear magnetic resonance (¹H-NMR), thermogravimetric/differential thermal analysis (TG/DTA), UltraViolet-Visible-Near InfraRed spectroscopy (UV-Vis-NIR), and other analyses. The bandgaps of the polymers, calculated from their absorption, were less than 0.5 eV. Post-crosslinking prevents resolubility and develops electron-conducting routes between the polymer chains for π-conjugated systems. Moreover, the post-crosslinked polymers maintain their narrow bandgaps. The electrical conductivities of the as-prepared polymers were two orders of magnitude higher than those before the crosslinking.

Construction of heteroaryl-bridged NIR AIEgens for specific imaging of lipid droplets and its application in photodynamic therapy

As a kind of subcellular organelle, lipid droplets (LDs) play a critical role in the body's normal metabolism. LDs have gained increasing attention as a fluorescent photodynamic target site. Near-infrared (NIR) organic light-emitting luminescent materials, with aggregation-induced emission (AIE)-active feature, preeminent LD-imaging ability, and effective reactive oxygen species (ROS) production property, have been widely used for photodynamic therapy (PDT) in diagnostic therapeutics, but its application remains challenging. In the present work, three novel NIR organic compounds with AIE-active feature, namely, TPET-Is, TPET-Fu, and TPEF-Is, were developed and synthesized. These heteroaryl-bridged molecules possess a donor-donor-π-acceptor structure and strong intramolecular charge transfer character. These AIEgens are capable of high-fidelity LD imaging in living cells (Pearson's coefficient values: 0.94, 0.96, 0.97) due to their biocompatibility, good photostability, and strong lipophilicity (LogP values: 9.39, 7.89, 8.03), respectively. Moreover, they can be also applied in bright imaging the LDs of oil-rich plant tissues, such as those of sunflower seeds. The respective AIEgens TPET-Fu of these compounds can also produce ROS in the condition of white light to effectively kill live Hela cells. The present study thus provides a potential strategy through heteroaryl-bridged molecular engineering for LD-targeted imaging and PDT application.

Redox-Neutral Ru(0)-Catalyzed Alkenylation of 2-Carboxaldimine-heterocyclopentadienes

A new Ru₃(CO)₁₂-catalyzed directed alkenylation of 2-carboxaldimine-heterocyclopentadienes has been accomplished. This process allows coupling of furan, pyrrole, indole, and thiophene 2-carboxaldimines with electron-poor alkenes such as acrylates, vinylsulfones, and styrenes. This regio- and chemoselective oxidative C-H coupling does not require the presence of an additional sacrificial oxidant. Density functional theory calculations allowed us to propose a mechanism and unveiled the nature of the H₂ acceptor.

Electrochemical oxidative dearomatization of 2-arylthiophenes

Herein we report a green and sustainable electrochemical oxidative dearomatization of 2-arylthiophenes. The variation of substitution patterns affords easy access toward both the C2/C3 and C2/C5 difunctionalized dearomative products. The...

Benzotriazole-EDOT electrochromic conjugated polymers perform sub-second response time and 774 cm2C-1 coloration efficiency

To investigate the effect of double fluorine substitution on the optical, electrochemical, thermodynamic, morphological and electrochromic properties of electrochromic polymers, two benzotriazole-EDOT electrochromic conjugated polymers of PBTz-E and P2F-BTz-E were...

Functional polydiynes prepared by metathesis cyclopolymerization of 1,7-dihalogen-1,6-heptadiyne derivatives

The MCP route used for the polymerization of 1,6-heptadiynes was successfully applied to the polymerization of 1,7-dihalogen-1,6-heptadiynes, and the target polymers were obtained in high yield with high molecular weight and unique UCST behavior.

Toward the Rational Design of Organic Solar Photovoltaics: Application of Molecular Structure Methods to Donor Polymers

Conjugated polymers are promising candidates in the design of polymer solar cell materials with suitable electronic properties. Recent studies show that the use of different functional groups as side chain in thiophene-based polymers changes the electronic and conformation structures. Here we design new thiophene-based molecules by replacing the hydrogen attached to the backbone of P3MT with electron-donating and electron-withdrawing groups. We then calculate the HOMO, LUMO, and HOMO-LUMO energy gap to quantify the theoretical merit of the new polymers as solar absorbers and their inter-ring torsional potential to understand their suitability to link together in high conductivity, extended conjugated systems. Calculations are done with first-principles density functional theory (DFT), implemented using B3LYP with dispersion function and 6-31G(d,p) as basis set. Our results show that the HOMO-LUMO gap is sensibly lowered by donating groups and we found that the substitution of the hydrogen with -NH2, and -F gives an energy gap lower than the energy gap of P3MT. The lowest energy gap was found when substituting with -NH2. Electron-withdrawing groups lower the HOMO, with the overall lowest found when -NO2 is used. -COCl, -CONH2, and -Cl give a steric hindrance greater than that of PTB7, which is set as reference. Our calculations show a possible approach to the rational design of donor materials when substituents are inserted systematically in a generic oligomer.

Overcoming intra-molecular repulsions in PEDTT by sulphate counter-ion

We set out to demonstrate the development of a highly conductive polymer based on poly-(3,4-ethylenedithia thiophene) (PEDTT), PEDOTs structural analogue historically notorious for structural disorder and limited conductivities. The caveat therein was previously described to lie in intra-molecular repulsions. We demonstrate how a tremendous >2600-fold improvement in conductivity and metallic features, such as magnetoconductivity can be achieved. This is achieved through a careful choice of the counter-ion (sulphate) and the use of oxidative chemical vapour deposition (oCVD). It is shown that high structural order on the molecular level was established and the formation of crystallites tens of nanometres in size was achieved. We infer that the sulphate ions therein intercalate between the polymer chains, thus forming densely packed crystals of planar molecules with extended π-systems. Consequently, room-temperature conductivities of above 1000 S cm-1 are achieved, challenging those of conventional PEDOT:PSS. The material is in the critical regime of the metal-insulator transition.

Molecular Design Strategies toward Improvement of Charge Injection and Ionic Conduction in Organic Mixed Ionic-Electronic Conductors for Organic Electrochemical Transistors

Expanding the toolbox of the biology and electronics mutual conjunction is a primary aim of bioelectronics. The organic electrochemical transistor (OECT) has undeniably become a predominant device for mixed conduction materials, offering impressive transconduction properties alongside a relatively simple device architecture. In this review, we focus on the discussion of recent material developments in the area of mixed conductors for bioelectronic applications by means of thorough structure–property investigation and analysis of current challenges. Fundamental operation principles of the OECT are revisited, and characterization methods are highlighted. Current bioelectronic applications of organic mixed ionic–electronic conductors (OMIECs) are underlined. Challenges in the performance and operational stability of OECT channel materials as well as potential strategies for mitigating them, are discussed. This is further expanded to sketch a synopsis of the history of mixed conduction materials for both p- and n-type channel operation, detailing the synthetic challenges and milestones which have been overcome to frequently produce higher performing OECT devices. The cumulative work of multiple research groups is summarized, and synthetic design strategies are extracted to present a series of design principles that can be utilized to drive figure-of-merit performance values even further for future OMIEC materials.

Indandione-Terminated Quinoidal Compounds for Low-Bandgap Small Molecules with Strong Near-Infrared Absorption: Effect of Conjugation Length on the Properties

Low-bandgap organic semiconductors have attracted much attention for their multiple applications in optoelectronics. However, the realization of narrow bandgap is challenging particularly for small molecules. Herein, we have synthesized four quinoidal compounds, i. e., QSN3, QSN4, QSN5 and QSN6, with electron rich S,N-heteroacene as the quinoidal core and indandione as the end-groups. The optical bandgap of the quinoidal compounds is systematically decreased with the extension of quinoidal skeleton, while maintaining stable closed-shell ground state. QSN6 absorbs an intense absorption in the first and second near-infrared region in the solid state, and has extremely low optical bandgap of 0.74 eV. Cyclic voltammetry analyses reveal that the lowest unoccupied molecular orbital (LUMO) energy levels of the four quinoidal compounds all lie below -4.1 eV, resulting in good electron-transporting characteristics in organic thin-film transistors. These results demonstrated that the combination of π-extended quinoidal core and end-groups in quinoidal compounds is an effective strategy for the synthesis of low-bandgap small molecules with good stability.

Ultrafast Aggregation-Induced Tunable Emission Enhancement in a Benzothiadiazole-Based Fluorescent Metal-Organic Framework Linker

Aggregation-induced emission enhancement (AIEE) is a process recently exploited in solid-state materials and organic luminophores, and it is explained by tight-molecular packaging. However, solution-phase AIEE and its formation mechanism have not been widely explored. This work investigated AIEE phenomena in two donor-acceptor-donor-type benzodiazole-based molecules (the organic building block in metal-organic frameworks) with an acetylene and phenyl π-conjugated backbone tapered with a carboxylic acid group at either end. This was done using time-resolved electronic and vibrational spectroscopy in conjunction with time-dependent density functional theory (TD-DFT) calculations. Fluorescence up-conversion spectroscopy and time-correlated single-photon counting conclusively showed an intramolecular charge transfer-driven aggregate emission enhancement. This is shown by a red spectral shift of the emission spectra as well as an increase in the fluorescence lifetime from 746 ps at 1.0 × 10^-11 to 2.48 ns at 2.0 × 10^-3 M. The TD-DFT calculations showed that a restricted intramolecular rotation mechanism is responsible for the enhanced emission. The femtosecond infrared (IR) transient absorption results directly revealed the structural dynamics of aggregate formation, as evident from the evolution of the C≡C vibrational marker mode of the acetylene unit upon photoexcitation. Moreover, the IR data clearly indicated that the aggregation process occurred over a time scale of 10 ps, which is consistent with the fluorescence up-conversion results. Interestingly, time-resolved results and DFT calculations clearly demonstrated that both acetylene bonds and the sulfur atom are the key requirements to achieve such a controllable aggregation-induced fluorescence enhancement. The finding of the work not only shows how slight changes in the chemical structure of fluorescent chromophores could make a tremendous change in their optical behavior but also prompts a surge of research into a profound understanding of the mechanistic origins of this phenomenon. This may lead to the discovery of new chemical strategies that aim to synthesize novel chromophores with excellent optical properties for light-harvesting applications.

Synthesis of Redox-Active Photochromic Phenanthrene Derivatives

A phenanthrene unit has been functionalized by several methylthiophene units in order to bring it a photochromic behavior. These compounds were characterized by NMR, absorption and emission spectroscopies, theoretical calculations as well as cyclic voltammetry. The association of a phenanthrene group with a photochromic center could open the door to a new generation of organic field-effect transistors.

Excited-state engineering of oligothiophenes via phosphorus chemistry towards strong fluorescent materials

Due to efficient intersystem crossing (ISC), combined with efficient non-radiative processes of the triplet excited state, oligothiophenes generally exhibit very weak photoluminescence. Phosphorus (P)-bridged terthiophenes (P-terThs) and phosphorus (P)-bridged bithiophenes (P-biThs) were synthesized. The diverse and well-defined P-chemistry has been applied to fine tune the photophysical properties of these materials. The asymmetric electronic coupling between the P-center and terThs suppressed the electronic interactions of two terTh and biTh moieties in the ground state S₀. Particularly, P-terThs and P-biThs having a positively charged P(+)-center induce pronounced asymmetric electronic environments on the two terThs and two biThs, respectively, which allows relaxation from the initial excited state via symmetry breaking charge transfer (SBCT) to give the charge separated state S_SBCT. P-terThs and P-biThs having a positively charged P(+)-center exhibit stronger SBCT than others, which may result in a weaker ISC of oligothiophenes, and consequently lead to the photoluminescence quantum yields (PLQYs) being as high as 71% and 39%, respectively. The current study uncovered detailed insights on the effects of phosphorus chemistry on the SBCT of oligothiophenes and their resulting effects on the photophysical properties of P-bridged oligothiophenes, which have not been previously addressed in oligothiophenes.

Revisiting the Heck Reaction for Fluorous Materials Applications

Installing fluoroalkyl chains on a molecule by the Heck reaction is a versatile method to transform the molecule’s properties that enable unique materials applications. This work further expands the scope of this reaction to thiophenes, which were able to undergo further functionalization and polymerization, highlighting the potential of these molecules in conjugated organic materials.

EQCM Analysis of the Insertion Phenomena in a n-Doped Poly-Alkyl-Terthiophene With Regioregular Pattern of Substitution

In the present work, we have undertaken the study of the n-doping process in poly-3,3″-didodecyl-2,2':5',2″-terthiophene (poly-33″-DDTT) employing the electrochemical quartz crystal microbalance (EQCM). The present study aims at understanding how cathodic charge in n-doped poly-33″-DDTT is compensated. For this purpose, the in situ analysis of the variations of the polymeric mass has been considered. Poly-33″-DDTT was obtained as a thin coating onto a metallic substrate via the anodic coupling of the corresponding monomer 3,3″-didodecyl-2,2':5',2″-terthiophene (33″-DDTT). When subjected to electrochemical n-doping in the polarization interval -2.5 ≤ E _appl ≤ 0 V vs. Ag/Ag⁺, the films of poly-33″-DDTT varied their mass according to a mechanism of cations insertion during n-doping and cations extraction during polymer neutralization. In fact, the electrochemical doping of polythiophenes requires the accompanying exchange of charged species to maintain the electroneutrality within the structure of the polymer in all states of polarization. At the end of a full electrochemical cycle (consisting of the n-doping and the successive neutralization of poly-33″-DDTT), the polymer retains a fraction of the mass acquired during n-doping, thus manifesting the phenomena of mass trapping. The combined analysis of electrochemical and microgravimetric data suggests that poly-33″-DDTT in the n-doped state undergoes (or electrocatalyzes) uncontrolled electrochemical reactions that are not accompanied by mass variations.

Oriented Thin Films of Insoluble Polythiophene Prepared by the Friction Transfer Technique

A thin film of unsubstituted polythiophene (PT), an insoluble conjugated polymer, with molecular chains uniaxially oriented in plane was prepared by the friction transfer method. The structure of highly oriented thin films of PT was investigated using grazing-incidence X-ray diffraction (GIXD), ultraviolet-visible (UV-vis) spectroscopy, and infrared (IR) spectroscopy. The polarized UV-vis and IR spectra and GIXD measurements showed the PT molecular chains were well aligned in parallel to the friction direction. The GIXD studies clarified that the polymer backbones were aligned with very narrow distribution, such that the half-width was about 4 degrees. The degree of orientation of the PT friction-transferred film was higher compared with those of regioregular poly(3-alkylthiophene)s. Moreover, the GIXD results show a preferred orientation where the a-axis is perpendicular to the substrate plane.

Europium(III), Terbium(III), and Gadolinium(III) Oxamato-Based Coordination Polymers: Visible Luminescence and Slow Magnetic Relaxation

The reaction of aqueous solutions of Eu^III, Tb^III, and Gd^III ions with Na₂Hpcpa [H₃pcpa = N-(4-carboxyphenyl)oxamic acid] afforded three new isostructural oxamate-containing lanthanide(III) coordination polymers of general formula {Ln^III₂(Hpcpa)₃(H₂O)₅·H₂O}_n [Ln = Eu (1),Tb (2), and Gd(3)]. Their structure is made up of neutral zigzag chains running parallel to the [101] direction where double syn-syn carboxylate(oxamate)-bridged dilanthanide(III) pairs (Ln1 and Ln2) are linked by three Hpcpa^2- ligands, one of them with the μ-κ²O,O':κO″ coordination mode and the other two with the μ₃-κ²O,O':κO″:κO'''. Additionally, two of those chains are interlinked through hydrogen bonding and π-π type interactions, resulting in a porous structure with channels where water molecules are hosted. The emission properties of 1 and 2 are evaluated as a function of the temperature, exhibiting an emission in red and green, respectively. The external quantum yield for 2 is approximately 7 times that obtained for 1, indicating that the oxamate ligand is a better sensitizer for Tb^III ions. The temperature dependence of the dc magnetic properties of 1-3 reveals a different magnetic behavior depending on the nature of the Ln^III ion. A continuous decrease of χ_MT occurs for 1 upon cooling, and finally χ_MT tends to vanish, as expected for the thermal depopulation of the six magnetic ⁷F_J excited states (J = 1-6) of the Eu^III ion with a nonmagnetic ⁷F₀ ground state. χ_MT for 2 decreases sharply with decreasing the temperature due to the depopulation of the splitted m_J levels of the ⁷F₇ ground state of the magnetically anisotropic Tb^III ion. A very weak antiferromagnetic interaction between the magnetically isotropic Gd^III ions across the double carboxylate(oxamate) bridge is responsible for the small decrease of χ_MT at low temperatures for 3. The dynamic (ac) magnetic properties of 2 and 3 reveal a slow magnetic relaxation with very incipient frequency-dependent χ_M″ signals below 6.0 K (2) and frequency-dependent χ_M″ peaks below 10.0 K (3) under nonzero applied dc magnetic fields, being thus new examples of field-induced single molecule magnets (SMMs).