Austel, the University of Ulm, for valuable discussions and also Boehringer Ingelheim Pharmaceuticals Inc., Ridgefield (USA), for apparatus support

Legion: An Instrument for High-Throughput Electrochemistry

Electrochemical arrays promise utility for accelerated hypothesis testing and breakthrough discoveries. Herein, we report a new high-throughput electrochemistry platform, colloquially called "Legion," for applications in electroanalysis and electrosynthesis. Legion consists of 96 electrochemical cells dimensioned to match common 96-well plates that are independently controlled with a field-programmable gate array. We demonstrate the utility of Legion by measuring model electrochemical probes, pH-dependent electron transfers, and electrocatalytic dehalogenation reactions. We consider advantages and disadvantages of this new instrumentation, with the hope of expanding the electrochemical toolbox.

Integrated System Built for Small-Molecule Semiconductors via High-Throughput Approaches

High-throughput synthesis of solution-processable structurally variable small-molecule semiconductors is both an opportunity and a challenge. A large number of diverse molecules provide a possibility for quick material discovery and machine learning based on experimental data. However, the diversity of the molecular structure leads to the complexity of molecular properties, such as solubility, polarity, and crystallinity, which poses great challenges to solution processing and purification. Here, we first report an integrated system for the high-throughput synthesis, purification, and characterization of molecules with a large variety. Based on the principle "Like dissolves like," we combine theoretical calculations and a robotic platform to accelerate the purification of those molecules. With this platform, a material library containing 125 molecules and their optical-electronic properties was built within a timeframe of weeks. More importantly, the high repeatability of recrystallization we design is a reliable approach to further upgrading and industrial production.

Hierarchical Nanomaterials Assembled from Peptoids and Other Sequence-Defined Synthetic Polymers

In nature, the self-assembly of sequence-specific biopolymers into hierarchical structures plays an essential role in the construction of functional biomaterials. To develop synthetic materials that can mimic and surpass the function of these natural counterparts, various sequence-defined bio- and biomimetic polymers have been developed and exploited as building blocks for hierarchical self-assembly. This review summarizes the recent advances in the molecular self-assembly of hierarchical nanomaterials based on peptoids (or poly-N-substituted glycines) and other sequence-defined synthetic polymers. Modern techniques to monitor the assembly mechanisms and characterize the physicochemical properties of these self-assembly systems are highlighted. In addition, discussions about their potential applications in biomedical sciences and renewable energy are also included. This review aims to highlight essential features of sequence-defined synthetic polymers (e.g., high stability and protein-like high-information content) and how these unique features enable the construction of robust biomimetic functional materials with high programmability and predictability, with an emphasis on peptoids and their self-assembled nanomaterials.

Spontaneous Combustion of 2-Bromo-3-Methoxythiophene: A Study on Reaction Pathways and Energetics by Quantum Chemical Calculations

Reaction pathways and energetics for the dimerization and trimerization reactions of 2-bromo-3-methoxythiophene (2Br-3Met) molecules are investigated using hybrid density functional theory (DFT) calculations to obtain insight into the oligomerization reaction observed in the spontaneous combustion of pure liquid 2Br-3Met. The calculations show that the carbon-bromine bond in a 2Br-3Met molecule elongates easily, and the trans addition of this C-Br bond to a double bond in the neighboring 2Br-3Met molecule occurs easily at room temperature, reflecting the evaluated activation energy of ΔH_a = 12.46 kcal/mol (enthalpy) or ΔG_a = 35.68 kcal/mol (Gibbs free energy, 298.150 K and 1 atm). The formation process of trimers is calculated in a similar way. A model for the explanation of spontaneous combustion is proposed; large oligomers of the 2Br-3Met molecule are produced spontaneously following the initial formation of dimers or trimers. UV-vis spectra and vibration spectra are obtained for related molecular species, which show reasonable agreement with the experimental results.

An integrated mass spectrometry platform enables picomole-scale real-time electrosynthetic reaction screening and discovery

The identification of new electrosynthetic pathways enables environmentally friendly synthetic applications. However, the development of miniaturized screening procedures/platforms to expedite the discovery of electrooxidation reactions remains challenging. Herein, we developed an integrated system that serves as a reactor and ion source in a single experimental step using only picomole-scale reactants to monitor electrooxidation in real-time. This reaction screening platform utilizes the intrinsic electrochemical capabilities of nano-electrospray ionization mass spectrometry. We validated the feasibility of this method by reproducing three known electrochemical reactions. We also discovered two new electroorganic reaction pathways: (i) C-N dehydrodimerization of 8-methyl-1,2,3,4-tetrahydroquinoline to construct a novel quinoline skeleton, and (ii) TEMPO-mediated accelerated electrooxidative dehydrogenation of tetrahydroisoquinolines. Moreover, the radical cations and key intermediates captured by this screening platform provided direct evidence for the mechanism of these novel electrochemical reactions.

Oxidative C-H/C-H Coupling Reactions between Two (Hetero)arenes

Transition metal-mediated C-H bond activation and functionalization represent one of the most straightforward and powerful tools in modern organic synthetic chemistry. Bi(hetero)aryls are privileged π-conjugated structural cores in biologically active molecules, organic functional materials, ligands, and organic synthetic intermediates. The oxidative C-H/C-H coupling reactions between two (hetero)arenes through 2-fold C-H activation offer a valuable opportunity for rapid assembly of diverse bi(hetero)aryls and further exploitation of their applications in pharmaceutical and material sciences. This review provides a comprehensive overview of the fundamentals and applications of transition metal-mediated/catalyzed oxidative C-H/C-H coupling reactions between two (hetero)arenes. The substrate scope, limitation, reaction mechanism, regioselectivity, and chemoselectivity, as well as related control strategies of these reactions are discussed. Additionally, the applications of these established methods in the synthesis of natural products and exploitation of new organic functional materials are exemplified. In the last section, a short introduction on oxidant- or Lewis acid-mediated oxidative Ar-H/Ar-H coupling reactions is presented, considering that it is a very powerful method for the construction of biaryl units and polycylic arenes.

Pd-Catalyzed one-pot sequential cross-coupling reactions of tetrabromothiophene

Unsymmetrical one-pot sequential cross-coupling reactions of sterically hindered tetrabromothiophene with arylboronic acid and an alkyne/alkene to afford selective bi-, tri-, and tetrasubstituted aryl/alkynyl-thiophenes with the aid of a palladium catalyst were described.

Intramolecular photostabilization via triplet-state quenching: design principles to make organic fluorophores “self-healing”

Covalent linkage of fluorophores and photostabilizers was recently revived as a strategy to make organic fluorophores “self-healing” via triplet-state quenching. Although Lüttke and co-workers pioneered this strategy already in the 1980s, the general design principles still remain elusive. In this contribution, we combine experiments and theory to understand what determines the photostabilization efficiency in dye–photostabilizer conjugates. Our results from single-molecule microscopy and molecular dynamics simulations of different Cy5-derivatives suggest that the distance and relative geometry between the fluorophore and photostabilizer are more important than the chemical nature of the photostabilizer, e.g. its redox potential, which is known to influence electron-transfer rates. We hypothesize that the efficiency of photostabilization scales directly with the contact rate of the fluorophore and photostabilizer. This study represents an important step in the understanding of the molecular mechanism of intramolecular photostabilization and can pave the way for further development of stable emitters for various applications.

Phenothiazine-Aromatic Hydrocarbon Acceptor Dyads as Photo-induced Electron Transfer Systems by Ugi Four-Component Reaction

A phenothiazinyl donor moiety can be covalently coupled to aromatic hydrocarbon acceptor units via Ugi four-component reaction in an efficient, rapid, and highly convergent fashion. These novel phenothiazine-acceptor dyads are electronically decoupled in the electronic ground state according to UV/Vis spectroscopy and cyclic voltammetry. In the excited state the inherent acceptor luminescence is substantially quenched. Calculations of the Gibbs energy of photo-induced electron transfer from readily available UV/Vis spectroscopic and cyclovoltammetric data according to the Weller approximation rationalizes the feasibility of the reductive electron transfer from phenothiazine to the aromatic hydrocarbon upon photoexcitation.

The Ugi four-component reaction as a concise modular synthetic tool for photo-induced electron transfer donor-anthraquinone dyads

Phenothiazinyl and carbazolyl-donor moieties can be covalently coupled to an anthraquinone acceptor unit through an Ugi four-component reaction in a rapid, highly convergent fashion and with moderate to good yields. These novel donor–acceptor dyads are electronically decoupled in the electronic ground state according to UV–vis spectroscopy and cyclic voltammetry. However, in the excited state the inherent donor luminescence is efficiently quenched. Previously performed femtosecond spectroscopic measurements account for a rapid exergonic depopulation of the excited singlet states into a charge-separated state. Calculations of the Gibbs energy of photo-induced electron transfer from readily available UV–vis spectroscopic and cyclovoltammetric data applying the Weller approximation enables a quick evaluation of these novel donor–acceptor dyads. In addition, the X-ray structure of a phenothiazinyl–anthraquinone dyad supports short donor–acceptor distances by an intramolecular π-stacking conformation, an important assumption also implied in the calculations of the Gibbs energies according to the Weller approximation.

One-pot three-component synthesis and photophysical characteristics of novel triene merocyanines

Novel triene merocyanines, i.e. 1-styryleth-2-enylidene and 4-(1,3,3-trimethylindolin-2-ylidene)but-2-en-1-ylideneindolones are obtained in good to excellent yields in a consecutive three-component insertion Sonogashira coupling-addition sequence. The selectivity of either series is remarkable and has its origin in the stepwise character of the terminal addition step as shown by extensive computations on the DFT level. All merocyanines display intense absorption bands in solution and the film spectra indicate J-aggregation. While 1-styryleth-2-enylideneindolones show an intense deep red emission in films, 4-(1,3,3-trimethylindolin-2-ylidene)but-2-en-1-ylideneindolones are essentially nonemissive in films or in the solid state. TD-DFT computations rationalize the charge-transfer nature of the characteristic broad long-wavelength absorptions bands.

Long-chain 3,4-ethylenedioxythiophene/thiophene oligomers and semiconducting thin films prepared by their electropolymerization

A series of soluble H-terminated conjugated oligomers incorporating 3,4-ethylenedioxythiophene (EDOT) combined with a small number of thiophene units and ranging in length from four to eight EDOT/thiophene groups was prepared with the ultimate goal to investigate if facile formation of a reactive trication radical species would enable electrochemical polymerization of such long-chain oligomers. Spectroscopic and electrochemical studies of the oligomers revealed some general dependencies of their electronic properties on the total number and position of EDOT groups. It was the number of consecutive EDOT units rather than total number of these units which was found to have the most profound effect on electronic energy gap and conjugation length. This influence originates from the especially strong planarization induced in the conjugated backbone by the incorporation of EDOT units. In contrast, incorporation of thiophene units was found to result in loss of the conformational stabilization. This phenomenon was analyzed using the natural bond orbital computational approach, which revealed the predominantly hyperconjugative nature of the EDOT-induced conformational stabilization. Whereas shorter oligomers, in agreement with the general consensus, were found to be inert toward electrochemical polymerization due to low reactivity of electrochemically generated cation radical and dication species, the longest oligomer showed an unprecedentedly efficient electropolymerization to yield a stable thin film of an electroactive polymer. The efficient electropolymerization of the long-chain oligomer was found to be in agreement with the formation of a reactive trication radical species. The electronic and spectral properties of the resulting semiconducting polymer film were studied by various electrochemical and spectroelectrochemical methods, as well as conductive probe AFM technique, and revealed a number of unusual features (such as electrical rectifying switching behavior) consistent with the possibility of increased molecular order in this material.

NMR landscapes for chemical shift prediction

The ability to reliably predict NMR chemical shifts plays an important role in elucidating the structure of organic molecules. Additionally, an intriguing question is how the multitude of variable factors (structural, electronic, and environmental) correlate with the actual electromagnetic shielding effect that determines the chemical shift value. This work presents NMRscape as a new tool for understanding these correlations by constructing the landscape that describes the relationship between the chemical shift value and the moieties bonded to a molecular scaffold. The scaffold may be as small as a single atom probed by NMR or a larger molecular framework containing the probed atom. NMRscape operates with only a list of the chemical moieties bonded to the scaffold, without utilizing any potentially biasing chemometric descriptors. The corresponding chemical shift landscape is constructed based on fundamental physical principles, which makes NMRscape a credible chemical shift prediction and analysis tool. As an illustration, we demonstrate that NMRscape can predict (13)C chemical shifts with an accuracy exceeding the substituent chemical shift (SCS) increment, hierarchical organization of spherical environments (HOSE), and neural networks (NN), methods for three distinct families of molecules sharing a common scaffold structure with moieties placed at two variable sites. The constructed NMR landscapes confirmed known empirical rules relating chemical shift values to the variation of chemical moieties on a scaffold, as well as uncovered hitherto hidden relationships. The practical importance of NMRscape is discussed.

Exploiting time-independent Hamiltonian structure as controls for manipulating quantum dynamics

The opportunities offered by utilizing time-independent Hamiltonian structure as controls are explored for manipulating quantum dynamics. Two scenarios are investigated using different manifestations of Hamiltonian structure to illustrate the generality of the concept. In scenario I, optimally shaped electrostatic potentials are generated to flexibly control electron scattering in a two-dimensional subsurface plane of a semiconductor. A simulation is performed showing the utility of optimally setting the individual voltages applied to a multi-pixel surface gate array in order to produce a spatially inhomogeneous potential within the subsurface scattering plane. The coherent constructive and destructive electron wave interferences are manipulated by optimally adjusting the potential shapes to alter the scattering patterns. In scenario II, molecular vibrational wave packets are controlled by means of optimally selecting the Hamiltonian structure in cooperation with an applied field. As an illustration of the concept, a collection (i.e., a level set) of dipole functions is identified where each member serves with the same applied electric field to produce the desired final transition probability. The level set algorithm additionally found Hamiltonian structure controls exhibiting desirable physical properties. The prospects of utilizing the applied field and Hamiltonian structure simultaneously as controls is also explored. The control scenarios I and II indicate the gains offered by algorithmically guided molecular or material discovery for manipulating quantum dynamics phenomenon.

Polymer-supported syntheses of thiophene-containing compounds using a new type of traceless linker

A new type of traceless linker is described for use in polymer-supported (PS) syntheses of thiophene-containing compounds. It is based on the cleavage of PS aryl 2-thienyl ketones by a mixture of potassium t-butoxide and water (typical mol ratio 10:3) in an ethereal solvent. Cleavage occurs to give the soluble thiophene-containing product. The method is used to prepare a range of eight thiophene-containing compounds including a terthiophene and a dialkylquaterthiophene. PS unsymmetrical diaryl ketones incorporating, for example, ortho-methoxyphenyl or pyrrole moieties could also serve as traceless linkers.

Selective mono- to perarylations of tetrabromothiophene by a cyclobutene-1,2-diylbisimidazolium preligand

(Cyclobut-1-ene-1,2-diyl)bis(1-methylimidazolium)tetrafluoroborate is applied as preligand in palladium-catalyzed cross-coupling reactions starting from tetrabromothiophene for the synthesis of mono-, bi-, tri-, and tetraaryl-substituted thiophenes bearing up to four different aryl rings. A synthetic kit for preparations of nine different substitution patterns of arylated thiophenes is presented by application of only one single catalyst system. In agreement with DFT calculations, which predict energetically low rotational barriers in triaryl-3-bromothiophenes and tetraarylthiophenes, no NOE effects between adjacent aryl rings are detectable. The regioselectivity of their syntheses has therefore been elucidated by reduction of triaryl-3-bromothiophene to 2,3,5-triarylthiophene followed by HMBC, HSQC, and NOESY NMR measurements. Additionally, results of an X-ray single structure analysis are presented.

Palladium catalyzed C-C coupling reactions of 3,5-dichloro-4H-1,2,6-thiadiazin-4-one

Palladium catalyzed Suzuki-Miyaura, Stille, and Sonogashira coupling reactions are reported for the electron-deficient heterocyclic scaffold 3,5-dichloro-4H-1,2,6-thiadiazin-4-one (1). Furthermore, 3,5-di(thien-2-yl)-4H-1,2,6-thiadiazin-4-one (7m) is further elaborated to afford the tetrathienyl 3,5-bis[(2,2'-bithien)-5-yl]-4H-1,2,6-thiadiazin-4-one (9). All compounds are fully characterized.

Why is chemical synthesis and property optimization easier than expected?

Identifying optimal conditions for chemical and material synthesis as well as optimizing the properties of the products is often much easier than simple reasoning would predict. The potential search space is infinite in principle and enormous in practice, yet optimal molecules, materials, and synthesis conditions for many objectives can often be found by performing a reasonable number of distinct experiments. Considering the goal of chemical synthesis or property identification as optimal control problems provides insight into this good fortune. Both of these goals may be described by a fitness function J that depends on a suitable set of variables (e.g., reactant concentrations, components of a material, processing conditions, etc.). The relationship between J and the variables specifies the fitness landscape for the target objective. Upon making simple physical assumptions, this work demonstrates that the fitness landscape for chemical optimization contains no local sub-optimal maxima that may hinder attainment of the absolute best value of J. This feature provides a basis to explain the many reported efficient optimizations of synthesis conditions and molecular or material properties. We refer to this development as OptiChem theory. The predicted characteristics of chemical fitness landscapes are assessed through a broad examination of the recent literature, which shows ample evidence of trap-free landscapes for many objectives. The fundamental and practical implications of OptiChem theory for chemistry are discussed.

Fluorogenic click reaction

Fluorogenic Cu(I)-catalyzed alkyne-azide cycloaddition (CuAAC) reactions have emerged as a powerful tool for bioconjugation, materials science, organic synthesis and drug discovery. This review highlights the design of the recent development of fluorogenic CuAAC reactions as well as their applications.

Conducting polymers in chemical sensors and arrays

The review covers main applications of conducting polymers in chemical sensors and biosensors. The first part is focused on intrinsic and induced receptor properties of conducting polymers, such as pH sensitivity, sensitivity to inorganic ions and organic molecules as well as sensitivity to gases. Induced receptor properties can be also formed by molecularly imprinted polymerization or by immobilization of biological receptors. Immobilization strategies are reviewed in the second part. The third part is focused on applications of conducting polymers as transducers and includes usual optical (fluorescence, SPR, etc.) and electrical (conductometric, amperometric, potentiometric, etc.) transducing techniques as well as organic chemosensitive semiconductor devices. An assembly of stable sensing structures requires strong binding of conducting polymers to solid supports. These aspects are discussed in the next part. Finally, an application of combinatorial synthesis and high-throughput analysis to the development and optimization of sensing materials is described.

Domino Reaction of 3-(2-Formylphenoxy)propenoates and Amines: A Novel Synthesis of 1,4-Dihydropyridines from Salicaldehydes, Ethyl Propiolate, and Amines

A novel synthesis of Hantzsch-type N-substituted 1,4-dihydropyridines from salicaldehydes, ethyl propiolate, and amines has been developed. Salicaldehydes were treated with ethyl propiolate in the presence of N-methylmorpholine to give ethyl 3-(2-formylphenoxy)propenoates. Three equivalents of ethyl 3-(2-formylphenoxy)propenoates reacted with 1 equiv of amines under trifluoroacetic acid (TFA) catalyst to furnish the corresponding N-substituted 1,4-dihydropyridines in good to excellent yields, recovering the starting material salicaldehydes. A possible mechanism for the domino process was proposed. Furthermore, the products can be easily derived via further transformations and three of them exhibited strong fluorescence (Phif = 0.36-0.63).