Evaluation of Tandem Mass Spectrometry Experiments in the Negative Ionization Mode for Phenolamide Regioisomer Characterization.

Phenolamides are abundant specialized metabolites found in nature and consist of hydroxycinnamic acids mono- or polyconjugated with polyamines. Their participation in flower development is well-documented, and their presence in pollen raises the question of their role in pollen/pollinator interactions. The structural characterization of phenolamides is complicated by the presence of positional isomers and stereoisomers. Liquid chromatography coupled to tandem mass spectrometry in the positive ionization mode is becoming very popular in phenolamide structural characterization. However, collision-induced transamidation processes that cause the swapping of side chains have been detected, making it difficult to distinguish regioisomers with this technique. In the present report, we explore the dissociation processes undergone by the [M - H]- ions of spermidine-based phenolamides as model compounds. We describe two original competitive dissociation routes, namely, the phenolate and imidate pathways, to account for the observed fragmentation reactions undergone by collisional activated standard phenolamide anions. Whereas the phenolate pathway is regioselective at the central position for spermidine, the imidate pathway, requiring a deprotonated amide, only occurs at the extremities. Tandem mass spectrometry experiments on negatively charged phenolamide ions may then outperform their positive ionization mode counterparts for the distinction between phenolamide regioisomers and globally for the identification of phenolamides in natural extracts.

Structural and Dynamic Disorder, Not Ionic Trapping, Controls Charge Transport in Highly Doped Conducting Polymers.

Doped organic semiconductors are critical to emerging device applications, including thermoelectrics, bioelectronics, and neuromorphic computing devices. It is commonly assumed that low conductivities in these materials result primarily from charge trapping by the Coulomb potentials of the dopant counterions. Here, we present a combined experimental and theoretical study rebutting this belief. Using a newly developed doping technique based on ion exchange, we prepare highly doped films with several counterions of varying size and shape and characterize their carrier density, electrical conductivity, and paracrystalline disorder. In this uniquely large data set composed of several classes of high-mobility conjugated polymers, each doped with at least five different ions, we find electrical conductivity to be strongly correlated with paracrystalline disorder but poorly correlated with ionic size, suggesting that Coulomb traps do not limit transport. A general model for interacting electrons in highly doped polymers is proposed and carefully parametrized against atomistic calculations, enabling the calculation of electrical conductivity within the framework of transient localization theory. Theoretical calculations are in excellent agreement with experimental data, providing insights into the disorder-limited nature of charge transport and suggesting new strategies to further improve conductivities.

Revealing the Organization of Catalytic Sequence-Defined Oligomers via Combined Molecular Dynamics Simulations and Network Analysis.

Similar to biological macromolecules such as DNA and proteins, the precise control over the monomer position in sequence-defined polymers is of paramount importance for tuning their structures and properties toward achieving specific functions. Here, we apply molecular network analysis on three-dimensional structures issued from molecular dynamics simulations to decipher how the chain organization of trifunctional catalytic oligomers is influenced by the oligomer sequence and the length of oligo(ethylene oxide) spacers. Our findings demonstrate that the tuning of their primary structures is crucial for favoring cooperative interactions between the catalytic units and thus higher catalytic activities. This combined approach can assist in establishing structure-property relationships, leading to a more rational design of sequence-defined catalytic oligomers via computational chemistry.

Helicity of Peptoid Ions in the Gas Phase.

Peptoids are attractive substitutes for peptides in several research areas, especially when they adopt a helical structure. The chain-size evolution of the secondary structure of the widely studied (S)-N-1-phenylethyl peptoids is here analyzed by means of the ion mobility mass spectrometry technique increasingly used as a powerful analytical tool and is further supported by theoretical modeling. We conclude that the helical shape of the peptoids prevailing in solution is lost in the gas phase by the need to screen the positive charge borne by the peptoid even though the collisional cross sections are close to the values expected for helical systems. We further illustrate that trend line analyses predicting molecular shapes from fits of the size evolution of cross sections can be very misleading since they critically depend on the range of polymerization degrees under study.

Luminescent Dinuclear Copper(I) Complexes Bearing an Imidazolylpyrimidine Bridging Ligand.

The synthesis and photophysical study of two dinuclear copper(I) complexes bearing a 2-(1H-imidazol-2-yl)pyrimidine bridging ligand are described. The tetrahedral coordination sphere of each copper center is completed through the use of a bulky bis(phosphine) ligand, either DPEphos or Xantphos. Temperature-dependent photophysical studies demonstrated emission through a combination of phosphorescence and thermally activated delayed fluorescence for both complexes, and an intense emission (ΦPL = 46%) was observed for a crystalline sample of one of the complexes reported. The photophysics of these two complexes is very sensitive to the environment. Two pseudopolymorphs of one of the dinuclear complexes were isolated, with distinct photophysics. The emission color of the crystals can be changed by grinding, and the differences in their photophysics before and after grinding are discussed.

Effects of electrospray mechanisms and structural relaxation on polylactide ion conformations in the gas phase: insights from ion mobility spectrometry and molecular dynamics simulations.

Recent advances in molecular dynamics (MD) simulations have made it possible to examine the behavior of large charged droplets that contain analytes such as proteins or polymers, thereby providing insights into electrospray ionization (ESI) mechanisms. In the present study, we use this approach to investigate the release of polylactide (PLA) ions from water/acetonitrile ESI droplets. We found that cationized gaseous PLA ions can be formed via various competing pathways. Some MD runs showed extrusion and subsequent separation of polymer chains from the droplet, as envisioned by the chain ejection model (CEM). On other occasions the PLA chains remained inside the droplets and were released after solvent evaporation to dryness, consistent with the charge residue model (CRM). Following their release from ESI droplets, the nascent gaseous PLA ions were subjected to structural relaxation for several µs in vacuo. The MD conformations generated in this way for various PLA charge states compared favorably to experimental results obtained by ion mobility spectrometry-mass spectrometry (IMS-MS). The structures of all PLA ions evolved during relaxation in the gas phase. However, some macroion species retained features that resembled their nascent structures. For this subset of ions, the IMS-MS response appears to be strongly correlated with the ESI release mechanism (CEM vs. CRM). The former favored extended structures, whereas the latter preferentially generated compact conformers.

Approaching disorder-free transport in high-mobility conjugated polymers.

Conjugated polymers enable the production of flexible semiconductor devices that can be processed from solution at low temperatures. Over the past 25 years, device performance has improved greatly as a wide variety of molecular structures have been studied. However, one major limitation has not been overcome; transport properties in polymer films are still limited by pervasive conformational and energetic disorder. This not only limits the rational design of materials with higher performance, but also prevents the study of physical phenomena associated with an extended π-electron delocalization along the polymer backbone. Here we report a comparative transport study of several high-mobility conjugated polymers by field-effect-modulated Seebeck, transistor and sub-bandgap optical absorption measurements. We show that in several of these polymers, most notably in a recently reported, indacenodithiophene-based donor-acceptor copolymer with a near-amorphous microstructure, the charge transport properties approach intrinsic disorder-free limits at which all molecular sites are thermally accessible. Molecular dynamics simulations identify the origin of this long sought-after regime as a planar, torsion-free backbone conformation that is surprisingly resilient to side-chain disorder. Our results provide molecular-design guidelines for 'disorder-free' conjugated polymers.

Comprehensive modelling study of singlet exciton diffusion in donor-acceptor dyads: when small changes in chemical structure matter.

We compare two small π-conjugated donor-bridge-acceptor organic molecules differing mainly in the number of thiophene rings in their bridging motifs (1 ring in 1; 2 rings in 2) with the aim of rationalizing the origin of the enhancement in the singlet exciton diffusion coefficient and length of 1 with respect to 2. By combining force field molecular dynamics and micro electrostatic schemes with time-dependent density functional theory and kinetic Monte Carlo simulations, we dissect the nature of the lowest electronic excitations in amorphous thin films of these molecules and model the transport of singlet excitons across their broadly disordered energy landscapes. In addition to a longer excited-state lifetime associated with a more pronounced intramolecular charge-transfer character, our calculations reveal that singlet excitons in 1 are capable of funneling through long-distance hopping percolation pathways, presumably as a result of the less anisotropic shape of the molecule, which favours long-range 3D transport.

Tackling saponin diversity in marine animals by mass spectrometry: data acquisition and integration.

Saponin analysis by mass spectrometry methods is nowadays progressively supplementing other analytical methods such as nuclear magnetic resonance (NMR). Indeed, saponin extracts from plant or marine animals are often constituted by a complex mixture of (slightly) different saponin molecules that requires extensive purification and separation steps to meet the requirement for NMR spectroscopy measurements. Based on its intrinsic features, mass spectrometry represents an inescapable tool to access the structures of saponins within extracts by using LC-MS, MALDI-MS, and tandem mass spectrometry experiments. The combination of different MS methods nowadays allows for a nice description of saponin structures, without extensive purification. However, the structural characterization process is based on low kinetic energy CID which cannot afford a total structure elucidation as far as stereochemistry is concerned. Moreover, the structural difference between saponins in a same extract is often so small that coelution upon LC-MS analysis is unavoidable, rendering the isomeric distinction and characterization by CID challenging or impossible. In the present paper, we introduce ion mobility in combination with liquid chromatography to better tackle the structural complexity of saponin congeners. When analyzing saponin extracts with MS-based methods, handling the data remains problematic for the comprehensive report of the results, but also for their efficient comparison. We here introduce an original schematic representation using sector diagrams that are constructed from mass spectrometry data. We strongly believe that the proposed data integration could be useful for data interpretation since it allows for a direct and fast comparison, both in terms of composition and relative proportion of the saponin contents in different extracts. Graphical Abstract A combination of state-of-the-art mass spectrometry methods, including ion mobility spectroscopy, is developed to afford a complete description of the saponin molecules in natural extracts.

Influence of Equilibration Time in Solution on the Inclusion/Exclusion Topology Ratio of Host-Guest Complexes Probed by Ion Mobility and Collision-Induced Dissociation.

Host-guest complexes are formed by the creation of multiple noncovalent bonds between a large molecule (the host) and smaller molecule(s) or ion(s) (the guest(s)). Ion-mobility separation coupled with mass spectrometry nowadays represents an ideal tool to assess whether the host-guest complexes, when transferred to the gas phase upon electrospray ionization, possess an exclusion or inclusion nature. Nevertheless, the influence of the solution conditions on the nature of the observed gas-phase ions is often not considered. In the specific case of inclusion complexes, kinetic considerations must be taken into account beside thermodynamics; the guest ingression within the host cavity can be characterized by slow kinetics, which makes the complexation reaction kinetically driven on the timescale of the experiment. This is particularly the case for the cucurbituril family of macrocyclic host molecules. Herein, we selected para-phenylenediamine and cucurbit[6]uril as a model system to demonstrate, by means of ion mobility and collision-induced dissociation measurements, that the inclusion/exclusion topology ratio varies as a function of the equilibration time in solution prior to the electrospray process.

Neutral Mononuclear Copper(I) Complexes: Synthesis, Crystal Structures, and Photophysical Properties.

Neutral green-emitting four-coordinate Cu(I) complexes with general formula POPCu(NN), where POP = bis[2-(diphenylphosphino)phenyl]ether and NN = substituted 2-pyridine-1,2,4-triazole ligands, were synthesized. The crystal structures of (POPCuMeCN)(+)(PF6)(-) (1), POPCuPhPtp (2a, PhPtp = 2-(5-phenyl-2H-[1,2,4]triazol-3-yl)-pyridine), and POPCu(3,5-2FPhPtp) (2d, 3,5-2FPhPtp = 2-(5-(3,5-difluorophenyl)-2H-1,2,4-triazol-3-yl)pyridine) were determined by single-crystal X-ray diffraction analysis. The electronic and photophysical properties of the complexes were examined by UV-vis, steady-state, and time-resolved spectroscopy. At room temperature, weak emission was observed in solution, while in the solid state, all complexes exhibit intense green emission with quantum yield up to 0.54. The electronic and photophysical properties were further supported by calculation performed at the (time-dependent) density functional theory level. One of the complexes was also tested as dopant in electroluminescent devices.

Energy-resolved collision-induced dissociation of non-covalent ions: charge- and guest-dependence of decomplexation reaction efficiencies.

Supramolecular chemistry, and especially host-guest chemistry, has been the subject of great interest in the past few decades leading to the synthesis of host cage molecules such as calixarenes, cyclodextrins and more recently cucurbiturils. Mass spectrometry methods are increasingly used to decipher at the molecular level the non-covalent interactions between the different associated molecules. The present article illustrates that the association between mass spectrometry and computational chemistry techniques proves very complementary to depict the gas-phase dissociation processes of ionic non-covalent complexes when subjected to collisional activation. The selected system associates a nor-seco-cucurbit[10]uril bitopic receptor with different amino compounds (adamantylamine, para-xylylenediamine, and para-phenylenediamine). When subjected to CID experiments, the ternary complexes undergo fragmentation via dissociation of non-covalently bound partners. Interestingly, depending on their charge state, the collisionally excited complexes can selectively expel either a neutral guest molecule or a protonated guest molecule. Moreover, based on energy-resolved CID experiments, it is possible to evaluate the guest molecule dependence on the gas phase dissociation efficiency. We observed that the relative order of gas phase dissociation is charge state dependent, with the adamantylamine-containing complexes being the weakest when triply charged and the strongest when doubly charged. The energetics of the gas-phase dissociation reactions have been estimated by density functional theory (DFT) calculations. We succeeded in theoretically rationalizing the experimental collision-induced dissociation results with a special emphasis on: (i) the charge state of the expelled guest molecule and (ii) the nature of the guest molecule.

Parameters influencing the photo-induced electron transfer from tryptophan-containing peptides to a Ru(II) complex: a systematic study.

Ruthenium(II) polyazaaromatic complexes have gained interest in recent decades as biomolecular tools, especially in the development of new phototherapeutic agents. These light emissive Ru complexes based on π-deficient ligands were first designed to allow a photo-induced electron transfer (PET) with the guanine base in DNA since their (3)MLCT state is highly photo-oxidizing. Later the field of research was extended to proteins with the highlighting of a PET process with the tryptophan residue. This paper reports the kinetics of the luminescence quenching of [Ru(TAP)2phen](2+) by several selected peptide sequences containing at least one tryptophan residue. By using a peptide library we highlight the important parameters influencing the kinetics of the photo-electron transfer process, such as the net electrostatic charge and the number of tryptophan residues. The best peptide candidates were selected to study the formation of photo-products by MALDI-ToF mass spectrometry. A high photoreactivity of the [Ru(TAP)2phen](2+) complex was observed and multiple photoadducts were characterized, among them inter-peptidic adducts as well as intra-peptidic adducts.

Ion mobility mass spectrometry of saponin ions.

RATIONALE: Saponins are natural compounds presenting a high structural diversity whose structural characterization remains extremely challenging. Ideally, saponin structures are best established using nuclear magnetic resonance experiments conducted on isolated molecules. However, saponins are also increasingly characterized using tandem mass spectrometry (MS/MS) coupled with liquid chromatography, even if collision-induced dissociation (CID) experiments are often quite limited in accurately determining the saponin structure. METHODS: We consider here ion mobility mass spectrometry (IMMS) as an orthogonal tool for the structural characterization of saponin isomers by comparing the experimental collisional cross sections (CCSs) of saponin ions with theoretical CCSs for candidate ion structures. Indeed, state-of-the-art theoretical calculations perfectly complement the experimental results, allowing the ion structures to be deciphered at the molecular level. RESULTS: We demonstrate that ion mobility can contribute to the structural characterization of saponins because different saponin ions present significantly distinct CCSs. Depending on the nature of the cation (in the positive ion mode), the differences in CCSs can also be exacerbated, optimizing the gas-phase separation. When associated with molecular dynamics simulations, the CCS data can be used to describe the interactions between the cations, i.e. H+ , Na+ and K+ , and the saponin molecules at a molecular level. CONCLUSIONS: Our work contributes to resolve the relationship between the primary and secondary structures of saponin ions. However, it is obvious that the structural diversity and complexity of the saponins cannot be definitively unraveled by measuring a single numerical value, here the CCS. Consequently, the structural characterization of unknown saponins will be difficult to achieve based on IMMS alone. Nevertheless, we demonstrated that monodesmosidic and bidesmosidic saponins can be distinguished via IMMS.

Spin relaxation of electron and hole polarons in ambipolar conjugated polymers.

The charge-transport properties of conjugated polymers have been studied extensively for opto-electronic device applications. Some polymer semiconductors not only support the ambipolar transport of electrons and holes, but do so with comparable carrier mobilities. This opens the possibility of gaining deeper insight into the charge-transport physics of these complex materials via comparison between electron and hole dynamics while keeping other factors, such as polymer microstructure, equal. Here, we use field-induced electron spin resonance spectroscopy to compare the spin relaxation behavior of electron and hole polarons in three ambipolar conjugated polymers. Our experiments show unique relaxation regimes as a function of temperature for electrons and holes, whereby at lower temperatures electrons relax slower than holes, but at higher temperatures, in the so-called spin-shuttling regime, the trend is reversed. On the basis of theoretical simulations, we attribute this to differences in the delocalization of electron and hole wavefunctions and show that spin relaxation in the spin shuttling regimes provides a sensitive probe of the intimate coupling between charge and structural dynamics.

Nanoscale doping of polymeric semiconductors with confined electrochemical ion implantation.

Nanoresolved doping of polymeric semiconductors can overcome scaling limitations to create highly integrated flexible electronics, but remains a fundamental challenge due to isotropic diffusion of the dopants. Here we report a general methodology for achieving nanoscale ion-implantation-like electrochemical doping of polymeric semiconductors. This approach involves confining counterion electromigration within a glassy electrolyte composed of room-temperature ionic liquids and high-glass-transition-temperature insulating polymers. By precisely adjusting the electrolyte glass transition temperature (Tg) and the operating temperature (T), we create a highly localized electric field distribution and achieve anisotropic ion migration that is nearly vertical to the nanotip electrodes. The confined doping produces an excellent resolution of 56 nm with a lateral-extended doping length down to as little as 9.3 nm. We reveal a universal exponential dependence of the doping resolution on the temperature difference (Tg - T) that can be used to depict the doping resolution for almost infinite polymeric semiconductors. Moreover, we demonstrate its implications in a range of polymer electronic devices, including a 200% performance-enhanced organic transistor and a lateral p-n diode with seamless junction widths of <100 nm. Combined with a further demonstration in the scalability of the nanoscale doping, this concept may open up new opportunities for polymer-based nanoelectronics.

Iridium(III) emitters based on 1,4-disubstituted-1H-1,2,3-triazoles as cyclometalating ligand: synthesis, characterization, and electroluminescent devices.

A series of blue and blue-green emitters based on neutral bis- and tris-cyclometalated Ir(III) complexes with 1-benzyl-4-(2,6-difluorophenyl)-1H-1,2,3-triazole (dfptrBn) as cyclometalating ligand is reported. The bis-cyclometalated complexes of the type [Ir(dfptrBn)(2)(L(^)X)] with different ancillary ligands, L(^)X = picolinate (pic) (2) or 2-(5-(perfluorophenyl)-2H-1,2,4-triazol-3-yl)pyridine (pytrF(5)) (3), are described and their photophysical properties compared with the analogous complexes containing the archetypal 2-(2,4-difluorophenyl)pyridinato (dfppy) as cyclometaled ligand (C(^)N). Complex 2 exhibits a marked solvatochromic behavior, from 475 nm in toluene to 534 nm in formamide, due to the strong MLCT character of its emissive excited state. Complex 3 displays a true-blue emission, narrower in the visible part than FIrpic. In addition, the homoleptic complex [Ir(dfprBn)(3)] (4) and the heteroleptic compounds with mixed arylpyridine/aryltriazole ligands, [Ir(dfptrBn)(2)(C(^)N)] (C(^)N = 2-phenylpyridinato (ppy) (5) or dfppy (6)), have been synthesized and fully characterized. The facial (fac) complex fac-4 is emissive at 77 K showing a deep-blue emission, but it is not luminescent in solution at room temperature similarly to their phenylpyrazole counterparts. However, the fac isomers, fac-5 and fac-6, are highly emissive in solution and thin films, reaching emission quantum yields of 76%, with emission colors in the blue to blue-green region. The photophysical properties for all complexes have been rationalized by means of quantum-chemical calculations. In addition, we constructed electroluminescent devices, organic light-emitting diodes (OLEDs) by sublimation of fac-6, and by solution processed polymer-based devices (PLEDs) using complexes fac-5 or fac-6 as dopants.

Free radical scavenging by natural polyphenols: atom versus electron transfer.

Polyphenols (synthetically modified or directly provided by human diet) scavenge free radicals by H-atom transfer and may thus decrease noxious effects due to oxidative stress. Free radical scavenging by polyphenols has been widely theoretically studied from the thermodynamic point of view whereas the kinetic point of view has been much less addressed. The present study describes kinetic-based structure-activity relationship for quercetin. This compound is very characteristic of the wide flavonoid subclass of polyphenols. H-atom transfer is a mechanism based on either atom or electron transfer. This is analyzed here by quantum chemical calculations, which support the knowledge acquired from experimental studies. The competition between the different processes is discussed in terms of the nature of the prereaction complexes, the pH, the formation of activated-deprotonated forms, and the atom- and electron-transfer efficiency. The role of the catechol moiety and the 3-OH group of quercetin as scavengers of different types of free radicals (CH3OO(•), CH3O(•), (•)OH, and (•)CH2OH) is rationalized. Identifying the exact mechanism and accurately evaluating kinetics is of fundamental importance to understand antioxidant behavior in physiological environments.

Quasi-alternating copolymerization of oxiranes driven by a benign acetate-based catalyst.

Alternating copolymers are distinctly unique in comparison with other copolymers. Herein, an in-depth investigation of the oxyanionic ring-opening copolymerization of propylene oxide (PO) and allyl glycidyl ether (AGE) from benzyl alcohol (BnOH) activated with potassium acetate (KOAc) complexed by 18-crown-6 ether (18C6) is described. We demonstrate that the 18C6/KOAc complex is an efficient and benign catalytic system to promote copolymerization of both oxirane monomers, leading to well-defined polyethers with varied comonomer content and low dispersity values (ƉM < 1.20). Kinetic analysis confirmed the controlled nature of the (co)polymerization process, and the determination of reactivity ratios revealed a quasi-alternating copolymerization profile, according to the Fineman-Ross method. The comparison between the quasi-alternating-type PO/AGE copolymerization and block or gradient copolymerization revealed significant differences, to confirm the different sequence incorporation in the different topological copolymers. These results highlight the great potential of 18C6/KOAc-mediated copolymerization process for the controlled sythesis of a series of copolymer topologies.

F-Center-Mediated Growth of Patterned Organic Semiconductor Films on Alkali Halides.

Organic semiconductors combine flexible tailoring of their optoelectronic properties by synthetic means with strong light-matter coupling, which is advantageous for organic electronic device applications. Although spatially selective deposition has been demonstrated, lateral patterning of organic films with simultaneous control of molecular and crystalline orientation is lacking as traditional lithography is not applicable. Here, a new patterning approach based on surface-localized F-centers (halide vacancies) generated by electron irradiation of alkali halides is presented, which allows structural control of molecular adlayers. Combining optical and atomic force microscopy, X-ray diffraction, and density functional theory (DFT) calculations, it is shown that dinaphthothienothiophene (DNTT) molecules adopt an upright orientation on pristine KCl surfaces, while the F-centers stabilize a recumbent orientation, and that these orientations are maintained in thicker films. This specific nucleation results also in different crystallographic morphologies, namely, densely packed islands and jagged fibers, each epitaxially aligned on the KCl surface. Spatially selective surface irradiation can also be used to create patterns of F-centers and thus laterally patterned DNTT films, which can be further transferred to any (including elastomer) substrate due to the water solubility of the alkali halide growth templates.