Zwitterionic Resonance Structures Research Articles

Synergistic resonant molecular passivator of various defects for high-performance perovskite solar cells

The defects and mismatched energy-level at interfaces of perovskite solar cells (PSCs) can weaken their performance and stability. In this paper, we introduce 3,8-dibromo-1,10-phenanthroline-5,6-dione (BPO) into PSCs to align perovskite energy levels and passivate defects. The theoretical and experimental results indicate that BPO molecules with zwitterionic resonance structure can accept electrons and change the charge state of perovskite surface to modify the energy-level. In addition, the multi-group passivation of BPO molecules, which contain “bipyridyl nitrogen” and bicarbonyl groups, can chelate the Pb defects such as Pb–I antisite (PbI) and iodine vacancy (VI). Notably, the BPO-doped PSCs achieve a power conversion efficiency of 23.18 % with enhanced efficiency retention ratio of 85 % (55 % for control sample) after 1000 h test at 65 % relative humidity. This study provides an effective choice for the exploration of novel additives for device performance improvement.

Local Spatial Polarization Induced Efficient Charge Separation of Squaraine‐Linked COF for Enhanced Photocatalytic Performance

AbstractThe efficiencies of semiconductor photocatalysis are always severely restricted by the rapid recombination of charge carriers in the bulk phase, while local polarization is an efficient scenario to conquer the above issue to boost the photocatalytic performance. Here, as a proof‐of‐concept demonstration, local charge polarization induced by the intrinsic zwitterionic resonance structure of photocatalysts is reported. A novel squaraine‐based covalent organic framework (SQ‐COF‐1) photocatalyst with an interesting zwitterionic resonance structure is creatively developed. Meanwhile, the comparison samples (SQ‐COF‐2 and PDA‐COF) based on similar COF structure with tunable local polarity are accordingly designed and prepared. The local charge polarization generated from the zwitterionic resonance structure profoundly redistributes and separates the charge carriers, as evidenced by experimental and theoretical results collectively. Benefiting by the largest local polarization, SQ‐COF‐1 exhibits superior visible‐light‐driven photocatalytic activities than SQ‐COF‐2, PDA‐COF, and commonly used polymeric carbon nitride photocatalyst. This work presents a local charge polarization protocol for engineering charge behavior to promote photocatalysis, which shows great promise for the future design of high‐performance photocatalytic materials.

Stability and Optical Absorption of a Comprehensive Virtual Library of Minimal Eumelanin Oligomer Models**

AbstractEumelanin is responsible for photoprotection in living organisms. It is made of 5,6‐dihydroxyindole (DHI) oligomers. However, lack of detailed structural knowledge limits understanding its function and exploiting its potential in material science. To uncover the relationship between structural stability and optical properties, we have studied a virtual library of 830 DHI dimers. We find a preference for oxidized, polycyclic structures which speaks in favor of graphite‐like structures for the larger oligomers, and propose an electrocyclic formation mechanism. Besides widely considered quinone oxidation patterns, also structures with interfragment double bonds and zwitterionic resonance structures are stable. Future theoretical melanine models will have to cover this diversity, and we introduce a new representative set of 49 stable dimers. Some stable oxidized dimers have absorption energies as low as 1.3 eV. They may be present as substructures in the naturally found oligomers and contribute to the absorption spectrum of the biopolymer.

An Ab Initio Computational Study of Electronic and Structural Factors in the Isomerization of Donor-Acceptor Stenhouse Adducts.

In this work, the photochemically and thermally induced isomerization of multiple donor-acceptor Stenhouse adducts (DASAs) of the first, second, and third generation is studied by means of state-of-the-art ab initio electronic structure methods leading to new insight into multiple facets of the reaction mechanism. Importantly, prior to any studies of the reaction mechanism, a set of test calculations demonstrate the suitability of the applied ADC(2) and CC2 methods in the present context. An important aspect in this regard is the availability of electronic energies and gradients under implicit consideration of solvent effects. On the basis of calculated reaction energies and barriers as well as a thorough analysis of the wave function compositions, interesting features of the reaction mechanism are deduced. For example, the closed form of second- and third-generation DASAs can be significantly stabilized by π - π interactions between the donor and acceptor termini when certain structural requirements are fulfilled. The central point of this work concerns the delicate balance between neutral and zwitterionic resonance structures that governs the relative barrier height for the crucial C2-C3 and C3-C4 bond rotations. Finally, a set of calculations on yet unreported derivatives highlights how this balance and hence the barrier heights can be tuned through variation of the donor-acceptor strength as well as the solvent polarity.

Donor‐Flexible Bis(pyridylidene amide) Ligands for Highly Efficient Ruthenium‐Catalyzed Olefin Oxidation

AbstractAn exceptionally efficient ruthenium‐based catalyst for olefin oxidation has been designed by exploiting N,N′‐bis(pyridylidene)oxalamide (bisPYA) as a donor‐flexible ligand. The dynamic donor ability of the bisPYA ligand, imparted by variable zwitterionic and neutral resonance structure contributions, paired with the redox activity of ruthenium provided catalytic activity for Lemieux–Johnson‐type oxidative cleavage of olefins to efficiently prepare ketones and aldehydes. The ruthenium bisPYA complex significantly outperforms state‐of‐the‐art systems and displays extraordinary catalytic activity in this oxidation, reaching turnover frequencies of 650 000 h−1 and turnover numbers of several millions.

Construction of Complex Bisether-Bridged Medium-Sized Cyclic Compounds from o-(1-(Acyloxy)propargyl)benzaldehydes under Base and Acid Catalysis.

We report herein our serendipitous discovery of the rapid and straightforward accesses to unprecedented diverse complex molecular structures from readily available starting materials. Catalyzed by 1,8-diazabicyclo[5.4.0]undec-7-ene under mild conditions, o-(1-(acyloxy)propargyl)benzaldehydes 1 underwent efficient and selective dimerization reactions to produce novel complex bisether-bridged tricyclic products 3 and 4. The reactions proceeded most probably through dimerizations between 3-methylene-3H-isochromene intermediate and its zwitterionic resonance structures which were generated from a concerted 6-π electrocyclic ring closure reaction from the initially formed (2-formylphenyl)allene intermediates derived directly from o-(1-(acyloxy)propargyl)benzaldehydes. Treatment of the resulting product simply with NaOEt in ethanol and aqueous HCl, respectively, enabled further development of complex molecular diversities.

Solvent Effects on the Stability and Delocalization of Aryl Dicyanomethyl Radicals: The Captodative Effect Revisited.

The captodative effect postulates that radicals substituted with both electron donating and accepting groups enjoy a special enhanced stabilization, a model given theoretical support by simple MO and resonance arguments. A key prediction from theory is that captodative stabilization of radicals is larger in polar solvents than in nonpolar solvents or the gas phase, which can be viewed in the resonance model as solvent stabilization of charge-separated resonance forms. Yet, several experimental studies have failed to observe a solvent effect on radical stability, casting doubt on key aspects of the captodative effect. Here, we examine in detail the effect of solvent on the stability of structurally related captodative aryl dicyanomethyl radicals. An attractive feature of these radicals is that they exist as stable steady state populations of radicals in equilibrium with their dimers, allowing us to directly characterize from experiment their thermodynamic stabilities and spin delocalization in solvents of varying polarity. In contrast to the prior studies, we find that captodative radicals are indeed stabilized by polar solvents, as measured by a shift in the radical-dimer association constants by up to 100-fold toward the radical upon going from nonpolar toluene to more polar DMF. Moreover, in polar solvents, the spin is shifted onto the donor substituent and away from the benzylic carbon. Within the resonance model, these results can be explained by the increased contributions of the zwitterionic resonance structures to the overall hybrid. These results provide experimental support to a key prediction from theory that had previously been dismissed.

Hyperconjugation effect on diene reactivity in 1-methyltetrazolo[5,1-a]isoindole-derived amides and thioamides

Optimized synthesis of 1-methyltetrazolo[5,1-a]isoindole-derived amides and thioamides was elaborated. Based on 13C NMR spectroscopy and X-Ray diffraction studies data, it was proposed that zwitterionic resonance structures contributed significantly to the structure of these compounds. Geometry optimization was performed in vacuo using m06-2x/cc-pvtz method taking into account polarizing effect of environment (PCM model) and specific intermolecular interactions. Electronic density distribution in these molecules was analyzed using NBO method. Using Δ(HOMOdiene–LUMOdienophile) for amides and thioamides in vacuo and in a protic solvent, possibility of Diels–Alder reaction was evaluated. Energies of π-π conjugation and n→σ* hyperconjugation for the amide derivative were estimated in vacuo, aprotic and protic solvents.

Metal-Ligand Cooperative Reactivity in the (Pseudo)-Dearomatized PNx(P) Systems: The Influence of the Zwitterionic Form in Dearomatized Pincer Complexes.

The concept of aromaticity in pincer ligands and complexes was discussed in order to provide insights into their metal-ligand cooperative activities. The aromatic PNx(P) and dearomatized PNx(P)* pincer ligands and the corresponding transition metal complexes were studied with the nucleus-independent chemical shift (NICSzz), anisotropy of the current (induced) density (ACID), isochemical shielding surfaces (ICSSzz), harmonic oscillator model of aromaticity (HOMA), MCBO, Shannon aromaticity, and natural bond order (NBO) analyses. The study on the model systems showed that for the dearomatized species the decrease of the NICS(1)zz value comes with the larger contribution of the aromatic zwitterionic mesomeric form. In all examples, the incorporation of the metal center into the pincer ligand decreases the NICS(1)zz values. The DFT calculations support the dearomatized pyridine ring in PNP* or PNN* ligand indeed being nonaromatic, in contrast to the PN3(P)* ligand which has partial aromatic character due to the larger contribution of the zwitterionic resonance structure. The difference in aromaticity between the rings contributes to the thermodynamic balance of the metal ligand cooperative reactions, changing the energetics of the process when different dearomatized pincer ligands are used. This was further exemplified by aromaticity analysis of the heterolytic hydrogen cleavage reaction of ruthenium PNN complexes of Milstein and the PN3 of Huang, with similar geometries but distinctive thermodynamic preference.

Rational Design of Dipolar Chromophore as an Efficient Dopant-Free Hole-Transporting Material for Perovskite Solar Cells.

In this paper, an electron donor-acceptor (D-A) substituted dipolar chromophore (BTPA-TCNE) is developed to serve as an efficient dopant-free hole-transporting material (HTM) for perovskite solar cells (PVSCs). BTPA-TCNE is synthesized via a simple reaction between a triphenylamine-based Michler's base and tetracyanoethylene. This chromophore possesses a zwitterionic resonance structure in the ground state, as evidenced by X-ray crystallography and transient absorption spectroscopies. Moreover, BTPA-TCNE shows an antiparallel molecular packing (i.e., centrosymmetric dimers) in its crystalline state, which cancels out its overall molecular dipole moment to facilitate charge transport. As a result, BTPA-TCNE can be employed as an effective dopant-free HTM to realize an efficient (PCE ≈ 17.0%) PVSC in the conventional n-i-p configuration, outperforming the control device with doped spiro-OMeTAD HTM.

Efficient Green Approach for the Synthesis of Spiro[indoline-3,4′-pyrazolo[3,4-b]quinoline]diones Using [NMP]H2PO4 and Solvatochromic and pH Studies

A mild and efficient protocol for the synthesis of spiro[indoline-3,4′-pyrazolo[3,4-b]quinoline]diones via a one-pot, three-component condensation of isatins, 1,3-dicarbonyls, and 5-amino-1-phenyl-3-methylpyrazole using [NMP]H2PO4 as a catalyst in EtOH/H2O is described. The catalyst could be recycled and reused four times without significant loss of activity. Spiro[indoline-3,4′-pyrazolo[3,4-b]quinoline]diones with stabilized zwitter ionic resonance structures showed feasible application as new fluorescent probes and pH indicators. These chemosensors have a good wavelength shift and showed excellent sensitivity in the range of pH from 11 to 13.

Theoretical Molecular Design of Heteroacenes for Singlet Fission: Tuning the Diradical Character by Modifying π-Conjugation Length and Aromaticity

A theoretical molecular design for efficient singlet fission (SF) is performed for several heteroacene models involving nitrogen (N) atoms based on the diradical character criterion of the energy level matching conditions. This criterion is found to be closely related to the relative contributions of diradical and zwitterionic resonance structures of the heteroacenes, i.e., the aromaticity of the central ring(s). From the analysis of the diradical characters of these heteroacene models, the increase in the aromaticity of the central ring(s) is found to prefer the diradical form to the zwitterionic form. From the comparison of the excitation energies evaluated by multireference second-order perturbation theory calculations, two promising candidates, chosen based on the diradical character criterion, are found to satisfy the energy level matching conditions and to possess high triplet energies of ∼1.1 eV, which are suitable for an application in organic photovoltaic cells. The proposed two candidates are shown to have mutually different types of the first excited singlet states, which are distinguished by the primary excitation configurations. These results suggest that the proposed two candidates exhibit different singlet fission dynamics due to the different amplitude of the electronic coupling.

Phase Equilibrium, Excess Enthalpies, and Densities of Binary Mixtures of Trimethylbutylammonium Bis(trifluoromethylsulfonyl)imide with Ethanol, 1-Propanol, and Dimethylformamide

The molecular interactions between the ionic liquid trimethylbutylammomium bis(trifluoromethylsulfonyl)imide, [N1114][Tf2N], and three common solvents, including short chain alcohols (ethanol and 1-propanol) and an aprotic solvent (N,N-dimethylformamide or DMF) were investigated through experimental properties. The excess enthalpies were measured by calorimetry at temperatures from 308.15 K to 323.15 K over the whole composition range and excess volumes were calculated from density measurements. The vapor–liquid equilibrium (VLE) measurements of the [N1114][Tf2N] + DMF mixture were obtained using a headspace gas chromatography instrument. It was determined that hydrogen bonds between the alcohol molecules contribute to the positive excess enthalpies, while the favorable interactions between DMF and [N1114][Tf2N] might be explained by electrostatic interactions between the ionic liquid and the zwitterionic resonance structure of DMF. The nonrandom two liquid (NRTL) equation was successfully used to correlate the excess enthalpy and VLE results.

Chirped-Pulse Fourier Transform Microwave Spectroscopy Coupled with a Flash Pyrolysis Microreactor: Structural Determination of the Reactive Intermediate Cyclopentadienone.

Chirped-pulse Fourier transform microwave spectroscopy (CP-FTMW) is combined with a flash pyrolysis (hyperthermal) microreactor as a novel method to investigate the molecular structure of cyclopentadienone (C5H4═O), a key reactive intermediate in biomass decomposition and aromatic oxidation. Samples of C5H4═O were generated cleanly from the pyrolysis of o-phenylene sulfite and cooled in a supersonic expansion. The (13)C isotopic species were observed in natural abundance in both C5H4═O and in C5D4═O samples, allowing precise measurement of the heavy atom positions in C5H4═O. The eight isotopomers include: C5H4═O, C5D4═O, and the singly (13)C isotopomers with (13)C substitution at the C1, C2, and C3 positions. Microwave spectra were interpreted by CCSD(T) ab initio electronic structure calculations and an re molecular structure for C5H4═O was found. Comparisons of the structure of this "anti-aromatic" molecule are made with those of comparable organic molecules, and it is concluded that the disfavoring of the "anti-aromatic" zwitterionic resonance structure is consistent with a more pronounced C═C/C-C bond alternation.

Chalcogen analogues of nicotine lactam studied by NMR, FTIR, DFT and X-ray methods

The selenoanalogue of nicotine has been synthesized and characterized by spectroscopic and X-ray diffraction methods. The crystals of selenonicotine are isomorphic with the thionicotine homologue and consist of molecules engaged in columnar π⋯π stacking interactions between antiparallely arranged pyridine moieties. These interactions, absent in other crystals containing nicotine fragments, seem to be induced by the presence of a lactam group. The molecular structures in the vacuum of the oxo-, thio- and selenonicotine homologues have been calculated by the DFT method and compared with the available X-ray data. The delocalized structure of thionicotine is stabilized by intramolecular CH⋯S hydrogen bond, which becomes weaker in the partial zwitterionic resonance structure of selenonicotine in favor of multiple CH⋯Se intermolecular hydrogen-bonds. The calculated data allow a complete assignment of vibration modes in the solid state FTIR spectra. The 1H and 13C NMR chemical shifts were calculated by the GIAO method with B3LYP/6-311G(3df) level. A comparison between experimental and calculated theoretical results indicates that the density functional B3LYP method provided satisfactory results for predicting FTIR, 1H, 13C NMR spectra properties.

Reactions of M2(CO)9L (M = Re, Mn; L = CO, MeCN) with thioacetamide and thiobenzamide: Facile metal-mediated nitrogen–hydrogen bond activation and subsequent carbon–nitrogen or sulfur–sulfur bond formation

Addition of thioacetamide or thiobenzamide to Re2(CO)10 at room temperature in presence of Me3NO gives low yields of the simple substitution products Re2(CO)9{к1-SC(R)NH2} (1, R = Me; 2, R = Ph) in which the acyclic amides are bound through sulfur and occupy an equatorial site. In contrast, Me3NO initiated reactions of Mn2(CO)10 with the same amides lead to the isolation of hexacarbonyl complexes, Mn2(CO)6{μ-S2C(R)NH}2 (3, R = Me; 4, R = Ph), crystallographic studies revealing a binuclear core in which Mn(CO)3 moieties are spanned by two RC(SS)NH ligands. When Mn2(CO)9(NCMe) is used as the starting material only the mononuclear species Mn(CO)4{к1:η1-RC(S)NHCO} (5, R = Me; 6, R = Ph) could be isolated. These contain a chelating RC(S)NHCO ligand formed as a result of coupling the deprotonated amides with CO. Crystallographic studies have been carried out on both complexes, a careful inspection of bond lengths and angles within the chelate ring suggesting that a zwitterionic acyl-thiolate resonance structure is most prevalent.

Multi-component one-pot synthesis of unsymmetrical dihydro-5H-indeno[1,2-b]quinolines as new pH indicators

Abstract A mild and efficient protocol for the synthesis of 7,8-dihydro-10-aryl-5 H -indeno[1,2- b ]quinoline-9,11-diones via a one-pot four component condensation of aromatic aldehydes, dimedone, 1,3-indandione and ammonium acetate using melamine trisulfonic acid as a catalyst in ethanol is described. The catalyst could be recycled and reused three times without significant loss of activity. Dihydro-5 H -indeno[1,2- b ]quinolines with stabilized zwitterionic resonance structures showed feasible application as new pH indicators. These chemosensors have a large wavelength shift (100 nm) and showed excellent sensitivity in the range of pH from 9.2 to 12.

An Acyclic Imino‐Substituted Silylene: Synthesis, Isolation, and its Facile Conversion into a Zwitterionic Silaimine

A new type of Si(II): A novel silylene stabilized by a Cp* and an imidazolin-2-iminato ligand has been prepared using two different methods. The X-ray crystallographic structure shows that the silicon(II) center is coordinated to an η(2)-Cp* ligand and the nitrogen atom of an imidazolin-2-iminato ligand. This silylene easily reacts with B(C(6)F(5))(3) to give a stable borane adduct having a zwitterionic resonance structure.

Synthesis and Structure of Amido‐ and Imido(pentafluorophenyl)borane Zirconocene and Hafnocene Complexes: NH and BH Activation

Treatment of Me(2)S·B(C(6)F(5))(n) H(3-n) (n=1 or 2) with ammonia yields the corresponding adducts. H(3)N·B(C(6)F(5))H(2) dimerises in the solid state through N-H···H-B dihydrogen interactions. The adducts can be deprotonated to give lithium amidoboranes Li[NH(2)B(C(6)F(5))(n)H(3-n)]. Reaction of the n=2 reagent with [Cp(2)ZrCl(2)] leads to disubstitution, but [Cp(2)Zr{NH(2)B(C(6)F(5))(2)H}(2)] is in equilibrium with the product of β-hydride elimination [Cp(2)Zr(H){NH(2)B(C(6)F(5))(2)H}], which proves to be the major isolated solid. The analogous reaction with [Cp(2)HfCl(2)] gives a mixture of [Cp(2)Hf{NH(2)B(C(6)F(5))(2)H}(2)] and the N-H activation product [Cp(2)Hf{NHB(C(6)F(5 )(2)H}]. [Cp(2)Zr{NH(2)B(C(6)F(5))(2)H}(2)]·PhMe and [Cp(2)Hf{NH(2)B(C(6)F(5))(2)H}(2)]·4(thf) exhibit β-B-agostic chelate bonding of one of the two amidoborane ligands in the solid state. The agostic hydride is invariably coordinated to the outside of the metallocene wedge. Exceptionally, [Cp(2)Hf{NH(2)B(C(6)F(5))(2)H}(2)]⋅PhMe has a structure in which the two amidoborane ligands adopt an intermediate coordination mode, in which neither is definitively agostic. [Cp(2)Hf{NHB(C(6)F(5))(2)H}] has a formally dianionic imidoborane ligand chelating through an agostic interaction, but the bond-length distribution suggests a contribution from a zwitterionic amidoborane resonance structure. Treatment of the zwitterions [Cp(2)MMe(μ-Me)B(C(6)F(5))(3)] (M=Zr, Hf) with Li[NH(2)B(C(6)F(5))(n)H(3-n)] (n=2) results in [Cp(2) MMe{NH(2)B(C(6)F(5))(2)H}] complexes, for which the spectroscopic data, particularly (1)J(B,H), again suggest β-B-agostic interactions. The reactions proceed similarly for the structurally encumbered [Cp''(2)ZrMe(μ-Me)B(C(6)F(5))(3)] precursor (Cp''=1,3-C(5)H(3)(SiMe(3))(2) , n=1 or 2) to give [Cp''(2)ZrMe{NH(2)B(C(6)F(5))(n)H(3-n)}], both of which have been structurally characterised and show chelating, agostic amidoborane coordination. In contrast, the analogous hafnium chemistry leads to the recovery of [Cp''(2)HfMe(2)] and the formation of Li[HB(C(6)F(5))(3)] through hydride abstraction.

DFT studies on tautomeric preferences. Part 3: Proton transfer in 2-(8-acylquinolin-2-yl)-1,3-diones

8-Acetyl- and 8-benzoyl-2-(quinolin-2-yl) derivatives of 1,3-diketones were studied from point of view of their tautomeric preferences. Effective electron delocalization and strong hydrogen bonds were found to be responsible for high stability of the enaminone form (the most stable tautomer). Density functional theory (DFT) calculations prove that enolimine and ketimine species are less favored. Contribution of the zwitterionic resonance structures to the enaminone form is most significant in 2-(quinolin-2(1H)-ylidene)-2H-indene-1,3-dione derivatives. Formation of the bifurcated hydrogen bonds was found to take place only in 8-acetyl derivatives. One of these bonds is of the RAHB (Resonance Assisted Hydrogen Bond) type. There is a steric interaction of the pyridine β-hydrogen and carbonyl oxygen atoms in the enaminone tautomers.