Π-electron System Research Articles

Substitution Effect of a Single Nitrogen Atom on π-Electronic Systems of Linear Polycyclic Aromatic Hydrocarbons (PAHs): Theoretically Visualized Coexistence of Mono- and Polycyclic π-Electron Delocalization.

We theoretically investigated the nitrogen substitution effect on the molecular structure and π-electron delocalization in linear nitrogen-substituted polycyclic aromatic hydrocarbons (N-PAHs). Based on the optimized molecular structures and magnetic field-induced parameters of fused bi- and tricyclic linear N-PAHs, we found that the local π-electron delocalization of subcycles (e.g., mono- and bicyclic constituent moieties) in linear N-PAHs is preserved, despite deviation from ideal structures of parent monocycles. The introduction of a fused five-membered ring with a pyrrolic N atom (N-5MR) in linear N-PAHs significantly perturbs the π-electronic condition of the neighboring fused six-membered ring (6MR). Monocyclic pyrrole exhibits substantial bond length alternations, strongly influencing the π-electronic systems of both the fused N-5MR and 6MR in linear N-PAHs, depending on the location of shared covalent bonds. A fused six-membered ring with a graphitic N atom in an indolizine moiety cannot generate monocyclic π-electron delocalization but instead contributes to the formation of polycyclic π-electron delocalization. This is evidenced by bifurcated diatropic ring currents induced by an external magnetic field. In conclusion, the satisfaction of Hückel's 4n + 2 rule for both mono- and polycycles is crucial for understanding the overall π-electron delocalization. It is crucial to consider the unique characteristics of the three types of substituted N atoms and the spatial arrangement of 5MR and 6MR in N-PAHs.

Synthesis and properties of doubly diphenylene-fused benzopyrrolo[1,4]diazocine with a [7-8-7] successive ring-fused structure

Abstract We explored a π-electronic system with a successively fused [7-8-7]-membered ring structure. X-ray analysis revealed its nonplanar structure with a mean plane deviation (MPD) of 1.53 Å. Compared with a reference compound with a [7-6-7]-membered ring structure (MPD: 0.38 Å), the incorporation of an 8-membered ring induced larger nonplanarity. On the other hand, replacing the 6-membered ring with an 8-membered ring induced a smaller strain in energy, as suggested by using DFT calculations. The π-orbital axis vector analysis showed that the nonplanarity was mainly dictated by torsion, not pyramidalization.

Topological Bistability of the π-System in a Helicene Carbon Nanohoop

AbstractMolecules with a π-system that can be mapped onto a Möbius strip may display Möbius aromaticity. Such molecules are difficult to synthesize because they have a twisted structure. Recently, we combined chiral [6]helicene and fluorescent [7]cycloparaphenylene, and synthesized the first helicene para-phenylene ([6,7]HPP) carbon nanohoop. We have demonstrated that this design strategy ultimately provides a Möbius topology of the molecular π-electron system and, therefore, offers the potential to study Möbius aromaticity experimentally. In addition, the synthesized nanohoop exists as a mixture of conformers in solution. Some of the conformers possess a different orientability of their π-systems, i.e., they differ in their topology. As a result, the recorded circularly polarized luminescence of isolated enantiomers displays both left- and right-handedness of the emitted light, each emanating from a conformer with a different π-system topology. Therefore, [6,7]HPP provided the first experimental evidence of such topological bistability in carbon nanohoops.

Hydrogen bond types which do not fit accepted definitions.

There are various interactions that either partially fit or do not fit the currently accepted definitions of the hydrogen bond. However, they possess characteristics of this interaction. It seems that it is partly connected to the fact that these definitions are not precise. The typical 3c-4e (three centres - four electrons) A-H⋯B hydrogen bond is characterized by the single-atom A and B centres that are highly electronegative. On the other hand, non-typical interactions that do not fit the hydrogen bond definitions well are characterised by uncommon proton donors and/or proton acceptors. The cases of multi-centre proton acceptors, π-electron or σ-electron systems are well known - such interactions are designated as A-H⋯π and A-H⋯σ hydrogen bonds, respectively. However, the cases of interactions with the multi-centre proton donors and proton acceptors do not fit the majority of definitions of hydrogen bond. The π⋯H+⋯π system in the proton-bound homodimer of acetylene is an example. This system can be classified as a hydrogen bond according to the two-sites hydrogen bond, 2sHB, definition. There are various types of interactions discussed in this review; among them, those that are undoubtedly unclassified as hydrogen bonds, i.e., hydride bonds, and charge inverted hydrogen bonds, CIHBs. Special emphasis is also put here on the proton sponges and other systems such as the [FHF]- anion or [NgHNg]+ cation (Ng is the noble gas centre).

Current and emerging trends of inorganic, organic and eco-friendly corrosion inhibitors.

Effective corrosion control strategies are highly desired to reduce the fate of corrosion. One widely adopted approach is the use of corrosion inhibitors, which can significantly mitigate the detrimental effects of corrosion. This systematic review provides a thorough analysis of corrosion inhibitors, including both inorganic and organic compounds. It explores the inhibition mechanisms, highlighting the remarkable inhibitive efficiency of organic compounds attributed to the presence of heteroatoms and conjugated π-electron systems. The review presents case studies and investigations of corrosion inhibitors, shedding light on their performance and application potential. Moreover, it compares the efficacy, compatibility, and sustainability of emerging environmentally friendly corrosion inhibitors, including biopolymers from natural resources as promising candidates. The review also highlights the potential of synergistic impacts between mixed corrosion inhibitors, particularly organic/organic systems, as a viable and advantageous choice for applications in challenging processing environments. The evaluation of inhibitors is discussed, encompassing weight loss (WL) analysis, electrochemical analysis, surface analysis, and quantum mechanical calculations. The review also discusses the thermodynamics and isotherms related to corrosion inhibition, further improving the understanding of inhibitor's behavior and mechanisms. This review serves as a valuable resource for researchers, engineers, and practitioners involved in corrosion control, offering insights and future directions for effective and environmentally friendly corrosion inhibition strategies.

Norcorroles as antiaromatic π-electronic systems that form dimension-controlled assemblies.

Norcorrole derivatives with 3,4,5-trialkoxyphenyl moieties at the meso positions were synthesized to form various stacking assemblies in single crystals and thermotropic liquid crystals (LCs) depending on aliphatic chain lengths. Triple-decker stacking structures were formed via the interactions between the antiaromatic systems formed for the butoxy and dodecyloxy derivatives in the single-crystal and LC states, respectively. In particular, the LC state exhibited discotic columnar structures comprising triple deckers to exhibit high electric conductivity, as supported by molecular dynamics simulations.

Porous crystalline conjugated macrocyclic materials and their energy storage applications.

Porous crystalline conjugated macrocyclic materials (CMMs) possess high porosity, tunable structure/function and efficient charge transport ability owing to their planar macrocyclic conjugated π-electron system, which make them promising candidates for applications in energy storage. In this review, we thoroughly summarize the timely development of porous crystalline CMMs in energy storage related fields. Specifically, we summarize and discuss their structures and properties. In addition, their energy storage applications, such as lithium ion batteries, lithium sulfur batteries, sodium ion batteries, potassium ion batteries, Li-CO2 batteries, Li-O2 batteries, Zn-air batteries, supercapacitors and triboelectric nanogenerators, are also discussed. Finally, we present the existing challenges and future prospects. We hope this review will inspire the development of advanced energy storage materials based on porous crystalline CMMs.

Synthesis and electronic properties of nitrogen-rich nanographene.

A polycyclic aromatic hydrocarbon displaying twelve edge nitrogen centers for a 42 π-electron system is reported. This compound was synthesized via Sonogashira coupling of pyrimidine precursors, [2+2+2] cycloaddition of bis(aryl) alkynes, and anionic cyclodehydrogenation. Spectroscopy, electrochemistry, and computational results suggest a narrowing of the HOMO-LUMO gap compared to the N-free analogue. Metal coordination affects the optical properties of the extended π system.

Ion-pairing assemblies of anion-responsive helical PtII complexes

Naphthylisoquinoline-appended dipyrrolyldiketone PtII complexes as helical π-electronic systems were synthesized. The PtII complexes, showing chiroptical properties in solution, exhibited anion-binding behaviour, resulting in the formation of anion complexes as pseudo...

Experimental and Theoretical Insights into a Novel Lightfast Thiophene Azo Dye

Thiophene ring-enhancing electron delocalization imparts unique properties to azoic chromophore tools. The novel TA-OH dye contains a push–pull π-electron system, including a thiophene-azo scaffold with a hydroxyl group at the ortho position to the azo bridge. The hydroxyl group is expected to lock the azo bridge in its trans conformation, concurring with the photostability and fastness of the dye. The single crystal analysis identified the molecule’s primary conjugation plane, and the theoretical analysis provided electronic pattern insights. The absorption behavior and the trans-to-cis conversion were examined from both experimental and theoretical perspectives. The effect of solvent polarity and the role of pH on the photophysical properties were explored. The solvent polarity strongly affects the absorbance spectrum of TA-OH, therefore potentially making NLO active. Additionally, TA-OH exhibited pH responsiveness akin to classic dichromatic pH indicators, with a noticeable color shift from red to blue observed as pH transitioned from neutral to alkaline. Absorbance titration experiments, along with experimental/theoretical determination of pKa, defined the pH sensing ability.

Facile Synthesis of Nitrogen Cation-Doped Polycyclic Aromatic Hydrocarbons by Anodic Intramolecular Pyridination

The bottom-up synthesis of heteroatom-doped polycyclic aromatic hydrocarbons (PAHs) has attracted attention in the development of models of doped graphene. Among PAHs, nitrogen-cation (N +)-doped PAHs are known to exhibit superior optoelectronic functionalities; however, practical methods for synthesizing N +-doped PAHs are limited and challenging [1]. In our previous study, the SNAr reaction was used for the intramolecular cyclization of phenylpyridine derivatives possessing an electron-deficient perfluoroaryl moiety to give the corresponding N +-doped triphenylene in high yields under moderate conditions without the use of a transition-metal catalyst [2,3]. However, one of the challenges in the synthesis of N +-doped PAHs via the SNAr reaction is poor functional-group tolerance.Here, a facile and selective synthesis method for N +-doped PAHs has been established via anodic intramolecular cyclization, inspired by the Yoshida’s protocol for C–H amination without a transition-metal catalyst or an oxidant [4]. The reaction mechanism, in particular the reaction selectivity, was elucidated using density functional theory (DFT) calculations. The N +-doped PAH products were found to have a donor–acceptor structure composed of an arene moiety and a pyridinium moiety, as evidenced by DFT calculations, UV–vis analysis, and electrochemical measurements. The proposed intramolecular anodic pyridination is a practical strategy for the late-stage introduction of N+ into π-electron systems and broadens the scope of molecular design of N +-doped PAHs.

Nonlinear and Emissive {[MIII(CN)6]3-···Polyresorcinol} (M = Fe, Co, Cr) Cocrystals Exhibiting an Ultralow Frequency Raman Response.

Optically active functional noncentrosymmetric architectures might be achieved through the combination of molecules with inscribed optical responses and species of dedicated tectonic character. Herein, we present the new series of noncentrosymmetric cocrystal salt solvates (PPh4)3[M(CN)6](L)n·msolv (M = Cr(III), Fe(III), Co(III); L = polyresorcinol coformers, multiple hydrogen bond donors: 3,3',5,5'-tetrahydroxy-1,19-biphenyl, DiR, n = 2, or 5'-(3,5-dihydroxyphenyl)-3,3″,5,5″-tetrahydroxy-1,19:3',1″-terphenyl, TriRB, n = 1) denoted as MDiR and MTriRB, respectively. The hydrogen-bonded subnetworks {[M(CN)6]3-;Ln}∞ of dmp, neb, or dia topology are formed through structural matching between building blocks within supramolecular cis-bis(chelate)-like {[M(CN)6]3-;(H2L)2(HL)2} or tris(chelate)-like {[M(CN)6]3-;(H2L)3} fragments. The quantum-chemical analysis demonstrates the mixed electrostatic and covalent character of these interactions, with their strength clearly enhanced due to the negative charge of the hydrogen bond acceptor metal complex. The corresponding interaction energy is also dependent on the geometry of the contact and size matching of its components, rotational degree of freedom and extent of the π-electron system of the coformer, and overall fit to the molecular surroundings. Symmetry of the crystal lattices is correlated with the local symmetry of coformers and {complex;(coformer)n} hydrogen-bonded motifs characterized by the absence of the inversion center and mirror plane. All compounds reveal second-harmonic generation activity and photoluminescence diversified by individual UV-vis spectral characteristics of the components, and interesting low-frequency Raman scattering spectra within the subterahertz spectroscopic domain. Vibrational (infrared/Raman), UV-vis electronic absorption (experimental and calculated), and 57Fe Mössbauer spectra together with electrospray ionization mass spectrometry (ESI-MS) data are provided for the complete description of our systems.

Photoinduced Electron Transfer in Inclusion Complexes of Carbon Nanohoops.

ConspectusPhotoinduced electron transfer (PET) in carbon materials is a process of great importance in light energy conversion. Carbon materials, such as fullerenes, graphene flakes, carbon nanotubes, and cycloparaphenylenes (CPPs), have unusual electronic properties that make them interesting objects for PET research. These materials can be used as electron-hole transport layers, electrode materials, or passivation additives in photovoltaic devices. Moreover, their appropriate combination opens up new possibilities for constructing photoactive supramolecular systems with efficient charge transfer between the donor and acceptor parts. CPPs build a class of molecules consisting of para-linked phenylene rings. CPPs and their numerous derivatives are appealing building blocks in supramolecular chemistry, acting as suitable concave receptors with strong host-guest interactions for the convex surfaces of fullerenes. Efficient PET in donor-acceptor systems can be observed when charge separation occurs faster than charge recombination. This Account focuses on selected inclusion complexes of carbon nanohoops studied by our group. We modeled charge separation and charge recombination in both previously synthesized and computationally designed complexes to identify how various modifications of host and guest molecules affect the PET efficiency in these systems. A consistent computational protocol we used includes a time-dependent density-functional theory (TD-DFT) formalism with the Tamm-Dancoff approximation (TDA) and CAM-B3LYP functional to carry out excited state calculations and the nonadiabatic electron transfer theory to estimate electron-transfer rates. We show how the photophysical properties of carbon nanohoops can be modified by incorporating additional π-conjugated fragments and antiaromatic units, multiple fluorine substitutions, and extending the overall π-electron system. Incorporating π-conjugated groups or linkers is accompanied by the appearance of new charge transfer states. Perfluorination of the nanohoops radically changes their role in charge separation from an electron donor to an electron acceptor. Vacancy defects in π-extended nanohoops are shown to hinder PET between host and guest molecules, while large fully conjugated π-systems improve the electron-donor properties of nanohoops. We also highlight the role of antiaromatic structural units in tuning the electronic properties of nanohoops. Depending on the aromaticity degree of monomeric units in nanohoops, the direction of electron transfer in their complexes with C60 fullerene can be altered. Nanohoops with aromatic units usually act as electron donors, while those with antiaromatic monomers serve as electron acceptors. Finally, we discuss why charged fullerenes are better electron acceptors than neutral C60 and how the charge location allows for the design of more efficient donor-acceptor systems with an unusual hypsochromic shift of the charge transfer band in polar solvents.

An ultra-stable and sensitive Ag-HP-β-CD/GO/NCF SERS substrate combine with the coffee-ring effect strategy for trace PAHs detection

This study proposes an ultra-stable and highly sensitive SERS substrate based on Ag nanoparticles/graphene oxide nanosheets hybrids decorated nanocellulose fiber (Ag/GO/NCF), which was further functionalized with hydroxypropyl-β-cyclodextrin (Ag-HP-β-CD/GO/NCF) for the trace detection of polycyclic aromatic hydrocarbons (PAHs). The Ag/GO hybrid structure enables the substrate to exhibit high SERS activity through the synergistic effect of electromagnetic and chemical enhancement. Simultaneously, the three-dimensional skeleton structure of NCF serves as a support matrix, ensuring the exceptional stability of the substrate. Furthermore, this study introduces a SERS measurement strategy utilizing the coffee-ring effect and explores parameters optimization to further enhance the detection sensitivity while maintaining the homogeneity and reproducibility. With the affinity effect of HP-β-CD and the enrichment effect of the delocalized π-electron system in GO, the hydrophobic PAHs could easily approach the SERS hotspots and achieve their trace detection. The limit of detection (LOD) for four representative PAHs, including pyrene, fluoranthene, phenanthrene and fluorene, are found to be 3.32 ug/L, 3.65 ug/L, 40.97 ug/L and 56.24 ug/L, respectively. These results highlight the potential application of this highly efficient SERS platform in environmental monitoring.

High-performance flexible organic gas sensor via alkyl side chain engineering of polyalkylthiophene

Poly(3-alkylthiophene) (P3AT) is a representative p-type organic semiconductor with a distinctive molecular structure that incorporates a π-electron system, promoting charge carrier mobility and providing exceptional electrical conductivity. The alkyl side chain linked to the π-electron backbone was found to have a significant effect on charge carrier transport and mechanical properties. This study presents a comprehensive comparison of the gas-sensing performances of various gases (NO2, SO2, and CO2) and the mechanical properties of gas sensors fabricated from P3AT with alkyl chain lengths of 6, 8, 10, and 12 (P3HT, P3OT, P3DT, and P3DDT). Our investigations revealed that as the alkyl side chain length increased, the gas sensor performance significantly improved owing to the increase in free volume, concurrently promoting improved flexibility. The P3DDT sensor exhibited a sensitivity approximately twice that of the P3HT sensor, along with superior mechanical flexibility owing to its free volume.

Theoretical Insight into the Effect of Phosphorus Oxygenation on Nonradiative Decays: Comparative Analysis of P-Bridged Stilbene Analogs.

Incorporation of the phosphorus element into a π-conjugated skeleton offers valuable prospects for adjusting the electronic structure of the resulting functional π-electron systems. Trivalent phosphorus has the potential to decrease the LUMO level through σ*-π* interaction, which is further enhanced by its oxygenation to the pentavalent P center. This study shows that utilizing our computational analysis to examine excited-state dynamics based on radiative/nonradiative rate constants and fluorescence quantum yield (ΦF) is effective for analyzing the photophysical properties of P-containing organic dyes. We theoretically investigate how the trivalent phosphanyl group and pentavalent phosphine oxide moieties affect radiative and nonradiative decay processes. We evaluate four variations of P-bridged stilbene analogs. Our analysis reveals that the primary decay pathway for photoexcited bis-phosphanyl-bridged stilbene is the intersystem crossing (ISC) to the triplet state and nonradiative. The oxidation of the phosphine moiety, however, suppresses the ISC due to the relative destabilization of the triplet states. The calculated rate constants match an increase in experimental ΦF from 0.07 to 0.98, as simulated from 0.23 to 0.94. The reduced HOMO-LUMO gap supports a red shift in the fluorescence spectra relative to the phosphine analog. The thiophene-fused variant with the nonoxidized trivalent P center exhibits intense emission with a high ΦF, 0.95. Our prediction indicates that the ISC transfer is obstructed owing to the relatively destabilized triplet state induced by the thiophene substitution. Conversely, the thiophene-fused analog with the phosphine oxide moieties triggers a high-rate internal conversion mediated by conical intersection, leading to a decreased ΦF.

Spectro-luminescent characterization of molecular complex based on tryptanthrin for design of new effective drugs

Optical absorption (at room temperature), fluorescence (at room temperature and T = 78 K) and phosphorescence (at T = 78 K) spectra of solution of the molecular complex based on tryptanthrin with platinum chloride and DMSO [Pt(Try)(DMSO)Cl2] (Try-Pt) was investigated. The positions of the first excited singlet (S1) and triplet (T1) energy levels were obtained. It was shown that in the excited state intramolecular singlet excitations energy transfer inside tryptanthrin π-electron system between two centers of optical absorption (that absorb equally) exists. It was found out the optical response of toxic influence of platinum chloride on tryptanthrin π-electron system inside this molecular complex as it is shown by us for a number of polynucleotides. Contrary to singlet energy transfer no intramolecular triplet electronic excitations energy transfer inside tryptanthrin π-electron system between both optical centers is observed. Taking into account the facts that such complexes can easy bind to telomeric G4 DNA the studied phenomena can be used for the design of new effective antitumor and anti-inflammatory drugs.

Asymmetrically Functionalized Electron-Deficient π-Conjugated System for Printed Single-Crystalline Organic Electronics.

Large-area single-crystalline thin films of n-type organic semiconductors (OSCs) fabricated via solution-processed techniques are urgently demanded for high-end electronics. However, the lack of molecular designs that concomitantly offer excellent charge-carrier transport, solution-processability, and chemical/thermal robustness for n-type OSCs limits the understanding of fundamental charge-transport properties and impedes the realization of large-area electronics. The benzo[de]isoquinolino[1,8-gh]quinolinetetracarboxylic diimide (BQQDI) π-electron system with phenethyl substituents (PhC2 -BQQDI) demonstrates high electron mobility and robustness but its strong aggregation results in unsatisfactory solubility and solution-processability. In this work, an asymmetric molecular design approach is reported that harnesses the favorable charge transport of PhC2 -BQQDI, while introducing alkyl chains to improve the solubility and solution-processability. An effective synthetic strategy is developed to obtain the target asymmetric BQQDI (PhC2 -BQQDI-Cn ). Interestingly, linear alkyl chains of PhC2 -BQQDI-Cn (n = 5-7) exhibit an unusual molecular mimicry geometry with a gauche conformation and resilience to dynamic disorders. Asymmetric PhC2 -BQQDI-C5 demonstrates excellent electron mobility and centimeter-scale continuous single-crystalline thin films, which are two orders of magnitude larger than that of PhC2 -BQQDI, allowing for the investigation ofelectron transport anisotropy and applicable electronics.

Nucleophile-Triggered π-Topological Transformation: A New Synthetic Approach to Near-Infrared-Emissive Rhodamines.

We describe a π-topological transformation-based synthetic method for the preparation of a new type of near-infrared (NIR)-emissive rhodamine dye called Polymethine-embedded Rhodamine Fluorophore (PeR Fluor). In contrast to conventional NIR-emissive dyes that require tedious synthetic steps and/or a high cost, linear fully π-conjugated PeR Fluor can be regioselectively prepared in one step by mixing different nucleophiles with ABPXs, a family of rhodamines with a cross-conjugated structure. PeR Fluor exhibits bright NIR fluorescence emission and high photostability owing to the cooperative π-electron system of rhodamines and polymethine scaffolds. Large bathochromic shifts of the absorption and fluorescence emission maxima can be achieved by modifying the N-substituted group to obtain NIR-absorbing/emitting PeR Fluor. We also demonstrate the stimulus-responsive functionality of PeR Fluor through the addition of chemicals (acid/base), which shows switchable NIR and visible fluorescence response. Our π-topological transformation-based synthetic method is a promising approach to produce new functionalized rhodamine dyes.

Formaldehyde interactions with oxygen- and nitrogen-functionalized carbonaceous surfaces in the presence of moisture: Computational approach vs experimental results

Theoretical studies of physical adsorption of formaldehyde and water (through a hydrogen bond formation), as well as chemisorption of formaldehyde (through a covalent CC bond formation) on the functionalized graphene surfaces were carried out. Various functional groups that are likely to exist at graphene surface edges, including hydroxyl, amine, carboxyl, amide, pyrrole, pyridine, lactam and imide groups were considered as interacting with HCHO. In the case of physical adsorption, hydroxyl, amine and amide groups showed the highest affinity towards formaldehyde, and lactam, carboxyl and hydroxyl groups towards water with stabilization energies in the range of 7.8–8.2 kcal mol−1 and 8.2–9.9 kcal mol−1 for formaldehyde and water, respectively. This effect was magnified when additional hydroxyl group(s), capable of forming hydrogen bonds with a specific target group, were present in its vicinity. For more than 50% configurations, physical adsorption favors water molecules. However, chemisorption of formaldehyde also occurred on surface functionalized with hydroxyl groups at room temperature since the electron rich graphene π-electron system initiated CHCHO-Cgraphene bond formation. This process requires overstepping the energy barrier of merely ca. 2.5 kcal mol−1 which means that it easily occurs at room temperature without an external catalyst. The opposite (desorption) reaction could not proceed as the energy barrier exceeded 30 kcal mol−1. Similar reaction, less energetically favorable though, was shown to take place in the case of amine functionality.