Evidence for side-chain π-delocalization in a planar substituted benzene: an experimental and theoretical charge density study on 2,5-dimethoxybenzaldehyde thiosemicarbazone

The charge density in 2,5-dimethoxybenzaldehyde thiosemicarbazone (1) has been studied experimentally using Mo-K(α) X-ray diffraction at 100 K, and by theory using DFT calculations at the B3LYP/6-311++G(2d,2p) level. The quantum theory of atoms in molecules (QTAIM) was used to investigate the extent of π-delocalization in the thioamide side-chain, which is virtually coplanar with the benzene ring. The experimental and theoretical ellipticity profiles along the bond paths were in excellent agreement, and showed that some of the formal single bonds in the side-chain have significant π-bond character. This view was supported by the magnitudes of the topological bond orders and by the delocalization indices δ(Ω(A), Ω(B)). An orbital decomposition of δ(Ω(A), Ω(B)) demonstrated that there was significant π-character in all the interchain non-H chemical bonds. On the other hand, the source function referenced at the interchain bond critical points could not provide any evidence for π-delocalization, showing instead only limited σ-delocalization between nearest neighbors. Overall, the topological evidence and the atomic graphs of the oxygen atoms did not provide convincing evidence for π-delocalization involving the methoxy substituents.     

Theoretical design of the biradical character in 1,3-diphosphacyclobutanediyl and homologous structures

The electronic nature of 1,3-diphosphacyclobutane-2,4-diyl is explored with wavefunction based and density functional methods. According to MCSCF calculations the singlet state of the title compound is a biradicaloid with closed shell character, the number of unpaired electrons, assigned upon the analysis of the natural orbitals, is close to one. The participation of closed shell contributions in the overall wavefunction arises from a strong mixing of canonical structures, which emphasizes (a) the phosphorane type of bonding as well as (b) π-delocalization within the ring system. The bonding situation changes when σ-attracting substituents, e.g. amino groups, are attached to the phosphorus atoms. They inhibit possible cyclic π-delocalization and enhance the biradical character within the ring system.     

A Thiophene Backbone Enables Two-Dimensional Poly(arylene vinylene)s with High Charge Carrier Mobility

Linear conjugated polymers have attracted significant attention in organic electronics in recent decades. However, despite intrachain π-delocalization, interchain hopping is their transport bottleneck. In contrast, two-dimensional (2D) conjugated polymers, as represented by 2D π-conjugated covalent organic frameworks (2D c-COFs), can provide multiple conjugated strands to enhance the delocalization of charge carriers in space. Herein, we demonstrate the first example of thiophene-based 2D poly(arylene vinylene)s (PAVs, 2DPAV-BDT-BT and 2DPAV-BDT-BP , BDT=benzodithiophene, BT=bithiophene, BP=biphenyl) via Knoevenagel polycondensation. Compared with 2DPAV-BDT-BP , the fully thiophene-based 2DPAV - BDT - BT exhibits enhanced planarity and π-delocalization with a small band gap (1.62 eV) and large electronic band dispersion, as revealed by the optical absorption and density functional calculations. Remarkably, temperature-dependent terahertz spectroscopy discloses a unique band-like transport and outstanding room-temperature charge mobility for 2DPAV-BDT-BT (65 cm² V⁻¹ s⁻¹), which far exceeds that of the linear PAVs, 2DPAV-BDT-BP , and the reported 2D c-COFs in the powder form. This work highlights the great potential of thiophene-based 2D PAVs as candidates for high-performance opto-electronics.     

Circularly Polarized Luminescence from Cyclic (Alkyl)(Amino) Carbene Derived Propellers

Organic circularly polarized luminescence (CPL)-active molecular emitters featuring dynamic propeller-like luminophores were prepared in one step from cyclic(alkyl)(amino) carbenes (CAACs). These molecules exhibit through-space arene-arene π-delocalization and rapid intramolecular inter-system crossing (ISC) in line with their helical character.     

Delocalization in Substituted Benzene Dications: A Magnetic Point of View

In this work, the induced magnetic field is analyzed for a series of substituted benzenes dications with formula C₆R₆²⁺ (R=I, At, SeH, SeCH₃, TeH, TeCH₃), presumably exhibiting concentric aromaticity. Previous studies concluded that in the carbon skeleton, just π-electrons are delocalized. However, our results support that both the σ- and π-electrons are delocalized in the carbon skeleton, combined with a σ-delocalization in the external ring. The role of the relativistic effects in these dications is discussed in detail.     

Proton Magnetic Resonance Studies of Heterocyclic Aromatic Amine Adducts of Nickel(II) and Cobalt(II) Acetylacetonate

The proton nmr isotropic shifts of pyridine type ligands coordinated to paramagnetic nickel(II) and cobalt(II) acetylacetonate are reported, and the mechanisms of unpaired electron spin delocalization in these complexes are discussed. It is found that a σ-delocalization mechanism is in dominant, but the π-contribution can not be rule out. The calculations of the geometric factor for Co(II) complexes are done. It is used in the ratio method to separate the contribution of pseudo-contact shift from isotropic shift for Co(II) complexes. The effect of pseudoaromatic chelate ring on contact shift is not so large as previously reported.     

Carbon-13 NMR studies of oxo-centered trinuclear chromium(III) complexes of the general formula [Cr3O(O2CR)6(L)3]+ (R = Me,Ph; L = H2O,py)

¹³C NMR resonances of a series of oxo-centered trinuclear chromium complexes have been observed and assigned. A π-delocalization mechanism dominates where ligands contain aromatic substituents or bridging carboxylate functionalities, consistent with the presence of a metal center where to first approximation unpaired electrons reside solely in t_2g. orbitals. A contrast is made between the delocalization mechanism in these complexes and those of their isostructural Mn and Fe analogues.     

Substituent Effect on the Electronic Structure of Sulfines: Molecular Orbital Treatment

The electronic structure and ground state properties of the gaseous sulfine H₂CSO and some of its derivatives were studied at the RHF, MP2, and B3LYP levels. The calculations showed that the Z-isomer is more stable than the E-one and their difference in energy depends on their level of calculation, basis set, and substituent. The factors affecting the isomer stability are the electrostatic interactions, the steric factor, and π-electrons delocalization. The substituent has little effect on the geometry of the CSO moiety but greatly affects its charge distribution and polarizability. The ΔE value of the E- to Z-isomerization process via rotation of the S═O bond was calculated. The substituent has an appreciable effect on both the geometry and energy barrier of isomerization depending on its electronegativity, electrostatic attraction, and effect on π-delocalization over the molecule.     

Bonding situations in tricoordinated beryllium phenyl complexes

The bonding situation in the tricoordinated beryllium phenyl complexes [BePh₃]⁻, [(pyridine)BePh₂] and [(trimethylsilyl-N-heterocyclic imine)BePh₂] is investigated experimentally and computationally. Comparison of the NMR spectroscopic properties of these complexes and of their structural parameters, which were determined by single crystal X-ray diffraction experiments, indicates the presence of π-interactions. Topology analysis of the electron density reveals elliptical electron density distributions at the bond critical points and the double bond character of the beryllium-element bonds is verified by energy decomposition analysis with the combination of natural orbital for chemical valence. The present beryllium-element bonds are highly polarized and the ligands around the central atom have a strong influence on the degree of π-delocalization. These results are compared to related triarylboranes.     

Electron-Deficient Polycyclic Molecules via Ring Fusion for n-Type Organic Electrochemical Transistors

The primary challenge for n-type small-molecule organic electrochemical transistors (OECTs) is to improve their electron mobilities and thus the key figure of merit μC*. Nevertheless, few reports in OECTs have specially proposed to address this issue. Herein, we report a 10-ring-fused polycyclic π-system consisting of the core of naphthalene bis-isatin dimer and the terminal moieties of rhodanine, which features intramolecular noncovalent interactions, high π-delocalization and strong electron-deficient characteristics. We find that this extended π-conjugated system using the ring fusion strategy displays improved electron mobilities up to 0.043 cm² V⁻¹ s⁻¹ compared to our previously reported small molecule gNR, and thereby leads to a remarkable μC* of 10.3 F cm⁻¹ V⁻¹ s⁻¹ in n-type OECTs, which is the highest value reported to date for small-molecule OECTs. This work highlights the importance of π-conjugation extension in polycyclic-fused molecules for enhancing the performance of n-type small-molecule OECTs.     

Study of electron spin crossover in iron(III) tris-dithiocarbamate complexes by carbon-13 NMR spectroscopy

¹³C and ¹H isotropic shifts have been measured for a series of Fe(III) tris-dithiocarbamate complexes. The ¹³C isotropic shifts may be interpreted as arising solely from contact hyperfine coupling and demonstrate that as the low-spin state of the metal is favoured there is an increase in metal-ligand π-bonding. σ-delocalization of unpaired spin density is more important in determining the ¹³C isotropic shifts than those of the contiguous proton.     

Chromophores in conjugated polymers--all straight?

What is the physical shape of the π-electron system in a large macromolecule such as a conjugated polymer? Although intuitively one may argue that any departure from rigidity by bending or twisting should disrupt conjugation, leading to the formation of discrete chromophores, single-molecule and ensemble time-resolved studies support the notion that the π-bond is remarkably persistent in space: even individual chromophores can be bent and twisted, so that caution is warranted when interpreting a wide range of polarisation-based spectroscopies.     

Synthesis of Robust MOFs@COFs Porous Hybrid Materials via an Aza-Diels–Alder Reaction: Towards High-Performance Supercapacitor Materials

Metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) have attracted enormous attention in recent years. Recently, MOF@COF are emerging as hybrid architectures combining the unique features of the individual components to enable the generation of materials displaying novel physicochemical properties. Herein we report an unprecedented use of aza-Diels–Alder cycloaddition reaction as post-synthetic modification of MOF@COF-LZU1, to generate aza-MOFs@COFs hybrid porous materials with extended π-delocalization. A a proof-of-concept, the obtained aza-MOFs@COFs is used as electrode in supercapacitors displaying specific capacitance of 20.35 μF cm⁻² and high volumetric energy density of 1.16 F cm⁻³. Our approach of post-synthetic modification of MOFs@COFs hybrids implement rational design for the synthesis of functional porous materials and expands the plethora of promising application of MOFs@COFs hybrid porous materials in energy storage applications.     

Electronic structures, intramolecular interactions, and aromaticity of substituted 1-(2-iminoethylidene) silan amine: a density functional study

The intramolecular hydrogen bond, molecular structure, π electrons delocalization, and vibrational frequencies in 1-(2-iminoethylidene) silan amine and its derivatives have been investigated by means of density functional method with 6-311++G** basis set, in gas phase, water, and carbon tetrachloride solutions. The obtained results showed that the hydrogen bond strength is mainly governed by resonance variations inside the chelate ring induced by the substituent groups. Furthermore, the topological properties of the electron density distributions for N–H···N intramolecular hydrogen bond were analyzed in terms of the Bader's theory of atoms in molecules. On the other hand, the aromaticity of the ring formed is measured using several well-established indices of aromaticity such as nucleus-independent chemical shift, harmonic oscillator models of the aromaticity, para-delocalization index, average two-center indices, aromatic fluctuation index, and π-fluctuation aromatic index. Natural population analysis data, the electron density and Laplacian properties, as well as γ(NH) and ν(NH) were further used for estimation of the hydrogen bonding interactions and the forces driving their formation.     

Structural studies of tetrazoles. Crystal and molecular structure and ab initio calculations of 1-phenyl-1H-tetrazole-5-thiolate,as its [diaqua(18-crown-6)sodium] salt: An anionic tetrazole free of direct metal interactions

The structure of the anionic heterocycle obtained by deprotonation of 1-phenyl-1H-tetrazole-5-thione was determined by X-ray diffractometry. The counterion is [Na(18-crown-6)(H₂O)₂]⁺. There are no direct interactions between the metal and the tetrazole, while hydrogen bonds between sodium ion coordinated water molecules and neighboring azole anions give cohesion to the crystal. The thiotetrazolato ion displays extensive π-delocalization, with all members of the ring and the exocyclic sulfur contributing to it; however, no interannular conjugation between the phenyl and azole rings is found. The structure of the heterocycle is best described as a resonance hybrid. The geometric parameters of the metal-free tetrazole could be used as the input of ab initio MO calculations at the 6-31G*//3-21G* level, which supported the proposal that the negative charge is mainly concentrated on the N₄-C₅-S portion of the tetrazolate. Some small but significant and consistent bond length differences are found in the uncoordinated tetrazole with respect to metal-bonded analogues. The coordination behavior of the crown ether, with five oxygens bonded to sodium in the complex cation, is rare. © 1997 John Wiley & Sons, Inc.     

8-AminoBODIPYs: cyanines or hemicyanines? The effect of the coplanarity of the amino group on their optical properties

The role of the amino group twisting ability in the BODIPY photophysics for nonsterically hindered and constrained molecular structures was studied. When a coplanar disposition of the amino and the BODIPY core is feasible, a hemicyanine-like delocalized π-system gives rise to novel blue and efficient BODIPY laser dyes. The key role of such rotamer is confirmed by newly synthesized derivatives where the amino and the BODIPY core are electronically decoupled by steric repulsions.     

Furan-Extended Helical Rylenes with Fjord Edge Topology and Tunable Optoelectronic Properties

Lateral furan-expansion of polycyclic aromatics, which enables multiple O-doping and peripheral edge evolution of rylenes, is developed for the first time. Tetrafuranylperylene TPF-4CN and octafuranylquaterrylene OFQ-8CN were prepared as model compounds bearing unique fjord edge topology and helical conformations. Compared to TPF-4CN , the higher congener OFQ-8CN displays a largely red-shifted (≈333 nm) and intensified absorption band (λ_max=829 nm) as well as a narrowed electrochemical band gap (≈1.08 eV) due to its pronounced π-delocalization and emerging of open-shell diradicaloid upon the increase of fjord edge length. Moreover, strong circular dichroism signals in a broad range until 900 nm are observed for open-shell chiral OFQ-8CN , owing to the excellent conformational stability of its central bis(tetraoxa[5]helicene) fragments. Our studies provide insights into the relationships between edge topologies and (chir)optoelectronic properties for this novel type of O-doped PAHs.     

Synthesis and spectral properties of N4, N3S, and N2S2 porphyrins containing one, two, three, and four meso-furyl groups

Porphyrins with N₄, N₃S, and N₂S₂ cores having one, two, three, and four furyl groups at the meso-positions were synthesized by following various methodologies and characterized by using mass spectrometry, NMR spectroscopy, elemental analysis, absorption, and fluorescence spectroscopic techniques. NMR studies indicated that by replacing the meso-aryl groups with meso-furyl groups, the β-pyrrole and β-thiophene protons of porphyrins experienced considerable downfield shifts, supporting the alteration of π-delocalization of porphyrins on the introduction of meso-furyl groups. The absorption and emission bands of porphyrins experienced red shifts on the introduction of meso-furyl groups and the magnitude of red shifts vary linearly with the number of meso-furyl groups. Thus, the spectral studies supported a systematic alteration in spectral properties on successive introduction of meso-furyl groups.     

Carbazole-based cyano-stilbene highly fluorescent microcrystals

Aggregation-induced enhanced emission (AIEE) is reported for 1-cyano-trans-1,2-bis-(4-carbazolyl)phenylethylene (CN-CPE). The weak luminescence of dilute CN-CPE solutions is enhanced upon aggregate formation into 2-3 μm sized crystals. In contrast to general observations, crystal formation of CN-CPE causes a blue-shift in emission and enhances the intensity. X-ray crystallographic analysis revealed that key factors causing high luminescence efficiency in the crystal are a lack of strong cofacial π-π alignment and the existence of the strong supramolecular interactions due to the intermolecular H-bonding. These factors seem to be responsible for the AIEE phenomenon as molecules of CN-CPE are held in a rigid twisted conformation, thereby increasing the fluorescence intensity in the solid or aggregated states. Accordingly, conformational twisting in the crystal packing process may be responsible for the unusual emission blue-shift in the aggregate.     

Experimental charge density analysis of a gallium(I) N-heterocyclic carbene analogue

The experimental electron density of the only known example of a four-membered Ga(I) N-heterocyclic carbene analogue has been determined by multipole modeling of 90 K X-ray diffraction data and compared to theoretical data. In order to obtain a satisfactory model, it is necessary to modify the radial dependency of the core electrons of Ga using two separate scaling parameters for s,p- and d-electrons. Evidence for significant lone-pair density on Ga is found in the electron density and derived properties despite the partial positive charge of this atom. Static deformation density and molecular electrostatic potential clearly show a directional lone pair on Ga, whereas the Laplacian of the total electron density does not; this feature is, however, present in the Laplacian of the valence-only density. The Ga center also acts as an acceptor in four intramolecular C-H···Ga contacts, whose nature is probed by density properties. Substantial covalent character is apparent in the Ga-N bonds, but no sign of donation from filled N p-orbitals to empty Ga p-orbitals is found, whereas π-delocalization over the organic ligand is evident. This study highlights the utility of experimental charge density analysis as a technique to investigate the unusual bonding and electronic characteristics of low oxidation state/low coordinate p-block complexes.