Through bond and through space interactions in dehydro-diazine radicals: a case study of 3c-5e interactions

Utilization of the through-space effect to design donor-acceptor systems of pyrrole, indole, isoindole, azulene and aniline

Molecular electrostatic potential (MESP) topology analysis reveals the underlying phenomenon of the through-space effect (TSE), which imparts electron donor-acceptor properties to a wide range of chemical systems, including derivatives of pyrrole, indole, isoindole, azulene, and aniline. The TSE is inherent in pyrrole owing to the strong polarization of electron density (PoED) from the formally positively charged N-center to the C3C4 bonding region. The N → C3C4 directional nature of the TSE has been effectively employed to design molecules with high electronic polarization, such as bipyrroles, polypyrroles, phenyl pyrroles, multi-pyrrolyl systems and N-doped nanographenes. In core-expanded structures, the direction of electron flow from pyrrole units towards the core leads to highly electron-rich systems, while the opposite arrangement results in highly electron-deficient systems. Similarly, the MESP analysis reveals the presence of the TSE in azulene, indole, isoindole, and aniline. Oligomeric chains of these systems are designed in such a way that the direction of electron flow is consistent across each monomer, leading to substantial electronic polarization between the first and last monomer units. Notably, these designed systems exhibit strong donor-acceptor characteristics despite the absence of explicit donor and acceptor moieties, which is supported by FMO analysis, APT charge analysis, NMR data and λmax data. Among the systems studied, the TSEs of many experimentally known systems (bipyrroles, phenyl pyrroles, hexapyrrolylbenzene, octapyrrolylnaphthalene, decapyrrolylcorannulene, polyindoles, polyazulenes, etc.) are unraveled for the first time, while numerous new systems (polypyrroles, polyisoindoles, and amino-substituted benzene polymers) are predicted to be promising materials for the creation of donor-acceptor systems. These findings demonstrate the potential of the TSE in molecular design and provide new avenues for creating functional materials.

Conjugation-Induced Spin Delocalization in Helical Chiral Carbon Radicals via Through-Bond and Through-Space Effects

A class of highly stable hydrocarbon radicals with helical chirality are synthesized, which can be isolated and purified by routine column chromatography on silica gel. These carbon-centered radicals are stabilized by through-bond delocalization and intramolecular through-space conjugation, which is evidenced by Density Functional Theory (DFT) calculation. The high stability enables to directly modify the carbon radical via palladium-catalyzed cross-coupling with the radical being untapped. The structures and optoelectronic properties are investigated with a variety of experimental methods, including Electron Paramagnetic Resonance (EPR), Ultraviolet Visisble Near Infrared (UV-vis-NIR) measurements, Cyclic Voltammetry (CV), Thermogravimetry Analysis (TGA), Circular Dichroism (CD) spectra, High-Performance Liquid Chromatography (HPLC), and X-ray crystallographic analysis. DFT calculations indicated that the 9-anthryl helical radical is more stable than its tail-to-tail σ-dimer over 13.2 kJ mol-1 , which is consistent with experimental observations.

Photochemistry of 3,6-Didehydropyridazine Biradical─An Untraceable Para Benzyne Analogue

We report matrix isolation infrared spectroscopic studies to characterize 3,6-didehydropyridazine 6, a heterocyclic analogue of para benzyne, combined with computations. In this regard, we have utilized 3,6-diiodopyridazine 11 as a photolytic precursor. The experiments toward the generation of the biradical are carried out in argon and nitrogen matrices at 4 K. Instead of the elusive biradical, we have observed a ring-opening product maleonitrile (Z)-7 upon irradiation at 254 nm. In contrast, prolonged irradiation at 254 nm leads only to Z-E isomerization, forming fumaronitrile (E)-7. The mechanistic aspects of ring-opening, product selectivity, and Z-E photoisomerization steps have been investigated in detail using high-level ab initio computations. These studies have found that 3,6-didehydropyridazine 6 is an untraceable intermediate, and the ring-opening step leading to maleonitrile is barrierless. In addition, we have proposed the involvement of the S₁ (π-π*) state via conical intersection in the Z-E photoisomerization of maleonitrile.

Instant Detection of Hydrogen Cyanide Gas and Cyanide Salts in Solid Matrices and Water by using CuII and NiII Complexes of Intramolecularly Hydrogen Bonded Zwitterions

A series of intramolecularly hydrogen-bonded zwitterionic compartmental ligands HL1-HL4, containing a pendent diamine arm that is monoprotonated and an aldehyde functionality at two different ortho-positions of a 4-halophenoxide, is reported herein. Single-crystal X-ray diffraction (SXRD) provides persuasive evidence for the identification of this class of proton-transferred zwitterions at room temperature. The solid-state photoluminescent nature of these zwitterions remains intact in aqueous and organic solutions. Grinding of HL1 and HL2 with Cu²⁺ /Ni²⁺ salts develop turn-on probes 1-4. Compounds 1 and 4 are dinuclear Cu^II and Ni^II species, respectively. Compound 2 is a tetranuclear Cu^II complex. Interestingly, compound 3 is a mononuclear Ni^II species in which both nitrogen atoms in the pendant diamine arm are protonated and, therefore, not coordinated to the Ni^II center. All these probes (1-4) display an instant response to the poison gas hydrogen cyanide (HCN) and cyanide salts present in both solid matrices and aqueous (100 % water) solution. Selective and rapid sensing of HCN gas and cyanide salts in solid/soil/water phases, without any interference, by the mechanosynthesized complexes 1-4 can be perceived easily by the naked eye under a hand-held UV lamp.

Deciphering Stability of Five-Membered Heterocyclic Radicals: Balancing Act Between Delocalization and Ring Strain

Computational studies on five-membered heterocycles with single heteroatom and their isomeric dehydro-borole 1a-1c, cyclopentadiene 2a-2c, pyrrole 3a-3c, furan 4b-4c, phosphole 5a-5c, and thiophene 6b-6c radicals have been carried out. Geometrical aspects through ground state electronic structures and stability aspects using bond dissociation energies (BDE) and radical stabilization energies (RSE) have been envisaged in this regard. Spin densities, electrostatic potentials (ESP), and natural bond orbital (NBO) analysis unveiled the extent of spin delocalization. The estimated nucleus-independent chemical shifts (NICS) values revealed the difference in aromaticity characteristics of radicals. Particularly the heteroatom centered radicals exhibit odd electron π-delocalized systems with a quasi-antiaromatic character. Various factors such as, the relative position of the radical center with respect to heteroatoms, resonance, ring strain and orbital interactions influence the stability that follows the order: heteroatom centered > β-centered > α-centered radicals. Among the influences of various factors, we confirmed the existence of a competition between delocalization and the ring strain, and the interplay of both decides the overall stability order.