Neutral Radical and Singlet Biradical Forms of Meso-Free, -Keto, and -Diketo Hexaphyrins(1.1.1.1.1.1): Effects on Aromaticity and Photophysical Properties

Hydrogen-Bonding-Assisted Substituent Engineering for Modulating Magnetic Spin Couplings and Switching in m-Phenylene Nitroxide Diradicals

Rational modification of the coupler for the theoretical design of molecular magnets has attracted extensive interest. Substituent insertion is a widely used strategy for adjusting molecular properties, but its effect and modulation on magnetic spin couplings have been less investigated. In this work, we predict the magnetic properties of the design m-phenylene nitroxide (NO) diradicals regulated by introducing substituents. The calculated results for those two pairs of diradicals indicate that the signs of their magnetic coupling constants J do not change, but the magnitudes remarkably change after substituent regulation in the range from 253 to 730 cm-1. Such noticeable magnetic changes induced by introducing subsituents are mainly attributed to different electronic effects of substituents, assisted by the proximity of two NO groups, good planarity, conjugation, and an intramolecular hydrogen bond. In particular, the insertion of intramolecular H-bonds not only indicates an electronic effect but also has greatly changed the spin density distribution. Further aromaticity of the coupler ring, spin densities, and molecular orbitals and energetics was evaluated to gain a better understanding of magnetic regulation. Interestingly, further protonation of some substituents (e.g., -NO2 and -CO2) can noticeably turn the spin coupling from ferromagnetic to antiferromagnetic, showing manipulable magnetic switching. This work provides a promising strategy based on substituent engineering for magnetic spin coupling modulation, not only turning the coupling magnitude but also enabling the magnetic switching, thus providing insights into molecular magnetic manipulation for spintronics applications.

One Carbon Ring Expansion of Bipyrrole to Bipyridine Enables Access to a π-Extended, Non-innocent, Corrole-like Ligand

A π-extended, diaza-triphenylene embedded, mono-anionic corrole analogue and its Ni^II complex were synthesized from a diaza-triphenylene precursor, which was obtained from a double one-carbon insertion into a naphthobipyrrole diester. Following conversion to the corresponding activated diol and acid-catalyzed condensation with pyrrole, subsequent reaction with pentafluorobenzaldehyde afforded mono-anionic, π-extended bipyricorrole-like macrocycle. Attempted Ni^II insertion with Ni(OAc)₂  ⋅ 4H₂ O resulted an ESR active, Ni^II bipyricorrole radical complex, which was converted to a stable cationic Ni^II complex upon treatment with [(Et₃ O)⁺ (SbCl₆ )^- ]. Both complexes were characterized by ¹ H and ¹³ C NMR, UV/Vis spectroscopy and single crystal X-ray diffraction analysis. The Ni^II bipyricorrole radical complex is converted to a cationic Ni^II complex by single-electron reduction using cobaltocene. Both the cationic Ni^II complex and the radical Ni^II complex exhibited ligand-centered redox behavior, whereas the Ni^II remains in the +2 oxidation state.

Controlling Diradical Character of Photogenerated Colored Isomers of Phenoxyl-Imidazolyl Radical Complexes

We propose a rational method for evaluating the diradical character of the photochromic phenoxyl-imidazolyl radical complex (PIC) derivatives based on their radical-radical coupling reaction rates. PIC consists of an imidazole ring, a phenoxyl ring, and a bridging unit that structurally connects them. The C-N bond formed between the imidazole and phenoxyl rings can be dissociated photochemically in a homolytic manner. The photochromism of PIC differs significantly from other photochromic molecules in that the transient colored open-ring isomer has a diradical character. The colored open-ring isomer returns promptly to the initial colorless closed-ring isomer by the intramolecular radical recombination reaction. By changing the aromaticity and substitution position of the bridging unit, it is possible to control the degree of contribution of the open-shell diradical and closed-shell quinoidal structures to the open-ring isomer. Systematic investigation of the photochromic reactions of several PIC derivatives revealed that the half-life of the open-ring isomers reflects the diradical character. Thus, the radical recombination reaction rate of the open-ring isomer of the PIC derivatives is an excellent parameter of the diradical character.

Fine-Tuning of Nonlinear Optical Contrasts of Hexaphyrin-Based Molecular Switches Using Inverse Design

In the search for new nonlinear optical (NLO) switching devices, expanded porphyrins have emerged as ideal candidates thanks to their tunable chemical and photophysical properties. Introducing meso-substituents to these macrocycles is a successful strategy to enhance the NLO contrasts. Despite its potential, the influence of meso-substitution on their structural and geometrical properties has been scarcely investigated. In this work, we pursue to grasp the underlying pivotal concepts for the fine-tuning of the NLO contrasts of hexaphyrin-based molecular switches, with a particular focus on the first hyperpolarizability related to the hyper-Rayleigh scattering (β_HRS). Building further on these concepts, we also aim to develop a rational design protocol. Starting from the (un)substituted hexaphyrins with various π-conjugation topologies and redox states, structure-property relationships are established linking aromaticity, photophysical properties and β_HRS responses. Ultimately, inverse molecular design using the best-first search algorithm is applied on the most favorable switches with the aim to further explore the combinatorial chemical compound space of meso-substituted hexaphyrins in search of high-contrast NLO switches. Two definitions of the figure-of-merit of the switch performance were used as target objectives in the optimization problem. Several meso-substitution patterns and their underlying characteristics are identified, uncovering molecular symmetry and the electronic nature of the substituents as the key players for fine-tuning the β_HRS values and NLO contrasts of hexaphyrin-based switches.

Dynamic Spin-Spin Interaction Observed as Interconversion of Chemical Bonds in Stepwise Two-Photon Induced Photochromic Reaction

Biradicaloids in π-conjugated organic molecules have been extensively studied in recent years because of the fundamental insights into the chemical bonds and unique optical, electrical, and magnetic properties. Several studies have reported that the spin-spin interactions of biradicaloids with flexible molecular frameworks dynamically evolve correlating with molecular structural changes. Although these dynamic behaviors will provide important insights into the relationship between molecular structures and spin properties, studies on such behaviors have been limited to two-spin systems. Here, we investigated the stepwise photochromic properties of biphotochromic molecules involving multiple spin interactions by double-pulse laser flash photolysis. The one-photon photochromic reaction generates the o-biradical form as the open-closed form, which thermally isomerizes to the o-quinoidal form and reaches the thermal equilibrium state between them. The additional absorption of a photon by the open-closed form leads to the photochromic reaction of the other photochromic unit, resulting in the generation of unpaired spins at the p-position of the central aromatic bridge of the biradical or quinoidal form. Under the situation, while the interaction between the unpaired spins and the o-biradical preferentially produces the p-quinoidal form in which the antiferromagnetic interaction at the p-position is dominant, that between the spins and the o-quinoidal form kinetically produces the bis(o-quinoidal) form followed by the thermal isomerization to the thermodynamically stable p-quinoidal form. These dynamic spin-spin interactions along with the rearrangement of chemical bonds will give a deeper understanding of the singlet biradicaloids and that to bridge organic multiradicals in molecular systems to cooperative spin behaviors in bulk materials.

A Robust Porphyrin-Stabilized Triplet Carbon Diradical

The synthesis of robust high-spin carbon radicals is an important topic in organic chemistry. Toward this end, several porphyrin-stabilized radicals have been systematically explored. A singly naphthalene-fused porphyrin radical was synthesized by a reaction sequence consisting of a Suzuki-Miyaura coupling of β-borylated porphyrin with 2-bromobenzaldehyde, addition of mesityl Grignard reagent, intramolecular Friedel-Crafts alkylation, and final oxidation with DDQ or tBuOK/O₂ . This strategy was also used to synthesize doubly naphthalene-fused porphyrins and syn- and anti-fused-anthracene-bridged porphyrin dimers. While singly naphthalene-fused porphyrin radical has been shown to be a stable monoradical, doubly naphthalene-fused porphyrins and anti-fused-anthracene-bridged porphyrin dimers have been shown to be closed-shell molecules. Finally, the syn-dimer was characterized as a surprisingly stable radical (t_1/2 =28 days under ambient air and at 80 °C) that is storable for more than several months, despite its high-spin triplet ground-state carbon diradical.

Nucleophilic Aromatic Substitution (SNAr) and Related Reactions of Porphyrinoids: Mechanistic and Regiochemical Aspects

The nucleophilic substitution of aromatic moieties (SNAr) has been known for over 150 years and found wide use for the functionalization of (hetero)aromatic systems. Currently, several "types" of SNAr reactions have been established and notably the area of porphyrinoid macrocycles has seen many uses thereof. Herein, we detail the SNAr reactions of seven types of porphyrinoids with differing number and type of pyrrole units: subporphyrins, norcorroles, corroles, porphyrins, azuliporphyrins, N-confused porphyrins, and phthalocyanines. For each we analyze the substitution dependent upon: a) the type of nucleophile and b) the site of substitution (α, β, or meso). Along with this we evaluate this route as a synthetic strategy for the generation of unsymmetrical porphyrinoids. Distinct trends can be identified for each type of porphyrinoid discussed, regardless of nucleophile. The use of nucleophilic substitution on porphyrinoids is found to often be a cost-effective procedure with the ability to yield complex substituent patterns, which can be conducted in non-anhydrous solvents with easily accessible simple porphyrinoids.

Air-Stable Oxyallyl Patterns and a Switchable N-Heterocyclic Carbene

Oxyallyl derivatives are typically elusive compounds. Even recently reported "stabilized" 1,3-diaminooxyallyl species are still highly reactive and have short lifetimes at room temperature. Herein, we report the synthesis and preliminary study of mesoionic pyrimidine derivatives that feature 1,3-bis(dimethylamino)oxyallyl patterns with an unprecedented level of stabilization. The latter are not only insensitive towards air and moisture, but they are also compatible with the formation of an ancillary stable N-heterocyclic carbene moiety. As the oxyallyl pattern is proton-responsive, it allows the reversible switching of the electronic properties of the carbene, as a ligand.

A Stable All-Thiophene-Based Core-Modified [38]Octaphyrin Diradicaloid: Conformation and Aromaticity Switch at Different Oxidation States

A soluble and stable core-modified [38]octaphyrin, MC-T8, containing eight thiophene rings was synthesized by Yamamoto coupling followed by oxidative dehydrogenation. X-ray crystallographic analysis revealed a nearly planar backbone, and the molecule is globally aromatic with a dominant 38π conjugation pathway. The dication MC-T8²⁺ is antiaromatic, and the backbone is distorted, with a different orientation of the thiophene rings. The tetracation MC-T8⁴⁺ becomes aromatic again, with a shallow-bowl-shaped geometry. Both the neutral compound and the dication demonstrated open-shell diradical character with a small singlet-triplet energy gap (-2.70 kcal mol^-1 for MC-T8 and -3.78 kcal mol^-1 for MC-T8²⁺ ), and they are stable owing to effective spin delocalization.

Propentdyopents: Brief history of a family of dipyrrolic pigments

Propentdyopents are naturally occurring dipyrroles deriving from the metabolism of heme and characterized by a dipyrrin-1,9-dione motif. The unusual name propentdyopent is due to the first colorimetric method (the Stokvis reaction) for the detection of these compounds, which were initially isolated from urine samples. Upon reduction in alkaline solutions, they produced red species that were termed pentdyopents to describe with Greek numerals their absorption maximum (525 nm) in the visible range. The precursors to the red pentdyopents were thus indicated as propentdyopents.Over the course of several decades, these dipyrrolic compounds have appeared in several studies of human physiology, typically associated to conditions of abnormal heme metabolism and/or oxidative stress. Concurrently, synthetic investigations have confirmed their chemical structure, reactivity, and ability to coordinate metals as bidentate monoanionic ligands. Notably, the planar dipyrrindione platform can undergo reversible one-electron redox processes and thereby act as an electron reservoir in metal complexes. In combination with the documented ability of the carbonyl groups to act as hydrogen-bonding acceptors, the coordination chemistry of propentdyopents could lead to new applications for this old class of pigments. Furthermore, the observation of these pigments in several clinical contexts could potentially delineate a role of propentdyopents as diagnostic biomarkers. This mini-review summarizes both the chemistry and biology of propentdyopents while highlighting the ample space for new discoveries.

Unexpected diradical character and large magnetic spin coupling in modified porphyrins induced by inverting pyrrole rings

Porphyrin derivatives with inverted pyrrole rings have been experimentally synthesized, and their relevant electronic and magnetic properties have great application prospects in terms of electronic devices. In this work, we rationally design the structures and computationally investigate the electronic properties of porphine and Mg/Zn-porphyrin derivatives with two inverted pyrrole rings, i.e. the dipyrrole-inverted porphine and Mg/Zn-porphyrin analogues (1NN-2H, 2NN-2H, 1NN-Mg, 2NN-Mg, 1NN-Zn and 2NN-Zn), at the B3LYP/6-311G(d,p) level. The main structural characters of these porphyrin derivatives are that the [double bond splayed left]NH units of two pyrrole rings are inverted outwards and the porphyrin-like macrocycles are distorted from square to diamond shapes. More interestingly, these dipyrrole-inverted porphyrin derivatives present diradical characters with noticeably large antiferromagnetic spin coupling constants, i.e.-982.2/-936.3 cm^-1 for 1NN-2H/2NN-2H, -796.3/-764.2 cm^-1 for 1NN-Mg/2NN-Mg and -1044.5/-1055.2 cm^-1 for 1NN-Zn/2NN-Zn, but their monopyrrole-inverted counterparts do not. Examinations of the orbital properties featuring large occupation numbers of the lowest unoccupied natural orbitals and two singly occupied molecular orbitals that are polarized in opposite directions also confirm these findings. These porphyrin derivatives have small singlet-triplet energy gaps and small energy gaps between the highest occupied molecular orbital and lowest unoccupied molecular orbital of the closed-shell singlet states. These are conducive to the emergence of diradical character and large spin coupling constants. Furthermore, the spin-alternation analyses show that each dipyrrole-inverted porphyrin derivative has two resonant structures featuring two spin-opposite single electrons, in agreement with the observed antiferromagnetic spin couplings. This work provides novel insights into the electronic structures and properties of the porphyrin derivatives with modified structures and also provides helpful information for the rational design, synthesis and characterization of new porphyrin-based magnets.

Magnetoelectric Radical Hydrocarbons

The molecular radicals, systems with unpaired electrons of open-shell electronic structures, set the stage for a multidisciplinary science frontier relevant to the cooperative magnetic exchange interaction and magnetoelectric effect. Here ferroelectricity together with magnetic spin exchange coupling in molecular radical hydrocarbon solids is reported, representing a new class of magnetoelectrics. Electronic correlation through radical-radical interactions plays a decisive role in the coupling between magnetic and charge orders. A substantial photoconductance and visible-light photovoltaic effect are found in radical hydrocarbons. The ability to simultaneously control and retrieve the changes in magnetic and electrical responses opens up a new breadth of applications, such as radical magnetoelectrics, magnets, and optoelectronics.

The Diradical-Dication Strategy for BODIPY- and Porphyrin-Based Dyes with Near-Infrared Absorption Maxima from 1070 to 2040 nm

Four stable boron dipyrromethene (BODIPY)- and porphyrin-based bis-arylamine diradical dications were synthesized by two-electron oxidation of their neutral molecules. The two BODIPY-based dications have open-shell singlet ground states. UV/Vis absorption spectra of all four dications showed large redshifts in the NIR region compared to their neutral precursors with absorption maxima at 1274 and 1068 nm for the two BODIPY-based dications and 1746 and 2037 nm for the two porphyrin-based dications. Thus, two new types of NIR dyes with longer wavelengths are provided by the diradical-dication strategy, which can be applied for the generation of other NIR dyes with a range of different chromophores and auxochromes.

Revisiting the intense NIR active bronzaphyrin, a 26-π aromatic expanded porphyrin: synthesis and structural analysis

Dithiabronzaphyrin, with intense absorption and fluorescence in the NIR region, was synthesized as its β-octaethyl analogue via a thiophene bridged terpyrrole. Solid state structural characterization obtained for the first time revealed inversion of the thiophene rings. Studies showed that there is no effect from protonation or temperature on the structural rigidity of the macrocycle. Crystal packing revealed four open dimeric trifluoroacetate [(CF3COO)2H]- moieties binding to the macrocycle in the protonated form via H bonding.

Proton-Coupled Redox Switching in an Annulated π-Extended Core-Modified Octaphyrin

Proton-coupled electron transfer (PCET) is an important chemical and biological phenomenon. It is attractive as an on-off switching mechanism for redox-active synthetic systems but has not been extensively exploited for this purpose. Here we report a core-modified planar weakly antiaromatic/nonaromatic octaphyrin, namely, a [32]octaphyrin(1.0.1.0.1.0.1.0) (1) derived from rigid naphthobipyrrole and dithienothiophene (DTT) precursors, that undergoes proton-coupled two-electron reduction to produce its aromatic congener in the presence of HCl and other hydrogen halides. Evidence for the production of a [4 n + 1] π-electron intermediate radical state is seen in the presence of trifluoroacetic acid. Electrochemical analyses provide support for the notion that protonation causes a dramatic anodic shift in the reduction potentials of octaphyrin 1, thereby facilitating electron transfer from halide anions (viz. I^-, Br^-, and, Cl^-). Electron-rich molecules, such as tetrathiafulvene (TTF), phenothiazine (PTZ), and catechol, were also found to induce PCET in the case of 1. Both the oxidized and two-electron reduced forms of 1 were characterized by X-ray diffraction analyses in the solid state and in solution via spectroscopic means.

Stable Nitrogen-Centered Bis(imino)rylene Diradicaloids

The synthesis of stable open-shell singlet diradicaloids is critical for their practical material application. So far, most reported examples are based on carbon-centered radicals, which are intrinsically reactive, and there are very few examples of stable nitrogen-centered diradicaloids. In this full paper, a series of soluble and stable bis(imino)rylenes up to octarylene were synthesized on the basis of newly developed dibromorylene intermediates. It was found that from hexarylene onward, these quinoidal rylenes showed open-shell singlet ground states and could be thermally populated to paramagnetic triplet aminyl diradicals. They are stable due to efficient spin delocalization onto the rylene backbone as well as kinetic blocking of the aminyl sites by the bulky and electron-deficient 2,4,6-trichlorophenyl groups. They exhibited very different electronic structures, diradical character, excited-state dynamics, one-photon absorption, two-photon absorption, and electrochemical properties from their respective aromatic rylene counterparts. These bis(imino)rylenes represent a rare class of stable, neutral, nitrogen-centered aminyl diradicaloids.

A Benzene-1,3,5-Triaminyl Radical Fused with ZnII -Porphyrins: Remarkable Stability and a High-Spin Quartet Ground State

A benzene-1,3,5-triaminyl radical fused with three Zn^II -porphyrins was synthesized through a three-fold oxidative fusion reaction of 1,3,5-tris(Zn^II -porphyrinylamino)benzene followed by oxidation with PbO₂ as key steps. This triaminyl radical has been shown to possess a quartet ground state with a doublet-quartet energy gap of 3.1 kJ mol^-1 by superconducting quantum interference device (SQUID) studies. Despite its high-spin nature, this triradical is remarkably stable, which allows its separation and recrystallization under ambient conditions. Moreover, this triradical can be stored as a solid for more than one year without serious deterioration. The high stability of the triradical is attributed to effective spin delocalization over the porphyrin segments and steric protection at the nitrogen centers and the porphyrin meso positions.

From open-shell singlet diradicaloids to polyradicaloids

In this Feature Article, we highlight our recent efforts toward stable open-shell singlet diradicaloids and polyradicaloids. A brief review on the historical works in the area is introduced first, followed by discussion on the fundamental electronic and physical properties of open-shell singlet diradicaloids. Then, the structure-diradical character relationships based on our recently developed diradicaloids are presented. Next, the challenges and solutions toward stable polyradicaloids and 3D π-conjugated diradicaloids are discussed. Finally, their preliminary material applications are introduced and a perspective view of the area is given.

Porphyrinoids as a platform of stable radicals

The non-innocent ligand nature of porphyrins was observed for compound I in enzymatic cycles of cytochrome P450. Such porphyrin radicals were first regarded as reactive intermediates in catabolism, but recent studies have revealed that porphyrinoids, including porphyrins, ring-contracted porphyrins, and ring-expanded porphyrins, display excellent radical-stabilizing abilities to the extent that radicals can be handled like usual closed-shell organic molecules. This review surveys four types of stable porphyrinoid radical and covers their synthetic methods and properties such as excellent redox properties, NIR absorption, and magnetic properties. The radical-stabilizing abilities of porphyrinoids stem from their unique macrocyclic conjugated systems with high electronic and structural flexibilities.

Stable meso-Fluorenyl Smaragdyrin Monoradical

The facile synthesis and physical characterization of a meso-fluorenyl smaragdyrin monoradical 4, which is stable due to efficient spin delocalization and kinetic blocking, is reported. It has a small energy gap and can be oxidized and reduced into the respective cation and anion, showing different charge distribution and electronic absorption properties.

Metalation Control of Open-Shell Character in meso-meso Linked Porphyrin meso-Oxy Radical Dimers

Control of open-shell character of meso-meso linked porphyrin meso-oxy radical dimers has been demonstrated by core metalation. Namely, Ni^II -porphyrin dimer 6Ni exhibits a clear ¹ H NMR spectrum and a distorted but rather coplanar quinonoidal structure consisting of two ruffled porphyrin rings, in accordance with the previous report. Freebase dimer 6H₂ shows a similar quinonoidal structure in the solid state but displays slightly broader and temperature-dependent ¹ H NMR spectra, indicating a partial diradical character in solution that increases at high temperature. In sharp contrast, bis-imidazole-coordinated Zn^II -porphyrin dimer 6ZnIm₂ exhibits a perpendicular structure consisting of two planar Zn^II -porphyrins and has been characterized as a distinct open-shell diradical on the basis of its non-observable ¹ H NMR signals, a clear ESR signal, and a characteristic absorption spectrum reaching about 1700 nm. Despite the distinct diradical character, 6ZnIm₂ is an extremely stable molecule.

Evidence for a [17] π-Electronic Full-Fledged Non-innocent Gallium(III)-Corrole Radical

One-electron oxidation of a Ga^III -corrole with N(4-BrC₆ H₄ )₃ SbCl₆ resulted in an air-stable non-innocent Ga^III -corrole radical. The single-crystal X-ray crystallography of the 2,17-bis-formyl-5,10,15-tris(pentafluorophenyl)corrolato gallium(III)(chloride) radical ([3-Cl]^. ) revealed delocalization of the unpaired electron, which was further confirmed by electron spin resonance (ESR) spectroscopy and spin density distribution plot. In addition, the nucleus-independent chemical shift (NICS), anisotropy-induced current density (AICD) and harmonic oscillator model of aromaticity (HOMA) supported a [17] π-electron-conjugated (or antiaromatic) radical.

Hetero Cu(III)-Pd(II) Complex of a Dibenzo[g,p]chrysene-Fused Bis-dicarbacorrole with Stable Organic Radical Character

Bis-dicarbacorrole (bis-H₃) with two adj-CCNN subunits was synthesized by incorporating a dibenzo[g,p]chrysene moiety into the macrocyclic structure. The two trianionic cores in bis-H₃ can stabilize two Cu(III) ions (bis-Cu) or concurrently a Cu(III) cation and a Pd(II) ion in the form of a hetero bis-metal complex (mix-Cu/Pd). As prepared, mix-Cu/Pd displays organic π radical character, as confirmed by various techniques, including electron paramagnetic resonance spectroscopy, cyclic voltammetry, femtosecond transient absorption measurements, and DFT calculations. Radical formation is ascribed to one-electron transfer from the dicarbacorrole backbone to the Pd center allowing the d⁸ Pd(II) center to be accommodated in a square planner coordination geometry. Nucleus-independent chemical shift and anisotropy of the induced current density calculations provide support for the conclusion that bis-H₃ and bis-Cu both display antiaromatic character and contain two formally 16 π-electron dicarbacorrole subunits. On this basis, we suggest that mix-Cu/Pd is best considered as containing a fused 15 π-electron nonaromatic radical subunit and a 16 π-electron antiaromatic subunit. The spectroscopic observations are consistent with these assignments.

Theoretical Study on the Open-Shell Singlet Nature and the Second Hyperpolarizabilities of Corannulene Derivatives with Two Phenoxyl Radicals

Using the spin-unrestricted density functional theory method, we investigate the interplay between the diradical character y and second hyperpolarizabilities γ (the third-order nonlinear optical (NLO) properties at the molecular scale) of corannulene derivatives with two phenoxyl radicals. This molecule in the singlet state exhibits intermediate y and thus displays a significantly larger γ value than the triplet state and the closed-shell bis-phenol analogue. We also examine the planar molecules involving a coronene moiety in place of the curved corannulene. The intermediate y and large γ values of the corannulene systems are found to originate not from their curved skeleton but from the equilibrium between benzenoid/quinoid resonance forms due to delocalization of the radical electrons of the terminal phenoxyl rings. The longitudinal γ value of the singlet state is found to be comparable to that of s-indaceno[1,2,3-cd;5,6,7-c'd']diphenalene, which is known to be one of the organic molecules with the largest two-photon absorption cross section in this size of pure hydrocarbons. The present system is thus expected to be a promising candidate for highly efficient open-shell NLO molecules.

A theoretical study on quasi-one-dimensional open-shell singlet ladder oligomers: multi-radical nature, aromaticity and second hyperpolarizability

Quasi-one-dimensional open-shell singlet ladder oligomers composed of indenofluorene-like units show strong correlation between multiradical nature, aromaticity and second hyperpolarizability.

Flexible Porphyrinoids

This review underscores the conformational flexibility of porphyrinoids, a unique class of functional molecules, starting from the smallest triphyrins(1.1.1) via [18]porphyrins(1.1.1.1) and concluding with a variety of expanded porphyrinoids and heteroporphyrinoids, including the enormous [96]tetracosaphyrin(1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0). The specific flexibility of porphyrinoids has been documented as instrumental in the designing or redesigning of macrocyclic frames, particularly in the search for adjustable platforms for coordination or organometallic chemistry, anion binding, or mechanistic switches in molecular devices. A structural prearrangement to coordinate one or more metal ions has been outlined. The coverage of the topic focuses on representative examples of geometry or conformational rearrangements for each selected class of the numerous porphyrinoids accordingly categorized by the number of built-in carbo- or heterocycles.

Role of a singlet diradical character in carbon nanomaterials: a novel hot spot for efficient nonlinear optical materials

Carbon atoms have the potential to produce a variety of fascinating all-carbon structures with amazing electronic and mechanical properties. Over the last few decades, several efforts have been made using experimental and computational techniques to functionalize graphene, carbon nanotubes and fullerenes for potential use in modern hi-tech electronic, medicinal, optical and nonlinear optical (NLO) applications. Since photons replaced electrons as a carrier of information, the field of NLO material design has drawn immense interest in contemporary materials science. There have been several reports of bridging the gap between the exciting fields of carbon nanomaterials and NLO materials by functionalizing carbon nanomaterials for potential NLO applications. In contrast to previous reports of the design of third-order NLO materials using conventional closed-shell materials, a novel strategy using open-shell diradical molecular systems has recently been proposed. Quantum chemically, diradical character is explained in terms of the instability of the chemical bonds in open-shell molecular systems. Interestingly, several carbon nanomaterials, which naturally possess open-shell singlet configurations, have recently gained momentum in the design of these classes of open-shell NLO materials with excellent NLO properties, stability and diversity. The present review establishes a systematic sequence of different studies (spanning over two decades of intense research efforts), starting from the simplest theoretical two-site diradical model, continuing to its experimental and theoretical realization in actual chemical systems, and finally applying the abovementioned model/rule to novel carbon nanomaterials to tune their NLO properties, particularly their second hyperpolarizability (γ). In the present report, we highlight several recent efforts to functionalize carbon nanomaterials by exploiting their open-shell diradical character to achieve efficient third-order NLO properties. Several issues and opportunities are discussed, including the inherited disadvantages of both experimental (the high reactivity and short life of diradical compounds) and quantum (need for multi-reference methodology) techniques when dealing with carbon nanomaterials. A comparative analysis of several quantum chemical investigations, along with contemporary experimental results, will be performed to emphasize the core issues and opportunities related to carbon nanomaterials with singlet open-shell diradical character. Thus, the present review will highlight carbon nanomaterials with diradical/biradical character for their prospective applications in the NLO field.

Highly planar diarylamine-fused porphyrins and their remarkably stable radical cations

Oxidative fusion of a meso-phenoxazino Ni(ii) porphyrin at high temperature gave a doubly phenoxazine-fused porphyrin as a highly planar diarylamine-fused porphyrin. One-electron oxidation of the corresponding β,β-dichlorinated compound gave a remarkably stable radical cation.

Oxidative fusion reactions of meso-phenoxazino Ni(ii) porphyrin were found to be temperature dependent, giving rise to either a doubly phenylene-fused product at room temperature or a singly phenoxazine-fused product at 70 °C. The latter was further oxidized to a doubly phenoxazine-fused Ni(ii) porphyrin, which was subsequently converted to the corresponding free base porphyrin and Zn(ii) porphyrin. Compared to previously reported diphenylamine-fused porphyrins that displayed a molecular twist, doubly phenoxazine-fused porphyrins exhibited distinctly different properties owing to their highly planar structures, such as larger fluorescence quantum yields, formation of an offset face-to-face dimer both in solution and the solid state, and the generation of a mixed-valence π-radical cation dimer upon electrochemical oxidation. One-electron oxidation of the phenoxazine-fused Ni(ii) porphyrin with Magic Blue gave the corresponding radical cation, which was certainly stable and could be isolated by separation over a silica gel column but slowly chlorinated at the reactive β-positions in the solid state. This finding led to us to examine β,β′-dichlorinated phenoxazine-fused and diphenylamine-fused Ni(ii) porphyrins, which, upon treatment with Magic Blue, provided remarkably stable radical cations to an unprecedented level. It is actually possible to purify these radical cations by silica gel chromatography, and they can be stored for over 6 months without any sign of deterioration. Moreover, they exhibited no degradation even after the CH₂Cl₂ solution was washed with water. However, subtle structural differences (planar versus partly twisted) led to different crystal packing structures and solid-state magnetic properties.