Properties Of Diradicals Research Articles

Control over electronic structures of organic diradicaloids via precise B/O-heterocycle fusion

Diradicaloid polycyclic hydrocarbons (PHs) own unique open-shell electronic structures and exhibit potential utility in the fields of organic electronics and spintronics. Herein, we disclose precise fusion of B/O-heterocycles onto PHs for control over their electronic structures and diradical properties. We designed and synthesized four B/O-containing diradicaloid isomers that feature the fluoreno[3,2-b]fluorene and fluoreno[2,1-a]fluorene π-skeletons, respectively. The precise B/O-heterocycle fusion modes along with the changed conjugation patterns lead to their modulated electronic structures and properties, such as diradical and aromatic structures, energy levels and band gaps, as well as magnetic, electrochemical and photophysical properties. Notably, the mode A may decrease the open-shell extent, whereas the mode B can enhance the diradical nature, leading to their well-tuned diradical characters in the range of 0.46‒0.70. Moreover, the mode A stabilizes the LUMOs and the mode B obviously increases the HOMO levels, which are remarkably contributed by the B and O atoms, respectively, further giving rise to the decreased band gaps and redshifted absorptions. This study clearly illustrates the electronic effects of B/O-heterocycle fusion on PHs and gains insight into B/O-type organic diradicaloids. These findings will provide an important guideline for the design of more fascinating heteroatom-containing diradicaloids.

Enhancing stability of diradical polycyclic hydrocarbons via P=O-attaching

Diradical polycyclic hydrocarbons (PHs) have unique open-shell structures and interesting physical properties. However, owing to high reactivity of unpaired electrons, such open-shell organic diradicaloids are usually less stable than closed-shell systems, limiting their practical applications. In this study, we report P=O-attaching of diradical PHs as a new strategy to enhance their stability while maintaining diradical properties. Three P=O-attached PHs containing the indeno[1,2-b]fluorene, fluoreno[3,2-b]fluorene and indeno[2,1-b]fluorene π-skeletons, respectively, were designed and synthesized. As theoretically and experimentally proved, two of them have the relatively large diradical characters and open-shell singlet diradical nature. In comparison to their all-carbon analogues, the attached electron-withdrawing P=O groups endow them with much lower LUMO/HOMO energy levels but preserved magnetic activities and physical properties, such as thermally accessible triplet species and multi-redox ability. Moreover, the P=O groups effectively decrease their oxidation activities and thereby lead to their remarkably excellent ambient stabilities. Thus, this P=O-attaching strategy will be applicable to other diradical PH systems and may promote the generation of stable organic diradicaloids for radical chemistry and materials.

Substituted effects on bonding characteristics of cyclopentane‐1,3‐diyl diradicals monitored by time‐resolved infrared spectroscopy

AbstractCyclopentane‐1,3‐diyl diradicals (DRs) provide excellent opportunities to study the properties of diradicals because their lifetimes can be significantly lengthened to up to milliseconds with the introduction of proper substituents. This study investigated the bonding characteristics of singlet and triplet DRs having C=O and p‐cyanophenyl groups (S‐DR3 and T‐DR3) by monitoring the photo‐induced formation of the diradicals from their precursor azo compounds using time‐resolved IR (TR‐IR) spectroscopy. Upon the formation of S‐DR3, a C=O stretching wavenumber was upshifted by 22 cm−1, whereas a C≡N stretching one was downshifted by 12 cm−1. The observed shifts indicate that the unpaired electrons increase and decrease the C=O and C≡N bond orders, respectively. The effects of the unpaired electrons in S‐DR3 were similar to those observed in our previous TR‐IR studies on a singlet cyclopentane‐1,3‐diyl diradical having C=O but no C≡N groups (S‐DR2) and on that having C≡N but no C=O groups (S‐DR1), respectively. Contrastingly, upon the formation of T‐DR3, the C=O wavenumber was downshifted by 16 cm−1, indicating that the unpaired electrons decrease the C=O bond order. More notably, no detectable shifts were observed in the C≡N stretching wavenumber. These observations are not clearly explained by a model suggested in the previous studies on S‐DRs. Here, we discuss and propose a more elaborated resonance hybrid of DRs that can explain the directions and relative magnitudes of the observed wavenumber shifts irrespective of spin multiplicities. We expect that the findings and suggestions presented here will stimulate research in both organic and theoretical chemistry.

Theoretical Study on the Formation and Decomposition Mechanisms of Coelenterazine Dioxetanone.

Bioluminescence has been drawing broad attention due to its high signal-to-noise ratio and high bioluminescence quantum yields, which has been widely applied in the fields of biomedicine, bioanalysis, and so on. Among numerous bioluminescent substrates, coelenterazine is famous for its wide distribution. However, the oxygenation reaction mechanism of coelenterazine is far from being completely understood. In this paper, the formation and decomposition mechanisms of coelenterazine dioxetanone were investigated via density functional theory (DFT) and time-dependent (TD) DFT approaches. The results showed that the oxygenation reaction first occurred along the triplet-state potential energy surface (PES), after the intersystem crossing (ISC), second jumped to the diradical-state PES, and ultimately formed coelenterazine dioxetanone. For the decomposition mechanism of dioxetanone, the computational results showed that the chemiexcitation of neutral dioxetanone was more efficient than that of other dioxetanone species. Moreover, the diradical properties and the degree of ionic character are modified by the counter ions.

Deprotonation-Induced and Ion-Pairing-Modulated Diradical Properties of Partially Conjugated Pyrrole–Quinone Conjunction

Quinoidal molecules based on dipyrrolyldiketone boron complexes (QPBs), in which pyrrole units were connected by a partially conjugated system as a singlet spin coupler, were synthesized. QPB, which was stabilized by the introduction of a benzo unit at the pyrrole β-positions, formed a closed-shell tautomer conformation that showed near-infrared absorption. The deprotonated species, monoanion QPB- and dianion QPB2-, showing over 1000 nm absorption, were formed by the addition of bases, providing ion pairs in combination with countercations. Diradical properties were observed in QPB2-, whose hyperfine coupling constants were modulated by ion-pairing with π-electronic and aliphatic cations, demonstrating cation-dependent diradical properties. VT NMR and ESR along with a theoretical study revealed that the singlet diradical was more stable than the triplet diradical.

Tuning the Diradical Character of Indolocarbazoles: Impact of Structural Isomerism and Substitution Position.

In this study, a set of 10 positional indolocarbazole (ICz) isomers substituted with dicyanomethylene groups connected via para or meta positions are computationally investigated with the aim of exploring the efficiency of structural isomerism and substitution position in controlling their optical and electronic properties. Unrestricted density functional theory (DFT), a spin-flip time-dependent DFT approach, and the multireference CASSCF/NEVPT2 method have been applied to correlate the diradical character with the energetic trends (i.e., singlet–triplet energy gaps). In addition, the nucleus-independent chemical shift together with ACID plots and Raman intensity calculations were used to strengthen the relationship between the diradical character and (anti)aromaticity. Our study reveals that the substitution pattern and structural isomerism represent a very effective way to tune the diradical properties in ICz-based systems with meta-substituted systems with a V-shaped structure displaying the largest diradical character. Thus, this work contributes to the elucidation of the challenging chemical reactivity and physical properties of diradicaloid systems, guiding experimental chemists to produce new molecules with desirable properties.

Air Stable Chalcogen-Doped Rubicenes with Diradical Character

Air Stable Chalcogen-Doped Rubicenes with Diradical Character

Tuning Diradical Properties of Boron-Containing π-Systems by Structural Isomerism.

Tuning diradical character is an important topic for organic diradicaloids. Herein, we report the precise borylation enabling structural isomerism as an effective strategy to modulate diradical character and thereby properties of organic diradicaloids. We synthesized a new B-containing polycyclic hydrocarbon that has the indeno[1,2-b]fluorene π-skeleton with the β-carbons bonding to two boron atoms. Detailed theoretical and experimental results show that this bonding pattern leads to its distinctive electronic structures and properties in comparison to that of its isomeric molecule. This molecule has the efficient conjugation between boron atoms and π-skeleton, resulting in downshifted LUMO and HOMO levels. Moreover, it exhibits smaller diradical character and thereby inhibited diradical properties, such as significantly blue-shifted light absorption, larger energy bandgap and weak para-magnetic resonance. Notably, this B-containing polycyclic hydrocarbon possesses much stronger Lewis acidity and its Lewis acid-base adducts display enhanced diradical character, demonstrating the positive effects of Lewis coordination on modulating diradical performance.

Learning how to fine‐tune diradical properties by structure refinement

AbstractThe literature has seen a large increase in the number of new carbon‐based organic diradicals/diradicaloids in recent years. While a plethora of new and exciting structures have been created, there seemingly is a gap in knowledge of what fundamental electronic parameters are in play and thus how to rationally manipulate said parameters to “fine tune” the resultant diradical properties. Since 2014, the Haley group has been exploring methods to systematically alter the diradical character and the singlet‐triplet energy gap in said class of molecules. Our entrance into organic diradicals began with the π‐expansion of the benzene core of indeno[1,2‐b]fluorene up to the anthracene core of diindenoanthracene (DIAn). DIAn possessed moderate diradical character (y = 0.62) with a surprising level of stability (more than 2 months in solution). From this molecular blueprint for producing stable diradicals, the Haley lab has investigated how to fine tune diradical properties via structural changes in two key positions: (a) the length of the acene core and (b) thoughtful exchange of the outer arenes. With this strategy at our disposal, we can make large scale changes to the diradical character index and singlet‐triplet energy gap through changing the core length, and these properties can be further fine‐tuned in a series of closely related diradicals by careful exchange of the outer arenes utilizing the straightforward methods described in this mini‐review.

Molecule Isomerism Modulates the Diradical Properties of Stable Singlet Diradicaloids.

Inclusion of quinoidal cores in conjugated hydrocarbons is a common strategy to modulate the properties of diradicaloids formed by aromaticity recovery within the quinoidal unit. Here we describe an alternative approach of tuning of diradical properties in indenoindenodibenzothiophenes upon anti → syn isomerism of the benzothiophene motif. This alters the relationship of the S atom with the radical center from linear to cross conjugation yet retains the same 2,6-naphtho conjugation pattern of the rearomatized core. We conduct a full comparison between the anti and syn derivatives based on structural, spectroscopic, theoretical, and magnetic measurements, showing that these systems are stable open-shell singlet diradicaloids that only access their triplet state at elevated temperatures.

Searching for Diradicaloid Chromophores with Efficient Singlet Fission: Cyano‐Group Substitution of Difuropyrene Systems

AbstractOn the basis of theoretical models and calculations, CN‐substituted difuropyrene systems have been investigated as potential singlet fission (SF) candidates for solar cell applications. Calculations indicate that the fusion of difuran and pyrene in different arrangements could introduce variable diradical properties, and the fused compound ab‐trans‐DFP has moderate diradical character (y0=0.45) while satisfying the main condition (ΔE1) of SF. Furthermore, CN‐substituted ab‐trans‐DFP (substituted at distinct positions) is used to adjust the frontier molecular orbital energy level to satisfy the SF secondary condition (ΔE2). Interestingly, the position of the cyano substituents has almost no effect on the diradical character, which can be explained by the insensitivity of the ring aromaticity to the cyano position. Both time dependent density functional theory (TD‐DFT) and multi‐configurational self‐consistent field (MCSCF) results confirm that the 4‐ and 5‐position cyano‐substituted ab‐trans‐DFP derivatives can satisfy the two energy conditions and may become potential SF candidates. This work provides an effective strategy for the rational design of new suitable diradical SF candidates, and it is expected that more polycyclic aromatic hydrocarbons such as pyrene can achieve SF conditions by chemical modification, such as fusion or substitution, to adjust the diradical character and the vertical excitation energy.

Meta‐ and para‐Phenylenediamine‐Fused Porphyrin Dimers: Synthesis and Magnetic Interactions of Their Dication Diradicals

Abstractmeta‐ and para‐Phenylenediamine‐fused nickel(II) porphyrin dimers were synthesized by SNAr reaction of meso,β,β‐trichloro nickel(II) porphyrin with meta‐ and para‐phenylenediamines and subsequent Pd‐catalyzed intramolecular C−H arylation. Their tetrachlorinated dication diradicals are very stable, allowing SQUID magnetometry and revealing clear open‐shell characters for both meta and para isomers with ferro‐ and anti‐ferromagnetic interactions, respectively. The nitrogen analogue of Thiele's hydrocarbon usually displays predominant closed‐shell nature but its hidden diradical characters increase either in a twisted conformation or upon insertion of an additional phenylene spacer. The observed distinct diradical nature of the para‐congener indicates that diradical properties can be enhanced also by efficient spin delocalization.

Meta- and para-Phenylenediamine-Fused Porphyrin Dimers: Synthesis and Magnetic Interactions of Their Dication Diradicals.

meta- and para-Phenylenediamine-fused nickel(II) porphyrin dimers were synthesized by SN Ar reaction of meso,β,β-trichloro nickel(II) porphyrin with meta- and para-phenylenediamines and subsequent Pd-catalyzed intramolecular C-H arylation. Their tetrachlorinated dication diradicals are very stable, allowing SQUID magnetometry and revealing clear open-shell characters for both meta and para isomers with ferro- and anti-ferromagnetic interactions, respectively. The nitrogen analogue of Thiele's hydrocarbon usually displays predominant closed-shell nature but its hidden diradical characters increase either in a twisted conformation or upon insertion of an additional phenylene spacer. The observed distinct diradical nature of the para-congener indicates that diradical properties can be enhanced also by efficient spin delocalization.

Effect of Structure on the Spin-Spin Interactions of Tethered Dicyanomethyl Diradicals.

Stable organic radicals with switchable spin states have attracted attention for a variety of applications, but a fundamental understanding of how radical structure effects the weak bonding interactions between organic radicals is limited. To evaluate the effect of chemical structure on the strength and nature of such spin interactions, a series of 14 tethered aryl dicyanomethyl diradicals were synthesized, and the structure and thermodynamic properties of the diradicals were investigated. These studies indicate that the nature of the dimer and the equilibrium thermodynamic parameters of the diradical-dimer equilibria are highly sensitive to the attachment point of the linker, the length of the linker, and the substituents on the radical itself. Values of the intramolecular Ka vary from as small as 5 to as high as 105 depending on these variables. An X-ray crystal structure for a linked ortho-substituted diradical shows that the diradical forms an intramolecular sigma dimer in the crystalline state with an elongated C-C bond (1.637 Å). Subtle changes to the radical structure influences the nature of the spin interactions, as fixing the dimethylamino substituent on the radical into a ring to make a julolidine-derived diradical leads to the weakest bonding interaction observed (Δ Gbonding = 1 kcal mol-1) and changes the spin-paired species from a sigma dimer to a diradical pimer. This work has implications for the design of stimuli-responsive materials that can reversibly switch between the dramatically different properties of closed-shell species and the unique properties of diradicals.

Synthesis, structure and properties of nitronyl nitroxide diradicals with fused‐thiophene couplers

AbstractFunctionalized diradicals are widely used in chemistry, physics, biology and material sciences because of their inherently high reactivity and magnetic activity, but the synthesis of planar diradical molecules is still a challenge. This paper describes the effective synthesis and properties of diradicals in which nitronyl nitroxide fragments and a fused‐thiophene bridge are almost coplanar. The diradicals DR1, DR2, DR4 were prepared in high yield by palladium‐catalyzed cross‐coupling of the (nitronyl nitroxide‐2‐ide)(triphenylphosphin)gold complex with 2,5‐diiodothieno[2,3‐b]thiophene, 2,5‐diiodothieno[3,2‐b]thiophene or 2,6‐diiododithieno[3,2‐b:2′,3′‐d]thiophene correspondingly. The torsion angles between planes of the nitronyl nitroxide fragments and fused‐thiophene systems lie in the range 2.6–8.5°. The planarity of DR2 and DR4 makes the Kekulé‐type coupler units effective bridges to transmit magnetic interactions by a spin polarization mechanism. According to the magnetic susceptibility measurements, supported by density functional theory calculations, the singlet–triplet energy splitting (2 J/kB) is −130 K for DR2 and close to −80 K for DR4. The singlet diradicals DR2 and DR4 are stable and demonstrate two one‐electron reversible electrochemical oxidation peaks and two‐electron quasi‐reversible reduction peaks resulting in the formation of the corresponding bis(oxoammonium) dication or bis(aminooxidanide) dianion. Copyright © 2016 John Wiley & Sons, Ltd.

DNA bases ring-expanded with a cyclopentadiene free radical: a theoretical investigation of building blocks with diradical character.

In this work, we computationally design radical nucleobases which possess improved electronic properties, especially diradical properties through introducing a cyclopentadiene radical. We predict that the detailed electromagnetic features of base assemblies are based on the orientation of the extra five-membered cyclopentadiene ring. Broken symmetry DFT calculations take into account the relevant structures and properties. Our results reveal that both the radicalized DNA bases and the base pairs formed when they combine with their counterparts remain stable and display larger spin delocalization. The mode of embedding the cyclopentadiene free radical in the structures has some influence on the degree of π-conjugation, which results in various diradical characteristics. Single-layered radical base pairs all have an open-shell singlet ground state, but the energy difference between singlet and triplet is not significant. For two-layered radical base pairs, the situation is more complex. All of them have an open-shell state as their ground state, including an open-shell singlet state and an open-shell triplet state. That is, the majority of radical base pairs possess anti-ferromagnetic or ferromagnetic characteristics. We present here a more in-depth discussion and analyses to study the magnetic characteristics of radical bases and base pairs. As an important factor, two-layered radical base pairs also have been carefully analyzed. We hope that all the measurements and results presented here will stimulate further detailed insights into the related mechanisms in modified DNA bases and the design of better ring-expanded DNA magnetic materials.

A theoretical study on magnetic properties of bis-TEMPO diradicals with possible application

Three different pairs of isomeric trans and corresponding cis forms of fluoroprotein chromophore (FPC) fragments coupled bis-2,2′,6,6′-tetramethylpiperidinyloxy (TEMPO) based diradicals have been designed, and subsequently the magnetic properties of these diradicals have been investigated. We have used blue shifted variants of green fluorescent protein (cyan fluorescent protein or CFP), orange variant of Discosoma coral (DsRed) and yellow mutant of Zoanthus sp. (zFP) chromophore fragments as spin couplers. Along with the gas phase magnetic exchange coupling constant (J), the zero field splitting parameters (ZFSs) of all the diradicals in water and also in blood plasma medium have been calculated. We have found the all molecules to be ferromagnetic in nature with trans diradicals having lower spin–spin ZFS parameter (|D|) and the higher magnetic exchange coupling constants values than their respective fluorescent cis isomers. This has also been noted that due to the presence of the fluoroprotein chromophores, the resulting diradicals can switch back and forth between the isomeric trans and cis forms upon irradiation of the proper wavelength of light. The time dependent density functional (TDDFT) technique is used to measure the actual wavelength of this conversion for these diradicals. A comparison between the evaluated ZFS values with other nitroxide based synthesized molecular systems which are in use as magnetic resonance imaging (MRI) contrast agents (MRICAs) indicates that if these diradicals are synthesized, they can be useful as MRICAs. A relation between the distance and the spin–spin ZFS parameter is also made. To support this fact, we have computed the solvent accessible surface area, molecular volume, the surface to volume ratio, and molecular diameter of each diradical species to address their potentiality to act as contrast agents in MRI. Finally the ability of these diradicals to act as MRICAs is further justified with the help of molecular electrostatic potential surfaces.

ChemInform Abstract: Quinoidal Oligothiophenes: New Properties Behind an Unconventional Electronic Structure

AbstractReview: 92 refs.

Quinoidal oligothiophenes: new properties behind an unconventional electronic structure

The main chemical, spectroscopic and material research done in tetracyano quinoidal oligothiophenes in the last 30 years has been reviewed. Their use as semiconducting substrates in organic electronic and their versatility to act as multifunctional materials have been highlighted. This tutorial review has been paralleled by the description of the main findings provided by Raman spectroscopy, in particular, associated with the discovery of the intrinsic diradical properties inherent to the pro-aromatic character of the quinoidal arrangement. It turns out that the fascinating properties of these materials are the manifestation of the diradical fingerprint behind a rather unconventional electronic structure.

ChemInform Abstract: Orbital Phase Design of Diradicals

AbstractReview: 164 refs.