Biradicaloid Character Research Articles

Charge transfer or biradicaloid character: assessing TD-DFT and SAC-CI for squarylium dye derivatives

Le Bahers's diagnostic indexes have been applied to study the electronic transition character of symmetrical squarylium dye derivatives through TD-DFT and SAC-CI methods.

Outside rules inside: the role of electron-active substituents in thiophene-based heterophenoquinones.

The biradicaloid vs. quinoidal character of the ground state of thiophene-based heterophenoquinones bearing donor or acceptor groups is investigated. Keeping the conjugation length fixed, namely, the 5,5'-bis(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadiene-1-ylidene)-2,2'-dihydroxy bithiophene backbone, an opposite effect occurs depending on the donating or withdrawing nature of the substituents. The character of the ground state depends not only on the electronic nature of the substituents but also on their position on the molecular skeleton: donor groups on the 3,3'-positions of the bithiophene central core stabilize a quinoidal ground state, whereas a biradicaloid electronic structure results from the introduction of the same donor groups onto the lateral phenones. Withdrawing groups behave similar to donors, but in the opposite direction.

Electronic Structure of N2P2 Four-Membered Rings and the Effect of Their Ligand Coordination to M(CO)5 (Cr, Mo, and W)

In this article the biradicaloid character of the ground-state structures of N2P2R2 (R = CH3) rings is studied using the DFT and CASSCF methods, and a satisfactory agreement of the B3LYP functional and CASSCF(6,6) ab initio method has been found. In order to obtain an adequate description of the biradicaloid character, we have combined two criteria: (i) singlet–triplet energy gaps and (ii) relative values of the occupation numbers for bonding and antibonding orbitals associated with the radical sites. We have analyzed how the biradicaloid character of the N2P2R2 ring changes upon coordination to M(CO)5 (M = Cr, Mo, and W) at the B3LYP/6-311+G* level of theory. Interestingly, in some cases the biradicaloid character increases dramatically upon complexation of the N2P2R2 ligands.

Importance of unpaired electrons in organic electronics

ABSTRACTThe first observation that PBBTPD, a low bandgap, ambipolar conjugated donor‐acceptor (DA) polymer based on benzobisthiadiazole (BBT), possesses an open‐shell singlet ground state as well as a thermally accessible triplet state is described. Similarly, interesting electronic behavior in semiconducting organic DA oligomers based on BBT is also observed. Theoretical predictions have suggested that such behavior is due to the biradicaloid character of BBT and we provide experimental evidence indicating that these predictions are correct. Furthermore, the open shell character strengthens as the conjugation length increases, as observed in the BBT‐based polymer, PBBTPD. We show that this biradicaloid structure is observed in each BBT moiety along the chain and that therefore PBBTPD is in fact a polyradicaloid. This observation will most likely aid in the development of better n‐type polymeric acceptors for organic semiconductor applications. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 287–293

Electronic Structure of N2P2 Four‐Membered Rings

AbstractThe biradicaloid character of the ground‐state structures of N2P2 rings is studied by using the high‐level ab initio multiconfigurational CASPT2/CASSCF method. In order to obtain accurate descriptors, we combine two criteria: 1) singlet–triplet energy gaps and 2) relative values of the occupation numbers for bonding and antibonding orbitals associated with the radical sites. The singlet–triplet energy gaps, the occupation numbers of antibonding‐like orbitals, and the weights of the main configuration state functions (CSFs) of the ground‐state wavefunctions, that is, Ψ(1A1), are used to derive the biradicaloid character that ranges from 10–15 %.

A Singlet Biradicaloid Zinc Compound and Its Nonradical Counterpart

Metal ions with radical centers in their coordination sphere are key participants in biological and catalytic processes. In the present study, we describe the synthesis of the cAAC:ZnCl2 adduct (1) using a cyclic alkylaminocarbene (cAAC) as donor ligand. Compound 1 was treated with 2 equiv of KC8 and LiB(sec-Bu)3H to yield a deep blue-colored dicarbene zinc compound (cAAC)2Zn (2) and the colorless hydrogenated zinc compound (cAACH)2Zn (3), respectively. Compounds 2 and 3 were well characterized by spectroscopic methods and single-crystal X-ray structural analysis. Density functional theory calculations were performed for 2 which indicate that this molecule possesses a singlet biradicaloid character. Moreover, we show the application of 2 in CO2 activation, which yields a zwitterionic cAAC·CO2 adduct.

Magnetism and the biradicaloid character of π-aromatic and antiaromatic systems in a strong magnetic field

The previously developed scheme of the full configuration interaction for magnetic perturbations of π systems is transformed into a scheme for calculations in the finite field. It helps create “magnetic portraits” of molecules, reflecting the essentially non-linear behavior of conjugated systems in a strong field. In particular, possible latent paramagnetism of aromatic systems and correspondingly latent diamagnetism of antiaromatic ones is easily detected. The degree of the π electron shell openness as well as the singlet-triplet splitting in the field are evaluated. From the data obtained it follows that in the strong magnetic field an aromatic molecule becomes as a rule biradicaloid and non-aromatic. Accordingly, an antiaromatic system dramatically reduces its initial biradicaloid character and thus loses its antiaromaticity.

Tuning the Quinoid versus Biradicaloid Character of Thiophene-Based Heteroquaterphenoquinones by Means of Functional Groups

A series of quinoidal bithiophenes (QBTs) with controlled variations in steric hindrance and electron activity of the substituents has been synthesized. Evidence of their quinoidal versus biradicaloid ground-state electronic character has been experimentally detected and coherently identified as fingerprints by spectroscopic methods such as NMR, UV-vis, multiwavelength Raman. From this analysis, alkoxy groups have been shown to strongly affect the electronic structure and the ground-state energy and stability of QBTs. Quantum-chemical calculations correctly predict the experimental spectroscopic response, even while changing the alkyl on phenone from a tertiary carbon atom to secondary to primary toward an unsubstituted phenone, further confirming the validity of the approach proposed. A control of the electronic structure accompanied by negligible variations of the optical gap of the molecules has thus been demonstrated, extending the potential use of quinoidal species in fields ranging from photon harvesting to magnetic applications.

A comparative study of semi-squaraine and squaraine dyes using computational techniques: tuning the charge transfer/biradicaloid character by substitution

Semi-squaraines (SMSQ) are known as donor-acceptor (D-A) type molecules whereas squaraines (SQ), which differs from SMSQ by an extra donor group, are more or less biradicaloids in nature. The effect of the additional donor group in SQ, which changes the nature of the molecule, on geometrical and electronic structure are studied here and compared with the corresponding SMSQ. It is noticed from the geometrical parameters that, a strong resonance exists in SQ whereas disparity in carbon-carbon bond lengths of central C4 ring is seen in SMSQ dyes. The increasing and decreasing of antibonding interactions between central C4 ring and side donor groups cause destabilization of HOMO and stabilization of LUMO respectively in case of SQ compared to SMSQ molecules. This leads to decreasing the HOMO-LUMO gap and promotes biradicaloid character of SQ. The absorption maxima obtained by using TD-DFT method with BLYP, B3LYP, BHandHLYP, CAM-B3LYP and M06-2X functionals are not in good agreement with experimental results. On the other hand SAC-CI method gives better results for all the molecules. From this work we can evolve a design principle of these molecules which play a role as sensitizers in dye sensitized solar cells.

Synthesis and Characterization of a Singlet Delocalized 2,4‐Diimino‐1,3‐disilacyclobutanediyl and a Silylenylsilaimine

The synthesis and characterization of a singlet delocalized 2,4-diimino-1,3-disilacyclobutanediyl, [LSi(μ-CNAr)(2)SiL] (2, L: PhC(NtBu)(2), Ar: 2,6-iPr(2) C(6) H(3)), and a silylenylsilaimine, [LSi(=NAr)-SiL] (3), are described. The reaction of three equivalents of the disilylene [LSi-SiL] (1) with two equivalents of ArN=C=NAr in toluene at room temperature for 12 h afforded [LSi(μ-CNAr)(2)SiL] (2) and [LSi(=NAr)-SiL] (3) in a ratio of 1:2. Compounds 2 and 3 have been characterized by NMR spectroscopy and X-ray crystallography. Compound 2 was also investigated by theoretical studies. The results show that compound 2 possesses singlet biradicaloid character with an extensive electronic delocalization throughout the Si(2)C(2) four-membered ring and exocyclic C=N bonds. Compound 3 is the first example of a silylenylsilaimine, which contains a low-valent silicon center and a silaimine substituent. A mechanism for the formation of 2 and 3 is also proposed.

Comparative Study of the Semiconducting Properties of Benzothiadiazole and Benzobis(thiadiazole) Derivatives Using Computational Techniques

Recent literature reports indicate that derivatives of benzothiadiazole (BT) and benzobis(thiadiazole) (BBT), which differs from BT by an extra thiadiazole ring, exhibit good semiconducting properties, such as high electron mobility and low-lying lowest unoccupied molecular-orbital (LUMO) levels. In this study herein, computational techniques like density functional theory (DFT), spin-flip DFT and valence-bond methods are used to analyze the semiconducting properties of these molecules. Calculations at the B3LYP/cc-pVTZ level reveal that all the BBT molecules, including the bare BBT ring, have lower lying LUMO energies (3.70-4.11 eV) compared to the BT derivatives (2.56-3.41 eV) with similar substitution. The reorganization energies (λ(+)/λ(-)) obtained at this level of theory of the BT derivatives are around (225-333)/(246-315) meV, while BBT derivatives have much smaller reorganization energies and these are in the range of (129-259)/(150-230) meV. We observe that the different behavior of BBT is due to the inherited biradicaloid character from the parent molecule tetramethylenebenzene (TMB), a disjoint non-Kekule biradical having non-bonding molecular orbitals (NBMOs) as the highest occupied molecular orbital (HOMO) and LUMO. Additionally, the perturbation of the orbitals of the biradical TMB to obtain BBT is the major cause for the BBT derivatives to have a larger electron affinity (EA) and a smaller HOMO-LUMO gap (HLG) compared to BT derivatives.

Substituents Destabilize the Molecule by Increasing Biradicaloid Character and Stabilize by Intramolecular Charge Transfer in the Derivatives of Benzobis(thiadiazole) and Thiadiazolothienopyrazine: A Computational Study

Keeping in view the possible applications of singlet open-shell molecules as semiconductors, non-classical derivatives of the heterocyclic rings benzobis(thiadiazole) (BBT) and its positional isomer thiadiazolothienopyrazine (TTP) are characterized using DFT methodologies. M06-2X, B3LYP and BHandHLYP functionals were used to optimize the geometries and estimate the vertical transition energies. It is observed that unlike the BHandHLYP functional (50% exchange), which gives rise to spin-contaminated solutions for all molecules in the series, M06-2X (54% exchange) affords a wavefunction either with no instability or negligible instability for most of the molecules. The results are compared with the earlier reported experimental data and those obtained herein using the spin-flip (SF)-5050 method. It is found that B3LYP does not fare well while on the other hand the M06-2X and SF-50-50 are in good agreement with the experimental results. It is seen that M06-2X TD-DFT for the molecules can be carried out without major spin contamination and also that the more time-consuming CI can be avoided for the calculation of transition energies. The biradical nature of the molecules is estimated by the singlet-triplet gap. Intramolecular charge transfer is calculated. It is found that the ring substituents donate charge in the ground state, creating a zwitterionic structure. Thus the substituents play an interesting dual role, decreasing the stability of the molecule by increasing the biradical character (small HOMO-LUMO gap), and stabilization of this ground state by intramolecular charge transfer.

On the nature of some biradicaloid and antiaromatic Vis/NIR‐chromophores

AbstractDyes derived from the biradicalic oxyallyl and cyclopentadienylium were calculated by time‐dependent density functional theory (TDDFT) and the characteristics of the prominent low‐energy transitions revealed by graphical means. According to theoretical and experimental studies, 4‐aminophenyl‐substituted dyes absorb intensely at long wavelengths up to the near infrared. If the amino groups are removed the absorption wavelengths are changed little. As found in the previous studies on the squaraine and croconaine oxyallyl dyes, the substituents play only a minor role in the spectral excitation. Charge‐transfer‐type excitations do not occur between the donor aryl substituents and the central oxyallyl or cyclopentadienylium acceptor group. This behaviour is exceptional since donor–acceptor compounds tend to produce charge‐transfer‐ or polymethine‐type electronic transitions. The hitherto rarely used electron density difference (EDD) maps clearly unveiled the spectral excitation features. The spectral excitation of the title compounds is predominantly localized at the oxyallyl and cyclopentadienylium groups, respectively. Characteristics of simple chromophoric compounds and of conventional CT‐ and polymethine dyes are given for comparison. The biradicaloid character of these dyes is supported by the calculated low singlet–triplet splitting energies. Spin properties were characterized in terms of expectation values of the S2‐operator and antiaromatic properties in terms of nucleus‐independent chemical shifts (NICS). According to the ΔES/T and <S2> criteria compounds with the acyclic oxyallyl fragment are more biradicaloid. The parent compounds oxyallyl, thioxyallyl and cyclopentadienylium, display extremely large <S2> values. These compounds are triplets in the ground state. The absorption wavelengths of selected biradicaloid species were also calculated by the multi‐reference SORCI method. Copyright © 2010 John Wiley & Sons, Ltd.

Biradicaloid Character of Thiophene‐Based Heterophenoquinones: The Role of Electron–Phonon Coupling

The quinoidal versus biradicaloid character of the ground state of a series of thiophene-based heterophenoquinones is investigated with quantum-chemical calculations. The role of the ground-state electronic character on molecular structure and vibrational properties is emphasized. The vibrational activities are experimentally determined and their analysis is performed by taking advantage of the definition of a collective vibrational coordinate (the R coordinate) maximizing the electron-phonon coupling, and connecting the quinoid and the aromatic biradicaloid resonance structures. The combined experimental and computational investigation supports the biradicaloid nature of the longer oligomers. The modulation of Raman intensities and frequency dispersion, experimentally observed by increasing the length of the chromophore, is shown to be reproduced well by model calculations on a single chromophore as a function of geometry displacements along the R-mode. These results underline the role of electron-phonon coupling in governing the structure-property relationship of highly conjugated organic compounds, underscoring the similarity of thiophene heterophenoquinone systems with other, more classical, oligophenylene and oligothiophene derivatives.

Biradicaloid and Polyenic Character of Quinoidal Oligothiophenes Revealed by the Presence of a Low-Lying Double-Exciton State

Evidence of the biradicaloid and polyenic character of quinoidal oligothiophenes is reported by proving at the CASSCF//CASPT2 computational level the presence of a low-lying double exciton state responsible for the weak features observed in the NIR absorption region of the longest members of this class of molecules. The energy lowering of this state, accompanying the length increase in the oligomers, causes a displacement of the ground-state equilibrium geometry toward more biradicaloid structures because of the more efficient S0-S1 state mixing. Furthermore, it is shown that the doubly excited state is strongly coupled to the ground electronic state, and the coupling is mediated by a collective mode dominated by the out-of-phase stretching of adjacent CC bonds, recently shown to govern the Raman activity. All together, this evidence offers a unified view of the low-lying electronic states for quinoidal oligothiophenes and polyenes.

Protonation‐Induced Formation of a Stable Singlet Biradicaloid Derived from a Modified Sapphyrin Analogue

Acid does the trick: A novel 1,10-phenanthroline-embedded sapphyrin analogue 1 (see scheme; R=COOEt, Ar=p-tolyl) bearing meso alkylidenyl double bonds has been synthesized. Interestingly, the protonated form of 1 ([1⋅3 H]3+) exhibits singlet biradicaloid character. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Theoretical Analysis of Intermolecular Covalent π−π Bonding and Magnetic Properties of Phenalenyl and spiro-Biphenalenyl Radical π-Dimers

Singlet-triplet splittings DeltaEST and intermolecular covalent pi-pi bonding characteristics of the prototypical phenalenyl pi-dimer and eight spiro-biphenalenyl radical pi-dimer structures are analyzed with the aid of restricted and unrestricted density functional theory calculations and paramagnetic susceptibility data fitted using the Bleaney-Bowers dimer model and the Curie-Weiss model. Single determinant approximations for DeltaEST as a function of transfer integrals and on-site Coulomb repulsion energy are presented for the two-electron two-site pi-dimers of phenalenyls and the two-electron four-site pi-dimers of spiro-biphenalenyl radicals. Within the range of intermolecular separation of 3.12<D<3.51 A, for the shorter separations, restricted theory works quite well and indicates the presence of a relatively strong intermolecular covalent pi-pi bonding interaction. For the longer separations, the singlet-triplet splittings are small; electron correlation plays a significant role, and only the unrestricted theory provides results that are in qualitative agreement with experiments. The bonding interactions in the pi-dimers are gradually weakened with increasing D, showing a transition from low D values with significant intermolecular pi-pi bonding and electron delocalization to high D values with localized spins and a biradicaloid character.

Synthesis and Biradicaloid Character of Doubly Linked Corrole Dimers

Directly linked corrole dimer 1, which has a planar cyclooctatetraene in its core, has been synthesized from 2-borylcorrole in excellent yield. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) oxidation of 1 provides its oxidized form 4, which has two NH protons in each corrole core. Dicorrole 4 and its Zn(II) complex 6 exhibit broad electronic absorption spectra, and magnetic measurement has revealed their biradicaloid character.

Synthesis and Biradicaloid Character of Doubly Linked Corrole Dimers

Directly linked corrole dimer 1, which has a planar cyclooctatetraene in its core, has been synthesized from 2-borylcorrole in excellent yield. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) oxidation of 1 provides its oxidized form 4, which has two NH protons in each corrole core. Dicorrole 4 and its Zn(II) complex 6 exhibit broad electronic absorption spectra, and magnetic measurement has revealed their biradicaloid character.

Biradicaloid character of phenalenyl-based aromatic compounds with a small HOMO–LUMO gap

Abstract Biradicaloid character of three Kekule aromatic compounds containing two phenalenyl moieties is discussed on the basis of the theoretical and experimental results. DFT calculation of the compounds reveals a small HOMO–LUMO gap with a large spatial overlap between them, leading to a singlet biradical character in a ground state and an excited triplet biradical state with a small ΔES–T. Singlet biradical character for 1 (see Fig. 1 ) is indicated by the X-ray crystallographic analysis, which shows dimeric pairs with substantially short non-bonding contacts of ∼3.1 A. The ESR measurements for 1 and 3 give typical spectra for triplet species and the temperature dependence of the half-field signal indicates the thermal excitation to the triplet states.