The diradical character of polyacenequinododimethides

Radenković, Slavko; Marković, Svetlana; Kuč, Ratko; Stanković, Nevena

doi:10.1007/s00706-011-0557-8

Cited by 9 publications

(8 citation statements)

References 38 publications

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“…Thus, it is evident by the larger BRE and CCE numbers given within the rings of the molecules in the series shown in Figure that the rings most remote to the exo -double bonds are the most aromatic by these measures. The upper two series in Figure were also theoretically studied in regard to diradical character …”

Section: Resultsmentioning

confidence: 99%

A Quantitative Metric for Conjugation in Polyene Hydrocarbons Having a Single Classical Structure

Dias

2014

J. Phys. Chem. A

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A quantitative measure of the extent of conjugation is introduced. The number of Dewar resonance structures (DS) in a conjugated hydrocarbon quantitatively determines its extent of conjugation interaction. The greater the degree of conjugation the more stable is the conjugated hydrocarbon. The number of single bonds joining the exo-double bonds of a purely cross-conjugated hydrocarbon is equal to its number of Dewar resonance structures (DS). Polyene cross-conjugated hydrocarbons are minimally conjugated and possess only one classical structure with a succession of fixed single and exo-double bonds. There are three increasing levels of conjugation for systems with a single classical structure according to whether they are cross-conjugated, linear extended two-way conjugated, and ring-enhanced conjugated. Numerous analytical expressions have been derived. Our valence-bond (VB) approach involves studying the interrelationship of several well-known series of conjugated hydrocarbons having only a single classical structure, i.e., the number of Kekulé resonance structures is one (K = 1). This work provides a totally new perspective to the concept of conjugation.

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Section: Resultsmentioning

confidence: 99%

A Quantitative Metric for Conjugation in Polyene Hydrocarbons Having a Single Classical Structure

Dias

2014

J. Phys. Chem. A

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“…[40][41][42][43] One can include analysis of other descriptors such as the singlet-triplet gap~E(ST), spin density or N FOD i. e. the integral over all the space of the fractional orbital density from finite-temperature DFT. [44][45][46][47][48] This work will focus only on results assessed by the NOON method using CASSCF as multiconfigurational methods are more naturally designed to describe fractional occupation numbers of orbitals. We note that diradical descriptors evaluated using PUHF may be different for the same molecule computed with CASSCF, however we have not investigated the structures using PUHF for comparison as it is not uncommon for triplet calculations to converge to an incorrect triplet state.…”

Section: Methodsmentioning

confidence: 99%

“…One can include analysis of other descriptors such as the singlet‐triplet gap ▵E(ST), spin density or N FOD i. e . the integral over all the space of the fractional orbital density from finite‐temperature DFT . This work will focus only on results assessed by the NOON method using CASSCF as multiconfigurational methods are more naturally designed to describe fractional occupation numbers of orbitals.…”

Section: Methodsmentioning

confidence: 99%

Elucidating the Relationship between Multiradical Character and Predicted Singlet Fission Activity

2020

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The diradical and multiradical characters of a large and diverse set of 241 predicted singlet fission candidates are determined by multiconfigurational wavefunction methods, specifically natural orbital occupation number (NOON) analysis using the CASSCF method. The ascertained multiradical character strongly supports the current prediction that a good singlet fission candidate, independent of its structure, will tend to be of the weak biradicaloid class as well as having very little tetraradical character. Although the rule remains robust, it is not absolute as a small portion of our potential candidates are predicted to be closed shell. Additionally, a smaller handful of outliers with apparent tetraradical characters are identified, suggesting there may be the possibility, albeit rare, of singlet fission amongst weak tetraradicaloids.

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“…Two related families of biradicaloids that illustrate this point are the o-xylylenes, 9, 1 and the linear acenes, 10. Recent u-B3LYP calculations on 9 through n = 8 58 and RASPT2 calculations through n = 6 129 find an apparent limiting preference for singlet ground states. The linear acenes (10) have been studied by a variety of high-level theoretical approaches.…”

Section: ■ Computational Methodsmentioning

confidence: 99%

“…2−4 The question is then whether such systems can be designed in which the exchange interaction favoring the triplet is sufficient to overcome the covalent character of the singlet. Most computational studies of the acenes 50−57 and the related o-xylylenes 58 have found that extending the conjugation via benzannelation does not produce a triplet ground state, nor is there experimental evidence of thermally populated triplets of the recently prepared octacene, nonacene, and nonacene derivatives. 59−62 Our initial design of the fluoranthene-based biradicaloids 1 (n = 1 and 2) 42,43 used simple resonance energy arguments to identify a core structure likely to have a large amount of biradical character, and the expectation that the necessary exchange coupling favoring the triplet would be provided by though-bond coupling via the CMe 2 linker 63,64 or via the embedded 1,8-naphthoquinodimethane 65,66 (2) unit.…”

Section: ■ Introductionmentioning

confidence: 99%

Rational Design of High-Spin Biradicaloids in the Isobenzofulvene and Isobenzoheptafulvene Series

Snyder

2012

J. Phys. Chem. A

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The relative energies of singlet biradicaloid and of triplet and singlet biradical electronic states for a series of benzannelated isobenzofulvenes and isobenzoheptafulvenes were calculated at the (u-)B3LYP/6-31G(d), full π-space CASSCF-CASPT2 (≤14 π-e(-)s), and full π-space RASSCF-RASPT2 (≤24 π-e(-)s) levels of theory. Both absolute and relative CASPT2 energies were reproduced quite well by the RASPT2 approach, which can be extended to much larger active spaces. RASPT2 (and DFT) calculations find that increasing benzannelation leads to triplet ground states in both hydrocarbon series, in violation of the classical principle of maximum bonding. This confirmed the expectations that the combined effects of resonance energy and aromaticity could compensate for the extra formal π-bond of the biradicaloid singlet, and that the strong exchange coupling inherent to the embedded trimethylenemethane (TMM) would manifest itself in the biradicaloids. The relative energy of the biradicaloid singlet rises rapidly upon benzannelation, as π-bonding between the high-energy delocalized GVB orbitals decreases. The underlying π-orbital topology is revealed when this weak π-bonding is artificially eliminated by a 1:1 mixing of the nondegenerate HOMO and LUMO to produce an overcorrelated valence bond (OCVB) orbital pair. For members of both biradicaloid series, the OCVB pairs are nondisjoint, revealing a limiting triplet preference with increasing benzannelation. Within the two-electron, two-orbital approximation, the effects of π-bonding in the singlet biradicaloids and orbital localization away from the acene π-system in the triplet biradicals can be analyzed as perturbations of the singlet OCVB biradicals. The application of a VB-based spin coupling scheme is discussed, in which the unpaired electrons of these species can be considered both ferromagnetically and antiferromagnetically coupled, with the strength of the latter strongly dependent on the acene subunit.

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The diradical character of polyacenequinododimethides

Cited by 9 publications

References 38 publications

A Quantitative Metric for Conjugation in Polyene Hydrocarbons Having a Single Classical Structure

A Quantitative Metric for Conjugation in Polyene Hydrocarbons Having a Single Classical Structure

Elucidating the Relationship between Multiradical Character and Predicted Singlet Fission Activity

Rational Design of High-Spin Biradicaloids in the Isobenzofulvene and Isobenzoheptafulvene Series

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