Curve Effect on Singlet Diradical Contribution in Kekulé-type Diradicals: A Sensitive Probe for Quinoidal Structure in Curved π-Conjugated Molecules

Abstract

Curved (non-planar) aromatic compounds have attracted significant research attention in the fields of basic chemistry and materials science. The contribution of the quinoidal structure in the curved π-conjugated structures has been proposed to be the key for materials functions. In this study, the curve effect on the quinoidal contribution was investigated in Kekulé-type singlet diradicals (S-DR1-4) as a sensitive probe for quinoidal structures in curved π-conjugated molecules. The quinoidal contribution in S-DR1-4 was found to increase with increasing the curvature of the curved structure, which was quantitatively analyzed using NBO analysis and the natural orbital occupation numbers computed by the CASSCF method. The curve effect on the singlet-triplet energy gap was examined by the CASPT2 method. The singlet-triplet energy gaps for the highly π-conjugated diradicals were determined for the first time using the CASPT2 method. Substantial quinoidal contribution was found in the curved structures of the delocalized singlet diradicals S-DR1-4, in contrast to its absence in the corresponding triplet states T-DR1-4.

Proquinoidal-Conjugated Polymer as an Effective Strategy for the Enhancement of Electrical Conductivity and Thermoelectric Properties

“…In a recent theoretical study on the curvature effect of quinoidal cycloparaphenylenes, a larger bent angle was found to lower y 0 . 33 Hence, the lowest y 0 of pBBTa26-4T is highly likely because of the large curvature in its backbone (Figure S7, Supporting Information) induced by the large dihedral angles between the thiophenes with the methyl group. Planarization of the polymer backbone is often known to occur in the solid state.…”

1,3-Diradicals Embedded in Curved Paraphenylene Units: Singlet versus Triplet State and In-Plane Aromaticity

“…In the late 2000s, pioneering studies in the synthesis of a new series of π-conjugated molecules, cycloparaphenylenes (CPPs), were reported; since then, these intriguing molecules have attracted much attention. − The most striking feature of these hoop-shaped molecules is the hypsochromic shift of the HOMO–LUMO energy gap with an increasing number of benzene rings in the CPP structures (Figure b). − This behavior is totally opposite to that of linear paraphenylenes. This unique behavior is rationalized as being a result of the quinoidal contribution of the small CPPs arising from the diradical character induced by the curved structure of the benzene rings . In the present study, we designed diradicals DR -( n +3) CPPs to investigate the quinoidal character in the hoop-shaped structures (Figure c).…”

“…This unique behavior is rationalized as being a result of the quinoidal contribution of the small CPPs arising from the diradical character induced by the curved structure of the benzene rings. 37 In the present study, we designed diradicals DR-(n+3)CPPs to investigate the quinoidal character in the hoop-shaped structures (Figure 1c). Among the precursors AZ-(n+3)CPPs featuring curved paraphenylene units, the novel azoalkane AZ-6CPP (n = 3) was synthesized to investigate its electronic character and the corresponding chemistry of the diradical DR-6CPP (n = 3).…”

Diradical Character of Neutral Heteroleptic Bis(1,2-dithiolene) Metal Complexes: Case Study of [Pd(Me2timdt)(mnt)] (Me2timdt = 1,3-Dimethyl-2,4,5-trithioxoimidazolidine; mnt2– = 1,2-Dicyano-1,2-ethylenedithiolate)

“…The triplet GS can generally be calculated by theoretical methods with unrestricted wave functions, such an unrestricted HF (UHF) or density functional theory (UDFT). The modeling of open-shell singlet diradicals requires multireference approaches: for instance, multireference coupled-cluster calculations, such as Mk-CCSD­(T), complete active-space self-consistent field (CASSCF), or the complete-active-space second-order perturbation theory (CASPT2) . In fact, although they are computationally very efficient, DFT calculations cannot accurately describe open-shell singlet states of diradicals, so that Δε ST and hence J values are largely uncertain.…”

“…The modeling of open-shell singlet diradicals requires multireference approaches: for instance, multireference coupled-cluster calculations, such as Mk-CCSD(T), 129 complete active-space self-consistent field (CASSCF), 94 or the complete-active-space second-order perturbation theory (CASPT2). 130 In fact, although they are computationally very efficient, DFT calculations cannot accurately describe open-shell singlet states of diradicals, so that Δε ST and hence J values are largely uncertain. The brokensymmetry (BS) DFT (DFT-BS) spin-unrestricted reference configuration with antiparallel spins has been proposed as a compromise to extend UDFT calculations to diradical species, 91,131−135 by correcting J and Δε ST for spin contamination 94,133,134 that affects the expectation value of = ε 0 + ZPE, kcal mol −1 ), Sum of Electronic Energies and Thermal Enthalpies (H corr , kcal mol −1 ), and Free Energies (G corr , kcal mol −1 ) Calculated for the 1,2-Dithiolene Ligands mnt q− and Me 2 timdt q− (q = 0, 1, 2) the total spin ⟨S 2 ⟩ with respect to ⟨S(S + 1)⟩.…”