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

Misaki Matsumoto,Manabu Abe,Ivana Antol

doi:10.3390/molecules24010209

Misaki Matsumoto, Manabu Abe + Show 1 more

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https://doi.org/10.3390/molecules24010209

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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.

Highlights

Curved aromatic compounds like fullerenes, buckybowls, and carbon nanotubes have attracted considerable attention from researchers in the fields of basic chemistry as well as materials science [1,2,3,4,5,6,7,8,9,10,11]
The quinoidal contribution is rationalized by the increase of diradical character in the bent structure of benzene, which is the intermediate structure for the formation of Dewar benzene [29,30,31,32]
S-DR1-4 [33,34,35,36,37] is investigated to design a sensitive probe for the quinoidal contribution (q) in curved π-conjugated molecules (Figure 1c) [35,36,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55]

Summary

Introduction

Curved (non-planar) aromatic compounds like fullerenes, buckybowls, and carbon nanotubes have attracted considerable attention from researchers in the fields of basic chemistry as well as materials science [1,2,3,4,5,6,7,8,9,10,11]. According to recent reports on cycloparaphenylenes ([n]CPPs, n being the number of benzene rings in the structure) [13,14,15,16,17,18,19], which are hoop-shaped carbon molecules, the absorption bands were blue-shifted with increasing number of benzene rings [20,21,22,23,24,25] This has been explained by the quinoidal characteristic of CPPs having small ring size, such as [6]CPP (Figure 1a). The quinoidal contribution is rationalized by the increase of diradical character in the bent structure of benzene, which is the intermediate structure for the formation of Dewar benzene [29,30,31,32]. Triplet and (T) forms of DR1 were optimized to obtain C2 symmetry at the UB3LYP/6-31G(d) level of

Results

Computations

Conclusions