Abstract
To explore the prospect of molecules involving silicon-silicon multiple bonds as nonlinear optical molecular systems, the relationship between the structure and the second hyperpolarizabilities γ of the oligomeric systems composed of carbon and silicon π-structures is investigated using the density functional theory method. It is found that these compounds indicate intramolecular charge transfer (ICT) from the silicon units to the carbon units together with nonzero diradical characters. The γ values of these compounds are shown to be 2–13 times as large as those of the carbon analogs. Although asymmetric carbon and silicon π-systems exhibit comparable enhancement to the corresponding symmetric systems, donor-π-donor structures exhibit remarkable enhancement of γ despite of their both-end short silicon π-chain moieties (donor units). Further analysis using the odd electron and γ densities clarifies that the intermediate diradical character also contributes to the enhancement of γ. These results predict that even short π-conjugated silicone moieties can cause remarkable enhancement of γ by introducing them into π-conjugated hydrocarbon structures.
Highlights
Multiple bonds are one of the essential elements for highly efficient functional molecules.Especially, multiple bonds between silicon (Si) atoms have been theoretically and experimentally investigated with great interest since Si containing compounds have several different chemical features from the carbon (C) analogs regardless of their belonging to the same 14 group
We investigate the effects of introducing Si–Si double bonds into C oligomeric π-structures on the γ values from the viewpoint of open-shell singlet nature and intramolecular charge transfer (ICT) nature
These compounds are composed of both ethylene units (C units) and disilene units (Si units)
Summary
Multiple bonds are one of the essential elements for highly efficient functional molecules.Especially, multiple bonds between silicon (Si) atoms have been theoretically and experimentally investigated with great interest since Si containing compounds have several different chemical features from the carbon (C) analogs regardless of their belonging to the same 14 group. Since the first synthesis of disilene in 1981 [4], modern synthesis technique has enabled the synthesis of a variety of molecules with Si-Si multiple bonds, such as disilyne [2,5,6], tetrasilabuta-1,3-diene [7], π-conjugated systems [8], oligomers with long main chains [9], and so on. Such compounds are expected to have unique properties due to their stimuli-responsibilities of Si-Si multiple bonds. For enhancement or Molecules 2016, 21, 1540; doi:10.3390/molecules21111540 www.mdpi.com/journal/molecules
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