Cooperation of σ–π and σ*–π* Conjugation in the UV/Vis and Fluorescence Spectra of 9,10-Disilylanthracene

Abstract

In 1996, we reported that silyl groups of 9,10-disilylanthracenes significantly affect the UV/Vis and fluorescence spectra. Although the results indicate that the silyl groups have strong electronic effects on anthracene, the details of the mechanisms responsible for this have not yet been clarified. This article describes the analysis of the UV/Vis and fluorescence spectra of 9,10-bis(diisopropylsilyl)anthracene by theoretical calculations. This study reveals that π conjugation of anthracene is extended by cooperation of σ–π and σ*–π* conjugation between the silyl groups and anthracene. This effect increases the transition moment of the π–π* transition of anthracene. As a result, the molecular extinction coefficient of the 1La band and the fluorescence quantum yield are increased.

Silapillar[n]arenes: Their Enhanced Electronic Conjugation and Conformational Versatility

“…π-Conjugated units can be installed into the rims of pillar­[ n ]­arenes, − however, the π electron communication between the units is very weak because the bridging methylenes disrupt the electronic communications between the π orbitals of the aromatic units. Incorporation of heteroatoms into organic π-conjugated systems is highly effective in tuning of the optoelectronic properties. − Silicon is tetravalent with a tetrahedral molecular geometry similar to carbon, while silylene-bridged π-conjugated systems provide quite different optoelectronic properties compared with their carbon analogues. − For instance, they often show σ–π conjugation between σ (Si–C) and π orbitals or σ*−π* conjugation between σ* (Si–C) and π* orbitals, leading to delocalization of π* orbitals via the bridging silicon atoms. − So far, several silylene-bridged macrocycles have been synthesized including meta-birdged silacalix[4]­arenes. − In general, the meta conjugation is weak due to the twisting of neighboring units and the inherent nodal nature of the frontier molecular orbitals of the benzene units. − Therefore, the para-linked structures of pillar­[ n ]­arenes are ideal for electronic conjugation. Moreover, because the Si–C bond is longer than the C–C bond (bond length: C–Si ∼ 1.87 Å vs C–C ∼ 1.53 Å), silylene-bridging can offer additional structural flexibility in the synthesis of strained molecules.…”