Conformation Dependence of Photophysical Properties of σ-π Conjugation as Demonstrated by cis- and trans-1, 2-Diaryl-1, 2-disilacyclohexane Cyclic Systems

Several conligurationally constrained cis- and trans-1,2-diaryl-l,2-dimethyl-l,2-disilacyclohexanes (3a-e) have been synthesized in order to measure their photophysical properties, such as UV absorption, magnetic circulardichroism (MCD), and photoluminescence. As has been well established, the UV absorption maximum ( 1 L a band) occurs at 230-270 nm, due to the effective σ-π conjugation between the aryl groups and the disilane moiety. The absorption maximum wavelength of the trans isomer is slightly longer than that of the cis isomer for all the diaryldisilanes employed in the present study. Although the MCD spectra of the cis- and trans-1,2-diphenyl-1,2-dimethyl-1,2-disilacyclohexanes (3a) show only a slight configuration dependence, this method is helpful to detect a weak 1 L b band. The disilanes 3a and 5a show a charge-transfer (CT) emission band. The quantum yields are dependent on the disilane configuration and the solvent polarity; the order of the quantum yield is cis-3a < 5a < trans-3a in 3-methylpentane, while the order is 5a < cis-3a < trans-3a in acetonitrile. These results demonstrate that the conformation constraint is effective for controlling the photophysical properties of the diaryldisilanes.