Gauche, Ortho, and Anti Conformations of Saturated A<sub>4</sub>X<sub>10</sub>Chains:  When Will All Six Conformers Exist?

Neumann, Frank; Teramae, Hiroyuki; Downing, J. W.; Michl, Josef

doi:10.1021/ja971406n

Cited by 61 publications

(91 citation statements)

References 35 publications

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“…Steric effects are also responsible for the observation of three conformational families of dihedral angles: gauche (G, ω ~ 60°), ortho (O, ω ~ 90°) and anti (A, ω ~ 180°) dominating in linear polysilanes [96][97][98]. Even more conformers are possible in branched and dendritic polysilanes due to the redundancy of silicon-silicon chains combined with strongly repulsive interactions between substituents.…”

Section: Conformational Effectsmentioning

confidence: 99%

Polysilane Dendrimers

Krempner

2012

Polymers

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Abstract:The synthesis, structure and electronic properties of polysilane dendrimers, a relatively new class of highly branched and silicon-rich molecular architectures is reviewed. After a detailed discussion of main synthetic strategies to well-defined single-core and double-core polysilane dendrimers, important structural and conformational features determined by single crystal X-ray crystallography and 29 Si-NMR spectroscopy are presented. The last part highlights the most interesting photochemical properties of polysilane dendrimers such as UV absorption and emission behavior, which are compared with those of linear and branched polysilanes.

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Section: Conformational Effectsmentioning

confidence: 99%

Polysilane Dendrimers

Krempner

2012

Polymers

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“…Emission from shorter oligosilane chains often peaks at the edge of the visible (5), and occasionally at wavelengths as long as 500 nm (6). The conformational behavior of these flexible chains is extraordinarily rich (7,8). Six potential energy minima are generally available for torsion about each internal Si-Si bond, many conformers tend to have comparable energies, and barriers to their interconversion are low.…”

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confidence: 99%

Magnetic circular dichroism of peralkylated tetrasilane conformers

Fogarty

Imhof

Michl

2004

Proc. Natl. Acad. Sci. U.S.A.

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Magnetic circular dichroism (MCD) of five peralkylated tetrasilanes (1-5) conformationally constrained to angles ranging from nearly 0°to 180°and of the open chain tetrasilane Si4Me10 (6) shows a clear conformational dependence and permits the detection of previously hidden transitions. In the tetrasilane CH2Si4Me8 (1), with the smallest dihedral angle, comparison of MCD with absorption spectra reveals four low-energy electronic transitions. In the tetrasilanes 2-4, three distinct transitions are apparent. In tetrasilanes 5 and 6, MCD reveals the very weak transition that has been predicted to be buried under the first intense peak and to which the anomalous thermochromism of 6 and other short-chain oligosilanes has been attributed. Linear oligosilanes and polysilanes, Si n R 2nϩ2 (R ϭ alkyl), are fully saturated hydrocarbon analogs with very unusual optical properties and have been of potential practical interest, for instance, as nonlinear optical materials, photoresists, and charge conductors (1, 2).They are of considerable theoretical interest as well. Their most striking property is long-wavelength absorption and luminescence. For long peralkylated polysilane chains, the first absorption peak is in the near-UV range, between 330 and 380 nm (3), and the emission peak is at somewhat longer wavelengths (4). Emission from shorter oligosilane chains often peaks at the edge of the visible (5), and occasionally at wavelengths as long as 500 nm (6). The conformational behavior of these flexible chains is extraordinarily rich (7,8). Six potential energy minima are generally available for torsion about each internal Si-Si bond, many conformers tend to have comparable energies, and barriers to their interconversion are low. In permethylated oligosilanes, the calculated preferred absolute values of backbone dihedral angles are Ϸ55°(gauche), Ϸ90°(ortho), and Ϸ165°(transoid) (9 -12). In oligosilanes with longer alkyl substituents, other angles (deviant and cisoid) can occur as well (13,14).The electronic absorption and emission spectra of these compounds are very sensitive to chain conformation, as manifested in the thermochromism (15, 16), piezochromism (17), solvatochromism (18), and related properties (19, 20) of polysilanes. They therefore represent an especially suitable vehicle for the study of the incompletely understood but clearly important phenomenon of -electron delocalization and particularly of its structural and conformational dependence. This dependence is important for many properties such as charge and energy transfer and substituent effect and spin-density propagation across saturated systems. Detailed analysis and assignment of electronic transitions of oligosilanes as a function of conformation are therefore important, but they are difficult. The absorption bands are broad and closely spaced, making the detection of weakly allowed transitions difficult. Essentially nothing is known about the triplet states of these molecules; they do not contribute significantly to ordinary absorption spectra, and we ...

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“…[29] The position of O1 [O1-P-Cl 178.33(5)°, P-O cation-anion contact 3.355(2) Å; cf. sum of the van der Waals radii r w (P) + r w (O) = 3.32 Å] [30] is envisaged as the associative site for DMAP in an S N 2 displacement of chloride. The cation in the solidstate structure of 3(OTf) is shown in Figure 3, and is comparable to that of the previously reported (DMAP·PPh 2 )(OTf).…”

Section: Introductionmentioning

confidence: 99%