Magnetic circular dichroism of peralkylated tetrasilane conformers

Abstract

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 ...

Conformational Dependence of σ‐Electron Delocalization in Linear Chains: Permethylated Oligosilanes

“…Keywords: density functional calculations · electronic structure · oligosilanes · silicon the sensitivity of s-electron delocalization to backbone conformation is provided by the profound changes in the UV spectrum of peralkylated tetrasilanes as the Si-Si-Si-Si dihedral angle is varied. [17,[34][35][36][37][38][39][40][41][42][43][44] The more important and nearly ubiquitous alkanes might appear to be a better choice of substrates for a study of s electron delocalization, but represent a much more difficult target both experimentally and computationally. The two primary reasons are ultimately both related to the more electronegative nature of carbon: 1) alkanes only absorb in the vacuum UV region, for which the choice of solvents is limited, whereas oligosilanes absorb in the near UV and in some instances even close to the visible region, [45] and 2) alkane electronic states are more closely spaced, since excitations from C À C and C À H bond orbitals have comparable energies, whereas in oligosilanes only excitations from Si À Si orbitals are important for the lowest excited states.…”

Electronic Transitions in Conformationally Controlled Tetrasilanes with a Wide Range of SiSiSiSi Dihedral Angles

“…Much effort has been devoted to the investigation of these conformational effects in chains of various lengths. These studies have been both computational (ladder C model,33–36 conformational analysis,27, 28, 37–41 excited state calculations on n ‐Si 4 H 10 ,42–44 n ‐Si 4 Me 10 ( 10) ,33, 45–47 and longer chains32) and experimental (matrix isolation of n ‐Si 4 Me 10 conformers,45, 48 conformational control based on cyclic carbosilanes,6, 8, 48–51 bicyclic disilanes,31, 51–55 bicyclic trisilane,56, 57 and bulky lateral substituents,58 hypervalent silicon,59–62 helicity induction by chiral terminal63–65 or lateral66, 67 substituents, and inclusion into cyclodextrin and other hosts68–73).…”

“…n ‐Tetrasilanes such as 10 have been frequently employed as model compounds,6, 8, 45–47, 49, 50 since they are the shortest oligosilanes that exhibit backbone conformational isomerism. Their backbone possesses a two‐fold axis of symmetry and their states can transform according to A or B irreducible representations.…”

“…Between the two limits, the two B states are allowed to mix and their crossing is avoided. The behavior of transitions into these states is reflected in the UV spectra of matrix‐isolated conformers of the parent permethylated tetrasilane and of peralkylated tetrasilanes whose conformation has been intentionally controlled 6. 45, 46, 49, 50, 52, 53 The situation is even more complicated, since in the vicinity of the syn geometry ( ω =0°) an avoided crossing with yet another state of B symmetry takes place 18.…”

“…The MCD spectra of 1 ‐ 4 , 8 , and 10 have been reported in a short communication,6 as have the MCD and LD of 6 53. The disulfone 9 and bis(methylthio)[2]staffane ( 11) are included in order to deal with potential complications from sulfur atom based excitations in the spectra.…”