Electronic Transitions in Conformationally Controlled Peralkylated Hexasilanes

Abstract

The photophysical properties of oligosilanes show unique conformational dependence due to σ-electron delocalization. The excited states of the SAS, AAS, and AEA conformations of peralkylated n-hexasilanes, in which the SiSiSiSi dihedral angles are controlled into a syn (S), anti (A), or eclipsed (E) conformation, were investigated by using UV absorption, magnetic circular dichroism (MCD), and linear dichroism spectroscopy. Simultaneous Gaussian fitting of all three spectra identified a minimal set of transitions and the wavenumbers, oscillator strengths, and MCD B terms in all three compounds. The results compare well with those obtained by using the symmetry-adapted-cluster configuration interaction method and almost as well with those obtained by time-dependent density functional theory with the PBE0 functional. The conformational dependence of the transition energies and other properties of free-chain permethylated n-hexasilane, n-Si Me , was also examined as a function of dihedral angles, and the striking effects found were attributed to avoided crossings between configurations of σσ* and σπ* character.

Intuitive Understanding of σ Delocalization in Loose and σ Localization in Tight Helical Conformations of an Oligosilane Chain

“…In peralkylated n ‐tetrasilanes, the spectra of which have been studied in considerable detail in a series of conformations, these additional transitions have been uncovered and are in impressive agreement with the results of TD‐DFT/TZ, CAS‐PT2, and SAC‐CI calculations . A less detailed study is available for a few conformers of peralkylated hexasilanes …”

“…Figure shows that the computations reproduce the behavior of the energy and the intensity of the first observed absorption maximum in samples with random sense of helicity quite well. It is also known that they perform well for the weak transitions in Si 3 Me 8 and a few tetrasilanes and hexasilanes, and it is reasonable to assume that the TD‐DFT B3LYP/TZ level of computation also describes the regular helical conformers correctly.…”

“…[4,22,25] Al ess detaileds tudy is availablefor af ew conformers of peralkylated hexasilanes. [49] The right-hand side of Figure 2s erves only for highlighting the contrast between s-delocalized and a s-localized behavior and will be dealt with in more detail elsewhere. It displays the reported UV absorption spectra of the alternating polycyclic oligosilanes all-[c]-2 [4] to all-[ca]-2 [10], [21] again with arandomly distributed helicity sense, in which the purpose of the additional alkane chains is to impose on the silicon backbone the geometry of the desired conformation.…”

Tunable SiN Hybrid Conjugated Materials

“…B3LYP/6-311G(d) was selected to allow for comparison to the literature, as Michl et al reported analysis of linear alkylsilanes with this basis set and functional. 46 Significant changes were not observed at higher levels of theory. Good agreement was observed between calculated and experimental HOMO energies (Figure 3a, compare the solid and dashed lines).…”

“…B3LYP/6-311G­(d) was selected to allow for comparison to the literature, as Michl et al. reported analysis of linear alkylsilanes with this basis set and functional . Significant changes were not observed at higher levels of theory.…”