σ Bonds: Electronic structure, photophysics, and photochemistry of oligosilanes

Abstract

The making and breaking of σ bonds is an integral part of almost all photochemical reactions. Yet, the electronic states of σ electrons are not nearly as well understood as the states of π-electron systems. Efforts in our laboratory to enhance the current state of their understanding are described, using the specific example of oligosilanes. We address the intrinsically cyclic nature of σ delocalization and its dependence on chain length and conformation, both in terms of theory and spectroscopic experiments, from the simplest disilane chromophore to the spectral properties of the individual conformers of permethylated heptasilane. We also describe a new low-energy luminescence from certain conformers of permethylated oligosilanes.

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

“…The left-hands ide of Figure2 compares the low-temperature UV absorption spectra of the linear permethylated oligosilanes Si n Me 2n + 2 (n = 4, 6, 8, and 10), [24,[42][43][44][45][46][47][48] believed to be due to the all-[t]c onformers with ar andomly distributed helicitys ense, [4] with the transition energies and oscillator strengths calculated (TD-DFT/TZ)f or the all-[t]c onformers all-[t]-1 [4] to all-[t]-1 [10]. The only observedi ntensea bsorption maximum gradually shifts from 43 100 cm À1 in Si 4 Me 10 to 34 100 cm À1 in Si 10 Me 22 , and gains af actor of nearly 10 in intensity in the process.…”

“…In peralkylated n-tetrasilanes, the spectra of which have been studied in considerable detail in as eries of conformations, these additional transitions have been uncovered and are in impressive agreement with the resultso fT D-DFT/TZ, CAS-PT2, and SAC-CI calculations. [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.…”

“…[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. Along with these spectra, Figure2 shows the resultso fT D-DFT/TZ calculations for the permethylated linear analogs of these polycyclic oligomers ([c]-1 [4] to all-[ca]-1 [10] in Figure 1, with uniform helical sense).…”

“…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. Along with these spectra, Figure2 shows the resultso fT D-DFT/TZ calculations for the permethylated linear analogs of these polycyclic oligomers ([c]-1 [4] to all-[ca]-1 [10] in Figure 1, with uniform helical sense). In this series of oligosilanes, there is no redshift in the observed primary absorption maximum with increasing chain length and it remains near 41 000 cm À1 ([c]-2 [4] is not at rue member of the series as it has only one backbone dihedral angle).…”

“…[1][2][3][4] They are sensitive to the backbone conformation and as ar esult lead to phenomena such as thermochromism, [5][6][7][8][9] piezochromism, [10,11] electrochromism, [12] ionochromism, [13,14] and solvatochromism. [15] Also their single-molecule electrical conductivity appearst od epend on conformation.…”