The Disilane Chromophore:  Photoelectron and Electronic Spectra of Hexaalkyldisilanes and 1,(n+2)-Disila[n.n.n]propellanes

Abstract

Photoelectron spectra and solution UV absorption and magnetic circular dichroism (MCD) of hexamethyldisilane (1), hexaethyldisilane (2), hexa-tert-butyldisilane (3), and the 1,(n+2)-disila[n.n.n]propellanes [n = 4 (4) and 5 (5)] were measured, as was the linear dichroism (LD) of 3 and 4 partially aligned in stretched polyethylene. The results support the assignment of the lowest energy electronic absorption band of the disilanes 1−5 to a doubly degenerate σSiSi(HOMO) → π*SiC(LUMO) transition and of the next band, observed in the solution spectra of 2−4 and in the gas-phase spectrum of 1, to a σSiSi → σ*SiSi transition. MP2/VTZ optimized geometries of 1−5 and ab initio molecular orbital energies (HF/VTZ//MP2/VTZ) and ionization potentials (ROVGF/VTZ//MP2/VTZ) of these disilanes reproduce the reported geometries and the trends observed in the photoelectron spectra, respectively. B3LYP/6-31G(d,p) calculations of the Kohn−Sham orbital energies and TD B3LYP/6-31G(d,p) calculations of transition energies and intensities of 1 as a function of Si−Si bond length suggest that many of the features of the UV absorption spectrum of 3, including the small energy difference between the two transitions observed and the large extinction coefficient of the band peaking at higher energy (σSiSi → σ*SiSi), are due to its very long Si−Si bond.

Polysilane Dendrimers

“…239 pm for some of the dendritic polysilanes. As demonstrated for various substituted disilanes by Michl et al, changes of the silicon-silicon distances may affect the energy of the σ-σ* transition and consequently the electronic properties of the bulk material quite dramatically [105]. Note, for example, that sterically crowded hexasilane dendrimer 51 (conformation EDE) clearly outperforms the conformationally rigid carbocyclic hexasilane 62 (conformation AAD) by 10 nm with respect to the wavelength of the absorption maximum (see Table 7).…”

“…Table 6 summarizes the overall conformation and average silicon-silicon distances of the longest silicon chains in selected polysilane dendrimers obtained from the X-ray data along with the measured UV absorption maxima. Note, that the average silicon-silicon distances of the longest silicon chains of all structurally characterized polysilane dendrimers (except for 51) lie within the relatively narrow range of 236-238 pm, suggesting only minimal influence of silicon-silicon bond stretching on the energy of the σ-σ* transition [105] and allowing for comparison of both conformational arrangements and UV absorption characteristics of individual dendrimers with each other.…”

Why do disilanes fail to fluoresce?

“…In an infinite chain, the most stable among them has no nodes across Si-Si bonds, and an excitation from the HOMO into it is forbidden by the local symmetry at each Si atom. In chains of finite length, chain-end effects cause minor modifications, but the first σσ* transition remains strongly allowed and the first σπ* transition extremely weak, except in disilane, where it is strong, since in this case the nodal properties of the σ and the π* MOs match 26,27 .…”

“…In the longer chains, the first vertical excited state is of σσ* nature, and in the shorter chains, it is of σπ* nature. In hexamethyldisilane (1), the two states differ considerably in their energies 26,27 . The first vertical σπ* state occurs at~52 000 cm -1 and states of σσ* nature are believed to be located above 62 000 cm -1 .…”

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

“…Numerous ab initio and density functional calculations of electronic excitation in alkylated oligosilanes have been published previously: disilanes, [46,61] trisilanes, [62,63] [17,34,35,[40][41][42][43][44] hexasilanes, [17] heptasilanes, [17] and series of oligosilanes in regularly repeating conformations, [32,64] and have provided a good account of the observed spectra. The DFT and TD-DFT procedures with a B3LYP functional and G-311GA C H T U N G T R E N N U N G (d,p) (TZ) basis [65] appear particularly suitable for use with the longer permethylated oligosilanes.…”