Coupling of Disilane and Trisilane Segments Through Zero, One, Two, and Three Disilanyl Bridges in Cyclic and Bicyclic Saturated Carbosilanes

Several six-membered cyclic and [2.2.2]­bicyclic organosilanes
with
varying proportions of silicon atoms in the bridges have been prepared
following a stepwise approach that exploits dianionic polysilanes.
Focus in our analysis was placed on the bicyclic compounds which all
have silicon atoms at the bridgehead positions. Quantum chemical calculations
of these compounds revealed the possibility to enhance the coupling
through a single cisoid tetrasilane cage segment by replacing one
or two of the other −SiMe2SiMe2–
bridges with −CH2CH2– bridges.
UV absorption spectroscopy revealed a red shift in the lowest visible
transitions when going from a bicyclo[2.2.2]­octane with three −SiMe2SiMe2– bridges to those with two or one
such bridge. However, these red shifts are deceptive, as the lowest
vertically excited singlet states, which are dark according to TD-DFT
calculations, do not display the same trend. Still, since these compounds
have (i) excellent structural rigidity, (ii) provide potentials for
functionalization through their exocyclic trimethylsilyl groups, and
(iii) display electronic structure variations with the number of −SiMe2SiMe2– bridges, they could be interesting
for further studies: e.g., in single-molecule electronics.