Conformations of oligosilanes with ethyl and methyl substituents

Conformations of cyclopentasilane stereoisomers control molecular junction conductance

Here we examine the impact of ring conformation on the charge transport characteristics of cyclic pentasilane structures bound to gold electrodes in single molecule junctions. We investigate the conductance properties of alkylated cyclopentasilane cis and trans stereoisomers substituted in the 1,3-position with methylthiomethyl electrode binding groups using both the scanning tunneling microscope-based break junction technique and density functional theory based ab initio calculations. In contrast with the linear ones, these cyclic silanes yield lower conductance values; calculations reveal that the constrained dihedral geometries occurring within the ring are suboptimal for σ-orbital delocalization, and therefore, conductance. Theoretical calculations reproduce the measured conductance trends for both cis and trans isomers and find several distinct conformations that are likely to form stable molecular junctions at room temperature. Due to the weakened σ-conjugation in the molecule, through-space interactions are found to contribute significantly to the conductance. This manuscript details the vast conformational flexibility in cyclopentasilanes and the tremendous impact it has on controlling conductance.

Electronic transitions in conformationally controlled tetrasilanes with a wide range of SiSiSiSi dihedral angles

Unlike π-electron chromophores, the peralkylated n-tetrasilane σ-electron chromophore resembles a chameleon in that its electronic spectrum changes dramatically as its silicon backbone is twisted almost effortlessly from the syn to the anti conformation (changing the SiSiSiSi dihedral angle ω from 0 to 180°). A combination of UV absorption, magnetic circular dichroism (MCD), and linear dichroism (LD) spectroscopy on conformationally controlled tetrasilanes 1-9, which cover fairly evenly the full range of angles ω, permitted a construction of an experimental correlation diagram for three to four lowest valence electronic states. The free chain tetrasilane n-Si4 Me10 (10), normally present as a mixture of three enantiomeric conformer pairs of widely different angles ω, has also been included in our study. The spectral trends are interpreted in terms of avoided crossings of 1B with 2B and 2A with 3A states, in agreement with SAC-CI calculations on the excited states of 1-7 and conformers of 10.

Calculation of Relative Energies of Permethylated Oligosilane Conformers in Vapor and in Alkane Solution

The geometries of 35 conformers of Me(SiMe2)nMe (n = 4, 1; n = 5, 2; n = 6, 3; n = 7, 4) were optimized at the MP2/VTDZ level, and CCSD(T) single-point calculations were done at three MP2/VTDZ conformer geometries of 1. The relative ground-state energies of the conformers of 1-4 in the gas phase were obtained from the MP2/VTDZ electronic energy, zero-point vibrational energy, and thermal corrections at 0, 77, and 298 K. Relative energies in an alkane solvent at 77 and 298 K were obtained by the addition of solvation energies, obtained from the SM5.42R model. The calculated energies of 26 of the conformers (n = 4-6) have been least-squares fitted to a set of 15 additive increments associated with each Si-Si bond conformation and each pair of adjacent bond conformations, with mean deviations of 0.06-0.20 kcal/mol. An even better fit for the energies of 24 conformers (mean deviations, 0.01-0.09 kcal/mol) has been obtained with a larger set of 19 increments, which also contained contributions from selected combinations of conformations of three adjacent bonds. The utility of the additive increments for the prediction of relative conformer energies in the gas phase and in solution has been tested on the remaining nine conformers (n = 6, 7). With the improved increment set, the average deviation from the SM5.42R//MP2 results for solvated conformers at 298 K was 0.18 kcal/mol, and the maximum error was 0.98 kcal/mol.