<p>The basic building blocks in molecular electronics often correspond to conjugated molecules. A compound class consisting of rigid rod-like staffane molecules with the heavier Group 14 elements Si, Ge, Sn and Pb at their bridgehead positions has now been investigated. Herein these oligomers are called heavy-core or Si-, Ge-, Sn- or Pb-core staffanes. These compounds benefit from interaction through their bicyclo[1.1.1]pentane monomer units. Quantum chemical calculations were performed to probe their geometries, stabilities and electronic properties associated with conjugation.</p><p>The stabilities of the bicyclo[<i>n.n.n</i>]alkane and [<i>n.n.n</i>]propellanes (1 ≤ <i>n</i> ≤ 3) with C, Si, Ge and Sn at the bridgehead positions were studied by calculation of homodesmotic ring strain energies. The bicyclic compounds with <i>n</i> = 1 and Si, Ge or Sn at bridgehead positions have lower strain than the all-carbon compound.</p><p>A gradually higher polarizability exaltation is found as the bridgehead element is changed from C to Si, Ge, Sn or Pb. The ratio between longitudinal and average polarizability also increases gradually as Group 14 is descended, consistent with enhanced conjugation in the heavier oligomers.</p><p>The localization of polarons in C-, Si- and Sn-core staffane radical cations was calculated along with internal reorganization energies. The polaron is less localized in Si- and Sn-core than in C-core staffane radical cation. The reorganization energies are also lower for the heavier staffanes, facilitating hole mobility when compared to the C-core staffanes.</p><p>The effect of the bicyclic structure on the low valence excitations in the UV-spectra of compounds with two connected disilyl segments was also investigated. MS-CASPT2 calculations of 1,4-disilyl- and 1,4-bis(trimethylsilyl)-1,4-disilabicyclo[2.2.1]heptanes and 1,4-disilyl- and 1,4-bis(trimethylsilyl)-1,4-disilabicyclo[2.1.1]hexanes revealed that although the bicyclic cage separates the two disilyl chromophores, there is a strong red-shift of the lowest valence excitations when compared to an isolated disilane.</p>

The research described herein includes synthesis, spectroscopy, and quantum chemical calculations with focus on the characteristic properties of compounds with bonds between carbon and the heavier Group 14 elements. The chapters based on the first four papers concern σ- and σ/π-conjugated compounds, although the focus of the first paper is on ring strain of bicyclo[1.1.1]pentanes with C, Si, Ge or Sn at the bridgeheads. The relationship between calculated homodesmotic ring strain energies and through-space distances between the bridgehead atoms was evaluated, and it was found that replacing one of the methylene bridges with phospha-methyl gave both low strain and short through-space distance. Two kinds of σ/π-interacting systems were analysed with the difference that the σ- and π-bonded segments were either allowed to rotate freely relative each other or frozen into a conformer with maximal σ/π-interaction. The freely rotating systems are star-shaped oligothiophenes linked by heavy alkane segments. Density functional theory (DFT) calculations of hole reorganization energies support the measured hole mobilites. In summary, longer central oligosilane linkages, when compared to shorter, facilitate intermolecular hole-transfer between oligothiophene units. In 1,4-disilacyclohexa-2,5-dienes, the strength of the π- and pseudo-π interaction depends on the substituents at Si. Vapour phase UV absorption spectroscopy of 2,3,5,6-tetraethyl-1,1,4,4-tetrakis(trimethylsilyl)-1,4-disilacyclohexa-2,5-diene reveals a strong absorption at 273 nm (4.50 eV). Time-dependent DFT calculations further indicate that octastannylated 1,4-disilacyclohexa-2,5-diene has is lowest excited state at 384 nm (3.23 eV). The electronic, geometric and optical properties of substituted 1,4-disilacyclohexa-2,5-dienes were compared with those of the correspondingly substituted siloles. It was found that the lowest excitations of siloles are less tunable than those of 1,4-disilacyclohexa-2,5-dienes. The final section concerns strongly reverse-polarised 2-amino-2-siloxysilenes formed thermally from carbamylpolysilanes, and their lack of reaction with alcohols. Instead, the carbamylsilane reacts with alcohols giving silyl ethers, leading to a new benign route for alcohol protection.

The basic building blocks in molecular electronics often correspond to conjugated molecules. A compound class consisting of rigid rod-like staffane molecules with the heavier Group 14 elements Si, Ge, Sn and Pb at their bridgehead positions has now been investigated. Herein these oligomers are called heavy-core or Si-, Ge-, Sn- or Pb-core staffanes. These compounds benefit from interaction through their bicyclo[1.1.1]pentane monomer units. Quantum chemical calculations were performed to probe their geometries, stabilities and electronic properties associated with conjugation. The stabilities of the bicyclo[n.n.n]alkane and [n.n.n]propellanes (1 ≤ n ≤ 3) with C, Si, Ge and Sn at the bridgehead positions were studied by calculation of homodesmotic ring strain energies. The bicyclic compounds with n = 1 and Si, Ge or Sn at bridgehead positions have lower strain than the all-carbon compound. A gradually higher polarizability exaltation is found as the bridgehead element is changed from C to Si, Ge, Sn or Pb. The ratio between longitudinal and average polarizability also increases gradually as Group 14 is descended, consistent with enhanced conjugation in the heavier oligomers. The localization of polarons in C-, Si- and Sn-core staffane radical cations was calculated along with internal reorganization energies. The polaron is less localized in Si- and Sn-core than in C-core staffane radical cation. The reorganization energies are also lower for the heavier staffanes, facilitating hole mobility when compared to the C-core staffanes. The effect of the bicyclic structure on the low valence excitations in the UV-spectra of compounds with two connected disilyl segments was also investigated. MS-CASPT2 calculations of 1,4-disilyl- and 1,4-bis(trimethylsilyl)-1,4-disilabicyclo[2.2.1]heptanes and 1,4-disilyl- and 1,4-bis(trimethylsilyl)-1,4-disilabicyclo[2.1.1]hexanes revealed that although the bicyclic cage separates the two disilyl chromophores, there is a strong red-shift of the lowest valence excitations when compared to an isolated disilane.

To understand the mechanism of a pressure-induced structural phase transition, it is important to know which bonding changes lead to the stabilization of the new structure. A useful approach in this regard is the quantum chemical topology, which provides a large variety of indicators for the characterization of interatomic interactions. In this work, a number of topological indicators are used to analyze the bonding changes during the pressure-induced phase transition from the cubic diamond (cd) to the β-Sn-type structure of the elements of the 14th group of the periodic table. The ability of these indicators to reflect the presence of the cd → β-Sn transition in experiment for Si, Ge and Sn and its absence for carbon is investigated. Furthermore, the effect of pressure on the interatomic interactions in the cd- and β-Sn-type structures is examined. It is observed that the energy change along the cd → β-Sn transformation pathway correlates with the evolution of certain parameters of the electron density and the electron localizability indicator (ELI-D). Accordingly, criteria of structural stability were formulated based on characteristics of interatomic interactions. These results can serve as guidelines for the investigation of other solid-state phase transformations by the topological methods.

To understand the mechanism of a pressure-induced structural phase transition, it is important to know which bonding changes lead to the stabilization of the new structure. A useful approach in this regard is the quantum chemical topology, which provides a large variety of indicators for the characterization of interatomic interactions. In this work, a number of topological indicators are used to analyze the bonding changes during the pressure-induced phase transition from the cubic diamond (cd) to the β-Sn-type structure of the elements of the 14th group of the periodic table. The ability of these indicators to reflect the presence of the cd → β-Sn transition in experiment for Si, Ge and Sn and its absence for carbon is investigated. Furthermore, the effect of pressure on the interatomic interactions in the cd- and β-Sn-type structures is examined. It is observed that the energy change along the cd → β-Sn transformation pathway correlates with the evolution of certain parameters of the electron density and the electron localizability indicator (ELI-D). Accordingly, criteria of structural stability were formulated based on characteristics of interatomic interactions. These results can serve as guidelines for the investigation of other solid-state phase transformations by the topological methods.