Silyl Groups as Dispersion Energy Donors

Abstract

The introduction of silyl groups as protecting groups for alcohols has substantially changed synthetic organic chemistry. Although heavily utilized, some aspects of silyl groups, especially their potential to function as so called ‘dispersion energy donors’ in catalyst and functional material design remains mostly untapped, despite their generally greater polarizability and bulkiness in comparison to known carbon based dispersion energy donors. In the first publication (section 2.1/2.2) of this work, we report an experimental and computational study on a rigid cyclooctatetraene molecular balance, a molecular system exhibiting a conformational equilibrium by means of which non covalent interactions can be quantified, substituted with common silyl protecting groups. We were able to quantify the relative steric demand of silyl groups and found conformational strain and Pauli exchange repulsion contributions to counteract significant stabilization from intramolecular London dispersion forces in the conformational equilibrium. The second publication (section 2.3/2.4) addresses the London dispersion dominated attractive potential of bulky silyl groups by employing a similar cyclooctatetraene molecular balance. Strain build up was avoided by introducing rotatable spacers between the silyl groups and the balance scaffold. We found our experimental findings to be in line with our theoretical predictions on the potential of silyl groups to function as dispersion energy donors, despite entropy mitigating the net effect in this particular case. In a third publication (section 2.5/2.6), we studied a cyclooctatetraene molecular balance featuring a spacer moiety and two hydroxy groups. We found the folded state of the balance to closely resemble the geometric arrangement of a cyclic water dimer transition state. By taking strain and solvation contributions into account, we were able to provide an estimate for the hydrogen bonding energy of the cyclic water dimer transition state, which proved to be in good agreement with high-level computational results. Furthermore, we were able to identify a significant contribution from London dispersion to the hydrogen bonding energy through computational energy decomposition analyses. In the final chapter (section 2.7), we report the preparation of a set of phosphorescent platinum(II) phenylacetylide complexes substituted with bulky silyl groups, and their respective carbon counterparts. We studied their photophysical properties in solution and thin film, and thus were able to identify the ideal silyl group substitution pattern for application in a pure blue hyperfluorescent device. Our experimental findings agreed well with the computational conclusions. So we are able to interrogate the details of the calculations to understand the interplay of London dispersion controlled aggregation and the electronic effects of the silyl groups resulting in improvements in photophysical properties over the parent unsubstituted compound.