Doctoral thesis defense: Mogens Brøndsted Nielsen

On March 16. at 13:15 Mogens Brøndsted Nielsen will defend his doctoral thesis titled 'Cross-Conjugation in Expanded π-Systems' at the H.C. Ørsted Institute Aud. 2. 


In freshman chemistry courses we teach students that alternating single and double bonds lead to lower excitation energies as the HOMO-LUMO gap decreases. But what if instead we have cross-conjugated double bonds where the double bonds branch off from the main chain? Is the π-electron delocalization smaller or larger? This is a key question that I have addressed together with my research group the last fourteen years from synthesis of a large selection of compounds that have been studied systematically for their optical and redox properties, chemical reactivities, and single-molecule conductance properties. Molecules include derivatives of extended tetrathiafulvalenes (TTFs), dihydroazulene/vinylheptafulvene (DHA/VHF) photo-/thermo-switches, and various acyclic and macrocyclic acetylenic scaffolds.

The work has been strongly synthesis-driven, and several protocols for achieving or functionalizing cross-conjugated molecules were developed. Synthesis by “acetylenic scaffolding” has played a key role, and the planar tetraethynylethene (TEE) unit was employed as a bridging unit between functional entities in several of the systems prepared and investigated. 

The main conclusion from the work presented in this dissertation is that π-electron delocalization is significantly less efficient via cross-conjugation than via linear conjugation, and cross-conjugation can therefore be employed as a structural tool to decouple two functional units and hence for tuning electronic properties of molecules.