Computational Design of Molecular Motors and Excited-State Studies of Organic Chromophores

Abstract

This thesis presents computational quantum chemical studies of molecular motors and excited electronic states of organic chromophores.The first and major part of the thesis is concerned with the design of lightdriven rotary molecular motors. These are molecules that absorb light energy and convert it into 360° unidirectional rotary motion around a double bond connecting two molecular halves. In order to facilitate potential applications of molecular motors in nanotechnology, such as in molecular transport or in development of materials with photo-controllable properties, it is critical to optimize the rates and efficiencies of the chemical reactions that produce the rotary motion. To this end, computational methods are in this thesis used to study two different classes of molecular motors.The first class encompasses the sterically overcrowded alkenes developed by Ben Feringa, co-recipient of the 2016 Nobel Prize in Chemistry. The rotary cycles of these motors involve two photoisomerization and two thermal isomerization steps, where the latter are the ones that limit the attainable rotational frequencies. In the thesis, several new motors of this type are proposed by identifying steric, electronic and conformational approaches to accelerate the thermal isomerizations. The second class contains motors that incorporate a protonated Schiff base and are capable to achieve higher photoisomerization rates than overcrowded alkene-based motors. In the thesis, a new motor of this type is proposed that produces unidirectional rotary motion by means of two photochemical steps alone. Also, this motor lacks both a stereocenter and helical motifs, which are key features of almost all synthetic rotary motors developed to date. Men för att fullt ut dra nytta av dessa och andra motorers potential att fungera som effektiva kraftkällor i olika sammanhang, t.ex. för transport och målspecifik leverans av läkemedel i kroppen, är det emellertid av största vikt att ytterligare förbättra befintliga motorers prestanda, samt även att designa helt nya motorer som har mer fördelaktiga egenskaper. I avhandlingen presenteras forskning där flera möjligheter att möta dessa mål undersöks med olika beräkningsmetoder inom teoretisk kemi.Avhandlingens andra del handlar också om ljusabsorberande molekyler, s.k.kromoforer, men fokuserar på design och utvärdering av nya tillvägagångssätt för att beskriva exciterade energitillstånd hos sådana molekyler mer noggrant, utan att nödvändigtvis ta till kostsamma beräkningsmetoder. Speciellt undersöks möjligheten att på detta sätt beskriva organiska kromoforer av en typ som är relevant för solceller och lysdioder.vii