Abstracts

This cumulative doctoraldissertation, based on three publications, is devoted to theinvestigation of several aspects of azobenzene molecular switches,with the aid of computational chemistry. In the first paper, theisomerization rates of a thermal cis trans isomerization ofazobenzenes for species formed upon an integer electron transfer,i.e., with added or removed electron, are calculated from Eyringstransition state theory and activation energy barriers, computed bymeans of density functional theory. The obtained results arediscussed in connection with an experimental study of the thermalcis trans isomerization of azobenzene derivatives in the presenceof gold nanoparticles, which is demonstrated to be greatlyaccelerated in comparison to the same isomerization reaction in theabsence of nanoparticles. The second paper is concerned withelectronically excited states of (i) dimers, composed of twophotoswitchable units placed closely side-by-side, as well as (ii)monomers and dimers adsorbed on a silicon cluster. A variety ofquantum chemistry methods, capable of calculating molecularelectronic absorption spectra, based on density functional and wavefunction theories, is employed to quantify changes in opticalabsorption upon dimerization and covalent grafting to a surface.Specifically, the exciton (Davydov) splitting between states ofinterest is determined from first-principles calculations with thehelp of natural transition orbital analysis, allowing for insightinto the nature of excited states. In the third paper,nonadiabatic molecular dynamics with trajectory surface hopping isapplied to model the photoisomerization of azobenzene dimers, (i)for the isolated case (exhibiting the exciton coupling between twomolecules) as well as (ii) for the constrained case (providing thevan der Waals interaction with environment in addition to theexciton coupling between two monomers). For the latter, theadditional azobenzene molecules, surrounding the dimer, areintroduced, mimicking a densely packed self-assembled monolayer.From obtained results it is concluded that the isolated dimer iscapable of isomerization likewise the monomer, whereas the sterichindrance considerably suppresses trans cis photoisomerization.Furthermore, the present dissertation comprises the generalintroduction describing the main features of the azobenzenephotoswitch and objectives of this work, theoretical basis of theemployed methods, and discussion of gained findings in the light ofexisting literature. Also, additional results on (i) activationparameters of the thermal cis trans isomerization of azobenzenes,(ii) an approximate scheme to account for anharmonicity ofmolecular vibrations in calculation of the activation entropy, aswell as (iii) absorption spectra of photoswitchsilicon compositesobtained from time-demanding wave function-based methods arepresented. Die vorliegende kumulativeDissertationsschrift basiert auf drei wissenschaftlichenPublikationen und beschftigt sich mit derAdvisors/Committee Members: Saalfrank, Peter (advisor).