Nenov, Artur (2012): Relation between molecular structure and ultrafast photoreactivity with application to molecular switches. Dissertation, LMU München: Faculty of Chemistry and Pharmacy 

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Abstract
Photoinduced ultrafast isomerizations are fundamental reactions in photochemistry and photobiology. This thesis aims for an understanding of the generic forces driving these reactions and a theoretical approach is set up, able to handle realistic systems, whose fast relaxation is mediated by conical intersections. The main focus is on the development of strategies for the prediction and accelerated optimization of conical intersections and their application to artificial compounds with promising physicochemical properties for technical applications as molecular switches. All calculations are based on advanced quantum chemical methods and mixed quantumclassical dynamics. In the first part of this thesis the twoelectron twoorbital theory by Michl and BonacicKoutecky used in its original formulation to rationalize the structure of conical intersections in charged polyene systems is extended by including the interactions of the active pair of electrons with the remaining closedshell electrons that are present in any realistic system. A set of conditions, called resonance and heterosymmetry conditions, for the formation of conical intersections in multielectronic systems are derived and verified by calculations on the basic units ethylene, cisbutadiene and 1,3cyclohexadiene at various geometries and functionalizational patterns. The quantitative results help to understand the role of geometrical deformations and substituent effects for the formation of conical intersections and to derive rules of thumb for their qualitative prediction in arbitrary polyenes. An extension of the rules of thumb to conical intersection seams is formulated. The strategy pursued is to divide the molecular system into basic units and into functional groups. Each unit and its intersection space are treated independently, thereby reducing the dimensionality of the search space compared to the complete molecule. Subsequently, the interconnectivity of the intersection spaces of the different units is determined and an initial guess for the complete seam is constructed. This guess is then fed into a quantum chemistry package to finalize the optimization. The strategy is demonstrated for two multifunctionalized systems, hemithioindigohemistilbene and trifluoromethylpyrrolylfulgide. In the second part of this thesis stateoftheart quantum chemical calculations and timeresolved transient and infrared spectroscopy are used to reconstruct the complex multichannel isomerization mechanisms of hemithioindigohemistilbene and trifluoromethylindolylfulgide. Both the cistrans isomerization in hemithioindigohemistilbene and the electrocyclic ring closure/opening in indolylfulgide are characterized by a charge transfer in the excited state. The ability of each system to stabilize this charge transfer is essential for the returning to the ground state. The relaxation to the ground state through extended regions of the seam is found to be the decisive step determining the reaction speed and the quantum yield. In the last part of this thesis mixed quantumclassical dynamics simulations at multiconfigurational perturbation theory (MSCASPT2) level, using Tully's fewest switches surface hopping approach, are performed to study the ultrafast photoreactivity of 1,3cyclohexadiene in the gasphase. For this purpose a numerical routine for the efficient calculation of nonadiabatic couplings at MSCASPT2 level is presented. The major part of the excited molecules are found to circumvent the 1B2/2A1 conical intersection and reach the conical intersection seam between the excited state and the ground state instantaneuosly. Time constants for the evolution of the wavepacket on the bright 1B2state, the relaxation into the 2A1state and the return to the ground state are extracted. It is demonstrated that the accessibility of the conical intersection seam depends on its energetic and spatial relation to the minimum energy path, as well as on the momentum which is accumulated during relaxation on the excited state potential energy surface.
Item Type:  Theses (Dissertation, LMU Munich) 

Keywords:  quantum chemistry, excited states, ultrafast photochemistry, conical intersections, mixed quantumclassical dynamics, molecular switches, multiconfigurational methods 
Subjects:  500 Natural sciences and mathematics 500 Natural sciences and mathematics > 540 Chemistry and allied sciences 
Faculties:  Faculty of Chemistry and Pharmacy 
Language:  English 
Date of oral examination:  28. September 2012 
1. Referee:  VivieRiedle, Regina de 
MD5 Checksum of the PDFfile:  06abbd327e5ff4f8aed53cd7be92f9ac 
Signature of the printed copy:  0001/UMC 20819 
ID Code:  15047 
Deposited On:  13. Dec 2012 14:29 
Last Modified:  24. Oct 2020 01:43 