Relation between molecular structure and ultrafast photoreactivity with application to molecular switches

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 quantum-classical dynamics. In the first part of this thesis
the two-electron two-orbital theory by Michl and Bonacic-Koutecky
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 closed-shell 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, cis-butadiene and 1,3-cyclohexadiene 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 multi-functionalized systems, hemithioindigo-hemistilbene
and trifluoromethyl-pyrrolylfulgide. In the second part of this
thesis state-of-the-art quantum chemical calculations and
time-resolved transient and infrared spectroscopy are used to
reconstruct the complex multi-channel isomerization mechanisms of
hemithioindigo-hemistilbene and trifluoromethyl-indolylfulgide.
Both the cis-trans isomerization in hemithioindigo-hemistilbene 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 quantum-classical
dynamics simulations at multi-configurational perturbation theory
(MS-CASPT2) level, using Tully's fewest switches surface hopping
approach, are performed to study the ultrafast photoreactivity of
1,3-cyclohexadiene in the gas-phase. For this purpose a numerical
routine for the efficient calculation of non-adiabatic couplings at
MS-CASPT2 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 1B2-state, the relaxation
into the 2A1-state 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.