Casting Light on the Fundamentals of Energy Conversion

The absorption of light by molecules can induce ultrafast dynamics of coupled electronic and nuclear vibrational motion. These dynamics take place on the femtosecond to picosecond timescale, which is the timescale of nuclear vibrational motion, and involve energy transfer processes which are fundamental to the very notion of chemistry. This thesis addresses the topic of intramolecular energy conversion using combined experimental and theoretical approaches.

More specifically, the project focusses on internal conversion by which electronic energy can be dissipated in conjugated molecules. By internal conversion the energy of an electronically excited state is given off to vibrational modes of the molecule. In other words, the excitation energy is transformed into heat.

As subject molecules were chosen seven cycloketones, three cyclopentadienes, and dithiane. These are all industrially relevant substances. For instance, several cycloketones are precursors for production of various polymers. All were investigated by time-resolved mass spectrometry and photoelectron spectroscopy supplemented by electronic structure calculations and quantum dynamics simulations.

The process of internal conversion was found to be non-ergodic. The ergodic hypothesis says that over a sufficiently long period of time, a particle will visit all regions within its phase space, and that the time spent by the particle in a given region is proportional to the volume of this region. However, for the processes investigated in the project it was shown, that the nuclear dynamics only sample a reduced space potentially resulting in localization of the dynamics in real space. In essence, this is a consequence of vibrational energy redistribution simply not being able to compete with the rate of internal conversion.

In the case of the cycloketones, the rate of internal conversion varies by more than an order of magnitude between the molecules. This non-ergodic process was found to primarily involve ring-puckering motion, and the different timescales observed could be rationalized on the basis of the vibrational frequency and the energy difference between the Franck-Condon and equilibrium geometries of the upper electronic state.

In the cylcopentadienes, the twisting of a single double bond is essential in reaching the conical intersection seam connecting the lowest excited state with the ground state. In dithiane, the coupling of stretching in the disulfide bond with torsion in the carbon backbone allows the molecule to repeatedly access the region near a conical intersection.

A common trait of the three types of molecules is that very few degrees of freedom participate in the investigated processes. By selectively modifying these modes, the rate of internal conversion can be significantly affected and the dynamics possibly tuned from non-ergodic to partly ergodic.