Modeling Intersystem Crossing in Transition Metal Complexes

The general aim of this project is to identify the major factor(s) which control the efficiency of intersystem crossing (ISC) in transition metal complexes. ISC is a photophysical process in which a system undergoes a spin-forbidden non-radiative transition to a state of different multiplicity. Transition metal complexes are used in many areas of research, e.g., organic light emitting diodes (OLEDs) and solar voltaics. Therefore, understanding (and controlling) the process of ISC is important in the rational design of complexes for specific purposes, providing means to improve device efficiencies. When considering ISC, and the timescale it occurs in, in molecular complexes spin-orbit (SO) coupling is often invoked. In transition metal complexes, SO coupling is considered to be the main contributor to the observed speed of ISC (10`s to 100`s femtoseconds). However, experimentally observed deviations of ISC rates in transition metal complexes, where large SO coupling is present, has indicated that one cannot simply use a simple set of indicators to predict ISC in transition metals. To this end, theoretical chemistry provides a method for studying ISC in transition metal complexes. Herein is proposed to use a combination of ab initio molecular dynamics (MD) and full TD-DFT, along with static calculations, to study the ultrafast dynamical mechanisms including ISC in transition metal complexes and thereby identify perturbations, other than SO coupling, which may contribute to the observed ISC timescales (e.g, solvent/environment effects, vibrational distortions, ligand effects). To perform the study an interface between the ab initio MD code (SHARC), capable of including both non-adiabatic and SO couplings, and a code capable of performing full TD-DFT (i.e., ADF or Dalton) calculations will be implemented. Since a number of experiments on transition metal complexes employ time-resolved X-ray spectroscopy as an experimental way of following excited state structural dynamics, the same methodology will also be adapted to calculate time-resolved X-ray spectra of selected transition metal complexes. By combining theory with experiment it may be possible to extract more information than previously managed from the experimental spectra alone. A series of transition metal complexes with potential applications in the fields of OLED development and dye-sensitized solar voltaic devices will be considered. Harnessing ISC in such complexes may provide a route toward a blue light emitting OLED with increased longevity and improving energy conversion efficiency in dye-sensitized solar voltaics.

The general goal of this project was to identify the major factor(s) which control the efficiency of inter- system crossing (ISC) in transition metal complexes. To this end we developed a new tool allowing one to study ISC dynamics in large molecular systems using theoretical chemistry, specifically utilizing time- dependent density functional theory (TD-DFT). This tool enables one to study the dynamical processes occurring in the system after excitation. Such processes are of interest in many fields of research, i.e. organic light emitting diode (OLED) development, improving the efficiency of dye-sensitized solar cells, photosensitizers for treating cancer or artificial photosynthesis. Gaining insights into the processes and what controls them is the first step towards rational design of complexes for an intended purpose. After the initial development of the new tool, it was applied to an archetypal system in the field of photosensitizers, ruthenium tris-bipyridyl, [Ru(bpy)3]2+. In this application we gained new insights into the process of ISC in this molecule, namely that the motion of the atoms in the molecule promote the speed of ISC (which occurs in <30 fs). Without motion ISC occurs over much longer timescales. Thus one cannot say, as has been previously stated, that the ultrafast ISC is due to the density of electronic states and the size of the spin-orbit coupling in [Ru(bpy)3]2+. This information can aid in the design of derivatives for use as photosensitizers, where ISC can be promoted or demoted from occurring by enhancing or restricting the movement of the atoms around the ruthenium core. The developed methodology was also applied to a simple test system, which has interest in the field of stellar chemistry. The small size of the molecule allowed for different levels of theory to be applied in order to understand the limitations of the different methods available to simulate ISC. Here it was found that the choice of the density functional is essential to get a correct qualitative or quantitative description of the ISC phenomenon of the molecule.Overall the project achieved its primary goal and provided a new tool allowing other researchers to study ISC in chemical systems that otherwise were previously computationally unfeasible. This will primarily have impact in the fields of renewable energy, green catalysis and light-based treatments in medicine.