Observing Charge Transfer Dynamics in Molecules: First Principles Simulation of Valence and Core Spectroscopies of DMABN and its Derivatives

Abstract

In order to understand the myriad of processes underlying photochemistry or photosynthesis, many of which are essential to the design of semiconductors or other technologies, it has always been important to directly observe the processes at play in the ultrafast excited state dynamics of molecules. The evolving nuclear and electronic character of excited states prepared by photo-excitation can be measured using a host of spectroscopic probe techniques, based on the valence or core excitation of molecules. Ultrafast photo-excitation generally leads to both electronic and geometrical (nuclear) rearrangements of a molecule's structure. One paradigmatic example is the twisted intramolecular charge transfer dynamics evinced by dimethylamino benzonitrile (DMABN). Here, the large amplitude motion, involving a twist and bend of the dimethylamino group, gives rise to a charge transfer state, characterized by a separation of charge between the benzene ring and the amino group, and giving rise to a large dipole moment. These nuclear and electronic rearrangements have interested physical chemists over the last 60 years, serving as a great benchmark for those testing the efficacy of their methods in detecting charge transfer states, and for those exploring the ability to achieve long lived charge separation in molecules in general. Naturally, the wealth of studies on DMABN has also led to many conflicting interpretations of the nuclear structures and electronic configurations important to the excited state dynamics of the molecule. By probing the electronic structure of DMABN with simulated valence absorption and photoelectron spectroscopies, along with new core absorption and photoelectron studies, a series of specific novel time-resolved experiments were proposed to more conclusively image the evolving charge transfer character in this system. Specifically, a dual X-ray Absorption and X-ray Photoelectron study on DMABN and its analogues is proven to be an efficient way to resolve signals attributed to charge transfer excited state minimums from other excited state minimums in this thesis. Furthermore, this thesis addresses some of these long standing controversies in the excited state dynamics of DMABN, by proposing a conclusive dynamical model that takes into account all previously performed experimental spectroscopies on the molecule. To develop the validity of this model, and encourage further time resolved studies to prove said model, this thesis also presents studies of a series of DMABN-like molecules. Using these analogues, this thesis proposes a host of unique time resolved spectroscopies on charge transfer systems, allowing for the earlier proposed dynamical model to be proven with several potential time-resolved experimental studies. Charge transfer states have proven uniquely difficult to analyze for most electronic structure methods even for static calculations, independent of dynamics. This work also demonstrates the effectiveness of a new ab initio quantum chemistry method in replicating experimental spectroscopies, DFT/MRCI(2). Furthermore, it was proven that said method is suitable for computationally inexpensive, core and valence absorption/photoelectron spectroscopies, while also retaining the ability to treat charge transfer states appropriately.