Felix Plasser: Understanding electronic excitation energies within and beyond the molecular orbital picture

Tuning the energies of electronic excitations is a central research theme in modern science playing a role for the design of light emitters, sensors, and fluorescent probes. Traditionally, the energies of excited states are rationalised based on the energies and shapes of the frontier orbitals. Whereas this proves to be very intuitive, it is a limited approach covering only the zeroth order description of the excitation process. The purpose of this talk is to explore how an intuitive understanding of excitation energies can be gained that goes beyond orbital energies. This approach builds on previously developed wavefunction analysis techniques [1, 2].

As a first example we consider excited states that can be described by a transition between only two orbitals. We show that in this case, the excitation energy is affected by the Coulomb attraction as well as the exchange repulsion between those orbitals. The corresponding two-electron integrals can visualised intuitively as an overlap between densities and their electrostatic potentials (see Fig. 1). We proceed to states that involve non-trivial correlations between different pairs of orbitals. As an example, we consider the ionic and covalent B3u states of naphthalene.

This molecule possesses for states, denoted 1B3u+, 1B3u-, 3B3u+, and 3B3u-, which all possess identical orbital transitions and, thus, densities. Nonetheless, these states differ vastly in their energies and properties. These differences are elucidated using a recently described technique for visualising excited-state electron correlation [3]. We proceed by showing that diagrammatic techniques, borrowed from many-body perturbation theory, can be used to keep track of the different contributions.

[1] F. Plasser, H. Lischka, J. Chem. Theory Comput. 2012, 8, 2777.

[2] F. Plasser, M. Wormit, A. Dreuw, J. Chem. Phys. 2014, 141, 024106.

[3] F. Plasser ChemPhotoChem 2019, 3, 702.