Toward an Understanding of Electronic Excitation Energies Beyond the Molecular Orbital Picture
Tuning the energies of molecular excited states is a central research theme in modern chemistry with high relevance for optoelectronic applications and chemical synthesis. Whereas frontier orbitals have proven as an intuitive and simple model in many cases, they can only provide a very rough approximation of the underlying wavefunctions. The purpose of this Article is to explore how our qualitative understanding of electronic excitation processes can be promoted beyond the molecular orbital picture by exploiting methods and insights from modern quantum chemistry. For this purpose, the physics of a correlated electron-hole pair is analysed in detail to show the origin of exchange repulsion and a dynamic Coulomb attraction, which determine its energy aside from the orbital energies. Furthermore, we identify and discuss the two additional effects of secondary orbital relaxation and de-excitations. Rules for reconstructing these four contributions from general excited states computations are presented and their use is exemplified in three case studies. First, the relative ordering of the singlet and triplet ππ∗ and nπ∗ states of uracil is explained. Second, the large differences between the energies of the first singlet and triplet states of the polyacenes are examined. Third, the identification of plasmonic states in the case of octatetraene is explored. Finally, we lay out some general ideas for how the knowledge gained could ultimately lead to new design principles for tuning molecular excitation energies as well as for diagnosing possible shortcomings of commonly used electronic structure methods.