Toward an Understanding of Electronic Excitation Energies Beyond the Molecular Orbital Picture

Abstract

<pre><div><div><div><p>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 to be an intuitive and simple model in many cases, they can only provide a very rough approximation of the underlying wavefunctions. The purpose of this Perspective 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-state computations are presented and their use is exem- plified in three case studies concerned with the relative ordering of the singlet and triplet ππ∗ and nπ∗ states of uracil, the large energetic differences between the first singlet and triplet states of the polyacenes, and the assignment of plasmonic states in octatetraene. 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.</p></div></div></div></pre>

A New Benchmark Set for Excitation Energy of Charge Transfer States: Systematic Investigation of Coupled Cluster Type Methods

“…For the criteria of the simple orbital picture to hold see the recent paper by Kimber and Plasser. 63 However, when dealing with a large number of excited states, the inspection of these orbitals can be tedious; therefore, in an automated procedure well-defined numerical descriptors are more useful.…”

Violation of Hund’s rule in molecules: Predicting the excited-state energy inversion by TD-DFT with double-hybrid methods

“…Phys. 10.1063/5.0076545 ture 30,31 which is however not possible within the linear-response (adiabatic) Time-Dependent Density Functional Theory (TD-DFT) method as recently shown. 32 Unsurprisingly, wavefunction methods for excited states can predict that excited-state energy inversion if at least double excitations are allowed into the excited-state wavefunction, [33][34][35] although at a normally more demanding cost than TD-DFT.…”

Organic Molecules with Inverted Gaps between First Excited Singlet and Triplet States and Appreciable Fluorescence Rates

“…[114][115][116] Within TOTEM, the singlet-triplet gap equals twice the exchange integral between HOMO and LUMO. [114][115][116] Hence, based on that model, the singlet-triplet gap can be minimized by reducing the spatial overlap between HOMO and LUMO, and this strategy proved effective leading to various designs for TADF emitters. 95,96,117 However, it has been shown recently that TOTEM is not sufficient to predict singlet-triplet gaps accurately, as the inclusion of additional states is necessary to account for the non-negligible contribution of double excitations.…”

“…Generally, the design of organic emitters has focused largely on minimizing singlet-triplet gaps based on a two-orbital two-electron model (TOTEM) using highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). [114][115][116] Within TOTEM, the singlet-triplet gap equals twice the exchange integral between HOMO and LUMO. [114][115][116] Hence, based on that model, the singlet-triplet gap can be minimized by reducing the spatial overlap between HOMO and LUMO, and this strategy proved effective leading to various designs for TADF emitters.…”