Classification and Analysis of Excited States

A new book chapter by Patrick and Felix just appeared online: “Classification and Analysis of Molecular Excited States“. Ultimately, this chapter will be part of the Comprehensive Computational Chemistry series published by Elsevier.

In this chapter we explore the various ways in which excited states are classified, that is, according to

  • the molecular orbitals involved,
  • valence bond resonance structures,
  • spatial and spin symmetry,
  • more fundamental wavefunction properties (double excitations, correlation, etc),
  • excited-state aromaticity, and
  • delocalisation and charge transfer.

The map below shows the different classes and highlights the multitude of ways that are used to discuss excited states in the literature.

It is the purpose of this chapter to discuss all these types of states, covering the mathematical and physical background as well as the consequences to spectroscopy and photochemistry.

Doubly excited states

Our new paper “Classification of Doubly Excited Molecular Electronic States” just appeared in Chemical Science.

The topic of doubly excited states has been discussed quite controversially in the literature over the last couple of years, see for example JACS, 139, 13770 (2017) and JCTC 14, 9 (2018), and it is often disputed whether to classify a state as doubly excited at all. To contribute to this discussion we worked on the development of a physically motivated definition of doubly excited character based on operator expectation values and density matrices, which works independently of the underlying orbital representation. We hope that this approach will provide new understanding on these issues.

Best-Practice DFT Protocols

I enjoyed this paper from the Grimme group:

Best-Practice DFT Protocols for Basic Molecular Computational Chemistry, Angew. Chem. Int. Ed., 2022, 61, e202205735.

The authors discuss the best modern methods for running a DFT computations – a guide through the jungle. For me the main conclusions regarding functionals are:

  • The composite method r2SCAN-3c offers great cost-benefit a ratio and we are starting to adopt it as a default method for ground-state optimisations.
    (This should be taken with a grain of salt, since it is the authors’ own method, but the arguments are sound.)
  • If you want to go beyond r2SCAN-3c, you have to try really really hard, as in range-separated hybrid meta-GGA with a triple/quadruple-zeta basis set. There is not really any reason to ever use double-zeta basis sets – better use a compound method.
  • B3LYP/6-31G* is outdated.

PhD position

A new funded PhD position is available in the group: Computational design of functional molecular materials.

This is a flexible position allowing you to apply state-of-the-art quantum chemistry along with sophisticated analysis methods. The goal is to develop new rules for designing functional materials going beyond the frontier orbital picture. The work will apply rules developed in PCCP, 22, 6058-6080 (2020) in connection with experimental partners.

This is a flexible studentship that can be adjusted to your needs and interests. Please apply by February 2023 if this interests you.

Non-Kasha fluorescence

Kasha’s rule states that fluorescence generally occurs from the lowest excited singlet state (S1). Exceptions to this rule are usually associated with a metastable S2 state that is separated from S1 not allowing for interconversion. In a recent article we outlined a different mechanism for non-Kasha fluorescence: If S1 and S2 are very close in energy, then S2 is populated in a dynamic equilibrium following Boltzmann statistics. This effect is particularly pronounced if there is a large amount of vibrational excess energy following excitation into a high-energy absorption peak. The full story, “Non-Kasha fluorescence of pyrene emerges from a dynamic equilibrium between excited states” was just published in J. Chem. Phys.

Eu(III) complexes

Our new paper “Impact of Varying the Phenylboronic Acid Position in Macrocyclic Eu(III) Complexes on the Recognition of Adenosine Monophosphate“, led by S. E. Bodman and S. J. Butler from Loughborough, just appeared in Organic Chemistry Frontiers. The paper is the second in a series studying the anion sensing properties of Eu(III) complexes with phenylboronic acids.

Aside from reporting the synthesis and anion binding, the paper presents new strategies for the computational analysis of such complexes. Aside from modelling the geometries by density functional theory, high-level multireference methods in OpenMolcas were applied to study the luminescence properties. These first principles computations offer a promising approach to access the emission spectra of lanthanide complexes, aiding the design of responsive lanthanide probes with specific photophysical properties