Planar chromophore design

It is by now fairly well understood how chromophore properties are affected by push/pull substituents and their degree of planarity. But how can we rationalise variations in the properties of planar chromophores that do not possess charge transfer character?

We were interested in understanding the apparent differences between these two isomeric molecules (called the Pechmann dyes).

Why is the T1 of PM5 (shown to the left) so much lower than the one of PM6 (shown to the right) making PM5 a powerful candidate for a singlet fission material whereas the S1/T1 gap of PM6 is too small for this purpose?

The answer is discussed in the new paper “Singlet Fission in Pechmann Dyes: Planar Chromophore Design and Understanding” that just appeared in JACS.

The overall lower excitation energies of PM5 vs PM6 can be understood by the fact that the S1 and T1 of the former are stabilised via excited-state aromaticity whereas the S0 of the latter profits from ground-state aromaticity.

The reason for the lower S1/T1 gap in PM5 is more subtle but also more fascinating pointing to a whole new way of viewing excitation energies. We were able to highlight the effect of the double bond conformation in influencing the exchange integral between the excited electron and hole, which ultimately leads to the variations in S1/T1 gap.

Excited states of diradicals

Computations on diradicals are not only difficult in terms of choosing an appropriate electronic structure method but in many cases it is also quite challenging to make sense of the results obtained. To tackle this problem we developed a detailed characterisation scheme for the excited states of diradicals in our new paper Classification and quantitative characterisation of the excited states of π-conjugated diradicals that just appear in Faraday Discussions.

Our paper builds on earlier work by Salem et al. and Stuyver et al. in terms of formally characterising the states as diradical and zwitterionic within a two-orbital two-electron model (TOTEM). On top of this, we have provided practical protocols for recognising these states in realistic computations. Using our tools in the case of the para-quinodimethane molecule as a model system, we show that it is possible to identify the states arising from the TOTEM but also that the π-conjugated bridge plays a crucial role producing more complicated states than one would have expected from the simple TOTEM.

Matrix-free hyperfluorescence

Hyperfluorescence is an emerging technique for generating highly efficient OLEDs by combining a triplet harvester with a bright emitter molecule. Current devices are overly complex due to the number of components involved hampering practical application. A new paper, led by Hugo Bronstein from the University of Cambridge presents an important step toward solving this problem. The idea is to encapsulate the emitter, thus, avoiding the need for a high-gap matrix. The approach is presented in the paper Suppression of Dexter transfer by covalent encapsulation for efficient matrix-free narrowband deep blue hyperfluorescent OLEDs, which just appeared in Nature Materials.

Ionic states

Our new paper Quantification of the Ionic Character of Multiconfigurational Wave Functions: The Qat Diagnostic just appeared in the Journal of Physical Chemistry A.

The paper deals with the fact that the widely used CASSCF method, if not used carefully, can yield large errors (1-2 eV) in vertical excitation energies. This problem arises for ionic states, as defined within valence bond theory. Within this work we developed a simple diagnostic to identify ionic states. We found a good correlation between the new diagnostic (Qta) and the error, as shown in the figure above.

We hope that the new diagnostic will be useful similar to analogous diagnostics identifying charge transfer states in TDDFT computations. This will give users the possibility to spot potential problems quickly.

On-going work is concerned with going from just diagnosing the problem to developing a numerical correction term to fix the problem.

HOMO/LUMO transitions

We just posted a preprint discussing a question I have been wondering about for a while: Why is the lowest excited state of a molecule not always the HOMO/LUMO transition? More generally we show how singlet and triplet state energies are affected in different ways by post-MO energy terms.

The preprint can be found here: Excited-state energy component analysis for molecules – Why the lowest excited state is not always the HOMO-LUMO transition

Update: the final published version is available here.

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.

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.