Photochemistry

Planar chromophore design

Felix

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.

Matrix-free hyperfluorescence

Felix

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.

Excited-state aromaticity in naphthalene

Felix

A recent JPCA article by Karadakov and Al-Yassiri highlights the differences in singlet and triplet aromaticity in naphthalene. To me this paper contains several striking observations:

  • The singlet HOMO/LUMO transition (S2, 1La) is shown to be strongly aromatic whereas the triplet HOMO/LUMO transition (T1, 3La) is antiaromatic. Does this mean states reached by the same kind of orbital transition behave differently depending on their spin-multiplicity?
  • The aromatic S2 lies above the antiaromatic S1 even though S2 is the HOMO/LUMO transition. Does this mean that singlet antiaromaticity is actually a stabilising effect?

We have discussed the excited states of naphthalene from an entirely different viewpoint in a recent J. Chem. Theory Comput. article. It would be fascinating to combine the two viewpoints.

HOMO/LUMO transitions

Felix

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.

Non-Kasha fluorescence

Felix

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.

Delayed fluorescence

Felix

Patrick’s first paper as first author just appeared in PCCP: The role of excited-state character, structural relaxation, and symmetry breaking in enabling delayed fluorescence activity in push-pull chromophores. Well done Patrick!

We were interested in understanding the difference in thermally activated delayed fluorescence (TADF) between two closely related donor-acceptor-donor systems using either an anthraquinone and benzodithiophenedione acceptor units, respectively. The first one was known to be an effective TADF emitter [JACS 2014, 136, 18070] whereas the second one had significantly lower quantum yield for TADF [PCCP 2019, 21, 10580].

Rather than just presenting energies, it was the purpose of this paper to shed detailed insight into the wavefunctions involved. Notable differences in the wavefunctions and charge-transfer character were found between the two molecules. Even more striking differences existed between different computational methods.

After evaluating electronic structure methods, we presented geometry optimisations in solution, highlighting the importance of symmetry breaking for producing an emissive lowest singlet state. The role of different solvation models was discussed as well.

Excited-state symmetry breaking

Felix

A recent study led by Zoltan Szakacs and Eric Vauthey from the University of Geneva explores excited-state symmetry breaking in donor-acceptor-donor systems. The associated paper just appeared in PCCP: Excited-state symmetry breaking in 9,10-dicyanoanthracene-based quadrupolar molecules: the effect of donor-acceptor branch length

The main idea behind this work is to use symmetry-selection rules and the associated forbidden transitions to probe how inversion symmetry is broken during the photodynamics. See [JPCL 2021, 12, 4067] for an initial discussion of the idea.