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.
Our new paper Using diketopyrrolopyrroles to stabilize double excitation and control internal conversion, led by Mariana do Casal and Mario Barbatti from Aix Marseille University, just appeared in PCCP. This work highlights how double excitation character can support internal conversion. Wavefunction analysis using TheoDORE sheds light into the wavefunctions involved.
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.
A new preprint is available, Patrick’s first manuscript as first author: The role of excited-state character, structural relaxation, and symmetry breaking in enabling delayed fluorescence activity in push-pull chromophores.
Well done, Patrick!
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.
A new paper on the photochemistry of fingerprinting reagents, led by L. Hunnisett and P. Goddard at Loughborough, just appeared in J. Chem. Phys.: Mechanistic Insight into the Fluorescence Activity of Forensic Fingerprinting Reagents. The paper investigates the reason for the different fluorescent activities observed in chemically related fingerprinting reagents.