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 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 JCTC14, 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.
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.
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
A recent study, lead by Florian Glöcklhofer from Imperial College London, explores the effect of methoxy and thiomethyl subtitutions on a formally antiaromatic macrocycle. The corresponding paper “[18.104.22.168]Paracyclophanetetraenes (PCTs): cyclic structural analogues of poly(p‑phenylene vinylene)s (PPVs)” is available via Open Research Europe, 1, 111, 2012.
The above figure compares the orbitals and aromaticity descriptors for different charge and spin states. Importantly, the symmetry is broken in the T1 state, inhibiting Baird aromaticity. By comparison, the symmetry is retained for the neutral singlet, dianion, and dication states all of which exhibit aromaticity.
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