A quantitative model for the design of molecules with tuned singlet- triplet energy gaps Felix Plasser Loughborough University, UK The gap between the first singlet (S 1 ) and triplet (T 1 ) excited states of a molecule plays a crucial role in a number of contemporary optoelectronic applications, such as singlet fission, thermally activated delayed fluorescence, and triplet-triplet annihilation. Despite their importance, no comprehensive theory for tuning singlet-triplet gaps has been developed yet. Indeed, most qualitative discussions are limited to discussions of the shapes and locations of the frontier orbitals. To overcome this problem, we have recently developed a quantitative theoretical model that gives deep insight into excitation energies beyond the molecular orbital picture by providing graphical representations as well as a quantification of different relevant post-MO terms [1-3]. This talk will describe how to proceed from this picture to actionable design rules for creating molecules with enhanced singlet-triplet gaps [4]. We discuss the effects of a number of parameters that are not captured with the existing models, e.g. delocalization, shape (linear vs bent), and whether the molecule is an alternant hydrocarbon. It is shown how the trends seen conform with the developed model. As an example, Figure 1 presents a comparison of the transition densities of ortho- and para- quinodimethane. Enhanced concentration of the transition density in the latter causes its increased singlet-triplet gap (2.65 vs 1.91 eV).
Figure 1: Comparison of the S 1 transition densities for ortho- and para-quinodimethane. Colouring indicates parts increasing the singlet-triplet gap (red, yellow) and lowering the singlet triplet gap (blue, cyan). References 1. P. Kimber, F. Plasser, Phys. Chem. Chem. Phys. 2020, 22, 6058–6080. 2. Z. Pei, Q. Ou, Y. Mao, J. Yang, A. D. La Lande, F. Plasser, W. Liang, Z. Shuai, Y. Shao, J. Phys. Chem. Lett. 2021, 12, 2712–2720. 3. F. Plasser, A. I. Krylov, A. Dreuw, WIREs Comput. Mol. Sci. 2022, 12, e1595. 4. W. Zeng, F. Plasser, H. Bronstein, in preparation .
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