Seminarium Optyczne

Almost one century ago, Friedrich Hund formulated his famous multiplicity rule according to which the term with maximummultiplicity has the lowest energy for a given electron configuration [1]. This rule provides the base for the Jablonski diagram [2]which is commonly used for discussions of molecular spectroscopy and photophysics. No stable organic molecules which violateHund’s Rule for the S1 and T1 excited states were definitely known until three years ago, when the existence of such moleculeswas discovered with wavefunction-based ab initio calculations [3,4] and was confirmed experimentally[3,5]. Further theoreticalinvestigations have shown that there exists a wide class of stable organic molecules in which the lowest excited singlet state liesbelow the lowest excited triplet state [6].This observation may have important implications in the field of organic light emitting diodes (OLEDs) because according to spinstatistics, the recombination of charge carriers generated by voltage in optoelectronic materials produces (emissive) singlet and(dark) triplet excitons in the ratio 1:3, so the luminescence quantum yield of such “standard” materials cannot nominally exceed25%. The discovery of stable organic molecules with inverted singlet-triplet ordering breaks a ground for creation of a newgeneration of organic optoelectronic materials where the triplet “traps” are spontaneously drained and allows in principle toachieve 100% yield of electron to photon conversion in OLEDs [7].In this presentation, the results of extensive computational explorations of novel classes of organic molecules exhibiting singlet-triplet inversion will be presented in order to provide understanding of the fundamental mechanisms which are responsible forthe violation of Hund’s rule and to determine the electronic and geometric factors that govern the phenomenon. Provided thatsuch molecules with appreciable fluorescence rates can be synthetized, they will become the next (forth) generation of OLEDmaterials.1. F. Hund, Zeitschrift f. Physik, 40 (1927) 742.2. A. Jabłoński, Nature 131 (1933) 839.3. J. Ehrmaier,E. Rabe, S. Pristash, K. Corp, C. Schlenker, A. L. Sobolewski, W. Domcke, J. Phys. Chem. A 123 (2019) 8099.4. P. de Silva, J. Phys. Chem. Lett. 10 (2019) 5674.5. N. Aizawa1, Y J. Pu1, Y. Harabuchi, A. Nihonyanagi, R. Ibuka, H. Inuzuka, B. Dhara1, Y. Koyama, K. Nakayama, S. Maeda, Nature609 (2022) 502.6. S.Pios, X. Huang, A. L. Sobolewski, W. Domcke, PCCP 23 (2021) 12968.7. A. L. Sobolewski and W. Domcke, J. Phys. Chem. Lett., 12 (2021) 6852