Haruta has been developing chemical theories for dynamical symmetry, mechanochemistry, and exciton
relaxation on the basis of a comprehensive understanding of structures, deformations, and electronicstates.
(1) Polyatomic clusters with as-yet-unknown symmetry
He has identified polyatomic clusters with catalytic activity [Nature Commun. 8, 688 (2017)], chemicalstability [Nature Commun. 8, 2046 (2017)], and spectroscopic feature [Science Adv. 5, eaax6455 (2019)] incollaboration with experimentalists. Such clusters have been predicted on the assumption that they arespherical, but it is effective just for subglobular clusters such as icosahedron. He proposed a symmetryadapted orbital model, which incorporates orbital splittings due to lower structural symmetries [NatureCommun. 10, 3727 (2019)]. This refinement predicts an abundance of clusters with various shapes, whichobey a certain periodicity. In fact, experimentally observed clusters are governed by the same rule.
Consequently, he constructed a periodic framework for clusters, which will be a useful compass to findmissing species.
Besides, he theoretically found special clusters. Spherical atoms have the highest geometrical symmetry,and hence atomic orbitals are highly degenerate. As for polyatomic species, an icosahedral structure is
the most symmetrical, yielding 5-fold degeneracy at the maximum. He discovered that it is possible toovercome these limitations. Tetrahedral clusters of Mg, Zn, and Cd exhibit higher-fold orbital degeneracy
than spherical or icosahedral symmetry [Nature Commun. 9, 3758 (2018)]. Remarkably, the origin of the“super-degeneracy” was identified as “dynamical symmetry”, the superordinate concept of point group.
This achievement will lead to supreme conductivity or magnetism utilizing super-degenerate orbitals.
(2) Mechanochemical reactions
He has tackled with mechanically-driven solid-state reactions [Tetrahedron Lett. 54, 5920 (2013);
ChemistrySelect 1, 984 (2016); Tetrahedron 73, 2305 (2017)], and successfully elucidated how mechanicalaction opens a new reaction path. Dibenzophenazine was recently synthesized in a ball-milling way at ahigh yield. This synthesis was expected to involve two distinct reaction paths. Actually, DFT calculations
show a stepwise lowest-energy path and also a concerted one with a higher barrier. However, the highest
energy point along the latter path is not a transition state but a higher-order saddle point. The concerted
path is therefore not generally accepted as a reaction path. He found that mechanical force causes
molecular deformation and removes the instability so that the higher-order saddle point is changed into atransition state [J. Phys. Chem. C 123, 21581 (2019)].
(3) A novel type of long-lived excitons
He has also solved a problem related to exciton dynamics in organic LEDs. A bisanthracene derivative isknown as an excellent luminescent material. Its high efficiency, however, cannot be explained byconventional thermally activated delayed fluorescence nor triplet-triplet annihilation. He focused on its
pseudo-degenerate frontier orbitals. His unexpected finding is that pseudo-degeneracy generally
suppresses nonradiative transitions, owing to the nontrivial cancellation of overlap between many-electron
wave functions [Sci. Rep. 7, 4820 (2017); J. Mater. Chem. C 7, 2541 (2019)]. As for the bisanthracene
derivative, electrically generated “higher triplets” are thus long-lived, and then converted into afluorescent singlet. His discovery tells us a novel category of efficient organic LEDs.