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Third‐Order Nonlinear Optical Properties of One‐Dimensional Open‐Shell Molecular Aggregates Composed of Phenalenyl Radicals
Author(s) -
Yoneda Kyohei,
Nakano Masayoshi,
Fukuda Kotaro,
Matsui Hiroshi,
Takamuku Shota,
Hirosaki Yuta,
Kubo Takashi,
Kamada Kenji,
Champagne Benoît
Publication year - 2014
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201402197
Subject(s) - diradical , intermolecular force , hyperpolarizability , stacking , chemistry , tetramer , dimer , open shell , singlet state , chemical physics , delocalized electron , density functional theory , computational chemistry , photochemistry , crystallography , molecule , atomic physics , organic chemistry , polarizability , physics , excited state , enzyme
The impact of intermolecular interactions on the third‐order nonlinear optical (NLO) properties of open‐shell molecular aggregates has been elucidated by considering one‐dimensional aggregates of π–π stacked phenalenyl radicals with different intermolecular distances and the long‐range corrected spin‐unrestricted density functional theory method. In the phenalenyl dimer, which can be considered as a diradicaloid system, the diradical character strongly depends on the intermolecular distance, and the larger the intermolecular distance is, the larger the diradical character becomes. Then, around the equilibrium stacking distance that corresponds to an intermediate diradical character, its second hyperpolarizability ( γ ) is maximized and its value per monomer exhibits about a 30‐fold enhancement with respect to the isolated phenalenyl monomer. This suggests that equilibrium is an optimal compromise between localization and delocalization of the radical electron pairs in such pancake bonding. No such effect was observed for the closed‐shell coronene dimer. Moreover, when going from the dimer (diradical) to the tetramer (tetraradical), the γ ‐enhancement ratio increases nonlinearly with the aggregate size, whereas switching from the singlet to the highest spin (quintet) state causes a significant reduction of γ . Finally, for the tetramer, another one‐order enhancement of γ is achieved for the dicationic singlet relative to its singlet neutral state. These results demonstrate the key role of intermolecular π–π stacking interactions and charge in open‐shell (supra)molecular systems to achieve enhanced third‐order NLO properties.