Nanotechnologies / Materials
Organic electronic materials, Sensors

Synthesis and investigation of aromatic-rings congested
π-cluster systems
Department of Chemistry, Graduate School of Science
Assistant Professor Tomohiko Nishiuchi
Researchmap https://researchmap.jp/t-nishiuchi

Abstract
Nowadays, variable structures of polycyclic aromatic hydrocarbons (PAHs) composed of benzene ring as a basic unit have beensynthesized and some of them have been developed for the organic devises such as organic semiconductors or organic light-emitting diodes. In this work, we have focused on the π-cluster system, having the faces of multiple aromatic rings being shorter than
3.4 Å, which is the sum of the van der Waals radii of carbons, andinvestigated the unique properties of the π-cluster system that isexpressed in congested situation.

methods for building π-cluster systems with various molecularframeworks for the development of highly versatile functional materials.

Nanotechnologies / Materials▍ Background & Results

Anthracene-based cyclic π-cluster possessing quite short distance at 2.8 Å between anthracene was effectively synthesized byemploying an intramolecular cyclization reaction and it was investigated for disclosing the nature of π-congestion effect. The cyclic
anthracene dimer exhibits multi-stimuli responsiveness due to highπ-congestion. For example, photoirradiation on the anthracene
affords its photoisomer having C–C bonds that are longer than 1.65
Å which can undergo thermal reversion under gentle heating. This
enabled mechanochromism of the photoisomer (colorless) to theoriginal anthracene dimer (red). Photoisomerization was also observed in the crystalline state, accompanied by crystal jumping orcollapsing, that is, the photosalient effect.
Since the discovery of the triphenylmethyl (TPM) cation 120
years ago, a variety of aromatic cations having various colors andluminescence properties have been rigorously studied. However,
the optical properties of non-substituted and highly reactive TPM
cation, which was observed to be very weakly luminescent, havenot been subjected to detailed investigation. Herein, we exploredthe optical nature of non-substituted TPM cation in the crystalline
state. Investigation of the optical properties revealed that TPM cation shows a crystalline state emission, a distinct color change toyellow upon cooling as a consequence of a change in the nature of
the charge transfer interaction between the cation and anion, andphosphorescence.

▍ Significance of the research and Future perspective
By bringing the two aromatic rings closer together at a distanceshorter than the sum of the van der Waals radii of the carbon atom,
two molecular orbitals of the aromatic rings are hybridized resulting
in small HOMO-LUMO gap. Therefore, this simple π-congestion
affects the electronic properties of aromatic compounds, leading
to the display of unique various properties that cannot be exhibitedin the monomeric state. For example, changes in absorption andemission spectra, in electron-donating and accepting properties,
isomerization of the molecular skeleton by light irradiation, heating,or mechanical grinding. On the other hand, the construction ofsuch highly congested molecules poses synthetic problems suchas low yields. Therefore, we are also focusing on effective synthetic

P a t e n t
Nishiuchi, Tomohiko; Kisaka, Kazuki; Kubo,Takashi. Synthesis of Anthracene-Based Cyclic π-Clusters and Elucidation of their Properties Originating fromCongested Aromatic Planes. Angew. Chem. Int. Ed. 2021; 60(10): 5400-5406. doi: 10.1002/anie.202013349
Nishiuchi, Tomohiko; Sotome, Hikaru; Kubo, Takashi et. al. Optical nature of non-substituted triphenylmethyl cation: Crystalline state emission, thermochromism, and phosphorescence. Aggregate. 2021; 2(6): e126. doi: 10.1002/agt2.126
UR
L http://www.chem.sci.osaka-u.ac.jp/lab/kubo/K e y w o r d polycyclic aromatic hydrocarbon, π-cluster, external stimulus responsiveness, luminescent properties, chromism
Tr e a t i s e