131Digest

J. Jpn. Soc. Colour Mater., 96〔4〕，131–136（2023）

Pyrazinacenes as a New Class of Highly Fluorescent REDOX Active Chromophores
Gary J. Richards＊,†, Keita Aoki＊, Daiki A kibayashi＊, Majid Tamboli＊, Jonathan P. Hill＊＊ and Akiko Hori＊＊Department of Applied Chemistry, Graduate School of Engineering and Science, Shibaura Institute of Technology,

307 Fukasaku, Minuma-ku, Saitama-shi, Saitama 337-8570, Japan＊＊Functional Chromophores Group, International Center for Materials Nanoarchitectonics, National Institute for Materials Science,

1-1 Namiki , Tsukuba, Ibaraki 305-0044, Japan†Corresponding Author, E-mail: richards@shibaura-it.ac.jp(Received December 1, 2022; Accepted December 27, 2022 )Abstract
Pyrazinacenes consist of rectilinearly fused 1,4-pyrazine units and can be considered as analogues of the well-studied acene family ofcompounds with the apical CH units replaced with nitrogen atoms. Herein we discuss the family of pyrazinacene compounds that have beendeveloped recently paying particular attention to their photophysical properties. We show that REDOX properties are dependent on thenumber of fused rings in the respective pyrazinacenes, and that in their reduced forms, pyrazinacenes are typically highly fluorescent species
with solution fluorescence quantum yields typically above 50%, even in extended or deprotonated systems where emission in the near infrared (NIR) region can be elicited.
Key-words: Pyrazinacenes, Acenes, REDOX, Near infrared, Fluorescence

1．Introduction
Acenes 1,2 ), consist of rectilinearly fused benzene rings
and are a class of compounds that has attracted significant
interest due to potential applications in molecular electronics
as organic field-effect transistors or organic photovoltaics3-6 ).
Many acene derivatives have been investigated in partto account for their intrinsic disadvantageous properties
including their low stability (especially of the higher acenes)
and processability issues involving their low solubility 7-9 ).
Acenes are electron-rich molecules whose aromaticity
decreases with increasing multiplicity of fused rings sothat higher acenes are susceptible to oxidative processes
leading to different degradation products depending on
the substrate 10 , 11 ). The low stability of acenes has beenameliorated to some extent by various strategies such as theintroduction of bulky substituents at reactive positions 12-14 )or by introducing heteroatoms 15 ), in particular, nitrogen 16 ),〔氏名〕 リチャーズ ゲーリー ジェームズ〔現職〕 Associate Professor, Department of AppliedChemistry, Shibaura Institute of Technology〔経歴〕 From 1997 - PhD in Organic Chemistry,
University of Hull, UK. 2001 – Post-doctoral
Research Associate, Oxford University, UK.
2007– Research scientist, National Instituteof Materials Science, Japan. 2013 - 2014 –Assistant Professor, Tsukuba University. 20142020 – Associate Professor, Ochanomizu
University. Current position from 2020.
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－5－leading to closely related families of compounds known as
the azaacenes 17 ) or N-heteroacenes 18,19 ). Here, we review
recent investigations into the photophysical propertiesof compounds in which all the apical C-H groups ofhydrocarbon acenes are replaced with nitrogen atoms.
This family of compounds, for which we have coined theterm “pyrazinacenes” 20 , 21 ), have properties that departsignificantly from those of conventional acenes or azaacenes.
The relative structures of pyrazinacenes as compared to theiracene congeners are illustrated in Scheme 1a.
There are many reports of acenes containing one, two or
three pyrazine rings and some of those most studied include
fluoflavine 22 ) and fluorubine 23 ) ( Scheme 1 b ) of which
various substituted derivatives have been reported 24,25 ).
However, there have been few reports of pyrazinacenes
consisting of more than three consecutively fused pyrazine
rings. Prior to our contribution 26 ) , the longest known
pyrazinacenes were the doubly reduced dodecaazahexacenes(Scheme 1c) reported by Stöckner and co-workers 27 ). The
synthetic route to these compounds leads directly to the tetraN-arylated species. Solutions of these compounds exhibited
fluorescence at λmax = 522 nm with fluorescence quantum
yields of 35% and a small Stokes shift of 5 nm.
Our synthetic route to pyrazinacenes 20 ) involves adouble nucleophilic aromatic substitution of electrondeficient 2 , 3 -dicyano- 1 , 4 , 5 , 8 , 9 -tetraazanaphthalene
moieties using aromatic o-diamines as shown in Scheme
1d. Using this approach, we have synthesized a large familyof pyrazinacene compounds that feature between 3 and 7© 2023 Journal of the Japan Society of Colour Material