94

Original Research Paper

J. Jpn. Soc. Colour Mater., 97〔4〕，94–102（2024）

Development of Intramolecular Exciton-Coupled Bis-Squaraine Dyes
for Application to Near-Infrared Organic Photodetectors
Ryuhei Sawada＊, Reiji H ama＊＊, Saki K inoshita＊, Takeshi M aeda＊＊,†, Naoya Suzuki＊＊ and Shigeyuki Yagi＊＊＊ Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University,

1-1 Gakuencho, Naka, Sakai, Osaka 599-8531, Japan＊＊ Department of Applied Chemistry, Graduate School of Engineering, Osaka Metropolitan University,

1-1 Gakuencho, Naka, Sakai, Osaka 599-8531, Japan†Corresponding Author, E-mail: tmaeda@omu.ac.jp(Received January 4, 2024; Accepted January 31, 2024 )Abstract
We synthesized two bis-squaraine dyes consisting of benzo[ 1,2-b:5,4-b’]dipyrrole central components and indolenine-based semisquaraine
or its dicyanovinylene analogue for application to electron donor materials for organic photodetectors with bulk heterojunction architecture.
These dyes exhibited split electronic absorption bands attributed to intramolecular exciton coupling. The lower energy absorption band of
the bis-squaraine dye with dicyanovinylene group was observed in the near-infrared region. In cyclic voltammograms of bis-squaraine dyes,
broad redox waves were observed in oxidative scans, indicating that one-electron oxidized dyes have delocalized electronic states throughout
two chromophores. The highest occupied molecular orbitals and the lowest unoccupied molecular orbitals of bis-squaraine dyes have suitableenergy levels for electron donor materials toward the fullerene-based electron acceptor. A composite film comprised of a fullerene acceptor
and a bis-squaraine dye showing near-infrared absorption exhibited a near infrared absorption with a maximum at ca. 780 nm, promising
photo-induced electron transfer upon the irradiation of near infrared light. An organic photodetector with the composite film demonstrated aspecific detectivity (D＊) of 4.6×1011 Jones under 750 nm illumination and 3.3×1011 Jones under 800 nm illumination. This indicated thatthe bis-squaraine dye can work as the donor material in organic photodetectors responsive to near-infrared light.
Key-words: Exciton coupling, Squaraine, Donor, Near infrared, Photodetector

1．IntroductionOrganic photodetectors (OPDs), that directly convert lightinto electrical signals enhanced by the organic photoactive
layer, have attracted considerable interest due to their key
advantages, including the elimination of toxic elements,
compatibility with flexible substrates, and lightweightcharacteristics.1,2) Particularly, a bulk-heterojunction (BHJ)
featuring an electron donor and an acceptor in a photoactive
layer facilitates the dissociation of photogenerated excitons
and the transport of holes and electrons to the respective
electrodes.3,4) This enhances the efficiency of photocurrentgeneration in OPDs.5) These features enable their applicationin various fields as a viable alternative to inorganic
compound-based photodetectors. Especially, OPDs designed
for near-infrared (NIR) light (NIR-OPDs) are expected to findapplications used for living tissues and organisms such as
biometric authentication technology, since NIR light exhibits
significant biological permeability due to the suppression ofabsorption, scattering, and autofluorescence in living tissues.6)【Figures and illustrations】Figures and illustrations published in black andwhite in the journal can be seen in color at our public website「J-STAGE」.
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© 2024 Journal of the Japan Society of Colour Material−2−To date, various compounds with extended π-electron systems
have been developed for integration into the photoactive layerof NIR-OPDs.7) These compounds exhibit strong absorption in
the NIR region and demonstrate semiconducting properties due
to a substantial overlap of π orbitals.
Squaraine dyes, derived from the condensation of squaric
acid and electron-rich heterocycles, emerge as strong
candidates for donor materials for NIR-OPD.8, 9) Theyexhibit electron-donating properties owing to their relatively
high oxidation potentials and display significant electronic
absorption strength with adjustable electronic transition energy.
This characteristic is primarily dependent on the structureof electron-rich heterocyclic components located at the
1,3- position of the central cyclobutenedione.10,11) Moreover,these dyes can be modified with diverse electron-poor
substituents at the 2- position of cyclobutenedione, enablingthem to exhibit intense NIR absorption properties.12,13) Theseversatile properties make them suitable for OPDs 14-17) andenable a wide range of applications, including chemosensors,18)
bioimaging,19,20) photoacoustic imaging,21) photothermaltherapy,22) and organic photovoltaics. 23 )The optimization of electronic absorption properties andoxidation potentials in squaraine dyes is typically achieved