333

Original Technical Paper

J. Jpn. Soc. Colour Mater., 96〔10〕，333–337（2023）

Aggregation-induced Emission and Solid-State Fluorescence of
2-(4-(N,N-diphenylamino)phenyl)tryptanthrin
Jun K awakami＊,†, Mizuki H arada＊, Yutaro A kagi＊, Masahiro Takahashi＊, Shun Ohta＊ and Shunji Ito＊
＊Graduate School of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki, Aomori 036-8561, Japan†Corresponding Author, E-mail: jun@hirosaki-u.ac.jp(Received July 8, 2023; Accepted August 15, 2023 )Abstract2-( 4-(N,N-diphenylamino)phenyl)tryptanthrin (T2PhNPh 2 ) was synthesized and its photophysical properties were examined. Amongthe seven examined solvents of various polarities, T2PhNPh 2 , which is non-planar only showed weak red fluorescence in non-polar
toluene and 1,4-dioxane. This was assumed to be due to thermal deactivation as a result of the rotation of the phenyl groups’ stimulation ofinternal conversion in calculation using the density functional theory. Although T2PhNPh 2 was non-fluorescent in tetrahydrofuran (THF),
aggregation-induced emission was shown to cause fluorescence in water/THF solutions with more than 65 vol% water content. Additionally,fluorescence was observed at ca. 650 nm in the solid state.
Key-words: Fluorescence, Aggregation-induced emission, Mechanochromic luminescence, Tryptanthrin

1．Introduction
Fluorescent analysis reagents, including fluorescentchemosensors and probes that are based on organic
fluorescent dyes, are simple and highly versatile. In
particular, these molecular tools have proven indispensable
for studying biological phenomena in life sciences 1 , 2 ).
However, under some conditions, such as those that resultin aggregation-caused quenching (ACQ), the advantageous
properties of these fluorescence analysis reagents can besomewhat limited 3 ). Many organic fluorescent dyes havea planar structure, and their fluorescence is dependent on
how the HOMO–LUMO gap changes when the region ofthe π-conjugated plane increases or decreases. For instance,
the typical organic fluorescent dye pyrene efficiently emits
fluorescence in a dilute solution. As the concentration
increases, however, the pyrene molecules are arranged in a
sandwich-type structure that shows excimer fluorescenceat a longer wavelength than that corresponding to themonomer, and the intermolecular distance increases from
3 to 4 Å 4 ). Furthermore, in high concentration solutions,the π-π stacking interaction becomes stronger, and themolecules form a layered structure that leads to fluorescencequenching. Thus, planar organic fluorescent dyes can beused only in dilute solutions, preventing their use in highconcentration conditions or the solid state. To address theissues in detection sensitivity and quantification associated【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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with ACQ, the development of novel methodologies forthe construction of fluorescent analysis reagents is highly
desirable. In 2001, B. Z. Tang and coworkers reported a
new aggregation-induced emission (AIE) type of organic
fluorescent dye, in which the fluorescence intensity rises
upon aggregation 5 ). Meanwhile, the competition between
the radiative process of light emission and the non-radiative
process of thermal release leads to consumption of excitationenergy. Tetraphenylethylene, a typical AIE dye undergoes
a non-radiative process with thermal release favored by thephenyl group’s rotational movement in a dilute solution andexhibits no fluorescence. Nevertheless, in high concentration
solutions, the rotational movement of the phenyl group issuppressed, favoring a light-emitting radiative process that
leads to the emission of fluorescence. The introduction of anelectron-donating group at the 2-position of tryptanthrin 6,7 ),an antimicrobial substance extracted from plants like indigo,is known to result in outstanding fluorescent properties 8-20 ).
However, in highly concentrated solutions, π-π stacking
interactions in fluorescent tryptanthrin compounds withplanar structures cause fluorescence quenching. Thus, forO

NNONFig. 1 Structural formula and X-ray crystal structure ofT2PhNPh 221）.

－1－© 2023 Journal of the Japan Society of Colour Material