185

Original Research Paper

J. Jpn. Soc. Colour Mater., 95〔7〕，185–188（2022）

Fluorescence Based on Excited-State Intramolecular Proton Transfer of
1-(2-Benzothiazolyl)-2-hydroxytryptanthrin
Jun K awakami＊,†, Masahiro Takahashi＊ 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 March 23, 2022; Accepted May 30, 2022 )Abstract1-( 2-Benzothiazolyl)-2-hydroxytryptanthrin (T2OH1BT) with benzothiazole as a proton acceptor at the 1-position of 2-hydroxytryptanthrin(T2OH) was synthesized and its photophysical properties were investigated. T2OH1BT was found to emit fluorescence at 630–670 nm
in polar solvents due to proton dissociation of the hydroxy group, and 575–610 nm in low polar solvents due to excited-state intramolecularproton transfer. T2OH1BT emitted fluorescence in the solid state, thereby differentiating it from common tryptanthrin derivatives whichcannot emit fluorescence in the solid state due to their planar structure.
Key-words: Fluorescence, Excited-state intramolecular proton transfer, Tryptanthrina)

1．Introduction
Organic fluorescent dyes, which are employed in organic
light-emitting diodes 1-3 ), dye-sensitized solar cells 4 ), dyelasers5), fluorescent probes6), and conventional dye materials,
have attracted increased research attention recently. We
explored the photophysical properties of tryptanthrin
derivatives as novel fluorescent dyes. Tryptanthrin is aweakly basic alkaloid present in several plant species 7 ).
Tryptanthrin shows antimicrobial activity against diverse
pathogenic bacteria and fungi 8 ). When an amino groupor other electron-donating substituent is introduced atthe 2 -position of tryptanthrin, intramolecular chargetransfer (ICT) occurs between the electron-withdrawing
carbonyl group at its 6 -position and the amino group,
resulting in strong fluorescence and positive fluorescencesolvatochromism 9 , 10 ). The fluorescence wavelengths offluorescent solvatochromic dyes vary according to thesurrounding environment (polarity difference); thus, various
life phenomena can be observed via the fluorescent color
change. The near-infrared ( NIR ) region ( 650 – 900 nm )is referred to as the “optical window” of cells and tissues
because of the lack of efficient endogenous absorbers in thisspectral range and the subsequent high penetration depth (inthe order of a few millimeters) in most tissues 11 ). Organic
fluorescent dyes that emit NIR fluorescence are preferable
for application in biological systems because they can reduce
autofluorescence and photodamage to living cells.
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－5－b)

ONNOOOHNNOOHSNFig. 1 Structures of (a) T2OH and (b) T2OH1BT.

Previously, we reported the photophysical properties ofan ICT dye, 2-hydroxytryptanthrin (T2OH, Fig. 1a) 12 ).
The absorption maxima (λa, max) of T2OH in a dimethyl
sulfoxide (DMSO)/H 2 O ( 1/9, v/v) solution at a pH range
from 1.3 to 7.2 and from 8.5 to 10.6 were at ca. 410 nm andca. 495 nm, respectively. The fluorescence maxima (λf, max)
were at ca. 660 nm regardless of the pH. T2OH was solublein polar solvents and showed two fluorescence peaks. One
was derived from undissociated T2OH at ca. 530 nm andthe second, which was not observed in nonpolar solvents,
was derived from the proton-dissociated tryptanthrin-2-olate
anion (T2O−) at ca. 660 nm. Therefore, we synthesized1-( 2-benzothiazolyl)-2-hydroxytryptanthrin (T2OH1BT,Fig. 1 b ) and investigated its photophysical properties,
assuming that the introduction of a proton-accepting
substituent into the molecule would allow it to exhibit only
long-wavelength fluorescence.
2．Experimental

Proton nuclear magnetic resonance (NMR) spectroscopy
was performed on a JEOL JNM–ECA 500 spectrometer withtetramethylsilane (TMS) as an internal standard. Ultraviolet© 2022 Journal of the Japan Society of Colour Material