1272 Bull. Chem. Soc. Jpn. Vol. 83, No. 10, 1272–1274 (2010)
Short Articles
Fluorescence Enhancement in
7-Hydroxyquinoline Analogs by
Methyl Substitution and Their
Spectroscopic Characteristics
in Aqueous Solution
Naoko Senda, Atsuya Momotake, and Tatsuo Arai*
Figure 1. Equilibrium among the prototropic species of
7-hydroxyquinoline (HQ) in water.
Graduate School of Pure and Applied Sciences,
University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8571
Received April 23, 2010
E-mail: arai@chem.tsukuba.ac.jp
Significant enhancement of the photosensitivity of
7-hydroxyquinolines in aqueous buffer solution has been
achieved simply by introducing two methyl groups at 2 and 4
positions. The product of the molar extinction coefficient and
fluorescence quantum yield (¾ Á ¯f) for 7-hydroxy-2,4-di-
methylquinoline is about 13-fold greater than that of original
7-hydroxyquinoline.
Figure 2. Chemical structures of 4-ethoxycarbonylmethyl-
7-hydroxy-2-methylquinoline (TG), 7-hydroxyquinoline
(HQ), and its analogs.
membrane, enhances their fluorescence. Very recently, we have
reported the new HQ-based fluorescent dye, Tsukuba-Green
(TG), 4-ethoxycarbonylmethyl-7-hydroxy-2-methylquinoline
(Figure 2).10 The photochemical behavior of TG might be
quite characteristic and distinctive when TG is used as a
fluorescence probe for live-cell imaging. Unlike those tradi-
tional probes, TG emits green fluorescence (-max = 509 nm,
¯f = 0.15) in aqueous solution, that dramatically changes to
blue fluorescence peaking around 360-380 nm in hydrophobic
media, due to the similarity of the excited-state dynamics of
TG to HQ. In addition, TG is found to be delocalized in the
cells. Fortunately, the photosensitivity, the product of the molar
extinction coefficient and fluorescence quantum yield (¾ Á ¯f)
of TG in neutral buffer solution is 4-5-fold greater than that of
the parent compound HQ, suggesting that TG possesses
advantages as a fluorescent probe compared to HQ. These
results let us assume the significant effect of substituent on its
photosensitivity of HQ. The main factor for the improvement
of the photosensitivity seems to be the methyl substituent.
However, since TG is the first HQ-based fluorescent probe for
living cells, an improvement of the fluorescence efficiency of
HQ families for biological use has not been explored yet. In
order to reveal the highly important properties of TG as a
fluorescent probe, we have studied the effect of methyl
substitution on the photochemical properties of HQ as a model
system. In this paper, we report the substituent effect on the
fluorescence efficiency using HQ analogs, 7-hydroxy-2-meth-
ylquinoline (HMQ) and 7-hydroxy-2,4-dimethylquinoline
(HDMQ) (Figure 2). pH Dependence of HQ, HMQ, and
HDMQ on their photochemical behavior is also investigated.
HMQ and HDMQ were prepared according to a previous
report.11 The photochemical data of HQ have been reported
elsewhere.10 Figure 3a shows the steady-state absorption
spectra of HQ, HMQ, and HDMQ, respectively, in water
(1% DMSO). The lowest absorption bands of prototropic
Photoinduced excited state proton transfer (ESPT) of
7-hydroxyquinoline (HQ) is a unique reaction system. In the
presence of water for example, HQ is proposed to form a
cyclical complex with water molecules,1 and undergoes excited
state proton transfer through a proton-relay system mediated by
water.2 The produced excited state species (Z*) emits a large
Stokes shifted fluorescence to give the ground state tautomer
followed by fast back proton transfer. Various models have
been proposed so far for solvent-assisted ESPT of HQ. Fang
investigated triple proton transfer within cyclic complexes
based on ab initio calculations.3 Kohtani et al. confirmed the
formation and 1:2 stoichiometry of HQ-alcohol complexes in
hexane on the basis of spectral analysis and semiempirical MO
calculations.4 Kwon et al. reported the intrinsic proton shuttling
dynamics of cyclically HQ-alcohol complexes which undergo
stepwise ESPT with the assistance of solvent fluctuations.5 Park
et al. recently reported the Grotthuss-type proton-transport
mechanism of water in cyclical HQ-water complex in diethyl
ether.6
In a neutral aqueous solution, HQ is in the equilibrium
configuration of four prototropic species: a normal form (N), an
anion form (A), a cation form (C), and a zwitterion form (Z) as
shown in Figure 1.7 HQ exist mostly as N (67%) and Z (29%)
with minor species of C (3%) and A (1%) at pH 7.7b In the
excited state, the pKa of the 7-hydroxy group decreases,
whereas that of the quinolinium nitrogen increases. Therefore
the 7-hydroxy group in HQ acts as a proton donor and the
nitrogen acts as a proton acceptor in the excited state.8
The fluorescence of many traditional fluorescent probes9 for
bio-imaging is usually very weak in water, but conjugation to
hydrophobic biomolecules such as a protein, DNA, or plasma