Sensitization of europium(III) luminescence by DTPA derivatives

Article abstract of DOI:10.1246/cl.2000.312

Novel ligands, diethylenetriaminepentaacetic acid bearing several aromatic amines, were synthesized and their chelates with a europium ion were prepared. The lanthanide luminescence of the chelates of 1-aminonaphthalene and 7-amino-4-methylcoumarin was largely enhanced by the energy transfer from the ligand to the europium ion.

Full text of DOI:10.1246/cl.2000.312

312
Chemistry Letters 2000
Sensitization of Europium(III) Luminescence by DTPA Derivatives
Hiroaki Ozaki, Emiko Suda, Toshihisa Nagano, and Hiroaki Sawai*
Department of Chemistry, Faculty of Engineering, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515
(Received December 27, 1999; CL-991101)
Novel ligands, diethylenetriaminepentaacetic acid bearing
able as a chelating moiety but their lanthanide chelates have
very low fluorescence intensity. In a previous study, Bailey et
al.7 reported that a conjugate of diethylenetriaminepentaacetic
acid (DTPA) and p-aminosalicylate enhanced lanthanide lumi-
nescence. Recently, Selvin et al.8 reported that a conjugate of
7-amino-4-methyl-2(1H)-quinolinone and polyaminocarboxylic
acid has been shown to be useful for the enhancement of lan-
thanide luminescence and as a luminescence donor of a lumi-
nescence resonance energy transfer experiment. In this paper,
we report the synthesis of conjugates of DTPA with several
aromatic amines and the luminescence property of their lan-
thanide chelates compared with that of p-aminosalicylate-
DTPA chelate. We found that the conjugate containing 7-
amino-4-methylcoumarin showed strong fluorescence intensity
which could be applied for the probe.
First, we carried out a preliminary experiment to gain
information on the luminescence intensity of the lanthanide
chelates of the conjugates of DTPA and several aromatic
amines as a light-absorbing chromophore. Diethylenetriamine-
pentaacetic dianhydride (DTPA anhydride) was allowed to
react with 1 equiv of several amines. p-Aminosalicylic acid, o-
aminophenol, m-aminophenol, p-aminophenol, 2-(aminoethyl)-
pyridine, 2-aminobenzophenone, 1-aminonaphthalene, and 7-
amino-4-methylcoumarin were used as amines (Scheme).
These synthesized ligands were used for chelating with a
europium ion, and the fluorescence spectra of the chelates were
measured in aqueous solution at 260 nm of excitation wave-
length. 1-Aminonaphthalene derivative and 7-amino-4-methyl-
coumarin derivatives showed large enhancement of europium
several aromatic amines, were synthesized and their chelates
with a europium ion were prepared. The lanthanide lumines-
cence of the chelates of 1-aminonaphthalene and 7-amino-4-
methylcoumarin was largely enhanced by the energy transfer
from the ligand to the europium ion.
Lanthanide chelates have unique luminescent properties
such as a millisecond lifetime and a spiked emission. They
have been used as a probe in biological applications.1
Especially, they give very high sensitivity by use of a time-
resolved fluorescence detection to eliminate short-lived fluores-
cence background signals.2 The disadvantages of lanthanide
chelates relative to organic fluorophores are their low quantum
yields in an aqueous system. For overcoming this problem,
several chelates were studied up to date.3-6 These chelates pos-
sess two functional units, a chelating moiety which coordinates
a lanthanide ion and an organic chromophore which absorbs
and sensitizes light. Polyaminocarboxylic acids such as DTPA,
ethylenediaminetetraacetic acid (EDTA), and triethylene-
tetraminehexaacetic acid (TTHA) form stable complexes with
lanthanide ions even at low concentrations. Thus, they are suit-
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
313
luminescence (618 nm). Thus, these two ligands were isolated
and their properties were investigated in detail. A p-aminosali-
cylic acid derivative was also isolated and examined as a refer-
ence. These ligands derived from 2a-c were purified by silica-gel
column chromatography to give two conjugates, monoamides
(3a-c) and bisamides (4a-c) in the ratio of about 2:1 from each
reaction. The yields were 69% for 3a, 31% for 4a, 36% for 3b,
19% for 4b, 46% for 3c, and 29% for 4c. Each compound was
characterized by ESI-MS and 1H NMR spectroscopy. The forma-
tion of the chelates with a europium ion was confirmed by ESI-
MS. In this analysis, each chelate shows a unique spectrum corre-
sponding to the isotopic abundance of europium (151Eu:153Eu =
47.8:52.2), to indicate the formation of the chelates.
The electronic absorption spectra of the ligands and their
chelates are summarized in Table 1. The europium chelates
containing p-aminosalicylate or 1-aminonaphthalene showed
very similar spectra with the corresponding ligands, indicating
that the europium ion did not give rise to perturbation of a chro-
mophore in the ground state. On the other hand, the spectra of
the chelate containing 7-amino-4-methylcoumarin were slightly
different from that of the corresponding ligand, which suggests
the coumarin moiety interacts with the europium ion.
spectra (data not shown). This energy transfer process likely
proceeds via an intersystem crossing from the first excited sin-
glet state to the triplet state of the ligand followed by energy
transfer to the europium ion. The synthesized chelates in this
study will have great potential for biological application. Thus,
work on the derivatization and the immobilization of the
chelate on biological molecules such as nucleic acids and pro-
teins is in progress.
Luminescence of the chelates was observed with a large
enhancement of the europium luminescence compared with the
europium chelate of DTPA. The emission of the chelates main-
We thank Dr. Seiji Tobita for helpful discussions on fluo-
rescence analysis.
5
7
5
ly arises from transition from D0 to F2 (615 nm) and D0 to
7F1 (594 nm). The luminescence data are shown in Table 2.
The emission intensity of these chelates at 615 nm under excita-
tion at λmax was in the following order: 7-amino-4-methyl-
coumarin-, 1-aminonaphthalene- and p-aminosalicylate-con-
taining chelates. Monoamide derivatives showed a slightly
larger or almost the same emission intensity compared with the
bisamide derivatives in the case of 1-aminonaphthalene- and p-
aminosalicylate-containing chelates, while the bisamide deriva-
tive showed larger emission intensity than the monoamide
derivative in the case of 7-amino-4-methylcoumarin-containing
chelate. The emission intensity of the europium chelate
reached 1000 times for 3c and 1800 times for 4c compared with
that for DTPA. This enhancement is caused by an energy trans-
fer from the ligand to the metal. Figure 1 shows the electronic
spectrum, excitation spectrum, and emission spectrum of [Eu3+
3b]. The excitation spectrum was similar to the electronic
spectrum, suggesting that the energy transfer takes place. Other
chelates also showed similar excitation spectra to the electric
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