Sensitization of the NIR emission of Nd(iii) by the α4 atropoisomer of a meso-tetraphenyl porphyrin bearing four 8-hydroxyquinolinylamide chelates

Article abstract of DOI:10.1039/b920676k

The α4 atropoisomer of the meso tetrakis 8-hydroxyquinolinylamide porphyrin and its Pd complex binds Nd(iii) and sensitizes efficiently its near infrared (NIR) emission when excited in the visible domain.

Full text of DOI:10.1039/b920676k

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Sensitization of the NIR emission of Nd(III) by the a4 atropoisomer
of a meso-tetraphenyl porphyrin bearing four 8-hydroxyquinolinylamide
chelateswz
a
a
b
bc
Fabrice Eckes, Veronique Bulach,* Aurelie Guenet, Cristian A. Strassert, Luisa De Colab
´
´
and Mir Wais Hosseini*a
Received (in Cambridge, UK) 6th October 2009, Accepted 2nd November 2009
First published as an Advance Article on the web 20th November 2009
DOI: 10.1039/b920676k
The a4 atropoisomer of the meso tetrakis 8-hydroxyquinolinyl-
amide porphyrin and its Pd complex binds Nd(III) and sensitizes
efficiently its near infrared (NIR) emission when excited in the
visible domain.
and Nd(III) (11 360 cmꢀ1). Moreover, by exploiting the binding
ability of the tetraaza core of the porphyrin, it should be
possible to tune the nature and the energy of the excited state
by the introduction of various transition metals (Fig. 1(c)).
Using the peripheral positions on the porphyrin backbone
such as the b-pyrrolic positions one may further adjust the
electronic levels of the antenna. Finally, by using aryl groups
bearing a bulky substituent at the ortho position connected to
the meso positions, one may take advantage of the formation
of atropoisomers12 and thus, in the case of the a4 isomer,
preorganise four chelates for binding the lanthanide ion
which usually adopts a coordination number in the range
of 8–12 (Fig. 1(a)). We have previously achieved this by
synthesising atropoisomers of a meso-(tetra-phenylcatecholamide)
porphyrin.13
For many optical devices such as for example amplifiers1 and
lasers,2 the luminescence properties3 of lanthanide ions are of
particular interest. Indeed, lanthanide ions emit narrow bands
ranging from visible to near-infrared.4 However, owing to the
low absorption coefficients of Laporte-forbidden-f–f transitions,
the emission must be sensitized by an energy transfer from
chromophoric ligands5–9 located in the proximity of the
lanthanide ion (antenna effect). Furthermore, in order to avoid
quenching of the emission by non-radiative deactivation, the
lanthanide cations must be protected by the presence of
multidentate ligands.10
As a chelating moiety, 8-hydroxyquinoline, a monoanionic
ligand when deprotonated, appeared to us as the candidate of
choice. For orientation and distance reasons, the connection
of the chelate was achieved using the position 7 of 8-hydroxy-
quinolinyl. Finally, the rather rigid amide group was used for
joining the chelate and the meso-arylporphyrin moiety. It is
worth noting that, based on CPK models, a distance of ca. 5 A
between the centroid of the four pyrrole moieties of the
porphyrin and the Ln cation may be expected (Fig. 1(b)).
Furthermore, the localisation of four chelates in close
proximity for binding of the lanthanide ion should protect
the latter from direct interactions with solvent molecules. Both
the short distance mentioned above and the protection of the
emitting centre should lead to efficient NIR luminescence.
For exploring the role played by transition metals complexed
by the porphyrin core (Fig. 1(d)), Pd(II) appeared as an
interesting candidate since Pd-porphyrin complexes possess a
very efficient intersystem crossing due to the heavy metal effect
and therefore the lowest excited state is a long lived triplet
state as can be easily demonstrated by its efficient quenching
by oxygen.11a
Here we report on the a4 atropoisomer of a meso-
tetraphenylporphyrin-based ligand bearing four 8-hydroxy-
quinolinyl amide chelates 2 (Scheme 1, Fig. 1) as an efficient
sensitizer for the NIR emission of Nd(III).
Our design principle was based on the use of the porphyrin
backbone as an antenna. The latter is perfectly suited for the
sensitization of NIR emission of lanthanide cations, since it
offers a triplet excited state4,11 typically in the range of
12 000–17 000 cmꢀ1 which is higher, but still close in energy
to the emitting levels of Er(III) (6500 cmꢀ1), Yb(III) (10 200 cmꢀ1
)
Fig. 1 Schematic representations of the a4 atropoisomer of the
porphyrin 2 (a), [2-Nd]ꢀ (b), 2-Pd (c) and [(2-Pd)–Nd]ꢀ (d).
a Laboratoire de Chimie de Coordination Organique, UMR CNRS
The synthesis of compound 2 (Scheme 1) was based on the
use of the a4 atropoisomer of meso-tetrakis(o-aminophenyl)-
porphyrin 1. The latter was prepared following the described
procedure by Lindsey et al.14 The condensation of 1 with
8-hydroxy-7-quinolinecarboxylic acid, prepared according to
a published procedure,15 in the presence of HBTU as a
coupling reagent16 afforded the compound 2 in ca. 80%
yield (see ESIz for detailed synthesis). It is worth noting that
the reaction must be performed at room temperature in order
to avoid atropoisomerisation. The synthesis of the Nd(III)
´
7140, Universite de Strasbourg, F-67000, Strasbourg, France.
E-mail: hosseini@chimie.u-strasbg.fr; Fax: 33 368851325;
Tel: 33 368851323
b Physikalisches Institut, Westfalische Whilhems-Universitat Munster,
¨
¨
¨
Mendelstrasse 7, D-48149, Munster, Germany.
¨
E-mail: decola@uni-muenster.de
c Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven,
The Netherlands
w Dedicated to Professor J.-C. Bunzli on the occasion of his 65th
¨
birthday.
z Electronic supplementary information (ESI) available: Experimental
section. See DOI: 10.1039/b920676k
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Fig. 2 Absorption spectra of 2, [2-Nd]ꢀ, 2-Pd, [(2-Pd)–Nd]ꢀ in THF
at room temperature.
in 73% yield upon addition of 1 eq. Nd(acac)3 and 1 eq.
TBAOH to a solution of 3 (4 eq.) in CH2Cl2–CH3CN
(see ESIz).
For the reference chelate 3, the electronic absorption
spectrum in THF shows an absorption maximum at 255 nm
assigned to a p–p* transition17 and a shoulder at 277 nm which
can be attributed to partial deprotonation of the ligand
(Table 1). When compared to 8-hydroxyquinoline,18 the
observed blue-shift of ca. 50 nm of the most intense band is
due to the presence of the electron withdrawing amide group
in position 7. For [(3)4Nd]TBA, a shift of ca. 20 nm in the
absorption maxima and the appearance of a new band at
350 nm attributed to an ILCT transition are observed
(Table 1, Fig. 2).
In THF, for all four porphyrin complexes 2, [2-Nd]ꢀ, 2-Pd
and [(2-Pd)–Nd]ꢀ, the UV part of the electronic spectra
(Table 1) presents a relatively intense absorption band centred
around 250–280 nm (Fig. 2) which can be assigned to p–p*
transitions of the quinolinyl moiety as discussed previously.
The visible part of the electronic spectra of 2 and [2-Nd]ꢀ is
characteristic of a free-base porphyrin with an intense Soret
band around 423 and 431 nm, respectively, together with four
Q bands between 510 and 650 nm (Table 1, Fig. 2).
Scheme 1
complex [2-Nd]ꢀ was achieved in 82% yield in CH2Cl2 upon
treatment of 2 by Nd(acac)3(H2O)2 in the presence of one
equivalent of tetrabutylammonium hydroxide (TBAOH). The
[2-Nd]TBA complex is soluble in most halogenated solvents
and in THF (see ESIz).
For the synthesis of the heterobimetallic species
[(2-Pd)Nd]TBA, in order to avoid the binding of Pd(II) by
the hydroxyquinolinyl chelates, a strategy based on a three-
step procedure was developed. Starting with a mixture of
atropoisomers of the porphyrin 1, the reaction, under Ar,
with Pd(OAc)2 in a refluxing mixture of CHCl3 and MeOH
afforded all the metallated atropoisomers of 1-Pd. The mixture
was first enriched in the a4 atropoisomer upon heating in
toluene in the presence of silica14 and subsequently purified by
column chromatography on alumina affording 1-Pd in 24%
yield. The complex 2-Pd was obtained in 67% yield upon
coupling of 1-Pd with 8-hydroxy-7-quinolinecarboxylic acid in
the presence of HBTU in CH2Cl2. Finally, the formation of
[(2-Pd)Nd]TBA was achieved in 55% yield as described above
for [2-Nd]TBA. However, owing to the low solubility of 2-Pd
complex, a mixture of CH3CN–CH2Cl2 was used.
Upon binding of Nd(III) ion, the Soret band is shifted by
8 nm and the relative intensity of the two Q bands Q(1,0) and
Q(0,0) changes (Q(0,0) less intense than Q(1,0)). This might be
due to a deformation of the porphyrin core induced by
the presence of Nd(III). The absorption spectra of the two
palladium complexes 2-Pd, and [(2-Pd)–Nd]ꢀ also display an
intense Soret band at 421 and 424 nm, respectively, and only
As a reference for the photophysical studies, the chelate 3
was synthesised and its Nd(III) complex [(3)4Nd]TBA prepared
Table 1 Spectral data at RT in THF
Absorption l/nm (e/Mꢀ1 cmꢀ1
)
Emission l/nm
2
284 (8.65 ꢂ 104), 423 (2.43 ꢂ 105), 517 (1.67 ꢂ 104),
548 (0.44 ꢂ 104), 591 (0.56 ꢂ 104), 646 (0.15 ꢂ 104)
280 (6.16 ꢂ 104), 431 (1.26 ꢂ 105), 522 (1.10 ꢂ 104),
560 (0.85 ꢂ 104), 591 (0.68 ꢂ 104), 654 (0.30 ꢂ 104)
254 (9.62 ꢂ 104), 421 (2.03 ꢂ 105), 527 (1.99 ꢂ 104),
558(0.40 ꢂ 104)
659; 723a
[2-Nd]ꢀ
2-Pd
665, 720, 1070, 1340a
570, 620, 705, 776a
570, 620, 700, 773, 1069, 1343a
[(2-Pd)–Nd]ꢀ
277 (4.97 ꢂ 104), 424 (0.92 ꢂ 105), 532 (1.12 ꢂ 104),
560 (0.26 ꢂ 104)
3
255 (3.20 ꢂ 104), 277 (1.90 ꢂ 104)
494
498, 1065, 1338
[(3)4Nd]ꢀ
275 (11.5 ꢂ 104), 350 (3.1 ꢂ 104)
a
Degassed solution RT.
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emission of Nd(III). A detailed study of the photophysics of
the system as well as the use of ligand 2 in conjunction with
other d and f metals are currently under investigation.
Many thanks to J. McB Harrowfield for helpful discussions.
Support from the Universite de Strasbourg, the International
´
Centre for Frontier Research in Chemistry (FRC),
Strasbourg, the C.N.R.S., the Institut Universitaire de France
and the Ministere de l’Enseignement Superieur et de la
´
Recherche (PhD fellowship to F. E.) is gratefully acknowl-
edged. The work of C. A. S. forms part of the research
programme of the Dutch Polymer Institute (DPI), project
#628. The work of A. G. was supported by the European
Commission (EC) through the Human Potential Programme
within the 6th framework programme (Marie Curie RTN
NANOMATCH, MRTN-CT-2006-035884).
Fig. 3 Excitation spectrum of [2-Nd]ꢀ and [(2-Pd)–Nd]ꢀ by monitoring
the emission at 1070 nm, and emission spectra (dotted line) of Nd(III)
porphyrin complexes while exciting at 429 nm in degassed THF at RT.
two Q bands around 530 and 560 nm are observed as expected
(Table 1, Fig. 2).19 The less pronounced shift of ca. 3 nm in the
Soret band for [(2-Pd)–Nd]ꢀ with respect to 2-Pd is probably
due to the rigidification of the porphyrin ring upon binding of
Pd(II). No change is observed for the relative intensity of the Q
bands upon binding of Nd(III).
Notes and references
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In THF, upon excitation at around 425 nm, both [2-Nd]ꢀ
and [(2-Pd)–Nd]ꢀ complexes display Nd-centred NIR lumi-
nescence, in addition to the porphyrin centred emissions in the
visible region (Fig. 3). However, the presence of Nd(III) causes
partial quenching of the porphyrin centred luminescence. At
RT, in degassed and aerated solution as well as at 77 K, the
luminescence spectrum (Fig. 3) in the 1000–1500 nm range
consists of two bands at 1065 and 1340 nm corresponding to
3 J.-C. Bunzli and C. Piguet, Chem. Soc. Rev., 2005, 34, 1048.
¨
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4 S. Comby and J.-C. G. Bunzli, Lanthanide Near-Infrared
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the expected f–f transitions (4F3/2 - 4I11/2 and 4F3/2 - 4I13/2
,
respectively). The excitation spectra of [2-Nd]ꢀ and [(2-Pd)–Nd]ꢀ
species while monitoring the emission from Nd at 1070 nm
(Fig. 3) are identical to the porphyrin absorption spectra
(Fig. 2) demonstrating thus the antenna effect of the porphyrin
moiety in both complexes.
¨
9 J. P. Leonard, C. B. Nolan, F. Stomeo and T. Gunnlaugsson,
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the exact same conditions in the presence and in the absence of
O2 while exciting at 350 nm. In all cases, enhanced emission
from the Nd(III) ion was observed in anaerobic conditions.
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emission could be estimated. These observations imply that:
(i) a triplet excited state is involved in the energy transfer
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i.e. energy transfer and quenching by oxygen of a triplet state,
are taking place and (iii) the intramolecular energy transfer is
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8-hydroxyquinolines organised in a convergent manner. The
difference in binding propensity of the two coordinating poles
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heterobinuclear ([(2-Pd)–Nd]ꢀ) Neodymium complexes. In
both types of complexes, the porphyrin backbone plays the
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Chem. Commun., 2010, 46, 619–621 | 621