Sensitized near-IR luminescence of lanthanide complexes based on push-pull diketone derivatives

Article abstract of DOI:10.1039/b915893f

Lanthanide complexes with two push-pull diketone derivatives as sensitizers have been developed as synthons for near-infrared emitting materials. The ligand substituents consist of a carbazole moiety with hole-transport properties and an aromatic or heteroaromatic unit. According to quantitative NMR analysis and complementary HPLC experiments, the diketones are predominantly in their enolic form in polar solvents such as THF and MeCN at room temperature. The preferred cis-enol form contributes strongly to the binding of lanthanide ions (Ln = Nd, Gd, Er). The resulting tris(diketonate) ternary complexes with terpyridine (Ln = Nd, Er) display sizeable near-IR emission with long luminescence lifetimes. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b915893f

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PAPER
www.rsc.org/dalton | Dalton Transactions
Sensitized near-IR luminescence of lanthanide complexes based on push-pull
diketone derivatives†
Nam Seob Baek,a Yong Hee Kim,a Yu Kyung Eom,b Jung Hwan Oh,b Hwan Kyu Kim,*b Annina Aebischer,c
Fre´de´ric Gumy,c Anne-Sophie Chauvinc and Jean-Claude G. Bu¨nzli*b,c
Received 4th August 2009, Accepted 26th October 2009
First published as an Advance Article on the web 14th December 2009
DOI: 10.1039/b915893f
Lanthanide complexes with two push-pull diketone derivatives as sensitizers have been developed as
synthons for near-infrared emitting materials. The ligand substituents consist of a carbazole moiety
with hole-transport properties and an aromatic or heteroaromatic unit. According to quantitative
NMR analysis and complementary HPLC experiments, the diketones are predominantly in their enolic
form in polar solvents such as THF and MeCN at room temperature. The preferred cis-enol form
contributes strongly to the binding of lanthanide ions (Ln = Nd, Gd, Er). The resulting tris(diketonate)
ternary complexes with terpyridine (Ln = Nd, Er) display sizeable near-IR emission with long
luminescence lifetimes.
In this work, we investigate further these phenomena by pre-
senting the synthesis of two push-pull diketone derivatives, CTPD
Highly luminescent lanthanide complexes are attracting attention
and CTNP (see Scheme 1) and by determining their keto-enol ratio
Introduction
by 1H-NMR and HPLC analysis, as well as their solvatochromic
behaviour. Furthermore, ternary tris(b-diketonate) complexes
with tpy are isolated and the photophysical properties of the
Nd(III) and Er(III) complexes are investigated on a quantitative
basis.
in a wide variety of photonic applications such as planar waveguide
amplifiers,1,2 light-emitting diodes3 and bio-inspired luminescent
probes.4 The design of organic photosensitisers has dominated the
development of smart lanthanide-based optical devices in view of
their high molar absorption coefficients, flexibility of molecular
design, as well as their efficient sensitization ability of the metal-
centred luminescence. In particular, this approach allows the
enhancement of the emission intensity and quantum yield of near-
infrared (NIR) emitting lanthanide ions.5 Among the numerous
ligands tested to date, b-diketonates6 appear to be adequate
sensitizers for tailoring luminescent lanthanide complexes in which
either visible or NIR emission is consecutive with photo-induced
energy transfer from the sensitizing ligand.7 In order to use lan-
thanide tris(b-diketonates) as emissive layers in electroluminescent
devices, “push-pull” ligands have been synthesized which feature
a carbazole substituent as electron donor and a naphthalene or
fused thiophene group as electron acceptor. Additionally, the
coordination sphere of the lanthanide ion is usually completed by
the use of an ancillary ligand such as 2,2¢:6¢,2¢¢-terpyridine (tpy).8,9
Although b-diketonates provide strong bidentate binding sites for
lanthanide ions, they commonly exist as keto-enol tautomers, a
proportion of which is intrinsically affected by the b-diketone
substituents.10,11 Furthermore, the characteristic electronic states
of the ligands is significantly influenced by the solvent polarity.12
Scheme 1 Synthesis of b-diketone ligands.
Experimental
Materials and methods
NMR spectra were measured at 25 ◦C on Bruker Biospin with
1
CryoProbeTM (1D, H, 800 MHz) and Bruker Avance DRX 400
1
(2D-COSY experiments, H, 400 MHz) spectrometers. Spectra
were recorded in CD3CN (99.8%, Aldrich) or THF-d8 (99.5%,
Armar chemicals); deuterated solvents were used as internal stan-
dards and chemical shifts are given with respect to TMS. Methyl
2-naphthoate was obtained from TCl Co. and used without fur-
ther purification. 3-Acetyl-9-ethylcarbazole13 and ethylthieno[3,2,-
b]thiophene-2-carboxylate were synthesized according to litera-
ture methods.14 HPLC experiments were performed on a Waters
600 apparatus (pump and controller) with a Waters 2487 dual
l absorbance detector using a reverse phase column (Waters
Symmetry C18, 3.5 mm, 4.6¥ 75 mm) with an acetonitrile–water
eluent starting from 0% acetonitrile and increasing by 1% per
minute.
aIT Convergence Technology Research Laboratory, Electronics and Telecom-
munications Research Institute (ETRI), Daejeon, 305-700, Korea
bCenter for Advanced Photovoltaic Materials (ITRC) and Department of
Advanced Materials Chemistry, Korea University, Jochiwon, Chungnam,
339-700, Korea. E-mail: hkk777@korea.ac.kr; Fax: +82-41-867-5396;
Tel: +82-41-860-1494
c
´
Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), Laboratory of
Lanthanide Supramolecular Chemistry, BCH 1401, 1015 Lausanne,
Switzerland. E-mail: jean-claude.bunzli@epfl.ch; Fax: +41 (0)21 693 98
25; Tel: +41 (0)21 693 98 21
† Electronic supplementary information (ESI) available: UV-Vis, visible
and near-IR emission spectra, and lifetime. See DOI: 10.1039/b915893f
1532 | Dalton Trans., 2010, 39, 1532–1538
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Photophysical data
(d, 1H, H6¢); 7.24 (d, 1H, H7); 4.76 (s, 2H, -CH2-); 4.47 (q, 2H,
-CH2-CH3), 1.42 (t, 3H, -CH2-CH3).
Luminescence spectra of the ligands and quantum yields of the
complexes were measured on a Fluorolog FL-3-22 spectrometer
from Horiba-Jobin-Yvon Ltd.; quartz cells with optical paths of
0.2 cm were used for rt spectra while low-temperature measure-
Preparation of sodium (1Z)-3-(9-ethyl-9H-carbazol-2-yl)-3-
oxo-1-thieno[3,2-b]thiophen-2-yl-prop-1-en-1-olate (CTPD sodium
salt). The deprotonated ligand was obtained as follows: 4 mg of
the ligand were dissolved in ethanol (0.5 mL) in a 5 mL flask and
2 equivalents of sodium hydroxide were added (0.1 M in ethanol);
the solution was stirred at rt for 1 h then the solvent was removed
and the solid was dried under vacuum (0.3 mbar) for 2 h and then
re-dissolved in the deuterated solvent (CD3CN or THF-d8).
1H-NMR (800 MHz in THF-d8): d = 8.78 (s, 1H, H1); 8.17 (d,
1H, 3J = 7.58 Hz, H8); 8.14 (d, 1H, 3J = 7.58 Hz, H5); 7.87 (s, 1H,
Henol); 7.45 (d, 1H, 3J = 7.33 Hz, H3,4); 7.43 (d, 1H, 3J = 5.13 Hz,
R
ments were carried out on samples in quartz Suprasilꢀ capillaries.
Detectors were a Hamamatsu R927 photomultiplier for the visible
range and a cooled InGaAs detector from Electro-Optical Systems
Inc. (DSS-16A02OL) for NIR measurements. Emission spectra
were corrected for the instrumental function regularly updated.
Quantum yields were determined on solid samples at 295 K,
under ligand excitation, according to an absolute method using
a home-modified integration sphere.15 Each sample was measured
several times under slightly different experimental conditions.
The estimated error for quantum yields is 10–20%. Luminescent
lifetimes were determined upon excitation at 355 nm provided by
a Quantum Brillant Nd:YAG laser equipped with a frequency
tripler; the emitted NIR light was analysed at 90◦ on a home-built
setup comprising a Spex 1870 single monochromator with 950
grooves/mm holographic gratings blazed at 900 nm. Light inten-
sity was measured with a Hamamatsu H9170-75 photomultiplier
cooled by the Pelletier effect at -60 ◦C and coupled to a Stanford
Research SR430 multichannel scaler. Lifetimes are averages of
three independent determinations.
3
H5¢); 7.38 (m, 2H, H3,4 and H6); 7.23 (d, 1H, J = 5.13 Hz, H6¢);
7.14 (t, 1H, 3J = 7.33 Hz, H7); 6.65 (s, 1H, H3¢); 4.40 (q, 2H, 3J =
7.09 Hz, -CH2-CH3), 1.38 (t, 3H, 3J = 7.09 Hz, -CH2-CH3)
Preparation of the CNPD ligand. 3-Acetyl-9-ethylcarbazole
(1.50 g, 6.32 mmol) and methyl 2-naphthoate (1.40 g, 7.59 mmol)
were dissolved in 50 mL anhydrous THF under a N2 atmosphere.
Sodium ethoxide (0.52 g, 7.59 mmol) was added. After stirring
for 24 h at 60 ◦C, hydrochloric acid (1.0 N) was added to the
solution. The crude mixture was extracted with CH2Cl2 and dried
over anhydrous magnesium sulfate. The residue was purified by
column chromatography (CH2Cl2) to give the final product as a
yellowish solid. Yield: 81%; EI-MS calcd for C27H21NO2 391.16,
Found [M+] 391; Anal. Calcd for C27H21NO2: C, 82.84; H, 5.41;
N, 3.58. Found C, 82.76; H, 5.52; N, 3.52.
Prepar◦ation of 3-acetyl-9-ethylcarbazole13. Yield 58%, mp =
116–117 C; 1H-NMR (300 MHz in CDCl3-d1): d = 8.76 (s, 1H,
Ar–H), 8.2 (t, 2H, Ar–H), 7.53 (s, 1H, Ar–H), 7.40–7.47 (m, 2H,
Ar–H), 7.31 (t, 1H, Ar–H), 4.38 (q, 2H, -CH2CH3), 2.74 (s, 3H,
-COCH3), 1.43 (t, 3H, -CH2CH3); Anal. Calcd for C16H15NO: C,
80.98; H, 6.37; N, 5.90, Found: C, 80.61; H, 6.43; N, 6.27.
Enol form: (2Z)-1-(9-ethyl-9H-carbazol-2-yl)-3-hydroxy-3-
naphthalen-2-yl-prop-2-en-1-one. d = 9.01 (s, 1H, H1); 8.72 (s,
1H, Henol); 8.30 (dd, 1H, 3J = 8.55 Hz, 4J = 1.47 Hz, H3¢,4¢); 8.25
3
3
4
(d, 1H, J = 7.58 Hz, H8); 8.18 (dd, 1H, J = 8.55 Hz, J =
1.47 Hz, H3¢,4¢); 8.05 (d, 1H, 3J = 7.83 Hz, H3,4); 7.98 (d, 1H, 3J =
8.55 Hz, H8¢); 7.94 (d, 1H, 3J = 7.83 Hz, H3,4); 7.62 (d, 1H, 3J =
Preparation of the CTPD ligand. 3-Acetyl-9-ethylcarbazole
(1.10 g, 4.64 mmol) and ethyl thieno[3,2,-b]thiophene-2-
carboxylate (1.18 g, 5.56 mmol) were dissolved in 25 mL anhydrous
THF under a N2 atmosphere. Sodium ethoxide (0.38 g, 5.56 mmol)
was added. After stirring for 24 h at 60 ◦C, hydrochloric acid (1.0
M) was added to the solution. The crude mixture was extracted
with CH2Cl2 and dried over anhydrous magnesium sulfate. The
residue was purified by column chromatography (ethyl acetate :
hexane = 1 : 3) to give the final product as a yellowish solid. Yield
65%, mp = 135 ◦C; EI-MS Calcd for C23H17NO2S2 403.07, Found
[M+] 403; Anal. Calcd for C23H17NO2S2 : C, 68.46; H, 4.25; N,
3.47; S, 15.89. Found C, 68.75; H, 4.45; N, 3.45; S, 16.02.
8.55 Hz, H5¢); 7.57 (m, 3H, H5, H6¢ and H7¢); 7.49 (d, 1H, J =
3
7.58 Hz, H6); 7.43 (s, 1H, H1¢); 7.27 (d, 1H, J = 7.58 Hz, H7),
3
4.93 (s, 2H, -CH2-, 0.02%),4.52 (q, 2H, 3J = 7.34 Hz, -CH2-CH3),
1.44 (t, 3H, 3J = 7.34 Hz, -CH2-CH3).
Preparation of sodium (2Z)-3-(9-ethyl-9H-carbazol-2-yl)-3-
hydroxy-1-naphthalen-2-ylprop-2-en-1-olate (CNPD sodium salt).
The product was synthesised with the same procedure as that
reported for the CTPD sodium salt. d = 8.82 (s, 1H, H1); 8.52
3
3
(s, 1H, Henol); 8.21 (d, 1H, J = 8.56 Hz, H8¢); 8.17(d, 1H, J =
8.32 Hz, H3¢,4¢); 8.12 (d, 1H, 3J = 7.58 Hz, H8); 7.88 (d, 1H, 3J =
7.82 Hz, H3,4); 7.79 (d, 1H, 3J = 7.82 Hz, H3,4); 7.77 (d, 1H, 3J =
8.32 Hz, H3¢,4¢); 7.45 (d, 1H, 3J = 7.33 Hz, H5¢); 7.38 (m, 4H, H5,
H6, H6¢ and H7¢); 7.11 (t, 1H, 3J = 7.58 Hz, H7); 6.82 (s, 1H, H1¢);
4.39 (q, 2H, 3J = 7.34 Hz, -CH2-CH3); 1.37 (t, 3H, 3J = 7.34 Hz,
-CH2-CH3).
Enol form: (2Z)-1-(9-ethyl-9H-carbazol-2-yl)-3-hydroxy-3-
thieno[3,2-b]thiophen-2-ylprop-2-en-1-one. 1H-NMR (800 MHz
in THF-d8): d = 8.90 (s, 1H, H1); 8.30 (s, 1H, Henol); 8.22 (d, 1H,
3J = 7.58 Hz, H8); 8.18 (d, 1H, J = 7.58 Hz, H5); 7.75 (d, 1H,
3
3J = 5.13 Hz, H5¢); 7.61 (d, 1H, 3J = 8.56 Hz, H3,4); 7.56 (d, 1H,
3J = 8.56 Hz, H3,4); 7.49 (t, 1H, J = 7.58 Hz, H6); 7.42 (d, 1H,
3
3J = 5.13 Hz, H6¢); 7.27 (t, 1H, J = 7.58 Hz, H7); 7.15 (s, 1H,
3
Preparation of [Ln(diketonate)3(tpy)]16. General Procedure
(see Scheme 2): a mixture of b-diketone (3.0 equiv.), and NaOEt
(3.3 equiv.) was stirred in freshly distilled THF at room tem-
perature overnight. After the completion of salt formation, the
methanol solution of anhydrous LnCl3 (1.0 equiv.) and terpyridine
(1.1 equiv.) was added to the reaction solution, and then stirred
for 2 days. The resulting solution was filtered and the solvents
H3¢); 4.49 (q, 2H, J = 7.09 Hz, -CH2-CH3), 1.45 (t, 3H, J =
3
3
7.09 Hz, -CH2-CH3)
Keto form: 1-(9-ethyl-9H-carbazol-2-yl)-3-thieno[3,2-b]thio-
phen-2-ylpropane-1,3-dione. Chemical shifts of H1, H5, H3,4, H3¢
cannot be distinguished from those of the enol form. d = 8.24 (d,
1H, H8); 7.78 (d, 1H, H5¢); 7.55 (d, 1H, H3,4); 7.47 (d, 1H, H6); 7.38
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Structure and keto-enol ratio for CTPD and its deprotonated form,
as determined by 1NMR
b-Diketones exist as keto-enol tautomers.10,11 Considerable at-
tention has been focused on this equilibrium and it has been
demonstrated that the ratio depends on the nature of the a-
substituents, the polarity of the solvent, and the presence of
acceptor or donor groups. In order to determine the structure
of the CTPD and CNPD molecules, 1H-NMR investigations
have been conducted in different deuterated solvents for both the
ligands and their deprotonated forms. Subsequently, the keto-enol
ratios (see Scheme 3) have been estimated from the peak areas of
the corresponding signals.
Scheme
2
Synthesis of [Ln(diketonate)3(tpy)] complexes (R
=
thieno[3,2-b]thiophene or naphthalene moiety).
were removed. The resultant solid was washed sequentially with
methanol, diethyl ether and hexane, yielding a yellowish solid.
Preparation of [Er(CTPD)3(tpy)]. A mixture of b-diketone
(0.60 g, 1.49 mmol) and NaOEt (0.11 g, 1.64 mmol) was stirred
overnight at rt in 50 mL distilled THF. After completion of the
reaction, 10 mL of a methanolic solution of anhydrous ErCl3
(136 mg, 0.50 mmol) and terpyridine (116 mg, 0.50 mmol) were
added to the reaction solution, which was then stirred for 2 days.
Yield 75%; Anal. calcd for C84H59N6O6S6Er and 1605.21: C, 62.74;
H, 3.70; N, 5.23; S, 11.96; Er, 10.40, Found C, 60.80; H, 3.77; N,
4.99; S, 12.08; Er, 10.83.
[Er(CNPD)3(tpy)] was prepared as [Er(CTPD)3(tpy)]. Yield
69%; Anal. calcd for C96H71N6O6Er and 1569.47: C, 73.35; H,
4.55; N, 5.35; Er, 10.64, Found C, 72.52; H, 4.37; N, 5.07; Er,
10.72.
Scheme 3 Potential keto-enol structures for CTPD.
Preparation of [Gd(CTPD)3(tpy)]. A mixture of b-diketone
(0.25 g, 0.62 mmol) and NaOEt (0.05 g, 0.75 mmol) in 50 mL
distilled THF was stirred overnight at rt. After completion of the
reaction, 10 mL methanolic solution of anhydrous GdCl3 (54 mg,
0.21 mmol) and terpyridine (53 mg, 0.23 mmol) were added to the
reaction solution, which was then stirred for 2 days. Yield 52%;
Anal. calcd for C84H59N6O6S6Gd and 1597.21: C, 63.13; H, 3.72;
N, 5.26; S, 12.04; Gd, 9.84, Found C, 62.65; H, 3.94; N, 4.98; S,
12.33; Gd, 10.02
[Gd(CNPD)3(tpy)] was prepared as [Gd(CTPD)3(tpy)]. Yield
60%; Anal. calcd for C96H71N6O6Gd and 1561.47: C, 73.82; H,
4.58; N, 5.38; Gd, 10.07, Found C, 73.06; H, 4.65; N, 5.04; Gd,
10.18
The NMR assignment of the molecular structures is based on
1D and 2D 1H-NMR spectra recorded at 800 MHz. Each molecule
exists in both its keto and enolic forms, the equilibrium being
displaced in favour of one enol form in deuterated THF. The
observed Henol (-CH-) and Hketo (-CH2-) peaks in CTPD appear
as singlets at 8.30 ppm and 4.76 ppm (see Fig. 1). The chemical
shifts observed for all protons are in agreement with the literature
values.17 Comparison of the integrated peak values of the CH2
protons on the a-carbon points to the enolic form being the
predominant molecular structure at room temperature. By inte-
grating the area corresponding to both species the keto/enol ratio
is found to be 5 : 95. The corresponding equilibrium constants in
Preparation of [Nd(CTPD)3(tpy)]. A mixture of b-diketone
(0.45 g, 1.12 mmol) and NaOEt (0.08 g, 1.23 mmol) was stirred
in 70 mL distilled THF overnight at rt. After completion of the
reaction, 10 mL methanolic solution of anhydrous NdCl3 (93 mg,
0.37 mmol) and terpyridine (87 mg, 0.37 mmol) were added to the
reaction solution, which was then stirred for 2 days. Yield 78%;
Anal. calcd for C84H59N6O6S6Nd and 1581.19: C, 63.65; H, 3.75;
N, 5.30; S, 12.14; Nd, 9.10, Found C, 62.86; H, 3.92; N, 5.44; S,
12.23; Nd, 8.97
[Nd(CNPD)3(tpy)] was prepared as [Nd(CTPD)3(tpy)]. Yield
69%; Anal. calcd for C96H71N6O6Nd and 1545.45: C, 74.44; H,
4.62; N, 5.43; Nd, 9.31, Found C, 74.17; H, 4.75; N, 5.49; Nd,
9.12.
Results and discussion
The two ligands, CTPD and CNPD were prepared in high yield
from 3-acetyl-9-ethylcarbazole and the carboxylic acid derivative
of the aromatic substituent. The corresponding one-pot synthesis
is summarized in Scheme 1.
Fig. 1 1H-NMR spectrum CTPD ligand in THF-d8. Resonances from
the keto form are marked by red arrows.
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Table 1 Chemical shift of the b-diketone ligands (HL) and their depro-
tonated forms (L-) in THF-d8
deuterated THF-d8 and CD3CN were found to be 19.0 and 5.7 at
rt, respectively.
As observed in the NMR spectrum, the most shielded aromatic
protons in both keto and enol forms are those corresponding to the
thieno[3,2-b]thiophen moiety (H5¢ and H6¢) which could indicate
that the preferred enol form is (2Z)-1-(9-ethyl-9H-carbazol-
2-yl)-3-hydroxy-3-thieno[3,2-b]thiophen-2-ylprop-2-en-1-one (1-
enol) and not (2Z)-3-(9-ethyl-9H-carbazol-2-yl)-3-hydroxy-1-
thieno[3,2-b]thiophen-2-ylprop-2-en-1-one (3-enol, see Scheme 3).
When CTPD is dissolved in CD3CN, the peaks are less resolved
than in the previous case, but the keto form can still be clearly
distinguished from the enol one and corresponds to ca. 15%.
If the ligand is totally deprotonated (by adding two equivalents
of sodium hydroxide in ethanolic 0.1 M solution, stirring and
evaporating the solvents), the observed form is the enol structure,
whatever the solvent in which the experiment is conducted
(THF-d8 or CD3CN), as shown in Fig. 2. All the protons are
up-shielded compared to the spectrum of the protonated ligand.
The most important chemical shift differences are observed for the
enolate proton (Dd = 0.43 ppm) as could be expected, but the H5¢,
H6¢ and H3¢ aromatic protons also display substantial shifts (Dd =
0.50, 0.32 and 0.19 ppm, respectively, see Table 1). These protons
belong to the thieno[3,2-b]thiophen moiety, which again indicates
that the preferred enolate is in the C3 and not the C1 position.
dCTPD-
CTPD
HL
CNPD
HL
dCNPD
Proton
L-
Dd
L-
Dd
HL
L-
H1
8.90 8.78 0.12 9.01 8.82 0.19
7.61 7.45 0.16 8.05 7.88 0.17
7.56 7.38 0.18 7.94 7.79 0.15
8.18 8.14 0.04 7.57 7.38 0.19
7.49 7.38 0.11 7.49 7.38 0.11
7.27 7.14 0.13 7.27 7.11 0.16
8.22 8.17 0.05 8.25 7.58 0.67
-0.11 -0.04
-0.44 -0.43
-0.38 -0.41
0.61
0.00
0.00
H3,4
H3,4
H5
0.76
0.00
0.03
H6
H7
H8
-0.03 0.59
H1¢
H3¢
H4¢
H5¢
H6¢
H7¢
H8¢
Henol
/
/
/
7.43 6.82 0.61
7.15 6.65 0.50 8.3 8.32 -0.02
8.18 7.77 0.41
/
/
/
7.75 7.43 0.32 7.92 7.45 0.47
7.42 7.23 0.19 7.57 7.38 0.19
/
/
/
/
/
/
7.57 7.38 0.19
7.98 8.21 -0.23
8.30 7.87 0.43 8.72 8.52 0.2
-CH2CH3 4.49 4.40 0.09 4.52 4.39 0.13
-CH2CH3 1.45 1.38 0.07 1.44 1.37 0.07
Fig. 3 1H-NMR spectrum of the CNPD ligand in THF-d8. The signal
from the keto form is indicated by an arrow. Large solvent resonances have
been removed for clarity.
Fig. 2 1H-NMR spectrum of deprotonated CTPD in THF-d8. * denote
resonances from residual ethanol.
substituent in CTPD vs. CNPD shows that the H3 and H4 aromatic
protons are down-shielded (Dd = 0.4 ppm) while H5 is up-shielded
(Dd = 0.6 ppm).
Structure and keto-enol ratio for CTPD and its deprotonated form,
as determined by 1NMR
One the other hand, there is no influence on H6 and H7. A similar
result is observed for H8 when comparing the neutral ligand while
a large effect occurs for the deprotonated form (Dd = 0.6 ppm).
This is due to important electronic effects induced by the formation
of the enolate. When comparing the protonated vs. deprotonated
forms of CNPD, one sees that the carbazolyl aromatic protons are
up-shielded by ca. 0.2 ppm, with the above-mentioned exception
of H8. On the other hand, the naphthalenyl moiety sustains much
larger chemical shift displacements (Table 1), in particular for
H1¢, H4¢, and H5¢. This indicates that the preferred enolate form
is probably on C3, thus inducing important electronic effects.
The chemical behaviour of both ligands is then the same, the
preferred enolate form being on the opposite side of the carbazolyl
The same experiments as for CNTP have been conducted
with CNPD. The ligand is also essentially present under its
enol form (1-(9-ethyl-9H-carbazol-2-yl)-3-(2-naphthalenyl)-1,3-
propanedione (see Table 1 and Fig. 3). The keto form amounts
to less than 2%, as calculated from the integrated intensity of the
CH2 singlet. The aromatic signals cannot be distinguished from
those of the enol form. In deuterated acetonitrile, the CH2 singlet
is much more intense (5–10%), demonstrating that the equilibrium
is slightly displaced in favour of the keto species in this solvent.
This effect is smaller than the one observed for CTPD. After
deprotonation of the ligand, the enol form is exclusively observed
(see Fig. 4). Comparing the chemical shifts of the carbazolyl
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Fig. 5 UV-Vis absorption spectra of CTPD and CNPD in acetonitrile
(MeCN), chloroform and cyclohexane.
indicates that CTPD and CNPD diketones are more conjugated in
polar solvents. As a result, their absorption bands are red shifted
in these solvents.
Fig. 4 1H-NMR spectrum of deprotonated CNPD in THF-d8.
In contrast to weak solvent-dependence of the absorption
spectra, the CTPD and CNPD ligands show remarkable solva-
tochromic emission behaviour as shown in Fig. 6. The fluorescence
spectra of CTPD and CNPD in polar solvents display broad
emission bands with large Stokes’ shifts while those in non-polar
cyclohexane exhibit a vibrational structure.17 This may indicate
that the nature of the excited electronic state in polar solvents is
different. In addition, the phospholuminescence spectra of CTPD
and CNPD in polar solvents are much alike, regardless of the
excitation wavelength (Fig. S3, ESI†), indicating that the emissive
states are similar.
substituents. The keto-enol equilibria of push-pull chromophores
are indeed affected by decreasing electron density at the a-position
and substitution with bulky groups results in an increase of steric
hindrance.9,10 For these reasons the two b-diketones of this study
appear mainly under the form of the cis enol tautomer, which is
stabilized by conjugation and intramolecular hydrogen bonds.
Determination of the keto-enol ratios by HPLC
Complementary experiments have been conducted to check the
keto-enol ratios of the neutral ligands. The latter were injected in
water–acetonitrile solution and the gradient was adjusted to reach
100% MeCN in 100 min. In each case, two peaks appeared upon
detection at 214 nm, the relative surface of them giving the keto-
enol ratios. The results are in good agreement with those obtained
previously by NMR spectroscopy (Table 2).
Photophysical properties of diketone ligands
Steady-state spectral properties. Absorption spectra of CTPD
and CNPD in polar and non-polar solvents are shown in Fig. 5.
CTPD and CNPD exhibit the absorption bands in the spectral
ranges 250–350 nm and 350–450 nm. The former are attributable
to the aromatic substituents, thieno[3,2-b]thiophene, naphthalene
and carbazole groups.
Fig. 6 Photoluminescence spectra of CTPD and CNPD.
The absorption bands around 350–450 nm are due to the
conjugated b-diketone. The absorption maxima of CTPD and
CNPD in polar solvent (CHCl3, MeCN) are slightly red shifted
compared to the absorption maximum in the non-polar solvent
cyclohexane. As mentioned above, CTPD and CNPD exist in their
keto and enol forms, the equilibria being displaced in favour of
one enol form in polar THF-d8 and CD3CN. The conjugation
length in the enol form is longer compared to the keto form. This
The HOMO and LUMO orbitals of the two b-diketone
derivatives have been calculated in order to evidence their electron
donor–acceptor (push-pull) nature. Geometry optimization was
carried out with the GAUSSIAN 03 W program, using the TD
B3LYP method with a 6-31G(d) basis set.18 The HOMO and
LUMO orbitals are represented in Fig. 7. The characteristic
feature of both HOMO orbitals is the p-density located on the
carbazole moiety. Upon photoexcitation, one electron moves into
the LUMO orbital, which results in the p-electronic density being
transferred towards the fused thiophene and naphthalene moiety
of CTPD and CNPD, respectively. Thus the theoretical modelling
substantiates the fact that charge transfer does indeed take place
between carbazole functioning as an electron donating group and
the fused thiophene or naphthalene unit of the ligand enol forms
acting as the accepting group. Intramolecular charge transfer
processes usually generate large Stokes’ shifts of the emission band
in polar solvents.12,19 This is in line with the structureless and red-
shifted emission band detected in the absorption spectra in the
Table 2 Percentage of keto form present in CD3CN and THF-d8 for
CTPD and CNPD as determined by NMR and HPLC
NMR
HPLC (214 nm)
MeCN
THF-d8
CD3CN
CTPD
Deprot. CTPD
CNPD
5
0
2
0
15
0
7
12
/
5
Deprot. CNPD
0
/
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Table 3 Lifetimes and quantum yields of the [Ln(diket)3(tpy)] complexes
sample
lex/nm
lan/nm
T/K
t1/ms
U [%]
CTPD-Er
CNPD-Er
CTPD-Nd
CNPD-Nd
355
355
355
355
1530
1530
1063
1063
295
295
295
295
1.44
1.17
0.95
0.85
0.008
0.007
0.1
0.05
Fig. 7 HOMO and LUMO orbitals of b-diketone derivatives.
range 350–450 nm arising from the intra-ligand charge-transfer
(ILCT) state.
Ligand-centred luminescence. In order to understand the ex-
cited state dynamics of CTPD and CNPD, we have measured
the time-resolved fluorescence decays in various solvents upon
excitation in the ILCT band around 390 nm. The fluorescence
decay profiles were monitored at the emission maxima. The
fluorescence decays of CTPD and CNPD in various solvents are
mono-exponential and are depicted in Fig. S3–S5 (ESI†). The
fluorescence lifetimes in cyclohexane are determined to be about
150 ps for both compounds. On the other hand, the fluorescence
lifetime of CTPD and CNPD in a polar solvent such as MeCN
increases 10-fold and reaches 1.5 ns, consistent with the fact that
ILCT states of organic molecules generally exhibit a long decay
time.19 After deprotonation of the ligands, the observed lifetime
is 1.78 ns for CTPD and 1.8 ns for CNPD in acetonitrile. As a
conclusion, the structureless and red-shifted emission band in the
fluorescence spectra of CTPD and CNPD in polar solvents as well
as the increased excited state lifetimes ascertain the presence of
ILCT states in these compounds.
Fig. 8 NIR emission spectra of CTPD and [Er(CTPD)3(tpy)] 2.0 ¥ 10-5
in MeCN (lex = 410 nm).
M
4
4
4
4
4F3/2 → I9/2 (890 nm), F3/2 → I11/2 (1060 nm), and F3/2
→
4I13/2 (1340 nm) transitions, respectively (Fig. 9).22 Upon laser
excitation at 355 nm, time-resolved luminescence measurements
showed that the lifetime of the Ln(III) excited states are in the
range 1.17–1.44 ms for Er(III) complexes and 0.81–1.01 ms for
Nd(III) complexes in aerated MeCN solution. The corresponding
quantum yields were found to be 0.007–0.008% for the Er(III)
complexes and 0.05–0.1% for the Nd(III) complexes (see Table 3).
The Gd(III) ion has no energy levels below 32 000 cm-1,20 and the
emission bands of CTPD are in the range 14300–25 000 cm-1. Thus,
Gd(III) cannot accept energy from the triplet or the singlet state
of CTPD. In degassed MeCN, no significant phosphorescence
is detected at room temperature for [Gd(CTPD]3(tpy)]. However
both a microcrystalline sample of this complex and the MeCN
solution display a structured phosphorescence band at 77 K with
a maximum at 520 nm (Fig. S6, ESI†). The luminescence decay
of the solution is biexponential with corresponding lifetimes of
25.1 0.3 ms (96%) and 1.88 0.04 ms (4%). This may indicate a
slight decomplexation in solution.
Fig. 9 Normalized NIR emission spectrum of a solid state sample of
[Nd(CTPD)3] at 10 K (lex = 400 nm).
Metal-centred luminescence of [Ln(CTPD)3(tpy)] and
[Ln(CTPD)3(tpy)] (Ln = Er, Nd). The NIR luminescence
spectra of the Ln(III) complexes have been measured in
acetonitrile. Upon excitation into the ILCT band of the
Conclusions
The two new push-pull b-diketone chromophores fitted with
charge-transport carbazole and aromatic moieties described in
this study, CTPD and CNPD, essentially appear under the ther-
modynamically stable enol form in solution displaying extended
resonance electronic structure: the cis-enol tautomer amounts to
95–98% of the speciation in THF at room temperature. These
ligands possess an ILCT electronic state, the energy of which is
sensitive to the polarity of the solvent and which allows convenient
sensitization of the luminescence of NIR emitting ions such as
ligands, the spectra of the Er(III) compounds display the
4
characteristic I13/2
→
4I15/2 transition of Er(III) at 1530 nm.21
The spectrum of [Er(CTPD)3(tpy)] is displayed in Fig. 8 as an
example. Simultaneously, the emission intensity of the ligands is
significantly diminished, compared to that of the corresponding
free ligands. This may be ascribed to energy transfer from
the ligands to the Er(III) ion. In Nd(III) complexes, weak and
sharp Nd(III) emission bands in the NIR are assigned to the
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Nd(III) and Er(III) in the visible spectrum. Presently, the reported
quantum yields for [Ln(diket)3(tpy)] (Ln = Nd, Er; diket = CTPD,
CNPD) remain modest in comparison with literature values23
but modification of the ligand by removing the C–H vibration
in the vicinity of the Ln(III) ion is feasible and should improve the
photophysical properties of the tris complexes.
6 L. R. Melby, N. J. Rose, E. Abramson and J. C. Caris, J. Am. Chem.
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This research was supported by grants from the Fundamental
R&D Program for Core Technology of Materials as well as from
the ITRC support program supervised by the NIPA (National
IT Industry Promotion Agency) (NIPA-2008-C1090-0804-0013)
funded by the Ministry of Knowledge Economy (MKE) (Re-
public of Korea) and from the World Class University program
from the National Research Foundation of Korea (Ministry of
Education, Science and Technology, R31-10035). JCB thanks the
Swiss National Science Foundation for financial support (Grant
200020_119866).
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1538 | Dalton Trans., 2010, 39, 1532–1538
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