Adducts of lanthanide β-diketonates with 2,4,6-tri(2-pyridyl)-1,3,5-triazine: Synthesis, structural characterization, and photoluminescence studies

Article abstract of DOI:10.1016/j.poly.2006.10.049

Adducts of lanthanide β-diketonates of the general formula LnL₃(TPTZ) were synthesized and structurally characterized by single crystal X-ray diffraction [Ln = Eu³⁺, Tb³⁺, Er³⁺; L is the conjugate base of dibenzoylmethane (DBM), 1-benzoylacetone (BA), thenoyltrifluoroacetone (TTA), or 4,4,4-trifluoro-1-phenyl-1,3-butanedione (BTFA); TPTZ = 2,4,6-tri(2-pyridyl)-1,3,5-triazine, a rigid Lewis base with a large π system]. The lanthanide ion in each of these complexes is nonacoordinate with six β-diketonate oxygen atoms and three TPTZ nitrogen atoms, forming a coordination polyhedron best describable as a monocapped square antiprism. Characteristic red, green, and near infrared luminescence was observed for the Eu³⁺, Tb³⁺, and Er³⁺ complexes, respectively. All complexes showed significantly enhanced luminescence quantum yields when compared with the corresponding aqua analogues, with one of the Eu³⁺ complexes displaying a quantum yield of 69.7% in chloroform.

Full text of DOI:10.1016/j.poly.2006.10.049

Polyhedron 26 (2007) 1229–1238
www.elsevier.com/locate/poly
Adducts of lanthanide b-diketonates with
2,4,6-tri(2-pyridyl)-1,3,5-triazine: Synthesis, structural
characterization, and photoluminescence studies
*
Channa R. De Silva, Jenine R. Maeyer, Alice Dawson, Zhiping Zheng
Department of Chemistry, University of Arizona, Tucson, AZ 85721, USA
Received 24 August 2006; accepted 12 October 2006
Available online 10 November 2006
Abstract
Adducts of lanthanide b-diketonates of the general formula LnL3(TPTZ) were synthesized and structurally characterized by single
crystal X-ray diffraction [Ln = Eu3+, Tb3+, Er3+; L is the conjugate base of dibenzoylmethane (DBM), 1-benzoylacetone (BA), thenoyl-
trifluoroacetone (TTA), or 4,4,4-trifluoro-1-phenyl-1,3-butanedione (BTFA); TPTZ = 2,4,6-tri(2-pyridyl)-1,3,5-triazine, a rigid Lewis
base with a large p system]. The lanthanide ion in each of these complexes is nonacoordinate with six b-diketonate oxygen atoms
and three TPTZ nitrogen atoms, forming a coordination polyhedron best describable as a monocapped square antiprism. Characteristic
red, green, and near infrared luminescence was observed for the Eu3+, Tb3+, and Er3+ complexes, respectively. All complexes showed
significantly enhanced luminescence quantum yields when compared with the corresponding aqua analogues, with one of the Eu3+ com-
plexes displaying a quantum yield of 69.7% in chloroform.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Lanthanide complexes; 2,4,6-Tri(2-pyridyl)-1,3,5-triazine; X-ray crystallography; Photoluminescence; Synthesis
1. Introduction
mechanism of energy transfer from the organic ligands to
a lanthanide ion is that of Crosby and Whan [3], which
involves light absorption by the organic ligand to produce
the singlet excited states, intersystem crossing to populate
the ligand triplet states, and energy transfer from the triplet
states to the lanthanide emissive levels for light emission.
Though not nearly as prevalent, other mechanisms are pos-
sible [4], including one involving direct energy transfer
from the ligand singlet state to the resonance levels of the
lanthanide ion [5]. The vibronic perturbation to the f orbi-
tals caused by ligand coordination is nevertheless limited as
these orbitals are well shielded by the filled 5s and 5p orbi-
tals. As such, characteristically narrow line-like emissions
are observed for luminescent lanthanide complexes. Tech-
nologically significant applications have been realized or
envisioned for these materials. These include their uses as
fluorescent probes for bio-immunoassays, efficient and
sharp-emitting materials in display technologies, and for
optical conversion and amplification [1].
A number of lanthanide ions display characteristic lumi-
nescence, a consequence of the f–f electronic transitions, in
the visible or near-infrared spectral regions upon ultravio-
let excitation [1]. Such transitions are forbidden by parity,
leading to long-lived excited states, in the micro- to milli-
second range. The forbidden nature of such transitions is
also reflected in the low molar absorptivities, making direct
photo-excitation of the lanthanide ions difficult [2]. Never-
theless, this can be overcome by energy transfer from
organic chromophores to the lanthanide ions; when ligands
containing organic chromophores with suitable photophys-
ical properties are employed, highly luminescent lanthanide
complexes can be obtained. The commonly accepted
*
Corresponding author. Tel.: +1 520 626 6495; fax: +1 520 621 8407.
E-mail address: zhiping@u.arizona.edu (Z. Zheng).
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.10.049

1230
C.R. De Silva et al. / Polyhedron 26 (2007) 1229–1238
Arguably the most extensively studied luminescent
characterized lanthanide complexes in which TPTZ acts
as a tridentate ligand [15,16]. However, TPTZ adducts of
lanthanide b-diketonates are unknown prior to our work.
In addition to the novelty of such complexes, the bulky
ligand, by offering three coordinating atoms, helps protect
the lanthanide ion from interacting with solvent molecules,
and therefore, suppressing solvent-based vibrational cou-
pling and luminescence quenching [17]. Furthermore, the
remaining nitrogen atoms allow for further coordination
to other metals, transition metals in particular. Thus, struc-
turally sophisticated heterometallic complexes may be envi-
sioned, for which novel magnetic as well as luminescence
properties can be expected [18,19].
As a first step to many of the research opportunities
elaborated above, we have prepared the TPTZ adducts of
several lanthanide b-diketonates, wherein the lanthanide
ion is Eu3+, Tb3+, or Er3+, and the b-diketone is dibenzoyl-
methane (DBM), 1-benzoylacetone (BA), thenoyltrifluoro-
acetone (TTA), or 4,4,4-trifluoro-1-phenyl-1,3-butanedione
(BTFA). The Eu3+ and Tb3+ complexes are synthesized as
potential red- and green-emitting materials, respectively,
while their Er3+ cognate is potentially useful in developing
integrated lasers or fiber amplifiers due to its near infrared
emission [20,21]. In this report, we describe in detail the syn-
thesis, structural characterization by single-crystal diffrac-
tion, and photoluminescence studies of these new
members of the lanthanide b-diketonate family.
lanthanide complexes are those with b-diketonate ligands
[6]. When three b-diketonate ligands coordinate a trivalent
lanthanide ion, an electrically neutral complex is produced.
Unless a sterically encumbering b-diketonate ligand is
used, additional ligands such as water and other Lewis
bases are necessary in order to fulfill the high coordination
requirement of these large metal ions. For practical appli-
cations, non-aqua neutral ligands are typically used, as
high phonon energy OH oscillators can bridge off at very
high rates the energy of the excited states of a lanthanide
ion, leading to low quantum efficiency of light emission [7].
The property of the Lewis base adduct of a lanthanide
b-diketonate depends on not only the diketonate but also
the accompanying neutral ligand(s). Specifically, while the
wavelength of light emission is inherently metal-dependent,
its quantum efficiency is heavily dependent on how well the
energetic levels of the ligand singlet, triplet, and the lantha-
nide emissive states are matched, a direct consequence of
the ligand-assisted transfer of excitation energy [3,4,8].
Other properties such as thermal stability, solubility, and
film-forming characteristics (for the fabrication of light-
emitting devices) are also strongly dependent on the nature
of the ligands.
We have been interested in the design and synthesis of
lanthanide b-diketonates for applications as light-emitting
materials in organic electroluminescent devices, as they
offer a number of advantages over the much studied con-
ducting polymers and other non-lanthanide materials [9].
Specifically, we have been looking at ligand design as a fun-
damental means of enhancing the thermal stability of a
complex and improving its charge-carrier transporting abil-
ity in a working device [10]. For example, a terbium com-
plex with acetylacetonate ligand modified with an
electron-withdrawing oxadiazolyl group has been shown
to possess higher thermal stability than its unmodified
counterpart. More importantly, significantly improved
electroluminescence device performance was also achieved
[10]. In comparison, the unmodified complex barely pro-
duced any green emission using an otherwise identically
configured device. Work by others has also validated this
ligand design approach for producing improved electrolu-
minescent lanthanide materials [11].
2. Experimental
Reagents were of commercial origin (Aldrich) and were
used as received. The recently reported procedure [12] for
the synthesis of Eu(DBM)3TPTZ was adopted with modi-
fications where necessary. Elemental analysis (CHN) was
performed by Desert Analytics Laboratory, Tucson,
Arizona.
2.1. Synthesis of Tb(DBM)3TPTZ
A mixture of TPTZ (0.312 g, 1.00 mmol) and TbCl3 Æ
6H2O (0.265 g, 1.00 mmol) in 15 mL of absolute ethanol
was stirred under reflux for 30 min to afford a clear solu-
tion. To this mixture was added over 10 min a solution
of KOBut (0.800 g, 7.130 mmol) and DBM (0.672 g,
3.00 mmol) in 20 mL of absolute ethanol, and the resulting
mixture was stirred at 50 ꢁC for 1 h, and then at room tem-
perature for an additional 30 min. Ethanol was removed
under vacuum, and the residue was washed with copious
de-ionized water. The crude product thus obtained was dis-
solved in a minimum amount of chloroform, and the solu-
tion was dried over anhydrous MgSO4. The filtrate was
collected, from which a yellow solid was obtained upon
removal of the solvent. Upon recrystallization from etha-
nol/chloroform (v/v 2:1), analytically pure product was
obtained as a yellow crystalline solid (0.73 g, 64.0%). Anal.
Calc. for C63H45N6O6Tb: C, 66.32; H, 3.98; N, 7.36.
Found: C, 66.44; H, 3.96; N, 7.08%.
The neutral ligands are equally important in modifying
the properties of luminescent lanthanide b-diketonates.
Just like the negatively charged ligands, neutral ligands
contribute to the overall stability of the resulting complex.
They may also serve to promote energy transfer for light
emission. Based on such considerations, we have recently
started the quest for adducts of lanthanide b-diketonates
with unique neutral ligands [12]. One of the ligands of
interest is 2,4,6-tri(2-pyridyl)-1,3,5-triazine (TPTZ),
a
bulky aromatic compound featuring three 2-pyridyl rings
fixed on a central 1,3,5-triazine platform [13]. This ligand
has been studied previously for its application in acti-
nide–lanthanide group separations in a synergistic extrac-
tion system [14]. There also exist a few structurally

C.R. De Silva et al. / Polyhedron 26 (2007) 1229–1238
1231
2.2. Synthesis of Er(DBM)3TPTZ
2.6. X-ray structure determinations
This compound was prepared by starting with ErCl3 Æ
6H2O (0.382 g, 1.00 mmol) and adopting otherwise identi-
cal preparative and purification procedures to those for
Tb(DBM)3TPTZ. The product was obtained as a pale yel-
low crystalline solid (1.02 g, 88.8%). Anal. Calc. for
C63H46N6O6.5Er: C, 65.32; H, 4.00; N, 7.25. Found: C,
65.45; H, 4.33; N, 7.28%.
Single crystals of Tb(DBM)3TPTZ and Er(DBM)3-
TPTZ suitable for X-ray diffraction studies were obtained
by directly layering their respective saturated chloroform
solutions with diethyl ether at room temperature. Single
crystals of Eu(BTFA)3TPTZ and Eu(TTA)3TPTZ were
produced by slow evaporation of their respective solutions
in ethanol:acetone (v/v 1:1), while single crystals of
Eu(BA)3TPTZ were obtained from its solution in etha-
nol:chloroform:hexane (v/v/v 2:1:1).
2.3. Synthesis of Eu(BA)3TPTZ
The crystals were mounted on a glass fiber in a random
orientation. Examination of the crystal was carried out on
a Bruker SMART 1000 CCD detector X-ray diffractometer
at 170(2) K and a power setting of 50 kV, 40 mA. Data
were collected on the SMART1000 system using graphite
A mixture of TPTZ (0.312 g, 1.00 mmol) and EuCl3 Æ
6H2O (0.366 g, 1.00 mmol) in 20 mL of absolute ethanol
was stirred at 50 ꢁC for 10 min to afford a clear solution.
To this mixture was added with stirring a solution of
KOBut (0.600 g, 5.347 mmol) and BA (0.487 g, 3.00 mmol)
in 15 mL of absolute ethanol. The resulting mixture was
stirred at 60 ꢁC for 0.5 h, and then at room temperature
under nitrogen for an additional 1.5 h. Purification of the
reaction mixture followed the procedures detailed above
for Tb(DBM)3TPTZ. Analytically pure product was
obtained as a pale yellow crystalline solid (0.57 g, 60.0%)
upon recrystallizing the crude product from ethanol:
chloroform:hexane (v/v/v 2:1:1). Anal. Calc. for C48H40-
N6O6.5Eu: C, 60.25; H, 4.11; N, 8.80. Found: C, 60.25;
H, 4.34; N, 9.01%.
˚
monochromated Mo Ka radiation (k = 0.71073 A). The
frames were integrated using the Bruker SAINT [22] software
package’s narrow frame algorithm, and empirical absorp-
tion and decay corrections were applied using the program
SADABS [23]. The structures were solved using SHELXS in the
Bruker SHELXTL (Version 5.0) software package [22].
Refinements were performed using SHELXL and illustrations
were made using XP.
Solution was achieved by using direct methods fol-
lowed by Fourier synthesis. Hydrogen atoms were added
at idealized positions, constrained to ride on the atom to
which they are bonded and given thermal parameters
equal to 1.2 or 1.5 times Uiso of that bonded atom.
Details on data collection and structure refinements are
given in Table 1.
2.4. Synthesis of Eu(BTFA)3TPTZ
EuCl3 Æ 6H2O was allowed to react with TPTZ as
described above. To this mixture was added with stirring
a solution of KOBut (0.336 g, 3.00 mmol) and BTFA
(0.648 g, 3.00 mmol) in 20 mL of absolute ethanol. The
resulting mixture was stirred at 60 ꢁC for 30 min, and then
at room temperature under nitrogen for an additional 3 h.
The mixture was filtered and the solid was washed with
copious de-ionized water to remove KCl byproduct. The
crude product thus obtained was washed with hexane
(3 mL), dissolved in acetone (50 ml), and the solution was
dried over anhydrous MgSO4. The filtrate was collected,
from which a pale yellow solid was obtained upon removal
of the solvent. Recrystallization from ethanol:acetone (v/v
1:1) afforded analytically pure product as a pale yellow
crystalline solid (0.63 g, 57.0%). Anal. Calc. for
C48H30N6O6F9Eu: C, 51.95; H, 2.72; N, 7.57. Found: C,
51.56; H, 2.54; N, 7.37%.
2.7. Photophysical studies
Electronic absorption spectra in the solid state and chlo-
roform solutions were recorded on a Perkin–Elmer
Lambda 10 spectrophotometer. Photoluminescence studies
were carried out with a Fluorolog-3 fluorometer. The mea-
sured solid-state and solution-phase luminescence for the
Tb3+ and Eu3+ complexes was excited by the light from a
Xe-Arc lamp and detected using a photo multiplier tube
at an angle of 90ꢁ with respect to the incident beam. Near
infrared emission spectrum of Er(DBM)3TPTZ was
obtained at room temperature by excitation using a 980-
nm laser diode (ADC Telecommunications), a grating
monochromator, and an InGaAs detector (Thorlabs). A
chopper and a lock-in amplifier were used for signal
amplification.
Photoluminescence quantum yields were measured and
calculated using cresyl violet perchlorate (quantum yield
u = 0.54 in methanol [24]) and rhodamin 6G (u = 0.95 in
ethanol [25]) as the standards. Corrections were made for
the instrumental parameters and differing refractive indices
of the solvents used [26,27]. The relative quantum yields
were calculated using the following established equation
where Abs, A, and n denote the absorbance at the excita-
tion wavelength, integrated area of the corrected emission
2.5. Synthesis of Eu(TTA)3TPTZ
Starting with EuCl36H2O (0.366 g, 1.00 mmol) and
adopting otherwise identical preparative and purification
procedures described for Eu(BTFA)3TPTZ, the product
was obtained as a pale yellow crystalline solid (1.00 g,
78.0%). Anal. Calc. for C42H24N6O6F9S3Eu: C, 44.72; H,
2.14; N, 7.45. Found: C, 44.38; H, 2.20; N, 7.45%.

1232
C.R. De Silva et al. / Polyhedron 26 (2007) 1229–1238
Table 1
Crystal data and structure refinement for Er(DBM)3(TPTZ), Tb(DBM)3(TPTZ), Eu(TTA)3(TPTZ) , Eu(BA)3(TPTZ), and Eu(BTFA)3(TPTZ)
Er(DBM)3(TPTZ)
65H50ErN6O6
1186.37
170(2)
0.71073
monoclinic
I2/a
Tb(DBM)3(TPTZ)
Empirical formula
C
C63H45TbN6O6
1140.97
170(2)
0.71073
monoclinic
P21/c
Formula weight (g molꢀ1
)
Temperature (K)
˚
Wavelength (A)
Crystal system
Space group
Unit cell dimensions
˚
a (A)
25.3385(12)
17.0616(8)
26.3956(13)
9.5880(10)
23.062(3)
24.482(3)
˚
b (A)
˚
c (A)
b (ꢁ)
Volume (A )
109.8140(10)
10735.7(9)
96.617(2)
5377.3(10)
3
˚
Z
8
4
Dcalc (mg/m3)
1.468
1.625
4816
0.19 · 0.19 · 0.18
1.45–28.32
1.409
1.374
2312
0.30 · 0.20 · 0.10
1.22–26.00
Absorption coefficient (mmꢀ1
F(000)
Crystal size (mm3)
h Range for utilized data (ꢁ)
Limiting indices
)
ꢀ33 6 h 6 33, ꢀ22 6 k 6 22, ꢀ35 6 l 6 35
ꢀ11 6 h 6 11, ꢀ28 6 k 6 28, ꢀ30 6 l 6 30
Reflections utilized
67549
57456
Independent reflections [Rint
]
13252 [0.1232]
10559 [0.1650]
Completeness to h = 28.32ꢁ (%)
Absorption correction
Maximum and minimum transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
99.1
100.0
semi-empirical from equivalents
0.7567 and 0.7470
full-matrix least-squares on F2
13251/250/730
semi-empirical from equivalents
0.8749 and 0.6834
full-matrix least-squares on F2
10559/243/685
0.905
0.980
Final R indices [I > 2r(I)]
R indices (all data)
R1 = 0.0346, wR2 = 0.0717
R1 = 0.0559, wR2 = 0.0765
1.260 and ꢀ0.586
0.109
R1 = 0.0634, wR2 = 0.1143
R1 = 0.1292, wR2 = 0.1356
1.162 and ꢀ1.425
0.134
ꢀ3
˚
Largest difference in peak and ꢀh3ole (e A
)
˚
RMS difference in density (e A
)
Eu(TTA)3(TPTZ)
Eu(BA)3(TPTZ)
Eu(BTFA)3(TPTZ)
Empirical formula
C
46H36EuF9N6O8S3
C48H39EuN6O6
947.81
170(2)
C50H36EuF9N6O7
1155.81
170(2)
Formula weight (g molꢀ1
Temperature (K)
)
1219.95
170(2)
˚
Wavelength (A)
0.71073
orthorhombic
P212121
0.71073
monoclinic
P21/n
0.71073
triclinic
Crystal system
Space group
Unit cell dimensions
ꢀ
P1
˚
a (A)
10.4683(8)
15.5897(12)
30.023(2)
12.265(3)
27.548(6)
12.472(3)
11.0603(10)
15.0588(13)
15.3027(14)
˚
b (A)
˚
c (A)
b (ꢁ)
Volume (A )
90
4899.7(7)
93.576(5)
4205.9(17)
92.666(2)
2418.8(4)
3
˚
Z
4
4
2
Dcalc (mg/m3)
1.654
1.5014
2440
0.294 · 0.174 · 0.071
1.36–26.47
1.497
1.549
1920
1.587
1.390
1156
0.269 · 0.310 · 0.370
1.33–26.42
Absorption coefficient (mmꢀ1
F(000)
Crystal size (mm3)
h Range for utilized data (ꢁ)
Limiting indices
)
0.228 · 0.223 · 0.101
1.80–23.27
ꢀ13 6 h 6 13, ꢀ28 6 k 6 30,
ꢀ13 6 l 6 12
25900
ꢀ13 6 h 6 13, ꢀ19 6 k 6 19,
ꢀ13 6 h 6 13, ꢀ18 6 k 6 18,
ꢀ37 6 l 6 37
ꢀ19 6 l 6 19
Reflections utilized
54175
26593
Independent reflections
10075
6045
9826
[Rint
]
[0.1213]
[0.1742]
[0.0329]
Completeness to h = 28.32ꢁ (%)
Absorption correction
Maximum and minimum
transmission
99.6
99.9 (h = 23.27ꢁ)
semi-empirical from equivalents
1.0 and 0.633047
98.8
semi-empirical from equivalents
1.0 and 0.895317
semi-empirical from equivalents
0.7062 and 0.6274
Refinement method
full-matrix least-squares on F2
10075/855/725
full-matrix least-squares on F2
6045/115/554
full-matrix least-squares on F2
9826/0/662
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
1.108
1.095
1.080
R1 = 0.0593, wR2 = 0.1352
R1 = 0.0663, wR2 = 0.1532
R1 = 0.0490, wR2 = 0.1120

C.R. De Silva et al. / Polyhedron 26 (2007) 1229–1238
1233
Table 1 (continued)
R indices (all data)
Eu(TTA)3(TPTZ)
Eu(BA)3(TPTZ)
Eu(BTFA)3(TPTZ)
R1 = 0.1236, wR2 = 0.1786
2.304 and ꢀ1.069
0.183
R1 = 0.1777, wR2 = 0.2247
2.004 and ꢀ1.680
0.172
R1 = 0.0633, wR2 = 0.1180
1.674 and ꢀ0.639
0.096
ꢀ3
˚
Largest difference in peak and ꢀh3ole (e A
)
˚
RMS difference in density (e A
)
spectrum, and refractive index of the solvent, respectively.
Subscript R and S refer to the reference and the unknown
sample, respectively [28,29]. The experiments were per-
formed using chloroform solutions (1–5 · 10ꢀ5 M) of the
title complexes whose absorbance values are less than 0.2.
The experimental uncertainty for the quantum efficiency
calculations was found to be 10%.
3. Results and discussion
3.1. Synthesis and structural characterization
As noted in our previous work, the Lewis base ligand
(TPTZ) is introduced prior to the coordination of DBM,
BA, TTA, or BTFA ligands (Scheme 1) to avoid possible
adventitious hydrolysis which has been shown to produce
unexpected polynuclear oxo/hydro complexes [30]. The
pre-occupation of part of the coordination sphere by the
rather bulky TPTZ ligand also helps prevent more than
three b-diketonate ligands from being incorporated [31].
Thus, electrically neutral and coordinatively saturated
complexes were obtained. All compounds were produced
in good to excellent yields. They are readily soluble in sol-
vents such as acetone, chloroform, and dichloromethane,
but sparingly soluble in alcohols. Product purification by
recrystallization from alcohol and one of these solubilizing
solvents proved to be facile. Satisfactory microanalysis
results (CHN) were obtained for all complexes.
The crystal structures of all five complexes have been
established by single crystal X-ray diffraction, and are
shown in Figs. 1–5, respectively. Selected metric values of
bond distances and angles are summarized in Table 2.
In each of these complexes, the lanthanide ion is situated
in a coordination sphere composed of six oxygen atoms of
the b-diketonate ligands and three nitrogen atoms of the
TPTZ ligand. The coordination polyhedron can be best
uS ¼ uRðAbsR=AbsSÞðAS=ARÞðn2S=nR2 Þ
The stoichiometry of each of the title complexes in the
solution phase was evaluated by titration of the specific
lanthanide chloride solution (1–5 · 10ꢀ5 M) with TPTZ
and the corresponding b-diketone. A stock solution of
the ligand was freshly prepared by deprotonation of the
b-diketone using a stoichiometric amount of KOBut.
Following each addition of the ligand (both TPTZ and
b-diketonate) solution, the integrated emission intensities
7
7
of the 5D0 ! F2 (for Eu3+ at 614 nm) and 5D4 ! F5
(for Tb3+ at 544 nm) transitions were monitored. Details
of the experiments can be found in Supporting Informa-
tion. The lanthanide (Eu3+ or Tb3+):TPTZ ratio was
found to be 1:1, while the ratio of Eu3+:diketonate
(BTFA, TTA, or BA) were found to be 1:3. However,
the sensitization of Tb3+ luminescence by DBM is too
weak to allow a reliable determination of the Tb3+:DBM
ratio. This finding is consistent with the results, presented
below, of the spectroscopic and luminescence studies of
Tb(DBM)3TPTZ in chloroform.
N
N
N
N
N
N
N
(i)
N
N
(ii)
N
N
N
MCl3 . 2H2O
N
N
N
+
R1
R
R
N
N
N
O
O
O
O
M
LnCl3 6H2O
R1
O
O
(i) EtOH, 50oC (ii) KOBut, X, 60oC
R
R1
Tb(DBM)3TPTZ: Ln=Tb; X=DBM; R=R1=C6H5
Er(DBM)3TPTZ: Ln=Er; X=DBM; R=R1=C6H5
Eu(TTA)3TPTZ: Ln=Eu; X=TTA; R=C4H3S, R1=CF3
Eu(BA)3TPTZ: Ln=Eu; X=BA; R=C6H5, R1=CH3
Eu(BTFA)3TPTZ: Ln=Eu; X=BTFA; R=C6H5, R1=CF3
Scheme 1. Synthesis of LnL3TPTZ (Ln = Eu, Tb, Er; L = DBM, BA, TTA, BTFA).

1234
C.R. De Silva et al. / Polyhedron 26 (2007) 1229–1238
Fig. 3. An ORTEP view of the crystal structure of Eu(TTA)3(TPTZ) with
partial atomic labeling. Thermal ellipsoids are drawn at the 50%
probability level.
Fig. 1. An ORTEP view of the crystal structure of Tb(DBM)3(TPTZ)
with partial atomic labeling. Thermal ellipsoids are drawn at the 30%
probability level.
others [14–16,33]. It is also of interest to note that the
˚
Eu–N(central) bonds [2.6757(16) A for Eu(DBM)3TPTZ;
˚
˚
2.618(11) A for Eu(BA)3TPTZ; 2.580(7) A for Eu(TTA)3-
˚
TPTZ; 2.607(3) A for Eu(BTFA)3TPTZ] are all noticeably
longer than that of [Eu(TPTZ)Cl3(MeOH)2] [2.555(4) A],
˚
the only other Eu–TPTZ complex [14]; the TPTZ ligand
is understandably forced further away from the metal cen-
ter by the bulkier diketonate ligands than if it is in a less
coordinatively congested environment. By a similar argu-
ment, the presence of a bulky TPTZ ligand causes notice-
ably longer Eu–O distance (average) in the present
adducts than those of closely related complexes with smal-
ler neutral ligands. For example, the average Eu–O
˚
˚
distance of Eu(DBM)3(TPTZ) is 2.384 A versus 2.348 A
of Eu(DBM)3L [L = 2-(40-dimethylaminophenyl)imidazo-
[4,5-f]1,10-phenanthroline] [34]; the corresponding value
˚
˚
for Eu(BTFA)3TPTZ is 2.405 A versus 2.360 A for
Eu(BTFA)3(2,20-dipyridyl) [35].
The non-coordinating ‘‘arm’’ of the TPTZ ligand in
Eu(BA)3TPTZ and Eu(BTFA)3TPTZ is close in space to
a corresponding arm of a second molecule. As such,
p-dimers are formed, with a separation between two TPTZ
Fig. 2. An ORTEP view of the crystal structure of Er(DBM)3(TPTZ) with
partial atomic labeling. Thermal ellipsoids are drawn at the 30%
probability level.
˚
ligands of 3.155 and 3.285 A, shown in Figs. 4 and 5,
respectively. The formation of such p-dimers raises an
interesting question of whether excimers can form in the
solid state, a point inviting further investigation.
described as a distorted square antiprism monocapped by
the central coordinating nitrogen atom. The rather
crowded coordination environment effectively protects the
lanthanide center from any substantial interactions with
potentially luminescence-quenching solvent molecules.
The aromatic rings of the TPTZ ligand are nearly copla-
nar in each of the five complexes. Within the same com-
plex, the Ln–N bond to the central coordinating N is the
shortest (indicated in bold face in Table 2), in agreement
with our previous observation [12,32] and those made by
3.2. Electronic spectroscopic and photoluminescence studies
Electronic spectroscopic and photoluminescence studies
have been carried out for the title complexes in both chlo-
roform solutions and the solid state, from which very sim-
ilar results have been obtained. In addition, luminescence
titration experiments utilizing solutions with concentra-
tions comparable to those for the luminescence quantum

C.R. De Silva et al. / Polyhedron 26 (2007) 1229–1238
1235
Fig. 4. The TPTZ ligands stack to form a centrosymmetric ‘‘dimer’’ in the crystal structure of Eu(BA)3TPTZ. The closest contact between the TPTZ
˚
ligands is 3.155 A.
Fig. 5. The TPTZ ligands stack to form a centrosymmetric ‘‘dimer’’ in the crystal structure of Eu(BTFA)3TPTZ. The closest contact between the TPTZ
˚
ligands is 3.285 A.
yield measurements were also conducted. The observed
ratio of 1:1:3 (TPTZ:Ln3+:diketonate) is consistent with
the molecular and crystal structures of the title complexes.
In other words, the complexes remained un-dissociated in
solution, even at the relatively low concentrations utilized.
The electronic absorption spectra of Er(DBM)3TPTZ
and Tb(DBM)3TPTZ in chloroform each show three max-
ima at 250, 285, and 353 nm, independent of the central
lanthanide metal. This observation is consistent with
ligand-based absorptions [DBM (250 and 352 nm) and
TPTZ (250 and 285 nm)] of these complexes. Complexes
Eu(BA)3TPTZ, Eu(TTA)3TPTZ, and Eu(BTFA)3TPTZ
also show ligand-based absorption in the range of 230–
385 nm (Supporting Information).
It is clear that the excitation of these complexes is mostly
ligand based. The maximum emission of the terbium com-
plex was observed when the complex was excited at
290 nm, suggesting that ligand-to-Tb3+ energy transfer is
mainly mediated by the TPTZ ligand (Fig. 6). In contrast,
the strongest emissions of Eu(BA)3TPTZ, Eu(TTA)3TPTZ,
and Eu(BTFA)3TPTZ were observed when the complexes
were excited at 345, 355, and 342 nm, respectively. These
excitation wavelengths match the absorption spectra of
the corresponding b-diketonate ligands [12] (Supporting
Information), suggesting that energy transfer originates
from the b-diketonate ligands to Eu3+ ion. Only the excita-
tion and emission spectra of Eu(BA)3TPTZ are shown in
Fig. 7 as representatives. Those for Eu(TTA)3TPTZ and
Eu(BTFA)3TPTZ are deposited in Supporting Informa-
tion. These observations are in line with the commonly
accepted notion of ‘‘antenna effect’’ for lanthanide lumines-
cence [36,37].
The electronic excitation and photoluminescence of
Tb(DBM)3TPTZ, Eu(BA)3TPTZ, Eu(TTA)3TPTZ, and
Eu(BTFA)3TPTZ were also studied in chloroform
solutions.

1236
C.R. De Silva et al. / Polyhedron 26 (2007) 1229–1238
Table 2
˚
Selected bond distances (A) and angles (ꢁ) of Tb(DBM)3(TPTZ), Er(DBM)3(TPTZ), Eu(BA)3TPTZ, Eu(TTA)3TPTZ, and Eu(BTFA)3TPTZ
Tb(DBM)3TPTZ
Er(DBM)3TPTZ
Tb(1)–O(1)
Tb(1)–O(2)
Tb(1)–O(3)
Tb(1)–O(4)
Tb(1)–O(5)
Tb(1)–O(6)
Tb(1)–N(1)
Tb(1)–N(2)
Tb(1)–N(3)
2.380(4)
2.335(4)
2.394(5)
2.327(4)
2.322(4)
2.421(4)
2.626(5)
2.596(5)
2.640(5)
Er(1)–O(1)
Er(1)–O(2)
Er(1)–O(3)
Er(1)–O(4)
Er(1)–O(5)
Er(1)–O(6)
Er(1)–N(1)
Er(1)–N(2)
Er(1)–N(3)
2.308(2)
2.373(2)
2.376(2)
2.314(2)
2.294(2)
2.326(2)
2.655(2)
2.568(2)
2.582(2)
Eu(BA)3TPTZ
Eu(TTA)3TPTZ
Eu(BTFA)3TPTZ
Eu(1)–O(34)
Eu(1)–O(14)
Eu(1)–O(24)
Eu(1)–O(12)
Eu(1)–O(32)
Eu(1)–O(22)
Eu(1)–N(41)
Eu(1)–N(412)
Eu(1)–N(418)
2.324(10)
2.349(9)
2.385(9)
2.406(9)
2.426(10)
2.466(9)
2.661(11)
2.618(11)
2.684(11)
Eu(1)–O(12)
Eu(1)–O(14)
Eu(1)–O(22)
Eu(1)–O(24)
Eu(1)–O(32)
Eu(1)–O(34)
Eu(1)–N(41)
Eu(1)–N(48)
Eu(1)–N(414)
2.602(7)
Eu(1)–O(1)
Eu(1)–O(2)
Eu(1)–O(3)
Eu(1)–O(4)
Eu(1)–O(5)
Eu(1)–O(6)
Eu(1)–N(1)
Eu(1)–N(3)
Eu(1)–N(2)
2.418(3)
2.397(3)
2.394(4)
2.433(3)
2.416(3)
2.373(3)
2.631(4)
2.607(3)
2.629(4)
2.347(6)
2.404(6)
2.397(6)
2.399(6)
2.425(6)
2.602(7)
2.580(7)
2.634(8)
Tb(DBM)3TPTZ
Er(DBM)3TPTZ
O(1)–Tb(1)–O(2)
O(3)–Tb(1)–O(4)
O(5)–Tb(1)–O(6)
O(1)–Tb(1)–N(2)
O(3)–Tb(1)–N(2)
O(5)–Tb(1)–N(2)
71.0(2)
73.0(2)
70.6(1)
113.5(2)
66.4(2)
O(1)–Er(1)–O(2)
O(3)–Er(1)–O(4)
O(5)–Er(1)–O(6)
O(1)–Er(1)–N(2)
O(3)–Er(1)–N(2)
O(5)–Er(1)–N(2)
70.7(1)
70.6(1)
75.1(1)
138.0(1)
69.7(1)
132.6(1)
114.0(2)
Eu(BA)3TPTZ
Eu(TTA)3TPTZ
Eu(BTFA)3TPTZ
O(12)–Eu(1)–O(14)
O(22)–Eu(1)–O(24)
O(32)–Eu(1)–O(34)
O(12)–Eu(1)–N(412)
O(22)–Eu(1)–N(412)
O(32)–Eu(1)–N(412)
73.3(3)
70.1(3)
70.1(3)
68.9(3)
109.6(3)
116.3(3)
O(12)–Eu(1)–O(14)
O(22)–Eu(1)–O(24)
O(32)–Eu(1)–O(34)
O(12)–Eu(1)–N(48)
O(22)–Eu(1)–N(48)
O(32)–Eu(1)–N(48)
72.5(2)
O(1)–Eu(1)–O(2)
O(3)–Eu(1)–O(4)
O(5)–Eu(1)–O(6)
O(1)–Eu(1)–N(2)
O(3)–Eu(1)–N(2)
O(5)–Eu(1)–N(2)
71.0(1)
68.0(1)
71.5(1)
138.7(1)
79.8(1)
82.4(1)
68.9(2)
70.6(2)
67.0(2)
72.9(2)
143.8(2)
1.0
0.8
0.6
0.4
0.2
0.0
Excitation
Emission
Excitation
Emission
1.0
0.8
0.6
0.4
0.2
0.0
250
300
350
400
450
500
550
600
650
300 350 400 450 500 550 600 650 700
Wavelength (nm)
Wavelength (nm)
Fig. 6. The excitation (---) and emission (—) spectra of Tb(DBM)3TPTZ
in CHCl3.
Fig. 7. The excitation (---) and emission (—) spectra of Eu(BA)3TPTZ in
CHCl3.

C.R. De Silva et al. / Polyhedron 26 (2007) 1229–1238
1237
effectively shielding the lanthanide ions from interacting
with luminescence-quenching solvent molecules.
1.0
0.8
0.6
0.4
0.2
0.0
Acknowledgements
This work was supported by NSF CAREER Grant No.
CHE-0238790. Acknowledgment is also made to the Do-
nors of The Petroleum Research Fund, administered by
the American Chemical Society, for partial support of this
research. We thank Professors S.S. Saavedra and K. Mir-
anda for the use of fluorometers, Mr. H. Gan for experi-
mental assistance, and Dr. G.S. Nichol for X-ray
crystallographic assistance. The CCD-based X-ray diffrac-
tometer was purchased through an NSF Grant (CHE-
96103474, USA).
1400
1450
1500
1550
1600
1650
1700
Wavelength (nm)
Appendix A. Supplementary material
Fig. 8. The emission spectrum of Er(DBM)3TPTZ in CHCl3.
Details of spectroscopic and photoluminescence studies
in both solution and the solid states. CCDC 238124,
238125, 618401, 618402 and 618403 contain the supple-
mentary crystallographic data for Tb(DBM)3TPTZ,
Er(DBM)3TPTZ, Eu(TTA)3TPTZ, Eu(BA)3TPTZ, and
Eu(BTFA)3TPTZ. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.poly.2006.
10.049.
The photoluminescence quantum yields of Tb(DBM)3-
TPTZ, Eu(DBM)3TPTZ, Eu(BA)3TPTZ, Eu(TTA)3TPTZ,
and Eu(BTFA)3TPTZ were found to be 3.4%, 17.4%,
15.5%, 40.2%, and 69.7% respectively. As expected, these
lanthanide b-diketonate adducts with TPTZ display higher
emission quantum yields than their aqua analogs [17,38]. In
terms of photoluminescence quantum yield, Eu(BTFA)3-
TPTZ is among the most efficient Eu3+ complexes that
are without substitution of H for D.
When the chloroform solution of Er(DBM)3TPTZ was
excited by a 980-nm pump laser, near infrared emission,
shown in Fig. 8, was observed. The maximum intensity
4
peak is located at 1527 nm, assigned to 4I13/2 ! I15/2, with
a full width at half-maximum (FWHM) of about 81 nm.
The value is greater than those of erbium-doped phosphate
glass (55 nm) [39] and an erbium complex doped in epoxy
resin polymers (47 nm) [40], but comparable to those of
erbium polydentate cage complexes (70 nm) in organic
solutions [41] and erbium tris(8-hydroxy)quinoline
(73 nm) doped in sol–gels [20]. A broad spectrum enables
a wide gain bandwidth for optical amplification in telecom-
munication systems.
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