Heavy atom effect driven organic phosphors and their luminescent lanthanide metal-organic frameworks

Article abstract of DOI:10.1002/cplu.201300121

This investigation demonstrates the heavy atom effect (HAE) concept in developing new organic phosphors and engineering the excited-state energy levels in lanthanide metal ion suprastructures. This was accomplished by coupling two independent energy-transfer photophysical processes: enhancing the electronic population in the excited triplet state through intersystem crossing (ISC) and transferring the triplet energy to the excited state of the lanthanide ions. A new series of iodo-substituted carboxylic ligands were synthesised through a tailor-made approach and complexes with Eu³⁺ ions to give one- and three-dimensional metal-organic frameworks (MOFs). Single-crystal structures of the europium complexes revealed the formation of a 1D linear coordination polymer for the monocarboxylate ligand and 3D MOFs for the dicarboxylate ligand. The HAE quenches the S₁→S₀ transition (self-fluorescence) in these ligands and promotes S₁→T processes for building enhanced excited triplet electronic states. Single-crystal structures of iodo-substituted complexes proved that the ligand molecules were held together by strong π stacking. The π stack restricted vibration relaxation and, as a result, these ligands turned into white or yellowish solid-state organic phosphors. In Eu³⁺ ion complexes, the solid-state phosphorescence of the ligands was completely quenched and the triplet excitation energy was channelled into ligand-to-metal energy transfer. Thus, the current approach opens up a new strategy for designing luminescent MOFs based on the HAE principle by controlling the excited-state energy levels. Copyright

Full text of DOI:10.1002/cplu.201300121

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DOI: 10.1002/cplu.201300121
Heavy Atom Effect Driven Organic Phosphors and Their
Luminescent Lanthanide Metal–Organic Frameworks
Ayyakkalai Balamurugan,[a, b] Arvind Kumar Gupta,[a] Ramamoorthy Boomishankar,[a]
Mundlapudi Lakshmipathi Reddy,[b] and Manickam Jayakannan*[a]
Dedicated to Professor K. N. Ganesh on the occasion of his 60th Birthday
This investigation demonstrates the heavy atom effect (HAE)
concept in developing new organic phosphors and engineer-
ing the excited-state energy levels in lanthanide metal ion su-
prastructures. This was accomplished by coupling two inde-
pendent energy-transfer photophysical processes: enhancing
the electronic population in the excited triplet state through
intersystem crossing (ISC) and transferring the triplet energy to
the excited state of the lanthanide ions. A new series of iodo-
substituted carboxylic ligands were synthesised through
a tailor-made approach and complexes with Eu3+ ions to give
one- and three-dimensional metal–organic frameworks (MOFs).
Single-crystal structures of the europium complexes revealed
the formation of a 1D linear coordination polymer for the
monocarboxylate ligand and 3D MOFs for the dicarboxylate
ligand. The HAE quenches the S1!S0 transition (self-fluores-
cence) in these ligands and promotes S1!T processes for
building enhanced excited triplet electronic states. Single-crys-
tal structures of iodo-substituted complexes proved that the
ligand molecules were held together by strong p stacking. The
p stack restricted vibration relaxation and, as a result, these li-
gands turned into white or yellowish solid-state organic phos-
phors. In Eu3+ ion complexes, the solid-state phosphorescence
of the ligands was completely quenched and the triplet excita-
tion energy was channelled into ligand-to-metal energy trans-
fer. Thus, the current approach opens up a new strategy for
designing luminescent MOFs based on the HAE principle by
controlling the excited-state energy levels.
Introduction
Energy-level engineering is emerging as an important scientific
problem to be addressed in the construction of luminescent
metal–organic frameworks (MOFs).[1] Lanthanide metal ion
complexes are one of the most important classes of lumines-
cent materials owing to their sharp, narrow band widths and
bright emissions from well-defined f!f orbital transitions.[2]
These classes of complexes/materials find wide applications in
imaging,[3] sensing[4] and opto-electronics.[5] The emission char-
acteristics of lanthanide complexes are enriched by coordinat-
ing organic chromophores as ligands that behave as antenna
molecules to channel the excitation energy to the metal
core.[6] The excitation energy-transfer pathway is schematically
described in Figure 1a. By absorbing light, organic ligands un-
dergo photoexcitation from the ground state (S0) to the singlet
excited state (S1). The excitation energy is often used for radia-
tive decay through the S1!S0 transition to give fluorescence
(see path 1 in Figure 1a).[7] Ligand-to-metal energy transfer
(LMET) occurred from the ligand triplet excited state (T) to the
5
metal ion (5D0 for Eu3+) via T! D transitions (as shown in
path 2 in Figure 1a).[7] Unfortunately, most of the organic mole-
cules are known to undergo vibration relaxation (as heat) from
the excited triplet state to the singlet ground state (via a T!S0
transition). Although a wide range of organic ligands have
been reported for coordination complexes with lanthanides,[8]
the manipulation of triplet-state energy levels and channelling
of the excitation energy to metal centre (see path 2, in
Figure 1) are two of the major problems yet to be addressed.
Hence, optimisation of the ligand chemical structure as an effi-
cient photosensitiser for LMET is one of the crucial factors for
achieving luminescence in lanthanides.
[a] A. Balamurugan, A. K. Gupta, R. Boomishankar, M. Jayakannan
Department of Chemistry
Indian Institute of Science Education and Research (IISER)
Dr. Homi Bhabha Road, Pune-411008, Maharashtra (India)
E-mail: jayakannan@iiserpune.ac.in
The investigation presented herein emphasises achieving
complete LMET through coupling of two independent energy-
transfer pathways: 1) enhancing the electronic population of
the excited triplet state through intersystem crossing (ISC)
from S1!T and 2) transferring these largely populated excited
[b] A. Balamurugan, M. Lakshmipathi Reddy
Chemical Sciences and Technology Division
National Institute for Interdisciplinary Science and Technology (NIIST)
Trivandrum-695019, Kerala (India)
5
Supporting information for this article is available on the WWW under
triplet electronic states to D0 of the lanthanide (Eu3+ ion). The
1
http://dx.doi.org/10.1002/cplu.201300121. It contains H and 13C NMR
first process, S1!T (ISC), is achieved by employing the heavy
atom effect (HAE) concept in ligand design. The HAE typically
quenches the S1!S0 transition (fluorescence) through heavy-
atom orbital couplings and enhances the S1!T process to
spectra of compounds; crystallographic images, unit cells, bond length,
bond angles, and hydrogen-bond lengths; absorption, excitation, emis-
sion, and phosphorescence spectra; luminescence lifetime decay pro-
files; energy-level diagram; and mass spectra.
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duce luminescent lanthanide MOFs.
Results and Discussion
New organic ligands based on 4-hydroxyphenyl propionic acid
were chosen based on our previous experience[10] and suitably
modified with iodide substitution on the aromatic rings. The
synthetic details pertaining to the ligands and their structural
characterisations are given in the Supporting Information (Fig-
ures S1 and S2). The phenyloxy group was functionalised with
2-ethylhexyl (Ia, IIa and IIIa), methyl (Ib and IIIb), 2-methylpyrid-
yl (IIIc) and carboxylic acid (IV) groups. These ligands were
complexed with Eu3+ ions to produce luminescent one-dimen-
sional coordination polymers and three-dimensional frame-
works. The absorption and emission properties of the carboxyl-
ate ligands were studied in solution and in the solid state (see
Figure 2). Ligands without iodo substitution (Ia and Ib) showed
absorption and emission maxima at l=277 and 295 nm, re-
spectively (see Figure S3 in the Supporting Information). Iodo
substitution of the aromatic ring leads to a redshift in the ab-
sorbance and fluorescence spectra with maxima at l=280 and
305 nm, respectively (see Figure S3 in the Supporting Informa-
tion). The solution fluorescence spectra of the iodo-substituted
ligands are shown in Figure 2a. The ligands showed almost
identical solution fluorescence spectra, irrespective of structur-
al variation in the monoiodo (IIa), bisiodo (IIIa, IIIb) or pyridinyl/
carboxyl substitution (IIIc and IV). The fluorescence quantum
yield of the ligands were determined by using 2-aminopurine
(F=0.68, in water) as a standard.[11] A quantum yield for
ligand Ia in methanol of 0.47 was obtained, whereas all other
ligands showed very low fluorescence (F<0.1, see Table S1 in
Figure 1. The HAE-assisted ligand-to-metal energy transfer (LMET) concept
(a) and the chemical structures of the organic ligands (b).
populate more triplet excited
states.[9] To the best of our
knowledge, no effort has yet
been made to apply the HAE
concept to achieve LMET in the
lanthanide-based metal–organic
frameworks. Herein, simple and
elegant organic carboxylate li-
gands with mono- and bisiodo
substitution (heavy atoms, see
Figure 1b) were designed and
synthesised for the above pur-
pose. In the solid state, these li-
gands were non-fluorescent and
turned into organic phosphors
through the T!S0 process.
Upon coordinating these solid-
state phosphors with lanthanide
metal ions, the excitation energy
was utilised for efficient LMET to
the metal centre. As a net result,
for the first time, both HAE-as-
sisted ISC (S1!T) and LMET (T!
Figure 2. Emission spectra of ligands in methanol (a; excitation wavelength of 275 nm) and in solid state (b; exci-
tation wavelength of 330 nm). Time-correlated single photon counting (TCSPC) decay profiles of fluorescence (c)
5D) were coupled in a single
photoexcitation process to pro- and phosphorescence (d) in the solid state. PL= photoluminescence.
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the Supporting Information). This clearly indicated that ligands
with heavy atom (iodide) substitution were weakly fluorescent
in nature. The solid-state excitation spectra of ligands showed
broad excitation maxima at l=320–380 nm. As shown in Fig-
ure 2b, the solid-state emission spectra of the ligands showed
remarkable changes with respect to the variation in their
chemical structures. Two kinds of emission features were ob-
served: 1) the appearance of the solid-state fluorescence at l=
370–480 nm and 2) the emergence of a new peak with respect
to the solid-state phosphorescence at l=550–650 nm. The in-
tensities of the solid-state fluorescence and phosphorescence
peaks largely varied, depending upon the structure of the li-
gands. The monoiodo-substituted ligand (IIa, with 2-ethylhexyl
substitution) showed only a fluorescence peak (at l=450 nm),
whereas its bisiodo counterpart (IIIa) showed an additional
shoulder at l=550 nm with respect to phosphorescence. A
similar trend was also observed in the emission spectrum of
methoxy-substituted bisiodo ligand (IIIb). Notably, the replace-
ment of the alkoxy substituents (methyl or 2-ethylhexyl) by
pyridyl or carboxylic acid moieties leads to large enhance-
ments in the phosphorescence characteristics. The variations in
the phosphorescence peak maxima among the bisiodo ligands
are attributed to their structural differences (see Figure 1b). To
quantify the exact luminescent colour, the solid-state emission
spectra of the ligands were fed into the CIE colour coordinates
and their chromaticity diagram is shown in Figure S17 in the
Supporting Information. The x and y colour coordinates are
given in Table 1. From the CIE coordinates, it is clear that li-
gands IIIa and IIIb produced white-light solid-state phosphor-
other samples). The decay curves (Figure 2c) revealed that the
fluorescent ligand (IIa) was less susceptible to decay than
those of phosphors IIIa and IIIc. The decay profiles in Figure 2c
were fitted biexponentially and their decay times were ob-
tained in the range of t1 =0.18–0.37 ns and t2 =0.73–1.50 ns
(see Table 1). The decay lifetimes are obtained in the nanosec-
ond range, which typically corresponds to the fluorescence of
organic molecules.[13] The decay profiles shown in Figure 2d,
collected with respect to phosphorescence, showed the oppo-
site trend to that of the plots shown in Figure 2c. The strongly
fluorescent ligand (IIa) experienced faster decay than that of
the solid-state phosphors (IIIa and IIIc). The decay lifetime
(Table 1) revealed that the solid phosphor ligands possessed
high lifetimes of t1 =1.47–3.11 ms. The higher lifetime in the
millisecond range confirmed that the emission peak at l=
550–650 nm in Figure 2b was nothing but solid-state phos-
phorescence.[14,9a,b] Interestingly, these ligands did not show
any phosphorescence in the solutions, which indicated that
packing of the ligands in the solid state played a crucial role in
the occurrence of phosphorescence.
In an effort to trace the solid-state packing of the ligands,
the pyridyl-substituted bisiodo ligand IIIc produced good quali-
ty crystals for structural determination. Single-crystal X-ray
analysis revealed that the pyridyl carboxylate ligand IIIc crystal-
lised as a solvent adduct of IIIc·H2O in monoclinic space group
C2/c (see Figure 3a). In the solid-state structure, the molecule
exists in the form of a 2D-sheet-like structure mediated by
both hydrogen-bonding and strong p–p interactions (see Fig-
ure 3b). A closer look at this structure reveals that a zigzag
double-chain structure is initially formed through the p–p in-
teractions between the pyridyl moieties of the adjacent ligands
and the hydrogen-bonding interactions of the solvated water
molecules (see the Supporting Information for molecular pack-
ing). Thus, water accepts hydrogen bonds from the carboxylate
protons and donates hydrogen bonds to the pyridyl nitrogen
and C=O groups of two IIIc units to complete the double-
chain network. Additionally, p–p interactions of the iodoaryl
units further connect neighbouring double chains, resulting in
the formation of a 2D-sheet-like structure. From Figure 3b, it is
evident that these ligand molecules are tightly packed in the
solid state through strong p–p interactions. Thus, during pho-
toexcitation, the bisiodo ligand IIIc is not flexible to undergo
structural changes in the solid state. Therefore, the occurrence
of solid-state phosphorescence is attributed to two important
factors (see Figure 1a): 1) HAE-assisted (iodide) ISC (S1!T) and
enhancement of the large triplet electronic-state population
and 2) suppression of vibration relaxation of the ligands (T!
S0) from the triplet state by the highly packed structures in the
solid state (through p–p stacking). Hence, the occurrence of
solid-state phosphorescence in the bisiodo ligands serves as
direct evidence for the enhancement of electronic population
in the triplet state by HAE.[9a,b] The absorption and excitation
spectra of iodine-substituted ligands were compared to find
out the role of iodide substitution on the S1 and T1 energy
levels (see Table 1). The energy difference between the S1 and
T1 levels of the ligands, and also the T1 level of the ligand to
the excited state of the lanthanide ions were known to deter-
Table 1. Fluorescence (FL) and phosphorescence (Phos) lifetime values,
and CIE x,y values of the ligands.
Ligand
t1(FL)
t(Phos)
Singlet
[cmꢀ1
Triplet
[cmꢀ1
[ns]
[ms]
]
]
Ia
0.17 (0.67)[a]
0.37 (1.50)[a]
0.37 (1.47)[a]
0.36 (1.45)[a]
0.18 (0.73)[a]
0.26 (1.06)[a]
–
34130
33000
33000
33000
33110
33110
20870
20600
20830
20120
20920
20610
IIa
IIIa
IIIb
IIIc
IV
0.01
1.47
1.57
3.11
1.5
[a] The t2 values of the ligands were obtained from biexponential fitting.
escence. The pyridyl and carboxylic acid substituted ligands
(IIIc and IV) produced yellowish emission owing to the shift in
their phosphorescence spectra towards higher wavelength.[12]
To further confirm the solid-state emission characteristics
(fluorescence or phosphorescence), the samples were subject-
ed to time-resolved luminescent decay measurements by
TCSPC techniques. The solid-state TCSPC decay profiles of li-
gands IIa, IIIa and IIIc with respect to fluorescence (collected at
l=370–470 nm by using a l=339 nm nano-light-emitting
diode (LED) laser as the excitation source) and phosphores-
cence (collected at l=550–650 nm by using pulsed tungsten
lamp as the excitation source) are shown in Figure 2c and d,
respectively (see Figure S5 in the Supporting Information for
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Figure 3. Single-crystal X-ray structures of ligand and metal complexes: (a) structure of IIIc, (b) the 2D-sheet-like structure in IIIc; the individual double chains
are differentiated by different bond colours, (c) and (e) show the ligand environment at the Eu centres, and (d) and f) show the formation of 1D-chain and
3D-framework structures in EuꢀIIIb and EuꢀIV, respectively.
mine the luminescence of the lanthanide complexes.[3] Accord-
ing to Reinhoudt’s empirical rule, the ISC process becomes ef-
fective when DE (1pp*–3pp*)ꢁ5000 cmꢀ1. The energy gaps DE
(1pp*–3pp*) of the ligands are in the range of 12170–
by FTIR spectroscopy (see Figure S6 in the Supporting Informa-
tion) and elemental analysis. The C=O bond stretching fre-
quency at around n˜ =1705 cmꢀ1 for ligands I, II, IIIb and IIIc
disappeared completely in all Eu3+ complexes and the appear-
ance of new peaks at n˜ =1600 and 1520 cmꢀ1 corresponding
to symmetric and antisymmetric EuꢀC=O vibrations,[15] respec-
tively, was observed (see Figure S6 in the Supporting Informa-
tion). Thermogravimetric analysis of the Eu3+ complexes con-
firmed their thermal stability up to 3008C (see Figure S7 in the
Supporting Information). Furthermore, good-quality single
crystals were obtained for complexes EuꢀIIIb and EuꢀIV and
their structures were determined by single-crystal X-ray diffrac-
tion. Structural analysis revealed that the complex EuꢀIIIb crys-
13260 cmꢀ1 [8o]
This indicates that ISC can occur in all ligands;
.
however, the spin-forbidden S1!T1 process is only facilitated
in the heavy atom (iodine) substituted ligands. Thus, phos-
phorescence in diiodo ligands is facilitated by HAE rather than
the variation in the energy-level differences (S1 or T1) among
the ligands.
All iodo ligands were complexed with Eu3+ ions in the pres-
ence of NaOH; synthetic details are provided in the Supporting
Information. The formation of Eu3+ complexes was confirmed
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tallised in the monoclinic space group P21/c as EuꢀIIIb·2CH3OH
(Table S2 in the Supporting Information). The coordination en-
vironment around the Eu3+ centre can be described as a dis-
torted trigonal prism that consists of five carboxylate ligand
moieties and two coordinated molecules of methanol (Fig-
ure 3c). Of the five carboxylates, three of them are unique:
one of them is bonded to a single EuIII centre in a chelating
fashion (O1 and O2) and the remaining two are involved in bi-
dentate bridging interactions. These bidentate interactions
result in the formation of two different types of ring structures:
a four-membered Eu2O2 ring (A), involving two bridging aniso-
bidentate carboxylate ligands (O4 and O5), and an eight-mem-
bered Eu2C2O4 ring (B) formed by isobidentate bridging inter-
actions (O7 and O9). The net effect is the formation of a one-
dimensional chain-like structure of these alternating rings run-
ning along the a axis (Figure 3d). Furthermore, the solid-state
packing of the molecule shows the formation of 2D square-
grid-like structure in which the corners of the grid are occu-
pied by Eu3+ chains and the rims are held by aryl p–p interac-
tions (see Figure S8 in the Supporting Information). The chan-
nels inside the grid are hydrophobic and are again filled with
p–p-stacked iodoaryl moieties of the carboxylate ligands.
framework. The Connelly surface pore analysis shows a narrow
pore along the B rings with only 6.7% of the unit-cell volume
available for guest water molecules.
The Eu3+ ion MOFs were subjected to excitation, emission
and luminescent decay measurements, as shown in Figure 4.
The emission spectra of the Eu3+ complexes exhibited sharp
metal emission peaks at l=580, 592, 615, 650 and 699 nm
5
7
5
7
5
7
5
7
with respect to the D0! F0, D0! F1, D0! F2, D0! F3, and
Similarly, the EuꢀIV complex crystallised in the centrosym-
metric space group P21/n as EuꢀIV·5H2O. The asymmetric unit
consists of a europium metal centre, two ligand motifs, two
coordinated and three free molecules of water (see Figure 3e).
The coordination environment around the Eu3+ cation can be
described as a distorted square antiprism, in which six of the
eight sites are occupied by carboxylate oxygen atoms, while
the remaining two sites (O11 and O12) are taken up by two
water molecules. The charge balance in the molecule is re-
stored by the presence of two dicarboxylate ligands, of which
one is dianionic and the other one is monoanionic with one
protonated carboxylate end (at O5). A closer look at carboxyl-
ate coordination to the Eu3+ centres shows the initial forma-
tion of an infinite 1D-chain comprised of contiguous eight-
membered Eu2(CO2)2 rings (A) assisted by m2-bridging interac-
tions of the carboxylate ions situated at the acetyloxy end.
Unlike the EuꢀIIIb complex, the 1D substructure in EuꢀIV is
comprised only of eight-membered rings in a zigzag manner.
These rings are further connected by carboxylate motifs situat-
ed at the other side of the phenyl rings (propionate end), re-
sulting in a closely knitted 3D-pillared coordination network
(see Figure 3 f). Notably, these linker propionate motifs are
monodentate and interact with the metal centres solely
through the double-bonded oxo functionality (see the Sup-
porting Information for molecular packing). The remaining two
oxygen atoms of the propionate groups are either protonated
(O5) or anionic (O10) and involved in hydrogen-bonding inter-
actions between each other (O5ꢀH5A···O10; see Figure S9 in
the Supporting Information). The pillared 3D network structure
in EuꢀIV is also accompanied by the formation of different
kinds of macrocycles (A, B and C; see Figure 3 f and Figure S9
in the Supporting Information). Although the eight-membered
rings (A) are involved in stitching the 1D zigzag chains, the 52-
membered Eu4C36O12 rings (B) and the 26-membered Eu2O6C18
rings (C) are responsible for the construction of the 3D pillared
Figure 4. Emission spectra of Eu complexes in the solid state (a, excitation
wavelength of 345 nm) and their TCSPC decay profiles of the Eu complexes
(b) and their lifetime values are given in the table. The photographs of sam-
ples in vials were taken by dispersing the MOFs in methanol and exposing
them to a hand-held UV lamp.
7
5D0! F4 transitions, respectively (for the excitation spectra, see
Figure S12 in the Supporting Information).[16] The emission in-
tensity of the Eu3+ complexes varied depending upon the
chemical structure of ligands. For example, the metal centre
emission intensity increases as follows: EuꢀIa<EuꢀII<Euꢀ
IIIb<EuꢀIIIa<EuꢀIIIc<EuꢀIV. Both ligands Ia (without iodo
substitution) and IIa (monoiodo substitution) largely showed
self-emissions along with a weak metal emission (see Fig-
ure S13 in the Supporting Information). As a result, the lumi-
nescence of the complexes typically obtained a bluish colour
(see Figure 4). On the other hand, the bisiodo ligands behaved
as excellent photosensitisers for Eu3+ ions and completely
transferred the excitation energy to produce a bright, sharp
metal-centred red emission. This emission colour is clearly visi-
ble in the photographs of the vials (see Figure 4). The CIE chro-
maticity x and y colour coordinates (table in Figure 4) con-
firmed that the Eu3+ complexes of the bisiodo ligands emitted
pure, strong, red emission.[17] Furthermore, to understand the
energy-transfer process, triplet (T) energy levels of the ligands
were determined based on their gadolinium complexes (see
Figure S14 in the Supporting Information). According to Latva’s
rule, the energy gap between the ligand triplets to the excited
state of the metal (5D) should be in the range of 2500–
4000 cmꢀ1 for effective LMET.[18] In the present case, all of the
5
ligands possessed optimal energy gaps (DE=T! D0) in the
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range of 3110–3500 cmꢀ1, which confirmed the possibility for
LMET (see Figure S15 in the Supporting Information).
duce red emission from the Eu3+ centre. The LMET photophys-
ical process was almost the same in both one- and three-di-
mensional frameworks (see Figure 3). Hence, the HAE concept
is a very good tool to engineer the population of excited-state
energy levels to obtain luminescent MOFs based on lantha-
nides.
The complexes were further subjected to luminescent decay
lifetime measurements by excitation at l=345 nm and collec-
tion at the metal emission (at l=612 nm). The decay profiles
of the complexes showed larger variation with respect to their
ligand structure (see Figure 4b). For example, the complexes
of ligands Ia and IIa showed fast decay with respect to the
weak metal-centred emission. On the other hand, the bisiodo
ligand complexes showed slow decay with respect to the for-
mation of stable excitons. Time-resolved luminescent decay
profiles were fitted monoexponentially and the values are
shown in the table in Figure 4. The luminescence decay life-
times of the EuꢀIa and EuꢀIIa complexes were much lower
than those of other Eu complexes. Furthermore, the lumines-
cent decay rate constants were determined from the lifetime
measurements (see detailed calculations in Figures S15 and
S16 in the Supporting Information). The radiative rate con-
stants (k) and intrinsic quantum yields (FLn) are given in the
table in Figure 4. The k values of the bisiodo complexes were
lower than those of EuꢀIa and EuꢀIIa; this indicated that exci-
tons generated in the excited state were more stable for bisio-
do Eu3+ MOFs. Lower intrinsic quantum yields of 2 and 6%
were obtained for EuꢀIa and EuꢀII, respectively. This decrease
is attributed to poor LMET. All bisiodo complexes showed in-
trinsic quantum yields in the order of 16–21%; this indicates
that the photoexcitation energy is completely transferred from
the bisiodo ligands to the excited state of the Eu3+ metal ion.
The quantum yield of the europium complexes of the diiodo
ligand were compared with earlier reports based on aromatic
carboxylate ligands.[8j,k,m] The observed quantum yields of the
present europium MOFs were higher (or comparable to) than
that of the reported non-iodinated aromatic carboxylate lan-
thanide complexes. Hence, the new Eu3+ complexes of the
iodine ligands have very good luminescence characteris-
tics.[8j,k,m] The quantum yields of the EuꢀIIIa, EuꢀIIIb and EuꢀIIIc
complexes were 17.0, 16.2 and 21.1% (see the table in
Figure 4). The differences in quantum yields among the ligands
only varied to a small degree. These differences were attribut-
ed to other factors, such as ligand packing and vibration
quenching of the alkyl groups. A comparison of the solid-state
PL of the ligands in Figure 2b with that of their Eu3+ com-
plexes in Figure 4 revealed the following conclusions: Strongly
self-fluorescent ligands Ia and IIa showed poor LMET to the
Eu3+ ions, and thus, these ligands, either alone or in the MOFs,
preferred to undergo the process shown by path 1 in Fig-
ure 1a to produce ligand-based self-emission. All bisiodo li-
gands are strong phosphors in the solid state and in the MOFs
(upon complexation with Eu3+ ions) the energy is completely
transferred from the ligand triplet state to the metal core (T!
5D). Thus, the solid-state phosphorescence of the ligands
quenched in the MOFs and a sharp and bright metal-centred
red emission emerged. This revealed that the emission charac-
teristics of the Eu3+ ion based MOFs were very selective to-
wards the structure of iodo-substituted ligands employed for
coordination. The HAE promotes the more triplet excited state
through ISC, which was subsequently utilised for LMET to pro-
Conclusion
This investigation demonstrated the HAE concept in ligand ge-
ometry to achieve complete control over the excited-state
energy levels for LMET in lanthanide complexes. The HAE con-
cept was exploited in ligands with aryl halide substitution
(iodide) for LMET. The bisiodo ligand structures provided the
opportunity to modify the functionality to produce lumines-
cent one-dimensional coordination polymers and three-dimen-
sional frameworks. In the bisiodo ligands, close packing
through p–p interactions restricted their molecular orientation
in the solid state and as a result they became solid phosphors
through T!S0 transitions. Upon complexation with Eu3+ ions,
the ligands underwent efficient LMET rather than self-phos-
phorescence in the solid state, which led to red emission from
the EuIII metal centre. The approach demonstrated herein de-
scribed in detail for Eu3+ ion complexes is not restricted to any
particular type of lanthanides and, in general, is applicable to
a wide range of luminescent MOFs. The current approach
gives a new insight into the ligand chemical structure based
on the HAE principle for excited-state energy-level engineering
in MOFs; this has great potential for breakthrough discoveries
in materials science.
Experimental Section
Materials
3-(4-Hydroxyphenyl)propionic acid, 2-ethylhexylbromide, 2-chloroe-
thylacetate, 2-bromomethylpyridine, Gd(NO3)3·6H2O, Tb(NO3)3·6H2O
and Eu(NO3)3·6H2O were purchased from Aldrich chemicals. K2CO3,
iodine, KI, methylamine, dimethylsulfate, and all other reagents
and solvents were purchased locally and purified following stan-
dard procedures. Compounds 2, 3, 7, 8 and 9 were synthesised by
following our earlier reported procedures.[10]
Characterisation
1H and 13C NMR spectra of all compounds were recorded by using
a 400 MHz Joel NMR spectrophotometer in CDCl3 containing
a small amount of tetramethylsilane (TMS) as an internal standard.
IR spectra of the ligands and their complexes were recorded by
using a Thermo-Scientific Nicolet 6700 FTIR spectrometer in the
solid state in KBr. The purity of the ligands and intermediate com-
pounds were determined by JEOL JSM600 fast atom bombard-
ment (FAB) high-resolution spectrometry (HRMS) by dissolving the
compound in THF and suspended in 3-nitrobenzylalcohol as
a matrix for FAB mass measurements. Elemental analysis was per-
formed by using an Elemental Vario EL-III CHN analyser. The ther-
mal stability of the lanthanide complexes was determined by using
a Perkin Elmer STA 600 model thermal analyser at a heating rate of
108Cminꢀ1 in a nitrogen atmosphere. The absorption and emission
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studies were performed on a Perkin–Elmer Lambda 45 UV/Vis spec-
trophotometer and a SPEX Fluorolog HORIBA Jobin Vyon fluores-
cence spectrophotometer with a double-grating 0.22 m Spex1680
monochromator and a 450 W Xe lamp as the excitation source at
room temperature. The excitation and emission spectra of the
complexes were corrected on the spectrometer. The PL lifetime
measurements were performed at room temperature by using
a SPEX Fluorolog HORIBA Jobin Vyon 1934 D phosphorimeter.
Single-crystal X-ray data of Eu3+ complexes were collected at 150 K
on a Bruker KAPPA APEX II CCD Duo diffractometer (operated at
1500 W, power: 50 kV, 30 mA) by using graphite-monochromated
MoKa radiation (l=0.71073 ꢁ). The crystals were mounted on
nylon CryoLoops (Hampton Research) with Paraton-N (Hampton
Research). Data integration and reduction were processed with
SAINT1 software. A multi-scan absorption correction was applied
to the collected reflections.
the washings was neutral. The product was purified by recrystalli-
sation from hot ethanol. Yield=1.89 g, 97%; 1H NMR (400 MHz,
CD3OD): d=7.71 (s, 2H; Ar-H), 3.81 (d, 2H; Ar-OCH2), 3.68 (s, 3H;
OCH3), 2.81 (t, 2H; Ar-CH2CH2), 2.55 (t, 2H; -CH2CH2-COOCH3), 1.93
ꢀ0.91 ppm (m, 15H; aliphatic H); 13C NMR (100 MHz, CD3OD): d=
175.7, 156.4, 141.1, 139.8, 90.1, 75.7, 40.3, 35.6, 30.0, 29.1, 23.5,
23.9, 13.3, 10.6 ppm; FTIR (KBr): n˜ =2928, 2853, 1710, 1530, 1442,
1381, 1245, 1147, 991, 866, 782, 701 cmꢀ1; HRMS: m/z calcd for
530.17; found: 553.97 [M++Na+].
3-{4-[(2-Ethylhexyl)oxy]phenyl}propanoic acid (Ia): Compound 2
(2.5 g, 0.86 mmol) was dissolved in methanol (15 mL) and NaOH
(0.05 g, 1.3 mmol) in water was added to the mixture. The rest of
the procedure was the same as that described for IIIa. Yield=2.3 g,
1
97%; H NMR (400 MHz, CDCl3): d=7.13 (d, 2H; Ar-H), 7.83 (d, 2H;
Ar-H), 3.83 (d, 2H; Ar-OCH2), 2.91 (t, 2H; Ar-CH2CH2), 2.65 (t, 2H;
-CH2CH2-COOH), 1.70–0.93 ppm (m, 15H; aliphatic H); 13C NMR
(100 MHz, CDCl3): d=179.1, 158.0, 132.0, 129.2, 115.4, 114.5, 70.1,
51.8, 39.5, 36.2, 30.59, 29.9, 29.2, 23.9, 23.2, 14.2, 11.20 ppm; FTIR
(KBr): n˜ =2950, 2860, 1710, 1570, 1530, 1440, 1380, 1290, 1240,
993, 864, 785, 706, 613 cmꢀ1 HRMS: m/z calcd for 278.38; found:
301.17 [M++Na+], 315.19 [M++K+].
Synthesis
Methyl 3-[3,5-diiodo-4-(pyridin-2-ylmethoxy)phenyl]propanoate
(10): Compound 7 (4.0 g, 9.2 mmol) and anhydrous K2CO3 (3.2 g,
23.1 mmol) were dissolved in dry acetonitrile (50 mL) and then
heated to 808C under a nitrogen atmosphere for 1 h. After addi-
tion of a catalytic amount of KI, 2-bromethypyridine (1.25 g,
10.2 mmol) was slowly added to the reaction mixture over 15 min.
The reaction was continued for 24 h at 808C under a nitrogen at-
mosphere. The reaction mixture was poured into water and ex-
tracted with ethyl acetate. The organic layer was washed with 5%
NaOH, brine, and water and then dried over anhydrous Na2SO4.
The solvent was removed to give a brown semisolid product,
which was purified by column chromatography on silica gel with
hexane and 45% ethyl acetate as the eluent. Yield=3.3 g, 90%;
1H NMR (400 MHz, CDCl3): d=8.65 (d, 1H; Ar-H), 7.95 (d, 1H; Ar-H),
7.88 (t, 1H; Ar-H),7.64 (s, 2H; Ar-H), 7.35 (t, 1H; Ar-H), 5.17 (s, 2H;
Ar-OCH2), 3.68 (s, 3H; OCH3), 2.85(t, 2H; Ar-CH2CH2), 2.60 ppm (t,
2H; -CH2CH2-COOCH3).
3-{4-[(2-Ethylhexyl)oxy]-3-iodophenyl}propanoic acid (IIa): Com-
pound 5 (1.0 g, 0.88 mmol) was dissolved in methanol (10 mL) and
NaOH (0.05 g, 1.3 mmol) in water was added to the mixture. The
rest of the procedure was the same as that described for IIIa.
1
Yield=0.94 g, 97%; H NMR (400 MHz, CDCl3): d=7.61 (s, 1H; Ar-
H), 7.52 (s, 1H; Ar-H), 7.20 (s, 1H; Ar-H), 3.85 (d, 2H; Ar-OCH2), 2.87
(t, 2H; Ar-CH2CH2), 2.67 (t, 2H; -CH2CH2-COOH), 1.84–0.91 ppm (m,
15H; aliphatic H); 13C NMR (100 MHz, CDCl3): d=178.1, 153.3,
139.5, 137.5, 130.5, 90.8, 40.3, 35.0, 30.0 (2C), 29.0, 28.1, 22.9, 14.0
(2C), 11.1 ppm (2C); FTIR (KBr): n˜ =2950, 2920, 2860, 2360, 1710,
1590, 1450, 1380, 1250, 1120, 1030, 862, 804, 744, 671, 625,
598 cmꢀ1
.
3-(3,5-Diiodo-4-methoxyphenyl)propanoic acid (IIIb): Com-
pound 7 (1.25 g, 2.8 mmol) was dissolved in methanol (10 mL) and
NaOH (0.17 g, 4.2 mmol) in water was added to the mixture. The
rest of the procedure was the same as that described for IIIa.
3-(3,5-Diiodo-4-methoxycarbonylmethoxyphenyl)propionic acid
methyl ester (11): Compound 7 (4.0 g, 9.2 mmol) and anhydrous
K2CO3 (3.2 g, 23.1 mmol) were dissolved in dry acetonitrile (50 mL)
and then heated to 808C under a nitrogen atmosphere for 1 h.
After the addition of a catalytic amount of KI, 2-chloroethylacetate
(1.25 g, 10.2 mmol) was slowly added to the reaction mixture over
15 min. The reaction was continued for 24 h at 808C under a nitro-
gen atmosphere. The reaction mixture was poured into water and
extracted with ethyl acetate. The organic layer was washed with
5% NaOH, brine, and water and then dried over anhydrous
Na2SO4. The solvent was removed to give a yellow solid, which was
purified by column chromatography on silica gel with hexane and
25% ethyl acetate as the eluent. Yield=4.5 g, 97%; 1H NMR
(400 MHz, CDCl3): d=7.56 (s, 2H; Ar-H), 4.51 (s, 2H; Ar-OCH2), 4.28
(q, 2H; -COOCH2CH3), 3.63 (s, 3H; OCH3), 2.80 (t, 2H; Ar-CH2CH2),
2.56 (t, 2H; -CH2CH2-COOCH3), 1.30 ppm (t, 15H; -COOCH2CH3);
13C NMR (100 MHz, CDCl3): d=172.6, 167.7, 155.2, 141.2, 139.8,
90.3, 69.0, 61.5, 51.9, 35.2, 29.1, 14.3 ppm; FTIR (NaCl, liquid state):
n˜ =2990, 2949, 1730, 1530, 1530, 1440, 1380, 1300, 1200, 1070,
1
Yield=1.9 g, 99%; H NMR (400 MHz, CD3OD): d=7.70 (s, 2H; Ar-
H), 3.79 (s, 3H; OCH3), 2.81 (t, 2H; Ar-CH2CH2), 2.53 ppm (t, 2H;
-CH2CH2-COOCH3); 13C NMR (100 MHz, CD3OD): d=174.9, 157.4,
141.1, 139.7, 89.6, 59.8, 34.9, 28.7 ppm; FTIR (KBr): n˜ =2884, 1691,
1535, 1454, 1409, 1303, 1243, 1138, 1052, 988, 927, 864, 781, 702,
656, 598 cmꢀ1; HRMS: m/z calcd for 431.99; found: 552.97 [M+
+Na+].
3-[3,5-Diiodo-4-(pyridin-2-ylmethoxy)phenyl]propanoic
acid
(IIIc): Compound 7 (2.29 g, 4.3 mmol) was dissolved in methanol
(15 mL) and NaOH (0.43 g, 10.9 mmol) in water was added to the
mixture. The rest of the procedure was the same as that described
1
for IIIa. Yield=2.15 g, 97%; H NMR (400 MHz, CD3OD): d=8.54 (d,
1H; Ar-H), 7.95 (m, 2H; Ar-H), 7.77 (s, 2H; Ar-H), 7.45 (q, 1H; Ar-H),
5.10 (s, 2H; Ar-OCH2), 2.85(t, 2H; Ar-CH2CH2), 2.61 ppm (t, 2H;
-CH2CH2-COOCH3); 13C NMR (100 MHz, CD3OD): d=175.4, 156.9,
156.3, 148.5, 142.3, 140.6, 138.3, 123.9, 90.5, 74.3, 35.6, 29.5 ppm;
FTIR (KBr): n˜ =2930, 285, 235, 1705, 1527, 1466, 1423, 1368, 1287,
1205, 1054, 1103, 864, 788, 751, 710, 612 cmꢀ1; HRMS: m/z calcd
for 508.08; found: 509.9 [M++H].
1040, 864, 700 cmꢀ1
.
3-{4-[(2-Ethylhexyl)oxy]-3,5-diiodophenyl}propanoic acid (IIIa):
Compound 8 (2.0 g, 3.7 mmol) was dissolved in methanol (20 mL)
and NaOH (0.22 g, 5.5 mmol) in water was added to the mixture.
The reaction mixture was heated at reflux for 12 h and then
poured into water. The product was obtained by neutralisation
with concentrated HCl and then washed with water until the pH of
3-[4-(Carboxymethoxy)-3,5-diiodophenyl]propanoic acid (IV):
Compound 7 (1.5 g, 2.9 mmol) was dissolved in methanol (20 mL)
and NaOH (0.35 g, 8.7 mmol) in water was added to the mixture.
The rest of the procedure was the same as that described for IIIa.
1
Yield=1.31 g, 95%; H NMR (400 MHz, CD3OD): d=7.73 (s, 2H; Ar-
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H), 4.54 (s, 2H; Ar-OCH2), 2.80 (t, 2H; Ar-CH2CH2), 2.57 ppm (t, 2H;
-CH2CH2-COOCH3); 13C NMR (100 MHz, CD3OD): d=174.8, 170.0,
155.0, 141.9, 139.9, 139.7, 89.6, 89.5, 68.1, 59.8, 34.8, 28.7 ppm;
FTIR (KBr): n˜ =2936, 2369, 1708, 1537, 1450, 1301, 1233, 1062, 948,
863, 806, 704, 667, 612, 574 cmꢀ1; HRMS: m/z calcd for 476.00;
found: 498.85 [M++Na+].
Eu3+ complexes: The compound (IIIa, IIIb, IIIc, IV, IIa or Ia; 3 equiv)
was dissolved in ethanol and NaOH (3 equiv) in water was added
to the solution. The whole mixture was stirred well until the solu-
tion became homogenous. Eu(NO3)3·6H2O (1 equiv) in ethanol
(1 mL) was slowly added to the mixture, precipitation took place
and stirring was continued for 12 h. The precipitate was filtered
and washed with excess water and ethanol. The white coloured
complexes were dried under vacuum.
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EuꢀIa: Yield=0.23 g, 34%; FTIR (KBr): n˜ =3390, 2960, 2920, 1540,
1510, 1430, 1230, 1180, 1100, 957, 827, 696, 532 cmꢀ1
.
EuꢀIIa: Yield=0.29 g, 33%; FTIR (KBr): n˜ =3410, 2960, 2920, 2860,
2380, 1540, 1440, 1380, 1250, 1160, 987, 862, 752 cmꢀ1
.
EuꢀIIIa: Yield=0.25 g, 74%; FTIR (KBr): n˜ =3744, 2918, 2858, 2348,
1534, 1429, 1240, 955, 860, 767, 697, 595 cmꢀ1; elemental analysis
calcd (%) for C52H76I6O11Eu: C 34.50, H 4.14; found: C 34.59, H 4.05.
EuꢀIIIb: Yield=0.38 g, 43%; FTIR (KBr): n˜ =3410, 2930, 2850, 2380,
1540, 1430, 1250, 1160, 987, 864, 710, 602, 540 cmꢀ1; elemental
analysis calcd (%) for C32H35I6O11Eu: C 25.47, H 2.34; found: C 25.22,
H 2.67.
EuꢀIIIc: Yield=0.63 g, 36%; FTIR (KBr): n˜ =2919, 2847, 233, 154,
1534, 1464, 1420, 1244, 1144, 999, 959, 874, 754, 713, 604 cmꢀ1; el-
emental analysis calcd (%) for C47H42I6O11N3Eu: C 32.44, H 2.55, N
2.41; found: C 32.19, H 2.20, N 2.30.
EuꢀIV: Yield=0.13 g, 52%; FTIR (KBr): n˜ =3861, 2346, 1539, 1457,
1423, 1332, 1237, 1026, 962, 870, 818, 752, 707, 585 cmꢀ1
.
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X-ray crystallography
CCDC 911330, 911331 and 911332 contain the supplementary crys-
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charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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Acknowledgements
Research grants from the Department of Science and Technology
(DST), New Delhi, India, under nano-mission initiative projects
SR/NM/NS-42/2009 and SR/FT/CS-014/2008 (R.B.) are acknowl-
edged. The research described in this manuscript was entirely
performed at the IISER-Pune, and A.B. thanks the Director of
IISER-Pune for extending the research facilities for his Ph.D. thesis
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Keywords: energy transfer · lanthanides · luminescence ·
metal–organic frameworks · photochemistry
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Received: April 4, 2013
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Published online on June 11, 2013
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ChemPlusChem 2013, 78, 737 – 745 745