Energy transfer at the zeolite l boundaries: Towards photo- and electroresponsive materials

Article abstract of DOI:10.1002/cplu.201300272

A series of iridium(III) compounds have been used as stopper molecules at the pore openings of zeolite L and act as effective donor units for transferring excitation energy to dye molecules entrapped within the zeolite channels. The synthesis and photophysical characterization of the new iridium(III) complexes are described, along with Fcrster resonance energy-transfer experiments. Transfer efficiencies for the studied systems are discussed on the basis of the role played by the localization of the donor excited state and the acceptor distribution in the channels. Because iridium(III) complexes can also be electrically excited, the electroluminescent behavior of donor-acceptor zeolite systems can be explored, by embedding them into a polymeric active layer and constructing light-emitting devices (LEDs). Novel hybrid LEDs can be fabricated with emission from the dyes entrapped into the zeolites and sensitized by the electro-responsive iridium(III) complex.

Full text of DOI:10.1002/cplu.201300272

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DOI: 10.1002/cplu.201300272
Energy Transfer at the Zeolite L Boundaries: Towards
Photo- and Electroresponsive Materials
Fabio Cucinotta,[a] Aurꢀlie Guenet,[a] Claudia Bizzarri,[a] Wojciech Mrꢁz,[c] Chiara Botta,[c]
BegoÇa Miliꢂn-Medina,[d] Johannes Gierschner,*[d] and Luisa De Cola*[a, b]
A series of iridium(III) compounds have been used as stopper
molecules at the pore openings of zeolite L and act as effective
donor units for transferring excitation energy to dye molecules
entrapped within the zeolite channels. The synthesis and pho-
tophysical characterization of the new iridium(III) complexes
are described, along with Fçrster resonance energy-transfer ex-
periments. Transfer efficiencies for the studied systems are dis-
cussed on the basis of the role played by the localization of
the donor excited state and the acceptor distribution in the
channels. Because iridium(III) complexes can also be electrically
excited, the electroluminescent behavior of donor–acceptor
zeolite systems can be explored, by embedding them into
a polymeric active layer and constructing light-emitting devi-
ces (LEDs). Novel hybrid LEDs can be fabricated with emission
from the dyes entrapped into the zeolites and sensitized by
the electro-responsive iridium(III) complex.
Introduction
The development of efficient assemblies made of organized
multichromophoric units might lead to the construction of
new artificial photonic antenna that are able to mimic the pho-
tosynthetic systems of the green plants, in which the excita-
tion energy is all delivered to a common acceptor compo-
nent.[1–6] In this regard, a wide variety of supramolecular sys-
tems have been designed and studied, including luminescent
metal complexes,[7] dyes in polymer matrices, and dendrim-
ers.[5,8] In most of them, the chromophoric units are organized
in a very primitive level and, although molecularly dispersed,
still show drawbacks related to photostability and aggregation
phenomena that might quench the luminescence.
from the harvesting units to the acceptors. In this regard,
channel-forming microporous host–guest compounds display
unique properties due their unidimensional character.[9,10] In
particular, zeolite L based materials were intensively investigat-
ed in the past, owing to the versatility and stability of the inor-
ganic host system.[11] Photoactive dye molecules can be entrap-
ped within the channels of zeolite L and, according to their
size, are mostly present as monomers, although aggregation
might occur under particular conditions that can partially
quench the fluorescence.[6,11,12] The resulting host–guest sys-
tems exhibit high internal ordering, owing to the strict orienta-
tion of the entrapped molecules in the narrow channels. This
leads to new functionalities, such as optical anisotropy and ef-
ficient unidirectional Fçrster resonance energy transfer
(FRET).[11,13,14] Several photonic antenna materials have been re-
ported that contain up to three different kinds of dyes.[15] Re-
cently, we studied a new FRET antenna, for which emission
from the inserted dye molecules was obtained by photoexcita-
tion of quantum dots decorating the whole zeolite L surface.[16]
We have also shown that energy transfer occurs from excited
polymer molecules inserted into a mesoporous material (SBA-
15) to cyanine dyes covalently linked to the surface of the
silica; moreover, we demonstrated that mesoporous silica,
upon appropriate synthetic modification, could be effectively
loaded with iridium complexes to give optical materials with
minimized molecular aggregation and excellent quantum
yields.[17] Such systems can, in turn, be assembled on a larger
scale onto conductive substrates to create ordered architec-
tures that exhibit patterns and perform specific functions.[18]
Furthermore, to interface the guests to the outer environment,
stopper molecules, that is, molecules that are able to enter
partly into the channels, are found to be a powerful tool, espe-
cially for injecting excitation energy into the channels or, alter-
natively, collecting it from inserted dye molecules.[19–21]
To circumvent these problems, a valid approach consists of
organizing the photoactive components in a hierarchical fash-
ion, by means of a host able to confer stability to the system,
as well as to address the vectorial transfer of excitation energy
[a] Dr. F. Cucinotta, Dr. A. Guenet, Dr. C. Bizzarri, Prof. L. De Cola
Physikalisches Institut
Westfꢀlische Wilhelms-Universitꢀt Mꢁnster
48149 Mꢁnster (Germany)
[b] Prof. L. De Cola
Current address:
Institut de Science et d’Ingꢂnierie Supramolꢂculaires (ISIS)
Universitꢂ de Strasbourg, 8 allꢂe Gaspard Monge
67083 Strasbourg (France)
E-mail: decola@unistra.fr
[c] W. Mrꢃz, Dr. C. Botta
Istituto per lo Studio delle Macromolecole, CNR
via Bassini 15, 20133 Milano (Italy)
[d] Dr. B. Miliꢄn-Medina, Dr. J. Gierschner
Madrid Institute for Advanced Studies
IMDEA Nanoscience, Calle Faraday 9
Campus de Cantoblanco, 28049 Madrid (Spain)
E-mail: johannes.gierschner@imdea.org
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201300272.
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In this regard, transition-metal complexes, particularly triplet
emitters, fulfill the requisites of effective closure molecules and
energy-transfer units. By using octahedral complexes, it is pos-
sible to design stopper systems made of a head moiety; the
coordinated metal ion, which is too big to enter the pore
openings of a zeolite channel; and a tail moiety, such as an ex-
tension of one of the coordinated ligands, which is narrow
enough to penetrate the channel. The use of transition-metal
complexes in photoinduced processes has proven that, when
properly designed, they display fast and efficient energy trans-
fer.[22–26] We have reported a luminescent ruthenium(II) com-
plex bound electrostatically to the channel entrances of zeoli-
te L, which acts as stopper energy donor,[27] whereas other au-
thors have demonstrated the use of a zinc phthalocyanine as
an acceptor unit.[9,28] So far, however, only photoexcitation has
been used to create the excited state of the donor molecule
despite the fact that the metal complexes can be electrically
excited. In particular, iridium(III) complexes with cyclometalat-
ed ligands have attracted much interest in recent decades
owing to their outstanding photophysical properties.[29,30] Be-
cause it is heavier than other d6 ions, the iridium(III) nucleus fa-
cilitates intersystem crossing more efficiently, leading to short
excited-state lifetimes and high emission quantum yields. The
sensitivity of the excited-state properties to the coordinated li-
gands allows the emission wavelengths to be tuned along the
whole visible spectrum.[31–33] Furthermore, because iridium(III)
complexes are electroluminescent, their use as stopper mole-
cules can lead to electrically addressable zeolite-based antenna
systems.[34–36] Together with high photostability, these advan-
tages render iridium(III) complexes suitable for applications
ranging from the design of supramolecular systems to the fab-
rication of optoelectronic devices.[37,38]
Scheme 1. Structures of the three iridium(III) complexes used as cationic
stoppers and their numbering; the highlighted area show the phenylpyri-
dine (ppy; blue), (1H-1,2,4-triazol-3-yl)pyridine (pta; red), and (1H-1,2-pyrazol-
5-yl)pyridine (ppa; orange) units.
moiety (1 and 2) or a ppa unit (highlighted with orange) with
a 3-methyl-1-propylimidazolium tail on position 3 of the pyra-
zole (3). To assist assembly, inside the negatively charged chan-
nels of the zeolites the tail is positively charged to favor inser-
tion by ion exchange in solution. Furthermore, compound 1 is
doubly charged because position 4 of the triazole moiety is
methylated.
To exploit the emissive properties of these complexes acting
as stopper molecules and to couple them with appropriate ac-
ceptor dye molecules, we designed a series of iridium(III) com-
pounds that possessed the two following prerequisites: 1) a
high-energy emitting excited state (i.e., emission in the blue or
green part of the electromagnetic spectrum) to act as an
energy donor with as many molecules as possible, and 2) a
“built-in” tail moiety able to penetrate the monodimensional
channels, leading to stable assemblies. Indeed the free cations
present in the channels of zeolite L, to balance the negatively
The performances of the presented stopper molecules as
energy-donor units in photoinduced processes with dye-
loaded zeolites have been investigated and are compared by
focusing on the role played by excited-state localization and
the donor–acceptor distance.
The isolation of the dyes inserted into zeolite crystals, and
the consequent decoupling of the charge recombination site
with the emission unit, represent a unique combination to
create a new generation of emitters for LEDs.[39] Electrochemi-
luminescence from a zeolite-encapsulated polymer has been
reported and coadsorption of hole and electron injection dop-
ants inside the zeolite pores has shown to increase device effi-
ciency.[40] However, to the best of our knowledge, hybrid light-
emitting diodes (LEDs) based on zeolites in which the emitting
species inside the zeolite pores is sensitized by a FRET process
have never been reported so far. Such a hybrid device is
driven as a conventional organic LED with the advantage that
degradation processes of the emitting species associated to
charge recombination could be avoided.[41] Herein, we demon-
strate that the principle works and electrical addressability of
the iridium(III)-based stoppers and the good efficiency of the
FRET process from the stopper to the dye into the zeolite
channels allow us to fabricate hybrid LEDs.
ꢀ
charged AlO4 units, can be partially replaced by the cationic
part of a tailed molecule.
Herein, we report on the design, synthesis, and characteriza-
tion of new positively charged iridium(III) complexes that can
be electrostatically plugged into the zeolite channels and act
as donor units for in-zeolite energy transfer.
Three heteroleptic iridium(III) complexes were synthesized
(see Scheme 1) and fully characterized. The compounds con-
tain two cyclometalated units, that is, ppy (highlighted with
blue in the scheme) ligands, which are either unsubstituted (2)
or substituted in positions 2 and 4 of the phenyl ring with fluo-
rine atoms (1 and 3), to shift the emitting excited state to
higher energy. The ancillary ligand is either a pta unit (high-
lighted with red) substituted in position 3 of the triazole with
a long conjugated tail terminated by a trimethylammonium
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Scheme 2. General procedure for the synthesis of the iridium(III) complexes 1, 2, and 3 (R=F for compounds 1 and 3, whereas R=H for 2). DMF=N,N-dime-
thylformamide.
in 72% yield. Compound 3 was then obtained in 42% yield by
nucleophilic substitution of the chloride atom 8 by 1-methyli-
midazole.[45]
Results and Discussion
Synthesis of the complexes
Compounds 1 and 2 were synthesized in three steps accord-
ing, to the synthetic route summarized in Scheme 2, starting
Photophysical properties in solution
from the respective dichloro-bridged iridium(III) dimers 4a and
The absorption spectra in the UV region of the three com-
4b. Compounds 4a, 4b, and 2-[5-(4-bromophenyl)-2H-1,2,4-tri-
pounds display a series of unstructured bands with maxima at
azol-3-yl]pyridine (5) were obtained by following reported pro-
cedures.[42] Coordination of 5 as an ancillary ligand to the iridi-
around l=270 and 350 nm (B and A bands, respectively, see
Figure 1), which can be assigned to intraligand p–p* transi-
um moiety was achieved by the reaction of 5 with 4a or 4b in
tions. Indeed, a general assignment of the observed bands can
a mixture of dichloromethane/ethanol at room temperatur-
e.[42b] Compounds 6a and 6b were thus obtained in 95 and
90% yield, respectively, and isolated by column chromatogra-
phy. The second step involves a Suzuki–Miyaura cross-coupling
reaction,[43] in which 6a or 6b is reacted with 4-(N,N-dimethyla-
minophenylboronic)pinacol ester to give 7a and 7b in 73 and
60% yield, respectively. Reaction of 7a with two equivalents of
trimethyloxonium tetrafluoroborate afforded compound 1, in
which both the terminal amino group and the triazole on posi-
tion 4 were methylated to yield a doubly charged compound.
When reacting 7b with only one equivalent of [(CH3)3O]BF4,
only the terminal amino group was quaternized and allowed
the formation of 2.
The synthesis of 3 was different because the introduction of
a positive charge on the tail implied a quaternization reaction
on the nitrogen atom of the methylimidazole involved in nu-
cleophilic substitution. Compound 8 was synthesized by chlori-
Figure 1. Absorption (black, left) and emission (normalized, blue, right) spec-
nation of (3-hydroxypropyl)-1H-1,2-pyrazol-5-yl)pyridine with
SOCl2 in CH2Cl2.[44] Reaction of 4a with 8 afforded compound 9
tra of 1 (c), 2 (a), and 3 (g) in deaerated CH3CN at 298 K. For the
emission spectra, lexc =380 nm.
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Figure 2. Calculated energies and topologies of relevant MOs for compounds 1–3 in acetonitrile.
be made on the basis of reported literature data for other phe-
nylpyridine iridium(III) complexes:[25,46–49] the highest energy
bands are ascribed mainly to the phenylpyridine ligands,
whereas the bands at l=350 nm correspond to transitions
that are most likely centered on the ancillary ligands. For com-
plex 2 the A band is particularly intense and this can be due to
electronic delocalization from the phenyl-substituted triazole
up to the quaternized amino group. Such delocalization
cannot occur for compound 1 because the presence of the
charge on the triazole traps the electronic density. At low
energy, in the region l=350–420 nm, we expect to see the
metal-to-ligand charge-transfer (MLCT) bands, which however
are masked by the tail of the intense UV absorptions. For
urations, built up from lower (higher) lying (un)occupied MOs
(see the Supporting Information). The topology of the frontier
orbitals is represented in Figure 2.
The majority of the frontier molecular orbitals (MOs) show
contributions from iridium d orbitals; thus the general descrip-
tion of the first set of singlet transitions that constitute the A
band includes mixtures of 1MLCT and ligand-centered (1LC)
transitions, which are comprised of both intra- (1LL) and interli-
gand charge-transfer (1LL’CT) excitations. The strong contribu-
!
tions of ligand-based p* p excitations give rise to rather
large molar extinction coefficients (em) of the A band; an esti-
mation of em from the sum of the calculated oscillator
strengths, ftot, in the spectral region of A (see the Supporting
Information) is obtained from Equation (1):
3
iridium(III) complexes, even the MLCT can be observed, owing
to the strong spin-orbit coupling (SOC) promoted by the
heavy atom.[34,50]
Z
f ¼ 4:319 ꢁ 10ꢀ9½m cm2ꢂ emdv ꢃ 4:319 ꢁ 10ꢀ9½m cm2ꢂ ꢁ emDv
Quantum-chemical calculations, by means of DFT, performed
on the charged compounds, embedded in a solvent (acetoni-
trile), confirm the above-mentioned assignments and prove
further insights on the complex nature of the manifold excited
states. The A band, centered at about l=350 nm, is composed
of a set of around nine singlet transitions (see also the Sup-
ð1Þ
in which Dn is the half-width of the band (ꢃ4000 cmꢀ1) and
gives roughly em,tot =1ꢄ104 mꢀ1 cmꢀ1 for
1 and 3 (4ꢄ
!
porting Information). Although the S1 S0 transition is mainly
104 mꢀ1 cmꢀ1 for 2), which is in reasonable agreement with ex-
perimental results. The larger em value of 2 is a direct impact
of the extended conjugation in the pta-bp (bp=biphenyl)
described by an excitation from the HOMO to the LUMO, the
higher states require a complex description of different config-
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Table 1. Lowest singlet and triplet states in compounds 1–3 calculated by time-dependent (TD) DFT in acetonitrile.
Compound
State
Energy [eV]
Transition type
Main CI contributions[a]
Orbital description[b]
Oscillator strength
[>20%] ([%])
[c]
1
T1
T2
2.716
2.872
3MLCT/3LL’CT
H!L (85)
pppy, pppy’, dIr!p*pta
–
–
–
[c]
3MLCT/3LL/3LL’CT
H!L+2 (50)
H-1!L+3 (22)
H!L (96%)
H!L+2 (87)
pppy, pppy’, dIr!p*ppy, p*pta
pppy, pppy’, dIr!p*ppy, p*ppy’
pppy, pppy’, dIr!p*pta
[c]
S1
S2
2.767
3.326
1MLCT/1LL’CT
1MLCT/1LL’CT
0.019
0.054
[c]
pppy, pppy’, dIr!p*pta
2
3
T1
T2
S1
2.708
2.709
3.031
3MLCT/3LL
H-1!L (65)
pptaꢀbp, dIr!p*pta-bp
,
–
–
3MLCT/3LL’CT/3LL
1MLCT/1LL’CT
H!L+1 (59)
H!L+1 (86)
pppy, pppy’, dIr!p*ppy, p*ppy’
ppta-bp, dIr!p*ppy
0.046
T1
T2
S1
2.843
2.875
3.171
3MLCT/3LL’CT/3LL
3MLCT/3LL’CT
1MLCT/1LL’CT
H-1!L (27)
H!L (25)
pppa, pppy, pppy’, dIr!p*ppy
pppa, ppta, dIr!p*ppy
pppa, dIr!p*ppy’
–
–
–
–
H!L+1 (29)
H-3!L (27)
H!L (82)
pppa, dIr!p*ppy
pppa, dIr!p*ppy
0.021
[a] CI=Configuration Interaction. [b] See also Figure 2. [c] According to experimental results, T1 (S1) should be higher in energy, above T2 (S2).
ligand, which gives rise to MOs centered in the pta-bp ligand
of widely delocalized character; this especially accounts for
HOMO-1, LUMO, and LUMO+3, see Figure 2 and the Support-
ing Information. The involvement of these MOs in the low-
lying singlet transitions not only results in a larger apparent os-
cillator strength, but also extends the A band to the red
(Figure 1); accordingly the S1 state of 2 is calculated to be
3.03 eV (404 nm; see Table 1), which is in reasonable agree-
ment with the experimental results.
the CT band, but not sufficiently to place it above the localized
S2 state of 1 at 3.33 eV (which according to experiment should
represent the lowest excited state); further evidence for this
assignment is given below.
The three iridium(III) complexes show bright photolumines-
cence (PL) at room temperature in deaerated acetonitrile, as
shown in Figure 1. The emission spectra have maxima at l=
453 and 455 nm for 1 and 3 respectively, whereas the emission
of 2 is redshifted by ꢀ0.15 eV to l=484 nm; the redshift is
well reproduced by theoretical calculations (ꢀ0.14 eV, see
Table 1). As in related iridium(III) complexes,[49] the PL is as-
signed to a phosphorescence process with decay times of
some ms under deaerated conditions (Table 2). Insight into the
character of the emitting state arises from the visible vibronic
structure of the compounds studied; this implies a strong
ligand-centered character of the emission. Indeed, in many re-
lated complexes, the lowest excited state is due to a strong
Although the same bp moiety is present in 1, here methyl
substitution in the pta unit leads to electronic decoupling, that
is, no common delocalized MO is formed (Figure 2). Instead,
the spectral features of 1 are much like those of 3 (see
Figure 1), owing to compensation of the hypsochromic shift in-
duced by the fluorine substituents in the ppy units of 1,[46a]
and the bathochromic shift as a result of the extended conju-
gation of the pta-bp ligand in 1 relative to the ppa ligand in 3.
Although the DFT calculations deal reasonably well with com-
pounds 2 and 3, the results for 1 are incorrect: the S1 state is
calculated to be 2.77 eV (448 nm) in acetonitrile; thus much
below the S1 of 3 (3.17 eV; 390 nm). The failure is due to the
3
mixing of the 3MLCT with the lowest LC state. We have report-
ed neutral and charged iridium(III) complexes possessing a tria-
zole as ancillary ligand and shown that, in most cases, the tria-
zole is not involved in the lowest excited state.[51,52] The only
exception is indeed when the triazole has been substituted
with a diphenyl group because, in this case, the lowest excited
state is localized on the ancillary ligand owing to extended de-
!
strong charge-transfer (CT) character of the S1 S0 transition in
1, which involves the delocalized positive charge in the pta
unit of 1. Inclusion of the solvent indeed strongly blueshifts
Table 2. Photophysical data for complexes 1, 2, and 3 at room and low temperature in solution.[a]
[c]
Complex
Absorbance [nm]
([103 mꢀ1 cmꢀ1])
Emission
298 K [nm]
F[b] aerated
t aerated
[ns] ([%])
F[b] deaerated
t deaerated
[ms] ([%])
kTS deaerated
Emission
77 K [nm]
t 77 K [ms]
(ꢄ104 sꢀ1
)
1
2
3
268 (58.2)
355 (6.2)
256 (49.2)
333 (34.4)
248 (45.1)
362 (5.2)
453
483
484
507
455
484
0.04
–
0.05
–
0.02
–
425
–
67 (40)
36 (60)
173
–
0.17
–
0.23
–
0.55
–
1.98
–
19.55 (70)
2.44 (30)
3.75
8.6
–
9.4
–
15
–
447
–
476
–
448
–
221.3
–
238.4
–
4.61
–
–
[a] All data for complexes were obtained in CH3CN at 298 K and butyronitrile matrix at 77 K. [b] Quantum yield values were calculated with the relative
method, with quinine sulfate in 1n H2SO4 as a reference, and also measured with an integrating sphere. [c] From Equation (2), with hISC =1, inserting the
values obtained at 298 K in deaerated solution. For 2, the shorter component of t was used for the calculation.
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localization promoted by the conjugated units.[52] In our cases,
however, also for complex 1, we could expect a great extent of
participation of the triazole moiety owing to the strong elec-
tron-withdrawing nature of the terminal group. The observed
crease in either the efficiency for intersystem crossing, hISC, or
the radiative rate, kTS. Because hISC is expected to be close to
unity in iridium complexes, owing to the high ISC rate towards
appropriate Tn states within the triplet manifold,[56] the change
upon cooling has to be attributed to a decrease in kTS, which is
in the order of 105 sꢀ1 at room temperature (Table 2).
!
photophysical behavior is supported by the calculated T1 S0
transition for 2 and 3. In 2, the transition is mainly described
by a HOMO-1!LUMO excitation, in which both MOs show
main contributions on the pta-bp ligands (Table 1), giving rise
to an intraligand transition (3LL), and thus, structured emission.
An intraligand transition is also observed for 3, here located
mainly in the ppy ligand, see Table 1. For 1, the structured
emission spectrum also suggests the intraligand character of
T1, which is not found in the calculation (Table 1), describing T1
as an interligand (3LL’CT) and T2 as an intraligand transition.
We thus suggest that T2 is the actual lowest triplet state. Clear-
An explanation could be excited-state switching upon cool-
ing, so that the nature of the emitting state is different. Alter-
natively, the reason for a decrease in kTS upon cooling could be
related to a reduced coupling of the T1 state to the ground
state S0. This can effectively occur because it depends on the
SOC of T1, which is indeed low, as estimated from a comparison
with the expected maximum value for SOC calculated from the
Strickler–Berg equation.[57–59]
3
Such an observation corroborates the LC character of the
3
ly, stabilization of the intraligand LL transition in the doubly
emission and the role played by the substitution of the triazole
moiety with the diphenyl tail. As mentioned above, the lowest
charged complex 1 by the polar solvent (acetonitrile) is not
sufficiently described by quantum-chemical calculations em-
ploying the (PCM) model.
3
emitting state has a mixed MLCT–3LC character (with a strong
contribution of the latter), which is also clearly visible at room
Notably, the emission of 2 appears to be somehow less
structured. At the same time, the PL lifetime is biexponential
(Table 2) and very different to the value of 1 and 3, in particu-
lar, for the presence of a very long lived component. We as-
cribe this effect to the thermal population of close-lying suble-
vels in the triplet manifold.[53,54] Indeed, according to the TD-
DFT calculations, T1 and T2 of 2 are very close in energy and
have different CT character, see Table 1. The LC state, which is
dominating, is most likely not in complete thermal equilibrium
with the CT state and this leads to a biexponential decay with
a very long excited-state lifetime, which is typical of a triplet
LC level, and a somewhat shorter decay that is representative
of the equilibration rate. At 77 K in a rigid matrix, the PL decay
of 2 becomes monoexponential and substantial line narrowing
is observed (see the Supporting Information), indicating that T2
is not thermally populated at 77 K, because it is shifted to
higher energy, owing to the CT nature of the state and there-
fore the lack of solvent stabilization in the rigid matrix.
temperature for complex 2. Upon cooling, mixing cannot
occur because the MLCT state will move to higher energy as
3
stabilization from the polar solvent (CH3CN) is lost in the rigid
butyronitrile glass. Consequently, the SOC significantly decreas-
es. Furthermore, at 77 K, the lack of rotational freedom of the
phenylene units reduces the radiationless deactivation chan-
nels, resulting in a rigid, fully conjugated ligand. Therefore, as
expected, the triplet state of this ligand has a long excited-
state lifetime.
The situation is different for complex 3, the excited state of
which is localized on the phenyl pyridine and decays as a classi-
cal iridium(III) complex with an excited-state lifetime in the
order of a few microseconds at low temperature.
Spectroscopic properties of the complexes at the channel
entrances of zeolite L
As mentioned above, crystals of zeolite L are very good host
systems primarily because of their channel-like structure,
which allows the insertion of small organic dyes and closing of
the channel entrances with so-called stopper molecules. More-
over, a well-established synthetic route permits the preparation
of crystals with different morphologies and sizes ranging from
30 nm to 10 mm.
Surprisingly, upon cooling to 77 K, the luminescence decay
time, t, of 1 and 2 strongly increases and reaches about
200 ms, which is rarely observed and only for complexes bear-
ing ligands with extended conjugation lengths.[43,55] The in-
creased excited-state lifetimes (nearly 100 times longer from
298 to 77 K) cannot be explained only by a concomitant in-
crease of the luminescence quantum yield, F, at low tempera-
ture, simply because such an increase is not possible (F values
for 1 and 2 at 298 K are already around 20% in deaerated con-
ditions; see Table 2). To understand the reason for the strong
increase in t, we might take a look at the definition of emis-
sion quantum yield for phosphorescent compounds, FPh, as
given by Equation (2):
For our experiments, we have used two types of zeolite L
crystals: small crystals with an average size of 50 nm were em-
ployed for the photophysical characterization, whereas cylindri-
cal crystals with a diameter of 1.5 mm and 3.5 mm in length
were suitable for fluorescence microscopy characterization.
Both types of material were synthesized according to a previ-
ously described procedure.[68]
Upon the addition of solutions of our complexes in acetoni-
trile to suspensions of zeolite L in toluene (see the Experimen-
tal Section for details), through an ion-exchange procedure,
the long-tailed positively charged ligands, pta-bp and ppa-pro-
pylimidazolium, coordinated to the iridium ion, are expected
to enter the channels and exchange with the potassium cat-
ions. The emission maxima and the vibronic structures for
FPh ¼ hISCkTStPh
ð2Þ
in which hISC is the S1!Tn intersystem crossing rate, and kTS is
the radiative rate constant for phosphorescence. Accordingly,
the increase in tPh must be mainly caused by a substantial de-
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compounds 1 and 3 (denoted herein as 1-Z and 3-Z, respec-
tively) do not change with respect to 1 and 3 in solution,
whereas those of 2-Z are somewhat blueshifted (see Figure 3,
Figure 4, and Figure 5).
However, the excited-state lifetime in zeolite L for com-
pounds 1-Z and 2-Z are clearly multiexponential and as sus-
pensions in aerated toluene the excited-state lifetimes of the
assemblies are much longer than those of the corresponding
free complexes in solution. This behavior can be easily ascribed
to the fact that the lowest excited state, which is localized on
the ligand now entrapped into the channels of the zeolites, is
less sensitive to dioxygen.[27] Indeed the excited-state lifetimes
in deareated solutions are similar to those of the free com-
plexes under the same conditions. The short decay component
can either be due to some iridium(III) complexes adsorbed on
the surface of the zeolites or to triplet–triplet annihilation of
close-by chromophores. Confocal microscopy analysis does not
reveal adsorption of the stoppers on the surface because no
emission is observed on the walls of the zeolites, but only at
the channel entrances. On the other hand, the close packing
of the complexes, which are crowded at the channel entrances,
could be responsible for the self-quenching of the iridium
emission and also for the broadening of the emission pattern,
which reflects the inhomogeneous distribution of the mole-
Figure 4. Top: Schematic representation and fluorescence microscopy pic-
tures of 3 mm long zeolite crystals, showing stopper 2 at the channel entran-
ces; a PyY-loaded zeolite; and a crystal loaded with both donor 2 and ac-
ceptor PyY (excitation filter: lexc =370–390 nm; scale bar=1 mm); the bor-
ders of the crystal are indicated along with the positions of the maximum
dye distribution and the inflection point. Bottom: Emission spectra of 2-PyY-
Z (black), 2-Z (green), and PyY-Z (red), recorded at lexc =360 nm as suspen-
sions in deaerated toluene at 298 K; the absorption spectrum of PyY-Z (dark
yellow) is also reported to show the overlap with the 2-Z emission.
Figure 5. Emission spectra of 3-PyY-Z (black), 3-Z (blue), and PyY-Z (red), re-
corded at lexc =360 nm as suspensions in deaerated toluene at 298 K; the
absorption spectrum of PyY-Z (dark yellow) is also reported to show the
overlap with the 3-Z emission.
Figure 3. Top: Schematic representation and fluorescence microscopy
images of 3 mm long zeolite crystals, showing stopper 1 at the channel en-
trances; a PyY-loaded zeolite; and a crystal loaded with both donor 1 and
acceptor PyY (excitation filter: lexc =370–390 nm; scale bar=1 mm); the bor-
ders of the crystal are indicated along with the positions of the maximum
dye distribution and the inflection point. Bottom: Emission spectra of 1-PyY-
Z (black), 1-Z (blue), and PyY-Z (red), recorded at lexc =360 nm as suspen-
sions in deaerated toluene at 298 K; the absorption spectrum of PyY-Z (dark
yellow) is also reported to show the overlap with the 1-Z emission.
cules. Another hypothesis, to explain a short excited-state life-
time component, is related to the different degree of insertion
of the tail for the terminally charged pta-bp. The fully penetrat-
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ing compounds will indeed give only the long excited-state
lifetimes, while the partially inserted complexes can still be
quenched by dioxygen and result in a short luminescence life-
time. This picture also fits for compound 3-Z, for which the T1
state arises from an intraligand transition located on the ppy
moiety. This state is thus located close to the metal center and
outside the zeolite, whereas the propylimidazolium tail enters
the channels, and therefore, is insensitive to the formation of
the assembly. Consequently, the decay is monoexponential
and similar to that in solution both in terms of lifetime and
sensitivity against oxygen, which is remarkably different to
compounds 1 and 2 (see Table 3).[60]
the dyes inside the crystal, PyY-Z; and 3) the energy-transfer
process in the 1-PyY-Z system. By comparison of the emission
profile of 1-Z with 1-PyY-Z at the same excitation wavelength
(l=360 nm), it is clear that quenching is observed for the iridi-
um component. Furthermore, when comparing the emission
of PyY-Z excited at l=360 nm with the emission in the assem-
bled system, 1-PyY-Z, a sensitization is observed. Similar results
were obtained for complex 2 (see Figure 4). In both systems,
we observe almost complete quenching of the donor emission,
along with a strong lifetime reduction and concomitant in-
crease in the emission intensity for the PyY dye, which is sensi-
tized by efficient excitation energy transfer. The efficiency of
the process, hET, can be obtained from experimental results by
using Equation (3):
Photoinduced energy-transfer processes
tDA
hET ¼ 1 ꢀ
ð3Þ
Complexes 1–3, which act as stopper molecules for the chan-
nels, were used as energy donors in photoinduced energy-
transfer processes. In particular, upon light excitation of the
complexes, electronic energy transfer occurs to the dye-loaded
zeolite L. To investigate the role played by the localization of
the excited states of the stopper and the corresponding
donor–acceptor distances, we have performed time- and spa-
tially resolved emission spectroscopy. As a suitable acceptor
unit, we chose an orange–red emitting molecule, the phenoxa-
zine dye pyronine-Y (PyY), which is strongly fluorescent (quan-
tum yield of ca. 0.9 in water) and can be inserted into zeolite L
by ion exchange. The photophysical properties of PyY inside
the zeolites are very similar to those in solution and, in particu-
lar, the absorption spectrum displays a good overlap with the
emission spectra of all complexes (Figures 3–5, yellow absorp-
tion line and overlap shown in grey), which is a fundamental
prerequisite for observing Fçrster-type energy transfer.[61]
Energy-transfer experiments were then performed, after estab-
lishing that selective excitation of the stopper molecules could
be obtained at l=360 nm, because almost negligible absorp-
tion of the dye was observed at this wavelength (see Fig-
ures 3–5).
tD
in which tDA and tD are, respectively, the quenched and the un-
quenched donor excited-state lifetimes (Table 3). The rate con-
stant for the transfer kET is accordingly calculated from Equa-
tion (4):
1
tDA tD
1
ð4Þ
kET
¼
ꢀ
By using averaged lifetimes under ambient conditions, the
transfer efficiency is about 70% for both compounds 1 and 2.
Different photophysical behavior was obtained for com-
plex 3, although the phenoxazine dye distribution in the chan-
nels was very similar for all systems. As already mentioned, the
excitation of complex 3-Z leads to the population of the excit-
ed state of the complex, which however is localized on the li-
gands placed outside the channels. Therefore, despite the
good spectral overlap, for such a complex, the distance be-
tween the donor and acceptor units is much larger. We indeed
observe a much lower quenching and, as a consequence,
weaker sensitization of the dyes inside the zeolites (see
Figure 5). The quenched emission is also mirrored in the life-
time values obtained from the assembled systems (Table 3)
that give us a transfer efficiency of 32% in air, which is much
smaller than that in systems containing stoppers 1 and 2.
Energy-transfer experiments were performed for all com-
plexes and Figure 3 shows the results for complex 1: 1) fluores-
cence microscopy images of the zeolite when complex 1 is
used as a stopper, 1-Z, demonstrating that indeed the complex
is mainly localized at the channel entrance; 2) the emission of
Table 3. Lifetime values, tDA,i (relative contributions xi), for the studied donor–acceptor systems (1-PyY-Z, 2-PyY-Z, and 3-PyY-Z), compared with the life-
times, tD,i, of the respective donor zeolite systems, along with PL quantum yields, FPL, of the donor systems and resulting FRET parameters (all measure-
ments at room temperature as a suspension in toluene).
[b]
[c]
[d]
[e]
System
t aerated [ns] ([%])
t deaerated [ns] ([%])
FPL[a] aerated
hET
kET
JDA
R0 [ꢅ]
RDA [ꢅ]
[ꢄ105 sꢀ1
]
[ꢄ10ꢀ10 cm6 molꢀ1
]
1-Z
1-PyY-Z
2-Z
2-PyY-Z
3-Z
3-PyY-Z
3140 (60), 585 (40)
962 (54), 250 (46)
1560 (36), 229 (64)
468 (27), 105 (73)
291
3230 (51), 808 (49)
1110 (54), 307 (46)
1630 (23), 348 (77)
515 (23), 114 (77)
1270
0.06
–
0.07
–
0.05
–
–
0.70
–
0.71
–
0.32
–
0.30
–
1.06
–
1.06
–
–
33
–
36
–
32
–
29
–
31
–
36
1.095
–
1.450
–
198
536
1.095
[a] Measured (integrating sphere). [b] Calculated from Equation (3) by using the value of t aerated. [c] Calculated from Equation (4). [d] Calculated from
Equation (6). [e] Calculated from Equation (5).
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To obtain a deeper understanding of the large differences in
the energy-transfer efficiencies, we calculated some of the pa-
rameters, for example, the donor–acceptor distance, RDA, that
govern the process by assuming a simple resonant transfer
mechanism based on point dipoles, as described by Fçrster in
his original formulation.[61] As we will see later, this approxima-
tion, although not generally valid in nanochannel-forming
host–guest systems,[62] is well justified in the present case with
not too close donor–acceptor separations. Also, examples of
of m for the ligand tail is assumed to be parallel to the channel
axis, owing to the localization of the emitting state in the tail.
Then the orientation factor is calculated to be k2 =sin2 q=0.90.
Once the Fçrster radius is obtained from Equation (6) and in-
serted into Equation (5), along with the measured hET values,
we obtain an average donor–acceptor distance, RDA, of 29 and
31 ꢅ for compounds 1 and 2, respectively, whereas a signifi-
cantly higher value (36 ꢅ) is found for 3 and reflects the much
lower energy-transfer efficiency of the latter. The differences in
the RDA values among the compounds can indeed be rational-
ized by the location of the emitting states. For 1 and 2, the ex-
cited state is closer to the acceptor, inside the channel, where-
as for compound 3 the excited state is on the ligand far from
the channels. The increase of the DA distance by this amount
is thus in good agreement with the Fçrster estimation. On the
other hand, comparing 1 and 3, the center-to-center distance
between the bp fragment and the ppy unit is roughly 8–9 ꢅ,
which again compares quite well with the difference of the
Fçrster estimations for 1 and 3.
1
triplet–singlet energy transfer (3D*+1A! D+1A*), such as that
in our systems in which donor units are triplet emitters, can be
handled in the same theoretical framework as that for singlet–
singlet transfer.[63] Following Fçrster’s theory, the transfer effi-
ciency is a function of the quenching distance RDA of a given
donor–acceptor pair [Eq. (5)]:
1
hET
¼
R6DA
R06
ð5Þ
1 þ
The absolute values of RDA reflect the average donor–accept-
or distance, although this is dominated by the nearest DA pair,
owing to the strong distance dependence of Fçrster transfer
[see Eq. (5)]. However, in the closest possible location of A
with respect to D (see Figure 6), the quenching distance
should be clearly smaller than 20 ꢅ and transfer efficiencies,
hET, close to unity are expected. Thus, the high values of RDA
(and therefore the rather moderate efficiencies) suggest that
the channels are not completely filled with acceptor molecules
towards the channel entrances, so that the average nearest-
neighbor distance is significant larger than the geometrically
possible one. Indeed, a closer inspection of the fluorescence
microscopy images (Figures 3 and 4) indicate an inhomogene-
ous distribution of the PyY molecules, as also observed in
other studies.[11] A low concentration of the chromophores in
the center of the crystal is observed; this might be attributed
to blocking of the channels by the chromophores and inhibit-
ing a homogeneous distribution of the dyes for the entire
length of the crystal.[65] The chromophore distribution might
vary;[65] nevertheless, the overall shape of the distribution to-
in which, in the present case, RDA is expected to represent
an average value of all transfer events from the stopper mole-
cule (donor) to the dye molecules (acceptor) in the channels.
The Fçrster radius, R0 (at which hET =50%), is calculated from
Equation (6):[61a]
9000 ln 10 FDk2
128p5NA n4
R60 ¼
JDA
ð6Þ
in which FD is the quantum yield of the donor, n is the re-
fractive index of the environment, and JDA is the numeric inte-
gral of the spectral overlap. The last one is calculated by con-
sidering the normalized donor emission spectrum, fD(n), and
the acceptor absorption in units of molar extinction coeffi-
cients, eA(n), to give JDA =sfD(n)eA(n)nꢀ4dn, in which k2 is the ori-
entation factor and depends on the angle between the donor
(D) and the acceptor (A) electronic transition dipole moments
m. While the electronic dipole moment, m, of PyY is known to
form an angle of 728 against the channel axis when sitting in
the cavity center of zeolite L[64] (see Figure 6), the orientation
Figure 6. Schematic representation of a single zeolite L channel, containing compound 3 and PyY as a donor–acceptor pair, along with the size parameters of
a unit cell. The arrow represents the direction of the energy-transfer process.
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wards the end of the crystal can be sufficiently described by
a simple sigmoidal shape, p(x)=1/[1ꢀeꢀ(xꢀa)/s], with an inflec-
tion point at about a=500 nm and a broad distribution of
about s=75 nm (i.e., 100 unit cells); this can even be roughly
seen by eye from the fluorescence images in Figures 3 and 4.
In a first approximation, we assume multiple transfer, kET =
SkET,i, of the donor to individual acceptors, i, located in the cav-
ities only within the same channel. Because in Fçrster theory
kET,i ꢃRDA,iꢀ6, the average quenching distance is calculated from
the individual weighted RDA,i from Equation (7):
the final concentration of stopper 3 at the channel entrances
of zeolite L in the polymer blend is quite low in comparison
with conventional LED structures.
As references for the 3-PyY-Z system, we fabricated LEDs
containing the polymer matrix without zeolites, with zeolites
containing only the PyY acceptor molecules, and zeolites con-
taining only the iridium complex stopper 3. The recorded elec-
troluminescence spectrum of the LED containing the assembly
3-PyY-Z and LEDs made with all of the reference systems are
shown in Figure 7. It is interesting to note that, while pyro-
1
rffiffiffiffiffiffiffiffiffiffiffiffi
RDA
¼
P
pi
R6DA;i
ð7Þ
6
i
For compound 3, for which the triplet state is only located
at the channel entrances, we obtain RDA =34 ꢅ, which is
indeed very close to the estimation obtained from the Fçrster
equation (RDA =36 ꢅ; Table 3). Although this picture is not
quantitative, owing to our (necessary) approximations, it con-
firms the importance of the insertion of the ligand tail inside
the channel and the effect of inhomogeneous dye distribution
(i.e., the incomplete filling of the channels) on the efficiency of
the energy-transfer process.
Electroluminescent devices
Figure 7. Electroluminescence spectra of 3-PyY-Z in the polymer matrix
(black solid line), relative to those of the undoped matrix (green line), the
matrix doped with PyY-loaded zeolites (PyY-Z, red line), and the matrix
doped with 3-loaded zeolites (3-Z, blue line), recorded upon applying a volt-
age of 9 V. The PL spectrum of 3-PyY-Z is also reported (black dashed line)
for comparison with the corresponding EL profile.
Even though the photoinduced energy transfer is rather effi-
cient for the systems containing 1 and 2 as donors, the system
3-PyY-Z displays a broad PL that results from partial energy
transfer from the blue emissive iridium complex to the red flu-
orescent dye. Therefore an electrical excitation of the system
and, in particular, the population, by charge recombination, of
the donor (iridium complex), could lead, by energy transfer, to
an overall white emission. We decided to test this appealing
possibility by constructing a simple LED.
nine-Y inserted into the zeolite and the matrix alone show only
a weak band, at lꢃ480–500 nm, characteristic of the exciplex
emission of PBD and TPD molecules in the polymer matrix,[66]
reference complex 3 inserted into the zeolite displays a weak
electroluminescence emission similar to its PL, even though it
is partially masked by the matrix emission. In the 3-PyY-Z
system, the EL spectrum displays a significant change in the
features, for which the sharp peak at l=590 nm can be attrib-
uted to dye emission. Because, as shown for reference PyY-Z,
the dye cannot be excited electrically, the red emission record-
ed for 3-PyY-Z must be due to energy transfer from the (elec-
trically) populated donor to the entrapped dye.
The zeolite L nanocrystals, with an average size of 20 nm,
were loaded with PyY and stopper 3, and the obtained system
was dispersed into
a polymer matrix, bisphenol A–poly-
carbonate (PC), to obtain a homogeneous film that was sand-
wiched between a transparent anode, indium tin oxide (ITO),
and a metal layer as the cathode. The emissive layer consists
of a PC/2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole
(PBD)/N,N’-diphenyl-N,N’-bis(3-methyl)-1,1’-biphenyl-4,4’-dia-
mine (TPD) (10:25:65) polymer blend, whereby PBD and TPD
act, respectively, as electron and hole transporters. In the so-
fabricated device, the stopper molecule mediates exciton
transfer from the matrix to the acceptor dye molecules within
the channels of zeolite L. When taking into account the rather
low molar content of stopper molecules per mg of zeolite
from Equation (8) (see the Experimental Section), it is necessary
to use a high amount of zeolite crystals to promote efficient
exciton transfer. On the other hand, the zeolite concentration
should be low enough to ensure electrical conductivity, de-
spite the insulating nature of the inorganic host. A zeolite con-
tent corresponding to 30% in weight was found to be suitable
and was used in the polymer blend. At this weight fraction,
We are aware that the results are not comparable with any
electroluminescent efficient system, but they demonstrate
a proof of principle for hybrid LEDs based on singlet–triplet
energy transfer, in which the iridium(III) complexes play a key
role as an interface between the electrodes and the dye
loaded inside the zeolites. Considering that optimization of the
device architecture and performance is required, that is, by in-
creasing the donor ratio through decoration of the entire zeo-
lite surface,[67] we believe that the described approach could
offer an interesting inspiration for the use of hybrid organic/in-
organic systems for optoelectronic applications.
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Supporting Information) and the percentage of loading was deter-
mined to be about 9% from the HF test.[15]
Conclusion
The synthesis and characterization of three new cationic cyclo-
metalated iridium(III) complexes have been described. The
complexes have been used as electro- and photoactive com-
ponents to block the channels of zeolite L, after insertion of or-
ganic dyes. Applying a combination of spectroscopic and
quantum-chemical techniques, we demonstrated that the
emitting state, of the metal complexes 1 and 2, was localized
in the long tail of the charged ligand inserted inside the chan-
nel.
Intercalation of the cationic stoppers at the channel entran-
ces of zeolite L
The number of channel entrances in moles of the weighed amount
of zeolite was calculated from Equation (8):
Xz
lz
ne ¼ 5:21 ꢁ 10ꢀ7
ð8Þ
Taking advantage of such a design, we have therefore dem-
onstrated that a vectorial energy transfer, from the excited
stopper molecules to the dyes inserted inside the crystal, takes
place. The efficiency, up to 70%, of the triplet-to-singlet
energy transfer was directly related to the average donor–ac-
ceptor distance in the system, which was triggered by the ex-
tended delocalization of the excited state of the donor (inside
the channel) and the inhomogeneous distribution of the dye
molecules in the channels.
in which Xz is the mass of zeolite in mg and lz is the average
length of the crystals in nm.[6] The molar amount of iridium(III)
complexes 1, 2, and 3 was calculated so that all channel entrances
were occupied by one complex. The following procedure was fol-
lowed: a 10 mL suspension of the weighed zeolites in toluene was
immersed in an ultrasonic bath for 30 min, after which time the
corresponding molar amount of a solution of complex 1, 2, or 3 in
acetonitrile (1 mL) was added. The suspension was stirred for 12 h
at 508C, then centrifuged and washed with fresh toluene to
remove the unbound stopper molecules. The described procedure
was followed to intercalate the stopper molecules at the channel
entrances of both unfilled and dye-loaded zeolite L samples.
Finally, one of the studied FRET pairs was employed as an
active component of a hybrid LED. The blending of the assem-
bly, containing the zeolites with the donor–acceptor systems,
in a polymeric matrix allowed the study of an electrically in-
duced energy-transfer process. The electroluminescence of
such LEDs has shown the effectiveness of the iridium(III) stop-
pers in mediating exciton transfer to the dye molecules entrap-
ped in the zeolite channels.
UV/Vis spectroscopy
UV/Vis absorption spectra were recorded on a Varian Cary 5000
double-beam UV/Vis–NIR spectrophotometer; all spectra were
measured from quartz cuvettes (1 cm, Hellma). Steady-state emis-
sion spectra in the spectral range of 300–800 nm were recorded
on a HORIBA Jobin–Yvon IBH FL-322 Fluorolog 3 spectrometer
equipped with a 450 W xenon arc lamp, double grating excitation
and emission monochromators (2.1 nmmmꢀ1 dispersion;
1200 groovesmmꢀ1), and a Hamamatsu R928 photomultiplier tube
or a TBX-4-X single-photon-counting detector. Emission and excita-
tion spectra were corrected for source intensity (lamp and grating)
and emission spectral response (detector and grating) by standard
correction curves. Time-resolved measurements up to approxi-
mately 5 ms were performed by using the time-correlated single-
photon counting (TCSPC) option on the Fluorolog 3 instrument. A
NanoLED (402 nm; full-width at half maximum (FWHM)<750 ps)
with repetition rates between 10 kHz and 1 MHz was used to
excite the sample. The excitation source was mounted directly on
the sample chamber at 908 to a double grating emission mono-
chromator (2.1 nmmmꢀ1 dispersion; 1200 groovesmmꢀ1) and col-
lected by a TBX-4-X single-photon-counting detector. Signals were
collected by using an IBH DataStation Hub photon-counting
module and data analysis was performed by using the commercial-
ly available DAS6 software (HORIBA Jobin Yvon IBH). For excited-
state lifetimes of >10 ms, a different experimental setup was used,
by equipping the Fluorolog 3 instrument with the FL-1040 phos-
phorescence module with a 70 W xenon flash tube (FWHM=3 ms)
with a variable flash rate (0.05–25 Hz). The signals were recorded
on the TBX-4-X single-photon-counting detector and collected
with a multichannel scaling (MCS) card in the IBH DataStation Hub
photon-counting module and data analysis was performed as de-
scribed above. Emission quantum yields were determined with
a Hamamatsu Photonics absolute PL quantum yield measurement
system (C9920-02, equipped with a L9799-01 CW Xenon light
source (150 W), monochromator, C7473 photonic multichannel an-
Experimental Section
Synthesis
All air- and water-sensitive experiments were performed under a ni-
trogen atmosphere by using standard vacuum-line techniques. All
chemicals were obtained commercially and used without further
purification. Dichloromethane was distilled over CaH2. Dry solvents
over molecular sieves were purchased and used as received. 1H
and 19F NMR spectra were acquired on either a Bruker AV300
(300 MHz) or a Bruker AV400 (400 MHz) spectrometer at 258C by
using deuterated solvent as the lock and residual solvent as the in-
ternal reference. ESI mass spectra were recorded on a Bruker Dal-
tonics (Bremen, Germany) MicroTof with loop injection. MS meas-
urements and NMR spectroscopy measurements were performed
in the Department of Organic Chemistry, University of Mꢆnster.
Complexes 4a, 4b, 6a and 6b were prepared as previously de-
scribed.[42,43] Compounds 5 and 8 were prepared by following re-
ported procedures.[44,45] Detailed synthesis and characterization in-
formation for the other iridium(III) complexes are given in the Sup-
porting Information.
Zeolite L crystals (cylinders with a diameter of 1.5 mm and 3.5 mm
in length and small crystals with average size of 50 nm) were syn-
thesized according to a previously described procedure.[68]
Insertion of pyronine-Y into zeolite L
The organic dye pyronine-Y was purchased from Molekula
Deutschland, Ltd. Insertion of the dye into zeolite L was performed
by following a well-established ion-exchange method (see also the
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alyzer, integrating sphere, and by employing U6039-05 PLQY mea-
surement software (Hamamatsu Photonics, Ltd., Shizuoka, Japan).
Keywords: donor–acceptor systems
iridium · luminescence · zeolites
·
energy transfer
·
Fluorescence microscopy
[1] V. Balzani, M. Venturi, A. Credi, Molecular Devices and Machines–A Jour-
ney Into the Nanoworld, Vol. 17, Wiley-VCH, Weinheim, 2003.
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ITO (15 W per square) substrates were cleaned by ultrasonic treat-
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´
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Acknowledgements
This study was supported by the European Commission through
the Human Potential Program (Marie-Curie Research Training
Network ‘Nanomatch’ Contract No. MRTN-CT-2006–035884) and
by the Deutsche Forschungsgemeinschaft in the framework of
the German–Chinese transregional research project TRR 61. J.G.
acknowledges the Spanish Ministerio de Economꢅa y Competitivi-
dad (MINECO; project RyC-2007-01559). WOLED Project of Cariplo
Foundation is acknowledged by W.M. for a grant. We thank Dr.
V. Vohra and Dr. U. Giovanella (Milan, Italy), as well as R. Costa
and E. Ortꢅ (Valencia, Spain) for helpful discussions.
´
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h
i
3
2pe2
ðvꢀDvÞ
cm2
s
ðvꢀDvÞ
~ ~
~
~
~
~
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n2
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