Effects of progressive halogen substitution on the photoluminescence properties of an erbium - porphyrin complex

Article abstract of DOI:10.1021/jp9119183

We have investigated the photoluminescence properties of porphyrin-based erbium and gadolinium complexes at different levels of halogen substitution. Both the intensity and the decay time of the erbium near-infrared emission correlate with the degree of t

Full text of DOI:10.1021/jp9119183

J. Phys. Chem. A 2010, 114, 4163–4168
4163
Effects of Progressive Halogen Substitution on the Photoluminescence Properties of an
Erbium-Porphyrin Complex
Roberto Pizzoferrato,*,† Roberto Francini,‡ Serena Pietrantoni,‡ Roberto Paolesse,§
Federica Mandoj,§ Angelo Monguzzi,⊥ and Francesco Meinardi⊥
Dipartimento di Ingegneria Meccanica - CNSIM, UniVersita` di Roma Tor Vergata, Via del Politecnico,
1-00133 Roma, Italy, Dipartimento di Fisica - CNSIM, UniVersita` di Roma Tor Vergata, Via della Ricerca
Scientifica, 1-00133 Roma, Italy, Dipartimento di Scienze e Tecnologie Chimiche, UniVersita` di Roma Tor
Vergata, Via della Ricerca, Scientifica 1-00133 Roma, Italy, and Dipartimento di Scienza dei Materiali,
UniVersita` di MilanosBicocca, Via Cozzi, 53-20125 Milano, Italy
ReceiVed: December 17, 2009; ReVised Manuscript ReceiVed: February 19, 2010
We have investigated the photoluminescence properties of porphyrin-based erbium and gadolinium complexes
at different levels of halogen substitution. Both the intensity and the decay time of the erbium near-infrared
emission correlate with the degree of the halogenation. Conversely, no clear correlation is found with the
triplet-state energy levels nor with the intensity of the residual visible emission. Such findings confirm that
the key role in the low efficiency of the near-infrared emission is played by the nonradiative quenching of the
erbium emitting level due to the vibrational modes of the surrounding C-H bonds.
1. Introduction
complexes based on fluorescein antenna chromophores and first
pointed out the importance of effective intersystem crossing for
The introduction of erbium-doped fiber amplifiers (EDFAs)1,2
for optical telecomm has given a great thrust to the research
for the exploitation of the 1.54 µm emission of Er(III). More
recently, the development of organic-based electrooptics has
pointed out the need to bind the erbium ion to organic ligands
in order to avoid clustering at high concentrations. Such
lanthanide coordination compounds present the further advan-
tage of combining the high spectral purity of the lanthanide
photoluminescence with the strong molar absorptivity and
tuneability of organic chromophores, thus compensating for the
small absorption cross section of the forbidden intra-4f transi-
tions of erbium. It is generally accepted that these features are
based on the efficient optical excitation of the “antenna” organic
ligand into its singlet state, followed by intersystem crossing
(ISC) to the lower-lying triplet state. ISC is greatly enhanced
by the presence of heavy atoms, such as the lanthanide itself,
in the organic molecule. Energy transfer from the ligand triplet
state to the upper levels of the erbium ion may then occur with
an efficiency depending the coordination geometry and the
matching between the energy levels.
obtaining strong emission bands. Reinhoudt et al. carried out
systematic studies using triphenylene5,6 or lissamine ligand
groups7,8 in polydentate hemispherand complexes. They con-
cluded that energy-transfer rates should be improved by direct
coordination of the antenna ligand to the lanthanide ion. On
the other hand, Hwan Kyu Kim et al.9,10 investigated intramo-
lecular processes in erbium-cored supramolecular complexes
based on platinumporphyrin ligands with no direct coordination
to the erbium ion. The authors’ calculations suggest that the
excitation is conveyed to the erbium ion through Fo¨rster resonant
transfer from the ligand first triplet excited state to the lanthanide
4
4F9/2 and I9/2 states.
Fo¨rster resonant transfer has also been investigated as the
main mechanism responsible for the low efficiency of the NIR
emission.11–15 In fact, coupling to the overtones of high-energy
oscillators, notably the O-H and C-H bonds of the ligands
and solvents, causes rapid vibrational quenching of the lumi-
nescence in the infrared region.16 This shows up in the reduction
of the emission decay time from the value τ ≈ 8 ms, in the
isolated ion to values in the range of 1-2 µs in typical lanthanide
complexes. Consequently, the overall radiative efficiency is
generally as low as φ ≈ 10-4. Halogenation of the hydrogen-
containing ligands has been suggested16 as a way to overcome
such effects and has been carried out in few cases17–20 with
different results. For example, a moderate increase of about 30%
in the emission efficiency has been found in erbium quinolinates
with a measured lifetime of τ ) 1.9 µs.17 Quite differently, a
much more noticeable growth to τ ) 741 µs has been achieved
in an imidodiphosphinate complex.19 The authors also found
evidence of a new low-lying ligand triplet state due to
fluorination which seems to quench the luminescence of visible
emitting lanthanides through intramolecular nonradiative pro-
cesses. A stretched exponential decay function with an average
lifetime of τ ) 300 µs was found in a similar erbium
compound.20 This latter result, however, was obtained upon
On such a basis, several erbium-containing compounds have
been synthesized and characterized over the past decade,3–25 with
the aim of enhancing the near-infrared (NIR) emission and
improving the physical processability and compatibility of the
materials. Specifically, great attention has been paid to the
investigation and engineering of the intramolecular energy
transfer through the choice of specific organic ligands, as well
as to reducing the quenching of the radiative emission. For
examples, Werts et al.3,4 synthesized luminescent erbium
* To whom correspondence should be addressed. E-mail: Pizzoferrato@
uniroma2.it.
† Dipartimento di Ingegneria Meccanica - CNSIM, Universita` di Roma
Tor Vergata.
‡ Dipartimento di Fisica - CNSIM, Universita` di Roma Tor Vergata.
§ Dipartimento di Scienze e Tecnologie Chimiche, Universita` di Roma
Tor Vergata.
2
⊥ Universita` di MilanosBicocca.
direct optical excitation into the H11/2 level of the Er(III) ion,
10.1021/jp9119183  2010 American Chemical Society
Published on Web 03/05/2010

4164 J. Phys. Chem. A, Vol. 114, No. 12, 2010
Pizzoferrato et al.
evaporated under reduced pressure. The product was directly
crystallized from CH2Cl2/hexane to obtain a green compound.
Spectroscopic data for Er(acac)BrFTPP. Yield: 27%. Found:
C, 31.5; H, 0.4; N, 3.0. ErC49H7O2N4F20Br8 requires C, 32.3;
H, 0.9; N, 2.4%. UV-vis: λmax(CH2Cl2), 433, 566, and 599 nm.
MS (MALDI): m/z 1870 (M+), isotope pattern in accord with
Br8 substitution.
Spectroscopic data for Gd(acac)BrFTPP. Yield: 32%. Found:
C, 31.6; H, 0.4; N, 3.0. GdC49H7O2N4F20Br8 requires C, 30.8;
H, 0.8; N, 3.5%. UV-vis: λmax(CH2Cl2), 431, 565, and 601 nm.
MS (MALDI): m/z 1860 (M+), isotope pattern in accord with
Br8 substitution.
2.2. Photophysical Measurements. For the optical measure-
ments in liquid solution, spectroscopic-grade deuterated tet-
rahydrofuran (THF) and chloroform were used as purchased
from Aldrich, either stabilized with amylene or ethanol. Thin
films for phosphorescence measurements were prepared by spin
coating chloroform solutions of PMMA/complex mixtures on
glass substrates with speed values in the range of 300-800 rpm
for 1 min. Vacuum annealing at 90 °C for 20 min was carried
out to remove residual solvent. Absorption spectra of the liquid
solutions were measured with a Perkin-Elmer Lambda 19
spectrophotometer by using quartz cells with a length of either
0.1 or 1 cm. Steady-state photoluminescence (PL) emission
spectra were recorded on a standard setup with a photomultiplier
for the visible range (Hamamatsu R3896) and a liquid-nitrogen
cooled Ge detector for the NIR spectra (EO817L, North Coast
Scientific Corp.) after passing the same dual-port 25 cm
monochromator (Oriel Cornerstone 260). Different light sources
were used for excitation, that is, the 458, 488, and 514 nm lines
of an Ar ion laser (with a power density of less than 50 mW/
cm2), the 410 nm line of a InGaN laser, and the monochroma-
tized output of a Hg-Xe cw discharge lamp. Phosphorescence
measurements were carried out by using a slightly different setup
equipped with a 30 cm monochromator (Acton Standard,
Princeton Instruments) and a Hamamatsu CCD for steady-state
spectra recording. The CCD was substituted with a Hamamatsu
R2949 photomultiplier and a gated photon counter/multiscaler
board (B&H PMS-300 time resolution of 200 ns) for phospho-
rescence lifetime measurements. In both PL and phosphores-
cence setups, the emission spectra were corrected for the
wavelength sensitivity of the systems.
Figure 1. Molecular structures of the erbium complexes (a) Er(a-
cac)TPP), (b) Er(acac)FTPP, and (c) Er(acac)BrFTPP.
which does not involve the typical antenna sensitizing mech-
anism. Moreover, in both of the cases with increased lifetime,19,20
no direct measurement of the NIR efficiency has been carried
out. Such promising, even though scattered, results demand
further investigations on the effects of halogen substitution on
the overall photophysics of the complex, with specific regard
to the intramolecular energy-transfer processes.
Within the framework outlined above, we have investigated
the effects on the photoluminescence properties of a two-step
progressive halogen substitution of porphyrin-based erbium and
gadolinium complexes. In detail, the following complexes have
been synthesized and characterized: unsubstituted (acetylaceto-
nate)Er(III)-5,10,15,20-tetraphenylporphyrin (hereafter referred
to as Er(acac)TPP); the partially fluorinated derivative (acety-
lacetonate)Er(III)-5,10,15,20-tetrakis(pentafluorophenyl)por-
phyrin Er(acac)FTPP with the substitution of only the aromatic
hydrogens of the external four phenyl rings; the completely
brominated on the beta pyrrolic positions (acetylacetonate)Er(III)-
2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(pentafluo-
rophenyl)porphyrin Er(acac)BrFTPP (see Figure 1).
Moreover, Gd(acac)TPP and the respective halogenated
derivatives have also been synthesized in order to investigate
the characteristics of the ligand triplet levels through phospho-
rescence measurements.
A third setup was used for decay time measurements of the
NIR emission by exciting the sample at 335 nm with a pulsed
Nd:YAG laser (Laser export mod. LCS-DTL-374QT) and
collecting the PL signal with a nitrogen-cooled PMT (Hamamat-
su R5509-73) coupled to a monochromator (Cornestone 2601/
4, ORIEL). The overall time resolution, operating in photon
counting mode by means of a multichannel scaler (ORTEC
9553), was better then 50 ns.
2. Experimental Methods
2.1. Synthesis. 5,10,15,20-Tetraphenylporphyrin (TPP),
5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (FTPP), and the
corresponding erbium and gadolinium metal complexes have
been prepared following the common synthetic procedures
reported in the literature.26
Er(acac)BrFTPP and Gd(acac)BrFTPP were obtained using
a slightly modified version of the Bhyrappa and Krishnan
approach.27
To a stirred solution of Ln(acac)FTPP (0.04 mmol) in 50
mL of CHC13 was added a solution of liquid bromine (48 µL;
1.0 mmol) in 15 mL of CHC13 over a period of 15 min at room
temperature. The stirring was continued for a further period of
1 h. Pyridine (80 µL, 1.0 mmol) in 15 mL of CHC13 was added
dropwise over a period of 15 min, and the solution was stirred
for another 1 h. Then, the reaction mixture was treated with
100 mL of 20% (v/v) aqueous sodium metabisulfite solution to
destroy the excess of bromine. The organic layer was separated
and dried over anhydrous sodium sulfate, and the solvent was
3. Results and Discussion
3.1. Absorption Spectra. The erbium complexes have first
been characterized as regards their optical properties in the
visible range. Figure 2 shows the absorption spectra of the
Er(acac)TPP complex (black line) and its fluorinated Er(a-
cac)FTPP (red line) and bromofluorinated Er(acac)BrFTPP
(blue line) derivatives in deuterated THF 10-5 M solutions. For
comparison, the absorption spectrum of the free base porphyrin
molecule H2TPP in THF is also reported (dashed green line).
Considering that the peak intensity values in the measured data
do not vary significantly over the different species, the reported
curves are normalized to the maximum intensity for an easier
comparison. The spectra of all of the complexes exhibit the

Photoluminescence Properties of an Erbium-Porphyrin Complex
J. Phys. Chem. A, Vol. 114, No. 12, 2010 4165
Figure 3. Photoluminescence spectra of the erbium complex and
halogenated derivatives along with the porphyrin free base H2TPP at
10-5 M in liquid THF. The spectrum of erbium quinoline ErQ3 is also
reported for comparison. All of the spectra are excited at λexc ) 410
nm.
Figure 2. Absorption spectra of the erbium complex and halogen
derivatives along with the porphyrin free base H2TPP at 10-5 M in
liquid THF.
with respect to Er(acac)TPP, the halogen substitution performed
in Er(acac)FTPP and Er(acac)BrFTPP produces noticeable
changes in the relative intensities of the two visible bands and,
to a lesser extent, in the positions of their peaks.
typical features of porphyrins, including the Soret band in the
range of 420-440 nm, representing the transition from the
ground state to the second excited singlet state (S2), and the
correspondingly weaker Q-bands (500-650 nm) corresponding
to the transitions to the first excited singlet state (S1). Comparing
Er(acac)TPP with the free base, we note that the coordination
to erbium affects two regions of the spectrum of the ligand in
different ways. Specifically, a moderate red shift (11 nm) is
observed in the peak position of the Soret band without any
appreciable change of the curve shape. More significantly, a
general reorganization of the relative intensities is observed in
the Q-bands. In particular, the first and fourth bands almost
disappear, while the Q(1,0) band at 552 nm grows by four times.
All of these effects are consistent with the pseudonormal nature
of the lanthanide monoporphyrinates.28 In such complexes, the
positions of absorption structures are mainly determined by the
π f π* transitions of the porphyrin ring, while the lanthanide
substituent only increases the macrocycle symmetry. As a
consequence, the splitting in four Q-bands due to the lower
symmetry of free base porphyrin is removed by the erbium
coordination, and only two Q-bands remain. From this point
on, we note that the substitution of the halogens for the
peripheral hydrogens does not promote further relevant changes
in the halogenated derivatives Er(acac)FTPP and Er(acac)BrFT-
PP (red and blue lines), apart from minor shifts and intensity
variations in both the Soret and Q-structures.
3.2. Visible Emission. Figure 3 compares the emission
spectrum of the free base H2TPP to the data of the Er(acac)TPP
complex and related halogenated derivatives in THF solutions.
All of the spectra are excited at λexc ) 410 nm. The free base
molecule shows the well-known visible fluorescence with two
peaks at 653 and 720 nm originating from the porphyrin singlet
states.28,29 This emission is strongly reduced in all of the erbium-
coordinating complexes due to ISC, which conveys the singlet
excitation into the ligand triplet state.28 Similarly to this
quenching and as a result of the energy transfer from the triplet
states to the lanthanide energy levels, the characteristic NIR
emission of the 4I13/2 f 4I15/2 transition of Er3+ appears at 1540
nm, as will be discussed later. Interestingly, the ratio of the
visible emission of the free base to that of the erbium complexes,
hereafter referred to as the quenching factor, varies in the range
from 10 to 100 over the different complexes, even without a
clear correlation with the degree of halogenation. Moreover,
In any case, it should be pointed out that the visible emissions
in the range of 600-720 nm of all of the complexes exhibit
quite fast decay times (i.e., shorter than 1 µs), as is demonstrated
by time-resolved measurements reported below. This is par-
ticularly relevant in comparison with the visible phosphores-
cence (with lifetimes as long as 40 µs) found at 665 and 730
nm in erbium complexe which include three platinum-
coordinated porphyrins as ligand groups.9,10 In the present case,
instead, the short decay times and the presence of other
emissions at lower energies (see next paragraph) strongly
suggest that the visible emissions of all of the complexes still
represent fluorescence from the ligand singlet states, as is the
case of the free base emission. Such a different behavior in the
emission properties of similar lanthanide coordination com-
pounds can be explained as follows. In platinum-coordinated
ligands, ISC is more efficiently induced by the heavier platinum
central substituent, independently from the presence of the
lanthanide, which is coordinated to another site of the complex.
This configuration enhances the transfer of the singlet excitation
to the triplet state, thus the growth of triplet population, while
the energy transfer to the erbium ion stands as a separate process
in competition with the triplet radiative decay (phosphores-
cence). As a result, fluorescence from the ligand singlet states
is not observed, and the phosphorescent emission is reduced
by only 13% upon erbium coordination to the platinum-
containing ligand.10 In the present complexes, instead, both ISC
and triplet depletion through energy transfer are caused by the
same central erbium substituent. Therefore, the optical excitation
in the Soret band is rapidly transferred to the lanthanide levels
without emission of phosphorescence, and the visible fluores-
cence is quenched by more than 90% in comparison to that of
the free base.
3.3. Phosphorescent Emission in Gd Complexes. In order
to investigate the role of the triplet state in the physical picture
outlined above, photoluminescence time decay measurements
have been carried out on the erbium complexes and on
specifically synthesized gadolinium-porphyrin complexes. Gd(I-
II) complexes are commonly used as model systems, owing to
their lack of energy transfer as a result of the lanthanide ion
emitting levels having higher energies compared to the ligand
singlet and triplet states. This prevents the depletion of the ligand

4166 J. Phys. Chem. A, Vol. 114, No. 12, 2010
Pizzoferrato et al.
below the 653 and 720 nm singlet levels. This confirms the
difference with respect to platinum-coordinated porphyrins10
where the phosphorescent bands at 665 and 730 nm situate the
triplet states very close to the singlet ones.
The band spectra of Figure 4 also show that the energy levels
of the triplet states in the present complexes are much more
sensitive to halogenation than the singlet ones, and such effects
should be considered with respect to the probability of resonant
4
transfer to the F9/2 level of Er(III). However, the position of
the triplet states does not show a clear correlation with the degree
of halogen substitution. In fact, the partially substituted Gd(a-
cac)FTPP seems to have the highest-energy onset of the triplet
emission, with the band peaked at 730 nm, which is blue-shifted
by as much as 64 nm with respect to the unsubstituted
Gd(acac)TPP and by 46 nm compared to Gd(acac)BrFTPP.
The particular behavior of Gd(acac)FTPP is also reflected in
its triplet experimental decay time (about 70 µs), which is
appreciably longer than those in the other two derivatives (50
µs). On the other hand, it is not easy to explain the relatively
strong bands with singlet emissions at 653 and 717 nm of
GdTPP. This could be due to the lower efficiency of ISC in
comparison with that of the halogen-substituted derivatives as
well as to the presence of impurities of the free base molecule,
which exhibits a much more efficient fluorescence. Further
investigations are in progress to make this point clear.
Figure 4. Photoluminescence spectra of the gadolinium complex and
halogenated derivatives along with free base H2TPP in solid solutions
in PMMA films. All of the spectra are excited at λexc ) 410 nm.
3.4. 1.5 µm Emission in Er Complexes. All of the emission
spectra (see Figure 3) exhibit the typical 1540 nm emission of
the 4I13/2 f 4I15/2 erbium transition with a line shape and fwhm
verysimilartothoseobservedinothererbiumcomplexes.8,10,21,24,25,32
The emission of ErQ, considered as the standard model for
erbium complexes,33 is also reported for comparison. The overall
NIR efficiency of the Er(acac)TPP complex was estimated to
approximately φIR ) 4 × 10-4 through absolute measurements
of the emitted intensity in a calibrated setup22 and is in the range
of the values reported in the literature, as discussed below. A
slight increase is observed in the partially halogen-substituted
Er(acac)FTPP, while the fully bromofluorinated Er(acac)BrFT-
PP approximately doubles the value measured in the unsubsti-
tuted complex. Such very low figures of efficiency cannot be
attributed to the incomplete ISC of the singlet excitation or to
the inefficient energy transfer to the erbium ion for a number
of reasons. First, the intensity of the 1540 nm band among the
different erbium derivatives does not correlate with the quench-
ing factor of the visible emission (Figure 3) that varies in a
range from 10 to 100. Second, even the lowest quenching factor,
that is, 10 in Er(acac)BrFTPP, corresponds to 10% only of
the absorbed energy emitted as visible radiation and thus lost
for transfer to the erbium ion. This could not account by itself
for the 0.04% efficiency of the NIR emission. Third, the present
value of φIR is not far from the value of 2 × 10-4 measured in
fluorescein-sensitized erbium complexes,4 7 × 10-5 of Er-
(DBM)334 and 1 × 10-3 of similar erbium porphyrinates.21 The
same order of magnitude was also estimated from the ratio
between the total and the radiative PL lifetime in terphenyl-
based6 and in pyridyltetrazine-based complexes.24 It is quite
unlikely that all of these different compounds have similarly
inefficient mechanisms of ISC and energy transfer to the erbium
ion. On the other hand, a recent study35 suggests that the
efficiency of the energy transfer to erbium is close to 100%.
To this regard, it should also be noted that the NIR emission
intensity in the present complexes seems to be independent of
the large variation of the triplet energy levels observed in the
different derivatives (see the band peak locations in Figure 4)
but increases with the degree of halogen substitution. All of
Figure 5. Decay of the photoluminescence signal in PMMA solid
solutions of some of the gadolinium complexes excited at λexc ) 410
nm.
triplet state and allows the emission of phosphorescence.
Therefore, on the basis of the fact that the substitution with
gadolinium does not alter the triplet state energies of the ligand,30
an estimate of the erbium complex energy levels can be
achieved. The Gd(III) complexes have been inserted into a rigid
polymer matrix of polymethylmethacrylate (PMMA) films in
order to restrict collisional deactivation. Figure 4 reports the
410 nm excited emission from samples of Gd(acac)TPP,
gadolinium halogenated derivatives Gd(acac)FTPP and Gd(a-
cac)BrFTPP, along with the freebase porphyrin in PMMA
films. While the spectra of the free base and erbium complexes
(see Supporting Information) are substantially similar to those
in liquid solutions, the spectra of the gadolinium ones extend
more toward the longer wavelengths with a series of additional
bands in the range of 730-1000 nm. Similarly structured
emissions have been reported31 for the gadolinium complexes
GdTPP and GdFTPP (without the acetylacetonate [acac] capping
ligand) and attributed to the vibronic replica of the ligand triplet
state. Such an attribution can also apply in the present case since
time-resolved measurements (Figure 5) report decay times
ranging from 50 to 70 µs for the bands in the range of 730-900
nm (see a summary in Table 1). In the present complexes,
therefore, the triplet states seem to be located at energies well

Photoluminescence Properties of an Erbium-Porphyrin Complex
J. Phys. Chem. A, Vol. 114, No. 12, 2010 4167
TABLE 1: Luminescence Parameters of the Free Base and the Lanthanide Porphyrin Complexes
emission band peaks (nm) and lifetimes
complex
H2TPP
Er(acac)TPP
GdTPP
Gd(acac)FTPP
Gd(acac)BrFTPP
singlet
triplet
654 (<1 µs)
655 (<1 µs)
653 (<1 µs)
649 (<1 µs)
656 (<1 µs)
718 (<1 µs)
717 (<1 µs)
717 (<1 µs)
794 (51 µs)a
730 (69 µs)a
776 (50 µs)a
895 (50 µs)a
818 (68 µs)a
845 (50 µs)a
910 (68 µs)a
900 (50 µs)a
708 (<1 µs)
a Uncertainties in the estimation of decay time are within (2 µs.
This could be due to the incomplete halogen substitution of the
present compounds since the acetylacetone capping ligand still
contains both C-H and O-H groups, even though at a larger
distance from the erbium ion than the aromatic groups.
4. Conclusions
We have investigated the effects produced on the optical
properties of an erbium-porphyrin complex by the progressive
substitution of the ligand hydrogens with halogens. While
achieving an exhaustive physical knowledge of the intramo-
lecular processes requires further quantitative analyses of the
energy-transfer mechanisms, some conclusions can still be made.
The efficiency of the NIR emission exhibits a clearly positive
correlation with the degree of halogen substitution, while seems
independent of the visible emission quenching factor and the
triplet level energies. Such a behavior can interestingly be
compared to the results reported in similar erbium complexes
with a different ligand configuration, that is, based on platinum
porphyrins. While the characteristic and nature of the visible
emission is quite different due to the different dynamics of
intramolecular energy transfer, the efficiency of the NIR
emission is comparably low. Moreover, a significant correlation
between the NIR intensity and lifetime has been found in the
present complexes at different levels of halogenation. All of
these findings confirm that the effective bottleneck of the
sensitized NIR emission lies in the quenching of the erbium
emitting level through Fo¨rster energy transfer to the second
overtone of the C-H vibration. From this point of view, the
substitution of the aromatic C-H bonds of the erbium-
coordinating porphyrin macrocycle apears to be the right
strategy. On the other hand, the relatively long distance
characteristic of Fo¨rster interaction requires that all of the
quenchers, that is, even the farther C-H and O-H groups of
the acac capping ligand, be substituted in order to achieve an
effective improvement of the quantum efficiency of the complex.
Figure 6. Decay of the NIR photoluminescence in the erbium complex
and halogen derivatives excited at λexc ) 355 nm.
these findings suggest that the effective bottleneck of the overall
efficiency does not lie in the intramolecular energy-transfer
processes.Morereasonably,asunderlinedbyseveralauthors,4,6,8,11–20
a strong quenching of the NIR emission may occur through
Fo¨rster resonant transfer to the second overtone of the C-H
vibration in the respective chemical groups of the porphyrinic
ligand.
Within this framework, the strategy of substituting all of the
aromatic hydrogens with halogens, presently achieved in Er(a-
cac)BrFTPP, seems to be a step in the right direction since it
doubles the intensity of the NIR emission regardless of the fact
that it gives the lowest quenching factor of the visible emission.
The improvement in efficiency is also reflected in the lifetime
of the 1540 nm emission, as reported in Figure 6. The decay
time progressively increases with the level of substitution from
the value τ ) 1.5 µs in Er(acac)TPP, to τ ) 2.1 µs in
Er(acac)FTPP, and up to τ ) 3.5 µs in Er(acac)BrFTPP.
Significantly, the measured lifetimes are in good linear correla-
tion with the intensities of the respective NIR emissions, in
agreement with the linear dependence predicted for the quantum
efficiency φobs of a quenched emission
Acknowledgment. F. Meinardi and A. Monguzzi thank
Fondazione Cariplo for the financial support.
Supporting Information Available: Figure S1 shows the
410 nm excited emission from samples of Er(acac)TPP,
Er(acac)FTPP, and Er(acac)BrFTPP in PMMA films. The
respective spectra are very similar to those in liquid solutions
(Figure 3) if the broadening and spectral shift due to the solid-
state environment are taken into account. As in the case of the
residual visible fluorescence in liquid solutions, the intensity
of the emission does not correlate with the degree of halogena-
tion. This material is available free of charge via the Internet at
http://pubs.acs.org.
τobs
ꢀobs
)
(1)
τrad
We think such an agreement strongly supports the hypothesis
that the low efficiency is definitely determined by the quenching
of the erbium ion emission.
However, despite the increase due to halogenation, the
observed lifetimes are still far from the typical value of the
isolated erbium ion (τrad ≈ 8 ms) and even farther from the
200-fold increase obtained in the imidodiphosphinate complex.19
References and Notes
(1) Mears, R. J.; Reekie, L.; Jauncey, I. M.; Payne, D. N. Electron.
Lett. 1987, 23, 1026.

4168 J. Phys. Chem. A, Vol. 114, No. 12, 2010
Pizzoferrato et al.
(2) Desurvire, E.; Simpson, J.; Becker, P. C. Opt. Lett. 1987, 2, 888.
(3) Werts, M. H. V.; Verhoeven, J. V.; Hofstraat, J. W.; Geurts, F. A. J.;
Verhoven, J. W. Chem. Phys. Lett. 1997, 276, 196.
(4) Werts, M. H. V.; Verhoeven, J. V.; Hofstraat, J. W. Chem. Soc.,
Perkin Trans. 2000, 2, 433.
(5) Klink, S. I.; Grave, L.; Reinhoudt, D. N.; Van Veggel, F. C. J. M.;
Werts, M. H. V.; Geurts, F. A. J.; Hofstraat, J. W. J. Phys. Chem. A 2000,
104, 5457.
(6) Klink, S. I.; Hebbink, G. A.; Grave, L.; Van Veggel, F. C. J. M.;
Reinhoudt, D. N.; Slooff, L. H.; Polman, A.; Hofstraat, J. W. J. Appl. Phys.
1999, 86, 1181.
(7) Slooff, L. H.; Polman, A.; Caialli, F.; Friend, R. H.; Hebbink, G. A.;
Van Veggel, F. C. J. M.; Reinhoudt, D. N. Appl. Phys. Lett. 2001, 78, 2122.
(8) Slooff, L. H.; Van Blaaderen, A.; Polman, A.; Hebbink, G. A.;
Klink, S. I.; Van Veggel, F. C. J. M.; Reinhoudt, D. N.; Hofstraat, J. W.
J. Appl. Phys. 2002, 91, 3955.
(9) Oh, J. B.; Kim, Y. H.; Nah, M.-K.; Kim, H.-K. J. Luminescence
2005, 111, 255.
(10) Nah, M.-K.; Oh, J. B.; Kim, H.-K.; Choi, H.-K.; Kim, Y.-R.; Jun-
Gill Kang, J.-G. J. Phys. Chem. A 2007, 111, 6157.
(11) Artizzu, F.; Deplano, P.; Marchio`, L.; Mercuri, M. L.; Pilia, L.;
Serpe, A.; Quochi, F.; Orru`, R.; Cordella, F.; Meinardi, F.; Tubino, R.;
Mura, A.; Bongiovanni, G. Inorg. Chem. 2005, 44, 840.
(12) Quochi, F.; Orru`, R.; Cordella, F.; A. Mura, A.; Bongiovanni, G.;
Artizzu, F.; Deplano, P.; Mercuri, M.; Pilia, L.; Serpe, A. J. Appl. Phys.
2006, 99, 053520.
(18) Mancino, G.; Ferguson, A. J.; Beeby, A.; Long, N. J.; Jones, T. S.
J. Am. Chem. Soc. 2005, 127, 524.
(19) Glover, P.; Bassett, A. P.; Nockemann, P.; Kariuki, B. M.; Deun,
R. V.; Pikramenou, Z. Chem.sEur. J. 2007, 13, 6308.
(20) Zheng, Y.; Pearson, J.; Tan, R. H. C.; Gillin, W. P.; Wyatt, P. B.
J. Mater. Sci: Mater. Electron. 2009, 20, 788.
(21) Foley, T. J.; Harrison, B. S.; Knefely, A. S.; Abboud, K. A.;
Reynolds, J. R.; Schanze, K. S.; Boncella, J. M. Inorg. Chem. 2003, 42,
5023.
(22) Pizzoferrato, R.; Ziller, T.; Paolesse, R.; Mandoj, F.; Micozzi, A.;
Ricci, A.; Lo Sterzo, C. Chem. Phys. Lett. 2006, 426, 124.
(23) Destri, S.; Porzio, W.; Meinardi, F.; Tubino, R.; Salerno, G.
Macromolecules 2003, 36, 273.
(24) Shavaleev, N. M.; Pope, S. J. A.; Bell, Z. R.; Faulkner, S.; Ward,
M. D. Dalton Trans. 2003, 808.
(25) Koppe, M.; Brabec, C. J.; Sariciftci, N. S.; Eichen, Y.; Nakmanov-
ich, G.; Ehrenfreund, E.; Epsten, O.; Heiss, W. Synth. Met. 2001, 121, 1511.
(26) Buchler, J. W.; Ng, D. K. P. In The Porphyrin Handbook; Kadish,
K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: Boston, MA, 2000;
Vol. 3, p 245.
(27) Bhyrappa, P.; Krishnan, V. Inorg. Chem. 1991, 30, 239.
(28) Dolphin D. The Porphyrins; Academic Press: New York, 1978.
(29) Gouterman, M.; Khalil, G.-E. J. Mol. Spectrosc. 1974, 53, 88.
(30) Crosby, G. A.; Whan, R. E.; Alire, R. M. J. Chem. Phys. 1961, 34,
743.
(31) Khalil, G. E.; Thompson, E. K.; Gouterman, M.; Callis, J. B.;
Dalton, L. R.; Turro, N. J.; Jockusch, S. Chem. Phys. Lett. 2007, 435, 45.
(32) Hebbink, G. A.; Reinhoudt, D. N.; Van Veggel, F. C. J. M. Eur. J.
Org. Chem. 2001, 4101.
(33) Gillin, W. P.; Curry, R. J. Appl. Phys. Lett. 1999, 74, 798.
(34) Kawamura, Y.; Wada, Y.; Yanagida, S. Jpn. J. Appl. Phys. 2001,
40, 350.
(13) Tan, R. H. C.; Motevalli, M.; Abrahams, I.; Wyatt, P. B.; Gillin,
W. P. J. Phys. Chem. B 2006, 110, 24476.
(14) Monguzzi, A.; Tubino, R.; Meinardi, F.; Biroli, A. O.; Pizzotti,
M.; Demartin, F.; Quochi, F.; Cordella, F.; Loi, M. A. Chem. Mater. 2009,
21, 128.
(15) Beverina, L.; Crippa, M.; Sassi, M.; Monguzzi, A.; Meinardi, F.;
Tubino, R.; Pagani, G. A. Chem. Commun. 2009, 34, 5103.
(16) Hasegawa, Y.; Wada, Y.; Yanagida, S. J. Photochem. Photobiol.,
C 2004, 5, 183.
(17) Van Deun, R.; Fias, P.; Driesen, K.; Binnemans, K.; Go¨rller-
Walrand, C. Phys. Chem. Chem. Phys. 2003, 5, 2754.
(35) Bertolo, L.; Tamburini, S.; Vigato, P. A.; Porzio, W.; Macchi, G.;
Meinardi, F. Eur. J. Inorg. Chem. 2006, 12, 2370.
JP9119183