Hierarchical self-assembly of luminescent EuIII complexes on silicon

Article abstract of DOI:10.1002/ejic.201402117

We have combined the metal-coordinating features of phenanthroline with the remarkable complexing properties of tetraphosphonate (Tiiii) cavitands towards N-methylammonium salts with the aim of assembling novel luminescent ternary complexes. The formation of such complexes was first tested in solution: the charged sarcosine derivative 1, bearing a phenanthroline moiety, was complexed by the cavitand Tiiii-A, followed by coordination of Eu^III- tris(β-diketonate) complex 2. The occurrence of the self-assembly has been proven by NMR spectroscopy, mass spectrometry and photophysical measurements. The transfer of this binding protocol to the surface showed the complete orthogonality of these interactions, as verified by control experiments on complexation-inactive surfaces. The formation of the ternary complexes on the silicon surface was monitored by means of X-ray photoelectron spectroscopy and luminescence spectroscopy. The emission properties of the silicon-bound Si-Tiiii-B·1·2 and the corresponding ternary complex Tiiii-A·1·2 in solution are similar, which indicates that the transfer of the self-assembly process onto silicon does not significantly perturb the Eu^III coordination environment. The self-assembly protocol illustrated here can be extended to a wide variety of lanthanide ions and can be implemented for applications in sensing, bioimaging and optoelectronic devices.

Full text of DOI:10.1002/ejic.201402117

FULL PAPER
DOI:10.1002/ejic.201402117
Hierarchical Self-Assembly of Luminescent EuIII
Complexes on Silicon
Kasjan Misztal,[a] Cristina Tudisco,[b] Andrea Sartori,[a][‡]
Joanna M. Malicka,[c][‡‡] Riccardo Castelli,[a][‡]
Guglielmo G. Condorelli,[b] and Enrico Dalcanale*[a]
Keywords: Supramolecular chemistry / Host–guest systems / Cavitands / Self-assembly / Lanthanides / Luminescence /
Silicon
We have combined the metal-coordinating features of phen-
anthroline with the remarkable complexing properties of tet-
raphosphonate (Tiiii) cavitands towards N-methylammonium
salts with the aim of assembling novel luminescent ternary
complexes. The formation of such complexes was first tested
in solution: the charged sarcosine derivative 1, bearing a
phenanthroline moiety, was complexed by the cavitand Tiiii-
tions, as verified by control experiments on complexation-
inactive surfaces. The formation of the ternary complexes on
the silicon surface was monitored by means of X-ray photo-
electron spectroscopy and luminescence spectroscopy. The
emission properties of the silicon-bound Si-Tiiii-B·1·2 and the
corresponding ternary complex Tiiii-A·1·2 in solution are
similar, which indicates that the transfer of the self-assembly
A, followed by coordination of EuIII–tris(β-diketonate) com- process onto silicon does not significantly perturb the EuIII
plex 2. The occurrence of the self-assembly has been proven
by NMR spectroscopy, mass spectrometry and photophysical
measurements. The transfer of this binding protocol to the
surface showed the complete orthogonality of these interac-
coordination environment. The self-assembly protocol illus-
trated here can be extended to a wide variety of lanthanide
ions and can be implemented for applications in sensing,
bioimaging and optoelectronic devices.
tested over the last three decades. Among them, cavitands[5]
are particularly versatile systems, the complexation ability
of which has been exploited not only for the recognition of
target molecules[6] but also for the fabrication of well-de-
fined 2D and 3D molecular assemblies.[7] Some examples of
cavitand-decorated surfaces of gold[8] and silicon[9] in which
the molecular-recognition properties of cavitands have been
exploited for the precise fabrication of a specific molecular
structure have been reported. Recently, β-cyclodextrins were
employed as molecular receptors for the self-assembly of
EuIII luminescent complexes on glass.[10] The antenna sensi-
tizer and the Eu complex were independently anchored on
the glass surface through hydrophobic adamantyl–cyclo-
dextrin complexation by using a multivalency approach.
The two components interact on the surface, thus produc-
ing localized sensitized emission.
In the present work, we describe an alternative, sequen-
tial self-assembly protocol aimed at building EuIII lumines-
cent coordination structures on silicon. Our approach is
based on the combination of the molecular-recognition
properties of a tetraphosphonate cavitand with the metal-
coordination ability of specific guests.
EuIII and TbIII complexes display exceptional photo-
physical properties[11] such as high photoluminescence
quantum yield (PLQY) in the visible spectral region, long
excited-state lifetimes, large Stokes shifts and sharp emis-
sion profiles related to f–f electronic transitions. The char-
Introduction
The formation of hierarchical monolayers and multilay-
ers on different surfaces by exploiting supramolecular inter-
actions represents a theme of great interest in contemporary
chemistry.[1,2] The idea behind this approach is to use the
thermodynamic control and reversibility of noncovalent in-
teractions for the error-free generation of specific architec-
tures directly on surfaces.[3] A substantial contribution to
the self-assembly protocols for 2D and 3D structures was
given by the development of molecular-recognition schemes
on surfaces,[4] which tapped into the large body of available
synthetic receptors that have been designed, prepared and
[a] Dipartimento di Chimica and INSTM UdR di Parma,
University of Parma,
Parco Area delle Scienze 17/A, 43124 Parma, Italy
E-mail: enrico.dalcanale@unipr.it
http://www.dalcanalegroup.it
[b] Dipartimento di Scienze Chimiche and INSTM UdR di
Catania, University of Catania, ISTM-CNR,
V.le A. Doria 6, 95125 Catania, Italy
[c] Molecular Photoscience Group, Istituto per la Sintesi Organica
e la Fotoreattività del CNR (ISOF-CNR),
Via P. Gobetti 101, 40129 Bologna, Italy
[‡] Current address: Dipartimento di Farmacia, University of
Parma,
Parco Area delle Scienze 27A, 43124 Parma, Italy
[‡‡]Current address: Consorzio MIST E-R, Via P. Gobetti 101,
40129 Bologna, Italy
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402117.
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acteristic red and green photoluminescence of trivalent Eu towards a sarcosine unit[17] connected to a phenanthroline
and Tb ions,[12] upon ultraviolet-light irradiation, has been ligand (1), followed by the coordination of 1 with the euro-
exploited from the fundamental and applicative point of pium β-diketonate complex 2 used as the luminescent
view. Accordingly, materials based on EuIII and TbIII are probe. The related nonluminescent and diamagnetic yttrium
extensively utilized in light-conversion molecular devices, as β-diketonate 3 was chosen as the reference compound for
phosphors in lighting technologies, as well as for sensing the NMR spectroscopic characterization in solution and for
purposes and memory devices.[13,14] Although some exam- photophysical control experiments on the surface. Cavit-
ples of EuIII complexes covalently attached on surfaces have and-decorated silicon surfaces were prepared by photo-
been reported,[15,16] to the best of our knowledge no exam- chemical grafting.[21] The entire process was fully tested in
ples of noncovalent self-assembly of lanthanide luminescent solution, and then successfully transferred to the solid state
probes on silicon surfaces have been described in the litera- where it was monitored by means of X-ray photoelectron
ture.
The main goal of the present work is the development of
spectroscopy (XPS) and luminescence spectroscopy.
a strategy for the precise and reversible hierarchical as-
sembly on silicon of a luminescent EuIII complex, which is
bound to a surface-anchored cavitand by means of a sarco-
sine moiety bearing a phenanthroline ligand. The design
of the connecting ligand was determined by the following
experimental observations: (i) the high association constant
of sarcosine with tetraphosphonate cavitands,[17] and (ii) the
well-known ability of phenanthroline[18,19] to serve as a
strong bidentate ligand and efficient sensitizer for EuIII–β-
diketonates.
Self-Assembly in Solution
The formation of ternary complexes was first carried out
in solution to prove its feasibility (Figure 1). The first step
was the complexation of the methylammonium guest 1 in-
side the Tiiii[C3H7, CH3, Ph] (hereafter referred to as Tiiii-
A) cavity in methanol, monitored by 31P and 1H NMR
spectroscopy measurements. Figure S1 in the Supporting
Information shows the 1H NMR spectrum of the host–
guest complex formed by 1 and Tiiii-A (Tiiii-A·1). Diagnos-
tic upfield shifts of the guest signals were observed, as ex-
pected for hosted species that experience the shielding effect
of the cavity.[17] In particular, the signals of the N-CH3 moi-
ety moved over 3 ppm upfield to δ = –0.53 ppm; other reso-
nances attributed to the sarcosine moiety as well as to the
phenanthroline unit were also shifted. The 31P resonance of
the four P=O groups moved downfield by almost 5 ppm
(from δ = 5.9 to 10.8 ppm; Figure S2 in the Supporting In-
formation). The upfield shift of the methyl residue is diag-
nostic of N-CH3 inclusion into the cavity, whereas the
downfield shift of P=O signals is a clear indication of the
Results and Discussion
Design and Strategy
The compounds used in the present work are shown in
Scheme 1. The precise and reversible hierarchical assembly
on silicon was obtained by using host–guest interactions
and metal–ligand coordination in sequence. This multistep
approach involves the recognition process of tetraphos-
phonate cavitands (Tiiii)[20] anchored on the silicon surfaces
Figure 1. Ternary complex formation in solution (10 mm of each
component): (a) host–guest Tiiii-A·1 complex formation in meth-
anol; (b) coordination between Tiiii-A·1 and 2 (or 3) in dichloro-
methane. The pink ball represents the chloride anion of the ammo-
nium-based ligand 1.
Scheme 1. Molecular structures of the compounds used in the pres-
ent work.
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participation of phosphonates in the complexation of the
guest.
All the investigated europium compounds, 2, 1·2 and
Tiiii-A·1·2, exhibit characteristic EuIII emission bands due
The ternary complexes between Tiiii-A·1 and the metal to the f–f transitions (Figure 2). The emission spectra con-
precursors 2 or 3 were formed quantitatively by simply mix- sist of 5D0 Ǟ7FJ transitions (J = 0–4) mainly dominated by
5
5
ing the three components in dichloromethane. The forma- the D0 Ǟ7F1 and D0 Ǟ7F2 sharp emission bands centred
tion of Tiiii-A·1·3 was monitored by means of H and 31P at 590 and 612 nm, respectively. The perfect overlapping of
1
NMR spectroscopy. In the proton spectrum of Tiiii-A·1·3 the emission spectra between 2, 1·2 and Tiiii-A·1·2 com-
the downfield shift of the peaks belonging to the phen- plexes shows that the coordination sphere of the europium
anthroline was observed, which is diagnostic of the metal cation remains virtually unchanged after encapsulation in
complexation (Figure S1 in the Supporting Information). the cavity. Notably, the PLQY of 2 is 15.5%, and increases
The phosphorus peak remained almost unchanged, which to 25.9% for 1·2. In the latter, the phenanthroline ligand
indicated that the metal ion does not compete with sarco- fills the coordination sphere around the europium cation
sine for the cavity. Because of the paramagnetic properties and eliminates vibrational quenching from water molecules
1
of EuIII, H NMR spectroscopy was not used to monitor and also serves as a light-harvesting antenna. PLQY of the
the formation of the Tiiii-A·1·2 complex, whereas the phos- ternary complex Tiiii-A·1·2 in solution is 20.0%, which is
phorus spectrum is similar to that of the diamagnetic yt- intermediate between those of 2 and 1·2. The trend of the
trium derivative Tiiii-A·1·3 (Figure S3 in the Supporting In- lifetimes values is in perfect agreement with that observed
formation), so it is safe to assume that the Tiiii-A·1·2 com- for the quantum yields (Table 1).
plex was formed. The self-assembly outcome changes dra-
matically by replacing 1 with bare phenanthroline: Tiiii-A
substitutes phenanthroline in the coordination sphere of
YIII, as indicated by a downfield shift in the 31P NMR spec-
trum (Figure S11 in the Supporting Information). There-
fore, the higher affinity of sarcosine for the cavity shifts the
equilibrium toward the exclusive formation of the ternary
complex Tiiii-A·1·2. Unfortunately, we have been unable to
precisely establish the coordination mode of Tiiii with LnIII
.
The presence of ternary species was also confirmed by high-
resolution mass spectrometry (HRMS). The HRMS spec-
trum of the analogous complex Tiiii-C·1·2 in acetonitrile,
in which the cavitand has a longer alkyl chain at the lower
rim, exhibited a peak at m/z corresponding to the complex
without the chloride anion (Figure S4 in the Supporting
Information).
Figure 2. Emission and excitation spectra of the compounds in
dichloromethane at 298 K. Excitation spectra: λem = 612 nm in all
cases but 1 for which λem = 410 nm; emission spectra: λex = 280 nm.
Absorption and Luminescence Spectra in Solution
The formation of the hierarchical complexes Tiiii-A·1·2
and Tiiii-A·1·3 is further supported by photophysical in- Table 1. Photophysical data of solutions of the compounds in
dichloromethane at 298 K.
vestigations. The UV/Vis absorption spectrum of 2 in
dichloromethane spans across the range 260–350 nm with a
maximum centred at 290 nm (Figure S5 in the Supporting
Information). The phenanthroline ligand 1 absorbs in the
PLQY [%]
Excited-state lifetime
1
2
5.7
15.5
25.9
20.0
2.7
1.9
1.7
4.90 ns[b]
0.75 ms[a]
0.85 ms[a]
0.80 ms[a]
3.70 ns[b]
3.10 ns[b]
3.50 ns[b]
same spectral range but its maximum is located at higher 1·2
Tiiii-A·1·2
energies (275 nm; Figure S5). In line with these findings, the
absorption spectrum of the compound obtained from the
phenanthroline system 1 and the Eu–β-diketonate complex
2 (1·2) is the sum of the spectral features of its building
blocks and shows a maximum at 285 nm (Figure S5). The
absorption spectrum of the analogous yttrium–β-diketonate
complex 1·3 is virtually identical to that of 1·2, which indi-
Tiiii-A·1
1·3
Tiiii-A·1·3
[a] EuIII-centred luminescence. [b] Phenanthroline-centred fluores-
cence.
YIII complexes (1·3 and Tiiii-A·1·3) do not exhibit metal-
cates the same electronic transitions in both compounds, as centred emission due to the closed-shell configuration of
expected (Figure S6 in the Supporting Information). The the ion. Therefore they were investigated as reference com-
Tiiii-A cavitand does not absorb light beyond 290 nm, and pounds. The only luminescence band observed was that of
the formation of Tiiii-A·1·2 leads only to a broadening of the sarcosine–phenanthroline ligand 1 at λmax = 440 nm.
the absorption band (Figure S5). Absorption spectra of 1·2, The emission profile of 1 in Tiiii-A·1·3 matches well with
1·3, Tiiii-A, and Tiiii-A·1 are presented in Figure S7 in the the emission of the ligand in Tiiii-A·1 and is shifted towards
Supporting Information.
longer wavelengths relative to free 1 (λmax = 290 nm) owing
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Figure 3. Si-Tiiii-B hierarchical self-assembly protocol with (a) guest 1 and (b) subsequently with metal complexes 2 or 3.
to the confinement effect of the cavitand. PLQY of free li- 400.0 eV assigned to the two phenanthroline nitrogen atoms
gand 1 was 5.7%. This value decreases to 2.7% in Tiiii-A, and the nitrogen atoms of the amidic bond (Nlig), and (ii) a
which suggests the presence of some electronic interaction less intense component at 402.0 eV assigned to the proton-
between the phenanthroline moiety and the cavitand. In the ated nitrogen of sarcosine (Nsar). The observed intensity ra-
case of the metal complex 1·3, the PLQY is decreased to tio Nlig/Nsar is about 3:1 as expected from the molecular
1.9%. The difference relative to free 1 suggests the presence structure. The successful coordination affording Si-Tiiii-
of some electronic perturbation induced by the yttrium cat- B·1·2 and Si-Tiiii-B·1·3, respectively, was corroborated by
ion.
the presence of Eu (or Y) XPS bands together with the F
1s signal due to the coordinated β-diketonate ligands
(Table 2).
Self-Assembly on the Tiiii-Functionalized Si Surface
Table 2. Elemental composition data from XPS measurements for
selected surfaces.
The noncovalent anchoring on the silicon surface is sche-
matically illustrated in Figure 3. The preliminary step was
the preparation of the Tiiii-B-functionalized Si surface (Si-
Tiiii-B) by covalent grafting of a mixed Tiiii-B/1-octene
monolayer, by means of photochemical hydrosilylation. Ac-
cording to previous studies,[9b,21] the use of 1-octene as a
spectator spacer in the mixed monolayers improves the pas-
sivation of the Si surface, thus minimizing substrate oxi-
dation due to aging. XPS measurements of the grafted sur-
faces showed the typical P 2p band of the silicon-anchored
Tiiii cavitand. The detailed characterization of the Si-Tiiii-
B surface is reported elsewhere.[9b]
Atomic fractions [%]
P
Si
O
C
N
F
Eu
Y
Si-Tiiii-B
1.1 16.3 31.9 50.4 0.3
1.0 12.9 30.5 53.9 1.7
–
–
–
–
–
–
–
0.32
–
Si-Tiiii-B·1
Si-Tiiii-B·1·2
Si-Tiiii-B·1·3
Si-MeCav·1·2
0.9 12.2 28.8 49.3 1.2 6.9 0.38
0.9 11.1 21.3 59.9 1.1 5.4
19.9 26.9 52.4 0.3 0.5 Ͻ0.03
–
–
The self-assembly protocol implemented in solution was
then transferred to the silicon surface (Figure 3). The first
step was the binding of 1 on the Si-grafted cavitand through
the complexation of the methyl ammonium moiety. This
was accomplished by dipping Si-Tiiii-B in a 1 mm solution
of 1 in methanol. The surface functionalized with the host–
guest adduct (Si-Tiiii-B·1) was then dipped in a 1 mm solu-
tion of 2 (or 3) in THF to bind the metal complex through
coordination with the phenanthroline moiety to yield Si-
Tiiii-B·1·2 (or Si-Tiiii-B·1·3, respectively).
Step-by-Step Monitoring of the Self-Assembly Process
Each self-assembly step was monitored by XPS. The
atomic compositions of Si-Tiiii-B, Si-Tiiii-B·1, Si-Tiiii-B·1·2
and Si-Tiiii-B·1·3 are reported in Table 2. A low N 1s signal
was already present in the Si-Tiiii-B sample owing to adven-
Figure 4. High-resolution N 1s XPS region of Si-Tiiii-B and Si-
Tiiii-B·1.
The Eu 3d XPS region of the Si-Tiiii-B·1·2 sample shows
titious N-containing species. However, after treatment with the characteristic doublet of Eu 3d5/2 and Eu 3d3/2 that is
guest 1, a significant increase of the N 1s signal was ob- centred at 1135.1 and 1164.8 eV, respectively; this is consis-
served. In addition, the shape of the XPS N 1s band (Fig- tent with the presence of EuIII (Figure 5, a).[16] It is worth
ure 4) is consistent with the molecular formula of 1. In fact, noting that a low-intensity doublet is observed at 1125.1
the band consists of two components: (i) a main feature at and 1155.1 eV, which could indicate the presence of reduced
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EuII.[22] The relative intensity of these bands (with respect of 2 and 3 on the cavitand-decorated surface does not pre-
to the main doublet) increases with X-ray irradiation time serve the original metal-coordination environment, but in-
and the amount of EuII is negligible for very short XPS duces ligand dissociation.
acquisition times (Figure S8 in the Supporting Infor-
mation). This suggests that europium is present only as
Photophysical Measurements on the Surface
EuIII in the monolayer and that reduction is produced dur-
To evaluate the presence of the free or uncomplexed
ing the XPS analysis. In the Si-Tiiii-B·1·3 sample, Y 3d5/2
phenanthroline ligand in the self-assembled systems, the lu-
and Y 3d3/2 peaks (Figure 5, b) appear at 158.8 and
minescence properties of Si-Tiiii-B·1, Si-Tiiii-B·1·2, Si-Tiiii-
B·1·3 and Si-Tiiii-B·2 surfaces have been examined in the
spectral window of the phenanthroline fluorescence band
(λex = 270 nm; Figure 6) for the various surfaces. Only Si-
Tiiii-B·1 showed the band of the phenanthroline ligand
(λmax = 440 nm), whereas in all the other samples no lumi-
nescence was detected (Figure 6). For Si-Tiiii-B·2 this result
is obvious, since phenanthroline is not present on the sur-
face. However, in the case of Si-Tiiii-B·1·2 the absence of
phenanthroline emission corroborates the occurrence of the
energy-transfer properties of the EuIII ion.
160.6 eV, respectively, consistent with the presence of a YIII
compound.
Figure 5. High-resolution XPS regions of (a) Eu 3d of Si-Tiiii-B·1·2
and (b) Y 3d of Si-Tiiii-B·1·3.
To demonstrate that the self-assembly on the surface is
driven by the molecular-recognition properties of the Tiiii-
B cavity and to rule out any physisorption phenomena of
the metal complexes on the surface, a specific control ex-
periment has been designed and executed. A blank sample
made of a complexation-inactive methylene-bridged cavit-
and grafted on silicon (Si-MeCav) was prepared. The re-
sulting Si-MeCav surface was treated with guests 1 and 2
(Si-MeCav·1·2) similarly to Si-Tiiii-B·1·2. XPS analysis of
Si-MeCav·1·2 (Table 2) did not show significant amounts of
nitrogen, europium or fluorine, which shows that physisorp-
tion on the surface is negligible, and the complexation capa-
bility of the Tiiii-B cavity is a key requirement for the self-
assembly process.
The atomic ratios Eu/N/F and Y/N/F of Si-Tiiii-B·1·2
and Si-Tiiii-B·1·3 are 1:3.2:18.2 and 1:3.4:16.9, respectively.
They are close to the theoretical expected ratio 1:4:18, al-
though a slightly lower amount of N is observed in both
cases. This behaviour can be due to a direct coordination
of the metal complexes in the Tiiii-B cavity, to a very limited
extent. To investigate this possible side-reaction pathway,
Si-Tiiii-B surfaces without guest 1 were treated with 2 and
3. XPS analyses of the samples thus obtained (Si-Tiiii-B·2
and Si-Tiiii-B·3) showed the presence of the metals and
fluorine in amounts comparable to that of Si-Tiiii-B·1·2 and
Si-Tiiii-B·1·3 (Figures S9 and S10, respectively, in the Sup-
porting Information), which indicated that the Tiiii-B cavity
can interact directly with complex 2 or 3. However, ob-
tained (Table S1 in the Supporting Information) Eu/F and
Y/F atomic ratios (1:7 and 1:8, respectively) are different
from the theoretical value (1:18) expected for intact com-
Figure 6. Emission spectra of different surfaces in the spectral win-
dow of the phenanthroline fluorescence.
EuIII emission was evaluated in the 480–750 nm range
(λex = 320 nm) for both the Si-Tiiii-B·1·2 and Si-Tiiii-B·2
surfaces. Si-Tiiii-B·1·2 exhibits emission bands related to
5
5
5
5D0 Ǟ7F1, D0 Ǟ7F2, D0 Ǟ7F3 and D0 Ǟ7F4 transitions,
at 592, 610, 648 and 690 nm, respectively, shifted a few
nanometres towards higher energies relative to the com-
pound in solution. For Si-Tiiii-B·2 only the strong signals
of D0 Ǟ7F1 and D0 Ǟ7F2 were observed (Figure 7). The
5
5
Figure 7. Fluorescence spectra of Si-Tiiii-B·1·2 (red trace) and Si-
Tiiii-B·2 (black trace) surfaces probed at five different points of the
plexes. This behaviour suggests that the direct complexation samples; λex = 320 nm.
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and self-assembly in solution and on a silicon surface; syntheses of
tetraphosphonate cavitands Tiiii-A[23] and Tiiii-B without the
methyl groups at the upper rim.[24] The NMR and MS spectra of
the formation of complexes can be found in the Supporting Infor-
mation.
emission intensity recorded for the Si-Tiiii-B·1·2 surfaces is
significantly higher than that of Si-Tiiii-B·2, thanks to the
sensitizing properties of the phenanthroline towards EuIII
,
which ultimately improves light absorption and, accord-
ingly, emission output. Moreover, the relative intensities of
the 5D0 Ǟ7F1 and 5D0 Ǟ7F2 transitions are different for the
two surfaces. In particular, for Si-Tiiii-B·1·2 the intensity
ratio between the 5D0 Ǟ7F1 and 5D0 Ǟ7F2 transitions (1:4)
is similar to that in solution (1:5), whereas for Si-Tiiii-B·2
it is remarkably different (1:2). These results suggest that
the self-assembly process in the presence of 1 occurs with-
out significant changes of the Eu coordination environment
relative to that of the solution process. By contrast, the di-
rect complexation of 2 in the Tiiii cavity leads to a signifi-
cant modification of the original Eu environment.
Bromo-N-(1,10-phenanthrolin-5-yl)acetamide (4): 5-Amino-1,10-
phenanthroline (0.20 g, 1.03 mmol) was placed in a round-bot-
tomed flask under argon. Acetonitrile (dry, 40 mL) and triethyl-
amine (0.11 g, 1.04 mmol, 0.15 mL) were added and the suspension
was stirred for 30 min at room temperature. The mixture was co-
oled to 0 °C before bromoacetylchloride (0.19 g, 1.23 mmol,
0.10 mL, 1.2 equiv.) in acetonitrile (5 mL) was added dropwise. The
mixture was left stirring for 16 h at room temperature and analysis
by TLC [CH2Cl2/MeOH(saturated with NH3) 9:1] showed no fur-
ther progress of the reaction. The solvent was removed under re-
duced pressure and the residue was solubilized in 5% K2CO3 aque-
ous solution (10 mL). Triple extraction with dichloromethane was
conducted and organic layers were collected and evaporated, yield-
ing a brown residue (0.25 g) that was next purified by silica column
chromatography using 95:5 CH2Cl2/MeOH(NH3) as eluent to give
a pale brown solid (0.166 g, 51%). 1H NMR (400 MHz, CDCl3,
298 K): δ = 9.26 (d, J = 4.2 Hz, 1 H), 9.18 (d, J = 5.6 Hz, 1 H),
8.81 (br. s, 1 H, NH), 8.39 (s, 1 H), 8.36 (d, J = 8.5 Hz, 1 H), 8.27
(d, J = 8.0 Hz, 1 H), 7.74 (dd, J = 4.0, 5.4 Hz, 1 H), 7.67 (dd, J =
5.8, 5.9 Hz, 1 H), 4.21 (s, 2 H, CH2) ppm.
Finally, no emission was observed for Si-MeCav·1·2. This
result matches with XPS findings, which indicates that no
EuIII complex was anchored on the surface in the absence
of specific Tiiii receptors.
Conclusion
Herein, we have reported the first example of the hierar-
chical self-assembly of luminescent lanthanide complexes
on silicon. Formation of these supramolecular structures on
a Si surface has been confirmed by two independent analyt-
ical techniques (XPS and fluorescence) and validated by
control experiments with surfaces decorated with a com-
plexation-inactive methylene-bridged cavitand. Self-as-
sembly on the cavitand-functionalized surface with a sarco-
sine–phenanthroline ligand followed by [Eu(hfac)3] (hfac =
hexafluoroacetylacetonate) coordination afforded a more
strongly emitting surface than direct coordination of
[Eu(hfac)3] by Tiiii due to the sensitization of the EuIII ion
by the phenanthroline ligand. The present approach can be
implemented for applications in sensing, bioimaging and
optoelectronic devices. The use of the universal bidentate
ligand 1 allows the complexation of other lanthanide ions,
as well as of metals in general. Moreover, the self-assembly
of these ternary complexes on silicon will widen the spec-
trum of possible applications, since silicon is a technologi-
cally important inorganic platform.
Ligand 1: Sodium iodide (1.2 equiv., 0.94 g; 0.63 mmol) was added
to 4 (0.166 g, 0.63 mmol) dissolved in dry acetonitrile (20 mL) (pale
yellow solution). The solution was stirred for 10 min at room tem-
perature until a white precipitate appeared (NaBr). Next, 30%
methylamine solution in ethanol (5 mL) was added in portions over
6 h of stirring at room temperature until analysis by TLC indicated
no starting compound was present. The reaction mixture was evap-
orated under reduced pressure and was further purified by silica
column chromatography using 95:5 CH2Cl2/MeOH(NH3) as eluent
1
to give a yellow-brown solid (0.126 g, 90%). H NMR (400 MHz,
CDCl3, 298 K): δ = 10.23 (br. s, 1 H, NH), 9.26 (d, J = 4.0 Hz, 1
H), 9.14 (d, J = 4.0 Hz, 1 H), 8.65 (s, 1 H), 8.34 (d, J = 8.4 Hz, 1
H), 8.26 (d, J = 8.0 Hz, 1 H), 7.73 (dd, J = 4.3, 4.3 Hz, 1 H), 7.67
(dd, J = 4.3, 4.3 Hz, 1 H), 3.58 (s, 2 H, CH2), 2.68 (s, 3 H, CH3)
ppm. 13C NMR (100 MHz, CDCl3, 298 K): δ = 170.2, 150.2, 149.6,
135.9, 130.2, 128.7, 123.5, 123.2, 122.8, 116.6, 55.4, 37.2 ppm.
HRMS: calcd. for C15H15N4O [M + H] 267.12; found 267.2.
Tiiii-C: Dichlorophenylphosphine (0.081 mL, 0.582 mmol) was
added slowly at room temperature to a solution of heptadecyl-
footed resorcinarene (130 mg, 0.090 mmol) in freshly distilled pyr-
idine (10 mL). After 3 h of stirring at 70 °C, the solution was cooled
to room temperature and aqueous 35% H2O2 (2 mL) was added.
The resulting mixture was stirred for 30 min at room temperature.
H2O (100 mL) was added and the solution was filtered. The solid
was suspended in water (100 mL), sonicated for 30 min and filtered
again resulting in a white powder (140 mg, 80%). 1H NMR
(400 MHz, CDCl3): δ = 8.10–8.04 (m, 8 H, PO–Ar–Ho), 7.64 (m,
4 H, POArHp), 7.55–7.51 (m, 8 H, POArHm), 7.31 (s, 4 H, Ar–
Hup), 7.02 (s, 4 H, Ar–Hdown), 4.82 (bt, 4 H, Ar–CH–Ar), 2.36 (bd,
8 H, –CH–CH2–CH2–), 1.47 (m, 8 H, –CH2–CH2–CH3), 1.28 [m,
112 H, –CH2–(CH2)14–CH3], 0.90 (m, 12 H, –CH2–CH3) ppm. 31P
NMR (160 MHz, CDCl3): δ = 7.6 (s, 4 P) ppm.
Experimental Section
General: All commercial reagents were ACS reagent grade and used
as received. Solvents were dried and distilled by using standard
procedures. 1H NMR spectra were recorded on Bruker Avance 400
(400 MHz) and Bruker Avance 300 (300 MHz) NMR spectrome-
ters. All chemical shifts (δ) were reported in parts per million (ppm)
relative to proton resonances resulting from incomplete deuteration
of NMR spectroscopy solvents. ESI-MS experiments were per-
formed on a Waters ZMD spectrometer equipped with an electro-
spray interface. The exact mass values were determined using a
LTQ ORBITRAP XL Thermo spectrometer equipped with an elec-
trospray interface. This section only includes the synthesis and
characterization of ligand 1 from commercially available 5-amino-
1,10-phenanthroline (5), through a modified literature procedure[14]
Cavitand–Guest–Ligand Complex Tiiii-A·1:
A
solution of
CH3COCl in methanol (0.752 m, 0.1 mL used) was added dropwise
to a solution of 1 in methanol (20 mg, 0.0752 mmol). Cavitand
Tiiii-A (89 mg, 0.0752 mmol) was added to this transparent solu-
tion. The mixture was stirred until everything dissolved (5 min,
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room temp.). Next, the solvent was removed under reduced pres-
sure and an orangish residue, Tiiii-A·1, was dried under vacuum.
1H NMR (400 MHz, CDCl3, 298 K): for 1: δ = 9.34 (br. s, 1 H),
9.26 (br. s, 1 H), 9.02 (br. s, 1 H), 8.43 (br. s, 1 H), 7.80 (s, 1 H),
7.71 (s, 1 H), 7.30 (br. s, 1 H), 6.83 (br. s, 2 H, NH2) 3.76 (s, 2 H,
CH2), –0.50 (s, 3 H, CH3) ppm; for Tiiii-A: δ = 8.15 (q, J = 7.2 Hz,
8 H, Ar–H), 7.82 (br. s, 4 H, Ar–H), 7.73 (t, J = 7.6 Hz, 4 H, Ar–
H), 7.60 (m, 8 H, Ar–H), 4.83 (t, J = 7.6 Hz, 4 H, Ar–CH–Ar),
2.63 (m, 8 H, CH–CH2–CH2), 2.20 (s, 12 H, Ar–CH3), 1.46 (m, 8
H, CH2–CH2–CH3), 1.10 (m, 12 H, CH2–CH3) ppm. 31P NMR
(160 MHz, CDCl3, 298 K): δ = 10.8 (s, 4 P) ppm.
mentioned mesitylene solution. The cell remained under UV irradi-
ation (254 nm) for 2 h. The sample was then removed from the
solution and was sonicated twice in dichloromethane for 10 min to
remove residual physisorbed material.
Host–Guest System on Silicon (Si-Tiiii-B·1): A Si-Tiiii-B wafer was
dipped in a 1ϫ10–3 m solution of guest 1 in CH3OH for 60 min,
sonicated in CH3OH for 5 min and dried in nitrogen flux.
Ternary Complex on Silicon with 2 and 3, Si-Tiiii-B·1·2 and Si-Tiiii-
B·1·3: Si-Tiiii-B·1 was dipped in solutions of complexes 2 and 3 in
THF (1ϫ10–3 m) for 30 min, sonicated in THF for 5 min and dried
in nitrogen flux.
Tiiii-C·1: A solution of CH3COCl in methanol (0.403 m, 0.1 mL
used) was added dropwise to a solution of 1 in methanol (11 mg,
0.040 mmol). Cavitand Tiiii-C (78 mg, 0.040 mmol) was added to
this transparent solution. The mixture was stirred for 5 min at
room temperature. The resulting orangish residue was dried under
Photophysical Measurements: Absorption spectra were recorded in
Hellma quartz cells (1 cm) with a Perkin–Elmer Lambda 950 UV/
Vis/NIR spectrophotometer. Steady-state photoluminescence spec-
tra and excitation spectra were recorded with an Edinburgh
FLS920 spectrometer (continuous 450 W Xe lamp), equipped with
a Peltier-cooled Hamamatsu R928 photomultiplier tube (185–
850 nm). Emission quantum yields of EuIII-based samples were de-
termined according to the approach described by Demas and
Crosby[25] by using [Ru(bipy)3Cl2] (Φem = 0.028 in air-equilibrated
water solution; bipy = 2,2Ј-bipyridine)[26] as standard.
1
vacuum. H NMR (400 MHz, CDCl3, 298 K): for 1: δ = 9.30 (br.
s, 2 H), 9.02 (br. s, 1 H), 8.49 (m, 4 H), 3.80 (s, 2 H, CH2), –0.64
(s, 3 H, CH3) ppm; for Tiiii-C: δ = 8.10 (m, 8 H, PO–Ar–H), 7.69
(m, 4 H, PO–Ar–H), 7.58 (m, 8 H, PO–Ar–H), 7.25 (br. s, 4 H,
Ar–H), 6.97 (br. s, 4 H, Ar–H), 4.78 (t, J = 6.6 Hz, 4 H, Ar–CH–
Ar), 2.85 (br. s, 8 H, CH-CH2-CH2), 1.51 (br. s, 8 H, CH–CH2–
CH2), 1.42 [m, 112 H, CH2–(CH2)14–CH3], 0.89 (m, 12 H, CH2–
CH2–CH3) ppm. 31P NMR (160 MHz, CDCl3, 298 K): δ = 12.1 (s,
4 P) ppm.
Excited-state lifetimes on the nanosecond timescale were measured
with a IBH 5000F time-correlated single-photon-counting spec-
trometer, by using pulsed NanoLED excitation sources at 278 nm;
analysis of the luminescence decay profiles was done with Decay
Analysis Software DAS6 provided by the manufacturer. Emission
decays on the millisecond timescale were measured with a Perkin–
Elmer LS-50B spectrofluorimeter equipped with a pulsed Xe lamp
and in gated detection mode. The phosphorescence decay analysis
was performed with the PHOSDecay software provided by the
manufacturer. Experimental uncertainties are estimated to be Ϯ2
and Ϯ5 nm for absorption and emission peaks, Ϯ20% for emission
quantum yields, and Ϯ8% for lifetimes.
Ternary Complex Tiiii-A·1·2: Complex 2 (14 mg, 0.017 mmol) was
added at once to a solution of Tiiii-A·1 in dichloromethane (25 mg,
0.017 mmol) and the mixture was stirred for 1 h at room tempera-
ture. The solvent was removed under reduced pressure and orange
residue Tiiii-A·1·2 was dried under vacuum. 31P NMR (160 MHz,
CDCl3, 298 K): δ = 11.3 (s, 4 P) ppm.
Ternary Complex Tiiii-A·1·3: Complex 3 (14.2 mg, 0.02 mmol) was
added at once to a solution of Tiiii-A·1 in dichloromethane (30 mg,
0.02 mmol) and the mixture was stirred for 1 h at room tempera-
ture. The solvent was removed under reduced pressure and orange
Supporting Information (see footnote on the first page of this arti-
1
residue Tiiii-A·1·3 was dried under vacuum. H NMR (400 MHz,
cle): It contains the monitoring data for the formation of complexes
1
by means of H and 31P NMR spectroscopy; the HRMS spectrum
CDCl3): for 1: δ = 9.25 (br. s, 1 H), 9.18 (br. s, 1 H), 8.70 (br. s, 1
H), 8.46 (s, 1 H), 8.23 (br. s, 1 H), 7.70 (br. s, 1 H), 7.09 (br. s, 1
H), 6.83 (br. s, 2 H, NH2), 3.58 (br. s, 2 H, CH2), –0.58 (s, 3 H,
CH3); for 2: δ = 6.07 (br. s, 2 H), 6.00 (br. s, 1 H) ppm; for Tiiii-
A: δ = 8.10 (q, J = 7.2 Hz, 8 H, Ar–H), 7.82 (br. s, 4 H, Ar–H),
7.75 (t, J = 7.04 Hz, 4 H, Ar–H), 7.59 (m, 8 H, Ar–H), 4.83 (t, J
= 7.2 Hz, 4 H, Ar–CH–Ar), 2.61 (m, 8 H, CH–CH2–CH2), 2.18 (s,
12 H, Ar–CH3), 1.47 (m, 8 H, CH2–CH2–CH3), 1.10 (m, 12 H,
CH2–CH3) ppm. 31P NMR (160 MHz, CDCl3, 298 K): δ = 11.0 (s,
4 P) ppm.
of the ternary complex; absorption spectra; XPS data; and compe-
tition experiments.
Acknowledgments
This work was supported by the European Union (contract PITN-
GA-2008-215399 - FINELUMEN). The authors also thank MIUR
for partial financial support through FIRB “RINAME” (RBA-
P114AMK). The authors thank Dr. Nicola Armaroli of Molecular
Photoscience Group, Istituto per la Sintesi Organica e la Fotoreat-
tività del CNR (ISOF-CNR) for support with the photophysical
measurements and for fruitful discussion of the results. J. M. M.
further thanks Istituto di Studi Avanzati, University of Bologna,
for the BRAINS-IN fellowship. Centro Interdipartimentale Misure
“G. Casnati” of the University of Parma is acknowledged for the
use of NMR spectroscopy and HR ESI-MS facilities. [Eu(hfac)3]
was kindly provided by Prof. Marek Pietraszkiewicz of the Institute
of Physical Chemistry of the Polish Academy of Science, Warsaw.
Ternary Complex Tiiii-C·1·2: Tiiii-C·1 (29 mg, 0.013 mmol) and
[Eu(hfac)3] (10.5 mg, 0.013 mmol) were dissolved in dichlorometh-
ane (10 mL) and the solution was stirred for 1 h at room tempera-
ture until everything was dissolved. The solvent was removed under
reduced pressure and the residue was dried under vacuum. 31P
NMR (160 MHz, CDCl3, 298 K): δ = 12.6 (s, 4 P) ppm. HRMS:
+
calcd. for [M – Cl]+ C150H190EuF18N4O19P4 2971.19277; found
2971.18155.
Synthesis of Si-Tiiii-B: The Tiiii-B/1-octene mixture (χcav = 0.05)
was dissolved in mesitylene (solution concentration = 50 mm) and
this solution (2.0 mL) was placed in a quartz cell and deoxygenated
by stirring in a dry box for at least 1 h. A Si(100) substrate was
dipped in H2SO4/H2O2 (3:1) solution for 12 min to remove organic
contaminants, then it was etched in a hydrofluoric acid solution
(1% v/v) for 90 s and quickly rinsed with water. The resulting hy-
drogenated silicon substrate was immediately placed in the above-
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Published Online: April 30, 2014
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