Cross-coupling of 5,11-dibromotetracene catalyzed by a triethylammonium ion tagged diphenylphosphine palladium complex in ionic liquids

Article abstract of DOI:10.1021/om2003943

Suzuki-Miyaura cross-coupling reactions of 5,11-dibromotetracene with arylboronic acids, using a triethylammonium-tagged palladium(II) diphenylphosphine complex as catalyst in a pyrrolidinium-based ionic liquid (IL), allowed the preparation of new 5,11-di

Full text of DOI:10.1021/om2003943

ARTICLE
pubs.acs.org/Organometallics
Cross-Coupling of 5,11-Dibromotetracene Catalyzed by a
Triethylammonium Ion Tagged Diphenylphosphine Palladium
Complex in Ionic Liquids
Antonio Papagni,† Claudio Trombini,‡ Marco Lombardo,*,‡ Stefano Bergantin,† Amani Chams,§
Michel Chiarucci,‡ Luciano Miozzo,*,†,§ and Matteo Parravicini†
†Scienza dei Materiali, Universitꢀa degli Studi di Milano Bicocca, Via R. Cozzi 53, 20125 Milano, Italy
‡Dipartimento di Chimica “G. Ciamician”, Universitꢀa degli Studi di Bologna, via Selmi 2, 40126 Bologna, Italy
§ITODYS, Cnrs-Umr 7086, Universitꢁe Paris Diderot, 15 rue Jean Antoine de Baïf, 75205 Paris Cedex 13, France
S Supporting Information
b
ABSTRACT:
SuzukiÀMiyaura cross-coupling reactions of 5,11-dibromotetracene with arylboronic acids, using a triethylammonium-tagged
palladium(II) diphenylphosphine complex as catalyst in a pyrrolidinium-based ionic liquid (IL), allowed the preparation of new
5,11-diaryl-substituted tetracenes in good to excellent yields. The synthesis of the new 5,11-diboronic-tetracene bis-pinacolate ester
and its use in SuzukiÀMiyaura cross-coupling reaction with aryl bromides in IL are also reported.
etracene, pentacene, and their derivatives are among the
most studied molecular organic semiconductors.1 A large
5,11-ditolyltetracene are reported in the literature,6 while the
preparation of 5,11-dinaphthyltetracene is claimed in a recent
patent.7 Although cross-coupling procedures look very attractive
as a general synthetic protocol for substituted tetracenes,8 only
recently the synthesis of substituted tetracenes has been report-
ed via a KumadaÀCorriu cross-coupling reaction between the
5,6,11,12-tetrachlorotetracene and methylmagnesium chloride,9
and using a SuzukiÀMiyaura cross-coupling reaction between
the 5,11-dibromotetracene and a perylene-based boronic deriv-
ative.10 This is quite surprising, since a SuzukiÀMiyaura cross-
coupling reaction is particularly appealing,11 due to the wide
commercial availability of stable boronic acids and esters, the
mild conditions required, and the high tolerance toward func-
tional groups. The difficulties related to the availability of iodo-
and bromoacenes and their low reactivity for steric demands
when the halides are at the 5-, 6-, 11-, and 12-positions partially
justify the scarce use of these substrates in transition-metal-
mediated cross-coupling reactions.
T
number of substituted acenes have been reported and used,
for example, for organic light-emitting diodes, field-effect tran-
sistors, and solar cells.2 Among them, substituted tetracenes,
such as 5,6,11,12-tetraphenyltetracene (rubrene) and 5,11-
dichlorotetracene,3 show very high charge mobilities. Indeed,
rubrene represents the state of the art in this field, showing
exceptional charge carrier mobility values up to 20 cm2/(V s).
The origin of these outstanding mobility values has been ascribed
to a favorable crystal packing,4 even though deeper investiga-
tions, aimed at understanding the origin of the electronic proper-
ties of these organic semiconductors, are in progress. Within this
framework, the improvement of mobility values is based on the
tuning both of electronic properties at the molecular level and of
the crystal packing, by acting on the nature of the substituents
present on the tetracene backbone. Unfortunately, few protocols
for the synthesis of substituted acenes are described in the
literature. They are traditionally based on multistep aryllithium
addition to naphthacenediones and DielsÀAlder-based reac-
tions, whose low yields and lack of generality are a limitation
to the preparation of a wide spectrum of derivatives.5 This is
particularly true for 5,11-diaryl-substituted tetracenes, and to the
best of our knowledge, only the syntheses of 5,11-diphenyl- and
Recently, an effective protocol for the SuzukiÀMiyaura cross
coupling in ionic liquids (ILs) has been developed, where the
ionic phosphine 1 (Scheme 1) acts as the palladium ligand (L).12
Received: May 13, 2011
Published: July 26, 2011
r
2011 American Chemical Society
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Organometallics
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Scheme 1. Synthesis of the Ion-Tagged Palladium Complex
PdL2Cl2 and SuzukiÀMiyaura Cross-Coupling Reactions in
1-Butyl-1-methylpyrrolidinium Bis(trifluoromethane)-
sulfonamide ([bmpy][NTf2])
arylboronic acids (aÀd) using the ligand 1 and [bmpy][NTf2] as
the IL at 60 °C. The reactions were very fast, and even though
the starting dibromotetracene 3a did not dissolve completely
in the ionic liquid, it was completely consumed after 2 h at 60 °C.
The reaction mixture turned from dark red to yellow when the
starting material was consumed, and the course of the reaction
can be easily monitored by TLC, since the formation of the
products was accompanied by the appearance of a very char-
acteristic bright yellow fluorescent spot. It is noteworthy that
both the reaction workup and the purification of the products are
very simple and fast, an important feature in order to limit the
formation of endoperoxides by the reaction of products with
oxygen (Scheme 3).
After purification by flash chromatography, 5,11-diaryltetra-
cenes 4aÀd were collected in good to excellent chemical yields
(Table 1). It is noteworthy that under our milder experimental
conditions, in comparison to those reported by M€ullen,14 better
yields and faster reactions are obtained.
With the aim of extending the potential and the scope of the
SuzukiÀMiyaura approach to disubstituted tetracenes, we also
synthesized 5,11-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)tetracene (6). Thus, 5,11-dibromotetracene (3a) was reacted
with bis(pinacolato)diboron 5 in the presence of Pd(AcO)2 as
catalyst at 80 °C in DMF, as described in the literature,16
affording 6 in 87% chemical yield. The tetracene derivative 6,
in turn, has been used in SuzukiÀMiyaura cross-coupling reac-
tions with bromobenzene and 4-cyanobromobenzene, under the
same experimental conditions used for 4aÀd, affording 4a,e in
38% and 37% yields, respectively (Scheme 4).
Due to the peculiar polarity properties of ILs, the cross-
coupling reactions performed in this medium seem to benefit
from the effect of high ionic strength or high polarity, indepen-
dent of the steric demand of the starting aryl halide. The high
yield of 6, synthesized in a high-polarity medium, seems to
confirm this hypothesis. It is reasonable that the polarity of the
medium plays an important role in the oxidative addition of
Pd(0) into a CÀBr bond: the Pd(II)ÀBr bond could have a
higher ionic character or it could even be dissociated under these
conditions, also favoring the transmetalation step in the catalytic
cycle proposed for these reactions. The low yield in derivative 4
from 6 could be explained by the higher steric demands of 6 in
comparison to the simpler boronic acids used in reaction with 3a,
which slow down the transmetalation step.
All the new compounds have been fully spectroscopically
characterized (see the Experimental Section,) and their optical
properties are reported in Table 2, together with the standard
potential values as determined by electrochemical techniques
using cyclic voltammetry. UVÀvisible absorption spectra of 5,11-
disubstituted tetracenes closely resemble that of tetracene, with
absorption maxima between 400 and 500 nm corresponding to
the π f π* transition in these compounds,17 characterized by a
series of vibronic replica, due to the coupling between the
electronic transition and the 0.18 eV intramolecular vibration
mode(duetoCÀCstretchingofaromaticrings). Clearlythenature
of the substituents does not affect the vibronic structure of the
tetracene core, whereas the electronic interaction between them
and the tetracene unit caused a small, systematic red shift of the
band, with a maximum shift for the 5,11-dibromotetracene. The
small red shift is due to the polarization of the electronic
transition that originated this absorption band; the introduction
of substituents in the 5,11- or 5,12-positions has a minor impact,
while the introduction of substituents on the short edge (positions
The resulting ion-tagged palladium complex PdL2Cl2 proved to
be suitable for SuzukiÀMiyaura cross-coupling reactions under
mild conditions, even using sterically hindered substrates. In
addition, it is noteworthy that the ionic phase containing the
catalytically active palladium species could be reused up to six
times without loss of activity. The ability of the ionic ligand to
stabilize the palladium catalyst, preventing precipitation of
inactive “Pd black”, is the key to success in catalyst recycling.
Here we report the application of this protocol to 5,11-
dibromotetracene, which allows the preparation of a series of
new 5,11-bis-aryl-substituted tetracenes in good to excellent
chemical yields. In addition, we also describe the synthesis of
the new 5,11-diboronic-tetracene bis-pinacolate ester and its use
in SuzukiÀMiyaura cross-coupling reactions with aryl bromides
in ILs. Among the possible ILs, we decided to use those pos-
sessing a pyrrolidinium ion, since an enhanced reactivity had
been previously observed with these ILs in palladium-mediated
cross-coupling reactions.13 On the other hand, imidazolium-
based ILs, although almost as efficient as pyrrolidinium-based
ILs, are known to afford N-heterocyclic carbene complexes
(NHC) on prolonged heating in the presence of a base.
’ RESULTS AND DISCUSSION
We synthesized 5,11-dibromotetracene (3a) by electrophilic
bromination with N-bromosuccinimide (NBS) in a CHCl3À
DMF mixture, as described by M€ullen,14 to overcome the low
solubility of tetracene (2) in DMF, normally used in the
bromination of polycyclic arenes.15 Both 5,11- (3a) and 5,12-
dibromotetracene (3b) are formed, but using the experimental
conditions described by M€ullen, the formation of 3b is less than
10% (Scheme 2). Pure 3a can be obtained by crystallization from
toluene. The crude bromination mixture is also contaminated by
some oxidation products (tetracene-quinone), which can be
easily removed by filtration over a bed of SiO2.
In a preliminary study, we tested the Suzuki coupling reaction
of 5,11-dibromotetracene (3a) with aryl- or heteroarylboronic
acids, using Pd(PPh3)4 as a catalyst in traditional toluene/water
mixtures, but only traces of the desired diarylated tetracene
derivatives were detected, in agreement with literature reports for
similar reactions.14 To overcome the reduced reactivity of 3a, we
carried out the SuzukiÀMiyaura cross-coupling reaction in ILs,
applying our original experimental conditions.12a 5,11-Dibromo-
tetracene 3a was allowed to react with differently substituted
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Scheme 2. Synthesis of 5,11-Dibromotetracene
Scheme 3. Suzuki Cross-Couplings of 5,11-Dibromotetra-
cene 3a in Ionic Liquids (L = 1)
Scheme 4. Synthesis and Suzuki Cross-Couplings of 5,11-
Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)tetracene 6
in Ionic Liquids (L = 1)
Table 1. Synthesis of 5,11-Diaryltetracenes 4aÀd
entrya
product
reacn time (h)
yield (%)b
1
2
3
4
4a
4b
4c
4d
2
2
4
7
97
94
93
85
Table 2. Optical Properties and Electrochemistry Data for
5,11-Diaryltetracenes 4aÀe
a Reaction conditions: PdL2Cl2 (3 mol %), [bmpy][NTf2] (0.5 mL),
arylboronic acid (3 equiv), K3PO4 (4 equiv), degassed water (0.25 mL),
3a (0.065 mmol), 65À70 °C. b Yield of isolated product after flash
chromatography.
absorption max
(nm)a
fluorescence max
std potential
(V)c
entry
product
(nm)b
1
2
3
4
5
6
7
8
4a
493
495
495
495
493
501
488
502
510
499
509
n.d.d
n.d.d
509
0.887
0.827
1.034
1.045
1.195
4b
2 and 3) has a greater impact on the position of this band. In the same
way, the introduction of substituents in the 5,11-positions has a minor
impact also on fluorescence spectra. Notably, compounds 3a and 4e
are not fluorescent, due to the heavy-atom effect of bromine (3a) or
to the presence of electron acceptors (4e). The standard potentials of
compounds 4aÀe, 6, and rubrene have been determined using cyclic
voltammetry (CV). Rubrene is considered as the reference molecule.
Voltammograms are presented in the Supporting Information. All
these molecules exhibit a reversible one-electron redox process with a
standard potential value in the range 0.810À1.24 V (Table 2). As
expected, rubrene shows the lowest standard potential, due to the
presence of four phenyl groups on the tetracene structure, whereas
higher standard potentials are measured for molecules 4aÀe
and 6, depending on the nature and structure of the 5,11-
substituents. The electron-withdrawing character of the cyano
groups induces the greater increase in the standard potential;
in contrast, the methoxyphenyl groups, known as electron
donors, induce a slight increase of the standard potential, in
comparison to that of rubrene. The HOMOÀLUMO gap is
slightly modified by the different substituents, as confirmed by
the positions of the absorption and fluorescence spectra.
Moreover, the significant anodic shifts measured by electro-
chemistry indicate that substituents cause a rigid shift of the
position of the HOMO and LUMO levels.
4c
4d
4e
3a
6
0.973
0.810
rubrene
a Position of the lowest energy absorption maximum, in 10À5 M CH2Cl2
solution. See the Supporting Information for full absorption spectra.
b In 10À5 MCH2Cl2 solution. c Standard potential in V vs SCE. d Compound
is not fluorescent.
’ CONCLUSIONS
We have reported here a palladium cross-coupling protocol in
ionic liquids for the synthesis of 5,11-diaryltetracenes 4aÀe
starting from 5,11-dibromotetracene (3a). The use of both ionic
liquids and ionic ligands turned out to be useful for the synthesis of
these challenging substrates in high chemical yields and under very
mild conditions. The synthesis of the 5,11-diboronyl-tetracene 6
derivative has been also described together with a preliminary
studyon itsreactivityinSuzukiÀMiyauracross-coupling reactions.
The physicochemical characterization of the products synthesized
showed that the physical properties of the tetracene can indeed be
tuned by installing different substituents onto its skeleton.
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’ EXPERIMENTAL SECTION
5,11-Bis(biphenyl-4-yl)tetracene (4c). Yield: 93%. 1H NMR (400 MHz,
CDCl3): δ (ppm) 8.48 (s, 2H), 7.94À7.91 (m, 4H), 7.88À7.79 (m, 4H),
7.73 (d, J = 7.8 Hz, 4H), 7.65 (d, J = 7.7 Hz, 4H), 7.60À7.52 (m, 4H), 7.45
(t J = 7.4 Hz, 2H), 7.36À7.29 (m, 4H). 13C NMR (100 MHz, CDCl3): δ
(ppm) 140.9, 140.3, 138.1, 136.7, 136.5, 132.0, 131.1, 129.4, 129.1, 129.0,
128.9, 128.5, 128.2, 127.5, 127.2, 125.4, 124.8. MS (EI): m/z 532 [M+].
HPLC (λ = 280 nm, method A): tR = 26.7 min.
General Methods. 1H and 13C NMR spectra were recorded at 400
and 100 MHz, respectively, on a Varian Inova spectrometer. Chemical
shifts (δ) are reported in ppm relative to TMS. HPLC studies were
carried out with a reverse-phase column (ZORBAX-Eclipse XDB-C8
Agilent Technologies) using the following conditions. Method A: H2O/
MeCN from 90/10 to 0/100 in 20 min, flow 0.5 mL/min, T = 30 °C.
Method B: H2O/MeCN from 70/30 to 20/80 in 8 min, flow 0.4 mL/
min, T = 30 °C. All starting materials were purchased from Aldrich,
Acros, or ABCR and used as received. Electrochemical analyses were
carried out in a one-compartment three-electrode cell using a Biologic
VSP Potentiostat (Grenoble, France). Dichloromethane (SDS, ProLa-
bo, HPLC grade) and the supporting electrolyte, tetrabutylammonium
hexafluorophosphate (TBAPF6, Aldrich, electrochemical grade), were
used without further purification. EC-Lab Express software supplied by
BioLogic was used for data acquisition. Electrochemical analyses were
performed using a platinum disk (outer diameter 1.6 mm) as the
working electrode, a stainless steel grid as the counter electrode, and a
saturated calomel electrode (SCE) as the reference electrode. Prior to
each analysis, the working electrode was carefully polished with diamond
paste (1 μm), rinsed with ethanol, and then air-dried. Cyclic voltam-
metry (CV) was carried out in dichloromethane containing the mono-
mer (5 Â 10À4 M) and tetrabutylammonium hexafluorophosphate
electrolyte salt (10À1 M), by varying the potential between 0 and 1.3 V at
a potential scan rate of 50 mV sÀ1. Absorption and fluorescence spectra
were measured on 10À5 M CH2Cl2 solutions. UVÀvis absorption
spectra were measured with a Varian CARY 500 spectrophotometer.
Fluorescence spectra were measured on a SPEX Fluorolog-3 instrument
(Jobin-Yvon). Compounds 4aÀe were characterized by NMR and mass
spectroscopy and judged >95% pure, but no satisfying elemental
analyses could be obtained, due to their instability toward air oxidation.
General Procedure for the Synthesis of 5,11-Diaryltetra-
cene. In an oven-dried Schlenk tube, under an argon atmosphere, the
Pd complex PdCl2L2 (2.8 mg, 0.002 mmol, 3 mol %) was dissolved in
0.5 mL of 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)-
imide ([bmpy][NTf2]) and the yellow, clear solution was stirred under
vacuum for a few minutes. Arylboronic acid (0.194 mmol, 3 equiv),
K3PO4 (0.260 mmol, 4 equiv), and 0.25 mL of degassed water were
added to the solution, and the reaction mixture was heated at 65À70 °C.
After a few minutes an orange homogeneous solution was formed. 5,11-
Dibromotetracene (3a; 25 mg, 0.065 mmol, 1 equiv) was added to the
reaction mixture, which was vigorously stirred at 65À70 °C until
complete consumption of the starting material (monitored by TLC).
After it was cooled to room temperature, the reaction mixture was taken
up with CH2Cl2, adsorbed on silica, and directly charged onto a silica gel
column. The desired product was isolated by elution with cyclohexane/
CH2Cl2 mixtures. During the whole process exposure to light and air
should be minimized in order to avoid the possible formation of oxidized
side products.
5,11-Bis(naphthalen-2-yl)tetracene (4d). Yield: 85%. 1H NMR
(400 MHz, CDCl3, mixture of two atropoisomers): δ (ppm) 8.43 (s,
2 H), 8.14 (s, 1.2 H), 8.18À8.14 (m, 2H), 8.09 (d, J = 10.5 Hz, 4H), 7.98
(d, J = 7.1 Hz, 2H), 7.80 (d, J = 8.5 Hz, 1H), 7.73À7.62 (m, 9H),
7.32À7.21 (m, 4H). 13C NMR (100 MHz, CDCl3, mixture of two
atropoisomers): δ (ppm) 137.0, 136.9, 136.70, 136.69, 133.5, 132.9,
131.10, 131.06, 130.5, 130.43, 130.40, 129.8, 129.60, 129.57, 129.3, 129.2,
129.02, 128.97, 128.4, 128.2, 128.12, 128.09, 128.07, 128.0, 127.1, 126.6,
126.5, 126.34, 126.28, 126.1, 126.0, 125.8, 125.4, 125.2, 124.9, 124.8. MS
(EI): m/z 480 [M+]. HPLC (λ = 280 nm, method A): tR = 25.3 min.
Synthesis of 5,11-Bis(4,4,5,5-tetramethyl-1,3,2-dioxabor-
olan-2-yl)tetracene (6). In an oven-dried round-bottom flask
equipped with a reflux condenser, under an argon atmosphere, Pd-
(OAc)2 (4.4 mg, 0.019 mmol), bis(pinacolato)diboron (5; 148 mg,
0.582 mmol), KOAc (86 mg, 0.873 mmol), and 5,11-dibromotetracene
(3a; 75 mg, 0.194 mmol) were dissolved in 15 mL of anhydrous DMF
and heated at 80 °C for 7 h. The red solution turned yellow-green during
the reaction. The reaction mixture was then cooled to 0 °C, and 30 mL of
water was added. The aqueous layer was extracted twice with ethyl
acetate (30 mL). The collected organic phases were washed with brine,
dried over Na2SO4, and concentrated under vacuum. The crude prod-
uct was purified by flash chromatography using cyclohexane/CH2Cl2
(80/20) as the eluent (yellow solid, yield 84%). 1H NMR (400 MHz,
CDCl3): δ (ppm) 9.21 (s, 2H), 8.42 (d, 2H, J = 9.2 Hz), 7.97 (d, 2H,
J = 8.0 Hz), 7.45À7.36 (m, 4H), 1.66 (s, 24H). 13C NMR (100 MHz,
CDCl3): δ (ppm) 135.8, 133.4, 131.0, 130.4, 129.6, 128.1, 125.7, 124.5,
84.5, 25.2 (carbon attached to boron was not observed).18 MS (EI): m/z
481 [M+]. HPLC (λ = 280 nm, method B): tR = 21.3 min.
Suzuki Reaction with 5,11-Bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)tetracene (6). In an oven-dried Schlenk tube,
under an argon atmosphere, the Pd complex PdCl2L2 (1.9 mg, 0.0013
mmol, 3 mol %) was dissolved in 0.5 mL of [bmpy][NTf2] and
the yellow, clear solution was stirred under vacuum for a few minutes. 6
(21 mg, 0.044 mmol, 1 equiv), K3PO4 (0.176 mmol, 4 equiv), and
0.25 mL of degassed water were added to the solution, which was heated
at 65À70 °C. The aryl bromide (0.132 mmol, 3 equiv) was added to
the reaction mixture, which was vigorously stirred at 65À70 °C until
complete consumption of the diboronic ester (monitored by TLC).
After it was cooled to room temperature, the reaction mixture was taken
up with CH2Cl2, adsorbed on silica, and directly charged onto a silica gel
column. The desired product was isolated by elution with cyclohexane/
CH2Cl2 mixtures. During the whole process exposure to light and air
should be minimized in order to avoid the possible formation of oxidized
side products.
5,11-Diphenyltetracene (4a). Yield: 97%. 1H NMR (400 MHz,
CDCl3): δ (ppm) 8.36 (s, 2H), 7.83 (d, J = 8.5 Hz, 2H), 7.71À7.61
(m, 8H), 7.59À7.53 (m, 4H), 7.35À7.29 (m, 2H), 7.28À7.23 (m, 2H).
13C NMR (100 MHz, CDCl3): δ (ppm) 139.1, 136.8, 131.5, 131.0,
129.4, 129.04, 128.96, 128.5, 127.6, 126.6, 125.9, 125.3, 124.7. MS (EI):
m/z 380 [M+]. HPLC (λ = 280 nm, method A): tR = 23.7 min.
5,11-Bis(4-methoxyphenyl)tetracene (4b). Yield: 94%. 1H NMR
(400 MHz, CDCl3): δ (ppm) 8.40 (s, 2H), 7.85 (d, J = 8.4 Hz, 2H),
7.68 (d, J = 8.8 Hz, 2H), 7.47 (d, J = 8.4 Hz, 4H), 7.34À7.24 (m, 4H),
7.21 (d, J = 8.4 Hz, 4H), 4.02 (s, 6H). 13C NMR (100 MHz, CDCl3): δ
(ppm) 159.1, 136.6, 132.6, 131.2, 131.0, 129.7, 129.5, 129.0, 126.7,
125.9, 125.2, 124.6, 114.0, 55.4. MS (EI): m/z 441 [M+]. HPLC
(λ = 280 nm, method A): tR = 23.0 min.
5,11-Bis(4-cyanophenyl)tetracene (4e). Yield: 37%. 1H NMR (400
MHz, CDCl3): δ (ppm) 8.25 (s, 2H), 7.99 (d, J = 8.4 Hz, 4H), 7.86 (d,
J = 8.4 Hz, 2H), 7.68 (d, J = 8.4 Hz, 4H), 7.49 (d, J = 9.2 Hz, 2H),
7.40À7.32 (m, 4H). MS (EI): m/z 430 [M+].
’ ASSOCIATED CONTENT
S
Supporting Information. Figures giving absorption and
b
fluorescence spectra, cyclic voltammograms, and H and 13C
1
NMR and mass spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
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ARTICLE
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: marco.lombardo@unibo.it (M.L.); luciano.miozzo@
unimib.it (L.M.).
’ ACKNOWLEDGMENT
L.M. thanks the Marie Curie Fellowship (FP7-PEOPLE, IEF-
2008, Proposal No. 236101, acronym TETRA-FUN) for finan-
cial support. The University of Bologna and the University of
Milano Bicocca are acknowledged for financial support.
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