Sequential multiphoton absorption enhancement induced by zinc complexation in functionalized distyrylbenzene analogs

Article abstract of DOI:10.1039/b615076d

Functionalized distyrylbenzene analogs 2 and 3, bearing a tris-(2-pyridylmethyl)amine-based receptor for Zn²⁺, were synthesized by a Horner-Emmons-Wittig coupling reaction. It has been found that Zn ²⁺ complexation induces changes in

Full text of DOI:10.1039/b615076d

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Sequential multiphoton absorption enhancement induced by zinc
complexation in functionalized distyrylbenzene analogsw
´
Graziano Fabbrini, Raffaele Ricco, Enzo Menna, Michele Maggini,*
Vincenzo Amendola, Mattia Garbin, Massimo Villano and Moreno Meneghetti*
Received 17th October 2006, Accepted 22nd November 2006
First published as an Advance Article on the web 11th December 2006
DOI: 10.1039/b615076d
Functionalized distyrylbenzene analogs 2 and 3, bearing a tris-(2-pyridylmethyl)amine-based
receptor for Zn21, were synthesized by a Horner–Emmons–Wittig coupling reaction. It has been
found that Zn21 complexation induces changes in the linear absorption spectrum that enhance a
nonlinear sequential two-photon absorption of nanosecond pulses at 532 nm. This absorption was
also found to depend on the nature of the substituent at the side benzene ring of the
styrylbenzene structure.
second pulses in the near infrared spectral region.23 However,
Introduction
the importance of sequential processes, that must be taken into
account for situations of quasi-resonance like those considered
in this work, has been stressed also for I-2PA.24 In the case of
derivative 1 (Chart 1), in which a DSB was functionalized at
one terminus with a cyclen ligand,13 it has been shown that a
large variation of its NL transmission characteristics could be
induced by Zn21 complexation. This result encouraged the
preparation and study of chromophores 2 and 3 in which a
styryl(vinylpyridyl)benzene (SVPB) structure is part of a
tris(2-pyridylmethyl)amine-based receptor (TPMA), known
for its Zn21 binding characteristics and whose coordination
chemistry has been studied in great detail.25–28 We have found
that, upon complexation with Zn21, 2 and 3 change their
linear absorption, as in the case of 1, thus allowing a control of
their multiphoton absorption properties. It is worth noting
that a variation in the linear absorption translates into a
different excited state population and, therefore, into a differ-
ent sequential multiphoton absorption. In the case of 2 and 3,
a higher NL transmission was recorded in the absence of
Molecular switches represent key components for the control
and processing of information for future technology.1,2 Effi-
cient schemes for molecular switching have been obtained
through changes of optical or magnetic properties in a variety
of molecular structures.3–7 In particular, second- and third-
order nonlinear optical (NLO) properties are becoming im-
portant issues for molecular switching: proton transfer reac-
tions,8 different redox states9–11 and molecular aggregates12
were used for NLO switching. We have shown that a functio-
nalized distyrylbenzene (DSB) derivative exhibits the switch-
ing of its multiphoton absorption properties upon Zn21
complexation.13 A model fitting of the experimental data
suggested to characterize the NL-absorbing process as a
sequential two-photon absorption (S-2PA) event where a
photon efficiently populates an excited state which, in turn,
absorbs a second photon. S-2PA have been observed also for
other classes of molecular structures, such as fullerenes,14–16
phthalocyanines,17–19 porphyrins20 and porphyrazines21 in
which NL absorption derives from the activity of a triplet
excited state. However, for these systems, the NLO properties
cannot be controlled by an external agent, as for our functio-
nalized DSB-based metal receptor13 or for other medium-
sensitive NLO probes that were recently reported in the
literature.22 S-2PA is operative with nanosecond pulses at
relatively low laser intensity, if compared to other NL absorp-
tion phenomena, such as instantaneous two-photon absorp-
tion (I-2PA), because of the high polarizability of excited
states involved in the sequential process. Recently, I-2PA
was observed for DSB-based Mg21 receptors with femto-
`
Dipartimento di Scienze Chimiche, Universita di Padova, Via Marzolo
1, 35131 Padova, Italy. E-mail: michele.maggini@unipd.it,
moreno.meneghetti@unipd.it; Fax: (þ39) 049-8275239
w Electronic supplementary information (ESI) available: Synthetic
details and characterization for compounds 4, 5, 7, 9–14; NMR and
IR spectra for 2 and 3; ESI-MS for 2, 3, 2-Zn21 and 3-Zn21; titrations
for stoichiometry and binding constant determination for 2-Zn21 and
3-Zn21. See DOI: 10.1039/b615076d
Chart 1 Molecular structures considered in this work.
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Zn21. In DSB 1, instead, the same variation was found
in the presence of Zn21. The effect that we observe for 2
and 3 is also opposite to that reported by Pond and
coworkers23 in which a lower I-2PA activity is observed upon
Mg21 complexation.
The results presented herein could also be of interest in view
of understanding the mechanism of NL absorption changes
induced by ion complexation in the frame of bio-ima-
ging,23,29–35 although other important factors, such as proto-
nation equilibria or solvent effects,36 should be considered.
Results and discussion
Synthesis
Scheme 2 Reagents and conditions: (a) NaH, THF, 0 1C, 3 h, 74%;
(b) PBr3, THF, 0 1C, 1 h, 84%; (c) P(OEt)3, D, 3 h, 87%.
Compounds 2 and 3 (Chart 1), have an electron-donor amine
or an electron-acceptor cyano substituent located at position 4
of the side benzene ring of the SVPB structure. It is reasonable
to assume that complexation with Zn21 induces a higher
localization of the electrons of the TPMA receptor and, in
particular, of the p electrons of the pyridine moiety which is
part of the SVPB structure, thus making this pyridine ring a
stronger acceptor. This change should, at least partially, affect
the intramolecular charge distribution of the molecules both in
their ground and excited states and could be useful for
obtaining a variation of their NLO activity. In this connection,
Pond and coworkers have demonstrated in a recent publica-
tion23 that a similar design scheme can be employed for the
development of NL molecular sensors for use in two-photon
laser scanning applications.
Phosphonates 4 and 5 were prepared by two different
routes. Derivative 4 was obtained by a three-step procedure,
starting from aldehyde–alcohol 7 (Scheme 2). A HEW cou-
pling of 7 with 4-cyanobenzyl-diethylphosphonate 8 gave
trans-stilbene 9 in 74% yield. Subsequent bromination with
phosphorus tribromide gave bromobenzyl derivative 10 in
84% yield. Phosphonate 4 was obtained from 10 and triethyl
phosphite by a Michaelis–Arbuzov reaction in 87% yield.
Aldehyde–alcohol 7 was prepared from bis-alcohol 12 in
90% yield by selective oxydation of one hydroxyl group with
2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ).39 Bis-
alcohol 12, in turn, was obtained in 89% overall isolated yield
through nucleophilic displacement of 2-(2-(2-methoxy)
ethoxy)ethyl-p-toluensulfonate and commercial ethyl 1,4-di-
hydroxy-2,5-terephthalate, followed by LiAlH4 reduction of
the ester functions (Scheme 3).
SVPBs 2 and 3 were synthesized through a Horner–Em-
mons–Wittig (HEW) coupling reaction of [6-formyl(2-pyridyl-
methyl)amine]bis(2-pyridylmethyl)amine 637 with a suitably-
functionalized diethylphosphonate 4 or 5 (Scheme 1) in 19 and
20% yield, respectively. The modest yields refer to isolated
compounds after column chromatography that revealed to be
a rather difficult task since the highly polar compounds 2 and 3
tend to irreversibly bind onto the stationary phase (either silica
gel or alumina), although a steric hindrance to the HEW
coupling by the oxyethylene chains cannot be ruled out.
HPLC analysis was employed to assess the purity of deriva-
tives 2 and 3 whereas NMR spectroscopy (Fig. S1–S4) and
exact mass determination to validate their proposed molecular
structure. The trans–trans configuration of the double bonds
in 2 and 3 was established via NMR, through the measurement
of the 3JHH of the olefin protons which was found to be around
16 Hz, as reported in the literature.38
Although the route illustrated in Scheme 2 afforded phos-
phonate 4 in good yields, it was not suitable for the production
of the corresponding N,N-dimethylamino derivative 5 because
the bromination steps gave mainly an intractable solid, prob-
ably produced by an intermolecular reaction of the amine with
the reactive bromobenzyl function. Phosphonate 5 was there-
fore obtained by an alternate route, in 24% yield, through the
monofunctionalization of bis-phosphonate 14 with 4-(N,N-
dimethylamino)benzaldehyde (Scheme 4). Compound 14 was
synthesized from dibromobenzyl derivative 13 that, in turn,
was prepared from bis-alcohol 12 and PBr3 in 71% yield,
followed by a Michaelis–Arbuzov reaction with triethyl phos-
phite in 84% yield.
The formation of a complex between 2 or 3 and the Zn21
cation (2-Zn21 and 3-Zn21) was clearly evidenced by mass
spectroscopic analysis of a 1 : 1 host/guest mixture in a MeOH
solution, showing the peak for the doubly-charged 1 : 1
adduct at m/z 453.7 (2-Zn21, Fig. S9) and m/z 462.3
(3-Zn21, Fig. S10). The binding ability of 2 and 3 toward
Zn21 was investigated by means of fluorescence titration
experiments through the addition of increasing amounts of
Zn(ClO4)2 hexahydrate to a 0.099 mM solution of 2 (or to a
0.104 mM solution of 3) in CH3CN. Fluorescence changes as a
function of [Zn21] showed a complete formation of the com-
plex up to one equivalent of Zn21. This prevented a precise
evaluation of the binding constant that, nevertheless, could be
Scheme 1 Reagents and conditions: NaH, THF, 0 1C, 12 h (2, 19%;
3, 20%).
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Fig. 1 Absorption spectra of 2 (dashed line) and 2-Zn21 (solid line)
1.21 ꢁ 10ꢂ3 M in CH3CN. Inset shows an expanded part of the
spectrum around 532 nm.
therefore, at 532 nm 2-Zn21 and 3-Zn21 absorb slightly more
than 2 and 3. This condition is fundamental for obtaining a
variation of sequential multiphoton absorption13 because a
stronger linear absorption makes it possible to populate the
first excited state easier, from which further absorptions may
occur.
Scheme 3 Reagents and conditions: (a) 2-(2-(2-methoxy)ethoxy)
ethyl-p-toluensulfonate, K2CO3, DMF, 60 1C, 48 h, 96%; (b) LiAlH4,
THF, 0 1C-reflux temperature, 0.5 h, 93%; (c) DDQ, CH2Cl2, 0–5 1C,
90%.
estimated as 4107.26 (for stoichiometry and binding constant
determination see Fig. S11).
NL absorption measurements at 532 nm of 2, 3, 2-Zn21 and
3-Zn21 are shown in Fig. 3a and 4a, respectively, where a
lower output intensity is determined by an intensity dependent
absorption. One finds that the NL absorption is very impor-
tant only for 2-Zn21 and 3-Zn21, whereas the almost linear
behaviour of 2 and 3 confirms their poor NL activity. This
shows that an enhancement of the NL absorption can be
obtained upon complexation. Fig. 3b and 4b report the same
experimental data of Fig. 3a and 4a, respectively, but directly
showing the absorption as a function of input intensities in a
logarithmic scale. These plots allow a better inspection of the
low intensity part of the data and are useful for controlling the
model fitting at these intensities as well. One also finds that
3-Zn21 displays a larger non-linearity than 2-Zn21, pointing
out the important role of the functional groups in the SVPB
structure.
Linear and nonlinear optical measurements
Solutions of metal complexes 2-Zn21 and 3-Zn21 for spectro-
photometric measurements were prepared in CH3CN by mix-
ing SVBP 2 or 3 with 2 equiv. of Zn(ClO4)2 hexahydrate at
room temperature. The absorption spectra of CH3CN solu-
tions of 2 and of 2-Zn21 are reported in Fig. 1. The corre-
sponding spectra of 3 and 3-Zn21 are shown in Fig. 2.
The spectra of 2 and 3 show that a variation of the linear
absorption follows the formation of the Zn21 complex, as it is
clear in the low energy part of the spectrum around 400 nm,
where 2-Zn21 and 3-Zn21 have a weaker absorption than their
uncomplexed counterpart. This can be ascribed to a reduced
number of backbone p electrons as a consequence of the
participation of the SVPB pyridyl ring to Zn21 complexation.
A similar effect was also observed earlier for a DSB-based
Explanation of the observed results could be provided by
considering the excited state population dynamics of the
molecules. In fact, the interaction between light and matter
can be described within a density matrix approach, and when
receptor for Mg21 23
However, it is interesting to note that a
.
stronger absorption of the Zn21 complexes is present in the
wing of the 400 nm band toward longer wavelengths and that,
Fig. 2 Absorption spectra of 3 (dashed line) and 3-Zn21 (solid line)
1.26 ꢁ 10ꢂ3 M in CH3CN. Inset shows an expanded part of the
spectrum around 532 nm.
Scheme 4 Reagents and conditions: (a) PBr3, THF, 0 1C-reflux
temperature, 5 h, 71%; (b) P(OEt)3, D, 3 h, 84%; (c) 4-(N,N-dimethy-
lamino)benzaldehyde, tBuOK, DMF, 0 1C, 12 h, 24%.
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Fig. 3 NL absorption at 532 nm of 2 (J) and of 2-Zn21 (K) 1.21 ꢁ 10ꢂ3 M in CH3CN. Solid lines are the results of model calculations (see text)
whereas the inset shows the model parameters (absorption cross sections sGS and sEX and decay rates k1 and k2), whose values are reported in
Table 1. (a) Recorded output intensities versus input intensities; (b) same data as in (a) but plotting the NL absorption as a function of input
intensities in a logarithmic scale.
the laser pulse duration is longer than the relaxation processes,
as in the present case (see below), one can use the diagonal part
of the density matrix, which corresponds to the populations of
the states of the system.40 Solutions of the rate equation for the
dynamics of these populations can be obtained according to
Ehlert et al.41 The model that has been used for the experi-
mental data description is reported in the inset of Fig. 3b. The
first excitation to be considered is in fact related to the linear
absorption, with an absorption cross section sGS, which
depends on the linear optical susceptibility, usually larger than
higher-order susceptibilities. After the first one-photon excita-
tion, a one- or two-photon absorption from the excited state
can occur.42 We have found that a one-photon absorption,
with an absorption cross section sEX, is sufficient for fitting the
experimental data. Four parameters (see inset of Fig. 3b and
Table 1) are needed for the fitting. Among them, sGS is
observed for organic molecules.43 These values and those of
the other two parameters (sEX and the decay rate for the first
excited state k1) that are obtained from the fitting of the
experimental data, are reported in Table 1.
It is worth noting that fitting of the data strongly depends on
the variation of the cross section for the one-photon excitation
from the ground state, which is experimentally evaluated from
the UV-Vis spectra. This shows the importance of the popula-
tion of the first excited state from which the second one-photon
absorption starts. Good fittings have been obtained both in the
high (Fig. 3a and 4a) and in the low (Fig. 3b and 4b) intensity
regions, indicating that the S-2PA model gives a good descrip-
tion of the excited states dynamics that contribute to the NLO
properties of the investigated molecules.
Conclusion
determined from the experimental linear absorption spectrum,
ꢂ1
whereas the decay time for the second excited state (t2 ¼ k2
)
Two DSB analogs (SVPB derivatives 2 and 3) bearing at one
can be considered very fast (of the order of ps) as it is usually
terminus the metal ion receptor tris(2-pyridylmethyl)amine,
Fig. 4 Corresponding data of Fig. 3 but for 3 (J) and 3-Zn21 (K) with a concentration of 1.23 ꢁ 10ꢂ3 M in CH3CN.
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Table 1 Parameters for the model of the excited states dynamics
University of Calabria, Arcavacata di Rende Mass Spectro-
metry Resource. Compounds 2 and 3, dissolved in MeOH and
diluted to obtain non-saturating ion in the molecular ion
region, were added to an a-cyano-4-hydroxycinnamic acid
matrix. The mass spectra were taken on a MALDI-TOF/TOF,
4700 proteomic analyzer from Applied Biosystem equipped
with a Nd:YAG pulsed laser at 355 nm. IR spectra were
recorded on a Perkin-Elmer FT-IR model 1720X. HPLC
analyses of 2 and 3 were carried out on a Shimadzu LC-8A
instruments, equipped with a Shimadzu SPD-6A UV/Vis
detector, working at 254 nm, using a Phenomenex Synergi
Polar-RP column (150 ꢁ 4.6 mm, 4 mm, eluent: H2O/CH3CN,
0.1% trifluoroacetic acid, flow rate: 2 ml minꢂ1; linear gradient
from 10 to 90% H2O in 20 min): SVPB 2 (Rf ¼ 16 min), SVPB
3 (Rf ¼ 11 min).
s
GS/cm2
s
EX/cm2
k1/sꢂ1
k2/sꢂ1
2
2.2 ꢁ 10ꢂ19
1.3 ꢁ 10ꢂ18
1.1 ꢁ 10ꢂ18
2.4 ꢁ 10ꢂ18
1.6 ꢁ 10ꢂ16
4.0 ꢁ 10ꢂ16
5.5 ꢁ 10ꢂ17
3.7 ꢁ 10ꢂ16
5.0 ꢁ 1010
5.0 ꢁ 1010
8.0 ꢁ 109
8.0 ꢁ 109
1.0 ꢁ 1013
1.0 ꢁ 1013
1.0 ꢁ 1012
1.0 ꢁ 1012
2-Zn21
3
3-Zn21
have been synthesized and characterized. It has been found
that a low absorption of nanosecond pulses at 532 nm is
enhanced upon complexation in the 2-Zn21 and 3-Zn21 form.
A model interpretation of the experimental data shows that
this NL behaviour can be described with a sequential two-
photon absorption (S-2PA). This mechanism allowed us to
observe the NL behaviour at low laser intensities, because of
the large polarizability of the first excited state from which the
absorption of the second photon occurs. The results presented
in this paper complement our previously reported study13 on a
DSB-based Zn21 ligand that showed an opposite behavior,
namely a larger NL activity for the uncomplexed molecule. In
the present case, it has also been found that 3-Zn21 displays a
larger nonlinearity than 2-Zn21, pointing out the important
role of the substituent (acceptor or donor) at the side benzene
ring of the SVPB structure. The observation of S-2PA phe-
nomena can be predicted also in spectral regions different from
that explored in the present work, provided that molecular
structures with appropriate variation of their linear absorp-
tion, upon complexation, are synthesized. To this end, the
coordination of metal cations different from Zn21, such as
Cu21 or lanthanide ions, could enhance the control of excited
state absorption thus offering further prospects for switching
and sensing.
NLO measurements
Multiphoton absorption properties of 2, 3 and of 2-Zn21 and
3-Zn21 were obtained by NL transmission measurements
using 9 ns pulses of a Nd:YAG laser (Quantel 982E) at 532
nm with solutions in 2 mm glass cells. The pulse energy was
controlled with a polarizing cube beamsplitter and a l\2 wave
plate, and measured with a pyroelectric detector (Scientech
SPHD25). An open aperture configuration (solid angle 0.1
sterad) was used to avoid NL scattering or refraction effects. A
calibrated photodiode was also employed to monitor pulse-to-
pulse energy variations. Control of the pulse energies and
recording of the detector signals were obtained with a home
made program based on Labviews.
Syntheses and selected analytical data for derivatives 2 and 3
Experimental
SVPB 2. To a solution of phosphonate 4 (212 mg, 0.31
mmol) in dry THF (20 ml) at 0 1C, NaH (15 mg, 0.37 mmol)
was added under a nitrogen atmosphere, and the resulting
solution was stirred for 20 min. Then, a solution of aldehyde 6
(100 mg, 0.31 mmol) in dry THF (10 ml) was added dropwise
over a period of 10 min. When the addition was complete, the
mixture was stirred at room temperature for 12 h. The solvent
was removed under reduced pressure and the crude product
purified by flash column chromatography (SiO2, eluent: ethyl
acetate/ethanol 8 : 2, then ethyl acetate/petroleum ether/iso-
propanol/aqueous ammonia 6 : 3 : 1 : 0.1). 50 mg (19%) of 2
were isolated as a fluorescent yellow-greenish oil. 1H-NMR
(250 MHz, CDCl3, 25 1C, TMS) d 8.51–8.49 (m, 2H),
7.94–7.88 (d, 1H, 3J(H,H) ¼ 16.0 Hz), 7.71–7.62 (m, 10H),
7.65–7.58 (d, 1H, 3J(H,H) ¼ 16.0 Hz), 7.47–7.44 (m, 1H), 7.34
Materials
All reagents, including dry solvents, were purchased from
Aldrich and used without further purification. [6-Formyl
(2-pyridylmethyl)amine]bis(2-pyridylmethyl)amine 6 was pre-
pared as reported in the literature.37 4-Cyanobenzyl-diethyl-
phosphonate 8 was prepared in 95% yield from 4-cyanobenzyl
bromide and triethylphosphite, following the standard
Michaelis–Arbuzov procedure. 2-(2-(2-Methoxy)ethoxy)ethyl-
p-toluenesulfonate was synthesized in 96% yield from triethy-
leneglycol monomethylether and tosyl chloride in the presence
of NaOH.
Instrumentation
3
Melting points were measured on a Leitz Laborlux 12 melting
point apparatus and are uncorrected. Thin layer (TLC) and
column chromatography were performed using a Polygram
SilG/UV254 (TLC plates) and silica gel MN 60 (70–230 Mesh)
by Macherey-Nagel. 1H (250.1 MHz) and 13C (62.9 MHz)
NMR spectra were recorded on a Bruker AC-F 250 spectro-
meter. The ESI-MS spectra were taken on a Thermo Finnigan
AQA LC/MS: ꢂ4 kV spray voltage; ꢂ10 V capillary voltage;
180 1C capillary temperature; nitrogen as nebulizing gas. The
samples were dissolved in methanol containing 1% trifluoro-
acetic acid. The exact mass determination was provided by the
(s, 1H), 7.24 (s, 1H), 7.21–7.15 (d, 1H, J(H,H) ¼ 16.0 Hz),
7.18–7.12 (m, 3H), 4.26–4.22 (t, 4H, J ¼ 4.0 Hz), 3.92–3.88 (t,
4H, J ¼ 4.0 Hz), 3.88 (s, 4H), 3.87 (s, 2H), 3.73–3.70 (m, 4H),
3.65–3.54 (m, 8H), 3.48–3.44 (m, 4H), 3.29 (s, 3H), 3.28 (s,
3H); 13C-NMR (62.9 MHz, CDCl3, 25 1C, TMS) d 155.42,
151.41, 148.95, 137.93, 137.25, 137.10, 136.80, 132.41, 129.68,
127.57, 127.36, 127.97, 126.89, 126.17, 124.10, 123.16, 122.27,
121.92, 121.19, 119.09, 111.78, 111.59, 110.25, 71.85, 71.82,
70.79, 70.77, 70.63, 70.62, 70.53, 70.47, 69.79, 69.16, 69.02; IR
(KBr) u 2917, 2222, 1591, 1102, 1046 cmꢂ1; ESI-MS: m/z 844
~
[M þ H]1. HR-MS: 844.4359, D ¼ ꢂ8.72 ppm.
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SVPB 3. To a solution of phosphonate derivative 5 (130 mg,
0.18 mmol) in dry THF (20 ml) at 0 1C, NaH (9 mg, 0.22
mmol) was added under a nitrogen atmosphere, and the
resulting solution was stirred for 20 min. Then, a solution of
tris(pyridyl)amino aldehyde 6 (59 mg, 0.18 mmol) in dry THF
(10 ml) was added dropwise over a period of 10 min. When the
addition was complete, the mixture was stirred at room
temperature for 12 h. The solvent was removed under reduced
pressure and the crude product purified by flash column
chromatography (SiO2, eluent: ethyl acetate/ethanol 8 : 2,
then ethyl acetate/petroleum ether/isopropanol/aqueous am-
monia 6 : 3 : 1 : 0.1). 31 mg (20%) of 3 were isolated as a
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1
fluorescent yellow-greenish oil. H-NMR (250 MHz, CDCl3,
25 1C, TMS) d 8.55–8.54 (m, 2H), 7.92–7.86 (d, 1H, 3J(H,H) ¼
16.5 Hz), 7.68–7.60 (m, 6H), 7.45–7.36 (m, 4H), 7.32–7.25 (d,
1H, 3J(H,H) ¼ 16.5 Hz), 7.18–7.16 (m, 4H), 7.09–7.03 (d, 1H,
3J(H,H) ¼ 16.5 Hz), 6.74–6.70 (d, 2H, J ¼ 8.7 Hz), 4.26–4.18
(m, 4H), 4.05 (s, 4H), 4.02 (s, 2H), 3.93–3.91 (m, 4H),
3.88–3.64 (m, 12H), 3.54–3.50 (m, 4H), 3.35 (s, 3H), 3.33 (s,
3H), 2.99 (s, 6H); 13C-NMR (62.9 MHz, CDCl3, 25 1C, TMS)
d 155.76, 151.55, 150.56, 150.09, 149.44, 149.09, 148.90,
137.76, 137.07, 136.71, 129.70, 129.17, 127.72, 126.16,
125.28, 123.47, 123.20, 122.47, 122.26, 120.88, 119.76,
118.59, 112.40, 111.79, 110.57, 71.87, 71.84, 70.88, 70.79,
70.64, 70.63, 70.52, 70.49, 69.88, 69.85, 69.23, 69.07, 59.90,
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58.98, 40.44; IR (KBr) n~ 2921, 1588, 1432, 1109, 1060 cmꢂ1
.
ESI-MS: m/z 862 [M þ H]1. HR-MS: 862.4733, D ¼ 2.54 ppm.
Acknowledgements
26 J. C. Mareque-Rivas, R. Prabaharan and R. Torres Martin de
Rosales, Chem. Commun., 2004, 76–77.
Thanks are due to G. Marcolongo and S. Crivellaro (U. of
Padova) for their help in carrying out some of the experi-
mental procedures, Dr L. Di Donna and Dr E. Marotta for
mass determinations and Dr F. Mancin for helpful discus-
sions. Funding from MIUR (GR/No. 2004035502,
RBAU017S8R, RBNE01P4JF and RBNE033KMA), the
University of Padova (GR/No CPDA012428) and from
ITM-CNR is gratefully acknowledged.
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