Synthesis and complexation properties of an oligooxaethylene-spacered porphyrin dimer - Toward the construction of a new switchable porphyrin array

Article abstract of DOI:10.1002/(SICI)1099-0690(199908)1999:8<1901::AID-EJOC1901>3.0.CO;2-R

A new linear switchable diporphyrin array has been synthesized in good yield by a Williamson coupling of 5-(4'-hydroxyphenyl)-10,15,20- triphenylporphyrin with tetraethylene glycol ditosylate. ¹H-NMR binding studies reveal that the title receptor binds to Na⁺ and K⁺ salts to give a supramolecular complex with stability constants (K) of 30 and 160 M^-1, respectively (CD₃CN/CDCl₃, 1:1). The formation of the host-guest complexes promotes a change of the geometry of the system toward a topology in which the two tetrapyrrolic macrocycles tend to face together. UV/Vis and fluorescence spectroscopy give indications of the actual geometry of the assembly. The system reported represents an important step toward the construction of a supramolecular model of relevant biological systems.

Full text of DOI:10.1002/(SICI)1099-0690(199908)1999:8<1901::AID-EJOC1901>3.0.CO;2-R

FULL PAPER
Synthesis and Complexation Properties of an Oligooxaethylene-Spacered
Porphyrin Dimer – Toward the Construction of a New Switchable
Porphyrin Array
[
a]
[a]
[b]
[b]
Donato Monti,* Mariano Venanzi, Giovanna Mancini, Federica Marotti,
[a]
[a]
Laura La Monica, and Tristano Boschi
Keywords: Porphyrinoids / Porphyrin receptors / Supramolecular chemistry / Cation binding / Cations / Host-guest
chemistry
A
new linear switchable diporphyrin array has been
the host-guest complexes promotes a change of the geometry
of the system toward topology in which the two
synthesized in good yield by a Williamson coupling of 5-(4Ј-
a
hydroxyphenyl)-10,15,20-triphenylporphyrin with tetraethyl-
tetrapyrrolic macrocycles tend to face together. UV/Vis and
fluorescence spectroscopy give indications of the actual
1
ene glycol ditosylate. H-NMR binding studies reveal that
+
+
the title receptor binds to Na and K salts to give a geometry of the assembly. The system reported represents an
supramolecular complex with stability constants (K) of 30 and
important step toward the construction of a supramolecular
model of relevant biological systems.
–1
1
3 3
60 M , respectively (CD CN/CDCl , 1:1). The formation of
Introduction
less demanding protocols but rests on assemblies which too
often show low stability in protic or coordinating media.[
7]
The synthesis and the study of porphyrin dimers is an The design of a switchable linear porphyrin array would rep-
attractive and challenging field of research yet to be fully resent a simple and elegant way for circumventing these
exploited. These molecules constitute interesting models of problems.[ We report here on the synthesis of a new por-
biological systems of vital importance, like the ones in- phyrin dimer 1a and its zinc derivative 1b by a facile syn-
volved in the photosynthetic machinery.[ The photosyn- thetic route which entails the functionalization of the basic
thetic reaction center of Rhodopseudomonas viridis and molecular frame of an oligooxaethylenic chain with two
Rhodobacter spheroides, for example, features two bacterio- tetrapyrrolic macrocycles. We also present evidences of a
chlorophyll moieties held at a precise distance and spatial conformational change of the system induced by a host-
8]
1]
[2]
orientation by a protein matrix. Moreover, cofacial met- guest interaction with alkali metal ions (Figure 1).
[
3]
allo-diporphyrins are regarded as promising mimics of re-
dox-active metalloproteins such as haemoglobin, myoglo-
bin, and cytochrome c oxidases. We have recently reported
on the synthesis and photophysical properties of unsym-
metrical porphyrin dimers,[ pointing out some interesting
features in terms of a directional, efficient energy transfer
4]
[4c]
between the covalently linked macrocycles.
Most of the synthetic strategies adopted so far for the
construction of the target architectures rely on either coval-
ent or supramolecular assemblies of the chosen porphyrin
subunits. The first approach,[ though it provides a good
5]
Figure 1. Cation-induced conformational change of diporphyrin
control over the geometric and electronic properties of the receptors
molecule, usually suffers the drawback of requiring time-
consuming and low-yield ending syntheses. The second ap-
proach, based on noncovalent interactions such as electro-
Results and Discussion
static interactions,[
metal coordination,
6aϪ6c]
6iϪ6r]
hydrogen bonding,
deals with elements achievable by
[6dϪ6h]
and
[
Synthesis of the Target Molecules
[
a]
The porphyrin dimer 1a and its metal derivative 1b have
been prepared according to the procedure outlined in
Scheme 1. Condensation of pyrrole and 4-methoxybenzal-
Dipartimento di Scienze e Tecnologie Chimiche, Universit a` degli
Studi di Roma “Tor Vergata”,
Via della Ricerca Scientifica s.n.c., I-00133 Rome, Italy
Centro C. N. R. di Studio sui Meccanismi di Reazione,
[
b]
Dipartimento di Chimica, Universit a` degli Studi di Roma “La dehyde in the presence of a ten-fold excess of benzaldehyde
Sapienza”,
[9]
in refluxing propionic acid gave a mixture of meso-tetra-
phenylporphyrin (H TPP) and 5-(p-methoxyphenyl)-
Ple. A. Moro 5, I-00185 Rome, Italy
Supporting information for this article is available on the WWW
2
under http://www.wiley-vch.de/home/eurjoc or from the author. 10,15,20-triphenylporphyrin (2). After chromatographic
Eur. J. Org. Chem. 1999, 1901Ϫ1906

WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
1434Ϫ193X/99/0808Ϫ1901 $ 17.50ϩ.50/0
1901

D. Monti, M. Venanzi, G. Mancini, F. Marotti, L. La Monica, T. Boschi
FULL PAPER
separation, porphyrin 2 was subjected to demethylation companied by a concomitant upfield shift (up to 150 Hz)
[
10]
with BBr3
to give the relative phenolic derivative 3, of the signals. This is a clear indication of the occurrence
which is in turn coupled with tetraethylene glycol ditosylate of a change of topology due to the incipient cofacialization
under Williamson conditions[ to give the title porphyrin of the tetrapyrrolic units caused by the wrapping of the
11]
1
a in good yield. The zinc derivative 1b was obtained in molecule around the cation. The induced conformational
quantitative yield by stirring a chloroform solution of 1a change of the receptor reflects itself in the mutual shielding
with an excess of a saturated methanolic solution of zinc of the proton of the porphyrin planes by effect of the ring
diacetate at reflux temperature.[
9]
current of the π-electron systems.
[13]
The protons of the
phenyl rings at the meso positions experience a similar up-
field splitting effect. The AB spin system of the C H O moi-
6
4
ety is instead downfield shifted. This result can be also ex-
plained in terms of the above anisotropic ring-current effect
of the porphyrin rings assuming that the protons are lo-
cated at the deshielding region. However, an effect of the
depletion of the electronic charge caused by the electrostatic
interaction of the cation with the oxygen atoms could not
be excluded a priori. The signals of the inner NϪH protons,
though to a lower extent, are also upfield shifted. The in-
variance of the chemical shift of some of the signals, upon
addition of salts, can be an indication of the geometry of
the porphyrin planes within the frame of the supramolecu-
Scheme 1. Synthesis of diporphyrin 1a, 1b, and 2; i) propionic acid,
reflux, 3 h; ii) BBr , CH Cl , 0°C then room temp. overnight; iii)
tetraethylene glycol ditosylate, anhydrous K CO
2 h; iv) MeOH/Zn(OAc) , CHCl , reflux, 3 h
3
2
2
2
3
, DMF, 80°C, lar complex (vide infra). The association constant for the
complexation of the receptors with the metal ions can be
evaluated by non linear least-squares regression of the
chemical shift values of the chosen signals as a function of
1
2
3
The UV/Vis spectra of 1a show features similar to the
the metal cation concentration. In all cases, a clear 1:1 bind-
ones observed for the reference porphyrin 2 ruling out the
occurrence of any detectable ground-state interaction be-
tween the appended tetrapyrrolic macrocycles.
ϩ
ing isotherm is obtained (Figure 2). K binds to receptors
ϩ
1
a somewhat more strongly than Na giving K(Kϩ) ϭ 160
Ϫ1
and K(Naϩ) ϭ 50 
.
NMR Studies
1H-NMR spectroscopy was found to be a convenient tool
for the study of the supramolecular cation-receptor interac-
tion. At room temperature, rapid exchange between com-
plexed and uncomplexed species is observed on the NMR
1
time-scale at 300 MHz. The H-NMR spectra of the bis-
(porphyrin) receptors feature a complex multiplet at δ ϭ
8
.4Ϫ8.8 (β-H pyrrole protons) and three set of signals for
the phenyl rings. The AX spin system relative to the aro-
matic protons of the ring linked to the spacer (C H O) is
6
4
clearly visible as a double doublet in the aromatic region
see Experimental Section). The resonances of the protons
of the oxyethylenic spacer lie in the range δ ϭ 3.4Ϫ4.2. The titration curves of different sets of signals for the binding of KSCN
(
1
Figure 2. H-NMR (300 MHz, CD
3
CN/CDCl
3
1:1 solvent mixture)
to 1a (᭹: β-H pyrrolic; ᭺: C OR aromatic protons); solid lines
represent the theoretical curves for a 1:1 complex formation
6
H
4
NϪH protons resonate, as expected, at δ ഠ Ϫ3. The ad-
ϩ
ϩ
Ϫ
dition of Na or K salts (SCN in CDCl /CD CN, 1:1
3
3
Ϫ4
mixture) to a 5·10  solution of the porphyrin dimer 1a
causes a gradual shift of some of the signals of the aromatic
This degree of selectivity between cations with different
part as well as the signals of the linker. The chemical shifts, ionic radii is in line with the values reported for related
however, did not reach a limiting value even at relatively open-chain podands bearing different aromatic ends,[
high concentration of the added guest, indicating the for- and can be accounted for by the intrinsic flexibility of the
mation of a weak complexes with low K values. The ex- ligand which can easily adapt itself to the required confor-
pected downfield shift (up to 50 Hz) of the ethylenic proton mational change. The binding constant values for the com-
14]
ϩ
ϩ
signals of the spacer is consistent with that observed in the plexation of Na and K cations to the zinc derivative re-
case of crown ethers and related open-chain compounds.[
12]
ceptor 1b are similar to the values obtained for the free-
More remarkable is the change observed for the β-H pyr- base 1a. This finding rules out the possibility that ion-pair-
rolic set of signals which undergo a pronounced change into ing with the central zinc ions plays a significant role for the
a set of a triplet and two doublets. The phenomenon is ac- observed conformational change.
1902
Eur. J. Org. Chem. 1999, 1901Ϫ1906

Oligooxaethylene-Spacered Porphyrin Dimer Ϫ Toward the Construction of a New Switchable Porphyrin ArraFy ULL PAPER
Negligible variations were observed in the case of the ref- plots relative to fluorescence quenching experiments of the
erence porphyrin 2, indicating that the complexation of the porphyrin emission by NaI and KI. In these experiments,
cation only takes place at the oligooxyethylenic site. More- the iodine quencher is constrained in proximity of the
1
over, neither the UV/Vis nor the H-NMR spectral pattern sample by the ion binding, affecting the quantum yield of
of the porphyrins 1a and 1b show any appreciable variation the porphyrin moieties. Interesting differences between the
ϩ
ϩ
upon addition of an excess of Bu NClO salt, excluding the binding of Na and K ions are shown by the quenching
4
4
occurrence of self-aggregation phenomena, like π-π stack- experiment results reported in Figure 3. The SternϪVolmer
[
15]
ing, that could be induced by the increased ionic strength plot relative to the NaI quencher is linearly dependent on
[
16]
of the medium. The formation of dimers or higher aggre- the NaI concentration, exhibiting only a dynamic quench-
gates is a common phenomenon, which is, however, usually ing of the porphyrin chromophore emission. KI quenching
seen in the case of charged derivatives in aqueous sol- experiments exhibit an upward curvature of the
[
17]
vents,
and is evidenced by both the broadening and the SternϪVolmer plot, indicating
a
complex quenching
batochromic shift of the Soret band.
mechanism arising form both static and dynamic (colli-
The complexation studies were also extended to 23Na- sional) processes.
[21]
Static contributions to the overall
NMR spectroscopy.[ When a solution of receptor 1a in quenching are usually ascribed to a ground-state complex
18]
ϩ
CDCl /CD CN (1:1) is added to a solution of Na in the formation, particularly evident in this case due to higher
3
3
ϩ
same solvent mixture at room temperature, the relaxation affinity of the K ion to 1a.
ϩ
time (T ) of the Na nucleus gradually decreases from 23
Interestingly the SternϪVolmer plot of the KI quenching
1
ϩ
ms (free cation) to 10 ms for a [1a]/[Na ] ratio of 0.2. This experiments on 2 (Figure 3), shows only a dynamic contri-
is ascribed to the formation of the more rigid molecular bution to the overall quenching process enhanced by prox-
complex [Na@1a] as a consequence of the wrapping of the imity effects.
[19]
oxyethylenic segment around the metal cation.
The same
behaviour is observed in the case of receptor 1b. UV/Vis
spectra of the porphyrin derivatives in the presence of an
ϩ
ϩ
excess of Na or K salts, however, show negligible varia-
tions. This observation sheds some light on the actual ge-
ometry of the supramolecular complexes. In the case of co-
facial diporphyrin derivatives with an interplanar distance
˚
ranging from 4.5 to 6.5 A, a hypsochromic shift of the Soret
band is usually observed[
5][20]
as a consequence of the elec-
tronic interaction of the π-electron systems of the two
macrocycles. In our case the two tetrapyrrolic planes are
not closely faced but more probably are somewhat dis-
placed apart in a more “open” configuration.
Fluorescence Quenching Experiments
Figure 3. SternϪVolmer plot for the quenching of porphyrin 1a
and 2 with KI and for the quenching of 1a with NaI (inset)
Preliminary fluorescence studies on porphyrin 1a were
carried out in acetonitrile solution by using several quench-
ers, i.e. NaI, KI, KSCN. The idea was to analyze how the
ϩ
ϩ
binding of Na or K ions to the sample, causing a struc-
tural rearrangement of the entire molecule, affects the pho-
tophysical properties of the two porphyrin moieties.
In all the experiments the emission spectrum of the por-
Gas-Phase Complexation Studies by FAB Mass
Spectrometry
Fast atom bombardment mass spectrometry has been
phyrin chromophore was strictly preserved (λem ϭ 648 nm, widely applied in the field of hostϪguest complexation
FWHM ϭ 21 nm), showing no additional radiative contri- chemistry.[ The mass spectra of bis(porphyrin) 1a in the
butions from Charge Transfer (CT) or excimer-like species. presence of KSCN provide clear evidence for the formation
This finding supports the idea that the ion-binding process of the complex. Peaks at 1416 and 1455 correspond to the
forces the molecule to attain a conformation that does not 1a and [K@1a] cation, respectively. The same result is ob-
involve strong intramolecular π-π interactions between the served in the case of the addition of an excess of NaSCN
22]
ϩ
two porphyrin moieties as also suggested by UV/Vis experi- to a slurry of 1a in NBA matrix. The fact that [M ϩ K]
ments. Accordingly, fluorescence intensity measurements at corresponds to the most intense peak of the spectrum
different KSCN concentrations show that the porphyrin (100% intensity), whereas in the analogous experiment the
ϩ
quantum yield is little affected by the ion binding, again a intensity of [M ϩ Na] presents a value of about 40%, gives
clear evidence of the weak intramolecular interaction be- an indication of the different stability of the molecular com-
ϩ
tween the porphyrin chromophores. The binding of Na
plexes in the gas phase which nicely parallels the trend ob-
ϩ
[23]
and K ions to 1a is clearly shown by the SternϪVolmer served in solution.
Eur. J. Org. Chem. 1999, 1901Ϫ1906
1903

D. Monti, M. Venanzi, G. Mancini, F. Marotti, L. La Monica, T. Boschi
FULL PAPER
showed the disappearance of the typical absorption band (λ ϭ
Conclusions
4
19Ϫ420 nm, Soret) of the title porphyrins in the eluent. The fil-
The chemistry of porphyrin dimers have attracted in- trate and the washings were collected, reduced to a small volume
creasing attention since these molecules represent good by rotary evaporation, and the residue applied to a silica gel col-
3 2
umn. The elution with CHCl gave H TPP as a first moving band.
The subsequent elution with a 1% methanol/chloroform mixture
gave 4.5 g of the porphyrin 2 (0.07 mol, 21% yield) which was used
models of key enzymatic systems. The approach we fol-
lowed for the construction of the required architectures en-
tails the functionalization of an oligo(oxaethylenic) chain
with two porphyrin platforms. The promising features of
the podand-like structure obtained resides on the possibility
of the change of topology toward a cofacial arrangement
induced by the supramolecular host-guest interaction of the
1
without further purification. Ϫ H NMR (CDCl
3
), δ ϭ 8.8Ϫ8.9
ϭ 7.1 Hz, J ϭ 2.3 Hz,
H, aromatics), 8.13 (d, J ϭ 9.2 Hz, 2 H, 3Ј-, 5Ј-H, C OMe),
.76 (m, 9 H, aromatics), 7.28 (d, J ϭ 9.2 Hz, 2 H, 2Ј-, 6Ј-H,
OMe), 4.09 (s, 3 H, OMe), Ϫ2.8 (br. s, 2 H, NH). Ϫ UV/Vis
ϭ 420 (Soret), 516, 550, 592, 650 nm. Ϫ FAB MS
(
m, 8 H, β-H pyrrolic), 8.20Ϫ8.24 (dd, J
1
2
6
7
6 4
H
6 4
C H
oxyethylenic segment with an alkali metal ion. Studies di- (CHCl ): λ
3
max
ϩ
ϩ
rected toward the elaboration of some of the structural fea- (NBA); m/z (%): 646 (100) [M] , 614 (15) [M Ϫ OMe] .
tures of the receptors aimed at the achievement of an opti-
mal geometry as well as higher binding constants and in-
creased selectivity toward different metal cations are in pro-
5
(
-(4Ј-Hydroxyphenyl)-10,15,20-triphenylporphyrin (3): Porphyrin 2
0.323 g, 0.5 mmol) was dissolved in 25 mL of carefully dried and
degassed CH Cl . The solution was cooled to Ϫ10°C and BBr
250 mL, 2.6 mmol) added dropwise. The bluish-green reaction
2
2
3
gress. A step along this direction could be, for example, the
(
introduction of the so-called “Donor End Groups”.[ This mixture was allowed to warm to room temperature and stirred
would result in a favourable enthalpic contribution to the overnight under nitrogen. The mixture was then quenched with
association as a consequence of the increased number of methanol (5 mL), cautiously poured into a mixture of ice and satu-
14]
rated aqueous solution of Na
to the suspension, the organic layer separated, washed with brine
until neutrality, and dried (Na SO ). The solution was reduced to a
small volume under reduced pressure and chromatographed (SiO
eluting with a 3% methanol/chloroform mixture to give 0.24 g
0.38 mmol, 75% yield) of the title compound which was used with-
2 3
CO . Chloroform (50 mL) was added
binding sites. The systems presented in this work may have
also important applications in the field of supramolecular
[
8a,24]
2
4
switches,
and ion recognition.
2
)
(
Experimental Section
1
out further purification. Ϫ H NMR (CDCl
H, β-H pyrrolic), 8.25Ϫ8.20 (dd, J ϭ 7.0 Hz, J
aromatic), 8.13 (d, J ϭ 8.7 Hz, 2 H, 3Ј-, 5Ј-H, C
H, aromatics), 7.28 (d, J ϭ 8.7 Hz, 2 H, 2Ј-, 5Ј-H, C
s, 3 H, OMe), Ϫ2.7 (br. s, 2 H, NH). Ϫ UV/Vis (CHCl
3
), δ ϭ 9.0Ϫ8.7 (m, 8
ϭ 2.0 Hz, 6 H,
OH), 7.76 (m,
OH), 4.1
Instrumentation: H- and 23Na-NMR spectra were recorded with a
Bruker AC 300 P (300 MHz) spectrometer. Chemical shifts are
given in ppm relative to tetramethylsilane (TMS) and are referenced
against residual solvent signals. Ϫ Routine UV/Vis spectra were
performed with a Varian Cary 1/E spectrophotometer. A Perkin
ϪElmer λ 18 instrument was used for more delicate UV/Vis meas-
urements. Ϫ Fluorescence quenching experiments were performed
with a SPEX Fluoromax spectrofluorimeter operating in Single
Photon Counting mode. Ϫ Mass spectra (FAB) were recorded with
a VG-Quattro spectrometer by using m-nitrobenzyl alcohol (NBA,
Aldrich) as a matrix. Ϫ Silica gel 60 (70Ϫ230 mesh, Merck) was
used for column chromatography. Ϫ Reagents (Aldrich, Fluka or
Carlo Erba) were of the highest grade available and were used with-
out further purification. Tetraethylene glycol ditosylate was synthe-
1
1
2
6
H
4
9
(
6 4
H
3
): λmax
ϭ
4
6
20 (Soret), 518, 551, 590, 650 nm. Ϫ FAB MS (NBA); m/z (%):
31 (100) [M ϩ H], 614 (15) [M Ϫ OH].
␣
,ω-Bis[5,10,15-triphenylporphyrinyl-20-(4-phenyloxyl)]tetra-
ethylene Glycol [H (P)TEG(P), 1a]: 50 mg of porphyrin
0.08 mmol), 20 mg of tetraethylene glycol ditosylate (0.04 mmol)
and 250 mg of anhydrous K CO in 10 mL of freshly dried and
4
3
(
2
3
distilled DMF were stirred at reflux temperature for 12 h under an
inert atmosphere. The solvent was then stripped off in vacuo to
give a purple residue which was dissolved in 50 mL of CHCl
washed with brine until neutrality. The organic solution was dried
(Na SO ), reduced to a small volume, and chromatographed (SiO
eluting with a 3% methanol/chloroform mixture to give 78 mg of
3
and
sized according to published procedures.[
26]
Solvents were dried,
[27]
2
)
degassed, and distilled before use, by using standard procedures.
2
4
˚
A small amount of activated 4-A molecular sieves was added to
NMR solvents in order to remove traces of water.
1
pure 1a (0.055 mmol, 68% yield). Ϫ H NMR, (CDCl
.9Ϫ8.8 (m, 16 H, β-H pyrrolic), 8.2Ϫ8.1 (m, 10 H, aromatic), 8.09
d, J ϭ 8.1 Hz, 4 H, C OR) 7.74Ϫ7.68 (m, 20 H, aromatic), 7.28
d, J ϭ 8.1 Hz, 4 H, C OR), 4.4 (br. t, J ϭ 4.8 Hz, 4 H,
CH O), 4.1 (br. t, J ϭ 6.0 Hz, 4 H, CH CH O), 3.9Ϫ3.8 (m,
H, CH O), Ϫ2.77 (br. s, 4 H, NH). Ϫ C NMR (CDCl ):
δ ϭ 158.6 (C-4Ј, C OR), 142.1 (C-1Ј, phenyl), 135.6 (C-2Ј,
OR), 134.7 (C-1Ј, C OR) 134.6 (C-2Ј, phenyl), 131.2 (C-4Ј,
phenyl), 130.0 (br., C-α), 129.0 (br., C-α), 128.5 (br., C-α), 127.7
3
) δ ϭ
8
(
(
CH
8
5-(4Ј-Methoxyphenyl)-10,15,20-triphenylporphyrin (2): To a solu-
6 4
H
tion of 35 g (0.33 mol) of benzaldehyde, 4.5 g of 4-methoxybenzal-
dehyde (0.033 mol) in 2 L of propionic acid at reflux temperature,
a solution of 28 g of freshly distilled pyrrole (0.42 mol) in 100 mL
of propionic acid was added dropwise over the period of 30 min.
The reaction mixture was stirred at reflux temperature for ad-
ditional 3 h, then cooled to room temperature. The solvent was
evaporated under reduced pressure to give a black tarry residue
which was dissolved in chloroform (250 mL) and a saturated
6
H
4
2
2
2
2
13
2
CH
2
3
6
H
4
C
6
H
4
6 4
H
(C-β), 126.7 (C-3Ј, phenyl), 120.1 (C-meso), 120.0 (C-meso), 119.9
(C-meso), 112.9 (C-3Ј, anisyloxy), 71.1 (CH CH O), 70.9
(CH CH O), 70.0 (CH CH O), 67.8 (CH CH O). Ϫ UV/Vis
(CHCl ): λmax ϭ 418 (Soret), 517, 552, 590, 645 nm. Ϫ FAB MS
(NBA); m/z (%): cluster centered at 1416 (100) [M ].
2
2
NaHCO
stirred until evolution of gas ceased and the organic phase sepa-
rated, washed with a 10% Na CO aqueous solution (3 ϫ 200 mL)
and then with brine until neutrality. The solution was dried with
anhydrous Na SO . In order to remove polymeric by-products sil-
3
aqueous solution was added. The mixture was vigorously
2
2
2
2
2
2
3
2
3
ϩ
2
4
Zn
2
(P)TEG(P) (1b): In a 25-mL round-bottomed flask, equipped
ica gel (200 g) was added to the solution and the resulting mixture with a reflux condenser and a nitrogen inlet tube was placed 50 mg
stirred for 30 min. The silica gel was filtered off and the solid resi- of porphyrin 1a (0.035 mmol) in 15 mL of chloroform. The mixture
due repeatedly washed with CHCl
3
until a UV/Vis check run was brought to reflux temperature and 1 mL of a saturated meth-
1904
Eur. J. Org. Chem. 1999, 1901Ϫ1906

Oligooxaethylene-Spacered Porphyrin Dimer Ϫ Toward the Construction of a New Switchable Porphyrin ArraFy ULL PAPER
anolic solution of Zn(OAc)
2
was added under stirring. The stirring
placed on the tip of a stainless steel probe. The spectra were re-
corded in the positive-ion mode.
and heating were prolonged for 3 h, the mixture was then allowed
to cool to room temperature, the solvent was removed in vacuo,
the cherry-red residue dissolved in 50 mL of chloroform, washed
with brine (3 ϫ 50 mL), and dried (Na
duced to a small volume and the residue chromatographed (SiO
2 4
SO ). The solvent was re-
Acknowledgments
2
)
on a short column eluting with a 3% methanol/chloroform mixture
to give 0.052 mg of the pure title compound (0.034 mmol, 97%
We wish to thank Mr. Alessandro Leoni and Mr. Giuseppe DЈAr-
cangelo for their valuable technical help. We are deeply indebted
with Professor G. Ercolani, University “Tor Vergata” for helpful
discussions. MURST and CNR are acknowledged for funding.
yield). The product can be further purified by crystallization from
1
dichloromethane/hexane. Ϫ H NMR (CDCl
3
): δ ϭ 8.95Ϫ8.86 (m,
1
8
6 H, β-H pyrrolic), 8,18Ϫ8.12 (m, 10 H, aromatic), 8.02 (d, J ϭ
.4 Hz, 4 H, C OR), 7.75Ϫ7.66 (m, 20 H, aromatic), 7.04 (d,
OR), 3.87Ϫ3.82 (m, 4 H, CH CH O), 3.53
O), 3.35Ϫ3.15 (m, 8 H, CH CH O). Ϫ
) δ ϭ 158.3 (C-4Ј, C OR), 142.9 (C-1Ј, phen-
yl), 135.4 (C-2Ј, C OR), 134.5 (C-1Ј, C OR), 132.1 (C-2Ј,
phenyl), 131.9 (C-2Ј, phenyl), 132.0 (C-α), 131.9 (C-4Ј, phenyl),
6
H
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