Synthesis, complexation properties and spectroscopic studies of the cation-induced conformational changes of some new oligooxaethylene-spacered diporphyrin arrays

Article abstract of DOI:10.1039/b008250n

New linear diporphyrin arrays and a heteroleptic diad have been synthesized in good yield. ¹H NMR, UV-visible and fluorescence studies reveal that the receptors bind to alkali and alkaline-earth metal ions to give a supramolecular complex in which the ion is nestled within the oligooxaethylene framework. The formation of the host-guest complexes promotes a change in the geometry of the system toward a topology in which the two tetrapyrrolic macrocycles tend to face together. This is evidenced by the mutual ring current anisotropy effect exerted by the porphyrinic platforms, and by the blue shift and broadening of the relative UV-visible bands (Soret). In particular, the changes observed allow the complexation process to be monitored at very low concentrations. The association constants lie within the range 25-1 × 10⁵ M^-1 (CD₃CN-CDCl₃ 1:1) depending on either the nature of the diporphyrinic receptors or that of the interacting metal ions. The heteroleptic diad shows a very efficient energy transfer process (>99%) between the two di erent macrocycles in a variety of solvents that is not altered by complexation with metal ions. The systems reported present interesting properties as switchable biomimetics.

Full text of DOI:10.1039/b008250n

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Synthesis, complexation properties and spectroscopic studies of the
cation-induced conformational changes of some new
oligooxaethylene-spacered diporphyrin arrays¤
Laura La Monica,a Donato Monti,*a Giovanna Mancini,b Marco Montalti,c Luca Prodi,*c
Nelsi Zaccheroni,c Giuseppe DÏArcangeloa and Roberto Paolessea
a Dipartimento di Scienze e T ecnologie Chimiche, UniversitaÏ degli Studi di Roma
“
“T or V ergataÏÏ, I-00133 Rome, Italy
b Centro C.N.R. di Studio sui Meccanismi di Reazione, Dipartimento di Chimica, UniversitaÏ degli
Studi di Roma ““L a SapienzaÏÏ, I-00185 Rome, Italy
`
c Dipartimento di Chimica ““G. CiamicianÏÏ, Universita di Bologna, I-40126 Bologna, Italy
Received (in Cambridge, UK) 3rd October 2000, Accepted 16th January 2001
First published as an Advance Article on the web 16th March 2001
New linear diporphyrin arrays and a heteroleptic diad have been synthesized in good yield. 1H NMR, UV-visible
and Ñuorescence studies reveal that the receptors bind to alkali and alkaline-earth metal ions to give a
supramolecular complex in which the ion is nestled within the oligooxaethylene framework. The formation of the
hostÈguest complexes promotes a change in the geometry of the system toward a topology in which the two
tetrapyrrolic macrocycles tend to face together. This is evidenced by the mutual ring current anisotropy e†ect
exerted by the porphyrinic platforms, and by the blue shift and broadening of the relative UV-visible bands (Soret).
In particular, the changes observed allow the complexation process to be monitored at very low concentrations.
The association constants lie within the range 25È1 ] 105 M~1 (CD CNÈCDCl 1 : 1) depending on either the
3
3
nature of the diporphyrinic receptors or that of the interacting metal ions. The heteroleptic diad shows a very
efficient energy transfer process ([99%) between the two di†erent macrocycles in a variety of solvents that is not
altered by complexation with metal ions. The systems reported present interesting properties as switchable
biomimetics.
1aÈ1c and 2 (Chart 1). The unsymmetrical diad 2 could allow
Introduction
the achievement of a system in which the di†erent substitution
pattern would result in interesting photophysical properties.1c
A systematic study of the interaction of such derivatives with
a series of alkali and alkaline-earth metal ions has been under-
taken in order to acquire information on the e†ect of the
structural features of the partners involved on the conforma-
tional changes of the diporphyrin systems investigated. The
strength of the ionÈreceptor interaction has been evaluated by
means of 1H NMR spectroscopy, which additionally gave
some indication of the actual change of geometry of the
systems. A nice parallelism has been evidenced by concomi-
tant UV-visible or Ñuorescence spectroscopy.
The rich chemistry of porphyrin dimers and oligomers consti-
tutes a challenging and stimulating arena of research devoted
to the construction of biomimetic models1 of the light harvest-
ing antenna apparatus of bacteria and green plants.2 More-
over, cofacial3a,b and the closely related, more Ñexible,
“
“Pac-ManÏÏ3c,d metallodiporphyrins are regarded as excellent
tools for multielectron redox activation of small molecules,
mimicking the activity of metalloenzymes involved in life-
sustaining cycles such as peroxidases, cytochrome P-450, cyto-
chrome c oxidase and nitrogenase.4
We recently reported5 on the synthesis and preliminary
binding properties of
a linear tetraoxaethylene-spacered
diporphyrin array, able to undergo a conformational change
toward a cofacial topology upon interaction with alkali metal
ions. This strategy, in which some critical covalent bonds are
replaced by non-covalent interactions, would represent a
promising alternative to either the non-covalent or the fully
covalent6 approaches usually pursued for the construction of
diporphyrin systems with cofacial topology. Moreover, the
possibility of tuning the geometry makes the systems investi-
gated particularly appealing and prone to applications, for
example in the Ðeld of molecular switches or sensors.7
Results and discussion
Synthesis
The systems studied are characterised by a di†erent number of
oxygen donor sites (e.g. length of the oligooxaethylene spacer)
as well as by the presence of two additional methoxy function-
alities on the aromatic part of the linkers, with respect to the
previously reported system.5 This should confer better coordi-
nation properties toward the investigated metal ions. The
diporphyrin arrays 1aÈ1c were prepared in a straightforward
way by following a modiÐed procedure reported earlier,5,8
which is outlined in Scheme 1(a). Condensation of pyrrole and
Prompted by the promising results obtained, we extended
the studies to a wider class of diporphyrin receptors, namely
4
-hydroxy-3-methoxybenzaldehyde (vanillin), in the presence
¤
Electronic supplementary information (ESI) available: 1H NMR
titration curves for receptor 1c and titration spectra of 1b. See http://
www.rsc.org/suppdata/nj/b0/b008250n/
of a tenfold excess of benzaldehyde in reÑuxing acetic acid,
a†orded, besides the expected meso-tetraphenylporphyrin
DOI: 10.1039/b008250n
New J. Chem., 2001, 25, 597È605
597
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

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(
2
H TPP), the desired 5-(4-hydroxy-3-methoxyphenyl)-10,15,
0-triphenylporphyrin 3a in satisfactory yield, after standard
3a was coupled with tri-, tetra- and penta-ethylene glycol
ditosylate under Williamson conditions9 to give in good yield
the targets 1aÈ1c, respectively. The procedure for the synthesis
of the unsymmetric diporphyrin 2 is slightly di†erent (Scheme
1b) from that above. Attempts to prepare 2 by direct coupling
of the oligoethylene glycol ditosylate in the presence of equi-
2
work-up and chromatographic separation. The use of an
excess of benzaldehyde was necessary in order to drive the
cyclisation toward selective formation of the monoaryl-
substituted porphyrinic derivative. The porphyrin derivative
Scheme 1 (a) Synthesis of diporphyrin derivatives 1aÈ1c. Reagents and conditions: (i) acetic acid, reÑux, 3 h, yield 12%; (ii) tri-, tetra-, or
penta-ethylene glycol ditosylate, anhydrous K CO , DMF, 80 ¡C, 12 h, 80%. (b) Synthesis of diporphyrin derivative 2. (iii) HO(CH CH O) -
2
3
2
2
3
CH CH OTs, anhydrous K CO , DMF, 80 ¡C, 12 h, 90%; (iv) TsCl, dry pyridine, r.t., 12 h, 80%; (v) 4, anhydrous K CO , DMF, 80 ¡C, 12 h,
2
2
2
3
2
3
2
0%. Ts \ p-Toluenesulfonyl.
598
New J. Chem., 2001, 25, 597È605

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molar amounts of both the aryl- and alkyl-porphyrin precur-
sors resulted in a mixture of hetero- and homo-diporphyrin
derivatives requiring tedious chromatographic separation.
Alternatively, the arylporphyrin precursor (3a) was coupled
with tetraethylene glycol monotosylate to give the correspond-
ing x-monoporphyrinyl derivative 3b. This was converted into
the corresponding tosyl derivative (p-toluenesulfonyl chloride,
CH Cl , pyridine) 3c, which was in turn coupled with the
according to this mechanism, the overlap integral between the
emission spectrum of the donor F(l) (normalised such that
/ F(l)dl \ 1) and the absorption spectrum of the acceptor e(l),
J , has to be calculated (eqn. 1), resulting, for this dimer, in
F
1.75 ] 10~14 cm6 mmol~1. The energy-transfer rate constant
is Ðnally expressed by eqn. 2 where n is the refractive index of
the solvent, U and q are the luminescence quantum yield and
lifetime of the donor, J is the overlap integral and K2 a geo-
2
2
F
octaalkylporphyrin (OAP) moiety 4 to give the desired hetero-
diad 2 in 15% overall yield. The alkylporphyrin 4 was
straightforwardly obtained by acid-catalysed cyclisation of the
relative biladiene-a,c precursor10 with vanillin.
metric factor, which depends on the relative orientation of the
dipoles and that, in our case, can be taken with conÐdence as
2/3.12a With a rate limiting constant of 1 ] 1010 s~1, it is pos-
sible to estimate, from eqn. 2, the distance between the two
chromophores in solution, which is lower than 12.5 AŽ .
The above porphyrin derivatives have been characterised by
UV-visible, 1H, 13C NMR and FAB-MS techniques. The
P
NP
proton NMR spectra (CDCl ) show features of both the oligo-
JF \ F(l)e(l)l~4 dl
F(l) dl
(1)
(2)
3
oxaethylenic spacers and of the porphyrinic platforms, within
the examined concentration window (up to 5 ] 10~4 M).
Details are reported in the Experimental section. FAB-MS
spectra provide further evidence for formation of the desired
architectures; showing molecular clusters at the expected m/z
values.
8.8 ] 10~25K2U
k \
J
e
n4qr6
F
Notably, the efficiency of the energy transfer did not change
on changing the solvent polarity, i.e. from cyclohexane to pure
acetonitrile, in contrast to what was reported14 for a system
having a similar poly(oxaethylene) bridge, and polypyridine
complexes of Ru and Os as energy donor and acceptor,
respectively. In our case the efficiency was [99% in all the
solvents examined, so that the photoinduced energy-transfer
process in 2 could not be used to probe solvent-dependent
conformational changes of the bridge.
Spectroscopic studies
The UV-visible spectra (1 ] 10~6 M CHCl ÈCH CN 1 : 1,
3
3
v/v) of the diporphyrin architectures are virtually superim-
posable on the algebraic sum of those of their parent mono-
mers, indicating that the electronic interaction between the
two macrocycles in the dimers is in all cases relatively weak.
The spectra feature the typical Soret band (402È419 nm), and
the four visible Q bands in the range 515È650 nm. The
Ñuorescence quantum yields and the excited states lifetimes
are typical of the parent monomers. In these systems the
only additional pathway responsible for a radiationless decay
to the ground state, due to the formation of dimeric struc-
tures, is an electron-transfer process between the two porphy-
rin units, which for such systems is thermodynamically
forbidden.1e,11 In the case of the unsymmetric diad 2, charac-
terised by the presence of both a TPP (triphenylporphyrin)
and an OAP (octaalkylporphyrin) unit, the typical Ñuores-
cence band of the latter macrocycle could not be observed,
even when the excitation light was prevalently absorbed by
this moiety. On the contrary, the typical band of the TPP
component is present, with an excited state lifetime equal to
that of the monomeric species. The excitation spectra were
very indicative for understanding the behaviour of this com-
Interaction with metal ions
1H NMR spectroscopy is a convenient technique for studying
the binding of the above systems with metal ions. The experi-
ments have been carried out in CD CNÈCDCl (1 : 1 v/v) in
3
3
order to achieve the optimal solubility of the species investi-
gated. The salts employed were perchlorates or iodides. The
typical downÐeld shift of the protons of the oxaethylene
spacers15 indicates the occurrence of binding at the ether sites.
The mass spectra (FAB) of the diporphyrin receptors in the
presence of an excess of metal salts (Mn`) gave further evi-
dence for formation of the complexes. Peaks at [1aÈ
1c ] M]n`, corresponding to the supramolecular adducts, are
clearly visible in the spectral pattern of all the receptors inves-
tigated.16
Typical chemical shift changes of the signals of the porphy-
rin macrocycles clearly evidence conformational change of the
receptors, promoted by interaction with the metal ions. A sig-
niÐcant example is shown in Fig. 1, which shows the chemical
shift changes of the porphyrin 1b protons with increasing con-
centration of potassium salt. The evident upÐeld shift of the
porphyrinic b-proton resonances, up to 200 Hz, is an indica-
tion of the change of topology toward cofacial geometry.
Negligible variations were observed in the case of the refer-
ence porphyrin 3a. Moreover, neither the UV-visible nor the
1H NMR spectral patterns of the diporphyrins studied show
any appreciable variation upon addition of an excess (up to
5 ] 10~2 M) of Bu NClO , ruling out the occurrence
pound. Positioning the monochromator emission at j \ 725
em
nm, where the Ñuorescence is almost entirely ([95%) due to
the TPP component, the excitation spectrum is almost the
same as the absorption spectrum of the whole compound,
showing bands of both the TPP and OAP components, with
their relative intensity unchanged. All these Ðndings clearly
indicate the occurrence of a thermodynamically allowed effi-
cient energy-transfer mechanism from the OAP to the TPP
moiety. A limiting value for the rate constant of the energy
transfer process (k ) of 1 ] 1010 s~1 can be calculated, on the
e
basis of a residual Ñuorescence intensity of the OAP moiety
lower than 1% of that observed for the unquenched chromo-
phore. This leads to the assumption that the whole process
occurs with an efficiency higher than 99%. An electron trans-
fer process was reported to be, in similar cases, slightly
endoergonic1e and for this reason it can not compete with a
very fast energy transfer process.
It is important at this point to compare the experimental
rate constants with theoretical ones for the energy-transfer
process. In particular, an energy-transfer process can be
4
4
of selfaggregation phenomena induced by the increased ionic
strength of the medium. It is possible to evaluate the strength
of the metalÈreceptor interaction by a non-linear least-squares
Ðtting analysis of the chemical shift variation of the selected
signals vs. the concentration of the added metal ions (see ESI
Fig. S1).¤ All the receptors investigated form a 1 : 1 complex
with the metal cation as clearly evidenced by the excellent
adherence of the experimental data to the calculated theoreti-
cal curve. The results obtained are shown in Table 1. As far as
alkali-metal ions are concerned, the association constants vary
within the range 25È6500 M~1, depending on the nature of
the cation and increasing with the number of oxygen coordi-
described in terms of a Coulombic (Fo
Dexter)12b mechanism. For similar diporphyrin systems, the
Foster mechanism was preferred for explaining the experimen-
tal results.1e,13 To estimate the energy-transfer rate constant,
Ž
ster)12a and an exchange
(
Ž
New J. Chem., 2001, 25, 597È605
599

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highest shielding is, in most cases, promoted by the sodium
cation which reaches its maximum e†ect in the interaction
with the tetra-spacered (n \ 3) 1b. A di†erent trend is found
for the smaller Li`, which features highest chemical shift
variations in the case of the shorter tri-segmented receptor 1a.
The vanishingly small chemical shift variations observed in
the case of the 1aÈCs` interaction can be ascribed to the poor
ability of the receptor to encircle the large cation, which,
evidently, results in a wider distance between the two porphy-
rinic platforms.
The unsymmetrical diad 2 features a somewhat increased
binding strength toward K`, with respect to its 1b counter-
part (Table 1). This is also accompanied by an increased
shielding e†ect on the corresponding proton resonances. This
is, in the Ðrst instance, a consequence of a relief of steric hin-
drance mutually exerted by the porphyrin platforms upon
cofacialisation. This interpretation is, however, in contrast to
the results obtained from both the UV-visible and Ñuores-
cence studies (see below) which indicate a substantially lower
degree of electronic interaction between the two macrocycles.
An alternative explanation considers the increased electron
density of the OAP moiety,19 which can account for the more
favourable cationÈreceptor electrostatic interaction, as well as
for the observed increased ring current anisotropy e†ect.
The binding constants for the interaction of several
alkaline-earth metal ions with the diporphyrin receptor 1b
span three orders of magnitude, within the range 4 ] 102È
1 ] 105 M~1, with a marked selectivity for the strontium
cation. The strength of the interaction is higher, by one order
of magnitude or more, with than that of monocationic
counterparts of similar ionic radius20 (Fig. 2) as a consequence
of the increased electrostatic interaction. The increased affinity
toward alkaline-earth metal ions is commonly encountered in
the case of crown ethers and other macrocycles,21 as well as for
related complex acyclics.22 In the latter cases, however, the
selectivity toward Sr2`, which is found in the case of 1b, van-
Fig. 1 1H NMR spectral variation (300 MHz, CD CNÈCDCl 1 : 1,
3
3
v/v) of the selected aromatic part (*) of compound 1b (3.5 ] 10~4 M)
in the presence of KClO : (a) 0, (b) 5.38 ] 10~3, (c) 9.68 ] 10~3, (d)
4
2
0.7 ] 10~3 M. (L) indicates residual solvent.
nation sites of the receptor. Not surprisingly, the presence of
an additional methoxy group on the aromatic end of the
spacer results in an appreciable enhancement of the affinity for
the considered metal ions by one order of magnitude.17 The
selectivity toward the inclusion of di†erent metal ions depends
on the nature of the receptors. Receptors 1b and 1c show the
largest affinity for potassium, whereas the smaller 1a prefer-
entially binds Na`. This Ðnding is evidently the consequence
of a Ðne balancing of enthalpic and entropic contributions, the
wrapping of a longer ether chain around a smaller cation
being disfavoured by the loss of conformational entropy.
The extent of the chemical shift variations, induced by
metal ion complexation, is dependent on the nature of the
partners involved and may give an indication of the geometry
achieved by the systems.18 Some selected data are given in
Table 1. They are not straightforwardly rationalisable; never-
theless, some key points may be identiÐed. It is important to
point out that the limiting values do not strictly depend on
the strength of the interaction, but rather on the size of the
interacting metal ion and on the length of the spacer. As far as
the peripheral porphyrinic b-protons are concerned, the
Fig. 2 Log K values (calculated from 1H chemical shift changes, see
text) for the interaction of alkali and alkaline-earth metal ions with
receptor 1b (CD CNÈCDCl 1 : 1, v/v) as a function of the ionic radii.
3
3
Table 1 Binding constant values (K/M~1)a for the interaction of diporphyrin receptors 1aÈ1c and 2, with alkali and alkaline-earth metal ions
Receptorc
Cation (r /A
Ž
)b
1a
1b
1c
2
i
Li`(0.76)
Na`(1.02)
K`(1.38)
Cs`(1.67)
Mg2`(0.72)
Ca2`(1.00)
Sr2`(1.18)
Ba2`(1.35)
85([120)
70([65)
160([80)
500([140)
280([20)
\25d
460([20)
890([80)
6500([45)
600([80)
1500([130)
340([50)
1850([150)
490([5)
6100([190)
[1 ] 105([100)
[4 ] 104([80)
a In CD CNÈCDCl 1 : 1, v/v, at 25 ¡C. b Data taken from ref. 20. c Limiting complexation-induced chemical shifts (*d/Hz, at 300 MHz) for the
3
3
peripheral b-H protons are reported in parentheses. Negative values indicate upÐeld shifts. d Not determined. Variations below the instrumental
detection limits.
600
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ishes in favour of other alkali earth metal ions such as calcium
or barium.
Sr2` and, to a somewhat smaller extent, Ba2` form complexes
for which the Soret band features the lowest intensity and the
largest broadening (Fig. 3). It is of note that these changes are
also accompanied by a new band at about 410 nm. This is
usually accounted for as the consequence of an electronic
interaction between porphyrin macrocycles held in a face-to-
face type conformation.3a,23 Conversely, in the case of the
longer penta-spacered 1c, the larger Ba2` induces the strong-
est perturbation.
A comparison between diporphyrin 1b and its ““hetero-
homologueÏÏ 2, characterised by the same tetraoxaethylene
unit, evidences the presence of a similar trend on changing the
metal ion, but the electronic interactions are unexpectedly
smaller. This decreased electronic interaction could be a con-
sequence of the achievement of an unfavourable conforma-
tion. One would expect, on the mere basis of the reduced
steric hindrance featured by the OAP unit, an increased elec-
tronic interaction as a consequence of the closer contact
between the porphyrinic frameworks. This favourable e†ect,
however, may be negatively counterbalanced by the increased
charge distribution on the OAP unit, which, in a face-to-face
geometry, would result in the onset of repulsive forces. The
system would then be forced to assume a more slipped, o†set
orientation.24 Further studies, however, are needed in order to
shed more light on this point.
UV-visible and Ñuorescence studies of metal ion complexation
UV-visible and Ñuorescence studies give further insights on
the cation-induced conformational changes of the diporphyrin
systems. As far as the absorption spectra are concerned, addi-
tion of alkali and alkaline earth metal ions to CHCl ÈCH CN
3
3
(
1 : 1, v/v) solutions of the diads causes an appreciable change
in intensity of the Soret band (380È440 nm, Table 2 and Fig.
). This is accompanied by a small but signiÐcant blue shift of
3
the absorption maxima, and an evident broadening, which
indicate cofacial arrangement of the porphyrinÈmetal complex-
es.3a As also observed by 1H NMR, the changes in the UV-
visible spectra strongly depend on the length of the bridging
unit, on the charge and size of the metal ion and on the nature
of the porphyrin systems. In the case of the tri-segmented 1a,
for example, smaller changes were usually observed as a con-
sequence of the steric strain su†ered on approaching a
pseudo-cyclic conformation. Diads featuring longer bridging
units, and hence a greater conformational mobility, experience
larger perturbations. In particular, for the tetra-spacered 1b,
In contrast to the absorption spectra, the Ñuorescence
properties, in terms of the energy of the transition, quantum
yield and excited state lifetime, are only slightly altered by the
complexation with alkali and alkaline earth metal ions (Table
3
). This is not, however, a completely unexpected result, since
the transition responsible for the Ñuorescence process
S ] S ) is exactly the opposite (S ] S ) of that responsible
(
1
0
0
1
for the Q bands in the absorption spectrum, that are also
slightly perturbed upon complexation. It is interesting that
the Ñuorescence of the unsymmetrical diad 2 is also perturbed
by complexation only to a marginal extent. In this case the
conformational changes caused by complexation, reducing the
distance between the donor and acceptor moieties, are
expected to increase the rate of the energy transfer process
Fig. 3 UV-visible spectroscopic variation (Soret band) of diporphy-
rin receptor 1b (1 ] 10~6 M, CD CNÈCDCl 1 : 1, v/v) in the pres-
3
3
ence of an excess of added metal ions.
(
eqn. 2). The efficiency of the process, however, can only slight-
ly be increased, since it has been estimated to be greater than
9% even in the ““freeÏÏ ligand; as a consequence, also the Ñuo-
9
Table 2 UV-visible spectral data (j )a of diporphyrin receptors
max
rescence spectrum does not undergo any change upon addition
of metal cations. These results do not exclude however the
possibility of using these systems as prototypes for Ñuorescent
chemosensors.25 Although the Ñuorescence quantum yield is
only slightly altered by complexation, it is still possible to
monitor the presence of metal ions by means of photolumines-
cence spectroscopy. The observed Ñuorescence intensity is in
fact proportional, when working in dilute conditions, both to
the Ñuorescence quantum yield and to the molar absorption
coefficient at the excitation wavelength. As a consequence, if
excitation is performed in the Soret band, whose intensity
decreases upon complexation, a decrease of the Ñuorescence
intensity is also observed. By virtue of the reasonably high
1
aÈ1c and 2, and their metal complexesb
Cation
1a
1b
1c
2
None
Na`
K`
Mg2`
Ca2`
Sr2`
Ba2`
418(1.00)
417(0.82)
417(0.93)
418(0.95)
416(0.97)
416(0.95)
418(0.93)
418(1.00)
417(0.88)
417(0.77)
418(0.95)
417(0.74)
416(0.64)
416(0.69)
418(1.00)
417(0.82)
417(0.83)
417(0.87)
416(0.95)
416(0.81)
416(0.77)
417(1.00)
418(0.85)
418(0.76)
417(1.00)
417(0.93)
419(0.79)
418(0.78)
a The relative absorbances (A ) are given in parentheses. b Porphyrin
concentration: 1 ] 10~6 M, (MeCNÈCHCl 1 : 1, v/v), 298 K.
rel
3
Table 3 Luminescence propertiesa of the investigated diporphyrin receptors and their metal ion complexesb
1a
1b
1c
2
Cation
j
U
q
j
U
q
j
U
q
j
U
q
max
max
max
max
None
Na`
K`
Mg2`
Ca2`
Sr2`
Ba2`
652
650
650
651
651
650
650
1.0
0.9
1.0
0.9
0.9
0.9
0.9
8.2
8.7
8.4
7.8
8.0
8.0
8.5
652
652
653
653
652
651
652
1.0
1.0
1.0
1.0
1.0
0.9
0.9
8.2
8.6
8.4
8.1
8.4
8.8
8.9
652
651
652
651
651
650
652
1.0
0.9
0.9
1.0
0.9
0.9
0.9
8.3
8.4
8.5
8.2
8.4
8.3
8.3
652
652
652
652
652
652
652
1.0
1.0
1.0
1.0
0.9
0.9
0.9
9.2
8.8
9.0
8.9
8.7
9.1
9.3
a j in nm, q in ns. b Porphyrin concentration: 1 ] 10~6 M, (MeCNÈCHCl 1 : 1, v/v), 298 K.
max
3
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Ñuorescence quantum yield of free base porphyrins, this could
allow one to monitor complexation even when the absorbance
due to the dimer is very low (\0.01), and changes can not
be observed by means of the less sensitive spectrophotometric
technique. Of course, in order to obtain efficient chemosensors
higher association constants and more efficient signal trans-
duction mechanisms should be achieved. In this context, the
case of the unsymmetric diad is very interesting. Unfor-
tunately in our system the energy transfer mechanism was so
e†ective even in the absence of metal ions that the approach
of the two porphyrin units caused by complexation does not
induce any further change in the Ñuorescence spectrum. This
indicates that, in order to have an e†ective switching o†/on of
the energy transfer mechanism, in the free species the distance
between the two chromophores, according to eqn. 2, should
tal response, inner Ðlter e†ects, and phototube sensitivity. A
correction for di†erences in the refraction index was intro-
duced when necessary. Mass spectra (FAB) were recorded on
a VG Quattro spectrometer by using m-nitrobenzyl alcohol
(NBA, Aldrich) as a matrix in the positive ion mode.
Materials
Silica gel 60 (70È230 mesh) was used for column chromatog-
raphy. Reagents (Aldrich, Merck or Fluka) were of the highest
grade available and were used without further puriÐcation.
Metal salts employed in the NMR titrations were of the
highest anhydrous grade available (Aldrich, Fluka), vacuum
dried and stored under a dry atmosphere. The a,x-ditosylates
were prepared according to published procedures.28 Solvents
were dried, distilled and degassed prior to use, by using stan-
be higher than 26 A
direction.
Ž and we are actually working in this
dard procedures.29 A small amount of activated 4 AŽ molecu-
lar sieves was added to NMR solvents in order to remove
traces of water. Solvents used for the spectrophotometric mea-
surements, acetonitrile and chloroform (Uvasol, Merck), were
stored over activated molecular sieves.
Conclusions and future perspectives
Diporphyrin derivatives constitute attractive models of the
key enzymatic apparatus involved in life sustaining cycles. The
systems we investigated possess the promising feature of a
tunable geometry by interaction with a series of alkali and
alkaline-earth metal ions. The promoted conformational
variations are substantially dependent on the ionic radii of the
interacting metal ions and on the lengths of the spacers. This
strategy may indeed represent a relevant step toward the
design and realisation of systems possessing a promising ver-
satility, arising from the possibility of tuning the geometry, the
Ñexibility and, consequently, the affinity of the resulting recep-
tor for substrate recognition26 and catalysis.27
In addition, it is worth underlining that the complexation
process induces noticeable changes in photophysical pro-
cesses, leading to the possibility to monitor the association
event between 1aÈ1c and 2 and metal cations even at low con-
centrations. Although a desirable onÈo† behaviour has not
been achieved with these systems, the results obtained are a
good starting point for the design of efficient chemosensors.
1H NMR titration
A CDCl ÈCD CN 1 : 1 v/v solvent mixture was used for solu-
3
3
bility reasons. The temperature was held constant, within
0.2 K, at 298 K. A general procedure for the 1H NMR titra-
tions is as follows. Solutions of metal ions (1 ] 10~3È
] 10~2 M) were prepared by dissolving in a 2 mL
^
5
volumetric Ñask the required amount of salt in a solution of
diporphyrin receptor. The resulting solution was added por-
tionwise via a microsyringe to 500 lL of the porphyrin solu-
tion (typically 5 ] 10~4 M) placed in a thermostatted NMR
sample tube. This procedure ensures a constant concentration
of the receptor throughout the NMR titration.
Data analysis
The stability constants (K) for 1 : 1 complexation were calcu-
lated from the chemical shifts of chosen signals. The values
obtained for di†erent sets of signals were the same within
experimental error (O5%). The equation used for analysing
NMR titration data was of the general form:30
Experimental
*d
\ *d K[Mn`]/(1 ] K[Mn`])
(3)
obs
i
Instrumentation
where *d \ d [ d , d is the observed NMR shift of a
obs
obs
0
obs
1
H NMR spectra were recorded with a Bruker AC 300 P (300
speciÐc acceptor proton in the equilibrium solution, d the
0
MHz) spectrometer equipped with a sample tube thermo-
station apparatus. Chemical shifts are given in ppm relative to
tetramethylsilane (TMS) and are referenced against residual
solvent signals. Numbering of carbon atoms follows the label-
ling scheme. Routine UV-visible spectra were measured on a
Varian Cary 1E spectrophotometer, whereas more delicate
measurements were performed on a Perkin-Elmer k40 spec-
trophotometer equipped with a thermostatted cell holder.
Uncorrected emission and corrected excitation spectra were
obtained with a Perkin-Elmer LS 50 spectroÑuorimeter. The
Ñuorescence lifetimes (uncertainty ^ 5%) were obtained with
an Edinburgh single-photon counting apparatus, in which the
shift of the free acceptor proton, *d \ d [ d , d is the shift
i
i
0
i
of the concerned proton of the Mn`Èreceptor complex, and
[Mn`] is the total concentration of the added salt. The
unknown parameters, *d and K, were obtained by the least-
i
(
squares curve Ðtting program Kaleidagraph 31 from six to
eight measurements. In the case of Ba2` and Sr2`, character-
ised by higher binding constants, an excess of salt was not
possible and eqn. 4 was necessarily used32
K \ (*d /*d )/([M2`][1 [ (*d /*d )])
obs obs
(4)
i
i
where [M2`] \ [M2`] [ (*d /*d )[P] ; [M2`] is the free
t
obs
i
t
metal concentration, [M2`] the total concentration of the
t
Ñash lamp was Ðlled with D . Luminescence quantum yields
added metal cation, and [P] the total concentration of the
2
t
(
(
uncertainty ^ 15%) were determined using H TPP in toluene
porphyrinic host. The regression values (r2) for the computer
2
/ \ 0.11) as a reference. In order to allow comparison among
emission intensities, we performed corrections for instrumen-
Ðtting were better than 0.996. Experiments were run in dupli-
cate and found to be in excellent agreement within experimen-
tal error (standard deviation O5%).
Preparations
5-(4-Hydroxy-3-methoxyphenyl)-10,15,20-triphenyl-
porphyrin, 3a. To a solution of 8.2 g (77 mmol) of benz-
aldehyde and 1.0 g (7.8 mmol) of 4-hydroxy-3-methoxy-
benzaldehyde (vanillin) in 400 mL of reÑuxing acetic acid, a
solution of 5.5 g (83 mmol) of pyrrole in 50 mL of acetic acid
602
New J. Chem., 2001, 25, 597È605

View Article Online
was added dropwise over a period of 30 min. The reaction
mixture was stirred at reÑux temperature for 3 h, then cooled
to room temperature and left overnight. The crystalline
residue, separated from the bulk of the reaction mixture by
Ðltration, was washed repeatedly with methanol. 1.5 g of shiny
dark purple crystals, consisting of a mixture of two products,
1,14-Bis[2-methoxy-4-(10,15,20-triphenyl-5-porphyrinyl)-
phenoxy]-3,6,9,12-tetraoxatetradecane, 1c. The procedure
employed for the preparation of compound 1a was followed.
Starting from 120 mg (0.182 mmol) of porphyrin 3a and 36.2
mg (0.0721 mmol) of pentaethylene glycol ditosylate, 215 mg
(0.145 mmol, 80% yield) of porphyrin 2c were obtained. 1H
NMR (CDCl ): d 8.9È8.8 (m, 8H, b-H), 8.2È8.1 (m, 5H, C H ),
was chromatographed on SiO . Elution with a hexaneÈ30%
2
3
6 5
chloroform mixture gave an initial moving band containing
7.7È7.6 (m, 12 H, C H ), 4.47 (brt, J \ 5.4 Hz, 2H,
6
5
the meso-H TPP co-product. The desired porphyrin was sub-
ArOCH CH O), 4.1 (m, ArOCH CH O), 3.95 (s, 3H, OCH ),
2
2
2
2
2
3
sequently eluted with a 1% methanolÈchloroform mixture to
3.9È3.8 (m, 4H, OCH CH O) and [1.49 (brs, 2H, NH). 13C
2
2
give 0.6 g (0.9 mmol, 12% yield) of 3a as a purple crystalline
solid. The product was used in subsequent Williamson coup-
NMR (CDCl ): d 148.2 (C-3@), 147.6 (C-4@), 142.1 (C-1A, C H ),
6 5
135.2 (C-1@), 134.5 (C-2A, C H ), 131.0 (C-a), 127.7 (C-b), 127.4
3
6
5
ling reactions without further puriÐcation. 1H NMR (CDCl ):
(C-4A, C H ), 126.7 (C-3A, C H ), 120.1 (C-meso), 119.9 (C-6@),
6 5 6 5
118.8 (C-5@), 111.6 (C-2@), 71.0 (ArOCH CH O), 70.8
3
d 8.9È8.8 (m, 8H, b-H), 8.3È8.2 (m, 5H, C H ), 7.8È7.7 (m, 12H,
6
5
2
2
C H ] aromatics), 7.31 (d, J \ 8.1 Hz, 1H, aromatics), 5.97
(ArOCH CH O), 69.9 (OCH CH O), 68.7 (OCH CH O) and
6
5
2
2
2
2
2
2
(
brs, 1H, OH), 4.0 (s, 3H, OMe) and [2.76 (brs, 2H, NH). 13C
56.1 (OCH ). UV-vis (CHCl ): j /nm (log e) 419 (6.16,
3
3
max
NMR (CDCl ): d 151.2 (C-a), 145.6 (C-3@), 144.9 (C-4@), 142.2
Soret), 516 (4.78), 551 (4.46), 590 (4.28) and 646 (4.13).
3
(
(
(
(
C-1A, C H ), 134.6 (C-1@), 134.2 (C-2A), 131.0 (C-b), 127.7
FAB-MS (NBA): cluster centred at m/z 1510, [M [ H]`.
6
5
C-4A), 126.7 (C-3A), 120.1 (C-meso), 120.0 (C-meso), 117.6
C-5@), 117.6 (C-2@) and 56.2 (OMe). UV-vis (CDCl ): j /nm
3
max
log e) 420 (5.48, Soret), 517 (4.13), 552 (3.82), 590 (3.64) and
2-[2-(2-(2-(10,15,20-triphenyl-5-porphyrinyl)-1-oxo-2-
648 (3.62). FAB-MS (NBA): m/z 660, [M]`.
methoxyphenyl)ethoxy)ethoxy)ethoxy]ethanol, 3b. 100 mg of
porphyrin precursor 3a (0.152 mmol), 58 mg of tetraethylene
glycol monotosylate (0.167 mmol) and an excess of anhydrous
K CO (250 mg) in 10 mL of freshly dried and distilled DMF
1,8-Bis[2-methoxy-4-(10,15,20-triphenyl-5-porphyrinyl)-
2
3
phenoxy]-3,6-dioxaoctane, 1a. 100 mg of porphyrin 3a (0.152
mmol), 34 mg of triethylene glycol ditosylate (0.075 mmol) and
an excess of anhydrous K CO (250 mg) in 10 mL of freshly
were stirred at reÑux temperature overnight under argon. The
mixture was then Ðltered and the solvent stripped o† in vacuo
to give a purple residue which was dissolved in chloroform (50
mL) and washed with brine (3 ] 100 mL) and then water until
neutral. The organic solution was dried (Na SO ), reduced to
2
3
dried and distilled DMF were stirred at reÑux temperature
overnight under argon. The mixture was then Ðltered and the
solvent stripped o† in vacuo to give a purple residue which
was dissolved in chloroform (50 mL) and washed with brine
2
4
a small volume and chromatographed (SiO ) eluting with a
2
3% methanolÈchloroform solvent mixture to give a purple
(
3 ] 100 mL) and then water until neutral. The organic
solid residue which was further crystallised (CHCl Èhexane) to
give 114 mg (0.136 mmol, 90% yield) of the desired product.
3
solution was dried (Na SO ), reduced to a small volume and
chromatographed (SiO ) eluting with
chloroform solvent mixture to give 180 mg (0.122 mmol, 82%
2
2
4
a
3% methanolÈ
1H NMR (CDCl ): d 8.9È8.8 (m, 8H, b-H), 8.2 (m, 5H), 7.5 (m,
3
2H), 4.4È3.6 (m, 16H, OCH CH O), 3.96 (s, 3H, OCH ), 2.21
2
2
3
yield) of the desired diporphyrin product. 1H NMR (CDCl ):
(brs, 1H, OH) and [2.75 (brs, 2H, NH). UV-vis (CHCl ): j
419 (Soret), 516, 551, 590 and 647 nm. FAB-MS (NBA): m/z
837 (35, [M]`) and 876 (100%, [M ] K]`).
3
3
max
d 8.9È8.8 (m, 8H, b-H pyrrolic), 8.2È8.1 (m, 5H, C H ), 7.8È
6
5
7
2
.7 (m, 11H, C H ] aromatics), 4.6È4.5 (brt, J \ 3.6,
H, ArOCH CH O), 4.2È4.1 (brt, J \ 3.6 Hz, 2H,
6
2
5
2
ArOCH CH O), 4.0 (s, 2H, OCH CH O), 3.9 (s, 3H,
ArOCH ) and [2.7 (brs, 2H, NH). 13C NMR (CDCl ): d
1
2
2
2
2
1-[2-Methoxy-4-(10,15,20-triphenyl-5-porphyrinyl)-
phenyoxy]-11-(p-tolylsulfonyloxy)-3,6,9-trioxaundecane,
3
3
48.2 (C-3@), 147.6 (C-4@), 142.1 (C-1A, C H ), 135.3 (C-1@) 135.5
3c.
6
5
(
1
C-2A, C H ), 131.1 (C-a), 127.7 (C-b), 127.4 (C-4A, C H ),
114 mg of porphyrin 3b and 130 mg of p-toluenesulfonyl chlo-
ride (61 mmol) were dissolved in 25 mL of dry pyridine. The
reaction mixture was stirred for 12 h under an argon atmo-
sphere, then diluted with chloroform (250 mL) and washed
with aqueous hydrochloric acid (3%), then with water until
neutral. The organic phase was dried (Na SO ) and evapo-
6
5
6 5
26.9 (C-3A, C H ), 120.1 (C-meso), 120.0 (C-meso), 119.9 (C-
6
5
meso), 118.8 (C-5@), 111.6 (C-2@), 71.1 (ArOCH CH O), 70.0
2 2
(
(
(
ArOCH CH O), 68.7 (OCH CH O) and 56.1 (OMe). UV-vis
2
2
2
2
CHCl ): j /nm (log e) 419 (6.16, Soret), 516 (4.76), 551
max
4.46), 590 (4.31) and 646 (4.12) FAB-MS (NBA): cluster
3
2
4
centred at m/z 1430, [M]`.
rated under reduced pressure to give 110 mg of compound 3c
(
0.10 mmol, 80% yield) which was used without further puriÐ-
cation in the subsequent coupling reaction. 1H NMR
1
,11-Bis[2-methoxy-4-(10,15,20-triphenyl-5-porphyrinyl)-
(CDCl ): d 8.9È8.8 (m, 8H, b-H pyrrolic), 8.3È3.2 (m, 5H,
3
phenoxy]-3,6,9-trioxaundecane, 1b. The procedure employed
for the preparation of compound 1a was followed. Starting
from 100 mg (0.15 mmol) of porphyrin 3a, and 39 mg of tetra-
ethylene glycol ditosylate (0.075 mmol), 180 mg of porphyrin
C H ), 7.8È7.7 (m, 14H, C H ] aromatics), 7.31 (d, J \ 8.1,
6
5
6 5
1H, aromatics), 7.30 (d, J \ 9.1 Hz, 2H, 3@-Ts), 5.97 (brs, 1H,
OH), 4.2È3.6 (m, 16H, OCH CH O), 4.0 (s, 3H, OMe), 2.13 (s,
3H, Ar CH ) and [2.77 (brs, 2H, NH). UV-vis (CHCl ): j
419 (Soret), 516, 551, 590 and 670 nm. FAB-MS (NBA): m/z
990 [M]`.
2
2
3
3
max
1
b (0.12 mmol, 80% yield) were obtained. 1H NMR (CDCl ):
3
d 8.9È8.8 (m, 8H, b-H pyrrolic), 8.2È8.1 (m, 5H, aromatics),
7
.7È7.6 (m, 11H, C H ] aromatics), 4.47 (brt, J \ 5.4 Hz, 2H,
6
5
OCH CH O), 4.10 (m, 2H, OCH CH O), 3.95 (s, 3H, CH O),
2
2
2
2
3
3
.9È3.8 (m, 4H, OCH CH O), and [1.49 (brs, 2H, NH). 13C
13,17-Diethyl-2,3,7,8,12,18-5-(4-hydroxy-3-methoxyphenyl)-
hexamethylporphyrin, 4. 0.5 g of 8,12-diethyl-2,3,7,13,17,18-
hexamethylbiladiene-a,c dibromide10 (0.87 mmol), 4-hydroxy-
3-methoxybenzaldehyde (0.12 g, 0.87 mmol), and p-toluene
sulfonic acid (1 g) were dissolved in 250 mL of ethanol. The
reaction mixture was reÑuxed for 4 h. An excess of sodium
acetate (1.5 g) was added in order to neutralise the solution.
The mixture was then evaporated under reduced pressure and
the solid dark residue dissolved in the minimum amount of
2
2
NMR (CDCl ): d 148.2 (C-3@), 147.6 (C-4@), 142.1 (C-1A, C H ),
6 5
35.2 (C-1@), 134.5 (C-2A, C H ), 131.0 (C-a), 127.7 (C-b), 127.4
3
1
(
1
(
6
5
C-4A, C H ), 126.7 (C-3A, C H ), 120.1 (C-meso), 119.9 (C-6@),
6
5
6 5
18.8 (C-5@), 111.6 (C-2@), 71.0 (ArOCH CH O), 70.8
2
2
ArOCH CH O), 69.9 (OCH CH O), 68.7 (OCH CH O) and
2
2
2
2
2
2
5
6.1 (CH O). UV-vis (CHCl ): j /nm (log e) 419 (6.12,
Soret), 516 (4.78), 551 (4.43), 590 (4.30) and 646 (4.13).
FAB-MS (NBA): cluster centred at m/z 1478, [M]`.
3
3
max
New J. Chem., 2001, 25, 597È605
603

View Article Online
K. M. Kadish, K. M. Smith and R. Guilard, Academic Press,
London, 2000, vol. 6, ch. 40, p. 1.
dichloromethane and chromatographed on neutral alumina
Grade III) eluting with CH Cl . A redÈviolet band was col-
lected and evaporated to a†ord 350 mg (0.6 mmol, 72% yield)
(
2
2
2
3
J. Deisenhofer and H. Michel, Science, 1989, 245, 1463.
(a) J. P. Collman, P. S. Wagenknecht and J. E. Hutchinson,
Angew. Chem., Int. Ed. Engl., 1994, 33, 1537, and references
therein; (b) J. P. Collman, Inorg. Chem., 1997, 36, 5145; (c) Y.
Deng, C. J. Chang and D. G. Nocera, J. Am. Chem. Soc., 2000,
of compound 4 which was used in the subsequent coupling
reaction without further puriÐcation. 1H NMR (CDCl ): d
3
1
0.15 (s, 2H, meso-H), 9.94 (s, 1H, meso-H), 7.6È7.3 (m, 3H,
122, 410, and references therein; (d) The synthesis of a ““face-to-
aromatics), 5.99 (s, 1H, OH), 4.07 (q, J \ 7.56, 2H, CH CH ),
2 3
.95 (s, 3H, OCH ), 3.63 (s, 3H, CH ), 2.56 (s, 3H, CH ), 1.86
faceÏÏ porphyrinÈcorrole, as a potential catalyst for four-electron
reduction of dioxygen, has recently been reported: F. Jerome,
C. P. Gros, C. Tardieu, J.-M. Barbe and R. Guilard, New J.
Chem., 1998, 22, 1327. For an overview of the role of porphyrins
as synthetic models of hemoglobin and myoglobin see: (e) J. P.
Collman and L. Fu, Acc. Chem. Res., 1999, 32, 455; ( f ) T. Ayashi
and H. Ogoshi, Chem. Soc. Rev., 1997, 26, 355; (g) M. Momen-
teau and C. A. Reed, Chem. Rev., 1994, 94, 659.
3
(
3
3
3
t, J \ 7.56 Hz, 3H, CH CH ) and [3.22 (brs, 1H, NH). 13C
2
3
NMR (CDCl ): d 146.1 (C-3@), 146.0 (C-4@), 145.4 (C-a), 143.8
3
(
(
C-a), 142.8 (C-a), 141.7 (C-a), 137.8 (C-b), 137.0 (C-b), 135.6
C-b), 134.7 (C-1@, C H OR), 118.5 (C-6@, C H OR),116.1 (C-
6
4
6 4
5
@, C H OR), 113.7 (C-2@, C H OR), 96.6 (C-meso), 95.3 (C-
6
4
6 4
meso), 56.3 (OCH ), 19.8 (CH ), 17.6 (CH ), 14.9 (CH ), 12.2
3
2
3
3
4
H. B. Dunford, Heme Peroxidases, Wiley-WCH Editions, New
York, 1999; P. R. Ortiz de Montellano, Cytochrome P-450:
Structure, Mechanisms and Biochemistry, 2nd edn., Plenum, New
(
CH ) and 11.6 (CH ). UV-vis (CHCl ): j /nm 404 (Soret),
02, 536, 571 and 623. FAB-MS (NBA): m/z 572 [M`].
3
3
3
max
5
York, 1995; M. Wikstrom, K. Krab and M. Saraste, Cytochrome
Ž
Oxidase: A Synthesis, Academic Press, London, 1981; B. K.
1
-[2,4-(13,17-Diethyl-2,3,7,8,12,18-hexamethyl-5-
porphyrinyl)-2-methoxyphenoxy]-11-[2-methoxy-4-
10,15,20-triphenyl-5-porphyrinyl)phenoxy]-3,6,9-
Burgess, Chem. Rev., 1990, 90, 1377.
5
6
7
D. Monti, M. Venanzi, G. Mancini, F. Marotti, L. La Monica
and T. Boschi, Eur. J. Org. Chem., 1999, 1901.
(
trioxaundecane, 2. 110 mg of porphyrin derivative 3c (0.11
mmol), 77 mg of 4 (0.13 mmol) and an excess of anhydrous
K CO (250 mg) in 25 mL of freshly dried and distilled DMF
For a critical approach to the topic, see: M. G. H. Vicente, L.
Jaquinod and K. M. Smith, Chem. Commun., 1999, 1771.
A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaughsson, A. J. M.
Huxley, C. P. McCoy, J. T. Rademacher and T. E. Rice, Chem.
Rev., 1997, 97, 1515; L. Fabbrizzi, M. Licchelli, P. Pallavicini, L.
Parodi and A. Taglietti, T ransition Metals in Supramolecular
Chemistry, Perspectives in Supramolecular Chemistry, ed. J. P.
Sauvage, John Wiley and Sons, Chichester, 1999, vol. 5, ch. 3,
pp. 93È134.
I. Tabushi, S. Kugimiya, M. G. Kinnaird and T. Sasaki, J. Am.
Chem. Soc., 1985, 107, 4192.
H. Feuer and J. Hooz, T he Chemistry of the Ether L inkage, Inter-
science Publishers, New York, 1967, ch. 10.
2
3
were stirred at 80 ¡C overnight under argon. The mixture was
then Ðltered and the solvent stripped o† in vacuo to give a
purple residue which was dissolved in chloroform (50 mL) and
washed with brine (3 ] 100 mL) and then water until neutral.
The organic solution was dried (Na SO ), reduced to a small
volume and chromatographed (SiO ) eluting with a 3%
methanolÈchloroform solvent mixture. The appropriate frac-
2
4
8
9
2
tion was collected and evaporated under reduced pressure to
give
chloroformÈhexane) to give 31 mg (0.02 mmol, 20% yield) of
the desired diporphyrin product as a purple crystalline solid.
a purple-red solid residue which was crystallised
(
10 J. S. Lindsey, I. C. Schreiman, H. C. Su, P. C. Kearney and M.
Marguerettas, J. Org. Chem., 1987, 52, 827.
1
1
1
2
K. Kalyanasundaram, Photochemistry of Polypyridine and
Porphyrin Complexes, Academic Press, London, 1991.
1
H NMR (CDCl ): d 10.2È9.9 (m, 3H, meso-H), 8.9È8.8 (m,
3
8
H, b-H), 8.2È8.1 (m, 5H, aromatics), 7.8È7.5 (m, 12H,
(a) T. H. Fo
Ž
ster, Discuss. Faraday Soc., 1959, 27, 7; (b) D. L. H.
aromatics), 6.89 (d, J \ 2.3 Hz, 1H, 3@-H), 4.5È4.4 (brt, J \ 5.0
Dexter, Chem. Phys., 1953, 21, 836.
Hz, 2H, ArOCH CH O), 4.2È3.6 (m, 13H, OCH CH O
13 A. Osuka, K. Maruyama, I. Yamazaki and N. Tamai, Chem.
Phys. L ett., 1990, 165, 392; J. L. Sessler, B. Wang and A. Harri-
man, J. Am. Chem. Soc., 1995, 117, 704.
2
2
2
2
]
OCH ] b-CH CH ), 3.5 (s, 3H, b-CH ), 2.50 (s, 3H, b-
3
2
3
3
CH ), 1.9È1.8 (m, 3H, b-CH CH ) and [2.8 (brs, 2H, NH).
UV-vis (CHCl ): j /nm (log e) 407 (5.97, Soret), 419 (6.03,
max
Soret), 511 (4.86), 536 (4.73), 550 (4.50), 586 (4.26), 624 (4.10),
3
2
3
14 N. C. Fletcher, M. D. Ward, S. Encinas, N. Armaroli, L. Flamigni
and F. Barigelletti, Chem. Commun., 1999, 2089.
3
1
5
D. Live and S. Chang, J. Am. Chem. Soc., 1976, 98, 3769. The
cationÈreceptor electrostatic interactions are evidenced by chemi-
cal shift variation of the oxaethylenic segments that closely paral-
lel the charge density of the cation (Li` B Na` [ K` [ Cs`).
The cation-labelling procedure is a powerful means for character-
isation of supramolecular structures and aggregates by FAB,
Ž Ž
MALDI-TOF, and ESI mass spectroscopy: M. Lamsa, J. Huus-
konen, K. Rissanen and J. Pursianen, Chem. Eur. J., 1998, 4, 84;
P. Timmerman, K. A. Jolli†e, M. C. Crego Calama, J.-L. Weid-
647 (4.13) and 675 (3.89). FAB-MS (NBA): m/z 1390 [M]`.
1
6
Acknowledgements
We are indebted to Mr Alessandro Leoni for his valuable
technical help. We also thank Professor Gianfranco Ercolani,
University of ““Tor VergataÏÏ, for helpful discussions. CNR
mann, L. J. Prins, F. Cardullo, B. H. M. Snelling-Ruel, R. H.
Ž
Fokkens, N. M. M. Nibbering, S. Shinkai and D. N. Reinhoudt,
(
9
STM programme 1998) and MURST (Project no.
8032774402) are gratefully acknowledged for funding.
Chem. Eur. J., 2000, 6, 4104; C. A. Schalley, J. M. Riveira, T.
Mart•
Chem., 1999, 1325.
n, J. Santamar•a, G. Siuzdak and J. Rebeck, Jr, Eur. J. Org.
 
1
7
Compare for example the association constant of compound 1b
to that of the earlier reported tetraspacered counterpart (ref. 5).
This e†ect, known as ““donor end group e†ectÏÏ, is usually encoun-
tered in the case of related podands bearing aromatic ends, and is
accounted for by a favourable enthalpic contribution exerted by
References and notes
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1
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19 The increased ring current e†ect, caused by the presence of the
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604
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rate constants (k ) are generally several orders of magnitude
out
smaller than the formation rate constants (k ). In our case a
in
rough estimate of the decomplexation rate constant k
B
out
1
03È104 s~1 is consistent with reported values.
2
2
3
R. Tahara, T. Morozumi, Y. Suzuki, Y. Kakizawa, T. Akita and
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1
“
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