Tweezers hosts for intercalation of Lewis base guests: Tuning physico-chemical properties of cofacial porphyrin dimers

Article abstract of DOI:10.1039/b100423i

The synthesis and both spectroscopic and electrochemical studies of a bis-porphyrinic tweezer are reported, as well as the insertion of Lewis base guests into the host bis-porphyrinic cavity, monitored through physico-chemical characteristics.

Full text of DOI:10.1039/b100423i

View Article Online / Journal Homepage / Table of Contents for this issue
Tweezers hosts for intercalation of Lewis base guests: Tuning physico-chemical
properties of cofacial porphyrin dimers
Julie Brettar, Jean-Paul Gisselbrecht, Maurice Gross* and Nathalie Solladié*
Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide, Université Louis Pasteur et CNRS, 4 rue Blaise
Pascal, 67000 Strasbourg, France. E-mail: nsolladie@chimie.u-strasbg.fr; gross@chimie.u-strasbg.fr,
nsolladie@chimie.u-strasbg.fr
Received (in Cambridge, UK) 10th January 2001, Accepted 8th March 2001
First published as an Advance Article on the web 30th March 2001
The synthesis and both spectroscopic and electrochemical
studies of a bis-porphyrinic tweezer are reported, as well as
the insertion of Lewis base guests into the host bis-
porphyrinic cavity, monitored through physico-chemical
characteristics.
compounds and 3 nm for metallated porphyrins) as well as a tiny
broadening of the Soret band could be noticed from the
monomer to the dimer, indicating that almost no coupling exists
between the two porphyrins.
Fluorescence measurements were carried out in CH2Cl2 on
both dimers 3a–b, and also on the spacer 1 and on the reference
porphyrins 4a–b. Upon almost selective excitation of the
anthracene moiety at 267 nm in the free-base dimer 3b, only the
fluorescence of the two porphyrins could be detected, indicating
a total quenching of the emission from the anthracenic moiety.
The assumption of an energy transfer from anthracene to the
adjacent porphyrins was corroborated by the registration of the
excitation spectrum of the dimer 3b, which matches the
absorption of the multicomponent system. A similar energy
transfer from anthracene towards the adjacent chromophores
was also observed upon irradiation of the Zn(II) dimer 3a but the
quantum yield was smaller and the fluorescence of the
anthracenic sub-unit still remains the main deactivation path-
way.
In natural photosynthetic systems, the energy contained in a
single photon is transferred in a very short time and with
minimal loss from the point where it is absorbed to where it is
needed.1
Increased efforts have been developed towards under-
standing this great efficiency and many multi-porphyrinic
devices have been synthesized as potential models of the natural
system.2 One possible approach for the preparation of such
devices is to force the cofacial orientation of the porphyrins via
the use of a rigid spacer.3 With the aim of identifying an
optimized spacer for the construction of photonic and electronic
wires, we first focused on the study of a cofacial porphyrin
dimer.4 We now report how the insertion of a pyrazine molecule
into the cavity of a Zn(II) porphyrin dimer through host–guest
interactions causes an electronic coupling between the two
porphyrin chromophores.
A 1,8-diethynylanthracenic spacer has been chosen for its
expected ability to hold two chromophores in a cofacial
orientation and for synthetic reasons as well. Indeed, the use of
a Sonogashira coupling reaction between the diacetylene 1 and
the iodoporphyrin 25 allowed the efficient synthesis of dimer 3a
in four steps starting from commercially available compounds
(Fig. 1).6 The free-base dimer 3b was obtained in 89% yield by
demetallation of the two porphyrins under acidic conditions.7
UV-visible spectra of the reference porphyrins, the Zn(II) and
free-base 5,10,15,20-tetra(3,5-di-tert-butylphenyl)porphyrins
4a–b, and both dimers 3a and 3b were recorded in CH2Cl2
solutions. Only a small blue shift (2 nm for free-base
We investigated also the possible generation of host–guest
complexes between the Zn(II) bis-porphyrin 3a and small
bidentate ligands bearing two nitrogen atoms.4 Pyrazine and
DABCO (1,4-diazabicyclo[2.2.2]octane) were chosen as first
examples because of their fairly different pKa values (re-
1
spectively 0.6 and 3). The coordination was monitored by H
NMR and UV-visible absorption spectroscopies in order to
determine, in particular, the stoichiometry of the complexes,
and the effects of such coordination on the characteristics of the
porphyrin dimer.
The complexation was first studied by 1H NMR (300 MHz)
titration of dimer 3a with pyrazine in CDCl3. Upon addition of
increasing amounts of pyrazine to a 2.1023 M solution of
porphyrin dimer 3a, changes in the chemical shifts were
observed up to a concentration of pyrazine equal to one equiv.
of 3a and no further changes beyond. This strongly suggests that
a 1+1 complex was created between pyrazine and porphyrin
dimer 3a. Further indications on this complexation were
obtained from UV-visible spectrophotometric titrations of a
2.1025 M solution of dimer 3a in CH2Cl2 with pyrazine (Fig. 2):
such a titration of 3a with pyrazine resulted in significant red-
shift of the Soret and Q bands in the porphyrins,8 and the
titration curves obtained from absorption changes at 562 nm (Q
band) exhibited a sharp saturation beyond the concentrations
ratio 1+1 for pyrazine–3a. The titration data and the observed
isosbestic points clearly indicated the existence of an equilib-
rium between two defined species, thus leading to the
conclusion that a 1+1 complex was formed between dimer 3a
and pyrazine, with an association constant of 105.6 M21.9 In
contrast, there was no experimental evidence of any appreciable
complex formation between the reference porphyrin 4a and
pyrazine at such low concentrations by either UV-visible or 1H
NMR spectroscopies. These results clearly indicated that the
pyrazine molecule was inserted into the cavity of the porphyrin
dimer 3a, yielding a 1+1 host–guest complex.
Fig. 1 (a) Zn, NH3aq., then HCl, i-PrOH, 36%; (b) TMSCCMgBr, PPh3,
Ni(acac)2, THF, reflux, 75%; (c)K2CO3, THF–CH3OH 85+15, 61%; (d)
Pd(PPh3)2Cl2, CuI, NEt3, 63%; (e) TFA in CH2Cl2, 89%. Ar = 3,5-di-tert-
butylphenyl.
Titration of 3a with DABCO and the absence of complexa-
tion between the latter and the reference porphyrin 4a until a
DOI: 10.1039/b100423i
Chem. Commun., 2001, 733–734
This journal is © The Royal Society of Chemistry 2001
733

View Article Online
first oxidation and the first reduction steps each became split
into two distinct one-electron reversible steps at respectively
21.87 and 21.95 V vs. Fc+/Fc and +0.28 and +0.40 V vs. Fc+/
Fc.10 Such a behavior is characteristic of a porphyrin–porphyrin
interaction. This experimental observation unambiguously
indicated that it is possible to modify the electron transfer
pattern of the dimer 3a by host–guest interactions, as docu-
mented for instance in the present paper, by intercalating one
pyrazine molecule into the cavity of the bis-porphyrinic tweezer
3a. Both spectroscopic and electrochemical studies on the
complexation of either pyrazine or DABCO Lewis bases by the
porphyrin dimer 3a revealed that no further spectral or
voltamperogram changes occurred beyond a concentrations
ratio Lewis base–3a = 1+1.
The above results clearly indicated that each bidentate Lewis
base (pyrazine and DABCO) was inserted into the cavity of the
porphyrin dimer 3a, generating a 1+1 host–guest complex. The
enhanced stability observed in the complexation of the bidentate
bases by the dimer 3a (if compared with the complexation of the
same bases by the reference porphryin 4a) may be ascribed to
the preorganization of the Zn(II) bis-porphyrin 3a. The
spectrometric and electrochemical results revealed that the 1+1
host–guest complex generated between 3a and the pyrazine
molecule enabled the two porphyrins to undergo an electronic
coupling. Such changes in the spectroscopic and electro-
chemical properties of cofacial porphyrin dimers by host–guest
interactions pave the way towards self-coordinated molecular
systems with predictable spectral and redox characteristics.
Work is in progress on this subject.
Fig. 2 UV-vis spectroscopic titration of 3a with pyrazine in CH2Cl2 at rt.
Spectral change of 3a on addition of pyrazine at rt: [3a] = 2.1025 M,
concentrations ratio = [pyrazine]+[3a] = 0; 0.2, 0.4, 0.6, 0.8, 1, 2. 3,5-Di-
tert-butylphenyl substituents on the porphyrins have been omitted for
clarity.
concentrations ratio of DABCO–4a = 10+1, produced similar
conclusions as above with pyrazine. A 1+1 host–guest complex
was created by insertion of one DABCO molecule into the
cavity of each porphyrin dimer 3a, with an association constant
This work was supported by the CNRS.
of 107 M21 9
.
Electrochemical studies were carried out on a glassy carbon
working electrode in CH2Cl2 + 0.1 M Bu4NPF6. Cyclic
voltammetry exhibited, for the porphyrin dimer 3a, two
reversible oxidation steps as well as a third, irreversible
oxidation and three reversible reduction steps at respectively
22.30 (2e2), 22.10 (1 e2), 21.86 (2 e2), +0.32 (2 e2), +0.64
(2 e2) and +0.95 V vs. Fc+/Fc. Assignments of the different
steps were made by comparison with the redox pattern of the
building blocks, namely anthracene 1 and porphyrin 4a, whose
redox characteristics are summarised in Table 1.
Analysis of the electrochemical results revealed that the
reversible one electron reduction of 3a at 22.10 V and the
irreversible oxidation at +0.95 V vs. Fc+/Fc occur on the
anthracene linker. The remaining four electron transfers each
involve two one-electron reversible transfers occurring on the
two porphyrin units. The peak shape and characteristics of the
cyclic voltammograms indicated that the two porphyrins behave
in 3a as independent redox centers as expected from the large
ring–ring distance (ca. 5.8 Å, according to molecular model-
ling). Such a distance makes unlikely any interaction between
the two porphyrin rings.
Notes and references
1 J. Barber and B. Andersson, Nature, 1994, 370, 31; W. Kühlbrandt,
Nature, 1995, 374, 497; G. McDermott, S. M. Prince, A. A. Freer, A. M.
Hawthornthwaite-Lawless, M. Z. Papiz, R. J. Cogdell and N. W. Isaacs,
Nature, 1995, 374, 517; T. Pullerits and V. Sundström, Acc. Chem. Res.,
1996, 29, 381.
2 R. W. Wagner, T. E. Johnson and J. S. Lindsey, J. Am. Chem. Soc.,
1996, 118, 11 166 and references cited therein; D. L. Officer, A. K.
Burrell and D. C. W. Reid, J. Chem. Soc., Chem. Commun., 1996, 1657;
M. S. Vollmer, F. Würthner, F. Effenberger, P. Emele, D. U. Meyer, T.
Stümpfig, H. Port and H. C. Wolf, Chem. Eur. J., 1998, 4, 260; A. Osuka
and H. Shimidzu, Angew. Chem., Int. Ed. Engl., 1997, 36, 135.
3 C. K. Chang and I. Abdalmuhdi, J. Org. Chem., 1983, 48, 5388; J. P.
Fillers, K. G. Ravichandran, I. Abdalmuhdi, A. Tulinsky and C. K.
Chang, J. Am. Chem. Soc., 1986, 108, 417; T. Nagata, A. Osuka and K.
Maruyama, J. Am. Chem. Soc., 1990, 112, 3054; J. P. Collman, J. E.
Hutchison, M. Angel Lopez, A. Tabard, R. Guilard, W. K. Seok, J. A.
Ibers and M. L’Her, J. Am. Chem. Soc., 1992, 114, 9869; H. A. Staab
and T. Carell, Angew. Chem., Int. Ed. Engl., 1994, 33, 1466; J. P.
Collman, D. A. Tyvoll, L. Leng Chng and H. T. Fish, J. Org. Chem.,
1995, 60, 1926.
4 C. A. Hunter, M. N. Meah and J. K. M. Sanders, J. Am. Chem. Soc.,
1990, 112, 5773; I. P. Danks, I. O. Sutherland and C. Hong Yap,
J. Chem. Soc., Perkin Trans. 1, 1990, 421; Y. Kuroda, K. Sugou and K.
Sasaki, J. Am. Chem. Soc., 2000, 122, 7833; M. R. Johnston, M. J.
Gunter and R. N. Warrener, Chem. Commun., 1998, 2739; V. V.
Borovkov, J. M. Lintuluoto and Y. Inoue, Org. Lett., 2000, 2, 1565; X.
Huang, B. Borhan, B. H. Rickman, K. Nakanishi and N. Berova, Chem.
Eur. J., 2000, 6, 216.
In the presence of increasing amounts of pyrazine, the cyclic
voltammograms of 3a were modified up to a pyrazine–3a ratio
of 1, in agreement with the UV-visible titrations. The resulting
complex gave well resolved cyclic voltammograms where the
Table 1 Redox potentials in CH2Cl2 + 0.1 M Bu4NPF6 obtained by cyclic
voltammetry on a glassy carbon working electrode. All potentials are given
vs. ferrocene used as internal standard. Pyrazine is not electroactive in the
available potential range
5 N. Solladié and M. Gross, Tetrahedron Lett., 1999, 40, 3359.
6 H. O. House, D. Koepsell and W. Jaeger, J. Org. Chem., 1973, 38, 1167;
H. E. Katz, J. Org. Chem., 1989, 54, 2179.
7 R. W. Wagner, T. E. Johnson and J. S. Lindsey, J. Am. Chem. Soc.,
1996, 118, 11 166.
Reduction
Oxidation
Species
E (V vs. Fc+/Fc)
E (V vs. Fc+/Fc)
8 C. A. Hunter and L. D. Sarson, Angew. Chem., Int. Ed. Engl., 1994, 33,
2313; C. C. Mak, N. Bampos and J. M. K. Sanders, Angew. Chem., Int.
Ed., 1998, 37, 3020; N. Armaroli, F. Diederich, L. Echegoyen, T.
Habicher, L. Flamigni, G. Marconi and J.-F. Nierengarten, New J.
Chem., 1999, 77.
9 J. R. Miller and G. D. Dorough, J. Am. Chem. Soc., 1952, 74, 3977; P.
Hambright, J. Chem. Soc., Chem. Commun. 1967, 470; C. H. Kirksey,
P. Hambright and C. B. Storm, Inorg. Chem., 1969, 8, 2141.
10 Y. Le Mest, M. L’Her and J.-Y. Saillard, Inorg. Chim. Acta, 1996, 248,
181.
1
22.08
+0.97
(irrev.)
(1 e2)
4a
3a
22.25
21.85 +0.35
(1 e2) (1 e2)
21.86 +0.32
(2 e2) (2 e2)
+0.67
(1 e2)
+0.64 +0.96
(2 e2) (irrev.)
(1 e2)
22.30 22.10
(2 e2) (1 e2)
3a +
pyrazine
22.07 21.95 21.87 +0.28 +0.40 +0.75 +0.95
(1 e2) (1 e2) (1 e2) (1 e2) (1 e2) (2 e2) (irrev.)
734
Chem. Commun., 2001, 733–734