Adjustable cavity for host-guest recognition in cofacial bis-porphyrinic tweezer

Article abstract of DOI:10.1039/b406680d

The synthesis of a cofacial bis-porphyrinic tweezer bearing a tris-anthracenic spacer is reported. Its behavior as host has been evidenced as well as the ability of its cavity to adjust to guests of various sizes.

Full text of DOI:10.1039/b406680d

Adjustable cavity for host–guest recognition in cofacial bis-porphyrinic
tweezer
Régis Rein, Maurice Gross and Nathalie Solladié*
Groupe de Synthèse de Systèmes Porphyriniques, Laboratoire d’Electrochimie et de Chimie Physique du
Corps Solide, Université Louis Pasteur et CNRS (UMR 7512), 4 rue Blaise Pascal, 67000 Strasbourg,
France. E-mail: nsolladie@chimie.u-strasbg.fr
Received (in Cambridge, UK) 4th May 2004, Accepted 27th May 2004
First published as an Advance Article on the web 8th July 2004
The synthesis of a cofacial bis-porphyrinic tweezer bearing a
tris-anthracenic spacer is reported. Its behavior as host has been
evidenced as well as the ability of its cavity to adjust to guests of
various sizes.
a large variety of guests. The ability of 6 to accommodate various
guests was investigated through binding studies carried out in
dichloromethane with three bidentate bases of different lengths
and pKa, namely pyrazine, DABCO and 4,4A-bipyridine. The
complexation was monitored by UV–visible spectrophotometric
titration of a solution of dimer 6 in CH2Cl2 with pyrazine (Fig. 1, [6]
= 6 1025 M), DABCO ([6] = 4.5 1025 M) or 4,4A-bipyridine ([6]
= 3 1025 M), which resulted as expected in significant red-shift of
the Soret and Q bands in the porphyrins.10 Clear isosbestic points
indicated in all cases an equilibrium between two defined
species.
Association constants (Ka) between tweezer 6 and pyrazine, 4,4A-
bipyridine and DABCO were calculated from UV–visible spectro-
scopic data and values of respectively 103.9 M21, 104.6 M21 and
105.6 M21 were found. These three association constants are
increased by one order of magnitude when compared to the
association constants of the same bidentate ligands with a reference
In 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. This extraordinary
efficiency is ascribed to the favored spacing and orientation of the
bacteriochlorophylls constituting the light harvesting pigments,
which are held in an appropriate parallel conformation by short
polypeptides.1 As part of our work on the synthesis of molecular
wires bearing pendant porphyrins for the study of exciton migration
and the obtaining of self-coordinated molecular systems with
predictable spectral and redox characteristics,2 we investigated the
modulation of the physico-chemical properties of cofacial bis-
porphyrinic tweezer by host–guest interactions.3 We recently
proved that an electronic coupling was generated between the two
porphyrins of such a small tweezer when a molecule of pyrazine
was inserted into the cavity generating a 1 : 1 host–guest complex.4
The enhanced stability observed in the complexation of pyrazine by
that dimer was ascribed to the preorganization of the Zn(II) bis-
porphyrin. We now report the synthesis of a new extended cofacial
bis-porphyrinic tweezer 6 bearing an adjustable cavity. Specifi-
cally, a tris-anthracenic spacer was chosen in order to facilitate a
cofacial orientation of the chromophores while allowing a free
rotation around the acetylenic axis, thus adjusting the cavity to a
large variety of guests.
The synthesis of dimer 6 is described in Scheme 1. It relies upon
the cross-coupling of two porphyrin–anthracene conjugates (5) on
a central 1,8-dibromoacetylene-anthracene. This synthesis is based
on the anchoring of two acetylenic functions at different positions
of the anthracene. This is exemplified in the central linker 4 which
bears dibromoacetylenes at the C-1 and C-8 positions, while the
precursor 2 has C-1 and C-5 substituents. Compound 4 was
obtained by bromination of 1,8-diethynylanthracene 1,5 which has
been previously reported.4 The dissymmetrical 1-ethynyl-5-tri-
ethylsilylethynyl-anthracene 2 was prepared from the commer-
cially available 1,5-dichloroanthraquinone in three steps. The latter
was first reduced using zinc powder.6 The resulting 1,5-di-
chloroanthracene was then reacted with triethylsilylacetylene
magnesium bromide in THF in the presence of PPh3 and Ni(acac)2
under reflux for 3 days.7 2 was finally obtained by statistical
deprotection of one triethylsilyl protecting group. This reaction
yielded 43% of the desired product 2, 16% of the fully deprotected
compound and 33% of unreacted starting material. A Sonogashira
coupling reaction was, at that stage, carried out between the free
acetylene 2 and the iodo-porphyrin 3.8 5 was obtained by
subsequent deprotection of the triethylsilyl group under classical
basic conditions. The desired tweezer 6 was finally obtained by
coupling two porphyrinic arms 5 to the central 1,8-dibromoacety-
lene-anthracene linker 4. Major difficulties were encountered
during this step due to the formation of the homo-coupling product.
The hetero-coupling conditions developed by Vasella and Cai gave
the best results for the present purpose.9 The desired bis-
porphyrinic tweezer 6 was finally isolated in 33% yield.†
Scheme 1 Reagents and conditions: a) Zn, NH3aq., then HCl, i-PrOH, 41%.
b) TESCCMgBr, PPh3, Ni(acac)2, THF, reflux, 25%. c) K2CO3, THF–
CH3OH 85 : 15, rt, 6 h 30, 43%. d) NBS (2.4 eq.), AgNO3 (0.12 eq.),
acetone, rt, 4 h, 87%. e) Pd(PPh3)2Cl2, CuI, NEt3, rt, 24 h, 58%. f) K2CO3,
THF–CH3OH 50 : 50, rt, 6 h, 87%. g) Pd2(dba)3 (0.02 eq.), CuI (0.025 eq.),
LiI (0.2 eq.), PMP (2.8 eq.), DMSO, rt, 3 days, 33%.
The tris-anthracenic spacer allows a free rotation around the
acetylenic axis thus contributing to adjust the size of the cavity to
1992
C h e m . C o m m u n . , 2 0 0 4 , 1 9 9 2 – 1 9 9 3
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Zn(II) porphyrin 5,10,15,20-tetra-di-tert-butyl-phenyl-porphyrin
A4 (102.7 M21, 103.6 M21 and 104.4 M21), in spite of the difference
in size and pKa of the guest involved in this recognition process.
This enforced stability of such complex may be ascribed to the pre-
organization of the bis-porphyrinic tweezer 6. It provides con-
vincing clues that the bidentate bases are inserted into the cavity of
the dimer via host–guest interactions.
Plotting the variation of the absorbance versus the number of
added equivalents of Lewis base per porphyrin evidenced that full
complexation was achieved faster for the complexes between
tweezer 6 and the three studied guests than for the complexes
between the same Lewis bases and the reference monoporphyrin A4
(Fig. 2). In particular, no further modification of the absorbance
was observed beyond the concentrations ratio 1 : 1 for both
DABCO–6 and 4,4A-bipyridine–6, thus providing further evidence
that the complexation occurred with an exact 1 : 1 stoichiometry
between host and guest in both cases. These results clearly confirm
that one guest molecule, either DABCO or 4,4A-bipyridine, was
inserted into the cavity of the porphyrin dimer 6, yielding a 1 : 1
host–guest complex, and they document the adjustability of the
host’s cavity to the size of the guest to be accommodated.
In summary, a new extended cofacial bis-porphyrinic tweezer 6
bearing an adjustable cavity was synthesized. A tris-anthracenic
spacer was chosen to facilitate a cofacial orientation of the
chromophores while allowing a free rotation around the acetylenic
axis, thus contributing to adjust the size of the cavity to guests such
as pyrazine, DABCO and 4,4A-bipyridine. The formation of 1 : 1
host–guest complexes 6–DABCO or 6–4,4A-bipyridine by insertion
of the Lewis base into the cavity of the dimer was demonstrated,
evidencing the feasibility of the recognition process via adjustment
of the host’s cavity to the size of the guest.
This work was supported by the CNRS and the French Ministry
of Research (ACI Jeunes Chercheurs).
Notes and references
† Selected spectroscopic data for 6: 1H NMR (CDCl3, 300 MHz) : d (ppm) :
9.78 (s, 1H, H9), 8.93 (d, 4H, Hb, 3J = 4.7 Hz), 8.91 (d, 4H, Hb, 3J = 4.8
Hz), 8.89 (d, 4H, Hb, 3J = 4.4 Hz), 8.88 (d, 4H, Hb, 3J = 4.6 Hz), 8.81 (s,
2H, H9A or 10A), 8.80 (s, 2H, H9A or 10A), 8.57 (s, 1H, H10), 8.25 (d, 4H, HoB, 3J
= 8.2 Hz,), 8.13 (d, 4H, HmB, 3J = 8.4 Hz), 8.13 (d, 2H, H8A, hidden), 8.13
(d, 2H, H4A, hidden), 7.99 (d, 4H, Ho, 4J = 1.7 Hz), 7.98 (d, 2H, H6A, 3J =
7.2 Hz), 7.92 (d, 8H, HoA, 4J = 1.4 Hz), 7.90 (d, 2H, H2A, 3J = 6.6 Hz), 7.89
(d, 2H, H4–5, 3J = 8.9 Hz), 7.75 (t, 2H , Hp, 4J = 1.7 Hz), 7.63 (t, 4H , HpA,
4J = 1.8 Hz), 7.57 (dd, 2H, H7A, 3J = 8.5 Hz, 3J = 7.0 Hz), 7.49 (dd, 2H,
H3A, 3J = 8.6 Hz, 3J = 7.0 Hz), 7.26 (d, 2H, H2–7, hidden), 6.95 (dd, 2H, H3,
3J = 8.5 Hz, 3J = 7.0 Hz), 1.49 (s, 36H, t-Bu), 1.35 (s, 72H, t-Bu); UV–vis
lmax (CH2Cl2)/nm : 265 (134700), 426 (505500), 552 (23700), 594 (10900);
MS (FAB+) : m/z = 2700.3 ([M]+ , calcd: 2700.3).
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Fig. 1 UV–vis spectroscopic titration of 6 with pyrazine in CH2Cl2 at rt.
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Fig. 2 Plot of the variation of the absorbance versus the number of
equivalents of Lewis base added per porphyrin. Full complexation is
reached for concentrations ratio 1 : 1 for tweezer–DABCO or tweezer–4,4A-
bipyridine (i.e. 2 : 1 for porphyrin–DABCO and porphyrin–4,4A-bipyr-
idine).
C h e m . C o m m u n . , 2 0 0 4 , 1 9 9 2 – 1 9 9 3
1993