Fig. 4 Cyclic voltammogram of 1.§
wavelength absorption (lmax = 1139 nm; e = 1.1 3 104
M21cm21). The unexpected blue shift on quinoidalization of 4
to 1 might be due to the greater bond-length alternation in
quinoidalized porphyrins.
While planarization of 2 to 1 results in a dramatic decrease in
the optical HOMO–LUMO gap, the electrochemical gap
Ox
between the first oxidation and reduction potentials (E1
2
E1Red) increases from 0.53 to 1.00 V on conversion of 2 to 1.
Ox
Compound 1 displays reversible oxidation (E1 = 0.35 V) and
Red
reduction (E1
= 20.65 V) waves, whereas the redox
processes of 2 are poorly reversible, but can be measured by
Ox
Red
square wave voltammetry (E1 = 0.10 V; E1
= 20.43 V).§
The easier oxidation and reduction of 2 probably reflects the
strain in the neutral form which would be released if oxidation
or reduction converts the central CNC link to a single bond,
allowing the macrocycles to twist to orthogonal orientations.
The first and second one-electron oxidations of 1 are separated
by 0.25 V, whereas the first and second one-electron reductions
almost coincide as seen in Fig. 4.
Fig. 2 Two views of the structure of 1·(MeOH)2, (A) omitting methanol and
(B) omitting aryl substituents (50% probability ellipsoids).
We thank EPSRC, DSTL and EOARD for supporting this
work, the EPSRC Mass Spectrometry Service (Swansea) for
mass spectra, and Dr K. J. McEwan (DSTL, Malvern) for
recording NIR spectra.
Notes and references
‡
Crystal data for 1: crystals grown from CHCl3–CH3OH,
¯
C102H96N12Zn2·8CH4O·3H2O, M = 1931.13, triclinic, space group P1, a =
14.6863(2), b = 17.3636(2), c = 22.3909(3) Å, a = 110.2640(4), b =
98.4021(4), g = 96.5091(9)°; V = 5215.6 Å3, Z = 2, m = 0.524 mm21, R
= 0.0648, Rw = 0.0676, Io = 13933 observed [I > 3.0s(I)] reflections out
of N = 19263 unique, GOF = 1.0537.
Crystal data for 2: crystals grown from CHCl3–pyridine–pentane,
¯
102H100N12Zn2·2C5H5N·4CHCl3, M = 2260.49, triclinic, space group P1,
C
Fig. 3 Absorption spectra of 1, 2 and 4 in 1% C5H5N–CH2Cl2.
a = 14.4288(3), b = 15.0483(3), c = 15.3810(4) Å, a = 82.9828(7), b =
73.3295(7), g = 63.312(1)°; V = 2858.4 Å3, Z = 1, m = 0.753 mm–1, R =
0.0534, Rw = 0.0645, Io = 6723 observed [I > 3.0s(I)] reflections out of
N = 10015 unique, GOF = 1.0480.
2 apparently facilitates its oxidation, making this the best route
to 1.
Both data sets were collected on an Enraf Nonius Kappa CCD
diffractometer; T = 150 K, l(Mo-Ka) = 0.71073 Å. CCDC reference
b204265g/ for crystallographic data in CIF or other electronic format.
§ Redox potentials were measured by square wave and cyclic voltammetry
(0.1 V s21) in CH2Cl2 with 0.1 M Bu4NPF6 and a carbon working electrode,
and are quoted relative to E1Ox of internal ferrocene.
The crystal structure of the methanol complex of 1 is shown
in Fig. 2.‡ This compound crystallizes as a tight bimolecular
aggregate; the two molecules in each aggregate are related via
an inversion centre. The AnaphthaleneA core of the dimer is
planar to within 0.04 Å. The central CmesoNCmeso bond length
is 1.43 Å, and although this is formally a double bond it is not
significantly shorter than the formally single Cmeso–Cmeso bonds
in OsukaAs triply linked dimers (analogs of 4 with aryl instead of
bromine substituents).4 The plane-plane separation at the centre
of the aggregate is 3.4 Å and the closest intermolecular contact
is between a nitrile nitrogen and the central carbon of the
dicyanomethylene unit (N…C distance = 3.13 Å).
1 J. Fabian, H. Nakazumi and M. Matsuoka, Chem. Rev., 1992, 92,
1197.
2 H. L. Anderson, Chem. Commun., 1999, 2323; M. G. Vicente, L.
Jaquinod and K. M. Smith, Chem. Commun., 1999, 1771.
3 I. M. Blake, L. H. Rees, T. D. W. Claridge and H. L. Anderson, Angew.
Chem., Int. Ed., 2000, 39, 1818.
4 A. Tsuda, H. Furuta and A. Osuka, J. Am. Chem. Soc., 2001, 123,
10304.
5 A. Tsuda and A. Osuka, Science, 2001, 293, 79.
6 I. M. Blake, H. L. Anderson, D. Beljonne, J.-L. Brédas and W. Clegg, J.
Am. Chem. Soc., 1998, 120, 10764.
The electronic absorption spectra of dimers 1, 2 and 4 are
compared in Fig. 3. As expected, the absorption of 1 (lmax
=
958 nm; e = 9.4 3 104 M21cm21) is sharper and more red-
shifted than that of 2 (lmax = 780 nm; e = 6.9 3 104
M21cm21), but it is surprising that dimer 4 exhibits the longest
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