Phthalocyanines substituted with dendritic wedges: Glass-forming columnar mesogens

Article abstract of DOI:10.1039/a801799i

Peripheral substitution of the phthalocyanine macrocycle with poly(aryl ether) dendritic wedges produces materials whose properties are dominated both by the columnar self-association of the phthalocyanine core and by the glass-forming character of the de

Full text of DOI:10.1039/a801799i

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Phthalocyanines substituted with dendritic wedges: glass-forming columnar
mesogens
Matthew Brewis, Guy J. Clarkson, Andrea M. Holder and Neil B. McKeown*†
Department of Chemistry, University of Manchester, Manchester, UK M13 9PL
Peripheral substitution of the phthalocyanine macrocycle
with poly(aryl ether) dendritic wedges produces materials
whose properties are dominated both by the columnar self-
association of the phthalocyanine core and by the glass-
forming character of the dendritic substituents.
the Pc core.6 This will help exploit the exciting electro-optical
properties displayed by Pcs, e.g. photoconductivity and third-
order harmonic generation. We describe here the synthesis of
some phthalocyanine (Pc) derivatives which are substituted by
poly(aryl ether) wedges and reveal the interesting properties of
these novel materials which are dominated both by the self-
association of the Pc subunit and by the glass forming character
of the dendritic substituents. The preparation of these phthalo-
cyanine-centred dendrimers involves a novel modification of
the convergent route to dendrimer synthesis in which the
formation of the core is used for dendrimer assembly, rather
than the attachment of the dendritic wedges to a pre-existing
core.
Dendrimers are currently the focus of much attention due to
their compact hierarchical structures. Although research into the
synthesis of novel dendrimers continues at a rapid pace,1 there
is also considerable interest in the functionalisation of existing
systems. For example, the versatile, convergent synthetic route
to the poly(aryl ether) dendrimers of Fre´chet2 has allowed the
incorporation of functional groups based on fullerene,3 pro-
phyrin4 and ferrocene5 moieties.
Phthalocyanine derivatives are best prepared from substi-
tuted phthalonitrile precursors. The aromatic nucleophilic
substitution reaction between 4-nitrophthalonitrile and the
benzylic alcohol group of the first, second and third generation
Our interests lie in the preparation of novel phthalocyanine
(Pc) derivatives in which control over the structure of the bulk
material is achieved by the attachment of suitable substituents to
[G-X]
O
N
N
N
CN
CN
CN
[G-X]
O
i
ii, iii
+
N H
H
[G-X]-OH
N
O
[G-X]
O2N
CN
[G-X]
O
N
N
N
1 X = 1
2 X = 2
3 X = 3
4 X = 1
5 X = 2
6 X = 3
O
iv, iii
[G-X]
R
O
R
O
O
O
O
O
O
O
O
O
O
O
O
O
N
N
N
N
R
R
N
H H
O
O
O
O
[G-X]
N
N
N
O
O
[G-1]
O
O
O
R
R
[G-2]
7 X = 1, R = O(CH2CH2O)2Me
8 X = 2, R = O(CH2CH2O)2Me
9 X = 3, R = O(CH2CH2O)2Me
[G-3]
Scheme 1 Reagents and conditions: i, anhydrous K2CO3, DMF, 50 °C; ii, C5H11OLi, C5H11OH, 135 °C; iii, AcOH; iv, 4,5-bis(1,4,7-trioxaoctyl)-
phthalonitrile, C5H11OLi, C5H11OH, 135 °C
Chem. Commun., 1998
969

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Table 1 Transition temperatures of dendrimer-substituted Pcsa
DSC analysis of 8 and 9 also shows a distinct reversible glass
transition. No change in the optical texture is observed on
cooling the mesophase below the glass transition temperature
and it is concluded that an anisotropic glassy state is obtained in
which the columnar structure is ‘frozen’.11 Small angle X-ray
diffraction studies of the resulting brittle solids are also
consistent with the retention of the columnar structure.
Materials such as 8 and 9 which display both an anisotropic
glassy phase and a readily aligned mesophase could be used to
fabricate monodomainal films suitable for optical or electronic
studies.
Transition/°C
Compound
Glass ? I
Glass ? oh oh ? I
115b 270b
D/Å
4
5
6
7
8
9
—
112
71
—
—
—
31.2
—
—
—
< 220b
115
—
—
> 320b
250–255
108–112
28.9
30.0
35.0
94
a
I
= isotropic liquid, oh = hexagonal columnar mesophase, D =
intercolumnar distance calculated from 1,0 diffraction ring. b Transition not
observed by DSC.
Notes and References
† E-mail: neil.mckeown@man.ac.uk
poly(aryl ether) wedges ([G-1]-OH, [G-2]-OH, [G-3]-OH)2
gives the required phthalonitriles 1–3, respectively, in 75–90%
yield (Scheme 1). Cyclotetramerisation of 1–3 using lithium
pentanolate in refluxing pentanol gives 4–6 in 10–20% yield,
respectively, as a mixture of four inseparable isomers. Alter-
natively, a mixed cyclotetramerisation between each of the
phthalonitriles 1–3 and a ten-fold excess of 4,5-bis(1,4,7-
trioxaoctyl)phthalonitrile7 produces unsymmetrical Pcs 7–9,
respectively. Each of these compounds is readily separated from
octakis(1,4,7-trioxaoctyl)phthalocyanine and other Pc by-prod-
ucts by column chromatography. The structures of 1–9 were
‡ Selected data for 1: n(KBr)/cm21 2229 (CN); dH(CDCl3, 500 MHz) 5.05
(4 H, s), 5.10 (2 H, s), 6.58 (1 H, t), 6.60 (2 H, d), 7.15 (1 H, dd), 7.25 (1 H,
d), 7.29–7.45 (10 H, m), 7.65 (1 H, d); m/z (EI) 446 (M+). For 2: n(KBr)/
cm21 2230 (CN); dH(CDCl3, 500 MHz) 4.97 (4 H, s), 5.02 (8 H, s), 5.08 (2
H, s), 6.52 (1 H, t), 6.57 (2 H, t), 6.59 (2 H, d), 6.67 (4 H, d), 7.14 (1 H, dd),
7.23 (1 H, d), 7.29–7.42 (20 H, m), 7.65 (1 H, d); m/z (EI) 871 (M+. For 3:
n(KBr)/cm21 2232 (CN); dH(CDCl3, 500 MHz) 4.97 (8 H, s), 5.03 (16 H,
s), 5.05 (2 H, s), 6.60–6.63 (7 H, m), 6.69–6.74 (14 H, m), 7.10 (1 H, dd),
7.23 (1 H, d), 7.29–7.46 (40 H, m), 7.64 (1 H, d); m/z (EI) 871 (M+). For 4:
l(CH2Cl2)/nm 718, 682, 654, 620, 422, 346; n(KBr)/cm21 3275 (NH);
dH(CDCl3, 500 MHz, 50 °C) 23.2 (2 H, br s), 5.06 (16 H, br s), 5.26 (8 H,
br s), 6.65 (4 H, br s), 6.80 (8 H, brs), 7.29–7.45 (40 H, m), 7.7–9.1 (12 H,
br M); m/z (FAB) 1788, (M+). For 5: l(CH2Cl2/nm 716, 680, 654, 620, 422,
346; n(KBr)/cm21 3276 (NH); dH(CDCl3, 500 MHz, 50 °C) 23.5 (2 H, br
s), 4.75–5.05 (48 H, br m), 5.26 (8 H, br s), 6.40–6.75 (28 H, br m), 6.90 (8
H, br s), 7.29–7.45 (80 H, m), 7.7–9.1 (12 H, br m); m/z (FAB) 3488, (M+).
For 6: l(CH2Cl2)/nm 715, 680, 654, 620, 422, 346; n(KBr)/cm21 3277
(NH); dH(CDCl3, 500 MHz, 50 °C 23.4 (2 H, br s), 4.66–5.05 (96 H, br m),
5.28 (8 H, br s), 6.30–6.70 (76 H, br m), 6.90 (8 H, br s), 7.29–7.45 (160 H,
m), 7.7–9.1 (12 H, br m). For 7: l(CH2Cl2)/nm 700, 664, 646, 398, 342;
n(KBr)/cm21 3433 (NH); dH(CDCl3, 500 MHz, 50 °C) 22.14 (2 H, br s),
3.47 (18 H, s), 3.70 (12 H, t), 3.95 (12 H, t), 4.20 (12 H, t), 4.74 (12 H, t),
5.18 (4 H, s), 5.33 (2 H, s), 6.69 (1 H, t), 7.05 (2 H, d), 7.22–7.50 (10 H, br
m), 8.55–8.70 (8 H, brm), 9.05 (1 H, d); m/z (FAB) 1542 (M+). For 8:
l(CH2Cl2)/nm 700, 664, 646, 398, 342; n(KBr)/cm21 3422 (NH);
dH(CDCl3, 500 MHz, 50 °C) 22.00 (2 H, br s), 3.46 (18 H, s), 3.70 (12 H,
t), 3.95 (12 H, t), 4.20 (12 H, t), 4.74 (12 H, t), 4.98 (8 H, s), 5.10 (4 H, s),
5.53 (2 H, s), 6.52 (2 H, t), 6.65 (1 H, t), 6.73 (4 H, d), 7.03 (2 H, d),
7.18–7.32 (20 H, br m), 8.52–8.70 (8 H, br m), 9.02 (1 H, d); m/z (FAB)
1989 (M+ + Na+). For 9: l(CH2Cl2)/nm 702, 664, 640, 400, 340; n(KBr)/
cm21 3429 (NH); dH(CDCl3, 500 MHz, 50 °C) 21.70 (2 H, br s), 3.42 (18
H, s), 3.70 (12 H, t), 3.95 (12 H, t), 4.20 (12 H, t), 4.74 (12 H, t), 4.88 (16
H, s), 4.98 (8 H, s), 5.06 (4 H, s), 5.50 (2 H, s), 6.48 (2 H, t), 6.49 (1 H, t),
6.52 (4 H, t), 6.69 (8 H, d), 6.72 (4 H, d), 7.00 (2 H, d), 7.15–7.35 (40 H, br
m), 8.62–8.75 (8 H, br m), 9.10 (1 H, d); m/z (FAB) 1989 (M+ + Na+).
1
confirmed by H NMR, UV–VIS absorption and IR spectros-
copy.‡ All compounds gave satisfactory elemental analyses and
all, except Pc 6, exhibited a parent mass ion using fast atom
bombardment (FAB) mass spectrometry.
Despite the steric bulk of the dendritic substituents, there is
evidence of self-association of the Pc cores of 4, 5 and 6 in dilute
solution. For example, the 1H NMR resonances corresponding
to the twelve protons attached to the Pc core of 4–6 are
broadened considerably even at a concentration of 1 3 1024
mol dm23 in CDCl3. Aggregation is also apparent in the UV–
VIS spectra of 4–6 in chloroform solution (1 3 1025 mol dm23
)
by the presence of a broad peak centred at 630 nm. Aggregation
is more evident in toluene at similar concentrations. The
solution behaviour of these materials is analogous to that of
1,3,4-oxadiazole-based dendrimers which were specifically
designed to produce columnar supramolecular structures.8
There is much less evidence of broadening due to aggregation in
the 1H NMR spectra of 7–9.
The thermal behaviour of 4–9, as measured by optical
polarising microscopy and differential scanning calorimetry
(DSC), is reported in Table 1. On cooling from the isotropic
melt, 4, 7, 8 and 9 display optical textures characteristic of a
hexagonal columnar mesophase, although the initially observed
‘sandy texture’ exhibited by 9 requires annealing at 105 °C for
several hours in order to obtain a recognisable texture. The
hexagonal columnar mesophase is commonly encountered in Pc
derivatives.9 It is remarkable that the presence of the large
dendritic wedge on Pc 9 does not prohibit columnar mesophase
formation but merely limits the thermal range over which the
mesophase is stable. A small angle X-ray diffraction analysis
(powder) of the mesophase of 4 and 7–9 reveals in each case a
single strong, sharp band which we believe originates from the
(1, 0) plane of the hexagonal lattice. Based on this assumption,
the calculated intercolumnar spacings are given in Table 1.
A potentially useful aspect of the thermal behaviour of these
materials is their tendency to form a glassy rather than a
crystalline solid phase, as indicated by DSC studies which show
distinct second-order glass transitions both on heating and
cooling. Thus, the non-mesogenic 5 and 6 produce clear, crack-
free solid films by cooling from the melt or by spin-coating onto
a glass substrate. UV–VIS absorption spectra of these non-
birefringent films (lmax = 620 nm) indicates strong cofacial
interactions of the Pc cores. The absence of light scattering from
domain boundaries is an attractive feature for optical stud-
ies.10
1 For a recent review see: G. R. Newcombe, C. N. Moorefield and F.
Vo¨gtle, Dendritic Molecules: Concepts, Syntheses and Perspectives,
VCH, Weinheim, 1996.
2 C. J. Hawker and J. M. J. Fre´chet, J. Am. Chem. Soc., 1990, 112, 7638;
K. L. Wooley, C. J. Hawker and J. M. J. Fre´chet, J. Am. Chem. Soc.,
1991, 113, 4252.
3 K. L. Wooley, C. J. Hawker, J. M. J. Fre´chet, F. Wudl, G. Srdanov, S.
Shi, C. Li and M. Kao, J. Am. Chem. Soc., 1993, 115, 9836.
4 R.-H. Jin, T. Aida and S. Inoue, J. Chem. Soc., Chem. Commun., 1993,
1260.
5 C.-F. Shu and H.-M. Shen, J. Mater. Chem., 1997, 7, 47.
6 N. B. McKeown, Phthalocyanine Materials: Synthesis, Structure and
Function, Cambridge University Press, Cambridge, 1998.
7 G. J. Clarkson, N. B. McKeown and K. E. Treacher, J. Chem. Soc.,
Perkin Trans. 1, 1995, 1817.
8 A. Kraft Chem. Commun., 1996, 77.
9 J. Simon and C. Piechocki, J. Am. Chem. Soc., 1982, 104, 5245; K.
Ohta, L. Jacquemin, C. Sirlin, L. Bosio and J. Simon, New J. Chem.,
1988, 12, 751.
10 R. D. George and A. W. Snow, Chem. Mater., 1994, 4, 209; M. Brewis,
G. J. Clarkson, V. Goddard, M. Helliwell, A. M. Holder and N. B.
McKeown, Angew. Chem., Int. Ed. Engl., in the press.
11 K. E. Treacher, G. J. Clarkson and N. B. McKeown, Liq. Cryst., 1995,
19, 887.
Received in Liverpool, UK, 4th March 1998; 8/01799I
970
Chem. Commun., 1998