Peripherally-substituted polydimethylsiloxane phthalocyanines: A novel class of liquid materials

Article abstract of DOI:10.1039/b100574j

Isotropic liquid phthalocyanine compounds with peripheral polydimethylsiloxane oligomer substitution were synthesized and found to have a unique combination of thermorefractive and nonlinear optical properties along with unusual metal substitution reactiv

Full text of DOI:10.1039/b100574j

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Peripherally-substituted polydimethylsiloxane phthalocyanines: a novel class of
liquid materials
Eva M. Maya,*ab James S. Shirk,a Arthur W. Snowa and Gerald L. Robertsb
a Naval Research Laboratory, Washington DC 20375-5342, USA. E-mail: emaya@ccf.nrl.navy.mil
b George Mason University, Department of Chemistry, 4400 University Drive, Fairfax, Virginia 22030-4444, USA
Received (in Irvine, CA, USA) 16th January 2001, Accepted 13th February 2001
First published as an Advance Article on the web 13th March 2001
Isotropic liquid phthalocyanine compounds with peripheral
polydimethylsiloxane oligomer substitution were synthe-
sized and found to have a unique combination of thermo-
refractive and nonlinear optical properties along with
unusual metal substitution reactivity and aggregation be-
havior.
silylation reaction. Next the allylphenoxyphthalonitrile–PDMS
adduct (4) is cyclotetramerized in the presence of lead oxide to
yield the phthalocyanine 1a. The alternate route is to first
conduct the cyclotetramerization yielding the tetrakis(allylphe-
noxy)phthalocyanine (5) then to perform the hydrosilylation to
yield the phthalocyanine product (1). This route has the
disadvantage of a more arduous purification (as a consequence
of a required large excess of 2 to ensure a quantitative
conversion of the allyl groups) and the use of a heterogeneous
non-acidic hydrosilylation catalyst (to avoid displacement of a
labile metal from the phthalocyanine cavity).
Structures 1–5 are consistent with spectroscopic character-
ization.† The phthalocyanine materials have both a mixed
isomer and polydisperse character. The phthalocyanine forming
reaction produces a statistical mixture of four possible isomers.5
The polydispersity is a result of the manner in which the butyl
capped PDMS oligomer (2) is prepared.6 These features are
regarded as advantages in discouraging liquid crystal formation.
In an analogous phthalocyanine system with polyethylene oxide
oligomer substituents it was found that a monodisperse system
is liquid crystalline7 while that with a polydisperse system is an
isotropic liquid.8 Liquid crystallinity is detrimental for applica-
tions where optical transparency is required.
The PDMS chains determine the liquid character of these
materials, which are viscous liquids at room temperatures. The
glass transition temperatures, Tg, for the Pb (1a), H2 (1b)
phthalocyanine compounds and the phthalonitrile precursor (4)
are 3, 16, and 14 °C respectively. This narrow range of glass
transition temperatures is a consequence of the dominant effect
of the siloxane chains. A similar trend of a slightly increasing
Tg, when progressing from lead to metal-free phthalocyanine
with epoxy substituted phthalocyanine glasses, has previously
been observed.9 This trend correlates with a greater tendency of
the metal-free phthalocyanine to aggregate relative to the lead
phthalocyanine.10 At rt these phthalocyanine materials will fill
short length ( ~ 4 mm) optical cells by capillary action over
several hours. At elevated temperatures ( ~ 100 °C) the viscosity
Polydimethylsiloxane oligomer substitution at the periphery of
a phthalocyanine ring generates a liquid material with excep-
tional optical properties and chemical behavior. This material is
designed to have the rheological and thermorefractive proper-
ties of a silicone fluid1 and the nonlinear optical properties
characteristic of the phthalocyanine chromophore.2 Structure 1
in Scheme 1 combines these molecular features coupled through
an aromatic ether linkage. The motivation for this design is to
combine two mechanisms important to an optical limiting
application2 into a single compound: a large fluence dependent,
refractive index;1 and a reverse saturable optical absorption.2
The aromatic ether linkage further incorporates an enhanced
photo-oxidative stability. Cloaking of the phthalocyanine ring
in a silicone covering has important implications for the
chemistries of aggregation and metal substitution. To our
knowledge, no examples of peripheral silicone substituted
phthalocyanines have been previously reported. In this com-
munication we report synthesis and preliminary character-
ization of a new class of liquid phthalocyanine (Pcs) materi-
als.
Two routes for the synthesis of the lead and metal-free
phthalocyanine compounds are depicted in Scheme 1. Starting
reagents for both routes are prepared in the first step. The butyl
capped hydrosilyl terminated poly(dimethylsiloxane) (PDMS)
oligomer (2) (DP = 9 in this example) is synthesized by an
anionic ring opening polymerization of hexamethylcyclo-
trisiloxane.3 The allylphenoxyphthalonitrile (3) is prepared by a
nitroaromatic displacement reaction of 4-nitrophthalonitrile4
with 2-allylphenol. In the preferred route, the PDMS oligomer
is first coupled to the allylphenoxyphthalonitrile by a hydro-
Scheme 1 Reagents and conditions: i, 3 drops of 0.1 N solution of H2PtCl66H2O in isopropanol, 60 °C, 1 h; ii, PbO, 165 °C, 16 h; iii, PbO or hydroquinone,
165–180 °C, 16 h; iv, 8 drops of platinium-divinyl tetramethyldisiloxane complex, toluene, 20 h; v, toluene, F3CCOOH, 10 min.
DOI: 10.1039/b100574j
Chem. Commun., 2001, 615–616
This journal is © The Royal Society of Chemistry 2001
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is much lower, the aggregation is reduced (spectroscopic
detection), and the filling of the cell is much more rapid.
The refractive index of a thin film of the lead phthalocyanine
compound (1a) was measured by ellipsometry and also by
measuring the angle of total internal reflection for a thin film on
a prism of known refractive index. Over a temperature range of
24 to 95 °C the refractive index at 1550 nm decreased from
1.492 to 1.465. This corresponds to a decrease of 4 ± 1 3 1024
per degree Centigrade and correlates well with the dn/dT
reported (4 3 1024 °C21) for linear dimethylsiloxane oligo-
mers.1 This decrease also indicates that the temperature
dependence of the refractive index of silicone substituted
phthalocyanines is dominated by PDMS chains. Relative to
other polymers, polydimethylsiloxane has an exceptionally
large dn/dT,11 and this combination with the phthalocyanine
structure demonstrates a successful coupling of this property
and this chromophore.
Z-scan and optical limiting measurements were performed on
the lead phthalocyanine compound (1a) to characterize the
nonlinear optical properties. The Z-scan of a 20.2 mM sample of
1a in a 50.5 mm sample cell at 532 nm is shown in Fig. 1. The
material is a strong reverse saturable absorber at 532 nm. An
estimate of the excited state cross section from this Z-scan and
from a nonlinear transmission experiment on the same sample
gave a value of the excited state cross section of ~ 20 times that
of the ground state at 532 nm. This implies that the material is
a very good reverse saturable absorber. The nonlinear absorbing
properties are similar to those found in lead tetrakis(cumylphe-
noxy)phthalocyanine, PbPc(CP)4.12 The strong nonlinear ab-
sorption, in combination with the large dn/dT reported above,
makes this a superior optical limiter material.
tion constants mentioned above. We attribute this result to the
nature of the siloxane chain and the ortho substitution of the
phenylene ether linkage to the phthalocyanine ring.
In summary, the incorporation of PDMS oligomers as
phthalocyanine peripheral substituents combines the desirable
rheological and refractive properties of a silicone fluid with the
nonlinear optical properties associated with the phthalocyanine.
The long silicone substituents further impart unique chemical
behavior with respect to the phthalocyanine synthesis and
aggregation.
Dr John Callahan is gratefully acknowledged for recording
the mass spectra, and the Office of Naval Research is
acknowledged for financial support.
Notes and references
† Selected data for 1a; l(toluene)/nm 721, 648, 365; n(NaCl)/cm21 2959
(CH), 1608 and 1492 (C–C), 1253 (SiCH3), 1091 and 1014 (SiOSi), 800
(SiC). For 1b l(toluene)/nm 703, 666, 638, 605, 346; n(NaCl)/cm21 3295
(NH), 2959 (CH), 1615 and 1479 (C–C), 1259 (SiCH3), 1091 and 1027
(SiOSi), 807 (SiC); m/z 4500–2200 consists of peaks separated by 74 amu
due to –Si(CH3)2O– degradation which is common to mass spectra of
PDMS oligomers.6 For 2: dH(CDCl3, 300 MHz)/ppm 0.04–0.07 (54H, br s,
SiCH3), 0.19 (6H, m, SiCH3), 0.53 (2H, m, SiCH2), 0.89 (3H, t, CH3), 1.32
(4H, m, CH2), 4.70 (1H, sept, SiH); n(NaCl)/cm21 2972 (CH), 2132 (SiH),
1272 (SiCH3), 1098 and 1027 (SiOSi), 800 (SiC). For 3: dH(CDCl3, 300
MHz)/ppm 3.23 (2H, d, CH2), 4.95 (2H, dd, NCH2), 5.78 (1H, m, NCH), 6.95
(1H, d, Harom), 7.12–7.33 (5H, m, Harom), 7.68 (1H, d, Harom); dC(CDCl3, 75
MHz) 34.0, 108.5, 114.9 and 115.4 (CN), 116.7, 117.6, 120.8, 120.9, 121.0,
126.7, 128.5, 131.6, 132.4, 135.3, 135.4, 151.1, 161.7; n(NaCl)/cm21 3082
(NCH2), 2229 (CN), 1615 (CNC), 1595 and 1486 (C–C), 1246; For 4:
dH(CDCl3, 300 MHz)/ppm 0.012–0.064 (60H, m, SiCH3), 0.51 (4H, m,
SiCH2), 0.86 (3H, t, CH3), 1.29 (4H, m, CH2), 1.58 (2H, m, CH2), 2.49 (2H,
t, CH2), 6.95 (1H, d, Harom), 7.14–7.31 (5H, m, Harom), 7.68 (1H, d, Harom);
n(NaCl)/cm21 2966 (CH), 2229 (CN), 1602 and 1492 (C–C), 1254 (SiCH3),
1098 and 1033 (SiOSi), 806 (SiC); For 5a l(toluene)/nm 721, 650, 346;
n(NaCl)/cm21 3076 (NCH2), 2919 (CH), 1638 (CNC), 1608, 1485 (C–C),
1239. For 5b l(toluene)/nm 703, 667, 639, 605, 350; n(NaCl)/cm21 3295
(NH), 3075 (NCH2), 1638 (CHNCH2), 1611 and 1467(C–C), 1228;
dH(CDCl3, 300 MHz)/ppm 24.1 (s, NH), 3.6 (m, CH2), 5.1 (m, NCH2), 6.1
(m, CHN), 6.8–7.7 (m, Harom); m/z 1091.
Finally, the silicone chains covering the phthalocyanine
chromophore impart some unusual chemical behavior, partic-
ularly with regard to metal substitution reactions and aggregate
formation. When the siloxane–phthalonitrile precursor (4) is
subjected to the Linstead conditions of lithium pentoxide–
pentan-1-ol for conversion to lithium phthalocyanine,13
a
product with a Q-band diagnostic of the dilithium substituted
phthalocyanine (675 nm) is obtained. However, it is very
difficult to displace the normally very labile lithium with
protons. Normally, this occurs under very mild acidic condi-
tions. We find that normal and progressively more severe acid
exchange conditions were unsuccessful.‡ Treatment with
concentrated HCl and heating resulted in conversion being first
observed at 90 °C which became quantitative after 2 h at this
temperature. However, once the lithium ion is displaced by the
proton, subsequent metal ion substitution reactions proceed
under normal conditions.§
‡ Unsuccessful lithium displacement conditions: aliquot addition of
trifluoroacetic acid (10 min); second aliquot addition of trifluoroacetic acid
(30 min); aliquot addition of conc. HCl (10 min).
§ Copper and lead ions were substituted into the metal-free phthalocyanine
using the acetate salts and refluxing for 2 h in pentan-1-ol–THF.
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The other aspect of unique chemical behavior is in the
aggregation tendency. Phthalocyanine compounds aggregate as
a concentration dependent association of phthalocyanine rings.
The dimerization constant is a useful measure of this aggrega-
tion tendency. Typical dimerization constants for phthalocya-
nine compounds range from 104 to 106 M21 14
Preliminary
.
analysis of concentration dependence of the Q-band absorption
using a monomer–dimer equilibrium model indicates that the
dimerization constant for 1b is 150 ± 100 M21. This is
significantly less than the range for phthalocyanine dimeriza-
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Phthalocyanines, 2000, 4, 518.
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J. Lightwave Technol., 1992, 10, 420; J. M. Cariou, J. Dugas, L. Martin
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Fig. 1 A Z-scan of a 20.2 mM sample of 1a at increasing input energy.
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