TETRAHEDRON
LETTERS
Tetrahedron Letters 44 (2003) 1967–1970 Pergamon
Preparation and photophysical properties of halogenated
silicon(IV) phthalocyanines substituted axially with
poly(ethylene glycol) chains
Pui-Chi Lo,
a
Shuangqing Wang,
a
Andre Zeug,
b
Matthias Meyer,
b
Beate Ro ¨ der
b
and
Dennis K. P. Ng
a,
*
a
Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, China
b
Institut fu ¨r Physik, Humboldt -Universita ¨t Berlin, Invalidenstraße 110, 10115 Berlin, Germany
Received 6 November 2002; revised 12 December 2002; accepted 20 December 2002
Abstract—A series of silicon(IV) phthalocyanines with two axial poly(ethylene glycol) chains SiPcX
8
[(OCH
2
CH
2
)
n
OCH
3
]
2
(X=H,
Cl, Br; n16) have been prepared of which the photosensitizing properties are enhanced as the periphery of the macrocycle is
substituted with heavier halogens. © 2003 Elsevier Science Ltd. All rights reserved.
Being a versatile class of functional dyes, phthalocya-
nines have been studied extensively for a vast number
of practical applications.
1
For those involving the pho-
tosensitizing properties of the macrocycles such as
photodynamic therapy
2
and photocatalytic degradation
of chlorinated phenols,
3
the molecules should exhibit
desirable photophysical properties including high triplet
(
T
) and singlet oxygen quantum yields (
) and long
triplet lifetimes (
T
). Nonaggregated phthalocyanines
with a closed-shell metal center such as Zn
II
, Al
III
and
Si
IV
are typical candidates for such applications.
Although replacing the usual metal center with a heavy
metal may increase the intersystem crossing quantum
yield (
isc
) as a result of heavy atom perturbation, the
resulting metallophthalocyanines usually have a very
short triplet lifetime which cannot trigger the formation
of singlet oxygen efficiently.
4
We report herein an alter-
native strategy to ameliorate the photosensitizing
efficiency of phthalocyanines by introducing heavy
halogens on the periphery of the ring. Although halo-
genated phthalocyanines have been known for some
time, the use of these heavy substituents to alter the
photophysical behavior of phthalocyanines has been
little studied.
5
We are particularly interested in sili-
con(IV) phthalocyanines because of their good photo-
physical properties and ease of preparation and
modification through the manipulation of the axial
substituents. To increase the hydrophilicity of the
macrocycles and reduce their aggregation tendency, two
poly(ethylene glycol) chains have been introduced to
the axial positions.
6
It is worth noting that phthalocya-
nines conjugated with these polymeric materials are
well-known for their mesogenic properties.
7
Reports on
their photosensitizing applications, however, remain
scarce in literature.
8
The method for preparation of the halogenated
phthalocyanines 3b and 3c was similar to that for the
unsubstituted analogue 3a (Scheme 1).
9
Starting from
dichloro- 1b or dibromo-phthalonitrile 1c, bubbling of
dry ammonia into a methanol solution in the presence
of sodium methoxide led to the corresponding 1,3-
diiminoisoindolines 2b or 2c, which upon treatment
with SiCl
4
in quinoline afforded the silicon(IV) phthalo-
cyanines 3b or 3c, respectively. Due to the poor solubil-
ity of these compounds in common organic solvents,
they could only be purified by Soxhlet extraction and
characterized with UV–vis spectroscopy.
10
Reactions of
3 (X=H, Cl, Br) with poly(ethylene glycol) methyl
ether (M
n
750) in the presence of NaH in refluxing
toluene resulted in axial substitution giving 4a–c in
moderate yields. These polymeric materials have high
solubility in many organic solvents and were purified
readily by column chromatography. Treatment of
diiodophthalonitrile 1d with ammonia also gave a
diiminoisoindoline 2d, but attempts to prepare the
octaiodo counterpart of 4 following the same pathway
were not successful. The
1
H NMR spectrum of the
resulting product gave many signals instead of a singlet
in the aromatic region showing that some of the iodo * Corresponding author. E-mail: dkpn@cuhk.edu.hk
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
PII:S0040-4039(03)00061-3