Preparation and photophysical properties of halogenated silicon(IV) phthalocyanines substituted axially with poly(ethylene glycol) chains

Article abstract of DOI:10.1016/S0040-4039(03)00061-3

A series of silicon(IV) phthalocyanines with two axial poly(ethylene glycol) chains SiPcX₈[(OCH₂CH₂)_nOCH ₃]₂ (X=H, Cl, Br; n≈16) have been prepared of which the photosensitizing properties are enhanced as the periphery of the macrocycle is substituted with heavier halogens.

Full text of DOI:10.1016/S0040-4039(03)00061-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 1967–1970
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¨derb and
Dennis K. P. Nga,*
aDepartment of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, China
bInstitut 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 SiPcX8[(OCH2CH2)nOCH3]2 (X=H,
Cl, Br; n:16) 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 therapy2 and photocatalytic degradation
of chlorinated phenols,3 the molecules should exhibit
desirable photophysical properties including high triplet
(FT) and singlet oxygen quantum yields (FD) and long
triplet lifetimes (~T). Nonaggregated phthalocyanines
with a closed-shell metal center such as ZnII, AlIII and
SiIV are typical candidates for such applications.
Although replacing the usual metal center with a heavy
metal may increase the intersystem crossing quantum
yield (Fisc) 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 SiCl4 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 (Mn: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 1H 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

1968
P.-C. Lo et al. / Tetrahedron Letters 44 (2003) 1967–1970
Scheme 1.
groups were replaced under the harsh reaction
conditions.
phthalocyanines in ethanol with the umax shifted slightly
to the red upon halogenation. The fluorescence emis-
sion occurred at 677–688 nm with both the intensity
and lifetime decreased along the series X=H>Cl>Br.
The intersystem crossing quantum yields (Fisc) of 4a–c
in toluene were also measured by transient absorption
spectroscopy. The value increased steadily from 0.35 to
0.56 along the series, which is in good agreement with
the heavy atom effect. Compound 4a, which is slightly
soluble in water, exhibited the Q band absorption at
682 nm and a fluorescence emission at 687 nm in water.
It is worth mentioning that due to the strong aggrega-
tion effects, fluorescence of phthalocyanines is rarely
observed in aqueous media.12 The chloro 4b and bromo
4c analogues have a lower solubility in water. Their
absorption spectra showed broad Q bands due to the
aggregated species and no fluorescence signal was
observed.
The poly(ethylene glycol)-containing phthalocyanines
4a–c were unambiguously characterized with various
1
spectroscopic methods.11 The H NMR spectrum of 4b,
for example, showed an intense aromatic signal at l
9.64 together with nine well-separated multiplets or
virtual triplets centered at l −1.94, 0.43, 0.88, 1.82,
2.57, 3.06, 3.27, 3.41 and 3.48 assignable to the nine
methylene groups nearest to the ring center. These
signals, in particular the former ones, were shifted
upfield due to the ring current generated by the
phthalocyanine ring. The methyl protons resonated at l
3.37 as a singlet, while signals for the remaining methyl-
ene protons overlapped in the region l 3.54–3.64.
Figure 1 shows the MALDI-TOF mass spectrum of 4a,
which consists of two major envelopes for the molecu-
lar ion M+ and [M−O(CH2CH2O)nMe]+ species, each
separated by 44 mass units, the molecular mass of the
repeating unit in poly(ethylene glycol). The distribution
of these two envelopes is rather broad leading to a
partial overlap. The spectra for the chloro 4b and
bromo 4c analogues also displayed these two envelopes,
but their intensities were much weaker, in particular
for 4c. Signals due to the Na+ adduct of the
−O(CH2CH2O)nMe fragment appeared as the base
peaks instead, showing that desorption of these
phthalocyanines becomes more difficult upon halogena-
tion.
One of the key parameters relating to the photosensitiz-
ing efficiency is the singlet oxygen quantum yield. The
values for these compounds were determined by both
steady-state and time-resolved methods giving a consis-
tent result (Table 1).13 The lifetimes of oxygen lumines-
cence (~D) were found to be very close (28.3–28.7 ns) for
all three compounds in toluene, which provided a fur-
ther support for the reliability of data. The FD values
increased from 0.27 to 0.42 and 0.55 as the b-peripheral
hydrogen atoms were replaced with chloro and bromo
groups, respectively. All these data showed that periph-
eral substitution of phthalocyanines with heavy halogen
atoms reduces the fluorescence emission and promotes
the intersystem crossing, leading to a higher singlet
oxygen quantum yield. The photosensitizing applica-
tions of these compounds are being investigated.
The electronic absorption and photophysical data of
4a–c are summarized in Table 1. All the compounds
showed typical UV–vis absorptions of nonaggregated

P.-C. Lo et al. / Tetrahedron Letters 44 (2003) 1967–1970
1969
Figure 1.
Table 1. Electronic absorption and photophysical data for 4a–c
Compound
umax (nm)a
uem (nm)a
Relative FF (%)a
~
(ns)a
FISC (%)b
FD (%)b,c
F
4a
4b
4c
355, 604, 638, 672
358, 609, 646, 678
360, 612, 650, 680
677
685
688
100
7593
4593
5.490.2
4.690.2
3.390.2
3592
4492
5695
2795
4295
5595
a In ethanol.
b In toluene.
c Using pheophoride a as standard (FD=58%).
Acknowledgements
4. Ferraudi, G. In Phthalocyanines—Properties and Applica-
tions; Leznoff, C. C.; Lever, A. B. P., Eds.; VCH: New
York, 1989; Vol. 1, pp. 291–340.
We thank Professor Dominic Chan and Yi-Man Fung
for recording the MALDI-TOF mass spectra. This
work was supported by The Chinese University of
Hong Kong (Direct Grant 2002-03).
5. Perry, J. W.; Mansour, K.; Lee, I.-Y. S.; Wu, X.-L.;
Bedworth, P. V.; Chen, C.-T.; Ng, D.; Marder, S. R.;
Miles, P.; Wada, T.; Tian, M.; Sasabe, H. Science 1996,
273, 1533–1536.
6. A silicon(IV) phthalocyanine with two axial poly(ethylene
glycol) chains (Mn:5000) has been briefly described. See:
Wo¨hrle, D. Macromol. Rapid Commun. 2001, 22, 68–97.
7. See for example: (a) McKeown, N. B.; Painter, J. J.
Mater. Chem. 1994, 4, 1153–1156; (b) Clarkson, G. J.;
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1H NMR (CDCl3, 300 MHz) l 9.83 (s, 8H, Pc-Ha),
3.58–3.64 (m, ca. 92H, CH2), 3.54–3.56 (m, 4H, CH2),
3.48–3.50 (m, 4H, CH2), 3.38 (s, 6H, Me), 3.27–3.29 (m,
4H, CH2), 3.05–3.07 (m, 4H, CH2), 2.55–2.57 (m, 4H,
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1
11. Selected spectroscopic data for 4a: H NMR (CDCl3, 300
MHz) l 9.60–9.64 (m, 8H, Pc-Ha), 8.33–8.36 (m, 8H,
Pc-Hb), 3.45–3.64 (m, ca. 104H, CH2), 3.37 (s, 6H, Me),
3.20 (vt, J=4.8 Hz, 4H, CH2), 2.96 (vt, J=4.8 Hz, 4H,
CH2), 2.44 (vt, J=4.8 Hz, 4H, CH2), 1.66 (vt, J=4.8 Hz,
4H, CH2), 0.39 (vt, J=6.0 Hz, 4H, CH2), −1.92 (vt,
J=6.0 Hz, 4H, CH2); MS (MALDI-TOF) isotopic clus-
ters peaking at m/z 2010.1 [calcd for M+ (n=16) 2012.0],
1276.2 [calcd for (M−O(CH2CH2O)nMe)+ (n=16)
1
1275.6]. 4b: H NMR (CDCl3, 300 MHz) l 9.64 (s, 8H,
Pc-Ha), 3.54–3.64 (m, ca. 92H, CH2), 3.47–3.49 (m, 4H,
CH2), 3.40–3.42 (m, 4H, CH2), 3.37 (s, 6H, Me), 3.26–
3.28 (m, 4H, CH2), 3.05–3.07 (m, 4H, CH2), 2.56–2.58
(m, 4H, CH2), 1.80–1.83 (m, 4H, CH2), 0.86–0.90 (m, 4H,
CH2), 0.43 (vt, J=6.0 Hz, 4H, CH2), −1.94 (vt, J=6.0
13. Spiller, W.; Kliesch, H.; Wo¨hrle, D.; Hackbarth, S.;
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