Pegylated phthalocyanines: Synthesis and spectroscopic properties

Article abstract of DOI:10.1016/j.tetlet.2011.10.002

The synthesis and spectroscopic properties of Zn(II)-phthalocyanines containing up to eight tri(ethylene glycol) groups are described. These pegylated-phthalocyanines are highly soluble in polar solvents and might find application as fluorescence imaging

Full text of DOI:10.1016/j.tetlet.2011.10.002

Tetrahedron Letters 52 (2011) 6675–6678
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Pegylated phthalocyanines: synthesis and spectroscopic properties
⇑
Hairong Li, Frank R. Fronczek, M. Graça H. Vicente
Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The syntheses and spectroscopic properties of a series of pegylated zinc(II)-phthalocyanines (Zn-Pcs) con-
taining one, two, or eight tri(ethylene glycol) chains are described. The single molecular structure of a
phthalonitrile precursor containing one hydroxyl and one PEG group, and its unique intermolecular
hydrogen bonding are presented. The pegylated Pcs are highly soluble in polar organic solvents and have
fluorescence quantum yields in the range 0.08–0.28.
Received 19 July 2011
Revised 30 September 2011
Accepted 2 October 2011
Available online 8 October 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
PDT
PEG
Phthalocyanine
Phthalonitrile
Fluorescence
Phthalocyanines (Pcs) are unique tetrapyrrolic macrocycles
containing an extended aromatic 18- electron system. Among
tend to minimize compound aggregation and improve tumor cellu-
lar uptake. Herein, we report the synthesis and photophysical
properties of a new series of PEG-substituted zinc(II)-Pcs contain-
ing one, two or eight tri(ethylene glycol) chains, which are highly
soluble in polar organic solvents. Furthermore, the new pegylated
Zn-Pcs are pure compounds, rather than statistical mixtures of
regioisomers, as previously reported.21,36
The synthetic route to tri(ethylene glycol) conjugated Pc 4–6 is
shown in Scheme 1. Mono-, di-, and octa-tri(ethylene glycol)
substituted Zn-Pcs 4–6 were obtained from the corresponding pht-
halonitrile precursors 1–3. Phthalonitrile 1 was synthesized from
4,5-bis(2,5-di-tert-butyl-4-hydroxyphenoxy)phthalonitrile9 and 2.3
equiv of 2-[2-(2-iodoethoxy)ethoxy]ethanol at 50 °C, in the pres-
ence of potassium carbonate, in 59% yield.27 2-[2-(2-Iodoethoxy)
ethoxy]ethanol was synthesized from the commercially available
2-[2-(2-chloroethoxy)ethoxy] ethanol and sodium iodide, by reflux-
ing in acetone for 24 h, in 90% yield. Pc 4 containing two PEG chains
terminated with hydroxyl groups (PEGOH) was synthesized by a
mixed macrocyclization using zinc(II) as template in the presence
of excess of unsubstituted phthalonitrile. The reaction was heated
to 140 °C and catalyzed by DBU to afford Pc 4 as a dark green solid
in 8% yield.28
p
the various applications of modified Pcs, they are promising photo-
sensitizers for photodynamic therapy (PDT) of cancer1,2 because of
their intense absorptions at long wavelengths (>670 nm) and
extraordinary ability for producing singlet oxygen. However, due
to Pcs’ intrinsic property of insolubility and high tendency for
aggregation in aqueous solutions, research has centered on the
synthesis of soluble Pcs and the tuning of their photophysical prop-
erties.3,4 A general approach consists of attaching multiple hydro-
philic or amphiphilic substituents to the periphery or the core of
Pcs, such as carboxylates,5 sulfonates,6 phosphonates,7 pyridinium
ions,8 hydroxyl groups,9 peptides,10 oligonucleotides,11 and poly-
ethylene glycol (PEG)12 groups. In particular, the use of PEG as
effective carriers13–15 or covalently attached for drug delivery to
target tissues,16,17 is well documented. PEG-functionalized photo-
sensitizers generally show improved serum life, reduced uptake
by the reticuloendothelial system, enhanced cellular permeability
and increased tumor accumulation.18 Lutetium texaphyrin or Lu-
texÒ, containing two short PEG chains and two free hydroxyl
groups, has demonstrated high photodynamic efficiency and has
been evaluated in phase I/II clinical trials.19,20 Recently, several
groups21–25,35,36 and us8,26 reported the synthesis of PEG-
substituted photosensitizers (porphyrins, chlorins, and Pcs) for
application in PDT. Although such molecules generally show
improved solubilities, their photophysical and biological properties
depend on the length and number of PEG chains at the macrocycle
periphery; two to four low molecular weight PEGs (<2000 amu)
Several examples of
a-substituted Pcs have been previously
reported.12,24,29–32 Such compounds are often observed to absorb
and emit at longer wavelengths compared with their b-substituted
analogs. This property could favor their application as photosensi-
tizers for PDT due to the deeper penetration of long wavelength
light into most human tissues. Furthermore,
a-substituted Pcs in
general show higher solubility in polar organic solvents and lower
aggregation tendency than the corresponding b-substituted Pcs.
The
⇑
Corresponding author. Tel.: +1 225 578 7405; fax: +1 225 578 3458.
E-mail address: vicente@lsu.edu (M.G.-H. Vicente).
a
-substituted octa-PEG-Pc 533 containing eight PEGOMe
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.10.002

6676
H. Li et al. / Tetrahedron Letters 52 (2011) 6675–6678
R3 R3
R1
R2
NC CN
R3 R3
R2
CN
L
L
L
L
N
N
N
M
N
L
L
R1
N
N
Zn(OAc)2, DBU or Li, 140 ºC
NC
N
N
L
L
tBu
tBu
O
tBu
(OCH2CH2)3OH
4: R1,R2=L=H; R3=
O
tBu
(OCH2CH2)3OH
1
2
3
1:
R ,R =H; R =
1
2
3
1
2
3
5:
R ,R =L=(OCH2CH2)3OCH3; R =H
2:
3:
R ,R =(OCH2CH2)3OCH3; R =H
6: R1=OH; R2=(OCH2CH2)3OCH3; R3=L=H
1
2
3
R =OH; R =(OCH2CH2)3OCH3; R =H
Scheme 1. Synthesis of Pcs 4–6.
chains at the
a positions of the Pc macrocycle, was synthesized
from phthalonitrile 2 using a lithium template, followed by anhy-
drous zinc(II) acetate.28 The precursor phthalonitrile 2 was synthe-
sized by di-pegylation of 2,6-dihydroxyphthalonitrile using
1.2 equiv CH3(OCH2CH2)3I8 by activation with 2 equiv potassium
carbonate in 90% yield.27 The synthesis of Pc 6 containing one hy-
droxyl group and one PEGOMe group at the
a positions of the Pc
was accomplished by mixed condensation of phthalonitrile 3 and
unsubstituted phthalonitrile, similar to the method used for the
synthesis of Pc 4.28 Phthalonitrile 3 was prepared by mono-pegyla-
tion of commercially available 2,6-dihydroxyphthalonitrile using
1 equiv CH3(OCH2CH2)3I8 in DMF at room temperature, in 43%
yield.27 The structures of the two independent molecules of pht-
halonitrile 3 in the crystal are shown in Figure 1. They are very
similar in conformation, except for the central ethylene glycol
group. In both structures, the magnitude of the torsion angle about
the C–C bond is the same, but opposite in sign, +65.5 and ꢀ65.9°.
However, the torsion angles about the neighboring C–O bonds
are such that the overall shape of the molecule does not change
significantly. Torsion angles about those C–O bonds are ꢀ176.8
and +172.5° in one molecule and ꢀ84.2 and ꢀ84.9° in the other
molecule. Each of the independent molecules forms an intermolec-
ular hydrogen-bonded dimer about an inversion center, as shown
in Figure 1. The acceptors are ether O, the Oꢁ ꢁ ꢁO distances are
2.621(2) and 2.655(2) Å, and each dimer is a 26-membered ring.34
The PEG-substituted Pcs 4–6 are soluble in polar organic sol-
vents, including DMF, DMSO, acetonitrile, and THF, as well as in
less polar solvents such as dichloromethane and diethyl ether.
The structures of these targeted Pcs were confirmed by NMR and
MS spectroscopies. The 1H NMR of Pc 4 in deuterated DMF shows
the characteristic aromatic protons above 7 ppm, as we have previ-
ously reported.9,31,32 Three sets of aromatic peaks at 9.45, 9.25, and
8.95 ppm in the downfield region were observed and assigned to
the
-protons at the two symmetrical isoindole subunits (A0A0);
9.25 ppm (doublet, 2H) for the -protons at the third isoindole
subunit (A) and 8.95 ppm (singlet, 2H) for the -protons at the
a-protons of this A3B-type Pc: 9.45 ppm (broad, 4H) for the
a
a
a
substituted isoindole subunit (B). The signals in the range of
8.21–8.17 ppm (multiplet, 6H) are due to the b-ring protons in
the ‘A’ part. The relative upfield protons at 7.31 (singlet, 2H) and
7.25 (singlet, 2H) ppm are due to the two b-substituted phenyl
rings. The aliphatic proton signals in the PEG chain appear between
3.56–4.38 ppm, and the OH signals overlap with the solvent peak
at around 3.5 ppm. Another two singlet peaks at 1.59 (18H) and
1.40 (18H) are assigned to the tert-butyl groups on the substituted
phenyl rings. The symmetrical octa-pegylated Zn-Pc 5 has a sim-
pler aromatic proton pattern due to the symmetry of the b-ring
Figure 1. Molecular structure of the two independent molecules of phthalonitrile 3
(above), showing hydrogen bonding (below).
protons, appearing at 7.76 ppm (doublet, 8H). The aliphatic proton
signals are in the range of 3.16–4.97 ppm. Pc 4 and 5 showed single
or double molecular ion modes, respectively, at m/z 1281.5361 (Pc

H. Li et al. / Tetrahedron Letters 52 (2011) 6675–6678
6677
4) and 965.9939 (Pc 5) with electrospray ionization method. Pc 6
containing one hydroxyl group and one PEGOMe group at the
fluorescence quantum yield (Table 1); 0.08 is similar to the quantum
yields reported for a series of cationic octa-PEG-alkylated-Zn(II)-Pcs
(0.04–0.08).8 Pc 6 containing one hydroxyl group and one PEGOMe
chain has an absorption peak at 676 nm and a broad emission peak
at 677 nm. Its fluorescence quantum yield is 0.13, probably as a re-
sult of intermolecular hydrogenbonding, as already observed in pht-
halonitrile 3 (Fig. 1). All of the Pcs 4–6 have small Stokes Shift of 1–
3 nm.
In summary, we have successfully synthesized three pegylated
zinc(II) Pcs 4–6 in 1–8% yields. All these PEG-Pcs are highly soluble
in polar organic solvents such as acetone, DMF, and DMSO. Pc 4
containing two PEGOH chains at the para-position of b-substituted
phenoxyl ring has similar absorption and emission peaks to the re-
ported 2,3-di-(2,5-di-tert-butyl-4-hydroxyphenoxy)-zinc(II) Pc,9
but significantly higher fluorescence quantum yield (0.28), proba-
bly due to its higher solubility in DMSO. Pc 5 containing eight PEG-
1,4-a-positons showed a molecular fragment ion at m/z 723.6
[MꢀOCH3]+ in MALDI-MS.
The absorption spectrum of Pc 4 containing two PEGOH moieties
in DMSO shows a characteristic strong Q band at 677 nm (Fig. 2) that
strictly follows the Lambert–Beer law. This result indicates that Pc 4
exists in the monomeric form in DMSO. In acetone, Pc 4’s Q band
absorption is blue-shifted to 669 nm. Its fluorescence emission
spectrumshowsa peakat680 nminDMSO. Theobservedabsorption
and emission properties of Pc 4 are almost identical to those of
2,3-di-(2,5-di-tert-butyl-4-hydroxyphenoxy)-zinc(II)-phthalocyanine,
previously reported by us.9 This result indicates that the attachment
of the PEGOH moieties at the phenoxy groups doesn’t significantly
change the photophysical properties of the Pc’s conjugated core sys-
tem, which is also in agreement with reports by others.24,37,38 How-
ever, the fluorescence quantum yield of Pc 4 was found to be 0.28
(Table 1), which is higher than that reported for 2,3-di-(2,5-di-tert-
butyl-4-hydroxyphenoxy)-zinc(II)-phthalocyanine (0.20).9 This is
probably due to the conjugation of the PEGOH chains, which makes
the disubstituted Pc 4 more soluble and consequently forming
monomeric species in DMSO. Interestingly, the OH group at the
end of the PEG chain does not make the intermolecular hydrogen
bonding a problem in this case. On the other hand, Pc 5 bearing eight
PEGOMe groups has a significantly red-shifted Q band to the near-
infrared (NIR) region at ꢂ750 nm (Fig. 2). This phenomena is due
OMe chains at the
a positions show pronounced red-shifted
absorption and emission to ꢂ750 nm. While Pc 5 has the lowest
fluorescence quantum yield (0.08), Pc 6 bearing only one PEGOMe
and one hydroxyl group shows similar absorption and emission
wave lengths to Pc 4, but significantly lower fluorescence quantum
yield (0.13), probably due to intermolecular hydrogen bonding.
Acknowledgments
The work described herein was partially supported by the Na-
tional Institutes of Health, Grant number R21 CA139385.
to the attachment of the eight PEGOMegroups at thePc’s apositions,
which reduces the Pc’s HOMO-LUMO gap and causes longer absorp-
tion wavelength compared with the unsubstituted Zn(II)Pc (about
References and notes
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Figure 2. Absorption spectra of PEG-substituted Zn(II)-Pcs 4 (solid line), 5 (dash
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Table 1
Spectroscopic properties of Pcs 4–6 in DMSO at room temperature
Pc
Abs (kmax, nm)
Ema (kmax, nm)
Ufb
SS (nm)
4
5
6
677
749
676
680
751
677
0.28
0.08
0.13
3
2
1
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7, 874–888.
Abs: absorption; Em: emission; SS: Stokes’ shift.
a
Excitation at 615 or 675 nm.
b
Calculated using Zn(II)-phthalocyanine as the standard.

6678
H. Li et al. / Tetrahedron Letters 52 (2011) 6675–6678
26. Sibrian-Vazquez, M.; Jensen, T. J.; Vicente, M. G. H. J. Photochem. Photobiol., B
2007, 86, 9–21.
(4.5) nm. Compound 6, 1H NMR (DMF-d7, 400 MHz): d 9.57–9.45 (m, 8H, Ar-H),
8.76–8.67 (m, 6H, Ar-H), 4.83 (br, 2H, OCH2), 4.19 (br, 2H, OCH2), 3.91–3.79 (m,
6H, OCH2), 3.56 (br, 2H, OCH2), 3.26 (s, 3H, OCH3). MS (MALDI-TOF) m/z 723.6
27. Selected spectroscopic data for phthalonitriles: Compound 1, 1H NMR (acetone-
d6, 250 MHz): d 7.32 (s, 2H, Ar-H), 7.06 (s, 2H, Ar-H), 6.91 (s, 2H, Ar-H), 4.23 (t,
J = 4.4 Hz, 4H, OCH2), 3.92 (t, J = 4.4 Hz, 4H, OCH2), 3.70-3.51 (m, 18H, OCH2,
OH overlap), 1.37 (s, 18H, C(CH3)3), 1.35 (s, 18H, C(CH3)3). 13C NMR (acetone-
d6, 63 MHz): d 155.7, 153.5, 146.3, 140.3, 138.5, 122.0, 120.6, 116.1, 112.7,
110.2 (Ar-C, CN), 73.4, 71.2, 71.1, 70.5, 68.5, 61.9 (CH2O), 35.2, 35.1, 30.6, 29.9
[MꢀOCH3]+, calcd for [C38H27N8O4Zn]+ 723.1. UV–vis (DMSO): kmax (log
e) 676
(3.6), 657 (3.3) nm.
29. Fukuda, T.; Homma, S.; Kobayashi, N. Chem. Eur. J. 2005, 11, 5205–5216.
30. Maqanda, W.; Nyokongo, T.; Maree, M. D. J. Porphyrins Phthalocyanines 2005, 9,
343–351.
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32. Li, H. R.; Fronczek, F. R.; Vicente, M. G. H. J. Organomet. Chem. 2009, 694, 1607–
1611.
(C(CH3)3). HRMS-ESI: m/z 833.4947 [M+H]+, calcd for [C48H69N2O10
]
+ 833.4946.
Compound 2, 1H NMR (CDCl3, 250 MHz): d 7.18 (s, 2H, Ar-H), 4.12 (t, J = 4.6 Hz,
4H, OCH2), 3.76 (t, J = 4.6 Hz, 4H, OCH2), 3.63–3.59 (m, 4H, OCH2), 3.54–3.49
(m, 8H, OCH2), 3.43–3.39 (m, 4H, OCH2), 3.23 (s, 6H, OCH3). 13C NMR (CDCl3,
63 Hz): d 155.0, 119.2, 112.8, 104.6 (Ar-C, CN), 71.5, 70.6, 70.2, 70.1, 69.7, 69.0
(OCH2), 58.6 (OCH3). HRMS-ESI: m/z 453.2230 [M+H]+, calcd. for [C22H33N2O8]+
453.2231. Compound 3, 1H NMR (acetone-d6, 250 MHz): d 7.49 (d, J = 9.4 Hz,
1H, Ar-H), 7.36 (d, J = 9.4 Hz, 1H, Ar-H), 4.30 (t, J = 4.5 Hz, 2H, OCH2), 3.84 (t,
J = 4.5 Hz, 2H, OCH2), 3.67–3.63 (m, 2H, OCH2), 3.59–3.54 (m, 4H, OCH2), 3.46–
3.43 (m, 2H, OCH2), 3.26 (s, 3H, OCH3). 13C NMR (acetone-d6, 63 MHz): d
155.80, 155.75, 123.66, 121.65, 114.37, 114.28, 104.30, 102.42 (Ar-C, CN),
72.53, 71.46, 71.14, 70.96, 70.81, 70.07 (OCH2), 58.70 (OCH3). HRMS-ESI: m/z
305.1140 [MꢀH]ꢀ, 306.1169 [M]ꢀ, calcd for [C15H17N2O5]ꢀ 305.1142,
[C15H18N2O5]ꢀ 306.1216.
33. Russell, D. A.; Cook, M. J. United States Patent 1998, 5834455.
34. Phthalonitrile 3:
a = 8.9144(5), b = 10.9803(10), c = 15.9181(15) Å,
= 87.909(5)°,V = 1544.3(2) Å3, Z = 4, Dx = 1.317 g cmꢀ3
k = 1.54178 Å, T = 90 K, hmax = 68.2°,
= 0.84 mmꢀ1
0.30 ꢃ 0.14 ꢃ 0.07 mm, Bruker Kappa Apex-II CCD diffractometer, 17,507
C
15H18N2O5, Mr = 306.31, triclinic space group P-1,
= 82.666(5), b = 89.322(5),
CuK radiation,
colorless lath
a
c
,
a
l
,
measured reflections, 5487 independent, 4228 with I >2r(I), Rint = 0.046,
R = 0.044, wR(F2) = 0.123 for 406 refined parameters and 5487 data, CCDC
826729.
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28. Selected spectroscopic data for ZnPcs: Compound 4, 1H NMR (DMF-d7, 400 MHz):
d 9.45 (br, 4H, Ar-H), 9.25 (d, 2H, J = 7.4 Hz, Ar-H), 8.95 (s, 2H, Ar-H), 8.21–8.17
(m, 6H, Ar-H), 7.31 (s, 2H, Ar-H), 7.25 (s, 2H, Ar-H), 4.38 (t, J = 4.3 Hz, 4H,
OCH2), 4.00 (t, J = 4.3 Hz, 4H, OCH2), 3.77 (t, J = 4.5 Hz, 4H, OCH2), 3.70 (t,
J = 4.5 Hz, 4H, OCH2), 3.66–3.55 (m, 4H, OCH2), 3.59–3.56 (m, 4H, OCH2), 1.59
(s, 18H, C(CH3)3), 1.40 (s, 18H, C(CH3)3). HRMS-ESI: m/z 1281.5361 [M+H]+,
calcd for [C72H81N8O10Zn]+ 1281.5362. UV–vis (DMSO): kmax (log
e) 677 (5.0),
611 (4.2) nm. Compound 5, 1H NMR (DMF-d7, 400 MHz): d 7.76 (d, J = 3.9 Hz,
8H, Ar-H), 4.97–4.91 (m, 32H, OCH2), 4.15 (t, J = 4.9 Hz, 16H, OCH2), 3.75 (t,
J = 4.9 Hz, 16H, OCH2), 3.58–3.56 (m, 16H, OCH2), 3.36–3.34 (m, 16H, OCH2),
3.16 (s, 24H, OCH3). HRMS-ESI: m/z 965.9939 [M+K+Naꢀ4H]2ꢀ
, calcd for
[C88H124N8O32ZnKNa]2ꢀ966.3582. UV–vis (DMSO): kmax (log
e) 749 (5.0), 674