Photophysical properties of microencapsulated phthalocyanines

Article abstract of DOI:10.1142/S1088424610002008

Lipophilic substituted zinc(II) phthalocyanines: 2,3,9,10,16,17,23,24- octakis[(N,N-dimethylaminoethylsulfanyl)]phthalocyaninatozinc(II) (S1) and tetrakis(N,N-dibutylaminoethoxy)phthalocyaninatozinc(II) (3) Were incorporated into soybean L-α-phosphatidylc

Full text of DOI:10.1142/S1088424610002008

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2010; 14: 278–283
DOI: 10.1142/S1088424610002008
Published at http://www.worldscinet.com/jpp/
Photophysical properties of microencapsulated
phthalocyanines
Virginia E. Diza, Gabriela A. Gaunab, Cristian A. Strasserta,b, Josefina Awruchb◊ and
Lelia E. Dicelio*a
a INQUIMAE, Departamento de Química Inorgánica, Analítica y Química Física, Facultad de Ciencias Exactas y
Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón II, 1428 Buenos Aires, Argentina
b Departamento de Química Orgánica, Facultad de Farmacia y Bioquímica, Universidad de Buenos Aires, Junín 956,
1113 Buenos Aires, Argentina
Received 9 June 2009
Accepted 14 July 2009
ABSTRACT: Lipophilic substituted zinc(II) phthalocyanines: 2,3,9,10,16,17,23,24-octakis[(N,N-
dimethylaminoethylsulfanyl)]phthalocyaninatozinc(II) (S1) and tetrakis(N,N-dibutylaminoethoxy)
phthalocyaninatozinc(II) (3) were incorporated into soybean L-α-phosphatidylcholine (SPC)
liposomes of 100 nm diameter. Liposomes were characterized by static light scattering (SLS),
transmission electronic microscopy (TEM), and differential scanning calorimetry. The fluorescence
quantum yield and singlet molecular oxygen production of 3 are ΦF = 0.15 and Φ∆ =0.24, respectively,
whereas the same photophysical parameters for S1 are ΦF = 0.13 and Φ∆ = 0.51. Higher values of
ΦF and Φ∆ are obtained in organic solvent for both dyes. Synthesis of compound 3 has also been
reported.
KEYWORDS: zinc(II) phthalocyanine, liposomes, fluorescence, singlet oxygen.
A great many investigations have shown the advantages
INTRODUCTION
of liposomal preparations over other formulations, such
Photodynamic therapy (PDT) is a clinically accepted
as simple aqueous dispersions of the drugs [4]. One of
treatment [1] which requires photosensitizers with
the expectations when incorporating photosensitizers into
absorption maxima above 600 nm and molar extinction
liposomes is to maintain the photosensitizer monomeric
coefficients ranging from 103 to 105 M-1.cm-1. Excitation
state since monomers are known to increase the oxygen
of the photosensitizer with light of an appropriate wave-
consumption and to be more efficient at photosensitizing
length leads to the formation of activated oxygen species
than aggregate species [5, 6].
capable of causing tumor destruction [2]. Phthalocya-
The aim of this project is to study the physicochemi-
nines have interesting photophysical and photochemical
cal characterization of two novel lipophilic zinc(II)
properties for utilization in PDT [3]. However, most of
phthalocyaninates incorporated into liposomes of soy-
these dyes are hydrophobic and solubility under physi-
ological conditions is a significant problem for intrave-
nous PDT drug delivery in vivo [4]. Therefore, liposomal
preparations are currently used as an effective deliv-
bean L-α-phosphatidylcholine as delivery drug carriers.
Particle size distribution and morphology were analyzed.
The photophysical properties of the 2,3,9,10,16,17,
23,24-octakis[(N,N-dimethylaminoethylsulfanyl)]
ery system in experimental studies and clinical trials.
phthalocyaninatozinc(II) (S1) and tetrakis(N,N-dibutyl-
aminoethoxy)phthalocyaninatozinc(II) (3) incorpo-
rated into liposomes and in homogeneous media were
SPP full member in good standing
analyzed. Results were evaluated in order to apply these
systems for photobiological purposes.
*Correspondence to: Lelia E. Dicelio, email: led@qi.fcen.uba.
ar, tel: +54 11-4576-3378/3380 (106), fax: +54 11-4576-3341
Copyright © 2010 World Scientific Publishing Company

PhOtOPhySICal PrOPertIeS Of mICrOenCaPSulateD PhthalOCyanIneS
279
Static light scattering (SLS) experiments were conducted
with an SLS 90 Plus/BI-MAS (MultiAngle Particle Siz-
ing Option) equipped with He-Ne laser operating at 632.8
nm, 15 mW. Transmission electron microscopy (TEM)
EXPERIMENTAL
Chemicals
liposome image were obtained by means of a Philips EM
301 electron microscope. Differential scanning calorim-
etry was performed with a Shimadzu DSC-50.
All reagents for organic synthesis were purchased
from Merck or Aldrich and used without further purifica-
tion. Chromatography columns were prepared with TLC
Kieselgel (Merck) and Aluminium Oxide 90 standard-
ized (Merck). N,N-dimethylformamide was dried over
3 Å molecular sieves during 72 h, filtered and freshly
distilled before utilization [7].
Tetra-t-butylphthalocyaninatozinc(II) was synthesized
in our lab [8], and 2,3,9,10,16,17,23,24-octakis[(N, N-
dimethylamino)ethylsulfanyl]phthalocyaninatozinc(II)
(S1) was prepared by the method described in the litera-
ture with some minor modifications (Fig. 1) [9].
Tetrahydrofuran (THF), toluene and anhydrous
chloroform were obtained from Sigma-Aldrich. Meth-
ylene Blue (Fluka p.a.) was used as supplied. L-α-
phosphatidylcholine from soybean (SPC), 99% (TLC)
lyophilized powder, buffer Tris (hydroxymethyl amino-
methano pKa = 8.06), Sephadex G50, N,N-diethyl-
4-nitrosoaniline; 1,3-diphenylisobenzofuran (DPBF),
imidazol were purchased from Sigma-Aldrich. If not
specified hereinbefore, all chemicals were reagent grade
and used without further purification. Distilled water
treated in a Milli-Q system (Millipore) was used.
Synthesis
Dibutyl-[2-(3,4-diisocyano-phenoxy)-ethyl]-amine
(2). A mixture of 4-nitrophthalonitrile (1) (0.100 g, 0.58
mmol), 2-dibutylamino-ethanol (0.4 mL, 1.92 mmol)
and K2CO3 (0.240 g, 1.74 mmol) in DMF (1.05 mL) was
stirred under argon at room temperature for 24 hours.
The reaction mixture was poured into water (30 mL)
and extracted with CH2Cl2 (4 × 30 mL), the combined
extracts being washed with water (4 × 30 mL) and dried
over Na2SO4. After evaporation in vacuo, the residue was
dissolved in a small volume of CH2Cl2-methanol (95:5)
and filtered through a silica-gel column packed and pre-
washed with the same solvent. After evaporation of the
solvent, an oil was obtained. Yield: 0.135 g (78%). IR
(KBr): ν, cm-1 2957, 2930, 2860, 2231(CN), 1599, 1561,
1492, 1466, 1377, 1321, 1254, 1170, 1089, 1025, 880,
1
835, 732, 523. H NMR (300 MHz, CDCl3): δΗ, ppm
0.95 (6H, t, -CH3), 1.30 (4H, m, -CH2-CH3), 1.68 (4H,
m, -CH2-CH2-CH3), 2.60 (4H, t, -N-CH2-), 2.93 (2H,
t, -CH2-N-), 4.34 (2H, t, -O-CH2-), 6.99 (1H, s, Ar),
7.32 (1H, d, Ar), 7.71 (1H, d, Ar). MS (ESI-TOF): m/z
300.3000 [M + H]+, calcd. for C18H25N3O [M]+ 299.1992.
Anal. calcd. for C18H25N3O: C, 72.21; H, 8.42; N, 14.03.
Found: C, 72.11; H, 8.39; N, 13.08.
Instrumentation
The 1H NMR of 2 and phthalocyanine 3 were recorded
on a Bruker MSL 300 spectrometer and on a Bruker
AM 500, respectively. ESI-TOF mass spectroscopy was
employed for the characterization of compounds 2–3.
Infrared spectra were obtained with a Perkin Elmer Spec-
trum One FT-IR spectrometer. Microanalyses were per-
formed with a Carlo Erba EA 1108 elemental analyzer.
Electronic absorption spectra were determined with a
Shimadzu UV-3101 PC spectrophotometer. Fluorescence
spectra were monitored with a QuantaMaster Model
QM-1 PTI spectrofluorometer. pH measurements were
obtained by means of a Termophmeter Altronix TPX-1.
Tetrakis(N,N-dibutylaminoethoxy)phthalocy-
aninatozinc(II) (3). A mixture of 2 (0.030 g, 0.10 mmol),
anhydrous zinc acetate (0.030 g, 0.16 mmol), and
DBU (0.45 mL, 3.0 mmol) was stirred and heated at
158 °C under argon during 5 min. The mixture was
cooled down and 10 mL of CH2Cl2 was added, it was
then washed with water (3 × 10 mL) and evaporated
in vacuo. The blue-green solid residue was dissolved in a
small volume of CH2Cl2 and filtered through a Al2O3 col-
umn packed and pre-washed with the same solvent. After
washing with MeOH, the title compound was eluted
with CH2Cl2-MeOH (95:5). After evaporation in vacuo,
phthalocyanine 3 was obtained (0.020 g, 63% yield). IR
(KBr): ν, cm-1 2928, 2856, 1648, 1619, 1490, 1458, 1384,
1326, 1262, 1091, 803, 744. 1H NMR (500 MHz, CDCl3):
δΗ, ppm 0.89–0.92 (24H, t, -CH3), 1.25–1.30 (16H, br,
-CH2-CH3), 1.57–1.64 (16H, br, -CH2-CH2-CH3), 3.28–
3.38 (16H, m, -N-CH2-), 3.46–3.52 (8H, m, -CH2-N-),
3.66–3.70 (8H, m, -O-CH2-), 6.19 (4H, s, Ar), 6.27 (4H,
d, Ar), 6.31 (4H, d, Ar). MS (ESI-TOF): m/z 1261.7354
[M + H]+, calcd. for C72H100N12O4Zn [M]+ 1260.7281.
Sample preparation. Stock solutions of phthalocya-
nines were prepared in THF for S1 as it aggregates in
DMF [10] and in DMF for 3, stored at 4 °C and carefully
R
R
N
N
N
N
N
R
R
R
R
N
Zn
N
N
R= SCH2CH2N(CH3)2
R
R
Fig. 1. Chemical structure of phthalocyanine S1
Copyright © 2010 World Scientific Publishing Company
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280
V.e. DIz et al.
protected from ambient light. Dyes and carrier concen-
tration are indicated in each experiment.
for light scattering by subtracting the spectra from empty
liposomes. Spectroscopic experiments were recorded at
concentrations in a range of 1.2 × 10-7 M–1.52 × 10-6 M
for S1 and 3 (see Table 2).
Fluorescence quantum yields. Fluorescence quantum
yields (ΦF) were determined by comparison with those of
tetra-t-butylphthalocyaninatozinc(II) (ΦF = 0.30 in tolu-
ene) [8] as a reference at λexc = 610 nm for S1 in THF and
3 in DMF and liposomal media for both dyes. Calcula-
tions were made using Equation 1;
Preparation of unilamellar liposomes. Large unila-
mellar vesicles (LUV) were prepared as follows: an ali-
quot of lipid chloroform solution was evaporated at 40 °C
to form a thin film of lipid and subsequently hydrated
with Tris-HCl buffer, pH 7.4 (10 mM), and 0.9% (w/v)
sodium chloride. All excess chloroform was removed
during evaporation. The resulting multilamellar lipo-
somes (MLV) suspension was sonicated (3 cycles for 30
min with a bath-type sonicator Testlab 450 W, 40 kHz),
to obtain an LUV suspension.
Basically, the above steps were followed to incorpo-
rate phthalocyanines into LUV. Both phthalocyanines
were solubilized in THF and added to an SPC-chloro-
form solution (8.45 × 10-4 mg S1/g SPC and 8.03 × 10-4
mg 3/g SPC).
R
2
IS 1−10− A


S






n
n
(1)
Φ
FS
= ΦFR
S
R
IR 1−10− A
where R and S superscripts refer to the reference and the
sample, respectively; I is the integrated area under the
emission spectrum, A is the absorbance of solutions at
the excitation wavelength, and (nS/nR)2 stands for the
refractive index correction.
Quantum yield of singlet oxygen production. The
quantum yield of singlet oxygen generation rates was
calculated by means of standard chemical monitor
bleaching rates [12]. DPBF was used as a singlet oxygen
chemical quencher [13] for Φ∆ studies in DMF and imi-
dazol (8 mM) and N,N-diethyl-4-nitrosoaniline (40–50
µM) in Tris buffer was utilized for liposomal phtha-
locyanines [14]. To prevent chain reactions induced by
DPBF in the presence of singlet oxygen, absorbance of
DPBF was under 1.9. DPBF decay was monitored at 410
nm, whereas N,N-diethyl-4-nitrosoaniline decay was at
440 nm.
Non-incorporated S1 and 3 were separated from
LUV-S1 and LUV-3, respectively, by gel-permeation
chromatography in a Sephadex G-50 column using the
mini-column centrifugation method [11].
Static Light Scattering measurements (SLS)
SLS was used to determine the average size of lipo-
somes. The scattering was detected at an angle of 90º. All
measurementswereperformedat25°Cand90°C.Measure-
ments were made employing different liposomal-buffers
Tris concentrations in order to obtain a more reproduc-
ible unimodal population.
Experiments were performed without photosensitizer
to prevent fluorescence interference in the light scattering
signals.
Polychromatic irradiation was performed using a
projector lamp (Philips 7748SEHJ, 24 V-250 W), and
a cut-off filter at 610 nm (Schott, RG 610) and a water
filter were utilized to prevent infrared radiation. Lipo-
somal samples of S1, 3 and reference (methylene blue:
Φ∆ = 0.56 in buffer) [12] and 3 and reference (S1:
Φ∆ = 0.69 in DMF) [10] were irradiated within the same
wavelength interval λ1–λ2, and Φ∆ was calculated accord-
ing to Equation 2;
Transmission Electronic Microscopy (TEM)
Negative staining electron microscopy images of lipo-
somes upon uranyl acetate staining were obtained with a
TEM Jeon 1210, 120 kV, equipped with an EDS analyzer
LINK QX 2000.
Differential scanning calorimetry
Temperature of phase transitions (Tm) and associated
change of enthalpy (∆H) of liposomes were determined
by differential scanning calorimetry, from -50 °C to 30 °C
at 10 °C/min rate.
λ2
R
I0 λ1−10− A λ dλ
rS
rR
∫
∫
λ1
λ2
Φ∆S = Φ∆R
(2)
S
I0  λ 1−10− A λ dλ
λ1
Photophysical parameters
where r is the singlet oxygen photogeneration rate and
the superscripts S and R stand for the sample and refer-
ence, respectively, A is the absorbance at the irradiation
wavelength, and I0(λ) is the incident spectral photon
flow (mol. s1. nm-1). When the irradiation wavelength
range is narrow, incident intensity varies smoothly
with the wavelength and the sample and reference have
overlapping spectra, I0 can be approximated by a con-
stant value which may be drawn out of the integrals and
cancelled.
Spectroscopic studies. Absorption and emission
spectra were recorded at different concentrations with a
10 × 10 mm quartz cuvette. All experiments were per-
formed at room temperature. Emission spectra of S1 and
3 were collected at an excitation wavelength of 610 nm
(Q band) and recorded between 630 and 800 nm; a cut-off
filter was used to prevent the excitation beam from reach-
ing the detector (Schott RG 630). Emission and absorp-
tion spectra of liposomal phthalocyanines were corrected
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PhOtOPhySICal PrOPertIeS Of mICrOenCaPSulateD PhthalOCyanIneS
281
R
spherical-shaped vesicles. In full
agreement with light scattering
size measurements, liposomes pre-
sented a uniform size of 100 nm
diameter.
N
N
R
R
N
NC
NC
NC
NC
N
N
N
Zn
N
NO2
R
Calorimetric measurements
N
2
1
Differential scanning calorim-
etry was availed to determine the
effect of the incorporation of S1
and 3 in the thermotropic behavior
of the liposomal bilayers. Thermo-
tropic profiles of aqueous suspen-
R
2, 3: R=OCH2CH2N(CH2CH2CH2CH3)2
3
Scheme 1. Synthetic route of phthalocyanine 3
sions of all liposomal bilayers show two endothermic
processes: one low peak between -24 and -18 °C, with a
maximum at -22 °C, and a second higher peak between
-3 and 0 °C, corresponding to fusion of aqueous medium
and other (Fig. 3). The transition temperature obtained
showed no significant differences due to incorporation
of both phthalocyanines in comparison with empty lipo-
somes. Thermodynamic parameters corresponding to
each thermogram are shown in Table 1. The incorpora-
tion of phthalocyanines into liposomes decreased the
phase transition enthalpy. These results indicate that
hydrophobic dyes, as expected, were incorporated into
the bilayers, therefore producing great changes in the
bilayer organization [15]. It should be noted that incorpo-
ration of 3 leads to a decrease in phase transition enthalpy
greater than that originated from the addition of S1 in
the liposomal system. The change in the lipidic bilayer
organization is greater for 3 than for S1; although both
RESULTS AND DISCUSSION
Synthesis
Tetrasubstituted phthalocyanine 3 was synthesized
as shown in Scheme 1. The reaction of 4-nitrophthalo-
nitrile with 2-dibutylamino-ethanol rendered precursor 2
in a 78% yield. Cyclotetramerization of phthalonitrile 2
employing 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)
and zinc acetate at 158 °C afforded phthalocyaninate 3 in
a 63% yield. When hexanol was employed as the reaction
solvent, compound 3 was obtained in a 42% yield whereas
the use of butanol as the solvent failed to obtain the desired
dye. Also reaction time was reduced from 30 min to 5 min
when the reaction was carried out without any solvent.
Intermediate 2 and dye 3 were characterized by IR and 1H
NMR spectroscopy while ESI-MS TOF spectroscopy was
employed for the characterization of both compounds.
Particle size distribution and morphology
The well-dispersed liposomes were characterized by
SLS and TEM. The average diameter for SLS studies
was 100 nm; a reproducible unimodal population of LUV
liposomes was obtained.
Figure 2 depicts electron micrographs of LUV
SPC liposomal samples, which appear as unilamellar
Fig. 2. Transmission electron microscopy following negative
staining
Fig. 3. Differential scanning calorimetry of empty and phthalo-
cyanines S1 and 3 into SPC liposomes
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282
V.e. DIz et al.
Table 1. Effect of incorporation of phthalocyanines S1 and 3 on
thermodynamic parameters of liposomes
in homogeneous media [8, 10, 16]. Spectra recorded at
[3] = 1.5 × 10-5 M–1.2 × 10-7 M show a linear behavior
obeying the Lambert-Beer equation, as only monomers
are present in the solution. The 3 absorption spectrum
shows characteristic Soret band centered around 346
nm, and Q band at a maximum of 675 nm and maximum
molar extinction coefficient of 6.2 × 104 M-1.cm-1.
The emission spectra do not depend on the excitation
wavelength; the excitation and absorption spectra coin-
cide and do not vary with concentration either in shape
or in wavelength. The same behavior for S1 in THF has
been reported elsewhere [10].
Sample
Tma, °C
∆Hcalb (J/g phospholipid)
Liposomes
-22.0
-21.3
-27.0
235.3
203.9
117.6
Liposomes-S1
Liposomes-3
a
Temperature corresponding to the maximum on the calori-
metric peak. b Calorimetric enthalpy calculated as area under
the curve.
Absorption spectra of 3 incorporated into SPC lipo-
somes show no relevant modifications in shape whereas
a red shift of 6 nm in wavelength was observed (Q band
λmax = 681 nm). The intensity ratio of the two bands of
S1 incorporated into liposomes correspond to the dimer
and monomer species [17]. This provides evidence that
S1 is aggregated, even at very low concentrations (i.e.
10-7 M); no significant wavelength shift of Q band mono-
mer was observed. The shape of S1 spectra in SPC
liposomes is the same as that of other zinc(II) phtha-
locyanines in organic solvents and in liposomes
[17]. The dimer spectrum shape in SPC lipo-
somes presents an intermediate behavior between
hydrophilic and lipophilic media, thus indicating
that the liposomal matrix does attenuate the abil-
ity of the bound phthalocyanine to interact with
the bulk aqueous phase (see band at 658 nm, Fig.
4a). This result agrees with that obtained in dif-
ferential scanning calorimetry experiments.
phthalocyanines are lipophilic, as the octasubstituted S1
molecule is bigger, it cannot be incorporated in the lipidic
bilayer as easily as tetrasubstituted phthalocyanine 3. S1
remains on the outside of the liposomes, thus leaving to
the formation of aggregates (Fig. 4)
Photophysical parameters
Spectroscopic results. The absorption spectrum of
3 in DMF in the 400–800 nm range is similar to those
previously reported for other zinc(II) phthalocyanines
The fluorescence emission spectra of phtha-
locyanines incorporated into SPC liposomes
are shown in Fig. 4. For both phthalocyanines,
in solution and in liposomes, neither significant
wavelength shifts nor relevant modifications in
spectral shape were observed with an increas-
ing dye concentration. In view of these results,
the fluorescence can be attributed to monomers
alone.
Phthalocyanine 3 fluorescence quantum yield
in homogeneous media (ΦF = 0.23) is similar to
that obtained for the monomer of tetrasubsti-
tuted zinc(II) phthalocyanines [8, 10, 16] since in
SPC liposomes ΦF value is 0.15, lower than that
obtained in organic solvents. A similar behavior
was observed for S1 (Table 2).
The difference in values in both media (organic
solvents and SPC liposomes) is due to the influ-
ence of two effects: the environment contribution
with a non-radiative process of electronic-to-
vibrational energy transfer, and the presence of
aggregates as in the case of S1 incorporated into
liposomes [5, 17, 18]. Singlet molecular oxygen
production for compound 3 is in the order of
Φ∆ = 0.60 in DMF whereas this value decreases to
Φ∆ = 0.24 in liposomal media.
Fig. 4. (a) Normalized absorption of S1 in THF (λmax = 704 nm) and in
liposomes (λmax = 658–704 nm); emission spectra of S1 (λexc = 610 nm)
in both media. (b) Normalized absorption of 3 in DMF (λmax = 675 nm)
and in liposomes (λmax = 681 nm); emission spectra of 3 (λexc = 610 nm)
in both media
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Table 2. Spectral and photophysical parameters of compounds S1 and 3
Compounda
ε
mon, M-1.cm-1
ΦF
Φ∆
λ
max, nm
S1
THF
704
1.1 × 105 b
—
6.2 × 104
—
0.26 0.05
0.13 0.08
0.69 0.08b
SPC liposomes
658–704
0.51 0.10
0.60 0.10
3
DMF
675
681
0.23 0.05
0.15 0.08
SPC liposomes
0.24 0.10
a Experiments were carried out in concentrations ranging between 1.2 × 10-7 and 1.5 × 10-6 M for S1 and 3.b [14].
4. Konan YN, Gurny R and Allémann E. J. Photo-
chem. Photobiol. B 2002; 66: 89–106, and refer-
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5. Rodriguez ME, Morán F, Bonansea A, Monetti M,
Fernández DA, Strassert CA, Rivarola V, Awruch J
and Dicelio LE. Photochem. Photobiol. Sci. 2003;
2: 988–994.
6. Damoiseau X, Schuitmaker HJ, Lagerberg JWM
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7. Burfield DR and Smithers RH. J. Org. Chem. 1978;
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CONCLUSION
In summary, we have characterized two lipophilic sub-
stituted zinc(II) phthalocyanines encapsulated into SPC
liposomes. The incorporation of zinc(II) phthalocyanines
S1 and 3 into liposomes result in a decrease in their Φ∆.
However, values obtained for both dyes seem to be suf-
ficiently high to use liposomal formulations of these pho-
tosensitizers to generate singlet molecular oxygen.
Acknowledgements
This work was supported by grants from the University
of BuenosAires, the Consejo Nacional de Investigaciones
Científicas y Técnicas (CONICET) and the Agencia
Nacional de Promoción Científica y Tecnológica. We
wish to thank the technical assistance as regards chro-
matography of Ms. Juana Alcira Valdez. The assistance
of Mr. Diego Cobice in ESI-TOF mass spectrometry is
also appreciated as well as language supervision by Prof.
Rex Davis.
8. Fernández DA, Awruch J and Dicelio LE. Photo-
chem. Photobiol. 1996; 63: 784–792.
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Supporting information
The NMR spectra and the ESI spectra of 3 are given
in the supplementary material. This material is available
free of charge via the Internet at http://www.worldscinet.
com/jpp/jpp.shtml.
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