Synthesis and photophysical studies of asymmetric zinc phthalocyanine-magnetic nanoparticle conjugates

Article abstract of DOI:10.1039/c7nj01716b

This work reports on the synthesis and amide bond linkage of carboxylic acid functionalized asymmetric zinc phthalocyanine (ZnPc) complexes to amino magnetic nanoparticles (AMNPs). The work further compares the photophysical and photochemical parameters o

Full text of DOI:10.1039/c7nj01716b

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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ARTICLE
Synthesis and Photophysical studies of asymmetric zinc
phthalocyanine - magnetic nanoparticles conjugates.
Gauta Gold Matlou, a Nagao Kobayashi, b* Mutsumi Kimurab and Tebello Nyokonga*
Received 00th January 20xx,
Accepted 00th January 20xx
This work reports on the synthesis and amide bond linkage of carboxylic acid functionalized asymmetric zinc
phthalocyanine (ZnPc) complexes to amino magnetic nanoparticles (AMNPs). The work further compares the
photophysical and photochemical parameters of the Pc complexes alone with the linked to form ZnPc-AMNPs with further
relation to the type of the spacer between the Pc and the AMNPs. Infrared spectroscopy confirmed the presence of the
amide bond formed between Pc complexes and AMNPs. Triplet quantum yields ranged from 0.62 to 0.87. However, low
singlet oxygen quantum yields were obtained due to competing pathways and the insufficient energy transfer from the
excited triplet state of the ZnPc molecules to the molecular oxygen.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Introduction
This work reports on the synthesis of carboxylic functionalized
Metalphthalocyanines (MPcs) are versatile macrocycles that
have attracted attention for various applications including in
mono zinc phthalocyanine complexes and their conjugation to
amino functionalized MNPs (AMNPs). Zinc phthalocyanines
(ZnPc) possess high triplet and singlet oxygen quantum yields
photodynamic
antimicrobial
therapy
(PDT)
[
1,2],
photodynamic
],
chemotherapy
(PACT)
[
3
[
16], hence ZnPc derivatives are employed in this work. The
complexes employed in this work are shown in Fig. 1.
Complexes and are reported in this work for the first time
whereas the synthesis of complex has been reported before
17]. The photophysical properties of conjugates of porphyrin
photoelectrochemistry and photovoltaics [4,5]. Depending on
the central metal, MPcs are efficient at generating singlet
5
6
oxygen
[6] which is essential for their application as
7
photosensitizers in applications such as in PACT and PDT.
[
(similar in structure to Pcs) with MNPs are affected by the
length of the spacer between the two [18], hence we compare
the effect of the extra double bond between the Pc and COOH
Studies on the conjugates between MPcs and nanoparticles
(NPs) are on the increase. The NPs improve the photophysical
parameters of the phthalocyanine complexes [7,8]. Iron oxide
(Fe2O3) magnetic nanoparticles (MNPs) have attracted
attention in biology and engineering due to their
biocompatibility, biodegradability and ease of functionalization
groups by comparing complexes
the effect of tert-butyl groups by comparing the photophysical
behaviour of with . Asymmetric phthalocyanines offer a
6 and 7. We further compare
5
7
more defined coordination to the nanoparticles because of
their one arm of attachment, this is the reason why this study
focuses on using asymmetric phthalocyanine. In addition,
previous reports have shown that asymmetry improves the
[9]. MPcs have been linked to Fe3O4 MNPs for photocatalytic
applications or photophysical studies [10–15]. MNPs are also
attractive because they encourage intersystem crossing (ISC)
to the triplet sate of the Pc, through the external heavy atom
effect of the metals contained in the MNPs. This enhances the
singlet oxygen generating ability of the Pc [13,14], which is
fundamental to applications of Pcs as photosensitizers. In
addition, the use of MNPs allows for magnetic retrieval of the
photocatalyst following photocatalysis [15].
photophysical properties of porphyrin-type complexes
The bulky tert-butyl substituents for help in improving
solubility and reducing aggregation. The COOH group for
and will be used to link the Pcs to AMNPs.
[19].
5
5, 6
7
Experimental
Materials
Zinc chloride, zinc acetate (Zn(OAc)2), dimethyl sulfoxide
(DMSO) and dimethyl formamide (DMF), 1,3-
diphenylisobenzofuran (DPBF), 1-pentanol, p-coumaric acid
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2b), N, N'-dicyclohexylcarbodiimide (DCC) and N-
Journal Name
(
Q band. The solution was then introduced into a
hydroxysuccinimide (NHS), and 1.2 dicyanobenzene (4b), were spectrophotometer cell of 1 cm path length and de-aerated
purchased from Sigma–Aldrich. Methyl 4-hydroxybenzoate using argon for 15 min. The complexes and conjugates were
(
2a),
tertbutylphthalonitrile
chemicals Tetrahydrofuran (THF) was purchased from by the MNPs at this wavelength.
MINEMA. 4-Nitro phthalonitrile ( ) was synthesized according Photocatalysis for singlet oxygen quantum yield studies were
to the method published in literature [20]. The syntheses of carried out using irradiation from a halogen lamp (300 W), 600
dimethylaminoethanol
(DMAE)
and
4- excited at the crossover wavelength of the Q band maxima of
(
4a), were purchased from Wako both the standard and the samples, and there is low scattering
.
1
amino functionalized magnetic nanoparticles (AMNPs) [21
and zinc mono carboxyphenoxy phthalocyanine (ZnMCPPc) (
17] have been reported.
]
nm glass (Schott) and water were used to filter off ultra-violet
and far infrared radiation respectively. An interference filter
(Intor, 670 nm with bandwidth of 40 nm) was placed in the
light path just before the sample. Light intensities were
measured with a POWER MAX 5100 (Molelectron detector
Incorporated) power meter and were found to be 9.42 x 1018
7)
[
Equipment
The ground state electronic absorption was measured using photons/cm2 s for singlet oxygen studies. For singlet oxygen
Shimadzu UV-2550 spectrophotometer. Fluorescence quantum yield determinations, samples or ZnPc standard at an
excitation and emission spectra were collected on a Varian absorbance of ~ 1.5 at the Q band were mixed with DPBF at a
Eclipse spectrofluorometer using a 360-1100 nm filter. The ratio of 1: 1 and the rate of degradation of the DPBF
excitation spectra were measured using the Q-band of the photobleaching agent for both the sample and the standard
emission maxima. Time Correlated Single Photon Counting were used to calculate the singlet oxygen quantum yields.
(TCSPC) setup (FluoTime 200, Picoquant GmbH) was used for
the fluorescence decay studies. The excitation source was a
Synthesis
diode laser (LDH-P-670 driven by PDL 800-B, 670 nm, 20 MHz
repetition rate, 44 ps pulse width, Picoquant GmbH).
Phthalonitriles (3a and 3b)
Bruker Alpha FT-IR spectrophotometer with universal
Scheme 1 illustrates the synthesis of 3a and 3b by base
attenuated total reflectance (ATR) was used to measure the
catalysed nucleophilic nitro displacement of nitro group from
FT-IR spectra. Bruker Autoflex III smartbeam TOF/TOF Mass
4-nitrophthalonitrile (1) with hydroxyl containing substituents
spectrophotometer was used to collect mass spectra data
using α-cyano-4-hydrocinnamic acid as the matrix in the
positive ion mode. Elemental analysis (CHNS microanalysis)
was recorded using the Vario-Elementar Microcube ELIII.
Bruker AMX 300 and Bruker AVANCE 600 MHz Hz nuclear
magnetic resonance (NMR) spectrometers were used to
measure the 1H NMR spectra.
(using 2a and 2b, respectively). Both reactions were carried
out at room temperature under inert gas with constant
stirring. Typically, for the synthesis of compound 3a, (Scheme
1, route (a))
(0.017 mol) of 2a and 3.52 (0.026 mol) of dry K2CO3 in 20 mL of
dry DMSO for 48 h to yield 3.58 g (75% yield) of compound 3a
Compound 3b, (Scheme 1, route (b)) was synthesized by
reacting 2.01 g (0.011 mol) of with 1.89 g (0.011 mol) of 2b
and 1.50 g (0.010 mol) of dry K2CO3 in 20 mL of dry DMSO for
48 h to yield 2.18 g (67% yield) of 3b 3a was washed and
, 3.0 g (0.017 mol) of 1 was reacted with 2.64 g
.
Transmission electron microscope (TEM) images were
obtained using a JEOL TEM 1210 transmission electron micro-
scope at 100 kV accelerating voltage. TEM samples were
prepared on the sample grid and allowed to dry before
measurements. X-ray powder diffraction (XRD) patterns were
recorded on a Bruker D8 Discover equipped with a Lynx Eye
detector, using Cu K α-radiation (λ = 1.5405 Å, nickel filter).
The XRD data were fitted using Eva (evaluation curve fitting)
software. Baseline corrections were performed on each
diffraction pattern. Dynamic light scattering (DLS) experiments
were conducted on a Malvern Zetasizer Nanoseries, Nano-
ZS90.
1
.
precipitated out of the reaction mixture by propanol and
finally recrystallized twice with hot methanol. 3b was
precipitated using ice water containing a few drops of
hydrochloric acid. The precipitate was filtered, washed with
water and recrystallized twice with methanol.
3a, Yield = 75%. IR (ATR): 2230 cm-1 (C=N), 2959, 3079 cm-1 (C-
1
H), 1710 cm-1 (C=O), 1240 cm-1 (Ar-O-Ar). H NMR (300 MHz,
CDCl3) δ 7.71 (d, J = 8.7 Hz, 2H), 7.42 – 7.11 (m, 4H), 6.88 (s,
1H), 2.11 (s, 3H). Calcd for C16H10N2O3: C (69.06); H (3.62); N
(10.07); Found C (68.34); H (3.05); N (11.02).
Triplet quantum yields were determined using laser flash
photolysis system. The excitation pulses were produced using
a tuneable laser system consisting of an Nd: YAG laser (355
nm, 135 mJ/4-6 ns) pumping an optical parametric oscillator
(OPO, 30 mJ/ 3-5 ns) with a wavelength range of 420-2300 nm
(NT-342B, Ekspla). Triplet lifetimes were determined by
exponential fitting of the kinetic curve using OriginPro 8
software. For laser flash photolysis studies, the samples and
the ZnPc standard solutions had an absorbance of ~ 1.5 at the
3b, Yield: 67 %. FTIR (ATR), cm-1. 3302 (Carboxylic OH), 3060
(Ar-H), 2232 (C=N), 1705 (C=O), 1622 (C=C), 1589 (Ar-C=C),
1
1253 (Ar-O- Ar). H NMR (300 MHz, DMSO) δ 12.46 (s, 1H, -
OH), 7.81 (d, J = 1.0 Hz, 2H), 7.48 (dd, J = 8.05, 3.24 Hz, 4H, Ar-
H), 7.42 (d, J = 1.0 Hz, 2H, Ar-H), 7.21 (s, 1H, Ar-H). Calcd for
C17H10N2O3: C (70.34), H (3.47), N (9.65); Found C (69.55), H
(3.63), N (9.22).
2 | J. Name., 2012, 00, 1-3
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Synthesis of asymmetric phthalocyanines
Scheme
1 illustrates the synthesis of asymmetrically
substituted Zinc phthalocyanine derivatives. Mixed
NMR (600 MHz, DMSO) δ 9.10 (s, 1H, -OH), 8.29 – 8.16 (m, 8H,
Pc-Ar), 7.97 (s, 3H, Pc-Ar), 7.81 (s. 1H, Pc-Ar), 7.53 (dd, J = 8.3,
2.2 Hz, 4H, subs-Ar), 1.34 (s, 27H, -CH3). MALDI-TOF MS (m/z):
Calcd 930.46: Found 931 [M + 1H] +. Calcd for C51H44N8O3Zn: C
(69.42); H (5.03); N (12.70); Found C (68.83); H (5.46); N
(11.22).
condensation reaction involving two phthalonitriles (4a and 3a
for
(b)) was used to achieve the desired asymmetrical zinc
phthalocyanine derivative. In a typical synthesis of 5a and
5(Scheme 1, route (a)); or 4b and 3b for 6 (Scheme 1, route
5,
compound 4a (0.460 g, 0.0025 mol) was reacted with 3a (0.440
g, 0.0015 mmol) in a glass tube reactor containing dry dimethyl
amino ethanol (DMAE) (1 mL) and dry zinc acetate (0.15 g,
0.008 mol). The reactions were carried out at 160 °C in an
argon atmosphere for 7 h with constant stirring to yield 62.28
mg (11%) of 5a after multiple column chromatography
separation from mixed products using silica gel as stationary
phase and 2% methanol in dichloromethane as eluting solvent.
Hydrolysis of 5a to yield mono functionalized carboxylic acid Pc
6, Yield = 17 %, UV/Vis (DMSO) λmax / nm (Log ε): 675 (5.54),
608 (4.77), 352 (5.01). IR (ATR): 3290 (Carboxylic O-H), 3090
(Ar-H), 1712 (Carboxylic C=O), 1475 (Ar-C=C), 1661 (C=C), 1233
(C-N). 1H NMR (600 MHz, DMSO) δ 12.45 (s, 1H, OH), 8.13 (d, J
= 8.8 Hz, 2H, Pc-Ar), 7.91 – 7.77 (m, 8H, Pc-Ar), 7.62 (d, J = 16.0
Hz, 2H, Pc-Ar), 7.50 – 7.43 (m, 3H, Pc-Ar), 7.22 (dd, J = 8.7,4.3
Hz, 4H, Subs-Ar), 6.57 (s, 1H, -C=C), 6.51 (s, 1H, -C=C). MALDI-
(
5
) was achieved by reacting 5a (50 mg) with 3 M KOH in 2 mL
dry tetrahydrofuran and 2 mL of methanol for 24 h at 70 °C.
The product ( ) was then precipitated and filtered out of the
reaction mixture by adding few drops of acetic acid and excess
amounts of water to yield 26 mg (52%) of . The synthesis of
+
TOF MS (m/z): Calcd: 740, Found: 742 [M + 2H] . Calcd for
C41H22N8O3Zn: C (66.54), H (3.00), N (15.14): Found C (65.48), H
(3.13), N (14.56).
5
5
6
was also carried out by using mixed condensation of two
phthalonitriles as follows: 4b (0.320 g, 2.5 mmol) and 3b
(0.435 g, 1.5 mmol) were mixed with zinc chloride (0.20 g,1.5
mmol) in 2 ml 1-pentanol. The reaction was carried at 160 °C
for 7 h catalysed with few drops of DBU. The desired product
Conjugation of Pc complexes to magnetic nanoparticles
Typically,
(20 mg, 0.028 mmol) were reacted with DCC (4.74 mg, 0.023
mmol for , 8.66 mg, 0.042 mmol for , 16.09 mg, 0.078 mmol
for ) and NHS (2.55 mg, 0.023 mmol for , 4.84 mg, 0.042
mmol for , 3.22 mg, 0.028 mmol for ) in dry DMF (2 mL) for
5 (20 mg, 0.023 mmol), 6 (20 mg, 0.027 mmol) and 7
(6) was separated from the mixture by extensive column
chromatography with silica stationery phase and 5 % methanol
in THF as the eluting solvent.
5
6
7
5
6
7
5a, Yield = 11%. UV/Vis (DMSO) λmax / nm (Log ε): 676(5.02),
610(4.41), 351(4.73). IR (ATR): 2963 cm-1 (C-H), 1711 cm-1
(C=O), 1493 cm-1 (Ar-C=C), 1242 cm-1 (C-N). 1H NMR (600 MHz,
DMSO) δ 8.31 (d, J = 2.0 Hz, 6H, Pc-Ar), 8.02 (s, 1H, Pc-Ar), 7.71
(s, 3H, Pc-Ar), 7.62 (d, J=2.8 Hz, 2H, subs-Ar), 7.32 (d, J = 1.8 Hz,
2H, subs-Ar), 7.29 (d, J = 2.3 Hz, 2H, Pc-Ar), 3.75 (s, 3H, -OCH3),
1.35 (s, 27H, -CH3). MALDI-TOF MS (m/z): Calcd 944.49; Found
940 [M - 4H]-. Calcd for C52H46N8O3Zn: C (69.68); H (5.17); N
(12.50); Found C (69.31); H (5.05); N (13.30).
48 h to activate the carboxylic group on the Pc complex. After
48 h, AMNPs (20 mg for all complexes) in dry DMF (2 mL) were
added and the mixture was left stirring for a further 72 h to
allow formation of the amide bond between NH2 groups of the
magnetic nanoparticles and the phthalocyanine carboxylic
group. The conjugates were purified from the reaction mixture
by centrifuging with DMF, twice with water, twice with
methanol and twice with ethanol.
Photophysical studies and Photochemical Parameters
5
, Yield = 52%. UV/Vis (DMSO) λmax / nm (Log ε): 677 (4.71),
611 (4.32),352 (3.99).IR (ATR): 3310 cm-1 (OH), 2961 cm-1 (C-
H), 1708 cm-1 (C=O), 1497 cm-1 (Ar-C=C), 1236 cm-1 (C-N).1H
The fluorescence (
quantum yields of
Φ
ꢀ), singlet oxygen (
5
Φ
△) and triplet (ΦT
-AMNPs, -AMNPs and 7-AMNPs
)
5,
6,
7,
6
were determined using comparative methods reported in
literature [22–25]. Unsubstituted ZnPc dissolved in DMSO was
F =0.2 [23] , Φꢂꢁꢃꢄ = 0.65 [24] and
used as reference standard (
Φ△ꢅꢃꢄ = 0.67 in DMSO [25]).
Φ
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appearance of the broad hydroxyl stretching band at 3310 cm-1
for , indicated the successful hydrolysis of 5a to . Also, Peaks
at 3290 cm-1 for , together with the 3310 cm-1 for
are
Results and Discussions
5
5
6
5
Synthesis and characterization
indicative of the presence of the carboxylic hydroxyl groups,
see Fig. 2. The carbonyl stretching bands were observed
The synthesis of the phthalonitrile complexes 3a and 3b is
illustrated on Scheme 1. The infrared spectra of compound 3a
and 3b indicated the presence of the nitrile (C=N) peaks at
2230 and 2232 cm-1, respectively. The carbonyl peaks (C=O)
were observed at 1710 cm-1 for 3a and 1705 cm-1 for 3b. Ar-O-
between 1708-1712 cm-1 for all complexes (5a
,
5
and
6). The
Pc core IR peaks were indicated by the aromatic carbon-carbon
(Ar-C=C) stretching bands at 1493 cm-1
(
5a), 1497 cm-1
) together with the carbon-nitrogen (C-N) peaks at
5a), 1236 cm-1 ) and 1233 cm-1
). The successful
(5) and
1475 cm-1
1242 cm-1
(
6
Ar stretch at 1240 cm-1 3a) and 1253 cm-1
( (3b) results from
(
(
5
(6
the ether bond between the two benzene rings and confirms
the structures of 3a and 3b. The 1H NMR spectra for
compounds 3a and 3b gave signals between 6.88 ppm to 7.81
ppm resulting from the two aromatic rings. Complex 3a had a
multiplet (7.42 – 7.11 ppm) which integrated into 4 protons
from the phenoxy ring, a doublet at 7.71 ppm and a singlet at
6.88 ppm which integrated into two protons and a single
proton, respectively, which completes the phthalonitrile
phenyl ring. The methyl protons gave a singlet peak at 2.11
which integrated into 3 protons. 3b had a broad singlet peak at
12.46 ppm representative of the carboxylic hydroxyl peak
which integrated into a singlet proton. The cinnamic acid
phenyl ring gave a doublet of a doublet at 7.48 ppm which
integrated into 4 protons, while the unsaturated carbons gave
a doublet peak at 7.42 ppm, which integrated to give 2 protons
completing the protons on the cinnamic acid substituent. The
phthalonitrile phenyl ring gave doublet peak at 7.81 ppm that
integrated into 2 protons and a singlet at 7.21 ppm which
integrated into a single proton which completes the protons of
the second ring for 3b. The elemental analysis of both the
phthalonitrile gave the values in agreement with analytically
calculated values.
synthesis of the Pc complexes was also confirmed by an
absence of C=N peaks between 2232 and 2240 cm-1 which
were present in the phthalonitriles (precursors).
1H NMR of the 5a and
appearance of the broad singlet peak at 9.10 ppm for
is absent in 5a, thus confirms the presence of hydroxyl groups
and the successful hydrolysis of 5a to . The Pc-core aromatic
5
complexes varied only by the
which
5
5
protons of 5a were between 7.29 ppm and 8.31 ppm all
integrating to 12 protons while the other protons resulted
from the aromatic ring of the substituents: 7.32 ppm, (2
protons) and 7.62 ppm (2 protons), the methoxy group (3.75
ppm, 3 protons) and the tertiary butyl groups (1.35 ppm, 27
protons), all accounting to 46 protons contained on 5a
complex. 5, gave a broad singlet peak at 9.10 ppm which
confirms the presence of the carboxylic acid after hydrolysis of
5a. The aromatic signals of the complex were indicated by the
presence of a multiplet peak at 8.29 - 8.16 ppm, a singlet at
7.81 ppm and another singlet at 7.97 ppm which completes all
12 protons of the Pc core aromatic rings. The aromatic ring of
the substituent groups gave a doublet of doublets (at 7.53
ppm) which integrated to four protons that completes the
aromatic ring of the substituent. The tertiary butyl groups gave
a sharp singlet peak at 1.34 ppm which integrated to 27
A3B type ZnPc were synthesized in this work. Tertiary butyl
protons, thus completing the 44 protons of the
The H NMR spectrum for complex is provided in supporting
information Fig. S1 and showed expected peaks. The
elemental analysis of the complexes (5a and ) gave
5 complex.
phthalonitrile (4a) and 1.2 dicyanobenzene (4b) were used as
6
the A3 while
diiminoisoindoline of the Pc complex. Scheme 1 illustrates the
synthesis of 5a and and phthalocyanine derivatives. The
FTIR spectra of the complexes 5a and indicate the
unsaturated (C-H) carbon stretching bands at 2963 cm-1 (for
5a) and 2961 cm-1 (for
) from the tert-butyl substituents. The
(3a) and (3b) were used as the B
,
5
6
5
6
percentages that are in close to the analytically calculated
percentages.
5
5
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5
6
7
Figure 1. Structure comparison of the phthalocyanines to be used in this work
Conjugation of Pc complexes to AMNPs.
Scheme 2 illustrates the method used to covalently link the The FTIR spectra of the complexes 5, 6
, 7, 5-AMNPs, 6-AMNPs
Mono-Pc complexes ( 6 and 7) onto the surface of the and 7-AMNPs are illustrated in Fig. 2. The peaks of interest
5,
AMNPs through an amide bond using DCC and NHS as the which suggest the successful linkage of the AMNPs to the
coupling and stabilizing agents respectively. Asymmetric Pc asymmetric carboxylic containing phthalocyanine complexes
complexes are only able to link through one attachment side. can be seen at 1540 and 1640 cm-1 which represent an amide
The number of Pc molecules bonded to the AMNPs was (N-H) spike and an amide carbonyl carbon respectively. It must
determined following literature methods, but using absorption be noted that the carboxylic hydroxyl groups (at ~ 3000 cm-1)
instead of fluorescence [26]. This involves comparing the Q of
band absorbance intensity of the Pc in the conjugate with that the formation of the covalent bond and loss of water during
of the initial Pc before the conjugation. The ratio of AMNPs: Pc the coupling reaction to give -AMNPs, -AMNPs and
is then determined and listed in Table 1. Approximately the AMNPs. The observation of an amide bond in the infrared
5, 6 and 7 disappear after conjugation to AMNPs because of
5
6
7-
same number of Pcs are linked to each MNP.
spectra provided evidence for the successful linkage of the Pc
complexes to the AMNPs.
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4a
160 °C , 7 h
DMAE, Ar, Zn (OAC) 2
5a
3a
Route (a)
K 2CO 3, Ar, DMSO
Rt , 48 h
3M KOH/H 2O
70°c, MeOH, THF
2a
1
5
K 2CO 3, Ar, DMSO
Rt , 48 h
2b
Route (b)
4b
3b
160 °C, 7 h
ZnCl 2, Pentanol
6
Scheme 1 : Schematic illustration of the synthesis of complexes
5
(Route (a) and
6
(route (b) and their precursors.
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HOOC
OR1
OR2
HOOC
OR2
HOOC
5
6
7
1) DCC, NHS
Rt, 48 h, DMF
SiH-O
SiH-O
SiH-O
Si
Fe2O3
NH2
2) AMNPs, DMF
Rt,72 h
O
SiH-O
Fe2O3 SiH-O
Si
N
H
SiH-O
OR1
5-AMNP
O
SiH-O
O
SiH-O
Fe2O3 SiH-O
Si
N
H
Fe2O3 SiH-O
Si
N
H
SiH-O
SiH-O
OR2
OR2
6-AMNP
7-AMNP
N
N
N
N
N
N
N
Zn
N
R2=
N
N
Zn
R =
1
N
N
N
N
N
N
Scheme 2: Schematic illustration of the conjugation of AMNPs to carboxylic
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7-AMNPs
7
6
OH
6-AMNPs
1640
1540
AMNPs
5-AMNPs
5
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)
Figure 2: Infrared spectra of complexes 5, 6 and 7 with their respective conjugates 5-AMNPs, 6-AMNPs and 7-AMNPs.
TEM images of the nanoparticles are illustrated on Fig.S2 typical of phthalocyanines due to their amorphous nature [28].
(supporting information). The images reflect spherically After conjugation, the MNPs diffraction peaks were clearly
shaped nanoparticles. The magnetic nature of the particles observable on the pattern of 5-AMNPs,
makes them to be attracted to each other forming clusters, AMNPs. The XRD patterns of the conjugates (
Fig. S2. The AMNPs, Fig. S2A, had an average size of 13.89 nm, AMNPs and
6
-AMNPs and
7
6
-
-
5-AMNPs,
7
-AMNPs) also shows the Pc diffraction peak at 2θ
Table 1. The TEM images of the conjugates (Fig S2 B-D,) = 15°. The sizes of the AMNPs and their conjugates were
showed an increase in size together with the presence of sheet determined using a Debye-Scherrer equation 1 [29].
like structures which can be attributed to the Pc complexes
ꢈꢉ
linked on the surface of the nanoparticles. Interactions
between the Pcs on adjacent NPs via π-π stacking may occur
ꢆ ꢇ ꢊꢋꢌꢅꢍ
1
leading to aggregation. Pcs are known for their π-π stacking to
form H aggregates [27].
where
ꢆ
is the crystal size,
ꢎ
is the wavelength of the X-ray
source (λ =1.5405˚A, nickel filter),
equal to 0.9, is the full width at half maximum of the
diffraction peak and is the diffraction angle of the crystal
ꢏ is an empirical constant
The XRD spectroscopy was used to show the changes in the
crystal structure of the AMNPs upon linkage to the Pc
ꢐ
ꢑ
complexes. The XRD patterns of the complexes (
example of the ZnPc derivatives) and the conjugates (
AMNPs, -AMNPs, -AMNPS) are illustrated in Fig. 3. The
5 as an
orientation peak. The sizes of the AMNPs and their conjugates
are listed in Table 1. Upon conjugating to Pc-complexes, the
crystal size of the AMNPs increases because of the Pc
incorporated on the surface of the nanoparticle, due to
aggregation as stated above.
5
-
6
7
AMNPs have a face centred cubic structure with diffraction
peaks at 2θ = 30°, 36°,43°, 54°,57° and 63°, corresponding to
hkl Milner indices of 220, 311, 400, 422, 511 and 440,
respectively. The broad peak near 2θ = 15° for Pc alone is
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311
220
400
511
440
5
5x103
4x103
3x103
2x103
5-AMNPs
7-AMNPs
AMNPs
1x103
0
6-AMNPs
10
20
30
40
50
60
70
80
90
2 (θ)
Figure 3: X-ray diffraction patterns of 5, 5-AMNPs, 7-AMNPs, AMNPs, and 6-AMNPs.
Figure 4: DLS graphs showing the average size distributions of A) AMNPs, B)
5-AMNPs, C)
6-AMNPs and D)
7
-AMNPs.
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2.0
1.5
1.2
1.0
0.8
0.6
0.4
0.2
1.2
6-AMNPs Exc
6-AMNPs EMs
6-AMNPs Abs
1.0
0.8
0.6
0.4
0.2
0.0
5-AMNPs
7-AMNPs
1.0
AMNPs
5
6-AMNPs
0.5
7
6
0.0
300
400
500
600
700
800
0.0
650
700
750
800
Wavelength (nm)
Wavelength (nm)
Figure 5: Electronic absorption spectra of the complexes 5,
6, 7, AMNPs, 5-AMNPs, 6-AMNPs and 7-AMNPs in DMSO.
Figure 6: Absorption, emission and excitation spectra of the
6-AMNPs in DMSO. Abs = Absorption; Exc = excitation; Ems
= emission
Dynamic Light Scattering (DLS) is a very sensitive technique There was a blue shift for 5-AMNPs compared to 5 alone. The
that provides the sizes of the particles in suspension and it was slight blue shift upon conjugation compared to Pc alone could
used in this work to determine the size of the AMNPs alone as be attributed to the electron deficiency induced on the Pcs
well for Pc-AMNPs conjugates in water. The size distributions upon coordination with NPs as reported before [30]. The
of the nanoparticles obtained from DLS are illustrated in Fig. 4 emission and excitation spectra of the conjugates are shown in
and listed in Table 1. These results show an increase in size Fig. 6. The emission and excitation spectra of the conjugates
from 15.15 nm for AMNPS to 19.31 nm (
5-AMNPs), 17.46 nm were found to be mirror images of each other, Fig. 6. Slight
(
6-AMNPs), 18.51 nm (7-AMNPs)) of the nanoparticles after differences in maxima between the absorption and excitation
linking the Pc complexes on the nanoparticle surface.
spectra could be due to different equipment used.
The electronic absorption spectra of the complexes are shown
in Fig. 5. Absorption spectra which are typical of metallated
phthalocyanines are observed in DMSO, with single Q-bands
with maxima ranging from 674 to 677 nm (Table 1), thus there
is not much difference in the Q band maxima of the three Pcs.
Photophysical and Photochemical parameters
.
Fluorescence quantum yields (ΦF) and lifetimes (τF)
The AMNPs showed
a abroad absorption Fig. 5. The
broadening below 550 nm for the conjugates is due to the
presence of AMNPs showing scattering. Upon conjugation of
the Pc complexes to the AMNPs, there were no significant
Fluorescence quantum yields and lifetimes of the complexes
are summarized in Table 1. A typical decay curve is illustrated
on Fig. S3 (supporting information) for complex
example). The F values are slightly lower compared to that of
the ZnPc that has ΦF = 0.2 [23]. The ΦF value for decreased
significantly from 0.18 to 0.086 upon linking to AMNPs. A slight
decrease was observed for -AMNPs compared to . The
linkage of AMNPs to did not have much influence on the ΦF
of the conjugates compared to
5 (as an
changes in Q band maxima for
1.
6-AMNPs and 7-AMNPs, Table
Φ
6
5
5
7
7
alone. The presence of the
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heavy central metal (through the heavy atom effect)
accompanied by the paramagnetic nature of AMNPs is
expected to encourage intersystem crossing to the excited
triplet state, hence decreasing the ΦF values of the Pc
complexes [31] when linked to AMNPs as is case for
6 and 6-
AMNPs.
Table 1: The photophysical and photochemical parameters of ZnPc complexes together with their conjugates in DMSO.
Complex
Size (nm)
TEM
Size (d)
XRD
(nm)
-
Size
(nm)
DLS
-
Loading
Ratio
ꢒꢓꢔꢕ
(nm)
ΦF
± 0.02
ꢖꢗ (ꢘꢕ)
ΦT
± 0.05
ꢙꢚ
± 0.02
ꢖꢛ (ꢜꢕ)
AMNPs: Pc
-
5
6
-
-
677
675
0.15
0.18
3.03
3.06
0.86
0.73
0.59
0.48
235
258
-
-
-
7
-
-
-
-
-
674
-
0.16
-
3.14
-
0.62
-
0.56
-
199
-
AMNPs
5-AMNPs
13.89
16.20
15.35
17.45
15.15
19.31
1: 6
673
0.11
3.62
0.87
0.47
279
6-AMNPs
7-AMNPs
16.28
15.85
18.15
17.56
17.46
18.51
1: 5
1: 5
676
675
0.086
0.14
4.66
3.02
0.65
0.69
0.38
0.51
224
256
The fluorescence lifetimes were determined from the time 0. 86 yielding the highest and
correlated single photon counting (TCSPc) setup. All complexes Upon linking the Pc complexes to AMNPs, a decrease in ΦT
gave a single lifetime. Table 1 shows that fluorescence values for -AMNPs compared to is observed, while
lifetimes lengthen for and , upon conjugation to AMNPs, AMNPs had a slight increase in ΦT value. There was no
but shorten for . It is expected that fluorescence quantum significant change for -AMNPs. It is expected that when ΦF
yields and lifetimes should change in unison, the observed decreases as is the case with -AMNPs and -AMNPs
lengthening of lifetimes for complex and in the presence of compared to and ΦT would increase. But this is not the
the AMNPs may be due to the protection of the former by the case in Table 1. It is also expected that the heavy atom effect
latter. Complex seems to behave differently with a decrease and paramagnetic nature of AMNPs would significantly
Magnetic cores normally quench the fluorescence of fluorophores increase ΦT due to the heavy atom effect [31]. This is observed
covalent bonding of fluorophore such as porphyrin for -AMNPs compared to , Table 1, but there is a decrease of
-AMNPs compared to and no change for -AMNPs
32]. Hence the differences in the behaviour of compared to . The length of the spacer between the MNPs
phthalocyanine in the presence of AMNPs could be related to and porphyrins has been reported to affect photophysical
the different interaction depending on orientation and the parameters [18,21]. This could also be the case for -AMNPs
which has a different spacer between the Pc ring and the
AMNPs compared to -AMNPs and -AMNPs. A longer spacer
7 yielding the lowest at 0.62.
6
6
7-
5
6
7
5
5
6
5
6
5
6,
7
in τF
.
and
a
7
7
to MNPS via an appropriate spacer was reported to reduce the ΦT for
quenching
6
6
5
[
5
6
length of the spacer.
7
5
Triplet quantum yields and lifetimes
may reduce the interaction between the AMNPs and the Pc,
resulting in the lowering of the heavy atom effect. However,
this is not expected to quench the triplet quantum yields. MPc
A comparative method using ZnPc as a standard was employed
aggregates form through
π
-π
stacking may quench the excited
to determine the ΦT of Pc complexes (
when linked to AMNPs (5-AMNPs, 6-AMNPs and 7
5
,
6
and
7
) alone and
triplet state [27].
-AMNPs),
Table 1. The Pc complexes alone gave high ΦT values with
5 =
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Thus, we show in this study that the heavy atom effect as well as
the paramagnetic nature of the MNPs, do not always result in
increased triplet state quantum yields, and that factors such as
spacing between the Pc and MNPs, and aggregation take
precedence.
molecules in the excited triplet state have a longer τT thus
minimizing the efficiency of energy transfer to the molecular
oxygen as indicated by the low Φꢀ of the conjugated ZnPc
complexes. It is also known that the presence of alkene double
bonds results in the scavenging of singlet oxygen [36], hence the
low value (Φꢀ) of 6 alone or in the presence of AMNPs (6-AMNPs)
could be due to the presence of the extra double bond. Even
though there is a decrease in Φꢀ following conjugation of ZnPc
derivatives to MNPs, the presence of MNPs will allow for magnetic
separation following use for applications of the conjugates for PACT
or for photocatalysis, allowing for re-usability of the catalyst. In
addition, MNPs have potential for use in various fields such as
magnetic resonance imaging (MRI), hyperthermia and drug delivery
[37]. Linking of phthalocyanines to MNPs allows for combination
therapy using hyperthermina and PDT and for possible MRI during
PDT. The Φꢀ values of the MNPs in Table 1 are still high enough for
PDT applications since PDT photosensitizers in clinical trials such as
LUTRIN have Φꢀ values as low as 0.11 [38].
The triplet yield lifetimes (τT) determine the residence time of the
excited molecules in the excited state. Photosensitizers with a
longer τT are excellent because longer τT allows sufficient time for
energy transfer from the triplet state of the Pc to the molecular
oxygen for the generation of the singlet oxygen species [33,34]. It is
expected that when ΦT decreases triplet lifetimes increase [31]. The
decrease of both ΦT and τꢁ values for 6-AMNPs could be due to
aggregation as discussed above. The lengthening in τꢁ in some
cases could be due to the protection of the Pc by AMNP.
Singlet oxygen quantum yields (Φꢀ
)
The ability of the Pc complexes and the Pc-AMNPs conjugates to
generate singlet oxygen species was evaluated using a chemical
method with ZnPc as a standard and DPBF as a singlet oxygen
quencher. The rate of photodegradation of DPBF was monitored by
UV-Vis spectra, Fig. S4 (supporting information) using 6-AMNPs as
an example. In all cases in Table 1, there is a decrease in singlet
oxygen quantum yield of the Pcs upon conjugation to AMNPs.
Conclusions
Asymmetric ZnPc complexes were successfully synthesized and
linked to AMNPs through an amide bond. The fluorescence
quantum yields decreased significantly for 6-AMNPs compared to 6
and minimally for 5-AMNPs and 7-AMNPs compared to 5 and 7
respectively. The triplet quantum yields slightly increased for 7-
AMNPs compared to 7 and did not change for 5-AMNPs compared
to 5. There was a decrease in triplet quantum yield for 6-AMNPs
compared to 6 alone (containing mono coumaric acid substituent),
due to aggregation and possibly due the length of the spacer
between the Pc and MNPs. Nonradiative relaxation in the excited
triplet state may have contributed to the limited energy transfers of
ZnPc molecules to the molecular oxygen as evidenced by low singlet
oxygen quantum yields especially in the presence of AMNPs. The
low oxygen quantum yields of Pc-AMNPS could also be due to the
screening effect.
Triplet and singlet oxygen quantum yields are related since the
latter are a result of the former interacting with ground state
oxygen. The decrease in the singlet oxygen quantum yield for Pc-
AMNPs conjugates could be due to the screening effect caused by
AMNPs which could have prevented the interaction of the excited
triplet state of the nanoconjugates and the ground state molecular
oxygen [35] resulting in the insufficient energy transfer from the
excited triplet state to the molecular oxygen. In addition, competing
pathways such as nonradiative relaxation of molecules from the
excited triplet state to the ground state are possible when
University. We thank NRF/Japan Society for the Promotion of
Science joint research program for funding.
Acknowledgements
Notes and references
This work was supported by the African Laser Centre (ALC), the
Department of Science and Technology (DST), CSIR National
Laser Centre, Rental Pool Program, National Research
Foundation (NRF) of South Africa through DST/NRF South
African Research Chairs Initiative for Professor of Medicinal
Chemistry and Nanotechnology (UID 62620), and Rhodes
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