Self-assembly of a symmetrical dimethoxyphenyl substituted Zn(II) phthalocyanine into nanoparticles with enhanced NIR absorbance for singlet oxygen generation

Article abstract of DOI:10.1016/j.jphotochem.2020.113123

Phthalocyanine (Pc) molecules have been investigated as photosensitizers (PS) for singlet oxygen (¹O₂) generation because of their intense absorption bands in the near-infrared region, which is desirable for applications such as photodynamic therapy (PDT). Here, we have designed a sterically crowded symmetrical Zn(II) phthalocyanine (PC-1), which contains 8 dimethoxyphenyl substituents, and studied its optical properties. PC-1 exhibits good thermal stability, appropriate theoretical HOMO and LUMO energy levels and band gap which are suitable for optoelectronic applications. PC-1 was converted into hydrophilic nanoparticles (PC-1NPs) via a solvent exchange based self-assembly approach, which exhibited good water dispersability, as well as uniform morphology and size. Optical studies revealed a bathochromic-shift in absorbance in the NIR wavelength region and ¹O₂ generation. These results suggest that PC-1 phthalocyanine is a potential candidate for photoinduced oxidative processes in biological and catalytic applications.

Full text of DOI:10.1016/j.jphotochem.2020.113123

Journal of Photochemistry & Photobiology, A: Chemistry 408 (2021) 113123
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Journal of Photochemistry & Photobiology, A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Self-assembly of a symmetrical dimethoxyphenyl substituted Zn(II)
phthalocyanine into nanoparticles with enhanced NIR absorbance for
singlet oxygen generation
,
,
,d,
Govind Reddya b, Enrico Della Gasperac, Lathe A. Jonesb **, Lingamallu Giribabua
*
a Polymers & Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Hyderabad, 500007, T.S., India
b Centre for Advanced Materials and Industrial Chemistry (CAMIC), School of Science, RMIT University, Melbourne, VIC, 3001, Australia
c School of Science, RMIT University, Melbourne, VIC, 3000, Australia
d Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, 201002, India
A R T I C L E I N F O
A B S T R A C T
Phthalocyanine (Pc) molecules have been investigated as photosensitizers (PS) for singlet oxygen (1O2) gener-
ation because of their intense absorption bands in the near-infrared region, which is desirable for applications
such as photodynamic therapy (PDT). Here, we have designed a sterically crowded symmetrical Zn(II) phtha-
locyanine (PC-1), which contains 8 dimethoxyphenyl substituents, and studied its optical properties. PC-1 ex-
hibits good thermal stability, appropriate theoretical HOMO and LUMO energy levels and band gap which are
suitable for optoelectronic applications. PC-1 was converted into hydrophilic nanoparticles (PC-1NPs) via a
solvent exchange based self-assembly approach, which exhibited good water dispersability, as well as uniform
morphology and size. Optical studies revealed a bathochromic-shift in absorbance in the NIR wavelength region
and 1O2 generation. These results suggest that PC-1 phthalocyanine is a potential candidate for photoinduced
oxidative processes in biological and catalytic applications.
Keywords:
Phthalocyanine
Singlet oxygen
Self-assembly
Nanoparticles
Photostability
1. Introduction
PDT due to their high molar extinction coefficients (εmax > 105 L molꢀ 1
cmꢀ 1), wide range of absorption (λmax > 660 nm) [7,8], and tunable
The phenomenon of photoinduced triplet to singlet oxygen conver-
sion in the presence of a photosensitizer has potential applications in
fine chemical synthesis, waste water treatment, and photodynamic
therapy (PDT) [1,2]. In this process, a photosensitizer absorbs light and
is excited to its singlet state, accompanied by intersystem crossing (ISC)
to a triplet state that can react with ground state triplet oxygen and
generate singlet oxygen via energy transfer [3]. In recent years, the
development of sensitizers has attracted significant attention to treat
cancer via photodynamic therapy (PDT) [4–6]. For PDT to be effective,
the photoactive molecule needs to exhibit a wide absorption range in the
Near-IR because light of this wavelength range has much deeper pene-
tration than visible. To this end, photoactive molecules such as por-
phyrins, BODIPY, chlorin, and phthalocyanine derivatives have been
used due to their suitable optical properties [3,4,7]. Among these,
phthalocyanine (Pc) based photosensitizers are potential candidates for
photophysical and photochemical properties by means of synthetic
modifications [9]. Recently, the clinical use of an Aluminium Pc deriv-
ative (Photosens®, Russia) has been approved [10]. Chen et al. reported
a pH-responsive photosensitizer (ZnPc(TAP)4) in which four 2,4,6-tris
(N,N-dimethylaminomethyl)phenoxy moieties are appended to ZnPc.
When the pH of the buffer medium was reduced from 8.0–6.0, the singlet
oxygen generation efficiency of (ZnPc(TAP)4) was enhanced from 0.32
to 0.55 [7,11,12]. However, one disadvantage of phthalocyanines is that
they tend to aggregate in solution due to strong π-π interactions. This
behaviour restricts to use of PCs in some applications as aggregation
decreases the singlet oxygen efficiency/reactive oxygen species (i.e.,
ROS), reducing the overall performance of these materials, specifically
in PDT. For this reason, the development of non-aggregating phthalo-
cyanine photosensitizers has emerged [4,13]. Taking into account that
aggregation of PCs occurs through interactions of adjacent conjugated
* Corresponding author at: Polymers & Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Hyderabad, 500007, T.S., India.
** Corresponding author.
E-mail addresses: r.govindreddy@gmail.com (G. Reddy), enrico.dellagaspera@rmit.edu.au (E.D. Gaspera), lathe.jones@rmit.edu.au (L.A. Jones), giribabu@iict.
res.in (L. Giribabu).
https://doi.org/10.1016/j.jphotochem.2020.113123
Received 31 October 2020; Received in revised form 26 December 2020; Accepted 27 December 2020
Available online 31 December 2020
1010-6030/© 2020 Elsevier B.V. All rights reserved.

G. Reddy et al.
Journal of Photochemistry & Photobiology, A: Chemistry 408 (2021) 113123
structures, we designed the sterically constrained Zinc (II) 2,391,016,
172,324-octakis(3,4-dimethoxyphenyl)phthalocyanine macrocyclic de-
rivative (PC-1 see Chart 1 ), as a potential photosensitizer for the gen-
eration of singlet oxygen. It has been previously observed that
diamagnetic metal ions (ex: Zn2þ, Al3+, and Ga3+) boost the singlet
oxygen production ability of phthalocyanine photosensitizers, leading to
a long lifetime and high triplet quantum yield [14]. Herein we report the
synthesis and characterization of nanoparticles of Zn(II) phthalocyanine
(PC-1), and study the physical and optical properties, including singlet
oxygen generation. Our findings unravel the influence of the dime-
thoxyphenyl substituents (sterically constrained) and diamagnetic Zn
(II) metal present in the phthalocyanine on singlet oxygen generation,
confirming the strategy of adding sterically constrained substituents to a
phthalocyanine can lead to materials suitable for applications such as a
photosensitizer for cancer treatment via photodynamic therapy. In
addition, self-assembly of PC-1NPs present strong NIR absorbance this
represents biological transparent window (550ꢀ 950 nm). Furthermore,
spectroscopy studies evidence that these NPs have the superior1O2
generation efficiency and good photostability.
2.3. Characterization techniques
2.3.1. XPS
X-ray photoelectron survey spectra of PC-1 powder were analysed
with a Thermo Scientific K-Alpha X-ray Photoelectron Spectrometer
(XPS). The binding energy (B.E) of the peaks of interest were adjusted
using an internal standard peak of C (1 s) predicted at 284.8 eV.
2.3.2. TGA
The thermal stability, and weight loss during heat treatment of the
PC-1 PC-1 were studied by thermo gravimetric thermal analysis (TGA;
Mettler Toledo, Star-e system) performed from 40 to 850 ◦C with scan
rate of 10 ◦C/min in a N2 atmosphere.
2.3.3. BET
The BET surface area, pore volume and pore sizes were determined
by using the N2 adsorption-desorption method using BEL Sorb II In-
struments, Japan at liquid nitrogen temperature. Before the measure-
ment the samples were degassed at 200 ◦C for 2 h.The surface area was
determined through the Brunauer ꢀ Emmett ꢀ Teller (BET) method. The
average pore volume and pore size were measured by Barrett ꢀ Joy-
nerꢀ Halenda (BJH) method.
2. Experimental section
2.1. Materials
2.3.4. Theoretical simulations
The optimized geometries of PC-1 photosensitizer was carried out
using the B3LYP hybrid functional and 6ꢀ 31(d,p) basis set in a gas phase
withGaussian 09 computational software. Ground state properties were
calculated using Density Functional Theory (DFT) in the gas phase and
excited state properties were implemented using Time Dependent
Density Functional Theory (TD-DFT) in PCM solvation model with THF
solvent. The software GaussSum3.0 was used to analyze the major
portions of the absorption spectra and to interpret the molecular orbital
contribution.
3,4-dimethoxyphenylboronic acid (≥95.0 %), 4,5-dicholorophthalo-
nitrile (99 %),1-pentanol (≥99 %), DMF (99.8 %), and [Pd(P(C6H5)3)4]
(99 %) were purchased from Sigma Aldrich. HCl, Li metal and THF were
supplied by TCI. These chemicals were used without further
purification.
2.2. Synthesis
A Zn(II) octa(1,2-dimethoxyphenyl) phthalocyanine system (PC-1)
was synthesized and characterized as per our previously reported pro-
cedure [15].
2.3.5. TEM
Transmission electron micrographs (TEM) images of the PC-1
nanoparticles drop casted on a carbon-coated copper grid were
captured at 100 kV on a JEOL 1010 TEM. The grids were prepared a day
before imaging and dried overnight under nitrogen.
2.2.1. Nanoparticle synthesis
PC-1 nanoparticles prepared via the solvent exchange approach,
which has been previously reported in the literature, with slight modi-
fication [16,17]. Approximately 1 mg of PC-1was dissolved in 1 ml of
THF solvent. This solution was added to10 ml of deionized water
(DI-Water) from a syringe under magnetic stirring. The THF in the sol-
vent mixture was then removed by nitrogen bubbling.
2.3.6. DLS
Dynamic Light scattering measurements were determined using
Zetasizer Nano ZS (Malvern Instrument Ltd., UK).
2.3.7. Optical measurements
Optical absorption and emission spectra were recorded on a Cary 60
UV–vis Spectrophotometer and Horiba FluoroMax® 4 fluorometer,
respectively.
2.3.8. Singlet oxygen quantum yield measurement
PC-1 or PC-1NPs was mixed with 1,3-Diphenylisobenzofuran (DPBF,
6ⅹ10ꢀ 5 mol/L) as the 1O2 probe in DMF (PC-1) or Water (PC-1NP)
respectively. The maximum absorption intensity of DPBF was around
1.2–1.40. The absorbance maximum of PC-1 or PC-1NPs was adjusted to
be ~0.3. Then the samples were irradiated with 625 nm LED (Thorlabs)
with power intensity 2.1 mW/cm2 at different intervals of time from 0 to
60 s. The absorbance change of DPBF at 418 nm was monitored by
UV–vis spectrophotometer over time. Methylene blue (ΦΔ = 0.49 in
DMF) is used as a reference to calculate ROS production yield of PC-1 or
PC-1NPs.
3. Result and discussion
The synthesis of Zn(II) 2,391,016,172,324-octakis(3,4-dimethoxy-
phenyl)phthalocyanine (PC-1 see Chart-1) was slightly modified from
our previous report [15]. PC-1 macrocycle was synthesized via
Chart 1. Chemical structure of PC-1 compound.
2

G. Reddy et al.
Journal of Photochemistry & Photobiology, A: Chemistry 408 (2021) 113123
cyclotetramerization of 4,5-bis(3,4-dimethoxy) phthalonitrile and Zn
(OAc)2 in 1-pentanol using DBU as a catalyst under reflux (8 h). The
metallophthalocyanine (PC-1) was isolated by silica gel column chro-
matography using DCM:MeOH (95:5) as the eluant. The structure was
confirmed by MALDI-TOF (see Figure S1(a) and isotropic pattern S1(b))
Table 1
XPS elemental composition of PC-1.
Elements in PC-1
Zn(2p)
N (1 s)
O (1 s)
C(1 s)
Binding energy (eV)
1044.33 (2p1/2
)
)
399.19 397.98
533.63
532.60
531.05
286.24
284.69
1021.35 (2p3/2
with
a
major peak at m/z-C96H80N8O16Zn: 1666.632 [calc.
m/z:1665.50]. Also, other calculated (see S1(c)) isotropic peaks at
m/z1664.514, 1665.514, 1666.5230, 1667.5094, 1668.5225,
1669.4957, 1670.5177 are good agreement with experimental values
m/z at 1664.600, 1665.409, 1666.636, 1667.5840, 1668.629,
1669.641, 1670.647 (see S1(b)).
peak at 286.24 eV [21,22]. Powder X-ray diffraction survey ofPC-1
revealed an amorphous peak at 2θ = 25,confirming a lack of crystallinity
in powder form, indicating that the bulky 3,4-dimethoxyphenyl sub-
stituents may hamper the close packing of the PC-1 units (Figure S3).
This is an initial indication the introduction of bulky 3,4-dimethoxyphe-
nylgroups at the peripheral positions of phthalocyanine are effective for
the minimization of PC-1 aggregation [23,24].
3.1. XPS and XRD studies
PC-1 was studied by XPS to confirm the presence of the expected
elements, the oxidation state of zinc metal in its central cavity, and the
effect of the peripheral units on the binding energies (Fig. 1(a)). Fig. 1
(b) displays the XPS spectra of the Zn 2p region to study the metal
present in the PC-1 ring, showing two intense peaks. The peak at lower
binding energy of 1021.35 eV represents 2p3/2, and the higher binding
energy peak at 1044.33 eV represents 2p1/2. The Zn 2p3/2 peak confirms
the existence of Zn metal in +2 state in the PC-1 macrocycle [18,19].
The main N (1 s) peak is at 399.19 eV. This main peak is accompa-
nied by a shoulder (see Fig. 1(c)) that represents the two chemical en-
vironments of N in PC-1 being (a) four nitrogen atoms bond with two
3.2. Thermal studies
We employed TGA to determine the thermal stability of the syn-
thesized phthalocyanine (PC-1), as shown in Fig. 2. It can be seen that
three-step weight loss occurs between 100 and 800 ◦C. The first weight
loss step from 200 to 300 ◦C is related to the loss of trapped solvent or
moisture present in the PC-1 powdered sample. The second weight loss
step from 430 to 530 ◦C is due to the core of the PC complex starting to
decompose and the third weight loss at >550 is due to complete
decomposition of PC-1 phthalocyanine [25]. Thus, PC-1 phthalocyanine
exhibited excellent thermal stability up to 450 ◦C and it thus suitable for
high temperature reactions.
–
carbon atoms (C N = C) and (b) the four nitrogen atoms coordinated
with the Zn metal [20]. Three distinctive oxygen O (1 s) peaks (as shown
in Fig. 1(d) and Table 1) were observed, attributed to meta and para
OMe’s of the peripheral substitution of dimethoxyphenyl group on PC
ring, with the other peak should be related to adsorbed water/moisture.
PC-1 shows the predominate C(1 s) peak at 284.69 eV, while the elec-
tron donor dimethoxyphenyl phthalocyanine shows (see Figure S2) a
Additionally, we undertook N2 sorption and thermal stability studies
for PC-1. The N2 adsorption–desorption isotherm of PC-1 is shown in
Figure S4, where a rapid N2 uptake in the pressure region P/P0 > 0.1 (of
0.45–0.98) was noted, typical of a type-IV isotherm, characteristic of a
Fig. 1. (a) XPS survey spectrum, (b) Zn2p core level spectrum (c) N1 s core level spectrum and (d) O1 s core level spectrum of the PC-1 phthalocyanine.
3

G. Reddy et al.
Journal of Photochemistry & Photobiology, A: Chemistry 408 (2021) 113123
planes of the phthalocyanine ring to hamper aggregation.
Density functional theory (DFT) geometry optimisations were car-
ried out using B3LYP/6ꢀ 31 G(d,p) level theory to understand the geo-
metric and electronic structure of the PC-1 molecule in the Gaussian 09
program package [29]. Top and side views of the PC-1 optimized mo-
lecular structures are shown in Fig. 3 (a), where it can be observed that
the dihedral angle between adjacent dimethoxyphenyl groups is ~5.4
◦C. The side view of PC-1 shows quiet clearly how
π-π stacking between
adjacent planar PC cores would be difficult due to steric hindrance of the
dimethoxyphenyl groups. The optimized frontier molecular orbitals as
demonstrated in Fig. 3(b), for PC-1 compound, its highest occupied
molecular orbitals (HOMOs) were mainly spread on the phthalocyanine
ring and dimethoxyophenyl donor group. While LUMOs were slightly
extended to the phenyl bridge besides the spreading on PC ring, due to
the presence of the electron deficit tetrapyrrolic cavity with the donor
and accepter, being dimethoxyphenyl moiety and phthalocyanine ring
respectively. PC-1 displayed the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) level of
ꢀ 4.75 and ꢀ 2.65 eV, respectively [30]. Consequently, the narrow
bandgap (2.1 eV) of PC-1 is indicative of photo absorption in the
long-wavelength region, which is confirmed by elucidation of the
TD-DFT absorption spectra of PC-1 simulated at b3lyp/6ꢀ 31 g(d,p)
level theory with the PCM solvation model in THF. Fig. 3 (c) shows both
the theoretical and experimental UV–vis spectra, confirming that PC-1
have the ability to absorb broadly in the visible spectrum. Simulated
DFT and TD-DFT parameters of PC-1 are presented in Table 2 and
Table S2. As depicted in Figure S5, the negative electrostatic potential
(ESP) is located on the methoxy (OMe) groups of the dimethoxyphenyl
moieties, whereas the most positive ESP is around the phthalocyanine
ring. All theoretical results suggest that a narrow HOMO–LUMO gap led
to significant shift of the Q and B bands to the visible region (see Fig. 3
(c)) and reveal the PC-1 is suitable for use as a photosensitizer in the
therapeutic window [31].
Fig. 2. Thermal stability (TGA/DTA plots) of PC-1.
porous structure (Figure S4(a)). The Brunauer ꢀ Emmett ꢀ Teller (BET)
[26] surface areas of the as synthesized material PC-1 was determined as
45 m2 gꢀ 1 with the pore volume estimated to be 0.197 cm3 gꢀ 1. The
average pore size distribution, as estimated by using the Barrett ꢀ
Joynerꢀ Halenda (BJH) approach, suggests narrow pore size distribu-
tions (Figure S4(b)) with the pore size estimated to be about 7.74 nm.
These parameters (see Table S1) are in good agreement with previously
reported (phthalocyanine based) heterogeneous and photo catalysts [27,
28]. In general, a high specific surface area of photocatalysts can create
additional surface-active sites and make charge carrier transport easier,
leading to an improvement in catalytic performance.
3.3. DFT calculations
DFT optimized structures were generated to probe the effect of the
sterically hindering groups on aggregation, and show that the OMe
groups on the 3,4-dimethoxyphenyl orientate either below or above the
Fig. 3. Theoretical studies of PC1(a)Optimized structure (b) Frontier molecular orbitals (c) TD-DFT UV–vis spectra comparison with experimental.
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G. Reddy et al.
Journal of Photochemistry & Photobiology, A: Chemistry 408 (2021) 113123
Table 2
Optical and theoretical properties of PC-1.
Property
aλmax (nm)
aλmax(nm)
aΦΔ
HOMO*
LUMO*
Eg*
Ref.
absorption
emission
(%)
ZnPC
670
681
52 (32)b
5.16
2.98
2.18
[39]
[10]
[10]
[40]
[40]
PcA1d
695
708
19
–
–
–
–
–
–
–
–
–
–
–
–
PcA2d
700
704
77
SiPc-Pt-HAe
SiPc-Pte
~700
~700
in THF
~705
~705
24
15
NPs in water
on film
83
PC-1
~550ꢀ 950
701
ꢀ 4.75
ꢀ 2.65
2.1
Present work
696
~550ꢀ 1050 (broad)
(5)c
(broad)
*Data from DFT, a in DMF, b 0.1 M CTAC/H2Oc H2O, dPc compounds peripherally substituted with amino groups (PcA1 and PcA2) eplatinum(II) conjugated silicon(IV)
phthalocyanine (SiPc–Pt) and its hyaluronic acid (SiPc–Pt–HA) formulated nanoparticles.
3.4. Nanoparticles synthesis and characterization
between 550 and 1000 nm. As shown in Fig. 5 (a), the Q bands are
viewed as doublets at 627 and 696 nm, which could be interpreted as
In general alkoxy substituted phthalocyanines are non-aqueous,
which is tough for biological applications. Nanoparticle (NP) carriers
have been shown to have increased water solubility and stability as
photosensitizers than pure compounds, especially for applications in
tumour tissue. Furthermore, NP carriers provided benefits including
longer circulation in the blood and higher accumulation of photosensi-
tizers at disease sites [32]. For this reason, we have converted PC-1 into
a NP suspension by the solvent-exchange approach as shown in Scheme
1. During this process, PC-1 would assemble into NPs as earlier reported
[16,17]. PC-1 powder was dissolved in tetrahydrofuran (THF) solution
and then quickly injected into DI-water under sonication. After 10 min,
the THF was removed from the aqueous media by nitrogen bubbling at
room temperature. A transparent solution was obtained which con-
tained hydrophilic PC-1NPs.
π-π* transitions between bonding and antibonding molecular orbitals [9,
30]. In solution the intensity of this band is very less, whereas in case of
thin film and nano particles the intensity of this band raises and merge
with the band at 697 nm. As a result a broad Q band was observed in thin
film and in nano particle, as was the case in our previous reports [35,36].
This was even predicted from DFT the optimized structures are shown in
Fig. 3 (a). When compared to PC-1 in THF, an extensive broadening can
be observed for the Q band of PC-NPs in DI-water, displaying a sub-
stantial red-shifted absorbance onset (~950 nm), which is in the bio-
logical transparent window. Similarly, absorption in the thin film
displays a broad Q-band with the wavelength region of 550ꢀ 1000 nm.
These spectra are indicative of strong intramolecular and intermolecular
π
ꢀ π
interactions of phthalocyanine macrocycles.
Furthermore, the fluorescence of the PC-1NPs was completely
Fig. 4 (a) shows the size distribution of the NPs, measured by using
dynamic light scattering (DLS) technique [33] of this PC-1NP solution,
where the average size of the NPs was found to be ~152 nm with a PDI
of 0.16. Additionally, the shape and size of the NPs were examined by
transmission electron microscopy (TEM). PC-1 molecules self-assembled
quenched (Figure S7) compared to PC-1 (in THF), which is in good
agreement with typical PCs in their aggregated state. Aggregation is bad
for stability/processability of the material, but good for its properties.
So, we found a way to get the best of both worlds, having a small extent
of controlled aggregation (NPs) avoiding large scale precipitates. This
aggregation behaviour can stop photothermal agents (PTAs) losing
absorbed energy via fluorescence emission and intersystem crossing
(ISC) energy transfer, which is advantageous for high photothermal
conversion efficiency [34].
through non-covalent interactions such as
substituents on PC-1 will reduce interactions preventing massive
aggregation, but some stacking will still occur, causing NP forma-
πꢀ π stacking (The bulky
π-π
π-π
tion) and hydrophobic ꢀ hydrophobic interactions between molecules,
which initiates nucleation and growth of PC-1 into well-defined and
homogeneous nanospheres with size around 100 nm as shown in Fig. 4
(b) and S6. The size and shapes of these nanoparticles should improve
the dispersibility of PC-1 in water, which would be beneficial, for
example, for the enhanced permeability and retention (EPR) of PC-1
NPs in a tumour site [17,34].
Singlet oxygen is one of the key cytotoxic agents that are generated
by irradiation of photoactive materials during PDT. Here, we measured
the singlet oxygen generation efficiencies (ΦΔ) of the PC-1 and its NPs in
DMF and water by using 1,3-diphenylisobenzofuran (DPBF) as a singlet
oxygen trapping reagent, and methylene blue (ΦΔ = 0.49 in DMF) as the
reference [37,38]. The phthalocyanine sensitizer with DPBF was irra-
diated with 625 nm LED with a power intensity of 2.1 mW/cm2 at
different intervals of time from 0 to 60 s. As shown in Fig. 5 (b–c), the
absorbance of DPBF at 420 nm for PC-1NPs and at 414 nm for PC-1
diminished significantly. From the Fig. 5(c), it can be observed that the
absorption intensity of DPBF declines drastically with the increasing
irradiation time, indicative of excellent 1O2 generation ability of PC-1,
superior to PC-NPs. The calculated singlet oxygen quantum yields of
PC-1 and PC-1NPs were determined to be 83 % and 5%, respectively
(see Table 2), which indicates that PC-1 and PC-1NPs present excellent
3.5. Optical and singlet oxygen studies
To study the optical properties of PC-1 and PC-NPs, both UV ꢀ Vis
and fluorescence (FL) spectra were collected. The absorption spectra of
PC-1 in THF, nanoparticles in DI-water, as well as PC-1 films on glass,
are shown in Fig. 5 (a), demonstrating the two typical distinctive bands
of PC-1 phthalocyanine macrocycle: the B band, situated in the UV-
region between 300 and 400 nm, and the Q band, in the region
Scheme 1. Solvent-exchange method for preparation of PC-1NPs.
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G. Reddy et al.
Journal of Photochemistry & Photobiology, A: Chemistry 408 (2021) 113123
Fig. 4. (a) The dynamic light scattering (DLS) histogram of PC-1NPs (Inset: The photographs of PC-1NPs in deionized water (c =100 μg/mL) (b) The transmission
electron microscopy (TEM) image of PC-1NPs.
Fig. 5. a)UV–vis spectra of PC-1 in THF(black), NPs in water (red)and on Film (blue) b) & c) Absorption changes during the DBPF oxidation in the presence of PC-
1&PC-1NPs under illumination with 625 nm LED (2.1 mW/cm2) d) DPBF absorbance PC-1&PC-1NPs solutions versus irradiation time (seconds).
singlet oxygen generation ability when compared to previous literature
data (see Table 2) [10,39,40]. For instance as in case of Zinc phthalo-
cyanines (Zn PC), the singlet oxygen quantum yield was reported 52 %
in DMF solvent [39]. On the other hand, axially substituted hyaluronic
acid formulated nanoparticles containing a platinum(II) conjugated
silicon(IV) phthalocyanine has showed poor singlet oxygen quantum
yields [40]. Singlet oxygen efficiency calculations of phthalocyanine
nanoparticles are rare in the literature due to inherent light scattering
phenomena associated with nanoparticles in solution (H2O) leads to
significant experimental error [31]. As a consequence of aggregation,
the 1ΦΔ values in water are lower since this decreases the lifetime of the
excited state and photosensitization efficiency, due to enhanced radia-
tionless decay, this may be a reason for the low singlet oxygen genera-
tion efficiency of PC-1NPs [41]. Even though, lutetium texaphyrin
exhibited a low 1ΦΔ of 11 % have been used for clinical application in
PDT [42]. At the end it should also be noted that our PC-1NPs exhibited
excellent photostability (see Figure S9) under continuous irradiation of
30 min with the same LED and power intensity.
4. Conclusions
In summary, PC-1 displayed a high surface area, pore volume, with
small pore size and acceptable thermal stability. Then, we have prepared
and self-assembled phthalocyanine nanoparticle photosensitizers using
PC-1 through the solvent-exchange method. The corresponding PC-
1NPs displayed appropriate size and morphology for PDT applications.
Furthermore, PC-1NPs showed significant red-shifted absorbance to the
biological transparent window (550ꢀ 950 nm). Additionally, PC-1NPs
6

G. Reddy et al.
Journal of Photochemistry & Photobiology, A: Chemistry 408 (2021) 113123
displayed excellent photostability, and 1O2 generation ability. This leads
to PC-1 being a promising candidate for photoinduced oxidative pro-
cesses in biological and catalytic applications.
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Author statement
Govind Reddy
Synthesis, characterization, Spectroscopy
Govind Reddy
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Synthesis, characterization, Spectroscopy
Enrico Della Gaspera
Singlet oxygen measurements
Lathe Jones
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Manuscript preparation and supervision
Lingamallu Giribabu
Manuscript preparation and supervision
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Declaration of Competing Interest
There are no conflicts to declare.
Acknowledgements
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G.R. thankful to IICT-RMIT joint Ph.D. program for a junior research
fellowship. Thanks to P.Koley for analysis of XPS and BET. Authors
thankful to CSIR-Indian Institute of chemical Technology (IICT) in India
and RMIT Microscopy and Microanalysis Facility-RMIT University in
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Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jphotochem.2020.
113123.
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