Graphene oxide–metallophthalocyanine hybrids with enhanced singlet oxygen generation

Article abstract of DOI:10.1016/j.materresbull.2019.02.018

Graphene oxide (GO) was added to zinc phthalocyanine (ZnPc) and tetra nitro substituted zinc phthalocyanine (ZnPc(NO₂)₄) solutions to evaluate the photophysical interaction of GO with ph- thalocyanines. Dimethylformamide (DMF) and acetonitrile (MeCN) were used as solvents. GO quenching is shown to be stronger with ZnPc(NO₂)₄, presenting a Stern-Volmer (K_sv) constant ap- proximately 10 times higher than ZnPc. The presence of GO and the increase in its concentration demonstrates a different pattern of absorbance for ZnPc(NO₂)₄. Singlet oxygen generation was found to be higher in DMF than MeCN enhaces with GO present. The overall singlet quantum yield of ZnPc is higher than ZnPc(NO₂)₄.

Full text of DOI:10.1016/j.materresbull.2019.02.018

Materials Research Bulletin 114 (2019) 45–51
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Graphene oxide–metallophthalocyanine hybrids with enhanced singlet
oxygen generation
⁎
Fellipy S. Rocha, Anderson J. Gomes, Claure N. Lunardi
Laboratory of Photochemistry and Nanobiotechnology, University of Brasilia, Brasilia, DF, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords:
Graphene oxide (GO) was added to zinc phthalocyanine (ZnPc) and tetra nitro substituted zinc phthalocyanine
(ZnPc(NO ) ) solutions to evaluate the photophysical interaction of GO with ph- thalocyanines.
Graphene oxide
Zinc phthalocyanine
Singlet oxygen
2
4
Dimethylformamide (DMF) and acetonitrile (MeCN) were used as solvents. GO quenching is shown to be
stronger with ZnPc(NO , presenting a Stern-Volmer (Ksv) constant ap- proximately 10 times higher than ZnPc.
The presence of GO and the increase in its concentration demonstrates a different pattern of absorbance for ZnPc
NO . Singlet oxygen generation was found to be higher in DMF than MeCN enhaces with GO present. The
overall singlet quantum yield of ZnPc is higher than ZnPc(NO
2 4
)
(
2 4
)
2 4
) .
1
. Introduction
living cells [10], and extraction of polycyclic aromatic hydrocarbons
from water samples using GO-polymer composites [11]. Also, sensors
[12], solar cells [13], fuel pro- duction [14], supercapacitors and en-
ergy storage [15] are promising applications of GO-based material.
Phthalocyanines are two-dimensional aromatic structures, consisting of
four isoindole subunits linked together through nitrogen atoms and
present chemical flexibility, making it pos- sible to tailor their physical,
optical, and electronic properties. Their electronic delocalization is
responsible for numerous features, making them valuable in different
fields of science and technol- ogy [16,17]. Phthalocyanines have been
extensively studied as sensors, low band gap molecular solar cells,
nonlinear optical materials, organic field effect transistors, optical in-
formation record- ing media, light-emitting devices, photocatalysts, and
photosensitizers for photodynamic therapy (PDT) [18].
Metallophthalocyanines exhibit several characteristic properties,
including an intense and broad visible light response (600–800 nm) and
high thermal stability [19]. Their photoactiv-ity arises from the ex-
tinction of excited triplet oxygen, forming singlet oxygen. Also, con-
juga- tion of phthalocyanines with nanoscale materials could enhance
their singlet oxygen yield, and this intensified feature is sought to im-
prove the efficiency of PDT treatments [2,20]. Zinc (II) phthalocyanine
(ZnPc) complexes have been studied extensively by their photo-
sensitizing prop- erties [21,22]. They present high fluorescent proper-
ties, which leads to their extensive use in photodynamic therapy.
Moreover, the particular chemical groups at the periphery of zinc
phthalo- cyanine compounds may be selectively preferred by some
tumor cells, and phthalocyanines with these structures are called third
Carbon based nanomaterials exhibit unique mechanical, structural,
and electronic properties, and these materials have attracted scientific
attention due to the wide applications range [1]. In particular, gra-
phene is a two-dimensional monolayer of sp2 carbon sheets that are
stacked, pre- senting a honeycomb (hexagonal) crystal lattice, which
demonstrates interesting electrical and op- tical properties [2]. Gra-
phene oxide (GO) is an oxidized form of graphene, containing func-
tional
groups exposed over the surface, expanding the capacity to link
functional molecules through non- covalent/covalent interaction [3,4].
GO also allows inorganic, organic, and bio-molecules to phys- ically
adsorb via strong π-π and electrostatic interactions [5]. Graphene oxide
sheets are used to attach biological and chemical drugs, demonstrating
potential as a nanocarrier of water-insoluble anticancer drugs. The two-
dimensional planar surface of GO confers a high capacity for drug
loading, better than other drug-delivery systems [6]. GO is considered
relatively biocompatible and is a good candidate for biological appli-
cations because it is primarily composed of carbon [7]. However, gra-
phene derivatives generally aggregate in biological or salt solutions,
shows a certain degree of cytotoxicity and the loading and release of
drugs can be difficult to control [8]. What makes GO more attractive for
studies of drug delivery is the presence of functional groups that can be
modified by conjugating nanomaterials, enhancing GO stability and
biocompatibility [9]. Another approach is based in developping GO
nanosheets for in situ simultaneous monitoring of ATP and GTP in
⁎
Corresponding author.
E-mail addresses: fellipysr@gmail.com (F.S. Rocha), ajgomes@unb.br (A.J. Gomes), clunardi@unb.br (C.N. Lunardi).
https://doi.org/10.1016/j.materresbull.2019.02.018
Received 9 October 2018; Received in revised form 17 February 2019; Accepted 18 February 2019
Available online 18 February 2019
0025-5408/ © 2019 Elsevier Ltd. All rights reserved.

F.S. Rocha, et al.
Materials Research Bulletin 114 (2019) 45–51
Scheme 1. Metallophtalocyanine and graphene oxide interaction in organic solvent.
IF0
IF
generation photosensitizers [23]. Tetra substituted phthalocyanines
may be preferred when their functionality and solubility are taken into
consideration because they offer higher stability as well. Functional
phthalocyanines are more favored than other forms due to their ad-
vanced features.
=
1 + Kqτ0 [Q] = 1 + Ksv [Q]
(1)
0
where I is the fluorescence intensity, I
the presence of the quencher (GO) at a concentration [Q], k
F
is the equivalent parameter in
F
q
is the
0
quenching rate constant of the bio-molecule, I is the average lifetime
F
Studies regarding the interactions between GO and phthalocyanines
0
of the fluorescent substance without any quencher, and I is the Stern-
F
(
Pc) in several medium are sparse, and understanding this process is a
Volmer quenching constant. Hence, Eq. (1) can be used to determine by
0
crucial step to support the GO–Pc hybrid material as a potential pho-
tosensitizer for PDT applications. Moreover, nature, specificity, and
mechanisms driving the GO–Pc interactions need to be elucidated. In
this study, we aimed to investigate sev- eral aspects of GO–Pc hybrid
formation (aggregation behavior, fluorescence quenching, binding ki-
netics, and oxygen singlet generation). Here, we provide a compre-
hensive study of GO inter- action with zinc phthalocyanine and tetra-
nitro substituted zinc phthalocyanine in organic solvents (Scheme 1).
linear regression of a plot of I against [Q] and Kq =
K
sv
F
[25]. The
τ0
IF
parameters of the association of ZnPc and ZnPc(NO
2
)
4
with GO were derived based on the Hill equation, as follows:
(I0 − I)
Q =
0
(2)
(3)
I
Q
[GO]n
(K + [GO] )
=
n
n
Q
max
D
where Qmax is the saturation value of Q, KD, is the equilibrium binding
dissociation constant, which describes the relative strength of the
GO–fluorophore interaction, and n represents the Hill coefficient, which
defines the cooperativity association [26].
2. Material and methods
GO (1 mg/mL), ZnPc (powder), and DPBF (powder) were obtained
commercially from Sigma- Aldrich. Before utilization, GO and ZnPc
solutions were submitted to sonication for 5–10 min.
2
.4. Singlet oxygen quantum yield measurements
ZnPc(NO
trophthalonitrile in the presence of 1,8-diazabicicle [5,4,0]undec-7-ene
DBU) and zinc acetate, as reported by Lunardi [24].
2 4
) was synthesized through the tetramerization of 4-ni-
Singlet oxygen quantum yield (ΦΔ) is typically carried out using the
(
chemical trapping method [27]. For assays, 30 μL of DPBF were added
to 2.5 mL of 5 μM solutions with ZnPc or ZnPc(NO ) and were irra-
2 4
diated at 637 nm using a laser with 25 mW. Experiments were per-
formed under air conditions and at ambient temperature. The kinetic
constant of DPBF photodegradation (KDPBF) was determined by ap-
plying the first order kinetic model equation, as follows:
2.1. Preparation of the GO–Pc hybrids
2 4
Phthalocyanine solutions (ZnPc and ZnPc(NO ) ) were fixed at
1
0 μM and mixed with in- creasing concentrations of the GO solution
(
0–12 μg/mL), then sonicated for 5–10 min before characterization.
(
(
A
0
− Ainf
)
)
kt = Ln
A − Ainf
(4)
2.2. Steady state spectral measurement
where A
0
and Ainf are the DPBF absorbances at zero and infinite time,
The absorption and emission spectra were recorded on a Hitachi
UV–vis U-900H spectropho- tometer and a spectrophotometer of
fluorescence from Hitachi High-Technologies Corporation , model F-
respectively. The singlet oxygen quantum yield (ΦΔ) was determined
by the following equation:
®
Pc
ΦPc
Δ
ΦStd
Δ
r
(1 − 10(A_637Std))
7000, respectively. All spectra were recorded with appropriate back-
DPBF
=
ground corrections. The concentrations of GO used in the different
experiments have been specified in the context. The experiments were
carried out at room temperature (300 K).
rDPBFStd (1 − 10(A_637Pc))
(5)
where ΦΔ represents the singlet oxygen quantum yield (ΦΔ = 0.5639
for the standard ZnPc in DMF), rDPBF Pc and rDPBF Std represent the
DPBF photobleaching rates (kinetic constants, k and kstd, respectively),
(1–10A637Std) and (1–10 A637 Pc) represent the fraction of absorbed
light at the irradiation wavelength, and A637Std and A637Pc represent
the absorbance at the irradiation wavelength of the standard ZnPc (Std)
and other phthalocyanines (Pc), respectively [28].
2.3. Fluorescence quenching analysis
For ideal cases of quenching, the following well-known Stern-
Volmer equation was applied:
46

F.S. Rocha, et al.
Materials Research Bulletin 114 (2019) 45–51
due to the π-π transitions from the highest occupied molecular orbital
(
HOMO) to the lowest unoccu- pied molecular orbital (LUMO) of the Pc
ring. The B band is observed at around 300–450 nm, arising from
deeper π levels to the LUMO transition [31]. The non-substituted Pc
(
ZnPc) shows (Fig. 1A and B) a typical electronic absorption spectrum
of a metallated Pc complex with monomeric behavior, evidenced by a
single (narrow) Q band at 665 nm for acetonitrile (Fig. 1A) and at
670 nm DMF (Fig. 1B). The presence of GO does not stimulate the
formation of dimers and aggregates due to no observed widening or
split of the Q band, which preserved the ZnPc monomeric behavior, and
the absorption intensity remained unaltered. Similar behavior was de-
scribed for phthalocyanines interaction with graphene quantum dots in
organic solvents [32].
On the other hand, tetra nitro substituted Pc (ZnPc(NO
Fig. 1C and D) a more broader Q band, with a Qx Qy split, char-
acterizing non-monomeric behavior, which was already expected since
the solution of ZnPc(NO was a mixture of structural isomers. The Pc
presentes a Qx band at 640 nm and a higher intensity Qy band at
80 nm in MeCN (Fig. 1C). In DMF (Fig. 1D), ZnPc(NO exhibits a Qx
band at 647 nm and a stronger Qy band at 685 nm with a shoulder at
00 nm. Initially, with GO addition in the MeCN solution tthe absorp-
2 4
) ) shows
(
2 4
)
6
2 4
)
7
tion intensity increases for the whole Q band with the Qx band pre-
senting the highest magnitude, while in DMF only the Qx band in-
creases and do not surpass the Qy band absorption. After increasing the
GO concentration in MeCN, the phthalocyanine Qx band weakens while
the blue-shifted Qy band tended to fade. In the DMF solution, the Qx
band also blue-shifts and decreases. Thus, these changes in the ab-
2 4
sorption spectrum of ZnPc(NO ) could be explained by the π-π stack of
tetra nitro substituted Pc isomers with GO.
Fluorescence quenching can serve as a valuable source of informa-
tion regarding biochemical systems. A significant number of recent
studies on fluorescence quenching of organic dyes by graphene have
emerged, while similar reports linked to GO remain sparse [33].
Fluorescence quenching measurements can reveal the accessibility of Pc
to GO groups, helping explain ligand- binding mechanisms, providing
clues to the nature of the binding phenomenon. It can also be used to
discover unique structural and dynamic information. The formation of
H-aggregates (face-to- face arrangement of the monomer) is char-
acterized by a blue-shift in the UV–vis spectra and a well- documented
fluorescence quenching, where the electron transition to the HOMO is
allowed and is followed by a rapid conversion of this excited state to the
LUMO, quenching the fluorescence as a result [34].
Fig. 1. Normalized absorption spectrum of ZnPc in (A) MeCN and (B) DMF and
ZnPc(NO ) in (C) MeCN and (D) DMF.
2 4
Fluorescence quenching is a process that decreases the fluorescence
intensity of a fluorophore. It is known that non-covalent attachment of
fluorophores to the GO surface results in fluorescence quenching, which
is induced by two simultaneous processes, energy transfer (ET) and
photo- induced electron transfer (PET), from the electron donor me-
tallophthalocyanines to the electron acceptor GO [35]. The fluores-
3. Results and discussion
ZnPc was obtained commercially from Sigma-Aldrich and ZnPc
NO was synthesized using a well-known route [24]. Characteriza-
tion of the interaction of ZnPc and ZnPc(NO with GO was performed
using two organic solvents, dimethylformamide (DMF) and acetonitrile
MeCN). We aimed to investigate the photophysical characteristics of
2 4
cence emission peak of ZnPc(NO ) (Fig. 2E and F) is re- markably
(
2 4
)
extinct under the presence of GO and nearly vanishes when GO con-
centration is in-creased, demonstrating immediate and strong adsorp-
tion onto the GO surface. In some studies, the strong π-π stacking effect
and electrostatic interaction between GO and fluorophores is at- trib-
uted to the high quenching efficiency of GO [36]. The effect of GO is
similarly observed for other dyes, and its role in the quenching process
has been discussed in detail [37,38]. Bagio et al. demonstrates that for
aqueous soluble phthalocyanine like AlClPc, its association to graphene
oxide quenches fluorescence emission and suppresses ROS generation.
This effect is attributed to electron donor-acceptor interactions and the
coordination of oxygenated groups in graphene oxides with the central
Al cation of phthalocyanine [39]. Similar behavior was also reported
for CoPc-GO nanocomposites [40].
2 4
)
(
the phthalocyanines interaction with GO, employing spectroscopic
techniques, such as ultraviolet-visible (UV–vis) for aggregation, singlet
oxygen evaluations and fluorescence emission for quenching and
binding kinetics. The UV–vis absorption spectroscopic technique can
characterize the intermolecular association of monomers [29]. The
amount of GO plays a critical role in the aggregation process; thus, the
determination of its optimal concentration is necessary [30]. We pre-
sent the absorbance intensity of the phthalocyanines (10 μM) with an
increasing GO ratio (Fig. 1).
The ground state electronic absorption spectra of phthalocyanine
compounds show two dom- inant absorption bands in the ultraviolet
and visible region. These absorption bands are known as B (Soret) and
Q bands, respectively. The Q band is observed at around 600–750 nm
However, in this study ZnPc, fluorescence intensity (Fig. 2E and F)
shows little changes under the presence of GO in organic medium. The
unexpected weak quenching effect for unsub- stituted zinc phthalo-
cyanine could be related to a slight rotational twist in the interaction
47

F.S. Rocha, et al.
Materials Research Bulletin 114 (2019) 45–51
Fig. 2. Stern-Volmer plot of the fluorescence quenching efficiency of (A) ZnPc (λex = 670 nm) and (B) ZnPc(NO2)4 (λex = 685 nm). The inset figures show the
fluorescence quenching efficiency.
48

F.S. Rocha, et al.
Materials Research Bulletin 114 (2019) 45–51
angle, disorienting the π-π stacking process [41].
We also noted shifts in the fluorescence emission maximum wave-
2 4
length λmax of ZnPc(NO ) , although no shifts were observed for ZnPc.
The inset of Fig. 2F shows that the GO presence in the DMF solution
induces a blue-shift of λmax from 708 nm to ˜705 nm. For the MeCN
solution (Fig. 2E), it is hard to determine if there was any shift because
of the peak format and a substantial depletion of fluorescence emission.
Relevant information may be revealed by a change of the λmax re-
2 4
garding the microenvironment of the GO–ZnPc(NO ) interaction. For
biological molecules, a blue-shift usually indicates that the fluorescent
agent is exposed to a more hydrophobic environment, and a red-shift
implies an increase in polarity and hydrophilicity of the local molecular
environment [25,26]. In theory, loading of the tetra substituted
phthalocyanine onto the GO surface by π-π stacking should enhance the
hydrophobicity of the environment of the fluorophore, and the pre-
sented blue-shift is a good indication of a successful load.
Stern-Volmer (SV) equation was applied to analyze fluorescence
emission data [42], assuming
2 4
Fig. 3. Hill plot of the fluorescence quenching of ZnPc and ZnPc(NO ) in the
presence of GO with increasing concentrations.
that the interaction of phthalocyanine with GO occurs under equi-
librium conditions. To explain the fluorescence quenching process, two
mechanisms are shown to exist mainly as dynamic and static
quenching, and this process can be due to the transfer of electrons in the
excited state from the dye to GO, which acts like an electron acceptor
different from 1, we suggest that the binding with GO exhibits negative
cooperativity due to the interaction angle rotational twist, and the
description of the interaction between monomer transition dipoles in
molecular dimers can help elucidate if a more complicated vibrational
[
38]. Dynamic quenching occurs when the lifetime of the excited
fluorophore decreases, and for static quenching the lifetime remains the
same [43]. It is known that fluorescence quenching is primarily driven
by diffusive transport at low nanomaterial concentrations [44]. A linear
pattern is observed in the SV plot (Fig. 2A and B), and there is sub-
stantial evidence that the fluorescence quenching process is mainly
driven by a dynamic (collisional) effect, which occurs due to weak
structure is taking place [47]. The solution of ZnPc(NO
exhibits the lowest K value of 1.015 ± 0.182 μM, while ZnPc shows
the highest K value of 11.000 ± 1.613 μM in the same solvent. ZnPc
NO and ZnPc in DMF solution presents similar K values of
2 4
) in MeCN
d
d
(
2
)
4
d
6
.692 ± 2.044 μM and 7.050 ± 1.712 μM respectively. However,
there is no systematic data available in the literature regarding the
number of binding sites and the association constants for GO interaction
with phthalocyanines. A similar system with tetrasulfonated phthalo-
coupling of ZnPc and ZnPc(NO
values of (1.18 ± 0.08) × 10
4.84 ± 0.17) × 10
2 4
) at the GO surface. ZnPc expresses Ksv
−2
−1
M
in
M−1 in MeCN, and ZnPc(NO
)
2 4
DMF
expresses Ksv
DMF and
in MeCN. These values show that the Ksv
and
−
2
(
cyanines binding to prion proteins presents a K
and another system with sulfonated aluminum phthalocyanine binding
in human serum albumin presents a K value of 2.5 μM [49].
d
value of 8.5 μM [48]
−2
−1
values
of
(14.43 ± 2.5) × 10
M
in
−
2
−1
(
50.86 ± 4.19) × 10
M
d
value of the tetra-substituted phthalocyanine is 10 times higher in both
solvents than the non-substituted Pc, and the Ksv value is about 4 times
higher in MeCN than in DMF solution for both dyes.
To evaluate the strength and cooperativity of the GO–ZnPc and
2 4
GO–ZnPc(NO ) interaction, we applied the mathematical model of Hill
because the amount of template material plays a critical role in the
molecular aggregation process. The GO sheets lead to a perfect inter-
action orientation as H aggregates under optimal conditions due to the
π-π and electrostatic cooperative interactions of the dye and GO [29].
Thus, it is necessary to quantify key parameters describing the associ-
ation between phthalocyanines and GO, i.e., the saturation value Q,
D
binding dissociation constant K , and the Hill coefficient n, assuming
that the binding of phthalocyanine to GO occurs under equilibrium
conditions [26].
Singlet oxygen is generated when an activated sensitizer in its ex-
cited triplet state interacts with oxygen in its ground triplet state.
Evaluation of the ability of novel phthalocyanines to produce reactive
oxygen species in organic solvents and aqueous solutions can be valu-
able for predicting in vitro activity against cancerous cells or micro-
organisms [50]. The photodynamic activities of free ZnPc and ZnPc
(
(
NO
NO
2
)
)
4
were evaluated and compared against GO–ZnPc and GO–ZnPc
hybrids. The ability of the phthalocyanines to produce singlet
2
4
2
oxygen (1O ) was tracked using 1,3- diphenylisobenzofuran (DPBF),
verifying the decrease in DPBF absorbance, monitored at 417 nm during
irradiation with a red laser, as shown in Fig. 4. The DPBF indirect
method has been widely used to provide a quantitative analysis of
singlet oxygen production since the reaction product (1,2-di-
benzylbenzene) does not absorb visible light. This technique correlates
the reduc-tion of DPBF absorbance and the amount of oxygen quantum
yield generated by a type II photo process [51].
The interaction of the tetra substituted dye with GO (Fig. 3) shows a
higher Q value than the
non-substituted dye, and for both phthalocyanines there is a higher
saturation value using MeCN as the solvent. ZnPc and GO interactions
in DMF suggest a sigmoidal curve, indicating that there is more than
one binding site that exhibits positive or negative cooperativity [45].
2 4
ZnPc and ZnPc(NO ) photobleaching was not verified since the Q
absorption band at 670 nm and 685 nm, respectively, remains un-
changed. DPBF photodegradation can be visually mon- itored by the
color change of the solution from yellow to colorless when irradiated by
a 637 nm red laser (25 mW). The photodegradation of DPBF by free
2 4
The interaction of GO with ZnPc in MeCN and ZnPc(NO ) in both
solvents presented a Hill coefficient around 1, and for ZnPc in DMF the
Hill coefficient was 5.34. When n is around 1 this indicates an in-
dependent binding process, and the interaction is probably happening
under optimal conditions, leading to the formation of H-type ag-
gregates, which corroborate with the blue-shift observed in the ZnPc
phthalocyanines and GO–ZnPc and GO–ZnPc(NO
first order kinetics, as illustrated in the insets of Fig. 4.
The singlet oxygen quantum yield (Φ ) is the most critical para-
meter to evaluate in photody- namic activity. The Φ is the ratio of the
number of produced 1O molecules to the number of photons absorbed
by the photosensitizer [52]. A precise determination of Φ is difficult
2 4
) followed typical
Δ
Δ
2
(
2 4
NO ) UV–vis spectra. For n values significantly different from 1.0 this
Δ
implies that the interaction is more complex, and a simple second order
reaction is inadequate description of the reaction and a more detailed
kinetic analysis may be required [46]. As ZnPc presented a weak
quenching effect and the sigmoidal Hill plot and n were significantly
due to the aggregation of the photosensitizer in solution, which reduces
its photobiological activity. Ag- gregation is associated with a red-shift
(
bathochromic displacement) or
a
blue-shift (hypochromic
49

F.S. Rocha, et al.
Materials Research Bulletin 114 (2019) 45–51
Fig. 4. Photodegradation of DPBF by red laser irradiation in the presence of ZnPc and ZnPc(NO
)
2 4
associated with GO in MeCN and DMF. The photodegradation
followed the same pattern of absorbance decrease for the free phthalocyanines. The inset figures show first order kinetics. (A) ZnPc in MeCN, (B) ZnPc in DMF, (C)
ZnPc(NO
article).
2 4 2 4
) in MeCN, and (D) ZnPc(NO ) in DMF (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this
displacement) of the Q band, and no shifts were observed during the
photobleaching tests [53].
In general, substituted derivatives show lower singlet oxygen
quantum yields [54]. As can be observed in Table 1, the overall values
the fluorescence quantum yield of one abnormal fluorescence peak
from the phthalocyanine triplet excited state. Other GQDs interaction
with ZnTPPcQ showed an increase of singlet oxygen quantum yield
suggested by the increase in π bonds [32]. High triplet quantum yield
with a corresponding low fluorescence quantum yield suggests a more
efficient intersystem crossing, which is desirable for a compound that
may be used as a photosensitizer. The ability of a molecule to be long-
lived and to populate the triplet state is essencial since this has a direct
bearing on singlet oxygen production [57]. Another feature is the sol-
vent effect in the singlet oxygen gen- eration, higher in DMF than
MeCN. As described solvent effects on photosensitizing ability could be
due to relative shifts in the positions of the photosensitizer’s singlet and
triplet excited states. In some solvents, the separation between the
singlet and triplet may be maximal, resulting in a low intersystem
crossing rate [58,59]. With increasing solvent polarity, the energy gap
between
of Φ
Δ
for ZnPc are higher than for ZnPc(NO
2
)
4
, as ex- pected. The
high pro-
singlet quantum yield of ZnPc was higher than ZnPc(NO )
2 4
duction of the excited triplet state, results in reasonably high singlet
oxygen quantum yields, though competing processes like phosphores-
cence and non-radiative decay may lower the yield for singlet oxygen
production. The literature [11] shows that unsubstituted ZnPc has a
relatively high triplet lifetime of 300 μs and a higher singlet oxygen
quantum yield as compared to the other complexes. How- ever, the
ZnPc(NO
Orbital) energy level. The 1O
HOMO energy level can be effectively reduced by polar solvents.
When the dyes interacts with GO, Φ increase, displaying excellent
2
)
4
compound has a high HOMO (Highest Occupied Molecular
2
production of the compounds with high
Δ
ability to generate sin- glet oxygen with relatively high potential to be
employed in PDT, and this effect has already been observed for me-
thylene blue [55]. The fluorescence enhancement of tetrasulfonated
zinc phthalo- cyanine by graphene derivatives has already been re-
ported [56], suggesting that graphene quantum dots (GQDs) enhance
the triplet state and the singlet state increases, and thereby reduces
intersystem crossing (MeCN is more polar than DMF). This, in turn,
reduces the yield of singlet oxygen produced by energy transfer from
the triplet state of the GO-phthalocyanine hybrid.
4. Conclusions
Table 1
DPBF photobleaching rate constants (kDPBF) and singlet oxygen quantum
yields (ΦΔ) for free ZnPc and ZnPc(NO ) , GO–ZnPc, and GO–ZnPc(NO ) .
2 4 2 4
In summary, we prepared and characterized two Go–Pc photoactive
hybrid materials by a simple method. The GO–ZnPc(NO ) hybrid
2 4
presented an immediate and strong π-π stacking interaction, confirmed
by absorption and fluorescence spectra, while no evidence of strong π-π
stacking was observed for the GO–ZnPc hybrid. The Stern-Volmer
mathematical method suggests that the interaction of GO is 10 times
stronger with ZnPc(NO ) compared to ZnPc, and its linear slope graph
2 4
plot showed that the interaction is mainly driven by a collisional effect.
The Hill coef-ficient calculated indicates an independent binding pro-
Free Phthalocyanines
kDPBF(s−1)
ΦΔ
−
−
3
3
ZnPc DMF
GO–ZnPc DMF
ZnPc MeCN
GO–ZnPc MeCN
ZnPc(NO2)4 DMF
GO–ZnPc(NO2)4 DMF
ZnPc(NO2)4 MeCN
GO–ZnPc(NO2)4 MeCN
(4.45 ± 0.20) × 10
(5.31 ± 0.18) × 10
(1.69 ± 0.001) × 10
(1.37 ± 0.001) × 10
(4.32 ± 0.10) × 10
0.5639
0.71 ± 0.008
0.50 ± 0.002
0.71 ± 0.012
0.25 ± 0.005
0.32 ± 0.001
0.27 ± 0.014
0.29 ± 0.007
−
3
3
−
−
3
3
3
3
−
(5.43 ± 0.24) × 10
(3.97 ± 0.38) × 10
(4.09 ± 0.29) × 10
−
−
cess for GO–ZnPc(NO
2 4
) hybrid formation, leading to the formation of
H-type aggregates by strong π-π stacking and a more tricky binding
50

F.S. Rocha, et al.
Materials Research Bulletin 114 (2019) 45–51
process for the GO–ZnPc hybrid. The phthalocyanines presents a higher
singlet oxygen quantum yield associated with GO and remained stable
during all tests. Future research on decay lifetimes might extend the
explanations of GO-Pc interaction mechanisms in different solvents.
Overall, our results demonstrate a strong effect of solvent and type of
substitution on metallophthalocyanines in the preparation and prop-
erties of formed hybrids. In conclusion, it would appear that the pre-
pared hybrids are promising photosensitizer candidates for PDT, spe-
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Acknowledgements
[
[
[
The authors acknowledge the financial support from the University
of Brasília, the Coordi- nation for the Improvement of Higher
Educational Personnel (CAPES code 001), the Federal District Research
Foundation (FAPDF), the Scientific and Technological Development
Founda-tion (FINATEC), and the National Council for Scientific and
Technological Development (CNPq) to conceive this work. F.S.R.,
A.J.G, and C.N.L.G. conceived and designed the research. F.S.R carried
out the exper- iments. F.S.R and C.N.L.G. interpreted the results and
wrote the manuscript. The authors declare no competing financial in-
terests.
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