A novel axially palladium(II)-Schiff base complex substituted silicon(IV) phthalocyanine: Synthesis, characterization, photophysicochemical properties and photodynamic antimicrobial chemotherapy activity against Staphylococcus aureus

Article abstract of DOI:10.1016/j.poly.2019.114135

In this study, a novel silicon(IV) phthalocyanine is reported for the first time as a phthalocyanine derivative bearing axially a palladium(II)-Schiff base complex. The photophysical and photochemical properties of the new Si(IV)Pc, such as absorption, fluorescence, singlet oxygen quantum yields, triplet state quantum yields and exited state lifetimes were measured in DMSO. The new silicon phthalocyanine displayed very low fluorescence, showing efficient intersystem crossing resulting in high triplet and high singlet oxygen quantum yields in DMSO. When compared with the unsubstituted SiPcCl₂, the singlet oxygen quantum yield value (Φ_Δ = 0.47) in relation to the triplet quantum yield (Φ_T = 0.82), which is an important determinant for PDT applications, increased. The photodynamic antimicrobial chemotherapy activity (PACT) of new Si(IV)Pc towards Staphylococcus aureus was determined in comparison to the unsubstituted SiPcCl₂. The results of the photodynamic antimicrobial effect study demonstrated that the Pd(II) complex substituted SiPc (5) possesses excellent photodynamic activity with a reduction percentage value of 99.94% and a log red value of 3.26.

Full text of DOI:10.1016/j.poly.2019.114135

Journal Pre-proofs
A novel axially palladium(II)-Schiff base complex substituted silicon(IV)
phthalocyanine: synthesis, characterization, photophysicochemical properties
and photodynamic antimicrobial chemotherapy activity against Staphylococcus
aureus
Pinar Sen, Tebello Nyokong
PII:
S0277-5387(19)30565-0
DOI:
https://doi.org/10.1016/j.poly.2019.114135
POLY 114135
Reference:
Polyhedron
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Accepted Date:
11 July 2019
21 August 2019
Please cite this article as: P. Sen, T. Nyokong, A novel axially palladium(II)-Schiff base complex substituted
silicon(IV) phthalocyanine: synthesis, characterization, photophysicochemical properties and photodynamic
antimicrobial chemotherapy activity against Staphylococcus aureus, Polyhedron (2019), doi: https://doi.org/
10.1016/j.poly.2019.114135
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A
novel axially palladium(II)-Schiff base complex substituted silicon(IV)
phthalocyanine: synthesis, characterization, photophysicochemical properties and
photodynamic antimicrobial chemotherapy activity against Staphylococcus aureus.
Pinar Sen,a*, Tebello Nyokonga*
aInstitute for Nanotechnology Innovation, Department of Chemistry, Rhodes University, PO
Box 94, Grahamstown, 6140, South Africa
Abstract
In this study, a novel silicon(IV) phthalocyanine is reported for the first time as a
phthalocyanine derivative bearing axially a palladium(II)-Schiff base complex. The
photophysical and photochemical properties of the new Si(IV)Pc, such as absorption,
fluorescence, singlet oxygen quantum yields, triplet state quantum yields and exited state
lifetimes were measured in DMSO. The new silicon phthalocyanine displayed very low
fluorescence, showing efficient intersystem crossing resulting in high triplet and high singlet
oxygen quantum yields in DMSO. When compared with the unsubstituted SiPcCl2, the singlet
oxygen quantum yield value (Φ = 0.47) in relation to the triplet quantum yield (ΦT = 0.82),
which is an important determinant for PDT applications, increased. The photodynamic
antimicrobial chemotherapy activity (PACT) of new Si(IV)Pc towards Staphylococcus aureus
was determined in comparison to the unsubstituted SiPcCl2. The results of the photodynamic
antimicrobial effect study demonstrated that the Pd(II) complex substituted SiPc (5) possesses
excellent photodynamic activity with a reduction percentage value of 99.94% and a log red
value of 3.26.
Keywords: Synthesis, Silicon(IV) phthalocyanine, Schiff base complex, Palladium,
Photodynamic antimicrobial therapy, S. aureus, Photophysicochemical properties
*Corresponding authors. Tel: + 27 46 6038260; Fax: + 27 46 6225109.
E-mail : t.nyokong@ru.ac.za. (T. Nyokong), sen_pinar@hotmail.com (P. Şen)

Introduction
After the World Health Organization reported that bacteria show multi-drug resistance,
which is encountered as the most important problem for human health, photodynamic
antimicrobial chemotherapy (PACT) has gained increasing interest as an alternative
bactericidal technique [1,2]. PACT is a light-controlled therapy and offers more promising
strategies to prevent and kill pathogenic bacteria compared with conventional antibiotic
approaches [3,4].
The mechanism of the PACT process is based on the photo-inactivation of
microorganisms through singlet oxygen produced with the reaction of non-toxic dyes called
photosensitizers (PS) and visible light in the presence of oxygen [5]. The singlet oxygen,
produced by energy transfer from the triplet state PS to molecular oxygen, reacts with the
biomolecules and performs a photodynamic process which is responsible for killing the
bacteria [6].
PSs play an important role in this process since they absorb the light and produce toxic
species such as singlet oxygen. Some of the features for ideal antimicrobial PS are high
singlet oxygen production resulting from high triplet-state quantum yields and a high molar
extinction coefficient, specifically in the red and near-infrared UV-vis spectral regions (600-
800 nm range), for a maximum light penetration [7].
Phthalocyanines, as heterocyclic macrocyclic aromatic compounds, are known as
potential PSs, meeting these mentioned requirements for photodynamic applications [8,9], in
addition to the use in various fields such as dye-sensitized solar, non-linear optical devices,
sensors, catalysis [10].
Considering the fact that the structure of PSs may be a key factor in determining their
antimicrobial activity, several kinds of phthalocyanines with different central metals and as
neutral, anionic and cationic structures have been reported [11-14]. Among the studied

phthalocyanines, axially substituted silicon(IV) phthalocyanines (SiPcs) show promising
antibacterial effects [15-17]. As SiPcs can provide axial substitution, they help to reduce
aggregation and increase the solubility in organic solvents, and this structural property
provides a significant advantage for PDT applications [18].
Schiff bases came into existence by the condensation reaction of a primary amine and
an aldehyde. They are the most studied ligands as building blocks in coordination chemistry
since they can generate very stable complexes with various heavy metal ions [19]. Metal
complexes of Schiff bases, providing metal-nitrogen bonding, are formed by the complexation
of Schiff bases and have had importance in medicinal chemistry for many years [20]. They
have gained considerable attention because of their biological activities against bacteria, fungi
and certain type of tumours [21].
The influences of metal ions on biological activities, such as antibacterial, antifungal,
anticancer and antioxidant, have been taken into consideration and well documented [22]. In
this context, palladium(II) complexes of Schiff base ligands have been reported as bio-active
agents following the success of cis-platin, which is used antitumor drugs worldwide [23-25].
In this study, we have prepared the axially Schiff base palladium(II) complex
substituted silicon(IV) phthalocyanine as a combination of two functional molecules, resulting
in a trinuclear phthalocyanine derivative. To the best of our knowledge, this kind of molecule
has not been reported to date. The photophysical-chemical properties of the obtained new
structure were determined and the photodynamic antibacterial activity against
Staphylococcus aureus was determined, comparing the results with the unsubstituted SiCl2Pc
to observe the effect of the metal complex when incorporated into the silicon(IV) Pc.
2. Experimental
2.1. Chemicals

4-Tert-butyl-phenol, SnCl4, 2,6-lutidine, paraformaldehyde, 1,3-diamino-2-propanol, PdCl2,
NaH, ethanol, acetonitrile, chloroform (CHCl3), triethylamine (TEA), toluene, cyclohexane,
diethylether and 1,3-diphenylisobenzofuran (DPBF) were purchased from Sigma Aldrich. All
solvents were dried and purified as reported by Perrin and Armarego [26] before use. Silica
gel (Merck grade 60) was used for the column chromatography. Phosphate-buffered saline
(PBS) solution pH 7.4 was prepared using the appropriate amounts of Na2HPO4 and KH2PO4
in ultra-pure water from Millipore water, ELGA, Veolia water PURELAB, flex
system (Marlow, UK). Nutrient agar and agar bacteriological BBL Muller Hinton broth were
purchased from Merck and S. aureus (ATCC 25923) was obtained Davies Diagnostics.
2.2. Instruments
Fluorescence emission spectra were recorded on a Varian Eclipse spectrofluorometer with a
1cm quartz cell at room temperature. Infrared spectra were recorded on a Bruker ALPHA FT-
IR spectrometer with a platinum attenuated total reflectance (ATR) sampling accessory.
Absorption spectra were recorded on a Shimadzu UV-2550 spectrophotometer. Fluorescence
lifetimes were measured using a time correlated single photon counting setup (TCSPC)
(FluoTime 300, Picoquant GmbH). The excitation source was a diode laser (LDH-P-670
driven by PDL 800-B, 670 nm, 20 MHz repetition rate, 44 ps pulse width, Picoquant GmbH).
Triplet quantum yield values were obtained using a laser flash photolysis system having a
LP980 spectrometer with a PMT-LP detector and an ICCD camera (Andor DH320T-25F03).
The signal from the PMT detector was recorded on a Tektronix TDS3012C digital storage
oscilloscope. The excitation pulses were produced by a tunable laser system consisting of a
Nd:YAG laser (355 nm, 135 mJ/4–6 ns), pumping an optical parametric oscillator (OPO, 30
1
mJ/3–5 ns) with a 420 to 2300 nm (NT-342B, Ekspla) wavelength range. Acquisition of H
NMR spectra was carried out on a Bruker AMX600 MHz NMR spectrometer. Mass analysis

was performed on a Bruker AutoFLEX III Smartbeam TOF/TOF mass spectrometer. Photo-
irradiations for singlet oxygen studies were done using a General Electric Quartz line lamp
(300W). A 600 nm glass cut off filter (Schott) and a water filter were used to filter off
ultraviolet and infrared radiation respectively. An interference filter (Intor, 670 nm with a
band width of 40 nm) was additionally placed in the light path before the sample. Light
intensity was measured with a POWER MAX5100 (Molelectron detector incorporated) power
meter. The Modulight ML7710-680-RHO laser system (680 wavelength probe) was employed
for the PACT studies. For the PACT studies, various irradiation periods at 0.5 W cm−2
irradiance, resulting in energy doses ranging from 0.9 kJ cm−2 to 3.6 kJ cm−2 corresponding to
time periods ranging from 30 to 120 min, were employed.
2.3. Synthesis
The syntheses of 5-tert-butyl-2-hydroxybenzaldehyde (1) and silicon phthalocyanine
dichloride (SiPcCl2) (4) have been reported elsewhere [27,28]. The obtained spectroscopic
results are in agreement with the literature data.
2.3.1. Synthesis of N,N’-bis(5-tert-butyl-salicylidene)-1,3-diamino-2-propanol (H3L) (2)
5-Tert-butyl-2-hydroxybenzaldehyde (1) (0.5 g, 2.8 mmoL) was dissolved in 20 mL ethanol,
then 1,3-diamino-2-propanol (0.126 g, 1.4 mmoL) dissolved in 20 mL of ethanol was added
dropwise to the solution. The reaction mixture was purged with argon and the reaction
mixture stirred under reflux for 8 h. The reaction for the formation of the Schiff base (2) was
monitored by thin layer chromatography (TLC) (CHCl3/EtOH 10/0.1). After the reaction
completed, the solution was evaporated to 1/3 volume. The formed precipitate was filtered
off, washed with ethanol and dried in a vacuum desiccator. Yield: 75% (0.430 g). FT-IR
(UATR-TWOTM)  max/cm-1: 3378 (OH), 3304 (OH), 3053 (Ar, C-H), 2954-2867 (Aliph, C-

H), 1634 (C=N), 1612-1513 (Ar, C-C), 1491-1392 (C-C), 1266 (tert, C-N), 1241 (Asym., Ar-
O-), 1179 (C-N), 1051 (Sym., Ar-O-), 828. 1H-NMR (DMSO) δ (ppm): 8.30 (s, 2H, -CH=N),
7.43 (s, 2H, ArH), 7.35 (d, 2H, ArH), 6.83 (d, 2H, ArH), 5.24 (s, 2H, OH), 4.59 (s, 1H, -OH),
3.77 (d, 4H, -CH2), 3.39-3.31 (m, 1H, -CH), 1.27 (s, 18H, -CH3). 13C-NMR (CHCl3) δ (ppm):
167.86, 155.44, 141.26, 129.85, 126.32, 123.55, 116.53, 69.97, 63.27, 34.15, 31.91. (MALDI-
TOF) m/z: 411.527 [M+1]+. Anal. Calc. for C25H34N2O3 (%): C, 73.14; H, 8.35; N, 6.82; O,
11.69; Found (%): C, 73.09; H, 8.38; N, 6.78
2.3.2. Synthesis of [N,N’-bis(5-tert-butyl-salicylidene)-1,3-diamino-2-propanol]palladium(II)
(3)
In a 100 ml round bottom flask, the ligand (2) (0.2 g, 0.48 mmoL) was dissolved in
acetonitrile (20 mL). A solution of PdCl2 (0.088 g, 0.50 mmol) in 20 mL acetonitrile was
added dropwise to the mixture, followed by stirring for 30 min at room temperature under an
argon atmosphere. After this, TEA (0.147 mL, 1.05 mmoL) was added to the resulting
solution; the reaction mixture was stirred under reflux for 12 h. The completion of the
complexation reaction was monitored by TLC (CHCl3/EtOH 10/0.1) and then it was allowed
to cool. The solution was evaporated to 1/3 volume and the precipitate formed by addition of
cyclohexane was collected by centrifugation. The collected compound was washed with
cyclohexane and diethylether, and dried. Yield: 70% (0.175 g). FT-IR (UATR-TWOTM) 
max/cm-1: 3335 (OH), 3062 (Ar, C-H), 2956-2868 (Aliph, C-H), 1614 (C=N), 1542 (Ar, C-
C), 1449-1363 (C-C), 1262 (tert, C-N), 1214 (Asym., Ar-O-), 1123 (C-N), 1061 (Sym., Ar-O-
1
), 827. H-NMR (DMSO) δ (ppm): 8.32 (s, 2H, -CH=N), 7.35 (s, 2H, ArH), 7.17 (d, 2H,
ArH), 6.98 (d, 2H, ArH), 4.45-4.41 (m, 1H, -CH), 3.61 (t, 4H, -CH2), 3.76 (s, 1H, OH), 1.69
13
(s, 18H, -CH3). C-NMR (CHCl3) δ (ppm): 164.97, 152.42, 141.67, 126.25, 124.07, 121.81,
114.84, 68.14, 42.48, 33.83, 31.68. (MALDI-TOF) m/z: 533.155 [M+H2O+1]+. Anal. Calc.

for C25H32N2O3Pd (%): C, 58.31; H, 6.26; N, 5.44; O, 9.32; Pd, 20.67; Found (%): C, 58.28;
H, 6.35; N, 5.39.
2.3.1. Synthesis of the silicon(IV) phthalocyanine (5)
A mixture of [N,N’-bis(5-tert-butyl-salicylidene)-1,3-diamino-2-propanol]palladium(II) (3)
(0.100 g, 0.194 mmoL) and silicon phthalocyanine dichloride (4) (0.04 g, 0.064 mmol) in
toluene (40 ml) was stirred and then sodium hydride (5 mg, 0.194 mmol) was added to this
mixture under an inert argon atmosphere. The resulting mixture was stirred under reflux at
o
110 C for 2 days. After cooling to room temperature, the mixture was filtered off and the
solvent was evaporated in a vacuum. The crude product was further purified by column
chromatography (silica gel, CHCl3/THF//TEA-85/15/1) to give 5 as a green solid. Yield: 18%
(0.019 g). FT-IR (UATR-TWOTM)  max/cm-1: 3053 (Ar, C-H), 2952-2865 (Aliph, C-H),
1612 (C=N), 1531 (Ar, C-C), 1439-1363 (C-C), 1260 (tert, C-N), 1208 (Asym., Ar-O-), 1121
1
(C-N), 1077 (Sym., Ar-O-), 1064 (Si-O), 829, 734. H-NMR (DMSO) δ (ppm): 8.45 (s, 4H),
7.95 (m, 8H), 7.35-7.04 (m, 8H), 6.87 (s, 4H), 6.68 (d, 4H), 6.58 (d, 4H), 3.78 (m, 8H), 3.14
(m, 2H), 1.29 (s, 36H). UV–Vis (DMSO-1x10-5 M) λmax (nm) (log ε): 680 (4.5), 355 (4.36).
(MALDI-TOF) m/z: 1582.718 [M+H2O]+, 1085.674 [M+H2O-C25H31N2O2Pd]+. Anal. Calc.
for C82H78N12O6Pd2Si (%): C, 62.79; H, 5.01; N, 10.72; O, 6.12; Pd, 13.57; Si, 1.79; Found
(%): C, 62.44; H, 5.06; N, 10.58
2.4. Antimicrobial studies
The photodynamic antimicrobial studies of the Pcs 4 and 5 against Staphylococcus aureus
were performed through the viable cell count method. Firstly, a stock culture of S. aureus
was grown aerobically in 6 mL of nutrient broth under incubation at 37 °C on a rotary shaker
(200 rpm) to obtain the bacteria culture. The incubation was maintained until the optical

density of the culture recorded at 600 nm was determined to be around 0.6. The resulting
microbial solution was centrifuged to remove the broth culture and the bacteria colonies were
washed three times with PBS. Then, a stock solution of the bacteria cultures was prepared by
diluting to 1/1000 (106 colony forming unit (CFU/mL). The antimicrobial photoactivity of the
phthalocyanines were determined at 2.5 μM bacteria concentration and each Pc (4/5) was
prepared by dissolving in water containing 5% DMSO, corresponding to 0.312–40 μM
concentrations in 6 mL of PBS. The prepared suspensions containing the bacteria and Pcs at
different concentrations were tested with the plate method over agar to observe the optimum
application concentration.
The optimum application concentrations of Pcs were determined as 10 μM for 4 and 0.625
μM for 5 from the agar plates as a result of the CFU/mL counting, depending on the degree of
reduction in bacteria colonies by exposure to light from 30 to 120 min.
After the bacteria/photosensitizer suspensions at the mentioned specific concentrations were
determined, each prepared bacteria/photosensitizer suspension was incubated in an oven
equipped with a shaker for 30 min in the dark at 37 °C before plating. The photoinactivation
with the Si(IV) Pcs (4 and 5) was evaluated with 30 min irradiation time intervals up to 120
min. While a part of the incubated bacterial suspensions was kept in the dark, the other part of
the suspension was irradiated at the Q-band maximum of the photosensitizers, using methods
previously reported [29]. The results of the performed experiments were determined by
plating over agar after irradiation and keeping the experiments in the dark for a certain period
of time. Also, control treatments were performed without photosensitizer, both with
irradiation and in the dark, in order to determine the effect of light and solvents. All
experiments were carried out three times. The data for CFU/mL were converted to the
logarithmic form.

3. Results and discussion
3.1. Synthesis and characterization
The reaction steps carried out for the target compound are summarized in Scheme 1.
Scheme 1.
The synthesis of 5-tert-butyl-2-hydroxybenzaldehyde (1) was achieved by starting with the
commercially available 4-tert-butyl-phenol and applying the procedure for obtaining
aldehydes in the ortho-position of aromatic phenols [28]. The new ligand (2), containing a
bis-imine group and acting as N2O2-chelating, was synthesised by the usual condensation
reaction of the aldehyde derivative (1) and 1,3-diamino-2-propanol in ethanol at reflux
temperature. The reaction of PdCl2 with the ligand (2) was conducted in acetonitrile in the
presence of TEA to give the palladium Schiff base complex (3). Finally, the reaction of
SiPcCl2 (4) and [N,N’-bis(5-tert-butyl-salicylidene)-1,3-diamino-2-propanol]palladium(II) (3)
provided the silicon(IV) phthalocyanine (5), using NaH in toluene.
13
1
Common spectroscopic methods, such as FT-IR, C-NMR, H-NMR, UV/Vis and
MS, together with elemental analysis, were employed for determining the proposed structures.
All the spectroscopic data confirmed the expected compounds.
In the FT-IR spectrum of the condensation product 2, the first significant difference is
that the carbonyl vibration of the aldehyde functional group at around 1640 cm-1 disappeared
and an imine vibration (-C=N) band appeared at 1634 cm-1. In the FT-IR spectrum of 3, the
C=N peak originating from compound 2 shifted to a lower frequency, appearing at 1614 cm-1
upon complexation, which is confirmation for the coordination of the ligand through the
azomethine nitrogen atom [30]. Also, the vibrational band observed at 3378 cm-1 due to Ar-
OH in the free ligand disappeared in the Pd(II) complex due to deprotonation. The aliphatic -

OH vibration band of diaminopropanol is seen in the Pd(II) complex (3), indicating that the –
OH group of the diaminopropanol does not take part in coordination [30].
In the FT-IR spectrum of 5, the -C=N peaks of the Pd(II) complex appeared at 1612
cm-1 and the -OH vibration belonging to compound 3 at 3335 cm-1 disappeared, providing
evidence for the formation of 5. The most significant result was the observation of a Si-O
band at 1064 cm-1 [31].
A MALDI-TOF mass spectrometer was utilized for acquisition of the mass spectra of
the products. The expected molecular ion peaks were obtained at m/z 411.527 [M+1]+ for 2,
m/z 533.155 [M+H2O+1]+ for 3 and 1582.718 [M+H2O]+ for 5. The fragment ion peak
belonging to SiPc (5) was also observed at m/z 1085.674 [M+H2O-C25H31N2O2Pd]+,
corresponding to the removal of one Pd(II) complex from the axial site, as seen in Fig. 1.
Fig. 1.
UV-Vis spectroscopy is one of the first methods used for the structural
characterization of phthalocyanines. Phthalocyanines, with an 18 π-electron conjugated
aromatic system, show characteristic bands in their absorption spectra. The strongest of these
is the Q band, which results from the chromophoric system and expresses an absorption in the
visible region between 600 and 700 nm. The other main absorption band, which is the so-
called B band, is in the UV region at about 300-350 nm. The Q band is attributed to the π-π*
transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied
molecular orbital (LUMO) of the Pc ring. The B-band arises from deeper π levels to the
LUMO [32].
The comparative absorption spectra of the Pd(II) complex 3 and Pc 5 were studied in
DMSO at 1x10-5 M concentrations (Figure 2). In the UV-vis spectrum of the 3, the most
distinctive absorption band appear at around 400 nm, which is ascribed to metal-to ligand

charge transfer transitions [33]. The electronic absorption spectrum of the SiPc (5) is typical
for non-aggregated phthalocyanines, showing an intense Q band at 680 nm, which is
indicative for the metallated phthalocyanines, as expected [32], together with two vibronic
bands at 613 and 654 nm, and the B (or Soret) band at 355 nm. Also, the metal-to ligand
charge transfer transition resulting from the Pd(II) complex was observed between 400 and
450 nm in the UV-vis spectrum of 5. When comparing to the UV-vis spectrum of the SiPc (5)
with the unsubstituted SiPcCl2 (4), the Q band of Pd(II) complex substituted SiPc is 8 nm red
shifted in DMSO (Table 1).
Fig. 2
1H-NMR and 13C-NMR spectra have supplied satisfactory information verifying the proposed
1
structures. In the H-NMR spectrum of 2, the disappearance of the HC=O proton signal as
distinct from 1 and the appearance of an imine proton peak at δ 8.30 ppm indicate that the
condensation reaction has occurred to give the imine compound. The phenolic OH and
aliphatic OH protons, originating from diamino propanol, were observed at δ 5.35 and 4.59
ppm, respectively. The methylene protons were observed at δ 3.77 and 3.39-3.31 ppm and the
methyl protons on the benzene ring were at δ 1.27 ppm.
In the 1H-NMR spectrum of 3, the disappearance of the signal due to the phenolic OH protons
originating from the free ligand (2) indicates the coordination of phenolic oxygen atom to the
Pd(II) ion. The azomethine –CH=N– proton of the ligand (2) shifted to high field in complex
3 at δ 8.32 ppm. In the 1H-NMR spectrum of compound 5, the appearance of the new peaks in
the aromatic region at δ 7.95-6.58 ppm are proof of axial ligation.
When the structures obtained were evaluated by 13C-NMR spectroscopy, the peak at δ
167.86 ppm belonging to the carbon atom of the imine group is characteristic proof for the
existence of Schiff base formation for compound 2. The aromatic and aliphatic carbon peaks

were observed in the ranges δ 155.44-116.53 and 69.97-31.91 ppm, respectively. The
azomethine carbon atom of complex 3 appeared at δ 164.97 ppm upon coordination to the
Pd(II) ion. The aromatic and aliphatic carbon peaks were observed in the ranges δ 152.42-
114.84 and 68.14-31.68 ppm, respectively.
3.2. Physicochemical properties
3.2. 1 Fluorescence quantum yields (ΦF), emission and lifetimes
The fluorescence emission, absorption and excitation spectra of the newly synthesized SiPc
compound are shown in Figure 3. The fluorescence behaviour of SiPc 5 was studied in
DMSO by comparing it with the unsubstituted SiPcCl2 (4) and the related data are listed in
Table 1.
Fig. 3.
Upon excitation at 608 nm, the fluorescence emission wavelength of the studied Pc (5) was
determined to be 682 nm. The emission profile is a mirror image of the absorption spectrum
for 5. The almost symmetric appearance of the emission and absorption spectra indicate that
the emission corresponds to similar absorption transitions [34].
The excitation spectrum of compound 5 is slightly blue-shifted compared to its
absorption spectrum. This observation could be regarded an inconsiderable shift in
comparison with its absorption spectra. The closeness of the wavelengths in the absorption
and excitation spectra could be attributed to the fact that there is no change in the molecule
upon excitation [35]. In addition, the observed Stokes shift was 7 nm for 5 in DMSO, which is
similar to the unsubstituted SiPcCl2 and is typical for MPc complexes (Table 1).
The fluorescence quantum yield (ΦF) and fluorescence lifetime (τF) are also important
for photodynamic applications. ΦF is described as the ratio of the number of photons emitted

through fluorescence, to the number of photons absorbed by the fluorophore, which indicate
the efficiency of the radiation-induced process. The fluorescence quantum yields of the
obtained Pcs were calculated using a method reported in the literature, using equation 1 [36]:
(1)
In this equation, F and FStd are the areas under the emission curves of the Pcs and
standards, respectively. A and AStd indicate the absorbance values corresponding to the
excitation wavelength of the sample and standard, respectively. The refractive indices of the
solvents used for the solution of the studied samples and standard are respectively expressed
as n2and n2Std. The unsubstituted ZnPc was utilized as a standard with the value ΦF = 0.2 [37].
The fluorescence quantum yield (ΦF) of the SiPc (5) was measured in DMSO. The ΦF
value of 5 was determined as 0.015 (Table 1). The ΦF value of 5 is much lower than those of
the ZnPc standard and unsubstituted SiPcCl2 (ΦF = 0.41) in DMSO (Table 1). This indicates
the presence of the Schiff base metal complex as an axial ligand, where Pd, as a heavy atom,
allows inter-system crossing [38].
The fluorescence lifetime (τF) of the axially-disubstituted Schiff base Pd(II) complex
silicon phthalocyanine (5) was determined from the exponential fitting of the fluorescence
decay curves by utilizing the TCSPC (time correlated single photon counting) method, which
is a counting process in DMSO (Figure S6 in the supporting information).
The determined τF value is 4.46 ns for 5, which is smaller than that for the
unsubstituted SiPcCl2 (τF = 5.06 ns). The fluorescence lifetime values are directly proportional
to the fluorescence quantum yields, meaning a fluorophore having a high fluorescent quantum

yield has a high fluorescence lifetime [39]. When comparing SiPc 5 with SiPcCl2 a good
correlation is seen.
3.2.2. Singlet oxygen quantum yields (Φ)
The most important factor that determines whether a photosensitizer is applicable to
PDT applications is the efficiency of its singlet oxygen generation. Singlet oxygen (1O2)
production is basically based on the energy transfer between a triple excited state and the
ground state of oxygen, in addition to substituents on the photosensitizer and solvents which
could quench singlet oxygen.
The singlet oxygen quantum yield (Φ) was determined by the comparative chemical
method, which is based on the chemical quenching of DPBF upon oxidation by singlet
oxygen generated during irradiation of 5 in DMSO (Fig. S7 in the supporting information).
The quenching of DPBF was monitored with an UV-Vis spectrophotometer, showing a
gradual decrease in absorbance upon light irradiation in the presence of SiPc 5. The same
procedure was applied for the unsubstituted ZnPc ( = 0.67 in DMSO) as a reference [40]
for comparison purposes, using the experimental set-up as described in the literature [41].
The novel Si(IV) Pc (5) bearing the Schiff base-Pd(II) complex produced higher
singlet oxygen generation in DMSO with a Φ value of 0.47 compared to the axially
unsubstituted SiPcCl2 (Φ = 0.18). This observation could be attributed to the heavy metal
effect of the palladium atom which enhances intersystem crossing to the triplet state, resulting
in high triplet and singlet oxygen quantum yields [35].
An electronic transition from a singlet to a triplet excited state within a molecule is a
spin-forbidden process and as such occurs inefficiently for many compounds. In order for an
effective transition between states to occur, spin-orbit perturbation is generally required [42].
Enhanced spin-orbit perturbations can be accomplished by adding a heavy atom directly to the

molecule (an internal heavy atom effect), by placing the molecule in an environment
containing heavy atoms (an external heavy atom effect) or by linking groups containing heavy
atoms to the main structure (an external heavy atom effect) [43,44].
When compared to previously published di-axially substituted Si(IV) phthalocyanine
studies, 5 was found to be more effective in terms of production of singlet oxygen [45,46]. At
the end of the chemical degradation experiment, while the singlet oxygen quenchers showed
degradation, there was no change in the Q-band, indicating the stability of 5.
3.2.3. Triplet quantum yield (ΦT) and lifetimes (τT)
One of the most common transitions of an excited photosensitizer is intersystem
crossing to populate its triplet state. The triplet quantum yield is a centrally important
parameter in photodynamic applications since the efficiency of a phthalocyanine as a
photosensitizer is dependent on its triplet state quantum yield (ΦT) and lifetime (τT). PSs
performing efficient intersystem crossing and generating a high triplet state population and
lifetime cause high singlet oxygen production, which is toxic for cells.
The triplet quantum yield (ΦT) was measured by exciting SiPc 5 and the axially
unsubstituted SiPcCl2 (4) at the Q-band wavelength by utilizing the unsubstituted ZnPc (ΦT =
0.65) [47] as a reference, as described in the literature [44]. The samples and the standard
were dissolved in deaerated DMSO. The Si(IV) Pc (5) exhibited a triplet quantum yield with a
value of 0.82, which is higher than the axially unsubstituted SiPcCl2 with a value of 0.38,
indicating that 5 exhibits a more efficient intersystem crossing as a result of the heavy atom
effect.
The triplet lifetime decay curve of the SiPc (5) (as an example) is shown in Figure S8
in the supporting information. An exponential decay curve was used to fit the experimental
data. The triplet lifetime (τT), which is much longer than the fluorescence lifetime, was

determined as 235 s for 5 and 155 s for 4. The triplet quantum yields and triplet state
lifetimes are shown in Table 1.
Table 1.
3.3. Antimicrobial activity
The photodynamic activities of 4 and 5 were studied against Staphylococcus aureus,
which is a gram-positive bacterium. The unsubstituted SiPcCl2 (4) was studied for
comparative purposes in order to identify the effect of the Pd(II)-Schiff base complex on the
photodynamic antimicrobial activity when it introduced as an axial ligand in 5.
The PACT experiment performed for 4 was not effective at lower concentrations. This
is why compound 4 was prepared at much a higher concentration than 5. The photodynamic
inactivation of the Pcs in terms of the decreased cells were quantified as Log reductions (Fig.
4.) and the reduction percentage as opposed to viable cell percentage (Fig. 5)
The reduction in bacteria colonies were calculated by using equation 2 [48]:
(2)
log reduction = log10(A) - log10(B)
where A and B are the number of viable microorganisms before and after treatment with light
or in the dark, respectively.
While the SiPcCl2 (4) did not exhibit dark toxicity, the SiPc (5) showed dark
cytotoxicity against S. aureus after being kept for 120 min (Fig. 5). This difference for
compound 5 could be attributed to carrying the Pd(II) complex in the axial position, since
palladium complexes are kinetically labile and naturally qualify as metallodrugs [49]. Control
experiments showed that the viability of bacteria was insignificantly affected by illumination
(data not shown).

The Pd-complex substituted SiPc (5) displayed the higher potency with a log reduction
value of 3.26, more than that of the unsubstituted SiPcCl2 (4) with a log reduction value of
1.94, as a result of radiation with light (Table 2).
Photodynamic antimicrobial therapy is based on the ability of singlet oxygen
production of PS. This is related to why SiPc (5) has a much better efficacy despite being
applied in a 16 times smaller concentration. It also relates to the antimicrobial effect of the
palladium complex itself [46].
Fig. 4.
The survival percentage (viable colonies) of the bacteria is stated by the number of
colonies remaining alive after treatment of the cells. Fig. 5 represents the bar graph of the
survival percentage of S. aureus performed with light and in the dark for compounds 4 and 5.
When the results were evaluated in terms of the reduction percentage of the cells, the
reduction percentage value of 5 reached 99.94% after exposure to light for 120 min (Table 2).
Fig. 5.
Table 2.
Conclusion
A new type of silicon(IV) phthalocyanine carrying a Pd(II)-Schiff base complex at the axial
position was successfully synthesized and characterized in this work. The new phthalocyanine
1
13
was characterized by common spectroscopic methods, such as H-NMR, C-NMR, FT-IR,
UV–Vis and MS and elemental analysis. After the photophysicochemical properties were
determined, the potential of the photodynamic antimicrobial effect of the novel Pc on

Staphylococcus aureus was investigated. The results of the antimicrobial activities indicated
that the newly synthesized SiPc (5) exhibit higher phototoxicity against bacterial pathogens at
much lower concentrations than that of the unsubstituted SiPc (4).
Acknowledgements
This work was supported by the Department of Science and Technology (DST) Innovation,
National Research Foundation (NRF) South Africa and Sasol Inzalo Foundation through
DST/NRF South African Research Chairs Initiative for Professor of Medicinal Chemistry and
Nanotechnology (UID 62620) as well as Rhodes University.
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A
novel axially palladium(II)-Schiff base complex substituted silicon(IV)
phthalocyanine: synthesis, characterization, photophysicochemical properties and
photodynamic antimicrobial chemotherapy activity against Staphylococcus aureus.
Pinar Sen,a*, Tebello Nyokonga*
aInstitute for Nanotechnology Innovation, Department of Chemistry, Rhodes University, PO
Box 94, Grahamstown, 6140, South Africa

O
O
Pd
O
N
N
N
N
N
N
Si
N
N
N
N
O
N
N
Pd
O
O
Staphylococcus
aureus
A novel silicon(IV) phthalocyanine bearing axially a palladium(II)-Schiff base complex is
reported for the first time as a phthalocyanine derivative. The photophysical and
photochemical properties were determined. The photodynamic antimicrobial chemotherapy
activity (PACT) of new Si(IV)Pc was determined towards Staphylococcus aureus in
comparison with the unsubstituted SiPcCl2.
Table Captions
Table 1. Photophysicochemical parameters of the complexes
Table 2. Percentage reduction and log reduction of S.aureus after irradiating for 120 min.

Table 1.
Compound
Stokes shift
Solvent
Abs
Exc.
Em.
ΦF
τF (ns)
Φ휟
ΦT
τT (s)

Stokes, nm
SiPc (5)
SiPcCl2 (4)
DMSO
DMSO
680
672
675
674
682
680
0.015
0.41
7
6
4.46 0.47 0.82
5.06 0.18 0.38
235
155
Table 2.
Compound
Concentration (M) Percentage reduction
Log reduction
SiPc (5)
0.625
10
99.94
98.86
3.26
1.94
SiPcCl2 (4)

Figure Captions
Scheme 1. Synthesis route: (i) SnCl4, 2,6-lutidine, toluene, paraformaldehyde, reflux, 12
hours, (ii) ethanol, reflux, 2 hours, (iii) PdCl2, acetonitrile, TEA, reflux, 12 hours, (iv) NaH,
toluene, reflux, 2 days.
Fig. 1. MALDI-TOF MS spectra of 2 (a), 3 (b) and 5 (c).
Fig. 2 Absorption spectra of the palladium complex (3) and the SiPc (5) in DMSO
(concentration 1x10−5 mol.L−1)
Fig. 3. Normalized absorption, excitation and emission spectra of the SiPc (5) in DMSO
(excitation wavelength: 608 nm)
Fig. 4. Survival curves of S. aureus in the presence of the SiPcs (4 and 5) with light and in the
dark.
Fig. 5. Phototoxicity studies S. Aureus in the presence of the SiPcs (4 and 5) with light and in
the dark

OH
OH
OH
OH
N
N
H2N
NH2
O
ii
i
OH
HO
2
iii
1
OH
N
N
Pd
3
O
O
iv
Cl
O
N
O
Pd
O
N
N
N
Si
N
N
N
N
N
N
Cl
N
4
N
N
N
Si
N
N
N
N
O
N
N
Pd
O
O
5
Scheme 1.

(a)
(b)
(c)
Fig. 1.

Fig. 2.
Fig. 3.

Fig. 4.
Fig. 5.