Dual-action platinum(II) Schiff base complexes: Photocytotoxicity and cellular imaging

Article abstract of DOI:10.1016/j.poly.2019.04.024

Nine photo-stable Pt(II) Schiff base complexes [Pt(O^N^N^O)] (Pt1–Pt9) containing tetradentate salicylaldimine chelating ligands have been synthesized and characterized as potential photosensitisers for photodynamic therapy (PDT). The effects of electron-withdrawing versus electron-donating substituents on their electronic spectral properties are investigated. Pt1–Pt9 show broad absorption bands between 400–600 nm, which makes them useful for green-light photodynamic therapy. The complexes showed intense phosphorescence with emission maxima at ca. 625 nm. This emission was used to track their cellular localization in cancer cells. Confocal cellular imaging showed that the complexes localized mostly in the cytoplasm. In the dark, the complexes were non-toxic to A549 human lung cancer cells, but exhibited high photo-toxicity upon low-dose green light (520 nm, 7.02 J/cm²) irradiation via photo-induced singlet oxygen generation. Thus, these photoactive Pt(II) complexes have the potential to overcome the problem of drug resistance and side effects of current clinical Pt(II) drugs, and to act as both theranostic as well as therapeutic agents.

Full text of DOI:10.1016/j.poly.2019.04.024

Accepted Manuscript
Dual-action Platinum(II) Schiff Base Complexes: Photocytotoxicity and Cellu-
lar Imaging
Samya Banerjee, Miles Capper, Guy J. Clarkson, Huaiyi Huang, Peter J. Sadler
PII:
S0277-5387(19)30271-2
https://doi.org/10.1016/j.poly.2019.04.024
POLY 13884
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
25 January 2019
1 April 2019
15 April 2019
Please cite this article as: S. Banerjee, M. Capper, G.J. Clarkson, H. Huang, P.J. Sadler, Dual-action Platinum(II)
Schiff Base Complexes: Photocytotoxicity and Cellular Imaging, Polyhedron (2019), doi: https://doi.org/10.1016/
j.poly.2019.04.024
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For submission to Polyhedron : Special Issue dedicated to Professor Akhil R. Chakravarty on
the occasion of his 65th birthday
Dual-action Platinum(II) Schiff Base Complexes: Photocytotoxicity and
Cellular Imaging
Samya Banerjee1, Miles Capper1, Guy J. Clarkson1, Huaiyi Huang1,2* and Peter J. Sadler1*
1Department of Chemistry, University of Warwick, Coventry, CV4 7AL, UK
2School of Pharmaceutical Science (Shenzhen), Sun Yat-sen University, Guangzhou, 510275,
China
E-mail: P.J.Sadler@warwick.ac.uk
Abstract
Nine photo-stable Pt(II) Schiff base complexes [Pt(O^N^N^O)] (Pt1-Pt9) containing
tetradentate salicylaldimine chelating ligands have been synthesized and characterized as
potential photosensitisers for photodynamic therapy (PDT). The effects of
electron-withdrawing versus electron-donating substituents on their electronic spectral
properties are investigated. Pt1-Pt9 show broad absorption bands between 400-600 nm,
which makes them useful for green-light photodynamic therapy. The complexes showed
intense phosphorescence with emission maxima at ca. 625 nm. This emission was used to
track their cellular localization in cancer cells. Confocal cellular imaging showed that the
complexes localized mostly in the cytoplasm. In the dark, the complexes were non-toxic to
A549 human lung cancer cells, but exhibited high photo-toxicity upon low-dose green light
(520 nm, 7.02 J/cm2) irradiation via photo-induced singlet oxygen generation. Thus, these
photoactive Pt(II) complexes have the potential to overcome the problem of drug resistance
and side effects of current clinical Pt(II) drugs, and to act as both theranostic as well as
therapeutic agents.
Keywords: Platinum(II), Schiff Base, Photocytotoxicity, Cellular Imaging, Singlet oxygen.
1. Introduction
Photodynamic therapy (PDT) is a non-invasive method used to treat several types of cancer
as well as bacterial or fungal infections, and various skin diseases [1]. PDT requires the
simultaneous presence of photosensitizers (PSs), oxygen and light [2]. Upon light activation,
the PSs generate reactive oxygen species (ROS) such as singlet oxygen and hydroxyl radicals
[3]. These ROS cause severe oxidative damage to biomacromolecules such as proteins and
1

DNA in cancer cells, which triggers cell death [4]. The current clinically approved PSs such
as Photofrin, Chlorin e6 and Foscan share a similar tetrapyrrolic scaffold [5]. As a result,
these PSs have similar limitations in clinical use, including 1) photo-instability; 2) high tissue
retention times; 3) severe skin sensitivity, and 4) side effects such as hepatitis [6,7]. Due to
the ‘heavy-atom effect’, the introduction of a heavy metal can significantly enhance the
efficiency of PDT. In recent years, metal-based tetrapyrrolic derivatives have been introduced
into clinical trials as PDT agents [8,9]. For example, the Pd(II) complex, WST11 shows high
absorbance at 763 nm with the 2-3x deeper light penetration depth than Photofrin [10].
However, due to the tetrapyrrolic scaffold, WST11 undergoes rapid photo-degradation which
can limit its applications [11].
To overcome the above limitations, polypyridyl metal complexes have recently attracted
significant attention as candidates for use in PDT with high photo-stability and enhanced
aqueous solubility [12-16]. Among these complexes, TLD-1433, a Ru(II) PS has recently
entered into clinical trials for non-muscle-invasive bladder cancer [17].
On the other hand, Pt(II) complexes such as cisplatin, carboplatin, and oxaliplatin are the
most wisely administered anticancer drugs in clinic, but their use can be accompanied by side
effects and drug resistance [18]. To overcome these restrictions, over the last decade, Pt(IV)
prodrugs which undergo reduction or light-induced activation have achieved promising
therapeutic effects [19-20]. The aim of the present work was to investigate novel Pt(II)-based
PSs for PDT which might have fewer side effects and a different mechanism of action to
circumvent resistance to Pt(II) chemotherapeutics, as well as the disadvantages of the current
organic PDT agents. Recently, Pt(II) complexes have been reported as PDT agents [21-24],
although Pt(II) Schiff base complexes have been rarely studied, despite their high
triplet-triplet annihilation (TTA) efficiency as a result of the efficient intersystem crossing
(ISC), strong visible light absorption, and high chemical and photo stability [25-28].
Herein, we report the synthesis, photocytotoxicity, and cellular imaging properties of nine
new Pt(II) Schiff base complexes (Pt1-Pt9). Two of the complexes have been characterized
by X-ray crystal structure determinations. The electronic absorption and emission properties
of the complexes are discussed. The complexes have a broad visible band at ca. 400-600 nm,
suitable for visible light PDT and show strong phosphorescence at ca. 625 nm with
enhancement of phosphorescence in viscous and hydrophobic solvents for near-IR cellular
imaging. The complexes mostly localize in the cytoplasm of cancer cells and exhibit
significant photo-cytotoxicity, while remaining non-toxic in the dark. Overall, these
complexes have potential for use in photodynamic anticancer therapy.
2. Experimental
2.1. Materials
All solvents were of analytical grade. All buffer components were of biological grade and
used as received. Benzene-1, 2-diamine, salicylaldehyde, and 5-methylsalicyclaldehyde, were
obtained from Acros, 4,5-dimethyl-1,2-phenyleneamine, 5-fluorosalicyclaldehyde, 1, 2-
diamino-4, 5-difluorobenzene, sulforhodamine B (SRB), Hoechst 33342, sodium acetate and
2

K2PtCl4, from Sigma-Aldrich, and MitoTracker deep red FM (MTR), and LysoTracker green
(LTG) from Life Technologies (USA).
2.2. Instruments and methods
2.2.1. X-ray crystallography
Single crystals of Pt5 and Pt9 were grown by slow vapour diffusion of diethyl ether into
N,N-dimethylformamide solutions of Pt5 or Pt9. A suitable crystal was selected and mounted
on a glass fibre with Fromblin oil and placed on an Xcalibur Gemini diffractometer with a
Ruby CCD area detector. The crystal was kept at 150(2) K during data collection. Using
Olex2 [29], the structure was solved with the ShelXT [30] structure solution program using
Intrinsic Phasing and refined with the ShelXL [31] refinement package using Least Squares
minimisation. X-ray crystallographic data for Pt5 and Pt9 have been deposited in the CCDC
under the accession numbers 1888366 and 1888367, respectively.
2.2.2. Electrochemistry
Cyclic voltammetry (CV) experiments were carried out using a CH Instrument model 600D
Electrochemical Analyzer/Workstation (Austin, TX). The electrochemical measurements
were performed on Pt1-Pt9 (1.0 mM) in acetonitrile solutions containing
tetrabutylammonium hexafluorophosphate (0.1 M) as supporting electrolyte. The solutions
were degassed under nitrogen and cyclic voltammograms were scanned from -2.3 V to +2.3
V (three complete scans for each experiment). In a typical electrochemical experimental set
up, the three-electrode system was used: a glassy carbon electrode as the working electrode,
Ag/AgCl in 3.0 M KCl as the reference electrode, and platinum wire as the counter electrode.
CV was performed at a scan rate of 100 mV/s. The excited state redox potentials were
calculated from the ground state redox potentials and the emission maxima to aid evaluation
of the photosensitization properties of Pt1-Pt9. Excited state reductive potentials: E([M*]+
/[M]2+) = Eox − Eλem, excited state oxidative potentials: E([M*]+ /[M]0) = Ered + Eλem
Eλem=1240/em [32].
.
2.2.3. NMR, UV-Vis, Phosphorescence spectroscopy, mass spectrometry (ESI-MS) and
elemental analysis
1H NMR spectra were acquired in 5 mm NMR tubes at 293 K or 310 K on either Bruker DPX
1
300 (1H = 300.13 MHz) or 400 (1H = 400.13 MHz) spectrometers. H NMR chemical shifts
were internally referenced to (d5H)-DMSO. All data processing was carried out using
MestReC or TOPSPIN version 2.0 (Bruker U.K. Ltd.).
Electronic absorption spectra were recorded on a Cary 300 UV-vis spectrophotometer using
1-cm pathlength quartz cuvettes (3 mL) and a PTP1 Peltier temperature controller. Spectra
were processed using UV Winlab software. Experiments were carried out at 293 K from 600
to 200 nm.
3

Phosphorescence emission measurements were obtained on a JASCO FP-6500
spectroflurometer. Complexes (10 M) in acetonitrile were diluted from a stock solution in
DMSO (1 mM). The complexes were excited at λex= 405 nm, 465 nm and 520 nm in a 1-cm
quartz cuvette at 298 K. Experiments were performed under air or nitrogen.
Electrospray ionization mass spectra were obtained by preparing the samples in 50%
methanol/50% H2O (v/v), or using NMR samples, for infusion into the mass spectrometer
(Bruker Esquire 2000). The mass spectra were recorded with a scan range of m/z 400–1000
or 800-2000 for positive ions.
CHN analyses were carried out on a CE-440 elemental analyzer by Warwick Analytical (UK)
Ltd.
2.2.4. Determination of singlet oxygen quantum yield
The singlet oxygen production quantum yield was determined by the
N,N-dimethyl-4-nitrosoaniline/imidazole assay based on the oxidation of imidazole by singlet
oxygen by following the reaction of oxidized imidazole with N,N-dimethyl-4-nitrosoaniline,
as reported [33]. The absorbance of the complex was adjusted to ca. 0.1 at the irradiation
wavelength. Complexes Pt1-Pt9 (1 mM) in DMSO were diluted in 4 mL acetonitrile solution
containing N,N-dimethyl-4-nitrosoaniline (20 M) and imidazole (2 mM), and irradiated in
1-cm pathlength quartz fluorescence cuvettes. Bleaching of N,N-dimethyl-4-nitrosoaniline
was followed by monitoring of absorbance at 520 nm. Negative control experiments were run
by repetition of the measurements in the absence of imidazole. The absorbance at 420 nm
was plotted as a function of irradiation time and the quantum yields of singlet oxygen
formation (Φ sample) were calculated using Rose Bengal as the standard (Φreference) and the
following formula:
S represents the slope of the absorbance vs. irradiation time, I the rate of absorption
calculated as the overlap of lamp emission spectra and the absorption spectra of the
compound according to the following formula:
where I0 represents the light-flux intensity of the lamp and A is the absorbance of the
compound.
2.2.5. Phosphorescence quantum yields and lifetimes
Phosphorescence quantum yields were measured on a Fluorolog spectroflurometer (Horiba
Jobin Yvon) equipped with a 405 nm pulsed diode NanoLED light source. Pt(II) complexes
were diluted from a stock solution in DMSO (1 mM) to achieve an absorbance = 0.1 at 405
nm in acetonitrile. The complexes were excited at λex= 405 nm in a 1-cm quartz cuvette at
298 K. Experiments were performed in air, and emission spectra were corrected for the
4

spectral sensitivity of the detection system by standard correction curves. Quantum yields in
aerated acetonitrile were determined by comparison with the emission of [Ru(bpy)3]Cl2 in
aerated water ( = 0.040). Phosphorescence lifetime measurements using the same
equipment and measurement procedure were continued until 1000 counts of excited state
species had been gathered. The raw data were analyzed with OriginPro 2016 software by
using exponential functions to obtain the phosphorescence lifetimes.
2.2.6. Synthesis of the Schiff base ligands and corresponding Pt(II) complexes
General procedure for the synthesis of Schiff base ligands: The respective salicylaldehyde
(2 mol equiv) in ethanol was added dropwise to a solution of the respective
1,2-phenyldiamines (1 mol equiv) in ethanol. The mixture was stirred overnight at room
temperature. The yellow precipitate was collected and washed with cold ethanol.
Ligand 1 was obtained from benzene-1, 2-diamine (216.2 mg, 2 mmol) and salicylaldehyde
(488.4 mg, 4 mmol). Product: 450.6 mg of yellow powder. Yield: 67%. 1H NMR (300 MHz,
DMSO) δ 12.96 (s, 2H), 8.94 (s, 2H), 7.70 – 7.63 (m, 2H), 7.44 (ddd, J = 15.9, 7.6, 2.7 Hz,
6H), 6.98 (t, J = 7.1 Hz, 4H).
Ligand 2 was obtained from benzene-1, 2-diamine (108.1 mg, 1 mmol) and
5-methylsalicyclaldehyde (272.3 mg, 2 mmol) Product: 191.6 mg of yellow powder. .Yield:
1
55%. H NMR (300 MHz, DMSO) δ 12.66 (s, 2H), 8.86 (s, 2H), 7.46 – 7.37 (m, 6H), 7.23
(dd, J = 8.4, 1.8 Hz, 2H), 6.86 (d, J = 8.4 Hz, 2H), 2.26 (s, 6H).
Ligand 3 was obtained from benzene-1,2-diamine (108.1 mg, 1 mmol) and
5-fluorosalicyclaldehyde (280.2 mg, 2 mmol). Product: 199.5 mg of yellow powder. Yield:
1
56%. H NMR (300 MHz, DMSO) δ 12.61 (s, 2H), 8.91 (s, 2H), 7.53 (dd, J = 9.0, 3.1 Hz,
2H), 7.48 – 7.36 (m, 4H), 7.28 (td, J = 8.8, 3.2 Hz, 2H), 6.99 (dd, J = 9.0, 4.5 Hz, 2H).
Ligand 4 was obtained from 4, 5- dimethyl-1,2-phenyleneamine (272.4 mg, 2 mmol) and
1
salicyclaldehyde (488.4 mg, 4 mmol). Product: 255 mg of yellow powder. Yield: 35%. H
NMR (300 MHz, DMSO) δ 13.11 (s, 2H), 8.92 (s, 2H), 7.64 (d, J = 6.9 Hz, 2H), 7.40 (t, J =
7.2 Hz, 2H), 7.28 (s, 2H), 6.97 (t, J = 7.0 Hz, 4H), 2.29 (s, 6H).
Ligand 5 was obtained from 4, 5-dimethyl-1, 2-phenyleneamine (136.2 mg, 1 mmol) and
5-methylsalicyclaldehyde (272.3 mg, 2 mmol). Product: 165.6mg of yellow powder. Yield:
1
44%. H NMR (300 MHz, DMSO) δ 12.81 (s, 2H), 8.85 (s, 2H), 7.43 (s, 2H), 7.26 (s, 2H),
7.22 (d, J = 8.4 Hz, 2H), 6.85 (d, J = 8.4 Hz, 2H), 2.30 (s, 6H), 2.27 (s, 6H).
Ligand 6 was obtained from 4, 5- dimethyl-1,2-phenyleneamine (136.2 mg, 1 mmol) and
5-fluorosalicyclaldehyde (280.2 mg, 2 mmol). Product: 188.3 mg yellow powder. Yield: 49%.
1H NMR (300 MHz, DMSO) δ 12.74 (s, 2H), 8.91 (s, 2H), 7.52 (dd, J = 9.0, 3.0 Hz, 2H),
7.31 – 7.21 (m, 4H), 6.98 (dd, J = 9.0, 4.5 Hz, 2H), 2.30 (s, 6H).
5

Ligand 7 was obtained from 1, 2- diamino-4,5-difluorobenzene (288.2 mg, 2 mmol) and
1
salicylaldehyde (488.4 mg, 4 mmol). Product: 461.6 mg yellow powder. Yield: 62%. H
NMR (300 MHz, DMSO) δ 12.65 (s, 2H), 8.93 (s, 2H), 7.69 (dd, J = 18.5, 8.5 Hz, 4H), 7.47
– 7.39 (m, 2H), 6.98 (t, J = 7.5 Hz, 4H).
Ligand 8 was obtained from 1, 2-diamino-4, 5-difluorobenzene (144.1 mg, 1 mmol) and
5-methylsalicyclaldehyde ((272.2 mg, 2 mmol)). Product: 257.5 mg yellow powder. Yield:
67%.1H NMR (300 MHz, DMSO) δ 12.37 (s, 2H), 8.87 (s, 2H), 7.68 (t, J = 9.9 Hz, 2H), 7.44
(s, 2H), 7.25 (d, J = 8.4 Hz, 2H), 6.87 (d, J = 8.4 Hz, 2H), 2.27 (s, 6H).
Ligand 9 was obtained from 1, 2-diamino-4,5-difluorobenzene (144.1 mg, 1 mmol) and
5-fluorosalicyclaldehyde (280.2 mg, 2 mmol). Product: 294.2 mg yellow powder. Yield:
1
75% . H NMR (300 MHz, DMSO) δ 12.29 (s, 1H), 8.91 (s, 1H), 7.69 (t, J = 9.9 Hz, 1H),
7.50 (dd, J = 8.9, 3.0 Hz, 1H), 7.30 (td, J = 8.7, 3.0 Hz, 1H), 7.00 (dd, J = 9.0, 4.4 Hz, 1H).
General procedure for the synthesis of platinum(II) complexes:
Sodium acetate (49.2 mg, 0.6 mmol) was suspended in a solution of ligand L1-L9 (0.3 mmol)
in DMF (8 mL). After stirring for 5 min at room temperature, K2PtCl4 (124.5 mg, 1 mmol)
and DMSO (4 mL) was added. The reaction mixture was heated to 343 K under nitrogen
overnight. Upon cooling to room temperature, diethyl ether (40 mL) was added to afford a
red precipitate. The precipitate was filtered off and washed with water, acetone and diethyl
ether, and dried under vacuum.
1
Pt1: 70.3 mg of red precipitate. Yield: 46%. H NMR (300 MHz, DMSO) δ 9.55 (s, 1H),
8.47 (dd, J = 6.1, 3.4 Hz, 1H), 7.88 (d, J = 8.1 Hz, 1H), 7.61 – 7.32 (m, 2H), 7.13 (d, J = 8.5
Hz, 1H), 6.82 (d, J = 7.8 Hz, 1H). Anal. calcd. for C20H14N2O2Pt: C, 47.15; H, 2.77; N, 5.50;
found: C, 47.33; H, 2.91; N, 5.41. ESI-MS: m/z [M+Na]+ 533.4, [2M+Na]+ 1041.0,
[3M+Na]+ 1551.2.
1
Pt2: 56.4 mg of red precipitate. Yield: 35%. H NMR (300 MHz, DMSO) δ 9.46 (s, 2H),
8.44 (dd, J = 6.3, 3.3 Hz, 2H), 7.65 (s, 2H), 7.44 (dd, J = 11.7, 5.7 Hz, 4H), 7.04 (d, J = 8.7
Hz, 2H), 2.30 (s, 6H). Anal. calcd. for C22H18N2O2Pt: C, 49.16; H, 3.38; N, 5.21; found: C, C,
49.02; H, 3.47; N, 5.35. ESI-MS: m/z [M+Na]+ 561.0, [2M+Na]+ 1098.1.
1
Pt3: 41 mg of red precipitate. Yield: 31%. H NMR (300 MHz, DMSO) δ 9.57 (s, 1H), 8.42
(dd, J = 6.1, 3.3 Hz, 1H), 7.76 – 7.32 (m, 3H), 7.13 (dd, J = 9.4, 4.8 Hz, 1H). Anal. calcd. for
C20H12F2N2O2Pt: C, 44.04; H, 2.22; N, 5.14; found: C, 43.87; H, 2.26; N, 5.32. ESI-MS: m/z
[M+Na]+ 568.4, [2M+Na]+ 1113.0, [3M+Na]+ 1658.4.
1
Pt4: 49.8 mg of red precipitate. Yield: 31%. H NMR (300 MHz, DMSO) δ 9.43 (s, 2H),
8.23 (s, 2H), 7.84 (d, J = 6.6 Hz, 2H), 7.55 (s, 2H), 7.10 (d, J = 8.5 Hz, 2H), 6.77 (s, 2H), 2.35
(s, 6H). Anal. calcd. for C22H18N2O2Pt: C, 49.16; H, 3.38; N, 5.21; found: C, 48.87; H, 3.21;
N, 5.33. ESI-MS: m/z [M+Na]+ 561.0, [2M+Na]+ 1098.0, [3M+Na]+ 1637.1.
6

1
Pt5: 59.4 mg of red precipitate. Yield: 35%. H NMR (300 MHz, DMSO) δ 9.35 (s, 2H),
8.21 (s, 2H), 7.60 (s, 2H), 7.39 (d, J = 9.0 Hz, 2H), 7.01 (d, J = 8.7 Hz, 2H), 2.35 (s, 6H), 2.29
(s, 6H). 13C NMR (126 MHz, DMF) δ 163.67, 150.08, 143.32, 137.10, 136.95, 134.13,
124.65, 121.96, 121.25, 116.89, 19.36, 19.22. Anal. calcd. for C24H22N2O2Pt: C, 50.97; H,
3.92; N, 4.95; found: C, 51.35; H, 3.78; N, 4.96.ESI-MS: m/z [M+Na]+ 588.1, [2M+Na]+
1153.1, [3M+Na]+ 1720.2.
1
Pt6: 77.4 mg of red precipitate. Yield: 45%. H NMR (300 MHz, DMSO) δ 9.33 (s, 1H),
8.07 (s, 1H), 7.55 (d, J = 9.5 Hz, 1H), 7.45 (t, J = 7.7 Hz, 1H), 7.07 (dd, J = 9.3, 4.7 Hz, 1H),
2.28 (s, 3H). 13C NMR (126 MHz, DMSO) δ 161.46, 154.25, 152.41, 150.33, 143.00, 137.66,
124.20, 122.84, 121.13, 118.10, 117.38, 20.04. Anal. calcd. for C22H16F2N2O2Pt: C, 46.08; H,
2.81; N, 4.89; found: C, 45.64; H, 2.66; N, 4.86. ESI-MS: m/z [M+Na]+ 595.0, [2M+Na]+
1169.0, [3M+Na]+ 1743.0.
P7: 57.2 mg of red precipitate. Yield: 34%. 1H NMR (300 MHz, DMSO) δ 9.34 (s, 2H), 8.57
(t, J = 9.8 Hz, 2H), 7.75 (d, J = 7.1 Hz, 2H), 7.57 (t, J = 7.0 Hz, 2H), 7.09 (d, J = 8.6 Hz, 2H),
13
6.77 (t, J = 7.2 Hz, 2H). C NMR (126 MHz, DMSO) δ 161.46, 154.25, 152.41, 150.33,
143.00, 137.67, 124.20, 122.85, 121.13, 118.10. Anal. calcd. for C20H12F2N2O2Pt: C, 44.04;
H, 2.22; N, 5.14; found: C, 43.81; H, 2.05; N,5.02. ESI-MS: m/z [2M+Na]+ 1113.0,
[3M+Na]+ 1659.0.
1
Pt8: 63.6 mg of red precipitate. Yield: 37%. H NMR (300 MHz, DMSO) δ 9.33 (s, 2H),
8.62 (d, J = 10.2 Hz, 2H), 7.55 (s, 2H), 7.43 (d, J = 8.7 Hz, 2H), 7.04 (d, J = 8.7 Hz, 2H), 2.29
(s, 6H). 13C NMR (126 MHz, DMSO) δ 163.48, 152.15, 151.12, 149.5, 147.6, 141.79, 138.08,
134.54, 125.14, 121.74, 121.61, 20.16. Anal. calcd. for C22H16F2N2O2Pt: C, 46.08; H, 2.81; N,
4.89; found: C, 45.88; H, 2.69; N, 4.83. ESI-MS: m/z [2M+Na]+ 1171.1, [3M+Na]+ 1744.0.
1
Pt9: 97.7 mg of red precipitate. Yield: 56%. H NMR (300 MHz, DMSO) δ 9.37 (s, 2H),
13
8.52 (t, J = 9.8 Hz, 2H), 7.51 (d, J = 8.7 Hz, 4H), 7.13 (s, 2H). C NMR (126 MHz, DMF) δ
154.49, 152.65, 151.65, 141.84, 128.41, 124.65, 123.04, 120.81, 117.55, 105.29. Anal. calcd.
for C20H10F4N2O2Pt: C, 41.32; H, 1.73; N, 4.82; found: C, 41.77; H, 2.05; N, 5.49.ESI-MS:
m/z [2M+Na]+ 1185.0, [3M+Na]+ 1766.9.
2.2.7 Cell culture and cell assays
Cell culture: the A549 human lung carcinoma cell line was obtained from the European
Collection of Cell Cultures (ECACC) and used between passages 5 to 18. Cells were grown
in Roswell Park Memorial Institute medium (RPMI-1640) supplemented with 10% fetal calf
serum and 1% penicillin and streptomycin. The cells were grown as adherent monolayers at
310 K in a 5% CO2 humidified atmosphere and passaged at ca. 70-80% confluence.
Dark- and photo-cytotoxicity assays: 5000 A549 cells were seeded per well in 96-well
7

plates at 310 K in a 5% CO2/95% air incubator (Thermo) for 24 h. Stock solutions of the test
compounds were firstly prepared in 5% DMSO (v/v) and a mixture 0.9% saline and
RPMI-1640 medium (1:1 v/v) following serial dilutions in RPMI-1640. The drug exposure
period was 4 h. After this, supernatants were removed by suction and each well was washed
with PBS. For light treatment, the cells were then irradiated in phenol-red-free cell culture
medium by 520 nm light (7.02 J cm-2). Then the cells were allowed to recover for a further 46
h in the incubator at 310 K. For dark-treated cells, after drug exposure, the cells were
incubated in drug-free cell culture medium at 310 K for 44 h. Finally, the cytotoxicity was
determined by sulforhodamine B (SRB) assay on a GloMax-Multi Microplate Multimode
Reader. IC50 values were determined as duplicates of triplicates in two independent sets of
experiments and their standard deviations were calculated.
Cellular localization assays: A549 cells were seeded in a glass-bottom dish (35 mm dish
with 20 mm bottom well (Costar) at 310 K in a 5% CO2/95% air incubator. The cells were
incubated with Pt1-Pt9 (5 M) at 310 K for 4 h and further stained with 100 M MTR and
LTR or Hoechst 33342 Cell imaging was carried out immediately by confocal microscopy
(LSM 710, Carl Zeiss, Göttingen, Germany) with a 63 oil-immersion objective lens.
Excitation/emission: 1 one-photon 405/562 nm and two-photon 760/562 nm, LTG 488/520
nm, MTR 633/660 nm.
3. Results and Discussion
3.1 Synthesis and characterization:
Scheme 1. Synthetic route for the Schiff base ligands H2L1-9 and Pt(II) complexes Pt1-Pt9.
To investigate the relationship between the photophysical, photochemical and
photobiological activity of the Pt(II) Schiff base complexes, we designed and synthesized
complexes Pt1-Pt9 (Scheme 1) with either electron-withdrawing or electron-donating groups
at R1 or R2 positions on the backbone phenyl ring. Schiff base ligands H2L1- H2L9 were
8

synthesised by amine-aldehyde condensation reactions from disubstituted (-H, -CH3, -F)
o-phenylenediamine and substituted (-H, -CH3, -F) salicylaldehyde in ethanol at room
temperature. All the ligands were characterised by 1H NMR and mass spectra.
Pt(II) complexes Pt1-Pt9 (Scheme 1) were synthesised by heating the Schiff base ligand,
sodium acetate and K2PtCl4 in DMSO and DMF (1:1 v/v) at 343 K overnight. All the
1
complexes were characterised by H, 13C, COSY NMR, elemental analysis and mass
1
spectrometry. The disappearance of the aromatic OH H NMR signal (between 12-13 ppm)
indicated the coordination of the ligand to Pt(II). The presence of fluorine at the R1 position
1
strongly influenced the H NMR peak splitting patterns. For instance, Pt7 showed a triplet
peak at 8.57 but without any proton coupling, confirmed by the COSY NMR (Figure S1, SI).
19
Thus the splitting of this proton signal was due to F coupling. Complexes were further
characterized by mass spectral data showing peaks corresponding to [2M + Na]+ and [3M +
Na]+.
X-Ray crystallography:
Crystals of Pt5 and Pt9 (Figure 1, Table S1) suitable for x-ray diffraction were obtained via
diffusion of ethyl ether into DMF solutions of the complexes. The complexes crystallized in
the monoclinic space groups P21/n and P21/c, respectively, with four molecules in the unit
cell. They are discrete mononuclear species with a PtN2O2 core formed by the tetradentate
N,N,O,O-donor Schiff base in a distorted square-planar environment. Pt-O bond distances are
similar in both the complexes (ca. 1.99 Å), slightly longer than the Pt-N bond distances (ca.
1.96 Å, Table S2). Both the Pt-O and Pt-N bond distance lies in the range of the previous
literature reports [22,23].
Figure 1. Side and top views of the X-ray crystal structures of complexes Pt5 and Pt9 with
50% thermal ellipsoids.
9

Photochemical and photophysical properties
Figure 2. The effect of substituents on (a) the UV-vis spectra of Pt1-Pt9 (10 M) in
acetonitrile, and b) normalized emission spectra of Pt1-Pt9 (10 M) in acetonitrile. Ex = 520
nm.
The convenient structural modifications and facile synthesis of ligands made it possible to
study the structure-activity relationships for 9 Schiff base-Pt(II) complexes. The UV-vis
absorption spectra of Pt1-Pt9 in acetonitrile are shown in Figure 2. All the solutions of Pt(II)
complexes in MeCN were red in colour with similar absorption spectral features between
200-600 nm. The intense absorption band at around 260 nm is assignable to the Schiff base
ligands, and those near 310 nm are π-π* ligand-based transitions [34]. The two strong
absorption bands between 350 and 400 nm are assignable to a mixture of metal-to-ligand
1[PtII→π*-(L)] transitions [34]. The absorption bands in the visible range (420 to 600 nm) are
metal-to-ligand-charge-transfer (MLCT) bands [25].
The substituents (CH3 or F) have a significant influence on the electronic absorption
spectra. For Pt1, Pt4 and Pt7 with different substituents at the R1 position, -CH3 induced a
hypsochromic effect (3 nm, in Pt4) and -F induced a bathochromic effect (4 nm, in Pt7)
compared with -H atom (in Pt1). For substitution at the R2 position, both the -CH3 (15 nm, in
10

Pt2) and -F (23 nm, in Pt3) groups induced bathochromic effects compared to Pt1. This
larger red-shift of Pt3 (ca 20 nm) compared with Pt7 (4 nm) indicates that -F substitution at
R2 significantly extends the absorption to longer wavelength. Pt9, where both R1 and R2 are
–F, exhibited 26 nm bathochromic shift compared with Pt1 and as a result, the spectrum tail
of Pt9 extended up to 600 nm which is ideal for near-IR light PDT [35]. Interestingly, Pt4
and Pt5 showed significant hypochromic effects and their molar extinction coefficients are
much smaller than thise for the other Pt(II) complexes.
Oxygen and viscosity sensitivity
Phosphorescence spectra of Pt1-Pt9 were recorded in acetonitrile at ambient temperature.
Since all the Pt(II) complexes exhibit strong absorption across the whole visible range, the
wavelength dependence of phosphorescence was investigated. Three excitation wavelengths
were selected: 405 nm, 465 nm and 520 nm. As shown in Figure 3, Pt1-Pt9 (10 M) all
exhibited red phosphorescence between 604-644 nm. The highest emission intensity was
obtained with excitation at 520 nm.
Similar to the UV-vis absorption spectra, substitution at the R1 or R2 position also
influenced the emission spectra (Figure 3). Pt1, Pt4 and Pt7 displayed the most blue-shifted
emission maximum at 604, 607 and 609 nm respectively compared with R2-substituted
complexes. Substitution with either -CH3 or -F group at R2 significantly shifted the
11

Figure 3. (a) Emission spectra of Pt2 (10 M) in acetonitrile in air. (b) Emission spectra of
Pt2 (10 M) under nitrogen. (c) Comparison of emission intensity of Pt1-9 (10 M) in air
and under nitrogen. (d) Concentration dependent phosphorescence intensity of Pt1 (black)
and Pt7 (red) in air-saturated acetonitrile.
emission maximum to longer wavelength (e.g. Pt1 604 nm, Pt2 625 nm, Pt3 633 nm) while
the –F substitutent showed more effective red-shift than -CH3. Taking the emission and the
UV-vis data into consideration, it is apparent that substituent at the R2 position has a stronger
effect on the emission and absorbance properties of Pt(II) Schiff base complexes than the R1
position.
The emission spectra were recorded under N2 in acetonitrile to investigate the effect of
oxygen on the emission. As shown in Figure 3a and 3b, the emission intensity of all the Pt(II)
complexes under a nitrogen environment increased significantly compared to the intensity
measured under air. Pt2 was the most oxygen-sensitive complex with a 32x phosphorescence
enhancement under nitrogen (Figure 3c). Pt1 showed the smallest emission enhancement (8x)
among all the Pt(II) complexes while it exhibited strongest emission in air. The
phosphorescence of the complexes can be used to track the complexes inside cells by cellular
imaging. Moreover, upon introducing functional groups, the phosphorescence intensity of
Pt7 showed a better linear relationship with increasing concentration than Pt1 (Figure 3d).
Thus the functional groups may introduce steric hindrance which might obstruct - stacking
between the planar Pt(II) complexes.
Effect of Solvent Polarity and Viscosity
12

Figure 4 (a) Excited state lifetime measurement for Pt7 in air-saturated acetonitrile and
glycerol. (b) Emission intensity of Pt1-Pt9 (3M) in solvents of varying polarity and
viscosity.
The influence of polarity and viscosity on the emission of the complexes Pt1-9 is important
for the possible use these complexes as cellular imaging or PDT agents as the intracellular
microenvironment is complicated with areas of varied polarity and viscosity [36]. As shown
in Figure 4 b, solvent polarity only has little effect on the emission intensities. However, the
viscosity of the solvent has a strong influence. The phosphorescence intensity was much
higher in viscous solvents like glycerol compared to the other low viscosity solvents as
depicted in Figure 4a,b. Furthermore, at different percentages of glycerol/water the
phosphorescence intensity was enhanced significantly with increased percentage of glycerol.
Complex Pt7, in particular displayed the highest intensity enhancement in the presence of
glycerol compared with all other Pt(II) complexes. The phosphorescence lifetime of 7 in
acetonitrile and glycerol indicated (Figure 4a) that Pt7 has a more than 40x longer
excited-state lifetime time in glycerol (3277 ns) than in acetonitrile (80.7 ns). These results
indicate that high viscosity and hydrophobic environments enhance the phosphorescence
intensity of these Pt(II) Schiff base complexes.
Cyclic voltammetry
Cyclic voltammograms of Pt1-Pt9 were recorded in degassed DMF (1 mM) with 100 mM
tetrabutylammonium hexafluorophosphate (TBAP) as supporting electrolyte. All potentials
are referenced to the ferrocenium–ferrocene [Eº(FeCp2+/0)] couple. In each case, there is one
irreversible anodic peak with Epa at 0.424–0.687 V and one irreversible cathodic peak with
Epc at -1.733 to -2.019 V (Figure S2). The anodic and cathodic peaks are attributed to the
oxidation and reduction of Schiff base ligands, respectively [25]. Introduction of the –CH3
group reduced the oxidation potential, while –F increased both the oxidation and reduction
potential in all of the complexes. The excited-state redox potentials of the complexes were
determined in order to assess the photosensitization properties of the complexes (Table 1).
Upon excitation by green light, these Pt(II) Schiff base complexes became more reductive
rather than oxidative.
Table 1. Electrochemical properties of Pt1-Pt9 and their calculated excited-state redox
potentials
Ground State
Epc/V
Excited State
Complexes
Pt1
Epa/V
0.581
0.456
0.525
0.517
*E(M/[M]+)/V
*E(M/[M]-)/V
0.106
-1.946
-1.982
-1.827
-1.997
-1.471
-1.528
-1.433
-1.525
Pt2
0.002
Pt3
0.131
Pt4
0.045
13

Pt5
Pt6
Pt7
Pt8
Pt9
0.424
0.612
0.648
0.517
0.687
-2.019
-1.857
-1.873
-1.911
-1.733
-1.556
-1.337
-1.388
-1.467
-1.238
-0.038
0.092
0.163
0.073
0.192
Singlet Oxygen Generation
The highly oxygen-sensitive emission of the Pt(II) complexes indicates that the complexes
probably interact with 3O2 in their excited states and may generate singlet oxygen (1O2) upon
photo-excitation. Thus the singlet oxygen quantum yields of Pt1-Pt9 were determined in
acetonitrile using both indirect and direct methods. The indirect method tracks the photo
bleaching of N, N-dimethyl-4-nitrosoaniline (RNO) induced by imidazole and singlet oxygen
over time. The direct method records the laser power dependence of near infrared singlet
oxygen phosphorescence spectra, with Rose Bengal as a reference photosensitizer.
Figure 5. a) Near-infrared singlet oxygen signal generated by Pt1-Pt9 after irradiation with
520 nm light in acetonitrile. b) Singlet oxygen generation quantum yields measured from the
absorption attenuation (ΔA = A0– At) of RNO under 520 nm light exposure in acetonitrile.
The slope corresponds to the absolute amount of generated 1O2.
Singlet oxygen phosphorescence spectra with emission maximum at 1270 nm were
observed for all the Pt(II) complexes (Figure 5a). Pt3, Pt6 and Pt7 with -F substitution
showed relatively higher singlet oxygen quantum yields of 46%, 43% and 42%, respectively.
Previous studies also showed that the presence of a fluorine substituent increases the
14

antitumor effects with enhanced singlet oxygen generation [37,38]. These Pt(II) complexes
exhibited excellent photostability and solubility in acetonitrile during photo-irradiation,
making them suitable as PDT agents.
Cell imaging and cellular localization
Based on the unique phosphorescence property of the complexes, confocal microscopy
imaging of Pt1-Pt9 treated living A549 lung cancer cells under 520 nm excitation was
carried out to track the complexes inside cancer cells. As shown in Figure 6, under 520 nm
excitation, Pt1-Pt3 showed much stronger intracellular phosphorescence than the other Pt(II)
complexes, especially Pt7-Pt9 whose intracellular phosphorescence was too weak to detect.
From the cell imaging data, it was evident that the Pt(II) complexes have similar intracellular
localization, mostly in the cytoplasm. Subsequently, to confirm whether these Pt(II)
complexes can target specific organelles in the cytoplasm, Pt1 was selected for co-staining
with commercial organelle dyes due to its strongest intracellular phosphorescence intensity.
A549 lung cancer cells were firstly incubated with Pt1 (5 M, 4 h) and then with specific
organelle dyes (30 min) in the dark, and finally imaged with a confocal microscope. As
shown in Figure 7, the red signal of Pt1 overlayed with both the fluorescent signal of
Mitotracker green (mitochondrion-staining dye) and of Lysotracker green (lysosome-imaging
dye). No overlay of luminescence was observed with the nucleus-staining dye Hoechst
33342. All these data indicate that these Pt(II) complexes target cell mitochondria and
lysosomes rather than the cell nucleus. Clinically approved Pt(II) drugs such as cisplatin and
carboplatin are known to target the cell nucleus and cross-link nuclear DNA [39]. Resistance
to these Pt drugs occurs due to the repair of such platinum-DNA cross links by a nucleotide
excision repair mechanism [40]. Hence these mitochondrion- and lysosome-targeting
complexes could provide a novel mechanism of anticancer activity to overcome Pt-drug
resistance.
15

Figure 6. Live A549 lung cancer cell imaging with the Pt(II) Schiff base complexes.
Excitation wavelength: 520 nm. Emission filter: 60040 nm.
16

Figure 7. Intracellular localization of Pt1 in A549 lung cancer cells. A549 cells were firstly
incubated with Pt(II) complexes (5 M, 4 h), and then with commercial dyes for another 30
min. Scale bar = 50 m. Excitation wavelengths: 520 nm for Pt1, 488 nm for Mitotracker and
Lysotracker, 405 nm for Hoechst 33342. Emission filter: 60020 nm for Pt(II) complexes,
52020 nm for Mitotracker and Lysotracker, 46020 nm for Hoechst 33342.
Photocytotoxicity:
After confirming the intracellular internalization and specific distribution of complexes Pt1-9
in A549 cancer cells, we explored the anticancer activity of these complexes, particularly as
1
PDT agents considering their excellent ability to generate O2. The anticancer activity of
these complexes was studied both in the dark and upon light irradiation. As shown in Table 2,
all the Pt(II) complexes were non-toxic in A549 cancer cells in the dark up to 20 µM
concentration, the cell survival percentages were all >80% after 4 h drug exposure followed
by 20 h recovery. However, upon irradiation with a low dose of 520 nm green light (7.02
J/cm2), significant photo-toxicity was observed, with several giving nanomolar IC50 values.
Pt2-Pt5 exhibited much better phototoxicity than Pt1 (which contained no substituents).
Table 2. Dark- and photo-toxicity of Pt1-Pt9 towards lung cancer cells (IC50/M).
Cell line
Compound
Pt1a
A549
Lighta
A549R
Lighta
Darka
>20
PI
>100
>159
>98
>134
>7
Darka
>20
PI
>66
>166
>57
>87
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
0.200.015
0.130.007
0.210.025
0.150.010
1.500.021
8.050.69
0.370.025
>10
0.300.010
0.120.011
0.350.023
0.230.015
n.d.
Pt2a
>20
>20
Pt3a
>20
>20
Pt4a
>20
>20
Pt5a
>10
n.d.
Pt6a
>10
>1
n.d.
n.d.
Pt7a
>20
>55
n.a.
n.a.
n.a.
n.a.
n.a.
n.d.
n.d.
Pt8a
>10
n.d.
n.d.
Pt9a
>10
>10
n.d.
n.d.
Cisplatina
Cisplatinb
5-ALA
>200
32.43.5
>200
>200
55.66.3
4.60.5
>200
54.74.4
4.70.3
34.02.9
>200
>200
aThe drug exposure time was 4 h. The cells were washed, replaced with fresh cell culture
17

medium and then received dark or light (520 nm, 7.02 J/cm2) treatment followed by 44 h
recovery. bThe drug exposure time was 48 h. n.d.: not detected. n.a.: not available.
Conclusions
A series of platinum(II) Schiff Base salicylaldimine complexes (Pt1-Pt9) was synthesized
with hydrogen, fluorine and methyl substituents on various positions on the phenyl rings of
the backbone. The effect of electron withdrawing and donating substituents on the UV-vis
and emission properties was investigated. These Pt(II) complexes show intense visible light
absorption between 400-600 nm due to metal-to-ligand-charge-transfer (MLCT).
Interestingly, the complexes showed viscosity and polarity-dependent strong emission under
green light excitation. Pt1-Pt9 showed excellent photo-stability under continuous light
irradiation and generated singlet oxygen with a quantum yield of ca. 50%.
In the dark, all the complexes were non-toxic to A549 lung cancer cells. However, strong
photo-toxicity was observed after excitation with low-dose green light (520 nm, 7.02 J cm-2).
Cell imaging indicated that the complexes mostly localized in the cytoplasm targeting
mitochondria and lysosomes efficiently. In general, this new series of green light activated
Pt(II) Schiff base complexes has potential as photosensitizers for photodynamic therapy with
mitochondria and lysosome targeting ability. The Pt(II) complexes show anticancer activity
only upon light irradiation while remaining dormant in the dark. All these properties indicate
that the complexes have the potential to overcome the problems of drug resistance and side
effects of current clinical Pt(II) drugs.
Appendix A. Supplementary data
CCDC 1888366 and 1888367 contain the supplementary crystallographic data for Pt5 and
Pt9, respectively. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
Acknowledgements
We thank the EPSRC (grants EP/G006792, EP/F034210/1 and EP/P030572/1 for P.J.S.,),
MRC (grant G0701062 for P.J.S.), The Royal Society (Newton-Bhahba International
Fellowship NF151429 for S.B. and Newton International Fellowship NF160307 for H.H.).
We thank Dr. L. Song and P. Aston for assistance with mass spectrometry; Dr. J. P. C.
Coverdale and H. E. Bridgewater for assistance with ICP experiments; and Dr. I. Prokes for
assistance with NMR spectroscopy.
Dedication
18

This paper is dedicated to Professor Akhil R. Chakravarty on the occasion of his 65th
birthday
Competing interests
The authors declare no competing financial interests.
Supporting information
1
SI contains crystallographic data for complexes Pt5 and Pt9, 1D and 2D COSY H NMR
spectra for Pt7, and cyclic voltammograms for Pt1-Pt9.
References
[1] D. E. Dolmans, D. Fukumura, R. K. Jain, Nat Rev. Cancer 3 (2003) 380.
[2] B. W. Henderson, T. J. Dougherty, Photochem. Photobiol. 55 (1992) 145.
[3] Z. Zhou, J. Song, L. Nie, X. Chen, Chem. Soc. Rev. 45 (2016) 6597.
[4] D. Nowis, M. Makowski, T. Stokłosa, M. Legat, T. Issat, J. Gołąb, Acta Biochim. Pol. 52
(2005) 339.
[5] H. I. Pass, J Natl Cancer I, 85 (1993) 443.
[6] S. B. Brown, E. A. Brown, I. Walker, Lancet oncol. 5 (2004) 497.
[7] M. Ethirajan, Y. Chen, P. Joshi, R. K. Pandey, Chem. Soc. Rev. 40 (2011) 340.
[8] F. Heinemann, J. Karges, G. Gasser, Acc. Chem. Res. 50 (2017) 2727.
[9] A. E. O’Connor, W. M. Gallagher, A. T. Byrne, Photochem. Photobiol. 85 (2009) 1053.
[10] O. Mazor, A. Brandis, V. Plaks, E. Neumark, V. Rosenbach‐ Belkin, Y. Salomon, A.
Scherz, Photochem. Photobiol. 81 (2005) 342.
[11] M. Price, L. Heilbrun, D. Kessel, Photochem. Photobiol. 89 (2013) 68.
[12] H. Huang, B. Yu, P. Zhang, J. Huang, Y. Chen, G. Gasser, L. Ji, H. Chao, Angew.
Chem. Int. Ed. 54 (2015) 14049.
[13] Z. Lv, H. Wei, Q. Li, X. Su, S. Liu, K. Y. Zhang, L. Wen, Q. Zhao, W. X. Li, Huang,
Chem. Sci. 9 (2018) 502.
[14] P. Zhang, C. K. C. Chiu, H. Huang, Y. P. Y. Lam, A. Habtemariam, T. Malcomson, P. J.
Sadler, Angew. Chem. Int. Ed. 56 (2017) 14898.
[15] L. K. McKenzie, H. E. Bryant, J. A. Weinstein, Coord. Chem. Rev. 379 (2019) 2.
[16] H. Huang, S. Banerjee, P. J. Sadler, ChemBioChem 19 (2018) 1574.
19

[17] S. Monro, K. L. Colon, H. Yin, J. Roque III, P. Konda, S. Gujar, R. P. Thummer, L.
Lilge, C. G. Cameron, S. A. McFarland, Chem. Rev. 10.1021/acs.chemrev.8b00211.
[18] E. Wong, C. M. Giandomenico, Chem. Rev. 99 (1999) 2451.
[19] T. C. Johnstone, K. Suntharalingam, S. J. Lippard, Chem. Rev. 116 (2016) 3436.
[20] J. S. Butler, P. J. Sadler, Curr. Opin. Chem. Biol. 17 (2013) 175.
[21] M. Obata, S. Hirohara, R. Tanaka, I. Kinoshita, K. Ohkubo, S. Fukuzumi, M. Tanihara,
S. Yano, J. Med. Chem. 52 (2009) 2753.
[22] J. Deng, J. Wang, M. Khan, P. Yu, F. Yang, H. Liang, J. Inorg. Biochem. 185 (2018) 10.
[23] F. Rahman, A. Ali, H.-Q. Duong, I. U. Khan, M. Z. Bhatti, Z.-T. Li, H. Wang, D.-W.
Zhang, Eur. J. Med. Chem. 164 (2019) 546.
[24] T. Chatzisideri, S. Thysiadis, S. Katsamakas, P. Dalezis, I. Sigala, T. Lazarides, E.
Nikolakakia, D. Trafalis, O. A. Gederaas, M. Lindgren, V. Sarli, Eur. J. Med. Chem. 141
(2017) 221.
[25] C. M. Che, C. C. Kwok, S. W. Lai, A. F. Rausch, W. J. Finkenzeller, N. Zhu, H.
Yersin, Chem. Eur. J. 16 (2010) 233.
[26] S. S. Razi, Y. H. Koo, W. Kim, W. Yang, Z. Wang, H. Gobeze, F. D’Souza, J. Zhao, D.
Kim, Inorg. Chem. 57 (2018) 4877.
[27] J. Zhang, L. Xu, W. Y. Wong, Coord. Chem. Rev. 355 (2018) 180.
[28] J. Zhang, G. Dai, F. Wu, D. Li, D. Gao, H. Jin, D. Han, J. Photoch. Photobio. A 316
(2016), 12.
[29] O.V. Dolomanov, L.J. Bourhis, R. J. Gildea, J. A. K. Howard, H. Puschmann, J. Appl.
Cryst. 42 (2009) 339.
[30] G. M. Sheldrick, Acta Cryst. A71 (2015) 3.
[31] G. M. Sheldrick, Acta Cryst. C71 (2015) 3.
[32] C. K. Prier, D. A. Rankic, D. W. MacMillan, Chem. Rev. 113 (2013) 5322.
[33] C. Mari, V. Pierroz, R. Rubbiani, M. Patra, J. Hess, B. Spingler, L. Oehninger, J. Schur,
I. Ott, L. Salassa, S. Ferrari, G. Gasser, Chem. Eur. J. 20 (2014) 14421.
[34] D. Babic, M. Ćurić, K. Molčanov, G. Ilc, J. Plavec, Inorg. Chem. 47 (2008) 10446.
[35] V. Shanmugam, S. Selvakumar, C. S. Yeh, Chem. Soc. Rev. 43 (2014) 6254.
[36] M. K. Kuimova, S. W. Botchway, A. W. Parker, M. Balaz, H. A. Collins, H. L.
Anderson, K. Suhling, P. R. Ogilby, Nat. Chem. 1 (2009) 69.
[37] T. Goslinski, J. Piskorz, J. Photoch. Photobio. C 12 (2011) 304.
20

[38] T. J. Dougherty,. Photodynamic therapy. Photochem. Photobiol. 58 (1993), 895.
[39] D. Wang, S. J. Lippard,. Cellular processing of platinum anticancer drugs. Nat. Rev.
Drug Discov. 4 (2005) 307.
[40] D. Fink, S. Nebel, S. Aebi, H. Zheng, B. Cenni, A. Nehmé, R. D. Christen, S. B. Howell,
Cancer Res. 56 (1996) 4881.
21

Intense phosphorescence of a family of square-planar Schiff base Pt(II) complexes is used to
map their distribution in cancer cells. Although non-toxic in the dark, they are potently toxic
to cancer cells on green-light irradiation via generation of singlet oxygen, with potential to
act as dual theranostic and photodynamic therapy sensitisers.
22