The effect of the regioisomeric naphthalimide acetylide ligands on the photophysical properties of N^N Pt(ii) bisacetylide complexes

Article abstract of DOI:10.1039/c4dt01732c

Two N^N Pt(ii) bis(acetylide) complexes Pt-1 and Pt-2 with regioisomeric amino NI acetylide ligands (L-1 and L-2, L-1 = 5-amino-4-ethylnaphthaleneimide; L-2 = 3-amino-4-ethylnaphthaleneimide) were prepared. The photophysical properties of the complexes were studied by steady state and time-resolved spectroscopy. The two complexes with regioisomeric ligands (Pt-1 and Pt-2) show different photophysical properties such as maximal absorption wavelength (485 nm vs. 465 nm), triplet excited state lifetimes (23.7 μs vs. 0.9 μs), and different solvent-polarity dependences of the emission properties. The absorption of the complexes is red-shifted as compared with the previously reported Pt(ii) complex containing the 4-ethylnaphthaleneimide ligand. The two complexes with regioisomeric NI ligands were used as triplet photosensitizers for triplet-triplet annihilation (TTA) upconversion; drastically different upconversion quantum yields (15.0% vs. 1.1%) were observed. Our results are useful for designing new visible light-harvesting Pt(ii) bisacetylide complexes as triplet photosensitizers which can be used in areas such as photocatalysis, photodynamic therapy and TTA upconversion.

Full text of DOI:10.1039/c4dt01732c

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The effect of the regioisomeric naphthalimide
acetylide ligands on the photophysical properties
of N^N Pt(II) bisacetylide complexes†
Cite this: Dalton Trans., 2014, 43,
13434
Lianlian Liu, Caishun Zhang and Jianzhang Zhao*
Two N^N Pt(II) bis(acetylide) complexes Pt-1 and Pt-2 with regioisomeric amino NI acetylide ligands (L-1
and L-2, L-1 = 5-amino-4-ethylnaphthaleneimide; L-2 = 3-amino-4-ethylnaphthaleneimide) were pre-
pared. The photophysical properties of the complexes were studied by steady state and time-resolved
spectroscopy. The two complexes with regioisomeric ligands (Pt-1 and Pt-2) show different photo-
physical properties such as maximal absorption wavelength (485 nm vs. 465 nm), triplet excited state life-
times (23.7 μs vs. 0.9 μs), and different solvent-polarity dependences of the emission properties. The
absorption of the complexes is red-shifted as compared with the previously reported Pt(II) complex con-
taining the 4-ethylnaphthaleneimide ligand. The two complexes with regioisomeric NI ligands were used
as triplet photosensitizers for triplet–triplet annihilation (TTA) upconversion; drastically different upcon-
version quantum yields (15.0% vs. 1.1%) were observed. Our results are useful for designing new visible
light-harvesting Pt(II) bisacetylide complexes as triplet photosensitizers which can be used in areas such
as photocatalysis, photodynamic therapy and TTA upconversion.
Received 11th June 2014,
Accepted 13th July 2014
DOI: 10.1039/c4dt01732c
www.rsc.org/dalton
intermolecular electron transfer or energy transfer. However,
conventional Pt(II) acetylide complexes show weak absorption
Introduction
N^N Pt(II) bis(acetylide) complexes (N^N stands for one bi- in the visible region and the triplet excited state lifetime is
dentate ligand coordinating through two nitrogen atoms, the short (a few microseconds).1,14,15,20,21 Therefore, much room is
typical example of these ligands is 2,2′-bipyridine) have left for preparation of visible light-harvesting Pt(II) bisacetylide
attracted much attention due to their readily tunable photo- complexes showing a long-lived triplet excited state and for
physical properties using either different acetylide ligands or full exploration of the potential applications of these interest-
the N^N coordination ligands.1–7 Recently these complexes ing compounds.
have been used for photocatalytic hydrogen (H2) production,8
In order to enhance the visible light absorption, a straight-
photoredox catalytic organic reactions,9 photo-induced forward method is to attach an organic visible light-harvesting
charge separation,10 phosphorescent molecular probes,2,11–13 chromophore to the Pt(II) coordination center.20,22–26 However,
non-linear optics,14,15 and triplet–triplet annihilation efficient intersystem crossing (ISC) or intramolecular energy
upconversion.16–19 Concerning most of these applications, transfer must be implemented otherwise the production of the
intermolecular energy or electron transfer is typical, and the triplet excited state will not be enhanced. Moreover, the photo-
visible light-harvesting ability of the complexes is crucial.20,21 physical properties of the complexes are elusive. For example,
Strong visible light-harvesting will enhance the production of previously perylenebisimide (PBI) was attached to the Pt(II)
the triplet excited state, because the concentration of the mole- coordination center. The complexes showed enhanced absorp-
cules at the excited state is proportional to the photons tion in the visible region. Interestingly, the triplet excited state
absorbed by the compound. On the other hand, the triplet lifetime was very short (246 ns).27 On the other hand, pyrenyl
excited state lifetime is also crucial for the applications acetylides were used to prepare the N^N Pt(II) bisacetylide
because a long-lived triplet excited state is able to enhance the complex, and a long-lived triplet excited state was observed
(48.5 μs),28 but the complex showed absorption only in UV and
blue regions. Recently we prepared the naphthalimide acety-
lide-containing Pt(II) complex, which shows a long-lived triplet
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian
University of Technology, E 208 Western Campus, 2 Ling-Gong Road, Dalian 116012,
excited state, but the complex shows absorption only in the
P.R. China. E-mail: zhaojzh@dlut.edu.cn; http://finechem.dlut.edu.cn/photochem
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
blue spectral region.29,30 We also used regioisomeric NI
ligands to prepare cyclometalated Ir(III) complexes.31
c4dt01732c
13434 | Dalton Trans., 2014, 43, 13434–13444
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In order to prepare Pt(II) complexes showing red-shifted show near IR phosphorescence due to the amino-NI ligand in
absorption and a long-lived triplet excited state, herein we the complex. A long-lived triplet excited state (23.7 μs) was also
report the preparation of N^N Pt(II) complexes with amino observed. Interestingly, Pt-1 and Pt-2 show different photo-
NI acetylide ligands (Scheme 1). Two regioisomers were physical properties, such as absorption wavelength, phosphore-
used, 5-amino-4-ethylnaphthaleneimide and 3-amino-4-ethyl- scence emission and lifetimes, as well as the dependence of
naphthaleneimide. The corresponding Pt(II) complexes Pt-1 the emission properties on the solvent polarity. The complexes
and Pt-2 show red-shifted absorption in the region were used as triplet photosensitizers for TTA upconversion.
450–550 nm as compared with the previously reported Pt(II) Improved upconversion was observed compared with the pre-
complexes. With steady state and time-resolved spectroscopy, viously reported NI-containing Pt(II) complexes. Our studies
as well as DFT calculations, we found that at RT the complexes indicated the significant effect of the regioisomers of the
Scheme 1 Synthesis of the complexes Pt-1 and Pt-2. (i) NBS, DMF, RT, 3 h; (ii) fuming nitric acid, AcOH, r.t., 10 h; (iii) Na2Cr2O7·2H2O, AcOH, reflux,
3 h; (iv) n-butylamine, ethanol, reflux, 40 min; (v) n-butylamine, CH3OCH2CH2OH, 80 °C, 1 h; (vi) trimethylsilylacetylene, Pd(PPh3)2Cl2, PPh3, CuI,
NEt3, Ar, reflux, 6 h; K2CO3, THF–MeOH, r.t., 0.5 h; (vii) Pt(dbbpy)Cl2, CuI, diisopropylamine, CH2Cl2, r.t., 20 h; (viii) 2-ethylhexylamine, ethanol,
reflux, 8 h; (ix) n-butylamine, CH3OCH2CH2OH, reflux, 8 h; (x) Br2, CH2Cl2, r.t., 12 h.
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amino-NI ligands on the photophysical properties of the com- tallization with glacial acetic acid to give 3 as a dark yellow
1
plexes, and the results will be useful for preparation of visible crystal (0.93 g), yield: 65.4%. H NMR (400 MHz, DMSO): 8.68
light-harvesting Pt(II) bisacetylide complexes and for the appli- (d, 1H, J = 4.0 Hz), 8.49–8.44 (m, 3H). EI-HRMS: calcd
cation of these complexes in areas such photocatalysis and ([C12H4NO5Br]+) m/z = 320.9273, found m/z = 320.9283.
TTA upconversion.
Compound 4. A solution of compound
3
(0.14 g,
0.43 mmol) in ethanol (13 mL) was heated to reflux. A
moment later the solution was cooled to 50 °C, and a solution
of n-butylamine (0.043 mL, 0.032 g, 0.43 mmol) in ethanol
(5 mL) was added dropwise. The mixture was refluxed and
stirred for additional 40 min. The solvent was removed under
vacuum and the residue was further purified by column
chromatography (silica gel, CH2Cl2) to give 4 as a yellow solid
Experimental
Materials and reagents
All the reagents were analytically pure and were used as
received. Solvents were dried and distilled before use.
1
(120.2 mg), yield: 74.0%. H NMR (400 MHz, CDCl3): 8.69 (d,
Analytical measurements
1H, J = 8.0 Hz), 8.50 (d, 1H, J = 8.0 Hz), 8.20 (d, 1H, J = 8.0 Hz),
7.91 (d, 1H, J = 8.0 Hz), 4.15 (t, 2H, J = 8.0 Hz), 1.75–1.67 (m,
2H), 1.47–1.36 (m, 2H), 0.97 (t, 3H, J = 8.0 Hz).
NMR spectra were recorded on a 400 MHz Varian Unity Inova
spectrophotometer (with TMS as the standard). Mass spectra were
recorded with a Q-TOF Micro MS spectrometer. UV-Vis absorption
spectra were measured with an Agilent 8453 UV-vis spectropho-
tometer. Fluorescence spectra and upconversion were recorded
on a Shimadzu RF-5301PC spectrofluorometer. Phosphorescence
quantum yields were measured with 4-(dicyanomethylene)-2-
methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) as the stan-
dard (Φ = 0.1 in CH2Cl2). Luminescence lifetimes were measured
on an OB920 luminescence lifetime spectrometer (Edinburgh
Instruments, UK) and an FLS920 spectrofluorometer (Edinburgh
Instruments, UK). The quantum yield of singlet oxygen was
measured with meso-tetraphenylporphyrin (TPP, ΦΔ = 0.62 in
CH2Cl2) and Rose Bengal (ΦΔ = 0.8 in methanol) as standards.
Compound 5. N-Butylamine (0.034 mL, 24.7 mg,
0.34 mmol) was added to a solution of 4 (106.2 mg,
0.28 mmol) in ethylene glycol monomethyl ether
(CH3OCH2CH3, 10 mL) at 30 °C. Then the reaction mixture
was heated to 80 °C and stirred for 1 h. The residue was fil-
tered, and further purified by column chromatography (silica
gel, CH2Cl2–CH3OH, 100 : 1, v/v) to give 5 as a yellow solid
(70.7 mg), yield: 62.6%. 1H NMR (400 MHz, CDCl3): 8.46 (d,
1H, J = 8.0 Hz), 8.31 (d, 1H, J = 8.0 Hz), 7.79 (d, 1H, J = 8.0 Hz),
7.58 (s, 1H), 6.71 (d, 1H, J = 8.0 Hz), 4.12 (t, 2H, J = 8.0 Hz),
3.34 (t, 2H, J = 8.0 Hz), 1.87–1.81 (m, 2H), 1.73–1.66 (m, 2H),
1.60–1.54 (m, 2H), 1.46–1.40 (m, 2H), 1.02 (t, 3H, J = 8.0 Hz),
0.95 (t, 3H, J = 8.0 Hz). MALDI-HRMS: calcd ([C20H23N2O2Br +
H]+) m/z = 403.1021, found m/z = 403.1043.
Synthesis
Compound 1. A solution of N-bromosuccinimide (NBS.
Compound L-1. Under an Ar atmosphere, Pd(PPh3)2Cl2
3.6 g, 20.2 mmol) in DMF (10 mL) was added dropwise to a (14.8 mg, 0.021 mmol), PPh3 (11.1 mg, 0.042 mmol) and CuI
solution of acenaphthene (3.1 g, 20.1 mmol) in DMF (10 mL) (8.2 mg, 0.043 mmol) were added to a solution of 5 (169.4 mg,
at room temperature (RT). After 3 h, the solution was poured 0.42 mmol) in a mixed solvent of THF–Et3N (12 mL/4 mL). Tri-
into ice water to give a yellow solid which was filtered, washed methylsilylacetylene (0.3 mL, 124.1 mg, 1.26 mmol) was added
with water, and then dried in air. The product was further pur- via a syringe. The mixture was stirred at 75 °C for 6 h. After the
ified by column chromatography (silica gel, petroleum ether) addition of water, CH2Cl2 was added to extract the product
to give 1 as a white solid (4.36 g), yield: 93.6%. EI-HRMS: calcd (3 × 25 mL). The organic phase was dried over anhydrous
([C12H9Br]+) m/z = 231.9888, found m/z = 231.9881. No 1H NMR Na2SO4, filtered and the solvent was removed under reduced
spectrum was recorded due to the poor solubility.
pressure. The residue was further purified by column chromato-
Compound 2. A solution of fuming nitric acid (1 mL) in graphy (silica gel, CH2Cl2–CH3OH = 100 : 1, v/v) to give a light
glacial acetic acid (2 mL) was added dropwise to a solution of red solid (143.8 mg), yield: 81.2%. K2CO3 (138.0 mg,
compound 1 (1.12 g, 4.4 mmol) in glacial acetic acid (4 mL) in 1.0 mmol) was added to a solution of the above trimethylsilane
an ice-water bath. Then the reaction mixture was stirred at RT protected intermediate (140.2 mg, 0.33 mmol) in a mixed
for 10 h. The product was filtered and purified by recrystalliza- solvent CH2Cl2–MeOH (10 mL/5 mL). The mixture was stirred
tion with glacial acetic acid to give 2 as a yellow acicular crystal at RT for 30 min. After the addition of water, CH2Cl2 was
1
(0.65 g), yield: 53.4%. H NMR (400 MHz, CDCl3): 7.83 (d, 1H, added to extract the product (3 × 30 mL). The organic phase
J = 8.0 Hz), 7.72 (d, 1H, J = 8.0 Hz), 7.32–7.26 (m, 2H), was dried over anhydrous Na2SO4, filtered and the solvent was
3.44–3.42 (m, 4H). EI-HRMS: calcd ([C12H8NO2Br]+) m/z = evaporated under reduced pressure. The residue was further
276.9738, found m/z = 276.9691.
purified by column chromatography (silica gel, CH2Cl2–
Compound 3. Compound 2 (1.21 g, 4.4 mmol) was added to CH3OH = 100 : 1, v/v) to give L-1 as a light red solid (98.3 mg),
a solution of Na2Cr2O7·2H2O (3.0 g, 10.1 mmol) in glacial yield: 85.5%. 1H NMR (400 MHz, CDCl3): 8.49–8.44 (m, 2H),
acetic acid (12 mL) at RT. The mixture was refluxed and stirred 8.05 (s, 1H), 7.75 (d, 1H, J = 8.0 Hz), 6.66 (d, 1H, J = 8.0 Hz),
for 3 h. Then the mixture was poured into ice water and fil- 4.13 (t, 2H, J = 8.0 Hz), 3.88 (s, 1H), 3.35 (s, 1H), 1.82–1.41 (m,
tered, and then the solid was washed with water and a little 8H), 1.04–0.95 (m, 6H). 13C NMR (100 MHz, CDCl3): 164.3,
glacial acetic acid. The product was further purified by recrys- 164.1, 151.1, 135.1, 133.0, 131.0, 130.0, 123.8, 121.6, 119.2,
13436 | Dalton Trans., 2014, 43, 13434–13444
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109.6, 105.2, 87.8, 84.4, 43.9, 40.2, 30.6, 30.4, 20.6, 14.1, (s, 1H), 8.59 (d, 2H), 8.50 (d, 2H), 7.65 (t, 1H), 4.03–4.14 (m,
14.0 ppm. EI-HRMS: calcd ([C22H24N2O2]+) m/z = 348.1838, 2H), 3.65 (t, 2H), 1.89–1.95 (m, 1H), 1.71–1.79 (m, 2H),
found m/z = 348.1848.
Complex Pt-1. Pt(dbbpy)Cl2 (25.0 mg, 0.047 mmol), CuI ([C24H31N2O2Br + H]+) m/z = 459.1647, found m/z = 459.1607.
(5 mg, 0.09 mmol) and diisopropylamine (2 mL) were dis- Compound L-2. The synthetic procedure is similar to that of
1.29–1.48 (m, 10H), 0.86–0.98 (m, 9H). MALDI-HRMS: calcd
solved in CH2Cl2 (5 mL); the mixture was stirred for 10 min. L-1, except that compound 8 (102.7 mg, 0.22 mmol) was used.
Under Ar, L-1 (49.1 mg, 0.14 mmol) was added and the The residue was further purified by column chromatography
mixture was stirred at RT overnight. After complete consump- (silica gel, CH2Cl2–petroleum ether = 2 : 1, v/v) to give L-2 as a
tion of the starting material, the solvent was evaporated under yellow solid (69.1 mg), yield: 77.6%. 1H NMR (400 MHz,
reduced pressure, and the residue was purified by column CDCl3): 8.56 (d, 1H, J = 8.0 Hz), 8.52 (s, 1H), 8.39 (d, 1H, J =
chromatography (silica gel, CH2Cl2–CH3OH = 60 : 1, v/v) to give 8.0 Hz), 7.63–7.59 (m, 1H), 5.54 (s, 1H), 4.12–4.06 (m, 2H),
Pt-1 as a dark red solid (21.8 mg), yield: 40.0%. 1H NMR 3.86–3.81 (m, 2H), 3.52 (s, 1H), 1.94–1.91 (m, 1H), 1.77–1.72
(400 MHz, CDCl3): 9.52 (d, 1H, J = 8.0 Hz), 8.46 (d, 1H, J = (m, 2H), 1.52–1.29 (m, 10H), 1.00–0.86 (m, 9H). 13C NMR
8.0 Hz), 8.40 (d, 1H, J = 8.0 Hz), 8.07 (s, 1H), 7.73–7.69 (m, 2H), (100 MHz, CDCl3): 164.9, 163.8, 153.8, 136.8, 132.0, 130.5,
6.59 (d, 1H, J = 8.0 Hz), 4.14 (t, 2H, J = 8.0 Hz), 3.21 (t, 2H, J = 130.0, 125.0, 123.3, 121.7, 112.2, 104.2, 84.0, 80.6, 49.4, 44.1,
8.0 Hz), 1.73–1.64 (m, 4H), 1.49–1.42 (m, 11H), 1.16–1.11 (m, 38.1, 33.6, 30.9, 28.9, 24.2, 23.3, 20.1, 14.3, 14.0, 10.9 ppm.
2H), 0.95 (t, 3H, J = 8.0 Hz), 0.57 (t, 3H, J = 8.0 Hz). 13C NMR EI-HRMS: calcd ([C26H32N2O2 + H]+) m/z = 405.2542, found m/z
(100 MHz, CDCl3): 164.9, 164.8, 164.3, 156.1, 152.5, 151.1, = 405.2556.
134.8, 131.4, 131.1, 130.1, 128.9, 125.3, 120.1, 119.3, 119.2,
Complex Pt-2. The synthetic procedure is similar to that of
108.1, 104.0, 103.6, 103.1, 43.5, 39.9, 36.1, 30.4, 30.3, 30.2, Pt-1, except that L-2 (52.0 mg, 0.13 mmol) was used. The crude
29.7, 20.5, 20.4, 13.9, 13.7 ppm. MALDI-HRMS: calcd product was further purified by column chromatography
([C62H70N6O2Pt]−) m/z = 1157.5106, found m/z = 1157.5027.
(silica gel, CH2Cl2–CH3OH = 150 : 1, v/v) to give Pt-2 as a red
1
Compound 6. 4-Bromo-1,8-naphthalene dicarboxylic anhy- solid (23.3 mg), yield: 42.3%. H NMR (400 MHz, CDCl3): 9.69
dride (1.0 g, 3.65 mmol) and 2-ethylhexylamine (0.8 g, (d, 1H, J = 8.0 Hz), 8.64 (s, 1H), 8.53–8.49 (m, 1H), 8.04 (s, 1H),
7.3 mmol) were dissolved in ethanol (100 mL). The mixture 7.72 (d, 1H, J = 8.0 Hz), 7.54–7.50 (m, 1H), 6.61 (s, 1H),
was refluxed for 8 h. Then the solvent was evaporated under 4.19–4.08 (m, 2H), 3.72–3.68 (m, 2H), 1.99–1.96 (m, 1H),
reduced pressure, and the residue was purified by column 1.67–1.60 (m, 2H), 1.49–1.18 (m, 19H), 0.96–0.87 (m, 6H), 0.53
chromatography (silica gel, CH2Cl2–petroleum ether = 1 : 2) to (t, 3H, J = 8.0 Hz). 13C NMR (100 MHz, CDCl3): 165.4, 164.6,
give 6 as a light yellow solid (1.1 g), yield: 80.0%. 1H NMR 164.3, 156.3, 154.0, 151.4, 135.4, 131.1, 130.9, 129.7, 125.2,
(400 MHz, CDCl3) 8.63–8.65 (d, 1H, J = 7.4 Hz), 8.53–8.55 (d, 123.7, 123.0, 120.9, 119.1, 111.2, 110.5, 96.9, 95.0, 49.8, 44.1,
1H, J = 8.5 Hz), 8.38–8.40 (d, 1H, J = 8.0 Hz), 8.01–8.03 (d, 1H, 38.1, 36.1, 33.3, 31.0, 30.4, 29.9, 29.0, 24.3, 23.3, 20.1, 14.3,
J = 7.9 Hz), 7.81–7.85 (t, 1H, J = 8.0 Hz), 4.05–4.16 (m, 2H), 13.6, 10.9 ppm. MALDI-HRMS: calcd ([C70H86N6O4Pt − H]+)
1.88–1.98 (m, 1H), 1.29–1.41 (m, 8H), 0.86–0.95 (m, 6H). m/z = 1268.6280, found m/z = 1268.6373.
MALDI-HRMS: calcd ([C20H22NO2Br + H]+) m/z = 388.0912,
found m/z = 388.0890.
Triplet–triplet annihilation upconversion
Compound 7. Compound 6 (1.0 g, 2.64 mmol) and n-butyl- A diode pumped solid state (DPSS) continuous laser (473 nm
amine (1.3 mL, 962.0 mg, 13.2 mmol) were dissolved in and 532 nm) was used as the excitation source for the up-
CH3OCH2CH2OH (18 mL). Then the mixture was refluxed for conversion. The diameter of the 473 nm and 532 nm laser spot
8 h. Water was added and CH2Cl2 was added to extract the is ca. 3 mm. The power of the laser beam was measured with a
product (3 × 25 mL). The combined organic layer was dried over VLP-2000 pyroelectric laser power meter. For the upconversion
anhydrous Na2SO4. After solvent was evaporated under reduced experiments, the mixed solution of the compound (triplet
pressure, the residue was purified by column chromatography photosensitizers) and 9,10-diphenylanthracene (DPA) was
(silica gel, CH2Cl2) to give 7 as a light yellow solid (42.7 mg), degassed with N2 for at least 15 min. The upconverted fluore-
1
yield: 40.2%. H NMR (400 MHz, CDCl3): 8.60 (d, 1H), 8.48 (d, scence of DPA was observed with a Shimadzu RF-5301PC spec-
1H), 8.10 (s, 1H), 7.61–7.65 (m, 1H), 6.74 (s, 1H), 4.05–4.15 (m, trofluorometer. In order to repress the scattered laser, a small
2H), 3.44 (t, 2H), 1.91–1.98 (m, 1H), 1.77–1.84 (m, 2H), black box was put behind the fluorescent cuvette to dump the
1.49–1.58 (m, 2H), 1.26–1.40 (m, 8H), 1.04 (t, 3H), 0.85–0.94 laser beam. The upconversion quantum yields (ΦUC) were
(m, 6H). EI-HRMS: calcd ([C24H32N2O2]+) m/z = 380.2464, determined with the prompt fluorescence of 4-dicyanomethy-
found m/z = 380.2471.
lene-2-methyl-6-(p(dimethylamino)styryl)-4H-pyran (DCM) as
Compound 8. Br2 (1.0 mL, 249.5 mg, 1.6 mmol) in anhy- the luminescence quantum yield standard (ΦF = 10% in
drous CH2Cl2 (10 mL) was added dropwise to a solution of 7 CH2Cl2). The upconversion quantum yields were calculated
(200.0 mg, 0.53 mmol) in CH2Cl2 (10 mL) at RT, and stirred for using eqn (1), where ΦUC, Asam, Isam and ηsam represent the
12 h. Then the solvent was removed under reduced pressure, quantum yield, absorbance, integrated photoluminescence
and the residue was purified by column chromatography intensity and refractive index of the solvents (where the sub-
(silica gel, CH2Cl2–petroleum ether = 2 : 1) to give 8 as a yellow script “std” stands for the standard used in the measurement
solid (296.4 mg), yield: 60.2%. 1H NMR (400 MHz, CDCl3): 8.64 of the quantum yield and “sam” stands for the samples to be
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measured). The equation is multiplied by a factor of 2 in order
to set the maximum quantum yield to unity.16 All these data
were independently measured three times (with different
sample solutions).
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
Astd
Asam
Isam
Istd
ηsam
ηstd
ΦUC ¼ 2Φstd
ð1Þ
All these data were independently measured three times.
The CIE coordinates (x, y) of the emission of the sensitizers
alone and the emission of the upconversion were derived from
the emission spectra with the software of CIE color matching
linear algebra.
Fig. 1 UV-Vis absorption of Pt(II) complexes and ligands L-1 and L-2.
c = 1.0 × 10−5 M, in toluene, 20 °C.
DFT calculations
Density functional theory (DFT) calculations were used for
optimization of both singlet and triplet states. The energy level
of the T1 state (energy gap between S0 and T1 states) was calcu-
lated with time-dependent DFT (TDDFT), based on the opti-
mized singlet ground state geometries (S0 state). All the
calculations were performed with Gaussian 09W.32
The UV-Vis absorption spectra of the ligands and the com-
plexes in solvents of different polarities were studied (Fig. 2),
no substantial variation was observed. Therefore the intra-
molecular charge transfer of the compounds in the ground
state is not significant.14
The photoluminescence spectra of the compounds were
studied (Fig. 3). L-2 shows emission at 422 nm and L-1 shows
emission in the red-shifted region (463 nm). Thus the regio-
Results and discussion
isomerism imposes
a significant effect on the photo-
Design and synthesis of the complexes
luminescence properties of the ligands. Pt-1 and Pt-2 show
different emission properties. The emission of Pt-1 is stronger
than that of Pt-2. The emission spectrum of Pt-1 is more struc-
tured than that of Pt-2. Both complexes give emission at a
similar wavelength (665 nm), which is slightly red-shifted com-
pared to Pt-0 (the energy difference is 0.12 eV).29
The amino-NI acetylides show much lower S1 state energy
levels than NI acetylide, by 0.85 eV and 0.46 eV for L-2 and L-1,
respectively. Interestingly, the T1 state energy levels of the com-
plexes Pt-1 and Pt-2 are only 0.12 eV lower than that of Pt-0.
Higher T1 state energy level is beneficial for the application of
Naphthalimide is a versatile chromophore and has been
widely used in construction of molecular arrays and fluo-
rescent molecular probes or fluorophores.33–36 In comparison,
the application of NI derivatives in the triplet excited state
manifold is rare.29–31,37 We noted that the amino group can be
introduced into the NI π-system. Normally the amino group
can induce red-shifted absorption.31 Regioisomers L-1 and L-2
were prepared via the Sonogashira coupling reaction.31 The
preparation of the N^N Pt(II) bisacetylide complexes Pt-1 and
Pt-2 followed the reported procedures. The known complex
Pt-0 was used as a reference compound in the study. All the
compounds were obtained in moderate to good yields (see the
Experimental section and ESI† for molecular structural charac-
terization data).
UV-Vis absorption and luminescence spectra of the
compounds
L-1 gives absorption at 463 nm (ε = 8900 M−1 cm−1). L-2 gives
absorption in the blue-shifted region at 422 nm (ε
=
23 600 M−1 cm−1) (Fig. 1).
This result indicates that the effect of the regioisomer is
substantial.31 Upon complexation with the Pt(II) coordination
center, Pt-1 shows intensified absorption as compared to the
ligand L-1, but the absorption wavelength is the same. For L-2,
however, the absorption band moved bathochromatically upon
coordination to the Pt(II) center. The absorption of both com-
plexes is red-shifted by 50 nm compared to that of Pt-0.29 To
the best of our knowledge, this is the first time that amino NI
acetylide is used for preparation of Pt(II) complexes. Previously
we used similar ligands for preparation of cyclometalated Ir(III)
complexes.31 A similar effect was observed.
Fig. 2 UV-Vis absorption of (a) Pt-1, (b) Pt-2, (c) L-1, (d) L-2 under
different atmospheres in CH3CN, DCM, methanol, THF and toluene. c =
1.0 × 10−5 M, 20 °C.
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Pt-2 in the near IR region is 78.3 μs and 20.1 μs, respectively.
The luminescence lifetime of the minor emission band in the
450 nm–550 nm region is 7.3 ns and 7.2 ns, respectively
(Table 1). The minor emission band of Pt-1 and Pt-2 in the
450 nm–550 nm region is unlikely due to the impurities of the
ligands because the ligands give longer fluorescence lifetimes
of 10.0 ns and 9.6 ns, respectively (Table 1).
The photoluminescence spectra of the complexes in sol-
vents of different polarities were studied (Fig. 5). L-1 and L-2
give different responses to the variation of solvent polarity. For
example, L-1 shows much weaker emission in CH3CN than
that in DCM. For L-2, however, the emission intensities in
DCM and CH3CN are similar. The luminescence of Pt-1 and
Pt-2 in different solvents is drastically different. Pt-1 shows a
decreased emission intensity in polar solvents, but the emis-
sion variation of Pt-2 in different solvents is more complicated.
These different emission properties in different solvents can
be attributed to the different intramolecular charge transfer
(ICT) features of the compounds.
Fig. 3 The normalized emission spectra of the Pt(II) complexes and
ligands: Pt-0 (λex = 420 nm), Pt-1 (λex = 495 nm), Pt-2 (λex = 490 nm),
L-1 (λex = 470 nm), L-2 (λex = 430 nm). c = 1.0 × 10−5 M in toluene,
20 °C.
triplet photosensitizers because the triplet–triplet-energy-
transfer (TTET) is the typical photophysical process involved in
the application of the triplet photosensitizers.5,20,38
The luminescence of the compounds under different
atmospheres was studied (Fig. 4). For both Pt-1 and Pt-2, the
emission in the near IR region was completely quenched in
aerated solution. On the other hand, the minor emission band
at 570 nm was not quenched. These results indicated that the
emission in the near IR region is phosphorescence and the
emission in the 500 nm–600 nm is fluorescence.
The emission of the ligands under different atmospheres
was also studied (Fig. 4c and 4d). Interestingly the emission
intensity is reduced in aerated solution. The assignment of the
fluorescence and the phosphorescence of the compounds was
supported by the measurement of the luminescence lifetimes
(Table 1). For example, the luminescence lifetime of Pt-1 and
77 K emission spectra
In order to study the properties of the emissive triplet excited
states of the complexes, the emission at 77 K was studied
(Fig. 6). The emission spectra of Pt-1 become more significant
vibrational propagation, and the thermally induced Stokes
shift is small. For Pt-2, similar emission spectra were observed
at 77 K. The minor emission band in the 500 nm region was
diminished. This may be due to the greatly intensified phos-
phorescence at 77 K as compared with that at RT. Pt-1 and Pt-2
share similar T1 state energy levels (1.89 eV and 1.92 eV,
respectively).
Nanosecond transient difference absorption spectroscopy
The nanosecond time-resolved transient difference absorption
spectra were used to study the triplet states of the complexes
(Fig. 7). Bleaching bands were found to be almost the same as
the steady state UV-Vis absorption bands upon pulsed laser
excitation. Moreover, similar transient absorption features
were observed for Pt-0, Pt-1 and Pt-2 in the range of
500–700 nm which indicate the triplet states present in the
ligands, while different positive transient absorption bands
indicate the triplet state’s difference of the ligands (Pt-2 is
much weaker than Pt-1). The bleaching bands of Pt-1 and Pt-2
are much weaker than that of Pt-0, which is proposed to be
decreased by the strong positive transient absorption bands.
DFT calculations
To assign the triplet states of the complexes, the spin density
surfaces were calculated by DFT methods.22,39–41 Generally the
triplet excited states are localized on the organic ligands,
instead of the Pt(II) coordination centers (Fig. 8). This is in
agreement with the long-lived triplet excited states of the
complexes.
Fig. 4 The emission spectra of the Pt(II) complexes and the ligands.
(a) Pt-1 (λex = 495 nm) (b) Pt-2 (λex = 490 nm), (c) L-1 (λex = 470 nm),
(d) L-2 (λex = 430 nm) under different atmospheres. c = 1.0 × 10−5 M in
toluene, 20 °C.
According to the calculation results of singlet–triplet state
energy gaps, for Pt-1, the electronic component of the tran-
3
sition is H → L, which can be treated as the IL transition (see
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Table 1 Photophysical parameters of Pt-0−Pt-2, L-1 and L-2
τF f (ns)
τP g (μs)
εb
λem
ΦF
ΦP
298 K
77 K
298 K
77 K
τT/μsh
ΦΔ
a
c
d
e
i
λabs
Pt-0
Pt-1
Pt-2
L-1
416
485
465
463
422
5.26
2.92
1.43
0.89
2.36
624
—
7.1%j
0.96%
0.01%
—
—
—
124j
78.3
20.1
—
205j
596.9
457.3
—
45.3
23.7
0.9k
—
0.99
0.84
0.63
—
560, 668
528, 653
536
0.01%
7.3
7.2
10.0
9.6
1.4
2.8
—
0.18 × 10−3
23.60%
47.21%
%
L-2
485
—
—
—
—
—
—
a In toluene (1.0 × 10−5 mol dm−3). b The molar extinction coefficient at the absorption maxima. ε: 104/cm−1 mol−1 dm3. c In toluene. d In toluene,
with DCM (Φ = 0.1 in CH2Cl2) as the standard. e In toluene, DCM (Φ = 0.1 in CH2Cl2) as the standard. f Luminescence lifetime, λex = 443 nm, at
RT (in deaerated toluene) and 77 K (in deaerated MTHF). g The phosphorescence lifetime of Pt-1 (λex = 495 nm) and Pt-2 (λex = 462 nm) at RT (in
deaerated toluene) and 77 K (in deaerated MTHF). h Triplet state lifetime, determined by nanosecond time-resolved transient difference
absorption spectroscopy, λex = 355 nm. 1.0 × 10−5 M in deaerated toluene. i The quantum yield of singlet oxygen (1O2), Pt-0 with TPP as the
standard (ΦΔ = 0.62 in CH2Cl2), 1.0 × 10−5 mol dm−3 in CH2Cl2. Pt-2–Pt-3 with RM as the standard (ΦΔ = 0.8 in methanol), 1.0 × 10−5 mol dm−3
in methanol. j Literature value. k c = 3.0 × 10−5 M.
Fig. 5 The emission spectra of the Pt(II) complexes and the ligands.
(a) Pt-1 (λex = 495 nm), (b) Pt-2 (λex = 490 nm), (c) L-1 (λex = 470 nm),
(d) L-2 (λex = 430 nm) in different solvents. c = 1.0 × 10−5 M, 20 °C.
Fig. 7 Nanosecond time-resolved transient difference absorption
spectra of (a) Pt-0, (c) Pt-1 and (e) Pt-2 (3.0 × 10−5 M). The decay traces
were monitored at (b) Pt-0 (540 nm), (d) Pt-1 (560 nm), (f) Pt-2 (550 nm,
3.0 × 10−5 M). In deaerated toluene after pulsed excitation (λex
355 nm). c = 1.0 × 10−5 M, 25 °C.
=
Fig. 6 Photoluminescence spectra of (a) Pt-1 (λex = 495 nm), (b) Pt-2
(λex = 462 nm) at RT and 77 K. c = 6.0 × 10−5 M in deaerated 2-methyl
tetrahydrofuran (MTHF).
results were also obtained for Pt-2, whose T1 energy level is
calculated to be 1.96 eV (the experimental value is 1.92 eV).
Triplet–triplet annihilation upconversion
ESI†). This is in agreement with the analysis of the spin
density surfaces. The predicted triplet energy level is 1.85 eV,
which is close to the experimental data (1.89 eV). Similar
The Pt(II) complexes show strong absorption of visible light
and long-lived triplet excited states, thus these complexes are
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Fig. 8 Spin density of the complexes and the ligands Pt-1, Pt-2, L-1, L-2.
Calculated based on the optimized triplet state by DFT at the B3LYP/6-31G(d)/
LanL2DZ and CAM-B3LYP/GENECP/LanL2DZ level using Gaussian 09W.
Fig. 9 Emission and upconversion of the complexes. (a) Emission of
the complexes alone and (b) the upconverted DPA fluorescence and the
residual fluorescence and phosphorescence of the complexes, c [DPA] =
4.0 × 10−5 M, in deaerated acetonitrile; (c) emission of the complexes
alone and (d) the upconverted DPA fluorescence and the residual fluor-
escence and phosphorescence of the complexes, c [DPA] = 1.4 × 10−4
ideal triplet photosensitizers for TTA upconversion.17,21 The
crucial step of TTA upconversion, i.e. the TTET from the triplet
photosensitizer to the triplet acceptor, can be enhanced by the M, in deaerated toluene. The asterisks in (a) and (b) indicate the scattered
473 nm excitation laser (5 mW), (c) and (d) indicate the scattered 532 nm
excitation laser (5 mW). c [sensitizers] = 1.0 × 10−5 M, 20 °C.
strong visible light-harvesting and the long-lived triplet excited
states.21,42
Previously a Pt(II) acetylide complex was used as a triplet
photosensitizer for TTA upconversion.43 However, the visible
same region (Fig. 9d). The lack of upconverted emission of
Pt-0 and Pt-1 is due to the inappropriate excitation wavelength.
These results indicated that using amino-NI acetylides, the
TTA upconversion of the complexes Pt-1 and Pt-2 is improved
as compared to the reference complex Pt-0.
light-absorption of that complex was weak. Recently we used a
pyrenyl acetylide Pt(II) complex for TTA upconversion.30b We
postulate that the TTA upconversion can be improved with the
new complexes Pt-1 and Pt-2.
Based on the T1 state energy levels of Pt-1 and Pt-2 (1.89 eV
and 1.92 eV, approximated by the 77 K phosphorescence emis-
sion wavelength of Pt-1 and Pt-2), 9,10-diphenyl anthracene
(DPA) was selected as the triplet acceptor/emitter for which the
triplet state energy level is 1.77 eV. It should be pointed out
that the triplet acceptor should not show any absorption in the
absorption region of the triplet photosensitizers.
Firstly the compounds were excited with a 473 nm laser
(Fig. 9a). Pt-2 shows very weak emission, but Pt-1 shows strong
emission. Pt-0 shows much stronger emission than Pt-2. Upon
addition of the triplet acceptor DPA, new emission bands in
the blue region (400–500 nm) were observed for Pt-1, Pt-2 and
the reference complex Pt-0 (Fig. 9b). However, the blue emis-
sion intensity of the three complexes varied substantially. Pt-1
gives the strongest emission and Pt-0 gives the weakest emis-
sion (Fig. 9b). Their different upconverted emission is attribu-
ted to their different absorption properties at the excitation
wavelength. Notably Pt-1 shows much stronger upconverted
emission than Pt-0, although Pt-0 has a longer T1 state lifetime
(Table 1). We also used a 532 nm laser for the photoexcitation
(Fig. 9c and 9d). Only Pt-1 can be excited and the emission at
667 nm was observed. Upon addition of DPA, Pt-1 shows
upconverted blue emission in the region of 400 nm–500 nm.
Pt-0 and Pt-1 did not show any upconverted emission in the
For the triplet–triplet upconversion quantum yields (ΦUC
)
(Table 2), when excited with a 473 nm laser in toluene, Pt-1
(ΦUC = 0.192) is similar to Pt-0 (ΦUC = 0.199), for which the lit-
erature value is 0.181 upon excitation at 445 nm in acetonitrile.
Pt-2 (ΦUC = 0.004) is much smaller than the other two com-
plexes. It is interesting to find that the ΦUC changed a lot in
acetonitrile which may be due to the variation of the solvent
polarity. We observed that ΦUC = 0.150 and 0.011 for Pt-1 and
Pt-2, respectively, with 532 nm excitation in toluene. These
Table 2 Triplet excited state lifetimes (τT), Stern–Volmer quenching
constant (KSV) and bimolecular quenching constants (kq) of the Pt(II)
sensitizers. In deaerated toluene solution, 20 °Ca
ΦUC
τT/ KSV
μs
/
kq/
103 M−1 b 109M−1 s−1 b Toluenec Acetonitrilec Toluened
e
Pt-0 45.3 402.5
Pt-1 23.7 50.5
Pt-2 0.9 29.2
6.4
2.1
32.4
19.9%
19.2%
0.4%
3.0%
2.7%
1.9%
—
15.0%
1.1%
a Upconversion quantum yields, with DCM as the standard (ΦUC = 10%
in CH2Cl2). b Excited at 355 nm. c 473 nm (5 mW) laser excitation.
d 532 nm (5 mW) laser excitation. e Cannot be excited at 532 nm.
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Fig. 10 Stern–Volmer plots for lifetime quenching of Pt-0, Pt-1
and Pt-2 with increasing concentration of DPA. c [photosensitizers] =
1.0 × 10−5 M, in deaerated toluene, 20 °C.
Fig. 11 Time-resolved emission spectra (TRES) of Pt-1 alone and the
TTA upconversion. With DPA as the triplet acceptor. (a) Pt-1 alone, the
phosphorescence region was measured (600 nm–830 nm, τ = 63.0 μs).
(b) TRES of Pt-1 in the presence of DPA. Upconverted emission in the
range of 400 nm–500 nm was observed (τ = 398.1 μs). (c) Pt-2 alone,
the phosphorescence region was measured (580 nm–830 nm, τ =
28.1 μs). (d) TRES of Pt-2 in the presence of DPA. Upconverted emission
in the range of 400 nm–500 nm was observed (τ = 234.5 μs). c [DPA] =
1.4 × 10−4 M; c [sensitizers] = 1.0 × 10−5 M. In deaerated toluene. Excited
with a nanosecond pulsed OPO laser synchronized with a spectrofluo-
rometer (λex = 532 nm), 25 °C.
results indicate different effects of the ligands on the photo-
physics of the complexes.
In order to study the TTET process of the TTA upconversion
of the complexes, the quenching of the triplet state lifetimes
of the complexes by the triplet acceptor DPA was studied
(Fig. 10). Pt-0 gives much larger quenching constants than Pt-1
and Pt-2. However, the upconversion of Pt-0 is the weakest
among the three complexes. These results indicated that the
visible light-harvesting ability of the triplet photosensitizer
is crucial for the triplet photosensitizers used for TTA
upconversion.
In order to unambiguously confirm that the TTA upconver-
sion is responsible for the blue emission band of the triplet
photosensitizer/DPA mixed samples, the time-resolved emis-
sion spectra (TRES) were recorded (Fig. 11). For Pt-1 alone,
long-lived emission at 667 nm was observed, with a lifetime of
63.0 μs. For the mixture Pt-1/DPA, however, the emission band
at 660 nm was quenched, and a new emission band in the
400 nm region developed. The lifetime of the emission is
398.1 μs. This long-lived fluorescence emission is the charac-
teristic feature of the TTA upconversion. Similar TRES was
observed for Pt-2. The luminescence lifetime of Pt-2 alone was
determined as 28.1 μs. In the presence of DPA, however, the
emission band at 410 nm with a lifetime of 234.5 μs was
observed.
The upconverted emission is visible to the unaided eye
(Fig. 12). The photographs of the emission of the triplet photo-
sensitizers alone and in the presence of DPA (TTA upconver-
sion) were taken (Fig. 12). With 532 nm laser excitation, only
Pt-1 shows phosphorescence emission. With addition of DPA,
strong blue emission was observed for Pt-1. For Pt-2, however,
no upconverted blue emission was observed.
The efficiency of the triplet excited state production is
studied by photosensitizing singlet oxygen (1O2) with the com-
plexes (Table 3 and Fig. 13). 1,5-Dihydroxylnaphthalene (DHN)
was photooxidized with the complexes as the photosensiti-
zers.44,45 The progress of the photooxidation is monitored by
UV-Vis absorption changes. The absorption of DHN at 301 nm
decreased as well as that of the oxidation product juglone at
427 nm increased due to the consumption of DHN and the
Fig. 12 (a) Photographs of the emission of sensitizers alone and (b) the
upconversion. (c) CIE diagram of the emission of sensitizers alone and
(d) in the presence of DPA (upconversion). λex = 532 nm (laser power:
5 mW). Pt-0 cannot be excited by a 532 nm laser. In deaerated toluene,
c[sensitizer] = 1.0 × 10−5 M, c[DPA] = 1.4 × 10−4 M, 20 °C.
production of juglone. More significant changes of UV-Vis
absorption were found for Pt-1 than that for Pt-2.
The photooxidation kinetics was studied by plotting the
ln(Ct/C0) against the irradiation time. Similar results were
obtained that Pt-1 has a steeper slope than Pt-2. These results
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Table 3 Photophysical parameters of the Pt(II) and the organic triplet
photosensitizersa
kobs/min−1 a
vi
Yieldc (%)
ΦΔ
b
d
Pt-0
Pt-1
Pt-2
MB
TPP
RB
Ir(ppy)2bpy
47.4
45.0
16.3
43.5
53.9
78.6
2.2
4.74
4.50
1.63
4.35
5.39
7.86
0.22
81.9
82.2
62.6
81.5
82.9
60.5
23.3
0.99
0.84
0.63
0.57e
0.62e
0.80e
—
Fig. 14 Photosensitizing of 1O2 with the complexes as photosensitizers.
Irradiation time-dependent decrease of absorbance at 301 nm of DHN
(1.0 × 10−4 M) with different 1O2 sensitizers. (a) Plots of ln (Ct/C0) vs.
irradiation time. (b) Plots of the chemical yield as a function of irradiation
time for the photooxidation of DHN. c [sensitizers] = 1.0 × 10−5 M. In
CH2Cl2–MeOH (9 : 1, v/v), 20 °C.
a Photoreaction rate constants, 10−3 min−1 b Initial rate, 10−6 M, in
.
min−1 c The yield of juglone after photoirradiation for 40 min. d The
.
quantum yield of singlet oxygen (1O2), with TPP (for Pt-0 ΦΔ = 0.62 in
CH2Cl2) and Rose Bengal (RB) (for Pt-1 and Pt-2 ΦΔ = 0.80 in
methanol) as standards. e Literature values.
two complexes show red-shifted absorption in the 450–550 nm
region, as well as a long-lived triplet excited state (23.7 μs) as
compared with the previously reported Pt(II) complexes (the T1
state lifetime is less than 5 μs). The complexes were used as
triplet photosensitizers for triplet–triplet annihilation (TTA)
upconversion. An improved upconversion quantum yield (up
to 15.0%) was observed. The two complexes with regioisomeric
ligands show distinctly different photophysical properties,
such as the maximal absorption wavelength (485 nm vs.
465 nm), triplet excited state lifetimes (23.7 μs vs. 0.9 μs),
solvent-polarity dependence of the emission properties and
TTA upconversion quantum yields (15.0% vs. 1.1%). Our
results are useful for designing new visible light-harvesting
Pt(II) bisacetylide complexes as triplet photosensitizers which
can be used in areas such as photocatalysis, photodynamic
therapy, and TTA upconversion.
Fig. 13 Photosensitizing of 1O2 with the complexes as photosensitizers.
Irradiation time-dependent decrease of absorbance at 301 nm of DHN
Acknowledgements
(1.0
×
10−4 M) with different 1O2 sensitizers (a) Pt-1, (b) Pt-2,
(c) Ir(ppy)2bpy, (d) MB. c [sensitizers] = 1.0 × 10−5 M. In CH2Cl2–MeOH
We thank the NSFC (21073028, 21273028 and 91127005), the
Royal Society (UK) and NSFC (Cost-Share-21011130154), the
Ministry of Education (SRFDP-20120041130005), the Program
for Changjiang Scholars and Innovative Research Team in
University [IRT_13R06], the State Key Laboratory of Fine
Chemicals (KF1203), the Fundamental Research Funds for the
Central Universities (DUT14ZD226) and the Dalian University
of Technology (DUT2013TB07) for financial support.
(9 : 1, v/v), 12 mW cm−2, 20 °C.
indicated different effects of the two ligands on the photo-
oxidation ability. What is more, Pt-1 gives a similar slope to
Pt-0, even steeper than the typical photosensitizer methylene
blue (MB). Pt-0, Pt-1 and MB gave similar results which are
much higher than Pt-2.
The ΦΔ values of 0.84 and 0.61 were determined for Pt-1
and Pt-2, respectively. The high ΦΔ values of Pt-1 indicate their
high photooxidation ability (Fig. 14).
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