Switching of the photophysical properties of bodipy-derived trans bis(tributylphosphine) Pt(ii) bisacetylide complexes with rhodamine as the acid-activatable unit

Article abstract of DOI:10.1039/c4dt03373f

A rhodamine moiety was used for the preparation of trans bis(tributylphosphine) Pt(ii) bisacetylide complexes (RH-BDPY-Pt-1 and RH-BDPY-Pt-2, with two different Bodipy acetylide ligands), which show acid/base-switchable photophysical properties. The rhoda

Full text of DOI:10.1039/c4dt03373f

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Switching of the photophysical properties of
Bodipy-derived trans bis(tributylphosphine) Pt(II)
bisacetylide complexes with rhodamine as the
acid-activatable unit†
Cite this: DOI: 10.1039/c4dt03373f
Poulomi Majumdar, Xiaoneng Cui, Kejing Xu and Jianzhang Zhao*
A rhodamine moiety was used for the preparation of trans bis(tributylphosphine) Pt(II) bisacetylide com-
plexes (RH-BDPY-Pt-1 and RH-BDPY-Pt-2, with two different Bodipy acetylide ligands), which show acid/
base-switchable photophysical properties. The rhodamine moiety undergoes reversible spirolactam ↔
opened amide structure transformation in the presence of an acid/base. Bodipy ligands are responsible
for strong visible light-harvesting. The photophysical properties of the Pt(II) complexes were studied with
steady state UV–Vis absorption, luminescence spectra, nanosecond transient absorption spectroscopy,
electrochemical characterization and DFT/TDDFT computations. In the absence of an acid, the com-
plexes show the absorption of Bodipy ligands at 580 nm and 500 nm, respectively. Both complexes show
fluorescence. A minor phosphorescence band was observed for RH-BDPY-Pt-1. In the presence of tri-
fluoroacetic acid (TFA), the spirolactam → opened amide transformation occurred and the absorption of
the rhodamine moiety at 570 nm appeared; colour changes were observed for the solutions of the com-
plexes. Moreover, the fluorescence of the complexes was switched on. Long-lived triplet excited states
were observed for the two complexes (35 μs and 423 μs, respectively, in dichloromethane). Upon the
addition of TFA, the triplet state lifetime of RH-BDPY-Pt-1 was substantially prolonged to 80 μs from
35 μs (the triplet state of RH-BDPY-Pt-1 is localized on the Bodipy moiety); for RH-BDPY-Pt-2, however,
the triplet state is switched from the Bodipy-confined triplet state to a triplet state delocalized on the
Bodipy and rhodamine moiety. Thus both the singlet excited state and the triplet state of the Pt(II) com-
plexes were switched upon the addition of an acid. The photophysical properties were rationalized with
DFT/TDDFT calculations. These results on tuning of the photophysical properties of Pt(II) complexes with
a rhodamine moiety may be useful for designing external stimuli-activatable transition metal complexes.
Received 3rd November 2014,
Accepted 8th January 2015
DOI: 10.1039/c4dt03373f
www.rsc.org/dalton
are fundamentally important to mimic photosynthesis, as well
Introduction
as for efficient photocatalysis.7
Triplet state photosensitizers are versatile compounds and
O’Shea et al.11 found that the singlet oxygen (1O2) photo-
have been widely used in photodynamic therapy (PDT) and sensitizing ability of the bromo-azaBodipy can be switched ON
oxygen sensing,1–4 photocatalysis,5–7 molecular logic gates,8,9 or OFF by changing the pH of the solution. This method has
etc. Recently the switching of the triplet state has become an been used for the preparation of targeting PDT reagents which
emerging research area because the controllable population of are able to be selectively activated by the acidic microenviron-
the triplet state is of particular interest for targeted PDT,10–12 ment of the tumour tissue.13 Ziessel et al. studied the photo-
and molecular devices.8 Controlling the vectorial energy trans- acid controlled vectorial fluorescence resonance energy transfer
fer (EnT) process,7 as well as the localization of the triplet state (FRET) with Bodipy-DPP triads (DPP = 1,4-oxo-3,6-diphenyl-
pyrrolo[3,4-c]pyrrole), but the study is on the singlet excited state
manifold.14 Akkaya et al. used acid-activated FRET to control
the production of the triplet excited state of Bodipy dyads, via
the protonation of an amino moiety attached on the Bodipy
chromophore.8 The photochromic moiety of diarylethene was
also used to modulate the photophysical properties of Ir(III),15
or Ru(II) and Pt(II) complexes,16–19 etc. However, much room is
still left to explore more mechanisms for the switching of the
State Key Laboratory of Fine Chemical, School of Chemical Engineering, Dalian
University of Technology, Dalian, 116024, P.R. China. E-mail: zhaojzh@dlut.edu.cn;
http://finechem2.dlut.edu.cn/photochem; Fax: (+86) 411-8498-6236
†Electronic supplementary information (ESI) available: 1H NMR, 13C NMR, and
HRMS spectra of the compounds, and photophysical data of the ligands and
complexes and coordinates of the optimized geometries of the complexes. See
DOI: 10.1039/c4dt03373f
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Scheme 1 Synthesis of the rhodamine-Bodipy-Pt(II) complexes. Compound B-6 and 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-
pyran (DCM) used as the standard for the fluorescence quantum yields are also presented. Reagents and conditions: (i) dry CH2Cl2, BF3·OEt2, NEt3;
(ii) NIS, RT; (iii) trimethylsilylacetylene, Pd(PPh3)2Cl2, PPh3, CuI, NEt3, reflux, 8 h; (iv) K2CO3, THF : MeOH (1 : 1, v/v), RT, 20 min; (v) K2CO3, MeOH, RT,
3 h; (vi) dry CH2Cl2, CF3COOH, BF3·OEt2, NEt3; (vii) under Ar, 1,2-dichloroethane, POCl3, 85 °C, reflux, 4 h. (viii) Under Ar, 1,2-dichloroethane, rhoda-
mine B acid chloride, RT, 24 h; (ix) distilled THF, NHEt2, CuI, −78 °C to RT, stirred; (x) NHEt2, 45 °C , 8 h; (xi) dry CH2Cl2, CuI, i-Pr2NH, 25 °C.
photophysical properties of the triplet photosensitizers with RH-BDP-Pt-2, Scheme 1). The two Bodipy ligands are with
diverse external stimuli.20
different coordination profiles, thus different absorption wave-
On the other hand, rhodamine is well known for its revers- lengths were observed.26 The rhodamine moieties in the com-
ible transformation between the two tautomers, i.e. the spiro- plexes are the acid-activatable units, thus we propose that the
lactam structure and the opened amide structure.21,22 The two photophysical properties of the complexes will be switched by
tautomers exhibit drastically different photophysical pro- the addition of an acid/base into the solutions. The photo-
perties, with the former showing no visible light absorption physical properties of the complexes were studied with steady
and no fluorescence. Conversely, a strong absorption of visible state UV–Vis absorption, luminescence spectra, nanosecond
light (ca. 555 nm) and fluorescence was observed for the transient absorption spectroscopy, electrochemical characteriz-
opened amide form. The two tautomers are reversibly inter- ation and DFT/TDDFT calculations. We found that the UV–Vis
changeable with a variation of the pH of solution. A wide absorption, luminescence and triplet state properties of the
variety of molecular sensors have been developed based on complexes were reversibly switched by an acid/base.
this reversible acid-activated tautomerism of rhodamine.21–24
However, this reversible structure/photophysical property
change was rarely used for the switching of the triplet excited
state. Previously, we prepared rhodamine-containing Pt(II)
Experimental section
Materials and reagents
complexes, but the structure of that specific rhodamine is
unable to undergo the spirolactam/open form transformation.25
All the chemicals are analytically pure and were used as
1
received. H and 13C NMR spectra were recorded on a Bruker
Herein, we prepared two rhodamine-containing Pt(II) com-
plexes with different Bodipy ligands (RH-BDP-Pt-1 and
400/500 MHz spectrometer (CDCl3 as a solvent, TMS as
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the standard, δ = 0.00 ppm). High resolution mass spectra (s, 12H), 1.43–1.35 (m, 30H), 0.89 (t, 18H, J = 8.0 Hz). 13C NMR
(HRMS) were determined with an ESI-Q-TOF MS spectrometer. (100 MHz, CDCl3): δ 158.4, 154.1, 142.0, 140.9, 140.7, 135.5,
Fluorescence spectra were measured on a RF-5301PC spectro- 131.5, 131.1, 129.2, 129.0, 128.2, 122.2, 120.7, 92.0, 90.0, 29.9,
fluorometer (Shimadzu, Japan). Fluorescence quantum yields 26.3, 24.4, 22.2, 22.1, 21.9, 14.4, 14.0, 13.3. MALDI-HRMS:
were measured with compound 5,10,15,20-tetraphenyl- calcd ([C45H72BClF2N2P2Pt]+), m/z = 981.4568, found, m/z =
porphyrin (TPP) as the standard (ΦF = 11% in toluene) and 2,6- 981.4590.
diiodo-1,3,5,7-tetramethyl-8-phenyl-4,4-difluorobora-diazainda-
Compound BDPY-Pt-2. Under an Ar atmosphere, a stirred
cene (diiodoBodipy, ΦF = 2.7% in MeCN). Fluorescence life- solution of compound 2 (34.8 mg, 0.1 mmol) and cis-Pt[P(n-
times were measured with an OB920 spectrometer (Edinburgh, Bu)3]2Cl2 (79.6 mg, 0.12 mmol) in Et2NH (8 mL) was refluxed
UK). Absorption spectra were recorded on an Agilent 8453A at 45 °C for 8 h. Then the reaction mixture was cooled to RT
UV–Vis spectrophotometer.
and the solvent was removed under reduced pressure. The
crude product thus obtained, was purified by column chromato-
Synthesis and characterization
graphy (silica gel, CH2Cl2–petroleum ether = 1 : 1, v/v) to
Compound (1),27 and compound (2)28 were synthesized accord- give an orange solid. Yield: 58 mg (59%). MP 119–121 °C. H
1
ing to the literature methods.
NMR (400 MHz, CDCl3): 7.35–7.08 (m, 4H), 5.97 (s, 2H), 2.55
Compound rhodamine spirolactam (RH). Under an Ar (s, 6H), 2.04 (s, 12H), 1.59 (m, 12H), 1.49–1.40 (m, 18H), 0.92
atmosphere, to a stirred solution of rhodamine B (402 mg, (t, 18H, J = 8.0 Hz). 13C NMR (100 MHz, CDCl3): δ 155.4, 143.3,
0.84 mmol) in 1,2-dichloroethane (10 mL), prop-2-yn-1-amine 142.3, 131.6, 129.8, 127.7, 121.2, 106.2, 86.2, 86.0, 85.9, 29.9,
(139 mg, 2.52 mmol) was added through a syringe at 0 °C. To 26.3, 24.6, 24.5, 24.4, 22.4, 22.3, 22.1, 14.7, 13.9. MALDI-
the mixture phosphorus oxychloride (0.3 mL, 3 mmol) was HRMS: calcd ([C45H72BClF2N2P2Pt]+), m/z = 981.4568, found,
added dropwise. The mixture was stirred at 0 °C for 15 min, m/z = 981.4577.
heated at 85 °C for 5 h, and then stirred at 25 °C for 24 h. The
Compound RH-Pt. Under an Ar atmosphere, to a stirred
solution was diluted with dichloromethane (20 mL) and acidi- solution of RH (28.8 mg, 0.06 mmol) and cis-Pt[P(n-Bu)3]2Cl2
fied with 2 M HCl (30 mL). The organic layer was washed with (40.2 mg, 0.06 mmol) in CH2Cl2 (10 mL), diisopropylamine
2 M HCl (2 × 30 mL), 2 M NaOH (3 × 30 mL), and brine (2 mL) and CuI (3.0 mg, 0.015 mmol) were added consecu-
(30 mL). The organic layer was dried over Na2SO4 and evapor- tively. The mixture was stirred at 25 °C for 18 h. Then the
ated under reduced pressure. The product was purified by solvent was removed under reduced pressure. The crude
column chromatography (silica gel, petroleum ether–EtOAc = product thus obtained was purified by column chromato-
3 : 1, v/v) to give a light pink solid RH. Yield: 305 mg (75%). graphy (silica gel, petroleum ether–EtOAc = 4 : 1, v/v) to give a
1
MP 188–190 °C. 1H NMR (500 MHz, CDCl3): δ 7.93–7.92 (m, colourless oil. Yield: 20 mg (30%). H NMR (500 MHz, CDCl3):
1H), 7.46–7.42 (m, 2H), 7.11–7.09 (m, 1H), 6.47 (d, 2H, J = 7.85–7.84 (m, 1H), 7.34 (m, 2H), 6.98–6.96 (m, 1H), 6.61 (d,
10 Hz), 6.39 (s, 2H), 6.27 (d, 2H, J = 10 Hz), 3.95 (s, 2H), 2H, J = 10 Hz), 6.38 (s, 2H), 6.25 (d, 2H, J = 10 Hz), 3.96 (s, 2H),
3.36–3.32 (m, 8H), 1.75(s, 1H), 1.16 (t, 12H, J = 10 Hz). 13C NMR 3.36–3.28 (m, 8H), 1.88–1.85 (m, 12H), 1.46–1.43 (m, 12H),
(125 MHz, CDCl3): δ 167.51, 153.91, 153.63, 149.02, 132.76, 1.40–1.33 (m, 12H), 1.16 (t, 12H, J = 10 Hz), 0.87 (t, 18H, J =
130.59, 129.25, 128.13, 123.93, 123.15, 108.16, 105.28, 97.97, 10 Hz). 13C NMR (125 MHz, CDCl3): δ 168.14, 154.84, 153.34,
78.45, 70.16, 64.93, 44.53, 28.67, 12.73. TOF HRMS ES+: calcd 148.68, 132.17, 130.32, 128.91, 127.68, 123.45, 122.84, 107.88,
([C31H33N3O7 + H]+), m/z = 480.2651, found, m/z = 480.2645.
106.17, 98.09, 93.78, 65.23, 44.46, 32.65, 29.87, 26.39, 26.16,
Compound BDPY-Pt-1. Under an Ar atmosphere, a round 24.45, 24.38, 24.31, 23.95, 23.78, 22.08, 21.92, 21.75, 14.00,
bottom flask containing a stirred solution of compound 1 12.87. MALDI-HRMS: calcd ([C55H86ClN3O2P2Pt + H]+), m/z =
(34.8 mg, 0.1 mmol) and cis-Pt[P(n-Bu)3]2Cl2 (80 mg, 1113.5610, found, m/z = 1113.5522.
0.12 mmol) in THF–Et2NH (1 : 1, v/v, 6 mL), was placed in a
Compound RH-BDPY-Pt-1. Under an Ar atmosphere, to a
Dewar flask (ethyl acetate–liquid nitrogen low temperature stirred solution of RH (24.0 mg, 0.05 mmol) and BDPY-Pt-1
bath). The flask was evacuated and back-filled with Ar several (49.1 mg, 0.05 mmol) in CH2Cl2 (10 mL), diisopropylamine
times. Then CuI (3.8 mg, 0.02 mmol) was added to the (2 mL) and CuI (3.8 mg, 0.02 mmol) were added consecutively.
mixture. The temperature of the mixture was allowed to rise The mixture was stirred at 25 °C for 24 h. Then the solvent was
from −78 °C to RT and stirred at RT. The reaction was moni- removed under reduced pressure. The crude product thus
tored by TLC (CH2Cl2–petroleum ether = 1 : 2 as an eluent, v/v). obtained, was purified by column chromatography (silica gel,
After the consumption of almost all of the starting materials, petroleum ether–EtOAc = 4 : 1, v/v) to give a purple solid. Yield:
deionized water was used to quench the reaction. The mixture 35 mg (49%). MP 82–84 °C. 1H NMR (500 MHz, CDCl3):
was extracted with dichloromethane (3 × 25 mL). The organic 7.93–7.87 (m, 1H), 7.47–7.46 (m, 3H), 7.41–7.34 (m, 2H),
phase was dried over Na2SO4, the solvent was evaporated 7.25–7.24 (m, 2H), 6.99–6.98 (m, 1H), 6.61–6.55 (m, 2H), 6.37
under reduced pressure, and the crude products thus obtained (s, 2H), 6.28–6.26 (m, 2H), 5.92 (s, 1H), 3.94 (s, 2H), 3.39–3.27
were purified by column chromatography (silica gel, CH2Cl2– (m, 8H), 2.61 (s, 3H), 2.52 (s, 3H), 1.94–1.91 (m, 12H),
petroleum ether = 1 : 2, v/v) to give BDPY-Pt-1 as a purple solid. 1.45–1.43 (m, 12H), 1.34–1.30 (m, 15H), 1.16 (t, 12H, J =
1
Yield: 75 mg (77%). MP 90–92 °C. H NMR (400 MHz, CDCl3): 10 Hz), 0.87–0.82 (m, 21H). 13C NMR (125 MHz, CDCl3): δ
7.48–7.28 (m, 5H), 5.93 (s, 1H), 2.64 (s, 3H), 2.53 (s, 1H), 1.93 167.95, 159.34, 154.75, 153.18, 152.84, 148.50, 140.94, 140.75,
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140.13, 135.47, 131.91, 131.04, 130.91, 130.26, 128.92, 128.71, via a semipermeable membrane. Dichloromethane was used
128.16, 127.44, 123.25, 122.90, 122.70, 120.08, 117.44, 107.74, as the solvent. Ferrocene was added as the internal reference.
106.23, 101.53, 98.96, 97.98, 94.69, 65.10, 44.29, 32.43, 31.94,
DFT calculations
29.71, 29.37, 29.68, 24.29, 24.23, 24.18, 23.73, 23.60, 23.46,
22.70, 14.40, 14.20, 13.84, 13.28, 12.71. MALDI-HRMS: calcd
([C76H104BF2ClN5O2P2Pt + H]+), m/z = 1425.7452, found, m/z =
1425.7397.
The density functional theory (DFT) calculations were used for
optimization of both singlet states and triplet states. The UV–
Vis absorption and the energy level of the T1 state were calcu-
lated with the time dependent DFT (TDDFT), based on the
optimized singlet ground state geometries (S0 state). The spin
density surface of the complexes was calculated based on the
optimized triplet state. All the calculations were performed at
the B3LYP/GENECP/LANL2DZ level with Gaussian 09W.29
Dichloromethane was used as the solvent for the calculations
(CPCM model).
Compound RH-BDPY-Pt-2. Under an Ar atmosphere, to a
stirred solution of RH (24.0 mg, 0.05 mmol) and BDPY-Pt-2
(49.1 mg, 0.05 mmol) in CH2Cl2 (10 mL) at 25 °C, diisopropyl-
amine (2 mL) and CuI (3.8 mg, 0.02 mmol) were added con-
secutively. The reaction was monitored by TLC (silica gel,
petroleum ether–EtOAc = 4 : 1, v/v). After consumption of all
the starting materials, the solvent was removed under reduced
pressure. The crude product thus obtained was purified by
column chromatography (silica gel, petroleum ether–EtOAc =
1
4 : 1, v/v) to give a sticky orange solid. Yield: 34 mg (48%). H
Results and discussion
Design and synthesis of the Pt(II) complexes
NMR (500 MHz, CDCl3): 7.86–7.82 (m, 1H), 7.35–7.32 (m, 4H),
7.05 (d, 2H, J = 10 Hz), 6.99–6.97 (m, 1H), 6.64 (d, 2H, J = 10
Hz), 6.38 (m, 2H), 6.25 (d, 2H, J = 10 Hz), 5.95 (s, 2H), 3.97 (s, 2-EthynylBodipy and meso-4′-ethylBodipy were used for the
2H), 3.38–3.29 (m, 8H), 2.54 (s, 6H), 2.03–2.00 (m, 12H), preparation of the complexes, in order to attain strong absorp-
1.52–1.48 (m, 12H), 1.43 (s, 6H), 1.40–1.33 (m, 12H), 1.17 (t, tion of visible light.30–32 Furthermore, the absorption wave-
12H, J = 10 Hz), 0.86 (t, 18H, J = 10 Hz). 13C NMR (125 MHz, length of the two coordinated ligands is different, thus the
CDCl3): δ 168.03, 155.11, 154.77, 153.21, 148.55, 143.28, singlet EnT process in the two complexes may be different.33
142.49, 131.94, 131.50, 131.43, 130.77, 130.20, 130.11, 128.75, The rhodamine moiety was connected to the Pt(II) coordi-
127.47, 127.39, 123.25, 122.68, 120.99, 112.13, 108.26, 107.74, nation centre via acetylide ligands. In order to feasibly attach
106.25, 101.60, 98.01, 94.68, 65.17, 44.29, 32.47, 31.93, two different acetylide ligands to the Pt(II) coordination centre,
29.70, 29.36, 26.31, 24.38, 24.32, 24.25, 23.97, 23.81, 23.64, the trans bis(tributylphosphine) Pt(II) bisacetylide coordination
22.69, 14.56, 14.12, 13.80, 12.73. MALDI-HRMS: calcd framework was selected.34–38 Complexes RH-Pt, BDPY-Pt-1
([C76H104BF2ClN5O2P2Pt + H]+), m/z = 1425.7452, found, m/z = and BDPY-Pt-2 were prepared as the reference complexes
1425.7539.
(Scheme 1). The preparation of the ligands and complexes are
based on routine methods. The compounds were obtained
with moderate yields.
Nanosecond transient absorption spectra
All the nanosecond transient absorption spectra were
measured on an LP920 laser flash photolysis spectrometer
(Edinburgh Instruments, UK) and recorded on a Tektronix
TDS 3012B oscilloscope. The lifetime values (by monitoring
the decay trace of the transients) were obtained with the LP900
software. All samples in flash photolysis experiments were de-
aerated with N2 for ca. 15 min before measurement and the
gas flow was maintained during the measurement. For the
samples with TFA added, the solution was allowed to stand for
8 min prior to measurement.
UV–Vis absorption spectra and luminescence spectra
RH-Pt shows an absorption band at 311 nm (Fig. 1). No
absorption in the visible spectral region was observed. For
BDPY-Pt-1 and BDPY-Pt-2, strong absorption bands at 570 nm
and 500 nm were observed, respectively. The UV–Vis absorp-
tion of RH-BDPY-Pt-1 shows an absorption spectrum which is
superimposition of the sum of the absorption spectra of the
Cyclic voltammetry
Cyclic voltammetry was performed using a CHI610D Electro-
chemical workstation (Shanghai, China). Cyclic voltammo-
grams were recorded at scan rates of 0.1 V s−1. The electrolytic
cell used was a three electrode cell. Electrochemical measure-
ments were performed at RT using 0.1 M tetrabutylammonium
hexafluorophosphate (Bu4N[PF6]) as a supporting electrolyte.
The solution was purged with N2 before measurement. The
working electrode was a glassy carbon electrode, and the
counter electrode was a platinum electrode. A non-aqueous
Ag/AgNO3 (0.1 M in acetonitrile) reference electrode was con-
Fig. 1 UV–Vis absorption spectra of BDPY-Pt-1, RH-BDPY-Pt-1,
BDPY-Pt-2, RH-BDPY-Pt-2 and RH-Pt. c = 1.0 × 10−5 M in toluene,
tained in a separate compartment connected to the solution 20 °C.
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Fig. 2 UV–Vis absorption spectra of complexes in the absence and in the presence of TFA. (a) RH-BDPY-Pt-1, RH-BDPY-Pt-1 + TFA (333 eq.) and
RH-BDPY-Pt-1 + TFA (333.3 eq.) + TEA (pure 150 μL). (b) RH-BDPY-Pt-2, RH-BDPY-Pt-2 + TFA (333 eq.) and RH-BDPY-Pt-2 + TFA (333 eq.) + TEA
(neat, 500 μL). (c) RH-Pt, RH-Pt + TFA (333 eq. and 8 min standing time), RH-Pt-1 + TFA (333 eq.) + TEA (neat, 250 μL). Color changes of the com-
plexes on the addition of an acid (TFA, 333 eq.) and a base (TEA): (d) RH-BDPY-Pt-1, (e) RH-BDPY-Pt-2, and (f) RH-Pt. c = 1.0 × 10−5 M in dichloro-
methane, 20 °C.
two components. Thus, we propose that there is no significant
interaction between the rhodamine (in the spirolactam struc-
ture) and the Bodipy moiety. Similar results were observed for
RH-BDPY-Pt-2 (Fig. 1). Notably the different coordination pro-
files of the Bodipy ligands in RH-BDPY-Pt-1 and RH-BDPY-Pt-2
give drastically different absorption wavelengths.26
The UV–Vis absorption of complexes does not show signifi-
cant solvent dependence, indicating that the ground state was
not affected by solvent polarity or hydrogen bonding (see ESI,†
Fig. S21).
The UV–Vis absorption of the complexes changed on the
addition of an acid (TFA), thus the spirolactam ↔ opened
amide form transformation occurred (Fig. 2). For RH-BDPY-Pt-
1, the absorption maxima were blue-shifted to 558 nm upon
the addition of TFA (Fig. 2a). For RH-BDPY-Pt-2, however, a
new absorption band at a longer wavelength (555 nm) (Fig. 2b)
was observed upon the addition of TFA, which is attributed to
the absorption of the opened amide form of the rhodamine
moiety. This postulation was supported by the change of UV–
Vis absorption of the reference complex RH-Pt (Fig. 2c).
The colour of the complex solutions changed upon the
addition of TFA (Fig. 2d–f). For example, RH-BDPY-Pt-1 gives a
purple solution whereas upon the addition of TFA, a magenta
solution was obtained. RH-BDPY-Pt-2 also shows different
Fig. 3 Emission spectra of the complexes (a) BDPY-Pt-1 (λex = 555 nm),
(b) RH-BDPY-Pt-1 (λex = 555 nm), (c) BDPY-Pt-2 (λex = 470 nm), and (d)
RH-BDPY-Pt-2 (λex = 470 nm) under different atmospheres of air, N2,
and O2. c = 1.0 × 10−5 M in toluene, 20 °C.
The photoluminescence spectra of the complexes were
colour changes. RH-BDPY-Pt-2 alone in solution shows a dark studied. Complex BDPY-Pt-1 gives a major emission band at
629 nm (Fig. 3a). The emission intensity is the same in both
the aerated and deaerated solution. Thus the luminescence is
yellow colour; upon the addition of TFA, a pinkish orange solu-
tion was obtained. These changes are different from RH-Pt,
which shows a colourless to light pink change upon the fluorescence. A minor emission band was observed at 800 nm
for BDPY-Pt-1. The intensity of this minor band was reduced
in the aerated solution, indicating that the emission band
addition of TFA. To the best of our knowledge, such tuning of
the colour of Pt(II) complexes is rarely reported, and the appli-
cation of the rhodamine chromophore in tuning the colour of most likely originated from an emissive triplet state, thus the
emission at 800 nm is phosphorescence.26 Similar results were
Pt(II) complexes has never been reported.22,39
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of BDPY-Pt-1 (Fig. 4a) decreased in polar solvents as compared
with that in nonpolar solvents. A similar profile was found for
RH-BDPY-Pt-1 (Fig. 4b). The emission intensity of BDPY-Pt-2
also decreased in polar solvents (Fig. 4c). More significant
solvent polarity-dependent photoluminescence was observed
for RH-BDPY-Pt-2 (Fig. 4d). This polarity dependent emission
indicates an intramolecular charge transfer feature of the
emissive state.
The photoluminescence spectra of the complexes upon the
addition of an acid were studied (Fig. 5). For RH-BDPY-Pt-1, a
new emission band at 598 nm developed upon the addition of
TFA. Notably the emission wavelength is shorter than the
fluorescence emission of the Pt(II) coordination centre of
RH-BDPY-Pt-1. The emission intensity of RH-BDPY-Pt-1 in
the presence of TFA was found to be weaker as compared to
RH-Pt in the presence of TFA, most probably due to FRET
(see ESI,† Fig. S22). For RH-BDPY-Pt-2, the emission of the
coordinated Bodipy at 500 nm was greatly enhanced upon
photoexcitation. Thus we propose that the non-radiative
decay channel of RH-BDPY-Pt-2 in DCM, such as intra-
molecular charge transfer, was inhibited upon the addition of
TFA.
Fig. 4 Solvent-polarity-dependence of the emission of the complexes
(c = 1.0 × 10−5 M): (a) BDPY-Pt-1 (λex = 555 nm), (b) RH-BDPY-Pt-1 (λex
555 nm), (c) BDPY-Pt-2 (λex = 470 nm), and (d) RH-BDPY-Pt-2 (λex
470 nm), 20 °C.
=
=
Furthermore, a new emission band at 573 nm was observed,
which is attributed to the opened amide structure of the rhod-
amine moiety. This conclusion was supported by the results
of RH-Pt (Fig. 5c). A switch-ON effect was observed for com-
plexes upon the spirolactam ↔ opened amide transformation.
These complexes may be developed for dual functional
materials for selective luminescence bioimaging and PDT
studies.21
In order to study the FRET in the complexes upon the
addition of TFA, the fluorescence excitation spectra of the
complexes were compared with their UV–Vis absorption
spectra (Fig. 6). For RH-BDPY-Pt-2, an excitation band at
observed for RH-BDPY-Pt-1 (Fig. 3b). For BDPY-Pt-2, however,
the emission profile is drastically different. An emission band
at 517 nm was observed (Fig. 3c). No phosphorescence emis-
sion band was observed. Similar results were found for
RH-BDPY-Pt-2 (Fig. 3d). These results indicate that the coordi-
nation profile of the acetylide ligands impose a substantial
effect on the photophysical properties of the complexes. Fur-
thermore, the rhodamine moiety in the spirolactam structure
does not contribute to the photoluminescence of the
complexes.
The solvent-dependence of the emission spectra of the com-
plexes was studied (Fig. 4). The fluorescence emission intensity
Fig. 5 Emission spectra of (a) RH-BDPY-Pt-1, RH-BDPY-Pt-1 + TFA (333 eq.) and RH-BDPY-Pt-1 + TFA (333 eq. and 8 min standing time) + TEA
(pure 150 μL) (λex = 550 nm). (b) RH-BDPY-Pt-2, RH-BDPY-Pt-2 + TFA (333 eq.) and RH-BDPY-Pt-2 + TFA (333 eq.) + TEA (pure 500 μL) (λex
=
470 nm). (c) RH-Pt, RH-Pt + TFA (333 eq. and 8 min standing time), RH-Pt-1 + TFA (333 eq.) + TEA (pure 250 μl) (λex = 510 nm). c = 1.0 × 10−5 M,
20 °C in dichloromethane. Luminescence colour changes of the complexes under a UV hand lamp (λex = 365 nm) on the addition of an acid (TFA,
333 eq.) and a base (TEA): (d) RH-BDPY-Pt-1, (e) RH-BDPY-Pt-2, and (f) RH-Pt. c = 1.0 × 10−5 M in dichloromethane, 20 °C.
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Nanosecond transient absorption spectra
The nanosecond transient absorption spectra of the complexes
were studied (Fig. 7).4,40,41 BDPY-Pt-1 gives a significant
bleaching band at 560 nm, and excited state absorption (ESA)
in the region of 300–500 nm (Fig. 7a). Thus the triplet state of
BDPY-Pt-1 is localized on the Bodipy ligand (3IL state).26,42 The
lifetime was determined as 57 μs (Fig. 7b). For BDPY-Pt-2,
however, the bleaching band is at 500 nm (Fig. 7c), and the
lifetime of the triplet state is up to 598 μs (Fig. 7d). The TA
spectra of the rhodamine-containing complexes RH-BDPY-Pt-1
and RH-BDPY-Pt-2 were also studied. RH-BDPY-Pt-1 shows a
transient spectrum (Fig. 7e) similar to that of BDPY-Pt-1. The
triplet state lifetime 52 μs (Fig. 7f) is also close to that of
BDPY-Pt-1. Therefore, the rhodamine moiety in the spiro-
lactam structure in RH-BDPY-Pt-1 does not impose any effect
on the triplet state of RH-BDPY-Pt-1. Moreover, based on
Fig. 6 Comparison of the normalized UV−vis absorption and the exci-
tation spectra of the complexes. (a) RH-BDPY-Pt-1 (λem = 600 nm), c
[RH-BDPY-Pt-1] = 5.0 × 10−6 M and (b) RH-BDPY-Pt-2 (λem = 580 nm), c
[RH-BDPY-Pt-2] = 3.3 × 10−6 M. In the presence of TFA (333 eq.). The
concentration was adjusted so that the absorbance at the maximal
absorption band is less than 0.2. In deaerated dichloromethane, 20 °C.
502 nm was observed, indicating the presence of FRET. We the bleaching band, we conclude that the triplet state of
noted a much weaker excitation band at 502 nm as compared RH-BDPY-Pt-1 is localized on the Bodipy moiety, and not on
to the UV–Vis absorption band of the complex in the same the spirolactam rhodamine moiety.43 For RH-BDPY-Pt-2, the
region. This discrepancy may be partially due to an inefficient transient absorption spectrum is similar to that of BDPY-Pt-2;
spirolactam ↔ opened amide structure transformation. The the triplet state lifetime of RH-BDPY-Pt-2 is up to 618 μs.
photophysical properties of the complexes are summarized in
The variation in the TA spectra upon the addition of an
Table 1.
acid (TFA) was also studied (Fig. 8). Dichloromethane (DCM)
Table 1 Photophysical properties of Pt(II) complexesa
λabs/nm
εb
λem/nm
τF c/ns
ΦF d/%
τT g/μs
ΦΔ/%
j
j
j
j
j
j
j
j
j
j
j
j
j
j
j
j
j
j
j
j
RH-Pt
Toluene
DCM
Acetonitrile
Methanol
Toluene
DCM
Acetonitrile
Methanol
Toluene
DCM
Acetonitrile
Methanol
Toluene
DCM
Acetonitrile
Methanol
Toluene
DCM
Acetonitrile
Methanol
DCM
311
311
311
311
432/583
428/570
427/562
425/562
503
500
496
1.19
1.22
1.17
1.22
1.38/3.0
1.58/3.10
1.70/3.3
1.63/3.521
9.26
8.78
8.58
BDPY-Pt-1
BDPY-Pt-2
RH-BDPY-Pt-1
RH-BDPY-Pt-2
629
641
641
638
517
514
509
510
643
653
659
653
516
512
510
509
575
598
513/573
0.3
0.2
0.2
0.2
2.6
0.8
1.4
1.1
0.3
0.2
0.2
0.3
2.4
1.0
1.4
0.5
3.5
3.0
1.79
1.6e
0.9e
0.6e
0.5e
25.4f
4.4f
0.9f
3.2f
0.6e
0.9e
0.3e
0.4e
16.3f
0.6f
0.6f
57
46
37
20
598
363
254
440
52
35
31
15h
48h
22h
7.8h
21i
60i
66i
497
8.73
34i
319/438/594
315/436/580
318/433/573
314/433/566
320/502
323/500
318/497
322/496
320/555
317/423/558
309/500/555
2.02/1.73/3.10
2.46/2.05/3.36
2.36/1.97/3.28
2.30/1.88/3.47
3.12/7.13
3.34/6.54
3.43/7.21
3.59/6.79
1.35/0.67
2.27/1.53/4.29
3.31/7.25/1.70
18h
35h
21h
6.7h
9.2i
72i
12
618
423
328
59i
f
0.9
331
22i
j
j
RH-Pt + TFA
RH-BDPY-Pt-1 + TFA
RH-BDPY-Pt-2 + TFA
54.8e
DCM
DCM
3.8e
80
406
34i
32.2e/9.4f
16h/51i
a c = 1.0 × 10−5 M. b Molar extinction coefficient at the absorption maxima. ε: 104 M−1 cm−1 c Fluorescence lifetimes in an aerated solution.
.
d Fluorescence quantum yields. e Fluorescence quantum yields with B-6 (ΦF = 10.5% in toluene) as the standard. f Fluorescence quantum yields
with DCM (ΦF = 10% in CH2Cl2) as the standard. g Triplet excited state lifetimes, measured by nanosecond transient absorption spectra under an
N2 atmosphere (c = 1.0 × 10−5 M in toluene). h Singlet oxygen quantum yield with methylene blue (MB, ΦΔ = 57% in DCM) as the standard. The
excitation wavelength, for BDPY-Pt-1 (λex = 592 nm), RH-BDPY-Pt-1 (λex = 592 nm), BDPY-Pt-2 (λex = 514 nm), RH-BDPY-Pt-2 (λex = 514 nm) in
toluene; for BDPY-Pt-1 (λex = 592 nm), RH-BDPY-Pt-1 (λex = 592 nm), BDPY-Pt-2 (λex = 488 nm), RH-BDPY-Pt-2 (λex = 488 nm) in CH2Cl2; for
BDPY-Pt-1 (λex = 588 nm), RH-BDPY-Pt-1 (λex = 588 nm), BDPY-Pt-2 (λex = 506 nm), RH-BDPY-Pt-2 (λex = 506 nm) in acetonitrile and methanol.
i Singlet oxygen quantum yield with 2,6-diiodo-Bodipy (ΦΔ = 83% in DCM) as the standard. The excitation wavelength, for BDPY-Pt-1 (λex
=
592 nm), RH-BDPY-Pt-1 (λex = 592 nm), BDPY-Pt-2 (λex = 514 nm), RH-BDPY-Pt-2 (λex = 514 nm) in toluene; for BDPY-Pt-1 (λex = 592 nm),
RH-BDPY-Pt-1 (λex = 592 nm), BDPY-Pt-2 (λex = 488 nm), RH-BDPY-Pt-2 (λex = 488 nm) in CH2Cl2; for BDPY-Pt-1 (λex = 588 nm), RH-BDPY-Pt-1 (λex
= 588 nm), BDPY-Pt-2 (λex = 506 nm), RH-BDPY-Pt-2 (λex = 506 nm) in acetonitrile and methanol. j Not determined.
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Fig. 7 Nanosecond transient absorption spectra and the decay curves of the complexes. (a) BDPY-Pt-1 and (b) the decay curve of BDPY-Pt-1 at
584 nm (λex = 532 nm), (c) BDPY-Pt-2 and (d) the decay curve of BDPY-Pt-2 at 504 nm (λex = 500 nm), (e) RH-BDPY-Pt-1 and (f) the decay curve of
RH-BDPY-Pt-1 at 584 nm (λex = 532 nm), (g) RH-BDPY-Pt-2 and (h) the decay curve of RH-BDPY-Pt-2 at 504 nm (λex = 500 nm). c = 1.0 × 10−5 M in
deaerated toluene, 20 °C.
Fig. 8 Nanosecond transient absorption spectra and the decay curves of complexes. Transient absorption spectra of (a) BDPY-Pt-1, (b)
RH-BDPY-Pt-1 and (c) RH-BDPY-Pt-1 + TFA (333 eq.), the corresponding decay curves of (d) BDPY-Pt-1 at 580 nm (λex = 532 nm), (e) RH-BDPY-Pt-1
at 584 nm (λex = 532 nm) and (f) the decay curve of RH-BDPY-Pt-1 + TFA (333 eq.) at 557 nm (λex = 532 nm). c = 1.0 × 10−5 M in deaerated dichloro-
methane, 20 °C.
was used as the solvent since the spirolactam ↔ opened amide observed for the TA spectra. However, the triplet state lifetime
structure transformation is retarded in toluene. The lifetime of was significantly extended to 80 μs. Thus switching of the
the triplet excited state of BDPY-Pt-1 in DCM (46 μs) is slightly triplet state lifetime was observed for RH-BDPY-Pt-1 upon the
shorter than that in toluene (57 μs). In the presence of TFA, no addition of TFA. Interestingly, the triplet state of RH-BDPY-Pt-1
significant change in the TA spectra and the lifetime of the is still localized on the Bodipy moiety, and not on the rhod-
triplet excited state (45 μs) was observed (see ESI,† Fig. S23).
amine moiety. This localization profile was attributed to the
For RH-BDPY-Pt-1, the triplet state lifetime in DCM is 35 μs. higher triplet state energy level of rhodamine than that of the
Upon the addition of TFA, no substantial changes were Bodipy moiety.41a
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Fig. 9 Nanosecond transient absorption spectra and the decay curves of complexes. (a) Transient absorption spectra of BDPY-Pt-2 and (b) the
corresponding decay curve at 504 nm (λex = 500 nm). (c) Transient absorption spectra of RH-BDPY-Pt-2 and (d) the corresponding decay curve at
504 nm (λex = 500 nm). (e) Transient absorption spectra of RH-BDPY-Pt-2 + TFA (333 eq.) and (f) the corresponding curve at 504 nm (λex = 500 nm),
(g) the decay curve of RH-BDPY-Pt-2 + TFA (333 eq.) at 557 nm (λex = 532 nm), (h) the decay curve of RH-Pt + TFA (333 eq.) at 554 nm (λex
532 nm). c = 1.0 × 10−5 M in deaerated dichloromethane, 20 °C.
=
BDPY-Pt-2 shows a much shorter triplet state lifetime in
DCM than that in toluene (Fig. 9). Similar results were
observed for RH-BDPY-Pt-2. This reduced triplet state lifetime
in a polar solvent may be due to an intramolecular charge
transfer.44 Notably the triplet state is still localized on the
Bodipy moiety. Upon the addition of TFA, a new bleaching
band at 553 nm was observed in the TA spectra of
RH-BDPY-Pt-2 (Fig. 9e). This band can be attributed to the
population of the rhodamine-localized triplet state. A major
bleaching band at 500 nm was observed, which can be attribu-
ted to the ground state bleaching of the Bodipy ligand. The
decay kinetics of the two bleaching bands were studied.
Different lifetimes of 406 μs and 35 μs were observed. This
result indicates that there is no efficient triplet state equili-
brium in RH-BDPY-Pt-2.41,45 The TA spectra of RH-Pt in the
presence of TFA were also studied. A very long-lived triplet
excited state with a lifetime up to 985 μs was observed. Based
on the triplet state lifetimes, the intramolecular triplet state
EnT rate constant in RH-BDPY-Pt-234 can be calculated as
Fig. 10 Cyclic voltammograms of (a) BDPY-Pt-1, (b) BDPY-Pt-2, (c)
RH-BDPY-Pt-1, (d) RH-BDPY-Pt-2. In deaerated CH2Cl2 solutions con-
taining 1.0 mM photosensitizers with the ferrocene (Fc) as an internal
reference, 0.10 M Bu4NPF6 as the supporting electrode, Ag/AgNO3 as
the reference electrode. Scan rates: 0.1 V s−1, 20 °C.
2.8 × 104 s−1 with eqn (1):
ꢀ
ꢁ
ꢀ
ꢁ
1
τe
1
τe°
kET
¼
ꢀ
ð1Þ
where, τe and τ°e are the triplet state lifetimes of the donor in
observed. For BDPY-Pt-1, a reversible oxidation wave at +0.62 V
was observed. A reversible reduction wave at −1.55 V was
observed. For BDPY-Pt-2, an irreversible oxidation wave at
+0.93 V was observed.
the presence and absence of an energy acceptor, respectively.
Electrochemical study: cyclic voltammetry
The reduction wave at −1.51 V is similar to that of BDPY-Pt-1.
For RH-BDPY-Pt-1, the oxidation waves are basically the sum
of the oxidation potential of BDPY-Pt-1 and RH-Pt, thus we
propose that there is no significant interaction between the
The redox potentials of the complexes and the references were
studied with cyclic voltammetry (Fig. 10). For RH-Pt, irrevers-
ible oxidation waves at +0.67 V, +0.79 V and +1.00 V were
observed (see ESI,† Fig. S24). No reduction waves were
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Table 2 Redox potentials of photosensitizers for the study of the potential intramolecular electron transfer by the calculation of free-energy
changes for charge separation ΔGCS (photoinduced electron transfer). The anodic and cathodic peak potentials are presenteda
E(OX) (V)
E(RED) (V)
ΔGs(eV)
ΔGcs c (eV)
ΔGCR c (eV)
ΔGcs d (eV)
ΔGCR d (eV)
b
b
b
b
b
b
RH-Pt
+0.67/+0.79/+1.00
+0.62
+0.93
—
—
—
—
—
—
—
—
—
—
—
—
b
b
b
b
b
BDPY-Pt-1
BDPY-Pt-2
RH-BDPY-Pt-1
−1.55
−1.51
−1.55
—
—
b
b
b
b
b
—
—
+0.94/+0.75/+0.55
−0.11c/+0.44d
−0.10c/+0.26d
+0.48e
+0.09 f
+0.56e
−0.26f
−1.99e
−1.99f
−2.16e
−2.16f
+1.03e
+0.64 f
+0.92e
+0.10 f
−2.54e
−2.54f
−2.52e
−2.52f
RH-BDPY-Pt-2
+0.93/+0.72
−1.54
a Cyclic voltammetry in deaerated CH2Cl2 containing 0.10 M Bu4NPF6 as the supporting electrolyte; Pt electrode as the counter electrode; glassy
carbon electrode as the working electrode; Ag/AgNO3 couple as the reference electrode. c [Ag+] = 0.1 M. 1.0 mM photosensitizers in DCM, 20 °C.
Conditions: 1.0 mM dyad photosensitizers and 1.0 mM ferrocene in DCM. b No reduction waves were observed, no ΔGcs and no ΔGCR values. c In
e
f
dichloromethane. d In toluene. E00 of complexes was approximated by the energy levels of the excited states of 3BDPY*. E00 of complexes was
approximated by the energy levels of the excited states of 1BDPY*.
two chromophores at the ground state. A reversible reduction 15.1 Å, RCC (RH-BDPY-Pt-2) = 16.6 Å, RD is the radius of the
wave at −1.55 V was observed, which can be attributed to the electron donor, RA is the radius of the electron acceptor, εREF
coordinated Bodipy ligand. Similar results were observed for is the static dielectric constant of the solvent used for the elec-
RH-BDPY-Pt-2. The redox potentials are listed in Table 2.
trochemical studies, and ε0 is the permittivity of free space.
The free energy changes of the intramolecular electron The solvents used in the calculation of the free energy of ET
transfer (ET) process, can be calculated using the Weller are dichloromethane (εS = 8.93) and toluene (εS = 2.4). The
equation (eqn (2) and (3)).41a
charge recombination energy state (ΔG(CR)) can be calculated
using eqn (4). The data are represented in Table 2.
ΔG0CS ¼ e½Eox ꢀ EREDꢁ ꢀ E00 þ ΔGs
ð2Þ
ð3Þ
ꢀ
ꢁꢀ
ꢁ
ΔGCR ¼ ꢀðΔGCS þ E00
Þ
ð4Þ
e2
e2
1
1
1
1
ΔGS ¼ ꢀ
ꢀ
þ
ꢀ
εREF εS
4πεSε0RCC 8πε0 RD RA
Based on the free energy changes (Table 2), ET is unlikely
where ΔGS is the static Coulombic energy, which is described to occur due to the large positive ΔGCR values. This conclusion
by eqn (3). e is the electronic charge, EOX is the half-wave is in agreement with the experimental results of the photo-
potential for one-electron oxidation of the electron-donor unit, luminescence study which show no quenching effect in
ERED = half-wave potential for one-electron reduction of the toluene as compared with that in other polar solvents. In
electron-acceptor unit; E00 = energy level approximated with DCM, however, either negative free energy changes or a small
the fluorescence emission (for the singlet excited state), εS
=
positive value were obtained, indicating that ET is possible for
static dielectric constant of the solvent, RCC = center-to-center both RH-BDPY-Pt-1 and RH-BDPY-Pt-2. Accordingly, lumines-
separation distance between the electron donor (rhodamine cence quenching was observed for RH-BDPY-Pt-1 and
unit) and the electron acceptor (BDPY-Pt-1), determined by RH-BDPY-Pt-2 in DCM as compared with that in toluene.
DFT optimization of the geometry, RCC (RH-BDPY-Pt-1) = Moreover, in DCM both RH-BDPY-Pt-1 and RH-BDPY-Pt-2
Scheme 2 Simplified Jablonski diagram illustrating the photophysical processes involved in (a) RH-BDPY-Pt-1 and (b) RH-BDPY-Pt-1 in the pres-
ence of TFA in dichloromethane. [BDPY-RH-c] stands for RH-BDPY-Pt-1 with the rhodamine part in the closed spirolactam structure. [BDPY-RH-o]
stands for RH-BDPY-Pt-1 with the rhodamine part in the opened amide structure. The component in the excited state was highlighted with red
colour. The number of the superscript designates either the singlet or the triplet excited state.
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Fig. 11 Electron density maps of the frontier molecular orbital of the
complex RH-BDPY-Pt-2 in dichloromethane. Based on the optimized
ground state geometry by the DFT calculations at the B3LYP/GENECP
level with Gaussian 09W.
Fig. 12 Electron density maps of the frontier molecular orbital of the
complex RH-BDPY-Pt-2 (open amide form) in dichloromethane. Based
on the optimized ground state geometry by the DFT calculations at the
B3LYP/GENECP level with Gaussian 09W.
show shorter triplet state lifetimes (Fig. 8 and Fig. 9) as com-
pared with that in nonpolar solvents such as toluene (Fig. 7).
The photophysics of RH-BDPY-Pt-1 and RH-BDPY-Pt-2 are
presented in Scheme 2. Based on the UV–Vis absorption/fluo-
rescence spectroscopy and the DFT/TDDFT calculated data,
both the singlet and triplet excited states of the closed lactam
form of the rhodamine part are with higher energy levels than
that of the BDPY part. In the presence of TFA, the energy level
of the S1 state of the Bodipy part is with a higher energy than
that of the S1 state of the opened rhodamine part.
identified as a charge transfer state. The main component of
the transition is HOMO → LUMO, which is ET from the rhod-
amine moiety to the Bodipy moiety, as well as HOMO−2 →
LUMO, which is charge transfer from the phenylacetylide
moiety to the Bodipy moiety. This ET feature supports the
experimental results that the fluorescence of RH-BDPY-Pt-2
was quenched in DCM as compared with that in toluene
(Fig. 4d).
DFT calculations: assignment of the excited states
The triplet state of RH-BDPY-Pt-2 was also studied with
TDDFT calculations (Table 3). The T1 state is a Bodipy-loca-
lized 3IL state (Fig. 11). The rhodamine unit and the Pt(II)
coordination centre do not contribute to the T1 state. This
The electronic states of the complexes were studied with DFT
calculations.19,46 The results of RH-BDPY-Pt-2 are presented in
Fig. 11 and Table 3. The frontier molecular orbitals are pre-
sented in Fig. 11. The S1 state of the complex (in DCM) was
Table 3 Electronic excitation energies (eV) and the corresponding oscillator strengths ( f ), main configurations and CI coefficients of the low-lying
electronic excited states of the complex RH-BDPY-Pt-2 in dichloromethane calculated by TDDFT//B3LYP/GENECP based on the DFT//B3LYP/
GENECP optimized ground state geometriesa
TDDFT//B3LYP/ GENECP
Electronic transition
Energya[eV nm−1
]
f b
Compositionc
CId
Character
Singlet
Triplet
S0 → S1
2.48/499
0.002
H−2 → L
H → L
0.535
0.419
0.700
0.121
0.497
0.221
0.710
0.575
0.346
0.601
0.651
MLCT
LLCT
ILCT
S0 → S5
S0 → S8
S0 → S15
2.89/429
3.42/363
3.97/312
0.589
0.064
0.498
H−3 → L
H−3 → L
H−2 → L+2
H−1 → L+2
H−3 → L
H−2 → L
H → L
ILCT
MLCT
MLCT
ILCT
MLCT
LLCT
LLCT
MLCT
S0 → T1
S0 → T2
1.54/805
2.48/501
0.000e
0.000e
S0 → T3
S0 → T4
2.51/494
2.62/473
0.000e
0.000e
H → L
H−2 → L
a Only the selected low-lying excited states are presented. b Oscillator strengths. c Only the main configurations are presented. d The CI
coefficients are in absolute values. e No spin–orbital coupling effect was considered, thus the f values are zero.
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Table 4 Electronic excitation energies (eV) and the corresponding oscillator strengths ( f ), main configurations and CI coefficients of the low-lying
electronic excited states of the complex RH(o)-BDPY-Pt-2 (the rhodamine part in the open amide form) in dichloromethane calculated by TDDFT//
B3LYP/GENECP based on the DFT//B3LYP/ GENECP optimized ground state geometriesa
TDDFT//B3LYP/GENECP
Electronic transition
Energy/eV nm−1 a
f b
Compositionc
CId
Character
Singlet
Triplet
S0 → S1
S0 → S4
S0 → S6
S0 → S15
S0 → S17
2.22/558
2.61/476
2.89/429
3.54/350
3.62/343
0.002
0.978
0.596
0.114
0.365
H → L
0.705
0.702
0.700
0.493
0.382
0.165
0.136
0.549
0.341
0.710
0.705
0.705
0.707
MLCT
ILCT
ILCT
ILCT
ILCT
MLCT
MLCT
MLCT
MLCT
ILCT
ILCT
MLCT
LLCT
H−2 → L
H−1 → L+1
H−11 → L
H−11 → L
H → L+2
H → L+3
H → L+3
H → L+2
H−1 → L+1
H−2 → L
H → L
S0 → S22
3.76/330
0.489
S0 → T1
S0 → T2
S0 → T3
S0 → T4
1.54/806
1.74/713
2.22/559
2.37/522
0.000e
0.000e
0.000e
0.000e
H−1 → L
a Only the selected low-lying excited states are presented. b Oscillator strengths. c Only the main configurations are presented. d The CI
coefficients are in absolute values. e No spin–orbital coupling effect was considered, thus the f values are zero.
conclusion is in agreement with nanosecond transient absorption physical properties of transition metal complexes with external
spectroscopy which shows that the T1 state of RH-BDPY-Pt-2 is stimuli. Herein an acid-activatable chromophore rhodamine
exclusively localized on the Bodipy moiety (Fig. 9c).
coordinated with Pt(II) was used for this purpose for the first
The excited states of RH-BDPY-Pt-2 with the rhodamine time. Moreover, Bodipy acetylide ligands were used for achiev-
moiety in the opened amide structure were also studied ing strong absorption of the visible light. The photophysical
(Fig. 12 and Table 4). Two low-lying singlet states were recog- properties of the complexes were studied with steady state UV–
nized for RH-BDPY-Pt-2, which are localized on the rhodamine Vis absorption spectra, luminescence spectra, nanosecond
moiety (S4 state) and the Bodipy moiety (S6 state), respectively. transient absorption spectroscopy, electrochemical characteriz-
We noted the discrepancy between the calculated excitation ation and DFT/TDDFT computations. The complexes show the
energy and the experimental UV-Vis absorption of the complex featured absorption bands of the coordinated Bodipy ligands.
in the presence of TFA (Fig. 2b), however, the trend is Upon the addition of an acid, new absorption bands assigned
correct.46
to the open amide structure of the rhodamine moiety were
The triplet states of RH-BDPY-Pt-2 with the rhodamine observed, accordingly colour changes were observed for the
moiety in the opened amide structure were studied with solutions. The fluorescence of the complexes was enhanced
TDDFT calculations. We found that the T1 state is localized on upon the addition of an acid. Furthermore, for one of the Pt(II)
the Bodipy moiety (1.54 eV), with HOMO−1 → LUMO+1 as the complexes the triplet state lifetime was extended upon the
component of the transition (Fig. 12). The T2 state is localized addition of an acid. For another complex, switching from a
on the rhodamine moiety with an opened amide structure, Bodipy-confined triplet state to a triplet state delocalized on
HOMO−2 → LUMO is involved in the transition (Fig. 12). both the Bodipy and the rhodamine ligands was observed. The
Notably the energy level of T2 (1.74 eV) is only slightly higher photophysical properties were rationalized with DFT/TDDFT
than that of T1 (1.54 eV), thus the population of both T1 and T2 calculations. Such tuning of the photophysical properties of Pt
states is possible. This postulation was confirmed by the nano- (II) complexes with a rhodamine moiety was reported for the
second transient absorption spectra of RH-BDPY-Pt-2 (Fig. 9e), first time and these results may be useful for designing exter-
in which both the bleaching bands of the Bodipy and the rho- nal-activatable triplet photosensitizers in future.
damine moiety were observed. Thus the photophysical pro-
perties of RH-BDPY-Pt-2 were fully rationalized by the DFT and
TDDFT calculations. Similar studies were carried out for
RH-BDPY-Pt-1 (please refer to ESI†).
Acknowledgements
We thank the NSFC (21273028, 51202207, 21473020 and
21421005), the Royal Society (UK) and the NSFC (China-UK
Cost-Share Science Networks, 21011130154), Program for
Conclusion
Changjiang Scholars and Innovative Research Team in Univer-
In conclusion, the UV–Vis absorption, photoluminescence and sity [IRT_13R06], State Key Laboratory of Fine Chemicals
the triplet state lifetimes of trans bis(tributylphosphine) Pt(II) (KF1203), the Fundamental Research Funds for the Central
bisacetylide complexes were switched by an acid (trifluoroacetic Universities (DUT14ZD226) and Dalian University of Technol-
acid, TFA). The aim of this research is to switch the photo- ogy (DUT2013TB07) for financial support.
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16 M. N. Roberts, C.-J. Carling, J. K. Nagle, N. R. Branda and
Notes and references
M. O. Wolf, J. Am. Chem. Soc., 2009, 131, 16644–16645.
1 A. Kamkaew, S. H. Lim, H. B. Lee, L. V. Kiew, L. Y. Chung 17 Y. Wu, Y. Xie, Q. Zhang, H. Tian, W. Zhu and A. D. Q. Li,
and K. Burgess, Chem. Soc. Rev., 2013, 42, 77–88.
Angew. Chem., Int. Ed., 2014, 53, 2090–2094.
2 S. G. Awuah and Y. You, RSC Adv., 2012, 2, 11169–11183.
3 (a) Y. You and W. Nam, Chem. Soc. Rev., 2012, 41, 7061–
18 L. Hou, X. Zhang, T. C. Pijper, W. R. Browne and
B. L. Feringa, J. Am. Chem. Soc., 2014, 136, 910–913.
7084; (b) Y. Feng, J. Cheng, L. Zhou, X. Zhou and H. Xiang, 19 J. Boixel, V. Guerchais, H. L. Bozec, D. Jacquemin, A. Amar,
Analyst, 2012, 137, 4885–4901.
4 J. Zhao, W. Wu, J. Sun and S. Guo, Chem. Soc. Rev., 2013,
42, 5323–5351.
A. Boucekkine, A. Colombo, C. Dragonetti, D. Marinotto,
D. Roberto, S. Righetto and R. D. Angelis, J. Am. Chem. Soc.,
2014, 136, 5367–5375.
5 D. P. Hari and B. Konig, Angew. Chem., Int. Ed., 2013, 52, 20 M. Irie, Chem. Rev., 2000, 100, 1685–1716.
4734–4743.
21 M. H. Lee, J. H. Han, J. H. Lee, N. Park, R. Kumar, C. Kang
and J. S. Kim, Angew. Chem., Int. Ed., 2013, 52, 6206–6209.
6 (a) L. Shi and W. Xia, Chem. Soc. Rev., 2012, 41, 7687–7697;
(b) G. Zhao, C. Yang, L. Guo, H. Sun, C. Chen and W. Xia, 22 J. Fan, M. Hu, P. Zhan and X. Peng, Chem. Soc. Rev., 2013,
Chem. Commun., 2012, 48, 2337–2339; (c) G. Zhao, C. Yang, 42, 29–43.
L. Guo, H. Sun, R. Lin and W. Xia, J. Org. Chem., 2012, 77, 23 X. Chen, Y. Zhou, X. Peng and J. Yoon, Chem. Soc. Rev.,
6302–6306; (d) D. Ravelli, M. Fagnoni and A. Albini, Chem.
Soc. Rev., 2013, 42, 97–113.
7 (a) T. Lazarides, T. M. McCormick, K. C. Wilson, S. Lee,
2010, 39, 2120–2135.
24 X. Zhang, Y. Xiao and X. Qian, Angew. Chem., Int. Ed., 2008,
47, 8025–8029.
D. W. McCamant and R. Eisenberg, J. Am. Chem. Soc., 2011, 25 L. Huang, L. Zeng, H. Guo, W. Wu, W. Wu, S. Ji and
133, 350–364; (b) R. P. Sabatini, B. Zheng, W.-F. Fu, J. Zhao, Eur. J. Inorg. Chem., 2011, 2011, 4527–4533.
D. J. Mark, M. F. Mark, E. A. Hillenbrand, R. Eisenberg and 26 W. Wu, J. Zhao, J. Sun, L. Huang and X. Yi, J. Mater. Chem.
D. W. McCamant, J. Phys. Chem. A, 2014, 118, 10663–10672. C., 2013, 1, 705–716.
8 (a) S. Erbas-Cakmak and E. U. Akkaya, Angew. Chem., Int. 27 G. Ulrich, R. Ziessel and A. Harriman, Angew. Chem., Int.
Ed., 2013, 52, 11364–11368; (b) S. Erbas-Cakmak, Ed., 2008, 47, 1184–1201.
O. A. Bozdemir, Y. Cakmak and E. U. Akkaya, Chem. Sci., 28 R. Ziessel and A. Harriman, Chem. Commun., 2011, 47,
2013, 4, 858–862. 611–631.
9 (a) Y. Sun, Z. M. Hudson, Y. Rao and S. Wang, Inorg. Chem., 29 M. J. Frisch, et al., Gaussian 09, Revision 01, Gaussian Inc.,
2011, 50, 3373–3378; (b) S. Silvi, E. C. Constable, Wallingford, CT, 2009.
C. E. Housecroft, J. E. Beves, E. L. Dunphy, M. Tomasulo, 30 A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson,
F. M. Raymo and A. Credi, Chem. Commun., 2009, 1484–
1486; (c) P. Wu, E. L.-M. Wong, D.-L. Ma, G. S.-M. Tong,
A. J. M. Huxley, C. P. McCoy, J. T. Rademacher and
T. E. Rice, Chem. Rev., 1997, 97, 1515–1566.
K.-M. Ng and C.-M. Che, Chem. – Eur. J., 2009, 15, 3652– 31 A. M. Soliman, M. Abdelhameed, E. Zysman-Colman and
3656; (d) S. K. Brayshaw, S. Schiffers, A. J. Stevenson, P. D. Harvey, Chem. Commun., 2013, 49, 5544–5546.
S. J. Teat, M. R. Warren, R. D. Bennett, I. V. Sazanovich, 32 C. K. M. Chan, C.-H. Tao, K.-F. Li, K. M.-C. Wong, N. Zhu,
A. R. Buckley, J. A. Weinstein and P. R. Raithby, Chem. –
Eur. J., 2011, 17, 4385–4395; (e) B.-B. Cui, C.-J. Yao, J. Yao
K.-W. Cheah and V. W.-W. Yam, Dalton Trans., 2011, 40,
10670–10685.
and Y.-W. Zhong, Chem. Sci., 2014, 5, 932–941; 33 C. K. M. Chan, C.-H. Tao, H.-L. Tam, N. Zhu, V. W.-W. Yam
(f) Y.-M. Zhang, S.-H. Wu, C.-J. Yao, H.-J. Nie and and K.-W. Cheah, Inorg. Chem., 2009, 48, 2855–2864.
Y.-W. Zhong, Inorg. Chem., 2012, 51, 11387–11395; 34 J. E. Rogers, J. E. Slagle, D. M. Krein, A. R. Burke,
(g) C.-J. Yao, Y.-W. Zhong and J. Yao, J. Am. Chem. Soc.,
2011, 133, 15697–15706.
10 T. Yogo, Y. Urano, Y. Ishitsuka, F. Maniwa and T. Nagano,
J. Am. Chem. Soc., 2005, 127, 12162–12163.
B. C. Hall, A. Fratini, D. G. McLean, P. A. Fleitz,
T. M. Cooper, M. Drobizhev, N. S. Makarov, A. Rebane,
K.-Y. Kim, R. Farley and K. S. Schanze, Inorg. Chem., 2007,
46, 6483–6494.
11 S. O. McDonnell, M. J. Hall, L. T. Allen, A. Byrne, 35 W.-Y. Wong and C.-L. Ho, Coord. Chem. Rev., 2006, 250,
W. M. Gallagher and D. F. O’Shea, J. Am. Chem. Soc., 2005,
127, 16360–16361.
2627–2690.
36 L. Yuan, W. Lin, Y. Yang and H. Chen, J. Am. Chem. Soc.,
2012, 134, 1200–1211.
12 J. Tian, L. Ding, H.-J. Xu, Z. Shen, H. Ju, L. Jia, L. Bao and
J.-S. Yu, J. Am. Chem. Soc., 2013, 135, 18850–18858.
13 (a) A. M. Bugaj, Photochem. Photobiol. Sci., 2011, 10, 1097–
1109; (b) P. Majumdar, R. Nomula and J. Zhao, J. Mater.
Chem. C, 2014, 2, 5982–5997.
37 (a) K. Feng, M.-L. Yu, S.-M. Wang, G.-X. Wang, C.-H. Tung
and L.-Z. Wu, ChemPhysChem, 2013, 14, 198–203;
(b) J. Chen, S. Li, L. Zhang, B. Liu, Y. Han, G. Yang and
Y. Li, J. Am. Chem. Soc., 2005, 127, 2165–2171;
(c) A. A. Rachford, R. Ziessel, T. Bura, P. Retailleau and
F. N. Castellano, Inorg. Chem., 2010, 49, 3730–3736.
14 D. Hablot, A. Harriman and R. Ziessel, Angew. Chem., Int.
Ed., 2011, 50, 7833–7836.
15 W. Tan, Q. Zhang, J. Zhang and H. Tian, Org. Lett., 2009, 38 (a) M. T. Whited, P. I. Djurovich, S. T. Roberts,
11, 161–164.
A. C. Durrell, C. W. Schlenker, S. E. Bradforth and
This journal is © The Royal Society of Chemistry 2015
Dalton Trans.

View Article Online
Paper
Dalton Transactions
M. E. Thompson, J. Am. Chem. Soc., 2011, 133, 88–96;
(b) N. D. McClenaghan, Y. Leydet, B. Maubert, M. T. Indelli
and S. Campagna, Coord. Chem. Rev., 2005, 249, 1336–
1350; (c) J. E. Yarnell, J. C. Deaton, C. E. McCusker and
F. N. Castellano, Inorg. Chem., 2011, 50, 7820–7830.
(b) J. A. G. Williams, Top. Curr. Chem., 2007, 281, 205–268.
39 W. Wu, H. Guo, W. Wu, S. Ji and J. Zhao, J. Org. Chem.,
2011, 76, 7056–7064.
40 W. Wu, J. Zhao, J. Sun and S. Guo, J. Org. Chem., 2012, 77, 43 C. Adamo and D. Jacquemin, Chem. Soc. Rev., 2013, 42,
5305–5312. 845–856.
41 (a) R. Ziessel, B. D. Allen, D. B. Rewinska and A. Harriman, 44 A. Vlcek Jr. and S. Zalis, Coord. Chem. Rev., 2007, 251, 258–
Chem. Eur. J., 2009, 15, 7382–7393; (b) J.-Y. Liu, 287.
M. E. El-Khouly, S. Fukuzumi and D. K. P. Ng, Chem. – 45 D. T. Chase, B. S. Young and M. M. Haley, J. Org. Chem.,
Asian J., 2011, 6, 174–179. 2011, 76, 4043–4051.
42 (a) S. Ji, W. Wu, W. Wu, P. Song, K. Han, Z. Wang, S. Liu, 46 W. Wu, L. Liu, X. Cui, C. Zhang and J. Zhao, Dalton Trans.,
H. Guo and J. Zhao, J. Mater. Chem., 2010, 20, 1953–1963; 2013, 42, 14374–14379.
–
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