Photophysical properties of rhenium(i) complexes and photosensitized generation of singlet oxygen

Article abstract of DOI:10.1039/c6pp00364h

fac-[Re(ampy)(CO)₃(NN)]⁺ complexes (ampy = 2-aminomethylpyridine and NN = 1,10-phenanthroline (phen), and 2,2′-bipyridine (bpy)) were synthesized, purified and characterized by proton nuclear magnetic resonance (¹H NMR), UV-visible and Fourier-transformed infrared (FT-IR) spectroscopies, and their photophysical properties were investigated using steady state and time-resolved emission spectroscopies. The electronic absorption spectra exhibit two main absorption bands: the higher energy band, which was assigned to intraligand transition (IL), and the lower energy band assigned to metal-to-ligand charge transfer (MLCT). Both complexes showed emission at room temperature in a CH₃CN solution (λ_max = 560 nm, ? = 0.091, τ = 560 ns for fac-[Re(ampy)(CO)₃(phen)]⁺; λ_max = 568 nm, ? = 0.024, τ = 100 ns for fac-[Re(ampy)(CO)₃(bpy)]⁺) and rigid media (λ_max = 530 nm, τ = 3300 ns for fac-[Re(ampy)(CO)₃(phen)]⁺; λ_max = 530 nm, τ = 853 ns for fac-[Re(ampy)(CO)₃(bpy)]⁺) arising from the lowest lying ³MLCT_Re→NN excited state. Both complexes along with fac-[Re(L)(CO)₃(NN)]^+/0 complexes, L = Cl or pyridine, were capable of efficiently photosensitizing the generation of singlet oxygen with quantum yield in the range of 0.59-0.28. These results highlight the potential application of fac-[Re(L)(CO)₃(NN)]^+/0 complexes in the development of sensitizers for the generation of singlet oxygen.

Full text of DOI:10.1039/c6pp00364h

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Page 1 of 21
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Photophysical properties of rhenium(I) complexes and
photosensitized generation of singlet oxygen
Luiz Duarte Ramos, Hugo Moreira da Cruz, Karina P. Morelli Frin*
Universidade Federal do ABC ꢀ UFABC, Av. dos Estados 5001, Santo AndreꢀSP – Brazil ꢀ
09210ꢀ170. *Email: karina.frin@ufabc.edu.br
Abstract
The facꢀ[Re(ampy)(CO)3(NN)]+ complexes, ampy = 2ꢀaminomethylpyridine,
NN = 1,10ꢀphenanthroline (phen), and 2,2’ꢀbipyridine (bpy) were synthesized, purified
and characterized by proton nuclear magnetic resonance (1H NMR), UVꢀvisible and
Fourierꢀtransformed infrared (FTꢀIR) spectroscopies, also their photophysical properties
were investigated using steady state and timeꢀresolved emission spectroscopies. The
electronic absorption spectra exhibit two main absorption bands: the higher energy
band, which was assigned to intraligand transition (IL), and the lower energy band
assigned to metalꢀtoꢀligand charge transfer (MLCT). Both complexes showed emission
at room temperature in CH3CN solution (λmax = 560 nm, φ = 0.091, τ = 560 ns for
facꢀ[Re(ampy)(CO)3(phen)]+; λmax = 568 nm, φ = 0.024, τ = 100 ns for facꢀ
[Re(ampy)(CO)3(bpy)]+) and rigid media (λmax = 530 nm, τ = 3300 ns for facꢀ
[Re(ampy)(CO)3(phen)]+; λmax = 530 nm, τ = 853 ns for facꢀ[Re(ampy)(CO)3(bpy)]+)
3
arising from the lowest lying MLCTRe→NN excited state. Both complexes along with
facꢀ[Re(L)(CO)3(NN)]+/0 complexes, L = Cl or pyridine, were capable of efficiently
photosensitize generation of singlet oxygen with quantum yield in the range 0.59ꢀ0.28.
These results highlight the potential application of facꢀ[Re(L)(CO)3(NN)]+/0 complexes
in the development of sensitizers for generation of singlet oxygen.
1

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Keywords: Rhenium(I) complexes; 3MLCT emission; singlet oxygen;
photosensitizers.
1. Introduction
Rhenium(I) complexes have been extensively investigated in several fields of
research due to their electronic and luminescent properties, some of them include the
development of technologies, such as Organic Light Emitting Diode Devices (OLEDs)
1ꢀ3, CO2 photoreduction 4ꢀ6, biosensors and cellular probes 7ꢀ12. The essence of the
luminescent properties of the polypyridinic Re(I) complexes is ascribed to the electronic
transition called metalꢀtoꢀligand charge transfer (MLCT), which can be modulated for
the exchange of the coordinated ligands 12, 13
.
14ꢀ17
Recently, investigation of Re(I) complexes
showed that they can be used
in cancer treatments as an alternative to the conventional ones, since many of them
showed cytotoxicity to different types of cancers cells when compared to some
previously studied compounds 17ꢀ19. A cancer treatment alternative is the generation of
O2 (1ꢁg) species, which are a highly reactive species and consequently can eradicate the
tumor of a specific region. In particular, the photosensitized production of singlet
oxygen has significance in photooxidation, DNA damage, photodynamic therapy (PDT)
of cancer. Most investigations of singlet oxygen photosensitization involve electronic
energy transfer from an organic molecule to the triplet ground state of molecular
oxygen. However, some transition metal complexes have also shown to be efficient
photosensitizers, especially those from second and third transition series, which
exhibited strong spinꢀorbit coupling allowing for efficient intersystem crossing (ISC)
that often will reflect to large Stokes shifts and longer lifetimes. In these cases, when
molecular oxygen quenches the complex emission, two species of 1O2 can be generated:
2

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O2 (1Σg+) with a short lifetime and/or O2 (1ꢁg) with a longer lifetime since deactivation
ꢀ
is accompanied by intersystem crossing. The energy gap between the ground state (3Σg
3
ꢀ O2) and the first excited state (1ꢁg ꢀ 1O2) is 22.5 kcal molꢀ1 corresponding to the near
infrared region (1270 nm) 20, 21. Consequently, the relative amount of singlet oxygen
produced can be monitored throughout its emission. Therefore, the energy transfer from
the triplet MLCT excited state of rhenium(I) complexes to the oxygen can be used to
photosensitize the generation of singlet oxygen, which can be suitable for photodynamic
therapy.
In this work, the novel Re(I) complexes, facꢀ[Re(ampy)(CO)3(NN)]PF6 ampy =
2ꢀaminomethylpyridine, NN = 1,10ꢀphenanthroline (phen) and 2,2’ꢀbipyridine (bpy),
Scheme 1, were synthesized, characterized and their photophysical properties have been
investigated by steadyꢀstate and timeꢀresolved emission spectroscopies in fluid and rigid
media. Also, it was investigated the generation of singlet oxygen in CH3CN solution by
the luminescence method in the near infrared region and compared with the wellꢀknown
facꢀ[Re(L)(CO)3(NN)]PF6 complexes, L = Cl or pyridine (py). The results were
3
rationalized in terms of their MLCT photophysical properties, which is easily tunable
and thus making these compounds interesting in the development of sensitizers for the
generation of singlet oxygen.
+
+
H2N
H2N
N
N
N
N
N
N
OC
OC
OC
OC
Re
Re
CO
CO
Scheme 1. Chemical structures of rhenium polypyridyl complexes.
3

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2. Experimental procedures
2.1.Methods
Proton nuclear magnetic resonance spectra (1H NMR) were obtained on a
Burker Avance III (500 MHz) spectrometer at 298 K using CD3CN as the solvent.
Residual CH3CN signals were used as internal standard. Absorption spectra were
recorded on an Agilent 8453 spectrophotometer using 1.00 cm optical length quartz
cuvette. The Fourierꢀtransformed infrared (FTꢀIR) spectra of all compounds were
measured by attenuated total reflectance (ATR) with a Spectrometer Spectrum Two,
Perkin Elmer, collected for 16 scans at 2 cmꢀ1 of resolution. Emission spectra at room
temperature were recorded with a Varian Cary Eclipse steady state spectrophotometer
using 1.00 cm optical length quartz cuvette for the fluid solution and a front face
arrangement for the polymer films. The spectra were not corrected for the
photomultiplier spectral response. Emission quantum yields of rhenium(I) compounds
22
were determined, as already described, using facꢀ[ReCl(CO)3(phen)] (ϕ = 0.017)
,
24
[Ru(bpy)3]2+ (ϕ=0.062)23 or facꢀ[Re(py)(CO)3(phen)] (ϕ= 0.18)
as standards in
deoxygenated acetonitrile solution and keeping their absorbance at excitation
wavelength between 0.1 and 0.2 (~10ꢀ5 mol Lꢀ1). Fluorescence decay times were
measured using a time correlated single photon counting (TCSPC) setup in a PicoQuant
FluoTime 300 fluorescence lifetime spectrometer using as excitation source a 375 nm
diode laser (LDHꢀPꢀCꢀ375B, 40 MHz repetition rate, 52 ps pulse width, PicoQuant
GmbH) driven by PDL 820 computer controller. The emission wavelengths were
isolated using a Uvꢀvisible monochromator. The solutions were degassed for 30 min
using argon atmosphere and care was taken to avoid any aggregation effect by using a
very dilute solutions (~10ꢀ5 mol Lꢀ1). The generation of singlet oxygen was monitored at
1270 nm using a Hamamatsu Near Infrared (NIR) photomultiplier. The solutions were
4

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airꢀequilibrated and the quantum yields were determined at room temperature, as
25ꢀ28
27, 28
already described,
using [Ru(bpy)3]2+ (ΦS´ =0.56 in CH3CN) as standard
and
Δ
according to eq. 1, where I and A are the integrated emission intensity and the
absorbance at the excitation wavelength, respectively.
ꢄ ꢆ
Φꢀ = Φꢀꢁꢂ ꢃ ꢅ ꢇꢈꢉ
1
ꢆ ꢄ
ꢇ ꢇꢈ
2.2.Materials
All solvents, from Aldrich or Synth, were reagent grade, except for those used
in the photophysical measurements, where HPLC grade solvents from LIChrosolv or
Aldrich were employed. [ClRe(CO)5], 1,10ꢀphenanthroline (phen), 2,2’ꢀbipyridine
(bpy), trifluoromethanesulfonic acid (tfms), pyridine (py), 2ꢀaminomethylpyridine
(ampy) and poly(methylmethacrylate) (PMMA, Mw = 110,000 g.molꢀ1) from Aldrich
were used as received.
2.3.Synthesis
The syntheses of Re(I) compounds were performed following previously
procedures 29ꢀ31. Briefly, [ClRe(CO)5] and a small excess of NN = phen or bpy ligand
were suspended in xylene and heated to reflux for 6 hours. The yellow product was
collected by filtration, and when necessary recrystallized from CH2Cl2 with slow
addition of nꢀpentane. The facꢀ[ReCl(CO)3(NN)] complex was converted to facꢀ
[Re(tfms)(CO)3(NN)] by adding an excess of trifluoromethanesulfonic acid in CH2Cl2
under inert atmosphere of argon during 2 hours and precipitated by addition of diethyl
ether. An excess of py or ampy was added to facꢀ[Re(tfms)(CO)3(NN)] in methanolic
solution and kept refluxing for about 6 hours. After cooling to room temperature,
NH4PF6 was added to precipitate the compounds as a salt, and then washed with water
and diethyl ether. facꢀ[ReCl(CO)3(NN)] and facꢀ[Re(py)(CO)3(NN)]PF6 complexes
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have already been synthesized previously following the procedure described elsewhere
and gave satisfactory purities.
facꢀ[Re(CO)3(phen)(ampy)]PF6. Yield 62%. Anal. Calc. for C21H16N4O3PF6Re C,
1
35.85%; N, 7.96%; H, 2.29%; found C, 35.88%; N, 8.04%; H, 2.38%. H NMR
(CD3CN, 500 MHz, δ/ ppm): 9.35 (dd, 2H), 8.02 (dd, 2H), 8.82 (dd, 2H), 8.25 (s, 2H),
8.10 (dd, 1H), 7.13 (dd, 1H), 7.60 (td, 1H), 7.03 (d, 1H), 3.97 (t, 1H), 3.75 (broad, 2H).
facꢀ[Re(CO)3(bpy)(ampy)]PF6. Yield 61%. Anal. Calc. for C19H16N4O3RePF6 C,
33.55%; N, 8.24%; H, 2.35%; found C, 33.63%; N, 8.01%; H, 2.25%.1H NMR
(CD3CN, 200 MHz, δ/ppm): 8.95 (d, 2H), 8.41 (d, 2H), 8.27 (m, 3H), 7.68 (m, 3H),
7.48 (t, 1H), 7.13 (d, 1H), 4.01 (t, 2H), 3.83 (broad, 2H).
1
Complete assignments of H NMR peaks are provided in the Electronic Supplementary
Information (Figures 1Sꢀ2S, Table 1S).
2.4.1
Preparation of poly(methyl methacrylate) films (PMMA). PMMA
films were prepared, as previously described 29, 32ꢀ34, by dissolving 250 mg of
PMMA (MW= 110,000 g molꢀ1) and 5.0 mg of the appropriated compound in 7.0
mL of acetonitrile HPLC and then left to dry in the absence of humidity.
3. Results and Discussion
All synthesis resulted in satisfactory yield, the complexes had been
1
characterized by H NMR and FTIR spectroscopies (1H NMR spectra are presented as
ESI along with the complete assignments) and gave satisfactory elemental analysis.
1
The H NMR spectra of facꢀ[Re(ampy)(CO)3(NN)]+ compounds and their
chemical shift values (Figures 1S and 2S at ESI, Table 1S) show that after coordination
to the metal center the proton signals of the polypyridyl ligands shifts to a region of
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high frequency compared to the protons of the uncoordinated ligand. In contrast, the
proton signals of the ampy ligand shift to a low frequency region after coordination,
indicating that its protons are more shielded. The same behavior was observed for
similar rhenium(I) polypyridyl compounds 30, 32, 34
.
Fourierꢀtransformed infrared (FTIR) spectra of the solid Rhenium(I)
complexes, Figure 1, shows a typical profile for rhenium(I) complexes with the
presence of intense bands around 2100ꢀ1800 cmꢀ1 ascribed to the CꢀO stretching
vibration characteristic of a facial geometry. For facꢀ[Re(ampy)(CO)3(NN)]PF6
complexes, these vibrations are in a higher frequency region than those with chloride
coordinated ligand. This behavior was reported for the similar complex facꢀ
[Re(ampy)(CO)3(cbz2phen)]PF6 and was ascribed to the decreased Re(I) πꢀbackbonding
to the CO ligand caused by electron withdrawing pyridine ligands 35. The high peak
intensity at 850 cmꢀ1 is characteristic to the counter ion PF6 30, 36ꢀ39 and additional bands
in the 1800 – 700 cmꢀ1 region were also observed and identified as ring vibrations of the
coordinated ligand.
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A
B
C
D
800 1000 1200 1400 1600 1800 2000 2200
Wavenumber (cm-1)
Figure 1: FTIR spectra of (A) facꢀ[Re(ampy)(CO)3(phen)]PF6 (─), (B) facꢀ
[ReCl(CO)3(phen)] (─), (C) facꢀ[Re(ampy)(CO)3(bpy)]PF6 (─) and (D) facꢀ
[ReCl(CO)3(bpy)] (─).
The electronic spectra of facꢀ[Re(ampy)(CO)3(NN)]+ compounds are shown in
Figure 2 and their spectral parameters are in Table 1. For comparison the complexes
40,
41
already reported in the literature facꢀ[ReCl(CO)3(NN)]
and facꢀ
[Re(py)(CO)3(NN)]+ 24, 42 were shown on the same graph.
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2
A
B
2
1
0
1
0
250
300
350
400
450
500
250
300
350
400
450
500
Wavelength (nm)
Wavelength (nm)
Figure 2: UVꢀVis spectra of (A) facꢀ[ReCl(CO)3(phen)] (…), facꢀ[Re(py)(CO)3(phen)]+
_
(ꢀꢀꢀ), facꢀ[Re(ampy)(CO)3(phen)]+ ( ) and (B) facꢀ[ReCl(CO)3(bpy)] (…), facꢀ
_
[Re(py)(CO)3(bpy)]+ (ꢀꢀꢀ), facꢀ[Re(ampy)(CO)3(bpy)]+ ( ) in CH3CN at 25 °C.
Table 1: Spectral parameters of Re(I) complexes.
λ
max (nm)
(ε (104 L.molꢀ1.cmꢀ1))
facꢀ[Re(ampy)(CO)3(phen)]+
facꢀ[Re(ampy)(CO)3(bpy)]+
facꢀ[Re(py)(CO)3(phen)]+
facꢀ[Re(py)(CO)3(bpy)]+
facꢀ[ReCl(CO)3(phen)]
facꢀ[ReCl(CO)3(bpy)]
220(3.3); 256(2.2); 275(2.3); 325(0.52); 365(0.35)
248(1.8); 260(1.6); 310(1.1); 320(1.2); 346(0.38)
234(2.3); 258(2.1); 278(2.4); 330(0.49); 384(0.33);
250(1.6); 263(1.5); 319(1.1); 366(0.27)
200(4.6); 216(4.1); 265(2.5); 290(1.2); 365(0.34)
234(1.8); 290(1.4); 315(0.8); 370(0.32)
The low energy band observed on the spectra of facꢀ[Re(ampy)(CO)3(NN)]+
complexes is hypsochromic shifted in comparison with the analogue complexes facꢀ
[ReCl(CO)3(NN)] due to the substitution of the axial ligand chloride, which is a πꢀ
electron donor, by a πꢀelectron acceptor ampy ligand. Furthermore, the similarity
between the spectra of pyridine compounds and of facꢀ[Re(ampy)(CO)3(NN)]+
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complexes indicates that the coordination of the ampy ligand occurs by the nitrogen of
the pyridine ring. The bands located in the less energetic regions (325ꢀ475 nm) of the
spectra for all rhenium(I) complexes can be assigned to metalꢀtoꢀligand charge transfer
1
24, 34, 35, 40, 42ꢀ44
transitions MLCTRe→NN as previously reported for similar complexes
.
An additional band centered at 325 nm can be observed for facꢀ
1
[Re(CO)3(phen)(ampy)]+ and can be assigned to a MLCTRe→ampy charge transfer
contribution. This band is precluded in the analogue bpy complex whereas it is not
possible to exclude this charge transfer contribution. Additionally, bands located in
34, 45
higher energy regions are assigned to ligand center transitions 1ILNN and 1ILampy
.
Additionally, the electronic spectra of facꢀ[Re(ampy)(CO)3(NN)]+ complexes
in PMMA matrix, Figure 3S and 4S at SI, is slightly bathochromic shifted when
compared with the fluid solution due to changes in the energy of MLCT. This excited
state is stabilized in polar environment due to a better interaction between the media
dipoles and those formed upon excitation of the complex40, 42. For facꢀ
[Re(ampy)(CO)3(bpy)]+ , it is also observed a red shift in the IL band. Ishitani and coꢀ
46
workers
observed this redꢀshift for the IL band in solid facꢀ[Re(PR3)(CO)3(bpy)]+
complexes, PR3 = triarylphosphines, and ascribed as the interaction between the axial
and the bpy ligands, which promotes a distortion of the bpy rings. Using the same
analogy, since facꢀ[Re(ampy)(CO)3(bpy)]+ was embedded into a rigid matrix, this shift
of around 3 nm could also be due to the influence of the ampy ligand over the bpy. In
addition, the fact that the phen ring is more rigid than the bpy one, this shift in the IL
band is almost not observed for the analogue phen complex.
The complexes embedded into a PMMA matrix can provide important
information on the excited state deactivation via emission, and can also provide a large
number of technological applications. Thus, the emission properties of rhenium
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compounds were investigated in a fluid and rigid medium, as shown Figure 3 and Table
2.
L
L
N
N
N
N
OC
OC
OC
OC
Re
Re
CO
CO
A
B
C
D
450
500
550
600
650
700
750
800
450
500
550
600
650
700
750
800
Wavelength (nm)
Wavelength (nm)
Figure 3: Emission spectra of facꢀ[ReL(CO)3(NN)]0/+ in CH3CN (A and B) and PMMA
(C and D) at room temperature. facꢀ[ReCl(CO)3(phen)] (…), facꢀ[Re(py)(CO)3(phen)]+
_
(ꢀꢀꢀ),
facꢀ[Re(ampy)(CO)3(phen)]+
( ),
facꢀ[ReCl(CO)3(bpy)]
(…),
_
facꢀ[Re(py)(CO)3(bpy)]+ (ꢀꢀꢀ), facꢀ[Re(ampy)(CO)3(bpy)]+ ( ).
Table 2: Emission data for rhenium(I) compounds in CH3CN and PMMA at 298 K.
λem(nm)
CH3CN
kr
knr
(x105 sꢀ1)
λem (nm)
PMMA
τ
φem
τ (ns)
(x105 sꢀ1)
(ns)
[Re(ampy)(CO)3(phen)]+
[Re(ampy)(CO)3(bpy)]+
560
568
0.091±0.006
0.024±0.001
0.179±0.004
0.1824
560
100
1.63
2.40
16.2
97.6
530
530
3300
853
1300
160024
232
[Re(py)(CO)3(phen)]+
[Re(py)(CO)3(bpy)]+
[ReCl(CO)3(phen)]
[ReCl(CO)3(bpy)]
550
566
600
611
1.38
2.38
1.20
0.69
6.32
40.7
71.3
344
515
511
550
562
4800
1350
1390
332
0.055±0.005
0.05942
24142
0.0166±0.005
0.0177 22
138
183 22
0.0020±0.000
4
29
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All compounds showed the characteristic broad and nonꢀstructured emission
3
24,
spectra assigned to the radiative decay from a longꢀlived MLCTRe→NN excited state
34, 45, 47ꢀ54. The spectra of both ampy complexes in a PMMA film are blue shifted in
3
comparison to the acetonitrile solution as the MLCT excited state is very sensitive to
the rigidity of the medium. This is similar to the behavior observed for other rhenium(I)
34, 53, 55,
complexes, which is known as the rigidochromic effect
56. Steady state
photoluminescence also revealed that the energy band showed a hypsochromic behavior
as the Clꢀ is substituted by the pyridinic ligands due to a higher backꢀbonding character
3
between the Re(I) atom and the axial ligand, which destabilized the MLCT excited
state. Using the same analogy, the difference observed between the pyridinic
compounds, facꢀ[Re(ampy)(CO)3(NN)]+ and facꢀ[Re(py)(CO)3(NN)]+ can be explained
,
by the increase in the π acceptor character (py > ampy). Therefore, this behavior
indicates that the electron density on the metal center influences the energy of the
luminescent excited states.
The emission quantum yield determined for facꢀ[Re(ampy)(CO)3(NN)]+
complexes provides information that the energy dissipates preferably by nonꢀradiative
pathways, as reported for other polypyridinic Re(I) compounds 45. Additionally, the
substitution of the phen ligand by bpy on facꢀ[Re(ampy)(CO)3(NN)]+ compounds in
acetonitrile solution results in small bathochromic shifts, lower emission quantum yields
and lifetimes (Figure 4, Table 2 and Figure 5S and 6S at ESI), which is consistent with
the energy gap law 20, 21, 55, 57. In spite of a slight different value of lifetime obtained for
facꢀ[Re(py)(CO)3(phen)]+ complex from that reported by Wallace and Rillema 24, our
results were highly reproducible. In addition, a good correlation can be observed
between these two series of rhenium(I) complexes, i.e. the nonꢀradiative rate constant
decrease as the energy of MLCT state increases, in agreement with the energy gap law.
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B
A
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0
0.5
1.0
Time (ꢁs)
Time (ꢁs)
Figure 4: Timeꢀresolved decay curves of (A) facꢀ[Re(ampy)(CO)3(phen)]+ and (B) facꢀ
[Re(ampy)(CO)3(bpy)]+ probed at maxima wavelength after 375 nm laser excitation in
acetonitrile solution at 298 K.
The emission band of facꢀ[Re(ampy)(CO)3(phen)]+ in acetonitrile solution,
Figure 5, is very sensitive to the presence of O2, being almost entirely quenched (~90%)
in O2 saturated solution. This significant quenching of the emission promoted by O2
corroborates with the triplet character of the MLCT excited states and suggests an
efficient photosensitization of O2 (1ꢀg) by the rhenium(I) complexes.
deoxygenated
air saturated
O2 saturated
450
500
550
600
650
700
750
800
Wavelength (nm)
Figure 5: Luminescence spectra of facꢀ[Re(ampy)(CO)3(phen)]+ in deoxygenated, air
saturated and O2 saturated acetonitrile solution.
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To explore Re(I) complexes ability to generate photosensitized O2 singlet the
luminescence intensity measurements were carried out following the O2 characteristic
emission at 1270 nm 25, 28. Using the [Ru(bpy)3]2+ as the standard in acetonitrile solution
the O2 (1ꢀg) quantum yields for facꢀ[ReL(CO)3(NN)]0/+1 complexes were determined, as
depicted in Table 3.
Table 3. Singlet oxygen quantum yields for facꢀ[ReL(CO)3(NN)]0/+1 complexes in
acetonitrile solution.
Φꢀ
facꢀ[Re(ampy)(CO)3(phen)]+
facꢀ[Re(ampy)(CO)3(bpy)]+
facꢀ[Re(py)(CO)3(phen)]+
facꢀ[Re(py)(CO)3(bpy)]+
facꢀ[ReCl(CO)3(phen)]
facꢀ[ReCl(CO)3(bpy)]
0.55±0.05
0.38±0.01
0.59±0.02
0.43±0.03
0.41±0.01
0.28±0.02
After 375 nm excitation of facꢀ[ReL(CO)3(NN)]0/+1 complexes the first excited
1
state is reached, which is MLCT since the ligands do not absorb in this region, and
yields the 3MLCT by intersystem crossing in a femtosecond scale 55, 57, 58. The latter can
be deactivated by radiative, by nonꢀradiative or by energy transfer process. The
mechanism of quenching by oxygen of the excited triplet state for rhenium(I) complexes
can be derived from organic sensitizers. The quantum yield of singlet oxygen
production, via sensitization, is given by the sum of contributions due to oxygen
quenching of singlet and triplet (Φꢀ = Φꢀ(Singlet) + Φꢀ(Triplet)). Thus, for rhenium(I)
14

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polypyridine compounds and for the ruthenium(I) complex, used as standard, it is
reported that the intersystem crossing is very close to unity due to the heavy atom effect
and, consequently, the singlet oxygen production via photosensitization is exclusively
by the energy transfer from the triplet excited state. Moreover, considering that
mechanism and the relative method, the ratio of fraction of triplet state quenched by O2
1
which yield O2(1ꢀq) from a sensitizer S relative to that observed from a suitable
standard sensitizer S´ should be close to unity. The choice of the standard sensitizer is
essential to use this approximation. Therefore, the quantum yields of singlet oxygen
25
production, Φꢀ, were determined using the relative measurements following the O2
characteristic emission at 1270 nm from the energy transfer of the triplet excited state of
rhenium(I) complexes and the standard in the same conditions (Figure 7S at ESI).
In spite of a slight different value of quantum yields of singlet oxygen
production obtained for facꢀ[Re(py)(CO)3(bpy)]+ complex from that reported by
Wolcan and coworkers (Φꢀ = 0.51 ± 0.05) 26 using a different method, our results were
highly reproducible. The investigated compounds showed values of Φꢀ similar to those
previously reported for other Re(I) complexes 26, 59. In addition, the compound exhibited
the same trend of the photophysical properties in the absence of oxygen, being it the
higher possibility of energy transfer process and consequently higher values of singlet
oxygen quantum yields for the complex with longer lived excited state. Therefore, the
facꢀ[Re(L)(CO)3(NN)]0/+ complexes can be used as triplet photosensitizers for the
generation of singlet oxygen, which makes them a potential phototoxic agent.
4. Conclusions
In this contribution, it was reported the excited state behavior of polypyridine
Re(I) complexes along with their ability to photosensitize O2 singlet formation in
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acetonitrile solution following the O2 characteristic emission band using the reference
method. Quantum yields of singlet oxygen production obtained for facꢀ
[Re(ampy)(CO)3(NN)]+ complexes, ampy = 2ꢀaminomethylpyridine, NN = 1,10ꢀ
phenanthroline (phen) and 2,2’ꢀbipyridine (bpy), where compared with facꢀ
[Re(L)(CO)3(NN)]PF6 complexes, L = Cl or pyridine (py), and rationalized in terms of
their 3MLCT photophysical properties. In other words, longer 3MLCTꢀlifetime enhance
the possibility of energy transfer process and consequently higher values of singlet
oxygen quantum yields are observed. The results shown here provides new insights on
the photophysical properties of Re(I) complexes and can be used in the development of
sensitizers for the generation of singlet oxygen in photodynamic therapy. Additionally,
the aminomethyl group attached to the coordinated py opens up a possibility to add a
polymer matrix or a pendant functional chain to better interact with cancer cells.
5. Acknowledgements
The authors would like to acknowledge financial support from Brazilian
agencies Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP/Grants
2011/23408ꢀ0, 2015/13149ꢀ8), Coordenação de Aperfeiçoamento de Pessoal de Nível
Superior (CAPES), Conselho Nacional de Desenvolvimento Científico e Tecnológico
(CNPq/Grant 402289/2013ꢀ7), Complexo Laboratorial Nanotecnológico (CLN) at
UFABC and Quantum Tech.
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fac-[Re(L)(CO)3(NN)]+ complexes were capable of efficiently photosensitize generation of
singlet oxygen (ΦΔ = 0.59-0.28). These results highlight their potential application in the
development of sensitizers for generation of singlet oxygen.