Article
pubs.acs.org/JPCC
Photoinduced Energy and Electron-Transfer Reactions by
Polypyridine Ruthenium(II) Complexes Containing a Derivatized
Perylene Diimide
Edjane R. dos Santos,† Joaõ Pina,‡ Tiago Venâncio,† Carlos Serpa,‡ José M. G. Martinho,*,§
and Rose Maria Carlos*,†
†
Departamento de Química, Universidade Federal de São Carlos, CP 676, 13565-905 São Carlos, SP Brasil
CQC, Universidade de Coimbra, Departamento de Química, 3004-535 Coimbra, Portugal
§
Centro de Química-Física Molecular e IN- Instituto de Nanociência e Nanotecnologia, Instituto Superior Técnico, Universidade de
Lisboa, 1049-001 Lisboa, Portugal
‡
S Supporting Information
*
ABSTRACT: The [Ru(II) (phen)2(pPDIp)]2+ complex, where pPDIp is the symmetric
bridging ligand phenanthroline−perylene−phenanthroline, shows strong electronic
absorption bands attributed to the pPDIp and {Ru(phen)2}2+ moieties in acetonitrile.
The charge-separated intermediate {Ru(III) (phen)2(pPDIp−•)} was detected by transient
absorption spectroscopy upon electronic excitation in either the pPDIp or the complex
moieties. The charge-separated intermediate species decays to generate the triplet state
3
*pPDIp-Ru(II) (τP = 1.8 μs) that sensitizes the formation of singlet molecular oxygen
with quantum yield ϕΔ = 0.57. The dyad in deaerated acetonitrile solutions is reduced by
triethylamine (NEt3) to the [Ru(II) (phen)2(pPDIp•−)] radical anion in the dark. The
electron-transfer reaction is accelerated by light absorption. By photolysis of the radical
anion, a second electron transfer reaction occurs to generate the [Ru(II)
(phen)2(pPDIp2−)] dianion. The changes of the color of solution indicate the redox states of complexes and offer a sensitive
reporter of each stage of redox reaction from start to finish. The reduced complexes can be converted to the initial complex, using
methyl viologen or molecular oxygen as an electron acceptor. The accumulation of electrons in two well-separated steps opens
promising opportunities such as in catalysis.
■
INTRODUCTION
Perylene diimides (PDIs) are fluorophores with exceptional
thermal and photochemical stability and strong absorption in
the visible and high fluorescence quantum yields.1−4 Due to
these properties, PDIs have been used in diverse applications
such as light-emitting diodes,5,6 field-effect transistors,7,8
sensing,9,10 and photovoltaic cells.11,12 However, dyads of
PDIs with other moieties, namely, transition metal complexes,
enlarge the processes of electronic excitation and decay of PDI,
broadening the scope of their potential applications.13−38 The
long-lived triplet state of PDI can be populated via strong
spin−orbit coupling induced by heavy metals, as observed in
Pt(II) square planar complexes with PDI covalently linked to
the metal center by an acetylide bond.14−19 However, in
palladium complexes where two Pd centers are attached to PDI
by metal−carbon σ bonds, the spin−orbit coupling is very
small, and PDI is highly fluorescent.20
PDI is a good electron acceptor, and therefore chargetransfer interactions are expected and indeed observed in dyads
comprising a PDI linked by the N-imide position to a porphyrin
and by a diphenylethyne linker to a Ru-porphyrin,21,22 Ruphthalocyanine,22,23 and Ru-polypyridine24,25 complexes. However, depending on the complex, other competitive deactivation
pathways of the electronic excited states, behind electron
© 2016 American Chemical Society
transfer, can occur. The binuclear Ru-tetraphenylporphyrin
(TPP), DPyPBI[Ru(TPP) (CO)]2, complex shows a rich
photophysics dependent on the excitation wavelength. Upon
electronic excitation of the DPyPBI moiety a fast photoinduced
electron transfer (ET) occurs from the Ru-TPP to the perylene
derivative (DPyPBI) with charge separation (CS) and charge
recombination (CR) lifetimes of τCS = 5.6 ps and τCR = 270 ps,
respectively. Contrarily, upon excitation of the Ru-porphyrin
moiety, an efficient triplet−triplet energy transfer process
occurs with the population of the DPyPBI triplet state that
decays to the ground state with lifetime τP = 9.8 μs.22 On the
other hand, upon photoexcitation of either chromophore of the
homologue phthalocyanine (Pc) ruthenium complex, BPyPDI[Ru(CO)Pc]2, the [Ru(CO)Pc•+-BPyPDI•−-RuCOPc] radical
ion pair with lifetime of 115 ns (ΔGET(Ru(CO)Pc) = −0.64 eV
and ΔGET(BPyPDI) = −0.88 eV) was generated.24 Similarly,
photoexcitation of either chromophore of the tetranuclear
[PDIpy4{Ru(CO)Pc}4] complex with the PDI located in the
center of the 4{Ru(CO)Pc} moieties generates a short-lived
radical ion pair (τ = 260 ps) that deactivates by charge
Received: July 4, 2016
Revised: September 22, 2016
Published: September 25, 2016
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recombination to yield the 3*{RuCOPc} triplet state.23
Nevertheless, the electron-transfer quenching is negligible for
complexes of PDI-pyridine/terpyridine ligands with several
metals such as Ru,26,27 Pt,13 Ir,28,29 Fe,30 Cu,31 and Zn,32−37
and so they are highly fluorescent.
Recently, attention has been focused on Ru(II)-polypyridine
complexes mostly because they give rise to 100% population of
the emissive, long-lived, redox-active triplet state, 3*MLCT, Ru,
dπ → π*, and α-diimine.39
The {Ru(PDI2-bpy) (tBuCN)2(Cl)2} complex containing
two pendant PDI-dicarboxydiimide chromophores conjugated
to bipyridyl (bpy) produces interesting results. Upon light
excitation on the PDI moiety, a fast electron transfer from the
Ru(II) moiety to 1*PDI generates the radical anion Ru(III)PDI•− with charge separation (ΔGCS = −1.0 eV) and charge
recombination (ΔGCR = −1.2 eV). The charge recombination
reaction resulted in the long-lived 3*PDI (τP = 39 μs) that
competes with the PDI fluorescence (τF = 4.55 ns).26
Subsequently, the incorporation in Ru(bpy) complexes of
PDI derivatives, functionalized in the bay positions (1,6,7,12positions of PDI) with different electron donor or acceptor,
shows that the 3*PDI triplet is reached (τP = 63−57 μs)
regardless of the electronic excited moiety, but the deactivation
pathways were dependent on the properties of the PDI
derivative.25
The bipyridine complexes of Ru(II) containing an azabenzannulated perylene bisimide, [Ru(bpy)2(abBpy)]2+, also
triggers the population of the 3*PDI (ϕP = 0.11) that decays
with a lifetime τP = 4.2 μs.38
These studies exemplify how the choice of the ligands around
the Ru(II) coordination sphere can be used to modulate the
photophysial behavior of the complexes.
These findings encouraged us to prepare the dyad cis[(phen)2Ru(pPDIp)]2+, where pPDIp is a perylene pendant
group functionalized with one of the coordinated phenanthroline ligands (Scheme 1).
guarantee a strong absorption to the 1*MLCT state that rapidly
decays to populate the 3*MLCT. The rigid structure of the
{Ru(phen)3}2+ with a strong ligand field coordination will
provide the stability of the complex that hampers its
photodissociation. We expected that optical excitation on the
MLCT (Ru, dπ → pPDIp,π*) or ILCT (pPDIp, π →
pPDIp,π*) absorption band of the dyad could generate the
charge-separated excited state {(phen)2Ru(III) (pPDIp•−)}
that can decay by charge recombination to generate the
3
*pPDIp triplet.
The dyad performance benefits from the properties of the
PDI and Ru(II)-polypyridine complexes and expands their
properties by: (1) generation upon electronic excitation of the
charge-separated intermediate {(phen)2Ru(III) (pPDIp•−)}
irrespective of the moiety being electronically excited; (2)
population of the long-lived triplet 3*PDI (τP = 1.8 μs) from
charge recombination of the intermediate that sensitizes the
formation of the singlet molecular oxygen (1Δg), with high
yield; (3) the dyad in the presence of triethylammine (NEt3) is
reduced to yield the [(phen)2Ru(pPDIp•−)]+ radical anion,
which generates upon light irradiation the very stable
[(phen)2Ru(pPDIp2−)] dianion.
■
EXPERIMENTAL SECTION
General. RuCl3·xH2O, 1,10′-phenanthroline (phen), perylene-3,4,9,10-tetracarboxylic dianhydride, lithium chloride, and
tetrabutylammonium hexafluorophosphate were obtained from
Aldrich. HPLC grade acetone and DMF were distilled just
before being used.
The electron transfer reactions and electrochemical and
spectroscopic experiments were conducted under nitrogen
atmosphere. The cis-[Ru(phen)2Cl2]·2H2O was prepared
according to the procedures described in the literature.40
The CHN elemental analysis of complexes was performed on
an EA 1110 CHNS-O Carlo Erba Instrument. FTIR spectra
were recorded on a Bomem-Michelson 102 spectrometer in
CD3CN solutions and for solids using KBr pellets.
The NMR experiments were acquired with a BRUKER DRX600 spectrometer in CD3CN or DMSO using tetramethylsilane
(TMS) as internal standard. 1H−1H g-COSY experiments were
performed with a spectral width of 8196.72 Hz, acquisition time
of 0.25 s, relaxation delay of 1 s, and 16 scans for each
increment (256 increments). The 1H−13C g-HSQC experiment
was performed using spectral widths of 8196.72 Hz for the 1H
dimension and 36 057.691 Hz for the 13C dimension. Longrange 1H−13C g-HMBC correlation maps were acquired with
spectral widths of 8196.72 Hz in the 1H dimension and
36 049.42 in the 13C dimension. For both 2D heteronuclear
correlation maps, the acquisition time was 0.25 s; the relaxation
delay was 1 s; and 80 scans were performed for each increment
(256 increments). TopSpin 3.0 software (Bruker BioSpin) was
used for data acquisition and processing.
Electrochemical measurements were recorded using a
μAutolab Type III potentiostat. Generally, the concentration
of the acetonitrile solutions was 10−3 mol L−1; the supporting
electrolyte was TBAPF6 (0.1 mol L−1); and the scan rate was
100 mV/s. A standard three-electrode configuration was
composed of a platinum disk as the working electrode (d = 2
mm, diameter), auxiliary electrodes (d = 4 mm), and Ag/AgCl
(KCl salt) wire as the reference electrode. This electrode was
calibrated with the internal reference Fc+/ Fc, which was found
at 0.54 V vs Ag/AgCl in CH3CN.
Scheme 1. Molecular Structure of Complex [(phen)2Ru(II)
(pPDIp)](PF6)2
The dyad was designed to improve the electronic coupling
between the PDI and the Ru(II)-polypyridine moiety so that
the 3*MLCT is able to tune the population of the 3*PDI triplet
excited states, by a fast electron injection in the forward
direction to give the charge-separated excited state,
{(phen)2Ru3+(pPDIp•−)}, that decays to populate the triplet
state of PDI.
Our strategy is to keep the trischelate structure around the
Ru(II) metal center using the complex [Ru(phen)3]2+, to
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Femtosecond Transient Absorption. The experimental
setup for ultrafast spectroscopic and kinetic measurements
consists of a broadband (350−1600 nm) HELIOS pump−
probe femtosecond transient absorption spectrometer from
Ultrafast Systems, equipped with an amplified femtosecond
Spectra-Physics Solstice-100F laser (displaying a pulse width of
128 fs and 1 kHz repetition rate), coupled with a SpectraPhysics TOPAS Prime F optical parametric amplifier (195−
22 000 nm) for pulse pump generation. Probe light in the Vis
range was generated by passing a small portion of the 795 nm
light from the Solstice-100F laser through a computerized
optical delay stage (with time window up to 8 ns) and focusing
on a sapphire plate to generate a white-light continuum in the
450−800 nm range. All measurements were obtained in a 2
mm optical path length quartz cuvette, with absorptions in the
range 0.3−0.5 at the pump excitation wavelength. To avoid
multiphoton absorption, the laser pump power was kept at
≤0.5 μJ. Transient absorption data were analyzed using the
Surface Xplorer PRO program from Ultrafast Systems.
Nanosecond Transient Absorption. The nanosecond
transient absorption measurements were recorded in an
Applied Photophysics LKS.60 laser flash photolysis using a
Nd:YAG laser (Spectra Physics Quanta-Ray GCR-130) and a
Tektronix TDS3052B Oscilloscope (5GS/s). A pulsed 150 W
Xe lamp was used to analyze the transient absorption that was
detected by a R928 photomultiplier at a right angle with the
excitation beam. The signal from the photomultiplier was fed
into the digital analyzer and transferred to an IBM RISC
computer and processed with the software furnished by Applied
Photophysics. The transient spectra were obtained by
monitoring the optical density (OD) change at intervals of
5−10 nm over the 300−800 nm range and averaging at least
five decays at each wavelength. The samples were irradiated
either with the third-harmonic pulse (355 nm, 8 ns fwhm) or
with the second-harmonic pulse (532 nm, 8 ns fwhm) of the
laser.
Singlet Oxygen Activation. Room-temperature singlet
oxygen phosphorescence was detected at 1270 nm using a
Hamamatsu R5509-42 photomultiplier cooled to 193 K in a
liquid nitrogen chamber (products for research, model
PC176TSCE-005). The excitation light of 355 or 532 nm
comes from the Nd:YAG laser (Spectra Physics Quanta-Ray
GCR-130). A 600-groove diffraction grating was used to extend
spectral response to the infrared. A Scotch RG1000 filter was
placed in the detection path to eliminate the first harmonic
contributions from the infrared sensitizer emission. 1Hphenalen-1-one (perinaphthenone) in toluene (λexc = 355
nm),43 ϕΔ = 0.93, rose Bengal in methanol (λexc = 532 nm),44
ϕΔ = 0.76, and [Ru(phen)32+] (λexc = 355 nm),45 ϕΔ = 0.71,
were used as standards.
Chemical and Photoinduced Electron-Transfer Reactions.
For this purpose, a specially designed cuvette was built. It
consists of a round-bottomed vessel (1 mL) connected to a
quartz cell (1 cm optical length) that is fixed to a gas/vacuum
manifold by a hose. The CH3CN solution of the complex was
placed in the cuvette together with NEt3, and the MV2+ was
introduced in the round-bottom vessel. After that, the system
was deaerated for 20 min using super pure N2 atm. The
absorption spectrum of the acetonitrile solution of the
complex−NEt3 mixture was measured and photolyzed with
420 or 520 nm light. When the irradiation was stopped, the
MV2+ was added, and the subsequent reaction in the dark was
followed by changes in the UV−vis absorption spectrum.
The electronic absorption spectra were recorded on an
Agilent 8453A UV−visible spectrophotometer or on a Jasco V660 UV−vis spectrophotometer.
Synthesis of cis-[Ru(phen)2(pPDIp)](PF6)2. cis-Ru(phen)2Cl2
(357 mg; 0.067 mmol) was dissolved in DMF (15 mL) at
which pPDIp (75 mg; 0.1 mmol) was added. The solution was
stirred under nitrogen atmosphere during 24 h under reflux.
Following that the volume of the solution was reduced, and a
stoichiometric amount of NH4PF6 and deoxygenated diethyl
ether were added to promote precipitation. The orange
precipitate of cis-[(phen)2Ru(pPDIp)](PF6)2 was cooled at 0
°C for 30 min, washed with water and ethyl ether, dried under
v a c u u m , an d fin a l ly r ec r y st al li z ed ( 8 0% y i el d ) .
C72H38F12N10O4P2Ru(1498.16): calcd C 57.72, H 2.55, N
9.35; found C 57.04, H 2.64, N 9.11.
Steady State Spectroscopy. The luminescence spectra were
recorded on a Shimadzu RF-5301PC spectrofluorometer or on
a Horiba Jobin Yvon Fluorolog 3−22 spectrofluorometer. The
luminescence quantum yields were calculated by the ratio
method using [Ru(bpy)3]2+ in acetonitrile (λexc = 436 nm, ϕem
= 0.0629) as a standard.41
Photolysis. Photolysis of the complex (∼10−6 mol L−1)
solutions (3.5 mL) were performed in deaerated solutions by
N2 bubbling under magnetic stirring. The solutions were
irradiated at 420 nm with a 300 W xenon lamp (model 6258
from Newport) or on an RMR-600 model Rayonet Photochemical reactor using RMR-4200 lamps. The reactions were
followed by UV−vis absorption in square quartz cells of 1 cm
optical path length.
Time-Resolved Photoluminescence. The fluorescence decay
curves with picosecond resolution were obtained by the singlephoton timing technique using the following excitation light
sources: (1) the fundamental and the second harmonic of a
cavity-dumped DCM dye laser (DCM = 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran dye (λexc
= 315−340 nm; 630−680 nm); (2) rhodamine 6G dye (λexc
= 284−310 nm; 568−620 nm) synchronously pumped by a
solid-state Nd:YVO4 laser (Vanguard, Spectra Physics)
delivering 2 Wof 532 nm light at a repetition rate of 76 MHz
with a pulse duration of ∼12 ps; (3) second harmonic of a
tunable wavelength (700−1000 nm) Ti:sapphire laser
(Tsunami, Spectra Physics, Mountain View, CA) pumped by
the second harmonic of a Nd:YVO4 laser (Milennia Xs, Spectra
Physics, Mountain View, CA), delivering 100 fs pulses at a
repetition rate of 80 MHz. Intensity decay measurements were
performed by alternate collection of the instrument response
function and decay curves, using an emission polarizer set at the
magic angle. The instrument response function was recorded at
the excitation wavelength with a scattering suspension. For the
decays, a cutoff filter was used to remove all excitation light.
The emission signal passed through a depolarizer, a Jobin Yvon
HR-320 monochromator with a grating of 100 lines/mm, and it
was detected with a Hamamatsu 2809U-01 microchannel plate
photomultiplier (MCP-PT). The instrument response had an
effective fwhm of 35 ps. Fluorescence intensity decay curves
were obtained by excitation light at 280, 340, and 532 nm using
the DCM and rhodamine 6G dye lasers, with emission
collected at 570, 600, and 620 nm, respectively. The decay
curves were analyzed using a homemade, nonlinear, leastsquares reconvolution software based on the Marquard
algorithm.42 The quality of the fit was evaluated by the reduced
χ2, the weight residuals, and the autocorrelation of the residuals.
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■
−1.17/−1.24 (E1/2 = −1.20) and −1.34/−1.57 (E1/2 = −1.45)
were attributed to the phenanthroline ligands coordinated to
the Ru(II) center. The redox couple at +1.50/+1.43 V (E1/2 =
+1.47) versus Ag+/AgCl was assigned to the Ru(II/III)
reversible couple.39,40,57
The UV−vis spectroelectrochemistry experiments in the
range of −1.57 to +1.57 V vs Ag/AgCl are shown in Figure S5.
The spectroelectrochemical oxidation of the Ru(II) complex at
constant potential +1.57 V shows the formation of the
electrogenerated Ru(III) product with absorption bands at
440, 280, and 250 nm. In accord, the solution color changed
from intense orange to yellow (Figure S5). The reduction of
the corresponding Ru(III) complex was not reversible. The
reductive electrolysis at −0.35 V shows the appearance of a
weak band at 704 nm attributed to [Ru(phen)2(pPDIp•−)]+
(λmax = 704 nm). When the reduction was performed at −0.76
V, the successive formation of the pPDIp dianion complex,
[Ru(phen)2(pPDIp2−)], with absorption at λmax = 565 was
observed (Figure S5). The changes in color of the solution
from orange to violet are depicted in Figure S5. The oxidation
of the corresponding anion radical and dianion pPDIp moieties
of complex was not reversible. These results are in accordance
with the reduction of the pPDIp moiety of the complex by one
electron and two electrons using light irradiation and NEt3 as
sacrificial agent, described below.
UV−Visible Spectroscopy. Figure 2A and 2B show the
absorption spectrum and the excitation and fluorescence
spectra of the pPDIp free ligand in dilute solutions (1.1 ×
10 −6 mol L −1 ), where aggregation is negligible. The
fluorescence spectrum of the pPDIp exhibited a mirror image
of the absorption spectrum with a Stokes shift of ∼360 cm−1.
The fluorescence spectrum, quantum yield ϕ = 0.97, and
lifetime τF = 3.5 ns are identical in aerated and deaerated
solutions and almost invariant with the excitation wavelength
(355, 450, and 525 nm) and temperature (25−75 °C), which is
in close agreement with other reports.1,2
The absorption spectrum of the complex (Figure 3A) can be
decomposed as the sum of the ligand pPDIp vibrational
resolved absorption spectrum (Figure 2A) plus the MLCT
broad absorption band of the [Ru(phen)3]2+ complex with
maximum at ∼450 nm. This indicates that the complex does
not behave as a super molecule but rather as a donor−acceptor
dyad.
Upon excitation at 413 nm, the complex exhibits a broad
emission with maximum at 600 nm (Figure 3B), whereas by
excitation at 450 nm (Figure 3C), the spectrum is composed of
a broad emission at 600 nm and a shoulder at 525 nm,
suggesting a different repartition of the excitation light between
the {Ru(phen)3}2+ and the pPDIp moieties at each excitation
wavelength. Consistent with these results, the excitation
spectrum recorded at 600 nm shows two overlapping
absorption bands characteristic of the {pPDIp} and {Ru(phen)3}2+ moieties.
RESULTS AND DISCUSSION
Synthesis and Characterization. The pPDIp ligand was
synthesized by the reaction of perylene-3,4,9,10-teracarboxylic
acid anhydride (PDA), with 5-amino-phenanthroline (1:3
molar ratio) in quinoline using zinc-acetate as catalyst,
following the procedure described in the literature.46 The
molecular structure of the ligand was confirmed by 1H NMR
(Figure S1 and Table S1). This was further supported by the
FTIR CO stretching band that was shifted from 1780 cm−1 in
the unreacted PDA to 1700 cm−1 inpPDIp (Figure S2).
The cis-[Ru(phen)2(pPDIp)](PF6)2 complex was synthesized by reacting cis-[Ru(phen)2Cl2]47 with 1.5 equiv of pPDIp
in DMF. The complex was isolated as a hexafluorophosphate
salt, whose composition and structure were verified by CHN
analysis and NMR, respectively. The 1H signals of the perylene
unit appear in the chemical shift range, 8.7 < δ < 9.0 ppm, while
those of phenanthroline appear at 7.5 < δ < 8.7 ppm (Figure
S3, Table S1). As expected, the phenanthroline protons are
composed of eight peaks instead of four of the free ligand
because the 1H protons of the ligands in the complex are not
equivalent.48
Electrochemistry. The cyclic voltammogram of the complex
in CH3CN using a Pt electrode is shown in Figure 1 and Table
Figure 1. Cyclic voltammogram of the cis-[Ru(phen)2(pPDIp)](PF6)2
complex in acetonitrile (0.1 M (nBu)4NPF6), platinum working
electrode (potential versus Ag/AgCl), and scan rate of 100 mV s−1.
E1/2 for Fc+/Fc couple = +0.54 V measured under the same
experimental conditions.
1. The voltammograms were independent of number of scans
and sweep rate (Figure S4). Using these data and the first-order
derivative of the voltammetric curves it was possible to identify
the redox couples at −0.235/−0.314 V (E1/2 = −0.27) and
−0.397/−0.702 (E1/2 = −0.55) vs Ag+/AgCl which were
ascribed to radical anion (pPDIp/pPDIp•−) and dianion
(pPDIp•‑/pPDIp−2) moieties of complex, respectively, as
reported for other PDI derivatives.49−56 The reductions at
Table 1. Redox Potentials (E1/2) and Energies of the Electronic Excited States (E0−0) in eV for pPDIp in DMF and
[Ru(phen)]32+ and [(phen)2Ru(II) (pPDIp)](PF6)2 Complexes in CH3CN
E1/2/eV
compound
RuII/III
[Ru(phen)3]2+
pPDIp
[(phen)2Ru(pPDIp)]2+
+1.30
+1.47
E0−0/eV
pPDIp0/•−
pPDIp•−/−2
phen
−0.43
−0.27
−0.70
−0.55
−1.05; −1.32
−1.20; −1.45
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1
*pPDIp
1.77
1
*MLCT
2.75
*MLCT
3
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Figure 2. (A) Absorption spectrum of pPDIp in DMSO. (B) Fluorescence (λexc = 355 nm, red) and excitation (λem = 590 nm, green) spectra of
pPDIp in CH3CN (1.1 × 10−6 mol L−1).
Figure 3. (A) UV−vis absorption spectrum (orange) of a 4.1 × 10−6 mol L−1 solution of [(phen)2Ru(II) (pPDIp)](PF6)2 in CH3CN and the
contributions of the pPDIp (red) and {Ru(phen)3}2+ MLCT absorption (green) bands. Luminescence spectra (red) by excitation at λexc= 413 nm
(B) and λexc= 450 nm (C) and excitation spectra (green) recorded at λem = 600 nm.
ligand even in a very small amount can make a substantial
contribution to the decay due to its very high quantum yield
(approximately 100 times higher than that of the spinforbidden 3MLCT {Ru(phen)3}2+ transition to the ground
state). The very short lifetime of ∼10 ps is absent in the free
ligand decay which suggests the presence of fast deactivation
processes owing to the presence of the {Ru(phen)3}2+ moiety
in the dyad.
By excitation at 450 nm, both the {Ru(phen)2}2+ and the
{pPDIp} moieties are electronically excited. The decays
recorded with a small time window (∼10 ns) are identical to
those obtained by electronic excitation at λexc = 532 nm. Using
a large time window (∼500 ns), it is possible to observe a long
decay component with a very small pre-exponential factor and
lifetime higher than 100 ns. This component of the decay was
obtained with a large uncertainty by the single-photon timing
(SPT) technique. These results indicate that at least upon
excitation at 450 nm a long-lived state in acetonitrile airequilibrated solutions should be reached.
Figure 5 shows the transient absorption spectra and the
decays of the complex in acetonitrile by excitation at 525 nm, at
which only the pPDIp moiety absorbs. The bleaching of the
ground state absorption of the pPDIp at 530 nm is
accompanied by the appearance of a broad band with maximum
at 700 nm, which was previously attributed to the pPDIp•−
radical anion (Figure 5A).24,26,49 This can be envisaged as the
charge-separated state *{(phen)2Ru(III) (pPDIp•−)} resulting
from the transfer of one electron from Ru(II) to pPDIp. Later,
this state decays (Figure 5B) and originates the triplet state
{(phen)2Ru(II) (pPDIp3*)}, with transient absorption at 505
nm. The kinetics of growth and decay of the pPDIp•− radical
This was confirmed by the time-resolved luminescence
decays of air-equilibrated dilute solution in CH3CN. The
electronic excitation at the {pPDIp} moiety of [(phen)2Ru(II)
(pPDIp)](PF6)2 at 532 nm requires a biexponential fit (Figure
4) with lifetimes of 10 ps (90%) and 3.5 ns (10%). The long
lifetime is identical to that of the free ligand suggesting the
presence of free ligand in the solution. The presence of the free
Figure 4. Luminescence decays (••••) of complex [(phen)2Ru(II)
(pPDIp)](PF6)2 in CH3CN at 22 °C by excitation at λexc = 532 nm
and emission recorded at λem = 570 nm. The decay was fitted (red)
with a sum of two exponentials by convolution with the instrumental
response function (blue): the quality of the fit was judged by χ2 = 1.21
and the plot of the weighted residues.
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Figure 5. Transient absorption spectra of the collected at 0.267 ps (blue), 0.587 ps (red) and 0.907 ps (black) (A) and 1.63 ps (pink), 29.5 ps
(blue), 61.1 ps (red line) to 213 ps (black) (B) after femtosecond-pulse excitation at 525 nm. The kinetics of growth and decay (••••) and fit (red)
of the radical anion pPDIp•− at 700 nm (C) and 3pPDIp* at 505 nm (D) and the corresponding weighted residues.
anion at approximately 700 nm can be fitted with a sum of
three exponentials (Figure 5C). A very fast decay component of
0.6 ps is followed by a growing component of 2.9 ps (negative
pre-exponential) that is followed by a decay component of 28
ps. The short component of 0.6 ps probably occurs due to the
cooling of the vibrational hot pPDIp•− radical anion after being
created; the growing component comes from the singlet excited
sate of 1*{(phen)2Ru (II) (pPDIp)}; and the decay component
of 28 ps is related to the decay of the pPDIp•− radical anion to
form the triplet state {(phen)2RuII(pPDIp3*)}. The growth of
the 3*pPDIp triplet state (Figure 5D) recorded in the transient
absorption band (475−775 nm)1,25 at 505 nm provides a risetime component of 22 ps. This lifetime is inferior to the decay
component of the pPDIp•− radical anion (28 ps) probably due
to the interference of the ground state depletion of the
1
*{(phen)2Ru(II) (pPDIp)} at approximately 530 nm. Indeed,
the transient absorption trace recorded in a new transient
absorption band of the triplet state (575−800 nm)1,25 of
{(phen)2Ru(II) (pPDIp3*)} at 635 nm provides a lifetime
component of 27 ps, which is in close agreement with the decay
of the pPDIp•− radical anion of 28 ps. This species presents a
long decay that cannot be recorded within the time window of
the fs transient absorption equipment. These results support
the population of the 3*pPDIp by a charge-transfer mechanism
instead of the intersystem crossing pathway that can be
envisaged due to the presence of the Ru heavy metal.
Upon excitation at 450 nm, both the {pPDIp} and the
{Ru(phen)3}2+ moieties absorb with a repartition of approximately 64% of the excitation energy being absorbed by the
{Ru(phen)3}2+ component. Although most of the excitation
radiation is absorbed by the {Ru(phen)3}2+ moiety we must
consider the influence of the decaying processes that occur after
excitation of the pPDIp moiety. Figure 6A and 6B shows the
femtosecond-transient absorption spectra upon excitation at
450 nm for several delay times after excitation of the complex.
The bleaching of both the {pPDIp} moiety (530 nm) and
{Ru(phen)3}2+ (450 nm) absorption bands is accompanied by
the appearance of a strong absorption band at 696 nm due to
the pPDIp•− radical anion. At longer delay times, we can clearly
identify a transient absorption species at 505 nm, which is
associated with the decay of the species at 696 nm and the
recovery of the shoulder with a red shift to 580 nm. The
difference absorption spectra versus time profiles at 700 nm and
at 500 nm are shown in Figure 6C and 6D. The 700 nm traces
exhibit a fast-growth rising component (τ = 8.3 ps) that decays
with a lifetime (τ = 25 ps) that matches the rise time
component of the species monitored at 500 nm (τ = 26.3 ps),
and it is attributed to the population of 3*pPDIp. These results
show that the triplet state {(phen)2Ru(II) (pPDIp3*)} is
reached irrespective of the moiety that is electronically excited
(pPDIp or the {Ru(phen)3}2+ moieties). By excitation of the
{Ru(phen)3}2+ moiety in the 1*MLCT state, a very fast process
(not detectable with the time resolution of the equipment)
occurs to generate the triplet state 3*MLCT. From this state, a
fast conversion to the pPDIp•− radical anion occurs with a rise
time of 8.3 ps that decays by two consecutive paths: a very fast
one with lifetime decay of 1.2 ps due to a conformation
relaxation process and another with a lifetime decay of 28.1 ps.
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Figure 6. Transient absorption spectra at 0.213 ps (gray), 0.320 ps (red), and 0.533 ps (blue) (A) and 0.867 ps (orange), 1.59 ps (red), and 30.2 ps
(yellow) (B) after femtosecond-pulse excitation at 450 nm. The kinetics of growth and decay (••••) and fit (red) of the radical anion pPDIp•− at
700 nm (C) and 3pPDIp* at 505 nm (D) and the corresponding weighted residues.
Figure 7. (A) Transient absorption spectra of the complex collected at 120 ns (red), 200 ns (green), 280 ns (blue), 400 ns (pink), 600 ns (orange),
and 1000 ns (gray) by nanosecond-pulse excitation at 355 nm. (B) The decay (blue) and a single-exponential fit (green) of transient absorption of
{(phen)2Ru(II) (pPDIp3*)} at 500 nm for deaerated dilute solution (4.0 × 10−6 mol L−1) in CH3CN and the weighted residuals (χ2 = 1.01).
Upon excitation with nanosecond pulses of 355 nm light
only a positive absorption due to the formation of 3*pPDIp is
observed (Figure 7A). The decay at 505 nm gives a lifetime of
1.8 μs attributed to the phosphorescence of pPDIp in the
absence of oxygen. This resembles the lifetime of 100 ns
observed by the SPT technique in aerated solutions of the
complex in acetonitrile.
Qualitative Energy Level Diagram of Excited States of
Complexes. The results presented herein demonstrate that
the triplet state of 3*pPDIp is reached irrespective of excitation
in the pPDIp or {Ru(phen)3}2+ moieties. In general, for Ru(II)
polypyridine complexes such as [Ru(phen)3]2+,39,40 the
population of the 1*MLCT is followed by an efficient ISC to
the 3*MLCT. Nevertheless, the difference absorption spectra
The latter decay corresponds to the rise-time component of the
3
*pPDIp triplet state with maximum at 505 nm (23 ps)
contaminated by the ground-state bleaching. Indeed, when
observed in the low energy transient absorption band of the
3
*pPDIp triplet at 635 nm, it gives a value of 28 ps that is equal
to the decay of the pPDIp•− radical anion. These results
support a charge-transfer mechanism instead of a resonance
energy transfer (RET)58 from the 3*MLCT to the pPDIp in the
ground state. RET processes can occur by Coulombic (Förster
mechanism) and/or exchange coupling (Dexter mechanism)
mechanisms: both are inefficient because the Fö r ster
mechanism is spin-forbidden and the wave function overlap
required for an efficient exchange process is very small.
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Scheme 2. Scheme of the Processes Occurring after Electronic Excitation of the [(phen)2Ru(II) (pPDIp)]2+ Complex
and the fast kinetics indicate the involvement of the chargeseparated intermediate state {(phen)2Ru(III) (pPDIp•−)}2+
regardless of the moiety of complex being electronically excited.
The driving force (ΔG°ET) for the intramolecular electron
transfer involved in both paths was estimated based on the
Rehm−Weller equation59 using the electrochemical data and
the lowest energy excited state (E0−0) values shown in Table 1.
A diagram displaying the energy levels of the states involved in
the processes upon electronic excitation of the dyad is shown in
Scheme 2 (Coulombic interactions were neglected). Upon
excitation of the dyad to the 1*MLCT state by light of
wavelength λ < 450 nm, a fast intersystem crossing occurs to
generate the 3*MLCT state in a nonequilibrium conformation.
After thermal equilibration, an electron is transferred from
Ru(III) to the pPDIp (τ = 8.3 ps) to yield the charge-separated
intermediate state {(phen)2Ru(III)pPDIp•−}2+ with charge
separation energy of +1.75 eV (ΔGCS = −0.35 eV). This
intermediate decays (τ = 23 ps) to generate the triplet state
3
*pPDIp.
Using excitation light with wavelength higher than 525 nm
the unique species that absorbs is the pPDIp moiety of the
dyad. The 1*pPDIp singlet decays rapidly to generate the
pPDIp•− radical anion (τ = 2.9 ps) by an electron transfer
reaction from Ru(II) to 1*pPDIp (ΔGCS = −0.61 eV).
Similarly, the charge-separated state decays (τ = 28 ps) to
generate the 3*pPDIp. The difference in the lifetime values is
related with the contamination of the decays as was discussed
before. The phosphorescence lifetime of the 3*pPDIp was
determined by nanosecond-flash photolysis as τP = 1.8 μs in
CH3CN deaerated by N2 bubbling solutions.
In conclusion, the spectroscopic properties of the complex
enable us to efficiently produce the triplet state of pPDIp in a
large optical window (350−525 nm) by excitation either in the
pPDIp or in the Ru(II) moieties. Furthermore, the chemical
and photochemical stability and the photophysical properties of
the dyad open the possibility for electron- and energy-transfer
processes with potential namely in catalysis.
Photoinduced Energy Transfer Processes. The longlived 3*pPDIp moiety of the complex was efficiently quenched
by ground state molecular oxygen (3Σg) to form the
corresponding singlet oxygen (1Δg) via an energy-transfer
process between the 3*pPDIp moiety and the triplet ground
state of molecular oxygen. Figure 8 shows the luminescence
decays collected at 500 nm by nanosecond flash photolysis in
Figure 8. Decay of the transient signal at 500 nm of the dyad upon
nanosecond-laser excitation at 355 nm for air equilibrated (black) and
deaerated solution (blue). The decays were fitted with a single
exponential function for air-equilibrated (red) and deaerated (green)
solutions. The quality of the fit was judged by the weighted residuals
(χ2 = 1.0, for both fits).
the presence of oxygen (air equilibrated solutions) and after
degassing with N2. The decays can be well fitted with a
monoexponential with 280 ns (air equilibrated) and 1.8 μs
(deaerated by N2 bubbling) solutions, respectively. The
quantum yields for singlet oxygen sensitization were obtained
by time-resolved measurements in aerated CH3CN solutions
(upon excitation at 355 or 532 nm), monitoring the
phosphorescence of the singlet oxygen at 1270 nm as a
function of laser intensity, using phenalenone, [Ru(phen)3]2+,
and rose bengal as reference sensitizers. The singlet oxygen
decay is monoexponential with lifetime of 70 μs. The 1O2
phosphorescence intensity varies linearly with the laser
intensity, thus discarding the occurrence of triplet−triplet
annihilation (Figures S6 and S7). The quantum yield of singlet
oxygen sensitization is ϕΔ = 0.57. A similar value was found
upon excitation in the pDPIp moiety at 532 nm. These values
are comparable to those obtained for the singlet oxygen
sensitization of PDI diimide derivatives.60−62
Chemical and Photoinduced Electron-Transfer Reactions. In this section, the ability of the dyad to participate in
intermolecular electron-transfer reactions using triethylammine
(NEt3) as a chemical sacrificial electron donor was studied.
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Figure 9. Variation of the absorption spectrum of the [Ru(phen)2(pPDIp)]2+ complex and NEt3 (2:1, complex:NEt3 ratio) in CH3CN. Reaction in
the dark (Top A) changes in the maximum absorbance of pPDIp → pPDIp•− moiety of complex over time (Top B). Photolysis with 420 nm light
irradiation (Bottom A) changes in the maximum absorbance of the pPDIp•− → pPDIp2− moiety of complex over time (Bottom B).
spectra show a progressive depletion of the absorption band of
the pPDIp moiety of the complex at 525, 489, and 476 nm
concomitant with growth of absorption at 704, 796, and 953
nm assigned to the [Ru(phen)2(pPDIp)•−]+ radical anion.
Isosbestic points at 545 nm were observed as the color of
solution changed from orange to green. The plots of changes in
the maximum absorption at 704 (consumption of green
solution) and 565 nm (formation of violet solution) versus
time were equivalent, suggesting a one-to-one correspondence.
The chemical reduction of the pPDIp0 moiety by NEt3 in
deaerated solutions is consistent with the electrochemical
reduction potential values of NEt363,64 and complex. Thus, the
complex in its ground state can oxidize NEt3 to its aminium
cation with the formation of [Ru(phen)2(pPDIp•−)]+, which is
found to be very stable in the absence of oxygen.65
In these experiments, dilute solutions of the complex (∼2.2
× 10−5 mol L−1) in acetonitrile were prepared to avoid
aggregation, and the reactions were followed by UV−vis
absorption. Typically, the procedure was performed in four
steps: (1) in the dark, NEt3 was added to the solution of
complex in the ratios of 1:1 or 1:2 under stirring, and oxygen
was removed from the solution by N2 bubbling during 20 min;
(2) the UV−vis spectral changes were followed in time until a
final stable spectrum was reached; (3) the same solution was
irradiated with continuous light at 420 or 520 nm; (4) the
intermolecular electron-transfer reaction was followed in time,
and the changes in the absorption spectrum were recorded until
a final stable spectrum was reached.
Reaction in the Dark. Figure 9(Top A and B) shows the
spectroscopic changes over time seen when NEt3 was added to
the solution of the complex (1:1 NEt3 to complex ratio). The
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Figure 10. Spectral changes during reaction in the dark observed after addition of MV2+ to the photolyzed solution of the complex−NEt3 mixture.
Scheme 3. Photographs Showing the Changes in the Color of the Complex during Reductive and Oxidative Cycling Processes
Photolysis. By irradiation of this solution with 420 nm light,
the absorbance of [Ru(phen)2(pPDIp•−)]+ (green solution)
increased until a plateau was reached. During prolonged
continuous light irradiation, the green solution turned violet
with the formation of [Ru(phen)2(pPDIp)2−]. The UV−vis
spectroscopic changes of this solution are shown in Figure 9
(Bottom A and B). The plots of changes in the maximum
absorption at 525 (consumption of orange solution) and 702
nm (formation of green solution) versus time were equivalent,
suggesting a one-to-one correspondence. If irradiation is ceased
after the pPDI•− and the pPDIp2− moieties of complex are
formed, the absorption spectrum does not change for at least 6
h in the absence of oxygen, showing the high stability of these
species. However, in the presence of oxygen, the absorption
spectra evolve to the initial spectrum of the complex−NEt3
solution, indicating that oxygen acts as an electron acceptor.
The same behavior was observed when this experiment was
repeated for the 2:1 complex:NEt3 ratio (Figure S8).
At 520 nm irradiation, using the 2:1 complex:NEt3 ratio, only
the green species is formed, indicating that the optical density
of the radical anion at this wavelength is very small to reach its
excited state (Figure S9).
Based on these results, a possible pathway for the generation
of the dianion [Ru(phen)2(pPDIp2−)] is through a second
reduction of the electronic excited state of the radical anion
[(phen)2Ru(pPDIp•−)]+. This process occurs only through the
electronic excited state and, consequently, only by photolysis
with 420 nm. The aminium cation (•NEt3+) is very reactive
and, as suggested by Whitten,66 can react with another
molecule of NEt3 to produce the alkyl radical [CH3Ċ HNEt2],
which can be the reductant of the electronically excited anion.
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opens the opportunity to use this complex as a catalyst in
organic synthesis.
It is interesting to emphasize that the complex is a good
oxidizing agent, whereas the {Ru(phen)2(pPDIp)2−} dianion is
a good reducing agent (Ered = −0.44 V vs Ag+/AgCl), which
indicates that the reverse process can occur in the presence of a
suitable electron acceptor. To confirm this possibility, the
procedure was repeated using the electron-acceptor N,Ndimethyl-4,4′-bipyridinium chloride (MV2+, Ered = −0.687 V vs
Ag+/AgCl).67
In this experiment, the MV2+ was added to the solution of
complex and NEt3 only after photolysis of the mixture, allowing
evaluation of NEt3−complex interaction without interference of
MV2+.
Figure 10 shows the UV−vis absorption spectral changes in
the dark after addition of MV2+ to the photolyzed solution
containing NEt3 and complex. The broad absorption band with
maximum at 605 and 394 nm appeared immediately after
addition of MV2+ to the solution and increased over time until a
stable spectrum was reached. This new absorption closely
matches the methylviologen radical absorption spectrum68,69
(MV2+ → MV•+, blue solution). The formation of MV•+ and
the consequent consumption of the dianion were too fast to be
observed with our equipment. However, the slow back reaction
to MV2+ in the dark was followed by UV−vis. The same
behavior was observed when this experiment was repeated for
the 1:1 complex:NEt3 ratio.
The changes in solution color during the reductive reaction
in the dark and under light irradiation of [Ru(phen)2(pPDIp)]2+ are depicted in Scheme 3.
Both the anion and dianion are very stable because the two
terminal phenanthrolines stabilize the double negative charges
on the PDI. The formation of stable perylene dianion
derivatives by chemical and electrochemical processes was
already observed.70,71 Indeed, to our knowledge, a perylene
dianion derivative generated through a photoinduced electrontransfer reaction was first reported by Wasielewski72 for a D−
A−D triad consisting of a PDI and two porphyrin moieties.
Recently, it was shown that the PDI dianion catalyzes the
reduction of stable aryl chlorides producing aryl radicals, which
were trapped by hydrogen atom donors or used in carbon−
carbon bond formation reactions.73,74
Herein, it was shown that, in the presence of NEt3, the
reduction of the complex occurs to form the radical anion in
the dark and that under light irradiation the reaction is faster.
The photolysis of the radical anion generates the dianion in
quantitative amounts in a controlled way. Moreover, the
electrons resulting from the reduction reaction and the energy
accumulated can be used to reduce MV2+ to MV•+ and recover
the complex−NEt3 mixture in a cyclic process.
■
ASSOCIATED CONTENT
* Supporting Information
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpcc.6b06693.
Detailed experimental results, IR and 1H NMR spectra,
and all the NMR data: 1D (1H, 13C) and NMR
characterization of complex:2D homonuclear (1H−1H
COSY) and heteronuclear (1H−13C-HSQC, 1H−13CHMBC, 1H−15-HMBC) (PDF)
■
AUTHOR INFORMATION
Corresponding Authors
*Rose Maria Carlos ([email protected]). Phone: +55 16 33518780. Fax: +55 16 3351-8350
*José Manuel Gaspar Martinho ([email protected].
pt). Phone: + 351 2184192500.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The authors would like to acknowledge FAPESP (process n°.
2011/10882-5 and 2013/23943-8, 2014/12538-8), CNPq, and
CAPES for the grants and fellowships received. The Fundaçaõ
para a Ciência e a Tecnologia (FCT, Portugal) is acknowledged
for financial support (UID/NAN/50024/2013 and PEst-OE/
QUI/UI0313/2014). J. Pina acknowledges a postdoc fellowship
(ref SFRH/BPD/108469/2015) from FCT. The authors are
indebted to Rafael Cavalieri Marchi, Isabele Aparecida Soares
de Campos, and Mariana Pigozzi Cali, for their help with the
conduction of the electron-transfer experiments.
■
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CONCLUSION
The dyad [Ru(II) (phen)2(pPDIp)]2+ expands the potential
applications of PDIs and PDI derivative complexes. Upon
electronic excitation in both the absorption bands of the dyad,
the triplet state of pPDIp is reached through the recombination
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■
NOTE ADDED AFTER ASAP PUBLICATION
The Table of Contents and Abstract graphics have been
updated and the revised version was posted on October 13,
2016.
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DOI: 10.1021/acs.jpcc.6b06693
J. Phys. Chem. C 2016, 120, 22831−22843