Emission Intensity Enhancement for Iridium(III) Complex in Dimethyl Sulfoxide under Photoirradiation

Article abstract of DOI:10.1021/acs.jpcb.1c03753

We found emission intensity enhancement for fac-Ir(ppy)3 (ppy = 2-(2′-phenyl)pyridine) in aerated dimethyl sulfoxide (DMSO) during photoirradiation for the first time. This phenomenon was concluded to be responsible for the consumption of 3O2 dissolved in DMSO through dimethyl sulfone production by photosensitized reaction using fac-Ir(ppy)3. A 3O2 adduct of DMSO molecule was detected by UV absorption measurement and theoretical calculation. We proposed a mechanism for the emission enhancement reaction including 1,3O2 molecules and 1,3O2-DMSO adducts and validated it through a simulation of emission intensity change using an ordinary differential equation solver.

Full text of DOI:10.1021/acs.jpcb.1c03753

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Article
Emission Intensity Enhancement for Iridium(III) Complex in
Dimethyl Sulfoxide under Photoirradiation
Shingo Hattori, Shuntaro Hirata, and Kazuteru Shinozaki*
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ABSTRACT: We found emission intensity enhancement for fac-Ir(ppy)3
(ppy = 2-(2′-phenyl)pyridine) in aerated dimethyl sulfoxide (DMSO)
during photoirradiation for the first time. This phenomenon was
3
concluded to be responsible for the consumption of O2 dissolved in
DMSO through dimethyl sulfone production by photosensitized reaction
using fac-Ir(ppy)3. A 3O2 adduct of DMSO molecule was detected by UV
absorption measurement and theoretical calculation. We proposed a
mechanism for the emission enhancement reaction including 1,3O2
molecules and 1,3O2-DMSO adducts and validated it through a simulation
of emission intensity change using an ordinary differential equation solver.
toxicity, a high polarity, and a capability of dissolving many
compounds,20−22 whereas there is no report on the role of
DMSO in the photosensitization process.
INTRODUCTION
■
Transition metal complexes attract great attention not only to
inorganic chemists but also material, medicinal, and organic
chemists because of the strong emission suitable for organic
light emitting diode (OLED),1−7 the photodynamic therapy
(PDT) using singlet oxygen,8,9 and the discovery of new
synthetic method using their photocatalytic ability,10−22
respectively. In particular, phosphorescent metal complexes
have been actively developed for application utilizing their
characteristics of the long-lived strong photoemission. fac-
Ir(ppy)3 (ppy = 2-(2-phenyl)pyridine), one of the typical
examples of them, has been vigorously investigated since the
demonstration of OLED green emitter by Baldo et al. in 1990.4
The long-lived triplet excited state of fac-Ir(ppy)3 is also
expected to be employed to the emission probe,23,24 the
photosensitization for photodynamic therapy and photo-
catalysis of the photochemical reactions.
Here, we report an emission intensity enhancement of fac-
Ir(ppy)3 in DMSO solution containing O2 under photo-
irradiation for the first time. The enhancement is not
confirmed in deoxygenated DMSO solution of fac-Ir(ppy)3
and other organic solvents. Moreover, when employing a
DMSO solution containing [Ru(bpy)3]2+ (bpy = 2,2′-
bipyridine) instead of fac-Ir(ppy)3, the emission enhancement
is observed as well. These results suggest that a photoreaction
of DMSO molecule with dissolved oxygen takes place by the
aid of excited fac-Ir(ppy)3 as a photosensitizer. 1H NMR
indicates that the product of a photochemical reaction is
dimethyl sulfone (DMS). 3O2-DMSO adduct is a key
intermediate of the photoproduction of DMS, supported by
experimental electronic absorption spectrum and theoretical
calculations using time-dependent density functional theory
(TD-DFT). A proposed mechanism may provide new insight
for understanding photocatalytic organic syntheses using fac-
Ir(ppy)3 in DMSO (Scheme 1).
The excited fac-Ir(ppy)3 effectively transfers its energy to
ground oxygen molecule (3O2) via the Dexter’s mechanism,
where the overlapping of electronic clouds between fac-
3
Ir(ppy)3* and O2 induces the production of singlet oxygen
molecule (1Δg) by the direct electron exchange. In 2019, Soni
et al. reported a new synthesis method to afford spiro-
1
azalactams analogues using the photosensitized O2 by fac-
Ir(ppy)3 in dimethyl sulfoxide (DMSO).20 Also, the same
1
Received: April 26, 2021
Revised: July 25, 2021
photosensitization system of fac-Ir(ppy)3 and O2 in DMSO
has been applied to PDT inducing a cell death.21 Although the
versatility of fac-Ir(ppy)3 in a photochemical reaction has been
demonstrated, the reaction mechanism has not been
investigated in detail so far. DMSO has been widely used as
a solvent of organic syntheses and PDTs because of a low
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Scheme 1. Reaction Scheme for Production of DMS from DMSO and O2 by the Photosensitization of fac-Ir(ppy)3
Figure 1. (a) Excitation and (b) emission spectra of fac-Ir(ppy)3 (2.20 × 10−5 M) in Ar (red), air (black), and O2 (blue) saturated DMSO at room
temperature. Inset is the emission spectra normalized at peaks, which are identical to each other.
dried using a rotary evaporator, and the solid was recrystallized
METHODS
■
from dichloromethane/hexane. The yield of fac-Ir(ppy)3 was
Materials. fac-Ir(ppy)3 was synthesized as reported in a
1
45% (47 mg, 0.07 mmol). H NMR (400 MHz, CD3CN) δ
previous method.6 A mixture of IrCl3·nH2O (101 mg, 0.34
mmol) and 2-phenylpyridine (108 mg, 0.70 mmol) in a mixed
solvent of water (1.5 mL) and 2-ethoxyethanol (4.5 mL) was
stirred at 105 °C under Ar atmosphere for 15 h. After cooling
to room temperature, the resulting precipitate was filtered off
and washed with methanol. The yield of [Ir(ppy)2Cl]2 was
65% (123 mg, 0.11 mmol). 1H NMR (400 MHz, CD2Cl2) for
[Ir(ppy)2Cl]2: δ 9.25 (d, 1H, J = 4.8 Hz), δ 7.94 (d, 1H, J = 8.0
Hz), δ 7.80 (t, 1H, J = 7.8 Hz), δ 7.56 (d, 1H, J = 8.0 Hz), δ
6.85−6.79 (m, 2H), δ 6.60 (t, 1H, J = 7.4 Hz), δ 5.87 (d, 1H, J
= 8.0 Hz). Following the previous methods, [Ir(ppy)2Cl]2 (85
mg, 0.08 mmol), 2-phenylpyridine (59 mg, 0.38 mmol),
acetylacetone (17 mg, 0.17 mmol), and triethylamine (21 mg,
0.21 mmol) were added to ethylene glycol (5 mL) and the
solution was degassed by Ar bubbling prior to heat at 175 °C
with stirring for 20 h. After cooling to room temperature, the
precipitate was filtered off, washed with water and ethanol, and
then purified by passing through a silica gel column followed
by elution with chloroform. The resulting yellow solution was
8.01 (d, 1H, J = 8.4 Hz), δ 7.75−7.71 (m, 2H), δ 7.60 (d, 1H, J
= 5.2 Hz), δ 7.00 (t, 1H, J = 6.4 Hz), δ 6.86 (t, 1H, J = 7.4
Hz), δ 6.75 (t, 1H, J = 7.0 Hz), δ 6.69 (d, 1H, J = 8.4 Hz).
Instrumental Techniques. 1H NMR spectra were
measured using a Bruker AVANCE III HD (400 MHz) and
analyzed by a TopSpin 3.5 pl7 software. UV−vis spectra were
measured using a JASCO spectrophotometer V-530ST.
Emission and excitation spectra were recorded on a JASCO
spectrofluorometer FP-6500ST equipped by a Peltier thermo-
controller. Sample solutions in a quartz cell attached with a
greaseless valve were aerated, degassed, or oxygenated by the
appropriate gas bubbling for 20 min prior to the UV and
emission measurements. Emission lifetimes were determined
through single-exponential curve-fitting of emission intensity
data recorded over time with a Tektronics TBS 1052B-EDU
digital oscilloscope after pulsed excitation at 355 nm using a
Continuum Surelite Nd:YAG laser. Detection of emission
intensity during photoirradiation was carried out using a
B
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JASCO spectrofluorometer FP-6500ST. During photoirradia-
tion with monochromatic 380 nm light from 150 W xenon
lamp, emission intensity change was measured while stirring
the gas-saturated solution and the temperature was adjusted
using a Peltier thermocontroller. White light of a USIO Xe
short-arc lamp (150 W) through water as a thermal-cut filter
was employed to prepare a photoproduct. The obtained
photoproduct was monitored with a Bruker NMR AVANCE
III HD (400 MHz).
(see Supporting Information for details on Stern−Volmer
plot).
Figure 2 shows time-profiles of emission intensity of fac-
Ir(ppy)3 at 523 nm under photoirradiation of O2 saturated
Computational Details. The geometry optimization was
carried out by density functional theory (DFT) calculation.
The B3LYP as a functional and the basis set of 6-31G* for H,
C, N, O, S and LANL2DZ for Ir were employed in the
calculation. Polarizable continuum model (PCM) was taken
into account for the solvent effect by DMSO on total energy.
The absorption spectra were simulated by time-dependent
density functional theory (TD-DFT) calculation using the
optimized structures. Here the B3LYP as a functional and the
basis set of 6-31G* for H, C, N, O, and S were employed in the
calculation. All calculations were carried out by Gaussian 16
package. The fitting simulation of differential equations was
carried out using MATLAB.
Figure 2. Time-profiles of emission intensity at 523 nm of fac-
Ir(ppy)3 in O2 saturated DMSO (red), acetonitrile (orange), ethyl
acetate (green), toluene solution (blue) under photoirradiation (2.02
× 10−5 M, 293 K, λex = 380 nm). Here, the intensities at t = 0 s are
normalized to unity.
DMSO, acetonitrile, ethyl acetate, and toluene solution (2.02
× 10−5 M) at 293 K for 10 800 s using UV light (λex = 380
nm). Surprisingly, the emission intensity of fac-Ir(ppy)3 was
enhanced under photoirradiation in O2 saturated DMSO
solution while that did not change in other solvents such as
acetonitrile, ethyl acetate, and toluene. The remarkable
emission intensity change observed after photoirradiation to
fac-Ir(ppy)3 in O2 saturated DMSO solution showed an almost
linear line as a function of t. From the slope of the straight line,
the rate in emission enhancement was determined to be 1.06 ×
10−3 s−1. For other solvents, the rates evaluated from the
respective slopes were negligibly small, 3.08 × 10−6 s−1
(acetonitrile), 3.14 × 10−6 s−1 (ethyl acetate), and −5.10 ×
RESULTS AND DISCUSSION
■
Emission Intensity Enhancement for fac-Ir(ppy)3.
Figure 1a,b shows excitation and emission spectra of 2.02 ×
10−5 M fac-Ir(ppy)3 in air, Ar, and O2 saturated DMSO
solutions at 293 K. The excitation peaks attributed to a
transition from metal to ligand charge transfer (MLCT) were
observed at 380 nm (Figure 1a), and the emission peaks
assigned to MLCT phosphorescence were observed at 523 nm
(Figure 1b).2,5 For emission spectra, the spectral profiles are
consistent with each other showing an emission peak at 523
nm although the emission intensities are observed to be in the
order of O2 < air < Ar saturated DMSO. Emission lifetimes
evaluated from curve fitting of emission decay curves as τ =
1.38 μs (Ar), 182 ns (air), 35.4 ns (O2). The variation in
emission lifetime is mainly responsible for the emission
10−6 −1 (toluene), which are 1000 times smaller than that for
s
DMSO solution. Since the entire solutions were O2 gas
saturated, emission from photoexcited fac-Ir(ppy)3 (fac-
Ir(ppy)3*) should be strongly quenched by the triplet−triplet
energy transfer, which are frequently observed for phosphor-
escent materials. This emission enhancement suggests that the
photoirradiation to fac-Ir(ppy)3 in DMSO solution can
suppresses the emission quenching by 3O2. The emission
intensity enhancement was observed even in a DMSO solution
containing [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) instead of fac-
Ir(ppy)3 (Figure S2).
Emission enhancement was observed with changing
excitation wavelength by every 20 nm from 280 to 480 nm
for an O2 saturated DMSO solution containing fac-Ir(ppy)3
(Figure 3a). Figure 3b shows action spectra of the photo-
reaction at 0, 2000, 4000, 6000, 8000, and 10000 s, which are
plots of emission intensity against the irradiation light
wavelength. The action spectrum at t = 0 corresponds to the
excitation spectrum of fac-Ir(ppy)3, consistent with absorption
spectrum of fac-Ir(ppy)3 (Figure 6). These plots clearly show
that the emission intensity increases along with irradiation
time. Here, the emission intensity was increased by irradiation
at emission peak, indicating that fac-Ir(ppy)3* as a photo-
sensitizer plays a key role of the emission enhancement and the
enhancement is accelerated with increasing the number of fac-
Ir(ppy)3*. Figure 4 shows plots of emission intensities against
irradiation time when the concentration of fac-Ir(ppy)3 was
varied from 4.00 × 10−6 to 1.00 × 10−4 M in O2 saturated
DMSO. Emission intensity was increased by increasing the
3
quenching by dissolved O2. Assuming the quenching is a
diffusion-controlled process where quenching rate constant
(kq) is typically kq = 1 × 109 to 1 × 1010 M−1 s−1,25,26 the
concentration of the quencher is roughly estimated as 3 × 10−3
to 3 × 10−2 M from the Stern−Volmer relation (see
Supporting Information for details on Stern−Volmer relation).
On the other hand, self-quenching of emission is well-known
for fac-Ir(ppy)3 because of the long-lived triplet excited state.
Emission decays of fac-Ir(ppy)3 in DMSO were measured
while changing the concentration from 1.01 × 10−5 M to 6.05
× 10−5 M. In polar solvent, the self-quenching of emission is
caused by the ground fac-Ir(ppy)3 but not triplet−triplet
annihilation.27−31 Evaluated emission lifetime from curve
fittings using single exponential functions are listed in Table
1. From a Stern−Volmer plot for the self-quenching (Figure
S1), the self-quenching rate and the lifetime at infinite dilution
are determined as ksq = 1.25 × 109 M−1 s−1 and τ0 = 1.45 μs
Table 1. Concentration Dependence in Emission Lifetime of
fac-Ir(ppy)3 in Ar Saturated DMSO
[fac-Ir(ppy)3]/M 6.05 × 10−5 4.03 × 10−5 2.02 × 10−5 1.01 × 10−5
τ/μs
1.31
1.35
1.41
1.42
C
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Figure 3. (a) Time-profiles of emission intensity at 523 nm of fac-Ir(ppy)3 at a variety of excitation wavelengths in O2 saturated DMSO, and (b)
action spectra for emission intensity enhancement of fac-Ir(ppy)3 (2.02 × 10−5 M) in O2 saturated DMSO upon photoirradiation with the
excitation spectrum (black line). In (a), the intensities at t = 0 s are normalized to unity.
100/1) solution without fac-Ir(ppy)3 (Figures S3 and S4).
These results clearly show that the photoirradiation of fac-
Ir(ppy)3 in O2 saturated DMSO provides DMS without
changing the structure of fac-Ir(ppy)3 (Scheme 1). The
emission intensity of fac-Ir(ppy)3 increases by the consump-
tion of O2 in this reaction.
Detection of 3O2 Adduct. Figure 6a shows UV−vis
absorption spectra of fac-Ir(ppy)3 (2.20 × 10−5 M) in air, Ar,
Figure 4. Time-profiles of emission intensity at 523 nm of fac-
Ir(ppy)3 at a variety of concentrations in O2 saturated DMSO (293 K,
λex = 380 nm).
concentration, which is consistent with the results of excitation
wavelength dependence in emission enhancement.
1H NMR measurement was measured for fac-Ir(ppy)3 in a
mixture of DMSO-d6 and DMSO-h6 (v/v = 100/1) before and
after photoirradiation as shown in Figure 5. The spectrum in
Figure 6. (a) UV−vis absorption spectra of fac-Ir(ppy)3 in Ar, air, O2
saturated DMSO. (b) UV absorption spectra of Ar, air, O2 and CO2
gas saturated DMSO. The spectrum of air saturated DMSO was
employed as a baseline. (c) A difference spectrum (black) between O2
saturated and Ar saturated DMSO and that (red) containing fac-
Figure 5. 1H NMR of fac-Ir(ppy)3 in 500 μL of DMSO-d6 containing
Ir(ppy)3.
5 μL of DMSO-h6 before (red) and after photoirradiation (blue), and
1H NMR of DMS in DMSO-d6 (green).
and O2 saturated DMSOs measured at room temperature
(method for details on preparation of variable saturated oxygen
the region of aromatic proton for fac-Ir(ppy)3 after photo-
irradiation was very similar to that before photoirradiation. On
the other hand, a new singlet peak was observed at 2.99 ppm.
This peak can be assigned to protons of dimethyl sulfone
(DMS), because it is in good agreement with that of
commercial DMS in DMSO-d6 (Figure 5, green). On the
other hand, the peak at 2.99 ppm was not observed after
photoirradiation of O2-saturated DMSO-d6/DMSO-h6 (v/v =
concentration in DMSO). The π−π* absorption bands at
around 280 nm and MLCT bands at around 380 nm were
observed for the entire solutions.18 Although these absorption
spectra resemble each other, absorption coefficients in the
shorter wavelength region (λ < 380 nm) are in the order Ar <
air < O2 saturated solutions. For air- and O2-saturated
solutions, a distinctive shoulder at 260 nm was observed
(Figure 6c, Figure S5). Peak wavelengths (λabs) and absorption
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coefficients (ε) at peaks for the absorption spectra are listed in
Table 2. When the once O2-saturated solution was purged with
Table 2. Absorption Coefficients (ε) at Peak in UV-vis
Spectra of fac-Ir(ppy)3 in Ar, air, O2 Saturated DMSO
ε/M−1 cm−1 (λabs/nm)
ε/M−1 cm−1 (λabs/nm)
Ar
air
O2
4.98 × 104 (284)
5.50 × 104 (283)
7.14 × 104 (281)
1.41 × 104 (377)
1.41 × 104 (377)
1.42 × 104 (376)
Ar gas to remove dissolved O2, the π−π* absorption band was
decreased in intensity to be consistent with that of the Ar
saturated solution. This trend in ε at around 280 nm is
associated with the concentration of dissolved O2 in DMSO.
However, O2 molecule does not absorb light in the region of
240−380 nm light but <240 nm. Since absorption spectra (λ >
380 nm) for the entire gas-saturated solutions are consistent
with each other, it is suggested that the change in absorption
spectra is not attributed to the O2 adduct formation of fac-
Ir(ppy)3 but some reaction of O2 with DMSO as a solvent.
UV absorption spectra for air, Ar, O2, and CO2 saturated
DMSO at room temperature using the spectrum of air
saturated DMSO as a baseline are shown in Figure 6b. For
Ar and CO2 saturated DMSO, the spectral profiles are
coincident with each other, and absorption coefficients below
350 nm are negative and peak wavelengths are both 264 nm.
On the contrary, for the O2 saturated DMSO the absorbance is
positive even below 350 nm and shows a peak at 262 nm.
These results strongly suggest that the absorption band at
around 260 nm rose from the adduct formation of O2 and
DMSO, which takes place even in the air saturated DMSO.
Since the absorption intensity at 260 nm is easily modulated by
the O2 concentration, O2 and DMSO would be loosely
attached with each other to produce an adduct via the
chalcogen-chalcogen interaction (O2-DMSO),32−34 and the
O2-adduct and O2 would be in an equilibrium state. Figure 6c
shows a difference spectrum by subtraction of the spectrum of
O2 saturated DMSO from that of Ar saturated one, and
another difference spectrum prepared by the subtraction of the
spectrum of O2 saturated DMSO containing fac-Ir(ppy)3 from
that of Ar saturated one. These difference spectra are very
similar to each other, suggesting that the O2-DMSO adduct is
stable even in the coexistence of fac-Ir(ppy)3 and the
interaction between fac-Ir(ppy)3 and the adduct is negligibly
weak. Assuming the ε = 100−1000 at 260 nm, the
1
Figure 7. Calculated absorption line spectra of DMSO (black) and
3O2-DMSO (red) in DMSO. No absorption band of 3O2 was
predicted to appear in the region of 200−300 nm. Red solid line is a
calculated absorption spectrum of 3O2-DMSO obtained by multi-
plying the line spectrum by a Gaussian function.
10−4, where HOMO-2 is a localized orbital on DMSO moiety
and LUMO is localized on O2 as shown in Figure S6. These
cacluations support the formation of O2-DMSO in O2-
saturated DMSO solution.
Reaction Mechanism. To clarify the detailed reaction
mechanism, a quantum chemical calculation based on the
density functional theory (DFT) was carried out. The energy
of O2, DMSO, O2-DMSO, and DMS in optimized structures is
shown in Figure 8. Here, the energy predicted by the
calculation is relatively described using the sum of these
energies as an offset (E(3O2, DMSO) = 0 kcal/mol). The
1
energy of singlet O2-DMSO (1O2-DMSO) was 34.0 kcal/mol
higher than the sum of the energy of singlet DMSO (1DMSO)
and triplet O2 (3O2). On the other hand, the energy of triplet
O2-DMSO (3O2-DMSO) was 0.41 kcal/mol lower than the
sum of the energy of 1DMSO and 3O2 (the equilibrium
constant is estimated as K = 1.04 M−1). This indicates that
1
3
3O2-DMSO is formed spontaneously from DMSO and O2.
This is consistent with the result that the observed UV
absorption in O2 saturated DMSO solution is due to the
formation of O2-DMSO. The singlet−triplet energy difference
in O2-DMSO was 34.4 kcal/mol. The lower energy difference
compared with the excitation energy of fac-Ir(ppy)3 (53.7 kcal/
mol) implies that the possibility of energy transfer from fac-
Ir(ppy)3*. The energy of singlet DMS was 23.3 kcal/mol lower
than that of 1O2-DMSO. Therefore, the reaction route (Figure
3
8, red) involving with O2-DMSO is proposed as follows: (1)
1
3
3O2-DMSO is spontaneously formed from DMSO and O2,
3
1
concentration of O2-DMSO adduct is estimated to be 6 ×
(2) O2-DMSO is generated by the energy transfer from fac-
10−5 −6 × 10−4 M.
Ir(ppy)3*, and (3) DMS is spontaneously produced from 1O2-
1
To clarify the origin of the absorption band observed at
around 260 nm in O2-DMSO, the simulation of absorption
spectrum of O2-DMSO using time-dependent density func-
tional theory (TD-DFT) calculation was carried out. Here, the
optimized geometry of O2-DMSO was employed for TD-DFT
calculation. Figure 7 shows calculated UV absorption spectra
DMSO. On the other hand, the reaction involving with O2 is
20,21
1
possible because fac-Ir(ppy)3 can generate O2,
and the
1
energy of O2-DMSO is 5.0 kcal/mol lower than the sum of
the energy of 1DMSO and 1O2. This reaction route is
illustrated as a blue broken line in Figure 8. (4) 1O2 is
1
generated by the energy transfer from fac-Ir(ppy)3*, (5) O2-
3
3
1
1
of O2-DMSO, DMSO, and O2 in DMSO. An absorption
DMSO is spontaneously formed from DMSO and O2.
To confirm the validity of the proposed reaction mechanism,
curve fitting of time-profiles of emission intensity at 523 nm of
fac-Ir(ppy)3 in O2 saturated DMSO using deferential equations
based on the proposed reaction mechanism (Scheme 1) was
carried out (Supporting Information for details on the
deferential equations). Figure 9a,b shows the emission
intensity enhancement curves as open circles depending on
the excitation wavelength and the concentration of fac-
3
band at 265.2 nm appeared for O2-DMSO alone, while the
other bands at 258.5, 239.2, and 226.5 nm of O2-DMSO are
very similar to those of DMSO. Any absorption bands of O2
3
1
3
does not appear in the region of 200−300 nm. The lowest
3
absorption at 265.2 nm of O2-DMSO is in good agreement
with the experimentally observed band at 265 nm for the O2
saturated DMSO. The electronic transition at 265.2 mn was
assinged to be mainly HOMO−2 → LUMO with f = 8.0 ×
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Figure 8. Schematic energy diagram for the photosensitization reaction affording DMS from DMSO and O2. Here, the energy predicted by the
1
calculation is relatively described using the sum of these energies as an offset (E(3O2, DMSO) = 0 kcal/mol).
Figure 9. (a) Excitation dependence in emission enhancement for fac-Ir(ppy)3 (2.02 × 10−5 M) in O2 saturated DMSO as open circles. Intensities
are normalized at t = 0. Simulation results are plotted as solid lines, which are changes in [Ir(ppy)3*] as a function of t. (b) Concentration
dependence in emission enhancement for fac-Ir(ppy)3 in O2 saturated DMSO as open circles. Intensities are normalized at t = 0. Simulation results
are plotted as solid lines, which are changes in [Ir(ppy)3*] as a function of t.
Figure 10. Simulation for the emission enhancement for fac-Ir(ppy)3 (1.00 × 10−4 M) in O2 saturated DMSO. Simulation results are displayed in
red lines against t, which are changes in concentration of species contributing to the photoreaction. The variations in concentration are obtained
using the rate constants of k0 = 2.40 × 107 s−1, k1 = 1.00 × 106 s−1, k2 = 6.69 × 108 M−1 s−1, k3 = 2.45 × 1010 M−1 s−1, k4 = 1.51 × 108 M−1 s−1, k−4
=
2.55 × 1010 s−1, k5 = 4.96 × 1010 M−1 s−1, k6 = 7.00 × 106 s−1, k7 = 9.00 × 108 M−1 s−1, k8 = 1.88 × 1010 M−1 s−1, k9 = 3.00 × 106 M−1 s−1, where
these rate constants are converted so that k1 matches the reciprocal of emission lifetime (t = 1 μs) of fac-Ir(ppy)3.
Ir(ppy)3, respectively, and their simulation results displayed as
solid lines. For both, the simulation and experimental result are
in very good agreement with each other. These results strongly
suggest the validity of proposed mechanism for the emission
intensity enhancement.
Figure 10 shows simulation results for emission intensity
enhancement in photoirradiation of 1.00 × 10−4 M fac-
Ir(ppy)3, where simulated values have been converted to
concentrations so as to be [Ir(ppy)3*] + [Ir(ppy)3] = 1.00 ×
10−4 M. [Ir(ppy)3*] increases its own concentration while
proceeding with the reaction, corresponding to the emission
intensity enhancement. This emission intensity enhancement is
reasonably accounted by the suppress of emission quenching
3
3
due to the decrease in concentrations of O2 and O2-DMSO
capable of quenching of emission from fac-Ir(ppy)3*. [3O2-
DMSO] is always three time larger than [3O2] during the
reaction, which indicates an equilibrium between them. The
3
3
concentrations of O2 and O2-DMSO suddenly decrease and
increase, respectively, at very early stage of simulated reaction.
These sudden changes in concentration are considered to
F
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correspond to the fast process toward the achievement of
equilibrium of the adduct formation of 3O2 with DMSO
Shuntaro Hirata − Graduate School of Nanobioscience,
Yokohama City University, Kanazawa-ku, Yokohama 236-
0027, Japan
1
1
molecule. For O2 and O2-DMSO species, being expectedly
very reactive, their concentrations are predicted to be almost
zero during the whole reaction process. This result suggests
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.jpcb.1c03753
1
1
that the O2 and O2-DMSO are the transient species and the
steady-state approximation can be applied to the d[1O2]/dt
and d[1O2-DMSO]/dt in the differential equations. [DMSO]
is considerably larger than those of other species and almost
constant during the reaction, which is agreement with the fact
that DMSO is solvent. The photoproduct DMS is gradually
increased in concentration with proceeding the reaction.
Therefore, we can summarize the emission enhancement
reaction during the photoirradiation of fac-Ir(ppy)3 in O2
saturated DMSO as Scheme 1.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by JSPS KAKENHI (Grants
JP17H06375 and JP21K14647).
REFERENCES
■
(1) Sprouse, S.; King, K. A.; Spellane, P. J.; Watts, R. J.
Photophysical Effects of Metal-Carbon Sigma Bonds in Ortho-
metalated Complexes of Iridium(III) and Rhodium(III). J. Am. Chem.
Soc. 1984, 106, 6647−6653.
(2) King, K. A.; Spellane, P. J.; Watts, R. J. Excited-state Properties
of a Triply Ortho-metalated Iridium(III) Complex. J. Am. Chem. Soc.
1985, 107, 1431−1432.
CONCLUSIONS
■
In this study, we found the emission intensity enhancement for
fac-Ir(ppy)3 in DMSO containing molecular oxygen. This
phenomenon was reasonably accounted by the 3O2 con-
sumption through dimethyl sulfone formation in the photo-
sensitized reaction due to fac-Ir(ppy)3*. To the best of our
(3) King, K. A.; Watts, R. J. Dual Emission from an Ortho-metalated
Iridium(III) Complex. J. Am. Chem. Soc. 1987, 109, 1589−1590.
(4) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. High-efficiency
Fluorescent Organic Light-emitting Devices Using a Phosphorescent
Sensitizer. Nature 2000, 403, 750−753.
(5) Hofbeck, T.; Yersin, H. The Triplet State of fac-Ir(ppy)3. Inorg.
Chem. 2010, 49, 9290−9299.
(6) Takayasu, S.; Suzuki, T.; Shinozaki, K. Intermolecular
Interactions and Aggregation of fac-Tris(2-phenylpyridinato-C2,N)-
iridium(III) in Nonpolar Solvents. J. Phys. Chem. B 2013, 117, 9449−
9456.
(7) Ito, W.; Hattori, S.; Kondo, M.; Sakagami, H.; Kobayashi, O.;
Ishimoto, T.; Shinozaki, K. Dual Emission from an Iridium(iii)
Complex/counter Anion Ion Pair. Dalton Trans. 2021, 50, 1887−
1894.
(8) Zamora, A.; Vigueras, G.; Rodríguez, V.; Santana, M. D.; Ruiz, J.
Cyclometalated Iridium(III) Luminescent Complexes in Therapy and
Phototherapy. Coord. Chem. Rev. 2018, 360, 34−76.
(9) Li, X.; Wu, J.; Wang, L.; He, C.; Chen, L.; Jiao, Y.; Duan, C.
Mitochondrial-DNA-Targeted Ir III -Containing Metallohelices with
Tunable Photodynamic Therapy Efficacy in Cancer Cells. Angew.
Chem., Int. Ed. 2020, 59, 6420−6427.
(10) Iqbal, N.; Jung, J.; Park, S.; Cho, E. J. Controlled
Trifluoromethylation Reactions of Alkynes through Visible-Light
Photoredox Catalysis. Angew. Chem., Int. Ed. 2014, 53, 539−542.
(11) Choi, S.; Chatterjee, T.; Choi, W. J.; You, Y.; Cho, E. J.
Synthesis of Carbazoles by a Merged Visible Light Photoredox and
Palladium-Catalyzed Process. ACS Catal. 2015, 5, 4796−4802.
(12) Chatterjee, T.; Iqbal, N.; You, Y.; Cho, E. J. Controlled
Fluoroalkylation Reactions by Visible-Light Photoredox Catalysis. Acc.
Chem. Res. 2016, 49, 2284−2294.
3
knowledge, we detected for the first time O2-DMSO adduct,
3
easily prepared at ambient condition when O2 is dissolved in
DMSO. Photoirradiation to 3O2 and triplet sensitizer in
3
DMSO system potentially has the capability of activating O2-
DMSO to labile 1O2-DMSO to oxidize some substrates as well
as 1O2. We proposed the reaction mechanism for the emission
intensity enhancement including transient species 1O2 and
1O2-DMSO adduct. The simulation predicting concentration
of Ir(ppy)3* in the photoreaction well reproduced the
emission intensity change even when the concentration of
fac-Ir(ppy)3 and the excitation wavelength were varied. As well,
the simulation provided the concentration change in other
1
1
species including the labile transients O2 and O2-DMSO of
which concentrations are almost zero during the photo-
1
reaction. It might be careful that O2-DMSO adduct could
participate in the oxidation of substrate in the organic
syntheses and/or in the cell death in PDTs using the system
of O2, triplet sensitizer, and DMSO.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.jpcb.1c03753.
Stern−Volmer plot, emission spectra of [Ru(II)(bpy)3]-
Cl2, 1H NMR of DMSO, UV−vis spectrum of fac-
Ir(ppy)3, DFT calculations, simulation details (PDF)
(13) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Visible Light
Photoredox Catalysis with Transition Metal Complexes: Applications
in Organic Synthesis. Chem. Rev. 2013, 113, 5322−5363.
(14) Shaw, M. H.; Twilton, J.; MacMillan, D. W. C. Photoredox
Catalysis in Organic Chemistry. J. Org. Chem. 2016, 81, 6898−6926.
AUTHOR INFORMATION
■
Corresponding Author
Kazuteru Shinozaki − Graduate School of Nanobioscience,
Yokohama City University, Kanazawa-ku, Yokohama 236-
0027, Japan; orcid.org/0000-0001-7571-9210;
Email: shino@yokohama-cu.ac.jp
(15) Marzo, L.; Pagire, S. K.; Reiser, O.; Kcņ ig, B. Visible-Light
Photocatalysis: Does It Make a Difference in Organic Synthesis?
Angew. Chem., Int. Ed. 2018, 57, 10034−10072.
(16) Romero, N. A.; Nicewicz, D. A. Organic Photoredox Catalysis.
Chem. Rev. 2016, 116, 10075−10166.
(17) Allen, L. J.; Cabrera, P. J.; Lee, M.; Sanford, M. S. N-
Acyloxyphthalimides as Nitrogen Radical Precursors in the Visible
Light Photocatalyzed Room Temperature C−H Amination of Arenes
and Heteroarenes. J. Am. Chem. Soc. 2014, 136, 5607−5610.
Authors
Shingo Hattori − Graduate School of Nanobioscience,
Yokohama City University, Kanazawa-ku, Yokohama 236-
0027, Japan; orcid.org/0000-0002-9385-961X
G
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J. Phys. Chem. B XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry B
pubs.acs.org/JPCB
Article
(18) Nakafuku, K. M.; Fosu, S. C.; Nagib, D. A. Catalytic Alkene
Difunctionalization via Imidate Radicals. J. Am. Chem. Soc. 2018, 140,
11202−11205.
(19) Cambié, D.; Bottecchia, C.; Straathof, N. J. W.; Hessel, V.;
Noël, T. Applications of Continuous-Flow Photochemistry in Organic
Synthesis, Material Science, and Water Treatment. Chem. Rev. 2016,
116, 10276−10341.
(20) Soni, V. K.; Hwang, H. S.; Moon, Y. K.; Park, S. W.; You, Y.;
Cho, E. J. Generation of N-Centered Radicals via a Photocatalytic
Energy Transfer: Remote Double Functionalization of Arenes
Facilitated by Singlet Oxygen. J. Am. Chem. Soc. 2019, 141, 10538−
10545.
(21) Liu, W. Q.; Lei, T.; Song, Z. Q.; Yang, X. L.; Wu, C. J.; Jiang,
X.; Chen, B.; Tung, C. H.; Wu, L. Z. Visible Light Promoted
Synthesis of Indoles by Single Photosensitizer under Aerobic
Conditions. Org. Lett. 2017, 19, 3251−3254.
(22) Tomita, R.; Yasu, Y.; Koike, T.; Akita, M. Combining
Photoredox-Catalyzed Trifluoromethylation and Oxidation with
DMSO: Facile Synthesis of α-Trifluoromethylated Ketones from
Aromatic Alkenes. Angew. Chem., Int. Ed. 2014, 53, 7144−7148.
(23) Yoshihara, T.; Yamaguchi, Y.; Hosaka, M.; Takeuchi, T.;
Tobita, S. Ratiometric Molecular Sensor for Monitoring Oxygen
Levels in Living Cells. Angew. Chem., Int. Ed. 2012, 51, 4148−4151.
(24) Kando, A.; Hisamatsu, Y.; Ohwada, H.; Itoh, T.; Moromizato,
S.; Kohno, M.; Aoki, S. Photochemical Properties of Red-Emitting
Tris(cyclometalated)Iridium(III) Complexes Having Basic and Nitro
Groups and Application to pH Sensing and Photoinduced Cell Death.
Inorg. Chem. 2015, 54, 5342−5357.
(25) Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Modern Molecular
Photochemistry of Organic Molecules; University Science Books:
Sausalito, CA, 2010.
(26) Martinez, L. J.; Scaiano, J. C. Characterization of the Transient
Intermediates Generated from the Photoexcitation of Nabumetone: A
Comparison with Naproxen. Photochem. Photobiol. 1998, 68, 646−
651.
(27) Kawamura, Y.; Brooks, J.; Brown, J. J.; Sasabe, H.; Adachi, C.
Intermolecular Interaction and a Concentration-Quenching Mecha-
nism of Phosphorescent Ir(III) Complexes in a Solid Film. Phys. Rev.
Lett. 2006, 96, 017404.
(28) Reineke, S.; Schwartz, G.; Walzer, K.; Falke, M.; Leo, K. Highly
Phosphorescent Organic Mixed Films: The Effect of Aggregation on
Triplet−Triplet Annihilation. Appl. Phys. Lett. 2009, 94, 163305.
(29) Song, D.; Zhao, S.; Luo, Y.; Aziz, H. Causes of Efficiency Roll-
Off in Phosphorescent Organic Light Emitting Devices: Triplet−
Triplet Annihilation versus Triplet−Polaron Quenching. Appl. Phys.
Lett. 2010, 97, 243304.
(30) Namdas, E. B.; Ruseckas, A.; Samuel, I. D. W.; Lo, S.-C.; Burn,
P. L. Photophysics of fac-Tris(2-Phenylpyridine) Iridium(III) Cored
Electroluminescent Dendrimers in Solution and Films. J. Phys. Chem.
B 2004, 108, 1570−1577.
(31) D ’ Andrade, B.; Forrest, S. R. Formation of Triplet Excimers
and Dimers in Amorphous Organic Thin Films and Light Emitting
Devices. Chem. Phys. 2003, 286, 321−335.
(32) Bleiholder, C.; Gleiter, R.; Werz, D. B.; Köppel, H. Theoretical
Investigations on Heteronuclear Chalcogen−Chalcogen Interactions:
On the Nature of Weak Bonds between Chalcogen Centers. Inorg.
Chem. 2007, 46, 2249−2260.
(33) Khanh, P. N.; Phan, C. D.; Ho, D. Q.; Vo, Q. V.; Ngan, V. T.;
Nguyen, M. T.; Trung, N. T. Insights into the Cooperativity between
Multiple Interactions of Dimethyl Sulfoxide with Carbon Dioxide and
Water. J. Comput. Chem. 2019, 40, 464−474.
(34) Lutkus, L. V.; Rickenbach, S. S.; McCormick, T. M. Singlet
Oxygen Quantum Yields Determined by Oxygen Consumption. J.
Photochem. Photobiol., A 2019, 378, 131−135.
H
https://doi.org/10.1021/acs.jpcb.1c03753
J. Phys. Chem. B XXXX, XXX, XXX−XXX