Bright and Long-Lived Emission from a Starburst-Type Arylborane-Appended Polypyridyl Ruthenium(II) Complex

Article abstract of DOI:10.1002/ejic.201700448

The starburst-type arylborane-appended polypyridyl Ru^II complex [Ru{4,7-bis({dimesityl}boryldurylethynyl)-1,10-phenanthroline}₃]²⁺ (1) was synthesized, and its spectroscopic and photophysical properties were evaluated. In the excited state of the complex, intramolecular charge transfer (CT) between the π orbital of the aryl group and the vacant p orbital of the boron atom [π(aryl)–p(B) CT] interacts synergistically with metal-to-ligand charge transfer (MLCT). The synergistic CT interactions in 1 are more effective than those in previously reported arylborane-appended polypyridyl Ru^II complexes and lead to bright (molar absorption coefficient of the MLCT band = 7.2 × 10⁴ m^–1 cm^–1, emission quantum yield = 0.29) and long-lived emission (8.7 μs) of 1 in CH₃CN at 298 K. The unique emission characteristics of 1 are discussed in detail together with its electrochemical properties and time-dependent DFT (TD-DFT) calculations.

Full text of DOI:10.1002/ejic.201700448

DOI: 10.1002/ejic.201700448
Full Paper
Polypyridyl Ruthenium Complexes
Bright and Long-Lived Emission from a Starburst-Type
Arylborane-Appended Polypyridyl Ruthenium(II) Complex
Atsushi Nakagawa,[a] Akitaka Ito,[b] Eri Sakuda,[c] Sho Fujii,[a,d] and Noboru Kitamura*[a,d]
Abstract: The starburst-type arylborane-appended polypyridyl interactions in 1 are more effective than those in previously
RuII complex [Ru{4,7-bis({dimesityl}boryldurylethynyl)-1,10- reported arylborane-appended polypyridyl RuII complexes and
phenanthroline}3]2+ (1) was synthesized, and its spectroscopic lead to bright (molar absorption coefficient of the MLCT band =
and photophysical properties were evaluated. In the excited 7.2 × 104
M
–1 cm–1, emission quantum yield = 0.29) and long-
state of the complex, intramolecular charge transfer (CT) be- lived emission (8.7 μs) of 1 in CH3CN at 298 K. The unique
tween the π orbital of the aryl group and the vacant p orbital emission characteristics of 1 are discussed in detail together
of the boron atom [π(aryl)–p(B) CT] interacts synergistically with its electrochemical properties and time-dependent DFT
with metal-to-ligand charge transfer (MLCT). The synergistic CT (TD-DFT) calculations.
Introduction
in the presence of triethylamine or coordinating metal ions.[9,10]
However, the development of complexes that simultaneously
show a high emission quantum yield (Φem) and a long emission
Polypyridyl RuII complexes such as [Ru(bpy)3]2+ (bpy = 2,2′-bi-
pyridine) have been utilized widely in a variety of photochemi-
cal applications as photosensitizers, triplet emitters, or both ow-
ing to their intense metal-to-ligand charge-transfer (MLCT) ab-
sorption in the visible region and clear phosphorescence from
the triplet MLCT (3MLCT) excited states.[1–6] To pursue further
the capabilities of these complexes for such applications, in-
tense visible-light absorption and a long-lived excited state
would be highly advantageous for photosensitizers, and high
emission efficiency is desired for light-emitting materials. There-
fore, the tuning of the spectroscopic and photophysical proper-
ties of polypyridyl RuII complexes is of primary importance. As
a typical example, polypyridyl RuII complexes with arene moie-
ties at the ligand periphery show quite long-lived emission
originating from the thermal equilibrium between the 3MLCT
lifetime (τem
)
is still challenging. [Ru(4,7-diphenyl-1,10-
phenanthoroline)3]2+ {[Ru(dp-phen)3]2+} shows remarkably in-
tense and long-lived emission [Φem = 0.366 and τem = 6.40 μs
in ethanol/methanol (4:1, v/v) at 293 K].[11] Furthermore, Tor
et al. reported that RuII complexes with 4- or 4,7-substituted-
phenanthroline ligands exhibited long-lived emission.[12] There-
fore, the chemical modification of the 4- or 4,7-positions of
phenanthroline (phen) ligands for RuII complexes is very fasci-
nating for the control of the photophysical properties of the
complexes.
For the synthetic tuning of the spectroscopic and photo-
physical properties, we studied polypyridyl RuII complexes with
triarylborane substituents at the peripheries of phen or bpy
ligands. Such complexes show quite intriguing photophysical
properties[13–15] owing to the participation of intramolecular
charge transfer (CT) from the π orbital of the aryl group [π(aryl)]
to the vacant p orbital of the boron atom [p(B)] in the triaryl-
borane groups [π(aryl)–p(B) CT],[16–24] which interacts synergis-
tically with MLCT in the excited state. In this paper, we focus
on the starburst-type [Ru{4,7-(DBDE)2-phen}3]2+ complex [1, PF6
salt, DBDE = (dimesityl)boryldurylethynyl; see Scheme 1].
3
excited state of the complex and the long-lived ππ* excited
state of the arene moieties in the ligands, as reported by Castel-
lano et al.[7] and Campagna et al.[8] On the other hand, a RuII
complex with an N-heterocyclic carbene unit in the bidentate
ligand shows intense emission, and the emission is enhanced
[a] Department of Chemical Sciences and Engineering, Graduate School of
Chemical Sciences and Engineering, Hokkaido University,
Kita-10, Nishi-8, Kita-ku, Sapporo 060-0810, Japan
[b] Graduate School of Engineering/School of Environmental Science and
Engineering, Kochi University of Technology,
185 Miyanokuchi, Tosayamada, Kami, Kochi 782-8502, Japan
[c] Division of Chemistry and Materials Science, Graduate School of
Engineering, Nagasaki University,
Among the various polypyridyl RuII complexes hitherto re-
ported except for those with anthryl[8,25] or pyrenyl[7,25–27] sub-
–
stituents, [Ru(4-DBDE-phen)(phen)2]2+ (2, PF6 salt) exhibits the
longest emission lifetime (τem = 12 μs in CH3CN at 298 K),[13]
–
whereas [Ru{4,4′-(DBDE)2-bpy}3]2+ (3, PF6 salt) in CH3CN at
298 K shows the brightest emission with a molar absorption
1-14 Bunkyo-machi, Nagasaki 852-8521, Japan
[d] Department of Chemistry, Faculty of Science, Hokkaido University,
Kita-10, Nishi-8, Kita-ku, Sapporo 060-0810, Japan
coefficient of the MLCT band [ε(MLCT)] of 5.6 × 104
M
–1 cm–1
and Φem = 0.43.[15] It is worth pointing out that such a large
ε(MLCT) value is not realized for [Ru(dp-phen)3]2+ [ε(MLCT) =
E-mail: kitamura@sci.hokudai.ac.jp
https://wwwchem.sci.hokudai.ac.jp/~bunseki/index_en.html
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201700448.
2.86 × 104 –1 cm–1]. Nonetheless, the Φem value of 2 is as low
M
as 0.11, and 3 shows only a moderately long emission lifetime
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–
Scheme 1. Chemical structures of 1–3 (PF6 salts).
of 1.7 μs. Hence, a polypyridyl RuII complex that shows simulta- summarized in Table 1. The 1MLCT absorption transition energy
neously bright [large ε(MLCT) and high Φem] and long-lived (νabs) was lowered by the introduction of the DBDE groups to
˜
emission in solution has not been realized to date. In the the peripheries of the three phen ligands [1, ν
= 21.2 ×
˜
abs
present study, we show that 1 simultaneously exhibits bright 103 cm–1 (λabs = 471 nm) with a shoulder at 19.8 × 103 cm–1
[ε(MLCT) = 7.2 × 104 –1 cm–1 and Φem = 0.29] and long-lived (λ ≈ 505 nm)] compared with that of [Ru(phen)3]2+ (νabs
M
=
˜
emission (τem = 8.7 μs) in CH3CN at 298 K and discuss the origin 22.5 × 103 cm–1, λabs = 445 nm). The intense and relatively sharp
of its unique emission characteristics.
1ππ* absorption band of the phen ligands in 1 (νabs = 35.2 ×
˜
103 cm–1, λabs = 284 nm) was also shifted to lower energy rela-
tive to that of [Ru(phen)3]2+ (ν
= 38.0 × 103 cm–1, λabs
=
˜
abs
263 nm), and the differences in the 1MLCT and 1ππ* absorption
Results and Discussion
energies (Δνabs) of 1 and [Ru(phen)3]2+ were almost comparable
˜
Synthesis of 1
with each other (2670 and 2810 cm–1, respectively). Therefore,
1
1
the low-energy shifts of the MLCT and ππ* absorption bands
observed for 1 relative to the band energies of [Ru(phen)3]2+
are ascribed mainly to energy stabilization of the π* orbital of
the 4,7-(DBDE)2-phen ligand. In practice, the first reduction
wave of one of the three ligands in 1 (–1.38 V vs. Fc/Fc+, Fc =
ferrocene) was more positive than that of [Ru(phen)3]2+ (–1.78 V
vs. Fc/Fc+), whereas the metal oxidation waves of the complexes
were less sensitive to the presence or absence of the DBDE
groups {+0.88 and +0.82 V for 1 and [Ru(phen)3]2+, respectively;
see Figure S1 and Table S1}. In addition to the low-energy
1MLCT transition, the molar absorption coefficient of the 1MLCT
The synthetic routes to 1 is shown in the Supporting Informa-
tion (Scheme S1). The complex was successfully synthesized
through the palladium-catalyzed Sonogashira–Hagihara cross-
coupling reaction between (ethynylduryl)dimesitylborane and
–
the PF6 salt of a dihalogen-substituted precursor complex
{[Ru(4,7-X2-phen)3]2+ (X = Br or Cl)}, similarly to the synthetic
procedure for 3.[15] The detailed synthetic procedures as well as
the chemicals and equipment used for the synthesis of 1 are
reported in the Supporting Information.
Absorption Spectra in CH3CN
band observed for 1 (ε = 7.2 × 104
enhanced significantly compared with that of [Ru(phen)3]2+ (ε =
1.7 × 104 –1 cm–1 at λ = 445 nm). The enhancement of the
M
–1 cm–1 at λ = 471 nm) was
The absorption spectra of 1 and [Ru(phen)3]2+ in CH3CN at
298 K are shown in Figure 1 together with those of 3 and
[Ru(bpy)3]2+ for comparison. The absorption maximum wave-
lengths (λabs) and corresponding ε values of the complexes are
M
1MLCT band of 1 can be explained by an increase in the dipole
moment of the absorption transition owing to the participation
of extended intramolecular CT from the metal ion to the elec-
tron-withdrawing arylborane units, that is, synergistic MLCT/
π(aryl)–p(B) CT interactions. In practice, 1 exhibits an intense
and characteristic absorption band at λ = 300–400 nm, which
1
can be assigned to the superposition of the ππ* transitions in
the durylethynylphenanthroline moieties and the π(aryl)–p(B)
CT transitions in the 4,7-(DBDE)2-phen ligands, that is, ligand-
centered (LC) absorption transition.[13]
Table 1. Absorption features of the complexes in CH3CN at 298 K.
Complex
λabs [nm] (ε/104 –1 cm–1
M )
1
284 (10.6), 382 (11.3), 471 (7.2), 505 sh (6.6)
263 (10.6), 416 sh (1.6), 445 (1.7)
311 (11.4), 375 (15.5), 460 sh (4.6), 488 (5.6)
287 (8.2), 424 sh (1.1), 451 (1.3)
[Ru(phen)3]2+
3[a]
Figure 1. Absorption spectra of 1 (red solid curve), [Ru(phen)3]2+ (red broken
curve), 3 (blue solid curve), and [Ru(bpy)3]2+ (blue broken curve) in CH3CN at
298 K. Inset: spectra normalized to the maximum ε values of the MLCT bands.
The spectrum of 3 was taken from ref.[15]
[Ru(bpy)3]2+
[a] Data taken from ref.[15]
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The comparison of the absorption spectrum of 1 with that Emission Properties
of 3 provided further significant insights into the synergistic
The emission spectra of the complexes in CH3CN at 298 K are
shown in Figure 2, and the emission parameters are summa-
rized in Table 2. The structureless emission from 1 indicates that
the emitting excited state of the complex is MLCT in character
MLCT/π(aryl)–p(B) CT interactions. The 1MLCT absorption bands
of both 1 and 3 were shifted to lower energy, and the ε(MLCT)
values were enhanced by the introduction of the six DBDE
groups to the peripheries of the three ligands. However, more
pronounced triarylborane effects on the ε(MLCT) value are ob-
(3MLCT*), and the maximum wavelength (λem) of 1 (λem
=
671 nm, ν = 14.9 × 103 cm–1) is shifted to a longer wave-
˜
em
served for 1, as the ε(MLCT) value for 1 (7.2 × 104
M
–1 cm–1 at
length compared with that of [Ru(phen)3]2+ (λem = 599 nm,
λ = 471 nm) is larger than that for 3 (5.6 × 104 –1 cm–1 at λ =
M
νem = 16.7 × 103 cm–1). The shift of the emission energy be-
˜
488 nm). The results demonstrate that the electronic coupling
between the phen and DBDE units in 1 is stronger than that
between the bpy and DBDE units in 3, and this stronger cou-
pling leads to more effective synergistic MLCT/π(aryl)–p(B) CT
interactions in 1. This could be explained by the rigid and pla-
nar π-extended structure of phen, which will be more favorable
for the synergistic CT interactions compared with the more
structurally flexible and less-π-extended bpy. The broader
1MLCT absorption band observed for 1 [the full width at half-
maximum (fwhm) is ca. 5300 cm–1] relative to that of 3
(3490 cm–1) might also indicate the effective and strong syner-
gistic MLCT/π(aryl)–p(B) CT interactions in 1.
tween 1 and [Ru(phen)3]2+ (1800 cm–1) was much larger than
the relevant value between 3 and [Ru(bpy)3]2+ (770 cm–1);
therefore, the effects of the arylborane substituents on the
emission spectrum are much larger for 1 than for 3. These re-
sults demonstrate that the triplet MLCT/π(aryl)–p(B) CT excited
state of 1 is largely stabilized in energy compared to that of
[Ru(phen)3]2+
.
Theoretical Calculations
The electronic absorption transitions of the complexes were
also evaluated by time-dependent density functional theory
(TD-DFT) calculations. To reduce computational costs, the het-
eroleptic complexes [Ru{4,7-(DBDE)2-phen}(phen)2]2+ (4) and
[Ru{4,4′-(DBDE)2-bpy}(bpy)2]2+ (5), which have one 4,7-(DBDE)2-
phen and 4,4′-(DBDE)2-bpy ligand, respectively, were employed
as model complexes (see Figures S3 and S4). The details of the
theoretical calculations, including the energies of the important
absorption transitions from the highest occupied molecular or-
bitals (HOMOs) to the lowest unoccupied molecular orbitals
(LUMOs) and the nuclear coordinates of 4, 5, [Ru(phen)3]2+, and
[Ru(bpy)3]2+, are reported in Tables S2–S11.
Figure 2. Emission spectra of the complexes in CH3CN at 298 K. The color
legend is the same as that for Figure 1, and the integrations of the spectra
in a wavenumber scale correspond to the relative emission quantum yields
of the complexes. The spectrum of 3 was taken from ref.[15]
Table 2. Emission properties of the complexes in CH3CN at 298 K.
[c]
λem
[nm]
τem
[μs]
kr[c]
knr
Complex
Φem
[105 s–1
]
[105 s–1
]
1[a]
671
599
651
620
0.29
0.045
0.43
8.7
0.42
1.7
0.33
1.1
2.5
0.82
23
3.4
10
[Ru(phen)3]2+[a]
3[a,b]
Our calculations demonstrated that 4 and 5 possess lower
energy and more intense singlet–singlet (S0–S1) absorption
transitions than those of [Ru(phen)3]2+ and [Ru(bpy)3]2+, respec-
tively. Hence, the calculations on 4 and 5 adequately reproduce
the absorption characteristics of 1 and 3. The lowest-energy
[Ru(bpy)3]2+[a]
0.095
0.89
1.1
[a] Data for a deaerated solution. [b] Data taken from ref.[14] [c] Calculated
with the equation Φem = kr/(kr + knr) = krτem
.
Complex 1 showed quite intense emission with Φem = 0.29,
singlet excited states (S1) of 4 and 5 are ascribed mainly to the which is 6.4 times larger than that of [Ru(phen)3]2+ (Φem
=
HOMO–1→LUMO transitions at λ = 487 [oscillator strength (f) = 0.045). As mentioned above, 3 shows the most intense emission
0.054] and 494 nm (f = 0.082), respectively. The LUMOs of both (Φem = 0.43) among the polypyridyl RuII complexes hitherto
complexes are distributed over the entire arylborane-substi- reported, and the present data demonstrate that the introduc-
tuted ligand [i.e., 4,7-(DBDE)2-phen or 4,4′-(DBDE)2-bpy]. Al- tion of six DBDE groups to the three ligands in [Ru(phen)3]2+
though both 4 and 5 have the HOMO–1 delocalized from the produces a large enhancement of Φem. Complex 1 also exhibits
RuII center to the duryl group, that of 4 is characterized by a long-lived emission as the decay profile shows in Figure 3 to-
larger contribution of density at the RuII center than that of 5. gether with that of [Ru(phen)3]2+. It is worth emphasizing that
The results demonstrate that the lowest-energy excited state of the τem value of 1 (8.7 μs) is 21 times longer than that of
4 (phen-type complex) is best characterized by synergistic [Ru(phen)3]2+ (0.42 μs). Thus, 1 shows simultaneously bright
MLCT/π(aryl)–p(B) CT, and the excited state possesses a rela- (Φem = 0.29) and long-lived emission (τem = 8.7 μs). As the
tively higher charge-separated character than that of 5 (bpy- phosphorescence transition of a molecule is a spin-forbidden
type complex). Furthermore, 4 shows an intense HOMO→LUMO process, the phosphorescence rate constant (kr) is generally
+ 1 transition at λ = 463 nm with f = 0.74. Such absorption very small. This leads to long τem (∝ 1/kr) and small Φem (∝ kr)
transitions predicted for 4 are also expected for 1 and can ex- values, as approximated by the equation Φem = kr/(kr + knr) =
1
plain the observed intense MLCT absorption band of 1.
krτem, in which knr is the nonradiative rate constant from the
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excited triplet state. The achievement of both high Φem and 105 s–1), and this suggests that the Φem value for 1 would be
long τem is generally difficult for an ordinary RuII complex with smaller than that of [Ru(phen)3]2+. However, this is not what is
small kr and large knr values; thus, the photophysical properties observed for 1. Thus, the Φem value of 1 is determined in bal-
of 1 are remarkable and worthy of a detailed discussion.
ance between kr and knr, as predicted from the relation Φem =
kr/(kr + knr). For [Ru(phen)3]2+, the knr value determines both
Φem and τem because knr (23 × 105 s–1) >> kr (1.1 × 105 s–1),
whereas the kr and knr values of 1 are almost comparable with
each other [knr (0.82 × 105 s–1) kr (0.33 × 105 s–1)]; thus, the
role of kr in determining Φem is more important for 1 than for
[Ru(phen)3]2+. In a recent study on the photophysical properties
of a series of [Ru(4-DBDE-bpy)3–n(bpy)n]2+ and [Ru{4,4′-(DBDE)2-
bpy}3–n(bpy)n]2+ (n = 0–3) complexes, we discovered that the kr
values of the complexes correlate linearly with the ε(MLCT) val-
ues (Strickler–Berg-type relation).[31] If a Strickler–Berg-type re-
lation is held, a larger ε(MLCT) value of a complex results in a
higher Φem value through the relation Φem = kr/(kr + knr). For
the present phen-type RuII complexes, in practice, the increas-
ing order of ε(MLCT) {1.7 × 104 ([Ru(phen)3]2+) < 2.6 × 104
Figure 3. Emission decay profiles of 1 (red) and [Ru(phen)3]2+ (brown) in
CH3CN at 298 K.
3
For an ordinary polypyridyl RuII complex, the MLCT* state
(2) << 7.2 × 104 –1 cm–1 (1)} agrees very well with that of Φem
M
undergoes nonradiative decay through thermal activation to
{0.045 ([Ru(phen)3]2+) < 0.11 (2) << 0.29 (1)}. The moderate bal-
ance between kr and knr results in the simultaneous bright and
long-lived emission of 1.
the nonemitting triplet dd excited (3dd*) state, in addition to
3
direct deactivation from MLCT* to the ground state.[28] As the
nonradiative decay of 3MLCT* occurs through the 3dd* state,
the τem value of a RuII complex in general decreases as the
temperature (T) increases. In contrast, the emission lifetime of
1 is almost independent of T (see Figure 4), similarly to those
of 2 and other DBDE-appended polypyridyl RuII and ReI com-
plexes.[13–15,29,30] Surprisingly, the τem value of 1 remains very
long even at 400 K (τem = 6.6 μs). As the oxidation potentials
of 1 and [Ru(phen)3]2+ are almost comparable (+0.88 and
Conclusions
We successfully developed a starburst-type arylborane-ap-
pended polypyridyl RuII complex (1) and demonstrated that it
showed bright [ε(MLCT) = 7.2 × 104 –1 cm–1 and Φem = 0.29]
M
and long-lived emission (τem = 8.7 μs) owing to the synergistic
MLCT/π(aryl)–p(B) CT interactions. As 1 possesses intense visi-
ble absorption, high Φem, and long τem in addition to compara-
ble oxidation potentials to those of [Ru(phen)3]2+, it is a promis-
ing candidate for applications as visible-light-driven photo/
redox sensitizer, a phosphorescence sensor, an emitting mate-
rial, and so forth.
3
+0.82 V vs. Fc/Fc+, respectively), the energies of the dd* states
of the two complexes are supposed to be rather similar. On the
other hand, the emission energy of 1 (14.9 × 103 cm–1 or
1.84 eV) is much lower than that of [Ru(phen)3]2+ (16.7 ×
103 cm–1 or 2.07 eV, energy difference = 230 meV). Therefore,
for 1, thermal activation from the 3MLCT* state to the nonemit-
ting 3dd* state is impeded owing to the larger 3MLCT*–3dd*
energy gap relative to that of [Ru(phen)3]2+, which leads to a
Experimental Section
very long τem value for 1 compared with that of [Ru(phen)3]2+
.
Electrochemical and Spectroscopic Measurements: N,N-Dimeth-
ylformamide (DMF) for nonaqueous titrimetry (Kanto Chemical Co.,
Inc.) was used for electrochemical measurements. Spectroscopic-
grade CH3CN (Wako Pure Chemical Ind., Ltd.) and Luminasol® (Do-
jindo Molecular Technologies, Inc.) CH3CN were used for the ab-
sorption and emission measurements, respectively, without further
purification. Propylene carbonate (Wako Pure Chemical Ind., Ltd.)
for T-controlled measurements was distilled before use.[32] Except
for the T-controlled experiments, all of the measurements were con-
ducted at 298 2 K.
In practice, the knr value of 1 (0.82 × 105 s–1) is 28 times smaller
than that of [Ru(phen)3]2+ (23 × 105 s–1). This will be the primary
reason for the long emission lifetime of 1 (τem = 8.7 μs at 298 K).
The cyclic voltammetry (CV) and differential pulse voltammetry
(DPV) of the complexes in DMF were conducted with a BAS ALS-
701D electrochemical analyzer with a three-electrode system con-
sisting of a glassy carbon working electrode, a Pt auxiliary electrode,
and a Ag/Ag+ reference electrode. A small amount of Fc was added
to the sample solutions as an internal standard for the correction
of the redox potentials of the samples. Tetra-n-butylammonium
Figure 4. Temperature dependences of the emission lifetimes of 1 (red boxes),
2 (purple boxes), and 3 (blue boxes) in propylene carbonate. The data for 2
were taken from ref.[13]
hexafluorophosphate (TBAPF6, 0.1
from ethanol three times, was used as the supporting electrolyte.
The concentration of the complex was ca. 5 × 10–4
. The sample
solutions were deaerated with an Ar stream for 20 min before the
M), purified by recrystallization
3
Although the nonradiative decay path of 1 through dd* is
M
essentially impeded, as discussed above, the kr value of 1
(0.33 × 105 s–1) is also smaller than that of [Ru(phen)3]2+ (1.1 × measurements. The potential sweep rate was 100 mV/s for the CV,
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and DPV was conducted with 50 mV height pulses (0.06 s duration)
stepped by 4 mV intervals (2.0 s intervals between the pulses).
The absorption spectra of the complexes were recorded with a Hita-
chi U-3900H spectrophotometer. Complex 1 was dissolved in
CH3CN at a concentration of 3.0 × 10–6
M, and each solution was
diluted to 10, 25, 50, and 75 % of the mother solution with CH3CN.
The absorption spectra of the diluted and mother solutions were
measured to enable the determination of the molar absorption co-
efficients (ε) of the complex at the maximum wavelength. The emis-
sion spectra of the complexes were measured with a Hamamatsu
PMA-11 multichannel photodetector (excitation wavelength =
355 nm), and the emission intensity at each wavelength was cor-
rected for the spectral response of the system so that the vertical
axis of the spectrum corresponded to the photon number at each
wavelength. The absolute emission quantum yields of the com-
plexes were determined with a Hamamatsu C9920-02 system
equipped with an integrating sphere and a PMA-12 red-sensitive
multichannel photodetector (excitation wavelength = 400 nm).[33]
The emission decay profiles were measured with a Hamamatsu
C4334 streak camera as a photodetector with excitation at λ =
355 nm with a nanosecond Q-switched neodymium-doped yttrium
aluminium garnet (Nd:YAG) laser (LOTIS TII, Ltd., LS-2137, fwhm ≈
16–18 ns, repetition rate = 10 Hz) as a light source. A liquid N2
cryostat (Oxford Instruments OptistatDN2 optical Dewar and Mer-
curyiTC temperature controller) was used for T-controlled measure-
ments. For emission spectroscopy, the absorbance of a sample solu-
tion was set to less than 0.05 at the excitation wavelength, and the
sample solutions were deaerated with an Ar stream for 30 min.
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Theoretical Calculations: Theoretical calculations for the com-
plexes were conducted with the Gaussian 09W software.[34] The op-
timizations of the ground-state geometries of the complexes were
performed with the B3LYP functional.[35,36] The LanL2DZ[37] and 6-
31G(d,p)[38] basis sets were used to treat the geometric structures
of the ruthenium and all other atoms, respectively. TD-DFT calcula-
tions were then performed to estimate the energies and oscillator
strengths of 10 lowest-energy singlet absorption transitions. All of
the calculations were performed in CH3CN with the polarizable con-
tinuum model (PCM).
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katsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G.
Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hase-
gawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A.
Montgomery Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Broth-
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Acknowledgments
N. K. acknowledges a Grant-in-Aid for Scientific Research from
the Ministry of Education, Culture, Sports, Science and Technol-
ogy (MEXT) of the Japanese Government for the support of the
research Nos. 25620035 (Grant-in-Aid for Exploratory Research)
and 26248022 [Grant-in-Aid for Scientific Research (A)].
Keywords: Ruthenium · Boranes · Photophysics · Charge
transfer · Phosphorescence
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Received: April 25, 2017
Eur. J. Inorg. Chem. 2017, 3794–3798
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