Two Emissive Long-Lived Excited States of an Imidazole-Functionalized Ruthenium Dipyridophenazine Complex

Article abstract of DOI:10.1021/acs.inorgchem.9b01372

A ruthenium(II) polypyridine-type complex based on the dipyridophenazine ligand with a directly fused imidazole unit (L1, dipyrido[3,2-a:2′,3′-c]phenazine-10,11-imidazole) has been synthesized, and its electrochemical and photophysical properties have been studied. The cyclic voltammogram of [Ru(tbbpy)₂(L1)]²⁺ (C1) (tbbpy is 4,4′-tert-butyl-2,2′-bipyridine) shows a cathodic shift of the phenazine-based reduction process compared to similar molecules, while the first detected reduction wave (-1.34 V vs Fc/Fc⁺) is assigned to the imidazole unit within the molecule. On the basis of the TD-DFT calculations, the strong visible absorption band exhibited by C1 is assigned to a metal-to-ligand charge transfer (MLCT) transition with a concurrent ligand-centered (LC) transition. At room-temperature, C1 features emission (φ = 0.04) from its lowest excited states with time constants of 1.2 and 18.3 μs. These lifetimes are assigned to emission processes from the ³MLCT and ³LC state, respectively. This is the first time that a long-lived dual emission has been observed for a ruthenium(II) complex bearing a directly fused extended π-system. Furthermore, the emission of C1 is quenched upon water addition. In contrast to related compounds based on a dipyridophenazine ligand, the excited state energy is not shifted, and the lifetime is drastically decreased to 169 ns.

Full text of DOI:10.1021/acs.inorgchem.9b01372

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Two Emissive Long-Lived Excited States of an Imidazole-
Functionalized Ruthenium Dipyridophenazine Complex
Dajana Isakov,† Robin Giereth,† Djawed Nauroozi, Stefanie Tschierlei,* and Sven Rau*
Ulm University, Institute of Inorganic Chemistry I, Albert-Einstein-Allee 11, 89081 Ulm, Germany
S
* Supporting Information
ABSTRACT: A ruthenium(II) polypyridine-type complex
based on the dipyridophenazine ligand with a directly fused
imidazole unit (L1, dipyrido[3,2-a:2′,3′-c]phenazine-10,11-
imidazole) has been synthesized, and its electrochemical and
photophysical properties have been studied. The cyclic
voltammogram of [Ru(tbbpy)2(L1)]2+ (C1) (tbbpy is 4,4′-
tert-butyl-2,2′-bipyridine) shows a cathodic shift of the
phenazine-based reduction process compared to similar
molecules, while the first detected reduction wave (−1.34 V vs Fc/Fc+) is assigned to the imidazole unit within the molecule.
On the basis of the TD-DFT calculations, the strong visible absorption band exhibited by C1 is assigned to a metal-to-ligand
charge transfer (MLCT) transition with a concurrent ligand-centered (LC) transition. At room-temperature, C1 features
emission (Φ = 0.04) from its lowest excited states with time constants of 1.2 and 18.3 μs. These lifetimes are assigned to
emission processes from the 3MLCT and 3LC state, respectively. This is the first time that a long-lived dual emission has been
observed for a ruthenium(II) complex bearing a directly fused extended π-system. Furthermore, the emission of C1 is quenched
upon water addition. In contrast to related compounds based on a dipyridophenazine ligand, the excited state energy is not
shifted, and the lifetime is drastically decreased to 169 ns.
the thermal depopulation of the bright state to the dark state is
reduced. Extensive studies were made in the past 30 years to
unravel the principle mechanism behind this light switch
effect.8−25 Besides the ability to function as a reversible DNA
light switch sensor,26−28 the extension of the dppz ligand by an
imidazole moiety should be also considered to provide long-
lived charge excited states in different solvents. Notably, the
extension of the dppz ligand, leads to strong effects on the
photochemistry. For instance, the loss of water sensitivity in
the excited state by fused pyrazine moieties29 or long-lived
dark charge separated states in fused benzene moieties (20
μs)30 and in substituted dppz ligands with electron-donating
triarylamine in 11-position (11 μs).31 Especially the long-lived
excited states of Ru(II) complexes of dppn (dppn is
benzo[i]dipyrido[3,2-a:2′,3′-c]phenazine)derivatives have re-
sulted in interesting properties like efficient light-driven ligand
dissociation and singlet oxygen formation30,32 and applications
as highly efficient photodynamic therapy agent.33 Instead, the
Ru(II) complex with a pyridoquinolinone unit fused to the
dppz ligand results in an emissive state with sub nanosecond
lifetimes.34,35
INTRODUCTION
■
Ruthenium polypyridine complexes play a prominent role in
functional molecular assemblies in solar energy conversion,
photodynamic therapy, and sensing.1,2 Tuning of excited state
properties of Ru(II) complexes, among others, plays a pivotal
role within all light driven applications. A crucial factor
determining the usability of ruthenium polypyridine complexes
is the lifetime of the excited state, since it determines the time
span where interactions with reactants may occur during
photocatalysis or photodynamic therapy. Adjusting these
properties may be accomplished by manipulating the ligand
sphere,3 immediate environment,4,5 or introduction of addi-
tional functional units that stabilize the triplet states.6,7
In that regard, ruthenium dipyridophenazine (dppz)
complexes are of particular interest. The principal dynamics
in the prototype complex [(bpy)2Ru(dppz)]2+ are associated
with the accessibility of several excited states. This causes the
light switch effect,8 that is, a strong dependence of the
luminescence characteristics on the solvent properties.9
Emission is not observed when these complexes are dissolved
in water, but the addition of DNA switches the luminescence
back on again. This effect can be explained by the existence of
two particular excited states known as the bright state and the
dark state, residing on the phenanthroline and phenazine part
of the dppz ligand, respectively. In protic solvents, the dark
state is stabilized to a certain extent, which favors radiationless
deactivation from the dark state over thermal depopulation of
the bright state.10−12 In presence of DNA, intercalation occurs
via the extended oligoaromatic units of the dppz ligand; hence,
However, dual emission in ruthenium complexes is mainly
known for asymmetric substituted 1,10-phenanthroline,3,36
terpyridine,37 or triazineligands38,39 and for dinuclear com-
plexes.40−42 In many cases, the dual emission is only
observable under 77 K38,39,43,44 or the emission decays in a
Received: May 10, 2019
© XXXX American Chemical Society
A
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few nanoseconds.36,40 One prominent example with a long-
lived dual emission of 11 μs is the Ru(II) complex where an
electron-rich p-methoxyphenyl unit was attached to the 4-
position of the 1,10-phenanthroline ligand via an alkynyl
bridge.45 Glazer et al. reasoned dual emission with the
prerequisite of having large deviations of the excited state
with respect to the ground state, which should only be possible
with asymmetric ligands.45 To the best of our knowledge, a
dual emission with microsecond lifetimes at room temperature
for a Ru(II) dppz complex and, thus, for a symmetric complex,
is unknown so far.
Scheme 1. Synthetic Route for [(tbbpy)2Ru(L1)]2+ C1
This study presents the electrochemical and photophysical
properties of a ruthenium complex with a modified dppz ligand
that contains a conjugated imidazole unit at the distal end. The
corresponding heteroleptic complex [(tbbpy)2Ru(L1)]2+ (C1)
(tbbpy is 4,4′-tert-butyl-2,2′-bipyridine and L1 is dipyrido[3,2-
a:2′,3′-c]phenazine-10,11-imidazole) (Figure 1) was prepared
and investigated using mass spectrometry, NMR, absorption,
and emission spectroscopy and cyclic voltammetry.
Figure 1. General structure of the ligands L0 and L1 with the applied
numbering scheme.
reversible redox couple assigned to RuII/RuIII and several LC
redox processes at more negative potentials (Figure 2 and
Supportive time-dependent DFT (TD-DFT) calculations
were also performed. The properties of C1 were compared
with C0, which is an analogous complex containing the
unsubstituted dipyrido[3,2-a:2′,3′-c]phenazine ligand L0 in-
stead of L1 (Figure 1).
Furthermore, the behavior of the triplet excited states of C1
were investigated in more detail via transient absorption
spectroscopy and emission lifetime measurements on the
nanosecond time scale. These measurements allowed the
nature of the triplet excited states to be characterized and the
impact of the incorporated imidazole unit on the underlying
processes to be elucidated.
RESULTS AND DISCUSSION
■
Synthesis. The π-extended dipyridophenazine complex C1
comprising an imidazole unit was obtained from the
condensation reaction of [(tbbpy)2Ru(phenO2)]2+ (Ru-
(phenO2); phenO2 is 1,10-phenanthroline-5,6-dione) with
5,6-diaminobenzimidazole (Scheme 1). Performing the re-
action at the complex circumvented the formation of the free
ligand, which would have required sophisticated purification of
the target molecule due to intermolecular π−π stacking.46−48
Complex C0 was synthesized for comparison following
literature procedures.47 Additionally [(MeCN)2Zn(L1)]-
(BF4)2 (ZnL1) was synthesized, allowing the characterization
of the poorly soluble ligand L1. Calculation of the ground state
structure of C1 confirmed that the ligand has an almost planar
structure in the complex. The details from this calculations and
the full characterization data for C1 and L1 are given in the
Supporting Information.
Figure 2. Cyclic voltammograms of solutions of C1 at room
temperature in acetonitrile (top) and acetonitrile with an excess of
sodium hydride (bottom), both referenced vs ferrocene/ferricenium
(Fc/Fc+) couple, respectively. Scan rate: 200 mV s−1. Supporting
electrolyte: 0.1 M Bu4NPF6.
Table 1). For C1 the RuII/RuIII redox couple is centered at
0.82 V vs Fc/Fc+, while the parent complex C0 has an
analogous redox couple at 0.85 V vs Fc/Fc+.47 Usually the
stepwise reduction of [(bpy)2Ru(L)]2+ complexes, where L is a
dppz derivative, follows the order: (i) phenazine part of L, (ii)/
(iii) bpy ligands, and (iv) phenanthroline part of L.49
Therefore, four LC reversible reductions are expected for
C1. By comparing the voltammetry recorded for C1 with that
Electrochemical Properties. The cyclic voltammogram of
C1 recorded in 0.1 M [Bu4N][PF6] acetonitrile displays a
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Table 1. Summary of the Electrochemical Properties of the
Table 2. Summary of the Photophysical Properties of the
Compounds C0, C1, and ZnL1
a
Compounds C0, C1, and the Deprotonated Form of C1
a
(C1depr
)
c
d
λabs [nm]
εMLCT [×103 M−1 cm−1
]
Φ
λem [nm]
E0ox [V]
E0red [V]
b
C0
C1
441
17.6
20.9
0.02
0.04
638
638
542
b
b
C0
0.85
0.82
0.81
−1.34, −1.86, −2.11, −2.48
433
c
C1
C1depr
−1.34, −1.69, −1.91, −2.12, −2.38
ZnL1
298, 389
c
a
−1.70, −1.91, −2.12, −2.52
Data were obtained from deaerated, anhydrous acetonitrile solution
b
c
at room temperature. 1MLCT absorption maximum. Quantum
yields were determined, using [Ru(bpy)3](PF6)2 as reference.57
a
The data were obtained from deaerated, anhydrous acetonitrile
solution at room temperature versus the ferrocene/ferricenium (F/
d
b
Fc+) couple. For reversible processes, E0 = (Epc + Epa)/2. From
Recorded at excitation wavelength of the maximum of the lowest
c
literature.47 Cathodic peak potential for irreversible processes.
energy absorption band.
bands in the UV region have a high intensity and are typical for
ligand centered transitions of ruthenium polypyridine com-
plexes.54 TD-DFT calculations showed that these bands do
indeed belong to several ligand centered (LC) transitions of
the tbbpy and the L1 ligand of C1 (compare Figure 3 and
Table S6.1 for details). A broad absorption band between 350
and 400 nm also emerges for C1, which is red-shifted
compared to the corresponding transition for C0. The reason
for this pronounced red-shift is that interligand π−π*
transitions occur from the benzimidazole unit of the L1 ligand
to the bpy ligand (Table S6.1).
The broad absorption between 400 and 575 nm displays a
maximum at 433 nm and a weakly pronounced shoulder at
approximately 478 nm. Remarkably, the extinction coefficient
of C1 at 433 nm is 20.9 × 103 M−1 cm−1, which is significantly
higher than the corresponding absorption value for C0 of 17.6
× 103 M−1 cm−1. The increased absorption coefficient for C1
at this wavelength is a result of overlaying MLCT and LC
transitions based on the L1 ligand. This is corroborated by
TD-DFT calculations, showing a contribution located at the
distal part of L1 that stems from the benzimidazole and
pyrazine unit, as depicted in the differential density plots
(Figure 4). Such LC transitions in the MLCT transitions range
recorded for C0 and similar compounds in the literature, one
can assign the reductions at −1.91, −2.12, and −2.38 V vs Fc/
Fc+ to the reduction of the tbbpy ligands and the phenanthro-
line part of L1, respectively.47,50,51 Accordingly, the reversible
reduction wave at −1.69 V vs Fc/Fc+ is attributed to the
phenazine part of the molecule. However, the voltammetry of
C1 shows a fifth reduction process at −1.34 V vs Fc/Fc+ that is
not reversible and occurs at a more positive potential than the
other ligand-centered redox couples. The irreversibility of the
process led us to believe that the redox couple at −1.34 V vs
Fc/Fc+ was due to the reduction of the imidazole moiety on
L1 in the C1 complex. Since imidazole moieties are known to
undergo protonation/deprotonation processes affecting the
reversibility of the corresponding redox couple,52,53 an excess
of sodium hydride was added to C1 in the electrolyte solution
to deprotonate the imidazole moiety. This led to complete
disappearance of the irreversible reduction process at −1.34 V
vs Fc/Fc+ (Figure 2), while the remaining reduction steps
remained almost unaffected during the in situ deprotonation.
Absorption and Emission Properties. The absorption
spectrum of C1 in acetonitrile (Figure 3 and Table 2) exhibits
two distinct maxima at 289 and 315 nm. These absorption
Figure 4. Differential density of C1 between the ground state and the
singlet excited state S8 (left) and S9 (right), respectively. Yellow
corresponds to a positive isovalue in the density difference between
the single-electron matrices of the ground and excited state. A decline
in electron density is shown in purple color corresponding to a
negative isovalue. More details about the transitions are summarized
in SI, section 6.
are unknown for complexes with the L0 ligand. Nevertheless,
such overlay behavior is comparable to metal complexes with
ligands bearing an extended π-system such as 3,6-dimethyl-
dipyrido[3,2-a:2′,3′-c]anthracene-9,10-dinitrile,55 12-bromo-
14-ethoxydipyrido[3,2-a:2′,3′-c]quinolino[3,2-h]-phenazine,34
or benzo[i]dipyrido[3,2-a:2′,3′-c]phenazine.56
Figure 3. Top: Normalized absorption (solid line) and emission
(dashed line) spectra of C0 (black), ZnL1 (blue), and C1 (red) in
deaerated, anhydrous acetonitrile; emission spectra were recorded at
excitation wavelengths of the maximum of the lowest energy
absorption band, respectively (see Table 2). Bottom: Calculated
absorption spectrum (B3LYP-D3(BJ)/def2-TZVP) of complex C1
(solid line), resulting from the convolution of the transitions above
560 nm (vertical bars) with a Gaussian function.
In addition to the TD-DFT calculations the absorption
spectrum of [(MeCN)2Zn(L1)](BF4)2 (ZnL1) was recorded
(Figure 3). L1 is insoluble in most solvents, but coordination
of the ligand to the redox inactive zinc(II) cation promotes the
solubility of L1. Spectra recorded for ZnL1 in solution only
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display the electronic transitions in the ligand L1.58 The
absorption spectrum of ZnL1 in acetonitrile solution shows
two maxima at 298 and 389 nm (Figure 3), which correlate
with the LC π−π* transitions of C1. Further, a very broad
contribution to lower energy is detected for ZnL1. These
experimental results and the TD-DFT calculations for C1
confirm that the LC transitions occur above 400 nm.
The excitation of C1 dissolved in degassed, anhydrous
acetonitrile solution at 433 nm results in an emission with a
maximum at 638 nm and a quantum yield of 0.04. While the
absorption properties of C1 are barely altered by changing the
solvent to dichloromethane or methanol (Figure S4.1), the
emission maxima and overall intensities vary with changing
solvent polarity (Figure S4.1 and Table S4.1). In dichloro-
methane and methanol, the emission of C1 occurs at a slightly
higher energy than in acetonitrile (Table S4.1). Similar
observations have been made for related complexes, such as
[(bpy)2Ru(L1)]2+, which emits at 625 nm in a water-based
buffer solution (pH 7), and [(phen)2Ru(L1)]2+, which emits at
610 nm in pure water.26,27 C0 and C1 display nearly the same
emission maxima in acetonitrile at 639 and 638 nm,
respectively. This indicates that the imidazole unit has a
weak influence on the bright state emission initiated from the
bpy part of the L0 scaffold.59
S4.11) show similar slopes, but the quenching effect is more
pronounced in case of C1. Additionally, the shift of the
emission maximum due to water content is less pronounced
for C1 (Figure 5 top) compared to C0.
Properties of Long-Lived Triplet Excited States. To
further investigate the excited state properties and the
underlying excited state deactivation processes of C1 in
more detail, time-resolved spectroscopy was performed on the
nanosecond time scale. C1 was excited at 355 nm in dry
acetonitrile under an argon atmosphere, and the decay of the
emission at 638 nm clearly revealed two long-lived emissive
states with lifetimes of 1.19 μs, τ1, and 18.25 μs, τ2 (Figure 6
For C0 and related complexes, the light switch mechanism
has been investigated in detail by quenching the emission with
the addition of water and, then, causing the emission to return
by adding DNA.59,60 We carried out a similar quenching
experiment to compare the light switch mechanisms of C1 and
C0. The emission of C0 is, in accordance to literature,24 clearly
quenched and shifted to higher energy due to addition of water
(Figure 5 and S4.9). C1 showed comparable behavior during
water addition, with the emission intensity considerably
quenched. The Stern−Volmer plots of C0 and C1 (Figure
Figure 6. Top: Time-resolved emission spectra of C1 in dry
acetonitrile under inert conditions after excitation at λex = 355 nm.
The cyan line represents a biexponential fit with the time constants τ1
= 1.19 μs ( 0.01 μs) and τ2 = 18.25 μs ( 0.18 μs). Bottom: Decay-
associated spectra of the biexponential fit with τ1 = 1.07 μs and τ2 =
13.40 μs. The amplitudes of the fit function F(λ, t) = ∑2i ci(λ)·exp(−t/
τi) + c0 are displayed in green squares (c1), red triangles (c2), and
black dots (c0, long-living component).
Table 3. Summary of the Excited State Lifetimes of C1 in
Acetonitrile and Acetonitrile/H2O (10% v/v) Solution at
Room Temperature after Excitation at λex = 355 nm
water
[% v/v]
transient absorption, τtA
[μs]
emission, τem [μs]
0
10
1.19 ( 0.01), 18.25 ( 0.18)
0.17 ( 0.01)
18.0
0.16
top and Table 3). The emission decay was also detected at
several other wavelengths between 570 and 700 nm, which
allowed a homogeneous kinetic emission map to be obtained
(Figure S7.1). Global analysis of the respective emission decays
resulted in amplitudes for the two time constants which are
almost same in energy (Figure 6 bottom). Comparable results
were obtained with an excitation wavelength of 532 nm
(Figure S7.6−8). This means that C1 possesses two excited
states which are deactivated via emissive radiation of
comparable energy. To the best of our knowledge this is the
first experimental evidence for long-lived dual emission in
mononuclear ruthenium complexes at room temperature,
bearing a dppz scaffold with a directly fused π-extension.
The short-lived component (1.19 μs, τ1) is in the same energy
range (630 nm) and time regime as reported for related
Figure 5. Top: Emission spectra of C0 (left) and C1 (right) in
degassed acetonitrile−water mixtures. Arrows indicate the shift of
emission maximum, upon increasing water concentrations. All spectra
were recorded at excitation wavelengths of the maximum of the
lowest-energy absorption band, respectively (see Table 2), and are
normalized to the respective emission maximum intensity. For
experimental details see SI, section 4.3. Bottom: Relative emission
area of C0 (black, triangles) and C1 (red, dots) as a function of water
concentration in deaerated acetonitrile solution.
10
[Ru(N∧N)L0]2+ complexes, such as [Ru(phen)2L0]2+ with
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660 ns and [Ru(bpy)2L0]2+ with 740 ns.61 Therefore, this
emission in C1 is also originated from a MLCT state located
nm) and fitted with a monoexponential decay where τ1 = 18.0
μs (Figure S8.2 and S8.4). This is in good agreement with the
long-lived emission lifetime as discussed above. The strong
ground state bleach is only weakly visible below 425 nm
because of the overlapping broad excited state absorption. In
contrast, [(bpy)2Ru(L0)]2+ and C0 exhibit a strong ground-
state MLCT absorption at 452 nm and a broad signal at λ >
3
on the bipyridine scaffold of L1, as assigned previously in
[Ru(N∧N)L0]2+ complexes.11 As a second emissive state in
transition metal complexes is highly uncommon, there are only
a few reports of such peculiar characteristics. For example,
Glazer et al. investigated a system where an additional
chromophoric unit was attached to the 4-position of the
phenanthroline ligand via an alkynyl bridge.41,45 In this and
related cases,44 the second emissive state was attributed to be a
MLCT process in nature. The time constant τ2 of C1 is,
however, unusually long-lived to be assigned to a pure 3MLCT
state. Similar observations were made for Ru(II) complexes of
dppn and pydppn (dppn is benzo[i]dipyrido[3,2-a:2′,3′-
c]phenazine, pydppn is 3-(pyrid-2′-yl)-4,5,9,16-tetraaza-
dibenzo[a,c]naphthacene), where the remarkably long lifetimes
500 nm.47,66 Such dominating excited state absorption of LC
3
excited states have already been described in the literature by
Dietzek et al. and Castellano et al.34,67 The shape of the
amplitude spectra (Figure S8.1) is surprisingly well represented
by the long-lived component alone. This implies that, when
3
3
compared to the LC state, the MLCT state has either (i) a
much higher absorption cross section or (ii) a much higher
population following excitation at 355 nm. Notwithstanding, it
3
is presumed that the LC state largely decays by radiationless
3
pathways due to the −NH bond on the imidazole ring and the
N-acceptor atoms on the phenazine moiety. Both moieties
offer an efficient pathway for radiationless deactivation
processes, for example, hydrogen-bonding to adjacent complex
or solvent molecules.
were attributed to a LC excited state, which was accessible
through absorption into the 1LC or 1MLCT state.30,56,62
Supporting TD-DFT calculations for C1 indicated the
presence of low-lying LC transitions are also responsible for
this emission (Figure 4 and Table S6.1). This is also
corroborated by electrochemical measurements and calcula-
tions performed on L0 and its derivatives, highlighting that
such electronically separated subunits within complexes
bearing π-extended diimine ligands are not uncommon.47,63−65
Similar behavior has recently been reported for heteroleptic
Cu(I) complexes with an extended π-system based on an
anthracene moiety.55 Although the excited states of the Cu(I)
complexes underwent nonradiative decay, this example high-
As stated above, upon addition of water (10% v/v), the
steady-state emission of C1 is considerably quenched (Figures
5 and S4.10). The emission signal decays monoexponentially
with a time constant of 169 ns (Figure S7.4), which is most
likely due to hydrogen bonding of the water molecules with the
pyrazine or imidazole nitrogen atoms leading to stabilization of
the 3LC state. Thus, it seems that the 3LC deactivates
nonradiatively (c.f., Figure S7.2). Therefore, it may be
concluded that the shortened lifetime is associated with the
proximal 3MLCT state instead of the 3LC state. This contrasts
the known light switch mechanism, where the dark state
(3MLCTdist) is populated from the bright state
(3MLCTprox).10,59,68 Because of the unincisive hypsochromic
shift in the emission spectra with addition of water it seems
3
3
lights the possibility of having MLCT and LC excited states
in parallel on one coordinated ligand with an extended π-
system.
Additionally, measurements of ZnL1 (Figure S7.5) were
performed to verify that all emission features originate from C1
and not from uncoordinated ligand in the solution. However,
no emission with time constants in the microsecond range
could be detected, confirming that the results of C1 described
above stem from the complex.
To corroborate the emission lifetime measurements,
transient absorption spectroscopy was performed. Here, the
spectra of C1 in deaerated acetonitrile were probed from 420
to 660 nm (Figure 7). The resulting spectra displayed a strong
and broad excited state absorption feature between 430 and
660 nm, which was rather structureless. Single kinetic-traces
were recorded with freshly prepared samples (to avoid
photodegradation) at characteristic bands (470, 500, and 570
3
that the distal and proximal MLCT states are only slightly
affected by the described hydrogen bonding. Thus, it is not
definite that the relaxation process of the light switch is present
in C1 as it is described for a Ru(II) complex bearing a
9,11,20,22-tetraazatetrapyrido[3,2-a:2′,3′-c:3″,2″-l:2‴,3‴]-pen-
tacene (tatpp) by MacDonnell and Coworkers.69 However,
this also implies that there is, if any, only weak coupling of the
two states. The nanosecond-transient absorption measure-
ments in acetonitrile/water (10% v/v) at λ = 470, 500, and 570
nm mirror the same behavior as observed in the emission
measurements: The intensity is strongly quenched and the
decay signal can be fitted monoexponentially with τ1 = 170 ns
(Figure S8.3).
CONCLUSIONS
■
The aim of this study was to investigate the electrochemical
and photophysical behavior of a complex bearing a
dipyridophenazine ligand with a directly fused imidazole
unit. For this purpose, a Ru(II) complex bearing a dipyrido-
[3,2-a:2′,3′-c]phenazine-10,11-imidazole ligand (C1) was
synthesized and structurally characterized. The cyclic voltam-
metry of C1 showed five ligand based redox couples. The
redox couple at the most positive potential belongs to the
imidazole unit and disappears upon deprotonation with a base.
Compared to the parent complex C0 bearing a dipyrido[3,2-
a:2′,3′-c]phenazine ligand, the absorption of C1 is red-shifted
and the extinction coefficient of the lowest lying absorption
band is increased. This is due to the overlay of metal-to-ligand
Figure 7. Transient absorption spectra of C1 in acetonitrile excited at
355 nm after 0.5 (red), 1.0 (yellow), 5.0 (green), 10.0 (blue), and
20.0 μs (magenta). The decay-associated amplitude spectra can be
found in the Supporting Information.
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charge transfer (MLCT) and ligand-centered (LC) transitions.
Thus, following excitation of C1, either a MLCT or a LC
state will be populated in the molecule (Figure 8). The
AUTHOR INFORMATION
■
1
1
Corresponding Authors
*E-mail: stefanie.tschierlei@uni-ulm.de.
*E-mail: sven.rau@uni-ulm.de.
ORCID
Robin Giereth: 0000-0003-0096-1728
Stefanie Tschierlei: 0000-0001-9441-7904
Author Contributions
†D.I. and R.G. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Figure 8. Simplified Jablonski diagram for the concurrent formation
of two different long-lived emissive excited states of C1 in acetonitrile
solution at room temperature. The green area indicates LC transition,
while the orange part corresponds to the MLCT state. It is unclear
whether a coupling between the respective MLCT and the LC states
exists.
S.T. grateful acknowledges the German Science Foundation
(DFG, TS 330/3-1) and the Fonds der Chemischen Industrie
(FCI) for funding. The authors acknowledge support by the
3
state of Baden-Wurttemberg through bwHPC and by the DFG
̈
(INST 40/467-1 FUGG). DFG is gratefully acknowledged for
the support of this work through Collaborative Research
Centre “CataLight” (Transregio SFB TRR234).
3
3
respective MLCT and LC states are populated by transfer
processes that occur faster than the ns time-scale explored
here. Unlike other reported Ru(II) complexes of dipyrido[3,2-
a:2′,3′-c]phenazine derivatives, both triplet excited states
discussed, herein, are emissive at room temperature with
time constants of 1.2 and 18.3 μs (λex = 355 nm). In this study,
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ASSOCIATED CONTENT
* Supporting Information
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chem.9b01372.
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