Long-Lived, Emissive Excited States in Direct and Amide-Linked Thienyl-Substituted RuII Complexes

Article abstract of DOI:10.1002/ejic.201501436

The excited state behavior of a new series of homoleptic and heteroleptic Ru^II complexes bearing thienyl groups appended to a 2,2′-bipyridine chelating ligand via direct, secondary and tertiary amide linkages is examined. The results of nanosecond transient absorption spectroscopy, emission lifetime measurements and bimolecular quenching experiments are correlated to determine that although the amide linkage does not act as a conjugated bridge to the peripheral substituents, it does not preclude possible electron transfer processes. Complexes bearing directly bound thienyl and bithienyl substituents exhibit long excited state and emission lifetimes (τ_em = 2 and 15 μs), with high emission quantum yields in solution (Φ = 0.35) and slow rates of non-radiative decay.

Full text of DOI:10.1002/ejic.201501436

DOI: 10.1002/ejic.201501436
Full Paper
Long-Lived Excited States
Long-Lived, Emissive Excited States in Direct and Amide-Linked
Thienyl-Substituted RuII Complexes
Marek B. Majewski,[a] Jeremy G. Smith,[a] Michael O. Wolf,*[a] and Brian O. Patrick[a]
Abstract: The excited state behavior of a new series of homo- does not act as a conjugated bridge to the peripheral substitu-
leptic and heteroleptic RuII complexes bearing thienyl groups
appended to a 2,2′-bipyridine chelating ligand via direct, sec-
ondary and tertiary amide linkages is examined. The results of
nanosecond transient absorption spectroscopy, emission life-
time measurements and bimolecular quenching experiments
are correlated to determine that although the amide linkage
ents, it does not preclude possible electron transfer processes.
Complexes bearing directly bound thienyl and bithienyl sub-
stituents exhibit long excited state and emission lifetimes (τem
=
2 and 15 μs), with high emission quantum yields in solution
(Φ = 0.35) and slow rates of non-radiative decay.
Introduction
sufficient amount of energy may be used to drive follow-on
reactions.[6–8] Many examples of RuII polypyridyl photosensitiz-
ers with peripheral donor and acceptor groups have been
shown to act as primary charge separation reaction centers for
artificial photosynthesis, among other applications.[9–12] In addi-
tion, significant efforts to increase the rate of forward electron
transfer (or decrease the rate of back electron transfer) by vary-
ing the substituents and connectivity to the central photosensi-
tizer in these types of systems have also been made.[4,13–15]
Mimicking photosynthesis is an enduring goal of research in
photochemical energy conversion and solar fuels research. One
of the primary steps in artificial photosynthetic systems is the
formation of a charge-separated (CS) electron-hole pair by pho-
toinduced electron transfer (ET). Photoexcitation of rationally
designed donor–acceptor (D–A) dyads or donor–chromophore–
acceptor (D–C–A) triads may lead to the formation of charge-
separated states via intramolecular electron transfer.[1–5] These
charge-separated states, if sufficiently long-lived and storing a
Scheme 1. Complexes and ligands discussed in this work.
[a] Department of Chemistry, University of British Columbia,
Vancouver, British Columbia, V6T 1Z1, Canada
One general strategy to extend the lifetime of these CS states
is to lengthen the bridge or linker between the ruthenium core
and the peripheral electron-donating/accepting moieties
thereby decreasing the rate of back electron transfer.[16–20]
Ruthenium based dyads and triads have been investigated with
E-mail: mwolf@chem.ubc.ca
https://groups.chem.ubc.ca/wolf/
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejic.201501436.
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both saturated and π-conjugated linkers between the core and ing the expected C2 symmetry of these ML2L′-type com-
the peripheral substituents, in an effort to mediate electron plexes.[37]
transfer.[4,21–23] As expected, it has been shown that both the
length and the nature of the linker greatly influence the proper-
ties of molecular wire-like electron and energy transfer behavior
within these assemblies.[4,14,24–27]
Solid State Characterization
X-ray quality crystals of heteroleptic complexes 1–4 were grown
from concentrated CH2Cl2 or CH3CN solutions layered with Et2O,
allowing for characterization of these complexes in the solid
state. Of particular interest in these complexes is the orientation
of the thienyl groups with respect to the pyridine rings. Coplan-
Previously, we have reported homoleptic RuII dyads that ex-
hibit long charge-separated state lifetimes resulting from the
formation of an excited state comprised of three equilibrating
states.[28] We have also reported a series of heteroleptic com-
plexes bearing laminate acceptor ligands that undergo direc-
tional photoinduced charge separation to give relatively long
lived charge-separated species.[29] In both of these instances,
the peripheral bithienyl donor groups (that act to both reduc-
tively quench the metal center, and provide an “energy reser-
voir”) are appended to the central chromophore via amide link-
ages. In general, the use of synthetically accessible amide link-
ers has also been previously reported in a number of other
systems with applications in photochemical energy conver-
sion.[13,30–36]
In this work, we report the preparation and characterization
of a series of new RuII complexes bearing thienyl groups ap-
pended by direct C–C, secondary amide and tertiary amide link-
ages (Scheme 1), and the corresponding ligands. The photo-
physical properties of these complexes are correlated in an ef-
fort to understand electronic contributions from the amide link-
age.
Results and Discussion
Synthesis and Characterization
The series of symmetrical ligands described in this work were
prepared using modifications of previously reported procedures
(Scheme S1, and S2). Ligand tL was prepared via Suzuki cou-
pling of 4,4′-dibromo-2,2′-bipyridine with thiophene-2-boronic
acid in the presence of catalytic Pd(PPh3)4. A bithienyl analog
of tL, btL was prepared by a modified Stille coupling.[37,38] Li-
gand sec-amL was prepared using a modified, previously re-
ported procedure where thiophene-2-carboxylic acid chloride
was treated with 4,4′-diamino-2,2′-bipyridine.[28] The sec-amL
ligand was converted into the tert-amL analog via deprotona-
tion of the amide by solid KOH in DMSO, followed by reaction
with CH3CH2I.[39]
Heteroleptic RuII complexes of all the ligands were prepared
by reaction of cis-Ru(bpy)2Cl2 with the desired ligand in EtOH/
H2O (Scheme S3). The target complexes were isolated as hexa-
fluorophosphate salts by addition of a saturated aqueous
NH4PF6 solution resulting in formation of precipitates that were
orange or red in color. These precipitates were purified by col-
umn chromatography with CH3CN/H2O/KNO3(aq) as eluent and
the final products were characterized using 1H and 13C NMR
spectroscopy, high resolution mass spectrometry and various
ground and excited state spectroscopic methods. Although Δ-
and Λ-isomers of these heteroleptic complexes are possible, no
efforts were made to isolate the isomers. 1H and 13C NMR spec-
tra are consistent with symmetrical species in solution, confirm-
Figure 1. Solid state structures of (a) 1, (b) 2, and (c) 3. Thermal ellipsoids are
drawn at 50 % probability. Hydrogen atoms, counterions and occluded sol-
vent molecules omitted for clarity.
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arity between these groups has significant implications in terms region of the spectrum and LC bands in the UV region. These
of electron delocalization to the periphery of the molecule.
In 1 (Figure 1, a), the thiophene rings are observed to be
almost entirely coplanar with respect to the bipyridine rings
with torsion angles of –11.2(5)° (C17–C12–C13–S2) and
10.5(15)° (C2–C3–C4A–S1A; Table S1). The solid-state structure
of 2 is similar to that of 1. In 2 the thiophene rings immediately
adjacent to the bipyridine portion of the ligand are nearly co-
planar to the pyridine rings with torsion angles of –176.6(8)°
(C15–C16–C17–S3) and –173.1(8)° (C12–C3–C4–S1; Table S2).
These rings are slightly out of planarity with respect to the
outer set of thiophene rings with torsion angles of –151(4)° (S3–
C20–C21–S4) and –178.8(10)° (S1–C7–C8–S2). The thienyl sulfur
atoms (S1–S4) on adjacent thiophene rings are in a transoid
arrangement relative to one another. The thiophene rings in 3
(Figure 1, c) are also relatively coplanar with the plane of the
amide linkage, and the pyridine rings (Table S3). This planarity
is reflected in the torsion angles; 1.6(11)° (O1–C11–C12–S1) and
–2.7(12)° (O2–C16–C17–S2). Interestingly, both the amide linka-
ges adopt conformations in the solid state wherein the amide
carbonyl oxygen is facing towards the inside of the bipyridine
plane; the two oxygen atoms are pointed towards each other.
As expected, replacement of the amide hydrogen with an
ethyl group in 4 (Figure 2) results in a dramatically different
geometry at the amide linkage (Table S4). In 4, the thiophene
rings are twisted out of the plane of the pyridine rings through
the C3–N2 and C8–N3 bonds. This results in a conformation
where the amide ethyl groups are oriented below the plane of
the pyridine rings, and the thienyl moieties are located above
the plane of the pyridine rings.
features, albeit bathochromically shifted (in the visible region),
are comparable to those observed in [Ru(bpy)3][PF6]2. In 2, an
additional transition centered at λmax = 400 nm (composed of
two maxima) is observed and attributed to π→π* transitions
localized on each of the bithienyl moieties.[37] The bathochro-
mic shift of the MLCT bands suggests electronic coupling be-
tween the central metal core and the peripheral bithienyl sub-
stituents. This increased degree of conjugation from the periph-
ery to the metal center is further evidenced in the emission
spectra of 1 and 2, where λem exhibits a bathochromic shift
from 610 nm {[Ru(bpy)3][PF6]2} to 638 nm (1) and 675 nm (2)
suggesting a narrowing of the HOMO–LUMO gap. Conversely,
ground state absorption spectra for the heteroleptic complexes
3 and 4 exhibit spectral features mirroring those observed in
[Ru(bpy)3][PF6]2 {a small bathochromic shift is noted in the
MLCT bands of these complexes relative to [Ru(bpy)3][PF6]2,
Δλmax = 10 nm}. In order to further characterize the photophysi-
cal properties of these complexes, a combination of emission
quantum yield (Φ), excited state lifetime (τes) and emission life-
time (τem) data were compared (Table 1).
Figure 3. Absorption and emission spectra for [Ru(bpy)3][PF6]2, 1, 3, 4 in
CH3CN (λex = 450 nm).
Although the emission quantum yield values vary, it is readily
apparent from the data in Table 1 that complexes with amide-
linked thiophene substituents (3, 4 and 6) have quantum yields
comparable to [Ru(bpy)3][PF6]2, while their directly bound ana-
logs have quantum yields that are approximately three times
higher (1 and 5). Similar findings have been reported by
McCusker et al. in homoleptic RuII complexes with 4,4′-di-
phenyl-2,2′-bipyridine (dpb) ligands.[40,41] The quantum yield of
[Ru(dpb)3]2+ was reported to be three times greater than that
observed for [Ru(bpy)3]2+, and these deviations originate from
a planarization of the phenyl rings with respect to the bipyrid-
ine rings once this complex was excited via the lowest energy
ground state absorption.[40] Similar excited state behavior is ex-
Figure 2. Solid state structure of 4. Thermal ellipsoids are drawn at 50 %
probability. Hydrogen atoms and occluded solvent molecules omitted for
clarity.
Photophysical Properties of Complexes 1–6
Complex 1 (Figure 3) and 2 (Figure S1) exhibit ground state pected in complexes 1, 2 and 5 regardless of the planarity of
absorption spectra with Ru dπ→π* (MLCT) bands in the visible the thienyl rings in the solid state.
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Table 1. Selected photophysical properties of the complexes 1–6.
kr[47] [105 s–1
]
knr
[105 s–1
]
[a]
[a,c]
[a]
[b]
[47]
λem [nm]
Φem
τem [μs]
τes [μs]
[Ru(bpy)3][PF6]2
610
638
675
623 (CH3CN)
620 (CH2Cl2)
623 (H2O)
634 (CH3CN)
620 (CH2Cl2)
644 (H2O)
647
0.095[48]
0.33
0.855[49]
1.98
0.925
2.3
15
0.73
1.7
–
11.0
3.4
–
1
2
3
0.11
0.12
0.876 (CH3CN)
1.016 (CH2Cl2)
0.502 (H2O)
0.824 (CH3CN)
0.728 (CH2Cl2)
0.315 (H2O)
1.28
0.836
1.3
10.2
4
0.724
1.5
10.7
5
6
0.35
0.08
1.03
0.450
2.7
1.8
5.1
20.3
642
0.451
[a] All measurements performed in CH3CN, unless noted; solutions were prepared in air and then purged with Ar for 15 min. In the case where measurements
were taken in different solvents, samples were first dissolved in CH3CN (13 % of final volume) and then made up to volume with the target solvent (CH2Cl2
or H2O). [a] λex = 453 nm. [b] λex = 355 nm. [c] Absolute quantum yield at room temperature.
The formation of an excited state with a greater degree of case of 4 it is also possible that replacing the hydrogen atom
electron delocalization results in a longer excited state (as well at the amide nitrogen with an ethyl group strengthens the elec-
as emission) lifetime. This is clearly observed in the case of 1 tron-donating character of the nitrogen, resulting in a greater
while in complex 2, in addition to the long lifetime expected perturbation of the MLCT acceptor orbital (and the observed
for the delocalized MLCT state, additional energy reservoir con- bathochromic shift of the emission maximum).[50]
tributions are also possible, due to the relatively low-lying trip-
let state localized on bithiophene (ET = 2.2 eV).[28,42,43] From
Excited State Properties of Complexes 1–4
nanosecond transient absorption (TA) spectroscopy (vide infra),
it becomes readily apparent that the excited state in 2 differs
significantly from that observed for 1. In stark contrast to 1, 2
and 5, amide-linked complexes 3, 4 and 6 exhibit lifetimes that
are comparable to that of [Ru(bpy)3][PF6]2. Despite the ob-
served lower energy emission maxima for these complexes, no
lifetime enhancement was observed (reflected by much higher
rates of non-radiative decay).
Comparison of the time-resolved excited state spectra of com-
plexes 1, 3, and 4 indicates that a different excited state may
ultimately be formed in all three of these complexes (Figure 4).
Complex 3 exhibits an excited state spectrum mirroring that of
[Ru(bpy)3][PF6]2, with a strong absorbance at ca. 375 nm attrib-
uted to the formation of a bipyridine anion in the MLCT excited
state and a bleaching of the ground state MLCT absorbance
band observed between 400–500 nm.[51–53] This is attributed to
the formation of RuIII, and concomitant depletion of the RuII
species. A weak positive feature in the TA spectrum is observed
centered at ca. 550 nm, and has been previously attributed to
π→π* transitions on a reduced bipyridine moiety.[54]
Of additional interest is the lower energy emission maximum
in 4 relative to 3. In complex 4, installing an ethyl group in
place of the proton at the amide nitrogen results in a “twisted”
amide bond arrangement where, regardless of the excited state
behavior, planarization of the thiophene with respect to the
pyridine is unlikely.[44] Moreover, measurement of the emission
lifetimes of 4 in solvents with differing dielectric constants re-
sults in no appreciable change in lifetime (Δτ = 0.096 μs). Com-
paratively in complex 3, varying the solvent from CH3CN to
CH2Cl2 results in an increase in lifetime, but little shift in the
emission maximum. Both complexes exhibit a shorter lifetime
in H2O than in CH3CN, however this may be attributed to
hydrogen bond formation between the amide linkage and sol-
vent environment.[13,45,46] One possible excited state that may
form in 4 is a ligand-to-ligand charge transfer state (LLCT)
where an electron is transferred from one of the peripheral li-
gands to the putative RuIII center formed on photoexcitation.
Based on the solid-state structure of 4, incorporating an N-ethyl
group into the thienyl-containing ligand decreases the cen-
troid-to-centroid distance of the thienyl group to the Ru core
to ca. 7.4 Å. Another possibility is an LLCT state where an un-
substituted 2,2′-bipyridine ligand is acting to quench RuIII (if the
amide substituted ligand is behaving as the acceptor in the
initial MLCT state).
Figure 4. TA difference spectra comparing 1 (blue), 3 (black), 4 (red) and
[Ru(bpy)3][PF6]2 (teal) in CH3CN (λex = 355 nm).
Although formation of a LLCT state is an attractive proposi-
tion, with implications in applications requiring vectorial charge
The same higher energy feature (centered at ca. 375 nm) is
separation or a large excited state intramolecular dipole, in the observed in the excited state spectrum of 4, concomitant with
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the formation and disappearance of a broad low energy feature unsubstituted [Ru(bpy)3][PF6]2 (Figure 6), however, in both com-
centered at ca. 650 nm. This feature is analogous to one previ- plexes, the lowest energy MLCT band (centered around 480 nm)
ously reported in oxidative spectroelectrochemistry of is bathochromically shifted. From these data, the homoleptic
[Ru(dpb)3]2+ attributed to the formation of a LMCT state.[40,55]
Complex 1 exhibits the most complicated excited state spec-
trum of the series, where outside of the higher energy absorb-
ance at ca. 390 nm and the depletion of the ground state RuII
species, two additional features are observed in the low energy
portion of the spectrum. One feature, centered at 550 nm, mir-
rors that observed in 3, while the low energy shoulder centered
at 650 nm mirrors that observed in 4. This suggests that the
excited state of 1 contains a combination of features observed
for the excited state species of 3 and 4: an LLCT band, and
transitions localized on a reduced bipyridine.[51] It is difficult to
compare 2 to complexes 1, 3, and 4 due to the possibility of
multiple, interfacing excited states arising from the low-lying
3LC states possible on the bithienyl moieties (Figure 5). We have
previously reported that contributions to positive absorption
features at higher energies (≈ 420 nm) may arise from triplet
states localized on peripheral bithienyl moieties, as well as
reduced phenanthroline groups.[56–58] By comparison, the
higher energy feature observed in the excited spectrum of
[Ru(bpy)2(btL)][PF6]2 (2) is also attributed to a triplet state local-
ized on bithienyl moieties appended to the bipyridine.[42] This
is additionally supported by the exceedingly long excited state
lifetime (τ = 15 μs) observed for this complex. Equilibration of
the 3MLCT state with a 3LC state localized on bithiophene gives
rise to an “energy reservoir” effect.[54,59–61]
complexes appear to behave in a similar fashion to their heter-
oleptic analogs. Of additional interest is the modest increase in
emission lifetime observed for 6 when compared to unsubsti-
tuted [Ru(bpy)3][PF6]2 and 1.
Figure 6. Emission and absorption spectra of 5, 6 and [Ru(bpy)3][PF6]2 in
CH3CN (λex = 490 nm).
The bathochromically shifted emission observed in [Ru(sec-
amL)3][PF6]2 (6) is expected based on previously reported stud-
ies of RuII complexes with similar secondary amide substituents.
Meyer and co-workers,[62] as well as George et al.[5] have re-
ported an analogous shift in the emission due to the slight
electron-withdrawing character of the amide linkage (in these
examples, the amide carbonyl was linked to the pyridine ring).
In the case of the sec-amL ligand, attachment of the amide
linkage to the pyridine ring via the amide N atom should result
in a slight increase in the energy level of the acceptor (bipyr-
idine) HOMO; producing a narrowing of the HOMO–LUMO gap,
and a corresponding bathochromic shift in the emission (and
in the ground state absorption spectrum). This narrowing of the
HOMO–LUMO gap, and lowering of the emission energy may
account for the shorter excited state lifetime observed in 6.
Conversely, the bathochromic shifts in the absorption and
emission spectra of 5 correlate to the same principles observed
in the analogous heteroleptic complex 1: a planarization of the
thiophene rings, and a corresponding delocalization of the pho-
toexcited electron.
Figure 5. TA difference spectra of 1, 2 and [Ru(bpy)3][PF6]2 in CH3CN (λex
355 nm, fwhm = 35 ps).
=
By comparing the excited state difference spectra for 5 and
6 (Figure 7) it becomes apparent that the spectrum for 5 closely
resembles that of 1. Examining the excited state spectrum in
the UV region reveals that complex 6 exhibits a narrow absorp-
tion centered at 325 nm that is absent in the spectrum of 5.
It is possible that a thiophene moiety may act to reductively
Photophysical and Excited State Properties of Homoleptic
Complexes 5 and 6
Homoleptic complexes 5 and 6 exhibit comparable emission quench the putative oxidized RuIII center resulting in formation
maxima (Δλem = 5 nm, Figure 6), that are both significantly of a hole on a thienyl moiety, as well as an electron localized
bathochromically shifted when compared to [Ru(bpy)3][PF6]2. on a bipyridine moiety. This LLCT state may be giving rise to
Much as in heteroleptic complexes 1 and 3, both 5 and 6 ex- this previously unobserved high energy feature (λmax
=
hibit a ground state absorption spectrum similar to that of the 325 nm). Transient absorption of 6 in the presence of low con-
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Conclusions
2+
Introducing an amide bridge between the central Ru(bpy)3
chromophore and peripheral thienyl groups alters the photo-
physical properties of both homoleptic and heteroleptic com-
plexes bearing bipyridine ligands with appended thienyl
groups. Despite the partial double bond character of the amide
linkage, and the planarity observed within the ligand in the
solid state, the amide linkage does not act as a direct conjuga-
tion path to the thienyl moieties. This is evidenced through the
relatively short emission and excited state lifetimes observed in
complexes 3, 4 and 6. In addition, the quantum yields of com-
plexes 1 and 5 with directly bound thienyl groups are much
higher than those of the amide-linked analogs 3 and 6. Incor-
poration of an amide bridge does not eliminate the possibility
of LLCT states, where the peripheral thienyl moieties may act
to reductively quench the RuIII formed on photoexcitation. This
is evidenced in the analysis of complex 6 via the short excited
state and emission lifetimes associated with this complex.
Creating a charge-separated state by photoinduced intramo-
lecular electron transfer is a long-standing goal of solar energy
research. In artificial photosynthetic reaction centers where an
amide linkage is used to bind peripheral electron-donating
groups to a central chromophore, it is important to understand
the role of the linkage. The results presented in this work sug-
gest that although the amide linkage may not be acting as a
conjugated linker between two chromophores, it also does not
hinder intramolecular electron transfer processes.
Figure 7. Comparison TA difference spectra of complexes [Ru(tL)3][PF6]2 (5,
red) and [Ru(sec-amL)3][PF6]2 (6, black) (t = 0–50 ns, CH3CN, λex = 355 nm).
centrations of an electron acceptor, methyl viologen (MV2+), re-
sults in quenching of the positive absorption band centered at
400 nm, while the band at 325 nm remains unchanged (Fig-
ure 8). The lack of quenching of the band at 325 nm suggests
that this band may arise from a thiophene cation radical formed
via an LLCT state. The feasibility of an LLCT intramolecular elec-
tron transfer can be approximated using the Rehm–Weller
equation;[63,64] where the potentials corresponding to reduction
of the acceptor and oxidation of the donor can be estimated
from the electrochemistry of complex 3 (Figure S2, Table S5)
with the oxdation of the thiophene moieties estimated from
literature values.[65] The result of this analysis indicates that
electron transfer from a thienyl moiety to RuIII is energetically
possible, ΔGET = –1.28 eV (–128 kJ mol–1). This quenching of
Experimental Section
General Procedures: Anhydrous THF was obtained via distillation
from Na/benzophenone, and anhydrous CH2Cl2 was obtained via
distillation from CaH. All other solvents were used as received. Silica
gel used in purification was purchased from Silicycle (SiliaFlash F60
or G60). 2,2′-Bipyridine and NH4PF6 (Alfa), thiophene-2-boronic acid,
methyl viologen dichloride (Sigma), Pd(PPh3)4 and RuCl3·xH2O
(Strem), were purchased from commercial sources and used as re-
ceived. Heterocyclic precursors 2,2′-bipyridine N,N′-dioxide,[66] 4,4′-
dinitro-2,2′-bipyridine N,N′-dioxide,[67] 4,4′-dibromo-2,2′-bipyr-
idine[68] and 4,4′-diamino-2,2′-bipyridine,[67] were all prepared as
previously reported. Metal-containing precursor Ru(bpy)2Cl2 was
also prepared according to published procedures.[69] Ru(tL)2Cl2 and
Ru(sec-amL)2Cl2 were prepared using a slightly modified procedure
where Ru(DMSO)4Cl2 (prepared as in ref.[70]) was combined with
2 equiv. of the desired ligand in EtOH or EtOH/H2O, in the presence
of LiCl. The resulting precipitates were collected, the expected struc-
ture was confirmed by MALDI-TOF MS, and used in the following
reactions without further characterization or purification due to
poor solubility and the instability of the complexes to recrystalliza-
tion.[71] [Ru(bpy)3][PF6]2 was prepared by addition of a saturated
NH4PF6(aq) solution to an aqueous solution of [Ru(bpy)3]Cl2 (Strem).
The resulting bright orange precipitate was collected by filtration
3
the excited state MLCT state by a LLCT state may account for
the shortened lifetime and lower quantum yield observed in
complex 6 as compared to [Ru(bpy)3][PF6]2 and complex 3.
1
and used without any further purification. H and 13C NMR spectra
were obtained on Bruker AV-300, Bruker AV-400Direct or Bruker AV-
400Indirect spectrometers. All chemical shifts in 1H and 13C spectra
were referenced to residual solvent, and the splitting patterns are
designated as s (singlet), d (doublet), t (triplet), m (multiplet) or br
(broad). NMR solvents (Cambridge Isotope Laboratories or Sigma)
were used as received. ESI MS (Waters ZQ with ESCI ion source)
Figure 8. Excited state spectra of [Ru(sec-amL)3][PF6]2 (5) in the presence of
MV2+ (1 m
M) in CH3CN (t = 0–50 ns, λex = 355 nm, fwhm = 35 ps).
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and HR-ESI MS (Bruker Esquire LC-MS) were obtained at the UBC
Microanalysis facility. Samples for spectroscopic measurements
heated at 100 °C for 3 d; during this time, the reaction was moni-
tored by TLC (98:2 CHCl3/MeOH). The reaction mixture was then
were prepared using HPLC-grade Fisher solvents. Electronic absorp- filtered to remove any insoluble solids that had formed, concen-
tion spectra were recorded on a Varian-Cary 5000 UV/Vis-near-IR trated under reduced pressure, and dissolved in 50 mL of anhydrous
spectrophotometer. Emission spectra and quantum yield measure- toluene with the addition of another 1 equiv. (0.48 mmol) of [2,2′-
ments were recorded on a Photon Technology International Quan- bithiophen]-5-yltri-tert-butylstannane and an additional 3 mol-% of
taMaster 50 fluorimeter fitted with an integrating sphere, double
monochromator and utilizing a 75W Xe arc lamp as the source.
Fluorescence lifetime measurements were carried out using a Hor-
Pd(PPh3)4. After an additional 24 h heating at reflux, TLC analysis of
the reaction mixture showed no starting material remained. The
reaction mixture was cooled to room temperature and reduced in
iba Jobin Yvon TBX Picosecond Photon Detection Module (Nanoled volume to approximately 3 mL under reduced pressure and left at
λex = 453 nm). Transient absorbance spectra were collected using
a Princeton Instruments Spectra Pro 2300i Imaging Triple Grating
Monochromator/Spectrograph with a Hamamatsu Dynamic Range
4 °C overnight, resulting in the formation a yellow/orange precipi-
tate, the title compound previously reported in literature.[35] 1H
NMR (400 MHz, CDCl3): δ = 7.04–7.09 (m, 1 H) 7.12 (d, J = 3.35 Hz,
1 H) 7.23 (d, J = 3.35 Hz, 1 H) 7.27–7.31 (m, 1 H) 7.51 (d, J = 3.65 Hz,
Streak Camera (excitation source: EKSPLA Nd:YAG laser, λex
=
355 nm, fwhm = 35 ps). Crystal structure data for complexes 1–4 1 H) 7.58 (d, J = 3.35 Hz, 1 H) 8.65 (br. s, 1 H) 8.69 (d, J = 3.96 Hz, 1
were obtained on a Bruker APEX DUO diffractometer with graphite
monochromated Mo-Kα radiation at 90 K. Data were collected and
integrated using the Bruker SAINT-69[72] software package. Data
were corrected for absorption effects using the multi-scan tech-
nique (SADABS or TWINABS[73]). The structures were solved by di-
rect methods.[74] All non-hydrogen atoms were refined anisotropi-
cally; hydrogen atoms were placed in calculated position. Diagrams
were drawn using CCDC Mercury 3.0.[75] Refinements for complexes
1, 2 and 4 were performed using SHELXL-97[76] via the WinGX[77]
H) ppm. 13C (100.63 MHz, CDCl3): δ = 117.23, 119.85, 124.68, 125.06,
125.47, 126.74, 128.34, 136.58, 137.19, 139.60, 150.07, 156.80 ppm.
ESI MS m/z 485.1 ([M + H]+).
N,N′-([2,2′-Bipyridine]-4,4′-diyl)bis(thiophene-2-carboxamide)
(sec-amL): Thiophene-2-carboxylic acid (120 mg, 0.939 mmol) was
added to 5 mL of anhydrous CH2Cl2. The mixture was cooled to
0 °C, and SOCl2 (0.2 mL, 2.818 mmol) was added dropwise followed
by 2 drops of DMF. This reaction mixture was warmed to room
temperature and then heated to reflux for 1 h. After 1 h, the solvent
was removed under reduced pressure and the residue (thiophene-
2-carbonyl chloride) was used in the next step without further puri-
fication or characterization. 4,4′-Diamino-2,2′-bipyridine (159 mg,
0.854 mmol) was dissolved in 40 mL of anhydrous THF, along with
Hünig's base (0.7 mL). This mixture was added to the acyl chloride
residue collected above, which had been cooled to 0 °C. A substan-
tial amount of white vapor formed (presumed to be HCl(g)), and
was removed from the reaction vessel with a steady stream of N2.
The reaction was warmed to room temperature, and then heated
to reflux overnight under N2. After heating, the solvent was re-
moved under reduced pressure, yielding a brown oily residue. On
addition of H2O to this residue, a white solid formed and was col-
lected by filtration. This white solid was recrystallized from MeOH/
H2O to yield the title compound, yield 139 mg (40 %). 1H NMR
(400 MHz, [D6]DMSO): δ = 7.27 (dd, J = 5.12, 3.75 Hz, 1 H), 7.94 (dd,
J = 4.95, 1.19 Hz, 1 H), 7.98 (dd, J = 5.46, 2.05 Hz, 1 H), 8.17 (dd, J =
3.76, 1.02 Hz, 1 H), 8.60 (d, J = 5.46 Hz, 1 H), 8.77 (d, J = 2.05 Hz, 1
H), 10.71 (s, 1 H, N-H) ppm. 13C (100.63 MHz, CDCl3): δ = 111.39,
114.70, 128.96, 130.76, 133.67, 139.78, 147.43, 150.66, 156.59, 161.30
ppm. ESI MS m/z 407.3 ([M + H]+).
interface. Refinements for complex
3 were performed using
SHELXL-2012[76] via the Olex2[78] interface.
Syntheses
4,4′-Di(thiophen-2-yl)-2,2′-bipyridine (tL): THF (10 mL), 4,4′-di-
bromo-2,2′-bipyridine (151 mg, 0.481 mmol), thiophene-2-boronic
acid (123 mg, 0.961 mmol) were loaded into a round-bottomed
flask, under N2. The mixture was purged with N2 for 20 min, after
which aqueous 2
M Na2CO3 (1 mL) was added. The biphasic reaction
mixture was purged with N2 for a further 5–10 min. Subsequently,
Pd(PPh3)4 (3 mol-%, 17 mg) was added and the mixture was heated
to 90 °C. The reaction progress was monitored by TLC (8:2 Hex/
EtOAc) and after 20 h, 4,4′-dibromo-2,2′-bipyridine was no longer
observed. A spot near the baseline of the TLC plate was observed,
and attributed to a mono-substituted product, so an additional por-
tion of thiophene-2-boronic acid (23 mg, 0.18 mmol) was added to
the reaction mixture. After 27 h of total reaction time, the reaction
was deemed to have reached completion by TLC (8:2 Hex/EtOAc
and 98:2 CHCl3/MeOH), and was cooled to room temperature. THF
was removed from the reaction mixture under reduced pressure,
and the residue was dissolved in CH2Cl2 then washed with water
(3 × 10 mL) to remove any residual salts. The organic layer was
separated and dried with anhydrous Na2SO4, then concentrated un-
N,N′-([2,2′-Bipyridine]-4,4′-diyl)bis(N-ethylthiophene-2-carbox-
amide) (tert-amL): Crushed KOH (224 mg, 3.99 mmol) was added
der reduced pressure. The concentrated mixture was passed to DMSO (2 mL). After 5 min of stirring this suspension, sec-amL
through a silica gel plug on a frit with 98:2 CHCl3/MeOH eluent. (63 mg, 0.155 mmol) was added, followed by CH3CH2I (0.33 mL,
The collected washings were combined, and the solvent was re- 4.0 mmol). The resulting suspension was stirred for 30 min and then
moved under reduced pressure to yield a grey solid; the title com-
pound, yield 75 mg (66 %). H NMR (400 MHz, CDCl3): δ = 7.16 (dd,
poured into 10 mL of a 1:1 MeOH/H2O mixture. This mixture was
left at 4 °C for 2 d, resulting in the formation of a small quantity of
white powder (the title compound) that was collected by filtration,
1
J = 4.95, 3.58 Hz, 1 H), 7.44 (dd, J = 5.12, 1.02 Hz, 1 H), 7.53 (dd, J =
5.12, 1.71 Hz, 1 H), 7.67 (dd, J = 3.75, 1.02 Hz, 1 H), 8.66–8.71 (m, 2
1
yield 10 mg (14 %). H NMR (300 MHz, [D6]DMSO): δ = 1.14 (t, J =
H) ppm. 13C (100.63 MHz, CDCl3): δ = 117.07, 119.73, 125.39, 126.91, 7.08 Hz, 3 H), 3.94 (q, J = 7.16 Hz, 2 H), 6.92 (d, J = 3.43 Hz, 2 H),
128.07, 141.07, 142.17, 149.34, 156.09 ppm. ESI MS m/z 321.2 ([M +
7.40 (dd, J = 5.25, 2.28 Hz, 1 H), 7.67–7.71 (m, 1 H), 8.25 (d, J =
1.83 Hz, 1 H), 8.65 (d, J = 5.25 Hz, 1 H) ppm. ESI MS m/z 485.1 ([M
+ Na]+).
H]+).
4,4′-Di([2,2′-bithiophen]-5-yl)-2,2′-bipyridine
(btL):
4,4′-Di-
bromo-2,2′-bipyridine (150 mg, 0.478 mmol) and 2.5 equiv. of
[2,2′-bithiophen]-5-yltri-tert-butylstannane[79] (546 mg, 0.45 mL,
1.20 mmol) were added to 50 mL of anhydrous toluene under N2.
The solution was purged with N2 for 20 min, after which 3 mol-%
of Pd(PPh3)4 (27 mg, 0.02 mmol) was added. The mixture was
General Procedure for Preparation of Heteroleptic RuII Com-
plexes: Ru(bpy)2Cl2 and 1.1 equiv. of the desired ligand were sus-
pended in a mixture of 1:1 EtOH/H2O (20 mL) and purged for
30 min with N2. The mixture was heated to reflux under N2, and
left heating at this temperature overnight. After heating, the bright
Eur. J. Inorg. Chem. 2016, 1470–1479
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Full Paper
orange to deep red solution was filtered hot through glass wool to
was then centrifuged to remove any insoluble impurities, and the
remove any insoluble impurities, and treated with a minimal supernatant was decanted. This dark red solution was filtered
amount (ca. 2 mL) of a saturated solution of NH4PF6(aq). The result- through glass wool into a saturated aqueous solution of NH4PF6 to
ing dark orange precipitate was collected by vacuum filtration, yield a deep red precipitate that was collected by vacuum filtration.
washed with small amounts (2–5 mL) of H2O and EtOH and left to
air dry. This crude product was further purified by column chroma- 4:1 EtOH/H2O and heated to reflux overnight after the addition of
tography. The product was dissolved in a small volume of CH3CN a second portion of tL (20 mg, 0.062 mmol). The same work up as
and eluted through a silica gel column using a 96:3:1 CH3CN/H2O/ above was performed, and a second portion of deep red product
The solid collected from centrifugation was suspended in 10 mL of
–
KNO3(aq) solvent mixture. The major orange band (a NO3 salt of
the target complex) was collected, and the solvent was removed
under reduced pressure. The residue was dissolved in a small vol-
was collected. The two products were combined and chromato-
graphed on silica gel with a 96:3:1 CH3CN/H2O/KNO3(aq) eluent. The
major dark red band was collected, concentrated under reduced
ume of MeOH, passed through glass wool (to remove insoluble pressure and treated with a solution of NH4PF6(aq) to yield the de-
KNO3) into an aqueous saturated solution of NH4PF6. The resulting
precipitate was collected by vacuum filtration, washed with small
portions (2 mL) of H2O, EtOH and Et2O (sequentially) and left to air
dry. X-ray quality crystals of these complexes were subsequently
obtained by preparation of a saturated solution in CH2Cl2 or CH3CN,
and layering this solution with a small volume of Et2O.
sired product, yield 39 mg (36 %). 1H NMR [400 MHz, (CD3)2CO]: δ =
7.32 (dd, J = 5.03, 3.81 Hz, 2 H) 7.79 (dd, J = 6.09, 1.83 Hz, 2 H) 7.84
(dd, J = 5.03, 0.76 Hz, 2 H) 8.04 (dd, J = 3.65, 0.91 Hz, 2 H) 8.19 (d,
J = 6.09 Hz, 2 H) 9.24 (d, J = 1.83 Hz, 2 H) ppm. 13C [150.91 MHz,
(CD3)2CO]: δ = 121.29, 124.69, 129.99, 130.64, 131.67, 140.14, 144.32,
153.30, 158.97 ppm. HR-ESI MS m/z [M]+ calcd. for C54H36N6F6PS6Ru,
1204.0044; found 1204.0027 (δ =1.4 ppm).
[Ru(bpy)2(tL)][PF6]2 (1): Yield 30 mg (37 %). 1H NMR (400 MHz,
CD3CN): δ = 7.29 (dd, J = 4.78, 3.76 Hz, 1 H) 7.38–7.45 (m, 2 H) 7.56
(dd, J = 5.80, 2.05 Hz, 1 H) 7.66 (d, J = 5.80 Hz, 1 H) 7.73 (ddd, J =
9.81, 5.38, 0.85 Hz, 2 H) 7.86 (dd, J = 5.80, 0.68 Hz, 1 H) 7.96 (dd,
J = 3.76, 1.02 Hz, 1 H) 8.07 (td, J = 8.02, 1.02 Hz, 2 H) 8.51 (d, J =
8.19 Hz, 2 H) 8.77 (d, J = 1.71 Hz, 1 H) ppm. 13C (100.63 MHz,
CD3CN): δ = 118.40, 121.45, 124.53, 125.62, 128.98, 129.95, 130.70,
131.64, 139.19, 140.18, 153.04, 158.34, 158.62 ppm. HR-ESI MS m/z
[M]+ calcd. for C38H28N6F6PS2Ru, 873.0524; found 873.0535
(–1.3 ppm).
[Ru(sec-amL)3][PF6]2 (6): Ru(sec-amL)2Cl2 (47.2 mg, 0.0509 mmol)
and sec-amL (20.7 mg, 0.0509 mmol) were suspended in 10 mL of
4:1 EtOH/H2O, purged with N2, and heated to reflux. The mixture
was heated to reflux overnight, and during this time the chloride
salt of the desired product precipitated from the solution. [Ru(sec-
amL)3]Cl2 was collected by vacuum filtration, and then dissolved in
1:1 THF/MeOH with heating. This solution was treated with a satu-
rated solution of NH4PF6(aq), and the mixture concentrated under
reduced pressure resulting in the formation of title compound as a
dark orange precipitate. This precipitate was suspended in a small
volume of H2O and filtered. This precipitate was eluted through a
basic alumina column using acetone. The major orange band was
collected, the solvent removed under reduced pressure. The result-
ing residue was treated with a saturated NH4PF6(aq) solution to yield
the title complex as a dark orange precipitate, yield 25 mg (30 %).
1H NMR (400 MHz, CD3CN): δ = 7.24 (dd, J = 5.03, 3.81 Hz, 2 H),
7.68–7.74 (m, 4 H), 7.81 (d, J = 4.87 Hz, 2 H), 7.93 (d, J = 3.65 Hz, 2
H), 9.04 (d, J = 1.52 Hz, 2 H), 9.39 (s, 2 H) ppm. 13C (100.63 MHz,
CD3CN): δ = 114.65, 118.23, 129.92, 131.58, 134.89, 139.80, 148.39,
153.31, 159.08, 162.57 ppm. HR-ESI MS m/z [M]+ calcd. for
[Ru(bpy)2(btL)][PF6]2 (2): The entire product obtained from the
synthetic procedure was used in preparing a crystallographic sam-
ple, characterization, and performing spectroscopic measurements.
No yield was calculated. 1H NMR (400 MHz, CD3CN): δ = 7.14 (dd,
J = 5.18, 3.65 Hz, 1 H) 7.37–7.45 (m, 5 H) 7.46–7.55 (m, 2 H) 7.65 (d,
J = 6.09 Hz, 1 H) 7.74 (d, J = 4.87 Hz, 1 H) 7.84–7.88 (m, 1 H) 7.92
(d, J = 3.96 Hz, 1 H) 8.07 (td, J = 7.92, 1.52 Hz, 3 H) 8.51 (d, J =
8.53 Hz, 3 H) 8.74 (d, J = 1.83 Hz, 1 H) ppm. HR-ESI MS m/z [M]+
calcd. for C46H32N6F6PS4Ru, 1040.0247; found 1040.0273 (–2.5 ppm).
[Ru(bpy)2(sec-amL)][PF6]2 (3): The crude product was purified by
silica gel chromatography using 50:6:2 CH3CN/H2O/KNO3(aq) as the
eluent, yield 62 mg (27 %). 1H NMR (400 MHz, CD3CN): δ = 7.23 (dd,
J = 4.95, 3.93 Hz, 1 H), 7.36–7.48 (m, 2 H), 7.54 (d, J = 6.49 Hz, 1 H),
7.66 (dd, J = 6.32, 2.22 Hz, 1 H), 7.72–7.82 (m, 2 H), 7.87–7.96 (m, 2
H), 8.05 (tdd, J = 7.89, 7.89, 5.03, 1.37 Hz, 2 H), 8.46–8.54 (m, 2 H),
9.03 (d, J = 2.39 Hz, 1 H), 9.49 (s, 1 H) ppm. 13C (100.63 MHz, CD3CN):
δ = 114.71, 118.28, 125.52, 128.82, 128.90, 129.91, 131.63, 134.89,
138.90, 139.75, 148.79, 153.07, 153.15, 158.44, 158.57, 158.76, 162.58
ppm. HR-ESI MS m/z [M]+ calcd. for C40H30N8F6PO2S2Ru, 962.0641;
found 962.0634 (δ =0.7 ppm).
C60H41N12O6S6Ru, 1316.0685; found 1316.0656 (δ =2.2 ppm).
X-ray Crystallography. 1: C39.5H31Cl3F12N6P2RuS2, M = 1151.18, tri-
clinic, space group P1 (No. 2), Z = 2, a = 11.372(2) Å, b = 13.702(3) Å,
¯
c = 16.258(3) Å, α = 68.688(3)°, ꢀ = 71.222(3)°, γ = 68.372(4)°, V =
2141.6(7) Å3, T = 90 K, 47661 reflections measured, 10188 unique
(Rint = 0.040), final R1 [I > 2.00σ(I)] = 0.048, wR2 [I > 2.00σ(I)] = 0.121.
2: C48H36Cl4F12N6P2RuS4, M = 1357.88, monoclinic, space group P21/
c (No. 14), Z = 4, a = 19.9343 (17) Å, b = 17.6009 (14) Å, c = 15.8490
(13) Å, ꢀ = 108.735 (2)°, V = 5266.2 (8) Å3, T = 90 K, 42110 reflections
measured, 9544 unique (Rint = 0.045), final R1 [I > 2.00σ(I)] = 0.085,
wR2 [I > 2.00σ(I)] = 0.196.
[Ru(bpy)2(tert-amL)][PF6]2 (4): Yield
5
mg (24 %). 1H NMR
(400 MHz, CD3CN): δ = 1.21 (t, J = 7.17 Hz, 3 H) 4.01 (qd, J = 7.11,
1.88 Hz, 2 H) 6.84 (dd, J = 4.95, 3.93 Hz, 1 H) 6.91 (dd, J = 3.76,
1.02 Hz, 1 H) 7.20 (dd, J = 6.14, 2.39 Hz, 1 H) 7.38 (ddd, J = 7.42,
5.89, 1.37 Hz, 1 H) 7.51–7.54 (m, 1 H) 7.56 (d, J = 6.14 Hz, 1 H) 7.73
(d, J = 5.80 Hz, 1 H) 7.77 (d, J = 5.46 Hz, 1 H) 8.05 (dd, J = 15.70,
1.37 Hz, 1 H) 8.11–8.16 (m, 2 H) 8.50 (t, J = 8.19 Hz, 3 H) ppm. HR-
ESI MS m/z [M]+ calcd. for C44H38N8O2F6PS2Ru, 1018.1259; found
1018.1260 (–0.1 ppm).
3: C44H36F12N10O P RuS , M = 1191.96, triclinic, space group P1 (No.
¯
2
2
2
2), Z = 2, a = 11.721 (3) Å, b = 13.510 (3) Å, c = 18.483 (4) Å, α =
94.061 (4)°, ꢀ = 98.273 (4)°, γ = 93.938 (4)°, V = 2880.0 (11) Å3, T =
90 K, 62120 reflections measured, 10593 unique (Rint = 0.067), final
R1 [I > 2.00σ(I)] = 0.087, wR2 [I > 2.00σ(I)] = 0.239.
4: C45H40Cl2F12N8O2P2RuS2, M = 1250.88, monoclinic, space group
C 2/c (No. 15), Z = 5, a = 39.552 (6) Å, b = 21.814 (4) Å, c = 11.961
(2) Å, ꢀ = 99.822 (2)°, V = 10169 (3) Å3, T = 90 K, 43504 reflections
measured, 9064 unique (Rint = 0.065), final R1 [I > 2.00σ(I)] = 0.069,
wR2 [I > 2.00σ(I)] = 0.187.
[Ru(tL)3][PF6]2 (5): Ru(tL)2Cl2 (68 mg, 0.080 mmol) and an excess
of tL (40 mg, 0.12 mmol) were suspended in 10 mL of 4:1 EtOH/
H2O which had been purged for 30 min with N2. The mixture was
heated to reflux and left at that temperature for 24 h. The mixture
Eur. J. Inorg. Chem. 2016, 1470–1479
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Keywords: Photophysics · Electron transfer · Donor–acceptor
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