Ruthenium(II)–Pyridylimidazole Complexes as Photoreductants and PCET Reagents

Article abstract of DOI:10.1002/ejic.201601403

Complexes of the type [Ru(bpy)₂pyimH]²⁺[bpy = 2,2′-bipyridine; pyimH = 2-(2-pyridyl)imidazole] with various substituents on the bpy ligands can act as photoreductants. Their reducing power in the ground state and in the long-lived³MLCT excited state is increased significantly upon deprotonation, and they can undergo proton-coupled electron transfer (PCET) in the ground and excited state. PCET with both the proton and electron originating from a single donor resembles hydrogen atom transfer (HAT) and can be described thermodynamically by formal bond dissociation free energies (BDFEs). Whereas the class of complexes studied herein has long been known, their N–H BDFEs have not been determined even though this is important in view of assessing their reactivity. Our study demonstrates that the N–H BDFEs in the³MLCT excited states are between 34 and 52 kcal mol^–1depending on the chemical substituents at the bpy spectator ligands. Specifically, we report on the electrochemistry and PCET thermochemistry of three heteroleptic complexes in 1:1 (v/v) CH₃CN/H₂O with CF₃, tBu, and NMe₂substituents on the bpy ligands.

Full text of DOI:10.1002/ejic.201601403

DOI: 10.1002/ejic.201601403
Full Paper
Proton–Coupled Electron Transfer
Ruthenium(II)–Pyridylimidazole Complexes as Photoreductants
and PCET Reagents
Andrea Pannwitz,[a] Alessandro Prescimone,[a] and Oliver S. Wenger*[a]
Abstract: Complexes of the type [Ru(bpy)2pyimH]2+ [bpy = Whereas the class of complexes studied herein has long been
2,2′-bipyridine; pyimH = 2-(2-pyridyl)imidazole] with various known, their N–H BDFEs have not been determined even
substituents on the bpy ligands can act as photoreductants. though this is important in view of assessing their reactivity.
3
Their reducing power in the ground state and in the long-lived Our study demonstrates that the N–H BDFEs in the MLCT ex-
3MLCT excited state is increased significantly upon deproton- cited states are between 34 and 52 kcal mol–1 depending on
ation, and they can undergo proton-coupled electron transfer the chemical substituents at the bpy spectator ligands. Specifi-
(PCET) in the ground and excited state. PCET with both the cally, we report on the electrochemistry and PCET thermochem-
proton and electron originating from a single donor resembles istry of three heteroleptic complexes in 1:1 (v/v) CH3CN/H2O
hydrogen atom transfer (HAT) and can be described thermo- with CF3, tBu, and NMe2 substituents on the bpy ligands.
dynamically by formal bond dissociation free energies (BDFEs).
Introduction
We report here on the ground- and excited-state properties
of a family of ruthenium–diimine complexes bearing a pyimH
ligand (Figure 1). The imidazole unit of the pyimH ligand can
be deprotonated, and this drastically influences the redox prop-
erties of the entire complex. Strong reductants are accessible
by photoexcitation, particularly when combined with deproto-
nation.
In the context of photochemistry and solar-energy conversion,
transition-metal complexes traditionally play an important role.
Especially d6 metal complexes such as Ru(bpy)3 are useful
2+
photosensitizers, due to the long-living 3MLCT excited states
that are accessible with visible light irradiation.[1,2] Redox proc-
esses from this state are energetically favored compared with
the ground state, and absorbed light energy can be trans-
formed into chemical energy, for example by photoredox catal-
ysis in organic synthesis,[3–5] or the generation of so-called solar
fuels.[6–8] In the general context of redox catalysis and charge
transfer, the coupling to proton transfer can play a crucial role,
because energy barriers can be lowered substantially by pro-
ton-coupled electron transfer (PCET), and reduction products
can be stabilized by protonation, whereas oxidation products
can be stabilized by deprotonation. It would be attractive to
combine the benefits of PCET with the principle of using light
as the principal energy input.[9–13] In some previous studies,
photons were used to generate reactive species that could sub-
sequently undergo PCET in the electronic ground state.[14–19]
A very promising approach is the use of photoexcited metal
complexes and external bases or acids.[10,11,20,21] PCET reactions
and even hydride transfer directly involving the excited-state
species are possible and have received increasing atten-
tion.[22–32]
Figure 1. Investigated [RuRpyimH]2+ complexes and their PCET reactivity.
Recently, we reported on the [RuRpyimH]2+ complex with R =
H, in which two unsubstituted bpy spectator ligands were
present.[27] Building on prior work,[33,34] we demonstrated that
the combined release of an electron and a proton makes this
complex a very strong (formal) hydrogen atom donor in the
long-lived 3MLCT excited state. Mechanistically, formal
hydrogen atom transfer (HAT) is a PCET process, but thermody-
namically the determination of a formal N–H BDFE is meaning-
ful.[35,36] For the [RuRpyimH]2+ complex with R = H, we deter-
mined 43 kcal mol–1 in 1:1 (v/v) CH3CN/H2O. In this work, we
explored to what extent the formal N–H BDFE is tunable by
altering the bpy spectator ligands. This seems important in view
[a] Department of Chemistry, University of Basel
St. Johanns-Ring 19 and Spitalstrasse 51, 4056 Basel, Switzerland
E-mail: oliver.wenger@unibas.ch
www.chemie.unibas.ch/~wenger/index.html
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.201601403.
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of tailoring the excited-state redox and PCET reactivity for spe-
cific applications.
Monocrystalline needles of [RuNMe2pyimH](PF6)2 were ob-
tained by slow diffusion of diethyl ether into a solution of the
compound in acetonitrile (Figure 2). It crystallized in space
In general, the BDFE for an X–H bond can be estimated from
the reaction free energies associated with the individual elec-
tron and proton transfer steps, as shown in the so-called square
scheme in Scheme 1. Proton transfer (PT) and electron transfer
(ET) are thermodynamically characterized by the acidity con-
stant (pKa) and the redox potential (E°), respectively. In aqueous
solution, the X–H BDFE can be calculated from Equation (1),
where E° must be entered in units of V vs. NHE, and the last
summand is a solvent-characteristic parameter describing solv-
ation of hydrogen atoms.[35]
–
group P1¯ with two PF6 counterions, 0.5 acetonitrile, and 0.5
diethyl ether molecules per complex in the asymmetric unit.
Crystallographic details are included in the Supporting Informa-
tion.
BDFE(X–H) = 1.37 pKa + 23.06 E° + 57.6 kcal mol–1
(1)
Figure 2. Crystal structure of [RuNMe2pyimH]2+. Thermal ellipsoids are drawn
at the 50 % probability level. Solvent molecules, counterions, and hydrogen
atoms, except N–H, are omitted for clarity.
Scheme 1. Thermochemical square scheme for the cleavage of X–H bonds
by individual deprotonation (pKa, pKaox) and oxidation steps (Eprot, Edep).
As noted above, the complex with R = H exhibits a formal
N–H BDFE of only 43 kcal mol–1,[27] which is comparable to
metal hydride complexes such as HV(CO)4(P-P) [with P-P =
Redox and Acid-Base Chemistry
Ph2P(CH2)nPPh2] or (Cp)Cr(CO)3H (with Cp = cyclopentadienyl). As described in the Introduction, the formal BDFE of the pyimH
The low N–H BDFE in our [RuHpyimH]2+ complex results N–H functionality can be calculated on the basis of redox and
from the conversion of light energy into chemical energy, and acid-base properties to predict formal HAT or PCET behavior in
it is manifested by the lowering of the N–H BDFE by ca. the ground and excited states.[27,35] For the [RuRpyimH]2+ com-
50 kcal mol–1 between the ground and the excited state. This plexes with R = CF3, tBu and NMe2, the relevant parameters
energy difference corresponds essentially to the absorbed visi- were determined analogously to our previously published ex-
ble photon. In this work, three derivatives of the parent com- ample of [RuHpyimH]2+ [27]
For the determination of the
.
plex with different chemical substituents at the 4- and 4′-posi- ground-state acidity constant (pKa), the spectral changes occur-
1
tions of the bpy spectator ligands are characterized in their ring in the spectral range of the MLCT absorption band were
ground and excited states in 1:1 (v/v) CH3CN/H2O mixture. We monitored as a function of pH (see the Supporting Information,
find that it is possible to tune the excited-state BDFEs between Figures S1–S3). Pourbaix diagrams (Figure 3) were established
52 kcal mol–1 (R = CF3) and 34 kcal mol–1 (R = NMe2), with the on the basis of cyclic voltammograms recorded at different pH
latter being an unusually low value.[10,37,38]
values (see the Supporting Information, Figures S4–S6). There
are three different regimes: At very basic pH, the redox poten-
tial Edep is pH-independent, because the complex is deproto-
nated in both the RuII and RuIII oxidation states. The redox po-
tential in the strongly acidic pH regime, Eprot, is also pH-inde-
pendent, because the complex is protonated irrespective of
whether the oxidation state is +II or +III. In the intermediate pH
Results and Discussion
Synthesis and Crystallographic Characterization
All commercially available chemicals for synthesis, including regime, the redox potential is expected to shift with a slope of
4,4′-di-tert-butyl-2,2′-bipyridine and RuCl3·3H2O, were used as –59 mV per pH unit. For R = tBu and NMe2, the oxidation proc-
received. The syntheses of the ligands 2-(2-pyridyl)imidazole ess is reversible and the slopes in the one-electron, one-proton
(pyimH) and 4,4′-bis(dimethylamino)-2,2′-bipyridine was regime are –52 mV pH–1 and –54 mV pH–1 respectively (Fig-
achieved by applying known procedures.[39,40] All [RuRpyimH]2+ ure 3b and c). For R = CF3, a slope of –74 mV pH–1 was deter-
–
complexes were isolated as PF6 salts. The preparation of mined (Figure 3a). This deviation from ideal behavior is attrib-
[RuCF3pyimH](PF6)2 followed the previously published proce- uted to the irreversible nature of the one-electron oxidation
dure.[41] The syntheses of [RutBupyimH](PF6)2 and [RuNMe2
-
process in this specific complex. In all three complexes, the
pyimH](PF6)2 were similar.[40–42] Synthetic procedures, including acidity constant decreases by 4–5 logarithmic units upon oxid-
complex and ligand syntheses are reported in the Experimental ation. [RuCF3pyimH]2+ is the most acidic (pKa 7.2) and
Section and in the Supporting Information.
[RuNMe2pyimH]2+ is the least acidic complex due to the elec-
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Figure 3. Pourbaix diagrams for [RuRpyimH]2+ in 1:1 (v/v) CH3CN/H2O with 0.05
M
buffer. The slopes for the one-electron, one-proton redox processes are
(a) –74 mV pH–1, (b) –52 mV pH–1, (b) –54 mV pH–1, established based on the cyclic voltammetry data shown in Figures S4–S6 of the Supporting Information.
Table 1. Thermodynamic parameters for the [RuRpyimH]2+ complexes in 1:1 (v/v) CH3CN/H2O: Acidity constants in the electronic ground state (pKa), in the
long-lived 3MLCT excited state (pKa*) and in the one-electron oxidized form (pKaox), oxidation potential in the ground state, 3MLCT energy and oxidation
potential in the excited state for protonated complex (Eprot, E00prot, *Eprot) and deprotonated complex (Edep, E00dep, *Edep).
ox
prot
dep
R
pKa
pKa*
pKa
Eprot
[V vs. SCE]
E00
*Eprot
[V vs. SCE]
Edep
[V vs. SCE]
E00
*Edep
[V vs. SCE]
[eV][a]
[eV][a]
H[b]
CF3
tBu
8.1 0.1
7.2 0.1
8.2 0.1
9.2 0.1
5.6 0.3
5.3 1[c]
5.4 0.3
8.4 0.5
3.6 0.1
3.2 0.1
3.2 0.1
4.8 0.1
1.00 0.05
1.28 0.05
0.94 0.05
0.41 0.05
2.1 0.1
1.9 0.1
2.1 0.1
2.0 0.1
–1.1 0.1
–0.6 0.1
–1.1 0.1
–1.6 0.1
0.73 0.05
0.98 0.05
0.66 0.05
0.17 0.05
1.9 0.1
1.8 0.1
2.0 0.1
1.9 0.1
–1.2 0.1
–0.8 0.1
–1.3 0.1
–1.7 0.1
NMe2
[a] From luminescence spectra recorded at 77 K in ethanol/methanol mixture, shown in the Supporting Information. [b] From ref.[27] [c] From ref.[41]
tronic influence of the substituents on the bpy spectator li- pKa* values were already known,[27,41] and for the complexes
gands (Table 1). The oxidation potential of all four pyimH com- with R = tBu and NMe2 the Förster method was applied. The
plexes shifts cathodically by ca. 0.3 V upon deprotonation, in resulting pKa* values were verified by pH titration monitoring
line with prior reports on iron and ruthenium complexes with steady-state emission as shown in the Supporting Information
deprotonatable ligands.[43–46] The highest oxidation potential is (Figures S8–S9). Because the π*-orbitals of the bpy ligands are
observed with the CF3 substituents (Eprot = 1.28 0.05 V vs. energetically lower lying than the π*-orbitals of the pyimH li-
SCE for the protonated and Edep = 0.98 0.05 V vs. SCE for gand, electron density is withdrawn from the acidic N–H func-
the deprotonated complex, respectively). The lowest oxidation tionality, and its acidity is increased in the emissive 3MLCT
potential is detected for the complex with NMe2 substituents states of all four complexes (Table 1) analogous to other photo-
(Eprot = 0.41 0.05 V and Edep = 0.17 0.05 V vs. SCE, respec- acids.[34,48] Excited-state acid-base equilibration takes place in
tively).
the pH range between pKa and pKa*, where the excited
[RuRpyimH]2+ complexes are deprotonated by solvent or buffer
molecules. Proton release then very quickly leads to the depro-
tonated ground state. This can unambiguously be seen in the
transient absorption spectra, which are essentially the differ-
ence between the UV/Vis spectra recorded before and after la-
ser excitation (for R = tBu in Figure 4). At pH 3 (Figure 4a) and
at pH 10 (Figure 4b), a bleach between 400 and 550 nm is
detected, which originates from disappearance of the 1MLCT
absorption. Under basic conditions, the bleach is redshifted, be-
cause the ground state is deprotonated, and the 1MLCT absorp-
tion band is redshifted compared with the protonated form. At
pH 6 (Figure 4c), a positive signal at around 500 nm becomes
detectable, which can be explained by deprotonation in the
excited state and rapid relaxation to the deprotonated ground
state. When subtracting the UV/Vis spectrum of the
Excited-state oxidation potentials (*Eprot, *Edep) were esti-
mated from Equations (2) and (3) based on the relevant ground-
state oxidation potentials (Eprot, Edep). MLCT energies were de-
3
termined at 77 K for the protonated (E00prot) and deprotonated
complexes (E00dep) (see the Supporting Information, Figure S7).
*Eprot = Eprot – E00
(2)
prot
*Edep = Edep – E00
(3)
dep
Photoacid Behavior and Excited-State Lifetimes
The acidity constant of the 3MLCT state (pKa*) was estimated
from the Förster equation based on the luminescence maxima [RutBupyimH]2+ complex from that of its deprotonated conge-
of protonated and deprotonated complex (λprot, λdep) at room ner, the spectrum in Figure 4d is obtained. This spectrum is
temperature in 1:1 (v/v) CH3CN/H2O.[47] For R = H and CF3 the very similar to the transient absorption spectrum in Figure 4c,
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from which we conclude that in the time-resolved laser experi-
ment the deprotonated complex in the ground state indeed
accumulates. This effect is less pronounced for the complex
with R = NMe2, in which case the deprotonated ground state
can only be detected after the excited stated has completely
decayed (see the Supporting Information, Figure S11). Presuma-
bly, this is due to the weaker driving force for proton release in
this case, and different complex–buffer interactions. The ex-
cited-state lifetimes are shortened by 50–90 % upon depro-
tonation (Table 2), ranging from 70 to 210 ns for the protonated
forms and from 17 to 75 ns for the deprotonated forms, respec-
tively, in the absence of oxygen. This effect can be explained
by the energy-gap law and by mixing of pyim–-based orbitals
with metal-centered d-orbitals. As a consequence, the resulting
excited state in the deprotonated complexes has a non-negligi-
ble ligand-to-ligand charge transfer character, and this can con-
tribute to the lifetime shortening relative to the protonated
complex.[34] The excited-state lifetimes under aerated condi-
tions are reported in the Supporting Information (Table S3).
Table 2. Excited-state properties of [RuRpyimH]2+ at 25 °C in 1:1 (v/v) CH3CN/
H2O at luminescence maxima of protonated and deprotonated complexes
(λprot, λdep) and lifetimes of protonated and deprotonated complexes (τprot
,
τ
dep) under deaerated conditions.
R
λprot [nm]
λdep [nm]
τprot [ns]
τdep [ns]
H[a]
CF3
tBu
NMe2
625
670
625
675
675
708
683
692
210 20
160 16
140 14
70
17
75
25
7
5
8
2
[b]
70
7
[a] Taken from ref.[27] [b] Taken from ref.[41]
Formal N–H BDFEs
Based on the acidity constants and redox potentials in Table 1,
formal N-–H BDFEs were calculated from Equation (1); the re-
sults are listed in Table 3. The values obtained for the ground
state were doubly determined by using pKa and Edep on the
one hand, and pKa and Eprot on the other hand. Both sets
ox
of data were measured independently from each other, and
ultimately they lead to an internally consistent picture. The
BDFEs for the electronic excited states (*BDFE) were also doubly
ox
determined by using pKa* and *Edep, and by using pKa and
*Eprot, respectively. The double determinations yielded the same
BDFEs within experimental accuracy. The general observation is
that for every complex the N–H bond cleavage is facilitated by
ca. 50 kcal mol–1 in the excited state. The ground-state BDFEs
are in the range from 79 1 kcal mol–1 (R = NMe2) to
96 1 kcal mol–1 (R = CF3), which is comparable in magnitude
to the N–H BDFEs of primary and secondary amines.[35,49,50]
Consequently, the BDFE can be tuned over a range of ca.
20 kcal mol–1 by substituent variation at the bpy spectator li-
gands. The complete thermochemistry regarding proton and
electron transfer in ground and excited states is summarized in
Scheme 2 in so-called “cube” schemes. In this representation,
the ground-state redox potentials (Eprot and Edep) for the proto-
nated and deprotonated complexes are found at the bottom
along with the acidity constants of the RuII and RuIII species
(pKa and pKaox). The bottom of Scheme 2 is, in fact, analogous
to the thermodynamic square scheme for the cleavage of X–H
bonds shown in Scheme 1. In Scheme 2, excitation to the
3MLCT state is represented by vertical arrows for both the pro-
Figure 4. (a)–(c) Transient absorption spectra of [RutBupyimH]2+ in 1:1 (v/v)
CH3CN/H2O at different pH values. Excitation occurred at 532 nm with laser
pulses of ca. 10 ns duration, the spectra were recorded without time delay
over a period of 200 ns. (d) Difference of ground-state UV/Vis spectra of
protonated and deprotonated complex. The asterisks mark laser stray light.
Table 3. X–H bond dissociation free energies in the ground (BDFE) and excited state (*BDFE) for some selected metal complexes, which can be considered
formal H-atom donors.
Complex
BDFE
*BDFE
Ref.
Comment
[kcal mol–1
]
[kcal mol–1
]
[27]
[RuHpyimH]2+
[RuCF3pyimH]2+
91
96
89
79
1
1
1
1
43
52
41
34
5
5
5
5
this work
this work
[RutBupyimH]2+
[RuNMe2pyimH]2+
this work
[33,53,54]
[a,b]
[a,b]
[Ru(bpy)2BiBzimH2]2+
[Ru(bpy)2BiimH2]2+
[Os(bpy)2BiBzimH2]2+
[Ru(hfacac)2pyimH]2+
[Ru(bpy){(OH)2-phen}2]2+
90
42
[33,53,54]
[33,53,54]
[55]
86
80
80
78
40
42
–
[a–c,e]
[56]
[a,b,d,f]
30
[a] BDFE calculated based on available redox potentials and acidity constants using both pathways illustrated in Scheme 1. [b] Excited-state bond dissociation
free energies (*BDFEs) calculated based on available pKaox, *Eprot values. [c] Redox potentials only in CH3CN available. [d] pKa determined from the Nernst
ox
equation. [e] E00 = 1.66 eV, approximated from [Os(bpy)3]2+, as reported in ref.[57] [f] E00 = 2.1 eV approximated from [Ru(bpy)3]2+ [58]
.
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Scheme 2. Thermodynamic “cube” scheme for [RuRpyimH]2+ in 1:1 (v/v) CH3CN/H2O based on the data in Tables 1 and 2. Horizontal/red: pKa values, vertical/
black: 3MLCT energy E00, pointing towards the reader in blue: oxidation potentials in V vs. SCE, diagonal in green: BDFEs.
tonated (E00prot) and deprotonated (E00dep) forms. In the excited Conclusions
state, the thermodynamically relevant processes are acid-base
All relevant thermodynamic parameters regarding proton-cou-
pled oxidation, i.e., formal H-atom donation, were determined
for three photoactive complexes in 1:1 (v/v) CH3CN/H2O. The
key findings are: (i) Deprotonation of the studied complexes
yields a gain in reducing power of 0.3 V in the ground state
and 0.1–0.2 eV in the emissive excited state. (ii) The formal N–
H BDFE can be tuned over a range of ca. 20 kcal mol–1 by vary-
ing the spectator ligands. (iii) Formal H-atom release is facili-
tated by roughly 50 kcal mol–1 upon excitation of these com-
plexes to their long-lived 3MLCT excites states, reaching excited-
state BDFEs down to 34 kcal mol–1. Thus, the photoexcited
[RuRpyimH]2+ complexes are very strong formal H-atom donors
even when compared to metal hydride complexes that are used
as hydrogenation catalysts.
equilibration (pKa*) (marked by a horizontal red arrow) and
oxidation of the *RuII complexes to give the respective RuIII
complexes in the electronic ground state, as represented by
blue face diagonals (*Eprot, *Edep). The combined loss of a pro-
ton and an electron from the excited state is shown on the
space diagonal from the back upper left corner to the lower
front right corner, indicated by *BDFE (green arrows).
Thus, for the excited state of [RuNMe2pyimH]2+ an exception-
ally low *BDFE of 34 5 kcal mol–1 is found, and this is caused
by the strongly electron-donating NMe2 groups, which lead to
a low potential for metal oxidation. The inverse effect accounts
for the behavior of [RuCF3pyimH]2+, which has a relatively high
*BDFE of 52 5 kcal mol–1. Nevertheless, even this *BDFE value
is still in the order of magnitude reported for M–H cleavage in
metal hydride catalysts that are used for thermal hydrogenation
reactions.[37,51]
In summary, the N–H BDFEs in [RuRpyimH]2+ complexes de-
crease with more electron-donating substituents on the bpy
spectator ligands. A similar finding was reported for the
ground-state N–H BDFEs of ruthenium(II)–2-(2-pyridyl)imid-
azole complexes when going from hexafluoroacetylacetonato
(hfacac) to acetylacetonato (acac) spectator ligands, which lead
to a decrease from 80 to 62 kcal mol–1.[52]
Experimental Section
Methods and Equipment: All commercially available chemicals for
synthesis were used as received. Acetonitrile for electrochemical
and photophysical measurements was HPLC grade, and water had
Millipore standard. Salts for buffers were used as received, and
aqueous buffer solutions (0.1
M concentration) were prepared ac-
cording to reported procedures.[59] The following buffers were used
for the various pH ranges: TsOH/TsONa (pH 1.0–2.0), citric acid
(pH 2.2–3.6), glycine/HCl (pH 2.2–3.6), acetate (pH 3.6–5.6), phos-
phate (pH 5.8–8.0), glycine/NaOH (pH 8.6–10.6), and phosphate
(pH 11.0–12.0). Unless otherwise noted, all measurements were per-
formed in 1:1 (v/v) CH3CN/H2O with a final buffer concentration of
The strong decrease of the N–H BDFEs upon photoexcitation
to values below 60 kcal mol–1 is in line with our prior report on
the [RuRpyimH]2+ complex with R = H.[27] In principle, this effect
is also expected for other metal complexes that can release
3
both a proton and an electron in the MLCT excited state, but
to date such excited-state BDFEs have not been reported. Based
on published acidity constants and redox potentials, we have correcting the measured pH value (pHmeas) in the mixture by using
tried to estimate some excited-state BDFEs for previously inves-
tigated complexes (Table 3), but these data should be consid-
0.05
M at 25 °C. The pH of the solvent mixture was determined by
the relationship pH = pHmeas – δ. For the 1:1 (v/v) CH3CN/H2O mix-
ture, the correction constant δ is –0.257.[60] All pH values reported
in this publication were corrected accordingly, and pKa values taken
ered with caution, because often different solvents were used
from ref.[41] were also corrected accordingly. H NMR spectra were
1
for determination of pKa values and redox potentials, and there
measured with a 400 MHz Bruker Avance III instrument. UV/Vis
are considerable uncertainties in their excited-state energies.
spectra were measured with a Cary 5000 instrument from Varian.
The general observation is that excited-state BDFEs are in the
Cyclic voltammetry was performed with a Versastat3-200 potentio-
range between 30 and 50 kcal mol–1. As noted before and as
stat from Princeton Applied Research using a glassy carbon disk
working electrode, a saturated calomel electrode as reference elec-
trode, and a platinum wire was used as counter electrode, 0.05
evident from Equation (1), the changes in redox potential have
more influence on the BDFE than changes of the acidity con-
stants.[52]
M
buffer served as supporting electrolyte. Prior to voltage sweeps at
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rates of 0.1 V s–1, the solutions were flushed with argon. For reversi-
ble cyclic voltammograms the average of reductive and oxidative
peak potentials was used to determine the redox potential; for irre-
Acknowledgments
We thank Martin Kuss-Petermann for the synthesis of [RutBu
-
versible oxidations, the inflection point of the oxidative sweep was pyimH](PF6)2. This work was supported by the Swiss National
used as an approximation for the oxidation potential. Steady-state
luminescence experiments were performed with a Fluorolog-3 ap-
paratus from Horiba Jobin–Yvon. Samples were excited at wave-
lengths corresponding to the isosbestic points observed in acid-
base titration experiments in Figures S2 and S3 of the Supporting
Information. Luminescence lifetime and transient absorption experi-
ments were conducted with an LP920-KS spectrometer from Edin-
burgh Instruments equipped with an iCCD detector from Andor.
The excitation source was the frequency-doubled output from a
Quantel Brilliant b laser. For aerated optical spectroscopic experi-
ments, quartz cuvettes from Starna and Helma were used. For all
deaerated optical spectroscopic experiments the samples were de-
oxygenated through three subsequent freeze–pump–thaw cycles in
home-built quartz cuvettes that were specifically designed for this
purpose. CCDC 1518357 {for [RuNMe2pyimH](PF6)2} contains the sup-
plementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre.
Science Foundation through grant number 200021_156063/1
and by the NCCR Molecular Systems Engineering.
Keywords: Photochemistry · Thermochemistry · Redox
chemistry · Substituent effects · Ruthenium
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Received: November 23, 2016
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