Photoinduced electron transfer in a triarylamine-organoboron-Ru(2,2′-bipyridine)32+ compound

Article abstract of DOI:10.1016/j.crci.2015.08.007

Long-range electron transfer reactions play a key role in biological photosynthesis, and they are likely to play an important role for future artificial photosynthetic endeavors as well. The possibility to control the rates for long-range electron transfer with external stimuli is of particular interest in this context. In the work presented herein, we explored a donor–bridge–acceptor compound in which intramolecular electron transfer from a triarylamine donor to a photoexcited Ru(bpy)₃²⁺ (bpy?=?2,2′-bipyridine) acceptor occurs across an organoboron bridge over a distance of approximately 22??. Fluoride has a high binding affinity to the organoboron bridge in apolar solutions, and the resulting organofluoroborate has a significantly different electronic structure. We explored to what extent the change from an electron-deficient organoboron wire to an electron-rich organofluoroborate bridge affects long-range electron transfer between the distant triarylamine donor and the Ru(bpy)₃²⁺ acceptor.

Full text of DOI:10.1016/j.crci.2015.08.007

C. R. Chimie xxx (2015) 1e7
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ꢀ
Full paper/Memoire
Photoinduced electron transfer in a triarylamine-
2þ
organoboron-Ru(2,20-bipyridine)3 compound
Luisa G. Heinz, Oliver S. Wenger*
Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
Long-range electron transfer reactions play a key role in biological photosynthesis, and
they are likely to play an important role for future artificial photosynthetic endeavors as
well. The possibility to control the rates for long-range electron transfer with external
stimuli is of particular interest in this context. In the work presented herein, we explored a
donorebridgeeacceptor compound in which intramolecular electron transfer from a tri-
Received 2 July 2015
Accepted 27 August 2015
Available online xxxx
Keywords:
2þ
arylamine donor to a photoexcited Ru(bpy)3 (bpy ¼ 2,20-bipyridine) acceptor occurs
Electron transfer
Time-resolved spectroscopy
Donoreacceptor systems
Photochemistry
Voltammetry
across an organoboron bridge over a distance of approximately 22 Å. Fluoride has a high
binding affinity to the organoboron bridge in apolar solutions, and the resulting organo-
fluoroborate has a significantly different electronic structure. We explored to what extent
the change from an electron-deficient organoboron wire to an electron-rich organo-
fluoroborate bridge affects long-range electron transfer between the distant triarylamine
Ruthenium
2þ
donor and the Ru(bpy)3 acceptor.
ꢀ
© 2015 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
anions were bound to the organoboron bridge, kET
decreased to ꢂ 106
s
ꢀ1. Here, we present results that we
Organoboron compounds have received significant
attention in recent years as (fluoride) sensor materials
[1e3], for opto-electronic applications [4e6], and for
fundamental studies of charge transfer phenomena [7e9].
In the vast majority of these charge transfer studies, the
boron center acted as a terminal electron acceptor, but the
efficiency of organoboron compounds as molecular bridges
(or “wires”) between a donor and a more potent acceptor
has not been investigated until very recently. In a study
published in 2015, we demonstrated that intramolecular
electron transfer between the triarylamine unit and
obtained on the structurally similar TAA-B-Ru2þ dyad
shown in Scheme 1b. We were curious to explore whether
the meta-linkage in the new dyad permits an equally effi-
cient switching of long-range electron transfer as in our
previously studied dyad.
The possibility to control the rates of long-range electron
transfer is of interest in the greater contexts of a future mo-
lecular electronics technology and artificial photosynthesis.
2. Results and discussion
2þ
photoexcited Ru(bpy)3 (bpy ¼ 2,20-bipyridine) across the
2.1. Synthesis
organoboron bridge of the dyad in Scheme 1a can be
controlled by fluoride anions [10]. Specifically, in the
absence of Fꢀ intramolecular electron transfer occurred
with a rate constant (kET) ꢁ 108 sꢀ1, but when two fluoride
The synthesis of the key ligand of the TAA-B-Ru2þ
compound is illustrated in Scheme 2. The triarylamine
donor moiety was introduced into the TAA-B-Ru2þ dyad
using the iodo-substituted triarylamine compound 4 which
was prepared following previously published protocols
[14,15]. The dimesitylboron-substituted bridging unit was
* Corresponding author.
E-mail address: oliver.wenger@unibas.ch (O.S. Wenger).
http://dx.doi.org/10.1016/j.crci.2015.08.007
ꢀ
1631-0748/© 2015 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Please cite this article in press as: L.G. Heinz, O.S. Wenger, Photoinduced electron transfer in a triarylamine-organoboron-
Ru(2,20-bipyridine)23þ compound, Comptes Rendus Chimie (2015), http://dx.doi.org/10.1016/j.crci.2015.08.007

2
L.G. Heinz, O.S. Wenger / C. R. Chimie xxx (2015) 1e7
2þ
Scheme 1. Chemical structures of two triarylamine-organoboron-RuðbpyÞ3 compounds: (a) Previously studied system [10]; (b) new system presented herein.
Scheme 2. Synthesis of the key ligand for the TAA-B-Ru2þ dyad from Scheme 1b: (a) P(tBu)3HþBFꢀ4 , Pd(dba)2, tBuOK, toluene; (b) C6H5I(CF3COO)2, I2, CH2Cl2; (c)
Et3N , CuI, PdCl2(PPh3)2; (d) n-BuLi, Bmes2F, Et2O; (e) NaH, toluene; (f) Pd(PPh3)4, THF [11]; (g) 2,5-dimethyl-4-trimethylsilyl-1-phenylboronic acid [12,13],
Pd(PPh3)4, Na2CO3, THF/H2O; (h) ICl, CH3CN/CH2Cl2.
prepared starting from 1,3,5-tribromobenzene (5) which
was reacted with 2 equivalents of 2-methyl-3-butyn-2-ol
(6) to afford compound 7 [16]. The latter was reacted
with dimesitylfluoroborane in order to obtain compound 8.
Subsequent deprotection with NaH gave the dialkynyl
compound 9. The iodo-xylene substituted bpy ligand unit
14 was prepared following our own published protocols
[11,15]. Ligand 15 was obtained by reacting triarylamine
compound 4, dimesitylboron-substituted bridging unit 9,
and bpy ligand unit 14 in 1:1:1 molar ratio using standard
Sonogashira coupling conditions. Subsequent reaction with
Ru(bpy)2Cl2 afforded the TAA-B-Ru2þ dyad.
2.2. Electrochemistry
Cyclic voltammetry of the TAA-B-Ru2þ dyad was per-
formed in CH3CN containing 0.1 M TBAPF6 as a supporting
electrolyte. Oxidative and reductive potential sweeps with
rates of 0.1 V/s were conducted separately because this
gave higher quality results. Typical scans are shown in
Fig. 1. Oxidation of the triarylamine unit is detected at
0.20 V vs. Fcþ/0 and oxidation of Ru(II) occurs at 0.77 V vs.
Fcþ/0, both in line with expectations [10,17,18]. Based on
previous studies, [18] we expect that the Ru(II/III) wave
overlaps with the wave associated with the oxidation of the
Please cite this article in press as: L.G. Heinz, O.S. Wenger, Photoinduced electron transfer in a triarylamine-organoboron-
Ru(2,20-bipyridine)23þ compound, Comptes Rendus Chimie (2015), http://dx.doi.org/10.1016/j.crci.2015.08.007

L.G. Heinz, O.S. Wenger / C. R. Chimie xxx (2015) 1e7
3
À
À
Á
À
ÁÁ
ꢀ
DGET ¼ e$ E0 TAAþ=0 ꢀ E0 bpy0=ꢀ ꢀ E00 ꢀ e2
0
(1)
ð4$p$ε0$εS$RDA
Þ
Photoinduced electron transfer leads to oxidation of the
triarylamine unit, hence the use of its oxidation potential in
Eq. (1). The reduction product is RuðbpyÞ3þ, with the
additional electron hosted by a bpy-localized orbital hence
the use of the first bpy reduction potential in Eq. (1).
Photoexcitation is taken into account by the E020þterm which
represents the 3MLCT energy of the Ru(bpy)3 sensitizer.
0
Eq. (1) yields
D
GET ¼ ꢀ0.2 eV which leads to the expec-
Fig. 1. Cyclic voltammograms of the TAA-B-Ru2þ dyad in CH3CN measured
in the presence of 0.1 M TBAPF6. Oxidative and reductive potential sweeps
with rates of 0.1 V/s were performed separately because this gave higher
quality results.
tation of photoinduced electron transfer upon excitation of
2þ
the Ru(bpy)3 unit of TAA-B-Ru2þ. This driving-force is
similar to that previously determined for the dyad from
Scheme 1a (
D
G0ET ¼ ꢀ0.3 eV).
Attempts to perform cyclic voltammetry in CH2Cl2 were
unsuccessful, the resulting data quality was very poor.
triarylamine monocation to its dicationic form. The latter is
unstable and can undergo carbazole formation. This ex-
plains both the poor reversibility of the wave at 0.20 V vs.
Fcþ/0 and the significantly stronger current associated with
2.3. UVeVis absorption and Fꢀ binding
The optical absorption spectrum of a 10ꢀ5 M solution of
the wave at 0.77 V vs. Fcþ/0
.
TAA-B-Ru2þ in CH2Cl2 is shown in Fig. 2 (solid line). The
Compared to the previously investigated compound
from Scheme 1a, oxidation of the ruthenium center in TAA-
B-Ru2þ is approximately 0.33 V easier to perform. Pre-
sumably this is due to the fact that in the new system the
2þ
1MLCT absorption band of the Ru(bpy)3
unit is the
lowest-energy absorption feature with a maximum at
460 nm. At 350 nm there is an absorption feature that can
be attributed to a N / B charge transfer band, as
commonly observed in triarylamine-triarylboron com-
pounds [4,6e8,26e30]. At 290 nm, one detects bpy-
2þ
Ru(bpy)3 unit is electronically more decoupled from the
electron-deficient organoboron unit by the additional p-
xylene spacer.
localized p-p* absorptions.
In the reductive sweep, one recognizes three quasi-
reversible reduction waves which are attributed to
consecutive one-electron reduction of each of the three bpy
ligands. Reduction of the boryl bridging unit would be ex-
pected to occur at ca. ꢀ2.2 V vs. Fcþ/0 based previously
reported values for comparable compounds [7,10,19e23],
but in Fig. 1 the respective reduction cannot be observed
unambiguously; we suspect that it is overlapped by one of
the bpy-related reduction waves.
Upon addition of TBAF, the N / B charge transfer ab-
sorption decreases (dotted lines), as expected when Fꢀ
binding suppresses optical charge transfer [7,26]. This kind
of observation forms the basis for many fluoride detectors
[1,26,31]. In Fig. 3 the absorbance at 370 nm is plotted as a
function of Fꢀ concentration. This titration curve is based
on the solution from Fig. 2 which has a nominal TAA-B-
Ru2þ concentration of 10ꢀ5 M. Given the presence of one
boron atom on the bridging unit of TAA-B-Ru2þ, one ex-
pects 1:1 association between Fꢀ and TAA-B-Ru2þ [1,2].
Consequently, the data in Fig. 3 was analyzed within the
framework of a 1:1 binding model to extract an association
constant (KA) between Fꢀ and the boron center (Eq. (2))
[32e34].
Given an energy (E00) of 2.1 eV for the emissive 3MLCT
excited state of the Ru(bpy)32þ unit and a distance (RDA) of
~22 Å between the triarylamine N atom and the Ru(II)
center in the TAA-B-Ru2þ dyad [24], the redox potentials
(E0) from Table 1 can be used to estimate the reaction free
energy (D
G0ET) for intramolecular electron transfer from
TAA to the photoexcited ruthenium complex [25].
Table 1
Reduction potentials (in volts vs. Fcþ/Fc) of the individual sub-
components of the TAA-B-Ru2þ dyad in CH3CN.a
E
0 [V]
TAAþ/0
RuIII/II
0.20
0.77
ꢀ1.82
ꢀ2.00
ꢀ2.29
ꢀ2.20b
bpy0/ꢀ
bpy0/ꢀ
bpy0/ꢀ
boryl0/ꢀ
a
Extracted from Fig. 1.
Fig. 2. Solid trace: UVeVis spectrum of 10ꢀ5 M TAA-B-Ru2þ in CH2Cl2 at
22 ꢃC. Dashed traces: Spectra measured from the same solution upon
addition of increasing amounts of TBAF.
b
From Ref. [10].
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4
L.G. Heinz, O.S. Wenger / C. R. Chimie xxx (2015) 1e7
Fig. 3. Titration curve based on the data in Fig. 2, displaying the absorbance
at 370 nm as a function of fluoride concentration.
ꢀ1
AðcFÞ ¼ A0 þ ½ðAlim ꢀ A0Þ=2$c0ꢄ$½c0 þ cF þ KA
2
1=2
ꢀ1
ꢀ ½ðc0 þ cF þ KA Þ ꢀ 4$c0$cFꢄ
ꢄ
(2)
In Eq. (2), A(cF) is the absorbance of the sample as a
function of Fꢀ concentration, and A0 is the absorbance of
the initial analyte solution prior to addition of any titrant.
Alim is the limiting absorbance value which is obtained in
the presence of a large excess of titrant, c0 is the concen-
tration of the analyte (TAA-B-Ru2þ). The dotted line in
Fig. 3 is the result of a fit with Eq (2) to the experimental
Fig. 4. (a) Transient absorption spectra measured by time-integrating over a
period of 40 ns following excitation at 532 nm with laser pulses of ~10 ns
duration. Black trace: 10ꢀ4 M TAA-B-Ru2þ in CH2Cl2. Grey trace: 10ꢀ4
M
TAA-B-Ru2þ in CH2Cl2 containing
2 equivalents of TBAF. (b) Spectro-
electrochemistry showing the UVeVis changes upon oxidation of TAA to
TAAþ in the TAA-B-Ru2þ dyad in CH3CN. (c) Spectro-electrochemistrþy
2þ
showing the UVeVis changes upon reduction of RuðbpyÞ3
in the TAA-B-Ru2þ dyad in CH3CN.
to RuðbpyÞ3
data, yielding KA ¼ (1.7
0.5),107 Mꢀ1
,
A0
¼
0.387,
Alim ¼ 0.327, c0 ¼ (3.5 1.4),10ꢀ5 M. The latter had to be a
freely adjustable parameter for successful fitting with this
binding model. Of key interest here is the apparent asso-
ciation constant, which is in reasonable agreement with
fluoride binding studies of chemically related organoboron
compounds in similarly apolar solutions [1,2,7,32,35,36]. It
should be kept in mind that trace amounts of water can
strongly affect fluoride binding due to the high hydration
enthalpy of Fꢀ.
2þ
of the Ru(bpy)3
unit [37,38]. In the spectro-
electrochemical experiments of Fig. 4b/c the UVeVis
spectrum of the TAA-B-Ru2þ dyad in CH3CN prior to
applying any electrochemical potential was used as a
baseline, and therefore the resulting difference spectra can
be directly compared to the transient absorption spectrum
in Fig. 4a (black trace). Based on this comparison we
conclude that the photoproduct observed in the transient
absorption spectrum contains oxidized triarylamine and
reduced Ru(bpy)32þ. This is direct evidence for intra-
molecular photoinduced electron transfer, as expected
based on the driving-force estimation made above
2.4. Transient absorption spectroscopy and spectro-
electrochemistry
(
D
G0ET ¼ ꢀ0.2 eV). In the following, the resulting photo-
Following the excitation of the TAA-B-Ru2þ dyad in
CH2Cl2 (10ꢀ4 M) with laser pulses of 532 nm wavelength
and ~10 ns duration, the transient absorption spectrum
shown in Fig. 4a (black trace) was obtained by time-
integrating over a period of 40 ns immediately after the
pulses. This spectrum exhibits absorption bands at 420,
540, and 730 nm, in addition to the (weak) bleach at
470 nm. All four of these spectral features can be under-
stood on the basis of the spectro-electrochemical data in
Fig. 4b/c.
For the series of UVeVis difference spectra in Fig. 4b, a
potential sufficient for oxidation of the triarylamine unit of
TAA-B-Ru2þ was applied to a Pt grid electrode (0.2 V vs.
Fcþ/Fc). Absorption bands at 420 and 760 nm were detec-
ted, as expected [18]. When applying a potential suffi-
ciently negative for bpy reduction (ꢀ1.5 V vs. Fcþ/Fc), the
UVeVis difference spectra shown in Fig. 4c were measured.
In this case, bands at 420 and 490 nm appear along with a
bleach at 455 nm, compatible with one-electron reduction
product will be abbreviated as TAAþ-B-Ruþ.
2.5. Electron transfer kinetics
In Fig. 5 the temporal evolution of the transient ab-
sorption signal at 750 nm is shown (black traces). The rise
time of the signal is instrumentally limited, indicating that
the formation of TAAþ-B-Ruþ occurs with a rate constant
kET ꢁ 108 sꢀ1. The decay is tri-exponential with lifetimes of
20 ns, 135 ns, and >1000 ns. This rather complex decay
behavior is most likely owed to the combination of reaction
pathways involving intramolecular reverse electron trans-
fer, bimolecular electron transfer, and
process.
a degradation
When recording the transient absorption spectra with
time delays ꢁ1000 ns, the typical spectroscopic signatures
þ
of TAAþ and RuðbpyÞ3 cannot be detected any more.
Consequently, the slowest time component of the tri-
Please cite this article in press as: L.G. Heinz, O.S. Wenger, Photoinduced electron transfer in a triarylamine-organoboron-
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L.G. Heinz, O.S. Wenger / C. R. Chimie xxx (2015) 1e7
5
conditions were identical as for the black trace in Fig. 4a.
While there are some spectral differences in the range from
400 to 620 nm between the measurements performed in
the absence (black) and presence of Fꢀ (grey), the overall
appearance of the spectra remains rather similar. Most
importantly, the triarylamine cation band at 740 nm is
clearly observed in both cases, and the spectral feature
between 480 and 600 nm (attributed above to the reduced
ruthenium complex) can still be detected. Thus one can
conclude that photoinduced electron transfer does still
occur when Fꢀ is bound to the triarylboron bridging unit.
The kinetics for TAAþ-B-Ruþ photoproduct formation and
disappearance are largely unaffected by fluoride binding
(grey traces in Fig. 5).
2.7. Comparison to a related organoboron dyad
In a recent study we investigated intramolecular elec-
tron transfer from the triarylamine unit to photoexcited
2þ
Ru(bpy)3 in the dyad shown in Scheme 1a [10]. The key
finding was that Fꢀ binding to the organoboron bridging
unit decreased kET from ꢁ108 sꢀ1 in the absence of Fꢀ to
ꢂ106 sꢀ1 with two bound fluorides. Here we find that
electron transfer across a similar bridge between the same
donor and the same acceptor occurs with kET ꢁ 108 sꢀ1
,
Fig. 5. Temporal evolution of the transient absorption signals at 750 nm
regardless of whether Fꢀ is bound or not. If any effect of Fꢀ
on the rate for photoinduced electron transfer is present
at all, it has to affect the kinetics on a very rapid (<10 ns)
timescale. Evidently, a deceleration of electron transfer to
the microsecond time range does not occur. Aside from
the presence of an additional p-xylene spacer in the dyad
from Scheme 1b, the main differences between the com-
pounds in Scheme 1a and Scheme 1b are the following:
The bridging unit in the compound from Scheme 1b
contains only one dimesitylboron-substituent which is in
the meta-position to the donor and acceptor units,
whereas in the compound from Scheme 1a there are two
dimesitylboron-groups which are in the ortho- and meta-
positions to the donor and acceptor units. Electronic
coupling between the ortho- and para-positions of ben-
zene rings is known to be significantly stronger than
electronic coupling between meta-positions [42e44].
Consequently, it can be argued that when Fꢀ binds to the
organoboron unit in the TAA-B-Ru2þ compound from
Scheme 1b, this does not affect the electronic coupling
pathway between the donor and the acceptor as much as
in the case of the previously investigated dyad from
Scheme 1a.
(from Fig. 4a) on a short (a) and a longer timescale (b). Black trace: 10ꢀ4
M
TAA-B-Ru2þ in CH2Cl2. Grey trace: 10ꢀ4 M TAA-B-Ru2þ in CH2Cl2 containing
2 equivalents of TBAF.
exponential decays is attributed to photo-degradation. The
fastest time component (20 ns) is most likely the one
associated with intramolecular reverse electron transfer
across the covalent backbone of the TAA-B-Ru2þ dyad; for
this process one expects comparatively strong electron
donoreacceptor coupling, and it seems plausible that this
results in rapid electron transfer. The 135 ns time compo-
nent is attributed to bimolecular processes. Due to the
weakness of the transient absorption signals, relatively
high sample concentrations (10ꢀ4 M) were used, and this
increases the probability for intermolecular electron
transfer significantly. Concentration dependence studies
were not possible because the signals became undetectable
upon dilution, and higher concentrations resulted in optical
densities which were too elevated.
Thus we find that intramolecular reverse electron
transfer occurs with a rate constant of krET ¼ 5,107 sꢀ1
.
Consequently, kET > krET, despite a significantly lower
driving-force for photoinduced (forward) electron transfer
3. Summary and conclusions
(
D
G0ET ¼ ꢀ0.2 eV) compared to thermal (reverse) electron
G0rET ¼ ꢀ1.9 eV). The inverted driving-force effect
Photoinduced electron transfer in the TAA-B-Ru2þ dyad
from Scheme 1b occurs with a rate constant kET ꢁ 108 sꢀ1
regardless of whether Fꢀ is bound to the organoboron
bridge or not. A deceleration of intramolecular electron
transfer to rates on the microsecond timescale in the
presence of bound fluoride, such as previously observed for
the dyad in Scheme 1a [10], is not detected for the new
TAA-B-Ru2þ compound. This contrasting behavior is
attributed to the fact that the dimesitylboron unit of the
TAA-B-Ru2þ dyad from Scheme 1b is electronically
transfer (
D
is therefore likely at play [39], but there might also be
differences in electronic coupling between forward and
reverse electron transfer [40,41].
2.6. Effect of Fꢀ binding on electron transfer
The grey trace in Fig. 4a is the transient absorption
spectrum obtained for 10ꢀ4 M TAA-B-Ru2þ in CH2Cl2 in the
presence of 2 equivalents of TBAF. The measurement
Please cite this article in press as: L.G. Heinz, O.S. Wenger, Photoinduced electron transfer in a triarylamine-organoboron-
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6
L.G. Heinz, O.S. Wenger / C. R. Chimie xxx (2015) 1e7
decoupled from the donoreacceptor electron transfer
pathway due to its attachment in the meta-position. This
strongly suggests that the kET switching observed for the
dyad from Scheme 1a is mostly an electronic coupling ef-
fect rather than a simple electrostatic effect which arises
from the binding of negative charges to the organoboron
bridge.
d
[ppm] ¼ 7.49 (d, J ¼ 1.4 Hz, 2H), 7.40 (t, J ¼ 1.4 Hz, 1H),1.98
(s, 2H), 1.60 (s, 12H).
4.2.2. Compound 8
Compound 7 (3.38 g, 10.52 mmol) was dissolved in dry
diethyl ether (50 ml) and cooled to ꢀ78 ꢃC under N2. 2.5 M
n-BuLi in hexane (13.9 ml, 34.7 mmol) was added dropwise.
Then the cooling bath was removed, and the solution was
stirred for 2 h. Prior to adding dimesitylfluoroborane
(5.64 g, 21.1 mmol) in dry diethyl ether (50 ml), the reaction
mixture was cooled again to ꢀ78 ꢃC. After stirring for
10 min at this temperature, the cooling bath was removed,
and the mixture was stirred at room temperature under N2
overnight. Then, de-ionized H2O was added and the phases
were separated. The organic phase was dried over anhy-
drous Na2SO4 and then evaporated. The resulting brown oil
was purified by chromatography on a silica gel column
using first pure pentane and then a 8:1 (v:v) mixture of
pentane and ethyl acetate as the eluent. This afforded the
product as a yellow crystalline solid (2.99 g, 6.1 mmol, 58%).
4. Experimental section
4.1. Materials and methods
Compounds 1, 2, 5, 6, 10, and 11 are commercial chem-
icals which were used as received. Compounds 3, 4, 12, 13,
and 14 were synthesized according to our own previously
published synthetic protocols [10,15]. All other compounds
were synthesized as described below.
Fluoride binding to the TAA-B-Ru2þ compound
occurred by the addition of commercial 1.0 M TBAF (tetra-
n-butylammonium fluoride) solution in THF. Cyclic vol-
tammetry was performed using a three-electrode setup
comprised of a platinum disk working electrode, a silver
wire as a counter electrode and a second silver wire as a
quasi-reference electrode. A Versastat3-200 potentiostat
from Princeton Applied Research was employed. Potential
scans were performed with sweep rates of 0.1 V/s, TBAPF6
(tetra-n-butylammonium hexafluorophosphate) was used
as a supporting electrolyte in dry, de-aerated CH3CN. A
platinum grid working electrode was used for spectro-
electrochemistry. NMR spectroscopy was performed using
a 400 MHz Bruker Avance III instrument. Mass spectra were
recorded on a Bruker Esquire 3000 plus instrument.
Elemental analysis was performed by Ms. Sylvie Mittel-
heisser in the Department of Chemistry at University of
Basel using a Vario Micro Cube instrument from Elementar.
UVeVis absorption spectra were measured on a Cary 5000
instrument from Varian. A Fluorolog-322 spectrometer
from Horiba Jobin-Yvon was used for steady-state lumi-
nescence spectroscopy. Time-resolved luminescence and
transient absorption experiments were performed using an
LP920-KS spectrometer from Edinburgh Instruments, using
the frequency-doubled output of a Quantel Brilliant b laser
as an excitation source.
1H NMR (400 MHz, CDCl3):
d
[ppm] ¼ 7.59 (t, J ¼ 1.7 Hz,1H),
7.47 (d, J ¼ 1.7H, 2H), 6.82 (s, 4H), 2.31 (s, 6H), 1.96 (s, 12H),
1.58 (s, 12H).
4.2.3. Dialkyne 9
Compound 8 (2.99 g, 6.1 mmol) was dissolved in dry
toluene (50 ml), NaH (731 mg, 18.3 mmol) was added, and
the mixture was heated to 100 ꢃC under N2 until comple-
tion of the reaction (reaction progress was monitored by
thin-layer chromatography). After cooling to room tem-
perature, the precipitate was filtered off, and the solvent
was evaporated. The solid residue was purified by chro-
matography on a silica gel column using pentane as the
eluent. This afforded the product as a pale yellow crystal-
line solid (0.72 g, 1.9 mmol, 31%). 1H NMR (400 MHz,
acetone-d6):
d
[ppm] ¼ 7.70 (t, J ¼ 1.6 Hz, 1H), 7.59 (d,
J ¼ 1.6 Hz, 2H), 6.82 (s, 4H), 3.04 (s, 2H), 2.31 (s, 6H), and
1.97 (s, 12H).
4.2.4. Ligand 15
Dialkyne compound
9
(100 mg, 0.267 mmol),
iodoxylene-bpy compound 14 (103 mg, 0.267 mmol) [15],
and iodo-substituted triarylamine 4 (115 mg, 0.267 mmol)
[15] were dissolved in dry triethylamine (20 ml). After de-
oxygenating, CuI (4 mol-%) and PdCl2(PPh3)2 (2 mol-%)
were added, and the reaction mixture was heated to reflux
under N2 for 1 h. Ethyl acetate was added after cooling to
room temperature. The mixture was washed with satu-
rated aqueous NH4Cl and with brine. The organic phase was
dried over anhydrous Na2SO4 and then evaporated to dry-
ness. Column chromatography on silica gel occurred first
with pure CH2Cl2 as the eluent, and then with a 100:10:1
(v:v:v) mixture of pentane, ethyl acetate and triethylamine.
This afforded the pure product as a pale yellow solid
(23 mg, 0.025 mmol, 9%). 1H NMR (400 MHz, acetone-d6):
4.2. Syntheses and product characterization data
4.2.1. Compound 7
Following a previously published procedure [16], 1,3,5-
tribromobenzene (5.00 g, 15.9 mmol) and 2-methyl-3-
butyn-2-ol (3.41 ml, 34.9 mmol) were dissolved in dry
triethylamine (85 ml). After de-oxygenating, CuI (4 mol-%)
and PdCl2(PPh3)2 (2 mol-%) were added, and the reaction
mixture was heated to reflux under N2 for 1 h. Then the
solution was cooled to room temperature, and ethyl acetate
was added. The mixture was washed with saturated
aqueous NH4Cl and with brine. After drying over anhydrous
Na2SO4 the solvents were evaporated. Chromatography on
a silica gel column occurred first with a 5:1 (v:v) and then
with a 3:1 (v:v) mixture of pentane and ethyl acetate as the
eluent. This afforded the product as a yellow crystalline
solid (3.59 g, 11.2 mmol, 70%). 1H NMR (400 MHz, CDCl3):
d
[ppm] ¼ 8.70 (d, J ¼ 4.1 Hz, 1H), 8.67 (d, J ¼ 1.7 Hz, 1H),
8.55 (dd, J ¼ 13.2, 8.1 Hz, 2H), 7.99e7.90 (m, 2H), 7.86 (s,
1H), 7.59 (s, 1H), 7.55 (s, 1H), 7.49 (s, 1H), 7.43 (dd, J ¼ 8.0,
4.2 Hz, 1H), 7.33 (ABq, JAB ¼ 8.9 Hz, 2H), 7.27 (s, 1H), 7.11
(ABq, JAB ¼ 9.0 Hz, 4H), 6.94 (ABq, JAB ¼ 9.0 Hz, 4H), 6.90 (s,
Please cite this article in press as: L.G. Heinz, O.S. Wenger, Photoinduced electron transfer in a triarylamine-organoboron-
Ru(2,20-bipyridine)23þ compound, Comptes Rendus Chimie (2015), http://dx.doi.org/10.1016/j.crci.2015.08.007

L.G. Heinz, O.S. Wenger / C. R. Chimie xxx (2015) 1e7
7
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12477e12484.
4H), 6.76 (ABq, JAB ¼ 8.9 Hz, 2H), 3.80 (s, 6H), 2.51 (m, 4H),
2.30 (m, 8H), and 1.97 (s, 12H).
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4.2.5. TAA-B-Ru2þ dyad
Ligand 15 (155 mg, 0.166 mmol) and Ru(bpy)2Cl2
(80 mg, 0.166 mmol) were heated to reflux in a mixture of
CHCl3 (5 ml) and CH3OH (16 mmol) under N2 overnight.
Then the solvents were removed on a rotary evaporator,
and purification occurred by column chromatography on a
silica gel stationary phase. At first pure acetone was used as
the eluent, then a 10:1 (v:v) mixture of acetone and de-
ionized H2O was employed, and finally a 100:10:1 (v:v:v)
mixture of acetone, H2O and saturated aqueous KNO3 so-
lution was used. The solvents were evaporated from the
desired chromatography fractions, and the product was re-
dissolved in CH3CN. Excess KNO3 was filtered off on a P4
frit. The concentrated CH3CN solution of the product was
added dropwise to a saturated aqueous KPF6 solution. The
resulting precipitate was washed with de-ionized H2O and
with diethyl ether. The pure product was obtained after
drying in a vacuum (130 mg, 0.074 mmol, 45%). 1H NMR
€
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ꢁ
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(400 MHz, acetone-d6):
d
[ppm] ¼ 8.90 (dd, J ¼ 12.3, 8.0 Hz,
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3H), 8.85 (t, J ¼ 8.8 Hz, 4H), 8.23e8.15 (m, 7H), 8.11e8.07
(m, 5H), 7.92 (d, J ¼ 1.6 Hz, 1H), 7.80 (t, J ¼ 1.6 Hz, 1H),
7.65e7.51 (m, 8H), 7.34 (s, 1H), 7.32 (ABq, JAB ¼ 8.9 Hz, 2H),
7.11 (ABq, JAB ¼ 9.0 Hz, 4H), 6.95 (ABq, JAB ¼ 9.0 Hz, 4H), 6.89
(s, 4H), 6.75 (ABq, JAB ¼ 8.9 Hz, 2H), 3.80 (s, 6H), 2.40 (s, 3H),
2.29 (s, 6H), 2.03 (s, 12H), 1.97 (s, 3H). ESI-MS (m/z): 675.1
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€
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(calc. 674.8 for
86H74N7O2BF12P2,4H2O,CH3CN: C, 60.31; H, 4.89; N, 6.39.
C
86H74N7O2BRu2þ). Anal. calcd for
C
Found: C, 60.56; H, 4.62; N, 6.26.
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This work was supported by the Deutsche For-
schungsgemeinschaft (DFG) through grant number
WE4815/3-1.
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Please cite this article in press as: L.G. Heinz, O.S. Wenger, Photoinduced electron transfer in a triarylamine-organoboron-
Ru(2,20-bipyridine)23þ compound, Comptes Rendus Chimie (2015), http://dx.doi.org/10.1016/j.crci.2015.08.007