scispace - formally typeset

Journal ArticleDOI

Electron Transfer between Hydrogen‐Bonded Pyridylphenols and a Photoexcited Rhenium(I) Complex

15 Apr 2013-ChemPhysChem (John Wiley & Sons, Ltd)-Vol. 14, Iss: 6, pp 1168-1176

TL;DR: Transient absorption spectroscopy provides unambiguous evidence for the photogeneration of phenoxyl radicals, that is, the overall photoreaction is clearly a proton-coupled electron-transfer process.
Abstract: Two pyridylphenols with intramolecular hydrogen bonds between the phenol and pyridine units have been synthesized, characterized crystallographically, and investigated by cyclic voltammetry and UV/Vis spectroscopy. Reductive quenching of the triplet metal-to-ligand charge-transfer excited state of the [Re(CO)3(phen)(py)]+ complex (phen=1,10-phenanthroline, py=pyridine) by the two pyridylphenols and two reference phenol molecules is investigated by steady-state and time-resolved luminescence spectroscopy, as well as by transient absorption spectroscopy. Stern–Volmer analysis of the luminescence quenching data provides rate constants for the bimolecular excited-state quenching reactions. H/D kinetic isotope effects for the pyridylphenols are on the order of 2.0, and the bimolecular quenching reactions are up to 100 times faster with the pyridylphenols than with the reference phenols. This observation is attributed to the markedly less positive oxidation potentials of the pyridylphenols with respect to the reference phenols (≈0.5 V), which in turn is caused by proton coupling of the phenol oxidation process. Transient absorption spectroscopy provides unambiguous evidence for the photogeneration of phenoxyl radicals, that is, the overall photoreaction is clearly a proton-coupled electron-transfer process.
Topics: Quenching (fluorescence) (60%), Kinetic isotope effect (53%), Spectroscopy (53%), Luminescence (51%), Electron transfer (51%)

Content maybe subject to copyright    Report

1
ARTICLES
DOI: 10.1002/cphc.200((will be filled in by the editorial staff))
Electron Transfer between Hydrogen-Bonded
Pyridylphenols and a Photoexcited Rhenium(I)
Complex
William Herzog,
[b]
Catherine Bronner,
[a]
Susanne Löffler,
[c]
Bice He,
[c]
Daniel
Kratzert,
[c]
Dietmar Stalke,
[c]
Andreas Hauser,
[b]
and Oliver S. Wenger*
[a]
Two pyridylphenols with intramolecular hydrogen bonds between the
phenol and pyridine units were synthesized, characterized
crystallographically, and investigated by cyclic voltammetry and UV-
vis spectroscopy. Reductive quenching of the
3
MLCT excited state of
the [Re(phen)(CO)
3
(py)]
+
complex (phen = 1,10-phenanthroline, py =
pyridine) by the two pyridylphenols and two reference phenol
molecules was investigated by steady-state and time-resolved
luminescence spectroscopy, as well as by transient absorption
spectroscopy. Stern-Volmer analysis of the luminescence quenching
data provides rate constants for the bimolecular excited-state
quenching reactions. H/D kinetic isotope effects (KIEs) for the
pyridylphenols are on the order of 2.0, and the bimolecular quenching
reactions are up to 100 times faster with the pyridylphenols than with
the reference phenols. This observation is attributed to the markedly
less positive oxidation potentials of the pyridylphenols with respect to
the reference phenols (ca. 0.5 V), which in turn is caused by proton-
coupling of the phenol oxidation process. Transient absorption
spectroscopy provides unambiguous evidence for the
photogeneration of phenoxyl radicals, i. e., the overall photoreaction
is clearly a PCET process
Introduction
The tyrosine Z (Tyr
Z
) / histidine 190 (His190) pair of photosystem
II is one of the best-known hydrogen-bonded phenol systems in
chemistry.
[1]
Numerous experimental and theoretical
investigations have been geared at understanding the proton-
coupled electron transfer (PCET) chemistry of the Tyr
Z
/ His190
reaction couple, many of them focusing on simple artificial model
compounds in which a phenol unit can form intramolecular
hydrogen bonds to a nitrogen base.
[2]
A wide range of
experimental methods has been applied, including EPR,
[3]
electrochemical,
[4]
and optical spectroscopic studies.
[5]
However,
in most cases the phenol oxidation process involves oxidants
which are in their electronic ground states, and there are
comparatively few studies in which the oxidant is an electronically
excited molecule.
[5b, 6]
Scheme 1. Molecular structures of the photosensitizer/quencher pairs with the
two hydrogen-bonded phenol molecules of central interest to this study; h =
light excitation; MLCT = metal-to-ligand charge transfer excitation; ET = electron
transfer; PT = proton transfer.
Against this background we deemed it interesting to explore
the photoredox chemistry between hydrogen-bonded phenol
molecules and a photoexcited rhenium(I) tricarbonyl diimine
complex which is known to be a potent excited-state oxidant.
[6e, 7]
The molecular structures of our model systems are shown in
Scheme 1. The rhenium(I) complex has a 1,10-phenanthroline
(phen) and a pyridine (py) ligand in addition to the three carbonyls,
the phenol reaction partners have pendant pyridine units that are
connected via a –CH
2
group in order to disrupt -conjugation
between the two aromatic subunits. One of the phenols contains
no further substituents (PhOH-CH
2
-py) while the other has tert.-
butyl groups at the 4- and 6-positions (
t
Bu
2
PhOH-CH
2
-py). We
anticipated that when excited to its long-lived
3
MLCT state the
rhenium(I) complex would be capable of inducing intermolecular
electron transfer (ET) with the phenol, and this process should be
accompanied by intramolecular proton transfer (PT) between the
phenol and the pyridine.
The PCET chemistry of
t
Bu
2
PhOH-CH
2
-py with various
[a] Dr. C. Bronner, Prof. O. S. Wenger
Departement für Chemie
Universität Basel
Spitalstrasse 51, CH-4056 Basel, Switzerland
Fax: +41 (0)61 267 09 76
E-mail: oliver.wenger@unibas.ch
[b] Dr. W. Herzog, Prof. A. Hauser
Département de Chimie Physique
Université de Genève
30 quai Ernest-Ansermet, CH-1211 Genève 4, Switzerland
[c] S. Löffler, Dr. B. He, D. Kratzert, Prof. D. Stalke
Institut für Anorganische Chemie
Georg-August Universität
Tammannstrasse 4, D-37077 Göttingen, Germany
Supporting information for this article is available on the WWW
under http://www.chemphyschem.org or from the author.

2
oxidants in their electronic ground states has been found
previously to occur via a concerted proton-electron transfer
(CPET) mechanism.
[2e, 5c-f]
Here we focus specifically on the
excited-state PCET chemistry between [Re(phen)(CO)
3
(py)]
+
and
PhOH-CH
2
-py or
t
Bu
2
PhOH-CH
2
-py. As reference phenols without
the possibility of forming intramolecular hydrogen bonds we used
ordinary phenol (PhOH) and 2,4-di-tert.-butylphenol (
t
Bu
2
PhOH).
Results and Discussion
Scheme 2. Synthesis of the two pyridylphenols from Scheme 1.
Synthesis
The synthesis of
t
Bu
2
PhOH-CH
2
-py had been previously
described,
[5c, 5d, 5f]
but in our hands a different procedure turned
out to be more convenient for obtaining the two pyridylphenols
from Scheme 1.
[8]
Our synthetic strategy is illustrated by Scheme
2 and begins with commercially available 2-bromophenols (1 and
3), which are methylated in order to protect the phenolic function
for the subsequent reaction step. The protected phenols (2 and 4)
are reacted with pyridine molecule 5 (which is accessible in one
step from 2-picoline and diisopropyl ketone) using a palladium
catalyst.
[8]
The coupling products (6 and 8) are deprotected using
aqueous HBr (in the case of 6)
[9]
or ethanethiol (in the case of
8)
[10]
in order to obtain the final pyridylphenols (7, PhOH-CH
2
-py
and 9,
t
Bu
2
PhOH-CH
2
-py).
Figure 1. Crystal structures of PhOH-CH
2
-py (left) and
t
Bu
2
PhOH-CH
2
-py (right).
Anisotropic displacement parameters are depicted at the 50 % probability level.
Selected bond distances and angles can be found in the supporting information.
Crystal structures
Figure 1 (left) shows the crystal structure of PhOH-CH
2
-py which
crystallizes in the monoclinic space group C2/c with one molecule
in the asymmetric unit. The molecules in the crystal lattice of
PhOH-CH
2
-py are connected through intermolecular hydrogen
bonds between the phenolic OH group and the nitrogen of the
pyridyl ring. These bonds generate a zigzag like chain along the b
axis of the crystal lattice. The position of the hydrogen atom was
modeled as a riding atom with a fixed distance of 0.84 Å and a
freely refined torsion angle. The resulting hydrogen bond is
slightly bent with 175° for the O-H-N angle and has a H-N
distance of 1.90 Å, resulting in a total O-H-N distance of
2.740(2) Å. Figure 1 (right) shows the crystal structure of
t
Bu
2
PhOH-CH
2
-py which crystallizes in the monoclinic space
group P2
1
/n with one molecule in the asymmetric unit. One of the
t
Bu groups happens to be disordered by 5%. This molecule only
forms an intramolecular hydrogen bond between H1 and N1. The
hydrogen H1 was found in the difference Fourier density. Position
and the isotropic vibration were refined freely with a distance
restraint of 0.84(2) Å to O1. The O-H distance refined to 0.88(2) Å
with a N-H distance of 1.827(16) Å and a total O-N distance of
2.6956(16) Å with an O-H-N angle of 169(2)°. Thus, there is clear
evidence for hydrogen-bonding interactions in the crystal
structures of PhOH-CH
2
-py and
t
Bu
2
PhOH-CH
2
-py.
[11]
A structure
of
t
Bu
2
PhOH-CH
2
-py had been previously published.
25
Intramolecular hydrogen-bonding in solution
1
H NMR spectra of the two pyridylphenols in CDCl
3
exhibit sharp
downfield resonances for the phenolic protons, specifically at
11.67 ppm for PhOH-CH
2
-py and at 11.40 ppm for
t
Bu
2
PhOH-
CH
2
-py, which is typical for intramolecularly hydrogen-bonded
phenols.
[12]
We conclude that intramolecular hydrogen-bonds are
not only present in one of our solid state structures but also in
aprotic solution
Cyclic voltammetry
Figure 2 shows cyclic voltammograms of (a)
t
Bu
2
PhOH, (b)
t
Bu
2
PhOH-CH
2
-py, and (c) PhOH-CH
2
-py in dry CH
2
Cl
2
in
presence of 0.1 M TBAPF
6
electrolyte. The reversible waves at
0.0 V vs. Fc
+
/Fc (dashed vertical line) are due to ferrocene, which
was added in small quantities for internal voltage calibration. The
voltammogram of the reference phenol (a) exhibits an irreversible
oxidation wave peaking at 1.05 V vs. Fc
+
/Fc which is typical for
ordinary phenols because the O-H proton is lost to the bulk
solution in the course of oxidation.
[13]
The
t
Bu
2
PhOH-CH
2
-py molecule, by contrast, exhibits a
voltammogram in which the oxidative peak current near 0.5 V vs.
Fc
+
/Fc is roughly 6 times larger than the corresponding reductive
peak current. Their voltage separation is 170 mV but depends on
voltage sweep rate. The voltammogram in Figure 2b is
qualitatively similar to that previously reported for the same
compound in CH
3
CN solution.
[5c]
The shape of this voltammogram
can be explained by the possibility of transferring the phenolic
proton to the pendant pyridine base in the course of oxidation and
back-transfer to the phenol unit during the subsequent reductive
potential sweep. The middle between the oxidative and reductive
peak currents in Figure 2b is taken as the oxidation potential of
t
Bu
2
PhOH-CH
2
-py (Table 1). Importantly, the oxidation potential
of
t
Bu
2
PhOH-CH
2
-py in CH
2
Cl
2
is about 0.5 V less positive than
the oxidation potential of
t
Bu
2
PhOH, a fact that has been
previously noted for CH
3
CN solution.
[5c, 5d]
It has been
demonstrated that the unusually low oxidation potential of
t
Bu
2
PhOH-CH
2
-py and related hydrogen-bonded phenols is a
direct manifestation of intramolecular proton transfer
accompanying electrochemical phenol oxidation; hydrogen-
bonding alone cannot account for the large magnitude of the
oxidation potential shift.
[5c, 14]
The cyclic voltammogram of PhOH-CH
2
-py in Figure 2c
exhibits an irreversible oxidation wave peaking at 0.66 V vs.
Fc
+
/Fc (Table 1). Despite the presence of an intramolecular
hydrogen bond phenol oxidation is clearly irreversible in this case,
possibly because of the absence of substituents at the 4- and 6-
positions of the phenol. Chemical substituents in ortho- and para-

3
position to the phenolic function are known to enhance the
stability of phenoxyl radicals.
[18]
By analogy to the other
pyridylphenol from Scheme 1 PhOH-CH
2
-py is oxidized at much
less positive potential than the PhOH reference molecule; in this
specific case the potential difference amounts to 0.6 V (Table 1).
Figure 2. Cyclic voltammograms of the hydrogen-bonded phenols from Scheme
1 in dry CH
2
Cl
2
in presence of 0.1 M TBAPF
6
. (a)
t
Bu
2
PhOH; (b)
t
Bu
2
PhOH-CH
2
-
py; (c) PhOH-CH
2
-py. The reversible waves at 0.0 V vs. Fc
+
/Fc are due to
ferrocene which was added in small quantities for internal voltage calibration;
the scan rate was 100 mV/s
Table 1. Electrochemical potentials (E) for oxidation of the four phenol
molecules and for reduction of the photoexcited [Re(phen)(CO)
3
(py)]
+
complex.
redox couple
E [V vs. Fc
+
/Fc]
PhOH
+
/PhOH 1.25
[a] [d]
PhOH-CH
2
-py
+
/PhOH-CH
2
-py 0.66
[b]
t
Bu
2
PhOH
+
/
t
Bu
2
PhOH 0.97
[b]
t
Bu
2
PhOH-CH
2
-py
+
/
t
Bu
2
PhOH-CH
2
-py 0.54
[b]
*[Re(phen)(CO)
3
(py)]
+
/[Re(phen)(CO)
3
(py)] 0.77
[c] [d]
[a] From reference
[15]
, converted from V vs. SCE to V vs. Fc
+
/Fc by
subtracting 0.38 V as described in ref.
[16]
; [b] Measured in this work, peak
potentials from Figure 2, 0.1 M TBAPF
6
electrolyte in CH
2
Cl
2
; [c] from
reference
[6f]
; [d] in CH
3
CN. The previously reported value for
t
Bu
2
PhOH-CH
2
-
py is 0.44 V vs. Fc
+
/Fc in CH
3
CN.
[5f]
The potential of
t
Bu
2
PhOH is in line with
the value reported in ref.
[17]
(0.519 V vs. NHE, addition of 0.624 V (according
to ref.
[16]
) gives 1.14 V vs. Fc
+
/Fc).
The electrochemistry of the [Re(phen)(CO)
3
(py)]
+
complex and
related rhenium(I) tricarbonyl diimines was explored extensively
in the past.
[7, 19]
In Table 1 we merely give the electrochemical
potential for one-electron reduction of
3
MLCT-excited
[Re(phen)(CO)
3
(py)]
+
as reported in the literature.
[6f]
Optical absorption
Figure 3 shows UV-vis spectra of the four phenols and the
rhenium(I) complex from Scheme 1 in CH
2
Cl
2
at 25 °C. The
important message from Figure 3 is that all four phenols are
spectroscopically innocent at wavelengths longer than 330 nm.
Between 270 and 280 nm they exhibit weak absorptions as
previously reported for other phenols; in presence of covalently
attached pyridine units the extinction between 270 nm and 280
nm increases because pyridine has itself weakly absorbing n-*
transitions occurring in this spectral range.
[20]
As reported
previously, the [Re(phen)(CO)
3
(py)]
+
complex exhibits a metal-to-
ligand charge transfer (MLCT) band with maxima at 380 nm and
336 nm, while the absorption maximum at 276 nm has been
attributed to phenanthroline-based electronic transitions.
[19a, 19b]
The most important observation from Figure 3 is that with light of
410 nm wavelength we can selectively excite the
[Re(phen)(CO)
3
(py)]
+
complex even in presence of large excess
of any of the four phenols. Furthermore, there is no phenol
absorption in the spectral range in which the rhenium(I) complex
emits (450 nm 700 nm); this is why in Figure 3 we show the
entire spectral range between 250 nm and 700 nm. We note that
phenol has a triplet energy (E
T
) of 3.55 eV,
[21]
while the
[Re(phen)(CO)
3
(py)]
+
complex has E
T
2.75 eV,
[7, 19a]
hence we
can a priori rule out the possibility of
3
MLCT excited-state
quenching by triplet-triplet energy transfer from
[Re(phen)(CO)
3
(py)]
+
to the phenols.
[22]
Figure 3. Optical absorption spectra of the four phenols and the rhenium(I)
complex from Scheme 1.
Luminescence quenching experiments
The solid trace in Figure 4a is the emission spectrum of
[Re(phen)(CO)
3
(py)]
+
in aerated CH
2
Cl
2
with 100 mM CH
3
OH at
25 °C. The excitation wavelength was set at 410 nm. The broad
and unstructured luminescence band is due to the typical
3
MLCT
emission of rhenium(I) tricarbonyl diimines.
[19a]
The solid trace in
Figure 4b shows the temporal evolution of the
3
MLCT
luminescence from Figure 4a after excitation with 10 ns laser
pulses at 410 nm; detection occurred at 530 nm. The
luminescence intensity decays in a single-exponential manner
over more than two orders of magnitude and one extracts a
3
MLCT lifetime of 1.2 s, in line with previous reports.
[19a, 19b]
The
dashed lines in Figure 4a/4b were recorded in presence of
variable concentrations (1 mM 10 mM) of PhOH. No significant
luminescence quenching is observed with PhOH, neither in
intensity (Figure 4a) nor in decay time (Figure 4b). Likewise,
when using deuterated phenol (PhOD), the emission intensity
stays virtually unchanged (Figure 4c) and the luminescence
decays are no faster than in the absence of PhOD (Figure 4d).
We conclude that the ordinary phenol is unable to quench the
3
MLCT excited state of [Re(phen)(CO)
3
(py)]
+
under the
experimental conditions chosen here.

4
Figure 4. (a) Luminescence of [Re(phen)(CO)
3
(py)]
+
in aerated CH
2
Cl
2
with 100
mM CH
3
OH in absence (solid line) and presence of increasing amounts of
PhOH (dotted lines; 1 mM 10 mM) after excitation at 410 nm; (b)
luminescence decays of [Re(phen)(CO)
3
(py)]
+
in the same solvent in absence
(solid line) and presence of increasing amounts of PhOH (dotted lines) after
excitation at 410 nm with laser pulses of 10 ns width (detection wavelength:
530 nm); (c) same experiment as in (a) but with deuterated phenol (PhOD); (d)
same experiment as in (b) but with deuterated phenol (PhOD). All y-axes are in
arbitrary units; the intensity of the unquenched emission in (a) and (c) is
normalized arbitrarily to 1; the intensity at t = 0 in (b) and (d) is normalized
arbitrarily to 1.
Figure 5 shows the results of an analogous series of
experiments performed with PhOH-CH
2
-py. From Figure 5a we
learn that the emission intensity of [Re(phen)(CO)
3
(py)]
+
is
significantly quenched in the presence of 1 mM to 10 mM PhOH-
CH
2
-py (dotted traces compared to solid trace). Similarly, the
luminescence decays are strongly dependent on the PhOH-CH
2
-
py concentration (Figure 5b). When using deuterated PhOD-CH
2
-
py the luminescence decays (Figure 5d) are noticeably slower
than for undeuterated PhOH-CH
2
-py at equal concentration
(Figure 5b). Likewise, in the luminescence intensity data of Figure
5c quenching at a given phenol concentration is noticeably
weaker than for the undeuterated quencher in Figure 5a. Thus,
there appears to be a significant H/D kinetic isotope effect (KIE).
Figure 5. (a) Luminescence of [Re(phen)(CO)
3
(py)]
+
in aerated CH
2
Cl
2
with 100
mM CH
3
OH in absence (solid line) and presence of increasing amounts of
PhOH-CH
2
-py (dotted lines; 1 mM 10 mM) after excitation at 410 nm; (b)
luminescence decays of [Re(phen)(CO)
3
(py)]
+
in the same solvent in absence
(solid line) and presence of increasing amounts of PhOH-CH
2
-py (dotted lines)
after excitation at 410 nm with laser pulses of 10 ns width (detection
wavelength: 530 nm); (c) same experiment as in (a) but with deuterated phenol
(PhOD-CH
2
-py) and 100 mM CH
3
OD; (d) same experiment as in (b) but with
deuterated phenol (PhOD-CH
2
-py) and 100 mM CH
3
OD. All y-axes are in
arbitrary units; the intensity of the unquenched emission in (a) and (c) is
normalized arbitrarily to 1; the intensity at t = 0 in (b) and (d) is normalized
arbitrarily to 1.
Figure 6a is a Stern-Volmer plot based on the luminescence
intensity data from Figures 4/5, and Figure 6b is a Stern-Volmer
plot based on the luminescence lifetime data from Figures 4/5.
[23]
The open circles in Figure 6a/6b represent data obtained using
PhOH-CH
2
-py, the open squares represent data obtained using
the deuterated analogue PhOD-CH
2
-py. Linear regression fits
yield the Stern-Volmer constants (K
SV
) given in the third (ordinary
phenols) and fourth column (deuterated phenols) of Table 2.
Figure 6. (a) Stern-Volmer plot based on the luminescence intensity data from
Figures 4/5; open circles: PhOH-CH
2
-py, open squares: PhOD-CH
2
-py, grey
filled circles: PhOH. (b) Stern-Volmer plot based on the luminescence lifetime
data from Figures 4/5; open circles: PhOH-CH
2
-py, open squares: PhOD-CH
2
-
py, grey filled circles: PhOH. (c) Stern-Volmer plot based on the luminescence
intensity data from Figures S1/S2; open circles:
t
Bu
2
PhOH-CH
2
-py, open
squares:
t
Bu
2
PhOD-CH
2
-py, grey filled circles:
t
Bu
2
PhOH, grey filled squares:
t
Bu
2
PhOD. (d) Stern-Volmer plot based on the luminescence lifetime data from
Figures S1/S2; open circles:
t
Bu
2
PhOH-CH
2
-py, open squares:
t
Bu
2
PhOD-CH
2
-
py, grey filled circles:
t
Bu
2
PhOH, grey filled squares:
t
Bu
2
PhOD.
The H/D KIE mentioned above shows up directly in the Stern-
Volmer constants: From the intensity data in Figure 6 one
extracts K
SV, H
= 691±7 M
-1
(for PhOH-CH
2
-py) and K
SV, D
= 355±5
M
-1
(for PhOD-CH
2
-py), the lifetime data in Figure 6b yield K
SV, H
=
701±3 M
-1
(for PhOH-CH
2
-py) and K
SV, D
= 334±6 M
-1
(for PhOD-
CH
2
-py). Based on the
3
MLCT lifetime of [Re(phen)(CO)
3
(py)]
+
(1.2 s in aerated CH
2
Cl
2
, see above) we calculate rate constants
for bimolecular excited-state quenching of k
Q, H
= (5.9±0.1)10
8
M
-1
s
-1
for PhOH-CH
2
-py and k
Q, D
= (2.8±0.1)10
8
M
-1
s
-1
for PhOD-
CH
2
-py (fifth and sixth column of Table 2).
[23]
The H/D KIE is the
ratio between k
Q, H
and k
Q, D
and amounts to 2.1±0.1 (last column
of Table 2).
[6g]
From the luminescence intensity data in Figure 6a
one extracts k
Q, H
= (5.9±0.1)10
8
M
-1
s
-1
for PhOH-CH
2
-py and k
Q,
D
= (3.0±0.1)10
8
M
-1
s
-1
for PhOD-CH
2
-py, yielding a value of KIE
(2.0±0.1) in accordance with the lifetime data.

5
Table 2. Results from luminescence quenching experiments
phenol Exp. type K
SV,H
[M
-1
] K
SV,D
[M
-1
] k
Q,H
[M
-1
.s
-1
] k
Q,D
[M
-1
.s
-1
] KIE
PhOH/D intensity 3.4±1.2 0.5±3.7 (2.9±1.0)10
6
(0.4±3.1)10
6
N/A
lifetime 8.4±0.4 13.8±1.0 (7.1±0.3)10
6
(11.7±0.8)10
6
N/A
PhOH/D-CH
2
-py intensity 691±7 355±5 (5.9±0.1)10
8
(3.0±0.1)10
8
2.0±0.1
lifetime 701±3 334±6 (5.9±0.1)10
8
(2.8±0.1)10
8
2.1±0.1
t
Bu
2
PhOH/D intensity 391±8 332±8 (3.3±0.1)10
8
(2.8±0.1)10
8
1.2±0.1
lifetime 437±11 309±10 (3.7±0.1)10
8
(2.6±0.1)10
8
1.4±0.1
t
Bu
2
PhOH/D-CH
2
-py intensity 1572±9 892±5 (13.3±0.1)10
8
(7.6±0.1)10
8
1.8±0.1
lifetime 1648±8 773±8 (14.0±0.1)10
8
(6.6±0.1)10
8
2.1±0.1
Stern-Volmer constants obtained from emission intensity or lifetime experiments with normal (K
SV, H
) and deuterated phenols (K
SV, D
). Rate constants for
bimolecular excited-state quenching with normal (k
Q, H
) and deuterated phenols (k
Q, D
); calculated from K
SV, H
and K
SV, D
values using the lifetime of
3
MLCT-excited
[Re(phen)(CO)
3
(py)]
+
in aerated CH
2
Cl
2
with 100 mM CH
3
OH (1177 ns). H/D kinetic isotope effect (KIE) calculated from the ratio of k
Q, H
and k
Q, D
The grey filled circles in Figures 6a/6b represent data
obtained for the undeuterated reference phenol PhOH. One
extracts K
SV, H
= 3.4±1.2 M
-1
from the intensity data in Figure 6a
and K
SV, H
= 8.4±0.4 M
-1
from the lifetime data in Figure 6b, which
in turn yields k
Q, H
values on the order of 10
6
M
-1
s
-1
. This order of
magnitude of k
Q, H
underscores what in principle is already
evident from the raw data in Figure 4: Reductive excited-state
quenching by PhOH is not kinetically competitive with other
(radiative and nonradiative) deactivation processes of
photoexcited [Re(phen)(CO)
3
(py)]
+
. Thus, even though k
Q, D
values for deuterated phenol are technically available from the
data in Figures 4c/4d, calculation of an H/D KIE is not meaningful
in the case of the simple reference phenol.
Figures 6c/6d show Stern-Volmer plots based on
[Re(phen)(CO)
3
(py)]
+
luminescence quenching experiments with
t
Bu
2
PhOH (grey filled circles),
t
Bu
2
PhOD (grey filled squares),
t
Bu
2
PhOH-CH
2
-py (open circles), and
t
Bu
2
PhOD-CH
2
-py (open
squares). The respective raw data are shown in Figures S1 and
S2 of the Supporting Information. The bimolecular rate constants
for excited-state quenching with
t
Bu
2
PhOH and its deuterated
congener extracted from this data are all around 310
8
M
-1
s
-1
(Table 2), for
t
Bu
2
PhOH/D-CH
2
-py the k
Q
-values are about a
factor of 3 larger. H/D KIEs range from close to 1.0 for
t
Bu
2
PhOH
to 2.0 for
t
Bu
2
PhOH-CH
2
-py.
All luminescence quenching experiments were performed in
presence of 100 mM CH
3
OH / CD
3
OD to ensure deuteration of
the phenol molecules for the KIE studies. Use of pure CH
2
Cl
2
or
CD
2
Cl
2
leads to markedly lower KIEs, presumably due to D/H
exchange of the deuterated phenols when brought into contact
with glassware / cuvettes.
Figure 7 shows a plot of the (average) k
Q
values versus
standard Gibbs free energy of reaction (G
ET
0*
) associated with
electron transfer from the individual phenols to
3
MLCT-excited
[Re(phen)(CO)
3
(py)]
+
. The free energies were calculated based
on the redox potentials from Table 1, using the previously
determined electrochemical potential for one-electron reduction of
photoexcited [Re(phen)(CO)
3
(py)]
+
of 0.77 V vs. Fc
+
/Fc (bottom
row of Table 1).
[6f]
Figure 7. Rate constant (k
Q, H
; from Table 2) for
3
MLCT excited-state quenching
of [Re(phen)(CO)
3
(py)]
+
versus driving-force for reductive excited-state
quenching (G
ET
0*
; estimated on the basis of the data in Table 1).
Transient absorption
Figure 8a shows the transient absorption spectrum obtained from
an acetonitrile solution with 6.710
-5
M [Re(phen)(CO)
3
(py)]
+
and
10 mM
t
Bu
2
PhOH-CH
2
-py. Selective excitation of the rhenium(I)
complex occurred at 355 nm (Figure 2) with laser pulses of 10
ns width. The data was time-averaged in a window ranging from 0
to 200 ns after the excitation pulse. The spectrum in Figure 8a
exhibits the signatures of the reduced rhenium tricarbonyl diimine
complex and neutral phenoxyl radical at the same time. The
intense narrow band centered around 315 nm and the weaker
featureless band extending from 340 nm to nearly 550 nm is
typical for the one-electron reduced form of the rhenium complex
considered here.
[6e, 24]
On the other hand, the narrow peaks at
390 nm and 409 nm (dashed vertical arrows) are due to the
phenoxyl radical as becomes evident from comparison to the
spectrum in Figure 8b. The latter spectrum was recorded after
355-nm excitation of a CH
3
CN solution containing 2 mM
t
Bu
2
PhOH-CH
2
-py, 5 mM 1,4-dicyanonaphthalene, and 0.3 M
biphenyl. These reaction conditions (making use of 1,4-
dicyanonaphthalene as a photosensitizer and biphenyl as a co-
donor) represent an efficient means for the photogeneration of
neutral phenoxyl radicals.
[25]
In presence of phenol the spectral
signatures of reduced 1,4-dicyanonaphthalene and oxidized
biphenyl have disappeared within 6 s, and hence when
detecting with a delay of 6.6 s after the 10-ns laser pulse one
obtains the spectrum shown in Figure 8b, representing the

Figures (10)
Citations
More filters

Journal ArticleDOI
Oliver S. Wenger1Institutions (1)
Abstract: The field of excited-state proton-coupled electron transfer (PCET) with d6 metal complexes is reviewed. This includes mostly work on Ru(α-diimine)32+ complexes and rhenium(I) tricarbonyl diimines. In many cases the metal complexes were designed such that they can exhibit acid/base chemistry in addition to their well-known photoredox behavior. Upon photoexcitation the resulting complexes can then act either as combined electron/proton donors or as combined electron/proton acceptors. This review aims to illustrate the usefulness of such complexes for mechanistic studies of excited-state PCET. In other studies the photoactive d6 metal complexes merely act as ordinary electron donors or acceptors, but their reaction partners undergo PCET chemistry, i.e., they release a proton upon oxidation or they are protonated in the course of reduction. This design principle permits the investigation of long-range electron transfer which is coupled to proton transfer, for example in phenol-ruthenium(II) or phenol-rhenium(I) dyads, and this represents another focal point of this review. PCET is an elementary reaction step in biological photosynthesis and plays an important role in water oxidation as well as in CO2 reduction or N2 fixation. Mechanistic studies of excited-state PCET are therefore of interest in the greater contexts of solar energy conversion and small molecule activation.

63 citations


Journal ArticleDOI
Abstract: Excited-state proton-coupled electron transfer (ES-PCET) is a promising avenue for solar fuel production and small molecule activation. Although less is known about ES-PCET reactions than related transformations involving exclusively thermal processes, ES-PCET holds promise as a simple and efficient reaction scheme for the formation of solar fuels, and it may provide access to new reactivity not accessible from ground electronic states. This review classifies ES-PCET into six categories based on the identity of the photoexcited reactant: excited-state H+/e– donors, excited-state H+/e– acceptors, photo-oxidants, photoreductants, photoacids, and photobases. A brief overview of each class of ES-PCET is presented. Recent advances and key discoveries within the six classes of ES-PCET are examined, and underexplored reaction systems and promising paths for future research are discussed.

60 citations


Journal ArticleDOI
TL;DR: Although the forward electron transfer reactions are very rapid, the charge-separated state recombines to the singlet ground state at a time of hundreds of nanoseconds because of the difference in spin multiplicity between the Charge- separated state and the ground state.
Abstract: Donor–chromophore–acceptor triads, (PTZ)2-Pt(bpy)-C60 and (tBuPTZ)2-Pt(bpy)-C60, along with their model compound, (Ph)2-Pt(bpy)-C60, have been synthesized and characterized; their photophysical and electrochemical properties have been studied, and the origin of the absorption and emission properties has been supported by computational studies. The photoinduced electron transfer reactions have been investigated using the femtosecond and nanosecond transient absorption spectroscopy. In dichloromethane, (Ph)2-Pt(bpy)-C60 shows ultrafast triplet–triplet energy transfer from the 3MLCT/LLCT excited state within 4 ps to give the 3C60* state, while in (PTZ)2-Pt(bpy)-C60 and (tBuPTZ)2-Pt(bpy)-C60, charge-separated state forms within 400 fs from the 3MLCT/LLCT excited state with efficiency of over 0.90, and the total efficiency with the contribution of 3C60* is estimated to be 0.99. Although the forward electron transfer reactions are very rapid, the charge-separated state recombines to the singlet ground state at ...

59 citations


Journal ArticleDOI
Wesley D. Morris1, James M. Mayer1Institutions (1)
TL;DR: The results imply a synchronous concerted mechanism, in which the proton and electron transfer components of the CPET process make equal contributions to the rate constants.
Abstract: Multiple-site concerted proton-electron transfer (MS-CPET) reactions were studied in a three-component system. 1-Hydroxy-2,2,6,6-tetramethylpiperidine (TEMPOH) was oxidized to the stable radical TEMPO by electron transfer to ferrocenium oxidants coupled to proton transfer to various pyridine bases. These MS-CPET reactions contrast with the usual reactivity of TEMPOH by hydrogen atom transfer (HAT) to a single e-/H+ acceptor. The three-component reactions proceed by pre-equilibrium formation of a hydrogen-bonded adduct between TEMPOH and the pyridine base, and the adduct is then oxidized by the ferrocenium in a bimolecular MS-CPET step. The second-order rate constants, measured using stopped-flow kinetic techniques, spanned 4 orders of magnitude. An advantage of this system is that the MS-CPET driving force could be independently varied by changing either the pKa of the base or the reduction potential (E°) of the oxidant. Changes in ΔG°MS-CPET from either source had the same effect on the MS-CPET rate constants, and a combined Bronsted plot of ln(kMS-CPET) vs ln(Keq) was linear with a slope of 0.46. These results imply a synchronous concerted mechanism, in which the proton and electron transfer components of the CPET process make equal contributions to the rate constants. The only outliers to the Bronsted correlation are the reactions with sterically hindered pyridines, which apparently hinder the close approach of proton donor and acceptor that facilitates MS-CPET. These three-component reactions are compared with a related HAT reaction of TEMPOH, with the 2,4,6-tri-tert-butylphenoxyl radical. The MS-CPET and HAT oxidations of TEMPOH at the same driving force occurred with similar rate constants. While this is an imperfect comparison, the data suggest that the separation of the proton and electron to different reagents does not significantly inhibit the proton-coupled electron transfer process.

39 citations


Journal ArticleDOI
TL;DR: A new protocol for the synthesis of ortho-functionalized electron-rich arenes from these boronic acids was developed using the b oronic acid moiety as a blocking group in the electrophilic aromatic substitution reaction, followed by the removal of the borony moiety via thermal protodeboronation.
Abstract: The metal-free thermal protodeboronation of various electron-rich arene boronic acids was studied. Several reaction parameters controlling this protodeboronation, such as solvent, temperature, and a proton source, have been investigated. On the basis of these studies, suitable reaction conditions for protodeboronation of several types of electron-rich arene boronic acids were provided. On the basis of this protodeboronation, a new protocol for the synthesis of ortho-functionalized electron-rich arenes from these boronic acids was developed using the boronic acid moiety as a blocking group in the electrophilic aromatic substitution reaction, followed by the removal of the boronic acid moiety via thermal protodeboronation. Mechanistic studies suggested that this protodeboronation might proceed via the complex formation of a boronic acid with a proton source, followed by the carbon-boron bond fission through σ-bond metathesis, to afford the corresponding arene compound and boric acid.

36 citations


References
More filters

Journal ArticleDOI
George M. Sheldrick1Institutions (1)
TL;DR: This paper could serve as a general literature citation when one or more of the open-source SH ELX programs (and the Bruker AXS version SHELXTL) are employed in the course of a crystal-structure determination.
Abstract: An account is given of the development of the SHELX system of computer programs from SHELX-76 to the present day. In addition to identifying useful innovations that have come into general use through their implementation in SHELX, a critical analysis is presented of the less-successful features, missed opportunities and desirable improvements for future releases of the software. An attempt is made to understand how a program originally designed for photographic intensity data, punched cards and computers over 10000 times slower than an average modern personal computer has managed to survive for so long. SHELXL is the most widely used program for small-molecule refinement and SHELXS and SHELXD are often employed for structure solution despite the availability of objectively superior programs. SHELXL also finds a niche for the refinement of macromolecules against high-resolution or twinned data; SHELXPRO acts as an interface for macromolecular applications. SHELXC, SHELXD and SHELXE are proving useful for the experimental phasing of macromolecules, especially because they are fast and robust and so are often employed in pipelines for high-throughput phasing. This paper could serve as a general literature citation when one or more of the open-source SHELX programs (and the Bruker AXS version SHELXTL) are employed in the course of a crystal-structure determination.

77,177 citations


Book
01 Jan 1963
Abstract: Presents a sequence of procedures for identifying an unknown organic liquid using mass, NMR, IR, and UV spectroscopy, along with specific examples of unknowns and their spectra,

11,605 citations



Journal ArticleDOI
Thomas Steiner1Institutions (1)
TL;DR: The hydrogen bond is the most important of all directional intermolecular interactions, operative in determining molecular conformation, molecular aggregation, and the function of a vast number of chemical systems ranging from inorganic to biological.
Abstract: The hydrogen bond is the most important of all directional intermolecular interactions. It is operative in determining molecular conformation, molecular aggregation, and the function of a vast number of chemical systems ranging from inorganic to biological. Research into hydrogen bonds experienced a stagnant period in the 1980s, but re-opened around 1990, and has been in rapid development since then. In terms of modern concepts, the hydrogen bond is understood as a very broad phenomenon, and it is accepted that there are open borders to other effects. There are dozens of different types of X-H.A hydrogen bonds that occur commonly in the condensed phases, and in addition there are innumerable less common ones. Dissociation energies span more than two orders of magnitude (about 0.2-40 kcal mol(-1)). Within this range, the nature of the interaction is not constant, but its electrostatic, covalent, and dispersion contributions vary in their relative weights. The hydrogen bond has broad transition regions that merge continuously with the covalent bond, the van der Waals interaction, the ionic interaction, and also the cation-pi interaction. All hydrogen bonds can be considered as incipient proton transfer reactions, and for strong hydrogen bonds, this reaction can be in a very advanced state. In this review, a coherent survey is given on all these matters.

4,729 citations


MonographDOI
31 May 2001
Abstract: 1. Introduction 2. Archetypes of the weak hydrogen bond 3. Other weak and non-conventional hydrogen bonds 4. The weak hydrogen bond in supramolecular chemistry 5. The weak hydrogen bond in biological structures 6. Conclusions Appendix

3,996 citations


Network Information
Related Papers (5)
Performance
Metrics
No. of citations received by the Paper in previous years
YearCitations
20211
20193
20175
20163
20152
20144