Probing the Proton-Coupled Electron-Transfer (PCET) Reactivity of a Cross-Conjugated Cruciform Chromophore by Redox-State-Dependent Fluorescence

Article abstract of DOI:10.1002/chem.201900268

Proton-coupled electron transfer (PCET) reactions are of great importance in synthetic chemistry and in biology, but the acquisition of kinetic information for these reactions is often difficult. Herein, we report the synthesis of a new PCET reagent, showing redox-state dependent fluorescence, by merging the concept of cross-conjugated cruciform chromophores with the strategy of imposing redox activity and Br?nsted basicity to aromatic compounds by substitution with guanidino groups. The compound is isolated and characterized in all stable states—reduced, twofold and fourfold protonated and twofold oxidized—and then applied in PCET reactions by using its redox-state dependent fluorescence signal for kinetic measurements.

Full text of DOI:10.1002/chem.201900268

A Journal of
Accepted Article
Title: Probing the Proton-Coupled Electron Transfer Reactivity of a
Cross-Conjugated Cruciform Chromophore by Redox-State
Dependent Fluorescence
Authors: Conrad Wagner, Olaf Hübner, Elisabeth Kaifer, and Hans-
Jörg Himmel
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To be cited as: Chem. Eur. J. 10.1002/chem.201900268
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Probing the Proton-Coupled Electron Transfer Reactivity of a
Cross-Conjugated Cruciform Chromophore by Redox-State
Dependent Fluorescence
Conrad Wagner, Olaf Hübner, Elisabeth Kaifer, Hans-Jörg Himmel*[a]
Abstract: Proton-coupled electron transfer (PCET) reactions are
of great importance in synthetic chemistry and in biology, but the
acquisition of kinetic information for these reactions is often
difficult. Herein, we report the synthesis of a new PCET reagent,
showing redox-state dependent fluorescence, by merging the
concept of cross-conjugated cruciform chromophores with the
strategy of imposing redox activity and Brønsted basicity to
aromatic compounds by substitution with guanidino groups. The
compound is isolated and characterized in all stable states –
reduced, twofold and fourfold protonated and twofold oxidized –
and then applied in PCET reactions, using its redox-state
dependent fluorescence signal for kinetic measurements.
compound can be protonated quantitatively with HBF4·OEt2 to
(BoTMG+H)+ (Scheme 1). Protonation reduces the quantum
yield for fluorescence (from 17% to 13%). On the other hand,
one-electron reduction of BoTMG to the radical monoanion
quenches the fluorescence. The reagent enables a reaction
sequence (Scheme 1) in which a hydrogen atom is transferred
to TEMPO.
In
recent
years,
Guanidino-Functionalized
Aromatics
(GFAs),[ 15 , 16 ] that combine redox activity with high Brønsted
basicity, were established by our group as a new class of PCET
reagents and redox-catalysts.[16-18] GFAs are generally readily
synthesized in good yield,[16] and represent valuable additions to
established PCET reagents such as quinones.[19] By merging the
“GFA concept” with the concept of cross-conjugated cruciform
fluorophores,[ 20 ] we report herein the synthesis of the new
Proton-coupled electron transfer (PCET) processes are often
key steps in synthetic chemistry and biology,[ 1 - 3 ] as well as
atmospheric chemistry.[4] Biology generally uses protonation to
adjust the redox potentials in redox-enzymatic reactions.
Mechanistic insight in these reactions is not trivial, since several
pathways are plausible, even if only one proton and electron are
involved.[5-9] Hence proton and electron transfer proceed either
simultaneously or subsequently. Then, hydrogen-atom transfer
(HAT) processes are possible (especially for nitrogen- or
oxygen-centered radicals), for which (in a concerted pathway)
proton and electron transfer involve the same orbitals of the two
reactants.[10-12] Moreover, PCET can occur from the electronic
ground state or from excited states.[13] If more than one proton
and electron are transferred, the situation becomes even more
complicated and a number of possible intermediates have to be
considered. An analysis via NMR or UV/Vis spectroscopy
becomes often challenging or impossible, since all involved
species contribute to the spectra. In the light of these difficulties,
it is highly desirable to use PCET reagents with redox-state
dependent fluorescence, allowing facile measurement of the
rates of electron transfer in these reactions. However, purely
organic reagents with such properties, that are stable in all
relevant states (reduced, protonated and oxidized), are almost
completely unknown. Very recently, Heiden et al. reported the
synthesis of the fluorescent 4,4-difluoro-4-bora-3a,4a-diaza-s-
indacene (BODIPY) derivative BoTMG (Scheme 1) with a
tetramethylguanidino (TMG) group in the 8-position.[14] Although
the BODIPY group reduces the guanidine basicity, the
chromophore
1,2,4,5-tetrakis(tetramethylguanidino)-3,6-bis-
((triisopropylsilyl)-ethynyl)-benzene, 1 (Scheme 1 and Figure 1).
Its redox-state dependent fluorescence is used to study the
kinetics of PCET reactions.
[a]
C. Wagner, Dr. O. Hübner, Dr. E. Kaifer, Prof. H.-J. Himmel,
Anorganisch-Chemisches Institut
Scheme 1. The fluorescent chromophore BoTMG and its chemistry, reported
by Heiden et al., and the first fluorescent redox-active tetrakisguanidine 1
reported in this work.
Ruprecht-Karls-University of Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg
E-mail: hans-jorg.himmel@aci.uni-heidelberg.de
Bright-yellow 1 is obtained in 43% isolated yield over two steps
starting with a benzothiadiazole precursor (see SI for details). It
Supporting information for this article is given via a link at the end of
the document.
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is soluble in less-polar solvents (THF, Et2O and toluene), but
poorly soluble in polar solvents (CH3CN). Crystals suitable for
structural analysis precipitate from an Et2O solution (Figure 1
and SI).
Temperature-dependent measurements showed that the
fluorescence intensity for (1+2H)2+ rapidly decreases with
increasing temperature, and increases at lower temperatures
(see Figure 3).
Figure 2. Comparison between the experimental UV/Vis spectrum of 1 in THF
and a simulation from TD-DFT (B3LYP/def2-TZVP). The lowest energy band
arises from a pure HOMO → LUMO excitation from the donor arm to the
acceptor arm (see plots of isodensity surfaces).
Figure 1. Structures of the new redox-active tetrakisguanidine in all stable
charge and protonation states. C-H hydrogens and counterions omitted for
clarity. Vibrational ellipsoids drawn at the 50% probability level. Color code: C
grey, N blue, Si yellow, H green. Photos of the luminescence (CH3CN solution
for (1+4H)4+, all others THF solutions) under UV light are included.
A broad, intense absorption appears at 433 nm in the UV/Vis
spectra of THF solutions (Figure 2). According to TD-DFT
calculations (B3LYP/def2-TZVP), this band originates from a
pure HOMO → LUMO transition (calculated at 429 nm, Figure 2).
While the HOMO is localized on the electron-rich arm of the
cruciform (guanidino groups - aromatic ring), the LUMO is
predominantly localized on its electron-acceptor arm (* of the
alkynyl groups - aromatic ring). Hence the lowest-energy
electronic transition has charge-transfer character from one arm
to the other, as characteristic for cruciform chromophores.
Figure 3. Left: UV/Vis spectra and (normalized) fluorescence spectra (dotted
lines) for 1 (ex = 433 nm), (1+2H)2+ (ex = 417 nm) and 12+ (fluorescence
silent) in THF. Right: Temperature dependent emission spectra for (1+2H)2+ in
CH3CN.
Table 1: Comparison between absorption and emission wavelengths (in nm),
with  in M−1cm−1 in parenthesis, fluorescence quantum yield  and lifetime 
(in ns) for all isolated compounds (in THF solution; (1+4H)4+: in CH3CN).
Notably, the compound shows fluorescence, with maximum of
emission at 502 nm in THF solution (Figure 3). The Stokes shift
measures 3332 cm1 and the quantum yield 18%.
Compound
1

max, abs. ()
max, em.
502
508
-


Reaction of 1 with NH4PF6 yields the diprotonated compound
(1+2H)2+ in an isolated yield of 92%, that crystallizes from a THF
solution (Figure 1 and SI). Protonation occurs at the imino N
atoms of two guanidino groups in para-position to each other,
and leads to characteristic structural changes of these groups
such as the elongation of the C=N imino bonds. In the UV/Vis
spectrum, the lowest-energy band is slightly blue-shifted from
433 nm in 1 to 419 nm in (1+2H)2+ (Figure 3), and exhibits a
slightly larger extinction coefficient (10900 M−1cm−1 for 1 and
13300 M−1cm−1 for (1+2H)2+). The wavelength of the
fluorescence maximum is almost unchanged (em = 502 nm for 1
and 508 nm for (1+2H)2+), but the quantum yield significantly
increases from 18% in 1 to 31% in (1+2H)2+ at room temperature.
332 (32200), 433 (10900)
293 (82100), 419 (13300)
346 (21200), 446 (32100)
279 (47000), 392 (4800)
0.18
0.31
-
2.3
3.9
-
(1+2H)2+
12+
(1+4H)4+
477
0.33
5.3
Even with an excess of NH4PF6, the twofold protonated
compound, (1+2H)2+, cannot be protonated further. Protonation
of all guanidino groups, leading to (1+4H)4+, is only achieved
with a stronger acid such as HCl (see Figure 1 and SI). In
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+ [21,22]
CH3CN solution, pKa = 10.43 for HCl and ca. 16 for NH4 .
Therefore, (1+4H)4+ is not relevant for the PCET reactivity. The
quantum yield for fluorescence increases upon protonation (from
 = 0.18 for 1 to 0.31 for (1+2H)2+ and 0.33 for (1+4H)4+), and
the fluorescence lifetime also increases (from  = 2.3 ns for 1 to
3.9 ns for (1+2H)2+ and 5.3 ns for (1+4H)4+, see SI).
the HOMO of 12+ involves primarily orbitals of the alkynyl group
() and the central C6 unit, while the LUMO is similar to the
HOMO of neutral 1. Hence the charge-transfer character (from
one cruciform arm to the other) of the lowest-energy electronic
excitation is now reversed, motivating the distinct change in the
optical properties upon oxidation.
The cyclic voltammogram (CV) of 1 in CH2Cl2 solution (Figure 4)
Next, the redox-state dependent fluorescence of the new
cruciform chromophore 1 is applied in test PCET reactions. The
reactions of 12+ with p-methoxy-thiophenol and with 3,5-
ditertbutyl-catechol were chosen for this purpose (Scheme 2),
since the similar reactions between related (non-fluorescent)
redox-active guanidines in their oxidized states proceed fast and
quantitatively.[18] First the reactions were followed by NMR
spectroscopy. In both cases clean reactions without any
byproduct formation were observed. The initial non-fluorescent
reaction mixtures turn into fluorescent solutions upon formation
of (1+2H)2+ in the course of the PCET reactions (see photos in
Figure 5). The NMR spectra showed that neutral 1 is not formed
in the course of the reaction. Hence the fluorescence signal from
the solution only arises from (1+2H)2+. Under the applied
conditions (see caption to Figure 5 and SI), 93% disulfide was
formed within 260 min. The catechol reaction is slower. A
catechol conversion of 92% is obtained after 1400 min reaction
time. The absence of signals from intermediate species in the
NMR spectra argues for a concerted transfer of electrons and
protons (concerted PCET process). The dication (1+2H)2+ does
not react with TEMPO (see SI), in line with the absence of
typical hydrogen-atom transfer (HAT) reactivity. Interestingly, the
reaction is markedly slower than the similar reaction of the
oxidized form of the non-fluorescent redox-active guanidine
1,2,4,5-tetrakis(tetramethylguanidino)-benzene.[18]
shows
a reversible two-electron wave at E1/2 = −0.65 V
(Eox = −0.60 V) vs. Fc+/Fc for the redox couple 12+/1, and a one-
electron wave at E1/2 = +0.65 V (Eox = +0.60 V), that is tentatively
assigned to the redox couple 1•3+/12+. As expected, the potential
needed for oxidation shifts to higher values upon addition of
NH4PF6. We tentatively assign oxidation waves at Eox = −0.13 V
to oxidation of the (unstable) monoprotonated species, (1+H)+,
and at Eox = +0.18 V to oxidation of the diprotonated species,
(1+2H)2+. Evidently, the protonated and oxidized molecules are
very strong Brønsted acids that immediately transfer their
protons to non-oxidized molecules.[23] Consequently, the redox
process is not reversible, and the reduction wave remains at Ered
= −0.75 V (the value for the non-protonated compound).
Chemical oxidation of 1 with 2 equivalents of FcPF6 in CH3CN
affords green 12+ in 70% isolated yield, that is crystallized by
slow diffusion of Et2O into saturated CH3CN solutions (Figure 1).
Since the HOMO does not involve orbitals of the alkynyl groups,
oxidation does not significantly change the wavenumber of the
(out-of-phase) (C≡C) stretching mode in the IR spectrum (2134
cm1 in 1 and 2137 cm1 in 12+, see SI). In the UV/Vis spectrum
of the oxidized compound, a very strong absorption appears at
max = 446 nm, being only slightly shifted from that of neutral 1,
but with three times higher extinction coefficient (Figure 3). The
similarity of the absorption wavelengths for 12+ and (1+2H)2+
hampers the monitoring of PCET reactions by UV/Vis
spectroscopy.
Scheme 2. Test PCET reactions (thiol - disulfide and catechol -
benzoquinone) carried out with the new redox-active tetrakisguanidine. The
product (1+2H)2+ is fluorescent, while the reactand 12+ is non-fluorescent.
According to quantum chemical calculations (B3LYP), the
reaction with the catechol is near thermoneutral (H = 7.8 kJ
mol1 at 0 K and G = +8.0 kJ mol1 at 298 K, 1 bar, without
inclusion of the solvent effect). To assess the practicability of
fluorescence measurements for the analysis of the kinetics of
PCET reactions, the concentration is decreased by a factor of
more than 100 compared to the NMR experiments, with an initial
ratio of catechol to 12+ of 10:1. As expected, in all experiments
the fluorescence signal starts to grow upon mixing the two
reactants together, signalling formation of fluorescent (1+2H)2+
from non-fluorescent 12+. In Figure 6, the normalized
fluorescence intensity IN (for ex = 417 nm) is plotted as a
function of reaction time for a CH3CN solution of 12+ (c = 3.6·105
M) with varying equivalents of catechol (see SI for details). The
lower concentration relative to the NMR experiment leads to a
Figure 4. Cyclic voltammogram of 1 (top) in CH2Cl2 and (1+2H)2+ (bottom) in
THF. The voltammograms in the middle show the effect of protonation with
NH4PF6 on the CV curves (THF). Bu4NPF6 as supporting electrolyte, scan rate
of 100 mV s1. Potentials are given vs. ferrocenium/ferrocene (Fc+/Fc).
Most importantly, the fluorescence is completely quenched upon
two-electron oxidation. According to DFT calculations (see SI),
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massive deceleration of the reaction, being now completed in
500 h (30000 min, Figure 6).
catechol (see SI). In additional experiments, solutions of the
PCET product (1+2H)2+ are titrated with the catechol reagent
(Figure 7). The band at 417 nm in the UV/Vis spectrum of
(1+2H)2+ starts to shift and split into two components for addition
of more than ca. 50 eq. of catechol (Figure 7). Upon addition of
400 eq. of catechol, a large UV/Vis band appears at 356 nm and
a small band at ca. 445 nm. The maximum of fluorescence shifts
to 405 nm (ex = 356 nm). These results indicate the formation of
a hydrogen-bonded complex, in which the two unprotonated
guanidino groups in (1+2H)2+ act as hydrogen-bond acceptor
and the OH groups of the catechol as hydrogen-bond donor.
Hence a simple kinetic analysis is only possible in a certain
concentration regime.
0.5
0 eq. ex = 417 nm
10 eq. ex = 417 nm
50 eq. ex = 414 nm
100 eq. ex = 397 nm
200 eq. ex = 386 nm
400 eq. ex = 356 nm
0 eq.
10 eq.
50 eq.
100 eq.
200 eq.
400 eq.
0.4
0.3
0.2
0.1
0.0
Figure 5. Conversion vs. time plots (see SI for details) from NMR experiments
3
on the PCET reaction of 12+ (c = 17.8·10 M) with 1.2 equivalents of 3,5-
ditertbutyl-catechol (red curve) or 2.0 equivalents of p-methoxy-thiophenol
(blue curve, c = 8.5·103 M); both in CD3CN.
Additional experiments show that the rate is highly solvent
dependent, being much faster in DMSO, where it is completed in
1500 min. Subsequently the catechol concentration was steadily
increased to obtain information about the reaction order with
respect to 12+ (Figure 6). The reaction with 100 eq. of catechol is
ca. 10 times faster than with 10 eq. After an initial period of ca.
800 min, the log(IN) vs. time plot follows a linear relationship (see
SI), in line with a first order kinetics with respect to 12+, with a
reaction rate of 5.9·104 s1. In the initial period the rate is slower,
indicating an autocatalytic effect of one of the products. Most
likely, the formation of an hydrogen-bonded complex between
the catechol and (1+2H)2+ (see below and Figure 7) weakens
the catechol O-H bonds and thereby supports the PCET process.
Experiments with different concentrations of 12+ and a constant
excess of 100 eq. catechol are in line with a pseudo-first order
kinetics (similar half-value times t1/2, see SI).
350 400 450 500 550 600 350 400 450 500 550 600 650 700 750
 / nm  / nm
Figure 7. Changes in the absorption (left) and emission (right) spectra of
(1+2H)2+ (c = 3.76∙105 M) upon addition of 3,5-ditertbutyl-catechol in CH3CN
solution.
In summary, we report herein a rational approach to new PCET
reagents with redox-state dependent fluorescence, merging the
concepts of cross-conjugated cruciform chromophores with the
strategy of imposing redox-activity and Brønsted basicity on
aromatic compounds by substitution with guanidino groups. The
new GFA cruciform (GFA
=
Guanidino-Functionalized
Aromatics) is stable in all relevant protonation and oxidation
states. It is fluorescent in its reduced (and protonated) state, and
the quantum yield is highly temperature-sensitive. On the other
hand, it is fluorescence-silent in its oxidized state. Its PCET
reactivity was shown by the examples of oxidative thiol coupling
and catechol oxidation experiments. Model PCET reactions with
3,5-ditertbutyl-catechol, using the redox-state dependent
fluorescence to follow the conversion, show that the reaction
rate increases with solvent polarity and concentration and argue
for simultaneous proton and electron transfer. These
experiments illuminate the strength of the fluorescence sensing,
but also the limitations arising from the formation of hydrogen-
bonded complexes at high substrate concentrations. Purely
organic PCET reagents showing comparable redox-state
dependent fluorescence were hardly known up to date. Based
on previous work by our group on redox-active GFAs, the
established strategies for tuning the electrooptic properties of
cross-conjugated cruciforms [20] can now be used to develop a
whole family of such compounds.
1.0
0.8
0.6
0.4
0.2
0.0
MeCN 10 eq. catechol
MeCN 50 eq. catechol
MeCN 100 eq. catechol
DMSO 10 eq. catechol
0
5000
10000
15000
t / min
Figure 6. Normalized fluorescence intensity (IN) as a function of reaction time
for PCET reaction between 3,5-ditertbutyl-catechol and 12+ (c = 3.6·105 M).
Keywords: proton-coupled electron transfer • guanidine •
oxidation• fluorescence • redox-active organic compounds
UV/Vis spectra, recorded immediately after mixing the two
reagents together, give no evidence for the formation of a
complex between 12+ and catechol, even for a large excess of
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Entry for the Table of Contents
COMMUNICATION
A novel PCET reagent with redox-
state depending fluorescence was
developed by merging the concept of
cross-conjugated cruciform
C. Wagner, O. Hübner, E. Kaifer, H.-J.
Himmel*
Page No. – Page No.
chromophores with the concept of
imposing redox-activity and high
Brønsted basicity on aromatics by
substitution with guanidino groups.
Probing the Proton-Coupled Electron
Transfer Reactivity of a Cross-
Conjugated Cruciform Chromophore by
Redox-State Dependent Fluorescence
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