Reactivity control of a photocatalytic system by changing the light intensity

Article abstract of DOI:10.1039/c9sc04584h

We report a novel light-intensity dependent reactivity approach allowing us to selectively switch between triplet energy transfer and electron transfer reactions, or to regulate the redox potential available for challenging reductions. Simply by adjusting the light power density with an inexpensive lens while keeping all other parameters constant, we achieved control over one- and two-photon mechanisms, and successfully exploited our approach for lab-scale photoreactions using three substrate classes with excellent selectivities and good product yields. Specifically, our proof-of-concept study demonstrates that the irradiation intensity can be used to control (i) the available photoredox reactivity for reductive dehalogenations to selectively target either bromo- or chloro-substituted arenes, (ii) the photochemical cis-trans isomerization of olefins versus their photoreduction, and (iii) the competition between hydrogen atom abstraction and radical dimerization processes.

Full text of DOI:10.1039/c9sc04584h

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Reactivity control of a photocatalytic system by
changing the light intensity†
Cite this: Chem. Sci., 2019, 10, 11023
*
*
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have been paid for by the Royal Society
of Chemistry
Christoph Kerzig
and Oliver S. Wenger
We report a novel light-intensity dependent reactivity approach allowing us to selectively switch between
triplet energy transfer and electron transfer reactions, or to regulate the redox potential available for
challenging reductions. Simply by adjusting the light power density with an inexpensive lens while
keeping all other parameters constant, we achieved control over one- and two-photon mechanisms,
and successfully exploited our approach for lab-scale photoreactions using three substrate classes with
excellent selectivities and good product yields. Specifically, our proof-of-concept study demonstrates
that the irradiation intensity can be used to control (i) the available photoredox reactivity for reductive
dehalogenations to selectively target either bromo- or chloro-substituted arenes, (ii) the photochemical
cis–trans isomerization of olefins versus their photoreduction, and (iii) the competition between
hydrogen atom abstraction and radical dimerization processes.
Received 11th September 2019
Accepted 8th October 2019
DOI: 10.1039/c9sc04584h
rsc.li/chemical-science
well-established under laser ash photolysis conditions,14,17–22
which however necessitates expensive pulsed lasers that are
1 Introduction
inaccessible for most synthetic laboratories. Catalytic cycles
comprising the consecutive absorption of two photons with an
excited state as intermediate (i.e., without converting the excited
state into a long-lived radical anion)23,24 were realized under
continuous wave (cw) excitation conditions only very recently.25
The feasibility of this new kind of two-photon chemistry on the
preparative scale with an inexpensive blue diode laser,26 whose
acquisition costs are practically identical to those of compa-
rable ready-to-use high-power LEDs, inspired the reactivity
control approach presented herein. As we will show, the exci-
tation of our recently developed water-soluble catalyst Irsppy27
(see Scheme 1 for its structure) with a single blue photon can
initiate several dehalogenation and isomerization reactions, but
when a second photon is absorbed within the lifetime of its
excited triplet state (3Irsppy) using higher light intensities pꢀer
irradiation area, highly aggressive hydrated electrons (eaqc )
that react with the same substrates via a completely different
pathway are produced, thereby allowing a fundamentally new
reactivity control strategy and its application to selective pho-
to(redox) applications.
The interplay of several photocatalyst properties such as molar
absorption coefficients, excited-state energies, redox potentials,
excited-state lifetimes and photostabilities determines the yield
and selectivity of a given photo(redox) reaction. Therefore, it
comes as no surprise that, under standardized conditions, every
photoreaction has one ideal catalyst that needs to be identied.
In direct consequence, a whole library of photoactive molecules
with tailor-made properties has been prepared over the last few
decades, by both organic1–4 and inorganic5–12 chemists.
To overcome the need for several, frequently expensive
catalysts with optimized properties, the concept of wavelength
dependence and wavelength selectivity13 has been exploited to
control the redox potential of single photocatalysts by regu-
lating the irradiation light color.14,15 In a somewhat related
approach, a photocatalytic allylation was combined with a light-
color selective isomerization to control product formation.16
While these recent chromoselective methods15,16 are very
elegant and clearly have an impact on photoredox catalysis, they
both rely on irradiation equipment consisting of at least two
high-power LEDs as light sources.
An attractive alternative, but yet underexplored strategy to
gain control over photochemical reactivity is provided by regu-
lating the light intensity while keeping all other parameters
constant. The approach of intensity dependent one- (low
intensity) and two-photon (high intensity) chemistry is rather
2 Results and discussion
With a triplet excited state energy of 2.65 eV and an oxidation
potential of 1.01 V vs. NHE,25 3Irsppy accessible by one blue
photon can either act as sensitizer for challenging triplet–triplet
energy transfer reactions28 or as photoredox catalyst for many
Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel,
Switzerland. E-mail: christoph.kerzig@unibas.ch; oliver.wenger@unibas.ch
one-electron substrate reductions (E1/2(Irsppy+/3Irsppy)
¼
† Electronic supplementary information (ESI) available: Comprehensive
experimental details, further mechanistic investigations, control experiments
and application-related details. See DOI: 10.1039/c9sc04584h
ꢀ1.64 V vs. NHE, see also le part of Scheme 1).25,27 As we re-
ported recently, the remarkable photostability of Irsppy
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Scheme 1 Reactivity control of our photocatalytic system with a continuous wave (cw) laser (447 nm, 1 W) using the natural laser beam for one-
photon excitation (left) or the collimated beam for consecutive two-photon excitation (right). Upper row, structure of the photocatalyst (PC)
Irsppy, reactive intermediates produced upon blue one- (left) or two-photon (right) excitation, their reactivities in terms of redox potentials
available for reductions (E1/2 vs. NHE) or triplet energy (ET), as well as photographs illustrating the experimental conditions. Lower row, one- and
twoꢀ-photon mechanisms for substrate (S) activation, and kinetic data showing the unquenched lifetimes of both reactive species (3Irsppy and
eaqc , respectively). For further explanations, see text and ESI.†
combined with its favorable excited-state properties, permits an chloro-5-uorobenzoic acid (which is present as carboxylate
efficient two-photon ionization producing the superreductant under our experimental conditions, see Chapter 3 of the ESI for
e
aqcꢀ with its standard potential of ꢀ2.9 V vs. NHE,29 even under details† and Fig. 1a for the structure). Efficient activation of the
cw excitation conditions (right part of Scheme 1). Interestingly, C–F bond of that substrate is unlikely to occur,29 but the uoro
19
the redox energy available for reꢀductions is almost doubled substituent allows us to exploit F NMR spectroscopy36 for
when going from 3Irsppy to eaqc , whereas the unquenched selective and sensitive detection of the reaction outcome.
lifetimes of both species are practically identical in our system
First irradiation experiments (3 h of 447 nm illumination)
(see kinetic data presented in Scheme 1 and Section 1.3 of the using our catalytic system (Irsppy and TEOA in alkaline water)
ESI for details†). During our recent investigations on the two- without the lens, i.e., under one-photon conditions, gave the
photon ionization of ꢀIrsppy25 using the chloroacetate debromination product 2-chloro-5-uorobenzoic acid in very
assay22,30,31 to probe eaqc formation, we did not observe any
e
aqcꢀ production with the natural beam of the 447 nm cw laser
operating at 1 W optical output (in Scheme 1, the beam of that
laser is visualized by the catalyst emission), but the biphotonic
ionization operates efficiently when the diode laser beam is
collimated to a spot smaller than 1 mm2 using a lens. In this
study, we put to good use a lens as inexpensive key element ꢀto
switch between one-photon (3Irsppy) and two-photon (eaqc )
induced lab-scale reactions under otherwise very similar
conditions. All photoreactions were carried out in 4 mL cuvettes
with septum caps (pathlength, 10 mm) employing triethanol-
amine (TEOA) as electron donor for catalyst regeneration and
hydrogen atom donor allowing the formation of well-dened
reaction products.25,32 A detailed description of our experi-
mental setup is given in Section 3.1 of the ESI.†
2.1 Redox-potential control for reductive dehalogenations
Fig. 1 Selective debromination and subsequent dechlorination carried
out with 4-bromo-2-chloro-5-fluorobenzoic acid (a) using the new
blue-light driven photocatalytic system in Ar-saturated solutions ((b),
one-photon conditions; (c), two-photon conditions) with the reaction
parameters given at the equations. Quantitative 19F NMR measure-
ments are displayed before (a) and after (b and c) irradiation. For further
Thermodynamically, the reductive activation of C–Cl bonds is
much more challenging than that of C–Br bonds,33–35 and wꢀe
speculated that 3Irsppy can merely debrominate, whereas eaqc
will certainly be able to dechlorinate29 as well. As rst substrate
to test our reactivity control approach, we selected 4-bromo-2- details, see text and Section 3.3 of the ESI.†
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low yields (4%) as observed by quantitative 19F NMR spectros-
We started our studies with unsubstituted trans-cinnamic
copy (for details, see Section 3.3 of the ESI†). Given that the rst acid, which was present in its carboxylate form under our
mechanistic step, the oxidative quenching of 3Irsppy by 4- experimental conditions (see ESI Section 3.4 for details†). This
bromo-2-chloro-5-uorobenzoate, is very slow (ꢁ3 ꢂ 105 Mꢀ1 substrate has a triplet energy of about 2.4 eV,45 i.e., more than
3
sꢀ1, Section 2.2 of the ESI†), a signicant increase in catalyst 0.2 eV lower than that of Irsppy, thermodynamically allowing
loading and reaction time was necessary to observe acceptable a TTET and subsequent double bond isomerization. Using
yields of the debromination product (55%, Fig. 1b). Impor- Irsppy as photocatalyst, photoinduced electron transfer can be
tantly, only traces of 3-uorobenzoic acid (<1%) could be excluded for thermodynamic reasons since the cinnamate
detected resulting in a debromination selectivity as high as 62 radical anion formation requires a signicantly higher (ꢁ0.8
3
(Fig. 1b and Section 3.3 of the ESI†).
V)46 excited-state oxidation potential than Irsppy can provide.
Simply by putting the lens between cw light source and We indeed observed fast quenching of 3Irsppy by the trans-
cuvette (top right photograph in Scheme 1), and irradiating our cinnamate ion. A Stern–Volmer analysis gave a rate constant of
catalytic system in the presence of the substrate 4-bromo-2- 1.20 ꢂ 109 Mꢀ1 sꢀ1 (see ESI, Fig. S2†), which is in line with
chloro-5-uorobenzoic acid under similar conditions as in diffusion-controlled quenching taking coulombic interactions
Fig. 1b, we observed complete consumption of the starting into account,47,48 and we detected the corresponding cis isomer
material and 3-uorobenzoic acid as main product aer six in preparative photolysis experiments employing 1H NMR
hours (Fig. 1c). The employed two-photon conditions that product analysis (one-photon conditions, 79% yield, details are
permit efficient eaqcꢀ production evidently initiate not only given in Section 3.4 of the ESI†). Collimating the cw lasꢀer output
debromination but also dechlorination. Compared to the one- with a lens to achieve two-photon conditions for eaqc genera-
1
photon reaction (Fig. 1a), the reduction of the reaction time to tion, a novel product was observed in the H NMR spectrum,
20% with an even higher yield of the desired two-fold dehalo- whose signals are in agreement with those of b-phenyl-
genated product (Fig. 1c) seems surprising at rst glance. propionate.46 However, a reliable quantication could not be
However, the diffusion controlled one-electron reduction carried out due to spectral overlap of the desired product peaks
kinetics of both bromo- and chloro-substituted benzoate with trans/cis-cinnamate (aromatic region of the spectrum) and
derivatives29 allowing fast reductive lab-scale dehalogenations TEOA-derived (aliphatic region) signals.
provide an explanation for these experimental ndings.
To circumvent these analytical issues, we turned to trans-3-
The results of this section clearly demonstrate that increasing uorocinnamate as substrate (Fig. 2). The addition of the uoro
the thermodynamic reactivity of a catalytic system while main- substituent to our substrate should only slightly modify the
taining a long lifetime, i.e., a high kinetic reactivity, of the initi- pertinent energetics (as the unchanged TTET kinetics displayed
ating species (longer than 1 microsecond, see Scheme 1) is very in the lower le part of Fig. 2 indicate), but allows straightfor-
benecial for photoredox applications: rst, we introduced the ward detection and quantication of both its cis isomer and the
light intensity as selectivity parameter to reductive photoredox corresponding hydrogenated product via 19F NMR spectros-
activations; second, our approach relying on eaqcꢀ as super- copy.49 Our irradiation studies with trans-3-uorocinnamate
reductant ensures relatively short reaction times for challenging employing this procedure gave 75% of the cis isomer with the
and consecutive photoreactions, which otherwise frequently unmodied laser beam (and 25% remaining trans isomer),
require more than one day of intense photoirradiation.15,35,37
whereas a yield of 62% for the hydrogenation product was
observed under two-photon conditions with otherwise unmod-
ied experimental parameters (upper reaction equations in
Fig. 2, NMR spectra are shown in Fig. S5 of the ESI†).
2.2 From triplet–triplet energy transfer (TTET) to electron
transfer (ET) chemistry
The observed cis–trans-ratio of 3 : 1 does not change upon
Having observed intensity dependent redox-potential regula- further irradiation which implies the existence of a photosta-
tion for photoreductions, we now turn to the question whether tionary state. Hence, in addition to the quenching rate constant
a completely different substrate activation mechanism can determination for the TTET reaction between 3Irsppy and trans-
operate under one- and two-photon conditions. A one-electron 3-uorocinnamate, we performed experiments to investigate
reduction is the only conceivable pathway for eaqcꢀ initiated whether the cis isomer can be sensitized by 3Irsppy as well.
reactions, whereas 3Irsppy with its high triplet energy (2.65 eV, Using the very same overall 3-uorocinnamate concentration,
see above) could additionally activate substrates through either present as pure trans isomer or as 3 : 1 cis-to-trans
a triplet–triplet energy transfer (TTET) event, which is quite mixture, the comparative 3Irsppy lifetime reduction was
promising given the ongoing rapid development of triplet- measured (lower right part of Fig. 2). With the well-dened
sensitized photocatalytic reactions.28,38,39
isomeric mixture as quencher, a longer 3Irsppy lifetime, i.e.,
Cinnamates, the salts and esters of cinnamic acids, have a slower quenching rate, was clearly detected. A kinetic analysis
3
a (photo)isomerizable double bond with the trans isomer being allowed us to estimate the TTET rate constant between Irsppy
more stable. Recent photochemical investigations on both the and cis-3-uorocinnamate, which is about 2.5 times slower than
triplet-sensitized trans–cis isomerization28,40–42 and the hydrated that with the trans isomer (see Section 2.1 of the ESI for
electron induced hydrogenation of double bonds43,44 prompted details†). We regard our indirect procedure for quickly esti-
us to apply our reactivity control approach to this substance mating the TTET rate with less-stable cis isomers as quenchers
class.
as
a
useful addition to the toolbox of mechanistic
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Fig. 2 trans–cis Isomerization (left, one-photon mechanism) and hydrogenation (right, two-photon mechanism) with the olefinic substrates
trans-3-fluorocinnamic acid and fumaric acid using the blue-light driven photocatalytic system in Ar-saturated solutions. The lower part of the
figure shows a Stern–Volmer analysis for the triplet–triplet energy transfer (TTET) between 3Irsppy and trans-3-fluorocinnamic acid (left); the
rate constant for the TTET with the corresponding cis isomer was estimated using a 3 : 1 (cis-to-trans) mixture of both isomers as quencher
(photostationary state, right). Further details are given in the text and Section 2.1 of the ESI.†
photochemistry, because this approach does not rely on the
The experiments presented in this section establish that our
prior isolation and purication of cis olens. These results novel reactivity control approach allows carrying out either one-
indicate that the triplet state of cis-3-uorocinnamate is slightly photon induced double bond isomerization reactions or two-
higher in energy than 3Irsppy, and they are in accordance with photon induced hydrogenation of the very same substrate
the cis isomer being the main product in the photoequilibrium. under comparable conditions. The underlying product forming
A plausible reaction mechanism for the formation of the key steps are fundamentally different although the same cata-
hydrogenated product (right part of the upper equation in lyst is employed.
Fig. 2) is provided by: ꢀ(i) initial one-electron reduction of the
cinnamate ion by eaqc ,44 (ii) protonation of the so-obtained
radical dianion,50 and (iii) hydrogen atom abstraction from
2.3 Differentiating between hydrogen abstraction and
electron donor (TEOA)-derived species by the protonated radical
anion32 to form the nal product.
dimerization
Finally, we address the question of whether the fꢀaster initiation
of a reaction with a more energetic species (eaqc ) produced by
a two-photon process might lead to a different nal product
distribution than a less reactive initiating species (3Irsppy)
available from single excitation. To explore this fundamental
aspect, we identied the photochemical reduction of benzyl
halides to produce benzyl radicals8,52–55 as a suitable test
ground, because this photoreaction can yield either the
hydrogen abstraction product toluene or dibenzyl as dimeriza-
tion product.
Trying to perform that reaction in aqueous solution, we rst
used the water-soluble benzyl bromide 4-(bromomethyl)benzoic
acid, but rapid hydrolysis occurred even at room temperature
and pH 7. To overcome this inherent hydrolysis problem of
benzylic C–Br bonds, we focused on the much more stable
benzyl chlorides and selected the substrate 4-(chloromethyl)
benzoic acid, because it is not only perfectly water-soluble in its
carboxylate form but also stable in alkaline solution on the
timescales of our experiments.
Next, we tried to apply our novel TTET-to-ET switch strategy
to the substrate fumaric acid, which exists as dianion under our
alkaline conditions (see Sections 3.2 and 3.4 of the ESI for
1
details† and H NMR spectra). Similar results for the hydroge-
nation of that substrate were obtained (lower reaction equations
in Fig. 2), but only very low yields of the desired cis isomer –
maleic acid – were observed under similar conditions (2%
catalyst loading and 3 h of irradiation) for the corresponding
one-photon reaction. The irradiation of our system for 20 h with
signicantly increased catalyst loading, however, resulted in
a good yield of maleic acid (Fig. 2). The much slower isomeri-
zation reaction rate for fumarate compared to cinnamates can
be rationalized by the high triplet energy of fumarate51 resulting
in a slightly endergonic TTET step with a concomitantly low
3Irsppy quenching efficiency under our experimental conditions
(see also Section 2.1 of the ESI†). The photoreduction under
two-photon excitation conditions proceeded very smoothly
within 3 h for the fumaric acid substrate.
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Fig. 3 3Irsppy (left) and eaqcꢀ (right) induced reductive activation of 4-(chloromethyl)benzoic acid resulting in the hydrogen abstraction and the
dimerization main product, respectively, together with rate constant determinations of the two substrate activation reactions. The conditions for
preparative irradiation experiments are given at the equations. For further details, see text and Section 1.3 of the ESI† (which enlarges on the two-
pulse experiments shown in the right part of the figure).
Studies on the emission quenching of 3Irsppy by 4-(chlor- in the presence of several 4-(chloromethyl)benzoate concentra-
omethyl)benzoate (le plot in Fig. 3) gave a rate constant that is tions and observed the expected superincrease of the substrate
about three orders of magnitude slower than the diffusion limit activation reaction rate in direct manner (right plot in Fig. 3,
(3.5 ꢂ 106 Mꢀ1 sꢀ1). This slow substrate activation is in line with details are givenꢀin Section 1.3 of the ESI†).
the expected low driving force for this reaction as the reduction
A related eaqc induced dimerization procedure in aqueous
of benzyl chlorides is known to be more challenging than that of solution has been published very recently.18 In that system, the
the corresponding bromides. In agreement with these ndings,
aqcꢀ formation step requires a pulsed UV laser and a sophisti-
e
the preparative photoreaction is very slow with our “one- cated micellar system to enable the formation of dimers.
photon” setup: aer 2 h of intense (1 W) irradiation (le part of However, owing to the micellar shielding of potential hydrogen
the equation in Fig. 3) we observed a conversion of 35% only atom donors, even the efficient dimerization of very reactive
(compare Section 2.2, in which we reached the reaction end radicals is possible with that approach.
point in less than 2 h under similar conditions), and there are
The experiments of this section illustrate the effect of the
still remaining substrate molecules aer 7 h reaction time (91% 3000-fold acceleration of a substrate activation rate constant
conversion). These two experiments gave the hydrogen (see rate constants included in Fig. 3). Changing the reactivity of
abstraction main product (4-methylbenzoic acid) in 66% and the initiating species, which we control with the photon ux per
70% conversion-normalized yields, respectively (details con- area, allows controlling the main product, when both hydrogen
cerning the analytics are given in Chapter 3 of the ESI†).
abstraction and dimerization pathways of intermediates are
Carrying out the 4-(chloromethyl)benzoate activation with conceivable.
the very same starting solution and light output of our cw laser,
but using the lens for beam collimation (“two-photon condi-
tions”), we observed a substantial change of the reaction
3 Conclusions
performance (right part of the equation in Fig. 3). The whole
substrate is consumed aer 2 h, corresponding to an increase in
the overall reaction rate by a factor of at least three, although
only a very small fraction of the reaction volume is illuminated
with the two-photon setup (compare, photographs displayed in
Scheme 1). In direct consequence, the local benzyl radical
concentration in the illuminated area has to be higher by orders
of magnitudes when the setup with the lens is employed, which,
as we found, promotes dimerization (54% yield). To sum up
these results: applying a higher photon ux per area to the
reductive activation of a water-soluble benzyl chloride – simply
by using a lens – increases the reaction rate drastically, and
allows to switch from the hydrogen abstraction to the dimer-
ization main product. As in the preceding sections, these results
provide clear evidence for a mechanistic change when the laser
beam is collimated. Under so-called one-photon conditions, the
reductive dechlorination is driven by ꢀ3Irsppy, but with the
collimated beam allowing efficient eaqc formation, the latter
reductant with its much higher reducing power operates. Using
our previously developed and optimized setup for two-pulse
laser ash photolysis,25,56–58 we generated hydrated electrons
As has emerged from this work, the combination of a single
photocatalytic system – i.e., photocatalyst, solvent and sacri-
cial reagent – and a blue cw light source can be exploited for
a variety of chemical reactions. Specically, using our system
the very same substrate can even be selectively converted into
different main products under very similar conditions. The key
for this reactivity control is the modulation of the light intensity
per area, which we realize with inexpensive optics for beam
collimation, thus allowing to switch from one- to two-photon
substrate activation chemistry. As has been noted recently,59
photocatalysis is becoming an increasingly mature eld in
which further innovation crucially depends on in-depth mech-
anistic investigations. Our study follows that philosophy.
The ndings presented herein on controlled substrate acti-
vation through light-power-density regulation might well stim-
ulate a broader application of this concept in the continuously-
growing eld of photo(redox) catalysis. Possible further direc-
tions include the incorporation of our approach into cascade-
type reactions, in which triplets28,38,60–66 or intermediate radi-
cals43,67–69 are trapped by further additives resulting in more
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valuable photochemical products, or the adaption of our 20 J. C. Scaiano, L. J. Johnston, W. G. McGimpsey and D. Weir,
methodology for reactivity control in several other solvents.
Acc. Chem. Res., 1988, 21, 22–29.
21 D. N. Nikogosyan, Int. J. Radiat. Biol., 1990, 57, 233–299.
22 T. Kohlmann, R. Naumann, C. Kerzig and M. Goez,
Photochem. Photobiol. Sci., 2017, 16, 1613–1622.
Conflicts of interest
¨
23 I. Ghosh, T. Ghosh, J. I. Bardagi and B. Konig, Science, 2014,
There are no conicts to declare.
346, 725–728.
24 M. Goez, C. Kerzig and R. Naumann, Angew. Chem., Int. Ed.,
2014, 53, 9914–9916.
25 C. Kerzig, X. Guo and O. S. Wenger, J. Am. Chem. Soc., 2019,
141, 2122–2127.
26 K. C. Harper, E. G. Moschetta, S. V. Bordawekar and
S. J. Wittenberger, ACS Cent. Sci., 2019, 5, 109–115.
27 X. Guo, Y. Okamoto, M. R. Schreier, T. R. Ward and
O. S. Wenger, Chem. Sci., 2018, 9, 5052–5056.
28 F. Strieth-Kalthoff, M. J. James, M. Teders, L. Pitzer and
F. Glorius, Chem. Soc. Rev., 2018, 47, 7190–7202.
29 G. V. Buxton, C. L. Greenstock, W. P. Helman and A. B. Ross,
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Acknowledgements
Financial support provided by the Swiss National Science
Foundation (grant number 200021_178760) and by the German
National Academy of Sciences Leopoldina (postdoctoral
fellowship LPDS 2017-11) is gratefully acknowledged. We thank
¨
Bjorn Pfund for skillful experimental support and Dr Xingwei
Guo for providing us with the catalyst Irsppy.
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