Energy Transfer from CdS QDs to a Photogenerated Pd Complex Enhances the Rate and Selectivity of a Pd-Photocatalyzed Heck Reaction

Article abstract of DOI:10.1021/jacs.9b11278

This Article describes the design of a colloidal quantum dot (QD) photosensitizer for the Pd-photocatalyzed Heck coupling of styrene and iodocyclohexane to form 2-cyclohexylstyrene. In the presence of 0.05 mol % CdS QDs, which have an emission spectrum that overlaps the absorption spectrum of a key Pd(II)alkyl iodide intermediate, the reaction proceeds with 82% yield for the Heck product at 0.5 mol % loading of Pd catalyst; no product forms at this loading without a sensitizer. A radical trapping experiment and steady-state and transient optical spectroscopies indicate that the QDs transfer energy to a Pd(II)alkyl iodide intermediate, pushing the reaction toward a Pd(I) alkyl radical species that leads to the Heck coupled product, and suppressing undesired β-hydride elimination directly from the Pd(II)alkyl iodide. Functionalization of the surfaces of the QDs with isonicotinic acid increases the rate constant of this reaction by a factor of 2.4 by colocalizing the QD and the Pd-complex. The modularity and tunability of the QD core and surface make it a convenient and effective chromophore for this alternative mode of cooperative photocatalysis.

Full text of DOI:10.1021/jacs.9b11278

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Article
Energy Transfer from CdS QDs to a Photogenerated Pd Complex
Enhances the Rate and Selectivity of a Pd-Photocatalyzed Heck Reaction
Zhengyi Zhang, Cameron Rogers, and Emily A. Weiss
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b11278 • Publication Date (Web): 10 Dec 2019
Downloaded from pubs.acs.org on December 10, 2019
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Energy Transfer from CdS QDs to a Photogenerated Pd Complex Enhances the Rate and
Selectivity of a Pd-Photocatalyzed Heck Reaction
Zhengyi Zhang, Cameron R. Rogers and Emily A. Weiss*
Department of Chemistry, Northwestern University, 2145 Sheridan Rd., Evanston, IL 60208-
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corresponding author. Email: e-weiss@northwestern.edu
Abstract
This paper describes the design of a colloidal quantum dot (QD) photosensitizer for the Pd-
photocatalyzed Heck coupling of styrene and iodocyclohexane to form 2-cyclohexylstyrene. In the
presence of 0.05 mol% CdS QDs, which have an emission spectrum that overlaps the absorption
spectrum of a key Pd(II)alkyliodide intermediate, the reaction proceeds with 82% yield for the
Heck product at 0.5 mol% loading of Pd catalyst; no product forms at this loading without a
sensitizer. A radical trapping experiment and steady-state and transient optical spectroscopies
indicate that the QDs transfer energy to a Pd(II)alkyliodide intermediate, pushing the reaction
toward a Pd(I) alkyl radical species that leads to the Heck coupled product, and suppressing
undesired β-hydride elimination directly from the Pd(II)alkyliodide. Functionalization of the
surfaces of the QDs with isonicotinic acid increases the rate constant of this reaction by a factor of
2.4 by co-localizing the QD and the Pd-complex. The modularity and tunability of the QD core
and surface make it a convenient and effective chromophore for this alternative mode of
cooperative photocatalysis.
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Catalytic transformations involving the photoexcited state of a transition metal (TM) complex
afford access to challenging substrates and unusual patterns of reactivity, but methods to
controllably generate TM species with excited states that can participate directly in bond-breaking
or bond-forming reactions are drastically less well-explored than ground state TM-catalyzed
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reactions. Previously reported excited state TM-catalyzed reactions operate either by direct
photoexcitation of the TM catalyst (Scheme 1A) or by cooperative photocatalysis, in which the
TM catalyst is instead excited through energy transfer (EnT) from a photosensitizer (PS) (Scheme
2
1
B). Among the class of excited state TM complex-catalyzed transformations, Pd-catalyzed
Heck-type reactions of alkyl halides with alkenes are fundamentally interesting because they
3
proceed through TM intermediates with radical character, and are particularly useful because they
3
4
achieve cross-couplings of un-activated sp alkyl halides. These photocatalytic reactions proceed
through direct excitation of the starting Pd(0) complex (Scheme 1A), followed by single electron
5
transfer (SET) to an alkyl substrate to generate a Pd(I) alkyl radical, which can then participate in
radical addition to an alkene. The photogenerated Pd(I) radical compounds rapidly relax to Pd(II)
alkyl species, and can subsequently undergo classical Pd(II) chemistry, including undesirable β-
hydride elimination. Recent investigations by Fu and co-workers however suggest a second
photoexcitation of the Pd(II) intermediate can re-form the Pd(I) alkyl excited state, analogous to
known bond-breaking photoexcitations used to generate reactive radicals from transition metal-
3g
alkyl or –acyl compounds. This result suggests a third model of TM-catalyzed excited state
chemistry (Scheme 1C), in which a sensitizer designed to selectively excite a photogenerated TM
intermediate dictates the reaction’s rate and selectivity. Here, we demonstrate this alternative mode
of cooperative photocatalysis by designing a colloidal quantum dot (QD) photosensitizer to
efficiently promote Pd(I) radical chemistry while suppressing β-hydride elimination from the
Pd(II) alkyl state in a Heck reaction between cyclohexyl iodide and styrene, Table 1 (top) and
Scheme 2.
Dedicated photosensitizers are frequently employed to improve challenging photochemical
transformations, especially when competing processes make direct excitation strategies
6
inefficient. This is especially relevant to the system we describe above, in which we propose to
sensitize a transient, rather than ground state, species. Illuminating a dedicated photosensitizer
with a finite exciton lifetime produces a steady-state concentration available excitons in solution.
When sensitizing a ground-state species, having this supply is unimportant, but a transient species
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is not certain to undergo direct excitation during its brief lifetime; the presence of excited
photosensitizers guarantees a larger fraction of these intermediates will successfully be excited per
unit time. We therefore hypothesized that transformations of the sort detailed above would proceed
much more efficiently with the use of a dedicated photosensitizer than when relying on direct
illumination.
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Colloidal QDs, wet chemistry-synthesized semiconductor nanocrystals with size- and material-
dependent bandgaps and redox potentials, are ideal photosensitizers because of their large
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extinction coefficients, stability upon photo-illumination with visible light (particularly when
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serving as energy donors as opposed to charge donors), tunable surface chemistry, and tunable,
narrow-linewidth emission spectra,10 which facilitates energy-selective photosensitization of
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species within a mixture. Our group has reported the first example of chemical catalysis triggered
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by triplet-triplet EnT from a QD, and the first use of energy relays (analogous to those operational
within the natural photosynthetic system) to enhance the quantum efficiency of the photocatalytic
reduction of H
.13
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Scheme 1. General mechanisms for TM photocatalysis with (A) direct excitation of a TM
photocatalyst, (B) EnT to a TM catalyst from a photosensitizer, and (C) direct excitation of a TM
photocatalyst plus photosensitization (through EnT) of an intermediate of the TM catalyst.
In the current work, we identify the visible spectrum absorption feature of a photogenerated
Pd complex intermediate – compound 7 in Scheme 2 – and synthesize a QD photosensitizer of
appropriate emission energy to efficiently promote re-excitation of this transient intermediate and
drive the formation of the Heck intermediate (alkyl radical species) 6. This second photoexcitation
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step – in addition to direct photoexcitation of the initial Pd(0) catalyst – leads to a dramatic increase
in reactivity and selectivity of the Heck coupling (Table 1, top) from that achieved without
photosensitizer, or using a photosensitizer that cannot perform EnT to 7.
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Table 1. Yield and Selectivity of the Pictured Photo-Heck Reaction upon Addition of
Photosensitizers to the Reaction Mixture.a
%
yield
% yield
cyclohexene
Spectral overlap
Photosensitizer (M cm µm )

em (nm)
Photosensitizer
Heck
product 3a
0
-1
-2
-6 c
b
4
None
.05 mol% CdS QDs (R = 2.1 nm)
.05 mol% CdS QDs (R = 2.4 nm)
-
-
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14
12
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15
33
28
22
16
2
0
0
435
450
463
535
560
506
476
625
-
100
216
156
14
65
82
54
6
0
.05 mol% CdS QDs (R= 2.7 nm)
0
0
.05 mol% CdSe QDs (R = 1.3 nm)
.05 mol% CdSe QDs (R = 1.5 nm)
12
2
0
.5 mol% Ir(ppy)
ppy) (dtbbpy)PF
(PF
3
6
2
190
102
20
48
33
0
0
.5 mol% Ir(dFCF
3
2
0
.5 mol% Ru(bpy)
3
6
)
No Pd-, no sensitizer
-
0
No Pd-, 0.05 mol% CdS QDs
(R= 2.4 nm)
Standard conditions: 25 µmol of styrene, 37.5 µmol of iodocyclohexane, 75 µmol of Cs CO , 0.125 µmol of Pd(dba) ,
2 3 2
450
216
0
a
0.375 µmol of Xantphos, and 0.0125 µmol of CdS QDs in 1 mL of benzene were illuminated with 390 nm blue LED
under Ar(g) atmosphere at room temperature for 24 h. The yields shown are the combined yield of E and Z isomers;
b
significant yields of the Z isomer were only observed when using Ir photosensitizers. NMR yields, reported with
c
respect to their limiting reagent: styrene for Heck product 3 and iodocyclohexane for cyclohexene. “Spectral overlap”
refers to the extent of overlap between the emission spectra of a sensitizer and the absorbance spectra of intermediate
6
, see the SI for the calculation.
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Scheme 2. Mechanism of Pd-photocatalyzed reaction between styrene and iodocyclohexane,
illustrating the desired radical-Heck pathway (right) and the competing elimination pathway (left).
The photoexcitation of 7 to re-form the Pd(I) alkyl radical species 6 is highlighted in blue. SET 
single electron transfer.
Scheme 2 describes the mechanism of the Heck reaction between cyclohexyl iodide and
3
d
styrene shown in Table 1. Upon absorption of a photon with minimum energy 2.8 eV, the first
excited state of the Pd(0) photocatalyst 5 – which, for our reaction mixture is putatively
Pd(0)(Xantphos)
2
– donates an electron to iodocyclohexane 1 (E = -0.85 V vs. SCE) to form
0
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4
intermediate 6. 6 then either (i) undergoes radical addition to styrene 2 to form intermediate 8,
which forms 9 and the Heck product 3 following β-hydride elimination, or (ii) rapidly decays to
the “oxidative addition product” 7, which undergoes β-hydride elimination to form 9 and the
cyclohexene byproduct 4. Following either pathway, the starting Pd(0) catalyst is regenerated from
9
in the presence of base (here, Cs
2
CO ).
3
Even though the Pd(0)(Xantphos)
2
compound directly absorbs the 390-nm light of our
illumination source, the Heck coupling between styrene and cyclohexyl iodide does not proceed
at 0.5 mol% catalyst loading after 24 h illumination with a 390-nm LED without a photosensitizer,
Table 1. The unsensitized reaction does however produce a 26% yield (with respect to cyclohexyl
iodide) of cyclohexene as the only product; we therefore conclude that the photoexcitation and
single electron transfer (SET) steps to form the Pd(I) alkyl radical 6 still occur without a
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photosensitizer present, but that the relaxation of 6 to the Pd(II)-alkyliodide 7 and subsequent β-
hydride elimination outcompetes the addition of radical 6 to styrene. We hypothesized that, if we
could form the excited state of 7 by EnT from a photosensitizer, we could push the Pd(I)-Pd(II)
equilibrium toward the Pd(I) radical intermediate 6 (blue arrow in Scheme 2) and promote the
corresponding radical pathway.
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The proposed photosensitizer for Förster-type EnT to 7 needs to have an emission spectrum
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that overlaps energetically with the absorption spectrum of the intermediate. Facile β-hydride
elimination however makes isolating the absorption spectrum of the Pd(II)-alkyl species
challenging (see the SI, Figure S1A); we therefore performed a model reaction where
(iodomethyl)trimethylsilane (TMS-CH
2
I), which lacks eliminable protons, was substituted for
cyclohexyl iodide. As shown by P-NMR spectra of the reaction mixture after 30 minutes of
illumination, Figure S1B of the SI, use of TMS-CH I in place of cyclohexyl iodide leads to a
3
1
2
comparatively long-lived intermediate, Pd(II)(Xantphos)alkyliodide, without formation of
HPd(II)(Xantphos)I.16 We could then study this intermediate with steady-state optical
spectroscopy to determine an appropriate FRET donor for this species.
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Figure 1. (A) Ground-state absorption spectra of Pd(dba)
2
with 3 eq. of Xantphos (black), and
the same sample plus 300 eq. of TMS-CH I at 0 (red), 10 min. (blue), 20 min. (pink), and 30 min.
2
(green) illumination with the blue LED. The samples were prepared in benzene and were purged
with Ar for 10 min before illumination. The main text describes the assignment of the absorbance
features. (B) Normalized absorbance (abs) of the Pd(II)CH
obtained from Figure 1A through a procedure described in the text, overlaid with the emission
em) spectra of CdS QDs (R = 2.4 nm), CdS QDs (R= 2.7 nm), CdSe QDs (R = 1.3 nm and 1.4
nm), Ir(ppy) , Ir(dFCF ppy) (dtbbpy)PF , Ru(bpy) (PF , and with the output spectrum of the
2
TMS(Xantphos)I (solid black),
(
3
3
2
6
3
)
6 2
LED. The emission spectra were collected with an excitation wavelength of 390 nm.
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Figure 1A shows absorbance spectra of the reaction mixtures following illumination of
Pd/Xantphos and TMS-CH
Pd(Xantphos) (arrow 1) decreases and the absorbance of Pd(Xantphos) (arrow 2) increases. In
addition, we observe the formation of a new absorbance feature at ~450 nm (arrow 3), which we
attribute to the Pd(II)CH TMS(Xantphos)I, since NMR indicates there are only three Pd-species
present in the mixture: Pd(0)(Xantphos) , Pd(0)Xantphos, and Pd(II)CH TMS(Xantphos)I, see
Figure S1B of the SI. To isolate the absorption spectrum of this intermediate, we subtracted the
absorbance of the Pd(0) species and Xantphos from the mixture using a titration curve of Pd(dba)
2
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2
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with different equivalents of Xantphos (see the SI, Figure S2). The absorbance feature at ~450 nm
is independent of the concentration of Pd(0)-species in the system or the equivalents of Xantphos
added (see the SI, Figure S3), consistent with our attribution of this feature to
Pd(II)CH TMS(Xantphos)I. We did not observe this absorbance feature when performing the
2
same experiment using cyclohexyl iodide, because its eliminable proton makes the corresponding
intermediate short-lived (see the SI, Figure S4).
Figure 1B shows the isolated absorption spectrum of the Pd(II)CH TMS(Xantphos)I obtained
2
from Figure 1A using the procedure described above, overlaid with both the emission spectra of
several molecular and QD photosensitizers and the spectrum of the LED used to drive the reaction.
The spectral overlap with the emission of CdS QDs with a radius of 2.4 nm suggests that these
QDs will best sensitize the Pd(II)cyclohexyl(Xantphos)I intermediate 7, assuming it has a similar
absorbance spectrum to that of the structurally analogous test species formed using TMS-CH I.
2
As shown in Table 1, we do observe dramatically improved yields of Heck product when
including 0.05 mol% 2.4 nm CdS QDs in the reaction mixture, compared with the yields of the
unsensitized case or when using photosensitizers having weaker spectral overlap with the
identified absorption feature. Figure 2A illustrates the marked change in the rates of reagent
conversion and product formation when the QD photosensitizer is included. The yield of the
desired Heck product is reasonably well predicted by the degree of spectral overlap with the
identified absorption feature of 7; note that although the emission of Ir(dFCF
3
ppy)
2
(dtbbpy)PF is
6
a closer energetic match than that of Ir(ppy) , poorer absorptivity at the excitation wavelength
3
results in a lower magnitude of spectral overlap, as shown in Table 1.
Both Ir sensitizers give the Heck product 3 predominantly as its Z isomer, suggesting that each
participates in a competing process of triplet-triplet energy transfer with the formed product,
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potentially lowering the efficiency of the reaction. We tested additional organic and inorganic
sensitizers having emission energies more similar to that of 2.4-nm CdS QDs, but we found that
the limited set of very well-matched sensitizers performed poorly due to poor solubility or short
fluorescence lifetime (Table S1). This result highlights an important advantage of using QDs as
photosensitizers: the facile ability to synthetically select a narrow emission spectrum at any desired
wavelength, while utilizing tunable surface chemistry to ensure good solubility and stability across
a range of reaction conditions.
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We performed a set of experiments employing a 467 nm LED as an additional light source,
chosen to match the identified absorption of 7, to test our hypothesis that a dedicated
photosensitizer would prove superior to direct illumination for exciting a transient species. (Table
S2) We observe that, as would be expected, addition of the second light source with 0.5 mol% Pd
and without QDs increases the production of Heck product from 0% to 5% yield with a
product/byproduct ratio of 0.1 after 24 h of illumination. Addition of the second light source with
10 mol% Pd and without QDs increases the production of Heck product from 33% to 55% with a
product/byproduct ratio of 0.6 after 24h illumination, in keeping with the reasonable yields
observed previously for this reaction when using high Pd loadings and broad excitation. Each of
our two-color direct excitation reactions was outperformed by the CdS QD-sensitized reactions
shown in Table 1, which featured only 0.5 mol% loadings of Pd. These results indicate that (i)
direct excitation of the identified Pd(II) intermediate 7 to reform 6 is possible, consistent with the
proposed mechanism and the identified absorption feature, and (ii) direct excitation is significantly
less effective for sensitizing the transient Pd(II)-alkyl species than including an energetically well-
matched photosensitizer.
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Figure 2. (A) Concentrations of cyclohexyl iodide (black), Heck coupling product (red), and
cyclohexene (blue) over the course of this reaction without (open symbols, dashed), and with
(
solid symbols, solid) the presence of 0.05 mol% CdS QDs. The numbers above the points are
the yields of Heck product. (B) Yield of the Heck coupling product, relative to styrene, vs.
illumination time when the reaction mixture includes 0.05 mol% oleate (OA)-capped CdS QDs
(black), 0.05 mol% OA-capped CdS QDs with 200 eq. iso-nicotinic acid (red), 0.05 mol% OA-
capped CdS QDs with 200 eq. pyridine (green), and 0.05 mol% OA-capped CdS QDs with 200
1
eq. benzoic acid (blue), determined by H-NMR using diglyme as the internal standard. The ratios
between product 3 and byproduct 4 are 3.7 for OA-capped QDs, 2.7 for OA/iNA-capped QDs,
2
.2 for OA/BA-capped QDs, and 1.9 for OA-capped QDs with pyridine. The samples were
prepared with 25 µmol of styrene, 37.5 µmol of iodocyclohexane, 0.0125 µmol of CdS QDs, 75
µmol of Cs CO , 0.125 µmol of Pd(dba) , and 0.375 µmol of Xantphos in 1 mL of benzene, and
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were illuminated with a blue LED under Ar(g) atmosphere at room temperature. The lines are
guides to the eye.
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When one equivalent of the stable radical TEMPO was added to the reaction system without
QDs, no TEMPO products were detected and the reaction proceeded as before. In contrast, addition
of TEMPO to the QD-sensitized system completely inhibited the reaction, and we observed the
TEMPO adduct of the cyclohexyl radical (see the SI, Figure S6). These results indicate that the
cyclohexyl radical is (i) a crucial intermediate in the photo-Heck pathway, (ii) not appreciably
present in the absence of the photosensitizer, perhaps because of rapid relaxation to the Pd(II) alkyl
species, and (iii) formed in the presence of CdS QD photosensitizer, consistent with the proposed
cooperative sensitization mechanism (Scheme 1C).
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It is possible, in principle, that the enhancement of Heck product yield in the presence of QDs
is due to either (i) electron transfer (eT) from the QDs to cyclohexyl iodide or (ii) EnT from the
QD to the Pd(Xantphos) catalyst, rather than through our proposed mechanism, sensitization of
2
intermediate 7. We have however eliminated these alternative mechanisms through steady-state
and time-resolved optical measurements of QD-cyclohexyl iodide and QD-Pd/Xantphos mixtures,
detailed in the SI, Figures S7, S8. A further proof that the QDs do not enhance the yield of Heck
product by photosensitizing the initial catalyst Pd(Xantphos)
reaction with 450-nm monochromatic excitation (which photoexcites the QDs but not
Pd(Xantphos) ) in place of the blue LED, we observe no formation of Heck product. When we use
05-nm monochromatic excitation (which photoexcites both the QDs and Pd(Xantphos) ), we do
2
catalyst is that, when we attempt the
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form 5% Heck product with respect to styrene, see Figure S9 of the SI. This result indicates that
direct excitation of the Pd(0) catalyst is necessary for the reaction to proceed, consistent with the
proposed mechanism of QD involvement by selective sensitization of the photogenerated
intermediate 7. In addition, we observe no change in the absorption features of the QDs during the
course of the reaction, indicating the photosensitizer is highly stable (see Figure S10 of the SI).
This is consistent with an energy transfer (as opposed to redox) mechanism of sensitization, which
avoids the deleterious buildup of charge in the QDs by removing both charge carriers from the QD
simultaneously.
We undertook a small exploration of substrate scope, to demonstrate the general applicability
of our results. (Table S3) We observe reasonable yields for styrene substrates having both
electron-withdrawing and electron-donating aryl substituents. We also tested a small scope of alkyl
halides, and found the reaction to be considerably slower when using an alkyl bromide rather than
an alkyl iodide (5% yield of desired Heck product), or when using a primary rather than secondary
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alkyl iodide (38% yield), in both cases presumably because of a less favorable SET from the
excited Pd(0) species to the substrate. This is reflected in the correspondingly low yield of the
elimination side product and the low overall consumption of the substrate. In contrast, the reaction
proceeds readily when using TMS-CH I because the alpha silicon effect facilitates SET to the
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halide, and we observe a 77% yield when using this substrate, similar to that obtained with the
original secondary alkyl iodide.
In addition to increasing the selectivity of the reaction for the Heck product over cyclohexene,
inclusion of the QD photosensitizer increases the overall rate of consumption of the
cyclohexyliodide starting material by a factor of three, Figure 2A. This result is not inconsistent
with, although not necessarily implied by, the proposed mechanism of QD involvement in this
reaction. The result could mean that the starting Pd(0) catalyst is re-formed directly from the
Pd(II)-alkyl species 7 at an appreciable rate. This elimination is plausible, but we do not have direct
evidence for it at this time.17
We note that, in contrast with the 0.5 mol% Pd case discussed above, higher loadings of Pd
catalyst do produce some Heck product 3 even without inclusion of a photosensitizer. In each case,
however, the yield improves dramatically when using 0.05 mol% CdS QDs photosensitizer, and
the selectivity diminishes as the Pd loading increases (Table S4 of the SI). These results suggest
that a mechanism other than that pictured in Scheme 2 is active at these higher catalyst loadings.
Given our conclusion that the QD promotes Heck coupling by transferring energy to the Pd(II)
intermediate 7 at low catalyst loading, we exploited the tunable surface chemistry of the QDs to
18
further enhance the reaction rate. We functionalized our CdS photosensitizer with iso-nicotinic
acid (iNA, see NMR spectra in Figure S11 of the SI), which binds to the QD surface through its
carboxylic acid, displacing some of the native oleate ligand, and is terminated with a pyridinic
nitrogen that could potentially coordinate to Pd-complexes. We envisioned that this ligand would
co-localize the QD and the Pd-catalyst and improve the yield of EnT to 7. As shown in Figure 2B,
the rate constant of Heck product formation is a factor of 1.4 times higher when using the iNA-
functionalized QDs than for the original oleate-capped QD (see the SI, Figure S12). This
enhancement does not occur when we instead add pyridine, which mimics the basicity of the iNA,
or benzoic acid (BA), which mimics the structure of the iNA on the surface of the QD. iNA-
functionalized QDs also however produce approximately 1.6 times the amount of the cyclohexene
byproduct 4 than do OA-capped QDs at 24 h. In addition to increasing the rate of reaction, iNA-
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capped QDs produce a better selectivity (ratio of desired Heck product 3 to byproduct 4 = 2.7)
when compared to reaction systems featuring BA (2.2), pyridine (1.9), or BA and pyridine together
(
2.0) (Table S5 of the SI).
In summary, we have identified a mode of cooperative photocatalysis for transition metal-
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catalyzed excited state reactions in which a photosensitizer interacts selectively with a
photogenerated transition metal intermediate. By choosing a CdS QD sensitizer with an emission
spectrum that overlaps with the absorption of a Pd(II) intermediate, we drive the formation of a
Pd(I) intermediate that promotes Heck coupling rather than to the direct β-hydride elimination
byproduct, and improved both the rate and selectivity of a Pd(0)-Pd(I)-Pd(II) photocatalytic cycle.
This strategy also drastically reduces the required Pd loading necessary for formation of the Heck
product, and significantly outperforms unsensitized systems even when the intermediate is directly
excited by a separate light source. Due to the electronically tunable colloidal QDs in this reaction,
the ability to design the surfaces of the QDs to have a high affinity for TM complexes, and the
versatile chemistry of the radical species the reaction generates, we view this strategy – selective
photosensitization of transition metal radical intermediates by QDs – as a general and
straightforward pathway to identify and promote reactive transition metal species with radical
character, which could be deployed for a diverse set of catalytic transformations.
Acknowledgements. The authors thank Dr. Mohamad S. Kodaimati for assistance with
transient absorption. Research primarily supported as part of the Center for Light Energy Activated
Redox Processes (LEAP), an Energy Frontier Research Center funded by the U.S. Department of
Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award # DE-SC0001059
(
synthesis, catalysis, data analysis, experimental design), and by the International Institute for
Nanotechnology at Northwestern University through a fellowship to C.R.R. (experimental design
and data analysis). This work made use of the IMSERC at Northwestern University, which has
received support from the NIH (1S10OD012016-01 / 1S10RR019071-01A1); Soft and Hybrid
Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the State of Illinois and
International Institute for Nanotechnology (IIN).
Supporting Information Available. Experimental details, NMR spectra, additional control
experiments, transient absorption and photoluminescence data, Tables S1-S5 and Figures S1-S12.
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