Design, Synthesis, and Application of Highly Reducing Organic Visible-Light Photocatalysts

Article abstract of DOI:10.1021/acs.orglett.8b02420

A series of inexpensive, easily tunable, and highly reducing organic photocatalysts (PCs) that exhibit strong absorption in the visible region are described. Time-dependent density functional theory calculations were applied to corroborate the experimental observations and explain the relationship between the structure and property. The photocatalytic efficiency of these PCs was demonstrated by catalyzing several challenging reactions and the radical cascade cyclization of 1,6-diyne with superior photocatalytic efficiency to other organic photocatalysts.

Full text of DOI:10.1021/acs.orglett.8b02420

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Design, Synthesis, and Application of Highly Reducing Organic
Visible-Light Photocatalysts
Dan Liu, Meng-Jie Jiao, Zhi-Tao Feng, Xing-Zhi Wang, Guo-Qiang Xu, and Peng-Fei Xu*
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou
730000, China
S
* Supporting Information
ABSTRACT: A series of inexpensive, easily tunable, and
highly reducing organic photocatalysts (PCs) that exhibit
strong absorption in the visible region are described. Time-
dependent density functional theory calculations were applied
to corroborate the experimental observations and explain the
relationship between the structure and property. The
photocatalytic efficiency of these PCs was demonstrated by
catalyzing several challenging reactions and the radical cascade
cyclization of 1,6-diyne with superior photocatalytic efficiency
to other organic photocatalysts.
isible light as a safe, inexpensive, abundant, and
light more efficiently and, therefore, are more efficient
Vsustainable energy source can provide sufficient chemical catalysts. Considering all of those facts, we aimed to develop
potential to facilitate many chemical reactions.1 Biosynthesis
highly reducing PCs with strong absorption in the visible
region to expand the toolbox for photosynthesis further.
We posited that the extension of conjugation would reduce
the energy gap between the HOMO and LUMO, which would
render PCs photoexcitable using visible light. Initially, cat. 115
and cat. 2,16 equipped with the extensive conjugation structure
of PTH, were synthesized, but little change in the absorption
property was observed (for cat. 1, λmax, abs = 371 nm; for cat. 2,
λmax, abs = 376 nm) (Figure 1a). We then hypothesized that the
replacement of naphthalene with quinoxaline might stabilize
the LUMO engaged in photon absorption, allowing low energy
visible light to photoexcitate the PCs. As a consequence, it was
found that when two nitrogen atoms were introduced into cat.
1, and the λmax, abs red-shifted to 419 nm (cat. 3)17 (see
Supporting Information (SI), Figure S1), proving cat. 3 to be a
better visible light absorber than PTH.11 DFT calculations
were employed to interpret the orbitals related to the
photoexcitation for cat. 1 and cat. 3 (Figure 1b). As expected,
the HOMOs of cat. 1 and cat. 3 were distributed in the whole
molecule, while the LUMOs were mainly localized on the
acceptor parts of the molecules (naphthalene or quinoxaline).
Therefore, the replacement of naphthalene with more electron-
withdrawing quinoxaline exerted a greater stabilization effect
on the LUMOs of PCs than the destabilization effect on the
HOMOs, with the net effect of λmax, abs red-shifted into the
visible regime (see Table S5). In addition to the red-shifting of
the λmax, abs, this replacement also enhanced the εmax of cat. 1
uses the visible portion of sunlight to provide energy for water
oxidation. It is highly desired that many more chemicals can be
prepared in the same way as the photosynthesis process;
however, the incapability of most organic molecules to absorb
visible light presents a major obstacle.2 Although the discovery
of ruthenium,3 iridium,4 and some earth-abundant transition-
metal-based complexes (Cu,5 Cr,6 Fe7 complexes) promoted
the development of visible-light-photocatalyzed organic syn-
thesis in the past decade,8 some challenging hurdles to remove
transition-metal photocatalysts from the final products have
impeded their further adaptation in organic synthesis.
To overcome these limitations, organic dyes9 have attracted
considerable attention as environmentally friendly photo-
catalysts (PCs) over transition-metal complexes. Although
extensive efforts have been devoted to the development of
organic PCs,10 visible-light organic PCs with strong reduction
potentials are still very rare. Some fluorescent dyes (e.g., PTH
(10-phenylphenothiazine)11 and perylene12) have also been
employed as PCs for organic reactions, while their weak
absorption in visible light limits their synthetic utility. Recently,
Miyake and co-workers also developed series of highly
reducing PCs for the photocatalyzed polymer synthesis via
O-ATRP (organocatalyzed atom transfer radical polymer-
ization), with most maximum absorption wavelengths
(λmax, abs) of these PCs being less than 400 nm.13 Most PC
design researchers believe that PCs absorbing visible light
rather than UV light are preferred because UV light can lead to
undesirable decomposition reactions.14 Meanwhile, PCs
possessing higher molar extinction coefficients (ε) can absorb
Received: July 30, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02420
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Letter
state reductive potential. The replacement of the sulfur atom
with the nitrogen atom, which is a stronger electron-donating
conjugate atom (cat. 5, ES 1/2* = −2.31 V) (Figure 3) resulting
1
Figure 3. Structures and photoredox potentials (vs SCE; all potentials
mentioned hereafter are against SCE).
in a higher HOMO energy, could make up the loss. Although
strong single-electron reductants such as cat. 5 are necessary
for some reactions, their weak positive ground-state oxidation
potentials will limit their application in some other trans-
Figure 1. (a) UV−vis light absorption of cat. 1 and cat. 2. (b) DFT
calculation of LUMO (left) and HOMO (right) localizations in cat. 1
and cat. 3.
1/2
formations (for cat. 5, Eox = 0.26 V vs SCE, Figure 3). To
overcome these limitations, we modified cat. 5 with N-
substituents to obtain cat. 6−cat. 9, which are easier to be
from 5671 to 11501 M−1 cm−1 of cat. 3. Replacing the sulfur
atom with a nitrogen atom further enhanced the εmax (cat. 5,18
εmax = 22274 M−1 cm−1) without appreciable change of λmax, abs
(see Figure S2). Furthermore, these N-substituent groups did
not appreciably perturb the λmax,abs, and all of those modified
PCs possessed excellent εmax’s typically greater than 10000 M−1
cm−1, as demonstrated by the UV−vis absorption of cat. 6−
cat. 9 (Figure 2), which suggested that N-substituent groups
reduced in the ground state. For those PCs with similar ES
1
(Table S4), modification of cat. 5 with an ethyl or Ph group
decreased the ES 1/2* to −2.02 V and −1.89 V for cat. 6 and
1
cat. 7, respectively, making them less reductive in the excited
state. Similar results were observed for other PCs, which could
be explained by the N-substituents acting as electron-
withdrawing groups decreasing the HOMO energy (Tables
S4 and S5), rendering them less reactive in the excited state.
Furthermore, the PCs modified with electron-donating groups
exhibited the opposite effect. For example, cat. 7, modified
with electron-donating groups (OMe, cat. 8), had a stronger
reducing ability than that with withdrawing groups (CF3, cat.
9) in their excited states (for cat. 8, ES 1/2* = −1.91 V; cat. 9,
1
ES 1/2* = −1.79 V vs SCE). In addition, we computationally
1
calculated the triplet energies and the excited-state reduction
potentials. All of the computationally calculated triplet energies
for these PCs exceeded 2.0 eV, and the triplet-excited-state
reduction potentials basically followed the same trend as the
singlet-excited-state reduction potentials. Notably, the triplet-
excited-state potential of cat. 6 was close to that of fac-
Ir(ppy)3,8 one of the most powerful reducing transition metal
PCs. Moreover, compared with PCs without N-substituents,
PCs with N-substituents exhibited reversible oxidation peaks,
which indicated that the N-aryl substituents improved the
stability of the radical cation species (PC•+) generated from
the oxidation of PCs.16
Recent applications of strongly reductive PCs include the
reduction of aryl iodides,20 aryl bromides,21 and aryl
chlorides,22 which possess negative reduction potentials and
high bond dissociation energies. With these PCs developed, we
expected them to catalyze these challenging reactions. Cat. 7
was successfully used to realize the intramolecular 1,5-H
transfer reaction of aryl iodide,23 in which a highly reductive
photocatalyst was necessary (Scheme 1a). Notably, cat. 5
Figure 2. UV−vis light absorption of cat. 5−cat. 9.
had little influence on the energy gap of HOMO and LUMO,
as supported by the DFT-calculated Eg of 5.64, 5.68, 5.68, and
5.70 eV for cat. 6, cat. 7, cat. 8, and cat. 9, respectively (Table
S5). Compared with structurally similar N,N-diaryldihydro-
phenazines, reported by Miyake and co-workers, cat. 5−cat. 9
typically absorb lower energy visible light to access excited
states.
The excited-state potentials were calculated with the
1/2
equation E1/2* = Eox − E0,0 (the energy of the excited
state), which was important for designing organic reactions.19
The singlet energy (ES ) was determined with the fluorescence
1
emission. Compared with cat. 1, the absorption profile of cat.
3 red-shifted to the visible regime with sacrificing the excited-
B
DOI: 10.1021/acs.orglett.8b02420
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Letter
a
Scheme 1. Application of the PCs in Some Challenging
Reactions
Table 1. Optimization of the Reaction Conditions
b
entry
PC
base
yield (%)
c
1
cat. 1
cat. 2
cat. 3
cat. 4
cat. 5
cat. 6
cat. 7
cat. 8
cat. 9
cat. 8
EY
23
18
16
41
51
56
58
60
58
77
38
40
80
c
2
3
4
5
6
7
8
9
10
KHCO3
KHCO3
KHCO3
KHCO3
d
11
12
13
4CzIPN
cat. 8
e
a
Reaction conditions: 1a (0.1 mmol), 2a (0.2 mmol), ox. 1 (0.2
mmol) cat. (1 mol %) in dry DMF under blue LED at 25 °C for 12 h.
could be applied to activate the more challenging C−Br bonds
and C−Cl bonds with 3 mol % catalyst loading at room
b
c
d
e
Isolated yield. White LED. Green LED. 48 h.
̈
temperature (Scheme 1b,c), which was achieved by Konig and
found that 1,6-diynes bearing aromatic rings substituted with
either electron-donating or electron-withdrawing as well as
multiple substituents delivered the desired products in good to
excellent yields (51−93%) (3a−k, Scheme 2). It should be
co-workers using 5−10 mol % perylene diimides (PDI), a class
of fluorescent dyes, at 40 °C.22 Meanwhile, cat. 8 could also
act as a PC to generate the acyl radical with slightly inferior
photocatalytic efficiency (Scheme 1d).24
Organophosphorus compounds have attracted researchers’
attention due to their potential as pharmaceutical agents.25 To
explore the photocatalytic activities of PCs further, we
designed a visible-light-mediated construction of organo-
phosphorus fluorene derivatives through the phosphorus
radical-mediated cascade cyclization reaction of 1,6-diynes,
which had been developed as highly versatile substrates for the
construction of the carbo- and heterocycles in a sustainable
and atom-economical fashion.26
a,b
Scheme 2. Substrate Scope of 1,6-Diynes
Initially, the radical cascade cyclization reaction was
performed using cat. 1 and cat. 2 as catalysts under white
LED irradiation. The fluorene derivative was obtained in poor
yields (Table 1, entries 1 and 2), which was due primarily to
the catalyst’s poor visible-light absorption. To investigate
further how the catalyst structure would influence the catalyst
performance, we tested the good visible light absorbers cat. 3
and cat. 4 in the reaction, which featured similar absorption
properties (with λmax, abs = 420 and 412 nm, respectively, and
εmax = 11501 and 10646 M−1 cm−1, respectively). Cat. 3
exhibited inferior performance to that of cat. 4 (Table 1,
entries 3 and 4), demonstrating the significance of the N-
substituent in the PCs, which may reduce the PC
decomposition. In addition, cat. 6−cat. 9 performed even
slightly better than cat. 5, which further corroborated that the
N-substituents were essential for the PCs. It was also found
that the addition of KHCO3 significantly increased the reaction
yield. Furthermore, we also applied the common organic PCs
(e.g., EY, RB, DCA, PDI, etc.; see Table S1) in this
transformation, which were able to catalyze this reaction albeit
with low to moderate yields, which confirmed the superior
photocatalytic efficiency of cat. 5−cat. 9. Control experiments
confirm that the light, oxidant and PC are essential for this
transformation.
a
Reaction conditions: 1 (0.1 mmol), 2a (0.2 mmol), ox. 1 (0.2
mmol), cat. 8 (1 mol %) and KHCO3 (0.15 mmol) in dry DMF
b
under blue LED at 25 °C for 48 h. Isolated yield.
noted that the electron-donating groups on the phenyl group
of the 1,6-diynes appeared to play a positive role in this
cascade reaction (e.g., 3p/3p′ = 2:1). We next evaluated 1,6-
diynes with different P-centered radicals in this photoredox-
mediated cascade cyclization, as shown in Scheme 3. As can be
seen, substrates possessing geminal substituents proved to be
To study the generality of this highly reducing PC in this
reaction, we extended the substrate scope of 1,6-diynes. It was
C
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Letter
Figure S5), which indicated that oxidant is the most likely
quencher for the excited state of cat. 8. In addition, the
quantum yield of the reaction revealed that the reaction
contains a radical-chain propagation process. According to the
analysis above, a plausible mechanism for this radical addition
tandem cyclization was proposed (Figure S9).28
Scheme 3. Substrate Scope of 1,6-Diynes and Phosphoryl
Radical Precursors
a,b
In summary, we have developed a series of organic dyes as
inexpensive, easily tunable, highly reducing, metal-free photo-
catalysts which exhibited good absorption of visible light and
excellent redox properties. These PCs successfully catalyzed
several challenging reactions in which highly reducing PCs
were necessary, and the cascade cyclization of 1,6-diynes shows
superior photocatalytic efficiency compared to common
organic PCs.
ASSOCIATED CONTENT
* Supporting Information
■
S
a
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02420.
Reaction conditions: 1 (0.1 mmol), 2 (0.2 mmol), ox. 1 (0.2 mmol),
cat. 8 (1 mol %) and KHCO3 (0.15 mmol) in dry DMF under blue
b
c
d
LED at 25 °C for 48 h. Isolated yield. 43 h. 45 h.
Experimental procedures and spectroscopic data for all
new compounds; electrochemical, UV−vis spectrometry,
and fluorescence studies; DFT calculations (PDF)
highly effective for the cyclization reaction (4e−i vs 4a,b),
which could be attributed to Thorpe−Ingold effect.
To demonstrate the efficiency of this PC further, we scaled
up the production of 3a to 2.8 mmol and obtained the product
in 60% yield. Compared with our previous work,27 this
protocol was an effective method to construct 3-phosphory-
lated coumarins, which was obtained in 90% yield even with
lower catalyst loading. Furthermore, this protocol could also be
applied to the synthesis of diphenyl(10-phenylindeno[2,1-
a]inden-5-yl)phosphine oxide 8a in a yield of 40% (Scheme 4).
Accession Codes
CCDC 1827446 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Scheme 4. Further Application of the Protocol
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: xupf@lzu.edu.cn.
ORCID
Peng-Fei Xu: 0000-0002-5746-758X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the NSFC (21632003 and 21572087), the
Key Program of Gansu Province (17ZD2GC011), and the
“111” Pro-gram from the MOE of PR China for financial
support.
To investigate the mechanism of the photoredox radical
cascade cyclization, several control experiments were per-
formed (Scheme 5). First, the reaction was conducted in the
presence of TEMPO, a radical quencher, and the reaction was
completely inhibited. Meanwhile, we also performed a series of
Stern−Volmer fluorescence quenching experiments (see
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