Photoinduced Charge-Transfer State of 4-Carbazolyl-3-(trifluoromethyl)benzoic Acid: Photophysical Property and Application to Reduction of Carbon?Halogen Bonds as a Sensitizer

Article abstract of DOI:10.1002/asia.201600538

The photoinduced persistent intramolecular charge-transfer state of 4-carbazolyl-3-(trifluoromethyl)benzoic acid was confirmed. It showed a higher catalytic activity in terms of yield and selectivity in the photochemical reduction of alkyl halides compared to the parent carbazole. Even unactivated primary alkyl bromides could be reduced by this photocatalyst. The high catalytic activity is rationalized by considering the slower backward single-electron transfer owing to the spatial separation of the donor and acceptor subunits.

Full text of DOI:10.1002/asia.201600538

CHEMISTRY
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Accepted Article
Title: Photoinduced Charge-Transfer State of 4-Carbazolyl-3-(trifluoro- me-
thyl)benzoic Acid: Photophysical Property and Application to Reduction of
Carbon-Halogen Bonds as a Sensitizer
Authors: Ryosuke Matsubara; Toshiyuki Shimada; Yasuhiro Kobori; Tatsushi Yabuta;
Toshiyuki Osakai; Masahiko Hayashi
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COMMUNICATION
Photoinduced Charge-Transfer State of 4-Carbazolyl-3-(trifluoro-
methyl)benzoic Acid: Photophysical Property and Application to
Reduction of Carbon–Halogen Bonds as a Sensitizer
Ryosuke Matsubara,* Toshiyuki Shimada, Yasuhiro Kobori,* Tatsushi Yabuta, Toshiyuki Osakai, and
[
a]
Masahiko Hayashi
Abstract: The photoinduced persistent intramolecular charge-
transfer state of 4-carbazolyl-3-(trifluoromethyl)benzoic acid was
confirmed. It showed a higher catalytic activity in terms of yield and
selectivity in the photochemical reduction of alkyl halides compared
to the parent carbazole. Even unactivated primary alkyl bromides
could be reduced by this photocatalyst. The high catalytic activity is
rationalized by considering the slower backward single electron
transfer owing to the spatial separation of the donor and acceptor
subunits.
respect to the two aromatic ring planes, which may have
hindered the primary charge-recombination (CR) process
between the
D and A moieties. Because acridinium ion
derivative 1 generates the CT state with a large positive
[
11]
oxidation potential, it has been utilized by Fukuzumi et al.
or
[
12]
others as a competent photocatalyst that accepts an electron
from demanding substrates to form a carbon radical (oxidative
mechanism).
On the other hand, most of the studies on the photochemical
reductions utilized pre-activated substrates and relied on the use
of transition-metal-containing photocatalysts. Therefore, an
organic photocatalyst that generates its CT state to catalyze the
formation of a carbon radical from demanding substrates via a
reductive mechanism should be developed. For this purpose, 4-
The generation of a photoinduced charge-transfer (CT) state is
the primary energy-absorbing step and crucial to light energy
[
1]
conversions such as natural photosynthetic reactions,
photovoltaic solar cells, and photocatalytic water splitting.
[2]
[3]
(
3,6-dimethoxycarbazol-9-yl)-3-(trifluoromethyl)benzoic acid (2a)
Small molecules that produce persistent CT states have gained
much attention because their photophysical properties and
catalytic activities can be tuned by a detailed molecular design.
Some transition-metal complexes, such as iridium and
ruthenium polypyridyl complexes, undergo metal-to-ligand CT
was designed and synthesized (Figure 1). Herein, we report its
structure, photophysical properties, and catalytic activity in the
reduction of alkyl halides (RXs). The catalytic activities of
several N-arylcarbazole (NAC) derivatives in the reduction of
RXs were compared; the results show that the perpendicular
conformation of the CT state is important for the high catalytic
activity.
3
upon irradiation with visible light, producing long-lived CT state;
they have been widely used as photocatalysts in diverse
[4]
chemical transformations. On the other hand, a persistent CT
state derived from transition-metal-free organic molecules is
attractive because the use of expensive transition metals can be
avoided. Indeed, the long-lived CT state of organic molecules
has been developed by spatially separating the donor (D) and
acceptor (A) to prevent the unproductive back-electron
[5]
transfer; however, those molecules have a relatively lower
excited energy and a large molecular weight, making it difficult to
use them as photocatalysts in the redox reactions of demanding
substrates.
Figure 1. Various NACs and their dihedral angles ()
Some classes of D–A molecules, where the D and A are
linked to each other by a single bond, can possess persistent
NAC 2a consists of carbazole/D and benzoic acid/A units,
covalently bonded at the nitrogen atom of the carbazole. We
chose carbazole having high LUMO and HOMO levels as a
donor unit because the CT state with a large negative reduction
potential which would be potent for reduction of demanding
3
CT states; they have received considerable attention in recent
[
6,7]
3
years
since the systematic investigation on CT state reported
[8]
by Kapturkiewicz et al. The studies on the development of
high-energy and long-lived CT states were culminated by
[9]
substrates
was
expected
to
be
generated.
Fukuzumi and Lemmetyinen et al. They reported that 9-
mesityl-10-methylacridinium ion (1) generates an intramolecular
charge-shifted state (corresponding to the CT state generated
from electronically neutral molecules) with an exceptionally long
lifetime (2 h at 203 K) and a high energy (2.37 eV). One of the
Trifluoromethylbenzoic acid unit was selected as an acceptor
because the intermolecular single electron-transfer (SET)
between carbazole and trifluoromethylbenzoate was known to
[13]
occur.
To secure the perpendicular connection between the
[
10]
two aromatic rings based on the molecular design of 1, the bulky
trifluoromethyl group is substituted on the benzoic acid moiety at
the ortho position to the carbazolyl group. Indeed, the X-ray
diffraction analysis of 2a shows that the two aromatic moieties
key structural features is the perpendicular conformation
with
[
a]
Prof. Dr. R. Matsubara, T. Shimada, Prof. Dr. Y. Kobori, T. Yabuta,
Prof. Dr. T. Osakai, Prof. Dr. M. Hayashi
Department of Chemistry
[14]
are perpendicular (Figure 2a). The HOMO and LUMO orbitals
of 2a based on the X-ray crystal structure were calculated by
DFT using the B3LYP/6-31G** basis set, confirming almost no
overlap between them (Figures 2b and c). The steady-state
absorption spectra of 2a (Figure S1a in the Supporting
Information) show a superposition of the peaks derived from the
Kobe University
Nada-ku, Kobe, Hyogo 657-8501 (Japan)
E-mail: matsubara.ryosuke@people.kobe-u.ac.jp,
ykobori@kitty.kobe-u.ac.jp
Supporting information for this article is given via a link at the end of
the document.
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Chemistry - An Asian Journal
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COMMUNICATION
3
D and A subunits with a slight perturbation caused by their
indicate that the lowest triplet excited state of 2a is CT with a
interactions. The weak shoulder peak at 420 nm can be
lifetime of 250 ns. The wavelength at the onset of the emission
1
1
attributed to the
CT←S
0
transition. The steady-state
peak shows that the energy of the CT state of 2a can be
3
[16]
photoluminescence spectrum of 2a (ex. 303 nm, in THF at 298
K) is distinct from that of N-ethyl-3,6-dimethoxycarbazole (4). In
addition to the weak peak with a max of 380 nm attributed to the
fluorescence from the local excited (LE) state of the carbazole
unit, a new luminescence peak was observed with a max of 530
evaluated to be ca. 2.95 eV, and that of CT is slightly less.
NACs 2b–e bearing different substituents were synthesized.
Figure 1 shows the dihedral angles () between the planes of
the carbazolyl and aryl groups obtained from the solid-state XRD
[
14]
structures. As expected, the dihedral angle increased as the
A-value of the substituent at the ortho position of the carbazolyl
group increased. Figure S1a shows the steady-state UV/Vis
absorption spectra of the NACs. In NACs 2b and 2c bearing
smaller dihedral angles than 2a, the intensities of the low-energy
band from 320 to 380 nm increased, indicating that the broad CT
absorption bands are superimposed on the vibronic transitions
1
of LE←S
0
. Such enhanced CT absorption is rationalized by the
increased orbital overlap between HOMO and LUMO, resulting
in the increased oscillator strength for the CT transitions. The
red shift of the CT absorption band in 2a compared to 2b and 2c
can be attributed to the decrease in the LUMO level on the
benzoic acid moiety caused by the electron-withdrawing
trifluoromethyl group and the resulting decrease in the HOMO–
Figure 2. Structure and frontier orbitals of NAC 2a. a) X-ray crystallographic
structure. b) HOMO. c) LUMO.
nm (Figure 3a), the mirror image of the shoulder absorption
peak. A significant red shift in the luminescence peak as the
solvent becomes polar indicates that the luminescence peak can
be attributed to the CT state (Figure S2).
[
17]
LUMO
gap.
Figure
3a
shows
the
steady-state
photoluminescence spectra of the NACs. The fluorescence
spectra of NACs 2b and 2c were characterized by strong peaks
with a max of 460 nm. Those peaks were attributed to the CT
fluorescence, as confirmed by the red shift of the peaks as the
solvent became polar (Figure S2). In contrast to 2a, TADF was
not observed for 2b and 2c. This suggests that quantum yield of
the long-lived CT state is decreased because of faster primary
Figure 3b shows the time profile of the fluorescence decay in
the region 580–600 nm. Two components with different time
constants (11 ns and 250 ns) were observed. The delayed
fluorescence spectra are in good agreement with the prompt
fluorescence spectra (Figure S1b), indicating that the delayed
luminescence can be assigned to the thermally activated
1
3
delayed fluorescence (TADF) emitted via CT← CT reverse
1
3
1
[
15]
0
CR (S ← CT) than CT← CT due to the increased HOMO–
intersystem crossing. The abovementioned observations
LUMO overlap as mentioned above. However, such increased
orbital overlap may induce the substantially larger singlet–triplet
energy gap in the CT states and thus inhibit the thermally
activation for the delayed fluorescence, even though a certain
3
amount of CT would be generated. The luminescence spectrum
1
of 2d was the superposition of LE fluorescence (max = 385
[
18]
nm)
and CT fluorescence (shoulder). NAC 2e showed only
1
[18]
LE fluorescence.
Next, the catalytic activities of the NACs in the reduction
reactions of RXs were investigated. First, we confirmed that the
catalyst-free photochemical reduction of a primary alkyl iodide
(
1°–RI) 6a was sluggish with the significant concomitant
formation of the byproduct, alkene 8 (entry 1, Table 1). The use
of 5 mol% of NAC 2a promoted the process, affording the
desired alkane 7 in 96% yield with much less formation of alkene
8
(entry 2). The similar catalytic activity of NAC 3 for 2a shows
that the acidic proton of the carboxylic acid of 2a does not affect
the reaction (entry 3). On the other hand, the use of NACs 2b–e
and carbazole 4 as photocatalysts resulted in the formation of a
significant amount of alkene 8 (entries 4–8). When a well-known
triplet sensitizer, benzophenone (5) was used, the reaction was
slower (entry 9). The similar tendency of less formation of alkene
using 2a was observed in the reaction of secondary RI (2°–RI)
(
entry 10 vs. 11). Next, the reaction of more demanding
substrate, primary alkyl bromide (1°–RBr) 6c, was investigated.
The reaction using 5 mol% of NAC 2a in THF for 48 h provided
product 7 in 52% yield (entry 13). In sharp contrast, the reaction
of carbazole 4 in THF for 48 h failed (entry 14). NACs 2b–e gave
lower yields than 2a (entries 15–18) with the exception that 2d
with a perpendicular conformation between the D and A subunits
Figure 3. (a) Photoluminescence spectra of 2a–e and 4 (5.0  105 M) in THF
measured at 298 K. ex = 303 nm. The intensity of 2a was multiplied by a
factor of 5 for clarity. (b) Transient photoluminescence decay curve of 2a in a
deaerated THF at 298 K, observed at 580―600 nm (ex = 355 nm).
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Chemistry - An Asian Journal
10.1002/asia.201600538
COMMUNICATION
resulted in a comparable yield as 2a. These results suggest that
both the formation of CT state and the perpendicular
arrangement are necessary for the high catalytic activity in the
reduction of 1°–RBr (vide infra). Notably, the metal-free
reduction of 1°–RBr bearing no activating group, such as an
photosensitizer in state 9 was assumed to be fast because the
forward and backward SETs can occur back to back in the same
[
24]
spatial region, forming a significant amount of alkene (eq. 5).
On the other hand, the backward SET in state 10, generated
from the CT state, would be slower because the D and A
subunits are spatially separated; thus, the carbon-centered
radical can abstract the hydrogen (eq. 8) before the backward
SET occurs (eq. 7).
[
19]
electron-withdrawing group, is rare.
Figure S3 shows the proposed mechanism for the formation
of alkanes and alkenes in the photocatalytic reduction of RX. A
[20]
SET from the excited photosensitizer
cleavage of the C–X bond affords a carbon-centered radical (eq.
, Figure S3). The radical thus formed abstracts a hydrogen
atom from 1,4-cyclohexadiene (CHD), affording an alkane (eq. 2, state of the catalyst failed (eq. 9), that involving the CT state of
to RX followed by the
In the reduction of a 1°–RBr, the backward SET would take
[
25]
place before the scission of the relatively strong C–Br bond.
Therefore, although the reduction of a 1°–RBr involving the LE
1
Figure S3). Instead, if the radical provides an electron before the
abstraction of a hydrogen atom, the corresponding cation would
the catalyst afforded the product (eq. 12). Although NACs 2b
and 2c generate the CT state, the SOMOs of their CT states
expand on the entire molecule because of their imperfect
perpendicular arrangement. Therefore, the backward SET would
be faster, leading to similar results as those of 4. This is a
unique example where the yield and selectivity of the chemical
reactions improved by the conformational arrangement of the CT
state in a small sphere. NAC 2d generates both the LE and CT
states with a perpendicular conformation. NAC 2d showed a
high catalytic activity in the reduction of RBr (entry 17, Table 1)
owing to the CT state, whereas the presence of the LE state
resulted in the formation of an alkene in the reduction of 1°–RI
[
21]
be formed, forming an alkene (eq. 3, Figure S3). The radical
cation of photosensitizer is converted to its ground state by the
[
22]
SET from tributylamine
S3).
or the radical of CHD (eq. 4, Figure
[
23]
[
a]
Table 1. Photochemical reduction of RXs.
(
entry 6, Table 1). NAC 2e generates only the LE state; thus, its
[
b]
catalytic activity is similar to that of 4.
Yield (%)
Selectivity
(7/8)
3.3
24.0
24.0
3.1
4.5
2.0
1.8
1.5
10.5
1.4
0.1
–
+
One may consider that the electron affinity of D –A affects
[
c]
Entry
RX
cat.
–
t (h)
24
7
7
8
RSM
1
2
3
4
5
6
7
8
9
6a
13
96
95
75
75
64
57
55
16
56
7
4
82
0
2a
3
4
3
4
0
2b
2c
2d
2e
4
6
25
16
31
32
37
1
0
6
0
3
0
4
0
4
0
5
4
85
0
10
11
12
13
14
15
16
17
18
19
6b
6c
2a
4
2
35
88
0
1
0
–
48
48
48
48
48
48
48
4
0
100
43
100
80
99
52
99
11
2a
4
52
0
1
>25
–
0
2b
2c
2d
2e
2a
18
2
1
–
0
–
39
1
1
>25
–
0
[
d]
6c
79
3
>25
[
3
a] A mixture of RX (1 equiv), catalyst (5 mol%), CHD (2 equiv), and Bu N (2
equiv) was irradiated with LED lamps (max = 365 nm) at 26 °C. [b] Determined
by GC analysis. [c] Recovered starting material. [d] The reaction was
conducted in DMF for 4 h.
The difference in the catalytic activity between 2a and 4
observed in the reduction of RXs can be rationalized as follows
(
Figure 4): The obtained photophysical data indicate that upon
irradiation, 2a forms CT state with perpendicular
a
a
conformation between the D and A. In the reduction of 1°–RIs
catalyzed by the LE state (entry 8, Table 1), the backward SET
from the carbon-centered radical to the radical cation of the
Figure 4. Competition between the productive and unproductive pathways and
the difference in the catalytic activities of LE and CT states.
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COMMUNICATION
the rate of the backward SET. However, the oxidation potentials
of NACs 2a–e and carbazole 4 showed no significant difference
among them (Figure S4). Furthermore, the reduction of a 1°–
RBr in a polar solvent (DMF) proceeded much faster (entry 19,
Table 1) than that in relatively nonpolar THF, probably because
of the facilitated dissociation of the radical ion pairs in 12. These
results do not contradict our assumption that the SET from the
CT state with a perpendicular conformation is important as the
first step of the reduction of RXs in hindering the unproductive
backward SET.
carbon-centered radical from RI and RBr with a variety of
substitution patterns (1°, 2°, and 3°) as well as from ArI (entries
1–10). When 4 was used as a catalyst in the reaction of 1°–RBr,
the yields were much lower than those using 2a (entries 2 and 5).
The cleavage of the C–Br bond of ArBr was slow (entry 11). The
C–Cl bond was not cleaved under the condition employed (entry
12).
In conclusion, the photo-irradiation of 4-carbazolyl-3-
3
(trifluoromethyl)benzoic acid 2a generated a persistent CT state
( = 250 ns at 293 K) that exhibits a significant photocatalytic
activity in the reduction of RXs, providing a higher yield and
selectivity than the parent carbazole. NAC 2a enabled even the
metal-free reduction of a 1°–RBr. The high catalytic activity of 2a
can be rationalized by considering the slower backward SET to
the radical cation of 2a owing to the spatial separation of the D
and A subunits. This information can be used for the future
design of photocatalysts, especially for demanding substrates in
redox reactions.
Finally, the scope and limitation of photochemical reduction
of RXs using 2a as a catalyst are shown in Table 2 (entries 1
and 6 are duplicate data of Table 1). NAC 2a could generate a
[
a]
Table 2. Photochemical reduction of several RXs using 2a as a catalyst
Entry
Substrate
Product
Solvent
Time
Yield
/
h
4
/%
96
[
b]
1
THF
[
b,c]
2
3
4
5
6
7
8
THF
DMF
DMF
DMF
THF
THF
THF
45
80[
0
Experimental Section
b,d]
48
The synthetic procedures and characterization of the compounds are
described in the Supporting Information.
[
b]
e]
48
24
24
90
[
69
Acknowledgements
Financial supports from a Grant-in-Aid for Scientific Research on
Innovative Areas “Advanced Molecular Transformations by
Organocatalysts” from MEXT, Japan, Special Coordination
Funds for Promoting Science and Technology, Creation of
Innovation Centers for Advanced Interdisciplinary Research
Areas (Innovative Bioproduction Kobe), MEXT, Japan, The UBE
Foundation, The Japan Prize Foundation, and Chugai
Pharmaceutical Co., Ltd. Award in Synthetic Organic Chemistry,
Japan are appreciated. Further acknowledgements are
represented in the Supporting Information.
[
e]
66[
5
d,e]
24
[
b]
b]
b]
2
56
[
42
66
[
0.5
60
Keywords: photocatalysis • radicals • reduction • charge
transfer • synthetic methods
[
[
b]
f]
9
THF
THF
12
19
71
[
[
[
[
1]
2]
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[
a] A mixture of RX (1 equiv), 2a (5 mol%), CHD (2 equiv), and Bu
was irradiated with LED lamps (max = 365 nm) at 26 °C. [b] GC yield. [c] 2a
10 mol%), CHD (4 equiv). [d] 4 (5 mol%) was used instead of 2a. [e] Isolated
yield. [f] NMR yield.
3
N (2 equiv)
(
[6]
A. Kapturkiewicz, J. Nowacki, J. Phys. Chem. A, 1999, 103, 8145.
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10.1002/asia.201600538
COMMUNICATION
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+
–
10] The phenomenon where the D and A moieties are connected with a
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[
[
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K. Ohkubo, T. Suenobu, Acc. Chem. Res. 2014, 47, 1455.
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A. Nicewicz, 2015, 137, 11340. For the reviews on the use of organic
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[20] For a review on photoredox sensitization, see: G. J. Kavarnos, N. J.
Turro, Chem. Rev. 1986, 86, 401.
[21] It was confirmed that no reaction occurred wihout irradiation, revealing
that the formation of alkene was not derived from the simple E2
elimination of alkyl halides by Bu
CO was used instead of Bu N under otherwise same conditions as
entry 2 in Table 1, resulting in much lower yield (<5%). This result
3
N.
[
[
13] I. Saito, H. Ikehira, R. Kasatani, M. Watanabe, T. Matsuura, J. Am.
Chem. Soc. 1986, 108, 3115.
[22]
K
2
3
3
14] CCDC 1442633–1442637 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The
3
suggests that Bu N works not only as a base to trap HX but also as an
electron donor.
Cambridge
http://www.ccdc.cam.ac.uk/data_request/cif.
15] a) H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi, Nature,
012, 492, 234; b) H. Tanaka, K. Shizu, H. Nakanotani, C. Adachi,
Crystallographic
Data
Centre
via
[23] At present we do not have a strong evidence to rule out the radical
chain mechanism where the photocatalyst acts just only as a radical
initiator, which the anonymous referee suggested. However, we think it
unlikely, especially in the reduction of RBr, because, if a radical chain
mechanism were operable, either a stabilized carbon-centered radical
[
2
Chem. Mater. 2013, 25, 3766; c) S. Y. Lee, T. Yasuda, Y. S. Yang, Q.
Zhang, C. Adachi, Angew. Chem. Int. Ed. 2014, 53, 6402; d) Q. Zhang,
B. Li, S. Huang, H. Nomura, H. Tanaka, C. Adachi, Nature Photon.
3
derived from CHD or Bu N would need to abstract a bromine atom from
RBr to generate a non-stabilized carbon-centered radical. See Figure
S6 in Supporting Information for details.
2
014, 8, 326; e) Q. Zhang, H. Kuwabara, W. J. Potscavage, Jr., S.
Huang, Y. Hatae, T. Shibata, C. Adachi, J. Am. Chem. Soc. 2014, 136,
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[24] The anonymous referee suggested the possibility that the alkene
formation might be derived from the hydrgen atom abstraction occurring
next to the carbon-centered radical. We have no experimental evidence
to deny it, but the possibility would be unlikely because the two short-
lived carbon-centered radicals need to react with each other (the
species which could abstract the hydrogen atom is, if any, only the
carbon-centered radical in this reaction system.)
1
[
[
16] Typically, an energy gap between the S
emitters is less than 0.3 eV.
1 1
and T (EST) of TADF
17] The order of the LUMO energy levels of NACs studied could be
experimentally determined by measuring their reduction potentials
using cyclic voltammetry (Figure S5).
[25] A. A. Isse, C. Y. Lin, M. L. Coote, A. Gennaro, J. Phys. Chem. B, 2011,
115, 678.
[
[
18] Confirmed by the dependency of the fluorescence spectra on the
polarity of the solvent. See Figure S2 in the Supporting Information.
19] The metal-free reductive dehalogenation reactions of
a slightly
activated RX (X is located at the benzylic position or close to the
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Chemistry - An Asian Journal
10.1002/asia.201600538
COMMUNICATION
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COMMUNICATION
The photoinduced persistent
Ryosuke Matsubara,* Toshiyuki
intramolecular charge-transfer state of
Shimada, Yasuhiro Kobori,* Tatsushi
Yabuta, Toshiyuki Osakai and Masahiko
Hayashi
4-carbazolyl-3-(trifluoromethyl)benzoic
acid was confirmed. It showed a
higher catalytic activity in terms of
yield and selectivity in the
Page No. – Page No.
photochemical reduction of alkyl
halides compared to the parent
carbazole. The high catalytic activity is
rationalized by considering the slower
backward single electron transfer
owing to the spatial separation of the
donor and acceptor subunits.
Photoinduced Charge-Transfer State
of 4-Carbazolyl-3-(trifluoro-
methyl)benzoic Acid: Photophysical
Property and Application to
Reduction of Carbon–Halogen Bonds
as a Sensitizer
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