Wavelength selective photoactivated autocatalytic oxidation of 5,12-dihydrobenzo[: B] phenazine and its application in metal-free synthesis

Article abstract of DOI:10.1039/d0ra01495h

Photochemical stability of 5,12-dihydrobenzo[b]phenazine (DHBP) was investigated with LEDs with central emission wavelengths in a range of 365 to 595 nm. Photochemical conversion of DHBP to benzo[b]phenazine (BP) was observed with wavelengths upto 516 nm. Light of 490 and 516 nm is not absorbed by DHBP, but photoactivated autocatalytic oxidation of DHBP to BP with these wavelengths was confirmed. The reaction rate is in a range of 111-208 μg min^-1 with these LEDs. The mechanism of the reaction was examined and the experimental results exclude the intermolecular interaction such as the F?rster resonance energy transfer, the intermolecular charge transfer, the photoinduced electron transfer and the formation of an exciplex. The formation of the reactive oxygen species was verified with electron paramagnetic resonance, which indicates its potential in the synthesis. When sunlight was used as the light source, the oxidation rate of 1 mg mL^-1 DHBP was 393 μg min^-1. Same autocatalytic oxidation was also observed on similar compounds and it can be used for producing metal-free organic substances for semiconductors.

Full text of DOI:10.1039/d0ra01495h

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Wavelength selective photoactivated autocatalytic
oxidation of 5,12-dihydrobenzo[b]phenazine and
its application in metal-free synthesis†
Cite this: RSC Adv., 2020, 10, 9949
Lei Bian,a Jie Ma,ab Xiaotong Feng,a Yuanhang Wang,a Lizhi Zhao,a Lei Zhao,a
c
a
Guangsheng Guoc and Qiaosheng Pu
*
Xiayan Wang,
Photochemical stability of 5,12-dihydrobenzo[b]phenazine (DHBP) was investigated with LEDs with central
emission wavelengths in a range of 365 to 595 nm. Photochemical conversion of DHBP to benzo[b]
phenazine (BP) was observed with wavelengths upto 516 nm. Light of 490 and 516 nm is not absorbed
by DHBP, but photoactivated autocatalytic oxidation of DHBP to BP with these wavelengths was
confirmed. The reaction rate is in a range of 111–208 mg minꢀ1 with these LEDs. The mechanism of the
reaction was examined and the experimental results exclude the intermolecular interaction such as the
Forster resonance energy transfer, the intermolecular charge transfer, the photoinduced electron
¨
transfer and the formation of an exciplex. The formation of the reactive oxygen species was verified with
electron paramagnetic resonance, which indicates its potential in the synthesis. When sunlight was used
as the light source, the oxidation rate of 1 mg mLꢀ1 DHBP was 393 mg minꢀ1. Same autocatalytic
oxidation was also observed on similar compounds and it can be used for producing metal-free organic
substances for semiconductors.
Received 16th February 2020
Accepted 2nd March 2020
DOI: 10.1039/d0ra01495h
rsc.li/rsc-advances
normally pretty high. On the other hand, heavy metal
compounds are also harmful to the environment and may be
1. Introduction
a threat to human health. It is reported that excess manganese
can damage the nervous system and cause a series of symptoms,
such as sleep disturbances, muscular pain and hypertonia,
hallucinations, and mental irritability.12 The accumulation of
Cr(VI) in the human body can lead to many diseases, such as
skin allergy, kidney disease and cancer.13 Therefore, it is of great
signicance to develop metal-free green synthesis methods for
the oxidation of the H2-NHAs.
NHAs exhibit some unique photochemical properties14 and
their photochemistry deserves investigation. Photochemistry is
common in nature, photosynthesis of oxygen by plants, isom-
erization of some organic molecules for the vision are all typical
photochemical processes which adopt the energy of light. The
importance of photochemistry in organic synthesis,15,16 CO2
reduction,17,18 and hydrogen production19 has again attracted
wide attention recently. Photoactivated oxidation is an
N-Heteroacenes (NHAs) are a group of well-known molecules
that are widely used in organic semiconductor devices.1 Their
utilization in chemical/bio sensing,2 OLEDs (as the thermally
activated delayed uorescence organic light emitters)3 and the
preparation of diradicals4,5 has also been reported. These
molecules can be prepared through the condensation of
aromatic o-diamines with aromatic o-dihydroxyarenes or acti-
vated o-dihalides,6 but the direct products of this condensation
are dihydro N-heteroacene (H2-NHA) derivatives, which nor-
mally exhibit good chemical stability and are oen used to
produce other H2-NHA derivatives. To get NHAs, oxidation of
the H2-NHA is frequently needed (Scheme 1). Oxidants, such as
MnO2 (ref. 7–10) and potassium dichromate,11 are suitable for
the oxidation of H2-NHAs to NHAs in most of cases. However,
residual metal ions in the product can be a big concern in the
semiconductor industry where the requirement of purity is
aState Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous
Metals Chemistry and Resources Utilization of Gansu Province, College of Chemistry
and Chemical Engineering, Lanzhou University, Lanzhou, 730000, China. E-mail:
puqs@lzu.edu.cn
bCollege of Chemical Engineering and Technology, Tianshui Normal University,
Tianshui, 741001, China
cDepartment of Chemistry and Chemical Engineering, Beijing University of Technology,
Beijing, 100124, China
Scheme 1 Synthesis of NHAs from aromatic o-diamines and aromatic
o-dihydroxyarenes or activated o-dihalides.
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra01495h
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important reaction of photochemistry, and related researches Japan, 400 MHz). Mass spectra were recorded on a microTOF
will help us to nd the substitutes of oxidants containing heavy mass spectrometer (Bruker Daltonics, Bremen, Germany)
metals. With a proper photocatalyst, metal-free active oxides equipped with an electrospray ionization source. Electron
with special oxidation ability can be produced in the system.20 paramagnetic resonance (EPR) spectra were recorded in quartz
Photobleaching is a well known photochemical phenom- tubes on a Bruker A300-9.5/12 device. Experimental conditions:
enon that is normally detrimental and is one of the shortcom- temperature 300 K, microwave frequency 9.445 GHz, microwave
ings of many organic dyes. Some of photobleaching reactions power 7.7 mW.
have been proved to be related to the photoactivated oxida-
tion.21 Photobleaching frequently produces complicated inter-
2.2 Photolysis and photoactivated autocatalytic oxidation of
mediates or by-products, barely be used for the synthetic
purpose. Therefore, fully understand of the photochemical
process of a photochemical substance is critical for its proper
application. Photoactivated autocatalysis route may accelerate
a photochemical reaction, where the product transfers the light
energy to the reactants.22 If this type of energy transfer is
smooth enough, it will efficiently prevent the damage of the
product and ensure the complete conversion of the starting
material.
As typical NHAs, phenazines, have been reported to be as
photocatalysts in atom transfer radicals polymerization.14,23
However, comprehensive investigation of their own photo-
chemical reaction has been scarcely reported. In this work,
possible reactions of H2-NHAs were systematically investigated
by using LEDs of different emission wavelengths. Photo-
activated oxidation of 5,12-dihydrobenzo[b]phenazine (DHBP)
to benzo[b]phenazine (BP) was conrmed for the wavelengths
that are within the absorption band of DHBP. More impor-
tantly, oxidation of DHBP was also conrmed with irradiation
wavelengths beyond the absorption edge of DHBP, which
showed typical autocatalytic characteristics. This implied the
possibility of using photons with lower energy for the reaction.
The process can be developed as a green synthesis way of metal
impurity-free BP or other similar products.
DHBP
Different concentrations of DHBP solution were prepared with
the mixed solvent, and the uorescence intensities of DHBP at
455 nm (lex ¼ 408 nm, Fig. S1†) were tested to obtain the rela-
tionship between uorescence intensities and concentrations.
LED photocatalytic oxidation experiment. 5 mL of DHBP at
a concentration of 1 mg mLꢀ1 was added to a weighing bottle
(30 mm i.d.) that was placed on a water bath of 10 ꢁC. The LED
was arranged on the top of the weighing bottle to illuminate the
reaction solution (35 mm above the surface of the solution). The
solution was stirred with a 5 mm magnetic stir bar at a speed of
200 rpm. Aer a given period of time, 50 mL of aliquot was taken
out from the bottle and diluted for the uorescence measure-
ment. The control experiment was similar to this procedure but
without light irradiation. Experimental conditions including
wavelength of the illumination, intensity of light, reaction
temperature, initial concentration of DHBP, and atmosphere
(N2, O2 or air) were investigated.
Sunlight catalytic oxidation. Sunlight (at noon) was used
instead of LEDs as the light source. The other experimental
operation was same as described above.
2.3 Synthesis
5,12-Dihydrobenzo[b]phenazine (DHBP).11 Naphthalene-2,3-
diol (320.0 mg, 2.0 mmol) and o-phenylenediamine (216.0 mg,
2.0 mmol) were mixed in an agate mortar and ground to
a uniform powder. Then the mixture was transferred into
2. Experimental section
2.1 Materials and methods
Chemicals were purchased from commercial sources and used a glass tube (10 mm ꢂ 75 mm) and heated under an atmo-
as received without further purication. 5,12-Dihydrobenzo[b] sphere of nitrogen at 180 ꢁC for 0.5 h. Aer cooled down to
phenazine (DHBP) and 5,12-dihydro-2,3-dimethylbenzo[b] room temperature, the material was then washed on a lter
phenazine (DMDHBP) were prepared according to the literature with methanol, acetone and diethyl ether. The resulting
procedures.11 The mixed solvent is a mixture of DMSO and material was collected and dried to give DHBP as a yellow solid
ethanol in a volume ratio of 4 to 1.
(371.0 mg, 80%). 1H-NMR (d in DMSO-d6) 8.05 (s, 2H), 7.14 (m,
LEDs and constant current source were purchased from 2H), 6.89 (m, 2H), 6.34 (m, 2H), 6.23 (s, 2H), 6.16 (m, 2H). 13C-
Taobao (www.taobao.com). Two crocodile clips were soldered to NMR (d in DMSO-d6) 135.03, 132.17, 131.33, 125.29, 123.30,
the constant current source output to facilitate replacement of 120.69, 112.21, 104.81. MS (ESI) [M + H]+ m/z ¼ 233.1 (meas.),
different LEDs. Working parameters of LEDs were showed in 233.1 (calc.).
Fig. S8, Tables S1 and S2.†
5,12-Dihydro-2,3-dimethylbenzo[b]phenazine (DMDHBP).11
The emission spectra of the LEDs were measured with Naphthalene-2,3-diol (80.0 mg, 0.5 mmol) and 4,5-dimethyl-1,2-
a PMS-80 UV-vis-near IR Spectrophotocolorimeter (Everne, phenylenediamine (68.1 mg, 0.5 mmol) were mixed in an agate
Hangzhou, China). Fluorescence spectra were recorded on mortar and ground to a uniform powder. Then the mixture was
a
F97Pro uorospectrophotometer (Shanghai Lengguang transferred into a glass tube (10 mm ꢂ 100 mm) and heated
Technology Co., Ltd., Shanghai, China), and UV-vis absorption under an atmosphere of nitrogen at 180 ꢁC for 0.5 h. Aer cooled
spectra were measured using a UV 2800SPC spectrophotometer down to room temperature, the material was then washed on
(Shanghai Sunny Hengping Scientic Instrument Co., Ltd., a lter with methanol, acetone and diethyl ether. The resulting
Shanghai, China). Nuclear magnetic resonance (NMR) spectra material was collected and dried to give DMDHBP (100.0 mg,
were obtained with a JNM-ECS spectrometer (JEOL, Tokyo, 77%). 1H-NMR (d in DMSO-d6) 7.89 (s, 2H), 7.13 (m, 2H), 6.88 (m,
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2H), 6.21 (s, 2H), 5.99 (s, 2H), 1.92 (s, 6H). 13C-NMR (d in DMSO-
d6) 135.29, 131.25, 129.65, 127.30, 125.13, 123.10, 113.88, 104.53,
19.03. MS (ESI) [M + H]+ m/z ¼ 261.1 (meas.), 261.1 (calc.).
Benzo[b]phenazine (BP). A solution of DHBP (5.0 mg, 21.6
mmol) in the mixed solvent (5 mL) was stirred in air under light
irradiation using LED-391. When the uorescence intensity at
455 nm fell to less than 1% of the initial value, an excess
amount of water was added to the reaction solution to obtain
precipitation. The resulting precipitate was separated, washed
with water and dried. DHBP was almost completely converted
into BP. 1H-NMR (d in CDCl3) 8.92 (s, 2H), 8.25 (m, 2H), 8.13 (m,
2H), 7.82 (m, 2H), 7.54 (m, 2H). 13C-NMR (d in CDCl3) 144.48,
139.95, 134.72, 130.97, 129.89, 128.67, 127.65, 127.08. MS (ESI)
[M + H]+ m/z ¼ 231.1 (meas.), 231.1 (calc.).
Table
1
Oxidation rate of DHBP at different temperatures and
concentrationsa
T (ꢁC)
DHBP (mg mLꢀ1
0.5 1.0 1.5
)
10
20
30
40
2.0
Rate (mg minꢀ1
)
208 235 254 289 343 208 146 113
a
Condition: LED-391, air.
oxidation rate decreased from 208 mg minꢀ1 to 74 mg minꢀ1
(Fig. S4†). When the initial concentration increased from 0.5 mg
mLꢀ1 to 2.0 mg mLꢀ1, the oxidation rate decreased from 343
mg minꢀ1 to 113 mg minꢀ1 (Table 1). At 10 ꢁC, the oxidation rate
of DHBP was 208 mg minꢀ1 with 1 mg mLꢀ1 of initial
concentration.
The reaction process was monitored through absorbance
(Fig. 1B) showed that in the rst 25 min the absorbance at
422 nm decreased linearly while the absorbance at 475 nm
increased correspondingly, which indicated the conversion of
DHBP to BP. When all DHBP converted to BP, further irradia-
tion caused decrease of the absorbance at 475 nm if there was
enough oxygen (Fig. S5†). When the irradiation time was
extended to 80 min, the absorbance at 475 nm reduced to 12%
of that at 25 min. As illustrated by its mass spectrum, the
product of photo-reaction of BP is a mixture (Fig. S6†).
For the synthesis purpose, when the concentration of DHBP
dropped to less than 1% of the initial, the reaction was termi-
nated by turning off the light and the product was precipitated
by water. Almost all DHBP was converted to BP with high purity.
2,3-Dimethylbenzo[b]phenazine (DMBP).
A solution of
DMDHBP (5.0 mg, 19.2 mmol) in the mixed solvent (5 mL) was
stirred in air under light irradiation using LED-391. When the
uorescence intensity at 455 nm fell to less than 1% of the
initial value, an excess amount of water was added to the
reaction solution to obtain precipitation. The resulting precip-
itate was separated, washed with water and dried. DMDHBP was
1
almost completely converted into DMBP. H-NMR (d in CDCl3)
8.85 (s, 2H), 8.11 (m, 2H), 7.94 (s, 2H), 7.52 (m, 2H), 2.56 (s, 6H).
13C-NMR (d in CDCl3) 135.14, 134.32, 129.90, 128.60, 128.29,
127.59, 127.20, 126.83, 20.98. MS (ESI) [M + H]+ m/z ¼ 259.1
(meas.), 259.1 (calc.).
3. Results and discussion
3.1 Photoactivated oxidation
The photochromism of DHBP was rst conrmed with an LED
with the maximum emission at 391 nm (LED-391). The detailed
procedures are given in the ESI.† The concentration of DHBP 3.2 Autocatalysis at longer wavelengths
was calculated based on the calibration curve shown in Fig. S2.†
LEDs of different color (labelled as LED-wavelength, Table S2†)
The results show that DHBP is photochromic in the presence of
oxygen (Fig. 1A). No change was observed without irradiation in
an atmosphere with oxygen, while in an atmosphere of
nitrogen, there is no change too, even with prolonged irradia-
tion (Fig. 1A). The reaction rate is dependent on the light
intensity, the temperature and the initial concentration of the
reactant (Table 1 and Fig. S4†). When the LED is connected in
parallel with a 10 U resistor, the oxidation rate decreases from
208 mg minꢀ1 to 96 mg minꢀ1 (Fig. S4†). Aer the distance from
the liquid surface to the LED increased from 3.5 cm to 7 cm, the
were used to examine the inuence of the wavelength on the
photoactivated oxidation of DHBP. The photographs of the
reaction solutions illuminated with different light sources were
shown in Fig. 2.
The results indicate that the photoreaction is wavelength
dependent (Fig. 3A). For wavelengths up to 452 nm, typical
photo-chemical conversion of DHBP to BP was observed. The
reaction rates are 141 mg minꢀ1 (for LED-365), 208 mg minꢀ1 (for
LED-391), 158 mg minꢀ1 (for LED-421) and 166 mg minꢀ1 (for
Fig. 1 (A) Normalized concentration change of DHBP under different
conditions; (B) normalized absorbance with irradiation time. Condi- Fig. 2 Photographs of the reaction solution under different lighting
tion: LED-391, 10 ꢁC, DHBP (1 mg mLꢀ1), air. Error bar: (ꢃ) SD.
conditions.
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Fig. 3 (A) Oxidation of DHBP under irradiation of LEDs with different
wavelengths. Condition: temperature, 10 ꢁC; distance, 3.5 cm; DHBP
conc. 1 mg mLꢀ1; atmosphere, air; (B) UV-vis absorption spectra of
DHBP and BP (solvent: DMSO/ethanol ¼ 4/1), and the emission spectra
of LEDs; (C) the relative concentration of DHBP versus time in the
presence of BP (light source LED-516); (D) normalized absorption
spectrum of BP and the fluorescence emission spectrum of DHBP.
Error bar: (ꢃ) SD.
Fig. 4 (A) Normalized spectra of DHBP (absorption) and BP (emission)
at lex ¼ 402 nm; (B) absorption spectra of DHBP, BP and DHBP + BP;
(C) emission spectra of DHBP, BP and DHBP + BP (lex ¼ 470 nm).
Conditions: the concentration of DHBP or BP were 10 mg mLꢀ1; (D) X-
band EPR spectra of the TEMPO under different conditions.
contrary, the uorescence emission band of DHBP overlaps with
the absorption band of BP (Fig. 3D). The energy of BP acquired by
absorption of photons of wavelengths of 490 and 516 nm cannot
transfer to DHBP through a FRET process.
In addition, the spectrum of the mixture of DHBP and BP was
exactly the sum of the spectra of DHBP and BP (Fig. 4B and C),
which basically indicates that there is no molecular interaction
between DHBP and BP either in the ground state or excited state
including the charge transfer, electron transfer or the formation
of exciplex.
LED-425) respectively. This was the consequence of the
absorption of photons by DHBP due to the overlap of these
emissions with the absorption spectrum of DHBP (Fig. 3B).
However, with LEDs of 490 and 516 nm, conversion of DHBP
was also observed although there is no absorption of DHBP at
these wavelengths (Fig. 3B). A typical inverse sigmoidal type of
the conversion curves was observed for these two wavelengths,
which indicates the presence of autocatalysis. Maximum reac-
tion rates are 172 mg minꢀ1 (for LED-490) and 111 mg minꢀ1 (for
LED-516) respectively. As illustrated by the absorption bands of
DHBP and BP (Fig. 3B), the irradiation of 516 and 490 nm are
absorbed by BP but not by DHBP, which means BP can act as
a kind of catalyst to convert DHBP to BP.
To verify the role of BP, 10 mol% BP was added to a DHBP
solution and irradiated with LED-516, the curve became same as
that irradiated with shorter wavelengths (Fig. 3C). As a contrast,
the DHBP solution containing BP stayed unchanged without
light irradiation. These results indicate a photo-autocatalysis of
oxidation of DHBP to BP.
The effect of concentration was also examined. The auto-
catalytic behaviour was observed at a very low concentration (10
mg mLꢀ1 DHBP) (Fig. S7†), which is another evidence that no
intermolecular interaction is involved.
To test if there are any oxidized products from BP act as the
oxidants to convert DHBP to BP, we irradiated 5 mL of BP
solution (1 mg mLꢀ1) with LED-391 under ambient atmosphere
until the concentration of BP dropped to 0.1 mg mLꢀ1, then
added 5 mg DHBP into the solution and the mixture was irra-
diated for another 25 min with LED-391 under a nitrogen
atmosphere. The content of DHBP in the solution only
decreased 1.1%, which was same as that of pure DHBP irradi-
ated under the same condition. This result indicates that none
of the photo-degraded products contributed to the conversion
of DHBP to BP.
Phenazines have been successfully used as photocatalysts in
atom transfer radicals polymerization14,23 and oxidative amida-
tion of aromatic aldehydes,20 reactive oxygen species or radicals
were proposed. To check if there were reactive oxygen species in
this system, 2,2,6,6-tetramethylpiperidine-1-oxyl radical
(TEMPO) was used as an indicator.26–28 As shown in Fig. 4D, the
presence of DHBP largely suppressed the EPR signal of TEMPO,
which was probably caused by the reduction of TEMPO radicals.
However, when the mixture was radiated with LED-516 for
3.3 Mechanism of autocatalysis
There are some reports on the photoactivated autocatalysis of
uorescein derivative,21 a-diketone24 and boron-dipyrromethene
(BODIPY) derivative.25 However, study of the effect of wave-
length on the photoautocatalytic reaction is scarce. In some of
the reported photoactivated autocatalysis,22 the autocatalytic
¨
process is derived from the Forster resonance energy transfer
(FRET) between the product and the starting material, where the
overlap between the uorescence emission band of the product
and the absorption band of the starting material is essential. But
in this case, there was no overlap between the absorption band of
DHBP and the uorescence emission band of BP (Fig. 4A). On the
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absorption in this range of wavelength, BP is therefore
responsible for the catalysis. It illustrates that photochemical
reaction may occur under the light irradiation that the reactants
do not absorb. A reaction rate of 208 mg minꢀ1 was achieved
with the inexpensive LED and it facilitates the reaction. Sunlight
can be also used to achieve green synthesis of BP, and the
oxidation rate of 1 mg mLꢀ1 DHBP was 393 mg minꢀ1 in
a 30 mm diameter glass vessel. With this photochemical
process, no metal ion containing oxidant or catalyst is needed,
which is advantageous for metal-free compounds used for
organic semiconductor devices. The reaction process can be
easily monitored with the uorescence, so nearly quantitative
conversion of H2-NHAs to NHAs can be achieved. In addition,
because of its oxygen activation capability, relatively inert
chemical property and uorescence emission of BP, this pho-
toactivated autocatalysis process may nd its usage in residual
oxygen detection and phototherapy with cyan to green light.
Fig. 5 (A) Changes of fluorescent intensities of the solution in different
atmosphere (LED-490); (B) the relative concentration of DHBP versus
time. The inset photos are the reaction solutions before and after
being exposed to the sunlight for 20 minutes; (C) sunlight-activated
oxidation of DMDHBP to DMBP. Error bar: (ꢃ) SD.
30 min, EPR signal resumed, indicating the formation of reac-
tive oxygen species under the light irradiation.
Conflicts of interest
There are no conicts to declare.
3.4 Applications
The photoactivated autocatalytic oxidation of DHBP could be
used as a sensor of the oxidizing substances (Fig. 5A). With
nitrogen, only slight change of the uorescent intensity at either
455 nm or 574 nm was observed, which indicates the slow
conversion of DHBP to BP, possibly by dissolved trace amount
of oxygen. When the system was lled with air, remarkable
change of uorescence in both channels was observed. When
the atmosphere switched back to nitrogen, the change of uo-
rescence reduced to the original level again.
Acknowledgements
The authors would like to acknowledge the nancial support
from the National Natural Science Foundation of China (No.
21575054, 21625501, and 21527808), the Beijing Municipal
High Level Innovative Team Building Program (No.
IDHT20180504), and the Beijing Outstanding Young Scientists
Program (BJJWZYJH01201910005017).
Sunlight can be readily used to oxidize DHBP (Fig. 5B). With
sunlight, the oxidation rate was calculated to be 393 mg minꢀ1
for the initial 5 min, which is 1.9 times faster than using LED-
391.
The method can be extended to other molecules. 2,3-Dime-
thylbenzo[b]phenazine (DMBP) was successfully synthesized
Notes and references
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was stirred in air under sun light. The reaction was monitored
by recording the uorescent intensity of the diluted solution.
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reaction solution to obtain precipitation. The resulting precip-
itate was separated, washed with water and dried. DMDHBP was
almost completely converted into DMBP.
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4. Conclusions
Photochemical oxidation of DHBP was systemically evaluated
with LEDs of the central emission ranging from 365–595 nm as
the light sources. The photoactivated autocatalytic oxidation of
DHBP with radiation of cyan to green light was conrmed.
Typical sigmoidal curves of the conversion were observed by
irradiation of reaction solution with LEDs with central emis-
sions at 490 nm and 516 nm. Because DHBP has virtually no
¨
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