Photocatalytic Systems Based on Acridine Salts Derivatives

Article abstract of DOI:10.1134/S1070363220070099

Abstract: The possibility of 10-methyl-9-phenylacridine iodide usage as a photocatalyst in the process of molecular hydrogen evolution in various aprotic solvents has been shown. A possible process scheme has been suggested. The influence of the acid nature and its concentration on the efficiency of the photocatalytic process has been revealed. It has been shown that the highest amount of molecular hydrogen is formed in presence of acids with intermediate pK.

Full text of DOI:10.1134/S1070363220070099

ISSN 1070-3632, Russian Journal of General Chemistry, 2020, Vol. 90, No. 7, pp. 1229–1234. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Zhurnal Obshchei Khimii, 2020, Vol. 90, No. 7, pp. 1040–1046.
Photocatalytic Systems Based on Acridine Salts Derivatives
a,
a
a
a
A. V. Dolganov *, A. V. Balandina , D. B. Chugunov , A. S. Timonina ,
a
a
a
a
a
Yu. I. Lyukshina , A. A. Ahmatova , A. D. Yudina , V. V. Shindina , V. O. Zhirnova ,
a
a
L. A. Klimaeva , and A. K. Osipov
a
N.P. Ogarev National Research Mordovian State University, Saransk, 430005 Russia
e-mail: dolganov_sasha@mail.ru
*
Received February 18, 2020; revised February 18, 2020; accepted February 27, 2020
Abstract—The possibility of 10-methyl-9-phenylacridine iodide usage as a photocatalyst in the process of mo-
lecular hydrogen evolution in various aprotic solvents has been shown. A possible process scheme has been sug-
gested. The influence of the acid nature and its concentration on the efficiency of the photocatalytic process has
been revealed. It has been shown that the highest amount of molecular hydrogen is formed in presence of acids
with intermediate pK.
Keywords: photocatalysis, photocatalyst, 10-methyl-9-phenylacridinium iodide, hydrogen, photocatalytic
properties
DOI: 10.1134/S1070363220070099
The interest to renewable and “green” energy sources
has rapidly emerged due to sharply increased energy
demand with the population growth, exhausting of the
fossil fuels, and ecological issues related to nuclear
energetics [1]. Molecular hydrogen is an energy source
meeting the demand. Photocatalytic reduction of water
is an environmentally friendly method of molecular
hydrogen generation [2]. Multicomponent systems
consisting of sensibilizer, mediator, catalyst, and electron
donor are used in this process. The major drawbacks
of conventional photocatalytic systems for molecular
hydrogen production are: photodegradation of the
sensibilizer (S), requirement of the match between redox
potentials of the mediator (M) and sensibilizer; two-
stage formation of molecular hydrogen involving the
bimolecular processes; low efficiency of the electron
transfer from the mediator to the catalyst (C) [3]. One
of the approaches to get rid of the said drawbacks is
the combination of the listed components in a single
molecule.
other conditions being the same [5]. A macrocyclic
complex containing bridging BF groups has been found
2
advantageous over the analogous complex with hydrogen-
bound bisdioximate ligand, since cobalt(II) ion in the
former compound is more prone to reduction and more
stable against the side reactions such as acid hydrolysis
and hydrogenation. This photocatalyst containing the
2
+
[Ru(bpy) (L-pyr)] fragment {L-pyr = (4-pyridine)-
2
oxazolo[4,5-f]phenanthroline} as photosensibilizer has
3
exhibited 10 turnovers in the presence of 100 eq. of
triethylamine and its tetrafluoroborate salt in acetone
during 15 h of irradiation [6]. The first three systems have
been practically inactive under irradiation with visible
light. The exchange of dimethylphenanthroline in the
ruthenium fragment with bipyridyl triggers the Ru,Co-
containing catalyst active under visible light, yet the
turnover number has remained low; the catalytic activity
has been significantly increased when the ruthenium-
based photocatalytic fragment has been replaced with
the heteroleptic iridium moiety [7].
Many hybrid photocatalytic systems have been
described [4]. A series of Ru-, Co-, Ir-, and Co-
containing heterobinuclear photocatalysts (Scheme 1)
has been suggested for the photochemical production
of hydrogen from triethylammonium tetrafluoroborate.
The Ru,Со-containing dyads are more efficient in the
hydrogen generation that other multicomponent systems,
Comprehensive study of the photochemical production
of molecular hydrogen in the presence of cobalt
clathrochelates and oxygen has revealed their efficiency
in the electron transfer in the photocatalytic systems. The
[Co(AMMEsar)] complex with anthracenecarboxylate
ion [A–Co(III)] has been used as photosensibilizer and
electron carrier in the reactions of water photodegradation
3+
1
229

1230
DOLGANOV et al.
Scheme 1.
with hydrogen evolution. Photoreduction of Со(III)
into Со(II) occurs under excitation of the anthracene
molecular hydrogen. The complexity of development
of such photocatalytic frameworks and their high price
have limited their industrial applications. Therefore,
the development of simpler and cheaper catalysts in an
important topical issue.
–
1
chromophore (ν <25 000 cm ). The A–Co(III) complex
–
4
practically does not show fluorescence (Φ ≤ 2×10 ),
whereas the A–Co(II) complex exhibits characteristic
violet fluorescence.
1
0-Methyl-9-phenylacridinium iodide (PhАсrI)
Two cage complexes of cobalt with covalently
linked anthracene chromophore have been used in
hydrogen generation. These compounds combine the
photosensibilizer (anthracene substituent) and the
electron carrier (cage fragment) in a single molecule
molecule (Scheme 4) can act as such framework.
Combining of the required components in a single
Scheme 2.
(Scheme 2). The efficiency of the electron transfer from
anthracene in the singlet excited state to the cage complex
of cobalt has been decreased with the increase in the
bridging fragment length.
3
+
The [Co((ClME)abcapten)] cage cation (E1/2
=
4
–1 –1
–
200 mV, k = 1.3×10 mol s ) has been also used
11
in the photocatalytic system for hydrogen generation
(Scheme 3).
The reactions of excited state quenching have
2
+
been studied using the [Ru(bpy)3] complex as well
as anthracenecarboxylate ion as sensibilizer. Both
excited states are capable of cobalt(III) sarcophaginate
reduction. The cage complexes of cobalt(II) can reduce
protons in the presence of Pt(PVA) with high rate of
electron interchange, which makes them promising
electron carriers in the photoinduced generation of
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 90 No. 7 2020

PHOTOCATALYTIC SYSTEMS BASED ON ACRIDINE SALTS DERIVATIVES
1231
Scheme 3.
Scheme 4.
molecule should also improve the process efficiency
due to the leveling off of two bimolecular stages which
often form the “bottleneck” limiting the overall process
efficiency.
is determined by the following conditions: chemical and
thermodynamic stability of the radical in time as well as
the matching redox potentials ensuring efficient electron
transfer from the donor (acridinyl radical). The formed
charge-separated complex with the electron localized at
the heterocyclic fragment is further protonated to afford
the cation-radical which can rapidly eliminate a hydrogen
atom.
The systems based on 10-methyl-9-phenylacridinium
iodide are efficient photocatalysts of various chemical
processes. The mechanism of photoexcitation of acridine
derivatives includes the light absorption by a substituent
in position 9 followed by fast electron transfer on the
heterolytic fragment [8]. The so formed charge-separated
complex is reduced into the radical with an external
electron donor. The following stage is the electron
transfer from the heterolytic fragment to the catalyst
Overall, it can be suggested that the photochemically
generated radical is equivalent to the radical (Scheme 2)
acting as the major intermediate in the electrochemical
generation of molecular hydrogen in the presence of
1
0-methyl-9-phenylacridinium iodide.
(Scheme 5). Hence, the second stage is a key step in the
However, efficient intramolecular electron transfer
generation of the catalytically active form of the catalyst.
The efficiency of the electron transfer to the cationic site
is possible if a set of requirements is met: the rate of
oxidation of the donor by the excited cation formed upon
Scheme 5.
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1232
DOLGANOV et al.
Fig. 2. Efficiency of the molecular hydrogen evolution for the
Fig. 1. Spectra of luminescence (1) and absorption (2) of
PhAcrI solutions in CH CN.
acids differing in nature. (1) CF3SO3H, (2) HBF4, (3) HCl,
(4) CH3COOH, (5) HClO4, and (6) p-TSA.
3
the intramolecular electron transfer should be faster than
the reverse electron transfer; the rate of protonation of
the excited radical should be close to the diffusion rate.
Meeting these requirements should favor the catalytic
generation of molecular hydrogen.
for efficient intramolecular energy transfer is the overlap
of the absorption and luminescence spectra of the donor
and acceptor parts of the molecule, respectively. That
feature of the absorption and luminescence spectra of
10-methyl-9-phenylacridinium iodide is demonstrated
in Fig. 1. The observed spectra overlap made the FRET
process theoretically possible, allowing the photochemical
production of molecular hydrogen in the presence of that
compound.
It is known that the rate of direct intramolecular
electron transfer from the substituent to the cationic
site during photoexcitation of acridine derivatives is of
1
1 –1
3
.6×10 s , whereas that of the reverse electron transfer
7
8
–1
is of 10 –10 s [8]. Furthermore, rate of the radical
protonation in the catalytic process shown in Scheme 6 is
Halogen lamp was used as the excitation source in
the study of the features of photocatalytic production
of molecular hydrogen in the presence of PhAcrI. A
test tube was used as the reactor, the reaction mixture
volume being 10 mL. The first series of the experiments
was performed to investigate the efficiency of the
molecular hydrogen production as function of the acid
nature and concentration. The following acids were
probed: HBF , HCl, CH COOH, HClO , СF SO H, and
8
–1
controlled by diffusion, the rate being of 10 s , whereas
the rate of reduction of the excited cation is of 2.4×
11
–1
1
0 s . Hence, kinetics of the intramolecular electron
transfer and the proton reduction processes satisfies the
requirements for the production of molecular hydrogen.
As stated above, the Förster's theory of resonance
energy transfer (FRET) claims that an important demand
4
3
4
3
3
4
-СН С H SO H, their concentration ranging between
3
6
3
4
3
–
–1
Scheme 6.
2.0×10 and 5.0×10 mol/L.
The obtained data revealed that the highest efficiency
of the molecular hydrogen generation was achieved in
the presence of medium-strength acids (Figs. 2 and 3).
Analysis of the key stages of the reaction mechanism,
the process rate should be limited by the protonation of
the intermediate radical. Since that fact was not supported
by the experiment, we suggested that the catalytic cycle
involved an additional rate-limiting stage.
Analysis of the electronic spectra of PhAcrI in the
presence of the probed acids (Figs. 4 and 5) revealed an
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PHOTOCATALYTIC SYSTEMS BASED ON ACRIDINE SALTS DERIVATIVES
1233
Fig. 3. Efficiency of the molecular hydrogen evolution as a
function of the acid concentration. (1) HCl, (2) CH COOH,
3
Fig. 4. Absorption spectra of the PhAcrI solutions in the
(
3) CF SO H, (4) HClO , (5) p-TSA, and (6) HBF .
3 3 4 4
presence of HBF at different concentration.
4
earlier unknown fact: irrespectively of the acid nature, its
addition led to the appearance of an additional absorption
band at λ = 290 nm. The increase in the absorbance at
that wavelength (А) was correlated with the acid pK value
and its concentration. The latter observation suggested
the formation of the 1 : 1 adduct.
of the evolved hydrogen was the highest in the cases of
CH COOH and HCl, and the corresponding concentration
3
of the dication form was the lowest.
In summary, it was demonstrated that the systems
based on 10-methyl-9-phenylacridinium iodide were
efficient photocatalysts. The efficiency of molecular
hydrogen generation was studied; it was revealed that
the process was the most efficient in the presence of
intermediate-strength acids.
The highest absorbance was achieved in the cases of
the acids with the lowest pK. The increase in pK weakened
the absorption band, the lowest intensity corresponding
to CH COOH. That band was assigned to the dication
formation (Scheme 7).
3
EXPERIMENTAL
Spectroscopic studies were performed using the
solutions of 10-methyl-9-phenylacridinium iodide
(
That stage was likely the rate-limiting one, which was
in good agreement with the above-shown data: the volume
PhAcrI) with concentration of 1.0×10–4 mol/L in
different solvents: acetonitrile, dichloromethane,
chloroform, and dimethylformamide. The following acids
–
3
(
5.0×10 mol/L) were used: HClO , HCl, CH COOH,
4
3
4
-СН С H SO H, СF SO H, and HBF .
3
6
4
3
3
3
4
The electronic absorbance spectra were recorded
using a Shimadzu UV-1800 spectrophotometer (Japan)
Scheme 7.
Fig. 5. Absorption spectra of the PhAcrI solutions in the
presence of HClO at different concentration.
4
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1234
DOLGANOV et al.
at 300–650 nm in the single-beam mode. The spectra
of fluorescence excitation and emission were recorded
using a Shimadzu RF-5301 PC spectrofluorimeter. The
solutions were deaerated via 3 min bubbling with argon
prior to the measurements. The fluorescence spectra were
recorded over 300–650 nm. The spectra were recorded
at room temperature.
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CONFLICT OF INTEREST
No conflict of interest was declared by the authors.
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