Radical Ions of 3-Styryl-quinoxalin-2-one Derivatives Studied by Pulse Radiolysis in Organic Solvents

Article abstract of DOI:10.1021/acs.jpcb.8b01004

The absorption-spectral and kinetic behaviors of radical ions and neutral hydrogenated radicals of seven 3-styryl-quinoxalin-2(1H)-one (3-SQ) derivatives, one without substituents in the styryl moiety, four others with electron-donating (R = -CH₃

Full text of DOI:10.1021/acs.jpcb.8b01004

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B: Liquids; Chemical and Dynamical Processes in Solution
Radical Ions of 3-Styryl-Quinoxalin-2-One Derivatives
Studied by Pulse Radiolysis in Organic Solvents
Konrad Skotnicki, Julio Ramón De la Fuente, Alvaro Cañete, Eduardo Berrios, and Krzysztof Bobrowski
J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b01004 • Publication Date (Web): 13 Mar 2018
Downloaded from http://pubs.acs.org on March 23, 2018
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Radical Ions of 3ꢀStyrylꢀquinoxalinꢀ2ꢀone
Derivatives Studied by Pulse Radiolysis in
Organic Solvents
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Konrad Skotnicki, Julio R. De la Fuente, Álvaro Cañete, Eduardo Berrios , Krzysztof
1
,4*
Bobrowski,
1
Institute of Nuclear Chemistry and Technology, 03ꢀ195 Warsaw, Poland
2
Departamento de Química Orgánica y Fisicoquímica, Facultad de Ciencias Químicas y
Farmacéuticas, Universidad de Chile, Casilla 223, Santiago 1, 6640750 Chile
3
Departamento de Química Orgánica, Facultad de Química, Pontificia Universidad
Católica de Chile, Casilla 306, Correo 22, Santiago, Chile
4
Notre Dame Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46556,
USA
*
2
Corresponding Authors: Julio De la Fuente: Phone: (56ꢀ2) 2978ꢀ2880, Fax: (56ꢀ2)ꢀ2978ꢀ
868, Eꢀmail: jrfuente@ciq.uchile.cl; Krzysztof Bobrowski: Phone (48ꢀ22)ꢀ504ꢀ1336, Eꢀ
mail: k.bobrowski@ichtj.waw.pl
Received date:
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Abstract
The absorptionꢀspectral and kinetic behavior of radical ions and neutral hydrogenated
radicals of seven 3ꢀstyrylꢀquinoxalinꢀ2(1H)ꢀone (3ꢀSQ) derivatives, one without
substituents in styryl moiety, four others with electron donating (R = ꢀCH , ꢀOCH , –
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N(CH ) ) or electron withdrawing (R = ꢀOCF ) substituents in paraꢀposition in benzene
3
2
3
ring, and remaining two with double methoxy substituents (ꢀOCH ), however, at different
3
position (meta/para and ortho/meta) have been studied by UVꢀvis spectrophotometric
pulse radiolysis in a neat acetonitrile saturated with argon (Ar) and oxygen (O ) and in 2ꢀ
2
propanol saturated with Ar, at room temperature. In acetonitrile solutions, the radical
•−
anions (4RꢀSQ ) are characterized by two absorption maxima located at λ
= 470–490
max
nm and λmax = 510–540 nm, with the respective molar absorption coefficients ε470ꢀ490
=
−1
−1
−1
−1
8
,500–13,100 M cm and ε
= 6,100–10,300 M cm , depending on the substituent
10ꢀ540
5
•
−
(R). All 4RꢀSQ decay in acetonitrile via firstꢀorder kinetics with the rate constants in the
6
−1
range (1.2 – 1.5) × 10 s . In 2ꢀpropanol solutions, they decay predominantly through
•
protonation by solvent forming neutral hydrogenated radicals (4RꢀSQH ) which are
characterized by weak absorption bands with λmax = 480–490 nm. Oxygen insensitive, the
•+
radical cations (4RꢀSQ ) are characterized by a strong absorption with λ
= 450 – 630
max
nm, depending on the substituent (R). They are formed in a charge transfer reaction
•
+
between a radical cation derived from acetonitrile (ACN ) and substituted 3ꢀstyrylꢀ
quinoxalinꢀ2ꢀone derivatives (4RꢀSQ) with a pseudoꢀfirstꢀorder rate constants k = (2.7 –
5
−1
4.7) × 10 s measured in solutions containing 0.1 mM 4Rꢀ3ꢀSQ. A Hammett equation
plot gave the very small negative slope (ρ = ꢀ0.08) indicating a very weak influence of the
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substituents in benzene ring on the rate of charge transfer reaction. The decay of 4RꢀSQ•+
in Arꢀsaturated acetonitrile solutions occurs with the pseudoꢀfirstꢀorder rate constants k =
4
−1
(
1.6 – 6.2) × 10 s and in principle is not affected by the presence of O suggesting
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chargeꢀspin delocalization over the whole 3ꢀSQ molecule. Most of the transient spectra
generated radiolytically are reasonably well reproduced by semiꢀempirical PM3ꢀZINDO/S
•
−
(
for 4RꢀSQ ) and DFT quantum mechanics calculations employing M06ꢀ2x hybrid
•
+
functional together with def2ꢀTZVP basis set (for 4RꢀSQ ).
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Introduction
Quinoxaline derivatives have received a wide attention in the past and in the
st
1
current decade of the 21 century due to their numerous and broad applications. They
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were found to be important in industry due to their ability to inhibit metal corrosion. The
quinoxaline structure acts as a precursor to assembly a large number of new compounds for
luminescent materials due to the πꢀdeficient character of the quinoxaline ring leading to
significant modifications of the photophysical properties of πꢀconjugated materials. The
electronꢀdeficiency of the quinoxaline ring can be used as a dipolar moiety, which favors
internal charge transfer. These molecules exhibit important solvatochromism and good
nonlinear optical properties. Moreover, the quinoxaline rings, due to the presence of
nitrogen atoms with lone electron pairs, can act as effective and stable complexing agents
3
making them good cation sensors or making them halochromism following protonation.
Some quinoxaline derivatives were also used as electronꢀtransporting or holeꢀblocking
4
ꢀ7
8
materials in organic lightꢀemitting devices, and in organic thin films transistors.
However, a recent update of literature has shown that quinoxaline derivatives, those
of quinoxalineꢀ2ꢀone in particular, attracted attention primarily because of their
1,9,10
tremendeously wide spectrum of biological activity.
They were found as antiꢀHIV
1
1,12
13,14
15
16
agents,
aldose reductase,
cyclinꢀdependent kinases, monoamine oxidase A, SR
1
7
18
proteinꢀspecific kinaseꢀ1 (SRPKꢀ1), glycogen phosphorylase inhibitors, angiotensin II
19,20
21
receptor and multipleꢀdrugꢀresistance antagonists,
5ꢀHT3 receptor agonists, and as
1
1,22ꢀ25
GABA/benzodiazepine receptor ligands.
Nearly all of the derivatives that have
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biological activity contain a substituent in the position 3 of the pyrazine ring of quinoxalinꢀ
2
ꢀone.
The biological activity of this class of compounds is closely related to their
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pharmaceutical applications. They are effective as antibacterial, antiparasite, antifungal,
analgesic, antimalarial, antitumor, antiamoebic, antiepileptic, anticonvulsant,
1
,9,10
antitubercular, antiproliferative and antiꢀinflammatory agents.
Recently, great attention has been paid to 3ꢀstyrylꢀquinoxaline derivatives
containing additional substituents in various position of the pyrazine ring. They were found
to be a class of compounds that can be potentially very effective in therapeutic medicine.
Selected examples include: two 3ꢀstyrylꢀquinoxaline derivatives containing pꢀ
methoxystyryl and pꢀfluorostyryl side chains with tetrazolo substituents at N1 and C2
26
which exhibit promising activity as potent anticonvulsants, and 3ꢀstyrylquinoxaline and
3ꢀ(2ꢀhydroxystyryl)ꢀquinoxaline with a thioether substituent at C2, that showed some
improved antitumor activity which qualified them for further study in antitumor drug
2
7
design.
Very recently attention has been paid to amphiphilic 3ꢀstyrylꢀquinoxalineꢀ2ꢀone
alkoxy derivatives, with different alkyl chain lengths at the 4 position of the alkoxy moiety,
which are widely used dye molecules owing to their acidichroism and photochemical
28ꢀ30
sensitivities.
They were found to undergo predominantly reversible transꢀcis
3
0
isomerization in solution, while in LangmuirꢀBlodgett films [2,2] photodimerization was
28
found as the preferred reaction pathway. Interestingly, it was observed that the
photodimerization kinetics was correlated with surface morphologies, and its rate constant
2
9
decreased significantly with the acidity.
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The 3ꢀstyrylꢀquinoxalinꢀ2ꢀone containing a pꢀdimethylaminestyryl side chain has
been recently tested as a new fluorescent probe for amyloidꢀβꢀfibrils which can be
considered as a promising staining tool for the detection and study of peptide/protein
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aggregation. This approach can be very useful for the diagnosis of neurodegenerative
diseases including prion, Parkinson’s, Huntington’s and Alzheimer’s disease in which
peptide/protein aggregation has been postulated.
Interestingly, four 3ꢀstyrylꢀquinoxalinꢀ2ꢀone derivatives (found to be very effective
in therapeutic medicine as aldose reductase inhibitors) expressed additionally very high
1
4
radical scavenging activity. These include: two 3ꢀstyrylꢀquinoxalinꢀ2ꢀone derivatives
containing pꢀhydroxystyryl side chains, a 3ꢀstyrylꢀquinoxalinꢀ2ꢀone derivative with a pꢀ
methoxystyryl side chain and a 3ꢀstyrylꢀquinoxalinꢀ2ꢀone derivative with a 3ꢀmethoxy,4ꢀ
hydroxystyryl side chain. This was shown especially for the latter derivative by the very
effective DPPH radical scavenging (comparable with Trolox) and by suppressing lipid
peroxidation. In particular, the pꢀhydroxystyryl side chain attached to the C3 atom was
identified as the key motif in quinoxalinꢀ2ꢀone compounds for their antioxidant activity.
Thus, these compounds can potentially increase intracellular antioxidant defense during
oxidative stress when there is an accumulation of excess Reactive Oxygen Species
resulting in the oxidative degradation of vital biomolecules. Moreover, a StructureꢀActivity
Relationship (SAR) analysis combined with docking studies showed that one of these 3ꢀ
styrylꢀquinoxalinꢀ2ꢀone derivatives containing a pꢀhydroxystyryl side chain was especially
14
tightly bound in the active site of aldose reductase. Interestingly, this side chain formed a
stable stacking interaction with the indole ring of the Trp111 and was well placed in the
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pocket formed by the side chains of Trp111, Leu300, Phe122, Cys303, Thr113, Phe115,
3
2
and Trp79. In addition, the quinoxalinꢀ2ꢀone moiety matched very well the hydrophobic
pocket that contains, on its surface, the side chains of Leu300, Trp219, Phe122, Trp20, and
Trp79. The fact that this compound is bound in such a specific position in the protein may
have serious consequences in their interaction with either amino acid residues or radicals
derived from them. Two amino acid residues tryptophan (Trp) and cysteine (Cys), are
particularly vulnerable to oxidation. Therefore, either the radical cations derived from 3ꢀ
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•
•
styrylquinoxalinꢀ2ꢀones can oxidize them to tryptophyl (TrpN ) and thiyl (CysS ) radicals,
respectively, or the radicals derived from the surrounding amino acid residues can act as
oxidants or reductants of quinoxalinꢀ2ꢀones intercalated into the protein matrix.
The examples given above, showing the broad application of 3ꢀstyrylꢀquinoxalinꢀ2ꢀ
one derivatives as electron transporting or holeꢀblocking materials as well their high
antioxidant activity and specific docking in proteins, strongly indicate the need to
characterize spectral and kinetic properties of radicals and radical ions derived from them.
In addition, knowledge regarding the spectral and kinetic behavior of radical anions and
hydrogenated radicals of 3ꢀstyrylꢀquinoxalinꢀ2ꢀone derivatives will become necessary in
interpreting some of the transient phenomena observed by us during photoreduction of
these compounds by selected amino acids.
In the last few years identification of radical and radicalꢀion species derived from
some selected quinoxalineꢀ2ꢀones was performed using pulse radiolysis in aqueous
33,34
solutions.
In turn, pulse radiolysis generation of radical cations of stilbene derivatives
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(
which can be also treated as styryl substituted benzenes), was successfully accomplished
3
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in 1,2ꢀdichloroethane and nꢀbutyl chloride.
In principle, pulse radiolysis offers a convenient method for generating radical ions
and excited states or preferentially one of them, depending of the solvent used and the
saturating gas. For instance, acetonitrile was used for the pulse radiolytic generation of
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radical cations, as well as radical anions derived from the appropriately chosen solutes.
Pulse irradiation of neat acetonitrile leads to the formation of acetonitrile positive ions, and
classical solvated electrons (dipoleꢀbound excess electrons) in dynamic equilibrium with
•−
44ꢀ47
dimeric radical anions (CH CN) (valenceꢀbound excess electrons).
Therefore, in this
3
2
solvent saturated with Ar both radical ions of the solute can be formed while in O2ꢀ
saturated acetonitrile only radical cations are formed. On the other hand, 2ꢀpropanol was
used for selective pulse radiolytic generation of radical anions derived from the
37,48,49
solutes.
In this protic, polar solvent radical anions are formed through the attachment
of the solvated electrons (e−solv) to the solute molecule. Solute radical cations, however, are
not formed because the alcohol derived positive radical cations disappear very fast
•
producing strongly reductive ketyl radicals ((CH ) C ꢀOH) and solvated protons
3
2
+
((CH ) CHOH . In turn, the radical anions derived from a solute can undergo protonation
3 2 2
5
0
involving either the alcohol molecule and/or solvated protons.
In the current study we generated and identified radical ions and hydrogenated
radical derived from seven 3ꢀstyrylquinoxalinꢀ2ꢀones (3ꢀSQ) (Chart 1) in acetonitrile and
2
ꢀpropanol solutions, respectively.
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Chart 1. Structures of 3ꢀstyrylꢀquinoxalinꢀ2(1H)ꢀone (3ꢀSQ) derivatives under study
The insertion of a styryl moiety at the carbon atom (C3) should stabilize the C3
hydrogenated radical by the captodative effect of the N4 atom and C=O group in the
pyrazine ring and by the resonance effect with the styryl moiety. This resonance could also
facilitate the delocalization of the radical site over the sequence of double bonds and thus
open the possibility for the formation of multiple stable products. Furthermore, the
substitution in the paraꢀposition either with electron donating (ꢀCH , ꢀOCH , ꢀN(CH ) ) or
3
3
3 2
with electron withdrawing (ꢀOCF ) substituents in the styryl moiety (Chart 1, structures 1b
3
–
1e) would modify the electronic distribution in the hydrogenated radical. Using seven 3ꢀ
styrylquinoxalinꢀ2ꢀ(1H)ꢀone derivatives, one without substituents in the styryl moiety
Chart 1, structure 1a), four others with electron donating or electron withdrawing
(
substituents (Chart 1, structures 1b – 1e), and two with different double methoxy
substituents (Chart 1, structures 1f – 1g) allowed for a detailed examination of substitution
on the absorption maxima of the ensuing radical ions as well their stability.
The experimental spectra of the radical anions and hydrogenated radicals will be
compared with the respective spectra calculated using ZINDO/S semiꢀempirical quantum
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2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
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5
5
6
mechanical methods applied earlier for oxoisoaporphine and quinoxalineꢀ2ꢀone
3
3,43,51
derivatives.
In turn, the experimental spectra of the radical cations will be compared
with the respective spectra calculated using Density Functional Theory (DFT).
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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5
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9
0
1
2
3
4
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8
9
0
1
2
3
4
5
6
7
8
9
0
Experimental section
Materials. Acetonitrile was purchased from J. T. Baker with 99.95 purity (Baker HPLC
analyzed) and 2ꢀpropanol was purchased from SigmaꢀAldrich with 99.8% purity (for
HPLC) and they were used as received.
Synthesis of 3ꢀstyrylꢀquinoxalinꢀ2ꢀones (1aꢀg). Substituted 3ꢀstyrylꢀ2(1H)ꢀquinoxalinꢀ
2
ꢀones (1a–g) were prepared by the termic reaction of the corresponding benzaldehyde (3
52
mM) and 3ꢀmethylꢀ2(1H)ꢀquinoxalinꢀ2ꢀone (1 mM) without solvent. Both reagents were
heated at 160°C for 3 h under nitrogen atmosphere in a pressure reactor vessel. After 3 h
the heating was stopped and the reactants were allowed to cool. The content of the flask
was washed with a minimal amount of cold absolute ethanol/ diethyl eter (1:1). The crude
solid component was purified by column chromatography eluted with dichloromethane /
acetonitrile (9:1) and finally crystallized from acetonitrile, to give the pure 3ꢀstyrylꢀ
quinoxalinꢀ2ꢀone derivatives.
Highꢀresolution mass spectrometry (HRMSꢀESI). Highꢀresolution mass spectra
(HRMSꢀESI) were obtained from Thermo Fisher Scientific Exactive Plus mass
spectrometer. The analysis for the reaction products was performed with the following
relevant parameters: heater temperature, 50 °C; sheath gas flow, 5; sweep gas flow rate, 0
and spray voltage, 3.0 kV at positive mode. The accurate mass measurements were
performed at a resolution of 140.000.
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1
13
Nuclear magnetic resonance (NMR). H and C NMR spectra were obtained at 25 °C
on Bruker Avance 400 MHz spectrometer using TMS as an internal standard. The NMR
spectra were processed with MestreNova software v9.0. The spectral characterization of
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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3
4
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0
1
2
3
4
5
6
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9
0
1
2
3
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9
0
1
2
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0
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9
0
1
2
3
4
5
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8
9
0
1
1
a–g by H NMR and HRMSꢀESI (+ mode) are in agreement with the expected structures.
1
13
DMSOꢀd H and C NMR for the substrates are provided in Supporting Infomation.
6
Preparation of Solutions. All solutions for pulse radiolysis experiments were prepared
freshly before experiments with organic solvent and contained 0.1 mM of 3ꢀstyrylꢀ
quinoxalinꢀ2ꢀone derivatives. Solutions were subsequently purged for at least 30 min per
2
00 ml of sample with the desired gas (Ar or O ) before pulse irradiation.
2
Pulse radiolysis. Pulse radiolysis experiments were performed with the INCT LAE 10
linear accelerator with typical pulse duration of 7 − 10 ns and with the Notre Dame Titan 8
MeV Beta model TBS 8/16ꢀ1S with typical pulse duration of 2 − 10 ns. Both data
acquisition systems allow for kinetic traces to be displayed on multiple time scales. A
detailed description of the experimental setups for optical measurements has been given
5
3ꢀ55
elsewhere along with the basic details of the equipment and the data collection system.
The irradiation cells were supplied with a fresh solution by continuous and controlled flow
o
of sample solutions at room temperature (~ 23 C). Experimental error limits are ± 10%
unless specifically noted.
.
All experiments for substituted 3ꢀstyrylꢀ2(1H)ꢀquinoxalinꢀ2ꢀones (1a–g) were
performed on the pulse radiolysis setup coupled with the INCT LAE 10, whereas for 3ꢀ
methylꢀ2(1H)ꢀquinoxalinꢀ2ꢀone on the pulse radiolysis setup coupled with the Notre Dame
Titan Beta model TBS 8/16ꢀ1S. The dose per pulse, which was determined by thiocyanate
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1
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2
2
2
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2
2
2
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3
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6
dosimetry, was on the order of 18 − 20 Gy with the INCT LAE 10 and 4 – 8 Gy for the
−
1
Notre Dame Titan 8MeV (1 Gy = 1 J kg ). Radiolytic yields are given in SI units as ꢀmol
J−1, i.e. the number of product species in micromoles that are generated for every joule of
energy absorbed by the solution.
0
1
2
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9
0
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9
0
Quantum mechanical calculations. Semiꢀempirical quantum mechanical calculations
•−
•
for radical anions (3ꢀSQ ) and their protonated forms (3ꢀSQH ) were performed using
HyperChemꢀ8.03 by HyperCube, Inc.. The structure geometries were optimized and the
respective formation enthalpies were calculated using the PM3 semiꢀempirical method
with the UHF approximation and the proper charge and multiplicity: ꢀ1 and 2 for the
•
−
•
radical anions (3ꢀSQ ) and 0 and 2 for the protonated radical anions (3ꢀSQH ). The
structures of these latter species were constructed by adding the H atom in the proper
position of the ground state of 3ꢀSQ molecule whose geometry were first optimized by the
+
Molecular Mechanic method MM followed by the PM3/UHF calculation. The respective
spectra were calculated by using ZINDO/S method applying the RHF approximation even
for radical species. The first five unoccupied and five occupied MO with weighting factor
values of 1.267 and 0.585 for σꢀσ and πꢀπ overlap were used for these calculations,
respectively. By including more MO (10 + 10) a greater number of absorption lines in the
highꢀenergy region were obtained, however, beyond the range of measurements performed.
What is important, this inclusion did not affect the spectral region of interest in this study.
All calculations were performed without taking into account any solvent effect that likely
can shift the calculated absorption by a few nanometers.
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•+
All calculations for radical cations (3ꢀSQ ) were carried out employing M06ꢀ2x
5
6
57
hybrid functional together with def2ꢀTZVP basis set for all atoms. Solvent effects were
5
8
implicitly taken into account using COSMO with ε =37.5 for acetonitrile. Geometry
optimizations were performed using an m5 grid to integrate the exchange and correlation
contribution to the electronic energy. Vibrational normal modes were calculated in order to
characterize nuclear configuration as a minimum in the PES. Finally, time dependent DFT
vertical excitation energies and oscillator strengths were obtained at the optimized ground
state geometry at the M06ꢀ2x/def2ꢀTZVP level of theory. All calculations were performed
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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0
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7
8
9
0
1
2
3
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5
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8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5
9
using TURBMOLE software unless stated otherwise. For selected radical cations
60
calculations were also carried out in vacuum employing CAMꢀB3LYP, and the same
basis set to compare with M06ꢀ2x vertical excitations. These calculations were performed
61
in NWChem.
Results
3
ꢀstyrylquinoxalinꢀ2(1H)ꢀone (SQ (1a)).
Argon saturated acetonitrile solutions. Pulse radiolysis of Arꢀsaturated acetonitrile
solution containing 3ꢀstyrylꢀquinoxalinꢀ2(1H)ꢀone (1a, Chart 1) yielded a complex series
of spectral changes (Figures 1A and 1B). A broad absorption was observed at 240 ns after
the pulse in the 420 – 550 nm range with two absorption maxima located at λmax = 475 nm
−3
3 −1
−1
−3
and 530 nm with the respective Gε equal to 1.4 × 10 dm J cm and 0.8 × 10
λ
3
−1
−1
dm J cm . (Figure 1A). Both absorptions buildꢀup within < 240 ns time domain,
however, the growth at λmax = 475 nm is more pronounced, and the transient absorption
spectrum is dominated by the 475ꢀnm band (Figure 1A). Moreover, the kinetic time
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profiles recorded at the 475ꢀnm and 530ꢀnm are different (Figure 1A, insert). The
6
−1
absorption at 530 nm decayed further by a rapid firstꢀorder process with k = 1.3 × 10 s
(insert in Figure 1A; Table 1). With the time elapsed when the 530ꢀnm absorption band
0
1
2
3
4
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9
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8
9
0
disappeared completely, the absorption spectrum underwent further changes, and 8 ꢀs after
the pulse is characterized by a broad and pronounced absorption band with λmax = 450 nm
−3
3 −1
−1
with Gε450 equal to 1.8 × 10 dm J cm (Figure 1A). The growth of the 450ꢀnm
5
−1
absorption band followed a firstꢀorder kinetics with k = 3.9 × 10 s (insert in Figure 1A).
Figure 1. Absorption spectra recorded in Arꢀsaturated (A and B) acetonitrile solutions containing 0.1 mM 3ꢀ
styrylquinoxalinꢀ2(1H)ꢀone. Spectra taken after the following time delays: (A) 240 ns (■), 800 ns (○), 2 ꢀs
(ꢀ), 4 ꢀs (ꢁ), 8 ꢀs (ꢀ) and insert: short time profiles representing growth and/or decays at λ = 450 (□), 475
(○) and 530 nm (•); (B) 10ꢀs (■), 24 ꢀs (○), 40 ꢀs (ꢀ), 100 ꢀs (ꢁ), 500 ꢀs (ꢀ) and insert: longꢀtime profiles
representing decays at λ = 450 (■), 475 (□), and 530 nm (ꢀ). Absorption spectra recorded in O ꢀsaturated (C
2
and D) acetonitrile solutions containing 0.1 mM 3ꢀstyrylquinoxalinꢀ2(1H)ꢀone. Spectra taken after the
following time delays: (C) 240 ns (■), 800 ns (○), 2 ꢀs (ꢀ), 4 ꢀs (ꢁ), 8 ꢀs (ꢀ) and insert: short time profiles
representing growth at λ = 450 (□) and 530 nm (□); (D) 10ꢀs (■), 24 ꢀs (○), 40 ꢀs (ꢀ), 100 ꢀs (ꢁ), 500 ꢀs
(ꢀ) and insert: longꢀtime profiles representing decays at λ = 450 (■) and 530 nm (□).
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However, in order to get the proper value for the k of the growth measured at λmax
=
450 nm, a correction is needed since in the first five microseconds after the pulse the
observed growth is perturbed by a concomitant simultaneous decay of the shortꢀlived
transient. Since the longꢀlived species does not absorb at λ = 550 nm, the observed decay
1
1
1
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1
1
1
1
1
1
2
2
2
2
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2
2
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0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
at that wavelength was recorded and multiplied by a factor equal to the ratio of Gε /Gε
4
50
550
measured at 240 ns. The normalized decay was subtracted from the recorded growth at λ =
50 nm (Figure 2). After correction the growth of the 450ꢀnm absorption band followed a
4
5
−1
firstꢀorder kinetics with k = 3.9 × 10 s .
Table 1. Spectral parameters of radical ions and hydrogenated radicals derived from 3ꢀ
styrylꢀquinoxalinꢀ2(1H)ꢀone derivatives (3ꢀSQ) in acetonitrile and 2ꢀpropanol solutions
λmax
•−
λmax
λmax
3ꢀSQ
2ꢀPropanol
460, 520
λmax •
•+
•−
Compound
ꢀSQ
3
(
ꢀSQ
3ꢀSQ
(MeCN)
450
3ꢀSQH
3
MeCN)
2ꢀPropanol
480, 440
*
SQ
475, 530
470, 520
470, 510
490, 530
475, 530
470, 510
490, 540
*
4
ꢀCH SQ
450ꢀ475
485ꢀ490
630
465, 520
470, 540
630
480, 450
3
*
4
ꢀOCH SQ
480, 450
3
4ꢀN(CH ) SQ
540
3
2
4
ꢀOCF SQ
450
470, 520
470, 520
475, 540
490, 435
3
*
3
,4ꢀ(OCH ) SQ
510
480, 460
3
2
*
2
,5ꢀ(OCH ) SQ
490
490, 470
3
2
*shoulder
This absorption band was still observed at longer times, however, at 500 ꢀs after the
pulse was close to the completion (Figure 1B). The decay of the 450ꢀnm band followed a
rather more complex kinetics which can be described as two firstꢀorder processes with k =
4
−1
3
−1
2.2 × 10 s and 2.8 × 10 s (insert in Figure 1B; Table 2). All these observations taken
together point out for the existence of at least two different transients.
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2
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0
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3
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8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 2. Kinetic traces recorded in Arꢀsaturated acetonitrile solutions containing 0.1 mM 3ꢀstyrylquinoxalinꢀ
2
=
(1H)ꢀone (SQ) at λ = 550 nm (○) and 450 nm (■). The normalized decay (ꢁ) and the corrected growth at λ
450 nm (ꢀ) using the procedure described in the text above.
Table 2. Kinetic parameters for the growth and decay of radical anions and cations derived
from 3ꢀstyrylꢀquinoxalinꢀ2(1H)ꢀone derivatives in acetonitrile solutions saturated either
with Ar or O2
Radical
cation
(3ꢀSQ )
Radical
cation
(3ꢀSQ )
Radical
anion
(3ꢀSQ )
Radical
cation
(3ꢀSQ )
Radical
cation
(3ꢀSQ )
•+
•+
•−
•+
•+
Compound
(3ꢀSQ)
growth
growth
decay
decay
decay
(
O )
(Ar)
(× 10 s )
(Ar)
(×10 s )
(O )
(s )
2.3 × 10
(Ar)
(s )
2.2 × 10
2
2
−1
5
−1
5
−1
6
−1
−1
(×10 s )
4
3
4
SQ
4.0
3.9
1.3
3
1
.9 × 104
2.8 × 104
4
ꢀCH SQ
3.8
4.1
4.7
3.8
2.7
3.9
3.8
3.8*
4.6
1.5
ꢀ
2.0 × 10
1.9 × 10
3
3
3
2.2 × 104
3.3 × 104
4
ꢀOCH SQ
2.4 × 10
2.0 × 10
3
3
3
2
.8 × 104
3.4 × 104
4
ꢀN(CH ) SQ
1.2
1.2
ꢀ
3.5 × 10
6.2 × 10
3
2
3
4
3.6 × 104
1.4 × 104
4
ꢀOCF SQ
3.9
2.8 × 10
2.0 × 10
3
3
3
6
.6 × 104
2.5 × 104
3
2
,4ꢀ(OCH ) SQ
3.6*
5.0*
2.0 × 10
1.6 × 10
3
2
3
3
2.9 × 104
2.9 × 104
,5ꢀ(OCH ) SQ
ꢀ
1.9 × 10
2.4 × 10
3
2
3
3
2
.7 × 10
3.6 × 10
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Oxygen saturated acetonitrile solutions. In order to sort out the possible
contributions of individual radical anion/triplet and radical cation derived from SQ, an
acetonitrile solution containing 3ꢀstyrylꢀquinoxalinꢀ2(1H)ꢀone (1a, Chart 1) was saturated
with oxygen as the scavenger for solvated electrons (e−solv) and/or dimeric radical anions
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
•−
(
CH CN) . In the presence of oxygen, the absorption spectra recorded at short times after
3 2
the pulse reveal different features (Figure 1C) with comparison to the spectra recorded at
the same time under anaerobic conditions (Figure 1A).
Formation of the absorption band with the maxima at λmax = 475 nm and 530 nm was
completely suppressed. Therefore, it is reasonable to assume tentatively that the 3ꢀstyrylꢀ
•
−
quinoxalinꢀ2(1H)ꢀone derived radical anions (SQ ) (Figure 1A, Table 1) are only
responsible for the absorption bands suppressed. The contribution of 3ꢀstyrylꢀquinoxalinꢀ2ꢀ
3
(
1H)ꢀone derived triplets ( SQ*) can be rather discarded based on flash photolysis
3
experiments in which the band assigned to SQ* is characterized by the absorption band
with λmax = 435ꢀnm (manuscript is under preparation).
The formation of the 450ꢀnm band is also fully developed within the range of 8 ꢀs
5
−1
(Figure 1C), and the formation kinetics recorded at λ = 450 nm with k = 4.0 × 10 s
(insert in Figure 1C) matches nearly the formation kinetics of the 450ꢀnm band in Arꢀ
saturated acetonitrile (after correction) (insert in Figure 1A). The absorption spectrum
recorded at 8 ꢀs after the pulse exhibits a strong and broad absorption band with a
−3
3 −1
−1
maximum located at λmax = 450 nm with Gε450 equal to 1.7 × 10 dm J cm (Figure 1C).
Since the intensity of that absorption band is not affected by the presence of O it is
2
reasonable to assign it to the radical cation derived from 3ꢀstyrylꢀquinoxalinꢀ2(1H)ꢀone
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2
2
2
2
2
2
2
2
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5
5
5
5
5
5
6
•+
(
SQ ) (Figure 1C, Table 1). With the further elapse of time, the spectrum observed after
40 ꢀs after the pulse is still characterized by the similar 450ꢀnm absorption band (Figure
D). Interestingly, the spectra observed at longer times (100 ꢀs and 500 ꢀs) after the pulse
1
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
are characterized by a very broad and flat absorption band in the range 440 – 470 nm
without a clearly pronounced maximum. The spectral and kinetic parameters of its
formation and decay are similar to those observed in Arꢀsaturated (Table 1 and 2)
suggesting the same species present in both Ar and O ꢀsaturated solutions. For instance,
2
the decay kinetics observed at λ = 450 nm followed again a complex kinetics which can be
4
−1
3
−1
described again as two firstꢀorder processes with k = 2.3 × 10 s and 1.9 × 10 s (insert
in Figure 1D; Table 2), similar to those measured in Arꢀsaturated solutions. Interestingly,
comparison of the kinetic traces at λ = 450 nm and 530 nm clearly shows that contrary to λ
=
450 nm, the decay kinetics observed at λ = 530 nm follows single firstꢀorder process
3
−1
with k = 7.4 × 10 s (insert in Figure 1D; Table 2). This last observation suggests the
existence of two different cationic species. The assignment of the flat absorption band
observed at longer times is not obvious, although very plausibly, it might be a dimeric
•
+
radical cation ((SQ) ) formed as a product of association of radical cations and parent SQ
2
35
62
molecules. Formation of such species in stilbene, and styrene derivatives is wellꢀ
documented in the literature. However, taking into account low concentration of SQ in the
system studied, one can expect its very low yield.
•−
Correction of the absorption spectrum of SQ . Subtraction of the absorption
spectrum recorded 240 ns after the pulse in O ꢀsaturated solutions from the respective
2
absorption spectrum obtained in Arꢀsaturated solutions should allow to take apart the
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•+
contribution of radical cation (SQ ), if any, leaving only that absorption spectrum that
•
−
corresponds to the radical anion (SQ ) at short times. Since the absorption signals
recorded 240 ns after the pulse over the whole wavelength range (i.e. 400 – 600 nm) in O2ꢀ
saturated solutions are very weak (insert in Figure 1C) and being on the noise level this
clearly suggests that the absorption spectrum recorded at 240 ns after the pulse in Arꢀ
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
•
−
saturated solutions can be assigned exclusively to radical anions (SQ ) (Figure 1A, Table
1
).
Ar-saturated 2-propanol solutions. Pulse radiolysis of Arꢀsaturated 2ꢀpropanol
solution containing 3ꢀstyrylꢀquinoxalinꢀ2(1H)ꢀone (1a, Chart 1) yielded a complex series
of spectral changes. A broad absorption was observed at 240 ns after the pulse in the 420 –
600 nm range with two absorption maxima located at λ = 460 nm and 520 nm and with a
rising absorption toward 600 nm with no defined maximum. (Figure 3A).
Figure 3. (A) Absorption spectra recorded in Arꢀsaturated 2ꢀpropanol solutions containing 0.1 mM 3ꢀ
styrylquinoxalinꢀ2(1H)ꢀone (SQ). Spectra taken after the following time delays: 240 ns (■), 480 ns (○), 10 ꢀs
ꢀ), 100 ꢀs (ꢁ), 200 ꢀs (ꢀ), (B) short time profiles representing decays at λ = 460 (■), 520 (○) and 600 nm
ꢀ), (C) short time profiles representing decays at λ = 440 (■), 480 (○) and 520 nm (ꢀ), (D) long time
(
(
profile representing decay at λ = 480 (■)
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2
2
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5
5
6
This latter absorption can be assigned to the solvated electrons which decay within
6
−1
submicrosecond time domain by a rapid pseudoꢀfirstꢀorder process with k = 5.5 × 10 s
(
Table 3). At the time when 600ꢀnm absorption disappeared completely, the absorption
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
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2
3
4
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7
8
9
0
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2
3
4
5
6
7
8
9
0
bands with λmax at 460 and 520 nm decreased also in intensity, however, the respective
time profiles recorded at those wavelengths reached a plateau with a substantial offset
(
Figure 3B). The absorption spectrum observed 500 ns after the pulse is characterized by a
distinct absorption maximum at λmax = 460 nm and a pronounced shoulder located at 500 –
540 nm (Figure 3A).
Table 3. Kinetic parameters for the decays of solvated electrons
−
•−
(e
), and radical anions (RSQ ) and hydrogenated radicals
solv
•
(RSQH derived from 3ꢀstyrylꢀquinoxalinꢀ2(1H)ꢀone derivatives
(3ꢀSQ) in 2ꢀpropanol solutions saturated with Ar
Solvated
electron
(e solv)
Radical anion Hydrogenated
•−
Compound
ꢀSQ
(3ꢀSQ )
decay
(× 10 s )
radical•
(3ꢀSQH )
decay
−
3
5
−1
decay
6
−1
3 −1
(×10 s )
(× 10 s )
SQ
5.5
5.0
7.1
9.7
4.6
3.0
5.6
6.6
6.4
4ꢀCH SQ
6.1
5.6
6.4
4.9
5.5
7.5
3.7
6.7
8.6
7.6
6.2
5.8
3
4
ꢀOCH SQ
3
4
ꢀN(CH ) SQ
3
2
4
ꢀOCF SQ
3
3,4ꢀ(OCH ) SQ
3
2
2
,5ꢀ(OCH ) SQ
3 2
It is noteworthy that this absorption spectrum is very similar to the analogous
spectrum recorded 240 ns in Arꢀsaturated acetonitrile (Figure 1A) and which was assigned
•−
to the 3ꢀstyrylꢀquinoxalinꢀ2(1H)ꢀone derived radical anions (SQ ). Moreover, the decays
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observed at λmax = 460 nm and 520 nm look differently (Figure 3C). At the time when 530ꢀ
5
−1
nm absorption disappeared in pseudoꢀfirst order process with k = 5.0 × 10 s (Table 3),
the spectrum underwent further changes, and 10 ꢀs after the pulse is characterized by two
distinct absorption maxima with λmax at 440 and 480 nm (Figure 3A). Very plausibly, the
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
•
−
decay observed at λ = 520 nm represents protonation of SQ by solvent (2ꢀpropanol) and
the spectrum obtained can be tentatively assigned to the respective hydrogenated radical
•
(SQH ). These two absorption bands are still observed at longer times (Figure 3A),
however, at 100 and 200 ꢀs after the pulse, the absorption band with a maximum at λmax
=
440 nm is transformed into a pronounced shoulder. The decay of the absorption band at
λmax = 480 nm (Figure 3D) occurs on the subꢀmillisecond time domain in a pseudoꢀfirst
3
−1
order process with k = 6.4 × 10 s (Table 3).
Argon and oxygen saturated acetonitrile solutions containing 3-styryl-quinoxalin-2-one
derivatives with single substituents.
Since substitution in paraꢀposition by the electronꢀdonating and electron
withdrawing groups does not affect significantly the time evolution of the spectral pattern
and kinetic parameters of transients formed, both in Ar and O ꢀsaturated acetonitrile
2
solutions in comparison to SQ, discussions of the results will be limited to those
parameters which were different and specific for the particular substituent.
3
-(4-methylstyryl)quinoxalin-2(1H)-one, (4-CH SQ (1b)). Substitution in paraꢀposition
3
63
by a weak electron donating methyl group (ꢀCH ) (σ = ꢀ0.17) in the styrene moiety
3
p
affects slightly a transient absorption spectrum obtained 240 ns after the pulse in Arꢀ
•
−
saturated solutions which was assigned to radical anions (4ꢀCH SQ ) derived from 3ꢀ(4ꢀ
3
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2
2
2
2
2
2
2
2
2
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3
3
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3
3
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3
4
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5
5
5
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5
5
5
6
methylstyryl)quinoxalinꢀ2(1H)ꢀone (4ꢀCH SQ) based on the earlier assignment in SQ. The
3
maxima of the respective absorption bands underwent a small hypsochromic shift of 5 and
−
3
10 nm and are located at λ = 470 and 520 nm with slightly lower Gε equal to 1.3 × 10
λ
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3 −1
−1
−3
3 −1
−1
dm J cm and 0.75 × 10 dm J cm , respectively (Figure 4A; Figure S1A in
Supplementary Information, Table 1). The absorption at λ =520 nm decayed by a rapid
6
−1
firstꢀorder process with k = 1.5 × 10 s which is slightly higher than analogous one in SQ
(insert in Figure S1A in Supplementary Information, Table 2).
Figure 4. Absorption spectra recorded 240 ns after the pulse in Arꢀsaturated acetonitrile solutions containing
.1 mM 3ꢀstyrylquinoxalinꢀ2(1H)ꢀone derivatives with single substituents: (■) SQ, (○) 4ꢀCH SQ, (ꢀ) 4ꢀ
0
3
OCH SQ, (ꢀ) 4ꢀN(CH ) SQ, (ꢁ) 4ꢀOCF SQ (A) and double methoxy substituents: (○) 3,4ꢀ(OCH ) SQ, (■)
3
3 2
3
3 2
2
,5ꢀ(OCH ) SQ, (B). Absorption spectra recorded 8 ꢀs after the pulse in O ꢀsaturated acetonitrile solutions
3 2
2
containing 0.1 mM 3ꢀstyrylquinoxalinꢀ2(1H)ꢀone derivatives with single substituents:. (■) SQ, (○) 4ꢀCH SQ,
3
(
ꢀ) 4ꢀOCH SQ, (ꢀ) 4ꢀN(CH ) SQ, (ꢁ) 4ꢀOCF SQ (C) and double methoxy substituents: (○) 3,4ꢀ
3 3 2 3
(
OCH ) SQ, (■) 2,5ꢀ(OCH ) SQ, (D).
3
2
3 2
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The absorption spectrum observed 8 ꢀs after the pulse both in Arꢀ and O ꢀsaturated
2
solutions is characterized by a broad and strong absorption band, however, with a weakly
pronounced maximum located in the region 450 – 475 nm with Gε450ꢀ475 ranged from 1.3 ×
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
−
3
3 −1
−1
−3
3 −1
−1
•+
1
0 dm J cm to 1.4 × 10 dm J cm and was assigned to the radical cation (MeSQ )
derived from 3ꢀ(4ꢀmethylstyryl)quinoxalinꢀ2(1H)ꢀone (MeSQ) (Figures 4A and 4C;
Figures S1A and S1C in Supplementary Information, Table 1).
The growth of the 450ꢀnm absorption band followed a firstꢀorder kinetics with k =
5
−1
3.8 × 10 s (after similar correction for the simultaneous decay of the absorption at λ =
4
50 nm performed earlier for SQ) in Arꢀsaturated solutions which matches nearly the
5
−1
formation kinetics in O ꢀsaturated solutions with k = 3.7 × 10 s (insert in Figure S1A in
2
Supplementary Information; Table 2). Again, the decay of the 450ꢀnm band, both in Ar
and O ꢀsaturated solutions followed a complex kinetics which can be described as two
2
firstꢀorder processes with the firstꢀorder kꢀvalues differing by the order of magnitude and
which are also comparable to the respective kꢀvalues measured for SQ (inserts in Figures
S1B and S1D in Supplementary Information; Table 2).
3
-(4-methoxystyryl)quinoxalin-2(1H)-one, 4-OCH SQ (1c). Substitution in paraꢀposition
3
6
3
by a stronger electron donating methoxy group (ꢀOCH ) (σ = ꢀ0.27) in the styrene
3
p
moiety practically does not affect the spectral parameters of the radical anions (4ꢀ
•
−
OCH SQ ) derived from 3ꢀ(4ꢀmethoxylstyryl)quinoxalinꢀ2(1H)ꢀone (4ꢀOCH SQ), except
3
3
a further small hypsochromic shift of 20 nm of the second absorption band maximum to
−3
3 −1
−1
λmax = 510 nm with the respective Gε470 equal to 1.2 × 10 dm J cm and Gε = 0.65 ×
510
10− dm J cm (Figure 4A, Figure S2A in Supplementary Information; Table 1), as
3
3 −1
−1
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1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
compared to SQ. Contrary to SQ and 4ꢀCH SQ, the fast decay of the second absorption
3
•
−
band at λmax = 510 nm of the 4ꢀOCH SQ is not observed because of the overlap with a
3
slower growth of a broad and strong absorption band with a weakly pronounced maximum
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
−3
3 −1
−1
at λmax = 485 nm and Gε485 equal to 1.2 × 10 dm J cm (insert in Figure S2A in
Supplementary Information; Table 1). Therefore, it is not possible to extract in a
straightforward way the rate constant (k) of this fast firstꢀorder decay. This band is also
present in O ꢀsaturated solutions with no clearly visible maximum located at λmax = 485ꢀ
2
−
3
3 −1
−1
490 nm with Gε490 equal to 1.3 × 10 dm J cm Figure 4C, Figure S2C in
Supplementary Information; Table 1). The growth of that band assigned to radical cations
•
+
(
4ꢀOCH SQ ) derived from 3ꢀ(4ꢀmethoxylstyryl)quinoxalinꢀ2(1H)ꢀone (4ꢀOCH SQ)
3
3
5
−1
followed a firstꢀorder kinetics with k = 3.8 × 10 s (in Arꢀsaturated solutions) (insert in
5
−1
Figure S2A in Supplementary Information) and k = 4.1 × 10 s (in O ꢀsaturated solutions)
2
(insert in Figure S2C in Supplementary Information) which are very close to those
•+
•+
measured for SQ and 4ꢀCH SQ (Table 2). Similarly to SQ and 4ꢀCH SQ , the decay of
3
3
•
+
4ꢀOCH SQ both in Ar and O ꢀsaturated solutions followed a complex kinetics described
3
2
again as two firstꢀorder processes with the firstꢀorder kꢀvalues differing by the order of
magnitude (inserts in Figures S2B and S2D in Supplementary Information; Table 2) and
•
+
•+
which are also comparable to the respective kꢀvalues measured for SQ and 4ꢀCH SQ
3
(Table 2).
3
-(4-N,N-dimethylaminestyryl)quinoxalin-2(1H)-one, 4-N(CH ) SQ (1d). Substitution in
3 2
6
3
paraꢀposition by a strong electron donating dimethylamine group (ꢀN(CH ) ) (σ = ꢀ0.83)
3
2
p
in the styrene moiety does not affect much the spectral and kinetic parameters of the
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•−
radical anions (4ꢀN(CH ) SQ ) derived from 3ꢀ(4ꢀN,Nꢀdimethylstyryl)quinoxalinꢀ2(1H)ꢀ
3
2
one (4ꢀN(CH ) SQ), except a small bathochromic shift of 15 nm of the first absorption
3
2
−
3
3 −1
−1
band maximum to λmax = 490 nm with the respective Gε490 = 0.9 × 10 dm J cm and
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
−3
3 −1
−1
•−
Gε530 = 0.8 × 10 dm J cm (at λ = 530 nm), as compared to SQ (Figure 4A, Figure
S3A in Supplementary Information, Table 1). Both absorption bands decayed by a rapid
6
−1
firstꢀorder process with k = 1.2 × 10 s (measured at λ = 490 nm) which is slightly lower
than analogous one for SQ (insert in Figure S3A in Supplementary Information, Table 2).
On the other hand, substitution affects substantially the spectral parameters of the longꢀ
lived transient, both in Ar and O ꢀsaturated solutions which is assigned to the radical
2
•
+
cations (4ꢀN(CH ) SQ ) derived from 3ꢀ(4ꢀN,Nꢀdimethylstyryl)quinoxalinꢀ2(1H)ꢀone (4ꢀ
3 2
N(CH ) SQ) (Figures 4A and 4C, Figures S3A and S3C in Supplementary Information,
3
2
•+
Table 1). The maximum of the absorption band of 4ꢀN(CH ) SQ underwent a significant
3
2
•
+
−3
bathochromic shift to λ = 630 nm as compared to SQ with Gε630 equal to 1.9 × 10
3 −1
−1
−3
3 −1
−1
dm J cm (in Arꢀsaturated solutions) (Figure 4A) and 2.2 × 10 dm J cm (in O2ꢀ
saturated solutions) (Figure 4C, Table 1). The growth of the 630ꢀnm absorption band
5
−1
followed a firstꢀorder kinetics with k = 4.6 × 10 s (in Arꢀsaturated solutions) (insert in
5
−1
Figure S3A in Supplementary Information; Table 2) and k = 4.7 × 10 s (in O ꢀsaturated
2
solutions) (insert in Figure S3C in Supplementary Information; Table 2) which are slightly
•+
higher than those measured for other SQ derivatives (Table 2). Similarly to SQ , 4ꢀ
•+
•+
•+
CH SQ , and 4ꢀOCH SQ , the decay of 4ꢀN(CH ) SQ both in Ar and O ꢀsaturated
3
3
3 2
2
solutions followed a complex kinetics described again as two firstꢀorder processes with the
firstꢀorder kꢀvalues differing by the order of magnitude (inserts in Figures S3B and S3D,
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1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
•+
Table 2) and which are also comparable to the respective kꢀvalues measured for SQ , 4ꢀ
•+
•+
CH SQ and 4ꢀOCH SQ (Table 2).
3
3
3
-(4-trifluromethoxystyryl)quinoxalin-2(1H)-one, 4-OCF SQ (1e). Substitution in paraꢀ
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
position by a relatively strong electron withdrawing trifluromethoxy group (ꢀOCF ) (σ = +
3
p
6
3
0.35) does not affect at all the time evolution of the spectral pattern and kinetic
parameters of transients formed both in Ar and O ꢀsaturated solutions in comparison to SQ.
2
In Arꢀsaturated solutions, the shortꢀlived species is formed within 240 ns time domain and
is characterized by the spectrum with two absorption bands located at λmax = 475 and 530
−3
3 −1
−1
−3
3 −1
−1
nm with the respective Gε475 = 1.4 × 10 dm J cm and Gε = 1.0 × 10 dm J cm
530
(Figure 4A, Figure S4A in Supplementary Information; Table 1). Similarly as for the other
•
−
derivatives, this absorption spectrum is assigned to the radical anions (4ꢀOCF SQ )
3
derived from 3ꢀ(4ꢀtrifluoromethoxystyryl)quinoxalinꢀ2(1H)ꢀone (4ꢀOCF SQ) (Table 1).
3
•
−
6
−1
The 4ꢀOCF SQ decayed by a rapid firstꢀorder process with k = 1.2 × 10 s (insert in
3
Figure S4A in Supplementary Information; Table 2). The absorption spectrum underwent
further changes and 8 ꢀs after the pulse is characterized by a broad and pronounced
absorption band with λmax = 450 nm, both in Ar (Figure 4A, Figure S4A in Supplementary
Information) and O ꢀsaturated solutions (Figure 4C, Figure S4C in Supplementary
2
Information, Table 1). This absorption band, by analogy to other SQ derivatives, has been
•
+
assigned to the radical cations (4ꢀOCF SQ ) derived from 3ꢀ(4ꢀtrifluoromethoxyꢀ
3
styryl)quinoxalinꢀ2(1H)ꢀone (4ꢀOCF SQ) (Table 1). The growth of the 450ꢀnm absorption
3
5
−1
band followed a firstꢀorder kinetics with k = 3.9 × 10 s (in Arꢀsaturated solutions) (insert
5
−1
in Figure S4A in Supplementary Information; Table 2) and k = 3.8 × 10 s (in O2ꢀ
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2
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saturated solutions) (insert in Figure S4C in Supplementary Information; Table 2) which
are similar to those measured for other SQ derivatives (Table 2). The decay of 4ꢀOCF SQ•+
3
both in Ar and O ꢀsaturated solutions followed a complex kinetics described again as two
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
firstꢀorder processes with the firstꢀorder kꢀvalues differing by the order of magnitude
(inserts in Figures S4B and S4D, Table 2) and which are also comparable to the respective
•
+
kꢀvalues measured earlier for other 3ꢀSQ radical cations with a single substituent (Table
2
).
Argon saturated 2-propanol solutions containing 3-styryl-quinoxalin-2(1H)-one
derivatives with single substituents.
Since substitution in paraꢀposition by the electronꢀdonating and electron
withdrawing groups does not affect significantly the time evolution of the spectral pattern
and kinetic parameters of transients formed in Arꢀsaturated 2ꢀpropanol solutions in
comparison to SQ (except 1d), the respective spectra and shortꢀ and longꢀtime profiles at
selected wavelengths are presented in the Supplementary Information (Figures S7ꢀS10).
Similarly as for SQ (1a), pulse radiolysis of Arꢀsaturated 2ꢀpropanol solutions containing
3
ꢀstyrylꢀquinoxalinꢀ2(1H)ꢀone derivatives (1b – 1e) with methyl (CH ꢀ) (1b), methoxy (ꢀ
3
OCH ) (1c), N,Nꢀdimethylamine (ꢀN(CH ) (1d) and trifluromethoxy (ꢀOCF ) (1e) groups
3
3 2
3
yielded a complex series of spectral changes. The spectra observed on the subꢀmicrosecond
•
−
time domain can be assigned to the respective radical anions (3ꢀSQ ) and those observed
•
on the microsecond time domain to the respective hydrogenated radicals (3ꢀSQH ). It is
•
−
noteworthy that the spectral parameters of radical anions (3ꢀSQ ) derived from 3ꢀ
(styryl)quinoxalinꢀ2(1H)ꢀone derivatives (1b, 1c, and 1e) match very well the spectral
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
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4
4
4
4
4
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5
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5
5
5
5
5
5
5
6
•−
parameters of 3ꢀSQ observed in acetonitrile solutions (Table 1). The only exception is 1d
•
−
where the maximum of the absorption band of 4ꢀN(CH ) SQ underwent a significant
3
2
bathochromic shift to λ = 630 nm (Figure S9 in Supplementary Information) as compared
to λmax = 490 and 530 nm observed in acetonitrile solutions (Table 1). On the other hand,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
•
the hydrogenated radicals (3ꢀSQH ), which were not observed in acetonitrile solutions, are
characterized by the absorption spectra with λmax = 480 – 490 nm, depending on the
substituent (Table 1). Again, the only exception is 1d where the maximum of the
•
absorption band of 4ꢀN(CH ) SQH underwent a bathochromic shift to λ = 550 nm (Figure
3
2
S9 in Supplementary Information). The rate constants of the respective processes occurring
for 3ꢀ(styryl)quinoxalinꢀ2(1H)ꢀone derivatives (3ꢀSQ) (1b – 1e) which were described in
detail for SQ (1a) are collected in Table 3.
Argon and oxygen saturated acetonitrile solutions containing 3-styryl-quinoxalin-2(1H)-
one derivatives with double methoxy substituents.
Since simultaneous substitution in para/meta and ortho/meta positions by relatively strong
electronꢀdonating methoxy groups does not affect significantly the time evolution of the
spectral pattern and kinetic parameters of transients formed, both in Ar and O ꢀsaturated
2
acetonitrile solutions in comparison to a single substituted methoxy 3ꢀstyrylꢀquinoxalinꢀ2ꢀ
one derivative (4ꢀOCH SQ), discussions of the results will be limited to those parameters
3
which were different and specific for the particular combination of substitutions.
3
-(3,4-dimethoxystyryl)quinoxalin-2(1H)-one, 3,4-(OCH ) SQ (1f). Substitution in paraꢀ
3 2
63
and meta positions by two electron donating methoxy group (ꢀOCH ) (σ = ꢀ0.27) in the
3
p
styrene moiety practically does not affect the spectral parameters of the radical anions (3,4ꢀ
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•−
(
OCH ) SQ ) derived from 3ꢀ(3,4ꢀdimethoxystyryl)quinoxalinꢀ2(1H)ꢀone (3,4ꢀ
3 2
−
3
3 −1
−1
−3
(OCH ) SQ), with the Gε equal to 1.3 × 10 dm J cm and Gε510 to 0.8 × 10
3
2
470
3 −1
−1
dm J cm (Figure 4B, Figure S5A in Supplementary Information; Table 1), as compared
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
to 4ꢀOCH SQ (Figure 4A, Figure S2A, Table 1). Similarly to 4ꢀOCH SQ, however,
3
3
contrary to SQ and other SQ derivatives, the fast decay of the second absorption band at
•−
λmax = 510 nm of the 3,4ꢀ(OCH ) SQ is not observed because of the overlap with a
3 2
slower growth of a broad and strong absorption band with a weakly pronounced maximum
−
3
3 −1
−1
at λmax = 510 nm and Gε510 equal to 1.3 × 10 dm J cm (insert in Figure S5A in
Supplementary Information; Table 1). Therefore, it is not possible to extract in a
straightforward way the rate constant (k) of this fast firstꢀorder decay. The growth of that
•+
band assigned to radical cations (3,4ꢀ(OCH ) SQ ) derived from 3ꢀ(3,4ꢀ
3
2
dimethoxystyryl)quinoxalinꢀ2(1H)ꢀone (3,4ꢀ(OCH ) SQ) followed a firstꢀorder kinetics
3
2
5
−1
with k = 3.6 × 10 s (in Arꢀsaturated solutions) (insert in Figure S5A in Supplementary
5
−1
Information, Table 2) and k = 2.7 × 10 s (in O ꢀsaturated solutions) (insert in Figure S5C
2
in Supplementary Information, Table 2) These rate constants differ slightly which is due to
the fact that the growth recorded at λ = 510 nm in Arꢀsaturated solutions and assigned to
•
+
formation of 3,4ꢀ(OCH ) SQ was not corrected by the simultaneous decay of 3,4ꢀ
3
2
•
−
•+
(OCH ) SQ . The decay of 3,4ꢀ(OCH ) SQ both in Ar and O ꢀsaturated solutions
3
2
3 2
2
followed a complex kinetics described again as two firstꢀorder processes with the firstꢀ
order kꢀvalues differing by the order of magnitude (inserts in Figures S5B and S5D in
Supplementary Information; Table 2) and which are also comparable to the respective kꢀ
•+
values measured earlier for other 3ꢀSQ radical cations with single substituents (Table 2).
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