Trans -A2B-corrole bearing 2,3-di(2-pyridyl)quinoxaline (DPQ)/phenothiazine moieties: Synthesis, characterization, electrochemistry and photophysics

Article abstract of DOI:10.1039/c8nj00606g

Two novel donor-acceptor systems (F10C-PTZ and F10C-DPQ) were designed and synthesized, in which corrole is an acceptor and either phenothiazine (PTZ) or 2,3-di(2-pyridyl)quinoxaline (DPQ) is an energy/electron donor. These dyads represent some of the few examples of photostable corrole dyads reported in the literature, but the first example of 2,3-di(2-pyridyl)quinoxaline (DPQ) containing meso substituted corrole (F10C-DPQ) also directly connected PTZ to the meso position of corrole (F10C-PTZ). Both the dyads were characterized by high-resolution mass spectrometry (HR-MS), NMR (¹H, ¹³C, ¹⁹F) spectroscopy, UV-Vis spectroscopy, steady state fluorescence, time-resolved fluorescence (TCSPC, MCP-PMT), electrochemical methods (CV, DPV) as well as theoretical calculations (DFT, TD-DFT). From absorption and electrochemical studies it is evident that there exist minimum π-π interactions between two hetero chromophoric units in the dyads. The steady state fluorescence studies indicate that an efficient quenching of emission corresponding to the PTZ unit in the F10C-PTZ dyad is attributed to the singlet-singlet energy transfer from PTZ to corrole. In addition to the spectral overlap, excitation spectra provided additional evidence for intramolecular energy transfer. Whereas in the F10C-DPQ dyad, a reductive electron transfer, from the ground state of DPQ to the excited state of corrole, was observed. The negative free energy (ΔG) for charge separation, anodic shift of reduction potential of corrole and the frontier molecular orbital (LUMO) location provide addition support for charge transfer. The solvent dependent rate of an energy and electron transfer was discussed.

Full text of DOI:10.1039/c8nj00606g

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Trans-A2B-corrole Bearing 2,3-di(2- pyridyl)
quinoxaline(DPQ)/phenothiazine Moiety’s: Synthesis,
Characterization, Electrochemistry and Photophysics
Jaipal Kandhadi,a Wei-Cong Yan,a Fan Cheng,b Hui Wang,*a and Hai-Yang Liu*b
Received 00th January 20xx,
Accepted 00th January 20xx
Two novel donor-acceptor systems (F10C-PTZ and F10C-DPQ) were designed and synthesized, in which corrole is an
acceptor and either phenothiazine (PTZ) or 2,3-di(2-pyridyl)quinoxaline(DPQ) are energy/electron donors. These dyads
represents one of the few examples of photostable corrole dyads reported in the literature, but the first example of 2,3-
di(2-pyridyl)quinoxaline(DPQ) containing meso substituted corrole (F10C-DPQ) also directly connected PTZ to meso
position of corrole (F10C-PTZ). Both the dyads were characterized by high-resolution mass spectrometry (HR-MS), NMR
(1H, 13C, 19F), UV-Vis, Study state Fluorescence, Time-resolved fluorescence (TCSPC, MCP-PMT), electrochemical methods
(CV, DPV) as well as theoretical calculations (DFT, TD-DFT). Absorption and electrochemical studies evident that, there
exist minimum π-π interactions between two hetero chromophoric units in the dyad. The steady state fluorescence
studies indicate an efficient quenching of emission corresponding to PTZ unit in F10C-PTZ dyad is attributed to singlet–
singlet energy transfer from PTZ to corrole. In addition to the spectral overlap, excitation spectra provided additional
evidence for intramolecular energy transfer. Whereas in F10C-DPQ dyad, the reductive electron transfer, from ground
state of DPQ to excited state of corrole was observed. Negative free energy (∆G) for charge separation, anodic shift of
reduction potential of corrole and frontier molecular orbital (LUMO) location providing addition support for charge
transfer. The solvent dependent rate of an energy and electron transfer was discussed.
DOI:10.1039/x0xx00000x
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elementary chemical process, it constitutes a key step in biological
processes of enormous relevance to life.6 Thus, identifying working
1. Introduction
Natural Photosynthesis is a physiological-biochemical process in
which the solar energy can be converted to the chemical energy.1
Harvesting of Sunlight by antenna complexes surrounding the
reaction center represents the first step of the natural
photosynthesis performed by plants, algae and cyanobacteria. The
complex of proteins and pigments embedded in the thylakoid
membrane is composed by carotenoids and (B)Chls, which are
responsible for light absorption, the captured light energy by the
antenna exhibit remarkable transport properties, which facilitate
highly efficient excitation energy transfer (PEnT) across the long
distances via a cascade of radiationless energy-transfer steps and
subsequently conveyed to the reaction center through resonant
energy transfer mechanisms, where the charge separation occurs.
This excited energy is necessary to perform (uphill process) such a
demanding step is water spilling in oxygenic photosynthesis, as a
result, the light-harvesting machinery must produce four electrons
for further processes.2-5 In addition to energy transfer, electron
transfer also considered as important chemical reaction, as an
principles that could serves as blueprints for the design of nature
inspired devices is an active area of research for the development
of future scalable technologies.7
In order to elucidate the complex events in energy transfer process
and to achieve the long lived charge separated (CS) state, much
effort has been devoted to the construction of artificial
photosynthetic model systems.8,9 In spite of the fact that, the
substantial progress has been achieved in this area, research
towards exploring for new models that differ from existing systems
in photosensitizer, spacer and position of substitution etc..., are in
large demand because of their applications in many scientific fields
for example materials chemistry,10 biochemistry,11 photodynamic
therapy,12 catalysis,13 DSSC.14 To mimic the key steps in the natural
process, e.g. Photo-induced electron (PET) and energy (PEnT)
transfer processes, researchers choose to use entities that have
great resemblance to natural systems both in terms of structure
and properties.15 Porphyrin and its analogues tetrapyrrole, i.e.,
corrole proved to be potent candidates for this purpose, because of
their similarities with chlorophyll.15 Porphyrin sensitizers appear to
be monopolies of the study due to their low cost, smooth synthetic
a.State Key Laboratory of Optoelectronics Materials and Technologies, Sun-Yat Sen
University, Guangzhou, Guangdong 510275, China E-mail:
stswh@mail.sysu.edu.cn
methods, easily tunable electrochemical and photophysical
properties by substituting different functional groups at their β
and/or meso position.16 Corroles are tetrapyrrole analogues of
porphyrin with a direct pyrrole–pyrrole linkage and three meso
carbons, the lack of one methine (=CH-) bridge is responsible for the
lower symmetry, higher fluorescence quantum yields, lower
oxidation potentials and more intense absorption in the low energy
b.Department of Chemistry, Key Laboratory of Functional Molecular Engineering of
Guangdong Province, South China University of Technology, Guangzhou,
Guangdong 510641, China. E-mail: chhyliu@scut.edu.cn
Jaipal Kandhadi (ORCID): 0000-0002-6621-8016
Electronic Supplementary Information (ESI) available: [NMR (1H, 13C, 19F), MASS,
Theoretical UV, Emission, Molecular orbital contribution and Flu. Int. ratio]. See
DOI: 10.1039/x0xx00000x
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region of visible spectrum. However corroles are poorly explored as reported D-A system based on tetrathiafulvalene and DPQ.38 In
compare to porphyrin due to their synthetic challenges and
moderate stability.17 After the synthetic breakthroughs by Gryko,
Gross and Paolesse,18 scientific community quest for stable
chromophores that comprise of corrole and other units is on-going
research. It is important to mention that the corrole core might be
addition to AIEE and sensor properties (beyond the scope of this
publication), DPQ shows relatively low oxidation potential (0.61 V vs
Ag/Ag+) might be the suitable as an electron donor in model
compounds for artificial charge separation and molecular electronic
devices. The advantage of DPQ over the other electron donors is
that: (1) there is no heavy-atom effect (like ferrocene) which means
there no formation of triplet state of DPQ via Intersystem crossing
(ISC) (2) availability two extra binding sites even after covalently
connecting to counter moiety. These properties turned our
attention to utilization of this ligand for construction of dyads.
stabilized
by
electron
withdrawoing
groups
like
petaflourobenzaldehyde at their meso position,19 by cosidaring this
property there are some stable donor-acceptor (D-A) systems have
been reported, however most of them are constituted with the
electron accepting moiety’s, for example fullerene20 and imide
derivatives.21 On the other hand the energy/electron donor groups One of the important and unique propertie of corrole is, the role of
like coumarin22 and bodipy23 have been reported but donor entities
either an energy/electron donor or acceptor depends on the nature
with stable corrole core are relatively low in the literature.
of counter entities. Introducing pentafluorophenyl units at 5 and 15
positions, the corrole core secure good photostability for
Phenothiazine (PTZ) is a tricyclic heteroaromatic chromophores, investigation of photophysical characterization (vide infra). All these
have strong tendency to donate electrons than their counter keeping in mind we have synthesized two new dyads based on PTZ
analogs such as triarylamines and carbazole chromophores, due to (F10C-PTZ) and 2,3-di(2-pyridyl)quinoxaline (F10C-DPQ), In which,
the presence of both electron-releasing nitrogen and sulphur
atoms.24 In addition to this, PTZ have gained much attention, due to
PTZ/DPQ are an energy/electron donor and corrole as an acceptor
and we have also explored the effect of solvent polarity on the
their interesting structural features is that, the torsion angle around rates of photoinduced energy and electron transfer in these dyads.
the N–S axis ( 147.1o) of the PTZ ring attains bowl-shaped
(nonplaner butterfly) conformation, which forbids the molecular
2. Experimental
aggregation, which in turn, prevents the formation of
intermolecular excimer even in highly concentrated solution and
solid state.25 By encasing these points PTZ have been employed in
development of various applications such as bulk heterojunction
solar cells (BHSCs),26 dye-sensitized solar cells (DSSCs)27 and organic
light emitting diodes(OLED).28 There are many donor-acceptor small
molecules based on PTZ have been reported,29 but so far
combination with corrole is very few,30 this might be due to the
instability of the composed dyads comprise of corrole and donor
entities(vide infra).
2.1. Synthesis of precursors
Synthesis of 10-methyl-10H-phenothiazine-3- carbaldehyde (PTZ-
30(a)
CHO),
2-perfluorophenyl (1H-pyrrol-2-yl)methyl)-1H-pyrrole
(DPM),39 both the dyads F10C-PTZ and F10C-DPQ was synthesized
40
according to reported and a bit modified procedures.
2.2. 6-bromo-2,3-di(pyridin-2-yl)quinoxaline(DPQ-Br): DPQ-Br was
synthesized according reported and slightly modified procedure41. 4-
bromobenzene-1,2-diamine (50 mg, 0.270 mmol) and 1,2-
di(pyridin-2-yl)ethane-1,2-dione (65 mg, 0.306 mmol) were dissolved
in ethanol (15 ml). The suspension was stirred and refluxed for 3-
hours under inert atmosphere. Reaction mixture cooled to room
temperature and ethanol was removed by distillation under reduced
pressure. The resulting crude product was directly purified by silica
gel chromatography (100－200) withCH2Cl2/MeOH (10∶1) used as
eluent, and the first yellow-orange band was collected. After
recrystallization from CH2Cl2 and hexaneDPQ-Br was obtained as
yellow solid (yield 90%).1H-NMR (400 MHz, CDCl3) δ(ppm): 8.32 (d, 1H,
J = 1.9 Hz), 8.27 (s, 2H), 8.01 (d, 1H, J = 8.9 Hz ), 7.88 (t, 2H, J = 7.37 Hz
), 7.80 (dd, 1H, J’ = 1.99 Hz, J’’ = 2.02 Hz ), 7.73 (t, 2H, J = 5.71 Hz), 7.16
(t, 2H, J = 5.06 Hz). 13C-NMR (CDCl3) δ: 157.10, 156.98, 153.16,
152.73,148.57, 148.54, 141.67, 139.83, 136.76, 136.66, 134.04, 131.64,
130.62, 124.51, 124.24, 124.16, 123.19, 123.13. Anal. Calcd for
2,3-bis(2-pyridy1)quinoxaline (DPQ), in which two pyridyl units are
attached to the 2 and 3 positions of the quinoxaline moiety through
a single bond. The coplanar arrangement of these two pyridine
rings with quinoxaline groups is impossible, due to steric forces
between protons that are close proximity to each other when the
two pyrinde rings attains planar geometry. To avoid this steric
hindrance the two pyridyl groups prefers to attain a twisted
geometry with the 140o out of plane of the quinoxaline ring.31 DPQ
is structurally related to pyrazine and 2,2’-bipyrimidine, their metal
complxes are well studied,32 the chelating properties of DPQ with
3d, 4d and 5d transition metal cations also reasonably studied.33
However the structure-property relation of this ligand is much
attracting, recently Ajay Misra et al. reported that the restriction of
intramolecular rotation of two pyridyl groups in their aggregataion
state, which will lead to the aggregation induced emission
enhancement (AIEE),31 in contrast to the most organic fluorophores
contain planar π-conjugated aromatic rings, in which aggregation
leads to fluorescence quenching due to non-radiative
deactivation.34 The molecules which are shows AIEE is of current
interest because their utilities in the development of OLEDs,35
organic field effect transistors (OFETs)36 and sensors.37 The same
group investigated its fluorescent sensor behavior of DPQ, based on
the pH value for the detection of acidic and basic medium.31 Jia et al
C
18H11BrN4: C, 59.52; H, 3.05; N, 15.43; found C, 59.51; H, 3.03; N,
15.44. ESI-Mass calcd for C18H11BrN4: m/z = 363.21, found m/z = 364.10
[M+1]+.
2.3. 2,3-di(pyridin-2-yl)quinoxaline(DPQ-Control compound): This
compound was synthesized by an analogous manner of the
preparation of DPQ-Br, in which, benzene-1,2-diamine was used
instated of its bromo-derivative. (yield 92%). 1H-NMR (400 MHz,
CDCl3) δ(ppm): 8.39 (d, 2H, , J = 4.5 Hz), 8.27-8.21(q, 2H, J = 3.5
2 | J. Name., 2012, 00, 1-3
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Hz), 7.97 (d, 2H, J = 7.8 Hz ), 7.85-7.79 (m, 4H), 7.27-7.22 (q, 2H, J =
7.25 Hz). 13C-NMR (CDCl3) δ: 157.35, 152.46, 148.66, 141.07,
136.70, 130.53, 129.39, 124.23, 122.94. Anal. Calcd for C18H12N4: C,
76.04; H, 4.25; N, 19.71; found C, 76.02; H, 4.24; N, 19.73. ESI-Mass
calcd for C18H12N4: m/z = 284.31, found m/z = 284.11 [M]+.
138.82, 136.59, 135.48, 130.20, 130.03, 128.91, 128.15, 127.72,
127.04, 126.45, 125.67, 125.16, 124.14, 122.95, 117.48, 112.71.
Anal. Calcd for C55H26F10N8: C, 66.80; H, 2.65; N, 11.33; found C,
66.78; H, 2.60; N, 11.30. HRMS calcd for C55H26F10N8: m/z =
988.2121, found m/z = 989.2215 [M+1]+.
2.4 4-(2,3-di(pyridin-2-yl)quinoxalin-6-yl)benzaldehyde(DPQ-
CHO): DPQ-CHO was synthesized according to reported and slightly
modified procedure.42 DPQ-Br (100 mg, 0.276 mmol), 4-
2.6.
Synthesis of corrole-PTZ dyad (F10C-PTZ): The similar
synthetic procedure (F10C-DPQ) was adapted to preparation of the
dyad, in which PTZ-CHO was used instead of DPQ-CHO. (yield
31%). 1H-NMR (400 MHz, CDCl3) δ(ppm): 9.08 (d, 2H, J = 4.2 Hz),
8.71 (dd, 4H, J’ = 4.72 Hz, J’’ = 12.01 Hz), 8.55 (d, 2H, J = 3.01 Hz),
7.94 (d, 2H, J = 8.94 Hz), 7.23-7.14 (m, 3H), 6.99-6.91 (q, 2H, J = 7.65
Hz), 3.61 (s, 3H),-2.48 (s,3H). 19F-NMR (CDCl3) δ(ppm): -137.74 (dd,
4F, J = 21.4 Hz), -153.12 (t, 2F, J = 18.8 Hz), -161.77(m, 4F). 13C-
formylphenylboronic acid (180 mg, 1.2 mmol) and Pd(PPh3)4 (30
mg, 0.026 mmol) was charged in a two necked round bottomed
flask equipped with condenser and nitrogen adaptor, then the
reaction flask was degassed with nitrogen for 10 min. The pre-
degassed dry tetrahydrofuran (20 ml) and 2 M Na2CO3 aqueous
solution (3 ml) was added into the reaction mixture and heated to
reflux with continuous stirring in the dark for 8 h, under the
nitrogen. After cooling to room temperature, the organic layer was
separated and the aqueous layer was extracted with CH2Cl2. The
combined organic layer was washed with water, brine, and dried
over sodium sulfate. The solvent was removed under reduced
pressure, and the crude product was purified by silica gel column
NMR (CDCl3)
δ:
145.78, 145.68, 139.36, 135.67, 133.92,
132.92, 127.73, 127.69, 127.39 ,123.27, 122.74, 122.59, 122.44,
121.53, 114.34, 113.03, 112.29. Anal. Calcd for C44H21F10N5S:
C, 62.78; H, 2.51; N, 8.32; S, 3.81 found C, 62.77; H, 2.49; N, 8.30; S,
3.79. HRMS calcd for C44H21F10N5S: m/z = 841.1358, found m/z =
+
842.1440 [M+1] .
chromatography eluting with CH2Cl2/MeOH (10∶2), the title
compound was obtained as pale-yellow solid (yield, 75%).1H-NMR
(400 MHz, CDCl3) δ(ppm): 10.05 (s, 1H), 8.43 (d, 1H, J = 6. 40 Hz ),
8.32 (d, 1H, J = 4.20 Hz), 8.25 (d, 1H, J = 9.42 Hz), 8.04 (dd, 1H, J’ =
2.10 Hz, J’’ = 1.79 Hz), 7.99-7.87(m, 4H), 7.77(t, 2H, J = 7.8 Hz ),
7.62-7.55(q, 1H, J = 7.1 Hz), 7.50-7.36(m, 2H), 7.19(t, 2H, J = 5.4 Hz ).
13C-NMR (CDCl ) δ: 191.70, 157.20, 153.19, 152.82,148.61, 145.53,
141.66, 141.25,140.94, 136.66, 135.93, 132.16, 132.04, 131.97,
130.51, 130.10, 129.82, 128.60, 128.45, 128.14, 127.66, 124.32,
124.23,123.16. Anal. Calcd for C25H16N4O: C, 77.30; H, 4.15;
N, 14.42; found C, 77.29; H, 4.13; N, 14.45. ESI-Mass calcd for C
H N O: m/z = 388.42, found m/z = 388.45 [M]+.
2.5 Synthesis of corrole-DPQ dyad (F10C-DPQ):
Pentafluorophenyl- dipyrrylmethane (DPM) (320 mg, 1.025 mmol)
and DPQ-CHO (200 mg, 0.512 mmol) were dissolved in MeOH (80
ml), Concentrated HClaq (5 ml) in water (80 mL) was then added,
and the resulting suspension was stirred at room temperature for 1
h. The reaction mixture was extracted with CH2Cl2 (2 × 50 mL), and
the organic phase was washed with water (3 ×50 ml) and dried over
anhydrous Na2SO4, and concentrated to 50 ml volume then 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (280 mg, 1.25 mmol)
was added, then the mixture was vigorously serried for 15 minutes
at room temperature. DDQ was removed by washing coloumn
through the silica gel (100-200) chromatography with CH2Cl2 as
eluent. The resulting crude compound was further purified by silica
gel chromatography (300-400) with CH2Cl2/MeOH (10:1) used as the
eluent. The title compound was obtained as a dark violet solid after
recrystallization from CH2Cl2 and hexane (yield 34%) H-NMR (400
MHz, CDCl3) δ(ppm): 9.11(d, 2H, J = 4.1 Hz), 8.79 (dd, 4H, J’ = 4.6
Hz, J’’ = 6.5 Hz), 8.57 (d, 2H, J = 3.52 Hz), 8.38 (dd, 6H, J’ = 12.03 Hz,
J’’ = 7.84 Hz), 8.22 (d, 2H, J = 7.83 Hz), 8.03 (d, 2H, J = 7.69 Hz), 7.86
(t, 2H, J = 7.35 Hz), 7.29 (t, 3H, J = 8.01 Hz), -2.51 (s, 3H). 19F-NMR
(CDCl3) δ(PPM): -138.72(dd, 4F, J’ = 16.7 Hz, J’’ = 5.6 Hz), -152.56 (t,
Fig. 1. Synthetic scheme of dyads along with their monomers.
3. Results and discussion
3.1. Synthesis and structure characterization
Fig.1. illustrates the synthesis of both the dyads (F10C-DPQ and
F10C-PTZ) including their formyl derivatives of hetero aromatic
hydrocarbons (PTZ-CHO and DPQ-CHO). There are two main
synthetic strategies for meso-substituted corrole dyads: (1) the
synthesis of corrole core molecule first, followed by the
modification of the peripheral groups; (2) the preparation of the
desired formyl derivative, which is then used in a condensation
reaction to produce the corrole dyad. On the basis of the results of
previous reports,20-23 we have fallowed the second strategy because
of the moderate stability of corroles.
2F, J = 20.9 Hz), -161.61 (m, 4F). 13C-NMR (CDCl ) δ: 157.33, 152.89,
3
152.30, 148.63, 147.26, 144.88, 142.73, 141.63, 141.46, 140.68,
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