Diarylpyrazine-based position isomers: A detailed study of optical properties and structure-property relationship

Article abstract of DOI:10.1016/j.dyepig.2020.108254

A versatile and expeditious synthetic route to pyrazine-based symmetric and asymmetric chromophores decorated with donor-acceptor (D-A) has been designed to study their structural effects on optical properties. Suzuki-Miyaura coupling of dihalopyrazine wi

Full text of DOI:10.1016/j.dyepig.2020.108254

Journal Pre-proof
Diarylpyrazine-based position isomers: A detailed study of optical properties and
structure-property relationship
Dong-Jin Park, Puttavva Meti, Young-Dae Gong
PII:
S0143-7208(19)32880-3
DOI:
https://doi.org/10.1016/j.dyepig.2020.108254
DYPI 108254
Reference:
To appear in: Dyes and Pigments
Received Date: 9 December 2019
Revised Date: 29 January 2020
Accepted Date: 2 February 2020
Please cite this article as: Park D-J, Meti P, Gong Y-D, Diarylpyrazine-based position isomers: A
detailed study of optical properties and structure-property relationship, Dyes and Pigments (2020), doi:
https://doi.org/10.1016/j.dyepig.2020.108254.
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Diarylpyrazine-based position isomers: A detailed study of optical properties
and structure-property relationship
†
†
Dong-Jin Park , Puttavva Meti , and Young-Dae Gong*
Innovative Drug Library Research Center, Department of Chemistry, College of Science, Dongguk University, 26, 3-ga, Pil-
dong, Jung-gu, Seoul 04620, Korea
Corresponding Author’s: Young-Dae Gong, Email: ydgong@dongguk.edu
Author Contributions
† Dong-Jin Park and Puttavva Meti contributed equally to this work.
Notes
The authors declare no competing financial interest.

Diarylpyrazine-based position isomers: A detailed study of optical properties and
structure-property relationship
†
†
Dong-Jin Park , Puttavva Meti , and Young-Dae Gong*
Innovative Drug Library Research Center, Department of Chemistry, College of Science, Dongguk University, 26, 3-ga, Pil-
dong, Jung-gu, Seoul 04620, Korea
Corresponding Author’s: Young-Dae Gong, Email: ydgong@dongguk.edu
Author Contributions
† Dong-Jin Park and Puttavva Meti contributed equally to this work.
Notes
The authors declare no competing financial interest.
High lights:
1) Diarylpyarzine-based linear and bent chromophores were synthesized by Suzuki cross-coupling
reaction.
2) The optoelectronic properties of both series were compared.
3) Linear series showed better conjugation with improved ICT interactions. Both series of dyes act as
reversible acid base sensors.
4) Linear chromophores 2f exhibited the largest first hyperpolarizibility of
β
= 11.7 x 10-30 esu.
1

Graphical abstract
Abstract
A versatile and expeditious synthetic route to pyrazine-based symmetric and asymmetric
chromophores decorated with donor-acceptor (D-A) has been designed to study their structural effects on
optical properties. Suzuki-Miyaura coupling of dihalopyrazine with various aryl boronic acids was
synthesized under microwave condition. Pyrazine functionalized at C-2, C-5 and C-6 serve as acceptor to
construct linear and angular push-pull chromophores. The photophysical, electrochemical and thermal
properties of all the target chromophores were systematically investigated and the results were correlated
theoretically by density functional theory computations. The emission wavelength was significantly red-
shifted by introduction of strong electron withdrawing group (CN). The permutation of terminal donor
acceptor units tunes the optoelectronic properties in a predictable way, aiding in the rational design of
small molecule for luminescent materials. These chromophores displayed multicolor change in different
solvents, exhibiting good solvatochromism with a large Stokes shift.
Keywords: Diarylpyrazine, cross-coupling, intramolecular charge transfer, solvatochromism.
2

1. Introduction
Fluorescent molecules have been attractive analytical tools and biosensors for centuries. Tuning the
photophysical properties of environment sensitive materials (solvatochromic dyes) has been intensively
studied in terms of fundamental research and practical applications in the fields of photovoltaic cells,
sensors, liquid crystals, multicolour lasers, and display devices [1-4]. Moreover numerous fluorescent
molecules exhibit a large range of biological activities, because their emission is highly sensitive. Thus
materials have been developed that are responsive to the acid, viscosity, or polarity of a solvent [5-9].
Thus stimuli-responsive fluorescence switching, such as vapochromism, piezochromism, and temperature
dependent fluorescent materials have received unprecedented attention [10]. Therefore, the predictive
design and synthesis of π-conjugated materials, that meet the requirements of optical devices, is an area of
intense research interest. Although there is a vast literature on small molecules, many organic materials
suffer from low thermal stability. The design and synthesis of new materials with high thermal stability is
necessary to enable the “molecular engineering” of electronic properties for realising high-performance
organic electronics.
In the last two decades, the palladium-mediated Suzuki-Miyaura cross-coupling reaction is
widely used in the academic and industrial sectors to synthesize drugs, agrochemicals, biologically active
and advanced materials [11]. It has grown into an indispensable tool for the construction of carbon-carbon
(C-C) bonds in organic synthesis. The salient features of these reactions include stability, commercial
availability, functional group tolerance, non-toxicity of boronic acids and easy isolation of product [12,
13]. These features have evidently extended its scope in synthetic chemistry and hence this reaction has
been extensively used in many pharmaceutical industries. Selective functionalization of heterocycles by
cross-coupling reactions has become the focus of many research groups. Heterocyclic compounds are
considered as exceptionally useful substrates for site-selective cross-coupling reactions due to heteroatom
induced electronic changes in the molecule.
3

Dipolar push-pull chromophores with donor-acceptor (D-A) arrangement guarantees efficient
intramolecular charge transfer (ICT) between donor and acceptor groups [14, 15] and expand the
absorption into higher wavelength. Such strategies have been widely studied and used in several
optoelectronic applications. Pyrazine core can efficiently act as π-bridge or acceptor. It offers many
nuclear sites for functional group incorporation thus it is an attractive building block in material
chemistry. Sylvain et al. reported many D-A functionalized pyrazine derivatives and studied their
protonation behavior [16-18]. Bures and co-workers synthesized pyrazine linkers and studies their NLO
properties [19, 20]. Pyrazine-based organic materials have emerged as a leading class of functionalized
organic chromophores and successfully implemented as active components in electronic devices such as
transistors [21-23], light-emitting diodes [24-27], organic photovoltaic cells [28-31], non-linear optical
materials [32] and sensors [33-35], due to their facile synthesis, strong electron-withdrawing ability,
versatility, and unique charge transporting properties.
In this manuscript, we have demonstrated the synthesis and characterization of a series of
thermally stable D-A functionalized diarylpyrazine-based position isomers under microwave condition
(MW). Optimised conditions under MW irradiation achieved shorter reaction time better yield than
conventional heating (70 oC 30 min versus 90 oC 24 h). The donor part of chromophore involves an amine
group, while the acceptor group involves -CF3, -CN and -NO2. The prepared isomers di er only in
molecular geometry yet exhibit strong end group dependent photophysical properties. This allowed us to
investigate the influence of molecular arrangement on the photophysical, thermal, and electrochemical
properties. The absorption, emission, solvatochromic behaviour, quantum mechanical calculations, and
thermal stabilities of these push-pull chromophores were systematically studied and compared to
understand their structure-property relationships. Our study o ers valuable insights into how subtle
changes in the molecular structure of materials result in tailored electronic structure and optical properties
and offers good reference for future molecular design to achieve high performance in optoelectronic
materials through accumulative structural modification.
4

2. Experimental section
2.1. Materials and instrumentation
All commercially available reagents were used for the reaction without further purification unless
otherwise stated. The progress of the reaction was monitored by thin layer chromatography (TLC), while
purification was carried out by silica gel column chromatography. NMR spectra were obtained on a
Bruker-500 MHz. Proton chemical shifts (δ) are relative to tetramethylsilane (TMS) as the internal
standard and expressed in ppm. Spin multiplicities are given as s (singlet), d (doublet), t (triplet), q
(quartet), and m (multiplet) and coupling constants (J) are given in hertz. High-resolution mass spectra
(HRMS) were obtained using a Q-TOF LC/MS system. The UV-vis absorption spectra were obtained by
Varian cary-50 spectrophotometer and photoluminescence (PL) spectra were measured by Cary Eclipse
Fluorescence Spectrophotometer. Both UV absorption and emission spectra was recorded using standard
1cm quartz cell.
Cyclic voltammetric measurements (CV) were recorded on a ZIVE MP1 Electrochemical
Workstation, with a conventional three-electrode configuration, with Pt as a working electrode, Pt wire
counter electrode, and Ag wire as reference electrode in degassed DCM solution with 0.1 M Bu4NPF6 at
RT at a scanning rate of 20 mV s−1. HOMO-LUMO energy gaps of the compounds were calibrated with
ferrocene. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were
performed with the STA 6000 and Perkin Elmer-Jade DSC under nitrogen with a heating rate of 10
min-1. The single crystal diffraction data were collected by Bruker SMART APEX
diffractometry with graphite-monochromated MoK radiation ( = 0.71,073 Å) at RT. The single crystal
X-ray
α
λ
structure was resolved via direct methods and refined by full-matrix least-squares on F2 utilizing the
program SHELX-2016
2.2. Synthetic procedures
2.2.1 General procedure for synthesis of linear chromophores (A).
5

A mixture of 2,5-dibromopyrazine (1) (200 mg, 0.42 mmol) was dissolved in toluene (2 mL)
and ethanol (1 mL), followed by the addition of Pd(PPh3)4 (0.021 mmol), 2M K2CO3 (2 mL) and the
subsequent addition of the corresponding boronic acid (0.88 mmol). The reaction mixture was irradiated
in MW at 70 for 30 min. After cooling, the solvent was evaporated in vacuum and the reaction mixture
was diluted with water and extracted with DCM twice. The combined organic layers were dried with
MgSO4, filtered and the solvent was evaporated under reduced pressure. The crude product was purified
using silica column chromatography to afford target cross-coupling products 2(a-d).
2,5-bis(4-diphenylaminophenyl)pyrazine (2a). Yield: 155 mg (65%); yellow powder, m.p: 216-218
1H NMR (500 MHz, DMSO-d6)
J = 7.4 Hz, 4H), 7.11 (d, J = 7.7 Hz, 8H), 7.04 (d, J = 8.7 Hz, 4H). 13C NMR (126 MHz, DMSO)
.
δ
9.16 (s, 2H), 8.07 (d, J = 8.7 Hz, 4H), 7.36 (t, J = 7.8 Hz, 8H), 7.13 (d,
δ
149.20, 148.94, 147.09, 140.80, 130.23, 129.41, 127.99, 125.37, 124.42, 122.26. HRMS (ESI) m/z:
[M+H]+ Calcd for C40H30N4, 567.2543; found 567.2539.
2,5-bis(4-dimethylaminophenyl)pyrazine (2b). Yield: 90 mg (68%); lime green powder, m.p: 332-
1
334 . H NMR (500 MHz, CDCl3)
δ 8.92 (s, 2H), 7.95 (d, J = 8.9 Hz, 4H), 6.82 (d, J = 8.9 Hz, 4H),
3.05 (s, 12H). 13C NMR (126 MHz, CDCl3)
HRMS (ESI) m/z: [M+H]+ Calcd for C20H22N4, 319.1917; found 319.1919.
δ 151.22, 149.28, 139.94, 127.39, 124.35, 112.37, 40.33.
1
2,5-bis(4-methoxyphenyl)pyrazine (2c). Yield: 70 mg (75%); pale yellow powder, m.p: 223-225 . H
NMR (500 MHz, CDCl3)
13C NMR (126 MHz, CDCl3)
[M+H]+ Calcd for C18H16N2O2, 293.1282; found 293.1289.
δ 8.98 (s, 2H), 8.01 (d, J = 8.8 Hz, 4H), 7.04 (d, J = 8.8 Hz, 4H), 3.89 (s, 6H).
δ
161.11, 151.08, 138.50, 129.21, 128.35, 114.37, 55.42. HRMS (ESI) m/z:
1
2,5-di(thiophen-2-yl)pyrazine (2d). Yield: 88 mg (86%); yellowish shiny crystals, m.p: 272-274 . H
NMR (500 MHz, DMSO-d6)
δ
9.16 (s, 2H), 7.97 (d, J = 3.0 Hz, 2H), 7.76 (d, J = 4.8 Hz, 2H), 7.25 (t, J =
6

4.1 Hz, 2H). 13C NMR (126 MHz, DMSO)
δ 144.92, 140.55, 138.92, 128.95, 128.26, 125.67. HRMS
(ESI) m/z: [M+H]+ Calcd for C12H8N2S2, 245.0202; found 245.0207.
The monosubstituted precursors 1(a-b) were prepared as previously described in general procedure (A)
using one equivalents of (4-(diphenylamino)phenyl)boronic acid and (4-(dimethylamino)phenyl)boronic
acid.
2-bromo-5-(4-diphenylaminophenyl)pyrazine (1a). Yield: 734 mg (87%); yellow liquid. 1H NMR (500
MHz, CDCl3)
δ 8.70 (s, 1H), 8.65 (s, 1H), 7.84 (d, J = 8.2 Hz, 2H), 7.30 (t, J = 7.4 Hz, 4H), 7.15 (d, J =
8.5 Hz, 4H), 7.12 (d, J = 5.1 Hz, 2H), 7.09 (d, J = 7.1 Hz, 2H). 13C NMR (126 MHz, CDCl3)
δ
150.95,
149.93, 146.99, 146.55, 141.08, 137.74, 129.48, 127.86, 127.65, 125.28, 123.92, 122.28. HRMS (ESI)
m/z: [M+H]+ Calcd for C22H16BrN3, 402.0600; found 402.0593.
1
2-bromo-5-(4-dimethylaminophenyl)pyrazine (1b). Yield: 490 mg (84%); yellow powder. H NMR
(500 MHz, CDCl3)
δ 8.67 (d, J = 1.2 Hz, 1H), 8.60 (d, J = 1.2 Hz, 1H), 7.89 (d, J = 8.9 Hz, 2H), 6.78 (d,
J = 8.9 Hz, 2H), 3.05 (s, 6H). 13C NMR (126 MHz, CDCl3)
δ
151.71, 151.53, 146.28, 140.55, 136.49,
127.78, 122.34, 112.16, 40.20. HRMS (ESI) m/z: [M+H]+ Calcd for C12H12BrN3, 278.0287; found
278.0289.
Asymmetrical chromophores 2(e-j) were synthesized following general procedure (A) using one
equivalent of boronic acid derivative with electron withdrawing groups.
2-(4-(trifluoromethylphenyl)-5-(4-diphenylaminophenyl)pyrazine (2e). Yield: 87 mg (76%); orange
powder, m.p : 142-145 . 1H NMR (500 MHz, CDCl3)
δ 9.05 (s, 1H), 9.05 (s, 1H). 8.18 (d, J = 8.2 Hz,
2H), 7.95 (d, J = 8.7 Hz, 2H), 7.77 (d, J = 8.3 Hz, 2H), 7.31 (t, J = 7.8 Hz, 4H), 7.17 (d, J = 8.3 Hz, 6H),
7.10 (t, J = 7.4 Hz, 2H). 13C NMR (126 MHz, CDCl3)
δ
151.27, 149.81, 148.04, 147.08, 141.23, 140.81,
139.81, 131.28 (d, J = 32.6 Hz), 129.48, 128.83, 127.74, 127.20, 126.84, 125.97 (q, J = 3.7 Hz), 125.27,
123.86, 122.41. HRMS (ESI) m/z: [M+H]+ Calcd for C29H20F3N3, 468.1682; found 468.1683.
7

2-(4-cynophenyl)-5-(4-diphenylaminophenyl)pyrazine (2f). Yield: 78 mg (74%); canary powder, m.p:
196-197 . 1H NMR (500 MHz, CDCl3)
7.81 (d, J = 8.4 Hz, 2H), 7.31 (t, J = 7.9 Hz, 4H), 7.18 – 7.16 (m, 6H), 7.11 (t, J = 7.4 Hz, 2H). 13C NMR
δ
9.05 (s, 2H), 8.19 (d, J = 8.4 Hz, 2H), 7.95 (d, J = 8.8 Hz, 2H),
(126 MHz, CDCl3)
δ 151.58, 149.97, 147.29, 147.00, 141.28, 140.88, 140.63, 132.80, 129.50, 128.53,
127.80, 127.00, 125.33, 123.95, 122.26, 118.66, 112.89. HRMS (ESI) m/z: [M+H]+ Calcd for C29H20N4,
425.1761; found 425.1761.
2-(4-trifluoromethylphenyl)-5-(4-dimethylaminophenyl)pyrazine (2g). Yield : 84 mg (81%); pale
1
yellow crystals, m.p : 252-254 . H NMR (500 MHz, CDCl3)
δ 9.03 (s, 1H), 9.01 (s, 1H), 8.16 (d, J =
8.1 Hz, 2H), 8.01 (d, J = 8.8 Hz, 2H), 7.76 (d, J = 8.1 Hz, 2H), 6.83 (d, J = 8.7 Hz, 2H), 3.07 (s, 6H). 13C
NMR (126 MHz, CDCl3) 152.09, 151.72, 149.67, 140.40, 139.82, 138.05, 131.35 (d, J = 32.5 Hz),
δ
128.00, 127.25, 125.82 (q, J = 3.7 Hz), 123.49, 123.02, 112.17, 40.25. HRMS (ESI) m/z: [M+H]+ Calcd
for C19H16F3N3, 344.1369; found 344.1368.
2-(4-cyanophenyl)-5-(4-dimethylaminophenyl)pyrazine (2h). Yield: 78 mg (62%); olive green powder,
1
m.p: 251-253 . H NMR (500 MHz, CDCl3)
δ 9.03 (s, 1H), 9.00 (s, 1H), 8.16 (d, J = 8.2 Hz, 2H), 8.01
(d, J = 8.8 Hz, 2H), 7.79 (d, J = 8.2 Hz, 2H), 6.82 (d, J = 8.8 Hz, 2H), 3.07 (s, 6H). 13C NMR (126 MHz,
CDCl3) 152.17, 151.78, 146.29, 141.17, 140.93, 140.39, 132.75, 127.98, 126.80, 123.03, 118.77,
δ
112.52, 112.21, 40.21. HRMS (ESI) m/z: [M+H]+ Calcd for C19H16N4, 301.1448; found 301.1451.
2-(4-nitrophenyl)-5-(4-diphenylaminophenyl)pyrazine (2i). Yield: 67 mg (61%); orange powder, m.p:
200-202 . HRMS (ESI) m/z: [M+H]+ Calcd for C28H21N4O2, 445.1659; found 445.1655.
2-(4-dimethylaminophenyl)-5-(4-nitrophenyl)pyrazine (2j). Yield: 72 mg (63%); red powder, m.p:
213-215 . HRMS (ESI) m/z: [M+H]+ Calcd for C18H17N4O2, 321.1346; found 321.1347.
8

2.2.2 General procedure for synthesis of angular chromophores (B).
A mixture of 2,6-dichloropyrazine (3) (0.67 mmol), Pd(PPh3)4 (0.034 mmol), 2M K2CO3 (2 mL) and
corresponding boronic acid derivative (1.4 mmol) was dissolved in toluene (2 mL) and ethanol (1 mL).
The reaction mixture was irradiated in a MW at 70 for 30 min. After cooling to RT, the solvent was
evaporated in vacuum and the reaction mixture was diluted with distilled water and extracted with DCM.
The combined organic layers were dried with MgSO4, filtered and the solvent was evaporated under
reduced pressure. The residue was purified using column chromatography to afford the desired cross-
coupling compounds 4(a-d).
2,6-bis(4-diphenylaminophenyl)pyrazine (4a). Yield: 258 mg (68%); yellow powder, m.p: 230-232
1H NMR (500 MHz, CDCl3)
4.2 Hz, 8H), 7.15 (d, J = 3.1 Hz, 4H), 7.08 (t, J = 7.3 Hz, 4H). 13C NMR (126 MHz, CDCl3)
.
δ
8.82 (s, 2H), 8.00 (d, J = 8.7 Hz, 4H), 7.29 (t, J = 7.8 Hz, 8H), 7.17 (d, J =
δ
151.03,
149.44, 147.20, 138.48, 129.81, 129.39, 127.79, 125.02, 123.58, 122.65. HRMS (ESI) m/z: [M+H]+
Calcd for C40H30N4, 567.2543; found 567.2543.
2,6-bis(4-dimethylaminophenyl)pyrazine (4b). Yield: 158 mg (74%); pale brown powder, m.p: 212-
1
214 . H NMR (500 MHz, CDCl3)
δ
8.74 (s, 2H), 8.06 (d, J = 8.5 Hz, 4H), 6.81 (d, J = 8.5 Hz, 4H),
151.39, 151.36, 137.06, 127.83, 124.53, 112.16, 40.28.
3.03 (s, 12H). 13C NMR (126 MHz, CDCl3)
δ
HRMS (ESI) m/z: [M+H]+ Calcd for C20H22N4, 319.1917; found 319.1920.
2,6-bis(4-methoxyphenyl)pyrazine (4c). Yield: 190 mg (97%); white crystals, m.p: 141-143 . 1H NMR
(500 MHz, CDCl3)
δ
8.85 (s, 2H), 8.11 (d, J = 8.8 Hz, 4H), 7.05 (d, J = 8.8 Hz, 4H), 3.89 (s, 6H). 13C
161.11, 151.08, 138.50, 129.21, 128.35, 114.37, 55.42. HRMS (ESI) m/z:
NMR (126 MHz, CDCl3)
δ
[M+H]+ Calcd for C18H16N2O2, 293.1285; found 293.1287.
9

1
2,6-bis(thiophene-2-yl)pyrazine (4d). Yield: 137 mg (84%); beige solid, m.p: 98-100 . H NMR (500
MHz, CDCl3)
δ 8.76 (s, 2H), 7.74 (d, J = 3.2 Hz, 2H), 7.50 (d, J = 4.9 Hz, 2H), 7.17 (dd, J = 4.8, 3.9 Hz,
2H). 13C NMR (126 MHz, CDCl3)
δ
147.40, 141.32, 137.76, 129.07, 128.33, 126.00. HRMS (ESI) m/z:
[M+H]+ Calcd for C12H8N2S2, 245.0202; found 245.0207.
Mono-coupled products 3(a-b) were synthesized according to the general procedure (B) using one
equivalent of boronic acid derivative with electron donating group.
1
2-chloro-6-(4-diphenylaminophenyl)pyrazine (3a). Yield: 120 mg (50%); yellow solid. H NMR (500
MHz, CDCl3)
δ 8.70 (s, 1H), 8.65 (s, 1H), 7.84 (d, J = 8.2 Hz, 2H), 7.30 (m, 5H), 7.15 (m, 5H), 7.09 (d, J
= 7.1 Hz, 2H). 13C NMR (126 MHz, CDCl3)
δ
150.95, 149.93, 146.99, 146.55, 141.08, 137.74, 129.48,
127.86, 127.65, 125.28, 123.92, 122.28. HRMS (ESI) m/z: [M+H]+ Calcd for C22H16ClN3, 358.1106;
found 358.1105.
1
2-chloro-6-(4-dimethylaminophenyl)pyrazine (3b). Yield: 84 mg (54 %); yellow solid, H NMR (500
MHz, CDCl3)
δ 8.81 (s, 1H), 8.32 (s, 1H), 7.94 (d, J = 8.9 Hz, 2H), 6.76 (d, J = 8.9 Hz, 2H), 3.05 (s, 6H).
13C NMR (126 MHz, CDCl3)
δ
152.96, 151.88, 148.61, 139.95, 138.15, 128.17, 121.90, 112.01, 40.14.
HRMS (ESI) m/z: [M+H]+ Calcd for C12H12ClN3, 234.0793; found 234.0798.
Asymmetrical chromophores 4(e-j) were synthesized following general procedure (B) using one
equivalent of boronic acid derivative.
2-(4-trifluoromethylphenyl)-6-(4-diphenylaminophenyl)pyrazine (4e). Yield: 112 mg (86%); yellow
crystals, m.p : 113-115 . 1H NMR (500 MHz, DMSO-d6)
δ 9.24 (s, 1H), 9.21 (s, 1H), 8.47 (d, J = 8.2
Hz, 2H), 8.17 (d, J = 8.8 Hz, 2H), 7.92 (d, J = 8.3 Hz, 2H), 7.38 (t, J = 7.9 Hz, 4H), 7.14 (dd, J = 6.3, 5.6
Hz, 6H), 7.07 (d, J = 8.8 Hz, 2H).13C NMR (126 MHz, DMSO)
δ
150.86, 149.67, 149.16, 147.02, 141.23,
140.55, 140.33 (d, J = 26.7 Hz), 130.52, 130.25, 129.10, 128.60, 128.08, 126.36 (q, J = 3.78 Hz), 125.45,
124.53, 123.57, 122.11. HRMS (ESI) m/z: [M+H]+ Calcd for C29H20F3N3, 468.1682; found 468.1682.
10

2-(4-cynophenyl)-6-(4-diphenylaminophenyl)pyrazine (4f). Yield: 106 mg (87%); yellow solid, m.p :
186-188 . 1H NMR (500 MHz, CDCl3)
δ
8.97 (s, 1H), 8.91 (s, 1H), 8.25 (d, J = 8.4 Hz, 2H), 8.00 (d, J =
8.7 Hz, 2H), 7.81 (d, J = 8.4 Hz, 2H), 7.31 (t, J = 7.8 Hz, 4H), 7.17 (d, J = 6.7 Hz, 6H), 7.11 (t, J = 7.3
Hz, 2H). 13C NMR (126 MHz, CDCl3)
δ
151.66, 149.95, 149.20, 147.00, 140.81, 140.59, 138.93, 132.71,
129.48, 128.66, 127.87, 127.45, 125.26, 123.91, 122.23, 118.57, 113.24. HRMS (ESI) m/z: [M+H]+
Calcd for C29H20N4, 425.1761; found 425.1752.
2-(4-trifluoromethylphenyl)-6-(4-dimethylaminophenyl)pyrazine (4g). Yield: 88 mg (64%); lime
yellow powder, m.p: 161-163 . 1H NMR (500 MHz, CDCl3)
Hz, 2H), 8.08 (d, J = 6.5 Hz, 2H), 7.78 (d, J = 5.6 Hz, 2H), 6.83 (d, J = 6.3 Hz, 2H), 3.07 (s, 6H). 13C
δ 8.94 (s, 1H), 8.84 (s, 1H), 8.25 (d, J = 5.1
NMR (126 MHz, CDCl3)
δ 152.09, 151.72, 149.67, 140.40, 139.82, 138.05, 131.35 (d, J = 32.5 Hz),
128.00, 127.25, 125.82 (q, J = 3.7 Hz), 123.49, 123.02, 112.17, 40.25. HRMS (ESI) m/z: [M+H]+ Calcd
for C19H16F3N3, 344.1369; found 344.1373
2-(4-cyanophenyl)-6-(4-dimethylaminophenyl)pyrazine (4h). Yield: 92 mg (61%); yellow solid, m.p :
181-182 . 1H NMR (500 MHz, CDCl3)
9.0 Hz, 2H), 7.82 (d, J = 8.5 Hz, 2H), 6.83 (d, J = 8.9 Hz, 2H), 3.07 (s, 6H). 13C NMR (126 MHz, CDCl3)
δ 8.96 (s, 1H), 8.83 (s, 1H), 8.26 (d, J = 8.5 Hz, 2H), 8.07 (d, J =
δ
152.21, 151.79, 149.00, 141.24, 140.20, 138.03, 132.67, 128.02, 127.46, 123.23, 118.71, 113.03,
112.16, 40.23. HRMS (ESI) m/z: [M+H]+ Calcd for C19H16F3N4, 301.1448; found 301.1449.
2-(4-nitrophenyl)-6-(4-diphenylaminophenyl)pyrazine (4i). Yield: 102 mg (81%); yellow solid, m.p:
208-210 . 1H NMR (500 MHz, CDCl3)
δ 9.00 (s, 1H), 8.96 (s, 1H), 8.38 (d, J = 8.4 Hz, 2H), 8.32 (d, J =
8.4 Hz, 2H), 8.01 (d, J = 8.3 Hz, 2H), 7.32 (t, J = 7.6 Hz, 4H), 7.18 (d, J = 7.8 Hz, 6H), 7.11 (t, J = 7.3
Hz, 2H). 13C NMR (126 MHz, CDCl3)
δ
151.73, 150.01, 148.88, 148.61, 147.01, 142.64, 140.87, 139.17,
129.50, 128.61, 127.90, 127.76, 125.30, 124.18, 123.94, 122.24. HRMS (ESI) m/z: [M+H]+ Calcd for
C28H20N4O2, 445.1659; found 445.1654.
11

2-(4-dimethylaminophenyl)-6-(4-nitrophenyl)pyrazine (4j). Yield: 83 mg (58%); red powder, m.p:
195-196 . 1H NMR (500 MHz, CDCl3)
8.2 Hz, 2H), 8.08 (d, J = 8.3 Hz, 2H), 6.83 (d, J = 8.3 Hz, 2H), 3.08 (s, 6H). 13C NMR (126 MHz, CDCl3)
δ
8.97 (s, 1H), 8.86 (s, 1H), 8.37 (d, J = 8.2 Hz, 2H), 8.32 (d, J =
δ
152.29, 151.82, 148.68, 148.50, 143.01, 140.37, 138.16, 128.04, 127.72, 124.12, 123.12, 112.16, 40.23.
HRMS (ESI) m/z: [M+H]+ Calcd for C18H16N4O2, 321.1346; found 321.1350.
3. Results and discussion
3.1 Synthesis
In a recent article we reported the synthesis of various 2,5- and 2,6-disubstituted pyrazine
derivatives using Sonogashira cross-coupling reaction [36]. A similar synthetic sequence was developed
to prepare a series of dipolar symmetric and asymmetric chromophores consisting of electron donating
and withdrawing groups on the terminal position via microwave assisted palladium-catalysed approach.
Symmetric chromophores 2(a-d) were obtained by twofold Suzuki-Miyaura cross-coupling reaction. The
reaction of 2,5-dibromopyrazine (1) with various arylboronic acids afforded target D-A-D chromophores
in one step with satisfactory yields. The synthetic pathway for the chromophores studied in this paper is
illustrated in Scheme 1.
The synthetic route towards angular chromophores 4(a-d) starts from 2,6-dichloropyrazine (3)
which underwent Suzuki-Miyaura reaction with appropriate arylboronic acid under MW conditions as
shown in Scheme 2. Asymmetrical chromophores 2(e-j) and 4(e-j) were prepared in a two-step fashion
using exact stoichiometric amounts of boronic acid (diphenylaminophenylboronic acid and
dimethylaminophenylboronic acid) to ensure exclusive mono-substitution product 1(a-b) and 3(a-b).
These precursor were further reacted with the boronic acid bearing electron accepting groups (-CF3, -CN,
and -NO2) to produce corresponding linear 2(e–h) and bent 4(e–j) chromophores in good (58%) to
excellent yields (97%). However, the insolubility of compounds 2i and 2j in common solvents prevented
their characterization by NMR, although the HRMS spectra could be registered. All compounds are
soluble in common organic solvents such as chloroform, acetone, methanol, DCM, THF, and DMSO. All
12

the intermediates and final compounds were characterized by 1H NMR, 13C NMR spectroscopy, and high-
resolution mass spectrometry.
Scheme 1 Synthetic pathway to linear chromophores 2(a-j).
13

Scheme 2 Synthetic route to angular chromophores 4(a-j).
3.2 X-ray crystallography
Single crystals of compound 4c suitable for X-ray crystallographic analysis was cultivated from
DCM solution at ambient temperature. The molecular structure of 4c presented in Fig. 2. The molecule
adopts a coplanar geometry in such a way that the pyrazine and phenyl units lie in one plane. The
compound 4c crystallized in orthorhombic system with space group P2(1)2(1)2(1) and the unit cell
dimensions of a) 5.3585(2) Å, b) 10.4584(3) Å, and c) 26.1186(7) Å, whereas αꢀ
=ꢀ90°, βꢀ=ꢀ90°, and
γꢀ 90°. The crystal parameters, data collection and refinement parameters for the compound 4c are
=
ꢀ
summarized in Supplementary information (SI). Crystallographic data of the compound 4c were
deposited with the Cambridge Crystallographic Data Centre with CCDC 1964953.
Fig. 2 a) Single crystal structure of 4c. b) Molecular packing of 4c viewed along a axis (hydrogen atoms
are omitted for clarity).
3.3 Photophysical properties
The electronic spectra of both linear and angular chromophores can be easily tuned by varying
the strength of donor and acceptor units. Analysis has been carried out in 10 µM DCM solution and the
representative spectra are shown in Fig. 3 and related photophysical parameters are compiled in
Table 1. All linear compounds displayed two characteristic broad absorption peaks in the UV-vis
14

spectra. The higher energy absorption band is due to π-π* electronic transition while the other in
the low energy region is due to the CT ascribed to D-A electronic structure formed between donor
to acceptor units. As expected, chromophore 2a exhibited bathochromic shift, which is in agreement
with the stronger donor effect and extended π-conjugation of the amine group. A comparison in the
linear series showed the expected red-shift by increasing the donor strength, but hypsochromic
shift results when thiophene is attached to the pyrazine core. Substitution of electron-withdrawing
groups in 2(e-h) shows a relatively large Stoke shift of about 108-141 nm indicating a larger ICT
character for these compounds.
In addition, we also investigated the influence of molecular structure on the absorption
spectra, by comparing the absorption spectra of linear and bent position isomers. The main
excitation band of the bent chromophore is blue-shifted compared to their linear isomers, due to
twisted structure leading to poor conjugation between pyrazine and the substitution located at 2-
and 6-position. Communication between electron donor and acceptor units can be evaluated by
comparing the λmax values, which range from 335-406 nm. Thus the position of donor-acceptor
and molecular geometry influence the optical properties of target chromophores.
Fluorescence measurements have been performed on both series after excitation at the maximum
of absorbance. Compared to the absorption spectra, the fluorescence profile varies dramatically with
respect to the end groups. The representative spectra are shown in Fig. 4. The presence of fluorescence in
solution as well as in solid-state was detected (Fig. 5). These yellowish-orange bright fluorescent
compounds exhibited broad emission spectra in the region of 391-547 nm with obvious bathochromic
shifts with the increase of solvent polarities. The introduction of acceptor moiety in compounds 2(e-h)
leads to bathochromic shift of the emission band. Compound 2f demonstrated a maximum wavelength at
547 nm among all chromophores due its longest conjugation between N,N-dimethyl aniline and acceptor
benzonitrile. The emission spectrum of 4g and 4h exhibited two peaks in the emission spectra in the range
396-409 nm and 512-533 nm unlike other compounds. The dual fluorescence could be attributed to
15

charge transfer (CT) and twisted intramolecular charge transfer (TICT) [37]. The normalized absorption
as well as emission spectra of remaining chromophores is represented in the SI data. The fluorescence
quantum yields of linear 2(a-h) and angular 4(a-h) compounds strongly depend on their molecular
structure (Table 1). The fluorescence e ciency of triphenylamine-substituted pyrazine 2e (ФF = 0.57) is
higher than angular counterpart 4e (ФF = 0.38).
Fig. 3 Absorption spectra of a) linear and b) angular chromophores in DCM (10-5 M),
Fig. 4 Fluorescence spectra of a) linear and b) angular chromophores in DCM, when excited at their
absorption maxima.
16

Fig. 5 Photographs of linear and bent chromophores, a) DCM solutions under UV light, b) solids under
UV light (365 nm), c) solids under natural light.
3.4 Solvatochromism
The D-A functionalised groups are expected to enhance the absorbance shifts in polar solvents
because of solvent induced perturbations of the ground states and excited states [38]. Solvent-dependent
changes in the spectra of these compounds provide a sensitive response to changes in the surrounding
environment. The solvents of various polarities were selected and absorption and emission for 2a and 4a
were recorded as shown in Fig. 6. The absorption spectra of 2a exhibited only 16 nm variation when the
solvent changed from nonpolar (toluene) to polar solvent (DMSO), signifying a weak solvent e ect in the
ground state. In contrast, the fluorescence spectra showed a dramatic bathochromic shift from 543 to 578
nm, with pronounced Stokes shift (Fig. 6b). This remarkable result indicates the strong dipole relaxation
of the solvent molecules around the photoexcited 2a, leading to emission at a longer wavelength. Parallel
investigation was carried for bent chromophores 4a, which displayed similar results (Fig. 7).
17

Fig. 6 a) Absorption and b) emission spectra of compound 2a recorded in solvents of varying polarities.
Fig. 7 a) Absorption and b) Florescence spectra of compound 4a recorded in different solvents.
3.5 Acidochromism
To assess how the absorption spectra change upon removal of the charge transfer absorbance,
protonation studies of 2a (linear) and 4a (bent) were undertaken. A representative illustration of changes
in absorption spectra 2a and 4a on gradual addition of trifluoroacetic acid (TFA) is shown in Fig. 8. As
the concentration of TFA is increased, the energy peaks at 295 and 406 nm decreases and a new peak at
547 nm grows in 2a, whereas in case of 4a initially peak at 385 nm decreases and then increases with new
peak at 500 nm. The addition of a large excess of TFA produces bathochromic shift [19]. Protonation by
TFA disrupts electron-donating capability of nitrogen atom for charge transfer [39, 40]. Finally, the
18

addition of triethylamine (TEA) to fully protonated 2a returns the original absorption spectrum as
pronated species is neutralized. This illustrates the reversibility of the process and is analogous to our
earlier protonation studies on pyrazine core [41-43]. The emission spectrum of chromophores 2a and 4a
on the gradual addition of TFA leads to decrease in the intensity, as observed in Fig. 9. The solution of
these chromophores underwent a significant color change in the presence of TFA and the original color is
regained upon the addition of TEA, which neutralize the e ect of TFA. On the basis of the above
observation, it is assumed that the chromophores exhibit acid-base equilibrium.
Fig. 8 Changes observed in the absorption spectra of 2a and 4a in DCM when protonated with TFA.
Fig. 9 Changes observed in the emission spectra of 2a and 4a in DCM when protonated with TFA.
19

3.6 Thermal properties
T
hermal stability of the chromophores is one of the key requirements for practical
applications
.
The thermal properties of chromophores 2(a-h) and 4(a-j) were investigated by
o
thermogravimetric analysis (TGA) under nitrogen atmosphere with heating rate of 10 C min-1. All
the compounds exhibited good thermal stabilities with the decomposition temperature (Td) higher than
o
o
200 C (201-316 C). Representative TGA plots are shown in Fig. 10 and remaining TGA plots are
included in SI data. Chromophores 4i functionalised with strong electron and acceptor group
o
exhibited higher decomposition temperature of 316 C (Table 1). The thermal behaviour of
compounds 2a, 2e, 4a, and 4e was studied by differential scanning calorimetry (DSC) as shown in
Fig. 10b. All compounds displayed exothermic transitions corresponding to the melting point. The
o
highest thermal stability was observed for 2e (Tm = 230 C) with sharp exothermic peak, while
o
compound 4e displayed the lowest thermal stability (Tm = 114 C). The thermal study indicates that the
modified groups influence the thermal properties of the chromophores and these flurophores can
have great application in organic electronics.
Fig. 10 Representative TGA (a) and DSC (b) curves of chromophores determined at a heating rate of 10
oC min-1 under nitrogen atmosphere
.
20

3.7 Electrochemical properties
.
Electrochemical measurements of all chromophores 2(a-h) and 4(a-j) were estimated by
measuring oxidation potentials using cyclic voltammetry in DCM solutions containing 0.1 M Bu4NPF6
electrolyte in a three electrode cell at a scan rate of 20 V/s. The acquired data are summarized in Table 1,
representative cyclic voltammogram are shown in Fig. 11 and SI data. The half-wave potential of the
first oxidation is recalculated to HOMO and LUMO energies. The CV analysis showed low oxidation
potential of the compounds which occurred between 0.53 and 1.49 V. Both linear and angular exhibit
clear oxidation and reduction peaks, However linear chromophores exhibit lower band gap (Eg) compared
to angular chromophores. In case of 2a, the compound exhibits two distinct oxidation peaks reflecting the
presence of two equivalent triphenylamine donor appended to pyrazine [44, 45]. In case of D-A-A
molecules 2(e-h) and 4(e-j), the variations in oxidation and reduction are predominantly function of the
acceptor moiety (-CF3, -CN, -NO2). The corresponding electrochemical gaps range from 2.62 to 3.28 eV,
which is affected by the variation in the first oxidation potential. The polarity and extent of ICT influence
the electronic behaviour of the chromophores. The orientation of the molecule significantly affect band
gap as observed from both optical and electrochemical measurements.
Fig. 11 Cyclic Voltammograms of chromophores in DCM solution with 0.1M nBu4NPF6 used as the
supporting electrolyte at a sweeping rate 20 m V/s.
21

Table 1 Summarized data of optical, electrochemical, and thermal properties of chromophores.
λabs (nm)a
295, 406
349, 398
308, 350
316, 366
294, 397
297, 406
278, 390
284, 402
305, 385
330, 381
286, 342
294, 356
292, 374
292, 373
250, 337
260, 335
302, 370
322, 356
ε (mM-1cm-1)a λem (nm)b
Stokes shift (nm)
Eox(V)
0.67
0.76
1.39
1.49
0.84
0.83
0.85
0.83
0.6
HOMO(eV)d
-3.73
-3.64
-3.01
-2.91
-3.56
-3.57
-3.55
-3.57
-3.8
LUMO(eV)e
-1.07
-0.91
-0.44
0.13
Td(oC)g
307
249
218
213
201
305
213
226
283
259
209
207
273
314
205
220
316
240
f
R
ФF
109.2
50.4
42.7
55.1
38.1
66.4
45.8
58.4
122.1
82.8
89.9
45.5
55.7
71.5
52.3
67.9
67.7
76.8
485
476
79
78
0.19
0.39
0.10
0.31
0.57
0.34
0.45
0.37
0.16
0.23
0.18
0.37
0.38
0.31
0.41
0.33
-
2a
2b
2c
2d
2e
2f
407
57
416
50
521
124
141
108
115
93
-0.86
-0.95
-0.83
-0.94
-0.95
-0.88
-0.4
547
498
2g
2h
4a
4b
4c
4d
4e
4f
517
478
466
85
0.65
0.72
0.73
0.57
0.56
0.71
0.7
-3.75
-3.68
-3.67
-3.83
-3.84
-3.69
-3.7
391
49
400
44
-0.5
509
135
163
175
198
-
-1.0
536
-1.05
-0.83
-0.89
-1.14
-1.07
396, 512
409, 533
4g
4h
4i
c
-
0.53
0.68
-3.87
-3.72
c
-
-
-
4j
a Maximal absorption peak in DCM. ε: molar absorption coefficient.
b Maximal emission peak in DCM, when excited at their absorption maxima.
c No signals were detected
d Electrochemical method was used to obtain the HOMO energy estimated from onset voltage of the first
oxidation potential. HOMO = - (Eox+4.4),
e LUMO = HOMO + Egopt. f ФF = Fluorescence quantum yield determined relative to Rhodamine 6G.
gTd observed from TGA loss at 10 oC min-1 under nitrogen.
22

3.8 Computational Modeling
In order to investigate the geometric and electronic properties quantum-chemical calculations
have been performed on a series of functionalized linear and bent chromophores. The spatial and electron
properties of the compounds were optimized by density functional theory (DFT) calculations with the
B3LYP exchange correlation functional under the 6-31G (d) basis set implemented in the Gaussian 09
program. The representative visualizations of the HOMO and LUMO levels are shown in Fig. 12. The
HOMO-LUMO energy gaps, dipole moment (µ), and first-order hyperpolarizability (β) of the
chromophores are summarized in Table 2.
The pyrazine core acts as acceptor in all chromophores. The calculated HOMO and LUMO
energies range from -5.88/-4.84 to -2.59/-0.98 eV. Their values in the series are dictated by the orientation
of the molecule. The band gaps (Eg) of linear and bent chromophores are in the range of 2.64- 4.32 eV.
The energy gaps obtained from the theoretical calculations are described in Fig. 13. The enhanced
electron density localization on the donor group is similar for both linear 2(a-d) and bent 4(a-d)
chromophores. The HOMO levels are localized on pyrazine and spread over the given donor group.
whereas, in chromophores 2(e-j) and 4(e-j) the HOMO level is associated with the donor group while the
LUMO level depends on the nature of the acceptor groups, because of the strong electron-withdrawing
ability of the -CF3, -CN and -NO2 moieties. The electron density is transferred from the donor to the
acceptor through the π-conjugated spacer acting as an electron relay. Thus, tailoring the nature and
structure of donor and acceptor allows tuning of the dipole formation. The stronger the electron-donating
strength of the donor leads to higher λmax and µβ value [46, 47]. The calculated dipole moments of the
chromophores range from 0 to 10.2 D. Chromophores 2c and 2d showed zero dipole moment. The
presences of a strong electron acceptor resulted in a large dipole moment and also possess highest
β value
as observed in 2(f-j), which is closely related to the strength of donor and acceptor. Apart from a slight
variation for each chromophore, there is a satisfactory correlation between data obtained from the
theoretical calculations and UV-vis absorption.
23

R
HOMO
LUMO
R
HOMO
LUMO
2a
4a
2b
2c
2d
2e
4b
4c
4d
4e
2f
2g
2h
2i
4f
4g
4h
4i
2j
4j
Fig. 12 Molecular orbital diagrams of linear and angular chromophores, calculated with B3LYP/6-31G
(d) wave function
24

Table 2 Calculated and experimental HOMO-LUMO band gap for linear and angular chromophores.
Calculated
Eg (eV)a
Experimental
Chromophore
HOMO (eV)a
LUMO (eV)a
µb
βtot (10-30 esu)c
Egopt (eV)d
-4.84
-4.69
-5.38
-5.64
-5.14
-5.21
-5.21
-5.29
-5.24
-5.34
-4.97
-4.89
-5.61
-5.88
-5.16
-5.22
-5.25
-5.33
-5.24
-5.35
-1.55
-1.12
-1.47
-1.97
-1.94
-2.19
-1.75
-2.02
-2.59
-2.48
-1.38
-0.98
-1.29
-1.72
-1.75
-2.06
-1.67
-1.95
-2.60
-2.51
3.28
3.56
3.90
3.67
3.19
3.01
3.46
3.26
2.64
2.85
3.58
3.91
4.31
4.16
3.40
3.15
3.58
3.38
2.64
2.84
0.03
0.62
0
0.03
0.31
0
2.66
2.73
3.20
3.04
2.70
2.62
2.72
2.63
-
2a
2b
2c
2d
2e
2f
0
0
5.08
7.71
7.18
9.74
8.27
10.26
2.08
4.03
0.17
1.49
4.0
2.48
11.7
3.8
2g
2h
2i
9.1
9.3
7.6
-
2j
1.1
2.85
2.87
3.28
3.17
2.83
2.79
2.86
2.81
2.73
2.65
4a
4b
4c
4d
4e
4f
1.6
0.45
0.57
0.91
7.23
2.3
6.16
5.81
7.73
6.48
8.02
4g
4h
4i
5.70
5.44
4.82
4j
a HOMO-LUMO and band gap from DFT calculation
.
b µ is the total dipole moment.
c βtot is the first-order hyperpolarizability calculated using DFT quantum mechanical method.
opt
d Estimated from the absorption edge of UV spectra Eg
=
1240/λonest
25

Fig. 13 Energy diagram of the frontier molecular orbitals of chromophores estimated by DFTcalculations.
4. Conclusions
In this work, we have synthesized two series of diarylpyrazine-based fluorescent dyes decorated
with electron donating and electron accepting moieties in a specific chromophoric position. These dyes
are outstanding by their bright emission regardless of the surrounding environment and displayed a
dramatic and reversible color change upon addition of acid. The rational design of such chromophores, in
terms of strength of donor and acceptor moieties and their simultaneous combination in the structure and
position in which they are anchored, modulates their photophysical properties to endow them with high
charge separation. The results of absorption spectroscopy, cyclic voltammetry, thermal properties and
theoretical calculations suggest that diarylpyrazine-based fluorophores can be regarded as excellent
materials for fluorescent material applications and luminescent pH sensors.
Acknowledgement
This research was supported by the Basic Science Research Program through the National Research
Foundation of Korea (NRF), funded by the Ministry of Education (No.2016R1D1A1B04932654) in
Korea.
26

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