Topochemically photoreacted fluorescent dimers of 2,3-dicyanopyrazines

Article abstract of DOI:10.1016/j.dyepig.2011.10.015

Novel fluorescent materials derived from 2,3-dicyanopyrazine were synthesized and subjected to photodimerization reaction. Styryl substituents were attached by Wittig reaction, and the [2+2] photocycloaddition of the 2,3-dicyanopyrazine, either in solutio

Full text of DOI:10.1016/j.dyepig.2011.10.015

Dyes and Pigments 94 (2012) 49e54
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Topochemically photoreacted fluorescent dimers of 2,3-dicyanopyrazines
*
Chun Keun Jang, Cheol Jun Song, Sung Jae Jung, Jae Yun Jaung
Department of Organic and Nano Engineering, Hanyang University, Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel fluorescent materials derived from 2,3-dicyanopyrazine were synthesized and subjected to pho-
todimerization reaction. Styryl substituents were attached by Wittig reaction, and the [2þ2] photo-
cycloaddition of the 2,3-dicyanopyrazine, either in solution or as a thin film, was studied with irradiation
under a high-pressure Hg lamp. The resulting compounds were characterized by 1H NMR, FT-IR and
elemental analysis. Spectral changes of UVevisible absorption intensity and fluorescent intensity were
Received 16 July 2011
Received in revised form
24 October 2011
Accepted 24 October 2011
Available online 2 November 2011
examined at specific exposure intervals. While the cyclobutane ring of dimers induced a discrete
p-
conjugation with aryl substituents to show a hypsochromic shift of absorption and emission spectra, the
fluorescence intensity was increased and the specific lowest unoccupied molecular orbital (LUMO) levels
were formed compared to monomers.
Keywords:
2,3-Dicyanopyrazine
Cyclobutane
[2þ2] Photocycloaddition
Wittig reaction
Fluorescence
Ó 2011 Elsevier Ltd. All rights reserved.
Photo-printing
1. Introduction
Our efforts in this study were focused on the new fluorescence
materials resulting from the 2,3-dicyanopyrazine system under-
Since the importance of functional dyes has increased with the
rapid progress of information and communication industries,
a variety of functional dye molecules have been reported and many
synthetic methods, mechanisms, and specific properties have been
demonstrated [1e3].
We are interested in the 2,3-dicyanopyrazine chromophores
because of their wide range of applications, such as biological active
materials, nonlinear optical materials, and emitters for electrolumi-
nescence(EL)devices. A greatdealofinterest hasbeenfocusedonthe
synthesis and characteristics for functional materials based on 2,3-
dicyanopyrazine chromophore because they have strong electron-
withdrawing ability due to the nitrile group on the pyrazine ring,
and they show a strong fluorescence even in solid state [4e7].
going a photocycloaddition reaction. The resulting compound has
advantages in terms of thermal and chemical stability. Specifically,
the introduction of a t-butyl unit to 2,3-dicyanopyrazine was
studied and tested to verify the improvement in structure stability
and solubility. The resulting photochemical properties were
studied for application as an optical waveguide and photoresist.
2. Experimental
Compounds were identified and their properties were
measured using the following techniques. Flash chromatography
was performed by using Merck-EM type 60 (230e400 mesh) silica
gel (flash). Melting points were obtained with a capillary melting
point apparatus and were uncorrected. Elemental analysis was
checked by using Flash EA-1112 analyzer. 1H NMR spectra were
recorded on a VARIAN UnityInova 300 MHz FT-NMR spectrometer.
UVevisible and fluorescence spectra were measured using SCINCO
S-4100 and SHIMADZU RF-5301PC spectrophotometers.
Electromagnetic radiation induces
a chemical change in
a molecule due to the absorption of a photon, which transforms the
state of molecular orbitals [8e11]. The photochemical [2þ2]
cycloaddition reactions have been the subject of extensive inves-
tigations and have played an important role in the development of
mechanistic organic photochemistry [12e16]. Stilbene has been
extensively studied as it has an absorption band in the near ultra-
violet (UV) region and only on the photochemical reactive site,
which yields simple reaction products.
The fluorescence quantum yield (F) was determined by optically
dilute measurement with Rhodamine B (Ff ¼ 0.69 in ethanol) as
a reference and using the following net equation [17].
!
!
ꢀ
ꢁ
nA2
n2ref
UVref
PLref
PLA
UVA
FA
¼
Fref
* Corresponding author. Tel.: þ82 2 2220 0492; fax: þ82 2 2220 4092.
E-mail address: jjy1004@hanayang.ac.kr (J.Y. Jaung).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.10.015

50
C.K. Jang et al. / Dyes and Pigments 94 (2012) 49e54
In this equation, PL is the integrated area under the corrected
emission spectrum, and UV is the absorbance of the solution at the
excitation wavelength . Subscripts A and ref refer to the unknown
2.4. General procedure to synthesize 5
l
Br2 (10.97 g, 0.07 mol) was added dropwise to a solution of 1-(4-
tert-butylphenyl)propane-1,2-dione 4 (20.0 g, 0.01 mol) in 100 ml
CHCl3 with stirring at 10 ꢀC. After 2 h at room temperature, the
mixture was diluted with ice water and extracted with 300 ml of
CHCl3. The organic layer was washed with water and dried with
anhydrous sodium sulfate. The mixture was concentrated under
reduced pressure and the crude product was purified by column
chromatography over silica gel with EA:n-hexane ¼ 1:3 as the
eluent to form 5 (yellow liquid, 94%); 1H NMR (300 MHz, CDCl3)
and reference solutions, respectively, and n is the refractive index of
the corresponding solutions. Cyclic voltammetry (CV) experiments
were performed by BAS-100 electrochemical analyzer with a three-
electrode configuration consisting of a Pt disk working electrode,
Ag/AgCl reference electrode and Pt wire counter electrode. All
measurements were in acetonitrile with 0.1 M tetra-n-buty-
lammonium tetrafluroroborate (TBABF4) as a supporting electro-
lyte. Their electronic level was calculated from the difference
between their onset redox potential and the half-wave potential of
ferrocene.
d
7.85 (d, J ¼ 8.7 Hz, AreH, 2H), 7.49 (d, J ¼ 8.7 Hz, AreH, 2H), 3.72 (s,
BreCeH, 2H), 1.35 (s, -t-Bu, 9H); Elemental Anal. Calcd. For
C13H15BrO2: C, 55.14; H, 5.34. Found: C, 55.71; H, 5.66; GCeMS(m/z)
283.06 Calcd. 283.16.
2.1. General procedure to synthesize 2
2.5. General procedure to synthesize 6
Br2 (89.47 g, 0.559 mol) was added dropwise to a solution of 1-
(4-tert-butylphenyl)propan-1-one 1 (152.0 g, 0.799 mol) in 300 ml
of CHCl3 with stirring at 10 ꢀC. After 2 h at room temperature, the
mixture was diluted with ice water and extracted with 500 ml of
CHCl3. The organic layer was washed with water and dried with
anhydrous sodium sulfate. The mixture was concentrated under
reduced pressure and the crude product was purified by column
chromatography over silica gel with EA:n-hexane ¼ 1:3 as the
eluent to form 2 (yellow liquid, 94%); 1H NMR (300 MHz, CDCl3)
A solution of 3-bromo-1-(4-tert-butylphenyl)propane-1,2-dione
(40 g, 0.14 mol), 2,3-diaminomaleonitrile (15.2 g, 0.14 mol) and
a small amount of p-toluenesulfonic acid as a catalyst in methanol
(200 ml) were refluxed for 2 h. After the reaction was complete, the
mixture was cooled to room temperature. Methanol was added into
the mixture, which was then stirred for 1 h at room temperature.
The generated precipitate was filtered and washed with methanol.
The crude product was purified by column chromatography over
silica gel with EA:n-hexane ¼ 1:3 as the eluent to form 6 (white
d
7.92 (d, J ¼ 8.7 Hz, AreH, 2H), 7.48 (d, J ¼ 8.7 Hz, AreH, 2H), 5.20
(q, BreCeH, 1H), 2.11 (d, J ¼ 6.8 Hz, eCH3, 3H), 1.40 (s, t-Bu, 9H);
Elemental Anal. Calcd. For C13H17BrO: C, 58.01; H, 6.37. Found: C,
58.19; H, 6.41; GCeMS(m/z) 269.04 Calcd. 269.18.
solid, 76%); mp 134 ꢀC; 1H NMR (300 MHz, CDCl3)
d 7.63 (d,
J ¼ 8.7 Hz, AreH, 2H), 7.59 (d, J ¼ 8.7 Hz, AreH, 2H), 3.81 (s,
BreCeH, 2H), 1.38 (s, -t-Bu, 9H); Elemental Anal. Calcd. For
C17H15BrN4: C, 57.48; H, 4.26; N, 15.77. Found: C, 57.14; H, 4.76; N.
15.39; GCeMS(m/z) 355.07 Calcd. 355.23.
2.2. General procedure to synthesize 3
2.6. General procedure to synthesize 7
Brominated precursor 2 (202 g, 0.75 mol) and potassium acetate
(257.6 g, 2.62 mol) in 500 ml of acetone were refluxed for 12 h. The
solution was filtered and the filtrate was evaporated using a rotary
evaporator. The acetoxy group-substituted precursor was hydro-
lyzed in 300 ml of 10% NaOH methanolic solution under reflux
condition for 1 h. After extracting with 500 ml ethyl acetate (EA)
and 600 ml of water, the organic layer was dried with sodium
sulfate and evaporated in vacuo. The crude product was purified by
column chromatography over silica gel with EA:n-hexane ¼ 1:3 as
the eluent to form 3 (yellow liquid, 86%); 1H NMR (300 MHz, CDCl3)
A solution of triphenylphosphine 44.3 g (0.16 mol) in toluene
(150 ml) was added dropwise into the mixture of 6 40 g (0.11 mol)
in toluene 200 ml at 10 ꢀC and the mixture was stirred for 1 h at
room temperature and refluxed for 5 h. After the reaction was
completed, the mixture was hot-filtered and washed by toluene.
The filtered cake was recrystallized by n-hexane. 7 (organic solid
57%): m.p. 207 ꢀC; 1H NMR (DMSO-d6)
d 7.65 (m, PPh3, 15H), 7.63 (d,
J ¼ 8.7 Hz, AreH, 2H), 7.59 (d, J ¼ 8.7 Hz, AreH, 2H), 4.02 (s, CH2,
2H), 1.38 (s, -t-Bu, 9H); Elemental Anal. Calcd. For C35H30BrN4P: C,
68.07; H, 4.90; N, 9.07. Found: C, 68.49; H, 5.37; N. 9.38; MALDI-
TOF-Ms (m/z) 617.34 Calcd. 617.52.
d
7.86 (d, J ¼ 9.0 Hz, AreH, 2H), 7.51 (d, J ¼ 8.7 Hz, AreH, 2H), 5.16
(m, 1H), 3.81 (s, eOH, 1H), 1.35 (s, -t-Bu, 9H), 0.90 (d, J ¼ 6.8 Hz,
eCH3, 3H); Elemental Anal. Calcd. For C13H18O2: C, 75.69; H, 8.80.
Found: C, 75.36; H, 9.01; GCeMS(m/z) 206.19 Calcd. 206.28.
2.7. General procedure to synthesize 8
A solution of [(3-(4-tert-butylphenyl)-5,6-dicyanopyrazin-2-yl)
methyl]triphenyl phosphonium bromide (4.8 g, 7.77 mmol) in
anhydrous N,N-dimethylformamide (DMF, 10 ml) under N2 atmo-
sphere was added dropwise for 30 min to sodium hydride (0.40 g,
9.32 mmol) in anhydrous DMF (7 ml) at 0e5 ꢀC and the mixture
was stirred for 1 h at room temperature. When the reaction was
completed, the same equivalent of benzaldehydes was slowly
added. The reaction mixture was refluxed under N2 atmosphere.
After the reaction was completed, the mixture was cooled and
methanol was added. The mixture was then filtered and washed
with methanol. The residue was chromatographed on silica gel
using EA:n-hexane ¼ 1:3 as the eluent to form 8a (yellow solid,
2.3. General procedure to synthesize 4
1-(4-tert-Butylphenyl)-2-hydroxypropan-1-one
3
(128 g,
0.62 mol) was added to a mixture of CuSO4$5H2O (334 g, 1.33 mol)
in 79 ml of pyridine and 18 ml of water. The mixture was refluxed
and the reaction progress was monitored using thin-layer chro-
matography (silica gel and EA:n-hexane ¼ 1:3). Then, the mixture
was extracted with 500 ml EA and 600 ml water. The organic layer
was dried with sodium sulfate and evaporated in vacuo. The crude
product was purified by column chromatography over silica gel
with EA:n-hexane ¼ 1:3 as the eluent to form 4 (yellow liquid,
75%); 1H NMR (300 MHz, CDCl3)
(d, J ¼ 8.7 Hz, AreH, 2H), 1.34 (s, -t-Bu, 9H), 0.90 (s, eCH3, 3H);
Elemental Anal. Calcd. For C13H16O2: C, 76.44; H, 7.90. Found: C,
76.64; H, 8.09; GCeMS(m/z) 204.04 Calcd. 204.26.
d
7.92 (d, J ¼ 9.0 Hz, AreH, 2H), 7.51
51%); m.p. 104 ꢀC; IR (KBr pellet):
n
(cmꢁ1) 2231 (C^N), 1616 (C]C),
1498, 1376; 1H NMR (300 MHz, CDCl3)
7.70 (d, 2H, J ¼ 8.7 Hz), 7.61 (d, 2H, J ¼ 8.7 Hz), 7.58 (m, 2H), 7.43 (m,
d
8.19 (d, 1H, J ¼ 15.6 Hz),
3H), 7.35 (d, 1H, J ¼ 15.6 Hz), 1.41 (s, t-Bu, 9H); 13C NMR(300 MHz,

C.K. Jang et al. / Dyes and Pigments 94 (2012) 49e54
51
CDCl3)
d
155.1, 154.6, 151.3, 137.5, 133.0, 130.7, 131.5, 131.2, 128.6,
N, 13.32. Found: C, 79.46; H, 6.95; N. 13.58 MALDI-TOF-Ms spectra
128.5, 127.9, 125.5, 125.4, 123.5, 117.1, 34.2, 31.3; Ff 0.011; Elemental
Anal. Calcd. For C24H20N4: C, 79.10; H, 5.53; N, 15.37. Found: C,
78.62; H, 5.88; N. 15.63; GCeMS(m/z) 364.12 Calcd. 364.44.
(m/z) 840.68 Calcd. 841.10.
9d: m.p.; 194 ꢀC; 1H NMR(300 MHz, CDCl3)
d 8.31 (d, AreH, 2H,
J ¼ 8.4 Hz), 7.51 (d, AreH, 4H, J ¼ 8.4 Hz), 7.27 (d, AreH, 2H,
J ¼ 8.4 Hz), 7.18 (d, AreH, 4H, J ¼ 8.4 Hz), 6.90 (d, AreH, 4H,
J ¼ 8.4 Hz), 5.01 (dd, J1 ¼ 10.0 Hz, J2 ¼ 6.9 Hz, 2H), 4.52 (dd,
J1 ¼10.0 Hz, J2 ¼ 6.9 Hz, 2H), 3.86 (s, OeCH3, 6H), 1.41 (s, t-Bu, 18H);
8b (yellow solid, 54%) m.p. 108 ꢀC; IR (KBr pellet): (cmꢁ1) 2227
n
(C^N), 1619, 1604 (C]C), 1498, 1375; 1H NMR (300 MHz, CDCl3)
d
8.16 (d, 1H, J ¼ 15.3 Hz), 7.70 (d, 2H, J ¼ 8.4 Hz), 7.61 (d, 2H,
J ¼ 8.4 Hz), 7.48 (d, 2H, J ¼ 8.4 Hz), 7.33 (d, 1H, J ¼ 15.6 Hz), 7.27 (d,
13C NMR(300 MHz, CDCl3)
d 160.0, 158.7, 157.3, 151.3, 133.4, 130.4,
2H, J ¼ 8.4 Hz), 2.4 (s, 3H), 1.41 (s, t-Bu, 9H); 13C NMR(300 MHz,
129.9, 128.8, 127.6, 125.5, 125.4, 117.1, 114.1, 55.8, 46.7, 45.1, 34.8,
32.9; Ff 0.76; Elemental Anal. Calcd. For C50H44N8O2: C, 76.12; H,
5.62; N, 14.20; Found: C, 75.97; H, 5.83; N. 14.44 MALDI-TOF-Ms
spectra (m/z) 788.45 Calcd. 788.94.
CDCl3)
d 155.0, 154.0, 150.9, 138.4, 137.6, 136.5, 133.0, 131.2, 130.7,
128.6,128.5, 126.6, 127.4,127.0,123.5, 122.5, 117.4, 34.2, 31.6, 21.3; Ff
0.021; Elemental Anal. Calcd. For C25H22N4: C, 79.34; H, 5.86; N,
14.80. Found: C, 78.94; H, 5.94; N. 14.65; GCeMS(m/z) 378.18 Calcd.
378.47.
3. Results and discussion
8c (yellow solid, 52%) m.p. 102 ꢀC; IR (KBr pellet): (cmꢁ1) 2237
n
(C^N), 1621, 1604 (C]C), 1500, 1375; 1H NMR (300 MHz, CDCl3)
3.1. Synthesis
d
8.15 (d, 1H, J ¼ 15.6 Hz), 7.70 (d, 2H, J ¼ 8.4 Hz), 7.61 (d, 2H,
J ¼ 8.4 Hz), 7.5 (d, 2H, J ¼ 8.4 Hz), 7.46 (d, 2H, J ¼ 8.4 Hz), 7.27 (d, 1H,
1-(4-tert-butylphenyl)propan-1-one 1 was synthesized using
a previously described method [18]. It was treated with one
J ¼ 15.6 Hz), 1.41 (s, 9H), 1.34 (s, t-Bu, 9H); 13C NMR(300 MHz,
CDCl3)
d
155.6, 154.6, 151.3, 150.5, 139.0, 133.0, 131.2, 131.5, 130.7,
equivalent of bromine in chloroform to produce an 85% yield of
brominated 4-(tert-butyl)alkyl phenone 2. The reaction of
a
-
-
128.2, 125.5, 124.9, 123.5, 117.1, 34.6, 30.8; Ff 0.017; Elemental Anal.
Calcd. For C28H28N4: C, 79.97; H, 6.71; N, 13.32. Found: C, 79.43; H,
6.86; N. 13.48; GCeMS(m/z) 420.28 Calcd. 420.55.
a
brominated ketone with an excess (3.5 equiv.) of anhydrous
potassium acetate in acetone afforded -acetoxylated 4-(tert-butyl)
alkyl phenone [19,20]. This compound was reacted with 10%
methanolic NaOH under reflux condition to produce the corre-
sponding compound of 1-(4-tert-butylphenyl)-2-hydroxypropan-
a
8d (yellow solid, 51%) m.p. 113 ꢀC; IR (KBr pellet):
n
(cmꢁ1) 2239
(C^N), 1598 (C]C), 1494, 1257 (CeO), 1172; 1H NMR (300 MHz,
CDCl3)
d
8.16 (d, 1H, J ¼ 15.6 Hz), 7.70 (d, 2H, J ¼ 8.4 Hz), 7.61 (d, 2H,
J ¼ 8.4 Hz), 7.52 (d, 2H, J ¼ 8.4 Hz), 7.27 (d, 1H, J ¼ 15.6 Hz), 6.95 (d,
1-one 3. a-Diketone 4 was obtained through the oxidation of a-
2H, J ¼ 8.4 Hz), 3.86 (s, 3H), 1.41 (s, t-Bu, 9H); 13C NMR(300 MHz,
hydroxyketone 3 with copper sulfate in aqueous pyridine solution.
The 1-(4-tert-butylphenyl)propane-1,2-dione 4 was treated with
one equivalent of bromine in chloroform at room temperature to
CDCl3)
d 159.8, 155.1, 154.6, 151.3, 133.0, 130.7, 131.5, 131.2, 130.2,
129.8, 125.5, 125.4, 123.5, 117.1, 114.2, 34.7, 31.3; Ff 0.018; Elemental
Anal. Calcd. For C25H22N4O: C, 76.12; H, 5.62; N, 14.20; Found: C,
76.01; H, 5.75; N. 14.33; GCeMS(m/z) 394.16 Calcd. 394.47.
produce an 89e95% yield of
propane-1,2-dione 5. 2,3-Dicyanopyrazine 6 was synthesized from
the condensation of -brominated 1-(4-tert-butylphenyl)propane-
a-brominated 1-(4-tert-butylphenyl)
a
2.8. General procedure to synthesize photodimer 9
1,2-dione 5 and diaminomaleonitrile (DAMN) in the presence of
a catalytic amount of p-toluenesulfonic acid in methanol. Treat-
ment of 6 with 1.5 equiv. of triphenylphosphine in toluene afforded
a good yield of {[3-(4-tert-butylphenyl)-5,6-dicyanopyrazin-2-yl]
methyl}triphenylphosphonium bromide 7 (70%).
Compounds 8ae8d (1 mmol) were dissolved in benzene (40 ml)
and solutions were irradiated with 365 nm light emitted from
a high-pressure Hg lamp (450 W of strength at 365 nm) at room
temperature for 7 h. The mixture was evaporated under reduced
pressure, and the product was purified by column chromatography
over silica gel with EA:n-hexane ¼ 1:5 as the eluent to form 9a: m.p.;
Reactions of 7withoneequivalentof aryl aldehydeinthe presence
of 1.2 equivalent of sodium ethoxide under reflux conditions
produced styryl derivatives 8aed. The synthetic route of this study is
summarized in Schemes 1 and 2. The new compounds were charac-
terized by FT-IR, UVevisible spectroscopy and 1H NMR spectroscopy.
From the FT-IR spectra of compounds 8aed, absorption bands were
observed at 2227e2237 and 1604e1621 cmꢁ1. The medium intensity
band in the 2227e2237 cmꢁ1 region was due to stretching vibration
of the CN groups. The strong intensity band at the 1604e1621 cmꢁ1
region was due to stretching vibration of the C]C groups.
180 ꢀC; 1H NMR(300 MHz, CDCl3)
d
7.60 (d, AreH, 4H, J ¼ 8.4 Hz), 7.45
(d, AreH, 4H, J ¼ 8.4 Hz), 7.26 (m, AreH, 6H), 7.18 (m, AreH, 4H), 5.23
(dd, J1 ¼10.1 Hz, J2 ¼7.2 Hz, 2H), 4.86 (dd, J1 ¼10.1 Hz, J2 ¼7.2 Hz, 2H),
1.41 (s, t-Bu, 18H); 13C NMR (300 MHz, CDCl3)
d 161.2, 159.1, 151.3,
136.5, 134.0, 131.7, 129.9, 128.9, 128.1, 125.9, 125.5, 125.3, 117.8, 46.4,
45.3, 34.5, 31.5; Ff 0.39; Elemental Anal. Calcd. For C48H40N8: C,
79.10; H, 5.53; N, 15.37. Found: C, 78.94; H, 5.64; N. 15.51; MALDI-
TOF-Ms spectra (m/z) 728.31 Calcd. 728.88 9b: m.p.; 187 ꢀC; 1H
The 1H NMR spectra of the 2,3-dicyanopyrazine compounds
8aed showed a doublet peak at 8.1e8.2 ppm, indicating the pres-
ence of the typical trans (J ¼ 15.6 Hz) confirmation of the proton in
the olefinic moiety. Additionally, the singlet proton peak appeared
at 1.40e1.34 ppm, corresponding to the terminal t-butyl group.
NMR(300 MHz, CDCl3)
d
8.32 (d, AreH, 2H, J ¼ 8.4 Hz), 7.54 (d, AreH,
4H), 7.25 (m, AreH, 2H, J ¼ 8.4 Hz), 7.05 (d, AreH, 4H, J ¼ 8.4 Hz), 6.89
(d, AreH, 4H, J ¼ 8.4 Hz), 4.50 (dd, J1 ¼10.1 Hz, J2 ¼ 7.2 Hz, 2H), 3.58
(dd, J1 ¼10.1 Hz, J2 ¼7.2 Hz, 2H), 2.29 (s, eCH3, 6H),1.30 (s, t-Bu,18H);
13C NMR(300 MHz, CDCl3)
d 161.2, 159.1, 151.3, 135.6, 133.5, 133.1,
130.0,129.9,128.8,128.0,125.5,125.4,117.1, 46.7, 45.1, 34.2, 31.3, 21.3;
Ff 0.43; Elemental Anal. Calcd. For C50H44N8: C, 79.34; H, 5.86; N,
14.80. Found:C, 79.03;H, 5.95; N.14.87 MALDI-TOF-Msspectra (m/z)
756.71 Calcd. 756.94.
3.2. Photoreaction of 2,3-dicyanopyrazines
Compound 8a in benzene solution was irradiated by UV light at
365 nm for 7 h to produce the photodimer 9a with a 40% yield. This
reaction route is summarized in Scheme 3. The product was iso-
lated by column chromatography using EA:n-hexane ¼ 1:5 as the
eluent. The configuration of dimerized structure of 9a was
confirmed by 1H NMR.
9c: m.p.; 186 ꢀC; 1H NMR(300 MHz, CDCl3)
d 8.31 (d, AreH, 2H,
J ¼ 8.4 Hz), 7.52 (d, AreH, 2H, J ¼ 8.4 Hz), 7.26 (d, AreH, 8H,
J ¼ 8.4 Hz), 6.91 (d, AreH, 4H, J ¼ 8.4 Hz), 4.50 (dd, J1 ¼ 10.0 Hz,
J2 ¼ 7.1 Hz, 2H), 3.56 (dd, J1 ¼ 10.0 Hz, J2 ¼ 7.1 Hz, 2H), 1.27 (d, t-Bu,
36H); 13C NMR(300 MHz, CDCl3)
d
161.6, 159.1, 151.3, 148.5, 133.8,
Fig. 1 shows the 1H NMR spectra of 8c and 9c. The assignment of
8c was conducted by the 1H NMR spectrum, which showed typical
trans (J ¼ 15.6 Hz) confirmation of the proton in the olefinic moiety
133.3, 130.2, 129.4, 127.7, 125.4, 125.3, 124.8, 117.6, 48.3, 46.7, 34.2,
31.3; Ff 0.78; Elemental Anal. Calcd. For C56H56N8: C, 79.97; H, 6.71;

52
C.K. Jang et al. / Dyes and Pigments 94 (2012) 49e54
Scheme 1. Synthetic routes of 2,3-dicyanopyrazine precursor 7.
with respect to that of the cis (J ¼ 12 Hz) [21,22]. Compared to 8c,
a common feature of chemical shifts for protons on the cyclobutane
ring in 9c is shown in Fig. 1. Two sets of doublet of doublets for the
two paired protons on the cyclobutane ring were observed at 4.50
and 3.56 ppm (J1 ¼10.2 Hz, J2 ¼ 7.2 Hz). The cyclobutane ring must
have received a steric hindrance between the aromatic substitu-
ents, and we can assume that the conformation during the cyclo-
emitted from the high-pressure Hg lamp (450 W of strength at
365 nm) at room temperature for 0 s, 30 s, 1 min, 5 min, 10 min,
20 min, and 30 min under a N2 atmosphere. During irradiation, the
changes of 8ae8d were observed by UVevisible and fluorescence
spectroscopy (Fig. 2). During irradiation using 365 nm UV light, the
absorption maximum intensity at 375 nm of 8a decreased and the
new absorption maximum of 351 nm increased. The isosbestic
points were observed at 324 nm, 341 nm and 362 nm, and equi-
librium mixtures will be included in benzene. The fluorescence
intensity at 445 nm increased and the wavelength was blue-shifted
to 410 nm during irradiation using 365 nm UV light.
addition reaction progressed to
a more stable form. Two
conformation models, which were determined from the coupling
tendency with adjacent protons, were introduced in Fig. S1 (see the
Supplementary materials), and it was concluded that the NMR
spectrum was mainly determined by the head-to-tail structure in
the case of 9c [22,23]. The 1H NMR spectra of the cyclobutane ring
in 9a, 9b and 9d showed a pattern similar to that of 9c, and were
assigned to a head-to-tail configuration.
3.3. Optical and electrochemical properties
Strong intramolecular charge-transfer chromophoric systems
were observed for compounds 8 and 9, with absorption and
emission maxima at 318e408 nm and 388e459 nm, respectively.
The electron-donating character of the substituents in compounds
8 and 9 strongly reflected their absorption spectra in producing
bathochromic shifts depending on their electron-donating ability.
On the other hand, compounds 8ae8d were dissolved in
benzene and the solutions were irradiated with 365 nm light
While the cyclobutane ring of dimers induced a discrete
p-conju-
gation with aryl substituents to show a hypsochromic shift of
absorption and emission spectra, the fluorescence intensity was
increased and the specific LUMO levels were formed compared to
monomers, as shown in Fig. 2. The Stoke’s shift (SS) indicates the
difference between Fmax and lmax and corresponds to energy loss in
the first excited singlet state. The SS increased gradually with
increasing electron-donating ability of substituents.
Scheme 2. Synthetic routes of 2,3-dicyanopyrazine derivatives (8ae8d).
Scheme 3. Synthetic route of photodimerization.

C.K. Jang et al. / Dyes and Pigments 94 (2012) 49e54
53
Table 1
Optical and electrochemical properties of compounds 8ae8d and 9ae9d.
almabasxðnmÞ lemmaxðnmÞ
bSS (nm) LUMO (eV) cHOMO (eV) dEg (eV)
CHCl3 Film
8a 358
8b 386
8c 389
8d 408
9a 318
9b 343
9c 348
9d 365
425
454
459
502
388
414
420
458
472 67
ꢁ3.54
ꢁ3.45
ꢁ3.45
ꢁ3.21
ꢁ2.83
ꢁ2.78
ꢁ2.74
ꢁ2.58
ꢁ6.32
ꢁ6.2
2.78
2.75
2.84
2.60
3.24
3.22
3.30
3.00
478 68
478 70
532 94
465 70
467 71
467 72
525 93
ꢁ6.29
ꢁ5.81
6.07
ꢁ6.0
ꢁ6.04
ꢁ5.68
a
In chloroform.
Stoke’s shift (SS).
Calculated data from cyclic voltammograms.
Estimated data from UV edge.
b
c
d
ꢂ
ꢃ
HOMOðeVÞ ¼ ꢁ4:8 ꢁ Eonset ꢁ E1=2ðFerroceneÞ
In this equation, Eonset is the onset oxidation potential (V) and E1/
2(ferrocene) is the half-wave potential (V) of ferrocene in the
solution. As the oxidation potentials decreased with increasing
electron-donating ability, the HOMO level was estimated in more
positive positions and narrow band gaps were simultaneously
exhibited in the optical results.
The spin-coated thin film containing 8c was irradiated by UV
light at 365 nm and the changes in the fluorescence spectra are
shown in Fig. 3. Solutions of compounds 8ae8d (5%) in benzene
were filtered through a polytetrafluoroethylene syringe filter (0.45-
mm pore size) prior to application to the glass substrate. Resist films
were prepared by spin coating on a glass substrate. The coating
speed was maintained for 20 s at 1000 rpm. The coated layer was
exposed under the high-pressure Hg lamp using a paper mask over
1 h. During irradiation, the changes of 8 to 9 were observed by
UVevisible and fluorescence spectroscopy. The irradiated film was
immersed in EA:n-hexane(1:5) as the eluent for 20 s. The film was
dried at room temperature for 24 h and further dried at 50 ꢀC in
a vacuum oven for 3 h. The fluorescence maximum at 478 nm was
blue-shifted to 467 nm during the irradiation. The irradiated film
comprised of 8 and 9 showed behavior typical of a negative-type
photoresist, as shown in Fig. 4. Compound 9 had different solu-
bility in common organic solvents compared to compounds 8aed.
Compounds 8 and 9 were very soluble in common solvents such as
acetone, chloroform and ethyl acetate, but they were insoluble in n-
hexane. We approached miscible co-solvents to control solubility
Fig. 1. 1H NMR spectra of 8c and 9c in CDCl3.
CV was used to investigate the electrochemical redox behaviors
of these compounds. The electrochemical properties of compounds
8 and 9 were recorded in acetonitrile using 0.1 M Bu4NBF4 as
a supporting electrolyte. The calculated HOMO energy levels were
obtained from the oxidation potentials using the following equa-
tion and the estimated bandgap (Eg) and LUMO levels from the UV
edge in the electronic absorption spectra are listed in Table 1
[24,25].
Fig. 2. Changes in the absorption and fluorescence spectra of 8a in solution under
irradiation.
Fig. 3. Procedure of the spin-casting process and changes in the fluorescence spectra
of 8c in a thin film under irradiation.

54
C.K. Jang et al. / Dyes and Pigments 94 (2012) 49e54
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Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.dyepig.2011.10.015.