The synthesis, electrochemical and fluorescent properties of monomers and polymers containing 2,5-diphenyl-1,3,4-thiadiazole

Article abstract of DOI:10.1016/j.dyepig.2009.07.007

Three, 2,5-diphenyl-1,3,4-thiadiazole-containing vinyl monomers and their polymers were synthesized and the HOMO and LUMO energies of the compounds, as estimated from cyclic voltammetry data, were -6.35 to -6.14 eV and -3.02 to -2.84 eV, respectively. The optical band gaps (E_g) were similar to those determined from cyclic voltammograms. The fluorescent emission spectra of all monomers and polymers displayed either blue or green light emission.

Full text of DOI:10.1016/j.dyepig.2009.07.007

Dyes and Pigments 84 (2010) 153–158
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The synthesis, electrochemical and fluorescent properties of monomers
and polymers containing 2,5-diphenyl-1,3,4-thiadiazole
,
,
a,b
Yongxin Tao a b, Qingfeng Xu a, Jianmei Lu a , Xuebo Yang
*
a Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Soochow University,
199 Renai Road, Suzhou 215123, China
b Key Laboratory of Fine Chemical Engineering of Jiangsu Province, Jiangsu Polytechnic University, Changzhou 213164, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three, 2,5-diphenyl-1,3,4-thiadiazole-containing vinyl monomers and their polymers were synthesized
and the HOMO and LUMO energies of the compounds, as estimated from cyclic voltammetry data, were
ꢀ6.35 to ꢀ6.14 eV and ꢀ3.02 to ꢀ2.84 eV, respectively. The optical band gaps (Eg) were similar to those
determined from cyclic voltammograms. The fluorescent emission spectra of all monomers and polymers
displayed either blue or green light emission.
Received 2 February 2009
Received in revised form
11 July 2009
Accepted 14 July 2009
Available online 21 July 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Thiadiazole
Vinyl monomer
Polymer
Fluorescent properties
Electrochemical properties
Synthesis
1. Introduction
2. Experimental
Five-membered, hetero-aromatic compounds containing imine
(–C]N–) groups, such as imidazole [1,2], thiazole [3–5], triazole
[6,7], oxadiazole [8,9], and thiadiazole [10–16] have long been of
interest as luminophores for optical application, owing to their
electron accepting nature. For example, a series of 1,3,4-oxadiazoles
derivatives areused forelectron accepting materials in organic light-
emitting diodes (OLEDs) [17,18]. Monomers containing oxadiazole
have been synthesized, such as 2-[4-(4-vinylbenzyloxy)phenyl]-
5-phenyl-1,3,4-oxadiazole and 1-(4-vinylbenzyloxy)-3,5-bis[5-
(phenyl)-2-oxadiazolyl]benzene[19,20], whilstpolymerscontaining
oxadiazole, either in the main- or side-chain have been reported
[21–23]; attention has been paid to 1,3,4-thiadiazole-containing
polymers [24,25]. Jansson et al. employed density functional theory
in a study of a thiadiazole system as electron transportation mate-
rials [26]. Thus far, the synthesis of vinyl monomers and their
polymers containing 2,5-dipheny-l,3,4-thiadiazole has not been
investigated. This paper concerns three, novel monomers and their
polymers and their electrochemical and fluorescent properties.
2.1. Characterizations
1H NMR spectra were measured with a Bruker ARX 300 MHz
and INOVA 400 MHz NMR spectrometer, CDCl3 or DMSO-d6 as
solution, TMS as standard. The IR spectra were recorded on an
AVATAR 380 FTIR spectrometer (KBr pellet, 4000–400 cmꢀ1).
Element analyses were obtained with Perkin-Elmer 2400II instru-
ment. Mass spectra were obtained with Mass spectrum Finnigan
HP5890. Molecular weight and the polydispersity relative to poly-
styrene were measured using a Waters1515 GPC with THF or DMF
as eluent and a column temperature of 30ꢁC. UV–visible spectra
were recorded on a Perkin-Elmer l-17 UV–vis instrument. Room
temperature emission and excitation spectra were carried out using
Edinburgh-920 fluorescence spectra photometer.
2.2. Synthesis of 2-(4-methylphenyl)-5-phenyl-1,3,4-thiadiazole
(MPPT) and 2-(4-methoxyphenyl)-5-phenyl-1,3,4-thiadiazole
(MOPPT)
All reagents and chemicals were purchased from Sinopharm and
were used without further purification. MPPT and MOPPT were
synthesized according to Scheme 1.
* Corresponding author. Tel.: þ86 512 65880367; fax: þ86 512 65882875.
E-mail address: lujm@suda.edu.cn (J. Lu).
0143-7208/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2009.07.007

154
Y. Tao et al. / Dyes and Pigments 84 (2010) 153–158
Scheme 1. Synthesis of 2-(4-methylphenyl)-5-phenyl-1, 3, 4-thiadiazole (MPPT) and 5-(4-methoxylphenyl)-2-phenyl-1,3,4-thiadiazole (MOPPT): (i) CH3OH, H2SO4, reflux; (ii)
excess hydrazine hydrate, reflux; (iii) benzoyl chloride, pyridine, 0 ꢁC; (iv) Lawesson’s reagent, dioxane, 80 ꢁC.
MPPT from 4-toluic acid as white crystals (69%): m.p. 140–
141 ꢁC; 1H NMR (300 MHz, CDCl3, dH, ppm), 2.43(s, 3H, –CH3),
7.29(d, 2H, J ¼ 8.0 Hz, Ar–H 1-H), 7.49–7.51(m, 3H, Ar-H 4-H & 5-H),
7.88(d, 2H, J ¼ 8.1 Hz, Ar–H 2-H), 7.99–8.02(m, 2H, Ar–H 3-H); MS
(m/z, %), 252.0 (Mþ, 100); FTIR (KBr, cmꢀ1), 1442 (1,3,4-thiodiazole).
MOPPT from 4-methoxybenzoic acid as white crystals (65%):
m.p. 132–134 ꢁC; 1H NMR (300 MHz, CDCl3, dH, ppm), 3.89(s, 3H,
–OCH3), 6.99(d, 2H, J ¼ 8.6 Hz, Ar–H 1-H), 7.49–7.51 (m, 3H, Ar–H
ethanol and purified by recrystallization from cyclohexanone to
yellowy crystals (6.30 g, 95%): m.p.193–195 ꢁC; 1H NMR (300 MHz,
CDCl3, dH, ppm), 4.54(s, 2H, CH2Br), 7.50–7.54(m, 5H, Ar–H 1-H &
4-H & 5-H), 7.99–8.03(m, 4H, Ar–H 2-H & 3-H); FTIR (KBr,
cmꢀ1),1440(1,3,4-thiodiazole).
2.3.2. 2-[4-(5-phenyl-1,3,4-thiadiazole-2-yl)-benzyloxy]
ethanol (PTBE)
4-H & 5-H),
d7.94–8.00 (m, 4H, Ar–H 2-H & 3-H); MS(m/z, %), 268.0
A mixture of BMPPT (3.31 g, 0.01 mol), sodium hydroxide
(0.40 g, 0.01 mol) and 20 mL glycol was stirred at 120 ꢁC for 30 min.
The mixture was poured into 100 mL ice water and the resultant
yellowy precipitate was filtered off. The crude product was purified
by recrystallization from acetone to white crystals (2.93 g, 94%):
m.p.94–95 ꢁC; 1H NMR (300 MHz, CDCl3, dH, ppm), 2.05(s, 1H, OH),
3.64(t, 2H, J ¼ 4.5 Hz, HOCH2), 3.81(t, 2H, J ¼ 4.5 Hz, CH2CH2OH),
4.64(s, 2H, Ar–CH2), 7.47–7.52(m, 5H, Ar–H 1-H & 4-H & 5-H), 8.00–
8.03 (m, 4H, Ar–H 2-H & 3-H).
(Mþ, 100); FTIR (KBr, cmꢀ1), 1441(1,3,4-thiodiazole).
2.3. Syntheses of the monomers and their model compounds
The synthetic route of the monomers and their saturated model
compounds are shown in Scheme 2.
2.3.1. 2-(4-bromomethyl phenyl)-5-phenyl-1,3,4-thiadiazole
(BMPPT)
2.3.3. 4-(5-phenyl-1,3,4-thiadiazole-2-yl) phenol (PTP)
A mixture of MPPT (5.04 g, 0.02 mol), N-bromosuccinimide
(3.56 g, 0.02 mol) and benzoyl peroxide in 120 mL benzene was
stirred at 80 ꢁC for 6 h, being irradiated by incandescent lamp. The
crude product was isolated by evaporating the solvent, washed by
MOPPT (3.00 g, 0.011 mol) was stirred in 100 mL 50% hydro-
bromic acid (caution: incompatible with strong bases, strong
oxidizing agents, ammonia, ozone, fluorine, water, metals; both air
Scheme 2. Synthesis of 2-[4-(5-phenyl-1,3,4-thiadiazole-2-yl)-benzyloxy] ethyl methacrylate (PTBEMA), 2-phenyl-5-(4-vinylphenyl)-1,3,4-thiadiazole (PVPT) and 2-[4-(5-phenyl-
1,3,4-thiadiazole-2-yl)-phenoxy]ethyl methacrylate (PTPEMA): (i) N-bromosuccinimide, benzene, 80 ꢁC; (ii) excess glycol, sodium hydrate, 130 ꢁC (iii) methylacryloyl chloride,
triethylamine, THF, 0 ꢁC; (iv) (1) triphenylphosphine; (2) formaldehyde, 20% sodium hydrate solution, 0 ꢁC; (v) 50% hydrobromic acid , 100ꢁC; (vi) chlorethyl alcohol, sodium hydrate
solution (vii) methylacryloyl chloride, triethylamine, THF, 0 ꢁC.

Y. Tao et al. / Dyes and Pigments 84 (2010) 153–158
155
and light sensitive) at 100 ꢁC for 20 h. The precipitate was filtered
off and washed by deionized water. The crude product was dis-
solved in NaOH solution and the insoluble residue removed by
filtration. 36% Hydrochloric acid was added to the filtrate and the
precipitate was filtered off, washed with deionized water and dried
under vacuum to give the product as a white powder (2.93 g, 93%):
1H NMR (400 MHz, DMSO-d6, dH, ppm), 6.98 (d, 2H, J ¼ 8.8 Hz, Ar–H
1-H), 7.62–7.64 (m, 3H, Ar–H 4-H & 5-H), 7.98 (d, 2H, J ¼ 8.8 Hz,
Ar–H 2-H), 8.10–8.12 (m, 2H, Ar–H 3-H), 10.38(s, 1H, OH).
(1.49 g, 5 mmol) and triethylamine (0.51 g, 5 mmol) in 10 mL
dichloromethane at 0–5 ꢁC. The solution was unceasingly stirred for
12 h. The crude product was isolated by evaporating the solvent,
and purified by gel chromatography (silica gel, mineral ether/ethyl
acetate ¼ 1:1) to yield white powder (1.66 g, 91%): m.p.93–94 ꢁC;
1H NMR (300 Hz, CDCl3, dH, ppm), 1.97(s, 3H, CH3), 4.31(t, 2H,
J ¼ 4.5 Hz, ArOCH2), 4.54(t, 2H, J ¼ 4.5 Hz, ArOCH2CH2),
5.61(s,1H,]CH2, cis), 6.16(s,1H,]CH2, trans), 6.95–7.04(m, 2H, Ar–
H 1-H), 7.40(Br, 3H, Ar–H 4-H & 5-H), 7.90–7.98 (m, 4H, Ar–H 2-H &
3-H); FTIR (KBr, cmꢀ1), 1725 (C]O), 1442(1, 3, 4-thiodiazole); Anal.
Calcd. for C21H20N2O3S (366.1): C 65.55; H 4.95; N 7.64; S 8.75.
Found: C 65.51; H 4.98; N 7.63; S 8.72.
2.3.4. 2-[4-(5-phenyl-1,3,4-thiadiazole-2-yl)-phenoxy]
ethanol (PTPE)
A mixture of PTP (2.65 g, 0.0104 mol), sodium hydroxide (0.42 g,
0.0105 mol), 0.7 mL 2-chloroethanol (caution: toxic; incompatible
with oxidizing agents) and 50 mL H2O was stirred at 100 ꢁC for
100 min. The precipitate was filtered off, washed with deionized
water and dried under vacuum to give the product as a white
2.5. Polymerizations
The polymerization of the 2,5-diphenyl-1,3,4-thiadiazole-con-
taining vinyl monomers was carried out in toluene with 2,20-azo-
bisisobutyronitrile (AIBN) as initiator. A mixture of the monomer
(1 mmol), AIBN (0.01 mmol) and 2 mL cyclohexanone was placed in
a round flask. The flask was sealed and cycled between vacuum and
N2 for four times, and immersed in an oil bath at 65 ꢁC for 6 h. After
polymerization, the products were dissolved in THF and precipi-
tated into a large amount of methanol. The precipitate was filtered,
washed by methanol and dried under vacuum. The crude polymers
were purified by Soxhlet extractor with ethanol to remove of
starting monomers.
powder (2.34 g, 75%): 1H NMR (400 MHz, CDCl3, dH
, ppm):
2.10(s, 1H, OH), 4.00(t, 2H, J ¼ 4.2 Hz, HOCH2), 4.15(t, 2H, J ¼ 4.2 Hz,
CH2CH2OH), 7.02(d, 2H, J ¼ 8.4 Hz, Ar–H 1-H), 7.49(br, 3H, Ar–H 4-H
& 5-H), 7.95 (d, 2H, J ¼ 8.4 Hz, Ar–H 2-H), 8.00(Br, 3H, Ar–H 3-H).
2.4. Monomers
2.4.1. 2-[4-(5-phenyl-1, 3, 4-thiadiazole-2-yl)-benzyloxy]
ethyl methacrylate (PTBEMA)
Methylacryloyl chloride (0.52 g, 5 mmol) in 5 mL dichloro-
methane was slowly added dropwise into a solution of PTBE (1.56 g,
5 mmol) and triethylamine (0.51 g, 5 mmol) in 10 mL dichloro-
methane at 0–5 ꢁC; the ensuing solution was stirred for 12 h. The
crude product was isolated by evaporating the solvent, and purified
using gel chromatography (silica gel, mineral ether/ethyl
acetate ¼ 2:1) to yield a white powder, (1.68 g, 88%): m.p.80–82 ꢁC;
1H NMR (300 Hz, CDCl3, dH, ppm), 1.98(s, 3H, CH3), 3.78(t, 2H,
J ¼ 4.7 Hz, ArCH2OCH2), 4.37(t, 2H, J ¼ 4.7 Hz, ArCH2OCH2CH2),
4.65(s, 2H, Ar–CH2), 5.61 (s, 1H, ¼ CH2, cis), 6.16(s, 1H, ¼ CH2, trans),
7.47–7.52(m, 5H, Ar–H 4-H & 5-H), 7.99–8.03(m, 4H, Ar–H 2-H &
3-H); FTIR (KBr, cmꢀ1), 1713 (C]O), 1450 (1,3,4-thiodiazole); Anal.
Calcd. for C21H20N2O3S (380.4): C 66.31; H 5.26; N 7.37; S 8.42.
Found: C 66.37; H 5.31; N 7.33; S 8.39.
Poly(PTBEMA) a white powder, yield: 0.351 g, 92.0%. 1H NMR
(300 Hz, CDCl3, dH, ppm): 1.00–1.11 (Br, CH3), 1.98 (Br, CH2), 3.57 (Br,
ArCH2OCH2), 4.09 (Br, ArCH2OCH2CH2), 4.40 (Br, ArCH2), (m, 2H,
Ar–H), 7.36 (Br, Ar–H 1-H & 4-H & 5-H), 7.82 (Br, Ar–H 2-H & 3-H);
FTIR (KBr, cmꢀ1): 1729 (C]O), 1442 (1,3,4-thiodiazole); GPC data
(THF as eluent): Mn ¼ 19,600, PDI ¼ 1.90.
Poly(PVPT) a white powder, yield: 0.235 g, 89.1%. 1H NMR
(300 Hz, CDCl3, dH, ppm): 1.3–1.9(Br, CH2), 1.9–2.6(Br, CH), 7.3(Br,
Ar–H), 7.50(Br, Ar–H), 7.68–7.73(Br, Ar–H), 7.94–7.97 (Br, Ar–H),
8.01–8.03(Br, Ar–H); FTIR (KBr, cmꢀ1): 2924 (–C–H–), 1443(1,3,4-
thiodiazole); GPC data (DMF as eluent): Mn ¼ 20,800, PDI ¼ 1.52.
Poly(PTPEMA) a white powder, yield: 0.344 g, 94.0%. 1H NMR
(400 Hz, CDCl3, dH, ppm): 0.93 1.10, 1.25 (Br, CH3), 1.92 (Br, CH2),
4.07 (Br, ArOCH2CH2), 6.77(Br, Ar–H), 7.30 (Br, Ar–H), 7.65(Br, Ar–
H), 7.75 (Br, Ar–H); FTIR (KBr, cmꢀ1): 1728 (C]O), 1443 (1,3,4-thi-
odiazole); no GPC data due to poor solubility.
2.4.2. 2-phenyl-5-(4-vinylphenyl)-1, 3, 4-thiadiazole (PVPT)
A mixture of BMPPT (3.31 g, 10 mmol) and triphenylphosphine
(caution: toxic; incompatible with oxidizing agents, acids) (2.63 g,
10 mmol) in 50 mL chloroform was stirred at 70 ꢁC for 5 h and
cooled to room temperature. 37% Formaldehyde solution (20 mL)
was poured into the solution and stirred. Subsequently, 30 mL 20%
sodium hydroxide solution was slowly added dropwise into the
solution and the mixture unceasingly stirred at room temperature
for 12 h. The chloroform solution was separated from water solu-
tion, washed by water, dried by anhydrous magnesium sulfate. The
crude product was isolated by evaporating chloroform, and purified
by recrystallization from absolute alcohol to yield white lamellar
3. Results and discussion
3.1. Synthesis and polymerization of the monomers
To prepare the 2,5-diphenyl-1,3,4-thiadiazole-containing
monomers, two inter-mediate compounds, MPPT and MOPPT were
synthesized from 4-toluic acid and 4-methoxybenzoic acid via
esterification, ammonolysis with hydrazine hydrate, amidation
with benzoyl chloride, annelation with Lawesson’s reagent and
white crystal formation in excess 65% overall yield. Because the
hydrogen of methyl group in the benzene ring could be easily
substituted by N-bromosuccinimide, BMPPT was synthesized from
MPPT via bromation. To improve on solubility, the reactive benzyl
bromine group could react via chain-extended reaction. PTBE was
synthesized from BMPPT via etherification with excessive glycol in
the presence of sodium hydroxide. It was a near quantitative
reaction. On the other hand, the vinyl monomer, PVPT could be
prepared from BMPPT via Wittig reaction with triphenylphosphine
and formaldehyde [27,28]. The methoxy group in the benzene ring
could be translated to phenolic hydroxyl group via demethylation.
PTP was synthesized from MOPPT with hydrobromic acid. PTPE was
synthesized from PTP via etherification with chloroethanol due to
crystals (2.24 g, 85%): m.p.157–159 ꢁC; 1H NMR(300 Hz, CDCl3, dH
,
ppm), 5.38(d, 1H, J ¼ 10.9 Hz, CH2], cis), 5.88(d, 1H, J ¼ 17.6 Hz,
CH2], trans), 6.72(dd,1H, J1 ¼17.6 Hz, J2 ¼10.9 Hz, CH2]CH), 7.50–
7.55(m, 5H, Ar–H 1H & 4-H & 5-H), d7.97–8.04(m, 4H, Ar–H 2-H & 3-
H); FTIR(KBr, cmꢀ1): 1689 (C]C), 1442(1,3,4-thiodiazole); Anal.
Calcd. for C16H12N2S(264.2.4): C 72.72; H 4.54; N 10.60; S 12.12.
Found: C 72.93; H 4.75; N 10.45; S 11.93.
2.4.3. 2-[4-(5-phenyl-1, 3, 4-thiadiazole-2-yl)-phenoxy] ethyl
methacrylate (PTPEMA)
Methylacryloyl chloride (0.52 g, 5 mmol) in 5 mL dichloro-
methane was slowly added dropwise into the solution of PTPE

156
Y. Tao et al. / Dyes and Pigments 84 (2010) 153–158
Table 1
UV–vis and fluorescent parameters of the monomers and polymers containing
2,5-diphenyl-1,3,4-thiadiazole.
sample
lmax (nm) lmax (nm) lmax (nm) Ff
lmax (nm) lmax (nm)
UV–vis
excitation emission
excitation emission
In DMF
In solid state
PTBEMA
PTPEMA
PVPT
poly (PTBEMA) 308
poly (PTPEMA) 321
311
321
323
354
365
368
358
328
384
390
398
390
410
405
448
0.047 388
450
412
425
450
414
515
0.049 350
0.054 392
0.049 389
0.050 367
0.352 430
poly (PVPT)
315
the improvement of solubility. The two methacrylate-type mono-
mers, PTBEMA and PTPEMA could be prepared from two model
compounds containing hydroxyl group via esterification by meth-
acrylic chlorine. The three monomers can be polymerized via the
initiation of AIBN. All monomers exhibit good solubility in common
organic solvent such as chloroform, THF and DMF, while the poly-
mers can be dissolved by chloroform, poly(PTBEMA) is easily
soluble in THF and DMF, and poly(PVPT) is slight soluble in THF and
DMF, but poly(PTPEMA) has very poor solubility in THF or DMF.
Thin films of poly(PTBEMA), poly(PVPT) and poly(PTPEMA) can be
prepared by spin coating from their solution in chloroform.
Fig. 1. Excitation and emission spectra of PTBEMA and poly(PTBEMA) in DMF.
poly(PTBEMA) compared to PTBEMA is 20 nm red-shifted, because
the chromophore of the monomer is completely dispersed in the
dilute solution whereas the chromophores within the polymer
exist intramolecular associating interactions. In solid state, the
emission spectra of PTBEMA and poly(PTBEMA) were obtained by
excitation at 400 nm. As shown in Fig. 2, both peaks are identical at
around 450 nm, pure blue light corresponds to coordinates
(x ¼ 0.15, y ¼ 0.13) of the1931 Commission Internationale De
L’Eclairage chromaticity diagram. In contrast within the solid state
and in DMF solution, the red-shift of the emission peak clearly
3.2. Characterization of the polymers, monomers and model
compounds
1,3,4-Thiadiazole is the electron-accepting five-membered
hetero-aromatic unit containing two imine –C]N– groups.
Because of the electron-withdrawing ability of the imine –C]N–
group, the 1H NMR data of 2,5-diphenyl-1,3,4-thiadiazaole deriva-
tives show the chemical shift of Ar–H (2-H, 3-H) (Scheme 2) at
around 8.0 ppm and the chemical shift of Ar–H (4-H, 5-H) at around
7.5 ppm. The chemical shift of Ar–H (1-H) is under the influence of
ortho substituent. The FTIR data exhibit absorption peak at around
1440 cmꢀ1 is assigned to 1,3,4-thiadiazole.
indicates chromophores p–p stacking interactions in the solid state.
In Fig. 3, we display the emission spectra of the PTPEMA and poly-
(PTPEMA), and similar result is also found. In DMF solution (see
Fig. 4), the emission maxima of PVPT and poly(PVPT) are 390 nm
and 448 nm respectively. The emission peak of poly(PVPT)
compared to PVPT shows 58 nm red-shifted, because PVPT has no
flexible chain between the chromophore and C]C double bond,
and the chromophores within poly(PVPT) leads to more intense
intramolecular interactions. In solid state (see Fig. 5), The emission
maximum of poly(PVPT) is 515 nm, green light corresponds to
coordinates (x ¼ 0.29, y ¼ 0.51), whereas emission maximum of
PVPT is 425 nm. Circa 90 nm red-shifted may be due to both
intramolecular and intermolecular interactions. To better under-
stand the fluorescent properties of the polymers and monomers,
the optical parameters are presented in Table 1. In solution, the
excitation maxima of PTBEMA, poly(PTBEMA), PVPT, poly(PVPT)
and PTPEMA are red-shifted by ca. 40–70 nm in comparison with
their absorption maxima, respectively. It points the formation of
static excimer. But poly(PTPEMA) is an exception, its excitation
maximum is only 7 nm red-shifted to its absorption maximum. We
conjecture that poly(PTPEMA) in dilute DMF trend to the formation
of dynamic excimer rather than static excimer.
3.3. Absorption spectra
The absorption spectra of the three monomers and their poly-
mers in DMF exhibit a broad band in the UV region. The bands are
ascribed to 2,5-diphenyl-1,3,4-thiadiazole group absorption. The
optical band gaps (Eg) are taken as the absorption onset of the UV–
vis spectra from the intersection of two tangents drawn at the
declining current and background current. The absorption maxima
and the onset are shown in Tables 1 and 2 respectively.
3.4. Fluorescence properties
The excitation and emission spectra of the monomers and
polymers were investigated in DMF as well as in the solid state.
Fig. 1 shows the excitation maxima of PTBEMA and poly(PTBEMA)
in DMF are 354 nm and 358 nm respectively, while their emission
maxima are 390 nm and 410 nm. The emission maximum of
Solution fluorescence quantum yield of the samples was
measured in DMF dilute solutions by using quinine bisulfate
Table 2
Electrochemical and optical parameters of the monomers and polymers containing 2,5-diphenyl-1,3,4-thiadiazole.
EOx eV
Ered eV
HOMO eV
LUMO eV
Eg (CV) eV
lonset (UV) nm
Eg (UV) eV
PTBEMA
PTPEMA
PVPT
poly (PTBEMA)
poly (PTPEMA)
poly (PVPT)
1.54
1.61
1.56
1.49
1.61
1.40
ꢀ1.90
ꢀ1.90
ꢀ1.76
ꢀ1.79
ꢀ1.72
ꢀ1.83
ꢀ6.28
ꢀ6.35
ꢀ6.30
ꢀ6.23
ꢀ6.15
ꢀ6.14
ꢀ2.84
ꢀ2.84
ꢀ2.98
ꢀ2.95
ꢀ3.02
ꢀ2.91
3.44
3.51
3.32
3.28
3.33
3.23
355
367
370
356
367
366
3.49
3.38
3.35
3.48
3.38
3.39
Eg (UV) ¼ hc/lonset (UV); Eg (CV) ¼ EOx ꢀ Ered.

Y. Tao et al. / Dyes and Pigments 84 (2010) 153–158
157
Fig. 4. Excitation and emission spectra of PVPT and poly(PVPT) in DMF solution.
Fig. 2. Emission spectra of PTBEMA and poly(PTBEMA) with lex ¼ 400 nm in the solid
state.
nitrogen in a three compartment cell with a platinum working
electrode, a platinum counter electrode, and a calomel reference
electrode without any electrolyte. The onset potentials were esti-
mated from the intersection of two tangents drawn at the rising
current and background current of the cyclic voltammogram. The
HOMO and LUMO energies of the monomers and their polymers
were estimated from anodic and cathodic onset of cyclic voltam-
metry data to be from ꢀ6.35 to ꢀ6.14 eV, and from ꢀ3.02 to
ꢀ2.84 eV respectively. The HOMO and LUMO levels are of impor-
tance to match the Fermi energy of an electrode. Our work provides
a new ideal to tune parameters for the HOMO and LUMO levels. The
optical band gaps (Eg) taken as the absorption onset of the UV–vis
spectra were similar to the band gaps determined from the cyclic
voltammogram. The electrochemical parameters are presented in
Table 2.
(Vf ¼ 0.546 in 0.1 N H2SO4) as standards [29]. Calculations are done
by Eq. (1), where Funk is the fluorescence quantum yield of the
sample, Vstd the fluorescence quantum yield of the standard, Iunk
and Istd the integrated emission intensities of the sample and the
standard, respectively, Aunk and Astd the absorbance of the sample
and the standard at the excitation wavelength, respectively, and
hunk and hstd the refractive indexes of the corresponding solution:
2
Funk
¼
FstdðIunk=AunkÞ ðAstd=IstdÞ ðhunk
=
hstd
Þ
(1)
Table 1 also summarizes the fluorescence quantum yield of the
monomers and polymers in dilute solution. We can see that the
values are around 0.05, except for poly(PVPT). The main reason for
higher fluorescence quantum yield of poly(PVPT) is that compact
structure within the polymer restrict the rotation between benzene
ring and thiadiazole.
4. Conclusions
3.5. Electrochemical properties
Three new monomers containing 2,5-diphenyl-1,3,4-thiadiazole
have been synthesized and their polymers which can emit strong
blue or green fluorescence have a good solubility in HCCl3 or DMF,
but poly(PTPEMA) is rather poor in DMF. Poly(PTBEMA), poly-
(PVPT) and poly(PTPEMA) thin films can be prepared by spin
Cyclic voltammetry (CV) measurements were carried out on an
electrochemical analyzer in DMF with 0.1 M tetrabutylammonium
perchlorate (TBAP) as the supporting electrolyte at a sweeping rate
of 100 mV/s. The electrochemical experiments were made under
Fig. 3. Emission spectra of PTPEMA and poly(PTPEMA) in DMF and in the solid state.
Fig. 5. Emission spectra of PVPT and poly(VPT) in the solid state.

158
Y. Tao et al. / Dyes and Pigments 84 (2010) 153–158
five-membered electron-accepting 1,3,4-thiadiazole, 1,2,4-triazole, or 3,4-
dinitrothiophene units: synthesis, packing structure, and optical properties.
Macromolecules 2005;38:4687–97.
coating from their chloroform solution. Poly(PTBEMA) in the solid
emits pure blue light and poly(PVPT) emits green light as well as
ploy(PTPEMA) emits indigo light. Chromophorous intramolecular
and intermolecular interaction can lead to fluorescence red-shift
and tune parameters for the HOMO and LUMO levels.
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Acknowledgments
Authors thank financial support from Chinese Natural Science
Foundation No. 20876101 and No. 20571054, Great Project of
Department of Education Jiangsu Province (08KJA430004) and The
Research Fund for the Doctoral Program of Higher Education No.
20070285003.
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Appendix. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.dyepig.2009.07.007.
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