The thermofluoric behavior of poly(fluorenetolyldiphenylamine)-oxadiazole pair in a polymer matrix

Article abstract of DOI:10.1039/c3ra42408a

Under UV irradiation, 1,4-bis(5-(4-octan-2-ylphenyl)-1,3,4-oxadiazol-2-yl) naphthalene (NOXD) and poly(fluorenetolyldiphenylamine) (PFT) are blue light emissive in solid film. When NOXD and PFT were blended to form a neat thin-film, yellowish exciplex emi

Full text of DOI:10.1039/c3ra42408a

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The thermofluoric behavior of
poly(fluorenetolyldiphenylamine)–oxadiazole pair
in a polymer matrix†
Cite this: RSC Adv., 2013, 3, 20227
Chin-Sheng Lee,a Cheng-Po Kuo,a Chiou-Ling Chang,a Ching-Nan Chuang,b
c
c
ab
*
Mandy M. Lee, Shih-Sheng Sun and Man-kit Leung
Under UV irradiation, 1,4-bis(5-(4-octan-2-ylphenyl)-1,3,4-oxadiazol-2-yl)naphthalene (NOXD) and
poly(fluorenetolyldiphenylamine) (PFT) are blue light emissive in solid film. When NOXD and PFT were
blended to form a neat thin-film, yellowish exciplex emission was observed. The emissive properties vary
when the materials were blended into different kinds of polymeric hosts; while polystyrene–NOXD–PFT
(5 : 1 : 1) and PMMA–NOXD–PFT (5 : 1 : 1) showed blue light emission, poly(acrylonitrile-co-methyl
acrylate) (P(AN-co-MA))–NOXD–PFT (5 : 1 : 1) showed blue and yellow dual emissions. When the film
was heated at 140 ꢀC for 2 min, the yellow light exciplex emission was enhanced. However, upon
cooling to ambient temperature, the exciplex emission intensity gradually dropped back. Similar
thermofluoric behaviors were observed when the thermally crosslinked polyepoxy-polymercaptan–
NOXD–PFT film (20 : 1 : 1) was heated at 160–170 ꢀC; the blue light emissive film showed yellow light
emission, and turned white when cooling down to ambient temperature. This thermofluoric
phenomenon is recyclable. We attributed the observation of the yellow emission to the segregation of
NOXD from the polymeric host at high temperature that allows NOXD to aggregate with PFT, leading
to exciplex emission.
Received 15th May 2013
Accepted 21st August 2013
DOI: 10.1039/c3ra42408a
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non-radiation decay and system crossing that could suppress
the exciplex formation. To form the exciplex, the D* (or acceptor
Introduction
Photoluminescence materials based on small molecules or
polymers have attracted a lot of attention due to their great
potential in modern technologies such as color displays,1–10
uorescence dyes and paints.11–14 There are several different
kinds of light-emitting mechanisms, including uorescence or
phosphorescence from excitons, excimers, electromers, exci-
plexes, or electroplexes, that govern the color and quantum
efficiency of the photoluminescence.15–33 An exciplex is one kind
of excited state complex that is formed by an electronically
excited state donor molecule, D* (or acceptor A*), with a
complementary acceptor molecule, A (or donor D), in their
ground state. There are many competition processes existing in
the medium, such as the normal uorescent process, thermal
A*) and A (or donor D) have to collide with each other prior to
any relaxation process. This would be strongly affected by the
diffusion rate of the molecules, which is governed by the
viscosity of the matrix and the effective concentration of D/A in
the media.
In principle, an exciplex is composed of the D–A pair with an
average separated distance of 0.3–0.4 nm,34–43 in which partially
charged components (0 < d < 1) with limited extents of charge
separation between the donor–acceptor pair would be formed.
Instead of having lower quantum yields, the exciplex emission
spectrum is usually red-shied so that a totally new emission
prole, that is different from that of D or of A could be observed.
During the past, many studies on enhanced exciplex emission
of polymer systems suggested the possibility of using LED
devices as a broadband emission source.15,30,31
Poly(uorenetriphenylamine)s44 form a class of hole-trans-
porting uorescent polymers that have been widely studied in
the area of polymeric light emitting diodes. Due to their high
uorescence quantum yields as well as the excellent hole-
injection and transport properties, this class of materials is
highly useful for organic optoelectronic applications.
aDepartment of Chemistry, National Taiwan University, 1 Roosevelt Road Section 4,
Taipei, Taiwan 10617, Republic of China. E-mail: mkleung@ntu.edu.tw;
d98223107@ntu.edu.tw;
d96549013@ntu.edu.tw;
d93223016@ntu.edu.tw;
easy2173@yahoo.com.tw; mmlee@gate.sinica.edu.tw; sssun@chem.sinica.edu.tw;
Fax: +886-2-23636359; Tel: +886-2-33661673
bInstitute of Polymer Science and Engineering, National Taiwan University, Republic of
China
cInstitute of Chemistry, Academia Sinica, Nankang, Taipei 115, Taiwan, Republic of
On the complementary side, oxadiazole (OXD) molecules are
among the most widely studied electron-transport materials for
organic electronics.45–47 Many different types of OXDs, including
China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra42408a
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dimeric,48–50 trimeric,51–53 starburst,54–56 tetraphenylmethane57
and arylsilane58-based OXDs have recently been reported. When
OXDs have been blended in an electron-rich or donating matrix,
with high-lying HOMO, photoluminescence from the exciplex
emission could be observed. For example, exciplex formation
was observed when 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadi-
azole (PBD) was blended into polyvinylcarbazole (PVK). Since
the exciplex formation phenomenon is not only an interesting
issue in photophysical studies, but also in many organic elec-
tronic applications such as OLED and organic solar cell
researches,59 we have carried out systematic studies of the
exciplex formation of the oxadiazole (OXD) derivatives in PVK
blends as emitter.60–62
Measurement
1H and 13C NMR spectra were recorded on a Varian Unity Plus
400 MHz spectrometer. Molecular weights and polydispersities
of polymers were determined by gel permeation chromatog-
raphy (GPC) analysis relative to polystyrene calibration (Waters
Styragel columns of HR3 and HR4E) using THF as eluent at a
ow rate of 1.0 mL minꢁ1. Differential scanning calorimetry
(DSC) analysis was performed under nitrogen at a heating rate
of 10 ꢀC minꢁ1. Thermogravimetric analysis (TGA) was deter-
mined under nitrogen by measuring weight loss while heating
at a rate of 10 ꢀC minꢁ1. Photoluminescence spectra were
obtained on a Hitachi F-4500 luminescence spectrometer.
There are many factors affecting the exciplex formation.
Besides the HOMO–LUMO matching factors, geometry factors
and diffusion rate would also be essential. Therefore, one might
expect that exciplex formation in a solid matrix would be
complicated. At least, unlike molecules in solution in which
conformational equilibrium is fast, conformational preferences
and restrictions of molecules in a solid or highly viscous glassy
matrix would be important factors to deal with. In the present
article, we are going to present our study of the exciplex
formation of 1,4-bis(5-(4-octan-2-ylphenyl)-1,3,4-oxadiazol-2-yl)-
naphthalene (NOXD) with conjugated poly(uorene-tolyldiphe-
nylamine) PFT5 in an poly(epoxy)-polymercaptan (PEPM) matrix
and the unexpected thermouoric phenomenon.63
Synthetic procedures
The synthesis of PFT and NOXD are denoted in Scheme 1. The
synthetic procedures are reported as follows.
4-Bromo-N-(4-bromophenyl)-N-p-tolylaniline (1). To an ice-
cooled solution of 4-methyl-N,N-diphenylaniline (2.5 g, 9.7
mmol, in 32 mL of DMF) was added dropwise NBS (N-bromo-
succinimide) solution (3.58 g, 20.3 mmol, in 10 mL of DMF).
Aer the addition, the mixture was stirred for a further 15 min.
Then, when the reaction was completed, water was added to
quench the reaction. The product was extracted with CH2Cl2,
dried over anhydrous MgSO4, and concentrated under vacuum.
The crude product was puried by column chromatography,
1
eluting with hexane to give 1 (3.6 g, 90%) as white crystals. H
NMR (400 MHz, CDCl3): d 7.32–7.29 (m, 4H), 7.08 (d, J ¼ 8.8 Hz,
2H), 6.98–6.95 (m, 2H), 6.93–6.89 (m, 4H). 13C NMR (100 MHz,
CDCl3): d 146.47, 144.07, 133.66, 132.05, 130.07, 125.03, 124.74,
Experimental section
Materials
All reagents were commercially available and were used without 114.82, 20.98. HRMS (FAB): calcd for C19H15Br2N, 414.9571
further purication. Polystyrene (PS, Mw: 250 000, Acros), (M+); obsd, 414.9551.
poly(methyl methacrylate) (PMMA, Mw: 97 000, Acros), and
2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dibu-
poly(acrylonitrile-co-methyl acrylate) (P(AN-co-MA), Aldrich) tyluorene (2). To a solution of 2,7-dibromo-9,9-dioctyluorene
were used as the host matrix.
(10.0 g, 23.0 mmol) in THF (400 mL) at ꢁ78 ꢀC was slowly
Scheme 1
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added, by syringe, n-butyllithium (1.6 M in hexane, 31.7 mL, chromatography on silica gel, using hexanes–CH2Cl2 (20/1) as
ꢀ
50.7 mmol). The mixture was stirred at ꢁ78 C for 1 h. 2-Iso- the eluent, followed by using hexane–CH2Cl2 (10/1) to give 4
1
propoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (11.4 mL, 55.8 (58.4 g, 59%) as a yellow liquid. H NMR (400 MHz, CDCl3): d
mmol) was rapidly added to the solution, and the resulting 8.12 (d, J ¼ 6.8 Hz, 2H), 7.30 (d, J ¼ 6.8 Hz, 2H), 2.76–2.82 (m,
mixture was warmed to room temperature and stirred for 24 h. 1H), 1.54–1.60 (m, 2H), 1.22–1.28 (m, 11H), 0.85 (t, J ¼ 6.8 Hz,
Aer reaction, the mixture was poured into water and extracted 3H); 13C NMR (100 MHz, CDCl3): d 155.62, 146.06, 127.63,
with ether. The organic extracts were washed with brine and 123.50, 40.13, 38.14, 31.78, 29.34, 27.61, 22.69, 22.05, 14.17; MS
dried over anhydrous MgSO4. The solvent was removed by (EI) 235 (M+); HRMS (EI) calcd for C14H21NO2 235.1572 (M+),
rotary evaporation, and the residue was puried by column obsd 235.1574.
chromatography, using hexane–CH2Cl2 (10 : 1) as the eluent, to
4-(Octan-2-yl)aniline (5). To a stirred suspension of 4 (10.0
afford 2 (6.5 g, 53%) as white crystals. 1H NMR (400 MHz, g, 43.0 mmol) and 10% Pd–C (2.6 g) in DMF (86 mL) was
CDCl3): d 7.78 (d, J ¼ 8.0 Hz, 2H), 7.72–7.69 (t, J ¼ 7.6 Hz, 4H), added anhydrous (NH4)+(HCO2)ꢁ (33.0 g, 0.5 mol) in a single
2.02–1.98 (m, 4H), 1.38 (s, 24H), 1.07–1.01 (m, 4H), 0.63 (t, J ¼ portion. The resulting solution was stirred at 80 ꢀC for 3 h,
7.2 Hz, 6H), 0.56–0.50 (m, 4H); 13C NMR (100 MHz, CDCl3): d extracted with ethyl acetate (200 ꢂ 2 mL) and washed with
150.24, 143.73, 139.49, 128.74, 119.26, 83.69, 55.15, 40.10, distilled water (200
ꢂ
2 mL) to remove any excess
25.93, 25.05, 23.06, 13.91; HRFAB m/z 530.3726, calcd for (NH4)+(HCO2)ꢁ. The catalyst was collected by ltration
33H48B2O4 530.3739. through a celite pad and rinsed with EtOAc (200 mL). The
Octan-2-ylbenzene (3). n-Hexylmagnesium bromide was collected organic layer was extracted with saturated NH4Cl(aq)
C
prepared as follows: to Mg (4.40 g, 184 mmol) was added a (200 ꢂ 2 mL) to remove DMF. The collected organic ltrate
small portion of n-hexyl bromide (8.70 g, 53.0 mmol) in ether was dried over anhydrous MgSO4 and concentrated under
(150 mL) to initiate the reaction. Aer initiation, an additional vacuum to give 5 (7.2 g, 83%). 1H NMR (400 MHz, CDCl3): d
amount of n-hexyl bromide (20.0 g, 122 mmol) in ether 6.98 (d, J ¼ 8.4 Hz, 2H), 6.64 (d, J ¼ 8.4 Hz, 2H), 3.53 (bs, 2H),
(300 mL) was added dropwise. The solution was heated at 2.55–2.60 (m, 1H), 1.51–1.56 (m, 2H), 1.19–1.28 (m, 11H), 0.90
reux for 2 h.
(t, J ¼ 6.8 Hz, 3H); 13C NMR (100 MHz, CDCl3): d 143.83,
To a solution of acetophenone (20.0 g, 166 mmol) in ether 138.01, 127.49, 115.05, 39.07, 38.70, 31.90, 29.49, 27.78, 22.75,
(200 mL) at ꢁ78 ꢀC was added the freshly prepared n-hexyl- 22.61, 14.20; MS (EI) 205 (M+); HRMS (EI) calcd for C14H23N
magnesium bromide and stirred for 12 h at room temperature. 205.1830 (M+), obsd 205.1840.
The reaction was quenched by addition of diluted HCl, and the
1-Iodo-4-(octan-2-yl)benzene (6). Sulfuric acid (6.0 mL) was
product was extracted with CH2Cl2 several times. The collected added dropwise to a suspension of 5 (4.0 g, 2.6 mmol) in H2O
ꢀ
organic layer was dried over anhydrous MgSO4 and concen- (80 mL) at 0 C. The reaction mixture was stirred for 30 min.
trated under vacuum to provide a mixture (crude alcohol and Acetone (30 mL) was then added to the slurry, and stirred for
acetophenone, 31.2 g). The mixture was heated with stirring at another 15 min. A solution of NaNO2 (4.0 g, 58 mmol) in water
ꢀ
130 C for 2 day to give dehydration product. Chromatography (40 mL) was added dropwise. The resulting solution was stirred
on silica gel, using hexanes as the eluent, gave a 2-substituted for 60 min. A solution of KI (16.0 g, 96.0 mmol) in water (40
and 3-substituted alkene mixture (21.6 g, 115 mmol). Catalytic mL) was added slowly to the reaction mixture. The resulting
hydrogenation of the alkene mixture (21.6 g, 115 mmol) in dark brown solution was allowed to warm to room temperature
methanol (380 mL), using 10% Pd/C catalyst (0.3 g) under overnight. The reaction mixture was extracted with ethyl
atmospheric hydrogen for 48 h gave alkane crude product that acetate (3 ꢂ 200 mL). The combined organic layers were
was further puried by chromatography on silica gel, using washed with HCl(aq) (1 N, 1 ꢂ 80 mL) to remove any residual
hexane as the eluent to get 3 as colorless liquid (18.6 g, 59% in 3 amine. The organic phase was then washed with aqueous
1
steps). H NMR (400 MHz, CDCl3): d 7.26–7.30 (m, 2H), 7.15– solution of Na2S2O3 (20% w/w, 3 ꢂ 200 mL), dried, and
7.18 (m, 3H), 2.65–2.70 (m, 1H), 1.54–1.59 (m, 2H), 1.23–1.29 concentrated under reduced pressure. The resulting residue
(m, 11H), 0.87 (t, J ¼ 7.2 Hz, 3H); 13C NMR (100 MHz, CDCl3): d was puried by chromatography on silica gel, using hexane as
1
147.45, 127.84, 126.60, 125.34, 40.21, 38.74, 32.13, 29.75, 28.05, the eluent to get 6 as brown liquid (5.8 g, 94%). H NMR (400
23.06, 22.72, 14.55; MS (EI) 190 (M+); HRMS (ESI) calcd for MHz, CDCl3): d 7.38 (d, J ¼ 8.4 Hz, 2H), 7.04 (d, J ¼ 8.4 Hz, 2H),
C
14H22 190.1722 (M+), obsd 190.1723.
2.58–2.63 (m, 1H), 1.49–1.55 (m, 2H), 1.19–1.28 (m, 11H), 0.85
1-Nitro-4-(octan-2-yl)benzene (4). A mixture of concentrated (t, J ¼ 6.8 Hz, 3H); 13C NMR (100 MHz, CDCl3): d 146.72,
nitric acid (46 mL, 65%, density: 1.40 g mLꢁ1, 0.67 mmol) and 131.14, 128.64, 119.16, 39.56, 38.38, 31.87, 29.44, 27.69, 22.76,
sulfuric acid (30 mL, 98%, density: 1.84 g mLꢁ1, 0.14 mmol) 22.38, 14.22; MS (EI) 316 (M+); HRMS (EI) calcd for C14H21
I
was added slowly to 3 (80 g, 0.42 mmol) with stirring at 0 C 316.0688 (M+), obsd 316.0685.
(ice cooling) for 30 min, The mixture was stirred at 30 ꢀC for
4-(Octan-2-yl)benzenenitrile (7). To a mixture of CuCN (10.4
ꢀ
4 h, quenched by pouring on crushed ice (0.2 kg), and g, 117 mmol), and DMF (290 mL) under argon was added 6 (18.5
extracted with toluene (200 ꢂ 3 mL). The combined organic g, 58.5 mmol) at room temperature. The mixture was stirred at
ꢀ
extracts were washed with aqueous NaHCO3 (5%, 200 ꢂ 3 mL) 150 C for 48 h and then cooled to room temperature, diluted
and distilled water (500 mL). The collected organic extracts with CH2Cl2 (500 mL), and washed with H2O (3 ꢂ 200 mL). The
were dried over anhydrous MgSO4 and concentrated under organic phase was dried over anhydrous MgSO4, concentrated,
vacuum. The undesired o-NO2 isomer was removed by and the residue was puried by ash chromatography on silica
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with hexane–EtOAc (15/1) to give 7 (9.7 g, 77%). H NMR (400 g, 50%). Mw ¼ 1.14 ꢂ 104 and Mw/Mn ¼ 1.90. 1H NMR (400 MHz,
MHz, d6-DMSO): d 7.73 (d, J ¼ 8.8 Hz, 2H), 7.40 (d, J ¼ 8.8 Hz, CDCl3, d, ppm): 7.75–7.47 (m, 12H), 7.23–7.13 (m, 6H), 2.36 (br,
2H), 2.74–2.80 (m, 1H), 1.51–1.56 (m, 2H), 1.03–1.25 (m, 11H), 3H), 2.03 (br, 4H), 1.12–1.08 (br, 4H), 0.71–0.59 (br, 10H).
0.82 (t, J ¼ 7.2 Hz, 3H); 13C NMR (100 MHz, d6-DMSO): d 152.34,
131.41, 127.20, 118.23, 108.25, 38.89, 37.30, 31.20, 28.72, 27.04,
Results and discussion
Preparation of NOXD and PFT
22.14, 21.65, 13.93; MS (EI) 215 (M+); HRMS (EI) calcd for
C5H21N 215.1674 (M+), obsd 215.1670.
5-(4-Octan-2-ylphenyl)-1H-tetrazole (8). A mixture of 7 (12.9
g, 20.0 mmol), NaN3 (7.7 g, 0.12 mol), and NH4Cl (6.5 g, 0.12
mol) in DMF (60 mL) was heated at 130 ꢀC for 24 h. Aer cooling
to room temperature, the solvent was removed under vacuum.
The residue was poured into diluted HCl(aq) (1 N, 13 mL). Aer
rinsing with distilled water, the resulting residue was puried
by chromatography on silica gel, using ethyl acetate as the
eluent to get 8 as white solid (10.6 g, 69%) m.p. 123–124 ꢀC. 1H
NMR (400 MHz, d6-DMSO): d 7.94 (d, J ¼ 8.4 Hz, 2H), 7.41 (d, J ¼
8.4 Hz, 2H), 3.40 (b, 1H), 2.72–2.77 (m, 1H), 1.52–1.57 (m, 2H),
1.06–1.21 (m, 11H), 0.81 (t, J ¼ 6.8 Hz, 3H); 13C NMR (100 MHz,
d6-DMSO): d 155.06, 150.45, 127.60, 126.83, 121.93, 38.90, 37.58,
31.26, 28.75, 27.12, 22.12, 22.03, 13.98; MS (EI) 258 (M+); HRMS
(EI) calcd for C15H22N4 258.1844 (M+), obsd 258.1846. Anal.
calcd for C15H22N4: C, 69.73; H, 8.58; N, 21.69. Found: C, 69.70;
H, 8.62; N, 21.92%.
Conjugated uorescent polymer PFT was prepared from Suzuki-
coupling of the corresponding bis(4,40-bromophenylamine) 1
and bis(boronic ester) 2 (Scheme 1). While the electron-rich
triphenylamine unit (with high lying HOMO) could function as
an electron donor (D) in the exciplex formation, the presence of
the uorene units would enhance the photoluminescence
quantum yield.
NOXD was prepared through a multi-step synthesis (Scheme
2). The synthesis was started from nitration of 3 to give 4, fol-
lowed by reduction with Pd catalyzed hydrogenation to give 5.
The arylamine 5 was then converted to 6 through diazonium
salt intermediate. On treatment of 6 with CuCN, the corre-
sponding nitrile 7 could be obtained in a reasonable yield.
Reaction of 7 with NaN3 gave the corresponding tetrazole 8, a
key intermediate for oxadiazole formation. Finally, two equiv-
alents of
8
were coupled ꢀ with naphthalene-1,4-diacyloyl
dichloride64 in pyridine at 90 C to give NOXD.
Synthesis of NOXD
Tetrazole 8 (1.1 g, 4.3 mmol) in pyridine (4 mL) was stirred at
room temperature under N2 till a homogenous mixture is
formed. To this mixture, naphthalene-1,4-dicarbonyl dichlor-
ide64 (0.5 g, 2.1 mꢀmol) was added slowly. The reaction mixture
was heated at 90 C for 24 h and cooled to room temperature.
Major portions of the solvent were removed under vacuum. The
precipitated white solid was ltered and washed thoroughly
with ethanol, and dried in oven to give NOXD (0.7 g, 53%). m.p.
Photophysical properties of NOXD and PFT
The UV-vis absorption and uorescent spectra are shown in
Fig. 1, with the data being summarized in Table 1. Bisoxadiazole
NOXD shows two UV absorption bands peaked at 278 nm and
1
180–182 ꢀC. H NMR (400 MHz, CDCl3): d 9.42–9.45 (m, 2H),
8.38 (s, 2H), 8.13 (d, J ¼ 8.4 Hz, 4H), 7.80–7.83 (m, 2H), 7.39 (d,
J ¼ 8 Hz, 4H), 2.78–2.83 (q, J ¼ 7.2 Hz, 2H), 1.60–1.66 (m, 4H),
1.19–1.32 (m, 22H), 0.88 (t, J ¼ 7.2 Hz, 6H); 13C NMR (100 MHz,
CDCl3): d 164.15, 163.08, 152.16, 130.19, 128.37, 127.51, 126.87,
126.67, 126.43, 123.75, 120.83, 40.41, 38.49, 32.10, 29.68, 27.98,
23.02, 22.48, 14.54; MS (ESI) 641 (M + 1)+; HRMS (ESI) calcd for
C
C
42H48N4O2 641.3777 (M + 1)+, obsd 641.3856. Anal. calcd for
42H48N4O2: C, 78.71; H, 7.55; N, 8.74. Found: C, 78.40; H, 7.61;
Scheme 2
N, 8.85%.
Copolymer synthesis of PFT
To a degassed solution of 1 (0.8 g, 1.9 mmol) and 2 (1.0 g, 1.9
mmol) in toluene (15 mL), K2CO3(aq) (2.0 M, 7.5 mL), Aliquat 336
(15 mg) and Pd(PPh3)4 (32.7 mg, 1.47 mol%) was added under
N2. The reaction mixture was heated at 90 ꢀC for 3 days. The
mixture was cooled and poured into aqueous MeOH (150 mL,
7 : 3 v/v). The crude polymer was collected and washed with
excess MeOH. The crude PFT was re-dissolved in CH2Cl2, and
then reprecipitated into MeOH. Finally, PFT was collected and
washed with acetone for 48 h using a Soxhlet apparatus, and
followed by washing with MeOH for 48 h to remove the oligo-
Fig. 1 Photophysical properties of NOXD and PFT: (a) UV-vis absorption (UV),
fluorescence (FL), and low-temperature fluorescence spectra (LTFL) of NOXD; (b)
meric portions. The polymer was nally dried to give PFT (0.52 UV and FL in CHCl3, FL and LTFL in THF, and UV and FL in the film state of PFT.
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Table 1 Photophysical properties of PFT and NOXD
PFT and NOXD in lm were also collected as reference for
comparison. The emission spectrum of the PFT–NOXD (1 : 1)
lm was obtained by UV excitation at l ¼ 365 nm. The excitation
spectrum of the PFT–NOXD (1 : 1) lm was obtained by moni-
toring the emission intensity at l ¼ 563 nm. For the blue light
emitting NOXD and PFT, the excitation spectrum of the neat
lms was monitored at 444 nm respectively.
Under UV irradiation at 365 nm, the photoluminescent
spectrum of PFT–NOXD (1 : 1) in Fig. 2 shows broad emission,
with the lmax appearing at 564 nm (blue line). This is a typical
exciplex emission, which is red-shied from that of PFT (lmax at
436 nm) or NOXD (lmax at 439 nm). The exciplex formation was
further conrmed with time-resolved measurements of the
photoluminescence, in which the data are well-tted by two
single-exponential decay components, rendering lifetimes of s1
ꢃ 18.3 ns and s2 ꢃ 80.0 ns. Note that the remarkably long
lifetime of s2 ꢃ 80.0 ns (64.5%) is comparable to that of the
other oxadiazole–carbazole exciplex systems.60
The highest occupied molecular orbital (HOMO) of PFT and
the lowest unoccupied molecular orbital (LUMO) of NOXD were
evaluated respectively by cyclic voltammetry (CV). The oxidation
half potential (Eo1/x2) of PFT was found to be 0.87 V, corre-
sponding to the HOMO of ꢁ5.12 eV. On the other hand, the
reduction half potential (Er1e/2d) of NOXD was found to be ꢁ1.20 V,
corresponding to the LUMO of ꢁ3.02 eV. The HOMO (PFT)–
LUMO (NOXD) gap of 2.1 eV is consistent with the yellowish
exciplex emission at lmax ¼ 564 nm (2.2 eV). The small deviation
may arise from the solvent and electrolyte effects in the
CV analysis.
To further understand the exciplex formation mechanisms,
excitation spectra of the above samples were examined (Fig. 2).
For NOXD, the excitation spectrum shows two peaks at 300 and
350 nm. On the other hand, the excitation spectrum of PFT
exhibits broad signals starting from 300 nm, with the band
peaked at 350 and 400 nm, and extending to over 430 nm.
However, the signal at around 300 nm is weak. These spectral
differences allow us to differentiate which absorptions are
important for the exciplex emission. For thin lm of PFT–
NOXD, the excitation spectrum covers the region from 300–400
nm with the spectral pattern similar to the overlay of the exci-
tation spectra of NOXD and PFT, indicating that both
UV
max
ex c
l
(nm)
lmax, QY
l
(nm)
max
PFTa
386
378
360
350
443, 0.65
436, 0.43
423, 0.90
439, —
395
399
365
357
PFT (lm)b
NOXDa
NOXD (lm)
a
Comparing against coumarin 1 (ref. 5) (FPL ¼ 0.85) in THF solution.
b
Comparing against poly(9,9-dioctyluorene)5 (FPL ¼ 0.55) in thin-
c
lm state. Peaked wavelength in the excitation spectrum.
360 nm respectively in THF. Theoretical calculations reveal that
the S1 ) S0 band peaked at 360 nm involves p–p* electronic
excitation from the HOMO to the LUMO (see ESI†). On the other
hand, the band peaked at 278 nm involves more complicated
state mixing. The uorescent emission peaked at 423 nm shows
moderate Stokes shi of 64 nm. The low temperature uores-
cent spectrum at 77 K in THF glass is almost identical, with only
small pattern change, in comparison to that in THF at room
temperature. This result indicates that the structural relaxation
of NOXD aer photo-excitation in the S1 is small.
Conjugated polymer PFT shows two absorption bands in
CHCl3; one peaking at 386 nm and the other peaking at 300 nm.
As discussed in previous publications, the absorption band at
386 nm is arising from the uorene chromophore while the
shoulder band peaking at about 300 nm originates from
the absorption of the triphenylamine moieties.5 The UV lmax of
the PFT lm is slightly blue-shied by about 8 nm to 378 nm.
This may be due to the phenomenon of conformational
restrictions that prohibit the polymer to align in coplanar
conformations and therefore reduce the weight of
p-conjugation.
PFT shows single strong emission band at 434 nm under UV
irradiation in CHCl3. However, the photo-emission becomes
relatively broad in THF with the emission maximum slightly
red-shied to 443 nm. When PFT is frozen in the organic glass
at 77 K, photoluminescence shows emission bands peaked at
436 nm with a shoulder at 460 nm. In the lm state, ne
features with the major emission at 438 nm, a shoulder band at
460 nm, and other bands at 450–520 nm could be observed.
This may be due to exciton migration and energy transfer in the
solid lm that favors the high energy excitons stepping down to
the low energy one, leading to emission bands in bathochromic
region.
PFT–NOXD exciplex formation
To examine the exciplex from PFT–NOXD, homogeneous lm of
PFT–NOXD (1 : 1) was prepared by spin-coating (600 rpm) of the
CHCl3 solution (PFT (2 mg) and NOXD (2 mg) in CHCl3 (1 mL))
onto a piece of ITO glass plate. We adopted an empirical weight
ratio of PFT–NOXD (1 : 1) because this ratio provides an optimal
performance about the thermouoric phenomenon in latter
studies. The emission spectra and excitation spectra of PFT–
NOXD (1 : 1) were collected by Hitachi F4500 equipped with a
Fig. 2 Excitation spectra of NOXD (monitored at 444 nm), PFT (monitored at
444 nm), and emission and excitation spectra PFT–NOXD blend (monitored at
lm holder for analysis. In addition, the excitation spectra of 563 nm).
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absorptions of NOXD and PFT have signicant contribution to
the exciplex emission at 563 nm.
The emission spectrum of the PFT/NOXD lm evolves with
time. Aer being allowed to stand at room temperature for 72 h,
the emission intensity at 444 nm gradually arouse as shown in
Fig. 3. This result indicates that the homogeneous blend,
initially obtained by the spin-coating method, is morphologi-
cally unstable; phase separation might gradually occur to lead
to domains of aggregates. Within these regions, NOXD and PFT
are separated so that blue emission from NOXD or PFT might
occur. Therefore, dual emissions at 444 and 561 nm were
recorded. It is noteworthy to mention that differences in the
excitation spectra for 444 nm and 520 nm could be found; the
excitation spectrum of 300 and 366 nm for 444 nm is well
matched with that of NOXD, indicating that NOXD were
segregated from the homogeneous lm and become the major
source of the blue light emission at 444 nm. On the other hand,
the emission spectrum of the exciplex contains two components
from the absorption of NOXD and PFT.
Fig. 4 Photoluminescence of PFT–NOXD films: in neat PFT–NOXD; in PMMA–
PFT–NOXD (5 : 1 : 1); in PS–PFT–NOXD (5 : 1 : 1); in P(AN-co-MA)–PFT–NOXD
(5 : 1 : 1).
PFT–NOXD exciplex formation in polymer matrix
To evaluate the behavior of the exciplex in polymeric hosts,
aprotic polymers spanning a wide range of polarity, including
polystyrene (PS), poly(methyl methacrylate) (PMMA), and
poly(acrylonitrile-co-methyl acrylate) (acrylonitrile 94%) (P(AN-
co-MA)), were adopted as the host matrix for PFT and NOXD. In
a typical procedure, the polymeric host and the uorescent
components in CHCl3 were stirred for 4 h. The solution was
spin-coated (600 rpm) onto a piece of ITO glass that has UV cut-
off at 300 nm. The plate was then dried at 50 ꢀC for 30 min and
Fig. 5 Schematic diagram for PFT–NOXD. Phase-separation model and the PFT–
NOXD entanglement models for PFT and NOXD in polymeric hosts. Exciplex
emission from the PFT–NOXD pairs is expected.
ꢀ
at 120 C for 10 min, during the course of which the polymer
lm will be solidied.
When PFT and NOXD were blended into relatively non-polar
PS and PMMA matrix, the major emission occurred at the range
of 400–500 nm that is corresponding to the independent emis-
sion from PFT and NOXD (Fig. 4). Small shoulder at 560 nm is
assigned to the PFT–NOXD exciplex emission. These results
indicated that PFT and NOXD might be phase separated and
isolated by the host matrix, as illustrated in Fig. 5 in the phase-
separation model, and hence the exciplex formation was
retarded. On the other hand, when PFT and NOXD were blended
into P(AN-co-MA) matrix, dual emissions at 450 nm and 550 nm
were observed. The stronger emission at 550 nm clearly arouse
from the exciplex emission; this result implies that certain
extents of PFT and NOXD were entangled in P(AN-co-MA),
allowing the exciplex formation to be effective under radiation.
As illustrated in the PFT–NOXD entanglement model in Fig. 5,
the p–p interactions between PFT and NOXD may indeed benet
their aggregation in a polar media. This will enhance the prob-
ability of exciplex formation under photo-excitation.
Thermouoric behavior of PFT–NOXD exciplex formation in
P(AN-co-MA) matrix
It is noteworthy to mention that when the PFT/NOXD/P(AN-co-
MA) (1 : 1 : 10) lm was heated at 140 ꢀC for 2 min, the exciplex
emission intensity at 550 nm was signicantly enhanced. This
may be due to the fact that more NOXD molecules diffuse from
the NOXD aggregates into the polymer matrix at high temper-
ature. This would increase the chances of PFT–NOXD exciplex
formation. Noteworthy to mention is that this exciplex emission
could also be reversed back slowly when cooling at ambient
temperature. The evolution of the photoluminescent spectra
Fig. 3 The photophysical properties of the film of PFT–NOXD blend after
standing at ambient temperature for 72 h.
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Fig. 7 The thermofluoric behavior of the NOXD–PFT–PEPM (1 : 1 : 20) film: (a)
before thermal annealing; (b) after thermal annealing at 170 ꢀC; (c) after standing
at ambient temperature for about 60 min.
cooling cycles. However, if the mixing ratio of NOXD–PFT–PEPM
was reduced to 1 : 1 : 40, blue emission maintained even aer
thermal annealing, indicating that the concentrations of NOXD
and PFT are too low to generate the exciplex.
Fig. 6 The evolution of the photoluminescent spectra of the 140 ꢀC annealed
PFT/NOXD/P(AN-co-MA) with time.
The spectral changes of NOXD–PFT–PEPM (1 : 1 : 20) with
time were recorded and shown in Fig. 8a. While the yellow
emission gradually (560 nm) faded away, the blue emission
intensity increased. The excitation spectra about the bands at
449 nm and at 560 nm are shown in Fig. 8b. Again, while the
emission band at 560 nm is strongly related to the PFT
absorption, the blue emission band at 449 nm corresponds to
the NOXD emission. These results are governed by the equi-
librium between the PFT–NOXD entanglement state and the
PFT–NOXD phase-separation state. At room temperature, NOXD
crystallized from PEPM matrix. The concentration of NOXD in
the PEPM matrix was below the critical concentration so that
exciplex emission could not be observed. When the lm was
annealed at 160–170 ꢀC, the high temperature environment
allowed the NOXD molecules to be segregated from the crys-
talline regions. Increasing of the concentration of NOXD in the
matrix led to higher probability of exciplex formation with PFT.
Under this situation, the yellow emission increased and the
blue emission decreased. Aer then, when the plate was cooled
at ambient temperature, the NOXD might gradually diffuse
back to the crystalline regions; this reverse process led to
decrease of the yellow emission and enhancement of the blue
emission.
with time were recorded as shown in Fig. 6. The exciplex
emission intensity dropped back to 1/3 of the initial intensity
aer 20 h of relaxation.
The thermouoric phenomenon of PFT–NOXD exciplex
formation in a cross-linked poly(epoxy) matrix
The photoluminescent behavior of uorescence compounds in
a cross-linked matrix is also interesting target to study. In the
following study, we adopted commercially available epoxy glue,
including an epoxy resin and a polymercaptan hardener (Chern-
Young 0367, PowerBon 2996, or Sellery 09-652) as the basic
material for the cross-linked matrix. In a typical procedure,
epoxy resin with polymercaptan hardener (PEPM, 1 : 1), uo-
rescent components and chloroform were mixed and stirred for
4 h. The solution was spin-coated (600 rpm) onto a piece of ITO
glass plate that has UV cut-off at 300 nm. The plate was then
ꢀ
ꢀ
dried at 50 C for 30 min and at 120 C for 10 min, during the
course of which the epoxy-glue could be solidied and reached
to the full strength. The absorption behavior and the uores-
cent properties of the plate were recorded. Aer then, the plate
was heated to 160–170 ꢀC and then cooled in the ambient
temperature. The spectral changes were monitored by photo-
luminescence, and excitation spectrum measurements at
specic time.
When NOXD and PFT were blended into the PEPM matrix, the
uorescence behavior is highly dependent on the mixing ratio.
ꢀ
Aer the cross-linking treatment at 120 C, all the PEPM lms
exhibited blue uorescence under UV irradiation. The result
indicated that at the beginning the NOXD and PFT are phase-
separated into the PEPM matrix. Upon thermal annealing at 160–
ꢀ
170 C for 2 min under nitrogen atmosphere, the NOXD–PFT–
PEPM (1 : 1 : 10) turned into yellow emission under UV irradia-
tion. The yellow emission status maintained even aer 24 h. On
the other hand, when the lm of NOXD–PFT–PEPM (1 : 1 : 20)
was annealed under the same conditions, the intensity of the
yellowish photoluminescence decayed with time (Fig. 7).
However, the yellowish emission could be resumed if the plate
Fig. 8 Thermofluoric studies of the NOXD–PFT–PEPM (1 : 1 : 20) film: (a) pho-
toluminescence spectral change of NOXD–PFT–PEPM (1 : 1 : 20) with time. (b)
Excitation spectrum of the NOXD–PFT–PEPM (1 : 1 : 20) film after thermal
ꢀ
was re-annealed again at 160–170 C for 2 min. The emission
color change could be repeated for several thermal annealing– annealing at 170 ꢀC.
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Scheme 3
Microcrystals formed in the matrix could be directly
observed by cross-polarized microscope. Fig. 10a shows an
image of NOXD–PFT–PEPM (1 : 1 : 10) under illumination by
cross-polarized light. Observation of the dark background sug-
gested that the lm of NOXD–PFT–PEPM (1 : 1 : 10) is mainly
homogeneous. However, small bright dots that correspond to
the crystalline NOXD with varies sizes could be clearly observed.
The lm was further explored by using scanning electron
microscope, from which sub-micron features could also be
observed. Fig. 10b revealed that formation of particles with the
size about hundred nanometers are evidenced.
Fig. 9 Comparison of the DSC analyses: (a) for NOXD:PFT:PEPM; (b) for the
PEPM:NOXD films against PEPM.
To further understand the details about the temperature-
dependent uorescence changes, the NOXD–PFT–PEPM lms
were examined by differential scan calorimetry (DSC). The
results are summarized in Fig. 9. In the absence of NOXD–
PFT, the PEPM does not show any Tg in this temperature
region (Fig. 9a). NOXD alone showed a sharp melting point at
176 ꢀC, indicating that NOXD crystallized in solid lm
(Fig. 9b). When the lm of NOXD–PFT–PEPM (1 : 1 : 20) was
subjected to DSC analysis, a small transition was observed at
167 ꢀC (Fig. 9a) that is consistent with the aforementioned
thermouoric temperature. Since neither PFT nor PEPM
shows any transition in the temperature range, we suspect
that the transition originates from the melting of NOXD
crystalline regions.
To further understand this observation, samples of NOXD in
PEPM were prepared for examination. A at line was obtained
from the sample of NOXD–PEPM (1 : 40), suggesting that
NOXD–PEPM (1 : 40) formed a homogeneous lm in this
formulation (Fig. 9b); no sign about crystallization of NOXD
from the matrix was evidenced. However, when the fraction of
NOXD in PEPM increased to (1 : 30), a small signal peaked at
173 ꢀC was observed. Increasing the amounts of the NOXD–PFT
component in PEPM led to more intense signal, indicating
crystallization of NOXD from the matrix becomes more signif-
icant. Perhaps due to the donor acceptor interactions, the
presence of PFT in PEPM would lead to reduction of melting
point of NOXD from 173 ꢀC in NOXD–PEPM (1 : 20) to 167 ꢀC in
NOXD–PFT–PEPM (1 : 1 : 20).
Conclusions
The use of exciplex formation in polymer blend has played an
important role in modern organic electronic applications.65 In
the present study, we reported the exciplex formation behavior
of NOXD and PFT in various polymer matrix. Our aforemen-
tioned observations are summarized in Scheme 3: when NOXD
and PFT were mixed with non-polar polymers such as poly-
styrene and PMMA, perhaps due to poor miscibility, NOXD and
PFT were separated so that the exciplex emission could not be
observed. On the other hand, dual-emissions were observed
when NOXD and PFT were blended into P(AN-co-MA). This
observation indicated that heterostructures exist in P(AN-co-
MA) matrix; when the blue emission arises from isolated NOXD
or PFT, the yellow emission arises from the exciplex of NOXD–
PFT. The equilibrium could be altered so that the yellow exci-
plex emission was enhanced by increasing temperature. This
process was found to be reversible, even though the process was
slow in the highly viscous polymer matrix. Similar thermo-
uoric phenomenon were observed in cross-linked poly(epoxy)
matrix, and the process can be recyclable for few times. In
summary, we have successfully demonstrated the thermouoric
behavior of NOXD–PFT in polymeric matrix. By controlling the
exciplex formation through phase-separation, we can create
thermouoric lm with blue-yellowish switching mechanism at
specic temperature.
Acknowledgements
The present work was supported by the Ministry of Education,
Ministry of Economic Affairs, National Taiwan University, and
National Science Council of Taiwan (NSC-101-2113-M-002-010-
MY3, 98-2119-M-002-006-MY3, 99-2119-M-002-007, and 102-EC-
17-A-08-S1-183).
Fig. 10 Images of NOXD–PFT–PEPM (1 : 1 : 10): (a) cross-polarized microscope;
(b) SEM image.
20234 | RSC Adv., 2013, 3, 20227–20236
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