Unique photoluminescence of diacetylene containing dendrimer self-assemblies: Application in positive and negative luminescence patterning

Article abstract of DOI:10.1021/cm300762j

A series of poly(benzyl ether) dendrimers with two benzene-1,3,5- tricarboxamide (BTA) units bridged by diacetylene were synthesized. The formation of hydrogen bonds between BTA units to form the supramolecular self-assembly was confirmed by FT-IR and AFM

Full text of DOI:10.1021/cm300762j

Article
pubs.acs.org/cm
Unique Photoluminescence of Diacetylene Containing Dendrimer
Self-Assemblies: Application in Positive and Negative Luminescence
Patterning
Joo-Ho Kim, Eunyoung Lee, Young-Hwan Jeong, and Woo-Dong Jang*
Department of Chemistry, College of Science, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Korea.
S
* Supporting Information
ABSTRACT: A series of poly(benzyl ether) dendrimers with
two benzene-1,3,5-tricarboxamide (BTA) units bridged by
diacetylene were synthesized. The formation of hydrogen bonds
between BTA units to form the supramolecular self-assembly was
confirmed by FT-IR and AFM measurements. When the
dendrimers in CH2Cl2 was excited with 280 nm irradiation, the
dendrimers exhibited an unexpected strong photoluminescence
(PL) around 450 nm in addition to 310 nm fluorescence upon
excitation at 280 nm. The PL intensity of each dendrimers has
shown generation dependency. With the continuous irradiation of 280 nm UV light, dendrimers exhibited decreased PL emission
at both 310 and 450 nm by photopolymerization of diaceytlene units. A poly(vinylidene fluoride) (PVDF) membrane saturated
with dendrimer solution were prepared to obtain a patterned PL image. Utilizing a photomask, a negative PL pattern was
successfully obtained. When the membrane was annealed to 100 °C, new anomalous PL was generated and opposite type PL-
patterned image could be obtained.
KEYWORDS: supramolecular polymer, dendrimer, topochemical porlymerizatin, polydiacetylene, fluorescence pattern
diacetylene. Interestingly, these dendrimers exhibited unique
photoluminescence (PL) from BTA units.8 Herein, we report
the topochemical polymerization of a luminescent supra-
molecular assembly of diacetylene-containing dendrimers for
dual patterned PL imaging through two-step excitation energy
transfer process from dendritic wedges to the core structure.
INTRODUCTION
■
Dendrimers are regularly branched macromolecules with well-
predictable three-dimensional morphology that are used as
building blocks for the construction of organized functional
materials in a wide range of materials chemistry applications.1
The solution properties of dendrimers depend primarily on
their peripheral functionality, and the core structure is
substantially insulated from the outer environment. Therefore,
three-dimensional warping of functional systems using
dendrimer frameworks has great potential to provide very
interesting properties.2 For example, three-dimensional warping
of conjugated polymers by the poly(benzyl ether) dendritic
framework provided high fluorescence emission by excitation
energy transfer from dendritic wedges to the polymer
backbone.3 Such excitation energy transfer phenomena provide
a motivation in the design of novel photofunctional materials.4
Polydiacetylenes (PDA) are conjugated polymers with
alternating ene-yne backbone structures, which can be prepared
by topochemical polymerization of diacetylene monomers.5 It is
noteworthy that well-organized self-assembly of diacetylene
monomers is essential for topological polymerization.6 It is
desirable to utilize sufficiently strong and directional
interactions for the design of such well-ordered structure.
Benzene-1,3,5-tricarboxamide (BTA) is a potential scaffold for
the construction of self-assembled structures, since the three
amide units can provide strong hydrogen bonding interactions.7
Based on the above considerations, we synthesized a series of
poly(benzyl ether) dendrimers with two BTA units bridged by
EXPERIMENTAL SECTION
■
Materials and Measurements. All commercially available
reagents were reagent grade and used without further purification.
Dichloromethane, n-hexane, tetrahydrofuran (THF), and ethyl acetate
were freshly distilled before each use. Recycling GPC was carried out
on a JAI model LC-9201 equipped with JAIGEL-2H and JAIGEL-3H
column using THF as eluent. UV−vis, FT-IR, and PL emission spectra
were recorded using a JASCO model V-660, JASCO model FT-IR 430
spectrometer, and JASCO model FP-6300, respectively. The solid
phase fluorescence measurement was carried out using below-400 nm-
1
cut off filter over detector. H NMR spectra were recorded using a
Bruker DPX 400 (400 MHz) spectrometer. Analytical GPC was
carried out on a Younglin model Acme 9000 HPLC equipped with
PLgel PL1110−650d column. MALDI-TOF MS measurements were
performed on a Bruker model LRF20 using dithranol as a matrix.
Atomic force microscopy (AFM) was performed using VEECO model
Multimode, where the sample was prepared by spin-coating of a
toluene solution of 1-G3 (1.0 × 10−4 M) onto freshly cleaved mica
surface at 2000 rpm for 1 min.
Received: March 9, 2012
Revised: May 18, 2012
Published: May 24, 2012
© 2012 American Chemical Society
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Bulk Polymerization. For the bulk polymerization of dendrimers,
1.0 mg of 1-G3, 1-G2, and 1-G1 were placed onto clean quartz plates,
and small amount of toluene was dropped to the dendrimers until the
diameter of solution become about 10 mm. After the evaporation of
toluene, UV-irradiation was carried out by mercury lamp for
determined intervals (1, 2, 3, 5, and 10 min). After UV irradiation,
the polymers were treated with various organic solvents. The only
soluble portion of 1-G1 in CH2Cl2 was subjected to analytical GPC.
Fabrications of PL Patterns. PVDF membrane filter (Gelman
Science FP-450) was put into the toluene solution of 1-G3 (0.5 mM).
After the saturation of 1-G3, the PVDF membrane was dried under
dark condition. Two different types of photomask were utilized for the
fabrications of PL patterns. Printed image onto polyethylene was
utilized for the UV irradiation by Handy-UV lamp (ex: 254 nm,
VILBER LOURMAT, VL-4LC). Heating processes were carried out as
putting the membrane into an oven with temperature setting at 100
°C. For the UV irradiation in mercury lamp, thermally stable metal
photomask have been utilized. PL images were captured under
irradiation of 254 nm UV light by handy-UV lamp.
6-G1: To mixture solution of 5-G1 (7.24 g, 15.19 mmol), PPh3 (4.0
g, 15.19 mmol), and phthalimide (2.23 mg, 15.19 mmol) in dry THF
(50 mL) was slowly added DIAD (8.0 mL, 1.9 M solution in toluene,
15.19 mmol), and the solution was stirred under N2 for 3 h at 0 °C.
The solvent was removed under reduced pressure, and residue was
purified with column chromatography to give 6-G1 (7.42 g) as white
solid in 80% yield. MALDI-TOF-MS: m/z calcd for C39H59NO4,
1
606.44 [M + H]+; found, 606.13. H NMR (400 MHz, CDCl3): δ =
7.82−7.80 (m, 2 H; ArH in phthalimide), 7.69−7.67 (m, 2 H; ArH in
phthalimide), 6.54−6.33 (m, 3 H; ArH), 4.76 (s, 2 H; N−CH2), 3.93
(t, 4 H; −O−CH2), 1.79 (m, 4 H; −O−CH2−CH2), 1.40−1.23 (m,
36 H; −(CH2)9−), 0.88 (t, 6 H; −CH3).
7-G1: To a solution of 6-G1 (2.29 g, 4.07 mmol) in THF/EtOH
(20 mL, 1:1 v/v) was added hydrazine monohydrate (5 mL), and the
solution was refluxed for 1 h. The solvent was evaporated and
extracted with ethyl acetate and water. The organic layer was collected
and dried to give 7-G1 (1.43 g) as colorless liquid in 80% yield.
MALDI-TOF-MS: m/z calcd for C31H57NO2, 476.44 [M + H]+;
found, 476.244. 1H NMR (400 MHz, CDCl3): δ = 6.44−6.32 (m, 3 H;
ArH), 3.93 (t, 4 H; −O−CH2), 3.44 (s, 2 H; H2N−CH2), 1.76 (m, 4
H; −O−CH2-CH2), 1.40−1.23 (m, 36 H; −(CH2)9−), 0.88 (t, 6 H;
−CH3).
8-G1: To a mixture solution of 7-G1 (1.43 g, 3.01 mmol), 2 (268
mg, 1.20 mmol), EDC (634 mg, 3.31 mmol), and HOBt (447 mg, 3.31
mmol) in CH2Cl2 (30 mL) was added TEA (2 mL), and the solution
was stirred for 24 h at 25 °C. The reaction mixture was poured into
distilled water and extracted with ethyl acetate. The combined extracts
were evaporated to dryness and purified with column chromatography.
After freeze-drying, 8-G1 (1.16 g) was obtained as white powder in
85% yield. MALDI-TOF-MS: m/z calcd. for C72H118N2O8: 1161.88
[M + Na]+; found: 1158.63. 1H NMR (400 MHz, CDCl3): δ = 8.54 (s,
2 H; ArH), 8.42 (s, 1 H; ArH), 6.70 (t, 2 H; −NH), 6.44−6.34 (m, 6
H; o, p-H in ArH), 4.54 (d, 4 H; −NH−CH2), 3.93 (m, 11 H; −O−
CH2 and −CO2CH3), 1.79 (m, 8 H; −O−CH2−CH2), 1.40−1.23 (m,
72 H; −(CH2)9−), 0.88 (t, 12 H; −CH3).
9-G1: To a solution of 8-G1 (1.16 mg, 1.02 mmol) in THF (20
mL) was added 3 M NaOH solution (20 mL), and the solution
refluxed for 12 h. The reaction mixture was poured into 0.2 N HCl
aqueous solution (100 mL) and extracted with ethyl acetate (100 mL).
The combined extracts were dried over anhydrous MgSO4 and freeze-
dried from benzene to give 9-G1 (1.12 g) as white powder in 97%
yield. 1H NMR (400 MHz, CDCl3): δ = 8.51 (s, 2 H; ArH), 8.47 (s, 1
H; ArH), 6.42−6.36 (m, 6 H; o, p-H in ArH), 4.54 (d, 4 H; −NH−
CH2), 3.93 (t, 8 H; −O−CH2), 1.79 (m, 8 H; −O−CH2−CH2),
1.40−1.23 (m, 72 H; −(CH2)9−), 0.88 (t, 12 H; −CH3).
Quantum Yields. The quantum yield can be calculated from
followed equation:
Fif ni2
Φif =
Φsf
s
Fsf ns2
i
where Φif and Φsf are the photoluminescence QY of the sample and
that of the standard, respectively. The subscript f is used because in
most cases one is dealing with fluorescence. Fi and Fs are the integrated
intensities (areas) of sample and standard spectra, respectively (in
units of photons). fx is the absorption factor (also known under the
obsolete term “absorptance”), the fraction of the light impinging on
12
the sample that is absorbed (fx = 1 − 10−A , where A = absorbance).
x
As a reference compound, naphthalene (7 × 10−5 M) was utilized
(ex:270 nm, Φ = 0.23 in benzene).
Synthesis. 2: Trimethyl benzene-1,3,5-tricarboxlate (3 g, 11.9
mmol) and NaOH (0.96 g, 23.9 mmol) were dissolved in MeOH (150
mL) and refluxed for 12 h. After that, the reaction mixture was
evaporated and extracted with diethyl ether and water. The organic
solvent was dried over anhydrous MgSO4, and recrystallized with n-
1
hexane to give 2 as a white powder (1.74 g) in 65% yield. H NMR
(400 MHz, DMSO-d6): δ = 8.78 (d, 1 H; ArH), 8.76 (d, 2 H; ArH),
3.95 (s, 3 H; −CO2CH3).
3: To a mixture solution of hexa-2,4-diyne-1,6-diol (1.5 g, 13.6
mmol), PPh3 (7.2 g, 27.2 mmol), and phthalimide (4.0 g, 27.2 mmol)
in dry THF (30 mL) was slowly added diisopropylazodicarboxylate
(DIAD; 14.3 mL, 1.9 M solution in toluene, 13.6 mmol), and the
solution was stirred under N2 for 3 h at 0 °C. The solution was filtered
and residue was washed with THF to give 3 as a white powder (4.23 g,
84%). MALDI-TOF-MS: m/z calcd for C22H12N2O4, 391.07 [M +
Na]+; found, 391.25. 1H NMR (400 MHz, CDCl3): δ = 7.86−7.84 (m,
2 H; ArH in phthalimide), 7.75−7.71 (m, 2 H; ArH in phthalimide),
4.52 (s, 4 H; −N−CH2).
1-G1: To a mixture solution of 9-G1 (465 mg, 0.413 mmol), 4 (34
mg, 0.19 mmol), EDC (108 mg, 0.57 mmol), and HOBt (76 mg, 0.57
mmol) in CH2Cl2 (10 mL) was added TEA (1.5 mL), and the solution
was stirred 24 h at 25 °C. The solvent was evaporated and purified
with column chromatography and recycling GPC. After freeze-drying,
3 (283 mg) was obtained as white powder in 65% yield. MALDI-TOF-
MS: m/z calcd. for C148H236N6O14: 2345.79 [M + Na]+; found:
2345.546. Elemental Anal. Calcd: C, 76.50; H, 10.24; N, 3.62. Found:
1
4: 3 (2 g, 5.43 mmol) was dissolved in MeOH (40 mL) and
hydrazine monohydrate (4 mL, excess) was added and refluxed for 1 h.
The reaction mixture was filtered and filtrate was evaporated. The
residue was dissolved in 1 N HCl/MeOH and recrystallized with water
and THF to give 4 (133 mg) as crystalline solid in 13% yield. 1H NMR
(400 MHz, D2O): δ = 3.92 (s).
C, 76.48; H, 10.37; N, 3.61. H NMR (400 MHz, DMSO-d6): δ =
9.17 (b, 6 H; -NH), 8.51 (s, 4 H; ArH), 8.47 (s, 2 H; ArH), 6.42−6.36
(m, 12 H; o, p-H in ArH), 4.54 (d, 8 H; −NH−CH2), 3.93 (t, 16 H;
−O−CH2), 3.41 (s, 4 H; C−CH2), 1.79 (m, 16 H; −O−CH2−
CH2), 1.40−1.23 (m, 144 H; −(CH2)9−), 0.88 (t, 24 H; −CH3).
6-G2: To a mixture solution of 5-G2 (2 g, 4.45 mmol), PPh3 (1.16
g, 4.45 mmol), and phthalimide (654 mg, 4.45 mmol) in dry THF (10
mL) was slowly added DIAD (2.34 mL, 1.9 M solution in toluene,
4.45 mmol), and the solution was stirred under N2 for 3 h at 0 °C. The
reaction mixture was evaporated in reduced pressure and purified with
column chromatography. After freeze-drying from benzene 6-G2 (3.38
g) was obtained as white powder in 99% yield. MALDI-TOF-MS: m/z
5-G1: To a mixture solution of 1-bromododecane (10 g, 40.12
mmol) and 3,5-dihydroxybenzyl alcohol (2.74 g, 19.6 mmol) in dry
acetone (50 mL) was added K2CO3 (10.8 g, 78.3 mmol), and the
solution was refluxed under N2 for 12 h. The solvent was evaporated
and extracted with ethyl acetate and water. The organic layer was
collected and dried over anhydrous MgSO4. After silica column
chromatography, 5-G1 (7.24 g) was obtained as colorless liquid in
1
calcd. for C33H31NO8: 608.17 [M + K]+; found: 608.78. H NMR
1
76% yield. H NMR (400 MHz, CDCl3): δ = 7.51−7.36 (m, 3 H;
(400 MHz, CDCl3): δ = 7.82−7.80 (m, 2 H; ArH in phthalimide),
7.69−7.67 (m, 2 H; ArH in phthalimide), 6.64−6.39 (m, 9 H; o, p-H
in ArH), 4.93 (s, 4 H; −O−CH2), 4.76 (s, 2 H; −N−CH2), 3.76 (s, 12
H; -OCH3).
ArH), 4.63 (d, 2 H; −CH2Ar), 3.93 (t, 4 H; -OCH2), 1.79 (m, 4 H;
−O−CH2−CH2), 1.40−1.23 (m, 36 H; −(CH2)9−, 0.88 (t, 6 H;
−CH3).
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a
Scheme 1. Synthesis of Diacetylene-Containing Dendrimers
a
Reagents and conditions: (i) NaOH, MeOH, reflux, 12 h; (ii) PPh3, phthalimide, DIAD, THF, 0 °C, 3 h; (iii) H2NNH2·H2O, MeOH, reflux, 1 h;
(iv) PPh3, phthalimide, DIAD, THF, 0 °C, 3 h; (v) H2NNH2·H2O, THF/EtOH (1/1), reflux, 1 h; (vi) 2, EDC, HOBt, TEA, CH2Cl2, 25 °C, 24 h;
(vii) THF, NaOHaq, reflux, 12 h; (viii) 4, EDC, HOBt, TEA, CH2Cl2, 25 °C, 12 h.
7-G2: To a solution of 6-G2 (2 g, 3.509 mmol) in THF/EtOH (30
mL, 1:1 v/v) was added hydrazine monohydrate (7 mL), and the
solution was refluxed for 1 h. The solvent was evaporated and
extracted with ethyl acetate and water. The organic layer was freeze-
dried from benzene to give 7-G2 (1.042 g) as white powder in 68%
yield. MALDI-TOF-MS: m/z calcd. for C25H29NO6: 440.20 [M + H]+;
found: 439.902. 1H NMR (400 MHz, CDCl3): δ = 6.58−6.40 (m, 9 H;
o, p-H in ArH), 4.97 (s, 4 H; −O−CH2), 3.78 (s, 14 H; −OCH3,
NH2−CH2).
1-G2: To a mixture solution of 9-G2 (300 mg, 0.29 mmol), 4 (25.2
mg, 0.14 mmol), EDC (59 mg, 0.31 mmol), and HOBt (41 mg, 0.31
mmol) in CH2Cl2 (10 mL) was added TEA (1.5 mL), and the solution
was stirred 24 h at 25 °C. The solvent was evaporated and purified
with column chromatography and recycling GPC. After freeze-drying,
1-G2 (297 mg) was abtained as white powder in 47% yield. MALDI-
TOF-MS: m/z calcd. for C124H124N6O30: 2200.83 [M + Na]+; found:
2200.39. Elemental Anal. Calcd: C, 68.37; H, 5.74; N, 3.86, found: C,
68.39; H, 5.66; N, 3.74. 1H NMR (400 MHz, DMSO-d6): δ = 9.17 (b,
6 H; -NH), 8.51 (s, 2 H; ArH), 8.45 (s, 4 H; ArH), 6.73−6.39 (m, 36
H; o, p-H in ArH), 4.94 (s, 24 H; −O−CH2), 4.40 (d, 8 H; -N−CH2),
3.41 (s, 4 H; C−CH2) 3.74 (s, 48 H; -OCH3).
8-G2: To a mixture solution of 7-G2 (1.04 g, 2.366 mmol), 2 (259
mg, 1.154 mmol), EDC (487 mg, 2.54 mmol), and HOBt (343 mg,
2.54 mmol) in CH2Cl2 (10 mL) was added TEA (2 mL), and the
solution as stirred for 24 h at 25 °C. The reaction mixture was poured
into distilled water and extracted with ethyl acetate. The combined
extracts were evaporated to dryness and purified with column
chromatography. After freeze-drying, 8-G2 (724 mg) was obtained
as white powder in 59% yield. MALDI-TOF-MS: m/z calcd. for
6-G3: To a mixture solution of 5-G3 (3 g, 3.04 mmol), PPh3 (798
mg, 3.04 mmol), and phthalimide (448 mg, 3.04 mmol) in dry THF
(15 mL) was slowly added DIAD (1.6 mL, 1.9 M solution in toluene,
3.04 mmol), and the solution was stirred under N2 for 3 h at 0 °C. The
reaction mixture was poured into distilled water and extracted with
CH2Cl2. The organic layer was evaporated under reduced pressure,
and the residue was purified with column chromatography to give 6-
G3 (3.38 g) in 99% yield. MALDI-TOF-MS: m/z calcd for
1
C60H62N2O16: 1067.42 [M + H]+; found: 1067.09. H NMR (400
MHz, CDCl3): δ = 8.54 (s, 2 H; ArH), 8.42 (s, 1 H; ArH), 6.62−6.19
(m, 18 H; o, p-H in ArH), 4.93 (s, 8 H; −O−CH2), 4.57 (d, 4 H;
−N−CH2), 3.90 (s, 3 H; −CO2CH3), 3.78 (s, 24 H; −OCH3).
9-G2: To a solution of 8-G2 (724 mg, 0.68 mmol) in THF (7 mL)
was added 5 M NaOH aqueous solution (2 mL) and MeOH (5 mL),
and the mixture was stirred for 12 h at 25 °C. The reaction mixture
was poured into 0.2 N HCl aqueous solution (100 mL) and extracted
with ethyl acetate (100 mL). The combined extracts were dried over
anhydrous MgSO4 and freeze-dried from benzene to give 9-G2 (548
mg) as white powder in 77% yield. MALDI-TOF-MS: m/z calcd for
1
C65H63NO16, 1136.40 [M + Na]+; found, 1138.19. H NMR (400
MHz, CDCl3): δ = 7.82−7.80 (m, 2 H; ArH in phthalimide), 7.69−
7.67 (m, 2 H; ArH in phthalimide), 6.64−6.39 (m, 21 H; o, p-H in
ArH), 4.93 (s, 12 H; −O−CH2), 4.76 (s, 2 H; -N−CH2), 3.76 (s, 24
H; −OCH3).
7-G3: To a solution of 6-G3 (1.5 g, 1.347 mmol) in THF/EtOH
(20 mL, 1:1 v/v), hydrazine monohydrate (7 mL) was added and
refluxed for 1 h. The solvent was evaporated and extracted with ethyl
acetate and water. The organic layer was freeze-dried from benzene to
give 7-G3 (846 mg) as white powder in 64% yield. MALDI-TOF-MS:
1
C59H60N2O16: 1075.38 [M + Na]+; found: 1076.029. H NMR (400
MHz, CDCl3): δ = 8.51 (s, 2 H; ArH), 8.47 (s, 1 H; ArH), 6.61−6.36
(m, 18 H; o, p-H in ArH), 4.90 (s, 24 H; −O−CH2), 4.52 (s, 4 H;
−N−CH2), 3.74 (s, 24 H; −OCH3).
1
m/z calcd for C57H61NO14, 984.41 [M + H]+; found, 984.739. H
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NMR (400 MHz, CDCl3): δ = 6.67−6.40 (m, 21 H; o, p-H in ArH),
4.97 (s, 12 H; −O−CH2), 3.78 (s, 26 H; −OCH3, NH2−CH2).
8-G3: To a mixture solution of 7-G3 (845 mg, 0.86 mmol), 2 (94
mg, 0.42 mmol), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide
(EDC; 177 mg, 0.92 mmol), and 1-hydroxy benzotriazole (HOBt;
125 mg, 0 92 mmol) in CH2Cl2 (7 mL) was added triethylamine
(TEA; 2 mL), and the solution was stirred for 24 h at 25 °C. The
reaction mixture was poured into distilled water and extracted with
ethyl acetate. The combined extracts were dried over anhydrous
MgSO4 and purified with column chromatography. After freeze-drying
form benzene, 8-G3 (666 mg) was obtained as white powder in 74%
yield. MALDI-TOF-MS: m/z calcd for C124H126N2O32, 2178.82 [M +
Na]+; found, 2280.34. 1H NMR (400 MHz, CDCl3): δ = 8.54 (s, 2 H;
ArH), 8.42 (s, 1 H; ArH), 6.62−6.37 (m, 42 H; o, p-H in ArH), 4.93
(s, 24 H; −O−CH2), 4.52 (d, 4 H; −N−CH2), 3.90 (s, 3 H;
−CO2CH3), 3.78 (s, 48 H; −OCH3).
coated on a mica substrate was observed by AFM to have a
fibrous network structure of regular thickness (ca. 2 nm)
(Figure 1).
9-G3: To a solution of 8-G3 (666 mg, 0.31 mmol) in THF (7 mL)
were added 5 M NaOH aqueous solution (2 mL) and MeOH (5 mL),
and the solution was stirred for 12 h at 25 °C. The reaction mixture
was poured into 0.2 M HCl aqueous solution (100 mL) and extracted
with ethyl acetate (100 mL). The combined extracts were dried over
anhydrous MgSO4 and freeze-dried from benzene to give 9-G3 (630
mg) as white powder in 95% yield. MALDI-TOF-MS: m/z calcd for
Figure 1. AFM image of 1-G3 on mica (1.0 × 10−4 M).
However, in addition to 310 nm emission, an unexpected
strong PL was observed around 450 nm (Figure 2b) upon
1
C123H124N2O32, 2164.81 [M + Na]+; found, 2165.620. H NMR (250
MHz, CDCl3): δ = 8.51 (s, 2 H; ArH), 8.47 (s, 1 H; ArH), 6.61−6.36
(m, 42 H; o, p-H in ArH), 4.90 (s, 24 H; −O−CH2), 4.52 (s, 4 H;
−N−CH2), 3.74 (s, 48 H; −OCH3).
1-G3: To a mixture solution of 9-G3 (300 mg, 0.14 mmol), 4 (12.4
mg, 0.068 mmol), EDC (29 mg, 0.15 mmol), and HOBt (20 mg, 0.15
mmol) in CH2Cl2 (10 mL) was added TEA (1.5 mL), and the solution
was stirred 24 h at 25 °C. The reaction mixture was evaporated and
purified with column chromatography and recycling GPC. After
freeze-drying form benzene, 1-G3 (72 mg) was obtained as white
powder in 24% yield. MALDI-TOF-MS: m/z calcd for C252H252N6O62,
4395.65 [M + K]+; found, 4395.533. Elemental Anal. Calcd: C, 69.47;
1
H, 5.83; N, 1.93. Found: C, 69.49; H, 5.78; N, 1.81. H NMR (400
MHz, DMSO-d6): δ = 9.17 (b, 6 H; −NH), 8.51 (s, 2 H; ArH), 8.45
(s, 4 H; ArH), 6.73−6.39 (m, 84 H; o, p-H in ArH), 4.94 (s, 48 H;
−OCH2), 4.40 (d, 8 H; −N−CH2), 3.41 (s, 4 H; C−CH2) 3.74 (s,
96 H; −-OCH3).
RESULTS AND DISCUSSION
A series of poly(benzyl ether) dendrimers (1-Gn, n = nth
generation; Scheme 1), with two BTA units bridged by
diacetylene, were synthesized and characterized by H NMR
■
1
and MALDI-TOF-MS analysis. All of the dendrimers were
highly soluble in various organic solvents such as CHCl3,
CH2Cl2, DMSO, and toluene. Although the dendrimers were
highly soluble in CDCl3, the 1H NMR spectra displayed slightly
complex peak patterns, which simplified to sharp peaks after the
addition of a drop of CF3CO2H (see Figure S1 in the
Supporting Infomraiton). These results suggest the formation
of a molecular assembly via hydrogen bonding and that
CF3CO2H promotes dissociation of the self-assembled
dendrimer structures.7a
Figure 2. UV−vis absorption and PL emission of 1-G3 (0.125 mM, 1
mm path length) in CH2Cl2. (a) UV−vis absorption and (b) PL
emission changes of 1-G3 upon 280 nm UV irradiation.
The formation of hydrogen bondings was confirmed by FT-
IR analysis. FT-IR spectra in solid state displayed relatively
broad absorption bands at 3320 cm−1 and sharp bands at 1653
and 1530 cm−1, characteristic stretching vibrations for the N−
H, and amide I and II, respectively (see Figure S3 in the
Supporting Information). Although the FT-IR absorption
bands deviated slightly from the typical C3 symmetric helical
arrangement of BTA, the values were in good agreement with
previously reported values of an octyl substituted BTA, which
indicate the formation of hydrogen bonds between BTA units
to form the supramolecular self-assembly even in solution
state.9 In fact, a toluene solution of 1-G3 (1.0 × 10−4 M) spin-
excitation at 280 nm. Interestingly, this blue PL gradually
decreased by successive addition of MeOH (Figure 3).
Similarly, the blue PL was almost completely absent in
DMSO. In sharp contrast, the anomalous PL around 450 nm
became much stronger after the addition of a nonpolar solvent,
n-hexane (Figure 3). Because hydrogen bonding interactions
become more stable under nonpolar conditions, BTA core
units readily self- assemble into hydrogen-bonded aggregate by
the addition of n-hexane. Therefore, the anomalous PL can
possibly be explained by aggregation-induced enhanced
emission (AIEE).10
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mM) displayed peak broadening after being subjected to
continuous UV irradiation, indicating that polymerization of
diacetylene units led to a restriction of conformation change for
1
1-G3. In sharp contrast, the H NMR spectrum of 1-G3 after
the addition of a drop of CF3CO2H did not change during
photoirradiation since the dendrimers did not have preor-
ganized structure in the presence of CF3CO2H (see Figure S1
in the Supporting Information). As aforementioned, well-
organized-structure is essential for topological polymerization
of diacetylene monomers.6 The supramolecular assembly
formation of BTA units would provide an optimum geometry
for topochemical polymerization of diacetylene units even
under very dilute conditions.
In general, the pristine PDAs obtained by photopolymeriza-
tion are nonfluorescent, but they often become fluorescent by
the exposure to external stimulus, such as heat, pH change, and
mechanical forces. Because our dendrimers exhibited unique PL
emission, the PL emission change of 1-G3 was monitored upon
continuous irradiation of 280 nm UV light. As a result, the PL
emissions of 1-G3 at both 310 and 450 nm were gradually
decreased by the photopolymerization (Figure 2b). The
decrease of PL emission can be explained by an additional
excitation energy transfer process. The elongation of π-
conjugation length by photopolymerization lowers the
HOMO−LUMO energy level of the PDA backbone, and
additional excitation energy transfer pathway can be generated
from dendritic wedges or self-assembled BTA units to the PDA
backbone (Scheme 2).13 Like other PDAs, the PDA backbone
Figure 3. PL changes of 1-G3 in CH2Cl2 by addition of n-hexane and
methanol. (a) PL emission spectra and (b) relative PL intensity
between 450 and 310 nm.
Scheme 2. Proposed Mechanism of PL Changes
In relation with this fact, PL emission of structural fragments
also has been measured. Dendrons without BTA core unit (5-
G3 and 5-G2) exhibited PL emission only at 310 nm, but 8-G3
and 8-G2 having amide units emitted the anomalous PL around
450 nm (see Figure S4 in the Supporting Information),
indicating the anomalous PL is emitted from amide bearing
benzene moiety in core.
The PL intensity of each dendrimers has shown generation
dependency. Large dendrimer emitted stronger PL than smaller
ones at same concentration. In relation with this, quantum yield
of each dendrimer was measured (see Figure S5 in the
Supporting Information).12 As a result, 1-G3 exhibited the
highest quantum yield. Because the PL emission of dendrimers
predominantly takes place from BTA units, the highest
quantum yield can be explained by excitation energy transfer
from benzene moieties to core BTA units. A large number of
benzene moieties in dendritic wedges might efficiently absorb
light energy so-called “antenna effect”11 and transfer the
excitation energy to the BTA core units. In addition to the
generation dependency, the PL emission was gradually
increased by the concentration increment of dendrimers,
reinforcing the AIEE phenomena of dendrimers.
Another interesting aspect of the dendrimers occurred with
the continuous irradiation of 280 nm UV light. Under these
conditions, dendrimers exhibited slightly increased absorption
in λ < 450 nm (Figure 2a), which can be explained by photo-
polymerization of diacetylene units. Through the photo-
polymerization reaction, dendronized-polymers with PDA
backbone can be obtained. Because the PDA backbone in
polymers has elongated π-conjugation length, absorption in
long wavelength region can be increased. The photopolyme-
rization of dendrimers was also evidenced by 1H NMR
of polymerized 1-G3 would be nonfluorescent. Therefore, the
additional energy transfer would eventually decrease the
intensity of PL emission. Photopolymerization of dendrimers
in cast film was also performed. For the photopolymerization of
each dendrimer, a toluene solution was drop cast onto a quartz
plate, and the PL changes were observed using a fluorescence
spectrophotometer under continuous irradiation with 280 nm
UV light. All of the solid films exhibited strong PL around 450
nm, which had a generation dependency with the highest
generation dendrimer 1-G3 exhibiting the strongest PL
1
experiments. The H NMR spectrum of 1-G3 in CDCl3 (0.2
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emission (Figure 4). The PL intensity around 450 nm gradually
decreased over time. To determine the degree of polymer-
Figure 5. Photopolymerization of 1-G1 in cast film. (a) UV exposure
time-dependent size exclusion chromatogram and (b) ratio of
polymeric substances.
Figure 4. (a) PL changes of 1-G3 (red) and 1-G2 (black) in cast film
upon 280 nm UV irradiation. (b) The maximum intensity of each
spectrum.
ization, we treated the polymerized film with various organic
solvents, such as CH2Cl2, CHCl3, ethyl acetate, THF, acetone,
DMSO, and toluene. However, within 1 min of UV irradiation
by mercury lamp, dendrimers 1-G3 and 1-G2 resulted in
insoluble films with all of the tested solvents. Only 1-G1, which
had the smallest dendritic wedges and long alkyl chains, was
partially soluble in CH2Cl2 after photopolymerization. The
solution thus obtained was subjected to size exclusion
chromatography to yield a new peak in the high molecular
weight region in addition to the monomeric peak. The
proportion of high molecular weight polymer gradually
increased with UV exposure time. After 10 min of UV
irradiation, more than 50% of monomeric 1-G1 was consumed
to form polymer (Figure 5). It is noteworthy that 1-G1 also
resulted in a partial insoluble mass, indicating the ratio of high
molecular weight to monomeric compound was actually much
greater than 50%.
Because the PL emission gradually decreased with photo-
irradiation, the resulting dendrimers could be utilized for
patterned PL imaging, an emerging technology in the areas of
display, memory, sensors, and molecular switches.14 In order to
obtain a patterned PL image, a poly(vinylidene fluoride)
(PVDF) membrane was immersed into a 0.5 mM solution of 1-
G3 in toluene, followed by drying. The resulting 1-G3-doped
PVDF membrane (1-G3/PVDF) exhibited strong blue PL
upon UV irradiation that gradually dissipated over time.
Utilizing a photomask, a negative PL pattern was successfully
obtained after 10 min of light irradiation at 254 nm with a UV
handy lamp. As shown in Figure 6a and left half of e, the
masked areas continued to emit a strong blue PL, but the UV-
exposed areas have dark contrast. On the other hand, when the
1-G3/PVDF membrane was exposed to a mercury lamp for 3
Figure 6. PL patterned images and PL changes of 1-G3/PVDF. (a)
Negative PL pattern obtained by 254 nm UV irradiation using metal
photomask, (b) PL pattern obtained by UV irradiation for 3 min using
a mercury lamp, (c) positive PL pattern obtained by 254 nm UV
irradiation to a, (d) Image of photomask, (e) negative (left half) and
positive (right half) PL pattern, (f) Bright field image of 1-G3/PVDF
film after photoirradiation.
min, the UV-exposed areas exhibited positive contrast with a
bright greenish white PL emission (Figure 6b). Because PDA is
one of the representative polymers with stress-induced
colorimetric and fluorescence transitions,5 we hypothesized
that thermal energy emitted from a mercury lamp could
influence on the PL emission. Therefore, we investigated the
influence of thermal annealing on the negative PL pattern
obtained from UV handy lamp irradiation. Interestingly, after
the PL patterned 1-G3/PVDF membrane was annealed to 100
°C, the dark contrast areas again emitted a bright greenish
white PL, indicating that 1-G3/PVDF could provide dual
patterned PL imaging. When the thermally annealed 1-G3/
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PVDF or the PL pattern obtained from mercury lamp
irradiation (Figure 6b) were exposed to 254 nm UV light,
the original PL disappeared and a perfect positive PL pattern
resulted (Figure 6c and right half of e).
For precise observation of PL changes, the 1-G3/PVDF
membrane was placed on a fluorescence spectrophotometer,
and PL emission around 450 nm was monitored upon
continuous irradiation with 280 nm UV light. As a result, the
PL emission around 450 nm was found to greatly decrease
within 5 min (Figure 7a). When this 1-G3/PVDF membrane
even after photopolymerization (Figure 6f). Because the two
BTA units in dendrimer are possible to provide a preorganized
structure for the diacetylene units, a much more dilute
concentration can be utilized for diacetylene polymerization.
In addition, because photopolymerization involves excitation
energy transfer mechanism, light absorption predominantly
occurs at the large number of benzene moieties in the dendritic
wedges. This characteristic may be useful for unique
applications such as security ink, which needs to be composed
of invisible substances.15 In addition, the patterned images
would be impossible to photocopy and would be readable only
under specific conditions. These new dendrimers satisfy all of
the above criteria and may also provide dual patterned images,
i.e., positive and negative PL patterning.
CONCLUSIONS
■
In summary, poly(benzyl ether) dendrimers with two BTA
moieties bridged with diacetylene were designed to obtain π-
conjugated polymeric materials through topochemical polymer-
ization. The dendrimers emitted strong anomalous PL around
450 nm by energy transfer from the dendritic wedges to the
self-assembled BTA units. Excitation energy transfer from the
dendritic wedges induced photopolymerization of the diac-
etylene units. Polymerization of the diacetylene units resulted
in a new energy transfer pathway with greatly decreased blue
PL emission. Furthermore, the PDA backbone again emitted
greenish white PL when it was thermally annealed. Using the
unique optical properties of this material, we successfully
generated dual-patterned PL images.
ASSOCIATED CONTENT
* Supporting Information
■
S
Spectral date and quantum yields of each dendrimers. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 7. PL emission changes of 1-G3/PVDF upon (a) continuous
280 nm UV irradiation and (b) successive thermal annealing.
AUTHOR INFORMATION
Corresponding Author
*Tel.: +82-2-2123-5636. E-mail: wdjang@yonsei.ac.kr.
Notes
■
material was then annealed at 100 °C for 5 min, PL was
recovered with a slightly red-shifted emission (Figure 7b). The
increase in PL emission occurred over the entire visible range.
This observation indicates that the thermal energy reorganizes
the core PDA backbone from a nonfluorescent state to a
fluorescent state.5
The plausible mechanism of PL emission changes can be
proposed as following (Scheme 2). (i) The self-assembled BTA
core emits blue PL because a large number of benzene moieties
in dendritic wedges absorb UV light and transfer their
excitation energy to self-assembled BTA core. (ii) By the
photopolymerization of diacetylene units, the excitation energy
of BTA core again transfers to PDA backbone. In this stage,
PDA backbone is nonfluorescent state. (iii) Nonfluorescent
PDA backbone reorganized to fluorescent state by thermal
annealing. The dendrimers emit strong greenish white PL
through two-step energy transfer.
Generally, the polymerization of diacetylene monomers
results in a new absorption band around 640 nm.5 However,
unlike typical diacetylene monomers, absorption changes of the
dendrimers only occurred in invisible UV ranges (Figure 1a),
whereas PL emission greatly changed, because the emission of
dendrimers relies on the energy transfer mechanism. Hence,
the 1-G3/PVDF membrane maintained its original white color
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government
(MEST) (2011-0001126). J.-H.K. and Y.-H.J. acknowledge
fellowships from the BK21 program from the Ministry of
Education, Science and Technology, Korea.
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