Synthesis of gem-tetraphenylethylene oligomers utilizing Suzuki reaction and their aggregation properties

Article abstract of DOI:10.1016/j.dyepig.2013.07.008

Four new gem-tetraphenylethylene-based oligomers were synthesized by Suzuki coupling reaction in good yields (62-80%) and characterized by NMR, mass spectrometer and elemental analysis. The absorption maxima of the synthesized oligomers in the solution possess similar absorption bands located at 328-332 nm. They are shown to be thermally stable and emit light in blue region (404-485 nm). All the oligomers are AIE-active, emit weakly in solutions with Φ_f values not exceeding 4%, become strong emitters in the aggregated states and the PL intensity values increase up to 18-fold. The results of CV measurements of the oligomers showed good reversibility, indicating that the compounds have good electrochemical stability. The HOMO values of the oligomers are in the range of -5.63 to -5.61 eV. The photoluminescence properties in aggregate-state and the high HOMO energy levels of these oligomers make them applicable as an active material for a light-emitting device.

Full text of DOI:10.1016/j.dyepig.2013.07.008

Dyes and Pigments 99 (2013) 740e747
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis of gem-tetraphenylethylene oligomers utilizing Suzuki
reaction and their aggregation properties
*
Debabrata Jana, Shatabdi Boxi, Binay K. Ghorai
Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711 103, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Four new gem-tetraphenylethylene-based oligomers were synthesized by Suzuki coupling reaction in
good yields (62e80%) and characterized by NMR, mass spectrometer and elemental analysis. The ab-
sorption maxima of the synthesized oligomers in the solution possess similar absorption bands located at
328e332 nm. They are shown to be thermally stable and emit light in blue region (404e485 nm). All the
oligomers are AIE-active, emit weakly in solutions with Ff values not exceeding 4%, become strong
emitters in the aggregated states and the PL intensity values increase up to 18-fold. The results of CV
measurements of the oligomers showed good reversibility, indicating that the compounds have good
electrochemical stability. The HOMO values of the oligomers are in the range of ꢀ5.63 to ꢀ5.61 eV. The
photoluminescence properties in aggregate-state and the high HOMO energy levels of these oligomers
make them applicable as an active material for a light-emitting device.
Received 12 June 2013
Received in revised form
1 July 2013
Accepted 3 July 2013
Available online 15 July 2013
Keywords:
gem-tetraphenylethylene
Suzuki coupling
Cyclic voltammetry
Aggregation-induced emission
Fluorescence
Ó 2013 Elsevier Ltd. All rights reserved.
Blue chromophore
1. Introduction
aggregated state due to the physical restriction of its intramolecular
rotations. The unique luminescence behavior of TPE and other ro-
The basic interest behind a new generation of optical and elec-
tronic materials is driven by their utility towards the manufacture
of cheap and efficient electronic devices, such as photovoltaic cells
and light-emitting diodes [1]. Often, the design of new chromo-
phores relies on tuning the molecular electronic structure, yet it is
the properties of molecular aggregates that ultimately dictate de-
vice performance. Generally, most organic chromophores are
highly emissive in solution, but become non-emissive in the solid
state due to aggregation-caused quenching [2]. Some chromo-
phores, however, display the opposite effect: they show no emis-
sion in dilute solutions, but are brightly fluorescent upon
concentration or solidification. This recent phenomenon of aggre-
gation-induced emission (AIE) [3] or aggregation-induced emission
enhancement (AIEE) [4] is characteristic of relatively strained
molecules whose emission manifold involves orbital on fast
rotating groups such as terminal phenyl rings. For instance, tetra-
phenylethylene (TPE) is a typical AIE-active material. It is non-
emissive in dilute solution, as its active intramolecular rotations
effectively dissipate the energy of the excitons through non-
radiative pathways. On the contrary, it emits intensely in the
tors have been harnessed for the development of solid state light-
ing materials [5], biological sensors [6], chem-sensors [7], explosive
detection [8], latent finger print [9] and luminescent polymers [10].
In all these cases, short intermolecular TPE contacts are responsible
for the turn-on luminescence effect.
Recently, an abundant amount of work has been directed to-
wards the design of conjugated TPE-based organic molecules with
attractive optical, optoelectronic, and conductive properties. The
most intensely studied TPE-conjugated molecules with all-carbon
frameworks have extended linearly, as found in oligomers and
polymers. However, the photophysical properties that result from
alternative modes of
gem-substituted TPE have been less frequently studied and the
potential utility of materials with a fully gem-conjugated -back-
p-electron communication, particularly in
p
bone has not at all been explored. Wang et al. [11] have synthesized
TPE-based steric oligomers involving FriedeleCrafts-type acylation
of TPE in the presence of a Lewis acid (AlCl3) and then reacting with
diphenylmethyllithium at lower temperature (0 ꢁC), followed by an
acid catalyzed dehydration. In this paper, we report the synthesis,
photophysical and aggregation-induced emission (AIE) properties
of new methoxy-functionalized gem-conjugated molecular mate-
rials based on a tetraphenylethylene moiety, utilizing Suzuki
coupling reaction. We have also studied the electrochemical
behavior of the synthesized molecules.
* Corresponding author. Tel.: þ91 33 26684561; fax: þ91 33 26682916.
E-mail addresses: bkghorai@yahoo.co.in, binay@chem.becs.ac.in (B.K. Ghorai).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.07.008

D. Jana et al. / Dyes and Pigments 99 (2013) 740e747
741
2. Experimental section
(MHþ, 100%); Anal. Calcd for C27H22O: C, 89.47; H, 6.12. Found: C,
89.44; H, 6.14.
2.1. General
2.5. 4-(1-bromo-2,2-diphenylvinyl)biphenyl (4)
All melting points are uncorrected. Unless otherwise noted, all
reactions were carried out under an inert atmosphere in flame dried
flasks. Solvents obtained from Spectrochem were dried and purified
by distillation before use as follows: tetrahydrofuran, dioxane and
diethyl ether from sodium benzophenone ketyl; dichloromethane
from P2O5; DMF from CaH2; triethylamine from solid KOH. After
drying, organic extracts were evaporated under reduced pressure
and the residue was column chromatographed on silica gel (Spec-
trochem, particle size 100e200 mesh), using an ethyl acetateepe-
troleum ether (60ꢀ80 ꢁC) mixture as eluent unless specified
otherwise. 4,40-Dimethoxybenzophenone, 4,40-dibromobenzophe-
none, phenylboronic acid, 4-methoxyphenylboronic acid, bis(pina-
colato)diboron, bis(triphenylphosphine) palladium(II) dichloride
[PdCl2(PPh3)2] and tetrakis(triphenylphosphine) palladium(0)
[Pd(PPh3)4] were purchased from Aldrich and used as received.
To a solution of 2 (1.0 g, 3.01 mmol) in chloroform (15 mL) was
added dropwise a solution of bromine (0.44 g, 2.75 mmol) in
chloroform (10 mL). After discoloration of the solution, the solvent
was distilled off, and the residue was crystallized two times from
ethanol to give
4 (767 mg, 62%) as colorless crystals. Mp
132ꢀ133 ꢁC; IR (KBr, cmꢀ1): 3012, 1601, 1498; 1H NMR (400 MHz,
CDCl3):
d
7.54 (d, 2H, J ¼ 8.0 Hz), 7.42e7.36 (m, 10H), 7.33e7.30 (m,
2H), 7.08e7.06 (m, 3H), 7.00e6.97 (m, 2H); 13C NMR (100 MHz,
CDCl3): 143.8, 143.6, 141.1, 140.5, 140.2, 139.9, 130.8, 130.4, 129.5,
d
128.8, 128.2, 127.9, 127.6, 127.5, 127.1, 127.0, 126.6; LCMS (EI): m/z
413 (MHþ þ 2, 10%), 411 (MHþ, 10%), 347 (100%), 345 (98%); Anal.
Calcd for C26H19Br: C, 75.92; H, 4.66. Found: C, 75.85; H, 4.71.
2.6. 4-(1-bromo-2,2-diphenylvinyl)-40-methoxybiphenyl (5)
2.2. 1-bromo-4-(2,2-diphenylvinyl)benzene (1) [12]
The synthetic procedure used was similar to that described for 4.
Starting from 3 (1.0 g, 2.76 mmol) and bromine (0.40 g, 2.50 mmol),
bromo compound 5 (730 mg, 60%) was obtained as a white solid.
Mp 80e81 ꢁC; IR (KBr, cmꢀ1): 3005, 2950, 1601; 1H NMR (400 MHz,
A solution of benzophenone (1.82 g, 10 mmol) and diethyl 4-
bromobenzylphosphonate (3.1 g, 10 mmol) in anhydrous THF
(40 mL) was stirred under argon atmosphere at 0 ꢁC. Potassium
tert-butoxide (1.1 g, 10 mmol) was added quickly and the mixture
was stirred for 2 h at room temperature. The reaction mixture was
poured into ethanol (10 mL) and a solid precipitated out. The pre-
cipitate was filtered and washed with ethanol (3 ꢂ 10 mL) to afford
1 as a white powder (2.41 g, 72%). Mp 52e54 ꢁC; IR (KBr, cmꢀ1):
CDCl3):
d
7.58 (d, 2H, J ¼ 8.8 Hz), 7.44 (d, 2H, J ¼ 8.8 Hz), 7.43e7.32
(m, 6H), 7.25e7.17 (m, 3H), 7.01e6.95 (m, 3H), 6.92 (d, 2H,
J ¼ 8.8 Hz), 3.83 (s, 3H); 13C NMR (100 MHz, CDCl3):
d 159.4, 144.0,
143.5, 141.2, 140.1, 139.4, 132.7, 130.9, 130.4, 129.6, 128.3, 128.1,
128.0, 127.8, 127.6, 127.1, 126.1, 122.2, 114.3, 55.4; LCMS (EI): m/z 443
(MHþ þ 2, 10%), 441 (MHþ, 10%), 363 (100%); Anal. Calcd for
3032, 1610, 1484; 1H NMR (300 MHz, CDCl3):
d
7.25e7.18 (m, 3H),
C27H21BrO: C, 73.48; H, 4.80. Found: C, 73.39; H, 4.78.
7.16e7.06 (m, 5H), 7.05e6.94 (m, 4H), 6.92e6.82 (m, 3H); LCMS
(EI): m/z 335 (MHþ, 100%).
2.7. 2-(1-biphenyl-4-yl-2,2-diphenylvinyl)-4,4,5,5-tetramethyl-
[1,3,2]dioxaborolane (6)
2.3. 4-(2,2-diphenylvinyl)biphenyl (2)
To a solution of 4-(1-bromo-2,2-diphenylvinyl)biphenyl (4)
(1.0 g, 2.18 mmol) in toluene (15 mL) was added PdCl2(PPh3)2
(77 mg, 0.10 mmol), Ph3P (57 mg, 0.20 mmol), bis(pinacolato)
diboron (664 mg, 2.61 mmol), and KOPh (fine powder, 431 mg,
3.27 mmol) under argon atmosphere. The mixture was stirred at
70 ꢁC for the period of 12 h. The reaction mixture was treated with
water (10 mL) at room temperature, extracted with dichloro-
methane (20 mL), washed with brine, and dried over anhydrous
Na2SO4. The product was purified by column chromatography
(silica gel, EtOAc/petroleum ether 5:95) to give 6 (648 mg, 65%) as
white solids. Mp 125 ꢁC; IR (KBr, cmꢀ1): 3005, 2950, 1601; 1H NMR
A mixture of 1-bromo-4-(2,2-diphenylvinyl)benzene (1) (2.0 g,
5.97 mmol) and phenylboronic acid (873 mg, 7.16 mmol) was dis-
solved in toluene (15 mL) under argon atmosphere. Aqueous so-
lution of K2CO3 (2 mL, 2 M), tricaprylylmethylammoniumchloride
(AliquatÒ 336) (3 drops) and Pd(PPh3)4 (10 mg) was added to the
mixture. The reaction mixture was heated at 90 ꢁC for 6 h. After
being cooled to room temperature, distilled water (5 mL) was
added and extracted with dichloromethane (3 ꢂ 10 mL). Organic
layer was dried over anhydrous Na2SO4. The solvent was removed
under reduced pressure, the residue was purified using column
chromatography (silica gel, ethyl acetate/petroleum ether 5:95)
afforded 2 (240 mg, 85%) as a white solid. Mp 97e98 ꢁC; IR (KBr,
(400 MHz, CDCl3):
7.27e7.20 (m, 4H), 7.08e7.02 (m, 5H), 6.98e6.93 (m, 2H), 1.08 (s,
12H); 13C NMR (100 MHz, CDCl3):
151.5, 144.6, 141.8, 140.8, 140.7,
d
7.49 (d, 2H, J ¼ 7.2 Hz), 7.36e7.29 (m, 6H),
cmꢀ1): 3015, 1601, 1493; 1H NMR (300 MHz, CDCl3):
J ¼ 7.5 Hz), 7.42e7.20 (m, 15H), 7.08 (d, 2H, J ¼ 7.5 Hz), 6.99 (s, 1H);
13C NMR (75 MHz, CDCl3):
143.2, 142.5, 140.4, 140.3, 139.1, 136.3,
d
7.53 (d, 2H,
d
138.2, 130.9, 129.8, 129.7, 128.6, 127.9, 127.5, 126.9, 126.8, 126.7,
126.6, 83.6, 24.5; MS (ESI): m/z 481 (Mþ þ Na, 100%), 559 (MHþ,
50%).
d
130.0, 129.9, 128.7, 128.6, 128.1, 127.6, 127.5, 127.45, 127.4, 126.7,
126.4; LCMS (EI): m/z 333 (MHþ, 100%); Anal. Calcd for C26H20: C,
93.94; H, 6.06. Found: C, 93.88; H, 6.09.
2.8. 2-[1-(40-methoxybiphenyl-4-yl)-2,2-diphenylvinyl]-4,4,5,5-
tetramethyl-[1,3,2]dioxaborolane (7)
2.4. 4-(2,2-diphenylvinyl)-40-methoxybiphenyl (3)
The synthetic procedure used was similar to that described for 6.
Starting from 5 (1.0 g, 2.26 mmol), bis(pinacolato)diboron (690 mg,
2.71 mmol), and KOPh (fine powder, 450 mg, 3.40 mmol), com-
pound 7 (794 mg, 72%) was obtained as a white solid. Mp 89e90 ꢁC;
The synthetic procedure used was similar to that described for 2.
Starting from 1 (2.0 g, 5.97 mmol) and 4-methoxyphenylboronic
acid (1.10 g, 7.16 mmol), compound 3 (1.77 g, 82%) was obtained
as a white solid. Mp 115e116 ꢁC; IR (KBr, cmꢀ1): 3030, 2942, 1605;
IR (KBr, cmꢀ1): 3005, 2950, 1601; 1H NMR (400 MHz, CDCl3):
d 7.47
1H NMR (400 MHz, CDCl3):
d
7.60e7.46 (m, 3H), 7.40e7.20 (m, 10H),
7.19e6.90 (m, 6H), 3.85 (s, 3H); 13C NMR (75 MHz, CDCl3):
158.7, 143.1, 142.0, 140.1, 138.5, 135.4, 132.7, 130.0, 129.6, 128.3,
127.8, 127.5, 127.4, 127.3, 127.1, 125.7, 113.8, 54.9; LCMS (EI): m/z 363
(d, 2H, J ¼ 8.8 Hz), 7.36e7.27 (m, 8H), 7.10e7.04 (m, 4H), 7.02e6.97
(m, 2H), 6.91 (d, 2H, J ¼ 8.8 Hz), 3.81 (s, 3H), 1.12 (s, 12H); 13C NMR
d
(100 MHz, CDCl3): d 158.7, 155.8, 151.4, 144.6, 141.7, 139.9, 137.8,
133.2, 130.8, 129.7, 129.6, 129.3, 127.8, 127.7, 127.5, 126.7, 126.0,

742
D. Jana et al. / Dyes and Pigments 99 (2013) 740e747
120.0, 115.2, 113.9, 83.7, 55.1, 24.4; MS (ESI): m/z 511 (Mþ þ Na,
the residue was purified using column chromatography (silica gel,
ethyl acetate/petroleum ether 5:95) afforded the compound 11
(265 mg, 80%) as a white solid. Mp >200 ꢁC; IR (KBr, cmꢀ1): 3002,
100%), 489 (MHþ, 50%).
2.9. 4,4,5,5-tetramethyl-2-triphenylvinyl-[1,3,2]dioxaborolane (8)
[14]
2985, 1598, 1501; 1H NMR (400 MHz, CDCl3):
d
7.12e6.90 (m, 30H),
6.76 (d, 4H, J ¼ 8.4 Hz), 6.70e6.58 (m, 8H), 6.54 (d, 4H, J ¼ 8.4 Hz),
3.76 (s, 6H); 13C NMR (100 MHz, CDCl3):
158.0, 143.8, 143.6, 142.2,
d
The synthetic procedure used was similar to that described for 6.
Starting from 1-bromo-1,2,2-triphenylethylene (1.0 g, 2.98 mmol),
bis(pinacolato)diboron (909 mg, 3.49 mmol), and KOPh (fine
powder, 590 mg, 4.47 mmol), compound 8 (751 mg, 66%) was ob-
tained as a white solid. Mp 78e79 ꢁC; IR (KBr, cmꢀ1): 2929, 2360,
141.4, 140.9, 140.6, 139.8, 138.9, 136.4, 132.6, 131.37, 131.34, 130.6,
130.5, 127.6, 127.5, 127.4, 126.3, 112.8, 55.0; MALDI-TOF MS: m/z
901.6 (MHþ, 20%), 900.5 (Mþ, 50%), 899.5 (90%), 898.5 (100%); Anal.
Calcd for C68H52O2: C, 90.63; H, 5.82. Found: C, 90.59; H, 5.80.
1596, 1494; 1H NMR (300 MHz, CDCl3):
d
7.37e7.27 (m, 5H), 7.17e
2.13. gem-TPE oligomer 12
7.01 (m, 8H), 6.98e6.92 (m, 2H), 1.12 (s, 12H); MS (ESI): m/z 405
(Mþ þ Na, 100%).
The synthetic procedure used was similar to that described for
11. Starting from dibromo-tetraphenylethylene derivative
1
2.10. 2-[1-(4-methoxyphenyl)-2,2-diphenylvinyl]-4,4,5,5-
tetramethyl-[1,3,2]dioxaborolane (9) [14]
(200 mg, 0.36 mmol) and boronic ester 10 (370 mg, 0.90 mmol),
compound 12 (275 mg, 77%) was obtained as a white solid. Mp
>200 ꢁC; IR (KBr, cmꢀ1): 2995, 1610, 1545; 1H NMR (500 MHz,
The synthetic procedure used was similar to that described for 6.
Starting from 1-(1-bromo-2,2-diphenylvinyl)-4-methoxybenzene
(1.0 g, 3.50 mmol), bis(pinacolato)diboron (1.06 g, 4.20 mmol),
and KOPh (fine powder, 693 mg, 5.25 mmol), compound 9 (793 mg,
55%) was obtained as a white solid. Mp 97e98 ꢁC; IR (KBr, cmꢀ1):
CDCl3): d 7.18e7.09 (m, 12H), 7.08e7.00 (m, 8H), 6.92 (d, 4H,
J ¼ 9.0 Hz), 6.90 (d, 4H, J ¼ 9.0 Hz), 6.77 (d, 4H, J ¼ 8.0 Hz), 6.73 (d,
4H, J ¼ 8.0 Hz), 6.67 (d, 4H, J ¼ 8.0 Hz), 6.65 (d, 4H, J ¼ 8.0 Hz), 3.81
(s, 6H), 3.76 (s, 6H); 13C NMR (125 MHz, CDCl3):
d 158.2,158.1, 144.3,
144.2, 142.4, 141.8, 140.6, 139.9, 139.4, 136.6, 136.1, 132.7, 132.6,
131.6,131.5,130.8,130.7,127.8,127.6,126.3,113.1,113.0, 55.24, 55.21;
HRMS (ESI): Calcd for C70H56NaO4 (Mþ þ Na): 983.4076, found:
983.4075.
2989, 2369, 1596, 1502; 1H NMR (300 MHz, CDCl3):
d 7.35e7.24 (m,
5H), 7.12e7.05 (m, 3H), 7.01e6.92 (m, 4H), 6.67 (d, 2H, J ¼ 8.7 Hz),
3.73 (s, 3H), 1.12 (s, 12H); MS (ESI): m/z 435 (Mþ þ Na, 100%).
2.11. Compound 10
2.14. gem-TPE oligomer 13
To a suspension of powder zinc (4.30 g, 66 mmol) in dry THF
(70 mL) was added titanium(IV) chloride (3.62 mL, 33 mmol)
dropwise under argon at 0 ꢁC. The mixture was allowed to reflux for
4 h and then cooled to room temperature. A solution of bis-(4-
bromophenyl)methanone (2.8 g, 8.25 mmol) and 4,40-dimethox-
ybenzophenone (2.0 g, 8.25 mmol) in THF (50 mL) was added to
this suspension at once. Reaction mixture was heated to reflux for
5 h. The mixture was cooled to room temperature, quenched with
10% aqueous K2CO3 solution (100 mL). The dispersed insoluble
material was removed by vacuum filtration using a CeliteÒ pad. The
organic layer was separated, and the aqueous layer was extracted
with CH2Cl2 (3 ꢂ 50 mL). Combined organic fractions were washed
with water (20 mL) and dried over anhydrous Na2SO4. The solvent
was removed under reduced pressure to afford the crude product,
which was purified by column chromatography (silica gel, ethyl
acetate/petroleum ether 1:5) to afford 10 (951 mg, 42%) as a white
solid. Mp 140e142 ꢁC; IR (KBr, cmꢀ1): 3005, 2942, 2834, 1610; 1H
The synthetic procedure used was similar to that described for
11. Starting from dibromo-tetraphenylethylene derivative
1
(200 mg, 0.36 mmol) and boronic ester 8 (416 mg, 0.90 mmol),
compound 13 (235 mg, 62%) was obtained as a white solid. Mp
>200 ꢁC; IR (KBr, cmꢀ1): 3005, 2985, 1600, 1498; 1H NMR
(500 MHz, CDCl3): d 7.52e7.25 (m, 26H), 7.18e6.92 (m, 24H), 6.86e
6.60 (m, 4H), 3.77 (s, 3H), 3.76 (s, 3H); 13C NMR spectral data was
not recorded due to very poor solubility; MALDI-TOF MS: m/z
1054.3 (MHþ, 95%), 1052.4 (100%); Anal. Calcd for C80H60O2: C,
91.22; H, 5.74. Found: C, 91.18; H, 5.71.
2.15. gem-TPE oligomer 14
The synthetic procedure used was similar to that described for
11. Starting from dibromo-tetraphenylethylene derivative
1
(200 mg, 0.36 mmol) and boronic ester 7 (440 mg, 0.90 mmol),
compound 14 (304 mg, 76%) was obtained as a white solid. Mp
>200 ꢁC; IR (KBr, cmꢀ1): 3002, 2995, 1598, 1502; 1H NMR
NMR (300 MHz, CDCl3):
d
7.22 (d, 4H, J ¼ 8.4 Hz), 6.91 (d, 4H,
J ¼ 9.0 Hz), 6.86 (d, 4H, J ¼ 8.4 Hz), 6.65 (d, 4H, J ¼ 9.0 Hz), 3.75 (s,
(500 MHz, CDCl3):
6.84 (m, 30H), 6.83e6.60 (m, 10H), 3.84 (s, 6H), 3.83 (s, 6H); 13C
NMR (125 MHz, CDCl3): 159.2, 158.1, 144.0, 143.8, 142.4, 142.3,
d 7.58e7.40 (m, 6H), 7.37e7.27 (m, 6H), 7.20e
6H); 13C NMR (100 MHz, CDCl3):
d 158.4, 142.7, 141.5, 136.5, 135.5,
132.9, 132.4, 130.9, 120.2, 113.2, 55.0; MS (ESI): m/z 552 (Mþ þ 4,
d
56%), 550 (Mþ þ 2, 100%), 550 (Mþ, 48%), 392 (72%); Anal. Calcd for
141.6,140.8,140.7,138.5,136.6,135.3,135.2,133.3,132.7,131.9,131.6,
131.5, 129.9, 129.7, 128.8, 128.5, 128.2, 128.0, 126.5, 126.3, 125.8,
125.7, 114.3, 113.0, 55.4, 55.1; MALDI-TOF MS: m/z 1113.1 (Mþ, 20%),
1112.1 (40%),1111.1 (90%),1110.1 (100%); Anal. Calcd for C82H64O4: C,
88.46; H, 5.79. Found: C, 88.39; H, 5.81.
C
28H22Br2O2: C, 61.11; H, 4.03. Found: C, 61.05; H, 4.06.
2.12. gem-TPE oligomer 11
To a solution of dibromo-tetraphenylethylene derivative 1
(200 mg, 0.36 mmol) and boronic ester 9 (345 mg, 0.90 mmol) in
dioxane (6 mL) was added tricaprylylmethylammoniumchloride
(AliquatÒ 336) (3 drops) and 2 M sodium carbonate aqueous so-
lution (2 mL). The mixture was stirred for 0.5 h under argon at-
mosphere. Pd(PPh3)4 (10 mg) was added to the reaction mixture
and stirred at 90 ꢁC for 16 h. After cooling to room temperature,
reaction mixture was poured into water (15 mL) and extracted with
ethyl acetate (3 ꢂ 10 mL). The organic layer was dried over anhy-
drous Na2SO4. Solvent was removed under reduced pressure and
3. Results and discussion
3.1. Synthesis
A Pd-catalyzed Suzuki coupling reaction was used to build gem-
linked TPE oligomers. The synthetic approach towards necessary
intermediates 1e10 is outlined in Scheme 1. 1-Bromo-4-(2,2-
diphenylvinyl)benzene (1) was prepared according to the litera-
ture procedure [12]. Suzuki coupling of 1 with phenylboronic acid

D. Jana et al. / Dyes and Pigments 99 (2013) 740e747
743
R
R
R
Br
O
B
a
b
c
Br
O
1
2, R = H, 85%
3, R = OCH3, 82%
6, R = H, 65%
7, R = OCH3, 72%
4, R = H, 62%
5, R = OCH3, 60%
H3CO
Br
OCH3
Br
R1
R1
O
O
d
Br
c
B
O
42%
Br
Br
10
8, R1 = H, 66%
9, R1 = OCH3, 55%
Scheme 1. Reagents and conditions: (a) ArB(OH)2, Pd(PPh3)4, K2CO3, toluene, H2O, AliquatÒ 336, 90 ꢁC, 6 h; (b) Br2, CHCl3, rt; (c) (PPh3)2PdCl2, PPh3, bis(pinacolato)diboron, KOPh,
toluene, 70 ꢁC, 12 h; (d) Zn (powdered), TiCl4, THF, reflux, 4 h; then 4,40-dimethoxybenzophenone, reflux, 5 h.
in presence of Pd(PPh3)4/K2CO3/toluene/H2O provided 2 (85%
yield), and subsequent treatment of 2 with bromine resulted in 4-
(1-bromo-2,2-diphenylvinyl)biphenyl (4) with 62% yield. Lithiation
of 4 with 1.2 equiv of n-BuLi at ꢀ78 ꢁC, followed by the addition of
1.4 equiv of triisopropylborate (1.5 equiv of pinacol is used after
acidic workup to stabilize the boronic species) gave 2-(1-biphenyl-
4-yl-2,2-diphenylvinyl)-4,4,5,5-tetramethyl-[1,3,2]dioxaborolane
(6) in moderate yield. Good yield was obtained in the last step using
H3CO
OCH3
13
62%
6
H3CO
OCH3
H3CO
OCH3
H3CO
OCH3
8
9
10
80%
77%
12
7
76%
11
H3CO
OCH3
H3CO
OCH3
14
Scheme 2. Synthesis of gem-TPE-based oligomers 11ꢀ14. Reagents and conditions: Pd(PPh3)4, Na2CO3, dioxane, H2O, AliquatÒ 336, 90 ꢁC, 16 h.

744
D. Jana et al. / Dyes and Pigments 99 (2013) 740e747
5
the conditions developed by Miyaura et al. [13], i.e., the conversion
of bromo derivative 4 to the boronic ester 6 using palladium-
catalyzed cross-coupling reaction with bis(pinacolato)diboron in
presence of KOPh as a base. Similar procedure was used for the
preparation of 2-[1-(40-methoxybiphenyl-4-yl)-2,2-diphenylvinyl]-
4,4,5,5-tetramethyl-[1,3,2]dioxaborolane (7) from 4-(2,2-dipheny-
lvinyl)-40-methoxybiphenyl (3) in 72% yield. We have previously
reported the procedure for preparation of compounds 8 and 9 [14].
The dibromo-substituted TPE 10 was obtained from 4,40-dime-
thoxybenzophenone and 4,40-dibromobenzophenone using con-
ventional titanium(0)-catalyzed cross-McMurry reaction in 42%
yield.
2.0x10
11
12
13
14
5
1.5x10
5
1.0x10
4
5.0x10
The synthetic protocol for the targeted oligomers is depicted in
Scheme 2. To achieve the synthesis of these oligomers Suzuki-type
coupling reactions have been explored. Products 11e14 were ob-
tained in good yields ranging from 62 to 80%. They were purified
conveniently by flash column chromatography rather than vacuum
sublimation, usually used for the purification of insoluble organic
semiconductors. 1H and 13C NMR spectra and MALDI-MS associated
with HRMS/elemental analysis were employed to confirm the
structure and purity of all the new compounds.
0.0
400
450
500
550
600
Wavelength (nm)
Fig. 2. Photoluminescence spectra of oligomers 11e14 in dichloromethane (C ¼ 1.0 ꢂ
10ꢀ5 M).
and 4% respectively in dichloromethane. The fluorescent quantum
yield is very low because phenyl rings or groups in TPE molecule
rotate more freely in solution [15]. Oligomer 14 has higher Ffl
compared to others. It is known that in solution, the molecular size
and steric hindrance affect the rotation; a larger molecule should
have lower freedom of rotation. The effective conjugation paths
have been attenuated due to the presence of aryl substituent on the
cross-conjugation position. Therefore, the data of Ffl are arranged
in the order of 14 > 11 z12 z13.
3.2. Optical properties
The photophysical properties of the compounds were measured
by UVevis absorption and photoluminescence (PL) spectroscopy in
dichloromethane. Fig. 1 outlines the absorption spectra of the
compounds 11e14. These spectra show that oligomers 11e14
possess similar absorption bands located at 328e332 nm. The
predominant feature of the absorption behavior is that the molar
absorptivity (ε) of these oligomers increases with the number of
chromophoric groups (OCH3) or aryl groups (phenyl or, anisyl)
attached to the oligomers backbone. Oligomers 11e14 featured a
broad emission in the dichloromethane. Fig. 2 shows the PL spectra
of compounds 11e14. The PL spectra for 11 and 12 show sky blue
emission at 484 and 474 nm (lmax) respectively, whereas 13 and 14
exhibit blue emission at 417 and 416 nm (lmax). Oligomer 13/14
showed a hypsochromic shift compared to the 11/12 due to the
presence of aryl group (phenyl for 13 and anisyl group for 14) in the
cross-conjugated position. An increase in the steric hindrance
affected the planar structure of the TPE core and inhibited effective
conjugation. The PL emission intensities of the compounds in
dichloromethane increased in the order of 11 > 12 > 13 > 14, and
the fluorescence quantum yields (Ffl) of 11,12,13 and 14 are 3, 2.9, 3
3.3. Thermal stabilities
The thermal properties of the tetraphenylethylene derivatives
were analyzed by thermogravimetric analysis (TGA) and are pre-
sented in Table 1. The thermal decomposition (Td) temperatures (5%
weight loss in nitrogen atmosphere) were observed in the range of
327e362 ꢁC. These compounds exhibited moderate thermal sta-
bilities. The decomposition temperatures with 5% weight loss un-
der N2 atmosphere (Td) for 11, 12, 13 and 14 were 327, 330, 352 and
362 ꢁC, respectively (Fig. 3). The Td values increased with the
increasing size of the linkers.
3.4. Cyclic voltammetric studies
The electrochemical properties of compounds 11e14 were
investigated by cyclic voltammetry (CV) in anhydrous dichloro-
methane (Fig. 4) at room temperature and the results have been
summarized in Table 2. Two quasi-reversible anodic oxidation
0.6
11
12
couples were observed with potential difference (
D
E ¼ E1pa ꢀ E2pa
)
170, 70, 190 and 160 mV for 11, 12, 13 and 14 respectively. The first
13
0.4
14
Table 1
Optical and thermal properties of compounds 11e14.
a
c
Compound
UVevis (ε ꢂ 107
PL (nm)
F
%
Eg b (eV)
Td (ꢁC)
0.2
Mꢀ1 cmꢀ1) (nm)
11
12
13
14
331 (3.3)
332 (3.6)
329 (4.2)
328 (4.8)
404, 483
407, 476
418, 482
417, 485
3.0
2.9
3.0
4.0
3.74
3.73
3.76
3.78
327
330
352
362
0.0
a
Fluorescence quantum yields (F), measured in dichloromethane solution using
300
350
400
450
pyrene (Ffl ¼ 0.32, in CH2Cl2) as standard, excited at 275 nm.
b
Wavelength (nm)
Estimated from the onset of the absorption spectra: 1240/lonset
.
c
Decomposition temperature (Td) obtained from thermogravimetric analysis
Fig. 1. Absorption spectra of oligomers 11e14 in dichloromethane (C ¼ 1.0 ꢂ 10ꢀ5 M).
(TGA).

D. Jana et al. / Dyes and Pigments 99 (2013) 740e747
745
Table 2
Electrochemical properties of compounds 11e14.
100
80
60
40
20
0
Compound E1pc, Ep1a Ep2c, E2pa E (mV)a Eox (onset) HOMO/
D
(V)
LUMO (eV)b
11
12
13
14
11
12
13
14
ꢀ0.97, ꢀ1.19 ꢀ1.15, ꢀ1.36 170
ꢀ0.94, ꢀ1.17 ꢀ1.13, ꢀ1.24 70
ꢀ0.99, ꢀ1.15 ꢀ1.16, ꢀ1.34 190
ꢀ0.97, ꢀ1.14 ꢀ1.14, ꢀ1.30 160
0.97
0.96
0.97
0.95
ꢀ5.63/ꢀ1.89
ꢀ5.62/ꢀ1.89
ꢀ5.63/ꢀ1.87
ꢀ5.61/ꢀ1.83
a
D
E ¼ E1pa ꢀ E2pa
.
b
HOMO ¼ Eox (onset) þ 4.66; LUMO ¼ HOMO þ Eg.
(5.80 eV) and indium tin oxide (ITO) (4.80 eV), suggests these
oligomers can acts as hole-transporting materials (HTL).
3.5. AIEE properties
100
200
300
400
500
600
0
Temperature, C
Oligomers 11e14 were markedly fluorescent in the solid state as
revealed qualitatively using a hand-held UV lamp. In THF solution,
however, oligomers 11e14 show no luminescence. Changes in the
PL peak intensities versus wavelength with addition of water
Fig. 3. TGA curves for oligomers 11e14 in nitrogen at a heating rate of 10 ꢁC/min.
and second oxidation potentials of the oligomers decrease with the
increase in the number of electron donating methoxy substituents
5
6x10
and
DE values are relatively larger for 11 and 13 (two methoxy
THF
groups) compared to 12 and 14 (four methoxy groups). Oligomers 12
and 14, both have four electron donating methoxy groups, however,
11
5
THF + 90% H O
2
5x10
the magnitude of the observed oxidation potential differences (DE)
5
4x10
for 12 is 2.2 times less compared to 14. The electronic interaction is
highly dependent on the planarity of the molecule. The electronic
coupling is prevented with the increase in the degree of torsional
freedom of electron donating moiety to the cationic charge centre
[16]. The HOMO levels were calculated by using the known equation
HOMO ¼ Eox (onset) þ 4.66 [17]. The calculated HOMO values are in
the range of ꢀ5.63 to ꢀ5.61 eV. Calculating the HOMO-LUMO gap
(measured from the absorption maxima), the LUMO levels are found
to be w1.83e1.89 eV. The HOMO-LUMO gaps (Eg) of samples 11e14
are 3.74, 3.73, 3.76 and 3.78 eV, respectively. The result clearly
5
3x10
5
2x10
5
1x10
0
demonstrates that there is almost no effective
p-conjugation be-
400
450
500
550
600
tween central TPE moiety and the substituents at cross-conjugated
positions. In addition, the results of CV measurements of oligomers
show good reversibility, indicating that compounds have good
electrochemical stability. The HOMO levels (5.63e5.61 eV) lying
between those of tris(8-hydroxyquinoline)aluminum (Alq3)
Wavelength (nm)
Fig. 5. PL spectra of 11 (10
mM) in THF and with 90% amounts of water. The inset
depicts the changes of emission image with water fraction. Excitation wavelength:
330 nm.
5
5x10
-5
1.5x10
THF
11
12
5
12
THF + 90% H O
2
4x10
-5
13
1.0x10
14
5
3x10
-6
5.0x10
5
2x10
0.0
5
1x10
-6
-5.0x10
0
-5
400
450
500
550
600
-1.0x10
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Wavelength (nm)
Potential (V)
Fig. 6. PL spectra of 12 (10
mM) in THF and with 90% amounts of water. The inset
Fig. 4. Cyclic voltammetry of oligomers 11e14 (w1 mM) measured in 0.1 M Bu4NPF6e
CH2Cl2, scan rate 100 mV/s.
depicts the changes of emission image with water fraction. Excitation wavelength:
330 nm.

746
D. Jana et al. / Dyes and Pigments 99 (2013) 740e747
5
4. Conclusions
7x10
THF
5
13
In the present work, we have synthesized four new gem-tetra-
THF + 90% H O
2
6x10
phenylethylene oligomers via symmetric double-fold Suzuki
coupling and investigated their optical and electrochemical prop-
erties. The molar absorptivity (ε) of the oligomers steadily increases
with the introduction of the chromophoric group (OCH3) or an aryl
group unit (phenyl or, anisyl) in the oligomers backbone. All the
oligomers are AIE-active and emit weakly in solutions with Ff
values not exceeding 4%. But these compounds are strong emitters
in the aggregated states (PL intensity increases up to 18-fold). The
AIEE behavior is sensitive to the substituent and comparatively
increases in the presence of aryl (phenyl/anisyl) group in the olig-
omer. The electron donating ability (HOMO energy level) of these
oligomers is nearly equal, i.e., the introduction of methoxy/anisyl
donor functionalities into the geminal position slightly induces the
donating ability of oligomers. The HOMO levels of oligomers lying
between ITO (4.80 eV) and Alq3 (5.80 eV) suggest that these com-
pounds can be used as a hole-transporting material (HTL). All the
oligomers enjoy high thermal stability, which increases with
the molecular size. Further studies addressing the electrolumi-
nescence of new gem-TPE oligomers are currently underway in our
laboratory.
5
5x10
5
4x10
5
3x10
5
2x10
5
1x10
0
400
450
500
550
600
650
Wavelength (nm)
Fig. 7. PL spectra of 13 (10
depicts the changes of emission image with water fraction. Excitation wavelength:
330 nm.
mM) in THF and with 90% amounts of water. The inset
fraction to the mixture for oligomers 11e14 were examined.
Gradual addition of water had little effect on the fluorescence
spectra until a mixture of 9:1 H2O/THF was reached. When the
water fraction reached 90%, the fluorescence intensity of 11 was
found to get enhanced 13-fold (Fig. 5). Similar PL enhancement was
observed for the remaining three compounds. The PL intensities of
12, 13 and 14 got enhanced 12-fold (Fig. 6), 14-fold (Fig. 7) and 18-
fold (Fig. 8), respectively. This increase in fluorescence intensity
was considered to be a result of the AIEE effect. The results indicate
that the aryl group substituted TPE oligomers show more AIE-effect
compared to the parent TPE compounds. The emission images of
the oligomers in pure THF and 90% water fraction mixtures under
365 nm ultraviolet (UV) illuminations are shown in the inset pic-
tures of the corresponding Figures. The pronounced fluorescence
signals were observed centered at 490, 491, 494 and 497 nm for 11,
12,13 and 14 respectively. Addition of a large amount of water (90%)
into the THF solution of the oligomers causes their molecules to
aggregate. The formation of these nano-aggregates restricts the
intramolecular rotations of the aromatic rings around the carbone
carbon single bonds, which might be the cause of enhancement of
light emission [18].
Acknowledgments
Financial support from CSIR, New Delhi is gratefully acknowl-
edged. We are thankful to Professor Anup Mondal of this Depart-
ment for his generous help with the CV studied.
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