Synthesis and photophysical properties of triphenylamine-based multiply conjugated star-like molecules

Article abstract of DOI:10.1039/c5nj00605h

Novel triphenylamine-based star-like molecules 11 were synthesized and characterized using FT-IR, ¹H, ¹³C, and DEPT-135 NMR spectroscopy, and MALDI-TOF mass spectrometry. The absorption and emission spectra of star-like molecules were studied in different solvents. The effect of solvent polarity and aggregation studies on the absorption and emission spectra has also been studied. The new star-like molecules are found to exhibit broad absorption and emission band along with intramolecular charge transfer character. The fluorescence spectra of triphenylamine derivatives shift from blue to green wavelength on increasing the extended conjugation of the molecule. The experimental results indicate that there is cooperative enhancement originating from the inter-branch coupling and an increase in the light-harvesting ability upon increasing the conjugated molecule size.

Full text of DOI:10.1039/c5nj00605h

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Synthesis and photophysical properties of
triphenylamine-based multiply conjugated
Cite this:DOI: 10.1039/c5nj00605h
star-like molecules†
Rajendiran Balasaravanan,a Kumaraguru Duraimurugan,a Jayaraman Sivamani,a
Viruthachalam Thiagarajanb and Ayyanar Siva*a
Novel triphenylamine-based star-like molecules 11 were synthesized and characterized using FT-IR, 1H,
13C, and DEPT-135 NMR spectroscopy, and MALDI-TOF mass spectrometry. The absorption and
emission spectra of star-like molecules were studied in different solvents. The effect of solvent polarity
and aggregation studies on the absorption and emission spectra has also been studied. The new star-
like molecules are found to exhibit broad absorption and emission band along with intramolecular
charge transfer character. The fluorescence spectra of triphenylamine derivatives shift from blue to
green wavelength on increasing the extended conjugation of the molecule. The experimental results
indicate that there is cooperative enhancement originating from the inter-branch coupling and an
increase in the light-harvesting ability upon increasing the conjugated molecule size.
Received (in Montpellier, France)
11th March 2015,
Accepted 14th July 2015
DOI: 10.1039/c5nj00605h
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hole mobility,9 optoelectronic properties,10 thermal and electro-
chemical stability,11 and a fluorophore for emission. They have
Introduction
Dendrimers are a new class of polymers, exhibiting highly found optical applications in optical power limiting properties,12
symmetric, globular, size monodisperse macro molecules with two photon fluorescence excitation microscopy,13 photodynamic
a central core moiety, which have been extensively investigated therapy,14 large area displays,15 two dimensional (2D) light
over the past decades, because of their fascinating structure sources16 and three dimensional (3D) optical data storages.17,18
and attractive distinguished properties in organic, bioorganic They are also extensively used as hole transporting materials19,20
and materials chemistry.1–3 The core is usually preferred as the in photovoltaic devices,21 field effect transistors,22 organic light-
emitting chromophore and the end groups are long chain emitting diodes,23 and dye sensitized solar cells.24 Generally, a
charge transporting units which increase the solubility4 of the simple strategy for obtaining longer wavelength emission is
dendrimers. Triphenylamine (TPA) is often used as a core through donor–acceptor conjugation, such as donor–acceptor
moiety due to its electron richness and fluorescence properties. dipolar dyes, donor–p–donor, and donor–acceptor–donor quad-
Further, TPA is a well-known compound which exhibits a rigid rupolar dyes, which shifts the fluorescence band to a longer
plane, three dimensional sterics, a long conjugated length, wavelength and increases the Stokes shift. Further, with all those
amorphous nature, hole transporting properties,5 good light molecules in polar solutions, the emission is quenched, which
harvesting properties,6 and a long life time. TPA related compounds reduces the fluorescence efficiency. This fluorescence quenching
are naturally used for 3D systems due to the non-coplanarity of its correlates with an increase in polarity, by which the transition
three aromatic rings.7,8
from a local excited state to a highly polarized excited state, such
Recently, TPA related dendrimers have gained considerable as an intramolecular charge transfer state, is facilitated.25,26 The
attention due to their potential large two photon absorption cross polarized excited state is stabilized by solvation from polar solvent
sections, which allow simultaneous absorption of two photons, molecules, which leads to increased nonradiative deactivation.
enhanced non-linear responses, excellent electron donating ability,
In this work, we design a new synthetic procedure for the
properties of phenylenevinylene based star-like molecules which
contain TPA as a central core and long alkyl chain as end groups
(Fig. 1), tris(4-((E)-4-((E)-3,5-bis(dodecyloxy)styryl)styryl)phenyl)-
amine (DTPA) and tris(4-((E)-4-((E)-3,4,5-tris(dodecyloxy)styryl)-
styryl)phenyl)amine (TTPA). These star-like molecules were prepared
via a convergent synthetic approach by Wittig reaction and followed
by a triple site Heck coupling reaction. These TPA derivatives with
a Department of Inorganic Chemistry, School of Chemistry, Madurai Kamaraj
University, Madurai-625 021, Tamilnadu, India. E-mail: drasiva@gmail.com
b Department of Chemistry, School of Chemistry, Bharathidasan University,
Tiruchirappalli-620 024, Tamilnadu, India
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5nj00605h
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Thermal analyses were recorded using a TGA Q50 V20. 13 Build 39.
Mass spectra were analysed using a Voyager DE PRO biospectro-
metry workstation (Applied Biosystems) matrix-assisted laser
desorption ionization time-of-flight (MALDI-TOF) mass spectro-
scopy (MS) instrument. A pulsed nitrogen laser of 337 nm was
used, and the TOF was operated in the delayed extraction mode.
General procedure A for alkylation (2)20
Methyl hydroxybenzoate 1 (5 g, 0.027 mmol) was dissolved in
dry DMF (75 ml) under a nitrogen atmosphere and cooled to
0 1C. K2CO3 (22.5 g, 0.1630 mmol) was added to the above
solution, followed by 1-bromododecane (22.3 ml, 0.090 mmol).
After addition, the suspension was allowed to warm and was
stirred until the reaction was complete. Then, the reaction
mixture was poured into ice-water and extracted with ethyl
acetate. The combined organic layers were washed with brine
solution, dried over Na2SO4 and evaporated. The crude product
was purified by silica gel (60–120 mesh) column chromatography
using petroleum ether as an eluent. The pure product was a
colorless solid.
Methyl 3,4,5-tris(dodecyloxy)benzoate (2a). Synthesized according
to general procedure A using methyl 3,4,5-trihydroxybenzoate 1a and
1-bromododecane. Yield: 90%, FT-IR (cmÀ1) 1082, 1630, 1725;
1H NMR (300 MHz, CDCl3) dH 7.30 (d, 2H), 4.03–3.99 (t, 6H), 3.89
(s, 3H), 1.86–1.69 (m, 6H), 1.47–1.44 (m, 6H), 1.37–1.17 (m, 54H),
0.94–0.86 (t, 9H); 13C NMR (75 MHz, CDCl3) dC 166.84, 152.91,
142.48, 124.72, 107.88, 73.58, 69.11, 52.03, 31.86, 30.85, 30.26, 29.63,
29.57, 29.33, 26.02, 22.63, 14.04.
Methyl 3,5-bis(dodecyloxy)benzoate (2b). Synthesized according
to general procedure A using methyl 3,5-dihydroxybenzoate 1b
and 1-bromododecane. Yield: 90%, FT-IR (cmÀ1) 1076, 1635,
1720; 1H NMR (300 MHz, CDCl3) dH 7.15 (d, J = 2.1 Hz, 2H), 6.63
(t, J = 2.2 Hz, 1H), 3.96 (t, J = 6.5 Hz, 4H), 3.89 (s, 3H), 1.86–1.69
(m, 5H), 1.43 (d, J = 7.5 Hz, 6H), 1.26 (s, 36H), 0.88 (t, J = 6.6 Hz,
7H); 13C NMR (75 MHz, CDCl3) dC 167.38, 160.56, 132.20,
108.02, 106.97, 68.70, 52.53, 33.33, 30.07, 30.04, 30.00, 29.77,
26.42, 23.09, 14.50.
Fig. 1 Triphenylamine-based D–p–D molecules.
donor–p conjugation–donor (D–p–D) structures have a lower
band gap due to the intramolecular charge transfer between
donor and donor through conjugation and it may enhance the
two photon absorption properties of TPA derivatives. The three
dimensional structure of these star-like molecules can improve
the carrier and charge transport to provide favourable electronic
properties. Therefore, these star-like molecules have a symmetrical
D–p–D structure with TPA as a core and long alkyl groups at the
periphery. Furthermore, the phenylene vinylene groups enhance
the conjugation, and broaden the absorption band of these
compounds when compared with the intermediate compound
10. These conjugated star-like molecules have good solubility in
all common organic solvents and they have very good ICT
properties. Using cyclic voltammetry, these star-like molecules
exhibit lower HOMO and LUMO energy band gaps.
Experimental section
Materials and methods
All of the chemicals and reagents used in this work were analytical
grade. Methyl 3,4,5-trihydroxybenzoate, methyl 3,5-dihydroxy-
benzoate, 1-bromododecane, LAH, PCC, PBr3, (4-bromophenyl)
methanol, POCl3, KtOBu and Pd(OAc)2 were obtained from Sigma
General procedure B for reduction (3)27
Aldrich. DMF, toluene, DCM, CHCl3 were obtained from Merck The ester 2 (1 eq.) in THF was added to a suspension of lithium
and all the solvents obtained were laboratory grade. aluminium hydride (1.5 eq.) in THF at 0 1C. The suspension
The 1H and 13C NMR and DEPT-135 NMR spectra were was allowed to warm to room temperature and was stirred until
recorded on a Bruker (Avance) 300 & 400 MHz NMR instrument the reaction was complete. After completion of the reaction, the
using TMS as an internal standard, and CDCl3 and DMSO as mixture was quenched by 2 N HCl and filtered through celite.
solvent. Standard Bruker software was used throughout. Chemical The filtrate was extracted with EtOAc and washed with brine
shifts are given in parts per million (d-scale) and the coupling solution. The combined organic layers were dried over anhydrous
constants are given in Hertz. Silica gel-G plates (Merck) were used Na2SO4 and purified by column chromatography using petroleum
for TLC analysis with a mixture of pet. ether and ethyl acetate as an ether/ethyl acetate (4 : 1) as an eluent, to afford the target compound.
eluent. Column chromatography was carried out in silica gel
(3,4,5-Tris(dodecyloxy)phenyl)methanol (3a). Synthesized
(60–120 mesh) using pet. ether and ethyl acetate as an eluent. UV according to general procedure B using methyl 3,4,5-tris-
visible absorption spectra were recorded using a JASCO V630 (dodecyloxy)benzoate 2a (4 g, 0.8 mmol). Yield: 94%, FT-IR
spectrophotometer. Photoluminescence spectra were recorded (cmÀ1) 1080, 1650, 3435; 1H NMR (300 MHz, CDCl3) dH 6.56
using an Agilent 8000. Lifetimes were measured by Time Correlated (s, 2H), 4.59 (s, 2H), 3.99–3.91 (m, 6H), 1.82–1.69 (m, 6H), 1.59–
Single Photon Counting (TCSPC). Electrochemical measurements 1.46 (m, 6H), 1.43–1.11 (m, 54H), 0.90–0.86 (t, 9H); 13C NMR
were recorded using a CH instruments model 680 Amp Booster. (75 MHz, CDCl3) dC 153.21, 137.79, 136.15, 105.34, 73.41, 69.17,
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65.58, 31.90, 30.29, 29.68, 29.64, 29.59, 29.40, 29.29, 26.08, (m, 6H), 1.47–1.41 (m, 6H), 1.40–1.26 (m, 54H), 0.90–0.86 (t, 9H); 13
22.62, 14.09. NMR (75 MHz, CDCl3) dC 153.19, 138.41, 136.95, 132.77, 112.68,
C
(3,5-Bis(dodecyloxy)phenyl)methanol (3b). Synthesized according 104.96, 73.49, 69.15, 31.92, 30.33, 29.75, 2970, 29.65, 29.42, 29.36,
to general procedure B using methyl 3,5-di(dodecyloxy)benzoate 26.10, 22.69, 14.10.
2b (5 g, 0.71 mmol). Yield: 91%. FT-IR (cmÀ1) 1075, 1638, 3450;
1,3-Bis(dodecyloxy)-5-vinylbenzene (5b). Synthesized according to
1H NMR (300 MHz, CDCl3) dH 6.49 (s, 2H), 6.37 (d, J = 1.8 Hz, general procedure D using 3,5-di(dodecyloxy)benzaldehyde 4b (1 g,
1H), 4.61 (s, 2H), 3.93 (t, J = 5.8 Hz, 4H), 1.84–1.71 (m, 4H), 1.42 0.21 mmol) and methyltriphenylphosphonium iodide (0.9 g,
(d, J = 6.8 Hz, 4H), 1.26 (s, 32H), 0.88 (t, J = 5.7 Hz, 6H); 13C NMR 0.25 mmol). Yield: 75%, FT-IR (cmÀ1) 1070, 1623, 1685; H NMR
1
(75 MHz, CDCl3) dC 100.95, 143.59, 105.46, 100.97, 68.48, 65.88, (300 MHz, CDCl3) dH 6.75–6.63 (m, 1H), 6.60 (s, 2H), 6.43 (s, 1H),
32.33, 30.04, 30.01, 29.76, 26.45, 23.09, 14.52.
5.76 (d, J = 17.5 Hz, 1H), 5.27 (d, J = 10.8 Hz, 1H), 3.98 (t, J = 6.4 Hz,
5H), 1.93–1.76 (m, 5H), 1.50 (d, J = 6.0 Hz, 6H), 1.34 (s, 39H), 0.95
(t, J = 6.1 Hz, 7H); 13C NMR (75 MHz, CDCl3) dC 160.40, 139.40,
General procedure C for aldehyde (4)28
PCC (1.5 eq.) was added to a solution of alcohol 3 (1 eq.) in 137.01, 113.85, 104.78, 100.93, 67.93, 31.94, 29.69, 29.66, 29.63,
CH2Cl2 at 0 1C under a nitrogen atmosphere. The reaction was 29.42, 2938, 29.30, 26.08, 22.69, 14.08.
allowed to warm to room temperature and was stirred until the
4-Bromobenzyl bromide (6)30
reaction was complete. After completion of the reaction, the
mixture was quenched by 2 N HCl and filtered through a bed of PBr3 (1.5 ml, 1.6 mmol) was added dropwise to (4-bromo-
celite. The filtrate was extracted with EtOAc and washed with phenyl) methanol (1 g, 0.53 mmol) in CHCl3 and was stirred
brine solution. The combined organic layers were dried over at 0 1C under nitrogen. The resulting mixture was stirred for
anhydrous Na2SO4 and purified by column chromatography 6 h. After completion of the reaction, the mixture was quenched
using petroleum ether/ethyl acetate (97 : 3) as an eluent, to with iced water and neutralized with NaHCO3 (aq). Then, the
afford the target compound.
solution was extracted with EtOAc, washed with water and
3,4,5-Tris(dodecyloxy)benzaldehyde (4a). Synthesized according brine, and dried over anhydrous Na2SO4 to afford white needles
1
to general procedure C using (3,4,5-tris(dodecyloxy)phenyl)- as a product without further purification. Yield: 91%. H NMR
methanol 3a (2.2, 0.33 mmol). Yield: 85%, FT-IR (cmÀ1) 1075, (300 MHz, CDCl3) dH 7.40 (d, J = 8.3 Hz, 1H), 7.20 (d, J = 8.3 Hz,
1642, 1705; 1H NMR (300 MHz, CDCl3) dH 9.83 (s, 1H), 7.19 (s, 2H), 1H), 4.37 (s, 1H); 13C NMR (75 MHz, CDCl3) dC 136.76, 131.94,
4.08–4.01 (m, 6H), 1.88–1.80 (m, 6H), 1.48–1.42 (m, 6H), 1.40–1.20 130.65, 122.44, 32.40.
(m, 54H), 0.94–0.86 (t, 9H); 13C NMR (75 MHz, CDCl3) dC 191.22,
153.47, 143.76, 131.40, 107.75, 73.57, 69.15, 31.89, 30.31, 29.66,
4-Bromobenzyl bromide Wittig salt (7)30
29.63, 29.60, 29.51, 29.34, 29.21, 26.03, 22.66, 14.07.
4-Bromobenzyl bromide (0.5 g, 0.2 mmol) and triphenylphos-
3,5-Bis(dodecyloxy)benzaldehyde (4b). Synthesized according to phine (0.58 g, 0.22 mmol), in CH2Cl2 (5 ml) were stirred for
general procedure C using (3,5-bis(dodecyloxy)phenyl)methanol 3b about 8 h. After completion of the reaction, the phosphonium
(3.6 g, 0.76 mmol). Yield: 88%, FT-IR (cmÀ1) 1060, 1645, 1710; salt was scratched with Et2O. The crude product was diluted
1H NMR (300 MHz, CDCl3) dH 9.89 (s, 1H), 6.98 (s, 2H), 6.69 (d, J = with CH2Cl2 and scratched with Et2O, filtered and washed with
1.6 Hz, 1H), 3.98 (t, J = 6.4 Hz, 4H), 1.84–1.73 (m, 4H), 1.44 (d, J = 7.0 Et2O to afford the target compound as a white solid. Yield: 98%.
Hz, 4H), 1.27 (s, 32H), 0.88 (t, J = 6.2 Hz, 6H); 13C NMR (75 MHz, 1H NMR (300 MHz, CDCl3) dH 7.77 (dd, J = 13.3, 7.5 Hz, 9H),
CDCl3) dC 192.49, 161.17, 138.72, 108.42, 107.99, 68.84, 32.32, 30.04, 7.68–7.56 (m, 6H), 7.22 (d, J = 7.9 Hz, 2H), 7.05 (d, J = 8.5 Hz,
29.99, 29.75, 29.53, 26.40, 23.09,14.51.
2H), 5.55 (d, J = 14.6 Hz, 2H); 13C NMR (75 MHz, CDCl3) dC
134.97, 134.21, 133.13, 131.62, 130.11, 126.34, 122.49, 117.89,
116.75, 30.05.
General procedure D for styrene (5)29
Methyltriphenylphosphonium iodide (1.2 eq.) and aldehyde 4
(1 eq.) were dissolved in 10 ml of DCM under a nitrogen
Bis-(4-formylphenyl)amine (8)20
atmosphere and then KtOBu (2 eq.) was added slowly. The To a mixture of phosphoryl chloride (12.4 ml, 13.3 mmol) and
mixture was stirred for about 8 h. After completion of the dimethylformamide (12 ml, 17.4 mmol) at 0 1C triphenylamine
reaction, the mixture was poured into 100 ml of water. The (2 g, 0.66 mmol) in THF (10 ml) was added dropwise while
crude product was extracted with EtOAc and washed with brine. stirring. The resulting mixture was stirred at 95 1C under
The combined organic layers were dried over anhydrous Na2SO4. nitrogen overnight. The mixture was then cooled to room
The crude material was purified by column chromatography temperature, poured into ice-water (300 ml), and neutralized
using petroleum ether/ethyl acetate (99 : 1) as an eluent, to afford with 40% aq. NaOH. The brown coloured solution was extracted
a white solid as a product.
with EtOAc, washed with water and the combined organic
1,2,3-Tris(dodecyloxy)-5-vinylbenzene (5a). Synthesized according layers were washed with brine, dried over anhydrous Na2SO4
to general procedure D using 3,4,5-tris(dodecyloxy)benzaldehyde 4a and concentrated. The crude product was purified by column
(2.84 g, 0.44 mmol) and methyltriphenylphosphonium iodide (2.1 g, chromatography with petroleum ether/ethyl acetate as an eluent
1
0.52 mmol). Yield: 73%, FT-IR (cmÀ1) 1075, 1620, 1690; H NMR (10 : 1) to give a pale yellow solid. Yield: 70%, 1H NMR (400 MHz,
(300 MHz, CDCl3) dH 6.60 (s, 2H), 6.64–6.55 (m, 1H), 5.61 (d, J = CDCl3) d 9.89 (s, 2H), 7.78 (d, J = 7.9 Hz, 4H), 7.40 (t, J = 7.4 Hz,
17.5 Hz, 1H), 5.17 (d, J = 10.8 Hz, 1H), 4.00–3.92 (m, 6H), 1.84–1.79 2H), 7.27 (t, J = 7.3 Hz, 1H), 7.19 (d, J = 7.5 Hz, 6H); 13C NMR
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(100 MHz, CDCl3) dC 190.57, 152.02, 145.51, 131.34, 131.27, 1658 (CQC olefin), 1594 (CQC, aromatic stretch), 1120 (C–O
130.19, 127.09, 126.30, 122.78.
stretch), 1054 (trans olefinic bond), 1322 (C–N stretch for TPA).
1H NMR (400 MHz, CDCl3) d 7.49 (d, J = 3.9 Hz, 2H), 7.45 (d, J =
6.8 Hz, 1H), 7.41 (d, J = 8.5 Hz, 2H), 7.36 (d, J = 8.5 Hz, 2H), 7.11
Tris-(4-formylphenyl)amine (9)20
To a mixture of phosphoryl chloride (12.4 ml, 13.3 mmol) and (d, J = 8.5 Hz, 2H), 7.08 (s, 1H), 7.04 (s, 1H), 7.01 (d, J = 8.4 Hz,
dimethylformamide (12 ml, 17.4 mmol) at 0 1C bis-(4-formyl- 1H), 6.95 (d, J = 16.2 Hz, 1H), 6.74 (s, 1H), 4.03 (dt, J = 17.4,
phenyl)-amine 8 (2 g, 0.66 mmol) in THF (10 ml) was added 6.5 Hz, 3H), 1.88–1.77 (m, 3H), 1.54–1.47 (m, 3H), 1.30
dropwise while stirring. The resulting mixture was stirred at (s, 27H), 0.91 (t, J = 6.6 Hz, 6H). 13C NMR (100 MHz, CDCl3)
110 1C under nitrogen overnight. The mixture was then cooled d 153.28, 146.80, 138.35, 136.42, 132.39, 131.73, 128.68,
to room temperature, poured into ice-water (300 ml), and 127.76, 127.52, 127.45, 126.67, 126.64, 126.07, 124.36,
neutralized with 40% aq. NaOH. The brown colored solution 124.17, 120.98, 105.19, 73.52, 69.16, 31.91, 29.74, 29.72, 29.69,
was extracted with EtOAc, washed with water and the combined 29.65, 29.60, 29.44, 2942, 29.37, 29.35, 26.11, 22.67, 14.09; MS
organic layers were washed with brine, dried over anhydrous (MALDI-TOF): m/z calcd for C174H267NO9: 2515.04; found
Na2SO4 and concentrated. The crude product was purified by 2516.99.
column chromatography with petroleum ether/ethyl acetate as
Synthesis of DTPA (11b). Synthesized according to general
an eluent (4 : 1) to give a pale yellow solid. Yield: 22.7%. procedure E using tris-(4-((E)-4-bromostyryl)phenyl)amine 10
1H NMR (400 MHz, CDCl3) dH 9.95 (s, 3H), 7.85 (d, J = 8.3 Hz, (0.3 g, 0.038 mmol) and 1,3-bis(dodecyloxy)-5-vinylbenzene 5b
6H), 7.26 (d, J = 8.3 Hz, 6H); 13C NMR (100 MHz, CDCl3) dC (0.54 g, 0.12 mmol). Yield: 65%, FT-IR (cmÀ1) 3026 (C–H stretch),
190.54, 151.21, 132.59, 131.53, 124.56.
1662 (CQC olefin), 1590 (CQC, aromatic stretch), 1122 (C–O
stretch), 1058 (trans olefinic bond), 1322 (C–N stretch for TPA).
1H NMR (300 MHz, CDCl3) d 7.50 (s, 6H), 7.45 (d, J = 9.9 Hz, 6H),
Tris-(4-((E)-4-bromostyryl)phenyl)amine (10)31
The phosphonium salt 7 (1.86 g, 0.36 mmol) and tris-(4-formyl- 7.42–7.32 (m, 6H), 7.12 (d, J = 8.6 Hz, 6H), 7.10–7.03 (m, 9H), 6.96
phenyl)amine 9 (0.4 g, 0.12 mmol) were dissolved in CH2Cl2 (d, J = 16.2 Hz, 3H), 6.69 (s, 6H), 6.42 (s, 3H), 4.00 (t, J = 6.4 Hz,
(10 ml) under a nitrogen atmosphere, and KtOBu (0.82 g, 12H), 1.84–1.79 (m, 12H), 1.48 (d, J = 7.5 Hz, 12H), 1.30 (s, 96H),
0.73 mmol) was added to it portionwise at 0 1C. After addition, 0.91 (t, J = 6.2 Hz, 18H). 13C NMR (75 MHz, CDCl3) d 160.47,
the mixture was stirred for 8 h at room temperature. Then, the 146.80, 139.15, 136.87, 136.43, 135.15, 131.72, 130.37, 128.51,
reaction mixture was poured into 100 ml of water. The crude 127.75, 127.50, 126.84, 126.63, 126.07, 124.16, 120.98, 105.09,
product was extracted with EtOAc and washed with brine. The 100.94, 68.05, 31.88, 29.63, 29.60, 29.57, 29.37, 29.31, 29.27,
combined organic layers were dried over anhydrous Na2SO4. 26.04, 22.64, 14.07; MS (MALDI-TOF): m/z calcd for
Purification was done by column chromatography by eluting
with petroleum ether/ethyl acetate (99 : 1) to afford a green solid
as a product. Yield: 42%. FT-IR (cmÀ1) 1060 (trans olefinic
bond), 1624 (CQC, aromatic stretch), 1322 (C–N stretch for
TPA). 1H NMR (300 MHz, CDCl3) dH 7.39 (d, J = 8.4 Hz, 6H), 7.33
(d, J = 8.6 Hz, 6H), 7.28 (d, J = 8.5 Hz, 6H), 7.03 (d, J = 8.5 Hz,
C138H195NO6: 1962.49; found 1964.04.
Results and discussion
Synthesis and characterization of star-like molecules
6H), 6.97 (d, J = 9.3 Hz, 3H), 6.87 (d, J = 16.3 Hz, 3H); 13C NMR The initial focus of our systematic approach was on the synthesis of
(75 MHz, CDCl3) dC 146.84, 136.52, 131.79, 130.44, 128.76, triphenylamine-based star-like molecules with optical properties.
127.82, 127.58, 126.28, 124.33, 121.08.
The synthetic routes of two star-like molecules (TTPA and DTPA)
are depicted in Schemes 1–3. These star-like molecules were
synthesized via a convergent synthetic approach. Briefly, the impor-
General procedure E for Heck coupling reaction (11)
Tris-(4-((E)-4-bromostyryl)phenyl)amine (1 eq.), vinylated aromatic tant intermediate 10 was synthesized by the Wittig reaction between
compound (3 eq.), Bu4NBr (6 eq.), and Pd(OAc)2 (8 mol%) were the 4-bromobenzyl bromide Wittig salt (7) and tris-(4-formylphenyl)-
dissolved in a dry DMF/toluene (1 : 1) mixture. The resulting amine (9) in DCM using KtOBu as a base. Compound 9 was
solution was purged under nitrogen for half an hour, then triethyl- prepared according to a previously reported procedure20 and
amine (0.23 mmol) was added as a base and the reaction mixture compound 8 was reacted with POCl3 and DMF at 110 1C.
was heated to 95 1C and maintained overnight. After completion of Another key reactant 5 was obtained by stepwise reaction. First,
the reaction, the resulting mixture was passed through a bed of the alkylation was carried out between methyl hydroxybenzoate
celite and washed with ethyl acetate. The organic layer was further 1 and 1-bromododecane in DMF under a nitrogen atmosphere
washed with saturated brine solution and dried over anhydrous using K2CO3 as a base, then the alkylated product was reduced
Na2SO4. The crude material was purified by column chromato- from an ester to alcohol 3, using LAH in THF at 0 1C. Then, the
graphy using petroleum ether/ethyl acetate (97 : 3) as an eluent, to alcohol was oxidised with PCC in DCM at 0 1C to afford the
afford a greenish semi-solid as a product.
corresponding aldehyde 4. Finally, the vinylated product 5 was
Synthesis of TTPA (11a). Synthesized according to general obtained by a Wittig reaction between the aldehyde and
procedure E using tris-(4-((E)-4-bromostyryl)phenyl)amine 10 methyltriphenylphosphonium iodide in DCM under a nitrogen
(0.13 g, 0.016 mmol) and 1,2,3-tris(dodecyloxy)-5-vinylbenzene 5a atmosphere, using KtOBu as a base. The triple site Heck
(0.32 g, 0.05 mmol). Yield: 64%, FT-IR (cmÀ1) 3026 (C–H stretch), coupling reaction of intermediate 10 and the vinylated
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13C, and DEPT 135 NMR and mass spectrometry and all of the
spectral data are given in the ESI.†
Photophysical studies
The photophysical studies of TTPA (11a) and DTPA (11b) in a
series of protic and aprotic solvents have been carried out.
Table 1 shows the absorption and fluorescence data of 11a and
11b in different solvents. The absorption and emission spectra
of triphenylamine-based molecules and their intermediate 10
in THF are depicted in Fig. 2. The absorption spectrum of
intermediate compound 10 displayed at 390 nm and DTPA and
TTPA in THF display two absorption peaks centred at around
350 and 420 nm. These bands are attributed to the localised
aromatic p–p* transition. The first absorption band in the
shorter wavelength region between 250–390 nm is attributed to
the triphenylamine moiety. The longer wavelength absorption
band between 390 to 420 nm is ascribed to the bromophenylene
and alkoxylated vinylene modified TPA. The emission spectra of
the TPA derivatives were recorded by exciting at the longer
wavelength absorption maximum. The observed red-shift in
the absorption and emission maximum of DTPA and TTPA, as
compared with the triphenylamine core-containing bromo-
phenylene, reflects that there is an obvious increase in p–p*
delocalization with the increase of the generation of star-like
molecules, as the whole molecule has acted as an electron rich
molecule (i.e. D–p–D). In solution the vibronic splitting is much less
obvious because the individual components are broadened. These
results also demonstrate that the effective conjugation length
improves with the increasing generation of the star-like molecules.
Scheme 1 Synthesis of vinylated aromatic compounds.
Scheme 2 Synthesis of the Wittig salt.
compound 5 catalyzed by Pd(OAc)2 and in ligand TBAB in basic
media gives desirable star-like molecules of DTPA and TTPA. All
of the compounds were unambiguously characterized using 1H,
Scheme 3 Schematic representation for the synthesis of star-like molecules (11).
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Table 1 Absorption and emission values of DTPA and TTPA in various
organic solvents
DTPA
TTPA
labs lem Stokes
labs lem Stokes
(nm) (nm) shift (cmÀ1) (nm) (nm) shift (cmÀ1
)
Entry Solvents ET30
1
Acetonea 42.2 404 514 5297
402 505 5074
409 464 2898
407 499 4530
407 517 5227
399 524 5978
380 498 6235
404 492 4427
2
3
4
5
6
7
Benzeneb 34.3 410 468 3028
DCMa
DMFa
40.7 405 507 4967
43.2 406 528 5691
DMSOa 45.1 407 534 5843
MeOHc 55.4 388 509 6127
THFa
37.4 406 500 4630
Fig. 4 Dimroth–Reichardt solvent polarity correlations for DTPA and
TTPA.
a
b
c
Polar aprotic. Non-polar aprotic. Polar protic.
wavelengths with increasing solvent polarity; this shift is due to
the stabilization of the charge-transfer transition in polar
solvents. The observed higher value of the Stokes shift on
moving from non-polar to polar solvents is indicative of the
fact that the intramolecular charge transfer transition is greater
when compared to the ground state. Further, upon increasing
the solvent polarity, the emission spectra increased gradually,
due to the charge separation upon excitation,32 which was
confirmed using Dimroth–Reichardt solvent polarity correla-
tions33 (Fig. 4). Fig. 5 shows the fluorescence decays of DTPA
and TTPA in DMF monitored at 529 and 522 nm, respectively.
DTPA and DTPA exhibit a single exponential decay in DMF with
a lifetime of 2.31 and 2.19 ns, respectively. This confirms that
both TPA derivatives exhibit a single conformation in the
excited state. Furthermore, we also investigated the PL quan-
tum yields (FF, Table 2) of the star-like molecules 11 (11a/11b)
by using quinine sulphate in 0.1 M H2SO4 (refractive index
Z = 1.33) as a standard having a quantum yield of 0.54,34 and
the FF values of TTPA and DTPA were found to be 0.37 and 0.40,
respectively. The quantum efficiency of DTPA (11b) is slightly
higher than that of TTPA (11a) due to the lower energy of the
emitting states, which may in turn facilitate nonradiative
pathways.35
Fig. 2 Absorption and emission spectra of TPA derivatives in THF.
Further, we compared both the star-like compounds’ behaviour
as solid thin films. The absorption and emission characteristics
of DTPA and TTPA are presented in Fig. S3 (ESI†). From the thin
film coated absorption spectra of all of the TPA derivatives, they
show a broad band compared to the solution state which might
be contributing to the enhanced photoactivity under visible
light. The edge of the TTPA compound shows a remarkable
shift to the visible range compared to the DTPA compound, a
broad absorption covering the range of 300–550 nm (UV to
visible) for TTPA and a broad band for the DTPA compound
appearing in the 320–500 nm range. This may be attributed to
the absorption tendency of conjugated triphenylamine moieties.
We assume that the main contribution to the absorption in the
spectral region from 300–500 nm to 300–550 nm is due to the
monomer form rather than due to the aggregated form of the
TPA derivatives. This behaviour confirms the low tendency of
conjugated molecules to form aggregates in the solid state. A
similar behaviour was also found in the emission spectra.
The absorption and emission spectra of TPA derivatives
recorded in various solvents are shown in Fig. 3 and Fig. S1
(ESI†). The emission band is found to shift towards longer
Fig. 5 Fluorescence decay profile of DTPA and TTPA in DMF solution.
Table 2 Photophysical properties of DTPA and TTPA in THF
Compounds
labs (nm)
lem (nm)
Stokes shift (cmÀ1
)
FF
DTPA
TTPA
406
404
500
492
4630
4427
0.40
0.37
Fig. 3 Absorption and fluorescence spectra of TTPA in different solvents.
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Aggregation studies
In general, highly conjugated star-like molecules show an aggrega-
tion induced fluorescence quenching/enhancement behaviour.
Further, the solubility fully depends on the side groups attached to
the backbone so that good solubility of the conjugated molecules in
common organic solvents can be facilitated and it also allows for
easy processing due to their size and relative flexibility. However,
conjugated star-like molecules are able to aggregate to lower their
energy due to the long alkyl chains present in the molecules. Hence,
we would like to study the aggregation induced fluorescence
behaviour of TPA derivatives having highly branched conjuga-
tions and terminal alkyl chains. Further, we studied the absorp-
tion and emission behaviour of the aggregation process of the
TPA derivatives (11) by increasing the percentage of water in the
THF solution. Fig. 6 and Fig. S2 (ESI†) show the absorption and
emission behaviour of TTPA in pure THF and in THF/water
mixtures. Interestingly, the increase in the percentage of water
led to a decrease in the absorbance along with a clear isobestic
Fig. 7 Cyclic voltammograms of (a) DTPA and (b) TTPA in 0.1 M Bu4NPO4/
CH2Cl2, scan rate of 100 mV sÀ1
.
Table 3 The energy level parameters and the electrochemical properties
of DTPA and TTPA
c,d
11
Eonset (V) Epox a (V) HOMOb (eV) LUMOc (eV) Eg (eV)
DTPA 0.674
0.86
0.848
À5.274
À5.27
À2.634
À2.60
2.64
2.67
point at 445 nm. Above 60% of water, the precipitation of the TTPA 0.67
TPA derivatives leads to scattering in the absorption spectrum
a
b
c
Onset oxidation potential. Peak potential. HOMO (eV) = Àe(Eonset
+
d
Fig. 6a. With a gradual increase in the percentage of water, there
is a decrease in the emission intensity, similar to the absorption
spectra Fig. 6b. The observed red-shift in the emission spectra of
the TPA derivatives with the addition of up to 80% of water is
due to the increase in polarity, and a further increase in the
water fraction leads to a blue-shift. The blue-shift is likely due
to the formation of p-stacked aggregates with a cis to trans
formation and also a scattering of the light due to the precipita-
tion occurs.
4.6); LUMO = HOMO À Eg. Determined from UV-Vis absorption
spectra.
which means that the electron donating ability of the TPA is
enhanced (Fig. 7). The LUMO energy levels of DTPA and TTPA are
À2.194 and À2.16 eV, respectively, calculated from the HOMO
energy level and energy band gap (Eg) determined from the UV-Vis
absorption spectra (LUMO = HOMO À Eg). The energy level para-
meters and the electrochemical properties data are listed in Table 3.
The calculated ground-state geometries demonstrate that intra-
molecular charge transfer (ICT) occurs in both molecules during
the procedure of charge excitation from HOMO to LUMO. Generally,
high charge mobility containing molecules (D–p–D) have strong
intramolecular charge transfer. Hence, this restricts charge conjuga-
tion throughout the p system of the TPA derivatives and favours
charge delocalization by a ‘‘through space’’ mechanism. The experi-
mental results indicate that there is cooperative enhancement
originating from inter-branch coupling and an increase in light-
harvesting ability upon increasing the conjugated molecule size.
Electrochemical experiments
Fig. 7, shows the cyclic voltammetry (CV) diagrams of the two
star-like molecules of DTPA and TTPA in CH2Cl2 in the
presence of Bu4N(PO4) as the supporting electrolyte using a
glassy carbon working electrode, a platinum wire counter
electrode and saturated calomel as a reference electrode at a
scan rate of 100 mV sÀ1. Fig. 7 shows the quasi reversible redox
peaks of DTPA and TTPA. The oxidation peak potentials of
DTPA and TTPA are 0.86 and 0.848 V, respectively, suggesting
that the successive formation of the cation radical should be
attributed to the removal of electrons from the triphenylamine
groups. On the basis of the roughly evaluated onset oxidation
potentials, the HOMO energy levels of DTPA and TTPA are estimated
as À5.274 and À5.27 eV, respectively (HOMO = Àe(Eonset + 4.6)),
Thermogravimetric studies
The synthesized triphenylamine-based star-like molecules of
TTPA and DTPA were investigated using thermogravimetric
analysis (TGA) under a nitrogen atmosphere and a heating rate
at 20 1C minÀ1. From the TGA curve (Fig. S4, ESI†) the TTPA-
based dendrimers have initial decomposition temperatures
from 86.98 1C to 183.88 1C, with a gradual increase in the
weight losses from 0.05 to 2.5%. Similarly, DTPA has an initial
decomposition temperature of 80.88 1C then the decomposi-
tion temperature gradually increased to 365.40 1C and with
weight losses of 0.3% (80.88 1C), 5.24% (232.71 1C) and 19.82%
(365.40 1C), respectively, indicating the greater thermal stability
of the TPA derivatives. From the TGA results (Fig. S4, ESI†) we
observed that the DTPA-based star-like molecule is more stable
than the TTPA one. Further, we measured the DSC (Fig. S5, ESI†)
Fig. 6 Aggregation induced quenching of DTPA in THF/water system
DTPA (a) absorption and (b) emission spectra.
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number of alkoxy chains at the periphery. In addition to that,
there is no glass transition temperature detected; the reason for
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To summarize, novel triphenylamine-based star-like molecules
have been synthesized and well characterised using various
spectral data. The photophysical properties of the newly synthesized
star-like molecules of DTPA and TTPA were carried out in various
solvents. Comparing the two compounds DTPA and TTPA, both the
compounds have similar photo behaviours and almost equal life-
times in the excited state. This may be due to inter-branch coupling,
as a result of cooperative enhancement. Furthermore, the two newly
synthesized molecules exhibit a broad absorption range cover-
ing the whole visible spectral region in solution as well as in the
solid thin film phase. Based on the thermogravimetric results,
DTPA is more stable than TTPA. Hence, these photophysical
and electrochemical properties highlight that these materials
are potential candidates for use as donor materials in solution-
processable organic photovoltaic cells.
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Acknowledgements
RB and AS acknowledge the financial support of the Department
of Science and Technology, New Delhi, India (Grant No. SR/F/
1584/2012-13), Council of Scientific and Industrial Research,
New Delhi, India (Grant No. 01(2540)/11/EMR-II) and University
Grants Commission, New Delhi, India (Grant No. UGC No.41-
215/2012 (SR). VT thanks UGC-FRP (Grant No. F. 4-5(24-FRP)/
2013(BSR)) for financial support.
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