Photophysical properties of isoelectronic oligomers with vinylene, imine, azine and ethynylene spacers bearing triphenylamine and carbazole end-groups

Article abstract of DOI:10.1016/j.dyepig.2013.01.025

Ten symmetrical oligomers containing either triphenylamine or carbazole substituents as end-groups and 1,4-phenylenevinylene, 1,4-phenylene imine, azine and 1,4-phenylene ethynylene as π-spacers, have been synthesized by polycondensation reactions between aromatic aldehydes (4- formyltriphenylamine and 3-formyl N-hexylcarbazole) or iodides (4-iodotriphenylamine and 3-iodo Nhexylcarbazole) with bisphosphonate derivative, 1,4-diaminobenzene, hydrazine or 1,4- diethynylbenzene, respectively. These oligomers are models for the corresponding conducting polymers, have a well-defined molecular structure, can be highly purified using common methods and processed as thin films by vacuum evaporation, dip or spin coating. The oligomers preserve all of the properties and potential applications of the corresponding polymers. The structural characterization of these oligomers was performed using usual spectroscopic methods (1H-RMN, FT-IR and DSC) and their photophysical properties were analyzed by UVeVis and fluorescence spectroscopy. Their redox properties were studied by cyclic voltammetry.

Full text of DOI:10.1016/j.dyepig.2013.01.025

Dyes and Pigments 98 (2013) 71e81
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Photophysical properties of isoelectronic oligomers with vinylene,
imine, azine and ethynylene spacers bearing triphenylamine and
carbazole end-groups
*
Mircea Grigoras , Loredana Vacareanu, Teofilia Ivan, Ana Maria Catargiu
“P. Poni” Institute of Macromolecular Chemistry, Electroactive Polymers Department, 41A Gr. Ghica Voda Alley, 700487 Iasi, Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
Ten symmetrical oligomers containing either triphenylamine or carbazole substituents as end-groups
Received 29 October 2012
Received in revised form
24 January 2013
Accepted 27 January 2013
Available online 13 February 2013
and 1,4-phenylenevinylene, 1,4-phenylene imine, azine and 1,4-phenylene ethynylene as
p-spacers,
have been synthesized by polycondensation reactions between aromatic aldehydes (4-
formyltriphenylamine and 3-formyl N-hexylcarbazole) or iodides (4-iodotriphenylamine and 3-iodo N-
hexylcarbazole) with bisphosphonate derivative, 1,4-diaminobenzene, hydrazine or 1,4-
diethynylbenzene, respectively. These oligomers are models for the corresponding conducting poly-
mers, have a well-defined molecular structure, can be highly purified using common methods and
processed as thin films by vacuum evaporation, dip or spin coating. The oligomers preserve all of the
properties and potential applications of the corresponding polymers. The structural characterization of
these oligomers was performed using usual spectroscopic methods (1H-RMN, FT-IR and DSC) and their
photophysical properties were analyzed by UVeVis and fluorescence spectroscopy. Their redox prop-
erties were studied by cyclic voltammetry.
Keywords:
Conjugated oligomers
Triphenylamine and carbazole dyes
Arylenevinylene, imine, azine, and
ethynylene oligomers
Spectral characterization
Photophysical properties
Electrochemical behavior
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
experimental and theoretical studies were carried out to correlate
their structure with spectral properties [16e20]. It is well known
Conjugated oligomers are an important class of electro- and pho-
toactive materials, investigated both in academic and industrial lab-
oratories [1e5]. The great interest for conjugated oligomers emerges
from their applications such as active components in organic elec-
tronic or electrochemical devices [6,7]. The advantage of using small
conjugated compounds is based on the possibility of tuning their
photophysical properties by changing the chemical structure, e.g. by
introduction of side substituents, end-capping groups, insertion of
certain specific functional groups and by changing the oligomer
length. Thus, conjugated oligomers are used as model compounds for
conducting polymers since their monodisperse, defectless structure
and better supramolecular organization in the solid state facilitate
their experimental and theoretical investigations [8].
that the central amine nitrogen atom is responsible for this donor
behavior and these arylamine compounds are commonly used, as
photoconductors in the Xerox process in laser printers and photo-
copiers [21e23]. From the structural point of view, the carbazole
molecule differs from diphenylamine by its planar structure, since it
can be further imagined as bonded diphenylamine in the ortho-
positions of phenyl rings, which induce the higher thermal stability
of carbazole-containing materials. Triphenylamine is a well-known
molecule that possesses useful functions such as redox activity,
fluorescence, ferromagnetism due to the ready oxidation of the
nitrogen center and hole-transporting properties via radical cation
species [24]. Owing to the noncoplanarity of the phenyl sub-
stituents, TPA derivatives can be viewed as 3D systems and the
During the last years, much attention has been paid to triphe-
nylamine (TPA) and carbazole compounds due to their electron
donating and hole-transporting properties, making these com-
pounds promising candidates for electroluminescent and photore-
fractive devices [9e12]. Many triphenylamine-based dyes are used
in dye-sensitized solar cells as sensitizers [13e15] and numerous
combinationwith linear
p-conjugated systems could be expected to
lead to amorphous molecular materials (molecular glasses) with
isotropic optical and charge-transport properties. Our interests in
conjugated arylamine oligomers [25e28] are justified by these
interesting properties, and in this paper we report the synthesis of
ten symmetrical and isoelectronic arylamine oligomers containing
either two triphenylamine or carbazole groups separated by
different
their photophysical properties.
p-spacers and investigate the influence of the spacer on
* Corresponding author. Tel.: þ40 232 217454; fax: þ40 232 211299.
E-mail address: grim@icmpp.ro (M. Grigoras).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.01.025

72
M. Grigoras et al. / Dyes and Pigments 98 (2013) 71e81
2. Experimental
vibration of HC]CH trans), 827 (CH p-substituted benzene ring),
697e695 (CeH phenyl rings). 1H NMR (CDCl3, 400 MHz)
, ppm:
d
2.1. Materials and instruments
7.47 (s, 4H), 7.40e7.38 (d, 4H, H vinyl, J ¼ 8.4 Hz), 7.28e7.24 (t, 8H,
J ¼ 8.4 Hz), 7.12e7.09 (d, 8H, J ¼ 7.6 Hz), 7.06e6.96 (m, 12H). Anal.
found: C, 89.40; H, 5.80; N, 4.65. C46H36N2 (616.808) requires C,
89.57; H, 5.88; N, 4.55.
Aldehyde monomers, 4-formyltriphenylamine [28] and 3-
formyl N-hexylcarbazole [26], and iodide derivatives; 4-
iodotriphenylamine and 3-iodo N-hexylcarbazole [29] were syn-
K1: 3,30-bis (N-hexylcarbazolyl)vinylbenzene was synthesized as
a yellow crystalline powder with 95% yield. M.p. ¼ 170 ꢀC. FT-IR
thesized by adaptation of known methods. Other chemicals: a, -
a0
dichloro-p-xylene (95%), 4,40-bis(chloromethyl)-1,10-biphenyl
(95%), 1,4-diaminobenzene, benzidine, hydrazine hydrate, 1,4-
diethynylbenzene, solvents and catalysts (all from Aldrich) were
obtained from commercial sources and used as received or purified
by standard methods.
(KBr) n
, cmꢁ1: 3435 (]CeH), 2956, 2856 (NeC), 1598, 1491, 1477
(CeC monosubstituted benzene) 1351 (]CeN), 1154 (eCeN
stretching vibration), 962 (out-of-plane bending vibration of
HC]CH trans), 814 (CH p-substituted benzene ring). 1H NMR
(CDCl3, 400 MHz)
d
, ppm: 8.26 (s, 2H), 8.16 (d, 2H, J ¼ 7.6 Hz), 7.69
Note: Benzidine has been linked to bladder and pancreatic
cancer and since August 2010 benzidine dyes have been included in
the environmental Protection Agency’s List of Chemicals of
Concern.
(dd, 2H, J ¼ 8.4 Hz, J ¼ 1.2 Hz), 7.50 (s, 2H), 7.42e7.23 (m, 6H), 7.36
and 7.18 (4H, H vinyl trans, J ¼ 16.4 Hz), 7.58 (4H, Hphenyl), 4.28 (4H,
eNeCH2),1.88 (4H, eCH2e),1.31e1.43 (12H, e(CH2)3), 0.89 (6H, t, e
CH3). Anal. found: C, 87.42; H, 7.60; N, 4.55. C46H48N2 (628.904)
requires C, 87.84; H, 7.69; N, 4.47.
The FT-IR spectra were recorded in KBr pellets on a DIGILAB-FTS
2000 spectrometer. 1H NMR spectra were recorded at room tem-
perature on a Bruker Avance DRX-400 spectrometer (400 MHz) as
solutions in CDCl3 and chemical shifts are reported in ppm and
referenced to TMS as internal standard. UVevisible and fluores-
cence measurements were obtained on a Specord M42 Carl Zeiss
Jena spectrophotometer using quartz cells (10 mm) and a Perkin
Elmer LS 55 apparatus (Perkin Elmer, Norwalk, CT, USA), in solution.
Thermal behavior was determined by DSC method with a Mettler
DSC-12E apparatus, in nitrogen atmosphere. Elemental analyses
were done with a 2400 II Perkin Elmer Elemental Analyzer. The
cyclic voltammograms (CV) reported in this article were recorded
using a Bioanalytical System, PotentiostateGalvanostat (BAS 100B/
W). The electrochemical cell was equipped with three electrodes: a
T2: 4,40-bis [4-(N,N0-diphenylamino)phenylvinyl]biphenyl was
obtained as yellow compound with 79% yield. M.p. ¼ 209 ꢀC. FT-IR
(KBr) n
, cmꢁ1: 3028e3020 (]CeH), 1591 (CeC monosubstituted
phenyl), 1586 (C]C, conjugated phenyl group), 1316 (eCeN
stretching vibration), 962 (out-of-plane bending vibration of
HC]CH trans), 821 (CH p-substituted benzene), 697e695 (CeH
benzene). 1H NMR (CDCl3, 400 MHz)
d, ppm: 7.56e7.84 (dd, 8H,
J ¼ 8.4 Hz, J ¼ 1.2 Hz), 7.41e7.39 (d, 4H, J ¼ 8.8 Hz), 7.28e7.24 (d, 8H,
J ¼ 8.0 hz), 7.12e7.10 (d, 8H, J ¼ 8.0 Hz), 7.08e7.00 (m, 12H). Anal.
found: C, 90.20; H, 5.70; N, 4.15. C52H40N2 (692.906) requires C,
90.13; H, 5.82; N, 4.05.
2.3. Synthesis of aryleneimine oligomers: general procedure
working electrode (disk shape Pt electrode,
F
¼ 1.6 mm), an
auxiliary electrode (platinum wire), and a reference electrode
(consisted of a silver wire coated with AgCl). Before experiments, Pt
electrode was polished between each set of experiments with
aluminium oxide powder on a polishing cloth, and then was soni-
cated in a mixture of detergent and methanol for 5 min and then
rinsed with a large amount of doubly distilled water. The reference
electrode (Ag/Agþ) was calibrated at the beginning of the experi-
ments by running the CV of ferrocene as the internal standard in an
identical cell without any compound in the system (E1/2 ¼ 0.52 V
versus the Ag/AgCl). Prior to the each experiment, the Bu4NBF4
solutions were deoxygenated by passing dry argon gas for 10 min.
All measurements were performed at room temperature (25 ꢀC)
under argon atmosphere.
In a 50 mL two-neck round bottom flask, equipped with a
magnetic stirrer and nitrogen inlet-outlet, aldehyde (1 mmol), 1,4-
phenylenediamine or benzidine (0.5 mmol) and absolute ethanol
(15 mL) were introduced and stirred at room temperature. The
solution turned on colored and a crystalline precipitate separated.
After several hours the crystalline precipitate was filtered, washed
with fresh solvent and purified.
T3: N,N0-bis[4-(diphenylamino)benzylidene]1,4-phenylenediamine.
Reaction was performed in ethanol, in absence of any catalyst by
stirring and heating at 50 ꢀC for 5 h. The product was precipitate in
water, filtered, dried and purified by repeated precipitations inwater
from ethanol solution. Yellow-orange powder. Yield ¼ 92.3%;
M.p. ¼ 176 ꢀC. FT-IR (KBr)
n
, cmꢁ1: 3410e3015 (]CeH), 2924, 2853
(NeC), 2730, 1615 (eCH]N), 1587 (C]C, conjugated phenyl group),
1506 (CeC monosubstituted phenyl), 1487 (CeC monosubstituted
benzene), 1420, 1317 (eCeN stretching vibration), 1268, 1193, 1164,
970 (out-of-plane bending vibration of CH]N), 876 (CH p-
substituted benzene), 752e615 (CeH phenyl rings). 1H NMR (CDCl3,
2.2. Synthesis of arylenevinylene oligomers. General procedure
In a 50 mL two-neck round bottom flask, equipped with a
magnetic stirrer, nitrogen inlet-outlet and dropping funnel, alde-
hyde (0.5 mmol) and chloroform (15 mL) were introduced. At the
resulted solution diphosphonium salt (0.25 mmol), dissolved in
ethanol (15 mL) was added. A solution of 0.5 M sodium ethoxide
(4 mL), freshly prepared, was added in excess to the mixture by
dropping. The reaction mixture was stirred at room temperature
under nitrogen atmosphere for 12 h. The precipitate was filtered
and washed with ethanol. After several purifications by precipita-
tion in methanol from toluene solution, oligomers were obtained as
bright colored powders with strong fluorescence in high dilute
solution.
400 MHz)
d
, ppm: 8.42 (s, 2H, eCH]Ne), 7.76 (d, 4H, phenyl, J ¼ 8.8
Hz), 7.08e7.35 (m, 28H, from triphenylamine group). Anal. found: C,
85.20; H, 5.61; N, 9.21. C44H34N4 requires (618.784) C, 85.40; H, 5.54;
N, 9.06.
K3: N,N0-bis(9-hexyl 3-carbazolylmethylidene)1,4-phenylenedia-
mine was obtained yellow-reddish crystals with 55% yield.
M.p. ¼ 130e134 ꢀC; FT-IR (KBr)
n
, cmꢁ1: 3430e3025 (]CeH), 2948,
2854 (NeC), 2170, 1686, 1611 (eCH]N), 1595(CeC mono-
substituted phenyl), 1477 (CeC monosubstituted benzene), 1415,
1382, 1323(eCeN stretching vibration), 1262, 1121, 1019, 888, 846,
830 (CH p-substituted benzene ring), 744e613 (CeH phenyl rings).
T1: 1,4-bis [4-(N,N0-diphenylamino)phenylvinyl] benzene was
obtained as
a
yellow crystalline powder with 94% yield.
1H NMR (CDCl3, 400 MHz)
d, ppm: 8.70 (s, 2H, eCH]Ne); 8.64 (s,
M.p. ¼ 196 ꢀC; FT-IR (KBr)
n
, cmꢁ1: 3023e2920 (]CeH), 1598, 1492
2H); 8.17 (d, 2H, J ¼ 8.6 Hz); 8.06 (d, 2H, J ¼ 8.6 Hz); 7.423e7.52 (m,
6H); 7.36 (s, 4H, phenyl); 7.28e7.30 (m, 2H); 4.22 (t, 4H, eNeCH2e);
1.77 (t, 4H; eNeCeCH2e); 1.17e1.31 (m, 12H, e(CH2)3e); 0.76 ppm
(CeC monosubstituted phenyl), 1589 (C]C, conjugated phenyl
group), 1328 (CeN stretching vibration), 960 (out-of-plane bending

M. Grigoras et al. / Dyes and Pigments 98 (2013) 71e81
73
(t, 6H, eCH3). Anal. found: C, 83.52; H, 7.43; N, 8.98. C44H46N4
(630.88) requires C, 83.76; H, 7.35; N, 8.89.
(1 mmol), 1,4-diethynylbenzene (0.5 mmol), PdCl2(PPh3)2
(0.005 mmol), CuI (0.03 mmol), PPh3 (0.02 mmol) and triethyl-
amine (10 mL) were introduced and stirred at 60 ꢀC for 12 h. Then
the reaction mixture was cooled, triethylamine removed at vacuum
and solid product washed with water and diluted HCl solution. The
product was purified by flash chromatography on silicagel using
CH2Cl2/hexane (1:5) as eluent.
T4: N,N0-bis[4-(diphenylamino)benzylidene]4,4’diaminobiphenyl
was obtained as pale-yellowsolid with 80% yield. M.p. ¼ 245 ꢀC. FT-IR
(KBr) n
, cmꢁ1: 3430e3025 (]CeH), 2923, 2852 (NeC), 2170, 1692,
1585 (C]C, conjugated phenyl group), 1486, 1422, 1315, 1282, 1164,
1027, 883, 818 (CH p-substituted benzene ring), 753, 695 (CeH
phenyl rings). 1H NMR (CDCl3, 400 MHz)
d, ppm: 8.43 (s, 2H, e
T6: 1,4-Bis(4-(diphenylamino)phenylethynylene)benzene was ob-
CH]Ne), 7.76 and 7.66 (d, 8H, from biphenyl, J ¼ 8.4 Hz), 7.08e
7.35 (m, 28H, from triphenylamine group). Anal. found: C, 86.20; H,
5.61; N, 8.23. C50H38N4 (694.882) requires C, 86.43; H, 5.51; N, 8.06.
tained as a yellow solid with 85.7% yield. M.p. ¼ 129e130 ꢀC. FT-IR
(KBr) n
, cmꢁ1: 3435 (]CeH), 3034, 2923, 2852, 2207 (CeC triple
bond), 1588, 1513, 1490, 1278, 833. 1H NMR (CDCl3, 400 MHz)
d,
ppm: 7.46 (s, 8H), 7.39e7.35 (t, 4H), 7.29e7.25 (m, 12H), 7.12e7.10
(d, 8H). Anal. found: C, 90.02; H, 5.11; N, 4.67. C46H32N2 (618.776)
requires C, 90.16; H, 5.26; N, 4.58.
2.4. Synthesis of azine oligomers: general procedure
K6: 1,4-Bis(N-hexyl 3-carbazolylethynylene)benzene was ob-
In a 50 mL two-neck round bottom flask, equipped with a
magnetic stirrer and nitrogen inleteoutlet, aldehyde (1 mmol) and
hydrazine hydrate 100% (0.5 mmol) were dissolved in absolute
ethanol (10 mL) and stirred at room temperature. After several
hours a crystalline precipitate separated. This crystalline compound
was filtrated, washed with fresh solvent and purified by recrystal-
lization from ethanol.
tained as pale-yellow solid. Yield ¼ 92.9%. M.p. ¼ 79e80 ꢀC, MS-
ESI ¼ 625.88 (M þ Hþ). FT-IR (KBr)
n
, cmꢁ1: 3449, 3047, 2964, 2926,
2205 (CC triple bond), 1622, 1587, 1474, 1443, 1344, 1271, 1231, 1124,
1082, 885, 789, 743, 719. 1H NMR (CDCl3, 400 MHz)
d, ppm: 8.64 (s,
2H), 8.22 (d, 2H, J ¼ 8.0 Hz) 7.73 (d, 2H, J ¼ 8.4 Hz), 7.64 (d, 2H,
J ¼ 8.4 Hz), 7.49e7.52 (m, 4H), 7.22e7.25 (t, 2H), 4.27 (q, 4H, eNe
CH2), 1.76 (m, 4H, eNeCeCH2) 1.20e2.27 (m, e(CH2)3e12H), 0.9
(t, 6H, eCH3). Anal. found: C, 88.28; H, 7.18; N, 4.61. C46H44N2
(624.872) requires C, 88.41; H, 7.10; N, 4.49.
T5: N,N0-bis[4-(diphenylamino)benzylidene]azine was obtained
as bright yellow crystals with 87% yield ¼ 87%. M.p. ¼ 223 ꢀC. FT-IR
(KBr) n
, cmꢁ1: 3445 (]CeH), 3034, 2923, 2850 (NeC), 1939, 1616,
1590 (CeC monosubstituted phenyl), 1489, 1425, 1331 (eCeN
stretching vibration), 1284, 1171, 1075, 966, 899, 832, 752, 697
(out-of-plane bending vibration of eHC]N). 1H NMR (CDCl3,
3. Results and discussion
400 MHz)
d
, ppm: 8.57 (s, 2H, eCH]Ne), 7.66 (d, 4H, J ¼ 8.4 Hz),
Ten symmetrical oligomers with arylenevinylene, aryleneimine,
aryleneazine and aryleneethynylene structures and ether two
groups at the termini have been synthesized by polycondensation
of aldehydes with bisphosphonate derivatives, diamines or hydra-
zine, or iodide derivatives with 1,4-diethynylbenzene (Sonogashira
coupling), respectively. The synthetic pathways are presented in
Scheme 1 and the chemical structures of the compounds are given
in Scheme 2.
The aldehydes, 4-(diphenylamino)benzaldehyde and 3-formyl
N-hexylcarbazole have been synthesized in good yields by a Vils-
meier formylation reaction of the triphenylamine or N-hex-
ylcarbazole, respectively. The monoaldehydes were separated as
pure compounds by flash chromatography using silicagel and
ethylacetate/hexane (1:10) as eluent. Arylenevinylene compounds
T1, T2 and K1 were obtained via Wittig condensation of the
bisphosphonium salts with the corresponding aldehydes using
sodium ethoxyde as base. Imine (T3, T4 and K3) and azine (T5 and
K5) compounds were obtained by condensation of 1,4-
phenylenediamine, benzidine and hydrazine hydrate, respectively,
with the requested aldehydes. Ethynylene compounds (T6 and K6)
were synthesized by Sonogashira coupling of 1,4-diethynylbenzene
with 4-iodotriphenylamine or 3-iodo N-hexylcarbazole. The
7.31e7.04 (m, 24H). Anal. found: C, 84.33; H, 5.32; N, 10.23.
C38H30N4 (542.686) requires C, 84.10; H, 5.57; N, 10.33.
K5: N,N0-bis(9-hexyl 3-carbazolylmethylidene)azine. Recrystal-
lized from CHCl3 as yellow crystalline needles with 84.5% yield;
m.p. ¼ 146e147 ꢀC. FT-IR (KBr)
n
, cmꢁ1: 3051 (]CeH), 2948, 2928,
2855 (NeC), 1616, 1596 (CeC monosubstituted phenyl), 1575 (C]C,
conjugated phenyl group), 1332 (eCeN stretching vibration), 1217,
1184, 969 (out-of-plane bending vibration of eHC]N), 891, 810 (CH
p-substituted benzene ring), 744, 730e608 (CeH phenyl rings). 1H
NMR(CDCl3, 400 MHz) d, ppm: 8.94 (s, 2H, eCH]Ne); 8.58 (s, 2H);
8.17 (d, 2H, J ¼ 8.0 Hz); 8.02 (d, 2H, J ¼ 8.0 Hz); 7.52e7.42 (m, 6H);
7.30 (t, 2H); 4.32 (t, 4H, eNeCH2e); 1.89 (4H, quintet, eNeCeCH2e
); 1.42e1.26 (m,12H, e(CH2)3e) and 0.87 (t, 6H, eCH3). Anal. found:
C, 82.11; H, 7.54; N, 10.26. C38H42N4 (554.782) requires C, 87.84; H,
7.69; N, 4.47.
2.5. Synthesis of ethynylene oligomers by Sonogashira coupling:
general procedure
In a 50 mL two-neck round bottom flask, equipped with a
magnetic stirrer and nitrogen inlet-outlet, iodine derivative
Scheme 1. Synthesis of arylenevinylene, aryleneimine, aryleneazine and aryleneethynylene oligomers.

74
M. Grigoras et al. / Dyes and Pigments 98 (2013) 71e81
Scheme 2. Chemical structures of arylenevinylene, aryleneimine, aryleneazine and aryleneethynylene oligomers.
structures of all compounds were proved by FT-IR, 1H NMR and
elemental analyses.
biphenyl unit may lead to a diminishing of the p-conjugation of the
whole system and a hypsochromic shift of the absorption bands.
UVevis absorption spectra of azine oligomers T5 and K5, pre-
sent absorption bands located in the range of 250e325 nm and
350e450 nm. With respect to azine-based carbazole oligomers
(K5), the triphenylamine-azine oligomer (T5) exhibited red-shifted
absorption maximum at 410 nm, and this behavior can be
explained by an increase of conjugation due to stronger triphe-
nylamine donor units. The same order was observed for ethynylene
oligomers, T6 > K6. The absorption data of all oligomers are sum-
marized in Table 1. The energy gaps (Eogpt) were determined from
the edges of the absorption spectra.
3.1. Absorption properties
All oligomers exhibit absorption bands in the wavelength range
from 250 to 550 nm assigned to electronic transitions in the tri-
phenylamine and carbazole chromophores, while almost no linear
absorption was observed beyond 550 nm. Fig. 1A presents the ab-
sorption spectra of the vinylene oligomers in dilute chloroform
solutions with absorption maxima located at around 392 nm (K1),
400 nm (T2) and 410 nm (T1), respectively. Absorption bands
located at around 300 nm can be assigned to the electronic
pe
p*
transitions from the aromatic rings, while the absorption maxima
3.2. Photoluminescence properties
located in the range of 390e410 nm can be assigned to electronic
p
e
p* transitions of the conjugated backbone. A clear hypsochromic
The photoluminescence spectra were recorded in chloroform
solution exciting the samples at maximum absorption peak (Fig. 2).
The blue shifting of absorption maxima of T2 with respect to
T1, observed in the UVeVis absorption spectra, can also be
distinguished in PL spectra and this may be explained by
decreasing of conjugation length due to nonplanarity of the
biphenyl unit. From the point of view of light emitting materials
applications, the non-coplanarity of T2 could be considered an
advantage because the electroluminescent materials has to be
thermally stable, to present a low tendency for aggregation and
excimer formation, crystallization and destruction. The non-
coplanar structure of the T2 inhibits the crystallization and ag-
gregation phenomena (observed by DSC studies). The carbazole-
based arylenevinylene oligomers exhibit a blue-yellow fluores-
cence while the triphenylamine compounds have a green-yellow
fluorescence in chlorinated solvents.
shift of the absorption maxima of oligomer T2 (400 nm) with
respect to T1 (410 nm) is observed in the UV spectra, as well as in
photoluminescence spectra.
The absorption spectra of aryleneimine oligomers (T3, T4 and
K3) exhibit bands in the range of 300 nm and 450 nm. In general, by
adding the biphenyl segment as a longer
p-spacer between the
triphenylamine units it should to increase the electronic conjuga-
tion pathway in T2 and T4 with respect to T1 and T3. However,
analyzing the absorption spectra of T4 with respect to T3 and T2
with respect to T1 it can be observed the exception from this rule.
The absorption bands of T4 and T2 are blue shifted to lower values
with respect to T3 or T1. This behavior could be explained by the
fact the two phenyl rings from the biphenyl spacer are twisted one
to another with a dihedral angle due to steric hindrance between
the ortho-hydrogen atoms. This non-coplanarity of the central

M. Grigoras et al. / Dyes and Pigments 98 (2013) 71e81
75
Fig. 1. UV spectra of arylamine oligomers, recorded in chloroform solution: (A) absorption spectra of arylenevinylene oligomers; (B) absorption spectra of aryleneimine oligomers;
(C) absorption spectra of aryleneazine oligomers; and (D) absorption spectra of ethynylene oligomers.
The fluorescence quantum yield was determinated for aryle-
nevinylene oligomers in chloroform solution using 9,10- diphe-
triphenylamine-based azine oligomers (Fig. 2C). Ethynylene oligo-
mers show emission maxima at 469 nm (T6) and 433 nm (K6).
As a conclusion, oligomers containing triphenylamine groups
show absorptions and emissions at longer wavelengths as
compared with carbazole homologues due to the higher donor
character of the triphenylamine group. On the other hand, the order
nylanthracene as reference standard
(
F
¼
0.90) [30]. The
fluorescence quantum yield values for T1, K1 and T2, are sum-
marized in Table 1, and are in the range of 0.375 and 0.707. For
imine and azine oligomers the fluorescence quantum yields have
low values (w0.1), due to the quenching of excited state by non-
radiative processes.
of absorption and emission maxima as a function of
arylamine oligomers is:
p-spacer of
In the aryleneimine oligomers series, T3 and T4 present a
bathochromic shift due to an increase of conjugation length caused
by the presence of triphenylamine units, with respect to oligomer
having carbazole units (K3) (Fig. 2B). Similarly to arylenevinylene
oligomers, the bathochromic effect of T3 besides T4, the presence of
biphenyl unit caused a blue shifting of the emission maximum to
lower values. In the azine oligomers series (T5 and K5), a hyp-
sochromic shift of carbazole-based azine oligomers was observed
with emission maximum value located at 428 nm, with respect to
vinylene > azine > arylenimine > ethynylene
3.3. Electrochemical behavior
The cyclic voltammetry was used to investigate electro-
chemical behavior and to estimate the HOMO levels of oligomers.
Oligomers containing triphenylamine units at both ends linked by
phenylenevinylene moieties exhibit potentials in the range from
0.0 V to 1.8 V, two anodic peaks located at 0.919 V, 1.113 V for
oligomer T1 and 1.053 V and 1.273 V for oligomer T2 (Fig. 3A).
These peaks are due to oxidation processes of the triphenylamine
units by losing electrons and thus generating cation radicals or
dications. On the reverse scanning the oligomers exhibit two
reversible cathodic peaks located at 0.973 V and 0.819 V for T1
oligomer and 0.967 V and 0.807 V for T2 oligomer. These cathodic
peaks are assigned to reduction processes of the oxidized species
that exist at the electrodeesolution interface. From the cyclic
voltammograms it can be noticed that the presence of the
biphenyl central unit linked through vinylene segments by two
triphenylamine units, generates more positive values of the
oxidation potentials compared to phenyl oligomer one. In addi-
tion, the oxidation peaks are very broad and do not have a well-
defined shape.
Table 1
Optical data for synthesized arylenevinylene, aryleneimine, azine and arylenee-
thynylene oligomers.
max
Oligomer
l
(nm)a
l
(nm)a
max
em
Eg (eV)b
F
abs
T1
K1
T2
T3
K3
T4
T5
K5
T6
K6
306; 410
244; 310; 392
306; 400
298; 404
294; 354; 380
297; 398
298; 410
288; 378
306, 388
300, 340, 356, 380
467; 500
434; 461
463
498
450
495
493
428
469
2.69
2.79
2.75
2.65
2.81
2.73
2.67
2.89
2.82
2.90
0.375
0.707
0.484
c
c
c
c
c
d
d
405, 433
a
Determined in diluted CHCl3 solution.
b
c
Obtained from UVeVis spectra: Eg ¼ 1240/lonset
.
As expected, replacing the vinylene structure with imine seg-
ments, the cyclic voltammograms of the corresponding oligomers
<0.1%.
d
Undetermined.

76
M. Grigoras et al. / Dyes and Pigments 98 (2013) 71e81
Fig. 2. Photoluminescence spectra of arylamine oligomers, recorded in chloroform solution: (A) emission spectra of arylenevinylene oligomers; (B) emission spectra of aryleneimine
oligomers; (C) emission spectra of aryleneazine oligomers; and (D) emission spectra of ethynylene oligomers.
are not so different from the previous cases and the curves retain
their shape, characteristic to triphenylamine oligomers. Thus, the
oligomers T3 and T4, with triphenylamine units at both ends and
iminic structures, show in the forward scanning two anodic peaks
located at 1.184 V and 1.388 V (for T3 oligomer) and 1.177 V and
1.401 V (for T4 oligomer) (Fig. 2B) which are correlated with the
oxidation processes of the triphenylamine units. It can be noticed
that the presence of the biphenyl central unit has the same effect as
for the vinylene oligomers and the anodic peaks are shifted to
positive values of potentials from T3 to T4. From the previous CVs
data, it can be said that these peaks can be attributed to the
oxidation of triphenylamine units at the ends of the oligomer. On
the reverse scanning, the cyclic voltammograms display broad
cathodic peaks at 1.069 V for T3 and 1.056 V for T4. These peaks are
due to the reduction processes of the oxidized species at the elec-
trode. Moreover, it can be observed that the oxidation and reduc-
tion processes of the triphenylamine units occur at more positive
values of the potential than vinylene counterparts and this can be
ascribed to electron withdrawing character of the imine segments
that is in accord with their electron-deficient nature.
smaller values than for imine oligomers. Thus, it can be concluded
that, regarding the oxidation onset values calculated from the
anodic sweep, the oxidation processes of the triphenylamine-based
oligomers fall in this order:
½T1<T2<T5ꢂh½T6ꢂ<½T3<T4ꢂ
onset
E
¼ð0:791VÞð0:824VÞð0:983VÞð1:000VÞð1:048VÞð1:077VÞ
ox
From these data it can be conclude that the imine segments
being more deficient in electrons, the withdrawing character is
more pronounced in oligomers T3 and T4 and this leads to a shift of
the oxidation processes to more positive values, making the imine
oligomers harder to be oxidized comparable with vinylene
oligomers.
Taking into account the onset values of the oxidation processes
we calculated the HOMO energy levels. As it is well known, the
onset oxidation potential observed in CVs is correlated with the
highest occupied molecular orbital (HOMO), which corresponds to
the ionization potential (IP), while the lowest unoccupied molec-
ular orbital (LUMO) level and electron affinity (EA) were estimated
from the onset reduction potential. The HOMO energy values were
estimated from the onset potentials of the first oxidation event,
after calibration of the measurements against Fc/Fcþ (E1/2 ¼ 0.52 V
versus the Ag/AgCl) Because the oligomers are not active on the
cathodic scanning CV, the corresponding LUMO levels were ob-
tained by the HOMO values and the energy gaps, which were
deduced from the edges of the absorption spectra.
The azineetriphenylamine oligomer exhibits two reversible
anodic peaks and two cathodic peaks in the range of 0.0 Ve1.800 V.
Because the azine structure is not active in the range of positive
potentials, these peaks can be attributed only to triphenylamine
units. The anodic and cathodic peaks correspond to the oxidation
(1.118 V and 1.350 V) and reduction (1.170 V and 1.082 V) processes
of the triphenylamine units.
The cyclic voltammograms of T6 is shown in Fig. 1C, and was
recorded in the same conditions as for other previous oligomers.
The cyclic voltamograms of the ethynyleneetriphenylamine olig-
omers present only one anodic peak located at 1.152 V. It can be
noticed that the oxidation process of triphenylamine groups
occurred at positive values than for vinylene oligomers and at
À
Á
onset
EHOMO ¼ ꢁe E
þ 4:28 eV
ox
ELUMO ¼ HOMO ꢁ Egopt:
All the electrochemical data are summarized in Table 2. The
values of HOMO and LUMO levels show these oligomers are good

M. Grigoras et al. / Dyes and Pigments 98 (2013) 71e81
77
Fig. 3. (A) Cyclic voltammograms of T1 and T2 oligomers containing triphenylamine and vinylene structure (10ꢁ3 M). (B) Cyclic voltammograms of T3, T4 and T5 oligomers
containing triphenylamine and imine structure (10ꢁ3 M). (C) Cyclic voltammogram of T6 oligomer containing triphenylamine and etynylene structure (10ꢁ3 M) in CH2Cl2 (10ꢁ3 M),
using Bu4NBF4, as support electrolyte (10ꢁ1 M). Scan rate: 50 mV sꢁ1 between 0.0 V and 1.6 V or 1.8 V, versus Ag/AgCl.
candidates for hole-transporting materials (EHOMO ¼ 5.07e5.35 eV)
having a small barrier (ELUMO ¼ 2.35e2.67 eV) for the electron in-
jection from cathodes made from common metals (barium,
calcium).
units, and on the reverse scanning, two cathodic peaks located at
0.973 V and 0.816 V which can be attributed to reduction processes
of the oxidized species at the electrodeesolution interface. First
oxidation peak located at 0.881 V is irreversible and is caused by the
formation of the cation radical which follows the typical ECE
mechanism, like triphenylamine oligomers. All the electrochemical
data recorded for oligomers containing carbazole units are sum-
marized in Table 3.
By repetitive scanning on the positive potentials, multiple cyclic
voltammograms for triphenylamine-based oligomers and
carbazole-based oligomers in CH2Cl2 solution and using Bu4NBF4 as
electrolyte support were recorded. The multiple cyclic voltammo-
grams recorded for triphenylamine oligomers are shown in Fig. 5.
In case of oligomers containing triphenylamine units T1 and T2,
the peak potential values are slightly shifted in the positive direc-
tion and the reduction potential peak are shifted in the negative
direction with increasing of the number of scans. In the case of
oligomer with iminic or azinic structure and having triphenylamine
units at both ends of the structure, new anodic peaks are formed
being located at lower potential values than for oligomer, with
increasing the number of the scans. This new peak suggested that
new species were formed at the electrode surface and based on the
correlation between the potential values of the monomer, dimer
and the other “mer” with the number of the structural units
increased, it could be said that these new peak belong to the
polymers that were formed on the electrode surface.
From the structural point of view, carbazole molecules differ
from diphenylamines by their planar structure, since it can be
further imagined as bonded diphenylamine in ortho-positions of
phenyl rings, which increase the thermal stability of carbazole-
containing materials. By submitting 10ꢁ3 M oligomer solution at
cyclic voltammetric measurements by swapping the potential
applied to working electrode raging in the range of 0.0e1.8 V or
2.0 V, with a scan rate of 50 mV sꢁ1, the recorded cyclic voltam-
mograms maintain the characteristic shape of carbazole units
(Fig. 4). From the cyclic voltammograms, it can be observed that the
oligomer K1 exhibits three anodic peaks located at 0.881 V, 1.069 V
and 1.599 V due to the multiple oxidation processes of carbazole
Table 2
Electrochemical data of the triphenylamine oligomers.
Oligomer E (V)
T1
T2
T3
T4
T5
T6
Eox1
0.919
1.113
0.791
5.07
1.053
1.273
0.824
5.10
1.184
1.388
1.048
5.32
1.177
1.401
1.077
5.35
1.118
1.350
0.983
5.26
1.152
1.337
1.000
5.28
Eox2
onset
E
ox
a
EHOMO (eV)
ELUMO (eV)
b
2.38
2.35
2.67
2.62
2.59
2.46
a
The electropolymerization process and the formation of an
electrodeposited thin polymeric films on the Pt electrode, was
Determined from the onset of the oxidation peak.
b
Calculated by the HOMO values and the energy gaps (Eogpt.).

78
M. Grigoras et al. / Dyes and Pigments 98 (2013) 71e81
Fig. 4. (A) Cyclic voltammograms of carbazole-based oligomer with vinyl structure (K1) and imine structure (K2) (10ꢁ3 M). (B) Cyclic voltammograms of carbazole-based oligomers
with azine (K5) and ethynylene (K6) structure (10ꢁ3 M) in CH2Cl2, using Bu4NBF4, as support electrolyte (10ꢁ1 M). Scan rate: 50 mV sꢁ1 between 0.0 V and 1.8 V, versus Ag/AgCl.
Table 3
electron transfer and chemical steps, a dimeric structure can be
formed. The cation-radical obtained in the first CV can react with a
Electrochemical data of the carbazole-based oligomers.
parent molecule or with another oxidated species generating tet-
raphenybenzidine or dicarbazolyl structures of oligomers. The
redox behavior of triphenylamine molecule was very well studied
[31,32]. Based on the literature data it can be assumed that the
triphenylamines undergo oxidation processes after which cation-
radicals are formed. These cations-radicals are not stable and by
chemical follow-up reactions tetraphenylbenzidine is formed by
tail-to-tail coupling. This step is accompanied by loss of two pro-
tons per dimer. When the phenyl groups are substituted at the para
positions the couple reactions are greatly prevented. In fact, p-
substituted triarylamines often give stable cation-radicals. In the
same manner, carbazole cation-radicals dimerizes to dicarbazolyl
structures [33].
Oligomer E (V)
K1
K3
K5
K6
Eox1
Eox2
0.881
1.069
1.599
0.792
5.07
1.207
1.381
1.689
1.112
5.39
1.254
1.501
1.690
1.143
5.42
0.889
1.431
e
Eox3
onset
E
0.80
5.08
2.18
ox
EHOMO (eV)
ELUMO (eV)
2.28
2.58
2.53
noticed, also, from the increase in peak currents attributed to the
monomer-type redox activity (Fig. 5). In all cases, at the end of the
repetitive scanning, a polymer film deposited on the Pt plate
electrode was obtained. Similar behavior was observed also for
carbazole oligomers (Fig. 6).
According to oligomer structures, the electrodeposited poly-
meric film has different colors, such as; for vinylene oligomers (T1
and T2), the deposited films have a dark-green color. The film
Having available para position on each two phenyl rings of tri-
phenylamine units or 6-position of carbazole ring, after the
Fig. 5. Multiple scans (10 cycles) for oligomers containing triphenylamine units (10ꢁ3 M) in CH2Cl2.

M. Grigoras et al. / Dyes and Pigments 98 (2013) 71e81
79
Fig. 6. Multiple scans (10 cycles) for oligomers containing carbazole units (10ꢁ3 M) in CH2Cl2.
Fig. 7. DSC scans of triphenylamine-based oligomers (1H, 1C and 2H are: first heating, first cooling and second heating run, respectively). Heating and cooling rate is 10 ꢀC/min,
under nitrogen.

80
M. Grigoras et al. / Dyes and Pigments 98 (2013) 71e81
deposited on the electrode surface obtained from the electro-
polymerization of the oligomer T3 had a bluish green color, and for
the film provided from electropolymerization of the oligomer T4
had a brick-red color. A blue colored film was obtained at the end of
the electropolymerization of the oligomer T6.
conducting polymer structures, since they have a well-defined
molecular structure and can be highly purified using common
methods known in organic synthesis.
From the analysis of absorption and emission spectra of these
oligomers it can be concluded that the triphenylamine-based
oligomers exhibit a bathocromic effect with respect to carbazole-
based oligomers, due to a stronger donating ability of the first
arylamine group. The introduction of longer spacer as biphenyl unit
instead of phenyl unit, led to a blue shifting of the absorption and
emission maxima of oligomers. The blue shifting of absorption
maxima may be explained by decreasing of conjugation length due
3.4. Thermal behavior
Thermal behavior of the oligomers was investigated using Dif-
ferential Scanning Calorimetry (DSC). The DSC curves were recor-
ded under nitrogen at a heating and cooling rate of 10 ꢀC/min and
Fig. 7 shows the DSc curves for triphenylamine-based oligomers.
With some exceptions (T2, T4, T6, K6) the DSC measurements
have revealed in the first heating run endothermic peaks associated
with melting process. For T1 an endothermic peak was observed at
195 ꢀC which can be assigned to the melting process. During the
cooling scan the crystallization process was evidenced by a broad
exothermic peak with maximum at 131 ꢀC while in the second
heating scan the melting temperature peak appeared at 196 ꢀC. If
the sample was rapid cooled, the DSC curve during the heating scan
shows the glass transition temperature at 85 ꢀC followed by a
crystallization process, and then by melting at 196 ꢀC (Fig. 7). T3
shows in the first run a melting at 177 ꢀC but by cooling the sample
was frozen in glass state and the second heating does not reveal the
melting process.
to noncoplanarity of the biphenyl central unit. Among
p-spacers,
phenylenevinylene has the more favourable effect on the absorp-
tion spectrum and redox properties.
Acknowledgments
Authors thank to the Romanian National Authority for Scientific
Research (gs1) (UEFISCDI) for financial support (Grant PN-II-ID-
PCE-2011-3-0274, contract 148/2011).
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