The synthesis and spectral characterization of red dyes containing biphenyl or fluorene conjugation and dicyanovinyl acceptors

Article abstract of DOI:10.1016/j.dyepig.2010.06.008

Novel dipolar compounds containing diarylamine donors, dicyanovinyl acceptors and either a fluorene or biphenyl linker were prepared in an attempt to unravel the similarities and differences between the biphenyl and fluorene segments in donor-acceptor arc

Full text of DOI:10.1016/j.dyepig.2010.06.008

Dyes and Pigments 88 (2011) 195e203
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The synthesis and spectral characterization of red dyes containing
biphenyl or fluorene conjugation and dicyanovinyl acceptors
*
Abhishek Baheti, Prachi Singh, K.R. Justin Thomas
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel dipolar compounds containing diarylamine donors, dicyanovinyl acceptors and either a fluorene or
biphenyl linker were prepared in an attempt to unravel the similarities and differences between the
biphenyl and fluorene segments in donoreacceptor architectures. The linker segments significantly
modify lmax and the redox potential of the dyes is influenced by the nature of both the amine and linker.
Density functional theory calculations performed at the 6-31G(d,p) level revealed that the higher
wavelength absorption contains a major contribution from the chargeetransfer transition between the
amine donor to the dicyanovinyl acceptor. The HOMO is mainly located on the triarylamine segment
while the LUMO is spread over the dicyanovinyl. The fluorene based dyes exhibit lower thermal stability
than their biphenyl analogues.
Received 22 March 2010
Received in revised form
19 June 2010
Accepted 21 June 2010
Available online 1 July 2010
Keywords:
Dipolar compounds
Fluorene
Ó 2010 Elsevier Ltd. All rights reserved.
Biphenyl
Red emission
Solvatochromism
Dicyanovinyl
1. Introduction
However, bifluorene [11] and fluorene [12] based dipolar
compounds containing diarylamine donors and dicyanoethylene
Red-emitting dyes containing donor and acceptor moieties have
received considerable attention over many years owing to their use
in organic light-emitting diodes and photovoltaics [1,2]. Red-
emitting dyes incorporating either low-band gap chromophores
such as benzothiadiazole [3], thienopyrazine [4], maleimide [5] or
fumaronitrile [6] and substituted with arylamines or polyacenes [7]
have been described. Molecules containing polyaromatic segments
acceptors, which show potential as emitting materials for electro-
luminescent devices, resist dipolar interactions in the solid state.
Moreover, diarylaminofluorene, dicyanoethylene and fullerene
triads exhibit a prolonged charge separated state owing to the
presence of dicyanovinyl group as electron-mediating bridge [13].
Among the triarylamine electron donors known to-date, fluorene
based materials are attractive for their thermal, chemical and redox
stability in addition to their excellent photoluminescence, electro-
luminescence and charge migration properties [14]. Combination of
these properties makes them one of the most frequently employed
chromophores in the design of materials suitable for electrolumi-
nescence and photovoltaic devices.
The present authors reported a series of diarylaminofluorene or
diarylaminobiphenyl and cyanoacrylic acid conjugates which
exhibited promising device performance parameters in dye-sensi-
tized solar cells [15]. The absorption characteristics of the dyes
were largely dependent on the nature of the bridging unit (fluorene
or biphenyl) and the donor strength of the diarylamine segment
tethered to the bridging segment [15]. As an extension to this work,
a series of dicyanovinyl derivatives have been prepared to explore
the differences in electronic properties arising from changes in
acceptor segment. This paper concerns the synthesis and spectral,
thermal and electrochemical characterisation of red dyes contain-
ing diarylaminobiphenyl or diarylaminofluorene electron donors
tend to aggregate due to the facile
pep stacking of the planar
segments which is detrimental for emission properties, though
beneficial for charge transport characteristics [8]. In contrast,
structures featuring donoreacceptor architecture inherit a strong
dipole which triggers dipoleedipole interactions in both the solid
state and polar solution [9].
For light-emitting devices, it is necessary to minimize the above
mentioned intermolecular interactions in order to secure optimal
emission. Several strategies have been reported for alleviating
concentration quenching of dipolar compounds in the solid state.
One method employs the doping of red-emitting materials in
a suitable host [10], the success of this physical method relying
upon the isolation of molecules and reduction of molecular contact.
* Corresponding author. Tel.: þ91 1332 285376; fax: þ91 1332 286202.
E-mail address: krjt8fcy@iitr.ernet.in (K.R. Justin Thomas).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.06.008

196
A. Baheti et al. / Dyes and Pigments 88 (2011) 195e203
and dicyanovinyl electron acceptors. Correlation of the acceptor
strength variation due to the dicyanovinyl and cyanoacrylic acid
units with the absorption, emission and electrochemical properties
were critically investigated.
2.2.2. Synthesis of 2-((40-(naphthalen-2-yl(phenyl)amino)biphenyl)
methylene)-malononitrile (D2)
D2 was synthesized from 40-(naphthalen-1-yl(phenyl)amino)
biphenyl-4-carbaldehyde (A2) by following the procedure
described above for D1 in 82% yield as red solid. mp 135 ꢀC. IR (KBr)
2. Experimental
n ( d: 6.99e7.04 (m,
: 2223 cmꢂ1 nC^N). 1H NMR (400 MHz, CDCl3)
3H), 7.14 (dd, J ¼ 8.4 Hz, 1.2 Hz, 2H), 7.22e7.26 (m, 2H), 7.34e7.39
2.1. General
(m, 3H), 7.44e7.50 (m, 4H), 7.66e7.70 (m, 3H), 7.80 (d, J ¼ 8.4 Hz,
1H), 7.88e7.92 (m, 4H). 13C NMR (125.770 MHz, CDCl3)
d: 159.12,
All reactions and manipulations were carried out under N2 with
the use of standard inert atmosphere and Schlenk techniques.
Solvents were dried by standard procedures. Column chromatog-
raphy purification was performed with the use of silica gel
(230e400 mesh, Rankem, India) as the stationary phase in
a column with 40 cm long and 3.0 cm diameter. The IR spectra were
recorded on a THERMO Nicolet spectrometer. The 1H and 13C NMR
spectra were measured with either a Bruker AMX500 or AMX400
spectrometer. Mass spectra were recorded on a JMS-700 double
focusing mass spectrometer (JEOL, Tokyo, Japan). Electronic
absorption spectra were obtained from either a Shimadzu spec-
trophotometer or Evolution UVevisible spectrophotometer. The
fluorescence spectra were measured on a Shimadzu spectrofluo-
rimeter. All the optical measurements were carried out using the
freshly prepared dye solutions. Spin coated thin films were
prepared from toluene solutions and used for the measurement of
solid state photoluminescence. The TGA analyses were performed
on a Perkin Elmer Pyris Diamond Analyzer using nitrogen as carrier
gas and at a heating rate 10 ꢀC/min.
149.59, 147.42, 146.88, 142.78, 135.37,131.59, 131.19, 130.56, 129.41,
129.04, 128.60, 127.95, 127.50, 127.14, 126.94, 126.74, 126.44, 126.38,
124.01, 123.29, 123.16, 120.54, 114.22, 113.11, 80.82, 53.48. HR MS
(ESI) m/z calcd. for C32H21N3: 447.1753; Found: 447.1734.
2.2.3. Synthesis of 2-((7-(diphenylamino)-9,9-diethyl-9H-fluoren-2
-yl)methylene)-malononitrile (D3)
D3 was obtained from 7-(diphenylamino)-9,9-diethyl-9H-
fluorene-2-carbaldehyde (A3) in 84% yield by following the general
procedure described above for D1. Red solid, mp 193 ꢀC. IR (KBr)
n:
2222 cmꢂ1
(
nC^N). 1H NMR (400 MHz, CDCl3)
d
: 0.35 (t, J ¼ 4.8, 6H),
1.82e2.0(m, 4H), 7.01e7.08 (m, 4H), 7.11e7.13 (m, 4H), 7.24e7.29
(m, 4H), 7.58 (d, J ¼ 8.4 Hz, 1H), 7.72 (d, J ¼ 8.0, 1H), 7.74 (s, 1H), 7.81
(dd, J ¼ 8.0 Hz J ¼ 1.2 Hz, 1H), 7.87 (s, 1H). 13C NMR (125.770 MHz,
CDCl3)
d: 159.87, 153.38, 151.07, 149.79, 148.66, 147.47, 133.56,
131.70, 129.48, 128.80, 124.88, 124.53, 123.65, 122.47, 122.12, 119.64,
117.46, 114.6, 113.67, 56.34, 32.40, 8.58. HR MS (ESI) m/z calcd. for
C33H2N3: 465.2205; Found: 465.2203.
Cyclic voltammetric experiments were performed with a CH
Instruments 603C electrochemical analyzer. All the measurements
were carried out at room temperature with a conventional three-
electrode configuration consisting of a glassy carbon working
electrode, a platinum wire auxiliary electrode, and a nonaqueous
Ag/AgNO3 acetonitrile reference electrode. The Eox values were
determined as ðEpa þ Epc Þ=2, where Epaand Epc are the anodic and
cathodic peak potentials, respectively. The potentials are quoted
against the ferrocene internal standard. Approximately
2.5 ꢁ 10ꢂ4 M acetonitrile solutions of the dyes were used in the
electrochemical measurements, and the 0.1 M tetrabutylammo-
nium hexafluorophosphate served as supporting electrolyte.
2.2.4. Synthesis of 2-((9, 9-diethyl-7-(naphthalen-2-yl(phenyl)
amino)-9H-fluoren-2-yl) methy-lene)malononitrile (D4)
D4 was derived from 9,9-diethyl-7-(naphthalen-1-yl(phenyl)
amino)-9H-fluorene-2-carbaldehyde in 79% yield as red solid. mp
194 ꢀC. IR (KBr) : 2224 cmꢂ1 nC^N). 1H NMR (400 MHz, CDCl3)
n ( d:
0.28 (t, J ¼ 7.2 Hz, 6H), 1.78e190 (m, 4H), 6.89e6.93 (m, 2H), 6.99
(t, J ¼ 7.6 Hz, 1H), 7.11e7.13 (m, 2H), 7.22e7.25 (m, 4H), 7.28e7.33
(m, 2H), 7.41e7.53 (m, 3H), 7.62 (d, J ¼ 8.0 Hz, 1H), 7.18 (s, 1H),
7.62e7.79 (m, 2H), 7.84e7.92 (m, 3H). 13C NMR (125.770 MHz,
CDCl3)
d: 159.83, 153.41, 150.93, 150.50, 148.86, 147.88, 143.20,
135.36, 132.67, 131.73, 130.87, 129.39, 128.58, 127.11, 126.90, 126.48,
126.38, 126.27, 124.42, 124.12, 123.24, 123.02, 122.14, 120.38, 119.43,
115.11 114.70, 113.70, 78.90, 72.15, 56.27, 50.62, 32.38, 8.47. HR MS
(ESI) m/z calcd. for C37H29N3: 515.2361; Found: 515.2369.
2.2. Synthesis
The dicyanovinyl derivatives of the aldehydes (A1eA5) [15] were
obtained by following an identical procedure involving the Knoe-
venagel condensation of malononitrile with appropriate aldehyde.
A representative procedure is illustrated below for D1 and the
remaining red dyes were obtained using the requisite aldehyde.
2.2.5. Synthesis of 2-((5-(9,9-diethyl-7-(naphthalen-1-yl(phenyl)
amino)-9H-fluoren-2-yl)-thiophen-2-yl)
methylene)malononitrile (D5)
It was obtained from 5-(9,9-diethyl-7-(naphthalen-1-yl(phenyl)
amino)-9H-fluoren-2-yl)thiophene-2-carbaldehyde in 81% yield as
dark red solid. mp 244 ꢀC. IR (KBr) : 2228 cmꢂ1 nC^N). 1H NMR
n (
2.2.1. Synthesis of 2-((40-(diphenylamino)biphenyl-4-yl)methylene)
malononitrile(D1)
(400 MHz, CDCl3)
d
: 0.30 (t, J ¼ 7.2, 6H), 1.55e1.86 (m, 2H),
1.92e1.99 (m, 2H), 6.91e7.00 (m, 3H), 7.08 (d, J ¼ 6.0, 2H), 7.19e7.23
(m, 2H), 7.28e7.33 (m, 2H), 7.41e7.52 (m, 5H) 7.57e7.68 (m, 3H)
7.76 (d, J ¼ 8.4 Hz, 2H), 7.86e7.90 (m, 2H). 13C NMR (125.770 MHz,
A 100 mL round bottom flask was charged with 40-(diphenyla-
mino)biphenyl-4-carbaldehyde (A1, 0.349 g, 1 mmol), malononi-
trile (0.099 g, 1.5 mmol), ethanol (10 mL) and a drop of piperidine.
The mixture was heated under reflux for 8 h with protection from
moisture. After the reaction was judged to be complete by TLC
examination the mixture was cooled to 0 ꢀC and the precipitate
collected by filtration and washed well with cold methanol.
CDCl3)
d: 158.02, 151.93, 151.06, 150.46, 149.17, 148.42, 144.24,
143.61, 140.34, 135.35,133.99, 133.56,130.93, 129.73,129.24,128.48,
126.92, 126.52, 126.34, 126.30, 126.16, 126.02, 124.30, 124.08,
122.49, 122.23, 121.01, 120.62, 119.56, 116.16, 114.49, 113.65, 75.55,
56.34, 32.62, 8.51. HR MS (ESI) m/z calcd. for C41H31N3S: 597.2239;
Found: 597.2252.
Reddish orange solid, yield 0.302 g (76%); mp 135 ꢀC. IR (KBr)
n:
2225 cmꢂ1 nC^N). 1H NMR (400 MHz, CDCl3)
(
d: 7.05e7.14 (m, 8H),
7.24e7.30 (m, 4H), 7.50 (d, J ¼ 8.8 Hz, 2H), 7.69e7.72 (m, 3H), 7.94
2.3. Theoretical calculations
(d, J ¼ 8.4 Hz, 2H). 13C NMR (125.770 MHz, CDCl3)
d: 159.16, 149.05,
147.16, 131.65, 131.53, 129.58, 129.51, 129.23, 128.01, 127.10, 125.22,
123.90, 123.16, 122.70, 114.24, 113.15, 81.00. HR MS (ESI) m/z calcd.
for C28H19N3: 397.1579; Found: 397.1577.
For theoretical dipole moment and electronic vertical transition
calculations, the structures of the dyes were optimized by applying
DFT with the hybrid B3LYP functional and 6-31G (d,p) basis set. In

A. Baheti et al. / Dyes and Pigments 88 (2011) 195e203
197
the case of fluorene derivatives, the 9-ethyl substituents were
replaced with hydrogen atoms to minimize the computational
time. With the optimized structure, the ground state dipole
moments were calculated with the DFT (B3LYP) models and the
6-31G (d,p) basis set. The electronic transitions were calculated
using time-dependent DFT (B3LYP) theory and the 6-31G (d,p) basis
set. Even though the time-dependent DFT method less accurately
describes the states with chargeetransfer nature, the qualitative
trends in the TDDFT results can still offer correct physical insight. At
least 10 excited states were calculated for each molecule. For all the
dyes, the first excited states were highly optically active as indi-
cated by the larger oscillator strengths; consequently, it was used as
basis in the elucidation of other states.
and the other is a strong absorption in the visible region around
450e479 nm, which can be assigned to an intramolecular charge
transfer (ICT) between the triarylamine donating unit and the
dicyanovinyl accepting moiety [16]. The assignment of the high
energy transition to chargeetransfer is also supported by the fact
that it is red-shifted (w50 nm) in the dye D5 owing to the presence
of more electron-rich conjugation due to the extra thiophene unit.
The chargeetransfer nature of the low energy transition is further
confirmed by the solvent dependence witnessed for this band
(vide supra). The broad absorption band in these compounds
suggests a good electronic delocalization throughout the entire
molecular system.
The spectra of the four new dyes are quite similar, but individual
biphenyl based dyes D1 and D2 display 27 and 28 nm hyp-
sochromic shifts respectively when equated to that of the fluorene
based analogs D3 and D4. This blue shift presumably arises from
the nonplanar linkage in the biphenyl unit, which declines the
efficient conjugation between the donor and acceptor segments. It
is also possible that the twisting in the excited state hinders the
donoreacceptor interaction in the biphenyl based dyes and as
a result leads to an inefficient charge transition. More efficient
charge polarization would lead to more pronounced solvation and
stabilization of the corresponding state. Additionally, as the fluo-
rene segment is more electron-rich than the biphenyl unit, it may
boost the donoreacceptor interaction by supporting the donor
strength of the amino group [17]. Within a category, there is no
difference in absorption wavelength observed for the diphenyl-
amine and 1-naphthylphenylamine donors (compare D1 with D2
and D3 with D4). This may be possibly due to the effective electron
withdrawing nature of the dicyanovinyl moiety. On the contrary, in
the dyes with cyanoacrylic acid acceptors the diphenylamine based
dyes exhibited red-shifted absorption when compared to that of 1-
naphthylphenylamine containing dyes [15]. These elucidations are
further supported by the following observations: (a) The biphenyl
based dyes (D1 and D2) possess comparatively lower molar
3. Results and discussion
3.1. Synthesis and characterization
The red dyes (Fig. 1) were prepared by treating the aldehydes
with malononitrile in the presence of piperidine as catalyst in
a Knoevenagel condensation reaction (Scheme 1). The aldehydes
required for the synthesis were available from an earlier investi-
gation [15]. The dyes were obtained in excellent yields (76e84%)
and characterized by 1H and 13C NMR spectroscopy and high
resolution mass spectrometry. The data were found to be consistent
with the expected structures. The vinyl protons of the dicyanovinyl
derivatives (D1eD4) are more shielded (7.63e7.72 ppm) when
compared to those in the analogous cyanoacrylic acid derivatives
(8.10e8.33 ppm) [15]. As the dicyanovinyl moiety is a stronger
electron withdrawing moiety than the cyanoacrylic acid segment,
the upfield shift observed for the dyes D1eD4 is probably arising
due to enhancement in electron density in the vinyl segment
caused by the effective delocalization of electrons from the amine
to dicyanovinyl unit. The dyes are orange or red in color and freely
soluble in common solvents including dichloromethane, tetrahy-
drofuran, methanol, dimethylformamide and toluene. The dilute
solutions of the dyes appear dark yellow and fluoresce in the orange
to red region depending on the solvent polarity (vide supra).
extinction coefficients (27.3
ꢁ
103 Mꢂ1 cmꢂ1 and
28.4 ꢁ 103 Mꢂ1 cmꢂ1) for the chargeetransfer band than the flu-
orene based analogues (D3 and D4) (37.3 ꢁ 103 Mꢂ1 cmꢂ1 and
38.5 ꢁ 103 Mꢂ1 cmꢂ1) when recorded in dichloromethane. The
decrease of extinction coefficient for the biphenyl dyes may be
credited to the decrease in the co-planarity between the electron
donor and the electron acceptor which would eventually lower the
magnitude of the transition probability. (b) The thiophene con-
taining dye (D5) displays bathochromism in the absorption profile
when compared to the congener (D4). Insertion of thiophene in the
conjugation pathway has been demonstrated to red-shift the
absorption in several donoreacceptor compounds [18]. The char-
geetransfer bands are broadened and less intense for D5 indicating
a possible overlap of the multiple electronic transitions of different
3.2. Optical spectra
The electronic absorption spectra of the dyes were examined in
different solvents to access their solution specific electronic prop-
erties. The absorption spectra recorded for the dyes in dichloro-
methane are displayed in Fig. 2 and the data are collected in
Tables 1 and 2.
All the dyes exhibit two distinct absorption bands. One in the
near-UV region around 307e311 nm which corresponds to the
pep
* transition localized at the aromatic segment, i.e., fluorene;
Fig. 1. Line drawings of the dyes (D1eD5) studied in this work.

198
A. Baheti et al. / Dyes and Pigments 88 (2011) 195e203
Scheme 1. Synthesis of the dyes (D1eD5).
origins. It is likely that the
p
e
p
* transition may share a wavelength
intensity and a narrow band for non-polar solvents and lower
intensity and a broader band for polar solvents (vide supra).
The effect of acceptor strength on absorption properties are
clearly understood when the absorption spectra of the precursor
amines (B1eB4) and aldehydes (A1eA4) are compared with the
dicyanovinyl dyes. The absorption profile progressively shifts to the
red region on going from the amines (B1eB4) to the dyes (D1eD4)
via the intermediate aldehydes (A1eA4). The amines display
absorption maxima below 350 nm while the aldehydes possess
peaks above 350 nm (Fig. 3). If we presume that there is no char-
geetransfer transition present in the parent amines (B1eB4) then
the red-shifted absorptions observed for the aldehydes (A1eA4)
region with the chargeetransfer transition in the higher energy
side which may remain unaltered during polarity change in the
medium. Such an alternation is expected to result in increased
and the dyes (D1eD4) suggest a contribution of
pep
* transition to
the prominent band in these molecules (vide supra).
Another interesting aspect of these dyes is their solvatochromic
behavior. The dyes exhibit negative solvatochromism for the char-
geetransfer transition from the amine donor to the dicyanovinyl
acceptor moiety which occurs at longer wavelength. Representative
absorption spectra of the dye D3 recorded in selected solvents with
varying dielectric constants are plotted in Fig. 4 and the relevant
parameters collected in Table 2. An illustrative variation of
absorption maxima vs. ET(30) parameter is plotted in Fig. 5 for the
dye D2. The observed trend indicates that the dyes are efficiently
solvated in the ground state in polar solvents. The ground state
solvation is attributed to the efficient charge polarization present in
the molecule triggered by the dipolar molecular structure. Such
a stabilization of the ground state may lead to a widening of the
band gap and lowering of the absorption wavelength in polar
solvents. However, it must be mentioned that there is no linear
correlation noticed for the absorption maximum with the solvent
polarity as indicated by the plot of ET(30) parameter of the solvent
against the absorption maxima (for example see Fig. 5). Larger
deviation from linearity observed for the dyes in dichloromethane
probably points to the fact that not only the dipolar interaction but
Table 1
Photophysical parameters of the dyes measured in toluene.
Dye lmax, nm (3max, Mꢂ1 cmꢂ1 ꢁ 103) lem, nm FF
Stokes shift, cmꢂ1
a
D1
D2
D3
D4
D5
307 (26.9), 449.5 (24.6)
306 (20.0), 447.5 (25.6)
309 (34.0), 473.5 (47.5)
306.5 (17.9), 473.5 (40.3)
355 (20.0), 485.0 (34.4)
550.0
546.0
550.0
546.0
575.0
0.21 4065
0.27 4031
0.33 2938
0.40 2804
0.14 3227
a
Fig. 2. Absorption spectra of the dyes (D1eD5) recorded in (a) toluene and (b)
Quantum efficiency of the dyes were measured relative to Rhodamine-6G
dichloromethane.
(FF ¼ 0.595).

A. Baheti et al. / Dyes and Pigments 88 (2011) 195e203
199
Table 2
Position of the charge transition observed for the dyes in different solvents.
Dye
lmax, nm (3max, Mꢂ1 cmꢂ1 ꢁ 103)
Toluene
THF
EA
DCM
DMF
ACN
D1
D2
D3
D4
D5
449.5 (24.6)
447.5 (25.6)
473.5 (47.5)
473.5 (40.3)
485.0 (34.4)
440.0 (13.2)
439.0 (22.6)
464.0 (26.9)
466.0 (39.1)
478.5 (32.1)
434.5 (23.0)
433.0 (22.5)
458.5 (33.9)
460.0 (26.6)
472.0 (37.6)
452.0 (27.3)
450.0 (28.4)
479.0 (37.3)
478.0 (38.5)
492.0 (27.8)
439.0 (12.9)
437.5 (13.3)
463.0 (25.7)
465.5 (30.8)
479.0 (31.1)
430.5 (21.9)
430.5 (21.9)
458.0 (37.0)
459.0 (56.6)
471.0 (40.9)
also the effective solvation of the dyes by the solvent influences the
absorption profile. It may be inferred from the solvatochromic data
that the hydrophobic toluene and bulkier dichloromethane are not
effectively solvating the dyes while in the polar solvents such as
acetonitrile and dimethylformamide pronounced solvation occurs.
based dyes are reversed in the TDDFT results. According to the
calculations fluorene based dyes are hypothesized to show blue-
shifted absorption when compared with the analogous biphenyl
based dyes. It is possible that the more-effective solvation of the
fluorene based dyes when compared to the biphenyl based dyes
would have resulted in a red-shift in the absorption profile for
solution measurements. The ground state dipole moments of the
dyes are appreciably larger, when compared to the parent amines
and the dipole moment significantly increased on introduction of
aldehyde and dicyanovinyl moieties which is attributable to the
enhancement of dipolar character.
3.3. Theoretical calculations
To determine the nature of the electronic transitions observed
for the dyes, TDDFT/B3LYP calculations were performed on the dyes
in the gas phase. The HOMO and LUMO orbitals predicted by the
calculations were largely in keeping with the expectations and the
electrochemical investigations (see below). The HOMO orbitals of
the dyes were invariably located on the triarylamine unit and the
LUMO orbitals spread over the dicyanovinyl acceptor segment
(Fig. 6). The electronic transitions calculated at the 6-31G(d,p) level
are displayed in Fig. 7 and the longer wavelength transitions are
listed in Table 3. The longer wavelength transition calculated for the
dyes are slightly red-shifted from the observed values for toluene.
This also corroborates the involvement of solvent effects and the
fact that in polar solvents, solvation stabilizes the molecules in the
ground state and reduces the absorption wavelength. More-
effective solvation might lead to a larger reduction in absorption
wavelength. Since the red-shifted absorption peaks in the dyes are
predicted to originate from the HOMO / LUMO transition they are
assigned to the chargeetransfer from the amine to dicyanovinyl
group. Lower wavelength transitions (not listed in Table 3) are
3.4. Emission spectra
The emission properties of the dyes were examined in different
solvents. Toluene was also used as solvent in the measurement of
fluorescence spectra in order to avoid the complications that may
arise due to the dipolar interactions and solvation effects. The
emission spectra of the dyes recorded in toluene are displayed in
Fig. 8 and the relevant parameters are collected in Table 1.
Irrespective of the difference in the nature of the amine and the
conjugating segment present in the dyes, all the dyes with the
exception of D5 emit at approximately the same wavelength. For
D5, the emission wavelength is red-shifted (w30 nm) when
compared to the other dyes. The larger Stokes shift observed for the
biphenyl based dyes (D1 and D2) when compared to the fluorene
containing dyes (D3 and D4) is likely to arise due to the twisting
occurring in the biphenyl conjugation on excitation while the
rigidity of fluorene will forbid thermal rotational relaxation [19].
The emission colors of the dyes D1eD4 in toluene may be described
as green while it is orange for D5. In the solid state the emission is
shifted to the higher wavelength. To ascertain the reason for this
arising between
p-type molecular orbitals located within the
phenyl, naphthyl or fluorenyl segments. It must be noted that the
trend which was observed between the biphenyl and fluorene
Fig. 3. Absorption spectra of the precursor amine and aldehydes recorded for
dichloromethane solutions.
Fig. 4. Absorption spectra of the dye D3 recorded in different solvents.

200
A. Baheti et al. / Dyes and Pigments 88 (2011) 195e203
Fig. 7. Calculated absorption spectra of the dyes D1eD5 in gaseous state.
compounds revealed a quasi-reversible oxidation couple attribut-
able to the removal of an electron from the amine segment. The
exact oxidation potential for D2 dye was obtained from the differ-
ential pulse voltammetry. The negatively shifted oxidation (Table 5)
observed for the fluorene based dyes is reasoned to the increased
electron-rich character of the fluorene unit, which may increase the
oxidation propensity of the amine unit. Insertion of the additional
fluorene segment in the conjugation pathway has been demon-
strated to significantly decrease the oxidation potential of the
amines [21]. Also, substitution of the fluorene unit for the other
aromatic units (phenyl, naphthyl) in the triarylamines has been
reported to decrease the oxidation potential dramatically [22]. The
oxidation potential of the present dyes is anodically shifted when
compared to the parent triarylamines without electron accepting
dicyanovinyl unit [15]. This clearly indicates the presence of an
electronic interaction between the amine donor and the dicyano-
vinyl acceptor.
Fig. 5. Correlation of absorption maximum of D2 with ET(30) values of the solvents
used for measurement.
red-shift we have measured the emission spectra of the dyes in
various solvents differing in dielectric constant (Table 4). In more
polar solvents such as dimethylformamide and acetonitrile the
dyes are non-emissive. However in dichloromethane and ethyl
acetate a weak red-shifted fluorescence was observed. All these
facts point the existence of pronounced dipolar interaction
between the dipoles of the dyes and solvents. Consequently, the
bathochromism observed for the film is assigned to the difference
in dielectric constant [20] of the medium rather than the aggre-
gation or stacking, which is quite unlikely in biphenyl systems due
to inherent non-planarity.
It is interesting to note that the oxidation potentials observed for
D3 and D4 are more positive than that observed for D5 which
contain a thiophene unit between the fluorene and acceptor units.
Cathodically shifted oxidation observed for D5 is ascribed to the
electron-rich character of the thiophene unit and decreased inter-
action between the amine donor and acceptor caused by the
elongation in the conjugation [23]. Stronger interaction between
the donor and acceptor will make the oxidation of the amine
difficult. Even though no shift in absorption profile is noticed for the
change in the diarylamine segment (compare D1 with D2 or D3
with D4), it substantially affected the redox potential of the amino
unit. Thus, the naphthylamine containing dyes displayed more
positive oxidation potentials than that of the diphenylamine based
dyes. This gives a route to design chromophores tunable in charge
transport properties while retaining the optical character.
3.5. Electrochemical properties
The bipolar character of the dyes was examined by using cyclic
voltammetry. The electrochemical measurements were carried out
in acetonitrile solution with 0.1 M tetrabutylammonium hexa-
fluorophosphate as the electrolyte and at a scan rate 100 mV/s.
The cyclic voltammograms recorded for the dyes (D1eD5) in
acetonitrile are displayed in Fig. 9. Except for D2 all of the
The orbital energies (Table 5) of the dyes were deduced by
comparing the observed redox potentials with that of the internal
Table 3
TDDFT calculated dipole moments, frontier orbital energies, low energy vertical
excitations and their assignments for the dyes.
Compd HOMO, LUMO, Dipole Moment lmax
eV eV (Debye) nm
,
f
Assignment
B1
A1
D1
D2
D3
D4
ꢂ5.031 ꢂ0.991 2.723
ꢂ5.128 ꢂ1.782 4.901
ꢂ5.318 ꢂ2.755 9.917
ꢂ5.347 ꢂ2.726 10.038
ꢂ5.282 ꢂ2.676 10.692
ꢂ5.308 ꢂ2.634 11.096
346.4 0.6537 HOMO / LUMO (90%)
412.1 0.4643 HOMO / LUMO (93%)
533.8 0.5375 HOMO / LUMO (91%)
522.2 0.5323 HOMO / LUMO (91%)
518.6 0.6724 HOMO / LUMO (88%)
505.3 0.6878 HOMO / LUMO (88%)
Fig. 6. Frontier molecular orbitals of the dyes (a) D2 and (b) D3.

A. Baheti et al. / Dyes and Pigments 88 (2011) 195e203
201
Fig. 8. Emission spectra of the dyes (D1eD5) recorded in toluene.
Fig. 9. Cyclic voltammograms recorded for the dyes (D1eD5) in acetonitrile. Sup-
porting electrolyte: TBAHFP; scan rate: 0.1 V/sec.
standard and the absorption edge measured in the same solvent.
The HOMO values were calculated from the oxidation potential by
referencing to ferrocene which was used as internal standard as
given below.
Table 5
Electrochemical data of the red dyes.
Compound
Eox
(
D
Ep), Va
HOMO, eVb
LUMO, eVb
E0-0, eV
E*0-0, Vc
HOMO ¼ Eox þ 4:8
While the LUMO was obtained from the optical band gap
derived from the absorption edge using the equation:
D1
D2
D3
D4
D5
0.678
0.682
0.620
0.647
0.587
5.478
5.682
5.420
5.447
5.387
3.179
3.204
3.009
3.060
3.268
2.299
2.278
2.411
2.387
2.119
ꢂ0.99
ꢂ0.97
ꢂ1.16
ꢂ1.11
ꢂ0.90
a
Measured for 2.0 ꢁ 10ꢂ5 M acetonitrile solutions at a scan rate of 0.1 V/s and
LUMO ¼ HOMO ꢂ E0ꢂ0
The HOMO and LUMO values realized for the compounds
suggest that they are potential to act as charge transporting emit-
ters in a light-emitting device.
0.1 M tetrabutylammonium hexafluorophosphate (TBAHP) was used as supporting
electrolyte.
b
Deduced from the equations HOMO ¼ Eox þ 4.8 and LUMO ¼ HOMO ꢂ E0-0
.
c
Excited state oxidation potential vs NHE.
3.6. Thermal properties
The decomposition temperatures of the dyes D1eD4 were
measured by thermal gravimetric analysis (TGA) under a nitrogen
atmosphere, with a heating rate of 10 ꢀC/min. TGA indicated that
these materials exhibit high decomposition temperatures (Td). All
the compounds exhibit a Td value ranging from 400 to 432 ꢀC
(Table 6). The highest value is observed for D2, while D3 shows the
lowest in the series. The thermal decomposition temperatures of
biphenyl based dyes (D1 & D2) are higher than those containing
fluorene chromophore (D3 & D4) which is attributed to the fragile
ethyl groups present in the molecule. Within the class, the dyes
containing diphenylamine donor (D1 & D3), display lower thermal
decomposition temperature when compared with those possessing
naphthylphenylamine donor (D2 & D4). The presence of fused
aromatic groups such as pyrene, carbazole and naphthalene has
Table 6
Thermal properties of dicyanovinyl derivatives.
Dye
Tm (ꢀC)
Tonset (ꢀC)
Td (ꢀC)
Tg (ꢀC)
D1
D2
D3
D4
D5
135.0
135.0
193.0
194.0
244.0
340
335
330
370
338
406
432
400
430
431
52.0
75.5
58.0
75.5
97.0
been demonstrated to increase the thermal stability of the organic
functional materials [24]. For the same arguments, thermal stability
difference observed between naphthyl and phenyl derivatives is
assigned to the rigidity of naphthyl units. Additionally, the dyes also
showed reasonable glass transition temperature (Tg) above 50 ꢀC.
The Tg did not differ very much between the biphenyl and fluorene
dyes, however the amine unit played a significant role. The naph-
thylamine based dyes (D2 and D4) exhibited higher Tg when
compared to the analogous diphenylamine based ones (D1 and D3).
Enhancement of Tg due to the presence of naphthyl segments is
Table 4
Emission peak wavelengths of the dyes in different solvents.
Dye
lem, nm
Stokes Shift
Toluene
Toluene
EA
DCM
Film
EA
DCM
observed earlier. For insta0nce, the commonly employed hole-
D1
D2
D3
D4
D5
550
546
550
546
575
588
580
581
578
633
613
525
612
603
612
616
614
631
597
666
4065
4031
2938
2804
3227
6008
5853
4599
4438
5389
5811
3175
4537
4337
3985
0
transporting material N4,N4 -di(naphthalen-1-yl)-N4,N4 -diphenyl-
[1,10-biphenyl]-4,40-diamine (C1) shows higher Tg (95 ꢀC) when
40 40
compared to the analogous N4,N4,N ,N -tetraphenyl-[1,10-
biphenyl]-4,40-diamine (C2) (70 ꢀC) [25].

202
A. Baheti et al. / Dyes and Pigments 88 (2011) 195e203
[6] Palayangoda SS, Cai X, Adhikari RM, Neckers DC. Carbazole-based
4. Conclusions
donoreacceptor compounds: highly fluorescent organic nanoparticles.
Organic Letters 2008;10(2):281e4.
In summary a series of red-emitting dicyanovinyl derivatives
featuring biphenyl or fluorene conjugation and diphenylamino or
naphthylphenylamino donor units have been synthesized. The
electron withdrawing dicyanovinyl moiety is stronger electron
withdrawing group than the cyanoacrylic acid unit and dictates the
absorption properties of the dyes D1eD4. Electron withdrawing
dicyanovinyl moiety dictates the absorption properties of the dyes
and they behave as strong electron withdrawing group when
compared to the cyanoacrylic acid unit. Due to this strong electron
withdrawing character the difference in donor strength between
the diphenylamine and naphthylphenylamine groups is not man-
ifested in the absorption and fluorescence properties significantly.
However, the electrochemical and thermal properties of the dyes
are affected by the nature of the amine unit. Also, the dyes exhibit
negative solvatochromism which indicate that the dyes display
charge polarization in the ground state. Due to the promising
properties such as reversible oxidation and red-shifted absorption,
we believe that these materials may be efficiently used as charge
transporting emitters in light-emitting diodes.
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This work was supported by Department of Science and Tech-
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initiation grant by the Indian Institute of Technology Roorkee.
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Appendix. Supplementary data
Absorption, 1H, and 13C NMR spectra and the thermogravimetric
traces of the dyes are provided in the Supporting Information.
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