An investigation on single crystal growth, structural, thermal and optical properties of a series of organic D-π-A push-pull materials

Article abstract of DOI:10.1039/c5ra04907e

We present the large single crystal growth of a series of donor-π-acceptor (D-π-A) push-pull chromophores; 4-N,N-dimethylamino-β-nitrostyrene (1), 2-(4-(dimethylamino)benzylidene)malononitrile (2), ethyl 2-cyano-3-(4-(dimethylamino)phenyl)acrylate (3) and

Full text of DOI:10.1039/c5ra04907e

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An investigation on single crystal growth,
structural, thermal and optical properties of a series
Cite this: RSC Adv., 2015, 5, 38591
of organic D–p–A push–pull materials†
Vinod Kumar Gupta and Ram Adhar Singh*
We present the large single crystal growth of a series of donor–p–acceptor (D–p–A) push–pull
chromophores; 4-N,N-dimethylamino-b-nitrostyrene (1), 2-(4-(dimethylamino)benzylidene)malononitrile
(2), ethyl 2-cyano-3-(4-(dimethylamino)phenyl)acrylate (3) and methyl 2-cyano-3-(4-(dimethylamino)
phenyl)acrylate (4). Single crystals with good optical transparency were grown from a mixed organic
solvent (1 : 1 acetone–methanol) applying slow evaporation techniques, and crystal dimensions up to
3
1
7 ꢀ 4 ꢀ 1 mm has been successfully achieved in the case of chromophore 4. A single crystal X-ray
diffraction study revealed that two (3 and 4) of them belong to the triclinic crystal system with space
ꢀ
group P1, while 1 and 2 crystallize in orthorhombic and monoclinic crystal systems with space groups
Pbca and P2
1
, respectively. The packing in all the chromophores (except 2) is stabilized by C–H/p
ꢁ
¼ 365 ꢁC) and 4
interaction. Thermal analysis shows higher thermal stability for 2 (T
d
¼ 354 C), 3 (T
d
(T
d
¼ 330 ꢁC) than 1 (T
d
¼ 240 C). The thermodynamic and kinetic parameters, such as heat and
entropy of fusion, and activation energy have been determined by exploiting DSC and TGA data,
respectively. A Lippert–Mataga plot reflects the highest change in dipole moment on excitation from
ground state to excited state for chromophore 3 (4.80 D) followed by 4 (4.45 D), 2 (3.59 D) and 1
ꢁ
Received 19th March 2015
Accepted 21st April 2015
(
3.17 D). We also report the quantum yield of all the chromophores in different organic solvents. In
DOI: 10.1039/c5ra04907e
acetonitrile, it is 0.007, 0.014, 0.023 and 0.019 for chromophores 1, 2, 3 and 4; respectively. These
studies indicate the potential opto-electronic application of these push–pull chromophores.
www.rsc.org/advances
applications such as OLEDs. In CT molecules, the redistribu-
tion of electronic charge occurs under the application of an
1
. Introduction
Molecular design and synthesis of organic charge transfer electric eld through p-conjugation. In the past, several D–p–A
(
OCT) materials have received great scientic attention because systems with different p-bridge and varying substituents
of their widespread application in light-emitting diodes, uo- (donors and acceptors) have been synthesized and their mate-
1
2
3
4
rescent dyes, nonlinear optics and optoelectronic devices.
Donor–p–acceptor (D–p–A) type chromophores are the simplest cally, in order to establish the structure–property
rial properties have been studied experimentally and theoreti-
5
8–10
example in this series. Such types of p-conjugated molecule relationship.
have an electron-donor functional group (D) at one end and an
Furthermore, there are two main forms in which organic ICT
electron-acceptor functional group (A) at other end, and exhibit materials can be isolated: polymers and single crystals. Among
1
,6,7
intramolecular charge transfer (ICT) from donor to acceptor.
them, single crystals are the most attractive materials because of
The advantage of such low energy ICT absorptions with much their typically large hyperpolarizability, high packing densities,
intensity is that they appear in the region of visible to near and superior long-term orientational and photochemical stabil-
infrared (NIR), which are in high demand for various optical ities as well as their better optical quality compared to polymeric
1
1,12
materials.
with appropriate size and high optical quality are required.
However, the growth of bulk single crystal of such charge transfer
But for practical applications, bulk single crystals
13,14
Department of Chemistry, Centre of Advanced Study, Faculty of Science, Banaras
Hindu University, Varanasi-221 005, India. E-mail: prrasingh@gmail.com
15,16
molecules still remain a big challenge.
This is because of their
†
Electronic supplementary information (ESI) available: Copies of NMR spectra
1
13
lower solubility in suitable organic solvent, quick nucleation
(
H and C) for the chromophores 1–4. Crystallographic data in CIF format for
1
s
, 2, 3 and 4. Plots of 1/T versus ln N . A comparative detail of bond lengths and during cooling/evaporation (in the case of solution growth) and
bond angles. View of C–H/p interactions. Thermal activation energy plots. The
UV-Vis absorption and uorescence spectra in different solvents.
Lippert–Mataga plots. CCDC 1044113, 1044114, 1020677 and 1020678. For ESI
and crystallographic data in CIF or other electronic format see DOI:
decomposition just aer melting during melt growth. Thus the
main goal is to synthesize the CT material with better thermal
stability and to improve the crystal quality by optimizing the
growth conditions. Also, there are several materials in literature
1
0.1039/c5ra04907e
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1
3
whose large single crystal growth, thermal and material proper-
C NMR (75 MHz, CDCl
3
): 140.25, 134.21, 132.15, 131.45,
ties have been explored few years aer their synthesis. Polyene 111.95, 105.83, 40.07 ppm.
17–19
and stilbazolium crystals
are benchmark examples of such
2.2.2. 2-(4-(Dimethylamino)benzylidene)malononitrile (2).
materials whose single crystal growth and material properties An equimolar amount of 4-N,N-dimethylaminobenzaldehyde
have been widely investigated in last two decades. Therefore, the (1.49 g, 10 mmol) and malononitrile (10 mmol, 0.66 g) mixed
large single crystal growth and exploration of material properties thoroughly and then 1 mmol of 1,8-diazabicyclo[5.4.0]undec-7-
of earlier reported materials has equal importance as the ene (DBU) (0.15 mL) was added. The reaction mixture was
synthesis of new materials. In this paper, we report the large ground in the mortar and pastel for 30 s. The reaction mixture
single crystal growth of 4-N,N-dimethylamino-b-nitrostyrene (1), was treated with cold water. The product was ltered, dried, and
2-(4-(dimethylamino)benzylidene)-malononitrile (2), ethyl 2- the crude compounds were recrystallized from a mixture of
cyano-3-(4-(dimethylamino)phenyl)-acrylate (3) and methyl 2- ethanol and acetone to get the desired compound in pure form
cyano-3-(4-(dimethylamino)phenyl)-acrylate (4) by solution (yield 1.90 g, 96%).
1
growth technique. To the best of our knowledge, no reports have
H NMR (300 MHz, CDCl
3
): 7.81 (d, 2H, Ar-H), 7.44 (s, 1H,
) ppm.
): 158.05, 154.22, 133.75, 119.27,
The grown single 115.93, 114.87, 111.57, 40.03 ppm.
been found related to large single crystal growth, thermal and H–C]C–), 6.70 (2H, d, Ar-H), 3.14 (s, 6H, –CH
3
1
3
optical properties of these materials to till date; however their
C NMR (75 MHz, CDCl
3
20–22
syntheses have been reported earlier.
crystals were structurally characterized for FT-NMR and single
Compounds 3 and 4 were synthesized by similar method.
crystal X-ray diffraction. Solubility of all the materials in a mixed
2.2.3. Ethyl 2-cyano-3-(4-(dimethylamino)phenyl)acrylate
organic solvent (1 : 1 acetone–methanol) has been measured in (3). 4-N,N-Dimethylaminobenzaldehyde (10 mmol, 1.49 g),
different range of temperature and various thermodynamic ethyl cyanoacetate (10 mmol, 1.06 mL), DBU (1 mmol, 0.15 mL),
parameters have been calculated for different solute/solvent grinding time 60 s (yield 2.20 g, 90%).
1
systems. Differential scanning calorimetry (DSC) and thermo
H NMR (300 MHz, CDCl
3
): 8.07 (s, 1H, H–C]C–), 7.94 (d,
–), 3.11 (s, 6H,
gravimetric-differential thermal analysis (TG-DTA) have been 2H, Ar-H), 6.70 (d, 2H, Ar-H), 4.36 (q, 2H, –CH
performed to investigate their thermal properties. The UV-Vis –CH ), 1.39 (t, 3H, –CH ) ppm.
C NMR (75 MHz, CDCl ): 164.24, 154.45, 153.52, 133.10,
in different organic solvents with varying polarity and the 119.34, 117.53, 111.45, 94.01, 61.84, 39.97, 14.26 ppm.
quantum yields reported. We also discuss the Lippert–Mataga 2.2.4. Methyl 2-cyano-3-(4-(dimethylamino)phenyl)acrylate
plots for all the chromophores and the change in dipole (4). 4-N,N-Dimethylaminobenzaldehyde (10 mmol, 1.49 g),
2
3
3
1
3
absorption and uorescence spectra have also been recorded
3
moment.
methyl cyanoacetate (10 mmol, 0.88 mL), DBU (1 mmol, 0.15
mL), grinding time 60 s (yield 2.10 g, 91%).
1
H NMR (300 MHz, CDCl
3
): 8.07 (s, 1H, H–C]C–), 7.95 (d,
), 3.88 (s, 3H,
2. Experimental
2
H, Ar-H), 6.70 (d, 2H, Ar-H), 3.11 (s, 6H, –CH
3
2.1. Materials and purication
–CH ) ppm.
3
1
3
C NMR (75 MHz, CDCl ): 164.76, 154.72, 153.63, 134.08,
19.31, 117.52, 111.48, 93.52, 52.76, 39.99 ppm.
3
4
-N,N-Dimethylaminobenzaldehyde (Avra Chemical Pvt Ltd)
1
was recrystallized from methanol before used. Ethyl cyanoace-
tate and methyl cyanoacetate were purchased from Spec-
trochem Pvt Ltd, while malononitrile, nitromethane and
ammonium acetate were received from S d Fine-Chem Ltd. All
these chemicals were used as received. 1,8-Diazabicyclo[5.4.0]-
undec-7-ene (DBU) (Sigma Aldrich, USA) was also used as
received. The solvents were distilled before used.
2
.3. Measurement and characterization
All the NMR spectra were recorded on a FTNMR-JEOL AL300
spectrometer using CDCl as a solvent. The single crystal of
3
all the chromophores were grown from the saturated solution
of acetone–methanol mixed solvent (1 : 1 by weight) at room
temperature by slow evaporation technique. X-ray diffraction
data of single crystals were collected using the Xcalibur
2.2. Synthesis of chromophores
The synthesis of all the four compounds (1 to 4) was performed oxford CCD diffractometer. Structure solution and rene-
23
according to literature procedures.
ment were carried out utilizing SHELXS and SHLEXL-97. DSC
2
.2.1. 4-N,N-Dimethylamino-b-nitrostyrene (1). 4-N,N- experiment was performed using Mettler TA 4000 machine
ꢁ
Dimethylaminobenzaldehyde (10 mmol, 1.49 g) was added to a under nitrogen atmosphere with a heating rate of 10
solution of ammonium acetate (24 mmol, 1.85 g) and nitro- min . However, thermo gravimetric-differential thermal
methane (60 mmol, 3.25 mL) in AcOH (12 mL) and reuxed for analysis (TG-DTA) was performed on a NETZSCH, STA 409 PC
C
ꢂ1
2
5 min. The mixture was poured into ice cold water, a red analyzer under a nitrogen atmosphere with a heating rate of
ꢁ
ꢂ1
ꢁ
colour precipitate was obtained and ltered. The precipitate 10 C min
from room temperature to 450 C. UV-Vis
Shimadzu
UV-1700 model. Fluorescence spectra were obtained using a
NMR (300 MHz, CDCl ): 7.98 (d, 1H, H–C]C–), 7.41–7.52 (m, JY Horiba uorescence spectrophotometer. Fluorescence
was washed properly with water and dried. The dried precipi- absorption spectra were acquired using
a
1
tate was recrystallized from ethanol (yield 1.65 g, 86%).
H
3
3H, two Ar-H and one –C]C–H), 6.69 (d, 2H, Ar-H), 3.07 (s, 6H, quantum yield was determined by using anthracene as the
–
CH ) ppm. reference.
3
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The solubility data is described by the van't Hoff equation
3
. Results and discussion
which correlates the mole fraction of solute (N ) with change in
s
3.1. Solubility measurement
enthalpy of the solution (DH ) and change in entropy of the
s
The knowledge of solubility of a solute in a particular solvent solution (DS ), when a solute is dissolve into the solvent/
s
24
provides information about the growth conditions and is useful solution.
for the thermodynamic analysis of solute–solvent interactions.
Solubility analysis of chromophores 1–4 was carried out in
acetone and methanol. It has been found that the solubility of
ꢀ
ꢁ
DH
s
DS
s
N
s
¼ exp
ꢂ
þ R
(1)
RT
where T is the absolute temperature and R is the gas constant.
The plot of ln N against 1/T results a straight line (Fig. S9†)
from which the values of DH and DS can be obtained from
slope and intercept of the straight line, respectively. The
obtained values of DHs and DSs for different solute
all the chromophores is very low in methanol while in acetone it
was good. However, due to low boiling point and fast evapora-
tion of acetone it was difficult to get large single crystals with
good optical quality. Therefore, we have checked the solubility
in various proportions of acetone–methanol. In 1 : 2 mixture of
acetone–methanol, the solubility was quite low (Table S1†) to
get a large size crystal. However, we avoided the major propor-
tion of acetone because of faster evaporation. That is why we
have not carried out solubility in 2 : 1 acetone–methanol mixed
solvent. The solubility in 1 : 1 mixture of acetone–methanol was
found to be good and therefore chosen for solubility measure-
ment and single crystal growth of chromophores. The solubility
curves in 1 : 1 acetone–methanol (by weight) for chromophores
s
s
s
(chromophore)/solvent systems are tabulated in Table 1. The
values of DH and DS are highest in the case of chromophore 3
s
s
which is attributed to the much higher solubility of chromo-
phore 3 into the solvent.
3.2. Crystal growth
The seed crystals of all the chromophores 1–4 were grown from
the saturated 1 : 1 acetone–methanol solution by slow evapo-
ration technique.
1–4 in the different temperature range are shown in Fig. 1.
For comparison, the solubility of all the chromophores at
For this purpose, saturated solutions of chromophores were
kept at room temperature for slow evaporation of solvent. Aer
a couple of days, small crystals were harvested and used for
seeding process. To grow large single crystals of all the chro-
mophores, seeds were inserted into the solutions which were
maintained at room temperature. With the slow evaporation of
solvent, nucleation started and large single crystals with
different dimensions have been obtained within 10 days. The
photographs of the grown single crystals of chromophores are
shown in Fig. 2. In the case of chromophore 4, crystal dimen-
ꢁ
3
7.0 C is reported in Table 1. In the case of chromophores 1, 2
and 4 the solubility is almost comparable, however in the case
of chromophore 3 it increased into large extent. The structures
of chromophores 3 and 4 are almost similar with only difference
in ester functionality. The chromophore 3 contains an addi-
tional –CH2– group and this group may facilitate interaction of
chromophore 3 with solvent and thus results increased
solubility.
Such type of interaction has been obtained in the crystal
packing of chromophore 3. In all the cases, solubility is high
enough to offer a fast growth rate from the solution and to get a
large single crystal.
3
sions of 17 ꢀ 4 ꢀ 1 mm has been successfully achieved.
3
However, for chromophore 3, 7 ꢀ 6 ꢀ 1 mm and for chromo-
3
phore 1, 4 ꢀ 2 ꢀ 1 mm have been obtained. Chromophore 2
showed poor growth characteristics and tiny crystals obtained.
This might be due to the poorer crystal packing and lack of
C–H/p interaction between the molecules. Plate like
morphology has been obtained in the case of chromophores 3
and 4; however 1 is thick needle-shaped with well-formed
crystallographic surfaces.
The correlations of between crystal growth and intermolec-
ular interactions have been discussed in Section 4.1. in detail.
Based upon these observations, it may be concluded that in a
Table 1 Solubility, change in heat and entropy of solution
a
Solubility
DH
(kJ mol
s
DS
s
(J K mol )
ꢂ1
ꢂ1
ꢂ1
Chromophore
(g per 100 g of solvent)
)
1
2
3
4
1.08
1.28
6.5
33.6
32.7
37.3
21.8
58.0
56.5
82.8
23.1
1.93
a
ꢁ
Solubility at 37.0 C in mixed 1 : 1 acetone–methanol (by weight)
Fig. 1 Solubility curves of chromophores 1, 2, 3 and 4 in 1 : 1 acetone– solvent.
methanol.
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crystallographic parameters for chromophores 1 and 2 show a
25
good agreement with their previous reported values.
The bond distances and bond angles within the molecules
show the usual values, the molecules being nearly perfectly
planar. In the crystals, molecules are linked by different types of
interactions. In chromophore 1 (C10–H10/O2) and chromo-
phore 2 (C2–H2/N2) only one type of interaction exist.
However, chromophore 3 exhibits C11–H11A/N1, C14–H2/
C2, C13–H13A/O1 and C5–H5/O1 types of interactions.
C13–H13A/C10, C6–H6/O1 and C7–H7/O1 types of inter-
actions have been found in chromophore 4. A comparative
detail of all the interactions, distances and angles are tabulated
in Table 3. However, these interactions are represented in Fig. 4.
In the case of chromophores 3 and 4, a view in projection down
Fig. 2 Grown crystals of (a) chromophore 1, (b) chromophore 2, (c) the c axis and down the b axis of the crystal packing are shown
chromophore 3 and (d) chromophore 4 from 1 : 1 acetone–methanol.
in Fig. 5 and 6, respectively. Apart from these interactions, the
packing in all the crystals (except chromophore 2) is stabilized
by intermolecular C–H/p interaction (Fig. S10–S12†). The
centroid to H distance is found to be 4.023, 3.967 and 3.725 A˚
for chromophores 1, 3 and 4, respectively. In the case of chro-
mophore 1, centroid of other benzene ring also shows interac-
tions with vinyl hydrogens of another molecule. These distances
are found to be 3.655 (centroid to a-H) and 3.637 (centroid to
b-H) A˚ .
particular push–pull system as we increase the strength of
electron acceptor groups (as nitrovinyl and dicyanomethylidene
in the case of chromophores 1 and 2, respectively) it will be
difficult to get large single crystals with good optical quality.
Furthermore, packing of molecules in a crystal lattice play an
important role in single crystal growth of materials. In the case
of chromophore 4, the molecules are connected in head–tail
fashion i.e., donor end of one molecule is connected with the
acceptor end of second molecule through intermolecular
interactions and results in a sheet structure (Fig. 6). In the
crystal packing of chromophore 3, the donor end of one
4
. Crystallographic studies
4.1. Crystal structure and packing
The crystal structure of chromophores 1, 2, 3 and 4 are reported
in Fig. 3. The lattice parameters as well as other crystallographic
parameters of the all the crystals are given in Table 2, while the
bond angles and bond distances are given in Table S2.† The
Fig. 3 The molecular structures of (a) chromophore 1, (b) chromophore 2, (c) chromophore 3 and (d) chromophore 4 showing displacement
ellipsoids at the 50% probability level.
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Table 2 Structural parameters for chromophores 1 to 4
1
2
3
4
Empirical formula
C
10
H
12
N
2
O
2
C
12
H
11
N
3
C
14
H
16
N
2
O
2
13 14 2 2
C H N O
M
r
192.22
197.24
244.29
230.26
Crystal size (mm)
Crystal system
Space group
0.3 ꢀ 0.2 ꢀ 0.1
Orthorhombic
Pbca
10.3118(10)
7.4555(8)
25.318(3)
90
90
90
1946.5(3)
8
1.312
0.3 ꢀ 0.2 ꢀ 0.1
0.4 ꢀ 0.3 ꢀ 0.2
0.4 ꢀ 0.3 ꢀ 0.2
Monoclinic
Triclinic
Triclinic
P2
1
P1ꢀ
P1ꢀ
˚
a [A]
3.9890(5)
14.0371(12)
9.5430(12)
90
100.572(12)
90
525.28(10)
2
1.247
7.6022 (17)
8.3944 (18)
11.5620(2)
106.836(18)
102.222(17)
102.304(19)
659.8(2)
2
1.230
293
0.71073
0.083
7.3225(14)
7.4824(14)
11.5556(16)
100.830(13)
104.334(14)
96.663(15)
593.64(18)
2
1.288
293
0.71073
0.088
˚
b [A]
˚
c [A]
ꢁ
a [ ]
ꢁ
b [ ]
ꢁ
g [ ]
3
˚
V [A ]
Z
ꢂ3
D [g cm
T [K]
l [A]
]
293
0.71073
0.093
293
0.71073
0.078
˚
ꢂ1
m [mm
]
F(000)
800
194
260
244
h
k
l
ꢂ14 to 13
ꢂ9 to 8
ꢂ10 to 34
3.47 to 29.18
4854
ꢂ5 to 4
ꢂ15 to 17
ꢂ8 to 12
4.58 to 29.14
2036
ꢂ10 to 9
ꢂ11 to 10
ꢂ15 to 14
3.78 to 29.09
5025
ꢂ9 to 9
ꢂ10 to 10
ꢂ14 to 15
3.69 to 29.11
4303
ꢁ
q range for data collection ( )
Reections measured
Reections unique
2192
1665
3538
2659
Data with F
o
o
> 2s(F )
1222
946
2017
1818
Parameters rened
Final R indices
Final wR indices
Goodness of t
CCDC number
127
138
166
0.0473
0.1177
1.029
154
0.0504
0.1089
1.031
0.0844
0.2121
0.952
0.0494
0.0539
0.878
1044113
1044114
1020677
1020678
molecule connect with the acceptor end of second molecule to found (as discussed above). Thus, in the case of chromophores
form a layer and this layer further connect with second layer in 3 and 4, during slow evaporation controlled nucleation will
head–tail fashion of the molecules. Thus give a 3D-sheet take place and that will grow in such a way to results a large
arrangement as shown in Fig. 5. However, in the case of single crystal.
chromophores 1 and 2, no such arrangements have been
4.2. Thermal properties
The thermal properties of all the chromophores were studied by
TG-DTA and DSC under a nitrogen atmosphere. The obtained
ꢁ
Table 3 Short-contact geometry ( A˚ , ) for chromophores 1 to 4
D–H/A
D–H
H/A
D/A
D–H/A
Chromophore 1
C10–H10A/O2
C10–H10B/O2
0.959
0.959
2.580
2.717
3.530
3.668
170.39
171.68
Chromophore 2
C2–H2/N2
0.930
2.673
3.385
133.89
Chromophore 3
C11–H11A/N1
C14–H2/C2
C13–H13A/O1
C5–H5/O1
0.97
2.593
2.894
2.606
2.672
3.503
3.782
3.542
3.445
156.32
160.14
164.80
141.07
0.931
0.960
0.930
Chromophore 4
C13–H13A/C10
C6–H6/O1
0.960
0.930
0.930
2.888
2.506
2.635
3.533
3.387
3.508
125.51
158.18
156.75
C7–H7/O1
Fig. 4 View of intermolecular interactions in (a) chromophore 1, (b)
chromophore 2, (c) chromophore 3 and (d) chromophore 4.
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Fig. 5 A view in projection down the c-axis of crystal packing in
chromophore 3.
Fig. 7 TGA curve of chromophores 1, 2, 3 and 4.
curves are shown in Fig. 7–9. The TGA data revealed that the
chromophores 2, 3 and 4 showed higher thermal stabilities as
compared to 1. Chromophore 1 showed 43% weight loss in
ꢂ ꢀ ꢁꢃ
ꢀ
ꢁ
ꢁ
1
E
a
1
the temperature range 208–261 C and about 16% in the
ln ln
¼
ꢂ
þ Constant
(2)
ꢁ
y
R
T
range of 262–443 C. In the case of chromophores 2, 3 and 4
almost similar single step weight losses have been observed.
From the TGA data, the amount of carbonized residue (char
where y ¼ (W
t
ꢂ W
N
)/(W
o
ꢂ W
N
) is the fraction of the number of
initial molecules not yet decomposed, E is the activation
a
2
6
yield) of 1 has been found (20%) higher than 2, 3 and 4 with
almost complete weight loss. This may be attributed to the
presence of entirely different acceptor moiety (nitrovinyl) in
the chromophore 1. DTA curve of all the derivatives reect
two peaks, one in the lower temperature region corresponds
to melting temperature and second one represent the
decomposition temperature of the materials. The decompo-
energy, R is the gas constant, T is the absolute temperature, Wt
is the weight at any time t, WN is the weight at innite time.
A plot of ln[ln(1/y)] vs. 1/T gives an good approximation to a
straight line (Fig. S14†) and the activation energy have been
calculated using the slope of the straight line. The obtained
values of activation energy in the particular decomposition
range for all the chromophores are also tabulated in Table 4.
The lower activation energy in the case of chromophores 2, 3
and 4 than chromophore 1 could be attributed to their higher
heat liability.
sition temperature obtained for chromophores 1 to 4 are 240,
ꢁ
3
54, 365 and 330 C, respectively. DSC curves exhibit sharp
endothermic peaks which are assigned as melting tempera-
tures of chromophores. Also, the heat of fusion and entropy
of fusion values were calculated using DSC data. The heat and
entropy of fusion values along with their melting and
decomposition temperatures are shown in Table 4.
4.3. Optical properties
4.3.1. Absorption spectral study. The electronic absorption
The activation energy associated with each stage of decom-
spectra of chromophores 1 to 4 have been compared in aceto-
nitrile and shown in Fig. 10. All the chromophores exhibit
27
position is described by the equation:
Fig. 6 A view in projection down the b axis of the crystal packing in chromophore 4.
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chromophores 1 to 4 occurs from the donor N,N-dimethylamino
group to acceptors nitrovinyl, dicyanomethylidene, cyano-
(ethoxycarbonyl)methylidene and cyano(methoxycarbonyl)-
methylidene, respectively. A decrease up to 18 nm in lmax value
has been observed as we move from chromophore 1 to 4. This
decrease in lmax value could be credited to a decrease in the
strength of acceptor group from nitrovinyl to cyano(methox-
ycarbonyl)methylidene for chromophores 1 to 4, respectively.
The electronic absorption spectra were also studied in
different solvents with varying polarity and found to be sensitive
to solvent polarity. In the case of chromophore 1 as the solvent
polarity increased from toluene to dimethyl formamide (DMF),
the absorption maximum is shied by 22 nm toward a longer
wavelength region. However, for chromophores 2, 3 and 4 the
shiing was found to be 11, 9 and 9 nm, respectively. The
electronic absorption spectra in various solvents for all the
chromophores are given in Fig. S15.† The lmax values along with
their molar extinction coefficient (3) for all the chromophores
are tabulated in Table 5.
Fig. 8 DTA curve of chromophores 1, 2, 3 and 4.
4.3.2. Fluorescence spectral studies. Fluorescence spectra
of all the chromophores have been recorded in both solution
and solid phases using their lmax values as excitation wave-
length, and are discussed separately as follows:
4.3.2.1. In solution. All the chromophores exhibit strong

uorescence in acetonitrile solution, except chromophore 1
which shows a weaker emission as reported in Fig. 11.
Strong emission in these chromophores arises from a state
which is more polar than the ground state and probably a
intramolecular charge-transfer (ICT) state. Like absorption, the

uorescence emission reects a red shi with increase in
solvent polarity from toluene to DMF (Fig. S16†). Also, the
uorescence behaviour is greatly affected by the nature of

solvent. Chromophores 2, 3 and 4 exhibit stronger uorescence
in DMF as compared to other solvent. In toluene, chromophore
1
surprisingly shows very strong emission while in methanol,
acetonitrile and DMF its emission is almost negligible. The
absence of CT uorescence in chromophore 1 in polar solvents
may be attributed to uorescence quenching by the rehybri-
28
dized intramolecular charge transfer (RICT) state.
Fig. 9 DSC curve of chromophores 1, 2, 3 and 4.
The change in dipole moment is an important parameter in
charge transfer processes and its evaluation is quite useful in
designing materials for various optical and nonlinear optical
applications. Several methods have been reported to evaluate
the change in dipole moment. In this connection, the Lippert–
Mataga equation that relates the Stokes shi to the dipole
intense absorption bands in the wavelength region 421 to
439 nm which are characteristic of highly p-conjugated mole-
cules and thus could be attributed to an intramolecular charge
transfer (ICT) p–p* transitions. The ICT transitions in
Table 4 Showing melting temperature, decomposition temperature, heat of fusion, entropy of fusion and thermal activation energy
a
m
ꢁ
b
ꢁ
ꢂ1
ꢂ1
ꢂ1
ꢁ
c
ꢂ1
Chromophore
T
( C)
T
d
( C)
DH
f
(kJ mol
)
DS
f
(J K mol
)
Decomposition range ( C)
a
E (kJ mol )
1
2
3
4
184.0
181.0
126.0
145.0
240.0
354.0
365.0
330.0
29.83
31.00
28.79
32.14
65.35
68.24
72.12
76.96
208–261
220–311
235–360
225–330
187.75
146.28
131.96
136.50
a
b
c
Melting temperature obtained from DSC. Decomposition temperature obtained from DTA. Activation energy has been calculated using TGA
data in the particular decomposition region.
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Fig. 11 Fluorescence spectra of chromophores 1, 2, 3 and 4 in
ꢂ5
acetonitrile with molar concentration 1.0 ꢀ 10 M.
M is the molecular weight of solute and N is the Avogadro
number] and Df ¼ Lippert–Mataga solvent polarity parameter
and is the function of static dielectric constant 3 and the
Fig. 10 Absorption spectra of chromophores 1, 2, 3 and 4 in aceto-
nitrile with molar concentration 1.0 ꢀ 10 M.
ꢂ5
31,32
refractive index of the solvent and dened as:
ꢂ
ꢃ
2
ð3 ꢂ 1Þ
ðn ꢂ 1Þ
moment of the excited state is a simple and widely used formula
as shown in eqn (3):
Df ¼
ꢂ
(4)
2
29,30
ð23 þ 1Þ ð2n þ 1Þ
À
Á
2
2
m ꢂ m
abs
max
emi
max
e
g
Dy ¼ y ꢂ y
¼
Df þ Constant
(3)
The plot of solvent polarity function versus stokes shi shows
a linear variation (Fig. S17†). The change in dipole moments
3
hca
(
m
e
ꢂ m
g
) for all the chromophore have been calculated from the
In the above equation (m
e
ꢂ m ) ¼ change in dipole moments
g
slope of the curve and found to be 3.17, 3.59, 4.80 and 4.45 D for
chromophores 1, 2, 3 and 4, respectively. Thus signicant
change in dipole moment on excitation has been observed. This
could be attributed to the presence of a non-classical twisted
intramolecular charge transfer (TICT) and increase in planarity
on excitation make the molecule more polar as compared to
ground state. The change was found to be highest for chro-
mophore 3 and lowest for chromophore 1 while chromophores
2 and 4 showed moderate change. The lowest change in case of
chromophore 1 might be due to its ground state is more polar
than the ground state of other chromophores.
of the excited and ground state, h ¼ Planck's constant, c ¼
velocity of light, a ¼ Onsager cavity radius of interaction
1
/3
[
¼ (3M/4pdN) , where d is the density of the solute molecule,
Table 5 Optical properties of chromophores 1 to 4 in different
solvents
abs
max
emi
max
l
3
l
D ꢀy st
ꢂ1
ꢂ1
ꢂ1
a
Compound Solvent (nm) (L mol cm
)
(nm) (cm
)
øF
1
1
1
1
2
2
2
2
3
3
3
3
4
4
4
4
DMF
MeCN
MeOH
447
439
434
81 008
32 642
16 305
34 128
73 731
87 930
44 622
94 972
61 984
50 645
50 613
85 064
79 114
72 497
63 131
51 391
557
536
527
503
487
483
473
460
485
480
474
455
485
482
474
456
4418
0.009
0.007
0.010
0.087
4.3.2.2. In solid. The recorded solid state uorescence
4122
4066
3649
2349
2497
2168
1735
2801
2920
2656
1945
2800
3004
2600
1994
spectra of all the chromophores differ in their emission
behavior compared to that in solution and a pronounced red-
0.022 shi has been noted. The spectra are shown in Fig. 12.
Toluene 425
DMF
MeCN
MeOH
437
431
429
0.014
0.014
0.012
0.036
0.023
0.018
This could be attributed to the inter-uorophore interactions
in the solid state. Such type of interactions in chromophores
has also been proved by the X-ray crystallographic studies.
Similar results have been observed for the several lumino-
Toluene 426
DMF
MeCN
MeOH
427
421
421
33
phores. Chromophores 1 and 2 reect a single broad emission
Toluene 418
0.017 band at 607.0 and 533.5 nm, respectively. However, in the case
DMF
MeCN
MeOH
427
421
422
0.034
0.019
0.018
0.018
of chromophores 3 and 4 dual uorescence has been observed.
In the case of chromophores 3 and 4 it is assumed that,
donor and acceptor groups have a minimum overlap in the CT
states and may results in dual uorescence as reported.
Furthermore, this interesting behavior of dual uorescence in
the case of chromophores 3 and 4 may be explained based upon
Toluene 418
a
Quantum yield of all the chromophores have been calculated using
36
anthracene as a reference (ø
F
¼ 0.20 in methanol).
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Acknowledgements
V. K. Gupta is thankful to the University Grant Commission
UGC), New Delhi for the award of BSR meritorious fellowship.
(
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