Fourier-transform infrared and optical absorption spectra of 4-tricyanovinyl-N,N-diethylaniline thin films

Article abstract of DOI:10.1016/j.saa.2007.03.032

Thin films of 4-tricyanovinyl-N,N-diethylaniline (TCVA) were prepared for the first time using thermal evaporation technique. The molecular structure and electronic transitions of TCVA films were investigated by Fourier-transform infrared (FTIR) and ultraviolet-visible (UV-vis) spectra. The observed vibrational wavenumbers in FTIR spectra were analysed and assigned to different normal modes of the molecule. UV-vis electronic absorption spectral measurements of TCVA films were analysed to obtain the electronic transitions and optical band gap (E_g). Other important optical parameters such as molar extinction coefficient (ε_molar), the oscillator strength (f), and the electric dipole strength (q²) were also reported.

Full text of DOI:10.1016/j.saa.2007.03.032

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Spectrochimica Acta Part A 69 (2008) 205–210
Fourier-transform infrared and optical absorption spectra of
4-tricyanovinyl-N,N-diethylaniline thin films
a
b
a
a
a,∗
M.M. El-Nahass , H.M. Zeyada , K.F. Abd-El-Rahman , A.A.M. Farag , A.A.A. Darwish
a
Physics Department, Faculty of Education, Ain Shams University, Rorxy Square, 11757 Cairo, Egypt
b
Physics Department, Faculty of Science at New Damietta, New Damietta 34517, Egypt
Received 23 February 2007; received in revised form 20 March 2007; accepted 20 March 2007
Abstract
Thin films of 4-tricyanovinyl-N,N-diethylaniline (TCVA) were prepared for the first time using thermal evaporation technique. The molecular
structure and electronic transitions of TCVA films were investigated by Fourier-transform infrared (FTIR) and ultraviolet–visible (UV–vis) spectra.
The observed vibrational wavenumbers in FTIR spectra were analysed and assigned to different normal modes of the molecule. UV–vis electronic
absorption spectral measurements of TCVA films were analysed to obtain the electronic transitions and optical band gap (E
g
). Other important
optical parameters such as molar extinction coefficient (εmolar), the oscillator strength (f), and the electric dipole strength (q ) were also reported.
2007 Elsevier B.V. All rights reserved.
2
©
PACS: 78.30.Jw; 87.64.Je; 8.20.Ae
Keywords: Organic thin film; FTIR spectra; Optical absorption
1
. Introduction
This dye belongs to a class of organic compounds known as
molecular rotors [5–7], which have a great importance as fluo-
rescence probes of the flexibility of surrounding media [5–12].
It is used for coloring synthetic polymer fibers [13]. Also, it is
applied as a sublimable dye in heat-transfer recording materi-
als [14] and in photoconductive recording materials [15]. It has
received some interest as a cytotoxic agent against tumours, as
an X-ray protective agent and as a stabilizer in plastics against
ultraviolet radiation [16].
The spectroscopic properties of organic electron-donor–
acceptor complexes have received considerable attention
[17–21]. Large changes in the polarity of molecules on elec-
tronic excitation had been early predicted by F o¨ rster [17] and
were then found by M u¨ lliken and Person [18] in charge-transfer
complexes and by Nagakura and Tanaka [19] in intramolecu-
lar charge-transfer complexes. Conjugation of the donor and
acceptor groups seemed to be the first condition for a significant
contribution of the intramolecular charge-transfer interaction.
For organic compounds, energy gap can be evaluated by
the energy interval between the highest occupied molecular
orbital (HOMO) and the first excited states in the lowest
unoccupied molecular orbital (LUMO) instead of the energy
interval between valence band and conduction band for tra-
ditional crystalline semiconductors. The absorption spectra in
Theelectronicandstructuralpropertiesofthedonor–acceptor
substituted ␲-conjugated organic compounds are of consider-
able interest because of their applicability to optoelectronic
devices and in fields of non-linear optics such as data storage [1].
Many donor–acceptor-conjugated organic molecules reported in
the literature fall into the following categories: substituted ben-
zene, biphenyls, stilbenes, azobenzenes ferrocenyl and schiff
bases [2,3]. Disubstituted benzenes of the type D–Ph–A consti-
tute a broad class of organic molecules with both technological
and biological interest where D is an electron donor and A is
an electron acceptor. Such compounds have been used in many
applications such as dyeing of fabric, coloring of toners, and per-
forming as dyes in photographic and optical information storage
technologies.
4
-Tricyanovinyl-N,N-diethylaniline (TCVA) is disubstituted
benzenes of the type donor–acceptor with an electron donor
diethylamino group) and an electron acceptor (tricyanovinyl
(
group) [4,5]. The molecular structure of TCVA is given in Fig. 1.
∗
Corresponding author. Tel.: +20 125430253.
E-mail address: aaadarwish@gmail.com (A.A.A. Darwish).
1
386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.03.032

2
06
M.M. El-Nahass et al. / Spectrochimica Acta Part A 69 (2008) 205–210
Fig. 1. The molecular structure of TCVA.
ultraviolet–visible (UV–vis) regions give important information
about interacting conjugated ␲-electrons which obviously are
also the basis of the relevant luminescent and electrolumines-
cent properties of the organic materials [22]. The absorption and
emission spectroscopic properties of TCVA have been investi-
gated in the vapour phase [23] and in liquid solutions [24].
Up to our knowledge, no study has been done on spectro-
scopic of TCVA thin films. This has given us the motivation
to study in detail the Fourier-transform infrared (FTIR) and the
UV–vis spectroscopes of TCVA in the form of thin films. In
this paper, TCVA thin films were prepared for the first time
using thermal evaporation technique. Also, we present the util-
ity of FTIR spectroscopy in the molecular structure analysis of
TCVA thin films and the spectra have been analysed in terms of
peak positions and intensities. Moreover, we studied its UV–vis
spectra in an attempt to obtain information about its electronic
transitions. We have also extended our measurements to exam-
ine carefully the main optical absorption edge and the optical
energy gap in the UV–vis spectral range.
Fig. 2. The differential scanning calorimeter, DSC, thermogram of TCVA com-
pound at a heating rate of 10 C/min.
◦
◦
that this compound is stable up to 150 C. Also, a sharp endother-
◦
mic peak at 168 C was observed, which corresponds to the
melting point of the material. The X-ray diffraction patterns of
TCVA in powder form show that there are several peaks with dif-
ferent intensities indicating that the material is polycrystalline.
TCVA films were prepared by thermal evaporation technique
using a high vacuum coating unit (Edwards, E306A). Thin films
were deposited from a titanium evaporator charged by TCVA in
−
4
a vacuum of 10 Pa. The material was heated up by passing a
suitable current through the titanium evaporator. The deposition
−
1
rate (5 nm s ) was controlled using a quartz crystal thickness
monitor (Edwards, FTM4). Some of these films were annealed
◦
at 150 C for 2 h in air. After annealing, the temperature was
slowly reduced to the ambient temperature. X-ray diffraction
patterns (not represented) recorded for the as-deposited TCVA
film, showed that there are several peaks with background of
amorphity indicating that the film is in a certain degree of crys-
tallinity. Also, X-ray diffraction patterns of the annealed film
showthatthepeaksincreasedintheirintensities, whichindicated
that annealing enhanced its degree of crystallinity.
2
. Experimental details
The TCVA compound was prepared by direct reaction
between N,N-diethylaniline and tetracyanoethylene [25] and the
synthesis was described as follows:
A solution of N,N-diethylaniline (10 mmol) was dissolved
◦
FTIR on TCVA films was performed using Bruker, Vector
in N,N-dimethylformamide, DMF, (20 ml) and cooled to 0 C.
2
4
2 infrared spectrophotometer in the infrared spectral range
00–4000 cm . For FTIR measurements of the powder form,
Tetracyanoethlene, TCNE, (10 mmol) was also dissolved in
DMF (10 ml) and it was then added drop wise to the aromatic
−
1
◦
1 mg of TCVA powder was mixed with 50 mg of vacuum dried
IR-grad KBr. However, for film form, a film of thickness 636 nm
was deposited onto optically flat KBr single crystal substrate.
The UV–vis absorption spectra of TCVA films deposited onto
optically flat fused quartz substrates were recorded on spec-
trophotometer (JASCO model V-570 UV–vis–NIR). All the
spectra were recorded at room temperature.
amine solution with stirring at 0 C. After it has been completely
◦
added, the solution was then heated at 90 C for 3 h with contin-
uous stirring. The solvent was removed under reduced pressure
and the crude dye was dissolved in chloroform and chromatogra-
phied on silica gel using a mixture of chloroform ethyl acetate
(
3:7) as eluent. Recrystallization from toluene gave TCVA as
1
dark brown crystals in 85% yield. H NMR (CDCL3): δ 8.06 (d,
J = 10 Hz), 6.73 (d, 2H, J = 10 Hz), 3.53 (q, 4H, J = 7 Hz, CH2),
1
.29 (q, 6H, J = 7 Hz, CH3). Elemental analysis: found C, 71.85;
3. Results and discussion
H, 5.72; N, 22.31; calculated for C H14N4: C, 72.00; H, 5.60;
15
N, 22.40.
3.1. Infrared spectra
In addition, thermal analysis is carried out by differen-
tial scanning calorimeter (DSC) using Shimadzu DSC-50. The
results of this investigation are presented in Fig. 2, which showed
The infrared spectrum is the unique characteristic of func-
tional groups stacking the molecule and is found to be the most

M.M. El-Nahass et al. / Spectrochimica Acta Part A 69 (2008) 205–210
207
Table 1
The position of band in FTIR spectra of different forms of TCVA
Powder
Film
Assignment
As-deposited
Annealed
3
3
2
2
2
1
1
1
1
1
1
1
1
1
1
1
432.7
100
3368.2
3100
3424.9
3100
O–H stretching vibration.
C–H aromatic
C–H asymmetric stretching in CH3
C–H asymmetric stretching in CH2
977.6
931.3
210.0
602.6
494.6
449.9
423.2
348.0
278.6
220.7
199.7
156.8
070.3
004.7
2967.9
2917.8
2206.2
1602.6
1488.8
1449.9
1421.3
1346.1
1276.7
1218.8
1191.8
1153.2
1068.4
1004.7
894.8
819.6
794.5
725.1
661.5
595.9
553.9
501.4
428.0
2979.5
2933.2
2210.0
1604.5
1490.5
1449.9
1423.2
1348.0
1278.6
1220.7
1185.4
1142.5
1070.3
1004.7
894.8
819.6
794.5
725.1
658.8
585.8
557.2
505.3
428.5
C
C
C
N stretching vibration
C stretching vibration
C bending vibration
CH3 asymmetric deformation
CH2 in-plane deformation
C–H symmetric deformation in CH3
C–H in plane bending
CH2 wagging
CH2 twisting
CH3 wagging
C–C stretching
C–C stretching
9
8
7
7
6
5
5
5
4
14.2
19.6
92.7
25.1
63.9
71.0
42.9
03.0
42.8
CH3 rocking
C–H out of plane bending
CH2 rocking
CH2 rocking
C–C in plane bending
C–C in plane bending
C–C in plane bending
C–C in plane bending
C–C in plane bending
Absorption frequencies are in cm 1.
−
Fig. 3. Infrared spectra of TCVA for (a) powder form, (b) as-deposited film and
This means that hydroxyl group could be interred due to the
absorption of water (H2O) by the material. However, the spec-
trum of TCVA annealed films shows that this peak is removed
indicating that when the material is heated and at a certain
◦
(
c) annealed film at 150 C for 2 h.
useful physical method of investigation in identifying functional
groups and to know the molecular structure. The absorption of
IR radiation causes the various bands in a molecule to stretch
and bend with respect to one another. The most important range
−1
temperature water will be removed. The 3100 cm absorption
−1
band is assigned to C–H aromatic. In addition, the 2977.6 cm
absorption band is assigned to an asymmetric C–H stretching
−1
−
1
mode of the CH3 group and the 2931.3 cm absorption can be
assigned to C–H asymmetric stretching in CH2.
(
400–4000 cm ) is of prime importance for the study of an
organic compound by spectral analysis [26]. The infrared spec-
trum has two regions. The fingerprint region is unique for a
moleculeandthefunctionalgroupregionissimilarformolecules
with the same functional groups.
Fig. 3 shows FTIR spectra of powder, as-deposited and
annealed film of TCVA and all the wavenumber values of peaks
values are listed in Table 1. A comparison of FTIR spectra of
powder and as-deposited film forms indicates that thermal evap-
oration technique is a good one to obtain undissociated TCVA
films. In addition, the FTIR spectrum of the annealed TCVA film
is similar to the spectra of powder and as-deposited film, which
indicated that TCVA film has good thermal stability.
−1
A sharp absorption band appearing at 2210 cm is assigned
to the stretching vibration of C N group that forms conju-
gated bond with benzene ring. The strong band appearing at
−
1
1
602.6 cm is due to C C stretching vibration. These bands
point to the incorporation of the tricyanovinyl group. The peaks
−
1
at 1450 and 1367.8 cm indicate to the CH3 bending modes.
The CH2 in-plane deformation and CH2 wagging were observed
−
1
at 1423.2 and 1344 cm , respectively. The C–H deforma-
tion bands of aromatic C–H groups are observed at 819.6 and
− −1
1
7
92.7 cm . The bands in the 792.7–679 cm region can be
assigned to C–H and C–C vibrations.
Absorption bands found in the high-frequency (4000–
−1
2
700 cm ) region of spectrum are usually associated with O–H
3.2. The electronic absorption spectra
and C–H stretching modes. Fig. 3 shows a broad intensity band
−
1
−1
at 3432.7 cm for TCVA powder and at 3368.2 cm for as-
deposited film. This band can be logically assigned to a hydroxyl
group O–H stretching vibration. The presence of the O–H band
in IR spectrum indicates that the material contains hydroxyl
group despite the molecular structure dos not contain this group.
The absorption spectra of as-deposited and annealed TCVA
films are shown in Fig. 4. The energies corresponding to the
absorption maximum presented in these spectra are listed in
Table 2. The electronic absorption spectra of TCVA display four
main bands. The first band (A) at 200–230 nm is assigned to the

2
08
M.M. El-Nahass et al. / Spectrochimica Acta Part A 69 (2008) 205–210
a molecule, electrons are promoted from HOMO to LUMO. In
a semiconductor bulk solid, the transitions occur between the
valence band and conduction band. The valence band and con-
duction band are the solid-state analogue of the HOMO and
LUMO of a molecule. Eg is the band gap or difference between
the valence and conduction bands. The optical absorption coef-
ficient (α) of a solid describes the exponential decay of light
−αd
intensity with the distance into solid (It = I0 e ). It can be
described using the expression:
2
.303 × Abs
2.303 × OD
α =
=
(1)
d
d
Here d is the film thickness and
ꢀ
ꢁ
ꢀ ꢁ
I0
It
1
Abs OD = log10
= log10
(2)
T
where T is transmittance and Abs is the absorbance or the optical
density OD.
Fig. 4. The absorption spectra of as-deposited and annealed TCVA films.
In many organic materials, it is usual to analyse the optical
absorption at the fundamental edge in terms of band-to-band
transitions theory [27–32]. In this treatment, the absorption data
follows a power law, which is given by [33]:
n–␴* transitions of N,N-diethylaniline group and second band
(
B) at 235–365 nm is assigned to the ␲–␲* transitions of the
aromatic rings; this band appears splitted. The third band (C) at
70–565 nm involves n–␲* transition of the C N groups. The
3
m
(
αhν) = g(hν − Eg)
(3)
longer wavelength band (D) at 570–900 nm can be assigned to an
intramolecular charge-transfer interaction. The charge transfer
will originate from N,N-diethylaniline group as an origin to the
C N group as a sink.
This absorption is in agreement with that obtained for 4-
tricyanovinyl-N,N-dimethylaniline whose absorption spectrum
was recorded in different solvents [25]. The bands are assigned
as a charge-transfer bands arising from the resonance of the lone
pair of nitrogen atom of the aniline to the tricyanovinyl sub-
stituents. An intense absorption band in the visible region was
also obtained for 4-dicyanovinyl-N,N-dimethylaniline (p-N,N-
dimethylaminobenzylidenemalononitrile) in different solvents
and yield values of the optical energy band (Eg). Here g is a
constant and m is an index which can assume values of 2, 1/2,
1/3 or 3/2 depending on the nature of the band-to-band electronic
transitions and the profile of the electron density in the valance
and conduction bands. The experimental data have been fitted
with the theoretical equation (3) for different values of m. As
expected, the best fit is obtained for m = 1/2. This behaviour
indicated that the transitions are indirect allowed transitions,
which are expressed by the following modified equation:
αhν)1/2 = g(hν − Eg ± Eph)
(
(4)
[
5] where they explained their results by ␲–␲* transition
where Eph is the energy of phonons assisted. Fig. 5 shows the
1/2
with a considerable amount of intramolecular charge-transfer
character.
functional dependence of (αhν) on hν for as-deposited and
annealed TCVA films. The obtained values of Eg and Eph from
these curves are listed in Table 2.
Generally the spectra of the molecule possessing phenyl–
cyan complexes demonstrate extremely large intra-molecular
charge transfer [22], which substantially changes both linear
as well as non-linear optical properties. So one can expect an
occurrence of the large range of the optical properties with the
enhanced non-linear optical susceptibilities. This is unambigu-
ously confirmed by the annealing spectral features presented in
Fig. 4, probably due to contribution of enhanced number of the
electron-vibration anharmonic modes.
The tail of the absorption edge is exponential indicating the
presence of localized states in the energy band gap and support-
ing the amorphous background obtained in the X-ray pattern.
The origin of this band tailing is still a matter of conjecture but
according to Dow and Redfield [34] it arises from the random
fluctuations of the internal fields associated with structural dis-
order which are considerable in many amorphous solids. The
amount of tailing can be estimated to a first approximation by
plotting the absorption edge data in terms of an equation origi-
nally given by Urbach [35] and which has been applied to many
The absorption bands are due to electronic transition from
the ground states to excited states. During such a transition in
Table 2
The electronic transitions in TCVA film
A (eV)
B
C
D (eV)
Eg (eV)
Eph (meV)
Et (meV)
B1 (eV)
B2 (eV)
B3 (eV)
C1 (eV)
C2 (eV)
As-deposited film
Annealed film
6.08
6.26
4.73
4.73
4.22
4.30
3.56
3.58
3.05
3.04
2.49
2.50
2.13
2.09
1.45
1.51
114
91
217
141

M.M. El-Nahass et al. / Spectrochimica Acta Part A 69 (2008) 205–210
209
Fig. 5. Plot of (αh␯)1/2 vs. h␯ for as-deposited and annealed TCVA films.
Fig. 6. Plot of ln(α) vs. h␯ for as-deposited and annealed TCVA films.
glassy materials. The absorption edge of non-metallic materials
gives a measure of the energy band gap and the exponen-
tial dependence of the absorption coefficient, ln(α), on photon
energy, hν, is found to hold over several decades for a glassy
material and takes the form [35]:
ꢀ
ꢁ
hν
ln(α) = ln(α0) −
(5)
Et
where α0 is a constant and Et is interpreted as the width of the tail
of localized states in the forbidden band gap. Fig. 6 represents
the linear dependence of the natural logarithm of absorption
coefficient, ln(α), on the photon energy, hν, for as-deposited and
annealed TCVA films. The reciprocal of the slope of each line
yields the magnitude of Et and its values for as-deposited and
annealed films are listed in Table 2. It is clear that annealing
process increases the energy gap of TCVA film and decreases
both phonon energy and width of the tail of localized states.
It is useful to relate the absorption coefficient, α, to the molar
extinction coefficient, εmolar, which is often used to describe the
absorption of light by non-solid molecular media. The spectral
distribution of the molar extinction coefficient was calculated
from the relation [36]:
Fig. 7. Plot of εmolar vs. h␯ for as-deposited and annealed TCVA films.
ε
for as-deposited and annealed TCVA films are shown in
molar
Fig. 7.
The two most important parameters are the oscillator
2
strength, f, and the electric dipole strength, q where their values
were calculated by using the following mathematical expression
ρ
3
α =
× 10 ln(10)ε
(6)
molar
[32]:
M
ꢂ
where ρ is the solid mass density; M the molecular weight;
f = 4.32 × 10−9
εmolar(ν) dν
(7)
− −1
1
ε
molar
is in units of L mol cm . The spectral distributions of
Table 3
The spectral parameters of TCVA film at different bands
−
1
−1
2
˚
Band
λmax (nm)
εmolar (L mol cm
)
q (A)
f
As-deposited
Annealed
As-deposited
Annealed
As-deposited
Annealed
As-deposited
Annealed
A
204
262
294
348
406
498
582
198
262
288
346
408
496
592
9515
8019
7518
4508
4914
4404
3876
6254
5313
4499
3483
3680
3354
2922
0.67
0.20
0.21
0.08
0.60
0.76
0.25
0.48
0.12
0.13
0.06
0.16
0.58
0.13
0.30
0.01
0.05
0.01
0.04
0.14
0.05
0.37
0.17
0.03
0.01
0.01
0.10
0.02
B1
B2
B3
C1
C2
D

2
10
2
M.M. El-Nahass et al. / Spectrochimica Acta Part A 69 (2008) 205–210
[7] M.S.A. Abdel-Mottaleb, R.O. Loutfy, R. Lapouyade, J. Photochem. Pho-
ꢀ
ꢁ
ꢀ
ꢁ
1
ꢀλ
λ
q =
ε
molar
(8)
tobiol. A 48 (1989) 87.
2
500
[
[
8] R.O. Loutfy, K.Y. Law, Macromolecules 14 (1981) 587.
9] K.Y. Law, Chem. Phys. Lett. 75 (1980) 545.
where ꢀλ is the absorption half bandwidth. In order to evaluate
the oscillator strengths and the electric dipole strength, a Gaus-
sian fit was performed to each peak. The calculated parameters
for as-deposited and annealed TCVA films are listed in Table 3.
A comparison of these results shows that all parameters vary in
the band regions, and have a slight change by annealing process.
[
[
[
[
10] K.Y. Law, Photochem. Photobiol. 33 (1981) 799.
11] R.O. Loutfy, B.A. Arnold, J. Phys. Chem. 86 (1982) 4205.
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. Conclusion
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17] R. F o¨ rster, Organic Charge-Transfer Complexes, Academic Press, London,
1969.
18] R.S. M u¨ lliken, W.B. Person, Molecular Complexes, Wiley-Interscience,
New York, 1969.
TCVAthinfilmswerepreparedforthefirsttimeusingthermal
evaporation technique. A comparison of FTIR spectra of powder
and as-deposited thin film forms indicated that the thermal evap-
oration technique is a good one to obtain undissociated TCVA
films. In addition, the IR spectrum of the annealed TCVA film
is similar to the spectra of powder and as deposited film, which
indicated that TCVA film has good thermal stability. According
to the optical absorption spectra, TCVA thin film has a wide
absorption range spectrum in UV–vis region, which is appropri-
ate for application of TCVA thin films in photovoltaic devices.
Furthermore, theanalysisofabsorptioncoefficientrevealedindi-
rect transitions with 1.45 eV as energy band gap.
[
[
19] S. Nagakura, J. Tanaka, J. Chem. Phys. 22 (1954) 236.
20] T.P. Carsey, G.L. Findley, S.P. McGlynn, J. Am. Chem. Soc. 101 (1970)
4502.
[21] Z.R. Grabowski, K. Rotklewicz, A. Siemiarczuk, D.J. Cowtey, W. Nouv.
Baumann, J. Chim. 3 (1970) 443.
[
22] E. Koscien, J. Sanetra, E. Gondek, B. Jarosz, I.V. Kityk, J. Ebothe, A.V.
Kityk, Spectrochim. Acta Part A 61 (2005) 1933.
[23] A.V. Deshpande, A. Beidoun, A. Penzkofer, G. Wagenblast, Chem. Phys.
1
42 (1990) 123.
[24] A.V. Deshpande, A. Beidoun, A. Penzkofer, G. Wagenblast, Chem. Phys.
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1
[
[
25] A.M. Asiri, J. Saudi Chem. Soc. 6 (1) (2002) 51.
26] J.R. Dyer, Applications of Absorption Spectroscopy of Organic Com-
pounds, Prentice-Hall of India (P), New Delhi, 1987.
Acknowledgements
[
[
27] M.M. El-Nahass, K.F. Abd-El-Rahman, A.A.M. Farag, A.A.A. Darwish,
Int. J. Modern Phys. B 18 (2004) 421.
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The authors are grateful to the referees for critical comments
on the paper. One of authors, Ahmed A. Darwish, wishes to
thank Sana’a University (Yemen) for the award of a scholarship
for PhD at Ain Shams University.
[
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