Synthesis, characterization and density functional theory study of low cost hydrazone sensitizers

Article abstract of DOI:10.4314/bcse.v29i1.13

The 2-{4-[2-benzylidenehydrazino]phenyl}ethylene-1,1,2-tricarbonitrile (System 1), 2-{4-[2-(1- naphthylmethylene)hydrazino]phenyl}ethylene-1,1,2-tricarbonitrile (System 2) and 2-{4-[2-(9-anthrylmethylene)- hydrazino]phenyl}ethylene-1,1,2-tricarbonitrile (System 3) were synthesized by direct tricyanovinylation of hydrazones. The bathochromic shift in absorption spectra has been observed by increasing the solvent polarity. The FTIR spectra of these new dyes exhibited three important absorption bands. The first band centered near 3260 cm^-1 in System 1 while 3208 cm^-1 and 3211 cm^-1 in System 2 and System 3 for the vNH absorption, respectively. The second band is a sharp absorption band in the region of 2212-2209 cm^-1, which was attributed to the cyano group absorption. The third is an absorption band in the region of 1611-1603 cm^-1 ascribed for the C=N. Density functional theory (DFT) calculation of relative energies, relative enthalpies and free energies shows that E isomers are the most stable except System 3 in which the most stable is Z isomers. The conformational energy profile shows two maxima near (-90 and 90°) while three local minima observed at (-180, 0 and 180) for N1-N2-C1-C2 torsional angle. The highest occupied molecular orbitals (HOMOs) are localized on the whole molecules while lowest unoccupied molecular orbitals (LUMOs) are distributed on the tricarbonitrile.

Full text of DOI:10.4314/bcse.v29i1.13

Bull. Chem. Soc. Ethiop. 2015, 29(1), 137-148.
Printed in Ethiopia
DOI: http://dx.doi.org/10.4314/bcse.v29i1.13
ISSN 1011-3924
 2015 Chemical Society of Ethiopia
SYNTHESIS, CHARACTERIZATION AND DENSITY FUNCTIONAL THEORY
STUDY OF LOW COST HYDRAZONE SENSITIZERS
Abdullah G. Al-Sehemi1,2,3*, Ahmad Irfan1*, Abdullah M. Asiri4,5, Yousry Ahmed Ammar1
1Department of Chemistry, Faculty of Science, King Khalid University, Abha 61413, P.O. Box
9004, Saudi Arabia
2Unit of Science and Technology, Faculty of Science, King Khalid University, Abha 61413,
P.O. Box 9004, Saudi Arabia
3Research Center for Advanced Materials Science, King Khalid University, Abha 61413, P.O.
Box 9004, Saudi Arabia
4Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203,
Jeddah 21589, Saudi Arabia
5The Center of Excellence for Advance Materials Research, King Abdulaziz University, P.O.
Box 80203, Jeddah 21589, Saudi Arabia
(Received December 22, 2013; revised September 26, 2014)
ABSTRACT. The 2-{4-[2-benzylidenehydrazino]phenyl}ethylene-1,1,2-tricarbonitrile (System 1), 2-{4-[2-(1-
naphthylmethylene)hydrazino]phenyl}ethylene-1,1,2-tricarbonitrile (System 2) and 2-{4-[2-(9-anthrylmethylene)-
hydrazino]phenyl}ethylene-1,1,2-tricarbonitrile (System 3) were synthesized by direct tricyanovinylation of
hydrazones. The bathochromic shift in absorption spectra has been observed by increasing the solvent polarity.
The FTIR spectra of these new dyes exhibited three important absorption bands. The first band centered near 3260
cm-1 in System 1 while 3208 cm-1 and 3211 cm-1 in System 2 and System 3 for the NH absorption, respectively.
The second band is a sharp absorption band in the region of 2212-2209 cm-1, which was attributed to the cyano
group absorption. The third is an absorption band in the region of 1611-1603 cm-1 ascribed for the C=N. Density
functional theory (DFT) calculation of relative energies, relative enthalpies and free energies shows that E isomers
are the most stable except System 3 in which the most stable is Z isomers. The conformational energy profile
shows two maxima near (-90 and 90o) while three local minima observed at (-180, 0 and 180) for N1-N2-C1-C2
torsional angle. The highest occupied molecular orbitals (HOMOs) are localized on the whole molecules while
lowest unoccupied molecular orbitals (LUMOs) are distributed on the tricarbonitrile.
KEY WORDS: Dye-sensitized solar cells, HOMO, LUMO, Absorption, FTIR spectra
INTRODUCTION
The basic design of today’s high performance dye sensitized solar cells (DSSC) was developed
in the early 1990’s by Grätzel et al. [1]. They have great potential applications in the fields of
electronics, optics, catalysis and energy storage/conversion [1]. The DSSC have been
extensively investigated due to their simple structure, low cost and environmental friendly [2-4].
It is assumed that the world energy demand will increase by about 70% between 2000 and 2030.
Fossil fuels are facing rapid resource depletion which is supplying 80% of all energy consumed
worldwide. The resource reserves of fossil fuels throughout the whole world in 2002 were
projected to last 40 years for oil, 60 years for natural gas and 200 years for coal. Because of a
growing demand for energy, there is an urgent need for environmentally sustainable energy
technologies. Among all the renewable energy technologies, such as wind turbines, hydropower,
wave and tidal power, solar cells, solar thermal, biomass-derived liquid fuels and biomass-fired
electricity generation, photovoltaic technology utilizing solar energy is considered as the most
promising one [5]. An ideal light harvesting system absorbs light efficiently in a relatively broad
region covering most of the sunlight spectrum. In order to achieve this, the integration of
multiple excellent chromophores is necessary, and the energy transfer within the system should
__________
*Corresponding author. E-mail: irfaahmad@gmail.com

138
Abdullah G. Al-Sehemi et al.
be efficient. The development of highly efficient light-harvesting systems remains a challenging
task. There is need in improvement like the choice of the chromophores, linker types,
connection sites, and synthesis strategy etc.
Dye is an important part in DSSCs, which absorbs the light and then generates electric
current. Up till now there are two kinds of dye sensitizers, metal-organic complexes and metal-
free organic dyes. In metal-organic complexes, especially the noble metal ruthenium
polypyridyl complexes, including N3 and black dye that were presented by Gratzel et al. [1]
have given best performance among the dye sensitizers with on the whole energy change
efficiency greater than 10%. On the other hand, metal-free organic dyes as sensitizers for DSSC,
together with cyanines, hemicyanines, triphenylmethanes, perylenes, coumarins, squaraines,
indoline, and azulene-based dyes have also been formed due to their high molar absorption
coefficient, relatively simple synthesis procedure, various structures and lower cost [6].
In contrast to the numerous experimental studies of dye sensitizers, the theoretical
investigations are comparatively inadequate. Only several groups focused on the electronic
structures, absorption properties of dye sensitizers, Ru-complexes and organic dyes coupled
TiO2 nanocrystalline as well as the electron transfer dynamics of the interface between dyes and
nanocrystalline. Until now, it is a severe challenge for both experiment and theory to make clear
the fundamental properties of the ultrafast electron injection and come up to the satisfied
efficiency of DSSC. Further developments in dye design will play a vital part in the continuing
optimization of DSSC and it depends on the quantitative knowledge of dye sensitizer. So the
theoretical investigations of the physical properties of dye sensitizers are very important in order
to disclose the relationship among the performance, structures and the properties, it is also
supportive to design and production of novel dye sensitizers with higher performance [7, 8].
The conjugated π-system contains asymmetrically positioned electron-donor and electron-
acceptor substituents have been the most widely studied [9]. The charge transfer between the
functional groups imparts a high degree of polarity to push-pull systems. The electronic and
structural properties of donor-acceptor substituted π-conjugated organic compounds are of
considerable interest because of their applicability to electrooptic devices and data storage [10,
11]. A wide variety of structural modifications on the donor-acceptor and conjugated moieties
have been carried out [12, 13]. It is well known that by increasing donor, acceptor strengths and
with increasing length of conjugation charge transfer properties can be enhanced [14]. Many
donor acceptors conjugated organic molecules reported in the literature fall into the following
categories: substituted benzene, biphenyls, stilbenes, azobenzenes, ferrocenyl and Schiff bases
[15-17]. In all these classes, benzene rings with or without the π-bridges have been employed as
the conjugated moieties to connect donor. We have synthesized different derivatives of
hydrazones. We report on the synthesis of the novel push-pull system based on hydrazones. By
substituting benzyl, naphthyl and anthryl groups as donor, the effect on electronic, spectroscopic
especially absorption and FTIR has been investigated. We have shed light on the electronic and
optical properties by density functional theory (DFT) as well. We have also discussed the
structure-property relationship.
The article describes the synthesis of the targeted materials, the framework of the Density
Functional Theory (DFT) and Time Dependent Density Functional Theory (TDDFT), the
geometries, electro-optical properties, thermo-dynamical properties, FTIR spectra and
conformational analysis.
EXPERIMENTAL
The hydrazone derivatives were prepared through direct condensation between the
corresponding aromatic aldehydes and phenyl hydrazine Scheme 1. Equimolar quantities of
phenylhydrazine and the aldehydes were boiled in ethanol for half an hour. The precipitated
hydrazones were filtered, washed and dried. The pure hydrazones were obtained after
recrystallization from ethanol.
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Synthesis, characterization and DFT study of low cost hydrazone sensitizers
139
NC
CN
H
H
CN
CN
NC
NC
N
H
+
N
H
N
N
CN
Ar
Ar
No
Ar
1a
1b
1c
No System 1
Ar
System 2
System 3
Scheme I
Scheme II
Scheme 1. Schematic diagram of investigated Systems.
o
Benzaldehyde phenylhydrazone [(C13H12N2 (196.25)]. M.p. 150-152 C, yield 93%, Rf 0.61
1
(9:1), H NMR (CDCl3): d 6.82 (t, 1H), 7.13 (d, 2H), 7.27 (d, 2H), 7.40-7.67 (m, 5H), 8.17 (s,
1H).
o
1-Naphthaldehyde phenylhydrazone [C17H14N2(246.31)]. M.p. 72-74 C, yield 91%, Rf 0.74
1
(8:2), H NMR (CDCl3): d 6.89 (bs, 1H, NH), 7.16-7.82 (m, 7H, naph-H), 7.84 (s, 5H, Ph-H),
8.0 (d, 1H, CH).
9-Anthraldehyde phenylhydrazone [C21H16N2(296.37)]. M.p. 199-200, yield 21%, Rf 0.35 (9:1),
1H NMR (CDCl3): d 7.31-7.63 (m, 14H, Ph-H), 9.13 (s, 1H, CH=N).
The new chromospheres were prepared by direct tricyanovinylation of hydrazones and this
was achieved by mixing together the appropriate hydrazones and tetracyanoethylene (TCNE), as
shown in Scheme 2. All the prepared chromospheres were obtained as dark violet solids. A
solution of the requisite hydrazone 1 (0.01 mol) and TCNE in DMF (20 mL) was stirred at 60-
90 oC for 5-7 h. The solvent was removed and the residual solid was collected and recrystallized
from toluene/chloroform mixture.
System 1: 2-{4-[2-Benzylidenehydrazino]phenyl}ethylene-1,1,2-tricarbonitrile [C18H11N5
o
(297.31)]. M.p. 285-288 C, yield 76%, UV-Vis (acetone): λmax (nm) 525, 333; FTIR (cm-1):
1
3262 (sec. NH), 2212 (CN), 1609 (C=N), 1337 (C-N); H NMR (DMSO-d6): 7.42-8.01 (m,
9H,Ar-H), 8.14 (s, 1H, CH=N), 11.84 (s, 1H, NH; cancelled with D2O). 13C NMR (DMSO-d6):
79.56, 144.48 (C=C), 3x114.8 (3CN), 112.95, 113.9, 119.4, 126.6, 2x128.8, 2x129.9, 132.7,
134.2, 137.3, 151.19 (12C-Ar), 162.29 (CH=N).
System
2:
2-{4-[2-(1-Naphthylmethylene)hydrazino]phenyl}ethylene-1,1,2-tricarbonitrile
o
[C22H13N5(347.37)]. M.p. 268-270 C, yield 87%; UV-Vis (acetone): λmax (nm) 529, 353; FTIR
(cm-1): 3208 (sec. NH), 2212 (CN), 1611 (C=N), 1341 (C-N); H NMR (CDCl3):  7.37-8.05
1
(m, 11H, Ar-H), 8.76(s, 1H, CH=N), 11.95(s, 1H, NH); 13C NMR (DMSO-d6): 78.15, 144.48
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Abdullah G. Al-Sehemi et al.
(C=C), 3x114.84 (3CN), 113.94, 2x119.52, 123.96, 125.61, 2x126.30, 127.52, 127.68, 128.88,
129.41, 129.62, 130.44, 132.84, 133.57, 137.32(16C-Ar), 162.27 (CH=N).
System
3:
2-{4-[2-(9-Anthrylmethylene)hydrazino]phenyl}ethylene-1,1,2-tricarbonitrile
[C26H15N5 (397.43)]. M.p. 247-248 oC, yield 74%; UV-Vis (acetone): λmax (nm) 540, 407; FTIR
(cm-1): 3211 (sec. NH), 2209 (CN), 1603 (C=N), 1334 (C-N); 1H NMR (DMSO-d6): 6.80-8.20
(m, 13H, Ar-H), 8.30(s, 1H, CH=N), 10.80 (s, 1H, NH). 13C NMR (DMSO-d6): 79.0, 143.40,
(C=C), 3x112.49 (3CN), 111.86, 118.93, 120.22, 124.18, 124.64, 124.90, 125.31, 125.48,
125.75, 126.87, 127.38, 128.17, 128.64, 128.96, 129.23, 129.70, 130.90, 132.31, 132.87, 135.03
(20C-Ar), 162.50 (C=N).
System 1-E
System 1-Z
System 1-azo
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Synthesis, characterization and DFT study of low cost hydrazone sensitizers
141
System 2-E
System 2-Z
System 2-azo
System 3-E
System 3-Z
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Abdullah G. Al-Sehemi et al.
System 3-azo
Figure S1. Three possible classes of isomeric molecules for E, Z and azo isomerism.
Computational methods
Recently, it has been shown that Density Functional Theory (DFT) calculations with B3LYP/6-31G(d)
level of theory is approach to reproduce the experimental data of the hydrazones [18]. The molecular
structures of dye compounds have been optimized using DFT calculations with B3LYP/6-31G(d) level of
theory [19-22]. The Time Dependent (TD-DFT) has been applied to compute the absorption spectra at the
B3LYP/6-31G(d) level of theory which has been proved to be accurate and reliable method [23, 24].
Thermodynamic parameters i.e., relative energies (E0), enthalpies (H) and free energies (G)
(kcal/mol) for E, Z and azo isomers (See Figure S1) of studied Systems have been computed at B3LYP/6-
31G(d) level of theory. All of the DFT calculations were done using Gaussian-03 program package [25].
These isomers arise by rotating around torsion angles N1-N2-C1-C2 for investigated dyes. Then by fully
optimization of the resulting structures the stable conformers were obtained.
RESULTS AND DISCUSSION
Geometries
The fully optimized structures of studied isomers are shown in Figure S1. The computed
geometrical parameters of all the conformers have been given in Table 1. The C-C, C-N and N-
N bond lengths of E and Z isomer of all the investigated systems are almost same while the C9-
C10 in Z isomer of System 2 and System 3 stretched compared to System 1. Similarly, the C9-C10
in azo isomer of System 2 and System 3 lengthened compared to System 1. By comparing the
bond lengths of the E, Z and azo isomer of System 1, we have observed that C9-C10 of Z isomer
lengthened 0.017 Å while azo isomer stretched 0.040 Å compared to E isomer. The C9-N1 and
N2-C1 of azo isomer elongated almost 0.191 and 0.059 Å than E and Z isomers while N1-N2
shortened 0.105 Å. In System2 the C9-C10 of Z isomer lengthened 0.019 Å and azo isomer
stretched 0.048 Å compared to E isomer. The C9-N1 and N2-C1 of azo isomer elongated almost
0.190 and 0.059 Å than E and Z isomers while N1-N2 shortened 0.105 Å. The change in the bond
lengths between E, Z and azo isomers in System 3 is nearly alike as System 1 and System 2. The
torsion of E isomers in System 1 and System 2 is smaller than Z and azo isomers.
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Synthesis, characterization and DFT study of low cost hydrazone sensitizers
143
Table 1. Optimized geometrical parameters of investigated systems at B3LYP/6-31G* level of theory.
Bond lengths (Å)
System
C9-C10
1.460
1.477
1.500
1.462
1.481
1.510
1.461
1.489
1.511
C9-N1
1.289
1.290
1.481
1.290
1.290
1.480
1.294
1.289
1.480
N1-N2
1.353
1.357
1.248
1.352
1.356
1.248
1.359
1.355
1.249
N2-C1
1.373
1.376
1.432
1.373
1.375
1.431
1.379
1.374
1.431
C1-C2
1.410
1.409
1.398
1.410
1.411
1.403
1.414
1.411
1.399
C1-C6
1.412
1.411
1.404
1.412
1.409
1.399
1.416
1.410
1.403
C4-C7
1.457
1.457
1.471
1.456
1.457
1.471
1.455
1.457
1.471
System 1-E
System 1-Z
System 1-azo
System 2-E
System 2-Z
System 2-azo
System 3-E
System 3-Z
System 3-azo
Dihedral angles (degrees)
C10-C9-N1-N2
-180.0
2.002
-1.003
-178.6
3.174
-4.348
179.8
-0.877
0.337
C9-N1-N2-C1
-179.9
177.1
-180.0
-179.2
176.6
-178.8
53.8
-178.8
179.9
N1-N2-C1-C2
-0.117
-173.1
178.5
0.415
-175.3
6.461
175.3
-2.956
-0.946
System 1-E
System 1-Z
System 1-azo
System 2-E
System 2-Z
System 2-azo
System 3-E
System 3-Z
System 3-azo
Electronic and optical properties
Figure 1 illustrates the distribution pattern of the highest occupied molecular orbitals (HOMO)
and lowest unoccupied molecular orbitals (LUMO). As can be seen clearly, HOMO is
concentrated on the whole molecules and lone pair of electron on the N atoms. The LUMO is of
antibonding character with * character distributed on the tricarbonitrile except System 1 in
which it is delocalized on whole of the molecule. It is might be due to donor group strength; as
in System 2 and System 3 naphthyl and anthryl groups are more donor than benzyl of System 1
due to which in System 2 and System 3 LUMO shifted towards tricarbonitrile. The HOMO-
LUMO energy gap of these dyes was calculated at the B3LYP/6-31G(d) level of theory, see
Table 2. The orbital energy level analysis at the B3LYP/6-31G(d) level of theory show HOMO
energy (EHOMO), LUMO energy (ELUMO) and HOMO–LUMO energy gap (Egap) has been used as
an indicator of kinetic stability of the molecule. A large HOMO–LUMO gap implies a high
kinetic stability and low chemical reactivity, because it is energetically unfavorable to add
electrons to a high-lying LUMO or to extract electrons from a low-lying HOMO. The trend of
Egap is System 1 > System 2 > System 3. Generally it is said that low Egap leads to red shift which
is in good agreement with experimental absorption tendency System 3 > System 2 > System 1.
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Abdullah G. Al-Sehemi et al.
Efficient DSSC sensitizers have narrow band gap with LUMO lying just above the conduction
band of TiO2 and HOMO below the redox couple. As a model for nanocrystallinity the HOMO
and LUMO energies of bare cluster (TiO2)38 are -7.23 and -4.1 eV, respectively, resulting in a
Egap of 3.13 eV [26]. Generally energy gap more than 0.2 eV between the LUMO of the dye and
the conduction band of the TiO2 is necessary for effective electron injection [27]. The LUMO
energies of studied dyes are above the conduction band of TiO2. The HOMO of the redox couple
-
(I-/I3 ) is -4.8 eV [28]. It can be found that HOMOs of the dyes are below the redox couple. The
smaller HOMO-LUMO energy gaps revealed that these dyes would be efficient for DSSC.
System 1-E
LUMO
System 1-E
HOMO
System 2-E
System 2-E
LUMO
HOMO
System 3-E
System 3-E
LUMO
HOMO
Figure 1. The distribution pattern of HOMOs and LUMOs of the System 1-3.
Table 2. HOMO energy (EHOMO), LUMO energy (ELUMO) and HOMO–LUMO energy gap (Egap) calculated
(eV) at B3LYP/6-31G* level of theory.
Dyes
EHOMO
-6.02
-5.84
-5.60
ELUMO
-3.31
-3.30
-3.29
Egap
System 1
System 2
System 3
2.71
2.54
2.31
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Synthesis, characterization and DFT study of low cost hydrazone sensitizers
145
The electronic absorption spectra of the new chromospheres are characterized by an intense,
low-energy band that is dependent on the nature of the substituent. System1 showed absorption
band at 523 nm in chloroform. A substitution of phenyl group by 1-naphthyl, e.g. System 2
showed a bathochromic shift of 12 nm compared with the System 1. System 3, in addition to
substitution of the phenyl group with 9-anthryl group caused a bathochromic shift of 20 nm
compared to System 1, see Table 3.
Table 3. The effects of solvents on the absorption maxima (nm) of investigated dyes.
Dyes
λmax
a
b
c
d
e
f
g
εmax
∆λ
System 1
System 2
System 3
525
333
529
353
540
407
522
550
316
528
350
527
407
523
317
535
354
543
411
516
315
522
350
524
405
507
558
318
570
359
593
411
45662
10564
21145
5568
21062
9406
51
528
354
535
405
517
354
519
407
53
74
a = CH3COCH3, b = CH3CN, c = CH3COOH, d = CHCl3, e = (C2H5)2O, f = PhCH3, g = (CH3)2NCHO.
The dyes were measured in various solvents having different polarity. In non-polar solvents
for instance in ether System 3 showed the most solvatochromic shift with absorption maxima at
524 nm while in the DMF the maxima is at 593 nm. The solvatochromic effect can be estimated
from the difference between the absorption maxima in polar and non-polar solvents, e.g. ∆λ.
The solvent’s effects can be rationalized on the basis of the dipole formed by charge migration
from the nitrogen to the acceptor tricyanovinyl (Scheme 2).
FTIR spectra
The FTIR spectra of hydrazones exhibited two important absorption bands; the first band
centered near 3300 cm-1 for the NH absorption. The second band is a sharp absorption band in
the region of 1598 cm-1 ascribed for the CH=N absorption.
The FTIR spectra of these new dyes exhibited three important absorption bands; the first
band centered near 3260 cm-1 in System 1 while 3208 cm-1 and 3211 cm-1 in System 2 and
System 3 for the NH absorption. The second band is a sharp absorption band in the region of
2212-2209 cm-1, which was attributed to the cyano group absorption. The third is an absorption
band in the region of 1611-1603 cm-1 ascribed for the C=N absorption. The tricyanovinylation
1
undoubtedly takes place at a position para to the hydrazine group as evidenced from the H
NMR signals for the doublet two hydrogens. The azomethine hydrogen of the synthesized dyes
was located in the region of 8.1-9.13 ppm.
Thermodynamic stabilities
Thermodynamic parameters and energy barrier for the dyes have been computed at B3LYP/6-
31G(d) level of theory, see Table 4. The relative energy, E0, is defined as a difference between
its zero-point corrected total energy and that of the most stable one E. Relative enthalpies and
free energies at 298 oK are also defined as the difference between the enthalpy or free energy (in
kcal/mol) of a given E/Z or azo isomers and that of E form. As shown in Table 2, the order of
relative stability of those isomers is the same when considering relative energy or relative free
energy. DFT calculation shows that E isomers are the most stable except System3 in which the
most stable is Z isomers. We have observed that dipole moment of Z isomer is highest while azo
isomer is lowest. The trend in different isomers is Z > E > azo.
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Abdullah G. Al-Sehemi et al.
Table 4. Thermodynamic parameters (kcal/mol) for E, Z and azo isomers of studied Systems at B3LYP/6-
31G(d) level of theory.
System
Dipole moment
12.0721
12.7074
7.1234
E0
0.00
4.41
16.4
0.00
2.54
14.82
7.82
0.00
13.8
H
0.00
3.76
16.5
0.00
2.37
14.76
7.98
0.00
13.9
G
0.00
6.02
15.8
0.00
2.90
14.86
7.54
0.00
12.8
E
0.00
4.10
17.1
0.00
2.12
15.26
7.78
0.00
14.5
System 1 E
System 1 Z
System 1 azo
System 2 E
System 2 Z
System 2 azo
System 3 E
System 3 Z
System 3 azo
12.502
13.0342
7.0346
10.9088
12.5737
7.1099
Conformation analysis
For modeling and scanning calculation the initial guess for the stable isomers E was first
obtained from the optimization using B3LYP/6-31G(d) and transformed into the Z-matrix
format with Babel program [29]. The starting geometry of E-isomers was obtained by driving
procedure in HyperChem [30]. To predict the rotation barriers around the N1-N2-C1-C2
torsional angle scanning calculations were done. To identify low energy conformations, the
potential energy surface shape has been examined at the B3LYP/6-31G(d) level of theory. The
potential energy surfaces of dihedral angle (N1-N2-C1-C2) from +180o to -180o in 5o or 10o
steps were performed (Figure 2). The conformational energy profile shows two maxima near
(-90 and 90o). The aromatic rings are nearly perpendicular at these values of selected torsion
angle. The energy barriers may be due to the steric interactions between the -electrons of the
two aromatic rings. It is clear from Figure 2, there are three local minima observed at (-180, 0
and 180) for N1- N1-N2-C1-C2 torsional angle and these are most stable conformers for this
torsion angle. The DFT optimized geometry of these dyes is coplanar at these values of selected
torsion angle.
14
12
10
8
6
4
2
0
-2
-200
-150
-100
-50
0
50
100
150
200
T (N1-N2-C1-C2)
Figure 2. Molecular energy profile using DFT against the selected torsional degree of freedom
of System 2.
Bull. Chem. Soc. Ethiop. 2015, 29(1)

Synthesis, characterization and DFT study of low cost hydrazone sensitizers
147
CONCLUSIONS
The DFT calculations of thermodynamic parameters revealed that the most stable isomers are E
with the exception of System 3 (Z isomer). The HOMOs are localized on entire molecule while
LUMOs distributed on the tricarbonitrile except System 1 (in which delocalization was observed
on entire molecule). By increasing the solvent polarity red shift in the absorption wavelengths
have been observed. The FTIR spectra of the investigated dyes showed three significant
absorption bands. Clear intra-molecular charge transfer has been observed in System 2 and
System 3. The LUMO energies of studied dyes are above the conduction band of TiO2 while the
HOMOs of the dyes are below the redox couple. The smaller HOMO-LUMO energy gaps
revealed that these dyes would be efficient for DSSC.
ACKNOWLEDGEMENTS
The present work has been carried out under project No. 08-NAN155-7 funded by KAUST
(King Abdulaziz City for Science and Technology) through the Long Term Comprehensive
National Plan for Science, Technology and Innovation program.
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