Synthesis, characterization and DFT study of methoxybenzylidene containing chromophores for DSSC materials

Article abstract of DOI:10.1016/j.saa.2012.01.016

Novel tricyanovinyl derived from hydrazones have been prepared by the reaction of tetracyanoethylene and phenylethylidene hydrazone, and these dyes showed absorption in the region of 539-650 nm. The dyes showed pronounced solvatochromic effects as the pol

Full text of DOI:10.1016/j.saa.2012.01.016

Spectrochimica Acta Part A 91 (2012) 239–243
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, characterization and DFT study of methoxybenzylidene containing
chromophores for DSSC materials
Abdullah G. Al-Sehemia,∗, Ahmad Irfana,∗, Abdullah M. Asirib,c, Yousry Ahmed Ammara
a Chemistry Department, Faculty of Science, King Khalid University, Abha, Saudi Arabia
b Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
c Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah 21589, P.O. Box 80203, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 October 2011
Accepted 8 January 2012
Novel tricyanovinyl derived from hydrazones have been prepared by the reaction of tetracyanoethylene
and phenylethylidene hydrazone, and these dyes showed absorption in the region of 539–650 nm. The
dyes showed pronounced solvatochromic effects as the polarity of the solvents changed. The torsion in
E isomer is smaller than Z and azo isomers of MBD1 and MBD2. The HOMOs are delocalized on whole
of the molecule while LUMOs are distributed on the tricarbonitrile. The LUMO energies are above the
conduction band of TiO2 and HOMOs of the dyes are below the redox couple of MBD1 and MBD2. The
HOMO energies, LUMO energies and HOMO-LUMO energy gap of MBD1 and MBD2 are almost same. The
absorption spectra of both the dyes in different solvents are approximately same except in cyclohexane.
© 2012 Elsevier B.V. All rights reserved.
Keywords:
Dye-sensitized solar cells
HOMO
LUMO
Absorption
1. Introduction
Many donor–acceptor conjugated organic molecules reported
in the literature fall into the following categories: substituted ben-
The conversion of solar energy to electricity appears as one
of the substitutes that can replace fossil fuels [1]. Dye-sensitized
solar cells (DSSCs) have attracted considerable attentions due to the
most promising low cost [2–4]. Although the efficiencies of DSSCs
have not yet approached the theoretical limit and are not competi-
tive with the silicon-based solar cells, organic dyes have their own
advantages [5], such as high absorption coefficient and easy con-
trol of redox potentials of the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO) lev-
els. Nonlinear optical have great potential [6,7] especially for use
in optical communication, information processing, frequency dou-
bling and integrated optics [8]. Organic NLO materials have many
advantages over inorganic materials, such as large nonlinear opti-
cal coefficients, greater ease for synthetic design, easy preparation
and lower cost [9,10].
zene, biphenyls, stilbenes, azobenzenes ferrocenyl and schiff bases
[11,12]. We have reported the synthesis of new tricyanovinyl based
on hydrazones such as dyes [13–15]. In this paper, we report herein
the synthesis of novel push–pull systems based on hydrazones and
methoxy groups as donor groups to study the effect on the activ-
ity of these chromophores. The effect on geometries, electronic,
spectroscopic especially absorption and IR spectra and confor-
mational analysis has been investigated. We have also discussed
the structure–property relationship. In addition, the synthesized
compounds were subjected to computational study to make a com-
parison between the experimental and theoretical data.
2. Experimental methods
Melting points were recorded on a Thomas–Hoover capillary
melting apparatus without correction. IR spectra were taken as
KBr disk on a Nicolet Magna 520 FTIR Spectrometer, 1H NMR were
recorded in DMSO-d6 on a Bruker DPX 400 Spectrometer using TMS
as internal standard. UV–vis spectra were recorded on a Shimadzu
260 Spectrometer for solutions.
The most common design of molecules with large ␲ values
comprises strong electron-donors and acceptors connected by
a ␲-conjugated system (donor–␲–acceptor or “push–pull” chro-
mophores). The majority of push–pull chromophores are neutral
organic molecules, with all three key components (donor, acceptor
and connecting ␲-system) varied very widely.
2.1. General procedure for the reaction of TCNE with hydrazones
A solution of aromatic amine (10 mmol) and TCNE in DMF
(25 ml) was stirred at 60–90 ◦C for 8 h. The solvent was removed
and the residual solid was collected and recrystallized from
toluene–chloroform mixture.
∗
Corresponding authors. Tel.: +966 72418632; fax: +966 72418426.
E-mail addresses: agmasq@gmail.com (A.G. Al-Sehemi), irfaahmad@gmail.com
(A. Irfan).
1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2012.01.016

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A.G. Al-Sehemi et al. / Spectrochimica Acta Part A 91 (2012) 239–243
Table 1
Optimized geometrical parameters of investigated systems at B3LYP/6-31G* level of theory.
.
Systems
Bond lengths (Å)
C5–C7
C7–N1
N1–N2
N2–C8
C8–C9
C11–C14
MBD1-E
MBD1-Z
MBD1-azo
MBD2-E
MBD2-Z
MBD2-azo
1.455
1.457
1.506
1.456
1.475
1.508
1.289
1.290
1.482
1.289
1.291
1.481
1.355
1.380
1.248
1.355
1.360
1.248
1.371
1.377
1.432
1.371
1.375
1.432
1.411
1.416
1.403
1.410
1.410
1.403
1.455
1.455
1.471
1.455
1.457
1.471
Dihedral angles (degrees)
C5–C7–N1–N2
C7–N1–N2–C8
N1–N2–C8–C9
MBD1-E
MBD1-Z
MBD1-azo
MBD2-E
MBD2-Z
MBD2-azo
−180.0
180.0
179.7
179.9
−2.342
0.591
179.9
45.90
−179.5
−179.9
−177.38
−179.6
0.162
178.2
8.172
−0.079
−8.963
5.928
(MBD1)
ethylene-1,1,2-tricarbonitrile:
2-{4-[2-4-Methoxybenzylidenehydrazino]phenyl}
time dependant density functional theory (TD-DFT) at the B3LYP
/6-31G(d) level of theory which has been proved to be accurate
and reliable method [21,22]. Thermodynamic parameters; relative
energies (ꢁE0), enthalpies (ꢁH) and free energies (ꢁG) (kcal/mol)
for E, Z and azo isomers of MBD1 and MBD2 have been computed
at B3LYP/6-31G(d) level. The calculations have been performed by
using Gaussian03 program package [23]. Three possible classes
of isomeric molecules for dyes are E, Z and azo are shown in
Fig. S1. These isomers arise by rotating around torsion angles
N1–N2–C3–C4 for investigated dyes. Then by fully optimization
stable conformers have been achieved.
[C19H13N5O(327.34)],
M.
P.
275–278 ◦C, yield 77%; UV–vis (ethanol): ꢀmax (nm): 550,
332; IR (cm−1): 3279 (sec. NH), 2217 (CN), 1614 (C N), 1343
(C N); 1HNMR (DMSO-d6): 3.82(s, 1H, OCH3), 7.02–8.01 (m,
9H,Ar H + NH), 8.11(s, 1H, CH N). 13C NMR (DMSO-d6): 55.3
(OCH3), 78.27, 151.28 (C C), 114.87(CN), 2 × 114.14, 2 × 114.38,
119.14, 126.76, 2 × 128.66, 2 × 133.77, 144.88, 160.82 (12C Ar),
162.29 (CH N).
(MBD2)
2-{4-[2-(3,4-Dimethoxybenzylidene)hydrazino]
phenyl}ethylene-1,1,2-tricarobo- nitrile: [C20H15N5O2(357.37)],
M. P. 280–282 ◦C, yield 70%; UV–vis (ethanol): ꢀmax (nm): 550,
347; IR (cm−1): 3226(sec. NH), 2212 (CN), 1609 (C N), 1341(C N);
1HNMR (DMSO-d6): 3.82, 3.86 (2 s, 6H, 2× OCH3), 7.02–8.01 (m,
7H,Ar H), 8.07(s, 1H, CH N), 11.80(s, 1H, NH). 13C NMR (DMSO-
d6): 55.49, 55.56 (2OCH3), 78.11, 145.07(C C), 114.86 (CN), 111.48,
114.16, 114.36, 119.16, 121.78, 126.90, 132.78, 136.75, 149.10,
150.75, 2 × 151.23(12C Ar), 162.27 (CH N), see Scheme 1.
4. Results and discussion
4.1. Geometries
The computed geometrical parameters of MBD1 and MBD2
have been tabulated in Table 1. The fully optimized structures of
studied isomers are shown in Fig. S1. The C C, C N and N
N
bond lengths of E and Z isomer of MBD1 and MBD2 are almost
˚
3. Computational methods
similar except the C5–C7 in Z isomer of MBD2 which is 0.019 A
longer compared to E isomer and N1–N2 of E isomer of MBD1
˚
The geometries of MBD1 and MBD2 have been optimized by
using density functional theory with B3LYP/6-31G(d) level of the-
ory [16–20]. The absorption spectra has been computed by using
is 0.025 A shorter than Z isomer. We have observed that there
is no significant change in bond lengths of azo isomers in MBD1
and MBD2. By comparing the bond lengths of the E, Z and azo
NC
CN
H
H
CN
CN
N
H
NC
NC
N
+
H
N
CN
Ar
N
2a-b
OMe
Ar
(1a-cb)
2 )Ar=
a
2 ) Ar=
b
OMe
OMe
Scheme 1. The new synthesized sensitizers 2a (MBD1) and 2b (MBD2).

A.G. Al-Sehemi et al. / Spectrochimica Acta Part A 91 (2012) 239–243
241
Table 2
HOMO energy (EHOMO), LUMO energy (ELUMO), HOMO–LUMO energy gap (Egap) and absorption spectra (ꢀmax) in solvents in eV.
Dyes
EHOMO
ELUMO
Egap
ꢀmax
a
b
c
d
e
f
MBD1
−5.76
−3.21
2.55
541
328
550
332
539
331
650
343
573
339
579
400
320
MBD2
−5.74
−3.20
2.54
544
346
550
347
543
344
580
360
575
357
581
375
325
a, CH3CN (37.5); b, CH3CH2OH (24.8); c, CHCl3 (4.7); d, cyclohexane (2.02); e, DMSO (46.8); f, TD-B3LYP/6-31G* level of theory in acetonitrile.
Table 3
Thermodynamic parameters (kcal/mol) for E, Z and azo isomers of studied sensitizers at B3LYP/6-31G(d) level.
ꢁE0
0.00
7.48
17.3
0.00
4.77
18.2
ꢁH
0.00
7.48
17.4
0.00
4.71
18.4
ꢁG
0.00
7.57
16.4
0.00
5.17
17.6
ꢁE
0.00
7.28
18.0
0.00
4.45
19.1
Dipole
MBD1 E
MBD1 Z
MBD1 azo
MBD2 E
MBD2 Z
MBD2 azo
14.5762
11.1691
7.8082
15.2058
12.3185
7.3037
isomer of MBD1, we have observed that C5–C7 of azo isomer
would strengthen the donor ability. Due to this, the LUMOs are
distributed on the 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-
˚
˚
stretched 0.051 A compared to E isomer and 0.049 A than Z isomer.
˚
The C7–N1 of azo isomer elongated almost 0.193 A than E and Z
˚
isomers. The N1–N2 of azo isomer shortened 0.107 and 0.132 A
compared to E and Z isomers. The N2-C8 of azo isomer lengthened
31G(d) level show HOMO energy (EHOMO), LUMO energy (ELUMO)
˚
0.061 and 0.055 A than E and Z isomers. In MBD2, C5–C7 of azo
and HOMO–LUMO energy gap (Egap) has been used as an indica-
tor of kinetic stability of the molecule. The EHOMO, ELUMO and Egap
of MBD1 and MBD2 are same which showed that di-substituted
methoxy would have no additional improvement.
˚
isomer stretched 0.033 A compared Z isomers. The C7–N1 of azo
˚
isomer elongated almost 0.192 and 0.190 A than E and Z isomers,
˚
respectively. The N1–N2 of azo isomer shortened 0.107 and 0.112 A
compared to E and Z isomers, respectively. The N2–C8 of azo isomer
lengthened 0.061 and 0.053 A than E and Z isomers, respectively.
The torsion in E isomer is smaller than Z and azo isomers of MBD1
and MBD2.
For good DSSC sensitizers following factors to be considered: a
narrow band gap, with LUMO lying just above the conduction band
of TiO2 and HOMO below the redox couple. As a model for nanocrys-
tallinity the HOMO and LUMO energies of bare cluster (TiO2)38 are
−7.23 and −4.1 eV, respectively, resulting in a HOMO–LUMO gap of
3.13 eV [24]. Usually an 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 [25]. The LUMO energies of MBD1
and MBD2 are above the conduction band of TiO2. The HOMO of the
redox couple (I−/I3−) is −4.8 eV [26]. It can be found that HOMOs
of the dyes are below the redox couple. The smaller HOMO–LUMO
energy gaps of MBD1 and MBD2 revealed that these dyes would be
efficient for DSSC.
˚
4.2. Electronic properties and absorption spectra
Fig. 1 illustrates the calculated spatial distributions of the HOMO
and LUMO levels. As can be seen clearly, HOMO is delocalized on
whole of the molecule; the oxygens of methoxy also take part in
the formation of HOMO. The strong electron withdrawing groups
CN attract the charge density while on other hand electron donat-
ing group methoxy is attached on left hand of the molecule which
Fig. 1. The HOMOs and LUMOs of the most stable isomers of MBD1 and MBD2.

242
A.G. Al-Sehemi et al. / Spectrochimica Acta Part A 91 (2012) 239–243
100
80
60
40
20
0
significant effect has been observed in absorption spectra toward
red shift by changing chloroform to CH3CN. It showed 11 nm red
shift in ethanol. Most significant effect toward bathochromic shift
has been observed in DMSO (34 nm) and cyclohexane (111 nm).
By substituting one more methoxy group as in MBD2 has approx-
imately same absorption spectra in different solvents alike MBD1
except in cyclohexane which has absorption spectra 580 nm.
The dyes were measured in various solvents having dif-
ferent polarity, see Table 2. The trend of absorption spectra
toward red shift of MBD1 and MBD2 in different solvents
is CHCl3 < CH3CN < CH3CH2OH < DMSO < cyclohexane. The absorp-
tion spectra of MBD1 and MBD2 in all the investigated sensitizers
are almost same except in cyclehexane in which MBD2 is 7 nm is
blue shifted. The maximum absorption spectra computed at TD-
B3LYP/6-31G* level of theory is 579 and 581 nm for MBD1 and
MBD2, respectively which are in reasonable agreement with the
experimental evidence.
1000
2000
3000
4000
Wavenumber[cm-1
]
4.3. IR spectra
80
60
40
20
0
The IR spectra of these new dyes exhibited three impor-
tant absorption bands; the first band centered at 3279 cm−1 and
3226 cm−1 for the ꢂNH absorption in MBD1 and MBD2, respec-
tively. The second band is a sharp absorption band in the region of
2217 cm−1 and 2212 cm−1, which was attributed to the cyano group
absorption in MBD1 and MBD2, respectively. The third is an absorp-
tion band in the region of 1614 cm−1 and 1609 cm−1 ascribed for
the C N absorption in MBD1 and MBD2, respectively, see Fig. 2. The
tricyanovinylation undoubtedly takes place at a position para to the
hydrazine group as evidenced from the 1H NMR signals for the dou-
blet two hydrogen. The azomethine hydrogen of the synthesized
dyes was located in the region of 8.1–9.13 ppm (see Supporting
information for detail).
4.4. Thermodynamic stabilities
1000
2000
3000
4000
Wavenumber[cm-1
]
Thermodynamic parameters and energy barrier for investigated
dyes have been tabulated in Table 3. The isomers (E, Z and azo)
relative energy, ꢁE0, is defined as a difference between its zero-
point corrected total energy and that of the most stable one E, in
each case. Relative enthalpies and free energies at 298 K are also
defined as the difference between the enthalpy or free energy of a
given E/Z or azo isomers and that of E form. As shown in Table 3,
the order of relative stability of those isomers is the same when
Fig. 2. IR spectra of MBD1 (top) and MBD2 (bottom).
The electronic absorption spectra of the new chromospheres are
characterized by an intense, low-energy band that is dependent on
the nature of the substituted on the aromatic rings. MBD1 showed
absorption band at 539 nm in chloroform and 541 nm in CH3CN, no
14
12
10
8
14
12
10
8
6
6
4
4
2
2
0
0
-2
-2
-200 -150 -100 -50
0
50
100
150
200
-200 -150 -100 -50
0
50
100
150
200
MBD1
MBD2
Fig. 3. Molecular energy profile using DFT against the selected torsional degree of freedom of MBD1 and MBD2.

A.G. Al-Sehemi et al. / Spectrochimica Acta Part A 91 (2012) 239–243
243
considering relative energy or relative free energy. DFT calculation
Acknowledgement
shows that E isomers are the most stable. We have observed that
dipole moment of E isomer is highest while azo isomer is lowest.
The trend in different isomers is E > Z > azo. The dipole moments of
all isomers of MBD2 are larger than MBD1.
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.
4.5. Conformation analysis
Appendix A. Supplementary data
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; the model starting geometry of E-isomers was
obtained by driving procedure in HyperChem [27]. To aid the
future design of isomerism, we set about to determining how
well quantum chemical methods calculation could predict the
rotation barriers around the, e.g. N1–N2–C8–C9 selected torsional
angle on these compounds. To identify low energy conforma-
tions, the potential energy surface shape has been examined at
the B3LYP/6-31G(d) level. The potential energy surfaces of dihe-
dral angle (N1–N2–C8–C9) from +180◦ to −180◦ in 5◦ or 10◦ steps
(Fig. 3). The conformational energy profile shows two maxima near
(−90◦ and 90◦). 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 Fig. 3, there are three local minima observed at
(−180, 0 and 180) for N1–N1–N2–C8–C9 torsional angle and these
are most stable conformers for this torsion angle. The DFT opti-
mized geometry of these dyes is coplanar at these values of selected
torsion angle.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2012.01.016.
References
[1] S. Kim, H. Choi, Ch. Baik, K. Song, S.O. Kanga, J. Koa, Tetrahedron 63 (2007)
11436–11443.
[2] L. Xiao, Y. Liu, Q. Xiu, L. Zhang, L. Guo, H. Zhang, Ch. Zhong, Tetrahedron 66
(2010) 2835–2842.
[3] M. Gratzel, Nature 414 (2001) 338.
[4] A. Hagfeldt, M. Gratzel, Acc. Chem. Res. 33 (2000) 269.
[5] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Muller, P. Liska, N.
Vlachopoulos, M. Gratzel, J. Am. Chem. Soc. 115 (1993) 6382.
[6] Sh.-Sh. P. Chou, Yu-Yeh, Tetrahedron Lett. 42 (2001) 1309–1311.
[7] P.N. Prasad, Y. Zhang, X. Gao, H. Pan, Chem. Mater. 7 (1995) 816–821.
[8] P.N. Prasad, D.J. Williams, Introduction to Nonlinear Optical Effects in Molecules
and Polymers, Wiley, New York, 1991, pp. 132–174.
[9] K.J. Drost, A.K-Y. Jen, V.P. Rao, Chemtech 25 (9) (1995) 16–25.
[10] S.R. Marder, J.W. Perry, Science 263 (1994) 1706–1707.
[11] C.W. Dirk, H.E. Katz, M.L. Schilling, L.A. King, Chem. Mater. 2 (6) (1990) 700.
[12] A.K.Y. Jen, V.P. Rao, K.Y. Wong, K.J. Drost, J. Chem. Soc. Chem. Commun. 1 (1993)
90–92.
[13] A.M. Asiri, J. Saudi, Chem. Soc. 7 (2003) 77.
[14] A.M. Asiri, I.M.I. Ismail, K.A. Al-Amry, N.A. Fatani, Dyes Pigments 67 (2005)
111.
[15] A.M. Asiri, I.M.I. Ismail, K.A. Al-Amry, N.A. Fatani, 71 (2006) 102–107.
[16] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
[17] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.
[18] P.J. Stephens, F.J. Devlin, C.F. Chabalowski, M.J. Frisch, Phys. Chem. 98 (1994)
11623.
[19] B.J. Lynch, P.L. Fast, M. Harris, D.G. Truhlar, J. Phys. Chem. A 104 (2000) 4811.
[20] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh, C.
Fiolhais, Phys. Rev. B 46 (1992) 6671.
[21] P.J. Walsh, K.C. Gordon, D.L. Officer, W.M. Campbell, J. Mol. Struct. Thechem.
759 (2006) 17–24.
[22] D.M. Cleland, K.C. Gordon, D.L. Officer, P. Wagner, P.J. Walsh, Spectrochim. Acta
A 74 (2009) 931–935.
[23] M.J. Frisch, et al., Gaussian 03, revision C.02, Gaussian, Inc., Pittsburgh, Walling-
ford, CT, 2004.
[24] M.P. Balanay, D.H. Kim, Phys. Chem. Chem. Phys. 10 (2008) 5121–5127.
[25] F.D. Angelis, S. Fntacci, A. Selloni, Nanotechnology 19 (2008) 424002.
[26] S. Ito, S.M. Zakeeruddin, R. Humphry-Baker, P. Liska, R. Charvet, P. Comte, M.K.
Nazeeruddin, P. Péchy, M. Takata, H. Miura, S. Uchida, Grätzel, Adv. Mater. 18
(2006) 1202–1205.
5. Conclusions
The DFT calculation of thermodynamic parameters revealed
that the most stable isomers are E. Three local minima have been
viewed at (−180, 0 and 180) for N1–N2–C8–C9 torsional angle
which revealed that these would be the most stable conformers for
this torsion angle. The HOMO is localized on intact molecule while
LUMO is distributed on the tricarbonitrile which showed best intra
charge transfer. The dyes were measured in various solvents having
different polarity. Generally increasing the solvent polarity gave a
bathochromic shift of the maximum absorption bands of dyes. The
IR spectra of the investigated dyes showed three significant absorp-
tion bands. The LUMO energies of MBD1 and MBD2 are above the
conduction band of TiO2. The HOMOs of the dyes are below the
redox couple and smaller HOMO–LUMO energy gaps revealed that
these dyes would be efficient for DSSC.
[27] L.M.A. Pinto, M.B. de Jesus, E. de Paula, A.C.S. Lino, J.B. Alderete, H.A. Duarte, Y.
Takahata, J. Mol. Struct. Theochem. 678 (2004) 63–66.