132
A.G. Al-Sehemi et al. / Journal of Molecular Structure 1019 (2012) 130–134
Table 2
HOMO energy (EHOMO), LUMO energy (ELUMO), HOMO–LUMO energy gap (Egap) and
absorption spectra (kmax) in different solvents in eV.
Dyes
EHOMO
À6.42
À6.11
À6.10
ELUMO
À3.61
À3.39
À3.39
Egap
Egap
a
b
c
d
e
NBHPET
CBHPET
BBHPET
2.81
2.72
2.71
512
400
263
523
321
519
417
262
530
325
509
393
262
520
324
533
427
252
573
238
528
415
290
537
367
321
539
375
322
522
322
530
326
521
324
563
254
aCH3CN, bCH3CH2OH, cCHCl3, dDMSO, eTD-B3LYP/6-31GÃ in CH3CN.
fect has been observed in absorption spectra toward red shift by
changing the solvent from chloroform to ethanol. Most significant
effect toward bathochromic shift has been observed in DMSO
(24 nm). In CBHPET and BBHPET the noteworthy red shifts
53 nm and 42 nm have been observed in DMSO compared to
chloroform, respectively. It has been examined that CBHPET and
BBHPET conferred red shift in different solvents compared to
NBHPET. The dyes were measured in various solvents having dif-
ferent polarity.
Fig. 1. The HOMOs (left) and LUMOs (right) of the most stable isomers of
investigated dyes.
The dyes were measured in various solvents having different
polarity, see Table 2. The trend of absorption spectra toward red
shift in different solvents is CHCl3 < CH3CN < CH3CH2OH < DMSO.
The absorption spectra of CBHPET and BBHPET in all the investi-
gated sensitizers are almost same while absorption spectra of
NBHPET are blue shifted. The maximum absorption spectra com-
puted at TD-B3LYP/6-31GÃ level of theory is 528, 537 and
539 nm for NBHPET, CBHPET and BBHPET, respectively which
are in good agreement with the experimental evidence, see Table 2.
of azo isomer lengthened 0.052 and 0.048 Å than E and Z isomers,
respectively. By substituting the chloro or bromo in place of nitro,
we have observed no significant effect in lengthening or shortening
of different bond lengths of the azo isomers. The torsion in E iso-
mer is smaller than Z and azo isomers of NBHPET, CBHPET, and
BBHPET.
4.2. Electronic properties and absorption spectra
4.3. IR spectra
The distribution pattern of the HOMOs and LUMOs has been
shown in Fig. 1. We have observed that HOMOs are delocalized
on entire molecule. The CN, Cl and Br of NBHPET, CBHPET, and
BBHPET also take part in the formation of HOMOs, respectively.
In NBHPET the LUMO is localized on whole molecule while in CBH-
PET and BBHPET the LUMOs are distributed on the tricarbonitrile.
It is might be due that strong electron withdrawing group CN at-
tract the charge density toward itself which leads to distribution
of LUMO in NBHPET on whole dye. While Cl and Br are weak with-
drawing groups compared to CN escort LUMOs on the tricarbonit-
rile. The HOMO–LUMO energy gap of these dyes was calculated at
the B3LYP/6-31G(d) level of theory. The HOMO energy (EHOMO),
LUMO energy (ELUMO) and HOMO–LUMO energy gap (Egap) of NBH-
PET, CBHPET, and BBHPET has been tabulated in Table 2.
The IR spectra of these new dyes exhibited three important
absorption bands; the first band centered at 3279 cmÀ1
,
3563 cmÀ1, and 3267 cmÀ1 for the
tNH absorption in NBHPET,
CBHPET, and BBHPET, respectively. The second band is a sharp
absorption band in the region of 2217 cmÀ1 and 2216 cmÀ1, which
was attributed to the cyano group absorption in NBHPET, CBHPET,
and BBHPET, respectively. The third is an absorption band in the
region of 1614 cmÀ1, 1610 cmÀ1 and 1599 cmÀ1 ascribed for the
C@N absorption in NBHPET, CBHPET, and BBHPET, respectively,
see Fig. 2. The tricyanovinylation undoubtedly takes place at a po-
sition para to the hydrazine group as evidenced from the 1H NMR
signals for doublet 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).
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 nano-
crystallinity the HOMO and LUMO energies of bare cluster
(TiO2)38 are À7.23 and À4.1 eV, respectively, resulting in a Egap 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 neces-
sary for effective electron injection [13]. The LUMO energies of
studied dyes are above the conduction band of TiO2. The HOMO
of the redox couple (I À =IÀ3 ) is À4.8 eV [14]. It can be found that
HOMOs of the dyes are below the redox couple. The smaller
HOMO–LUMO energy gaps of NBHPET, CBHPET, and BBHPET re-
vealed that these dyes would be efficient for DSSC.
4.4. Thermodynamic stabilities
Thermodynamic parameters of isomers (E, Z and azo); relative
energies, relative enthalpies and free energies for NBHPET, CBH-
PET, and BBHPET dyes have been tabulated in Table 3. The relative
energy,
DE0, is defined as a difference between its zero-point cor-
rected 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 considering
relative energy or relative free energy. DFT calculation shows that E
isomers are the most stable.
The NBHPET showed absorption band at 509 nm in chloro-
form, 512 nm in CH3CN and 519 nm in ethanol. No significant ef-