A. Karakaßs et al. / Journal of Molecular Structure 877 (2008) 152–157
155
Examination of the bond lengths suggests that there is
an extended series of p bonds through the whole molecule.
Except for the C7–N1 bond connecting and phenyl groups,
all the other bonds between non-H atoms show p + r char-
acter; the C–C bond lengths in the phenyl groups range
between 300 and 360 nm involving mainly p fi p* transi-
tions. The band at around 400 nm is frequently suggestive
of a large hyperpolarizability. In this paper, the vertical
transition energies and oscillator strengths from the ground
state to each excited state have been computed, giving
OPA, i.e., the UV–vis spectrum. The calculated wave-
lengths (kmax) and oscillator strengths (f) for the maximum
OPA of the investigated molecule are shown in Table 3.
The molecule in Fig. 1 has four OPA peaks in its spectrum.
As can be seen from Table 3, the optical spectra exhibit rel-
atively intense four bands involving p fi p* transitions cen-
tered between 220 and 405 nm. The values of all absorption
maxima are located in the UV region estimated to be
shorter than 450 nm, being transparent in the visible
region.
˚
from 1.355(6) to 1.409(4) A. The C7–N1, C1–C2 and C5–
C6 bond lengths are shorter than typical single r bonds,
and these bond lengths are in good agreement with related
ligands in literature [28]. The Csp2–O bond associated with
the oxygen atom is clearly a single bond, while the C7–N1
˚
[1.276(4) A] bond length shows a partial double bond char-
acter [28,29], which is also an evidence for the conjugation.
4.2. Computational results and discussion
NLO techniques are considered as among the most
structure-sensitive methods to study molecular structures
and assemblies. Since the potential of organic materials
for NLO devices have been proven, NLO properties of
many of these compounds have been investigated by both
experimental and theoretical methods [30]. In the past
5 years, the efforts on NLO have been largely devoted to
preparing third-order NLO materials using theoretical
methods and exploring the structure–property relation-
ships. Quantum chemical calculations have been shown
to be useful in the description of the relationship between
the electronic structure of the systems and its NLO
response [6]. The computational approach allows the deter-
mination of molecular NLO properties as an inexpensive
way to design molecules by analyzing their potential before
synthesis and to determine high-order hyperpolarizability
tensors of molecules.
It can be very helpful in the investigation of NLO mate-
rials making it possible to check, apart from NLO
responses, also spectroscopic absorbance in the appropri-
ate wavelength. Thus, the wavelengths obtained by UV–
vis spectral analysis can be helpful in planning the synthesis
of the promising NLO materials only [31]. Since it is neces-
sary to know the transparency region, the electronic
absorption spectral studies of compounds designed to pos-
sess NLO properties are important. Albert et al. [32] have
reached the conclusion that with the correct substitution
of the push–pull system in the porphyrin ring, character-
ized by strong intramolecular p fi p* charge transfer tran-
sitions found through UV–vis spectral analysis, some
specific electronic and structural properties of this system
could produce high NLO responses. Zhou et al. [33] have
found that the kmax results of novel para-phenylenealkyne
macrocycles are not accelerated with the odd number of
unit, even with 10 units the value of kmax is 360 nm.
Though the kmax was estimated to be shorter than
400 nm in an enough large sample, a strong increase in
the hyperpolarizability value is obtained with a sizeable
increase. Di Bella et al. [34] have reported that bis(salicyl-
aldiminato) nickel(II) compound exhibited interesting lin-
ear optical features which will be seen to be related to the
NLO response. There is a broad band in the region
Once conceived, the idea can be first pursued by theoret-
ical means, and promising results would justify experimen-
tal efforts to obtain the envisioned compounds
synthetically as well. One could determine the hyperpolar-
izability tensors of molecules using a suitable computa-
tional approach. These tensors describe the response of
molecules to an external electric field. At the molecular
level, the NLO properties are determined by their dynamic
hyperpolarizabilities. TDHF is a procedure generally used
to find out approximate values and can be a means of
understanding both static and dynamic hyperpolarizabili-
ties of organic molecules. We present here a comprehensive
ab-initio study on the NLO properties of the title molecule
using the TDHF method. In this study, in addition to
the static linear polarizabilities a(0;0) and second hyperpo-
larizabilities c(0;0,0,0), the following processes for
dynamic (hyper)polarizabilities have been considered: fre-
quency-dependent linear polarizabilities a(ꢀx;x), THG
c(ꢀ3x;x,x,x). Some significant calculated magnitudes
of the static and frequency-dependent linear polarizabilities
and second hyperpolarizabilities are shown in Tables 4–7,
respectively.
The values of c depend on the halogen substitution in
the molecular structure. Further, the donor capacities of
Br atoms are ordered in terms of their r donation ability.
So, one would expect the bromo compounds to exhibit lar-
Table 3
Calculated the maximum UV–vis absorption wavelengths (kmax, nm) and
oscillator strengths (f) of the title compound
kmax
f
405.32
381.23
249.46
219.71
2.0015
1.7366
1.4471
1.1269
Table 4
Some selected components of the static a(0;0) and Æaæ(0;0) (·10ꢀ24 esu)
value of the title compound
axx
ayy
azz
Æaæ
34.410
14.624
2.355
17.130
Karaka?, Asli
