Substituent effects on the absorption and vibrational spectra of some 2-hydroxy Schiff bases: DFT/TDDFT, natural bond orbital and experimental study

Article abstract of DOI:10.1134/S0022476615030038

The electronic structure of salicylideneaniline (SA) and some of its derivatives are investigated both experimentally and theoretically. The equilibrium geometric structures of the studied compounds are determined at the B3LYP/6-311++Glevel of theory. A set of 12 substituted SA derivatives is considered in the present work. The choice of these substituents aims to create a push-pull system on the SA basic structure which would shade light onto its photo physics. The electronic absorption spectra of SA are recorded in the UV-VIS region, in both polar and nonpolar solvents. Assignments of the observed electronic transitions are facilitated via time-dependent density functional theory (TDDFT) computations at the same level of theory. Electronic configurations contributing to each excited state are identified and the relevant MOs are characterized. The extent of delocalization and intramolecular charge transfer are estimated and discussed in terms of natural bond orbitals (NBO) analysis and second order perturbation interactions between donor and acceptor MOs. Solvent effects on the electronic absorption spectra are discussed in terms of the difference in polarizabilities of the ground and excited states. FTIR spectra of SA and its derivatives are measured in KBr platelets. Detailed vibrational assignments are given based on the calculated potential energy distributions. "IR marker bands" that characterize the SA framework are identified. The effect of substituents, the nature of the characteristic "marker bands", and intensity quenching of some bands are discussed.

Full text of DOI:10.1134/S0022476615030038

Journal of Structural Chemistry. Vol. 56, No. 3, pp. 414-427, 2015.
Original Russian Text © 2015 S. A. Elroby, S. Aboud, S. G. Aziz, R. Hilal.
SUBSTITUENT EFFECTS ON THE ABSORPTION
AND VIBRATIONAL SPECTRA OF SOME 2-HYDROXY
SCHIFF BASES: DFT/TDDFT, NATURAL BOND
ORBITAL AND EXPERIMENTAL STUDY
1,3
S. A. Elroby , S. Aboud , S. G. Aziz ,
1
1
UDC 541.65:548.737
1,2
and R. Hilal
The electronic structure of salicylideneaniline (SA) and some of its derivatives are investigated both
experimentally and theoretically. The equilibrium geometric structures of the studied compounds are
determined at the B3LYP/6-311++G** level of theory. A set of 12 substituted SA derivatives is considered
in the present work. The choice of these substituents aims to create a push-pull system on the SA basic
structure which would shade light onto its photo physics. The electronic absorption spectra of SA are
recorded in the UV-VIS region, in both polar and nonpolar solvents. Assignments of the observed
electronic transitions are facilitated via time-dependent density functional theory (TDDFT) computations at
the same level of theory. Electronic configurations contributing to each excited state are identified and the
relevant MOs are characterized. The extent of delocalization and intramolecular charge transfer are
estimated and discussed in terms of natural bond orbitals (NBO) analysis and second order perturbation
interactions between donor and acceptor MOs. Solvent effects on the electronic absorption spectra are
discussed in terms of the difference in polarizabilities of the ground and excited states. FTIR spectra of SA
and its derivatives are measured in KBr platelets. Detailed vibrational assignments are given based on the
calculated potential energy distributions. “IR marker bands” that characterize the SA framework are
identified. The effect of substituents, the nature of the characteristic “marker bands”, and intensity
quenching of some bands are discussed.
DOI: 10.1134/S0022476615030038
Keywords: electronic spectra, vibrational spectra, DFT/TDDFT, solvent and substituent effects, NBO
analysis, 2-hydroxy Schiff bases.
INTRODUCTION
Aromatic Schiff bases have been [1-4] and still are the subject of numerous studies because of possible applications,
e.g. in molecular memories and switches [5-7]. This class of molecular systems is of fundamental interest because it shows
two interesting phenomena, namely, the excited state intramolecular proton transfer (ESIPT) and photochromism. The best
1
2
Chemistry Department, Faculty of Science, King Abdullaziz University, Jeddah, Saudi Arabia. Chemistry
3
Department, Faculty of Science, Cairo University, Giza, Egypt; rhilal@kau.edu.sa. Chemistry Department, Faculty of
Science, Benisuef University, Benisuef, Egypt. The text was submitted by the authors in English. Zhurnal Strukturnoi Khimii,
Vol. 56, No. 3, pp. 444-456, May-June, 2015. Original article submitted March 05, 2014.
414
0022-4766/15/5603-0414 © 2015 by Pleiades Publishing, Ltd.

known photochromic Schiff base is salicylideneaniline (SA). This molecule and its derivatives have been widely studied over
a wide span of time [8-24]. In SA and its derivatives, the intramolecular hydrogen bonding opens a possibility of fast proton
transfer between different tautomers followed by isomerization changes, leading to a metastable photochromic modification
of the initial structure. Numerous experimental studies [25-28] on SA have been devoted to the analysis of the mechanism
and dynamics of the proton transfer reaction from the primary excited enol form to the keto tautomer and the creation of the
photochromic tautomer in the ground electronic state. Nevertheless, the results are not fully consistent and much controversy
is still present.
The absorption spectrum of SA shows a quite prominent band at 300-400 nm attributed to the photoexcitation to
a ππ* excited state [29]. The fluorescence spectrum also presents a single largely Stokes shifted band centered at around 570
nm. This band is believed to come from the cis-keto tautomer and is also a ππ* state. The quantum yield of the fluorescence
is quite low [24]. A global picture of photochromism in SA has been reached based on the time- resolved experiments
[24, 29]. This picture starts with photoexcitation to the ππ* excited state followed by a quick transfer of the hydroxylic proton
to the nitrogen imino atom resulting in the fluorescent cis-keto tautomer. This species is the precursor of the photochromic
transient, believed to be a ground state keto tautomer. This transient has a long lifetime of the order of milliseconds and
a maximum absorption band at 474 nm [24]. However, the detailed mechanism about the species involved in the whole
process and the time of the different steps is not a settled question. In fact, several different decay mechanisms have been
proposed, namely, thermal deactivation and internal conversion (IC) through conical intersections (CIs) or intersystem
crossing.
The present work attempts to provide a detailed experimental and quantum chemical study of the electronic and
vibrational spectra of SA at high level DFT and TDDFT methods. The origin of each absorption band is identified and the
contributing configurations and MOs are characterized. The magnitude and direction of the intramolecular charge transfer is
also identified and evaluated. In this respect, natural bond orbital analysis (NBO) is carried out to identify the extent of
delocalization and the charge transfer of the electron density in the studied molecular systems. The effect of substituents on
the vibrational spectra is also investigated. An attempt is made to identify and assign the “IR marker bands” for SA. Solvent
and substituent effects are quantitatively analyzed and discussed.
EXPERIMENTAL
Compounds. SA and its derivatives were prepared by conventional methods where equimolar quantities of aniline
and salicylaldehyde were heated in spectroscopically pure dry ethanol and then the solution was allowed to cool. Compounds
were then recrystallized from chloroform.
Solvents. Spectroscopic-grade solvents (Merck) were used without any further purification.
Measurements. Infrared spectra of the compounds were recorded on a Shimad24 820/PC FT-IR spectrometer
-1
within the range 4000-400 cm . For samples in the solid state, the IR spectra were measured in KBr pellets at a resolution of
-1
1 cm . All UV-VIS spectral measurements were carried out using a Perkin–Elmer UV-Visible spectrophotometer (model
Lambda 2).
Computational methods. All computations were carried out using the Gaussian 09W software package [30].
Molecular geometries of all of the studied compounds were fully optimized using the 6-311++G** basis set using Beck’s
three-parameter-hybrid (B3LYP) DFT method [31-33]. No symmetry constrains were applied during the geometry
optimization. Calculations of the electronic spectra and excited states were carried out at the TDDFT/6-311++G** level of
theory [34]. The choice of this basis set was due to its flexibility, and the fact that diffuse p functions on the hydrogen atoms
tend to compensate the anharmonic effects of the CH and NH stretches. The force constants were analytically computed at
the fully optimized geometries. NBO calculations have been performed at the B3LYP/6-311+G** level using the NBO 3.1
program as implemented in the Gaussian09W software package in order to quantitatively measure the intramolecular
delocalization in the studied systems. NBO theory [35, 36] proved to be extremely useful in analyzing hyper conjugative [37]
(2)
interactions through the second-order perturbation energy (E ), which is estimated as
415

(2)
2
= ΔEij = qi (Fij) /Δ ,
E
where qi is the donor orbital occupancy, Δ is the difference between the energy of an acceptor orbital (j) and a donor orbital
(i), and Fij is the off-diagonal NBO Kohn–Sham matrix. Throughout this work MOs were constructed using the Gauss View
5.0.8 visualization program [38].
RESULTS AND DISCUSSION
Electronic and vibrational spectra of SA. Scheme 1 displays the equilibrium geometric structure and the
numbering system of SA adopted in the present work.
Scheme 1. Equilibrium geometric structure and
atom labeling of SA adopted in the present work.
Fig. 1 displays the electronic absorption spectrum of SA in cyclohexane and ethanol as solvents. The spectrum is
composed of low intensity diffuse bands. One can identify five overlapping bands and a shoulder, spanning the 350-200 nm
range. An increase in the solvent polarity causes a marked blue shift of all observed transition bands. Table 1 presents the
electronic spectral characteristics of SA in both polar and nonpolar solvents. It is instructive at this point to identify the origin
of these observed transition bands. A comparison between the observed and theoretically computed spectra is given in
Table 1. The correspondence is excellent. The first transition which is predicted at 353 nm (observed 350 nm) is composed of
two main configurations: 51→53 (–0.18) and 52→53 (0.68). MOs involved in these two configurations are displayed in
Fig. 2. Three MOs involved are the delocalized π orbitals, and hence, the respective transition band corresponds to a typical
ππ* state. The second transition appears as a diffuse overlapping band observed at 322 nm. This transition is not predicted
theoretically for the trans-enol structure.
The third transition is observed at 305 and is predicted theoretically at 304 nm. The same two configurations 51→53
(0.67) and 52→53 (0.19) contribute to the S3 state indicating its ππ* nature. The fourth band is observed in the 280-270 nm
range and is predicted theoretically at 282 nm. There is only one configuration contributing to the S4 state. This configuration
is 49→53 (0.70). MO 49 is localized completely on the N-phenyl moiety, whereas vacant MO 53 is a delocalized orbital.
Fig. 1. Electronic absorption spectra of SA, in
(–) cyclohexane and (– – –) ethanol as solvents.
416

TABLE 1. Experimentally Observed and Theoretically Predicted Spectral Characteristics of SA
Theoretically predicted for SA enol
Theoretically predicted for SA cis-keto
Observed λ, nm
Cyclohexane Methanol
E, eV
f
Contribution
E, eV
f
Contribution
λ, nm
λ, nm
2.96
3.21
419
386
0.3
0.0
52-53 (0.77)
51-53 (0.70)
353
345
3.51
353
0.4
51-53 (0.18)
52-53 (0.68)
322
305
327
300
3.91
4.18
317
297
0.4
0.0
50-53 (0.70)
49-53 (0.22)
42-54 (0.66)
4.07
305
0.5
51-53 (0.67)
52-53 (0.19)
49-53 (0.70)
50-53 (0.63
51-5 (0.13)
52-54 (0.27)
48-53 (0.44)
52-54 (0.36)
48-53 (0.32)
52-54 (0.52)
50-53 (0.24)
4.26
4.40
291
282
0.0
0.0
270
283
4.56
271
0.01
49-53 (0.63)
50-54 (–0.17)
240
249
4.92
5.00
252
248
0.0
0.1
4.75
262
0.1
48-53 (0.22)
52-55 (0.66)
227
Therefore, the S4 state is ππ* involving a considerable charge transfer character. This charge transfer takes place against the
dipole moment vector, and consequently, the corresponding excited state S4 has a lower dipole moment, and hence, is much
less polarized. This would suggest that polar solvents stabilize the more polar ground state to a greater extent and thus cause
Fig. 2. MOs involved in the electronic states of SA (52 and 53 represent HOMO and LUMO, respectively).
417

a red shift of the observed band maxima. The fifth excited state S5 is observed at 249 nm and predicted theoretically at
252 nm. The correspondence is excellent.
An increase in the solvent polarity has but little effect on the spectrum of SA. Thus, the first absorption band suffers
a blue shift of 8 nm whilst the second band suffers a red shift by almost the same amount. This increases the overlap between
the two bands. This small solvent effect is expected in view of the similar polarities of both ground and excited states for
these two transitions. Furthermore, there seems to be a general trend of enhancing intensity with increasing solvent polarity.
Table 1 presents a comparison between the experimentally observed and theoretically computed spectral characteristics of
SA.
Although the present high level TDDFT calculations are able to predict well all the observed transitions, yet the
second 322 nm band is not predicted theoretically at all. This might suggest the presence of another absorbing species in the
solution, presumably a cis-keto form of SA. Table 1 displays the computed spectral band positions and intensities of the cis-
keto form of SA. This form seems to have long wavelength ππ* absorption bands at 418 nm which are not observed in the
experimental spectra. The second S2 transition predicted theoretically at 317 nm is in good correspondence with the observed
322 nm band. However, the correspondence in the short wavelength region seems much less satisfactory than that of the enol
form.
The present study focuses on the effect of substituents on the electronic and vibrational spectra of SA. In order to
fulfill this goal it is instructive to analyze and discuss the NBO results for SA. The NBO analysis provides the electron
density distribution on atoms and in bonding regions. By comparing a set of molecules you can, for instance, see the donor or
acceptor effect of the substituent. Table 2 presents the most relevant perturbation interactions within the natural orbitals of
SA. In this table, the reported relevant interaction values are ≥10 kcal/mol. Careful inspections of the data in this table reveals
that:
1. The C–N π orbital interacts equally well with both phenyl rings. In fact, its interaction with the OH–Ph ring is
greater. Furthermore, the lone pair orbital of the nitrogen atom enjoys hyperconjugation with the C1–H2 orbital.
2. The oxygen lone pair orbital, on the other hand, interacts essentially with the π framework of the phenyl ring.
The interaction of the oxygen atom lone pair orbitals with the hydrogen virtual and Rydberg orbitals is pronounced.
3. It is surprising to notice a decrease in the population of the NBO C15–C17, C16–C18, and C20–C21 reflecting
a charge transfer away from the phenyl ring. This is also evident in the case of the population of the oxygen lone orbital
LP(3), which is reduced by almost 0.5 e.
In conclusion, SA enjoys the linear conjugation that is responsible for the observed spectrum. No specific part of the
molecule manifests itself in the observed spectrum due to the extended π system. The interaction between the oxygen and
hydrogen atomic orbitals is of a covalent character, and thus, a proton transfer toward the nitrogen atom would be opposed by
a high energy barrier.
It would be reasonable at this point to analyze and discuss the vibrational spectrum of SA. Fig. 3 displays the FTIR
spectrum of SA measured as a KBr pellet. The spectrum is composed of three main regions. A broad low intensity envelope
–1
spans the 3600-3200 cm range. This envelope encloses the aromatic symmetric and asymmetric CH stretching and OH
stretching modes. The middle region is of much higher intensity and is composed of several intense sharp absorption peaks
–1
corresponding to the aromatic ring vibration and the C=N stretching mode that can be identified at 1613 cm . The third
–1
region spans the 1000-500 cm range and is composed of several sharp peaks of variable intensities corresponding to the
coupling and combination bands.
A comparison of the experimentally observed and theoretically computed vibrational bands is given in Table 4 along
with their assignments. In the present discussion, we select one band from each of the three regions. The selected bands
correspond to the main functional groups characterizing the SA framework, namely, OH, C=N, and the aromatic ring where
substituents will be introduced. Thus, in this part and the subsequent discussion, we focus on three bands which can be
considered as “IR marker bands” for SA. These are the OH stretching predicted as strong (relative intensity 334) at
–1
3175 cm , the C=N stretching and coupling vibrational band of the C=N vibrational mode, which is observed and predicted
–1
in the 1600 cm region, and the phenyl ring breathing modes observed at 750 cm and predicted at 860 cm .
–1
–1
418

TABLE 2. Second Order Perturbation Interaction Energy Values Computed in the NBO Basis for SA and Some of its
Derivatives
(2)
E , kcal/mol
Compound
SA
Donor orbital
Acceptor orbital
NBO
Population
C1–N3
C4–C5
*C4–C5
*C1–N3
*C1–N3
*C1–H2
*C15–C17
*C4–C5
*C15–C17
*H26
13.68
15.15
22.61
11.62
37.10
47.59
136.02
17.84
456.60
25.91
139.93
17.50
32.53
15.03
475.69
10.78
7.09
C15–C17
C16–C18
C20–C21
LP(1)N3
LP(2)O25
LP(3)O25
1.5666
1.7081
1.6965
1.8643
1.8103
1.5647
C15–C17
LP(1) N3
LP(2)O25
*C1–N3
*C1–N3
C17–O25
LP(3)O25
C14–C15
*C1–N3
C16–O24
LP(1)N3
LP(1)O24
LP(3)O24
C1–N3
*H26
o-NO2
*C1–N3
*C14–C15
*H25
C4–C5
C6–C9
1.6227
1.6625
1.6352
1.5893
1.6225
1.4371
1.60999
1.6669
1.6438
1.6669
1.6437
1.6305
1.6158
1.5532
C14–C15
C16–C19
C17–C20
LP(3)O24
C4–C6
*H25
*H25
*H25
p-NO2
C4–C6
C1–N3
C14–C15
*C1–N3
*C1–N3
*C1–H2
*C16–C19
*C4–C6
*C14–C15
*H25
C5–C7
C4–C6
7.77
C9–C11
C5–C7
C14–C15
LP(1)N3
LP(2)O24
*C1–N3
*C1–N3
C16–O24
C1–N3
26.18
10.99
35.87
92.72
124.98
17.51
15.25
10.61
22.09
37.00
131.01
10.43
11.75
21.70
11.31
16.50
45.49
172.86
17.63
37.18
14.4
C9–C11
C14–C15
C17–C20
LP(3)O24
o-OCH3
*C4–C6
*C1–N3
*C1–N3
*C12–C14
*C4–C6
*C4–C5
*C1–N3
*C1–N3
*C1–H2
*C14–C16
*C4–C5
*C14–C16
*LPH25
*LPH25
*LPH25
LP(1)H25
C4–C6
C5–C7
1.6197
1.6759
1.6712
1.6991
1.5696
1.6429
1.7209
1.6562
1.5717
1.7062
1.6969
1.5616
C4–C6
C12–C14
LP(2)O22
*C1–N3
C1–N3
C–8–C9
C17–C18
LP(3)O22
C4–C5
p-OCH3
C4–C5
C6–C9
C14–C16
LP(1)N3
LP(2)O24
*C1–N3
*C1–N3
C16–C24
LP(1)N3
LP(1)O24
LP(3)O24
C7–C11
C14–C16
C15–C17
C19–C20
LP(3)O24
459.78
Substituent effect. The substituted SA derivatives studied in the present work are presented in Scheme 2. Two sets
of substituents have been considered. A set of electron withdrawing substituents, namely –Cl, –Br, and –NO2 substituted in
the ortho- and para-positions of the N–Ph moiety. Another set is composed of three electron donating substituents, namely,
–OH, –NH2, and –OCH3 also substituted in both ortho- and para-positions. The choice of these substituents aims to create
a push-pull system on the SA basic structure, which hopefully would shade light onto its photo physics.
419

Fig. 3. FTIR spectrum of SA measured in the KBr pellet.
Scheme 2. Series of substituted SA derivatives
studied in the present work.
Fig. 4 displays the electronic absorption spectra of the o-Cl, o-Br, and o-NO2 SA derivatives, in both ethanol and
cyclohexane as solvents. The general features of the spectra resemble those of the parent compound. The perturbation effect
of the nitro group seems to be pronounced. Thus, a new absorption band appears at 375 nm (ε = 3300) while the second and
Fig. 4. Electronic absorption spectra of (3) o-Cl, (7) o-
Br, and (9) o-NO2 measured in cyclohexane (C) and
ethanol (E) as solvents.
420

third transitions are now diffuse and appear as low intensity (ε = 2700) shoulders under the short wavelength tail of the new
long wavelength band. The main configuration contributing to the first excited state is 63-64 configuration (0.7).
It is worth noting that the full geometry optimization of the o-nito derivative of SA at the B3LYB/611++G** level
of theory reveals the non-coplanrity of the molecule. This is expected due the steric hindrance caused by the presence of
a bulky NO2 group in the ortho position. This steric hindrance forces the Ph–NO2 moiety to be out-of-plane by an angle of
83°. The second order perturbation energy analysis of the o-NO2 derivative (Table 2) reveals a dramatic change in the
bonding as compared to parent SA. Thus, the interaction of the C1–N3 NBO with the π system of the nitrophenyl moiety is
now reduced to very small values. The interaction of the C1–N3 NBO with the π system of the o-hydroxy phenyl moiety is
more pronounced. One should also note that a pronounced interaction energy term between the C–N nitrogen lone pair orbital
and the H25virtual orbital has been observed. Therefore, Table 2 and 4 data and the electronic spectra of the o-nitro phenyl
derivative reveal that the charge redistributions might facilitate and lower the potential energy barrier for the proton transfer
from the OH group to the nitrogen atom of the C–N group leading to a keto structure. There is also a reduction in the
population of the NBO′s of the phenyl moiety due to the strong withdrawing effect of the nitro group.
Fig. 5 displays the optimized geometric structure of the o-nitro derivatives and some of the molecular orbitals
involved in the electronic transitions. The first excited state is composed of one configuration 63-64. HOMO 63 is
a delocalized MO (in spite of the non-coplanarity of the molecule) whereas LUMO 64 is localized over the Ph–OH moiety.
Therefore, the long wavelength band is a ππ* transition having a charge transfer character. The second and third excited
states each involve three configurations; all involve the charge transfer to the Ph–OH moiety (Table 2). The electronic
absorption spectrum of the o-Cl derivative of SA resembles to a great extent that of parent SA. However, all observed bands
exhibit a marked increase in the intensity. This molecule demonstrates co-planarity. The general features of the spectrum of
the o-Br derivative, on the other hand, are different. Thus, there is a considerable reduction in the intensity and a blue shift in
the position of the long wavelength absorption band and a considerable intensification of the 275 nm band. The second and
third bands (in the 320-290 nm region) are not observed in the spectrum of the o-Br derivative.
The vibrational spectrum of the o-NO2 derivative of SA reflects the major geometry changes introduced by the nitro
group. Thus, the o-NO2 group seems to have a marked effect on SA IR marker bands. Thus, the OH stretching vibration is
Fig. 5. Fully optimized geometry of the o-NO2 derivative of SA and MOs involved in the electronic transitions.
421

Fig. 6. FTIR spectrum of the o-Br derivative of SA measured in the KBr pellet.
–1
shifted to 3316 cm and is intensified. The C=N stretching mode suffers also the same trend where it is shifted to 1680 cm
–1
and is considerably intensified. The C=N and phenyl ring breathing combination band shows also the same trend (Table 4).
This same trend of shifting to higher wavenumbers and the intensification of the three IR marker bands are observed for o-Br
and o-Cl derivatives. Fig. 6 displays the FTIR spectrum of the o-Br derivative of SA. The bulk o-Br group causes the splitting
and intensification of the OH and aromatic CH stretching modes. These stretching bands are also shifted to longer
wavenumber values.
In conclusion, the substitution in the ortho position of the N–Ph moiety by electron withdrawing substituents such as
NO2, Cl, and Br causes non-coplanarity of the SA molecular skeletal structure and all observed ππ* transitions have
a considerable charge transfer character in the direction of the Ph–NO2 moiety. However, these types of substitution do not in
general create new transitions, except the case of the NO2 derivative where a long wavelength band at 380 nm is observed.
Furthermore, electron withdrawing groups in the ortho position also have the general effect of shifting the IR marker bands of
SA to higher wavenumbers and appreciably increase their intensities.
The substitution by the same electron withdrawing groups but in the para-position of the N–Ph moiety does not
introduce as much perturbation as that observed in the case of ortho substitution (Table 3). Thus, the p-NO2 derivative is still
non-coplanar, but the dihedral angle between the plane of the Ph–NO2 moiety and the rest of the molecule is much smaller
(38°) than that in the case of the o-NO2 derivative. The origin of the non-coplanarity in the case of the para derivative is the
dimensioned charge density accumulation in the NO2Ph–N bond region. The NO2 group acts as an electron sink withdrawing
the charge density away from this region. NBO analysis of the p-NO2 derivative (Table 2) indicates that it retained the
extended conjugation of SA as revealed by the interaction of C1–N3 NBOs with those of both phenyl rings. Furthermore, the
interaction of the nitrogen lone orbitals with the hydroxyl group hydrogen atom is marked. Table 3 data indicate that the
electronic spectrum, however, resembles to a great extent that of the ortho derivative yet more diffuse and of a lower
intensity. The p-Br substitution causes a marked red shift of the first absorption band to 356 nm whilst the second and third
bands appear overlapping with the peak maxima at 328 nm and 312 nm. One should note that the band appearing at 270 nm
in SA seems to be persistent and appears in almost the same region in all derivatives studied. This band is a conjugation band
characterizing the SA π framework.
422

TABLE 3. Experimentally Observed and Theoretically Computed Electronic Spectral Characteristics of SA Derivatives
Studied in the Present Work
Experi-
mental
Experi-
mental
Theoretical
Theoretical
Com-
Com-
pounds
pounds
Molecular orbital
contribution
Molecular orbital
contribution
f
f
λ, nm
λe λc
362 366
318 319
λ, nm
λe λc
2
0.69964
385 0.3
8
0.67771
360 0.43
313 0.47
290 0.0001
279 0.01
258 0.01
254 0.07
381 0.01
346 0.043
320 0.13
319 0.003
307 0.003
303 0.015
346 347
331
H→L
H→L
0.62101
0.6778
332 0.01
329 0.5
0.66816
0.70268
0.47585
0.56654
0.49281
0.69784
0.60003
0.63254
0.44848
0.6417
H-4→L
H-1→L
H-4→L+1
H→L+1
H-3→L+1
H→L+2
H→L
H-1→L
0.63103
0.49278
0.57715
310 0.02
295 0.04
290 0.01
H-1→L+1
H-2→L+1
H-3→L+1
227 228
219 220
385 355
354 334
333 232
267 266
230 231
371 372
3
9
H-1→L
H→L+1
H-3→L
H-2→L
H-2→L+1
0.66692
0.66105
0.62149
0.57736
0.63134
0.57893
0.69393
0.68269
0.54786
0.46621
0.43115
0.5335
334 0.30
316 0.01
279 0.01
261 0.30
253 0.02
239 0.02
354 0.60
318 0.04
279 0.02
270 0.20
255 0.01
252 0.05
349 0.40
320 0.01
290 0.02
270 0.30
252 0.02
243 0.09
361 0.46
316 0.45
291 0.001
280 0.01
262 0.04
254 0.07
357 0.31
306 0.0001
305 0.50
291 0.08
263 0.46
259 0.05
H→L
H-1→L
H-2→L
H-3→L
H→L+1
H-1→L+1
H→L
272 272
0.66115
267 269
225 227
359 369
228 229
385 359
334 333
4
5
6
7
10
0.67988
0.69732
0.66737
0.55166
0.57345
0.66735
0.69666
0.67978
0.63693
0.70233
0.69382
0.53052
349 0.33
H→L
323 0.0001 334 335
298 0.50
H-1→L
H→L+2
H-4→L
H-4→L
H-3→L
H→L
H-2→L
H-1→L
H-3→L
H→L+1
H-5→L
H→L
270 272
293 296
224 227
349 353
329
279 0.002
254 0.04
248 0.00
372 0.75
322 0.05
294 0.04
284 0.0001
278 0.002
267 0.04
263 265
227
0.6889
11
364 365
326
0.68264
0.59137
0.54747
0.60048
0.58548
0.67877
0.67028
0.70143
0.4764
H-1→L
H-2→L
H-3→L
H→L+1
H→L+2
H→L
H-1→L
H→L+1
H-4→L
H→L+2
H→L+3
269 270
225 228
353 355
328 329
310 313
272 273
265 266
229 230
12
0.67693
0.64759
0.61882
0.70225
0.58127
0.67465
349 0.39
307 0.26
285 0.24
292 0.00
256 0.03
251 0.001
H-1→L
H-4→L
H→L+1
H-2→L
H→L+1
H→L
H→L
H-1→L
H-2→L
H-3→L
H→L+1
H→L+2
0.55738
0.50277
0.68297
0.69869
0.66536
0.62409
0.66453
0.64169
230 232
345 350
265 266
252
223 224
H-3→L
H-1→L
H-2→L
H→L+2
H→L+1
275 277
233 234
* Only main configurations.
423

Fig. 7. Electronic absorption spectra of the (4) -o-OCH3
and (5) p-OCH3 SA derivatives measured in
cyclohexane (C) and ethanol (E) as solvents.
Electron withdrawing substituents in the para position do not have the same marked effect noted in the case of the
ortho substitution. In fact, the IR marker bands do not show any significant shift in peak positions. However, peak intensities
show a reduction in intensities. Nevertheless, the main observation to be made here is the coupling between the OH and
aromatic CH stretching modes with a consequent reduction in the intensity.
Fig. 7 displays the electronic absorption spectra of the o-OCH3 and p-OCH3 derivatives of SA in cyclohexane and
ethanol as solvents. Electron donating groups cause a marked shift of the electron density onto the –N=C–phenol moiety. The
o-methoxy substitution forces the N–Ph moiety out of the plane of the molecule; the dihedral angle is 38°. However, the
general features of the spectrum of SA is unaffected by the o-OCH3 substitution. The first absorption band is slightly red
shifted and all observed bands suffer a reduction in the intensity. The first absorption band observed at 358 nm and predicted
theoretically at 360 nm are in excellent agreement. Two configurations contribute to this S1 state, with 69-70 (0.7) as the main
configuration. Thus, as Table 3 indicates, this transition corresponds to a ππ* state delocalized over the entire molecule. The
o-NH2 substitution causes not only a red shift of the observed transitions but also the intensification, especially for the second
absorption band that became more intense than the first. This band seems to be more affected by the substitution. In the case
of the o-OH derivative (Table 3), this band is red shifted to the extent that it became obscured under the broad long
wavelength band.
Inspection of Fig. 7 and Table 3 clearly indicates that the general features of the spectra of the para substituted
derivatives are essentially the same as those of the corresponding ortho derivatives. In general, the electron donating
substitution causes a red shift and the intensification of the observed transitions. The second absorption band originally
appearing at 320 nm is most affected by the substitution. It is a red shift to 340 nm and may become obscured by the broad
long wavelength band. It is interesting to note that the hydroxyl group substitution either in the ortho or para positions causes
the coupling of the C–N stretching mode with the asymmetric aromatic C=C stretching and OH bending modes. These two
–1 –1
coupling modes appear at 1660 cm and 1608 cm . The two OH groups, on the other hand, do not couple with each other
–1 –1
and appear at 3800 cm and 3210 cm (Table 4).
CONCLUSIONS
The present experimental and DFT/TDDFT investigations of the electronic structure and spectra of SA and some of
its derivatives reveal that SA enjoys linear conjugation which is responsible for the observed spectrum. No specific part of the
molecule reflects itself in the observed spectrum due to the extended π system. The interaction between the oxygen and
hydrogen atomic orbitals is of a covalent character, and thus, a proton transfer toward the nitrogen atom in the ground state
would be opposed by a high energy barrier. A set of 12 substituted SA derivatives has been investigated. The choice of these
substituents aims to create a push-pull system on the SA basic structure, which would shade light onto its photo physics. The
424

TABLE 4. Experimentally Observed and Theoretically Computed Vibrational Spectral Characteristics and Assignments of
IR Marker Bands of SA and Some of its Derivatives
Relative
–1
–1
Compound
Assignment
νexp, cm
νcalc, cm
intensity
SA
3600-2800(w)
1613(s)
3175
334
43, 160
140
8.9
OH str.
1663, 1660
1606
859
C=N str., C=Nstr+OH bending
C=N str.+ring breathing vibration
751 (vs)
3600-2800(w)
1599 (s)
p-NO2
3276
1664
1660
790
514
212
383
5
OH str.
C=N
1561(s)
C=Nstr.+OH bending
C=Nstr.+ring breathing
OH str.
759 (vs)
o-Cl
p-OCH3
o-OH
3600-2800 (w)
1608(s)
3252
1670
918
575
362
36
C=N str.
C=Nstr.+ring breathing
OH str.
3600-2800 (w)
1608 (s)
3203
1664
919
507
316
15
C=Nstr. Coupled with ring modes
C=Nstr.+ring breathing
OH str.
833 (vs)
3600-2800 (w)
3120
3800
1660
1608
854
452
48
OH str. (salisaylidene)
C=N str. Coupled with ring modes
C=Nstr.+OH bending
C=N Str.+ring breathing
OHstr. Coupled with arom.CH
C=Nstr.
1613 (s)
1590 (s)
156
143
25
736 (vs)
p-Br
3600-3200 (s)
1606 (s)
3197
1659
792
280
287
7
828 (vs)
C=Nstr.+ring breathing
OHstr.
o-Br
3600-3200 (w)
3156
3136
1656
1000
865
370
72
OHstr.Coupled with.CH(C=N)
C=Nstr.
1611(s)
1185 (s)
752 (vs)
3059 (s)
2938 (s)
1615 (w)
744 (vs)
3375 (w)
3344 (w)
1623 (s)
738
242
172
14
C=Nstr. Oupled with OH bending
C=Nstr.+ring breathing
OHstr. +arom.CH
OHstr.+arom.CH
C=N str.
p-Cl
o-NO2
p-OH
3200
3190
1665
780
170
148
716
19
C=Nstr.+ring breathing
OHstr.
3316
600
541
32.4
C=Nstr.
1680
845
C=Nstr.+ring breathing
3389 (s)
3300(w)
1612(w)
835 (vs)
3835
3206
1665
871
107
494
300
8.7
OH str.
OH str. (salisaylidene)
C=N str. Coupled with ring modes
C=N Str.+ring breathing
substitution by bulky groups in the ortho position of the N–Ph moiety forces it out of the plane of the molecules, yet the π
conjugation and hyperconjugation across the C=N bond minimized the effect of non-coplanarity on the observed spectrum.
The observed transitions are sensitive to both solvent and substituent effects. The extent of the charge transfer character and
425

its direction enabled the interpretation of the observed shifts in band positions and intensities. The analysis of the
experimentally determined and theoretically computed FTIR spectra of SA enabled the identification of three “IR marker
–1 –1
bands”, namely, the OH stretching mode at 3175 cm , the C=N stretching mode at 1660 cm , and the C=N and ring
–1
breathing combination band at 850 cm . These bands are persistent in all substituted SAs studied in the present work and are
sensitive to the type and position of the substitution.
REFERENCES
1. G. O. Dudek and E. P. Dudek, J. Am. Chem. Soc., 88, 2407 (1966).
2. T. Rosenfeld, M. Ottolenghi, and A. Y. Meyer, Mol. Photochem., 5, 39 (1973).
3. J. W. Lewis and C. Sandorfy, Can. J. Chem., 60, 1727 (1982).
4. R. S. Becker, C. Lenoble, and A. Zein, J. Phys. Chem., 91, 3509 (1987).
5. T. Elsässer and H. J. Bakker, Ultrafast Hydrogen Bonding Dynamics and Proton Transfer Processes in the Condensed
Phase, Kluwer Academic Publ. (2002).
6. E. Hadjoudis and I. M Mavridis., Chem. Soc. Rev., 33, 579 (2004).
7. M. Irie, Chem. Rev., 100, 1685 (2000).
8. V. Vargas and L. Amigo, J. Chem. Soc., Perkin Trans. 2, 1124 (2001).
9. T. Dziembowska, E. Jagodzińska, Z. Rozwadowski, and M. Kotfica, J. Mol. Struct., 598, 229 (2001).
10. K. Ogawa, J. Harada J., T. Fujiwara, and S. Yoshida, J. Phys. Chem. A, 105, 3425 (2001).
11. A. Mandal, D. Fitzmaurice, E. Waghorne, A. Koll, A. Filarowski, D. Guha, and S. Mukherjee, J. Photochem.
Photobiol., A, 153, 67 (2002).
12. M. Rospenk, I. Król-Starzomska, A. Filarowski, and A. Koll, Chem. Phys., 287, 113 (2003).
13. A. Ohshima, A. Momotake, and T. Arai, J. Photochem. Photobiol., A, 162, 473 (2004).
14. W. M. Fabian, L. Antonov, D. Nedeltcheva, F. S. Kamounah, and P. J. Taylor, J. Phys. Chem. A, 108, 7603 (2004).
15. P. Barbara, P. Rentzepis, and L. Brus, J. Am. Chem. Soc., 102, 2786 (1980).
16. K. Kownacki, A. Mordzinski, R. Wilbrandt, and A. Grabowska, Chem. Phys. Lett., 227, 270 (1994).
17. A. Grabowska and K. Kownacki, J. Lumin., 60, 886 (1994).
18. M. Z. Zgierski and A. Grabowska, J. Chem. Phys., 112, 6329 (2000).
19. M. Knyazhansky, A. Metelitsa, M. Kletskii, A. Millov, and S. Besugliy, J. Mol. Struct., 526, 65 (2000).
20. H. Joshi, F. S. Kamounah, C. Gooijer, G. van der Zwan, and L. Antonov, J. Photochem. Photobiol., A, 152, 183 (2002).
21. N. Otsubo, C. Okabe, H. Mori, K. Sakota, K. Amimoto, T. Kawato, and H. Sekiya, J. Photochem. Photobiol., A, 154,
33 (2002).
22. S. Mitra and N. Tamai, Phys. Chem. Chem. Phys., 5, 4647 (2003).
23. M. Ziółe, J. Kubicki, A. Maciejewski, R. Naskręcki, and A. Grabowska, Phys. Chem. Chem. Phys., 6, 4682 (2004).
24. C. Okabe, T. Nakabayashi, Y. Inokuchi, N. Nishi, and H. Sekiya, J. Chem. Phys., 121, 9436 (2004).
25. S. Mitra and N. Tamai, Chem. Phys. Lett., 282, 391 (1998).
26. S. Mitra and N. Tamai, Chem. Phys., 246, 463 (1999).
27. V. Vargas, J. Phys. Chem. A, 108, 281 (2004).
28. Rodríguez-Córdoba, W. J. S. Zugazagoitia, E. Collado-Fregoso, and J. Peon, J. Phys. Chem. A, 111, 6241 (2007).
29. M. Ziółek, J. Kubicki, A. Maciejewski, R. Naskr cki, and A. Grabowska, J. Chem. Phys., 124, 124518 (2006).
30. G. W. T. M. J. Frisch, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin,
V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi,
426

M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma,
V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman,
J. V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian Inc., Wallingford, CT (2009).
31. T. Schaefer, T. A. Wildman, and S. R. Salman, J. Am. Chem. Soc., 102, 107 (1980).
32. T. Schaefer, S. R. Salman, T. A. Wildman, and P. D. Clark, Can. J. Chem., 60, 342 (1982).
33. A. D. Becke, J. Chem. Phys., 98, 5648 (1993).
34. R. Bauernschmitt and R. Ahlrichs, Chem. Phys. Lett., 256, 454 (1996).
35. S. E. Ulic, C. O. D. Védova, A. Hermann, H.-G. Mack, and H. Oberhammer, J. Phys. Chem. A, 112, 6211 (2008).
36. A. E. Reed and F. Weinhold, J. Chem. Phys., 78, 4066 (1983).
37. X. Chen, F. Yan, M. Wu, H.-B. Tian, S. X. Li, X. Shan, K. Wang, Z. Li, and K. Xu, Chem. Phys. Lett., 472, 19 (2009).
38. H. P. H. Frisch, A. E…….., R. D. Dennington II, et al., Gaussian Inc., Wallingford, CT (2009), p. 235.
427