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Spectrochimica Acta Part A 85 (2012) 85– 91

Contents lists available at SciVerse ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

j ourna l ho me page: www.elsev ier .com/ locate /saa

ynthesis, spectroscopic characterizations and quantum chemical computationaltudies of (Z)-4-[(E)-(4-fluorophenyl)diazenyl]-6-[(3-ydroxypropylamino)methylene]-2-methoxycyclohexa-2,4-dienone

igdem Albayraka,∗, Mustafa Odabas oglub, Arzu Özekc, Orhan Büyükgüngörc

Faculty of Education, Sinop University, 57100 Sinop, TurkeyChemical Technology Program, Pamukkale University, 20070 Kınıklı-Denizli, TurkeyDepartment of Physics, Faculty of Arts and Sciences, Ondokuz Mayıs University, 55139 Kurupelit-Samsun, Turkey

r t i c l e i n f o

rticle history:eceived 3 February 2011eceived in revised form 7 September 2011ccepted 8 September 2011

eywords:chiff Basezo dyeon-linear optical properties

a b s t r a c t

In this study, the molecular structure and spectroscopic properties of the title compound were charac-terized by X-ray diffraction, FT-IR and UV–vis spectroscopies. These properties were also investigatedusing DFT method. The most convenient conformation of title compound was firstly determined. Thegeometry optimizations in gas phase and solvent media were performed by DFT methods with B3LYPadding 6-31G(d) basis set. The differences between crystal and computational structures are due to crys-tal packing in which hydrogen bonds play an important role. UV–vis spectra were recorded in differentorganic solvents. The results show that title compound exists in both keto and enol forms in DMSO, EtOHbut it tends to shift towards enol form in benzene. The polar solvents facilitate the proton transfer by

FTD-DFTpectral characterization

decreasing the activation energy needed for Transition State. The formation of both keto and enol forms inDMSO and EtOH is due to decrease in the activation energy. TD-DFT calculations starting from optimizedgeometry were carried out in both gas and solution phases to calculate excitation energies of the titlecompound. The non-linear optical properties were computed at the theory level and the title compoundshowed a good second order non-linear optical property. In addition, thermodynamic properties wereobtained in the range of 100–500 K.

© 2011 Elsevier B.V. All rights reserved.

. Introduction

Azo compounds have received much structural interest in chem-stry due to their versatile applications in many different areas suchs polyester fiber [1], disperse dyes [2], as well as their use in manyiological reactions and in analytical chemistry [3]. Furthermore,heir application as industrial dyes and in biological systems whereome may be used as inhibitor for tumor growth [3] is of greatmportance. Azobenzene is one of the most representative classesf photochromic molecules with two geometric isomers, trans andis [4–6]. The trans-to-cis isomerization occurs by photo irradiationith UV light and cis-to-trans isomerization proceeds with blue-

ight irradiation or heating. It is generally accepted that their transorms are thermodynamically more stable than their cis forms [7].

∗ Corresponding author. Tel.: +90 368 2715526; fax: +90 368 2715530.E-mail address: calbayrak@sinop.edu.tr (C . Albayrak).

386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2011.09.020

Intramolecular proton transfer plays an important role in manyfields of chemistry [8,9]. Intramolecular proton transfer mechanismwhich occurs in both excited and ground states is a subject of inten-sive research [10–12]. Molecules exhibiting intramolecular protontransfer are used as laser dyes, in higher energy radiation detectors,memory storage devices, fluorescent probes and polymer protec-tors [13–15]. Hence, many molecules such as o-hydroxy Schiff Basesexhibiting intramolecular proton transfer have attracted consider-able attention from both experimental and theoretical points ofview [16–21].

The DFT and TD-DFT (time-dependent density functional the-ory) are of particular interest owing to give satisfactory results withexperiment by costing low computational demands compared tothe computational methods developed for the calculation of theelectronic structure and excitation energies of molecular systems

[22,23].

In this work, the crystal structure of (Z)-4-[(E)-(4-fluorophenyl)diazenyl]-6-[(3-hydroxypropylamino)methylene]-2-methoxycyclohexa-2,4-dienone (I) was determined by single

86 C . Albayrak et al. / Spectrochimica A

Table 1Crystal data, data collection and refinement details.

Chemical formula C17H18FN3O3

Crystal system, space group, Z Monoclinic, P 21/c, 4a 19.1832(8) Ab 7.4944(4) Ac 11.3173(5) Aˇ 103.002(4)◦

V 1585.32(14) A3

Dx 1.388 Mg m−3

Radiation, � Mo K�, 0.71073 A� 0.11 mm−1

T 293 KTmin, Tmax 0.481, 0.872F(0 0 0) 696Diffractometer STOE IPDS IIScanning mode ωScan range −23 < h < 23, −9 < k < 9, −13 < l < 13�min, �max 2.2◦ , 26.0◦

Number of measured/independentreflections, Rint

22,015/3107, 0.046

Number of reflections with 2�(I) 2399Number of refined parameters 221S 1.04R[F2 > 2�(F2)] 0.037

ctuc

2

2

ata

2

aatstbraenaaor

2

tCCa+

wR(F2) 0.101��max, ��min 0.18, −0.19 e A−3

rystal X-ray diffraction study. The structure of I was experimen-ally characterized by FT-IR, UV–vis spectroscopies, investigated bysing DFT and excitation energies were carried out using TD-DFTalculations starting from optimized geometry.

. Experimental and computational methods

.1. Instrumentation

The melting point was determined by StuartMP30 melting pointpparatus. FT-IR spectrum of I was recorded on a Bruker 2000 spec-rometer in KBr disk. UV–vis absorption spectra were recorded on

Thermo scientific BioGenesis UV-Vis spectrometer.

.2. X-ray crystallography

All diffraction measurements were performed at room temper-ture (293 K) using graphite monochromated Mo K� radiation and

STOE IPDS 2 diffractometer. Reflections were collected in the rota-ion mode and cell parameters were determined by using X-AREAoftware [24]. Absorption correction was achieved by the integra-ion method via X-RED software [24]. The structure was solvedy direct methods using SHELXS-97 [25]. The refinement was car-ied out by full-matrix least-squares method on the positional andnisotropic temperature parameters of the non-hydrogen atoms, orquivalents corresponding to 221 crystallographic parameters. Allon-hydrogen atom parameters were refined anisotropically andll H atoms except for H1 were located in their idealized positionsnd refined using a riding model with C–H distances in the rangef 0.93–0.96 A. The data collection conditions and parameters ofefinement process are listed in Table 1.

.3. Supplementary data

Crystallographic data (excluding structure factors) for the struc-ure in this paper have been deposited with the Cambridge

rystallographic Data Centre as the supplementary publication no.CDC 787956. Copies of the data can be obtained, free of charge, onpplication to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax:44 1223 336033 or e-mail: deposit@ccdc.cam.ac.uk).

cta Part A 85 (2012) 85– 91

2.4. Computational procedures

All computations were performed by using Gaussian 03Wprogram package [26]. Full geometry optimization of the titlemolecule was performed by using DFT method with Becke’s three-parameters hybrid exchange-correlation functional (B3LYP) [27]employing 6-31G(d) basis set [28] as implemented in Gaussian03W.

The most convenient conformation of I was firstly investigatedin the gas phase. To determine the conformational energy pro-files for keto form, the values of DFT energies were calculated asthe functions of the torsion angles �1(C2C1N1N2), �2(C8C7N2N1),�3(C15C16C17O3) and �4(C9C14N3C15) from −180◦ to 180◦ var-ied every 10◦, �5(C10C11O2C13) from 0◦ to 360◦ varied every 10◦

and the molecular energy profiles were obtained. The geometryoptimization of the most convenient conformation was performedby DFT methods with B3LYP adding 6-31G(d) basis set for calcula-tions [29,30]. The optimized geometry of molecule, total molecularenergy and dipole moment were obtained from the optimizationoutput. Solution phase geometry optimizations were performedwith the same level of theory by the polarizable continuum model(PCM) [31,32]. TD-DFT calculations starting from optimized geome-tries for gas phase and solution phase were carried out at the sametheory level in order to calculate the excitation energies of enol andketo tautomers. In addition, thermodynamic properties of I wereobtained by applying same level of theory.

2.5. Synthesis

A mixture of 4-fluoroaniline (0.7 g, 6.5 mmol), water (20 mL)and concentrated hydrochloric acid (1.6 mL, 19.7 mmol) wasstirred until a clear solution was obtained. This solution wascooled to 273–278 K and a solution of sodium nitrite (0.6 g,8.7 mmol) in water was added dropwise while the temperaturewas maintained below 278 K. The resulting mixture was stirred for30 min in an ice bath. o-Vanillin (1 g, 6.5 mmol) solution (pH 9) wasgradually added to a cooled solution of 4-fluorobenzenediazoniumchloride, prepared as described above, and the resulting mixturewas stirred at 273–278 K for 60 min in ice bath. The prod-uct was recrystallized from ethanol to obtain solid (E)-2-hyd-roxy-3-methoxy-5-(4-fluorophenyldiazenyl)benzaldehyde. Thecompound (Z)-4-[(E)-(4-fluorophenyl)diazenyl]-6-[(3-hydroxy-propylamino)methylene]-2-methoxycyclohexa-2,4-dienone wasprepared by refluxing a mixture of a solution containing (E)-2-hydroxy-3-methoxy-5-(4-fluorophenyldiazenyl)benzaldehyde(0.5 g, 1.82 mmol) prepared as described above in 20 mLethanol and a solution containing 3-hydroxypropylamin(0.137 g, 1.82 mmol) in 20 mL ethanol. The reaction mixturewas stirred for 2 h under reflux. The crystals of (Z)-4-[(E)-(4-fluorophenyl)diazenyl]-6-[(3-hydroxypropylamino)methylene]-2-methoxycyclohexa-2,4-dienone suitable for X-ray analysiswere obtained by slow evaporation from ethanol (yield 85%, m.p.428–430 K).

3. Results and discussion

3.1. Structure determination

The crystal data and the refinement details of (Z)-4-[(E)-(4-fluorophenyl)diazenyl]-6-[(3-hydroxypropylamino)methylene]-2-methoxycyclohexa-2,4-dienone (I) compound are given in

Table 1. The selected bond lengths and angles are given in Table 2.The molecular structure of I is shown in Fig. 1 with the atomnumbering scheme. o-Hydroxy Schiff Bases exhibit tautomerismby intramolecular proton transfer from the oxygen atom to the

C . Albayrak et al. / Spectrochimica Acta Part A 85 (2012) 85– 91 87

Table 2The selected bond lengths, angles and torsion angles ( ´A,◦).

X-ray DFT/B3LYP enol form DFT/B3LYP keto form

C10–O1 1.2573(16) 1.3339 1.2600N3–C14 1.2871(18) 1.2835 1.3171N1–N2 1.2605(16) 1.2645 1.2678N1–C1 1.4287(17) 1.4152 1.4146N2–C7 1.4056(17) 1.4073 1.3996C14–N3–C15 124.07(12) 119.8978 128.5611O1–C10–C9 122.94(12) 122.5454 123.2298O1–C10–C11 121.59(11) 118.1173 121.0173C9–C14–N3–C15 −179.08(14) −179.4368 −169.0142

Fs

nisifIaditdctifi

H

nRpiomaCi

Table 3Hydrogen bonding geometry ( ´A,◦).

D–H· · ·A D–H H· · ·A D· · ·A ∠D–H· · ·AN3–H1· · ·O1 0.91(2) 1.98(2) 2.6774(15) 132.1(17)N3–H1· · ·O3 0.91(2) 2.25(2) 2.7897(16) 117.4(16)

ig. 1. A partial packing diagram for I with N–H· · ·O and O–H· · ·O hydrogen bondshown as dashed lines [symmetry code: (i) −x + 2, −y + 1, −z + 1].

itrogen atom. As a result of this, o-hydroxy Schiff Bases can existn two tautomeric structures as enol and keto form in the solidtate. As shown in Fig. 2, depending on their forms, two types ofntramolecular hydrogen bonds are possible (a) N–H· · ·O in ketoorm and (b) O–H· · ·N in enol form. As it can be seen in Fig. 1,

exists in keto form. The C14–N3 bond length of 1.2871(18) Ånd C10–O1 bond length of 1.2573(17) Å are consistent with theistances of the C–N single bond and the C–O double bond reported

n previous studies [33,34]. While C9–C14, C7–C8, C11–C12 dis-ances are 1.4152(18), 1.3657(19), 1.3566(19) Å; C9–C10, C10–C11istances are 1.4410(18), 1.4581(18) Å (Supplementary data). Theontraction of C10–O1, C9–C14, C7–C8, C11–C12 distances andhe elongation of C9–C10, C10–C11 distances show that I existsn keto form. As an another way of confirming if I exists in ketoorm, the harmonic oscillator model of aromaticity (HOMA) indexs calculated by using Eq. (1) for rings [35,36].

OMA = 1 −[

˛

n

n∑i=1

(Ri − Ropt)2

](1)

is the number of bonds in ring, is the constant equal to 257.7 andopt is equal to 1.388 A for CC bonds. For the purely aromatic com-ounds HOMA index is equal to 1 but, for non-aromatic compounds

t is equal to 0. The HOMA indices in the range of 0.900–0.990r 0.500–0.800 show that the rings are aromatic or the non aro-

atic, respectively [37,38]. We calculated HOMA index of C1–C6

nd C7–C12 rings. The calculated HOMA indices for C1–C6 and7–C12 rings are 0.908 and 0.528, respectively. These results also

ndicate that the title compound exists in keto form.

Fig. 2. Keto and enol tautomeric forms of I.

O3–H3A· · ·O1i 0.82 2.01 2.8092(13) 164

Symmetry code: (i) −x + 2, −y + 1, −z + 1.

In the title compound, the aromatic rings adopt a trans con-figuration around the azo bridges. The dihedral angle betweenthe planes of two aromatic rings is 27.08(8)◦. The N N length of1.2605(16) A corresponds to double bond character and agrees wellwith previously reported values [39,40].

Two significant intramolecular interactions are noted betweenatom O1 and nitrogen atom N3 and between atom O3 and nitro-gen atom N3. These interactions constitute a six-membered ringS(6). The O1· · ·N3 distance of 2.6774(14) A and O3· · ·N3 distanceof 2.7897(16) A are indicative of strong intramolecular hydrogenbonding (Table 3). This length is clearly shorter than the sum of thevan der Waals’ radii for N and O [41].

The intermolecular O3–H3a· · ·O1 hydrogen bond is importantin the crystal packing of I, giving rise to the formation R6

6(8) motif(Fig. 1) [42]. The intermolecular hydrogen bond geometry and thedetails of O3–H3a· · ·O1 interactions are given in Table 3. Further-more, the distance between the ring centroids from Cg1 and to Cg1ii

is 3.746 A. These weak �–� interactions formed between aromaticrings further stabilize the structure (Fig. S1 in Supplementary data).

In order to determine the conformational energy profiles forketo form, the values of DFT energies were calculated as the func-tions of selected torsion angles �1(C6C1N1N2), �2(C12C7N2N1),�3(C15C16C17O3), �4(C9C14N3C15) and �5(C10C11O2C13). Thecalculated molecular energy profiles as the functions of torsionangles �1, �2, �3, �4 and �5 are given in Supplementary data. Themolecular energy profiles as a function of �1 and �2 show two max-ima at −90◦ and +90◦ because of decreasing of conjugation betweenaromatic rings. The molecular energy profile as a function of �3shows three low-energy conformers at +50◦ (conformer 1), −30◦

(conformer 2), and −80◦ (conformer 3). The energies of three con-formers are close to each other. The conformer 1 has the lowestenergy. The energies of conformer 2 and conformer 3 are higherthan that of conformer 1 by 0.264 kcal/mol and 0.075 kcal/mol,respectively. The molecular energy profile as a function of �4 showstwo minima at −180◦ and 180◦. The calculated molecular energyprofile as a function of �5 has two maxima at 120◦ and 240◦ whichhave 3.8 kcal/mol energy barriers. In additional, there are two localminima at 60◦ and 300◦ and a minimum at 180◦ which corre-sponds to conformer with the lowest energy. After determiningthe most convenient conformation, the geometry optimizationspertaining to enol and keto forms of I were performed again byDFT methods with B3LYP adding 6-31G(d) basis set for calcu-lations to compare each other. The optimized geometry of ketoform (Fig. S2 in Supplementary data) deviates from experimen-tal geometry with an r.m.s. of 1.575 A. While the dihedral anglebetween the rings is 0.64◦ in the optimized geometry, it is 27.08◦

for experimentally obtained geometry. This means that the opti-mized geometry of I is planar while X-ray crystal structure of I isnon-planar. This result can be attributed to the presence of crys-tal packing effects. The hydrogen-bonding properties in the crystalpacking are fundamental in determining the crystallographicallyobserved conformation. The selected bond lengths and angles forthe optimized structure and X-ray geometry of the molecule are

listed in Table 2 and Supplementary data. As shown in Table 2,C10–O1 and C14–N3 distances for the keto form at the optimizedgeometry are 1.26 A and 1.3171 A while these distances for theenol form are 1.3339 and 1.2835 A. In addition, while C9–C14,

88 C . Albayrak et al. / Spectrochimica Acta Part A 85 (2012) 85– 91

C1f(tTfe

3

sstdbaXofp3rsiarCdbTtsat

svuafvc

Table 4The experimental and the calculated vibrational frequencies (cm−1).

Assignments Experimental DFT/B3LYP

N–H str. 3353 3117C–H str. (aromatic) 3057 3085, 3070C–H (CH2) str 2918, 2836 3007, 2956, 2952, 2935C–H (CH3) str 2957, 2883 3037, 2962, 2906C10–O1 + C14–N3 str. 1647 1645C C str. (aromatic) 1607, 1575 1611, 1598, 1579C C str. (aromatic) 1546, 1527 1562, 1531C C str. (aromatic) 1497 1491N N str. 1418 1460C–H bend. (aromatic) 1138 1132C–H bend. (aromatic) 833 984CH2, CH3 bend. 1454 1450, 1441CH2 bend. 1389 1405, 1341

spectra in EtOH are also recorded by dropping H2SO4 and addingNaOH. When the solution was exposed to acid, the absorption bandat 368 nm followed by shoulder at 406 nm disappeared (Fig. 5) anda new absorption band appeared at 328 nm. Keto form cannot exist

Fig. 3. FT-IR spectrum of I.

7–C8, C11–C12, C9–C10, C10–C11 distances for enol form are.4582, 1.3896, 1.3827 A, 1.4163, 1.4271 A, these lengths for ketoorm are 1.4086, 1.3772, 1.3679 A, 1.4642, 1.4684 A, respectivelyTable S1 in Supplementary data). The bond lengths obtained fromhe optimization for keto form are near to the experimental ones.he differences between the crystal and optimized geometries ariserom inter and intramolecular hydrogen bonds and crystal packingffects.

.2. FT-IR absorption spectrum

FT-IR spectrum of I is given in Fig. 3. The (N–H) and (O3–H)tretching vibrations which broaden owing to the formation oftrong intramolecular and intermolecular hydrogen bonding inhe structure are at 3353 cm−1. The result obtained from X-rayiffraction study indicates that C10–O1 bond of I is the doubleond in character. As a result of this, the sharp absorption bandt 1647 cm−1 corresponds to (C O) stretching. Depending on the-ray and IR results, I in the solid state exists in keto form. Thether characteristic IR absorption band of I is (N N) stretchingrequency at 1418 cm−1. The aromatic C–H stretching, C–H in-lane bending and C–H out-of-plane bending vibrations appear in000–3100 cm−1, 1100–1500 cm−1 and 800–1000 cm−1 frequencyanges, respectively [43]. The absorption band at 3057 cm−1 corre-ponds to the aromatic C–H stretching vibrations of I. In addition,n plane bending and out-of-plane C–H vibrations were observedt 1138 cm−1 and at 833 cm−1 for I, respectively. The asymmet-ic and symmetric stretching vibrations of the aliphatic CH2 andH3 group of I were observed at 2957, 2918, 2883, 2836 cm−1. Theeformation modes of these groups were observed at 1454 cm−1,ending modes were observed at 1389 cm−1, 1022 and 674 cm−1.he absorption bands observed at 1600–1400 cm−1 are assignedo C–C stretching vibrations of the aromatic compounds. The C–Ctretching modes of aromatic rings of I are observed at 1607, 1575,nd 1546, 1527 and 1497 cm−1. These results are in agreement withhe literature [43].

The vibrational frequencies of I were calculated by using theame level of theory. The scale factor of 0.9613 was applied toibrational frequencies [44]. Vibrational bands have been made bysing Gaussview. The experimental and the calculated frequenciesre given in Table 4. The calculated results by frequency analysis

or N N and N–H stretching show deviations from experimentalalues due to the differences between the calculated structure andrystal structure.

CH3, CH2 bend. 1022, 674 1139, 739

Str., stretching; bend., bending.

3.3. UV–vis absorption spectra

o-Hydroxy Schiff Bases can exist in two forms as keto formcontaining N–H· · ·O intramolecular hydrogen bond and enol formcontaining O–H· · ·N hydrogen bond in the solid state. The previ-ous studies indicated that o-hydroxy Schiff Bases containing azogroup can exist in keto and enol forms in both the solid [40,45]solution states [46]. To investigate the behavior of I in solution,its UV–vis electronic spectra in three organic solvents with dif-ferent polarity (DMSO, EtOH and benzene) were measured in thewavelength range 200–600 nm at room temperature. The charac-teristic UV–vis absorption bands of I in DMSO, EtOH and benzeneare given in Table 5. UV–vis spectra are shown in Fig. 4. Exami-nation of the results indicates that the UV–vis electronic spectraof I are largely dependent on the nature of the solvent. The UV–visabsorption spectra of I display mainly two absorption bands arisingfrom � → �* transitions. These absorption bands are observed at350–390 nm followed by a shoulder above 400 nm in benzene. Theabsorption band in the range of 350–390 nm shows bathochromicshift while the one above 400 nm rises with the increase in thepolarity of solvent.

To investigate the effect of pH on tautomeric forms, the UV–vis

Fig. 4. The solvent effect on UV–vis spectra of I in (- - - -) DMSO, (—) EtOH, (– · · –)benzene.

C . Albayrak et al. / Spectrochimica Acta Part A 85 (2012) 85– 91 89

Table 5For enol and keto forms wavelength, oscillator strength, major contributions of calculated transitions.

Experimental Calculated

Keto form Major contribution Enol form Major contribution� (nm) � (nm) (f) � (nm) (f)

Gas phase– 387.48 (0.5087) H → L + 1 (70%) H → L (8%)

428.57 (0.2716) H → L (70%) H → L + 1 (9%) 377.43 (0.7014) H → L (77%)

Benzene358 397.10 (0.3911) H → L + 1 (79%)

434.83 (0.6688) H → L (78%) 393.70 (0.9041) H → L (81%)

EtOH368 394.05 (0.2947) H → L + 1 (75%) H → L (6%)406 425.08 (0.7777) H → L (74%) H → L + 1 (7%) 391.84 (0.8840) H → L (80%)

→ L + → L (

ia4kcts

eiaiadTataeE

tebvust

can say that the absorption band in the range greater than 400 nm

DMSO386 395.37 (0.2710) H410 427.50 (0.8411) H

n acidic media because both phenolic oxygen and nitrogen atomsre protonated. Therefore, the absorption band disappeared above00 nm in acidic media corresponds to absorption band formed byeto form. The addition of grainy NaOH to the solution caused smallhanges in the absorption spectrum in EtOH (Fig. 5). The absorp-ion band at 368 nm showed bathochromic shift and intensity ofhoulder at 406 nm increased.

The keto form is more polar than enol form, it can be stabilizedasily by polar solvents as EtOH and DMSO. As a result of this, I existsn both keto and enol forms in EtOH and DMSO. Since benzene is

solvent of low polarity a shift towards enol form was observedn the absorption band of I. The previous computational studieslso show that the polar solvents facilitate the proton transfer byecreasing the activation energy needed for Transition State [30].S structure of the title compound is stabilized by DMSO and EtOH,nd as a result of this, the activation energy needed to overcomehe barrier of intramolecular proton transfer between keto formnd TS structures decreases. This decrease in the activation energynables the formation of both keto and enol forms in DMSO andtOH.

For the comparison of the experimental and theoretical results,he total energy, dipole moment and frontier molecular orbitalnergies are calculated by using DFT/B3LYP method with 6-31(d)asis set by adding the polarizable continuum model (PCM) in sol-ent media for both enol and keto form. Results obtained from

sed solvents having different polarities are shown in Table 6. Ashown in Table 6, the total energies decrease with the increase inhe polarity of the solvent, in other words, the stability of molecule

Fig. 5. UV–vis spectra of I (- - - -) with acid and (– · · –) with NaOH in EtOH.

1 (77%)76%) H → L + 1 (6%) 394.95 (0.9127) H → L (81%)

increases. While the dipole moment also increases, the energy gap(�E) between the highest occupied molecular orbital (HOMO) andthe lowest-lying unoccupied molecular orbital (LUMO) increasesfor keto form with the increase in the polarity of the solvent. HOMOand LUMO called Frontier molecular orbitals (FMO) are the mostimportant orbitals because they play important roles in reactionsbetween molecules and in electronic spectra of a molecule. Thefrontier molecular orbitals for both enol and keto form of I areshown in Fig. 6 for gas phase.

In addition, the first 10 spin-allowed singlet–singlet excita-tions for both enol and keto forms of I were calculated by TD-DFTapproach. TD-DFT calculations were started from optimized geom-etry using the same level of theory and carried out in gas phaseto calculate excitation energies. The percentage contributions ofmolecular orbitals to formation of the bands were extracted fromoutput by using SWizard Program [47]. For both enol and keto formof I, wavelength (�), oscillator strength (f) larger than 0.25, majorcontributions of calculated transitions are given in Table 5. FromTable 5, in view of TD-DFT calculations corresponding excitationenergy at 377.43 nm arises from HOMO → LUMO (77%) transitionin gas phase for enol form. The excitation energy from HOMO toLUMO (70%) and HOMO to LUMO + 1 (9%) is at 428.57 for keto form.The experimental absorption bands are observed at 350–390 nmand above 400 nm in all solvents. From TD-DFT calculations, we

belongs to HOMO → LUMO and HOMO → LUMO + 1 transitions ofthe keto form and the absorption band in the range of 350–390 nmarises from that of enol form of I.

Fig. 6. The molecular orbital surfaces of I for enol and keto forms in gas phase.

90 C . Albayrak et al. / Spectrochimica Acta Part A 85 (2012) 85– 91

Table 6The energies and dipole moments of keto and enol forms in gas phase and solution.

Gas phase Benzene EtOH DMSO

Keto formEtot (a.u.) −1148.34313885 −1148.35408718 −1148.36990069 −1148.37049854� (D) 3.7313 5.4692 6.5582 6.6853ELUMO (eV) −1.917 −1.900 2.010 2.009EHOMO (eV) −5.082 −5.056 5.186 5.186�E (eV) 3.165 3.156 3.176 3.177

Enol formEtot (a.u.) −1148.34190617 −1148.35034933 −1148.36243628 −1148.36298190� (D) 1.7548 1.7811 1.8649 1.8667

−2.216 −2.225−5.633 −5.6393.417 3.414

3

dpndologidpipbidebgIaAtutc�etn(

iE

�

˛

ˇ

iBg

ao

Table 7Calculated dipole moments (D), polarizability and first hyperpolarizability compo-nents (a.u.) for the title compound.

�x −1.0389341 ˇxxx 4397.8559237�y −0.9760553 ˇxxy −728.5872868�z 0.3507316 ˇxyy −365.79141

ˇyyy −273.5402732˛xx 509.0315178 ˇxxz 146.4112794˛xy 21.0290349 ˇxyz −58.2468345˛yy 234.7209847 ˇyyz −25.2137522˛xz 14.575595 ˇxzz −20.5961183

temperature from 100 K to 500 K. As the results show increase oftemperature increases heat capacities, entropies and enthalpiesdue to increasing intensities of molecular vibration.

Table 8Thermodynamic properties of title compound at different temperature.

T (K) H0m (kcal/mol) S0

m (cal/mol K) C0p,m (cal/mol K)

100 2.328 99.744 35.054200 7.280 133.090 59.785298.15 14.521 162.225 83.800300 14.680 162.757 84.251400 24.494 190.829 107.667

ELUMO (eV) −2.034 −2.106

EHOMO (eV) −5.478 −5.536

�E (eV) 3.444 3.430

.4. Non-linear optical (NLO) properties

The non-linear optical properties play an important role for theesign of materials in modern communication technology, signalrocessing, optical switches and optical memory devices [48]. Theon-linear optical properties of the organic molecules arise fromelocalized � electrons that move along molecule. The increasef the conjugation on molecule leads to an increase in its non-inear optical properties. One another way to increase non-linearptical properties is to add donor and acceptor groups. Acceptorroup is opposite of donor group in the organic molecules contain-ng donor and acceptor groups and � electron cloud moves fromonor group to acceptor group. If the donor and acceptor groups areowerful, delocalization of � electron cloud on organic molecules

ncreases and as a result of this the polarizability and first hyper-olarizability of organic molecules increase [49]. The energy gapetween HOMO and LUMO has an important role in getting polar-

zability of a molecule [50]. The increment of the strength of theonor and acceptor groups increases the non-linear optical prop-rties of organic molecules due to the decrease the energy gapetween HOMO and LUMO. The molecules having a small energyap are more polarizable than molecules having a large energy gap.n addition, UV–vis spectra can be used to correlate with polariz-bility. A small HOMO–LUMO gap means small excitation energy.bsorption bands of molecules having a small energy gap shift

owards the visible region. Quantum chemical calculations can besed to describe the relationship between the electronic struc-ure of molecules and their non-linear optical properties. The titleompound, a Schiff Base containing azo group, has the delocalized

electron system, electron-donating substituent –OH group andlectron-withdrawing substituent –F atom. In order to investigatehe effect of � electron system and donor–acceptor group on itson-linear optical property I was computationally studied by DFTB3LYP) theory level.

The total static dipole moment �, the average linear polarizabil-ty ˛, and the first hyperpolarizability can be calculated by usingqs. (2)–(4), respectively [48].

= (�2x + �2

y + �2z )

1/2(2)

= 13

(˛xx + ˛yy + ˛zz) (3)

= [(ˇxxx + ˇxyy + ˇxzz)2 + (ˇyyy + ˇxxy + ˇyzz)2 + (ˇzzz + ˇxxz + ˇyyz)2]1/2

(4)

The dipole moment, polarizability and the first hyperpolar-zability were calculated using polar = ENONLY at the level of3LYP/6-31G(d) and the results obtained from calculation were

iven in Table 7.

The calculated polarizability and first hyperpolarizability of Ire 41.603 A3 and 35.76 × 10−30 cm5/esu that are greater than thosef urea ( and of urea of 3.8312 A3 and 0.37289 × 10−30 cm5/esu),

˛yz 7.4710347 ˇyzz −4.2741836˛zz 99.3389395 ˇzzz 42.6944248

respectively [51]. In addition, it is found that the first hyperpolariz-ability and polarizability of I are greater than those of related SchiffBases [51–53]. That the energy gap between HOMO and LUMO ofI is 3.165 eV for gas phase shows that the title compound has asmaller energy gap, thus, the absorption bands in the electronicspectra are shifted towards the visible region. The movement of �electron cloud from electron donating −OH group to electron with-drawing −F substituent (at opposite side of donor group) throughN N bridge and the increase of � electron system by adding C Nbond to molecule bond lead to an increase of the conjugation andconsequently an increase in its non-linear optical properties. Theseresults show that the title compound can be used as a good non-linear optical material.

3.5. Thermodynamic properties

The heat capacity (C0p,m), entropy (S0

m) and enthalpy (H0m) that

are the standard thermodynamic functions were performed usingDFT/B3LYP method with 6-31G(d). The results obtained fromthe basis of vibrational analysis are shown in Table 8. The heatcapacities, entropies and enthalpies were obtained by increasing

500 36.504 217.543 127.936

C0p,m = 7.92782 + 0.27564T − 6.99837 × 10−5T2, R2 = 0.99974.

S0m = 65.44074 + 0.35617T − 1.0472 × 10−4T2, R2 = 0.99988.

H0m = −0.33858 + 0.01463T + 1.1823 × 10−4T2, R2 = 0.99998.

mica A

eFc

4

dcohtdTfbdcTBcwmciats

A

t

R

[[

[[

[[

[

[

[

[

[[

[[[

[[

[

[[[[[[

[

[[[[

[

[[[[

[

[

[

[

[[

[[51] Y.X. Sun, Q.L. Hao, W.X. Wei, Z.X. Yu, L.D. Lu, X. Wang, Y.S. Wang, J. Mol. Struct.

(Theochem) 904 (2009) 74–82.[52] M. Jalali-Hevari, A.A. Khandar, I. Sheikshoaie, Spectrochim. Acta A 55 (1999)

C . Albayrak et al. / Spectrochi

The correlation equations between heat capacities, entropies,nthalpies and temperature are shown in Table 8 andigs. S8–S10 in Supplementary data can be used for analyzing heatapacities, entropies and enthalpies in different temperature.

. Conclusion

In this work, the compound (Z)-4-[(E)-(4-fluorophenyl)iazenyl]-6-[(3-hydroxypropylamino)methylene]-2-methoxycy-lohexa-2,4-dienone was experimentally characterized by meansf X-ray diffraction, FT-IR and UV–vis spectroscopic techniques. Itas been determined that the title compound is in keto form inhe solid state both on the basis of X-ray and FT-IR spectroscopicata. UV–vis spectra were recorded in different organic solvents.he results show that compound exists in both keto and enolorms in DMSO, EtOH but it tends to shift towards enol form inenzene. Since the polar solvents facilitate the proton transfer byecreasing the activation energy needed for Transition State, titleompound exists in both keto and enol forms in DMSO and EtOH.he geometry optimization was performed using DFT method with3LYP applying 6-31G(d) basis set. There are differences betweenrystal and computational structures because of crystal packing inhich the hydrogen-bonding properties are fundamental in deter-ining the crystallographically observed conformation. TD-DFT

alculations starting from optimized geometry were carried outn both gas and solution phase to calculate excitation energies. Inddition, the non-linear optical properties were computed andhe changes of thermodynamic properties were obtained with theame level of theory.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.saa.2011.09.020.

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