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Vibrational Circular Dichorism and Non Linear optics of Azobenzene

Derivatives; A Theoretical Study

Article  in  International Journal of Chemistry · April 2014

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Nasarul Islam and Altaf Hussain PandithInt. J. Chem.

Vol 3 (2) (2014) : pp 184 - 194

ISSN 2249 - 2119

International Journal of Chemistry184

Vibrational Circular Dichorism and Non Linear optics of AzobenzeneDerivatives; A Theoretical Study

Nasarul Islam1 and Altaf Hussain Pandith,1

1Department of Chemistry, University of Kashmir, Srinagar, Kashmir-190006, IndiaEmail: altafpandit23@gmail.com

AbstractThe present work reports a theoretical study on vibrational spectra and nonlinear optical properties of azobenzenederivatives. The theoretical geometrical parameters in the ground state have been investigated by density functionalmethod using B3LYP functional and 6-311G++ (d, p) basis set. The analysis of calculated IR and VCD indicatesthat the binaphthyl and alkoxy side chain of azobenzenes derivatives contribute differently to spectral line shape. Theout-of-phase bending and twisting of azobenzene rings, out-of-phase stretching of binaphthyl substituent and wag-ging of hydrogen atoms present on the alkoxy side chain vibrational modes dominate display distinct bands inspectra. To investigate nonlinear optical behavior, the electric dipole moment, the polarizability and thehyperpolarizability were computed using DFT/B3LYP/6-311G++ (d,p) level of theory. The results reveal that thereis a potential rise in nonlinear optical character of azobenzene derivatives on account of binaphthyl substitution,due to extend in p-conjugation length.

Keywords: Density functional theory, Vibrational circular dichorism spectra, IR spectra, Non linear optics

Introduction

Azobenzene are good examples of assuming inherenthelicity, (P) or (M), generated by the steric hindrance ofeach of the two benzene rings 1. According to Takaishniet. al 2, the properties of the helicity of twisting patternsof the Azobenzene derivatives is induced byintramolecular chirality transfer from the axial chiralityof the binaphthyl units. Azobenzenes have got muchattention from last decade, due to their switchableabsorption spectra, fluorescence properties, andassociation constants and are frequently used asphotochromic components 3-16. Synthesis and propertiesof a series of optically active compounds (binaphthyl andimidazole tetrapeptide) with azobenzene adducts have

been reported during the last decade 1,17-24. Azobenzenederivatives impart nonlinear optical behavior, because oftheir large dipole moments and the stability of theazolinkage25-27. These properties in conjunction with theelectrooptic switching properties of liquid crystals canlead to interesting materials for nonlinear optical andphotoswitching applications. The presence of electrondonor and acceptor groups in these systems inducebathochromic shift which may facilitate resonanceenhancement effects and which can be chemicallyfunctionalized with ease. Amino derivatives ofazobenzenes act as colorants, being dichoric compoundswith a large permanent dipole moment and long molecularstructure, giving them the potential to be applied in non-linear optical devices and LCDs28. They have high

Vol. 3 (2) April - June 2014 185

electrochemical and photochemical stability with highdichoric ratios29. Some cholesterol- linked azobenzenederivatives have been synthesized and their photoinducedionic conductivity and gelation properties have beeninvestigated 30,31. The nonlinear optical properties ofcholesterol- linked azobenzene were studied by electricfield poling32 as well as hyper-Rayleigh scatteringtechiniques33. Lin et. al. showed the relationship betweendipole moments, hyperpolarizabilities and structures andthree kinds of substituent constants were applied tocorrelate with both ground state dipole moment andhyperpolarizabilities34. These systems display sizeablecircular dichorism (CD) and circularly polarizedluminescence (CPL) spectra. Chiral molecules showdifferential absorption of left versus right circularlypolarized IR radiation by a molecular vibration transition.The analysis of the photoswitchable optical properties,including absorption, circular dichorism and opticalrotation, using substituted binaphthyl-azobenene dyadsand their regioisomers was done by Takaishni et. al 2.According to them, change in absorption near 360nmconformed the cis-trans isomerization of the azobenzenemoiety. In their CD spectrum, reversible positive-negativeand zero-positive (zero-negative) chiroptical signals weredetected. For the chiral systems mentioned above i.e.,substituted binaphthyl-azobenezes their vibrationalcircular dichorism (VCD) has not been studied yet. Hereinwe report ab initio studies on the IR, VCD and Non linearresponse of these substituted binaphthyl-azobenezes usingDensity Functional Theory (DFT). The main objective ofthis study is to provide a complementary understandingof the chiral behavior of these molecules. IR is a usefultechnique to understand the structure and dynamics ofhelical systems as the line intensity and frequencies givedirect information about the interactions holding themolecule intact. The main advantage of VCD comparedto ECD is that all (3N-6) vibrational modes of a chiralmolecule, N being the number of atoms in the molecule,can give rise to circular dichroism 35. VCD (vibrationalcircular dichroism) is simply an extension of ECD(electronic circular dichroism) into the infrared and nearinfrared regions of the spectrum used as probes to identifythe absolute configuration and solution phase confor-mations of different molecular systems 36-42. The

advancement in determination of absolute configurationof chiral molecules in the solution state using vibrationalcircular dichorism (VCD) studies have been reviewed byFreedman et al 43.

Materials and Methods

Computational DetailsThe quantum chemical computations were performedusing the Gaussian 03 quantum chemistry package44. Theinitial geometries were optimized by the DFT method byemploying Becke’s three parameter hybrid functional(B3LYP) and 6-311G++ (d, p) basis set 45,46 . IR and VCDspectra of all M- and P-isomeric forms were calculatedat the same level of theory. All the geometries werecharacterized as minima with Zero imaginary frequencies.The VCD intensities were calculated with the magneticfield perturbation method47 implemented in Gaussianprogram package using Gauge Invariant Atomic Orbitals(GIAO). B3LYP in numerous studies have been found tobe efficient and reasonably accurate in predicting groundand excitation energies of a broad range of organicmolecules and inorganic complexes 36-43. In order to obtainthe real time spectral frequencies, all the calculatedfrequencies were uniformly scaled by 0.97 for all theisomers.

The electronic and photophysical properties of azobenezesare very attractive for the design of new organic light-emitting and semiconductor materials. We studied theelectronic dipole moment (µ), molecular polarisability (á)and hyperpolarizability (â) of investigated hetero-helicenes. The total static dipole moment (µ), averagelinear polarisability (á) and first-order hyperpolarizability(â) were calculated using the x, y, z components fromGaussian 03 W output by Equations 1, 2 and 349-53 and aregiven in Table 1.

2 2 2 1/ 2x y z

xx yy zz

μ = (μ μ μ ) 11(a) = (a a a ) 23

+ +

+ +

2 2xxx yyy zzz yyy yzz yxx

2 1/ 2zzz zxy zyy

β=[(β β β ) (β β β )

(β β β ) ] 3

+ + + + + +

+ +

Vibrational Circular Dichorism and Non Linear optics of Azobenzene Derivatives; A Theoretical Study

International Journal of Chemistry186

Results and Discussion

The IR, VCD and NLO studies were carried out for

substituted binaphthyl-azobenzenes given in scheme1.The M and P isomeric forms of substituted binaphthyl-azobenzenes were found to be atropisomers.

N N

N NO O

OO

NO

O

N

OR1

OR2

P-1 P-2P3; R1=R2= OH:P4; R1= OC6H5, R2= OH:P5; R1= R2= OC6H5:P6; R1= R2= OCH2 (C6H5)2

Scheme 1: Sketches of P-isomers of binaphthyl-azobenzene derivatives 1, 2, 3, 4, 5 and 6.

The lowest energy optimized structures among the M andP configurations of substituted binaphthyl-azobenzenesare shown in Figure 1.The relative energies show that theP- atropisomers are more stable than the M-atropisomerfor binaphthyl-azobenzenes 3, 4, 5 and 6. Due to highrigidity of binaphthyl-azobenzenes, the transit from oneconformation to another needs to cross a high energybarrier. Thus, calculation of the transition state was notattempted. We have assigned the skeletal vibration as wellas trends in absorption in mid-IR region to eachcharacteristic group of the molecule, in addition tocalculating the normal modes of vibrations for allatropisomers. Upon inspection of the infrared spectrum,no significant dissimilarities were found in the IRabsorptions between two isomeric azobenzene derivativesof 1 2, 3, 4, 5 and 6 except shifts in frequency. Howeverwe found characteristic feature of the particularenantiomers/atropisomers, M or P, in VCD spectra ofthese molecules that can give diagnostic information fordetermination of the absolute configuration. In the mid-IR region (1300-1700cm-1) the calculated IR and VCDspectra display peaks, with characteristic features,

involving normal mode vibrations and provide importantinformation regarding the configuration identifications.

The IR spectrum of 1 and 2 azobenzenes (Figure 2(a))normally contains many peaks of varying intensities, manyof which are not readily identifiable. The IR absorptionbands are due to excitation of normal modes at signalswhich can be seen clearly are due to the twisting ofazobenzene rings and the stretching of N-N bond, whichis a very strong peak around 1609.28 cm-1 (P-1/M-1 ) and1581.61cm-1(P-2/M-2). In case of P-2/M-2, the peaks at1343.23 cm-1 and 1474.90 cm-1 are due to wagging of hy-drogen atoms present on alkoxy side chain. In compari-son to P-1/M-1 the N-N stretching occur at lower fre-quency in P-2/M-2, which may be due to the presence ofalkoxy chain at the ortho position of azobenzene ring.The IR absorption peaks in the region 1300-1700cm-1 (Fig-ure 2(b)) are due to excitation of normal modes at 1357.71cm-1,1426.24 cm-1, 1489.53 cm-1, 1507.93 cm-1, 1557.19cm-1, 1611.31 cm-1, 1675.36 cm-1, 3710.58 cm-1 for M-3and P-3 configurations and at 1369.83 cm-1, 1428.88cm-1, 1432.60 cm-1, 1462.23 cm-1, 1481.96 cm-1,

Vol. 3 (2) April - June 2014 187

P-1 P-2 P-3

P-4

P-5

P-6Fig. 1. Optimized geometries of (a) P-1, (b) P-2, (c ) P-3 (d)P-4, (e) P-5 and (f) P-6 isomeric forms of binaphthyl-

azobenzene derivatives.

Vibrational Circular Dichorism and Non Linear optics of Azobenzene Derivatives; A Theoretical Study

1526.34 cm-1 ,1573.34 cm-1 ,1616.42 cm-1 ,1655.91 cm-1

,1681.17 cm-1 and 3726.14 cm-1 for P-4 and M-4 con-figurations. These absorptions are due to out-of-phasestretching of the two azobenzene rings, binaphthyl ringand N-N bond stretching. The peaks in P3(P4) IR spec-trum at 1357.71 cm-1, 1426.24 cm-1 are due to the wag-ging of alkoxy side chain, while singles at 1489.53 cm-1,1507.93 cm-1 are assigned to out-of phase stretching ofbinaphthyl ring. The peaks at 1514.93 cm-1 and 1557.19cm-1 are due to twisting of azobenzene rings, with N-Nstretching occurring at 1611.31 cm-1(1573.34 cm-1) forP-3(P-4). In P3 (P4) the peak at 3710.58 cm-1 correspondsto OH stretching of hydroxyl substituent present onbinaphthyl component of azobenzene derivative. The IRspectra of P-5/P-6 in the region of 1300-1700 cm-1 dis-plays a number of peaks of varying intensity and are notfully resolved (Figure 3). In P-5(P-6) normal mode exci-tations observed at 1354.65 cm-1, 1419.23 cm-1, 1460.67cm-1 and 1478.11 cm-1 are assigned to out-of phase stretch-ing of binaphthyl ring. In case of P-5/M-5 the low inten-sity peak at 1339.20 cm-1 is due to stretching of O-C bondof phenyl ring attached to binaphthyl substituent. The lowresolution peaks at 1373.97 cm-1 and 1436.15 cm-1 aredue to the wagging of alkoxy group present onazobenzenes ring. The characteristic peaks due to thetwisting of azobenzenes ring occur at 1501.34 cm-1,1523.11 cm-1, 1617.25 cm-1, 1655.98 cm-1 for P-5(P-6),with a high intensity peak of N-N stretching at 1573.32cm-1. In P-6/M-6, an addition peak appear at 1538.25cm-1 on account of C-C stretching due to presence of twophenyl rings attached with the binaphthyl ring.

Fig. 2. IR spectral intensity in the mid-IR region (1300-1700 cm-1) of binaphthyl-azobenzene derivatives (a) M-1,

M-2 and (b) M-3, M-4. A scale factor of 0.97 is used. Thesignals seen in the spectra are 1331.38 cm-1, 1357.16 cm-1,1485.88 cm-1, 1519.87 cm-1 , 1609.28 cm-1, 1629.12 cm-1 and1648.16 cm-1 for P-1/M-1 and 1343.23 cm-1, 1360.04 cm-1,

1433.82 cm-1, 1474.90 cm-1, 1524.86 cm-1, 1581.61 cm-1,1648.82 cm-1, and 1654.12 cm-1 for P-2/M-2 enantiomers.

The normal modes of vibrations due to Azobenzene ring,alkoxy side chain and groups present on binaphthyl partof Azobenzene structure generate VCD signals which areremarkably efficient markers for configuration identifi-cation of these chiral molecular systems. The VCD spec-trum for P-1 and M-1 (Figure 4 (a)) shows asymmetricintensity pattern with negative polarization at 1331.36cm-1, 1357.16 cm-1 , 1485.88 cm-1 , 1609.28 cm-1 for P-1

International Journal of Chemistry188

Fig. 3. IR spectral intensity in the mid-IR region (1300-1700 cm-1) of M-5, M-6 of binaphthyl-azobenzene deriva-

tives. A scale factor of 0.97 is used.

configuration (positive polarization for M-1), with changein polarization at 1512.33 cm-1 and 1574.99 cm-1 . For P-1the VCD signal shows a positive polarization at 1519.87cm-1 and 1648.16 cm-1 opposite to that of M-1 configura-

tion. Interestingly in the region of N-N stretching (1609.28cm-1), the P-1 configuration show right polarization, whileM-1 configuration shows left polarization. The VCD sig-nals of P-2 and M-2 [Figure 4(b)] configurations showopposite polarizations at 1294.72 cm-1 and 1355.66 cm-1

with change in polarization at 1333.00 cm-1. For P-2 con-figuration VCD signals at 1482.00 cm-1, 1515.62cm-1,1581.61 cm-1, 1613.76 cm-1, 1654.12 cm-1 show right(negative) polarization with high intensity peaks at1515.62cm-1, 1613.76 cm-1due to wagging of alkoxy chainand twisting of azobene ring respectively. The M-2 con-figuration in N-N stretching region show positive polar-ization having low intensity at 1518.61 cm-1. In deriva-tive 3, [Figure 5 (a)] P-3 configuration show left polar-ization at 1374.23 cm-1, 1426.24 cm-1 , 1640.83 cm-1and1654.75 cm-1 (right polarization M-3) and right polariza-tion at 1489.53 cm-1 , 1507.93 cm-1 , 1527.47 cm-1, 1624.04cm-1 without change in polarization.

An additional doublet appears at 1390.15 cm-1 and1410.21 cm-1 with negative polarization of P-3. In theN-N stretching region a weak band appears at 1654.75cm-1 for M-3 with right polarization (left for P-3). TheVCD signal at 1383.95cm-1 [Figure 5(b)] shows left (posi-tive ) polarization for P-4 configuration and right (nega-tive) polarization for M-4 configuration, whereas at1399.41 cm-1 the polarization for the two configurationschanges sign. Interestingly the VCD signals of P-4 andM-4 configurations again change polarization at 1426.11cm-1 and 1498.23cm-1, with enhanced intensity, displaystwo doublets of opposite polarization at 1438.37 cm-1 ,1486.61 cm-1 (positive polarization for P-4) and -1505.70cm-1, 1529.38 cm-1.

Fig. 4. VCD spectral intensity in the mid-IR region (1300-1700 cm-1) of binaphthyl-azobenzene derivatives (a) M-1/ P-1and (b) M-2/ P-2. A scale factor of 0.97 is used.

Vol. 3 (2) April - June 2014 189

Vibrational Circular Dichorism and Non Linear optics of Azobenzene Derivatives; A Theoretical Study

Fig. 5. IR spectral intensity in the mid-IR region (1300-1700 cm-1) of binaphthyl-azobenzene derivatives (a) M-3/P-3, M-4/P-4 and (b) M-5/P-5, M-6/P-6. A scale factor of 0.97 is used.

International Journal of Chemistry190

In the region of N-N (1573.43 cm-1) stretching the peak isnot clearly visible for both the configurations. In the VCDspectra P-3 (P-4) an additional peak at 3710.58 cm-1

(3726.14 cm-1) corresponding to OH stretching ofhydroxyl substituent present on binaphthyl component ofazobene derivative appears with negative polarization ofP-3(P-4).

The VCD signals for P-5(P-6) and M-5(M-6) (Figure 5(b))shows asymmetric intensity pattern with negativepolarization of 1354.65 cm-1 and 1396.06 cm-1 and posi-tive polarization at 1377.35cm-1 , 1415.57 cm-1 , 1434.36cm-1 and 1478.21 cm-1 for P-5 configuration with changein polarization at 1388.41 cm-1 and 1412.34 cm-1. TheVCD signals of P-5 and M-5 configurations at 1396.06cm-1 and 1478.21 cm-1 have high intensity due to doubletat 1415.57 cm-1 and 1434.36 cm-1. The VCD spectra ofP-5 and M-5 configuration display negative polarizationpeaks for P-5 at 1500.12 cm-1, 1526.02 cm-1, and 1611.25cm-1 with change in polarization in N-N stretching regionat 1573.38 cm-1. An additional peak at 3190.97 cm-1 withright polarization for P-5 appears due to hydrogenstretching of phenyl ring. The corresponding VCD signalsfor the P-6 and M-6 configurations show low intensitynegative polarization of P-5 at 1351.97 cm-1, 1404.93cm-1 1466.00 cm-1 and 1646.99cm-1 and inversepolarization at 1372.78 cm-1, 1432.76 cm-1. At 1478.84cm-1, 1522.82 cm-1 and 1622.13 cm-1 VCD spectra displaysleft polarization for P-6 without change in polarization,followed by change in polarization in N-N stretchingregion at 1646.99 cm-1. An additional peak at 3060.11cm-1 with right polarization for P-5 appears due to out-of–phase stretching of substituent phenyl ring. The VCDsignal of P-5(M-5) have high intensity as compared toP-6(M-6) which may occur due to the less rigidity ofP-5(M-5).

We have extended this study to the determination ofelectric dipole moment, the isotropic polarizability andthe first hyperpolarizability in order to understand themicroscopic NLO mechanism of the above compounds.Theoretically calculated results of polarizabilities andhyperpolarizability obtained from Gaussian 03 output fileusing Equations 1, 2 and 3 are reported in atomic units

(a.u.), for the more stable P-isomeric form. The calculatedvalues have been converted into electrostatic units (esu)(á: 1a.u. = 0.1482 ×10"24 esu; â: 1 a.u. = 8.6393×10"33

esu).and are listed in Table 1. The corresponding valuesof dipole moment of Azobenzene derivatives are higherthan urea (µ= 1.528 A3) except P-5, with maximum of P-3 (6.07) and the hyperpolarizability value of theinvestigated azobenzene derivatives are all higher thanthose of urea. (â= 343.27x 10-33esu)49-53. Thus, bysubstituting the azobenzene by binaphthyl groups, theNLO character of molecule increases as â increases withan increase in donor and acceptor strength and withextending π- conjugation length. It is evident that themagnitude of the polarisability and the hyperpolarizabilityof these large molecular frame works is dependent onextension of π-conjugation length or structuralmodification due to incorporation of substituent inazobenzene system. The pz orbital of each sp2 hybridizedcarbon atom in the helical system form a ð-network withthe neighboring pz electrons. This ð-networking is ofdelocalized nature, resulting in high electronicpolarisability and hyperpolarisabilities as reflected fromthe enhancement of the hyperpolarizability. In additionto it, the substitution on binaphthyl ring also enhancesthe π-delocalization over the ring system. Thus, thecompounds probably are good candidates of NLOmaterials.

Conclusions

DFT calculations at the B3LYP/ 6-311++G (d, p) levelhas been used to study vibrational circular dichroism andnon linear response of Azobenzene derivatives. Analysisof the data revealed that, M and P isomeric forms ofderivatives 1 and 2 are enantiomers, whereas P-atropisomers are more stable than the M-atropisomers incase of 3, 4, 5 and 6 azobenzene derivatives. The infraredspectrum displays no significant dissimilarities betweentwo isomeric forms of azobenzene derivatives 1 2, 3, 4, 5and 6 except shifts in frequency. The characteristic IRand VCD signals arise due to the twisting of azobenzenerings and the stretching of N-N bond and somecontribution from wagging of hydrogen atoms present onalkoxy side chain. Theoretically calculated results of

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Vibrational Circular Dichorism and Non Linear optics of Azobenzene Derivatives; A Theoretical Study

polarisabilities of Azobenzene derivatives are higher thanurea (µ= 1.528 A3) expect P-5, with maximum for P-3(6.07) and the hyperpolarizability value of the investigatedazobenzene derivatives are all higher than those of urea.(â= 343.27x 10-33esu) 49-53. The result reveals that themagnitude of the polarisability and the hyperpolarizabilityof these large molecular Azobenzene derivatives isdependent on extend of π-conjugation length or structuralmodification due to incorporation of substituent inAzobenzene skeleton. Thus, the Azobenzene derivativesstudied display a significant non linear response.

Acknowledgements

The authors are thankful to Prof. G. M. Peerzada, Head,Department of Chemistry, University of Kashmir,Srinagar, Jammu and Kashmir, India for his constant

Table 1. The calculated dipole moments (Debye), static polarizability components (a.u.) and first hyperpolarizabilitycomponents (esu.) of Azobenzene derivatives.

encouragement and inspiration.

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International Journal of Chemistry192

P-1 P-2 P-3 P-4 P-5 P-6 Polarizability 1024 esu. 1024 esu. 1024 esu. 1024 esu. 1024 esu. 1024 esu.

αxx 27.870 39.971 80.213 78.563 82.355 101.331 αxy 0.001 0.000 2.580 3.455 -9.081 8.131 αyy 18.003 33.405 54.893 77.845 105.720 104.789 αxz 3.551 1.176 0.000 -5.825 0.000 0.000 αyz 0.000 0.000 0.000 -1.446 0.000 0.000 αzz 14.936 24.518 63.016 75.793 77.760 107.738 αtotal 20.282 32.649 66.077 77.436 88.649 104.665

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