chapter iv a. optical properties, solvent...

73

CHAPTER IV

A. OPTICAL PROPERTIES, SOLVENT EFFECT,

FLUORESCENCE STUDIES OF BENZOINS AND ITS

DERIVATIVES

4.1. INTRODUCTION

The study of solvent effects on the structure and spectroscopic behavior of

a solute is essential for the development of solution chemistry [1-5]. The presence of

specific and non-specific interaction between the solvent and the solute molecules

are responsible for the change in the molecular geometry, electronic structure and

dipole moment of the solute. These solute/solvent interactions affect the solute’s

electronic absorption spectrum and this phenomenon is referred to as

solvatochromism [6]. Moreover, the behavior of a solute in a neat solvent is very

different from the behavior in mixed binary solvent systems. In these kinds of

systems, the solute may induce a change in the composition of the solvents in the

cybotactic region compared to that in the bulk leading to preferential solvation. This

situation commonly results from specific (hydrogen bonding) and non-specific

(dielectric effects) interactions.

In the present work, the 1H NMR, IR and UV/Vis spectra, fluorescence

spectra of substituted benzoins and derivatives are considered. The UV/Vis

absorption spectra of benzoins and derivatives as well as the fluorescence spectra are

investigated in organic solvents of different polarity and are discussed with respect

to different solvent parameters.

4.1.1. Effect of solvent on the Fluorescence spectra of benzoins and its

derivatives

The maximum intensity (240) of fluorescence occurs at 410 nm for

benzoin in ethanol [Fig.4.1]. On the other hand the introduction of a methoxy

74

substituent in the chromophoric benzene ring decreases the intensity to 180

increasing the wavelength of fluorescence to 420 nm. For the 2C4MB the intensity

falls to 130 and the maximum florescence occurs at 440 nm. The increase in

wavelength of fluorescence can be attributed to the lowering of n→π* excitation

energy by the +E effect of methoxy group. The decrease in intensity can be

explained on the basis of the decrease in population in the excited state. The

influence of the chlorine (-I) group in the other ring may be attributed to both steric

and electronic factors. [Table 4.1a, 4.1b, 4.1c]

Table 4.1.a. Fluorescence spectra of benzoins in Ethanol

Compound (Ethanol) λmax (nm) Intensity

4-methoxy benzoin 420 180

2-chloro-4’methoxy benzoin 440 130

Benzoin 410 240

Table 4.1.b. Fluorescence spectra of benzoins in acetonitrile

Compound (Acetonitrile) λmax (nm) Intensity



Benzoin 412 133

Table 4.1.c. Fluorescence spectra of benzoins in Chloroform

Compound (CHCl3) λmax (nm) Intensity



Benzoin 309 239

75

Figure 4.1. Fluorescence spectra of BEN

When compare the wavelength of fluorescence of 2C4MB compound in

three different solvents of different polarity, it is observed that lower the polarity

shows the higher intensity of fluorescence [Fig.4.2]. This can be explained on the

basis of higher population of excited molecules in the less polar solvent, when

compared to the more polar solvent. It shows that less polar solvent stabilizes

molecules in the ground state while more polar solvent stabilizes the molecule in the

excited state.

Figure 4.2. Fluorescence spectra of 2C4MB

76

This can be further explained on the basis of the role of solvent solute

interaction. The polar solvents have attractive interaction with excited state of the

molecules both by dipole dipole interaction and dipole induced interaction. The

intensity of fluorescence for 4MB in acetonitrile is far higher than the fluorescence

in ethanol and chloroform. In acetonitrile being aprotic and polar solvent is capable

of stabilization polar excited state of 4MB more than ethanol and chloroform

[Fig.4.3].

Figure 4.3. Fluorscence spectra of 4MB

The λmax of fluorescence of benzoin in chloroform occurs at the

comparatively very low wavelength (309 nm) though with high intensity. This can

be explained on the basis of the fact that the excited state of benzoin is much less

stable when compared to the excited state of other substituted benzoin. The

excitation energy is higher hence the wavelength of fluorescence is lower

[Scheme 4.1].

Scheme 4.1.Ground and excited states of benzoin compound

77

4.1.2. Effect of solvent on the IR spectra of benzoins and its derivatives

In the IR spectrum of 4MB taken in KBr pellet indicates the presence of

intermolecular hydrogen bonded –OH group [Fig.4.4]. The molecule does not seem

to exist in a keto and enol tautomeric form. The intermolecular hydrogen carbonyl

bonding seems to affect the absorption and –OH stretching frequency. The carbonyl

group appears at 1715 cm-1 and –OH group appears at as a broad band at 3369 cm-1.

Figure 4.4. IR spectra of 4MB in solid KBr

In ethanol benzoin prefers to exhibit keto-enol tautomerism [scheme 4.2]

as it observed from the weakening of the intensity of carbonyl absorption and in

high dilution it disappears. [Fig.4.5a & 4.5b] The weak and absence of –OH

stretching absorption can be explained on the basis of possible cis-trans

isomerisation of the enol form [scheme 4.3a & 4.3b]. For example in the enol form

the following two forms can co-exist [scheme 4.2].

78

Figure 4.5a. IR spectra of 4MB in ethanol at higher concentration

Figure 4.5.b. IR spectra of 4MB in ethanol at and lower concentration

This can explained by the weak absorption of carbonyl at lower frequency

region as the –OH group is almost held strongly in a cyclic structure as shown in the

diagram 4.1.

79

'Ar

O

'C C

O

Ar

H

H

Enol form ( in solition) Keto form ( in solid)

C C

O

Ar Ar

OH H

Scheme 4.2. Keto-enol tautomerism of benzoin

C CAr Ar

O

'

OH H

C CAr ArO

'

O

H

H

Cis formTrans form

C C

O

Ar Ar

OH H

C C

O

Ar Ar

HO

H

''

Scheme 4.3.a & 4.3.b. Cis and trans isomerisation of enol form

Cyclic form( intra)

C C

O

Ar Ar

H

OH

'

Diagram 4.1. Cyclic form of benzoin compound

In less aprotic solvent like dioxane the keto-enol tautomerism exist as

shown by the weak carbonyl absorption and comparatively strong -OH absorption

[Fig.4.6]. The strong -OH absorption at 3514 cm-1 can be explained by the trans

form of the enol form.

80

Figure 4.6. IR Spectra of 4MB in dioxane

Benzoins belonging to α-ketol function are found to exhibit keto-enol

tautomerism. The tautomersim is found to be more prominent in 2C4MB

[Scheme 4.2]. In the solid state 2C4MB exhibits intramolecular hydrogen bonding

[Fig.4.7]. This is supported by the appearance of –OH as a sharp intense band at

3475 cm-1 and CO stretching frequency is found to have been decreased to

1666 cm-1. The intramolecular hydrogen bonding is expected to weaken the CO

double bond shown in Fig.4.8a & 4.8b.

Figure 4.7. IR spectra of 2C4MB in solid KBr

81

Figure 4.8a & 4.8 b. IR spectra of 2C4MB in ethanol at higher and lower

concentration

Non protic solvent like n-hexane, cannot favour intermolecular hydrogen

bonding [Fig.4.9].This is further supported by the sharpening the ‒OH band in IR

spectra. In ethanol, the tautomeric equilibrium exists between keto and enol forms as

the dilution increases, the intensity of CO absorption decreases, while the –OH

absorption increases.

Figure 4.9. IR spectra of 2C4MB in n-hexane

In less a protic solvent like dioxane the keto-enol tautomerism exist as

shown by the weak CO absorption and comparatively strong -OH absorption.

[Fig.4.10] The strong OH absorption at 3503cm-1 can be explained by the trans form

of the enol form.

82

Figure 4.10. IR spectra of 2C4MB in dioxane

In the IR spectrum of 2C4MBS in the KBr pellet the absorption is in the

range of 3540 cm-1. It seems to be very broad and multiplet [Fig.4.11]. This can be

attributed to the presence of –OH and –NH groups in the derivative. The N-H

appears at 3455 cm-1. In ethanolic solution the –OH absorption seems to be intense,

broad and a clear doublet indicating the presence of the intermolecular hydrogen

bonding [Fig.4.12a & 4.12b]. The intensity of N-H absorption is found to have been

decreased possibly because of the keto-enol tautomerism, in which the trans form is

more favoured [Scheme 4.4].

Figure 4.11. IR spectra of 2C4MBS in solid KBr

83

Figure 4.12a & 4.12b. IR spectra of 2C4MBS in ethanol of higher and lower

concentration

In dioxane solution the –OH absorption seems to be little sharp and

doublet indicating the presence of –OH and –NH groups [Fig.4.13]. This is

explained by the cis form of the tautomer of derivative [Scheme4.4]. The absence of

CO group in solution is very much supported by the absence of any signal in the

range of 170-180 ppm in the 13C NMR spectrum [Fig.4.14] of all the benzoins and

its derivatives.

Ar-C-C-Ar'

H

HO NNH-CO-NH2\

Ar-C C-Ar'H

OH

NNH-CO-NH2

Ar-C C-Ar'

O N-NH-CO-NH2HAr-C C-Ar'

NH-NH-CO-NH2

HO

HO

C-Ar'Ar-C

H N-NH-CO-NH2

H

CIS FORMTRANS FORM

Scheme 4.4 Cis-trans tautomer of 2C4MBS

84

Figure 4.13. IR spectra of 2C4MBS in dioxane

The IR spectrum of 22’CD in KBr pellet shows broad –OH band in the

range of 3332 cm-1 indicating presence of –OH group and absence of any absorption

in the range of 1620-1720 cm-1 indicates that the carbonyl groups of 22’dichloro

benzil, have disappeared during the borohydride reduction [Fig.4.14].The IR

spectrum of ethanolic solution, diol does not differ very much from that of the IR

spectrum taken in KBr pellet. This indicates the presence of intermolecular

hydrogen bonding both in solid and in solution.

Figure 4.14. IR spectra of 22’CD in solid KBr

85

[Fig.4.15a & 4.15b] shows the IR spectra of 22’CD in ethanol and di-

oxane. The polarity of the solvent and concentration of solution has minimum effect

on IR spectrum of diol.

Figure 4.15a. IR spectra of 22’CD in ethanol

Figure 15b. IR spectra of 22’CD in di-oxane

86

4.1.3. Effect of solvent on the UV spectra of benzoin and its derivatives

In 4MB, π→π* transition was observed in all the solvents in the range of

205 nm [Table 4.2]. In CHCl3 it is observed at 204 nm, in acetonitrile λmax is slightly

increased to 204.5 nm, while in highly polar protic solvent it has decreased to

203nm.These absorptions may be attributed to π→π* transition [Fig. 4.16a, 16b,

16c].

Figure 4.16a. UV spectra of 4MB in ethanol

Figure 4.16b. UV spectra of 4MB in chloroform

87

Figure 4.16.c. UV spectra of 4MB in acetonitrile

Table 4.2. UV spectra of 4MB in different solvent

Solvent π→π* (nm) n→π* (nm)

Ethanol 203 324

Acetonitrile 204.5 278

Chloroform 204 265

The solvent effect in the intensity and λmax of n→π* transition is higher

than that of π→π* transition. In the case of 2C4MB, less polar solvent like CHCl3

both n→π* and π→π* transition occurs comparatively at lower wavelength. In more

polar solvents like ethanol the λmax of both π→π* and n→π* transition appears

comparatively at higher wavelength. In the case of non-protic polar solvent like

acetonitrile the λmax for both π→π* and n→π* transition appear at wavelength lower

than that observed for both CHCl3 and ethanol solvents. This abnormality in 2C4MB

can be explained by assuming that the steric effect due to the ortho chloro

substituent inhibits a co-planar conformation which inhibits the excitation to occur.

The steric effect is more favoured in acetonitrile as shown in the table 4.3.

88

In less polar solvent CHCl3 the π→π* transition shows the hypsochromic

shift by the introduction carbonyl group in benzene of the benzoin, while the n→π*

transition shows the bathochromic shift by the introduction of the methoxy group.

This indicates that the –OCH3 group lowers the energy of the π-level of benzoin so

that the π→π* transition requires more energy. The –OCH3 group lowers the π*

level in the 4MB so that the energy requires for n→π* transisiton is lowered by

increasing the λmax. A similar bevaiour is observed in more polar non-protic solvent

acetonitrile as shown in the table 4.2.

In the case of 2C4MB, the λmax for both n→π* and π→π* transition in

acetonitrile solution is lower than that expected [Table 4.3]. This abnormality can be

explained on the basis of the steric factor becoming enhanced in acetonitrile

solvent so that the excitation energy is increased leading to decrease in λmax

[Fig.4.17a,4.17b,4.17c].

Figure 4.17a. UV spectra of 2C4MB in ethanol

89

Figure 4.17b. UV spectra of 2C4MB in acetonitrile

Figure 4.17c. UV spectra of 2C4MB in chloroform

Table 4.3.UV spectra of 2C4MB in different solvents

Solvent π→π*(nm) n→π*(nm)

Ethanol 285 293

Acetonitrile 205 270

Chloroform 240 293

90

BEN and 4MB, both in acetonitrile and chloroform solution, show a total

of four transitions appear two for π→π* transition and two for n→π* transition

[Tables 4.2 & 4.4].This can be explained by assuming that the BEN and 4MB exist

in two different conformation in solution. Each conformation give rise to one set of

n→π* and π→π* transition leading to a total of four transitions. From the table 4.2,

it is observed that the n→π* transition of 4MB in acetonitrile is slightly higher than

that in CHCl3. This shows that the π level is more stabilized in less polar solvent

than in more polar solvent (acetonitrile).This is obvious from the fact that the

excited state is more polar than ground state. The more polar state will be stabilized

by more polar solvent than by less polar solvent [Fig.4.18a, 4.18b].

Figure 4.18a. UV spectra of benzoin in acetonitrile

Figure 4.18b. UV spectra of benzoin in chloroform

91

Table 4.4. UV spectra of BEN in different solvents

Solvent π→π*(nm) n→π*(nm)


Chloroform 213 265

Where as, in 2C4MB has only two transitions, i.e., n→π* and π→π*

transitions are observed. This can be explained by the preferential presence of one

conformation of 2C4MB, as a consequence of the steric influence of ortho chloro

substituent.

In UV spectrum of 2C4MBS in different solvents both n→π* and π→π*

transition are observed. The values are given in table 4.5 n→π* transition shows

bathochromic shifts by changing the solvent from ethanol to acetonitrile [Fig.4.19a,

4.19b, 4.19c]. This can be explained by assuming that in hydroxylic solvent ethanol,

the trans form is more favoured while in non-hydroxylic solvent, the cis form is

more favoured. In the trans form the –OH is free [Scheme 4.5] such that the C=C

character is obtained, while in the cis form because of intramolecular hydrogen

bonding, the C=N character is slightly reduced to C-N, leading to the lowering of the

excitation energy, giving rise to increase in the wavelength of absorption

[Table 4.5].

'

N N

H

C

O

NH2OH

C C Ar

H

ArAr C ' C Ar

O

N NH

C

O

NH2 H

H

Cis form Trans form

(ketoform) (enol form)

Scheme 4.5. Cis and Trans forms of 2C4MBS

92

Figure 4.19a. UV spectra of 2C4MBS in ethanol

Figure 4.19.b. UV spectra of 2C4MBS in acetonitrile

Figure 4.19c. UV spectra of 2C4MBS in diethylether

93

Table 4.5. UV spectra of 2C4MBS in different solvents

Compound(2C4MBS) π→π*(nm) n→π*(nm)

Ethanol 225 270


Diethylether 210 280

4.1.4. Effect of solvent by NMR of benzoins and its derivatives

PMR spectra of all the benzoins and their derivative were taken in

solvents differing in polarity both with protic and aprotic nature. The chemical shifts

of aromatic proton and –CH proton of the ethylene bridge were taken for

comparison and shown in the table 4.6. It is clear that there are not much appreciable

changes in the chemical shift by changing the solvents. [Fig.20a, 20b, 20c, 20d]

[Figure 4.21a & 4.21b] [Figure 4.22a, 4.22b, 4.22c, 4.22d].

Figure 4.20a. 1HNMR of 4MB in acetone d6

94

Figure 4.20.b. 1HNMR of 4MB in DMSO

Figure 4.20.c. 1HNMR of 4MB in MeOD

95

Figure 4.20.d. 1HNMR of 4MB in chloroform

Figure 4.21.a.1H NMR of BEN in MeOD

96

Figure 4.21.b.1H NMR of BEN in acetone d6

Figure 4.22.a.1H NMR of 2C4MB in MeOD

97

Figure 4.22.b. 1H NMR of 2C4MB in acetone d6

Figure 4.22.c. 1H NMR of 2C4MB DMSO

98

Figure 4.22.d. 1H NMR of 2C4MB in chloroform

Table 4.6. NMR of benzoins with different solvents

Compound Solvent -CH protons Aromatic protons

4MB

Acetone 6.1(d) 7(d)

MeOD 6.1(d) 6.9(d)

Chloroform 5.9(d) 6.8(d)

DMSO 5.9(d) 6(d)

2C4MB

Acetone 6.3(s) 7(d)

MeOD 6.4(s) 6.9(d)

Chloroform 6.3(s) 6.8(d)

DMSO 6.2(s) 7(d)

Benzoin Acetone 6(d) 7.2(d)

MeOD 6.1(d) 6.1(d)

99

B. OPTICAL NON-LINEARITY OF BENZOINS AND ITS

DERIVATIVES

4.2. INTRODUCTION

With the rapid development of optical communication, the novel

materials with large and ultrafast nonlinear optical responses are needed for

fabricating the ultrafast optical switching and processing devices. For these

purposes, many materials, including semiconductors, polymers, nanomaterials and

inorganic materials have been researched. In recent years, -conjugated organic

materials have received considerable interest for their high nonlinear optical (NLO)

properties and fast response time of the nonlinearity. Especially, the organometallic

and coordination materials have been attracting a great deal of attention in the field

of nonlinear optics because they can combine the advantages of architectural

flexibility, ease of fabrication, tailoring and high NLO properties of organics with

good transmittancy and thermal stability of inorganics.

The study of nonlinear optical properties of the synthesized complexes

will lead to the development of new NLO materials. The results of research may be

useful in understanding the NLO properties of material and their applications in

various optoelectronic technologies such as optical signal processing, all-optical

switching, optical computing and other NLO devices. In this regard, the present

work is aimed to design and characterizes suitable materials for NLO applications.

In recent years conjugated organic nonlinear optical (NLO) materials have

been attracting attention because of their second or third-order hyperpolorizabilities

compared to those of inorganic NLO materials [7]. Many investigations are being

done to synthesize new organic materials with large second-order optical

nonlinearities in order to satisfy day-to-day technological requirements [8]. They

have innumerable potential applications including telecommunications, optical

computing, optical data storage, etc. The conjugated molecules consist of a skeleton

containing conjugated π- electrons; the conjugated bridge is linked to two end

groups with electron donor (D) and electron acceptor (A) character, respectively.

100

The electron acceptor group withdraws electronic charge from the donor through the

conjugated bridge: as a consequence the π -electrons of the skeleton become

polarized, giving rise to a relevant molecular dipole moment which defines a charge

transfer axis roughly coincident with the chain axis of the conjugated system. These

molecules are known as push-pull molecules [9, 10]. The basic strategy of using

electron-donor and electron-acceptor substituents to polarize the π -electron system

of organic materials has been illustrious for developing the NLO chromophores

possessing large molecular non-linearity, good thermal stability, improved solubility

and processability [11, 12]. Recently, much effort is being devoted to understand the

origin of non-linearity in large systems and to relate the nonlinear optical (NLO)

responses to electronic structure and molecular geometry for designing and

fabricating the NLO materials of large molecular hyperpolarizability [13, 14].

In the present investigation, the growth of 4-methoxy benzoin (4MB)

single crystals and the detailed vibrational spectral analysis of the molecule in the

crystalline state are taken up to understand the correlation of the NLO activity.

4-methoxy benzoin (4MB) is a potential organic NLO material. The molecular

design of 4-methoxy benzoin, containing one electron donor (methoxy) and one

electron acceptor (carbonyl) moiety, provides it with a push-pull configuration,

which is a well-known way of enhancing the optical non linearities. 4MB

crystallizes in the orthorhombic system, with space group, Pca21 having lattice

parameters:

a = 14.4691(10) Å, b = 14.0978(10) Å, c = 5.8468(4) Å = = = 90 deg.

4.2.1. Second harmonic generation efficiency

Preliminary study of the powder SHG conversion efficiency was carried

out with Nd:YAG laser beam of wavelength 1064 nm, using Kurtz and Perry

method [15]. Nd:YAG laser beam of wavelength 1064 nm was used with an input

power of 6.1 mJ per pulse. The crystals of 4MB were ground to a uniform particle

size of about125–150 m and packed in capillaries of uniform bore and exposed to

the laser radiation. A powder of KDP with same particle size was used as the

reference material in the SHG measurement. The output from the sample was

101

allowed through a monochromater to collect the intensity of 532 nm components

and eliminate fundamental. The generation of second harmonic was focused by a

lens and detected by the photomultiplier tube. The generation of the second

harmonic was confirmed by the emission of green light (532 nm). The SHG signal

for 4MB of 100 mV was obtained. The standard potassium dihydrogen phosphate

(KDP) crystal gave a SHG signal of 25.6 mV per pulse for the same input energy.

The conversion efficiency of the 4MB crystal is found to be 4 times that of standard

potassium dihydrogen phosphate (KDP) crystal.

4.2.2. Introduction to open aperture Z-scan

The Z-scan technique is a popular method for the measurement of optical

non-linearity of the material. It has the advantage of high sensitivity and simplicity

[16-18]. One can simultaneously measure the magnitude and sign of the non-linear

refraction and non-linear absorption, which are associated with the real part χR(3) and

imaginary part χ I(3) of the third order non-linear susceptibilities. The Z-scan

technique has been used to measure the third order non-linear optical properties of

semiconductors [19] dielectrics [20] organic or carbon-based molecules [21] and

liquid crystals [22].

In Z-scan technique is based on the variation of light intensity by altering

geometrical parameters of light-matter interaction. It is a simple yet highly sensitive

technique to measure the nonlinear optical coefficients based on the transmittance as

a function of z position. For an optical material exhibiting a third-order optical

nonlinearity, both the sign and magnitude can be determined easily by using this

technique. In this technique, the sample moves along the axis of a focused laser

beam through its focal plane and the transmission of the sample is measured for each

z position, this technique is called Z-scan technique. As the sample experiences

different electric field strengths at different z positions, the recording of the

transmission as a function of the z coordinate provides accurate information about

the nonlinear effects present in the sample. Hence the method has been referred as

Z-scan. Z-scan provides simplicity and high sensitivity. This technique enables one

to quickly determine both the sign and magnitude of nonlinear refraction and

102

nonlinear absorption. Due to these advantages, Z-scan technique has been widely

utilized as a standard tool to characterize various nonlinear optical properties

[Fig.4.24].

Figure 4.24. Block diagram of the open aperture z-scan experiment.

The block diagram of the Z-scan set-up is shown in Fig.4.24. The

incoming laser light is first split by a beam splitter. The reflected light is recorded by

detector D1 as a reference. The transmitted light is focused by a lens and the power

of the transmitted light passing through the sample is measured by detector D2.

Then the information of the nonlinearity can be derived from the ratio of D2 to D1

as a function of the position z. With or without an aperture in front of D2, the

nonlinear refraction or nonlinear absorption can be extracted by fitting the Z-scan

theory to the measured data. Two types of Z-scan can be performed: (a) the open

aperture Z-scan: where all the transmitted light is collected, which provides

information about the nonlinear absorption of the sample; and (b) the closed aperture

Z-scan: where only the light transmitted through a pinhole placed in front of the

detector (D2) is detected. The closed aperture Z-scan provides information about the

nonlinear phase variation, resulting from focusing or defocusing of the transmitted

beam. In this study, used the open-aperture Z-scan has been applied to investigate

the nonlinear absorption in the nanostructures.

103

4.2.3. Open aperture Z-scan

In this case, no aperture is kept between sample and detector. Therefore,

the detector will measure all the light transmitted by the sample, excluding scattering

losses. In an open aperture Z-scan experiment, by moving the sample along the z-

axis through the focus the intensity dependent absorption is measured as the change

of the transmission through the sample using a detector in the far field. On

approaching the focus the intensity increases by several orders of magnitude relative

to the intensity away from focus, inducing nonlinear absorption in the sample. Thus,

the change in the transmission due to various nonlinear absorption phenomena will

be seen in the detector. When the sample is at the focal point, its transmission will

be either maximum or minimum, depending on the sign of the dominant nonlinear

absorption process. For RSA it will be a minimum, and for saturable absorption it

will be a maximum.

In general, an open aperture Z-scan trace will be symmetric with respect

to the focus. The normalized transmittance of the medium for a third order optical

nonlinearity is given by

dqq

LRT )]exp(1ln[)exp()1( 20

0

2

With,

)exp(1 LLeff

, effLIRq 00 )1(

Where I0 is the peak intensity at the focal point, β denotes the nonlinear

absorption coefficient. For a medium that is transparent at the excitation wave

length, β will be the two-photon absorption coefficient. If the medium has some

absorption, then β will contain contributions from both SA and RSA.

104

Figure 4.25. Schematic representation of the Z-scan set-up

Fig. 4.25 shows the schematic diagram of open aperture Z-scan

experiment. A plano-convex lens of 10.5 cm focal length was used to focus the laser

beam. For investigating the nonlinear transmission properties of the samples 5ns

pulses from a Q-switched, frequency doubled Nd:YAG laser(Minilight, Continuum)

emitting at the wave length of 532 nm (2.33 eV) was used. Samples in liquid form

taken in a 1 mm glass cuvette (Hellma GmBH) were loaded as such on a

programmable linear translation stage. The input energy reaching the sample and the

energy transmitted by the sample were measured using two pyroelectric energy

probes (RjP 735, Laser Probe Inc.). Open aperture Z-scan measurements were

performed in order to calculate the nonlinear absorption coefficients. Nonlinear

optical absorption reveals the information about photo-excited structure of state and

carrier dynamics. The z-scan measurements on these organic compounds in

suspensions at same input intensities show a reverse saturable absorption behavior:

the transmittance of the sample decreases with the increase of the laser intensity and

reaches the minimum as the sample moves into the beam focus symmetric about the

focus (Z=O), which indicates that the higher order nonlinear processes are involved

as seen. The observed reverse saturable absorption is an indicator of the optical-

limiting effect.

Nonlinear optical absorption (NOA) measurements of the two compounds

4MB and 2C4MB were carried out at nanosecond scale by open aperture Z-scan

105

technique. Figs. 4.26 & 4.27 exhibit the normalized Z-scan transmittance of

2x10-4 mol/L solutions of compounds 4MB and 2C4MB in DMF. The nanosecond

laser source was a Q-switch locked Nd:YAG (1064 nm) with a second harmonic

generation of 532 nm, pulse width of 12 ns, repeat frequency of 10 Hz, average

pulse energy of 0.114 mJ and peak irradiance of 0.670 GW/cm2. As the sample was

moved away from the focus point, the transmittances of both compounds were

nearly a flat line, which displayed the linear absorption under weak light irradiation.

As the samples were moved close to the focus point, the transmittances decreased as

the laser irradiance increased. At the focus point (Z¼0) where the laser irradiance

reached maximum, the normalized transmittance decreased to a minimum. These

results indicated that both compounds 4MB and 2C 4MB were of obvious reverse

saturation absorption (RSA). For conjugated donar-acceptor systems, the distortion

of molecular cloud under strong laser irradiation was the reason for nonlinear

phenomena. For conjugated π systems of organic compounds, the conjugation

behavior of the molecular bone can be easily characterized by UV–Vis

spectroscopy. For this purpose, the UV–Vis absorption spectra of compounds 4MB

and 2C4MB were measured [Fig.4.27]. It was obvious that the λ max of both

compounds appeared in the UV region, and the λmax of compounds 4MB and

2C4MB were at 324 nm and 293 nm, respectively.

Figure 4.26. Non linear absorption coefficient of 4MB

106

Figure 4.27. Non linear absorption coefficient of compound 2C4MB

4.2.4. Optical limiting effect

The nonlinear optical limiting effect, or simply the optical limiting effect,

refers to the fact that the transmittance of a material decreases with increase of the

input light fluence or intensity, which has attracted considerable attention because of

its potential application in the fields of laser production and optical communication.

Using the Nd:YAG laser with pulse width of 12 ns as the light source, the optical

limiting properties of both compounds were measured under three different

concentrations. The input–output energy plots are shown in Figure 4.28. When the

input laser energy was weak, the output energy increased linearly as the input energy

increased. The absorption coefficient was independent on the laser irradiance (I0).

As the input laser irradiance (I0) increased continuously, the increase in output

energy deviated from the linear relationship, which was less than that predicted from

the slop of the linear region. This meant that the absorption coefficients of the

samples increased as the input laser irradiance (I0) increased. Transmittance of the

sample was obtained by dividing the output energy by input energy. The variation of

transmittance (T) against light irradiance (I) provides a measurement of optical

limiting property of the sample. Figs. 4.26 & 4.27 exhibit the plots of transmittance

of compounds 4MB and 2C4MB against light irradiance at different concentrations.

At low light irradiance, the T–I plot was flat, that indicated a linear optical behavior.

107

The transmittance had no dependence on the irradiance of the laser. As light

irradiance increased continuously, transmittance started to decrease. In other words,

both compounds showed optical limiting effect.

For compound 2C4MB, the optical limiting threshold value (EL), defined

as the light irradiance at which transmittance turns down in the T–I plot, was about

3.7 mJ. For compound 4MB, the EL value was less than 1 mJ. Except for the

difference in the EL values, the strengths of optical limiting effects for the two

compounds were different too. At light irradiance of 7 mJ, the transmittance

decrease for compound 4MB was about 40 % as compared with that of the linear

region. For compound 2C4MB, the decrease was only 14.6 %. The optical limiting

effect for compound 4MB was far more significant than that of compound 2C4MB.

This behavior was consistent with the strength of their reverse saturation absorption

behavior. For the two compounds at same concentration, the transmittance for

compound 4MB was larger than that of compound 2C4MB in the linear region.

When light irradiance increased, the trend of the change of transmittance with

intensity for a given compound was about the same. Based on the results obtained

above, we concluded that compound 4MB was a better optical limiting material

among the two compounds synthesized. Comparing the molecular structures of

compound 4MB with 2C4MB, both of them are of the same electron acceptor, the

difference is that compound 2C4MB has got electron donoar, methoxy group and

chloro group as the electron acceptor. The increase of the electron-donating group is

favored to the delocalization of π-electron, and the electron cloud is looser, which is

susceptible to distortion when excited by laser light. So the nonlinear optical

absorption effect of compound 4MB is more remarkable. As for compound 2C4MB,

although a methoxy group is present in the benzene ring as the chromophore, both of

the reverse saturation absorption and optical limiting effects are worse than that of

compound 4MB, probably because of the presence of a chloro substitutent which has

electron accepting property also. Clearly, increase in number of the electron-donating

group may be more effective for promotion of the nonlinear optical absorption

properties of the compounds, while the increase in the number of electron attracting

group may be effective in decreasing non-linear optical properties.

108

In the compound 2C4MB the ring having the –OCH3 group is attached to

electron attracting carbonyl group in addition to that electron attracting group is

present in the other benzene ring. There by the electron donating effect of –OCH3

group is reduced and hence the delocalization and polarisability of the compound

2C4MB is less than that of compound 4MB.

References

[1] K J De Almeida, T C Ramalho, Z Rinkevicius, O Vahtras O, H Agren and A Cesar J.Phys.Chem.A.115 1331 2011

[2] D Kosenko , L V Slipchenko J.Phys.Chem.A.115 392 2011

[3] Y G Sidir, E Taal and E Ermi Spectrochim.ActaA.78 640 2011

[4] O A Adegoke and S I Olakunle, Spectrochim.Acta A.75 719 2010

[5] B M Baughman, E Stennett, R E Lipner,A C Rudawsky and S J Schmidtke J.Phys.chem.A 113 8011 2009

[6] W Han, T Liu, F Himo, A Toutchkine, D Bashford,K M Hahn and L A Noodleman Chemphyschem.4 1084 2003

[7] S S Gupte, A Marcano, R D Pradhan, C F Desai and J Melikechi Appl. Phys. 89 4939 2001

[8] S P Karna (Ed.) J. Phys. Chem. A 104 2000.

[9] D R Kanis, M A Ratner and T S Marks Chem. Rev. 94 195 1994

[10] H S Nalwa, S Miyata Nonlinear Optical Properties of Organic Molecules and Polymers. (Boca Raton, FL: CRC Press)(1996)

[11] D S Chemla and J Zyss Non-linear optical properties of organic molecular crystals,Vol 1 and 2 (London: Academic Press) (1987)

[12] Peter Gunter Nonlinear Optical Effects and Materials. (New York: Springer-VerlagBerlin Heidelberg) (2000)

109

[13] Ch Bosshard, R Spreiter, L Degiorgi and P Gunter Phys. Rev. B. 66 205107 2002

[14] P N Prasad and D J Williams Introduction to Nonlinear Optical Effects in Organic Molecules and Polymers. (New York :John-Wiley & Sons Inc) (1991)

[15] S K Kurtz and T T Perry J. Appl. Phys. 39 3798 1968

[16] M Sheik–Bahe, A A Said, T H Wei, D J Hagan and E W Van Stryland J. Quantum Electron. 26 760 1990

[17] W Zhao and P Palffy-Muhoray Appl. Phys. Lett. 63 1993 1613

[18] M Yin, H P Li, S H Tang and W Ji Appl. Phys. B 70 587 2000

[19] T D Krauss and F W Wise Appl. Phys. Lett. 65 1739 1994

[20] R Rangel-Rojo,T Kosa, E Hajito, Ewen, A E Owen, A K Kar and B S Whereett Opt. Commun. 109 145 1994

[21] M Sheik-Bahe, J Wang J and E W Van Stryland J. Quantum Electron. 30 249 1994

[22] L Li, H J Yuan, G H Hu and P Palffy-Muthoray Liq. Cryst. 16 703 1994.

chapter iv a. optical properties, solvent...

Documents