chapter 4 spectral characteristics of organic...

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CHAPTER 4

SPECTRAL CHARACTERISTICS OF ORGANIC DYES

4.1 INTRODUCTION

Materials with large and fast nonlinear optical response are required for the development of photonics devices such as optical switches and optical limiters .The essential requirements of good photonic materials are high non linearity, synthetic flexibility and ease of processing. Extremely large numbers of organic compounds with delocalized electron, conjugated double bond systems and a large dipole moment have been synthesized to realize the susceptibilities far larger than the inorganic optical materials. It is thus important to understand the influence of various photo processes of the dyes on their nonlinear response. To study the nonlinear optical processes of the dyes, their lasing behavior must be elucidated. The spectral characteristics of these dyes give an insight about their lasing characteristics.

Solvents play an important role in the absorption and emission

characteristics of dyes. The solvent medium can cause displacement of the

fluorescence and absorption spectra of the dyes. Hence, the position and

structure of the absorption and emission spectra of molecules in solutions

are dependent on the solvent medium used. The solvent medium can force the

solute particles to get distributed in a spectral fashion in space through solute-

solvent interactions (Fleming and Wolynes 1990). So by recording the

absorption spectra, fluorescence spectra and the lifetime of the dyes, the effect

of medium on their spectral characteristics can be studied (Dyumaev et al

1992).

95

Time- resolved measurements are widely used in fluorescence spectroscopy because they contain more information than the steady- state data. Information on the number of fluorescence entities and number of different sites of binding a single fluorophore is available through time-resolved measurements. Lifetime τ, is measured as the average time the molecule spends in the excited state prior to the return to the ground state. Generally, fluorescence lifetimes are in the range of nano to pico seconds. The fluorescence lifetime can function as a molecular stopwatch to observe a variety of molecular events which occur in the above timescale.

In this chapter a study of the absorption and fluorescence spectra and lifetimes of the dyes in liquid environment are studied. The absorption curve does not mention the nonlinear effects as the light source in the spectrophotometer is insufficient to cause these effects, yet the absorption measurements is used to determine the suitable wavelength of the light source for which the optical material can act as a optical limiter. The molecular structures of dyes chosen are shown in Figure 2.1(a-f). Therefore, thorough investigations of these dyes in liquid media are necessary to understand the lasing characteristics dyes in solvent. The solvent medium can force the solute particle to get solute-solvent interactions. Physical properties of the solvents that have been used in this study are shown in Table 4.1.

Table 4.1 Physical properties of the solvents that have been used in

this study

Solvent Polarity

index Dipole

moment Linear ref. index (no)

Dielectric constant

Density gm/lit

Viscosity

Ethanol 5.2 1.69 D 1.361 24 0.789 0.949

Methanol 5.0 1.69 D 1.328 33 0.791 0.593

Acetonitrile 5.8 3.44D 1.344 37.5 0.786 0.380

96

4.2 SPECTRAL CHARATERISTICS OF ANTHRAQUINONE

DYES

The anthraquinone dyes ( alizarin, alizarin red and alizarin cyanin

green) are obtained from Loba Chemie, India. The molecular structures of

these dyes are given in Figure 2.1. (a) - (i-iii). Thin layer chromatography

(TLC) test confirms the absence of any impurities. These dyes were used as

received. Acetonitrile was chosen as solvent since these dyes were readily

soluble in it.

4.2.1 Absorption and Fluorescence Spectra of Anthraquinone Dyes

The absorption spectra of the anthraquinone dyes of 0.01mM

concentration in acetonitrile are recorded and shown in Figure 4.1.The

fluorescence spectra of the dyes in acetonitrile for 0.01mM concentration are

recorded and shown in Figure 4.2 a-c. The peak wavelengths of the absorption

and fluorescence spectra are measured and the spectral parameters such as

molecular absorption coefficient ((λ)), oscillator strength (f), absorption

bandwidth ()1/2, fluorescence bandwidth (FWHM) and Stoke’s shift are

calculated and given in Table 4.2.

4.2.2 Fluorescence Lifetime Measurements

The fluorescence decay measurements of anthraquinone dyes

were recorded using IBH time correlated single photon counting spectrometer

(TCSPC). The fluorescence decay profile and the residuals of the dye alizarin

red, alizarin cyanin green and alizarin in the solvent acetonitrile are shown in

Figure 4.3 a-c. The residuals shown in the Figure are well within the error

limits. Alizarin red, alizarin cyanin green exhibits bi-exponential decay

while alizarin exhibits a tri – exponential decay. The average life time values

of the anthraquinone dyes are shown in Table 4.2.

97

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

400 500 600 700 800

A

B

C

Wavelength (nm)

Ab

sorb

anc

e

Figure 4.1 Absorption spectra of anthraquinone dyes in acetonitrile

(A) Alizarin (B) Alizarin Red (C) Alizarin Cyanin Green

0

2000

4000

6000

8000

10000

12000

14000

16000

600 650 700 750 800 850

Wavelength (nm)

Fluo

resc

ence

Int

ensi

ty

Figure 4.2.a Fluorescence spectra of alizarin dye in acetonitrile

98

0

5000

10000

15000

20000

25000

500 600 700 800 900Wave length (nm)

Fluo

resc

ence

Int

ensi

ty

Figure 4.2.b Fluorescence spectra of alizarin red dye in acetonitrile

0

0.2

0.4

0.6

0.8

1

1.2

650 700 750 800 850Wavelength (nm)

Fluo

resc

enc

e In

tens

ity

Figure 4.2.c Fluorescence spectra of alizarin cyanin dye in acetonitrile

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-6

-4

-2

0

2

4

6

4 9 14 19

Residuals

Time (ns)

Sta

ndar

d D

evia

tion

-3

-2

-1

0

1

2

3

3 8 13 18

Residuals

Time (ns)Sta

ndar

d D

evi

ati

on

1

10

100

1000

10000

100000

0 10 20 30 40 50 60

Prompt

Decay

Fit

ALIZARIN

Log

(Cou

nts)

Time (ns)

-6

-4

-2

0

2

4

2 12 22 32 42

Residuals

Time (ns)

Sta

ndar

d D

evi

atio

n

1

10

100

1000

10000

0 10 20 30 40

Prompt

Decay

Fit

ALIZARIN RED

Time (ns)

Log

(Cou

nts)

1

10

100

1000

10000

0 10 20 30 40 50

Prompt

Decay

Fit

ALIZARIN CYANIN GREEN

Time (ns)

Log

(Co

unts

)

Figure 4.3.a Fluorescence decay profile and residuals of alizarin dye

Figure 4.3.b Fluorescence decay profile and residuals of alizarin red dye

Figure 4.3.c Fluorescence decay profile and residuals of alizarin cyanin

green dye

100

Table 4.2 Spectral parameters of antharquinone dyes in acetonitrile

Dye

Absorption spectra

Fluorescence spectra Average

life time (ns)

Peak wavelength

nm

(λ) 104 L mol-1 cm-1

()1/2 cm-1

Oscillator Strength fx10-24 L mol-1

cm-2

Peak wavelength

nm

FWHM nm

Stoke’s shift cm-1

Alizarin 643 6.65 1100 0.317 715 113 1513 1.00 Alizarin Red

600 5.59 2500 0.605 659 133 1485 0.52

Alizarin Cyanin Green

588 6.29 4000 1.089 689 36 1000 0.68

4.2.3 Results and Discussion

The longest wavelength absorption band of the anthraquinone dyes

are shown in Figure 4.1. The absorption maxima in nm are shown in Table

4.2. The absorption wavelength maxima decrease in the order of alizarin >

aizarin red > alizarin cyanin green. This variation is related to the conjugation

of π electronic cloud in the fused aromatic system, which is further altered by

the substituents on anthraquinone nucleus. In alizarin, two – electron releasing

OH- groups are bound to anthraquinone nucleus. In alizarin red there is an

electron withdrawing SO3Na grouping. As a result the absorption maximum is

blue shifted to a lower wavelength value (600 nm) in alizarin red compared to

alizarin (643nm). Hence, in alizarin red, the electron cloud will be more

tightly held than in alizarin. In alizarin cyanin green, the electron releasing

hydroxyl groups of alizarin are replaced by aniline groupings, but both the

aniline groupings carry electron withdrawing SO3Na grouping. As a result the

electron release from aniline groupings to anthraquinone nucleus is largely

101

retarded by SO3Na grouping so the electronic cloud is still more tightly held

than in alizarin red. Hence, the absorption maximum is shifted to shorter

wavelength (588 nm) than in alizarin red.

The maximum absorption wavelength of anthraquinone dyes is

largely influenced by the substituents bounded to anthraquinone nucleus. The

electron releasing substituents make the electron cloud of anthraquinone

nucleus less tightly held, thus shifting the absorption maximum to a longer

wavelength, while electron withdrawing grouping shift the absorption

maximum towards shorter wavelength.

The fluorescence spectra of the dyes are shown in Figure 4.2 a-c.

All the three dyes show fluorescence emission at longer wavelength than their

respective absorption maxima. Hence, loss of energy by vibrational cascade

leads to occupation of lower energy level. Generally, the vibrational cascade

will be more significant for flexible molecular systems than rigid molecular

systems. It is evident from the fluorescence spectrum of alizarin cyanin green,

as it is more flexible than other two dyes, due to two bulky anilino derivatives

bound to anthraquinone nucleus.

The average lifetime of anthraquinone dyes are shown in Table 4.2.

Alizarin with its rigid molecular structure shows larger lifetime than other two

dyes. Alizarin cyanin green shows longer lifetime than alizarin red. Hence,

the anilino derivatives bound to anthraquinone nucleus in alizarin cyanin

green has less influence on the lifetime of the excited species, compared to

alizarin red, where the substituents, SO3Na is directly bound to the

anthraquinone nucleus. The solvent acetonitrile can solvate SO3Na of alizarin

red and aids rapid vibrational cascade. In alizarin cyanin green SO3Na groups

are very far from anthraquinone nucleus and hence, it has less influence on

the lifetime of anthraquinone nucleus.

102

4.3 SPECTRAL CHARACTERISTICS OF AZO DYES

The azo dyes (oil red o and metanil yellow) are obtained from loba

chemie, India. The molecular structure of these dyes were given in Figure 2.1.

(b) - (i-ii). Thin layer chromatography (TLC) test confirms the absence of any

impurities. These dyes were used as received. Acetonitrile was chosen as

solvent, since these dyes were readily soluble in it.

4.3.1 Absorption and Fluorescence Spectra of Azo Dyes

The absorption spectra of azo dyes in acetonitrile for 0.01mM

concentration are recorded and shown in Figure 4.4.The emission spectra of

azo dyes in acetonitrile for 0.01mM concentration are recorded and shown in

Figure 4.5.The peak wavelengths of the absorption and fluorescence spectra

are measured and the spectral parameters are calculated and are shown in

Table 4.3.


The fluorescence decay profile and the residuals of the dyes oil red

o and metanilyellow in solvent acetonitrile are shown in Figure 4.6 a-b. The

residuals shown in the Figure are well within the error limits. The average life

time values of these dyes are shown in Table 4.3. Oil red o exhibits bi-

exponential decay while metanil yellow exhibits a tri – exponential decay.

103

0

0.2

0.4

0.6

0.8

300 400 500 600 700 800 900Wavelength (nm)

Abs

orba

nce

Oil Red O

0

0.4

0.8

1.2

300 400 500 600 700Wavelength (nm)

Ab

sorb

an

ce

Metanil Yellow

Figure 4.4 Absorption spectra of azo dyes in acetonitrile

0

20

40

60

80

540 560 580 600 620 640Wavelength (nm)

Fluo

rese

cnce

Inte

nsit

y

Oil Red O

0

20

40

60

80

550 575 600 625 650 675

Wavelength (nm)

Flu

ores

cenc

e In

tens

ity

Metanil Yellow

Figure 4.5 Fluorescence spectra of azo dyes in acetonitrile

104

1

10

100

1000

10000

100000

0 10 20 30 40 50

Prompt

Decay

Fit

METANILYELLOW

Log

(Cou

nts)

Time (ns)

-6

-4

-2

0

2

4

6

3 8 13 18 23

Residuals

Time (ns)Sta

ndar

d D

evia

tio

n

-4

-2

0

2

4

3 13 23 33

Residuals

Time (ns)

Sta

ndar

d D

evia

tion

Figure 4.6.a Fluorescence decay profile and residuals of oil red o dye

Figure 4.6.b Fluorescence decay profile and residuals of metanil yellow

dye

Table 4.3 Spectral parameters of azo dyes in acetonitrile

Dye

Absorption spectra Fluorescence spectra Average

life time (ns)

Peak wave-length

nm

(λ) 104 L mol-1 cm-1

()1/2 cm-1

Oscillator Strength f x 10-24 L mol-1

cm-2

Peak wave-length

nm

FWHM nm


Oil red O 362, 523 6.40 3900 1.10 556 35 1120 1.00 Metanil- yellow

352, 521 11.05 4900 2.34 589 58 2136 3.00

1

10

100

1000

10000

100000

0 10 20 30 40 50 60

Prompt

Decay

Fit

OIL RED O

Log (C

ounts

)

Time (ns)

105


The absorption spectra of azo dyes are shown in Figure 4.4. The

absorption maxima in nm are shown in Table 4.3. The absorption wavelength

maxima decrease in the order of oil red O > metanil yellow. In each spectrum

there are two absorption maxima, the longer wavelength absorption maxima

is due to the excitation of electron in azo group (π -π* electronic transition).

The shorter wavelength, absorption maxima are due to electronic excitation in

the aromatic ring. As the π -π* is an allowed transition, the (λ) values for

both the dyes are very high.

The fluorescence spectra of azo dyes are shown in Figure 4.5. The

fluorescence maxima of oil red O, occurs at shorter wavelength than metanil

yellow dye. In metanil yellow the SO3Na grouping is more solvated than

methyl substituents in oil red O. Hence, there will be much enhanced

vibrational cascade in metanil yellow than in oil red O. As a result the

fluorescence maxima of metanil yellow lies at longer wavelength than in oil

red O. It is also reflected in the fluorescence lifetime values of the dyes.

Metanil yellow slowly relaxes and emits fluorescence, whereas, oil red O

relaxes very slowly than metanil yellow and hence, rapidly fluoresce. So the

lifetime of the oil red O is less than metanil yellow.

4.4 SPECTRAL CHARACTERISTICS OF THIAZIN DYES

The thiazin dyes (Azure I, Azure II and Toludine Blue O) are

obtained from CDH , India. The molecular structures of these dyes were

given in Figure 2.1. (c) - (i-iii).Thin layer chromatography (TLC) test

confirms the absence of any impurities. These dyes were used as received.

Acetonitrile was chosen as solvent, since these dyes were readily soluble in

it.

106

4.4.1 Absorption and Fluorescence Spectra of Thiazin Dyes

The absorption spectra of thiazin dyes in acetonitrile for 0.01mM

concentration are recorded and shown in Figure 4.7.The emission spectra of

the thiazin dyes in acetonitrile for 0.01mM concentration are recorded and

shown in Figure 4.8.The peak wavelengths of the absorption and fluorescence

spectra are measured and the spectral parameters are calculated and shown in

Table 4.4.


The fluorescence decay profile and the residuals of the dyes azure I,

azure II and toludine blue O in solvent acetonitrile are shown in Figure 4.9

a-c. The residuals shown in the Figure are well within the error limits. The

average life time values of these dyes are shown in Table 4.4. All the dyes

exhibit a bi-exponential decay.

Table 4.4 Spectral parameters of thiazin dyes in acetonitrile

Dye


life time (ns)

Peak wavelength

nm

(λ) 104 L

mol-1 cm-1

()1/2 cm-1


cm-2

Peak wavelength

nm

FWHM nm


Azure I 638 1.25 1999 0.11 685 62 1074 3.28

Azure II 654 1.06 1074 0.05 706 62 1125 1.70

Toluidine Blue O

624 0.81 1852 0.07 680 40 1294 1.20

107

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

400 500 600 700 800

Azure I

Azure II

Abs

orba

nce

Wavelength (nm)

0

0.2

0.4

0.6

0.8

350 450 550 650 750

Toludine Blue O

Abs

orba

nce

Wavelength (nm)

Figure 4.7 Absorption spectra of thiazin dyes in acetonitrile

0

200000

400000

600000

800000

1000000

640 690 740 790

Azure I

Azure II

Wavelength (nm)

Fluo

resc

enc

e In

tens

ity

0

20

40

60

80

600 650 700 750 800 850 900

Wavelength (nm)

Flu

ore

sce

nc

e In

ten

sity

Toludine Blue O

Figure 4.8 Fluorescence spectra of thiazin dyes in acetonitrile

108

-3

-2

-1

0

1

2

3

3 8 13 18

Residuals

Time (ns)Sta

nda

rd D

evia

tion

-3

-2

-1

0

1

2

3

3 8 13 18 23 28

Residuals

Time (ns)

Standar

d Dev

iation

Time (ns)

-4

-3

-2

-1

0

1

2

3

4

4 8 12 16 20

Residuals

Time (ns)

Sta

nda

rd D

evi

ati

on

1

10

100

1000

10000

0 10 20 30 40 50

Prompt

Decay

Fit

AZURE I

Time (ns)

Log

(Coun

ts)

Figure 4.9. a Fluorescence decay profile and residuals of azure I dye

1

10

100

1000

10000

0 10 20 30 40 50

Prompt

Decay

Fit

AZURE II

Time (ns)

Log

(Cou

nts)

Figure 4.9.b Fluorescence decay profile and residuals of azure II dye

1

10

100

1000

10000

0 10 20 30 40 50

Prompt

Decay

Fit

Touldine Blue O

Time (ns)

Log

(Cou

nts)

Figure 4.9.c Fluorescence decay profile and residuals of toludine blue O

dye

109


The absorption spectra of thiazin dyes are shown in Figure 4.7. The

absorption maxima in nm are shown in Table 4.4. The absorption wavelength

maxima decrease in the order of azure II > azure I > Toludine blue O. The

absorption maximum of azure I is less than azure II. The difference between

azure I and azure II is the difference in the charge balancing ions. In azure I,

BF4- and in azure II Cl- are the charge balancing ion. As BF4

- is bulkier than

Cl- the BF4- cannot stabilize the quinonoid structure and hence, the absorption

maxima of azure I is less than azure II. The absorption maxima of toluidine

blue O is less than azure II. As the difference between azure II and toluidine

blue O is the replacement of one methyl group in azure II by hydrogen in

toluidine blue O. But the ring carries the methyl substituent.The fluorescence

lifetime of thiazin dyes illustrates a decrease from azure I > azure II >

toluidine blue O. The difference in the lifetime of thiazin dyes is attributed to

the difference in the charge balancing ions.

4.5 SPECTRAL CHARACTERISTICS OF

SULPHONAPHTHALEIN DYES

The sulphonaphthalein dyes (thymol blue and xylenol orange) are

obtained from loba chemie and fischer chemic, India. The molecular

structures of these dyes were given in Figure 2.1. (d) - ( i-ii).Thin layer

chromatography (TLC) test confirms the absence of any impurities. These

dyes were used as received. Ethanol was chosen as solvent since the dyes

were readily soluble in it.

110

4.5.1 Absorption and Fluorescence Spectra of Sulphonaphthalein

Dyes

The absorption spectra of sulphonaphthalein dyes in ethanol for

0.01mM concentration are recorded and shown in Figure 4.10.The emission

spectra of the sulphonaphthalein dyes in ethanol for 0.01mM concentration

are recorded and shown in Figure 4.11.The peak wavelengths of the

absorption and fluorescence spectra are measured and the spectral parameters

are calculated and shown in Table 4.5.

4.5.2 Fluorescence lifetime measurements

The fluorescence decay profile and the residuals of the dyes thymol

blue and xylenol orange in solvent ethanol are shown in Figure 4.12 a-b.

The residuals shown in the Figure are well within the error limits. The

average life time values of these dyes are shown in Table 4.5. Both the

dyes exhibit bi-exponential decay.

Table 4.5 Spectral parameters of sulphonaphthalein dyes in ethanol

Dye


life time (ns)

Peak wave-length

nm

(λ) 104 L mol-1 cm-1

()1/2 cm-1


cm-2

Peak wavelength

nm

FWHM nm


Thymol Blue 505 6.13 4014 1.06 573.5 39 3035 4.00

Xylenol Orange 521 5.89 1641 0.42 605 84 2661 1.75

111

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

350 450 550 650 750

Thymol Blue

Xylenol Orange

Wavelength (nm)

Ab

sorb

an

ce

Figure 4.10 Absorption spectra of sulphonaphthalein dyes in ethanol

0

40000

80000

120000

160000

200000

550 600 650 700 750 800Wavelength(nm)

Fluo

resc

ence

Int

ensi

ty

Xylenol Orange

0

1

2

3

4

5

6

7

540 560 580 600 620 640

Wavelength (nm)

Fluo

resc

enc

e In

ten

sity

Thymol Blue

Figure 4.11 Fluorescence spectra of sulphonaphthalein dye in ethanol

112

-4

-2

0

2

4

3 13 23 33

Residuals

Time (ns)

Stan

dard

Devi

atio

n

-4

-2

0

2

4

3 8 13 18 23

Residuals

Time (ns)

Sta

nda

rd D

evi

ation

1

10

100

1000

10000

0 10 20 30 40 50

Prompt

Decay

Fit

Time (ns)

Log

(Cou

nts)

THYMOL BLUE

Figure 4.12.a Fluorescence decay profile and residuals of thymol blue

dye

1

10

100

1000

10000

0 10 20 30 40 50

Prompt

Decay

Fit

XYLENOL ORANGE

Log

(Cou

nts)

Time (ns)

Figure 4.12.b Fluorescence decay profile and residuals of xylenol

orange dye


The absorption spectra of sulphonaphthalein dyes are shown in

Figure 4.10. The absorption maxima in nm are shown in Table 4.5. The

absorption wavelength maxima decrease in the order of xylenol orange >

thymol blue. The absorption spectra of thymol blue and xylenol orange

indicates the quinonoid structure, which is due to phenolic hydroxyl group. In

the quinonoid structure the SO3 will be free and cannot be bounded to

tetrahedral carbon. The average lifetime of thymol blue is higher than that of

xylenol orange. It is attributed to direct binding of bulky iso propyl grouping

113

to the aromatic ring in thymol blue. In xylenol orange the iso propyl group is

replaced by bulky substituents, via CH2 grouping. The bulky substituents

cannot much influence on the vibrational cascade of the base nucleus as it is

separated by CH2 grouping.

4.6 SPECTRAL CHARACTERISTICS OF NATURAL DYE

The natural dye (carmine) was obtained from CDH, India. The

molecular structure of the dye was given in Figure 2.1. (e). - ( i). Thin layer

chromatography (TLC) test confirms the absence of any impurities. The dye

was used as received. Methanol was chosen as solvent since the dye was

readily soluble in it .

4.6.1 Absorption and Fluorescence Spectra of Natural Dye

The absorption spectra of carmine dye in methanol for 0.01mM

concentration was recorded and shown in Figure 4.13. The emission spectra

of the carmine dye in methanol for 0.01mM concentration was recorded and

shown in Figure 4.14.The peak wavelength of the absorption and fluorescence

spectra are measured and the spectral parameters are calculated and shown in

Table 4.6.

4.6.2 Fluorescence Lifetime Measurement

The fluorescence decay profile and the residuals of the dye carmine

in solvent methanol are shown in Figure 4.15.The residuals shown in the

Figure are well within the error limits. The average life time value for the

dye is given in Table 4.6. Carmine dye exhibit a bi-exponential decay.

114

-5

-3

-1

1

3

5

3 13 23

Residuals

Time (ns)

Sta

nd

ard

Dev

iati

on

0

0.2

0.4

0.6

0.8

1

1.2

300 400 500 600 700 800 900

Wavelength (nm)

Abs

orba

nce

Figure 4.13 Absorption spectra of carmine dye in methanol

0

20000

40000

60000

80000

100000

120000

140000

500 550 600 650 700 750 800

Wavelength (nm)

Fluo

resc

ence

Inte

nsit

y

Figure 4.14 Fluorescence spectra of carmine dye in methanol

1

10

100

1000

10000

0 10 20 30 40 50

PromptDecayFit

CARMINE

Log

(Cou

nts)

Figure 4.15 Fluorescence decay profile and residuals of carmine dye

115

Table 4.6 Spectral parameters of carmine dye in methanol

Dye

Absorption spectra Fluorescence spectra Average life

time (ns)

Peak wavelength

nm

(λ) 104 L

mol-1

cm-1

()1/2 cm-1


cm-2

Peak wavelength

nm

FWHM nm


Carmine 515 10.48 6000 2.72 582 89 2234 1.13


The absorption spectrum of the carmine dye is shown in Figure

4.13 The absorption maxima in nm are shown in Table 4.5. The carmine dye

has a absorption maximum at 515 nm, with the lifetime of 1.13 ns. As the

glucose is directly bound to anthraquinone nucleus, its lifetime is short.

4.7 SPECTRAL CHARACTERISTICS OF FLUORONE DYE

The fluorine dye (mercurochrome) was obtained from S.D fine

chem, India. The molecular structure of the dye was given in Figure 2.1. (f)

- ( i).Thin layer chromatography (TLC) test confirms the absence of any

impurities. The dye was used as received. Methanol was chosen as solvent

since the dye was readily soluble in it.

4.7.1 Absorption And Fluorescence Spectra of Fluorone Dye

The absorption spectra of mercurochrome dye in methanol for

0.01mM concentration are recorded and shown in Figure 4.16.The emission

spectra of the mercurochrome dye in methanol for 0.01mM concentration

was recorded and shown in Figure 4.17.The peak wavelength of the

absorption and fluorescence spectra are measured and the spectral parameters

are calculated and shown in Table 4.7.

116

-5

-3

-1

1

3

5

2 12 22 32

Residuals

Time (ns)

Stan

dard

Dev

iation

Time (ns)

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

300 400 500 600 700 800Wavelength (nm)

Abs

orb

anc

e

Figure 4.16 Absorption spectra of mercurochrome dye in methanol

0

100000

200000

300000

400000

500000

600000

550 600 650 700 750 800Wavelength (nm)

Flu

ore

sce

nt

Inte

nit

y

Figure 4.17 Fluorescence spectra of mercurochrome dye in methanol

1

10

100

1000

10000

0 10 20 30 40 50

Prompt

Decay

Fit

MERCUROCHROME

Time (ns)

Log

(Cou

nts)

Figure 4.18 Fluorescence decay profile and residuals of mercurochrome

dye

117

4.7.2 Fluorescence Lifetime Measurement

The fluorescence decay profile and the residuals of the dye

mercurochrome in solvent methanol are shown in Figure 4.18. The

residuals shown in the Figure are well within the error limits. The average

life time value of the dye is shown in Table 4.7. The dye exhibits a bi-

exponential decay.

Table 4.7 Spectral parameters of mercurochrome dye in methanol

Dye


life time (ns)

Peak wave-length

nm

(λ) 104 L mol-1 cm-1

()1/2 cm-1


cm-2

Peak wavelength

nm

FWHM nm


Mercuro-chrome

516 5.94 2320 0.60 773 20 6433 1.40


The absorption spectrum of mercurochrome dye is shown in Figure

4.16. The mercurochrome dye gives a wavelength maximum at 516 nm, with

the lifetime of 1.40 ns. It also has a less lifetime similar to carmine because of

direct attachment of benzoic acid grouping attached to the main nucleus.

4.8 CONCLUSION

The spectral characteristics of the dyes were studied and the

spectral parameters were calculated. It is observed that all the dyes studied

except azo dyes, have a single band absorption and that their fluorescence

intensity profiles though sharp are weak in intensity. The azo dye molecules

studied displayed two absorption bands in the range 300nm to 650nm.

118

The fluorescence profile for anthraquinone dyes and thiazin dyes

shows a red shift which can be attributed to intermolecular charge transfer

(ICT) which occurs when these molecules are excited. Internal conversion is a

mechanism that is mostly responsible for the low fluorescence efficiency in

organic dyes. It is a direct nonradiative decay of the lower excited state singlet

state S1, to the ground state S0. As this mechanism results in heat transfer ,

which may facilitate thermal nonlinearity.

From the above studies it is to be concluded that electron releasing

substituents, bounded to the main skeleton of the dye, shift the wavelength

maxima to a longer value. But the electron withdrawing substituents, shift the

wavelength maxima to a shorter value. The bonding of the bulky substituents

and readily solvatable substituents to the main nucleus increases the lifetime.

chapter 4 spectral characteristics of organic...

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