Fluorescence Lifetime

Media

5.0. Introduction

5.2.4. Fluorescence Lifetime

Fluorescence Lifetime 151

deprotonation occurs in the excited state irrespective of w0 value, whereas the charged photoacid sulfonato derivatives of 2-naphthol are localized around the center of the water pool and show significant proton transfer activity.^391,392 In the previous section, we have seen that in DMSO, HPBI in presence of β-CD and nitrogen substituted analogues even in absence of β-CD form anion even in the ground state. Since the excitation spectra give a better picture, the excitation spectra were recorded for all three molecules monitored at both bands.

The fluorescence excitation spectra of HPBI (Figure 5.17) show quite differently from those of HPIP-b (Figure 5.18) and HPIP-c (Figure 5.19). The excitation spectra of HPBI monitored at the normal band show an increase in intensity (Figure 5.17a). This indicates that not only the increase in the nonradiative decay of the tautomer but also the rise in relative population of enol conformers is responsible for the enhancement in the normal band rather than increase in the nonradiative decay. But the spectra monitored at the tautomer band initially decreases till w0 = 12.4 (Figure 5.17b) then increases on increase in size of the nanopool following the similar trend as observed in the tautomer band of emission spectra of HPBI (Figure 5.15b). However, in case of HPIP-b and HPIP-c, same as emission spectra the intensities of the excitation spectra monitored at both normal and tautomer bands decrease.

While the excitation spectra monitored at normal band of HPIP-b reach saturation point at w0

= 12.4, the spectra monitored at tautomer band reach saturation point at w0 = 9.3. On the other hand, both excitation spectra of HPIP-c get saturated at w0 = 9.3. However the excitation spectra do not indicate the formation of any additional band due to deprotonation.

Thus, ruling out the existence of anionic species.

Fluorescence Lifetime 152

that is the trans-enol rotamer which is responsible for the normal band of HPBI in AOT reverse micelle at w0 = 0. On increasing the size of the water nanopool, the contribution from the tautomer of HPBI is not observed in the decay profile of the normal band. Instead apart from the trans-enol, a new species is observed in the biexponential decay of the normal band.

The new species has lifetime different from that of the tautomer and trans-enol and can be assigned to solvated enol.⁹⁵ At w₀ = 12.4, the solvated enol form has the major contribution of 53.2% with lifetime of 1.81 ns while the trans-enol form has a contribution of 46.8% with 1.14 ns lifetime. The decrease in lifetimes of both normal and tautomer bands is not so significant in HPBI.

Table 5.2. Fluorescence lifetime of HPBI, HPIP-b and HPIP-c in AOT reverse micelles monitored at normal and tautomer band maxima.

Normal Band Tautomer Band

wo

λλλλEm (nm) τ1

N (f1) τ2

N (f2) χ² λλλλEm (nm) τ1

T (f1) χ² HPBI

0 350 1.47 (90.8) 4.20 (9.2) 1.08 470 4.30 (100) 1.14

12.4 355 1.14 (46.8) 1.81 (53.2) 1.00 465 4.12 (100) 1.18 HPIP-b

0 355 1.10 (85.4) 2.20 (14.6) 1.00 500 4.84 (100) 1.25

3.1 355 0.95 (85.4) 1.97 (14.6) 1.01 500 3.80 (100) 1.17 6.2 355 0.80 (78.0) 1.65 (22.1) 1.00 500 3.19 (100) 1.16 9.3 355 0.70 (68.8) 1.44 (31.2) 1.03 500 2.91 (100) 1.26

HPIP-c

0 390 0.97 (17.5) 2.73 (82.5) 0.98 485 4.83 (100) 1.04

0.36 385 0.76 (23.9) 2.44 (76.1) 1.00 485 4.68 (100) 1.02 0.9 390 0.71 (29.2) 2.21 (70.8) 1.00 480 4.41 (100) 1.00 3.1 390 0.65 (48.3) 1.85 (51.8) 1.00 485 3.66 (100) 1.00 9.3 385 0.58 (69.0) 1.88 (31.0) 1.01 485 2.86 (100) 1.04 1λexc = 308 nm.

In the case of HPIP-b and HPIP-c, both the components in the normal emission have different lifetimes than that of the tautomer band even at w0 = 0. This shows that the normal band has contribution from two different excited species – one is the trans-enol and the other is the solvated open enol form. The presence of extra nitrogen might have lead to this solvated enol, where the fluorophores should have formed hydrogen bonding with surfactant.

The relative population between the trans-enol and the solvated open enol also changes to a great extent. The trans-enol has lifetime of 1.10 ns with contribution of 85.4 % in dry AOT while the solvated open enol form has 2.20 ns lifetime with contribution of 14.6 %. trans- Enol has lifetime of 0.27 ns in water (pH 7.0). The lifetimes of trans-enol in acetonitrile and ethanol are 0.65 and 0.76 ns, respectively.¹⁷⁴ At w₀ = 9.3, the lifetimes of trans-enol and solvated enol form are 0.70 ns and 1.44 ns, respectively. The relative contribution of trans- enol reduces to 68.8 % and that of solvated enol rises to 31.2%. trans-Enol of HPIP-c in dry

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Fluorescence Lifetime 153

AOT has lifetime of 2.73 ns with relative contribution of 82.5 % and the solvated enol form has 0.97 with contribution of 17.5 %. trans-Enol of HPIP-c has lifetime of 0.05 ns in water (pH 7.1). The lifetimes of trans-enol of HPIP-c in acetonitrile and methanol are 1.0 ns and 0.34 ns, respectively.¹⁷⁵ At w₀ = 9.3, the lifetimes of trans-enol and solvated enol form are 1.88 ns and 0.58 ns, respectively. The solvated enol form becomes a major contributor with relative contribution of 69.0 %. All these show the shifting of equilibrium to towards solvated enol.

S₀ S₁

N N

O H

H N *

N O H

H

*

h

ν

2

Keto Band

N N

O H

H

cis-Enol

N N

O H

H

Keto

h

ν

^'

Cyclic process of Four energy

levels Normal

Band

N N

H O H

*

N N

H O H

trans-Enol

h

ν

^'' h

ν

1

N N

O

H H Solvent *

h

ν

^'''

h

ν

3

N N

O

H H Solvent

Solvated open enol

S₀ S₁

N N

O H

H N *

N O H

H

*

h

ν

2

Keto Band

N N

O H

H

cis-Enol

N N

O H

H

Keto

h

ν

^'

Cyclic process of Four energy

levels S₀

S₁

S₀ S₁

N N

O H

H N *

N O H

H

*

h

ν

2

Keto Band

N N

O H

H

cis-Enol

N N

O H

H

Keto

h

ν

^'

Cyclic process of Four energy

levels

N N

O H

H N *

N O H

H

*

h

ν

2

Keto Band

N N

O H

H

cis-Enol

N N

O H

H

Keto

h

ν

^'

N N

O H

H N *

N O H

H

*

h

ν

2

Keto Band h

ν

2

Keto Band

N N

O H

H

cis-Enol

N N

O H

H

cis-Enol

N N

O H

H

Keto

N N

O H

H

Keto

h

ν

^' h

ν

^'

Cyclic process of Four energy

levels Normal

Band

N N

H O H

*

N N

H O H

trans-Enol

h

ν

^'' h

ν

1

N N

O

H H Solvent *

h

ν

^'''

h

ν

3

N N

O

H H Solvent

Solvated open enol

Normal Band

N N

H O H

*

N N

H O H

trans-Enol

h

ν

^'' h

ν

1

N N

H O H

*

N N

H O H

trans-Enol

N N

H O H

trans-Enol

h

ν

^'' h

ν

1

h

ν

^'' h

ν

^'' h

ν

1

h

ν

1 N

N O

H H Solvent *

h

ν

^'''

h

ν

3

N N

O

H H Solvent

Solvated open enol

N N

O

H H Solvent *

h

ν

^'''

h

ν

3

h

ν

^''' h

ν

^'''

h

ν

3

h

ν

3

N N

O

H H Solvent

Solvated open enol

N N

O

H H Solvent

Solvated open enol

Figure 5.20. Schematic diagram of the photophysical processes of HPBI occurring in AOT reverse micelle.

Based on the experimental results a scheme was formulated for the photophysical processes of the fluorophores occurring in AOT reverse micelle and is depicted in Figure 5.20. cis-Enol is the main component present in dry AOT with some population of trans-enol and small contribution from solvated enol (in pyridine nitrogen substituted analogues) and all are in equilibrium. Addition of water increases the formation of trans-enol and solvated enol from cis-enol. Initially, the molecules reside at the interfacial region between the polar head group and water nanopool. With further addition of water, formation of solvated enol conformers indicates that the molecules drift away from the interfacial region and reside between the interfacial region and the center of the bulk water nanopool. When the fluorophore molecules are photoexcited, the cis-enol conformers undergo the ESIPT process

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Introduction 154

and give the tautomer band while the trans-enol and open enol emit the normal band.