Two-proton Phototautomerization Dynamics of 7-Azaindole Complexes

-Tapas Chakraborty

Indian Association for the Cultivation of Science Calcutta, India

TK02, OSU ICMS 2013

Tautomeric forms of 7-azaindole Tautomeric forms of 7-azaindole

N N

H

N N

H

S0

S1

4.2

eV

2.8

eV

Normal Tautomer

0.65 eV (~ 15 kcal/mol )

Double proton transfer phenomenon Double proton transfer phenomenon

UV light

300 nm

Taylor, El-Bayoumi and Kasha, PNAS, 63 (1969) 253

Taylor, El-Bayoumi and Kasha, PNAS, 63 (1969) 253UV light

< 300 nm

DPT 1.1 ps

260 280 300 320

Wavelength (nm)

N N

H

N N

NN

HH

~470 nm

Tautomer emission

Fluorescence spectra

Wavelength (nm)300 350 400 450 500 550

~320

nm

>350

nm

Normal emission

Absorption spectra10-5 M

10-3 M

Double proton transfer phenomenon Double proton transfer phenomenon

UV light

< 300 nm

DPT 1.1 ps

Key Issues

•Double Proton Transfer sequence :Concerted vs Stepwise

•Role of exciton interactions

•Medium effect

•Double Proton Transfer Barrier (1.5 kcal/mol)

Double proton transfer phenomenon Double proton transfer phenomenon

UV light

< 300 nm

DPT 1.1 ps

Key Issues

•Double Proton Transfer sequence :Concerted vs Stepwise

•Role of exciton interactions

•Medium effect

•Double Proton Transfer Barrier (1.5 kcal/mol)

Double proton transfer phenomenon Double proton transfer phenomenon

Double proton transfer barrier (original expt.) Double proton transfer barrier (original expt.)

Estimated barrier: 1.4 kcal/mol

F2

F1

N N

H

Tautomerization under jet-cooling condition (Tautomerization under jet-cooling condition (10 K) 10 K)

(7-azaindole)(7-azaindole)22 is synthesized in Ar matrix is synthesized in Ar matrix

Inert gas (Ne, Ar, N2, etc.) crystals pack in Face-Centered cubic lattice structures

Pimentel (1922-1989)

(7-azaindole)(7-azaindole)22 is synthesized in Ar matrix is synthesized in Ar matrix

3600 3400 3200 3000 2800 2600

Wavenumber (cm-1)

a

b

c

d

Dimer in Ar Matrix

Dimer in a supersonic jet expansion

Dimer in CCl4 solution

N N

H

(7-azaindole)(7-azaindole)22 formation in Ar matrix is probed using infrared spectroscopy formation in Ar matrix is probed using infrared spectroscopy

Mukherjee and Chakraborty, CPL 546 (2012) 74

Qe NORMAL

S0

S1

Qe TAUTOMER

Q

V(Q

)N N

NN

HH

NN

H

N N

H

N N

NN

HH

NN

H

N N

H

X

DPT of (7-azaindole)DPT of (7-azaindole)22 is inhibited in Ar matrix is inhibited in Ar matrix

Mukherjee and Chakraborty, CPL 546 (2012) 74

300 400 500

Wavelength (nm)

Inte

nsity ex = 291.5 nm

ex = 310 nm

330

315

338 N N

NN

HH

NN

H

N N

HX

300 400 500

Annealed matrix

Inte

nsity

Wavelength (nm)

DPT of (7-azaindole)DPT of (7-azaindole)22 is inhibited in Ar matrix is inhibited in Ar matrix

3600 3400 3200 3000 2800 2600Wavenumber (cm-1)

d

N N

H

Pre-annealed matrix

7AICyclic amides

O N

NN

H

HC

Thymine Adenine

7-azaindole∙∙∙amide complexes7-azaindole∙∙∙amide complexes

O

NN

N

7-azaindole∙∙∙7-azaindole∙∙∙δδ-valerolactum complex-valerolactum complex

(7AIVL)Normal (7AIVL)Tautomer

S0

S1 6.0 kcal/mol

18.5 kcal/mol

~67

.0 k

cal/m

ol

91.7

kca

l/mol

(7AI2)Normal (7AI2)Tautomer

S0

S1 10.0 kcal/mol

14.5 kcal/mol67

.5 k

cal/m

ol

92.2

kca

l/mol

7AI-valerolactam (VL)

Normal form Tautomeric form

CIS/6-311++G**

Wavelength (nm)

ex = 312 nm

10-5M 7AI10-5M 7AI + 10-3 M VL

12

1

2

7AI+ VL

350 400 450 500 550

0 2000 4000 6000

50

100

150

200

250

Wavelength (nm)

1/[VL]1/

A

Ka = 1.64 103 M-1

7AI+VL

260 280 300 320 340

Efficient tautomeric conversion is observed in hydrocarbon solution at room temperature

Tautomerization of 7-azaindole∙∙∙Tautomerization of 7-azaindole∙∙∙δδ-valerolactum complex-valerolactum complex

Mukherjee et al, JPCA, 2012

Wavelength (nm)

7AI+VL

260 280 300 320 340

32200 3240032100 32300Wavenumber (cm-1)

22.5

69 76

114.

5

149

137

153

171

prob = total fluorescence

45 92 99

145

000

Wavelength (nm)311.5 310.6 309.6 308.6

Ene

rgy

Reaction Co-ordinate

v = 0v =1v = 2

v = n

v = 0v = 1v = 2

v = n

S0

S1

X

X

Tautomerization of the complex is inhibited under a jet-cooling conditionTautomerization of the complex is inhibited under a jet-cooling condition

200 300 400 500 600

0

50

100

Wavelength (nm)

T(%

)

22.5

45

114.

5

149

69 76

prob = 300 – 400 nm (local emission)000

32200 3240032100 32300Wavenumber (cm-1)

22.5

69 76

114.

5

149

137

153

171

prob = total fluorescence

45 92 99

145

000

prob = 400 – 750 nm (tautomer emission)

Wavelength (nm)

T(%

)

No tautomeric conversion in jet cooled condition

3205

4 cm

-1

S0

S1

Mukherjee et al, JPCA, 2012

200 300 400 500 600

0

40

80

Wavelength (nm)

UV32+ U330

200 400 600 800

0

40

80

120

Wavelength (nm)

UV+ IR cut filter

Tautomerization of the complex is inhibited under a jet-cooling conditionTautomerization of the complex is inhibited under a jet-cooling condition

Photophysics of 7-azaindolePhotophysics of 7-azaindolePyrazole complexPyrazole complex

S0

S1

S1

S08.2 kcal/mol

13.4 kcal/mol

99.6 kcal/mol

78.0 kcal.mol

16 kcal/mol

Karmakar and Chakraborty, CPL, 2013

Tautomerization of 7-azaindoleTautomerization of 7-azaindolePyrazole complex is inhibited in a supersonic jetPyrazole complex is inhibited in a supersonic jet

Karmakar and Chakraborty, CPL, 2013

Complex vs homodimer Complex vs homodimer

7-azaindole7-azaindolePyrazole complex Pyrazole complex

7-azaindole7-azaindolecyclic amide complex cyclic amide complex

Concluding remarks Concluding remarks

• Excited state tautomerization of 7-azaindole dimer is easily affected by external perturbations

• Compared to homodimer, the tautomerization barriers of mixed dimeric complexes appear much larger, and further studies are essential to understand the origin.

Acknowledgement:

Co-workers:

Dr. Moitrayee Mukherjee

Ms. Shreetama Karmakar

Funding:

Department of Science and Technology, Govt. of India