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Double proton transfer phenomenon UV light 300 nm Taylor, El-Bayoumi and Kasha, PNAS, 63 (1969) 253
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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