non-radiative decay pathways of the first excited electronic states of 1:1 hydrogen bonded complexes...
TRANSCRIPT
Non-radiative decay pathways of the first excited electronic states of 1:1 hydrogen bonded complexes of 7-azaindole with phenol and
formamide
IACS
Moitrayee Mukherjee, Anamika Mukhopadhyay, Tapas Chakraborty
Physical Chemistry DepartmentIndian Association for the Cultivation of ScienceJadavpur, Kolkata 700032, West Bengal, India
In doubly hydrogen-bonded dimeric structure, tautomerization is the primary non-radiative decay channel in the excited electronic state
300 350 400 450 500 550
0.0
3.0x106
6.0x106
Wavelength(nm)
Flu
ores
cen
ce
HN N
H
NN
N N
HH
NN
7-azaindole: A prototype of DNA bases from photophysical viewpoints
H
NN
AcceptorDonor
Electronic excitation results in reorganization of charge distribution leading to enhanced acidity of pyrrolic
N–H group and basicity of the Pyridinic nitrogen
En
ergy
Reaction Co-ordinate
Normal form
Tautomeric form
NNH
OO
H
C
R
NNH
OH
R
NH
N
OO
H
C
R
O
NN
HH
R
Catalytic tautomerzation in complexes with alcohols and carboxylic acid
Tautomerization by carboxylic acid catalyst is very efficient, and the dimer shows no local emission following UV excitation in hydrocarbon liquids.
In alcohols, tautomerization is slow, exhibits primary kinetic isotope effects as well as a classical Arrhenius type activation barrier that has been associated to formation of specific cyclic structure with an alcohol molecule.
Goal of our studies
HN N
ONH
H C
H
N N
H
N
H
OH
H C
Photophysics of the mixed dimer between 7-azaindole and formamide, particularly the following tautomeric conversion in the excited state
The specific question that we have addressed is whether the relaxation of the locally excited state to a tautomeric state could still be the major non-radiative decay pathway.
Significance: Here the tautomerization of 7AI leads to a simultaneous tautomerization of FM, and the system mimics the base pairs more realistically.
En
ergy
Reaction Co-ordinate
10 kcal/mol
FM
Energetics of tautomeric conversion of 7AI---FM mixed dimer
En
ergy
Reaction Co-ordinate
6.5 kcal/mol(7AI)2
In case of (7AI)2, electronic excitationis delocalized over the entire dimer,and tautomerization in S1 is exothermic
En
ergy
Reaction Co-ordinate
7AI-FM complex
?
Excitation energy is localized only on 7 AI moiety. The question is, whether the tautomerization in S1 of the mixed dimer could effectively be an exothermic process
A 1:1 mixed dimeric complex is synthesized in a supersonic jet expansion
and the laser-induced fluorescence excitation and dispersed fluorescence
spectra following excitation to several single vibronic levels in S1 are
measured.
Photophysical studies, i.e., absorption, emission and fluorescence
excitation spectra are also performed by dissolving the two solutes in
hydrocarbon solvents, and the spectra were recorded both at room
temperature and 77 K hydrocarbon glass.
Measurement schemes:
158 16
4
000
2834
55
102
178
17
0
(32335 cm-1)
32300 32350 32400 32450 32500 32550
3226
8 cm
-1
000
3230
8 cm
-1
000
119
98 218
23832
268
cm-1
FE Spectrum of (7-AI)2 probing visible fluorescence
Wavenumber(cm-1)
Inte
nsi
ty
FE Spectrum of 7AI-FM
FE Spectrum of (7-AI)2
Beyond the displayed energy range the spectrum shows no distinct vibronic feature
7AI-FM complex absorbs in the same spectral region of the 7AI homodimer
LIF of 7AI-FM complex in a Supersonic Jet Expansion
3233
5 cm
-1
7AI-
FM
S0
S134
631
cm-1
7AI
2296
cm-1
No vibronic feature appears in the spectrum when a UV cutoff/visible pass filter is placed in front of the PMT detector
DF spectrum of 7AI…FM; band
DF spectrum of 7AI monomer; band
000
2847
5686
114
148
176
195 23
4 294
440
624
710
771
799
857
906
915
000
434
554
628
759
832
0 200 400 600 800 1000
Wavenumber(cm-1)
0ex 00
0ex 00
LIF of 7AI-FM complex in a Supersonic Jet Expansion
28
86 148
771
0 47 86 148
294
624
771
919
2828
86
148
771
799
Wavenumber(cm-1)200 400 600 800 10000
Analysis of the vibrational features of both the spectra reveals that emission originates from the level excited, and no relaxed fluorescence is detected
The absence of visible fluorescence confirm that the excited state tautomerization does not occur on excitation to S1 origin region of the mixed dimer in a cold supersonic jet expansion
cm-1
band
0ex 00 34
0ex 00 102
LIF of 7AI-FM complex in a Supersonic Jet ExpansionDispersed Fluorescence Spectra
cm-1
band
The spectral change signify either a general solvation shift, or formation of the 1:1 complex in the ground state
400350300250
0.0
0.05
0.1
0.15
312nm
Ab
s.
Wavelength(nm)
7AI(10-5M)
7AI(10-5M) +FM(10-3M) 7AI(10-5M)
7AI(10-5M) +FM(10-3M)
7AI(10-5M) +FM(510-3M)
ex=295nm
300 400 5000.
0
1.0107
2.0107
3.0107
4.0107
Wavelength(nm)
7AI-FM complex in a hydrocarbon solution at room temperature
The visible fluorescence is the distinct signature of 7-AI tautomerization by FM at room temperature
7AI(10-5M)
7AI(10-5M) +FM(10-2M)
7AI(10-5M) +FM(10-1M)
312nm
em=500nm
280 300 320 340
1.0
0.50
1.5
2.0
0.0
Wavelength(nm)
7AI-FM complex in a hydrocarbon solution at room temperature
Fluorescence excitation spectra for detection of tautomer fluorescence
280 300 340320Wavelength(nm)
1.0
0.0
Normalization shows that the detailed features of the pure and mixed dimer spectra appear different even in the solution phase
The first absorption band of the mixed dimer appears at ~312 nm
7AI-FM heterodimer shows no local emission in ultraviolet indicating that tautomerization is very efficient
7AI-FM complex in a hydrocarbon solution at room temperature
ex=312nm
350 400 450 500 550
0.0
1.0
2.0
3.0
4.0
5.0
Wavelength(nm)
7AI(10-5M)
7AI(10-5M) +FM(10-2M)
7AI(10-5M) +FM(10-1M)
Some of the low frequency inter- and intramolecular vibrational modes significant for the tautomerization process are populated to higher levels at room temperature, which assist in crossing the energy barrier.
Thermal motion of the solvent molecules allows orientational relaxation, internal rotation between two dimer moieties and quick exchange between the normal and tautomeric forms of FM in the complex.
HN N
NH
H C
H
O
N N
H H
N HC
H
O
In room temperature liquids a small quantity of the tautomeric form of FM exists in equilibrium with the normal form, and such distribution could promote the tautomeric conversion.
Origin of photophysical distinctions in cold jet and hydrocarbon soutions
Photophysics of 7AI-FM complex in a rigid hydrocarbon glass at 77K
7AI(10-5M)(300K)
7AI(10-5M) +FM(5×10-3M) (300K)
7AI(10-5M)(77K)
7AI(10-5M) +FM(5×10-3M) (77K)
ex=295nm
In the cold rigid matrix the complex shows only local UV fluorescence
Rigidity of the medium inhibits tautomerization
300 350 400 450 500
0.0
2.0
4.0
6.0
8.0
10.0ex=312nm
0.0
2.0
4.0
6.0
8.0
10.0
300 350 400 450 500Wavelength(nm)Wavelength(nm)
Summary:
Photophysical studies of the 1:1 complex of 7AI and FM are performed in Supersonic jet expansion, in hydrocarbon solution at room temperature and in a hydrocarbon glass at 77K.
In hydrocarbon solution tautomerization has been found be the dominant nonradiative decay channel of the complex from the locally excited state (S1). However, the process does not occur in the cold jet expansion on excitation to S1 origin region of the 7AI moiety of the complex. On the other hand, the process shows restricted behavior in the cold hydrocarbon glass, and the complex shows red-shifted local fluorescence.
The observations indicate that no straightforward correlation of this type of a photophysical process occurring in a cold supersonic jet and liquids at room temperature can be established .
Acknowledgement
Co-worker: Dr. Montu K HazraMr. Amit K SamantaMr. Prasenjit Pandey
Mr. Biman Bandyopadhyay
Financial Support: Department of Science and Technology, Government of India
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