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The Ohio State University Department of Chemistry
Ultrafast Vibrational Cooling Dynamics in 9 Methyladenine Observed with UV Pump/UV Probe Transient Absorption Spectroscopy
Chris Middleton, Boiko Cohen and Bern Kohler
The Ohio State University Department of Chemistry
- The bases in DNA and RNA absorb damaging UV light
- UV initiated photochemistry can lead to skin cancer
NHN
NN
NH2
Adenine Guanine
NHN
NNH
O
NH2NH
NH
O
O
CH3
Thymine
NH
N
O
NH2
Cytosine
20
15
10
5
0
/
103
300280260240220
/ nm
adenine cytosine guanine thymine
- IC places large amount of energy into the vibrational modes of ground state (> 1000 K)
INTRODUCTION: DNA PHOTOPHYSICS
Intramolecular Vibrational Redistribution (IVR) - 100’s of femtoseconds
Vibrational Cooling (VC) - 2 – 20 picoseconds
VIBRATIONAL RELAXATION IN SOLUTION
1
1
T•Vibrational population decay times can be predicted by Landau-
Teller formula if the vibrational friction spectrum () is well-known
•Many mechanism for vibrational relaxation are available to polyatomic molecules
Deak et al. J. Raman Spec. 2000, 31, 263
•VC is well described by a two stage model in many systems
Visible probe monitors S1 population decay
Visible probe monitors S1 population decay
UV probe monitors S0 population recovery
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TRANSIENT ABSORPTION
1.2
0.8
0.4
0.0300230
255 nm 285 nm
Ground State Absorption of 9MA:
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3 0 0
3 5 00
24
1 0
2 0
3 0
4 0
5 0
/ nmTime / ps
Pecourt, et al. J. Am. Chem. Soc. 2000, 122, 9348
Pecourt, et al. J. Am. Chem. Soc. 2001, 131, 10370
- Dynamics observed in UV are much slower than those observed in the visible vibrational cooling?
- VC is very fast (0.4 – 2 ps) VC mediated by hydrogen bonds?
UV PROBE TA OF ADENOSINE IN H2O
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TRANSIENT ABSORPTION OF 9MA IN H2O
001 SSS VCIC k*k
-1.0
-0.5
0.0
0.5
1.0
No
rma
lize
d
OD
14121086420-2
Time Delay / ps
285 nm = 0.63 ± 0.10 ps
= 1.94 ± 0.16 ps
255 nm = 2.23 ± 0.16 ps
570 nm = 0.220 ± 0.03 ps
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SOLVENT EFFECT ON UV PROBE SIGNALS
•Large effect for aprotic vs. protic solvent (factor of 7)
•H/D Kinetic Isotope Effect (KIE) ~ 2
-1.0
-0.5
0.0
0.5
1.0
No
rma
lize
d
OD
403020100
Time Delay / ps
H2O = 2.23 ± 0.16 ps
D2O = 3.8 ± 0.2 ps
MeCN = 14.9 ± 1.9 ps
H2O = 1.94 ± 0.16 ps
D2O = 4.4 ± 0.5 ps
MeCN = 15.0 ± 0.7 ps
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Cohen, et al. J. Am. Chem. Soc. 2003, 125, 13594
Solvent 9MA S1 Lifetime
H2O 220 ± 20 fs
D2O 220 ± 20 fs
CH3CN 350 ± 20 fs
* Internal conversion rate is independent of solvent *
(LACK OF A) SOLVENT EFFECT ON S1 LIFETIME
560 nm probe
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MACROSCOPIC PROPERTIES
Solvent H2O D2O MeCN
Vibrational Cooling Time (ps) 2.23 3.8 14.9
Ratio to H2O Value 1 1.7 6.7
Heat Capacity (J/mol*K) 75.3 85.0 91.7
Ratio to H2O Value 1 1.128 1.217
Thermal Conductivity (W/m*K) 0.607 0.595 0.188
Ratio to H2O Value 1 1.02 3.23
•Lack of a strong correlation with macroscopic properties suggests a microscopic mechanism is important
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KINETIC ISOTOPE EFFECT – FRICTION EFFECT?
Vibrational friction spectrum from Chorny et al. J. Chem. Phys. 2002, 116, 8904
9MA frequencies from Xie and Yan Int. J. Quant. Chem. 2000, 76, 686
9MA-d0
9MA-d2
•Magnitude of vibration friction is relative unchanged for modes associated with hydrogen bonding amino group
•Large increase in D2O at 600 cm-1 – 1200 cm-1
•Importance of high frequency region suggest IP theory regime
•Vibrational population decay time is proportional to reduced mass
•Vibrational friction for amino group modes is the same in H2O and D2O
•Good agreement with KIE for 9MA = 1.72 ± 0.08
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KINETIC ISOTOPE EFFECT – HYDROGEN BONDING?
1
1
T
H
D
H
D
T
T
87.1NH
ND
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EFFECT OF INITIAL TEMPERATURE
9MA: 3N – 6 = 48 TIVR ~ 1800 K
AMP: 3N – 6 = 81 TIVR ~ 1200 K
9MA KIE = 1.72 ± 0.08 AMP KIE = 1.4 ± 0.1
-1.0
-0.5
0.0
0.5
No
rma
lize
d
OD
14121086420-2
Time Delay / ps
AMP / H2O = 1.77 ± 0.19 ps
9MA / H2O = 2.23 ± 0.17 ps
AMP / D2O = 2.5 ± 0.3 ps
9MA / D2O = 3.8 ± 0.2 ps
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CONCLUSIONS
•Long dynamics observed in the UV are assigned to vibrational cooling of the hot ground state
•Large acceleration VC rate in protic solvents suggests the hydrogen bonds strongly mediate VC
•A KIE isotope effect for VC has been observed with two possible mechanisms
QUESTIONS RAISED
•Solute-solvent vs. solvent-solvent hydrogens bonds?
•Importance of lower frequency vs. higher frequency modes?
The Ohio State University Department of Chemistry
Dr. Bern Kohler
Dr. Boiko Cohen (Wasielewski Group, Northwestern University)
Kohler Group
Gustafson Group
NIH
International Symposium on Molecular Spectroscopy
ACKNOWLEDGEMENTS