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

The Ohio State University Department of Chemistry

TRANSIENT ABSORPTION

1.2

0.8

0.4

0.0300230

255 nm 285 nm

Ground State Absorption of 9MA:

The Ohio State University Department of Chemistry

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

The Ohio State University Department of Chemistry

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

The Ohio State University Department of Chemistry

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

The Ohio State University Department of Chemistry

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