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Pyrimidine Photophysics in Solution
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The Ohio State University Department of Chemistry
Femtosecond Mid-infrared Spectroscopy Of Pyrimidine DNA Bases: Intramolecular Vibrational Redistribution, Vibrational Cooling And Excited State Photophysics
Chris T. Middleton, Patrick M. Hare†, Kristin I. Mertel and Bern Kohler
† Current address: University of Notre Dame Radiation Laboratory, Notre Dame, Indiana, 46556
Nucleic acid bases
UV initiated photochemistry can lead to mutations and skin cancer
(photocarcinogenesis)
Pyrimidine Photophysics in Solution
Hare, et al., JPCB 110 18641 (2006)
Acetonitrile H2O
UV/Visible Transient Absorption of 1CHU
NH
O
ON
1-cyclohexyluracil
(1CHU)
- both the lifetime of 1nπ* state and the triplet quantum yield are dramatically reduced in protic solvents
- broad electronic absorption spectra of 1nπ* and triplet strongly overlap
Transient mid-IR Spectra of 1CHU
*
Kinetics of 1nπ* band
a Hare, et al., JPCB 110 18641 (2006)
- solvent dependence of lifetime confirms assignment to 1nπ*
- no ground state modes between 1750 cm-1 and ~3000 cm-1
- excited state vibrational spectra provide information about the electronic structure, gauge of theoretical calculations
Thymine Triplet Dynamics
excellent agreement between short and long time spectra
confirms that triplet is formed rapidly (< 10 ps)
vibrationalvibrationalcoolingcooling
Vibrational cooling
NH
NH
O
O
T ≈ 2000 K
N
NN
N
NH2
Me
T ≈ 1800 K
Solvent Isotope Effects on Vibrational Cooling
-1.5
-1.0
-0.5
0.0
A
/ 10
-3
6050403020100
Time Delay / ps
Thymine (250 nm) acetonitrile acetonitrile-d3
-3
-2
-1
0
A /
10-3
6050403020100
Time Delay / ps
9-methyladenine (250 nm) acetonitrile acetonitrile-d3
-1.00
-0.75
-0.50
-0.25
0.00
Nor
mal
ized
A
1614121086420-2Time Delay / ps
9-methyladenine (255 nm) H2O D2O
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
Nor
mal
ized
A
12080400
Time Delay / ps
Thymine (1684 1/cm)AcetonitrileAcetonitrile-d3
Comparison of UV and IR Probing of Vibrational Cooling
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0N
orm
aliz
ed
A
100806040200
Time Delay / ps
thymine / acetonitrile 250 nm 1684 1/cm
NH
NH
O
O
O
HOH
HHHH
HO
N
NH
O
O
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
Nor
mal
ized
A
6050403020100
Time Delay / ps
thymidine / acetonitrile 250 nm 1694 1/cm
Conclusions
- 1nπ* state of 1CHU characterized by a unusually blue-shifted vibrational band
- Triplet band lies at 1600 cm-1 and appears within 10 ps, providing further support for the current model of ISC in pyrimidine bases
- Solvent isotope effects for vibrational cooling suggest the energy transfer between high frequency solute and solvent modes is important
- Differences between vibrational cooling rate measured by UV and mid-IR probing suggests that IVR may take place significantly slower than expected
The Ohio State University Department of Chemistry
AcknowledgementsBern Kohler
Patrick Hare – UV/visible measurements on 1nπ* and triplet, ns-TRIR
Kristin Mertel – ns-TRIR
Terry Gustafson, Matt Platz
Gotard Burdzinski, Jacek Kubicki
Kohler Group & Gustafson Group
Funding: NIH
Instrumentation: