Solvent Effects on the Excited State Dynamics of 1-cyclohexyluracil

Patrick M. HareBern Kohler

The Ohio State University Department of Chemistry

100 West 18th AvenueColumbus, OH 43210

International Symposium on Molecular Spectroscopy 23 June 2005

Environment 9H-Adenine Thymine Uracil

Aqueous solution1 τ = 0.18 τ = 0.54a τ = 0.21a

Acetonitrile2 τ = 0.44

Gas phase3,4 τ = 1.0τ = 6.4

τ = >100b

Gas phase5τ = 22000

τ = 12000c

Gas phase6τ = 0.100

τ = 1.1

τ = 0.105

τ = 5.12

τ = 0.130

τ = 1.05

1 Crespo-Hernandez, et al. Chem. Rev. 2004, 104, 1977.2 Cohen, et al. J. Am. Chem. Soc. 2003, 125, 13594.3 Kang, et al. J. Chem. Phys. 2003, 118, 6717. 4 Kang, et al. J. Am. Chem. Soc. 2002, 124, 12958. 5 He, et al. J. Phys. Chem. A 2004, 108, 943. 6 Canuel, et al. J. Chem. Phys. 2005, 122, 074316

a ribonucleoside b assigned to 3Thy c Thy(H2O)1

Base S1 Lifetimes in ps

Introduction: Nucleic Acid Photophysics

ThymineUracil

Adenine

Cytosine

n

Introduction: Uracil Photophysics

S0

T0

- Highest triplet yield of the bases- ΦISC reported in solvents with

dielectric constants of 80 (H2O) – 25 (EtOH)

- 10-fold increase in ΦISC in less polar solvents

- μs lifetime quenched by O2, self-quenching

- Peaks at ~370 nmOur Tasks: Investigate the excited state

dynamics, especially in low polarity solvents and aprotic solvents

Determine the triplet yieldsInvestigate pump wavelength

dependence

The Triplet State in Uracil The Singlet State in Uracil

- ~200 fs lifetime in aqueous solution

- ESA peaks at ~600 nm

n

Gas phase / nonpolar solution

n

Aqueous Solution

Tools: Transient Absorption, 1-cyclohexyluracil

12

3NH

45

O

ON

6

O

OHOH

HH

HH

HO

NH

N

O

O

Uridine (Urd) 1-cyclohexyluracil (1CHU)

Pump266 nm

I.C.

S1

S0

SN

Probe (visible)

I.S.C.

T0

Probe (visible)

T1

Probe (UV)

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

A *

10-3

1086420-2

Time / ps

400 nm 500 nm 570 nm 610 nm 650 nm

Results: Transient Absorption

Acetonitrile

1Cohen, et al. Faraday Disc. 2004, 127, 137

Uridine in aqueous solution: τSinglet = 0.21 ± 0.03 ps1

τSinglet = 0.27 ps

τTherm. = 4.5 ps

1.2

0.8

0.4

0.0

A *

10-3

3000200010000

Time / ps

τTrip = 5 ns

5

4

3

2

1

0

-1

-2

A *

10-3

620600580560540520500480460440420400

Wavelength / nm

2nd order pump

1CHU in ACNpump: 282 nmt = 50 ps

From Wood, P. D. and R. W. Redmond, JACS, (1996) 118, 4256

Results: Transient Triplet Spectrum

Results: UV Probe

1.0

0.5

0.0

-0.5

-1.0

No

rma

lize

d

A

50403020100

Time / ps

100 1000

log (Time)

252 nm 570 nm

τVC = 12 ps

Acetonitrile

τTrip = 5 ns

τSinglet = 0.27 ps

τTherm. = 4.5 ps τTrip = 5 ns

-1.5

-1.0

-0.5

0.0

A *

10-3

25002000150010005000

Time / ps

Open N2 purged

700

600

500

400

300

200

100

0

A *

10-6

25002000150010005000

Time / ps

Open Ar purged

Results: Oxygen Quenching in Acetonitrile

400 nm probe

250 nm probe

Solvent Dielectricconstant

Singlet (ps) 2 (ps) Trip (ns)

pH 7 buffer* 80.10 0.3 ± 0.1 17 ± 4 2.5 ± 5

acetonitrile 36.64 0.27 ± 0.08 4.5 ± 1 5 ± 1

methanol* 33.0 0.2 ± 0.1 200 ± 100 10 ± 20

n-butanol 17.84 0.20 ± 0.01 4 ± 2

n-pentanol 15.13 0.22 ± 0.01 4 ± 1

chloroform 4.81 0.21 ± 0.02 110 ± 70 5 ± 2

* Single wavelength fitsProtic solvents

Results: Lifetimes from Global Fits of Visible Probe Transients

Solvent Dielectricconstant

Singlet (ps) 2 (ps) Trip. (ns)

pH 7 buffer* 80.10 0.3 ± 0.1 17 ± 4 2.5 ± 5

acetonitrile 36.64 0.27 ± 0.08 4.5 ± 1 5 ± 1

methanol* 33.0 0.2 ± 0.1 200 ± 100 10 ± 20

n-butanol 17.84 0.20 ± 0.01 4 ± 2

n-pentanol 15.13 0.22 ± 0.01 4 ± 1

chloroform 4.81 0.21 ± 0.02 110 ± 70 5 ± 2

Results: Relative Quantum Yields / H-Bonding

1 Salet, C., R. Bensasson, et al. Photochem. Photobio. 1979, 30, 325.

4

3

2

1

0

-1

-2

A *

10-3

100806040200

Time / ps

1CHU in ACN MeOH

250 nm

nn

tripletISC A

A

Dielectric Constant

ΦISC(exp.) 1CHU

ΦISC(lit.)1 Uracil

Aq. Solution 80.10 0.04 0.02

Methanol 33.0 0.075 0.06

Acetonitrile 36.64 0.25 0.2

n

t

nneAfit

Dielectric Constant

ΦISC(exp.) 1CHU

ΦISC(lit.)1 Uracil

Aq. Solution 80.10 0.04 0.02

Methanol 33.0 0.075 0.06

Acetonitrile 36.64 0.25 0.2

2.0

1.5

1.0

0.5

0.0

No

rmalize

d

A

50403020100

Time / ps

5 6 7100

2 3 4 5 6 71000

2

Log(Time)

250 nm 259 nm 275 nm 287 nm

Results: Variable Pump Wavelength

No observable changes in dynamics with variable pump wavelength

Probe: 400 nmτTherm. = 5.0 ± 1.2 psτTrip = 5 ± 1 ns

1.6

1.2

0.8

0.4Ab

sorb

ance

300275250

Wavelength / nm

Conclusions

1CHU is soluble in solvents with ε >5

ns lifetime observed in 1CHU is the triplet state

Triplet is formed very rapidly

Excited state lifetime shows no appreciable solvent effects

Hydrogen bonding is important for decreasing ΦISC

No change in lifetimes with varying pump wavelength

Future work

Determine ΦISC in more solventsFurther investigation of the variation of ΦISC with pump wavelength

Acknowledgements

Dr. Bern Kohler

Funding National Institutes of Health

Dr. Terry Gustafson

Kohler Group

C hirp e d Pulse Am p lifie dTi:Sa p p hire La se r Syste m

PD

1 .234 1 .234

790 nm , 600 J, 90 fs

395 nm , 790 nm

Variab le tim e de lay

1 m m FS flow ce ll

C om puter-contro lledw aveplate

O pticalC hopper

1 cm H O ce ll2

Lock-in A m plifie r

.5 m m B BO.3 m m B BO

polarizer

Pulse c o m p re sso r

Obligatory Laser System Slide 1

Obligatory Laser System Slide 2

Mira, Evolution, Legend

2.9W, 800 nm, 40 fsOpera-DFG 230-300 nm

LockinAmplifier

MonochrometerPD/PMT

PrismCompressor

DelayStage

1mmFlowCell

400 nm

266 nm1cmwatercell

WLC

mm BBO

mm BBO

OpticalChopper

Polarizer

ComputerControlledWavePlate

Results: Relative Amplitudes in Acetonitrile

0.8

0.6

0.4

0.2

0.0

Re

lativ

e A

mp

litu

de

650600550500450400

Probe Wavelength / nm

hot triplet triplet singlet

266 nm

τIC = 0.2 psS1

S0

SN

τISC ~ 0.2 ps

T0

T1

~600 nm

τISC ~ 5000 ps

~370 nm

τTherm ~ 5 ps

τVC = 12 ps

Results: Summary of Dynamics in Acetonitrile