Study of Solvent Dependent Excited State Energy Flow

Study of Solvent Dependent Excited State Energy Flow in DANS Probed with Ultrafast fs/ps-CARS Mikhail N. Slipchenko, Benjamin D. Prince, Beth M. Prince, Alex J. Blom and Hans U. Stauffer Department of Chemistry, Iowa State University

Prof. Hans U. Stauffer Acknowledgments Ames Laboratory ACS - Petroleum Research Fund Benjamin D. Prince Beth M. Prince Alex J. Blom

Motivation: Push-Pull Stilbenes - A • Charge Transfer + D • Molecular wire template Ref: . abruna. chem. cornell. edu/Mol%20 elec. html • Twisted Intramolecular Charge Transfer (TICT) D + hv A - D + A - D+ A-

Intramolecular Charge Transfer twisted intramolecular charge transfer (TICT) LE (Polar Solvent) Adopted from Oberlé, et al. , J. Raman Spectrosc. , 31 (2000), 311

Dynamics in PC vs ACN Transient absorption 400 nm pump 515 nm probe Slower rise and decay of DANS transient absorption in propylene carbonate vs acetonitrile

Optical Configuration

Simultaniously time- and frequencyresolved Raman probe - fs/ps CARS B. D. Prince et. al. J. Chem. Phys. 125, 044502 (2006). fixed

Simultaniously time- and frequencyresolved Raman probe - fs/ps CARS B. D. Prince et. al. J. Chem. Phys. 125, 044502 (2006). fixed frequency

Simultaniously time- and frequencyresolved Raman probe - fs/ps CARS B. D. Prince et. al. J. Chem. Phys. 125, 044502 (2006). fixed frequency time t

Suppression of nonresonant contribution 0

Suppression of nonresonant contribution Raman shift, cm-1 Large nonresonant contribution 0

Suppression of nonresonant contribution Raman shift, cm-1 Free of nonresonant contribution 0 pr

DANS in propylene carbonate Static and Transient Absorption Spectra

DANS in propylene carbonate Static and Transient Absorption Spectra ex t 1 2 CARS 3 pr Time delay t is scanned, while pr is fixed at optimal value

DANS in PC Ground vs Excited states Ground state

DANS in PC Ground vs Excited states NO 2 sym. str. Ground state * ring defs. ring CH rocks C-NO 2 str. olefinic C=C Assignments from work of Moran et. al.

DANS in PC Ground vs Excited states NO 2 sym. str. Ground state * ring defs. ring CH rocks C-NO 2 str. olefinic C=C Excited t=0. 5 ps Assignments from work of Moran et. al.

DANS in PC Ground vs Excited states NO 2 sym. str. Ground state * ring defs. ring CH rocks C-NO 2 str. olefinic C=C Excited t=0. 5 ps Excited t=10 ps Assignments from work of Moran et. al.

DANS in PC Ground vs Excited states NO 2 sym. str. Ground state * ring defs. ring CH rocks C-NO 2 str. olefinic C=C Excited t=0. 5 ps Excited t=10 ps Assignments from work of Moran et. al.

DANS in PC Ground vs Excited states NO 2 sym. str. Ground state * ring defs. ring CH rocks C-NO 2 str. olefinic C=C Excited t=0. 5 ps Excited t=10 ps Assignments from work of Moran et. al.

DANS in PC Ground vs Excited states NO 2 sym. str. Ground state * ring defs. ring CH rocks C-NO 2 str. olefinic C=C Excited t=0. 5 ps Excited t=10 ps Assignments from work of Moran et. al.

acetonitrile propylene carbonate DANS in PC and ACN 2 D spectrum Assignments from work of Moran et. al.

DANS in PC and ACN 2 D spectrum NO 2 sym. str. Assignments from work of Moran et. al.

DANS in PC and ACN 2 D spectrum NO 2 sym. str. ring CH rocks Assignments from work of Moran et. al.

Vibrational dynamics ring CH rocks NO 2 sym. stretch ? - NO 2 sym. str. shifts 5 cm-1 to the blue with 3 ps time constant - Other two modes do not change their frequency Assignments from work of Moran et. al.

Vibrational dynamics ring CH rocks NO 2 sym. stretch combina tion - NO 2 sym. str. shifts 5 cm-1 to the blue with 3 ps time constant - Other two modes do not change their frequency Assignments from work of Moran et. al.

Results outline w. NO 2 sym. stretch mode is 50 cm-1 red shifted in DANS excited state vs ground state. w. NO 2 sym. stretch experience 5 cm-1 blue shift in DANS excited state. w The slower dynamics of mode at around 1300 cm-1 could be explained by Fermi resonance with NO 2 sym. stretch.

Conclusions w fs/ps CARS is a promising tool for studying vibrational dynamics with sub picosecond time resolution w Vibrational dynamics of DANS excited state is slower in the propylene carbonate vs. acetonitrile in accordance with larger viscosity of propylene carbonate. w The conformational changes in DANS upon electronic excitation in polar solvents occur on sub 100 fs time scale and most probably involve twisting around Ph-NO 2 bond consistent with present picture of twisted intramolecular charge transfer in DANS

END Thank you for your attention

END Thank you for your attention

Suppression of nonresonant contribution

NO 2 sym. stretch shift

From zwitterion to neutral stractures LE (weaker vibration) A*5 (stronger vibration)

DANS in PC 2 D spectrum Ground state Raman shift, cm-1

DANS in PC 2 D spectrum NO 2 sym. str. Ground state Raman shift, cm-1 Assignments from work of Moran et. al.

DANS in PC 2 D spectrum NO 2 sym. str. ring CH rocks Ground state Raman shift, cm-1 Assignments from work of Moran et. al.

Optical Configuration