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

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

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

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

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

Optical Configuration

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

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

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

Suppression of nonresonant contribution  0

 0 Large nonresonant contribution Raman shift, cm -1

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

DANS in propylene carbonate Static and Transient Absorption Spectra

 ex 11 22 33  CARS DANS in propylene carbonate Static and Transient Absorption Spectra t  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 Ground state NO 2 sym. str. ring CH rocks olefinic C=CC-NO 2 str. ring defs. * Assignments from work of Moran et. al.

DANS in PC Ground vs Excited states Ground state Excited t=0.5 ps NO 2 sym. str. ring CH rocks olefinic C=CC-NO 2 str. ring defs. * Assignments from work of Moran et. al.

DANS in PC Ground vs Excited states Ground state Excited t=0.5 ps Excited t=10 ps NO 2 sym. str. ring CH rocks olefinic C=CC-NO 2 str. ring defs. * Assignments from work of Moran et. al.

DANS in PC Ground vs Excited states Ground state Excited t=0.5 ps Excited t=10 ps NO 2 sym. str. ring CH rocks olefinic C=CC-NO 2 str. ring defs. * Assignments from work of Moran et. al.

DANS in PC Ground vs Excited states Ground state Excited t=0.5 ps Excited t=10 ps NO 2 sym. str. ring CH rocks olefinic C=CC-NO 2 str. ring defs. * Assignments from work of Moran et. al.

DANS in PC Ground vs Excited states Ground state Excited t=0.5 ps Excited t=10 ps NO 2 sym. str. ring CH rocks olefinic C=CC-NO 2 str. ring defs. * Assignments from work of Moran et. al.

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

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

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

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 combina tion 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.

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

Conclusions  fs/ps CARS is a promising tool for studying vibrational dynamics with sub picosecond time resolution  Vibrational dynamics of DANS excited state is slower in the propylene carbonate vs. acetonitrile in accordance with larger viscosity of propylene carbonate.  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 (weaker vibration) (stronger vibration) LE A5A5 *

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

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

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

Optical Configuration