Research Interests

My research interests lie in developing the femtosecond stimulated Raman spectroscopy (FSRS) technique within the Wasielewski Group and using it to study the structural dynamics of the bridge molecules in donor-bridge-acceptor (D-B-A) assemblies. D-B-A systems with efficient long distance charge transport mechanisms are good candidates for photoconversion of solar energy, and the optimization of these assemblies requires a fundamental understanding of the charge transfer process on the molecular level. While it has been suggested that structural dynamics play an important role in achieving wire-like transport, structural changes of the bridge during the charge transfer processes have not been probed directly with time-resolved methods.

FSRS is a relatively new vibrational spectroscopy technique that has high spectral and temporal resolution, high signal-to-noise, and short data acquisition times. Time-resolved FSRS is a three-pulse technique as shown in Figure 1. The femtosecond actinic pump initiates the photophysics. The Raman spectrum of the system at a time Dt after the actinic laser flash is collected with two pulses overlapped in time: the Raman pump, which is a narrow-bandwidth (~20 cm -1) pulse with a duration of ~1 ps, and a broadband ~50 fs Raman probe pulse.

Selective resonance enhancement (up to 2-3 orders of magnitude) of the transient Raman modes can be achieved by selecting a Raman pump wavelength that coincides with an electronic transition of the transient species. To that end, a Noncollinear Optical Parametric Amplifier (NOPA) has been implemented to produce the sub-20 cm -1, tunable Raman pulses needed in FSRS. b-carotene was used as an initial model system (Figure 2): after a initial 390-nm excitation into the S 2 state, the ~130-fs appearance of the S 1 C=C vibrational feature due to direct internal conversion from S 2 and its ~10-ps decay to S 0 were observed, in good agreement with recent literature values [Shim and Mathies, J. Phys. Chem. B 112, 4826-4832 (2008)].

The D-B-A molecules currently under investigation contain a donor unit consisting of a 3,5-dimethyljulolidine molecule attached to anthracene (DMJ-A), fluorenone (Fn) or phenylene ethynyl (PE) units as the bridge, and naphthalenediimide (NI) as the acceptor (Figure 3). I hope to use FSRS to elucidate the effects and importance of vibrational modes associated with the charge transfer process by directly observing structural changes of selected resonantly enhanced states and bridge intermediates during photochemical reactions.

Figure 1. Schematic representation of time-resolved FSRS. (a) Energy-level diagram for a typical time-resolved FSRS experiment where the Raman pump is resonant with the S 1 state. (b) Pulse durations and relative timing of the actinic, Raman pump, and Raman probe pulses.

Figure 2. Kinetics of the S 1 C=C vibrational Raman band of b-carotene at 1790 cm -1 after excitation at 390 nm. Early time behavior is presented in the inset.

Figure 3. D-B-A molecules of interest where the donor (DMJ-A) is in red, bridges (PE or Fn) in black, and the acceptor (NI) in blue.