News & Events

Correlating electronic and nuclear motions in ultrafast photoinduced charge transfer reactions with femtosecond multidimensional spectroscopies

Date: 
Wednesday, March 13, 2013 - 16:00
Speaker: 
Dr. Michael Lynch
Affiliation: 
University of Washington
Host: 
Undergraduate Chemistry Society
Notes: 
Refreshments provided.
Location: 
Chemistry D215

The University of Washington has given me the opportunity to give a seminar to my alma mater. I graduated from UBC (Honours Chemistry) in 2007 where my honours thesis under Prof. Grant involved investigating the Jahn-Teller conical intersection in the gas phase with “frequency domain” electronic spectroscopies involving a 10 ns (1 ns = 10–9 s) dye laser system. My world was turned upside down when I arrived at the UW and started working with Prof. Khalil, as our solid-state laser has a pulse duration of 35 fs (1 fs = 10–15 s). The switch from incoherent to coherent spectroscopy changed my spectroscopic perspective into the time domain, which opened the door to the direct measurement of photochemical reaction dynamics in solution.

 

My research revolves around electronically exciting a molecule in solution and subsequently probing the non-equilibrium relaxation of the excited state with femtosecond multidimensional infrared (IR) spectroscopies in order to elucidate the role of high-frequency vibrations in condensed phase charge transfer processes. The system of interest is a trinuclear cyano-bridged mixed-valence complex of the form [(NC)5FeII–CN–PtIV(NH3)4–NC–FeII(CN)5]4– in D2O. We use the four high-frequency CN stretching (nCN) vibrations to explore metal-to-metal charge transfer (MMCT) dynamics occurring on the femtosecond time scale. It is imperative to understand the electronic ground state before inducing the MMCT process, which is why we initially performed experiments such as 2D IR spectroscopy in order to determine anharmonic coupling constants, vibrational energy relaxation (VER) time scales, and solvation relaxation time scales. That knowledge is then used to assign transient IR features that appear when the MMCT is induced with a l = 400 nm photon and subsequent back-electron transfer (BET) dynamics are monitored with first- and third-order nonlinear IR spectroscopies. We find that BET occurs in 110 fs due to the high density of high-frequency vibrational states available at the crossing between the ground and electronic states. Our novel fifth-order visible–IR technique measures non-equilibrium VER rates and also suggests that nuclei are coherently coupled to the BET reaction.

 

The talk will conclude with a brief discussion on recent developments in our lab towards a coherent IR–Raman technique aimed at measuring couplings between low- (<1500 cm–1) and high-frequency (>1500 cm–1) vibrational modes.

Website development by Checkmark Media. Design by Armada