| As
mentioned in the generic group description, our ultrafast studies are
aimed at understanding the processes occurring in transition metal
complexes after photoexcitation of the molecule. My research
project focuses on the more fundamental questions: what is
the cascade of relaxation events in the system and can we find a
synthetic “handle” to systematically modify the rate or order in which
they happen? Much of the existing ultrafast research on
transition metal complexes has focused on compounds with MLCT excited
states, such as Ru(II) polypyridyls and Cu(I) phenanthroline
compounds. Even amongst these complexes there are clear
discrepancies in the timescales of relaxation processes. Outside
of our research group, there has been very little emphasis on the
spectroscopy of ligand-field excited states. My research utilizes Cr(acac)3
compounds due to their simple electronic structure and the ability to
synthetically modify the acac ligands. Due to the nature of the
metal center, the visible transitions are ligand-field in nature, with
charge transfer excitations lying in the UV region. Research by
previous group members has laid the foundation for these studies,
including the ultrafast dynamics of Cr(acac)3 and the
synthesis of a beautiful series of homoleptic substituted-acac
compounds. Previously, only >150 femtosecond (fs) to
sub-nanosecond measurements were made on these new compounds due to
instrument limitations; however, we have since purchased new equipment
which will allow us to investigate these blind areas and reveal the
efficacy of these ligand modifications on controlling excited state
relaxation. Of particular interest are the dynamics in the
sub-picosecond range utilizing <50 fs pulses, which in Cr(acac)3
have shown vibrational coherence. This coherence has allowed us
to single out a mode which might be responsible for the <100 fs
intersystem crossing time in this compound. This data was
collected on a collaborator’s instrument prior to the purchase of our
own 35 fs pulse laser system, so I am very excited to continue these
coherence studies on the remainder of the compounds, as well as at a
variety of excitation wavelengths.
|
|
