The research in the Bradforth group centers on understanding
the ultrafast dynamics of chemical reactions in solution and in complex condensed
phase systems. The common strand in our experiments is to explore the coupling
of the dynamics of the nuclei (e.g. vibrations, bond formation/breaking and
solvent motion) to a given non-adiabatic electronic process. The time scales
for these non-adiabatic processes are on the order of 100 fs; likewise the timescales
of nuclear motions are 10 fs - 1 ps. In a condensed environment, this allows
only a short time window to explore the system evolution and catch the newly-formed
products before complete relaxation. Femtosecond spectroscopy is therefore the
appropriate experimental technique to elucidate the dynamics.
Most small molecule photochemistry that is well characterized
in the gas phase takes place at wavelengths shorter than 350 nm. Our group follows
reactions of such molecules when embedded in a liquid. However, the ultraviolet
is traditionally a difficult area for ultrashort pulse generation. We have recently
harnessed developments in hollow core fiber technology to achieve breakthrough
tunable deep UV pulses of ~ 25 fs. When combined with fully dispersed spectral
probing we carry out solution photochemistry experiments exploiting this unprecedented
Experimental projects using ultrafast spectroscopy in the Bradforth
group include (i) probing the primary pathways for electron ionization and detachment
in liquid water (ii) investigating chemistry initiated by ejection of an ultrafast
electron, (iii) the influence of solvent on bond dissociation dynamics and on
rotational relaxation initiated by photodissociation, (iv) determining pathways
of energy migration of an electronic excitation through an organized architecture
of chromophores such as DNA. Strong connection with current theory is emphasized
in all projects. In addition, we are engaged in several collaborative projects
with theoreticians in the areas of electronic structure and quantum dynamics
of solutes in bulk liquids.
Solution photodetachment and photodissociation
and photodetachment are well-understood processes in vacuum. Our experiments
build a molecular level picture of the dynamics of electron ejection in liquids.
We have now firmly established mechanisms for the resonant detachment of many
simple anionic systems. These experiments have allowed development of solution-based
photodetachment spectroscopy for exploring reaction dynamics in liquids, such
as descent from a bimolecular barrier and contact reactions of bimolecular partners
who would normally meet by diffusion. In addition we are actively exploring
the photophysics of liquid water, the nature of the conduction band in water
and mechanisms of liquid photoionization. This fundamental work is of crucial
importance in understanding the effects of radiation on animal tissue, and hazardous
Photodissociation studies of benchmark systems such as ICN
in polar liquids reveal unusually long-lived free rotor behavior in the very
hot product fragment. This allows a new probe of liquid rotational friction.
Our experiments characterize a reaction-induced change in the liquid structure
which leads to a breakdown in the linear response type dissipative behavior
of the solvent. In addition, curve-crossing dynamics in the photodissociation
can be disentangled in a full liquid environment.
DNA and Aromatic photophysics: Energy Transfer in Chromophore Arrays
crystallography reveals that nature uses cyclic architectures of chlorophyll
pigments to achieve phenomenal efficiency in photosynthetic light harvesting.
Polymer analogs to the supramolecular pigment architectures employed in photosynthesis
have been synthesized by Prof. Hogen-Esch of this department. These chromophore
arrays successfully harvest electronic energy by efficient energy transfer between
chromophores. In contrast, UV excitation of DNA – another ordered chromophore
array – leads to damage and mutagenesis. Patterns of damage in oligonucleotides
may be connected with energy transfer along base stacks in DNA and the increased
excited state lifetime in the polymer. Using time-domain spectroscopy, our group
is studying energy transfer dynamics in both macrocyclic polymers and in DNA.
Fluorescence and time resolved absorption anisotropy and annihilation are being
used to analyze energy migration pathways and timescales. We have recently observed
via dispersed pump-probe spectroscopy that there are sub-100fs pathways for
electronic relaxation in constituent DNA bases.
"Nuclear uptake of ultrasmall gold-doxorubicin conjugates imaged by fluorescence lifetime imaging microscopy (FLIM) and electron microscopy", X. Zhang, S. Shastry, S. E. Bradforth, and J. L. Nadeau, Nanoscale, in press (2015).