Picosecond Studies of Molecular Energy Transfer, Reorientation, and Internal Motion Dynamics

Citation

Millar, David Philip (1982) Picosecond Studies of Molecular Energy Transfer, Reorientation, and Internal Motion Dynamics. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/NSA9-8028. https://resolver.caltech.edu/CaltechETD:etd-04042008-131831

Abstract

This thesis describes the development and characterization of mode- locked CW dye lasers and their application to time-resolved studies of molecular reorientation in liquids, electronic energy transfer in solution, and the internal motions of nucleic acids. Both passive and synchronous mode-locking are found to produce slightly structured pulses with coherence times Δt_c ~ 0.8 psec, and pulse envelope widths Δt_p ~ 2 to 3 psec.

The dynamics of reorientation of cresyl violet in alcohol solvents is directly studied on the picosecond time scale. The observed rotational correlation function decays as a single exponential, with a rotational relaxation time that is linearly dependent on solvent viscosity. Rotation times calculated from hydrodynamics are a factor of 2 smaller than observed. The discrepancy is attributed to hydrogen-bonding interactions with the solvent that cause additional friction.

Electronic energy transfer between cresyl violet donors and azulene acceptors in solution is studied. The excited donor decay is in excellent agreement with the Rirster dipole-dipole model over the time range from 2 psec to 10 nsec. The critical transfer distance inferred from the decay R_o = 26Å agrees well with the value calculated from the spectral properties, R_o = 27Å. The increased energy transfer rate in fluid solutions due to translational motion is accurately predicted by the approximate solution to a diffusion equation for the donor excitation. However, the energy transport due to donor-donor transfer was nondiffusive on the picosecond time scale.

The dynamics of the torsional and bending motions of nucleic acids are studied by sub nanosecond time-dependent fluorescence depolarization of intercalated ethidium bromide. The non exponential relaxation behavior is observed for the first time, and is in excellent agreement with the theoretical predictions of an elastic model for DNA internal motions. The intrinsic torsional rigidity of calf thymus DNA is C = 1.43 ± 0.11 x 10^-19 erg.cm. The torsional rigidity is shown to be sensitive to details of primary, secondary, and tertiary nucleic acid structure. The polyelectrolyte contribution to the torsional rigidity of DNA has been measured for the first time.

Thesis Files