Molecular reorientational relaxation and excited-state dynamics probed by time-resolved fluorescence spectroscopy

Picosecond time-resolved fluorescence anisotropy and lifetime measurements using the time-correlated single photon counting technique have been applied to study various systems of photophysical, photochemical and biophysical interest.; We have chosen trans-4,4′-diphenylstilbene (DPS) as a probe molecule for solvent-solute interactions. The fluorescence lifetimes do not have a significant solvent or temperature dependence, indicating that isomerization via activated barrier crossing is not a dominant decay pathway for S₁ DPS. However, the temperature dependence is greater in polar solvents than nonpolar solvents. Arrhenius plots for polar solvents give two different decay channels in the low-temperature and high-temperature regions. Solvent-solute coupling represented by the friction coefficient tends to decrease with increasing solvent viscosity in nonpolar solvents, and the trend seems to be reversed in polar solvents. Transient absorption spectra of S₁ DPS feature two distinctive bands assigned to different electronic transitions. While both bands show vibrational relaxation, only the lower-wavelength band displays prominent conformational dynamics. Both vibrational and conformational relaxations are complete within 60 ps after photoexcitation. Vibrational relaxation lifetimes follow the trend in solvent thermal diffusivity. Conformational dynamics demonstrate solvent mediation via dielectric constant and viscosity.; We have explored the molecular reorientation and photophysics of the intramolecular hydrogen-bonded chiral dendrons. Time-resolved fluorescence anisotropy yielded bi-exponential decays consisting of a slow and fast depolarization component, which is respectively assigned to the global rotation of the whole dendron molecule and the local rotation of the chiral terminal group. While the fast component dominates in the lower generation dendrons, the slow component becomes much more significant in the higher generation dendrons. The hydrodynamic volume calculated from the global rotation indicates molecular folding in the higher generation dendrons. The decrease in the initial limiting anisotropy for the higher generation dendrons suggests the presence of an ultrafast relaxation process beyond the time resolution of the instrument.; The methyl viologen (MV2+) excited state dynamics and carbaryl photodegradation were investigated using fluorescence lifetime measurements. MV2+ in acetonitrile alone at room temperature has a fluorescence lifetime of ∼1 ns. The fluorescence is efficiently quenched by electron transfer from added quenchers with gas-phase ionization potentials lower than ∼10.8 eV The lifetime of S₁ carbaryl was essentially unchanged in going from a low pH to a high pH buffer solution, ruling out the possibility of base-promoted photodegradation via bimolecular quenching of the singlet excited state.