Polarization Sensitive CARS Investigations of Controlled Molecular Rotations

The emerging field of nanotechnology requires new methodologies to be developed in order to address the needs of fabrication, manipulation, control, and measurement of motions of molecules and devices on the nanoscale. Optical excitation and control of translational, vibrational, and rotational motions can offer a unique way to achieve the desired goals by using a sequence of short laser pulses, e.g., tuned to initiate certain nuclear motions of well studied photo active proteins (e.g. PYP or Rhodopsin ) or engineered larger molecular structures.3 Measuring nuclear motions requires structurally sensitive spectroscopy methods, and therefore spontaneous Raman and coherent Raman spectroscopies can be applied to qualitatively test and quantitatively measure such motions. Polarization Sensitive Resonance Coherent anti-Stokes Raman Spectroscopy (PSCARS), steady state or time-resolved, can measure controlled dipole moment reorientation through the anisotropic third order nonlinear optical susceptibility. The measured vibrational spectra are sensitive to photo induced isomerization5 or internal molecular rotation if the rotating chemical group is coupled to or interacting with internal normal modes or to the surrounding molecules e.g. in solutions. In our study, the method was applied to measure the vibrational manifold and reorientation of DCVJ for the first time. DCVJ is a charge transfer molecular rotor4 showing a viscosity dependent fluorescence quantum yield. This dependence is attributed to the 'friction'type interaction between the 90 degree internal rotation of the molecule5 upon photo-excitation and the solvent cage surrounding it. Quantum chemical studies using DFT and normal mode calculations support our findings related to the conformational evolution of the molecule. Theoretical and experimental principles associated with CARS with an emphasis on points relevant to the interpretation of the results will be presented. The design and operation of an all solid-state broadband nanosecond CARS system will be also presented7. Finally, an overview of applications of molecular rotors in biology, nanotechnology and perhaps information technology will be outlined.