Photophysics of Molecular Semiconductors Probed by Single Molecule Spectroscopy and Near-Field Optics

Dr. Steve Buratto
Department of Chemistry and Biochemistry
University of California, Santa Barbara

I will discuss the techniques of near-field scanning optical microscopy (NSOM) and single molecule spectroscopy and their application to molecular semiconductors.  I will focus mainly on our results from probing the fluorescence of a new class of oligo(phenylenevinylene) (OPV) molecules, where four OPV “arm” molecules are liked via a single sp3 carbon center.  Our results show that these so called tetrahedral molecules contain multiple chromophores with limited inter-arm coupling, but significant molecular motion about the central carbon results in fluctuations in the both the polarizability axis of the molecule and the fluorescence intensity on the timescale of 100 ms to 10 s.  We have also examined the photon statistics of single tetrahedral molecules using photon pair correlation spectroscopy (PPCS). The second order correlation function g(2) obtained from PPCS is used to quantify the number of chromophores involved in emission. Our results show that the tetrahedral molecules are able to sustain simultaneous emission from multiple arms. Finally, we describe, for the first time, direct comparisons of the detailed structures of the OPV “arm” molecules and their luminescence properties on a single molecule level.  This data originates from a combination of two powerful diagnostic tools in physical chemistry: gas-phase ion mobility and single molecule fluorescence spectroscopy.  The results show that the structures observed in the gas phase are strongly correlated to the categories of molecules observed in the single molecule polarization anisotropy measurements with nearly identical distributions for the two OPV molecules studied. These categories are determined by the number of cis vinylene linkages in the OPV structure and are shown to directly influence the fluorescence intensity, the structure of the tetrahedral molecule and the morphology of films made from these materials.