Research

Research

Single Molecule Chiroptical Spectroscopy

The focus of single molecule chiroptical studies is to understand how different isolated molecules interact with chiral light. The desire is to understand the physical cause of why single molecules (if they do) vary from ensemble measurements. By understanding the variation from molecule to molecule, we in a way learn about the trees that make up the forest instead of just the forest. Properties we are trying to understand are how environment, orientation, conformation, among other factors affect chiroptical response. Our studies have found the chiroptical response from molecule to molecule does vary. Instead, a wide range of response is observed. Figure 2 shows the distribution of responses. The shape of the distribution suggests different orientations of the molecule on the surface. Below the distribution are possible orientations.

Normally, when one observes a slide of fluorescent molecules (or anything else!) in an inverted microscope, one would of course focus the object sharply. However, for single emitters such as helicen molecules, it turns out that some more information can be obtained by a slight defocusing of the microscope objective lens, which results in a complicated image that betrays the nature and spatial orientation of the microscopic light source. While images of helicene molecules obtained in this way are somewhat similar to images of linear, 1-d dipole emitters commonly observed in many moleculer systems, important differences suggest the possibility that the inherent “handedness” in the emission properties manifests as a significant perturbation to the image. We are currently investigating the utility of this assumption in building models of the microscopic radiation source associated with emission from the chiral molecule. One simple “ansatz” model is that of non-collinear 1-d dipoles emitting in such a way as to produce slightly elliptically polarized light. Numerical simulation of images resulting from this model (See figure 3 for comparison to experiment) yield a lack of bi-lateral symmetry and outer-ring distortion that is evident in the experimental images (Figure 3, A-F). Ongoing work in our laboratory seeks to validate and improve upon this model.

CdSe Nanocomposite Spectroscopy

Processibility of quantum dots is crucial to their widespread incorporation in discrete optoelectronic devices. To meet this demand, functionalization of CdSe nanocrystals with electroactive ligands serves two purposes: enhanced packing in the condensed phase and improved optoelctronic characteristics relative to bare quantum dots. The optical properties of CdSe-oligo(phenylene vinylene) composite nanostructures (pictured) are vastly modified from those of either the CdSe and organic consituents.

Recent experiments in our group have shown a reduction in fluorescence intermittency (aka “blinking”), near-unity ligand-to-nanocrystal energy transfer, and a strongly one-dimensional tra

nsition dipole moment in CdSe-OPV (shown at left), all of which arise only in the presence of these semiconducting ligands, which we probe utilizing single molecule polarization anisotropy and wide-field defocused imaging. This change in transition dipole moment, which appears as a degenerate 2D transition orthogonal to the crystalline axis in non-functionalized nanocrystals, allows control of nanostructure emission by tuning the polarization of the excitation field, making these nanostructures promising as optoelectronic components.