Resonance Raman Spectroscopy and Scanning Probe Microscopy of Light-Harvesting Chromophore Aggregates

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In this project supported by the Experimental Physical Chemistry Program, Professors Jeanne McHale and Ursula Mazur will perform spectroscopic and scanning probe measurements to investigate the link between excited state properties of light harvesting aggregates (LHAs) and their supramolecular structures. Resonance Raman spectroscopy, atomic force microscopy, and scanning tunneling microscopy (STM) will address the following questions: (1) How do details of molecular structure (nonplanarity, presence of metal ions, charge, etc.) influence LHA morphology (nanorods, nanorings, helices, etc.) on nano- and meso-scopic length scales? (2) How do optical and electronic properties depend on molecular and supramolecular structure? (3) For fractal aggregates, how are the sub-units assembled and what is the nature of the electronic communication between them? (4) What is the extent of structural heterogeneity in the ensemble of LHAs and how is it reflected in the optical spectra? In this collaborative research, the McHale lab will measure and analyze Raman excitation profiles of LHAs to reveal excitonic coupling. The surface-enhanced resonance Raman signal of LHAs on noble metals will be optimized in order to obtain Raman spectra of individual LHAs. The Mazur lab will obtain high-resolution STM images of self-assembled aggregates composed of molecules with varying degrees of nonplanarity to probe its role in directing the formation of closed rings similar to those found in photosynthetic organisms. In addition, they will use orbital mediated tunneling spectroscopy to image the delocalized orbitals associated with electron transport at molecular and submolecular resolution. The LHAs of interest in this work have applications in molecular electronics, as sensors, and in solar energy conversion. Fundamental studies of the link between aggregate morphology, molecular structure, and optical and electronic properties of the assembly will enable these properties to be tuned for a particular application, borrowing ideas from nature where the structures of LHAs are optimized to collect and funnel solar energy to the reaction center. In addition to training diverse graduate and undergraduate students as they have in the past, the PI and co-PI will use the proposed research as a vehicle for outreach to area Native American and Hispanic students by participating in WSU's "Pumping up the Math and Science Pipeline". This program engages high-risk high school students in research projects in plant science and alternative energy.