Vibrational and electronic spectroscopy, especially resonance Raman spectroscopy, for the study of electron transfer, solvent dynamics, chromophore aggregation, and solar photoconversion. Optical and electronic properties of nanomaterials and self-assembled dye aggregates, studies of interfacial electron transfer, optical and electronic properties of semiconductor nanoparticles. Light-harvesting plant pigments for solar energy conversion.

Biography

Prof. McHale received her Ph.D. in physical chemistry in 1979 from the University of Utah, where she worked with Prof. Jack Simons. She was a member of the chemistry faculty at the University of Idaho from 1980 until 2004, when she joined the chemistry faculty at Washington State University. She is a fellow in the American Association for the Advancement of Science and the author of Molecular Spectroscopy (Prentice-Hall, 1999).

The McHale lab specializes in the use of resonance Raman spectroscopy and other optical techniques for the study of molecules and nanomaterials with interesting optical and electronic properties. Fundamental quantum mechanical aspects of electron transfer in solution and in interfacial systems are a major focus of our experiments. We pioneered the use of resonance Raman spectroscopy to study molecular aspects of solvent dynamics in electron transfer, using solvatochromic dyes as probes of the solvent response. In this work, resonance Raman excitation profiles were determined by gathering Raman spectra as a function of excitation wavelength using tunable lasers. These profiles were then analyzed using time-dependent spectroscopy theory. This research has established the potential for resonance Raman spectroscopy to probe the early time (sub-picosecond) solvent motion that accompanies photo-induced electron transfer. Experiments using resonance Raman intensity analysis in isotopically substituted solvents suggest that vibrational dynamics and nonlinear solvent response, generally neglected in analysis of excitation profiles, can be important considerations.

Our interest in the spectroscopy of electron transfer has led to our current researcch emphasis on interfacial electron transfer in dye-sensitized solar cells (DSSCs). Developed by Prof. M. Gratzel in Switzerland, these novel photovoltaic cells are based on wide band gap semiconductors in nanoparticulate form, coated with visible-light absorbing dyes. DSSCs offer some potential advantages over the current silicon-based devices, but many questions remain concerning the quantum mechanical basis for interfacial electron transfer and its dependence on molecular details of the sensitizer, the nanoparticle surface, and the contacting electrolyte. We are using spectroscopy and photocurrent measurements to understand the microscopic details of electron transfer and transport in dye-sensitized titanium dioxide nanoparticles.

We are also interested in finding alternatives to the expensive ruthenium-based dye sensitizers presently used in these solar cells, by exploring natural dyes from plants and flowers as potential sensitizers in solar photoconversion. We are also elucidating the basis for solvent effects on the efficiency of converting solar to electrical energy, and developing photoluminescence probes of the surface properties of semiconductor nanoparticles.

Dyes which absorb visible light strongly often have a tendency to self-aggregate, which can alter the optical and electronic properties. We are interested in the unique optical properties of J aggregates of water-soluble porphyrins. We recently discovered that aggregates of positively charged tetra(p-carboxyphenyl) porphyrin diacid (H ₂ TCPP ²⁺ ) have a structure that depends on the identity of the negatively charged counter ion. Atomic force microscopy reveals rod-like aggregates in the presence of nitrate ion and rings in the presence of chloride ion. Further studies of these dye aggregates will explore the relation between structure and optical and electronic properties.