Willem R. Leenstra

Physical Chemistry and Spectroscopy

Associate Professor

Willem.Leenstra@uvm.edu
phone: 802-656-0273
office: Cook Rm A119

In our group, interest is focused on applying various spectroscopic methods to problems in chemical physics, biophysics and solid-state chemistry. Additionally, individual students' ideas may be developed into viable projects as long as molecular spectroscopy is a key feature of the project. For example, we have developed a novel analytical technique: low-level detection of insect pheromone components through the kinetic analysis of bioluminescence signals of the pheromone aldehyde with luciferase (1998 Ph.D. dissertation of S. Gander: "Trace Determination of Aldehydes in Mixtures, an Application of Bioluminescence and Kalman Filtering").

A biophysical extension of our interest in double molecules is a project in which bilirubin is studied regarding its behavior as a folded, internally hydrogen-bonded "double molecule" in equilibrium with the unfolded, monomer chromophore. The spectroscopic investigations have recently been complemented with molecular mechanics calculations that track the geometry of bilirubin in the environment of a layer of water, as a function of temperature (1999 M.S. dissertation of F. Dieudonne: "A Molecular Dynamics Calculational Study of the Solution Structure of Bilirubin").

An ongoing theme has been that of "double molecules". For example, in systems such as biquinoline, biquinoxaline, and bipyridine we have used optically detected magnetic resonance of the excited triplet state, as well as conventional optical spectroscopy to characterize the nature of localization (on one of the two chromophores) vs. delocalization (over the entire molecule) of triplet energy for such molecules. Also we have utilized this class of molecules as a basis to understand the details of the effect of the nitrogen quadrupolar nucleus on the photoexcited molecular triplet state.

Most recently we have embarked on a project in materials science. A number of zirconium phosph(on)ate systems with a variety and a mix of interlayer pendant groups (both alkyl moieties and chromophores) have been designed and synthesized. We have determined the underlying principles that govern the principal, structural parameters in these materials: With p-aminobenzyl groups as the large pendant, interlayer distance increases uniformly with large-group content. Furthermore, a number of interesting photophysical processes have been observed, including singlet energy transfer and excimer formation (1999 Ph.D. dissertation of J. Amicangelo: "Synthesis, Characterization, Structural Investigation, and Photophysical Study of Arene-Derivatized Zirconium Phosphonates").

A study focused on pyrene-derivatized zirconium phosphonates has revealed that d-spacing dependence on stoichiometry can also show an abrupt collapse in interlayer distance. Also, in these systems two types of excimers can be discerned: side-by-side chromophores, and across-the-divide chromophore pairs (2004 Ph.D. dissertation of Thomas Castonguay: "Synthesis, Characterization, Structural Investigation, and Photophysical Study of Pyrene-Derivatized Zirconium Phosphonates").

Selected Publications

Amicangelo, J.C.; Leenstra, W.R. "Zirconium Arene-Phosphonates: Chemical and Structural Characterization of 2-Naphthyl- and 2-Anthracenylphophonate Systems", Inorg. Chem., 2005, 44, 2067.

Amicangelo, J. C.; Leenstra, W. R.; Rosenthal, G. L. "Molecular Modeling of Interlayer Catalytic Sites for Aniline Polymerization in a Zirconium Mixed Phosphonate Phosphate", Chem. Mater. 2003, 15, 390.

Amicangelo, J. C.; Leenstra, W. R. "Excimer Formation in the Interlayer Region of Arene-Derivatized Zirconium Phosphonates", J. Am. Chem. Soc., 2003, 125, 14698.

Leenstra, W. R.; Amicangelo, J. C. "Synthesis, Characterization, and Interlayer Distance Study of Zirconium Phosphonates with Stoichiometric Variation of Methyl and p-Aminobenzyl Pendant Groups, Inorg. Chem. 1998, 37, 5317.

Amicangelo, J. C.; Leenstra, W. R. "A Novel Staged Form of Layered Zirconium Phosphonates with Methyl and p-Aminobenzyl Pendant Groups," J. Am. Chem. Soc. 1998, 120, 6181.