Unraveling the mechanism of chemical reactions often requires a detailed understanding of the role of potential reaction intermediates. These intermediates can be quite difficult to characterize because they are often unstable, highly reactive and short-lived. Professor Goodman's research goals involve the experimental challenge of preparing such reactive intermediates and accurately describing their physical and chemical properties (structure, energetics and kinetics). Radicals, biradicals, carbenes, and ion radicals are all highly reactive species that have been postulated as reactive intermediates in organic reactions.
Stereochemistry, effects and kinetics are all diagnostic methods commonly employed. In addition, several complimentary time-resolved experimental techniques, pico- and nanosecond absorption spectroscopy and pulsed time-resolved photoacoustic calorimetry are used. Photoacoustic calorimetry is a new technique that effectively listens to chemical reactions. The sound waves can be used to determine the dynamics (10 ns - 50 ms) and energetics of reactive intermediates. The experimental thermochemical and dynamic information can provide valuable mechanistic insight into their intermediacy in chemical reactions.
One area of current interest is the chemistry of ion radicals in solution. Ion radicals are now being recognized as extremely important reactive intermediates in organic reactions. However, little is known about their chemistry in solution. Photoinduced electron transfer is a convenient, efficient method by which to generate these species. Interestingly, their chemistry and properties are often quite remarkable. They are often more reactive and more selective than their neutral precursors. In addition, the energetics of ion radical reactions are often quite different than that of the neutral molecules. This relationship can be exploited to synthesize interesting new novel compounds.
In recent years, we have investigated new strategies to increase the efficiencies of photoinduced electron transfer reactions by decreasing the rate of return electron transfer (RET), or increasing the rates of useful competing processes such as ion pair separation or follow-up ion radical reactions. A recent alternative approach we have explored is to have RET itself do useful work when it is coupled to a chemical reaction such as bond dissociation, Figure 1. This process, termed dissociative return electron transfer (DRET), is highly efficient in several systems we have studied.
- She, C. X., McGarrah, J. E., Lee, S. J., Goodman, J. L., Nguyen, S. T., Williams, J. A. G., Hupp, J. T. "Probing Exciton Localization/Delocalization: Transient dc Photoconductivity Studies of Excited States of Symmetrical Porphyrin Monomers, Oligomers, and Supramolecular Assemblies," J. Phys. Chem. A. 2009, 113, 8182.
- Ikeda, H., Hoshi, Y., Namai, H., Tanaka, F., Goodman, J. L. and Mizuno, K. "Evidence for significant through-space and through-bond electronic coupling in the 1,4-diphenylcyclohexane-1,4-diyl radical cation gained by absorption spectroscopy and DFT calculations," Chemistry-A European Journal 2007, 13, 9207.
- Al-Kaysi, R. O., Goodman J. L. "Bond-Coupled Electron Transfer Processes: Cleavage of Si-Si Bonds," J. Am. Chem. Soc. 2005, 127, 1620.
- Hoshi, Y., Ikeda, H., Goodman, J. L., Miyashi, T. "Substituent Effects on the Absorption Spectra of 1,4-Diarylcyclohexane-1,4-diyl and 1,3-Diarylcyclohexane-1,3-diyl Radical Cations: Spectroscopic Evidence and Theoretical Explanation for Through-Space and Through-Bond Electronic Couplings," J. Am. Chem. Soc. 2003, submitted.
- Wang Y. S, Luttrull D. K., Dinnocenzo J. P., Goodman J. L., Farid S., Gould I.R. "Associative Return Electron Transfer. A Bond-coupled Electron Transfer in the Photoreactions of Cyclopropylamines," Photochemical & Photobiological Sciences 2003, 2, 1169-1176.
- Ikeda, H., Akiyama, K., Takahashi, Y., Nakamura, T., Ishizaki, S., Shiratori, Y., Ohaku, H., Goodman, J. L., Houmam, A., Wayner, D. D. M., Tero-Kubota, S., Miyashi, T. "Spectroscopic and Calorimetric Studies on the Mechanism of Methylenecyclopropane Rearrangements Triggered by Photoinduced Electron Transfer," J. Am. Chem. Soc. 2003, 125, 9147.
- Gould, I.R., Boiani, J. A., Gaillard, E. B., Goodman, J. L., Farid, S. "Intersystem Crossing in Charge Transfer Excited States," J. Phys. Chem. A 2003, 107, 3515.
- Goodman, J. L. "Photoacoustic Calorimetry of Carbenes," in Advances in Carbene Chemistry, Brinker, U. ed., Elsevier, Amsterdam, 2001.
- Cuppoletti, A., Dinnocenzo, J. P., Goodman, J. L., Gould, I. R. "Bond-Coupled Electron Transfer Reactions: Photoisomerization of Norbornadiene to Quadricyclane," J. Phys. Chem. A 1999, 103, 11253.