The Inorganic, Bioinorganic, and Organometallic Chemistry Program and the Experimental Physical Chemistry Program support the work of Professor Jeffery I. Zink of the University of California-Los Angeles to examine the excited electronic states and reactivities of metal-containing molecules. His research program has four complementary objectives: 1) to spectroscopically characterize the excited electronic states of metal-containing compounds, 2) to understand the effects of potential surface coupling on electronic and resonance Raman spectra, 3) to study large-amplitude molecular motions, and 4) to explore spectra, photochemistry and photodeposition from volatile molecules in the gas phase. These studies may contribute to the development of photoactivated molecular machines and nanovalves that can trap useful molecules such as drugs and release them on demand. The research group recruits undergraduate students from UCLA's Student Research Participation Program and the Center for Academic Excellence (CARE) Program. The CARE Program focuses on the retention students from educationally or socioeconomically disadvantaged backgrounds. These students as well as other graduate, undergraduate and high schools student and teachers also participate in the California NanoSystems Institute to introduce students to various aspects of science and technology in the core curriculum set by the state of California. Professor Zink also promotes communication with the public as he regularly participates in the American Chemical Society's Speaker Tour, giving public lectures on Excited State Distortions.
The research program was a multi-faceted examination of excited electronic states of molecules. A molecule in its stable low energy ground state can absorb a photon to produce a higher energy excited state. In the excited state, the bonding properties are different from those in the ground state, and bond lengths and bond angles are correspondingly changed. These changes are typically small – bond lengths change by a few percent – but they can be very large and used as the molecular machines described below. The information gained by measuring the changes (using electronic absorption and emission spectroscopy, resonance Raman spectroscopy, multiphoton ionization spectroscopy and theory) provides details about the locations of electrons in the molecule and can be used to explain new reactivity (photochemistry). We are particularly interested in the concept that we developed of excited state mixed valence. It exists when a system possesses two or more interchangeably equivalent sites that have different oxidation states in an excited electronic state but a symmetrical charge distribution in the ground electronic state. Some of the most commonly encountered molecular systems having these characteristics contain two identical charge bearing units M separated by a bridge B and are represented by the generic M-B-M symbol. The mixed valence excited state occurs when the charge is transferred from the center to one M or the other which produces two localized configurations: M+-B-M and M-B-M+. Because the two M groups communicate with each other (are "coupled") through the bridge, the excited state is not completely localized and the spectra contain information about both configurations simultaneously. We used bond length changes and new spectroscopic features to develop a mathematical and physical picture called the Neighboring Orbital Model of the coupling. We have also investigated other types of compounds including very small molecules (containing two atoms) and measured how their vibrations change in unusual oxidation states, and very large mechanically interlocked molecules (such as a cyclic molecule pierced by a dumbbell-shaped stalk) and found that photons cause electrons to move from the dumbbell to the cyclic component. Large amplitude bond length and angle changes have a very important application as nanomachines. Our research program investigated two types. The first involves molecules that undergo trans-cis photoisomerization (a large amplitude angle change, i.e. a change of configuration where two functional groups move from opposite sides of the molecule to an excited state where they are on the same side). When attached to a solid support (such as a nanoparticle), these moving parts can caused other molecules to move. We attached these molecules to the inside of tubular pores, and showed that these "nanoimpellers" could trap smaller molecules inside the structure in the dark but move them out of the pores when photoexcited. As another example, we studied a system consisting of a long-chain stalk that is threaded through a cyclic molecule. The cyclic molecule moves along the stalk when excited by photons. When this system is bonded to the opening of a nanopore filled with cargo molecules, the valve is closed (and the cargo trapped) when the cyclic molecule is near the pore but opened when cyclic molecule moves away from the pore (and the cargo is released). Both of these types of excited-state structural changes have potential applications in electronics (switches) and medicine (drug delivery). The research was carried out primarily by chemistry graduate students. They learned fundamental experimental spectroscopy (including operation of high tech laser instruments), theoretical skills to interpret the spectra (including molecular orbital calculations and the time-dependent theory of spectroscopy), synthetic methods for making the molecules, and creative ways of using their discoveries for practical applications.
