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Mechanisms of Molecular Spin-Photoswitches Studied with New Algorithm

09-08-2005

Our research group has implemented an efficient algorithm to investigate spin-forbidden transitions in inorganic complexes and biochemical reactions.

Spin is a quantum property of electrons. Chemists and physicists have long known that many iron-containing molecules often have a "net spin" associated with their outermost electrons. Most often, iron complexes that are excited with light will make a transition from a low energy electronic configuration to a higher energy configuration but will eventually return to their lowest energy state. Chemists and physicists call this lowest energy state the "molecular ground state". The Rodriguez group has been using quantum physics, quantum chemistry, and supercomputers to study a remarkable class of iron complexes which, upon excitation with light, do not return to their spin ground state.

Our research group has developed techniques of computational quantum mechanics to study iron-containing "spin-photoswitches". Contrary to common iron complexes, spin photoswitches can be excited with light but, after making a transition to a higher spin and higher energy state, these do not return to their original ground state. Instead, these remarkable molecular crystals remain in a metastable high spin state as long as their temperature is below some 50 Kelvin. This phenomenom, which has been previously discovered by experimentalists, is called it light-induced excited-state spin trapping "LIESST".

Rodriguez used time-dependent density functional theory in conjunction with supercomputers to elucidate the initial excitations that take place at the onset of the LIESST process [J. Chem. Phys., Vol. 123, 094709, 2005]. In addition, Chachiyo and Rodriguez published a paper [J. Chem. Phys., Vol. 123, 094711, 2005] that describes a computational algorithm to follow the structural rearrangements and changes in spin state during the LIESST process. The computational studies show that there is a sequence of spin states which the photoswitches can go through, forming a pathway which takes them from the ground state to the metastable high spin state. This is a novel description of the mechanism by which iron spin photoswitches work. These theoretical results will likely aid to the development of better quality molecular photoswitches which, by remaining trapped in a metastable spin state at high temperatures, may be used in molecular-level memory storage of liquid display technological applications. Molecular-level memory storage is seen as a powerful futuristic way to store huge amounts of information in very small (nanoscale) regions of space. The work done by the Rodriguez group is an example of how computational quantum physics and quantum inorganic chemistry can contribute to the development of novel nanoscale technological devices.

The research was supported in part by a NSF CAREER award CHE-0349189 (JHR).

FIGURE CAPTION: The iron complex shown in the figure is a spin photoswitch that, upon excitation with green light, can change its spin from S=0 to S=2. The Rodriguez group has used supercomputers, quantum physics, and quantum chemistry to discover novel mechanisms about the spin-photoswitching process.