Photoisomerization of (E and Z)
Daniel J. Phillips, Thomas W. Bell*, Vincent J. Catalano
Chemistry Department, University of Nevada, Reno,
Reno, NV 89557 USA
This is an abstract
for a presentation given at the
Foresight Conference on Molecular Nanotechnology.
There will be a link from here to the full article when it is
available on the web.
The synthesis of a molecular motor molecule is the long range objective of this research project. The initial process was to find synthetic references which would lead toward an unsubstituted version of the switch to prove the synthesis works and then poceed toward making the substituted molecule. With the synthesis of the unsubstituted model compound done, the next step was to place a substituent in the fluorene ring to break up the symmetry and to be useful for studying the photophysics of the compound. For this part a t-butyl group was put in the 2 position of fluorene to give products. The final step will be the synthesis of a chiral base.
The unsubstituted model molecule was found to have interesting properties. The X-ray crystallography showed a trisubstituted alkene with angle strain only on the vinyl double bond. This is thought to be due to a "gearing" of the trityl group about the fluorene ring. One phenyl group is on one side of the ring and another phenyl group is on the other side with the third phenyl ring away from the fluorene. The effect of the gearing is to prevent twisting of the double bond. The closeness also caused the resonances of the adjacent protons on the fluorene ring to occur at a high field in the proton NMR spectrum. The successful synthesis of this molecule led us to next make an unsymmetrical version, the E and Z isomers of a 2 position on the fluorene of a tertiary butyl group.
Results and Discussion:
Synthesis of the 2-t-butylfluorene compound was easy and quick to reach the E and Z isomers of the bromomethylene compound. These isomers were also new compounds. Attempts to separate the viscous oil products were unsuccessful, so the next step was done to form the trityl switch. Initially this was unsuccessful, but scaling up showed the product was made only in a smaller yield than expected. Characterization was similar to the unsubstituted compound. Infrared spectroscopy, UV/Vis and fluoremetry were only slightly different from the unsubstituted compound. Single crystal X-Ray analyses was also similar exhibiting the same angle-strained only feature and gearing.
The E and Z isomers were tested individually for photoisomerization when exposed to ultraviolet radiation. They both changed from pure isomer to a mixture of the two after exposure. This is the first test of switching ability. Other compounds have also been made in a similar vein and work on the chiral anchor has begun.
The E and Z isomers of the 2-t-butyl compound have been made and characterized. They were found to be similar to the previously made compound, but were the first unsymmetrical allylidenefluorenes made. They showed photoisomerization and are one step closer to our goal of a new molecular motor.
Thomas W. Bell
Chemistry Department, University of Nevada, Reno
MS/216, Reno, NV 89557 USA