Now this is just neat. For a while I’ve been following research on “molecular architectures”–miniature structures created from interlocked molecules. UCLA has published a large amount of great research on the topic in recent months. This one comes from the lab of Fraser Stoddart.
Rotaxanes are dumbbell-shaped molecules with a ring around the central portion of the dumbbell that can slide back and forth. “Caps” on the dumbbell prevent the central ring from falling off. Typically, the central portion of the dumbbell will contain some functionality that can complex with the ring, so that the entire assembly can act as a sort of switch. When the ring and dumbbell are complexing, the switch is “on,” and when they aren’t, it’s “off.”
Stoddart’s lab has used the rotaxane concept to posit and explore electrochemically switchable dyes. Complexing between a ring and dumbbell with a HOMO/LUMO gap in the visible region leads to visible light emission. His lab has previously found good red and green complexes, but was never able to find a solid blue complex that worked with the same ring he used for red and green complexes. If the right blue dumbbell molecule was discovered, the red, green, and blue complexes could be combined to form a completely switchable display unit for a television screen or monitor.
Stoddart’s most recent paper describes the discovery and synthesis of the elusive blue complex. The ring (“guest”) he has used previously is cyclobis(paraquat-p-phenylene), CBPQT4+. It has a great tendency to bind to host pi systems through charge-transfer interactions, but previously no host had been found with the right HOMO energy to give blue light upon complexing. The jackpot molecule, as determined by theoretical energy calculations is bis-N-oligoethyleneglycol 3,3′-difluorobenzidine. Stoddart’s lab synthesized the four-EG molecule, bis-N-tetraethyleneglycol 3,3′-difluorobenzidine (DFBZ-TEG).
The next step, says the paper, is the integration of red, green, and blue dumbbell molecules with CBPQT4+ to form a completely switchable dye. The nifty figure below shows a catenane structure (two interlocked rings) with red, green, and blue functionality on different parts of the host ring. Applying different voltages to the molecule gives different colors.