Molecular machines shift into gear

A set of molecular gears based on porphyrin complexes that can be chemically engaged and disengaged using acid or base has been created by Japanese researchers. The work is an important step towards nano-sized machines that can be put to work in miniature devices.

The team, led by Masayuki Takeuchi from the National Institute for Materials Science in Tsukuba and Seiji Shinkai at Kyushu University, combined two different types of metal-porphyrin complex to make two toothed gears set perpendicular to each other. ’We chose porphyrins because they are highly symmetrical and relatively easy to synthesise,’ says Takeuchi, ’and we can easily design the number and length of the teeth on each gear.’

The main (top) rotor is based on a double-decker lanthanum porphyrin - two flat porphyrin rings with a lanthanum atom sandwiched in between them. The top ring of the sandwich is the rotor gear, with four teeth made from benzene rings separated by alkynes. Attached to either one or two edges of the bottom deck porphyrin ring are pyridine groups. These coordinate rhodium atoms at the centres of the second set of porphyrin rotor gears, which each also have four teeth.

Takeuchi explains that the two types of rotors have quite different natural rotation speeds. ’The double-decker porphyrin rotor is slow enough to observe by NMR spectroscopy,’ he says, ’but the other one is much faster.’ When the two mesh together, the top rotor controls the speed of the side gears, slowing them down enough to observe. The fact that the gears interlock so smoothly is a big step, Takeuchi adds, ’sometimes when you design such machines, steric crowding completely changes the original conformation, so they don’t mesh together.’

The team also found that when they added triethylamine (TEA) to the rotor complexes in solution, the side gears disengaged and spun more rapidly. When they then added trifluoroacetic acid (TFA), the side rotors meshed back in with the top rotor and slowed back down. Takeuchi attributes this to a conformational change in the porphyrin base when its final pyrrole N-H group is deprotonated.  

’It’s technically and intellectually a beautiful piece of work,’ says James Tour from Rice University in Texas, US, who has built a variety of nanoscale machines including cars, trucks and trains. He was particularly impressed by how well the gears interlock and affect each other’s rotation. 

However, Tour adds that to really get any useful work out of this kind of machine, they need to be attached to a surface. ’The pieces are all there,’ he says, ’if they attached some branches to the bottom to pin it down, you could start thinking about using it to pick things up or push them around.’ 

Phillip Broadwith