Linear and unanticipated second-order nonlinear optical properties of benzylic amide [2]Catenane thin films: Evidence of partial rotation of the interlocked molecular rings in the solid state

Ordered thin films of the prototypical benzylic amide [2]catenane 1 were obtained which had good light propagation properties. The vacuum evaporated thin films were characterised by a surprisingly large refractive index (nTE = 1.613 , nTM = 1.595 at  = 1314 nm) and low, for polycrystalline films, propagation losses of (2.8  0.1) dB/cm. An order parameter  in the range 0.020.03 was estimated from the optical birefringence. The linear electro-optical coef-ficient at the wavelength  = 633 nm was found to be r33 = (1.2  0.2) pm/V. By generating the second harmonic frequency with the fundamental wave at the wavelength of f = 1064 nm, we measured a SHG-coefficient of d33 = 0.012 pm/V at a film thickness of 106 nm. From theoretical considerations a ratio of r33 / d33 = 1.6 was expected by taking into account only electronic contributions to the electro-optic coefficient. In contrast, we measured a ratio of r33 / d33 = 120. This suggests that intra or inter-ring motions of the catenane rings in the fre-quency range from DC to the MHz range may contribute to the electro-optic coefficient. A comparison of the results of both measurements shows that ring rotations of catenanes with a small resonance frequency play an important role in the linear electro-optic measurement. This is the first evidence for at least partial rotation of the catenane rings in the solid state by an external applied time dependent electric field - a key requirement for the development of nanodevices based on mechanically interlocked architectures.