Paper Introduction Amrutha A.S. 19th May 2015

Synthetic design The rigid rod polymer P1 incorporates Az chromophores in a poly(p-phenylene) backbone with two dodecyl side chains per repeat unit. Presence of o-methyl groups large dihedral angles between the azobenzene units in order to decouple these units and break the electronic conjugation. This point ensures that Z-rich state can be reached upon UV irradiation.

In thin films P1 self assembles into ‘molecular zippers’ In thin films P1 self assembles into ‘molecular zippers’. (Molecular zippers – molecular structures where molecules are interlocked in two directions through non covalent interactions, along the length of the zip and across it.) Thin films of P1 were formed on the freshly cleaved muscovite mica substrates – film thickness 20nm (by AFM)

Results Layered island structures as revealed by microscopy Homogeneous sample morphology Most frequently occurring step-height 3-3.5nm suggests a predominant edge – on orientation of the polymers wrt the mica substrate

No out of plane Bragg reflections of the polymer film

Light controlled molecular interdigitation as shown by x-ray diffraction 3 main diffraction features.. in-plane scattering feature at a wavevector transfer 2,3.

Kinetics of E to Z isomerization as judged by optical spectroscopy Upon UV irradiation (20±2)% of all azobenzenes Within the film are switched from E to Z Film thickness has no significant effect on the relative amount of switched azobenzenes Differential reflectance spectroscopy (DRS) UV irradiation optical reflectivity increases in the region between 3.5-4.2eV, decreases in the region between 2.5- 3.2eV E Z isomerization is indeed occuring within the polymer film

Discussion

Conclusion This is the first demonstration of of a rigid main chain polymer- based molecular zipper undergoing reversible light controlled opening and closing. Fast E Z isomerization of a small fraction (20%) of all azobenzene chromophores within the polymer film is sufficient to disrupt the long-range ordering of the protruding dodecyl side chains and to induce amorphization of the film. This amorphization occurs on a much larger time scale (τ=90s). Findings of a delay between azobenzene switching and the structural effects are important for optochemically active or self healing materials that make use of a reversible light controlled phase transition.