Synthesis, Structure, and Nonlinear Optical Properties of Cross‐Conjugated Perphenylated iso ‐Polydiacetylenes

Monodisperse, cross‐conjugated perphenylated iso‐ polydiacetylene ( iso‐ PDA) oligomers, ranging from monomer 15 to pentadecamer 25 , have been synthesized by using a palladium‐catalyzed cross‐coupling protocol. Structural characteristics elucidated by X‐ray crystallographic analysis demonstrate a non‐planar backbone conformation for the oligomers due to the steric interactions between alkylidene phenyl groups. The electronic absorption spectra of the oligomers show a slight red‐shift of the maximum absorption wavelength as the chain length increases from dimer 17 b to pentadecamer 25 , a trend that has saturated by the stage of nonamer 22 . Fluorescence spectroscopy confirms that the pendent phenyl groups present on the oligomer framework enhance emission, and the relative emission intensity consistently increases as a function of chain length n . The molecular third‐order nonlinearities, γ , for this oligomer series have been measured via differential optical Kerr effect (DOKE) detection and show a superlinear increase as a function of the oligomer chain length n . Molecular modeling and spectroscopic studies suggest that iso‐ PDA oligomers ( n >7) adopt a coiled, helical conformation in solution.