Structure and properties of cyano-substituted poly(2,5-dialkoxy-p-phenylene vinylene)s

Soluble, crystallizable cyano-substituted poly(2,5-dialkoxy-p-phenylene vinylene)s (RO-CNPPV)s having the alkoxy groups hexyloxy, octyloxy, and decyloxy were synthesized by use of the Knoevenagel-type condensation method and characterized by use of X-ray diffraction, differential scanning calorimetry, and W-vis and photoluminescence (PL) spectroscopy measurements. In the ordered phase, the polymers have a two-layer structure with side chain aligned in the all-trans conformation lying on the same plane of the coplanar main chains. However, the presence of the bulky cyano group on the vinylene segment leads to a deviation from coplanarity and poor stacking of the main chains and therefore the absence of vibronic transitions in the UV-vis and PL spectra (which usually appear in the PPV and RO-PPVs). In addition, as temperature increases, the extent of aromatic ring distortion increases gradually, causing a continuous increase in the d-spacing between two neighboring stacking subchains (d(m)), which increases up to 16% at the end of melting, but the side chain retains the same orientation and the d-spacing between two successive layers (d(s)) expands by only about 3%. This is opposite to that of poly(3-dodecylthiophene) having a vibronic transition, for which d(m) remains constant and d(s) increases by 22% mainly in the melting region. In the melting range, although the aromatic rings distort to a higher extent, and in the meantime the side chains become more coil-like and randomly oriented, both can recover to their original alignments after cooling down to below the melting region. In the entire thermal process, a significant thermochromism occurs with the optical absorption maximum shifting by 81 nm compared to that of poly(3-dodecylthiophene), 100 nm, while the emission maximum blue-shifts by 94 nm. As temperature rises well above T-m (by more than 40 degrees C), the aromatic rings distort to a high extent such that the side chains intermingle with each other, causing a strongly hindered relaxational motion of the main chains after the cooling. They can recover to their original state only by redissolving and then recasting. No liquid crystalline state in the RO-CNPPVs is observed.