STRUCTURE AND PROPERTIES OF CYANO-SUBSTITUTED POLY(2,5-DIALKOXY-P-PHENYLENE VINYLENE)S

Soluble, crystallizable cyano-substituted poly(2,5-dialkoxy-p-phenylen e vinylene)s (RO-CNPPV)s having the alkoxy groups hexyloxy, octyloxy, and decyloxy were synthesized by use of the Knoevenagel-type condensat ion method and characterized by use of X-ray diffraction, differential scanning calorimetry, and W-vis and photoluminescence (PL) spectrosco py measurements. In the ordered phase, the polymers have a two-layer s tructure with side chain aligned in the all-trans conformation lying o n the same plane of the coplanar main chains. However, the presence of the bulky cyano group on the vinylene segment leads to a deviation fr om coplanarity and poor stacking of the main chains and therefore the absence of vibronic transitions in the UV-vis and PL spectra (which us ually appear in the PPV and RO-PPVs). In addition, as temperature incr eases, the extent of aromatic ring distortion increases gradually, cau sing a continuous increase in the d-spacing between two neighboring st acking subchains (d(m)), which increases up to 16% at the end of melti ng, 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 transi tion, for which d(m) remains constant and d(s) increases by 22% mainly in the melting region. In the melting range, although the aromatic ri ngs distort to a higher extent, and in the meantime the side chains be come more coil-like and randomly oriented, both can recover to their o riginal alignments after cooling down to below the melting region. In the entire thermal process, a significant thermochromism occurs with t he optical absorption maximum shifting by 81 nm compared to that of po ly(3-dodecylthiophene), 100 nm, while the emission maximum blue-shifts by 94 nm. As temperature rises well above T-m (by more than 40 degree s C), the aromatic rings distort to a high extent such that the side c hains intermingle with each other, causing a strongly hindered relaxat ional motion of the main chains after the cooling. They can recover to their original state only by redissolving and then recasting. No liqu id crystalline state in the RO-CNPPVs is observed.