Thermochromatism and structural evolution of metastable polydiacetylenic crystals.
Topochemically polymerized sodium 10,12-pentacosadiynoate (PCDA-Na) microcrystals show an irreversible red-to-blue chromatic transition accompanied by a distinct structural evolution upon initial thermal treatment, and show a subsequent completely reversible blue-to-red chromatic transition upon further thermal stimuli. Visible absorption spectroscopy, X-ray diffraction (XRD), and differential scanning calorimetry (DSC) are used to investigate the thermochromatic transition behavior of the polydiacetylenic microcrystals. Brief quantum mechanical geometry optimization is employed to explain the lattice dimensional change during the irreversible red-to-blue chromatic transition of the metastable polydiacetylenic crystals.
Chromatic polydiacetylene with novel sensitivity.
Conjugated polymers have been investigated for a number of applications in optoelectronics and sensing due to their important electronic and optical properties. For instance, polydiacetylene (PDA) may change color in response to external stimuli and has been extensively explored as a material for chromatic sensors. However, the practical applications of PDA materials have been largely hampered by their irreversible chromatic transitions under limited stimuli such as temperature, pH, and chemical. As a result, much effort has been paid to improve the chromatic reversibility and increase the scope of external stimuli for PDA. In this tutorial review, the recent development of PDA materials which show reversible chromatic transition and respond to new stimuli including light and electrical current has been described.
Synthesis and properties of a new polydiacetylene: Poly[1,6‐di(N‐carbazolyl)‐2,4‐hexadiyne]
The solid-state synthesis and properties are reported for a new polydiacetylene: poly(1,6-di(N-carbozolyl)-2,4-hexadiyne). The monomer crystals polymerize quantitatively with ..gamma.. irradiation or thermal annealing. An autocatalytic effect is observed in both ..gamma..-ray polymerization and thermal polymerization and is attributed to an increase in chain propagation length at about 5% conversion. The activation energy for thermal polymerization is about 25 kcal/mole, independent of the degree of conversion to polymer. The exceptional thermal stability of the polymer crystals allowed a thermomechanical analysis over a large temperature range, -50 to 300/sup 0/C. With increasing temperature, the polymer contracts in the chain direction linearly with temperature over the entire range, yielding a thermal expansion coefficient of (-2.32 +- 0.02) x 10/sup -50/C/sup -1/. Photoconductivity action spectra are reported for the polymer crystals. The energies for the photoconductivity onset (ca. 2.3 eV) and for the lowest energy optical transition (1.89 eV) are the lowest reported for the polydiacetylenes. The photoconduction onset is blue-shifted with respect to optical absorption--a result which is consistent with the excitonic assignment for the lowest energy optical transition in the polydiacetylenes.