Nonthermochromic Forms of the Polydiacetylene ETCD
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ABSTRACT Subjecting crystals of the thermochromic polydiacetylene(PDA) from the bis (ethylurethane) of 5,7-dodecadiyne-1,12-diol(ETCD) to the solvents chlorobenzene and N,N-dimethylformamide above the thermochromic transition temperature of PDA-ETCD irreversibly converts the material to nonthermochromic forms. These new forms have been characterized by their structural, thermal, and spectral properties. The materials isolated after exposure to chlorobenzene are highly crystalline with a structure that has expanded and disordered, yet is still related to that of the pristine polymer. The observed properties are discussed in the framework of the crystal structure-thermochromism relationship for urethane-substituted PDA. INTRODUCTION Among the polydiacetylene(PDA) macroscopic single crystals whose anisotropic optical properties have been studied by reflectance, 1-3 the urethane substituted systems which exhibit thermochromism 1 and photochromism 4 are of particular interest. The urethane substituted PDA such as ETCD(Ia) and IPUDO(lb) have room temperature electronic spectra which exhibit maxima at 635 and 647 nm 1 , respectively. These wavelengths are comparable to those observed 1 in other PDA systems that have ir-electron systems nearer to the backbone. Since the polarizability of the environment surrounding the conjugated PDA chain is widely held 1,5-7 to be important in determining the absorption energy, the similarity of the spectra of the urethane systems(with 4 CH2 groups between the backbone and substituent) to those of PDA with polarizable it-systems one CH2 group from the backbone is definitely noteworthy. In recent years, we have devoted considerable attention to the relationship between crystal structure and thermochromism in urethane-substitued PDA. 8 ,9 We note that PDA such as lab are the best-defined thermochromic polymers. Thermochromism in la is associated with an endothermic first-order phase transition involving an increase in unit cell volume. Hydrogen bonding and backbone planarity do not appear to be significantly disrupted in the course of the transition. The conformer population of the CH2 side groups changes as la is heated from 23 to 130"C. Calorimetric studies1 0 have excluded the possibility that monomer melting plays a significant role in lb's thermochromic transition. The suggestion 11 that molecular distortion, implying a mechanical strain on the backbone, is required for the formation of the hydrogen bonds in urethane-substituted PDA has been supported by quantum chemical computations. 12 R
R = -(CH 2)4-O-C(O)-NH-R'
~C-CCC R
R= -CAH, EITCD, la R'= i-C 3 H7 , IPUDO 1lb
n
Polydiacetylene(PDA) repeat structure. 101 Mat. Res. Soc. Symp. Proc. Vol. 328. ©1994 Materials Research Society
In the course of attempting to prepare thin films of la from solution, it was observed that when samples of Ia were held in solvents at temperatures above the thermochromic transition temperature, they turned red as expected. Upon cooling to ambient temperature, they remained red. When such materials w
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