Optical and Magnetic Processes and the Side Chains of Diacetylenic Materials
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OPTICAL AND MAGNETIC PROCESSES AND THE SIDE CHAINS OF DIACETYLENIC MATERIALS
DANIEL J. SANDMAN, GREGORY P. HAMILL, MARK LEVINSON, PHILIP G. ROSSONI, AND ELIZABETH A. YOST
GTE Laboratories Incorporated, 40 Sylvan Road, Waltham, Massachusetts 02254 GEORGIA C. PAPAEFTHYMIOU Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
ABSTRACT Similarities in the molecular energy levels of alkylcarbazoles and aliphatic nitroxides motivate the detailed study of 4,4'-(butadiyne-l,4-diyl)-bis-(2,2,6,6-tetramethyl-4-hydroxypiperidin-1oxyl) (4) as a precursor to a ferromagnetic polymer. A more convenient synthetic route to 4 is reported, and an X-ray-induced transformation of the IS-phase of 4 to the cc-phase has been observed. The magnetic properties of 4 and derived polymers have been studied using a SQUID system. The observed level of ferromagnetism found in samples of polymerized 4 is comparable to the amount of trace iron materials detected in these samples. INTRODUCTION
The polydiacetylenes (PDA, 1) are the best known class of polymers available as macroscopic polymer single crystals [1-3]. Associated with the PDA RCH 2 In
CH2R conjugated backbone is an intense visible absorption which manifests a degenerate third-order nonlinear susceptibility (X(3 )(())greater than 10-10 esu at transparent wavelengths with a temporal response in the femtosecond regime [4-51. The numerous roles of side groups (R in 1) in PDA structure, spectroscopy, and processes have been discussed [6]. The role of side group polarizability in the crystal structure - solid state spectroscopy relationship is of particular interest [5,7-9], and has been offered [9] as a reason that poly-DCH (La), with a polarizable carbazole side chain, absorbs at lower energy than other fully polymerized PDA single crystals. The results of both electroreflectance [10] and photoconductivity Mat. Res. Soc. Symp. Proc. Vol. 173. ©1990 Materials Research Society
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[11] studies in poly-DCH have been interpreted [10,11] to imply a significant coupling of the carbazole group to the i-electrons of the PDA chain. Additional motivation for further inquiry into electronic interactions between the PDA conjugated backbone and side groups comes from reported values of the crystal dipole strengths, D, a measure of the intensity of a spectral transition. For poly-DCH [12], D = 1.93 A2, while reported [12,131 values for other PDA fall in the range D = 1.66-1.76 A2. This reported modest enhancement of the intensity of the PDA excitonic transition is reminiscent of enhancements of molecular transitions in conformationally rigid 3,y-unsaturated ketones [ 141 and homoconjugated styrenes [15] where the intensity enhancement has been attributed [16] to mixing of locally excited states of the interacting chromophores. N-Ethylcarbazole (2) is a useful model for the side group energy levels (IG, vertical ionization energy; E1 , solution radiation potential; Xmax, position of maximum absorption of lowest energy electronic transition
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