Novel Glass-Forming Organic Materials. 2. Structure and Fluorescence of Pyrene- and Carbazole-Containing Cyclohexane, Bicyclooctene, and Adamantane

A series of novel glass-forming organic materials consisting of pyrenyl and carbazolyl groups attached to cyclohexane with a 1-axial-2-equatorial configuration, bicyclo[2.2.2]oct-7-ene with an all-exo configuration, and adamantane were synthesized and characterized. On the basis of proton NMR spectra, it was found that the rotation of pendant pyrenyl and carbazolyl groups is restricted in the bicyclic system presumably because of steric hindrance in the all-exo configuration. In contrast, free rotation was found to prevail in cyclohexane- and adamantane-based systems. Fluorescence spectra gathered in solution at room temperature show evidence exclusively for intramolecular excimer formation in pyrene-containing compounds up to a concentration of 10-4 M. On the contrary, carbazole-containing compounds are not prone to excimer formation in the concentration range 10-6−10-3 M, presumably because of the more stringent requirements of interchromophoric distance and orientation. Although both pyrene and carbazole are highly crystalline on their own, attachment to cyclic, bicyclic, and tricyclic central cores was found to contribute to an ease of vitrification of the hybrid systems with a T g ranging from 43 to 132 °C. Moreover, the quenched glasses of all seven model systems were found to possess morphological stability in view of the absence of recrystallization upon heating from 0 to 200 °C at a heating rate ranging from 0.2 to 20 °C/min. Morphological stability was further supported by the absence of recrystallization upon prolonged thermal annealing at temperatures above T g.