The synthesis of a new diamine monomer, N-n-butyl 3,12-diamino-5,6,9,10-tetrahydro[5]helicene-7,8-dicarboxylic imide (4), that contains a helically locked, U-shaped 4',4"-o-terphenyl moiety is described. The monomer was polymerized with 3,3',4,4'-oxydiphthalic dianhydride and 2,2-bis[4-(4-aminophenoxy)phenyl] propane to form a series of copoly(ether imide)s (5a-e). The incorporation of 4 into the poly(ether imide)s varied the glass-transition temperature of the copolymers of which it was a part. There was a tendency to form macrocyclic materials at higher molar percentages of 4 during polymerization. The fluorescence of all the copoly(ether imide)s gradually decreased as the content derived from monomer 4 increased in the polymer.