3)21t,3'-Dimethyl-p-quaterphenyl (I), 2", 3', 5'. 6" -tetramethyl-P-quaterphenyl (II), 6.7dihydro-6-methyl-3,9-diphenyl-SH-dibenz[c, elazepine (m), 5.7dihydro -3 , 9diphenyldibenzo[c, elthiepin (IV), 5,7dihydro-3, g-diphenyldibenzo[c, elselenepin (V), and 6,6dicarbethoxy -6 , 7dihydro -3 , 9 -diphenyl -5H -dibenzo[a, clcycloheptene (VTj have been synthesized and screened a s potential primary liquid scintillation solutes. The scintillation properties of I, II, III, N, V, and VI have been compared with 2,7-diphenylfluorene (VII) (4), 9,lO-dihydr0-2,7-diphenylphenanthrene (Vm) (5), and 5,7-dihydro-3, g-diphenyldibenz[c, eloxepin (M) (5). The relative pulse-heights of I, VI, VII, Vm, and I X at 3 mM in toluene gave a linear relationship when plotted against the cos2 of the respective estimated angles of torsion about the 1". 4I-bond in the P-quaterphenyl system. The azepine 111, thiepin N, selenepin V, and tetramethyl-P-quaterphenyl I1 were very poor scintillators in toluene solution.In previous communications (4,5,6) it was shown that 2,7'-diphenylfluorene (VII), 9,10 -dihydro-2.7diphenylphenanthrene (VIE), and 5,7dihydro-3, 9diphenyldibenz[c, eloxepin (nr) were excellent liquid scintillation solutes. Molecular models of VIII indicate that the CH2-CH2-bridge in this compound causes the bridged benzene rings to be twisted at an angle of about 31" with respect to one another, while the -CH2 -0 -C H 2 -bridge in M causes the benzene rings to be twisted at an angle of about 47" (Figure 1). The C H 2 -bridge in the fluorene VII causes the two benzene rings to be essentially coplanar but not colinear. The resonance interaction between the benzene rings should be greatest in VII and least in M, since the P-orbital overlap between the directly bonded carbon atoms of the bridged benzene rings should decrease in the order VII > VIII > M. The high I-max values obtained for VIII and M (1.15 and 1.14, respectively (7)) as compared
