Liquid scintillators. XIII. steric inhibition of resonance in liquid scintillators

Abstract

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

How the Central Torsion Angle Affects the Rates of Nonradiative Decay in Some Geometrically Restricted p-Quaterphenyls

“…19 The compound displays relatively intense fluorescence in fluid solution that is known to originate from the long-axis 1 L a f 1 A transition. Interestingly, Taber 20 showed in 1965 that the scintillator efficiency for p-QT could be controlled by altering the planarity of the external rings in the molecule. Subsequently, Daub et al 21 prepared other constrained derivatives, but no systematic study of their electronic behavior was made.…”

Photophysical properties of thiophenes and 2,2′-bithiophenes containing alicyclic moieties

“…The fact that aldehyde 9a has more efficient electron charge transfer due to rigid and planar structure is demonstrated by the moderate quantum yield value in comparison with 9b. Although compounds 9a and 9b have small structural difference such as an additional methylene group in the alicyclic unit the possible explanation of low quantum yield for 9b can be attributed to non-planar geometry of the molecule [75,78]. The presence of an extra methylene group in the olefin fragment of 5-monosubstituted bithiophene 9b provides greater conformation flexibility that may cause loss of energy by external or internal process [75] and decrease quantum yield.…”

“…The possible explanation for the results observed in ethanol and diethylene glycol is strong hydrogen-bond interaction in the excited state of the molecules (9a, 9b, 11ce11e). The Kamlet p*-values consider the polarity and polarizability parameters for solvents; however the relation of these parameters on the fluorescence maxima were presented in the Lippert equation [78,79].…”

“…The orientation polarizability (DG), which consists two parameters the dielectric constant of solvent and refraction index, may be used for estimation of influence of solvent polarity on Stokes shift [78,79].…”