‘ be 8 . te ES ded dS oa ee els ter ta a ed lel lt etaleee a 196 erease in the values of Stokes loss and FWRE (FI). These effects of bridging can be noted in comparison of the data and figures of bridged and unbridged com- pounds, ie. XV and XVI versus V. Compounds XII and XIII also possess the characteristics of planar compounds. In comparing effects on the fluorescence characteristics by a methylene group versus an eth- would not interfere with the measured fluorescence characteristics. An alkyloxy substituent when in the para position as in XVII (Figure 154) and XVIII is effective in in- creasing the value of ema, and the static dipole moment of the ground and first excited singlet state. As ylene group, it is noted that the quantum yield is noted in Figures 154-156, the fluorescence spectra become more diffuse and are shifted toward the red in are larger when an ethylene group is employed. Since of the structure in the fluorescence spectrum is lost. smaller and the values of Stokes loss and FWRE (Fl) AIT and XIII can be called 2-phenyl-fluorene and 2phenyl-9,10-dihydrophenanthrene, respectively, it is apparent that information as to the structure of these compounds can be obtained from studies on fluorene and 9,10-dihydrophenanthrene. From x-ray data on these latter compounds in the erystalline state, it is concluded that fluorene is planar‘’), whereas in the case of 9,10-dihydrophenanthrene the planes containing the phenyl]rings are estimated to be at an angle of about 20° with respect to each other.) On comparing the spectroscopic data on these latter compounds, it is found that the fluorescence quantum yield of fluorene is larger and the values of Stokes loss and FWRE (Fl) are smaller than those of 9,10-di- hydrophenanthrene.®) From these results on comparing bridging by a methylene chain vis-a-vis an ethylene chain, it is concluded that pheny! rings are held in a more planoar conformation when bridged by a methylene chain, and the fluorescence characteristics are optimized better. It is apparent from looking at the absorption spec- tra of the bridged oligophenylenes that the long wave- length absorption bands are produced by two separate transitions. In the case of the most planar compound tested, XIV, the absorption spectrum is very structured and the bands due to the two transitions are well separated. Since ro for each of the compounds in Table 67 is computed by integrating over the whole absorption curve and since a meaningful value of ro is obtained only when the integration is performed over the bands corresponding to the transition leading to fluorescence, the ro values for the bridged compounds are questionable. Therefore, these values are bracketed. The absorption spectra of all the other compounds may possibly contain, also, bands lying at shorter wavelengths than the main band, but in these cases the secondary bands are not readily apparent. Such a masked band in an absorption spectrum would contribute to the very large measured values of FWRE, yet the transition responsible for this band polar solvents such as ethanol. Even in benzene most This spectral shift and the loss of structure are inter- preted by Eisinger and Nayon® as being produced by an interaction between the dipole moment of the solute and that of the solvent. On the other hand, when two alkyloxy substituents are positioned at op- posite para positions of the chromophor as im XIX and XX, the dipole moment becomes negligibly small but emax becomes larger, almost as large as that produced by an additional phenyl group. These effects are also shown in Table 67 and Figures 157 and 158. Benzenehas little effect on this compound. Since XTX is practically insoluble in ethanol, no figure is available for this solution. It is believed that the alkyloxy substituent is particularly effective in affecting the value of emax and the dipole moment when substi- tuted on the para position because the transition moment is along the long axis of the molecule. In conclusion, efficiency and speed are two useful features of a good scintillator and both of these characteristics depend on large values of the molar ex- tinction coefficient. Not only are a large number of rings desirable, but they should be in a linear and planar conformation. When substituents and bridging groups are employed, they should be so positioned so as to enhance, not interfere, with this arrangement. REFERENCES 1. Berlman, I, B. Mol. Cryst. 4, 157 (1968). 2. Wirth, H. O., Herrmann, F. U., Herrmann, G., and Kern, W. Mol. Cryst. 4, 321 (1968). 3. Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic Molecules. Academic Press Inc., New York, 1965, p. 13-26. 4. Berlman, I. B. Chem. Phys. Letters 3, 61 (1969). 5. Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic Molecules. Academic Press, Inc., New York, 1965, pp. 188 and 141. 6. Suzuki, H. Bull. Chem. Soc. Japan 33, 109 (1960). 7. Burns, M. D., and Iballs, J. Proc. Roy. Soc. (London) A227, 200 (1955). 8. Beaven, G. H., Hall, D. M., Leslie, M.S., and Turner, E. EB, J. Chem. Soc. 864, (1952). 9. Berlman, I. B. Unpublished data. 10. Eisinger, J. and Navon, G. J. Chem. Phys. 50, 2069 (1969).