ve Bo ee rennet a dei, ella ae OR 183 ON THE FLUORESCENCE CHARACTERISTICS OF THE p-OLIGOPHENYLENES AND THEIR SUBSTITUTED ANALOGS I. B. Beriman and O. J. Steingraber The p-oligophenylenes have very desirable fluorescence characteristics, such as high quantum yields and_ short decay times, that make them very useful as scintillators. A systematic investigation has been made of the fluorescence characteristics of variously substituted and bridged compounds with the hope of understanding better the relationship between molecular structure and these characteristics. When large alkyl chains are employed as substituents to enhance the solubility of a compound, it is important that these substituents be positioned in the proper place. When alkyl substituents are placed on the para or meta positions of terminal rings, the effect on the fluorescence characteristics is minimal; but if the substituents are placed on other posi- tions, the characteristics are adversely affected by steric (FWRE) of the maximum value of the spectra have been explained elsewhere.'* 4+) Compound IV (Table 67) was kindly sent to us by Prof. Dr. W. Ried* and the remaining compounds except for I and V were generously given to us by Dr. H. O. Wirth.+ RESULTS Unsubstituted p-oligophenylenes such as p-terphenyl (I) and p-quaterphenyl (V)“> and some of the substituted compounds such as 3,3’-dimethyl-pterphenyl (II) display band structure in their fluorescence spectra but not in their absorption spectra as crowding. Moreover, an alkyloxy group substituted on the para position will enhance the dipole moment of the ground and first excited state, as well as the molar extinction coefiicient. curve is structureless because a planar configuration in the ground state is impeded by steric crowding. In INTRODUCTION ing adjacent phenyl rings, acquire some double bond shown in Figure 140. It is believed that the absorption the excited state the essential bonds, those bonds join- In a previous publication’) it was shown that the character, thus becoming stronger and moreeffective in forcing the pheny! rings into a more planar configura- p-oligophenylenes (see Table 67) are a most useful set of compounds possessing very fast fluorescence de- tion. Supporting evidence for the above contentions the molar extinction coefficient « was increased, the natural fluorescence lifetime decreased, and both the cussed below, analogous compounds with substituents cay times. It was reported that as the number of phenyl rings in the basic chromophor was increased, absorption and fluorescence spectra shifted toward longer wavelengths. An experimental problem that arises when the molecular length is increased is that the solubility of these compounds in aromatic sol- vents is drastically decreased. Wirth, et al. have demonstrated that by adding large alkyl chains to the p-oligophenylenes, their solubility is dramatically enhanced. For example, the solubility of p-quater- phenyl in toluene at room temperature is about 0.1 g/l, whereas that of tetramethyl-p-quaterphenyl (VII) is over 500 g/). Substituents not only change the solubility of a compound, but they may also interfere with the planarity of the chromophoror affect its dipole moment in either the groundstate or first excited state or both. The effect of various types of substituents, their number, and positions on the chromophor on the fluorescence characteristics have been systematically investigated; the data are tabulated in Table 67 and some of the spectra are shown in Figures 140-158. The procedures employed in measuring the absorption and fluorescence spectra, the fluorescence decay time, natural lifetime, and quantum yield, Stokes loss, and the full width at reciprocal epsilon may be found in the observation that the values of the FWREof the fluorescence spectra are much less than those of the absorption spectra (Table 67). As disthat interfere with the planarity of the chromophor do not have fluorescence spectra with structure. It has been known for some time”) that the maxi- mum value of the molar extinction coefficient emax of the p-oligophenylenes increases monotonically with the number of phenyl rings. Since the value of the natural fluorescence lifetime 7» can be approxi- mated") by integrating over the long wavelength ab- sorption bands as follows Le. Av | e dp TO . where vo is the wave number of the line of mirror symmetry between the absorption and fluorescence spectra and ¢« is the molar extinction coefficient (liter mole~tem~*), it is apparent that the longer the chain of rings the shorter is the computed lifetime. It has already been pointed out™) that the p-oligophenylenes have the added advantage of having their emission spectra at relatively short wavelengths (large vo values) as compared to other conjugated * Goethe University. Frankfurt, Germany. + Present address: Deutsche Advance Produktion GMBH, 6140 Marienberg. Post Bensheim, Germany.