195 1.00 4700 4500 a | 1 4300 T i Ly 4100 i + WAVELENGTH hal 4. (A) oe at00 4s 4" -D] HEXAHY DROFRANESOXY -P-TEAPHENTL 0.90 | SOLVENT ao o 4 32000 EXCITING WAVELENGTH 3040A ) s oa 0027 G/L . 4 28000 ~~ DECAY TIME 0.95 NSEC WAVELENGTH (AVE) Qo Ww 0.09 MM WAVELENGTH (CG) -| 24000 2700 CM! 4 20000 PHOTON INTENSITY +d STOKES LOSS J649A 3677A 16000 s 0.40 0,30 L EMISSION ABSORPTION 12000 0.20 | 0.10 L 0.00 21000 L 22000 i 23000 1 24000 7 25000 rn 26000 1 27000 L 28000 1 29000 k 30000 WAVE NUMBER HaCiso-—{_)—{_)—{(_))—00Hs 31 000) (CM 3 33000 33000 34000 35000 3e000 + 8000 4 4000 MOLAR EXTINCTION COEFFICIENT =] Qa a ' T n Oo SLIT WIDTH T (ARBITRARY SCALE) T CONC. BENZENE 37000 158.—Compound XIX, 4,4”-dihexahydrofarnesoxy-p-terphenyl, solvent benzene systems of the similar size. Among the compounds tested, XI is the longest molecule and, as expected, has the shortest computed lifetime, 0.8 nsee. It has been postulated by Suzuki‘® that the larger readily noted in comparing the data for II with III, VI with VII, and VII with IX, as shown in Table 67 and Figures 140, 141, and 143-146. If it were not for steric crowding by the substituents with the hydrogen mophor in the ground state. In support of this as- atoms on the basic chromophor in VII, X (Figure 147) and XI (Figure 148), the natural fluorescence become smaller as the chromophor becomes larger as IV (Figure 142) can be considered as tetraphenyl-p- the p-oligophenylene, the more planar is the chro- sumption, the FWREvalues of the absorption spectra shown in Table 67 for compoundsII, VI, and VIII. Alkyl chains substituted in the para and meta posi- lifetimes would be even shorter than those tabulated. terphenyl and is an illustration of an extreme case of steric hindrance. For this latter compound the quan- tions of terminal rings are effective in increasing the tum yield is very small, the intense transition that is planarity of the chromophor or the strength of the transition. For these reasons, these substituted com- ward shorter wavelengths, and Stokes loss is very solubility of the compound and yet do not affect the pounds possess characteristics very similar to those of the unsubstituted compounds. On the other hand, if these same substituents are positioned on the ortho position of the end rings or on meta or ortho positions of phenylene rings, the planarity of the basic chromophoris disturbed, and its fluorescence characteristics are affected in the followmg manner: The long wavelength absorption bond is shifted toward shorter wavelengths, the value of enax Is reduced, the value of FWRE of the absorption curve is increased, the fluorescence spectrum becomes less structured, the value of Stokes loss is increased, and the quantum yield is reduced. These effects of steric hindrance are the hallmark of the p-oligophenylenes is shifted tolarge. Surprisingly FWRE (Fl) is abnormally small. This result may be explained by the fact that the six phenyl rings make large angles with the central benzene rings so that there is no effective conjugation between the rings and the fluorescence characteristics correspond to a 1L, — 1A transition of the basic chromophor, the central benzene ring. The angle between successive phenyl rings can be held fixed by means of a bridging group as in XII, XIII, XIV, XV, and XVI, Figures 149-153, respec- tively. This added planarity is responsible for the structure in both the absorption and fluorescence spectra, and their shift toward longer wavelengths, for the increase in the value of emax, and for the de-