LOW CONCENTRATION ANOMALIES IN THE ELECTRONIC-ENERGY TRANSFER BEHAVIOR OF SATURATED-HYDROCARBON LIQUIDS

The transfer of electronic energy to 2,5-diphenyloxazole (PPO) from th e neat liquids cyclohexane, n-hexadecane, and 2,3-dimethylbutane, and from dilute solutions of cis-decalin in the nonfluorescent solvents 2, 2,4-trimethylpentane (isooctane), 2,2,4,4,6,8,8-heptamethylnonane (HMN ), and 2,2-dimethylbutane has been studied for excitation at 161 nm ov er a PPO concentration range from 10(-2) to 10(-5) M. The results conf irm and generalize earlier measurements showing that in saturated hydr ocarbon liquids there exists an anomalously efficient sensitization of PPO fluorescence that is mainly manifested at low PPO concentrations and derives from a state, T, other than the normal fluorescing state. The T state process is shown not to depend on the viscosity of the sys tem or on the concentration of the donor liquid. A mechanism is sugges ted involving the generation of an ion-pair state and the migration of its charge separation to PPO.