The mechanism for the quenching by oxygen of the singlet and triplet states of several anthracene derivatives in methylcyclohexane (MCH) under pressures of up to 700 MPa was investigated. The value for the rate constant of fluorescence quenching, k s, at 0.1 MPa is found to vary from (3.2 f 1.1) X lo9 M-' s-* for 9,10-dicyanoanthracene (DCNA) to (2.88 f 0.27) X IOy0 M-l s-I for 9-methylanthracene (MEA), whereas values for the rate constant of the triplet-state quenching process, k at 0.1 MPa are similar for each of the anthracenes, being in the range (3.0-3.8) X IO9 M-' s-I. The values for the pressure increase. A linear relation between In k," and In 7 is found for anthracene (A) and MEA, with slopes of -0.57 f 0.04 and -0.64 f 0.02, respectively. However, plots of this relation show a distinct downward curvature for 9,lO-dichloroanthracene (DCLA) and DCNA. It is also found that In kqT does not vary linearly with In 7 for any of the derivatives examined. The activation volumes of k," for A and MEA are estimated to be in the range 12-14 cm3 mol-I. These values are about 2 times larger than those determined for k T, but are only half of the value reported for the activation volume of the viscosity of MCH. The ratio of k, ' to k: for MkA and DCLA at 0.1 MPa is reasonably close to the predicted value of 1/9 and increases with pressure, reaching a value of approximately 4 9 for DCLA at 700 MPa. These results suggest that, is not conserved, may come to play an important role in the quenching of the triplet state as the pressure is increased. Dynamic aspects of the fluorescence quenching are also discussed in terms of the transient decay feature characterized by the function predicted by the Smoluchowski model. k: and k, P' decrease with increasing pressure, mainly as a result of the increase in viscosity of the solvent that accompanies in addition to '(AOz)*, encounter complexes of the form 3(AOz)* or / (A02)*, for which the total spin angular momentum
IntroductionMolecular oxygen is an efficient quencher of the electronically excited states of many organic molecules. In most cases, the quenching by oxygen is so eMicient that the reaction rate is believed to be diffusion-limited. However, quenching rate constants reported in the literature vary from compound to compound, for example, 3.3 X IO'O M-' s-' for p-methoxybiphenyl and 4.4 X lo9 M-l s-I for flu0ranthene.I Despite this wide range of magnitudes,