Pressure effects on fluorescence and photodimerization of anthracene and 9-methylanthracene in solution

Abstract

The relative values of photodimerization yields Φ. and fluorescence yields Φ_f with anthracene and 9-methylanthracene were measured in n-hexane at room temperature under pressures up to 3, 000kg/cm^2. In anthracene where Φ_f has a slight pressure dependence, the apparent first order rate constant k_o_b_s for photodimerization yield was approximately inversely proportinal to the viscosity of the solvent. On the other hand, in 9-methylanthrareae where Φ_f increases steeply with increasing pressure, k_o_b_s did not decrease with pressure so much as predicted from the increase of the viscosity of the solvent. Considering the changes of Φ_f as the changes of lifetime. τ_f of the lowest excited singlet state S_1, the pressure dependences of photodimerization rate constants k_r were obtained according to the relation k_o_b_s = k_r.τ_f. In 9-methylanthracene as well as anthracene k_r were nearly inversely proportional to the viscosity of the solvent. The remarkable effects of pressure on fluorescence yields also were discussed in view of the intersystem crossing from the S_1 state to the second triplet state T_2.