Solvent viscosity effects on the fluoresence lifetimes of 9-phenylanthracene

This study is concerned with a microscopic look at viscosity, generally considered a macroscopic quantity. Viscosity effects on a molecular scale were probed by studying the fluorescence decay times of a nonpolar dye (9-phenylanthiacene) as a function of viscosity in 10 nonpolar linear alkanes. These times increased from 3.45 to 5.75 ns for viscosities from 0.300 to 2.57 cp. This is attributed to "frictional" forces between fluorescing and solvent molecules during isomerization of the fluorescers. Excitation and deexcitation require the rotation of the phenyl ring from a direction perpendicular to the anthracene moiety in the ground state. Solvent viscosity tends to hinder this rotation. Hence the fluorescence decay rates are influenced by viscosity on a molecular scale. An empirical model that relates the decay rates to a reciprocal function of viscosity is discussed. The fluorescence was excited at 362 nm by a pulsed N2-pumped dye laser, and emission was monitored at 420 nm. Our fast analog technique was used in the acquisition and analysis of the time-resolved data.