Time-resolved measurements of electron and energy transfer of rhodamine B monolayer on the surface of organic crystals

Abstract

Fluorescence decays of a monolayer of rhodamine B on single crystals of anthracene, phenanthrene, and naphthalene have been measured for the first time with a picosecond laser and a streak camera. The fluorescence decays were not single exponentials. Two different decay characteristics have been observed corresponding to an exo‐ and endo‐energetic electron transfer reaction between excited dye and substrate. The short decay of 35±7 ps on an anthracene crystal is explained by the electron‐transfer kinetics from anthracene to excited rhodamine B. In the cases of naphthalene and phenanthrene,the electron transfer reaction becomes endo‐energetic and slower. The decays within several tens of picoseconds are analyzed in terms of two‐dimensional Förster‐type energy transfer to quencher sites where dyes are suggested to be in contact with each other. In connection to this phenomenon, the concentration quenching of the fluorescence of rhodamine B in aqueous solution was elucidated in terms of the Förster‐type energy transfer to the nonfluorescent dimer leading to a nonexponential decay.