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

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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.