In the life sciences Theodor Fçrster’s description of non-radiative, resonant energy transfer (FRET)[1, 2] continues to be widely applied in measurements of protein–protein interactions.[3–6] In addition to his work on describing energy transfer, Fçrster also contributed greatly to the understanding of proton transfer and the formation of excimers.[7, 8] Herein we discuss another, perhaps less widely known, element of Fçrster’s work on molecules in the excited state. In their paper from 1971, Fçrster and Hoffmann outlined their motivation for studying the quantum yield of some triphenyl methane dyes such as auramine O and crystal violet as a function of viscosity. They highlighted concerns regarding deficiencies of an existing theory based on the rotational diffusion of the phenyl rings, which predicted a proportional relationship between the inverse of the fluorescence quantum yield and the inverse of the viscosity. Indeed, the quantum yield data they obtained of crystal violet in various solvents, methanol–glycerol solutions and glycerol at different temperatures from À35 to+ 358C showed poor agreement with this theory over several orders of magnitude. In their paper they developed a model based on the motion of the phenyl rings in the forces of the potentials given by the π electron systems and the steric hindrance, and described the relationship between the quantum yield, Ff, of a dye and the viscosity, h, of the medium surrounding it as Ff/ha, where a= 2/3.[9]

Although there has not been the same explosion in the number of applications of this relationship compared with FRET, it remains fertile ground for further exploration, particularly for fluorescence imaging applications. In the past 40 years there have been examples of the use of the Fçrster–Hoffmann