Slow photochemical transformations of single dye molecules in polymer environment at room temperature

Wide-field epi-fluorescence microscopic images of single dye molecules in thin films have been studied with 1 s time resolution using a CCD camera. The fluorophores at a concentration of 10−7 M were doped in a 12 nm thin polystyrene film layered by spin coating on a surface of fused silica. Terrylene, N,N′-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide and a heterocyclic derivative of the latter were chosen as dyes. The solvents, polymers and the quartz surface were purified carefully in order to get rid of uncontrolled fluorescence. The time dependence of fluorescence intensity for about 100 molecules in each sample has been followed at an excitation intensity of 500 W cm−2. Both the constant or one-step bleaching patterns of fluorescence intensity were found to be rather an exception than a rule for many systems. Most of the molecules exhibit multiple on–off switching, fluctuating or creeping behavior. The population shelving on the T1 level and the rotational diffusion of the transition dipole cannot serve as universal explanation of such fluorescence intensity dynamics. The fluctuations depend on the composition of the gas atmosphere (air, N2 or neat O2), and the presence of electron donors (N,N,N′N′-tetramethyl-p-phenylenediamine) or acceptors (C2Cl6). It has to be surmised that additional phenomena, such as photoisomerisation may be involved in the blinking effect if it persists in nitrogen atmosphere.