Emission of the contact ion pair of rhodamine dyes observed by single molecule spectroscopy

Spectra of single molecules of different rhodamine dyes deposited on a glass surface and on a methyl-terminated silanised glass surface were recorded by means of confocal fluorescence microscopy. The observation of an emission, blue shifted in comparison to the solution spectrum of these rhodamine dyes, is reported. It is suggested that this shorter wavelength emission is related to the contact ion pair formation of the single rhodamine dye molecule.