Room temperature observation of quantum jumps of single molecule into dark states

We monitored the fluorescence emission rate of single dye molecules on a dry surface with the near-field scanning optical microscope (NSOM). A Texas Red fluorophore was covalently attached to a short strand of a DNA molecule and bound to an amino-propyl-silanized glass coverslip. After imaging, the NSOM probe was stationed over a single molecule and its emission rate was recorded as function of time until its photodestruction. Counts were integrated for 520 ms and typical count rates were 5-30 kHz. A typical emission time trace is shown. In addition to the irreversible photodestruction event at 26s, reversible jumps to background level around 4s and 9s are noticeable. These long-lived dark states last few seconds. The two curves are polarization-resolved emission time traces recorded simultaneously with circular polarization excitation.