ABSTRACT
Figure 14A shows a histogram of fluorescence photocounts of individual CV molecules. A distinct bimodal distribution is found in this histogram. To ex amine whether the bimodal distribution is found in this histogram. To examine whether the bimodal distribution is ascribable to the flexible structure of CV, we implemented a control experiment using Texas Red (TR) under the same experi mental conditions as those in the CV experiment. Because of the oxygen bridge between two phenyl rings and immobilization of amino groups TR has a rigid molecular structure. Figure 14B shows a histogram of fluorescence photocounts of individual TR molecules. Only one maximum is found in this histogram. The sharp contrast between the two histograms is expected to be due to the flexibility (or rigidity) or the molecular structures. Because of the flexible structure CV is extremely sensitive to the local viscosity of a PMMA film. On the other hand, the fluorescence efficiency of TR is close to unity [92]; thus, less sensitive to the local viscosity. It is not surprising that the photocounts of the maximum of the histogram for TR is smaller than those of the second maximum in the histogram of CV. The long-pass filter used cut a large portion of the fluorescence of TR,
while almost all of the fluorescence from CV passed the filter because of a large strokes shift of the CV fluorescence.
