| Abstract |
We report on single-molecule fluorescence of an aluminum phthalocyanine dye associated with PAMAM dendrimers at a glass surface. The fluorescence decays of the phthalocyanine-dendrimer complex deposited from a solution at pH 3 are monoexponential and with lifetimes closely distributed around its natural fluorescence lifetime. However, in the absence of buffer (pH 6), the distribution of fluorescence lifetimes is broadened toward shorter decay times. Fluorescence quenching is attributed to electron transfer between excited-state phthalocyanine and the dendrimer s tertiary amines, which are deprotonated at higher pH and therefore can act as efficient electron donors. Single-molecule fluorescence measurements give direct access to static heterogeneity as revealed by the broad lifetime distributions for the quenched systems. However, it also gives evidence of dynamic heterogeneity from the fluorescence time traces. In sonic traces, it was possible to identify clear intensity changes that are accompanied by lifetime changes and to define discrete intensity lifetime levels. This behavior was attributed to phthalocyanine dendrimer conformational dynamics that occur on the millisecond-to-second time scale affecting the efficiency of fluorescence quenching by electron transfer. Faster dynamics are not distinguished as discrete levels but give rise to single-molecule multiexponential fluorescence decays. This rich behavior is not accessible in ensemble measurements, whereby only an average lifetime distribution can be obtained. |
