Single molecule study of Rhodamine/ATO nanoparticle junctions

Abstract

Single rhodamine B (RhB) molecule-Sb:SnO 2 (ATO) nanoparticle junctions were studied using two-photon excitation single molecule fluorescence spectroscopy. For each molecule-nanoparticle junction, fluorescence decay of RhB was found to be single exponential, indicating a static heterogeneous distribution of junction properties. The measured fluorescence lifetime is shorter than RhB on inert substrates but longer than that expected on ATO based on ensemble averaged electron transfer rates. Possible origins for the shorten single molecule lifetimes and distribution are discussed.

Single-Molecule Interfacial Electron Transfer in Donor-Bridge-Nanoparticle Acceptor Complexes

“…Under these conditions, the molecules detected by single molecule fluorescence measurements account for a few percent of the excited state population, and these molecules either inject electrons at a slow rate (≪1/(3 ns)) or do not undergo IET at all. Similar observation has been reported before for RhB molecules on Sb−SnO 2 (ATO) …”

“…On the single molecule level, fluorescence decay was found to be single exponential, although there existed a distribution of lifetimes, suggesting a static inhomogeneous distribution of IET rates. Similar static heterogeneous distribution of ET rate was observed in our previous study of rhodamine B (RhB) on ATO . We noted that the observed single molecule lifetimes are much longer than the ensemble averaged lifetimes due to incomplete sampling of molecules undergoing fast ET in the single molecule study.…”

“…Therefore, the observed distribution and fluctuation of lifetimes cannot be attributed to a fluctuation in nonradiative decay rate. For a fluorescence molecule at an interface, its radiative lifetime depends on the refractive index of the media and the orientation of the molecule relative to the interface normal. , A distribution of single molecule lifetimes on glass due to variation of orientation has been observed in previous studies. , We attribute the observed lifetime distribution to the distribution and fluctuation of the orientation of SRhB−silane molecules relative to surface normal. This orientation change may increase or decrease the emission intensity, depending on how it affects the projection of the transition dipole relative to the polarization of the linearly polarized excitation pulse.…”

“…Single molecule fluorescence spectroscopy has been used to study electron transfer (ET) processes in molecules, , in conjugated polymers, at interfaces − and in biological systems. − Studying ET process by single molecule fluorescence is still technically challenging, because it shortens the fluorescence lifetime and reduces fluorescence quantum yield of the dye molecules. As shown in Figure , the relationships of relevant deactivation processes and emission quantum yield can be described by eqs − 1 τ = k r + k nr = k 0 1 τ ′ = k r + k nr + k et = k 0 + k et Φ 0 = k r k 0 , Φ I = k…”

“…As a result, these IET events could not be directly observed. So far, there have been only a few published reports of single molecule IET. − In their pioneering work, Lu and Xie measured single molecule fluorescence lifetime of cresyl violet on ITO (Sn/In 2 O 3 ) . They observed shorter fluorescence lifetimes on ITO than on glass and attributed it to IET from the excited dye to ITO.…”