Electron transport in the long-range charge-recombination dynamics of single encapsulated dye molecules on TiO2 nanoparticle films

The interfacial electron transfer and subsequent charge-recombination between single carbo-rhodamine Atto647N molecules and TiO₂ nanoparticle and between single cucurbit[7]uril encapsulated Atto647N molecules and TiO₂ nanoparticle are studied. Power-law distributions for the lifetimes of the dark charge-separated states are obtained. This is attributed to the broad time range taken for the injected electron to diffuse between traps on the TiO₂ nanoparticle before undergoing back electron transfer (BET) to the oxidized dye. A significantly smaller power-law exponent for the inclusion complex suggests that the spatial separation, created by the macrocyclic host, between the dye molecule and metal oxide results in a slower BET rate and a longer electron transport process. Computational simulations based on the continuous-time random walk are used to model the electron transport process, and the power-law exponent values obtained from the computational simulations are consistent with the experimental observations.