Excitonic behavior of rhodamine dimers:: A single-molecule study

The optical behavior of a dimer of tetramethylrhodamine-5-isothiocyanate has been investigated by means of single-molecule measurements. Bulk absorption and fluorescence spectra show the existence of two populations of the dimer molecule that exhibit distinct excitonic interactions (strong and weak coupling). Fluorescence confocal scanning microscopy has been employed to analyze the behavior of the weakly coupled dimers at the single-molecule level. Stepwise photodamage and collective on/off behavior have been observed in real-time fluorescence trajectories of the dimer. By polarization-sensitive detection, we distinguished between two conformationally different subpopulations within the weakly interacting dimers. Correlation between the fluorescence intensity and fluorescence lifetime of the dimer recorded in time has revealed the competition between photobleaching and trap formation as photodamaging processes of the chromophores in the dimer. The results obtained demonstrate the capability of single-molecule techniques to provide detailed insight into the exciton dynamics in multichromophoric systems, which is a key point in understanding the behavior of relevant natural aggregates and, eventually, in allowing for a rational design of molecular photonic devices.