Electrostatic Formation of Quantum Dot/J-aggregate FRET Pairs in Solution

We report the formation and photonic properties of CdSe/ZnS quantum dot (QD)/J-aggregate donor-acceptor Forster resonant energy transfer (FRET) pairs that are electrostatically bound and dispersed in water. Assembly occurs when the positively charged dye, 1,1',3,3'-tetraethyl-5,5',6,6'-tetrachlorobenzimidazolocarbocyanine (TTBC), binds electrostatically to QDs that are coated with a negatively charged amphiphilic polymer. QD/J-aggregate FRET pairs display the broadband absorption in the visible and the ultraviolet (UV) part of the spectrum typical of quantum dots, along with the narrow emission linewidths characteristic of J-band emitters (similar to 12 nm full width at half-maximum). We use dynamic light scattering, transmission electron microscopy, photoluminescence spectroscopy, and photoluminescence lifetimes to conclude that the size of the aggregates formed in the presence of QDs is between 1-6 nm. We find the FRET radius of the QD/J-aggregate to be 5.1 nm. We further demonstrate electrostatic binding as a general synthetic strategy for these types of FRET pairs by attaching a negatively charged J-aggregate (BIC) to a quantum dot coated with positively charged ligands.