FRET and ligand related NON-FRET processes in single quantum dot-perylene bisimide assemblies

Abstract

Nanoassemblies are formed via self-assembly of ZnS capped CdSe quantum dots (QD) and perylene bisimide dyes (PBI). Upon assembly formation with functionalized dye molecules the QD photoluminescence (PL) is quenched. Quenching has been assigned partly to FRET (fluorescence resonance energy transfer) and NON-FRET processes. By means of time resolved single particle spectroscopy of immobilized QD-dye assemblies, it is demonstrated that NON-FRET processes are due to new non-radiative decay channels caused by the assembly formation process itself. Immobilized (single) assemblies exhibit the same processes as ensembles of assemblies in toluene solution. Only one dye molecule on a QD quenches the PL up to 50%, which is much stronger than is expected when replacing a volume related number of ligands. NON-FRET processes are distinct from photoinduced charge and/or energy transfer. A combination of a Stern-Volmer and FRET analysis of ensemble experiments supports the investigation of the dynamics of assembly formation at extremely low concentration ratios of PBI to QD. This allows us to distinguish between the effects of PBI and ligands on PL quenching on a single molecule level which is not possible in conventional ligand dynamic experiments.

Ligand Exchange Dynamics and Temperature Effects upon Formation of Nanocomposites Based on Semiconductor CdSе/ZnS Quantum Dots and Porphyrins: Ensemble and Single Object Measurements

“…It means that the exciton relaxation dynamics in QD initiated by a single titration step is not only due to the added H 2 P molecules themselves, but also to a local change in the capping ligand shell on QD surface upon nanocomposite formation as well as to a local replacement of TOPO by H 2 P molecules as suggested earlier. [78,79] This argumentation is in line with the finding that dilution of a QD solution reduces the average coverage of the QDs with TOPO, [51,52] giving rise to both an intrinsic reduction of QD PL [80] as well as to increased accessibility of the QD surface to the quencher molecules. It should be mentioned also that the capping ligand coverage (TOPO or amines), and thus, the number of accessible attachment sites are controlled by the solvent polarity.…”

“…We assume that a similar situation holds also for H 2 P just after titration step. Indeed, QD PL quenching is still increasing in the presence of H 2 P while also FRET increases at long waiting times (see Figure 5C) up to > 24 h. [51,52] It means that at very high H 2 P concentrations and long waiting times the TOPO capping shell becomes nearly completely replaced by H 2 P molecules.…”

“…At ambient conditions, single particle experiments (PL spectra and decays) were performed in a home built laser scanning confocal microscope [51,52,54] depicted schematically in Figure 2. Shortly, the main features of this experimental setup are as follows.…”

“…(6) Finally, it follows from ensemble and single object experiments for "QD-perylene-diimides" nanocomposites, [51,52] that the number of attached dye molecules to a QD is much less than that given by the molar ratio x. We assume that a similar situation holds also for H 2 P just after titration step.…”

“…[31][32][33][34][35][36][37][38] Inspired by our earlier work on self-assembled multiporphyrin arrays [39][40][41][42][43][44][45] we have elaborated the experimental approach in the direct labelling of trioctylphosphine oxide (TOPO)-and amino (AM) -capped semiconductor quantum dots (QD) CdSe/ZnS with functional ligands (dyes) of two types (pyridyl substituted porphyrins and heterocyclic pyridyl functionalized perylene diimide molecules) in liquid solutions and polymeric matrixes. [46][47][48][49][50][51][52][53][54][55] We have shown that depending on redox and electronic properties of interacting subunits as well as anchoring groups (connecting organic and inorganic counterparts) the formation of "QD-Dye" nanocomposites allows for a controlled realization of mutually relative (spatial) orientations and electronic energy scales in order to optimize intended photoinduced processes such as charge transfer, [56,57] fluorescence Foerster energy transfer (FRET) [20,46,47,50,52] or electron tunnelling in the conditions of quantum confinement (non-FRET process). [48,50,53] Typically, all these processes lead to the pronounced quenching of QD photoluminescence (PL) in nanocomposites that may be used as an indicator of complex interface phenomena selectively depending on attached ligands (porphyrins, perylene diimides, etc.…”