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

Abstract

Dye molecules with pyridyl side substituents (porphyrins and heterocyclic perylene diimides) coordinatively attached to semiconductor CdSe/ZnS quantum dots (QDs) surface form quasi-stable "QD-Dye" nanocomposites of various geometry in the competition with capping molecules (tri-n-octyl phosphine oxide or long chain amines) exchange. This results in photoluminescence (PL) quenching of the QDs both due to Foerster resonance energy transfer and formation of non-radiative surface states. QD surface is inhomogeneous with respect to the involved attachment and detachment processes. The formation of "QD-Porphyrin" nanocomposites is realized at least two time scales (60 and 600 s), which is attributed to a reorganisation of tri-n-octylphosphine oxide capping shell. In a low temperature range of 220÷240 K related changes in QD absorption and emission reveal a phase transition of the capping shell (tri-n-octyl phosphine oxide and amine). In "QD-Dye" nanocomposites, this phase transition is enhanced considerably by only a few attached

Deactivation of Excited States in Nanostructures Containing Cu-Porphyrin Subunit

“…In this case, like for multiporphyrin complexes (described above), a controllable formation of "QD-Porphyrin" nanoassemblies have been realized as a surface passivation of CdSe/ZnS ODs by tetrameso-pyridyl substituted porphyrins (free base and/or Cu-complex) in titration experiments. [29,[90][91][92][93][94][95][96][97][98][99] It is well-known from chemical background that the 3d transition metal Zn 2+ ion (of ZnS shell) has empty 3d 10 orbital while heteroatom N-pyr of the porphyrin mesopyridyl ring is a very good e-donor having an unshared electron pair. Thus, in this case a "key-lock" principle is realized via oneor two-fold non-covalent coordination Zn….N-pyr.…”

“…[97,100,101] We have quantitatively discriminated for the first time, that the major part of the observed PL strong quenching for CdSe/ZnS QD in "QD-Porphyrin" nanoassemblies can be understood in terms of the electron tunneling across the ZnS shell in the conditions of quantum confinement, and the minor part (10-15 %) of the QD PL quenching is caused by Foerster resonance energy transfer (FRET) QD→porphyrin. [90,91,[93][94][95][96][97]123] Instead, the interaction of porphyrin molecules with QD leads to the inhomogeneous surface dynamics for semiconductor QDs and the non-FRET quenching of QD PL in nanoassemblies might be related to few possible reasons: i) depletion of capping ligand TOPO molecules by the respective porphyrin molecules, ii) creation or redistribution of QD surface trap states, iii) low temperature reconstruction of capping ligand layer perturbed by attached porphyrin, etc. Correspondingly, using porphyrins (especially meso-pyridyl substituted H 2 P and CuP) attaching with QD surface, one may tune the electronic states of CdSe/ZnS QD and control luminescent properties of nanoassemblies.…”

“…Time and spectrally resolved single QD experiments have been carried out with a home-built setup. [97,100,101] All photophysical measurements were completed within 1-2 hours following preparation. Additional experimental details are presented in the corresponding papers cited for every class f compounds being studied.…”

“…[29,[90][91][92][93][94][95][96][97][98][99]123] Structures of QDs and porphyrin molecules as well as necessary details explaining the formation of nanoassemblies "QD-Porphyrin" are shown in Figure 9. The results described below concern to the analysis of the interaction between semiconductor CdSe/ZnS quantum dots and surface attached porphyrin molecules (free base and Cu-complex) in order to evaluate the influence of tetrapyrrolic macrocycle on pathways and mechanisms of exciton relaxation in "QD -Cu-porphyrin" nanoassemblies.…”

From Porphyrin Chemical Dimers to Complex Multicomponent Nanoassemblies: Some Rare Phenomena and Relaxation Processes

“…[75] Using a combination of ensemble and single molecule spectroscopies of "QD-Porphyrin" nanoassemblies, we showed that single functionalized molecules can be considered as extremely sensitive probes for studying the complex interface physics and chemistry and related exciton relaxation processes in QDs. [76][77][78][79][80][81][82][83][84] It was quantitatively shown that the major part of the observed QD photoluminescence (PL) quenching in nanoassemblies is caused by electron tunneling to the QD surface in conditions of quantum confinement, which is influenced by the attachment dynamics of porphyrin molecules. In these nanoassemblies, Foerstertype energy transfer (FRET) QD→dye is often only a small contribution to the PL quenching and is already effectively suppressed in slightly polar solvents.…”