Designed Long-Lived Emission from CdSe Quantum Dots through Reversible Electronic Energy Transfer with a Surface-Bound Chromophore

Abstract

The size-tunable emission of luminescent quantum dots (QDs) makes them highly interesting for applications that range from bioimaging to optoelectronics. For the same applications, engineering their luminescence lifetime, in particular, making it longer, would be as important; however, no rational approach to reach this goal is available to date. We describe a strategy to prolong the emission lifetime of QDs through electronic energy shuttling to the triplet excited state of a surface-bound molecular chromophore. To implement this idea, we made CdSe QDs of different sizes and carried out self-assembly with a pyrene derivative. We observed that the conjugates exhibit delayed luminescence, with emission decays that are prolonged by more than 3 orders of magnitude (lifetimes up to 330 μs) compared to the parent CdSe QDs. The mechanism invokes unprecedented reversible quantum dot to organic chromophore electronic energy transfer.

Keywords: energy transfer; luminescence; nanoparticles; phosphorescence; triplet sensitization.

Figure 1

Schematic representation of the QD–pyrene conjugates and the investigated processes.

Figure 2

Energy‐level diagram for CdSe QDs decorated with 1‐PCA. CdSe‐2 and CdSe‐3 exhibit reversible electronic energy transfer (REET) involving their excitonic level and the energy‐matched triplet excited state of 1‐PCA. The excitonic levels of CdSe‐1 and CdSe‐4 are too high and too low, respectively, for REET to occur at RT.

Figure 3

Absorption (dashed lines, left scale) and luminescence (full lines, right scale; λ _exc=500 nm) spectra of CdSe‐3 (gray traces) and CdSe‐3@1‐PCA (green traces) in air‐equilibrated heptane at RT (C=2.1×10⁻⁷ mol L⁻¹). Each nanoparticle is decorated with 90±7 pyrene units on average.

Figure 4

Luminescence decay, monitored at 600 nm, of CdSe‐3 (gray trace) and CdSe‐3@1‐PCA (green trace) in deoxygenated heptane solution at RT, as measured by a) time‐correlated single‐photon counting (log plot, λ _exc=405 nm) and b) gated streak camera (log‐log plot, λ _exc=465 nm). c) Luminescence decay of CdSe‐3 QDs, with and without 1‐PCA, in air‐equilibrated heptane. d) Luminescence decay of CdSe‐1 QDs, monitored at 540 nm, with and without 1‐PCA, in deoxygenated heptane. The data in panels (c) and (d) were obtained by single‐photon counting upon 405 nm excitation at RT.

Figure 5

Luminescence spectra of optically matched solutions of CdSe‐3@1‐PCA (green traces) and CdSe‐3 (gray traces) in deoxygenated (a) and air‐equilibrated (b) heptane, recorded at different delay times (full lines, 40 μs; dashed lines, 80 μs; dotted lines, 100 μs) upon pulsed excitation at 500 nm.