Förster electronic excitation energy transfer upon adsorption of meso-tetra(3-pyridyl)porphyrin on InP@ZnS colloidal quantum dots
Introduction
Owing to the discovery of Murray, Norris, and Bawendi in 1993 [1] of a simple and very efficient high-temperature synthesis of colloidal quantum dots (QDs), not only new opportunities for their study have opened up, but also real prospects for their practical use have become clearer. The further development of this method is aimed at the search for new high-temperature solvents that effectively stabilize the surface of QDs, and on the development of methods for post-synthetic modification of QDs, which permits achieving high photoluminescence quantum yields [2].
QDs are a new class of luminophores [3], which, unlike organic molecular luminophores, potentially have much higher thermal stability and photostability. In addition, QDs have a number of functional advantages due to the unique ability to vary their spectral characteristics; hence, they are now being used in light-emitting diodes, displays, solar cells, optical amplifiers, lasers, chemo- and biosensors, biomedical diagnostics.
An important property of QDs as luminophores is that their spectra exhibit considerable inhomogeneous broadening, because of which their solutions do not obey the usual laws of luminescence: the photoluminescence spectra depend on the excitation wavelength, the photoluminescence excitation spectra depend on the emission wavelength and are not proportional to the absorption spectrum, the universal thermodynamic Kennard-Stepanov relation between the absorption and photoluminescence spectra does not hold [4]. This significantly complicates the analysis of the QD optical spectra.
Of particular interest for a variety of applications are hybrid luminophores formed by binding of molecular luminophores with QDs. The binding can be carried out due to physical or chemical adsorption of a dye molecule on the surface of a quantum dot, with an important role played by the processes of rearrangement of the ligand shell of the quantum dot and competition for the adsorption site between the dye and ligand molecules. The ligand shell not only prevents coagulation of the nanoparticles, but also is necessary for high photoluminescence of QDs, compensating for surface defects and surface trap states being QD photoluminescence quenchers. Therefore, the rearrangement of the ligand shell upon the introduction of dye molecules can lead to a significant change in photoluminescence of the quantum dot. In addition, upon absorption of light in hybrid luminophores, photoinitiated electron transfer processes and Förster resonance energy transfer (FRET) can occur. All these factors ultimately lead to changes in the optical characteristics: they change the optical absorption and emission spectra, the luminescence quantum yield and lifetimes of both the quantum dot and the luminophore molecules.
In the hybrid system nanoparticle–dye, the quantum dot, having a large absorption cross section, is an antenna, whereas the dye possesses certain functional properties determining a specified field of application of such systems. For example, hybrid systems are actively being studied in the direction of creating various luminescent chemosensors [5], [6], increasing the efficiency of solar cells [7], [8], and expanding the capabilities of photodynamic therapy [9], [10], [11], [12].
In these applications, one of the important classes of dyes are porphyrin derivatives. In the case of photochemosensors, the use of porphyrins is related to their ability to bind ions of various metals, with changing their optical properties [13], [14]. For solar cells, of interest are their high extinction coefficients in a wide range of wavelengths [15]. For photodynamic therapy, it is important that porphyrins in the electronically excited state effectively generate singlet oxygen.
In a large number of works, Zenkevich et al. studied CdSe and CdSe@ZnS (core@shell) QDs with adsorbed meso-pyridyl-substituted porphyrins [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. The main effect, which was observed with confidence in these studies, was QD photoluminescence quenching by substituted porphyrins.
In [16], experiments were carried out with a variety of modes of pyridyl substitution in the porphyrin moiety: the number of pyridyl substituents, their mutual positions, and the position of nitrogen with respect to the site of attachment to the porphyrin macrocycle were varied. Tetra-pyridine-substituted porphyrins with nitrogen atoms in the meta and para positions were found to possess the maximum efficiency of QD photoluminescence quenching. Porphyrins with the substituents having nitrogen atoms in the ortho position, as well as pyridine itself, unsubstituted porphyrin, and tetraphenylporphyrin did not quench photoluminescence at comparable concentrations. As a result of these studies, it was concluded that the attachment of tetrapyridylporphyrin to the surface of the quantum dot occurs via the two nitrogen atoms of the nearest pyridyl substituents, with the porphyrin macrocycle occupying a position perpendicular to the adsorbing surface. It was also experimentally shown in [16] that upon adsorption of the dye on QDs, FRET becomes possible between them, which leads to a small increase in the porphyrin photoluminescence intensity. Calculations showed that the characteristic time of the energy transfer is of the order of hundreds of picoseconds [18] and, hence, FRET may be completely responsible for quenching QD photoluminescence. However, it is argued in the experimental works [17], [19], [20] that FRET cannot completely explain the quenching. Furthermore, it was concluded [21] that only 10–15% of the total quenching of QD photoluminescence was due to FRET. It was noted in [23] that the kinetics of adsorption of the studied dyes on the surface of QDs, which was measured by the kinetics of photoluminescence quenching, did not have single characteristic time. Photoluminescence quenching was observed after the addition of the dye at times shorter than 1 min and longer than 10 min, and even for several hours a slow decrease in QD photoluminescence continued. It was suggested that the broad distribution of the adsorption rates was due to inhomogeneity of the surface of QDs, leading to a difference in the binding energies of individual molecules of the ligand shell of the quantum dot, which are displaced by the dye molecules. The reason for the photoluminescence quenching can also be slow reorganization processes of the ligand shell due to adsorption of the dye.
However, the reliable interpretation of the observed phenomena described in the works of Zenkevich et al. cited above is significantly complicated by the considerable instability of the samples of CdSe and CdSe@ZnS QDs used. As noted by the authors, the luminescence quantum yield of CdSe and CdSe@ZnS QDs can decrease considerably within a few tens of minutes even without an addition of porphyrin molecules. Furthermore, the quantum yield undergoes significant changes even upon dilution of the colloidal solution, and the quantum yield strongly depends on the purity of the solvent and other experimental conditions. [23]. Despite the fact that the authors of these works took various measures to eliminate the influence of such changes, carefully selecting the conditions of the experiment, it was not possible to completely eliminate the influence of QD instability. Therefore, a number of fundamental questions concerning the mechanism of QD photoluminescence quenching are still unanswered.
In this work, we studied the processes of QD photoluminescence quenching by meso-tetra(3-pyridyl)porphyrin (TPyP) molecules. We used here much more stable InP@ZnS QDs with a ligand shell consisting of tetradecylamine molecules. The luminescence properties of solutions of these QDs practically do not change on keeping for a long time or upon dilution of the colloidal solutions. Furthermore, unlike cadmium-containing QDs, these QDs possess low toxicity. While this is not of fundamental importance for the studies, in the long run, this can be useful for practical applications.
Section snippets
Experimental
To synthesize InP@ZnS core–shell QDs with tetradecylamine as a stabilizing ligand, the following reagents were used: indium(III) chloride (99.995%, Acros), zinc chloride (anhydrous, 98%, Sigma–Aldrich), tetradecylamine (TDA, 95%, Aldrich), tris(dimethylamino)phosphine (TDMAP, 97%, Aldrich), 1-dodecanethiol (DDT, 98%, Aldrich), chloroform (99.5%, Sigma–Aldrich, with amylene content of 0.01–0.02%), toluene (99.99%, Fisher Scientific), methanol (reagent grade, Chimmed), and acetonitrile (high
Results and discussion
Fig. 1 shows the absorption and photoluminescence spectra of InP@ZnS QDs and meso-tetra(3-pyridyl)porphyrin (TPyP) used in this work. The absorption spectrum of TPyP was normalized to the value of the molar extinction coefficient ε516 = 1.93 × 104 M–1cm–1 for the peak of the most intense Q band (λ = 516 nm) taken from the work of Zenkevich et al. [16].
The absorption spectrum of the InP@ZnS QDs was normalized to the value of the extinction coefficient at a wavelength of λ = 350 nm, calculated by
Conclusions
It has been shown that after the addition of a solution of TPyP to a solution of InP@ZnS QDs in chloroform, adsorption of TPyP molecules on InP@ZnS QDs occurs with logarithmic kinetics, which indicates the presence of a wide distribution of the activation energy of the process. The adsorption of the dye leads to two effects. First, a shift in its absorption and photoluminescence bands and an increase in the photoluminescence lifetime are observed. Second, FRET from the QDs to dye molecules
Acknowledgments
This work was supported by the Russian Science Foundation, project no. 14-13-01426.
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