Quantitative Analysis of Singlet Oxygen (1O2) Generation via Energy

ARTICLE pubs.acs.org/JPCC

Quantitative Analysis of Singlet Oxygen (1O2) Generation via Energy Transfer in Nanocomposites Based on Semiconductor Quantum Dots and Porphyrin Ligands Eduard I. Zenkevich,*,† Evgenii I. Sagun,‡ Valery N. Knyukshto,‡ Alexander S. Stasheuski,‡ Victor A. Galievsky,‡ Alexander P. Stupak,‡ Thomas Blaudeck,§,|| and Christian von Borczyskowski|| †

National Technical University of Belarus, Nezavisimosti Avenue 65, 220013 Minsk, Belarus B.I. Stepanov Institute of Physics, National Academy of Science of Belarus, Nezavisimosti Avenue 70, 220072 Minsk, Belarus § Department of Science and Technology, Organic Electronics, Link€oping University, Bredgatan 34, 60174 Norrk€oping, Sweden Center for Nanostructured Materials and Analytics, Institute of Physics, University of Technology Chemnitz, Reichenhainer Strasse 70, 09107 Chemnitz, Germany

)

‡

ABSTRACT: We report on the results of a detailed quantitative experimental evaluation of exciton relaxation pathways as well as direct measurement of singlet oxygen (1O2) generation efficiencies for CdSe/ZnS quantum dot (QD)
porphyrin nanocomposites in toluene at 295 K. QD photoluminescence quenching in nanocomposites is caused by two main factors: electron tunneling in the quantum confined QD (efficiency 0.85
0.90) and F€orster resonance energy transfer (FRET) QDfporphyrin (quenching efficiency 0.10
0.15). Efficiencies of 1O2 generation γΔ by nanocomposites are essentially higher with respect to those obtained for QDs alone. For nanocomposites, the nonlinear decrease of 1O2 generation efficiency γΔ on the laser pulse energy is caused by nonradiative intraband Auger processes, realized in the QD counterpart. Finally, FRET efficiencies found from the direct sensitization data for porphyrin fluorescence in nanocomposites (ΦFRET = 0.14 ( 0.02) are in good agreement with the corresponding values obtained via the direct 1O2 generation measurements at low laser excitation (ΦΔ FRET = 0.12 ( 0.03). The obtained quantitative results provide for the first time strong evidence that a FRET process QDfporphyrin is the reason for singlet oxygen generation by nanocomposites.

1. INTRODUCTION Over the past years, advances in the synthesis of semiconductor quantum dots (QDs) with controllable size, shape, and optical properties as well as the hybridization of QDs with functional organic ligands make them promising materials for a diverse range of applications including photovoltaics and light emitting devices,1
5 quantum computing,6 and biomedicine.7
13 In this respect, nanocomposites consisting of inorganic QDs (for instance, CdSe/ZnS and/or CdTe, TiO2) and attached photosensitizing organic molecules (ligands) are considered as a perspective way in nanotechnology for using the so-called photodynamic therapy (PDT) effect for the selective destruction of malignant tissues, such as cancer cells.14
19 PDT is based on localized photoinduced generation of cytotoxic forms of molecular oxygen (available in a tissue under treatment) following the activation of a nontoxic photosensitizer (PS) with light.20,21 The commonly used in PDT practice organic PSs belong to the class of biologically important tetrapyrrolic pigments. They may be accumulated in cancer tissues and are known to populate the long-living triplet state with high efficiency and are therefore characterized by high efficiencies of singlet oxygen generation.20
22 For PDT purposes, QDs seem to be attractive for the following reasons:2,23 (i) QDs are efficient r 2011 American Chemical Society

photon harvesters since their absorption spectra are broader than the spectra of typical organic dyes used in PDT and are characterized by high molar decimal extinction coefficients in a wide spectral range; and (ii) the photon absorption of QDs can be tuned to the spectral transparency window of human skin. Although metallic QDs can be less toxic,24 semiconductor QDs have important advantages for medical applications. Nevertheless, the direct measurements of the efficiencies of singlet oxygen generation by semiconductor QDs are relatively rare or contradictory,14,15,25
27 thus QD full capabilities for PDT have yet to be harnessed. The anchoring of functional organic molecules to tunable wide gap semiconductor colloidal QDs (using various approaches, such as blends, substitution of surfactant molecules by appropriate organic ligands based on self-assembly processes via covalent bonding, coordination and electrostatic interactions) is of considerable scientific and practical interest.8,12,18,28
31 Concerning PDT, clear potential advantages of nanocomposites based on semiconductor QDs and organic Received: April 29, 2011 Revised: October 4, 2011 Published: October 04, 2011 21535

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The Journal of Physical Chemistry C PSs in comparison with individual organic PSs are as follows: (i) the use of the total visible region for excitation, (ii) the ability to target drugs of interest; (iii) imaging abilities, and, most importantly, (iv) the nonradiative effective F€orster resonance energy transfer (FRET) QDfPS, depending on the absorption/ emission properties and intercenter distances between coupled interacting QD
PS moieties. In this respect, nanocomposites based on semiconductor QDs and tetrapyrrolic molecules have been intensively studied during the past years.14,15,18,25,27,32
42 From the photochemical point of view, the generation of singlet oxygen by QD
PS nanocomposites should involve two steps: (i) the energy transfer QDfPS followed by the nonradiative transition into the PS triplet state T1 and (ii) energy transfer PS(T1)foxygen(3O2) resulting in 1O2 generation. In this respect, it should be mentioned that the photoluminescence (PL) of QDs in solution are subject to various dynamic processes that are related to QD interface properties.2,13,43
48 This complex exchange dynamics is influenced by temperature, dielectric properties of the solvent, and functional groups of ligands or coordinated molecules. It should be mentioned that the quenching of QD PL upon interaction with PS molecules serves as the indicator of the nanocomposite QD+PS formation. Nevertheless, the evaluation of the real reason of QD PL quenching in QD-PS nanocomposites as well the role of the energy transfer is not a trivial question, and thus needs a thorough experimental and theoretical verification for each system.49
57 As was mentioned above, the formation of QD
dye composites is followed by QD PL quenching and is commonly interpreted as being due to photoinduced charge transfer (CT)58
60 and/or the excitation energy transfer processes QDfdye.32
36,61
75 To date, although in most cases ample qualitative evidence for the presence of such quenching processes is given, only a few reports quantitatively unravel that the PL quenching (full or in some cases partly, at least) can uniquely be assigned to CT76,77 or FRET for bulk solutions61,71,72 and for single QD
dye nanocomposites.65,74,75,78 With respect to nanocomposites based on semiconductor QDs and tetrapyrrolic sensitizers, the detailed analysis of FRET in QD PL quenching has been presented in a few papers.18,32,35,41 In fact, PL quenching may be induced by other non-FRET processes.32
36,39,42,66,75 A possible reason for the observation of non-F€orster behavior in such nanocomposites is connected with the involvement of QD surface states47,72,79,80 or is related to the inherent photoinduced blinking of single QDs.46 Finally, the influence of the dynamical attachment process on QD PL efficiency in QD
PS nanocomposites should also be taken into account.81
86 The research presented here is focused on a detailed photophysical study of excited state relaxation dynamics for CdSe/ZnS QD
porphyrin nanocomposites based on steady-state and PL picosecond time-resolved measurements as well as on the direct measurements of the efficiencies of the singlet oxygen (1O2) generation by these complexes. The aim of this study is not devoted to the optimization of QD
dye nanocomposites with regard to high FRET efficiencies; instead a quantification of FRET and non-FRET quenching of the QD PL is under special investigation in this paper. In addition, we concentrate not only on the experimental elucidation of the efficiencies of 1O2 generation by these nanocomposites but analyze some factors thay may influence this process in solutions. Finally, we show that the direct detection of the emission of singlet oxygen generated by CdSe/ZnS QD
porphyrin nanocomposites seems to be

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considered as an additional route for the quantitative verification of FRET realization in organic
inorganic complexes.

2. MATERIALS AND EXPERIMENTAL METHODS 2.1. CdSe/ZnS QDs. Colloidal tri-n-octyl phosphine oxide (TOPO)-capped CdSe/ZnS core
shell semiconductor QDs (CdSe/ZnS core/shell TOPO-Evidots Test Kits) were obtained from Evident Technologies, Inc., Troy, NY, USA. The molar absorption coefficients and core diameters of QDs were calculated from the first exciton absorption peak on the basis of wellproven experimental dependences between the position of the first excitonic maximum in absorption and the nanoparticle diameter.87
89 The core diameters dCdSe of QD vary between 2.1 and 5.2 nm, while two capping ZnS monolayers have been applied. Here, the detailed spectral-kinetic information will be analyzed and discussed for QDs of one size (Evidot abbreviation Hops Yellow, dCdSe = 3.0 nm, 2 ZnS monolayers). In steady-state and time-resolved spectral measurements, the absorbance of the QD starting solutions was adjusted to be lower than 0.1 OD at excitation and emission wavelengths in order to avoid nonlinear absorption and reabsorption effects. The concentrations varied in the range (0.5
10)  10
7 M. The stability and purity of the QD solutions were checked by measuring the quantum yield stability at least over 3 h after preparation. 2.2. PS: Pyridyl-Substituted Porphyrin-Free Base. 5,10,15,20Tetra-metapyridyl-porphyrin H2P(m-Pyr)4 was synthesized and purified according to known methods.90,91 It is known from our recent publications32,35,36,39,67 that among a series of pyridylsubstituted free-base porphyrins and chlorins, H2P(m-Pyr)4 was found to exhibit the most effective PL quenching of the CdSe QD under the same titration conditions. The porphyrin stock solution was prepared in toluene under ultrasonic treatment at 40
C at concentrations in the range (3
30)  10
5 M. 2.3. Titration Experiments. We have shown previously92
95 that selectively replacing phenyl rings in tetraphenylporphyrins with pyridyl rings opens the possibility of a controllable formation (by titration or one-step mixing) of multiporphyrin assemblies with Zn
porphyrin dimers via a key-hole principle. Thus, taking into account the existence of an inorganic ZnS shell covering a CdSe core, we have succeeded to realize the surface passivation of CdSe/ZnS QDs by pyridyl substituted porphyrins, H2P(m-Pyr)n (n = 1, 2, 3 or 4) and chlorins32,35,36,39,67 as well as by other organic molecules with the corresponding anchoring groups35,74,75 using the self-aggregation approach. For all titration experiments, toluene (Spectroscopic grade, Fluka SeccoSolv dried over a molecular sieve) was used. The optical cuvettes (Hellma QS-111, path length: 1 cm) and other glassware were flushed with acetone and ethanol, chemically cleaned with aqueous H2SO4:H2O2, flushed with deionized water, dried in a nitrogen flow, and purged with toluene. Aliquots of porphyrin were added in steps of 10
20 μL to QD dispersed in 2.6 mL toluene obtaining molar ratios of x = [H2P(mPyr)4]/[QD]) = 0.1
20. For the PL spectra, in order to keep direct excitation of H2P(m-Pyr)4 as low as possible, the QD were excited at 465 nm (range of the porphyrin absorption minimum). All PL spectra were corrected for both detector sensitivity and dilution effects during the titration. The residual QD emission intensity was numerically separated. The PL quantum efficiencies were determined in comparison with a quinine sulfate dihydrate standard. 21536

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Figure 1. Structures of the porphyrin ligand (A), TOPO molecule (B), and schematic presentation of QD
porphyrin nanocomposite (C). The scales of CdSe core, ZnS shell, porphyrin, and TOPO molecules as well as intercenter distances correspond to relative sizes of the main components of the arrays:32 the thickness of one ZnS layer l = 5 Å; parameters for conical TOPO molecules rbottom = 5.5 Å and hcon = 9.9 Å were used; rm = 7.5 Å is the radius of porphyrin molecule with opposite pyridyl rings having nitrogens in meta-positions, h = 10 Å is the mean distance between meta-nitrogens of adjacent pyridyl rings (HyperChem software, release 4.0 geometry optimization with semiempirical PM3 method). On the basis of experimental data32 and calculations of the optimized geometry (ab initio density functional theory (DFT) simulations, the VASP code56) it has been proven that the porphyrin molecule is attached to the QD being presumably perpendicular to its surface.

At ambient temperature, the titration of CdSe/ZnS QD toluene solution by a comparable amount of meso-pyridylsubstituted porphyrin molecules H2P(m-Pyr)n (i.e., upon increase of molar ratio x) manifests itself in QD PL quenching (relative intensity decrease and decay shortening).32,35,36,39,67 This has been interpreted as being due to the formation of nanoassemblies via anchoring porphyrin molecules on the ZnS surface. It follows from the detailed spectral observations32,67 that at low molar ratios x, all H2P(m-Pyr)4 molecules can be attached to the CdSe/ZnS surface. Thus from the experimental titration data on the basis of a bimolecular reaction scheme (valid for a dynamic equilibrium between complexed and free constituents like in the case of multiporphyrin arrays93), the values of the complexation constant KC may be estimated using the following expression:67 K C ¼ ð1
βÞ 

1 CQD 3 βðx þ β
1Þ

ð1Þ

where CQD is the QD initial concentration, x is a molar ratio of the porphyrin ligand in the solution; β = I/I0 is the portion of the uncomplexed QDs that can be estimated from the measurements of the uncomplexed QD PL; I and I0 are the measured and initial integral PL intensities, respectively, for the uncomplexed QD at every step of the titration. For CdSe/ZnS QDs and H2P(m-Pyr)4 ligands at CQD = 5  10
8 M, the complexation constant was measured to be KC = 2.6  107 M
1. Like for multiporphyrin self-assembled complexes with similar complexation constants,92,93 the importance of two-point interacting domains is also evident upon formation of the CdSe/ZnS QD
porphyrin nanocomposites. Figure 1 shows a schematic presentation of such hetero-nanoassemblies consisting of a QD with TOPO surfactant layer and H2P(m-Pyr)4 molecule attached via two meso-pyridyl rings nearly perpendicular to the QD surface. 2.4. Spectral and Time-Resolved Measurements. The absorption spectra of QD and H2P(m-Pyr)4 solutions were recorded with a Shimadzu 3001 UV/vis and Cary-500 M Varian spectrometers, and emission spectra were measured with a Shimadzu RF-5001PC spectrofluorophotometer and a homebuilt high-sensitive laboratory setup,22 respectively. Timeresolved PL measurements were performed in the time-correlated

single photon counting (TCSPC) mode under right-angle geometry using a homemade experimental setup32 based on a cavitydumped dye laser (Spectra-Physics Models 375B and 344S) synchronously pumped by a mode-locked argon-ion laser (Spectra-Physics Model 171) for the excitation and a Peltiercooled R3809U microchannel-plate photomultiplier tube (MCPPMT Hamamatsu) with monochromator and computer photon counting board for the emission detection. In addition, TCSPC emission decays were also measured using a laboratory spectrofluorometer equipped with computer module TCC900 (Edinburg Instruments) and light emitting diodes (PicoQuant GmbH): PLS-8-2-130 (λmax = 457 nm, fwhm ∼ 713 ps) and PLS-8-2-135 (λmax = 409 nm, fwhm ∼ 990 ps). Measurements of spectra and decays (TCSPC) of the singlet oxygen emission (λmax = 1270 nm) as well as quantum efficiencies of 1O2 generation (γΔ) have been performed on a laboratory highly sensitive laser near-infrared (NIR) spectrometer:96a laser excitation by STA-01SH Nd:LSB laser (λexc = 531 nm, energy of 4 μJ, fwhm = 0.7 ns, repetition rate of 1 kHz. STANDA Ltd.), monochromator MS2004i, (registration range of 950
1400 nm, SOLAR TII Ltd.), computer photon counting board P7888-2 board (time resolution of 1 ns/channel, FAST ComTec GmbH), PMT Hamamatsu H10330-45, (experimental response Δt1/2 = 1 ns). The efficiencies γΔ were determined with respect to Pd-mesoporphyrin as a standard compound (γ0Δ = 1.0 in toluene at ambient temperature22). In these experiments, toluene (Spectroscopic grade, Fluka SeccoSolv dried over a molecular sieve) was additionally distilled, thus the content of possible impurities and their influence on measured emission intensities of singlet oxygen were reduced to a minimum. At λexc = 531
532 nm, the absorption of toluene is smaller by a few orders of magnitude with respect to that measured for QDs. Thus direct excitation of toluene has low probability in these conditions. A noticeable generation of singlet oxygen by pure toluene upon nanosecond laser excitation in the visible region (especially at 531
532 nm) was not observed in our experiments. At laser excitation, parameters being used for measurements of singlet oxygen generation efficiencies (λexc = 531 nm, energy of 1
4 μJ, fwhm = 0.7 ns, repetition rate of 1 kHz), two-photon absorption effects, as well as the formation of radicals as possible additional reasons leading to the singlet 21537

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oxygen formation were excluded in these conditions. Typically, two-photon absorption effects for both QDs96b and porphyrins96c may be detected only when using 120
150 fs laser pulses (fwhm) of 0.8
1.0 mJ per pulse with a repetition rate of 1 kHz. Such powerful laser excitation conditions have not been used in our case. The main procedure for the measurement of 1O2 generation quantum efficiency γΔx is based on the comparison of emission intensity of singlet oxygen (λmax = 1.27 μ) photosensitized by a standard compound (intensity I0) and by the compound under investigation (intensity Ix) in the same solvent: γxΔ ¼ γ0Δ

I x  β0 I 0  βx

ð2Þ

where γ0Δ is the quantum efficiency of singlet oxygen generation by a standard, β0 = (1
10
D0) and βx = (1
10
Dx) are fractions of absorbed exciting light by the standard and the object under study, respectively, at a given excitation wavelength. Ix and I0 values were averaged and extrapolated to the maximal pulse intensity after more than 32 laser pulses for every measurement. The detection limit equivalent for 1O2 luminescence with a total quantum efficiency has been evaluated to be 6  10
9.96 Optical densities did not exceed OD e 0.2 at optical length of l = 10 mm. A relative experimental error of γ0Δ measurements was estimated to be (15% for nanocomposites and be (30% for individual QDs.

3. RESULTS AND DISCUSSION 3.1. Spectral and Kinetic Properties of Nanocomposites. Typically, the titration of CdSe/ZnS QD toluene solution by a comparable amount of porphyrin molecules H2P(m-Pyr)n (i.e., upon increase of molar ratio x) manifests itself in the QD PL quenching.32,35,36 In titration experiments at C = (0.5
10)  10
7 M, the QD PL dynamic quenching due to collisions of interacting moieties is low within QD mean PL decay (∼20 ns).32,36 Thus the observed QD PL quenching is due to the formation of nanoassemblies. Figure 2 shows typical absorption (A) and PL (B) spectra for CdSe/ZnS QDs solutions as a function of added tetrameso-pyridyl substituted porphyrins H2P(m-Pyr)4 at well-defined molar ratios x = [H2P]/[QD]. It is seen that upon increase of x, (excitonic maximum of QD at λmax = 548 nm), a linear increase of the porphyrin absorption bands takes place. In all cases, QD absorption remains constant, while the PL (at λmax = 555 nm) is considerably quenched upon titration by H2P(m-Pyr)4 molecules. Time-resolved PL measurements show32,36 that the PL of pure QD without porphyrin ligand is characterized by a nonexponential decay, and the interaction with anchored porphyrin molecules manifests itself in the appearance and rise of two additional short time components (∼7 ns and ∼700 ps). In addition, the fluorescence decay analysis for H2P(m-Pyr)4 molecules upon λexc = 410 and 460 nm and at a detection wavelengths of 650 and 719 nm (porphyrin Q(0,0) and Q(0,1) fluorescence bands) in toluene at 295 K shows that for individual porphyrin solutions the S1-state decay is τ0S = 8.7 ns as is well-known for porphyrinfree bases in nondegassed liquid solutions at 295 K.22 Being attached to the QD surface, the H2P(m-Pyr)4 ligand is characterized by slightly increased S1-state decay τS = 9.5 ns under the same excitation and detection conditions. This decay increase

Figure 2. Absorption (A) and emission (B, λex = 465 nm) spectra of CdSe/ZnS QD (CQD = 4  10
7 M) and H2P(m-Pyr)4 molecules in toluene at 295 upon molar ratio x = [H2P]/[ QD] increase. Inset in A: peak intensity of the Soret band as a function of the nominal concentration. Deviations from linearity represent the uncertainty in the amount of added substance (i.e., 5.0 ( 2.5%). Circle in B shows the existence of an isosbestic point in the emission spectra upon titration.

may be attributed to the screening action of a relatively large TOPO-capped QD subunit in a QD
porphyrin nanocomposite, thus limiting the access of oxygen molecule (quencher) to the excited extra-ligand, as shown also for porphyrin triads and pentads.97,98 All these facts indicate that in QD
porphyrin nanocomposites the PL quenching of CdSe counterpart is a dynamic process caused by the increased nonradiative relaxation channels for exciton. The observed QD PL quenching seems to be the result of the complex effective electronic interactions in QD
porphyrin nanocomposites, e.g., via energy and/or photoinduced CT between the interacting counterparts. A short analysis of both of these reasons and other quenching processes will be discussed later. 3.2. FRET Analysis. The conclusions based on experimental results61,62,65,99,100 and theoretical considerations56,101 allow one to apply the F€orster point dipole
dipole model52,102 for the FRET analysis in QD-dye composites. However, we have shown recently32,36,75 that, under the presented experimental conditions, the theoretical FRET efficiencies Φthear FRET is always much larger than Φexp FRET. We have assigned this observation to non-FRET processes possibly resulting in ligand-induced charge localization at the QD surface accompanied by new nonradiative decay channels.36,75 As was discussed in the Introduction, in the FRET case for QD
organic dye nanocomposites, the direct quantitative verification of the energy transfer process as a real reason of QD PL 21538

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Figure 3. QD integrated PL intensities I/I0 (open circles (1) left scale) and FRET efficiencies based on H2P(m-Pyr)4 fluorescence excitation spectra (filled circles (2), right scale) as a function of the molar ratio x = [H2P(m-Pyr)4]/[QD] in toluene at 295 K.

quenching is the comparison of the experimental values of FRET efficiencies obtained, on one hand, via the donor (QD) PL quenching and, on the other hand, via the sensitization of the acceptor (dye or porphyrin ligand) fluorescence. This is often missing in most publications, resulting in incorrect assignments of processes and erroneous data evaluation. With these ideas in mind, we have also carried out in the present case a direct comparison of PL quenching of QD and fluorescence sensitization for porphyrin using a complete set of titration experiments. PL quenching of QD as well as porphyrin fluorescence excitation spectra (recorded at λem = 720 nm) have been obtained at every titration step. The corresponding FRET efficiencies ΦFRET have been evaluated from the comparison of the absorption spectra and fluorescence excitation spectra of the QD
H2P solution using an approach presented in refs 32, 36, 61, and 103 and the following expression:   IDA ðλex ¼ 465 nmÞ
IA ðλex ¼ 465 nmÞ ΦFRET ¼ I A ðλex ¼ 590 nmÞ   ODDA ð465 nmÞ
ODA ð465 nmÞ : ODA ð590 nmÞ ð3Þ Here, IDA corresponds to the porphyrin fluorescence intensity at λem = 651 nm for QD
porphyrin nanocomposites, IA is the fluorescence intensity of porphyrin at the same molar ratio at two different excitation wavelengths (465 and 590 nm). OD values are the corresponding optical densities of the solution at a given molar ratio x. The difference IDA(λex = 465 nm)
IA(λex = 465 nm) reflects the increase of the acceptor emission due to FRET. λex = 590 nm corresponds to the wavelength with negligible QD absorption. For all titration steps, ΦFRET data obtained according to eq 3 are included in Figure 3 together with the QD PL quenching efficiency I(x)/I0 in QD
porphyrin nanocomposites. At each molar ratio x, ΦFRET values calculated from fluorescence enhancement (right scale) are significantly smaller than those estimated from QD PL quenching efficiency ΦQ = 1
I(x)/I0. In the total titration range ΦFRET values are at least 4% and do not exceed 14% even at high x values (Absolute ΦFRET data vary depending on QD from various Test Kits delivered by Evident Technologies). As has been concluded by us

previously,32,36 FRET is not the only reason of QD PL quenching for QD
porphyrin nanocomposites. In addition, the independence of QD PL quenching efficiency on redox properties of porphyrin ligands and the absence of the ligand fluorescence quenching in QD
porphyrin nanocomposites35,67 rules out a dominant role of photoinduced CT. With this physical background in mind, we have comparatively analyzed the interaction of individual QDs and QD
porphyrin nanocomposites with molecular oxygen in liquid solutions. 3.3. Singlet Oxygen Generation by CdSe/ZnS Semiconductor QDs. The process of T1-state (3PS) quenching by O2 is realized via nonradiative transitions between electronic states in an intermediate short-lived diffusion-controlled complex formed by the given excited organic molecule 3PS with an oxygen molecule.104 Taking into account Wigners spin conservation rule (transitions between terms of the same multiplicity are spin-allowed),105,106 the quenching of the T1-state of the organic PS (3PS) followed by the formation of singlet oxygen can be described as 3

PS þ 3 O 2 f 1 ½3 PS 3 3 3 3 O 2  f 1 ½1 PS0 3 3 3 1 O 2 ð1 Δ g Þ f 1 PS0 þ 1 Δ g

ð4Þ

We have succeeded in detecting the 1O2 emission (spectra and kinetics, Figure 4) generated with a quantum efficiency γΔ = 1.5% by CdSe/ZnS QDs of two sizes (dCdSe = 2.5 and 3.0 nm, Table 1). At 295 K in toluene solutions and in the presence of 1 atm of air, the 1O2 emission decay was measured to be τ(1Δg) = 30.0 μs. These results coincide with the corresponding data obtained for CdSe QDs (dCdSe = 5 nm) in oxygen-saturated toluene at ambient temperature (γΔ ∼ 1.5%, τ(1Δg) = 28.5 μs27). Table 1 shows that for CdSe/ZnS QDs with dCdSe= 5.2 nm, no singlet oxygen was observed when these QDs were excited at the same conditions. In contrast to CdSe/ZnS QDs of smaller sizes (dCdSe = 2.5 and 3.0 nm), Maple Red Orange QDs (dCdSe = 5 nm) are characterized by essentially lower PL quantum efficiency (Table 1), caused by many deep trap surface states with their low level emission in the red and IR spectral range overlapping with the 1O2 emission region. Correspondingly, these competitive PL quenching processes seem to be the main reason for the absence of the 1O2 generation by large QDs in our case. In this respect, it should be mentioned that singlet oxygen generation was not detected for water-soluble CdTe QDs.25,107 It follows also from Figure 5 and Table 1 that, for CdSe/ZnS QDs, the 5-fold increase of the dissolved oxygen concentration leads to an increase of the quantum efficiency γΔ by 1.7 times, while the 1O2 emission decay τ(1Δg) remains unchanged. Simple calculations using the formula C = [(4π/3)  R3]
1, connecting the concentration C and mean distances R between molecules,108 show that the 5-fold increase of C corresponds to the increase of R by 1.7. The last value is in good coincidence with the increase of γΔ (see Table 1). This means that the interaction of the excited QD with oxygen is realized via the diffusion-controlled formation of a short-lived collision complex. It is noteworthy that the 5-fold increase of dissolved oxygen does not lead to a shortening of the Æτ0Dæ value for the QD PL. In fact, like in our case, low efficiencies of 1O2 sensitization were also mentioned in the literature.14,15,25
27,31 Some physical reasons may be considered as being responsible for the low quantum efficiency of singlet oxygen generation by QDs in liquid solutions at ambient temperature. It is well documented109,110 that the lowest energy exciton (formed by the band-edge 21539

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Figure 4. (A) Spectrum of singlet oxygen 1O2 emission directly sensitized by CdSe/ZnS QDs in air-saturated toluene at 295 K. Spectrum obtained by integrating the kinetic signals in the range 10
60 μs and fitted by the sum of Gaussian and Lorentzian functions. An average noise level equal to 69  104 counts was subtracted from this spectrum. (B) 1O2-sensitized luminescence decays in toluene solutions in the presence of 1 atm of air (1) and 1 atm of pure oxygen (2). An average noise level equal to 14  103 counts was subtracted from both kinetic signals.

Table 1. TOPO-Capped CdSe/ZnS QD Parameters and Their Quantum Efficiencies of Singlet Oxygen Generation (Toluene at 295 K)a QD type (2 ZnS monolayers)

a

gas conditions

λPL max nm

dCdSe, nm

Æτ0Dæ, ns

jD0

γΔ

Catskill Green (CG)

1 atm air

2.5

540

14.9

0.43

0.015 ( 0.005

Hops Yellow (HY)

1 atm air

3.0

558

14.7

0.63

0.015 ( 0.005

Hops Yellow (HY)

1 atm O2

3.0

558

14.5

0.63

0.025 ( 0.005

Maple Red Orange (MRO)

1 atm air

5.0

620

11.7

0.075

0.0

j0D is the QD PL quantum efficiency. Æτ0Dæ is the weighted average QD lifetime.

Figure 5. (A) Comparative presentation of singlet oxygen 1O2 luminescence decays sensitized by CdSe/ZnS QDs (1), by H2P(m-Pyr)4 (2) and by nanocomposites QD
porphyrin (3). Molar concentration of QDs 1 and 3 and of porphyrin 2 and 3 were kept constant. The molar ratio x = [H2P(m-Pyr)4]/[QD] = 4 in air-saturated toluene solutions at 295 K. A noise level of up to 1600 counts was subtracted from all kinetic signals. (B) Normalized kinetic signals of 1O2 luminescence observed for H2P(m-Pyr)4 (2) and Pd-mesoporphyrin (4). Laser excitation at λexc = 531 nm, pulse energy W = 1 μJ observation at λreg = 1270 nm.

1S(e)
1S3/2(h) transition) is split into five sublevels caused by the crystal shape asymmetry, the intrinsic crystal field (in hexagonal lattice structures), and the electron
hole exchange interaction. Two of the five states with lower energy, including the ground state, are optically passive (a total angular momentum N = 2) and form the so-called “dark” exciton. The high-energy states being optically active (a total angular momentum N = 1) form the so-called “bright” exciton. According to theoretical considerations,109 the energy distance between the first optically active state and the optically forbidden ground exciton state is estimated by tens of millielectron volts (∼200 cm
1) and increases with decreasing QD size, leading to an increase of the Stokes shift in PL spectra.

In its turn, the thermal redistribution of excitons between states with different total angular momentums (Nm = 1L and 2) is the main factor that leads to the strong temperature dependence of single-exciton intrinsic recombination dynamics in CdSe QDs.13,110,111 At low temperatures, exciton recombination occurs primarily via the low-oscillator-strength “dark” state and is characterized by slow (submicrosecond to microsecond) PL decay. Upon temperature rise, the higher-oscillator-strength short-living “bright” states become thermally populated from the “dark” state, which leads to faster PL dynamics. Finally, at sufficiently high temperatures (e.g., thermal energy kT is comparable or larger than the energy gap ΔE = Ebright
Edark) exciton population becomes distributed equally between the bright and dark states leading to the saturation of PL decay at a value of twice the bright-exciton lifetime (in the region of ∼20 ns for CdSe QDs). It follows from the analysis of the size dependence of the bandedge exciton splitting (fine structure) for hexagonal CdSe QDs (presented in ref 108) that for QDs with dCdSe = 3.0 nm, this energy distance ΔE = Ebright
Edark is in the range of 30 meV (∼240 cm
1). As far as the transition from the ground to the dark state it is not observable in the QD steady-state absorption spectra, the low-oscillator-strength dark state is often referred as the triplet state.27 It is also considered112,113 that the exciton fine structure of QDs109
111 is an analogous spectral quantity to the singlet
triplet manifold of molecules. In this respect, two principal features should be taken into account for QDs under study: (i) at room temperature ΔE ∼ 240 cm
1 is comparable with kT ∼ 210 cm
1, and dark states are shortened down to ∼15 ns due to a effective thermal distribution between the bright and dark states; (ii) both for bright exciton (a total angular 21540

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momentum N = 1) and for dark exciton (N = 2), the reaction between molecular oxygen and QD in these states does not fully correspond to the Wigner spin conservation rule. In addition, on the basis of femtosecond time-resolved studies, it has been suggested27 that the low quantum efficiency of 1O2 generation by QDs may be due to carrier trapping and nonradiative carrier recombination occurring on the early picosecond time scale. Then, like for organic PSs,104,105 spin statistics also diminish the fraction of the QD
3O2 contact complexes capable of generating singlet oxygen. Finally, for some CdSe/ZnS QDs based on PL decay time distribution analysis, it was shown that trap or defect states exhibit at room temperature a lifetime of about 80 ns and are red-shifted by about 110 meV (∼890 cm
1),114 which may effectively compete with 1O2 generation. Thus, taking into account all these considerations, we conclude that at ambient temperature QDs are hardly perspective for the direct generation of singlet oxygen. 3.4. Singlet Oxygen Generation by QD
Porphyrin Nanocomposites. In any titration experiment with use of steady-state optical spectroscopy, the total QD PL intensity I(x) obtained from an ensemble of QD
(porphyrin)n nanocomposites with a statistical distribution of n at a given ratio x = [H2P]/[QD] is the weighted sum of the contributions of each individual nanocomposite. Assuming a quasi-infinite number n* of binding sites available for porphyrin molecules per one QD, the probability of the existence of a nanocomposite QD
(porphyrin)n with particular n porphyrin molecules follows the Poisson distribution (e.g., given in ref 115) Pn ðxÞ ¼ xn 3 expð
xÞ=n!

ð5Þ

Thus, free porphyrin molecules are subject to a redistribution over the free binding sites with respect to their availability, with the probability of filling a vacancy on a particular nanocomposite QD
(porphyrin)n proportional to the probability of its occurrence in the ensemble. This partial redistribution picture occurred in a good agreement with the experimental results on time-resolved fluorometry of nanocomposites QD
(porphyrin)n, showing also that the fraction of porphyrin molecules remaining uncomplexed in solution increases with growing porphyrin-to-QD molar ratio x.116 Using the experimental quenching data upon titration (Figure 3) and taking into account the analysis of a Poisson distribution in the molar ratio range of 0 e x e 10,116 we analyze the singlet oxygen generation by nanocomposites based on CdSe/ZnS QDs and porphyrin molecules H2P(m-Pyr)4 at the molar ratio x = [H2P(m-Pyr)4]/[QD] = 4. The nanocomposite with a given x value was prepared by one-step mixing. At this molar ratio PL quenching and FRET are detected simultaneously. (Figure 3 and Table 1): The efficiency of QD PL quenching is I(x)/I0 = 0.65, QD emission decay shortening from Æτ0Dæ =14.7 ns down to ÆτDæ = 13.7 ns is observed and a FRET efficiency ΦFRET = 0.14 ( 0.02 (eq 4). We have carried out a comparative analysis of the singlet oxygen (1O2) luminescence decays and intensities sensitized by CdSe/ZnS QDs, H2P(m-Pyr)4 molecules and solutions of QD
porphyrin nanocomposites at x = [H2P(m-Pyr)4]/[QD] = 4. Figure 5A evidently shows that upon laser excitation the nanocomposite QD
porphyrin does indeed produce singlet oxygen with a higher efficiency in comparison with single QDs under the same experimental conditions in air-saturated toluene solutions. On the other hand, it demonstrates that under these

conditions, the efficiency of 1O2 generation by the nanocomposite QD
porphyrin is higher than that measured for the solution of porphyrin molecules at the same molar porphyrin concentration. Normalized kinetic signals of Pd-mesoporphyrin and H2P(m-Pyr)4 are shown in Figure 5B (curve 4). In all cases, the radiative relaxation of 1O2 detected at λreg = 1270 nm is characterized by the same decay time of τ(1Δg) = 30 ( 0.5 μs that is typical for the singlet oxygen emission in protoncontaining liquid organic solvents (such as toluene, benzene, etc.).18
22,27 From these findings we conclude that QD
porphyrin nanocomposites produce singlet oxygen more effectively with respect to their counterparts alone. Nevertheless, for a clear understanding of the contribution of energy transfer and related pathways involved in the two-step process of singlet oxygen generation by the given nanocomposites, we perform a quantitative comparison of the experimental data. Before doing so, test experiments were conducted to elucidate the role of excitation conditions. The experimental dependencies of the normalized PL intensities on the energy of laser excitation are depicted in Figure 6. It is evident that the QD PL intensity (λem = 558 nm) is not a linear function of the laser pulse energy for both QDs (A) and QD in nanocomposites (B). The nonlinearity is more pronounced for QDs alone. In the case of single porphyrin solutions (λem = 720 nm) the dependence is almost linear (C), while, by contrast, this dependence is nonlinear for the porphyrin counterpart in the nanocomposite (D). The linear dependence (Figure 6C) over the total energy range for single porphyrin solutions can be explained (C) by the negligible role of singlet
singlet and triplet
triplet annihilation processes22,117 at low solute concentrations (g10
5
10
6 M in our case). The nonlinear dependence obtained for single QDs (Figure 6A) reflects the well-documented nonradiative intraband Auger processes:13,111,118 While the intrinsic mechanism for depopulation of singly excited exciton is a radiative electron
hole recombination, the decay of multi-e
h pair states in QDs is dominated by nonradiative Auger processes mediated by Coulomb electron
electron interactions in the conditions of the spatial confinement.111 The less pronounced nonlinear dependence of QD PL intensity observed in QD
porphyrin nanocomposites (Figure 6B) may be taken as a signature of the influence of two deactivation processes (non-FRET and FRET36) leading to an additional quenching of the exciton in QD. This conclusion is in agreement with other results showing that biexciton Auger recombination of CdTe QDs is not only a function of the QD diameter but is strongly dependent on the QD surface trapping, capping reagents, and/or surface conditions.118 Thus, it may be concluded that the nonlinear dependence of fluorescence measured for porphyrin being attached to the QD surface (Figure 6D) is direct proof of the realization of a QDfporphyrin FRET process competing with both radiative electron
hole recombination and nonradiative Auger processes at high excitation energies. Notably, in a very recent publication119 the two-photon excitation (λexc = 800 nm) of QDs was used for singlet oxygen generation by biocompatible CdSe QDs with water-soluble porphyrins. However, the indirect chemical method for singlet oxygen detection being used in these experiments is not so sensitive and precise with respect to our approach based on the direct measurements of singlet oxygen emission, including detection of both amplitudes and decays of singlet oxygen 21541

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Figure 6. Dependencies of the normalized PL intensities on the energy of laser excitation for CdSe/ZnS QDs (A) and QDs in nanocomposite at x = 4 (B) as well as for H2P(m-Pyr)4 (C) and porphyrin molecules attached to QDs (D) in air-saturated toluene at 295 K. QD molar concentrations were the same in cases A and B, while in cases C and D porphyrin molar concentrations were kept equal. Optical densities were