Energy Transfer from Colloidal Quantum Dots to Near-Infrared

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Energy Transfer from Colloidal Quantum Dots to Near Infrared Absorbing Tetraazaporphyrins for Enhanced Light Harvesting Zhihua Xu, Feng Gao, Elena A Makarova, Ahmed A Heikal, and Victor N. Nemykin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b01603 • Publication Date (Web): 15 Apr 2015 Downloaded from http://pubs.acs.org on April 23, 2015

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The Journal of Physical Chemistry

Energy Transfer from Colloidal Quantum Dots to Near Infrared Absorbing Tetraazaporphyrins for Enhanced Light Harvesting

Zhihua Xu,a∗ Feng Gao,a Elena A. Makarova,b Ahmed A. Heikal,b Victor N. Nemykinb*

a

Department of Chemical Engineering, University of Minnesota-Duluth, Duluth, MN 55812

b

Department of Chemistry and Biochemistry, University of Minnesota-Duluth, Duluth, MN 55812

∗

Corresponding authors, emails: [email protected], [email protected]

1 ACS Paragon Plus Environment


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Abstract In this report, we investigate the mechanisms of energy transfer from CdSe quantum dots (QDs) to porphyrin derivatives as a potential antenna system with enhanced light-harvesting efficiency. Two

ferrocenyl-containing

tetraazaporphyrins

derivatives,

2(3),7(8),12(13),17(18)-tetraferrocenyl-5,10,15,20-tetraazaporphyrin magnesium

namely,

magnesium

(TAPFcMg)

and

2(3),7(8),12(13),17(18)-tetracyano-3(2),8(7),13(12),18(17)-tetraferrocenyl-

5,10,15,20-tetraazaporphyrin (TAPFcCNMg), are used as energy acceptors in this proposed antenna system along with size-dependent QDs as donors.

Our approach includes Förster

resonance energy transfer (FRET) calculations as well as photoluminescence (PL) intensity and lifetime quenching measurements. Our FRET calculations indicate that higher energy transfer efficiency can be achieved with smaller quantum dot size. However, PL intensity and lifetime measurements suggest that energy transfer efficiency in QD/tetraazaporphyrin complexes is regulated by a competing trap-assisted ultrafast quenching mechanism, which is more dominant with smaller QD size. Furthermore, it is found that the trap-assisted quenching process is more active in QD/ TAPFcMg than QD/ TAPFcCNMg complexes. As a result, high efficiency energy transfer can be achieved in the complexes combining large QDs and TAPFcCNMg, where trapassisted quenching mechanism is suppressed. Our study suggests that CdSe quantum dots can be promising energy transfer donors for NIR-absorbing tetraazaporphyrins to form antenna systems with enhanced light-harvesting efficiency.


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Introduction In the past decade, synthetic porphyrins have emerged as important light harvesting materials for low-temperature, solution-processed photovoltaic cells such as dye-sensitized solar cells (DSSCs).1 A record high power conversion efficiency of 13.3% for DSSC was recently achieved by utilizing a donor-π-acceptor porphyrin as light absorbing sensitizer. 2 However, porphyrins only show strong absorption for a small portion of the sunlight spectrum, and thus limit their light harvesting efficiency. For example, although porphyrins possess a strong UV-blue light absorbing Soret band (B-band) and red absorbing Q-bands, they generally do not absorb in the near infrared (NIR) spectral region, which accounts for significant part of the energy spectrum of the sunlight. Fortunately, this problem might be eventually solved by the spectral tunability of porphyrins through chemical functionalization at the four meso or eight β-pyrrolic positions in the porphyrin ring. For example, Zn-porphyrin functionalized by N-annulated perylenes (NP) exhibits an extended light absorption up to 900 nm.3 The relatively low absorption coefficient between the strong absorbing B-band and Qbands in metal-linked porphyrins represents another challenge for usage in light-harvesting materials for DSSCs. As a consequence, there will be a weak absorption for the blue-green region of the visible light where solar light has the highest intensity. A general strategy to address this problem is to co-sensitize multiple dyes, with complementary spectral features, on metal-oxide nanoparticles, in order to achieve broader absorption of the visible light.4 But the limited sensitizing positions on metal-oxide nanoparticle surface for the dyes restrict the overall enhancement of light absorption through co-sensitization. An alternative approach is to utilize the so-called the “antenna” effect, which enhances light absorption of the sensitizer by energy transfer mechanisms. In this approach the energy donor, usually an organic dye, could be either



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linked or separated from the sensitizer, i.e. the energy acceptor, and thus eliminates the competition for sensitizing positions at metal oxide nanoparticle surface.5 Recently, several studies on the “antenna” systems adopt colloidal quantum dots (QDs) as a donor, which has broad absorption spectrum in the visible light to enhance light capture of an organic dye (an acceptor) via a Förster resonance energy transfer (FRET) mechanism.6-8 As inorganic semiconductor nanocrystals, QDs have gained a growing interest as the sensitizers in DSSCs due to their broad absorption spectra, photostability, and solution processibility.9, 10 In order to form an effective “antenna” system, however, the photon energy absorbed by the quantum dots must be efficiently transferred to the sensitizer. Although high energy transfer efficiency from quantum dots to organic dyes have been reported, 11-13 multiple studies have suggested alternative excited-state mechanisms such as photoinduced charge transfer that may dominate in QD/porphyrins complexes.14,15 In this work, we investigated the energy transfer mechanisms between the CdSe QDs of several sizes and two NIR-absorbing tetraazaporphyrins. In particular, we elucidated the size effect of QDs on energy transfer efficiency in QD/tetraazaporphyrin. The choice of two tetraazaporphyrins discussed in this paper was two-fold. First, it was already demonstrated that the tetraazaporphyrins used in this report have close to panchromic absorption spectra between UV and NIR regions thus significantly enhancing potential light harvesting efficiency. 16 - 17 Second, it was shown earlier that the ferrocene group(s) in ferrocene-porphyrinoid dyads with direct ferrocene-to-porphyrinoid 18-30 or ferrocene-to-central ion 31-36 bond(s) are very effective electron donors, which might be useful for light harvesting37-41 as well as electro-42-46 and photocatalytical 47 - 48 processes. The result of this work provides useful guidance for designing QD/tetraazaporphyrin “antenna” systems for solar energy harvesting.


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Materials and Experimental Methods The two ferrocenyl-containing tetraazaporphyrins, namely magnesium 2(3),7(8),12(13),17(18)tetraferrocenyl-5,10,15,20-tetraazaporphyrin

(TAPFcMg),

and

magnesium

2(3),7(8),12(13),17(18)-tetracyano-3(2),8(7),13(12),18(17)-tetraferrocenyl-5,10,15,20tetraazaporphyrin (TAPFcCNMg) were prepared using template condensation reaction between magnesium butoxide and (Z)-dicyanovinylferrocene49 and tricyanovinylferrocene,50 respectively, in boiling butanol, followed by a standard chromatographic purification steps (see reference 16 for details).16 The octadecyl amine (ODA) capped CdSe quantum dots dispersed in toluene were purchased from NN-Labs, LLC. Titration experiments were carried out by adding various volume of 0.2 mM TAPFcMg and TAPFcCNMg solutions into 1 nM quantum dots solution in toluene. Absorption spectra were measured with Agilent 8453 UV-Vis spectrophotometer, and PL spectra were measured with Horiba Fluoromax-4 Spectrofluorometer. Time-resolved PL lifetime measurements were carried out using a time-correlated single-photon-counting (TCSPC) system, which was described in detail elsewhere.51 In short, a titanium sapphire laser system (Mira 900-F, Coherent), a pulse picker (Mira 9200, Coherent), and a second-harmonic generator were used to generate the femtosecond excitation laser pulses (460 nm) used in these studies. The samples were prepared in a deep-well slide and a coverslip, sealed with nail polish, and positioned in the focal plane of a 1.2NA microscope objective (Olympus) in an inverted IX81 microscope (Olympus). The fluorescence signal was collected by a microchannel plate photomultiplier tube (MCP-PMT; R3809U, Hamamatsu) and a histogram of fluorescence photon arrival times (i.e., a fluorescence decay) was recorded using a SPC 830 module (Becker and Hickl, Berlin, Germany) and analyzed using SPCImage software.



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Results and Discussion Steady-State Spectroscopy of QDs and Tetraazaporphyrins Solutions: The chemical structures and UV-Vis-NIR absorption spectra of tetraazaporphyrin derivatives (TAPFcMg and TAPFcCNMg) are shown in Figure 1. In addition to the classic B- and Q-bands absorptions in UV- visible region, the two tetraazaporphyrins used in this work exhibit an additional broad absorption in NIR range which has been assigned to several metal-to-ligand charge transfer (MLCT) dominated excited states on the basis of TDDFT calculations.16 It is especially worth noting that the presence of four strong electron-withdrawing cyano groups in the TAPFcCNMg complex creates a very strong low-energy MLCT band and extends its absorption spectrum over 1200 nm. Although UV-vis-NIR spectra of TAPFcMg and TAPFcCNMg complexes are close to panchromic behavior, the spectra of both tetraazaporphyrins clearly show weak absorption in the visible region between 400 and 550 nm, which is important region for light-harvesting as the sun light has very high intensity in this area. Thus, it was very tempting for us to test the possibility to use quantum dots as the energy donors for TAPFcMg and TAPFcCNMg, in order to improve lightharvesting. The quantum dots used in this work have a CdSe core, which is capped by octadecyl amine (ODA) ligands, of diameter 2.2 nm (QD1), 3.2 nm (QD2), and 5.7 nm (QD3), respectively. Figure 2a shows that as the CdSe core size increase from QD1 to QD3, both the absorption and emission peaks of the quantum dots shift to lower energies (i.e., longer wavelength), which is expected for semiconductor quantum dots. Combining the broad visible absorption of the QDs and the NIR absorption of the TAPFcMg and TAPFcCNMg complexes, the QD/tetraazaporphyrin mixtures in solutions show continuous and extended UV-NIR absorption. As shown in Figure


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2b, when QD2 was added into the TAPFcMg solution, the visible absorption at 400-550 nm increased gradually. In addition, we have not observed any significant shift of (or formation of new) absorption band upon QDs and tetraazaporphyrins mixing, suggesting that there is no ground state interaction or complex formation between tetraazaporphyrins and quantum dots. As discussed above, in order to form an effective “antenna” system, the photon energy absorbed by quantum dots should be efficiently transferred to the tetraazaporphyrins. Figure 1 shows significant spectral overlaps between the emission spectra of QDs and the absorption spectra of tetraazaporphyrins, suggesting potential energy transfer from QDs to TAPFcMg and TAPFcCNMg. Furthermore, the degree of spectra overlap for efficient energy transfer clearly depends on the QD size. For example, the PL emission peak of QD1 is located at the weak absorption range between the B- and Q-bands of tetraazaporphyrins, leading to very low level spectra overlap with the absorption of tetraazaporphyrins. Emission peak of QD2 has slightly lower energy and reasonable spectral overlap with Q-band of TAPFcMg and TAPFcCNMg, while the emission peak of QD3 shows significant overlap with the strong absorbing NIR MLCT band of TAPFcMg and the Q-band of TAPFcCNMg. Therefore, it is reasonable to hypothesize that the energy transfer efficiency in QD/tetraazaporphyrin complexes can be tuned by manipulating the QD size in this proposed antenna system.

FRET Calculation: Here we analyze the energy transfer in the QD/tetraazaporphyrin assemblies based on FRET theory. 52 The rate of energy transfer (kT) for a donor and an acceptor with separation distance r is given by Equation (1).  Φ Dκ 2   9000(ln 10 )  ∞ ×  × ∫ FD (λ )ε A (λ )λ4 dλ kT ( r ) =  6   5 4  r 128 N n τ π A  D    0

(1)



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where ΦD and τD are the PL quantum yield and life time of the donor in the absence of acceptor respectively, κ2 is the factor that describes the relative orientation of the transition dipoles of the donor and acceptor in space, NA is Avogadro's number, n is the refractive index of the medium, and r is the donor-acceptor distance. The integral term represents the spectral overlap, J(λ), between the normalized donor emission FD(λ) and the acceptor absorption εA(λ), expressed in terms of the extinction coefficient. And the efficiency of energy transfer (E), which is the ratio between the photon energy transferred from the donor to the acceptor to the total energy absorbed by the donor alone, is given by Equation (2)

E=

kT ( r ) τ + kT ( r )

(2)

−1 D

As shown in Equation (1) and (2), the energy transfer rate and efficiency from the donor (QDs) to the acceptor (tetraazaporphyrin) are determined by multiple parameters including donoracceptor distance r, PL quantum yield (ΦD) and lifetime (τD) of QDs, and spectral overlap J(λ) between QD emission and tetraazaporphyrin absorption. Among all the parameters, the donoracceptor separation distance r, which represents the distance from the center of QD core to the tetraazaporphyrin molecule, plays the most significant role in determining energy transfer rate and efficiency. Although the separation distance in the QD/tetraazaporphyrin assembly can be reasonably considered as the sum of QD core radius (rQD) and the length of ODA ligands, accurate estimation of r is not straightforward due to the flexible conformation of ODA molecule in solution. Although it may not be an integral part of the fluorescent moiety of QDs, the ODA ligand region can be considered as “an excluded volume” for the acceptor. In our FRET calculations, we considered two distinct conformations: one with fully extended ODA ligands (donor-acceptor distance, r1) and another with coiled ligand (donor-acceptor distance, r2). The orientation parameter κ2 is assumed to be 2/3 for a random orientation of the transition dipoles of 8 ACS Paragon Plus Environment

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both the donor (quantum dots) and the acceptor (tetraazaporphyrins). Because of the spherical symmetry of the emitting QDs, the exact orientation parameter may be slightly larger than the assumed value here for randomly oriented donor and acceptor fluorophores, each of which has well-defined dipole moment. The estimated energy transfer rate and efficiency for the six QD/tetraazaporphyrin complexes are shown in Table 1, assuming a refractive index of 1.5 for toluene. Based on FRET theory, QD size has mixed effect on energy transfer efficiency. In general, increase of quantum dot size from QD1 (2.2 nm) to QD3 (5.7 nm) significantly enhances spectral overlap integral J (λ) due to the red shift of the QD emission spectra as the size increases, which enhances the energy transfer efficiency. As shown in Table 1, J (λ) increases from 3.33×1014 nm4M-1cm-1 for QD1/ TAPFcMg to 22.5×1014 nm4M-1cm-1 for QD3/ TAPFcMg, and increases from 5.46×1014 nm4M-1cm-1 for QD1/ TAPFcCNMg to 33.7×1014 nm4M-1cm-1 for QD3/ TAPFcCNMg. On the other hand, larger QD size leads to larger separation distance r and lower QD PL quantum yield (ΦD), both of which may reduce the corresponding energy transfer efficiency as the size increase. Based on Equation 1 and 2, energy transfer efficiency should linearly increase with J (λ) and ΦD, but decrease in the 6th power law with r. Therefore, separation distance is the most dominant factor in determining energy transfer efficiency. As a result, FRET calculation leads to higher energy transfer efficiencies for QD/tetraazaporphyrins with smaller QD size. As shown in Table 1, assuming coiled ligands conformation, the theoretical energy transfer efficiency for QD1/ TAPFcMg (89.8%) and QD1/ TAPFcCNMg(93.5%) are significantly higher than those of QD3/ TAPFcMg (59.9%) and QD3/ TAPFcCNMg (71.4%). In addition, changing the separation distance from r1 (extended ligand conformation) to r2 (coiled



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conformation) enhances the energy transfer rates by about one order of magnitude and almost doubles energy transfer efficiencies. It is worth noting that due to the multiexponential decays of the fluorescence of QDs (Figure 4), the average PL lifetimes are used for τD in FRET calculation. However as shown in the Supplementary Information (SI), there are two definitions of average lifetime, namely intensity weighted lifetime and amplitude weighted lifetime, both of which have been adopted under different circumstances. For example, the intensity average lifetime is preferred for the analysis of dynamic PL quenching by Stern-Volmer equation, which is the case for the QD/tetraazaporphyrins used in our work. 53 However, amplitude average lifetime is generally used for the calculation of energy transfer efficiency. In this work, both types of lifetimes are used for FRET calculation and the following PL lifetime quenching measurement. The results calculated from intensity average lifetime are shown in the Table1, while the results obtained with amplitude average lifetime are shown in the Table S1 in SI. The results indicate the definition of average lifetime has a significant effect on theoretical energy transfer rate, but negligence effect on theoretical energy transfer efficiency, which can be derived from the equation 1 and 2.

Photoluminescence Quenching: Since energy transfer is expected to compete with the radiative emission of the donor, PL quenching measurement is an effective method to evaluate energy transfer efficiency. Indeed, we observed that the PL intensity of QD solutions was gradually quenched by the titration of tetraazaporphyrins, as shown in Figure 3a for QD1/TAPFcMg. Once again, we did not observe significant shift in the peak position or the formation of new emission band in the PL spectra of QDs during titration, further eliminating the possibility of complex


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formation. The PL quenching of QDs is expressed as Stern-Volmer plots shown in Figure 3b and 3c, and analyzed using the Equation (3):

I o / I = 1 + k SV [Q ]

(3)

where I0 and I is the PL intensity of QDs in the absence and presence of tetraazaporphyrins, respectively, [Q] is the concentrations of the quencher (e.g., the tetraazaporphyrins), and kSV is the Stern-Volmer quenching constant. If energy transfer is the only PL quenching mechanism, the value of kSV should be independent of quencher concentration, and consequently the value of I0/I should increase linearly with the [Q]. Our results show that I0/I deviate from the linear dependence on [Q] as shown in Figure 3. Furthermore, the degree of PL quenching does not follow the QD size dependence as predicted by our FRET calculations (Table 1). For example, FRET calculations predict higher energy transfer efficiency in QD1/TAPFcMg than QD2/TAPFcMg. However, we observed higher PL intensity quenching in QD2/TAPFcMg than in QD1/TAPFcMg (Figure 3). As a result, the PL quenching data strongly suggest that energy transfer is not the only quenching mechanism in QD/tetraazaporphyrin solutions. Other mechanisms may also contribute to the PL quenching in QD/tetraazaporphyrin solutions, leading to complex QD size effect on PL intensity quenching as shown in Figure 3.

PL Lifetime Measurements: To further understand the PL quenching mechanisms in QD/tetraazaporphyrin assemblies, PL lifetime measurement was carried out on both QDs and QD/tetraazaporphyrin solutions. The PL lifetime of QDs (1 nM) without (τ0) and with (τ1) the presence of porphyrins (14 µM) are obtained from the transient PL decay curves shown in Figure 4. It is known that the PL lifetimes of quantum dots (τ0) are determined by the radiative (kR) and nonradiative (kNR) recombination rate shown in the Equation (4):



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τo =

1 k R + k NR

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(4)

The observed multiexponential PL decays of quantum dot solutions (Figure 4) indicate the existence of a distribution of nonradiative recombination rates, which is not uncommon for colloidal QDs. 54 The PL lifetimes of QD/tetraazaporphyrin mixture (τ1) are expected to be reduced as compared with τ0 due to the quenching effects of tetraazaporphyrin on the QDs excited-state dynamics. If energy transfer is the sole quenching mechanism with rate kT, the corresponding PL lifetime τ1 is determined by Equation (5):

τ1 =

1 k R + k NR + kT

(5)

However, PL intensity quenching measurements suggest the existence of an additional quenching process in QD/tetraazaporphyrin solutions that takes place at a rate kQ. In this scenario, the PL lifetime of QD/tetraazaporphyrin mixture (τ1) will be modified accordingly as described by Equation (6):

τ1 =

1 k R + k NR + kT + kQ

(6)

If the PL lifetime reflects all the recombination processes and τ1 is determined by Equation (6), the quenching of PL lifetime, Qτ = (τ0- τ1 )/τ0, should be equal to the quenching of PL intensity, QI = (I0- I1 )/I0),. However, as shown in table 2, the values of Qτ are significantly smaller than QI. It is worth noting that the TCSPC system used to conduct the PL lifetime measurement has time resolution greater than 0.1 ns. While this high temporal resolution is capable of resolving the energy transfer process (ns time scale, Table 1), recombination process can take place on a much faster time scale that may be beyond our temporal resolution. The results shown in Table 2 indicate the existence of an ultrafast quenching process in QD/tetraazaporphyrin


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complexes that cannot be directly resolved by the PL lifetime measurement using TCSPC. As a result, PL lifetime τ1 of QD/tetraazaporphyrin assemblies obtained by our measurements only reflects the quenching from energy transfer and it is therefore determined by the Equation (5), instead of Equation (6). Based on Equation (2) and (5), the energy transfer efficiency (E) in QD/tetraazaporphyrin assemblies is derived to be equal to the quenching of PL lifetime Qτ.

The Ultrafast PL Quenching Mechanism: Since the quenching of PL intensity QI, originates from both energy transfer and the ultrafast quenching mechanism, and the quenching of PL lifetime Qτ, results from energy transfer, the corresponding difference, (QI - Qτ), can be reasonably attributed to an ultrafast quenching mechanism or process. As shown in Table 2, although QI does not show clear QD size dependence, the Qτ-based energy transfer efficiency increases with QD size, which is in contradiction with the FRET calculations shown in Table 1. In addition, the quenching efficiency of the ultrafast process, (QI Qτ), decreases with increasing quantum dot size. These findings suggest that the energy transfer efficiency in QD/tetraazaporphyrin assemblies is largely determined by the ultrafast queching mechanism. The theoretical limit of energy transfer efficiency in QD/tetraazaporphyrin assemblies seems unlikely to be reached when the ultrafast queching process dominates in the excited state dynamics of QDs. As a result, the highest energy transfer efficiency is found with QD3/TAPFcCNMg, where the ultrafast mechanism has the minimum PL quenching efficiency. We note that the PL lifetime quenching in Table 2 are calculated from intensity average lifetime, the results obtained with amplitude average lifetime are shown in Table S2. The definition of the average lifetime does influence the values of energy transfer efficiency calculated from PL



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lifetime quenching, but the data in two tables lead to a consistent QD size effect on the ultrafast PL quenching mechanism and energy transfer efficiency. The ultrafast quenching mechanism in QD/tetraazaporphyrin assemblies is not fully understood. Photoinduced charge transfer has been reported to be responsible for the PL quenching of CdSe quantum dots with surface-functionalized organic dyes.14 The tetraazaporphyrins used in this report, however, are not directly linked to QD surface. As a result, the spacing from the ODA ligands renders a relatively large distance between CdSe core and tetraazapophyrin molecules, and thus may prohibit efficient charge transfer. Alternatively, we propose a trap-assisted PL quenching mechanism in this CdSe/tetraazaporphyrin system. It is well known that the surface defects of CdSe quantum dots form shallow traps for charge carriers and thus opens an ultrafast charge–exciton interaction mechanism in the picosecond to femtosecond time scale and efficiently quenches the PL of QDs. The PL blinking of single quantum dots has been attributed to a similar mechanism.55 Surface passivation with organic ligands like ODA or high bandgap semiconductor layer (e.g., ZnS) is an effective approach to suppress the trap-related PL quenching mechanism. In the tetraazaporphyrin/QD solutions, it is likely that the dynamic collisions or static associations between tetraazaporphyrin and the surface-ligated quantum dots generate new surface trap states. For example the extended π-electrons in associated tetraazaporphyrin molecules may change local surface energy distribution of quantum dots and form new low energy defects. It is also known that the charge carriers, including electrons and holes that are confined in CdSe core, may tunnel through the confinement barrier and reach the outer surface and are captured by the surface traps. And consequently the trapped charges form effective quenchers for PL of QDs through ultrafast charge-exciton interaction. The possibility of tunneling increases with


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decreased QD size.15 As a result, the PL quenching through this trap-assisted mechanism is more significant in small quantum dots. This is consistent with size-dependence result of (QI - Qτ) shown in Table 2. Additionally, we observed that TAPFcCNMg induces less ultrafast PL quenching than TAPFcMg. It may result from their difference in electronic structure and interaction with the QD ligands, and hence differing in trap generation on QD surface.

Conclusions The energy transfer from CdSe quantum dots to two NIR-absorbing tetraazaporphyrins has been investigated, as a function of the nanocrystal size, using both theoretical and experimental methods. FRET calculations predict higher energy transfer efficiency with smaller quantum dot size. Experimentally, however, the PL intensity and lifetime measurements suggest an ultrafast trap-assisted PL quenching mechanism coexisting with energy transfer process in QD/tetraazaporphyrin solutions. Based on our results, we propose that the trap-assisted quenching process is more dominant in the case of smaller QD sizes and with TAPFcMg. The energy transfer efficiency is significantly below the theoretical limit when the ultrafast quenching process dominates the excited state dynamics of QDs. As a result, the most efficient energy transfer is obtained with the combination of large quantum dot size and TAPFcCNMg, which has the minimum quenching from trap-assisted mechanism. Our study suggests that CdSe quantum dots are promising energy transfer donor for NIR-absorbing tetraazaporphyrins to form antenna systems for enhanced light-harvesting materials if the ultrafast trap-related quenching mechanism can be effectively suppressed.



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Acknowledgements This work is supported by the Grant-in-Aid program of the University of Minnesota, UMD Chancellor’s Small Grant, and UMD Small Seed Research Grant to ZX. Research carried out in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-SC0012704. Generous support from the NSF CHE-1110455 and CHE-14010375 to VNN is greatly appreciated. We would like to thank Seb Pont for his help at the early stage of this project.


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Table 1: Energy transfer calculations for QD/tetraazaporphyrin assemblies. QD/

J (λ)

τD

tetraazaporphyrin

(1014 nm4

(ns)

ΦD

RQD

r1

r2

kT (r1)

kT (r2)

(nm)

(nm)

(nm)

(ns-1)

(ns-1)

E(r1)

E(r2)

M-1cm-1)

QD1/TAPFcMg

3.33

28.6

0.40

11

36

24

0.027

0.307

0.435

0.898

QD2 /TAPFcMg

11.5

21.0

0.30

16

41

29

0.043

0.347

0.477

0.879

QD3/TAPFcMg

22.5

17.1

0.25

28

53

41

0.019

0.087

0.243

0.599

QD1/TAPFcCNMg

5.46

28.6

0.40

11

36

24

0.044

0.503

0.558

0.935

QD2/TAPFcCNMg

9.49

21.0

0.30

16

41

29

0.036

0.287

0.430

0.858

QD3/TAPFcCNMg

37.7

17.1

0.25

28

53

41

0.031

0.146

0.349

0.714

J(λ): spectral overlap between the QDs PL spectra and the tetraazaporphyrins absorption spectra τD: PL life time of QDs without the presence of tetraazaporphyrins ΦD: PL quantum yield of QDs without the presence of tetraazaporphyrins RQD: radium of the CdSe core of QDs r1: donor-acceptor separation distance assuming extended ODA ligand conformation r2: donor-acceptor separation distance assuming coiled ODA ligand conformation kT (r1): energy transfer rate with separation distance r1 kT (r2): energy transfer rate with separation distance r2 E(r1): energy transfer efficiency with separation distance r1 E(r2): energy transfer efficiency with separation distance r2



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Table 2. Data summary for both the PL intensity quenching QI = (I0- I1 )/I0 and PL lifetime quenching Qτ = (τ0- τ1 )/τ0 for QD/tetraazaporphyrin solutions QD/ tetraazaporphyrin

QI

Qτ

QI - Qτ

QD1/TAPFcMg

0.82

0.08

0.74

QD2/TAPFcMg

0.86

0.12

0.74

QD3/TAPFcMg

0.56

0.23

0.33

QD1/TAPFcCNMg

0.9

0.21

0.69

QD2/TAPFcCNMg

0.97

0.77

0.2

QD3/TAPFcCNMg

0.85

0.78

0.07


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Figure Captions Figure 1. (a) Chemical structures and (b) absorption spectra (solid lines) of TAPFcMg (red) and TAPFcCNMg (blue), PL spectra of QDs (dash lines).

Figure 2. a) Absorption (solid lines) and PL (dash lines) spectra of quantum dots; b) absorption spectra of TAPFcMg and TAPFcMg /QD2 solutions.

Figure 3. a) PL spectra of QD1 in the absence and in the presence of different concentration of TAPFcMg; b) Stern-Volmer plots of QDs/ TAPFcMg; c) Stern-Volmer plots of QDs/ TAPFcCNMg.

Figure 4. PL decay curves of QDs (QD1, QD2, and QD3) without and with the presence of tetraazaporphyrins (TAPFcMg and TAPFcCNMg).



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Figure 1


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Figure 2



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Figure 3


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Figure 4



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TOC Image


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Energy Transfer from Colloidal Quantum Dots to Near-Infrared

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