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Accelerating FRET between Near-Infrared-Emitting Quantum Dots Using a Molecular J-aggregate as an Exciton Bridge Chen Wang, and Emily A. Weiss Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b02559 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 9, 2017
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Accelerating FRET between Near-Infrared-Emitting Quantum Dots Using a Molecular Jaggregate as an Exciton Bridge Chen Wang and Emily A. Weiss* Department of Chemistry, Northwestern University, 2145 Sheridan Rd., Evanston, IL 602083113 *corresponding author. Email:
[email protected] Abstract: Fast energy transfer (EnT) among quantum dots (QDs) with near-infrared (NIR) emission is essential for fully exploiting their light harvesting and photon downconversion (multiexciton generation) abilities. This paper demonstrates a relayed EnT mechanism that accelerates the migration of NIR excitons between PbS QDs by a factor of 20 from that of onestep EnT through a polyelectrolyte, and even a factor of ~2 from that of one-step EnT from QDs in direct contact, by employing a J-aggregate (J-agg) of a cyanine dye as an exciton bridge. The donor QDs, acceptor QDs and J-agg are electrostatically assembled into a sandwich structure with layer-by-layer deposition. Estimates of EnT rate and yield from transient and steady-state absorption and photoluminescence spectroscopies show that the rate-limiting step in the relay is EnT from the donor QD to the J-agg, while EnT from the J-agg to the acceptor QD occurs in 95% of incoming photons, or at 991 nm (herein denoted “exc,J”), where the J-agg absorbs >98% of incoming photons, Figure 2A. When we excite the DJ film at exc,QD, we observe nearly complete quenching of DQD emission and a sharp PL peak at 1055 nm corresponding to the emission of the J-agg (Figure 2B, blue, and SI). We know that this J-agg emission is due to EnT from the DQDs to the J-agg, since the same excitation for the JA film results in no J-agg emission (Figure 2B, red). By comparing the intensity of PL from the J-agg within the DJ film obtained with exc,QD to that obtained exc,J we estimate an EnT yield of 70% from DQDs to the J-agg within DJ, see the SI and Table 1. This result is consistent with our previous observation of efficient EnT between PbS QDs and J-agg in the solution phase.21 When we excite the JA film at exc,J (Figure 2C, red), we find that the J-agg emission is weaker and shifted to slightly higher energy than that from J-agg within DJ films (Figure 2C, blue), and the emission from AQDs is enhanced, consistent with EnT from the J-agg to AQDs. We estimate the yield of this EnT to be 25%, by comparing the emission intensities of the AQDs within JA using exc,J vs. exc,QD, see the SI and Table 1. The limited yield of EnT from J-agg to AQD is due to competition with the fast non-radiative exciton decay rate of the J-agg, as we discuss below. We note that the emission from AQDs within JA is attenuated by ~70% compared to the AQD-only film (see the SI, Figure S4) due, most likely, to charge transfer from the AQDs to IR-140-Cy-, but we account for this attenuation in our EnT yield calculation by comparing the emission of the AQDs at the two excitation wavelengths, see the SI.
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Figure 2. Steady-state Optical Spectra of PbS QD/J-agg Composite Films. A) Ground state absorption spectra of D/PSS/A, DJ, JA, and DJA films. The arrows indicate the excitation wavelengths for the PL spectra in B and C. B) PL spectra acquired with 460 nm excitation (exc,QD), where 95% of photons excite the DQDs and AQDs, and 5% excite the J-agg. “*” indicates scattered light from the Xe lamp. C) PL spectra acquired with 991 nm excitation (exc,J), where 98% of photons excite the J-agg.
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The PL spectrum of the DJA film, after excitation at exc,QD, is dominated by AQD emission, Figure 2B, green. A small J-agg emission signal can also be identified at 1047 nm. The PL from AQDs within DJA is almost twice as intense as that from AQDs within JA with the same excitation, which indicates that excitons created by DQD absorption are contributing to the AQD emission. Additionally, the emission from J-agg within DJA is a factor of four less intense than from J-agg within DJ, which indicates that the J-agg acts as an energy shuttle from DQDs to AQDs rather than emitting itself. Given that, according to the absorption spectra in Figure 2A, the optical densities of AQDs and DQDs in the DJ and JA films are nearly identical at 460 nm, it follows that the two species have nearly identical absorptions at 460 nm in the DJA film. These steady-state spectra therefore allow us to conclude that the DJA architecture acts as an exciton relay. The overall DQD-to-AQD EnT yield in the DJA film can be estimated by taking the product of the respective EnT yields from DQD to J-agg (in the DJ film) and from J-agg to AQD (in the JA film): 70%×25% = 18%, Table 1. A priori, it seems reasonable to calculate this yield from the differences in the intensity of AQD emission between JA and DJA films, but that calculation is misleading because the PL quantum yields of AQDs are different in the two films, see the SI. When the DQD and AQD layers are separated by a layer of PSS with a DQD-AQD center-tocenter distance of 6.62.5 nm, rather than a layer of J-agg with a DQD-AQD distance of 7.43.0 nm, the yield of DQD-to-AQD EnT is 78%. As we shall see below, however, this higher yield is achieved by eliminating the non-radiative decay of exciton population inside J-agg, and occurs despite the fact that direct EnT is a factor of 20-30 slower than the relayed EnT in the DJA system. We then monitored the EnT dynamics within the composite films using transient absorption (TA) spectroscopy. The spectra in the window of 850-1300 nm are dominated by the ground state bleach of the J-agg peaked at 1060 nm (Figure 3A), even when we excite at 460 nm (exc,QD) where 95% selective excitation of the QDs at 460 nm. These spectra include the ground state bleaches of the J-agg at 1060 nm and the AQD at 1160 nm; the latter is largely obscured by the J-agg signal but is more apparent in the magnified spectra in the inset. (B) Normalized kinetic traces extracted from the NIR-TA spectra of J, DJ, JA and DJA films at 1060 nm (the peak of the J-agg bleach), after pumping the QDs at 460 nm. (C) VisTA spectra of DJA at a series of time delays after >98% selective excitation of the J-agg at 990 nm showing the absorption of the S1 state of IR-140-Cy-. (D) Normalized kinetic traces extracted from the Vis-TA spectra of DJ and DJA films at 555 nm (the peak of the S1 absorption), after pumping the J-agg at 990 nm. The kinetic traces are fit with multi-exponential functions as described in the text, with parameters listed in the SI.
Figure 3B, black, shows that, upon 460-nm direct excitation of the J-agg within the J-only film, the J-agg bleach decays with three distinct exponential components with time constants of 1.78, 24 and 510 ps (see the SI); this decay can be attributed to many- and bi-exciton annihilation,
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and intrinsic single exciton decay processes.30-33 The overall, amplitude-averaged exciton lifetime in the J-agg is only 2.4 ps in the J-only film (or 2.6 ps in the DJ film). We cannot deconvolve the exciton dynamics of the DJA film into two rate constants for the two EnT steps in the relay, so to understand these dynamics, we compare the dynamics of the Jagg excited state within DJA to that within DJ and JA. First, upon excitation of DJA at exc,QD (Figure 3B, green), the J-agg bleach has a growth, appearing as a delayed decay, between ~10 and 300 ps, that is not present in the trace for the J-only film (black). This formation of J-agg is also present, and more easily distinguished, in the trace for the DJ film (blue), and is attributable to EnT from the DQD to the J-agg. We do not attempt to extract a time constant for EnT from the DQDs to the J-agg,
→
, from these kinetics, however, because such a fit would be ambiguated
by overlapping timescales of exciton formation and decay in the J-agg, and variation of J-agg →
lifetime with exciton density. Instead, we obtain
from the PL decay of DQDs within DJA, as
discussed below. After ~300 ps, the J-agg bleach for the DJA film (Figure 3B, green) decays more quickly and has a smaller final amplitude (at 3 ns) than in the DJ film (blue). This behavior is consistent with an additional decay pathway for the J-agg excited state provided by the AQDs. We confirm this additional pathway by selectively exciting the J-agg within the DJA film or the JA film at 990 nm, and monitoring the excited state of the J-agg via its photoinduced absorption at 555 nm21, 34 (because the J-agg bleach overlaps with our pump), Figure 3C. The excited state of the J-agg indeed decays faster in the DJA and JA films than in the DJ film (Figure 3D). We observe no signals from the radical cation or anion of IR-140-Cy- in the visible TA spectra, so we can rule out charge transfer from the J-agg to the QDs as this decay pathway. This piece of evidence, along with the steady-state enhancement of AQD PL within JA and DJA films when exciting the J-agg, allows us to conclude that J-agg excitons in the DJA film are decaying through EnT to the AQDs. We estimate the EnT rate from the J-agg to the AQDs within DJA, 1/
→
, from the difference
between the average J-agg excited state lifetime, d, within DJ and DJA: 1/ 1/
→
. This calculation yields
→
1/
8 ps, Table 1. This number is predictable from a FRET
analysis for this system based on the radiative lifetime of the cyanine J-agg, see the SI. Despite this fast time constant for J-to-A EnT, the yield of this process is still limited to 25% because of the strong competition from the rapid exciton decay in the J-agg ( 9
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Table 1. EnT Yields and Time Constants in Various Composite Films. Measurement
EnT
Film
Yield
D-to-J
DJ, DJA
70%
1.70.5 ns
J-to-A
JA, DJA
25%
82 ps a
DJA
18% b
2.72.3 ns
AQD bleach growth
D/PSS/A
78%
AQD PL growth DQD PL decay
D/A
-
644 ns 581 ns 5.40.2 ns 4.31.4 ns
D-to-A
DQD PL decay J-agg ESA decay
AQD PL growth DQD PL decay
a
Calculated from the difference between the amplitude-averaged rate constants for the decay of the J-agg ESA signals in the DJA and DJ samples. b Calculated as the product of the DQD-to-J-agg EnT yield (in the DJ film) and the J-agg-to-AQD EnT yield (in the JA film).
Figure 4A shows time-resolved PL measurements of D/PSS/A, DA and DJA films monitored at 900-1100 nm (selective for DQD emission). The decay of DQD PL is accelerated in the DJA film relative to the two control films, due to EnT to the J-agg, and fits to a time constant
→
1.7 ns. This 1300 nm (AQD emission) (B) after exciting at 450 nm. (C) Normalized kinetic traces for the change of the population of the AQD exciton as extracted from the NIR-TA spectra of D/PSS/A, DA and DJA films at 1160 (the peak of the AQD
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ground state bleach), after >95% selective excitation of the QDs at 460 nm The kinetic traces are fit with multi-exponential functions as described in the text, with parameters listed in the SI. We have demonstrated a two-step FRET relay between donor and acceptor PbS QDs in a composite film using a J-agg of cyanine dye as an exciton shuttle. Through this NIR exciton relay, we accelerate exciton migration from DQDs to AQDs by a factor of 20 from that of the direct EnT process where the DQDs and AQDs are separated by a polyelectrolyte, and by a factor of two from that of the direct EnT process where the DQDs and AQDs are separated only by their ligands. The EnT rate using the molecular exciton bridge is also at least one order of magnitude faster than previously reported EnT processes between PbS QDs, which are on the order of tens to hundreds of nanoseconds, with one exception. In our most recent measurement of EnT rates within solutionphase aggregates of PbS QDs, we record a time constant of 2.1 ns for EnT between donor and acceptor PbS QDs, but we had to bring the DQD/AQD pair to within a distance of 3.8 nm centerto-center, about half of the distance as they are in the DJA films, to achieve this rate.35 The relay provides a strategy for accelerating EnT without minimizing ligand length or precisely controlling interparticle distance. The rate-limiting step for the relayed EnT process is EnT from the DQD to the J-agg (1.7 ns). By comparing this time constant to that we observed in solution-phase complexes of glutathionecoated PbS QDs and adsorbed J-agg (800 ps),21 we can propose to increase the rate of relayed EnT in DJA by (i) depositing more ordered J-aggregates, to achieve a higher oscillator strength transition, which will improve the spectral overlap for EnT, and (ii) eliminating the CdS shell on the QDs, which introduces more dielectric screening than that of a purely organic capping layer. Increasing the intermolecular ordering in the J-agg will also improve the yield of relayed EnT (currently, ~18%) by decreasing the conformational fluctuations of the dye molecules and therefore the rate of vibrationally mediated non-radiative decay of the J-agg excited state. Although the rate and yield of relayed EnT in our system can be improved, this system is a demonstration of a powerful strategy for accelerating exciton migration between donor and acceptor chromophores by transforming 1-step FRET to multi-step FRET with an exciton bridge. Supporting Information. Synthesis of PbS/CdS core/shell QDs, preparation of PbS QDs with charged surface, PDDA structure, preparation of DA film, preparation of the J-agg, steady state spectroscopy, EnT quantum yield estimation, AFM, TA setup, TCSPC, radiative relaxation of the J-agg, estimation of the J-to-A EnT rate. 13
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Acknowledgements. This material is based on upon work supported by the Air Force Office of Scientific Research, under AFOSR award No. FA-9550-14-1-0005, and the David and Lucile Packard Foundation through a Packard Foundation Fellowship for Science and Engineering. The authors thank Dr. Chengmei Zhong and Dr. Mark Hersam for their assistance with AFM measurement and data analysis. REFERENCES (1) Rogach, A. L.; Klar, T. A.; Lupton, J. M.; Meijerink, A.; Feldmann, J. J. Mater. Chem. 2009, 19, 1208-1221. (2) Choi, H.; Santra, P. K.; Kamat, P. V. ACS Nano 2012, 6, 5718-5726. (3) Kramer, I. J.; Levina, L.; Debnath, R.; Zhitomirsky, D.; Sargent, E. H. Nano Lett. 2011, 11, 3701-3706. (4) Chou, K.; Dennis, A. Sensors 2015, 15, 13288. (5) Díaz, S. A.; Menéndez, G. O.; Etchehon, M. H.; Giordano, L.; Jovin, T. M.; JaresErijman, E. A. ACS Nano 2011, 5, 2795-2805. (6) Díaz, S. A.; Giordano, L.; Jovin, T. M.; Jares-Erijman, E. A. Nano Lett. 2012, 12, 35373544. (7) Díaz, S. A.; Giordano, L.; Azcárate, J. C.; Jovin, T. M.; Jares-Erijman, E. A. J. Am. Chem. Soc. 2013, 135, 3208-3217. (8) Huang, Z.; Li, X.; Mahboub, M.; Hanson, K. M.; Nichols, V. M.; Le, H.; Tang, M. L.; Bardeen, C. J. Nano Lett. 2015, 15, 5552-5557. (9) Wu, M.; Congreve, D. N.; Wilson, M. W. B.; Jean, J.; Geva, N.; Welborn, M.; Van Voorhis, T.; Bulović, V.; Bawendi, M. G.; Baldo, M. A. Nat. Photon. 2016, 10, 31-34. (10) Selmarten, D.; Jones, M.; Rumbles, G.; Yu, P.; Nedeljkovic, J.; Shaheen, S. J. Phys. Chem. B 2005, 109, 15927-15932. (11) Smith, C.; Binks, D. Nanomater. 2014, 4, 19-45. (12) Nozik, A. J. Physica E: Low Dimens. Syst. Nanostruct. 2002, 14, 115-120. (13) Hanna, M. C.; Nozik, A. J. J. Appl. Phys. 2006, 100, 0745101-8. (14) Robel, I.; Gresback, R.; Kortshagen, U.; Schaller, R. D.; Klimov, V. I. Phys. Rev. Lett. 2009, 102, 177404. (15) Padilha, L. A.; Stewart, J. T.; Sandberg, R. L.; Bae, W. K.; Koh, W. K.; Pietryga, J. M.; Klimov, V. I. Acc. Chem. Res. 2013, 46, 1261-1269. (16) Clark, S. W.; Harbold, J. M.; Wise, F. W. J. Phys. Chem. A 2007, 111, 7302-7305. (17) Corricelli, M.; Enrichi, F.; Altamura, D.; De Caro, L.; Giannini, C.; Falqui, A.; Agostiano, A.; Curri, M. L.; Striccoli, M. J. Phys. Chem. C 2012, 116, 6143-6152. (18) Xu, F.; Ma, X.; Haughn, C. R.; Benavides, J.; Doty, M. F.; Cloutier, S. G. ACS Nano 2011, 5, 9950-9957. (19) Litvin, A. P.; Ushakova, E. V.; Parfenov, P. S.; Fedorov, A. V.; Baranov, A. V. J. Phys. Chem. C 2014, 118, 6531-6535. (20) Wang, C.; Kodaimati, M. S.; Schatz, G. C.; Weiss, E. A. Chem. Commun. 2017. (21) Wang, C.; Weiss, E. A. J. Am. Chem. Soc. 2016, 138, 9557-9564. (22) Halpert, J. E.; Tischler, J. R.; Nair, G.; Walker, B. J.; Liu, W.; Bulovic, V.; Bawendi, M. G. J. Phys. Chem. A 2009, 113, 9986-9992. 14
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