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Direct Triplet Sensitization of Oligothiophene by Quantum Dots

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Direct Triplet Sensitization of Oligothiophene by Quantum Dots Direct triplet sensitization of oligothiophene by quantum dots Zihao Xu, Emory University Tao Jin, Emory University Yiming Huang, Rice University Karimulla Mulla, Rice University Francesco Evangelista, Emory University Eilaf Egap, Rice University Tianquan Lian, Emory University Journal Title: Chemical Science Volume: Volume 10, Number 24 Publisher: Royal Society of Chemistry: Open Access | 2019-01-01, Pages 6120-6124 Type of Work: Article | Final Publisher PDF Publisher DOI: 10.1039/c9sc01648a Permanent URL: https://pid.emory.edu/ark:/25593/v0zb4 Final published version: http://dx.doi.org/10.1039/c9sc01648a Copyright information: © 2019 The Royal Society of Chemistry. This is an Open Access work distributed under the terms of the Creative Commons Attribution-Noncommercial 3.0 Unported License (http://creativecommons.org/licenses/by-nc/3.0/). Accessed September 28, 2021 1:23 PM EDT Chemical Science View Article Online EDGE ARTICLE View Journal | View Issue Direct triplet sensitization of oligothiophene by quantum dots† Cite this: Chem. Sci.,2019,10,6120 a a b b All publication charges for this article Zihao Xu, Tao Jin, Yiming Huang, Karimulla Mulla, have been paid for by the Royal Society Francesco A. Evangelista, *a Eilaf Egap *b and Tianquan Lian *a of Chemistry Effective sensitization of triplet states is essential to many applications, including triplet–triplet annihilation based photon upconversion schemes. This work demonstrates successful triplet sensitization of a CdSe quantum dot (QD)–bound oligothiophene carboxylic acid (T6). Transient absorption spectroscopy provides direct evidence of Dexter-type triplet energy transfer from the QD to the acceptor without populating the singlet excited state or charge transfer intermediates. Analysis of T6 concentration Received 3rd April 2019 dependent triplet formation kinetics shows that the intrinsic triplet energy transfer rate in 1 : 1 QD–T6 Accepted 12th May 2019 complexes is 0.077 nsÀ1 and the apparent transfer rate and efficiency can be improved by increasing the DOI: 10.1039/c9sc01648a acceptor binding strength. This work demonstrates a new class of triplet acceptor molecules for QD- rsc.li/chemical-science based upconversion systems that are more stable and tunable than the extensively studied polyacenes. Creative Commons Attribution-NonCommercial 3.0 Unported Licence. F ¼ F F F F Introduction UC ISC TET TTA FL (1) ffi F Triplet excitons in organic materials exhibit long lifetime, The overall upconversion e ciency ( UC) is the product of 1–3 ffi extended diffusion length and low-lying energy levels, and e ciencies in each step involved, namely the ISC of the sensi- 23 4 F have found promising applications in photon upconversion. tizer ( ISC), the triplet energy transfer (TET) from the sensitizer F F For example, an optical upconversion layer on a solar cell can to transmitter to emitter ( TET), TTA of the emitter ( TTA), and F capture sub-bandgap photons and emit above bandgap the emitter's uorescence ( FL). For a speci c sensitizer and This article is licensed under a 5 F F F photons, increasing the efficiency of the conventional single- emitter, ISC, TTA and FL are determined by the material's junction devices beyond the Shockley–Queisser limit.6 Various properties. A promising area for performance improvement is ffi strategies for improving the overall efficiency of upconversion the design of QD/transmitter complexes to enable e cient 24 Open Access Article. Published on 13 May 2019. Downloaded 6/20/2019 2:08:09 PM. systems have been developed.7–10 In recent years, quantum dot TET. (QD) sensitized upconversion systems11–14 have attracted In most reported QD sensitized upconversion systems, the intense interest as a versatile and promising approach because transmitter/emitter molecules are solely limited to acenes and of their large absorption coefficient,15 small singlet-to-triplet their derivatives, which have limited structural and energetic 11,13,18–22,25 energy gap and fast intersystem crossing (ISC) rate,16 and tunability and poor stability. Oligothiophenes have 26 tunable band gap and band alignment.17 wide range of tunability in energetics and molecular structure, In a typical photon-upconversion system, triplet excitons are making them a very desirable class of triplet acceptors/ generated through a sensitizer that undergoes intersystem transmitters in QD-organic hybrid TTA upconversion systems. crossing from an excited singlet state to a triplet state. This Although an example of oligothiophene phosphonic acid graf- 27 process is followed by a sequential triplet energy transfer rst to ted to cadmium selenide (CdSe) QDs was previously reported, the transmitter then to the emitter. The latter, can undergo the photoluminescence quenching was attributed to charge triplet–triplet-annihilation (TTA) and emit a higher energy transfer and TET was not observed. We hypothesize that with photon.4,11,13,18–22 Thus, the efficiency of a typical sensitizer- appropriate design of the energetics of QDs and oligothiophene ffi emitter-based upconversion system can be represented by the acceptors, e cient TET transfer can be facilitated while following equation:4 competing single energy and charge transfer pathways may be suppressed (Fig. 1). aDepartment of Chemistry, Emory University, 1515 Dickey Dr, Atlanta, GA, 30322, USA. E-mail: [email protected]; [email protected] bDepartment of Materials Science and NanoEngineering, Department of Chemical and Results and discussion Biomolecular Engineering, Rice University, 6100 Main St, Houston, TX, 77005, USA. E-mail: [email protected] In this work, we demonstrate for the rst time a successful † Electronic supplementary information (ESI) available. See DOI: Dexter-type TET from CdSe QDs to carboxylic acid functionalized 000 00 0 0 00 00 000 000 0000 0000 00000 10.1039/c9sc01648a oligothiophene (3 ,4 -dihexyl-[2,2 :5 ,2 :5 ,2 :5 ,2 :5 ,2 - 6120 | Chem. Sci.,2019,10,6120–6124 This journal is © The Royal Society of Chemistry 2019 View Article Online Edge Article Chemical Science bound T6 shows a red-shi of 20 nm, which is attributed to a change in the dielectric environment and has been observed in a similar system.30 The total amounts of T6 in the samples were obtained by subtracting QD only spectra from QD–T6 complex spectra. However, the total T6 amount contains both QD-adsorbed and free T6 molecules, which cannot be easily differentiated by their absorption spectra alone. The amount of QD-bound T6 is determined by tting the transient kinetics of triplet formation in QD–T6 complexes (see below). QD photo- luminescence (PL) intensity decreased with increasing T6 concentration (Fig. 2), suggesting possible energy or charge Fig. 1 Cartoon of QD–T6 complexes showing the energetics of the transfer from the QD to T6. relevant states and associated excited state decay processes. To understand the uorescence quenching mechanism in QD–T6 complexes, we studied pristine T6 rst to characterize its excited states spectral signature via TA spectroscopy. The TA sexithiophene]-5-carboxylic acid or T6, Fig. 1). In this system, only spectra of T6 measured with 400 nm excitation (Fig. 3A and B) the triplet energy transfer from the CdSe QD to T6 is energetically show clear evolution from a singlet to triplet excited state on the favored while the charge and singlet transfer are energetically sub-nanosecond time scale. The TA spectra at <0.5 ns (Fig. 3A) uphill. We used transient absorption (TA) spectroscopy to rst show: (i) a ground (S0) state bleach (GSB) centered at 400 nm caused by the decrease of ground state T6 molecules; (ii) identify the lowest energy singlet (S1) and triplet (T1) state spectral features of free T6 molecules in solution. Then TA study of QD– a stimulated emission from the T6 singlet excited state – T6 complexes provide direct evidence for triplet sensitization of appearing as a negative signal at 500 600 nm; and (iii) a broad Creative Commons Attribution-NonCommercial 3.0 Unported Licence. T6 by CdSe QD. We also employed Density Function Theory (DFT) singlet (S1) excited state absorption (ESA) from 600 nm to computations to characterize the low-lying excited states involved 900 nm. The assignment of GSB and stimulated emission of T6 in the triplet–triplet transfer mechanism. is based on comparison with the steady state absorption and † T6 (shown in Fig. 1) was synthesized following a literature emission spectra shown in Fig. S4. The (S1) ESA peak is formed 28,29 – – procedure and characterized by 1H-NMR, 13C-NMR and high at an early delay time (2 5 ps) and its amplitude grows from 2 5 – resolution mass spectrometry (see ESI and Fig. S1–S3† for ps (red) to 10 50 ps (yellow) as the stimulated emission band details). The absorption spectra of four QD–T6 complex samples shi s to longer wavelength (by 20 nm) at the same time, which with increasing T6 concentrations ranging from 12.5 to 50 mM is attributed to a fast relaxation from the initial excited state to ¼ are shown in Fig. 2. These plots show a T6 band at 415 nm and the v 0 vibrational level of the S1 excited state. Similar fast This article is licensed under a the rst excitonic peak of CdSe QDs at 584 nm. In contrast to the relaxation processes have also been observed in other oligo- 26,31–34 absorption spectrum of T6 in toluene (Fig. 2 and S4†), the QD- thiophene molecules. Both the S1 ESA and stimulated emission decay on the sub-nanosecond time scale to form the Open Access Article. Published on 13 May 2019. Downloaded 6/20/2019 2:08:09 PM. rst triplet excited state (T1) with an absorption peak (T1 to Tn transition) centered at 715 nm (Fig. 3B). The lifetime of this species is longer than 10 ms and can be shortened by more than 1000 times in the presence of oxygen (Fig.
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