Visible-Light-Driven Sensitization of Naphthalene Triplets Using

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Spectroscopy and Photochemistry; General Theory

Visible-Light-Driven Sensitization of Naphthalene Triplets using Quantum-Confined CsPbBr3 Nanocrystals Yaoyao Han, Xiao Luo, Runchen Lai, YuLu Li, Guijie Liang, and Kaifeng Wu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00597 • Publication Date (Web): 13 Mar 2019 Downloaded from http://pubs.acs.org on March 13, 2019

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The Journal of Physical Chemistry Letters

Visible-Light-Driven Sensitization of Naphthalene Triplets using Quantum-Confined CsPbBr3 Nanocrystals Yaoyao Han,†,‡ Xiao Luo,† Runchen Lai,† Yulu Li,† Guijie Liang,§ and Kaifeng Wu†* †

State Key Laboratory of Molecular Reaction Dynamics and Dynamics Research Center

for Energy and Environmental Materials, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning 116023, China ‡ University

§

of the Chinese Academy of Sciences, Beijing 100049, P.R. China

Hubei Key Laboratory of Low Dimensional Optoelectronic Materials and Devices,

Hubei University of Arts and Science, Xiangyang, Hubei 441053, China

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Paragon Plus Environment

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Polycyclic aromatic hydrocarbons (PAHs) are very old and well-known organic compounds that are ubiquitous on the earth and in the universe. Their optical and electronic properties, such as singlet and triplet transition energies and redox potentials, have been well-documented in the literature.1 Recently there are been renewed interests in these compounds because of potential applications of their singlets and triplets in solar energy, photocatalysis and light emission. For example, tetracene and pentacene have been known to undergo an efficient excitation multiplication process called singlet fission (SF), i.e., the generation of two triplets by the fission of one singlet, which provides an exciting opportunity to enhance the efficiency of solar cells and light emitting devices if these triplets can be efficiently harvested.2-3 SF is a unique spin-conserving process generating triplets in tetracene and pentacene. In other PAHs where SF cannot occur, triplets are difficult to be directly generated by optical excitation because of their “dark” nature and because of inefficient intersystem crossing (ISC) in PAHs. Triplets of these “non-SF” PAHs, however, are also technologically important because they can be applied to photon upconversion,4-5 photoredox catalysis,6-7 room temperature phosphorescence,8 etc. Triplets of PAHs are often generated using a so-called sensitization approach (Fig. 1, top right).4 With only a few exceptions where boron-dipyrromethene (BODIPY) dyes9-10 and fullerenes11-12 were employed as the sensitizers, heavy-metal-containing organometallic dyes are frequently used because their singlets populated by light absorption can undergo efficient ISC to triplets, enabled by a strong spin-orbital coupling. The triplets of sensitizers can in turn transfer their energy to PAHs via Dexter-type triplet energy transfer (TET). Previous studies have mainly focused on PAHs with relatively low triplet 3 ACS Paragon Plus Environment


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energy (ET) in the red and near-infrared (NIR) part of the spectra,13-19 mainly including pyrene (ET ~ 2.0 eV), antracene (ET ~ 1.83 eV), perylene (ET ~ 1.53 eV), tetracene (ET ~ 1.25 eV), and their derivatives such as diphenylanthracene (ET ~ 1.77 eV) and rubrene (ET ~ 1.15 eV). This is because these PAHs can be paired with common heavy-metalcontaining sensitizers such as tris(2-phenylpyridine) iridium(III) [Ir(ppy)3; ET ~ 2.5 eV],13 tris(bipyridine)

ruthenium(II)

octaethylporphyrin

[Ru(bpy)32+;

(PtOEP;

ET

ET ~

~ 1.9

2.0 eV),17,

eV],6

palladium(II)

20

platinum(II)

tetraphenyltetrabenzoporphyrin (PtTPBP; ET ~ 1.6 eV),15 Palladium(II) phthalocyanine (PdPc; ET ~ 1.3 eV)21-22, Pt(II)/Pd(II) octabutoxynaphthalocyanine (PtNac/PdNac, ; ET ~ 1.0 eV)23 and their derivatives. Note that triplet acceptors other than PAHs, such as the BODIPY dyes 24, diketopyrrolopyrroles derivatives 25 and conjugated polymers26-27, have also been developed, but they are still paired with the common sensitizers mentioned above. Good molecular sensitizers often have near unity ISC yields, but on the other hand they are in general associated with a large intrinsic energy loss in the ISC process,17 on the order of ~0.5 eV or higher, resulting in several practical issues with their applications. It reduces attainable photon upconversion margins, defined as the energy differences between emitted and absorbed photons. For this reason, demonstrated upconversion examples are usually from NIR to visible or from visible to visible, while visible-toultraviolet (UV) upconversion remains relatively rare and inefficient.14, 28 The large ISC energy loss also prohibits sensitization of very energetic triplet species using common sensitizers. For example, naphthalene, the simplest form of PAHs, has a triplet energy of ~2.6 eV.1 In the traditional sensitization scheme, a sensitizer with a singlet energy of at 4 ACS Paragon Plus Environment

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least 3.1 eV would be required (Fig. 1, top right), higher than all aforementioned sensitizers. Relatedly, UV light sources would be required to drive the sensitization process. Recently, inorganic semiconductor nanocrystals (NCs) have been developed as alternative triplet sensitizers for PAHs.17, 29-34 The exchange interaction in NCs are only a few to 10s meVs,35-36 resulting in negligible bright-dark state energy splitting compared to the singlet-triplet splitting in dye sensitizers. As such, sensitization using NCs can, in principle, occur at the limit of energy conservation (Fig. 1, top left). Among various types of NCs, the recently-introduced lead halide perovskite NCs, although have already proven attractive for many light-harvesting and -emitting applications,37-39 remain relatively underexplored in the field of triplet sensitization.40 In particular, we realize that CsPbBr3 NCs,39 with a bulk bandgap of ~2.4 eV, would be an ideal sensitizer for naphthalene triplets (Nap3*). With the aid of the quantum confinement effect, the energetics requirement for TET can be met. In addition, CsPbBr3 NCs with very high emission quantum yields (QYs) approaching unity can be routinely synthesized, thereby minimizing nonradiative recombination channels that potentially compete with the interfacial TET process. In contrast, traditional CdSe and CdS NCs that satisfy the energetics requirement for Nap sensitization have very low QYs unless specially engineered or shell-coated.




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dictated by the donor-acceptor electronic coupling rather than the driving force, because NC size-dependent TET rates approximately scale linearly with the amplitudes of squared wave function at the surfaces of NCs. Sensitized Nap3* can find important applications in visible-to-UV upconversion, photoredox catalysis and room temperature phosphorescence (Fig. 1, bottom). In order to meet the energetics requirement for Nap3* sensitization, we synthesized a series of quantum-confined CsPbBr3 NCs; see Supporting Information (SI) for details.41 In light of a Bohr exciton diameter of ~7 nm for CsPbBr3,39 the sizes of these NCs were controlled to be < 7 nm. Fig. 2 shows representative transmission electron microscopy (TEM) images of the six samples used in this study. These NCs are in general cubeshaped and the average NC edge lengths (L) vary from 3.2 nm to 5.1 nm. According to the statistical histograms in Fig. S1, the size distributions of these samples (standard deviations) are Q 15%.




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are particularly well-defined, which range from 453 nm (2.74 eV) to 483 nm (2.57 eV). The samples are labelled by this peak position. Given a triplet energy of ~2.6 eV for Nap, the apparent TET driving forces from ~0.14 eV to ~-0.03 eV can be estimated. The absorption spectra of NC-PCA complexes contain not only excitonic features of NCs but also an intense absorption band around 300 nm that can be assigned to Nap. Because 1naphthalene carboxylic acid is not soluble in hexane, this absorption band can only be attributed to NC-surface-bound Nap molecules. Based on the absorption spectra, the Nap:NC ratio in these complexes can be determined; see SI for details.

1.0

4 0.5

0.0

1

450 500 550 Wavelength(nm)

3 2

459 nm

c

0.0


1.0

3 2

467 nm

e

6

1.0

5 0.5

0.0


1

400 500 Wavelength (nm)

PL intensity

PL intensity

4

4

600

473nm

f


600


3 0.0

2



5

1.0

4 0.5

3 2

600

483 nm

0.5

0.0


1 0

0 300

0.0

300

1

0

2

0.5

0 300

Abs (OD)

d

600

Abs (OD)


3

463 nm

1

0 300

1.0

4 0.5

1 0

5 PL intensity

5

PL intensity

b

Abs (OD)

PL intensity

Abs (OD)

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453 nm

PL intensity

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Abs (OD)

a

Abs (OD)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300


600

300


600

Figure 3. Absorption and emission (inset) spectra of CsPbBr3 NCs (gray solid lines) and NC-Nap complexes (colored solid lines). The NCs have their first excitonic absorption peaks ranging from ~453 nm (a) to ~483 nm (f). The emission spectra were acquired under 400 nm excitation.



The photoluminescence (PL) spectra of NCs and NC-PCA complexes, acquired using 400 nm light which selectively excites NCs, are shown in Fig. 3 insets. Quenching of NC emissions by Nap is observed for all samples, but with the quenching efficiency decreases from 66.1% for NC-453 nm to 4.4% for NC-483 nm (Table S1). Because the reduction and oxidation potentials of Nap are ~ 2.5 V and 6.2 V (vs. vacuum)1 whereas the conduction and valence band edges of bulk CsPbBr3 are ~ 3.6 and 6.0 V (vs. vacuum)45, quenching of CsPbBr3 NC excitons via electron and/or hole transfer to Nap can be ruled out based on energetics considerations. Thus, TET from NCs to Nap should be the dominant quenching mechanism for NC excitons. To gain more insights into the TET process, we directly measured the TET dynamics from NCs to Nap using pump-probe transient absorption spectroscopy. Free NCs and NC-Nap complexes were measured under exactly the same conditions: they were excited using 400 nm pump pulses and pump-induced absorption changes were recorded at variable delays using white light probe pulses; see SI for experimental details. Pump energy densities were kept low such that the average exciton number per NC () was

Visible-Light-Driven Sensitization of Naphthalene Triplets Using

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