Interface-Dependent Aggregation-Induced Delayed Fluorescence in

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Interface-Dependent Aggregation-Induced Delayed Fluorescence in Bottlebrush Polymer Nanofibers Christopher M. Tonge and Zachary M. Hudson* Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada

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S Supporting Information *

ABSTRACT: Bottlebrush copolymers provide a covalent route to multicompartment nanomaterials that remain nanosegregated regardless of environmental conditions. This is particularly advantageous when combining polymers for optoelectronics, where the ability to control the interface between multiple chemically distinct polymers can be key to a device’s function. Here we prepare bottlebrush nanofibers from an acridine- and triazine-based donor/acceptor pair, which have been shown to exhibit thermally activated delayed fluorescence (TADF) via through-space charge transfer (TSCT). By controlling the morphology of the donor and acceptor domains within the bottlebrush, random, miktoarm, and block bottlebrush morphologies are obtained. Using these materials, nanofibers may be prepared which (i) strongly exhibit TSCT TADF; (ii) exhibit switchable TSCT TADF based on aggregation of the fibers; or (iii) preserve the properties of the original donor and acceptor components. This work demonstrates that a bottlebrush strategy may be used to either force or prevent interactions between chemically dissimilar optoelectronic polymers in blended thin films. In this way, we establish a convenient method for either maximizing or minimizing donor−acceptor interactions in semiconductor polymer blends, using different arrangements of the same building blocks within a bottlebrush nanofiber.

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INTRODUCTION The ability to create complex and hierarchical nanomaterials from soft matter has paved the way for many new applications of nanotechnology. Recent advancements in noncovalent selfassembly have provided access to nanomaterials with unprecedented control over their morphology, composition, and function.1−7 Self-assembly methods do, however, typically require the nanostructure generated to exist in either a local or absolute free energy minimum.8,9 This is achieved by tuning the structure, van der Waals forces, and solvophilicity of the building blocks such that a self-assembled structure is stabilized relative to its dispersed parts. As such, the potential metastability of these structures can pose a challenge when self-assembled nanomaterials are investigated under nonideal conditions, and a robust approach to generating multicompartment, covalently bound nanostructures would accelerate their adoption in broad applications. Bottlebrush copolymers (BBCPs) are one such route to covalently bound nanostructures with multiple well-defined compartments.10−13 Consisting of a linear backbone with many polymeric side chains, they typically adopt extended, wormlike conformations due to the steric demands of these closely packed pendant groups.14−16 Their unique morphology and reduced chain entanglement have enabled many applications, including as photonic crystals,17−20 templates for lithography or metal deposition,21−23 and as imaging agents for nanomedicine.24−27 Their block-by-block synthesis provides facile access to multicompartment structures that remain nanosegregated regardless of solvent or temperature, as well as © XXXX American Chemical Society

providing a simple route to noncentrosymmetric nanofibers that are otherwise difficult to obtain.28 We and others recently proposed that BBCPs presented a strategy for preparing multicompartment nanofibers from a diverse range of optoelectronic materials.29−31 Using a bottomup approach, we have shown that structures analogous to organic p−n junctions and multilayer electroluminescent devices can be prepared in which the electronic properties of each component of the fiber are observed. This versatile technique now allows us to prepare nanofibers with photophysical properties that were only recently discovered, and to study how these properties behave at nanoscale interfaces. The discovery of thermally activated delayed fluorescence (TADF) has revolutionized organic light-emitting diode (OLED) technology, by demonstrating that carefully designed emitters can harvest both singlet and triplet excitons without the use of noble metals.32 By facilitating reverse intersystem crossing (RISC) from their lowest triplet excited state (T1) to the singlet state (S1) with a small singlet−triplet energy gap (ΔEST), 100% internal quantum efficiency can be achieved in OLEDs using TADF emitters.33,34 TADF polymers, in particular, have received considerable recent attention due to their suitability for low-cost solution processing.35−37 In addition to their applications in OLEDs, TADF materials have also been explored for their applications in fluorescence lifetime imaging38,39 as well as oxygen sensing.40 Received: July 5, 2019

A

DOI: 10.1021/jacs.9b07156 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society Wang, Shao, and co-workers recently introduced a new paradigm for TADF polymer design, exploiting through-space charge transfer (TSCT) to achieve the TADF effect.41 By preparing copolymers of donor (D) and acceptor (A) along a nonconjugated backbone, overlap between the donor HOMO and acceptor LUMO was minimized, creating the low ΔEST necessary for TADF to be observed. The use of nonconjugated but spatially proximate D and A chromophores also avoids the large red shifts typical of charge-transfer polymers, enabling access to blue TADF materials. This approach has also been used in blends of polymer with small molecules,42 and has been recently extended for full color TSCT TADF by tuning the structures of the D/A pairs.43 Here we prepare bottlebrush nanofibers exhibiting the TSCT TADF effect and demonstrate that this effect is highly dependent on the interface between D and A within the nanofiber by controlling their bottom-up architecture (Chart 1). In particular, we find that the brush-like structure of the

ature with dispersities from 1.15 to 1.21 (Figure 1). This controlled-radical polymerization facilitates the preparation of

Chart 1. Architectures of TADF Polymers

heterotelechelic polymers functionalized at the α and/or ω termini asymmetrically, allowing us to quantitatively install a norbornene handle for ring-opening metathesis polymerization (ROMP). Using Grubbs’ third generation catalyst,47 these macromonomers were then used to generate a series of bottlebrush nanofibers by grafting-through ROMP with distinct D/A interface types and target backbone lengths of 150 (Figure 2 the suffix “BB” denotes a bottlebrush). These included: (1) a bottlebrush homopolymer of the random macromonomer ACRTRZ-MM, giving a nanofiber ACRTRZ150-BB with a fully random D/A interface; (2) miktoarm copolymer ACR75-coTRZ75-BB of ACR-MM and TRZ-MM, giving a bottlebrush with homopolymer side chains but D/A interfaces consisting of blended strands; and (3) a diblock bottlebrush copolymer, ACR75-b-TRZ75-BB, created by polymerization of ACR-MM followed by the addition of TRZ-MM. We also prepared bottlebrush homopolymers ACR150-BB and TRZ150-BB consisting of only donor and acceptor chromophores, respectively, to demonstrate that the photophysical properties of these organic semiconductors do not change substantially upon incorporation into bottlebrush nanofibers. These polymers were purified by precipitation into methanol followed by preparatory SEC in THF to yield high-purity bottlebrush polymers with molecular weights ranging from 604 kDa to 812 kDa and dispersities from 1.18 to 1.38 (Table 1). The relatively high dispersity of these ROMP procedures is attributable to the steric bulk and poor solubility of the triazine-functionalized macromonomer, and (co)polymers thereof.46 This methodology highlights the utility of a bottlebrush framework for generating multicomponent nanostructures with explicit control over the interface between optoelectronic components on length scales from tens to hundreds of nanometers. Such nanostructures allow for tunable control at three levels of hierarchy: (1) the structures of the donor and acceptor monomers used; (2) the interfacial contact of these monomers within macromonomer strands; and (3) the interpolymer contact of macromonomer strands attached to a common bottlebrush backbone. To confirm that the bottlebrush morphology itself imposes minimal change on the photophysical properties of the ACR and TRZ chromophores, cyclic voltammetry data for all five bottlebrushes were obtained. As shown in Figure 3, ACR150-

Figure 1. Synthesis of norbornene-functionalized macromonomers from a series of organic semiconductors, and their reaction to form bottlebrush polymers by grafting-through ROMP.

BBCP enables interstrand CT from all-donor side chains to allacceptor side chains, due to the inability of these chains to phase separate when bound to a common backbone. In this way, miktoarm-type (“mixed-arm”) nanofibers exhibit TSCT TADF in much the same manner as fully random D−A copolymers. TSCT can further be activated by aggregation of the nanofibers into a globular form in solution, resulting in a dramatic enhancement in TADF.

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RESULTS AND DISCUSSION To create a series of bottlebrush copolymers with TSCT properties, the choice of donor and acceptor is key to efficient emission. Lixiang Wang and co-workers recently demonstrated that dimethylacridine- and 1,3,5-triphenyltriazine-based chromophores make an ideal donor/acceptor pair for TSCT copolymers, with good color tunability for the resulting CT emission depending on the substituents at each monomer.41,43 Two acrylic monomers were thus synthesized based on these moieties, a dimethylacridine-based monomer (ACR) and a triazine based monomer (TRZ) (see Supporting Information, SI). Using these monomers, we sought to explore the effect of interfacial mixing on the TSCT TADF effect, as well as how distinct interface types within a bottlebrush nanofiber impact TSCT. Using norbornene-functionalized initiator 1, a series of three macromonomers were prepared using a facile Cu(0) reversible-deactivation radical polymerization (Cu(0)-RDRP) procedure.44−46 Using this method, macromonomers entirely composed of dimethylacridine chromophore (ACR-MM), triazine chromophore (TRZ-MM), or equal parts of the two by mass (ACRTRZ-MM) can be prepared at room temperB

DOI: 10.1021/jacs.9b07156 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

Figure 2. Schematic illustrations and atomic force microscopy height traces (scale bar = 100 nm) of polymers ACRTRZ150-BB (a), ACR75-coTRZ75-BB (b), and ACR75-b-TRZ75-BB (c), where the ACR and TRZ chromophores are represented in teal and purple, respectively. Inset: photographs of the respective BBCP powders excited with 365 nm light.

BB displays two reversible oxidation peaks at 0.79 and 1.03 V, and TRZ150-BB shows a single reversible reduction peak at −1.73 V, relative to FeCp 2 0/+ . All three bottlebrush copolymers, however, clearly show reduction and oxidation peaks corresponding to both chromophores. Furthermore, these peaks do not shift relative to the homopolymer bottlebrushes or macromonomers (Figure S16), implying little to no modulation of the HOMO and LUMO levels of the individual donor and acceptor groups when combined with different interface types. In contrast, the fluorescent properties of these polymers depend strongly on the donor−acceptor interface in each

Table 1. Synthesis of Bottlebrush Polymers entry

Mn,a (kDa)

DPa

Đa

conv. (%)a

ACR150-BB TRZ150-BB ACRTRZ150-BB ACR75-co-TRZ75-BB ACR75-b-TRZ75-BB

763 812 619 778 604

150 138 104 142 66/52b

1.18 1.35 1.24 1.37 1.38

86 78 81 82 81

a Determined by SEC in THF using triple detection. bBlock 1/Block 2. DP (B2) determined by 1H NMR.

Figure 3. a) Cyclic voltammetry of BBCPs at 2 mg mL−1 in in 1,2-difluorobenzene with 0.2 M tetrabutylammonium hexafluorophosphate; b) illustration of energy transfer process involved in TSCT; c) solid state fluorescent emission of BBCPs excited at 313 nm; lifetimes in solid state for d) ACRTRZ150-BB, e) ACR75-co-TRZ75-BB, and f) ACR75-b-TRZ75-BB, measured as thin films on glass slides under inert conditions and under ambient atmosphere, excited at 313 nm. Note the difference in time scales for d) and e) vs f). C

DOI: 10.1021/jacs.9b07156 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society Table 2. Fluorescent Lifetimes of Bottlebrush Copolymers solutiona entry ACR150-BB TRZ150-BB ACRTRZ150-BB ACR75-co-TRZ75-BB ACR75-b-TRZ75-BB

solid state

τ, air (ns)

τ,inert (ns)

τp,air (ns)

2.6, 1.6, 32.0, 29.7, 0.8,

2.9, 1.8, 66.0, 30.3, 0.8,

5.3, 2.8, 33.9, 30, 3.9,

8.9 6.0 56.7 130 4.4

11.0 7.6 126 132 5.1

41.7 39.4 137 132 40.7

τd,air (μs)

1.8, 33.7 6.8, 59.9

τp,inert (ns) 5.5, 3.0, 43.1, 30.2, 3.3,

42.1 40.2 157 132 39.7

τd,inert (μs)

23.0, 129 5.8, 75.5

a

Fluorescence on a microsecond time scale was not observed in solution for all samples.

environment will vary both in its proximity to other chromophores, its exposure to solvent (if applicable), and the likelihood of interdigitation with another nearby bottlebrush. In light of these factors, the complex lifetimes we observe are not unexpected, particularly for a charge-transfer process. Several significant conclusions can be drawn from these data. First, the block bottlebrush demonstrates that the steric demands of the bottlebrush morphology enforces nanosegregation on the individual donor and acceptor domains, minimizing interpenetration and allowing the D and A chromophores to retain their individual photophysical properties even in the solid state. Second, interchain TSCT is not only significant, but dominant in the emission spectra of miktoarm-type bottlebrush nanofibers, suggesting that the TSCT TADF effect can be achieved in mixed-arm polymers without the need for copolymer synthesis. This is in contrast to the fluorescence of a thin film formed from a 1:1 mixture of the ACR and TRZ macromonomers, which exhibits both monomer emission and TSCT (Figure 4). In effect, the bottlebrush morphology is able to enforce strong interaction

bottlebrush (Table S1). The emission spectra of both the random copolymer ACRTRZ150-BB and the miktoarm ACR75co-TRZ75-BB in the solid state are dominated by green TSCT emission, with λmax = 490 nm in both cases. Interestingly, the quantum yield of TSCT fluorescence is substantially higher in the fully blended ACRTRZ150-BB (0.47 in toluene at 0.01 mg mL−1, 0.12 in the solid state) compared to the miktoarm (0.15 and 0.07, respectively). This is likely due to less complete mixing in the case of the miktoarm polymer leading to decreased fluorescence quantum yield when compared to ACRTRZ150-BB. The charge transfer nature of the TSCT process is confirmed by the significant positive solvatochromism of the fluorescence of both ACRTRZ150-BB and ACR75co-TRZ75-BB (Figure S12), resulting in large red shifts in emission in polar solvent. ACR75-co-TRZ75-BB also shows a larger degree of residual monomer emission, attributable to the larger average D−A spacing expected for the miktoarm-type polymer. This larger average D−A spacing also results in a slight reduction in the relative ratio of delayed fluorescence to prompt fluorescence observed for the miktoarm bottlebrush, decreasing from 60.3% delayed fluorescence for the random copolymer to 57.4% delayed fluorescence for the miktoarm. In contrast, the diblock bottlebrush ACR75-b-TRZ75-BB however shows almost no TSCT, with its emission spectra in solution and the solid state appearing as essentially a superposition of the ACR and TRZ chromophores, with blue fluorescence at λmax = 414 nm. This is accompanied by almost no solvatochromic response, indicating minimal charge-transfer character in the emission of this bottlebrush. We next examined the fluorescence lifetimes of these bottlebrushes under air and inert atmosphere. In solution, the two homopolymers ACR150-BB and TRZ150-BB exhibit only prompt fluorescence (90%). In a similar manner, Wang and Li have also shown that

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CONCLUSIONS In summary, grafting-through ROMP of donor- and acceptorfunctionalized macromonomers has been shown as an effective strategy for generating organic nanofibers exhibiting the TSCT TADF effect. The morphology of the donor/acceptor interface was found to play a key role in TSCT, and can be easily controlled using covalent synthesis via a bottlebrush strategy. In particular, we found that interchain TSCT was dominant in the case of miktoarm-type bottlebrush polymers, giving highly stimuli-responsive nanofibers with enhanced TSCT relative to a mixture of linear polymers, and can be readily controlled by aggregation. Finally, a block bottlebrush morphology preserved the properties of individual chromophores within the polymer nanofibers, with minimal donor−acceptor TSCT even in the solid state. These investigations open the door to further studies on multicompartment nanoscale wires with donor/ acceptor properties, including nanomaterials with exciplex-type E

DOI: 10.1021/jacs.9b07156 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society emission and stimuli-responsive fluorescence. Furthermore, a bottlebrush strategy may be used to either force, or prevent, interactions between chemically dissimilar optoelectronic polymers using either a miktoarm or block morphology. This may prove to be an effective method for preparing blended, multicomponent films from organic semiconductors, where maximizing D−A interactions or preserving the individual properties of each component of a blend may be necessary for example, in organic photovoltaics or ambipolar films for charge-transport.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b07156.

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Experimental details, synthetic procedures, additional photophysical data, and additional electrochemical data (PDF)

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Zachary M. Hudson: 0000-0002-8033-4136 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Foundation for Innovation (CFI), the British Columbia Knowledge Development Fund (BCKDF), and the Canada Research Chairs Program for support of their research. We are also grateful to Materia Inc. for the donation of ruthenium catalyst.

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DOI: 10.1021/jacs.9b07156 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX