Light-intensity switch enabled nonsynchronous growth of fluorinated raspberry-like nanoparticles

Raspberry-like (RB) nanoparticles hold potential for diverse applications due to their hierarchical morphology. Here we developed a novel tandem synthetic approach of nonsynchronous growth based on photo-mediated reversible-deactivation radical polymerization, enabling simple, efficient and bottom-up synthesis of RB nanoparticles of uniform sizes at quantitative conversions of fluorinated monomers. Chain transfer agents of different chain lengths, concentrations and chemical compositions were varied to tune the diameter of RB particles. Importantly, fluorinated RB nanoparticles obtained with this method allow facile post modifications via both covalent bond formation and intermolecular physical interactions without disrupting the RB morphology. The facile nature of this method and versatility of the obtained fluorinated RB materials open new opportunities for the development of functional materials using nanoparticles.


Introduction
Nonspherical colloidal particles with anisotropic structures have been widely investigated due to their importance in fabricating functional materials. 1 Among them, raspberry-like (RB) particles have gained considerable attention due to their surface roughness, large surface areas and high level of scattering ratios, 2,3 leading to applications such as drug delivery, 4 catalyst supports, 5 photonic materials 6 and superhydrophobic coatings. 7 While reversible-deactivation radical polymerization (RDRP) has enabled the generation of polymeric particles with various morphologies 8-10 ranging from spheres, worms, and vesicles to ordered inverse mesophases through the operationally simple polymerization induced self-assembly (PISA) pathway, [11][12][13][14][15] it still remains challenging to generate RB particles based on PISA. Conventionally, RB particles have been obtained via the combination of small corona and large core particles via physical or chemical interactions, 16,17 as well as emulsion polymerization. 18,19 However, these strategies oen suffer from complex operating processes or driving force of multiphase separation. Moreover, while post-synthetic modication represents an attractive approach to functionalize polymeric colloids, its utility for RB particles was conned due to lack of modiable substituents and requirement of multistep processes. 20,21 Therefore, the development of a facile PISA approach to prepare versatile RB particles would provide improved opportunities for materials engineering.
The employment of light as a low-cost and environmentally sustainable stimulus in RDRP 22-32 (i.e., photoinduced electron transfer-reversible addition-fragmentation chain-transfer (PET-RAFT) polymerization) [33][34][35] has aroused a lot of interest, promoting the preparation of polymeric colloids without intermediate purication. [36][37][38][39] Motivated by the outstanding physicochemical properties of uoropolymers, 40,41 we have developed photo-RDRP of uorinated alkenes. 42,43 While the uorine-uorine (F-F) interaction has been adopted to generate various morphologies, 44-47 the preparation of uorinated RB particles remains unexploited. Recently, kinetic mediation by light intensity in photo-RDRP has been demonstrated by Boyer 24 and Miyake. 25 We envisioned that by regulating the polymerization rate through visible light, selfassembly of uorous substances would lead to Scheme 1 Synthesis of PPFS RB nanoparticles via a tandem photomediated approach based on photo-RDRP. nonsynchronous growth of successively-generated particles, resulting in polymeric colloids of different sizes, which would fuse to provide the RB morphology.
Herein, we report a one-pot, bottom-up synthesis of uorinated RB particles through tandem photo-mediated nonsynchronous growth for the rst time (Scheme 1), which represents a convenient and practical pathway to control the morphology of nanoparticles via the photo-RDRP of pentauorostyrene (PFS). Moreover, the poly(PFS) (PPFS) backbone enables post modication of RB nanoparticles via both chemical and physical interactions, as demonstrated by cross-linking, substitution and aggregation-induced emission (AIE) experiments, providing an attractive platform for materials engineering.

Results and discussion
At the beginning of our study, poly(dimethylacrylamide) (PDMA 54 , M n ¼ 5.64 Â 10 3 Da, Đ ¼ 1.06) terminated with a trithiocarbonate group was prepared as a chain-transfer agent (CTA), and employed in the photo-RDRP of PFS using tris ( Fig. S1 †) at room temperature. Dimethyl sulfoxide (DMSO) was adopted as solvent. In the formed copolymers, the PDMA block is solvophilic and the PPFS block is solvophobic. Aer optimization, light intensities of 33 and 0.56 mW cm À2 have been employed for the strong-and weak-light irradiations, respectively (Table S1 †).
As shown in Fig. 1a, while the strong-light irradiation enables faster polymerization compared to weak-light (k p ¼ 0.77 vs. 0.25 h À1 ), both processes exhibit rst-order kinetics during propagations. Although a two-stage regime of kinetics has been detected in PISA, 48,49 our observations are consistent with RAFT polymerization of PFS under thermal conditions, 44 where the absence of the rst stage for homogeneous polymerization is attributed to the fast nucleation which occurred at very low degrees of polymerization (DPs) of the uorinated core-forming block as driven by the F-F interaction.
The reaction mixtures were analyzed by size-exclusion chromatography (SEC). The peak molecular weight (M p ) was adopted to indicate the changing trend of the molar mass of PPFS. The    37 suggests that a secondary nucleation could take place during the photo-RDRP process.
Next, the particles generated under both conditions were characterized by transmission electron microscopy (TEM). As shown in Fig. 1d, spheres of hydration diameter ¼ 90 AE 10 nm were obtained using strong-light irradiation. Similarly, when other ratios of [PFS]/[PDMA 54 -CTA] (from 100/1 to 500/1) were employed to provide copolymers of different lengths of the uorinated segment, only a spherical morphology was observed (diameter ¼ 83 to 119 nm, M p ¼ 24.3-123 kDa, Fig. S6, S7 and Table S4 †), which is different from the morphology evolution trend detected in other photo-PISA processes. 8,36 In comparison, when weak-light intensity was used under otherwise identical conditions, large particles (341 nm) and a few small spheres ($30 nm) have been simultaneously observed by TEM at 82% conversion of PFS (Fig. 1e).
Under weak-light irradiation, photoredox catalysis would generate relatively fewer propagating chains and lead to slower monomer consumption rates compared to strong-light irradiation. As a result, a portion of PDMA-CTA would rst grow into uorinated polymers and lead to the generation of uorous particles via self-assembly. These particles would absorb PFS in solvent, subsequently provide uoropolymers with high molar masses due to the increased molar ratio of the monomer to growing chain, 42 and evolve into large particles during the relatively long-time polymerization. Meanwhile, the reaction between remaining PDMA-CTA and PFS dissolved in solvent would generate new uorous particles of small sizes, which would have time to further fuse with large ones (Fig. S9 †). In contrast, strong-light irradiation at the beginning of the reaction would promote the generation of more propagating chains, and enable high initiation efficiencies of PDMA-CTA ( Fig. S2 and Table S2 †). The formation of lots of uorinated chains at the early stage would accelerate the formation of uorous particles, which don't have enough time or remaining PFS to allow their growth into large ones.
Although the fusion of small and large particles could take place under weak-light irradiation, the remaining PFS absorbed in fused particles would act as plasticizers to increase the mobility of polymers. 45 Before the complete consumption of PFS, the slow polymerization rate under weak-light irradiation could provide enough time for polymers to undergo chain movement, which would reduce the surface roughness of fused particles (Fig. S11 †). We envisioned that accelerating the monomer consumption at the late-stage of polymerization could benet maintaining the surface roughness.
Next, we attempted to apply weak-and strong-light irradiation successively to photo-RDRP in a one-pot fashion (Fig. 2ad). Aer optimization of the switching timing ( Fig. S10 †), we found that when the light intensity was changed at high conversions (84-95%) of PFS, RB particles of uniform sizes were successfully obtained (327-341 nm, particle size distribution (PSD) ¼ 1.05-1.14, Table S5 †). During the reaction, formation and entanglement of uoropolymers between different particles could stabilize the fused particles. 50 According to the hypothesis, the uorous cores should have higher PFS concentrations than that in solvent. Therefore, this method should lead to different populations of molar masses for polymers grown in uorous cores and generated from CTA and PFS in solvents. When the reaction mixtures were analyzed by SEC, bimodal distributions were observed (M p ¼ 455-956 and 33.7-51.6 kDa, Fig. S8 †), and the proportions of higher M p values increased with the exposure time.
An advantage of PPFS is the ease in post-synthetic modication by nucleophilic aromatic substitution on the -C 6 F 5 group, facilitating the preparation of functional polymers. 53,54 In our studies, when RB particles were treated with 1,2-ethanedithiol, cross-linked colloids were generated without a clear morphological change (Fig. 5a and b). Compared to original RB particles, thermal stability of the rough surfaces of the crosslinked counterparts has dramatically increased for over 30 C (80 vs. 115 C, Fig. S20 †).
Moreover, three types of nucleophiles including alkyl, benzyl and aryl thiols have been used to react with RB particles at room temperature. As determined by 19 F nuclear magnetic resonance, the para-uorine atom of the -C 6 F 5 group could be efficiently converted in the nucleophilic aromatic substitution, while the RB morphology was maintained ( Fig. 5c and Table S10 †), furnishing opportunities for loading various molecules onto RB particles under mild and metal-free conditions via C-S bond formation.
Compounds with tetraphenylethylene (TPE) units show AIE behavior, and have stimulated biomedical and optoelectronic applications. 55 Upon mixing a carboxyl substituted TPE (TPE-COOH) with RB particles, TPE-COOH was encapsulated into RB particles as indicated by the promoted AIE expression   ( Fig. 5d, e and S22 †), exhibiting an enhanced uorescence at 430 nm and a blue-shi of 40 nm compared to TPE-COOH due to restricted intramolecular rotation. 56,57 The successful incorporation of TPE-COOH is probably due to the polar-p interaction 58,59 between -C 6 F 5 from the host particle and phenyl from the TPE guest (Fig. 5f).

Conclusion
In conclusion, we have developed a tandem approach based on nonsynchronous growth via photo-RDRP that enables the onepot and in situ synthesis of RB nanoparticles with uniform sizes at quantitative monomer conversion and under mild conditions. The successive visible-light irradiation from weak-to strong-light intensity has allowed the generation and fusion of large and small colloids, facilitating the synthesis of RB particles of a variety of sizes. Moreover, the uorinated RB particles could be modied by both chemical and physical interactions, broadening opportunities to access uorinated nanomaterials with various physicochemical properties. Given the broad interest in nonspherical colloids, uorinated materials and photochemistry, we expect this method to be useful for providing versatile PPFS RB particles for advanced materials engineering.

Conflicts of interest
There are no conicts to declare.