Living Light-Induced Crystallization-Driven Self-Assembly for Rapid

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Living Light-Induced Crystallization-Driven Self-Assembly for Rapid Preparation of Semiconducting Nanofibers Suyong Shin,† Florian Menk,‡ Youngjin Kim,§ Jeewoo Lim,§,∥ Kookheon Char,§ Rudolf Zentel,‡ and Tae-Lim Choi*,† †

Department of Chemistry, Seoul National University, Seoul, 08826, Korea Institute for Organic Chemistry, Johannes Gutenberg University, Duesbergweg 10-14, 55128, Mainz, Germany § Department of Chemical Engineering, Seoul National University, Seoul, 08826, Korea ‡

S Supporting Information *

ABSTRACT: Well-defined nanostructures composed of conjugated polymers have attracted significant attention due to their intriguing electronic and optical properties. However, precise control of the size and uniformity of these semiconducting nanostructures is still rare and challenging, despite recent advances in strategies to obtain self-assembled nanostructures with narrow dispersions. Herein, we demonstrate the preparation of fluorescent conjugated block copolymers by one-shot polymerization and rapid formation of nanofibers in a few minutes via light-induced crystallization-driven self-assembly, driven by facile cis-to-trans photoisomerization of its poly(p-phenylenevinylene) blocks. Furthermore, living self-assembly was possible, allowing not only nanofibers with excellent length control and narrow size distribution but also ABA triblock comicelles and gradient comicelles, to be produced by seeded growth. Lastly, the seeded growth could be activated and deactivated repeatedly by switching the light on and off, analogous to light-induced living radical polymerization.

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advantage of the strong π−π interactions of insoluble conjugated polymers, spontaneous self-assembly readily occurs during the living polymerization of BCPs, producing kinetically trapped nanostructures. Although various unique nanostructures have been prepared with high stability,25,26 rapid aggregation during the INCP process hinders the generation of well-ordered structures with uniform sizes. In particular, conjugated block copolymers synthesized by ring-opening metathesis polymerization (ROMP) contain a mixture of cisand trans-olefins in the conjugated backbone, reducing the crystallinity of the polymers.23,26,27 Recently, we reported the in situ formation of unique nanostructures by preparing BCPs containing poly(pphenylenevinylene) (PPV)28 via the ROMP of [2.2]paracyclophane-1,9-diene (PCPDE) without any side chain.26,27 Nanostructures containing PPV are attractive materials due to their useful optoelectronic properties22,29 and the interesting cis- to trans-PPV isomerization induced by light irradiation.30,31 This photoisomerization can significantly lower the solubility of PPV because cis-rich PPV has a twisted and coiled configuration, but the extended trans-rich PPV has a conjugated rod-like structure, which increases its π−π interactions and crystallinity, thereby facilitating its packing.32 However, the photoisomerization-induced self-assembly of PPV-containing

INTRODUCTION Self-assembly of amphiphilic block copolymers (BCPs) in solution is a general method to prepare various nanostructures,1,2 ranging from simple spherical micelles3 to anisotropic cylinders, platelets,4−9 and more complex hierarchical structures.10,11 These nanostructures have been widely utilized as functional materials for drug delivery,12 catalytic reactors,13 and templating agents.14 One of the most important requirements for their practical application is precise control of their nanostructures to obtain uniform size, which remains highly challenging. Fortunately, the recent development of an elegant strategy called crystallization-driven self-assembly (CDSA),15 wherein BCPs having a crystallizable block undergo precise selfassembly to form uniform cores, has made this possible. Intriguingly, CDSA allows for not only seeded growth, in which epitaxial growth of BCPs from small seed micelles produces nanostructures with uniform sizes,16 but also the formation of block comicelles and supermicelles from two different BCPs, just as in living polymerization.17 However, only a few examples of living CDSA using conjugated BCPs, such as poly(3hexylthiophene),18 poly(3-decylselenophene),19 and pentamers of dihexyloxy-p-phenylenevinylene,20 have been reported, in spite of their high potential as functional nano-optoelectronic materials.21,22 On the other hand, in situ nanoparticlization of conjugated polymers (INCP) is an efficient method to prepare nanostructures composed of conjugated polymers.23,24 Taking © XXXX American Chemical Society

Received: February 16, 2018

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DOI: 10.1021/jacs.8b01954 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society BCPs has not been achieved, although analogous photoisomerizations have been used for the self-assembly of small molecules containing stilbene and azobenzene moieties.33−39 Herein, we report successful light-induced crystallization-driven self-assembly (LI-CDSA) involving the initial preparation of a soluble cis-rich PPV BCP synthesized by one-shot ROMP, followed by its facile photoisomerization to more crystalline trans-rich PPV. This triggered self-assembly into not only uniform-length nanofibers but also block and gradient comicelles by living seeded growth. Notably, LI-CDSA occurred rapidly, taking only a few minutes, and its living epitaxial growth was easily modulated by turning the light on and off.

Table 1. Synthesis of MeO-PPV Block Copolymers by OneShot ROMP

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RESULTS AND DISCUSSION 1. Synthesis and Characterization of BCPs. For successful LI-CDSA, the cis-rich PPV block synthesized by ROMP should have good solubility, but should become insoluble after light-induced isomerization to the transdominant configuration, thereby triggering assembly into the PPV crystalline core. In our previous report on INCP, nonsubstituted PPV BCPs were insoluble even with a high content of cis-olefin even before isomerization.26,27 Therefore, we introduced short dimethoxy substituents to PPV to provide the solubility for the cis form. The dimethoxy group was chosen because larger substituents would result in undesirably high solubility even after isomerization. 30 Therefore, polynorbornene-b-poly(p-phenylenevinylene-2,5-dimethoxy-pphenylenevinylene) (PNB-b-MeO-PPV, P1-b-P2) was synthesized by the ROMP of norbornene (NB) derivatives (1a and 1b) and dimethoxy substituted [2.2]paracyclophane-1,9-diene (MeO-PCPDE, 2) using a fast-initiating third-generation Grubbs catalyst in THF at rt for 5 min and then increased temperature to 40 °C (Table 1). Notably, simple one-shot feeding of both monomers produced P1-b-P2 directly, due to the significantly different reactivities of the two monomers (fast-reacting NB vs slow-reacting PCPDE).27,40 The conversion of each monomer (monomer-to-initiator ratio of 50:20:1 ([1a]:[2]:[cat])) during the one-shot ROMP was monitored by 1H NMR at 0 °C to ensure reliable detection of the highly reactive 1a. The ROMP of 2 proceeded only after the full consumption of 1a, thereby producing P1a50-b-P220 (equivalent to PPV40) with a BCP microstructure, presumably due to the significant retardation of the propagation of 2 by both steric hindrance and the coordination of the methoxy group to the propagating Ru carbene (Figures 1a and S1− S2).41,42 The size-exclusion chromatography (SEC) trace of P1a50-b-P220 in THF exhibited a signal from only free polymer chains without other signals in the high molecular weight region that could correspond to aggregates23 (Mn = 25.7 kDa, polydispersity (Đ) = 1.13, Table 1, entry 1 and Figure 1b), indicating that the resulting BCP was soluble in THF, as intended. Comparing this SEC trace with that of the P1a50 homopolymer (Mn = 17.6 kDa, Đ = 1.03, Table 1, entry 2), a clear peak shift was observed for the 1a block. Furthermore, the two similar signals in the trace of P1a50-b-P220 synthesized using ROMP via conventional sequential addition of two monomers (Mn = 24.1 kDa, Đ = 1.08, Table 1, entry 3) verified that the BCP was successfully prepared by one-shot ROMP (Figure 1b). Another copolymer, P1b50-b-P215, was prepared by one-shot ROMP using oxanorbornene (ONB) derivatives containing biocompatible polyethylene glycol (PEG, 1b)27,43 which showed good solubility in various solvents (Mn = 29.5

entry

NB

NB:2:cat.

conv. (%)a

yield (%)

Mn (kDa)b

Đb

1 2 3c 4 5

1a 1a 1a 1b 1a

50:20:1 50:0:1 50:20:1 50:15:1 20:15:1

>99 >99 >99 >99 >99

98 97 98 95 90

25.7 17.6 24.1 29.5 11.3

1.13 1.03 1.08 1.22 1.13

a

calculated by 1H NMR. bDetermined by THF SEC calibrated using polystyrene standards. cSynthesized by conventional sequential addition of two monomers.

Figure 1. (a) Conversion vs time plots for the one-shot ROMP of 1a and 2 at 0 °C with a feed ratio of [1a]:[2] = 50:20 at 0.1 M, based on the concentration of 2. (b) SEC traces of P1b50-b-P215 prepared by one-shot ROMP (entry 1), P1b50 (entry 2), and P1b50-b-P215 prepared by conventional sequential monomer addition (entry 3).

kDa, Đ = 1.22, Table 1, entry 4). This simple one-shot protocol to prepare BCPs containing easily functionalized PNB44,45 and fluorescent and semiconducting PPV22 will further broaden the scope and preparation of functional polymer materials. 2. LI-CDSA of P1b50-b-P215. To demonstrate the feasibility of LI-CDSA, P1b50-b-P215 in THF (a good solvent for both blocks) was analyzed using various spectroscopic methods before and after photoisomerization (Figure 2). First, UV/vis analysis indicated that the initial cis-rich P2 block successfully and rapidly isomerized to a trans-dominant configuration. A red shift in the UV/vis absorbance spectrum (from a λmax of 400 to 486 nm) appeared in only 5 min of 5 W LED exposure (Figure 2a), while it took about 1 h under fluorescent light (the light from ceiling lamp, Figure S3). After isomerization, the emission peak at 524 nm in the fluorescence spectrum was significantly reduced, and the emission peak at 554 nm was relatively stronger compared to the initial spectrum, implying some aggregation via self-assembly of the BCPs after isomerization (Figure 2b).46 The isomerization of the P215 block could be also visually identified by the change in the color of the B

DOI: 10.1021/jacs.8b01954 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Figure 2. Various analyses of P1b50-b-P215 at a concentration of 0.1 mg/mL in THF before and after photoisomerization. (a) UV/vis absorbance spectra before and after 5 W white LED light exposure for various durations. (b) Fluorescence emission spectra. Photographs of the solution before (left vial) and after (right vial) isomerization (c), and the same solutions under UV light (d). (e) 1H NMR spectra in d2-DCM. (f) FT-IR spectra, including a spectrum of the P1b50 homopolymer. (g) DLS profiles before and after irradiation with 5 W white LED light for up to 5 min. (h) AFM image after isomerization. Inset image shows height profiles along the white lines in the AFM image. (i) TEM images after isomerization. (j) Proposed mechanism for the formation of nanofibers through the LI-CDSA of P1n-b-P215.

change even for 3 days (Figure S5). Atomic force microscopy (AFM) images revealed that P1b50-b-P215 did indeed selfassemble into fiber-like nanostructures with micrometer lengths and a consistent height of 4.5 nm after isomerization (Figures 2h and S6). The same nanofibers were also observed in transmission electron microscopy (TEM) images. The nanostructures could be detected without the staining process, revealing that the second block of the P215 core had a width of 37.6 nm, due to the high electron density of the conjugated block (Figure 2i).23,24,27 After staining the first block shell using phosphotungstic acid,47 the overall width of the whole nanofiber increased to 48.5 nm (Figure S7). Cryo-TEM images revealed that the nanofibers had a ribbon shape, showing both a thinner height and thicker width that matches the results obtained by both AFM and TEM images respectively (Figure S8). To determine the crystallinity of the nanofibers, film state XRD spectra of the P1b50-b-P215 before and after isomerization were compared to those of P1b50 and isomerized P215 homopolymers (Figure S9). The absence of a signal originating from the interchain distance between PEG side chains of P1b50 (0.44 nm)48 in the P1b50-b-P215 spectrum indicated that the P1b50 shell formed an amorphous structure in the selfassembled structures. The signal at 0.73 nm observed in both the P215 homopolymer and P1b50-b-P215 was in agreement with the distance between methoxy groups on the same phenyl ring (Figure S9). After the formation of the nanofibers by isomerization, a new signal appeared at 0.46 nm, which was consistent with the reported PPV interchain distance in wormlike nanostructures,46,49 thereby providing further evidence of

solution, from yellow to orange and from green to yellow emission under a UV lamp (Figure 2c−d). Furthermore, the isomerization to trans-PPV was also confirmed by the 1H NMR and FT-IR spectra. The 1H NMR spectra of P215 (Supporting Information) showed that the polymer synthesized by ROMP contained 60% cis-configuration (3.43−3.50 ppm, corresponding to protons from methoxy groups on alternating-cis and allcis-P2) and 40% trans-configuration (3.85 ppm, corresponding to protons from methoxy groups on alternating-trans-P2). Then, after isomerization of P1b50-b-P215, the signal at 3.85 ppm disappeared and the signal at 4.00 ppm, corresponding to protons from methoxy groups on all-trans-P215, emerged30 (Figure 2e and S4). Additionally, the aromatic proton signals (6.5−7.7 ppm) were shifted significantly downfield. In addition, the integration of these signals decreased to 56% of the original area (calculated by setting the olefin signal from the P1b50 block as a standard, Figure S4) after isomerization, implying a certain degree of aggregation from P215. Lastly, the FT-IR spectra showed a decrease in the signal corresponding to the cis-olefins (850 cm−1) and an increase in the signal from the trans-olefins (965 cm−1), further confirming the isomerization (Figure 2f).32 Evidence of the light-induced self-assembly of P1b50-b-P215 via isomerization was provided by dynamic light scattering (DLS) and imaging analysis. Before the isomerization, the average hydrodynamic diameter (Dh) of the BCP in THF was 13 nm, which presumably reflected the size of single polymer chains. After irradiation with a white LED for 1 min, the Dh dramatically increased to over 450 nm (Figure 2g). Without light, isomerization did not occur so the DLS profile did not C

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solution of nonisomerized P1b50-b-P215 unimers was added to the seed micelle solution to give unimer-to-seed ratios of 2, 5, 10, 20, 35, and 50. After irradiating each solution with a white LED for 5 min, DLS analysis indicated a gradual increase in Dh with increasing unimer-to-seed ratio, from 92.5 to 285 nm (Figure S12). More precisely, TEM images revealed that the Ln of the nanofibers increased linearly from 125 nm to 227, 337, 631, 929, and 1210 nm with increasing unimer-to-seed ratio and, more surprisingly, that a narrow length distribution (Lw/Ln = 1.08−1.14) was successfully obtained in all cases (Figures 3 and S11−12). In order to investigate the influence of the isomerization temperature or rate on the controlled growth of the nanofibers, the same experiments were conducted at different temperatures (10 and 30 °C) using a fluorescent light and a 5 W white LED as the light sources. The seeded growth was found to be reproducible in these conditions (Figures S13−S21), indicating that the LI-CDSA of P1b50-b-P215 was not sensitive to the isomerization conditions. Therefore, we successfully controlled the seeded growth to obtain nanofibers with controllable lengths and a narrow length distribution by easy and rapid LI-CDSA of P1b50-b-P215 in just a few minutes at room temperature, in contrast to other CDSA processes that require a long aging time at a specific optimized temperature.16,20 4. On−Off Controlled Growth of Nanofibers. The living growth of the nanofibers by irradiation provided the possibility of controlling the growth by switching the light on and off, just as in light-induced living radical polymerization (LI-LRP).50,51 In other words, turning the light on and off could control the degree of isomerization or driving force for self-assembly, thereby controlling the activation and deactivation of the seeded growth (Figure 4a). The same procedures used for the seeded growth of P1b50-b-P215 with a unimer-to-seed ratio of 50 (seed: Ln = 78 nm, Lw = 94 nm, Lw/Ln = 1.21, σ/Ln = 0.46) was repeated, this time with multiple cycles of switching the light on and off. The 5 W white LED was used to irradiate the polymer solution for a short period of time (30, 30, 60, and 120 s for each cycle) and was then turned off to give dark conditions for 30 min (Figure 4b). The isomerization and seeded growth were monitored for each on-and-off cycle using UV/vis absorbance spectra and TEM images, respectively (Figures S22−24). Irradiation led to a gradual red shift in the spectrum and an increase in the Ln value of the nanofibers, but under dark conditions, no significant changes were observed in the absorbance spectrum and the Ln values of the micelles, indicating that CDSA and isomerization were suppressed in the dark. Repeating the on-and-off photoisomerization cycle resulted in a stepwise increase in Ln from 69 to 158, 299, 490, and 692 nm, with the length remaining constant in the dark (Figures 4b and S23−24). This result demonstrated that the seeded growth was driven only by photoisomerization, and that the living growth of the micelles could be deactivated or become dormant in the light-off state, with growth being reactivated in the light-on state (Figure 4a). This on−off control of the seeded growth could provide a new platform to prepare precise hierarchical nanostructures. 5. Preparation of ABA Triblock and Gradient Comicelles. It was also expected that living CDSA could be used to prepare block comicelles (BCMs), which are composed of two types of micelles having the same crystalline core but different outer shells (Figure 4c).17 Therefore, to prepare BCMs by LI-CDSA, another BCP, P1a20-b-P215, was first prepared using one-shot ROMP (Mn = 11.3 kDa, Đ = 1.13,

LI-CDSA via photoisomerization. Additionally, differential scanning calorimetry (DSC) measurement of P1b50-b-P215 nanofibers exhibited a transition temperature at around 65 °C, which was not observed from P1b50 or isomerized P215 homopolymers (Figure S10). This implied that the transition temperature was originated from interaction of the P215 block forming core of the micelle. This was further supported by monitoring changes in the sizes of the nanofibers in THF by DLS analysis which showed a decrease in Dh from 430 to 240 nm at 60 °C (Figure S10). Based on various analytical data, we propose a plausible mechanism for LI-CDSA (Figure 2j). The soluble P215 synthesized by ROMP has a high cis-olefin content, resulting in a coil−coil BCP conformation. After irradiation with light, cis-P215 readily isomerizes to the trans-configuration, which rigidifies its backbone, thereby decreasing its solubility and increasing its crystallinity. The transformation from a coil−coil to a coil−rod BCP induces rapid self-assembly, i.e.,the epitaxial growth of a nanofiber. The resulting nanofiber has a ribbon shape, having a core with a width of 37.6 nm which corresponds to twice the length of a fully extended trans-P215 unit (Figures 2i and S9). 3. Seeded Growth of Nanofibers. If epitaxial growth occurred during the LI-CDSA, we expected that it would be possible to control the length of the P1b50-b-P215 nanofibers by seeded growth through the addition of unimers of the single polymer chain, just as in living polymerization.16 The seed micelles were prepared (Figure 3a, number-average length, Ln = 60 nm, weight-average length, Lw = 78 nm, Lw/Ln = 1.3, σ/Ln = 0.55) by sonicating a THF solution of the original nanofibers (0.1 mg/mL) at 0 °C for 30 min. Then, a 0.1 mg/mL THF

Figure 3. Seeded growth of P1b50-b-P215 via LI-CDSA. TEM images of nanofibers prepared by applying LED irradiation for 5 min to mixed solutions of P1b50-b-P215 seed micelles and unimers in THF (0.1 mg/ mL) with various ratios. [unimer]/[seed] = (a) 0 (seed only), (b) 2, (c) 5, (d) 10, (e) 20, (f) 35, and (g) 50. (h) Plot of the Ln values vs the unimer-to-seed ratio. Error bars indicate standard deviation (σ), and the solid gray line and red line represent theoretical Ln values and linear plot of experimental Ln values, respectively. D

DOI: 10.1021/jacs.8b01954 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Figure 4. (a) Schematic illustration of nanofiber growth controlled by light on−off modulation via LI-CDSA. (b) Plot of Ln versus time for light on− off cycles (yellow regions: light on; gray regions: light off; light source: 5 W white LED). (c) Schematic illustration of the formation of ABA triblock comicelles by seeded growth via LI-CDSA. (d) DLS profiles of the seed micelles from P1a20-b-P215 and ABA triblock comicelles. TEM images of seed micelles from P1a20-b-P215 (e) and ABA triblock comicelles obtained by adding a THF solution of P1b50-b-P215 to the seed micelles followed by LED irradiation (f). The TEM image of ABA triblock comicelles was stained with phosphotungstic acid.

Figure 5. (a) Schematic illustration of the preparation of gradient comicelles by on−off controlled seeded growth of two different BCPs via LICDSA. AFM images of ABA block comicelles (b) and gradient comicelles (c). Top left: full scale image. Right: magnified image of an individual comicelle. Middle left: overlay of the height profiles along the white lines shown in the full-scale image. Bottom: 3D image.

nm, Lw = 255 nm, Lw/Ln = 1.17, σ/Ln = 0.42). To these seed micelles, a solution containing 4 equiv of P1b50-b-P215 unimers was added, and the mixture was irradiated for 5 min. The successful preparation of BCMs was confirmed by the increase in Dh from 140 to 300 nm in the DLS measurements (Figure 4d). Furthermore, TEM imaging also confirmed the elongation

Table 1, entry 5). Then, after LED irradiation followed by sonication to form the initial P1a20-b-P215 seed micelles (Figure S26, Ln = 69 nm, Lw = 77 nm, Lw/Ln = 1.13, σ/Ln = 0.36), seeded growth by LI-CDSA with a unimer-to-seed ratio of 2 produced the first short nanofibers, which were used as seed micelles for the BCMs (Figure 4d and e, Dh = 140 nm, Ln = 218 E

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Journal of the American Chemical Society of the nanofibers, from an Ln value of 218 to 802 nm (Figure S26, Lw = 841 nm, Lw/Ln = 1.05, σ/Ln = 0.2). Finally, selective staining of the PEG side chain of P1b50 using an aqueous solution of phosphotungstic acid showed a clear difference in the electron density, revealing darker fibers at the two ends than in the unstained central block of the comicelles (Figure 4f). Additionally, the measured Ln value of the central block of the comicelles was almost identical to that of the initial seed micelles (Figure S26, about 220 nm), confirming that the ABAtype triblock comicelles were successfully prepared by LICDSA of P1b50-b-P215 from P1a20-b-P215 seed micelles. Building on the of successful preparation of BCM by the unique on−off controlled living LI-CDSA process, we attempted to prepare a gradient comicelle52 with a composition gradient in the shell (Figure 5a). Analogous to the previous preparation of BCM, seeded growth of P1a20-b-P215 with a unimer-to-seed ratio of 10 was carried out with just 15 s of LED irradiation. The partial isomerization of P215 and the resulting formation of short nanofibers were confirmed by UV/vis analysis (as a red shift from 400 to 465 nm), DLS (Dh = 260 nm), and TEM imaging (Figure S28). To this mixture of nanofibers and partially isomerized unimers, the same amount of P1b50 -b-P2 15 unimers (total ratio of seed:P1a 20 -bP215:P1b50-b-P215 = 1:10:10) was added, followed by LED irradiation for 5 min. The full isomerization of the BCPs was confirmed by UV/vis analysis (as a red shift from 465 to 485 nm), and the production of elongated nanofibers with a Dh of 380 nm and an Ln of 1120 nm was observed (Figures S28−30, Lw = 451 nm, Lw/Ln = 1.16, σ/Ln = 0.40). Because of difficulty of distinguishing the shell gradient using TEM imaging even after selective staining of the PEG of P1b50 (Figure S29), AFM imaging was carried out instead. The height of the nanofibers composed of P1a20-b-P215 (∼6 nm) was found to be about 2 nm higher than those composed of P1b50-b-P215 (∼4.5 nm, Figure S6), thereby enabling the composition of the comicelles to be observed. The comicelles demonstrated a gradual height increase along the nanofibers, with the lowest height (∼4 nm) at both ends and the highest height (∼6 nm) at the center, while the height profile and 3D images of the ABA block comicelles showed a clear height difference between each block (Figure 5b−c). This result implied that the gradient comicelles had a gradually increasing P1a20-b-P215 content toward the center of nanofibers, due to faster photoisomerization and LICDSA of P1a20-b-P215 which was added earlier and partially isomerized already, than cis-rich P1b50-b-P215, which was added later. To confirm that the formation of gradient comicelles was not from the intrinsic isomerization rate or CDSA rate difference of the two BCPs, photoisomerization of the mixture having the same ratio (seed:P1a20-b-P215:P1b50-b-P215 = 1:10:10) was carried out, and as a result, random comicelles were generated with the height of 5 nm (Figure S31).

was possible, allowing the length of the nanofibers to be controlled linearly by varying the unimer-to-seed ratio; ratios from 2 to 50 produced lengths from 125 to 1210 nm with a narrow distribution. Notably, the seeded growth could be activated and deactivated by simply turning the light on and off, analogous to the LI-LRP process. Taking advantage of this living process, ABA triblock and gradient comicelles were successfully prepared by seeded growth using two different BCPs. This self-assembly process can be extended to a broader scope of copolymers, because the LI-CDSA completes in a few minutes using a 5 W white LED, and the BCPs are easily prepared by one-shot ROMP. In addition, since the PNB shell block can be easily modified, and the core contains fluorescent PPV, various functional polymeric materials with precise nanostructures can be designed for various applications.

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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.8b01954. Experimental methods, synthetic procedures, characterization, and supporting tables and figures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected]. ORCID

Jeewoo Lim: 0000-0002-7962-5163 Rudolf Zentel: 0000-0001-9206-6047 Tae-Lim Choi: 0000-0001-9521-6450 Present Address ∥

Department of Chemistry, Kyung Hee University, 26 Kyungheedae-ro, Dongdaemun-gu, Seoul, 02447, Korea. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This paper is dedicated to Professor Young Keun Chung at SNU on his retirement and his lifelong achievement in research and education at SNU. We are thankful for financial support from the Creative Research Initiative Grant and the Creative Material Discovery Program through NRF, Korea.

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REFERENCES

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CONCLUSION In summary, the formation of fluorescent nanofibers by ultrafast LI-CDSA was successfully achieved by LED irradiation of solutions of P1-b-P2, which was easily prepared by simple one-shot ROMP. Introducing a dimethoxy group on the PCPDE monomer not only improved the one-shot block copolymerization by slowing its propagation via coordination to the catalyst but also modulated the solubility of the resulting second MeO-PPV block, which was initially soluble but became insoluble after photoisomerization to the trans-configuration, thereby enabling rapid LI-CDSA. Furthermore, living LI-CDSA F

DOI: 10.1021/jacs.8b01954 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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DOI: 10.1021/jacs.8b01954 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX