Spherical Assemblies from π-Conjugated Alternating Copolymers: Toward Optoelectronic Colloidal Crystals

Adachi, Taeko; Tong, Lewei; Kuwabara, Junpei; Kanbara, Takaki; Saeki, Akinori; Seki, Shu; Yamamoto, Yohei

doi:10.1021/ja3106626

Cited by 76 publications

(65 citation statements)

References 49 publications

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“…Figures 1e–1g show scanning electron microscopy (SEM) images of the self-assembled microspheres of polymers 1 – 4 , respectively. The sizes of the spheres were typically 1–10 μ m in diameter, which depended on the precipitation rate, e.g ., the solubility of the polymers, the initial concentration and the setting temperature26. Cross-section scanning transmission electron microscopy (XSTEM) shows that the spheres are homogeneously filled with the composed polymer (Fig.…”

Section: Resultsmentioning

confidence: 99%

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Self-assembled conjugated polymer spheres as fluorescent microresonators

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Braam

Kushida

et al. 2014

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Confinement of light inside an active medium cavity can amplify emission. Whispering gallery mode (WGM) is one of mechanisms that amplifies light effectively by confining it inside high-refractive-index microstructures, where light propagates along the circumference of a sphere via total internal reflection. Here we show that isolated single microspheres of 2–10 μm diameter, formed from self-assembly of π-conjugated alternating copolymers, display WGM photoemission induced by laser pumping. The wavelengths of the emission peaks depend sensitively on the sphere size, position of the excitation spot and refractive index of each polymer. The Q-factor increases with increasing sphere diameter and displays a linear correlation with the reciprocal radius, indicating that the small curvature increases the efficacy of the total internal reflection. WGM photoemission from π-conjugated polymer microspheres is unprecedented and may be of high technological impact since the microspheres fulfill the role of fluorophores, high-refractive-index media and resonators simultaneously, in addition to their simple fabrication process.

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Section: Resultsmentioning

confidence: 99%

“…In this context, we recently reported that several π -conjugated alternating copolymers such as compounds 1 – 3 (Figs. 1a–1c) self-assemble to form well-defined spheres quantitatively with diameters ( d ) of sub- to several-micrometers2627. These polymers consist of two different arylene groups in the repeating unit.…”

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Self-assembled conjugated polymer spheres as fluorescent microresonators

Tabata

Braam

Kushida

et al. 2014

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“…17,18 The slow diffusion of a polar nonsolvent vapor into a solution of a low-crystallinity polymer results in the formation of spheres with diameters in the submicrometer to several micrometers range. Interestingly, upon laser excitation of a single sphere, clear whispering gallery modes (WGM) were observed in photoemission.…”

Section: ■ Introductionmentioning

confidence: 99%

Whispering Gallery Resonance from Self-Assembled Microspheres of Highly Fluorescent Isolated Conjugated Polymers

et al. 2015

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Self-assembly of highly fluorescent isolated conjugated polymers (ICPs), comprising alternating phenylene moieties with an insulating cyclic side chain and different arylene moieties, was comprehensively studied. Two out of nine ICPs were identified to form well-defined spheres of 1−6 μm diameter. The degree of twisting of the main chains was found to be an important structural factor enabling formation of spheres, for which dihedral angles >50°between the neighboring arylene moieties were required. A single microsphere with high sphericity exhibited whispering gallery mode (WGM) photoemission upon excitation with a focused laser. In this emission, sharp and periodic emission lines were superimposed on a broad photoemission spectrum. The WGM spectral profiles were very sensitive to the integrity of the spherical geometries and surface smoothness, which depends on the self-assembling condition as well as the structure of the polymer backbone. Microspherical optical resonators consisting of such highly fluorescent conjugated polymers are novel. They also present advantages in that (i) there is no need for a light waveguide and fluorescent-dye doping, (ii) its high refractive index is beneficial for light confinement, and (iii) the fabrication process is simple, not requiring sophisticated, costly microfabrication technology. ■ INTRODUCTIONπ-Conjugated polymers possess charge transport, photoemission, and redox properties, which are useful for applications in electronic and optoelectronic devices such as light-emitting diodes, transistors, solar cells, and electrochromic devices. 1−6 Recently, spherical colloids formed from π-conjugated polymers have attracted attention for additional optical and biomedical applications involving fluorescence imaging, drug and gene delivery, and colloidal photonic crystals. 7−9 However, in general, π-conjugated polymers are hard to form into well-defined spheres because of their rigid and planar backbones. Thus, there have been few examples of π-conjugated polymer spheres reported so far. 8,10−16 Moreover, most of these were prepared using miniemulsion polymerization and dispersion polymerization methods.In this context, we have recently reported several π-conjugated alternating copolymers that form well-defined microspheres quantitatively via self-assembly in a thermodynamic solution process. 17,18 The slow diffusion of a polar nonsolvent vapor into a solution of a low-crystallinity polymer results in the formation of spheres with diameters in the submicrometer to several micrometers range. Interestingly, upon laser excitation of a single sphere, clear whispering gallery modes (WGM) were observed in photoemission. 19 Because the refractive indices (η) of these polymers are in the range of 1.6− 1.8, which is high enough in comparison with the refractive index of air (η air ∼ 1.0), fluorescence generated inside the microspheres is efficiently confined via total internal reflection at the polymer/air interface. As a result, sharp and periodic emission lines appear in their photolumine...

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“…Polyfluorene (PF) derivatives have attracted much attention as promising materials in organic electronic devices such as light-emitting diodes (LEDs), [1][2][3][4][5] photovoltaic cells, [6][7][8][9] and field-effect transistors (FETs) [10][11][12][13] for their high quantum yields, excellent solubility, and film-formation ability. However, it is difficult for PF to obtain pure and stable blue light emission due to the presence of undesirable green emission upon exposure to heat or during device operation.…”

Section: Introductionmentioning

confidence: 99%