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.
Strong coupling plays a significant role in influencing chemical reactions and tuning material properties by modifying the energy landscapes of the systems. Here we study the effect of vibrational strong coupling (VSC) on supramolecular organization. For this purpose, a rigid-rod conjugated polymer known to form gels was strongly coupled together with its solvent in a microfluidic IR Fabry-Perot cavity. Absorption and fluorescence studies indicate a large modification of the self-assembly under such cooperative VSC. Electron microscopy confirms that in this case, the supramolecular morphology is totally different from that observed in the absence of strong coupling. In addition, the self-assembly kinetics are altered and depend on the solvent vibration under VSC. The results are compared to kinetic isotope effects on the self-assembly to help clarify the role of different parameters under strong coupling. These findings indicate that VSC is a valuable new tool for controlling supramolecular assemblies with broad implications for the molecular and material sciences.
Low‐threshold organic microlasers are demanded for advanced optical applications such as nano/micrometer scale memory, sensing, and communication tools, and further valuable for future electrically driven laser applications. In this paper, it is demonstrated that various highly fluorescent conjugated polymers self‐assemble to form single‐component microspheres that exhibit, upon femtosecond pumping to a single microsphere, whispering gallery mode (WGM) lasing with blue, green, and red emission colors. In particular, the microsphere consisting of polyfluorene shows the lowest threshold fluence as low as 1.5 µJ cm−2 and high photostability against successive pumping of >105 pulse. The threshold fluence is further reduced by one fourth (0.37 µJ cm−2) by mounting the microspheres on an Ag‐coated substrate, where a mirror effect of the Ag layer enhances efficiency of the photoluminescence confinement with a minor effect of plasmonic near‐field. Considering the intrinsic charge injection and transport properties, π‐conjugated polymer microsphere resonators will be possible materials for electrically pumped WGM luminescence.
Highly luminescent π-conjugated polymeric microspheres were fabricated through self-assembly of energy-donating and energy-accepting polymers and their blends. To avoid macroscopic phase separation, the nucleation time and growth rate of each polymer in the solution were properly adjusted. Photoluminescence (PL) studies showed that efficient donor-to-acceptor energy transfer takes place inside the microspheres, revealing that two polymers are well-blended in the microspheres. Focused laser irradiation of a single microsphere excites whispering gallery modes (WGMs), where PL generated inside the sphere is confined and resonates. The wavelengths of the PL lines are finely tuned by changing the blending ratio, accompanying the systematic yellow-to-red color change. Furthermore, when several microspheres are coupled linearly, the confined PL propagates the microspheres through the contact point, and a cascade-like process converts the PL color while maintaining the WGM characteristics. The self-assembly strategy for the formation of polymeric nano- to microstructures with highly miscible polymer blends will be advantageous for optoelectronic and photonic device applications.
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...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.