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

Abstract

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...

Applications of Optical Microcavity Resonators in Analytical Chemistry

“…Various organic polymer materials have been used for optical microcavity fabrication (54, 55, 67, 74–85). The principal advantage of these materials is low-cost, simple manufacturing, and many of the resulting devices, such as the polymer microgoblets, maintain remarkable optical qualities with Q-factors as high as 10 6 (54).…”

“…Polymers doped with fluorescent dyes and quantum dots have also been used to coat the inner walls of optofluidic resonators (46, 47). Isolated conjugated polymers (ICPs) are a particularly interesting material for WGM sensing, as they possess the advantages of free-space coupling (discussed above), inherent fluorescence (i.e., no doping needed), and simple synthesis/fabrication methods (67). However, additional developments are needed to realize full functionality as optical resonator sensors.…”

Color-Tunable Resonant Photoluminescence and Cavity-Mediated Multistep Energy Transfer Cascade

“…μ-PL measurements of single microspheres were carried out using a μ-PL measurement system ( Figure S6). 17 An optical microscope was used with a long-distance 100× objective (NA = 0.8) to identify suitable particles and determine their diameters (d). For measurements, a WITec μ-PL system was used with a model Alpha 300S microscope combined with a Princeton Instruments model Action SP2300 monochromator (grating: 300 grooves mm −1 ) and an Andor iDus model DU-401A BR-DD-352 CCD camera cooled to −60°C.…”

Fabrication of anisotropic janus composite particles based on natural renewable urushiol

“…Polymer microspheres have shown important application and scientific research value in many fields, such as material science [1][2][3], biochemistry [4,5], biology medicine [6,7], physics, and so on. In many polymer microspheres, the Janus particles with heterogeneous surface chemistries attract the interests of researchers [8][9][10].…”