Multifunctional Chiral π-Conjugated Polymer Microspheres: Production and Confinement of NLO signal, Detection of Circularly Polarized Light, and Display of Laser-Triggered NLO Emission Shifts

Abstract

Two chiral π-conjugated polymers with optical nonlinearity (NLO), abbreviated as R- and S-BP, are prepared by copolymerization of R- and S-6,6'-dibromo-2,2'-diethoxy-1,1'-binaphthyl with 1,4-diethynyl-2,5-bis(octyloxy)benzene by Sonogashira coupling. The R- and S-BP copolymers self-assemble in a tetrahydrofuran/H2O/ethanol mixture producing microspherical particles. Remarkably, the obtained polymer microspheres support excitation of whispering gallery mode resonances (500–720 nm) in one- and two-photon excited luminescence or two-photon luminescence (TPL) with a quality factor (Q) of up to ≈700. As a result of a high Q value, a single (R- or S-BP) microsphere displays a circular dichroism (CD) effect in the TPL signal. At high laser pump-power (≥10 mW), the TPL spectral features of microspheres change significantly, suggesting alterations of the polymer packing. Similar spectral variations are observed for microspheres heated thermally or with high power continuous-wave laser. Photoluminescence lifetime imaging microscopy down to the single microsphere level corroborate transformations of polymer packing from a J type to a mixture of J, H, and H-like (with disordered side chains) types. This work demonstrates the tremendous potential of tailor-made chiral polymer microspheres exhibiting multifunctional properties, such as i) light-confinement, ii) NLO-CD, and iii) laser-driven optical emission shifts, useful for many optoelectronic and photonic device applications.