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.