AIEgen containing side-chain liquid crystalline polymers: Photoluminecence or photothermal, which dominate?

Why many luminescent liquid crystalline polymers (LLCPs) containing aggregation-induced emission luminogen (AIEgen) show weak emission is a question still to be answered. Herein, a series of LLCPs (α-Pns, n = 4, 8, and 12) with polynorbornene as main chain and two α-dicyanodistyrylbenzene (α-DCS) as side chain are successfully synthesized to solve this issue. Differential scanning calorimetry (DSC), polarized light microscopy (PLM), one-dimentional (1D), two-dimentional (2D) middle-angle and wide-angle X-ray scattering (MAXS and WAXS) results demonstrate that the polymers form smectic A (SmA) phase with the side chains interdigitated packed within the smectic layers. Meanwhile, the photophysical properties of α-Pns were investigated by ultraviolet–visible (UV–vis) absorption, steady state and time-resolved spectroscopy, and photothermal effect. Results show that the polymers are AIE active, but emit weak emission. The emission peak of α-Pns film red-shift from 473 to 531 nm, the quantum yield gradually increases from around 1.6% to 14.7%, and the photothermal conversion efficiency decreases from 39% to 19% with the alkyl tail length increased. The photothermal effect, but not photoluminescence, dominates the excited state relaxation.