Multistimuli-ResponsiveEmissive Liquid Crystals withHigh-Contrast Mechanochromic and Solvatochromic Emission Based onCyclometalated Platinum(II) Complexes with Fluoroalkylated Ligands

Abstract Luminescent liquid crystals based on cyclometalated Pt complexes have attracted considerable interest owing to their excellent emission properties that can be tuned in a targeted manner. Herein, we propose a robust approach for a simple design of multistimuli-responsive liquid-crystalline Pt complexes. Seven square-planar cyclometalated Pt-ppy (5a-d) and Pt-dfppy complexes (6c,d and 7c,d), where ppy = 2-phenylpyridine and dfppy = 2,4-difluorophenylpyridine, with mesogenic bidentate benzoylthiourea auxiliary ligands containing either alkyl (BTUCn, n = 6, 8, 12, 3a-d) or perfluoroalkyl chains (BTUF8, 4c,d) were synthesized and characterized. Pt complexes with alkyl chains lack mesogenic properties. Liquid crystalline properties with smectic phases are achieved only when one alkyl chain of the BTU ligands is replaced with a perfluorooctyl chain, as revealed by polarized optical microscopy (POM) observations and powder X-ray diffraction (XRD). A glassy state is exhibited by all Pt-BTUF8 complexes (6c,d and 7c,d) at temperatures lower than 45 °C. These Pt complexes are able not only to exhibit emission in solid, solution, mesophase, and glassy states but also to exhibit attractive reversible mechanochromism or solvatochromism upon exposure to different solvents (CH2Cl2, acetone, or EtOH). The chromic shift response of 6c and 6d is about 120 nm, which is higher than that found for other Pt metallomesogens. Density functional theory (DFT) and time-dependent DFT investigations (TDDFT) have been employed to rationalize the photophysical properties of Pt complexes.