Axially Chiral Spiro-Conjugated Carbon-Bridged p-Phenylenevinylene Congeners: Synthetic Design and Materials Properties

Abstract

Spiro-conjugated systems are attracting considerable interest for their chiroptical properties and because of their compact structure the small reorganization energy upon electronic excitation or ionization. We report here a modular and convergent synthesis of axially chiral spiro-conjugated carbon-bridged p-phenylenevinylenes (spiro-CPVs) in a racemic and optically active form where two carbon-bridged p-phenylenevinylene molecules are connected by a spiro carbon atom. Our synthetic design focuses on the C2 symmetry of the spiro-CPV molecules, relying on coupling of two 3-lithio-2-arylindene molecules on a carbon monooxide molecule that serves as the spiro carbon center in the target molecule. We prepared derivatives including those possessing phenol groups that facilitate optical resolution and also serve as a platform for the synthesis of a variety of optically active derivatives, which exhibit circularly polarized photoluminescence with high fluorescence quantum yields, large dissymmetry factors, and high photostability. For example, a bis(phenylethynyl) derivative exhibited a fluorescence quantum yield of 0.99 and a dissymmetry factor in luminescence of |glum| = 2.7 × 10-4, values highest among and comparable to those of reported CPL compounds, respectively. A tetrakis-diarylamine derivative shows hole mobility (μh = 3.84 × 10-5 cm2 V-1 s-1; space charge-limited current measurement of a spin-coated film) comparable to that of a popularly used hole-transporting material, spiro-OMeTAD (μh = 2.6 × 10-5 cm2 V-1 s-1), as well as high thermal and phase stability (T5d = 382 °C, Tg = 171 °C).