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 C 2 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 |g lum| = 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 (T 5d = 382 °C, T g = 171 °C).

Two planar conjugated systems perpendicularly connected via a quaternary sp3 carbon center interact with each other through the spiro linkage. Such spiro-conjugated π-systems show useful optoelectronic properties. Herein, we present a unified modular synthetic strategy for a series of chiral spiro-conjugated carbocycles utilizing a C–H activation reaction as a key step. The chiral spiro-conjugated carbon/heteroatom-bridged p-phenylenevinylenes (spiro-X compounds) possessing C, O, N, S, and SO2 as bridging groups are thermally stable, and exhibit circular dichroism and circularly polarized luminescence (CPL). Spiro-SO2 undergoes intramolecular charge transfer. It exhibits CPL with a large |glum| of 2.1 × 10−3 and shows rarely reported solvatofluorochromism for CPL luminophores. The racemic spiro-O compound solidifies as a racemic solid solution (or pseudoracemate). It did not form X-ray-quality crystals. This therefore provided us with a unique opportunity to examine the potential of the microcrystal electron crystallography popularized recently. The diffraction data sets, merged over many micrometer-sized solid particles, afforded the 3D molecular structure and the crystal packing of the spiro compound.

We have studied electron acceptor and electron donor spiro-conjugated dimers based, respectively, on bis(benzoquinoidal tetracyano carbon-bridged para-phenylene-vinylene) and on bis(benzo-aromatic bis(aryl)amino carbon-bridged para-phenylene-vinylene)s. Of the latter, two molecules either with identical units of carbon-bridged para-phenylene-vinylene or featuring a carbon-bridged para-phenylene-vinylene and a fluorene unit are considered. The radical forms of their charged species have been studied to explore the existence of SOMO-HOMO energy level inversion. It turns out that the radical trianion of the quinoidal and the radical cation of asymmetric aromatic show non-Aufbau SOMO-HOMO energy property. Spiro-conjugation and accumulation of electron-electron repulsion in the trianion and para-phenylene-vinylene/fluorene segmentation or localization of the frontier orbitals in the cation both explain the “anomalous” electronic configurations. Quantum chemical calculations and UV-Vis-NIR absorption spectroscopy have been used.

A new type of molecule tetrakis(benzo[b]furyl)ethene (TBFE) undergoes two-wavelength ON–OFF–ON three-stage fluorescence switching triggered by aggregation.

Solid solution (mixed crystal) is a crystal containing a second constituent (with a variable ratio) which fits into and is distributed in the lattice of the host crystal (IUPAC). It is widely found among inorganic substances, but rarely among organic compounds because it generally forms an amorphous solid or, at best, tiny crystals with a low degree of crystalline order. We examined the potential of microcrystal electron crystallography for the study of a solid solution of a racemic mixture (pseudoracemate) of a new molecule that we synthesized recently. Diffraction data of rather poor quality were collected for numerous crystal-like particles, and optimized for various parameter including the R(F) value. The datasets collected from approximately 20 particles each containing R and S isomers in different ratios afforded the molecular structure as well as the crystal packing of the specimen of interest. In this lecture, we will also touch on our recent analysis of a supramolecular organic polymer that defied X-ray analysis.