A 1,6-Diphenylpyrene-Based, Photoluminescent Cyclophane Showing a Nematic Liquid-Crystalline Phase at Room Temperature

Mechanochromic aromatic hydrocarbons that bear one simple substituent

Structurally simple aromatic hydrocarbons that possess only one isocyano group show luminescent mechanochromism. The structural isomers of these aromatic hydrocarbons exhibit blue- and red-shifted emission bands upon mechanical stress. Their low molecular weight enables their sublimation under mild conditions.

Pyrene-Fused Poly-Aromatic Regioisomers: Synthesis, Columnar Mesomorphism, and Optical Properties

π-Extended pyrene compounds possess remarkable luminescent and semiconducting properties and are being intensively investigated as electroluminescent materials for potential uses in organic light-emitting diodes, transistors, and solar cells. Here, the synthesis of two sets of pyrene-containing π-conjugated polyaromatic regioisomers, namely 2,3,10,11,14,15,20,21-octaalkyloxypentabenzo[a,c,m,o,rst]pentaphene (BBPn) and 2,3,6,7,13,14,17,18-octaalkyloxydibenzo[j,tuv]phenanthro [9,10-b]picene (DBPn), is reported. They were obtained using the Suzuki-Miyaura cross-coupling in tandem with Scholl oxidative cyclodehydrogenation reactions from the easily accessible precursors 1,8- and 1,6-dibromopyrene, respectively. Both sets of compounds, equipped with eight peripheral aliphatic chains, self-assemble into a single hexagonal columnar mesophase, with one short-chain BBPn homolog also exhibiting another columnar mesophase at a lower temperature, with a rectangular symmetry; BBPn isomers also possess wider mesophase ranges and higher mesophases' stability than their DBPn homologs. These polycyclic aromatic hydrocarbons all show a strong tendency of face-on orientation on the substrate and could be controlled to edge-on alignment through mechanical shearing of interest for their implementation in photoelectronic devices. In addition, both series BBPn and DBPn display green-yellow luminescence, with high fluorescence quantum yields, around 30%. In particular, BBPn exhibit a blue shift phenomenon in both absorption and emission with respect to their DBPn isomers. DFT results were in good agreement with the optical properties and with the stability ranges of the mesophases by confirming the higher divergence from the flatness of DBPn compared with BBPn. Based on these interesting properties, these isomers could be potentially applied not only in the field of fluorescent dyes but also in the field of organic photoelectric semiconductor materials as electron transport materials.

Fluorescent cyclophanes and their applications

With the serendipitous discovery of crown ethers by Pedersen more than half a century ago and the subsequent introduction of host-guest chemistry and supramolecular chemistry by Cram and Lehn, respectively, followed by the design and synthesis of wholly synthetic cyclophanes-in particular, fluorescent cyclophanes, having rich structural characteristics and functions-have been the focus of considerable research activity during the past few decades. Cyclophanes with remarkable emissive properties have been investigated continuously over the years and employed in numerous applications across the field of science and technology. In this Review, we feature the recent developments in the chemistry of fluorescent cyclophanes, along with their design and synthesis. Their host-guest chemistry and applications related to their structure and properties are highlighted.

Light-Emitting Chiral Nematic Dimers with Anomalous Odd-Even Effect

In this report, based on the results derived from the extensive study into the thermal and photophysical properties, an anomalous mesomorphic behavior of photoluminescent, chiral nematic (N*) liquid crystalline dimers, belonging to two different series has been revealed. They comprise cholesterol and fluorescent three-ring Schiff base or salicylaldimine core interlinked via an ω-oxyalkanoyloxy spacer of varying length and parity. The effect of molecular structure on the liquid crystal (LC) behavior and photophysical properties of both the series has been probed by varying the length of the terminal n-alkoxy tails for a fixed (odd or even) parity of the spacer. The detailed investigations using complementary techniques not only evidenced the existence of the N* phase in all the dimers synthesized but also the occurrence of an intriguing odd-even effect; blue phases (BPs) exist in all the dimers comprising even-membered spacer, which surprisingly remains totally absent in their odd-membered counterparts. While the results reported hitherto are exactly opposite to the aforesaid findings, this atypical behavior has been interpreted in terms of the over-all shape of the dimers rendered by the orientation of terminal tails. Photophysical studies carried out clearly revealed the intrinsic light emitting feature of the dimers not only in their dilute solutions but also in their three condensed states viz., solid, N* phase, and isotropic liquid state; the emission intensities of the N* phase varies with the change in temperature, as expected. CD spectra of the N* phase recorded as a function of temperature show bisignate CD band characteristically, signifying large chiral correlations in the molecular self-assembly, while the origin of bands from positive to negative region suggests a right-handed twist of the N* helix.