ortho-Fluorination of azophenols increases the mesophase stability of photoresponsive hydrogen-bonded liquid crystals

Abstract

We present a photoresponsive supramolecular liquid-crystalline (LC) system with enhanced stability of the LC phase due to ortho-fluorination of the bond-donating hydroxyazobenzene derivative, an important characteristic for their future use in photonics.

Fluorinated azobenzenes as supramolecular halogen-bonding building blocks

“…Octafluorinated A2 forms [2 + 2] boxes in the solid state and possibly in solution, whereas tetrafluoroiodoethynyl A3 is as donor strong enough to reliably permit the characterization of the boxes formed in solution and in the solid state [35]. Electrostatic interaction plays a dominant role in halogen bonding [1,12–13]. Therefore, we calculated the molecular electrostatic potentials of the halogen bond donors A1 – 3 to visualize their capabilities to form halogen bonded architectures (Fig.…”

“…By introduction of fluorine atoms ortho to the azo bond, the two n→π* of E - and Z -state become sufficiently separated to address them individually using visible light sources. Along with averting UV light for the photochemical reaction, high PSS ratios can be observed, which is very desirable for application in supramolecular systems [12–13 35]. Tetra- and octafluorinated A1 and A2 show clear spectral separation of the n→π* bands, whereas the extended π-system of iodoethynyl A3 lead to a bathochromic shift of the π→π* band by 24 nm, now partially overlapping with the, also broadened, n→π* band of Z - A3 .…”

The crystal structure of 1,8-bis(pyridin-4-ylethynyl)anthracene-1,2,4,5-tetrafluoro-3,6-diiodobenzene (2/1), C62H32F4I2N4

“…The halogen bond is an attractive non-covalent interaction between a polarized halogen atom (donor) and a Lewis base (acceptor) [3]. Halogen bonding was successfully used to construct three-dimensional supramolecular networks [4,5] and halogen-bonded liquid crystals [6] that show enhanced stability and enable control over mesophase formation by varying the degree of fluorination accompanied by a change of the halogen bond strength. Clarke et al [7] describe the cocrystallisation of the halogen bond donor 1,4diiodotetrafluorobenzene (tfib) and the halogen bond acceptor 4,4′-bipyridyl (bipy) on a graphite surface.…”

Recent synthetic advances in pyridine-based thermotropic mesogens

“…These polymers are attractive to adsorb and sense or separate negatively charged molecules and anions. Saccone et al 57 prepared een supramolecular complexes using stilbazoles and azophenols as proton acceptors and proton donors, respectively, and studied the effects of ortho-uorination of the azophenol on the mesogenic behavior. By diazotization and azo-coupling reactions, they synthesized di-uorinated (128a-b) azophenols.…”