Halogen bonding stabilizes acis-azobenzene derivative in the solid state: a crystallographic study

Abstract

Crystals of trans- and cis-isomers of a fluorinated azobenzene derivative have been prepared and characterized by single-crystal X-ray diffraction. The presence of F atoms on the aromatic core of the azobenzene increases the lifetime of the metastable cis-isomer, allowing single crystals of the cis-azobenzene to be grown. Structural analysis on the cis-azobenzene, complemented with density functional theory calculations, highlights the active role of the halogen-bond contact (N...I synthon) in promoting the stabilization of the cis-isomer. The presence of a long aliphatic chain on the azobenzene unit induces a phase segregation that stabilizes the molecular arrangement for both the trans- and cis-isomers. Due to the rarity of cis-azobenzene crystal structures in the literature, our paper makes a step towards understanding the role of non-covalent interactions in driving the packing of metastable azobenzene isomers. This is expected to be important in the future rational design of solid-state, photoresponsive materials based on halogen bonding.

Halogen Bonding in N-Alkyl-3-halogenopyridinium Salts

“…To date, various Lewis bases (neutral molecules or charged species) have been employed as halogen bond acceptors in constructing halogenbonded supramolecular assemblies. These have most commonly been organic [13,14] and metal-organic [15] molecules containing electron-rich nitrogen [16][17][18][19][20][21][22][23][24][25][26] and oxygen atoms [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41], as well as inorganic anions such as halogenides [42][43][44][45][46][47][48][49][50][51][52][53][54]. The most commonly used halogen bond donors have traditionally been neutral organic molecules where a halogen atom is bonded to electron-withdrawing molecular residues.…”

Challenges for Incorporating Optical Switchability in Organic-Based Electronic Devices

“…The trans form can be converted into the metastable cis form (less stable by ∼50 kJ/mol compared to the trans form) under an illumination in the UV range, while the back conversion can be induced by an illumination in the visible or upon heating (Figure a) . The dynamics of the isomerization processes as well as the stability (and hence lifetime) of the metastable cis form , can be modulated by functionalizing the azobenzene core. These derivatives undergo severe structural changes upon isomerization, since the length of the molecule evolves from 9 Å down to 5.5 Å going from the trans to the cis form; a strong loss of planarity also takes place in the cis form in order to limit the steric hindrance between the two adjacent phenylene rings.…”

Fine-Tuning the Electronic Properties of Azo Chromophore-Incorporated Perylene Bisimide Dyads

“…54,59 It is known that based on the molecular structures, some Z-isomers can be stabilized under ambient conditions. 60,61 After 1 week at room temperature under dark conditions, the Z-isomer concentration of 3 was not changed significantly (i.e., 83.7−83.2%) and became 82.5% at the end of 1 month. The same is the case with compound 6.…”

“…Most importantly, the Z -isomer of 3 was stable over 1 month (Figure S49) at room temperature, contrary to the other azobenzene derivatives that are able to relax readily to the E -configuration. , It is known that based on the molecular structures, some Z -isomers can be stabilized under ambient conditions. , After 1 week at room temperature under dark conditions, the Z -isomer concentration of 3 was not changed significantly (i.e., 83.7–83.2%) and became 82.5% at the end of 1 month. The same is the case with compound 6 .…”