title: Halogen-Bond Effects on the Thermo- and Photochromic Behaviour of Anil-Based Molecular Co-crystals.
abstract: N-Salicilideneanilines are among the most studied thermo- and photochromic systems in the solid state. Although thermochromism is a general property of crystalline N-salicilideneanilines, photochromism is known in a limited number of cases. As a method for the construction of thermo- and photo-responsive molecular architectures, the co-crystallisation of 1,2,4,5-tetrafluoro-3,6-diiodobenzene (I2F4) with three selected imines of o-vanillin, named 1, 2 and 3, obtained through a condensation reaction with 3-aminopyridine, 4-bromoaniline and 4-iodoaniline, respectively, is reported herein. All crystals and co-crystals have been characterised by means of solid-state complementary techniques (X-ray diffraction, solid-state NMR spectroscopy, absorption and emission spectroscopy). The role of halogen bonding and crystal packing in the optical and chromic properties of all solid materials is discussed. All solids exhibit thermochromic behaviour, and three of them (2, 22⋅I2F4 and 32⋅I2F4) are also photochromic. Imine derivative 3 crystallises in two different polymorphic forms (3 A and 3 B) and a solvate (3Solv). The bromo and iodo derivatives, 2 and 3 B, are isomorphous and form isomorphous co-crystals with I2F4, but behave differently when exposed to UV light because only crystalline 2 is photochromic. Interestingly, the replacement of bromine with iodine seems to turn off the photochromism because crystalline 3 A and 3Solv, and even the 20.730.3solid solution, do not manifest photochromic behaviour.
authors.affiliation: Department of Chemistry, University of Namur (UNamur), Rue de Bruxelles 61, Namur, 5000, Belgium.
authors.forename: Andrea
authors.identifier: http://orcid.org/0000-0003-2865-1596
authors.initials: A
authors.lastname: Carletta
authors.affiliation: Università di Bologna, Dipartimento di Chimica G. Ciamician, Via Selmi 2, 40126, Bologna, Italy.
authors.forename: Floriana
authors.initials: F
authors.lastname: Spinelli
authors.affiliation: Università di Bologna, Dipartimento di Chimica G. Ciamician, Via Selmi 2, 40126, Bologna, Italy.
authors.forename: Simone
authors.initials: S
authors.lastname: d'Agostino
authors.affiliation: Istituto per la Sintesi Organica e la Fotoreattività (ISOF)-CNR, Via P. Gobetti 101, 40129, Bologna, Italy.
authors.forename: Barbara
authors.identifier: http://orcid.org/0000-0002-8207-1659
authors.initials: B
authors.lastname: Ventura
authors.affiliation: Dipartimento di Chimica and NIS Centre, Università di Torino, via Giuria 7, 10125, Turin, Italy.
authors.forename: Michele R
authors.initials: MR
authors.lastname: Chierotti
authors.affiliation: Dipartimento di Chimica and NIS Centre, Università di Torino, via Giuria 7, 10125, Turin, Italy.
authors.forename: Roberto
authors.initials: R
authors.lastname: Gobetto
authors.affiliation: Department of Chemistry, University of Namur (UNamur), Rue de Bruxelles 61, Namur, 5000, Belgium.
authors.forename: Johan
authors.initials: J
authors.lastname: Wouters
authors.affiliation: Università di Bologna, Dipartimento di Chimica G. Ciamician, Via Selmi 2, 40126, Bologna, Italy.
authors.forename: Fabrizia
authors.initials: F
authors.lastname: Grepioni
keywords: NMR spectroscopy; halogen bonds; photochromism; solid-state reactions; thermochromism