title: Photoinduced reversible switching of porosity in molecular crystals based on star-shaped azobenzene tetramers.
abstract: The development of solid materials that can be reversibly interconverted by light between forms with different physico-chemical properties is of great interest for separation, catalysis, optoelectronics, holography, mechanical actuation and solar energy conversion. Here, we describe a series of shape-persistent azobenzene tetramers that form porous molecular crystals in their E-configuration, the porosity of which can be tuned by changing the peripheral substituents on the molecule. Efficient E→Z photoisomerization of the azobenzene units takes place in the solid state and converts the crystals into a non-porous amorphous melt phase. Crystallinity and porosity are restored upon Z→E isomerization promoted by visible light irradiation or heating. We demonstrate that the photoisomerization enables reversible on/off switching of optical properties such as birefringence as well as the capture of CO2 from the gas phase. The linear design, structural versatility and synthetic accessibility make this new family of materials potentially interesting for technological applications.
authors.affiliation: Dipartimento di Chimica 'G. Ciamician', Università di Bologna, via F. Selmi 2, 40126 Bologna, Italy.
authors.forename: Massimo
authors.initials: M
authors.lastname: Baroncini
authors.affiliation: Dipartimento di Chimica 'G. Ciamician', Università di Bologna, via F. Selmi 2, 40126 Bologna, Italy.
authors.forename: Simone
authors.initials: S
authors.lastname: d'Agostino
authors.affiliation: Dipartimento di Chimica 'G. Ciamician', Università di Bologna, via F. Selmi 2, 40126 Bologna, Italy.
authors.forename: Giacomo
authors.initials: G
authors.lastname: Bergamini
authors.affiliation: Dipartimento di Chimica 'G. Ciamician', Università di Bologna, via F. Selmi 2, 40126 Bologna, Italy.
authors.forename: Paola
authors.initials: P
authors.lastname: Ceroni
authors.affiliation: Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, via R. Cozzi 55, 20125 Milano, Italy.
authors.forename: Angiolina
authors.initials: A
authors.lastname: Comotti
authors.affiliation: Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, via R. Cozzi 55, 20125 Milano, Italy.
authors.forename: Piero
authors.initials: P
authors.lastname: Sozzani
authors.affiliation: Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, via R. Cozzi 55, 20125 Milano, Italy.
authors.forename: Irene
authors.initials: I
authors.lastname: Bassanetti
authors.affiliation: Dipartimento di Chimica 'G. Ciamician', Università di Bologna, via F. Selmi 2, 40126 Bologna, Italy.
authors.forename: Fabrizia
authors.initials: F
authors.lastname: Grepioni
authors.affiliation: Dipartimento di Chimica 'G. Ciamician', Università di Bologna, via F. Selmi 2, 40126 Bologna, Italy.
authors.forename: Taylor M
authors.initials: TM
authors.lastname: Hernandez
authors.affiliation: Dipartimento di Chimica 'G. Ciamician', Università di Bologna, via F. Selmi 2, 40126 Bologna, Italy.
authors.forename: Serena
authors.initials: S
authors.lastname: Silvi
authors.affiliation: 1] Dipartimento di Chimica 'G. Ciamician', Università di Bologna, via F. Selmi 2, 40126 Bologna, Italy [2] Istituto per la Sintesi Organica e la Fotoreattività, Consiglio Nazionale delle Ricerche, via Gobetti 101, 40129 Bologna, Italy.
authors.forename: Margherita
authors.initials: M
authors.lastname: Venturi
authors.affiliation: Dipartimento di Chimica 'G. Ciamician', Università di Bologna, via F. Selmi 2, 40126 Bologna, Italy.
authors.forename: Alberto
authors.initials: A
authors.lastname: Credi