title: Surface-Induced Phase of Tyrian Purple (6,6'-Dibromoindigo): Thin Film Formation and Stability.
abstract: The appearance of surface-induced phases of molecular crystals is a frequently observed phenomenon in organic electronics. However, despite their fundamental importance, the origin of such phases is not yet fully resolved. The organic molecule 6,6'-dibromoindigo (Tyrian purple) forms two polymorphs within thin films. At growth temperatures of 150 °C, the well-known bulk structure forms, while at a substrate temperature of 50 °C, a surface-induced phase is observed instead. In the present work, the crystal structure of the surface-induced polymorph is solved by a combined experimental and theoretical approach using grazing incidence X-ray diffraction and molecular dynamics simulations. A comparison of both phases reveals that π-π stacking and hydrogen bonds are common motifs for the intermolecular packing. In-situ temperature studies reveal a phase transition from the surface-induced phase to the bulk phase at a temperature of 210 °C; the irreversibility of the transition indicates that the surface-induced phase is metastable. The crystallization behavior is investigated ex-situ starting from the sub-monolayer regime up to a nominal thickness of 9 nm using two different silicon oxide surfaces; island formation is observed together with a slight variation of the crystal structure. This work shows that surface-induced phases not only appear for compounds with weak, isotropic van der Waals bonds, but also for molecules exhibiting strong and highly directional hydrogen bonds.
authors.affiliation: Institute of Solid State Physics, NAWI Graz, Graz University of Technology , Petersgasse 16, 8010 Graz, Austria.
authors.forename: Magdalena
authors.initials: M
authors.lastname: Truger
authors.affiliation: School of Chemistry, University of Southampton, Southampton SO17 1BJ, United Kingdom; Dipartimento di Chimica Industriale "Toso Montanari", Università di Bologna, viale Risorgimento 4, 40136 Bologna, Italy.
authors.forename: Otello M
authors.initials: OM
authors.lastname: Roscioni
authors.affiliation: Institute of Solid State Physics, NAWI Graz, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria; Institute of Pharmaceutical Sciences, Department of Pharmaceutical Technology, University of Graz, Universitätsplatz 1, 8010 Graz, Austria.
authors.forename: Christian
authors.initials: C
authors.lastname: Röthel
authors.affiliation: Department of Condensed Matter Physics, Charles University Prague , Ke Karlovu 5, Prague 12116 2, Czech Republic.
authors.forename: Dominik
authors.initials: D
authors.lastname: Kriegner
authors.affiliation: Institute of Solid State Physics, University of Bremen, Otto-Hahn-Allee 1, 28359 Bremen, Germany; Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz, Altenbergerstraße 69, 4040 Linz, Austria.
authors.forename: Clemens
authors.initials: C
authors.lastname: Simbrunner
authors.affiliation: Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz, Altenbergerstraße 69, 4040 Linz, Austria; National Center for Physics, Quaid-e-Azam University Campus, Islamabad, Pakistan.
authors.forename: Rizwan
authors.initials: R
authors.lastname: Ahmed
authors.affiliation: Physical Chemistry, Johannes Kepler University Linz , Altenbergerstraße 69, 4040 Linz, Austria.
authors.forename: Eric D
authors.initials: ED
authors.lastname: Głowacki
authors.affiliation: Institute of Solid State Physics, NAWI Graz, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria; Division of Neuroradiology, Medical University of Graz, Auenbruggerplatz 9, 8036 Graz, Austria.
authors.forename: Josef
authors.initials: J
authors.lastname: Simbrunner
authors.affiliation: Department of Physics, Humboldt Universität zu Berlin , Brook-Taylor Straße 6, 12489 Berlin, Germany.
authors.forename: Ingo
authors.initials: I
authors.lastname: Salzmann
authors.affiliation: Institute of Solid State Physics, NAWI Graz, Graz University of Technology , Petersgasse 16, 8010 Graz, Austria.
authors.forename: Anna Maria
authors.initials: AM
authors.lastname: Coclite
authors.affiliation: Institute of Solid State Physics, NAWI Graz, Graz University of Technology , Petersgasse 16, 8010 Graz, Austria.
authors.forename: Andrew O F
authors.initials: AO
authors.lastname: Jones
authors.affiliation: Institute of Solid State Physics, NAWI Graz, Graz University of Technology , Petersgasse 16, 8010 Graz, Austria.
authors.forename: Roland
authors.initials: R
authors.lastname: Resel