Surface-Induced Phase of Tyrian Purple (6,6 '-Dibromoindigo): Thin Film Formation and Stability

M Truger and OM Roscioni and C Rothel and D Kriegner and C Simbrunner and R Ahmed and ED Glowacki and J Simbrunner and I Salzmann and AM Coclite and AOF Jones and R Resel, CRYSTAL GROWTH & DESIGN, 16, 3647-3655 (2016).

DOI: 10.1021/acs.cgd.6b00104

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 degrees C, the well-known bulk structure forms, while at a substrate temperature of 50 degrees 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 comparison of both phases reveals that pi-pi 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 degrees 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.

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