Journal
CRYSTAL GROWTH & DESIGN
Volume 16, Issue 7, Pages 3647-3655Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.cgd.6b00104
Keywords
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Funding
- Austrian Reasearch Promotion Agency (FFG) [842496]
- Austrian Science Foundation (FWF) [25154, 25887]
- Austrian Science Fund (FWF) [P25154, P25887] Funding Source: Austrian Science Fund (FWF)
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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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