Randomization and Constraint of Molecular Alignment and Orientation: Temperature-Dependent Anisotropy and Phase Transition in Vapor-Deposited Thin Films of an Organic Cross-Shaped Molecule

We observe potential randomization and constraint of molecular alignment and orientation in an organic semiconductor molecule with increasing temperature up to the phase-transition temperature. Variable-angle spectroscopic ellipsometry and second-harmonic generation are used to study the changes in the molecular alignment in vapor-deposited organic thin films as samples are heated and cooled in a cycle from room temperature to the phase-transition temperature. The films consist of sterically bulky and cross-shaped molecules, 2-cyano-9,10-di(2-naphthyl)anthracene, and the anisotropy of its two moieties is probed. Anisotropic molecular alignment with respect to the surface normal in as-deposited amorphous films changes with the film thickness, which increases slightly with increasing substrate temperature. Moreover, the axis near the long axis of the anthracene moiety changes significantly with respect to the surface normal from the magic angle to isotropic alignment, showing monotonically decreasing anisotropy. Interestingly, the anisotropy of the axis near the long axis of the anthracene moiety disappears before the phase-transition temperature. In contrast, the axis near the short axis of the anthracene moiety exhibits a notable characteristic change in the temperature-dependent alignment during the heating process; although the anisotropy initially decreases, it significantly increases as the temperature approaches the phase transition. At a certain temperature during heating, the film thickness shows a discontinuous jump, similar to a first phase transition, while the anisotropic molecular alignment completely disappears. During the cooling process after the phase transition, however, the properties of the films are irreversibly changed, and anisotropic molecular alignment is no longer observed; thus, the samples become completely isotropic.