Tilt orientationally disordered hexagonal columnar phase of phthalocyanine discotic liquid crystals

The structures of the discotic liquid crystalline (LC) phase of metal-free octa-substituted phthalocyanine (Pc) derivatives were investigated using molecular dynamics (MD) simulations. Special attention was paid to the LC phase structure of the non-peripheral octa-hexyl substituted Pc-derivatives that were recently found to show very high carrier mobilities for the discotic LCs. We obtained spontaneous transition to the columnar hexagonal $\left(\mathrm{Col}{}_h\right)$ LC phase in a melting simulation from the crystal structure obtained using an x-ray diffraction study. In this simulated $\mathrm{Col}{}_h$ structure, the Pc-core normal vectors were tilted $47{}^{\circ }$ from the column axis in parallel within each column, but the tilting directions are disordered between columns. We also found that the inter-core distance was not as large as previously suggested (0.4–0.5 nm) but similar to the common value (0.36 nm). This may resolve the contradiction between the high carrier mobility of the non-peripheral substituted Pcs, because larger inter-core separations degrade the mobilities.

Figure 1

Peripheral $\left(R={\mathrm{C}}_n{\mathrm{H}}_{2n+1},R^{\prime }=\mathrm{H}\right)$ and non-peripheral $\left(R=\mathrm{H},R^{\prime }={\mathrm{C}}_n{\mathrm{H}}_{2n+1}\right)$ octa-alkyl substituted phthalocyanine.

Figure 2

Time variations of the cell parameters in the melting simulation.

Figure 3

Snapshot after a 30 ns MD run at LC temperature (438 K). Upper: view along the crystal $a$ axis. Lower: view perpendicular to the crystal $a$ axis.

Figure 4

Comparison of the single column structure along the crystal $a$ axis (dashed line). Left: initial crystalline state. Right: final state in Fig. 3.

Figure 5

Averaged 2-D structure function with the incident wave vector parallel to the column-axis.

Figure 6

The Pc-core normal vector configurations. Upper: for the initial crystalline structure. Lower: for the snapshot in Fig. 3.

Figure 7

Intracolumnar (solid line) and intercolumnar (dashed line) orientational correlation functions of the Pc-core normal vectors.

Figure 8

Averaged orientational distribution function of the Pc-core normal directions.

Figure 9

Time variations of the Pc-core normal tilts from the column axis. Black and blue lines: the $\mathrm{H}{}_2\mathrm{Pc}(\mathrm{C}{}_6$$\mathrm{H}{}_{13})$${}_8^{np}\mathrm{s}$ystem from the crystal and the non-tilted initial structure, respectively (note the convergence of them). Green line: the $\mathrm{H}{}_2\mathrm{Pc}(\mathrm{C}{}_6$$\mathrm{H}{}_{13})$${}_8^p\mathrm{s}$ystem. Red line: the $\mathrm{H}{}_2\mathrm{Pc}(\mathrm{COOC}{}_5$$\mathrm{H}{}_{11})$${}_8^p\mathrm{s}$ystem.

Figure 10

Averaged 1D structure functions. Black and blue lines (with symbol “$+$”): the $\mathrm{H}{}_2\mathrm{Pc}(\mathrm{C}{}_6$$\mathrm{H}{}_{13})$${}_8^{np}\mathrm{s}$ystem from the crystal and the non-tilted initial structure, respectively (note the agreement between them). Green line (with symbol “$*$”): the $\mathrm{H}{}_2\mathrm{Pc}(\mathrm{C}{}_6$$\mathrm{H}{}_{13})$${}_8^p\mathrm{s}$ystem. Red line (with symbol “$\times$”): the $\mathrm{H}{}_2\mathrm{Pc}(\mathrm{COOC}{}_5$$\mathrm{H}{}_{11})$${}_8^p\mathrm{s}$ystem.

Figure 11

Averaged radial distribution functions. Blue lines (with symbol “$+$”): atom-atom (solid line) and molecular-molecular (dashed line) RDF for the $\mathrm{H}{}_2\mathrm{Pc}(\mathrm{C}{}_6$$\mathrm{H}{}_{13})$${}_8^{np}\mathrm{s}$ystem, respectively. Red lines (with symbol “$\times$”): atom-atom (solid line) and molecular-molecular (dashed line) RDF for the $\mathrm{H}{}_2\mathrm{Pc}(\mathrm{COOC}{}_5$$\mathrm{H}{}_{11})$${}_8^p\mathrm{s}$ystem, respectively.