Birefringent Stable Glass with Predominantly Isotropic Molecular Orientation

Birefringence in stable glasses produced by physical vapor deposition often implies molecular alignment similar to liquid crystals. As such, it remains unclear whether these glasses share the same energy landscape as liquid-quenched glasses that have been aged for millions of years. Here, we produce stable glasses of 9-(3,5-di(naphthalen-1-yl)phenyl)anthracene molecules that retain three-dimensional shapes and do not preferentially align in a specific direction. Using a combination of angle- and polarization-dependent photoluminescence and ellipsometry experiments, we show that these stable glasses possess a predominantly isotropic molecular orientation while being optically birefringent. The intrinsic birefringence strongly correlates with increased density, showing that molecular ordering is not required to produce stable glasses or optical birefringence, and provides important insights into the process of stable glass formation via surface-mediated equilibration. To our knowledge, such novel amorphous packing has never been reported in the past.

Figure 1

(a) Thickness vs temperature for $\alpha$,$\alpha \text{−}A$ deposited at $T_{\mathrm{dep}}=0.80$, $T_g=288\mathrm{K}$. Heating and cooling rates were $1\mathrm{K}/\min$. Dashed lines are linear fits to SG, liquid-quenched glass, and SCL regimes used to evaluate $T_g$ and relative density $\mathrm{\Delta }\rho$. (b) Density difference between the SGs and transformed liquid-quenched glasses, $\mathrm{\Delta }\rho$, as a function $T_{\mathrm{dep}}$. Filled symbols were reported in Ref. [35]. Half-filled symbols are obtained in this work. The dashed line is the extrapolated SCL line and the solid line is the guide to the eye. The inset is the molecular structure of $\alpha$,$\alpha \text{−}A$.

Figure 2

(a) $n_{xy}$ (green) and $n_z$ (orange) of the same film as shown in Fig. 1 as a function of temperature. (b) Calculated $n_{xy}$ (green) and $n_z$ (orange) of as-deposited (filled) and transformed (open) films as a function of $T_{\mathrm{dep}}$. Two individual depositions were carried out at each $T_{\mathrm{dep}}$. All indices were measured at $T=296\mathrm{K}$ and $\lambda =632.8\mathrm{nm}$. Solid lines are guides to the eye.

Figure 3

(a) Orientational nematic order parameter, $S_z$, vs $T_{\mathrm{dep}}/T_g$ as determined from simulated fits to angle- and polarization-dependent PL data of $\alpha$,$\alpha \text{−}A$ films. All samples indicate isotropic orientation of the anthracyl substituents. (b) Single crystal unit cell of $\alpha$,$\alpha \text{−}A$.

Figure 4

(a) Correlation of $\mathrm{\Delta }n$ to density change (black) and $n_z$ to density change (orange). Values of all 16 measured samples were reported. (b) Schematic molecular packing in the most ($T_{\mathrm{dep}}=0.73$ $T_g$) and least ($T_{\mathrm{dep}}=0.95$ $T_g$) anisotropic $\alpha$,$\alpha \text{−}A$ glasses with random orientation.