# Dual Thermal-/Electrical-Responsive Luminescent 'Smart' Window
## 3 Results  
The coumarin derivative (Dye 1) and perylene derivative (Dye 2) were individually dissolved at 1 wt% in 8 CB. Three distinct liquid crystal phases are identified (SmA 30-35 \({}^{\circ}\)C N 40-45 \({}^{\circ}\)C I: the DSC may be found in Figure S1), similar to those found in the literature. [25]. In the planar cells, the fluorescent dyes can absorb a significant fraction of the incoming light as they are aligned with absorption axis perpendicular to the incoming light. The application of a voltage reorients the liquid crystals, which in turn reorient the dyes to a homeotropic state, to a position with the absorption axis parallel to the incoming light. Due to the dichroism of the dyes, light absorption decreases in the homeotropic orientation, resulting in a more transparent state. Increasing the temperature decreases the LC order, as seen in Table 1; the order parameters (\(S\)) of the dyes were calculated according to [26]  
\[S=\frac{A_{par}-A_{per}}{A_{par}+2A_{per}} \tag{1}\]  
where \(A_{par}\) and \(A_{per}\) are the peak absorbance when the film is exposed to light polarized parallel and perpendicular to the LC alignment direction, respectively. As the dyes become more disordered, absorption increases, as shown in Figure 2. The order parameters (see Table 1) clearly show that, as the temperature increases, the alignment of the dyes decreases as the system becomes more isotropic, finally resulting in an order parameter of 0 when the LC is fully isotropic. In Figure 3, transmission at the peak absorption wavelengths of the dyes at an increased temperature and voltage for planar cells are depicted: Figure 3a shows Dye 1, and Figure 3b Dye 2. Initially, in the smectic A phase at 24 \({}^{\circ}\)C, a higher voltage (123 Vpp (peak-to-peak voltage)) is required to overcome the LC alignment and switch the dye to a homeotropic orientation, resulting in an increased light transmission. On the other hand, increasing the temperature to the nematic state increases the mobility and disorder in the system, resulting in a lower transmission and lower switching voltage to the homeotropic state (82 Vpp at 26 \({}^{\circ}\)C and even 41 Vpp at 30 \({}^{\circ}\)C). Increasing the temperature further to the isotropic state results in a minimum transmission but prevents electrical switching, as the LC phase is randomized and loses its switching properties [27]. Emission spectra from \(5\times 5\) cm\({}^{2}\) cells from the edge parallel to the LC alignment direction were measured under a solar simulator by an integrating sphere as a function of temperature and applied voltage: the results are depicted in Figure 3c,d. It can be seen that the emissions for Dye 1 (Figure 3c) are higher compared to Dye 2 (Figure 3d). As the temperature increases, the emission parallel to the absorption increases as the transmission decreases. However, increasing the temperature too much (\(>\)35 \({}^{\circ}\)C) reduces edge emission as dye orientation becomes less favorable. Appling an electric potential across the cell reorients the LC/dye, at a cost of reduced emission as a result of less light absorption (even though the emission direction of the dye is more favorable). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Temperature \({}^{\circ}\)C** & **24** & **27** & **30** & **33** & **37** & **41** \\ \hline
**S, Dye 1** & 0.25 & 0.24 & 0.27 & 0.24 & 0.11 & 0.00 \\ \hline
**S, Dye 2** & 0.27 & 0.24 & 0.20 & 0.12 & 0.00 & 0.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Order parameter of 1 wt% Dye 1 and Dye 2 in 8 CB in planar cells at different temperatures. Figure 3: Transmission at the peak absorption of the dye at increased temperature and voltage for planar cells containing (**a**) Dye 1; (**b**) Dye 2. Summation of emission spectra (from the edge with dyes aligned parallel to the LC alignment) between 400–800 nm at increased temperature and voltage for planar \(5\times 5\) cm\({}^{2}\) containing (**c**) Dye 1; (**d**) Dye 2.