**3. Results and Discussion**

Compounds **1**–**4** were crystalline solids at room temperature. The powder X-ray diffractograms were measured on a Malvern PANalytical Empyrean system, with a Cu K-α source with a wavelength of 1.5406 Å, measuring with a PIXcel3D detector (Malvern analytical, Eindhoven, Noord-Brabant, The Netherlands). The measurements were done at room temperature. The XRD (and POM) measurements confirmed the crystal nature of the samples (Figure S2).

The absorbance spectra of **1** and **2** showed a maximum absorbance of around 350 nm with a corresponding high FR of several hundred thousand ◦/Tm. This is not surprising since the FR response is enhanced near resonances. However, what is surprising is that even far away from resonance strong Faraday rotation was observed. For example, for compound 1, the Verdet constant in the wavelength region 525 to 700 nm nanometers was still on the order of 50,000 to 70,000◦/Tm, while compound 2 exhibits Verdet constants over 150,000◦/Tm around 500 nm. Similar behavior has been observed for other crystalline acetylenes [5]. Both molecules had an electron acceptor group (–NO2 and –CF3) within a conjugated π-system. They showed very similar FR spectra with several peaks and valleys in the visible part of the spectrum (Figure 3A). Tolane 2 exhibited a higher Faraday response over most part of the spectrum (Figure 3B).

**Figure 3.** (**A**) and Faraday rotation spectrum (**B**) of **1** and **2**.

Our results for compounds **4** and **5**, nicely illustrate the importance of their macroscopic structure. Compound **5** was an isotropic liquid at r.t. temperature, while compound 4 was solid at room temperature. Therefore and in agreement with earlier findings, **4** exhibited a Faraday response that was orders of magnitude higher over the entire ◦/Tm, even outside regions of absorption. For 5, we measured a featureless and low-intensity Faraday spectrum that gradually decreased towards the IR region (Figure 4B). Note also that the Faraday spectrum of 4 resembles that of **1** and **2** with a maximum Verdet constant near resonance and a very feature-rich shape in the visible part of the spectrum. This seems to indicate that molecular structure does have an impact on the shape of the Faraday spectrum. Moreover, the supramolecular organization (either in a crystalline or liquid form) is a necessary requirement to observe strong FR activity.

Of all the samples, the strong increase in FR towards the UV part of the spectrum was due to the presence of the absorption band as well as the wavelength dependency of the Verdet constant (V~λ−2). We do not know the origin of the peaks and valleys, but recent work on Faraday rotation in other organic molecules suggests crediting them to the presence of spin-forbidden or hidden singlet and/or triplet states [4]. The non-substituted diphenylacetylene 3 exhibited too much birefringence and scattering to perform reliable measurements. Its UV-Vis absorption spectrum can be found in the supporting information (Figure S3). POM images can be found in the supporting information (Figure S4).

**Figure 4.** UV-Vis (**A**) and Faraday rotation spectrum (**B**) of 4 and 5.

### **4. Conclusions**

We have investigated the FR response of different phenylacethylene derivatives. It is clear that macroscopic organization within the bulk material is a key factor in obtaining a high Faraday response. The molecular structure dictates the macroscopic organization of the building units, which, in consequence, determines the FR response of the resulting bulk material. The detailed shape of the Faraday spectrum is a result of this (i.e., the molecular structure), but is in itself not sufficient to create a high Faraday response. Once again, we have to emphasize the aforementioned duality (single molecule vs. macroscopic bulk) that interferes with the design of efficient FR materials.

The tolanes that were crystalline at room temperature showed very high Verdet constants—much higher than typically observed for diamagnetic materials—in regions of the spectrum where there is no optical absorption, making them potentially useful for applications.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-6412/9/10/669/s1, Figure S1: Verdet constant measurements at 400 nm: Verdet constant of samples 1, 2, 4 and 5 turned azimuthal 0◦, 30◦, 60◦ and 90◦. The Verdet constant was measured at 400 nm. No dependence of Verdet constant on the rotation of azimuthal angle was observed; Figure S2: X-ray diffractograms: tolanes (1–3) and n-hetero-tolane derivatives 4; Figure S3: UV-Vis absorbance spectrum of the unsubstituted diphenylacetylene (3); Figure S4: Polarized optical microscopy: polarized optical microscopy images of the materials in the LC cells.

**Author Contributions:** Conceptualization, T.V. and G.H.; methodology, M.E.; validation, M.E.; formal analysis, M.E.; investigation, M.E.; Resources: LC: T.V. and M.E., Synthesis, I.L.-D. and G.H.; data curation, M.E.; writing—original draft preparation, M.E. and T.V.; writing—review and editing, M.E., T.V. and G.H.; visualization, M.E. and G.H.; supervision, M.E. and G.H. and T.V., project administration, M.E. and G.H.; funding acquisition, G.H and T.V. All authors have given approval to the final version of the manuscript.

**Funding:** This research was funded by the Spanish government (MINECO, project CTQQ2016-7557-R) and the KULeuven (C1 project).

**Acknowledgments:** The authors would like to thank B. Goderis for using his POM equipment and M. Rouffaers and L. Clinckemalie for the X-ray data analysis.

**Conflicts of Interest:** The authors declare no conflict of interest.
