Liquid‐Crystalline Squaraine Dyes with Anisotropic Absorption of Near Infrared Light

Abstract

CommuniCationconformational change of the squaraine backbone from a transoid to a cisoid planar π-scaffold (for DFT calculations for SQ 1, see Figure S1, Supporting Information) leading to several advantageous properties, namely, increase in stability, increase in fluorescence quantum yields and red-shift of the S 0 →S 1 (S 0 ; ground state, S 1 ; excited state) optical transition.[37] Squaraines with this molecular design have already found application as NIR emitters, [37] as conformation-sensitive exciton transfer systems, [38] in solar cells, [30,39,40] tumor imaging and photothermal therapy. [34,41] Interestingly, while none of the previously reported more than 20 dyes [37] showed LC properties, much to our surprise we discovered that SQ 1 bearing the most extended π-scaffold provided by two benzoquinolinium moieties ( Figure 1a) entered a mesophase above 153 °C.SQ 1 was prepared by a slight modification of our previously described synthetic methodology (Supporting Information) [37] and isolated as a dark crystalline powder. The liquid-crystallinity of SQ 1 was examined by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) measurements. POM image of SQ 1 sandwiched in between two glass plates show a Schlieren texture when cooled to 170 °C from the isotropic melt (Figure 2a). This texture indicates the formation of a nematic phase. POM and DSC measurements revealed that SQ 1 exhibits upon heating an exothermic transition corresponding to a crystal-crystal transition at 117 °C and an LC phase from 153 °C until isotropization at 182 °C (Figure 2b). On cooling process, the isotropic to nematic phase transition occurs at 182 °C and freezes into a glassy phase at 60 °C ( Figure 2b). It is noteworthy that the transitions between the isotropic state and the LC phase are of first order with low enthalpy values ( Figure 2b). This indicates a phase transition between low ordered phases (nematic-isotropic). The wide angle X-ray scattering pattern of SQ 1 at 170 °C shows two broad peaks at 23 and 4 Å, respectively ( Figure S2, Supporting Information). The first reflection in the small angle region most likely corresponds to the average distance between π-conjugated units. The broad nature of this peak suggests a low degree of order in the LC phase where the molecules organize with different distances between π-units ( Figure 1b). The second peak in the wide angle region corresponds to the diffuse halo generated by the alkyl chains. This XRD pattern is in accordance with a low ordered nematic phase. The absence of a sharp peak in the wide angle region corresponding to a particular π−π stacking distance (≈3-4 Å) discards a nematic discotic phase, despite the fact that the molecule exhibits a large π-conjugated core. The unusual shape of SQ 1 likely prevents the organization as a discotic nematic liquid crystal and promotes the formation of a calamitic-type nematic phase. Remarkably, the sample exhibits a similar XRD pattern after cooling the sample from 170 to 50 °C ( ...

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