Azobenzene-functionalized spirobichromane-based derivatives

All spirobichromane derivatives synthesized in this work could be processed to amorphous thin films of very good optical quality. The rigid core promotes the formation of a stable amorphous phase as packing is hindered. However, the perfluorohexyl-substituted spirobichromane compound (1d) could not be processed to a thin film since it crystallized immediately after spin coating.

Therefore, no SRG or azo-NIL experiments were performed with this compound.

In the following the spirobichromane derivatives featuring unsubstituted azobenzene moieties (1a), derivatives with CF3(1b), perfluoropropyl (1c) and methoxy (1e) substituents were investigated. As in previous experiments, micrometer-scaled L-shaped patterns were imprinted in the spin coated thin films using a PDMS stamp and biphotonic exposure on the ‘vertical setup’. All spirobichromane compounds showed a good solubility in common organic solvents (here THF and cyclopentanone) and the film thickness achieved was around the optimum of 500 nm. Figure 59 shows the structure heights attained as a function of the exposure time for the different spirobichromane compounds.

Figure 59: Heights of imprinted structures in films of spirobichromane compounds 1a-1e (film thicknesses:

505 nm (1a), 625 nm (1b), 500 nm (1c) and 560 nm (1d)) as function of the exposure time. The structures were imprinted using biphotonic exposure at wavelengths of 365 nm and 455 nm at a power of 0.3 mWcm^-2 and 0.42 mWcm^-2, respectively. The chemical structures of the molecule core, as well as the azobenzene moieties with their substituents are depicted besides the plot. The inset shows the first 700 s of the imprinting process.

In this set of experiments the exposure time was chosen in a way that any compound could attain the maximum theoretical structure height of the replica of about 100 nm. Like in the case of the trisamide compounds, the derivatives featuring perfluoroalkyl chains have a lower imprinting performance in terms of speed. However, compound 1e, which has already proven to be very efficient and quick when it comes to SRG formation experiments, has performed well in azo-NIL, too.^[92,132] In The methoxy substituted compound is nearly twice as fast (τ = 64 s) as the unsubstituted spirobichromane equivalent (τ = 112 s). This finding could be explained by the significant red shift of the π-π*-absorption maxima caused by the electron donating MeO group.

This shift causes an overlap of the two absorption bands (n-π* and π-π*) and efficiently triggers isomerization cycles. Moreover, the maxima of the absorption bands is nearer to the emitting wavelengths of the two LEDs contributing to a more efficient imprinting process. Compared to the azobenzene-functionalized homopolymer 4e, which carries the same methoxy substituent as molecular glass 1e does, the imprinting speed is 5 times higher. Table 7 depicts the data of the performance of the investigated spirobichromanes in azo-NIL.

Table 7: Summarized experimental series with film thickness, maximum filling height h0, build-up constant τ and the corrected build-up constant τcorr.

Compound Film thickness / nm

Maximum filling height / nm

Build-up τ

/ s Build-up τcorr / s

1a 505 95 112 118

1b 625 99 266 269

1c 500 89 699 785

1e 560 102 65 64

Figure 60 depicts exemplary AFM images and vertical sections of the micrometer-scaled patterns imprinted in thin films of the spirobichromane compounds 1a-e using azo-NIL.

Figure 60: AFM images of imprinted micrometer-scaled L-shaped patterns in thin films of spirobichromane compounds 1a (film thickness: 505 nm, top left), 1b (film thickness: 625 nm, top right), 1c (film thickness:

500 nm, bottom left) and 1e (film thickness: 560 nm, bottom right). The structures were imprinted for 600 s (1a and 1b), 1200 s (1c) and 300 s (1e) using two LEDS at wavelengths of 365 nm and 455 nm at a power of 0.3 mWcm^-2 and 0.42 mWcm^-2, respectively. The substituent attached to the azobenzene moiety is depicted on the bottom right corner of each image. The white line indicates the area in which the vertical section was generated. Bottom: Vertical sections were generated from the AFM images above.

5 µm

Apart from dents in the film of compound 1b, the L-shaped 1 µm-sized patterns in all thin film of the spirobichromane compounds are qualitatively very good imprints. As it is the case of the 1,3,5-benzenetrisamide-based compound 2a, crystallization can be excluded as reason for the phenomenon and most probably dust particles or similar has caused the dents. In contrast to the 1,3,5-benzenetrisamide-based perfluoropropyl substituted equivalent, the spirobichromane compound 2c does not dewet from the substrate and spin coating yields a perfectly clean film, which is not negatively affected by the exposure during the imprinting process. From the vertical profiles, it can be concluded that all compounds wet the PDMS material of the stamps. The vertical profiles are similar to the ones observed in case of the trisamide compounds.

4.6.4.1 Micrometer-scale imprinting – summary

In the previous section, the influence of the resist material on the azo-NIL process, especially the imprinting speed, has been investigated. As resist materials, a photo-orientable homopolymer and 9 different azobenzene molecular glasses based on three different molecule cores were chosen.

The three molecule cores triphenylamine, 1,3,5-benzenetrisamide and spirobichromane increase in their rigidity in this order, hence, their azobenzene-functionalized derivatives become more likely to yield stable amorphous phases.

The most flexible derivatives, which are based on the triphenylamine core 3f and 3g crystallized upon exposure. As a consequence these compounds are no suitable resist materials for azo-NIL.

The azobenzene-functionalized 1,3,5-benzenetrisamide derivatives feature a more rigid core and additional secondary interactions through H-bonds. Consequently, these compounds did not crystallize and showed good imprinting results. The derivative, which carries unsubstituted azobenzene moieties 2a featured a corrected imprinting time constant τcorr. is 381 s. The derivatives with CF3 (2b) and C3F7 (2c) substituents in the para position of the azo-group feature much higher time constants of 704 s and 2155 s. Obviously, substitution with the electron-withdrawing CF3

group has a negative impact. The introduction of a stiff, fluorinated alkyl chain additionally decreased the imprinting performance. As the spirobichromane-based molecular glasses feature the most rigid core, the amorphous phase of the compounds was stable throughout the imprinting process. The missing secondary interactions have led to a significant improvement of the imprinting speed, as the unsubstituted azobenzene-functionalized derivative 1a features a τcorr of just 118 s, which is an improvement of a factor of 3 compared to the corresponding 1,3,5-benezenetrisamide derivative 2a. As in case of the trisamides, the electron-withdrawing CF3 group and a fluorinated alkyl chain decreases the imprinting speed. The electron-donating methoxy group of compound spirobichromane-based compound 3e features the best imprinting performance. Its corrected time constant is 64 s, hence better by more than a factor of 2 than the unsubstituted derivative 2a. The methoxy-substituted azobenzene moiety carrying homopolymer 4e featured a surprisingly good imprinting speed. Despite the high molecular weight of 3.75∙10⁵ g/mol, the homopolymer 4e the time constant τcorr. is just 308 s. However, compared to the corresponding low-molecular-weight compound with the same MeO substituent, this is slower by a factor of 5.

4.6.5 Imprinting of nanopatterns using azobenzene-functionalized