Optical characterization

Since in the following chapters (4-6), primarily the photo-isomerization of the azobenzene moieties is exploited, the knowledge absorption in thin film architectures is of crucial importance. Thus, the optical characterization of all compounds is performed via UV/Vis spectroscopy in THF solutions and thin films of the respective compounds. The concentration of the solutions was approx.

0.01 mg/ml. The thicknesses of the transparent homogenous films range from about 20 - 50 nm.

Azobenzene-functionalized spirobichromane derivatives

The UV/Vis characterization of THF solutions and thin films of the spirobichromane-based compounds 1a-1e is shown in Figure 22.

The absorption spectrum of the solution of spirobichromane derivative 1a features a π-π*-absorption band with a shoulder and a maximum at 325 nm and an n-π*-transition maximum at ca 461 nm. In comparison, the π-π*-absorption bands of the molecular glasses featuring fluoroalkyl-substituted azobenzene moieties (1b-1d) are slightly blue shifted to 319 nm, which can be explained by the electron-withdrawing characteristics of the substituents. Methoxy-substituted compound 1e is characterized by a significant red-shift of the π-π*-absorption band to a wavelength of 359 nm. Since the MeO group at the para position of the azobenzene moieties is an electron-donating group, this shift was expected. The shift results in a stronger overlap of the π-π*- with the n-π*-absorption band at about 443 nm.

The films feature similar trends and broadened peaks owing to the higher degree of disorder and the possible formation of H- and J-aggregates in the respective system compared to the solutions.

The maxima of the π-π*-absorption band of compound 1a is still at 325 nm, while the perfluoroalkyl-substituted compounds show a slight trend to further shifting to lower wavelength with increasing chain length from 315 nm (CF3 substituent) to 309 nm (perfluorohexyl substituent).

A trend, which was almost not observable in the films. This observation could be explained by worsening solubility of the molecules in THF with increasing chain length, leading to the formation of aggregates, which could, in case of H-aggregates, lead to a blue-shift of the absorption maximum.

As expected, the maximum of the n-π*-absorption peak, however, remains almost unchanged at a wavelength of ca. 455 nm for each compound as this absorption represents the non-bonding state.

Figure 22: Normalized UV/Vis-spectra of solutions of non-substituted and perfluoroalkyl-substituted azobenzene functionalized spirobichromane-based derivatives (compounds 1a-1e) in solution (top) and as films (bottom). Film thicknesses range from 20 to 45 nm. Solution concentrations: 0.01 mg/ml.

Azobenzene-functionalized 1,3,5-benzenetrisamide derivatives

Figure 23 shows the UV/Vis characterization of THF solutions and thin films of the 1,3,5-benzenetrisamide-based compounds 2a-2d.

The solutions of the 1,3,5-benzenetrisamide-based compounds feature a π-π*-absorption maximum at around 330 nm and an n-π*-transition at 455 nm. Compared to the width of the π-π*-absorption bands in the thin films of the 1,3,5-benzenetrisamide-based compounds, the width of the π-π*-absorption bands in the solutions is smaller, implying more separated molecules in solution than it is the case in the films. As a consequence of the narrower π-π*-bands, the overlap with the n-π*-transition is decreased. In contrast to the spirobichromane derivatives, the absorption maxima of the π-π*-transition of the perfluoroalkyl-substituted compounds are shifted slightly to lower wavelength compared to the unsubstituted 2a.

Compared to thin films of the spirobichromane-based compounds 1a-1e, the π-π*-absorption

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n*

Norm. absorption / a.u.

Wavelength / nm

1a (R: -H) 1b (R: -CF₃) 1c (R: -C₃F₇) 1d (R: -C₆F₁₃) 1e (R: -OCH₃)

* solution

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n

Norm. absorption / a.u.

Wavelength / nm

1a (R: -H) 1b (R: -CF₃) 1c (R: -C₃F₇) 1d (R: -C₆F₁₃) 1e (R: -OCH

3)

 solid state

which might be attributed to the molecule core. The maxima of the π-π*-absorption bands are located at a wavelength of about 322 nm, is broadened and has a distinct overlap with the n-π*-absorption band at ca. 450 nm. As in case of the spirobichromane derivatives, the substitution of the azobenzene moieties with a perfluorinated alkyl chain has no substantial influence of the absorption spectrum of the molecular glasses.

Figure 23: Normalized UV/Vis spectra of solutions of non-substituted and perfluoroalkyl-substituted azobenzene functionalized 1,3,5-benzenetrisamides-based compounds (compounds 2a-2d) in solution (top) and as films (bottom). Film thicknesses range from 30 to 50 nm. Solution concentrations: 0.01 mg/ml.

Azobenzene-functionalized triphenylamine derivatives

The UV/Vis characterization of THF solutions and thin films of the triphenylamine-based compounds 3f and 3b are depicted in Figure 24.

The solutions of the triphenylamine-based compounds feature a large absorption peak originating from the molecule core at 357 nm (as evidenced by a reference compound), which overlaps the π-π*-absorption maximum at 318 nm. The n-π-π*-absorption band is located at 442 nm. In solution, the π-π*-absorption bands is not significantly overlapping with the n-π*-absorption band. Unlike in

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n

Norm. absorption / a.u.

Wavelength / nm

2a (R: -H) 2b (R: -CF₃) 2c (R: -C₃F₇) 2d (R: -C₆F₁₃)

 solid state

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n

Norm. absorption / a.u.

Wavelength / nm

2a (R: -H) 2b (R: -CF₃) 2c (R: -C₃F₇) 2d (R: -C₆F₁₃)

 solution

substituted compounds are not significantly shifted to a lower wavelength compared to the unsubstituted compound 3f. This result is not surprising as the shift is more distinct with increasing chain length.

The π-π*-absorption bands of both triphenylamine derivatives 3f and 3g feature a shoulder. The maxima are located at a wavelength of about 366 nm, and therefore are located closer to the n-π*-absorption band at ca. 450 nm and both bands overlap. As in case of the other molecular glasses investigated so far, the substitution of the azobenzene moieties with the CF3 group has no substantial influence of the absorption spectrum of the molecular glasses.

Figure 24: Normalized UV/Vis-spectra of solutions of non-substituted and perfluoroalkyl-substituted azobenzene functionalized triphenylamine-based compounds (compounds 3f, 3g) in solution (top) and as films (bottom). Film thicknesses are 40 nm (3f) and 20 nm (3g). Solution concentrations: 0.01 mg/ml.

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solution triphenylamine



Norm. absorption / a.u.

Wavelength / nm

3a (R: -H) 3b (R: -CF

3)

n

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n

solid state

Norm. absorption / a.u.

Wavelength / nm

3a (R: -H) 3b (R: -CF₃) triphenylamine

