We present a series of small-molecular trisazobenzene chromophores, including, for instance, 1,3,5-tris{[4-[4-[(4-cyanophenyl)azo]phenoxy]butyryl]amino}benzene that feature a remarkably stable light-induced orientation in initially amorphous thin-film architectures. It is demonstrated that for optimal performance it is critical to design chemical structures that allow formation of both an amorphous and a liquid-crystalline phase. In the present approach, the liquid-crystalline feature was introduced by inserting decoupling spacers between a trisfunctionalized benzene core and the three azobenzene moieties, as well as adding polar end groups to the latter. To compensate for the deleterious reduction of the glass transition temperature associated with the spacers in the compounds, polar units were incorporated between the benzene core and the side groups. Intriguingly, the molecular glasses that feature a latent liquid-crystalline phase display a remarkable "postdevelopment", i.e., an increase of the amplitude of refractive index modulation in holographic experiments after writing of optical gratings is arrested, exceeding 20% for the previously mentioned derivative. Thus, these nonpolymeric, azobenzene-containing compounds presented in this work appear to be attractive candidates for fabrication of stable holographic volume gratings.

This paper presents a comprehensive study of the formation of surface relief gratings in a series of photoresponsive molecular glasses. Holographic experiments were performed on films of the azobenzene-containing molecular glasses. Seven relevant polarization configurations of the writing beams were systematically applied, and simultaneously the diffraction efficiency was monitored during the process of inscription. The temporal evolution of the diffraction efficiency can be precisely simulated with a model which takes both the surface relief and the phase grating in the volume into account. From the measured diffraction efficiencies, the modulation heights can be directly calculated and they were independently confirmed by atomic force microscopy. We found that all experimental results can be explained with the gradient force model, and we suggest that the grating heights generated with different writing polarizations can be ascribed to the varying strengths of the gradient force. For materials with different substituents at the azobenzene chromophore, the optical susceptibility at the writing laser wavelength and, therefore, the gradient force varies. By applying the most efficient polarization configuration in combination with the best material, we were able to reach modulation heights of up to 600 nm, which is a factor of 2 higher than modulations usually reported for azobenzene-containing polymers.

This paper reports on the synthesis and properties of a new series of photochromic molecular glasses and their structure–property relations with respect to a controlled and efficient formation of surface relief nanostructures. The aim of the paper is to establish a correlation between molecular structure, optical susceptibility, and the achievable surface relief heights. The molecular glasses consist of a triphenylamine core and three azobenzene side groups attached via an ester linkage. Structural variations are performed with respect to the substitution at the azobenzene moiety in order to promote a formation of a stable amorphous phase and to tune absorption properties and molecular dynamics. Surface relief gratings (SRGs) and complex surface patterns can easily be inscribed via holographic techniques. The modulation heights are determined with an equation adapted from the theory for thin gratings, and the values are confirmed with AFM measurements. Temperature‐dependent holographic measurements allow for monitoring of SRG build‐up and decay and the stability at elevated temperatures, as well as determination of the glass transition temperature. SRG modulation heights of above 600 nm are achieved. These are the highest values reported for molecular glasses to date. The surface patterns of the molecular glasses are stable enough to be copied in a replica molding process. It is demonstrated that the replica can be used to transfer the surface pattern onto a common thermoplastic polymer.