4 4 bis diethylamino benzophenone

Lockyball structures were produced by two-photon polymerization (2PP) of Zr-based hybrid photopolymer. The description of the material synthesis has been comprehensively reported by Ovsianikov et al. [19 (link)] and Oubaha et al. [20 (link)]. For the present experiments 0.2 wt.% of the photoinitiator (4,4'-bis(diethylamino) benzophenone (Sigma-Aldrich) was added to the material. For 2PP a Ti:sapphire laser (Femtotrain EC-800-100FS, HighQ) delivering 100 fs pulses at a repetition rate of 73 MHz at approximately 810 nm was used. The final steps were as previously reported [17 (link)]. The laser beam was focused into the material by a conventional 20 x microscope objective (NA = 0,8; Carl Zeiss). The structures were produced in a layer-by-layer fashion, with the CAD model (STL format) sliced into 1 μm thick layers. Each layer was produced by patterning in a linear scanning fashion at a distance of 0,5 μm between the neighboring scans. An average laser power of 400 mW at the scanning speed of 5 mm/s was used to induce 2PP. Approximately 2,000 lockyballs were fabricated.

The hybrid organometallic polymers were synthesised according to published protocols [21 (link),25 (link)] Briefly, the Al-based material was prepared by dissolution of aluminium isopropoxide (AIP, ≥98%, Merck, Kenilworth, NJ, USA) in toluene (ACS, ISO, Reag. Ph Eur, Merck, Kenilworth, NJ, USA). In parallel, 3-(trimethoxysilyl)propyl methacrylate (MAPTMS, 98%, Sigma-Aldrich, St. Louis, MO, USA) was hydrolysed using HCl (0.1 M, Applichem, Darmstadt, Germany). Methacrylic acid (MAA, 99%, Sigma-Aldrich, USA) was then added to the solution of aluminium isopropoxide in toluene at a 1:1 molar ratio and subsequently, hydrolysed MAPTMS was added to the mixture at a 1:1:4 AIP:MAA:MAPTMS molar ratio. Finally, 1% of photoinitiator (4,4′-bis(diethylamino)benzophenone, Sigma-Aldrich, St. Louis, MO, USA) was added to the weight of AIP, MAA and 3-(trihydroxysilyl)propyl methacrylate (the product of MAPTMS hydrolysis) and stirred, while shielding from ambient light to prevent undesired crosslinking.
The Zr-based material was prepared in an analogous manner, with molar ratios of zirconium (IV) propoxide, MAA and MAPTHS being 1:1:4 with 1% of photoinitiator by weight (excluding solvents).

All of the chemicals were used as received and were not purified any further. Pentaerythritol triacrylate (PETA), ethanol ≥ 99.8%, and 4,4′-bis(diethylamino) benzophenone (DEABP) were obtained from Sigma Aldrich (St. Louis, MO, USA).