Milan Klikar

When the solvent dictates the fate of aggregation: one chromophore, different solvent pairs, different aggregates – a journey from ACQ to AIE.

The design, synthesis, and comprehensive characterization of six novel halogenated azobenzene acrylate monomers bearing fluorine, chlorine or bromine substituents is reported. The monomers were prepared via a facile three-step synthetic route involving azo-coupling, O- alkylation and O -acylation. Apart from the steady electrochemical properties, halogen substitution proved to be a very useful tool to tune the thermal and optical properties, light-induced switching in particular. Monohalogen derivatives exhibited up to 93% E → Z photoconversion efficiency in solution, whereas the efficiency of dihalogen analogues is lower by 20%, which is ascribed to their nonplanar arrangement. Kinetics studies identified the most stable Z -isomer of the difluoro derivative ( τ _1/2 = 3.74/6.59 h at 60 °C in DCE/CDCl ₃ ). The monofluoro derivative embedded in a polystyrene film demonstrated photoresponsive behavior and remarkable stability by maintaining a macroscopically visible color change for over 90 days. These findings demonstrate that ortho -halogenation is a powerful tool for tuning the properties of photoresponsive materials for potential applications in colorimetric thermal sensing and light-controlled functional systems.

A new cyano-substituted olefin-linked COF was synthesized and utilized as a support for palladium nanoparticles. The resulting Pd@CN-COF was demonstrated as a versatile heterogeneous catalyst in cross-coupling reactions, cyanation, and allylic substitution with the productivity exceeding the commercial homogeneous precatalyst [Pd(PPh₃)₄]. The catalytic activity of Pd@CN-COF was further evaluated in synthetically valuable transformations towards industrially, agrochemically, and pharmacologically relevant products.

Fourteen novel tripodal fluorophores based on a central triphenylamine donor, electron‐rich, and polarizable divinylthiophene linker, and eight different fluorine‐based substituents have been designed and prepared via a straightforward four‐step sequence. Altering the peripheral F‐substitution has been demonstrated to largely affect their fundamental properties such as thermal robustness (210–420 °C), the LUMO energies ( E _LUMO = −2.35 to −3.11 eV), the HOMO–LUMO gap (Δ E = 2.07–2.66 eV), and the absorption/emission maxima ( λ _max ^A/E = 442–478/521–678 nm). The experimental data, corroborated by DFT calculations, further revealed twofold and tunable ICT employing both central triphenylamine and auxiliary thiophene donors, and the peripheral F‐substitution either boosting or switching‐off two‐photon absorption activity. Whereas the ─SF ₅ groups impart an exceptional cross‐section of 1930 GM, the ─COCF ₃ group may completely suppress the nonlinear optical response.

Tripodal push–pull chromophores with D–(π–A)₃ arrangement were synthesized using 1-methyl-2,4,5-triphenyl-1H-imidazole as a central electron donor, and their thermal, electrochemical, photophysical and non-linear optical properties were studied and corroborated with quantum-chemical calculations. Their facile synthesis involved Suzuki–Miyaura and Knoevenagel reactions, allowing the installation of various peripheral electron acceptors such as formyl, cyano, ester, trifluoromethyl and more complex moieties such as malonic/acetic acid derivatives, indan-1,3-dione and rhodanine. All phenyl rings appended at the central imidazole core were more or less twisted depending on the peripheral substitution. Although imidazole undergoes reversible one-electron oxidation, peripheral acceptors are reduced irreversibly in a multi-electron process. This behaviour is further seen as a variation of the LUMO, while the HOMO remained almost unaltered across the whole series. TD-DFT calculations revealed centrifugal charge transfer from the central imidazole to all C2, C4 and C5 branches occupied by the LUMO, LUMO+1 and LUMO+2. The HOMO–LUMO gap is tuneable within the range of 3.55–2.31 eV, while the longest-wavelength absorption/emission maxima were found within the broad range of 304–448/393–612 nm. Although the absorption spectra are solvent-independent, the emission depends strongly on the solvent polarity and the electron-withdrawing power of the peripheral acceptors. Extended chromophores with complex electron acceptors were investigated as two-photon absorbers, revealing relatively good cross-section values of up to 521 GM and a figure-of-merit (Φ_F × δ_2PA) of around 190 GM.

Centripetal chromophores based on the s-triazine core with tuneable properties and excellent two-photon absorption properties were used for the construction of high-quality and well-transparent emissive polyimide films.

A photochemical cross-coupling protocol towards bi(hetero)aryls has been developed. The coupling reactions were mediated by dicyanopyrazine photoredox catalyst, while a photoinduced disproportionation process has been identified as an accompanying mechanism, especially for pyrrole derivatives. The developed method allows the cross-coupling of five-membered rings such as pyrrole, imidazole, thiazole and oxazole as well as various diazines (pyridine and pyrimidine) and benzene derivatives. A plausible mechanism of the reaction has also been disclosed. The practical application and relevance of the developed method were demonstrated by constructing an atorvastatin core or by the gradual functionalization of benzo[c][1,2,5]thiadiazole. In total, twenty-one bi(hetero)aryls were prepared in yields ranging from 19 to 95%.