Photoisomerization of a maleonitrile-type salen Schiff base and its application in fine-tuning infinite coordination polymers

Controlling Two-Photon Action Cross Section by Changing a Single Heteroatom Position in Fluorescent Dyes

The optimization of nonlinear optical properties for "real-life" applications remains a key challenge for both experimental and theoretical approaches. In particular, for two-photon processes, maximizing the two-photon action cross section (TPACS), the figure of merit for two-photon bioimaging spectroscopy, requires simultaneously controlling all its components. In the present Letter, a series of difluoroborates presenting various heterocyclic rings as an electron acceptor have been synthesized and their absorption, fluorescence, photoisomerization, and two-photon absorption features have been analyzed using both experimental and theoretical approaches. Our results demonstrate that the TPACS values can be fine-tuned by changing the position of a single heteroatom, which alters the fluorescence quantum yields without changing the intrinsic two-photon absorption cross section. This approach offers a new strategy for optimizing TPACS.

Spectroscopic studies of the intramolecular hydrogen bonding in o-hydroxy Schiff bases, derived from diaminomaleonitrile, and their deprotonation reaction products

The structural study of five Schiff bases derived from diaminomaleonitrile (DAMN) and 2-hydroxy carbonyl compounds was performed using ¹H, ¹³C and ¹⁵N NMR methods in solution and in the solid state as well. ATR-FTIR and X-Ray spectroscopies were used for confirmation of the results obtained by NMR method. The imine obtained from DAMN and benzaldehyde was synthesized as a model compound which lacks intramolecular hydrogen bond. Deprotonation of all synthesized compounds was done by treating with tetramethylguanidine (TMG). NMR data revealed that salicylidene Schiff bases in DMSO solution exist as OH forms without intramolecular hydrogen bonds and independent on the substituents in aromatic ring. In the case of 2-hydroxy naphthyl derivative, the OH proton is engaged into weak intramolecular hydrogen bond. Two of imines (salDAMN and 5-BrsalDAMN) exist in DMSO solution as equilibrium mixtures of two isomers (A and B). The structures of equilibrium mixture in the solid state have been studied by NMR, ATR-FTIR and X-Ray methods. The deprotonation of three studied compounds (salDAMN, 5-BrsalDAMN, and 5-CH₃salDAMN) proceeded in two different ways: deprotonation of oxygen atom (X form) or of nitrogen atom of free primary amine group of DAMN moiety (Y form). For 5-NO₂salDAMN and naphDAMN only one form (X) was observed.

Synthesis, characterization, redox, and Hg2+ optical ion sensing properties of ferrocenyl-containing maleo- and fumaronitrile derivatives

Mono-((Z)-2-amino-3-((E)-ferrocenylideneamino)butenedinitrile) (1) and bis-((E)-2,3-bis-((E)-ferrocenylideneamino)butenedinitrile) (2) substituted Schiff bases obtained by the reaction between ferrocenecarbaldehyde and diaminomaleonitrile were prepared and characterized by 1H and 13C NMR, IR, and UV-vis-NIR spectroscopy methods, elemental analysis, and X-ray crystallography. Density functional theory and time-dependent density functional theory were used to elucidate the electronic structure and the origin of observed excited states in both compounds. Electrochemical techniques were employed to investigate the possibility for the formation of a mixed-valence state of the iron centers in 2+, which are connected by a conjugated π-system. Two reversible oxidation potentials were observed in cyclic voltammetry, and spectroelectrochemical experiments revealed the presence of a low-energy intervalence charge transfer band in 2+, corresponding to class II of the mixed-valence systems. Both complexes were tested for cation-recognition properties using a variety of main group and transition-metal cations. It was shown that complex 1 can be used as a selective optical sensor in recognition of Hg2+ ions. It was also shown by NMR spectroscopy and X-ray crystallography that one of the final reaction products between Hg2+ ions and complex 1 is 2-ferrocenyl-4,5-dicyanoimidazole (3).