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Although organic compounds account for more than 99% of currently approved clinical drugs, the established clinical use of cisplatin in cancer or auranofin in rheumatoid arthritis have paved the way to several research initiatives to identify metal-based drugs for a wide range of human diseases. Nitrogen and sulfur donor ligands, characterized by different binding motifs, have been the subject in recent years of one of the main research areas in coordination chemistry. Among the nitrogen/sulfur compounds, very little is known about thiocarbohydrazones (TCH), the higher homologues of the well-known thiosemicarbazones (TSC), and their metal complexes. The extra hydrazine moiety provides the ligands of variable metal binding modes, structural diversity and promising biological implications. The interesting coordination chemistry of TCH has mainly been focused on symmetric derivatives, which are relatively simple to synthesize while few examples of asymmetric ligands have been reported. This informative review on TCHs and their metal complexes will be helpful for improving the design of metal-based pharmaceuticals for applications ranging from anticancer to antinfective therapy.

State-of-the-art femtosecond spectroscopies and quantum-chemical methods were used to investigate the excited-state dynamics of D−π–A+ (C1) and D−π–A+–π–D (C2) methylpyridinium (acceptor unit, A) derivatives bearing dibutylamino groups as strong electron donors (D) and bithiophenes as highly effective π-rich spacers. The absorption spectra of C1 and C2 are broad and shifted to the red side of the visible spectral range. A significant negative solvatochromism was observed for the absorption bands of the investigated salts with increasing solvent polarity that was rationalized in terms of the change in electron density upon excitation. The absorption spectra of C2 are red-shifted with respect to those of C1, whereas the emission bands of the two compounds overlap, suggesting a localization of the excitation on just one branch of the quadrupolar compound, which becomes the fluorescent portion. This is in agreement with our quantum-mechanical calculations, which predict that the symmetry of C2 is broken in the relaxed S1 geometry. Excited-state symmetry breaking was observed in all of the investigated solvents regardless of their polarity. Femtosecond transient absorption and fluorescence up-conversion measurements revealed that the excited-state dynamics of C1 is essentially dominated by solvent relaxation, whereas in the case of C2, two distinct excited singlet states were detected in polar solvents, where an intramolecular charge-transfer (ICT) state is efficiently produced. The main photoinduced decay pathway of both compounds was found to be internal conversion in all of the investigated media. High two-photon-absorption cross sections of 500 and 1400 GM for C1 and C2, respectively, were obtained by means of femtosecond-resolved two-photon excited fluorescence measurements, thus demonstrating the enhancement in the nonlinear optical properties of the quadrupolar compound over its dipolar counterpart, in agreement with the more efficient ICT observed in the case of C2.

No abstract

We report here a successful attempt to test a solvatochromic method to estimate the hyperpolarizability (β) of cationic push–pull chromophores. This represents a simple method, alternative to the sophisticated spectroscopic techniques often employed, which can be easily and quickly applied through equipment commonly available in a typical chemistry laboratory. The case study taken into consideration consists of nine donor−π–acceptor derivatives exhibiting the rarely observed negative solvatochromism. In these dyes the electron acceptors are positively charged methylpyridinium or quinolinium rings and the electron donors are electron rich thiophene rings eventually coupled with the strongly electron donating dibuthylamino group or piperidine. The obtained β values are enhanced in this molecular series upon increasing molecular dimensionality and conjugation as well as by increasing the donor/acceptor strength. The highest hyperpolarizability is estimated for the chromophore bearing methyl quinolinum and piperidine where the most efficient photoinduced intramolecular charge transfer is also revealed by means of state of the art femtosecond transient absorption measurements.