A series of symmetrically substituted diphenylacetylene derivatives possessing electron-donating or electron-accepting character were prepared and studied with respect to their spectral and photophysical properties.

The influence of the stable 2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO) nitroxide and its six C4-substituted derivatives, as well as two C3-substituted analogues of 2,2,5,5-tetramethylpyrrolidynyl-N-oxyl (PROXYL) nitroxide on the chosen fluoroquinolone antibiotics (marbofloxacin, ciprofloxacin, danofloxacin, norfloxacin, enrofloxacin, levofloxacin and ofloxacin), has been examined in aqueous solutions by UV absorption as well as steady-state and time-resolved fluorescence spectroscopies. The mechanism of fluorescence quenching has been specified and proved to be purely dynamic (collisional) for all the studied systems, which was additionally confirmed by temperature dependence experiments. Moreover, the selected quenching parameters—that is, Stern–Volmer quenching constants and bimolecular quenching rate constants—have been determined and explained. The possibility of electron transfer was ruled out, and the quenching was found to be diffusion-limited, being a result of the increase in non-radiative processes. Furthermore, as the chosen nitroxides affected the fluorescence of fluoroquinolone antibiotics in different ways, an influence of the structure and the type of substituents in the molecules of both fluoroquinolones and stable radicals on the quenching efficiency has been determined and discussed. Finally, the impact of the solvent’s polarity on the values of bimolecular quenching rate constants has been explained. The significance of the project comes from many applications of nitroxides in chemistry, biology and industry.

Pull–push molecules containing dimethylaniline and aromatic hydrocarbon linked by an acetylene unit were studied using spectroscopic methods and DFT theoretical calculations. Spectral and photophysical properties of N,N-dimethyl-4-(naphthalen-1-ylethynyl)aniline, N,N-dimethyl-4-(naphthalen-2-ylethynyl)aniline, 4-(athracen-9-ylethynyl)-N,N-dimethylaniline were studied in 26 solvents (non-polar, polar aprotic and protic). The influence of solvent properties on the spectral and photophysical properties of the compounds studied was analyzed using empirical Riechardt's solvent polarity index as well as Catalán and Kamlet–Tatf multi-parameter solvent scales. The change of the dipole moment in the excited state was estimated based on Bilot–Kawski theory. Using the solvatochromic method, the change of hydrogen bond energy in the excited state was calculated. Theoretical calculations confirm the conclusions arising from experimental studies.

A series of symmetrically substituted diarylacetylenes and diaryl-1,3-butadiynes were prepared and studied with an emphasis on their spectral and photophysical properties. The photophysical characteristics of these compounds were studied in relation to their structures and the influence of solvent or temperature. The observed spectral and photophysical properties are explained using potential energy maps of the ground and excited states obtained from density functional theory calculations. The structure–property relationships of all of the compounds are discussed and compared with the appropriate diphenylacetylene and diphenyl-1,3-butadiyne derivatives.