Photoisomerization and photocyclization of styryl derivatives of 1,10-phenanthroline

Derivatives of 1,10-phenanthroline with various substituents are actively synthesized and studied in the search for promising materials for nonlinear optics, fluorescent probes for DNA G-quadruplexes, chemotherapeutic agents for cancer treatment, and fluorescent chemosensors for ions and antibiotics. Bis-styryl derivatives of 1,10-phenanthroline (BSPs) have enlarged conjugated π-systems, which leads to a shift of the absorption spectra towards longer wavelengths. This allows the use of soft UV and even visible light to excite fluorescent labels, which is very important in biochemistry and medicine. Moreover, these compounds are believed to exhibit relative inertness to chemical reactions, with the exception of salt formation and chelation.

We investigated several BSPs with substituents at the para-position of the styryl moiety and found that these compounds are photochemically active. When BSPs solutions were exposed to light, we observed characteristic changes in the absorption spectra. Principal component analysis of the spectral matrices revealed the sequential formation of at least three new chromophores. Initially, the starting synthetically available EE isomer undergoes photoisomerization to the ZE isomer. Next, the ZE isomer undergoes an intramolecular C-N photocyclization reaction, which results in the formation of a dihydrophenanthrene derivative. The latter is oxidized to a 10-aza-10c-azonia-dibenzo[c,g]phenanthrene derivative (ADBP), which has a long-wavelength absorption band in the region of ~500 nm.

A study of the reaction products by electrospray ionization (ESI) mass spectrometry and NMR spectroscopy showed that in the case of the nitro derivative, under the influence of light, a dissociative photorearrangement of one of the nitro groups into a hydroxy group first occurs, and then the styryl fragment with the hydroxy group undergoes a photocyclization and oxidation reaction to form the ADBP derivative.

By processing the kinetic data, the quantum yields of the reactions were calculated: the photoisomerization reaction of BSPs proceeds with quantum yields of φ_tc ~ 0.1; in the C-N photocyclization reaction, the quantum yields are two orders of magnitude lower, equal to φ_pc ~ 10^-3. It is worth noting that the photoisomerization reaction is reversible, while C-N photocyclization followed by oxidation is irreversible.

The photochemical activity of phenanthroline derivatives must be considered when using their optical and complexing properties in optoelectronics, analytical chemistry, biochemistry, and medicine. The study was performed in accordance with the State task No. 124013000686-3