N. V. Biktimirova

Electronic and steric effect manifestations in the structure of 9-Azidoacridine

The crystal structure of 9-azidoacridine (9AA) was determined by X-ray structure analysis (the compound crystallizes in the rhombic system). The crystallographically independent fragment of the structure of 9AA was found to contain two molecules. Both molecules were nonplanar, and the azido group was displaced out of the acridine nucleus plane by 34.6° (molecule A) and 28.6° (molecule B). The barriers to azido group rotations about the C-N bond were calculated by the semiempirical PM3 and nonempirical DFT B3LYP quantum-chemical methods. According to the B3LYP/6-31G* calculations, the structures with the azido group situated in the acridine nucleus plane and perpendicularly to this plane are 0.21 and 1.66 kcal/mol, respectively, higher in energy than the completely optimized structure, in which the dihedral angle between the azido group and acridine nucleus planes is 32°. The PM3 method overestimates the steric strain energy of 9AA and underestimates the energy of azido group conjugation with the acridine nucleus compared with B3LYP calculations.

Photochemical properties of amino and nitro derivatives of 2-and 4-styrylquinolines and their hydrochlorides

The trans-cis and cis-trans photoisomerization reactions of a double bond in nitro and amino derivatives of 2-and 4-styrylquinolines in the neutral and protonated (hydrochloride) forms were studied. Protonation in nitrostyrylquinolines was shown to have no effect on the photoisomerization quantum yields. In aminostyrylquinolines, the photoisomerization reaction is “switched off” for the monocationic form as a result of the competitive process of intramolecular charge transfer and is “switched on” again for the dication. In the latter case, the quantum yield of trans-cis photoisomerization decreases by a factor of 2–2.5 and the quantum yield of cis-trans photoisomerization remains practically unchanged as compared to that of the neutral compound. Upon long-term irradiation of 4-(4′-nitrostyryl)quinoline, the photocyclization reaction of the cis-isomer was observed.

Synthesis and properties of azidostyrylquinolines

Isomeric azidostyrylquinolines with a 4-azidostyryl group in position 2 or 4 of the quinoline ring have been synthesized. In the neutral form the azidostyrylquinolines absorb in the near UV but the protonated species absorb in the short wavelength region of the visible spectrum. In both forms the azides are lightsensitive and decompose under the influence of light with a quantum yield close to 1.

Photochemistry of azidostyrylquinolines: 3. Study of the photolysis products by electrospray mass spectrometry

The products of the photolysis of 2-and 4-(4′-azidostyryl)quinolines and their hydrochlorides were analyzed by electrospray mass spectrometry. Corresponding amino-, nitroso-and nitrostyrylquinolines were detected as major products and azo, hydrazo, and azoxy compounds as minor products. All these compounds are formed as a result of photodissociation of the N-N2 bond and the subsequent reactions of nitrene. In addition, a number of unidentified products, as well as unsubstituted styrylquinolines, were detected. This finding indicates the photodissociation of the C-N3 bond, a process that is unconventional in the photochemistry of aromatic azides.

Photochemistry of azidostyrylquinolines: 2. Photodissociation of azido group

It was found that the quantum yield of azido group photodissociation in 2-and 4-(4′-azidostyryl)quinoline isomers is 0.7–0.9 for both neutral and cationic forms. In the protonated form, the absorption spectra are bathochromically shifted to the visible region; therefore, hydrochlorides of these compounds are sensitive to visible light in the region up to 470 nm. The primary photodissociation products of the azido group are light sensitive as well and are subject to further photolysis.

Photochemical properties of 9-azidoacridine in neutral and protonated forms

It was found that the quantum yield of 9-azidoacridine photodissociation was equal to 0.95 (in acetonitrile) and remained unchanged upon protonation. Quantum-chemical calculations on the structures of the azide and its cation in the ground (S0) and the lower single excited (S1) state were performed using semiempirical (PM3) and ab initio (HF, B3LYP) methods. The σNN* antibonding orbital at the N-N2 bond was occupied in both of the azides in the S1 state; this fact is consistent with the photochemical activity of these compounds. Because of the presence of absorption bands in the visible region of the spectrum, 9-azidoacridinium hydrochloride is sensitive to visible light, and, among all of the currently known arylazides, it is sensitive to light with the longest wavelength: the quantum yield of its photodissociation is 0.65 on irradiation with 470-nm light.