Solvent effects on the photoinduced [6]-electrocyclization reactions of mono-, di- and trisubstituted arylamines. Photophysical, preparative photochemistry and mechanistic investigations

IVAN ROMERO CORDERO; POSTIGO J. ALBERTO; BONESI SERGIO M

JOURNAL OF ORGANIC CHEMISTRY

AMER CHEMICAL SOC

Lugar: Washington; Año: 2023

0022-3263

The substituent and solvent effects on the spectroscopic behavior and on the photoinduced [6]-electrocyclization reaction of substituted triphenylamine derivatives have been investigated. Direct irradiation of triphenylamines bearing electron-donor substituents in different solvents have provided for the first time the substituted exo/endo carbazole derivatives from modest to good yields whereas triphenylamines bearing electron-withdrawing substituents did not provide the carbazoles due to the formation of CTCs. A corollary of the experiments purports that the photoreaction is favored with weak electron-acceptor groups in polar solvents. The lowest frequency absorption bands of the triarylamines (* electronic transitions) displayed bathochromic shifts as the solvent polarity is increased. The fluorescence emission spectra of triarylamines bearing electron-donor substituents behave as mirror images of the lowest absorption bands, showing dependence on the solvent polarity. Conversely, triarylamines bearing formyl, acetyl and nitro groups formed charge transfer complexes (CTC) behaving as good fluorescence chromophores in polar solvents. Hammett correlations on the E(0,0) energies of monosubstituted amines showed a bell-shape behavior where the  values depended on the solvent polarity. The physical quenching of the photoreaction of triarylamines have demonstrated for the first time that the triplet excited state is univocally the photo reactive state leading to exo/endo carbazole derivatives.