Photochemical and photophysical properties of aza[6]helicenes synthesized via EDA complex initiated visible-light-mediated photocyclization.

We have synthesized three new six-membered fluorescent azahelicenes of naphthofuroquinoline (NFQ) series via blue LED-induced and electron donor-acceptor (EDA) complex mediated photocyclization of 1-naphthyl naphtho[2,1-b]furans. The concise and effective photochemical approach to the synthesis of aza[6]helicenes (Hel) was made possible by the ability of the starting naphthofuran acyl oxime to form EDA complexes with tertiary amines. Performed synthetic studies and 1H NMR monitoring showed that acyl oximes of the naphthofuran series form a very effective EDA complex with diazabicyclo(5.4.0)undec-7-ene (DBU) with a bathochromic shift of the absorption band, which opens the possibility of using blue LED radiation for their photocyclization. The photophysical and photochemical properties of Hels in typical organic solvents were described. Both electronic absorption and fluorescence spectra were found to be acid-dependent. In the most interesting case of 8-(4-(trifluoromethyl)phenyl)benzo[f]naphtho[1',2':4,5]furo[2,3-c]quinolone in CH3CN, the fluorescence quantum yield is modulated by acidification from moderate (25%) to quite high (63%). Acidochromism in the presence of trifluoroacetic acid is caused by the formation of two successive complexes, [HelH+…CF3COO-] and [HelH+…CF3COO-…CF3COOH]. The equilibrium constants are determined. Hels are photochemically stable in chlorine-free solvents. In chloroform and dichloromethane (DCM) photoprotonation was observed, occurring from both singlet and triplet excited states. The studied helicenes promoted the formation of singlet oxygen in acetonitrile solutions with a moderate (30-40%) quantum yield.