Polycyclic Aromatic N-Ethoxycarbonyl Thioamide S-Oxides and Their Triflic Acid Promoted Cyclization to Fluorescent Thiophene Imine-Fused Arenes

We present the synthesis of a series of polycyclic aromatic-N-ethoxycarbonylthioamide S-oxides and their triflic acid-promoted cyclization to thiophene imine-fused arenes having 2H-naphtho[1,8-bc]thiophen-2-imine, 3H-pyreno[10,1-bc]thiophen-3-imine, 4H-pyreno[1,10-bc]thiophen-4-imine, 3H-pyreno[10,1-bc]thiophen-3-imine, 4H-pyreno[1,10-bc]thiophen-3-imine, 3H-pyreno[10,1-bc]thiophen-3-imine, and 4H-peryleno[3,4-bc]thiophen-4-imine cores. The proposed reaction mechanism involves the intermediacy of a novel type of electrophilic sulfur species, namely protonated iminosulfenic acid or iminosulfenium cations. These species may attack the peri- or ipso-position of the arene, leading in some cases to regioisomeric products. The reaction affords in high yields novel polycyclic fluorophores emitting in the range of 500–606 nm with quantum yields of 0.025–0.64. Comparison with the parent arenes reveals that the fused iminothiophene moiety brings about significant bathochromic shifts of the absorption and emission bands.