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.