Conjugation Extension and Halochromic Behaviors of S‐Fused Polycyclic Aromatic Hydrocarbons Bearing Cyclopenta[b]thiopyran Moieties

Abstract Three S‐fused polycyclic aromatic hydrocarbons (PAHs) bearing cyclopenta[ b ]thiopyran moieties have been designed and successfully synthesized. With the conjugation extension, the absorption onset of the longest PAH reaches 1110 nm. All the three S‐fused PAHs exhibit significant halochromic properties in both solution and solid states. Upon protonation, the proton is incorporated on the cyclopentadiene ring while the positive charge is localized on the thiopyrylium ring. Moreover, no significant difference can be found for the two shorter PAHs upon the protonation by different organic acids, such as trifluoroacetic acid (TFA) and trifluoromethanesulfonic acid (TfOH), while the longest PAH can be only mono‐protonated by TFA but di‐protonated by stronger TfOH. Furthermore, after protonation, the non‐emissive S‐fused PAHs exhibit strong fluorescence and can be regenerated by simply neutralization with triethylamine. The enhanced emission of mono‐protonated products stem from S 2 →S 0 transitions, which disobey the Kasha's rule.