Regioselective Acylation of 2,3,4,6‐Tetrahydro‐1,6‐dithia[3‐ a ]azaphenalene toward Donor–Acceptor‐Type Benzophenones

2,3,4,6‐Tetrahydro‐1,6‐dithia[3‐ a ]azaphenalene (THDTAP) is a heteropolycycle possessing two reactive sites for acylation. The regioselective acylation of THDTAP was achieved at the para and meta positions with respect to the nitrogen atom under conditions of AlCl 3 (1.3 equiv, 25 °C) and AlCl 3 (4 equiv, 40 °C), respectively. A series of para ‐ and meta ‐benzophenones having a donor–acceptor‐type structure were then synthesized, in which the THDTAP and benzoyl moieties serve as donor and acceptor, respectively. Under typical McMurry reaction conditions, para ‐ and meta ‐benzophenones show different reactivity, and the electronic nature of the aryl group also plays a pivotal role. Theoretical, electrochemical, and photophysical studies indicate that the electronic structures of para ‐ and meta ‐benzophenones are distinct because the electronic communication between donor and acceptor subunits are different for the two regioisomers. The para ‐benzophenones show off/on fluorescence upon acidification/neutralization and hence might be useful as chemosensors.