A simple one-pot synthesis of solvatofluorescent push-pull thiophenes

Depending on the substitution patterns in the sulfanes 1, two different types of thiophenes 3 and 4 can be obtained upon cyclization with a bis-imidoyl chloride 2. Whereas thiodiglycolic acid diester yielded a thiophene derivative with the expected symmetrical structure, thiodiacetonitrile gives a derivative with an asymmetrical substitution pattern (2,3-aryl-amino/4,5-dicyano). This unexpected finding was confirmed by single-crystal X-ray analysis and possibly originates from a 1,2-rearrangement of a thietanium salt intermediate. These new push-pull thiophenes are easily accessible in a single step from simple starting materials. Unlike in the case of the derivatives 3, the substitution pattern in 4 was not previously known. A simple hydrolysis of compounds 4 offers a new and efficient route to derivatives of thiomaleic acid anhydrides 6. The 2,3-dicyano-4,5-bis(arylamino)thiophenes 4 exhibit strong solvatofluorescence with unusually large Stokes shifts. Quantum chemical calculations explain the large Stokes shifts as most probably being due to strong structural relaxation upon excitation to the S, state. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).