Substituent dependent tunable fluorescence in thieno[3,2-c]pyrans

A series of thieno[3,2-c] pyrans were designed and synthesized by L-proline catalyzed reaction of 6-aryl/5,6-diaryl-4-methylthio-2H-pyrane-2-one-3-carbonitriles or 4-(methylthio)-2-oxo-5,6-dihydro-2H-benzo [h] chromene-3-carbonitrile and methylthioglycolate in good yields. These thieno[3,2-c] pyrans exhibit substituent dependent fluorescence. The 6-aryl-thieno[3,2-c] pyrans 3a-3e exhibit high fluorescence quantum yields (95%) with large Stokes shifts, whereas the 6,7-di-substituted-thieno[3,2-c] pyrans 3f-3h show poor fluorescence in solution and exhibit an aggregation-induced emission (AIE). Interestingly, fused 6,7-di-substituted-thieno[3,2-c] pyran is highly fluorescent in the solution state, which reveals that restricted intramolecular rotation is the cause for AIE in 3f-3h.