Metal‐Free Synthesis of Thermally Stable Fluorescent p‐Terphenyls by Ring Transformation of 2H‐Pyran‐2‐ones

Abstract A metal‐free approach for the synthesis of p ‐terphenyls 7 a – i and cyclic p ‐terphenyls 9 a – d and 11 a – d is described via carbanion induced ring transformation reaction of 6‐biphenyl‐2 H ‐pyran‐2‐ones 5 with malononitrile 6 , cyclohexanone 8 and 1,4‐cyclohexanedione monoethylene ketal 10 respectively. Additionally, the base‐mediated ring transformation reactions were working smoothly under mild reaction conditions and ring transformation products 7 , 9 and 11 were isolated in good to excellent yields. The synthetic approach provides the flexibility of introducing of both electron‐withdrawing and ‐donating functionalities in p ‐terphenyl architecture. Moreover, the photo physical properties of compounds 7 , 9 and 11 were analyzed using UV‐visible and Fluorescence Spectroscopy. p ‐Terphenyls 7 c showed cyan fluorescence in chloroform (λ max (em): 508 nm) and acetonitrile (λ max (em): 420 nm) while cyclic p ‐terphenyl 9 a showed blue fluorescence in chloroform and 1,4‐dioxane (λ max (em): 470 nm). Similarly, compound 11 a showed blue fluorescence in chloroform (λ max (em): 468 nm) and 1,4‐dioxane (λ max (em): 473 nm). Additionally, the thermal stability of synthesized cyclic p ‐terphenyls 11 a , 11 c and 11 d were also studied using TG and DTA techniques.