“…Recent theoretical studies showed that this bathochromic shift is due to a better interaction between the HOMO of phosphole with the HOMO of thiophene, compared to that of pyridine [6b,8]. It is worth to note that the value of k max recorded for 3a (412 nm) [6a ,b] is considerably more red-shifted than that of the related tert-thiophene (355 nm) [9]. This observation is in agreement with theoretical studies predicting that heterocyclopentadienes with a low aromatic character are optimal building blocks for the synthesis of extended p-conjugated systems with low HOMO-LUMO gap [1h,6d].…”