The Quest of Excited‐State Intramolecular Proton Transfer via Eight‐Membered Ring π‐Conjugated Hydrogen Bonding System

Searching for eight‐membered ring π‐conjugated hydrogen bonding (8‐MR H‐bonding) systems with excited‐state intramolecular proton transfer (ESIPT) property is seminal and synthetically challenging. In this work, a series of π‐conjugated molecules (8‐HB‐1, 8‐HB‐L1 and 8‐HB‐2) potentially possessing 8‐MR H‐bonding are strategically designed, synthesized and characterized. The configurations of these three potential molecules are checked by their X‐ray structures, among which 8‐HB‐L1 (a structurally locked 8‐HB‐1 core chromophore) is proved to be an 8‐MR H‐bonding system, whereas 8‐HB‐1 and 8‐HB‐2 are too sterically hindered to form the 8‐MR intramolecular H‐bond. The ESIPT property of 8‐HB‐L1 is confirmed by the dual fluorescence consisting of normal and proton‐transfer tautomer emissions. The insight into the ESIPT process of 8‐HB‐L1 is provided by femtosecond fluorescence upconversion measurements together with computational simulation. The results demonstrate for the first time a successful synthetic route to attain the 8‐MR H‐bonding molecule 8‐HB‐L1 with ESIPT property. Mr. Lock: In searching for seminal eight‐membered ring π‐conjugated hydrogen bonding (8‐MR H‐bonding) systems, we strategically designed and synthesized a series of compounds (8‐HB‐1, 8‐HB‐L1, 8‐HB‐2) potentially possessing 8‐MR H‐bonding, among which the structurally locked 8‐HB‐L1 is affirmed to be an authentic 8‐MR H‐bonding system with prominent excited‐state intramolecular proton transfer.