Mesityl‐Substituted Azuleno[1′,2′:4,5]pyrrolo[2,3‐ b ]quinoxalines: Synthesis, Functionalization, Photophysical, and Electrochemical Properties

Azulene‐fused heterocycles have attracted attention because they induce an effective donor–acceptor interaction, leading to a smaller highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap that exhibits characteristic photophysical and electronic properties. The synthesis of mesitylene‐substituted azuleno[1′,2′:4,5]pyrrolo[2,3‐ b ]quinoxaline was established in high yield by the condensation of azulene‐analog of isatin derivative with 1,2‐phenylenediamine. Functionalization of the azuleno[1′,2′:4,5]pyrrolo[2,3‐ b ]quinoxaline framework was successfully accomplished by N ‐substitution at the pyrrole N ‐atom and electrophilic substitution at the five‐membered ring site of the azulene moiety. Introduction of electron‐withdrawing tetracyanobutadiene (TCBD) units at both the pyrrole N ‐atom and the azulene ring was accomplished via formal [2 + 2] cycloaddition–retroelectrocyclization (CA–RE) with tetracyanoethylene. The electronic characteristics of the resulting azuleno[1′,2′:4,5]pyrrolo[2,3‐ b ]quinoxalines were elucidated by spectroscopic and voltammetric analyses, supported by density functional theory calculations.