has a large dipole moment of 1.08 D. [2] By contrast, naphthalene, which only contains two fused six-membered rings, has a dipole moment of 0 D. Azulene, which was discovered and named by Piesse in 1863, has attracted considerable interest since its discovery in petroleum exploitation. [3] In 1937, Plattner and Pfau published their pioneering report on the synthesis of azulene. [4] However, their method was not practical due to its low azulene yield. In the 1950s, Ziegler and Hafner formulated a highly efficient and practical method for synthesizing azulene and its derivatives. [5] Because azulene is difficult to synthesize and has a low natural abundance, research on azulene-based molecules and materials is less advanced relative to research on its isomer. Direct functionalization of 1-and/ or 3-positions is a common and effective method of producing azulene derivatives. However, direct functionalization of other positions is much harder, e.g., bromination of 6-and 1,3-positions, [6] nucleophilic addition to obtain 6-subsituted azulene, [7] borylation at 2-position, [8] arylation at the 2-position, [9] and so on. The electron distribution in the front-line molecular orbitals (MOs) must usually be considered when functionalizing azulene. Because of resonance delocalization, the oddnumbered position of azulene, which is electron-rich, easily reacts with electrophilic compounds. Among the odd-numbered azulene varieties, 1-and 3-azulene have the highest activity. [10] By Azulene has a considerably larger dipole moment than its isomer naphthalene; it also has unique physicochemical properties, including different reactivities on five-and seven-membered rings, a dark color, a narrow gap between the highest occupied molecular orbital and lowest unoccupied molecular orbital, and stimulus response. Although azulene was discovered more than 100 years ago, the use of azulene-based derivatives can be traced back to five centuries ago. Due to its unusual structure, azulene-based molecules and materials are widely used in various fields. However, studies on azulene-based materials have long been hindered by difficulties in the synthesis. Considerably fewer studies have been conducted on azulene-based derivatives than on naphthalene-based derivatives. This perspective paper mainly introduces recent reports on the synthesis of azulene-based aromatic molecules, conjugated polymers, and coordination polymers, in addition to azulene-based frameworks. The structure-property relationship, optoelectronic applications, and energy-related applications of azulene-based derivatives are reviewed, including their use in near-infrared absorption, organic field-effect transistors, organic solar cells, and microsupercapacitors.
