Synthesis, crystal structure and photophysical properties of 1,4-bis(1,3-diazaazulen-2-yl)benzene: a new [pi] building block

A dimerized 1,3-diazaazulene derivative, namely 1,4-bis(1,3-diazaazulen-2-yl)benzene [or 2,2'-(1,4-phenylene)bis(1,3-diazaazulene)], C22H14N4, (I), has been synthesized successfully through the condensation reaction between 2-methoxytropone and benzene-1,4-dicarboximidamide hydrochloride, and was characterized by 1H NMR and 13C NMR spectroscopies, and ESI-MS. X-ray diffraction analysis reveals that (I) has a nearly planar structure with good π-electron delocalization, indicating that it might serve as a π building block. The crystal belongs to the monoclinic system. One-dimensional chains were formed along the a axis through π-π interactions and adjacent chains are stabilized by C--H...N interactions, forming a three-dimensional architecture. The solid emission of (I) in the crystalline form exhibited a 170nm red shift compared with that in the solution state. The observed optical bandgap for (I) is 3.22eV and a cyclic voltammetry experiment confirmed the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The calculated bandgap for (I) is 3.37eV, which is very close to the experimental result. In addition, the polarizability and hyperpolarizability of (I) were appraised for its further application in second-order nonlinear optical materials.