Reaction Mechanism of One-step Synthesis, Optical Absorption in Solid, and Substituent Effect for Dihydrohexaazatrinaphthylene

The one-step synthesis of 1,6-dihydro-3,4,9,10,15,16-R6-1,6,7,12,13,18-hexaazatrinaphthylene (R6HATN–H2: in R=H, HATN–H2) via the reaction of hexaketocyclohexane (HKC) with 1,2-phenylenediamine (PDA) was recently reported. In this study, the reaction mechanism, the optical absorption in the solid state, and the substituent effect (R = Me and Cl) were investigated. Firstly, the reaction mechanism was examined through experiments and DFT calculations. Rather than HATN, HKC and/or its condensates are hydrogenated by PDA. Moreover, to isolate HATN–H2 under air atmosphere, crystallization is required to inhibit its oxidation to HATN. Next, HATN–H2 showed an optical absorption at 667 nm in the solution, owing to its compact triangular donor/acceptor structure. Surprisingly, in the diffuse reflection of the crystal, the similar absorption was observed at 647 nm but a different broad peak apperaed at 810–1010 nm. The latter peak is attributed to HATN–H* radicals on the crystal surface, based on electron spin resonance measurement and DFT calculation. The radicals remained very stable for a few years under air atmosphere. Finally, two derivatives, Me6HATN–H2 and Cl6HATN–H2, were synthesized via the one-step synthesis. Single-crystal analysis showed columnar stacking structures. The dihydropyradine unit with two N–H groups are arranged in the same direction for Me6HATN–H2 and HATN–H2, but in the opposite direction for Cl6HATN–H2. The Me6HATN–H2 crystal showed a broad radical peak (770–960 nm) in the diffuse reflection, but no such peak was observed for Cl6HATN–H2. This differences in the crystal structure and the reflection absorbance are probably attributed to the Cl substituent.