Amino-Substituted 1,3-Diazaazulenes: Synthesis, Structural Studies, Reactivity, and Photophysical Properties

We report a facile method for the functionalization of the 1,3-diazaazulene core at the C4, C6, and C8 positions via regioselective bromination and subsequent substitution with amine nucleophiles. Both steps can be controlled to enable selective mono-, di-, or trisubstitution at C4, C6, or C8. Crystallographic studies revealed significant distortion from planarity in the aromatic core of the highly substituted 2,4,6,8-tetra-amino-1,3-diazaazulenes, further confirmed by density functional theory (DFT). The tetra-aminosubstituted diazaazulenes displayed unusual hydrolytic reactivity in acidic media, a reaction that is rare with aminobenzenes and appears to be promoted in this case by structural distortion. The combinatorial functionalization of 1,3-diazaazulene resulted in soft-tunable optical properties observed in both solution and poly(methyl methacrylate) (PMMA) films. Strong low-energy absorptions (ε = 11,800-38,500 M^-1·cm^-1), arise from π-π* transitions, as supported by our 2^nd order approximate coupled-cluster (CC2) calculations. Consequently, the fluorescence relaxation mechanism involves a π*-π transition mixed with charge transfer, producing emission spanning from deep blue to yellow (λ_{em_max} = 428-535). We anticipate that the modular synthesis, tunable substitution patterns, and consequent modulation of both reactivity and photophysical properties renders the 1,3-diazaazulene core an intriguing molecular platform for a variety of applications.