Amino-substituted 1,3-diazaazulenes: Synthesis, structural studies, reactivity, and photophysical properties

We report a facile method for the functionalization of 1,3-diazaazulene core at 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 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 PMMA films. Strong low-energy absorptions (ε = 11800–38500 M⁻¹·cm⁻¹), arise from π→π* transitions, as supported by our CC2 calculations. Consequently, the fluorescence relaxation mechanism involves a π*→π transition mixed with charge transfer, producing emission spanning from deep blue to yellow (λem_max = 428–535), with a quantum yield reaching 38%. We anticipate that the modular synthesis, tunable substitution patterns, and consequent modulation of both reactivity as well as photophysical properties, renders the 1,3-diazaazulene core an intriguing molecular platform for a variety of applications.