Achiral nonlinear optical (NLO) chromophores 1,3-diazaazulene derivatives, 2-(4'-aminophenyl)-6-nitro-1,3-diazaazulene (APNA) and 2-(4'-N,N-diphenylaminophenyl)-6-nitro-1,3-diazaazulene (DPAPNA), were synthesized with high yield. Despite the moderate static first hyperpolarizabilities (beta(0)) for both APNA [(136 + 5) x 10(-30) esu] and DPAPNA [(263 + 20) x 10(-30) esu], only APNA crystal shows a powder efficiency of second harmonic generation (SHG) of 23 times that of urea. It is shown that the APNA crystallization driven cooperatively by the strong H-bonding network and the dipolar electrostatic interactions falls into the noncentrosymmetric P2(1)2(1)2(1) space group, and that the helical supramolecular assembly is solely responsible for the efficient SHG response. To the contrary, the DPAPNA crystal with centrosymmetric P-1 space group is packed with antiparalleling dimmers, and is therefore completely SHG-inactive. 1,3-Diazaazulene derivatives are suggested to be potent building blocks for SHG-active chiral crystals, which are advantageous in high thermal stability, excellent near-infrared transparency and high degree of designing flexibility. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 649-656, 2011