Abstract
Achiral nonlinear optical (NLO) chromophores 1,3-diazaazulene derivatives, 2-(4'-aminophenyl)-6-nitro-1,3-diazaa- zulene (APNA) and 2-(4'-N,N-diphenylaminophenyl)-6-nitro-1,3- diazaazulene (DPAPNA), were synthesized with high yield. Despite the moderate static first hyperpolarizabilities (β 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 com- pletely 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.
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